Although less heralded, cellulosic ethanol has a fairly rich history. One of the first recorded attempts at commercializing a cellulosic ethanol process was made in Germany as early as in 1898. The process was based on the use of dilute acid to hydrolyze the cellulose to glucose and the subsequent fermentation of glucose to ethanol. The reported productivity was 7.6 liters of ethanol per 100 kg of wood waste, equivalent to 18 gallons per short ton. As an early process, the conversion of wood waste into ethanol was quite remarkable; the process was further enhanced in Germany to yield about 50 gallons of ethanol per short ton of biomass. In the United States this pro­cess was further enhanced during World War I by adopting a single-stage dilute sulfuric acid hydrolysis process, by which the overall ethanol yield per input biomass was about 50% lower than the original German version, but the throughput of the process was much higher. This American process was short-lived, due to a significant decrease in lumber production in the post­war era. However, this process was brought to commercial operation again during World War II for production of butadiene by ethanol conversion to ultimately produce synthetic rubber. Even though the process achieved an ethanol yield of 50 gallons/dry ton of wood cellulose, this level of productiv­ity was far from profitable and the process was halted after the war. Even though commercial production had been stopped, active research on cellu — losic ethanol continued throughout the world, intensifying even more as a result of several rounds of petroleum crises, booming bioethanol consump­tion, and rapid advances in biotechnology.

In 1978, Gulf Oil researchers [33] designed a commercial-scale plant pro­ducing 95 x 106 liters per year of ethanol by simultaneous enzymatic hydro­lysis of cellulose and fermentation of resulting glucose as it is formed, thereby overcoming the problem of product inhibition. The process con­sisted of a unique pretreatment which involved the grinding and heating of the feedstock followed by hydrolysis with a mutant bacterium, also specially developed for this purpose. Mutated strains of the common soil mold tricho- derma viride were able to process 15 times more glucose than natural strains. Simultaneous hydrolysis and fermentation reduced the time requirement for the separate hydrolysis step, thus reducing the cost and increasing the yield. Also, the process did not use acids which would increase the equipment costs. The sugar yields from the cellulose were about 80% of what was theo­retically achievable, but the small amount of hemicellulose in the sawdust was not converted. This fact demonstrated a need for an effective pretreat­ment to cause hemicellulose separation.

As advances in enzyme technology have been realized, the acid hydrolysis process has been gradually replaced by a more efficient enzymatic hydro­lysis process. In order to achieve efficient enzymatic hydrolysis, chemical or biological pretreatment of the cellulosic feedstock has become necessary to prehydrolyze hemicelluloses in order to separate them from the lignin. The researchers of the Forest Products Laboratory of the U. S. Forest Service (USFS) and the University of Wisconsin-Madison developed the sulfite pre­treatment to overcome recalcitrance of lignocellulose (SPORL) for robust enzymatic saccharification of lignocellulose [34].

Cellulase is a class of enzyme that catalyze cellulolysis, which breaks cellu­lose chains into glucose molecules. In recent years, various enzyme compa­nies and biotechnology industries have contributed significant technological breakthroughs in cellulosic ethanol technology through the development of highly potent cellulase enzymes as well as the mass production of these enzymes for enzymatic hydrolysis with economic advantages. Research, development, and demonstration (RD&D) efforts in cellulase enzyme by many international companies such as Novozymes, Genencor, Iogen, SunOpta, Verenium, Dyadic International, and national laboratories such as National Renewable Energy Laboratory (NREL), are quite significant.

Cellulosic ethanol garnered strong endorsements and received significant support from the U. S. President George W. Bush in his State of the Union address, delivered on January 31, 2006, that proposed to expand the use of cellulosic ethanol. In this address, President Bush outlined the Advanced Energy Initiative (AEI 2006) to help overcome America’s dependence on for­eign energy sources and the American Competitiveness Initiative (ACI 2006) to increase R&D investment and strengthen education. The Renewable Fuel Standard (RFS) program was originally enacted under the Energy Policy Act of 2005 (EPAct 2005) and established the first renewable fuel volume mandate in the United States. The original RFS is referred to as RFS1. RFS1 required 7.5 billion gallons of renewable fuel to be blended into gasoline by 2012. The origi­nal timeline and renewable fuel volume mandate were revised and expanded.

The Energy Independence and Security Act (EISA) of 2007 established long­term renewable-fuels production targets through the second Renewable Fuel Standard (RFS2). The RFS2 expanded upon the initial corn-ethanol produc­tion volumes and timeline of the original RFS, under which the U. S. EPA is responsible for implementing regulations to ensure that increasing volumes of biofuels for the transportation sector are produced. The U. S. EPA released its final rule for the expanded RFS2 in February 2010, through which its statutory requirements established specific annual volumes, for the total renewable fuel volume, from all renewable fuel sources [35]. As a mandate potentially affecting the long-term future of corn ethanol, the RFS2 man­dates that the country as a whole is required to blend 36 billion gallons of renewable fuels into the transportation fuel sector by 2022, of which 16 bil­lion gallons is expected to come from non-corn based ethanol. The U. S. EPA implementation of the RFS2 would position the United States for making significant improvements in the greenhouse gas footprint due to the trans­portation sector. In February 2010, the White House under the leadership of President Barack Obama released "Growing America’s Fuel," which is a comprehensive roadmap to advanced fuels deployment [36].

In 2004, the researchers at the National Renewable Energy Laboratory, in collaboration with two major industrial enzyme manufacturers (Genencor International and Novozymes Biotech), achieved a dramatic reduction in cellulase enzyme costs, which was one of the major stumbling blocks in the commercialization of cellulosic ethanol. Cellulases belong to a group of enzymes known as glycosyl hydrolases, which cleave (hydrolyze) chemical bonds linking a carbohydrate to another molecule. The novel technology involves a cocktail of three types of cellulases: endoglucanases, exogluca — nases, and beta-glucosidases. These enzymes synergistically work together to attack cellulose chains, pulling them away from the crystalline structure, breaking off cellobiose molecules (two linked glucose residues), splitting them into individual glucose molecules, and making them available for further enzymatic processing. This breakthrough work is claimed to have resulted in twenty — to thirtyfold cost reduction and earned NREL and col­laborators an R&D 100 Award [37].

This is certainly a milestone accomplishment in cellulosic ethanol tech­nology development; however, further cost reductions are required in cel- lulase enzyme manufacture, new routes need to be developed to enhance enzymatic efficiencies, the development of enzymes with higher heat toler­ance and improved specific activities is highly desired, better matching of enzymes with plant cell-wall polymers needs to be achieved, a high-solid enzymatic hydrolysis process with enhanced efficiency needs to be devel­oped, and more. The U. S. Department of Energy Workshop Report summa­rizes, in scientific detail, the identified research needs in the area (Biofuels Joint Roadmap [26]).

In recent years, major advances have also been made utilizing genetic engineering and advanced microbiology in the development of robust microbe systems that are capable of efficiently co-fermenting both C5 and C6 sugars and that are resistant to inhibitors and tolerant against process variability.

Another effort for production of cellulosic ethanol is via catalytic conver­sion of gaseous intermediates produced by thermochemical conversion of cellulosic materials without the use of enzymes. Certainly, there is a trade­off between the purely chemical route and the enzymatic route in various aspects, including conversion efficiency, product selectivity, reaction speed, capital cost, overall energy efficiency, raw material flexibility, and more. A large commercialization effort launched by Range Fuels in 2007, based on catalytic conversion of thermochemical intermediates derived from biomass, was shut down in 2011 without meeting its original goals.