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
24 November 2010
|
|
| 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.
Resources
Hari C. Mantripragada and Edward S. Rubin (2010) CO2 reduction potential of coal-to-liquids (CTL) process: Effect of gasification technology (GHGT-10)
I think you are wise to use compressed NatGas rather than coal; the ash products left by processing coal is nasty stuff and there are no plans to contain it but by piling it up as a mountain, just waiting for a change to pollute the watersheds.
Posted by: Lad | 24 November 2010 at 11:13 AM
I don't think what is left can be returned to the land. There is so much mercury and other harmful substances, the plants are more likely to die than grow.
Posted by: SJC | 24 November 2010 at 01:24 PM
China is extracting uranium from the ash and using it to maje electricity. ..HG..
Posted by: Henry Gibson | 25 November 2010 at 05:10 AM
The mercury can be extracted and partially collected. Volcanoes put out CO2 and SO2 and mercury. Humans are not the only thing to put stuff into the air and on the ground. Nature, besides humans, puts 30 times as much CO2 into the air and about an equal amount of mercury. From the statistics of an ever increasing population, whatever humans are doing seems to be healthier for humans than ever before when only a few thousand of them were runnning away from wolves in the forests.
All humans have radioactive potasium in them and always have had it along with all other live things. It is not possible to get rid of all the dangers of life, and if anybody wants to start doing it anyway they should start with automobiles or cigarettes. And then build caves powered with fission power plants away from the sun and wind and cold and bugs and waves and volcanic ash and automobiles etc. ..HG..
Posted by: Henry Gibson | 25 November 2010 at 05:24 AM
Ash problems are one reason to use a full-slagging gasifier; the solidified slag pellets are far less leachable than fine ash and cannot create problems like the flood in Tennessee.
I've long said that CCS from syngas will be cheaper than from combustion gases, though I haven't had time to do the full analysis. It's good to see this done.
Posted by: Engineer-Poet | 25 November 2010 at 08:39 AM
How is plant emission reduced with CCS? CO2 emission is not really reduced and does not disappear with CCS. It is simply captured and sent somewhere else. There is NO real reduction of emissions. Clean coal is a joke.
Posted by: HarveyD | 25 November 2010 at 08:49 AM
The most likely GTG plants will be as front ends on ICGCC electric generation plants.
These are the both cleanest fossil fueled electric generation plants, and also the highest most thermally efficient plants existent.
They turn coal into gas & steam, making water gas, extract the gas, cleanse it of things like ash, mercury and CO2, and then burn the cleansed gas, cleaner than NG,in a gas turbine, and extract additional energy from the combined cycle, by exhausting the gas turbine into steam turbines.
In such CTG, water is is a necessary ingredient, to the gasification process, and I doubt the Shell process would help much, or at all.
As NG supplies increase from tapping Shale gas and eventually hydrates, there is less of a demand for CTG processes, but more for CTL conversion. For such processes the Shell process may apply.
Posted by: Stan Peterson | 25 November 2010 at 09:09 AM
The argument that "nature causes pollution also so why shouldn't we do it" falls weakly on the ground. The idea of dodging all the environmental dangers by hiding seems a bit disingenuous and a far-fetched amplification of the argument.
We in the U.S. have a pool of energy that is far safer for our environment that coal...NatGas...Use it instead of coal until we are independence from foreign-oil and DirtyCoal(one word) and have sufficient clean energy resources in place to reduce our need for NatGas.
DirtyCoal must be phased out of our lives as soon as possible.
Posted by: Lad | 25 November 2010 at 09:20 AM
HarveyD, if you call sending a pure (98-99%) CO2 steam to deep saline formations 2km underground emission, then yes, there is no reduction of emissions. But the idea is to reduce the emission of CO2 into the atmosphere!
So the carbon came from the underground and is sent back to the underground - with some oxygen added (with no effect on the atmosphere).
As for safe storage for millennia that is not even relevant. The objective is to reduce the concentration of GHG in the atmosphere while utilizing cheap and reliable resources. Should the sequestered CO2 somehow slowly seep back to the surface, it is a shame for the wasted effort, but not a problem for the atmosphere.
I agree that humans should not pollute. But there is no pollution coming out of a coal plant with CCS. The fly ash (pulverized coal plant) can be used in cement and slag (from gassifiers) can be used as road-fill and is otherwise inert.
It's going to take some decades to learn how to truly cope with renewable energy as the only source of power. In the mean time we will likely need some dispachable fossil power to balance wind energy, and nuclear to serve as base load for solar energy. Using coal (and gas) with CCS along with fast-starting gas turbines, nuclear and renewable energy will drastically reduce the CO2 (and pollutant) emissions to the atmosphere.
We need solutions, not zealotry.
Let dirty coal be a thing of the past and clean coal part of the future!
Posted by: Thomas Pedersen | 25 November 2010 at 10:56 AM
Good discussion.
"when the CO2 price exceeds $12/tonne."
They talk about a price for carbon, which is a factor.
Some day we may need a source of CO2 rather than extract it from the air. If we put it in spent natural gas wells we know where to find it.
Posted by: SJC | 25 November 2010 at 11:05 AM
"Nature, besides humans, puts 30 times as much CO2 into the air and about an equal amount of mercury." HG
Sometimes Henry's logic suggests a view beyond human norms. Thank you Henry.
No one suggests that because "nature causes pollution" man should also. Rather, it is illuminating to consider natural emissions an integral part of nature itself. A nature evolving over the course of 4 billion years and which will be around long after man is not.
Posted by: Reel$$ | 27 November 2010 at 09:37 AM