BioCycle September 2006, Vol. 47, No. 9, p. 59
Overview of several technologies provides insights into the effect of the quality of landfill and anaerobic digester gas on equipment to generate electricity.
OPPORTUNITIES to produce electricity from methane are being improved continuously as manufacturers introduce new and upgraded equipment to address market needs and varying characteristics of raw biogas. In the renewable energy from organics recycling arena, the two primary types of electricity generation equipment are microturbines and reciprocating gas engines. This article provides a brief overview of some of the systems and equipment available to methane-generating projects that want to convert biogas into electricity. Questions were posed to several manufacturers to gain a better understanding of their specific technologies and applications in the world of generating power from organics recycling.
Microturbines are small gas turbines that burn methane, mixed with compressed air. The hot pressurized gases that result from combustion are forced out of the combustion chamber and through the turbine wheel, causing it to spin and turn the generator. In addition, most microturbines include a compressor, recuperator (a gas to gas heat exchanger that uses some of the heat from the exhaust to preheat the incoming air), and various devices for controlling combustion and converting electricity to the desired form. They are also known for their relatively clean combustion and low exhaust emissions, particularly NOx components.
“The true uniqueness of microturbines is in their small size,” noted an article in BioCycle on the inner workings of microturbines. “All of the components are housed in a self-contained unit roughly the size of a large household appliance. Microturbines can be scaled to accommodate small power generators and users. Two or three units can generate electricity from biogas produced on a 500-cow dairy farm. Typical units range from 30 to 100 kW in power generation capacity. Multiple units can be combined to increase the total power output to several megawatts.”
Reciprocating gas engines, on the other hand, are essentially natural gas engines that have been transformed into machines that can handle larger volumes of fuel due to the CO2 in the fuel, and have been modified to accept higher levels of contaminants in the incoming air stream (versus the consistency of natural gas). According to a paper written by Michael Devine, Gas Product Marketing Manager in the Electric Power Group of Caterpillar, Inc., the latest generation gas engines incorporate technologies developed with the U.S. Department of Energy Advanced Reciprocating Engine Systems (ARES) program, and attained commercial status in 2002. “These lean-burn, electronically controlled units …. deliver 1 to 2 MW of capacity and are capable of 43 percent mechanical efficiency, with NOx emissions rated as low as 0.5 g/bhp-hr without exhaust after treatment,” writes Devine in “New Paradigms In Efficiency, Emissions and Power Cost in Landfill Gas-Fueled Generators.”
In an interview with Devine, he explains that the machines “accept higher levels of CO2 gases as well as other contaminants.” Contaminants of greatest concern in landfill gas, he adds, are sulfur compounds, halides, water vapor, silicon and siloxanes. “There are also site to site, and even hour to hour differences in the amount of methane and CO2 in the biogas and in some cases, the landfills have leaky gas collection pipes, so there may be excess air (oxygen and nitrogen) in the fuel,” says Devine. “As you go to different landfills around the world, the level of contaminants and methane in the gas can change site by site depending upon the source of the refuse, ambient conditions and the site management.” In another paper, “Dealing With Landfill Fuel: Evaluating Fuel Treatment Options,” he explains that “the effects of contaminants on the engine depend on a number of factors, including engine component metallurgy, exposure time and rate, engine operating temperature and the brake mean effective pressure of the engine.”
Capstone Turbine Corporation: Currently, over 3,500 Capstone microturbines have been sold worldwide in a wide variety of applications, ranging from natural gas and oilfield gas to biogas and liquid fuels, reports Rick Wade, Director of Sales for Capstone’s Renewable Energy Systems. Within the biogas area, there are hundreds installed at landfills, wastewater treatment plants (WWTP), and in agricultural waste and industrial food processing applications throughout North America, Europe, and Asia. At the beginning of 2006, the company introduced a 65-kW microturbine that uses waste flare gases from landfills or WWTPs to create renewable energy. “The new CR65 is available with an optional stainless steel integrated heat exchanger for heat recovery that can provide for a system efficiency of up to 65 percent,” says Wade. “Another development in 2006 was that Capstone’s 30- and 65-kW renewable energy systems became the first generators to be classified by Underwriters Laboratories to the UL2200 standards for Stationary Engine Generators, under the new category of Engine Generators Fueled by Biogas or Raw Natural Gas. This along with the UL 1741 certification allows greatly simplified interconnection to the power grid for Capstone’s microturbine systems.”
