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EfW: could biogas be used to power fuel cells?


One of the major problems for fuel cells is the fuelling infrastructure itself. Hydrogen – the fuel of choice from an environmental or energy efficiency perspective – is not yet widespread enough to drive fuel cells towards the mainstream, and other fuel choices such as natural gas are equally problematic. But one fuel cell company in the USA has found an answer by powering fuel cells with biogas derived from a number of different applications.

FuelCell Energy's commercial fuel cell products were originally developed for distributed generation using pipeline natural gas as the main fuel, with the knowledge that the technology was inherently fuel flexible. But as the commercialisation effort proceeded, it became apparent that biogas, in particular anaerobic digester gas (ADG), was one of the most important alternate fuels that needed to be considered in the product mix.

The major market drivers for evaluating biogas as a fuel are well known. Biogas is commonly produced as a by-product of wastewater treatment, food processing, or the decomposition of organic wastes in a landfill. Previously, the biogas was simply vented into the atmosphere or flared if there was concern for ignition but as energy costs began to rise, the use of this renewable fuel source increased – it was initially used in boilers to heat water for the digesters and then used in combustion generation to produce power.

The potential of biogas

Biogas is the gaseous product generated by the breakdown of organic matter by micro-organisms. It typically contains 50%–80% methane, 20%–50% CO2, and traces of gases such as hydrogen, water vapour, carbon monoxide, and nitrogen. Depending on the source of the feedstock used, biogas will have contaminants such as compounds of sulphur, siloxanes, halogens, and heavy metals.

Some of the common sources of biogas are:

  • Municipal wastewater treatment;
  • Farm animal waste;
  • Food/beverage processing waste;
  • Landfills.

Municipal wastewater treatment

According to a 2007 US Environmental Protection Agency (EPA) report, there are more than 16,000 municipal wastewater treatment facilities (WWTFs) in the USA alone, and they represent a significant source of biogas. In a single day, for every person served, a typical WWTF processes 100 gallons of wastewater and generates approximately 1 ft3 of digester gas by anaerobic digestion.

What is biogas?

Biogas is mostly methane, diluted with 20% to 50% CO2. As a greenhouse gas, methane is over 20 times more potent than CO2. Environmental pressure is mounting on plant operators to find a more benign approach than simply releasing raw methane into the atmosphere, or flaring of the gas, which produces NOx, SOx and CO2 emissions.

Using biogas to produce electricity or usable heat is considered carbon neutral since the carbon released is from a non-fossil source, and was sequestered in the growing of the organics that comprise the biomass. Many of the biomass fuels represent methane that would be released into the atmosphere (with 24 times the greenhouse impact compared to CO2) if not captured and used as fuel. An energy generation technology capable of utilising biogas in an efficient and clean manner for small-scale distributed generation is needed, and fuel cell power plants meet this need.

Biogas applications are a good fit for fuel cell products developed with natural gas as the target fuel. The major fuel constituent in both fuels is the same: methane. Biogas generally consists of 50% to 80% methane, with the balance CO2. The impurity levels of biogas are higher than natural gas, but this can be dealt with using auxiliary fuel cleanup systems. The project economics are also very attractive in these applications, since the cost of creating the fuel and collecting it are already covered as part of the core business economics, and, in many cases, economic incentive programmes help pay for the auxiliary cleanup equipment.

Based on an electrical efficiency of 47% for a FuelCell Energy Direct FuelCell (DFC) for example, and assuming an average methane content of 60%, this amounts to a power production of approximately 3.2 W per person. Typically, the amount of gas generated in municipal WWTFs is roughly proportional to the population served, and it is possible to predict the power generating capacity based on growth trends in the population.

With this in mind, municipal WWTFs represent an attractive market for the utilisation of fuel cells. Currently, approximately one third of municipal WWTFs use their biogas for heat, power, or combined heat and power (CHP) applications, leaving a vast reserve of untapped biogas available for fuel cells. Even at facilities which currently use biogas in fired applications (engines or boilers), increasing emissions constraints are forcing the evaluation of cleaner technologies, such as fuel cells.

The temperature in a wastewater treatment digester depends on the design of the system but is typically 95°F for mesophilic digesters, and 130°F for thermophilic digesters. For an optimum performance of anaerobic digesters, it is important that the temperature in the digester be maintained constant. This enables the microorganisms to grow and continue to break down the organic matter contained in the incoming waste stream.

Heat input is required to heat the incoming waste stream to the required digester temperature, and to compensate for heat losses from the digester system to the surroundings. This heat input can be obtained from the waste heat of the fuel cell, thereby offsetting the fuel that would otherwise be required.

