Numerical Investigation of Heat Generation and Accumulation Contributing to Elevated Temperature in MSW Landfills

Landfills are complex geostructures which contains organic and inorganic municipal, and in some cases industrial, wastes and are expected to remain operational for long times. Due to the complex nature of physical, chemical, biological, and thermal reactions that carry on within the depths of a landfill, unexpected incidents such as elevated temperatures could become inevitable. While uncommon to happen, elevated temperatures cause health and environmental issues such as odors, rapid settlements, slope instabilities. In addition, elevated temperatures can negatively impact engineered components in composite bottom liners, cover systems, leachate collection, and gas extraction and recovery systems. Air intrusion into municipal solid waste landfills can cause a localized switch from anaerobic to aerobic biodegradation adjacent to the intrusion. Air intrusion can cause a quick switch from anaerobic to aerobic conditions and as a result, cause a fast increase in temperature of up to 30 ◦C associated with stimulation of aerobic biodegradation reactions. Associated with the change to aerobic conditions is a decrease in CH4/CO2 (v/v) ratio in the landfill gas. Depending on the air flow rate intruding or injecting into the landfill, localized aerobic biodegradation is stimulated and as a result heat generation rate of 10 to 150 W/m3 leads to temperature increase. Pyrolysis leads to charring of MSW and decomposes MSW more quickly than anaerobic decomposition, causing high rates of settlements within shorter periods of times. For example, MSW could lose 40% volume under 220°C and generate pyrolysate gas of more than 200 m3/kg of MSW within the first couple of months. The initiation of smoldering highly depends on the oxygen concentration and external heat source. Lowest oxygen concentration of 4% (v/v) and initial heat radiation of 5kW/m2 provide conditions to activate exothermic oxidation reaction of smoldering. Exothermic reaction increases MSW temperature to ~300°C and if oxygen and heat supply remain in place, secondary oxidations activate and increase temperature to ~800°C. Heat generation from smoldering can generate 30 to 300 kW/m3 of MSW, which is approximately two orders of magnitude higher than that of other biochemical reactions in a landfill