Dry distillation of coal and gasification of coal

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Coal carbonization is one of the important processes in coal chemical industry. It refers to the process in which coal is heated and decomposed in an air-free environment, producing products such as coke (or semi-coke), coal tar, crude benzene, and gas. Depending on the final heating temperature, it can be divided into three types: 900–1100°C corresponds to high-temperature dry distillation, that is, coking ； 700–900°C is medium-temperature dry distillation ； 500–600°C is considered low-temperature carbonization (see low-temperature carbonization of coal). The coal carbonization process involves the following changes: when the temperature of the coal is above 100°C, the moisture in the coal evaporates ； When the temperature rises above 200°C, the bound water in coal is released ； Above 350°C, cohesive coals begin to soften and further form viscous colloids (this phenomenon does not occur in peat and lignite, etc.) ； At 400–500°C, most of the gas and tar are distilled off; these are known as the primary thermal decomposition products ； At 450–550°C, thermal decomposition continues, and the residue gradually thickens and solidifies to form semi-coke ； Above 550°C, the semi-coke continues to decompose, with the remaining volatile substances (primarily hydrogen) being released; as the semi-coke loses weight, it contracts and develops cracks ； When the temperature is above 800°C, the semi-coke shrinks and hardens to form porous coke. When dry distillation is carried out in a chamber-type dry distillation furnace, the primary thermal decomposition products come into contact with the red-hot coke and the high-temperature furnace walls, resulting in secondary thermal decomposition and the formation of secondary thermal decomposition products (coke oven gas and other coking chemical products). The products of coal carbonization are coal, coal tar, and coal gas. The yield and composition of coal dry distillation products depend on the quality of the raw coal, the furnace design, and the processing conditions (mainly temperature and time). Depending on the final temperature of carbonization, the coal carbonization products also vary. The solid product of low-temperature dry distillation is a loosely structured black semi-coke; the gas yield is low while the tar yield is high ； The solid product of high-temperature dry distillation is a structurally dense silver-gray coke, with a high gas yield and a low tar yield. The yield of the medium-temperature carbonization products lies between that of low-temperature and high-temperature carbonization. The gas produced during coal carbonization consists mainly of hydrogen and methane, which can be used as fuel or as raw materials for the chemical industry. High-temperature dry distillation is mainly used to produce metallurgical coke; the tar obtained is a mixture of aromatic compounds and heterocyclic compounds, and it serves as an important source of aromatics in industry ； Coal tar obtained through low-temperature carbonization contains more alkanes than tar produced at high temperatures, and it is one of the important sources for synthetic oil. Coal gasification refers to the process in which coal, within specific equipment and under certain temperatures and pressures, undergoes a series of chemical reactions with gasifying agents such as steam/air or oxygen, thereby converting solid coal into combustible gases like CO, H2, and CH4, as well as non-combustible gases such as CO2 and N2. For coal gasification, three conditions are necessary: a gasifier, a gasifying agent, and heat supply; none of these can be missing. As shown in the figure, the reactions that occur during the gasification process include the pyrolysis, gasification, and combustion reactions of coal. The pyrolysis of coal refers to the process in which coal transforms from a solid state into gaseous, solid, and liquid products. The gasification and combustion reactions of coal involve two types of reactions, namely heterogeneous gas-solid reactions and homogeneous gas-phase reactions. Different gasification processes have varying requirements regarding the properties of the feedstock; therefore, when selecting a coal gasification process, it is extremely important to take into account the characteristics of the coal used for gasification and their impacts. The properties of coal used for gasification mainly include its reactivity, caking property, slagging tendency, thermal stability, mechanical strength, particle size distribution, as well as the contents of moisture, ash, and sulfur. Coal gasification processes can be classified based on pressure, gasifying agent, and the method of heating during the gasification process. The most common classification is based on the way in which the coal and the gasifying agent come into contact within the gasifier. The main types are as follows: 1) Fixed-bed gasification: During this process, coal is fed into the gasifier from the top, while the gasifying agent is added from the bottom. The coal and the gasifying agent come into contact in a counter-current manner; compared to the upward movement of the gas, the coal moves very slowly, or even appears to remain stationary, which is why it is called fixed-bed gasification ； In reality, the coal moves downward at a very slow pace during the gasification process; it is more accurate to refer to this as moving-bed gasification. 2) Fluidized bed gasification: It uses small coal particles with a particle size of 0–10 mm as the feedstock for gasification. These particles are suspended and dispersed in a vertically rising stream of air within the gasification furnace, where they undergo gasification reactions in a boiling state. This approach ensures uniform temperature throughout the coal layer, making it easier to control the process and improving the efficiency of gasification. 