Renewable natural gas (RNG), also known as biomethane, is a biogas which has been upgraded to a quality similar to fossil natural gas and has a methane concentration of 90% or greater.[1] By removing CO2 and other impurities from biogas, and increasing the concentration of methane to a level similar to fossil natural gas, it becomes possible to distribute RNG via existing gas pipeline infrastructure. RNG can be used in existing appliances, including vehicles with natural gas burning engines (natural gas vehicles).
The most common way of collecting biogas with which to produce biomethane is through the process of anaerobic digestion. Multiple ways of methanizing carbon dioxide/monoxide and hydrogen also exist, including biomethanation, the Sabatier process and a new electrochemical process pioneered in the United States currently undergoing trials.[2]
Renewable natural gas can be produced and distributed via the existing gas grid, making it an attractive means of supplying existing premises with renewable heat and renewable gas energy. Renewable natural gas can also be converted into liquefied natural gas (LNG) or compressed natural gas (CNG) for direct use as fuel in transport sector.
In the United States, projections of the ultimate supply potential for RNG vary. An analysis conducted in 2011 by the Gas Technology Institute determined that renewable gas from waste biomass including agricultural waste has the potential to add up to 2.5 quadrillion Btu annually, being enough to meet the natural gas needs of 50% of American homes.[3][4] The Environmental and Energy Study Institute estimated that renewable natural gas could replace up to 10% of all natural gas used in the United States,[5] and a study by the National Association of Clean Water Agencies and the Water Environment Federation found that the quantity of biosolids removed from wastewater could be turned into enough biogas to potentially meet up to 12% of America's national electricity demand.[6]
More recently, a study commissioned by the American Gas Foundation and executed by ICF in 2019 projected that between 1.6-3.78 trillion cubic feet of RNG could be produced annually for pipeline injection in the U.S. by 2030.[7]
The UK's National Grid believes that an estimated 18% of all gas consumed could be made from matter such as sewage, food waste such as food thrown away by supermarkets and restaurants and organic waste created by businesses such as breweries.[8][9][failed verification]
In combination with power-to-gas, whereby the carbon dioxide and carbon monoxide fraction of biogas are converted to methane using electrolyzed hydrogen, the renewable gas potential of raw biogas is approximately doubled.[10]
A biomass to RNG efficiency of 70% can be achieved during the production process.[11][12] Costs are minimized by maximizing production scale and by locating an anaerobic digestion plant next to transport links (e.g. a port or river) for the chosen source of biomass. The existing gas storage infrastructure would allow the plant to continue to manufacture gas at the full utilization rate even during periods of weak demand, helping minimize manufacturing capital costs per unit of gas produced.[13]
Renewable gas can be produced through three main processes:
In North America, most RNG development has historically occurred in the municipal solid waste (MSW) sector.[15] The first commercial RNG facility was launched at the Fresh Kills landfill near New York City in 1982. As of 2023, more than 300 RNG facilities are currently operational in North America,[16] with more than 70% of supplies drawn from the MSW and landfill sectors, according to the U.S. trade group RNG Coalition.[17]
Göteborg Energi opened the first demonstration plant for large scale production of bio-synthetic natural gas (SNG) through gasification of forest residues in Gothenburg, Sweden within the GoBiGas project. The plant had the capacity to produce 20 megawatts-worth of bioSNG from about 30 MW-worth of biomass, aiming at a conversion efficiency of 65%. From December 2014 the bioSNG plant was fully operational and supplied gas to the Swedish natural gas grid, reaching the quality demands with a methane content of over 95%.[18] The plant was permanently closed due to economic problems in April 2018. Göteborg Energi had invested 175 million euro in the plant and intensive attempts for a year to sell the plant to new investors had failed.[19]
It can be noted that the plant was a technical success, and performed as intended.[20] However, it was not economically viable, given the prices of natural gas at the time. It is expected the plant is to re-emerge around 2030 when economic conditions may be more favorable, with the possibility of a higher carbon price.[21]
SNG is of particular interest in countries with extensive natural gas distribution networks. Core advantages of SNG include compatibility with existing natural gas infrastructure, higher efficiency that Fisher-Tropsch fuels production and smaller-production scale than other second generation biofuel production systems.[22] The Energy Research Centre of the Netherlands has conducted extensive research on large-scale SNG production from woody biomass, based on the importation of feedstocks from abroad.[23]
Renewable natural gas plants based on wood can be categorized into two main categories, one being allothermal, which has the energy provided by a source outside of the gasifier. One example is the double-chambered fluidized bed gasifiers consisting of a separate combustion and gasification chambers. Autothermal systems generate the heat within the gasifier, but require the use of pure oxygen to avoid nitrogen dilution.[24]
In the UK, NNFCC found that any UK bioSNG plant built by 2020 would be highly likely to use "clean woody feedstocks" and that there are several regions with good availability of that source.[25][26]
In the UK, using anaerobic digestion is growing as a means of producing renewable biogas, with nearly 90 biomethane injection sites built across the country.[27] Ecotricity announced plans to supply green gas to UK consumers via the national grid.[28] Centrica also announced that it would begin injecting gas, manufactured from sewage, into the gas grid.[29]
In Canada, FortisBC, a gas provider in British Columbia, injects renewably created natural gas into its existing gas distribution system.[30]
A company called Divert, which also reduces food waste through donation, says it will use a $1 billion investment from Canadian pipeline operator Enbridge to scale its existing network of food waste anaerobic digesters to cover all major markets of North America.[31][32]
Biogas creates similar environmental pollutants as ordinary natural gas fuel, such as carbon monoxide, sulfur dioxide, nitrogen oxide, hydrogen sulfide and particulates. Any unburned gas that escapes contains methane, a long lived greenhouse gas. The key difference from fossil natural gas is that it is often considered partly or fully carbon neutral,[33] since the carbon dioxide contained in the biomass is naturally renewed in each generation of plants, rather than being released from fossil stores and increasing atmospheric carbon dioxide.
A major concern is that the potential biogas yield would only represent a small percentage of existing supplies of fossil gas (also called natural gas). This fact has led existing natural gas suppliers to push back against measures to increase the use of electricity as an energy supply - decreasing demand for gas. This reality prompted Southern California Gas Company (SoCalGas) to covertly support the creation of a nonprofit: Californians for Balanced Energy Solutions (C4Bes) which then went on to lobby for the gas sector and against the momentum in favor of electrification. The Sierra Club exposed the hand of SoCalGas in the formation of C4Bes (astroturfing) and so C4Bes curtailed its lobbying activities, although it continued to promote demand for gas.[34][35][36]
