Advances in gas engine technology and rising transmission costs are raising the financial appeal of on-site power production with gas-fuelled reciprocating engines. Case studies from Canada, China and Spain highlight the fast-changing economic reality, finds Michael A. Devine.

Cat CG132-8 gas generator set

Since the 1960s, government incentives have largely driven the market for natural-gas-fuelled combined heat and power (CHP). Today, market forces are conspiring to make CHP – and gas-fuelled distributed generation in general – financially attractive solely on its merits, especially in North America.

An abundance of natural gas brought about by growth in hydraulic fracturing (fracking) in shale formations has driven North American gas prices down sharply – and fuel represents 70–90% of lifetime owning and operating costs for a CHP system.

Meanwhile, emissions regulations affecting coal-fired power plants and the rising cost of siting transmission and distribution lines, an increase in higher-cost renewable sources in utility portfolios, and other factors are pushing grid power prices up. This makes on-site power production with gas-fuelled reciprocating engines highly cost-competitive – and likely to become more so in the long run.

Furthermore, while government austerity measures in Europe have curtailed incentives for CHP, such incentives are on the rise in the United States. A search on the US Department of Energy Database of State Incentives for Renewables and Efficiency (www.dsireusa.org) reveals 34 states in which government- or utility-sponsored CHP incentives exist – as tax incentives, grants or loans.

Agricultura y Exportación, a pepper producer in Balsicas, Spain, operates a CHP system that also uses exhaust carbon dioxide for fertilization

Finally, advanced fuel-efficient, low-emission gas engines are readily available – and engine technology continues to improve fuel efficiency, enhance reliability and reduce maintenance and operating costs for gas-fueled on-site generation. The addition of heat recovery further enhances the economics of gas-fuelled generation.

Technology at hand

Advances in gas engine technology have been essential to progress in CHP and distributed generation. By nature, gas engines serve well as isolated or grid-connected on-site power sources. They offer high power density, low first cost per kilowatt, and quick, simple installation.

Fuel and operating costs are competitive. Emissions are clean. Service is readily available with an abundance of technicians worldwide. The basic technology is highly reliable. The engines readily handle full or part loads, tolerate varied altitude and ambient conditions, and can be brought on-line or taken off-line quickly.

Engine technology took a significant leap forward in the early 2000s with the US Department of Energy’s Advanced Reciprocating Engine Systems (ARES) program, which challenged manufacturers to develop a new breed of gas engines specifically for distributed generation with significantly higher thermal efficiency, significantly lower NOx emissions, and much longer service intervals than traditional units.

Power of electronics

The first ARES-derivative engines became available in 2003, achieving simple-cycle mechanical efficiencies up to 43.5%, versus 32–37% just a few years before. These engines, now proven in multiple applications worldwide, use a variety of digital microprocessor-based monitoring and control technologies that include:

  • Air/fuel ratio control based on charge air density, maintaining NOx within the tightest available tolerance under all ambient and load conditions, irrespective of changes in air temperature and humidity.
  • Detonation sensing by individual cylinder with automated control to retard timing if detonation occurs.

The engines also have intake systems that enable efficient airflow and minimize heating of charge air, increasing the air/fuel charge to the cylinders for optimum performance. An open-chamber cylinder design and low-pressure fuel system (0.5 to 5 psi) eliminate the need for a fuel compression skid.

Longer maintenance intervals are enabled by high-energy, long-life spark plugs, along with lower oil temperatures that reduce oil nitration and oxidation. Some engines in this general family apply advanced materials that provide “hardening” against wear, for use with impure, low-Btu fuels such as landfill and digester gas.

Custom solutions

While the ARES-derivative engines provide cost-effective off-the-shelf solutions for numerous applications, a second production model has emerged: customization of individual engine-generator sets to fit the application.

