Table 5 196 Waste incineration: sub-categories with primary pathways of releases of mercury and recommended inventory approach
Chapter
Sub-category
Air
Water
Land
Product
Waste/
residue
Main inventory approach
5.7.1
Incineration of municipal/
general waste
X
x
x
x
X
PS
5.7.2
Incineration of hazardous waste
X
x
X
PS
5.7.3
Incineration of medical waste
X
x
X
PS
5.7.4
Sewage sludge incineration
X
X
X
PS
5.7.5
Informal waste incineration
X
X
X
OW
Notes: PS = Point source by point source approach; OW = National/overview approach;
X - Release pathway expected to be predominant for the sub-category;
x - Additional release pathways to be considered, depending on specific source and national situation.
5.7.1Incineration of municipal/general waste
5.7.1.1Sub-category and process description
The mercury content in the general waste stream originates from three main groups of inputs: 1) intentionally used mercury in discarded products and process waste; 2) natural mercury impurities in high volume materials (plastics, paper, etc.) and minerals; and 3) mercury as a human-generated trace pollutant in high volume materials. The mercury concentrations are directly dependent on the inputs of mercury to the waste, and will therefore likely vary much between different countries and circumstances.
Refuse or municipal solid waste (MSW) consists primarily of household garbage and other non-hazardous commercial, institutional, and non-manufacturing industrial solid waste. In some countries, sewage sludge and pathogenic medical waste incinerated along with municipal waste.
MSW is sometimes incinerated (under controlled conditions as described here), while waste fractions dominated by mineral materials is generally deposited in landfills. The quantitative split between incineration and other treatments of combustible waste vary between countries.
The MSW may be burned without pre-treatment or may be treated for production of so-called 'refuse-derived fuel'. In the USA, refuse-derived fuel incinerators burn MSW that has been processed to varying degrees, from only removal of large, bulky and non-combustible items, to extensive processing to produce a well separated fuel suitable for co-firing in pulverized coal-fired boilers. Processing MSW to refuse-derived fuel generally raises the heating value of the waste because many of the non-combustible items are removed (US EPA, 1997a).
In some types of incinerators a part of the mercury may remain in part of the waste not fully incinerated and leave the incinerator with the grate ash. Generally, however, virtually all of the mercury present in the waste is converted to a vapour because of the high temperatures of the combustion process. The major part of the mercury leaves with the exhaust gas and the share of mercury input that is released as air emissions through the stack will be largely dependent on the control devices present. Poorly controlled facilities will have most releases going out through the stack in the form of mercury air emissions whereas in well controlled facilities, most mercury input will end up in the flue gas residues. The effectiveness of various controls is discussed below.
5.7.1.2Main factors determining mercury releases and mercury outputs
Table 5 197 Main releases and receiving media from incineration of municipal/general waste
Phase of life cycle
Air
Water
Land
Products
General waste
Sector specific
treatment/disposal
Controlled waste incineration
X
x
x
x
X
X
Notes: X - Release pathway expected to be predominant for the sub-category;
x - Additional release pathways to be considered, depending on specific source and national situation.
Important factors determining releases of mercury from this sub-category are the concentration of mercury in the wastes and the efficiency of the control devices (if present) to reduce mercury emissions.
The incineration technology and particularly the flue gas cleaning systems applied, determine the distribution of the output of mercury between air emissions, accumulation in solid incineration residues (grate ash) and gas cleaning residues, and releases to water (only indirectly to water via some flue gas cleaning technology types). Post-combustion equipment for flue gas cleaning, applied widely in many countries today, retains parts of the otherwise released mercury. The flue gas cleaning systems used are similar to those described for large coal combustions plants (mentioned in section 5.1.1), except for a possible additional (integrated) step involving injection and subsequent capturing of activated carbon (which adsorbs/absorbs some mercury). The activated carbon technology is used in some countries, for example the USA, Germany, Sweden, Denmark and Austria.
