Given the variability of both the physical as well as chemical characteristics of the different feedstocks shown in Figure 2.2, robust conversion technologies must be developed that can economically accommodate this resource diversity. Additionally, feedstocks tend to be geographically diverse, i. e., softwoods in the Southeastern United States, and corn stover in the Midwest. Accommodating this diversity implies that the conversion technologies of future integrated biorefineries will be a function of the locally available feedstock resources. For this reason, cellulosic ethanol production technology must be sufficiently robust to optimize the conversion of multiple biomass resources to fuel.

Although there are a multitude of conversion technology routes under development for converting biomass to fuels and chemicals (15, 16), the predominant differentiation is the primary catalysis system (Figure 2.2).

Biochemical conversion uses biocatalysts (such as enzymes and microbial cells), heat, and chemicals to convert biomass first to an intermediate sugar stream and then to ethanol or other fermentation-produced fuel and co-products such as heat, power, and chemicals (17).

On the other hand, thermochemical conversion technologies use heat and/or physical catalysts to convert biomass to an intermediate product, and then a chemical transforma­tion step to convert that intermediate product into fuels and chemicals. Thermochemical conversion technologies tend to be grouped into two distinct categories for fuel production (18): gasification and pyrolysis. Gasification conversion reduces biomass to a fundamental chemical building block, syngas (carbon monoxide and hydrogen), that canbe reconstructed into ethanol and other fuel products through catalytic fuel synthesis processes (19). There are several gasification technologies capable of converting biomass to syngas via a network of chemical reactions, which can include partial oxidation, pyrolysis, and steam reforming, among others. There are several biomass gasification technologies under development that offer various pros and cons. A good recent review of the different types of biomass gasification technologies and their current state of development is provided by Spath and Dayton (19).

Pyrolysis is another thermochemical conversion technology but, unlike gasification that converts biomass to a syngas, pyrolysis converts the biomass to a liquid intermediate. Fast pyrolysis produces a pyrolysis oil or “bio-oil” in a short residence time process (0.1-2 s) in the absence of air at intermediate reaction temperatures typically in the range of 400-650° C. A good recent review on fast pyrolysis technologies and their current status is provided by Bridgwater and Peacocke (20, 21). In its produced form, the bio-oil is unacceptable for use directly as a transportation fuel because of its instability, high viscosity, and highly corrosive nature. Unfortunately, the highly heterogeneous nature of bio-oil makes the economical conversion to a fuel a very difficult challenge. However, many groups are currently research­ing the process of converting bio-oil to transportation fuels inside a petroleum refinery (22), and preliminary indications are that this work looks promising for the economical conversion of bio-oils to transportation fuel.

In addition to the base biological and thermochemical conversion routes of biomass to fuels listed above, there are a number ofhybrid processes that take advantage ofthe synergies of both biochemical and thermochemical technologies for innovative and economically promising options for biorefineries. Some examples of such innovative approaches are syngas fermentation (23) and aqueous phase processes for converting sugars, sugar alcohols, and polyols into alkanes ranging from C to C15 (24-26). In fact, it can be argued that even the biological approach described in the next section is a synergistic biochemical and thermochemical approach in the fact that a thermochemical pretreatment process is the first step required in the process. Huber and coworkers (16) provide a good recent comprehensive recent review of biorefinery conversion technology options and their current status.

In the context of individual biorefineries there really is no clear single technology choice. Biorefinery developers will need to best match the conversion technology with the charac­teristics of the locally available feedstock(s) as well as match co-product options with local chemical markets to develop the best match for their particular set of conditions. Therefore, to realize the ultimate potential of biofuels for supplying the largest possible percentage of transportation fuels, the suite of conversion technologies must be capable of accommodating the diversity of feedstocks. Hence, it is necessary to look closely at the feedstock conversion technology interface.

Thermochemical conversion technologies, particularly gasification approaches that re­duce the biomass to a syngas, are robust to feedstock physical and chemical diversity (27). Forest residues — from small-wood forest thinnings or residues such as “hog fuel” from the forest products industry-are considered primarily for thermochemical conversion because of their compositional variability and lack of control over this diversity. Additionally, these types of feedstocks tend to have higher lignin content than herbaceous feedstocks, making them more suitable for thermochemical gasification conversion. Agricultural residues, in contrast, can be considered better suited for biochemical conversion technologies. These resources are expected to have a more uniform chemical composition because they are de­rived from cultivated crops that can be genetically engineered or selected for properties more amenable to biochemical conversion technologies (such as low recalcitrance or high cellulose or xylan content); this also holds for energy crops. The more uniform chemical composi­tion of the feedstocks is in the macro sense, recognizing that in the micro sense there can be considerable variability. Biomass grown specifically for transportation fuel production can be engineered or selected to have the most desirable chemical and physical properties for a conversion technology. In addition, increasing the biomass resource base that can be biochemically converted to fuels provides an additional resource: lignin-rich fermentation residues that can be used for combined heat and power production or converted to biofuel in advanced, integrated biochemical-thermochemical biorefineries.

Although the long-term attractiveness of both biochemical and thermochemical ligno — cellulosic biomass conversion technologies looks very good, they are not yet economically competitive with either petroleum-derived gasoline or starch-based ethanol. To achieve economic viability of Phase III biorefineries, parallel efforts need to be undertaken to re­duce both the feedstock cost component as well as the conversion cost component. The next three sections describe the R&D and technical challenges of achieving near-term economical competitiveness of Phase III biorefineries. The final section discusses long-term technology and R&D needs to realize the ultimate potential of biorefineries in supplying a significant portion of transportation fuel needs.