Combined use of anaerobic digestion and Fischer-Tropsch reaction to convert organic wastes into biodiesel

Abstract

Transportation sector consumes a high amount of energy (e.g., gasoline and diesel) and is the main responsible for a large part of the CO2 and other pollutants emissions. Replacing the energy derived from fossil fuel required in this sector with that derived from a renewable resource, such as biomass, is a solution that can relieve global warming and other environmental problems. This work focuses the attention on the overall process that includes the anaerobic digestion of organic wastes, and the conversion of the biogas produced to Biofuels by Fischer-Tropsch (FT) synthesis. The first aim of the present work is the optimization of the anaerobic digestion processes for biogas production from Municipal Solid Wastes (MSW) under mesophilic conditions, the anaerobic co-digestion of Municipal Solid Wastes (MSW) with lignocellulosic biomasses from Giant reed (GR), the effect of mineral solution M9 10x and 400x salts addition, and the effect of trace metals addition in individual and mixed form. The results show that the highest amount of biogas as well as the highest methane fraction were obtained adopting a suitable combination of the operating parameters (15 wt. % of TS, 10 V/V% of inoculum, co-digestion of 75% GR and 25% MSW, with the addition a mineral and salt solution and addition of 5mg/L from individual elements of Ni, Co and Zn, however higher production was with the addition a mixture of three above elements at concentration 5mg/L for each one). The second aim is the optimization of the FT synthesis reaction for the exploitation of the synthesis gas (H2 and CO) obtained by a reforming step of the methane produced by anaerobic digestion. Under the operating conditions adopted, the main product of the FT reaction were liquid hydrocarbons. The FT reaction was studied under diluted conditions with H2/CO ratio equal to 2 and extruded cylindrical pellets (d = 2 mm) of Cobalt-Alumina based catalyst. Working with diluted condition (4% H2, 2% CO, 94% He) drastically reduces the influence of the temperature, since the FT reactions are highly exothermic, allowing to hypothesize a kinetic mechanism at isotherm conditions. The catalyst was prepared by impregnation technique under vacuum condition with 15% wt. of Cobalt, and it has been characterized using a temperature-programmed reduction (TPR) technique and a N2 adsorption isotherm. The experimental investigation on FT reaction was conducted varying the GHSV, from 370 to 820 h-1, and the temperature from 220 to 250 °C, using a fixed bed reactor with 5 g of catalyst. Liquid and gaseous phases products were analyzed by gas-chromatography techniques, in the range C1, C2-C4, C5-C11, C12-C20 and C21+ according to the number of C atoms in the chains. The results indicated that changes in the temperatures and GHSV do not have a significant effect on the conversion of diluted syngas. On the other hand, the results show that the value of CO conversion obtained at steady state with lowest temperatures and GHSV is about 27%, which is higher than the values reported in the literature with the same H2/CO ratio for a not diluted condition. The optimum conditions were obtained adopting the lowest values of temperatures and GHSVs. Under these conditions, the liquid hydrocarbon yield at steady state was about 26% and the selectivities towards gasoline and diesel hydrocarbons were about 40.30% and 47.18% respectively.