Lean-Burn Or Rich-Burn?

It depends on what meets the customer's application needs Lean‐burn or rich‐burn? GE's Gas Engines business develops lean‐burn and rich‐burn technologies that have proven themselves in minimizing emissions and delivering strong operational performance. The basic differences between lean‐burn and rich‐burn engines, and how to decide which is best for you, are neatly summarized by Christian Trapp, head of performance engineering for Jenbacher gas engines. While lean‐burn gas engines are more economical at increased exhaust gas temperatures allow the use of certain emissions calibration levels and can operate a three‐way catalyst. The resulting high conversation at higher loads, rich‐burn engines can achieve lower rates (for NOx above 99 percent) significantly reduce emission levels with a single after treatment, are all three major types of engine‐out emissions ‐ NOx, more tolerant of broad fuel ranges and ambient CO and HC ‐ and destroy inferior but hazardous conditions, and generally have better transient load pollutants like formaldehyde (CH20). In this way, rich‐ capability," says Trapp. "Neither technology is burn engines can reach a system‐out emission limit inherently superior: Choosing the right one depends below 50 mg/Nm3 (@ 5 percent 02 in the exhaust gas on requirements for fuel flexibility, reliability, power < 0,1 g/bhph) NOx and ultra‐low total hydrocarbon density, gas costs, and compliance with local emissions, leaving a decreased overall greenhouse emissions standards." gas footprint. When it comes to meeting high power‐ density needs or achieving the highest possible BASIC DIFFERENCES AND ADVANTAGES. efficiency at moderate emission limits of 500 or 250 mg/Nm3 NOx (@ 5 percent 02 in the exhaust gas) ‐ Essentially, rich‐burn engines operate at an almost such as those stipulated in the German TA Air or the stoichiometric air/fuel ratio (AFR), which is exactly Gothenburg Protocols ‐ lean burn engines can enough air to burn all of the fuel. leverage this advantage: At an adequate gas quality This allows a simple three‐way (NSCR or Nonselective they deliver BMEP levels of up to 24 bar with Catalytic Reduction) catalyst (TWC) like in a gasoline electrical efficiencies up to 46.5 percent (type 6 passenger car to be applied to reduce nitrogen oxides engine) without the need for a NOx or THC after (NOx), carbon monoxide treatment system. To (CO), hydrocarbons (HC), "While lean‐burn gas engines are usually more lower the NOx emissions and HAPS (Hazardous Air economical and powerful and operate at higher toward levels reached by Pollutants), all in one loads, rich‐burn engines can achieve lower rich‐burn engines with a after treatment system. emission levels with a single after treatment three‐way‐catalyst, lean‐ Lean‐burn engines use a and show a higher flexibility regarding transient burn engines require lot of excess air. Usually loads and ambient conditions" selective catalytic up to twice the amount ‐ Christian Trapp, head of performance engineering for converters with urea needed for complete fuel Jenbacher gas engines injection to maintain combustion. This air engine efficiency. dilution effectively cools down the peak combustion Oxidation catalysts perform most of the CO reduction temperatures in the cylinder; that reduces the NOx in lean‐burn engines but, as with other catalytic production and allows low engine‐out emissions systems, the fuel gas must be very pure. These without the need for an after treatment system in catalysts also can reduce CH20 emissions ‐ again, if many applications. This lean combustion process has the gas is pure ‐ but their low exhaust temperature the additional advantage of reducing the knock limits hydrocarbon conversion efficiency. (detonation) probability and, therefore, allowing higher BMEP (Brake Mean Effective Pressure) levels OPERATIONAL FLEXIBILITY. (loads) and an optimized combustion phasing. This results in higher power density and usually produces While rich‐burn engines can operate on a broad better fuel efficiency. variety of natural gas fuels, alternative gases like biogas, sewage gas, or landfill gases cannot be used CUTTTING DOWN EMISSIONS. because they could poison the three‐way catalyst. The potential for "poisoning" the catalyst makes the Rich‐burn engines operate at engine‐out emissions of TWC solution suitable only for clean fuels such as 12‐16 g/bhph‐hr (5,000 ‐ 6,500 mg/Nm3@ 5 percent natural gas, and not for sewage gas, biogas, or landfill 02 in the exhaust gas) NOx, but the almost gas. High combustion temperatures restrict specific stoichiometric exhaust gas composition and the output and the BMEP, so there is lower efficiency than with lean‐burn engines operating at typical controlled amount of urea into the catalyst to air/fuel ratios. If lean burn engines are calibrated to convert NOx to nitrogen. Being able to operate at a operate at extremely low NOx levels (ultra‐lean), more optimal AFR with an SCR system makes the their efficiency begins to