Wed 17 Sept 2008 – The US Defense Advanced Research Projects Agency (DARPA) has announced a research programme to exploit the country’s huge coal resources to extract liquid fuel that could satisfy its military JP-8 jet fuel needs for “several thousand years” and reduce its reliance on foreign petroleum-based fuels. Not only is the challenge to produce a fuel that is cost competitive with existing fuels, it must also be environmentally friendly.
Under the Broad Agency Announcement (BAA) process, DARPA is awarding total funding of up to $4.56 million, limited to one government fiscal year, to organizations with “innovative” research proposals.
DARPA states that the US military’s dependence on foreign oil imports constitutes a vulnerability that could adversely impact on national security should a disruption to supplies occur. “Given the abundance of US coal reserves, it is reasonable to assume that coal derived fuels could play an increased role in meeting future US energy needs,” it says.
The US Department of Defense (DoD) currently uses 300,000 barrels of petroleum-based liquid fuels per day. There are estimated US coal reserves of over 275 billion tonnes. Therefore, says DARPA, it is a reasonable expectation that with existing CTL technologies, DoD’s liquid fuel needs could be met from using these reserves “for several thousand years”.
However, DARPA estimates the current cost of implementing a 100,000 barrel per day (bpd) CTL facility would be in excess of $6 billion – four times the cost of implementing a similar capacity crude oil refinery – with end-user costs of over $4.50 per gallon. It is seeking proposals that can meet a similar capacity for a capital cost not exceeding $1.5 billion and end-user costs of less than $3 per gallon.
Not only are the two existing methods of extracting fuel from coal extremely expensive to implement, they consume large amounts of water and produce unacceptable amounts of atmospheric CO2 and other pollutants. The first, using an indirect process whereby the coal is first gasified and then converted to the desired hydrocarbon content via a water gas shift reaction followed by Fischer-Tropsch (F-T) synthesis, can convert 120,000 tonnes of coal into about 150,000 bpd of oil. Although direct approaches exist to liquefy coal without first producing syngas, none of them are currently being exploited in a commercial oil production facility.
Every kg of coal that is converted via the indirect F-T liquefication process produces just 0.27kg of oil but 1.3kg of carbon dioxide, says DARPA. The life cycle environmental cost of fuels derived from coal results, therefore, in a net increase of 80% or more in terms of CO2 emissions as compared to the use of petroleum-based fuels. While the use of proposed carbon capture technologies might mitigate the increased CO2 emissions, the agency says the scalability, reliability and economics of these technologies are still unknown.
Existing CTL technologies rely on large amounts of water as a source of hydrogen due to the fact that although high in carbon content, coal is comprised of only 5% hydrogen compared to JP-8’s 15% hydrogen composition by weight. Water is a natural source for hydrogen (11% hydrogen by weight) but the water-based reactions for converting coal to liquid fuels result in excess carbon and oxygen reacting to produce the additional unwanted CO2 – a major disadvantage of existing CTL technologies. However, DARPA believes that innovative CTL concepts may exist that result in zero CO2 emissions by avoiding the production of CO2.
Another challenge is that the need for steam in conventional CTL processes requires that for each kg of coal converted to oil, as much as 1kg of water is consumed. DARPA suggests that by using the hydrogen available in the coal feedstock, alternative hydrogen sources and/or recycling part of the water consumed during the process, the amount of water required could be halved.
In summary, DARPA says proposals must therefore meet a requirement for zero CO2 emissions up to the 100,000 bpd production level and consume less than 235kg of water for every barrel of fuel produced – equivalent to under 0.5kg of water for every 0.27kg of fuel. It also says that proposals that include incremental improvements to existing Fischer-Tropsch or other existing CTL processes will not be considered.
So, could coal-derived fuels eventually find their way into commercial airline jet fuel? The Air Transport Association of America (ATA), which represents most US airlines, published its Commercial Aviation Alternative Fuel Commitment earlier this year. It states that “any alternative jet fuel must be more environmentally friendly than traditional jet fuel, in particular resulting in a reduced emissions profile on a lifecycle basis, without compromising critical uses of relevant feedstocks.”
ATA is a major sponsor of the Commercial Aviation Alternative Fuel Initiative (CAAFI), a coalition of airlines, airports, aerospace manufacturers, government agencies, fuel companies and research institutions that is seeking economically, technically and environmentally friendly alternatives to traditional petroleum-based jet fuel.
CAAFI’s Executive Director, Richard Altman, says that CO2-sequestered coal/biomass blended (CBTL) fuels (comprising, for example, switchgrass and prairie grasses) are a more favourable option than petroleum fuels on a GHG lifecycle basis – an airline prerequisite – and are currently undergoing scrutiny by the coalition. The technology involved requires Fischer-Tropsch synthesis, a process DARPA is seeking to avoid on environmental grounds, but Altman points to a Princeton University study that indicates dramatically reduced GHG emission rates for CBTL fuels (see graph below).
Greenhouse gas emission rates for fuel production use (source: Princeton University studies, W. Harrison & R. Altman, March 2007)
The use of coal as an energy source is an emotive issue amongst environmentalists and of great concern to climatologists. A report just published by Science Daily quotes researchers Pushker Kharecha and James Hansen from the NASA Goddard Institute for Space Studies, a member of Columbia University’s Earth Institute, as saying the ongoing rise in atmospheric carbon dioxide from burning fossil fuels might be kept below harmful levels if emissions from coal are phased out within the next few decades. They say that less plentiful oil and gas should be used sparingly as well but, because of far greater supplies, coal must be the main target of reductions.
The phase out, they maintain, could come either from reducing coal consumption or by using current or near-term technologies to capture and trap CO2 from coal burning before it reaches the air.
Professor Hansen is credited with warning the world ten years ago that he was “99% certain” that human activities were heating up the planet, thereby putting global warming on the political agenda.
It was Hansen’s research, now widely accepted, that suggested a dangerous level of global warming may occur if levels of CO2 exceeded a concentration of around 450 parts per million, a 61% increase on the pre-industrial level of 280 parts per million, but only 17% more than the current, and rising, 385 parts per million.
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