Toolkit for Identification and Quantification of Releases of Dioxins, Furans and Other Unintentional POPs
PART II Emission Factors
Source Group 3 Power Generation and Heating
This source group includes power stations, industrial firing places (furnaces) and installations for providing space heating, which are fired with fossil fuels (including up to 1/3 co-combustion of waste), biogas including landfill gas, and biomass only. Table II.3.1 outlines the five categories within this source group. The main release vectors are air and residue. Land is considered as a release vector only in the case of domestic heating and cooking using either biomass (mostly wood) or fossil fuels. Releases to land can occur if residues are dumped on the ground.
As generation of heat or power is the aim of these plants, in the case of the combustion of biomass or fossil fuels, the amount of PCDD/PCDF cannot easily be equated to mass (in tons) or energy input (in Joule) of fuel burned. The preferred basis to report emissions of PCDD/PCDF would be the energy input of the fuel. As the heat or power output is the “product” of the processes in this group, the default emission factors derived from the available data are linked to the heating value of the fuel. Thus, instead of reporting default emission factors in μg TEQ/t of fuel, these factors are given in μg TEQ/TJ of heat input. The reason for this approach is the high variety of fuels used for power generation. The range of heating values of various coals from various parts of the world stretches over more than one order of magnitude. To recalculate heating values into masses, conversion tables are provided in Annex 28.
Table II.3.1. Overview of source categories included in group 3 - Power Generation and Heating
3 - Power Generation and Heating Potential release route Source categories Air Water Land Product Residue a Fossil fuel power plants (coal, oil, gas, shale oil, and co-combustion of waste) x x b Biomass power plants (wood, straw, other biomass) x x c Landfill, biogas combustion x x d Household heating and cooking with biomass (wood, other biomass) x (x) x e Household heating and cooking with fossil fuels (coal, oil, gas) x (x) x
Table II.3.2. Relevance to Article 5, Annex C of the Stockholm Convention
No Toolkit source category Part II Part III Relevant source category in Annex C 3a Fossil fuel power plants (coal, oil, gas, shale oil, and co-combustion of waste) X Co-incinerators of waste 3a Fossil fuel power plants (coal, oil, gas, shale oil, and co-combustion of waste) X Fossil fuel fired utility and industrial boilers 3b Biomass power plants (wood, straw, other biomass) X Co-incinerators of waste 3b Biomass power plants (wood, straw, other biomass) X Firing installations for wood and biomass 3c Landfill, biogas combustion X Co-incinerators of waste 3d Household heating and cooking with biomass (wood, other biomass) X Co-incinerators of waste 3d Household heating and cooking with biomass (wood, other biomass) X Residential combustion sources 3e Household heating and cooking with fossil fuels (coal, oil, gas) X Co-incinerators of waste 3e Household heating and cooking with fossil fuels (coal, oil, gas) X Residential combustion sources 3e Household heating and cooking with fossil fuels (coal, oil, gas) X Fossil fuel fired utility and industrial boilers
An example of elaborating source inventory and release estimate for this source group is included in the example inventory 4.
Six classes are defined within this category according to the types of fuels used, namely coal, heavy fuel oil, shale fuel oil, peat, light fuel oil and natural gas, as well as any type of fossil fuel in a combination with the co-combustion of any kind of waste or sludge. For all classes, it is assumed that reasonably well-operated and maintained power steam generators are employed in order to maximize power output. In all cases, air and residue are the only release vectors under consideration.
Fossil fuel is burned in a wide array of devices for power generation ranging from small stoker fired furnaces to large and highly sophisticated boiler/burner systems with extensive air pollution control plants at the back end. Coal combustion for power generation takes place in two general types of boilers distinguished by the way the ash is extracted from the system. The so-called dry bottom boilers use stokers or pulverized coal burners arranged in an opposed wall, which burn coal in a highly efficient manner leaving the majority of the ash as a dry residue at the bottom of the boiler. The so-called wet bottom boilers use pulverized burners in a cyclone or U-fired arrangement, which leads to much higher combustion temperatures resulting in the ash melting and collected as a liquid slag at the bottom of the boiler. Typical flue gas cleaning devices for large coal-fired power plants consist of units for NOx control (e.g. SCR technology), particulate matter control (e.g. electrostatic precipitators) and desulphurization (e.g. lime stone scrubbing). These devices can reduce PCDD/PCDF emissions as a side effect.
