Gas engine development continues to focus on lean-burn technology and modification for biofuels, writes Drew Robb, with manufacturers in the US and elsewhere managing to inch forward on overall efficiency too.
Lean-burn technologies have been making news in the engine field for some time. And 2006 was no different. But 2006 was also the year when biofuel R&D also moved into the fast lane. With natural gas prices remaining relatively high, some are looking at alternative fuel sources as a means of keeping costs down. Digester gas from wastewater plants and dairy farms, methane from landfill sites, and various biomass fuels such as wood are growing in popularity across the cogeneration sector.
Paul Bautista of Discover Insights LLC, however, points out that biofuels come with a catalogue of design and operational challenges – fuel collection and emissions, for example.
LEAN BURN MOVES ON
Lean combustion basically means that the engine has excess air introduced into it along with the fuel. By reducing combustion temperature, the extra air cuts down significantly on the amount of NOx produced. Further, additional power is produced due to the presence of more oxygen.
According to Chris Farmer, turbine product manager of Cummins Power Generation’s Energy Solutions, lean-burn reciprocating engine generators offer several advantages when compared with gas-powered combustion turbines.
These include higher overall efficiency, greater electrical output per unit of fuel input, shorter delivery/commissioning periods and generally lower installed costs. As a result, lean-burn units are often favoured in applications in the 300 kW to 20 MW range. He cites recent advances in Cummins lean-burn gensets in terms of size, efficiency, reliability and performance.
The most current Rolls-Royce KV G4 gas engine, too, includes several lean-burn editions. The G4 achieves 46.2% efficiency compared with the 39% of the earlier G1 version. By exploiting heat energy regained from the cooling system and exhaust gases in cogeneration applications, these engines are said to achieve a total efficiency of 93%.
Rolls-Royce states that its KV-series engines are based on a marine, medium-speed design that offers low fuel and maintenance costs as well as high efficiency. The Bergen KV-G4.2 comes in 12, 16 and 18-cylinder models with 2500 kW, 3335 kW and 3750 kW respectively.
Further lean-burn innovation is being accomplished under the auspices of the US Department of Energy’s (DOE) Advanced Reciprocating Engine Systems (ARES) programme. ARES was begun at the start of this decade to develop a higher standard of engine technology.
Original equipment manufacturers (OEMs) Waukesha, Caterpillar and Cummins are working with the DOE to improve fuel efficiency and flexibility, lessen overseas fuel dependency, achieve ultra-low emissions goals, lower power costs and improve power system and grid reliability.
ARES is primarily targeting engines in the 500 kW to 6500 kW range used in power generation applications. Each OEM is focusing upon a different platform, though they are collaborating on development. Waukesha is using its VGF Series, Caterpillar is using its 3500 Series, and Cummins is using a mix of its QSK and QSV gas engines. Eleven universities are also working with the DOE on various aspects of the programme.
Caterpillar C-175 centerline engine
When ARES began, the thermal efficiencies of internal combustion engines were averaging between 37% and 40%. Nowadays some vendors are boasting numbers in the mid-forties and higher. The target is 50% by 2010.
Engine manufacturers are already rolling out the benefits of their involvement in the ARES programme. Waukesha, for example, announced some performance enhancements to its 12-cylinder VHP turbo-charged engines in the middle of last year. This includes an extended oil change interval of 3500 hours. This is accomplished via a deep sump oil pan, improvements to the oil filtration system and a new air cleaner.
In addition, the VHP has a single wiring harness and fewer connection points in order to facilitate the process of packaging the engine with a compressor. These units should begin shipping in February 2007.
Caterpillar, meanwhile, has introduced a family of diesel gensets based upon its successful Cat 3500 Series gensets. Operating in the 2 to 4 MW range, they are powered by the C-175 centerline engine (see picture and table above) and intended for standby, prime, continuous and load management applications requiring a large amount of power.
The engine incorporates improvements to components such as the crankshaft, bearings, piston and camshaft. The C175 air system was designed to enhance engine breathing using a tall cross-flow cylinder head that offers greater power density and reduced cooling needs. This effectively minimizes heat transfer between the intake and exhaust ports, as well as the manifolds.
Recent work on ARES is also paying dividends in the ignition department. The DOE’s Argonne National Lab in Illinois has been conducting ignition studies in its rapid compression machine (RCM). The RCM is an opposed piston machine that is designed for minimal vibration, a compression ratio of 10.0 and maximum pressure of 362 bar.
Sustaining reliable ignition, after all, has been one of the bugbears of the lean-burn movement. Conventional coil-based ignitions have struggled in some lean-burn environments, so ARES researchers have been pursuing various alternatives. One of the more promising ones is laser ignition as it appears to work reliably at high pressures and under lean conditions.
