Thermochemical conversion of biomass is the most widespread technological aspect of bioenergy, since it includes several different technologies from simple combustion to the most modern gasification and fast pyrolysis, and also allows obtaining a wide range of useful products. It should be noted that biomass combustion dominates in the modern industry, while more advanced technologies account for only a few percent of the world bioenergy production [1].
At the heart of each of the technological processes are fundamentally different options for the chemical transformation of organic raw materials, determined primarily by the presence of oxygen or the nature of interaction with it. In the most uncompromising cases, the combustion of biomass occurs in the free presence of oxygen (air), during gasification of biomass there is a limited and controlled presence of oxygen, while pyrolysis is carried out in the absence of oxygen [2].
Of course, in practice, deviations in the options for using oxygen, as well as combinations of the processes mentioned, are possible.
The combustion between oxygen and other main participants in the process - carbon, hydrogen and methane, results in water and carbon dioxide:

С + О₂ = CO₂
H₂ + 1/2O₂ = H₂0
CH₄ + 2O₂ + CO₂ + 2H₂O

The main product of biomass incineration is heat, which is used for industrial or residential buildings, as well as for generating electricity.

Gasification makes it possible to obtain syngas, i.e. a mixture of predominantly carbon monoxide and hydrogen, which can be used both as a fuel and as a chemical raw material to obtain a wide range of various valuable products, including synthetic natural gas or liquid fuels. The main reactions during gasification are [3]:

C₆H₁₂O₆ + O₂ + H₂O → CO + CO₂ + H₂ + other species
CO + H₂O → CO₂ + H₂ (+ small amount of heat)

During pyrolysis, depending on the temperature conditions and the duration of the process, liquid, gaseous and solid reaction products can be obtained.
Another important feature of using biomass as an energy raw material is its high heterogeneity, moisture content, low density and extreme non-technological, especially when grinding, transporting and dispensing, which is especially noticeable when comparing biomass with the traditional type of solid energy raw material – coal. The most stringent requirements for bioenergy raw materials are imposed by more complex technologies, primarily gasification. It also does not contribute to their competitive distribution.
Any technologies for thermochemical processing of biomass inevitably lead to significant emissions of carbon dioxide and the formation of other hazardous gases and solids. Combustion of biomass in this part is the least safe. However, thermochemical biomass conversion is often regarded as a zero-emission option, since after its equivalent reproduction in wildlife, for example, in the form of forest planting, complete absorption of the produced carbon dioxide is expected. Nevertheless, this interpretation raises reasonable doubts, taking into account the time range of these events.

As follows from the data in Table 1, the percentage composition of the final energy product largely depends on the temperature conditions and time of the process, and also, as noted above, on the degree of oxygen or air use. To ensure uniform and timely heating, the biomass is preliminarily subjected to grinding, down to millimetre particles and even smaller. Different types of process reactors are used depending on the composition of the feedstock and the requirements for the finished product. First of all, they are characterized by the degree of biomass mobility (fixed-bed, fluidized bed, entrained flow), the way of heating the biomass (autothermal gasifiers or direct gasifiers, and allothermal or indirect - external heating), the type of heat carrier (solid heat-carriers, gaseous heat- carriers).

The main types of final products resulting from thermochemical conversion of biomass are thermal and electrical energy, much less often syngas, biochar or bio-oil. Statistical data on the production of electricity from biomass are shown in Fig.3 and 4. In total, according to [12], in 2018, the total capacity of bioenergy plants in the world approached 120 GW, and electricity generation was about 523 TWh. At the same time, bioenergy, which uses solid biomass and renewable waste as a feedstock, accounted for more than 80% of the total capacity - 101.4 GW, with electricity generation exceeding 425 TWh.

The leading countries using biomass for electricity generation in 2018 were Finland and Thailand, where the share of this type of feedstock in the total electricity generation balance reached or exceeded 15%. It should be emphasized once again that throughout the world electricity production in most cases is carried out using biomass combustion technologies, especially in combination with coal.
More detailed statistical information on the production and consumption of heat and electricity from bioenergy raw materials can be found in [13}.

