Anaerobic-digester Gas Powers 200-kW Fuel Cell

The first fuel cell to capture a por-tion of aerobic digester gas (ADG) and convert it to electricity and heat completed its initial year of rated operation in September. Located in Yonkers (Westchester County, NY) the fuel cell helps the county reduce air pollution from flaring, efficiently use on-site resources, and comply with Clean Air Act requirements.

Figure 1 depicts the fuel cell's concept. The fuel cell consumes methane and emits negligible amounts of NOx and VOC, thereby effectively solving the problems associated with ADG^*.

The Yonker's cell is owned and operated by the New York Power Authority (NYPA)^**. All electric and thermal energy produced by the cell is consumed on site.

The Yonker's cell is a standard commercial 200-kW^*** unit modified and con-verted from natural gas fuel to ADG.

To facilitate ADG use required the development of many new parts and implementing substantial changes. The latter included the cell stack assembly, reformer unit, gas-fired heater, thermal-management system, piping, valves, controls, and other components.

In particular, ADG's use in the fuel cell required surmounting three potential impediments:

1. Mechanical components in the reac-tant gas supply system were modified to accommodate the larger volume flow rates that result from use of dilut-ed methane fuel;

2. Corrosion protection was effected to protect the fuel cell from halides and sulfur compounds.

3. A low heat rate (high efficiency) was achieved for the diluted methane fuel.

Construction of the ADG-powered fuel cell consisted of three modules: a gas pre-treatment unit (GPU) module, a power module, and a cooling module.

The GPU module, Figure 2, was specifically devel-oped for NYPA's application. It uses a non-regenerable potassium-hydroxide-impregnated activated-carbon bed to remove hydrogen sulfide (H2S) from the ADG.

Power generation began on April 14, 1997, with the following adaptations having to be made during the initial months of operation:

• Fuel-to-air ratios over the entire oper-ating range were adjusted to wider-than-usual limits to compen-sate for broader-than-anticipated methane-concentration variations in the ADG.

• Additional drains were installed in the fuel line to remove periodically the entrained water blocking the ADG supply to the GPU.

• An additional blower was installed to compensate for lower-than-anticipated ADG pressure.

Through the end of 1997 the fuel cell power plant had operated continuously in excess of 4000 hours and had produced almost 600 MW-hr. NYPA's share of the project cost should be more than recovered over the life of the project through the sale of electricity produced.

^* Released to the atmosphere without being burned, ADG's contribution to the greenhouse effect is more than 600 times that of carbon dioxide alone. Excess ADG therefore typically is flared-a not wholly satisfactory solu-tion, since ADG combustion generates photoreactive ozone precursors. As a result, wastewater-treatment plants are regulated as stationary sources of air pollution under the Clean Air Act. This requires installation of expensive con-trol and monitoring technologies.

^{* *}The NYPA joined forces with Westchester County, the New York State Energy Research & Development Authority (NYSERDA), the Electric Power Research Institute (EPRI), the U.S. EPA and the U.S. Department Of Energy to install the fuel cell.

^{* * *}Model PC25C, fabricated by the Intemational Fuel Cell (IFC/ONSI) Corporation, a division of United Technologies.