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Environ Eng Res > Volume 28(3); 2023 > Article
Sosa, Laines, García, Hernández, Zappi, and Espinosa de los Monteros: Activated Carbon: A Review of Residual Precursors, Synthesis Processes, Characterization Techniques, and Applications in the Improvement of Biogas


The energy growing demand and the international environmental policies contribute to the use of renewable energy sources. Among these sources, biogas has acquired great relevance due to its energetic similarity to fuels such as liquefied petroleum gas (LPG) and natural gas (NG). However, biogas needs to be upgraded by removing CO2 and trace gases to obtain biomethane (>85% CH4). This review identifies and classifies seven techniques used in biogas upgrading, reported in academic and scientific publications. A 13-year review period (2008–2021) was considered. Lineal regression was used to analyze the publications number per year. Membranes use represents the largest proportion of publications (33%), while activated carbon (AC) reaches only 22%. However, the use and application of AC obtained the best trend as a publication topic, with a maximum value of R2 = 0.7882. This review documents publish works on obtaining and applying AC in biogas enrichment processes. It includes a review of the characteristics and generation rates of various residual lignocellulosic materials used in the synthesis of AC, the synthesis processes, the characterization techniques, and the final adsorption capacities.

1. Introduction

The residues use such as activated carbon (AC) precursors is an environmentally sustainable and economically viable method. The carbon atom (C) has an electronic configuration 1s2 2s2 2p2 that gives it the possibility of bonding with itself, with many other elements and facilitating allotropic forms. There are three types of carbon’s orbital hybridization, sp3, sp2 and sp, equivalent to three basic structures as diamond, graphite and carbyne. Carbon has different presentations with multiple applications, including graphite, graphene, carbon nanotubes, carbon fibers, carbon black and AC. Graphite has a laminated carbon structure, as condensed rings which are joined by low-energy metal-type bonds. AC’s have a three-dimensional structure of carbon atoms in flat sheets, but with no crystallographic order in the third dimension, as there is cross-linking of the sheets, causing a greater surface area [1], the spaces between each sheet are called “pores” [2]. The microcrystalline structure of AC differs slightly from that of graphite since it has a spacing of 0.335 nm and in AC it varies from 0.34 – 0.35 nm [3]. AC is generally described as an amorphous form of carbon processed to have a high surface area, high internal porosity, and certain chemical properties on its surface, such as a wide range of oxygenated functional groups that allow it to absorb or react chemically with specific elements [46]. Actually AC has high demand due to its multiple applications, such as the removal of organic contaminants, ions, heavy metals and dyes in both liquid and gas phases, as well as in the manufacture of high-yield capacitors and adsorption refrigeration systems [78]. González-García et al. [5] publish the carbon weight percentage of most common applications in the removal of pollutants such as heavy metals (59.95), dyes (8.47), organic compounds (7.41), carbon dioxide (5.82), ammonia (4.16), methane (3.66), hydrogen sulfide (1.27), nitrogen dioxide (0.61); also its use in special applications such as catalysis (6.77), capacitors (1.48), sensors (0.50), lithium batteries (0.34), pharmacists (0.30), all these based on a review of 193,250 publications reported from 1985 to 2016.
The AC world market in 2018 was equivalent to 3,100 million dollars, with a projected growth of 4.2% by 2022, with a global demand of 457.60 thousand tons [9]. In Mexico there is a monetary deficit in AC trade, since 8,687 t are exported, equivalent to 32.98 million dollars and represents 58% of national coal exports; and 14,374 t are imported at a cost of 79.32 million dollars [10], 2.4 times more than the export cost. However, the AC can be obtained in mineral form or from virgin or residual lignocellulosic materials through a synthesis process. In this context, the use of agricultural and timber waste, including agroforestry waste, is one clean and efficient option for producing plant-based carbon. Materials with high carbon content such as lignite, peat, and other biomass resources are suitable for obtaining AC [7, 1115] Furthermore, the use of agricultural wastes as precursors to other processes is considered a renewable and relatively affordable method [16], especially considering that agricultural production in the 1969–2015 period has tripled. In 2018, the global production of cereals was 2,609 million tons, yet, by 2050, it is estimated that 50% more food, forage, and biofuel will be required to sustain the growing population [17]. In addition, there are great losses in crop production during both production and processing, and these wastes could potentially be destined for other uses. In Africa, for example, it is estimated that 15% of cereals are lost during post-harvest operations [18]. During the different stages of agricultural production (harvest, post-harvest, washing, husking, pressing or extraction and packaging), different types of waste such as stems, branches, husks, fibers, seeds and bagasse are generated as a result of quality control or food safety. In Mexico, an increase of 27.8% in agricultural production is expected for 2030, for which the National Agricultural Plan has recommended the growing of 38 strategic crops along with 14 basic crops and 24 additional crops with market potential [19]. Notably, the expected increase in agricultural production in Mexico will also translate into an increase in agricultural and agro-industrial wastes. It is important to consider treatment options along with the possibility that these wastes be added value assigned for their potential applications, which could represent an extra income source for producers or processors. In addition, the abundance and availability of agricultural byproducts and some industrial and agro-industrial wastes mean that these could potentially be a good source of raw material in the AC production [13, 2021]. However, the AC properties depend on numerous factors, such as the precursors type and characteristics, and the synthesis method and activation processes. Other factors influencing the precursors selection are purity, easy activation, minimum and economic preparation and pretreatment requirements, easy supply, and low cost.
Moreover, at the global level, energy demand grew by 2.2% compared to the average in the last 10 years (1.7%), with natural gas (NG) being the energy source with the highest growth (3% annual) due to the coal substitution to NG in China, the Middle East, and Europe [22]. The United States [23] projects that the demand for NG will increase to 5,349 billion cubic meters (3,752 MMcm) by 2040. Since 2016, following the Paris Agreement, the fossil fuels substitution for cleaner energy sources, the fuel efficiency improvement and the energy rational use were outlined as clear strategies for reducing greenhouse gas (GG) emissions and mitigating climate change [24]. NG is considered one of the least contaminating and most efficient primary energy sources. These characteristics have led to increase its use all over the world; however, it is still a non-renewable resource, that’s why some countries are in process of substituting this gas for renewable sources with similar potential in permanent and environmentally friendly production systems. Jury et al. [25] and Qian et al. [26] mention that, based on life-cycle analysis, it is difficult for biogas to compete with NG, although it is competitive in terms of its effect on climate change on natural resources and its reduction in the demand for fossil energy. Kacem, Pellerano, and Delebarre [27] mention that the biogas produced by the anaerobic digestion of manure, agricultural residues, wastewater, and landfills has the potential to replace up to 50% of the NG demand in the United Kingdom. However, to achieve this goal, systems or processes for the biogas enrichment are necessary to reduce up to 94% v/v the concentration of carbon dioxide (CO2) and thereby enhance the gas energy value to obtain a fuel similar to NG but with the benefit of being produced from renewable sources. Yousef et al. [28] mention that the biogas cleaning consists in the gases elimination and acid impurities, while the enrichment process is based on the separation of CO2.
This review documents and multiple organize publications related to the generation and chemical characteristics of lignocellulosic residues used as CA precursors. The generation and chemical characteristics of lignocellulosic residues used as CA precursors are documented. The synthesis processes were classified into physical and chemical processes. The types of processes and the final characteristics obtained by kind of precursor were summarized. The techniques generally used in the characterization of AC are described in a simplified way. Physicochemical and energetic characteristics of biogas are compared with conventional fuels. The conditions of the biogas enrichment techniques at an industrial level, the percentages of improvement and their final applications are described and compared. Techniques reported in academic and scientific publications related to biogas cleaning and enrichment were documented and organized. It was possible to observe the growth and weighting trends for each technique. Finally, the main reports related to the improvement of biogas, through the application of CA, are summarized.

2. Agricultural and Agro-industrial Waste

The plant biomass is generally composed of cellulose, hemicellulose, and lignin. Cellulose has the highest value in the market because of its multiple applications, such as in paper production. It is a linear polymer formed by glucose molecules [29]. Meanwhile, hemicellulose contains heteropolymers composed of xylose and mannose monomers. Finally, lignin is a complex, three-dimensional structure formed by the polymerization of phenylpropane with hydroxyl and methoxy substitutes. Someone widely used cellulose and hemicellulose in the production of paper, sugars, and biofuels [3032]. Cellulose and many of its products are considered innocuous given their capacity to reincorporate into the environment by the action of decomposing microorganisms. However, large volumes of cellulosic materials currently generated in agricultural or industrial activities are wasted because their economic value is underestimated [33]. In addition, lignin is an abundant, low-cost resource in the market that is used as a renewable precursor of different added-value products, although less than 2% of the lignin produced annually is re-utilized.
Lignin is one of the main plants components and represents 15% to 40% of dry weight [34, 35]. Lignin provides rigidity and cell wall cohesion, confers impermeability to water, and forms a physical and chemical barrier against microbial attack [36, 37]. Currently, the most common application of lignocellulosic materials in agro-industrial processes is combustion for energy production. Although its use as fuel can save costs, it is necessary to add value. In this regard, it is important to advance in technology to increase the lignin potential as a versatile raw material [34]. Research has been performed on the conversion of lignin in synthetic resins and surfactants, although it would be interesting to explore its use in the fine chemical products production as a substitute for different substances usually obtained from petroleum, which would help to reduce the fossil fuels consumption of and decrease environmental contamination [34]. It is important in further studies to explore the lignin different applications, its fundamental chemistry and physicochemical transformation during different processes [35]. Lignin has been identified as the main component of the lignocellulosic biomass responsible for adsorption [38].
Agricultural or agro-industrial wastes are commonly used to generate heat and vapor as well as synthetic fuels, oils, methane (CH4), ethanol, biodiesel, methanol, and vegetal carbon. The waste conversion into other materials is a general strategy to support or to improve the production processes sustainability [3941]. Several common agricultural crops and their associated wastes are listed in Table 1.
It is important to know waste yields or rates generated per production unit to quantify or estimate the potential use of residual biomass. Ajewole et al. [46] report that, from a metric ton of recently harvested cocoa pods, 240 kg of dry beans and 506 kg of shells or dry pods are obtained. Which would represent a 24% product yield per ton and around 50% waste per ton. White et al. [47] reports residual biomass generation rate of 496.9 kg ha−1 in sugarcane crops; extrapolating to 395 million t of residues, with respect to the global cultivation area. Other authors report that the amount of residual biomass in sugarcane cultivation represents 15 to 25% of production [48, 49]. In the case of plantain cultivation, a 220 t ha−1 waste generation is estimated [50]. Subramaniam et al. [51] calculate a residual biomass generation of 13.55 t ha−1 a−1 in the oil palm industry. The rice straw generation is equivalent to 45% (227.78 million tons) of the total rice production in the main producing countries [52]. In soybean processing (milk and tofu production) 1.2 kg are produced for each kilogram of product [53]. In the production of 1 kg of coffee generates 1 kg of residual coffee husk [54].
Some of the factors that enable the crop waste use feasibility are the yield, the moisture content, and easy degradation (for post-harvest crops). If these factors are discouraging (low quantity, high moisture content, and easy degradation), wastes are usually used as food for animals, burned (with or without energy recovery), or re-used in crop fields. Lower quantities wastes generated from agro-industrial processes are often used in fuel boilers. In Mexico, Secretary of Energy of Mexico (SENER) [55] reported that, 79 bioenergy plants that utilize agro-industrial and solid urban wastes were registered with an installed electrical generation capacity of 996 MW, and the electrical energy production was 1,884 GWh, 81% with sugar cane residues use and 13% with biogas at the end of 2017.

2.1. Composition of Agricultural and Agro-industrial Waste: Precursors of Activated Carbon

The lignocellulosic composition plays an important role in AC synthesis, mainly the lignin content. Since the complexity and high resistance make biomasses with high lignin content attractive for use in AC synthesis. Lignin is mainly composed of three elements: carbon (C), hydrogen (H) and oxygen (O) [34], although its complexity lies in the phenylpropane units, mainly syringyl, guaiacyl and p-hydroxy phenol [56], being complex amorphous polymers with three-dimensional spatial structure [57, 58], this allows lignocellulosic materials to be resistant to chemical and biological conversions [59]. Materials with high lignin content are more attractive for use in AC synthesis.
The composition or compounds present in lignocellulosic materials can be analyzed to evaluate their possibility of use or synthesis (Table 2). Two types of analysis are necessary for precursors: proximate analyses, such as humidity content, organic matter, ash [60], and fixed carbon (100% minus the sum of the previous contents); and ultimate analyses, including the elemental content of C, H, O, nitrogen (N) and sulphur (S). The proximate analyses enable the AC yield from synthesis processes to be calculated considering that a high amount of volatile material indicates carbonization losses, high content of fixed carbon indicates good yield, and a high moisture content indicates the need for drying processes due to the low yield of usable dry matter. For the elemental analysis, the use of automated equipment is currently the most employment technology for these analyses, although equipment such as LECO® elemental analyzer (CHNS 932) still requires the oxygen content to be calculated by subtracting the percentages of ash, C, H, N, and S, from 100%. For instance, the low presence of O in the carbonized material indicates the reduction of oxygenated functional groups, which has an impact in more alkaline materials or those with greater hydrophobicity [61, 62].
It is important to know the methods and the conditions under which these analyzes are carried out. For example, the proximal analysis can take as reference the ASTM D 2974 standard. It is important to report the specific use of times, temperatures, quantities, required pretreatment and type of material (soils, organic matter, organic waste, etc.). For the elemental analysis method, ASTM-D 5291 serves as a reference with the use of an elemental analyzer CHN-LECO 800, based on combustion in a furnace. For lignocellulosic determination, ASTM D 1106 is used to determine acid-insoluble lignin. The determination of cellulose is made based on its solubility in a sodium hydroxide (ASTM D 1696). however, prior to these lignocellulosic tests, it is necessary to carry out two extractive tests, one of them using the Soxhlet extraction apparatus (ASTM D 1107) and a mixture of solvents such as ethanol/toluene or acetone/hexane (2:1 ratio). The second extraction with hot water is extremely important (ASTM D 1110). The extraction stages are used for the removal of dyes, sugars, gums, tannins, and other substances soluble in both water and solvent. The determination of hemicellulose is generally carried out by difference of the sum of the concentration percentages of lignin and cellulose with respect to 100%. The lignin result must be adjusted with the ash content.
One relevant technique for the precursors characterization is thermogravimetric analysis (TGA). TGA is a thermal analysis technique, in which the change in the amount of mass of material is analyzed as a function of temperature and/or time, under a controlled atmosphere. The procedure is to expose small samples (less than 5 mg), to temperature increase between 5 to 20 °C min−1, until temperatures greater than 1000 °C or more are reach, using inert gases (N2, Ar, He), oxidants (air, O2) or reducers, meanwhile the equipment balance continuously determines the sample mass [87]. Miranda et al. [88] report the use of TGA on orange peels under heating velocities of 1, 5, 10, 20, and 40 °C min−1, range temperatures 25 to 1000 °C, and N2 flow of 60 ml min−1.

