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MAY 2007

The “Better Business” Publication Serving the Exploration / Drilling / Production Industry NSCR, SCR Systems Reduce Emissions By Wilson Chu can be particularly challenging because gine cycles between rich- and lean-burn of the vast array of applications, with dif- such that the three-way catalyst converts MALVERN, PA.–The implementation ferent duty cycles, durability demands, CO and hydrocarbons during one cycle, of the federal government’s new source packaging constraints and regulatory re- and converts the NOx on the other cycle. performance standards (NSPS) and even quirements. Some catalysts even contain materials stricter regulations in some air quality dis- However, reliable solutions are available that store oxygen during the lean cycle tricts administered by state and local gov- for both rich- and lean-burn stationary en- for release during the rich cycle. ernments are significantly tightening con- gines that provide high conversion efficien- Stationary gas compression engines trol requirements on exhaust emissions cies to reduce emissions of CO, hydrocar- usually are not as sophisticated as auto- from stationary sources. bons, VOCs, HAPs, NOx, and where mobile engines. For instance, the air/fuel Emissions control from stationary en- applicable, particulate matter. Although ratio controllers on stationary natural gines has been mandated to varying degrees lean-burn engines may offer operational ad- gas-fired engines typically have a single over the years, depending on factors such vantages to the owner, such as improved fuel oxygen sensor. However, the basic oper- as engine size, location, site limits, operat- efficiency, compared to a rich-burn design, ation of the three-way converter essen- ing hours, annual emissions rates, and re- the cost to install emissions control can be tially is the same. The oxygen sensor gional nonattainment status. Emissions of significantly higher for lean-burn engines. measures the amount of O2 in the exhaust, nitrogen oxides (NOx), carbon monoxide and the air/fuel ratio controller governs NSCR For Rich-Burn Engines (CO) and volatile organic compounds the O2 in the fuel mixture (maintaining (VOCs) specifically have been targeted. In The fundamental difference between a ±0.5 percent oxygen concentration for addition to the proposed NSPS, the En- rich-burn engine and a lean-burn engine is rich-burn engines) to ensure that the en- vironmental Protection Agency already has oxygen concentration in the exhaust. With gine runs close to the stoichiometric point issued the National Emission Standards for a rich-burn engine, oxygen concentration and that there is enough O2 in the exhaust Hazardous Air Pollutants (NESHAP) to is typically 0.5 percent, and no more than to allow the catalyst to eliminate un- control formaldehyde, acetaldehyde, a- 1 percent. Nonselective catalytic reduction burned hydrocarbons and CO. crolein and methanol emissions from sta- (NSCR) technology is the most economi- tionary internal combustion engines. cal and accepted emissions control method SCR For Lean-Burn Engines Stationary reciprocating internal com- for rich-burn engines. A three-way NSCR Selective catalytic reduction (SCR) is bustion engines provide the horsepower to catalyst with an air/fuel ratio controller can used to reduce NOx emissions from lean- compress natural gas in a variety of appli- achieve very high conversion efficiencies burn engines, which generally have oxy- cations at every stage of the gas supply (more than 98 percent) on natural gas-fired gen concentrations in the 8-12 percent chain–from production and gathering, to rich-burn engines. range. Lean-burn designs also modify the processing and storage, to natural gas trans- The three-way catalysts used on station- combustion process through exhaust gas mission and distribution. The spark-ignited ary gas engines to reduce CO, hydrocar- recirculation (EGR) or some other method engines packaged with gas compressors bon, VOC, HAPs and NOx emissions orig- of lowering the temperature in the com- typically burn natural gas as the feedstock inally were developed for automotive bustion chamber, since the higher the tem- in both rich-burn (stoichiometric) and lean- catalytic conversion. The gasoline-fired perature, the greater the amount of NOx burn configurations. stoichiometric engines being installed to- generated. Primary measures to reduce exhaust day in cars and light trucks obviously are Rather than using a three-way cata- emissions involving in-cylinder modifi- quite sophisticated, with computer controls lyst in a low-oxygen environment, SCR cations can be effective, but they also can and multiple oxygen sensors, but they use systems inject ammonia or a compound adversely impact performance. the same basic three-way catalyst technol- such as urea, which is decomposed into Increasingly, secondary measures such as ogy as a gas compressor engine: essential- ammonia, into the lean-burn exhaust installing catalytic conversion technolo- ly, simultaneous reduction of NOx and ox- stream as a reducing agent. Pure anhy- gies are being specified to meet local and idation of CO and unburned hydrocarbons drous ammonia, aqueous ammonia or national emissions standards. Applying with a platinum and rhodium catalyst. urea can be used as the reductant, but in exhaust gas treatment to stationary sources In an automotive application, the en- stationary gas engine applications, urea

