Abstract:Bringing down the cost of SO2 and NOx removal

A twist on an old technique, flue gas recirculation, helps prevent slagging in the upper furnace and convective pass, according to pilot testing recently completed by APTECH CST and the Southern Research Institute. The technology—along with a companion technology for furnace sorbent and urea injection for SO2 and NOx control—could help owner/operators of smaller, older coal-fired plants meet emissions limits at a reasonable cost.Last year, POWER described APTECH CST’s novel technology that promised significant reduction in key air emissions (see April 2007 “Focus on O&M”); the company said that at least 50%, and possibly up to 80%, reduction in SO2 and NOx could be achieved. In this article, we review the details and results of initial pilot-scale testing. The results met all of the company’s expectations when testing Powder River Basin (PRB) and Galatia coals. Testing continues, and a full-scale project is planned for operation in 2009.
Technology push meets regulatory pull
Air pollution control authorities worldwide continue to tighten power plant and industrial boiler emission standards for SO2 and NOx emissions, and U.S. federal regulatory proposals for controlling mercury, although recently placed in limbo by the courts (see POWER, September 2008, “EPA’s air program: Still hazy after all these years”), are sure to have a return engagement next year. As a result, the need for increasingly effective—and cost-effective—control technologies is a given.
Sulfur in the fuel is the source of sulfur oxides. The combustion process unavoidably results in the production of sulfur dioxide (SO2) and sulfur trioxide (SO3). The favored approach for reducing sulfur emissions is either to burn low-sulfur coal (thus reducing, but not eliminating, emissions) or pass the flue gas through scrubbers using flue gas desulfurization (FGD) technology.
But current FGD technologies are expensive—$300 to $450/kW is becoming the norm (see POWER, July 2007, “What’s that scrubber going to cost?”)—and often cost more than the original plant. FGD units also have a relatively large footprint, and, in the case of wet FGD, an additional land commitment is needed for the ponding, storage, and stabilization of FGD by-products. Some plants are under the gun to install an FGD, but they just don’t have the space available.
1. Injection and recirculation are the keys. Generalized arrangement of a boiler outfitted with APTECH CST technology. Actual configuration is based on site- and fuel-specific parameters. Source: APTECH CST
Staging and limiting peak flame temperatures by installing low-NOx burners and/or recirculating flue gas to the burners is the favored approach for limiting the production of NOx in coal-fired boilers. Though this approach does achieve a limited reduction in NOx, it is lacking when fuel flexibility and other future requirements are considered.
Selective noncatalytic reduction (SNCR) involves the injection of urea or ammonia into the flue gas to remove NOx, achieving up to 50% NOx removal. When SNCR is not enough, selective catalytic reduction (SCR), at a cost of perhaps $200 to $300/kW (see POWER, Jan./Feb. 2006, “Estimating SCR installation costs”), must be used.
There are numerous vendors in the field of SO2, NOx, and mercury control, but none has offered an effective, low-cost technology to simultaneously remove all three pollutants from industrial and small to midsize utility boilers—until now, thanks to APTECH CST’s patented and patent-pending technologies for emissions reduction (CST is short for Clean Stack Technologies).

Taken from: http://powermag.com/environmental/Bringing-down-the-cost-of-SO2-and-NOx-removal_1435.html Powermag.com