Production of Advanced Biobased Hydrogen Enriched Methane from Waste Glycerol in a Two-stage Continuous System

In the present work, a continuous process was developed for the production of advanced biobased hydrogen enriched methane, from crude glycerol in a two-stage reactor system. In the first step, biohydrogen production was studied, using attached mixed acidogenic consortia in an up-flow column bioreactor. Cylindrical ceramic beads with porosity corresponding to 600 mL were used as attachment matrix of bacterial cells. The hydrogen yield and the substrate consumption were investigated for a hydraulic retention time of 24h, with feed pH values 6, 6.5 and 7 and a concentration of 20g/L. The effluent of the hydrogenic reactor was fed to a methanogenic continuous stirred reactor (CSTR) in which the effect of organic loading on the methane yield was studied. The gaseous phase of the reactors was mixed for the production of the final gasous biofuel (hythane). At glycerol concentration of 20 g/L, hydrogen was produced with a yield of 0.051, 0.070 and 0.094 L/g COD feed with feed pH values 6, 6.5 and 7 respectively. Additionally, methane was produced with a yield of 0.257 L/g COD feed (commercial glycerol in the feed), 0.283 L/g COD feed (crude glycerol in the feed) , 0.198, 0.242 and 0.273 L/g COD feed (effluents from the hydrogenogenic (1st stage), diluted with water to 5, 7.5 and 10 g COD / L) respectively. INTRODUCTION The replacement of natural gas in internal combustion engines by a blended gas of hydrogen 10–60% (v/v) with methane was shown to highly improve the combustion efficiency, decrease the fuel consumption, and reduce significantly the emissions of carbon monoxide, carbon dioxide, and nitrous oxides [1]. The production of this hydrogen enriched methane, sometimes called hythane® as it was trademarked by Hydrogen consultants Inc., USA [2], is mostly based on the catalytic conversion of natural gas [3], which is inherently an inefficient and unsustainable process. The challenge of producing low cost, sustainable, environmentally friendly and highquality hydrogen enriched methane is a key factor that will allow it to attain its potential market position. A very promising approach embracing all the above requirements is the combined biological production of hydrogen and methane, which can be performed through a two-stage biological fermentation process through anaerobic dark fermentation of carbohydrate based wastes in the first-stage, and anaerobic digestion of the effluent in the second-stage. Hydrogen-enriched methane, sometimes known as hythane®, offers a significant number of advantages. In the 1990’s, it was demonstrated that the blending of hydrogen and methane led to a reduction of NOx and greenhouse gas emissions as well as to an overall improved combustion when compared with the combustion of methane. However, the challenge of producing low-cost, sustainable, environmentally friendly and high-quality hydrogen-enriched methane is a key factor that will allow it to attain its potential market position. A very promising approach, adressing all the above requirements, is the combined biological production of hydrogen and methane, which can be performed through a two-stage biological fermentation process. In the two-stage process, hydrogen is produced through anaerobic dark fermentation of carbohydrates in the first-stage, while the effluent of the first-stage reactor is converted to methane in the second-stage reactor [4-7]. However, in most of these studies published so far, the individual stages are not coupled, with control and mixing of the gaseous stream effluents, implying that the hydrogenogenic and methanogenic reactors have not been really integrated in a targeted producing process. Previous results demonstrate that blending hydrogen with methane leads to a more environmentally friendly biofuel than methane alone. By using a two-stage process i.e. dark fermentation for hydrogen production (in the