Bioenergy is one of the most important renewable energy industries, based on the transformation of various types of biomass into a variety of useful energy products. Biomass includes natural organic materials of plant or animal origin, as well as specially grown energy crops, agricultural or forestry waste, food waste, wastewater and other industrial organic matter. Biomass is the oldest energy source known to mankind, and the heat and light generated from the combustion of biomass were fundamental in shaping human society. The vast majority of biomass energy on Earth is concentrated in green vegetation but is also found to a much lesser extent in terrestrial and marine living organisms.

Bioenergy is a renewable resource capable of replacing fossil fuels at a significant scale. Moreover, modern methods of converting biomass without direct combustion make it possible to switch to carbon-free energy; these methods also help to dispose of numerous organic wastes that release methane into the atmosphere during uncontrolled decomposition, which is the most aggressive source of the greenhouse effect.

Currently, bioenergy is a highly developed technology that has advanced in many countries around the world. However, the centuries-old tradition of using biomass for heat production, which continues today, does not correspond to modern ideas of energy efficiency and requires improvement. The practice of using food vegetation for energy purposes when the problem of hunger has not yet been universally solved has also been subjected to harsh criticism from various organizations. Other negative factors include the widespread use of agricultural land for biomass cultivation, resulting in competition for land; fossil-fueled farm vehicles, used, for example, for harvesting do not make this technology 100% green. In this regard, the most promising technologies in the development of bioenergy are second-generation biofuel production that use raw materials from non-food biomass/non-food crops, for example, from algae or wood; and deeper biomass processing technologies – gasification and pyrolysis, as opposed to incineration [1,2].

According to [3], in 2018, the share of traditional biomass as a percentage of total energy consumption was around 7%, with the share of more modern or advanced types accounting for a little more than 5%.

A huge advantage of bioenergy is it allows for the possibility of obtaining a wide range of energy products in solid, gaseous and liquid form, which contributes to the satisfaction of various energy needs in electricity, heat supply or transport. In addition, by-products of bioenergy can be used as feedstock for the chemical, construction and agricultural industries. Another equally important advantage of bioenergy is that many of its products can be accumulated, stored long-term and easily transported, which at this stage of technological development is practically impossible in the case of   solar and wind energy.

Bioenergy has a relatively modest share in global power generation, its total generation capacity is slightly over 5%, which is insignificant when compared to other renewable energy sectors, including hydropower. At the same time, bioenergy is dominant in the production of heat energy and vehicle fuels. As a result, the share of bioenergy in the global consumption of energy from renewable sources is about 50% [4], taking into account the consumption of traditional biomass. Moreover, today more than 70% of processed biomass is used for thermal energy production, while the share of transport and electricity accounts for about 20 and 8%, respectively [3].

There are a large number of different technologies for the conversion of biomass into energy products, which take into account the characteristics of the feedstock and are mainly based on three types of technological impact – chemical, biochemical or thermochemical.

Fig. 1 shows a simplified diagram of biomass processing options that are either most widely used industrially or are at the design stage. It should be noted that due to the wide variety of technological processes it is difficult to simultaneously arrange them on a small diagram in accordance with the main stages of production. As a result, certain processes can be considered as preliminary processing, and others, as main processing, etc. Nevertheless, in general the diagram reflects the main types of feedstock and final products, as well as the dominant technological processes for direct biomass conversion.

Fig. 1. The main methods of processing biomass for energy production

Technological applications of biomass processing begin with the collection of raw materials, for example, in the form of agricultural waste, or special energy crops, such as rapeseed, corn or sugarcane. The biomass feedstock is then pre-treated. This process includes the following stages: drying, size reduction, crushing and milling, densification, and sorting. For certain types of raw materials, mixing, washing, exposure to steam, torrefaction, etc. are used. It should be noted that these procedures are expensive and largely determine the effectiveness of the subsequent stages of biomass processing. Unlike other types of raw energy materials including liquid and gaseous hydrocarbons or coal – biomass is a technologically complex substance, often unsuitable for efficient subdivision or transportation. Therefore, increased attention is paid to methods of preliminary treatment of biomass, as well as the development of the methods themselves.

