Brief introduction of coal gasification development?

Coal gasification refers to the process that the organic matter in coal reacts with gasifying agent (such as steam/air or oxygen) at a certain temperature and pressure in a specific equipment to convert solid coal into combustible gas containing carbon monoxide, H2 and methane and nonflammable gas such as carbon dioxide and N2. Coal gasification must meet three conditions: gasifier, gasifying agent and heat supply, all of which are indispensable.

As shown in the figure:

The reactions in the gasification process include pyrolysis, gasification and combustion of coal. Pyrolysis of coal refers to the process that coal changes from solid phase to gas, solid and liquid three-phase products. The gasification and combustion reactions of coal include two kinds of reactions, namely heterogeneous gas-solid reaction and homogeneous gas-phase reaction.

Different gasification processes require different properties of raw materials, so it is extremely important to consider the characteristics and effects of gasification coal when selecting gasification processes. The properties of coal for gasification mainly include reactivity, caking, slagging, thermal stability, mechanical strength, particle size composition, moisture, ash and sulfur content.

The coal gasification process can be classified according to pressure, gasifying agent and heating mode during gasification. Usually, it is distinguished according to the contact mode of coal and gasification agent in the gasifier, which mainly includes:

1) fixed bed gasification: in the gasification process, coal is added from the top of the gasifier, and the gasifying agent is added from the bottom of the gasifier, and the coal material contacts the gasifying agent in countercurrent. Compared with the rising speed of gas, the coal material decreases slowly, which can even be regarded as fixed bed gasification, so it is called fixed bed gasification; In fact, coal material moves downwards at a very slow speed in the process of gasification, which is more accurately called moving bed gasification.

2) Fluidized bed gasification: small coal particles with particle size of 0- 10mm are used as gasification raw materials, which are suspended and dispersed in the vertically rising airflow in the gasifier, and the coal particles undergo gasification reaction in boiling state, so that the temperature in the coal bed is uniform, easy to control and the gasification efficiency is improved.

3) entrained-flow gasification. It is co-current gasification, in which the pulverized coal with the particle size below 100um is brought into the gasifier together with the gasifying agent, or the pulverized coal can be made into coal water slurry first, and then pumped into the gasifier by a pump. At the temperature higher than the melting point of coal ash, coal reacts with gasifying agent, and coal ash is discharged from gasifier in liquid form.

4) Gasification in the molten bath bed. It injects pulverized coal and gasifying agent into the high-temperature and high-stability molten pool at high speed along the tangential direction, and transfers part of kinetic energy to the slag, so that the molten materials in the molten pool do spiral rotation and gasify. At present, this gasification process is no longer developed.

All of the above are ground gasification and underground gasification.

Coal gasification technology is widely used in the following fields:

1), as an industrial gas with a general calorific value of 1 100- 1350, can be produced in both atmospheric fixed bed gasifier and fluidized bed gasifier. Mainly used in steel, machinery, health, building materials, textiles, food and other departments to heat various furnaces, or directly heat products or semi-finished products.

2) As the general calorific value of civil gas is 3000-3500 kcal, CO is required to be less than 10%. Besides coke oven gas, it can also be obtained by direct gasification, and Lurgi furnace is more suitable. Compared with direct coal burning, civil gas can not only significantly improve coal utilization efficiency and reduce environmental pollution, but also greatly facilitate people's lives, with good social and environmental benefits. For the sake of safety, environmental protection and economy, it is required that the content of hydrocarbon combustible gases such as H2 and CH4 in civil gas should be as high as possible to improve the calorific value of gas. CO is toxic and its content should be as low as possible.

3) As the feed gas for chemical synthesis and fuel oil synthesis, as early as World War II, Germany and others used Fischer-Tropsch to synthesize aviation fuel oil. With the development of synthesis gas chemistry and carbon chemical technology, the route of producing synthesis gas by coal gasification and then directly synthesizing various chemicals has become the basis of modern coal chemical industry, mainly including synthetic ammonia, synthetic methane, synthetic methanol, acetic anhydride, dimethyl ether and synthetic liquid fuel.

Chemical synthesis gas does not require high calorific value, but mainly requires components such as CO and H2 in the gas. Generally Texaco gasifier and Shell gasifier are more suitable. At present, more than 50% of the methanol output of synthetic ammonia in China comes from coal gasification synthesis process.

4) As metallurgical reducing gases, CO and H2 in coal gas have strong reducing effect. In metallurgical industry, iron ore can be directly reduced to sponge iron by reducing gas; In nonferrous metal industry, metal oxides such as nickel, copper, tungsten and magnesium can also be melted with reducing gas. Therefore, the metallurgical reducing gas has requirements for the CO content in the gas.

5) Integrated gasification combined cycle power generation with gas (IGCC for short) means that coal is gasified under pressure, the generated gas is purified and burned, and the high-temperature flue gas drives the gas turbine to generate electricity, and then the high-pressure superheated steam is generated by using the waste heat of flue gas to drive the steam turbine to generate electricity. The gas used in IGCC does not require high calorific value, but requires high gas purity, such as dust and sulfide content. Generally, fixed-bed pressurized gasification (Lurgi furnace), entrained-flow gasification (Texaco) and pressurized gas flow (Shell gasifier) are used for coal gasification supporting IGCC. The calorific value of gas is about 2200-2500 kcal.

6) Coal gasification fuel cell is a chemical power generation technology that directly converts fuel (chemical energy) such as H2, natural gas or coal gas into electric energy through electrochemical reaction. At present, it is mainly composed of phosphate (PAFC), molten carbonate (MCFC) and solid oxide (SOFC). IG-MCFC and IG-SOFC are power generation technologies that combine them with high-efficiency coal gasification, and their power generation efficiency can reach 53%.

7) Hydrogen produced by coal gasification is widely used in electronics, metallurgy, glass production, chemical synthesis, aerospace, direct coal liquefaction and hydrogen energy batteries. At present, 96% of the hydrogen in the world comes from the conversion of fossil fuels. Hydrogen production from coal gasification plays an important role. Generally, coal is converted into carbon monoxide and H2, and then carbon monoxide is converted into H2 and H2O through shift reaction. Hydrogen can be obtained from hydrogen-rich gas by low temperature separation or pressure swing adsorption and membrane separation technology.

8) The gas source of coal liquefaction, whether it is direct liquefaction or indirect oxidation, is inseparable from coal gasification. Coal liquefaction needs coal gasification to produce hydrogen, and optional coal gasification processes also include fixed bed pressurized Lurgi gasification, pressurized fluidized bed gasification and pressurized entrained flow gasification.