Can ammonia scrubbing cut the cost of CO2 capture?

19 July 2004

Ammonia scrubbing, as used in Powerspan's ECO process for SO2, NOx and Hg, could also be applied to carbon dioxide capture - promising a cheaper alternative to "conventional" MEA technology.

Powerspan and the US Department of Energy's National Energy Technology Laboratory have entered into a co-operative research and development agreement aimed at developing a cost effective carbon dioxide (CO2) removal process for coal-based power plants. The regenerative process uses an ammonia solution to capture CO2 in flue gas and release it for subsequent sequestration; after regeneration the ammonia solution is recycled.

The CO2 removal process is expected to be readily integrated with Powerspan's ECO technology, which uses aqueous ammonia to absorb high levels of sulphur dioxide (SO2), nitrogen oxides (NOx), and mercury. The CO2 processing steps are situated downstream of ECO's SO2, NOx, and mercury removal steps.

Ammonia scrubbing of CO2 potentially offers several advantages over the commercially available amine process for CO2 capture from coal-fired power plants. According to research conducted by the National Energy Technology Laboratory and others on the use of aqua ammonia for absorption of CO2:

{blob}The traditional monoethanolamine (MEA) process for CO2 removal has disadvantages of low CO2 loading capacity (kg CO2 absorbed per kg absorbent); high equipment corrosion rate; amine degradation by other flue gas constituents, which requires a high absorbent makeup rate; and high energy consumption during absorbent regeneration.

{blob}By comparison, aqua ammonia has higher loading capacity; does not pose a corrosion problem; does not degrade in a flue gas environment, minimising absorbent makeup; requires much less energy to regenerate; and costs much less than MEA.

{blob}The aqua ammonia process compared with MEA:

{indent}demonstrated CO2 absorption capacity up to two times greater;

{indent}required 49% to 64% less heat to release CO2 and regenerate absorbent; and

{indent}had reagent makeup costs of approximately one-sixth.

{blob}Removal of other pollutants (principally SO2 and NOx) could be integrated with the aqua ammonia process for CO2 removal.

Although CO2 capture technologies are commercially available for use on coal-fired power plants, they have substantial capital and operating costs. According to a 2002 study conducted by the Electric Power Research Institute and Parsons Infrastructure & Technology Group, a new 490 MW pulverised coal plant with a commercially available amine-based CO2 capture system designed to remove 90% CO2 requires 27% of plant capacity to capture CO2 and compress it for sequestration. Capital costs represent 42% of the cost of the new coal-fired power plant. The 27% power draw for this process is in sharp contrast to commercially available equipment that removes SO2, NOx, and mercury. Combined, these systems require approximately 3-5% of plant power to operate. The incremental cost of power production associated with CO2 removal is $0.04/kWh, and the cost per ton of CO2 removed is $47.

Test plans

The scope of the three-year co-operative research and development agreement includes laboratory testing, pilot testing, and detailed studies of the CO2 capture process economics.

Powerspan has conducted initial laboratory testing of the process, which demonstrated 90% CO2 removal under conditions comparable to a commercial-scale absorber. These test results confirm those previously obtained by the Department of Energy under similar conditions. Further testing at both the Department of Energy and Powerspan facilities under the co-operative agreement will be conducted to fully characterize the process capability and to optimise process efficiency. In particular, Powerspan will perform testing to integrate CO2 capture and regeneration into a continuous operation of the ECO process, demonstrating SO2, NOx, mercury, and fine particulate matter removal in addition to CO2 removal.

The three-year agreement is expected to culminate in a pilot test on an actual power plant that would confirm process design and cost estimates. Both Powerspan and the Department of Energy will generate economic analyses comparing the CO2 capture process with a comparable scrubbing system.

Initial cost estimates indicate that the aqueous ammonia process would cost less than half of the best CO2 capture technologies currently available for use on coal-fired power plants.

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