Gas Naturally

California, according to some dairy commercials, is home to happy cows. So many cows, in fact, that Pacific Gas and Electric Co. estimates that dairy manure makes up 20 percent of the state's available waste biomass for conversion into renewable fuels. The company is aggressively courting developers of anaerobic digestion and biomass gasification projects to provide biomethane for its millions of natural gas customers.
By Jerry W. Kram
Biomethane, also called biogas, would seem to be a natural, easy-to-obtain renewable fuel. Take some manure or other biomass, cover it to catch the gas and let innumerable methane generating bacteria do the work for you. But raw biogas isn't ready for the pipeline. Along with the valuable methane, raw biogas is a mix of water, sulfur, carbon dioxide and possibly even pathogenic bacteria. These components lower the heat value of the biogas and can even damage natural gas pipelines.

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.

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.
To move beyond the limits of anaerobic digestion, PG&E is exploring a project it calls biomethanation. "[Anaerobic digestion] is pretty traditional stuff," Brennan says. "It's been done in Europe for the past 10 or 15 years. Biomethanation is a more emerging technology." The company began the biomethanation project in the first quarter of 2008 by issuing a request for proposals. At this point, the project is not limiting itself to any specific feedstock other than manure, or technology or process, as long as it makes a pipeline-ready gas out of biomass. "Dairy manure is about 20 percent, give or take, of the available biomass in the state, so we are looking for technologies that can turn the other 80 percent into renewable energy for us," he adds. "That includes anything from ag waste to food waste to municipal waste to woody biomass from forests, anything organic, really."

PG&E issued a request for information on new biomethanation projects earlier this year. The company is looking for innovative technologies, primarily using gasification, to increase the percentage of renewables in its natural gas supply. "We want to encourage people to develop projects to convert these hard organics into methane and put that into our pipeline system," Boschee says. The company will review the submissions in May, and Brennan says the company hopes to sign up the first projects before the end of 2008.

PG&E will not act as the developer for the biomethanation projects. Rather, the company will be acting as facilitator to bring developers, investors and regulators together to expedite the development of the projects. "We are trying to bring the parties together, the people who have the technology, the people who have waste streams, people who like to site those facilities and people who have money for investing," Boschee says. "All of those parties are part of the process. We will work with them and help to find financing to develop demonstration projects somewhere here in California."

With the resources of a major company, PG&E can help smaller developers overcome financial and regulatory barriers. "We work with project developers and dairymen to facilitate the development," Brennan says. "There are a lot of hurdles these projects have to face, whether it is permit or process related. Smoothing the way, spreading information and making good contacts for developers are very important roles that PG&E is playing in facilitation of these projects."

PG&E will also consider projects that will improve the efficiency of anaerobic digestion. One of the ways this could be accomplished is by reducing the time it takes to produce biogas in the digesters. Brennan says the traditional digestion process takes 20 to 40 days to complete, whereas new methods complete the process in less than a week.

Adding Value to Ag Waste
Projects such as PG&E's biomethanation initiative have other benefits as well. Agricultural waste is an environmental headache in California. Nutrients and bacteria in manure threaten to contaminate water supplies. Air quality regulations prevent farmers from burning straw and other ag waste, leaving them with a disposal problem. "We have been trying to encourage farmers to realize the energy potential of these waste streams," Boschee says. "They can capitalize on these waste streams and capture the methane gas and use it in the most efficient way possible."

PG&E is not using any biogas in its system currently, although the first dairy-based project is about to come on line. The company's goal is to aggressively grow the supply of biomethane in the state to provide for 10 percent of its consumption. That would be equivalent to more than 200,000 Mcf per day. Biomethane is an important part of PG&E's plans to increase its renewable portfolio. Unlike solar and wind energy, biomethane can be stored and easily dispatched to areas where it's needed. "You don't need to use it right away, where as electrical energy must be," Brennan says. "With gas, you can stick it in the pipeline and store it until it can be used most efficiently. It provides a great deal more flexibility than other renewable options."

Brennan also sees other advantages from the local production and control of biomethane. "Because it is being manufactured right in our service territory, it reduces our reliance on outside sources of gas," he says. "It reduces our need to reinforce our pipeline system to bring gas into the state. It also reduces our need to reinforce our local transmission systems. [Gas from] these dairies is going into existing pipelines that are already serving our customer base."

California recently passed legislation calling on utilities to reduce their greenhouse gas emissions. While this legislation calls for the ambitious development of renewable fuels, Brennan points out that PG&E started its biomethane program well before the bill was passed. "The impetus for our program came before Assembly Bill 38 was passed," he says. "But it does set a very ambitious goal of 20 percent renewables by 2010. So we were forced to put the gas pedal down on something we were already doing."

Jerry W. Kram is a Biomass Magazine staff writer. Reach him at or (701) 738-4962.