The past five years have witnessed an explosion in the laboratory effort put into finding an economical way to develop pretreatment processes for biomass feedstocks in order to prepare them for conversion to sugar and ethanol. The next step requires taking that base of laboratory knowledge and converting it to on line processes. Because of the high temperatures and high desired-solids levels required for most pretreatment techniques, direct steam injection is the most practical approach to heating the slurry. The following introduces the challenges associated with scaling the lab pretreatment process to production levels, and some practical advantages of developing successful pilot strategies.

All structural plant matter is a combination of cellulose, hemicellulose and lignin. Only the direct cellulose is readily convertible to fermentable products. Hemicellulose must be converted to a fermentable form of sugar, and the lignin is generally not convertible and must be removed. Cellulose is the part of the carbohydrate portion of plants such as grass, corn stover, straw and trees. Like conventional starch conversion to ethanol, hemicellulosic materials can be converted to sugars and fermented to create ethanol, biodiesel or other useful energy products.

The Process
In all biomass processing cases, the main technological problem is to free the cellulose material in the plant to allow it to be converted without significantly reducing the yield of the existing cellulose material. This process is generally referred to as “pretreatment” of the biomass.

In the pretreatment step, a slurry of feedstock is treated with heat, time and some type of chemical to convert the hemicellulose to a sugar. Pretreatment could also be used to change the nature of the hemicellulose in order to allow a secondary agent, such as an enzyme, to hydrolyze the cellulose. This step is conducted in either a batch or continuous process. In the batch process, high-solids (20 percent to 25 percent) slurry of feedstock, usually corn stover, is fed to a high temperature reactor and subjected to high temperature (more than 300 degrees Fahrenheit). A strong chemical such as sulfuric acid, caustic or a solvent may also be present in the reactor. At the conclusion of the pretreatment step an acid or enzyme is added to hydrolyze the cellulose and form sugars. These sugars are then further processed and fermented to create ethanol.

The continuous process is another approach to pretreatment taking a pump-able slurry of feedstock and subjecting it to heat and time to soften the hemicellulosic structure. The softened slurry is then treated with acid or alkaline to break down the slurry to a form that can be hydrolyzed with an enzyme to form sugars. This process would be in-line as opposed to batch.

Transition from Lab to Production
Most of the current biomass research work has focused on laboratory techniques to determine the effects of temperature and pH (among others) on the conversion rates. These lab settings resemble the chemistry labs one might have experienced in high school and college. Pretreatment laboratory work is almost exclusively batch-driven given the complexities involved in controlling low flow processes. As a result, there is a general lack of knowledge in the best approaches and potential problems with continuous heating of the biomass feedstock stream during pretreatment in a production process.

Factors to consider when scaling up the lab process include:

› Flow rates will increase and add complexity to fluid transfer

› Residence times will change from a relatively fixed-hold vessel to a continuous flow

› The flowability of the slurry is an important factor

› Piping design and flow dynamics can add and/or change fluid velocities and impact the slurry flow.

Pilot Scale Considerations
As with all new process development, technologies need to evolve from the lab stage to production-level processes. This is a significant leap as there is more focus on the chemistry than the mechanical process in most lab settings. The goal is to develop production-level processes that maintain the unique design technology and can be scaled to reach economically feasible production-level processes. For most transitions, a pilot plant stage allows companies to test out actual process components such as conveyors, heat transfer, mixers and pumps.

Considerations for developing a pilot plant include:

› Design to mimic full scale process layouts

› Use equipment similar to full scale processes

› Be careful on the compromises from full scale

› Determine what you are trying to learn

› Make sure production-level equipment exists similar to pilot scale.

Unlike grain mash ethanol, there are significant differences in the pretreatment of corn stover, switchgrass and wood fiber. Challenges associated with fiber slurry heating include:
› Heat exchangers are generally not viable because of processing temps of 300 degrees Fahrenheit or greater

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