CONTROL OF INDUSTRIAL ENGINE EXHAUST GAINS IMPORTANCE

Walter R. Taber Jr.
Houston Industrial Silencing
Houston

Modifying combustion chambers and using catalytic converters help reduce emissions from heavy-duty oil field engines. Environmental regulations from the Environmental Protection Agency (EPA) and various state air quality agencies are requiring operators to decrease the amount of noxious exhaust from industrial engines, such as those used on drilling rigs, offshore production platforms, and onshore production facilities.

The strategies to reduce harmful exhaust emissions from combustion engines fall into two broad categories: engine combustion modification and exhaust after-treatment (catalytic conversion).

Historically, government agencies have made emissions regulations more strict over time. Thus, the engine operators should select emission control systems that meet current requirements and can be upgraded to meet possible stricter requirements in the future.

EMISSIONS REGULATIONS

In the 1990 Clean Air Act amendments, the EPA- divided the U.S. into attainment areas and nonattainment areas. The emission requirements are stricter in the nonattainment areas.

These regulations are rather complicated, but in general, in the attainment areas exhaust emissions from engines of 500 hp or more are required to meet the following values:

• CxHy - 1.0 g/hp/hr

• CO - 3.0 g/hp/hr

• NOx - 2.0 g/hp/hr.

In the nonattainment areas, the emission levels are the same, but the rules apply to engines of 150 hp or more.

In the South Coast Air Quality Management District (Scaqmd) of southern California, which encompasses the Los Angeles and San Diego areas, the requirements are stricter:

• CxHy - 0.6 g/hp/hr

• CO - 0.5 g/hp/hr

• NOx - 0.3 g/hp/hr.

These stricter standards will shortly be imposed in the Bakersfield, Calif., area also because of the large number of boilers that are being installed for heavy oil recovery in that area. This Scaqmd engine standard approximately corresponds to the emissions obtainable from a boiler with standard combustion controls; some experts predict this standard may become the engine standard nationwide.

COMBUSTION

When hydrocarbons are burned in air at atmospheric pressure in an unconfined space, the carbon and hydrogen are oxidized, but the nitrogen is essentially unchanged.

When combustion takes place in a confined space under pressure, such as in the combustion chamber of a reciprocating engine or a gas turbine, the increased pressure and temperature cause additional reactions, creating compounds harmful to the environment. In high-temperature, high-pressure combustion some of the incompletely oxidized carbon forms carbon monoxide, and some of the nitrogen in the air combines with the oxygen to form nitrogen oxides, principally NO and NO2, which are commonly referred to as NOx.

Some of the fuel does not oxidize and is carried through unburned. Various regulatory agencies refer to these unburned hydrocarbons as volatile organic compounds, reactive organic compounds, or nonmethane hydrocarbons. The terms essentially have the same meaning and for simplicity are referred to here as CxHy"

NOx helps produce ozone, which is toxic, and CxHy helps produce smog. The EPA has therefore decided that all CxHy COx and NOx emissions from industrial engines should be reduced.

Basically engine exhaust emissions can be reduced by internal engine modifications and exhaust after-treatment, which involves the use of catalytic converters on the exhaust system.

ENGINE MODIFICATIONS

In general, engines that run with an extremely lean air/fuel ratio produce lower emissions than engines that run at a stoichiometric mixture, in which the fuel and air mixture is theoretically correct for complete combustion (Fig. 1).

The excess air causes the air/fuel mixture to exceed the ignition limit of a spark plug. Thus, the mixture must be ignited by some other means to get this extremely lean mixture to bum with the low emissions.

A common strategy is to use a very lean mixture in the cylinder combustion chamber and a richer mixture, which is ignited by the spark plug, in a small prechamber in the cylinder head. This prechamber then shoots a flame out into the combustion chamber with sufficient energy to ignite the lean mixture and cause it to burn.

The same effect can be accomplished with stratified charge technology. The air/fuel mixture in the combustion chamber is stratified to provide a rich mixture adjacent to the spark plug and a lean mixture in the rest of the chamber.

These lean bum strategies generally fall into one of the following four categories:

• New engines

  Incorporating lean burn technology in a new engine at the time of manufacture is not a large expense. The cylinder heads can be designed with the prechamber in them, and a more efficient turbocharger can provide the additional combustion air required. Many new engines are being built with this design, and they can generally satisfy most current emission regulations.

• Retrofit by engine manufacturers

  Most engine manufacturers also furnish a retrofit conversion for their older engines to bring them up to the lean burn standards of their new engines.

