Coal gasification refers to a series of reactions between organic matter in coal and gasification agent (such as steam/air and/or oxygen) under certain temperature and pressure in specific equipmentchemical reactionTo convert solid coal into CO, hydrogen, methane, etccombustible gas andcarbon dioxide, nitrogen and other non combustible gasesSyngas(Syngas, or synthesis gas).During coal gasification, gasification furnace, gasification agent and heat supply are indispensable.
Coal gasification refers to the process of transforming organic matter in coal into gas by thermochemical processing of coal with gasification agent under certain temperature and pressure.Its meaning is a multiphase reaction process in which coal, semi coke or coke is used as raw material and air, oxygen rich, steam, carbon dioxide or hydrogen is used as gasification medium to make coal undergo partial oxidation and reduction reaction and convert its carbon, hydrogen and other substances into carbon monoxide, hydrogen, methane and other combustible components as the main gas products.Further processing of this gas product can produce other gas, liquid combustion materials or chemical products.After gasification, the latent heat of coal will become the latent heat of gas as much as possible.
On October 27, 2017, the list of carcinogens released by the International Agency for Research on Cancer of the World Health Organization (WHO) was preliminarily sorted out for reference, and coal gasification was included in the list of Class 1 carcinogens.[2]
Coal gasification is a production process that takes coal as raw material and converts solid coal into combustible gas by a series of chemical reactions between organic matter and gasification agent in coal under high temperature and pressure in specific equipment (such as gasifier).Oxygen, steam or hydrogen are usually used as gasifying agents, and carbon monoxide, hydrogen and methane are the main components of the generated combustible gas.Coal gasification includes high temperature drying and dehydration of coal, heating to release volatile matter, and gasification reaction between volatile matter and remaining coal.
The direct combustion of coal will bring serious environmental problems, such as the generation of sulfur dioxide, nitric oxide and other harmful gases. The accumulation of a large number of harmful gases at high altitude will lead to the formation of acid rain, which will seriously harm buildings, crops and human health.It is impossible to make full use of coal resources by direct combustion. The furnace smoke takes away a lot of heat, and the slag still contains carbon that is not fully burned.These problems cannot be solved economically and effectively, so people consider whether coal can be converted into clean gas or liquid fuel for reuse. Coal gasification technology has been vigorously developed. The combustible gas generated after coal gasification will only generate water and carbon dioxide after combustion, greatly reducing the pressure on the environment caused by coal utilization,It can be said to be the technical basis for the clean utilization of coal in the future.The gasification process of coal only generates a small amount of carbon dioxide and water, and most of the carbon is converted into combustible gas, which greatly improves the utilization efficiency of coal.The gasification product of coal gas can be widely used in power production, urban heating, liquid fuel, chemical raw material synthesis and other aspects, and the coal resources can be effectively and fully utilized accordingly.[1]
Main categories
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(1) It is classified based on the original form, including solid combustion gasification, liquid fuel gasification, gas combustion material gasification and solid/liquid mixed fuel gasification.
Coal gasification
(2) The classification is based on the particle size of the fired coal, including block gasification (6~50mm), pulverized coal gasification (less than 0.1mm), etc.In addition, those formed by coal/oil slurry or coal/water slurry in furnace combustion are included in small coal and pulverized coal gasification method.
(3) Classification is based on the operating pressure of gasification process, including atmospheric or low-pressure gasification (0~0.35MPa), medium pressure gasification (0.7~3.5MPa) and high-pressure gasification (7MPa).
(4) It is classified based on gasification medium, including air blast gasification, air steam gasification, oxygen steam gasification and hydrogenation gasification (the process of producing high calorific value gas from non coal with hydrogen as the chemical agent).
(5) The classification is mainly based on slag discharge, including dry or wet slag discharge gasification, solid or liquid slag discharge gasification, continuous or intermittent slag discharge gasification, etc.
(6) It is classified according to the heat supply mode of gasification process, including externally heated gasification (the heat required for gasification is released from the inside of the gasifier with the help of an external heating device) and heat carrier (gas, solid or liquid slag carrier) gasification.
