Renewable Natural Gas
Renewable natural gas (RNG) is a topic that does not get a lot of attention in the debate over renewable energy, but it represents some of the best untapped resources available today.
Pipeline quality natural gas is primarily methane, and liquefied natural gas (LNG) is almost pure methane (99%), and methane is a renewable molecule. Natural gas can be produced biologically and synthetically from abundant waste biomass and also excess electric power from intermittent renewable sources.
Currently there is very little policy to support the integration of RNG into the natural gas market and very little awareness of the opportunity. The Bioenergy Association of California (BAC) was formed to support sustainable bioenergy development in California and they have recently come out with a series of reports on biogas resources in California.
In their recent report, “Decarbonizing the Gas Sector,” BAC states that California currently imports more than 90% of the natural gas it uses, costing the state billions of dollars per year. California consumes more than 2 trillion cubic feet of natural gas every year and it provides more than half of the state’s electricity, heating, and cooling and a growing share of transportation fuels. Natural gas is also the source of more than a quarter of California’s greenhouse gas emissions.
Organic waste converted into biogas could meet more than 10% of California’s natural gas demand. According to BAC total organic waste in CA could be used to produce 284 billion cubic feet (bcf) of renewable natural gas. This RNG is equal to 2.5 billion gge (gasoline gallon equivalents) of transportation fuel, enough to replace ¾ of all the diesel fuel used in the state. Alternatively, the RNG could produce 5,000 – 6,000 MW of flexible electric power generation.
California, like much of the world, has abundant resources to produce RNG. More than 16 million tons of organic waste are landfilled every year in the state. Additionally there are over 500 wastewater treatment plants, 278 landfills, 1600 dairies, and extensive forests. It is estimated that food waste could provide 82 bcf of gas, landfill gas could provide 53 bcf, livestock manure could provide 43.4 bcf, sewage treatment could provide 23 bcf, and forest waste could provide 82.4 bcf every year.
In addition to providing many jobs and economic opportunities there are many environmental benefits to producing RNG. RNG directly displaces fossil natural gas resulting in reduced greenhouse gas emissions, methane emissions and black carbon. RNG production reduces the amount of waste going into landfills and helps reduce wildfires by using by fuel from the forest floor. RNG helps improve energy security and offers flexible and reliable power generations opportunities.
Renewable natural gas has the lowest carbon content of any transportation fuels available. Gasoline and diesel have nearly 100 grams of CO2 per megajoule of energy. Traditional biofuels such as ethanol from corn and sugarcane are not much better since producing agricultural crops is very fossil fuel intensive and the conversion processes are not very efficient. Natural gas has 68 grams of CO2 and hydrogen derived from natural gas has 39.42 grams of CO2. Landfill gas has only 11.26 to 15.56 grams of CO2 and the numbers drop from there for dairy biogas (13.45), wastewater biogas (7.89) and most remarkably, biogas from food and green waste is rated as having negative carbon emissions (-15). Biogas produced from food and green waste is the only carbon negative transport fuel available today.
There is a major effort across transportation sectors to ramp up the use natural gas to replace diesel fuel in high horsepower applications such as truck transport, marine, rail and off road vehicles. The reasons for push are two-fold, to take advantage of natural gas’s lower costs compared to diesel, but also to reap the environmental benefits. Diesel and other liquid fuels are responsible for major shares of toxic criteria pollution, such as NOx, SOx, and particulates.
Fossil natural gas is clean and using it results in the virtual elimination of SOx and particulates and substantial reductions in NOx. But fossil natural gas still has significant carbon emissions that must be reduced. RNG amplifies the emissions benefits of fossil natural gas by also reducing carbon emissions because renewable carbon is substituted for fossil carbon.
Renewable natural gas can be produced in a variety of ways. The most common is through the process of anaerobic digestion (AD) where microbes in an oxygen starved container decompose organic materials. AD is commonly deployed at wastewater treatment plants to break down sewage and on farms to help dispose of animal manure.
Landfills produce methane naturally through the decomposition of organic materials and in many areas this methane effluent is regulated. So the landfill industry has been collecting the methane in many landfills for years and this is the leading commercial source for renewable natural gas today. The waste disposal industry have been leaders in converting their garbage trucks to run on landfill gas which is typically cleaned up and upgraded to road quality CNG.
There is now also a growing anerobic digestion (AD) industry focused on diverting organics from going to landfills. Large digesters are being built throughout the country as governments recognize the importance of diverting organic waste from landfills. The biosolids, or digestate, that is left over in AD process are a valuable fertilizer that helps replace the use of fossil fuel derived chemical fertilizers.
Finally, there is a new emerging technology platform for producing renewable natural gas from renewable electricity. Power-to-gas (P2G) is a concept that is being deployed in Germany and elsewhere in Europe but has barely broken through in energy policy discussions in the USA. Power grid operators have major technical challenges with integrating large proportions of intermittent renewable energy sources such as wind and solar into the grid. Traditionally, supply and demand of electricity are balanced by grid operators who can instruct power plants to ramp up or down. The challenge with wind and solar is that they produce power when the resources are available without regards to demand, so sunny afternoons or breezy nights can see a large ramp up in power production that exceeds demand and forces other power plants to shut down quickly and inefficiently. As the quantity of intermittent renewables climbs this integration challenge grows and has caused a great deal of interest in creating energy storage solutions such as large batteries, flywheels, pumped hydro and others.
Power to gas offers an alternative solution for energy storage by converting excess electricity into hydrogen or methane for storage in the natural gas pipeline networks. The major advantage of P2G is that it leverages existing infrastructure that is endlessly scalable and allows for long term energy storage today. By using P2G, grid operators can manage electric and gas networks simultaneously, creating electricity from gas and vice versa as needed, and overcoming bottlenecks arising from trying to constantly match power demand and supply in a dynamic production landscape.
Renewable natural gas offers robust opportunities to mitigate waste problems while producing high quality low carbon fuels that are suitable for use in heat, power and transportation applications.