Bruce Portz
Western Gas Resources
Denver
Western Gas Resources, Denver, has installed additional compression at its Midkiff, Tex., gas plant within stringent emissions constraints on NOx, CO, and unburned hydrocarbons imposed by state and federal agencies.
To satisfy regulatory ceilings, Western Gas modified older Ingersol KVS (Fig. 1) compressor engines. Emissions of the converted engines compare favorably with the performance of more contemporary units.
Additionally, the conversions were economical compared with competing mechanical arrangements and avoided the fuel and chemical costs associated with other alternatives.
EMISSIONS SOURCE
Western Gas purchased the 1950s vintage Midkiff lean oil plant (Fig. 2) from El Paso Natural Gas more than 4 years ago and converted it to a modern turboexpander operation (OGJ, Mar. 4, 1991, p. 41).
In the meantime, the plant has been recovering liquids in volumes that exceeded design targets. In the last few months, an increase in volumes available to the plant for processing have prompted a need for additional plant and field compression.
The additional volumes of gas to be handled required supplementing the compression at the plant and at one of the outlying booster sites. The additional compression requirement amounted to about 2,000 hp in the main plant and 2,000 hp at the Driver field booster location.
Deciding what type and capacity compressors to add was relatively simple. How they could be added economically within emissions constraints imposed by the U.S. Environmental Protection Agency and the Texas Air Control Board was more difficult.
Midkiff is considered a major emission source for NOx and CO emissions. Under the language of the TACB's PI 7 approval process, holding incremental increases of NOx to less than 40 tons/year, CO to 100 tons/year, and unburned hydrocarbons to 40 tons/year would accelerate the regulatory approval process.
Therefore, Western Gas chose to limit emissions to these levels.
For the size drivers Western Gas required, these limits restricted NOx to 2.0 g/hp hr, CO to about 5.0 g/hp hr, and C3+S to 2.0 g/hp hr per site. Each of these levels allows a safe margin within the PI 7 language.
Several reciprocating engines are available that reliably give these levels of specific emissions. In Western Gas' case, however, the company would not be purchasing new units for these services.
Availability within Western Gas and existence of compressor blocks at both locations made the re use of vintage Ingersol KVS integrals appealing. Use of these engines, however, required that they be cleaned up considerably if they were to fall within emission guidelines.
THREE METHODS
Western Gas began a search for acceptable, economical technology. During late 1992, the company concluded that three basic methods were available to clean up emissions of the old KVS units it was to use.
The methods reviewed were selective catalytic reduction (SCR), catalytic incineration, and stratified combustion technology. Variations on the stratified combustion technology, are available from several suppliers.
AMMONIA USE
Selective catalytic reduction involves fitting a reducing catalyst bed on the exhaust of the engine involved. A controlled amount of ammonia is injected upstream of the bed, resulting in reduced emissions.
The cost of this unit includes the catalyst bed, the NOx analytical and ammonia injection instrumentation, and ammonia storage and handling.
Although this process was quite cost competitive on a capital expenditure basis, three major disadvantages, however, were present:
- Cost and safe handling of the ammonia
- Concerns over ammonia as a hazardous air pollutant
- Concern for the analytical instrumentation required to monitor NOx content of the exhaust and regulate ammonia injection.
Economic and environmental reasons dictate injection of the minimum amount of ammonia. Western Gas found that the instrumentation required continuously to monitor the exhaust stream for low part-per million levels of NOx was expensive, relatively new, and unproven in an application such as Midkiff.
The advantages did not seem to outweigh the disadvantages.
LNCINERATION
Catalytic incineration, another option for emission abatement, requires installation of both reducing and oxidizing catalyst beds in the engine exhaust.
Engine exhaust is combined with fuel gas; this mix is passed over a reducing catalyst bed. Air is added to the effluent from this bed; this mix is then passed over a bed containing an oxidizing catalyst.
This arrangement yields the lowest emission levels of any of the technologies Western Gas reviewed. The capital cost for this installation would have been somewhat more than SCR installation.
Working against this technology was the utility consumption. The annual cost for supplemental fuel required by the converters amounted to about half of the capital cost for the entire initial installation.
Unless some sort of heat-recovery system is incorporated, these supplemental fuel costs will certainly restrict this approach in most applications.
STRATIFIED CHARGE
The last approach Western Gas reviewed involved stratified charge technology. Several companies offer such technology, each with its own variation on the theme.
Basically, a small pocket of rich fuel air mixture is ignited by the spark plug. This in turn ignites the much larger pocket of relatively lean fuelair mixture.
The overall lean mixture leads to relatively low combustion temperatures and consequently low NOx emission levels.
The differences between the different offerings of stratified charge technology involved how the relatively lean and rich mixtures are produced in the engine and how the engine is provided with the large amount of combustion air required.
The most conventional technologies contain the rich mixture in a separate small chamber attached to the cylinder head. When the mixture in this separate chamber is ignited by the plug, it torches into the main combustion chamber igniting the bulk lean charge.
Typically, this type of conversion requires extensive engine head modifications.
An alternative stratified-charge technology Western Gas looked at avoided the complicated head modifications of its competitors.
