By Steve Blankinship, Associate Editor
Hundreds of megawatts of electricity hide from us in plain sight. It’s in the garbage you put in the alley and in those tree limbs you put out for the monthly trash collection. It’s in that pile of debris that was an old house or building before its demolition.
 Construction debris offers one highly visible source of biomass that can be readily converted into megawatts. |
It’s in the farm that raises chickens, cattle or pigs. It’s also in the wastewater treatment plants that process every drop of water that goes down your household drains.
But the full potential for generating electric power from these resources is not close to being fully realized, even though most of the pieces necessary for making it happen are in place. The economics for energy projects using a wide range of biofuels get better every day. Natural gas prices are high and will remain so. Environmental issues encourage using as many renewable resources as possible while eliminating as many environmental liabilities as possible. Then there’s the cornucopia of federal and state financial and tax incentives, increased need for power and robust markets for the renewable energy credits (RECs) that such power generation produces.
What’s lacking, however, is consistency when it comes to viewing, valuing and applying such incentives. Addressing those inadequacies will spur further use of such renewable resources, especially smaller-scale resources that many experts believe represent the largest amount of potential energy in plain view.
Making Lemonade
Making power with biogas is a little like making lemonade when life hands you a lemon. Case in point: Municipal solid waste landfills are the largest source of human-related methane (CH4) emissions in the United States, accounting for about 25 percent of the methane produced in 2004. Landfill-generated CH4 emissions also represent an opportunity to capture and use a significant energy resource. Landfill gas is created as solid waste decomposes. It’s about 50 percent CH4 (the primary component of natural gas) and about 50 percent carbon dioxide (CO2).
The gas is extracted through wells drilled into the landfill. This system directs the gas to a central point where impurities are removed that might affect the engine or turbine that will consume the biogas. The biogas also can be upgraded to pipeline quality. The sealed nature of a landfill allows anaerobic digestion to occur underground without the need for above-ground digester tanks.
For almost 25 years, Lebanon Methane in Pennsylvania produced 1.2 MW of power using several reciprocating engines at the Greater Lebanon County Refuse Authority landfill. PPL Renewable Energy, a unit of PPL Corp., recently upgraded the site to produce 3.2 MW, using two 1,600 kW Caterpillar engines. Lebanon is one of four PPL landfill projects in Pennsylvania and New Jersey. The newly upgraded Lebanon plant produces 24 million kWh/year.
Steve Gabrielle, who is responsible for PPL’s renewable generation portfolio, said PPL can use the green credits produced by the renewable generation or re-market them. By capturing CH4 and using it to make electricity, the plant will prevent the equivalent of 19,500 tons of CO2 per year—equal to removing 27,000 cars from the road or planting 38,000 acres of new forest.
“Until recently, about 30 percent of the gas produced at the landfill was being flared,” said Gabrielle. “Today, we use all of it. We make about three times the power with just 30 percent more gas.” The company continuously reconfigures the gas flows and air infiltration in the field (using a process known as “tuning up” the field) to produce more gas to keep the engines fed.
Parasitic power used to vacuum methane from the landfill takes 5 percent to 10 percent of the gross output, depending on seasonal temperature. Other than dewatering, no gas pre-treatment is needed for the engines. Nor is selective catalytic reduction (SCR) used on the back end. “We test the gas to make sure it does not have too much siloxane (a contaminant found in biofuels from landfills and wastewater facilities that can damage engine parts) or other significant contaminants,” Gabrielle said. “Other than that we just let the engines run and maintain them.”
Jenbacher engines power a portfolio of landfill energy applications in the United States, Mexico and Canada. The engines designed to run on such gas currently range from 350 kW to 2.4 MW. In anticipation of bigger landfill applications in the West, Northeast, Midwest and Florida, the company said it will boost output next year to as much as 2.8 MW for landfill gas.