What is critical to the operating success of the total microturbine energy system is proper pretreatment (or conditioning) of the biogas prior to its entering the microturbine. “The gas must be dry, compressed to a minimum pressure and have minimal amounts of siloxanes and hydrogen sulfides, for a long-term, reliable installation,” emphasizes Wade.
There are a number of case studies available on the Capstone website (www.microturbine.com). For example, the Sauk County Landfill in Wisconsin installed 12 30-kW microturbines, which along with the system’s gas conditioning skid, is turning 260,000 cu.ft./day of biogas into about 6,600 kWh/day of electrical energy. The energy, enough to power about 150 homes, is sold back to the local utility. More recently, 10 30-kW microturbines were installed at the Sheboygan, Wisconsin WWTP to process digester gas. The cost of the microturbines was $300,000; the city paid $200,000, and Alliant Energy, which will own the microturbines and maintain them for six years, paid the balance. After the six-year period, Alliant Energy will sell the equipment to the city for $100,000. Officials expect the electricity and heat generated will save the city about $70,000/year by cutting the WWTP’s electric and natural gas bill by 40 percent and in addition, will earn renewable energy and emissions credits.
Ingersoll Rand: Through its Energy Systems Division, Ingersoll Rand (IR) markets two microturbine units – the MT70 series with capacity of 70 kW of continuous on-site electrical power, and the MT250 series, with capacity of 250 kW. In an August 2006 report on its website (www.irenergysystems.com), the company notes that its microturbines in service around the world have a total capacity of 14 MW, enough to power 9,000 homes. The microturbines feature a patented recuperator and combustor. In anaerobic digester applications, the thermal energy recovered from the microturbine engine can help maintain optimum digester temperature, reducing the need for an outside fuel source.
The first field test microturbine units went into operation in mid-2000. The first commercial 70 kW unit was shipped in 2002, followed by the 250 kW in 2004. Last year, says the company, the MT250 became the first (and only) microturbine to be certified as meeting the California Air Resource Board’s 2007 emission standards for distributed power generation technologies.
An innovative project underway in Grove City, Ohio will include an IR microturbine. The Solid Waste Authority of Central Ohio (SWACO) is working with FirmGreen Energy, Inc. to develop the Green Energy Center. FirmGreen is installing its patented CO2 Wash technology to clean and process SWACO’s landfill gas for green electricity generation and production of renewable compressed natural gas (CNG). The microturbine will generate electricity for the CNG plant, the CNG fueling station, and SWACO’s administrative and maintenance buildings.
Caterpillar, Inc.: Caterpillar has been very involved in the design of engines fueled by landfill and anaerobic digester gas to minimize the effects from fuel impurities. Features in the Caterpillar reciprocating gas engines include specific component modifications and use of corrosion-resistant materials. Another feature installed in engines fueled by landfill gas is a water temperature regulator. “The warmer jacket water temperature inhibits water vapor entrained in the exhaust gas and blow-by gases from condensing on the cylinder liners and on other internal engine surfaces, thus limiting the formation of acids and attendant corrosion,” explains Michael Devine of CAT. “It also helps keep condensation and acid formation from reaching the lubricating oil, further protecting components and helping to extend oil change intervals.” A low-pressure air pump draws warm, filtered air into the crankcase and removes harmful gases.
