Treatment and use of air pollution control residues from MSW incineration: An overview

Abstract

This work reviews strategies for the management of municipal solid waste incineration (MSWI) residues, particularly solid particles collected from flue gases. These tiny particles may be retained by different equipment, with or without additives (lime, activated carbon, etc.), and depending on the different possible combinations, their properties may vary. In industrial plants, the most commonly used equipment for heat recovery and the cleaning of gas emissions are: heat recovery devices (boiler, superheater and economiser); dry, semidry or wet scrubbers; electrostatic precipitators; bag filters; fabric filters, and cyclones. In accordance with the stringent regulations in force in developed countries, these residues are considered hazardous, and therefore must be treated before being disposed of in landfills. Nowadays, research is being conducted into specific applications for these residues in order to prevent landfill practices. There are basically two possible ways of handling these residues: landfill after adequate treatment or recycling as a secondary material. The different types of treatment may be grouped into three categories: separation processes, solidification/stabilization, and thermal methods. These residues generally have limited applications, mainly due to the fact that they tend to contain large quantities of soluble salts (NaCl, KCl, calcium compounds), significant amounts of toxic heavy metals (Pb, Zn, Cr, Cu, Ni, Cd) in forms that may easily leach out, and trace quantities of very toxic organic compounds (dioxin, furans). The most promising materials for recycling this residue are ceramics and glass–ceramic materials.

The main purpose of the present paper is to review the published literature in this field. A range of studies have been summarized in a series of tables focusing upon management strategies used in various countries, waste composition, treatment processes and possible applications.

Introduction

The solid particles produced during municipal solid waste incineration (MSWI) in mass burning units may be grouped into bottom ashes (BA) and fly ashes (FA). Fly ashes are defined by the International Ash Working Group (IAWG, 1997) as “the particulate matter carried over from the combustion chamber and removed from the flue gas stream prior to addition of any type of sorbent material”. Incorrect terms are sometimes used in the literature when it is appropriate to identify the ash according to the unit that contributes to the waste (Sabbas et al., 2003). In our assessment, only studies using the following terminology have been taken into account:

• Heat recovery system ash, which is collected in hoppers below the boiler, superheater and economiser (sometimes referred to as fly ash – FA);

• Electrostatic precipitator ash (ESP);

• Fabric filter or baghouse ash (FF);

• Dry scrubbing residues (DS), semi-dry scrubbing residues (SDS) or wet scrubbing residues (WS), with sorbents such as Na₂CO₃, Ca(OH)₂ and activated carbon;

• Cyclone ash (CA);

• Air pollution control (APC) residues, which may include FA and the solid material captured downstream from the acid gas treatment units and before the gases are released into the atmosphere.

Therefore, the solid waste considered here consists mainly of particles produced in mass burning units, which are carried over by flue gases with or without chemical additives (sorbents). It includes combinations from different units, which will be referred to in the next sections as FA, ESP, FF, DS, SDS, WS, CA and APC residues.

Environmental legislation is becoming increasingly restrictive, and consequently industrial plants have to be constantly adapted to enhance control of gas emissions, in particular. Depending on the units used for post-combustion control, the resulting solid waste may have different characteristics, which means that different management strategies may be used. The next sections examine the strategies adopted in some countries where incineration processes are generally used for the management of MSW, and discuss the treatment and application of possible wastes (FA, ESP, FF, DS, SDS, WS, CA, APC). Some directions for future research in this field will also be suggested.