Algae Reactor
The cultivation of microalgae in the algae bioreactor system provides excellent prospects for renewable energy production. Microalgae are the most primitive plant form—typically a single-cell plant. Because of this simple structure, algae are very efficient in converting sunlight, carbon dioxide, and nutrients into oil (for biodiesel) and starch (for ethanol).
Algae are suspended in water acquired from the anaerobic digesters and process water from the coal plant. To grow, the algae will use micro-nutrients from the digesters, along with carbon and nitrogen from the coal plant. A significant portion of the carbon dioxide and some of the nitrogen oxides from the power plant fl u gas are consumed in the bioreactor by algae through photosynthesis. Algae will be harvested daily, sent through a dewatering process, and then processed into co-products including solids and oils.
The outputs from the algae reactor are numerous. The carbon-enriched algae biomass can be dried and fed back into the power plant as renewable fuel or further processed to produce transportation fuels and other high-value products. The lipid oils in the algae can be processed into biodiesel. The carbohydrates can be fermented into ethanol, and the proteins can be used in the production of feed and fertilizers for crops. Most of the water can be recycled and used in the coal plant cooling system or returned to the reactor for additional algae growth.
Algae systems have been researched for decades, most notably by the National Renewable Energy Laboratory and NASA. Production was found to be viable, but most work was done when fuel prices were half of what they are today. No large-scale algae reactor is in operation today, but significant venture capital investment has been made recently to develop this technology.
Two of the Holcomb Expansion Project partners, Sunflower Electric and Tri-State Generation and Transmission Association, have contracted with Greenfuel Technologies to begin a study at Holcomb to determine the appropriate algae strains for the algae reactor subsystem at the Center.
Anaerobic Digestion
An anaerobic digester, a system that harnesses the naturally occurring process of decomposition, is used to process waste and produce biogas and other co-products. The digester will process wastewater and manure from the dairy, thin stillage from the ethanol plant, and possibly glycerol from the biodiesel plant. Bacteria in the digester will produce methane that can be used by the ethanol and/or the power plant. Other co-products from the digester could include ammonia, water and nutrients (nitrogen and phosphorus) for the algae reactor, water for the power plant, and treated sludge that can be used as fertilizer.
Digesters have been used throughout the world for many decades, but they have recently gained popularity because of advances in anaerobic microbiology, reactor technology, and the potential to generate value-added end products.
Biodiesel Plant
Biodiesel is a fuel that has many of the same characteristics as normal petroleum diesel, including similar energy content, improved lubricity, and higher fl ash points. Biodiesel is derived from “cutting” triglycerides found in vegetable oils and animal fat, using simple alcohol in the presence of an alkali catalyst (transesterification). The biodiesel plant will be a multi-feedstock facility and will receive shipments of vegetable oil (including soy), animal fat, possibly extracted corn oil from the ethanol plant, and eventually algae oil from the algae reactor.
Although the use of biodiesel as a fuel for machinery dates back to the 1930s, the awareness in biodiesel as an alternative fuel has grown dramatically over the past several years. Some experts believe that growth in the biodiesel industry may be limited by the access to or cost of oil sources. Many of the crops that produce oil require large acreages to produce a significant volume of oil. Soybeans, for example, produce around 50 gallons (depending on location) of biodiesel per acre per year. In contrast, one acre of algae could produce 8,000 gallons of biodiesel per year.
Coal-Based Power Plant
The Holcomb site offers several resources for an integrated bioenergy facility. These include access to land, water, rail, natural gas, and carbon dioxide from power plant emissions. The emissions produced by the power plant, when passed through the algae reactor, optimize algae growth by utilizing the warm fl u gas that includes carbon dioxide.
Dairy
The dairy brings important components to the Center. The dairy will provide manure and wastewater to an anaerobic digester where it will be converted to methane. The starch and wet distillers grain from the ethanol plant and possibly solids from the algae reactor will be used by the dairy for cattle feed.
Ethanol Plant
Ethanol is a high octane, clean-burning, renewable fuel that is produced by converting cereal starches found in grain into sugar. The sugar is then converted to ethanol through fermentation. The ethanol plant will consume local corn and milo as well as grain railed from other parts of the country, possibly starch from the algae reactor, and methane (displacing natural gas) from the anaerobic digester. Co-products that will likely be provided include extracted corn oil to the biodiesel plant, thin stillage to the anaerobic digester, and distillers grain to the dairy and surrounding livestock industry.
Project developers are also exploring options to power the thermal needs of the ethanol plant with over 90% renewable fuels. This achievement would enable the ethanol to qualify for additional credits under the Renewable Fuels Standard.