Plant-Based Waste and Residues into Biofuel – Geeking out over Lignocellulose

by Richard Pallardy (Neste) Plant biomass, or lignocellulose, is the most abundant renewable carbon source on the planet. It has been around forever, but only now are we starting to grasp its full potential in replacing fossil resources. What makes lignocellulose so attractive as a raw material, and what does the future hold for one of the most important building blocks of Mother Nature?

Each year, plants convert some 100 billion tons of carbon dioxide into biomass. More than 90 percent of that carbon combines with other elements and is converted into lignocellulose—trunks, stems, leaves, and other plant structures.

The most abundant organic material on Earth, lignocellulose is composed of three tightly bound substances: cellulose, hemicellulose, and lignin. Early land plants, which appeared some 470 million years ago, adapted to dry land by amping up their production of lignocellulose, producing thicker cell walls that supported their vertical growth.

Without lignocellulose, our tulips would flop and tree trunks would snake along the ground like gigantic, limp noodles.

Neste, the world’s leading producer of renewable diesel and sustainable aviation fuel, estimates based on analysis by McKinsey & Company that 300 million metric tons of oil equivalent could be produced every year from lignocellulosic biomass in agricultural and forestry residues alone.

“Large amounts of waste and residues from existing forestry and agricultural production remain underutilized and could be transformed into valuable and highly sustainable new raw materials,” says Markus Rarbach, Vice President, Business Platform, Lignocellulosics at Neste.

Second generation biofuels, such as the ones utilizing lignocellulosic biomass wastes and residues, improve upon the technologies used in the development of conventional biofuels, which are distilled from grains like corn and sugarcane.

The raw materials for these advanced fuels are derived from a huge range of forestry and agricultural manufacturing processes.

“That’s what makes lignocellulose an attractive raw material from a sustainability perspective,” Rarbach explains. “We are not using the edible part of the biomass; instead, we can upgrade waste and residues into highly sustainable, new products."

Waste and residue wood products such as sawdust, branches, and treetops as well as corn stover, wheat straw and bagasse from sugarcane production all contain lignocellulose that can be reduced to compounds from which value can be extracted. Other options, such as switchgrass and Miscanthus, can be grown on land not suitable for food crops.

Thermochemical methods use heat and pressure to convert plant matter to fuel. They mimic the geologic pressures that result in crude oil and other fossil resources, resulting in bio-based oil and syngas.

Biochemical methods employ biological catalysts to do the same. They use enzymes to release the sugars contained in lignocellulose which are then fermented by microorganisms into usable fuels like ethanol.

Sometimes they are deployed in hybrid form; biochemical processes to produce intermediates and thermochemical techniques converting them into finished products. Rarbach is confident that both will eventually be applicable to even the toughest lignocellulosic raw materials.

How lignocellulosic biomass can be sourced sustainably has been a matter of debate. Some have suggested that the move toward lignocellulosic fuels might incentivize timber harvesting and thus damage forest ecosystems.

Neste is intent on ensuring that this doesn’t happen. The company relies on products from sustainably managed forests—especially those feedstocks that might not be otherwise used. And they are confident that a diversity of forest ecosystems and mindful harvesting will result in a viable, environmentally friendly economy.

“Sustainably managed forests grow more wood than is harvested and their biodiversity is maintained. Also, continuous growth of forests ensures sustained carbon sequestration,” says Rarbach.

Partnerships with projects such as Treesearch, a Swedish initiative that investigates novel utilization of forest resources, ensure that Neste has constant access to innovations in this sector.

Crop, forestry, and other waste residues have a further edge. They utilize inevitable byproducts and thus the ecological downsides are minimal.

Even the byproducts of lignocellulosic refinery have proven useful. Lignin, the most recalcitrant of the compounds in lignocellulose, can fuel additional processing.It is also used in the manufacture of adhesives, carbon fiber and resins on a small but growing scale. Hence, more wastes are converted into sustainable products. READ MORE