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Substance Flow Analysis for Dioxin 2002

3 Formation and turnover by energy production activities

In Denmark energy production is based on a mixture of sources, primarily coal, natural gas, oil and biomass, besides also waste incineration, wind and sun energy. This chapter focuses on fossil fuels and biomass. Emission of dioxin from combustion processes involving such materials is well documented by several studies. As no studies, to the best of knowledge, so far have indicated any natural content of dioxin in these materials, dioxin emission must be assumed entirely to be due to "De Novo synthesis" during the combustion process and flue gas treatment operations.

Since the substance flow analysis from 2000 the information from the source /dk-TEKNIK 2000/ has been completed and published under the name of: "Measurements of dioxin emissions from selected secondary sources" /Schleicher et al. 2001/. The remaining information from this investigation is included in this update of the substance flow analysis, and the data that was already included have been revised because of minor differences between the temporary data used and the data published in the report /Schleicher et al. 2001/. In this report is therefore referred to /Schleicher et al. 2001/.

3.1 Coal power plants

Consumption of coal and coke in Denmark in 2001 accounted for approx. 175,000 TJ or approx. 6.9 million tonnes /Energistyrelsen 2002/, of which approx. 95% was used for production of electricity and heat by central power plants. The remainder was primarily used for energy supply for manufacturing purposes.

Coal incineration will result in around 13 - 15 % residuals, primarily fly ash and to a lesser extent slag, bottom ash, gypsum and other desulphurization products.

Dioxin formation and disposal

Previous Danish measurements of dioxin formation by coal combustion dates back to before 1990 and did not detect dioxin /Nielsen and Blinksbjerg 1989/. In /Jensen 1997/ dioxin emission to air from coal combustion in Denmark has been estimated at 2 g I-TEQ/year corresponding to an emission factor of 0.2 µg I-TEQ/ton, whereas an estimated 40 g I-TEQ was collected and deposited as production residues. The 40 g collected as residues was estimated based on rather old measurements of total dioxin in fly ash transformed by analogy considerations to N-TEQ.

From an investigation in 1999 at a Danish coal powder power plant an emission to air of 4.7 pg I-TEQ/m³ (n,t at 5.8% oxygen) was reported /Fyns Amt 2000/. This emission corresponds to an emission factor of 33 ng I-TEQ/ton coal. The plant in question (Section 7 at Fynsværket) can with respect to the temperature pattern over the flue gas treatment system and dust removal in general be taken as representative to around 99% of the Danish consumption of coal for energy generation /Elsam-Projekt 2000a/.

At the same power plant, but in another section, dioxin measurement was made in July 2001. This section is firing with a mixture of coal, fuel oil, natural gas, meat and bone meal and straw. The sample from July was only made on coal and meat and bone meal. The result of the measurement shows an emission of 0.0006 ng I-TEQ/Nm³. The yearly emission and the emission factor have not been calculated because of the different fuel types used.

The European Dioxin Inventory (section on Germany) gives air emission factors for electricity generation by coal power plants in the range of 1.06 - 7.01 µg I-TEQ/TJ. Assuming a conversion factor of 25 GJ/tonne of coal this equals a range of 0.027 - 0.18 g I-TEQ/t. For residential heating the Inventory (section on Germany) states air emission factors of (Landesumweltamt Nordrhein-Westfalen 1997):

Measurements from the Netherlands on a coal power plant and an industrial coal combustion plant gave air emission factors as follows /Bremmer et al 1994/:

However the given international figures are substantially higher than the Danish measurement from Section 7 at Fynsværket. It is therefore chosen to consider the Danish measurement as valid for the Danish combustion of coal in power plants. To comply with the uncertainty from this estimate, the emission level will be used with an uncertainty of ± factor 10.

The amount of residues from coal combustion generated in Denmark in 2000 was approx. 1.2 million tons which are 100 % recycled for cement, concrete, filling and other purposes. /Danish EPA, 2001/.

No measurements of dioxins in residues from Denmark exist, and literature figures are scarce. /Dyke et al 1997/ quote figures of 0.02 - 13.5 ng I-TEQ/kg for grate ash and 0.23 - 0.87 ng I-TEQ/kg for filter dust from cyclones and bag filters. The residues measured originate from industrial plants and may thus not be representative to residues from large coal power plants.

