The science of capturing gas from wastewater treatment plants to be used as fuel for vehicles and generators has been well developed. However, some utilities are turning to fuel cells to produce electricity directly from biogas — without relying on combustion.

Fuel cells operate on the same principle as common batteries used in consumer electronics. Batteries create an electric current when charged particles move between two dissimilar materials — for example, nickel and cadmium — through a medium known as an electrolyte.

The Direct FuelCell offered by Connecticut-based FuelCell Energy works by converting methane to hydrogen and carbon dioxide using a nickel-based catalyst. The cell creates a chemical reaction between hydrogen on one side and the atmosphere on the other through an electrolyte consisting of potassium and lithium carbonates.

“Fuel cells have been around for a long time and were used in the space program, running off bottled hydrogen,” says Tony Leo, vice president of applications engineering and new technology development for FuelCell Energy. “Our company has been developing this technology for decades and offered the first commercial unit in 2003. We’ve worked to lower the cost of the technology by about 60 percent since then.”

Leo notes that when a typical turbine-based electrical generating system uses biogas as fuel, the combustion can create nitrogen oxide, sulfur dioxide, carbon monoxide and particulate matter. The byproducts of a Direct FuelCell are water and heat, with the heat returned to the treatment plant to aid anaerobic digesters in producing biogas.

The largest application of FuelCell Energy’s fuel cells at a wastewater treatment plant is a 2.8-megawatt facility located at the Inland Empire Utilities Agency treatment plant in Ontario, California. The plant was dedicated in late 2012 and replaced two internal combustion engines to help meet the stringent emissions regulations issued by the South Coast Air Quality Management District.

The IEUA’s fuel cell plant is owned and operated by project developer and investor Anaergia Inc., which sells the electricity and heat to the agency under a 20-year power purchase agreement. Under the agreement, at least 75 percent of the fuel used at the plant must come from biogas, with natural gas making up any shortfall.

“The only gas quality provisions that IEUA is obligated to meet per the power purchase agreement are for heating value and hydrogen sulfide concentration,” says Jesse Pompa, senior associate engineer with the IEUA. “The digester gas that we provide to the fuel cell plant is first treated with an iron sponge to remove H2S. The iron sponge has shown to be effective in removing H2S to the required level.”

Anaergia operates and maintains the gas conditioning system, which further treats the gas before introduction into the fuel cell. The equipment is also remote-monitored by the company. A full-time Anaergia operator is assigned to the site to maintain compressors and instrumentation and to ensure an acceptable pH and moisture content in the biogas.

The multiphase conditioning system includes the use of regenerable activated carbon, first-stage activated carbon and metal-impregnated activated carbon to remove volatile organic compounds, sulfides and siloxanes. Leo notes, however, that the removal of siloxanes is incidental because they won’t damage fuel cells in the same way they harm engines and turbines.

Andrew Dale, project engineering lead for Anaergia, notes that communications with IEUA and wastewater treatment plant staff are generally limited to issues surrounding changes to the fuel supply.

“They’ll let us know if there’s a change in gas pressure or production or whether they’re down for maintenance,” says Dale. “We’ll also coordinate with them if we have a shutdown.”

Unlike a regular battery, fuel cells continue to generate electricity as long as fresh biogas and air are introduced into them.

“The nickel catalyst will wear out about every five years,” says Leo. “Once the fuel cells are refurbished, the modules operate just like they did at the beginning of their lives.”

FuelCell Energy has outfitted plants in other parts of California — including San Jose — and in South Korea. A demonstration project was also recently initiated in Cheyenne, Wyoming.

“Any utility considering fuel cell power generation needs to primarily look at how much gas they can supply through anaerobic digestion,” says Leo. “A wastewater treatment plant serving a few hundred thousand people should produce enough biogas.”

Power plant location and available space are also important. FuelCell energy estimates its technology can produce about 10 megawatts of electricity per acre of land. The fuel cell plant would ideally be located as close to the source of biogas and an electrical connection as possible.

“Depending on the application, we estimate the payback period for such a biogas project at between four and seven years,” says Leo.