How forest and agricultural residues can fuel our future

Forestry, forest products industry, and agriculture produce waste and residue at several stages of their respective value chains. When a forest is harvested for saw log or pulp wood production, treetops and branches are usually left behind. While it's important to leave enough residues in the forest to maintain nutrients and support biodiversity, there is plenty of surplus raw material to be utilized globally. Pre-commercial thinning, a process commonly used in forest management, removes small and low-quality trees to allow the remaining trees more room to grow. In addition, wood processing such as sawmilling creates sawdust and bark residue. Similarly, agricultural processes leave behind crop residues such as stalks, straw or leaves. And finally, end-of-life wood products such as old pallets or wood used on construction sites often end up in the waste stream.

All this waste and residue from forestry and agriculture has plenty of potential as a raw material for renewable fuels.

“It's a valuable resource that is still under-utilized today,” says Juha-Erkki Nieminen, Head of Lignocellulose at Neste, the world’s leading producer of sustainable aviation fuel (SAF) and renewable diesel. When collected, forest and agriculture waste is still mainly combusted to produce heat and power. But, Nieminen says, “there is an opportunity to increase the added value of these streams and use them in applications where fossil fuels are more difficult to replace, such as in aviation.”

Creating renewable fuels from lignocellulose

Forestry and agricultural wastes and residues consist mainly of lignocellulose – a collective term for the components that give plant cell walls their rigidity: cellulose, hemicellulose and lignin. But what makes lignocellulose from plant waste a good candidate as a raw material for lower-emission renewable fuels and chemicals?

The main advantage is that there is a lot of lignocellulosic waste and residue available. “Based on Neste's estimate, the global available volume that could be sustainably used corresponds to at least 350 million tonnes of oil equivalent,” says Nieminen. That is a large untapped resource.

Another advantage is that lignocellulosic components already have the carbon backbone that is required for renewable fuels, so they are a good starting point. But the chemistry of converting lignocellulose to renewable fuels is not a straightforward process. Lignocellulose polymers have more oxygen atoms than the oils and fats that are commonly used in renewable fuel production. This excess oxygen needs to be removed. Another challenge is that lignocellulose is a solid material. “We need to do a fair amount of processing to turn it into a liquid hydrocarbon,” says Nieminen.

The demand for renewable solutions is growing rapidly and new raw materials and solutions are needed to meet this demand. Neste is working on developing the required technologies for converting lignocellulose to renewable fuels and feedstock for polymer and chemical production. But it is also important to ensure that the process is sustainable, as well as technologically and economically viable. This means for instance careful selection of the raw materials and suppliers, strict adherence to certified sources and production chains, and collection practices that preserve biodiversity and soil quality. It’s also important to consider the full life cycle of the waste and residue streams.

Life cycle assessment of forestry waste and residue

“Life cycle assessment can help organizations improve their systems, by showing what happens to their carbon footprint if they make changes,” says Ilkka Leinonen, Research Professor at the Bioeconomy and Environment research unit of the Natural Resources Institute Finland (Luke). “It can also help policymakers to provide incentives for actions that create environmental benefits.”

Leinonen and his team recently calculated how different practices affect the environmental impact of forestry products. This is part of a larger mandate to develop life cycle assessment (LCA) methods for biobased materials.

Life cycle assessment is an established method to quantify the environmental impact of a product. However, while the LCA methods are widely used to report on environmental sustainability, there are some aspects in the methodology that are not yet standardized. “What was still missing is how specific features of bio-based products, mainly the carbon uptake from the atmosphere, are handled in this process,” says Leinonen.

That was the driving force behind the Bio-LCA project, which further developed the LCA methodology and demonstrated it in case studies. For example, a new report shows how different methods of forestry management, such as pre-commercial thinning, affect greenhouse gas emissions of forestry products.

Neste also participated in this life cycle assessment analysis. “Sustainability is a fundamental building block of our business,” says Nieminen, “so we wanted to understand in more detail what the impact is over the lifecycle of the forest if some of the harvesting residues are collected.”

As part of the study, the Bio-LCA team found that collecting harvesting residues for use as raw material for biofuels creates a net benefit for the climate when replacing fossil fuels and chemicals. Even though the collection of harvesting residues leads to some reduction in carbon sequestration in the soil, this reduction has a relatively small effect when compared to the climate benefits of lignocellulose-based fuels and chemicals.

“We can create more climate benefits by using this material than by leaving it in the forest,” Leinonen summarizes.

That means that it’s worth continuing to explore how to scale up the use of lignocellulose from forestry and agricultural waste as a source for renewable fuels.

The challenge of scaling up the use of lignocellulose

“There is currently still limited commercial production of renewable fuels from solid biomass,” says Nieminen. While the chemistry of the conversion processes is robust, the challenge lies in large scale operation.

Nieminen explains the first hurdle: “We have to have reliable operations. The production facility needs to operate continuously over many months without major issues.”

The other major challenge of scaling up is maintaining the quality of the final product. “We need to ensure that we can produce a consistently high-quality hydrocarbon that is a drop-in solution into the existing distribution channels, be it road transportation fuels or aviation fuels.”

While there is still a lot of work to be done, the life cycle assessment and the on-going developments are encouraging. Nieminen is optimistic about the future: “The ambition is that in the 2030’s we will be producing fuel products, including renewable diesel and sustainable aviation fuel, from forestry and wood processing waste and residues, as well as from agricultural waste and residues and end-of-life wood products.”

By: Eva Amsen & Neste