The rich mixture is produced by feeding a small quantity of fuel through a tube installed in the body of the spark plug. This produces a rich mixture local to the plug and consequently the stratified charge effect without mechanical separation.
This region of rich mixture is ignited by the spark plug and in turn ignites the bulk lean mixture.
Of all the abatement technologies reviewed, stratified charge tended to be the most initially expensive, requiring extensive modifications to the engines and combustion-air supply systems.
In their favor, stratified-charge abatement technologies also tended to be the most economical in the long run because they did not tend to increase fuel consumption or require additional utilities or chemicals.
ECONOMICAL, QUICK
Of the three offerings reviewed, Emissions Plus Inc., Houston, offered the most economical package and one that could be completed most quickly.
Emission Plus' offering did not require the extensive head modifications required by the other two packages. Unfortunately, Emission Plus' was the least proven of the three offerings, although the company had been successful in converting other types of engines.
After much discussion and research, Western Gas finally decided to let Emission Plus perform the conversion of the engines.
The economics of the catalytic processes suffered from mostly chemical or fuel consumption as well as safety and control concerns. Although proven, two of the three stratified charge offerings required expensive, exensive engine modifications and lots of time.
Emission Plus, although unproven in this application, offered an economical package and a track record in converting other types of engines.
The conversion offered by Emission Plus included the following:
- Revisions to the turbochargers substantially to increase available combustion air
- Mechanical modifications to produce the stratified charge in the combustion chamber
- A controller to monitor and regulate the system.
To ensure adequate air to the converted engines, the standard Elliot turbochargers were replaced with German MAN units (Fig. 3). These offered better technical support, larger air flow potential, and more tunability.
Two turbos were required because the KVS engine is fitted with two turbochargers, one at each end.
In order to help these turbos deliver the air flow required, turbo discharge coolers were fitted. Water for these coolers was provided by a new dedicated, closed loop, pumped system.
SPARK PLUG MODIFIED
The heart of Emission Plus' stratified charge arrangement lies in the company's patented precombustion chamber (Fig. 4).
This consists of a modified spark plug fitted with stainless steel tubing and a checkvalve assembly. Fuel to these assemblies is regulated by a programmable controller provided as part of the conversion package (Fig. 5).
Fuel that enters the combustion chamber through this assembly stays in the vicinity of the spark plug providing the rich mixture local to the spark plug.
This rich charge is easy to ignite and upon ignition tends to torch into and fire the lean mixture in the bulk of the cylinder (Fig. 4).
In addition to the fuel introduced at the spark plug, fuel is also fed through the normal poppet fuel valve into the cylinder.
The piping to the poppet valve is also modified slightly to provide more complete mixing of the bulk lean mixture. Better mixing helps to ensure more complete combustion.
The supply to each cylinder's fuel poppet is fitted with a small volume reservoir and a globe valve (Fig. 6).
The intent is that when the fuel poppet valve is closed, fuel flows into and pressurizes the fuel reservoir. The lobe valve is used to control the charging rate to the reservoir.
When the fuel poppet valve opens, the fuel pulses deeply into the cylinder making for a thorough mix. When the fuel poppet valve opens, the fuel reservoir depressurizes quickly while the lobe valve restricts flow sufficient]y to avoid oversupply of fuel.
The required size for the fuel reservoir is calculated for the specific engine, and the position of the globe valve is determined during tuning of the engine.
ENGINE CONVERSION
The actual field conversion of the engines included all the usual work required for a normal engine rebuilding which was undertaken by Western Gas' personnel.
The entire project went smoothly with the exception of repeated expansion joint failures around the turbocharger.
The most serious problem encountered during the tuning phase of the engines involved selecting the correct nozzle rings for the turbo charger. MAN was helpful in offering a range of pieces that allowed Western Gas to tailor the turbos performance to match the requirements of the engine.
The driver unit was set up and tuned before the Midkiff plant's unit; consequently the Midkiff tuning went much faster.
After several months' running, Western Gas is pleased with the performance of the engine. Since this conversion was the first of its kind, the company had initially settled for guaranteed performance corresponding to only 85% of the engine's rated horsepower.
The guaranteed specific emission rates were NOx at 2.0 g/hp hr, CO at 2.5 g/hphr, and unburned hydrocarbons at 1.0 g/hp hr.
As it turned out, Western Gas was able to meet the specific emission rates for NOx and unburned hydrocarbons at 100% of the engine's rated capacity.
It has proven difficult, however, to meet the specific emission rates for CO at full rated capacity. So far, the CO content has run about 4.5 g/hp hr. This rate for CO is considerably higher than guaranteed but corresponds to 100% of rated capacity, not the 85% guarantee basis.
Even at the 4.5 g/hp hr level, the engine is still within the increment allowed by, a PI 7 filing. Western Gas has amended the PI 7 filing.
The engine so far yields specific emission rates as follows: NOx
Spark plug life is running between 1,000 and 1,500 hr. This is in the normal range of expected plug life but far less than the 4,000 hr mentioned in the early stages of Western Gas' discussions with Emission Plus.
This would not normally be a major concern except for the high price of the modified plugs. The price of the plugs is high enough for them to become a major operating cost contributor.
Because the plugs are patented, the options are limited.