The choice of technology for biomass processing is primarily determined by the feedstock type, the desired end product and the availability of the necessary infrastructure. The commitment of manufacturers to modern environmental and social standards will also play a factor. For example, bioethanol can be obtained from wheat, corn, sugarcane or other food plants via traditional acid hydrolysis and fermentation technologies. Bioethanol can also be obtained from more complex biomass, based on lignocellulose, using more complex second generation technologies such as, enzymatic hydrolysis or high-temperature gasification followed by catalytic synthesis.

Today, the most prevalent end products of biomass conversion are biodiesel and bioethanol, which are used as liquid fuel; and other products, such as biogas, wood chips and pellets. Biodiesel and bioethanol are predominantly obtained using first-generation technologies. Biogas is produced from agricultural waste, and fuel chips and pellets from woodworking industry waste, i.e. not from food products. Therefore, in this sense, they can be classified as second generation technologies. However, these products are then still subjected to traditional and often ineffective incineration. As a result of additional purification of biogas to biomethane or processing of pellets by special torrefaction, high-quality fuel can be obtained. This fuel can be considered a product of modern bioenergy.

First generation diesel fuel is produced from vegetable oil, most often obtained from rapeseed, or oil and fat production waste, by reacting fats with methyl alcohol with the addition of a catalyst. This process is called transesterification. As a result, diesel and glycerin are formed. Due to the simplicity of the production cycle and the low temperature conditions, this method was popularity with the agricultural sector, and even gained state support in some countries. However, the intensive use of agricultural areas for rapeseed cultivation provoked protests, and the high sensitivity of economic indicators of production to external market conditions significantly restrain the development of this technology. According to one estimate [3], 47 billion liters of biodiesel were produced globally in 2020, primarily in Europe, the USA, Brazil, Argentina, Indonesia and Thailand.

Over the last decade new technologies for the production of second-generation biofuels in cooperation with oil refineries have become very widespread. For example, OMV has launched a “Bio CRACK plant” pilot project for the production of biodiesel in Schwechat which uses wood waste and heavy mineral oil produced at a local refinery as feedstock. The processing of raw materials include liquid-phase pyrolysis at a temperature of 420⁰C, which makes it possible to obtain diesel fuel with a 20% biogenic content [5].

Finnish Neste Oil has launched facilities to produce diesel fuel using the NExBTL technology at several of its own refineries. Any kind of vegetable oil or animal fat can be used as raw materials in this process. Hydrogen, which is widely used in oil refining, is used here for hydrocracking and hydrogenation of feedstock, instead of the traditional esterification method [6]. In 2019, the annual production of HVO (HVO-hydrotreated vegetable oil) reached 6.5 billion liters globally [3].

One of the most promising prospects for the production of biodiesel lies in the processing of certain types of microalgae, as lands unsuitable for agricultural production can be used for algae cultivation and their oil content per unit mass exceeds that of many food crops. This technology will be presented in more detail below. Bioethanol is another popular first-generation biofuel produced worldwide on an industrial scale.

The report by the American association RFA states that in 2019 the production of bioethanol was mainly concentrated in the USA (about 60 billion liters) and Brazil (about 33 billion liters). Other countries and regions accounted for about 16%, including other large producers - the European Union, China and India [7]. According to [8], by 2027 the production of bioethanol in the world will reach 130 billion tons. The volumes of production of second generation bioethanol from cellulosic biomass are still small. For comparison, in 2019 the production of cellulosic ethanol in Brazil was estimated at 45 million liters, and in Europe in 2017 at 85 million liters [9-10]. The annual list of ethanol producers in the United States in 2019 included only 11 enterprises out of almost 250 use cellulosic biomass as feedstock [7]. The main crops for the production of first generation ethanol are sugarcane/beets and maize. The first generation ethanol technology is based on a fermentation processes that breaks down glucose molecules under anaerobic conditions. More complex technologies for the production of ethanol from cellulosic biomass are discussed in a separate chapter.

