Last week, the gas distributor Énergir announced that new buildings connecting to its network will have to buy only renewable natural gas (RNG). What is GNR? Is it really a greener solution than natural gas of fossil origin? Overview.
What is Renewable Natural Gas (RNG)?
Renewable natural gas is produced from organic matter, not fossil. It is made up of the exact same molecule — methane (CH4) — found in conventional natural gas. RNG can therefore be injected into the gas network without any problem and replace all uses of natural gas.
When you burn methane, you release thermal energy and generate CO2. If not burned, methane is itself a powerful greenhouse gas: after a century, one kilo of this gas heats the atmosphere as much as 28 kilos of CO2. From a climatic point of view, it is therefore better to burn the methane than to let it go skyward.
How to produce GNR?
RNG, also called biomethane, comes from organic residues: municipal waste, compostable materials, animal manure, agricultural residues, sludge from wastewater treatment plants, forestry residues, etc. The decomposition of this material in an oxygen-poor environment generates methane.
In practice, to produce RNG, the organic residues are macerated in “biodigesters”, airtight containers, from which the gas is taken. We can also capture methane that escapes from landfills where organic matter decomposes. In any case, the gas must be purified, as it also contains a portion of CO2.
After the process, there remains a solid material called “digestate” that farmers use as fertilizer. “The great thing about biomethanation is that the nutrients — nitrogen and phosphorus — are completely retained [dans le digestat]. It is even better than manure, because the nutrients have been digested a little, and therefore the nitrogen is more easily assimilated by the plants”, explains biomethane specialist Lorie Hamelin, from the National Research Institute for agriculture, food and the environment in France.
How is the GNR distributed in Quebec?
This year, RNG represents only about 1% of the natural gas distributed in Quebec. This fraction will be expected to increase in the coming years: Quebec regulations require that in 2030, at least 10% of the natural gas distributed in the province be from a renewable source.
Note that GNR is mixed throughout the gas network. Énergir subscribers can buy this more virtuous type of gas by paying a surplus, but they receive, in practice, the same mixture of natural gas – with a very high fossil content – as the rest of the company’s customers.
What is the carbon footprint of GNR?
Whether the methane is from a fossil or renewable source, when it is burned, it generates the same emissions: 1.9 kg of CO2 per cubic meter of natural gas. However, GNR is part of a “short cycle”: the carbon sent into the atmosphere will quickly be recovered by new plants that grow, which will in turn be eaten by pigs or thrown into the compost. As long as the wheel turns, the sum is zero.
On the contrary, natural gas of fossil origin is part of a “long cycle”. The underground reserve from which we are currently drawing has been accumulated over several million years. Burning this methane now leads to a net increase in CO2 in the air.
Additional benefits can be attributed to the use of GNR. Indeed, capturing fumes from pig manure, for example, reduces methane emissions. These saved emissions can be subtracted from the already zero carbon footprint of biomethane. From this perspective, RNG can be “carbon-negative”.
In reality, the carbon footprint of RNG varies greatly depending on the type of organic matter used. The climate benefit is particularly large with manure, and rather modest with municipal waste. The exact values depend a lot on the assumptions made: what would have happened to the organic matter if it had not gone to the biodigester?
Why should leaks be taken into account?
To clearly define the carbon footprint of GNR, it is also necessary to take into account methane leaks. These leaks, also called “fugitive emissions”, occur in biogas plants, in the distribution network and at customers.
Remember that methane is a powerful greenhouse gas: if it escapes before being burned, it greatly warms the atmosphere. Leakage rates vary greatly from one biogas plant to another, but they can exceed 10%.
In the worst cases, fugitive GNR emissions can completely undo the gains made. It is even worse if the organic matter used does not generate methane in the reference scenario. Think of plant residues that decompose in the open field, in an oxygen-rich environment.
From a decarbonization perspective, what is the best use of RNG?
Globally, only a tiny fraction of available organic matter is still used to make biomethane. Even if we increased the rate, it would not be possible to replace all the gas of fossil origin that we consume. According to the International Energy Agency (IEA), exploiting the full “sustainable potential” of RNG would meet 20% of current natural gas needs.
“From a decarbonization perspective, it is best to use biomethane in sectors where few low-carbon alternatives exist, such as in high-temperature industrial heating, the petrochemical industry, trucking heavy and maritime transport,” reads an IEA report.
“If we are serious as a society and want to move towards a low carbon fossil economy, biomethane is such an obvious and practical vector! » argues Mme Hamelin, a Quebec native who did a doctorate on energy from agricultural biomass in Denmark. It also pleads for the use of GNR where electrification is impossible.