Flyash from electrostatic filters and bag filters is the dominant residue developed in Denmark. Assuming a figure of 0.2 - 0.9 ng I-TEQ/kg to be valid to the total amount of residues generated in Denmark, the amount of dioxins collected with these residues may be roughly estimated at 0.2 – 1.1 g I-TEQ/year. It is noted that this estimate is considerably below the previous estimate for Denmark (40 g I-TEQ – see the beginning of this section) which is due to different data sources. Recognizing that neither of the data sources likely are representative of the coal types and operating conditions found at coal power plants in Denmark today, the choice is made here to accept a range of 0.2 – 40 g I-TEQ/year as the best estimate of the dioxin amount collected with residues from coal combustion in Denmark.

3.2 Other fossil fuels

Other fossil fuels cover natural gas and oil products. The consumption of these energy products in Denmark in 1998 can be summarised as follows /Energistyrelsen 2002/:

Around 80% of the consumption of natural gas was used for industrial processes, power generation and other large-scale uses, whereas the remaining 20% mainly was used for residential heating /Energistyrelsen 2002/.

The Danish consumption of orimulsion is approximately 33,400 TJ or 1,094,000 tons /Energistyrelsen, 2002/ (included in the 3.95 million tonnes)

Dioxin formation and disposal

In connection with the data collection for the update of the substance flow an analysis for dioxin a measurement from combustion of orimulsion from 1997 has shown up.

The measurement shows a dioxin emission of 0.0124 ng I-TEQ/Nm³, which correspond to an annual emission from combustion of orimulsion at approximately 38 - 343 mg I-TEQ/year, assuming an uncertainty of ± factor 3 (reference is made to section 1.5). The emission factor is approximately 0.0001 mg I-TEQ/ton orimulsion used.

No other measurements of dioxin emission related to combustion of oil and natural gas has been undertaken in Denmark.

The European Dioxin Inventory (section on Germany) gives air emission factors for electricity generation by natural gas in the range of 0.02 - 0.03 g I-TEQ/TJ. Assuming a conversion factor of 40 GJ/1000 Nm³ this equals a range of 0.0008 - 0.0012 ng I-TEQ/Nm³. For residential heating the Inventory (section on Germany) states air emission factors of /Landesumweltamt Nordrhein-Westfalen 1997/:

Assuming that these data are representative to the qualities and processes used in Denmark, and an average density for oil products of 0.9 kg/l can be applied, the dioxin emission to air can be estimated at:

Adding the emission for orimulsion the total emission from other fossil fuels is equal to 0,4 - 1,3 g I-TEQ/year.

The total emission for other fossil fuels is a little higher than the estimate from SFA 2000 /Hansen, 2000/, where the total emission was 0.14 - 0.46 g I-TEQ/year.

No knowledge concerning the content of dioxins in soot/ash from combustion of natural gas or oil products seems to exist. The amount of soot/ash generated is, however, small. Soot/ash will be directed to landfills. Emissions to wastewater should be regarded as negligible.

3.3 Biomass

The major biomass fuels are straw and wood. There are following types of wooden fuels: firewood, forest wood chips, wood pellets, wood briquettes and wood waste, including bark.

Straw and woods are used as fuels mainly in private homes, district-heating plants and in central and decentral, combined heat and power plants (CHP).

The total energy production by biomass fuels was estimated at 33,601 TJ in 1998 (see table 3.1).

Table 3.1
Energy production in Denmark 1998 based on biomass / Energistyrelsen 2000/

An ongoing study by the Center of Biomass Technology for the Danish Energy Agency has estimated the number of biomass installations in 1998 as shown in table 3.2:

Table 3.2:
Rounded numbers of biomass installation in Denmark 1998.

The knowledge and assessments related to the different types of installations are presented in the following.

3.3.1 Private wood stoves

The number of wood stoves in private homes in Denmark is estimated to be about 400,000 stoves. An investigation from the beginning of the 1990s /Houmøller 1995/ showed that 33% of the woods consumed in these stoves were good qualities of hardwood from forestry. The rest included wood from private gardens, replacement of old hedges, industrial surplus wood etc. Paper, cardboard, milk cartoons, painted and impregnated wood waste (reference is made to section 3.6.1) and perhaps also plastics are known to be used to a certain degree, but there are no available studies and therefore no precise knowledge of these partly illegal customs in Denmark. Attention should e.g. be paid to the fact that the ordinary blue colours used in newspapers, on milk packaging etc. are typically based on copper pigments, which can act as catalysts of the dioxin formation. It should also be noted (see below), that the typical temperatures present in the stoves as well as the chimney belongs to the interval more or less optimal for dioxin formation.