Torrefaction is an example of slow low-temperature pyrolysis, which effectively removes moisture and a significant part of volatiles from biomass and leads to a significant improvement in its properties, including an increase in calorific value and embrittlement.This, among other things, allows the combination of biomass subjected to such a treatment with other types of raw materials, such as pulverized coal, for further joint combustion. According to [6], the calorific value of different solid fuels of wood chips and wood pellets is 9-12 and 15-16 MJ / kg, respectively, and after torrefaction pellets already have 20- 24 MJ / kg, which is comparable to the heat output of coal. At the same time, torrefied pellets have a power density similar to coal and significantly lower moisture content [7]. In this sense, torrefaction makes it possible to obtain a new type of bioenergy product called bio-coal. In addition, torrefaction provides a decrease in the ability to absorb moisture and a decrease in the volume of the original biomass, which is very important during its transportation and storage.

Torrefaction is usually carried out in the temperature range of 200-300^oС [4,6,14,15]; however, some sources report that the upper temperature can be even higher up to - 400^oС [10]. Wood chips are most often used as raw materials; less often other woodworking products, including wood pellets. For pelletizing wood chips after torrefaction, a higher compaction pressure is required to ensure the required pellet strength. Among the types of torrefaction reactors, proven technologies include, first of all, the screw reactor and the rotary drum reactor. Also used are a fluidized bed reactor, a moving bed reactor, a microwave reactor, a torbed reactor, and others. Reactors can be continuous reactors, with direct or indirect heating (directly or indirectly heated), in the latter case, heating is carried out most often due to flue gas.
The Screw reactor and Rotary drum reactor allow for the best possible process and optimum temperature control. Mixing of fuel is possible in the Rotary drum reactor and Fluidized bed reactor. The moving bed reactor and the Fluidized bed reactor have good potential for upscaling. Finally, the reactor with the lowest conversion costs is the moving bed reactor [6].
The economic value of torrefaction pellets in comparison with the basic version of the production of conventional pellets is discussed in detail in [7]. Despite the need for significantly higher capital and operating costs in the case of torrefaction (capital investments, electricity consumption, etc.), the final cost of the product can be about 30% lower due to significant savings in logistics operations.

Figure 5 shows the location of the largest regional enterprises for torrefaction of biomass, as well as data on patent activity of countries over the past decade (including fast pyrolysis technologies). Torrefaction has become widespread in the United States and Western Europe; in other regions of the world, this technology has not yet been implemented.

Figure 5. Examples of large factories and patent activity of countries around the world in the torrefaction and fast pyrolysis sector

Fast pyrolysis is another promising area of ​​thermochemical biomass conversion that has gained commercial acceptance in recent years. This technology is implemented at moderate temperatures, usually at 500^oС, in the absence of oxygen, and a very short residence time of several seconds (Table 1). As a result of this treatment, the biomass decomposes with the formation of a relatively small amount of charcoal, combustible gases and oily vapour, from which, after subsequent cooling and condensation, about 75% of a bioenergy liquid called bio-oil can be obtained. The peculiar qualities of this technology inevitably require fine preliminary grinding of the biomass and the provision of severe conditions for uniform and, in essence, instant heat transfer from the coolant to the biomass particles inside the reactor.
In general, the sequence of technological operations of fast pyrolysis includes (Figure 6): reception and storage; drying and grinding; preparation and loading of the heat transfer agent; biomass feed and fast pyrolysis; removal of pyrolysis products; condensing of pyrolysis gas and liquids collection; separation of flue gas and ash. Pyrolysis by-products in the form of flue gas can be used in the drying stage or transferred through heat exchangers in the form of heat to other consumers.
A very important advantage of the fast pyrolysis technology is the possibility of obtaining bio-oil, which can be stored or transported over a long distance using traditional infrastructure, as well as used as a fuel or chemical raw material to obtain more valuable products, including automobile fuel.