3. Synthesis of AC

Promdee et al. [89] highlighted that AC is produced in three stages: a) pre-activation, which involves the determination of the required quality and correct size; b) activation, or the process in which precursors are converted to AC; and c) post-activation, which consists of the output parameters, quality control and final AC characteristics. There are two general methods for the preparation of AC: physical and chemical processes. In physical treatments, the precursors are first carbonized and then activated under inert gas atmospheres such as water vapor (H2O), CO2, or nitrogenated compounds (N2 or NH3). In contrast, in chemical treatments, the precursors are first impregnated by a reagent and then submitted to a heating process in an inert atmosphere [90]. In particular, the reactive agents promote the formation and extension of cavities and reticulated structures [91, 92]. For this, some of the most utilized chemical agents are H3PO4, ZnCl2, KOH, NaOH, H2SO4, CaO, and HF.

3.1. Pyrolysis

Pyrolysis is a thermochemical conversion method in which complex hydrocarbons are reduced to added-value products by heating in the presence of low levels or absence of oxygen. In this process, the biomass is heated from 300 to 800 °C, and three products are obtained: a carbonized solid (biochar), a liquid phase product, and a non-condensable gas-phase product, such as CO, CO2, CH4 or H2 [93, 94]. Pyrolysis can be classified into four types (Table 3) according to temperature, residence time, and heating velocity.
During thermal decomposition, biomass components undergo reactions such as dehydration, crosslinking, repolymerization and depolymerization, fragmentation, rearrangement, condensation, and carbonization [9698]. Slow pyrolysis involves longer residence times and slower heating rates, producing similar composition products, and is carried out at near-atmospheric pressure conditions [96]. Slow pyrolysis is the most widely method used to obtain AC since it generates the largest amount of charred material. New methods using microwaves, for example, can be used to generate hot spots in carbon particles and bulk solids, representing a promising method to obtain AC under conditions from 0.91 to 2.45 GHz [99]. However, the carbonaceous product (biochar) obtained from the biomass thermal conversion under poor or zero oxygen conditions has a less efficient surface compared to AC, although biochar is less expensive to produce than AC [62].
Chen et al. [100] describe described and classify the carbonization process into four temperature ranges. In the first stage (25–150°C), the physical desorption of approximately 12% of the absorbed water occurs with a small degree of change in order; second stage (150–240°C), the dehydration of the =H and -OH fragments occurs on an intramolecular scale; third stage (240–400°C), thermal rupture of the glycosidic bond in other C-O bonds and some C=C, through a reaction of free radicals, forming large amounts of tar, H2O, carbon monoxide (CO) and CO2; fourth stage (>700°C), a graphite-like structure is obtained, with disordered layers that tend to align with increasing temperature.

3.2. Activation

The porosity and selectivity are great interesting characteristics in the production and use of AC. These characteristics dependent on the disorganization level in the monocrystalline structure, therefore, they depend on the precursor type, synthesis process and activation method used [101]. Pallarés et al. [63] highlighted that the precursor carbonization (300–800 °C) causes the less stable bonds rupture, liberating the precursor volatile fraction (gases and tars), resulting in carbonaceous residues enriched with aromatic carbon rings (plant carbon) with low adsorption capacity due to the tars repolymerization and their deposition on the material surface, filling in the formed pores. Therefore, a post-activation step is necessary to remove tar deposits and increase both porosity and adsorption capacity. For this reason, the temperature, time, and agents used for activation are essential for developing and enhancing the microporosity of AC [102].
The activation process generally requires high temperatures (700–1000 °C) and an activating agent presence. During this stage, several reactions occur, such as the tar deposits removal and the opening and new pores formation [63]. There is debate that higher temperatures favor CA adsorption by increasing its porosity [102]. Some authors indicate that a larger pore volume is obtained at low combustion temperatures and larger pores at higher combustion temperatures [103]. Pallarés et al. [63] highlights that, in the activation, the pores become larger with longer times, however, the new pores formation and the existing pores depth are severely reduced. Consequently, with prolonged times, more meso and macropores develop, as well as surface area (BET method, Brunauer, Emmett and Teller), however, the pore volumes decrease [104]. The formation and increase of porosity in the morphology of ACs directly influences the functionality and application of certain types of AC. Qambrani et al. [96] explain that the micropores are responsible for the surface area and the high absorption capacity; Mesopores are important in liquid-solid adsorption processes, and macropores influence aeration, solid-gas adsorption, and hydrology.
Physical activation is carried out primarily with steam and gas to produce significant physical changes in surface area, pore volume, and pore structures or to affect surface chemical properties, including surface functional groups, the surface, hydrophobicity and polarity [105]. The use of CO2 and H2O is endothermic in nature, which allows better control of the process. Ahmad et al. [103] point out that the use of CO2 favors the microporosity formation during the first stages of activation. Pallares et al. [63] concluded that the use of CO2 increases the microporosity by 43% and that the application of high temperature water (800 °C) reduced the BET surface area (−3%) and the micropores volume (−5.4 %), increased the total pore volume (9.6%) due to enlargement of the mesopores, and increased the activation time from 1 to 2 h, which led to a reduction in microporosity due to the enlargement and collapse of the mesopores. Table 4 presents a summary of the different synthesis methods organized by physical and chemical activation techniques.
Both temperature and time are vital to optimize AC functionalities, especially when there are prepared for special needs or applications. Therefore, the activation process not only influences the AC morphological modification but also affects the chemical surface. During each stage of the synthesis, the formation of important compounds that promote better adsorption capacities occurs. but they are also affected by time and temperature conditions. An important step is the cleaning of the synthesized ACs. In the use of chemical activators, the AC must be washed with water or alcohol, but in physical activations, it is recommended to apply a vacuum devolatilization process, at relatively low temperatures, from 100 to 250° C, at −1 ATM. This procedure removes impurities in the materials, causing pore saturation and, in some cases, reducing the adsorption capacity of the materials.

4. Surface Chemistry

The AC surface has a great advantage, which is the ability to attract and make simple bonds with certain types of substances or compounds due to its electrostatic power. On the AC surface, the adsorption mechanism can be by physisorption and/or chemisorption. These adsorption mechanisms usually favor the contaminants’ removal in liquid or gas phases, into a solid phase. The AC modified surface increases the selectivity and the efficiency of certain compounds’ immobilization. On the AC surface, the presence of functional groups is related to the presence of heteroatoms (C and H atoms) and O, N, S, and P [106]. These functional groups develop from the chemical interaction between the AC network (the external surface or at the edge of the basal plane) and the radicals supplied by the chemical agent used for synthesis [107]. The delocalized heteroatoms and electrons determine the acid or basic AC surface character [108]. Finally, the surface chemistry influences the AC adsorption capacity of, as shown below.

4.1. Acidic Surfaces

The surface complexe formation such as carbon-oxygen, carbon-hydrogen, carbon-nitrogen, carbon-sulfur, and carbon-halogen has been demonstrated by X-ray diffraction analysis [3]. Oxygen groups come from a wide variety of groups such as carboxylic, hydroxyl, carbonyl, lactone, phenol, pyrone, chromene, quinone, and ether [106, 109111] Oxygen complexes provide electron-rich properties, which can enhance the interaction between the carbon absorbers surface and CO2, due to the introduction of several polar groups, such as hydroxyl (OH) and carboxyl (COOH) groups [112]. The carbon-oxygen complex has various classifications such as acid, base or neutral [108]. There is an effect of the synthesis temperature and activation in the formation of carboxylic groups considered strong acids at low temperatures, while phenolic groups are considered weak acid groups at high temperatures. The temperature effect on carbonization and activation is linked to the presence or absence of functional groups. Bansal and Goyal [3] indicate that the surface oxygen groups developed in the carbonization stages are desorbed by high temperatures in the activation stage. For example, CO2 is released by the decomposition of carboxylic and lactone groups in the temperature range of 350 to 750°C; CO due to decomposition of quinones and phenolic groups in the temperature range of 500 to 950°C; H2O from the decomposition of carboxyl, phenols in the temperature range from 200 to 600°C.
The AC precursors chemical characteristics influence the formation of oxygen functional groups, as well as the oxidation mechanisms. In gas phase synthesis, an increase in hydroxyl and carbonyl functional surface groups has been shown, while in the liquid phase, carboxylic, quinone and phenolic groups increase [113, 114]. The gaseous agents that promote oxygen groups are air, O2, CO2, and H2O [106]. The liquid agents are HNO3 and H2SO4 [111114], these particularly favor the formation of carboxylic, lactone, and phenolic hydroxyl groups [109, 115]. Functional groups containing oxygen have a higher affinity for alkaline gases [107]. Chemisorption by oxygen complexes achieves the decomposition of oxidizing gases (O3, NO2, CO2) and various aqueous solutions (FeCl3, K2S2O8, (NH4)2S2O8, NaClO, KMnO4, K2Cr2O7, Na2S2O3, H2O2, HNO3), mainly by O2 chemisorption [3]. Therefore, the oxidation mechanisms are important in the AC synthesis.

4.2. Alkaline Surfaces

In general, the basic surface AC characteristics result mainly from delocalized beta electrons in the graphene layers [116, 117], which can act as Lewis bases [118]. However, nitrogen functional groups are associated with basic functionalities on the AC surface. The reaction of compounds such as NH3, HNO3, and amines, or by activation with nitrogen-containing precursors [106, 108, 120]. Another formation route is when the carbon surface is freed from its oxygen groups by thermal treatment in vacuum or in inert atmospheres at high temperatures (1000°C) and cooled to room temperature, with contact with oxygen gas [3].
The functional groups associated with the AC basicity are amid, imide, lactam, pyrolic, and pyridine [35, 38, 91]. Other groups, such as chromene [120], ketone [119], and pyrone [117] (functionalities containing oxygen), contribute to the AC basicity. The AC basic property increases the CO2 adsorption capacity [118, 121124] and H2S [125]. AC with basic properties is related to the removal or the equilibrium reaction between basic and acidic species.

5. AC Characterization Techniques

It is important to characterize changes on the surfaces and in the AC structures and to identify the presence and new compounds formation giving characteristics as adsorption and absorption’s capacity and selectivity. Different techniques can be used depending on the availability of technology; however, they can be classified into two types, physical and chemical techniques.
The physical techniques consider the structure and AC morphology according to its surface area, size, and pore volume. Below, the main techniques for the characterization of AC are described.

5.1. Scanning Electron Microscope

Scanning electron microscope (SEM) is one of the most utilized methods for visualizing the surface morphology of different materials. The images are obtained by scanning with a very narrow electron beam through a sample, measuring the scattered electrons and mapping them in space, achieving magnifications of 10 – 50 kX, and resolutions of 50 – 100 nm [150]. Depending on its electron source, SEM can be classified into thermal and field emission, and depending on the chamber conditions, at high vacuum or ambient pressures. When materials with high electron absorption capacity are available, it is necessary to cover the samples with thin films of metals such as platinum or gold, achieving resolutions of up to 5 nm [124]. Ronix et al. [151] mention the use of SEM in carbonized samples of coffee husks, managing to observe the development of porosity with the formation of cavities of different sizes and their distribution on the surfaces, in comparison with samples of precursors, which present a rigid morphology. and compact, with no apparent cavities and no pore structure.
This technique enables the surfaces and pore structures of AC to be characterized. However, to clearly view the surface morphology of materials with high electron absorption capacity, they must be first coated with gold [124]. Although there are techniques with higher resolution power such as scanning probe microscopy (SPM) or atomic force microscopy (AFM), which have been applied in the characterization of black carbon, they managed to observe structures at a scale of 5 – 10 nm (SPM) and 1–2 nm (AFM) [152].

5.2. Adsorption Capacity

Among the common techniques to determine the AC adsorption capacity, there is the iodine number method and the methylene blue method. Both methods determine the carbons’ relative activation level based on the quantity of adsorbed iodine or methylene blue solution in milligrams per gram of carbon [153, 154]. In some cases, the use of techniques such as ultraviolet rays (UV) to determine the absorbance allows more accurate readings of the initial and final concentrations. Adsorption isotherms are another commonly utilized technique for representing the interaction between the adsorbate and adsorbent in addition to the distribution of the adsorbate between the liquid phase and solid phase under equilibrium conditions. In particular, the isotherms of Langmuir and Freundlich are frequently used. The first is based on the affinity of the binding sites and is used to identify the saturation point, or moment, in which all active centers are occupied (monolayer). Meanwhile, the second is based on an empirical model that describes non-ideal multilayer adsorption on the heterogeneous surface of an adsorbent [154]. Chemical techniques are considered qualitative or quantitative according to the available procedures and applications. Qualitative techniques are based on the identification of the chemical species present, whereas the quantitative techniques are used to determine the number of chemical species present on the carbon surface [107].
The BET method utilizes the multilayer adsorption framework under two principals: The first assumes that the surface is energetically homogeneous, assuming that there is no energy variation in adsorption across a single layer, whereas the second assumes that the adsorption-desorption velocities are similar according to a model of kinetic equilibrium. This method utilizes the multilayer adsorption framework under two principles: The first assumes that the surface is energetically homogeneous, assuming that there is no energy variation in adsorption across a single layer, where the second assumes that the adsorption-desorption velocities are similar according to a model of kinetic equilibrium. The BET equation describes the full course of the isotherm (Eq. 1), including the areas of monomolecular adsorption, polymolecular adsorption, and capillary condensation.
  • q : adsorption capacity (mg adsorbate adsorbed per g adsorbent)

  • B : Constant expressing the interaction of energy with the surface.