The Danish Environmental Protection Agency has in 2001 introduced a campaign on private wood stoves and dioxin. The effect of this campaign can however not be evaluated as measurements on flue gas from private wood stoves are not present on a larger scale.

Plant activity

No flue gas cleaning is installed for this type of equipment.

Dioxin formation and disposal

In the first Danish study /Dyrnum et al. 1990/ the total dioxin emission was estimated at 32 g N-TEQ/year with an uncertainty range of 10 - 50 g N-TEQ/year based on an annual wood consumption of 222,000 tons. The flue gas concentrations were <200 ng total dioxin/Nm³ for hardwood, about 1000 ng total dioxin/Nm³ for waste briquettes and about 65,000 ng total dioxin/Nm³ for PCP-treated wood. It was assumed that burning 1kg wood would generate 8.6 Nm³ flue gas. N-TEQ was assumed to correspond to 1.5% of total dioxin.

In a more recent Danish study /Hansen et al. 1994/ the emission concentration from burning hardwood and softwood under controlled representative conditions in commonly sold Danish wood stoves ranged 5.8 - 53 ng total dioxin/Nm³ or quite similar to the previous study. The average was 12 ng total dioxin/Nm³ or 0.18 ng N-TEQ/Nm³. The emission factor was 1.9 µg N-TEQ/tonnes wood. The total consumption of wood for stoves was 214,000 ton/year in 1992 and based here upon the total emission was estimated at <0.4 g N-TEQ/year ± 60%. In 1995 the Danish consumption of firewood had increased to 578,231 tons and the dioxin emission correspondingly to 1.1 g N-TEQ/year.

In a new Danish investigation clean birch and dried clean excess wood from manufacturing was fired in a new stove /Schleicher et al. 2001/. The testing covered for both types of wood ordinary firing as well as night firing. Night firing covers the practice of adding a large amount of wood at one time and adjusting the air supply to a minimum in order to allow the fire to continue the night over. In all cases 6-hours' sampling covering lightning as well as operation was performed. Ordinary firing gave a dioxin emission (to air) of 5.1 µg I-TEQ/tonnes wood for birch and 1.9 µg I-TEQ/tonnes wood for excess wood. Night firing gave emissions factors of 0.61 µg I-TEQ/tonnes wood for birch and 0.64 µg I-TEQ/tonnes wood for excess wood. The results for ordinary firing and night firing are mean values of two samples made at the same time. There is no official explanation why ordinary firing with clean birch results in a higher dioxin emission than the ordinary firing with excess wood.

In 1993 the Swedish Environmental Protection Agency reported an emission factor for stoves of 0.13 - 0.3 g N-TEQ/tonnes wood burned /Swedish EPA 2000/.

In the Netherlands the emission factors for wood stoves and open wood fire places ranged 1.0 - 3.3 µg I-TEQ/tonnes dry clean wood and 13 - 29 g I-TEQ/tonnes dry clean wood, respectively /Bremmer et al. 1994/. In Switzerland wood stoves were estimated at emit 0.77 (open door)-1.25 (closed door) µg I-TEQ/tonnes clean wood and 3,230 g I-TEQ/tonnes household waste /Schatowitz et al 1994, quoted by Swedish EPA 2000 and US Dioxin Inventory 1998/.

In the most comprehensive German study /Bröker et al. 1992/ the emission factor for stoves burning clean wood was typically 0.71 g I-TEQ/tonnes wood and ranged 0.53 - 0.94 g I-TEQ/tonnes wood. Burning of wood at open fireplaces resulted in a lower typical value of 0.46 g I-TEQ/tonnes wood and a range of 0.07 - 1.25 g I-TEQ/tonnes wood. In another study with inclusion of 30% paper as fuel the dioxin emission concentrations raised about five times /Launhardt et al. 1996/.

The European Dioxin Inventory has assessed the existing investigations published up to the middle of the nineties (including the investigations described above) and has adopted the following default air emission factors for domestic wood combustion /Landesumweltamt Nordrhein-Westfalen 1997/:

This assessment is here accepted as a reasonable illustration of the variations caused by different types of combustible materials used in wood stoves. The Danish results from day and night firing correspond quite well to the value for clean wood. Considering that the dominant part of the material burned in Denmark is clean wood, but that other materials to some extent will also be included, it is deemed fair to expect the overall picture to be somewhat between a clean wood situation and a slightly contaminated wood situation. An activity of approx. 430,000 tonnes/year burned and an air emission factor of 1-50 µg I-TEQ/ton equals a total air emission of 0.43 – 22 g I-TEQ/year.

