With applications that range from firming intermittent resources to providing combined heat and power solutions, reciprocating engine manufacturers have made strides to improve their machines’ efficiency and environmental profile.
Reciprocating gas engines find work in multiple power generation applications, from providing fast-start backup generating capacity for intermittent renewable resources to offering scalable and increasingly efficient solutions for commercial and industrial combined heat and power (CHP) systems.
Major global manufacturers such as GE-Jenbacher, Cummins, and Wärtsilä have focused engineering efforts in recent years on improving their machines’ efficiency and meeting environmental regulations, particularly nitrogen oxide (NOx) emission rules.
Another factor driving interest in reciprocating engines for industrial CHP applications in the U.S. is the long-term prospect for plentiful natural gas supplies and relatively stable prices. Commercial and industrial customers are finding in some instances that they can generate their own electricity for less money than if they were to buy power from the grid, said Scott Nolen, product line management leader, GE Gas Engines for Power Generation. Creative uses of reciprocating engine technology that include not only power generation but also thermal energy and even captured emissions are gaining traction in some parts of the country.
For example, in August 2012, GE and Houweling’s Tomatoes, a leading California-based greenhouse, completed work on a CHP project that also captures carbon dioxide (CO2) from the engine exhaust for plant fertilization. Using two GE 4.36-MW Jenbacher J624 two-staged turbocharged natural gas engines and a GE-designed CO2 fertilization system, the plant provides heat, power, and CO2 to Houweling’s 125-acre tomato greenhouse in Camarillo, Calif. The system provides 8.7 MW of electrical power and 10.6 MW of thermal energy (hot water) for heating the greenhouses. The system offers a total thermal efficiency of nearly 90%. When considering the avoided energy that would be required to externally source the CO2 and the recovery of water from the exhaust, GE said the overall system efficiency (effectiveness) exceeds 100%.
This past July, the largest power plant running exclusively on gas engines on the African continent—and among the first of its kind in the Republic of South Africa—entered service. The Sasol Gas Engine Power Plant, located south of Johannesburg, is powered by 18 Wärtsilä 34SG gas-fired generating sets with a combined operating capacity of 140 MW (see the Top Plant story on this plant in the September issue). Electricity produced by the plant is used by Sasol’s nearby chemical factory, with about half of the electricity production fed to the national grid.
One of Wärtsilä’s largest U.S. installations is the 202-MW Pearsall Power Plant owned by South Texas Electric Cooperative. The plant includes 24 reciprocating engines and is used to firm intermittent wind generation resources.
And on Long Island, N.Y., about 60 miles east of New York City, the William Floyd School District, with a student population of approximately 11,000, faced steadily rising on-peak electric rates that were straining operational budgets. The district decided to install a CHP system from Cummins Power Generation, which supplies on-peak electricity as well as heating and cooling to three school buildings. In the first three years of operation, the CHP system saved the school district more than $1.2 million. The CHP system consists of two PowerCommand 1.25-MW reciprocating engine generators with a combined capacity of 2.5 MW. The generator sets feature a QSV91G lean-burn natural gas engine, which is a 91-liter engine with a high exhaust temperature in relation to the amount of electricity produced, making it a good fit for CHP applications with large heating or cooling loads.
“It is the best performing plant I’ve been involved with,” said Peter Schroeck, manager ESB North America/Caribbean Business Development for Cummins Engines. He said the growth potential for CHP applications has benefited from prospects for stable and more predictable natural gas prices. Gas price volatility in the past made it difficult to build a financial model that would hold up with any certainty.
A barrier to growth for reciprocating engines that still must be addressed is the lower heat rate common with many technologies. System efficiencies typically range from 36% to 45%, Schroeck said, with a 1 percentage point gain in efficiency considered “tremendous.” Much of the current effort aimed at improving efficiency focuses on reducing friction in the engine and working to reduce exhaust temperatures, both of which can help produce more high-value product, he said.
A 2011 report by the North American Electric Reliability Corp. (NERC) said that reciprocating engine–based generators are among the most mature distributed generation (DG) technologies available, accounting for about 90% of current DG installations in North America. The generators themselves are typically synchronous machines that can provide dynamic reactive power and voltage control. Applications include utility power generation, peak shaving, and remote customer and backup power. Reciprocating engines can run on a range of fuels including diesel, natural gas, gasoline, propane, and methane. (This article focuses on natural gas–fueled machines.) Reciprocating engines also are typically fast starting, fast ramping, and have good part-load performance. The NERC report said these characteristics make reciprocating engines a good reliability resource when they are responsive to system operator commands.
CHP, also sometimes known as cogeneration, is one form of DG that generates both electric power and thermal energy. According to the Department of Energy, at the end of 2011 nearly 70 GW of CHP generating capacity was installed across the U.S. and accounted for almost 7% of total U.S. installed capacity. Of that 70 GW, 25 GW was in the industrial sector, 2 GW in the commercial sector, and 43 GW in the electric power sector. The average capacity factor for generators at industrial CHP plants was around 57%.
