Challenges of Cellulosic Ethanol Conversion

Cellulose is the world’s most abundant organic material and its use in production of biofuels could improve longer term outlooks for increasing energy supply.  However, conversion of cellulosic materials to biofuels is more difficult and costly compared to direct fermentation of sugars and starches from grains.

Cellulose is the primary source of sugar in fibrous biomass, but it is tightly bound with hemicellulose and lignin, which form a rigid protective sheath.  Pretreatment of cellulosic biomass is necessary to break apart and release the sugars for fermentation to ethanol (figure 4.7).  Mechanical (e.g., crushing) and thermochemical (e.g., hydrolysis) pretreatments, can successfully break up hemicellulose and lignin. These processes are applied sequentially. Pre-treatment processes tend to  form by-products that inhibit fermentation.  Detoxification is often necessary to remove these inhibiting by-products.  This adds costs to the process, and  many pretreatments are reportedly ineffective on forest biomass (i.e., woody feedstocks with high lignin content).

Figure 4.7. Cellulosic ethanol production involves a multi-step process for releasing sugars from hem-cellulose and lignin, in addition to fermentation and refinement into biofuels. (CL Williams, 2011).

A key pretreatment technology for cellulosic ethanol fermentation is hydrolysis – the transformation of cellulose to glucose.

  • In chemical hydrolysis, acids are used to break down cellulose, sometimes accompanied with application of heat and increased atmospheric pressure.  These inputs and processes increase the costs of biofuel production as well as time required for producing the biofuel end product.
  • Enzymatic hydrolysis involves use of cellulase enzymes like those present in the stomachs of ruminant animals.  These enzymes are produced by bacteria.  Cellulase enzymes are currently limited in supply (and costly) for cellulosic ethanol to move beyond pilot projects to commercial scale production.  However, researchers are seeking technological breakthroughs for mass production of hydrolysis enzymes at competitive prices.

Once sugars are released from cellulosic materials through mechanical pretreatment and hydrolysis, they can be fermented and then refined into biofuels. Lignin residues from fermentation contain carbon; these residues can be recovered and used in thermal and thermochemical conversion processes.

Thermochemical technologies (e.g. gasification or pyrolysis) for conversion of cellulosic materials are alternatives to fermentation in the production of liquid and gaseous fuels.  However, gasification and pyrolysis do not directly produce liquid transportation fuels suitable for current passenger vehicles).  Therefore, additional processing is necessary, adding to overall biofuel production costs.  Much effort is being directed toward technological advances necessary to reduce costs in order to move cellulosic biofuel production from pilot scales to commercial scales.