The National Environmental Research Institute has in Novewmber 2002 conducted a dioxin measurement of air in a housing area with several private wood stoves. The measurement showed a dioxin content of 70 femtogram I-TEQ/m³ (femtogram = 10^-15g), which is almost three times as much as the measured level in rural area /Vikelsøe, 2003/.

Residues

The National Environmental Research Institute has in 2002 made a measurement of the dioxin content in ash from a wood stove. This measurement shows a dioxin content of 0.03 ng I-TEQ/kg dry matter /The Environmental Research Institute, 2002. /Dumler-Gradl et al 1993 & 1995 quoted by Dyke et al 1997/ gives figures of 75 – 500 ng I-TEQ/kg ash and 500 – 9000 ng I-TEQ/kg soot for a wood based household heating system. They also give soot values of 4 – 42000 ng I-TEQ/kg for a household heating system using a mixture of wood, coal and waste. In the last case the maximum value is related to wood burning only. The mean concentrations of dioxin in soot from various wood stoves and ovens were 1.4 - 3.5 g I-TEQ/kg soot /Dumler-Gradl et al. 1995 quoted by US Dioxin Inventory 1998/. In Canada the dioxin content in soot from wood stoves was 211 ng/kg / US Dioxin Inventory 1998/.

Assuming an amount of ash of approx. 4,300 tons and a dioxin content of 0.03 – 500 ng I-TEQ/kg ash, the amount of dioxin to be disposed of with ash is estimated at 0.0001 – 2.2 g I-TEQ/year. This ash will be disposed of with other household waste or spread in gardens.

Soot from chimneys will normally be removed by the chimney sweeper. The amount of dioxin collected and disposed in this context has not been estimated, but most of the soot will be directed to incineration, as it is normally disposed of with the household waste /Schleicher, 2003a/.

3.3.2 Other biomass combustion plants

A significant amount of other biomass combustion plants is operating in Denmark partly as a result of a Danish policy to develop the utilisation of biomass for energy generation. Generally, the materials combusted in biomass plants will be clean materials. However, it must be assumed that a number of plants will also use materials to some extent contaminated by glue, paint or plastics or perhaps disposable pallets or other types slightly contaminated by PCP. No precise knowledge on this issue is available, and it is not possible to quantify the extent to which the materials combusted are contaminated.

Plant activity

The activity of other plants for energy generation from biomass in Denmark is summarised in table 3.3. It may be noted that the activity of farm boilers has been reduced in the last 10 years, as 580,000 tonnes straw was burned at 11,000 farms in 1989.

Dioxin formation and emission factors

As the raw materials, operation conditions as well as flue gas cleaning varies between the different types of biomass combustion plants, it should be expected that dioxin formation and emission would likely vary also. However, as indicated in the following only few investigations on the different types of plants are available. The data available are presented in the following:

Concerning farm boilers for straw an early investigation by /Nielsen and Blinksbjerg 1989/ reported the very low emission concentration of 0.016 ng Eadon-TEQ/Nm³ and an emission factor of 5 ng Eadon-TEQ/GJ. That will correspond to about 3 ng I-TEQ/GJ. The translation factor from Eadon-TEQ to I-TEQ for this source was about 0.6. In this study the dioxin emission was hundred times greater by burning straw bales than loose straw.

Table 3.3
Activity of other plants generating energy from biomass in Denmark

In a newer study (Jensen & Nielsen 1996) three farm boilers using full bales, sliced bales and grated bales, respectively, were investigated. The emission concentrations from the two first mentioned boilers were below the detection limit (<0.02 ng N-TEQ/Nm³). From the grated bale boiler the air emission concentration was calculated to 16 ng N-TEQ/m³ (n, t) at 10% O₂. The airflow was about 600 m³/h and the load was 41 kg/h, thus the emission flux was about 9.6 µg N-TEQ/h corresponding to an air emission factor of 230 g N-TEQ/tons straw. As an average for all 3 plants investigated, the emission factor can be calculated to 77 g N-TEQ/tons straw.

A new Danish investigation from 2000 on one farm boiler using full bales gave air emission factors of 5.3 - 9.2 g I-TEQ/tons straw /Schleicher et al. 2001/.

