Value-added Processing of Biofuel Residues/Wastes


Value-added processing of the biofuel residues/wastes is a downstream process to produce additional high-value products from residues/wastes generated from biofuel production. This approach is called “biorefinery” when it is adopted in biofuel industries. According to National Renewable Energy Laboratory (NREL), a biorefinery is a facility that integrates biomass conversion processes and equipment to produce fuels, and chemicals from biomass. The biorefinery concept is analogous to today’s petroleum refineries, which produce multiple fuels and products from petroleum. Industrial biorefineries have been identified as the most promising route to the creation of new domestic bio-based industries. By producing multiple products, a biorefinery can take advantage of the differences in the biomass components and intermediates and maximize the value derived from the biomass feedstock while also being able to adapt to changing market conditions. The treated effluent following the value-added processing requires no or minimal treatment; therefore, the value-added processing can lower the overall biofuel production cost by as much as 20%. Additional revenue sources from the high-value products enhance profitability and possibly could also offset the cost for producing biofuels. Consequently, this approach could serve as a viable and sustainable alternative for the biofuel plants.



Sugarcane Ethanol Production


Sugarcane feedstock mainly consists of sugar in the form of disaccharide (sucrose), which is readily fermented into ethanol by S. cerevisiae. The process of making ethanol from sugarcane starts when cane stalks are crushed to extract a sugar-rich cane juice. When cane stalks passed through extractor/expeller, cane juice is collected and delivered to a fermentation tank where the yeast fermentation reaction occurs to generate ethanol. The leftover fibrous residue called bagasse (45-50% moisture content) after juice extraction process is commonly combusted to generate heat/electricity for in-plant use. After fermentation, the fermentation broth containing approximately 5-12% ethanol by weight is now called beer. The beer is delivered to distillation column where the ethanol is recovered and the liquid residue known as vinasse is co-generated at the bottom of distillation column. At this process, the purity of ethanol can be up to 92-95% therefore further water separation process is required. Commonly, dehydration of the residual water is carried out using molecular sieves resulting in the final product, a fuel-grade anhydrous ethanol (200 proof or >100% ethanol).



Sugarcane Ethanol Residues


There are two principal residues from sugarcane-to-ethanol production including bagasse (solid residue) and vinasse (liquid residue). Normally, bagasse is used to provide heat/steam/electricity for the ethanol plant. However, vinasse could not be used as an energy source and has to be treated before disposal. Therefore, vinasse is still a major burden for sugarcane ethanol production.

Vinasse


Vinasse also known as stillage or distillery spent wash is generated as a leftover at the bottom of distillation column following ethanol recovery process. Vinasse characteristics are:

  • Dark colored
  • Low pH (4.0-4.5)
  • High total solids
  • Extremely high organic content
  • High nitrogen content (derived from yeast cells leftover from fermentation process)




  • Fig. 1 The schematic diagram of sugarcane ethanol production




    Fig. 2 Vinasse from sugarcane-ethanol industry