Product Recovery from a Pyrolysis Biorefinery. Image cite: Robert Brown, Bioeconomy Institute, Iowa State University
Lignin is one of the most abundant, yet least commercialized plant materials on earth. Lignin, along with cellulose and hemicellulose, are molecules that are woven together in a kind of plant wall fabric that give plants their structure. Cellulose and hemicellulose are attractive as sources of sugar, but less obvious is what to do with lignin. In an effort to use the entirety of biomass, researchers have been working to find commercial uses for lignin.
“Lignin in plant cell walls is like the plastic sheath surrounding a copper wire” said Robert Brown, CenUSA co-director.
“Nature intended for lignin to be hard to break down,” said Brown. This is a challenge when it comes to creating commercial applications for lignin. Chemical treatments and fungi are two known ways of breaking lignin down. One of the easiest ways is through pyrolysis. Pyrolysis rapidly heats sawdust sized biomass particles to temperatures of 400°C, or greater, in an oxygen-free environment. This releases so-called pyrolytic lignin, a substance that is chemically and physically different than lignin from which it is formed but still has some similarities.
Pyrolytic lignin is very reactive, causing it to solidify when heated. Mixed with aggregate, gravel, it forms an asphalt substitute. It can be used as a solid fuel, known as Lignocol™ directly substituting for coal.
The higher heating value and lower moisture content of Lignocol™ allows it to burn very efficiently. From an air quality standpoint, when burned for fuel Lignocol™ has extremely low sulfur content and only about a third of the nitrogen of leading coals.
One could argue that the low sulfur content alone makes products like Lignocol™ worth the extra money. However, market trends favor coal unless legislation encourages use of fuels with low greenhouse gas emissions.
What will be the first break-away commercial product made from lignin? Brown believes rather than biofuels, it will be one of a number of high-value products currently in the works; cosmetics, sunscreen, and plastics. Simply put, producing lignin-based fuels for less than what a gallon of gas sells for is difficult to pull off, but scientists see promise in creating non-toxic water bottles out of lignin to address solid waste problems and an increasing concern that some types of plastic can contaminate drinks in plastic bottles. Lignin-based plastics are one of the most lucrative options and are valued at over $10,000 per ton.
However, lignin has plenty of options that can create clean energy, safer products, and possibly make farmers some extra cash in the process.