TOP BILLING on the biogas to electricity stage typically goes to the microturbines and reciprocating gas engines. But in the theater of renewable energy from organics recycling, the show can’t go on without the supporting cast of gas clean-up and cooling equipment and the gear to connect to the power grid. This month’s “What’s New” column focuses the spotlight on the full spectrum of equipment and systems needed to generate electricity from anaerobic digesters and landfill gas recovery projects.
At the risk of oversimplifying, the biogas power equipment marketplace seems to be divided into several categories: manufacturers of microturbines and reciprocating gas engines, manufacturers of components that are needed to efficiently operate these engines and make grid connections, and biogas power “packagers,” whose business is to assemble the complete system needed to connect the biogas line to the power line.
In addition to methane, the gas stream from landfills and anaerobic digesters contains moisture and a variety of contaminants (e.g., siloxane, hydrogen sulfide, dust and particulates) that need to be removed both to maximize the efficiency of and protect the turbines and generators. Gas pretreatment components that perform these tasks include cooling and contaminant removal systems.
At the heart of any biogas power system is the combustion engine that produces electricity. The two types of units used primarily in the biogas marketplace are microturbines and reciprocating gas engines. These are defined as follows:
Microturbines: Small gas turbines that burn methane, mixed with compressed air. In most applications, a gas pretreatment process cleans the methane to remove impurities, and it is then mixed with compressed air and combusted to produce a hot pressurized gas that spins a turbine wheel, and an electrical generator. Microturbines are differentiated by either recuperated, those that recover exhaust heat in order to increase the temperature of the combustion process and its overall efficiency; and unrecuperated microturbines, which do not recover exhaust heat, are less efficient, but lower cost.
Reciprocating Gas Engines: 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).
The major reason reciprocating gas engines are selected over microturbines is for larger scale power production. But for smaller capacity systems, the advantages of microturbines is just that — they are small. Requiring less space than a household refrigerator, units are sized to produce from 25 to 500 kW each, and are usually located at the point where the energy will be utilized, thus eliminating transmission losses. In some cases, microturbines are selected over gas engines because of air emissions requirements (the turbines are known to have lower emissions levels).
“The sweet spot for microturbines is four to 12 units,” says Jan Scott of Unison Solutions, a company that packages biogas to energy systems. “That is a manageable size, they are efficient and easy to service, and the footprint isn’t unreasonable. As a rule of thumb, a four turbine, 120 kW system can service an anaerobic digester at a 750 head dairy operation that doesn’t take additional waste streams, producing about 80 cubic feet/minute of gas. A 12-unit system at a landfill can burn a couple hundred cfm of gas.”
Most of the large agricultural manure digestion facilities produce at least 850 kW, with much higher output common. Mike Casper, Regional Manager for Microgy, Inc., explains that for his company to install one of its anaerobic digester systems on a dairy farm with 1,000 head of cattle, producing 850 kW of electricity, would require eight or nine 100kW microturbines, compared to installing just one 850 kW reciprocating gas engine. “There may be less moving parts with microturbines, and reduced O&M costs, but there is some advantage to only having one unit to operate,” explains Casper.
Increasingly, biogas power installations are taking advantage of the cogeneration potential (CHP or combined heat and power). Waste heat from the microturbines and gas engines can be transferred via a heat exchanger to produce steam or provide hot water for a greenhouse, maintain needed digester temperatures, heat dairy barns, etc.
BIOGAS POWER PACKAGERS
Martin Machinery, Inc.: Based in Latham, Missouri, Martin Machinery engineers, packages and assembles biogas power systems — essentially integrating all the necessary equipment for the generator set between the gas line and the power line. “We have been building systems since the late 1970s and early 1980s, starting with my great uncle’s dairy farm in Lancaster County, Pennsylvania,” says Marcus Martin. “One of the first installations we did was with a small Caterpillar engine for a digester, and it still is running there today.” He adds that interest in biogas production from anaerobic digesters on farms started to really pick up in the mid-1990s, and continues today. While it utilizes reciprocating gas engines from the original equipment manufacturers, Martin Machinery makes its own cooling and heat recovery systems, base rails and frames and skid assemblies. It formed a separate company, Gen-Tec, to manufacture and assemble controls and switch gear to tie into the grid. It imports the Huegli gas mixing engine control technology that adjusts the engine to accept varying fuel qualities. Gen-Tec also offers a digester gas pressure tracking system so that the engine adjusts its power output to match the biogas being produced.
When asked “What’s New”, Martin said on the biogas engine side, the lean burn engines are “a big deal. Basically, you are getting more electricity for the same amount of cows.” He explains that the engines have higher compression ratios that make them more efficient (using 4 valves/cylinder). “Essentially, a lean burn engine has the capability of converting one unit of biogas into 1.25 units of electricity, whereas the standard engine produces one unit of electricity from one unit of biogas.” Martin Machinery had to redesign its cooling system to remove moisture to prepare gas for the lean burn engines. Modifications also were made to its particulate filter. In terms of maintenance, the company has trained factory personnel that service its installations in North America.
