Why perennial grasses for 2nd generation biofuels?

Perennial grasses offer some benefits over traditional grain crops when used for biofuel production.  Perennial grasses are long-lived and thus do not need to be planted each year.  They also require less fertilizer, water, and energy for production, reduce greenhouse gas emissions, and build soil carbon through extensive belowground biomass production.  With perennial grasses, the entire aboveground portion of the plant can be harvested and used to generate biofuel.  After the plant matures, undergoes senescence, and soluble nutrients are recycled to belowground storage organs for winter, the aboveground portion of the plant will consist mainly of non-living plant material or cell wall.  Cell walls are composed mainly of cellulose, hemicellulose, and lignin.  Cellulose and hemicellulose are long-chain sugar molecules that can be used for biofuel production.  Various methods are being examined to produce second generation biofuels including enzymatic processes, and a burning process, called pyroloysis.  Each of these processes have their own advantages and disadvantages, which will be examined in a later lesson.

The graph above shows the average percent of each of the three molecules that compose the cell wall in a plant cell.  Image credit: DOE, Office of Biological and Environmental Research

Opportunity for farmers and growers in the Midwest

Perennial grasses can be grown for biofuel production in the Midwest. Production of these grasses can offer farmers an opportunity to enter into a new and different market. Perennial grasses offer farmers an alternative crop for marginal or highly erodible land.  If traditional row crops do not perform well on marginal land, it may be cost effective to use that land for perennial grasses instead. Due to challenges with transporting large quantities of biomass, refinery facilities have to be located nearby farms which can be an additional benefit to local rural communities. 

Governmental policy and Implications for perennial grass feedstocks

The Energy Independence and Security Act (EISA) of 2007 set a limit of how much 1st generation biofuels can be produced to meet the domestic needs for energy.  As grain-based ethanol production has improved and expanded, the United States has reached the ceiling on mandated 1st generation biofuel production. This means that other biofuels now need to come from a difference source, such as cellulosic-biomass based or 2nd generation biofuels.  This is where perennial grasses can fit in to produce biofuel production mandates of EISA. The graph below shows ethanol production data from 1995-2010 on the left, as well as the mandates through 2022 on the right. Notice that the mandated quantities of biofuels exceed the caps on 1st generation biofuel production.  

The graph above illustrates the ethanol production data from 1995-2010, and the mandates through 2022. Image from Congressional Research Services, Agriculture-based biofuels: Overview and emerging issues (R41282, Schnepf).

Perennial grasses for biofuel and forage

Breeding and selection research on native perennial grasses such as switchgrass, big bluestem, and Indiangrass started in 1936 when the USDA-Agricultural Research Service (ARS) at Lincoln, Nebraska began breeding native grasses to revegetate land damaged by the drought of the 1930s. Since that time, scientists from the USDA-ARS and land-grant universities have evaluated, selected, and bred cultivars with improved aboveground biomass yield and digestibility from native collections of perennial grasses.  Perennial grasses used in 2nd generation biofuel production will have a close relationship with its conventional use as forage for livestock.  While improved digestibility and persistence of perennial grasses have been primary traits selected for in early breeding efforts, improved biomass and energy yields will be traits selected for in efforts to improve use of perennial grasses in 2nd generation biofuels production. 

The image above depicts a dust storm in the 1930's on land damaged by drought.  Image credit: Library of Congress

Perennial grasses can help restore damaged land and prevent erosion. Image credit: USDA-ARS