Concerning district heating the study of /Jensen & Nielsen 1996/ also covered three district heating plants using straw. The emission concentrations from two of them were above the detection limit (0.01 ng N-TEQ/m³). The two plants in question can be briefly characterised as follows:

Measured dioxin levels:     0.01/0.44 ng N-TEQ/m³ (n, t) at 10% O₂,

All three plants had cyclone and bag filters installed.

Based on these few data, emission factors of an average 1.7 g N-TEQ/ton and min./max. factors of 0 and 5 g N-TEQ/ton can be calculated for straw at district heaters with flue gas cleaning.

A new Danish investigation from December 1999 on a straw-based district heating plant (6.3 MW) equipped with cyclone and bag filter for flue gas cleaning gave air emission factors of 24, 21 and 24 ng I-TEQ/ton straw /Schleicher et al. 2001/. The investigation was originally based on 4 samplings, but one of the samplings was discarded, so only results from 3 samplings, each lasting for 6 hours, are recorded. The temperature of the flue gas over the filter ranged from 110 to 120ºC.

Combustion of wood chips (dry excess wood from furniture manufacturing) and crushed chipboards (inclusive glue, plastic or paper coating and misc. additives) in a district heating plant (6.3 MW) was investigated in summer 2000 /Schleicher et al. 2001/. The plant was equipped with electrostatic filter for flue gas cleaning, and the temperature over the filter ranged within 110 - 120ºC. For each type of fuel 4 samplings each lasting 6 hours were undertaken. However, 2 out of the total 8 samplings were later assessed as contaminated. For wood chips the air emission factors for the remaining 3 samplings were determined as 18, 19 and 46 ng I-TEQ/ton chips respectively, whereas the factors for crushed chipboards were determined as 19, 23 and 35 ng I-TEQ/ton chipboard /Schleicher et al. 2001/.

One Danish investigation for small stoker boilers based on wood pellets is available. The investigation was carried out on a new boiler using 6 hours' sampling. The dioxin emission (to air) reported ranged within 0.21- 0.53 g I-TEQ/ton pellets /Schleicher et al. 2001/.

Measurements of dioxin emission from 3 Danish central or de-central CHP-plants were undertaken in autumn 1999. From each plant 3 measurements of 2 hours representing normal operation were undertaken. The fuel was mainly straw, but 2 of the plants also used wood chips. Based on energy content, wood chips accounted for up to 35% of the fuel consumption. The flue gas temperatures ranged between 99 and 129ºC. All plants are undertaking flue gas cleaning by electrostatic filter (1 plant) or bag filter (2 plants). The dust emission of all plants is 10 mg/ Nm³ or below. The dioxin concentrations reported range between 0.4 and 5.3 pg I-TEQ/Nm³ /ELSAMprojekt 2000b/. Assuming 10 Nm³/kg of straw or chips, emission factors of 4 – 53 ng I-TEQ/ton can be calculated.

For stokers burning wood slightly contaminated by glue, PUR and other kinds of plastics and operating cyclones for flue gas cleaning Dutch investigations / Bremmer et al. 1994/ reports air emission factors of 3 - 8 g I-TEQ/ton wood with a best estimate of 5 µg I-TEQ/ton wood.

For industrial wood combustion including combustion in boilers, gas turbines and stationary engines the European Dioxin Inventory – section on Germany - reports air emission factors of 1 – 500 g I-TEQ/TJ for clean wood and 0.75 – 6,200 µg I-TEQ/TJ for contaminated wood /Landesumweltamt Nordrhein-Westfalen 1997/. Assuming a conversion factor of 20 GJ/ton wood these emission factors can be expressed as 0.02 – 10 g I-TEQ/ton wood for clean wood and 0.015 – 125 µg I-TEQ/ton contaminated wood.

The European Dioxin Inventory /Landesumweltamt Nordrhein-Westfalen 1997/ – section on United Kingdom – reports air emission factors of 17 – 50 µg I-TEQ/ton for straw burning. These factors are partly based on /Nielsen and Blinksbjerg 1989/ referred above. Furthermore air emission factors of 1-2 µg I-TEQ/ton for clean wood burning and 9 – 19 g I-TEQ/ton for burning of treated wood are reported.

The US air emission factors for various industrial wood-fired boilers were between 0.5 - 1.3 µg I-TEQ/tonnes. Regards burning of wood stored in seawater the emission factor rose to 17 g I-TEQ/tonnes (EPA draft report 1998).

