A fractal-like kinetics equation to calculate landfill methane production

摘要

Landfill appears as a convenient choice to get rid of municipal solid waste while providing energy, due to methane generated through anaerobic fermentation. However, without capture and treatment landfill gas is considered an important source of atmospheric methane. The control and use of this gas require knowledge of both, current yield and long-term accumulative production. These values are usually calculated with mathematical expressions that consider 100% of conversion, and homogeneous chemical reactivity inside the fill. Nevertheless, fermentation in landfills is erratic and spatially heterogeneous. This work introduces a fractal-like chemical kinetics equation to calculate methane generation rate from landfill, Q_(CH_4) (m~3/year), in the way: Q_(CH_4) = L_0∑_j ∑_i ∑_(ij)C_(ij)~0k_i(t_j)~(-d_s/2) exp[-k_it_j], where fermentable wastes are partitioned in readily, moderately and slowly biodegradable categories, L_0 is the potential of methane yield of refuse (m~3/tonne under standard conditions), d_s is the solid-phase fracton dimension, k_i, is the reaction kinetics constant of waste category i (year~(-1)), and t_j is the time from the year of burying j (year), C_(ij)~0 (kg/tonne) and M_(ij) (kg) are the initial concentration and the mass of waste category i landfilled in year j, respectively. The idea behind this equation is that methane production kinetics is limited by the diffusion of hydrolyzed substrate into a heterogeneous solid-phase towards discrete areas, where methanogenesis occurs. A virtual study for a hypothetical case is developed. The predictions from this fractal approach are contrasted with those coming from two equations broadly used in the industrial work. The fractal-like kinetics equation represents better the heterogeneous nature of the fermentation in landfills.