Biodiesel biorefinery: opportunities and challenges for microbial production of fuels and chemicals from glycerol waste

Abstract

The considerable increase in biodiesel production worldwide in the last 5 years resulted in a stoichiometric increased coproduction of crude glycerol. As an excess of crude glycerol has been produced, its value on market was reduced and it is becoming a "waste-stream" instead of a valuable "coproduct". The development of biorefineries, i.e. production of chemicals and power integrated with conversion processes of biomass into biofuels, has been singled out as a way to achieve economically viable production chains, valorize residues and coproducts, and reduce industrial waste disposal. In this sense, several alternatives aimed at the use of crude glycerol to produce fuels and chemicals by microbial fermentation have been evaluated. This review summarizes different strategies employed to produce biofuels and chemicals (1,3-propanediol, 2,3-butanediol, ethanol, n-butanol, organic acids, polyols and others) by microbial fermentation of glycerol. Initially, the industrial use of each chemical is briefly presented; then we systematically summarize and discuss the different strategies to produce each chemical, including selection and genetic engineering of producers, and optimization of process conditions to improve yield and productivity. Finally, the impact of the developments obtained until now are placed in perspective and opportunities and challenges for using crude glycerol to the development of biodiesel-based biorefineries are considered. In conclusion, the microbial fermentation of glycerol represents a remarkable alternative to add value to the biodiesel production chain helping the development of biorefineries, which will allow this biofuel to be more competitive.

Figure 1

A: World biodiesel (bars) and crude glycerol (lines) production between 2005 and 2010. Biodiesel production was grouped by continents; whereas crude glycerol represents the total production in the world over the years. B: Top ten biodiesel producing countries in 2010. Their production corresponds to approximately 71.3% of the total 19.21 million cubic meters of biodiesel. Production percentage is shown for each country. The production of crude glycerol was estimated assuming 0.106 L of crude glycerol per liter of biodiesel. The above figures were derived from an interactive table generated on January 11, 2012 from U.S. Energy Information Administration, International Energy Statistics, Biofuels Production ( http://www.eia.gov/cfapps/ipdbproject/iedindex3.cfm?tid=79&pid=81&aid=1&cid=regions,&syid=2005&eyid=2010&unit=TBPD).

Figure 2

Examples of chemicals produced by microbial fermentation of crude glycerol. Circles/positions indicate the aerobiose conditions in which these chemicals can be produced by microbial fermentation and the main microbial producing groups.

Figure 3

Different metabolic pathways for metabolism of glycerol. Production of 1,3-PDO by Enterobacteriaceae family members is shown in red. DHA production by G. oxydans is shown in blue. Ethanol and formate production pathways in the fermentative utilization of glycerol by E. coli are shown in gray. Proposed pathway for the conversion of glycerol to glyceric acid is shown in green. Dashed lines indicate multiple steps or unknown enzyme(s) (GA pathway). Main products are highlighted in color- filled boxes. Abbreviations: GLY, glycerol; GLY-Dhd, GLY dehydrogenase; DHA, dihydroxyacetone; DHA-Kin, DHA kinase; DHAP, DHA phosphate; PYR, pyruvate; GLY-Dht, GLY dehydratase; 3HPA, 3-hydroxyapropionaldehyde; 1,3-PDO-Dhd, 1,3-PDO dehydrogenase; GLY-DhdE, membrane-bound GLY-Dhd; GLY-Kin, GLY kinase, G3P, glycerol 3-phosphate, G3P-Dhd, G3P dehydrogenase; PPP, pentose phosphate pathway; PFL, pyruvate formate-lyase; mAdh: membrane-bound alcohol dehydrogenase.