## 4 Waste Conversion for Fuel Gas Production
### Influence of External Conditions on the Characteristics of the Pyrolysis and  
_Gasification Products_  
Many studies confirm that temperature is the most important factor influencing the quantitative and qualitative characteristics of pyrolysis products. [PERSON] et al. [67] studied the gasification of corn and clover grass in supercritical water. A continuous flow reactor was used. The temperature varied from 500 to 700 \({}^{\circ}\)C. Under supercritical water gasification conditions, biomass reacts with water according to the reaction C\({}_{6}\)H\({}_{12}\)O\({}_{6}\) + 6H\({}_{2}\)O \(\rightarrow\) 6 CO\({}_{2}\) + 12H\({}_{2}\). [PERSON] et al. [67] found that an increase in temperature from 500 to 700 \({}^{\circ}\)increased the gasification efficiency indicator up to 2.24 times. The gas yield increased by 20%. The temperature rise also influenced the gas composition. At higher temperatures, CO\({}_{2}\) content decreased, while the methane concentration, on the contrary, increased to 20 vol% (compared to 10 vol% at 500 \({}^{\circ}\)C). Additional methane could be generated both from CO/CO\({}_{2}\), and directly from the reacting biomass [67]. H\({}_{2}\) concentrations also increased with increasing temperature. Hydrogen is released from both water and biomass. However, CO formation was inhibited due to the water-gas shift reaction (CO + H\({}_{2}\)O \(\rightarrow\) CO\({}_{2}\) + H\({}_{2}\)) dominating at high temperatures. Thus, at 500 \({}^{\circ}\)C, the CO concentration was 15 vol%, and at 700 \({}^{\circ}\)C, it was less than 1 vol%. [PERSON] et al. [98] investigated the kinetics of a water-gas shift reaction under non-catalytic conditions in supercritical water and found that the CO conversion increased by more than 2.5 times with an increase in temperature by 60 \({}^{\circ}\)C.  
The study [54] is devoted to finding an effective way to use semicoke powder. This component is a finely dispersed by-product of the coal chemical industry and accounts for 10% of the total amount of semicoke. Direct combustion of semicoke powders in a boiler is complicated by the low content of volatiles and high ignition temperatures. [PERSON] et al. [54] suggested using semicoke powders in supercritical water gasification technologies. The semi-coke was ground to a size less than 100 \(\upmu\)m, and 0.1 wt% xanthan gum was used as a stabilizer. Deionized water and alkaline catalysts were then added to the mixture to obtain a uniform and stable water-semicoke slurry. The experiments were carried out in a supercritical water fluidized bed reactor system. The temperature range was 540-660 \({}^{\circ}\)C. The pressure in the system was 23 MPa. [PERSON] et al. [54] showed that, with an increase in temperature from 540 \({}^{\circ}\)C to 660 \({}^{\circ}\)C, the carbon gasification efficiency and hydrogen yield increase from 27.83% to 95.26% and from 17.53 mol/kg to 85.90 mol/kg, respectively. The high temperature facilitated the steam reforming reaction. With an increase in temperature, the proportion of carbon monoxide increased from 1.84% to 2.75%, and the proportion of carbon dioxide decreased from 37.01% to 30.01%, since the higher temperature promoted the reverse reaction of water gas conversion (CO + H\({}_{2}\)O \(\rightarrow\) CO\({}_{2}\) + H\({}_{2}\)). As the concentration of semicoke powders increased from 10 to 30 wt%, the hydrogen concentration decreased from 55.06% to 50.26%, and the methane concentration increased from 8.14% to 12.57%. The authors assumed that there is a competition of the hydrogen element between H\({}_{2}\) and CH\({}_{4}\). It is mainly because the lower concentration of water inhibits the water gas shift reaction and the methane steam reforming reaction [54]. The pyrolysis of lubricating oil wastes was studied by [PERSON] et al. [63]. Gases were analyzed by gas chromatography (GC), liquids--by gas chromatography/mass spectrometry (GC/MS). [PERSON] et al.