# Microwave Pyrolysis of Biomass: The Influence of Surface Area and Structure of a Layer
## 1 Introduction
### Microwave Pyrolysis of Biomass and Waste: A Review of Modern Researches  
The following factors and processes are often analyzed when studying microwave and thermochemical pyrolysis: fuel characteristics (particle size, sample distribution, humidity and density of a layer) [13; 26; 35; 36; 37; 38]; the influence of metal catalysts (Al, Ni, Fe, Cu, etc.) [33; 37; 38] and absorbers [38; 39]; heating characteristics (power and temperature) [37; 38; 40]; carrier gas properties (air, CO\({}_{2}\), H\({}_{2}\)O, reaction gas) [26; 40]; fuel pretreatment methods [13]. These factors and processes are considered in the context of their influence on the pyrolysis efficiency, yield and distribution of solid, liquid and gaseous end products. When choosing and designing microwave reactors for research, it is necessary to take into account several important criteria that also affect the characteristics of pyrolysis [41; 42; 43]: (1) operating mode: periodic, semi-continuous or continuous; (2) type of reactor: fixed bed, fluidized bed or continuous flow; (3) orientation of the magnetron: side, top or bottom; (4) microwave frequency: 916 MHz or 2.45 GHz; (5) coating material: quartz or ceramic; (6) microwave resonator mode: single- or multi-mode; (7) the scale of the reactor: laboratory, pilot-scale or industrial. The studies [26; 32; 36] present the results of numerical simulation of microwave pyrolysis. The data obtained [26; 32; 36] make it possible to evaluate various heating scenarios (for example, changing the structure and size of the object, observing the moving object, varying the characteristics of materials, etc.) and to compare the results with experimental data. In [26], [PERSON] et al. studied spiral continuous microwave pyrolysis of sewage sludge. In comparison with periodic microwave pyrolysis, this technology provides instantaneous heating of the sludge and overcomes the challenge of producing large amounts of tar. In the experiment, the microwave power varied in the range of 1800-4200 W, the pyrolysis temperature varied from 300 to 600 \({}^{\circ}\)C, and fuel samples (moisture content 0-60%) were continuously fed into the reactor at a feed rate of 4 rpm. Nitrogen at a flow rate of 100 l/h was used to blow down the installation, and carbon dioxide was the medium for pyrolysis. The key factors for the analysis were pyrolysis temperature, fuel sample moisture content and CO\({}_{2}\) concentration. They were considered in terms of their influence on biogas yield, and H\({}_{2}\) and CO concentrations. It was found that an increase in temperature (400-600 \({}^{\circ}\)C) leads to an increase in the biogas yield (from 17.23 wt% to 53.09 wt%). At the same time, the combined use of H\({}_{2}\)O and CO\({}_{2}\) had a positive effect on the biogas yield (40.65-55.67 wt%), which is significantly higher than the biogas yield obtained by conventional pyrolysis (18-20 wt%) or periodic microwave pyrolysis (25-42 wt%) under identical temperature conditions. With an increase in humidity from 0 to 60 wt%, the biogas yield gradually decreased from 55.54 wt% to 40.72 wt%, which is associated with a lower quality of organic matter of the sewage sludge. However, water molecules reacted with light hydrocarbon compounds through steam reforming, increasing H\({}_{2}\) concentration (from 5.34 vol% to 31.51 vol%). CO concentration gradually increased with increasing temperature and CO\({}_{2}\) concentration. At a pyrolysis temperature of 600 \({}^{\circ}\)C and CO\({}_{2}\) concentration of more than 80 vol%, the CO concentration reached 30.92-37.47 vol%. Various types and ratios of purge gases as well as catalysts have been used in many studies aimed at increasing the yield and quality of the generated gas. For example, in [6], the thermochemical catalytic pyrolysis of rice straw (particles less than 0.1 \(\upmu\)m, the supplied fuel mass of 1.5 \(\pm\) 0.02 g) was studied using various pyrolysis plants (one- and two-stage). CO/SiO\({}_{2}\) was used to improve the reaction kinetics. In [6], [PERSON] et al.