An attributional life cycle analysis was developed for two
waste-to-energy units: a moving grate combustor and a vertical
shaft gasifier coupled with direct melting of ashes. Data from
several units in operation were processed by means of mass and
energy balances, and a specific material and substance flow
analysis.
The assessment of the life cycle environmental impacts highlighted
the sustainable environmental performances of both the
thermal treatment technologies. The level of emissions into the
atmosphere appears low for both the units, and the burdens
related to the chemicals utilized in the plant operations appear
of limited relevance. The moving grate WtE unit shows a clearly
better environmental performance for almost all the impact
categories.
The details of the contributions from all the stages of the two
process life cycles indicated the stages of each specific technology
where further improvements in technological solutions and management
criteria could produce remarkable positive environmental
results. The combustion WtE unit can achieve a great further
reduction of its environmental burdens by sending a larger percentage
of bottom ashes to an up-to-date process for recovery of
ferrous and non-ferrous metals and inert materials. The gasification
WtE unit should instead focus the attention on the utilization
of renewable biomass coke: this could strongly reduce the ‘‘Global
Warming’’ impact and give further value to the important advantages
of reduced amount of residues to final disposal in landfill
and immediate recovery of slags and metals.