“Although emission standards in California are very tough, even for landfills, Jenbacher can meet them now,” said Roger George, sales growth leader for GE Energy’s Jenbacher gas engine business.
On a recently installed unit at a California landfill, Jenbacher’s CLAIR gas after-treatment is reducing carbon monoxide (CO) to 0.3 parts per million (ppm), lower than the required 2.1 ppm. Cost of electricity is 5.5 cents to 7.5 cents/kWh without incentives, said George. Capital cost for a landfill plant is $450/kW to $800/kW, the higher figure indicating a unit where cogeneration (involving combined heat and power) is used.
Moo, Oink, Cheep-Cheep
Methane-based animal farm waste can be placed into anaerobic digester tanks where microbes convert it into a gas having enough heating value to power engines or turbines. Today, livestock waste contributes about 8 percent of human-related CH4 emissions in the United States.
Benson, Minn., is the site of one of the country’s first poultry litter power plants. Among the largest such plants in the world, the $200 million, 55 MW facility was developed by Fibrominn LLC and entered service in early 2007 fueled by waste produced by 16,000 turkeys. Poultry litter—meaning animal waste mixed with wood chips, feed hulls, feathers and spilled feed—is a traditional source of fertilizer. The problem with such a mix, however, is that it adds nitrates and sulfates to soil and groundwater, which can prove to be environmentally damaging.
Minneapolis-based Xcel Energy has a 21-year offtake contract for the turkey-based power to help it meet a state requirement to derive 110 MW annually from biomass. The balance of that obligation is met with power plants that burn wood waste. Fibrowatt, which has developed three smaller poultry litter power stations in Great Britain, plans similar plants in other major poultry-producing states.
Municipal wastewater affords a third waste-to-energy opportunity. More than 16,000 municipal wastewater treatment facilities operate in the United States. However, fewer than 550 currently use anaerobic digestion to help treat the waste. Of those, about 100 use the biogas produced by their anaerobic digesters to generate electricity or thermal energy.
The City of Los Angeles has been doing just that for years. “The capital cost of using biogas is slightly higher than for natural gas,” said Omar Moghaddam, manager of Los Angeles Department of Power & Water’s sanitation regulatory affairs division. He said the additional cost comes largely from various gas conditioning systems needed for biogas to fire an engine or turbine. That’s because biogas from digester systems has far more sulfur than natural gas (100 ppm compared to 10 ppm), along with more volatile organic compounds (VOCs) and particulates. On the plus side, there are fewer NOX emissions from biogas than from natural gas. “On balance, the overall capital costs are a little more because the higher sulfur-control costs exceed the lower costs of NOX control,” said Moghaddam.
There’s also a biosolids component to wastewater treatment. Dewatered human waste consists of 60 percent to 70 percent carbon-based hydrocarbons, plus silica and heavy metals. Dehydration produces pellets or powder having the consistency of coal and a heating value of 7,500 to 7,800 Btu/lb. That equals or exceeds the heating value of low-grade coal such as lignite. As a result, biosolids can be burned in boilers to make heat and electricity. Similarly, the gas from wastewater effluent can be combusted in engines or turbines.
Los Angeles is now exploring a new combined cycle or simple cycle power block to take the digester gas currently being produced at the Hyperion waste water treatment plant and use it at the adjacent Scattergood Generating Station. Scattergood already produces 18 MW from 7.5 MMcf/day of biogas and sends steam back to the wastewater plant for use in the digesters. “With the much better heat rates produced by today’s power blocks, we should be able to generate 23 or 25 MW from that same amount of digester gas,” said Moghaddam.
The city hopes to have the new plant in service by 2010. Although California’s South Coast Air Quality Management District imposes a 2.5 ppm limit on NOX from natural gas plants, it will allow 25 ppm NOX for digester gas (and 100 ppm for CO) to encourage the use of renewable gas. “I’m not sure we could meet 2.5 ppm on digester gas without selective catalytic reduction or ammonia injection,” said Moghaddam. “You can put an SCR on the back of a natural gas unit and not worry about catalyst fouling. But even though digester gas is a low NOX emitter, there may be some particulate and other contaminants that may not be suitable for SCR.” Iron-based chemistry reduces sulfur to 10 ppm. The gas then goes through two parallel granulated activated carbon towers that remove particulate and moisture.