Figure 6. Basic technological operations of fast pyrolysis

The factors that determine the efficiency and composition of fast pyrolysis products are [4] - reaction temperature, feedstock characteristics, reactor design, additives and catalysts, hot vapour and solids residence time, pressure.
Wood is most often used as a raw material for pyrolysis, although, according to [16], many other types of biomass, including agricultural waste, can be used.
The most critical part of the fast pyrolysis technology is the reactor, despite the fact that the cost of its construction is only 10-15% of the total capital costs [4].
Among the various reactor options for fast pyrolysis, there are bubbling fluidized bed reactors, circulating fluidized bed reactors, rotating cone reactor, auger or screw reactor [2,11]. Bubbling fluidised bed reactors allow for good temperature control, and easy scaling; the technology is well studied, but requires small particle sizes [2]. Circulation reactors of this group allow the use of larger particles, allow for large-scale production, but have more complex hydrodynamics of the process. The main auger or screw reactor is the presence of moving parts in the hot zone, heat transfer at larger scale may be a problem and lower bio-oil yields [2].

BTG-BTL has developed a commercial fast pyrolysis technology based on its own patented technology [17]. They applied the design of a rotating cone reactor and sand as a circulating heat carrier material; combustion of solid reaction products and heating of sand is carried out in a combustion chamber with a fluidized bed combustor. The resulting excess heat can be used, among other things, to generate electricity (Figures 7 and 8).
Fortum in 2013 in Joensuu, Finland put into operation one of the world's first industrial Integrated pyrolysis oil production technology and combined heat and power plant [18]. The plant uses Valmet technology with a circulation fluidized bed boiler reactor and uses sand as a heat transfer medium. Low-temperature heat from condensation is used to dry biomass before pyrolysis [19]. The annual production of bio-oil in Joensuu is 50,000 tons, with processing from 300,000 to 450,000 m³ of wood [18]. Examples of plants for the conversion of biomass using fast pyrolysis technology can be seen in Fig. 5.
Among the main disadvantages of bio-oil production are [2] - high cost, in some cases significantly higher than fossil fuel; incompatibility with conventional fuels; lack of standards; limited supplies for testing.
Figure 5 shows examples of plants for the conversion of biomass by fast pyrolysis and data on patent activity of the countries around the world over the last decade (including torrefaction technologies). Table 2 lists the main patent holders which have received patents in the torrefaction and fast pyrolysis technology sector over the last 10 years between 2009 and 2018. In total, about 1100 patents were selected for consideration, in which the authors directly indicated that the proposed technical solutions belong to these technologies. Data on the shares of patent holders in the general register of intellectual property are also shown below.

Gasification is a process of high-temperature conversion of biomass with the formation of producer gas or synthesis gas (syngas) as the main product, as well as thermal energy, biochar, and useful mineral residues. Biomass gasification is a more advanced technological process compared to combustion in terms of biomass conversion efficiency, variety and value of the finished product.
Wood is primarily used as feedstock for gasification, but agricultural, municipal and industrial waste, marine biomass, etc. can also be used. The properties of the feedstock can have a serious impact on the gasification process. These primarily include: its physical characteristics, including moisture content, particle size, density and porosity, and some others; and also, to the same degree of importance, chemical indicators, for example, the content of lignin, cellulose and hemicellulose content, the content of gases, solids, metals – carbon, oxygen, nitrogen, hydrogen, sulfur, chlorine, alkali and other metals. Variations of the noted indicators for different types of biomass can be significant. For example, Rice straw can contain 50.0-80.0% moisture, and Wheat straw only up to 20%. Softwood contains 41% Cellulose, 24% Hemicellulose, 28% Lignin, and Birch wood 35.7%, 25.1%, 19.3%, respectively [20]. It is also very important that the oxygen content in carbohydrates from biomass is quite high, while the amount of the main fuel elements – carbon and hydrogen, is significantly lower compared to fossil fuels.
All gasifiers are divided into two large groups – autothermal and allothermal. In the first, heat for the conversion of biomass is produced due to its partial oxidation in the reactor with a controlled supply of air or oxygen. However, in this case it is necessary to ensure the possible uniformity of heating through intensive heat transfer. In addition, when air is used as an oxidizer, the resulting syngas will contain higher nitrogen concentrations than when oxygen is supplied, which makes it unsuitable for the subsequent production of energy products. On the other hand, the use of oxygen requires the construction of expensive facilities for its production.
In indirectly-heated gasifiers, heat is supplied to the reactor from an external heat source. Most often, sand is used for these purposes, which is heated in a separate reactor and circulated between it and the gasification reactor. In addition, the heat of the hot flue gas is used for indirect heating. In recent years, numerous unconventional technologies of indirect heating with the use of electric heaters, microwave generators, and solar reflectors have been encountered in patent documents. There are two initial drivers pushing developers for such innovations – firstly, the use of such heating options dramatically improves the environmental performance of the gasification process while significantly reducing the cost of technical equipment for cleaning flue gases, and secondly, indirect heating makes it possible to obtain syngas with a permissible nitrogen content without using oxygen.
The main chemical reactions during gasification with an agent in the form of water vapour are (steam reforming) [20]:

