Onion Processor Uses AD, Fuel Cells To Convert Waste Into Energy
Gills Onions, one of the largest fresh onion producer/ processors in the world, was disposing of 1.5 million pounds of onion waste every week by spreading it as compost across 15,000 acres it farms in Oxnard, Calif. It was an arduous, time-consuming process that cost the company approximately $400,000 annually.
Gills had also long-faced two even potentially costlier concerns: high electricity costs (12 to 13 cents per kilowatt hour plus a 15 percent rate hike for 2010) and electricity blackouts. After experiencing a series of blackouts that had jeopardized the extensive refrigeration required to process and protect its fresh-cut onions, the 27-year-old family owned company began to aggressively pursue a long-held desire to convert its onion waste into energy.
Determining how to efficiently convert onion waste into methane gas through anaerobic digestion presented a challenge, according to Bill Deaton, president of Deaton & Associates LLC, the Kayenta, Utah, chemical engineering and energy consulting firm retained by Gills to study the concept and develop a plan.
“It works well at breweries where beer waste is used to run boilers and on dairy farms where manure is converted into fuel. But until Gills decided to try it, no one had used microorganisms to generate digester gas from onion waste before,” Deaton says.
After testing by Ruihong Zhang at the University of California, Davis confirmed that the sugar content in onion waste was ideal food for methane-producing microbes, Deaton assembled a team of engineers and contractors to develop an efficient waste-to-energy system they named the Advanced Energy Recovery System.
In late winter 2009, following a multiyear research and design process, a new energy recovery system went fully on line at Gills, converting the company’s onion waste to methane gas that feeds two 300-kilowatt fuel cells. Today this system is offsetting 100 percent of the company’s baseload power costs and its expensive waste hauling and disposal operation is no longer necessary.
During its fresh cut operations, Gills removes the top, tail and skin, which is roughly 35 to 40 percent of the onion. This waste, generated at a rate of approximately 300,000 pounds daily, now goes through grinding and pressing equipment where 30,000 gallons of onion juice (approximately 75 percent of the total waste) is separated from the solids. The juice is diluted and fed to the energy recovery system’s fully automated anaerobic digestion process. Microorganisms in the digester convert the juice’s sugar content into methane and carbon dioxide. The remaining leftover solids are sold as high-value cattle feed.
In the first step, the onion juice flows to the anaerobic reactor’s equalization tank that ensures the influent is pumped to the anaerobic digester at a constant, continuous flow. The equalized water is pumped to a rapid mix tank, where the influent is mixed with a dilution stream and conditioned for efficient anaerobic digestion by adjusting temperature, pH and nutrients (including phosphoric acid and nitrogen). A heating recirculation loop maintains the necessary temperature. From the rapid mix tank, the stream is pumped through a static mixer, which injects ferric chloride and micronutrients into the stream prior to entering the digester.
“The temperature of the feed flow is between 35 and 40 degrees Celsius (95 to 104 degrees Fahrenheit),” Deaton says. “To obtain this mesophilic condition, we brought waste heat from other processes in the plant to heat the digester feed.”
The system’s upflow anaerobic sludge blanket (UASB) reactor has a working capacity of 550 cubic meters and a hydraulic retention time of just 29 hours. During this period, anaerobic bacteria within the digester convert the biodegradable onion waste material into methane, carbon dioxide and new biomass.
Feed flow from the rapid mix tank splits and enters the bottom of the digester through two feed headers. A proprietary feed distribution-piping system is used in the distribution of the digester feed across the bottom of the digester. The distribution system is specifically designed to create good initial flow distribution of the conditioned onion juice through the sludge bed.
The feed flows upwards through the digester. At the top of the reactor, effluent flows over weirs that spill equally into a continuous trough system and combine into a settler drop box. Once collected in the drop box, it exits the digester and flows to a standpipe. An anaerobic effluent composite sampler located in the recycle piping upstream of the rapid mix tank automatically collects samples of the effluent to determine digester performance.
Biogas exits the digesters through a gas nozzle at the top of each digester vessel. Excess biomass is periodically removed from the digester and pumped to a tanker truck. Vent gases from the equalization tank, rapid mix tank, digester and standpipe are collected via a vent air blower and purified in a biofilter.
One initial challenge the team faced was foaming generated in the sludge bed due to the high protein content of the onion waste. “But by making several modifications and changing some of the gas piping, we eliminated the foaming issue,” Deaton says.
