Pre-Treatment

• First, biomass is processed by chopping it up. If the feedstock is wood, for example, it can be diced up into smaller chunks, which makes it easier to transport and also increases the surface area for chemical reactions. Next the biomass is pretreated through either steam explosion or acid hydrolysis. In the steam explosion technique, water is injected into the plant fibers and rapidly heated so it turns to steam, bursting open the fiber bundles. Alternatively, strong acids can break down the fiber bundles through acid hydrolysis.
  
  

Enzymatic Hydrolysis

• Next, the pretreated fibers are exposed to a solution of enzymes that break the cellulose down into glucose sugars. Endocellulase, exocellulase and beta-glucosidase are three types of enzymes that each play a part in this process. Endocellulase disrupts the structure of cellulose, while exocellulase makes cuts at the end of cellulose chains. Beta-glucosidase breaks the bonds between neighboring molecules of glucose. Unfortunately, this process does not break down the lignin also found in plant cell walls, which is more refractory to treatment.
  
  

Fermentation

• Next, the glucose solution is fed to microbes such as yeast, which will consume it and produce ethanol as a byproduct of their metabolism. This part of the process is no different from the fermentation process that turns grapes into wine. Yeast cannot grow if the ethanol concentration gets too high, however, so the product of fermentation must subsequently be distilled to increase the ethanol concentration. Distillation can only achieve an ethanol content of about 97 percent, so molecular sieves or other techniques are necessary to remove the remaining water content.
  
  

Challenges

• The main obstacle to commercialization of cellulosic ethanol is cost. Cellulosic ethanol must compete with gasoline and corn ethanol, both of which are cheaper to manufacture. Scientists are working to improve the process in several different ways. Plant cell walls contain hemicellulose, a polymer that incorporates both five- and six-carbon sugars, but yeast can only ferment the six-carbon sugars. Some researchers are genetically engineering yeast to overcome this problem. Others are working to reduce the lignin content of plants such as switchgrass and miscanthus, which would make it easier to process them efficiently. Possibly the most important challenge, however, is the fairly high cost of the enzymes.