Biochemical Conversion Processes
The diagram below depicts a high-level view of the primary units of operation in the biochemical conversion process. Specific process operation conditions, and inputs and outputs within and between each unit, vary in practice. These process variations can impact the key performance outcomes (titer, rate, and yield), which determine economic viability when the process is scaled up. The following descriptions highlight issues in each key process step.
During pretreatment, biomass feedstock undergoes a process to mechanically or chemically fractionate the lignocellulosic complex into soluble and insoluble components. Soluble components include mixtures of five- and six-carbon sugars (mainly xylose, arabinose, mannose, galactose, and glucose) and some sugars oligomers. Insoluble components include cellulosic polymers and oligomers and lignin (and any other components that may be linked to the constituents). Depending on the exact chemistry chosen for this step, variable amounts of the biomass may be solubilized. The main purpose of this step is to open up the physical structure of the plant cell walls to permit further deconstruction during the hydrolysis stepyea. The more open structure of the resulting material makes the remaining carbohydrate polymers more accessible for hydrolytic conversion to soluble sugars by enzymes or chemicals. The specific mix of sugars and oligomers released depends on the feedstock used and the pretreatment technology employed.
In some process configurations, the pretreated material goes through a hydrolysate conditioning and/or neutralization process to adjust the pH of the biomass slurry and remove undesirable by-product from pretreatment that are toxic to the downstream fermenting microorganism. In some cases, this step and hydrolysis, the next step, are combined into a single process.
In hydrolysis, the pretreated material, with the remaining solid carbohydrate fraction, primarily cellulose, is guided through a chemical reaction that releases the readily fermentable sugar, glucose. This can be accomplished with enzymes, such as cellulases, or with strong acids. Addition of other enzymes in this step, such as xylanases, may allow for less severe pretreatment conditions, potentially resulting in a reduced overall pretreatment and hydrolysis cost. Depending on the process design, enzymatic hydrolysis requires several hours to several days, after which the mixture of sugars and any unreacted cellulose is transferred to the fermenter. Current processes use purchased enzymes or enzymes manufactured on site, based on the economics of the specific process. For technologies using strong acids, acid recovery is important for the economics to be viable.
Currently, the most common approach to biological processing is to employ a fermentation step, wherein an inoculum of a fermenting microorganism is added to the biomass hydrolysates. Fermentation of all sugars is then carried out, and after a few days of continued saccharification and fermentation, nearly all of the sugars are converted to biofuels or other chemicals of interest. The resulting aqueous mixture or two-phase broth is sent to product recovery. Some processes combine the hydrolysis and fermentation steps (i.e., simultaneous saccharification and fermentation [SSF]).
Chemical or catalytic conversion can be used in place of, or in addition to, fermentation to convert the hydrolysis products, such as sugars, alcohols, or a variety of other stable oxygenates, to desired end products. The addition of a catalyst makes the reaction less energy intensive, thus making the entire process more efficient. Different reactions achieve different yields and intermediates while targeting different end fuels and chemicals, so current research is aimed at identifying optimal process combinations with respect to efficiency, feedstock utilization, cost, sustainability, finished product characteristics, and anticipated market demands.
Product Upgrading and Recovery
Product upgrading and recovery varies based on the type of conversion used and the type of product generated, but in general, involves any biological and chemical transformations, distillation or any other separation and recovery method, and some cleanup processes to separate the fuel from the water and residual solids. Residual solids are composed primarily of lignin, which can be burned for combined heat and power generation or chemically converted to intermediate chemicals or intermediates for other uses.