  A retrofit conversion is more expensive than the lean-burn design of a new engine because it requires the purchase of new cylinder heads with the prechambers built in, new more efficient turbochargers, new insulated exhaust manifolds to replace the water-cooled manifolds, new ignition systems, increased intercooler capacity, and more-sophisticated engine control systems.

  These rather expensive conversions will typically meet current emission regulations, but there may be little assurance of meeting probable stricter regulations in the future.

• After-market conversions

  Some of the conversions for older engines can be less expensive from companies other than the engine manufacturers. These conversions are less expensive because they generally involve remachining the existing cylinder heads for the prechamber and using a positive displacement compressor to furnish the additional air required by this modification. This type of air compressor is less expensive than a new turbocharger. These companies assert that the parasitic load imposed by the compressor is more than offset by a net gain in engine fuel economy and a higher available horsepower rating on naturally aspirated engines because of the supercharging effect of the air compressor.

• Prestratified charge

  In the prestratified charge (PSC) modification, air is introduced into the intake manifold to each cylinder just upstream of the inlet valve. When the inlet valve opens on the intake stroke, this air is sucked into the cylinder prior to the air/fuel mixture in the manifold. It is thought that this excess air remains adjacent to the piston on the compression stroke, while the normal air/fuel mixture is stratified in the combustion chamber and remains closer to the cylinder head where the spark plug is located.

  This process gives the spark plug a richer mixture to ignite, and the lean mixture then burns in a manner that reduces the generation of harmful emissions.

  The PSC modification is a less expensive way to provide lean bum combustion but it has the disadvantage of derating the power available from the engine because of the dilution with air. Engine deratings on the order of 25-30% have been reported on naturally aspirated engines and about 10% on turbocharged engines.

  The derating of naturally aspirated engines can be reduced by the addition of a turbocharger, but the turbocharger adds to the expense of the conversion. Realistically, during an economic analysis of the PSC system, the cost per horsepower of the derating needs to be considered as part of the expense of the conversion.

AIR/FUEL RATIO CONTROLLER

Each type of engine exhaust emission control depends on the engine air/fuel ratio. Under steady state operating conditions, the engine can be adjusted to the proper air/fuel ratio manually. As conditions change, however, the air/fuel ratio deviates from the manual set point. This deviation can be caused by a change in the fuel composition, engine load or speed, or ambient air temperature (which can vary substantially from night to day).

A normal carburetor cannot hold a constant air/fuel ratio under these changing conditions. Thus, the best conversion efficiency is achieved with an automatic air/fuel ratio controller.

Air/fuel ratio controllers are driven by an oxygen sensor installed in the exhaust stream of the engine. (A sensor must be installed on each bank of a "vee" engine.)

The sensor, identical to that used in automobile engines, sends an electric signal to the computer in the air/fuel ratio controller. The signal varies from 0 to I volt; the ideal set point for a three-way catalytic converter is around 700-900 millivolts, depending on the other engine operating parameters.

The set point can be easily picked with a portable exhaust analyzer. The analyzer can determine the millivolt reading at which the proportions of CO and NOx are balanced for correct conversion in the catalyst. The computer then varies either the fuel flow or air flow to the engine to maintain the proper air/fuel ratio by one of following methods:

• Trim fuel can be introduced downstream of the carburetor with a lean adjustment. This additional fuel will mix with the flow coming out of the carburetor to give the proper air/fuel ratio.

• The effective fuel pressure to the carburetor can be varied by changing the set point on the fuel gas regulator with a pneumatic signal to raise or lower the fuel pressure to the carburetor or by using a valve installed between the fuel gas, regulator and the carburetor to restrict the fuel flow and reduce the effective fuel pressure. In these instances the carburetor is set rich initially, and the fuel pressure is reduced by one of those two methods.

• The air can bypass the carburetor. Filtered air is taken upstream of the carburetor, routed around the carburetor through a modulating valve, and introduced back downstream of the carburetor. The carburetor is set rich, and the bypassed air is used to dilute the flow coming from the carburetor back to the proper set point.

The necessary differential pressure to cause this air flow is generated by the butterfly valve in the carburetor, which is rarely fully open. The pressure differential between the manifold and the inlet to the carburetor is generally at least 4-6 in. of a mercury column. This differential exists in a turbocharged engine where the carburetor operates under positive pressure from the turbocharger.

All three of these approaches seem to work equally well, and there seems to be no theoretical advantage of any one method. However, some engines respond better to one method than to another.

EXHAUST AFTER-TREATMENT

• Nonselective (three-way) catalytic converters

  The basic and most widely used catalytic converter technology is the three-way catalytic converter, which simultaneously reduces CxHy, COx and NOx. This system is used on many automobile engines and on rich-burn (stoichiometric) four-cycle gas engines.