  • qmax : solute adsorbed per unit weight of adsorbent in forming a complete monolayer on the surface (Ms/Mc

  • Ss : Solute saturation coefficient (Ms L3−1)

  • Se : equilibrium adsorbate concentration, (Ms/L3)

The BET theory starts from inferring that only the first adsorbed layer is strongly attracted to the surface, the second layer is essentially adsorbed by the first layer, and finally, the adsorption propagates to result in multilayer adsorption, although it is not necessary. That a layer is completely formed before the beginning of subsequent layers and Van der Waals forces would be responsible for adsorption [2]. The technique used in the analyzers generally begins with degassing at 25°C, from 12 to 24 h, subsequently, a saturation of the material with nitrogen gas (N2) is carried out, as a sorbent at temperatures of 77 K and desaturation pressures of <660 mmHg [91]. Invariably, the BET technique has been the most widely used in the characterization of adsorption capacities.

5.3. Raman Spectroscopy

Raman spectroscopy (RS) is a technique to identify and analyze the structural footprint of functional groups on AC surfaces by measuring the photon scattering energy. In this technique, AC is exposed to monochromatic light from a laser. Although some beams are elastically scattered, RS depends on the inelastic scattering of photons. Samples may be then observed through revolution or in their different rotational or vibrational states [155, 156].
The energy gradient between the incident and reflected beam is measured in terms of the wave number (cm−1) [157], which corresponds with the molecular vibration within the adsorbent, enabling the chemical species in a sample to be identified [158160]. This technique presents advantages because of the minimal spectral manipulation, simple data interpretation, high spectral recording speed, high spatial resolution, and relative ease of preparation. Also, it allows the simultaneous and clear surface species identification without interference from water and serves as a complementary method to infrared spectroscopy [161, 162]. However, degradation effects may affect the sample, including the fluorescence and sub-development, possibly leading to an incomplete understanding of the provided information [107].

5.4. X-ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy (XPS) is one of the most utilized techniques in the quantification and characterization of the elemental composition, chemical state, and oxidation of elements and radicals of the surface structures of AC. It is also called electronic spectroscopy for chemical analysis [106, 163165]. In this technique, the study material is subjected to radiation by x-rays, and the electrons liberated from the material are then measured, analyzing the upper (0 nm) to lower portions (10 nm) of the material and their kinetic energies [107]. Susi et al. [165] described the method in three stages: (1) Photons from x-rays are absorbed, and their energies are transferred to a nucleus electron. (2) Select atomic species are excited, emitting photoelectrons, and resulting in an empty core state. (3) Emitted photoelectrons are transported to the surface of the material before escaping the void where the energy content is measured with the help of an electron analyzer. When using XPS in the identification, concentration, and binding of doping groups, measurements of the samples prepared for analysis should be considered, specifically the intrinsic photoemission response and presence of relevant spectra. These determinations can reduce the risk of making incorrect conclusions about the material or its characterization, especially in the assignment of linking energies to the respective atomic configurations during the composition analysis [107].
Tongpoothorn et al. [166] performed XRD analysis on AC obtained from Jatropha Curcas husks. They exhibit broad peaks and the absence of a sharp peak, revealing a predominantly amorphous structure. In addition, they mention that the frequency of broad peaks around 26° and 43° shows signs of the formation of a crystalline carbonaceous structure, equivalent to an alignment of graphene layers. Keiluweit et al. [167] classified the structural changes in cellulose (grass) and lignin (wood), in CA synthesis at different temperatures. The XRD data emerge that from 300 to 400 °C the cellulose crystallinity of chars disintegrates and randomizes, while turbostratic crystallites evolve at char temperatures above 400 °C. There are some variants, such as X-ray photoelectron spectroscopy (EFRX) and analysis of structures near the X-ray absorption edge (ECBARX), which have contributed to improving the quality of the analyses.

5.5. Fourier Transformed Infrared Spectroscopy

Fourier transformed infrared spectroscopy (FTIR) consists of passing a light beam at different frequencies on a sample to measure the absorbance or transmittance of infrared light [168, 169]. The obtained spectra is graphically represented as the infrared density as either transmittance or absorbance (cm−1) against the wave number. This technique has several advantages: It is highly sensitive, has a good signal-noise relationship, produces high-quality spectra, has a non-destructive focus, can detect species at low concentrations, and can be used to analyze multiple gas species at the same time. However, it is only suitable for compounds with a dipole moment, as compounds without a dipole moment do not absorb infrared rays and, therefore, cannot be characterized by FTIR [170, 171]. In the quantitative evaluation and infrared spectra interpretation, errors may be generated since absorption bands do not always represent a specific functional group, but may be formed as a result of the overlapping of several bands representing different groups. However, obtaining infrared spectra from AC is often a challenge due to irregular light scattering caused by the large adsorbent particle size, poor transmission, or problems associated with sample preparation [107, 172].

5.6. Boehm Titration

Boehm titration (BT) is a wet characterization technique. It is an acid-base titration method that was originally developed to identify and quantify the acidic oxygen functional groups on the surface of modified and AC carbonized according to their different acid strengths according to the acidity constant, Ka [173, 174]. The basic principle is that strong acids potentially react with all types of bases, whereas weak acids only donate protons to the conjugated bases of acids with higher pKa values; therefore, functional groups with different acidity can be measured against suitable bases of different strength [107]. The technique classifies acid groups according to three ranges of pKa strength, 5.0–6.4, 6.4–10.3, and 10.3–13.0, which correspond with the carboxylic, lactone, and phenolic groups, respectively. In the titration of AC with three different bases (NaOH, Na2CO3, and NaHCO3), each functional group will react with the bases that have a conjugated acid with greater pKa [175, 176]. During titration, it is assumed that NaOH easily reacts with the carboxylic, phenolic, and lactonic functional groups and that Na2CO3 reacts with carboxylic and lactonic groups. Meanwhile, NaHCO3 only reacts with carboxylic groups [107, 175, 177179]. Also, the basic functional groups can be identified and quantified by reaction with hydrochloric acid based on the quantity of HCl consumed during the evaluation compared to the original AC sample [178, 180, 181].

6. Biogas Enrichment and Upgrading Processes

Biogas by itself does not have the energy capacity that allows it to compete directly with fuels such as LP gas or natural gas (Table 5, part A). However, the importance of upgrading and enrichment processes is aimed at increasing or improving these characteristics. Another important factor is the substrates used to obtain biogas, since it has different characteristics in their composition, for example, substrates from farms, sewage sludge and landfill. The particular characteristics of each substrate result in different ranges in the biogas constituent compounds (Table 5, Part B). These differences in the biogas “quality” is a key piece for the selection and implementation of upgrading and enrichment systems and processes, with the aim of achieving the necessary criteria for the use of biogas or biomethane, in combined heat and power systems or in its injection to grid gas.
The heat capacity of the presented fuels has important differences. However, analyzing the specific heats of each hydrocarbon, CH4 has the highest specific heat (1,708 kJ kg−1), followed by C4H10 (1,551 kJ kg−1), C3H8 (1,480 kJ kg−1), and C2H6 (kJ kg−1). However, the presence of CO2 in biogas reduces its heat capacity total, thence, the need to remove the CO2 immersed in the biogas, to reach similar values to NG.

6.1. International Enrichment and Upgrading of Biogas, and AC Application

The case of Denmark is one of the most important examples of the biogas use in local heat and energy generation plants (combined cycle). The government has aimed to increase the manure use, currently at 10%, to 50% by 2025, which would correspond with an increase in energy production of 7 to 17 PJ. The use of manure represents a good strategy because of its constant production throughout the year, especially considering that its storage is expensive [182]. It has been shown, that biogas produced from manure is a good substitute for NG when quality is improved.
The International Energy Agency [183], with 37 member countries lists up to 2019, 602 biogas upgrading and enrichment plants. 70% of these plants are in countries such as Germany (203*), the United Kingdom (96*), Sweden (69*) and Netherlands (54*). The main uses of biogas upgrading, and enrichment plants are injection and conduction in the gas red (271*), vehicle gas and fuel (25*), CHP, electricity, and heat. The methane increase reaches ranges of 88 – 99% v/v. The main processes used are membrane (184*), scrubber (176*), chemical scrubber (103*), adsorption by pressure variation (79*) and organic physical scrubber (17*), catalytic methanation (1*), biological methanation (1*). Therefore, multiple biogas enrichment techniques have been implemented, with characteristics and requirements as shown in Table 6.
Historically, adsorption processes using different media have been developed under two operating schemes, a liquid-gas or solid-gas contact, at both industrial and commercial scales. The combination of these two interfaces is common, for example, packed bed adsorption columns, spray columns, bubble columns, rotating beds, mini-channels, and contact membranes, to mention several. One of the additional advantages of using AC for CO2 capture compared to other technologies is its low cost. Farooq et al. [184] reported that AC has a lower cost in terms of capital and operation cost in comparison to washing with water, which has costs of 1.5 and 0.9 p kWh−1, respectively. In addition, the large amount of wastewater that is usually generated can be reduced. Therefore, this method represents a more sustainable form of biomethane production.
The most important variables in the use of adsorption columns in the capture of CO2 are column height and diameter as well as the packing material, elevated surface area of the packing material, resistance to wear under operating conditions, low energy consumption, and easy regeneration. In Table 7, a summary of the diverse applications of AC in the capture of CO2 is provided, along with their key characteristics.

6.2. Art State of Topics Upgrading and Enrichment from Biogas

To know the state of the art, research articles published by Elsevier® and EBSCO® host were reviewed. Through the corresponding online databases, articles on topics related to biogas upgrading and enrichment were searched during the 2008–2021 period, comparing the number of articles for each method (dependent variable) with respect to period time (independent variable). 164 articles were found and organized by six techniques, as shown in Fig. 1.
Most articles were related to membranes (32.93%), AC (21.95%), gas scrubbing (13.41%), and zeolites (10.37%). Linear regression was performed to determine the trend in the number of publications per year. As a result, the technique or use of AC obtained the greatest tendency with a value of R2 = 0.6260, indicating continual growth in the study of biogas for the topics of interest. However, a decrease in publications is observed in the years 2020 and 2021. Discarding these two years, the trends present a better fit, for example AC increase in R2 = 0.7882, membranes in R2 = 0.5641 and gas scrubbing in R2 = 0.4149.
Absorption and adsorption processes have been frequently implemented in the cleaning and enrichment of biogas. Although absorption processes have a high separation capacity, they also have a high cost because of the high energy requirement of solvent regeneration in addition to requiring large quantities of water [195]. An alternative approach is adsorption in which contaminants are eliminated from mixtures by solid, porous absorbents such as AC. In the case of AC, the use of low temperatures is most functional for the capture of CO2 because its removal capacity decreases as temperature increases. In addition, AC can be regenerated by varying the temperature or pressure or vacuuming [125, 196].

7. Conclusion

  • AC synthesis is an affordable and economically viable option, for handling large volumes of lignocellulosic waste, generated in the southeastern region of Mexico. This option would represent an alternative income for agricultural producers and agro-industries, helping to balance the trade balance of AC in the country. Certain synthesis and characterization techniques are standardized, favoring production on a commercial scale. The multiplicity of applications that AC has is a commercial market guarantee.

  • The need to produce Biomethane as an alternative to NG promotes the development of more efficient and economic cleaning and enrichment technologies. The use of AC as a biogas improvement technique closes a cycle of simultaneous use of residual biomass.

  • The positive trend in research on the use of AC in biogas upgrading is important. Although the membrane process is the most widely used industrially, would be related to the greater number of publications on this technique. However, the abundance, low cost, good yields, and capacities of lignocellulosic precursors help to increase the use of CA in biogas upgrading processes.

List of Nomenclature




Percent volume on volume


Relative or approximate


Register mark


Degrees centigrade

°C min−1

Degrees centigrade per minute


Pressure in bar


Wave number

cm s−1

Centimeter per second


cubic centimeter per gram




Gigawatt per hour




Acidity constant


Pressure in kilopascal

kg ha−1

Kilogram per hectare

kg air/kg fuel

Kilogram of air, per kilogram of fuel

kWh m−3

kilowatt hour per cubic meter

m2 g−1

Square meters per gram

mg g−1

Milligrams per gram



ml min−1

Milliliters per minute




Billions of cubic meters

mmol g−1

Millimole per gram

mol kg−1

Mole per kilogram


Pressure in megapascal

MJ m−3

Mega Joule per cubic meter

MJ kg−1

Mega Joule per kilogram







p kWh−1

Cost per kWh


Peta Joule


Dissociation constant


Part per million


Regression value



t ha−1

Tons per hectare

t ha−1 a−1

Tons per hectare per annual


Multiplication symbol


Temperature delta


We appreciate the support of the professor from the University of Louisiana helping with the manuscript redaction.


