Plant-based waste and residues into biofuel – geeking out over lignocellulose

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.

Ultimately, the diversification of plants—due in part to the remarkable evolutionary innovation of their cell walls—profoundly altered the atmosphere and drove the evolution of other lifeforms. One could argue that we wouldn’t even be here were it not for lignocellulose.

This fantastic product of evolutionary engineering has long shown potential as a highly sustainable, renewable source of fuels and materials.

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.

Response to the urgency of the climate crisis

Although lignocellulose has been around forever and processes for extracting ethanol fuel from cellulose were developed already in the early 1800s, industrial conversion into fuels and chemicals took time to grow to maturity.

While minor levels of industrial fuel production from lignocellulosics emerged during the world wars and the oil shortages of the 1970s, interest waned as these crises faded. Petroleum thus maintained its dominance—until now.The urgency of the climate crisis has renewed interest in developing new means by which to extract plant-based energy. The European Green Deal has set the goal of carbon neutrality by 2050, and the United States has committed to halving its emissions by 2030. Consumer demand for lower-emission fuels has further incentivized the trend.

“Neste has been actively exploring and developing technologies to address the potential of lignocellulosic feedstocks for the past 20 years,” says Rarbach.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.

“We’re seeing technologies enabling us to utilize various lignocellulosic waste and residues becoming mature enough to be scaled up and make the transition from a development environment into a commercial environment,” Rarbach claims. “That creates the technology push.”

The process: Turning plant matter into fuel

The lignocellulosic fuel technology took so long to develop in part due to the nature of the substance. Lignocellulosic materials are recalcitrant—they are highly resistant to extraction of the molecules and energy that they have stored.This makes perfect evolutionary sense. Plants have developed these complex matrices of cellulose, hemicellulose, and lignin in response to brutal environmental pressures. They are not going to give up the goods easily.

“Mother Nature has made it quite demanding to use them as raw materials,” Rarbach laughs.Two separate means of extracting value have been developed in response. 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.

Neste is constantly exploring and refining the processes that drive these reactions and investigating the possibilities of new ones in its quest to scale up the technology.

These innovations will allow for increased exploitation of diverse lignocellulosic feedstocks from an ever-growing variety of sources—different materials require different treatments in order to generate maximum yield. Product offerings are expected to expand accordingly.

Towards a sustainable and multi-solution future

Neste is hopeful that these plant-based waste and residue materials can make a significant contribution to carbon neutrality in the coming decades.

Neste believes that a diversity of options in the energy and mobility space will be key. No single technology is going to facilitate this goal. Rather, a combination of technical solutions and feedstocks will be required, leveraging waste oils, novel vegetable oils, and algae in addition to lignocellulosics.

“We are going to see a huge diversity of technical solutions in the transport sector, where electric vehicles will play a part. Of course, it's also important that other forms of transport energy contribute to achieving sustainability in the mobility sector” Rarbach says.

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. Rarbach is hopeful that waste will eventually be eliminated entirely. “That would be obviously highly desirable from a market perspective and from a customer perspective,” he enthuses. “Because carbon neutrality is what we are striving for.”

Credits:Richard Pallardy, science writer Richard Pallardy has worked as an editor for Encyclopedia Britannica. His writing has also appeared in National Geographic Learning publications, on Earth.com and Areo Magazine.