Air Pollution as a Climate Forcing: A Workshop
Day 3 Presentations
NOx Controls for the Power Industry
Energy and Environmental Strategies for the Clean Air Task Force
With electric generation accounting for approximately 25-35% of total NOx emissions, NOx reductions in the central station, power sector will impact ozone and nitrate production.
There are two categories of NOx emission control, those that minimize the formation of NOx during the combustion process (e.g., smaller quantities of NOx are formed), and those that reduce the amount of NOx formed prior to exiting the stack into the atmosphere. In industry "language" the first is referred to under the "umbrella" of Combustion Modifications; technologies in the second category are termed "Post-Combustion Controls."
Selective Catalytic Reduction (SCR) is a post combustion technology capable of large NOx reductions (90+%) and represents at present the most effective NOx control technology. It uses an ammonia-containing reagent to react with the NOx produced in the boiler, and converts it to harmless nitrogen and water. Capital costs range from about $50 to $100/kW. Operating costs are driven primarily by the consumption of the chemical reagent — usually ammonia.
SCR technology was first used on utility boilers in the 1970s in Japan, followed by extensive deployment in Japan and Germany in the mid to late 1980s. By 1990, the number of commercial units in Japan and Germany had grown to about 200 units or about 40,000 MW of capacity. Most applications in Japan and Germany were designed to achieve 60%-80% NOx removal efficiencies. Technical development/assessment activities in the U.S. geared up at this time as well. In California, anticipating the requirements of Rule 1135, RECLAIM, etc., SCR-related developments for gas-fired applications were underway. The status of SCR for coal-fired boilers in the U.S. made a major leap in the mid 1990s when a number of new coal-fired boilers were permitted and went online using SCR technology, and technical research and innovation have continued.
In the US, market projections suggest many SCR retrofits over the next several years to comply with emissions requirements. For coal-based boilers, it is projected that some additional 80,000 -100,000 MW of SCR capacity will be deployed by 2006-07. For gas-fired generation (gas turbine combined cycle), the equivalent projection is about 75,000-80,000 MW of new SCR. This represents a significant fraction (>30%) of the current fossil generation capacity in the US.
Today gas-fired generation can achieve NOx emissions in the single digit (PPM) level. In the next couple of decades, advanced coal technologies (gasification) will make it possible to achieve similar results for coal-based generation.