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Doubly renewable

Innovative anaerobic digester project in California supports another renewable energy facility: ethanol.

March 16, 2016  by Treena Hein

 Pixley Biogas uses a two-stage, mixed plug flow digester, designed by Wisconsin-based DVO and built by Washington-based Regenis. Photo by Photo by Ryan Krauter/4Creeks Creative

Like most areas of California, Tulare County – located in the world-renowned San Joaquin Valley – is pulled in many directions recreationally, socially and environmentally. Air pollution in the valley has publicly been called “some of the country’s worst” by the California Energy Commission (CEC). Within Tulare, which is inland roughly halfway between San Francisco and Los Angeles, there are six large-scale cheese and dairy producers that need lots of power and inherently produce waste. And, like most of California, Tulare is packed with people; people who generally expect effective environmental protection and progressive thinking.

It’s no great surprise then that Tulare is home to the innovative and doubly renewable Pixley Biogas Project, which just finished its first year in operation in October 2015. Pixley Biogas is the state’s first anaerobic biodigester to power another renewable energy facility. It cost $10 million to build, including $4.6 million granted by the CEC from a surcharge on vehicle registration fees (a fund that produces about $100 million each year for California to support the use of low-carbon fuels such as ethanol). Twenty-three construction jobs were created and there are two full-time ongoing operational jobs. Pixley Biogas is owned by Pixley Cogen Partners (an affiliate of an ethanol company called Calgren). It’s a two-stage, mixed plug flow digester, designed by Wisconsin-based DVO and built by Washington-based Regenis.

The entire process begins at Four J Farm Dairy, where 2,000 dairy cows produce 55,000 gallons of solid and liquid manure every day. This manure is pumped through a mile of eight-inch diameter PVC pipe to the Pixley digester. Insiders note that while the pipeline wasn’t cheap, it’s considered more environmentally friendly than trucking and more convenient as well, allowing the farm to send manure along at any time.

Added to the Pixley digester every day is 10,000 gallons of food waste, trucked in from local food processors, renderers and companies that pump grease-traps for restaurants, food processors and other food service outlets.

“The food waste is very important for gas production,” says Pixley project engineer Craig Hartman. He works for 4Creeks Inc., an engineering, planning and surveying firm in nearby Visalia, Calif. “It was easier to get the permits for food waste and the other parts of the project than it’s been before. It wasn’t a big deal, but there were lots of questions to answer. It’s the first digester project in California to use the state’s streamlined permitting process for co-digestion.”

Holding time in the 1.2 million gallon digester tank is about three weeks. Bacteria work on the raw manure (solids content about three percent) and food waste (solids content up to 20 percent) to produce about 21,000 cubic feet of biogas every hour. That biogas is then used at the adjacent Calgren Renewable Fuels ethanol plant, which has been producing approximately 58 million gallons of ethanol each year from corn and sorghum since 2008. The firm has another ethanol plant in Kansas as well.

The biogas is used to heat the hot water needed for ethanol creation, allowing the plant to shave down its use of natural gas by about five percent. Media reports say the eventual goal is to use 10 percent biogas, which would provide tens of thousands of dollars in savings on natural gas bills every month.

For its ongoing manure contributions, Four J Dairy receives digester solids to use as cow bedding and effluent for its fields, saving money on both fronts. The effluent is pasteurized first using waste heat from the Calgren plant, and waste heat from ethanol production is also used to heat the digester when required. Hartman notes that Calgren also collects carbon dioxide from the ethanol production process, which is transported by rail to a customer company that makes soda pop.

“It’s an energy company fully vested in the future and it’s awesome in my opinion,” Hartman observes. “It’s good forethought for Calgren to diversify and invest in things that haven’t been done. It’s the first in the area that I know of, and it’s excellent energy and resource management.”

He explains that, while a site-specific Environmental Impact Report was requested by neighbors, it worked out well and there have been no complaints.

“I think it’s a very good demonstration that helps people see the mitigation that can be done. It mitigates emissions from the cows, prevents food waste from going into landfill, reduces the need for fertilizer, reduces waste management efforts at the dairy and makes the production of ethanol even more renewable.”

According to CEC, carbon dioxide reductions from Pixley are equivalent to removing 4,200 cars from the road. Hydrogen sulfide is also removed from the biogas, helping improve air quality. The biogas produced from the Pixley project results in 17 percent less greenhouse gases than what is released by other California ethanol manufacturers. Pixley Biogas reclaims millions of gallons of odor-free water from the manure, which is re-used on the farm.

“The Pixley project demonstrates that waste-based feedstocks – dairy manures in this case – can be integrated into existing commercial-scale biorefineries to displace the fossil natural gas used to fire the boilers, which in turn reduces the carbon footprint of the final ethanol product,” says Jim McKinney, program manager of the CEC’s Alternative and Renewable Fuel and Vehicle Technology Program.

 “The Calgren biorefinery connected to the digester now has the lowest carbon intensity score among California ethanol producers,” McKinney says.

In addition to four ethanol plants, California has eight biodiesel plants and two renewable diesel plants. McKinney believes they could all “presumably” follow Pixley’s lead.

“We have about 14.6 billion cubic feet per year in manure resources, which could be converted to biogas for transportation use and displace over 100 million dge [diesel gallon equivalents] per year.”

He notes that while this is a large number, it’s important to point out that California’s total diesel consumption is about 3.6 billion dge per year.

Several initiatives are underway in California to try to develop more dairy digester projects for transportation fuels.

“The main challenge in California to convert waste manures to transportation fuels has been cost,” McKinney explains. “Large-scale anaerobic digestion projects using diverted municipal organic waste or wastewater treatment plant sludge are more economically competitive because the feedstock transportation costs are already incorporated into the system.”

One of these projects is already running in Tulare. A city biodigester handles the industrial wastewater produced by several large dairy product plants, including Nestlé, Saputo and Land O’Lakes. The city recently called on Geomembrane Technologies to install a floating gas collection system over the digester, and about 600,000 cubic feet of biogas is used daily to generate what’s considered a “significant” portion of the plant’s electricity requirement.





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