CLEARWATERS: Energize with state-of-the-art technologies

Situated behind the digester building of the Yonkers Wastewater Treatment Plant, a large gray box about the size of a pickup truck quietly converts anaerobic digester gas (ADG) into electricity and heat. This ordinary-looking box is a fuel cell, and it is actually an exciting technological breakthrough in the future of power generation. The fuel cell is manufactured by ONSI and installed by the New York Power Authority (NYPA) in collaboration with the Electric Power Research Institute. It can produce 200 kW of electricity, about enough to power one hundred average homes, with only water and CO₂ as significant by-products. This project, along with other examples of new distributed generation technologies dotting the country, demonstrates that electricity can be produced with little effect on the environment. Furthermore, it is another example of how wastewater treatment plants can insulate themselves from volatile energy costs.

Energy is always a focus of any discussion regarding sustainability. The acquisition and use of energy, along with the associated costs, dramatically affect society. Wastewater treatment plants and collection systems rely on energy for their operation. In fact, energy costs are second only to labor costs in the average wastewater treatment plant budget.

Many wastewater treatment plants are fortunate to have anaerobic digester gas available as a good source of energy. Most of these plants already have engine generators in place to convert ADG to electricity, but concerns about air emissions have often limited the use of those systems. Now, however, new technologies are available to solve these problems. At the forefront are fuel cells and microturbines. Both have recently been field-tested at wastewater treatment plants, and they promise to become invaluable tools for distributed generation.

Advantages of distributed generation

The on-site generation of electricity, also known as distributed generation, offers several advantages over centrally located, large-scale power generation. Distributed generation replaces the traditional backup generator by offering a reliable source of power in the event of a disruption in the regional transmission and distribution power grid. In addition, it can improve power quality by protecting against periods of low voltage, which can damage electric equipment.

Distributed generation can efficiently utilize local sources of methane that might otherwise be wasted. As the price of natural gas rises, free sources of methane from oil fields, landfills, and sludge digesters have become an increasingly important resource. At an average of 600 Btu/ft³, ADG has 60% of the energy content of commercial natural gas.

Another advantage is that heat produced as a by-product of electrical generation can be recovered and used locally at the facility for heating or cooling. The practice of using the excess heat associated with electrical generation is known as cogeneration, and it can double the efficiency of a system, subsequently reducing pollution levels by half.

Also, electricity produced locally is not lost in the resistance of transmission wires, aiding the overall efficiency of converting fuel into useful work. In some cases the step of creating electricity can be skipped altogether by directly driving pumps or blowers with the mechanical power produced by a turbine or engine.

Finally, and most importantly for the energy sector, smaller distributed generation systems are easier to locate. Local resistance to new, large-scale power plants and transmission lines has made their planning and construction extremely costly and time consuming. The demand for more electricity can be met by smaller clean distributed generation technologies without community disruptions. Wastewater treatment plants can provide an optimal location for distributed generation technologies. Moreover, the new generation of high-efficiency and low-emission distributed generation technologies can replace older generators.

Fuel cells

Fuel cells, such as the one installed at the Yonkers Wastewater Treatment Plant, work in a manner similar to batteries. Unlike a battery, however, the fuel supply to a fuel cell is constantly replenished. Fuel cells generate electricity through an electrochemical reaction that transfers electrons from the hydrogen fuel source to oxygen molecules taken from the air.

Diagram of the basic operation of a fuel cell

One of the main advantages of fuel cells is that they do not rely on combustion. This allows them to achieve much higher fuel efficiencies than flame-based technologies. Current fuel cell electrical efficiency is in the range of 40-60%, as opposed to 25-35% for turbines and reciprocating engines.

The electrochemical process also results in virtually zero emissions, and a noise level as quiet as a room air conditioner. ONSI / International Fuel Cells estimates that the Yonkers fuel cell will eliminate 40,000 lb/yr of air emissions when compared with an equivalent engine-based generator. For wastewater treatment plants that are constrained by air quality emissions requirements, this is a major advantage. Low noise levels and near-zero emissions will also allow siting of fuel cells in virtually any location where there is an available fuel supply, including pump stations located in residential neighborhoods.

Selecting a fuel cell

Heat quality Fuel cell technologies that operate at higher temperatures usually provide better quality heat for cogeneration applications.

Life span The main drawback of the current generation of phosphoric acid fuel cell is that the electrolyte needs an expensive replacement after about 5 years of operation.