5.7.1.3Discussion of mercury inputs
Table 5 198 Overview of activity rate data and mercury input factor types needed to estimate releases from incineration of municipal/general waste
The mercury content of the MSW will depend on the use of mercury-added products in the country and the presence of specific collection systems for mercury containing waste products. Known sources of mercury in MSW include, among others, batteries, discarded electrical equipment and wiring, fluorescent lamps, teeth and other dental amalgam waste, paint residues, and plastics. Depending on the life-time of the products the sources of mercury in the waste will reflect the use of mercury for the different products a number of years before the assessment of mercury in the waste.
In the USA the sources of mercury to the waste stream have changed over time as a consequence of the changes in the mercury use pattern. Mercury batteries have, during the period from 1980 to 2000, accounted for the major part of mercury in products in the MSW in the USA (Table 5 -199). In 1989, it was estimated that about 88% of the total discard of mercury was from batteries. Of the 88%, about 28% was from mercuric oxide batteries and the remainder from alkaline and other batteries (US EPA, 1997a). However, the number of mercury-containing batteries consumed since the late 1980s has decreased significantly in the USA and probably many other countries, but as the total mercury content of the waste has decreased significantly, batteries in 2000 still accounted for more than 50% of the mercury in products in the waste stream (Table 5 -199).
As of 1989, 644 metric tons of mercury was reported discarded in the municipal solid waste stream in the USA, and the concentration of mercury in solid waste is reported to be in the range of less than 1 - 6 ppm by weight with a typical value of 4 ppm by weight (ppm = g mercury per metric tons waste). However, because of changes in mercury consumption, the quantity of mercury discarded in the municipal solid waste stream has decreased dramatically since 1989 to a level of about 157 metric tons in 2000 (Table 5 -199).
Mercury concentration in MSW in New Jersey in 2001 has been estimated to be in the range of 1.5 - 2.5 ppm (NJ MTF, 2002).
Table 5 199 Mercury in products in the MSW stream in the USA in 1980, 1989 and 2000 projected (based on Franklin Associates, Ltd. (1989), as cited by Yep et al., 2002)
The sources of mercury in MSW in Denmark in 1992/93 and 2001, respectively, are shown in Table 5 -200. In 1992/93 batteries accounted for more than half of the total content, similar to the results from the USA shown above. In 2001 the batteries accounted for only 27%, mainly due to a decrease in the content of mercury in alkaline and 'other' batteries. In 2001 mercury present as a natural impurity of the waste (natural trace element) accounted for 28% of the total mercury content in the waste. Please note that this contribution is not included in the sources of mercury in MSW in the USA shown in (Table 5 -199). As illustrated, the uncertainty of the estimates for each waste group is quite high even though the estimates are based on detailed substance flow analyses. The total mercury content of the waste decreased in the period from 0.4 - 1.2 ppm to 0.1 - 0.6 ppm (the actual mercury content is according to the studies most probably in the high end of the estimated ranges).
Table 5 200 Sources of mercury in MSW disposed of for incineration in Denmark 1992/93 and 2001 (Maag et al., 1996; Skårup et al., 2003)
Waste type
1992/93
2001
kg Hg/year
% of total
kg Hg/year
% of total
Teeth and miscellaneous dental waste
200 - 310
18
64 - 180
12
Light sources
4 - 20
1
19 - 110
6
Switches and relays
0 - 120
4
75 - 380
22
Thermometers
80 - 200
10
19 - 38
3
Monitoring equipment
0 - 40
1
19 - 47
3
Batteries
420 - 1,100
53
52 - 510
27
Mercury as impurity (trace element)
20 - 370
14
28 - 560
28
Total (rounded)
700 - 2,200
100
280 - 1,800
100
5.7.1.4Examples of mercury in releases and wastes/residues
Atmospheric mercury emissions from municipal waste combustors (MWCs) can to some extent be reduced by removing mercury adsorbed to particles from the flue gas by electrostatic precipitators (ESPs) and fabric filters (FFs). The mercury removal efficiency of the filters depends on the filter's capability for removal of small size particles. Acid gas reduction in the flue gas may also contribute to the mercury retention.
