A major activity of the Capture Group has been exploring coal/biomass to liquids (CBTL) with CCS systems that produce synfuels + electricity. A new focus this year was using mixed prairie grasses (MPGs) grown via low-intensity techniques on carbon-depleted soils (as proposed in a 2006 Science article by David Tilman and collaborators) as the biomass feedstock. Energy and carbon balances as well as capital costs have been estimated, taking into account for the latter recent escalations in construction costs. Costs for the synfuels and electricity produced have been estimated as a function of the economic value of GHG emissions. This activity has been led by Williams and Larson in collaboration with Stefano Consonni and Giulia Fiorese at the Politecnico di Milano, extending earlier analysis led by Williams and Larson on systems with CCS that make synfuels + electricity from coal plus switchgrass (a monoculture energy crop). In this collaboration Giulia Fiorese, one of the Politecnico graduate students who visited PEI during 2007 (see Collaboratiion with Politecnico di Milano, below), had the major responsibility for developing a comprehensive model to evaluate the logistic chain of biomass supply, its costs and the overall carbon/energy balances of gasification energy with CCS systems that make synfuels and electricity from coal and MPGs.

This MPG strategy would help address biodiversity loss concerns for monoculture energy crops while significantly increasing the negative emissions potential of biomass by complementing geological storage of photosynthetic CO2 (the focus of earlier work on switchgrass) with root and soil carbon buildup during MPG growth. A system producing 17,000 barrels per day (1032 MW) of Fischer-Trospch (F-T) liquids + 452 MWe of electricity was designed with just enough MPGs to reduce the net GHG emission rate for the F-T liquids to zero, while allocating to electricity a GHG emission rate for a stand-alone IGCC plant with CCS (see Figure 1). For a crop consisting of a mixture of 16 prairie grass species it was estimated that zero net GHG emissions could be realized when 21% of input energy is in the form of MPGs (see Figure 1). (If no credit could be realized for the soil/root carbon buildup, the GHG emission rate for the F-T liquids would instead be 46% of the rate for the crude oil-derived products displaced.)

Figure 1. Carbon and Energy Balances for a System Making Low GHG-Emitting F-T Liquid Fuels and Electricity from Coal and Mixed Prairie Grasses (MPGs) Grown on Carbon-Depleted Soils. The system captures and stores as CO2 85% of the C not contained in liquid products. Assuming 16 MPG species, the estimated 30-year average soil/root C storage rate is 0.6 tC for each tC in harvested biomass. Some 106 dry tonnes of MPG are required annually. For a hypothetical case study plant located in Southern Illinois, the estimated MPG yield is 10.4 dry tonnes per hectare per year on lands now planted in corn, and the average MPG transport distance is 43 km. The captured CO2 is transported 53 km by pipeline for storage in the Mt. Simon aquifer.

For a such a system sited in southern Illinois using MPGs grown on land currently planted in corn, the amount of liquid fuel energy provided would be 3.3 times the amount of ethanol energy that could be produced from the corn now grown on this land (see Figure 2). Alternatively, this same land might be planted in switchgrass for conversion (via use of technologies yet to be demonstrated commercially) to cellulosic ethanol; so doing would reduce the GHG emission rate by an almost order of magnitude relative to that for corn ethanol, but the liquid fuel yield would be about the same (see Figure 2).

Figure 2. Liquid Fuel Yields for Alternative Technologies. The bar on the left is for the zero net GHG-emitting coal/biomass F-T liquids with CCS case shown in Figure 1 based on use of 16 mixed prairie grasses (MPGs) at a site in southern Illinois where MPGs are grown with low intensity inputs on land now growing corn. The second bar is for ethanol from corn (current corn yield on this land: 7.6 dry tonnes (dt) per hectare per year, 0.47 liters of ethanol per dt). The third bar is for cellulosic ethanol (0.38 liters per dt) derived from switchgrass grown at the same yield as estimated for MPGs (10.4 dt per hectare per year).

A paper describing a case study for the State of Illinois is being prepared, to show under which conditions and which strategies are needed to make the production of FT fuels from co-gasification with CCS competitive with other strategies to mitigate GHG emissions.

If the 19 million acres currently used to grow corn in the US for ethanol were converted 100% to CBTL with CCS using MPG biomass, some 22.5 billion gallons of ethanol-equivalent liquid fuel could be produced annually—more than the total amount of advanced biofuels targeted for production in the US by 2022 under the Energy Independence and Security Act of 2007—some 21 billion gallons per year. Moreover, if all this fuel were used in US light-duty vehicles (LDVs), the result would be 10% lower oil use and a 10% lower GHG emissions intensity for LDVs in that year, assuming no other low carbon fuels would be used to displace oil. For perspective, this emissions reduction potential can be considered in the context of the Low Carbon Fuel Standard that Governor Arnold Schwarzenegger launched via executive order for California in January 2007—which calls for reducing the average carbon intensity of fuels for light-duty vehicles in California at least 10 percent by 2020.

