displays the total normalized and weighted impacts
against the total costs (all measured in terms of per ton of product
gas produced). In this manner, a simple comparison can be made
against the costs and environmental performance of the waste conversion
technologies. The lowest impacts lie on the far left of the
chart while the lowest costs are at scattered the bottom. The most
favorable candidates in terms of environmental performance are
observed to be the pyrolysis–gasification of MSW and the steam
gasification of wood. Where cost-effectiveness is given the top priority,
the CFB gasification of organic waste, followed by the combined
pyrolysis, gasification and oxidation of MSW, would be the
preferred two technologies. The least favorable – highest environmental
impact as well as highest costs – appear to be the thermal
cracking gasification of granulated MSW and the gasification of
shredded tyres. For these two methods, the pre-processing of
waste take up a significant portion of the costs involved.
6. Final remarks
Singapore is a highly populated, industrialized city with limited
land area that can be used as landfills (Tan and Khoo, 2006). This
makes effective waste management highly challenging for the
small island-city state that intends to maintain its status as a
‘Green Garden City’. Gasification and pyrolysis, or a combined approach,
are widely available for the conversion of MSW, biomass,