Caterpillar gas engines (www.cat.com) have been installed at numerous landfill gas recovery projects, as well as some anaerobic digestion facilities. “We learned a lot with our first low energy generator set installations at Waste Management landfills in the early 1980s,” recalls Devine. “That’s where we discovered the effect of siloxanes, and why landfills can generate so many different variations of gas content. Digester gas from agricultural operations, on the other hand, tends to be more predictable. There are more consistent levels of methane and CO2 based in part on the protein fed to the livestock.”
One of the decisions that biogas recovery projects need to make when evaluating different combustion technologies is putting capital into a pretreatment system versus purchasing an engine capable of consuming gases containing contaminants. “It’s a ‘pay me now or pay me later’ situation,” says Devine. “You can put all kinds of contaminant-limiting devices on an engine upfront to remove halides, sulfur compounds, siloxanes, etc. Those devices cost money, and maintaining the equipment costs money.” The bottom line, he notes, is that it really boils down to the configuration that provides the best cost/kilowatt hour. The best answer may be a combination of both limited fuel cleaning and engine technology.
Among the installations using the Caterpillar equipment is Modern Landfill in Model City, New York. Landfill gas is combusted in seven CAT engine generator sets (Model G3516) producing 5.6 MW of electricity. Heat captured from the jacket water and engine exhaust is used by an adjacent hydroponic greenhouse operation, H2Gro, LLC. The facility houses 7.5 acres of hydroponic, temperature-controlled greenhouses, producing an average of 10,000 lbs/day of tomatoes. The energy needs of H2Gro are supplied without using any fossil fuels. Annual fuel savings are estimated at $700,000, and due to the low heating costs, the greenhouse can operate year-round, providing 40 local jobs.
GE Energy’s Jenbacher: General Electric Energy’s Jenbacher division manufactures gas-fueled reciprocating engines, packaged generator sets and cogeneration units for power generation. Its engines range from the Type 2 unit’s 250 to 350 kW range up to its Type 6 engine that is in the 1.8 to 3 MW power range. Jenbacher patented the LEANOX® lean mixture combustion system to manage fluctuations in the methane content of gases being combusted and minimize NOx emissions. The company began developing engines for landfill gas in the 1970s, and for biogas from wastewater treatment plants in the 1980s. “We have a long history working with these gases,” says Michael Wagner with GE Energy Jenbacher. “Our focus from the beginning was to ensure reliable operation when burning gases of low heating value and with varying composition. With all of these gases, the methane content varies, while at the same time, the customer wants reliable and constant power generation at constant low emissions levels. So it is up to the engine management system, in our case the Leanox control system, to make that happen.”
From its base in Jenbach, Austria (www.gepower.com), the company is seeing rapid growth in the installation of biogas digesters. Germany is seeing a lot of that activity. Under its Renewable Energy Law, Germany established fixed feed-in tariffs for electricity fed into the grid, creating an incentive for power generation projects. There is an emphasis in the incentives for biogas generated from dedicated energy crops. In 2005, a new combined heat and power biogas plant using GE Energy’s Jenbacher gas engine system began operating near the city of Soltau, about 60 miles south of Hamburg. The facility uses corn and rye as biomass; the plant is designed to generate 4.2 MW of electricity and 4.3 MW in thermal energy. The electricity is fed into the regional grid; the thermal energy supports a yeast production process.
Landfill gas recovery is quite common throughout Europe, as is electricity generation from digester gas at wastewater treatment plants. In North America, a Jenbacher 1 MW cogeneration system was installed at a 36,000-head feedlot in northern Alberta, as part of a pilot-scale project by Highmark Renewables to anaerobically digest manure. Another market being tapped by GE Energy’s Jenbacher is congeneration from pyrolysis gas produced at domestic waste gasification plants (35 percent water content) and gas from wood chip gasifiers. The key, according to Jenbacher literature, is that the gas meets certain requirements related to its lower heating value, methane content and the laminar flame speed. Gas purification also is critical to successful combustion of these types of gases.