Reproduced for Johnson Matthey Inc. with permission from The American Oil & Gas Reporter page 117-118.qxp 5/23/2007 2:54 PM Page 118

is most common because of its ease of proach is to use chemical technologies Progressively More Stringent use. As it hydrolyzes, each mole of urea to absorb NOx from the exhaust and in- As the NSPS standards for stationary en- decomposes into two moles of ammonia. ject it into a reducing catalyst. However, gines are phased in over the next decade, The ammonia then reacts with the NOx neither method so far has proven capable emission control regulations will become to convert it into nitrogen and water. of economically achieving high conver- progressively more stringent. Beginning this An oxidation catalyst must be added sion efficiencies, particularly in smaller- year, all newly manufactured stationary en- to the SCR design if hydrocarbons and CO horsepower stationary gas engines. gines must comply with Tier 1-Tier 4 stan- need to be controlled in addition to NOx Horsepower Is Key dards-49-99 horsepower engines must meet on a lean-burn engine. The oxidation cat- Tier 2 regulations, 100-751 horsepower en- alyst first oxidizes the exhaust stream to As some of the newer engine designs gines must meet Tier 3 regulations, and en- are demonstrating, it certainly is possi- convert CO to CO2 and hydrocarbons to gines larger than 751 horsepower must meet ble to reduce emissions from a natural CO2 and water. The CO2, water and NOx Tier 2 standards until 2011, when Tier 4 re- then enter the SCR catalyst, where the gas-fired engine while optimizing its per- quirements become effective for 3,000 NOx reacts with the ammonia. formance in gas compression applica- horsepower and larger engines. SCR systems can attain efficiencies of tions. The key is horsepower. In the drive One issue that remains unresolved at 95 percent or greater, but ammonia/urea to reduce emissions, no operator wants the moment relates to NSPS certification. requirements tend to increase with high- to derate a compressor by sacrificing NSPS certification is voluntary, and al- er NOx conversion efficiencies, creating horsepower. Under some circumstances, though engines can be certified by the man- the potential to slip more ammonia. a three-way catalytic converter could ufacturer, certification also may be left up Ammonia cleanup catalysts are a relative- have a negative impact on horsepower by to the dealer, the packager or even the end ly new technology that can be installed increasing system backpressure, al- user. The engines must be certified for a behind the SCR catalyst to collect any ex- though NSCR technology has evolved to useful life of 10 years or 8,000 hours of cess ammonia that slips through (convert- the point where the backpressure prob- operation. However, the engine owner will ing it into nitrogen and water). lem pretty much has been conquered. be required to perform a source test on non- The biggest problem with SCR is that Installing a three-way converter on an certified engines. Furthermore, noncerti- it is very expensive compared to NSCR. existing engine can be accomplished fair- fied engines of 500 horsepower or larger The cost to install SCR on a lean-burn ly quickly and easily. Depending on en- will require a source test every three years engine can be a factor of five times high- gine size, an NSCR installation can be or 8,760 hours of operation. That could be- er than installing a three-way catalytic completed within a day in most cases. come a very strong selling point, because converter on a rich-burn engine. A typi- The process involves removing the si- if an end user buys an engine certified by cal SCR system plus oxidation catalyst lencer, installing the proper flanges, the manufacturer, the end user would not for a 500-horsepower engine might cost adding supports for the converter, and have to pay for source testing. $100,000 or more, compared to perhaps then reinstalling the silencer. Another issue for new lean-burn engines $20,000 for a three-way converter plus In some gas-producing regions, noise an air/fuel ratio controller for a compa- reduction is a growing issue, especially rably sized rich-burn engine. in metropolitan areas. A number of fac- Moreover, SCR systems have a fairly tors affect the noise transmitted from a high cost of entry because any