The removal efficiency of the controls may be enhanced by adsorbing the mercury vapours from the combustion chamber onto acid gas adsorbent material or other adsorbents and then removing the particle-phase mercury. The PM control devices most frequently used in the USA are electrostatic precipitators (ESPs). To achieve high mercury control, reducing flue gas temperature at the inlet to the control device to 175 ºC (or lower) is beneficial. Typically, newer MWC systems use a combination of gas cooling and duct sorbent injection (DSI) or spray dryer (SD) systems upstream of the particle removal device to reduce temperatures and provide a mechanism for acid gas control (US EPA, 1997a).
Under incineration conditions at temperatures above 850ºC and O2 content of 8-10% vol., the prevailing mercury species will be mercury chlorides (I and II) and elemental mercury (Velzen et al. 2002). The thermodynamically calculated chemical equilibrium for mercury in a typical flue gas containing HCL and SO2 shows that the major product between 300 and 700ºC is HgCl2, whereas above 700ºC elemental mercury is the dominant species. A summery of mercury removal efficiencies for different flue gas cleaning equipment in incinerators is shown in Table 5 -201 (Velzen et al., 2002). For the estimation it is assumed that the HgCl2/Hg(0) ratio is between 70/30 and 80/20. 'Special absorbents' (or adsorbents) added may be absorbents impregnated with sulphur or sulphur compounds or active carbon based adsorbents, which increase the sorption of mercury on particles.
Table 5 201 Mercury removal efficiencies of flue gas cleaning systems for waste incinerators
Circulating fluidized bed + fabric filter (special absorbents added) *1
130
90 - 99%
80 - 95%
87 - 98%
Velzen et al., 2002
ESP or FF + carbon filter beads
99%
Pirrone et al., 2001
ESP or FF + carbon injection
50 - >90%
Pirrone et al., 2001
ESP or FF + polishing wet
scrubber
85%
Pirrone et al., 2001
Notes - *1 Special absorbents may be absorbents impregnated with sulphur or sulphur compounds or active
carbon based absorbents, which increase the sorption of mercury on particles.
As shown in the table, simple electrostatic precipitators sometimes only have very low mercury removal efficiencies. Wet scrubbers or spray absorbents using limestone for acid gas removal has efficiencies of 55-65% and 44-52%, respectively. For obtaining high removal efficiency, >90%, the addition of special absorbents/adsorbents, most often activated carbon, is a requisite.
According to compliance tests recently conducted at 115 of the 167 large municipal solid waste incinerators in the USA, the average mercury control efficiencies for large municipal incineration plants was 91.5%. The average control efficiency at each site was based on a 3-test average determined by measuring the total flue gas concentration of mercury both before and after the control system at each site (injection of powdered activated carbon upstream of either a spray dryer and fabric filter baghouse, or a spray dryer and electrostatic precipitator) (UNEP, 2002).
The mercury eliminated from exhaust gases is retained in incineration residues and, for some types of filtering technology, in solid residues from wastewater treatment (from the scrubbing process). These residues are generally sent to landfills or – depending upon their content of hazardous materials and other characteristics – used for special construction purposes (gypsum wallboard, roadbeds or similar). In some cases such solid residues are stored in special deposits for hazardous waste, which are additionally secured with a membrane or other cover that eliminates or reduces releases by evaporation and leaching.
Some examples of the distribution of mercury in the different outflows from municipal waste incinerators are shown inTable 5 -202. Compared to the typical removal efficiencies shown in Table 5 -201, the ESPs of these incinerators have relatively high removal efficiency, through the retention of a larger part of the small-size particles.
Table 5 202 Examples of mercury removal efficiencies of flue gas cleaning systems for waste incinerators
Notes - *1 Indicated in the reference as "gas cleaning water"; it is not mentioned if the waste water is filtered,
and if the filter cake disposed of separately.
