Coal Gasification
Coal gasification is a thermochemical process that converts coal into a gaseous mixture known as synthesis gas (or syngas), primarily composed of carbon monoxide and hydrogen. This technology allows syngas to be utilized as a fuel for power generation or as a feedstock for producing synthetic fuels and fertilizers. The process operates under high temperature and pressure by reacting coal with steam and oxygen, facilitating the breakdown of coal into its fundamental chemical components. Coal gasification has evolved since its initial development around 1780, transitioning from its early use for municipal lighting to modern applications that aim to mitigate environmental impacts typically associated with coal combustion.
While conventional coal-fired plants emit substantial carbon dioxide, coal gasification produces cleaner syngas with lower emissions, making it a potential step toward a more sustainable energy future. Various gasification technologies exist, including entrained flow, moving bed, and fluid bed gasifiers, each with distinct operational efficiencies and suitability for different coal types. An integrated approach called Integrated Gasification Combined Cycle (IGCC) maximizes electricity generation while minimizing environmental impact, using both gas and steam turbines. Ongoing research focuses on enhancing coal gasification efficiency, exploring alternative feedstocks, and developing cost-effective technologies to compete with natural gas.
Coal Gasification
Summary
Coal gasification is the chemical and physical process of converting coal into coal gas, a synthesis gas (syngas) composed of varying amounts of carbon monoxide and hydrogen gas. The syngas is subsequently used as fuel for power generation or as feedstock in chemical processes such as producing synthetic fuels and fertilizers.
Definition and Basic Principles
Coal gasification is a method of converting coal into a combustible gas that can be used for heating, power generation, and manufacture of hydrogen, synthetic natural gas, or diesel fuel. Coal gasification plants are in operation throughout the world. Unlike conventional coal-fired power plants, gasification involves a thermochemical process that breaks down coal into its basic chemical constituents. This is accomplished in modern-day gasifiers by reacting coal with a mixture of steam and air or oxygen under high temperature and pressure. The end product is a gaseous mixture of carbon monoxide, hydrogen, and other gas compounds. Some experts argue that coal gasification is an important step toward a clean-energy future. Whereas burning coal contributes to global warming by increasing the concentration of carbon dioxide in the atmosphere, coal gasification produces lean gas because pollutants or impurities such as sulfur and mercury are removed in the system. Coal-derived syngas produce smaller amounts of carbon dioxide gas than coal itself does.
Background and History
Coal gasification was first developed around 1780, but the technology and its applications have evolved significantly. When coal gasification was implemented, the carbon monoxide produced was used as an energy source for municipal lighting and heating because industrial-scale natural gas production was not yet available. This coal gas was called blue, producer, water, town, or fuel gas. Natural gas became widely available in the 1940s, and by the 1950s, coal gas was nearly replaced with natural gas because it burned cleaner, had a greater heating value, and was safer. When the price of alternative fuels like oil and natural gas was low, interest in coal gasification fell further. However, renewed interest in coal gasification solutions arose in the early twenty-first century because of the high cost of oil and gas, energy security, and environmental considerations.
How It Works
Coal Selection and Preparation. The first step in successful coal gasification is carefully selecting, analyzing, and preparing coal. Coal is analyzed for its percentage of sulfur, fixed carbon, oxygen, ash, and other volatile content. Generally, using a coal feedstock with low sulfur, low moisture, high fixed-carbon content, and low ash content will result in low oxygen consumption, a high volume of syngas, and a small volume of waste-product generation. Some adjustments to gasification systems can be made to accommodate different coal qualities.
Coal Gasifier. Central to coal gasification is the gasifier. A gasifier converts hydrocarbon feedstock into gaseous components by providing heat under pressure. Processes used for gasification include steam-oxygen gasification (which is most common), water-gas shift reaction, pyrolysis, methanation, hydrogasification, and catalytic steam gasification. A gasifier, unlike a combustor, relies on careful regulation of the quantity of air or oxygen permitted to enter the reaction so that only a small portion of the fuel, the coal, burns completely. At a high temperature (about 900 degrees Celsius) and high pressure, the oxygen and water in the gasifier partially oxidize the coal. During this stage, called the steam-forming reaction, rather than burning, the coal feedstock is converted into a syngas mixture of carbon dioxide, carbon monoxide, molecular hydrogen, and water in the form of vapor. To get the syngas out of the gasifier, the syngas cools to room temperature using exhausts and filters that remove solid particles.
Types of Gasifiers. There are three main types of gasification technologiesentrained flow, moving bed, and fluid bed. Of the three, the entrained-flow gasifier possesses the greatest efficiency. Because it operates at a high temperature, nearly 99 percent of the coal is converted into high-purity syngas, as most tar and oil in the coal are destroyed with high heat. The one drawback is that the entrained-flow gasifier has a high oxygen demand. This is easily exacerbated by using a coal feedstock with high ash content. The moving-bed gasifier has the lowest oxygen demand. In this system, coal moves slowly in a downward fashion and is gasified by a counter-current blast. The low operational temperature tends to inhibit the reaction rate, resulting in lower syngas purity and volume generation. The fluid-bed gasifier, named so because of the fluidlike manner in which coal particles behave in the gasifier, facilitates good interaction between the coal feedstock and oxygen without requiring a membrane, leading to lower overall cost to implement and operate. The fluid-bed gasifier is the best option for low-rank (low-quality) coals. Overall, it possesses the lowest carbon conversion rate, resulting in low-purity syngas.
