BioCycle February 2007, Vol. 48, No. 2, p. 41
Pioneering projects have generated positive results with fuel cells converting digester methane into electricity, reducing both air emissions and on-site energy demand.
Wilson Rickerson and Sander Cohan
ACCORDING to the National Renewable Energy Laboratory, approximately 465,000 tons of methane are emitted annually by the nation’s 16,000 wastewater treatment plants (WWTPs). While wastewater methane is both a greenhouse gas and a potentially valuable fuel source, the U.S. EPA reports that only two percent of the nation’s WWTPs capture their methane to produce useful energy (e.g. heat or electricity). Despite these statistics, the WWTP sector has become an unlikely incubator for emerging energy technology.
As discussed in a 2002 report from KEMA Inc., several wastewater treatment plants are experimenting with advanced fuel cells to convert digester methane into electricity and heat for use on-site. Fuel cells have significant advantages over standard combustion technologies in that they typically operate at higher efficiencies and emit a comparatively negligible amount of pollution. However, their high capital costs have proven to be a barrier to their widespread adoption. This article presents case studies of how treatment plants in New York City and Washington State have pioneered the use of on-site fuel cells.
FUEL CELLS: HYPE OR OPPORTUNITY?
At the turn of the millennium, fuel cell cars and the hydrogen economy were being touted as the next energy revolution. Despite support from the Bush administration for the Hydrogen Fuel Initiative and the FreedomCAR, however, the furor over fuel cells has subsided somewhat. One of the big challenges to fuel cell cars is the investment required to build hydrogen infrastructure nationwide. Another obstacle to the federal hydrogen plan is that it envisions using fossil fuels, instead of renewable energy, to manufacture hydrogen fuel. While the Bush vision for a nation of hydrogen fuel cars may be decades away, wastewater treatment plants could be a near-term niche market for stationary fuel cells. After all, WWTPs manufacture their own fuel on-site, and they use a renewable resource to do it.
Unlike microturbines and engines, fuel cells do not rely on combustion to generate electricity. Instead, they rely on a chemical reaction involving hydrogen. Fuel cells are made up of two electrodes (the anode and the cathode) separated by a permeable material containing an electrolyte. Hydrogen atoms are stripped of their electrons in the anode, and the positively charged hydrogen ions pass through the electrolyte. The electrons cannot pass through the electrolyte and instead travel through an external circuit, producing electricity. In the cathode, the electrons and ions reunite and combine with oxygen from the air to produce water. This reaction also generates heat.
Fuel cells are distinguished primarily by the electrolyte they use. The four major fuel cell types currently in use or under development are phosphoric acid fuel cells (PAFC), molten carbonate fuel cells (MCFC), proton exchange membrane fuel cells (PEM), and solid oxide fuel cells (SOFC). All of these fuel cells are powered by hydrogen. Pure hydrogen is challenging to produce and store, however, and many operating fuel cells use a fuel reformer to strip hydrogen from other hydrocarbon fuels (e.g. natural gas).
Digester methane, for example, contains hydrogen and can be fed into a fuel reformer for use in a fuel cell. Although the variable chemical composition of digester gas can pose challenges to its use, Kema states that only the PEM is incompatible with digester methane. To date, PAFCs and MCFCs have been successfully deployed at wastewater treatment plants. This article reviews the experience of the New York Power Authority (NYPA), which has pioneered the use of PAFC fuel cells at wastewater plants, and the King County South Treatment Plant in Renton, Washington, which has installed the first wastewater methane MCFC fuel cell in the U.S.
THE NEW YORK POWER AUTHORITY
The New York Power Authority is a state-owned public power enterprise founded in 1931 to develop New York State’s large hydropower resources. NYPA’s roles and responsibilities have expanded over the years and it is now the nation’s largest state-owned power organization. In total, NYPA owns 18 hydropower, natural gas, and oil-fired power plants totaling 6,260 MW of capacity. NYPA sells comparatively low-cost electricity to public agencies in New York State, and to the state’s municipal utilities and rural electric cooperatives. Since the early 1990s, NYPA has played a leadership role in installing distributed clean energy technologies in New York State, including 663 kilowatts of photovoltaic systems and 21 fuel cells totaling over 4.5 megawatts. Of NYPA’s fuel cells, nine have been installed at wastewater treatment plants.
In 1997, NYPA installed its first fuel cell at the Yonkers Joint Wastewater Treatment Plant in Westchester County, New York. The 200 kW phosphoric acid fuel cell was the first in the United States to be powered by anaerobic digester gas. The fuel cell manufacturer, UTC Fuel Cells, had to modify its standard PAFC model to accommodate digester gas. The primary concerns were pretreating the gas to eliminate contaminants that would degrade fuel cell performance, and adjusting system pressure to compensate for the fact that digester gas is only 60 percent methane, rather than 95 percent like natural gas.
The primary motivation for the system was to reduce the emissions associated with digester gas flaring. Although flaring digester gas eliminates methane’s contribution to the greenhouse effect, it contributes to ground-level ozone and exposes WWTPs to regulation under the Clean Air Act. Yonkers processes 95 million gallons of wastewater per day and generates 17,400 cubic feet of digester gas per hour. Of this, 70 percent was used in engines and boilers, and 30 percent was flared. The fuel cell installation has halved the amount of flared digester gas and reduced the WWTP’s regulatory risk. The system was supported with demonstration funds from the Electric Power Research Institute, the New York State Energy Research and Development Authority (NYSERDA), and the U.S. Department of Energy.
