Operation of any fuel-fired power generating equipment results in emissions of exhaust gases. Principal among these are carbon dioxide (CO2), water vapor (H2O), oxides of nitrogen (NO and NO2, generally referred to as NOx), oxides of sulfur (SOx), carbon monoxide (CO), unburned hydrocarbons (UHC), and particulates. The environmental permitting requirements for on-site generation impose restrictions on emissions of NOx, SOx, CO, and particulates because of their contributions to smog and acid rain. Regions of the U.S. with significant air quality problems are classified as "Non-Attainment Zones" and severe limits are placed on annual emissions of these pollutants in those areas. As a consequence, requirements for pollution abatement equipment are more stringent in those areas.

The rates of emissions depend on the quantities of fuel consumed, the type of fuel used, and the temperature of combustion. "Thermal" NOx emissions are a consequence of the high combustion temperatures; the higher the temperature level, the greater the formation rate for NOx. This is true no matter what fuel is used. "Fuel based" NOx emissions are negligible in systems using natural gas, but they can be a significant source of pollution when fuel oil is used. SOx formation is a consequence of sulfur contained in the fuel and is insignificant for natural gas but must be considered when fuel oil or other fuels are used. Generally, technologies for reducing NOx and SOx emissions increase emissions of CO and UHCs.

Emission rates for equipment can be reported in ppmv (parts per million, volume), pounds per million Btu of fuel (lb/MMBtu), or milligrams per mega-Joule of fuel (mg/MJ) and they are generally regulated in terms of tons per year. The conversion between units is not entirely straightforward, however, particularly when changing from ppm to lb/MMBtu or mg/MJ. This change is complicated because ppm incorporates the air flow rate which is not the same for all equipment. The amount of air required to oxidize a specific fuel is fixed (stoichiometric requirement), but different engine types use different amounts of "excess" air. Lean burn internal combustion engines may operate with around 100% excess air (200% of the stoichiometric rate) while gas turbines use 300 to 400% excess air; microturbines may use more the 800% excess air.

Emission Control Technologies are a development of technologies to remove SO2 (sulfur dioxide), SO3 (sulfur trioxide), NOx (nitrous oxide), and other particulate matter from coal-fired power plants and other industrial sources. The research focuses on the development, demonstration, and commercialization of technologies to minimize the environmental impact of mercury and other air toxic metal pollutants on the environment worldwide. The task could be implemented by retrofitting the original devices or engines. Retrofitting refers to the installation of non-original parts or devices on a vehicle or device after the engine's manufacture.

The least expensive mechanisms for reducing NOx emissions are based on lowering the combustion temperature to lower thermal NOx. This can be accomplished by injecting water or steam with the combustion air or by specialized designs of the combustion chambers. Exhaust gas treatment can be performed with non-selective or selective catalytic reduction (NSCR or SCR). NSCR causes CO to react with NOx in the presence of a catalyst to form CO2 and N2. In the case of SCR, an ammonia or urea solution is sprayed into the exhaust gases from the power generator where NH3 reacts with NOx in the presence of a catalyst to form nitrogen (N2) and water vapor (H2O). NSCR is commonly used in conjunction with rich-burn IC-engines while SCR is applied more often to gas turbines. Efficient operation of SCR requires careful control of the ammonia spray and the exhaust gas temperature. SCR can add $500 to $900 per kW to the cost of small gas turbines (<5 MW) and on the order of $250 per kW or less to larger turbines. Low NOx burners cost about the same as water or steam injection. Scrubbers can be used to reduce SOx emissions. This is accomplished by injecting calcium carbonate (CaCO3) in the form of a lime or limestone solution with SO2 in the exhaust gases to produce CaSO3 and CO2. Carbon monoxide can be forced to react with oxygen in the exhaust using a catalyst to form CO2. Wet and dry equipment are available to reduce particulates in the exhaust.

SCR and other catalytic processes can be added to reciprocating engine generators to reduce their emissions, as is commonly done with gas turbines. In both cases the reduced emissions come at the cost of increased maintenance and operating costs and may affect operating efficiencies.

Sample emission control technologies are listed below.

Diesel oxidation catalysts are devices that use a chemical process to breakdown pollutants in vehicle exhaust into less harmful components. More specifically, a DOC has a porous ceramic honeycomb-like structure that is coated with a material that catalyzes a chemical reaction to reduce harmful air pollutants. DOCs rarely require maintenance, come with a 100,000 to 150,000 mile warranty, can last for 7 to 15 years, and can be installed on a variety of diesel vehicles. When used with the ultra-low sulfur diesel (ULSD) fuel, DOCs reduce emissions of carbon monoxide by 70 percent, hydrocarbons by 60 percent, and particulate matter by 25 percent. DOCs do not require the use of Ultra-low Sulfur fuel though. Reductions will still be significant when DOCs are used with regular diesel fuel but not as high as with the use of ULSD fuel. In general a DOC cost between $1,000 and $1,600. DOCs take about 1 to 3 hours to install.

Diesel particulate filters are ceramic devices that collect particulate matter in the vehicle exhaust stream. The heat produced by the vehicle exhaust heats the ceramic filter and allows the particles to break down (or oxidize) into less harmful components. Unlike diesel oxidation catalysts (DOC), DPFs must be used in conjunction with ultra-low sulfur diesel (ULSD) fuel. DPFs work best on diesel engines built after 1995. Therefore, knowing the age and type of each engine in the fleet as well as the exhaust temperature is an important part of any retrofit project. Manufactures recommend that DPFs be cleaned about every 100,000 miles. Most DPFs come with a 100,000 to 150,000 mile warranty. A vehicle that uses ULSD fuel and has a DFP installed can reduce emissions of carbon monoxide, hydrocarbons, and particulate matter by approximately 90 to 95 percent. The filters cost from $7,000 to $8,000 per vehicle and work best on engines built after 1995. DPFs typically take about 6-8 hours to install.

Exhaust gas recirculation technology has great potential for reducing emissions of NOx from diesel engines. There are two distinctively different types of EGR available. High-Pressure EGR re-circulates exhaust between the two high pressure points - exhaust manifold and inlet manifold. The Low-Pressure EGR re-circulates exhaust between the two low pressure points - exhaust system tail pipe and turbocharger inlet.

Certain compounds can adsorb NOx in the exhaust gases. Catalysts containing barium oxide were tested and the results showed that adsorption was most effective at temperatures close to 750°F (approx. 400°C). The catalysts have a limited storage capacity and needs to be regenerated in order to regain activity. The regeneration is done under stoichiometric conditions and the adsorbed NOx dissociates to nitrogen. This type of catalyst is interesting for the development of petrol lean-burn engines which run with excess air except at high loads when the engine runs stoichiometric. To obtain regeneration conditions in diesel exhaust gases is a great technical challenge which is enabled by the use of electronic fuel injection and injection of fuel to the exhaust gases. Also, the catalysts tested are sulphur sensitive.

Selective catalytic reduction of NOx means reducing it to nitrogen (N2) and oxygen (O2) over a specially formulated catalyst in the reaction with a reductant. The most commonly used reducing agent is ammonia (NH3) in the form of urea. Although high levels of NOx reductions can be achieved this technology presents several practical challenges - filling and storage of urea, limited distribution network, etc. The technical challenges include complicated urea handling and dosing system, requirement for high temp exhaust for a catalyst to be efficient, control of ammonia slip during transient conditions and large volume of catalyst required.