Fly ash in Waste-to-Energy plants consists mainly of Air-Pollution-Control (APC) residues, usually mixed with boiler ash. This mixture amounts to 2 to 3 percent by weight of the original waste. It consists of inert, mineral particles, variably soluble salts (for example, NaCl), and heavy metal compounds (of which CdCl2 is readily soluble). The grain size is very fine; thus fly ash is very dusty.
The finest, incombustible particles of incinerated waste pass along with the flue gas out of the furnace and into the boiler. Since the flue gas velocity in the boiler is lower than in the furnace, some of the particles settle as boiler ash and are removed from the bottom hoppers of the boiler. The finest particles, however, pass on to the APC installation. When the flue gas cools in the boiler, various gaseous compounds—for example, evaporated heavy metals and their compounds, including zinc, lead, and cadmium chloride (ZnCl2, PbCl2and CdCl2), formed from hydrogen chloride (HCl) in the flue gas—condense into particles to form fly ash. Fly ash is either collected on its own, perhaps in an Electrostatic Precipitator (ESP), or together with the reaction products of APC processes.
APC residues from waste incineration facilities exist in a number of different varieties depending on the type of incinerator and flue gas cleaning equipment installed. The chemical composition of the residues also depends on the waste incinerated. Typically, however, APC residues are a very fine grained powder, ranging from light grey to dark grey.
Overall, two different types of residues exist, which apply to most modern waste incinerators worldwide:
Slaked lime is injected into the flue gas, either in dry form or as slurry. This is done to neutralize acidic components in the flue gas, and is typically done before removing the fly ash from the flue gas. Fly ash, reaction products and unreacted lime are typically removed in fabric filters. Activated coal may be injected for dioxin removal and removed together with the fly ash. Dry and semi-dry systems typically generate a single residue. Maximum byproducts are 3-5% per mass of MSW feed.
Fly ash is typically removed before neutralizing acidic components. After this, the flue gas is scrubbed in one, two, or a multistage arrangement of scrubbers. The scrubber solutions are then treated to produce sludge and gypsum. Wet systems typically generate more than one residue. Maximum byproducts are 2-3% per mass of MSW feed.
The main environmental concern with respect to APC residues is leaching of:
1. Easily soluble salts such as Chloride (Cl) and Sodium (Na): Although they are not toxic for humans in typical concentration levels, these components may significantly affect ecosystems and pollute drinking water resources.
2. Heavy metals such as Cadmium (Cd), Chromium (Cr), Copper (Cu), Nickel (Ni), Lead (Pb), and Zinc (Zn): Heavy metals and trace elements can potentially be in concentrations which are harmful for humans as well as for ecosystems.
3. Dioxins: Although dioxins and furans do not easily leach, release of these contaminants is of major concern because of their toxicity. In order to minimize environmental impacts, the release of the above contaminants should be reduced as much as possible.
Available techniques of reducing leaching produced by residues after final placement may be grouped as follows according to the main principle of operation:
Processes involving extraction and removal of specific components in the residues.
The main advantage of extraction and separation processes is the use of relatively simple techniques. The main disadvantage is the generation of metal and salt containing process water; this may, however, be utilized for further recovery.
Processes involving binding and immobilization of contaminants by means of chemical reactions.
The main advantage of chemical stabilization processes is a significant improvement of the leaching properties of the residues and the use of relatively simple techniques. The main disadvantage is the production of process water containing heavy metals and salts.
Processes involving physical binding and encapsulation of residues, and in some cases also chemical stabilization.
The main advantages of solidification techniques are the decrease of leaching and the improvement of the mechanical properties. Solidification techniques also often make use of relatively simple technology. The main disadvantages are that the physical integrity of the product may - depending on the choice of binder - deteriorate over time, and that mass and volume increase with the treatment.
Processes involving heating of the residues and changes of their physical and chemical characteristics
A main advantage is the production of a very dense and stable product with good leaching properties. Another very important aspect is the destruction of Persistent Organic Pollutants (POP’s) such as dioxins. The main disadvantages relate to high energy demands for the process and generation of flue gas containing volatile metals. Three major types of thermal treatment exist: plasma gasification/vitrification, melting, and sintering. Also, A pyrolysis solution is currently under research.
The overall aim has been to treat the residues so that landfill acceptance criteria are fulfilled, but also various material-related criteria may have to be fulfilled, for example in the case of utilization. The above techniques reflect different approaches to meeting these goals. The reasons for development of these different types of techniques, rather than using a single process worldwide, have been differences in local traditions, regulations, market conditions, and political focus.
However, even after the most sophisticated treatment process, a risk of leaching and thereby future release of contaminants from the processed residues remains. To minimize such a risk, the treated fly ash should be safely landfilled under controlled conditions (in a special cell/ash monofill). Only a limited number of recovery and utilization solutions for fly ash exist today. The main reason for the lack of commercially available recovery and utilization technologies is likely to be difficulties related to achieving satisfactory technical qualities of products based on APC residues (due to high content of salts and heavy metals) compared to readily available virgin materials.