Environmental performance of alternatives to treat fly ash from a waste to energy plant

Abstract

Incineration has been adopted by many developed countries as an alternative to treat municipal solid waste due to its capacity to reduce the amount of waste and recover energy. Waste to energy plants produce two waste streams: bottom ashes and fly ashes (FA). FA are classified as hazardous waste, and they cannot be utilised or landfilled without prior treatment. Stabilisation with cement solidification is the most used method to treat FA because it achieves the immobilisation of pollutants at a relatively low cost. However, the accelerated carbonation of FA, which allows the encapsulation of certain mobile metals under alkaline conditions, has recently been proposed as an alternative to the solidification/stabilisation process. To determine the environmental performance of FA stabilisation and carbonation, a life cycle assessment (LCA) was conducted. The LCA results of the carbonation and stabilisation processes were compared, and multiple carbonation scenarios were analysed: carbonation with different CO2 sources (incineration flue gas and flue gas from the combustion of natural gas), and different pressures (1–5 bar) and percentages of CO2 excess (10%, 55% and 100%) in the flue gas stream. Stabilisation had higher environmental impacts than carbonation due mainly to cement production and consumption. The best operating conditions of the carbonation process were found at flue gas pressures between 3 and 5 bar, since the total energy consumption decreases as the pressure increases. Moreover, the environmental benefits associated with the substitution of electricity from the grid mix made the scenarios based on the combustion of natural gas perform better than those that use the incineration gases as a CO2 source.