Not all of that DG capacity represented reciprocating engines, by any means. Combustion turbines and even coal-fired boilers are in use. Even so, natural gas is the most common primary energy source used in CHP applications, followed by coal and biomass (often in the form of waste products at paper mills). Indeed, more than 65% of CHP capacity currently operating in the U.S. uses natural gas as a primary fuel source. CHP generators in the U.S. and elsewhere are more likely than other generators to burn a variety of fuels (such as different types of biomass and industrial byproducts).
For example, in July, the Middle Eastern country of Lebanon rolled out its first landfill gas-to-energy project near Beirut. The on-site power project is powered by a GE Jenbacher J312 landfill gas engine and potentially will generate 637 kW. That capacity could expand in the future if this initial application proves successful. GE said that landfill gas typically consists of approximately 55% methane, which the Jenbacher engine uses for fuel.
In Germany, the entity Stadtwerke Rosenheim supplies water, electricity, gas, process steam, and district heating for residential and local industrial customers. Using combined reciprocating engines in a CHP application offered the city of Rosenheim a more flexible energy supply infrastructure. A J920 FleXtra gas engine (Figure 1) combined with heat storage was combined with a waste incineration plant and four existing GE engines to create a CHP plant that generates about 40% of the electricity and 20% of the heat required by the city.
CHP facilities often are built at factories or at educational or corporate campuses that have heat or steam demands. Applications are widespread across Europe but less so in the U.S. However, a concentration of CHP units may be found along the Gulf Coast, where cogeneration plants are located near refineries and chemical plants. A number of smaller-scale CHP installations are located near pulp and paper mills in the South, in northern Wisconsin, and in Maine; these burn wood waste byproducts as fuel. CHP installations are also common in states with a history of utility regulation that favors the application, such as Massachusetts, New Jersey, North Carolina, California, and New York.
In addition, CHP was the focus of an August 2012 Executive Order signed by President Obama. That order set a national goal of 40 GW of new CHP capacity by 2020, a 50% increase. The order said meeting that goal would save energy users $10 billion a year, result in $40 billion to $80 billion in new capital investment in manufacturing and other facilities, and reduce emissions equivalent to 25 million cars.
Energy and Environmental Analysis, an ICF company, produced a white paper for the U.S. Environmental Protection Agency that mentions a widely cited reference that reciprocating internal combustion (IC) engines are one of the most common engines used for DG. With more than 9,000 MW of installed capacity in the U.S., the IC engine makes up around 75% of all fossil fuel–driven DG units. Diesel and natural gas are used in these engines—the former is used in compression ignition engines, the latter in spark ignition engines. Other spark ignition fuels include biogas, landfill gas, and propane.
IC generators are also known as “generator sets” or “gensets.” As such, they combine an IC engine, a generator, and various ancillary devices that form a DG unit. The technology benefits from low relative cost, high reliability, long operating life, short startup times, and readily available fuel sources. The IC engine also has high part-load efficiency, meaning it can match or follow the electric load demand within a 30% to 100% load range fairly cost effectively and with little decrease in efficiency.
Current generation natural gas engines offer low first cost, fast startup, and reliable service when properly maintained, in addition to excellent load-following characteristics and heat recovery potential. Electric efficiencies of natural gas engines range from 30% lower heating value (LHV) for small stoichiometric engines (<100 kW) to more than 40% LHV for large lean-burn engines (>3 MW). Waste heat recovered from hot engine exhaust and from the engine cooling systems produces either hot water or low-pressure steam for CHP applications.
Reciprocating engine technology has improved over the past several decades, driven in large part by economic and environmental pressures for power density improvements (more output per unit of engine displacement), increased fuel efficiency, and reduced emissions. Computer systems have also advanced reciprocating engine design and control, accelerating advanced engine designs and making possible more precise control and diagnostic monitoring.
Heat in the engine jacket coolant accounts for up to 30% of the energy input and is capable of producing 200F to 210F hot water. Some engines, such as those with high-pressure cooling systems, can operate with water jacket temperatures up to 265F. Engine exhaust heat represents 30% to 50% of the available waste heat, and exhaust temperatures of 850F to 1,200F are typical. By recovering heat in the cooling systems and exhaust, approximately 70% to 80% of the fuel’s energy can be used to produce both power and thermal energy.
Manufacturers also have introduced generator sets that feature “lean-burn” technology. The combustion is considered “lean” when excess air is introduced into the engine along with the fuel, in this case natural gas. This produces two effects. First, the excess air reduces the temperature of the combustion process, which in turn reduces the amount of NOx produced by nearly half, compared to a conventional natural gas engine. Second, since excess oxygen is available, the combustion process is more efficient and more power is produced from the same amount of fuel.