A cost of electricity (COE) study done when the Hyperion/Scattergood project went into service in 1995 priced the gas at $4.50/mmBtu. Moghaddam said the bus bar COE is 3.5 to 4 cents/kWh. Population growth will no doubt create more sewage so planners see it as a sustainable fuel.
Meeting California Emission Standards
Because California air quality regulations make it increasingly difficult for reciprocating engines or boilers to operate (especially in the South Coast Air Quality Management District) fuel cells are making inroads into wastewater treatment-to-power facilities.
 Most states view biomass as carbon neutral and generally sustainable. Emissions remain an issue. |
“Fuel cells are CARB 2007 certified and exempt from air permitting,” said William Karambelas, vice president of Fuel Cell Energy (FCE)’s western region. In addition to electrical efficiencies approaching 50 percent, cogeneration using fuel cells produces waste heat that can provide heat needed for the digester.
Karambelas said multiple sites exist where reciprocating engines are unable to run because air quality concerns and/or increased costs to meet the requirements of an air permit are now uneconomical. “Utilizing a fuel cell in place of the reciprocating engine allows them to utilize the renewable fuel source,” he said.
Since fuel cells don’t operate by combustion, there are no concerns regarding NOX, SO2 or particulate matter. But a thorough gas cleanup is needed before digester gas may be used. Chevron Energy Solutions provided engineering and construction of Rialto, Calif.’s, wastewater treatment facility that runs on fuel cells. The Southern California city’s 900 kW fuel cell plant makes power from wastewater sludge and kitchen grease from local restaurants. The fuel cell plant converts methane into hydrogen, which produces power electrochemically. Residual waste heat from the fuel cells is used to heat the digesters.
The plant is expected to increase municipal revenues, reduce landfill wastes and lower greenhouse emissions by nearly 5.5 million tons annually. It will also reduce the city’s energy costs by about $800,000 a year. The fuel cell plant and other energy efficiency improvements will reduce greenhouse gas emissions by 11 million pounds of CO2 annually. The $15 million project is eligible for a $4 million rebate on the fuel cell plant cost from California’s Self-Generation Incentive Program. The remaining cost will be self-funded through energy cost savings and power station revenues.
Wood Is Good
Wood is both renewable and, in most places, regarded as carbon-neutral. For Foster Wheeler, producing power with wood involves using the company’s fuel flexible circulating fluidized bed (CFB) technology to cleanly burn biomass and convert it to steam and power. “The CFB is capable of burning nearly all types of biomass as well as tires, demolition wood and waste paper,” said Bob Giglio, director of marketing for Foster Wheeler’s Global Power Group.
Foster Wheeler recently supplied boilers for three wood-fired projects in Germany, each about 30 MW in size. Two use construction and demolition wood, while the third uses wood chips, sawdust and bark from a local paper mill and from forestry operations. The company is also building one of the world’s largest biomass plants, a 125 MW wood-fired plant in Finland.
Giglio said most biomass plants have been built in Europe where they are supported by the European Union’s renewable energy and landfill directive. He said the United States is not seeing much activity, due mainly to a narrow focus on wind energy. And because of their relatively small size, most renewable biomass power plants tend to be more expensive than coal, gas and nuclear plants and need government programs to compete economically.
Technology for renewable plants also tends to cost more, too, because it’s more elaborate. “A biomass plant has to be very flexible because it will not have a 30-year supply of uniform biomass,” said Giglio. Instead, it may have to burn straw grass, agricultural products or demolition wood that may contain construction materials and come in many sizes
He said the biofuels infrastructure isn’t as highly developed as that for coal, which can be delivered in bulk shipments. So while the cost of biofuel itself is low, transportation can represent a large percentage of the total cost because the materials’ heating value is low compared to conventional fuels.
