With operators already facing stricter emissions standards imposed by the Air Quality Standards Act, last October the U.S. Environmental Protection Agency instituted the Compliance Assurance Monitoring (CAM) rule under the Clean Air Act. Among other things, CAM establishes minimum thresholds for emission testing, and requires the owner/operator of a stationary source to monitor the performance of emissions control devices themselves. So, not only are the regulations compelling operators to apply pollution control on their equipment to meet tougher standards, but they are mandating operators to take corrective action on any emissions control equipment that does not perform up to snuff.

Although the federal government has instituted minimum emissions thresholds, lawmakers left the door open to allow state governments and/or state agencies impose more stringent requirements by enforcing lower minimum threshholds. A few states have already clamped down with tougher standards-most noticeably California-and others will no doubt follow suit. All signs indicate that controlling exhaust emissions is quickly becoming a standard operating practice for the oil and gas industry.

When it comes to environmental regulation, of course, the industry has recognized the value of acting proactively, making moderate investments today in order to preempt potential problems that could prove very costly later. The best time to implement an emissions control solution may well be right now. But what can a compressor operator do today to help ensure that his equipment will meet tomorrow's increasingly stricter emissions requirements?

CRB Technology

One solution is controlled rapid-burn (CRB) precombustion chamber technology. When used in conjunction with a turbocharger, which supplies additional air for leaner fuel/air mixtures, the CRB precombustion chamber modifies the internal combustion process to completely burn the fuel fed into the cylinder and discharge fewer pollutants into the atmosphere.

When igniting very lean air-to-fuel ratios, much more energy must be available to light the lean mixture, and provide dependable and stable combustion. Precombustion chambers provide several orders of magnitude more energy than standard multispark or continuous-duration ignition systems. They ensure that the engine will fire at exactly the desired point of ignition (optimally 10 degrees before top dead center), and that enough ignition energy is available to fully burn the fuel on each cycle.

The advantages of the technology include:

* Reduced emissions;

* Enhanced engine operating efficiency, eliminating cylinder misfires;

* Potential reduced fuel consumption in some cases;

* Complete retrofit capability (Precombustion chambers are available for virtually all models of integral compressor engines, and components are specifically designed for each model and application.); and

* Lower cost than alternative clean-burn conversions, including rebuilding engines with new cylinder heads and other major components.

The fact is that with the new emission standards, a significant portion of the compressors in service today require some type of emissions control. However, as any compressor operator knows, there is a fine line between reducing emissions and maintaining performance.

The worst-case scenario for an operator is to invest in upgrading a compressor engine with emissions control equipment, only to have that equipment negatively impact the compressor's ability to move gas. As demonstrated in actual field applications, with CRB precombustion chamber technology, operators do not have to sacrifice engine performance in order to comply with low emissions regulations.

Southern California Gas

In 1995, Southern California Gas Company performed comparison testing between three emissions reduction packages on a Dresser Clark TLA-6, 2,000-horsepower gas compressor engine at its South Needles transmission station in the Mojave Desert. The testing was prompted by the Mojave Desert Air Quality Management District's (MDAQMD) Rule 1160, which established emission limits of 2.0 grams per brake horsepower-hour of oxides of nitrogen (NOx) and 1.2 grams/bhp-hour of volatile organic compounds (VOCs).

Time was also a factor. The short deadline the MDAQMD established for complete conversion meant the three companies supplying the clean-burn packages for testing had to have production packages available as soon as the company made its selection. In addition, Southern California Gas demanded the best fuel economy possible after conversion was complete.

The three systems tested were the CRB precombustion chamber, a multistrike ignition system providing multiple spark strikes per revolution, and an extended-duration spark system utilizing a high-frequency, high-energy ignition spark that could be programmed to fire continuously up to 45 crank-angle degrees.

The CRB precombustion chamber test package also featured high-pressure "high-flow" fuel injection valves. These cartridge-type valve assemblies replaced the existing fuel valves without modifying the engine, enhancing fuel delivery and improving in-cylinder fuel/air mixing for more efficient combustion. The basic theory behind the multistrike system is to provide more opportunities to fire the lean fuel/air mixtures in the cylinder. The same is true for the extended-duration system, with the additional advantage of providing a higher energy source than a traditional spark.

Testing was performed on the No. 2 engine at South Needles, a turbocharger TLA-6 two-stroke engine. Test data were gathered through three sources: an emissions source testing van, a PFM 2000 engine analyzer, and Kaye Industries' DIGI IV engine controller. Pacific Environmental Services was contracted to provide the testing van, which contained analyzers to measure NOx, oxygen, carbon monoxide, and total hydrocarbon concentration levels. VOC levels were determined using a modified EPA Method 25.1.

The PFM 2000 provided real-time, in-cylinder pressure information. Installed on the power cylinders, the pressure curves provided information on combustion stability and engine balance. Installed on the compressor cylinders, the pressure curves helped verify engine load data. Engine parameter and fuel usage data were taken directly off the DIGI IV panel. The DIGI gathered information provided by various thermocouples, pressure transducers, flow meters, etc., and used the data to record engine operating conditions and control operating parameters such as air manifold pressure (AMP).