One of the main areas of bioenergy is the production and use of wood chips and pellets as raw materials for the generation of heat and electricity. According to [11], about 500 million m³ of various sawn wood and 37 million tons of pellets were produced in the world in 2018. The main pellet producers are the USA, Canada, Vietnam, Latvia and Russia, and the largest consumers are Great Britain, Denmark, South Korea and Italy. Drying is almost always included in the list of technological operations for the pretreatment of woody biomass, since further processing of wet wood would be impossible. Wood chips are usually produced by mechanically shredding forestry waste in chippers and shredder machines. To produce pellets the feedstock is finely ground in a Hammer Mill and then passed through a wood pellet machine where the ground biomass is configured into dense granules. This raw material is consumed widely both by large enterprises and households.

Recently, woody biomass has been replacing part of the coal feedstock at thermal power plants, potentially reducing carbon dioxide emissions. Further, after reforestation it is hypothesized that emissions from biomass combustion will be offset, leading to zero emissions. However, this approach has been increasingly subjected to criticism [12]. There is a significant time gap between biomass combustion and reforestation and, in addition, owing to the significantly lower calorific value of biomass compared to fossil fuels with the same mass (as well as the low density of the finished raw material - wood chips or pellets), their transportation over long distances leaves a serious ecological footprint, which reduces the attractiveness of this particular area of bioenergy.

Therefore, in modern bioenergy more efficient thermochemical methods of using biomass are being intensively developed, such as gasification and pyrolysis, which, like biogas production, will be discussed in more detail below.

As noted above, the progressive development of bioenergy benefits by relying on the use of non-food biomass. These primarily include agricultural and forestry waste. Direct indicators of bioenergy resource supply can be specific quantitative data; for example, the volume of municipal organic waste or biomass burned, the volume of sewage, the volume of wood chips production, etc. Indirect indicators such as agricultural production, arable land area and forest area also provide useful auxiliary material for assessing biomass resources in a particular region. Figures 6 and 7 show maps of the resource potential of the biomass of agricultural activity and forestry for the 20 most advanced countries of the world for each of the detailed indicators shown on the maps. The information is based on data from [13, 14].

Fig. 6. Top 20 countries in the world in terms of agricultural activity

Clearly, with an increase in the size of farmland or livestock the level of organic waste also increases. This can provide extensive raw material for bioenergy, although, this of course largely depends on the level of applied technologies and the specific type of agricultural products. As follows from the data in Fig. 6, countries with large territories are not always among the leaders in terms of agricultural land or arable land per capita. For example, large countries such as Algeria, the Republic of Congo, Libya, Egypt, and Pakistan are not included in the top 20 list. Canada is in the second half of this list in terms of total agricultural land area, but among the leaders in terms of arable land per capita. The largest areas of agricultural land are in the United States, Russia, China, Kazakhstan, Australia and Brazil. The leading countries in terms of arable land per capita are Canada, Kazakhstan and Australia. Many types of animal waste can be used as raw materials for the production of biodiesel or biogas, which is directly related to the livestock population. Countries such as the United States, Brazil, India and China have the largest livestock population. Pakistan, Nigeria, Ethiopia and Australia also have significant livestock resources. In [14] data is provided on the direct combustion of biomass; according to the source, the USA, Brazil, India, Russia, Indonesia and China annually burn more than 10 million tons of biomass each.

A somewhat different picture emerges when analyzing the countries of the world in terms of forest area and the level of timber processing. The absolute leader based on the four indicators shown in Fig. 7 is Canada. Each of these indicators (Forest area, Forest area per capita, sawn wood production, Chips and Particles production), to a certain degree demonstrates the resource potential of biomass from waste from the forest and timber processing industries. Among the leaders, as in the previous example with agricultural indicators, are the countries with the largest forest area: the USA, Russia, China, Australia, and Brazil.

Fig. 7. Top-20 countries in the world in terms of forest area and level of timber processing