## 3 Results
### Characteristics of Semi-Coke  
Table 2 summarizes the characteristics and elemental composition of semi-coke obtained from the steam pyrolysis of OS. Table 2 shows that the semi-coke sample exhibits a high ash content (A\({}^{\text{d}}\) = 63.7 wt%), which is associated with the origin and storage of the initial OS. In this case, the obtained heating value (LHV = 14.6 MJ/kg) is comparable to that for brown coal [33] and biomass [34]. The volatile yield in terms of dry ash-free mass is rather high (V\({}^{\text{daf}}\) = 62.7 wt%), which is characteristic of high reactive fuel. A high sulfur content (S\({}^{\text{d}}\) = 1.7 wt%), which causes SO\({}_{2}\) release during combustion, should be taken into account. The main ash components include silicon oxide (63.4%), aluminum (12.1%), and iron (10.4%). The study of ash fusibility characteristics showed that the ash residue obtained from the combustion of semi-coke is of the medium-melting type (t\({}_{\text{c}}\) > 1250 \({}^{\circ}\)C). The content of ash and SO\({}_{2}\) formed during combustion can be decreased by mixing semi-coke with wood wastes [35], which exhibit a low ash content (up to 2 wt%) and no sulfur [34]. Figure 7 presents the results of the scanning electron microscopy of the semi-coke sample particles. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Humidity W\({}^{\text{a}}\), wt\% & 0.8 \\ Ash content A\({}^{\text{d}}\), wt\% & 63.7 \\ Volatile yield V\({}^{\text{daf}}\), wt\% & 62.7 \\ Heating value LHV, MJ/kg & 14.6 \\ \hline \hline \multicolumn{3}{c}{Elemental composition\({}^{\text{d}}\), wt\%} \\ \hline C & 25.3 \\ H & 2.0 \\ N & 0.3 \\ S & 1.7 \\ O & 1.0 \\ \hline \hline \multicolumn{3}{c}{Elemental composition of the ash residue\({}^{\text{d}}\), \%} \\ \hline SiO\({}_{2}\) & 63.4 \\ Al\({}_{2}\)O\({}_{3}\) & 12.1 \\ Fe\({}_{2}\)O\({}_{3}\) & 10.4 \\ SO\({}_{3}\) & 2.4 \\ CaO & 4.7 \\ MgO & 2.9 \\ K\({}_{2}\)O + Na\({}_{2}\)O & 4.1 \\ \hline \hline \end{tabular} Indices: \({}^{\text{a}}\)—analytical mass of the sample, \({}^{\text{d}}\)—dry basis of the sample, \({}^{\text{daf}}\)—dry ash-free basis of the sample.  
\end{table} 
Table 2: Physical and chemical characteristics of semi-coke. The semi-coke sample particles are characterized by a spherical shape and inhomogeneous uneven surfaces with numerous depressions. A large number of smaller fragments less than 10 \(\upmu\)m in size can be found on the particle surface. The particle sizes did not exceed 80 \(\upmu\)m.  
Figure 8 shows the results of thermal analyses in inert and oxidizing atmospheres in the form of TG and DTG curves. Due to the high content of volatile compounds (Table 2), the initial temperature T\({}_{i}\) and the final temperature T\({}_{e}\) of the intensive oxidation attain 248 and 518 \({}^{\circ}\)C, respectively. The total mass loss in the oxidation mode is 38.6 wt%. Generally, oxidation of the sample can be divided into two stages. The first one is removal of residual volatile compounds (up to 365 \({}^{\circ}\)C). The second one is subsequent oxidation of the carbon residue. Moreover, the monomodal DTG profile has an additional shoulder in the temperature range of 248-365 \({}^{\circ}\)C. The maximum reaction rate w\({}_{\rm max}\) is 3.1 wt%/min at T\({}_{\rm max}\) = 462 \({}^{\circ}\)C.  
Figure 8: TG, DTG, and MS data for thermal conversion of the semi-coke sample in oxidizing (air) and inert (argon) atmosphere. Heating rate is 10 \({}^{\circ}\)C/min, gas flow rate is 150 ml/min, sample weight is \(\approx\)15 mg.  
Figure 7: Micrograph of the semi-coke sample. The MS analysis data show that during oxidation of the semi-coke sample, SO\({}_{2}\) releases with the profile that can be determined in the temperature range of 294-497 \({}^{\circ}\)C. The profile of CO\({}_{2}\) release, the main product of oxidation of combustible matter, is in fact a mirror image of the corresponding DTG profile.