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SRI developing process for co-gasification of methane and coal to produce liquid transportation fuels; negligible water consumption, no CO2
20 December 2011
|Top: Conventional F-T process. Bottom: SRI process. Click to enlarge.|
Researchers from SRI International (SRI) are developing a methane-and-coal-to-liquids process that consumes negligible amounts of water and does not generate carbon dioxide. Based on data from bench-scale tests, SRI engineers estimate that the capital cost for a full-scale plant using SRI’s process would be less than half that of a conventional coal-to-liquids (CTL) plant using Fischer-Tropsch synthesis (FTS). FTS produces only a small fraction of the hydrocarbons needed for fuel and requires extensive recycling.
The SRI CTL plant design offers a lower CO2-emitting fuel then conventional diesel; a lifecycle analysis by SRI put conventional diesel at 389 gCO2/mile, conventional F-T coal-to-liquids diesel at 830 gCO2/mile; and the SRI synthetic fuel at 326 gCO2/mile (when using carbon-neutral electricity. If biogas is substituted for conventional natural gas, total GHG emissions can further significantly reduced (190 gCO2/mile).
|Lifecycle GHG comparison. Data: SRI. Click to enlarge.|
The cost per gallon for the SRI fuel is higher than F-T fuel, however: a calculated $2.81 per gallons vs. $2.14.
SRI’s new process, developed in response to a DARPA (Defense Advanced Research Projects Agency) solicitation (DARPA-BAA-08-58), is based on the co-gasification of coal and methane. The coal first decomposes into volatiles and char while CH4 is converted into CO/H2 mixtures; the char is converted into CO/H2 mixtures via steam gasification on longer time scales.
The syngas is converted into methanol, which is then processed to make transportation fuels—in the case of the DARPA challenge, JP-8 (military distillate fuel). The use of natural gas (CH4) eliminates the need to add water as a source of hydrogen, reduces the need to add energy to drive the gasification reaction, and results in the use of a smaller gasifier.
As described in a presentation by Ripudaman Malhotra, associate director of SRI’s Chemical Science and Technology Laboratory at the 28th Annual International Pittsburgh Coal Conference, SRI uses process intensification (all C to product) to reduce capital cost; adjusts the syngas ratio to produce CO + 2H2, ideal for methanol; and uses efficient COTS (commercial off-the-shelf) technology for the methanol to JP-8 conversion.
In conventional CTL approaches, energy is supplied by burning a portion of the coal feed, which then produces carbon dioxide. SRI’s approach makes it economical to use carbon-neutral electricity, such as nuclear, hydro, or solar as a source of additional energy.
|Process flow diagram. Source: SRI. Click to enlarge.|
SRI estimates the efficiency of its CTL plant at 67%—significantly higher then traditional CTL plants predominately because it is converting 100% of the carbon feed into product and it utilizes electricity generated off-site. Accounting for the heat rate of generating that electricity from a traditional coal plant would result in a plant efficiency of 47%.
The implications of this research are expansive, including enhancing US energy security through the use of domestic carbon sources. The process can also dramatically reduce the environmental footprint associated with alternative transportation fuels.—Robert Wilson, Ph.D., director, Chemical Science and Technology Laboratory, SRI International
SRI performed a series of analyses to examine the environmental impact of the technology under several scenarios. Based on these analyses, if diesel were produced using biogas as the source of methane, the resulting product would qualify as an alternative fuel under the revised Renewable Fuels Standard of the Energy Independence and Security Act of 2007. The Act requires alternative fuels to meet a standard of 50% reduction of greenhouse gas emissions compared to other fuels.
The work was supported by DARPA under Contract No. HR0011-10-0049.
DARPA solicitation. The DARPA solicitation set goals for a coal-to-liquids process for JP-8 of:
- Process scalable to 100,000 bbl/day
- Production cost of JP8 less than $3.00/gallon
- No CO2 emissions during process
- Water consumption less than 235 kg/barrel
- Capital cost less than $15,000/daily barrel
- (The availability of CO2-free electricity was assumed.)
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