fuel
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
PHOTO: PG&E
Manure and Then Some
Before biomethane can be injected into pipelines using this pipeline injection system, it has to be cleaned to remove carbon dioxide, water and hydrogen sulfide, as well as any microbial contamination.
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
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5|2008 BIOMASS MAGAZINE 37