Digester Gas & Landfill Gas

When you bury trash at a landfill, you create an oxygen-free environment under the capping soil layer. With relatively dry conditions, landfill waste produces significant amounts of gas as it decomposes -- mostly methane. With Californians dumping 33 million tons of waste per year, the total amount of landfill gases produced in California is tremendous.

If these gases were just released to the atmosphere, they could add to global climate change problems. They could also be potentially a fire or explosion hazard if not collected and gotten rid of. So, a good solution to the landfill gas problem is to collect it and burn it to produce electricity.

The gas can be collected by a collection system, which typically consists of a series of wells drilled into the landfill and connected by a plastic piping system. The gas entering the gas collection system is saturated with water, and that water must be removed prior to further processing.

The typical dry composition of the low-energy content gas is 57 percent methane (natural gas), 42 percent carbon dioxide, 0.5 percent nitrogen, 0.2 percent hydrogen, and 0.2 percent oxygen. In addition, a significant number of other compounds are found in trace quantities. These include alkanes, aromatics, chlorocarbons, oxygenated compounds, other hydrocarbons, and sulfur dioxide.

After the water is removed, the landfill gas can be used directly in reciprocating engines. Or the carbon dioxide can be removed with further refining, and purer methane can be used for electricity generation applications such as gas turbines and fuel cells. For example, Southern California Edison and Los Angeles Department of Water and Power operate a 40 kilowatt phosphoric acid fuel cell using processed landfill gas at a hotel/convention center complex in the City of Industry.

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Digester Gas

Anaerobic digestion is a biological process that produces a gas principally composed of methane (CH4) and carbon dioxide (CO2) otherwise known as biogas. These gases are produced from organic wastes such as livestock manure, and food processing waste.

Anaerobic processes could either occur naturally or in a controlled environment such as a biogas plant. Organic waste such as livestock manure and various types of bacteria are put in an airtight container called digester so the process could occur. Depending on the waste feedstock and the system design, biogas is typically 55 to 75 percent pure methane. State-of-the-art systems report producing biogas that is more than 95 percent pure methane.

The process of anaerobic digestion consists of three steps.

The first step is the decomposition (hydrolysis) of plant or animal matter. This step breaks down the organic material to usable-sized molecules such as sugar. The second step is the conversion of decomposed matter to organic acids. And finally, the acids are converted to methane gas.

At Royal Farms No. 1 in Tulare, Calif., hog manure is slurried and sent to a covered lagoon for biogas generation. The collected biogas fuels a 70-kilowatt (kW) engine-generator and a 100 kW engine-generator. The electricity generated on the farm is able to meet monthly electric and heat energy demand.

Given the success of this project, three other swine farms (Sharp Ranch, Fresno, and Prison Farm) have also installed floating covers on lagoons. The Knudsen and Sons project in Chico, Calif., treated wastewater which contained organic matter from fruit crushing and wash-down in a covered and lined lagoon. The biogas produce is burned in a boiler. And at Langerwerf Dairy in Durham, Calif., cow manure is scraped and fed into a plug-flow digester. The biogas produced is used to fire an 85-kW gas engine. The engine operates at 35-kW capacity level and drives a generator to produce electricity. Electricity and heat generated is able to offset all dairy energy demand. That system has operated since 1982.

Many anaerobic digestion technologies are commercially available and have been demonstrated for use with agricultural wastes and for treating municipal and industrial wastewater. Where unprocessed wastes cause odor and water pollution such as in large dairies, anaerobic digestion reduces the odor and liquid waste disposal problems and produces a biogas fuel that can be used for process heating and/or electricity generation.