The electric power and waste heat signature of a fuel cell lends itself particularly well for applications in WWTFs using anaerobic digestion, especially for the more common mesophilic-type digesters.

A digesters treating, say, 40 million gallons per day (MGD) could generate enough biogas to produce 1.4 MW with a highly-efficient fuel cell, but less than 900 kW with a less efficient generator (such as a micro turbine performing at 28% efficiency). Micro turbines produce excessive waste heat, some of which occurs after a portion is used as part of the digester.

Waste heat produced by the fuel cell is a better match for the digester heat requirement, and more of the energy input is converted to useful electricity. This also increases the value of fuel cell-derived electricity, since the higher power output of the fuel cell drives much better project economics.

Food/beverage processing waste

Food and beverage processing represents a large and diverse market for the implementation of fuel cells.

Anaerobic digesters are used in the food and beverage, pulp and paper, chemical, dairy, brewery, winery, and pharmaceutical industries. The size of the available biogas market for food and beverage processing can be determined by the number of digesters put into service within these industries. FuelCell Energy currently has installations in the brewery, food waste, and dairy industries, and sees potential in other allied industries.

Farm animal waste

Anaerobic digestion is increasingly being used to treat the animal waste generated on farms. In a single day, the amount of digester gas generated by one head of swine is 4 ft3; for dairy 46 ft3, for beef 28 ft3, and per bird 0.29 ft3. Based on an electrical efficiency of 47% for a DFC and assuming an average methane content of 60%, this amounts to a power production per head as 12.8 W for swine, 147 W for dairy, 90 W for beef, and 1 W for poultry.

By April 2008, there were about 114 farm-scale anaerobic digesters in operation in the USA alone treating wastes from swine, dairy, beef, and poultry. About 80% of the energy is used in the form of electricity, and the balance is used as a fuel gas for process heating or injection into the natural gas pipeline after treatment.

While most farm-scale digesters tend to be too small to operate a fuel cell, three factors will contribute to future opportunities for fuel cell applications; the large farm animal population, a trend towards bigger community digesters, and blending of higher-energy wastes with animal wastes in these digesters. Due to the consistent feedstock and the absence of siloxanes, the digester gas from farm digesters may be easier to treat than that produced from municipal WWTFs.

Landfill gas (LFG)

According to the US EPA, approximately 60% of all municipal solid waste (MSW) generated in the USA is currently being disposed of in roughly 1800 operational MSW landfills. Landfills are the largest human source of methane emissions in the USA, accounting for 25% of all methane sources. As of December 2007, approximately 445 landfill gas energy projects were operational in the USA. The EPA estimates that approximately 535 additional landfills present attractive opportunities for project development.

Landfill gas presents an attractive market in terms of size, but processing of this biogas to make it suitable for use in a fuel cell can be challenging. The main issues with using LFG as a fuel source for fuel cell power plants are the gas clean-up requirements resulting from the variability of its oxygen content, low methane levels, variability of the gas, and the variety of contaminants.

LFG can have methane contents that fall below 50%, and outside the variability limits for Fuel Cell Energy's fuel cells. LFG can have high concentrations of contaminants that are detrimental to fuel cells, such as chlorides (both organic and inorganic chlorides), aromatic compounds (benzene, toluene, and xylene), high total sulphur, metals, dust, and siloxanes. A cleanup system can be designed to deal with any contaminant that is present if it is found in consistent quantities. In landfills, these contaminants can vary greatly over time. Due to these complications, DFC applications for landfills are not emerging as quickly as digester gas applications.

Technology in action

Sierra Nevada beverage facility

Sierra Nevada Brewing Co. installed a 1 MW fuel cell power plant from FuelCell Energy in its brewing facility in Chico, California, to provide a reliable, environmentally-friendly source of onsite power.

Sierra Nevada's fuel cell system provides baseload power to the brewery using a mix of natural gas and digester gas generated in the brewing process. Waste heat from the fuel cell is used for brewing processes and heating needs, providing enhanced efficiency to the system. The facility was named one of 12 “Top Plants” worldwide by Power Magazine in 2006.

Tulare wastewater treatment facility

The regional wastewater treatment facility in Tulare, California installed a 900 kW power system in order to reduce pollutant emissions and reliance on the local power grid.

By using digester gases from the wastewater treatment process as a source fuel, the fuel cells are providing economical 24/7 power while addressing severe emissions' non-attainment restrictions in place throughout California's San Joaquin Valley. This ensures that Tulare's Regional wastewater treatment facility will remain compliant, efficient, and environmentally friendly for many years to come.

