BioCycle January 2010, Vol. 51, No. 1, p. 46
Stahlbush Island Farms, a sustainable fruit and vegetable grower and food processor in Oregon, turns its residuals streams into heat, power, hot water and crop nutrients.
Dean Foor and Matt Coen
STAHLBUSH Island Farms, founded by Bill and Karla Chambers in Corvallis, Oregon in 1985, held an open house for its new anaerobic digester biogas plant on July 2, 2009. Stahlbush Island Farms is committed to the practice of sustainable agriculture throughout its approximately 5,000 acres of farmland and its processing facility. Minimizing the company’s carbon footprint and supporting renewable energy are both considered integral to this commitment.
Anaerobic digestion (AD) is applicable to a wide range of situations, but synergy is most realized at facilities like Stahlbush that: 1) Have access to sizeable organic feedstock at little to no cost; 2) Require electricity and heat which can be provided by a biogas-powered combined heat and power (CHP) unit; and 3) Can utilize the digester effluent as compost, liquid fertilizer and irrigation water. Macronutrients contained in the processing waste, which would have otherwise been exported off site, can be recovered for land application.
Until installation of the digester, processing waste from Stahlbush Island Farms was trucked offsite for animal feed or land application, resulting in costs for disposal, transportation and labor. Food waste from their canning facility, such as sweet corn, berries, broccoli and pumpkin, serves as an excellent source of feedstock for the anaerobic digestion system. The residual “waste” portion of each crop can exceed 50 percent of the raw weight, in the case of corn, or be less than 5 percent, as with some berries.
Currently, residuals from over 30 different fruit and vegetable crops amount to approximately 20,000 tons/year. Sweet corn cannery waste referred to as “corn silage” amounts to an additional 16,000 tons/year, exceeding all other waste streams combined on an annual basis. Pumpkin/squash and potato make up over 90 percent of the remaining “processing waste” streams. The vast majority of the processing residue (sweet corn and pumpkin) is generated from September through December.
Because of the seasonality of the waste flow, Stahlbush Farm’s digester system was designed to utilize crops as well. Crop cover planted over approximately one-third of the acreage results in 16,000 tons/year of “green chop” (chopped straw). The corn silage and green chop are ensiled in Ag Bags together for processing in the digester throughout the year. Ensiling is a well-documented process that takes place in an anaerobic environment where naturally occurring bacteria produce lactic acid from carbohydrates, which lowers pH and prevents spoilage. In fact, the silage process largely mimics the first two steps of the AD process, hydrolysis and acidification, effectively acting as a pretreatment. If done properly, ensiling can preserve and even enhance methane yield relative to fresh crops.
At Stahlbush Farms, the ensiled material is removed from the bags and fed via the dry material process to the mix tank (described below). The optimum total solids (TS) content range for silage is 35 to 40 percent. Ensiling pumpkin/squash residuals (10% TS) requires a significant amount of straw (90% TS) to reach that optimal range. The blending is accomplished in a “feed wagon” similar to what dairies use to deliver mixed rations.
FEASIBILITY STUDY AND SYSTEM SELECTION
Before moving ahead with the AD project, Stahlbush Island Farms contracted with Essential Consulting Oregon, LLC (EC Oregon) in Eugene in August 2007 to perform a technical and financial feasibility study. It entailed analysis of waste streams, assessment of potential codigestion substrate, digestion trials, estimation of biogas yields, assessment of technology options, biogas utilization calculations, utility interconnection options, effluent treatment and use, permitting requirements, and financial modeling. The study concluded the project was technically feasible due to the AD technology expertise available. In addition, based on the industry information gathered by EC Oregon, data provided by Stahlbush, and some relatively conservative assumptions, the project was also determined to be financially feasible. The “processing waste” sources were considered cumulatively for the purposes of methane potential and financial cost/benefit, using weighted averages where appropriate.
The complete mix or continuously stirred tank reactor (CSTR) technology was recommended as a proven and effective technology for feedstocks with a wide range of total solids. Complete mix systems run at steady state with continuous flow of reactants and products; the feed assumes a uniform composition throughout the reactor and the exit stream has the same composition as in the tank. This homogenization ensures maximum contact between substrate and microbe, enhancing the digestion process and biogas quality.
Vendor selection for the digester technology and combined heat and power (CHP) unit was determined through a request for proposal process. The Austrian firm AAT-Biogas was selected to supply the complete mix digester design, process engineering and equipment for the biogas digester system. The CSTR design employs a hydraulic mixing method that further improves net energy yields. The multichambered tanks utilize differential pressure created via biogas production, an automatically controlled valve system and engineered baffles for highly efficient mixing with no internal moving parts or electricity demand.
EC Oregon was retained by Stahlbush to apply for the necessary permitting, business energy tax credit and electrical interconnection. The project took 14 months to complete at a cost of $10 million. The plant was commissioned in the second quarter of 2009 and is processing nearly 55,000 wet tons/year of mixed biomass (primarily fruit and vegetable waste) through two anaerobic digestion tanks (each with 900,000 gallons of capacity) to produce biogas.
DIGESTER AND CHP UNIT OPERATIONS
All dry material is ground through a hammermill and blended with wet material or dilution water in a mix tank, where initial hydrolysis takes place, before being fed to the primary digesters. Ensiled material also is processed in the hammermill. Each digester is fed from the mix tank on a cyclical schedule, four to six times per day. Average TS in the digesters is 10 percent or less. During the harvest season, food processing residuals are loaded directly to the digester at a rate equal to its design capacity; the excess is ensiled. Retention time in the digester tanks is about 25 days.
Hydrogen sulfide (H2S) is removed from the biogas by biological desulphurization, where small amounts of oxygen are injected into the biogas. Microorganisms oxidize H2S into elemental sulfur, which precipitates back into the post digestate.
Stahlbush Farms installed a 1.6 MW Caterpillar CHP unit. The biogas plant is estimated to produce enough electricity for approximately 1,200 homes, nearly twice what the farm and food processing plant uses in a year. It began producing electrical power in June 2009. Electricity is sold to the PacifiCorp utility.
The CHP supplies jacketed steam and hot water to the processing plant (and to heat the digesters) while air from the building housing the engine and boiler is exhausted to a drying application (for pumpkin seeds). The Stahlbush Island Farms Biogas Plant currently generates 1,600 kW approximately 16-plus hours per day, with biogas production at 55 to 65 percent methane content. The biogas yield is expected to be at full design capacity – about 350 MCF methane/day – in the first half of 2010.
A screw press is used to separate/dewater the digestate fiber to a solids content of about 25 percent. Stahlbush Farms is experimenting with both composting and direct land application of the fiber to cultivated land. The final processing method has not yet been determined. Separated liquid is being used for irrigation. A membrane technology is being pilot-tested to determine the effectiveness of concentrating the liquid/nutrients to facilitate transportation.
The current renewable energy environment in Oregon is as attractive as any in the United States. The Energy Trust of Oregon provided financial assistance for the initial feasibility study and the Oregon Department of Energy (DOE) Business Energy Tax Credit supported up to 50 percent of the cost of the project. Without the financial assistance received, the Stahlbush Island Farms Biogas Plant would not be a reality. The plant is expected to see a return on investment in five to seven years.
Dean Foor and Matt Coen are Project Engineers with Essential Consulting Oregon, LLC (www.ecoregon.com) in Eugene. EC Oregon was founded in 2005 to provide energy analysis services to the agricultural and business community with an emphasis on anaerobic digestion of organic waste and biogas production.
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BioCycle January 2010, Vol. 51, No. 1, p. 46