In this model, rather than purchase an engine and accessory package based on published ratings and a price list, users provide a fuel sample, describe the ambient and conditions and altitude, and specify the application and key operating objective (e.g. top fuel economy, lowest emissions, block loading capability). The manufacturer then custom designs an engine-generator system to fit those specific criteria.

The breadth of customization is considerable. For example, the manufacturer’s application engineers can select from a variety of compression ratios, pistons with different swirl capability for specific fuel types, different turbochargers and nozzle or ring configurations, and site-specific air system operating and engine timing maps. The custom units are delivered with no significant difference in lead time or cost per kilowatt versus the traditional model.

Taking technology further

The customized units come with further advances in engine technology that expand the limits of engine control and reach new heights of efficiency – up to 44% electrical efficiency in the generator sets alone and up to 90% total plant efficiency in CHP or combined heat, cooling and power (trigeneration) service. Advances include:

Optimized air and exhaust flow. Both the inlet and the exhaust system are tuned to enable highly efficient, laminar flow. A technology called Pulsed Energy Advanced Recovery Line (PEARL) uses flow-optimized exhaust pipes that convey a constant exhaust mass flow to the turbochargers. Each PEARL module evacuates the exhaust of two cylinders. Exhaust flow is timed to keep the turbocharger spinning at the optimum speed over the engine’s entire operating load range. Consistent exhaust mass flow leads to uniform backpressure so that all cylinders operate at the same power level. Precise ignition timing, automatically adjusted by cylinder for fuel quality changes, augments this entire process.

Cat CG170-12 gas generator set

Miller cycle. This adjustment in the combustion cycle by itself increases fuel efficiency by about 1%. The Miller cycle differs from the more traditional Otto cycle in that the intake valves close not when the piston reaches bottom dead centre but at typically 10 to 15 degrees before bottom dead centre (in the early inlet close version).

In the Miller cycle, as the piston continues downward with the intake valves closed, the air/fuel mixture expands and thus cools, increasing the detonation margin. A greater detonation margin in turn enables a higher compression ratio of 14:1 or 15:1, versus 11:1 or 12:1 for the Otto cycle. The higher compression ratio allows for a higher expansion ratio, making better use of fuel to increase fuel efficiency. (Delivering a full air-fuel charge in a shortened intake interval requires more efficient turbochargers that deliver rapid airflow; for this reason, Miller cycle engines tend to derate more quickly at altitude and high ambient temperatures.)

Unified control. A Total Electronic Management (TEM) system optimizes generator set performance by measuring the temperature of each cylinder and adjusting combustion characteristics to minimize fuel consumption and engine emissions and prevent detonation.

In addition, if the user wishes, the system can be configured to control functions beyond the engine itself, such as radiator motors, downstream electrical breakers and systems, and even complete CHP plant operations. It can be adjusted to fit site-specific conditions and requirements, avoiding the complexity and cost of a power plant master control or auxiliary drive controls. It also interfaces easily with SCADA systems. Remote access options allow users to control from a single touchscreen in the control room or from any internet-connected PC.

Optimizing the combustion chamber. Piston designs can be tailored to the application, providing the optimum turbulence for complete combustion depending on fuel characteristics. Cylinder dead space is minimized, allowing more complete combustion and reducing emissions of unburned hydrocarbons and other pollutants.

Blowby recirculation. Un-burned hydrocarbons and oil mist that bypass the piston rings and enter the crankcase are extracted by a crankcase breathing system. Special filtration separates oil and returns it to the crankcase; unburned fuel then can be rerouted to the fuel system, adding to fuel efficiency.

High-energy ignition. Pre-chamber spark plugs have been improved to optimize their geometry. These plugs admit air and fuel through small orifices and upon ignition eject flame through those same orifices. This high-energy ignition enables the engine to operate on an extremely lean fuel mixture without risking lean misfire, thus sustaining high power output and low emissions over extremely long lifespans.

Lower maintenance costs

Among their benefits, these technology advances contribute to a new paradigm in maintenance intervals. The intervals for spark plug replacement and oil changes are designed to be the same, at 4000 hours – some six months of continuous operation, or more than double the interval expected with traditional technology.

