2.2.2.1 Introduction

Basically, biochemical conversion is the liberation and fermentation of sugars from biomass feedstocks. The challenge is to efficiently convert the carbohydrate portion of the biomass to sugars, or “saccharify” it, and ferment the impure sugars to ethanol with a robust mi­croorganism. In this process, the lignin component of the biomass provides the heat and power needs of the process. This process shows great promise for producing ethanol cost effectively with high yields and minimal environmental impact.

There are two primary routes for saccharification: 1) acid hydrolysis, with either concen­trated or multiple stages of dilute; and 2) pretreatment followed by enzymatic hydrolysis. In the 1980s, DOE evaluated the long-term potential of each process (31) and although at the time acid hydrolysis technology was further developed and appeared less expensive, com­paring progress and future potential suggested that enzymes offered greater opportunity for ethanol cost reduction in the long run (32). Acid hydrolysis technologies are certainly feasible and in proper niche situations they are being pursued to commercialization.

Enzyme hydrolysis requires a pretreatment to generate an intermediate material that can be effectively digested by enzymes. Dilute acid pretreatment of corn stover followed by enzymatic hydrolysis can achieve more than 90% conversion of cellulose to glucose (33). Various pretreatment methods have been suggested; most use heat coupled with a chemical catalyst such as an acid, base, or other solvent. Recent advances (34) suggest that “accessory”

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enzyme systems such as hemicellulases could lead to low-severity and low-cost pretreatment processes in the future. Although currently it appears that dilute-acid-based approaches give the best overall performance over the range of feedstocks envisioned for biochemical conversion, other approaches such as alkaline approaches also show considerable promise; and more development is needed in the pretreatment area to meet cost performance goals. Wyman and coworkers (35) give a good recent review on the comparative performances of the leading pretreatment technologies under development.

A representative block flow diagram of a biochemical conversion route to convert lignocel — lulosic biomass to ethanol using dilute acid pretreatment followed by the enzymatic hydroly­sis approach is shown in Figure 2.4. The process also includes ancillary supporting operations such as feedstock interface handling and storage, product recovery, wastewater treatment, residue processing (lignin combustion), and product storage not shown in Figure 2.4.

The feedstock is delivered to the feed-handling area for size reduction and storage. From there, the biomass is conveyed to pretreatment and conditioning. In this area, the biomass is treated with a dilute sulfuric acid catalyst (the current leading pretreatment technology) at a high temperature for a short time. This hydrolyzes the hemicellulose to a mixture of sugars (i. e., xylose, arabinose, galactose, mannose, and a small amount of glucose) and other compounds. In addition, the pretreatment step makes the remaining biomass more accessible for later enzyme saccharification. A conditioning process then removes byproducts from the pretreatment process that are toxic to the fermenting organism.

In hybrid saccharification and co-fermentation (HSF), the pretreated solids (now pri­marily cellulose) are saccharified with cellulase enzymes to form monomeric glucose. This requires a couple of days, after which the mixture of sugars and any unreacted cellulose is transferred to a fermenter. An inoculum of fermenting microorganism is added, and the sugars are fermented to ethanol. Meanwhile, the enzymes are used for further glucose pro­duction from any remaining biomass, which is now at conditions optimal to fermentation. After a few days of fermentation and continued saccharification, nearly all the sugars are converted to ethanol. The resulting beer (or low-concentration ethanol) is sent to product recovery.

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Product recovery involves distilling the beer to separate the ethanol from water and resid­ual solids. An azeotrope of water and ethanol is converted to pure ethanol using vapor-phase molecular sieves. Solids from the distillation bottoms are separated and sent to the boiler (called residue processing). Distillation bottoms liquid is then concentrated by evaporation using waste heat. The evaporated condensate is returned to the process, and the concentrated syrup is sent to the burner.

Part of the evaporator condensate, along with other wastewater, is treated by anaerobic and aerobic digestion. The biogas (which is high in methane) from anaerobic digestion is sent to the burner for energy recovery. The treated water is suitable for recycling and is returned to the process.

The solid distillates — the concentrated syrup from the evaporator and biogas from anaer­obic digestion — are burned in a fluidized bed combustor to produce steam for process heat. The majority of the steam demand is in the pretreatment reactor and distillation areas. Generally, the process co-generates enough electricity to use in the plant and to sell to the grid. A detailed description of the conversion process described above is provided by Aden and coworkers (17).