NYPA’s next round of digester gas fuel cell projects came in 2001. That summer, New York City faced serious power supply shortages. In order to meet the projected shortfall, NYPA installed 10 small natural gas turbines at six sites within New York City. As part of the project, NYPA pledged to offset 100 percent of the emissions from the new plants. To achieve its targeted emissions reductions, NYPA installed eight fuel cells totaling 1.6 MW at four New York City Department of Environmental Protection wastewater treatment facilities. This second generation of 200 kW PAFCs incorporated many of the lessons learned through the Yonkers project and achieved the targeted emissions reductions. Furthermore, grants received from NYSERDA and the U.S. Department of Defense allowed NYPA to cost-effectively sell electricity directly from the fuel cells to the WWTPs at NYPA’s standard power rates.
It is estimated that there is enough wastewater methane in New York City to power five to ten megawatts of fuel cells. Although NYPA has been pleased with the performance and availability (approximately 90 percent) of its digester gas fuel cells, its next planned methane recovery project will involve a Stirling engine.
KING COUNTY SOUTH TREATMENT PLANT
Like NYPA, King County, Washington, has had a long history of energy innovation at its wastewater facilities. Since 1985, the County has captured methane at its West Point Treatment Plant to produce electricity and heat using reciprocating gas engines.
In 2004, the Department of Natural Resources and Parks in King County began a two-year demonstration of a 1 MW molten carbonate fuel cell fueled by wastewater methane from its South Treatment Plant in Renton, Washington. This is one of the larger MCFC installations in the country (most previous MCFCs have been 250 kW), and the first to be powered with a digester gas supply. The project cost $22.8 million, and was funded by a $12.5 million grant from the USEPA, and contributions from King County and Fuel Cell Energy, the technology provider.
The MCFC’s 650°C operating temperature melts carbonate salts within the cell’s electrolyte to produce carbonate ions (CO3) that conduct from the cathode to the anode ends of the fuel cell. Combining with hydrogen, the ensuing reaction creates water, CO2 and electrons. At the end of the cycle, these electrons react with oxygen in the air and recycled CO2 to create new carbonate ions that replenish the fuel cell.
The MCFC has a number of advantages over other fuel cell designs. First, MCFCs are more tolerant of gas impurities than other fuel cells. Carbon monoxide, for example, can poison the reformers of lower temperature fuel cells. MCFCs operate a temperature high enough to convert carbon monoxide into useful energy. The technology is also well suited to the high CO2 content of digester gas because the CO2 can be used to replenish the fuel cell’s electrolyte. Finally, the high operating temperature of the King County fuel cell lends it to combined heat and power applications as well. While overall electricity efficiency of a typical MCFC is 50 percent, its efficiency increases to 80 percent when waste heat is recovered.
The primary disadvantage of the MCFC is the fact that it degrades quickly as a result of its high operating temperature. The King County project designers estimate that, although the MCFC is projected to run reliably in the medium-term, the fuel cell stacks will need to be replaced every two to five years.
In its first year of operation, the King County fuel cell performed well, meeting or exceeding expectations. As of December 2004, it reached efficiency levels of 45 percent on digester gas and an overall availability of 91 percent. Emissions were low and well below the thresholds set by the California Clean Air Resources Board.
The implementation of the fuel cell, however, has revealed other challenges. The system first experienced problems with gas supply switching. One goal of the program was to use three different sources of methane to power the fuel cell: scrubbed digester gas, raw digester gas, and natural gas from the local utility (Puget Sound Energy). Significant problems occurred with maintaining appropriate gas flow while switching between these gas supplies and the plant experienced delays of up to six hours during fuel switches. While initially a roadblock, Fuel Cell Energy solved this problem by calibrating the system to automatically switch fuels when PSE diverts gas flow.
A second issue, related to all new technologies that connect to the electrical grid, was interconnection. Owing to unfamiliarity with the technology, and an overall lack of performance data, negotiations with Puget Sound Energy (PSE) to establish an interconnection contract were lengthy and difficult. Similarly, utility representatives found themselves dealing with an unfamiliar generation technology, leading to delays in certification and required interconnection inspections.
Overall, the King County fuel cell remains a successful experiment, producing grid-viable power within expected thresholds. Managers intend to continue operating the plant after the pilot period ends, but future expansion will be limited by digester gas availability. In addition to the fuel cell, the South Plant also scrubs digester gas for sale to PSE and combusts gas in an eight megawatt cogeneration facility.
POWERING THE FUTURE?
The pioneering efforts in New York and Washington have generated positive results. Fuel cells have been successfully deployed using a readily available, renewable source of fuel and have reduced both air emissions and on-site energy demand at WWTPs. Wastewater treatment plant fuel cells have been proven to be a viable and (in the case of New York) a replicable niche technology application. The future of this technology remains uncertain, however. The primary hurdle to the spread of WWTP fuel cells is that so few wastewater treatment plants employ anaerobic digesters or capture the methane they produce. Moreover, the high capital cost of fuel cells makes them less attractive than other conversion technologies for facilities that do utilize methane.
Finally, digester methane fuel cells remain a young technology and can face a number of project development road blocks, especially with regard to heterogeneous gas quality and interconnection barriers. In spite of these obstacles, it is possible that fuel cells could increasingly be deployed at wastewater plants in the future as demand for domestic sources of renewable energy grows and as concern over greenhouse emissions mount.
Wilson Rickerson is a Boston-based energy consultant focusing on renewable energy markets, policies, and technologies. Sander Cohan is an Energy Analyst at KEMA Inc. in Burlington, Massachusetts.
February 21, 2007 | General
Renewable Hydrogen From Fuel Cells At Wastewater Treatment Plants
BioCycle February 2007, Vol. 48, No. 2, p. 41