Heavy fuel oil is also combusted for power generation purposes. It is usually burned in specially designed burners incorporated in the boiler walls. The formation of PCDD/PCDF is favored during co-combustion of liquid or sludge wastes such as waste oil and/or used solvents.
Light fuel oil and natural gas are always fired in specially designed burners and are not likely to generate large amounts of PCDD/PCDF since both are highly calorific, clean burning fuels with little or no ash at all. Only if liquid or sludge waste is co-fired, higher releases of PCDD/PCDF may be formed.
In some countries such as Australia, Brazil, Canada, China, Estonia, France, Russia, United Kingdom (in Scotland), South Africa, Spain, Sweden, and the USA large quantities of oil shale exist, which can be converted to shale oil, a substance similar to petroleum. In Estonia, for example, more than 90% of the country’s electricity is generated from shale oil (Schleicher 2004a). In some countries peat is a domestic energy resource and is used for heat and/or power generation, e.g. in Finland or Ireland (McGettigan et al.2009).
Like in all combustion processes, PCDD/PCDF are usually formed after the combustion process is completed and the flue gas cools down. The remaining soot particles and the chlorine contained in the coal recombine in the presence of the metal-chloride catalysts to form PCDD/PCDF. Releases to water, land and product are normally negligible. Major release routes are to air and residue, especially to fly ash. Releases to water may occur at plants where wet scrubbers are installed, where water is not recirculated within the scrubbers. In such cases, releases to water have to be included. Sludge from such scrubbers, when separated from the effluents, will occur under “Residues”. In the case of wet limestone scrubbing for desulfurization, the resulting gypsum is used in building industries and may be considered as “Product”.
In some countries, catalysts are marketed for the combustion of soot and boiler cleaning. These catalysts contain copper salts and lead to a significant increase of PCDD/PCDF formation to air and residues. Measurements from Poland show an increase of emissions in such cases by a factor of 1,000.
PCDD/PCDF emission factors for six source classes are listed in Table II.3.3. The emission factors apply to the operation of boilers in general and therefore include the combined heat and power production as well as the production of heat only. Revised or newly added emission factors are highlighted in red. Detailed information on how these emission factors have been derived can be found in Annex 30.
Table II.3.3 PCDD/PCDF emission factors for source category 3a Fossil Fuel Power Plants
|3a||Fossil Fuel Power Plants||Emission Factors (µg TEQ/TJ fossil fuel burned)|
|1||Fossil fuel/ waste co-fired power boilers||35*||ND||NA||NA||ND|
|2||Coal fired power boilers||10**||ND||NA||NA||14|
|3||Peat fired power boilers||17.5||ND||NA||NA||ND|
|4||Heavy fuel fired power boilers||2.5||ND||NA||NA||ND|
|5||Shale oil fired power boilers||1.5||ND||NA||NA||***|
|6||Light fuel oil/natural gas fired power boilers||0.5||ND||NA||NA||ND|
* including co-firing of biomass (range: 30-50 µg TEQ/ TJ).
** high range depending on fuel quality and combustion conditions (3-100 µg TEQ/TJ).
*** Releases with residues can be calculated on a mass basis (see Annex 30, Section on Release in Residues).
The classification of sources is according to the type of fossil fuel used. A further split depending on the size and type of technology in place is not proposed here due to lack of reliable information. Such a split may be introduced at the national level depending on available data. The source allocation shall fit to the corresponding activity rates (see below). Therefore, national energy statistics and their source category split are essential for source classification.
Class 1 For the co-firing of waste, the allocation to this class depends on the main purpose of the process (here: heat and power generation, not waste incineration). Co-firing usually occurs with solid fuels (coal, lignite) together with sewage sludge, biomass, organic waste from industry or other waste-derived fuels. The co-combustion of different types of gases (e.g. coke oven gas and blast furnace gas) is not considered here. The combustion of sewage sludge together with liquid fuels or natural gas should be allocated to waste incineration.
Class 2 The emission factors refer to the combustion of hard coal. In case of information lacking at the national level, this factor may be transferred to lignite fired boilers. Emission factors can be significantly higher in the case of unfavorable combustion conditions (Grochowalski and Konieczynski 2008, see Annex 30).
