by amanda m. klemp
With 136 077 kg of onion scraps a day to dispose of, Gills Onions of Oxnard, California, U.S.A., found a tearfree way to put that waste to use. Whereas the scraps used to be hauled and spread on the fields for fertilizer, the grower and producer of packaged onion products installed an anaerobic digester and scalable 600 kW fuel cellpowered power plant to generate electricity that can be used at the facilities.
FuelCell Energy Inc. (FCE), Danbury, Connecticut, U.S.A., provided the fuel cell power plants for the system at Gills. The system comprises the digester to break down onion waste and create methane, and two FCE 300 kW fuel cell systems to convert the methane to electrical and thermal energy. By designing the system in a modular approach, its capabilities can be expanded if Gills Onions expands.
Not only does using digester gas in the fuel cells save on energy costs around US$700 000 a year it also reduces the expense of storing, composting and hauling the waste around the facilities. Gills estimated a US$400 000 annual savings by reducing the amount of waste that is handled and moved around the facilities.
The system works by first convertand ing the onion waste into the biogas. The waste goes into a reactor vessel and mixes with bacteria that digests the organic solids into methane, reducing the volume of waste, said Tony Leo, vice president of OEM and application engineering at FCE. The remaining solid waste is then sold as cattle feed.
Gills Onions said the minimal emissions from the digester convertand fuel cells are much less compared to emissions from transporting and spreading the decomposing onion waste. “Methane comes off this sludge. It builds up in the head space of the reactor, and from that reactor there’s a little bit of a storage container to provide surge capacity,” explained Leo. “Then it flows into what we call the digester gas pretreatment system.” The pretreatment system is required to treat the biogas before it enters the fuel cells to rid it of impurities, such as sulfur compounds and moisture, and bring it closer to natural gas standards.
The FCE systems were originally designed to operate on natural gas, which comprises about 95% methane naturally. Biogas formed in a digester is about 60 to 80% methane, depending on the source and the digester. However, because of the electrochemical conversion process, the fuel cells can be more forgiving to varying qualities of gas, said FCE.
The systems also come equipped with a natural gas blending system, which is completely automatic. “We can blend in as much natural gas or digester gas as the customer wants. In the case of Gills, they want it to be all digester gas,” said Leo. “After that pretreatment system, it flows into the fuel cell and the fuel cell has a flow controller on it.
It demands a certain amount of gas depending on the current it wants to run at,” said Leo. The fuel cell is designed as a stack of 400 individual cells. The fuel reacts with the air across an electrolyte membrane in the fuel cell to create heat, generating the energy through an electrochemical process.
The system is about 47% electrically efficient, reaching up to 90% efficiency when used in combined heat and power applications, said FuelCell Energy. In the case of Gills Onions, the waste heat will be used in another food process. The energy created in the fuel cell stacks 350 V of dc current is directed to a power conversion unit to convert the dc power to ac power and then sent to the grid.
Gills Onions’ fuel cell system generates 35 to 45% of the energy to operate the facilities. Leo said, “If you were to put engines there, he’d be producing maybe 500 kW worth of gas, but because our systems are more efficient, he can produce about 600 kW worth of electricity from the same amount of gas.” In addition to reducing operation and energy costs, Gills Onions received nearly US$6 million in grants and tax incentives for developing and implementing the power plant.
The payback period is expected to be around six years for the company’s US$10.8 million investment. Each fuel cell unit is about 10 m by 7 m, and the digester is about 10 m by 20 m. Leo said plant-based taste is a potential growth opportunity for fuel cell power. Additional fuel sources come from other types of agricultural waste, wastewater treatment plants or any processing that produces organic wastes, such as hops from breweries.