(7) It is classified according to the process dynamics of the coal in the furnace, including moving bed gasification, liquefied bed gasification, gas flow (entrainment) bed gasification and molten bed (slag or molten salt, molten iron) gasification.
(8) According to the relative movement direction of solid coal and gas medium, there are co gasification or co flow gasification, counter flow gasification, etc.
(9) Classification is based on the type of reaction, including thermodynamic process, catalytic laboratory process.
(10) The classification is mainly based on the stage of the process, including single stage gasification, two stages (single stage, double stage) or multi-stage gasification.
(11) It is classified mainly by process operation mode, including continuous intermittent gasification or cyclic gasification.
Implementation principle
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Gasification is a thermochemical process of coal.It is a process in which coal or coal char is used as raw material, oxygen (air, oxygen rich or industrial pure oxygen) and steam are used as gasifying agents, and the combustible part of coal or coal char is converted into combustible gas through chemical reaction under high temperature and pressure.The combustible gas obtained during gasification becomes coal gas. The coal gas used as chemical raw material is generally called syngas (syngas can also use natural gas, heavy oil components, etc. as raw materials in addition to coal). The equipment for gasification is called gas producer or gasifier.
Coal gasification involves a series of physical and chemical changes.Generally, it includes four stages: drying, combustion, pyrolysis and gasification.Drying is a physical change. With the increase of temperature, the moisture in coal evaporates by heating.Others are chemical changes, and combustion can also be considered as part of gasification.After the coal is dried in the gasifier, with the further increase of temperature, the coal molecule will undergo thermal decomposition reaction, generating a large number of volatile substances (including dry distillation gas, tar, pyrolysis water, etc.), and at the same time, the coal will be bonded into semi coke.The semi coke formed after coal pyrolysis will react with the gasifying agent fed into the gasifier at a higher temperature to generate gaseous products with carbon monoxide, hydrogen, methane, carbon dioxide, nitrogen, hydrogen sulfide and water as the main components, i.e. raw gas.Gasification reaction includes many chemical reactions, mainly the reactions among carbon, water, oxygen, hydrogen, carbon monoxide and carbon dioxide. The reaction between carbon and oxygen is also called combustion reaction, which provides heat for gasification process.
The main reactions are:
1. Steam reforming reaction
C+HtwoO=CO+Htwo-131kJ/mol
2. Water gas shift reaction
CO+ HtwoO =COtwo+Htwo+42kJ/mol
3. Partial oxidation reaction
C+0.5 O2=CO+111kJ/mol
4. Complete oxidation (combustion) reaction
C+Otwo=COtwo+394kJ/mol
5. Methanation reaction
CO+3Htwo=CHfour+HtwoO+206.3kJ/mol[3]
6. Boudouard reaction
C+COtwo=2CO-172kJ/mol
nature
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It mainly includes reactivity, cohesiveness, slagging, thermal stability, mechanical strength, particle size composition, moisture, ash and sulfur content of coal.
technology
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The coal gasification process can be classified according to pressure, gasification agent, heating mode of gasification process, etc. The common classification is based on the contact mode of coal material and gasification agent in the gasifier, mainly including:
(1) Fixed bed gasification: in the gasification process, coal is added from the top of the gasifier, and the gasification agent is added from the bottom of the gasifier. The coal material is in contact with the gasification agent countercurrent. Compared with the rising speed of the gas, the coal material falls slowly, even can be regarded as fixed, so it is called fixed bed gasification;In fact, coal material moves downward at a very slow speed during gasification, which is more accurately called moving bed gasification.
(2) Fluidized bed gasification: it uses small particles of coal with particle size of 0-10mm as gasification raw materials, suspends and disperses them in the vertical upward airflow in the gasifier, and the coal particles conduct gasification reaction in the boiling state, so that the temperature in the coal bed is uniform, easy to control, and improve the gasification efficiency.
(3) Entrained flow gasification.It is a kind of parallel flow gasification, in which pulverized coal with particle size below 100um is brought into the gasifier with gasifying agent, or pulverized coal can be made into coal water slurry first, and then pumped into the gasifier.The coal material has combustion reaction and gasification reaction with the gasification agent at a temperature higher than its ash melting point, and the ash is discharged from the gasifier in liquid form.
