## 1 Introduction
### Technologies of the Use of Wood for Energy Purposes
In addition, the engine power is limited by the intensity of the heat transfer. On the other hand, updraft gasification processes can be used, which are less sensitive to the moisture content of the feedstock [38; 39]. The efficiency estimates for the Stirling engine running on biofuels are given in [27] (from 10% at 10 kW to 20% at 200 kW). Internal combustion engines are generally more efficient, although maximum power is usually reduced by 20-30% with producer gas compared to a specification fuel (natural gas, diesel, or gasoline) [31; 40; 41]. Optimal operating modes are shifted to the area of lower air-flow rates, and due to the lower combustion temperature, nitrogen oxide emissions are reduced [42]. In [43], the thermal efficiency of about 35% with the combustion efficiency of 93-97% was achieved by burning gas with a tar content of about 20 mg/Nm\({}^{3}\). However, there were condensation deposits on the piston rings despite the low tar concentration. In [37], the thermal efficiency of a gasoline engine running on producer gas was estimated at 15-20%. In [44], experimental studies have shown the maximum thermal efficiency of internal combustion engines of the order of 25%. Authors of [45; 46] obtained the thermal efficiency of 19-22% at a power of 5 kW by using agricultural and household waste as fuel. Thermal efficiency values of 12% to 24% (for power of 2-20 kW(e)) are given in [40]. Higher efficiency values are promised by manufacturers of gasification-based mini-CHPs [47] (about 30%, if one estimates it as the total efficiency of the plant). This is probably due to the scale effect: as the power increases, the efficiency tends to increase [48]. Engine exhaust gases are generally used for heating the air, and for drying and torrefying the fuel [49]. In multi-stage gasification processes, the heat of the exhaust gases is used to perform pyrolysis of biofuel [50]. As mentioned above, sometimes, this heat is used to implement the organic Rankine cycle. Finally, it can be used for house heating [38; 51]. The fuel cells make it possible to significantly enhance the efficiency of electricity generation. This, however, requires thorough gas cleaning, not only from tar and particulate matter but also from some sulfur and nitrogen compounds. For example, in [52], the efficiency of a plant with a fuel cell and organic Rankine cycle at the exhaust is almost 55%. In [53], the combination of the Stirling engine and fuel cell at optimal gas distribution yields the efficiency of the plant of about 40%. With a fuel cell used without engines, the estimated efficiency of power generation is 30% [54; 55; 56; 57]. The authors [58] conducted a three-day experiment on the operation of a fuel cell using the producer gas obtained from the multi-stage gasification of biofuel, with the cell efficiency of about 40%. Combination with other renewable energy sources can, in some cases, increase the reliability of the energy supply. For example, a gasifier plant can operate in the basload state, smoothing out power output dips in solar and wind installations (for example, due to the accumulation of combustible gas in dedicated storage facilities) [10; 59; 60]. Solar concentrators and thermal energy storage systems can be used to heat a gasification reactor [61; 62], including operation in hybrid operating modes, when the blowing agent in excess heat is water steam, and in heat shortage, it is air [63]. According to estimates [64], the auxiliary needs of a gasification-based power plant are about 16-18% of the generated power. This energy is mainly used for fuel conditioning and compressor operation. A summary of the data available in published research is shown in Figure 2. The bulk of the points indicate power plants using combustible gas in an internal combustion engine, but already at a capacity of about 1 MW(th), the efficiency values of such power plants are close to the limit (about 30%). Fuel cells, as mentioned above, demonstrate better performance but prove to be the most demanding in terms of operating conditions.