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The gasification process is the transition of the organic components to the gas phase by exposure to the thermochemical transformation process and to obtain volatile, flammable components as a result of the secondary reactions that occur. As a result of the gasification process, in addition to these volatile components, a semi-char (char) and a tar are formed that will give the energy necessary for the process if they are burned by air. Gasification of biomass is the process of turning solid fuels into a combustible gas. The product contains dense carbon monoxide, carbon dioxide, hydrogen, methane, water and nitrogen as well as ash and tar [98].

Biogas is a flammable gas obtained as a result of processing biomass. Unlike other flammable gases (e.g., natural gas), biogas is obtained only from animal or vegetable, i.e., organic raw materials. Biological wastes, organic wastes originating from the food industry, energy plants such as corn or sugar beet and animal feces in animal husbandry can be used as a substrate in biogas [99].

Various types and groups of microorganisms are involved in the conversion of complex organic compounds into biogas in oxygenated biogas production processes. The decomposition of these complex organics in an oxygen-free environment takes place in four stages: hydrolysis, acid production, acetic acid production and biogas as a result of methane production. In Figure 1.4, the decomposition process is schematized.

As a result of anaerobic decomposition, 50-80% CH₄ (methane) and 20-50% CO₂ (carbon dioxide) and a mixture of gases containing very small amounts of hydrogen, carbon monoxide, nitrogen, oxygen and hydrogen sulfide are formed. This biologically produced gas is defined as biogas. The gas composition depends on the raw materials used and environmental conditions. The thermal value of biogas (natural gas) containing 99% CH₄ is 37.3 MJ/m³, and the thermal value of biogas containing 65% CH₄ is 24.0 MJ/m³ [99, 100].

Anaerobic degradation is the conversion of biomass into other products and by-products by microorganisms in an oxygen-free environment. Anaerobic processes have been used for years to produce energy from biomass in both developed and developing countries. The energy obtained by burning agricultural and animal wastes in developing countries is used as a source of warming and conventional energy. In developed countries, these wastes are fermented in central biogas production facilities and significant amounts of energy are produced and used from methane gas [101].

The main processes of processing biomass as biogas can be described independently of the composition of the substrates used as follows: Using bacteria and other microorganisms, biomass is decomposed in a biogas facility. As the final products of this multi-stage fermentation process, methane (45-70%) and carbon dioxide (25-55%) are formed, especially in a humid environment free from air and light [102].

Figure 1.4 The decomposition process of biogas [99].

The availability of biogas as energy depends primarily on the ratio of methane in biogas. The produced biogas is generally converted into electrical energy, which can be used locally or delivered to the electricity grid in combined heat and power station (cogeneration). It is also possible to use the heat generated during the combustion phase to heat the buildings or greenhouses near the facility, dry the straw, cool the milk or climate the barns [103].

Biogas is predominantly methane and carbon dioxide gas formed as a result of biodegradation (anaerobic fermentation) of organic substances in oxygen-free conditions. Conversion of various organic materials to methane and carbon dioxide is carried out by mixed microbiological flora. As a result of this oxygen-free degradation, methane gas is formed by a threestep process. Oxygen-free degradation (anaerobic fermentation) basically has three stages; fermentation and hydrolysis, the formation of acetic acid and finally the formation of methane gas [104].

In the first stage, high molecular weight solid and dissolved organic materials (cellulose, starch, hemicellulose, fat, protein, etc.) are hydrolyzed with the extracellular enzymes of the bacteria and converted into lower molecular weight organic substances. The second phase of acid production is converted into volatile fatty acids and then to acetic acid by the acid bacteria of low molecular weight organic substances. In the final stage, CH₄ methane is produced by breaking down the acetic acid produced during the acid production phase or by synthesizing CO₂ and H₂ [105].

Syngas, also known as synthetic gas or synthesis gas, can be produced from carbon-containing biomass (wood gas), plastics, coal and urban waste or similar materials. Synthesis gas is created by gasification or pyrolysis of carbon-containing materials. Gasification includes materials subjected to high temperatures to maintain the reaction by providing thermal energy with limited combustion in the control of oxygen. Gasification operation can be performed in a gasifier reactor or alternatively it can be carried out in places where there are underground coal mines. If the raw material that turns into gas is a recently obtained biological resource such as wood or organic waste, the gas produced by the gas converter is considered to be renewable fuel [106].

Synthesis gas has half the energy content of natural gas. It cannot be burned directly but can be used as a fuel source. Another usage area of syngas is hot steam or electricity generation. Generally coal, petroleum-based materials and wastes containing carbon can be used in gasification. In these reactions, carbon combines with water to form CO₂, CO and H₂ [107]. Figure 1.5 shows the conversion of natural gas into a synthesis gas; a mixture of hydrogen, carbon monoxide and carbon dioxide. The quantities of these compounds in the mixture vary according to the selected natural gas conversion process and the type of synthetic fuel to be obtained [108].

The pyrolysis method, which is the thermal degradation of the raw material in an oxygen-free environment with the thermal conversion methods applied to the biomass, the liquid product obtained is an option for petrochemical derivative fuels and petrochemical industry input with its high thermal value, measurable, movable and storable properties [109]. The main parameters obtained in pyrolysis studies, which affect liquid, solid, gas product yields are heating rate, pyrolysis temperature, particle diameter, dripping gas flow rate and particle size. In addition, the effect of secondary reactions in which the heating rate is over 100 °C min^-1, occurring under static pyrolysis and static retorting conditions, limiting the yield and quality of the liquid is not reduced and it is possible to obtain a high-yield liquid product [110].

Figure 1.5 Production of synthesis gas from natural gas flow chart [108].

Gasification of biomass is one of the recommended methods for direct hydrogen production from renewable energy sources, although natural gas costs more than conventional vapor reformation [111]. Advanced and current technology is an integrated gasification combined cycle (IGCC) system in which hydrogen and electricity are produced from coal together. In the gasification section, the main system unit is a gasifier. Steam and oxygen are combined with coal under a certain temperature and pressure. Only a small part of the raw material burns out (also called partial oxidation). As a result of the process, a mixture of hydrogen, carbon monoxide and other gases called synthesis gases are produced. The minerals in the raw material are removed from the bottom of the gasifier, substances such as sulfur and ammonia are removed in the later stages of the process. The combined cycle part of the process refers to the use of the combustion turbine followed by the steam turbine used to generate electricity from synthesis gas, which increases system efficiency. The benefit of this integrated system is to use synthesis gas as a source of hydrogen. This can be used electrically by means of transportation fuel or fuel cells [112].