Atmospheric mercury emissions from MWCs in the USA have declined significantly over the past decade. These reductions were partly due to reduction of mercury in the wastes, but also partly due to improvement/enhancement of control technologies. In the early 1990s about 40 metric tons were released from MWCs, and by 2001 the atmospheric emission had declined to about 4 metric tons mercury (US EPA, 2001).
Current emission controls on New Jersey (USA) solid waste incinerators, which primarily consist of the injection of carbon into the particulate control device, remove an estimated 95% or more of the mercury from the exhaust gas. The carbon is eventually mixed with the ash. Based on information from the New Jersey task force, mercury remains adsorbed on the injected carbon and mercury releases from this residue are likely to be low (NJ MTF, 2002).
The US EPA developed atmospheric emission factors (EFs) for MWCs for the year 1994, as shown in Table 5 -203. The EFs for early years would likely be higher, and EFs for more recent years would likely be lower due to the decreased concentrations of mercury in the wastes.
Table 5 203 Average emission factors for municipal solid waste incinerators in the USA for 1994-1995 (based on US EPA, 1997a)
Combustor Type
Mercury concentration
g/dry m3 at
standard conditions,
at 7% O2
Average emission factors in
g/metric ton waste
MSW without acid gas control
340
1.4
MSW with acid gas control
205
0.83
MSW with acid gas control + carbon
19
0.077
Refuse-derived fuel without acid gas control
260
2.6
Refuse-derived fuel with acid gas control
35
0.34
Notes: Acid gas control includes SD, DSI/FF, SD/ESP, DSI/ESP, SD/FF, and SD/ESP configurations);
SD = spray dryer; DSI = duct sorbent injection; ESP = electrostatic precipitator.
5.7.1.5Input factors and output distribution factors
Based on the information compiled above on inputs and outputs and major factors determining releases, the following preliminary default input and distribution factors are suggested for use in cases where source specific data are not available. It is emphasized that the default factors suggested in this Toolkit are based on a limited data base, and as such, they should be considered subject to revisions as the data base grows. Also, the presented default factors are expert judgments based on summarized data only. In many cases calculating releases intervals will give a more appropriate estimate of the actual releases.
The primary purpose of using these default factors is to get a first impression of whether the sub-category is a significant mercury release source in the country. Usually release estimates would have to be refined further (after calculation with default factors) before any far reaching action is taken based on the release estimates.
Actual data on mercury levels in the waste - for example established through the procedures of this Toolkit - will lead to the best estimates of releases.
If no indications is available on the mercury concentration in the municipal waste, a first estimate can be formed by using the default input factors selected in Table 5 -204 below (based on the data sets presented in this section). Because concentrations vary so much, it is recommended to calculate and report intervals for the mercury inputs to this source category. The low end default factors has been set to indicate a low end estimate for the mercury input to the source category (but not the absolute minimum), and the high end factor will result in a high end estimate. The low end input factor is expected to be relevant for a situation where substantial parts of the waste products with high mercury concentration (thermometers, batteries, dental amalgam wastes, switches etc.) have been sorted out of the waste for separate treatment, and will therefore be present in lower numbers in the municipal waste. The high end input factor is expected to be relevant for situations where no such sorting takes place and most of the product waste with high mercury concentrations is therefore present in the municipal waste. As mentioned, the mercury levels in waste are of course also directly dependent on the consumption of mercury-containing products and materials in the country investigated.
