Compared to fossil fuels including natural gas, biogas — which is made from waste — is a cleaner, greener alternative.
The International Energy Agency estimates that biogas (and biomethane) could meet up to 20 per cent of the world's gas requirements during the transition away from fossil fuels, and help us get closer to net zero.
But a new study, published in the journal One Earth, claims emissions from biogas could be higher than we've been estimating, and there is significant work to be done to get its emissions down.
The researchers looked at data from European biogas producers.
They found the rate (not the total volume) of emissions along the biogas supply chain — from production through to supply — was comparable and generally higher than for natural gas, according to lead author Semra Bakkaloglu from the Imperial College London.
"The level of emissions was higher than we thought it would be," Dr Bakkaloglu said.
It's important to point out here that even with high emissions along the supply chain, biogas is still the greener alternative to natural gas.
That's because biogas is created out of waste that would have otherwise released emissions as it decomposed naturally.
However, that's not to say that biogas is always climate neutral.
To create biogas, you start with a feedstock, which might be, for instance, cattle manure or food scraps.
That's broken down anaerobically by microorganisms. They create mostly methane and some carbon dioxide as by-products, which are captured as gas — but some of it escapes into the atmosphere.
Compared to carbon dioxide, which is released during aerobic digestion, methane is a much more potent, albeit shorter-lived, greenhouse gas, according to Peter Ashman, a professor of chemical engineering at the University of Adelaide, who wasn't involved with the research.
"What they're highlighting here is important because if you take CO2 from the atmosphere and you emit it back into the atmosphere as methane, that methane has a much higher potency as a greenhouse gas than CO2," Professor Ashman said.
Unlike biogas, the International Energy Agency has warned there can be no new coal, oil, or natural gas developments if the world is to reach net zero by 2050.
The researchers argue that if we're going to use it as a transition fuel, we need to know biogas emissions for nations to accurately gauge their greenhouse gas footprints.
"We believe that with the proper design, detection, measurement, and repair techniques, much of the observed emissions can be avoided."
'Super emitters' in the crosshairs
Once the gas is captured during the production of biogas, what's left of the feedstock at the end is what's known as digestate.
According to the research, the greatest source of emissions along the biogas supply chain was at the digestate storage stage.
The researchers found that methane emissions from digestate were up to 23 per cent higher than had been previously reported.
The next highest source of emissions was found to be during the biogas production stage, mostly from the anaerobic digester.
Professor Ashman said he wasn't shocked that supply-chain emissions might be higher from European biogas producers, as they lacked the scale and financial backing of most natural gas operators.
"Some of these operations are pretty agricultural and they probably don't put much effort into capturing those emissions."
The researchers said there was a silver lining to their work.
First of all, they found there were "super emitters" — a few operations responsible for the vast majority of escaping methane.
In this case, just 5 per cent of biogas producers were responsible for more than 60 per cent of all methane emitted.
Dr Bakkaloglu said it was a similar case for the fossil fuel gas industry.
"Biomethane and biogas supply chains exhibit similar emissions characteristics to oil and natural gas, with super emitters present at all stages," she said.
For biogas, what that means is that targeting those few worst offenders can put a big dent in the overall greenhouse-gas footprint of the industry.
And there are some fairly straightforward steps to plugging leaks in the supply chain.
"We do not want to discourage the production and use of biomethane," Dr Bakkaloglu said.
"Rather, we want to highlight those emissions and encourage people to take immediate action to tackle them.
"Biomethane is an important renewable source, but it has the potential to be even better."
Bioenergy Australia chief executive officer Shahana McKenzie said the research pointed to compliance matters rather than "an inherent feature of the sector".
"Like many other industrial processes, anaerobic digestion will operate under the best environmental standards if effective technologies and processes are used," she said in a statement.
"Our members are developing the biogas and biomethane sector in Australia with best-practice methods and technologies for effective management of supply chain emissions.
"Biogas from anaerobic digestion is an essential part of our green energy future, particularly in making rapid changes in our hard-to-abate sectors.
"It offers renewable gas for our gas grids, renewable industrial heat for many businesses, and sustainable fuels for aviation and other sectors. Capturing methane is essential in driving down emissions in Australia."