Microgy, Inc.: Microgy builds, owns and operates anaerobic digestion facilities that process agricultural and food by-product residuals to produce biogas — and generate carbon offset credits. The company, a subsidiary of Environmental Power Corporation, imports a European digester technology and works with original equipment manufacturers for the generator sets. Microgy has two farms that use Waukesha reciprocating gas engines, and one using an 850 kW GE Jenbacher engine. “Because we codigest fats, oil, and grease (FOG) with dairy manure at these farms, we are generating more biogas than if we were just digesting manure,” explains Mike Casper. “This is due to the significantly greater fat content of FOG — up to 80 percent — compared to manure at 1 to 2 percent, resulting in more biogas, and more power generation.”
The Microgy business model is to increase power generation and profits for the host farm and the company by sourcing FOG to supplement the manure. The three Wisconsin dairy farms currently under contract to Microgy were financed by Microgy and owned by the farmers. New projects in development will be owned by Microgy with a business arrangement to share revenues with the farmer. With this financial incentive to increase codigestion of FOG, Microgy expects to be operating on farms that produce more than 1,000 kW. In addition, the company announced that it is constructing a facility (Huckabay Ridge) near Stephenville, Texas that will produce gas to be treated and compressed to pipeline-grade methane and sold as a commodity directly into a nearby natural gas pipeline. The facility is to be located adjacent to Producers Compost Incorporated, a composting site that receives manure from over 20,000 cows.
Unison Solutions, Inc.: The company custom engineers and fabricates biogas conditioning and distributed generation systems. Notes Jan Scott, “we build everything between the gas source and the generator or other end-use,” including gas treatment and filtration components, e.g., for moisture, H2S and siloxane removal. The company assembles the gas treatment and compression systems on skids — using either microturbines or gas engines depending on the scale and type of project — and then will do the installation if required by the customer. It also offers maintenance and service contracts. In terms of “What’s New”, Scott says Unison is offering a siloxane filtration system it manufactures for Applied Filter Technology (AFT) that is continuously self-regenerating. SWOP™ utilizes the energy in a fraction of the biogas to operate itself, completely removing siloxanes and other organosilicons and VOCs that can damage emission catalysts, according to AFT’s literature. In a landfill gas application, the SWOP process utilizes some of the purified landfill gas (around 0.5% to 1.5%) for continuous self-regeneration. VOCs and siloxanes are removed and concentrated into a small gas stream that is sent to an enclosed ground flare for destruction.
One area that is growing for Unison Solutions is direct-use of digester and landfill gas after it goes through a cleaning process. “Basically, we install a system that cleans and compresses the gas that is then sent to a boiler,” explains Scott. “These applications are used primarily to displace natural gas use, e.g., at food processing and manufacturing facilities.”
The two primary pro-viders of microturbines in North America are Capstone Turbine Corporation and Ingersoll Rand. The following are highlights on “What’s New” in 2007 for these companies.
Capstone: Last year, Capstone introduced a 65-kW microturbine to service higher output biogas projects (the company also makes a 30-kW unit). The CR65 is available with an optional stainless steel integrated heat exchanger for heat recovery. Rick Wade, Director of Sales for Capstone’s Renewable Energy Systems, notes that the company is seeing a lot more interest from the agricultural sector. “We are getting a lot of calls about the larger unit from large dairy farmers and cattle farmers, as well as inquiries about digesting waste associated with distillers grain from ethanol production,” says Wade. “Interest is growing as well in the heat exchanger unit. Potential users are understanding that if they are going to create energy out of waste, they should also take advantage of the thermal energy generated. Ranchers, farmers and wastewater treatment plant operators are finding ways to use that heat, e.g., for the digesters or the dairy barns. Using the heat recovery module makes the overall system more cost-efficient.”
Increasingly, anaerobic digesters built to process one primary feedstock, e.g., manure or wastewater treatment solids, are being used to process other feedstocks, e.g., food processing residuals such as cheese whey or fats, oils and grease. Adding these higher energy feedstocks ultimately boosts the heating value of the gas produced, which in turn can increase power generation from the same volume of gas. “Instead of getting 60 percent methane in the biogas, which is pretty typical for digesters, some plants are getting 65 to 75 percent methane,” notes Wade. “The higher the methane content, the less overall biogas the turbine needs to burn to produce electricity. We also are seeing more fluctuation in the hydrogen sulfide levels, which our equipment can tolerate up to specified limits.” Critical to the equipment handling the contaminants in the biogas stream is maintenance according to Capstone’s recommendations. “The industry has come a long way in recognizing and dealing with the havoc created by siloxane and hydrogen sulfide in the biogas stream,” he adds. “You have to clean the gas of those contaminants and maintain the equipment annually, and then the turbines will run 24/7 as designed.”
Ingersoll Rand: Through its Energy Systems Division, Ingersoll Rand (IR) markets two microturbine units — the MT70 series with capacity of 70 kW and the MT250 series, with capacity of 250 kW. The primary distinction between the 70 kW and 250 kW microturbine systems is that the smaller unit has an induction generator and the larger unit has a synchronous generator, notes Holly Emerson of IR. Both units have an integral fuel gas booster and heat recovery. “We are developing a fuel system and combustor for the MT 250 that will be able to handle high hydrogen content gaseous fuels (10 to 15 percent range), such as gas formed by pyrolysis,” says Emerson. “That should be available by the last quarter of 2007. She adds that the MT250 is the first and only microturbine to be certified as meeting the California Air Resources Board 2007 emissions standards, “the most stringent in the U.S. for distributed generation technologies.”