Call for analysis and quote 916-368-4770

Compact SCR

Compact SCR™ for gas engines

Biogas • Natural gas • Gasified biomass • Propane

Lean-burn spark-ignition engines are reliable, efficient, and economical prime movers for electric generators, pumps, and other mechanical-drive needs up to about 3 MW (4,000 horsepower). By using the waste heat from the engine, combined heat-and-power (CHP) systems can reach overall efficiencies much greater than the best central power plants. That saves money and reduces greenhouse emissions. Even greater savings and emission reductions can be achieved using renewable fuels such as biogas and gasified biomass.

Although lean-burn engines can lower greenhouse emissions, they have relatively high emissions of oxides of nitrogen (NOx), an important contributor to local air pollution or smog. Increasingly strict emission limits have largely stymied the spread of CHP and other distributed generation technologies in California and other states with serious air-pollution problems.

Selective catalytic reduction (SCR) is the most effective NOx control technology for lean-burn engines. Until recently, though, the only SCR systems available were designed for large power plants. These systems were too big, too complicated, and too expensive to be practical for most gas engines.

Engine, Fuel, and Emissions Engineering, Inc. (EF&EE) has pioneered the application of compact, high cell-density SCR catalysts to non-road and stationary engine applications. Originally developed for diesel trucks and equipment, EF&EE’s Compact SCR™ systems are about one-fifth the size and weight and considerably less expensive than systems using conventional power-plant technology.

How SCR works

SCR works by adding a small amount of a reductant chemical to the exhaust upstream from a catalytic converter. Commonly used reductants are ammonia and urea (which breaks down to form ammonia in the hot exhaust). The catalyst in the converter selectively promotes the chemical reaction between the ammonia and the NOx. This reaction converts the ammonia and the NOx into harmless nitrogen gas and water vapor.

The amount of reductant needed for NOx control varies with the amount of NOx in the exhaust. Too little reductant means that some NOx escapes un-reacted, while too much means left-over ammonia in the exhaust (referred to as ammonia “slip”). 

In our Compact SCR™ systems, a programmable controller varies the reductant injection rate to match the engine’s NOx emission rate. A metering pump injects the right amount of reductant into a stream of compressed air that goes to an atomizing nozzle in the exhaust. The compressed air cools the nozzle, keeps the reductant from caking, and aids in atomization. Solid-state NOx sensors upstream and downstream of the SCR catalyst enable EF&EE's Optimin™ self-tuning. In this advanced control method, the controller “learns” and remembers the optimum injection rate, minimizing NOx and ammonia emissions at each engine load point. 

The SCR catalysts also function as oxidation catalysts, thus reducing formaldehyde and volatile organic (VOC) emissions as well. 

Typical Compact SCR™ system sized for 800 kW gas engine

Emissions performance

EF&EE's Compact SCR™ systems are guaranteed to meet “best available control technology” (BACT) requirements for stationary internal-combustion engines under California regulations. They have demonstrated emission levels comfortably below the California BACT limit of 0.15 g/BHP-hr (11 ppmvd) of NOx in engines running on natural gas, biogas, syngas, and LPG. EF&EE can also supply systems guaranteed to meet 0.07 lb NOx/MWH, as required by California SGIP and AB1613 programs, and by SCAQMD Rule 1110.2. Continuous self-monitoring and Optimin™ self-tuning ensure that these low NOx levels are maintained year in and year out, and that any malfunction is flagged before it results in a violation. 


The relatively small size of EF&EE’s Compact SCR™ systems means lower cost for materials and fabrication. That plus the advantages of modular design mean that EF&EE can offer Compact SCR™ systems at prices well below those of competing systems.

Compact size also means a smaller system footprint and lower cost of installation and supporting structure. Technical support for startup and commissioning is included in the equipment price. Solid-state NOx sensors upstream and downstream allow us to verify that the engine is meeting its NOx emissions guarantee, and that we are meeting ours. Customers can be confident that their systems will pass before scheduling the independent source test.

Low Cost of Ownership

Low initial cost would be no bargain if it meant higher costs for operation and maintenance in the long run.  EF&EE Compact SCR™ systems and supporting services are designed to minimize long-run owning and operating costs.  Features include:

  • Self-monitoring. The Compact SCR™ controller performs system checks upon startup, then continuously monitors system performance to identify and flag problems before they result in an air-pollution violation. The system can even be programmed to send e-mail when it finds a problem!

  • Remote access. The system front panel allows password-protected access via LAN and Internet. This allows remote monitoring, diagnosis, and software updates, eliminating the need for most service visits.

  • SCADA interface. The Compact SCR™ controller supports most SCADA protocols, including MODBUS and MODBUS/TCP, allowing ready integration into plant control and monitoring systems.

  • Serviceability. Solenoid valves, electronic modules, injectors, and other components are designed for easy removal and replacement using hand tools.

  • Choice of urea or ammonia solution. For the redactant, Compact SCR™ systems can use 32 percent urea solution (also known as diesel exhaust fluid or AdBlue™) or 19 percent ammonia solution. Urea solution is non-hazardous and easier to store and handle, but ammonia solution is typically less expensive in large quantities.