Class 3 The emission factor refers to peat combustion in boilers for heat and/or power production. Peat is used in countries where it is domestically available.
Class 4 The emission factor refers to heavy fuel oil combustion in boilers for heat and/or power production. Heavy fuel oil is a fraction from mineral oil refining with standardized properties. Residual oils or other residues from the refining process are not considered here.
Class 5 The emission factor refers to shale oil combustion in boilers for heat and/or power production. Shale oil is used in countries where it is domestically available.
Class 6 The emission factor refers to the combustion of natural gas or light fuel oil in boilers for heat and/or power production. This factor may be transferred to the combustion in gas turbines or in combined cycle power plants as well.
The activity rates for this category can be derived from national energy statistics. For PCDD/PCDF release estimates, only a split according to the fuel type is proposed in the Toolkit. Combustion in boilers for heat and/or power production occurs in various economic sectors. Here, energy industries are the most important ones. In other industrial sectors, combustion may occur in boilers or in other types of process furnaces which need to be distinguished (e.g. drying of products, other heating furnaces). A detailed knowledge of technologies in place may be necessary for an appropriate allocation of the energy input to such processes. The respective industries’ associations may provide such information.
Co-firing of waste usually cannot be found in energy statistics, which only include the total amount of incinerated waste. In most cases, the actual quantity needs to be directly collected from the power plant operators. If no statistical data exist, estimates may be based on a smaller sample at the local scale, with an extrapolation of results to the whole country.
The levels of confidence were assigned based on a lack of knowledge of the sources and the variability of the emissions from a given source. The latter is linked to fuel quality and operating conditions. The variability of emissions becomes lower with higher fuel quality (e.g. natural gas compared to co-firing of waste) and optimized plant operation (e.g. large power plants compared to small boilers). The level of confidence is high for natural gas (due to high range of data from many literature results, high variability of emissions observed) and decreases to low for solid fuels, in particular for the co-firing of waste (low geographical coverage).
The results should be cross-checked with regard to the consistency of the overall inventory (share of the sector in total emissions; per capita emissions compared to countries with a similar structure).
Many countries and regions are heavily dependent on the combustion of biomass for power and heat production. Biomass fuels may include wood including twigs, bark, saw dust, wood shavings, peat, and/or agricultural residue (e.g., straw, citrus pellets, coconut shells, poultry litter, camel excretes, etc.). In most cases, biomass is burned directly and without any addition of fossil fuels in small, continuously operated steam boilers. For the Toolkit, four classes are defined within this category according to the type of biomass used, namely wood-fired boilers using clean wood or mixed biomass, and other types of herbaceous biomass-fired boilers, namely straw and other agricultural residues. Agricultural residues like straw or rice husk are an important fuel type in many countries. This herbaceous biomass often has higher chlorine content compared to wood, leading to problems during combustion (e.g. slagging) but also to potentially higher formation of PCDD/PCDF. Therefore emission factors are distinguished from those for wood combustion.
For all classes, it is assumed that reasonably well-operated and maintained power steam generators are employed in order to maximize power output. In all cases, air and residue are the only release vectors. This category does not address firing of contaminated wood waste, which is covered by category 1f Waste Wood and Waste Biomass Incineration.
Biomass is burned in a wide array of devices for power generation ranging from small stoker fired furnaces to large and highly sophisticated boiler/burner systems with extensive air pollution control plants at the back end. The various types of biomass furnaces with typical applications and fuels are presented in the BAT&BEP Guidelines.
PCDD/PCDF emission factors for four source classes are listed in Table II.3.4. Revised or newly added emission factors are highlighted in red. Detailed information on how these emission factors have been derived can be found in Annex 31.
Table II.3.4 PCDD/PCDF emission factors for source category 3b Biomass Power Plants
|3b||Biomass Power Plants||Emission Factors (µg TEQ/TJ biomass burned)|
|1||Mixed biomass fired power boilers||500||ND||NA||NA||ND|
|2||Clean wood fired power boilers||50||ND||NA||NA||15|
|3||Straw fired boilers||50||ND||NA||NA||70|
|4||Boilers fired with bagasse, rice husk, etc.||50**||ND||NA||NA||50|
* Total of bottom ash and fly ash.