Because the goal of the testing was to determine which system performed best in this specific application, the test plan was simple and flexible. A test point consisted of three, five-minute runs. During each run, emission levels were recorded continuously, and engine parameters were recorded once a minute. The PFM was run through the combustion cylinders once during each test point to determine engine stability, and through the compressor cylinders to confirm the horsepower as indicated by the DIGI IV.

Test Parameters

In order to determine the performance and emissions levels, Southern California Gas tested the affect of changing air manifold pressure, ignition timing, and engine load. At rated load (2,000 horsepower), AMP was tested from 14 to 19 inches Hg, and ignition timing was tested from 8 to 12 degrees before top dead center. Load was tested at 1,800, 2,000 and 2,200 horsepower. The common reference point between the three systems and the baseline engine configuration was 2,000 horsepower, 11 degrees before top dead center, and 19 inches Hg AMP.

The extended-duration spark system was tested at reference point, and then AMP was increased until combustion stability and heat rate (fuel burned per brake horsepower-hour) became unacceptable. The system performed comparable to the baseline engine configuration at the reference condition.

To date, the company has installed the equipment on six Clark TLA-6 compressor engines in its Mojave Desert operating area. The compliance targets for emission limits established by the MDAQMD's Rule 1160 were achieved for each of the six units converted, demonstrating the effectiveness of CRB precombustion chamber technology. The conversion of the first unit was completed in October 1995, and the conversion of the last unit was in November 1996, allowing Southern California Gas Company to fully comply with Rule 1160 within the prescribed time frame.

Columbia Gas Transmission

Columbia Gas Transmission Corporation installed CRB precombustion chamber systems on two compressors equipped with Dresser Clark TLA-6 compressor engines at its Artemas compressor station in Bedford County, Pa. The engines were installed in 1970 with a rating of 2,000 brake horsepower at 300 rpm. Under Title I of the Clean air Act, the station is classified as a major source of NOx subject to "reasonable available control technology" (RACT) regulation. The RACT determination requires that the engines meet an emission rate of 3.0 NOx grams/bhp-hour. To comply with the RACT permit, Columbia decided to use lean-burn control technology.

Each compressor's turbocharger, exhaust system, engine control system, and ignition system were modified to produce and ignite a lean fuel/air mixture. Both engines have a turbocharger mounted off-engine. The turbochargers were modified with a new exhaust turbine wheels to provide increased air. For this high air-to-fuel ratio application, Columbia considered precombustion chambers the best technology for an ignition source. To avoid the high cost of replacing cylinder heads with new heads featuring built-in precombustion chambers, Columbia began investigating alternative retrofit systems, eventually field testing a high-energy ignition system.

Although the engine could meet the emission requirements using the high-energy ignition system, several operating problems were encountered. Analysis of the engine's combustion cycle showed that all cylinders were exhibiting intermittent misfires, late ignition and detonation, and excessive variation in the location of peak pressures. Additional testing was conducted after modifications were made to the engine's control system and different fuel valves were installed, but satisfactory performance was never achieved. It appeared the ignition system could not consistently ignite the lean fuel/air mixture at the same point in the combustion cycle, and the location of peak firing pressure changed from cycle to cycle.

CRB System Selected

Columbia decided to change out the system to prevent possible mechanical damage to the engine. After evaluating several proposals, Diesel Supply Company's CRB precombustion chamber and high-flow fuel valves were selected based on performance, price and delivery/installation time. The high-energy ignition system was removed, and the new system was installed on both engines. The system includes:

* Screw-in precombustion chambers and check valves

* Auxiliary cooling water and pilot fuel gas systems

* An Altronic CPU-II ignition system

Testing showed acceptable engine operation while meeting emissions requirements over the entire operating range. The significant improvements observed by Columbia were:

* Improved cylinder-to-cylinder balance

* Decreased cycle-to-cycle peak pressure variability

* Decreased cylinder temperature

The CRB precombustion chambers allowed Columbia to use the existing power cylinder heads, which reduced both initial cost and installation time. Installation was completed in only three days, and the converted engines were returned to operation in June 1997. Columbia is very satisfied with this low-cost emissions control system. Both engines continue to operate satisfactorily, and no additional modifications are planned.

Either CRB component-the precombustion chamber or the high-flow fuel valve-can be used alone in applications requiring small to moderate emission reductions. For instance, looking to reduce emissions from seven Dresser Clark TLA-6 compressors to offset emissions for installing a new turbine at the Delhi compressor station in Richland Parish, La., Columbia Gulf Transmission Company installed Wesco-style fuel valve bodies with Diesel Supply Company's high-flow fuel valves. Even without installing CRB precombustion chambers or modifying the turbochargers, NOx emission were reduced by more than 40 percent while providing improved cylinder balance, decreased cycle-to-cycle peak pressure deviations, and no increase in fuel consumption.