Emission Control of Natural Gas Generator Set
2.5.2 Emissions Control Options
Emissions from natural gas SI engines have improved significantly in the last decade through better design and control of the combustion process and through the use of exhaust catalysts. Advanced lean burn natural gas engines are available that produce NOx levels as low 1.8 lb/MWh and CO emissions of 8.1lb/MWh before any exhaust gas treatment. Adding selective catalytic reduction (SCR) and a CO oxidation catalyst can allow lean burn reciprocating engines to meet the very stringent California South Coast emissions standards of 0.07 lb/MWh for NOx and 1.0 lb/MWh for CO.
NOx control has been the primary focus of emission control research and development in natural gas generator sets. The following provides a description of the most prominent emission control approaches.
2.5.2.1 Combustion Process Emissions Control
Control of combustion temperature has been the principal focus of combustion process control in gas engines. Combustion control requires tradeoffs – high temperatures favor complete burn up of the fuel and low residual hydrocarbons and CO, but promote NOx formation. Lean combustion dilutes the combustion process and reduces combustion temperatures and NOx formation, and allows a higher compression ratio or peak firing pressures resulting in higher efficiency. However, if the mixture is too lean, misfiring and incomplete combustion occur, increasing CO and VOC emissions.
Lean burn engine technology was developed during the 1980s as a direct response to the need for cleaner burning gas engines. As discussed earlier, thermal NOx formation is a function of both flame temperature and residence time. The focus of lean burn developments was to lower combustion temperature in the cylinder using lean fuel/air mixtures. Lean combustion decreases the fuel/air ratio in the zones where NOx is produced so that peak flame temperature is less than the stoichiometric adiabatic flame temperature, therefore suppressing thermal NOx formation. Most lean burn engines use turbocharging to supply excess air to the engine and produce the homogeneous lean fuel-air mixtures.
Lean burn engines generally use 50 to 100 percent excess air (above stoichiometric). The typical uncontrolled emissions rate for lean burn natural gas engines is between 1.5-6.0 lb/MWh.
As discussed above, an added performance advantage of lean burn operation is higher output and higher efficiency. Optimized lean burn operation requires sophisticated engine controls to ensure that combustion remains stable and NOx reduction is maximized while minimizing emissions of CO and VOCs.
Table 2-9 shows data for a large lean burn natural gas engine that illustrates the tradeoffs between NOx emissions control and efficiency. At the lowest achievable NOx levels (45 to 50 ppmv), almost 1.5 percentage points are lost on full rated efficiencyCombustion temperature can also be controlled to some extent in reciprocating engines by one or more of the following techniques:
• Delaying combustion by retarding ignition or fuel injection.
• Diluting the fuel-air mixture with exhaust gas recirculation (EGR), which replaces some of the air and contains water vapor that has a relatively high heat capacity and absorbs some of the heat of combustion.
• Introducing liquid water by direct injection or via fuel oil emulsification – evaporation of the water cools the fuel-air mixture charge.
• Reducing the inlet air temperature with a heat exchanger after the turbocharger or via inlet air humidification.
• Modifying valve timing, compression ratio, turbocharging, and the combustion chamber configuration.
Water injection and EGR reduce diesel NOx emissions 30 to 60 percent from uncontrolled levels. The incorporation of water injection and other techniques to lean burn gas engines is the focus of ongoing R&D efforts for several engine manufacturers and is being pursued as part of the Department of Energy’s Advanced Reciprocating Engine Systems (ARES) program. One of the goals of the program is to develop a 45 percent efficient (HHV) medium sized natural gas engine operating at 0.3 lb NOx/MWh (0.1 g NOx/bhph).
2.5.2.2 Post-Combustion Emissions Control
There are several types of catalytic exhaust gas treatment processes that are applicable to various types of reciprocating engines. Table 2-10 shows the methods in use today, the applicable engine types, and the pollutant reduction achievable.
2.5.2.3 Oxidation Catalysts
Oxidation catalysts generally are precious metal compounds that promote oxidation of CO and hydrocarbons to CO2 and H2O in the presence of excess O2. CO and non-methane hydrocarbon analyzer (NMHC) conversion levels of 95 percent are achievable. Methane conversion may approach 60 to 70 percent. Oxidation catalysts are now widely used with all types of engines, including diesel engines. They are being used increasingly with lean burn gas engines to reduce their relatively high CO and hydrocarbon emissions.
