Syngas: Coal Gasification

    Coal gasification is the process of producing syngas–a mixture consisting primarily of carbon monoxide (CO), hydrogen (H2), carbon dioxide (CO2) and water vapor (H2O)–from coal 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.

    In the past, coal was converted to make coal gas, which was piped to customers to burn for illumination, heating, and cooking. High prices of oil and natural gas are leading to increased interest in “BTU Conversion” technologies such as gasification, methanation and liquefaction. The Synthetic Fuels Corporation was a U.S. government-funded corporation established in 1980 to create a market for alternatives to imported fossil fuels (such as coal gasification). The corporation was discontinued in 1985.
    The Flemish scientist Jan Baptista van Helmont (1577–1644) discovered that a “wild spirit” escaped from heated wood and coal, and, thinking that it “differed little from the chaos of the ancients”, he named it “gas” in his Origins of Medicine (c. 1609). Among several others who carried out similar experiments, were Johann Becker of Munich (c 1681) and about three years later John Clayton of Wigan, England, the latter amusing his friends by lighting, what he called, “Spirit of the Coal”. William Murdoch (later known as Murdock) (1754–1839) (partner of James Watt) is reputed to have heated coal in his mother’s teapot to produce gas. From this beginning, he discovered new ways of making, purifying and storing gas; illuminating his house at Redruth (or his cottage at Soho) in 1792, the entrance to the Manchester Police Commissioners premises in 1797, the exterior of the factory of Boulton and Watt in Birmingham, England, and a large cotton mill in Salford, Lancashire in 1805.
    Professor Jan Pieter Minckeleers lit his lecture room at the University of Louvain in 1783 and Lord Dundonald lit his house at Culross, Scotland, in 1787, the gas being carried in sealed vessels from the local tar works. In France, Philippe le Bon patented a gas fire in 1799 and demonstrated street lighting in 1801. Other demonstrations followed in France and in the United States, but, it is generally recognized that the first commercial gas works was built by the London and Westminster Gas Light and Coke Company in Great Peter Street in 1812 laying wooden pipes to illuminate Westminster Bridge with gas lights on New Year’s Eve in 1813. In 1816, Rembrandt Peale and four others established the Gas Light Company of Baltimore, the first manufactured gas company in America. In 1821, natural gas was being used commercially in Fredonia, New York. The first German gas works was built in Hannover in 1825 and by 1870 there were 340 gas works in Germany making town gas from coal, wood, peat and other materials.
    Working conditions in the Gas Light and Coke Company’s Horseferry Road Works, London, in the 1830s were described by a French visitor, Flora Tristan, in her Promenades Dans Londres:
    Two rows of furnaces on each side were fired up; the effect was not unlike the description of Vulcan’s forge, except that the Cyclops were animated with a divine spark, whereas the dusky servants of the English furnaces were joyless, silent and benumbed. … The foreman told me that stokers were selected from among the strongest, but that nevertheless they all became consumptive after seven or eight years of toil and died of pulmonary consumption. That explained the sadness and apathy in the faces and every movement of the hapless men.
    The first public piped gas supply was to 13 gas lamps, each with three glass globes along the length of Pall Mall, London in 1807. The credit for this goes to the inventor and entrepreneur Fredrick Winsor and the plumber Thomas Sugg who made and laid the pipes. Digging up streets to lay pipes required legislation and this delayed the development of street lighting and gas for domestic use. Meanwhile William Murdoch and his pupil Samuel Clegg were installing gas lighting in factories and work places, encountering no such impediments.
    Early history of coal gas production by gasification
    In the 1850s every small to medium sized town and city had a gas plant to provide for street lighting. Subscribing customers could also have piped lines to their houses. By this era, gas lighting became accepted. Gaslight trickled down to the middle class and later came gas cookers and stoves.
    The 1860s were the golden age of coal gas development. Scientists like Kekulé and Perkin cracked the secrets of organic chemistry to reveal how gas is made and its composition. From this came better gas plants and Perkin’s purple dyes, such as Mauveine. In the 1850s, processes for making Producer gas and Water gas from coke were developed.
    Unenriched water gas may be described as Blue water gas (BWG).
    Mond gas, developed in the 1850s by Ludwig Mond, was producer gas made from coal instead of coke. It contained ammonia and coal tar and was processed to recover these valuable compounds.
    Blue water gas (BWG) burns with a non-luminous flame which makes it unsuitable for lighting purposes. Carburetted Water Gas (CWG), developed in the 1860s, is BWG enriched with gases obtained by spraying oil into a hot retort. It has a higher calorific value and burns with a luminous flame.
    The carburetted water gas process was improved by Thaddeus S. C. Lowe in 1875. The gas oil was fixed into the BWG via thermocracking in the carburettor and superheater of the CWG generating set. CWG was the dominant technology in the USA from the 1880s until the 1950s, replacing coal gasification. CWG has a CV of 20 MJ/m³ i.e. slightly more than half that of natural gas.
    Development of the coal gas industry in the UK
    The advent of incandescent gas lighting in factories, homes and in the streets, replacing oil lamps and candles with steady clear light, almost matching daylight in its colour, turned night into day for many—making night shift work possible in industries where light was all important—in spinning, weaving and making up garments etc. The social significance of this change is difficult for generations brought up with lighting after dark available at the touch of a switch to appreciate. Not only was industrial production accelerated, but streets were made safe, social intercourse facilitated and reading and writing made more widespread. Gas works were built in almost every town, main streets were brightly illuminated and gas was piped in the streets to the majority of urban households. The invention of the gas meter and the pre-payment meter in the late 1880s played an important role in selling town gas to domestic and commercial customers.
