Ruminal fermentations to produce liquid and gaseous fuels

Abstract

The ruminant animal has evolved to effectively utilize relatively recalcitrant cellulosic biomass to fulfill its needs for growth and reproduction. Although the animal has developed a unique biomass pretreatment process based on mastication and rumination, the key driver of biomass utilization is the microbiota residing in the rumen. The ruminal microflora, largely comprised of anaerobic bacteria, can ferment all of the non-lignin biomass components, primarily to volatile fatty acids (VFAs), methane, and carbon dioxide. Methane is a well-known and widely utilized fuel for human activities, but most of the energy in the fermentation products resides in the VFA, which serves as the primary energy source for the animal. Because VFA can be converted by chemical and electrochemical routes to hydrocarbons, alcohols, or esters, they are the central components of the “carboxylate platform” for biofuel production. The ruminal fermentation can readily be conducted in bioreactors (i.e., extraruminally) and shares several attractive features with carboxylate platform processes based on “stuck” anaerobic digestion: Feedstock fl exibility, high solid loading rate, non-aseptic operation, and wide substrate range. The extraruminal fermentation has the additional benefi t of a short run time (days, as opposed to weeks). This chapter summarizes current knowledge in the potential for fuel production using the ruminal fermentation, along with the potential for certain members of the ruminal community to be used for production of specific chemicals in pure culture or defi ned mixed culture.