## 4 Energy Efficiency Assessment of Wood Fuel Gasification
We propose to use criterion \(C_{g}\) for estimating the efficiency of the gasification process for a given fuel composition and technological parameters. It is obvious that the maximum efficiency of the process is achieved under the condition \(C_{g}=1\). In this case, the minimum fraction of the heat value of the initial fuel is spent on maintaining the gasification reaction: in the terminology of [51], this process is called \"thermoneutral\" gasification. Under given technological conditions, the optimal parameters of the gasification process are regulated by adding steam to the reactor to reduce the outlet gas temperature (\(C_{g}>1\)), or by increasing the air stoichiometric ratio (\(C_{g}<1\)) [52]. The results of calculations of the cold gas efficiency and heating value of syngas obtained from some solid fuels are shown in the Table 3. Upper rows of the cells of the table contain the initial values of the corresponding values, lower rows contain optimal (limiting) values. A feature of the staged gasification process is heat recovery, which allows to reduce the loss of useful energy with the sensible heat of the exhaust gases [53]. This heat is returned to the gasifier after the gas combustion gas in the engine; therefore, for the gasification process, such recovery is equivalent to external heating, i.e., an additional term \(\Delta q_{r}\) appears in the heat balance equation:  
\[q^{{}^{\prime}}_{h}\ =\ H_{air}(T_{c})+H_{prod}(T_{c})+H_{ash}(T_{c})-H_{air}(T_{ b})-H_{prod}(T_{f})-H_{ash}(T_{f})+\Delta q_{r}\]  
when this heat is used for heating and carbonization of fuel, the production of useful heat energy for supplying the consumer is reduced. However, the fraction of recovered heat is usually low: our estimates show that no more than 10-15% of the heating value of the fuel is to be recovered [31]. With a high electricity price, the unit will primarily work for electricity production. Therefore, the use of waste heat to increase electricity production will be more efficient than heat production. Table 3 presents the results of thermodynamic calculations. For wood and brown coal, the calculations were carried out with different values of moisture content. A less humid fuel has better gasification characteristics in autothermal gasification (\(C_{g}=1\)). However, if heat recovery is possible, a more humid fuel is preferred. This is due to the use of evaporated moisture as a gasifying agent [34; 54]. The syngas is enriched with hydrogen and its specific heating value increases. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \multirow{2}{*}{**Fuel**} & **Moisture Content \%** & **Ash Content \%** & \multicolumn{3}{c}{**Organic Matter Composition \%**} & \multicolumn{3}{c}{**Heat Recovery \%**} & \multicolumn{3}{c}{**Heat Value of Syngas**} \\ \cline{3-14}  & **Moisture Content \%** & **Ash Content \%** & **C** & **H** & **O** & **N** & **S** & **MoJy/Asg** & **MoJy/Nm3** & **MoJy/kg of fuel** \\ \hline Wood (dried) & 6.8 & & & & & & & & 0 & 2.76 & 0.44 & 1.97 & 7.21 \\ \hline Wood (humid) & 18.25 & & 0.4 & 49 & 6 & 45 & 0 & 0 & 1.0 & 0.80 & 5.74 & 13.29 \\ \hline Brown coal (dried) & 10 & & & & & & & & 5.91 & 1.0 & 1.23 & 11.68 & 20.26 \\ \hline Brown coal (humid) & 28.9 & & & & & & & & 0 & 1.97 & 0.48 & 2.42 & 6.62 \\ \hline Peat & 50 & 10 & 56 & 6 & 35 & 1 & 2 & 0.84 & 1.0 & 0.79 & 5.20 & 7.16 \\ \hline \end{tabular}
\end{table} 
Table 3: Limit characteristics of gasification of some low-grade fuels. Calculations show that during gasification of wet biomass with heat recovery of 30%, the CGE can exceed 100% (due to the recovery of heat in the form of chemical energy [55]). Naturally, if we take into account all the components of the heat balance, then the gasification efficiency will be lower (for example, with high-temperature steam gasification, the thermal efficiency is usually less than 40% [48; 56]). The obtained values of the CGE are, apparently, only the upper estimate of the thermodynamic efficiency.