An Emissions Solution On Two-Stroke Lean-Burn Engines

January 2010 Vol. 237 No. 1

The internal combustion engine (ICE) fueled by natural gas has moved in the direction of efficiency and environmental friendliness with lean-burn technology. Cameron’s AJAX®LE uses a CleanBurn™ prechamber type of combustion system that allows for operation with very stable combustion at lean air fuel mixtures.

The lean air fuel mixture reduces the temperature in the combustion process, which greatly reduces the formation of NOx. The lean mixture also provides a more complete combustion process resulting in reduced CO and hydrocarbon levels in the exhaust.

However, in recent years stationary engines have been faced with increasingly stringent air quality regulations. Although lean-burn technology has lowered exhaust emissions, further reductions are now required. One common method for reducing such emissions is the use of a catalytic converter in the exhaust system of the engine. This has been proven effective on 4-stroke lean-burn engines, but until recently the 2-stroke lean-burn (2SLB) engine was only able to demonstrate desired removal efficiencies for short periods of time. Cameron has developed the oxidizing catalytic converter for the AJAX LE 2-stroke engine, lowering carbon monoxide, formaldehyde and VOC emissions for long-term emission removal efficiencies. This provides a solution to further reduce NOx levels with the AJAX exhaust expansion chamber.

Federal And State Regulations
National Emissions Standards for Hazardous Air Pollutants (NESHAP) are emissions standards set by the federal Environmental Protection Agency (EPA). The current standard requires the use of Maximum Achievable Control Technology (MACT) to achieve the following requirements for 2SLB engines:

1. A 58% reduction in CO through a catalytic converter (where CO serves as a surrogate to indicate satisfactory removal of formaldehyde (H2CO) from the exhaust), or
2. Limit the formaldehyde in the exhaust stack to 12 ppmvd at 15% O2 in the exhaust.

This applies to engines rated at +/-500 BHP. The H2CO levels for engines rated at less than 500 BHP have recently been restricted via compliance with NSPS regulations because the regulation of volatile organic compounds (VOC) acts as a surrogate to indicate satisfactory removal of H2CO. New Source Performance Standards (NSPS) are control standards issued by the EPA for criteria pollutants. The standards set forth by the NSPS effective July1, 2008 require Reciprocating Internal Combustion Engines (RICE) rated at 100+ BHP to achieve NOx levels of < 2.0 gm/BHP-hr, CO < 4.0 gm/BHP-hr and VOC < 1.0 gm/BHP-hr. New regulations to take effect July 1, 2011 require RICE rated at 100+ BHP to achieve NOx levels < 1.0 gm/BHP-hr, CO < 2.0 gm/BHP-hr and VOC < 0.7 gm/BHP-hr. Many states require NOx < 2.0 gm/BHP-hr now and a few states such as California and Wyoming already require NOx levels < 1.0 gm/BHP-hr. Testing
Cameron’s oxidizing catalytic converter is capable of removing more than the required carbon monoxide, formaldehyde and VOC levels with a long duration between cleaning of the catalyst elements for 2SLB engines, which was previously achieved only with four-stroke stationary engines. Successful emissions removal was first confirmed during a 550-hour R&D lab test of the AJAX® oxidizing catalytic converter with a DPC-2802LE engine rated at 384 BHP (286 KW) and 440 RPM.

Next, the catalytic converters were placed into service with two 700 BHP engines at a gas transmission plant in Colorado. To ensure continuous compliance with the emissions regulations, it was guaranteed that the CO removal efficiency with the original catalyst elements would not fall below 60% during the first six months of operation.

The challenges associated with the use of converters on 2SLB engines include catalyst fouling from the products of combustion of the force-fed cylinder oil, relatively low exhaust temperatures and occasional rapid pressure fluctuations in the exhaust system. Two major advancements implemented to enhance the commercial viability of the converters include:

1. Development of a catalyst box that can be retrofitted to existing exhaust silencers.
2. Formulation of a lube oil that avoids catalyst fouling and which can be successfully used for cylinder and crankcase lubrication.

Cameron standardized on one catalyst element for the entire line of AJAX® engines. This was done by sizing the common catalyst element based on one-half of the exhaust flow for the test two-cylinder engine. The result was one catalyst element per power cylinder for all of the engines involved. The catalyst elements are designed like desk drawers which slide into the catalyst rack (Figure 1).

