How is coal used in gasification?
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How is coal used in gasification?


The dirtiest fossil fuel is coal. Its burning results in emissions that contribute to global warming, acid rain, and water pollution. You could be excused for assuming that dirty coal was finally going away with all the buzz surrounding nuclear power, hydropower, and biofuels. However, coal is not the smoky remnant of the industrial revolution; it produces half of the nation’s electricity as long as it is accessible and affordable. By using gasification technology to clean the coal and regulate its emissions, coal gasification aims to lessen the damaging effects of coal on the environment.

How does coal gasification work?
A number of physicochemical changes occur during the complicated thermochemical process known as coal gasification. Coal initially goes through pyrolysis and drying at a specific temperature and pressure, and then complex chemical processes take place with gasification agents, reaction intermediates, reaction products, and numerous components in between. Coal quality (coal rank, coal type, coal’s mineral content, internal surface area, etc.) and reaction circumstances are the key elements influencing the coal gasification reaction (reaction temperature, pressure, gasification furnace type, oxygen-coal ratio, steam-coal ratio, etc.).

The coal gasification process varies with different gasification furnace types. In fixed-bed gasification, the oxygen or air sprayed into the lower part of the gasifier reacts with the residual carbon in the coal in an oxidative combustion reaction, producing a large amount of heat flow gas rising along the gasifier to provide heat for the coal seam through which it flows. distribution. In the process of fluidized bed and air bed gasification, coal particles or pulverized coal are in a highly mixed state of discontinuous phase in the gasifier, the residence time of coal particles is short, the evaporation of water in coal, coal pyrolysis and chemical reaction between coal particles and gasification agent coexist, and the purpose is completed in a very short time, coal in the gasifier occurs oxidation combustion, reduction, conversion, water gas conversion, methanation and other reactions successively or simultaneously.

What is the classification of coal gasification?
Gasifiers are the main piece of equipment used in coal gasification. Fixed bed (moving bed), fluidized bed (boiling bed), air bed, and other types of gasification are all based on how the coal is moved through the gasifier.

-Fixed-bed (moving-bed) gasification

Coal and gasification agent are added from the top and bottom of the gasifier respectively, and the two are in contact with each other counter-currently for reaction. After the gasification reaction, the residue formed by the ash in the coal is finally discharged from the bottom of the furnace, and the syngas is pushed out from the top of the gasifier.

Fixed bed gasification is separated into atmospheric fixed bed gasification and pressurized fixed bed gasification depending on the gasification pressure.

  • Atmospheric pressure fixed bed gasification, which mostly produces low calorific value gas, uses coke or coal as the source material and often feeds it via a screw conveying mechanism. The primary procedures are the Wellman incinerator and atmospheric pressure fixed bed gasifier, among others.
  • Pressurized fixed-bed gasification, which generally feeds coal by locking hopper and uses steam and oxygen as gasification agent to gasify coke or coal to produce medium and high calorific value gas, the main processes are Lucci pressurized gasification technology and Zemak slag gasification technology. Pressurized fixed-bed gasification has larger capacity in a single furnace than atmospheric gasifier, higher gasification temperature, higher intensity, lower residual carbon in slag, and has certain advantages in coal to natural gas due to higher methane content in crude syngas than other gasification technologies. Pressurized fixed bed gasification includes process units such as pressurized gasification, slag removal, gas cooling, coal lock gas compression, gas water separation and phenol ammonia recovery.
  • Fixed-bed gasification has certain requirements for incoming coal particle size (about 6~50 mm) to ensure the permeability of the bed; the bed cloth coal to be uniform, to avoid the emergence of ditch flow, resulting in uneven bed reaction; mechanical strength, adhesion, slagging and thermal stability and other coal parameters directly affect the permeability of the bed, raw material selection is subject to certain restrictions; syngas contains phenols and tar, which need to be separated and recovered: wastewater in The organic components are complex and high in content, and the treatment cost is high.-Fluidized bed (boiling bed) gasification

