Conversion refers to the collection of processes employed to modify a feedstock into desired product(s).

Given that LB is composed of a number of distinct components, there are a variety of treatment options available that one can use to change these components into fuel, chemicals and other products.

With a harsh condition (high temperature, strong acid/base, strong solvents, or a combination of these agents), LB can be turned into small molecular units (such as C, CO2, CO, H2 and H2O) and then further con­verted to a desired product. Thermochemical conver­sion technologies usually employ this strategy to break down LB unselectively to accommodate further conver­sions either catalytically or biologically. Therefore, ther­mochemical conversion technologies can be versatile.

The structure of wood (or LB in general) is sufficiently strong and complex that it is not feasible to attack the whole complex at mild conditions in a single step, nor is it feasible to isolate the components and attack them individually. When mild conditions are desired, one must attack at least one portion of the whole structure and weaken it. Follow up with another treatment to break down the first component or attack a second component. Continue to treat the biomass until the desired composition is obtained. These treatment op­tions are classified into mechanical, thermal, chemical, or biological processes. These are not discrete classifica­tions. In other words, a process can be considered to belong to more than one category. For example, if one were to saturate LB with water, then heat it under high pressure and rapidly release the pressure, the hot water could vaporize into steam and thus explode apart the woody cells it had penetrated. This is called steam explosion and it uses a thermal process to accomplish a mechanical breakdown of the woody material. Steam explosion will be discussed later in this chapter.

While there are several LB conversion technologies available, this chapter will focus on biochemical conver­sion technologies with some discussions in thermo­chemical conversions.

These treatments may be applied at various points across the process. The typical process to acquire fuel products using a bioconversion methodology is gener­ally described in four parts: pretreatment, hydrolysis, fermentation and distillation, separation and filtration.

As discussed previously, cellulose, hemicellulose and lignin are strong, stable structures. These structures are challenging for one to convert into fermentable compo­nents (Mielenz, 2001). Of these three components, hemi — cellulose is the most vulnerable and easiest to degrade. Recall that compared to cellulose, hemicellulose is a lower molecular weight and is less uniform as it is composed of a variety of sugar polymers and residual units.

In bioconversions, the objective of pretreatment is to, as efficiently as possible, prepare LB for fermentation into products. The amount of energy required to break

Thermal-

mechanical

pretreatment

Gasification

Syngas

>“Coke

FIGURE 27.3 Schemes of biochemical conversion to materials, chemicals and fuels: (1) sequential incremental deconstruction; (2) two-step saccharification and fermentation; (3) simultaneous saccharification and fermentation; (4) gasification and fermentation. Source: Wang et al., 2012. (For color version of this figure, the reader is referred to the online version of this book.)

down LB is fixed. No matter what suite of treatment options used to convert LB to product, the thermody­namic barrier is the same. It requires the same amount of energy to completely biologically degrade wood as it does to chemically treat it or gasify it. One trades off time for allowing organisms to invest energy on one’s behalf versus applying heat or concentrated chemicals to accomplish the same task more quickly. Additionally, biological methods allow for greater selection of the portion of biomass to convert it into product, and usu­ally by selecting a descending route of molecular chem­ical energy or intermediates that are not down to simplistic building blocks if possible (green chemistry). By selecting only a portion of the LB to convert, one lowers the amount of investment energy required. Biochemical processes operate at moderate or low tem­peratures. These milder conditions may be slightly more efficient than their thermochemical counterparts. However, a burden of biological or biochemical pro­cesses could arrive for the need of detoxification. One must often remove toxic components resulting from the pretreatment methods employed.

Figure 27.3 illustrates a set of four treatment path­ways to convert LB into various products. These are not the only methods available but merely an example of commonly used methods. This pathway represents one of the most popular biorefinery designs used to bio­chemically convert LB into biofuels and bioproducts.

Pathway 4 shows a gasification process to produce syn­gas. This is a thermochemical process. The sugars in syn­gas are subsequently fermented into liquid fuels similar to those produced by the more biochemical methods. The four pathways shown vary in the number of steps, or time, required to acquire the product.

Figure 27.4 provides a slightly more detailed look at pathway 1 from Figure 27.3. Different pretreatment methods have different desired characteristics (Limayem and Ricke, 2012). A summary of pretreatment methods to be discussed in this chapter and their characteristics is shown in Table 27.2.