3) Fluidized bed gasification. It is a co-current gasification process in which a gasifying agent is used to introduce coal powder with a particle size of 100 um or less into the gasifier; alternatively, the coal powder can first be converted into coal slurry and then pumped into the gasifier. At temperatures higher than its ash fusion point, the coal reacts with the gasifying agent through combustion and gasification reactions, and the ash is discharged from the gasifier in a liquid state. 4) Vaporation in the melt bath bed. It involves injecting pulverized coal and a gasifying agent at high speed in a tangential direction into a molten pool that is at a high temperature and maintains a stable level; this transfers some of the kinetic energy to the slag, causing the molten material in the pool to move in a spiral motion and to vaporize. This gasification process is no longer being developed. The above are all surface gasification methods; there is also an underground gasification process. Coal gasification technology is widely applied in the following fields: 1) As industrial fuel gas, gas with a normal calorific value of 1100–1350 kcal per unit volume can be produced using either atmospheric-pressure fixed-bed gasifiers or fluidized-bed gasifiers. It is mainly used in industries such as steel, machinery, sanitation, building materials, light textiles, and food processing, to heat various furnaces and kilns, or to directly heat products or semi-finished goods. 2) As domestic gas, its normal calorific value ranges from 3000 to 3500 kcal, with a requirement that the CO content be less than 10%. It can be obtained through direct gasification as well as from coke oven gas; the Ruhr process is particularly suitable for this purpose. Compared with direct coal combustion, household gas can not only significantly improve coal utilization efficiency and reduce environmental pollution, but also greatly facilitate people’s lives, offering good social and environmental benefits. For reasons related to safety, environmental protection, and cost, it is necessary to keep the levels of H2, CH4, and other hydrocarbon combustible gases in household gas as high as possible, in order to increase the calorific value of the gas ； Since CO is toxic, its concentration should be kept as low as possible. 3) As a raw material gas for chemical synthesis and fuel oil production, as early as World War II, countries such as Germany used the Fischer-Tropsch process to synthesize aviation fuel. With the development of syngas chemistry and carbon-chemistry technologies, the route of producing syngas through coal gasification and then directly synthesizing various chemicals has become the foundation of modern coal chemical industry, including the production of synthetic ammonia, synthetic methane, synthetic methanol, acetic anhydride, dimethyl ether, and synthetic liquid fuels. Chemical synthesis gas does not require a high calorific value; the main requirements are related to components such as CO and H2 in the gas. Generally, Texaco gasifiers and Shell gasifiers are suitable for this purpose. At present, over 50% of the methanol produced from synthetic ammonia in our country comes from coal gasification synthesis processes. 4) As metallurgical reducing gases, CO and H2 in coal gas possess strong reducing properties. In the metallurgical industry, reducing gases can be used to directly reduce iron ore into sponge iron ； In the non-ferrous metals industry, metal oxides such as nickel, copper, tungsten, and magnesium can also be smelted using reducing gases. Therefore, metallurgical reduction gas has requirements regarding the CO content in coal gas. 5) As gas for combined-cycle power generation, integrated gasification combined cycle power generation (abbreviated as IGCC) refers to the gasification of coal under pressure; the resulting gas is purified and then burned. The high-temperature flue gases drive a gas turbine to generate electricity, while the residual heat from these flue gases is used to produce high-pressure superheated steam that drives a steam turbine to generate electricity. The gas used in IGCC does not require a high calorific value, but it has strict requirements regarding the purity of the gas – such as low levels of dust and sulfides. The gasification used in conjunction with IGCC generally employs fixed-bed pressurized gasification (Ruhrgas furnace), fluidized-bed gasification (Texaco process), or pressurized fluidized-bed gasification (Shell gasifier); in Guangdong Province, the pressurized fluidized-bed gasification process is used, with the calorific value of the gas ranging from 2200 to 2500 kcal per unit volume. 6) As a coal gasification fuel cell: A fuel cell is a chemical power generation technology in which fuels such as H2, natural gas, or gas (chemical energy) are directly converted into electricity through electrochemical reactions. Currently, they are mainly of the phosphate-type (PAFC), molten carbonate-type (MCFC), solid oxide-type (SOFC), and others. The power generation technologies that combine them with efficient gasification are IG-MCFC and IG-SOFC, which can achieve a power generation efficiency of 53%. 7) Hydrogen production via coal gasification. Hydrogen is widely used in fields such as electronics, metallurgy, glass production, chemical synthesis, aerospace, direct coal liquefaction, and hydrogen fuel cells. Currently, 96% of the hydrogen produced worldwide comes from the conversion of fossil fuels. Coal gasification for hydrogen production plays a very important role; generally, coal is converted into CO and H2, and then CO is transformed into H2 and H2O through shift reactions. Hydrogen-rich gases can be processed using low-temperature separation, pressure swing adsorption, or membrane separation techniques to obtain hydrogen. 8) Gas source for coal liquefaction: Whether it is direct coal liquefaction or indirect oxidation, coal gasification is essential. Coal liquefaction requires coal gasification to produce hydrogen, and the available coal gasification processes include fixed-bed pressurized Lurgi gasification, pressurized fluidized bed gasification, and pressurized pneumatic bed gasification processes.