NETL Report Concludes CTL Plus Carbon Capture Results in Fuel with 5-12% Less Lifecycle GHG Than Petroleum Diesel; Modest Biomass Additions Lower GHG Further
|Lifecycle GHG emissions of CTL/CBTL/BTL compared to 2005 petroleum diesel baseline. Background colors of the cells represent the crude oil price required for economic feasibility. Tarka et al. (2009) Click to enlarge.|
A new report from the US Department of Energy (DOE) National Energy Technology Laboratory (NETL) concludes that coupling a Coal to Liquids (CTL) process with carbon capture and sequestration (CCS) yields a fuel with 5-12% less lifecycle greenhouse gas (GHG) emissions compared to the average emissions profile of petroleum-derived diesel, based on the US national average in 2005. These synthetic fuels are economically competitive with petro-diesel when the crude oil price (COP) is at or above $86 per barrel (based on a 20% rate of return, in January 2008 dollars, with a carbon price of zero).
Adding biomass to the coal in the CTL process (Coal and Biomass to Liquids, CBTL) can reduce the GHG emissions further, according to the study. A mixture of 8% (by weight) biomass and 92% coal can produce fuels which have 20% lower life cycle GHG emissions than petroleum-derived diesel and which are economically competitive when crude prices are equal to or above $93/bbl.
According to the NETL report, the addition of CCS to CTL is relatively inexpensive, adding only 7 cents per gallon to the Required Selling Price (RSP) of the diesel product. Increasing the percentage of biomass in the feed further reduces the life cycle GHG emissions of the fuel, but also increases capital and operating costs due to the higher cost of biomass feedstock and reduced economies of scale, the NETL report notes.
Diesel produced in a biomass only—i.e. Biomass to Liquids (BTL)—process only becomes economically competitive when the GHG emission value exceeds $130/mt CO2 equivalents (CO2e) and does not result in greater reductions in net GHG emissions than if the biomass were used in a CBTL plant, according to the report.
Based on these findings, it is anticipated that CTL and CBTL with modest biomass percentages (less than thirty percent by weight) would, as a part of the United States’ energy portfolio, provide a balanced solution to the nation’s transportation fuel dilemma, providing affordable fuels from domestic feedstocks, and enabling significant reductions in GHG emissions.—Tarka et al. (2009)
The study evaluated the performance and cost of three types of plants (CTL, CBTL and BTL) in 11 different configurations at capacities up to 50,000 bpd. Feeds of 8, 15 and 30 wt% biomass were evaluated for the CBTL cases.
The study used two types of CCS configurations: a default, or “simple CCS” configuration, and a second “aggressive CCS” configuration in which equipment is added (at additional cost and performance penalty) in order to further reduce CO2 emissions from the CBTL process.
The “simple CCS” is a low incremental cost option for CCS, as it is functionally identical to the “without CCS” cases— CO2 is already captured within the CTL/CBTL/BTL plant as part of the process. Adding a CCS function requires the addition of CO2 compression, transport and storage capital and operating costs. The default option results in the capture of 91% or more of the CO2 produced by the plant.
The aggressive CCS configurations also utilize ATR (auto-thermal reformer) and WGS (water gas shift) reactors to attain increased levels of CO2 capture. The ATR unit partially oxidizes the light hydrocarbons (C2-C4) in the tail gas to CO, producing H2 as a by-product and making it possible to capture carbon which would otherwise be combusted and emitted as CO2. The aggressive CCS configuration can capture more than 95% of the CO2 produced by the plant, although this additional level of capture incurs both an efficiency and cost penalty which in many cases makes this plant configuration not preferred economically, the study found.
|GHG Emissions of CTL/CBTL Plants Compared to Petro-Diesel|
|WTW GHG Emissions
|Change from petroleum||+147%||-5%||-12%||-20%||-33%||-63%||-42%||-75%||-9.2%||-321%||-358%|
|* Plant capacity reduced from 50,000 bpd due to a scenario in which there is limited availability of biomass (4,000 dry tons per day.)|
The study took no credit for soil root carbon, i.e. the accumulation of carbon in the soil and roots of energy crops, as there is some question as to the appropriate accounting method which should be used for this carbon. The authors note that the report therefore may understate the potential GHG benefits of biomass usage. While that has little effect on the overall economic findings, it could result in CBTL fuels which produce net zero GHG emissions with as little as 35-40 wt% biomass, according to the authors.
The study used one feedstock of each type—bituminous coal and switchgrass—to evaluate the CTL/CBTL/BTL processes. These were chosen as representative feedstocks for a Midwest plant location. Other coal and biomass feedstocks will be evaluated in a later study.
EPA and EISA 2007. The NETL findings are in contrast to an earlier analysis by the Environmental Protection Agency (EPA) which found that fuel produced by a CTL plant equipped with CCS had GHG emissions which were 3.7% greater than petroleum-derived diesel fuel, using fuel produced in the year 2017 as a basis of comparison. NETL, however, concluded that diesel fuel from CTL with CCS has life cycle GHG emissions which are 9% to 15% below that of petroleum-derived diesel, when a petroleum base year of 2017 is assumed (as in the EPA study).
The difference is critical for the development of CTL/CBTL fuels.
These preliminary findings from the EPA led lawmakers to insert language into the Energy and Independence & Security Act (EISA) of 2007 to preclude the use of fuels with a higher GHG footprint than those produced from petroleum, effectively discouraging domestic CTL development. This study clearly demonstrates that the use of the EPA feasibility study resulted in a misguided characterization of the life cycle GHG benefits of CTL with CCS.
...In CTL plants, a fuel can be produced which has 5% and 12% less life cycle GHG emissions than petroleum-derived diesel, using carbon sequestration, and sequestration coupled with aggressive capture, respectively. Therefore, CTL with CCS clearly meets the EISA 2007 criteria of producing a fuel with less life cycle GHG emissions than petroleum-derived diesel and federal agencies will be able to procure this fuel.
Furthermore, by co-gasifying a modest amount of biomass—8% by weight—the GHG emissions profile of the fuel is reduced to 20% below that of petroleum-derived diesel. Co-gasification of 15% and 30% (by weight) biomass results in emissions reductions of 33% to 63%, respectively. Additional reductions in GHG emissions can also be achieved through the use of aggressive CCS.—Tarka et al. (2009)
Rentech study. Separately, Rentech, Inc. released summary findings from a third-party lifecycle assessment of the carbon footprint of synthetic fuels to be produced at its proposed CTL Natchez plant. (Earlier post.)
The WTW greenhouse gas analysis of the proposed Natchez facility, which includes CCS, concluded that the fuels from the facility would produce 11% to 23% fewer carbon dioxide emissions than would result from fuels produced from conventional crude refining.
Rentech proposes using petroleum coke as feedstock with the Rentech Fischer-Tropsch process to produce approximately 30,000 barrels per day of synthetic fuels, specialty waxes and chemicals. The facility is designed to capture approximately 80% of the carbon dioxide generated in the production process, which will be sold under a long-term agreement with Denbury Resources for enhanced oil recovery in the region.
Rentech says that it will publish the full study when it is available.
Tarka et al. (2009) Affordable, Low-Carbon Diesel Fuel from Domestic Coal and Biomass (DOE/NETL-2009/1349)