Accumulation of heat in large, deep, and wet landfills is being observed at U.S. landfills. Heat accumulation can lead to higher temperatures deep in a landfill, which can disrupt normal biological degradation processes and adversely affect the systems for collecting and processing landfill gas.
Landfill gas generation from organic matter in a landfill is a natural phenomenon with varying degrees of gas generation depending on the age and the mix of organic matter in the waste. Over the past four decades, solid waste engineers have successfully developed and constructed various types of gas extraction and control systems (GCCS), to effectively collect, remove, and dispose of landfill gases under normal operating conditions at Subtitle D landfills. The number of technologically sophisticated systems converting landfill gas to reusable energy continue to grow significantly over the last decade.
Various types of gas extraction systems with certain accompanying controls for the destruction of gas after extraction (flaring), conversion systems to generate electricity, or produce medium or high Btu gas from the extracted gas are commonplace. However, high temperatures developing inside landfills do raise the questions of the survivability and sufficiency of these systems to meet compliance requirements in the long run, and if the existing systems can be modified to extract enough heat. Solid waste engineers need to address heat removal from landfills and design to control temperatures below the landfill surface in a much more direct and practical way.
In the past, some solid waste engineers thought that high levels of gas extraction from a landfill could remove sufficient accumulated heat to mitigate elevated temperature conditions. However, the current consensus of many solid waste professionals is that the extraction of hot gas from the landfill removes heat from the interior of the landfill, but thermodynamic evaluation of heat transfer has shown that the amount of heat extracted from landfills via hot gas through the GCCS is insignificant compared to the amount of heat that accumulates under elevated temperature landfills.
Due to the low molecular weight of landfill gas, the amount of heat that can be extracted from the interior of an elevated temperature landfill is limited. The rest of the heat needs to be transferred out of the landfill by other means. There are a few pathways for heat transfer out of the landfill under natural conditions, namely heat transfer through the exposed surface of the landfill (above ground surface), heat transfer into the ground below the waste, and heat transfer to liquids traveling vertically through the waste column and eventually extracted in the form of leachate. There are also chemical reactions taking place in the landfill that absorb heat from the surrounding environment to complete the reaction process. There are also numerous chemical reactions, as well as biological reactions, that are exothermic and add heat to the waste mass.
Solid waste operators and engineers cannot solely rely on the amount of heat extracted through gas wells to control high temperatures in elevated temperature landfills. Watered out gas wells with reduced gas productivity further contribute to a reduction in heat extraction through gas collection wells. Thermodynamic computations have indicated that an unrealistically high number of gas extraction wells per acre may be needed to extract the accumulated heat in elevated temperature landfills, which means increasing the number of gas wells cannot be the sole solution to address excess heat removal from a landfill.
Every landfill is unique and requires a custom solution, but our general advice would be to construct a reasonable number of gas wells per acre, maintain existing gas extraction wells to the best of your ability, expedite movement of liquids through the waste vertical column supplemented by an effective leachate collection system, and look into developing systems to manage heat in the landfill.
SCS is an expert in the management of elevated temperature landfills and has been promoting the development of heat management systems over the past several years. As a result, we are highly qualified to address heat accumulation in landfills and development of heat removal systems to control temperatures below the landfill surface. If you have an elevated temperature landfill at your facility or a landfill that seems to be progressing in the direction of becoming an elevated temperature landfill in the future, contact us and let us review field data from your facility and develop means to control temperatures below the landfill surface.
Author: Ali Khatami, PhD, PE, LEP, CGC, is a project director and a vice president of SCS Engineers. He is also our National Expert for Landfill Design and Construction Quality Assurance. He has nearly 40 years of research and professional experience in mechanical, structural, and civil engineering.
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