Cellulosic ethanol production in the United States: Conversion technologies, current production status, economics, and emerging developments

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Abstract

Details of existing conversion technologies for cellulosic ethanol production, both hydrolysis and thermochemical, have been discussed along with their present adoption status. Furthermore, economics of ethanol production by using different conversion technologies has been discussed. Emerging conversion technologies and other developments which might affect the cellulosic ethanol production are also characterized. Based on current estimates, it was found that about 400 million gallons of cellulosic ethanol will be produced in the country in coming years using different conversion technologies. It was noticed that out of several available conversion technologies, thermochemical-based technologies are gaining popularity and it is projected that the use of these conversion technologies will reduce the cellulosic ethanol production cost significantly. Similarly, recent advancements in hydrolysis-based technologies have also helped in reducing the production cost of cellulosic ethanol. However, more resources will be needed in coming years to meet the policy goal of producing 21 billion gallons of cellulosic ethanol by the year 2022. It is expected that this review will be helpful in efficient allocation of resources for facilitating future technology development and in streamlining the whole initiative of cellulosic ethanol production in the United States.

Introduction

The United States (U.S.) is the largest consumer of petroleum products in the world and is dependent on imports for meeting this demand. The U.S. consumed about 20.7 million barrels/day of petroleum products in 2007 out of which about 58% i.e., 12 million barrels/day was imported (EIA, 2008a). It is predicted that the gasoline consumption will further rise along with the rising population, as it is a primary energy source for meeting non-commercial transportation demand (EIA, 2008b). Due to increased use of petroleum products like gasoline, the amount of greenhouse gases released into the atmosphere has also shown a rising trend (EIA, 2008c). It was found that the transportation sector alone emitted about 34% of the total carbon dioxide (CO2) released into the atmosphere in 2005 i.e., 2007 million Mg (EIA, 2008d).

Because of rising energy dependency, increasing emissions of greenhouse gases, and risks associated with the price fluctuations in the international energy markets, federal and various state governments have started to evolve new energy strategies in which the role of various renewable energy sources is emphasized. Out of many renewable energy resources (biomass, solar, wind, geothermal, tidal, etc.), biomass is given a high priority as it is the only source which can be directly utilized for production of various alternative transportation fuels especially ethanol.

Using food crops for ethanol production raises concerns of food security (Mitchell, 2008) and environmental degradation (Pimentel and Patzek, 2005). Therefore, majority of the petroleum importing countries (including U.S.) are interested in utilizing cellulosic biomass as a feedstock for ethanol production. As U.S. has a large cellulosic biomass production base (Perlack et al., 2005), production of ethanol from cellulosic feedstock and utilizing it as a substitute for gasoline could help in promoting rural development, reducing greenhouse gases, and achieving energy independence. Therefore, the federal government has announced various policy targets and incentives to promote the production of cellulosic ethanol in the country. For instance, the Energy Independence and Security Act of 2007 set a target of producing 21 billion gallons of biofuels from cellulosic feedstocks by 2022. Additionally, the recently enacted Farm Bill of 2008 provides a subsidy of $1.01 on every gallon of cellulosic ethanol produced.

Several technologies have been proposed to convert different cellulosic feedstocks into ethanol. These technologies range from fermentation (Lin and Tanaka, 2006) to gasification (Perkins et al., 2008). However, doubts exist among various stakeholders about the commercial viability of existing conversion technologies (Waltz, 2008, Tan et al., 2008, Ruan et al., 2008, Wright and Brown, 2007). As a result, federal and various state governments are providing funding support to several private companies and public institutions to develop a suitable conversion technology using which cost of ethanol production from cellulosic feedstock can be brought down significantly. Already, federal government has provided a funding of $1 billion for promoting research in developing a commercial viable conversion technology for producing cellulosic ethanol (Curtis, 2008). It is expected that the successful demonstration of at least one conversion technology on a commercial scale will help in increasing the confidence of investors in cellulosic ethanol production and thus, will help in achieving the policy target of producing 21 billion gallons of cellulosic ethanol by the year 2022.

In light of the importance given to the commercial viability of a conversion technology, it is essential to review the existing conversion technologies to ascertain their performance in terms of adoption status and economics. Emerging technological alternatives should also be analyzed to understand the future trajectory of technology development. Such an attempt will help in creating a baseline for the emerging conversion technologies and in guiding policy makers to streamline funding and institutional support.

In the next section, the composition of cellulosic feedstock is briefly discussed. In the third section, two major conversion technologies or base technologies that are commonly used for converting cellulosic feedstocks into ethanol namely hydrolysis and thermochemical conversion are explained. An attempt has also been made to capture the existing versions of both the base technologies. In the fourth section, the adoption status of existing conversion technologies is discussed to evaluate the current status of cellulosic ethanol production. In the fifth section, economics in terms of unit ethanol production cost of the existing conversion technologies is discussed. In the sixth section, emerging trends in the technology development and alternate uses of cellulosic biomass are discussed and finally study is concluded in the seventh section.

Section snippets

Cellulosic feedstock composition

Cellulosic feedstock is composed of cellulose, hemi-cellulose, lignin, and solvent extractives. Lignin acts as a cementing material and binds all other constituents together. It is also responsible for providing structural rigidity to a cellulosic feedstock. Cellulose is a polymer of repeating β-d-glucopyranose units and is a chief constituent of the feedstock. Hemi-cellulose, like cellulose, is a polysaccharide but is less complex and easily hydrolysable. Soluble materials or extractives in

Base technologies

At present, several technologies are in use for converting cellulosic feedstocks into ethanol. However, all these technologies can be grouped into two broad categories namely hydrolysis and thermochemical conversion. In hydrolysis, the polysaccharides (cellulose and hemi-cellulose) present in a feedstock are broken down to free sugar molecules (glucose, mannose, galactose, xylose, and arabinose).1 These free sugar molecules are then fermented to

Current production using different conversion technologies

Inspired by the subsidies offered by federal government, many private entrepreneurs have ventured into cellulosic ethanol production. Fig. 3 shows the details of the total quantities of cellulosic ethanol currently or expected to be produced within the country by employing different conversion technologies (RFA, 2009).

As observed from Fig. 3, it is expected that about 405 million gallons of cellulosic ethanol will be produced by the end of 2012 and three conversion technologies (enzymatic

Economics of cellulosic ethanol production

Production of ethanol from cellulosic feedstocks is costly when compared to its production from starch-based agricultural feedstocks (McAloon et al., 2000). Therefore the goal of the several research agencies is to bring down the cost of production of cellulosic ethanol to $1.33/gal by the end of 2012 by improving overall efficiency of conversion technologies (EERE, 2009). Sassner et al. (2008) have analyzed the cost effectiveness of three cellulosic feedstocks (namely salix, corn stover, and

Emerging developments

The importance given to the commercial viability of ethanol production from cellulosic feedstocks has attracted many scholars. It has been found that irrespective of the technology applied, the costs of the plant are correlated with the overall energy loss of the plant (Lange, 2007). Therefore, several new ideas are being tried at different levels for ensuring commercial production of ethanol from cellulosic feedstocks. Some of these emerging technologies are discussed below.

Conclusions

Production of cellulosic ethanol presents a challenge in terms of development of a commercially viable conversion technology. However with the rising interest of policy makers and researchers, it is expected that such a technology will soon be developed. It is more likely that the developed conversion technology will be based on the thermochemical platform rather than sugar platform as embedded technologies like gasification and catalytic conversion are already quite mature and only small

Acknowledgement

This study was a part of Pinchot Institute of Conservation's project titled “Wood Bioenergy and Forest Sustainability”.

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