FIGURE 3-1 Contribution to minimum selling price of corn-derived ethanol from each process area.
SOURCE: Humbird et al. (2011). NREL/TP-5100-47764 (Somerville presentation).
performance. Batch processes produce dilute fuel that needs to be concentrated, catalysts and microorganisms are basically burned after each batch is processed, and wastewater generation is substantial. In addition, the entire process is shut down after each batch is completed, during which time capital equipment sits unused.
In the ideal process scheme, sugars are loaded into the fermentation tanks at an optimal rate and fuel is removed continuously. One scheme for reaching this optimum is to use a plugged reactor concept based on the continuous removal of fuel (see Figure 3-2). This concept relies on liquid/liquid extraction or an ethanol-selective membrane to achieve fuel separation under continuous operating conditions, and depending on the concentration of the sugar input, there will be little or no waste water to handle. Developing liquid/liquid extraction processes and new ethanol-selective membranes are areas in which the chemical sciences can make substantial contributions.
In a hypothetical continuous process that Somerville discussed, biomass would be ground and fed into a lignin removal process that feeds polysaccharides into a polysaccharide depolymerization process using enzymes or chemical catalysts that can be recycled. He made note of ongoing research that has produced heterogeneous platinum on carbon catalysts with extremely high activities. A concentrated sugar solution would then be fed in a fermentation reactor, with fuel separation and volume adjustment being performed continuously. And while there are challenges remaining to optimize this process, the only real stumbling block today is implementing lignin removal at the beginning of the process in a way that minimizes polysaccharide loss and recycles the lignin solved at very high efficiency.
In reviewing some of the approaches being explored today, Somerville said that each has limitations. Acid pretreatment is inexpensive, but it removes valuable hemicelluloses as well as lignin and it also produces some downstream inhibition. Various ionic liquids will dissolve cellulose and also separate polysaccharide-lignin mixtures, but ionic solvents are currently too expensive and would require recycling efficiencies of 99.999 percent to work economically. A more promising approach, one that needs the attention of the chemical sciences, is to develop catalysts that will partially depolymerize lignin. He noted that there have been some successes reported with model systems, but that this is still an area of research that is underexplored. Studies using supercritical water also look promising, though the engineering challenges of working at the required high pressures are substantial.
Studies on continuous pretreatment technologies have examined weak acid/high temperature and strong acid/low temperature combinations, as well as the use of aqueous bases, with or without ammonia and with or without added oxidants. So far, the strong acid/low temperature approach appears to be the most promising, and it produces a very pure sugar solution after a solid/liquid separation step. Both hydrochloric acid and the extraction solvent are recycled efficiently. Somerville said a major issue is building durable and safe process equipment that works with strong, concentrated acids such as 40 percent hydrochloric acid.