3.2.2. Effects of variables on response
The three dimensional response surfaces plotted for three factors on bio-oil yield are shown in Fig. 5. The linear and quadratic terms of N2 flow rate was observed to influence bio-oil yield. The bio-oil yield increases with increased N2 flow rate up to 5LPM, beyond which the bio-oil yield decreases with higher N2 flow rate (Fig. 5, (b) and (c)). This difference is mainly due to the heat carrying capacity of N2 gas, which controls the final TUBL and TMBL by effecting pyrolysis conditions. At fairly low N2 flow rate, high TUBL facilitates volatile removal and low TMBL impedes pyrolysis conditions. However, increase in N2 flow rate controls the TUBL by carrying heat and partly raising the TMBL assisting pyrolysis conditions. Further increase in N2 flow rate reduces the TUBL to incomplete pyrolysis conditions (see Fig. 2, Run 6, 7–12 and 13). The interaction effects of CAC loading and microwave power shown in Fig. 5, (a) revealed that the bio-oil yield linearly increases with increasing CAC loading at high microwave power, whereas it decreases with increasing CAC loading at low microwave power. The increased bio-oil yield with increased CAC loading and microwave power can be explained by higher microwave absorption capacity of CAC solids and fast layer-to-layer heat transfer by maintaining high TUBL and TMBL final pyrolysis temperature resulting pyrolysis process to completion. The decrease in bio-oil yield with increasing CAC loading at low microwave power is mainly the result of self-heating of carbon solids by the insufficient amount of microwave energy (Salema and Ani, 2012a).