Conflict-of-Interest Statement

The authors declare that there is no conflict of interest in the development of the topic and the compilation of information

Autor’s Contribution

J.S.O. (Ph D. Student). Contribution: Review bibliographic of synthesis and characterization of AC. https://orcid.org/0000-0001-6786-0521

J.R.L. (Head of pilot plant 3 - treatment of air and solid waste). Contribution: Review bibliographic of lignocellulosic waste. https://orcid.org/0000-0002-6770-5596

D.S.G. (Head of the nanotechnology laboratory). Contribution: Review characterization technical. http://orcid.org/0000-0001-9953-9142

R.H. (Professor and Department Head of Chemical Engineering). Contribution: Review application of upgrading and enrichment of biogas.

M.Z. (Professor and Department Head of Chemical Engineering). Contribution: Review application of upgrading and enrichment of biogas. https://orcid.org/0000-0002-7608-076X

A.E.E.M. (Professor) Contribution: Review of general technics and writing). https://orcid.org/0000-0003-1319-5380


1. Álvarez MÁ, Carrasco FM, Hódar JF. Carbones Activados. Desarrollo y aplicaciones de materiales avanzados de carbón. UNIA; 2014. p. 18–45.

2. Cecen F, ÖzgÜr A. Activated Carbon for Water and Wastewater Treatment Integration of Adsorption and Biological Treatment. Weinheim: Wiley-VCH Verlag & Co. KGaA; 2012. p. 365

3. Bansal RC, Goyal M. Activated Adsorption Carbon. Taylor & Francis Group. 3:Chromatographia. 2005. p. 38–40.

4. Demirbaş A, Arslan G, Pehlivan E. Recent studies on activated carbons and fly ashes from Turkish resources. Energy Sources, Part A Recover. Util. Environ. Eff. 2006;28(7)627–38.

5. González P. Activated carbon from lignocellulosics precursors: A review of the synthesis methods, characterization techniques and applications. Renew. Sustain. Energy Rev. 2018;82:1393–414. http://dx.doi.org/10.1016/j.rser.2017.04.117

6. Wong S, Ngadi N, Inuwa IM, Hassan O. Recent advances in applications of activated carbon from biowaste for wastewater treatment: A short review. J. Clean. Prod. 2018;175:361–75. https://doi.org/10.1016/j.jclepro.2017.12.059

7. Konwar LJ, Boro J, Deka D. Review on latest developments in biodiesel production using carbon-based catalysts. Renew. Sustain. Energy Rev. 2014;29:546–64. http://dx.doi.org/10.1016/j.rser.2013.09.003

8. Sah RP, Choudhury B, Das RK. A review on adsorption cooling systems with silica gel and carbon as adsorbents. Renew. Sustain. Energy Rev. 2015;45:123–34. http://dx.doi.org/10.1016/j.rser.2015.01.039

9. Freedonia. Global activated carbon [Internet]. 2018. https://hubs.ly/H0fTC5K0%0ATable

11. Tancredi N, Cordero T, Rodríguez-Mirasol J, Rodríguez JJ. Activated Carbons from Eucalyptus Wood. Influence of the Carbonization Temperature. Sep. Sci. Technol. 1997;32(6)1115–26.

12. Khalkhali RA, Omidvari R. Adsorption of mercuric ion from aqueous solutions using activated carbon. Polish J. Environ. Stud. 2005;14(2)185–8.

13. Ioannidou O, Zabaniotou A. Agricultural residues as precursors for activated carbon production—A review. Renew. Sustain. Energy Rev. 2007;11(9)1966–2005. https://doi.org/10.1016/j.rser.2006.03.013

14. El Hannafi N, Boumakhla MA, Berrama T, Bendjama Z. Elimination of phenol by adsorption on activated carbon prepared from the peach cores: modelling and optimisation. Desalination. 2008;223(1–3)264–8. https://doi.org/10.1016/j.desal.2007.01.229

15. Heo YJ, Park SJ. A role of steam activation on CO2 capture and separation of narrow microporous carbons produced from cellulose fibers. Energy. 2015;91:142–50. http://dx.doi.org/10.1016/j.energy.2015.08.033

16. Hirunpraditkoon S, Tunthong N, Ruangchai A, Nuithitikul K. Adsorption capacities of activated carbons prepared from bamboo by KOH activation. World Acad. Sci. Eng. Technol. 2011;78:711–5.

17. FAO. Crop prospects and food situation [Internet]. 2019. https://www.fao.org/3/ca6057en/ca6057en.pdf

18. FAO. The future of food and agricultura Trends and challenges [Internet]. 2017. https://www.fao.org/3/i6583e/i6583e.pdf

19. SAGARPA. Planeación Agrícola Nacional 2017–2030 [Internet]. 2017. https://www.gob.mx/cms/uploads/attachment/file/255627/Planeaci_n_Agr_cola_Nacional_2017-2030-_parte_uno.pdf

20. Kadirvelu K, Namasivayam C. Activated carbon from coconut coirpith as metal adsorbent: Adsorption of Cd(II) from aqueous solution. Adv. Environ. Res. 2003;7(2)471–8.

21. Malik R, Ramteke DS, Wate SR. Adsorption of malachite green on groundnut shell waste based powdered activated carbon. Waste Manag. 2007;27(9)1129–38.
crossref pmid

22. British Petroleum. Informe Estadístico Mundial de energía de BP 2018 [Internet]. 2018. https://www.aop.es/wp-content/uploads/2019/05/bp-stats-review-2018-full-report.pdf

23. Energy, Information, Administration. Annual Energy Outlook 2019 [Internet]. 2019. https://www.eia.gov/outlooks/ieo/pdf/ieo2019.pdf

24. Svenfelt Å, Alfredsson EC, Bradley K, et al. Scenarios for sustainable futures beyond GDP growth 2050. Futures. 2019;111:1–14. https://doi.org/10.1016/j.futures.2019.05.001

25. Jury C, Benetto E, Koster D, Schmitt B, Welfring J. Life Cycle Assessment of biogas production by monofermentation of energy crops and injection into the natural gas grid. Biomass and Bioenergy. 2010;34(1)54–66. https://doi.org/10.1016/j.biombioe.2009.09.011

26. Qian Y, Sun S, Ju D, Shan X, Lu X. Review of the state-of-the-art of biogas combustion mechanisms and applications in internal combustion engines. Renew. Sustain. Energy Rev. 2017;69:50–8. http://dx.doi.org/10.1016/j.rser.2016.11.059

27. Kacem M, Pellerano M, Delebarre A. Pressure swing adsorption for CO2/N2 and CO2/CH4 separation: Comparison between activated carbons and zeolites performances. Fuel Process. Technol. 2015;138:271–83. http://dx.doi.org/10.1016/j.fuproc.2015.04.032

28. Yousef AM, Eldrainy YA, El-Maghlany WM, Attia A. Upgrading biogas by a low-temperature CO2 removal technique. Alexandria Eng. J. 2016;55(2)1143–50. http://dx.doi.org/10.1016/j.aej.2016.03.026

29. Mahmood N, Yuan Z, Schmidt J, Xu C. Depolymerization of lignins and their applications for the preparation of polyols and rigid polyurethane foams: A review. Renew. Sustain. Energy Rev. 2016;60:317–29. https://doi.org/10.1016/j.rser.2016.01.037

30. Younas R, Zhang S, Zhang L, et al. Lactic acid production from rice straw in alkaline hydrothermal conditions in presence of NiO nanoplates. Catal. Today. 2016;274:40–8.

31. Antonyraj CA, Haridas A. A lignin-derived sulphated carbon for acid catalyzed transformations of bio-derived sugars. Catal. Commun. 2018;104:101–5.

32. Chen PA, Cheng HC, Wang HP. Activated carbon recycled from bitter-tea and palm shell wastes for capacitive desalination of salt water. J. Clean. Prod. 2018;174:927–32. https://doi.org/10.1016/j.jclepro.2017.11.034

33. Suhas , Gupta V, Carrott P, Singh R, Chaudhary M, Kushwaha S. Cellulose: A review as natural, modified and activated carbon adsorbent. Bioresour. Technol. 2016;216:1066–76. http://dx.doi.org/10.1016/j.biortech.2016.05.106
crossref pmid

34. Cao L, Yu IK, Liu Y, et al. Lignin valorization for the production of renewable chemicals: State of the art review and future prospects. Bioresour. Technol. 2018;269:465–75. https://doi.org/10.1016/j.biortech.2018.08.065
crossref pmid

35. Shu R, Long J, Yuan Z, et al. Efficient and product-controlled depolymerization of lignin oriented by metal chloride cooperated with Pd/C. Bioresour. Technol. 2015;179:84–90.
crossref pmid

36. Dietrich Fengel GW. Wood: chemistry, ultrastructure, reactions. 2nd ed.Berlin - New York: 1989. p. 626

37. Kumari D, Singh R. Pretreatment of lignocellulosic wastes for biofuel production: A critical review. Renew. Sustain. Energy Rev. 2018;90:877–91. https://doi.org/10.1016/j.rser.2018.03.111

38. Mohamad Nor N, Lau LC, Lee KT, Mohamed AR. Synthesis of activated carbon from lignocellulosic biomass and its applications in air pollution control - A review. J. Environ. Chem. Eng. 2013;1(4)658–66. https://doi.org/10.1016/j.jece.2013.09.017

39. Elshaarani M, Yaakob Z, Dahlan K, Mohammad M. Jatropha deoiled cake filler-reinforced medium-density polyethylene biocomposites: Effect of filler loading and coupling agent on the mechanical, dynamic mechanical and morphological properties. Polym. Compos. 2013;34(5)746–56.

40. Mohammad M, Yaakob Z, Abdullah SR. Carbon derived from jatropha seed hull as a potential green adsorbent for cadmium (II) removal from wastewater. Materials (Basel). 2013;6(10)4462–78.
crossref pmid pmc

41. Asim N, Emdadi Z, Mohammad M, Yarmo MA, Sopian K. Agricultural solid wastes for green desiccant applications: An overview of research achievements, opportunities and perspectives. J. Clean. Prod. 2015;91:26–35. http://dx.doi.org/10.1016/j.jclepro.2014.12.015

42. Papong S, Yuvaniyama C, Lohsomboon P, Malakul P. Overview of Biomass Utilization in Thailand. In : ASEAN, Biomass meeting; Bangkok. 2004.

43. Elshaarani T, Yaakob Z, Zaman MD, Mohammad M, Abdullah SR. Mechanical properties, morphology, flammability, and thermokinetic investigation of high-density polyethylene/Jatropha deoiled cake composites. J. Appl. Polym. Sci. 2012;126:78–88.

44. Ishak MR, Sapuan SM, Leman Z, Rahman MZ, Anwar UM, Siregar JP. Sugar palm (Arenga pinnata): Its fibres, polymers and composites. Carbohydr. Polym. 2013;91(2)699–710. https://doi.org/10.1016/j.carbpol.2012.07.073
crossref pmid

45. Llorach P, Lopez E, Peña J, Rieradevall J, Montero JI, Puy N. Technical feasibility and carbon footprint of biochar co-production with tomato plant residue. Waste Manag. 2017;67:121–30. https://doi.org/10.1016/j.wasman.2017.05.021
crossref pmid

46. Ajewole TO, Elehinafe FB, Okedere OB, Somefun TE. Agro-residues for clean electricity: A thermo-property characterization of cocoa and kolanut waste blends. Heliyon. 2021;7(9)1–9. https://doi.org/10.1016/j.heliyon.2021.e08055
crossref pmid pmc

47. White PM, Viator RP, Webber CL. Temporal and varietal variation in sugarcane post-harvest residue biomass yields and chemical composition. Ind. Crops Prod. 2020;154:1–6. https://doi.org/10.1016/j.indcrop.2020.112616

48. Renó M, Lora E, Palacio J, Venturini O, Buchgeister J, Almazan O. A LCA (life cycle assessment) of the methanol production from sugarcane bagasse. Energy. 2011;36(6)3716–26. http://dx.doi.org/10.1016/j.energy.2010.12.010

49. Krishnan C, da Costa Sousa L, Jin M, Chang L, Dale BE, Balan V. Alkali-based AFEX pretreatment for the conversion of sugarcane bagasse and cane leaf residues to ethanol. Biotechnol. Bioeng. 2010;107(3)441–50.
crossref pmid

50. Ahmad T, Danish M. Prospects of banana waste utilization in wastewater treatment: A review. J. Environ. Manage. 2018;206:330–48. https://doi.org/10.1016/j.jenvman.2017.10.061
crossref pmid

51. Subramaniam V, Loh SK, Aziz AA. GHG analysis of the production of crude palm oil considering the conversion of agricultural wastes to by-products. Sustain. Prod. Consum. 2021;28:1552–64. https://doi.org/10.1016/j.spc.2021.09.004

52. Singh R, Srivastava M, Shukla A. Environmental sustainability of bioethanol production from rice straw in India: A review. Renew. Sustain. Energy Rev. 2016;54:202–16. http://dx.doi.org/10.1016/j.rser.2015.10.005

53. Khare SK, Jha K, Gandhi AP. Citric acid production from Okara (soy-residue) by solid-state fermentation. Bioresour. Technol. 1995;Jan 54(3)323–5. https://linkinghub.elsevier.com/retrieve/pii/0960852495001557

54. Gouvea BM, Torres C, Franca AS, Oliveira LS, Oliveira ES. Feasibility of ethanol production from coffee husks. Biotechnol. Lett. 2009;31(9)1315–9.
crossref pmid pdf

55. SE. Renewable Energy Prospect 2018–2032 [Internet]. Mexico: 2018. 73 https://base.energia.gob.mx/Prospectivas18-32/PER_18_32_F.pdf

56. Taherzadeh M, Karimi K. Pretreatment of Lignocellulosic Wastes to Improve Ethanol and Biogas Production: A Review. Int. J. Mol. Sci. 2008;9(9)1621–51. https://doi.org/10.3390/ijms9091621
crossref pmid pmc

57. Zhu G, Qiu X, Zhao Y, Qian Y, Pang Y, Ouyang X. Depolymerization of lignin by microwave-assisted methylation of benzylic alcohols. Bioresour. Technol. 2016;218:718–22. https://doi.org/10.1016/j.biortech.2016.07.021
crossref pmid