** Estimate based on straw combustion, Thailand: Installation with APC (ESP, cyclones, Venturi scrubbers): ca. 20 µg TEQ/ TJ.
Class 1 includes boilers firing wood waste which is not contaminated by paints or coatings. In some countries, classifications of wood waste exist which refer to the level of contamination. Here, mixed biomass refers to the category characterizing low contamination. This type of wood waste is frequently used in CHP boilers e.g. in wood industries. Incineration of contaminated wood waste shall be allocated to category 1f (waste wood and waste biomass incineration).
Class 2 includes boilers firing log wood, wood chips or pellets as a high quality fuel allowing optimized combustion conditions.
Class 3 includes boilers firing straw for heat or power production. Straw-fired boilers need to be adapted to this fuel with regard to ash properties (slagging) and combustion conditions.
Class 4 includes boilers firing various types of herbaceous biomass such as rice husk or bagasse. Especially in Asian countries a wide range of agricultural residues is used for heat generation. Nevertheless, information on PCDD/PCDF emissions from this source is still scarce.
Biomass used for heat and power generation should be indicated in the national energy statistics. Nevertheless, biomass is frequently merged with other fuels and allocation to the four classes will require additional information such as the installed capacity of biomass-fired boilers, statistics on agricultural production and waste statistics. Additional assumptions may be necessary such as the share of straw used for energy recovery.
In class 2 the level of confidence is high with high quality fuels burned in good operation conditions (clean wood), due to the wide range of data and availability of many literature results. Low confidence is linked with class 4, for which fuels are not well defined, operating conditions may be unknown and experimental data on PCDD/PCDF emissions is scarce. For classes 1 and 3 the level of confidence is estimated medium due to lower data range.
Landfill gas and biogas are both generated from anaerobic digestion of organic matter. The resulting gas is a mixture of carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), and ammonia (NH3), and smaller fractions of combustible gases as well as a large fraction of water (H2O). The combustible portion of the gas is usually around 50% and the heating value is 15–25 MJ/kg depending on the origin of the gas. The combustion of landfill gas and biogas either occurs in a flare, in gas motors or turbines and other power generating devices.
The combustion of these gases for power generation takes place predominantly in either gas-fired boilers or gas motors/turbines. Both systems closely resemble to those firing natural gas. The combustion process is virtually residue-free.
One PCDD/PCDF emission factor for a single source class is listed in Table II.3.5. Detailed information on how this has been derived can be found in Annex 32.
Table II.3.5 PCDD/PCDF emission factors for source category 3c Landfill Biogas Combustion
|3b||Landfill Biogas Combustion||Emission Factors (µg TEQ/TJ gas burned)|
|1||Boilers, motors/turbines, flaring||8||ND||NA||NA||NA|
Class 1 This class includes the combustion of biogas resulting from anaerobic digestion (see above).
This class includes various activities requiring different sources of information:
The quality of the biogas (and potentially the variability of emissions) depends on the origin of the gas. Landfill gas may have contaminants from volatile compounds of the surrounding waste leading to higher POPs emissions. Gas quality can be better controlled in dedicated installations e.g. for the digestion of agricultural residues. The assigned LoC is medium, due to limited number of datasets available.
Heating and cooking with biomass in residential households is a common practice in many countries. In most cases the fuel of preference is wood, however, other biomass fuels may be used such as straw, peat, etc. Six individual classes are defined within this category, with the main difference being the quality of the fuel and the appliances used. This differentiation comes from the need for representing simple stoves or 3-stone stoves which are widely used especially in developing countries. Air, residue, and in some cases land are the release vectors under consideration.
Biomass for residential heating and cooking is burned in a wide array of devices ranging from small, open pit stoves and fireplaces to large and highly sophisticated wood burning stoves and ovens. The latter are addressed in the “advanced technology” class. The combustion of biomass for household heating and cooking takes place predominantly in devices of increasing combustion efficiency as the gross national product and the state of development of individual countries increase.
PCDD/PCDF are formed as a result of incomplete combustion, typical in these small devices with no or limited combustion controls. Releases to water and product are negligible. Releases to land can occur only if the combustion process takes place directly on the ground (this case is addressed in Group 6 Open Burning Processes) or when residues are disposed of to the land. Thus, the only significant release routes are to air, land, and residue.
