from the energy recovery system contributes to the highest GHG offsets. The use of MSW to generate electricity in AIF provides better GHG offsets compared to LFG (i.e., recovered CH4) to generate heat and electricity in LFE. This can be partly attributed to the fact that landfill CH4 has a lower heating value than MSW combustion, and only the biodegradable portion of MSW in a landfill contributes to the CH4 generation. Furthermore, it is assumed that the CH4 emissions
are not fully recovered due to inefficiencies in the landfill gas collection system, and the aforementioned landfill operating systems indicate that not all recovered CH4 is used for electricity and heat production. Fig. 3a and b also indicates that the contribution of GHG emissions from the transport process is relatively insignificant as compared to the other individual sub-processes. This is mainly due to the small land area of Hong Kong, where the distances traveled between RTS and the respective waste disposal facilities are relatively
short. A summary of GHG emissions or reductions from individual sub-processes for all four scenarios are shown in Table A.1 (Supplementary data). The results in Fig. 3 provide valuable information to policy makers to improve the performance of facility by reducing the GHG emissions. The results could serve as guidelines for improvement of processes from the respective waste disposal facilities which significantly release or reduce the GHG emissions.
3.3. Comparison of LFE and AIF with and without energy recovery system As previously stated, the relative GHG reductions from LFE and AIF, with or without an energy recovery system, are investigated in this study. The results of all four scenarios are illustrated in Fig. 4. As
expected, net GHG emissions for waste disposal facilities with energy recovery systems are lower compared to those facilities without these systems. However, this phenomenon is more significant for AIF. AIF with an energy recovery system emits 435.2 kg CO2e tonne−1 less compared to AIF without this system, while LFE with an energy recovery system emits 72.4 kg CO2e tonne−1 less than LFE without this system. Apart from this result, it is interesting to note that scenarios
without an energy recovery system, in which BAU (Scenario 1) and Scenario 4 are the best and worst case respectively, exhibit a reverse ranking order in terms of GHG emissions. In other words, without the energy recovery systems, LFE releases less GHG emissions as compared to AIF. The remarkable GHG emission reductions for AIF indicate that the energy recovery system in AIF plays a more crucial role in contributing to GHG offsets as compared to LFE. This is owing to the fact that AIF is capable of generating an order of magnitude more electricity than LFE, given the same amount and composition of MSW. Hence, it provides a huge advantage on GHG reductions and fossil fuel offsets. As a result, policy makers are advised to provide more
incentives and enhance efficiency of the technology of energy recovery since it provides a promising technique for reducing GHG emissions and fossil fuels consumption