58. Watanabe M, Kanaguri Y, Smith RL. Hydrothermal separation of lignin from bark of Japanese cedar. J. Supercrit. Fluids. 2018;133:696–703. https://doi.org/10.1016/j.supflu.2017.09.009

59. De Bhowmick G, Sarmah AK, Sen R. Zero-waste algal biorefinery for bioenergy and biochar: A green leap towards achieving energy and environmental sustainability. Sci. Total Environ. 2019;650:2467–82. https://doi.org/10.1016/j.scitotenv.2018.10.002
crossref pmid

60. ASTM D2974. Standard Test Methods for Moisture, Ash, and Organic Matter of Peat and Other Organic Soils [Internet]. 2000. 4 https://www.astm.org/Standards/D2974.htm

61. Chen B, Zhou D, Zhu L. Transitional Adsorption and Partition of Nonpolar and Polar Aromatic Contaminants by Biochars of Pine Needles with Different Pyrolytic Temperatures. Environ. Sci. Technol. 2008;42(14)5137–43. https://doi.org/10.1021/es8002684
crossref pmid

62. Masebinu SO, Akinlabi ET, Muzenda E, Aboyade AO. A review of biochar properties and their roles in mitigating challenges with anaerobic digestion. Renew. Sustain. Energy Rev. 2019;103:291–307. https://doi.org/10.1016/j.rser.2018.12.048

63. Pallarés J, González A, Arauzo I. Production and characterization of activated carbon from barley straw by physical activation with carbon dioxide and steam. Biomass and Bioenergy. 2018;115:64–73. https://doi.org/10.1016/j.biombioe.2018.04.015

64. Yang W, Liu Y, Wang Q, Pan J. Removal of elemental mercury from flue gas using wheat straw chars modified by Mn-Ce mixed oxides with ultrasonic-assisted impregnation. Chem. Eng. J. 2017;326:169–81. https://doi.org/10.1016/j.cej.2017.05.106

65. Zanzi R, Bai X, Capdevila P, Björnbom E. Pyrolysis of biomass in presence of steam for preparation of activated carbon, liquid and gaseous products. In : 6th World Congress of Chemical Engineering; Melburne, Australia. 2001. p. 23–7.

66. Olupot PW, Candia A, Menya E, Walozi R. Characterization of rice husk varieties in Uganda for biofuels and their techno-economic feasibility in gasification. Chem. Eng. Res. Des. 2016;107:63–72. https://doi.org/10.1016/j.cherd.2015.11.010

67. Bundhoo ZM. Microwave-assisted conversion of biomass and waste materials to biofuels. Renew. Sustain. Energy Rev. 2018;82:1149–77. http://dx.doi.org/10.1016/j.rser.2017.09.066

68. Carrier M, Hardie AG, Uras Ü, Görgens J, Knoetze J. Production of char from vacuum pyrolysis of South-African sugar cane bagasse and its characterization as activated carbon and biochar. J. Anal. Appl. Pyrolysis. 2012;96:24–32. http://dx.doi.org/10.1016/j.jaap.2012.02.016

69. González JF, Román S, Encinar JM, Martínez G. Pyrolysis of various biomass residues and char utilization for the production of activated carbons. J. Anal. Appl. Pyrolysis. 2009;85(1–2)134–41. https://doi.org/10.1016/j.jaap.2008.11.035

70. Grillo G, Boffa L, Binello A, et al. Cocoa bean shell waste valorisation; extraction from lab to pilot-scale cavitational reactors. Food Res. Int. 2019;115:200–8. https://doi.org/10.1016/j.foodres.2018.08.057
crossref pmid

71. Daud W. Comparison on pore development of activated carbon produced from palm shell and coconut shell. Bioresour. Technol. 2004;93(1)63–9. https://doi.org/10.1016/j.biortech.2003.09.015
crossref pmid

72. Köseoğlu E, Akmil-Başar C. Preparation, structural evaluation and adsorptive properties of activated carbon from agricultural waste biomass. Adv. Powder Technol. 2015;26(3)811–8. https://doi.org/10.1016/j.apt.2015.02.006

73. Florian TD, Villani N, Aguedo M, et al. Chemical composition analysis and structural features of banana rachis lignin extracted by two organosolv methods. Ind. Crops Prod. 2019;132:269–74. https://doi.org/10.1016/j.indcrop.2019.02.022

74. Ozbay N, Yargic AS, Yarbay Sahin RZ, Yaman E. Valorization of banana peel waste via in-situ catalytic pyrolysis using Al-Modified SBA-15. Renew. Energy. 2019;140:633–46. https://doi.org/10.1016/j.renene.2019.03.071

75. Prahas D, Kartika Y, Indraswati N, Ismadji S. Activated carbon from jackfruit peel waste by H3PO4 chemical activation: Pore structure and surface chemistry characterization. Chem. Eng. J. 2008;140(1–3)32–42. https://doi.org/10.1016/j.cej.2007.08.032

76. Okutucu C, Duman G, Ucar S, Yasa I, Yanik J. Production of fungicidal oil and activated carbon from pistachio shell. J. Anal. Appl. Pyrolysis. 2011;91(1)140–6. https://doi.org/10.1016/j.jaap.2011.02.002

77. Tsai WT, Chang CY, Wang SY, Chang CF, Chien SF, Sun HF. Cleaner production of carbon adsorbents by utilizing agricultural waste corn cob. Resour. Conserv. Recycl. 2001;32(1)43–53. https://doi.org/10.1016/S0921-3449(00)00093-8

78. Aqsha A, Tijani MM, Moghtaderi B, Mahinpey N. Catalytic pyrolysis of straw biomasses (wheat, flax, oat and barley) and the comparison of their product yields. J. Anal. Appl. Pyrolysis. 2017;125:201–8. https://doi.org/10.1016/j.jaap.2017.03.022

79. Suárez F, Martínez A, Tascón JM. Pyrolysis of apple pulp: effect of operation conditions and chemical additives. J. Anal. Appl. Pyrolysis. 2002;62(1)93–109. https://doi.org/10.1016/S0165-2370(00)00216-3

80. Kilic M, Apaydin E, Pütün AE. Adsorptive removal of phenol from aqueous solutions on activated carbon prepared from tobacco residues: Equilibrium, kinetics and thermodynamics. J. Hazard. Mater. 2011;189(1–2)397–403. https://doi.org/10.1016/j.jhazmat.2011.02.051Get
crossref pmid

81. Pap S, Šolević T, Radonić J, Maletić S, Igić SM, Turk M. Utilization of fruit processing industry waste as green activated carbon for the treatment of heavy metals and chlorophenols contaminated water. J. Clean. Prod. 2017;162:958–72. https://doi.org/10.1016/j.jclepro.2017.06.083

82. Shahul K, Muthirulan P, Meenakshi M. Adsorption of chromotrope dye onto activated carbons obtained from the seeds of various plants: Equilibrium and kinetics studies. Arab. J. Chem. 2017;10:S2225–33. https://doi.org/10.1016/j.arabjc.2013.07.058

83. Okman I, Karagöz S, Tay T, Erdem M. Activated Carbons From Grape Seeds By Chemical Activation With Potassium Carbonate And Potassium Hydroxide. Appl. Surf. Sci. 2014;293:138–42. https://doi.org/10.1016/j.apsusc.2013.12.117

84. Ozdemir I, Şahin M, Orhan R, Erdem M. Preparation and characterization of activated carbon from grape stalk by zinc chloride activation. Fuel Process. Technol. 2014;125:200–6. https://doi.org/10.1016/j.fuproc.2014.04.002

85. Tiryaki B, Yagmur E, Banford A, Aktas Z. Comparison of activated carbon produced from natural biomass and equivalent chemical compositions. J. Anal. Appl. Pyrolysis. 2014;105:276–83. https://doi.org/10.1016/j.jaap.2013.11.014

86. Kurniawan A, Ismadji S. Potential utilization of Jatropha curcas L. press-cake residue as new precursor for activated carbon preparation: Application in methylene blue removal from aqueous solution. J. Taiwan Inst. Chem. Eng. 2011;42(5)826–36. http://dx.doi.org/10.1016/j.jtice.2011.03.001

87. Kodal M, Karakaya N, Wis AA, Ozkoc G. Thermal properties (DSC, TMA, TGA, DTA) of rubber nanocomposites containing carbon nanofillers. Carbon-Based Nanofillers and Their Rubber Nanocomposites: Fundamentals and Applications. Elsevier Inc; 2019. p. 325–66. http://dx.doi.org/10.1016/B978-0-12-817342-8.00011-1

88. Miranda R, Bustos D, Blanco CS, Gutiérrez MH, Rodriguez ME. Pyrolysis of sweet orange (Citrus sinensis) dry peel. J. Anal. Appl. Pyrolysis. 2009;86(2)245–51. https://doi.org/10.1016/j.jaap.2009.06.001

89. Promdee K, Chanvidhwatanakit J, Satitkune S, et al. Characterization of carbon materials and differences from activated carbon particle (ACP) and coal briquettes product (CBP) derived from coconut shell via rotary kiln. Renew. Sustain. Energy Rev. 2017;75:1175–86. http://dx.doi.org/10.1016/j.rser.2016.11.099

90. Yahya MA, Al-Qodah Z, Ngah CWZ. Agricultural bio-waste materials as potential sustainable precursors used for activated carbon production: A review. Renew. Sustain. Energy Rev. 2015;46:218–35. http://dx.doi.org/10.1016/j.rser.2015.02.051

91. Campbell QP, Bunt JR, Kasaini H, Kruger DJ. The preparation of activated carbon from South African coal. J. South. African Inst. Min. Metall. 2012;112(1)37–44. http://www.scielo.org.za/pdf/jsaimm/v112n1/v112n1a10.pdf

92. Danish M, Ahmad T. A review on utilization of wood biomass as a sustainable precursor for activated carbon production and application. Renew. Sustain. Energy Rev. 2018;87:1–21. https://doi.org/10.1016/j.rser.2018.02.003

93. Alonso DM, Wettstein SG, Dumesic JA. Bimetallic catalysts for upgrading of biomass to fuels and chemicals. Chem. Soc. Rev. 2012;41(24)8075–98. https://doi.org/10.1039/C2CS35188A
crossref pmid

94. Papari S, Hawboldt K, Helleur R. Pyrolysis: A theoretical and experimental study on the conversion of softwood sawmill residues to biooil. Ind. Eng. Chem. Res. 2015;54(2)605–11. https://doi.org/10.1021/ie5039456

95. Jahirul MI, Rasul MG, Chowdhury AA, Ashwath N. Biofuels production through biomass pyrolysis-A technological review. Energies. 2012;5(12)4952–5001. https://doi.org/10.3390/en5124952

96. Qambrani NA, Rahman MM, Won S, Shim S, Ra C. Biochar properties and eco-friendly applications for climate change mitigation, waste management, and wastewater treatment: A review. Renew. Sustain. Energy Rev. 2017;79:255–73. http://dx.doi.org/10.1016/j.rser.2017.05.057

97. Lua AC, Yang T, Guo J. Effects of pyrolysis conditions on the properties of activated carbons prepared from pistachio-nut shells. J. Anal. Appl. Pyrolysis. 2004;72(2)279–87. https://doi.org/10.1016/j.jaap.2004.08.001

98. Quan C, Gao N, Song Q. Pyrolysis of biomass components in a TGA and a fixed-bed reactor: Thermochemical behaviors, kinetics, and product characterization. J. Anal. Appl. Pyrolysis. 2016;121:84–92. https://doi.org/10.1016/j.jaap.2016.07.005

99. Ao W, Fu J, Mao X, et al. Microwave assisted preparation of activated carbon from biomass: A review. Renew. Sustain. Energy Rev. 2018;92:958–79. https://doi.org/10.1016/j.rser.2018.04.051

100. Chen S, Liu Z, Jiang S, Hou H. Carbonization: A feasible route for reutilization of plastic wastes. Sci. Total Environ. 2020;710:136250 https://doi.org/10.1016/j.scitotenv.2019.136250
crossref pmid

101. Cagnon B, Py X, Guillot A, Stoeckli F. The effect of the carbonization/activation procedure on the microporous texture of the subsequent chars and active carbons. Microporous Mesoporous Mater. 2003;57(3)273–82. https://doi.org/10.1016/S1387-1811(02)00597-8

102. Chang CF, Chang CY, Tsai WT. Effects of Burn-off and Activation Temperature on Preparation of Activated Carbon from Corn Cob Agrowaste by CO2 and Steam. J. Colloid Interface Sci. 2000;232(1)45–9. https://doi.org/10.1006/jcis.2000.7171
crossref pmid

103. Ahmad F, Ashri WM, Ahmad MA, Radzi R, Azmi AA. The effects of CO2 activation, on porosity and surface functional groups of cocoa (Theobroma cacao) - Shell based activated carbon. J. Environ. Chem. Eng. 2013;1(3)378–88. http://dx.doi.org/10.1016/j.jece.2013.06.004

104. Zhang YN, Ye ZF, Yang K, Dong SL. Antenna-predominant and male-biased CSP19 of Sesamia inferens is able to bind the female sex pheromones and host plant volatiles. Gene. 2014;Feb 536(2)279–86.
crossref pmid

105. Tan X fei, Liu S bo, Liu Y guo, et al. Biochar as potential sustainable precursors for activated carbon production: Multiple applications in environmental protection and energy storage. Bioresour Technol. 2017;227:359–72. http://dx.doi.org/10.1016/j.biortech.2016.12.083
crossref pmid

106. Shafeeyan MS, Ashri WM, Houshmand A, Shamiri A. A review on surface modification of activated carbon for carbon dioxide adsorption. J. Anal. Appl. Pyrolysis. 2010;89(2)143–51. http://dx.doi.org/10.1016/j.jaap.2010.07.006

107. Abdulrasheed AA, Jalil AA, Triwahyono S, Zaini MA, Gambo Y, Ibrahim M. Surface modification of activated carbon for adsorption of SO2 and NOX : A review of existing and emerging technologies. Renew. Sustain. Energy Rev. 2018;94:1067–85. https://doi.org/10.1016/j.rser.2018.07.011

108. László K, Szűcs A. Surface characterization of polyethyleneterephthalate (PET) based activated carbon and the effect of pH on its adsorption capacity from aqueous phenol and 2,3,4-trichlorophenol solutions. Carbon N. Y. 2001;39(13)1945–53. https://doi.org/10.1016/S0008-6223(01)00005-7

109. Menéndez JA, Phillips J, Xia B, Radovic LR. On the Modification and Characterization of Chemical Surface Properties of Activated Carbon: In the Search of Carbons with Stable Basic Properties. Langmuir. 1996;12(18)4404–10. https://doi.org/10.1021/la9602022

110. Ravenni G, Sárossy Z, Ahrenfeldt J, Henriksen UB. Activity of chars and activated carbons for removal and decomposition of tar model compounds – A review. Renew. Sustain. Energy Rev. 2018;94:1044–56. https://doi.org/10.1016/j.rser.2018.07.001

111. Pérez AF, Maldonado FJ, Moreno C. On the nature of surface acid sites of chlorinated activated carbons. Carbon N. Y. 2003;41(3)473–8. https://doi.org/10.1016/S0008-6223(02)00353-6

112. Bai BC, Kim EA, Lee CW, Lee YS, Im JS. Effects of surface chemical properties of activated carbon fibers modified by liquid oxidation for CO2 adsorption. Appl. Surf. Sci. 2015;353:158–64. http://dx.doi.org/10.1016/j.apsusc.2015.06.046

113. Figueiredo L, Pereira M, Freitas M, Orfaó J. Modification of the surface chemistry of activated carbons. Carbon N. Y. 1999;37:1379–89.

