## 2 The Technology of Multi-Stage Gasification of Wood Fuel  
A detailed analysis of the prerequisites for using biomass gasification technologies and their main development trends is presented in reviews [17; 18; 19]. It is shown that the main factors stimulating to the development and implementation of gasification technologies are:  
* Increase in energy consumption in developing and densely populated countries (China, India, Brazil);
* Significant fluctuations in world hydrocarbon prices;
* Countries' policies aimed at diversifying energy structures in the fuel and energy balance;
* Development of distributed energy generation, which allows reducing the distance of energy transport through the use of local fuels;
* The pursuit of environmental friendliness (through the use of renewable solid fuel resources, including waste). According to the International Energy Agency, in the countries included in the Organization for Economic Cooperation and Development (OECD), there are about 160 projects at various stages of design and construction of energy facilities using biomass gasification technologies. Typically, power facilities with a capacity of up to 300-500 kW use fixed-bed reactors and those with a capacity above 300 kW use fluidized-bed reactors [20; 21]. However, despite a large number of projects, their practical implementation is slow due to some problems. This is confirmed by the fact that 46 projects have been suspended [22]. Although biomass is one of the most reliable and accessible renewable energy sources [23], it has specific features that make it difficult to be used in gasification plants. Biomass is characterized by high humidity, high reactivity [24], and low ash content. At the same time, ash is characterized by increased corrosion and slagging properties due to the increased content of alkali metals [25]. Biomass gasification produces a large amount of tar [26]. It is worth noting that the capacity of biomass gasification plants is limited by the area of collection and logistic problems of transportation [27]. These circumstances worsen the technical and economic efficiency of the power plants. The producer gas may contain a significant amount of tar, which can lead to fouling of technological equipment and damaging of engines. The maximum permissible tar content in the gas supplied to the internal combustion engine is 50 mg/nm\({}^{3}\)[28]. For a gas turbine, this value is 5 mg/nm\({}^{3}\), and for a fuel cell-1 mg/nm\({}^{3}\)[29]. Two methods are usually used to clean the producer gas from tar: 1) direct removal (decomposition) of tar in gasifier; and 2) the use of special cleaning devices after gasifier, including cyclones, various filters, and scrubbers. In addition, catalysts for tar cracking can be used. Direct removal of tar in the reactor is carried out by changing the process parameters (temperature, pressure, fuel residence time, the use of various additives), as well as by using special gasifier designs [30]. One of the methods to significantly reduce the tar in the producer gas and increase the efficiency of gasification is the organization of a staged process [31]. In this process, the exothermic stage of internal combustion takes place in a separate reactor (or zone), the produced combustible gas is completely or partially burned in the combustion chamber, and the combustion products are used as a gasifying agent in the second reactor, where charcoal from the first reactor is supplied. Such an organization of the process makes it possible to burn the tarry products released in the first stage, use their energy to force the gasification of charcoal and stabilize the composition of the final gas, as well as to obtain producer gas with a minimum amount of tar [32; 33]. Figure 1 shows a diagram of a multi-stage gasifier. It is worth noting that the combustion chamber of the tarry pyrolysis gas can be either organized inside the reactor (shaft type gasifiers) or designed as a separate reactor. Figure 1: A diagram of a multi-stage gasifier.