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A landfill resource assessment typically involves characterising the site by carrying out predictive modelling and calibrating these results by undertaking a pumping trial. The objective of a landfill gas pumping trial is to determine the volumes and qualities of LFG that can be sustainably extracted from a landfill. The information generated from the trial gives assurance that the correct management approach is selected e.g. passive management or active extraction. Where active extraction can be shown to be sustainable, the trial will provide information on the required design parameters of a permanently installed extraction system to ensure the system is adequate in scale so that all the can be controlled correctly.
As part of the resource assessment a Biogas Systems’ proprietary predictive gas generation model is used and the results compared against an Australian inventory model such as NGERs Solid Waste Calculator and GasSim. The models are based on the degradable carbon content in the landfill and if no waste records are held, default values can be selected (e.g. the methane degradation rate and the proportion of carbon in the waste). If the waste input volumes are not known, bore logs and survey data (or satellite imagery) can be used to estimate the fill volume. Biogas Systems uses GasSim and is very familiar with the model. GasSim is an independent resource and risk management tool not a proprietary gas resource assessment model, therefore we believe it represents an objective estimate.
Pumping trials are completed in accordance with the approved methodology and program over a period of usually 12 weeks sometimes less. During this time, we will ensure the trial is up and running satisfactorily and attend site regularly to balance the GCS and monitor levels of carbon monoxide remotely using state of the art telemetry systems. We attend site following drawdown of the concentration of methane and to undertake radius of site tests and recovery tests.
In addition to the main objectives we often assess the rate and concentration of methane generation from the selected ‘typical’ zone within the landfill and extrapolate this across the remaining parts of the site and provide recommendations, based on the results of the pumping trial, for ongoing LFG extraction at the site. The results are calibrated and validated against the model outputs and calculations carried out previously. These results include investigation the radius of influence being effected by individual extraction wells and draw down and recovery rate for assessing waste permeability and LFG flow which can help refine LFG well spacing in the future.
Pumping trials also assist in defining well-head gas flow characteristics, which in turn help describe the behaviour of individual wells under passive and active conditions, quantifying probable LFG production rates that are obtained by active abstraction over a period, increasing flow rate until balance gas increases above the selected set point and the quality of the recovered methane falls. This test can, however, be dependent on the quality of the capping. It is also critical to allow for seasonal variation as gas production may be higher during some climatic times.
Finally, the abstraction pressures are determined, which in-turn enables correct sizing of the permanent abstraction equipment. The final sizing of the equipment will be a function of gas production rate versus flow rate, which should be extrapolated to include the whole site at the post-trial stage.
Biogas Systems have the equipment and experienced personnel available to undertake resource assessments from any landfill across Australia and internationally. This includes elevated temporary flares which meet compliance requirements, installation equipment such as drill rigs and trenching equipment, and welders to connect pipework. A selection of our elevated flares are also telemetry enabled which allows data to be received in real time.