Coal gasification is the process of producing syngas–a mixture consisting primarily of carbon monoxide (CO), hydrogen (H2), carbon dioxide (CO2), methane (CH4), and water vapor (H2O)–fromcoal and water, air and/or oxygen.
Historically, coal was gasified using early technology to produce coal gas (also known as “town gas”), which is a combustible gas traditionally used for municipal lighting and heating before the advent of industrial-scale production of natural gas.
In current practice, large-scale instances of coal gasification are primarily for electricity generation, such as in integrated gasification combined cycle power plants, for production of chemical feedstocks, or for production of synthetic natural gas. The hydrogen obtained from coal gasification can be used for various purposes such as making ammonia, powering a hydrogen economy, or upgrading fossil fuels.
Alternatively, coal-derived syngas can be converted into transportation fuels such as gasoline and diesel through additional treatment via the Fischer-Tropsch process or into methanol which itself can be used as transportation fuel or fuel additive, or which can be converted into gasoline by the methanol to gasoline process. Methane from coal gasification can be converted intoLNG for use as a fuel in the transport sector.
Scheme of a Lurgi gasifier
During gasification, the coal is blown through with oxygen and steam (water vapor) while also being heated (and in some cases pressurized). If the coal is heated by external heat sources the process is called “allothermal”, while “autothermal” process assumes heating of the coal via exothermal chemical reactions occurring inside the gasifier itself. It is essential that the oxidizer supplied is insufficient for complete oxidizing (combustion) of the fuel. During the reactions mentioned, oxygen and water molecules oxidize the coal and produce a gaseous mixture of carbon dioxide (CO2), carbon monoxide (CO),water vapour (H2O), and molecular hydrogen (H2). (Some by-products like tar, phenols, etc. are also possible end products, depending on the specific gasification technology utilized.) This process has been conducted in-situ within natural coal seams (referred to as underground coal gasification) and in coal refineries. The desired end product is usually syngas (i.e., a combination of H2 + CO), but the produced coal gas may also be further refined to produce additional quantities of H2:
- 3C (i.e., coal) + O2 + H2O → H2 + 3CO
If the refiner wants to produce alkanes (i.e., hydrocarbons present in natural gas, gasoline, and diesel fuel), the coal gas is collected at this state and routed to a Fischer-Tropsch reactor. If, however, hydrogen is the desired end-product, the coal gas (primarily the CO product) undergoes the water gas shift reaction where more hydrogen is produced by additional reaction with water vapor:
- CO + H2O → CO2 + H2
Although other technologies for coal gasification currently exist, all employ, in general, the same chemical processes. For low-grade coals (i.e., “brown coals”) which contain significant amounts of water, there are technologies in which no steam is required during the reaction, with coal (carbon) and oxygen being the only reactants. As well, some coal gasification technologies do not require high pressures. Some utilize pulverized coal as fuel while others work with relatively large fractions of coal. Gasification technologies also vary in the way the blowing is supplied.
“Direct blowing” assumes the coal and the oxidizer being supplied towards each other from the opposite sides of the reactor channel. In this case the oxidizer passes through coke and (more likely) ashes to the reaction zone where it interacts with coal. The hot gas produced then passes fresh fuel and heats it while absorbing some products of thermal destruction of the fuel, such as tars and phenols. Thus, the gas requires significant refining before being used in the Fischer-Tropsch reaction. Products of the refinement are highly toxic and require special facilities for their utilization. As a result, the plant utilizing the described technologies has to be very large to be economically efficient. One of such plants called SASOL is situated in the Republic of South Africa (RSA). It was built due to embargo applied to the country preventing it from importing oil and natural gas. RSA is rich in Bituminous coal and Anthracite and was able to arrange the use of the well known high pressure “Lurgi” gasification process developed in Germany in the first half of 20-th century.
“Reversed blowing” (as compared to the previous type described which was invented first) assumes the coal and the oxidizer being supplied from the same side of the reactor. In this case there is no chemical interaction between coal and oxidizer before the reaction zone. The gas produced in the reaction zone passes solid products of gasification (coke and ashes), and CO2 and H2O contained in the gas are additionally chemically restored to CO and H2. As compared to the “direct blowing” technology, no toxic by-products are present in the gas: those are disabled in the reaction zone. This type of gasification has been developed in the first half of 20-th century, along with the “direct blowing”, but the rate of gas production in it is significantly lower than that in “direct blowing” and there were no further efforts of developing the “reversed blowing” processes until 1980-s when a Soviet research facility KATEKNIIUgol’ (R&D Institute for developing Kansk-Achinsk coal field) began R&D activities to produce the technology now known as “TERMOKOKS-S” process. The reason for reviving the interest to this type of gasification process is that it is ecologically clean and able to produce two types of useful products (simultaneously or separately): gas (either combustible or syngas) and middle-temperature coke. The former may be used as a fuel for gas boilers and diesel-generators or as syngas for producing gasoline, etc., the latter – as a technological fuel in metallurgy, as a chemical absorbent or as raw material for household fuel briquettes. Combustion of the product gas in gas boilers is ecologically cleaner than combustion of initial coal. Thus, a plant utilizing gasification technology with the “reversed blowing” is able to produce two valuable products of which one has relatively zero production cost since the latter is covered by competitive market price of the other. As the Soviet Union and its KATEKNIIUgol’ ceased to exist, the technology was adopted by the individual scientists who originally developed it and is now being further researched in Russia and commercially distributed worldwide. Industrial plants utilizing it are now known to function in Ulaan-Baatar (Mongolia) and Krasnoyarsk (Russia).
