Bioenergy production from glycerol in hydrogen producing bioreactors (HPBs) and microbial fuel cells (MFCs)

The supply of glycerol has increased substantially in recent years as a by-product of biodiesel production. To explore the value of glycerol for further application, the conversion of glycerol to bioenergy (hydrogen and electricity) was investigated using Hydrogen Producing Bioreactors (HPBs) and Microbial Fuel Cells (MFCs). Pure-glycerol and the glycerol from biodiesel waste stream were compared as the substrates for bioenergy production. In terms of hydrogen production, the yields of hydrogen and 1,3-propanediol at a pure-glycerol concentration of 3 g/L were 0.20 mol/mol glycerol and 0.46 mol/glycerol, respectively. With glucose as the co-metabolism substrate at the ratio of 3:1 (glycerol:glucose), the yields of hydrogen and 1,3-propanediol from glycerol significantly increased to 0.37 mol/mol glycerol and 0.65 mol/glycerol, respectively. The glycerol from biodiesel waste stream had good hydrogen yields (0.17-0.18 mol H₂/mole glycerol), which was comparable with the pure-glycerol. In terms of power generation in MFCs, pure-glycerol was examined at concentrations of 0.5-5 g/L with the highest power density of 4579 mW/m³ obtained at a concentration of 2 g/L. The power densities from the biodiesel waste glycerol were 1614-2324 mW/m³, which were likely caused by the adverse effects of impurities on electrode materials. An economic analysis indicates that with the annual waste stream of 70 million gallons of glycerol, the expected values generated from HPBs and MFCs were $311 and $98 million, respectively.