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.

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!)

Resources:

• Increasing Security and Reducing Carbon Emissions of the US Transportation Sector: A Transformational Role for Coal with Biomass