Argonne, therefore, has been comparing both methods using the RCM. After an initial MIT design was significantly improved, testing was done at around 490˚C and 80 bar to simulate ignition conditions on methane-air mixtures. Researchers concluded that laser ignition improves the reliability of ignition systems in lean-burn engines, provides faster combustion and fewer combustion delays. These results were then confirmed during a single-cylinder engine test. Look for manufacturers to begin incorporating such ignition systems in commercially available models.
MEETING STRICTER EMISSION STANDARDS
There is plenty of progress being made, however, outside of the ARES perimeter. Wärtsilä’s 50DF lean-burn dual-fuel engine incorporates both low emissions and high-fuel efficiency features. According to Wärtsilä, it is available with up to 18 cylinders that supply up to 16.64 MW in continuous base-load operation.
The 50DF features a 7980 kJ/kWh heat rate which is equivalent to an overall thermal efficiency of 45%. These engines can automatically change from gas to back-up liquid fuel instantly if the gas supply is interrupted, while continuing to deliver full power. When the gas supply is restored, it is then possible to immediately switch back to gas mode.
The 50DF was selected to deliver a 163 MW gas-fired power plant to Pacific Gas and Electric company – the Humboldt Bay plant is near Eureka in northern California and should enter commercial operation by mid-2009. This repower project will comprise 10 Wärtsilä 18V50DF lean-burn dual-fuel engines. The new facility is expected to be 35% more efficient with 90% fewer air emissions.
Alturdyne is another player that offers some more efficient gas engine options (see box below). It has developed, for instance, a dual-fuelled conversion kit for a Cummins 5.9 L diesel engine so it can burn natural gas. Heat recovery is optional. The company also offers a line of cogeneration models that are modified versions of seven Caterpillar engines ranging from the Cat G3304NA (65 kW) up to the Cat G3518 (810 kW).
In tandem with lean-fuel models, engine manufacturers have been turning out an increasing number of biofuel-burning machines. Previously uneconomic technologies in the US have received a legislative boost from such sources as the federal Production Tax Credit (PTC) and state-sponsored Renewable Portfolio Standards (RPS).
In all, 22 states as well as the District of Columbia have enacted RPS statutes. In addition, the US Department of Agriculture’s most recent Farm Bill gives loans/grants to farmers, ranchers and rural small businesses and these have stimulated interest in bio-energy systems.
Bio-energy opportunities encompass several areas such as digester gas, landfill gas, biomass, coal-bed methane, wood and wood waste. The DOE’s Oak Ridge National Laboratory estimates that these sources can potentially generate around 120 GW of capacity via CHP application.
These fuel resources are available in a surprising array of places. Digester gas, for example, can be found at wastewater treatment facilities, landfill sites, and farms. Engines for biogases, however, need to be able to burn the low-methane gas and sometimes cope with highly corrosive elements. Landfill gas contains sulphur, silicon (in the form of siloxane), ammonia and other contaminants that can plate out on engine surfaces, pollute the lube oil, corrode metals and cause other problems. But that hasn’t stopped people rushing to enter the field.
Such is the demand, in fact, that the US Environmental Protection Agency (EPA) records about 400 operating projects that encompass most states with 600 more candidate landfills lining up to join them. As a result, the EPA has launched a Landfill Methane Outreach Program with a mission to reduce methane emissions by promoting the development of cost-effective landfill gas energy projects.
In addition, methane from US coal beds amounts to another 37 billion cubic meters each year. Add that to the 94 billion cubic meters that the European Commission estimates that landfill sites in Europe already produce and you see why methane is becoming big business.
The OEM community is rapidly responding to demand. Cummins low-Btu engines, for instance, are now being designed to be more tolerant of corrosive contaminants such as siloxane, ammonia and acids. They can function with a dilute mixture of 40 percent methane or above. They are fully compliant with existing UK emissions regulations without the need for after-treatment catalytic exhaust systems.
These Cummins machines incorporate a carbon cutting ring at the top of the cylinder wall which breaks up carbon and silicate deposits, more durable cast iron pistons, bearings that are less susceptible to corrosion and a more alkaline form of engine lubricating oil. The whole idea is to extend lifespan and reduce maintenance in what can be a challenging environment for an engine.
Wood no longer is something quaint to burn in a winter cabin while trying to evoke a romantic image. Instead, it is emerging once again as an economically viable fuel source.
Wood and by-products like sawdust, bark and shavings are being used in CHP applications. Discovery Insights notes that both back pressure and condensing steam turbines have been utilized in this regard.
