Coal-chemicals are the process by which coal is used as a feedstock for chemical transformation into gas, liquid fuels, chemicals and new materials. The rationale is to modify the molecular structure of coal by using chemical reactions (mainly thermochemical transformation) to extract or synthesize valuable components. The following are the basic principles and main process processes of the coal industry:
I. Rationale
Hydrocarbon restructuring: coal consists mainly of carbon (c), hydrogen (h), oxygen (o) and a small amount of sulphur (s), nitrogen (n), with a complex molecular structure and a large molecular mass. At the core of the coal industry is the destruction of the large molecular structure of coal through processes such as cracking, gasification, liquefied, etc., and its transformation into intermediates with smaller molecular volumes, easier use or further processing (e. G. Synthetic gas, tar, liquid fuel, essential chemicals)。
2. Depuration of impurities: coal contains ash fractions (inorganic minerals) and harmful elements such as sulphur, nitrogen, etc. The coal-chemical process must include efficient purification, desulphurization and denitration steps to protect downstream equipment, meet environmental requirements and obtain pure products。
Catalytic transformation: many key steps (e. G., feto synthesis, methanol synthesis, methanol-based alkyl, etc.) require specific catalysts to accelerate response, increase target product selectivity and reduce response conditions。
Energy balance: coalchemicals are energy-intensive industries, and many reactions (especially gasification) require high temperature and pressure. Process design needs to take into account the efficient use of energy (e. G. Heat recovery, cogeneration)。
Ii. Main processes (technical routes)
The modern coal chemical industry consists mainly of the following main technical routes:
1. Coal distillation (focalization):
Rationale: under conditions of isolation from air (or inert atmosphere), coal is heated to high temperature (usually 900 - 1100°c) and is thermally decomposition。
Main products: mellurgical coke (for iron refining in furnaces), coal tar, corrosive benzene, coke gas。
Core processes:
Coal preparation: reception, storage, co-operation, shredding of raw materials。
Coal heated dry distillation in the coke chamber。
Cutting out: cooling hot coke (wet or dry)。
Scavenger: catalytic coke。
Chemical recovery: refrigeration, purification, recovery of tar, ammonia, benzene, hydrogen sulfide, etc., from waste gas (gas that escapes from the oven) and purification of the stove gas。
Gas purification and refining: caustic gas desulphurization, desamination, debenzene, etc., are provided with pure gas or chemical raw gas; coal tar and coarse benzene are processed in depth (distillation, hydrogen, etc.) to produce a variety of chemical products (benzene, tobenzo, diphenyl, phenol, phenol, asphalt, etc.)。
Gasification: (centres and foundations of modern coal chemicals)
Principle: under high temperature, pressure (or constant pressure), coal and gasification agents (oxygen/air, water vapour) are partially oxidized to convert solid coal into a mixture of co and h2 (synthetic gas)。
Main product: synthetic gas (co + h2)。
Core processes:
Coal preparation: coal fragmentation, sifting, drying. For gas-basket furnaces, coal usually needs to be grinded into fine powder (dry powder or made of water sour)。
Gasification response: in gasification furnaces (types such as fixed beds, fluidized beds, gaseous beds, etc.), coal and gasification agents react under high temperature (1200-1600°c) pressure (2. 0-8. 5 mpa)。
Synthetic gas purification: complex purification following irritation or waste heat recovery (generation of vapour):
Dust removal: remove solid particles (fly ash, slag)。
Water washing/change: adjusting the h2/co ratio (reactive through co + h2o → co2 + h2)。
De-sulphurized carbon: removal of acid gases such as h2s, cos, co2 (common method: cryogenic methanol washing recisol, selexol, mdea, etc.)。
Precision desulphurization: deep devulcanization up to the ppb level to protect downstream catalysts。
Sulphur recovery: sulfur recovery from devulcanization units (e. G. The klaus process)。
Coal liquefied:
Principle: the large molecular structure of coal is either directly or indirectly disassembled, hydrogenized and converted into liquid fuels (like oil) and chemicals through hydrogenization, with high temperature pressure and catalysts。
Main products: liquid fuels (oil, diesel, coal, etc.), chemical raw materials。
Core processes (in two main categories):
Direct liquefied:
Plasma preparation: coal powder mixed with recycling solvents (or initial solvents) into magma。
