Investigation of reuse of coal fly ash to mitigate landfill methane gas

Abstract

Methane (CH4) is a powerful greenhouse gas (GHG), contributing 32% of current climate forcing (US EPA, 2015). In 2014 the White House released the “Strategy to Cut Methane Emissions” as part of the Climate Action Plan. Landfills are one of the strategy’s 4 key sectors, contributing 18% of US methane emissions (US EPA, 2013). Even when landfills capture methane and burn it to produce electricity, around 25% of methane still leaks through landfill covers (US EPA, 2005). In natural environments, iron oxides, manganese oxides, nitrates, and sulfates have been shown to stimulate microbial anaerobic oxidation of methane (AOM) (Sivan et al., 2014). Adding fly ash, which can contain these components, to the soil of a landfill cover would thus be expected to increase AOM in the lower layers of the cover. The current work is a preliminary study to determine the effect of fly ash on microbial activity of methanotrophs in landfill’s final cover. Two fly ash samples were selected based on their chemical components and adsorption capacity to be mixed with two final cover soil samples. Soil alone and 75% soil- 25% fly ash mixtures were put in batch reactors in contact with a CH4 and CO2 mixture, similar to the bio gas produced in landfills. To compare the performance of different samples, the concentration of CH4 and CO2 in the reactors was monitored for 40 days, via a gas chromatograph. The results indicate that adding fly ash to the soil would decrease the performance of microbes to oxidize methane. The concentrations of elements in the fly ash with the potential to oxidize methane (e.g. iron, manganese) were low; in addition, the large fraction of fly ash used (25%) substantially reduced the microbes available for methane oxidation. Fly ash or other wastes with a higher percent of constituents like iron and manganese should be tested, in lower fractions.