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Modeling Thermal Changes at Municipal Solid Waste Landfills: A Case Study of the Co-Disposal of Secondary Aluminum Processing Waste
Tolaymat, T., A. El-Badawy, J. Smith, M. Barlaz, P. Jain, S. Luettich, AND X. Huang. Modeling Thermal Changes at Municipal Solid Waste Landfills: A Case Study of the Co-Disposal of Secondary Aluminum Processing Waste. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-16/174, 2016.
The objective of the project was to assess the impact of salt cake disposal on the waste temperature distribution in MSW landfills. The impact of salt cake disposal on MSW landfill temperature distribution was numerically modeled. Heat is released from the reaction of salt cake with water (US EPA 2015). Salt cake heat-generation-potential data based on a US EPA (2015) laboratory-scale study were used to estimate the heat-generation rate from salt cake. These estimated heat generation rates were used as inputs for modeling thermal dynamics resulting from salt cake disposal in MSW landfills using TEMP/W® , a finite element computer program used for modeling heat transport in porous media. TEMP/W® simulations were also coupled with a companion program, AIR/W® (Geo-Slope International Ltd.) to assess the impact of landfill gas flow on waste temperature distribution in landfills; AIR/W® is a finite element software that models air flow in porous media. The impacts of heat generation rate, thermal conductivity, heat capacity, and moisture content on the temperature distribution in a MSW landfill were analyzed. The impacts of different salt cake disposal strategies (e.g., placement near the landfill surface, placement in several discrete pockets scattered throughout the landfill) were analyzed as well.
The reaction of secondary aluminum processing waste (referred herein to as salt cake) with water has been documented to produce heat and gases such as hydrogen, methane, and ammonia (US EPA 2015). The objective of this project was to assess the impact of salt cake disposal on MSW landfill waste temperature distribution. Literature-reported properties of MSW and data from salt cake reactivity testing by the United States Environmental Protection Agency were used in a finite-element model to assess the magnitude of impact on waste temperature resulting from adjusting the individual thermal properties of the materials over the reported ranges, and from various salt cake waste placement scenarios.The modeling results from various MSW and salt cake placement disposal scenarios presented in this report suggest that, over the relatively low literature-reported range used for modeling, waste thermal conductivity had a limited impact on the waste temperature for the conditions modeled (e.g., elevated heat generation rate). Ambient and ground temperatures (independent of salt cake placement) were found to have a moderate impact on waste temperature. The specific heat capacity and the heat generation rate of the mixed waste (i.e., containing both salt cake and MSW) were found to have the most significant effect of all the properties evaluated in this assessment. The landfill temperature increase following the co-disposal of salt cake with MSW was found to be directly proportional to the heat generation rate and inversely proportional to the heat capacity of the surrounding MSW fraction; heat capacity of a material is the amount of heat needed to increase its temperature by a unit degree. Several factors such as of the amount of salt cake (relative to MSW) and heat release timeframe influence the heat generation rate within the landfill. Because of its wider range, the heat generation rate is expected to have a greater influence on waste temperature within a landfill than any other factor. Apart from material properties (e.g., heat generation rate, heat capacity), the model simulations predicted that the material placement strategy would have a significant impact on the spatial distribution of waste temperature within a landfill. The simulated placement of salt cake in single or multiple discrete pockets resulted in localized high heat generating zones with temperatures in excess of 190°C (374°F). As a point of reference, Title 40 of the Code of Federal Regulations, Part 60, Subpart WWW requires the operation of landfill gas collection systems to maintain landfill gas well temperatures below 55°C (131°F). Temperatures above 55°C may indicate air infiltration and can lead to an elevated risk of a landfill fire. The placement of the material at the surface or uniformly mixed with MSW resulted in a lower maximum predicted waste temperature of 50-70°C (122-158°F). However, surface placement of salt cake may adversely impact closure cap integrity and fugitive gas emission. The role of leachate and landfill gas movement (i.e., convective modes of heat transport) in heat removal from and the resultant temperature distribution within a MSW landfill was found be insignificant. Stabilizing (i.e., reacting with water) the salt cake and exhausting its heat and gas generation potential before co-disposal with MSW should be explored. This approach may provide opportunities to beneficially recover the heat and combustible gas generated during salt cake waste reactions while mitigating the unintended consequences to landfill infrastructure that may occur at an MSW landfill.