  The three-way catalytic converter requires that the air/fuel ratio be slightly richer than the stoichiometric point where the quantity of CO and NOx are equal (Fig.1). The CO acts as a reducing agent for the NOx in the following reaction:

  [SEE FORMULA]

  The small amount of oxygen remaining in the exhaust oxidizes the CxHy in the following reaction:

  [SEE FORMULA]

  This simple, cost-effective emission reduction strategy is used on many automobile engines built worldwide today.

• Two-stage catalytic converters

  In southern California and several other areas, the emission reduction requirements are very stringent and cannot be met with a three-way catalytic converter. In these instances, a two-stage catalytic converter is used. An air/fuel mixture that is more rich than the stoichiometric point is introduced into the first catalyst for a more thorough conversion of the NOx. The following reaction occurs:

  [SEE FORMULA]

  Air is then introduced downstream of this catalyst to provide a lean mixture passing a second oxidation catalyst, producing the following reaction:

  [SEE FORMULA]

  Because two catalysts are required, this process is more expensive than the three-way catalytic converter. However, the two-stage catalytic converter can reduce emissions to meet all current and many foreseeable environment requirements.

  A catalytic converter can be designed to use a three-way catalyst initially, with space available for the later addition of a second oxidation catalyst in case of a somewhat stricter emission requirement in the future.

• Oxidation catalysts

  In some cases a lean-burn engine can be designed to reduce the level of NOx during combustion, but the CxHy- and CO emissions are excessive (Fig. 1). This problem can be solved by the addition of only an oxidation catalyst. The oxidation catalyst uses the excess oxygen in the exhaust to reduce the CO and C,H, by the same reaction as the second stage described above.

• Selective catalytic reduction

  In an oxygen-rich exhaust (such as that produced by a lean-burn, four-cycle engine, a two-cycle engine, a gas turbine, or a diesel engine), it is not currently possible to reduce NO with a catalyst alone. In this case, the &10, can be reduced by introducing ammonia (NH3) into the exhaust upstream of a catalysts A reaction between the NOx and NH3 takes place in the catalyst, producing the following reaction:

  [SEE FORMULA]

  In a premium selective catalytic reduction system, the addition of an oxidation catalyst downstream causes the CO and C,H,. to react with the excess oxygen:

  [SEE FORMULA]

  This process is the most effective method to reduce CxHy, COx and NOx emissions in an oxygen-rich exhaust, but it involves the extra expense of the ammonia. A catalytic converter can be designed for only an oxidation catalyst for initial application, and space can be allowed for the addition of another catalyst to convert it to a selective catalytic reduction unit at a later date in case emissions requirements become more stringent.

CATALYSTS

The catalyst in a three-way catalytic converter consists of the precious metals platinum, palladium, and rhodium plated onto a substrate that supports the catalyst and provides a flow path for the exhaust gas. These metals cause the necessary catalytic reactions. The catalyst stimulates the reaction but is not consumed during the reaction.

Several substrates have been developed over the years with varying degrees of successful use. The first substrates used in the early automobile and industrial engine catalytic converters were ceramic beads. The ceramic beads contain the precious metals and are packed in a basket through which the exhaust gas flows. This method was popular because mass production of the beads was inexpensive, and additional beads could be easily added to the basket when needed. Pulsation in the exhaust system, however, causes the beads to rub together, thereby reducing the volume of catalyst by abrasion.

This problem was solved by using a ceramic honeycomb structure with the catalyst plated onto the ceramic surface. This system has been used in many automotive and industrial catalytic converters. The ceramic honeycomb substrate, however, has a relatively large surface area for industrial engines, and the large surface area exposed to the pulsating flow in the exhaust of a slow-speed engine causes fatigue damage to the catalyst module. The ceramic substrates are also vulnerable to damage from engine backfire. Backfire relief valves are usually used with these units to reduce damage, but it is difficult for such a valve to open quickly enough to relieve a pressure wave traveling at sonic velocity.

Thus, some catalytic converter manufacturers now use metal substrates instead of ceramic. The catalyst is plated on a metal foil of stainless steel. Some foils are crimped to prevent nesting when wound into a roll. Other foils use two strips of metal, one flat and one corrugated, wound into a cylinder shape to allow axial flow.

The metal substrates have proven to be more durable 'than ceramics in industrial engines because of their more resilient nature and greater strength. Because of these advantages, 3 automobile manufacturers began using metallic substrates on their new vehicles in 1987. By 1989, 10 auto manufacturers were using metallic substrates on new models, and as of 1992, 19 auto manufacturers worldwide were using metallic substrates on new vehicles. This trend is expected to continue.