Any air/fuel reaction requires an energy source to initiate combustion. In natural gas engines, the spark plug performs this function. In lean-burn engines, the combustion process is enhanced by premixing the air and fuel upstream of the turbocharger before it is introduced into the cylinder. This creates a more homogenous mixture in the combustion chamber and reduces the occurrence of “knocking” or early detonation. To prevent either knocking or misfiring, the combustion process must be controlled within a narrow operating window. Charges in air temperatures and volume, together with air/fuel ratio, are constantly monitored by microprocessor-based engine controllers that regulate not only the fuel flow and air/gas mixture but also the ignition timing.
Lean-burn engines marketed by Cummins, for example, are designed to operate at a lean air/gas ratio of lambda = 1.7. (Traditional stoichiometric natural gas engines have an air/gas ratio of lambda = 1.0.) A richer mixture (stoichiometric) can potentially produce knocking and higher NOx emissions; a leaner mixture than lambda 1.7 may not combust reliably and may cause misfiring, which raises hydrocarbon emissions.
One benefit of lean-burn engine technology is reduced emissions in the exhaust. For example, Cummins’ lean-burn gas engine generators have NOx emissions as low as 0.85 grams/brake horsepower (BHP)-hr, and produce low amounts of hydrocarbons, carbon monoxide, and particulate matter. This allows the generator sets to meet air quality regulations without after-treatment devices in the exhaust stream. For still lower emissions, lean-burn gas engine generator sets may be coupled with after-treatment options such as selective catalytic reduction systems and oxidation catalysts. These can result in NOx levels at or below 0.15 grams/BHP-hr, capable of meeting most prime mover power emissions regulations worldwide.
Another advantage of lean-burn technology with full-authority electronic engine controls is the ability to operate on gas with a wide range of quality. A measurement called the methane number (MN) is used to determine fuel gas suitability as an engine fuel. Most natural gas has an MN from 70 to 97, and pipeline quality gas typically has an MN of about 75. Resource recovery gas from landfills or sewage treatment facilities is typically of lower quality, but is often suitable for use in lean-burn engines. Cummins’ lean-burn gas engine generators operate on gas with an MN of 50 or greater. Gas with an MN below 70, however, may require derating the generator output.
In addition to these advancements, work also is under way to increase plant net efficiency, optimize cooling systems, pursue alternator improvements, and enhance combustion systems, said Thomas Hägglund, vice president of Power Plant Technology for Wärtsilä Corp. Hägglund said Wärtsilä has developed advanced engine control systems that optimize each cylinder individually to enhance combustion performance. Automated systems check combustion during startup, and engine lubrication is optimized for fast startups. The effect is to improve the machine’s efficiency during ramp-up as well as ramp-down, two critical phases when a machine is following an intermittent renewable energy load. “There are a lot of factors to monitor,” Hägglund said.
And GE Jenbacher has worked to improve combustion in the cylinders, the fuel/air ratio, and pressures, which Nolen said have risen 50% through a combination of stronger connecting rods, steel pistons, and spark plug improvements. The net effect has been to increase engine efficiencies from around 36% to in excess of 45%. In 2010, GE Jenbacher also introduced a two-stage turbocharger system that increases power density by around 10%.
Leading Prime Mover
These improvements are in addition to previously existing features that have made reciprocating engines a leading prime mover for CHP and other DG applications, according to the Energy and Environmental Analysis report. These features include:
■ Size range: Reciprocating engines are available in sizes from 10 kW to over 10 MW and may be linked together to provide even larger generating capacities.
■ Thermal output: Reciprocating engines can produce both hot water and low-pressure steam.
■ Fast startup: In peaking or emergency power applications, reciprocating engines can quickly supply electricity on demand.
■ Black-start capability: In the event of an electric utility outage, reciprocating engines require minimal auxiliary power requirements. Generally, only batteries are required.
■ Availability: Reciprocating engines have typically demonstrated availability in excess of 95% in stationary power generation applications.
■ Part-load operation: The high part-load efficiency of reciprocating engines ensures economical operation in load-following applications.
■ Reliability and life: Reciprocating engines have proven to be reliable power generators, given proper maintenance.
■ Emissions: Diesel engines have relatively high emissions levels of NOx and particulates. However, natural gas spark ignition engines have improved emissions profiles.
The economics of engines in on-site power generation applications often depend on effective use of the thermal energy contained in the exhaust gas and cooling systems. As a rule of thumb, this generally represents 60% to 70% of the inlet fuel energy. Most of the waste heat is available in the engine exhaust and jacket coolant, while smaller amounts can be recovered from the lube oil cooler and the turbocharger’s intercooler and aftercooler. ■
— David Wagman is content director for ELECTRIC POWER, which will be held in New Orleans April 1–3, 2014.