Study finds that dry-feed gasification for coal-to-liquids is more efficient, lower-emitting and cheaper than slurry-feed; CCS cost-effective for reduction of CO2
|Comparison of coal consumption and CO2 emissions for co-production and separate production of liquids and power. The liquids output capacity of CTL plants is 50,000 barrels/day. Source: Mantripragada and Rubin. Click to enlarge.|
A study comparing the performance, greenhouse gas emissions and costs of Fischer-Tropsch (FT) coal-to-liquids (CTL) processes using two different coal-feeding methods—slurry (based on GE gasifier designs) and dry (based on Shell gasifier designs)—found that dry-feed gasification is more efficient, emits less CO2 and has lower capital and product costs compared to a CTL plant using a slurry-feed GE gasifier.
The main reason for the better performance and economics of the dry-feed [Shell] system is the larger fraction of CO and H2, the main reactants in a FT reaction, in the gasifier products compared to the GE system. For both cases, the cost of liquid product from both a liquids-only plant and for a plant that co-produces electricity are comparable to crude oil prices seen in the past two-three years, according to the paper presented at the recent international Greenhouse Gas Control Technologies (GHGT) conference by a team from Carnegie Mellon University’s Department of Engineering and Public Policy.
Conventional CTL plant gasifies coal to produce a syngas which is then converted in a Fischer-Tropsch reactor to products. In a liquids-only CTL plant, unconverted syngas from the FT reactor is recycled to the reactor to increase the production of liquids. A co-production scenario—yet to be commercial—would take unconverted syngas from the FT reactor and combust it in a combined cycle power plant to generate electricity that is sold to the grid.
Although co-production plants are much more costly than liquids-only configurations in terms of capital cost, Hari Mantripragadaa1 and Edward Rubin found, because of the high electricity revenues the cost of liquid product is lower than that of the liquids-only case, at market prices of electricity. Co-production is also much more efficient than the separate production of liquids and power and the difference in efficiency increases with the addition of CCS.
Plant-level CO2 emissions can be greatly reduced by using the CCS technology, the study found, without much increase in capital cost. Even with CCS, the liquid product costs are comparable to recent crude oil prices.
For a liquids-only configuration, CCS is a cheaper option when the CO2 price exceeds $12/tonne. However, for a co-production plant, the CO2 price has to be more than $35/tonne to make CCS cost-effective, when the electricity prices are in the range of $0 - $100/MWh. Without CCS, there will be a huge increase in CO2 emissions. Thus, even though co-production plants mitigate financial risk, they also cause environmental risk in the form of increased CO2 emissions, unless there is a sufficiently high price on CO2 emissions.
However, co-production is more efficient than the separate production of liquids and power and, the difference in efficiency increases with the addition of CCS. This advantage can be utilized to achieve a net reduction in overall CO2 emissions.—Mantripragada and Rubin
GHGT-10 took place 9-23 September 2010 in RAI, Amsterdam, The Netherlands. The Greenhouse Gas Control Technologies (GHGT) conference series was formed in 1997 following the merger of the earlier series of International Conference on Carbon Dioxide Removal (ICCDR) and the Greenhouse Gas Mitigation options conference. The IEA Greenhouse Gas R&D Programme (IEA GHG) is the organizer of the GHGT conferences which are held every two years. The conference series rotates between, North America, Europe and Asia. GHGT-11 will be held 18- November 2012 in Kyoto.
Hari C. Mantripragada and Edward S. Rubin (2010) CO2 reduction potential of coal-to-liquids (CTL) process: Effect of gasification technology (GHGT-10)