CH₄ + H₂O → CO + 3 H₂ 
CH₄ + ½O₂ → CO + 2H₂

and water-gas shift reaction:

CO + H₂O → H₂ + CO₂

Depending on the type of reactor, gasification is usually carried out in a temperature range from 700  to 1600^oС, although, for example, in [5,23], lower temperatures are indicated, and in some patent documents, in contrast, upper temperature conditions can reach up to 2000^oС.
Gasification technology inevitably includes Pre-treatment of biomass; high-temperature impact on biomass with the supply of activating agents; cooling, separation, gas cleaning, gas cooling, separation, tar removal, utilization Heat.
The general design of an autothermal plant for biomass gasification with steam reforming is shown in Figure 9. A significant share in the total metal consumption of the plant is taken up by equipment elements related to the treatment of off-gases – a purge gas cooler, a flue gas cooler, a purge gas filter, a flue gas filter, and a gas scrubber. Conventionally, the installation in the figure includes useful syngas consumers – a gas generator, a hotwater boiler, an electricity transmitter from the generator, an electric transformer to the power grid, and a peak load boiler. The details of the equipment are shown in the figure in proportion to the real dimensions.

Figure 9. General design of a biomass gasification plant

Lahti Energia’sKymijärvi II power plant in Lahti, Finland is one of the world's largest gasification waste and wood conversion plants. The plant was intended to replace a thermal coal station. The power plant has a capacity of 50 MW of electricity and 90 MW of thermal energy, making it one of the largest in the world. The plant uses a Valmet circulating fluidized bed gasifier, as well as a special cleaning and cooling system, steam boiler and environmental protection system [26].
 
Figure 12. Examples of the largest biomass gasification plants in the world

Most of the large biomass gasifiers are located in Germany, USA, Finland and Sweden.
More detailed information on various designs of reactors for gasification of biomass, as well as on production facilities, can be seen in the following scientific papers and databases [27-31].
Table 3 below lists the top ten patent holders for biomass gasification over the past 10 years (2009 -2018), as well as their shares in the intellectual property register in relation to all patent holders. About 1,500 patents were selected for analytical evaluation, in which the authors indicated that the proposed technical solutions belong to biomass gasification technologies at any stage of the production process, including raw material preparation, waste gas treatment, sludge removal, etc.

Syngas (synthesis gas), which is predominantly a mixture of hydrogen and carbon monoxide, is the most important chemical raw material for the production of liquid and gaseous environmentally-friendly fuels. There are several methods for the synthesis of simple syngas molecules into long fossil fuel molecules. Since these processes occur at the intermolecular level, very stringent requirements are imposed on the purity of syngas, which was mentioned above, when considering the gasification processes. Therefore, purification of syngas prior to synthesis processes is of paramount importance. The most common method for producing synthetic fuels from syngas is the Fischer-Tropsch process (FT), first developed about 100 years ago and named after its creators. Typical parameters of the process are a temperature in the range 150-300^oС pressure – up to several tens of atmospheres, catalysts – mainly cobalt and iron [36]. The Fischer-Tropsch conversion of biomass to liquid (BTL), has much in common with related processes, gas to liquid (GTL) and coal to liquid (CTL). However, there is a significant difference due to the fact that syngas obtained from biomass has a lower hydrogen to carbon monoxide ratio and a greater number of pollutants in comparison with syngas produced from natural gas [37]. If we also take into account that natural gas and coal are more technological types of raw materials, it becomes clear why GTL and CTL have received larger development in comparison with BTL technologies. For example, the world's largest power plants – Pearl GTL and Oryx GTL in Qatar have capacities of 140,000 and 130,000 barrels per day (bpd), respectively, and the total capacity of all BTL factories in the world is much lower. For comparison, the capacity of one of the largest plants in the world for the production of second generation bioethanol run by a Canadian company, Enerkem, in Alberta (using syngas obtained from municipal waste and biomass as feedstock), is only 38 million litres per year [38].
The formation of multidimensional hydrocarbon molecules during the implementation of the conditions for the Fischer-Tropsch reaction occurs on the surface of catalysts, while the nature of this process and, moreover, the creation of its rigorous mathematical model is the subject of numerous and longstanding research. But it is well known that this reaction is exothermic, creating conditions for overheating of catalysts, therefore the creation of a stable temperature in reactors is of paramount importance [39]. The most common are four types of reactors – fixed-bed multitubular reactors, fluidized-bed reactors, slurry-bed reactors, microchannel reactors [37]. Each of these has its own advantages and disadvantages. For example, fixed-bed multitubular reactors are quite simple to operate and easily scalable, however they require long downtime when replacing catalysts [39]. Slurry-bed reactors and microchannel reactors ensure the best use of catalysts [37], etc.

