# Microwave Pyrolysis of Biomass: The Influence of Surface Area and Structure of a Layer
## 4 Results and Discussion
### Distribution of Biomass on the Crucible Bottom
This result was caused by a  
Figure 4: The concentrations of the components of the gas mixture with varying distribution of the sample in the crucible: along the edges (**a**); uniform distribution over the entire crucible bottom (**b**); on half of the crucible bottom (**c**); the comparison of concentrations of major gases (**d**). different contact area of the biomass with the crucible surface. When the sample was located along the edges as separate fragments, the biomass was heated more intensively, receiving thermal energy from microwave radiation and the walls of the crucible. The fuel was pyrolyzed unevenly due to the lack of heat exchange between biomass particles. The minimum concentrations of CO, CO\({}_{2}\), NO and CH\({}_{4}\) were 8.6, 6.31, 2.77 and 1.8%, respectively. When the biomass was located over the entire crucible bottom, the maximum values of CO\({}_{2}\) and H\({}_{2}\) amounted to 18.74% and 3.8%, respectively (Figure 5b). With this distribution of the sample, it received thermal energy from microwave radiation and the walls of the crucible, and heat was better transferred between the biomass particles. The analysis of the appearance of the solid residue (Figure 6) showed that the distribution of fuel on the crucible bottom and relative to the magnetron affects the uniformity of heating and the completeness of thermal decomposition. The distribution of fuel on a half the crucible bottom provided a more complete conversion. In this case, the mass of the solid residue relative to the initial mass of the sample was minimal (3.4%). When the fuel was distributed along the edges and over the entire crucible bottom, this indicator was 4.2% and 4.5%, respectively. Figure 5: Average (**a**) and maximum (**b**) concentrations of gas mixture components for different distributions of biomass. Figure 6: Solid residue after microwave pyrolysis of biomass with different options for the initial distribution of a sample in the crucible: along the edges (**a**); uniform distribution over the entire crucible bottom (**b**); on half of the crucible bottom (**c**). When the biomass is distributed along the crucible walls, heating is uneven since heat transfer between the particles is difficult. The areas located on the side of the magnetron begin to warm up first, and then the rest does. In the case of a uniform distribution over the surface, heat and mass transfer also proceeds more uniformly. Table 4 shows the gas flow rates obtained during the experiments. Calculations were made for normal conditions of 20 \({}^{\circ}\)C and 1 atm. The average values of the concentrations of the main gases were converted to mg/m\({}^{3}\), after which the obtained values were multiplied by the time of gas yield. The obtained data were compared with the results of numerical and experimental studies presented in [49; 56; 57]. It has been shown that the characteristics of sample heating during microwave pyrolysis strongly depend on the geometry and dimensions of the sample, electromagnetic and thermal parameters [56], and the distribution of the fuel inside the reactor [57]. In particular, [PERSON] and [PERSON] [49] compared the experimental and numerical data obtained during the pyrolysis of biomass in the form of a layer and granules. The simulated surface temperature profile showed that most of the layer and biomass granules had a lower temperature compared to the edge parts of the fuel sample. The calculated data obtained agree with the experiment. As soon as a high temperature region (hot spot) is formed in the sample, it begins to expand, covering the surrounding mass of fuel. The biomass around the hotspot turns into a carbonaceous material, which absorbs large amounts of microwave energy and transfers heat to the environment. The temperature in the area of the hot spot is 165 \({}^{\circ}\)C higher than in the center of the biomass layer or granules. Such a temperature gradient may be due to the low thermal conductivity and diffusion capacity of the biomass.