Effects of Concentration and Catalyst on the Kinetics of Biogas Production from Cattle Dung at Thermophilic Temperature

Abstract

Five different reactors (Ra, Rb, Rc, Rd and Re) of equal capacity (500cm³ each) were constructed and connected to a gas-collecting device. For the investigation of the effect of concentration on biogas production (uncatalysed reaction), different concentrations of reactant (20g/250cm³, 30g/250cm³, 40g/250cm³, 50g/250cm³ and 60g/250cm³) prepared from cattle dung were respectively placed in Ra, Rb, Rc, Rd, and Re, and anaerobically fermented at thermophilic temperature (37⁰C) for 16days. For the investigation of the combined effect of concentration and catalyst (catalysed reaction), parallel set ups were made, but in this case, 1.0g, 1.5g, 2.0g, 2.5g, and 3.0g of yeast were respectively added to the reactants (slurries) in Ra, Rb, Rc, Rd, and Re, which were also anaerobically fermented at 37⁰C for 16days. The reactors and their contents for both the catalyzed and uncatalyzed reactions were made in triplicate and the mean (average) volume of biogas collected in each of the reactors were evaluated. The average volumes of biogas obtained were used for the kinetics studies, where the rate of biogas production, the rate constant for biogas formation from the substrate (cattle dung), the rate of catalyst substrate unstable complex formation, the fraction of total catalyst that involved in the formation of complex with substrate, the concentration of excess (free) catalyst in the slurry, the rate of substrate transformation into biogas and the saturation constant of the catalyst added to each slurry was evaluated using the appropriate equations. The research revealed that, addition of catalyst to the slurries,  enhanced biogas production; at a certain point, the volume of biogas produced, the rate of biogas formation, the rate of catalyst substrate unstable complex formation and the rate of substrate transformation into biogas were directly proportional to the concentrations of the reactants and catalyst; the rate constants for reactions (catalyzed and uncatalyzed) were inversely proportional to the concentrations of reactants and the catalyst added; the fractions of the total catalyst that involved in the formation of unstable complex  with substrate in Ra, Rb, Rc, Rd, and Re were 0.95g, 0.97g, 0.98g, 0.98g, and 0.98g, respectively; the concentrations of the excess catalyst in Ra, Rb, Rc, Rd, and (g/cm³) were 0.05, 0.53, 1.02, 1.52 and 2.02, respectively and; the saturation constants (g/g) of the catalyst added to the slurries in Ra, Rb, Rc, Rd, and Re were 0.0476, 0.0484, 0.0488, 0.0490, and 0.0492, respectively. The research also revealed that the reactions (catalyzed and uncatalyzed) carried out obeyed first order rate law equation.

Key-words: Biogas; cattle dung; anaerobic fermentation; concentration; catalyst; thermophilic temperature.