Gasification agent from the bottom of the gasifier through the distributor evenly into the furnace, fine particles of coal (less than 6 mm) from the upper part of the furnace to join, in the furnace in parallel or countercurrent movement, so that the solids are suspended to form a fluidization, while the gasification reaction occurs, fluidized bed gasification of the raw coal into the furnace activity has certain requirements, low reaction temperature, high residual carbon content in the fly ash, gasifier furnace temperature is not easy to control, the operating flexibility is small. In order to ensure the fluidization of coal in fluidized bed and sufficient reaction time, the fluidization number (ratio of operating flow rate to critical flow rate) should be ensured to be 1.4~2.0 at the best flow rate. Fluidized bed gasification technology can be used for gasification of lignite, long-flame coal and weak viscous coal, etc., while in gasification of poor coal and anthracite coal, it is necessary to increase the gasification temperature and increase the residence time of particles in the gasifier. Typical technologies include high temperature Winkler circulating fluidized bed gasification, ash fusion gasification and conveyor bed gasification.

-Fluid bed gasification

The water-coal slurry or dry pulverized coal and the gasification agent enter the gasifier through the burner and atomize under the entrainment of the gasification agent in order to fully carry out the gasification reaction. The gas flow bed gasifier has a short residence time, so it is necessary to reduce the inlet particle size of coal (less than 0.1 mm), increase the contact area between the gasifier and coal particles, and increase the furnace temperature to enhance the gasification reaction rate, so that the ash is discharged in a molten state. The gas flow bed gasification reaction temperature is high, the gasification intensity is high, the adaptability of coal type is wide, and the conversion rate of carbon can reach more than 99%. Gas flow bed gasifier usually adopts refractory bricks or water-cooled wall structure, and the high temperature syngas coming out of the gasifier is usually cooled by means of excitation cooling or waste pot.

According to the different feeding methods, gas flow bed gasification is divided into coal-water slurry gasification and dry powder gasification.

①Water slurry gasification, belongs to the wet feed, gasification pressure is 2.0~8.7 MPa, gasification temperature is about 1350℃, gasification plant is divided into coal slurry preparation and transportation, gasification and scrubbing, black water flash and gray water system and other units, water slurry gasification technology mainly has the United States Tongli by gas water coal offender gasification technology, multi-nozzle opposed water coal slurry gasification technology, SF flat push flow water coal slurry gasification technology, etc.

Compared with coal-water slurry gasification, dry pulverized coal gasification has a wider range of coal types, higher gasification temperature and carbon conversion rate, and lower oxygen consumption. The main process units of dry pulverized coal gasification include coal grinding and drying, pulverized coal pressurization and transportation, gasification (waste pot or radical cooling), slag removal, ash removal (dry ash removal and wet scrubbing ash removal) and syngas scrubbing, ash water vapor extraction and clarification.

What are the advantages and disadvantages of coal gasification?
Advantages of coal gasification
The introduction of coal utilization process brings many advantages, which include:
1.The gasifier, which warms water, air, and coal under pressure to create syngas, a mixture of hydrogen and carbon monoxide, is the heart of the gasification process. The minerals in the fuel are separated, and the gasifier’s bottom is either entirely reduced to ash or takes the form of a glassy, inert slag that may be recycled into products like concrete and road fill.
2. After exiting the hot gasifier pipeline, the crude syngas is contaminated with ammonia, some dangerous particles, mercury, and hydrogen sulfide. The majority of the particulates are removed from the syngas by using a combination of particulate filters, quench chambers, and heat exchangers to chill it from room temperature.

3.To remove more than 90% of the hazardous metal, the syngas is next run over a small bed of charcoal to trap the mercury. The used charcoal, which still contains some mercury that was captured, will be disposed of in a hazardous landfill.
4. The removal of sulfur impurities in the acid gas removal unit, where the impurities are transformed to sulfuric acid or elemental sulfur, which are both useful by-products, is the last stage of cleaning in gasification.