Table 5 204 Preliminary default input factors for mercury in municipal solid waste
Material
Default input factors;
g Hg/metric ton waste;
(low end - high end) *1
Municipal solid waste (general "household" waste) *1
1 - 10
Notes: *1 The low end input factor is expected to be relevant for a situation where substantial
parts of the waste products with high mercury concentration (thermometers, batteries,
dental amalgam wastes, switches etc.) have been sorted out of the waste for separate
treatment, and will therefore be present in lower numbers in the municipal waste. The
high end input factor is expected to be relevant for situations where no such sorting
takes place and most of the product waste with high mercury concentrations is therefore
present in the municipal waste. As mentioned, the mercury levels in waste are of course
also directly dependent on the consumption of mercury-containing products and materials in the country investigated.
b) Default mercury output distribution factors
Table 5 205 Preliminary default mercury output distribution factors for municipal solid waste incineration
Emission reduction devices *1
Default output distribution factors, share of Hg input
Air
Water
Land
Products
General waste
Sector specific treatment/
disposal *4
None
1
?
?
PM reduction with simple ESP,
or similar
0.9
*3
?
?
0.1
Acid gas control with limestone (or similar acid gas absorbent) and downstream high efficiency FF or ESP PM retention
0.5
*3
?
?
0.5
Mercury specific absorbents and
downstream FF
0.1
*3
?
?
0.9
Notes: *1 PM = particulate material; FF = fabric filter; ESP = electrostatic precipitator; *2 Depending on the specific flue gas cleaning systems applied, parts of the mercury otherwise deposited
as residue may follow marketed by-products (for example road bed slags/ashes and fly-ash for cement
production);
*3 In case residues are not deposited carefully, mercury in residues could be considered released to land;
*4 May be landfilled at general waste landfill or at specially secured hazardous waste landfills.
c) Links to other mercury sources estimation
For the waste treatment sub-categories it is very important to keep in mind that the mercury content in the waste originates from 1) intentionally used mercury in discarded products and process waste; 2) natural mercury impurities in high volume materials (plastics, paper, etc.) and minerals; and 3) mercury as a human-generated trace pollutant in high volume materials. Note that parts of these mercury inputs may be directed to municipal, hazardous and medical waste.
The mercury releases to the environment and waste deposits from these sub-categories should therefore be seen as a consequence of mercury being present in the products used in society.
Similarly, the estimated mercury inputs to waste treatment sub-categories can be qualified through the quantification of mercury inputs to society with products and materials, as described in sections 5.4 - 5.6.
Calculated input totals from waste related mercury sources: To avoid double counting of mercury inputs with waste products in the input total in the Inventory Level 2 spreadsheet, only 10% of the mercury input to waste incineration sources, general waste deposition and informal dumping is included in the total for mercury inputs. These 10% represent approximately the mercury input to waste from materials which were not quantified individually in this Toolkit. These materials include such things as food wastes, paper, plastic, etc. which generally have very low mercury concentrations but very high volumes. The actual fraction of mercury from such materials, of the total inputs of mercury to waste, will vary between regions and very little data on this issue is available in the literature. Limited data from a Danish substance flow analysis (Skårup et al., 2003) for mercury indicate however, that this mercury fraction is small, in the range of some 2-20% of total mercury inputs to general waste.
5.7.1.6Source specific main data
The most important source specific data would in this case be:
In case mercury inputs to waste (through products etc.) can be estimated quite accurately, these input data can be used in the quantification of mercury releases from waste incineration. Note, however, that mercury inputs to incineration from mercury trace concentrations in high volume materials (plastics, paper, etc.) are not quantified individually in this Toolkit, and quantification of total inputs would therefore tend to be underestimated when using this approach.
As mercury inputs in waste are typically difficult to measure, or otherwise quantify accurately, the following data may likely give the best estimates of mercury releases/outputs from waste incineration:
Atmospheric releases: Measurements of average mercury concentrations in the flue gas combined with measurements of flue gas produced (per year) at average conditions;
Outputs to solid residues: Measurements of average mercury concentrations and amounts of residues produced per year for each relevant residue output stream (ashes/slags, flue gas cleaning residues, gypsum boards etc.);
Aquatic releases (if any): Measurements of average mercury concentrations in the aquatic discharges combined with measurements of the amounts discharged (per year) at average conditions.
See also advice on data gathering in section 4.4.5.