The team selected Biothane to build the specialized reactor to produce the methane. Biothane, a Veolia Water Solutions & Technologies company, is a leading biotechnology company that focuses on highly efficient, cost-effective biological methods to treat wastewater while creating energy and reducing pollution.
“We liked the idea of the UASB reactor having a sludge bed system rather than a fixed bed,” Deaton says. “Initially we had considered using a fixed-bed reactor because of the high surface area for the bacteria to form. But after further consideration, we decided we didn’t want to deal with the blockage, cleaning, or any of the other issues that can arise with fixed-bed reactors when treating a waste material containing pulp, like diluted onion juice,” says Deaton. Biothane’s UASB reactor forms a blanket of granular sludge that suspends in the tank.
“By suspending the bacteria, you can mix the feed with it and get good contact, and you don’t have to deal with the problems of the fixed media in the bed. Ultimately, the simplicity of the UASB played a big part in our decision to select it,” Deaton says. “Plus, there was a lot of engineering we did outside of the reactor—we transferred waste heat from other processes in the plant with the onion juice, then diluted the onion juice and added micronutrients when necessary.”
Before the methane could be safely fed into the system’s fuel cells, the team had to first overcome a serious obstacle. The methane produced by the feed flow has high sulfur content—part of what makes onions smell and makes people’s eyes water. “You can’t send high-sulfur gas to these fuel cells because they have very stringent gas quality specifications for displacing natural gas as a fuel source,” Deaton says. A grant to fund research by the Gas Technology Institute helped solve the issue. Consequently, the reactor was modified to allow for the collection and conditioning of the biogas.
“With this modification, the biogas flows from the digester into a conditioning process that purifies, dehumidifies and compresses the gas, making it acceptable for use in these high-efficiency fuel cells,” Deaton says. “As a result, the system produces about a 70 percent methane gas, which is very high quality for biogas.”
Fuel Cells=Clean Energy
Because Gills wanted an energy recovery system that produces clean heat and electricity, it ruled out engines, combustors or boilers, choosing fuel cells instead. Although relatively new to commercial industry, fuel cells are increasingly being adopted because they generate a highly clean form of energy, run quietly, and produce power and heat at high efficiency.
The new energy recovery system at Gills uses two 300-kilowatt fuel cells manufactured by Fuel Cell Energy. The methane produced by the anaerobic digester feeds the fuel cells, which re-form the methane to hydrogen and carbon. The hydrogen and carbon recombines with air in the fuel cells to generate electricity in a classic oxidation/reduction reaction. The end-products are electricity, water and carbon dioxide. The two fuel cells generate enough combined electricity to power the equivalent of 460 homes.
“A standard power plant runs at about 30 percent efficiency,” Deaton says. “These fuel cells are currently running at 47 percent efficiency. And, because this is an industrial site, Gills can take advantage of all the heat being generated, too—using it to heat water, evaporate water, and refrigerate or chill plant water. So, the system is actually running at about 80 percent efficiency and negligible emissions.” The emissions avoided by using fuel cells, plus the emissions avoided by not trucking away and land applying the onion waste, yields an approximate 15,000 ton-per-year reduction in the company’s carbon emissions.
Since the fuel cells are designed to supply base-load power and do not ramp up and down quickly, Gills Onions is able to offset 100 percent of its base-load power costs. Therefore, the company is able to bring the power up and hold it steady 24/7.
“In a large fresh-cut produce plant like Gills, there are high, around-the-clock refrigeration demands. You don’t ever want to lose the cold chain in maintaining fresh onions. This constant demand is now satisfied with the AERS process on-site,” Deaton says.
Quick Payback, Flexibility Gained
The entire project cost $9.5 million. Deaton says the savings of $400,000 a year from eliminating waste hauling, $700,000 a year in deferred electricity costs, plus $2.7 million in incentives for the project from Southern California Gas Co. (as part of the state’s Self-Generation Incentive Program) and tax credits should enable Gills to receive a five-year payback.
“Meanwhile, we’re working on another process to take the remaining 25 percent of the waste left after the grinding and pressing process and reduce that by half. This should add another 20 to 50 percent to energy production. When you’re offsetting Edison Power at about 12 to 13 cents per kilowatt hour, the payback comes fairly quickly,” he says.
As the system’s project manager, Deaton says he is pleased with what has been accomplished, but he also says the work is not yet finished. Future plans include further reducing the carbon footprint through energy storage, water re-use, and extracting quercetin from the onion waste for use as an anti-oxidant supplement for humans.
Authors: Denise Johnston
Vice President Marketing and Sales, Biothane LLC
Project Manager, Biothane LLC
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