Hydrogen liquefied: coal plasma reacts with hydrogen gas at high temperatures (400-470°c), pressure (15-30 mpa) and catalysts to crack and saturate coal molecules。
Solid separation: separation of reaction products (liquidated oils, gases, unreactive coal and minerals) (frequently decompressive distillation, solvent extraction, critical solvent ash removal, etc.)。
Gravity processing: hydrogen refining, hydrogen fragmentation, restructuring, etc., of liquefied crude oil obtained from separation, production of eligible oils。
B. Indirect liquefied: (based on gasification)
Synthetic gas production: first gasification (see gasification process above) produces pure synthetic gas。
Fito synthesis: the synthetic gas (co + h2) reacts as a polymer at lower temperatures (200-350°c) and pressure (2-3 mpa) with catalysts (iron, cobalt-based) and growth chain hydrocarbons (axes, oils) and water。
Production modification: refinement of crude oils (axes) resulting from the synthesis of feto, such as hydrogen fissure, isomerization, re-engineering, etc., to produce liquid fuels (diesel, prune oils, etc.) and chemicals (olefins, paraffins)。
4. Coal-based chemical synthesis: (mainly based on gasification)
Rationale: basic chemicals are produced by catalytic synthetic reaction using purified synthetic gas as a feedstock。
Main products and core processes:
Synthetic ammonia: co in synthetic gas is fully converted to co2 and h2 by transformation, and h2 is highly pure after removal of co2. H2 and n2 in air synthesized ammonia (nh3) with high temperature high pressure catalysts. Ammonia is the basis for fertilizers (urea, etc.) and a large number of chemicals。
Methanol synthesis: synthetic gas (adjusted h2/co ratio to ~2. 2) under catalyst (copper) at moderate temperature (200-300°c), medium pressure (5-10 mpa) synthetic methanol (ch3oh). Methanol is an important base chemical material and fuel。
Lower-water methanol products (coal-based olefins/arocarbons, etc.):
Methanol-based hydrocarbons: methanol is dehydrated on specific molecular sift catalysts (e. G. Sapo-34), producing low-carbon ethylene (c2h4) and acrylate (c3h6) alkyl (mto/mtp process)。
Methanol aromatic hydrocarbons: methanol is converted into aromatic hydrocarbons (mta processes) such as benzene, tobenzo, and diphenyl (btx) on specific catalysts。
Methanol-based acetic acid/ethylene acetylene etc.: other important chemicals are produced through processes such as thallium synthesis。
Ethylene: synthetic gas (co) produces ethanol (meg) through the ester or direct synthesis method。
Gas substitution (coal-based gas): synthetic gas (adjusted h2/co ratio to ~3) produced as a catalyst for methaneization (sng, main component ch4)。
Iii. Key features and technical challenges
Resource dependence is strong: heavy dependence on coal resources, whose economic nature is affected by coal prices。
Technical complexity and high investment: process length, high equipment requirements (high temperature, high pressure, resistant to corrosion), especially gasification and large composite devices。
High energy, high water consumption: requiring large amounts of energy (electricity, steam) and water resources (especially gasification and cooling processes)。
Environmental challenges stand out:
High carbon intensity: the release of large quantities of co2 during production (mainly from gasification, transformation, combustion, etc.) is a priority area for achieving the “two carbon” target。
Three waste treatment pressures are high: generation of large amounts of wastewater (complex and difficult to treat), sludge (gasification ash, biochemical sludge, etc.) and exhaust gas (sulphur, nitrogen-containing pollutants)。
There is a high demand for system integration: there is a need for co-production of coal, electricity, petrochemicals, heat, etc., to improve energy efficiency and resource utilization。
Summary
Coalification is based on the conversion of coal into gaseous fuels, liquid fuels, chemicals and materials through thermal chemical transformation (drying, gasification, liquefied) and catalytic synthesis. Gasification is the cornerstone of modern coal-chemicals, providing critical synthetic gas raw materials for many routes such as synthetic ammonia, methanol, feto-synthetic oil, coal-based natural gas, coal-based alkyl and aromatic hydrocarbons. While coal-chemicals are strategic in ensuring energy security and diversification of chemical materials, the challenges they face, such as high carbon emissions, high energy consumption, environmental pressures and huge capital investments, have also led the industry to move in the direction of ** bigification, zoranization, baseing, multiple production, cleaner and efficient** and to actively explore carbon capture and storage (ccus) technologies to reduce carbon emissions。