Recent studies showed comparatively low emission factors for open fire simple stoves (Cardenas et al. 2011). Nevertheless, simple stoves can lead to high exposure rates with negative impacts on human health through indoor air pollution.
In some countries, catalysts are marketed for the combustion of soot and boiler cleaning. These catalysts contain copper salts and lead to a significant increase of PCDD/PCDF formation to air and residues. Measurements from Poland show an increase of emissions in such cases by a factor of 1,000 (Grochowalski 2010, 2012).
PCDD/PCDF emission factors for six source classes are listed in Table II.3.6. Revised or newly added emission factors are highlighted in red. Emission factors for other unintentional POPs are listed in Annex 33. Detailed information on how default emission factors have been derived can also be found in Annex 33.
Residues from biomass combustion are generated with a rate of 0.5 – 5% per mass of biomass burned. Ash contents for different types of wood vary from 0.1 up to 3%. Details on various types of wood can be found in Annex 28.
Table II.3.6 PCDD/PCDF emission factors for source category 3d Household Heating and Cooking with Biomass
|3d||Household Heating and Cooking with Biomass||Emission Factors (µg TEQ/TJ biomass burned)||Concentration (ng TEQ/kg ash)|
|1||Contaminated biomass fired stoves||1,500||ND||ND||NA||1,000|
|2||Virgin biomass fired stoves (advanced technology)||100||ND||ND||NA||10|
|3||Straw fired stoves||450||ND||ND||NA||30|
|4||Charcoal fired stoves||100*||ND||ND||NA||0.1|
|5||Open-fire 3-stone stoves (virgin wood)||20**||ND||ND||NA||0.1|
|6||Simple stoves (virgin wood)||100||ND||ND||NA||0.1|
* Preliminary expert estimate; Emissions from barbecuing are not included.
** Expert estimate derived from a field test in Mexico (Cardenas et al. 2011).
Class 1 includes all types of stoves firing contaminated biomass such as wood waste, painted wood, etc. The actual emissions will depend on the degree of contamination and the combustion conditions.
Class 2 includes ovens and stoves with controlled air supply and optimized combustion conditions firing virgin wood. This class applies usually to residential heating with biomass in modern appliances. Lower emissions are expected from automatic furnaces using wood chips or pellets.
Class 3 applies to all types of residential combustion using herbaceous biomass as a fuel such as straw. In case of mixed fuels (e.g. wood and straw) the class with the higher emission factor shall apply.
Class 4 applies to all types of residential combustion using charcoal as a fuel.
Class 5 applies to residential combustion of wood without control of combustion conditions and without ducts for the evacuation of flue gases. Traditional 3-stone stoves are a typical example.
Class 6 applies to simple stoves for heating or cooking with limited combustion control and with a duct for the evacuation of flue gases.
Biomass use in the residential sector is often not covered by statistical data. Especially informal wood markets are not registered. If possible local studies should be conducted on the amount of biomass used, as well as technologies in place. Results from such studies may be extrapolated to the national level. In case of absence of such data, results from countries with similar structure may be transferred e.g. via per capita consumption of biomass. The use of waste in residential appliances is an illegal practice in many countries. Here, expert estimates have to be made to quantify emissions from this source. Some countries have developed case studies on this topic and results may be used as a first indication.
There are multiple sources of uncertainty associated with the emissions from the residential sector. Activity rates are uncertain due to incomplete coverage of statistical data (see above). PCDD/PCDF emissions are strongly dependent on fuel quality and combustion conditions. Both parameters are largely varying and are often unknown at the national level. Therefore, the level of confidence is estimated low for all classes (due to limited data availability but wide range of values) except class 2 with clean fuel and controlled combustion conditions (confidence: medium). For the latter, emission factors are derived based on many studies available, including a wide range of values.
Fossil fuel is used extensively for domestic heating, especially in developed countries and in countries with economies in transition. Coal, (light fuel) oil and (natural) gas are the main sources of fossil fuel used for domestic heating. For these three classes, it is assumed that reasonably well-operated and maintained heating ovens are employed in order to maximize heat output. In the case of co-firing of waste and/or biomass, combustion conditions may degrade due to lower fuel quality. In all cases, air is the release vector under consideration. In the case of coal combustion, residue must also be considered as a potential release vector.