application compressor package, including engine will require fixed cost items such as a urea size, muffler type, cooler size, etc., but the injection system, urea control system and most important is the noise frequency of storage tanks. In addition, urea must be the engine exhaust. In general, lower fre- stored at temperatures above 11 degrees quencies are harder to abate than higher Fahrenheit or it can solidify, adding cost frequencies. In fact, a three-way catalyst and complexity in cold weather areas. actually reduces some of the higher fre- WILSON Because of the higher costs associat- quencies, but the lower frequencies must CHU ed with SCR, the technology typically is be removed through a series of baffles in installed only on large engines, such as an exhaust silencer. Wilson Chu is marketing and new those used in power generation in non- Integrated catalyst/silencer systems are business manager, stationary source attainment areas. Three-way NSCR nor- available as self-contained units that meet emissions control, for Johnson Matthey mally is the more economic solution for critical- or hospital-grade noise require- Inc. in Malvern, Pa. He is involved in smaller horsepower applications, partic- ments. Combining a three-way converter all aspects of marketing and new busi- ularly engines under 350 horsepower. For with an exhaust silencer into a single hous- ness development, and has extensive ex- this reason, it often makes sense for oil ing results in a simpler installation, small- perience in developing new markets and and gas companies to replace lean-burn er overall package size, less backpressure commercializing new technologies. Chu engines with rich-burn models for small- on the engine, and lower installed cost. The has 21 years of experience with Johnson er-horsepower compressor applications section of a converter/silencer unit that Matthey in emissions control catalysts in nonattainment and other areas with houses the three-way catalyst typically has and systems for mobile and stationary low emissions requirements. a hatch to allow the catalyst to be serviced sources, gas purification systems and Obviously, an effective, low-cost NOx or removed. Units are commercially avail- fuel cells. His industry expertise spans control system for smaller-horsepower able today for the full range of engine sizes. the environmental, semiconductor and lean-burn engines would be of signifi- Three-way catalytic converters can be power sectors. Chu is a member of the cant interest to the compressor market. economically scaled down to meet almost Manufacturers of Emission Controls There are a variety of ways in which NOx any application. In fact, catalysts are even Association, the Institute of Clean Air could be reduced from a lean-burn en- available for pumpjack engines. For very Companies and the California Clean gine’s exhaust stream, at least theoreti- small engines, the biggest limiting factor Distributed Generation Coalition. He cally. For instance, a fuel source could in terms of economics tends to be the cost holds a B.S. in chemical engineering be injected into the exhaust to react with of the air/fuel ratio controller, as opposed from Northeastern University. the NOx across a catalyst. Another ap- to the cost of the catalytic converter itself. page 117-118.qxp 5/23/2007 2:54 PM Page 119

is meeting HAPs requirements under NSPS particulate matter from stationary engine a fuel cell or some other type of zero-emit- regulations. If a stationary engine emits ei- exhaust streams, emissions control tech- ting engine–which will not be any time ther 10 tons a year of a single HAP nology is becoming increasingly impor- soon–the natural gas compression indus- (formaldehyde, acetaldehyde, acrolein or tant in the daily operation of natural gas try, like other industries, will continue to methanol) or 25 tons a year of a group of compression equipment and facilities. rely on the internal combustion engine to HAPs, the new standards require HAPs The truth is that no solution can elimi- provide the horsepower to get the job control. A three-way catalyst will provide nate 100 percent of emissions. The inter- done. And that, in turn, means operators some HAPs reduction on rich-burn engines nal combustion process will generate of gas compressors will need NSCR, SCR because it will reduce hydrocarbons in the some amount of pollutants no matter and other catalytic control technologies exhaust, but an oxidation catalyst is the only what type of fuel is used or how sophis- to reduce the emissions generated in pro- way to effectively reduce HAPs on lean- ticated in-cylinder or exhaust gas treat- ducing, gathering, processing, storing and burn engines. ment technologies become. transporting natural gas.  With heightened concerns about the But until the day arrives when the in- emissions of gaseous air pollutants and ternal combustion engine is replaced with