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NETL and USAF Release Study for Conceptual Coal+Biomass-to-Liquids Facility; Offers Lower Carbon Footprint than Petroleum Refining

Block flow diagram for the corn stover scenario. The study analyzes the use of three biomass feedstocks: stover, switchgrass, and woody biomass. Click to enlarge.

The US Department of Energy’s National Energy Technology Laboratory (DOE/NETL) and the US Air Force have released a study that concludes that a facility capable of producing 7,500 barrels per day or more of jet fuel or diesel from coal and biomass (coal + biomass-to-liquids, CBTL), with accompanying 20% lower life-cycle emissions of CO2 compared to conventional petroleum refining processes, is feasible.

The Air Force intends to supply 50% of its CONUS (Continental United States) fuel requirements from domestic synthetic sources by 2016. (Earlier post.) One option under serious consideration is the indirect coal-to-liquids (CTL) process (gasification and Fischer-Tropsch (FT) synthesis). To reduce the heavy CO2 output of the conventional CTL process (approximately 1.8 times that of conventional petroleum processes), the Department of Defense (DoD) will require synthetic fuel producers to use carbon capture and sequestration (CCS).

Although studies conclude that conventional CTL coupled with CCS could limit CO2 emissions to a level approximately equivalent (+4% to -5%) to that of the existing petroleum-based fuel supply chain, the DoD wants to explore options that will further improve its environmental performance by reducing the carbon footprint of the plant to be below that of a conventional petroleum refinery.

The conversion of coal and biomass to liquid fuels (CBTL) has been recently proposed as a possible option to accomplish this.

The option to use various process (including algae) for reuse/reform of CO2 emissions with CTL/CBTL process has been proposed, but is not in the scope of the current report. This option will be considered in future research.

The just-published report, commissioned by NETL/USAF and produced by Nobilis, Increasing Security and Reducing Carbon Emissions of the US Transportation Sector: A Transformational Role for Coal with Biomass, had two specific objectives:

  • First, to develop a coal-biomass-to-liquids plant design capable of co-gasifying mixtures of coal and biomass to produce a clean synthesis gas that can then be sent to Fischer-Tropsch units for synthesis of diesel, jet and naphtha liquid fuels. The goal of this CBTL plant was to determine the appropriate mixture of coal and biomass that would produce these fuels with a net carbon footprint 20% lower than would occur from the production of low sulfur diesel from an existing conventional petroleum refinery. This also assumes the use of carbon capture and sequestration.

    The reference plant studied was a 7,500 BPD diesel plant located in southern Illinois. The plant configurations call for the capture of about 88% of the process CO2 emissions. The captured gas is compressed to 2,200 psi and piped from the plant.

    The size was based on a preliminary and approximate estimate of the amount of biomass required. The report does not suggest that 7,500 BPD is either the maximum or optimum size for a CBTL plant—depending upon resource availability, plants as large as 30,000 BPD could be feasible.

  • Second, to develop a CBTL pathway for diesel fuel production that has the potential for meeting the DoD goal of 100,000 BPD of synthetic fuel with the requirement that carbon dioxide emissions should be less than those from conventional petroleum. The study examined three biomass types—woody biomass, switchgrass, and corn stover—which are relatively abundant and the use of which would not directly affect food supplies.

Most of the estimates for CO2 emissions associated with the production, transportation, and processing of feedstocks and end products were obtained from the Argonne National Laboratory (ANL) Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation (GREET) Model version 1.7.

In the conceptual designs, based on a 7,500 BPD (diesel) capacity, coal and biomass are gasified in entrained flow gasifiers and the raw synthesis gas is cleaned of impurities. The syngas then flows to slurry phase FT reactors—the same type of reactor being used by Sasol at the Oryx GTL plant in Qatar. (The authors note that although slurry reactors have excellent heat transfer characteristics and allow high conversions of synthesis gas per pass, there has as yet not been much commercial experience with these reactors.)

In addition to the 7,500 barrels per day of jet fuel or diesel, the plant would also produce more than 3,500 barrels per day of liquid naphtha products that can be shipped to a refinery for further upgrading to commercial-grade products or sold as chemical feedstock and 11.1 megawatts of electricity that can be exported to the grid, in addition to the electricity generated for internal use.

According to the report, there are issues relating to hydrodynamics and separation of the wax produced in the FT process from the fine catalyst. (Wax, produced to maximize the distillate yield, is hydrocracked to produce additional distillate product.)

The study estimates the amount of each type of biomass that would need to be co-fed with the coal (Illinois #6 bituminous coal) to reach the target of 20% reduction of CO2 emissions.

Biomass Portion Required for
20% CO2 Reduction in CBTL
Type% by Weight% by Energy
Woody biomass 10-15% 7-10%
Switchgrass 12-18% 7-10%
Corn stover 12-18% 7-11%

The results of the study indicated that FT diesel can be produced at the target CO2 reduction level by co-gasifying coal with a relatively modest amount of biomass. (See table at right.)

