Synthetic diesel fuel from coal
Interest has renewed in coal-derived diesel fuels for several reasons. The surge in diesel vehicles has led to a desire to find a long-term low-carbon fuel for diesels. Several of the nations that will be key to any action on global warming have vast coal resources, including the United States, China, and India. And there has been growing policy interest in as well as R&D funding for carbon capture and storage, which may make it possible to utilize coal resources for making fuels (and electricity) without increasing greenhouse gas emissions. However. even with all the benefits, it still doesn't come close to having low enviromental impact that biodiesel fuels have.
America's coal resource base is vast. Recoverable coal reserves amount to about 270 billion tons, or 250 years at current production rates. 69 The demonstrated reserve base is almost 500 billion tons, and the total resource base may exceed 4 trillion tons. 70 Coal is also relatively inexpensive compared to other fossil fuels, a key reason it generates over 50% of U.S. electricity. Numerous considerations constrain the use of coal as an energy resource. Mining can have significant impacts on land and water. When burned, coal produces sulfur oxides (SOx), nitrogen oxides (NOx), mercury, and particulate matter (PM). Coal also produces more carbon dioxide per unit of energy produced than oil or natural gas.
Technological innovations can ameliorate many of the difficulties associated with the use of coal as an energy resource. By converting coal to liquid or gaseous fuels, it will be possible to use it for transportation, greatly reducing the risks of price shocks associated with increasing demand for oil outpacing supply. Gasification and liquefaction can remove nearly all of the sulfur and mercury from the coal, dramatically reducing pollutant emissions. If carbon sequestration is proven to be effective, coal-derived liquid fuels could have lower CO2 emissions than conventional diesel or gasoline. Production of liquid fuels from coal offers a viable option for energy security for the U.S., China, and other coal-rich countries.
Synthetic fuels development was a focus of U.S. energy independence efforts in the 1970's. Efforts were largely abandoned, but then resumed in the 1990's. In 2001, the National Energy Technology Laboratory held a series of "Clean Liquid Fuels" workshops. One coal-to-liquids facility is under development in Gilberton, PA.71 The project was awarded a DOE grant for a feasibility study in 2000; the facility has since been funded for development through the Clean Coal Power Initiative. 72 The facility will seek to produce electricity and liquid fuels from waste coal; CO2 sequestration is a possibility.
Coal can be converted to liquid or gaseous fuels through a number of processes. Some of these were pioneered in the Second World War to provide fuel for airplanes and tanks. Germany in particular employed a number of technologies, including lignite distillation, Fischer-Tropsch processes, and hydrogenation.
Indirect coal liquefaction first combines the coal with oxygen and steam or water to produce synthesis gas (also known as syngas), which is most often a mix of carbon monoxide and hydrogen. The syngas can then be processed into liquid or gaseous fuels such as Fischer-Tropsch liquids, dimethyl ether (DME), or methanol. Toxic metals can be removed from the syngas through a carbon filter. Direct coal liquefaction skips the syngas step, and includes technologies such as distillation and hydrogenation (adding hydrogen to a coal-water slurry). The addition of hydrogen improves the H/C ratio, bringing the resulting product closer to lighter hydrocarbons such as those found in gasoline or diesel fuel. It also removes sulfur, allowing for clean-burning fuel. Direct liquefaction results in a crude oil, which is then be further refined.
For purposes of mitigating climate change, synthetic diesel fuels seem to present an appealing option. Diesel compression-ignition engine (CIE) cars are 20% to 30% more efficient than gasoline spark-ignition engine (SIE) cars, and hybrid technology offers the potential for further efficiency gain. As noted above, however, using conventional diesel fuel, black carbon (soot) can offset the greenhouse gas benefits-even if vehicle particulate matter standards are tightened by a factor of eight down to 0.01 grams per mile.76 Although some assessments dispute this conclusion (because particulate matter also includes light-colored matter that produces some cooling effects), it is widely recognized that at the very least particulate matter has a number of harmful health effects. For any replacement fuel, low PM emissions are an advantage.
