Cellulosic ethanol is identical to first generation bio ethanol except that it can be derived from agricultural residues, other lignocellulosic raw materials or energy crops.
These lignocellulosic raw materials are more widely available than the standard material used for ethanol. They are also considered to be more sustainable, however they need to be broken down (hydrolysed) into simple sugars prior to distillation, a much more complex process than the first generation bioethanol. It first must go through pretreatment,hydrolysis then a conversion. Research since the 1970s and large investments are being made in the US and Europe to speed up development of this route to bioethanol. Biomass refineries like Inbicon in Denmark are producing cellulosic ethanol and are encouraging it for the rest of the world.
To break down cellulose you have to first get past hemicellulose and Lignin, which surround the cellulose in a protective sheath. This is the job of pretreatment. Typically a moderately high-temperature, high-pressure dilute acid pretreatment process is used to break down hemicellulose and disrupt or dissolve the lignin. This process also creates another source of soluble sugars that will later be used for fermentation to ethanol. NREL is investigating potentially cheaper, but still effective, pretreatment methods. NREL employed enzymes to enable milder pretreatment. Using a milder pretreatment process could cut process costs dramatically and eliminate sugar degradation losses. The challenge is to maintain a high level of effectiveness with the milder process, which is accomplished by using enzymes to further break down the hemicellulose after pretreatment. Pretreatment
For cellulosic ethanol production, the primary challenge is breaking down (hydrolyzing) cellulose into its component sugars. Cellulases are used to breakdown the cellulose. Once cellulases make contact with cellulose, the real work begins. Cellulases act very slowly. That’s why dead trees take years to decompose in the forest. To accelerate cellulose conversion, it is critical to start with the best enzymes nature has to offer. The most active known cellulases are in the cellobiohydrolase I (CBH I) family, derived from fungi. But not all CBH I enzymes are equal. NREL recently confirmed the existence of CBH I enzymes that are twice as active as those from industrial sources.
During fermentation, microorganisms(primarily fungi and bacteria)convert the sugars in biomass to ethanol.Yeasts are currently the fermentation organisms of choice for the corn ethanol industry. They are reasonably tolerant of ethanol, acid, and moderately high temperatures.
Ethanol can also be produced thermochemically from any form of biomass. In this approach, heat and chemicals are used to break biomass into syngas (CO and H2) and reassemble it into products such as ethanol. This method is particularly important because up to one third of cellulosic biomass—the lignin-rich parts—cannot be easily converted biochemically. A thermochemical biomass conversion process is complex, and uses components, configurations, and operating conditions that are more typical of petroleum refining.