After about eight hours of planned downtime for routine service, the engine is ready to run for six months more. Cylinder head overhaul intervals are up to 32,000 hours (up to 16,000 hours even on unpurified biogases) and major overhaul intervals are 64,000 hours regardless of fuel quality.

Operating costs are further reduced by low oil consumption: technology innovations have improved oil consumption by up to 50%, a savings of 500 gallons per year on a 20-cylinder engine-generator rated at 2 MW.

Well suited for CHP

Today’s advanced gas engines have proven themselves in the field, often with uptime approaching 98% year after year. They are built for flexibility to operate on gases of varying quality, including natural gas, landfill and digester gas, coke gas and coal mine methane. The latest configurations develop high power output in a footprint up to 50% smaller than traditional units, providing an excellent fit in small engine rooms or containerized power plants.

CHP enhances the equipment’s inherent efficiencies. Economically viable CHP configurations can range from coolant circuit and exhaust gas heat recovery systems optimized for large, continuous process heating demands, to systems with low-cost coolant circuit heat exchangers for limited domestic water or space heating, essentially as bonus energy where full-time or part-time power generation is the driving factor.

Seldom if ever have market conditions been more favourable for on-site gas-fuelled power generation and CHP. The time is right for industrial and commercial facilities, wastewater treatment plants, institutions, utilities, food processing facilities and other large power users and producers to explore the economic possibilities of generation with today’s gas engine technology.

Cases in point

These three examples illustrate how advanced gas engines are enabling cost-effective CHP in diverse applications and climate conditions worldwide.

Guangzhou Zhujiang Brewery Group Co. Ltd

In 2005, this Chinese brewer looked for a combined cooling, heating and power (CCHP) system to enhance its power capabilities, in part using seasonally produced biogas from grain and yeast by-products.

A cost analysis showed that a CHP system could use up to 95% of the brewery’s biofuel methane to generate power and supply heat from engine coolant and exhaust. To compensate for a variable biogas supply, the company installed advanced engine-generators with different output ratings: one 460 kW Cat G3508 and one 960 kW Cat G3516.

The generator sets’ combined efficiency reached up to 80%, and costs savings on the system were $58,000 per month, for an estimated projected payback of 18 months.

City of Lethbridge, Alberta

The city’s wastewater utility since 2005 has used a CHP system to consume all digester gas and meet the wastewater treatment plant’s thermal demand. The system is built around two Cat G3516 16-cylinder lean-burn gen sets, each rated at 825 kW.

A dual heat-recovery system captures heat from engine jacket water and exhaust, adding heat to the existing heating loop. The gensets operate in parallel with the local utility grid, delivering up to 1100 kW to the grid while also fulfilling about 70% of the treatment plant’s electrical load. The generators typically burn a digester gas blended with 10–15% pipeline natural gas. The gensets have consistently met demanding sound-level criteria affecting both indoor and outdoor environments. The project included a 10-year maintenance agreement with a 94.5% uptime guarantee.

Agricultura y Exportación

This pepper producer in Balsicas, Spain, operates a CHP system that also uses exhaust carbon dioxide for fertilization. The company’s 43 ha of greenhouses produce 5500 to 6500 tonnes of peppers for export each year.

The company invested €3.2 million ($4.1 million) in a turnkey natural gas-fuelled CHP plant supplied by Germany-based MWM, a Caterpillar company. The facility uses an MWM TCG 2020 V20 engine-genset with an electrical output of 2 MW at 42.5% electrical efficiency.

Heat energy recovered from engine coolant and an exhaust gas exchanger is stored in a buffer tank. This allows heat energy to be stored when the greenhouses do not need heating and released when heat is demanded.

Michael A. Devine is gas product/marketing manager with the Electric Power Gas Group of Caterpillar Inc., based in Lafayette, Indiana, US Devine_Michael_A@cat.com.

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