Clean Coal - Underground Coal Gasification
by Marvin Pirila
According to the Underground Goal Gasification (UCG) Partnership and School of Energy Resources (SER), the estimated available coal reserves and corresponding gas reserves exist for underground coal gasification.
-Estimated available coal reserves for UCG (billion tonnes): 138.1
-Potential gas reserves from UCG (as Natural Gas) (trillion m3): 41.4
-Current natural gas reserves (end-2005) (trillion m3): 5.9
UCG allows access to more of the physical global coal resource than would be included in current economically recoverable reserve estimates. Where mining is no longer taking place, for economic or geological reasons, UCG permits exploitation of deposits by the controlled gasification (reaction of coal to form a syngas) of coal seams in situ (underground). Carbon dioxide from the process can safely be returned to the gasified seam, resulting in zero emissions and very little ground disturbance.
Gasification (a thermo-chemical process) breaks down coal into its basic chemical constituents without burning gas directly. In modern gasifiers, coal is exposed to steam and carefully controlled amounts of air or oxygen under high temperatures and pressures. Under these conditions, molecules in coal break apart, initiating chemical reactions that typically produce a mixture of carbon monoxide, hydrogen, and other gaseous compounds.
The environmental benefits of gasification stem from the capability to achieve extremely low SOx, NOx and particulate emissions from burning coal-derived gases. During the process, sulfur converts to hydrogen sulfide and captured by current chemical industry processes. In some methods, the extracted sulfur is either a liquid or a solid form that is sold commercially. In an Integrated Gasification Combined-Cycle (IGCC) plant, the syngas produced is virtually free of fuel-bound nitrogen. Selective Catalytic Reduction (SCR) reduces levels comparable to firing with natural gas.
If oxygen is used in a coal gasifier instead of air, carbon dioxide is emitted as a concentrated gas stream in syngas at high pressure. In this form, it costs less to capture and sequester. By contrast, when coal burns or is reacted in air, 79% of which is nitrogen, the resulting carbon dioxide is diluted and more costly to separate.
Typical coal combustion-based power plants use heat from burning coal to boil water, making steam that drives a steam turbine-generator. As little as a third of the energy value of coal is converted into electricity.
A coal gasification power plant, however, typically gets dual duty from the gases it produces. First, coal gases cleaned of impurities, burn in a gas turbine to generate one source of electricity. The hot exhaust of the gas turbine and some of the heat generated in the gasification process, generate steam for use in a steam turbine-generator. This dual source of electric power, called a "combined cycle," is much more efficient in converting coal's energy into usable electricity. The fuel efficiency of a coal gasification power plant may potentially rise 50% or more by this type of combined cycle.
Future concepts that incorporate a fuel cell or a fuel cell - gas turbine hybrid could achieve efficiencies nearly twice that of today's typical coal combustion plants. If any of the remaining heat is used to process steam or heat, perhaps for nearby factories or district heating plants, the overall fuel use efficiency of future gasification plants could reach 70 to 80%.
Coal gasification power processes under development by the Energy Department could cut the formation of carbon dioxide by 40% or more, per unit of output, compared to today's conventional coal-burning plant.
A gasifier differs from a combustor in that it controls the amount of air or oxygen so a relatively small portion of the fuel burns completely. This "partial oxidation" process provides the heat. Most of the carbon-containing feedstock is broken apart chemically, rather than burnt, by the gasifier’s heat and pressure, setting into motion chemical reactions that produce "syngas." Syngas is primarily hydrogen and carbon monoxide, but can include other gaseous constituents; the composition of which can vary depending upon the conditions in the gasifier and the type of feedstock.
Minerals components in the fuel do not gasify like carbon-based constituents and leave the gasifier either as an inert glass-like slag or in a form useful to marketable solid products. A small fraction of the mineral matter blows out of the gasifier as fly ash and requires removal downstream.
Sulfur impurities in feedstock are converted to hydrogen sulfide and carbonyl sulfide, from which sulfur can be extracted as elemental sulfur or sulfuric acid, both valuable byproducts. Nitrogen oxides, another potential pollutant, is absent in the oxygen-deficient (reducing) environment of the gasifier; instead, ammonia is created by nitrogen-hydrogen reactions and easily stripped out of the gas stream.
In Integrated Gasification Combined-Cycle (IGCC) systems, the syngas is cleaned of its hydrogen sulfide, ammonia, and particulate matter and is burned as fuel in a combustion turbine. The combustion turbine drives an electric generator. Exhaust heat is from the combustion turbine is recovered and used to boil water, creating steam for a steam turbine-generator.
The use of a combustion turbine and a steam turbine, in combination, is known as a "combined cycle." This cycle is one reason why gasification-based power systems can achieve high power generation efficiencies. Current commercially available gasification-based systems operate at about 40% efficiency; in the future, some IGCC systems might achieve efficiencies approaching 60% with the deployment of advanced high-pressure solid oxide fuel cells. (A conventional coal-based boiler plant, by contrast, employs only a steam turbine-generator and is typically limited to 33-40% efficiencies.)
A 60% efficient gasification power plant can cut the formation of carbon dioxide by 40% compared to a typical coal combustion plant.
Clean syngas is useful as:
• Chemical "building blocks" to produce a broad range of higher-value liquid or gaseous fuels and chemicals;
• A fuel producer for highly efficient fuel cells or perhaps in the future, hydrogen turbines and fuel cell-turbine hybrid systems;
• A source of hydrogen, when separated from the gas stream can be used as a fuel or as a feedstock for refineries (which use the hydrogen to upgrade petroleum products).
Another advantage of gasification-based energy systems is that when oxygen is used in the gasifier (rather than air), the carbon dioxide produced by the process is in a concentrated gas stream, making it easier and less expensive to separate and capture. Once captured, the carbon dioxide can be sequestered - that is, prevented from escaping to the atmosphere, where it could otherwise potentially contribute to the "greenhouse effect."
Coal gasification offers one of the most versatile, clean ways to convert coal into electricity, hydrogen, and other valuable energy products.
In conclusion, current regulations for controlling CO2 emissions is adequate, making additional regulations unwarranted. Coal-generation plants should be replaced when companies need to build new, not as the government wishes to mandate. UCG is a viable alternative to coal reductions, and is highly desirable as the U.S. has the largest coal reserves in the world. UCG is determined site by site, and when properly situated, poses little risk to the environment, particularly water aquifers. Likewise, drastic cuts in the overall production are unwarranted and harmful to the U.S. goal of energy independence.