Green or Not?
One current issue in the United States is whether or not to allow biomass to qualify fully for green energy credits because the biomass fuel releases CO2 that has been stored only relatively recently. Giglio said that in Europe even peat doesn’t qualify for green credits because its 200-year absorption/release cycle is considered insufficient.
He estimated that capital cost for a stoker or fluidized bed biomass plant at $2,700 to $3,000/kW, compared to coal, which can be about $2,000/kW. Overall, fuel and operation costs are about the same for either fuel. Fuel prep costs are included in the capital cost, but little or no need exists for scrubbers. In most cases ammonia can be injected into the cyclone area of the CFB. Giglio estimated this can result in a 30 percent reduction in NOX, unless lower levels are needed to qualify for renewable energy credits. In that case, alternate technologies are now available. Electricity from a new biomass plant will cost 8 to 9 cents/kWh, he said.
Clean construction and demolition biomass (with nails, plastic, aluminum, wallboard and other materials removed) is also a potentially good source of renewable energy. “There’s no good place to get rid of this material and nobody wants it,” said Kevin Toupin of Babcock Power. “So if you use it in an environmentally sound manner, it’s really a good renewable fuel.”
And the cost of construction and demolition biomass can even be negative; after all, construction sites typically pay to have it hauled away. The biggest variable in construction and demolition and other biomass fuels is moisture. Rich Abrams of Babcock Power said that because of high moisture content, heating value can range anywhere from 3,800 Btu/lb up to 6,500 Btu/lb.
Babcock Power’s Riley Power subsidiary provides biomass stoker-fired boilers, a number of which burn construction and demolition fuels. It also offers its regenerative selective catalytic reduction system (RSCR) to remove NOX, the primary pollutant associated with wood-based fuels. The system can produce outlet NOX emissions as low as 0.065 lb/mmBtu. RSCR can be installed at the cold end of the gas stream leaving the boiler and downstream of the particulate collection device. It can also be modularized and installed quickly to avoid a long shutdown.
Demolition wood can produce some heavy metals, a small amount of SO2 and hydrochloric acid. “We are looking at several projects here in the U.S. where they would be burning 100 percent” construction and demolition biomass, said Toupin. “That would entail a boiler and a dry scrubber followed by our RSCR system for controlling NOX and potentially CO.”
Renewable? Yes. Green? No.
In terms of CO2, most states view biomass as carbon neutral and in most cases, sustainable, said Abrams. Issues over whether wood fuels such as construction and demolition are regarded as green may not be so much related to CO2 as to concerns about other potential emissions. Emissions technology such as that offered by Babcock Power can achieve high removal rates for pollutants unique to C&D, making its emissions as low as those from conventional wood. “Our Turbosorp CFB dry scrubber removes acid gas and other pollutants including more than 99 percent of the arsenic contained in old pressure treated wood,” said Abrams.
There’s also a category of fuel consisting of new lumber scraps left at construction sites. At least one California facility burns these scraps along with other renewable biomass materials, including tree limbs and other plant waste from residential lawns and gardens. Burning such material to make power may be a good way to mitigate greenhouse gas formation. A proposed 100 MW biofuels plant in East Texas could potentially co-fire wood from Houston, home to a large local forestry and paper industry.
However, as long as U.S. energy costs remain low compared to other parts of the world, using all of the energy that exists in plain sight remains something that might happen sometime in the indefinite future.
Perhaps Finland-based Wärtsilä’s strategy regarding the U.S. market sums up where things stand today. Wärtsilä has developed many solid and liquid biofuel technologies that have been successful in Europe—where renewable energy subsidies are sufficient to support the economics of such projects. The company says it anticipates introducing the technologies into North America. But only “as the market supports their development.”