Wärtsilä markets a line of bio-energy CHP engines known as BioPower that typically use wood-based fuels, such as sawmill residues, forest harvesting residues, sawdust, wood chips and bark. The company has added two sizes of biomass-fuelled plants to its BioPower product range.
The latest additions, the BioPower 3 and BioPower 7 plants, deliver approximately 3 and 7.5 MW of electricity respectively. They join the existing BioPower 2 and 5 plants. All incorporate Wärtsilä BioGrate combustion technology to burn biomass fuels with high combustion efficiency and low NOx and CO emissions.
BioPower plants operate on a closed steam-feed water system which is separate from the heating water system. Steam is generated in a water-tube boiler, and supplied to a back-pressure steam turbine driving an alternator. Turbine exhaust steam then heats the heating water, and the condensate is returned as feed water to the boiler.
MORE TO COME
Environmental regulations, these days, have taken on the same inevitability as death and taxes. It appears that for the foreseeable future, the regulatory framework will continue to favour CHP applications that make use of lean-burn mixtures and biofuels. And if gas prices continue to stay in a generally higher range, economics will drive more and more cogeneration developers to adopt such technologies.
For the remainder of the decade, at least, lean burn and biofuel are set to remain front and center in the minds of reciprocating engine designers.
Drew Robb is a Los Angeles-based freelance writer specializing in engineering and technology issues.
Full-authority engine controls
The latest generation of Cummins gensets is designed to operate with an optimum mixture of air and gas. In reality, a very small operating window of peak efficiency and low NOx exists. Too much gas can lead to knocking and higher emissions. Too lean a gas mixture can lead to lower combustion and misfiring.
A range of Cummins models in the 315 kW to 2MW (50 Hz and 60 Hz) incorporates full-authority electronic engine controls, sensors and microprocessors to maintain combustion within the appropriate boundaries. An open combustion chamber is housed in the combustion crown, which is shaped to create turbulence in the air/fuel mixture. Combustion efficiency is raised as by exposing more of the gas to the advancing flame front.
The Cummins QSV91G, for example, has a 1.75 MW output. This 91-litre, 18-cylinder engine is spark ignited and lean burn.
Alturdyne has been using Mazda gas rotary engines with heat recovery features to build a DOE-sponsored 40 kW natural gas fuelled rotary genset for Tampa Electric (TECO) – People’s Gas in Florida. It includes jacket water heat recovery. Frank Verbeke, president of Alturdyne says this unit was installed at a Walgreens Drug Store in St. Petersburg, but the local utility (Progress Energy) won’t let it be connected to the grid due to demands made about a long-term power purchase agreement.
40 kW rotary genset using jacket water heat recovery
A second unit is being prepared, to be used on a dairy farm to consume methane derived from manure process. Waste heat from jacket water and the exhaust will be used on the farm. Verbeke believes that if the rotary engine can be successfully run on this farm gas, there are hundreds of applications where it could be used, including landfill and digester gas.
New 60 Hz engine can use variety of gases
GE Energy last year introduced a new Jenbacher gas engine to serve the power generation, cogeneration and on-site power segment in North America. The J420 GS gas engine (pictured right), said to be ideal for 60 Hz applications running at 1800 rpm, is suitable for natural gas or a variety of other gases, such as landfill gas, biogas, sewage gas and coal mine gas.
The J420 at 50 Hz was introduced in 2002 to fill a power output niche that existed between the Jenbacher 1 MW J320 GS model and its larger unit, the 1.8 MW J612 GS engine – while also launching this engine platform as the next generation of gas engine efficiency, the Jenbacher ‘High-Efficiency Concept.’ More than 300 of the 50 Hz J420 GS units have been installed around the world, says the company, operating on a variety of fuels.
The new 60 Hz model offers customers very high efficiency at high specific outputs, having completed rigorous endurance and performance testing at GE’s manufacturing facilities for Jenbacher engines in Austria.
GE says the engine is characterized by a very high power density (kW/weight), due to its highly compact, robust design and small footprint, resulting in low peripheral investments for foundations and buildings. Advanced efficiency features include swirl-optimized, four-valve cylinder heads, a Miller timing combustion system and high-performance spark plugs developed in-house by GE. In turn, this leads to lower fuel costs, and longer operating periods for components, including cylinder heads for up to 30000 hours and spark plugs for up to 15000 hours, resulting in overall attractive costs of electricity.
Electrical output of the new model is 1.426 MW, while thermal output will be about 5,700 MBTU/hr when fuelled by natural gas and 5,400 MBTU/hr with biogas, says GE. Electrical efficiency is 40.8%, with a total engine efficiency of up to 86.6%.
The first J420 for 60 Hz applications were shipped in late 2006 and commissioned in early 2007, with the first field installation entering service during the first quarter of 2007.