(4) Melt bath bed gasification.It is to inject pulverized coal and gasification agent into a high temperature and highly stable molten pool at a high speed in tangent direction, transfer part of the kinetic energy to the slag, and make the molten substance in the pool rotate in spiral motion and gasify.This gasification process is no longer developed.
All of the above are ground gasification and underground gasification processes.
application area
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Coal gasification technology is widely used in the following fields:
Industrial gas
Generally, gas with a calorific value of 1100-1350 can be produced by using atmospheric fixed bed gasifier or fluidized bed gasifier.It is mainly used in steel, machinery, health, building materials, textile, food and other departments to heat various furnaces, kilns, or directly heat products or semi-finished products.
Civil gas
Generally, the calorific value is 3000-3500 kcal, and CO is required to be less than 10%. In addition to coke oven gas, it can also be obtained by direct gasification, and Lurgi furnace is more suitable.Compared with direct coal combustion, civil gas can not only significantly improve coal consumption efficiency and reduce environmental pollution, but also greatly facilitate people's life, with good social and environmental benefits.In consideration of safety, environmental protection, economy and other factors, it is required that Htwo、CHfourThe content of, and other hydrocarbon combustible gases 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.
Synthetic feed gas
As feed gas for chemical synthesis and fuel oil synthesis
As early as the Second World War, Germany and others used Fischer Tropsch process to synthesize aviation fuel oil.With the development of syngas chemical industry and carbon chemical technology, the route of producing syngas from 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 has low requirements for calorific value, mainly for CO and H in coal gastwoSuch components are required, and Texaco gasifier and Shell gasifier are generally suitable.More than 50% of methanol output of synthetic ammonia in China comes from coal gasification synthesis process.
Metallurgical reducing gas
CO and H in gastwoIt has strong reducing effect.In metallurgical industry, iron ore can be directly reduced to sponge iron by using reducing gas;In nonferrous metal industry, nickel, copper, tungsten, magnesium and other metal oxides can also be smelted by reducing gas.Therefore, metallurgical reduction gas has requirements for CO content in gas.
combined cycle
Integrated gasification combined cycle power generation (IGCC for short) refers to the gasification of coal under pressure, the generated gas is purified and burned, the high-temperature flue gas drives the gas turbine for power generation, and the waste heat of flue gas is used to generate high-pressure superheated steam to drive the steam turbine for power generation.The gas used for IGCC has low requirements for calorific value, but high requirements for gas purification, such as dust and sulfide content.The gasification matching with IGCC generally adopts fixed bed pressurized gasification (Lurgi furnace), entrained flow gasification (Texaco), pressurized airflow (Shell gasifier) Guangdong pressurized fluidized bed gasification process, with the gas calorific value of about 2200-2500 kcal.
fuel cell
The fuel cell is composed of Htwo, natural gas or gas and other fuels (chemical energy) are directly converted into electricity through electrochemical reaction.Coal gasification fuel cell mainly consists of phosphate type (PAFC), molten carbonate type (MCFC), solid oxide type (SOFC), etc.Their power generation technologies combined with efficient coal gasification are IG-MCFC and IG-SOFC, and their power generation efficiency can reach 53%.
Hydrogen production
Hydrogen is widely used in electronics, metallurgy, glass production, chemical synthesis, aerospace, direct coal liquefaction, hydrogen energy batteries and other fields. 96% of the world's hydrogen comes from fossil fuel conversion.However, coal gasification plays an important role in hydrogen production, generally converting coal into CO and HtwoAnd then convert CO and HtwoO converted to HtwoAnd COtwoThe hydrogen rich gas can be obtained by low-temperature separation or pressure swing adsorption and membrane separation technology.
Coal liquefaction
Both direct liquefaction and indirect oxidation of coal are inseparable from coal gasification.Coal liquefaction requires coal gasification to produce hydrogen, and the optional coal gasification processes also include fixed bed pressurized Lurgi gasification, pressurized fluidized bed gasification and pressurized fluidized bed gasification.