“Carbon debt” concerns have recently been raised about US plans to expand corn ethanol production from 6.4 billion gallons in 2007 to 12 or 15 billion gallons per year over the next decade. Rapid reduction in U.S. corn growing for food is expected to lead to bringing new lands into food production elsewhere in the world that would release large quantities of carbon to the atmosphere from standing biomass and soil carbon now stored in these lands. Calculations indicate that typically several decades to hundreds of years of biofuel production would be required to “pay off” the carbon debt associated with these carbon releases. This carbon debt problem might be mitigated if corn lands now used for alcohol production could be shifted to CBTL with CCS systems using MPGs. This would be of interest to farmers, however, only if the shift would lead to incomes that are at least as great as for growing corn for ethanol production.

In our economic analysis, the breakeven crude oil price was first estimated to be ~ $70 per barrel for coal to liquids (CTL) plants for the plant scales considered. It was assumed that the CBTL plants would have to sell their products at the same prices as CTL plants. This condition determines the synfuel producer’s willingness to pay (WTP) for biomass—a price that depends sensitively on the market value of GHG emissions. At low GHG emission prices CBTL technology would not be economic, but at CO2-equivalent GHG emission values greater than ~$30-$35/tCO2, favorable economics would be realized for systems getting credit for soil/root carbon buildup at WTP levels that would lead to farmer incomes from growing MPGs that are greater than from growing corn. This emissions value is of the order of the minimum GHG emission value needed to induce CCS for new coal power plants. Thus a carbon policy stringent enough to induce CCS for coal power plants would make MPG-based CBTL technology economic in a world with oil prices of the order of $70 a barrel.

A shift from corn to MPGs could plausibly get started in this time frame. MPGs can be established as harvestable crop systems in a few years time. And one technological variant of the CBTL concept—based on dry-feed entrained-flow gasifiers—is commercially ready. Baard Energy is planning to construct a 50,000 barrels per day CBTL plant at Wellsville, Ohio, using this approach. Plans are to co-fire the plant with bituminous coal and 30% biomass (dry-weight basis) and to capture CO2. Captured CO2 would be used either for enhanced oil recovery in a nearby oil field or stored in a deep saline formation. Baard is targeting bringing on line the first part of the plant in the 2011-2012 timeframe. The CBTL with CCS concept is attracting considerable attention, to a large extent as a result of CMI studies and many presentations.1

The Air Force has set a goal of meeting ½ of its jet fuel needs in 2016 with F-T liquids. A study for the Air Force and the National Energy Technology Laboratory (NETL)2 addressed the question: could these F-T liquids be provided with an average CO2 emissions rate that is 20% less than that for the crude oil-derived products displaced by exploiting both CCS and CBTL technologies? Also, Williams is a technical advisor for a major follow-on NETL study on the CBTL with CCS concept that will be published in early 2008.

In September 2007, Williams was invited to submit written testimony for the Hearing on the Future of Coal under Carbon Cap and Trade before the Select Committee on Energy Independence and Global Warming of the US House of Representatives, Chaired by Congressman Edward Markey. In part of his testimony Williams sketched out the strategic importance of the CBTL with CCS option, discussed the major challenges, and proposed off-budget incentive schemes for encouraging CBTL and competing low-carbon fuels technologies that would substitute the market for the government as the “picker of the winning technologies.”

 


1. For example, a December 2006 panel presentation by Williams (“Toward Cost-Competitive Synfuels from Coal and Biomass with Near-Zero Well-to-Wheels GHG Emissions by Simultaneous Exploitation of Two Carbon Storage Mechanisms,” Alternative Fuels Seminar: Carbon to Liquids, Center for Strategic and International Studies, Washington, DC) sparked interest in CBTL with CCS technology by NETL Director Carl Bauer, who made a presentation on the same panel. Also, a July 2007 luncheon keynote address by Williams to the Washington Coal Club ( “Synfuels From Coal + Biomass with CO2 Capture and Storage (CCS)”) brought the CBTL with CCS concept to the attention of many interested in coal issues in Washington. The Washington Coal Club convenes a regular seminar attended by coal industry lobbyists, DOE coal staffers, and Congressional staffers interested in coal issues.

2.  DOE/NETL and DoD/Air Force, Increasing Security and Reducing Carbon Emissions of the US Transportation Sector: A Transformational Role for Coal with Biomass, DOE/NETL-2007/1298, 24 August 2007.