Heavy metals are mainly produced during combustion/incineration processes. Heavy metals exert a range of chronic and acute toxic health effects including carcinogenic, neurological, hepatic and renal. Toxic effects associated with heavy metals generally occur at higher concentrations. Based on the source of heavy metals type of the metal emitted to the ambient air varies. These heavy metals are given in detailed below:
Mercury and mercury compounds:
Mercury can currently still be found in municipal waste, notably in the form of batteries, thermometers, dental amalgam, fluorescent tubes or mercury switches. Separate collection of these can help reduce overall loads in mixed MSW but collection rates of 100 % are not achieved in practice.
Mercury is a highly toxic metal. Without adequate air pollution controls, the incineration of mercury containing wastes can give rise to significant emissions.
In hazardous waste incineration, there are several specific streams that may contain increased concentrations of mercury in the received waste:
- Tars from coking plants,
- Waste from chlorine alkaline electrolysis (amalgam process),
- Caustic oil sludge from refineries,
- Chemicals containing mercury
The form of the mercury emissions depends strongly on the chemical environment in the flue gas.
A balance between metallic mercury (Hg) and HgCl2 normally develops. Where there is a sufficiently high concentration of HCl in the flue-gas (in relation to the reduction agent SO2) mercury will mainly be contained in the flue-gas as HgCl2. This can be separated from the flue gas significantly more easily than metallic mercury. If, however, HCl is contained in the flue gas at lower concentrations (e.g. in sewage sludge incineration plants) mercury exists in the flue-gas mainly in metallic form and is then more difficult to control.
The combustion temperature also influences HgCl2 formation.
Metallic mercury is virtually insoluble in water (59bg/l at 25 °C). Mercuric (II) chloride is much more soluble at 73 g/l. Mercury (II) chloride can therefore be separated in wet scrubbers, whereas the separation of metallic mercury requires further flue-gas treatment stages.
Cadmium and thallium compounds:
Common sources of cadmium in municipal waste incineration plants are electronic devices (including accumulators), batteries, some paints and cadmium-stabilised plastic. Thallium is virtually non-existent in municipal waste.
Hazardous wastes may contain high concentrations of Cd and Tl compounds. Effluent treatment sludge and drummed wastes from metal plating and treatment may be significant sources. Cadmium is highly toxic and can accumulate in the soil.
Other heavy metal compounds:
This term comprises the heavy metals antimony, arsenic, lead, chromium, cobalt, copper, manganese, nickel, vanadium, tin and their respective compounds. European and many national regulations, thus, group them together for emission measurement requirements. This group contains carcinogenic metals and metal compounds such as arsenic and chromium (VI) compounds, as well as metals with toxicity potential.
The retention of these metals depends largely on an effective separation of dust as they are bound in dust due to the vapor pressures of their compounds, as contained in the flue-gas (mainly oxides and chlorides).
Techniques for the reduction of mercury emissions:
Mercury is highly volatile and therefore almost exclusively passes into the flue-gas stream. The limit value set in the EU waste incineration directive is 0.05 mg/m3. Limit values as low as 0.03 mg/m³ as a daily average value (with continuous monitoring) have been set in some European Member States.
Metallic mercury can be removed from the flue-gas stream by:
- Transformation into ionic mercury by adding oxidants and then deposited in the scrubber,
- Direct deposition on activated carbon, hearth furnace coke, or zeolites,
High chlorine total loads (approx. 4 % w/w input) and a therefore high interim Cl2 supply lead to high Hg chlorination levels and Hg deposition of close to 100 %. With lower chlorine loads the Hg deposition degree reduces rapidly.
Techniques for the reduction of other emissions of heavy metals:
Other heavy metals in incineration are converted mainly into non-volatile oxides and deposited with flue ash. Thus, the main techniques of relevance are, therefore, those applicable to dust removal. Activated carbon is reported to be also used for reducing heavy metals emissions.
As it is described above, Santes offers two basic treatment methods for the removal of heavy metal:
- Dry Scrubber
- Wet Scrubber
Dry scrubber system is composed of two steps. First one is application of the activated carbon application through the gas originating from combustion process for the removal of heavy metals. At this stage dioxin and furan can be removed from the flue gas stream as well. Second one is the bag filter which holds unreacted reagent and the reacted hazardous content of the combustion gas with the use of Bag Filters. Therefore, for an effective heavy metal removal, there are two units used in Santes Flue Gas Treatment Systems; namely:
- Bag Filter
The reactor relies on active carbon as absorption medium for the reaction to adsorb heavy metals and dioxin &furans. Dioxins and furans, having been destroyed in the post combustion chamber, can form again in the subsequent heat exchanger because of the cooling procedure. Activated carbon is added to the lime to absorb dioxins and Hg.
In dry sorption processes the absorption agent (activated carbon) is fed into the reactor as a dry powder. The dose rate of activated carbon may depend on the temperature as well as on reagent type and brand. The reaction products generated are solid and need to be deposited from the flue-gas as dust in a subsequent stage, normally a bag filter. Therefore, bag filters are placed to Santes Flue Gas Treatment Systems in order to remove reacted heavy metals and unreacted activated carbon from the Flue Gas.
Within Jet Pulse cleaning mechanism of the Bag Filter, diaphragm valves are used for each filter bag. There are also timers and pressure difference mechanism for the bag filters to manage the Jet Pulse period of the filter bags with the information taken by pressure difference mechanism.
Some of the heavy metals can be removed from the flue gas stream. Heavy metal removal efficiency is less than those in Dry Scrubbers in practice. However, in some cases wet scrubbers are applied for the removal of these compounds. Wet scrubbers especially used if other types of pollutants such as SO2 and HCl exist in the flue gas stream. There are 3 main types of scrubbers that can be used in the removal of heavy metals:
- Spray Tower
- Tray Tower
- Packed Bed Tower