    The education and training of the large workforce, the attempts to standerdise manufacturing and commercial practices and the moderating of commercial rivalry between supply companies prompted the founding of associations of gas managers, first in Scotland in 1861. A British Association of Gas Managers was formed in 1863 in Manchester and this, after a turbulent history, became the foundation of the Institute of Gas Engineers (IGE). In 1903, the reconstructed Institution of Civil Engineers (ICE) initiated courses for students of gas manufacture in the City and Guilds of London Institute. The IGE was granted the Royal Charter in 1929. Universities were slow to respond to the needs of the industry and it was not until 1908 that the first Professorship of Coal Gas and Fuel Industries was founded at the University of Leeds. In 1926, the Gas Light and Coke Company opened Watson House adjacent to Nine Elms Gas Works. At first, this was a scientific laboratory. Later it included a center for training apprentices but its major contribution to the industry was its gas appliance testing facilities, which were made available to the whole industry, including gas appliance manufacturers. Using this facility, the industry established not only safety but also performance standards for both the manufacture of gas appliances and their servicing in customers’ homes and commercial premises.
    During World War I, the gas industry’s by-products, phenol, toluene and ammonia and sulphurous compounds were valuable ingredients for explosives. Much coal for the gas works was shipped by sea and was vulnerable to enemy attack. The inter-war years were marked by the development of the continuous vertical retort which replaced many of the batch fed horizontal retorts. There were improvements in storage, especially the waterless gas holder, and distribution with the advent of 2–4 inch steel pipes to convey gas at up to 50 psi (340 kPa) as feeder mains to the traditional cast iron pipes working at an average of 2–3 inches water gauge (500–750 Pa). Benzole as a vehicle fuel and coal tar as the main feedstock for the emerging organic chemical industry provided the gas industry with substantial revenues. Petroleum supplanted coal tar as the primary feedstock of the organic chemical industry after World War II and the loss of this market contributed to the economic problems of the gas industry after the war.
    A wide variety of appliances and uses for gas developed over the years. Gas fires, gas cookers, refrigerators, washing machines, hand irons, pokers for fire lighting, gas-heated baths, remotely controlled clusters of gas lights, gas engines of various types and, in later years, gas warm air and hot water central heating and air conditioning, all of which made immense contributions to the improvement of the quality of life in cities and towns world wide. The evolution of electric lighting made available from public supply extinguished the gas light, except where colour matching was practised as in haberdashery shops.
    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 (H2S; 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 or Selexol). 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 H2S 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.
    Carbon capture for use in enhanced oil recovery
    By far the most extensive use of CO2 in the United States is for enhanced oil recovery. The Global CCS Institute has reported on recent market demand for CO2, noting that the global demand was estimated at 80Mtpa, of which 50Mtpa is utilized for Enhanced Oil Recovery, almost exclusively in North America. Carbon dioxide enhanced oil recovery (CO2 EOR) is a technique used to recover oil, typically from mature fields that have ceased being productive through traditional primary and secondary recovery methods. CO2 EOR is an established technique in the United States, and is the only oil recovery technique that has shown any growth since the 1980s. In fact, CO2 EOR now accounts for 6% of the Nation’s oil production. It can extend the productive life of an existing oilfield by several decades, and it can lead to recovery of millions of barrels of additional oil. A well-known example of CO2 EOR is the use of the Great Plains Synfuels Plant’s captured CO2 for EOR at Canada’s Weyburn oil fields (see the Weyburn-Midale Carbon Dioxide Project).
    CO2 EOR is not only cost effective, but environmentally advantageous because of its effectiveness and its ability to sequester gasification-created CO2 emissions. Following CO2 EOR operations, the CO2 can remain underground in the reservoir, and is thereby prevented from entering the atmosphere.
    Until recently, most CO2 EOR has depended on naturally-occurring underground deposits as the source of the CO2. In recent years, however, industry has begun to utilize CO2 that has been captured as a by-product of fossil fuel gasification or other industrial processes. Because a ton of CO2 costs up to $30 delivered, and each ton can yield 2-3 barrels of oil, there is a strong economic incentive to re-use industrially-sourced CO2 from gasification plants and gas processing facilities.
    Several new IGCC-based projects in the United States will be expanding the scope of CO2 capture and use/storage:

    • Kemper County Energy Facility – 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 CO2 generated using the Selexol process. The CO2 will be sent by pipeline to depleted oil fields in Mississippi for enhanced oil recovery operations.
    • Hydrogen Energy California (HECA) Project – 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 Texas Clean Energy, LLC: Texas Clean Energy Project – 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 McKibben who view any continued use of fossil fuels as counterproductive.