3.1.1. Effects of N2 flow rate
The heating rate and final pyrolysis temperature reached in biomass layers were found to vary with N2 flow rate. The upper biomass solids demonstrated significant increase in heating rate and final pyrolysis temperature, when pyrolysis was carried out by maintaining fairly low N2 flow rate of 2.4LPM. However, increase in N2 flow significantly reduced the upper biomass solids heating rate and final pyrolysis temperature. The final pyrolysis temperature of upper biomass solids (TUBL) was observed 496 °C at 2.4LPM, which reduces to 354 °C and 270 °C using 6LPM and 9LPM, respectively (Fig. 3, Run 6, 7–12 and 13). The differential heating nature in upper biomass solids suggests that the heat carried with N2 gas from the surface carbon solids plays an important role in controlling heat to the supported biomass solids. As carbons are considered good MWA and respond quickly to microwave energy (Menéndez et al., 2010). Therefore, the microwaves reflected by the cavity walls could be absorbed more frequently near the surface carbon solids than the carbons in the middle of the pyrolysis sample and is converted to thermal energy, which can then be transmitted to the supported biomass solids. As a result of high N2 flow rate, the heat carried with inert gas is higher than the heat generated by the surface carbon solids, which can reduce the heating rate and final pyrolysis temperature in upper biomass solids (Mushtaq et al., 2014c). The heat carrying capacity of N2 gas was reported to influence microwave pyrolysis reaction temperature (Huang et al., 2013).