Gaseous Biofuels Production from Sweet Sorghum and Olive Pulp

Biomass from energy crops and agroindustrial wastes can be biologically converted to liquid or gaseous fuels, such as ethanol, methanol, methane and hydrogen, which was recently characterized as the fuel of the future. Hydrogen is a clean and environmentally friendly fuel, which produces water instead of greenhouse gases when combusted. It can be produced by renewable raw materials, such as organic wastes, and possesses a high-energy yield (122 kJ/g) due to its light weight. Furthermore, hydrogen could be directly used to produce electricity through fuel cells. Biological hydrogen production, one of the several ways to produce hydrogen, has received special attention during the last decade. Biohydrogen may be produced by cyanobacteria and algae through biophotolysis of water, or by photosynthetic and chemosynthetic fermentative bacteria. Anaerobic fermentative bacteria produce hydrogen without photoenergy, and so the cost of hydrogen production is 340 times lower than the photosynthetic process. In addition, carbohydrates are the main source of hydrogen during fermentative processes and therefore wastes/wastewater or agricultural residues rich in carbohydrates can be considered as potential sources of hydrogen. The hydrogen yield varies depending on the final metabolic products, mainly volatile fatty acids (acetic, propionic and butyric acids), lactic acid and ethanol. The effluent from a hydrogen producing reactor may be subjected to a subsequent anaerobic digestion step with the conversion of the remaining organic content to biogas (mainly methane and carbon dioxide), which may also be used as a fuel for the production of electricity. In this study, a two-step continuous process is developed for the biological hydrogen production and the subsequent production of biogas from energy crops and agroindustrial wastes. The process feasibility is examined for the cases of (a) sweet sorghum and (b) olive pulp, as feed materials. Sweet sorghum is rich in readily fermentable sugars and thus it can be considered as an excellent raw material for fermentative hydrogen production. The fermentative production of hydrogen from the sugars contained in a sorghum extract in a continuous stirred tank type bioreactor was examined at various hydraulic retention times. The effluent from this reactor was subsequently fed to a continuous stirred tank type anaerobic digester for the production of methane. Furthermore, the methane potential of the solids remaining after the extraction process was determined and the overall potential of sweet sorghum biomass for hydrogen and methane production was assessed. It was shown that continuous fermentative hydrogen production from sweet sorghum extract is possible and stable using the indigenous microflora without a preheating step. The highest biogas and hydrogen production rate was obtained at the HRT of 6h while the highest yield of hydrogen produced per kg of sorghum biomass was achieved at the HRT of 12h. The second application considered was the production of gaseous fuels from olive-pulp, a semi-solid residue generated from two phase olive-mills. The replacement of three-phase olive mills by their two-phase counterparts is a very promising perspective from an environmental point of view, but its feasibility depends on the ability to exploit the generated olive-pulp. Again it was shown that a two-step process based on biohydrogen production followed by production of biogas is indeed stable and feasible. The performance of the process for the two different substrates is compared and discussed. Moreover, the present study showed that sweet sorghum extract could be used for hydrogen and methane production in a two-stage process. It was proved that the effluent from the hydrogenogenic reactor is an ideal substrate for methane production. This work demonstrated that biohydrogen production can be very efficiently coupled with a subsequent step of methane production and that biomass from energy crops and agroindustrial wastes could be ideal substrates for a combined gaseous biofuels production.