The study authors concluded that because the percentage of biomass required is relatively low and within the range of the limited demonstration test data available for coal and biomass co-feeding to pressurized gasifiers, the proposed CBTL process is potentially feasible.

The also concluded that biomass availability would not be a major limiting factor for CBTL plants in the 7,500 BPD diesel capacity range. This size CBTL facility would require a sustainable annual supply of biomass of about 1,000 TPD. For switchgrass and poplar with dry yields per acre of about 5-6 tons, the total land area required would be about 1,440 square miles (a radius of about 22 miles).

For corn stover with a lower crop yield of about two dry tons per acre (half of the crop is left on the land for soil conditioning), the area required for sustained operations to produce 1,000 TPD would be about 920 square miles (radius of about 17 miles) because the land available for production is assumed to be as high as 31%.

While this study concludes that it is practical to attain the desired CO2 emissions reduction target it must be cautioned that, because the amount of actual field data available on gasification of biomass in pressurized entrained flow gasifiers is so limited, considerable RD&D will be needed to determine the pretreatment necessary and the optimum type of feed system needed to enable reliable feeding of these biomass types to these high pressure gasification systems.

Biomass gasification using high temperature and pressure entrained flow gasifiers would be preferable to eliminate tar and methane formation from the biomass. Also the CBTL plants would be simpler and less costly if the same gasifier could be used to process both the coal and the biomass. Separate feed systems for coal and biomass may also be preferable so that, if there are problems with the biomass feed system, the gasifier can be kept in operation using coal. Another potential option is separate gasification of biomass. This option is out of the scope of this report but will be considered in future work.

The study examined multiple scenarios for the ramp up to the production goal of 100,000 BPD. The timeline ranges from an aggressive scenario with 7 CBTL plants producing 100,000 BPD by 2016, to a more conservative scenario of not attaining the production goal until 2026.

The aggressive scenario assumes multiple plants being constructed simultaneously, with the first two being small 7,500 BPD facilities that use corn stover. Future plants would be larger in capacity (up to 22,500 BPD) and use mixtures of switchgrass, corn stover, and woody biomass.

(A hat-tip to Laurens and Biopact!)



Roger Arnold

Note that the presumed low carbon footprint requires capture and sequestration of the CO2 from the CBTL process. That part might or might not happen.

The military interest in synthetic fuels is understandable. Fuel is the lifeblood of the modern military, and assurance of supply is always a top concern. FT synthetic fuels offer the very desirable property of being indefinitely storable.

In the future, it's likely that nuclear-powered aircraft carriers will include their own fuel production plants, using CO2 extracted from the air and hydrogen extracted from sea water. An ironic combination of green and black technologies.


If the coal and biomass are gasified together, the ashes will be mixed also. The ashes of biomass contain the anorganic salts of the biomass, mainly K, Na, Ca, P, Fe, Mg,... which could be returned to the fields.
The ashes of the coal contain also Cd, Pb, Hg, U,... which should definitely not be thrown on the fields.
I would prefer not to mix it.


I thought of the same thing, but after consideration I doubt that flattops have enough room to hold the required equipment.  You also have to balance the shorter supply line against the vulnerability of your fuel source by putting it in the middle of a battle group.


So what is wrong with just biomass gasification?, why do they need coal in the process? Or did I read it wrong and they are just trying to make a gasifier that can use either coal or biomass together or individually.


Viva USAF! There's enormous untapped potential with the USAF research labs that could be unleashed on other renewable energy issues.

At the moment, ecology is not an argument for investing in alternative fuels (sadly) ; national security is. Since coal is cheaper than biomass, the economics dictate using coal. Also, coal is more secure than biomass. Because public opinion would turn against dirty coal, they want to make it a little greener by lowering the carbon footprint to the same level as petroleum-fuels. All the army wants is a secure fuel-source. Once this kind of installations is operational, all it takes to make it greener is producing more biomass and making it cheaper (or making coal more expensive). If 100% biomass is used, together with the proposed carbon sequestration, a net carbon sequestration can be obtained by using biofuels. In that case, we can clean the atmosphere by burning fuel. (provided enough biofuel can be produced, which is way-of at the moment)

Rafael Seidl

If the CO2 is clean enough, it could be used to support intensive algaculture, especially in summer. This would reduce the required CO2 sequestration capacity. Turning waste CO2 from coal into biofuel indirectly sequestrates carbon in that it displaces petroleum distillates in the market - IFF the price is right. That means more of the carbon already in the ground in the form of crude oil stays there.

Algaculture requires a lot of land, but there are over 1000 superfund sites in the country - more than a few of them former military installations. Siting racetrack ponds with sealed bottoms there would put this otherwise worthless contaminated land to good use and reduce clean-up costs: in-situ bioremediation is relatively cheap but usually takes a long time.

Jim G.

Producing "biofuel" from coal flue gas is the equivalent of money laundering with CO2 emissions. The carbon is still moving from coal seams deep in the earth out to the open air, but by the time it meets the consumer, it appears to him to be biologically sourced, renewable green fuel.

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