Emissions Control Technology

Selective Catalytic Reduction

Overview

Selective catalytic reduction is the most common aftertreament method used to control NOx emissions from natural gas lean-burn reciprocating engines and natural gas combustion turbines. The lean burn technology is used for higher engine electrical efficiency and lower generated emissions. Lean-burn engines can be controlled to produce 0.5 grams/bhp-hr with no aftertreament controls. However, to meet the California standard and the standards for larger engines in other states, selective catalytic reduction is required. Selective catalytic reduction can cost from $100 - $400/KW, depending on the size of the engine.

Operation

An SCR system consists of ammonia storage, feed, and injection system, and a catalyst and catalyst housing. Selective catalytic reduction systems selectively reduce NO x emissions by injecting ammonia (either in the form of liquid anhydrous ammonia or aqueous ammonium hydroxide) into the exhaust gas stream upstream of the catalyst. Nitrogen oxides, NH 3 , and O 2 react on the surface of the catalyst to form N 2 and H 2 O. For the SCR system to operate properly, the exhaust gas must be within a particular temperature range (typically between 450 and 850 ° F). The catalyst determines the temperature range. Exhaust gas temperatures greater than the upper limit (850 ° F) will pass the NO x and ammonia unreacted through the catalyst. Ammonia emissions, called NH 3 slip, are a key consideration when specifying a SCR system. SCR systems on lean-burn engines operated at constant loads can achieve control efficiencies as high as 90 percent.

SCR on Reciprocating Engine

Natural Gas Turbines

The primary pollutants from gas turbines burning natural gas are nitrogen oxides (NOx ) , carbon monoxide (CO), and to a lesser extent, volatile organic compounds (VOC). Emissions of NOx are in the range of 9 to 25 ppm at 15% O2 with new efficient combustion methods employed on new turbines for industrial applications (generally under 50 MW). When required by the regulatory agencies, these levels can be reduced over 80% with the addition of selective catalytic reduction.

Nitrogen oxide formation is strongly dependent on the high temperatures developed in the combustor. Selective catalytic reduction (SCR) systems selectively reduce NOx emissions by injecting ammonium (NH 3 ) into the exhaust gas stream upstream of a catalyst. Nitrogen oxides, NH 3 , and O 2 react on the surface of the catalyst to form N 2 and H 2 O. The exhaust gas must contain a minimum amount of O 2 and be within a particular temperature range (typically 450 ° F to 850 ° F) in order for the SCR system to operate properly. The temperature range is dictated by the catalyst material, which is typically made from noble metals, including base metal oxides such as vanadium and titanium, or zeolite-based material. The removal efficiency of an SCR system in good working order is typically from 65 to 90 percent. Exhaust gas temperatures greater than the upper limit (850 ° F) cause NOx and NH 3 to pass through the catalyst unreacted. Ammonia emissions, called NH 3 slip, may be a consideration when specifying an SCR system and are often limited by air permitting. Ammonia, either in the form of liquid anhydrous ammonia, or aqueous ammonia hydroxide is stored on site or injected into the exhaust stream upstream of the catalyst.