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Engine Drive Systems: Considerations with Engine Applications - Emissions / Permitting

Even though IC engines are efficient combustion devices, products of incomplete combustion lead to the formation of some hazardous air pollutants (HAPs). The following HAPs have been found in IC engine exhausts at levels above the method detection limits: acetaldehyde, acrolein, 1,3-butadiene, benzene, beryllium, chromium, ethyl benzene, formaldehyde, lead, manganese, mercury, methylene chloride, naphthalene, nickel, PAHs (Polynuclear Aromatic Hydrocarbons), selenium, toluene, and xylene. The concentrations of these HAPs are low, generally less than 30 ppm, with many HAPs very close to the test method detection limits. A large engine could emit about 10 ton/yr of combined HAPs, with formaldehyde accounting for the bulk of the HAP emissions.

There are no specific controls for HAP emissions from existing IC engines. However, non-selective catalytic reduction and oxidation catalyst systems installed on some IC engines to reduce NOx or CO also reduce HAP emissions. About 20% of the existing rich-burn engines are equipped with non-selective catalytic reduction systems. Most of the lean-burn engines are currently uncontrolled for HAP emissions.

The only significant concern applicable to natural gas engines is NOx emissions. The NOx emitted by natural gas engines is primarily "thermal NOx" which is formed when nitrogen and oxygen combine at high temperature. "Fuel NOx", which is formed when burning nitrogen-containing fuels, is never a problem when burning natural gas.

There are two ways to reduce an engine's NOx emissions: lower combustion temperature so NOx is not formed or treat the exhaust to destroy it before it is released. Among the most promising NOx reduction technologies are lean-combustion engines, precombustion chambers, prestratifed charge combustion and Selective Catalytic Reduction.

Selective Catalytic Reduction (SCR) - SCR is normally used with relatively large (>2 MW) lean-burn reciprocating engines. In SCR, a NOx-reducing agent such as ammonia is injected into the hot exhaust gas before it passes through a catalytic reactor. The NOx can be reduced by about 80-95%.

Oxidation Catalysts - promote the oxidation of CO and unburned hydrocarbons to CO2 and water. CO conversions of 95% or more are readily achieved.

The EPA, other Federal agencies, and over 300 state, local and tribal air quality agencies have worked since the enactment of the Clean Air Act to develop an effective partnership to achieve reductions in emissions of toxic air pollutants nationwide. As government co-regulators, EPA's headquarters and regional offices and state, local and tribal agencies recognize the importance of teamwork for rulemakings, implementation and program review activities to ensure a successful program to protect public health and the environment.

As part of these coordination efforts, EPA has developed a website, http://www.epa.gov/ttn/atw/wks/mainwks.html, to share information on community capacity building, tools for understanding local air toxics and improving air quality.