This study was conducted to investigate thermal aspects of municipal solid waste landfills as a function of operational conditions and climatic region. Spatial and temporal distributions of waste temperatures were determined at four landfills located in North America �Michigan, New Mexico, Alaska, and British Columbia�. Temperatures of wastes at shallow depths �extending to 6 to 8 m depth� and near the edges of a cell �within approximately 20 m� conformed to seasonal temperature variations, whereas steady elevated temperatures �23 to 57°C� with respect to air and ground temperatures were reached at depth and at central locations. Waste temperatures decreased from the elevated levels near the base of landfills, yet remained higher than ground temperatures. Thermal gradients in the range of approximately −30 to +22°C / m with average absolute values typically less than 5°C / m were measured within the wastes. Heat content �HC� of wastes was determined as the difference between measured waste mass temperatures and unheated baseline waste temperatures at equivalent depths. Peak HC values ranged from 12.5 to 47.8°C day/ day. The peak HCs were directly correlated with waste placement rates and initial waste temperatures, and they occurred at a specific average precipitation �2.3 mm/ day� beyond which further precipitation did not contribute to heat generation. HC was determined to conform to exponential growth and decay curve relationships as a function of climatic and operational conditions. Heat generation was determined based on HC using 1D heat transfer analysis. The heat generation values ranged from 23 to 77 MJ/ m 3 without losses and were significantly higher than biochemical prediction models, yet lower than values from incineration analyses. Overall, the highest values for temperatures, gradients, HC, and heat generation were observed in Michigan, followed by British Columbia, Alaska, and New Mexico. Integrated analysis of temperature and gas composition data indicated that temperature increases and HC values were greater during anaerobic decomposition than aerobic decomposition. Sustained high temperatures and heat generation occurred in wastes under anaerobic conditions.
A numerical modeling approach has been developed for predicting temperatures in municipal solid waste landfills. Model formulation and details of boundary conditions are described. Model performance was evaluated using field data from a landfill in Michigan, USA. The numerical approach was based on finite element analysis incorporating transient conductive heat transfer. Heat generation functions representing decomposition of wastes were empirically developed and incorporated to the formulation. Thermal properties of materials were determined using experimental testing, field observations, and data reported in literature. The boundary conditions consisted of seasonal temperature cycles at the ground surface and constant temperatures at the far-field boundary. Heat generation functions were developed sequentially using varying degrees of conceptual complexity in modeling. First a step-function was developed to represent initial (aerobic) and residual (anaerobic) conditions. Second, an exponential growth-decay function was established. Third, the function was scaled for temperature dependency. Finally, an energy-expended function was developed to simulate heat generation with waste age as a function of temperature. Results are presented and compared to field data for the temperature-dependent growth-decay functions. The formulations developed can be used for prediction of temperatures within various components of landfill systems (liner, waste mass, cover, and surrounding subgrade), determination of frost depths, and determination of heat gain due to decomposition of wastes.
The effects of placement practices on decomposition of wastes were investigated at Anchorage Regional Landfill (Anchorage, Alaska) since 2002. Temperatures and gas concentrations of wastes placed at various seasons were monitored. Wastes were placed at sub-freezing temperatures during cold seasons. Waste temperatures generally increased upon placement.High variation was observed in waste temperatures near the surface whereas steady temperatures were obtained at depth. High maximum stable temperatures resulted from warm placement conditions. Steady temperatures between approximately -1 to +35°C were observed. The central portion of a frozen waste band (with a total initial thickness of 7 m at placement, currently between depths of approximately 8 m to 15 m) remains frozen 2 years after placement. Both the top and bottom regions of the frozen waste band have thawed. Heat Content (HC) varied between -8.2 (for 2-year-old waste at a depth of 11.9 m in frozen wastes) to +25.9°C-day/day (for 13-year-old waste at a depth of 32 m for waste placed in summer). The measured frost depths in waste ranged from 0.7 to 1.3 m and were less than that for native soil at the landfill site. Instantaneous thermal gradients ranged from -73 to +60°C/m. Gas concentrations were similar to air at the time of waste placement. Anaerobic decomposition conditions and onset of landfill gas production started within 3 to 4 years of placement for wastes placed during warm seasons. Virtually no decomposition or gas generation were observed in the frozen wastes. A 1-D numerical model was used to investigate distribution of temperatures for placement at varying temperatures and for varying lift thicknesses. It is recommended to minimize frozen lift thicknesses to obtain higher temperatures.
Analytical and numerical approaches have been developed for modeling temperatures in municipal solid waste landfills. Steps for model formulation and details of boundary conditions are described. The formulation was based on a transient conductive heat transfer analysis. Conventional earth temperature theories were modified for landfill systems by incorporating heat generation functions representing biological decomposition of wastes. Finite element analysis was used for general modeling and parametric evaluations. Thermal properties of materials were determined using field observations and data reported in literature. The boundary conditions consisted of seasonal temperature cycles at the ground surface (established using near-surface field measurements) and constant temperatures at the far-field boundary (established using field measurements and maps of regional groundwater temperatures). For heat generation, first a step-function was developed to provide initial (aerobic) and residual (anaerobic) conditions. Second, an exponential growthdecay function was established; and third, the function was scaled for climatic conditions. The formulations developed can be used for prediction of temperatures within various components of landfill systems (liner, waste mass, cover, and surrounding subgrade), determination of frost depths, and determination of heat gain due to decomposition of wastes.
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