114. Mangun CL, Benak KR, Daley MA, Economy J. Oxidation of Activated Carbon Fibers: Effect on Pore Size, Surface Chemistry, and Adsorption Properties. Chem. Mater. 1999;11(12)3476–83. https://doi.org/10.1021/cm990123m

115. Otake Y, Jenkins RG. Characterization of oxygen-containing surface complexes created on a microporous carbon by air and nitric acid treatment. Carbon N. Y. 1993;31(1)109–21.

116. Jiang W, Xing X, Li S, Zhang X, Wang W. Synthesis, characterization and machine learning based performance prediction of straw activated carbon. J. Clean. Prod. 2019;212:1210–23. https://doi.org/10.1016/j.jclepro.2018.12.093

117. Montes MA, Suárez D, Menéndez JA, Fuente E. On the nature of basic sites on carbon surfaces: An overview. Carbon N. Y. 2004;42(7)1219–25. https://doi.org/10.1016/j.carbon.2004.01.023

118. Arenillas A, Smith KM, Drage TC, Snape CE. CO2 capture using some fly ash-derived carbon materials. Fuel. 2005;84(17)2204–10. https://doi.org/10.1016/j.fuel.2005.04.003

119. Contescu A, Vass M, Contescu C, Putyera K, Schwarz JA. Acid buffering capacity of basic carbons revealed by their continuous pK distribution. Carbon N. Y. 1998;36(3)247–58. https://doi.org/10.1016/S0008-6223(97)00168-1

120. Garten VA, Weiss DE. A new interpretation of the acidic and basic structures in carbons ii the chromene-carbonium ion couple in carbon. Aust. J. Chem. 1957;10(3)309–28. https://doi.org/10.1071/CH9570309

121. Plaza MG, Pevida C, Arenillas A, Rubiera F, Pis JJ. CO2 capture by adsorption with nitrogen enriched carbons. Fuel. 2007;86(14)2204–12. https://doi.org/10.1016/j.fuel.2007.06.001

122. Mercedes M, Lu Z, Zhang Y, Tang Z. Sorbents for CO2 capture from high carbon fly ashes. Waste Manag. 2008;28(11)2320–8. https://doi.org/10.1016/j.wasman.2007.10.012
crossref pmid

123. Plaza MG, Pevida C, Arias B, et al. Different approaches for the development of low-cost co2 adsorbents. J. Environ. Eng. 2009;135(6)426–32. http://dx.doi.org/10.1061/(ASCE)EE.1943-7870.0000009

124. Mopoung S, Nogklai W. Chemical and surface properties of longan seed activated charcoal. Int. J. Phys. Sci. 2008;3(10)234–9.

125. Bamdad H, Hawboldt K, MacQuarrie S. A review on common adsorbents for acid gases removal: Focus on biochar. Renew. Sustain. Energy Rev. 2018;81:1705–20. https://doi.org/10.1016/j.rser.2017.05.261

126. Ratan JK, Kaur M, Adiraju B. Synthesis of activated carbon from agricultural waste using a simple method: Characterization, parametric and isotherms study. Mater. Today Proc. 2018;5(2)3334–45. https://doi.org/10.1016/j.matpr.2017.11.576

127. Hernández M, Yperman J, Carleer R, et al. Adsorption of Ni(II) on spent coffee and coffee husk based activated carbon. J. Environ. Chem. Eng. 2018;6(1)1161–70. https://doi.org/10.1016/j.jece.2017.12.045

128. Pereira MS, Panisset CM, Martins AL, Sá CH, Barrozo MA, Ataíde CH. Microwave treatment of drilled cuttings contaminated by synthetic drilling fluid. Sep. Purif. Technol. 2014;124:68–73. https://doi.org/10.1016/j.seppur.2014.01.011

129. Tovar AK, Godínez LA, Espejel F, Ramírez RM, Robles I. Optimization of the integral valorization process for orange peel waste using a design of experiments approach: Production of high-quality pectin and activated carbon. Waste Manag. 2019;85:202–13. https://doi.org/10.1016/j.wasman.2018.12.029
crossref pmid

130. Tobi AR, Dennis JO, Zaid HM, Adekoya AA, Yar A, Fahad U. Comparative analysis of physiochemical properties of physically activated carbon from palm bio-waste. J. Mater. Res. Technol. 2019;8(5)3688–95. https://doi.org/10.1016/j.jmrt.2019.06.015

131. Nasri NS, Hamza UD, Ismail SN, Ahmed MM, Mohsin R. Assessment of porous carbons derived from sustainable palm solid waste for carbon dioxide capture. J. Clean. Prod. 2014;71:148–57. http://dx.doi.org/10.1016/j.jclepro.2013.11.053

132. Ghani ZA, Yusoff MS, Zaman NQ, Zamri MF, Andas J. Optimization of preparation conditions for activated carbon from banana pseudo-stem using response surface methodology on removal of color and COD from landfill leachate. Waste Manag. 2017;62:177–87. https://doi.org/10.1016/j.wasman.2017.02.026
crossref pmid

133. Sulaiman NS, Hashim R, Mohamad Amini MH, Danish M, Sulaiman O. Optimization of activated carbon preparation from cassava stem using response surface methodology on surface area and yield. J. Clean. Prod. 2018;198:1422–30.

134. Garg D, Kumar S, Sharma K, Majumder CB. Application of waste peanut shells to form activated carbon and its utilization for the removal of Acid Yellow 36 from wastewater. Groundw. Sustain. Dev. 2019;8:512–9. https://doi.org/10.1016/j.gsd.2019.01.010

135. Berhe T, Wang S, Nam H. Adsorption of H2S, NH3 and TMA from indoor air using porous corncob activated carbon: Isotherm and kinetics study. J Environ Chem Eng. 2019. 74 https://doi.org/10.1016/j.jece.2019.103234

136. Wu XX, Zhang CY, Tian ZW, Cai JJ. Large-surface-area carbons derived from lotus stem waste for efficient CO2 capture. Xinxing Tan Cailiao/New Carbon Mater. 2018;33(3)252–61. http://dx.doi.org/10.1016/S1872-5805(18)60338-5

137. Yu Q, Zhao H, Zhao H, et al. Preparation of tobacco-stem activated carbon from using response surface methodology and its application for water vapor adsorption in solar drying system. Sol Energy. 2019;324–36. https://doi.org/10.1016/j.solener.2018.11.029

138. Enniya I, Rghioui L, Jourani A. Adsorption of hexavalent chromium in aqueous solution on activated carbon prepared from apple peels. Sustain. Chem. Pharm. 2018;7:9–16. https://doi.org/10.1016/j.scp.2017.11.003

139. Li M, Xiao R. Preparation of a dual Pore Structure Activated Carbon from Rice Husk Char as an Adsorbent for CO2 Capture. Fuel Process. Technol. 2019;186:35–9. https://doi.org/10.1016/j.fuproc.2018.12.015

140. Hameed B, Din A, Ahmad A. Adsorption of methylene blue onto bamboo-based activated carbon: Kinetics and equilibrium studies. J. Hazard. Mater. 2007;141(3)819–25. https://doi.org/10.1016/j.jhazmat.2006.07.049
crossref pmid

141. Loredo M, Soto E, Cerino FJ, García RB, García AM, Garza MT. Determining optimal conditions to produce activated carbon from barley husks using single or dual optimization. J. Environ. Manage. 2013;125:117–25. https://doi.org/10.1016/j.jenvman.2013.03.028
crossref pmid

142. Van Thuan T, Quynh BTP, Nguyen TD, Ho VTT, Bach LG. Response surface methodology approach for optimization of Cu2+, Ni2+ and Pb2+ adsorption using KOH-activated carbon from banana peel. Surfaces and Interfaces. 2017;6:209–17. https://doi.org/10.1016/j.surfin.2016.10.007

143. Elisadiki J, Jande YAC, Machunda RL, Kibona TE. Porous carbon derived from Artocarpus heterophyllus peels for capacitive deionization electrodes. Carbon N. Y. 2019;147:582–93. https://doi.org/10.1016/j.carbon.2019.03.036

144. Treviño H, Juárez LG, Mendoza DI, Hernández V, Bonilla A, Montes MA. Synthesis and adsorption properties of activated carbons from biomass of Prunus domestica and Jacaranda mimosifolia for the removal of heavy metals and dyes from water. Ind. Crops Prod. 2013;42(1)315–23. https://doi.org/10.1016/j.indcrop.2012.05.029

145. Arroyo JJ, Villarroel D, de Freitas KC, Martínez CA, Sapag K. Applicability of activated carbon obtained from peach stone as an electrochemical sensor for detecting caffeine. J. Electroanal. Chem. 2018;822:171–6. https://doi.org/10.1016/j.jelechem.2018.05.028

146. Pezoti O, Cazetta AL, Bedin KC, et al. NaOH-activated carbon of high surface area produced from guava seeds as a high-efficiency adsorbent for amoxicillin removal: Kinetic, isotherm and thermodynamic studies. Chem. Eng. J. 2016;288:778–88. https://doi.org/10.1016/j.cej.2015.12.042

147. Heraldy E, Lestari WW, Permatasari D, Arimurti DD. Biosorbent from tomato waste and apple juice residue for lead removal. J. Environ. Chem. Eng. 2018;6(1)1201–8. 10.1016/j.jece.2017.12.026

148. Hsu SH, Huang CS, Chung TW, Gao S. Adsorption of chlorinated volatile organic compounds using activated carbon made from Jatropha curcas seeds. J. Taiwan Inst. Chem. Eng. 2014;45(5)2526–30. https://doi.org/10.1016/j.jtice.2014.05.028

149. Pongener C, Bhomick PC, Supong A, Baruah M, Sinha UB, Sinha D. Adsorption of fluoride onto activated carbon synthesized from Manihot esculenta biomass - Equilibrium, kinetic and thermodynamic studies. J. Environ. Chem. Eng. 2018;6(2)2382–9. https://doi.org/10.1016/j.jece.2018.02.045

150. Baldelli A, Trivanovic U, Sipkens TA, Rogak SN. On determining soot maturity: A review of the role of microscopy- and spectroscopy-based techniques. Chemosphere. 2020;252:126532 https://doi.org/10.1016/j.chemosphere.2020.126532
crossref pmid

151. Ronix A, Pezoti O, Souza LS, et al. Hydrothermal carbonization of coffee husk: Optimization of experimental parameters and adsorption of methylene blue dye. J. Environ. Chem. Eng. 2017;5(5)4841–9. https://doi.org/10.1016/j.jece.2017.08.035

152. Paredes JI, Gracia M, Martínez A, Tascón JM. Nanoscale investigation of the structural and chemical changes induced by oxidation on carbon black surfaces: A scanning probe microscopy approach. J. Colloid Interface Sci. 2005;288(1)190–9. https://doi.org/10.1016/j.jcis.2005.02.081
crossref pmid

153. ASTM. Standard Test Method for Determination of Iodine Number of Activated Carbon 1. ASTM Int. 2006;94( Reapproved)1–5. http://compass.astm.org.acces.bibl.ulaval.ca/download/D4607.6656.pdf

154. Kumar S, Nehra M, Kedia D, Dilbaghi N, Tankeshwar K, Kim KH. Carbon nanotubes: A potential material for energy conversion and storage. Prog. Energy Combust. Sci. 2018;64:219–53. https://doi.org/10.1016/j.pecs.2017.10.005

155. Pincus G. Introduction to Infrared and Raman Spectroscopy. J. Am. Chem. Soc. 1965;87(5)1155–6. https://doi.org/10.1021/ja01083a063

156. Wepasnick KA, Smith BA, Bitter JL, Howard Fairbrother D. Chemical and structural characterization of carbon nanotube surfaces. Anal. Bioanal. Chem. 2010;396(3)1003–14. https://doi.org/10.1007/s00216-009-3332-5
crossref pmid

157. Dalton JS, Janes PA, Jones NG, Nicholson JA, Hallam KR, Allen GC. Photocatalytic oxidation of NOx gases using TiO2: A surface spectroscopic approach. Environ. Pollut. 2002;120(2)415–22. https://doi.org/10.1016/S0269-7491(02)00107-0
crossref pmid

158. Yung MM, Holmgreen EM, Ozkan US. Cobalt-based catalysts supported on titania and zirconia for the oxidation of nitric oxide to nitrogen dioxide. J. Catal. 2007;247(2)356–67. https://doi.org/10.1016/j.jcat.2007.02.020

159. Irfan Malik M, Malaibari ZO, Atieh M, Abussaud B. Electrochemical reduction of CO2 to methanol over MWCNTs impregnated with Cu2O. Chem. Eng. Sci. 2016;152:468–77. https://doi.org/10.1016/j.ces.2016.06.035

160. France LJ, Yang Q, Li W, et al. Ceria modified FeMnOx—Enhanced performance and sulphur resistance for low-temperature SCR of NOx. Appl. Catal. B Environ. 2017;206:203–15. https://doi.org/10.1016/j.apcatb.2017.01.019

161. Larkin PJ. Infrared and Raman Spectroscopy: Principles and Spectral Interpretation Infrared and Raman Spectroscopy: Principles and Spectral Interpretation. Elsevier Inc; 2017. p. 1–286.

