— by Cristina Deptula
Last month for our volunteer enrichment Deepti Tanjore and Ning Sun of the Lawrence Berkeley National Laboratory came to share with us about the next generation of biofuels.
According to Tanjore and Sun, last year the United States spent $274 billion on biofuels and created 14.5 billion gallons and 280,000 jobs. These substances hold promise for environmental conservation, reducing greenhouse gas emissions by more than 60 percent. Yet technical issues have slowed progress on biofuel development and made it difficult to scale biofuel production up to commercially useful levels. This talk focused on ways to improve the process to make biofuels more practical.
The first generation of biofuels were produced from cereal grains and other plants containing sugar and starch. These plants competed with food crops for land and water, so researchers looked into second generation sources of fuel, such as algae, straw, manure, nut shells, and crude glycerine. However, these had a complex chemical structure that was hard for microbes to break down into fuel. Now, as Tanjore and Sun discussed, advances in pre-treatment and the addition of enzymes to do some preliminary digestion before adding the microbes for fermentation helps break the rougher material down.
Pretreatment is necessary to break down the matrix of carbohydrates and lignin holding many plant cells together and to turn the long chains of cellulose molecules into smaller, simpler glucose sugars.
There are two basic categories of pretreatments for modern biofuels: mechanical, where raw material gets chopped or milled to reduce its size, and chemical, where substances are added to change the chemical structure of the material.
Chemical pretreatment via the application of dilute acids (biocatalysis) has been tried but it has been difficult to recover much fuel from this process. Sugar also degrades at the high temperatures needed for this process. Ionic liquids and ‘liquid salts’ that melt at low temperatures may prove more successful as solvents, especially since these liquids are safely non-flammable and can break down cellulose by swelling and dissolving the cell walls. Aspergillus, clostridium, saccharomyces and other bacteria can also break down plant tissue for use as a biofuel.
Large commercial biofuel operations such as Iowa’s Project Liberty venture illustrate the potential commercial viability of this technology. A project even exists through Livermore National Laboratories to produce the high-density fuel needed for aviation from organic materials. Dr. Taek Soon Lee is working on using microbes to produce bisabolene, a chemical found in plants, as an alternative to diesel jet fuel.
The next generation of biofuels may come from algae, which has the advantage of being able to grow everywhere and not just in Midwestern cornfields and farms. Processes exist for recovering usable fuel material from algae by diluting the algae with water and extracting the fuel with hexane in a two-cycle process and then filtering out useful molecules by size. Currently, though, this process does not yield enough fuel to be scalable. Stressing the algae cells may increase fuel production.
While taking audience questions, Tanjore and Sun pointed out that we must wait a few years before harvesting biomass from a field of crops. ‘Waste’ material such as husks, dead leaves, etc fall back into the soil as mulch, replenishing the ground with nitrogen and other nutrients. Using switchgrass and other plants might also be a good idea as they put nitrogen into the soil more quickly.
Overall, these researchers pointed out that biofuels represent a promising new class of technology that can benefit our planet and our economy. Technical challenges still exist regarding efficient fuel production but researchers are finding and improving methods to deal with them and make these fuels scalable and commercially viable.