September 28, 2008

Carbon Capture: Coming to a Power Plant Near You?

By Shelley, Suzanne

When it comes to managing criteria pollutants (such as SO^sub X^, NO^sub X^, mercury and particulates), IGCC has an inherent advantage over conventional coal-fired power plants, because it relies on relatively compact, energy-efficient separation systems that capture pollutants from the coal-derived syngas before it is used to fire the gas turbines. The pre-combustion syngas stream has a much smaller net volume - typically just 1/100th that of the post- combustion fluegas of a comparably sized coal-fired power plant - and is already at elevated pressure coming out of the gasifier. By comparison, coal-fired power plants routinely employ larger, more- energy-intensive equipment trains (such as wet scrubbers, baghouses, electrostatic precipitators and selective catalytic reduction [SCR] systems) at the back end of the facility, to capture pollutants from the larger-volume, lower-pressure post-combustion fluegas stream. When it comes to CO2, the pre-combustion capture from IGCC syngas versus the post-combustion capture from coal-fired fluegas provides similar advantages (in terms of the ability to use more-compact equipment trains that provide capital and operating cost savings). Once CCS is required, the typical revised IGCC flowsheet will likely incorporate a catalytic water-gas shift reactor (to convert the CO in coal-derived syngas to CO2), followed by an absorption column (using either physical or chemical solvents) to separate the CO2 from the "shifted" syngas. The concentrated CO2 would then be pressurized for industrial use, enhanced oil recovery or long-term subsurface geological sequestration. The remaining hydrogen-rich syngas stream would then be used to fire the gas turbines.

"The above-ground part of the CCS equation for IGCC is ready," says Amick of ConocoPhillips. "All of the technologies are already in commercial use - just not together in one place. This was one of the demonstration goals of the now-defunct FutureGen project." For instance, EPRI's Phillips notes that the catalytic water-gas shift process followed by solvent-based CO2 recovery has been used extensively since the early 1980s at gasification-based coal-to- chemical and fertilizer plants operated by Eastman Chemical, Ube Ammonia and Dakota Gasification. Similarly, Selexol UOP's widely used absorption-based process - is already being used for CO2 capture and acid gas cleanup in more than 30 industrial installations.

By comparison, the post-combustion capture of CO2 at coal-fired power plants would likely use large-scale solvent-based absorption towers. While proven in other industrial settings on a smaller scale, this approach "has never been implemented at the scale that would be needed for a 500-MW power plant, so there are a lot of unknowns there," says Phillips of EPRI. "That doesn't mean we shouldn't work on post-combustion CO2 capture - it just means we have a lot of work to do."

The cost penalty

CO2 capture - once mandated - will increase the cost of all forms of fossil-fuel-based power generation. However, according to Phillips, numerous studies show that an IGCC plant can capture and compress CO2 at one-half the incremental cost of a traditional pulverized-coal-fired power plant.

"The ability to capture CO2 from the pre-combustion syngas using commercially proven technologies provides operational and cost advantages, and makes IGCC the lowest-cost, most-efficient option for capturing CO2 compared to any other type of fossil fuel-based power generation," adds GTC's Childress.

Specifically, according to EPRI, 90% CO2 capture would initially increase an IGCC facility's cost of electricity (COE) by 35-40% - but would increase the COE of a coal-fired facility by roughly 70%. Similar data from DOE/NETL are shown in the table below. Industry stakeholders agree that ongoing technology advances, and the broader commercial deployment of IGCC, will help to drive down the cost of IGCC+CCS in the long run.

Anticipating the inevitability of CO2 with subsurface sequestration in the power-generation sector, GE Energy and Schlumberger Carbon Services (Houston, TX) formed an alliance in May to combine GE's IGCC expertise with Schlumberger's expertise in subsurface geological and geophysical characterization.

Copyright American Institute of Chemical Engineers Sep 2008

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