Established in 1979 by the Lester family, Dixon Ridge Farms grow, package and sell organic walnuts. Each year they process around 685,000 kilograms (kg) of walnuts – resulting in 910,000 kg of walnut shells that need to be disposed of.

Concern about future fuel costs, as well as the environmental benefits of renewable power, drove Dixon Ridge Farms to look into energy self-sufficiency. In 2007, the company set a goal of being energy self-sufficient by the year 2012, while also being carbon-neutral or negative. The Lester family also wanted its energy to come from non-food sources.

Russ Lester and his family were already selling their walnut shells to a biomass power plant for between $10–20 per tonne, so they were familiar with the concept of using biomass as a renewable energy source.

In 2007, the company began working with the Community Power Company (CPC) of Colorado. CPC secured a California Energy Commission grant to place one of their BioMax® 50 downdraft gasifier systems at Dixon Ridge Farms to convert their walnut shells into electricity or heat.

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CPC’s gasifier converts walnut shells into a low Btu combustible fuel gas. The gasifier is also capable of converting other biomass feedstocks including woodchips and tree prunings. Project Manager Brad Roberts says a number of agricultural residues in California, such as olive pits, grape marc and almond shells, would also be well suited to gasification.

The BioMax 50 gasifier produces enough syngas to fuel a generator outputting 50 kilowatts (kW) of utility-grade electricity with around 45 kg of shells per hour. The combustible gas is then used in an engine generator to produce electricity, or combusted to produce heat for the Lester’s walnut drying system. The electricity is used to power a 1,115 square metre freezer that previously cost them approximately $US5,000 per month to operate.

Previously, the Dixon Ridge Farm used about 34,000 litres of propane per week to fuel six heaters during the month-long walnut drying season. Now they use producer gas from the gasifier to displace 30 per cent of the propane used in their heaters.

“We estimate that the walnut shells that we would normally sell for $US20 per tonne are worth $US150 per tonne when gasified and used to offset our onsite heat and electricity costs,” says Russ Lester.

Brad Roberts believes that, having now clocked over 11,000 hours of operating time – including one endurance run where the system operated for 732 hours out of 745 hours – the BioMax 50 has the highest availability record of any small modular gasification plant in the world. The plant is normally shutdown every two weeks for scheduled maintenance. New units being designed by CPC will have additional self-cleaning features to reduce the need for operator intervention.

The biochar, a by-product from the gasification process, is 47 per cent carbon and is produced at a rate of around 55 gallons (208.197 litres) every two days. This is being added to compost and incorporated into the organic farming system. The Lester’s BioMax 50 is a prototype which is tied in with various energy and agricultural research projects at the University of California, Davis, and is providing valuable information back to CPC. Brad monitors the gasifier’s performance, collects research data, performs CPC-designed experiments, and trouble shoots when necessary.

He feels that the average mechanically minded person would be able to run the system, and it’s getting easier to do as improvements are incorporated through his activities.