Dublin San Ramon wastewater treatment facility

In order to provide 600 kW of 24/7 power at its regional wastewater treatment facility in Pleasanton, CA, the Dublin San Ramon Services District (DSRSD) installed two DFC power plants.

Pleasanton's fuel cells have helped reduce constraints on the local power grid by providing as much as 50% of the facility's required power, allowing the rest of the community to keep their lights on, even in blackout conditions.

While it is impractical to design a cleanup process to deal with all of the possible contaminants that may be present in landfills, a well characterised LFG site with the proper fuel treatment could be made to work well with fuel cells. It is anticipated that increasing environmental pressures and economic conditions will lead to a growing interest in the use of fuel cells in landfills.

Biogas as a fuel source in DFC power plants

Fuel cells are electrochemical devices that combine fuel with oxygen from the ambient air to produce electricity and heat, as well as water. The non-combustion, electrochemical process is a direct form of fuel-to-energy conversion, and is much more efficient than conventional heat engine approaches. CO2 is reduced, due to the high efficiency of the fuel cell, and the absence of combustion avoids the production of NOx and particulate pollutants.

As FuelCell Energy began to consider digester gas projects, one of the first issues that needed to be evaluated was the impact of CO2 on fuel cell performance. This included actually running laboratory tests on simulated digester gas (methane diluted with CO2). A unique aspect of the carbonate fuel cell chemistry is that CO2 is produced in the anodes and consumed in the cathodes.

The presence of CO2 in the fuel will reduce anode performance and improve cathode performance. Fuel Cell Energy's models indicate that that the cathode gain is roughly equal to the anode penalty – the DFC power plants perform about the same in the presence of the CO2 diluent, as long as methane content is above 50%.

These results indicate that the DFC can efficiently use digester gas, because it is so insensitive to the CO2 diluent. This product match, plus the basic market forces described above, has resulted in an increasing involvement in ADG fuel based projects, to the point where more than 20% of FCE's installed fleet is now installed at ADG sites (see box – Technology in action).

Economic benefits of biogas powered fuel cells

As described in the above sections, fuel cells are a good technical fit in biogas applications. Apart from the technical and environmental benefits, a strong economic case can be made for biogas-powered fuel cells.

The capital and operating costs for a biogas-powered fuel cell exceed those for one powered by natural gas due to the additional biogas treatment equipment. However, these increased costs are offset over the lifecycle of the plant since the raw biogas is essentially free, and treating the gas costs only US$1 – US$3/MMBtu. In a market of volatile natural gas prices, using biogas as a fuel becomes an attractive alternative.

Power generated from biomass fuel is not only lower in cost than power generated from traditional fuels, it is also less expensive than other renewable energy sources, such as wind and solar, once capacity factor is considered.

The intermittent nature of wind and solar tend to result in higher power costs per kWh generation than baseload generation from biomass fuel. Wind is also a central generation technology, resulting in additional costs and NIMBY issues related to power transmission and distribution.

Figure 1 on the right shows the comparison of the cost of electricity from natural gas and biogas powered fuel cells. The cost of electricity is comprised of three major components:

  • Net installed capital costs which include equipment costs, labour, and material for equipment installation. In the case of ADG, this includes the cost of gas pre-treatment;
  • O&M costs include the costs incurred to operate and maintain the power plant, such as periodic maintenance of equipment, and catalyst and fuel cell stack replacements;
  • Fuel costs (delivered costs of natural gas, or cost to maintain the biogas treatment system).

It can be seen that the cost of electricity is lower for biogas-powered fuel cells. This advantage is amplified when the Investment Tax Credit (ITC) is taken into consideration. The ITC offsets 30% the cost of fuel cell equipment, up to a maximum of US$3000/kW.

The cost of electricity for a biogas powered fuel cell with the ITC is less than 10 Image /kWh, which is competitive in many States. Certain States (such as California) have additional incentive programmes for fuel cells, such as the SGIP (Self Generation Incentive Program), further reducing the cost of electricity.

Conclusions

FuelCell Energy has had considerable success with its biogas applications, which have become one of the most important market segments addressed by its DFC products.

The product enhancements from early projects have resulted in the development of features which address the specific needs of the biogas market. Because of the carbon neutrality and low cost of the fuel, this is a very attractive power generation feedstock.

Successfully exploiting this feedstock requires a technology which can operate at full load despite the presence of the CO2 diluent, adjust to the changing fuel composition and quantity, and operate with minimal emissions and with minimal operator intervention.

The Direct FuelCell has been shown to meet these criteria, which bodes well for seeing increased penetration of fuel cells using biogas for industrial applications.
 

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Bioenergy  •  Energy storage including Fuel cells

 

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