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.