162. Smith E, Dent G. Modern Raman Spectroscopy - A Practical Approach Modern Raman Spectroscopy - A Practical Approach. John Wiley and Sons; 2005. p. 1–210.

163. Starr DE, Liu Z, Hävecker M, Knop A, Bluhm H. Investigation of solid/vapor interfaces using ambient pressure X-ray photoelectron spectroscopy. Chem. Soc. Rev. 2013;42(13)5833 https://doi.org/10.1039/C3CS60057B
crossref pmid

164. Fadley CS. X-ray photoelectron spectroscopy and diffraction in the hard X-ray regime: Fundamental considerations and future possibilities. Nucl. Instruments Methods Phys. Res. Sect. A Accel. Spectrometers, Detect. Assoc. Equip. 2005;547( 1)24–41. https://doi.org/10.1016/j.nima.2005.05.009

165. Susi T, Pichler T, Review PA. X-ray photoelectron spectroscopy of graphitic carbon nanomaterials doped with heteroatoms. Beilstein J. Nanotechnol. 2015;6:177–92. https://doi.org/10.3762/bjnano.6.17
crossref pmid pmc

166. Tongpoothorn W, Sriuttha M, Homchan P, Chanthai S, Ruangviriyachai C. Preparation of activated carbon derived from Jatropha curcas fruit shell by simple thermo-chemical activation and characterization of their physico-chemical properties. Chem. Eng. Res. Des. 2011;89(3)335–40. http://dx.doi.org/10.1016/j.cherd.2010.06.012

167. Keiluweit M, Nico PS, Johnson MG, Kleber M. Dynamic Molecular Structure of Plant Biomass-Derived Black Carbon (Biochar). Environ. Sci. Technol. 2010;44(4)1247–53. https://doi.org/10.1021/es9031419
crossref pmid

168. Boehm HP. Surface oxides on carbon and their analysis: A critical assessment. Carbon N. Y. 2002;40(2)145–9. https://doi.org/10.1016/S0008-6223(01)00165-8

169. Kim UJ, Furtado CA, Liu X, Chen G, Eklund PC. Raman and IR spectroscopy of chemically processed single-walled carbon nanotubes. J. Am. Chem. Soc. 2005;127(44)15437–45. https://doi.org/10.1021/ja052951o
crossref pmid

170. Smith BC. Fundamentals of Fourier Transform Infrared Spectroscopy [Internet]. SecondCRC Press; 2011. p. 207 https://doi.org/10.1201/b10777

171. Baker MJ, Trevisan J, Bassan P, et al. Using Fourier transform IR spectroscopy to analyze biological materials. Nat. Protoc. 2014;9(8)1771–91. https://doi.org/10.1038/nprot.2014.110
crossref pmid pmc

172. Fuente E, Menéndez JA, Díez MA, Suárez D, Montes MA. Infrared Spectroscopy of Carbon Materials: A Quantum Chemical Study of Model Compounds. J. Phys. Chem. B. 2003;107(26)6350–9. https://doi.org/10.1021/jp027482g

173. Barkauskas J, Dervinyte M. Investigation of the functional groups on the surface of activated carbons. J. Serbian Chem. Soc. 2004;69(5)363–75. https://doi.org/10.2298/JSC0405363B

174. Dąbrowski A, Podkościelny P, Hubicki Z, Barczak M. Adsorption of phenolic compounds by activated carbon - A critical review. Chemosphere. 2005;58(8)1049–70. https://doi.org/10.1016/j.chemosphere.2004.09.067
crossref pmid

175. Fidel RB, Laird DA, Thompson ML. Evaluation of modified boehm titration methods for use with biochars. J. Environ. Qual. 2013;42(6)1771–8.
crossref pmid pdf

176. Singh B, Camps MA, Lehmann J. Biochar: A Guide to Analytical Methods. First. CRC, press; 2017. p. 320

177. Salame II, Bandosz TJ. Surface chemistry of activated carbons: Combining the results of temperature-programmed desorption, Boehm, and potentiometric titrations. J. Colloid Interface Sci. 2001;240(1)252–8. https://doi.org/10.1006/jcis.2001.7596
crossref pmid

178. Figueiredo JL, Pereira MFR. The role of surface chemistry in catalysis with carbons. Catal. Today. 2010;150(1–2)2–7. https://doi.org/10.1016/j.cattod.2009.04.010

179. Tsechansky L, Graber ER. Methodological limitations to determining acidic groups at biochar surfaces via the Boehm titration. Carbon N. Y. 2014;66:730–3. http://dx.doi.org/10.1016/j.carbon.2013.09.044

180. Zhang X, Zhang Y, Wang S, Zhang J, Zhou W. Effect of activation agents on the surface chemical properties and desulphurization performance of activated carbon. Sci. China Technol. Sci. 2010;53(9)2515–20. https://doi.org/10.1007/s11431-010-4058-5

181. Kalijadis A, Vukcevic M, Jovanovic Z, Lausevic Z, Lausevic M. Characterization of surface oxygen groups on different carbon materials by the Boehm method and temperature programmed desorption. J. Serbian Chem. Soc. 2011;76(5)757–68. https://doi.org/10.2298/JSC091224056K

182. Jensen IG, Skovsgaard L. The impact of CO2-costs on biogas usage. Energy. 2017;134:289–300. http://dx.doi.org/10.1016/j.energy.2017.06.019

183. International Energy Agency (IEA). IEA Bioenergy Task 37 updated list of biogas upgrading plants [Internet]. 2019. http://task37.ieabioenergy.com/plant-list.html

184. Farooq M, Bell AH, Almustapha MN, Andresen JM. Bio-methane from an-aerobic digestion using activated carbon adsorption. Anaerobe. 2017;46:33–40. http://dx.doi.org/10.1016/j.anaerobe.2017.05.003
crossref pmid

185. Angelidaki I, Treu L, Tsapekos P, et al. Biogas upgrading and utilization: Current status and perspectives. Biotechnol. Adv. 2018;36(2)452–66. https://doi.org/10.1016/j.biotechadv.2018.01.011
crossref pmid

186. Sarkar SC, Bose A. Role of activated carbon pellets in carbon dioxide removal. Energy Convers. Manag. 1997;38:S105–S110. https://doi.org/10.1016/S0196-8904(96)00254-3

187. Yi H, Li F, Ning P, et al. Adsorption separation of CO2, CH4, and N2 on microwave activated carbon. Chem Eng J. 2013;215–216:635–42. http://dx.doi.org/10.1016/j.cej.2012.11.050

188. Gu M, Zhang B, Qi Z, et al. Effects of pore structure of granular activated carbons on CH4 enrichment from CH4/N2 by vacuum pressure swing adsorption. Sep. Purif. Technol. 2015;146:213–8. https://doi.org/10.1016/j.seppur.2015.03.051

189. Pino L, Italiano C, Vita A, Fabiano C, Recupero V. Sorbents with high efficiency for CO2 capture based on amines-supported carbon for biogas upgrading. J. Environ. Sci. (China). 2016;48:138–50. http://dx.doi.org/10.1016/j.jes.2016.01.029
crossref pmid

190. Álvarez N, Gil MV, Rubiera F, Pevida C. Kinetics of CO2 adsorption on cherry stone-based carbons in CO2/CH4 separations. Chem. Eng. J. 2017;307:249–57. https://doi.org/10.1016/j.cej.2016.08.077

191. Álvarez N, Gil M V, Rubiera F, Pevida C. Simplistic approach for preliminary screening of potential carbon adsorbents for CO2 separation from biogas. J. CO2 Util. 2018;28:207–15. https://doi.org/10.1016/j.jcou.2018.10.001

192. Piechota G. Multi-step biogas quality improving by adsorptive packed column system as application to biomethane upgrading. J. Environ. Chem. Eng. 2021;9(5)105944 https://doi.org/10.1016/j.jece.2021.105944

193. Durán I, Álvarez N, Rubiera F, Pevida C. Biogas purification by means of adsorption on pine sawdust-based activated carbon: Impact of water vapor. Chem. Eng. J. 2018;353:197–207. https://doi.org/10.1016/j.cej.2018.07.100

194. Al Mamun MR, Karim MR, Rahman MM, Asiri AM, Torii S. Methane enrichment of biogas by carbon dioxide fixation with calcium hydroxide and activated carbon. J Taiwan Inst Chem Eng. 2016;58:476–81. https://doi.org/10.1016/j.jtice.2015.06.029

195. Yu CH, Huang CH, Tan CS. A review of CO2 capture by absorption and adsorption. Aerosol Air Qual. Res. 2012;12( 5)745–69. https://doi.org/10.4209/aaqr.2012.05.0132

196. Sreedhar I, Nahar T, Venugopal A, Srinivas B. Carbon capture by absorption – Path covered and ahead. Renew. Sustain. Energy Rev. 2017;76:1080–107. http://dx.doi.org/10.1016/j.rser.2017.03.109

197. Rasi S, Veijanen A, Rintala J. Trace compounds of biogas from different biogas production plants. Energy. 2007;32(8)1375–80. https://doi.org/10.1016/j.energy.2006.10.018

Fig. 1
Tendencies in techniques for the upgrading and enrichment of biogas in published scientific articles
Table 1
Main Agricultural Crops and Their Associated Wastes [39, 4245]
Crop Generated waste
Cotton Stems
Rice Husk, straw
Oats Grass, straw
Bamboo Stems, leaves
Coffee Husks, pulps
Sugarcane Bagasse, molasses, cane trash, filter mud and vinasse,
Barley Grass, shells, bran
Plum Seeds
Coconut Shells, fiber, copra
Jatropha Shells
Maize Cobs, stubble, stems, leaves
Peanuts Shells
Apple Pulp
Orange Rinds
Nuts Shells
Oil palm Stems without fruits, fiber, shells
Potato Shells
Pineapple Shells and crown
Banana Peels, pseudostem, rachis
Soy Hulls, meal
Tobacco Fallen leaves
Tomato Leaves, stems
Wheat Grass, hulls, bran
Grapes (vines) Seeds, stems
Table 2
Summary of the Components of Different Crop Wastes
Crop waste Proximate analysis (%) Ultimate analysis (%) Lignocellulosic composition (%) Author’s

Moisture Ash Volatile material Fixed carbon C H O N S Cellulose Hemicellulose Lignin
Peach pits 9.3 1.1 71.7 17.9 45.92 6.09 47.38 0.58 0.03 46 14 33 [5]
Guava seeds 6.5 0.3 28 65.2 88.3 1.6 8.4 1.3 0.09 28 15.5 41.7 [5]
Barley husks 9.0 5.0 77.20 17.30 45.40 6.10 41.92 0.70 0.07 36 24 6.3 [59, 63]
Wheat straw 6.30 5.97 66.96 20.77 46.5 6.3 46.3 0.9 - 30 50 15 [59, 64, 65]
Rice husks 10.57 22.93 61.68 15.4 29.98 4.46 42.31 0.42 0.005 42.5 24.5 9.2 [66, 67]
Coffee husks 7.3 4.9 76.1 19 38.7 5.4 - - - - - - [68]
Sugarcane bagasse 8.8 5.6 82.5 11.9 45.8 5.1 45 1 - 45 20 30 [68]
Almond shells 10 0.6 80.3 9.1 50.5 6.6 42.69 0.2 0.01 32.50 25.50 24.80 [69]
Walnut shell 11 1.3 71.8 15.90 45.1 6.00 48.60 0.3 0.00 40.1 20.7 18.2 [69]
Olive seeds 10.4 1.4 74.4 13.8 44.8 6.00 49.09 0.1 0.01 30.8 17.1 32.6 [69]
Cocoa bean shells 6.66 9.0 - - 50.30 5.70 40.60 3.50 - 13.24 10.81 16.43 [70]
Barley straw 9.0 5 77.2 17.2 45.4 6.1 41.92 0.7 0.07 - - - [63]
Coconut shells 8.21 0.1 73.09 - 48.63 6.51 44.64 0.14 0.08 19.8 68.7 30.1 [71]
Oil palm shells 7.96 1.1 72.47 18.7 50.01 6.90 41 1.9 0 29 47.7 53.4 [71]
Orange peel 9.2 3.09 76.52 20.39 46.63 6.04 47.05 0.23 0.05 - - - [72]
Banana rachis - 28.50 - - 63.80 7.3 - 2.5 - 35.30 17.90 76.5 [73]
Banana peel 9.49 13.36 62.62 14.53 39.95 7.06 52.28 0.71 - 40.15 10.50 24.28 [74]
Jackfruit rind 10 4 50 36 - - - - - - - - [75]
Pistachio shells 6.99 0.09 80.01 12.08 42.41 5.64 51.87 0.07 0.01 53.98 20.10 25.25 [76]
Corn cobs 4.3 0.9 78.7 - 46.8 6 46.3 0.9 - 33.6 37.2 19.3 [77]
Oat straw 4.38 3.21 74.04 18.37 43.6 6.12 45.39 0.88 1.14 37.6 23.7 12.9 [78]
Apple pulp 5.7 2.63 79.03 3.28 49.56 8.43 38.22 0.97 0.05 43.99 - 17.32 [79]
Tobacco waste 8.13 11.73 67.55 12.59 40.95 5.21 47.85 5.99 - 42.3 - 8.75 [80]
Plum pits 2.3 0.8 - - 45.6 6.7 46.5 0.3 - 22.9 9 25.4 [81]
Tamarind seeds 10 4.5 36.5 48.9 - - - - - - - - [82]
Grape seeds 8.5 4.7 72.8 14 48.7 6.5 43.7 1.1 - - - - [83]
Grape stems 7 8.7 63.13 21.17 41.58 4.92 52.16 1.18 0.16 - - - [84]
Tomato stems 3.58 10.6 - - - - - - - 27.03 21.1 16.01 [85]
Jatropha curvas meal 7.3 2.6 50.9 39.2 47.8 6.7 41.2 4.3 - - - - [86]
Table 3
Types, Conditions, and Products of Pyrolysis Methods [95, 96].
Types of pyrolysis Temperature (°C) Residence time Carbonized product (% weight) Liquid (% weight) Gas (% weight)
Slow 300–700 Hours or days 35 30 35
Intermediate ~500 10–20 20 50 30
Fast 850–1250 0.5–10 12 75 13
Flash 1050–1300 <0.5 20 50 30
Table 4
Summary of Synthesis Process Used to Obtain AC from Crop Wastes
Crop residues Particle size (0 mm) Carbonization/pyrolysis Activation Yield (%) Surface area (m2 g−1) Pore volume (cm3g−1) Pore diameter (nm) Absorption capacity (mg g−1) Author’s
Physical activation