Figure 1: View of Catalyst box

See Figure 2 at article head.

Initial Field Application
More than 95% of the applications for this kind of engine are the vertical configuration (Figure 2), but an application in Colorado had horizontal exhaust silencers, which the customer wanted to use for the field-converter project. Therefore, a horizontal version of the silencer/converter was designed ( Figure 3). The configuration required the catalyst to be mounted on top of the horizontal section, downstream from the second volume chamber of the silencer.
Figure 3: Field Site - Two Horizontal Converters.

For these silencer/converter units, the exhaust pipes enter the first volume chamber of the silencer between the building wall and silencer. The rectangular box on top of the horizontal section contains the catalyst elements and serves as the third volume chamber of the silencer. The pipes entering the building above the silencer/converter units are for intake air. At 5,600 feet, the site rating for these engines is 702 BHP at 440 RPM.

Field Monitoring Requirements
The key operating parameters which need to be monitored include engine RPM and BHP. The other data to be recorded include:

  • Pre- and post-catalyst CO.
  • Pre- and post-catalyst H2CO.
  • Post-catalyst NOx.
  • % oxygen in the exhaust.
  • Pre- and post-catalyst exhaust temperatures.
  • ?P across the catalyst.

The above data have been measured and recorded many times during field testing with the silencer/converter units. During the entire operating period (over five years with two of the converters), the converters have been removing more of the emissions than the guaranteed reduction levels.

The CO removal efficiency degraded from 98% to 93% during 15,000 operating hours. The H2CO removal efficiency dropped from about 95% to about 87%. The pre-catalyst temperature is well above the MACT requirement of 450ºF, which is important because the lab testing indicates that the best removal efficiencies are for exhaust temperature of > 500ºF. The catalyst ?P increased by only a couple of tenths of an inch of water during the first 20 months.

The NOx levels of 0.5 to 0.8 gm/BHP-hr are substantially less than the standard NOx levels at the design rating for these engines. If these engines were running at the site rating of 440 RPM and 702 BHP, then the expected NOx would be 1.8 gm/BHP-hr. However, these tests were conducted at about 92% of the site rating at which point the expected NOx would be about 0.8 gm/BHP-hr.

Since this initial field test, numerous additional silencer/converter units have been installed with 2SLB AJAX engines in multiple states and various applications. Performance has exceeded expectations.

The AJAX LE engine-scavenging process removes the residual gases from the previous cycle out of the power cylinder, pushing these gases into the exhaust pipe. It also fills the power cylinder with fresh air prior to the closing of the intake and exhaust ports, after which fuel is directly injected into fresh air that is in the cylinder. With the standard engine, immediately after the residual gases have been scavenged from the cylinder, there is some loss of fresh air through the open exhaust ports.

Figure 4: Expansion Chambers.

The exhaust expansion chamber (Figure 4) was developed for the AJAX engine to speed up the process of removing the residual gases from the cylinder and to avoid the loss of fresh air into the exhaust after the residual gases are scavenged. The result is a large increase in the mass of air trapped in the power cylinder, which further reduces the NOx level in the exhaust. Figure 5 shows expansion chambers on a DPC-2804LE operating at a high elevation site in New Mexico.

Figure 5: AJAX 2804LE with Expansion Chambers in New Mexico.

The expansion chamber features diverging and converging sections of pipe using exhaust pulsations to optimize the scavenging of the power cylinders while providing a substantial increase in fresh air trapped in the cylinder. The results are improved performance and cooler combustion, which reduce the NOx levels. This process is accomplished strictly by the shapes and lengths of the various sections of the exhaust system. This emissions control technique is built into the design of the exhaust system and it adds no moving parts or maintenance requirements to the package.

Conclusion
Based on the analyses of data from the lab and field-test sites, it is concluded that Cameron’s oxidizing catalytic converters can be applied effectively to 2SLB engines to meet current NSPS standards. When combined with Cameron’s expansion chamber, results are better than required by even the 2011 NSPS standards. The amount of emissions reduction is acceptable for nearly all gas-plant sites and the duration between cleaning and/or replacement of the catalyst elements will be acceptable to nearly all engine users.