Efficiency Newer higher efficiency technologies will have lower overall fuel costs.

Reformer technology Some fuel cell designs, such as solid oxide, use an internal reformer process that saves space and simplifies operation.

Sensitivity to impurities Especially with ADG, it is important to determine the sensitivity of a technology to impurities such as SO₂.

Microturbines

In addition to the Yonkers fuel cell project, NYPA helped install microturbines, another new power generation tool, at the Lewiston, NY wastewater treatment plant near Buffalo. This project uses two 30-kW microturbines, manufactured by Capstone, to convert ADG into electricity.

Microturbines use very small turbines, rotating at high speeds, to convert a variety of fuels into electricity. Adapted from turbine technology used in turbochargers and aerospace auxiliary power units, microturbines can produce a large amount of electricity from a unit as small as a home refrigerator.

Advanced bearing technology is used in microturbines because of the high speeds needed (in the range of 40,000 to 90,000 rpm). Some manufacturers, including Capstone and Honeywell, rely on airfoil bearings that use a layer of air—only microns thick—to lubricate the turbine shaft. Systems that rely on air bearings do not require ancillary oil pumping equipment and, as a result, have few moving parts. In addition, the rotary motion of the turbine limits vibration, when compared to an internal combustion-based generator. This combination of characteristics reduces maintenance requirements and also limits noise production.

Another advantage microturbines offer is the low emission of air pollutants such as NO_x (NO and NO₂), CO, and unburned hydrocarbon fuel. Microturbines can operate on a lean mixture of air to fuel, thereby lowering combustion temperature and reducing the creation of NO_x. Microturbines also reduce unburned hydrocarbon fuel and CO levels by providing good air and fuel mixing in the combustion chamber.

Microturbines incorporate advanced electronics to maintain a high-quality power output. The system works by converting the high-frequency motion of the turbine shaft into a high-frequency AC signal. Then the high-frequency AC signal is converted into DC power and finally back into 50- or 60-Hz AC using an inverter. This process is carefully controlled, and the resulting power is a high-quality signal.

Before installing a microturbine at Lewiston, a Capstone microturbine was installed at a wastewater treatment plant in Jeanette, Pennsylvania. According to Capstone, the Jeanette project proved valuable because it demonstrated that microturbines could run using ADG. The installation also identified the need to remove siloxane compounds from the ADG before combustion. Siloxanes are a family of compounds commonly found in ADG. They become problematic when ADG is burned in any type of engine because siloxanes form a sand-like by-product in combustion that can erode engine parts.

The future

At present, the “free” fuel available at wastewater treatment plants makes distributed generation applications attractive. Natural gas is expensive now and may continue to be expensive if demand outpaces supply. The newer distributed generation technologies allow operators to use ADG without the past problem of emissions constraints.

In addition, grant funding opportunities from sources such as the New York State Energy Research and Development Authority (NYSERDA) make the initial capital cost of fuel cells and microturbines attractive. NYSERDA announced a distributed generation grant program in the spring of 2001, and it intended to repeat that program in the fall of 2001.

As fuel cells and microturbines are more widely adopted, their production prices will drop. These state-of-the-art technologies will replace backup generators in wastewater treatment facilities and collection systems. So, instead of generators sitting idle for most of their life span, every pump station may soon have an active fuel cell or microturbine creating energy as its backup power and for daily use. These technologies can provide economical power in support of the electricity grid and generate revenues for their owner—advantages not possible with yesterday's technologies.

Evaluation matrix

Fuel cells (6-24 months to deploy)

Emissions and permitting	O&M, reliability*	Cost	Construction notes
Near zero emissions. Permit often not required	Depends on technology. Few examples	Most expensive option** but grants available	May not require enclosed building

* Testing of digester gas is recommended to identify any impurities (for example, siloxane and sulfide compounds).
** December 2000, Los Angeles Department of Water & Power announced a $5 million project to install two 250-kW fuel cells. January 2001, King County Washington announced a $19 million project to test a 1-MW experimental fuel cell. Yonkers Wastewater Treatment Plant installed the 200-kW fuel cell at a cost of approximately $1 million.
*** Cost figures are based on a March 1999 Gas Research Institute report.
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Brian R. Klett is a project engineer with Malcolm Pirnie, Inc. He specializes in energy management, operations, and financial analysis. Phone 914-641-2706.

Rande J. Wilson is an associate at Malcolm Pirnie. He leads the firm's energy practice. Phone 914-641-2531.