Fossil fuel is burned in devices ranging from small stoker fired furnaces to large elaborate highly sophisticated boiler/burner systems for central heat generation in large multi unit residential buildings.
Combustion for domestic heating takes place in two general types of boilers distinguished by the way the heat is transported and released. The so-called central heating systems, which use oil or gas as a fuel, include one large furnace to heat water, which then is circulated through the building to release its heat in numerous decentralized radiators. These modern systems are typically highly efficient and fairly clean, leaving little or no residue for disposal. The second type of heating system is mostly based on solid fuels (coal) and consists of individual stoves, which are located in each room of the building or inside the wall to provide direct access to several rooms at the same time. These stoves consist of fairly small furnaces but provide a system for air to circulate inside the stove around the furnace. These systems are typically older, less efficient and less clean. In addition, bottom ash resulting from the inert content of the fuel is generated and must be disposed of. Some of these systems are also capable of burning oil.
In some countries catalysts are marketed for the combustion of soot and boiler cleaning. These catalysts contain copper salts and lead to a significant increase of dioxin formation for both release routes air and residues. Measurements from Poland show an increase of emissions in such cases by a factor of 1,000.
PCDD/PCDF emission factors for six source classes are listed in Table II.3.7. Revised or newly added emission factors are highlighted in red. Detailed information on how these emission factors have been derived can be found in Annex 34.
Table II.3.7 PCDD/PCDF emission factors for source category 3e Household Heating and Cooking with Fossil Fuels
|3e||Household Heating and Cooking with Fossil Fuels||Emission Factors (µg TEQ/TJ fossil fuel burned)||Concentration (ng TEQ/kg ash)|
|1||High chlorine coal/ waste/biomass co-fired stoves||1,700*||ND||NA||NA||5,000|
|2||Coal/waste/biomass co-fired stoves||200||ND||NA||NA||NA|
|3||Coal fired stoves||100||ND||NA||NA||5|
|4||Peat fired stoves||100||ND||NA||NA||NA|
|5||Oil fired stoves||10||ND||NA||NA||NA|
|6||Natural gas fired stoves||1.5||ND||NA||NA||NA|
* Pandelova et al. 2005
Class 1 applies to domestic stoves firing coal with high chlorine content (chlorine salt content above 0.5% mass). High chlorine salt contents are a specific property of certain domestic coals. Information on properties of coals and briquettes marketed in a country for domestic use needs to be taken into account.
Class 2 applies to domestic stoves using mixed solid fuels. In most cases this category applies to the simultaneous or alternating firing of coal and biomass. Nevertheless, co-firing of waste in residential appliances is an illegal practice in many countries.
Class 3 applies to domestic stoves, ovens and boilers firing coal or coal briquettes with low chlorine content.
Class 4 applies to domestic stoves, ovens and boilers firing peat. The use of peat as a fuel in the residential sector is closely linked to its local availability.
Class 5 applies to domestic stoves, ovens and boilers firing light fuel oil. The use of heavy oil fractions in the residential sector is often banned.
Class 6 applies to domestic stoves, ovens and boilers firing natural gas. The same factor can be applied to light petroleum gas and similar fractions.
Classes 1, 2, 3, 5 and 6 are usually covered by national energy statistics. Information on coal properties may be available from associations of coal suppliers or from case studies on this topic. Specific investigations may be necessary to quantify domestic peat consumption as it may be produced in an artisanal way. Expert estimates have to be made to quantify emissions from the combustion of mixed solid fuels. Energy statistics do not address the share of co-firing in total fuel consumption. The use of waste in residential appliances is an illegal practice in many countries. Some countries have nevertheless developed case studies on this topic and results may be used as a first indication.
In this category, the level of uncertainty is directly linked with the fuel quality. The level of confidence is high in the case of natural gas combustion (class 6). This is due to the use of clean fuel and high stability of the process. Low confidence levels can be attributed to the combustion of mixed solid fuels in particular in the case of co-firing of waste (classes 1 and 2), due to the low stability of the process and wide range of data. Medium levels are estimated for classes 3, 4 and 5, due to the better defined fuel composition but wide range of data.