Coal has been considered a dirty fuel for a long time. It caused the famous 19th century London fogs, and it killed 19 people in Donora, PA in 1948 when a temperature inversion trapped smoke from the surrounding steel mills against the ground. The latter incident helped to get pollution control regulations off the ground, even though there had been sporadic attempts as far back as 1306.
Clean coal technology is the use of coal without the release of pollutants, although there is some division of opinion as to whether carbon dioxide (CO2) should be classed as a pollutant or considered separately. Aside from CO2, the main pollutants from coal burning are particulates, sulfur dioxide (SO2), oxides of nitrogen (NOx) and metals (chiefly mercury).
The first pollutants to be controlled were particulates, via electrostatic precipitators and baghouses, and SO2, via scrubbers and fluidized-bed combustion. Methods for NOx reduction include staged combustion, fluidized beds and scrubbers.
Mercury control measures are more recent: The EPA estimated in 1999 that U.S. power plants emitted approximately 50 tons of mercury per year. EPA’s Clean Air Mercury Rule (adopted in 2005) aimed to reduce utility emissions of mercury to 15 tons a year using a cap-and-trade system. The 15-ton goal is supposed to be met some time after 2018.
Clean coal demonstration projects
In 1998 the 50-megawatt Healy clean coal power plant went on line in Alaska. The plant used the TRW Clean Coal Combustion System, multistage slagging combustors with injection of limestone, plus a spray dryer absorber. It was indeed clean — nitrogen oxide emissions were only 0.26 pounds per million BTUs and sulfur dioxide emissions were 0.01 pounds per million BTUs. There were, however, problems with safety, reliability and economics, and the plant was shut down in 2000. It was recently sold and may yet go on line.
Tampa Electric Company’s 250 MWe Polk Power Station demonstration project, built in 2001, ran for five years. It employed an IGCC (Integrated Gasification Combined Cycle) system using the Texaco process gasifier technology. Results seemed promising at first, but carbon conversion was less than anticipated, there were equipment problems, and the economics were just not there. The best case estimate to build a new plant was $1300/kW in 2001 dollars, and in October of 2008 the company scrapped plans for a 630 MW clean coal plant in favor of one running on natural gas, despite a higher price for gas. According to the St. Petersburg Times the coal plant would have cost $2900/kW versus $1000/kW for natural gas, and would have emitted much more carbon dioxide.
One approach to pollution control is gasification; the idea is that by converting the coal to a gaseous fuel the pollutants can be removed before combustion. Coal gasification dates back to the early 19th century, with the development of water gas (C + H2O → CO + H2) for illumination. The original method used atmospheric air and the resulting product contained large amounts of nitrogen; it was replaced mid-century by the blue water gas (BWG) process, which involves blowing air over the coal to burn and heat it, then switching to high-temperature steam to produce the product. Syngas (synthesis gas) is still made from coal using the same reaction, sometimes supplemented with CO2 + C → 2CO if more CO is wanted, and sometimes with the water gas shift reaction, H2O + CO → H2 + CO2, when more hydrogen is wanted. Some processes (the Healy plant, for example) add oxygen to the combustion air, to nitrogen dilution of the syngas.
Coal to liquid
Coal-to-liquid processes have been around since the early 20th century. Germany developed two methods: direct coal hydrogenation (the Bergius, Pott-Broche and IG Farben processes, suitable for producing diesel fuel and lubricants) in 1910 to 1925, and the Fischer-Tropsch process (suitable for producing high-quality aviation and motor gasoline) in 1923. Germany used these during WWII when crude oil imports were cut off. South Africa first used the Lurgi process when embargoes in protest of that country’s apartheid policies did the same, and has been using the Fischer-Tropsch process since; a principal player in that space is Sasol Limited.
The Fischer-Tropsch process starts with syngas (CO + H) and produces liquid hydrocarbons (chiefly alkanes). The water gas shift reaction (H2O + CO → H2 + CO2) is often used with it as a source of additional hydrogen.
In the Bergius process finely-ground coal is mixed with a heavy oil and reacted with hydrogen at high temperatures and pressures in the presence of a catalyst. Further processing is required, and equipment operating conditions are harsh, with a combination of high temperatures and pressures along with the abrasive and viscous coal slurry. While the process had not been used for many years, in 2003 the Shenhua Group in China, working with Headwaters, Inc. began construction of a plant in Majata, Inner Mongolia for direct conversion of coal to liquid fuels; there are reports the product costs $67 to $82/bbl to produce, so with the present low price of oil and the economic downturn we will have to see what happens. Original plans called for work to continue through 2010.
Rentech, Inc. has a Product Demonstration Unit in Colorado that it says is the only operating synthetic transportation fuels facility in the United States. It is designed to produce about 10 bbl/day of ultra-clean diesel, aviation fuels, naphtha and specialty waxes and chemicals.
In 2006 DKRW Energy began a Fischer-Tropsch process coal-to-liquids project in Medicine Bow, WY. Siting and air permits were received in February and March of this year, and the company expects to begin commercial operation in 2013, with output of 15,000 to 20,000 bpd.
In 2007 Baard Energy began work on the 53,000 bpd Ohio River Clean Fuels project near Wellsville, OH. The facility, designed to produce diesel fuel, jet fuel and electricity using coal and biomass as feedstock, recently received its final air permit.
Silverado Green Fuel Inc. has been working on development of what it calls Low-Rank Coal-Water Fuel, which would be produced from low rank coal. It would be used to power oil-fired power plants or as refinery feedstock.
Watch for part 2 of “Exploring Clean Coal Technologies,” where we discuss carbon dioxide and the growing pressure to reduce it, and conclude our article with a look at clean coal’s future.