Classification of gasification processes

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In the gasifier, coal reacts with a gasifying agent under high temperatures, converting the solid fuel into gaseous fuel, with only ash-containing residue remaining. Typically, the gasifying agents are water vapor, oxygen (air), and carbon dioxide. The products in raw gas are carbon dioxide, hydrogen, and methane; the accompanying gases include carbon dioxide, water vapor, etc. In addition, there are sulfides, hydrocarbon products, and other trace components. The composition of various gases depends on the type of coal, the gasification process, and the composition of the gasifying agent, affecting the thermodynamic and kinetic conditions of the gasification reaction. There are various ways to classify gasification methods, as follows: First, classification by the calorific value of the produced coal gas. The coal gasification processes are divided into 3 categories based on the calorific value of the coal gas produced under standard conditions: 1. Methods for producing coal gas with a low calorific value, where the calorific value of the coal gas is below 8347 kJ/m3 (2000 kcal/m3) ； 2. Method for producing medium calorific value gas, with a gas calorific value of 16,747~33,494 kJ/m3 (4,000~8,000 kcal/m3) ； 3. Method for producing high-calorific-value gas, with a calorific value of over 33,494 kJ/m3 (8,000 kcal/m3). II. Classification by heating method: The overall heat balance of the gasification process shows that the overall reaction is endothermic; therefore, heat must be supplied. The heat required for various processes varies, and this is primarily determined by the design of the process as well as the properties of the coal. Generally, 15% to 35% of the calorific value of the coal used for gasification is needed; the upper limit applies to co-current gasification, while the lower limit applies to counter-current gasification. There are several ways in which heat can be supplied: 1. Autothermal gasification – This is a direct method of heat supply, also known as partial gasification. In this approach, no external heat is required; the heat necessary for the gasification reaction between coal and steam is provided by the heat generated from the combustion of another portion of coal with oxygen in the gasifying agent. This is the most commonly used heating method in various industrial gasification furnaces at present. The oxygen-containing gas can be industrial oxygen or oxygen-enriched air, as well as air. The gasification process can be intermittent regenerative or continuous autothermal gasification. 2. Indirect heating gasification method: In this method, coal undergoes gasification reaction only with water vapor, with heat being supplied from outside the gasifier through the wall of the reactor. Therefore, such processes are also referred to as externally heated (or heat-supplied) steam gasification of coal. Such technologies mostly employ fluidized bed and gasification bed gasification methods. External heat can be provided by electric heating or nuclear reaction heat. 3. Combination of steam gasification and hydrogen gasification of coal: The reaction in which coal reacts with hydrogen at temperatures ranging from 800 to 1800 degrees Celsius under pressure to produce methane is an exothermic reaction. This reaction can be used directly to supply heat for the steam gasification of coal. The principle of this process is that coal is first hydrogasified; the residue resulting from this hydrogasification then reacts with steam, and the syngas produced serves as a hydrogen source for the hydrogenation stage. 4. Heat supply using a heat carrier: In a separate reactor, coal or coke is burned together with air to heat a heat carrier, which can be a solid (such as limestone), a liquid molten salt, or slag. III. Classification by vaporizer: 1. Air-steam vaporization – uses air (or oxygen-enriched air) and steam as the vaporization agents. Among these, there are two methods: intermittent gas production with internal air-vapor heat storage, and continuous gas production using oxygen-enriched air-vapor self-heating. Gas produced using air as a vaporizing agent is generally called air gas, with its main components being large amounts of nitrogen and carbon dioxide, along with a certain amount of carbon monoxide and oxygen. Gas produced using water vapor as a vaporizing agent is called water gas, with its main components being hydrogen, carbon monoxide, carbon dioxide, and methane. Gas produced using a mixture of air and water vapor as a vaporizing agent is called producer gas. Furthermore, in the ammonia synthesis industry, gas with a ratio of (carbon monoxide + hydrogen) ≈ 3:1 is called semi-water gas. 2. Oxygen-vapor gasification uses industrial oxygen and water vapor as the gasifying agents. Modern gasification technologies almost always use industrial oxygen and high-pressure steam as gasifying agents. 3. Hydrogen gasification: Using hydrogen or hydrogen-rich gases as a gasifying agent in the coal gasification process can produce gas rich in methane; this method is also known as hydrogenation gasification. IV. Classification by the movement state of solid fuel 1. Moving-bed (fixed-bed) gasification method 2. Fluidized-bed gasification method 3. Gas-flow bed gasification method