5.Contaminants are easily eliminated after the coal is transformed into gas. Mercury, sulfur, and particulates can all be extracted and then sold on the open market. The “clean” gas can then be used in various fuels, including oil to power vehicles or heat homes.

Disadvantages of Coal Gasification
Coal gasification can also have many disadvantages. These disadvantages include, among others, the following.
1. There are many drawbacks to coal gasification. There won’t be enough waste coal in the nation to support the beast once gasification starts. As a result, it’s possible that individuals will resume mining coal, which is never a good decision.

2.It is evidently unproven and untested for coal gasification. Gasification will be significantly reliant on subsidies because IGCC plants are more expensive than traditional coal-fired plants.

3.More CO2 is produced by coal gasification, which ultimately contributes to global warming. Even if people utilize IGCC plants, a lot of CO2 will still be produced.

What is the future development direction of coal gasification?
Under the conditions of a “double carbon” and “double control” industrial strategy, the “large-scale and high-pressure” coal gasification technology that is already in use can be improved, the coal types can be expanded, and the gasifier itself can be made better. The development direction of coal gasification technology is to increase the overall efficiency of the gasifier, broaden the adaptability of coal types, improve the single furnace capacity of the gasifier, reduce the environmental impact of gasification technology, and strengthen the technical integration of coal gasification and new coal chemical industry against the backdrop of “double carbon” and “double control” industrial policy.

1) Improve heat recovery and use while maximizing the recovery of high temperature sensible heat from coal gasification. The high temperature heat of gasification can be recovered by carefully choosing and optimizing the excitation and cooling process as well as the waste heat boiler in conjunction with the plant resources to achieve energy savings and consumption reduction.

2) Major coal chemical companies use “in-situ conversion at the pit,” which can dramatically lower costs, boost productivity, and boost an organization’s competitiveness while realizing the best use of resources. Choose the proper gasification technology while also selecting the suitable coal allocation and low-quality coal pretreatment technologies to increase the gasifier’s ability to adapt to different types of coal, depending on the product scenario and coal quality.

3) In terms of large-scale development, air-bed gasification technology is the preferred technology direction to improve the processing capacity of a single furnace by increasing the gasification reaction temperature, gasification pressure and enhanced mixing, which is conducive to improving the economy of the overall coal chemical industry chain. Secondly, in the air bed gasification technology, the mixing process can be strengthened by setting a reasonable number and position of nozzles to form a more reasonable flow field structure in the furnace, which can further improve the processing capacity of the plant and meet the demand of large-scale.

4) Optimize the coal allocation standard of incoming coal to ensure the stability of ash melting temperature and viscosity temperature characteristics, and also realize the long-cycle safe and reliable operation of gasification plant through the breakthrough of key equipment, materials and protection technology of key parts; combine with the energy supply and consumption of the whole plant, optimize the gasification process from upstream and downstream, finely manage the energy flow and material flow, and optimize the efficiency of the whole gasification system.

5) Develop green coal gasification technology for pollutants in the process of coal gasification, such as the wastewater problem in coal gasification is a major problem in the development of gasification technology, develop targeted process for wastewater treatment, and promote new technology in solving the problem of wastewater treatment in coal gasification according to the maturity of technology. Through engineering demonstration, we promote relevant basic research results to reduce the emission of pollutants in the process of coal gasification; for the existing equipment, we adopt new waste treatment and utilization technologies, such as biological method to treat wastewater, waste residue resource utilization and other new technologies to reduce the emission of pollutants from coal gasification plant and finally achieve near zero emission.

To reach the objective of carbon peaking and carbon neutrality, it is crucial to utilize coal resources in an effective and clean manner. Coal gasification technology is a very common type of technology that is combined with the current double carbon target in terms of energy savings and consumption reduction, improving the adaptability of coal type, and improving and optimizing the coal gasification plant in the process of achieving the double carbon target.