Principles of gasification process

Coal gasification consists of two parts: coal pyrolysis and coal gasification reactions. When heated, coal undergoes a series of physical and chemical changes, which mainly depend on the type of coal, temperature, pressure, heating rate, and the design of the gasification furnace. The gasification reaction of coal refers to the reaction between a gasifying agent (air, water vapor, oxygen-enriched air, industrial oxygen, and their corresponding mixtures, etc.) and a carbonaceous feedstock, as well as the chemical reactions between the reactants and the feedstock and the reaction products. When coal is heated, it undergoes a complex series of physical and chemical changes, a process known as pyrolysis. The gasification reaction in a gasifier is a highly complex system. Due to the complex structure of coal, which contains carbon, hydrogen, oxygen, and other elements, when discussing gasification reactions, it is always based on the assumption that only carbon, the main element in coal, is considered. Gasification reactions refer primarily to the reactions between carbon in coal and oxygen, water vapor, and hydrogen in the gasifying agents, as well as reactions between carbon and the reaction products. Gasification reactions are classified into heterogeneous reactions and homogeneous reactions depending on the phase state of the reactants; in the former case, the gasifying agent or gaseous reaction products react with solid coal or coal coke ； The latter refers to the reactions between gaseous reaction products or their reactions with the gasifying agent. In the gasification unit, different gasification reactions occur depending on the gasifying agent, and there are both parallel reactions and sequential reactions. *Vaporization reactions are conventionally divided into three types: carbon-oxygen reactions, water vapor decomposition reactions, and methane production reactions. Reactions between carbon and oxygen: The reactions between carbon and oxygen include: C + O2 = CO2 2-1-1; 2C + O2 = 2CO 2-1-2; C + CO2 = 2CO 2-1-3; 2CO + O2 = 2CO2 2-1-4. Among these reactions, 2-1-3 is known as the CO2 reduction reaction; it is a strong endothermic reaction that requires high temperatures to occur. The remaining three reactions are exothermic. 1.2.2 Reaction of carbon with water vapor At a certain temperature, the following reactions occur between carbon and water vapor: C + H2O = CO + H2 2-1-5 C + 2H2O = CO2 + 2H2 2-1-6 These are the main reactions used in the production of water gas; they are also known as steam decomposition reactions. Both of these reactions are endothermic. The CO produced in the reaction can further react with water vapor as follows: CO + H2O = CO2 + H2. This reaction is known as the carbon monoxide shift reaction; it is also referred to as the homogeneous water-gas reaction or water-gas equilibrium reaction, and it is an exothermic reaction. In the relevant process, in order to convert all or part of the carbon monoxide into hydrogen, this reaction is often utilized outside the gasifier. 1.2.3 Methane formation reaction: The methane in coal gas originates partly from the thermal decomposition of volatiles in coal, and partly from the reaction between carbon in the gasifier and hydrogen in the coal gas, as well as from reactions among the gaseous products. C + 2H2 = CH4 2-1-8 CO + 3H2 = CH4 + H2O 2-1-9 2CO + 2H2 = CH4 + CO2 2-1-10 CO2 + 4H2 = CH4 + 2H2O 2-1-11 All of the reactions listed above that produce methane are exothermic reactions. 1.2.4 Reactions of other elements in coal with gasifiers Coal also contains small amounts of the elements nitrogen (N) and sulfur (S). The possible reactions they can undergo with the gasifying agents O2, H2O, H2, and the gaseous reactants formed during the reaction are as follows: S + O2 = SO2 2-1-12; SO2 + H2 = H2S + 2H2O 2-1-13; 2H2S + SO2 = 3S + 2H2O 2-1-14; C + 2S = CS2 2-1-15; CO + S = COS 2-1-16; N2 + 3H2 = 2NH3 2-1-17; N2 + H2O + 2CO = 2HCN + 1.5O2 2-1-18; N2 + XO2 = 2Nox 2-1-19. These reactions result in the formation of sulfur- and nitrogen-containing compounds in the gas. These products can cause corrosion and contamination, and must be removed during gas purification. The sulfur-containing compounds among them are mainly hydrogen sulfide. COS, CS2, and other sulfur-containing compounds play only a minor role. Among nitrogen-containing compounds, ammonia is the main product, while NOX and HCN are secondary products; the aforementioned reactions play no significant role in the chemical equilibrium of the gasification reaction. The gasification reactions listed above are the basic chemical reactions in coal gasification; different gasification processes are formed by combining these reactions, or some of them, in series or in parallel. 2. Overview of the development and classification of coal gasification technologies 2.1 Introduction Coal gasification technology was developed quite early on – pressure-free fixed-bed gasifiers existed already in the 1920s. From the 1930s to the 1950s, pressurized fixed-bed Ruhr reactors, pressure-free Winkler bubbling reactors, and pressure-free fluidized-bed K-T reactors for coal gasification were brought online industrially. These types of gasifiers are generally referred to as the first generation of coal gasification technologies. The development of second-generation coal gasification technologies began in the 1960s. At that time, rapid progress was being made internationally in the extraction and utilization of oil and gas resources for the production of syngas, which reduced the investment and production costs associated with syngas manufacturing. As a result, natural gas and oil became the main raw materials used for syngas production worldwide, thereby hindering the advancement of new coal gasification technologies. However, the oil crisis that occurred globally in the 1970s spurred further efforts to develop new coal gasification technologies. By the 1980s, some of these new technologies had been put into industrial use, while others had completed pilot plant tests. Notable examples of such technologies include the Texaco pressurized water-coal slurry gasifier, the slag-based Ruhr process furnace, the high-temperature Etiw furnace, and the dry pulverized coal pressurized gasifier. The main features of second-generation gasification technology are: increasing the operating pressure and temperature of the gasifier, boosting the production capacity per unit gasifier, expanding the range of coal types and particle sizes that can be used, improving the technical and economic indicators of production, and enhancing environmental quality to meet environmental protection requirements. In recent years, the development of gasification technology abroad has shown a trend toward using coal powder and water-coal slurry as raw materials, with fluidized bed and gasized bed reactors operating under high temperature and pressure.

The most basic reactions: C+H2O=CO+H2, C+2H2O=CO2+2H2, C+O2=CO2