Contributed by Mark Holtzapple, Department of Chemical Engineering, Texas A&M University
In 1996, we received the first Green Chemistry Challenge Award (Academic) for our process that converts waste biomass to animal feed, chemicals, and fuels. This award came at a very opportune time. Less than one year prior to receiving the award, a start-up company was formed to commercialize our process. The award validated the investment, which continues to this day.
Our process is an example of an entire class of biomass conversion processes, which we call the carboxylate platform. It uses carboxylic acids and their salts as key intermediates to make industrial chemicals and transportation fuels (e.g., gasoline, jet fuel). Potential feedstocks include municipal solid waste, sewage sludge, animal manure, agricultural residues, and energy crops.
Figure 1 shows an overview of the carboxylate platform. Biomass components (e.g., cellulose, hemicellulose) are biologically converted to carboxylate salts (e.g., acetate, propionate, butyrate) via a mixed culture of microorganisms. The process is similar to classical anaerobic digestion, except that methanogens are inhibited, which allows carboxylate salts to accumulate rather than being converted to methane.
Using well-established chemical routes (Figure 2), the carboxylates are transformed into a wide variety of products, many of which are hydrocarbons commonly employed in gasoline and jet fuel. In some cases, hydrogen is required in the chemical conversion step. The hydrogen can be produced by gasifying undigested residues, or by reforming abundant natural gas.
Figure 3 is a schematic of the process. If the biomass lignin content is high, it is pretreated to enhance its digestibility. If the lignin content is low, pretreatment is not required. Using a mixed culture of microorganisms derived from soil, the biomass is fermented to carboxylate salts, which are recovered and chemically converted to the products shown in Figures 1 and 2.
Table 1 compares the capital cost and selling price of hydrocarbon fuels from the three biomass conversion platforms. Because of its simplicity, the carboxylate platform has a substantially lower capital cost, which allows for a low selling price for hydrocarbon fuels.
The carboxylate platform is being performed in a demonstration plant that can process up to 1 ton per day of biomass feedstock (Figures 4 to 6).
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