Barley straw 0.045 – 0.5 500 – 600 °C / 60 – 120 min (N2) 600 – 000 °C / 60 – 120 min (N2 and CO2) 2.21 – 36.56 160 – 788 (BET) 0.0849 – 0.3495 1.773 – 2.235 - [63]
Sugarcane bagasse - 460 °C / 60 min (Vacuum 8 kpa) 800 °C / 60 min (H2O) 61.7 570 (BET) 0.356 25 – 26.1 (BET) 5.22 g g−1 (methylene blue) [68]
Almond shells 1 – 2 600 °C / 60 min (N2) 850 °C / 30 min (H2O) - 601 (BET) 0.341 (BET) - 0.309 (CO2) [69]
Nut shells 1 – 2 600 °C / 60 min (N2) 850 °C / 30 min (H2O) - 42 (BET) 0.071 (BET) - 0.458 (CO2) [69]
Olive seeds 1 – 2 600 °C / 60 min (N2) 850 °C / 30 min (H2O) - 53 (BET) 0.028 (BET) - 0.309 (CO2) [69]
Coconut shells 1 – 2 850 °C / 60 min (N2) 850 °C / 5 – 80 min (CO2) - 183 - - 120 [71]
Pistachio shells 500 °C / 60 min (N2) 900 °C / 300 min (CO2) 57.5 708.6 0.359 2.02 - [76]
Coconut shells - 400 – 800 °C / 60 min (N2) 700 °C / 60 min (H2O) 18.90 21.40 0.36 0.65 - [101]
Cocoa bean shells Pellets (10 × 1 mm 0) 800 °C / 60 min (N2) 850 °C / 5 – 80 min (CO2) 73.5 558.3 (BET) 0.166 2.3 - [103]
Rice husks - Direct flame (firewood) NaCl (4:1) 34.1 281.7 0.14 - 72.4 [126]
Coffee husk - 500 °C / 60 min (N2) 800 °C / 30 min (H2O) - 383 0.176 0.84 (N2)
0.47 (CO2)
57.14 [127]
Cocoa bean shells - 500 °C / 40 min (N2) 800 °C / 30 min (H2O) - 642 - - - [128]
Orange peels 0.4 400 – 600 °C (H3PO4-HCl) / 60 – 180 mm (N2) - 29.62 – 49.00 0.103 – 62.919 - - 2342.91 [129]
Shells oil palm oil palm 3 – 5 700 °C (Ar) 700 °C / 60 min (N2) - 700 0.862 3.28 - [130]
Bagasse oil palm - 700 °C (Ar) 700 °C / 60 min (N2) - 592 0.657 2.00 - [130]
Branches of oil palm - 700 °C/(Ar) 700 °C/60 min (N2) - 823 0.985 4.20 - [130]
Oil palm shells 0.5 – 1.18 700 °C/120 min (N2) 800 °C (N2 / CO2) - 167.08 (BET) 0.089 21.47 0.60 – 6.30 (mmol g−1) [131]
Banana rachis - - 761 °C / 87 min 27 - - - 1101 (Iodine number) [132]
Cassava stems 10 – 20 300 °C / 30 min 900 °C / 150 min 4.7 674.40 0.2966 1.879 - [133]

Peanut shells 0.833 – 1.65 - 650 °C / 120 min 37.2 965.67 BET) 0.204 - - [134]
Corn cobs - 450 °C / (KOH) 800 °C/180 min (Ar) 1618 0.8117 0.9404 164.5 (H2S); 190.68 (NH3); 323.56 (TMA) [135]
Lotus stems - 450 °C /120 min (N2) 800 °C / 60 min (N2 80 ml min−1) 45 2893 1.59 2 – 5 385 – 6.17 mmol g 25 – 0 °C [136]

Chemical activation

Plum pits - 180 °C / 45 min (H3PO4) 500 °C /120 min 76.5 829 (BET) 0.418 1.008 172.43 [81]
Tamarind seeds - 300 – 400°C (NaHCO3) 700 – 900 °C (HNO3) - - - - 4.94 [82]
Grape seeds - 800 °C / 60 min (K2CO3 - KOH) (N2) - 1222 – 1238 (BET) 0.47 - - [83]
Tobacco waste 20 – 60 (mesh) 70 °C / 420 h (ZnCl2) 621 °C / 60 – 73 min (N2) - 1736.90 - - 576.7 (H2O) [137]
Apple peels - - 619 °C / 56 min (H3PO4) - - - - 36.01 [138]
Rice husk - 900 °C / 60 min (N2) K2CO3 - 1097 0.34 – 0.49 - 3.1 – 7.6 (CO2) [139]
Bamboo 1 – 2 700 °C / 60 min (N2) 850 °C /120 min (CO2 / KOH) - 1896 (BET) 1.109 (BET) 2.34 (BET) 454.20 (Methylene blue) [140]
Barley husk 0.6 – 1 300 – 700 °C / 20 – 180 min (ZnCl2) 436 °C / 20 min 48.48 811.44 (BET) - - 901.86 (Iodine number) [141]
Banana peels 1 500 °C / 60 min (N2) 500 °C / 30 min (KOH / N2) - 63.5 (BET) 0.014 11.1 - [142]
Dragon fruit rind - 400 °C / 60 min 700 °C / 60 min (KOH / N2) - 1955 1.35 3.05 - [143]
Wheat straw 0.18 – 0.3 140 °C / 60 min (H3PO4) 450 – 550 °C / 60 min (N2) 47.51 522.89 0.3849 3.48 - [116]
Sorghum straw 0.18 – 0.3 140 °C / 60 min (H3PO4) 450 – 550 °C / 60 min (N2) 39.88 487.58 0.3315 2.88 - [116]
Plum pits 1 70 °C / 60 min (H3PO4 /CaCO3 from eggs) 800 °C / 240 min - 329 0.155 - 0.01 – 0.25 [144]
Peach pits 2 80 °C /120 min (ZnCl2/FeCl3) 550 °C /180 min (N2) - 760 – 1020 (BET) 0.35 – 0.45 - - [145]
Guava seeds 425 – 500 pm 500 °C /120 min (N2)
130 °C / 240 min (NaOH)
750 °C / 90 min (N2) - 2573.6 (BET) 1.260 1.06 570.4 [146]
Tomato waste 100 (mesh) Room temperature / 24 h (NaOH) - - 11.13 0.0081 1.49 152 (Pb II) [147]
Apple juice pulp 100 (mesh) Room temperature / 24 h (NaOH) - - 45 0.0044 1.08 108 (Pb II) [147]
Jatropha curcas 500 μm 400 °C / 60 min (N2) 800 °C /120 min (NaOH-N2) - 1758 (BET) 0.9238 23.39 - [148]
Cassava stems and leaves 2 600 °C /180 min (N2) 120 – 180 min (H3PO4) - 812 (BET) 0.498 - 1.568 [149]
Table 5
Characteristics of Gaseous Fuels (Part A); Characteristics of Biogas by Source and the Requirements for Its Use (Part B).
Características LPG* NG* Biogas Author's
Part A Composition (% v/v) 85 (CH4) 57 (CH4) [26]
30 (C3H8) 7 (C2H6) 41 (CO2)
70 (C4H10) 2 (C3H8) 0.18 (CO)
1 (N2) 0.18 (H2)
5 (CO2) Traces gases
Lower heating value at 1 atm and 15 °C (MJ kg1) 45.7 50 17
Density at 1 atm and 15 °C 2.26 0.79 1.2
Flame speed (cm s−1) 44 34 25
Stoichiometric A/F (kg air/kg fuel) 15.5 17.3 5.8
Flammabiltiy limits (% Vol in air) 2.15 – 9.6 5 – 15 7.5 – 14
Octane number 103 – 106 120 130
Autoignition temperature (°C) 406 – 450 540 650

Part B Type plant / Requirement’s biofuel CH4 ' CO2 ' O2 ' N2 ' H2 ' H2S HHV* (MJ m−3) Wobbe Index (MJ m−3)

Landfill 37 – 67.9 24 – 40.1 < 1 < 1 – 17 - 15.1 – 427.5 - - [197]
Farm biogas plant 55 – 58 37 – 38 < 1 <1 – 2 - 32 – 169 - -
Sewage digester 57.8 – 65 33.9 – 38.6 0 ≤ 1 < 2 – 8.1 - 24-1 – 62.9 - -

Biogas requirement in unit (manufacturers limits) 50.0 38.2 1.1 1.9 0.05 1100 26.5 25.0 [192]
Biomethane in gas grid 95.5 1.1 0.05 1.5 0.05 5.0 39.0 50.2

* Liquefied petroleum gas;

* Higher Heating Value.

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Table 6
Comparison of Different Biogas Enrichment Techniques [184, 185].
Variables Membrane separation Physical washing Chemical adsorption Cryogenics Water washing Adsorption by pressure variation Physical adsorption (AC)
Energy consumption (kWh m−3) 0.18–0.20 0.20–0.30 0.05–0.15 0.76 0.25–0.30 0.23–0.30 0.11
Heat demand (°C) - 55–80 100–180 −196 - - 20
Loss of CH4 (%) < 0.6 2–4 < 0.1 2 < 2 < 4 -
Pre-purification Suggested Suggested Yes Yes Suggested Yes No
Operating pressure (bar) 5–8 4-8 Atmospheric 80 4–10 3–10 1–7
Cost High Medium High High Medium Medium Low

[i] Note: (*) number of plants

Table 7
Relevant Studies on the Capture of CO2 by AC.
Characteristics References

[136] [184] [186] [187] [188] [189] [190] [191] [192] [193] [139] [194]
Type of AC Granular - Pellets - Granular Granular - - - (Commercial) Granular (Commercial) Pellets - -
Origin Lotus stems Commercial Coconut shells - Anthracite Commercial Cherry stems Commercial bituminous carbon Coconut shells Bituminous carbon phenolic resin Coconut Pine sawdust Rice husk Commercial
Activation H2SO4 / N2 - Carbonized Microwaves Physical vapor Impregnated with tetraethylenepentamine Vapor CO2 Vapor Vapor - CO2 CO2 and K2CO3 -

Pressure (Bar) 1 - 1.5 100 (kPa) 0.90 (MPa) - 1 500 (kPa) - 135 (kPa) 1 >1
Temperature (°C) 25 25 - 12 (Δt exothermic) 23 25 70 30 30 23.9 30 25 25
Cycles - 3 2 1 - - 20 6 6 - 6 7 ~ 25 (environmental)
Gases CO2 CO2 CO2/H2S/N2 (43:1:56 %v/v) CO2 /air CO2/CH4/N2 CH4/N2 (30:70 %v/v) CO2 (80 %v/v) CH4 / CO2 (1.5 molar ratio) CO2:CH4 (65:35 %v/v) CO2/CH4 (50:50 %v/v) CH4/CO2/N2/O2 (62.5/35.44/1.79/0.25 %mol/mol) CO2/CH4 (65:35 %v/v) CO2 pure CO2/CH4 (62:34 %v/v)
Sorption capacity (mmol g−1) 3.85 - - - 2.13 (CO2) 5.74 - 2.12 (mol kg−1 1.98 (mol kg−1) 1.73 (mol kg−1) 13.484 10.219 13.2687 - 1.64 3.1 -
Saturation capacity (mmol g−1) - - - - - 6.90 - - - - - - - - - 3.1 -

Operating time (min) - - 196 (min) 2 - 30 (s) 34 - 60 26.85 16.87 29.05 4 – 18 60 2 24 (h)
Efficiency - 0.067 0.34 - - 0.21 0.37 - - - - - - 99.76 - 95 81
Selectivity - - - - - - 24 - - - - - - organic impurities 3.09 7.6 -

Saturation time - - 7–9 min - - - 162 320 8–16 - - - 310 4.36 20 20
Desorption conditions 200 °C / 6 h - 40 ml min−1 (N2) / 4 min - - 100 min / 120 °C 180 °C / 60 min / He 50 ml min−1 100 (kPa) - - 25 °C -
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