The two basic shapes of ceramic and metallic substrates are round and rectangular. The rectangular shape has the advantage of modular construction, in which catalyst modules of a standard dimension and thickness can be stacked side by side and to various depths to provide the required catalyst volume for a given application. The disadvantage of this configuration is that the majority of the flow tends to go through the central portion of the catalyst module, with less flow in the corners.

The round configuration does not lend itself to modular construction but has the advantage of allowing a more uniform flow through the entire volume of the catalyst.

CONVERTER LOCATION

Fig. 2 shows the catalyst installed in the exhaust line with no exhaust silencer. This arrangement has the advantage of minimal exhaust back pressure on the engine, but the sound attenuation within the catalytic converter is also minimal and not adequate for many applications.

Fig. 3 shows a catalytic converter located upstream of the exhaust silencer. This arrangement has the advantage of providing maximum exhaust temperature at the catalyst for improved catalytic conversion. In general, the higher the temperature (up to a point), the better the catalyst operates. This setup has the disadvantage of subjecting the catalyst to full pulsation of the engine exhaust, which can cause fatigue and eventual deterioration of the catalyst module.

Fig. 4 shows a catalytic converter located downstream of the exhaust silencer. This arrangement has the advantage of protecting the catalyst module from exhaust pulsation. However, the catalyst is less effective because much heat radiates away from the exhaust silencer, reducing the temperature of the exhaust entering the converter.

Fig. 5 shows the optimum arrangement with the catalyst located approximately in the center of the exhaust silencer. The first reaction chamber of the silencer greatly reduces the exhaust pulsation for minimal fatigue damage to the catalyst module, vet the exhaust is still very hot for a good catalytic reaction. The exhaust gas is flow-conditioned by the first chamber. Thus, there is less pulsation in the exhaust, and the exhaust is distributed more evenly across the entire face of the catalyst for maximum catalytic reduction. This setup usually has a lower total pressure drop than a separate converter and silencer in series, resulting in less exhaust back pressure to the engine for cooler, more economical engine operation.

MAINTENANCE

A properly maintained catalyst can give many years of trouble-free service and effective emissions reduction. In addition to the possible pulsation and backfire damage, there are two other main Ways in which a catalyst can be rendered less effective: overheating and buildup.

• If an engine misfires because of ignition problems or bad spark plugs, the unburned fuel and air from the misfiring cylinders can flow into the exhaust stream. This fuel/air mixture will bum on the catalyst, greatly raising the temperature of the catalyst.

  Most emission-reducing catalysts operate in the range of about 800-1,100 F.; however, a misfiring engine can create temperatures exceeding 2,000 F. Sustained temperatures in excess of about 1,400 F. can destroy the catalyst and sometimes the substrate.

  Thus, it is prudent to install a thermocouple downstream of the catalyst and connected to an automatic shut-down device on the engine in case the catalyst overheats. Although the down time can be inconvenient and add to operations expense, the cost may be less than replacing an expensive catalyst.

• The buildup of combustion products on the catalyst surface can adversely affect performance. The buildup can "blind" the catalyst such that the exhaust gas cannot reach the catalyst surface for it to perform the necessary chemical reaction.

  The principal contaminant is sulfated ash from lubricating oil that works past the piston rings and into the combustion chamber. The use of good piston rings and a low ash oil will generally allow an engine to run about 1-2 years before the ash buildup becomes excessive and the reduction of the exhaust emissions decreases.

  After the first time buildup occurs, the catalyst can be rejuvenated with a minimum expense by removing it from the converters blowing the soot out with an air hose, and exposing a fresh face to the exhaust flow. Most metallic and some ceramic modules are reversible for reinstallation in the converter-the trailing edge of the module is inserted as the leading edge to expose a fresh surface to the exhaust flow. Some ceramic modules are stacked in series; the tiles from the rear of the converter can be moved to the front, and those originally in front can be moved to the rear of the stack to expose a fresh surface to the flow.

  After another 1-2 years of operation, the buildup will probably require a more thorough cleaning. The catalyst module may have to be soaked in a solvent recommended by the manufacturer and washed to remove the ash buildup. If the buildup is severe, the catalyst may require a more thorough cleaning in which the catalyst is immersed in a solvent with ultrasound waves used to assist in the cleaning process. Most large cities have an ultrasonic cleaning company that can perform this service at reasonable cost. After an ultrasonic cleaning, the catalyst is essentially like new.

  Earlier catalytic converters required some time and effort for disassembly and catalyst access; however, virtually all converters built today have some sort of catalyst access port to facilitate the periodic cleaning.

Copyright 1993 Oil & Gas Journal. All Rights Reserved.