Abstract:
The aim of this paper is to address the need for effective capacity expansion management in the landfill gas extraction environment. Landfills expand as waste is deposited over time, which creates the need for the capacity expansion of Landfill Gas Extraction Projects (LFGEPs)
LFGEPs start off small and expand throughout the lifetime of the project with the addition of more gas wells to extract the maximum quantity of Landfill Gas (LFG). The extracted LFG is combusted in generators to produce electricity. Extracted LFG, that cannot be used to generate electricity because of the limited number of generators, is flared. This results in decreased electricity production and loss of income for the project.Capacity expansion in LFGEPs is made up of two parts.Firstly it should be determined when a specific landfill cell should be developed and which type of gas well (vertical gas well or horizontal gas collector) should be installed to maximise the net income for this cell. The cell development sequence is of critical importance as the extracted LFG flow rate should match the electricity generating capacity of the plant throughout the life time of the project to maximise the return on investment. An integer programming approach is followed in this study to develop a cell development sequence plan that guarantees a constant LFG flow to the generators.
Secondly, the optimal electricity generating capacity is calculated to accommodate the extracted LFG. LFGEPs generate income by selling the generated electricity to Eskom and by trading in Carbon Emission Reductions (CERs). The LFG flow rate fluctuate as gas wells are installed in newly developed landfill cells. Every cell contains different types of waste that is at different stages of decomposition. This creates variation in the flow rate of the extracted LFG. The generators' capacity must be adapted to handle such a variation. The news vendor problem approach is used to calculate the number of generators needed at every stage of the project.
The capacity expansion of the two areas must match through the whole life-cycle of the project to maximise the quantity of LFG used for electricity production. This paper describes the procedure to achieve this with the techniques described above. The two methods are interlinked as one model's output will be used as input to the other model and vice versa. This forms a continuous cycle throughout the life time of the project.The Weltevreden landfill is used as a real-world case. The results from the two models described above are compared to the current capacity expansion plans of the landfill. Results are promising as the modeled capacity expansion plan showed an increase in the return on investment of 4% compared to the current plan.