Pressurized airflow bed gasification technology created through the joint development between Wison Group and Shell (Hybrid). For example: Hybrid is an advanced pulverized coal gasification technology, this technology combined with the existing advantages of Shell SCGP waste heat boiler, includes more than just a conveying system, pulverized coal pressurized gasification burner arrangement, lateral jet burner membrane type water wall, and the intermittent discharge has been fully validated in the existing SCGP plant such as mature and reliable technology, at the same time, it removed the existing process complications and in the syngas cooler (waste pan) and [fly ash] filters which easily failed, and combined the current existing gasification technology that is widely used in synthetic gas quench process. It not only retains the original Shell SCGP waste heat boiler of coal characteristics of strong adaptability, and ability to scale up easily, but also absorb the advantages of the existing quench technology.
Underground coal gasification
Underground coal gasification is an industrial gasification process, which is carried out in non-mined coal seams using injection of a gaseous oxidizing agent, usually oxygen or air, and bringing the resulting product gas to surface through production wells drilled from the surface. The product gas could to be used as a chemical feedstock or as fuel for power generation. The technique can be applied to resources that are otherwise not economical to extract and also offers an alternative to conventional coal mining methods for some resources. Compared to traditional coal mining and gasification, UCG has less environmental and social impact, though some concerns including potential for aquifer contamination are known.
Carbon capture technology
Carbon capture, utilization, and sequestration (or storage) is increasingly being utilized in modern coal gasification projects to address the greenhouse gas emissions concern associated with the use of coal and carbonaceous fuels. In this respect, gasification has a significant advantage over conventional coal combustion, in which CO2 resulting from combustion is considerably diluted by nitrogen and residual oxygen in the near-ambient pressure combustion exhaust, making it relatively difficult, energy-intensive, and expensive to capture the CO2 (this is known as “post-combustion” CO2 capture).
In gasification, on the other hand, oxygen is normally supplied to the gasifiers and just enough fuel is combusted to provide the heat to gasify the rest; moreover, gasification is often performed at elevated pressure. The resulting syngas is typically at higher pressure and not diluted by nitrogen, allowing for much easier, efficient, and less costly removal of CO2. Gasification and integrated gasification combined cycle’s unique ability to easily remove CO2 from the syngas prior to its combustion in a gas turbine (called “pre-combustion” CO2 capture) or its use in fuels or chemicals synthesis is one of its significant advantages over conventional coal utilization systems.
CO2 capture technology options
All coal gasification-based conversion processes require removal of hydrogen sulfide (H2
S; an acid gas) from the syngas as part of the overall plant configuration. Typical acid gas removal (AGR) processes employed for gasification design are either a chemical solvent system (e.g., amine gas treating
systems based on MDEA, for example) or a physical solvent system (e.g.,Rectisol
). Process selection is mostly dependent on the syngas cleanup requirement and costs. Conventional chemical/physical AGR processes using MDEA, Rectisol or Selexol are commercially proven technologies and can be designed for selective removal of CO2
in addition to H2
S from a syngas stream. For significant capture of CO2
from a gasification plant (e.g., > 80%) the CO in the syngas must first be converted to CO2
and hydrogen (H2
) via a water-gas-shift
(WGS) step upstream of the AGR plant.
For gasification applications, or IGCC, the plant modifications required to add the ability to capture CO2 are minimal. The syngas produced by the gasifiers needs to be treated through various processes for the removal of impurities already in the gas stream, so all that is required to remove CO2 is to add the necessary equipment, an absorber and regenerator, to this process train. In combustion applications, modifications must be done to the exhaust stack and because of the lower concentrations of CO2 present in the exhaust, much larger volumes of total gas require processing, necessitating larger and more expensive equipment.
IGCC-based projects in the United States with CO2 capture and use/storage
Mississippi Power’s Kemper Project is in late stages of construction. It will be a lignite-fuel IGCC plant, generating a net 524 MW of power from syngas, while capturing over 65% of CO2generated using the Selexol process. The technology at the Kemper facility, Transport-Integrated Gasification (TRIG), was developed and is licensed by KBR. The CO2 will be sent by pipeline to depleted oil fields in Mississippi for enhanced oil recovery operations.
Hydrogen Energy California (HECA) will be a 300MW net, coal and petroleum coke-fueled IGCC polygeneration plant (producing hydrogen for both power generation and fertilizer manufacture). Ninety percent of the CO2 produced will be captured (using Rectisol) and transported to Elk Hills Oil Field for EOR, enabling recovery of 5 million additional barrels of domestic oil per year.
Summit’s Texas Clean Energy Project (TCEP) will be a coal-fueled, IGCC-based 400MW power/polygeneration project (also producing urea fertilizer), which will capture 90% of its CO2 in pre-combustion capture using the Rectisol process. The CO2 not used in fertilizer manufacture will be used for enhanced oil recovery in the West Texas Permian Basin.
Plants such as the Texas Clean Energy Project which employ carbon capture and storage have been touted as a partial, or interim, solution to climate change issues if they can be made economically viable by improved design and mass production. There was opposition by utility regulators and ratepayers due to increased cost and by some environmentalists such as Bill McKibbenwho view any continued use of fossil fuels as counterproductive.