As a result of the technology of conversion of syngas by the FT method, various types of products can be obtained – diesel fuel, paraffins, gaseous fuels, wax. Regulation of the percentage yield of products can be ensured by temperature conditions, by choosing the optimal type of reactor and catalysts [39]. The hydrocarbons obtained, if necessary, are subjected to additional processing – separation, purification, and hydrocracking.
One of the biomass to liquid plants was launched in Finland in Varkaus in 2009 [40] (Figures 13-14). The demonstration unit has a capacity of 12 MW and includes a drying section, a gasifier, gas purification and catalyst testing units (including drying of bio-mass, gasification, gas cleaning and testing of Fischer-Tropsch catalysts).
FT biomass to liquid technology is more capital intensive than other biomass-to-fuel conversion methods. A detailed technical and economic analysis of the production of various types of liquid fuel by thermochemical conversion methods is given in [41]. The cost of diesel fuel produced by the FT method is estimated here as the highest – 4.29-4.85 USD / gallon. To reduce the cost of the process, both technical and organizational improvements are made. In [42], a new BTL concept is described, aimed at reducing the cost of the final product by up to 35%. The concept provides for the conversion of biomass by the FT method in medium-sized plants, integrated with third-party facilities through heat supply. At the same time, according to this concept, FT units should mainly produce synthetic oil, distillates and wax, which are then sent to nearby refineries for the production of final types of standardized fuels.
Another area of production of various fuels from syngas is the intermediate production of methanol and its derivatives, as well as methanation.
Methanol is obtained from synthesis gas at temperatures from 220^oC to 300^oC, pressure from 50 to 100 bar and with a catalyst of copper and zinc oxide [28, 39, 43]. The main reactions in the conversion of syngas to methanol are:

СО + 2 Н₂ → СН₃ОН
CO₂ + 3 H₂ → CH₃OH + H₂O

Methanol is used in various forms, both as an additive to traditional fuels and as a feedstock for the production of gasoline, diesel, and dimethyl ether (DME).

Methanation is carried out at temperatures from 700^oC to 1000^oC; nickel is used as the catalyst material. The main reaction [44]:

CO + 3H₂ → CH₄ + H₂O

The process was first implemented in Güssing, Austria. The demonstration methanization unit was combined with an existing commercial biomass gasifier and has been successfully operating for several years (Figures 15-16).
Examples of the largest facilities for the production of synthetic fuels from synthesis gas obtained from biomass gasification are shown in Figure 17.

Figure 17. Examples of objects of thermochemical processing of biomass and patent activity of the countries of the world in the BTL area

Table 4 shows the leading BTL technology patent holders that were issued by world patent offices in 2009-2018, as well as their shares in the intellectual property register in relation to all patent holders. For the analysis, about 500 patents were selected, in which the authors unambiguously expressed the applicability of the proposed technical solutions in the field of BTL. The undisputed leader in this area is Shell Internationale Research Maatschappij B.V., from the Netherlands, whose share in the general register of intellectual property was almost 13%.

Table 4. Leading patent holders of BTL technologies between 2009 and 2018 and their shares in the register of intellectual property in relation to all patent holders