In Europe and elsewhere, these problems have been handled by using biogas in small-scale combined-heat-and-power (CHP) generators designed to handle the impurities in the gas. These CHP facilities typically only serve the farms where the biomass is produced, and the surrounding local area. Biogas would be a more useful energy source if it could be integrated into the existing natural gas distribution system and dispatched to wherever it is needed.
Pacific Gas and Electric Co. owns one of the largest of these gas distribution systems. The company provides gas and electricity to approximately 15 million people in California. Its gas transmission “backbone” is 6,128 miles long and reaches 4.2 million homes and businesses. The company has a long-held interest in alternative fuels and environmental protection. In May 2006, the company adopted a policy statement to the effect that it would not only seek to minimize its greenhouse gas emissions, but would also become a leader in addressing global climate change with responsible policies and programs. “Our commitment to renewable energy is pretty solid,” says Ken Brennan, a senior project manager in PG&E’s business development division. “We are trying to get any kind of nonfossil-fuel-based renewable energy we can into our portfolio.” One way to expand its efforts to reduce its greenhouse gas emissions is the integration of biogas into its gas distribution system.
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The first stage in the process is a project in California’s dairy country involving the anaerobic digestion of manure. “We have been working with dairies in the San Joachin Valley to connect them with our transmission system,” Brennan says. The initial project centered on converting the manure into biomethane and creating a system that could clean up the gas at the farm level so it could be injected into the existing gas pipeline system. “Some dairies are already capturing methane from their covered digestion ponds,” says Rod Boschee, manager of PG&E’s business development division. “They are burning it on-site in combustion engines to produce electricity to use on the farm. That is certainly a step in the right direction but we feel a more efficient use of that gas is to clean it up and put it in a pipeline.”
The first dairy in the project began biomethane production in April, and the gas it produced is being tested to ensure that it meets the standards for pipeline gas. The dairy is expected to produce about 600 Mcf of gas per day, and plans call for three or four neighboring dairies to eventually tie into the same system.
Manure and Then Some
The next stage of the project will be to investigate codigestion, where agricultural waste and other biomass is placed in the digester along with the manure. “You add to the dairy waste soft waste such as food waste, cheese whey, grape pomace, all types of other material that can enhance the volume of gas from the digestion process,” Brennan says. “We see this as the first step of the evolutionary process of using additional waste streams that can generate gas.” Future projects could look at wastewater processing plants and landfills as additional waste streams to convert.
The problem with digester gas is that it’s more than just methane. It can contain carbon dioxide along with a corrosive mix of sulfur compounds and water. The company also has to be aware of potentially pathogenic bacteria being introduced into the pipeline system. “There is technology that can remove the main components of the biogas,” Brennan says. “They have to remove the hydrogen sulfide and carbon dioxide. So the final product should be pretty much pure methane gas. But the real concerns that utilities have is biological. Is there any material in the gas, microbes and pathogens that could be harmful? That’s the big unknown that needs to be evaluated.”
Brennan says PG&E will be taking numerous samples of the biomethane from its initial dairy project to check for biological contamination. He doesn’t expect there will be any problems because during the cleaning process and in compressing the gas for injection into the pipeline the gas is heated to several hundred degrees Fahrenheit, high enough to kill most bacteria. “If the gas meets our pipeline quality, we anticipate that it will be a good, clean product,” he says.
Boschee says injecting the biomethane in a pipeline is better than producing electricity on-site because PG&E’s large combined-cycle gas-fired power plants are much more efficient than the farm-based combustion generators. “You can get even greater utilization of that energy to get even more power for the electricity demands here in California,” he says.
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