### 1. Introduction

### 2. Materials and Methods

### 2.1. Materials

### 2.2. Sample Analysis

### 2.3. Experimental Setup

_{vinegar}, Y

_{tar}, Y

_{biochar}, and Y

_{non condensable gases}[6]. They were determined on a wet mass basis (wb, wt.%) of the initial dried biomass feed, m

_{biomass}, as illustrated by the Eq. (1)–(3) [28]. However, bio-oil yield was taken as the total liquid products (vinegar and tar) collected from the initial biomass feed content [29]. The vinegar and tar compounds were characterized via gas chromatography (GC) and physicochemical properties analyses while for biochar the calorific values were examined [30].

_{0}= weight of biomass feed, M

_{1}= Weight of empty measuring cylinder, M

_{2}= Weight of measuring cylinder with vinegar, M

_{3}= Weight of measuring cylinder with tar, M

_{R0}= weight of empty reactor, and M

_{R1}= Weight of reactor with biochar while the non-condensable gas (NCG) yields were determined from the difference [29].

### 2.4. Experimental Design

^{n}); m center point experiments (0, 0) that are augmented with a group of axial/star points (2.n) was employed. The span from factorial point design space to the center point was taken as either +1 or −1 and from the center to the star points as alpha |

*α*| > 1 [33]. The total number of experiments, N, was calculated by Eq. (4) [26], where n is the numbers of factors and m center points. Consequently, a series of 20 experimental runs, 8 factorial (cube) points, 6 axial points and 6 center points (m = 6) replicate based on 3 factors (variables) and 3 base block experiments were performed. The number of axial/star points in a CCD was twice the total number of the design factors and they denoted the extreme values (low or high) for each design factor. Thus, the CCD was experimented in multiple blocks to create orthogonality within the blocks, enabling factors and block effects to be estimated solely while minimizing the variation in the regression coefficients [32].

### 2.5. Statistical and Optimization Analysis

*p*≤ 0.05 [35]. The optimum conditions for the three variables, heating temperature (A), sample mass (B) and residence time (C) was obtained using data from the statistical analysis. Similarly, Minitab software was used to fit the equations developed and to prepare the response surfaces and contour plots.

### 3. Results and Discussion

### 3.1. Experimental Results

### 3.2. Statistical and Regression Analysis of Vinegar, Tar and Char Models

*p*) values and

*F*-tests at 95% confidence level generating regression coefficients and ANOVA for the quadratic model of the response surfaces. The larger the

*F*-value the more significant the variable while

*p*-values less than 0.05 indicated a significant model term. Again, the relatively smaller

*p*-values of the squared and the interactions terms denoted a firm possibility of a response surface curvature. Both the ANOVA and regression analysis compared the linear, quadratic and interaction terms and tested their

*p*-values for significance [36].

*R*

*values as well as the*

^{2}*F*and

*p*values from which the statistical significance of the experimental factors was investigated. Table 4 outline the estimated regression coefficients and ANOVA for reduced regression models for banana peels vinegar, tar and biochar yields.

^{2}values (vinegar = 0.84, tar = 0.80 and biochar = 0.91) and the smaller error terms (vinegar, S = 4.75, tar, S = 4.38 and biochar, S = 1.47). Thus the models sufficiently fitted the data and there was an immense reduction of the factors variability. Furthermore, the high

*F*-values and the low

*p*-values for the models lack of fit indicated no significant lack of fit for banana peels responses and proved that the models were statistically significant [26]. Therefore, sample mass (B)

*p*= 0.020, residence time (C)

*p*= 0.012 and the quadratic terms of sample mass (B

^{2})

*p*= < 0.001 and residence time (C

^{2})

*p*= 0.015, had a significant effects on banana peels vinegar yield. Equally, sample mass (B)

*p*= < 0.001 also had significant effects on banana peels tar response. Moreover, temperature (A)

*p*= < 0.001, sample mass (B)

*p*= < 0.001 and residence time (C)

*p*= <0.001, quadratic term for sample mass (B

^{2})

*p*= 0.009 as well as the interactions of temperature and residence time (AC)

*p*= 0.030 and sample mass and residence time (BC) terms

*p*= 0.045, also had significant impact to the banana peels biochar yields. Conversely, the quadratic terms of temperature (A

^{2})

*p*= 0.723, quadratic terms for residence time (C

^{2})

*p*= 0.991 and the interaction of terms between temperature and sample mass (AB)

*p*= 0.139, showed no significant influence on the yields of banana peels vinegar, tar, and biochar, respectively.

##### (5)

$$\begin{array}{ll}{\mathbf{Y}}_{\text{vinegar}}=\hspace{0.17em}\hfill & 29.62+0.14\text{A}-0.06\text{B}-1.06\text{C}-\hfill \\ \hspace{0.17em}\hfill & 4.76\text{E-}05{\text{A}}^{2}+6.8276\text{E-}05{\text{B}}^{2}+0.0076{\text{C}}^{2}-\hfill \\ \hspace{0.17em}\hfill & 6.78\text{E-}05\text{AB}-8.92\text{E-}04\text{BC}+0.00051\text{AC}\hfill \end{array}$$##### (6)

$$\begin{array}{ll}{\mathbf{Y}}_{\text{tar}}=\hspace{0.17em}\hfill & 110.39-0.233\text{A}-0.057\text{B}-0.312\text{C}+\hfill \\ \hspace{0.17em}\hfill & 0.00016{\text{A}}^{2}-6.19\text{E-}05{\text{B}}^{2}+2.742\text{E-}05{\text{C}}^{2}+\hfill \\ \hspace{0.17em}\hfill & 0.0001\text{AB}+0.00044\text{BC}+0.00022\text{AC}\hfill \end{array}$$##### (7)

$$\begin{array}{ll}{\mathbf{Y}}_{\text{biochar}}=\hspace{0.17em}\hfill & 81.49-0.126\text{A}-0.046\text{B}-0.223\text{C}+\hfill \\ \hspace{0.17em}\hfill & 5.20\text{E-}05{\text{A}}^{2}+1.44\text{E-}05{\text{B}}^{2}-8.83\text{E-}04{\text{C}}^{2}+\hfill \\ \hspace{0.17em}\hfill & 2.78\text{E-}05\text{AB}+0.00058\text{BC}+0.0018\text{AC}\hfill \end{array}$$**Y**is the response yield expressed in percentage weight of the fed biomass weight, while A, B and C are the coded terms representing the three variables of the experiment, that is, temperature (A), sample mass (B) and residence time (C).