degrade so that the lean‐burn engine very efficient and allows high break difference between rich‐burn and lean‐burn fuel mean effective pressures. consumption is minimized. Since lean‐burn engines Oxidation catalysts are used to provide most of the have a much higher AFR ‐ with about 10 percent CO and NMHC reduction in lean‐burn engines but, as excess oxygen in the exhaust ‐ their engine‐out NOx with other catalytic systems, the fuel gas has to be emissions are only 5 percent to 10 percent of the very pure. These catalysts also can reduce CH20 amount discharged by a rich‐burn engine. Lean‐burn emissions ‐ again, if the gas is pure ‐ but their low engines require selective catalytic reduction (SCR) exhaust temperature limits hydrocarbon conversion treatment to obtain the lowest possible NOx efficiency. emissions levels in the exhaust gas. SCR injects a GE'S GAS ENGINES BUSINESS HAS THE TECHNOLOGY SOLUTIONS TO MAKE THE BEST USE OF THE LEAN‐BURN AND RICH‐BURN CONCEPTS CONTRASTING LEAN‐BURN COMBUSTION acid‐producing components in the exhaust gas. Other CONTROLS. methods use sensors mounted in the combustion chamber to measure combustion temperature, but Controlling the AFR is essential for controlling the their exposure to high temperatures, peak pressures, combustion and, therefore, NOx emissions. One and fouling by oil ash and trace component deposits technology for controlling lean‐burn combustion from the fuel gas can throw off the temperature applies Lambda sensors to detect exhaust gas signals, leading to an offset in the AFR measurement. oxygen, but readings can be distorted by sensor LEANOX*, the GE lean‐burn concept, is a vastly exposure to comparatively high temperatures and different approach from Lambda sensors. Without resorting to costly exhaust‐gas after treatment levels can be easily pushed below 0.4 g/BHP‐hr, systems, LEANOX controls NOx emissions to@ 5 which is lower than lean‐burn engines without percent O2‐dry, which equals ~ 0.55 g/BHP‐hr, by exhaust gas after treatment systems. However strict measuring engine output, intake pressure, and air‐ the clean air requirements become, GE is keeping fuel mix temperature after the intercooler, and pace with or even exceeding them. Rich‐burn engines feeding these values into a controller that adjusts the from GE's Waukesha product line are reliable gas mixer to produce the appropriate AFR. This performers in a variety of circumstances, such as hot combustion and control system keeps the thermal or fluctuating fuel conditions; when there are loading and mechanical stresses on related engine parts at capabilities with more than 50 percent load steps; low levels. With no sensors located in critical areas, when service intervals are extended; or when the LEAN OX reliably complies with exhaust emission limits under volatile operating conditions. CRITICAL CONSIDERATIONS. As emissions standards become more exacting the natural gas industry must develop technologies that reduce the levels of these substances as much as possible. Those rules require low NOx and CO emissions on a national level, but some states are getting even tougher than that and mandating NOx levels of 0.5 g/BHP‐hr or less. That especially impacts businesses with large fleets of engines that require mobility and application flexibility. In these cases, their fuel and application flexibility and very low emissions levels make rich‐burn engines a good choice. Rich‐burn engines with TWC technology are LEANOX LOWERS NOx EMISSIONS BV CONTROLLING THE AFR preferable when lowest emissions with highest operating flexibility are the requirements. The NOx same continuous operation for gas compression has Finally, rich‐burn engines operate with a wide margin proven to be reliable. Also, while lean‐burn engines for knock and misfire. TWC‐equipped rich‐burn can have altitude engines featuring the limitations on their GE's Gas Engines team has the technology Waukesha Engine System performance and require solutions to make the best use of the lean‐burn Manager* control derating above 1,500 and rich‐burn concepts, and we can help you system can work with feet, the flexible rich‐ determine which engine is the best choice for higher loads and lower burn technology of your application. fuel quality ‐ up to l, 700 engines such as the BTU/ft3 with 99 percent Waukesha L5794GSI allows full power at up to 8,000 ethane ‐ so customers don't have to store, transport, feet. flare, or sell the ethane. MEETING YOUR GAS ENGINE NEEDS. "GE's Gas Engines team has the technology solutions to make the best use of the lean‐burn and rich‐burn concepts, and we can help you determine which engine is the best choice for your application," sums up Trapp. The GE portfolio includes Waukesha rich‐ burn gas engines, which perform reliably in remote and harsh applications, and highly efficient, economical Waukesha and Jenbacher lean‐burn engines. All have proven themselves in tackling combustion control challenges.

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