2.5.2.4 Diesel Particulate Filter
While not an issue for spark ignition engines firing gaseous fuels, compression ignition engines fueled by diesel or heavy oil produce particulates that must be controlled. Diesel particulate filters can reduce over 90 percent of particulate (soot) emissions from diesel engines. There are a variety of filter materials and regeneration strategies used. Currently, there are no commercially available particulate control devices available for large, medium speed diesel engines.31
2.5.2.5 Three–Way Catalyst (Non Specific Catalytic Reduction)
The catalytic three-way conversion process (TWC) is the basic automotive catalytic converter process that reduces concentrations of all three major criteria pollutants – NOx, CO, and VOCs. The TWC is also called non-selective catalytic reduction (NSCR). NOx and CO reductions are generally greater than 90 percent, and VOCs are reduced approximately 80 percent in a properly controlled TWC system. Because the conversions of NOx to N2, the conversion of CO and hydrocarbons to CO2 and H2O will not take place in an atmosphere with excess oxygen (exhaust gas must contain less than 0.5 percent O2), TWCs are only effective with stoichiometric or rich-burning engines. Typical “engine out” NOx emission rates for a rich burn engine are 10 to 15 gm/bhp-hr. NOx emissions with TWC control are as low as 0.15 g/bhp-hr.
Stoichiometric and rich burn engines have significantly lower efficiency than lean burn engines (higher carbon emissions) and only certain sizes (<1.5 MW) and high speeds are available. The TWC system also increases maintenance costs by as much as 25 percent. TWCs are based on noble metal catalysts that are vulnerable to poisoning and masking, limiting their use to engines operated with clean fuels (e.g., natural gas and unleaded gasoline). In addition, the engines must use lubricants that do not generate catalyst poisoning compounds and have low concentrations of heavy and base metal additives.
Unburned fuel, unburned lube oil, and particulate matter can also foul the catalyst. TWC technology is not applicable to lean burn gas engines or diesels.
2.5.2.6 Selective Catalytic Reduction (SCR)
This technology selectively reduces NOx to N2 in the presence of a reducing agent. NOx reductions of 80 to 90 percent are achievable with SCR. Higher reductions are possible with the use of more catalyst or more reducing agent, or both. The two agents used commercially are ammonia (NH3 in anhydrous liquid form or aqueous solution) and aqueous urea. Urea decomposes in the hot exhaust gas and SCR reactor, releasing ammonia. Approximately 0.9 to 1.0 mole of ammonia is required per mole of NOx at the SCR reactor inlet in order to achieve an 80 to 90 percent NOx reduction.
SCR systems are considered commercial today and represent the only technology that will reduce NOx emissions to the levels required in Southern California and the Northeast U.S. Still, SCR adds significantly to the capital and operating cost of a reciprocating engine CHP system. As shown previously in Table 2-4, SCR with oxidation catalyst and associated continuous energy monitoring system adds between $150-$700/kW to the capital cost for a lean burn reciprocating engine CHP installation. The cost burden is higher for smaller engines.
2.5.3 Gas Engine Emissions Treatment Comparison
Table 2-11 shows achievable emissions for each of the five representative gas engine systems. The emissions presented assume available exhaust treatment. System 1, the 100 kW engine, is a high speed, rich burn engine. Use of a TWC system with EGR provides NOx emissions of just under 0.07 lb NOx per MWh after credit is taken for the thermal energy provided.32 The Lean burn engine systems use an SCR/CO system providing emissions reduction that meets the CARB 2007 emissions limits without consideration of the thermal energy credit.
With current commercial technology, highest efficiency and lowest NOx are not achieved simultaneously. Therefore many manufacturers of lean burn gas engines offer different versions of an engine – a low NOx version and a high efficiency version – based on different tuning of the engine controls and ignition timing. With the addition of SCR after-treatment, described below, some manufacturers tune engines for higher efficiency and allow the SCR system to remove the additional NOx. Achieving highest efficiency operation results in conditions that generally produce twice the NOx as low NOx versions (e.g., 3 lb/MWh versus 1.5 lb/MWh). Achieving the lowest NOx typically entails sacrifice of 1 to 2 points in efficiency (e.g., 38 percent versus 36 percent). In addition, CO and VOC emissions are higher in engines optimized for minimum NOx.
Note: If you are interested in diesel generator, please contact us by email: sales@dieselgeneratortech.com