How to Rescue Biofuels from a Sustainable Dead End

November 23, 2022

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by Peter Fairley (Nature) An environmentally friendly path forwards for liquid fuel derived from plants will depend on smarter agriculture and smarter regulation. -- ... If filling fuel tanks with these plant-derived liquids reduces carbon emissions by decreasing the demand for fossil fuels, it would help to tackle the climatic shifts that threaten humanity and biodiversity.

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To make agriculture smarter, farmers need to pay close attention to what crops work best where, and how those crops are grown. Embracing regenerative farming methods, such as reduced tilling of the soil, can retain carbon and nutrients. So, too, can planting an emerging set of winter oilseeds that can be grown seasonally between food-crop rotations. This would generate revenues that could pay for a soil-saving practice called cover cropping that few farmers have embraced so far.

“We cover crop less than 2% of our land. If you go to 40–50%, you’re meeting this huge global demand for low-carbon feedstocks,” says Glenn Johnston, referring to the process of growing a crop to protect and improve the soil — a crop that, in this case, can also be used to make biofuel. Johnston leads regulatory and sustainability programmes for agribusiness firm Nuseed at its research centre near Sacramento, California.

Despite this promise, the new era of biofuels still poses environmental concerns. Researchers argue that regulation needs to be much improved to ensure that the industry arcs towards sustainability. Tracking carbon is a complex process full of pitfalls. Get it wrong and biorefineries could end up as one more environmental panacea that bites the dust.

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Digging deeper

A decade ago, a transition to better biofuels seemed imminent. A new generation of commercial-scale biorefineries was coming online in the United States, Brazil and Europe. They were designed to make ethanol from fibrous cellulose-rich feedstocks such as agricultural leftovers, grasses or fast-growing trees that generally thrive on marginal farmlands and require less intensive cultivation than corn or soya beans. By now, these cellulosic biofuels made from sustainable feedstocks were supposed to be gushing into the fuels market, trimming transport emissions — the fastest-growing source of CO₂ worldwide.

Alas, the flow of cellulosic fuel is barely a trickle. Processing equipment proved hard to operate, petrol prices fell and governments eased mandates designed to force the pricier cellulosic fuels into the market. “Ultimately all of those facilities struggled. Most are either producing at very low levels today or not producing at all,” says John Field, who studies the climate mitigation potential of bioenergy systems at Oak Ridge National Laboratory in Tennessee.

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Furthermore, finding reliable data isn’t easy. Soil carbon, for example, varies greatly across short distances. And variability over time means it can take up to a decade before sampling detects important changes in soil carbon. “It’s time-consuming and costly to do it right,” says Rebecca Rowe, who studies soil carbon at the Centre for Ecology and Hydrology in Lancaster, UK.

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That makes assessing biofuel sustainability “daunting” according to Pedro Piris-Cabezas, director for sustainable international transport based in London at the Environmental Defense Fund. “It quickly becomes crazy,” he says. But Piris-Cabezas thinks that tools and methods exist to reliably cut through the complexity, and these will show that some biofuels do reduce carbon emissions without degrading ecosystems and communities. Piris-Cabezas has written a handbook (see go.nature.com/3s6hco2) on tracking methods that can ensure that alternatives to aviation fossil fuels have “high integrity”.

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At the same time, most mitigation pathways that limit global warming in keeping with the Paris climate agreement require an outright reduction in agricultural land use.

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“Short of returning land to a completely wild state, we will always be balancing impacts against the needs of society,” says Rowe, whose work is helping the UK government to implement plans to expand the planting of bioenergy crops from close to nothing to about 3% of the UK’s land area by 2050.

And Field’s research suggests that biofuels still have the potential to be more than a necessary evil. In a 2020 paper³ he and his colleagues showed through simulation that, under certain conditions, cellulosic ethanol can rival or exceed the climate benefits of ecosystem restoration. The best results occurred for the case of land use transitioning from food crops or pasture to the cultivation of switchgrass (Panicum virgatum), a popular feedstock for cellulosic biofuel. In those cases, Field and his co-authors estimated that the carbon mitigation potential was comparable to that for reforestation. If crop yields and bioprocessing technologies can be improved, and if CO₂ from biorefineries can be permanently sequestered deep underground, the researchers predict that supplying cellulosic feedstocks could ultimately store up to four times more carbon than does reforestation. “It’s aspirational, but these are areas where there’s a lot of research and development attention right now,” says Field.

Companies are already developing CO₂ pipelines in North Dakota and Illinois, and they’re in line for enhanced tax breaks under the US Inflation Reduction Act that was passed in August. Of course, these companies also face significant pushback, including from farmers whose land might be in the pipelines’ path.

For the UK bioenergy crop scale-up, Rowe says Miscanthus (a crop akin to switchgrass) and other perennial feedstocks are the preferred option. The UK government expects that these crops will help to cut emissions from biorefineries by the 2030s — especially when coupled with deep sequestration. The key, says Rowe, is to use the lessons learnt from biofuels development to work out the most sustainable places to cultivate. That generally means avoiding high-carbon soils such as peatlands, biodiversity hotspots and high-value agricultural croplands.

The best candidates for sustainability are the cover crops in development that seem to be a good response to arguments against dedicating land to biofuels. Soil in fallow fields tends to compact, and is susceptible to erosion by wind and rain. A cover crop puts roots down to secure the soil and its nutrients, and creates channels that help water to sink in rather than drain off. Farmers might be convinced to plant oilseed cover crops because the crop can pay for itself by producing oils that can be supplied to biorefineries.

Nuseed’s crop carinata — adapted from Brassica carinata, a towering cousin of rapeseed (Brassica napus) — produces an energy-rich, inedible oil. And it packs a punch: Johnston says carinata excels at storing carbon in soil and contains about 2.5 times more oil than soya beans, the dominant crop for renewable diesel. Most importantly, he says, carinata does not compete with food supplies or cause climate-harming land-use changes. The latter advantage means that although land-use effects alone add an extra 4–26 grams of CO₂ emissions per megajoule of energy delivered from soya-based fuels, according to Field, carinata cuts 9–13 grams of emissions per megajoule from fuels. “Land-use change goes from being a highly uncertain but potentially large liability to having a small-but-positive effect,” says Field, who is part of a consortium partnered with Nuseed on carinata research and development.

A 2022 report⁴ by Field and his colleagues shows that carinata could support a major biofuels industry in the southeastern United States. Simulating application of carinata every third year across southern Georgia, southern Alabama and northern Florida — a few percent of US cropland — they project annual harvests exceeding 2 million tonnes. That’s enough seed to make about one billion litres of aviation fuel.

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For carinata to occupy a larger role in the biofuels scene smarter policies are needed, says Johnston. Government programmes for biofuels, he says, lack the breadth and specificity to recognize and reward the crop’s benefits. READ MORE

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carbon offsets Agricultural Policy environmental policy Transportation Fuels Policy UK (United Kingdom) cover crops regenerative farming oil seed crops winter crops California Sustainability soil carbon carbon/CO2 sequestration carbon capture and storage (CCS) carbon capture and utilization (CCU) BioRefineries/Renewable Fuel Production Brazil European Union (EU) cellulosic biofuel Agricultural waste/residue perennial grasses energy grasses Grasses marginal land soybeans corn/maize National Laboratory Tennessee ILUC (Indirect/Induced Land Use Change) land use land use change energy crops carbon pipeline pipelines North Dakota Illinois miscanthus Switchgrass carinata Investing government investment Argentina France

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