Gasification
See also: Underground Coal Gasification High prices of oil and natural gas are leading to increased interest in "BTU Conversion" technologies such as
gasification, methanation and liquefaction.
Coal gasification breaks down the coal into smaller molecular weight molecules, usually by subjecting it to high temperature and pressure, using steam and measured amounts of oxygen. This leads to the production of
syngas, a mixture mainly consisting of
carbon monoxide (CO) and
hydrogen (H2).
In the past, coal was converted to make
coal gas, which was piped to customers to burn for illumination, heating, and cooking. At present, the safer
natural gas is used instead.
South Africa still uses gasification of coal for much of its petrochemical needs.
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.
Gasification is also a possibility for future energy use, as the produced syngas can be cleaned-up relatively easily leading to cleaner burning than burning coal directly (the conventional way). The cleanliness of the cleaned-up syngas is comparable to natural gas enabling to burn it in a more efficient
gas turbine rather than in a boiler used to drive a steam turbine. Syngas produced by gasification can be CO-shifted meaning that the combustible CO in the syngas is transferred into carbon dioxide (CO2) using water as a reactant. The CO-shift reaction also produces an amount of combustible hydrogen (H2) equal to the amount of CO converted into CO2. The CO2 concentrations (or rather CO2 partial pressures) obtained by using coal gasification followed by a CO-shift reaction are much higher than in case of direct combustion of coal in
air (which is mostly nitrogen). These higher concentrations of carbon dioxide make
carbon capture and storage much more economical than it otherwise would be.
[edit] Liquefaction - Coal-To-Liquids (CTL)
Coals can also be converted into
liquid fuels like
gasoline or
diesel by several different processes. The
Fischer-Tropsch process of indirect synthesis of liquid
hydrocarbons was used in
Nazi Germany for many years and is today used by
Sasol in South Africa. Coal would be gasified to make syngas (a balanced purified mixture of CO and H2 gas) and the syngas condensed using Fischer-Tropsch
catalysts to make light hydrocarbons which are further processed into gasoline and diesel. Syngas can also be converted to
methanol, which can be used as a fuel, fuel
additive, or further processed into gasoline via the
Mobil M-gas process.
A direct liquefaction process
Bergius process [8] (liquefaction by
hydrogenation) is also available but has not been used outside
Germany, where such processes were operated both during
World War I and
World War II. SASOL in South Africa has experimented with direct hydrogenation. Several other direct liquefaction processes have been developed, among these being the SRC-I and SRC-II (Solvent Refined Coal) processes developed by
Gulf Oil and implemented as pilot plants in the United States in the 1960s and 1970s.
[9]
Another direct hydrogenation process was explored by the NUS Corporation in 1976 and patented by Wilburn C. Schroeder. The process involved dried, pulverized coal mixed with roughly 1wt%
molybdenum catalysts. Hydrogenation occurred by use of high temperature and pressure
synthesis gas produced in a separate gasifier. The process ultimately yielded a synthetic crude product,
Naphtha, a limited amount of C3/C4 gas, light-medium weight liquids (C5-C10) suitable for use as fuels, small amounts of NH3 and significant amounts of CO2.
[10]
Yet another process to manufacture liquid hydrocarbons from coal is low temperature
carbonization (LTC). Coal is coked at temperatures between 450 and 700°C compared to 800 to 1000°C for metallurgical coke. These temperatures optimize the production of coal tars richer in lighter hydrocarbons than normal coal tar. The coal tar is then further processed into fuels. The
Karrick process was developed by Lewis C. Karrick, an oil shale technologist at the
U.S. Bureau of Mines in the 1920s.
All of these liquid fuel production methods release carbon dioxide (CO2) in the conversion process, far more than is released in the extraction and refinement of liquid fuel production from petroleum. If these methods were adopted to replace declining petroleum supplies, carbon dioxide emissions would be greatly increased on a global scale. For future liquefaction projects,
Carbon dioxide sequestration is proposed to avoid releasing it into the atmosphere, though no pilot projects have confirmed the feasibility of this approach on a wide scale. As CO2 is one of the process streams, sequestration is easier than from flue gases produced in
combustion of coal with
air, where CO2 is diluted by
nitrogen and other gases. Sequestration will, however, add to the cost.
The reaction of coal and water using high
temperature heat from a nuclear reactor offers promise of liquid transport fuels that could prove carbon-neutral compared to petroleum use. The development of a reliable nuclear reactor that could provide 900 to 1000 deg C process heat, such as the pebble bed reactor, would be necessary.
Coal liquefaction is one of the
backstop technologies that could potentially limit escalation of oil prices and
mitigate the effects of transportation energy shortage that some authors have suggested could occur under
peak oil. This is contingent on liquefaction production capacity becoming large enough to satiate the very large and growing demand for petroleum. Estimates of the cost of producing liquid fuels from coal suggest that domestic U.S. production of fuel from coal becomes cost-competitive with oil priced at around 35 USD per barrel,
[11] (break-even cost). This price, while above historical averages, is well below current
oil prices. This makes coal a viable financial alternative to oil for the time being, although current production is small.
[12]
Among commercially mature technologies, advantage for indirect coal liquefaction over direct coal liquefaction are reported by Williams and Larson (2003). Estimates are reported for sites in China where break-even cost for coal liquefaction may be in the range between 25 to 35 USD/barrel of oil.[
citation needed]'
Intensive research and project developments have been implemented from 2001. The
World CTL Award is granted to personalities having brought eminent contribution to the understanding and development of Coal liquefaction. The 2008 presentation ceremony took place at the
World CTL 2008 Conference (3 & 4 April, 2008).