Integrated Gasification Combined Cycle. The technology that combines coal gasification and the subsequent burning of the gas is called integrated gasification combined cycle (IGCC). IGCC combines a coal gasifier with a gas turbine and a steam turbine to produce electric power. The hydrogen-rich syngas from the gasifier is purified to remove acidic compounds and particulates. It then enters the first turbine (the gas turbine) to generate electricity, and the waste heat from the gas turbine works to power the second turbine (the steam turbine), which produces additional electricity. Because steps are taken to remove the majority of acidic compounds and particulates, the resulting combustion-exhaust gas, or flue gas, that leaves the gas turbines has minimal effect on the environment. The combined use of a gas turbine and steam turbine, which both work to produce electricity, makes IGCC the preferred technology in this carbon-constrained world. It is more energy-efficient and generates less carbon dioxide per ton of coal used compared with conventional coal-fired power plants.
Applications and Products
Underground Coal Gasification. Coal gasification can be applied to coal in situ— in underground coal seams. Gasification of coal contained underground is called underground coal gasification (UCG). It is particularly well suited for technologically or economically unmineable coal deposits. The traditional method of extracting coal is through the excavation of open pits to expose coal-containing seams or through the excavation of underground mines. The design, construction, equipment, and labor to build such mines are costly. UCG-candidate coal seams possess one or more features that render it unsuitable, from a technical and economic standpoint, for conventional coal miningfaulting, volcanic intrusions, complex depositional and tectonic features, and environmental constraints.
The UCG process uses injection and production wells drilled from the surface to access the underground coal, and the coal is not mined to the surface. A horizontal connection underground between the injector and extractor is made normally by hydraulic fracturing, or fracking, which is a process that uses high-pressure water to break up the rock or coal. Through the injection well, oxidants (such as water and air or a water and oxygen mixture) are sent down into the coal seam. As in the case of conventional coal gasification, the coal is heated to temperatures that would normally cause the coal to burn, but through careful control of the oxidant flow, the coal is separated into syngas. The product syngas is drawn out through the second well and then can be used to create methanol, dimethyl ether, synthetic natural gas, or Fischer-Tropsch diesel.
Gasification By-Products. Some by-products of coal gasification have commercial value, so they are isolated and set aside to be sold or used on-site for industrial use. Mineral components in the coal that are unable to gasify leave the gasifier or fall to the bottom of the gasifier as an inert, glasslike material, known as slag, which can be used in cement or road construction.
Mercury that is isolated by passing the syngas through a bed of charcoal has no commercial value, but the final cleaning step that follows in the acid gas removal units handles sulfur impurities, which are converted into valuable by-products. Sulfur impurities are converted to hydrogen sulfide (H2S) and carbonyl sulfide (COS), which are isolated in the form of elemental sulfur or sulfuric acid, which can be used for industrial processes such as incorporation into fertilizer.
Nitrogen, in the form of ammonia, is also extracted from the product gas stream for industrial use. Other usable by-products include tar and phenols.
Carbon dioxide emissions from UCG can be sequestered and stored in below-ground reservoirs via carbon dioxide-enhanced oil recovery techniques.
Careers and Course Work
Courses in advanced engineering such as process engineering, chemical engineering, petroleum engineering, civil engineering, mechanical engineering, electrical engineering, mathematics, and physics are foundational requirements for students interested in pursuing careers as gasification or process engineers. Software programs specific to engineering, such as CAD and AutoCAD, are also essential mastery areas. Earning a bachelor’s of science or applied science will prepare a student for graduate studies in a similar field. A professional engineer (PE) license, usually earned through the fulfillment of work in a field of engineering for a prescribed number of years and successful completion of an engineering ethics examination and a comprehensive engineering examination, will facilitate career advancement. A master's degree or doctorate equips students to pursue advanced career opportunities in industry. To obtain an upper management or executive role in industry, one would likely be required to hold a management or finance degree, such as a master of business administration (MBA).
An advanced degree is typically not required for a technician or administrator position. A coal gasification engineer or manager in the field would work closely with various other engineering specialties to build, monitor, and maintain coal gasification plants. They may also work with vendors and customers to procure equipment, feedstock, or other industrial materials and supplies.
Social Context and Future Prospects
Fuel flexibility is important in an increasingly carbon-constrained world. Future technology could accommodate the economical use of various feedstocks, such as municipal waste, biomass, and recycled materials. Coal gasifiers could be used to retrofit coal-burning power plants or in manufacturing plants to provide syngas as a raw material for fertilizer production. Existing coal gasification technologies perform best on high-rank, costly coal or petroleum refinery products. They are inefficient and expensive to operate when using poorer-quality coal. Researchers continually investigate the co-gasification of low-quality coal with biomass under high pressure and various techniques to produce cleaner, more efficient fuel.
Technological advancements to lower the cost of constructing, operating, and maintaining coal gasification plants will be necessary to compete with natural gas-fired power plants. One cost-cutting method is reducing the cost of oxygen used in the gasification process. Oxygen is obtained through an expensive cryogenic process. Research involving ceramic membranes has demonstrated promising results for separating oxygen from air at higher temperatures. Innovations in oxygen separation—via ion transport membranes, magnetic gradients, redox swing, sorbents, or biomimicry—may enhance the utility of coal gasifiers. Developing inexpensive membranes that can readily separate hydrogen from syngas would also be useful. The economic sequestration of hydrogen could help drive the use and advancement of hydrogen fuel cells and hydrogen-powered vehicles. Changes to turbine design, advances in heat recovery, and modularization could further drive adoption of IGCC power plants.
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