Producing liquid fuels from coal with no carbon sequestration would increase net CO2 emissions relative to petroleum-derived fuels. Because much of the CO2 generated in the coal-to-liquids process is a central waste stream, capture and storage may be a viable strategy, which would significantly reduce this impact and allow such coal-derived fuels to have the coal or lower net greenhouse gas emissions than regular diesel fuel. Further, blending in some biomass with the coal before it is gasified and the CO2 is sequestered can sharply reduce net emissions, as the biomass pulled CO2 out of the air while it was growing and the sequestration process then permanently stores it underground.
DIMETHYL ETHER (DME)
DME can be made from biomass, natural gas, or coal. If it is made from biomass, the life-cycle CO2 emissions are 25% that of diesel. If it is made from natural gas, emissions are comparable to diesel. And, if made from coal, CO2 emissions would be 90% higher than diesel if the CO2 were vented, but could be reduced to 20% less than diesel with sequestration of CO2 and H2S.77 By one calculation, carbon sequestration might add only 15% to the cost of DME.
Used in a diesel engine, DME provides substantial reductions in particulate matter (PM) and nitrogen oxides (NOx). As noted by Dr. Bob Williams, "For DME used in a heavy duty CIE, uncontrolled emissions of NOx and PM have been measured to be 58% and 75% less than for conventional Diesel."79 Black carbon soot is virtually eliminated due to the absence of carbon-19 carbon bonds. However, DME is a gas at standard temperature and pressure. For use as a vehicle fuel, it would require moderate compression (similar to propane) to liquefy it for distribution. This would entail considerable infrastructure investments.
FISCHER-TROPSCH (F-T) LIQUIDS
The Fischer-Tropsch process can produce fuel out of syngas. It also offers a way to recover "stranded" natural gas (resources that would be uneconomical to transport in gaseous form). While diesel fuel is easily produced, the lighter components can be processed into gasoline with substantial refining. Fischer-Tropsch processes are widely used in South Africa to produce liquid fuels from coal.
Fischer-Tropsch liquids can be transported through the existing infrastructure and used in existing engines. They offer a substantial opportunity to reduce dependence on petroleum, and offer modest environmental benefits. In a 1998 study, emissions from trucks were measuring using California diesel and Fischer-Tropsch diesel. F-T emissions were lower by 12% for NOx, 24% for PM, CO by 18%, and hydrocarbons by 40%.80 Fischer-Tropsch diesels are also very low in aromatics, a class of compounds that includes hazardous chemicals such as benzene.81 However, life-cycle CO2 emissions from Fischer-Tropsch liquids are higher than for petroleum-derived fuels unless sequestration is employed.82
Indirect coal liquefaction through the Fischer-Tropsch process provides a "safety valve" against oil price shocks or supply disruptions. Its emission characteristics are superior to conventional diesel, and its cost may be competitive. The National Academy of Sciences estimated an achievable cost of F-T diesel at $35/bbl, or $30/bbl with electricity co-production.83 This cost is for commercialized technology, not pioneer plants. The Academy also notes that "experience with sustained R&D indicates that DOE's goal of $25/bbl (1991 dollars) for coal-based liquids may be attainable with continued research and systems studies."
The challenges to widespread use of coal-derived fuels are significant. As Bob Williams explains, "the major drawback of DME is that requires a new gaseous fuel infrastructure."85 Thus it faces most of the same barriers that have proved intractable for other alternative fuels in the United States. DME may prove a more viable fuel in areas countries without in existing gasoline infrastructure, such as China.
Fischer-Tropsch diesel does not have the infrastructure problems. Price, however, remains a problem and, as with cellulosic ethanol, someone will have to take the risk with the first several pioneer plants. Also, particulate matter, while lower than conventional diesel, is still high enough to be a significant concern for both global warming (black carbon) and direct human health impact. Finally, until sequestration is demonstrated to be politically, economically, and environmentally viable on a large-scale, neither F-T diesel nor DME will make sense from a global warming perspective.