MICROBIAL METHANE OXIDATION PROCESSES AND TECHNOLOGIES FOR MITIGATION OF LANDFILL GAS EMISSIONS
CHAPTER ONE
INTRODUCTION
BACKGROUND OF THE STUDY
Biodegradable organic material present in landfilled waste undergoes microbial degradation and produces landfill gas. Landfill gas is a mixture, predominantly made up of methane and carbon dioxide, with small amounts of hydrogen. These are typically referred to as bulk gases because they are often present at percentage concentrations (e.g. a landfill gas composition of 60% methane and 40% carbon dioxide). Landfill gas will also contain a wide variety of trace components at low concentrations (Environment Agency 2010a). When collected, the landfill gas may also contain varying amounts of nitrogen and oxygen derived from air that has been drawn into the landfill or collection system. Control of landfill gas is necessary both to manage local environmental issues and safety risks, and to limit emissions of greenhouse gases. The carbon dioxide emissions from landfills are part of the natural carbon cycle as they derive from the degradation of organic material and are not from a fossil carbon source. Methane emissions from landfills are not part of the natural cycle and the global warming potential of methane is 28 times that of carbon dioxide, over a 100-year lifetime (Myhre et al. 2013). Therefore, efficient collection of the landfill gas and oxidation of the methane to carbon dioxide is necessary to mitigate greenhouse gas emissions. The established best practice for managing landfill gas from biodegradable waste landfills in the UK is to actively collect the gas. The gas is drawn from vertical pipes in the waste through connecting pipework to a gas management facility. The collected gas is either used as an energy source, normally to generate electricity, or is flared. A high temperature flare is required as a back-up to electricity generation or can be used in parallel where the gas yield exceeds the electricity generation capacity of installed plant. Combustion, either for energy generation or in a flare, oxidises the methane to carbon dioxide. The detailed requirements for the best practice for the management of landfill gas are set out in Environment Agency guidance LFTGN 03 (Environment Agency 2004) and in an Industry Code of Practice (Environmental Services Association 2012). Non-binding EU guidance (European Commission 2013) sets out the most important criteria in ensuring effective management of landfill gas, which is required until gas production becomes negligible. The established approaches to use for electricity generation and flaring gradually become less effective as gas yields and methane concentrations decline over time. Other methods of landfill methane oxidation exist and will need to be adopted to ensure the continued effective management of landfill gas.
SIGNIFICANCE OF THE STUDY
The purpose of this report is to provide evidence to allow operators of biodegradable waste landfills to select appropriate methane oxidation techniques over the whole life-cycle of a landfill, in particular when landfill gas production has declined past its peak. In addition, implementing methane oxidation techniques at older, closed landfill sites without active gas control measures presents a challenge, which requires investment (often significant) to install and operate the necessary technology. This report provides evidence for those responsible for such sites on the most appropriate and cost-effective techniques which may be used to mitigate significant passive methane emissions. This report describes these methane oxidation techniques and provides a framework setting out what each can achieve and what conditions each technique is suitable for. The appropriate use of these techniques will help minimise methane emissions from landfills.