Thermodynamic Study of Heavy Metals Behaviour During Municipal Waste Incineration

Abstract

T he incineration of municipal solid waste (MSW) contributes significantly to the presence of heavy metals in urban area aerosols. It is thus important to ascertain the quantities and chemical forms of the heavy metals (HM) that are emitted from the incineration plant stacks. The behaviour of HM, which depends strongly on the thermal and chemical environments, was investigated herein with a modelling approach, consisting of several parts. First, a refuse bed combustion model was developed for simulating on-grate MSW incineration. It describes most of the physico-chemical and thermal phenomena occurring during waste combustion. Second, results from the bed model were taken as boundary conditions to perform 3D simulations of the post-combustion zone and of the boiler. The case studied was of the Strasbourg incineration plant. Finally, the local thermal conditions and the local elementary compositions of gas and solid phases obtained from these simulations were used to carry out thermodynamic calculations of the speciation of HM at each point in the incinerator. The results for four metals (Cd, Zn, Pb, Cr) are presented, discussed and compared to available data. Predicted species are in agreement with observations for volatile metals, except lead, whose volatilization seems overestimated.

A particulate model of solid waste incineration in a fluidized bed combining combustion and heavy metal vaporization

“…Cooper and Hallett [3] modeled coal particle combustion in a fixed bed, which later inspired Menard et al's model [4]. This model takes into account the following heterogeneous reactions: coal combustion, CO gaseous oxidation, and mass transfer between the gas and solid phases and within the gaseous phase.…”

“…During the combustion of a waste particle, whether or not it contains HM, four phases can be distinguished [4]: pyrolysis, pyrolysis gas combustion, residual carbon combustion, and residual carbon gasification.…”

“…It uses an effective stoichiometry from the classical combustion reactions that lead to a CO-CO 2 mixture as the product. Arthur's law is used to calculate the ratio C CO =C CO 2 [4].…”

“…The enthalpy of pyrolysis is assumed to be zero; it is negligible in comparison to other thermal processes [4].…”

“…This can be done because of the uncertainty in quantifying the pyrolysis emissions and the weak importance of this step in the model. Kinetic relationships and parameters are identical to those used by Menard et al [4] (see Table 3). …”