In December 2000 measurements were carried out on a Danish 51 MW combined heat and power station, which fires with industrial wood waste. The flue gas is purified through an electrostatic precipitator. The two measurements resulted in a mean dioxin emission of 0.016 ng I-TEQ/Nm³. The average emission factor of two simultaneous measurements was 95 ng I-TEQ/ton wood./Schleicher et al., 2001/

Based on these data the following air emission factors are adopted for the current situation in Denmark:

These factors are deemed appropriate for assessing the total emission in Denmark, but it may well be the case that the emission for some biomass plants will be outside the range stated. The factors are argued as follows:

For straw burning without flue gas cleaning which mainly addresses farm boilers the emission factors reflect the actual Danish experience as described above. For straw burning with flue gas cleaning, one is considering partly CHP plants and partly district heating plants. Again the emission factors reflect the actual Danish experience as described above.

Wood burning without flue gas cleaning deals with small stoker boilers operated by small companies and individuals and fired with pellets, chips and for some boilers also crushed chipboards and larger pieces of wood. The emission factors adopted are partly based on the Danish measurement described above and recognise that the dominant type of fuel will be wood pellets, but do also pay respect to the possibility that part of the chips or wood otherwise used could be contaminated.

Wood burning with flue gas cleaning is relevant to district heating plants and CHP plants, in particular industrial CHP plants. The flue gas cleaning facilities relevant will be cyclones and bag filters and to a lesser extent electrostatic precipitators, whereas real dioxin abatement is not assumed to be used. Generally district heating plants will be designed and operated to maximise heat extraction, and the temperature of the flue gas over the filter will typically be close to 100ºC and will certainly not exceed 200ºC. The same applies to most CHP plants. Furthermore, the plants should be expected to be in control of the materials burned. It has not been investigated to what extent district heating plants have permission to burn contaminated materials. However, as burning of materials classified as waste (e.g. chipboards) is financially less attractive due to the Danish waste fee system, it seems fair to assume that this practise is not widespread for district heating plants. Industrial CHP plants will be designed to burn wood waste from the manufacturing activities that may include chipboards, sawdust, bark etc. but occasionally also other materials like paper depending on the design of the individual plant /Schleicher et al. 2001/. The emission factors adopted reflects the few Danish measurements described above besides paying respect to the possibility that combustion of contaminated materials could take place to a limited extent.

Combustion of biomass and air emission of dioxin can based on table 4.3 and the emission factors adopted above be summarised as follows:

The total emission of dioxins to air from biomass combustion plants in Denmark can thus be estimated at 0.3 – 19.4 g I-TEQ/year, corresponding to 0.3 -15 g I-TEQ/year for smaller plants and 0.03 - 4.4 g I-TEQ/year for larger plants.

It is noted that the air emission of dioxins from combustion of biomass (other sources than wood stoves) in Denmark in a previous report /Jensen, 1997/ has been estimated at 0.07 - 6.6 g I-TEQ/year (straw burning) and 0.25 g I-TEQ/year (wood burning).

Residues

Seven measurements of dioxin content in ash from four straw furnaces have been made by the National Environmental Research Institute in 2002. The highest measurement shows a dioxin content of 12.4 ng I-TEQ/kg dry matter and the lowest result is 0.2 ng I-TEQ/kg dry matter. Assuming that the data are normal distributed the mean value equals 3.7 ng I-TEQ/kg dry matter and the true average can be expected to be found in the range 0.38 - 7 ng I-TEQ/kg dry matter, using a 90 % confidence level.

Furthermore two measurements have been made on ash from two district heating stations firing with straw showing a dioxin content of 0.1 and 0.7 ng I-TEQ/kg dry matter respectively. Two measurements of the dioxin content in ash from two woodchip-fired district heating stations have also been carried out. The dioxin content in the ash from the two stations was measured to be 0.03 ng I-TEQ/kg and 1.4 ng I-TEQ/kg dry substance respectively. The 90 % confidence level of these four measurements corresponds to <0.01- 1.3 ng I-TEQ/year.

The new Danish measurements indicate that the dioxin concentration in residues from smaller plants, as farm boilers and smaller stoker boilers, is higher than the dioxin concentration in residues from larger plants.

The international data available are presented in the following:

The only study available on residues from straw combustion is a UK study concerning a whole bale straw combustor from which a concentration of 10 ng I-TEQ/kg grate ash was reported /Dykes et al. 1997/. The concentration was considered very low and caused by a high temperature and long residence time on the grate causing destruction of dioxins. As an estimate for assessing the situation in the UK covering both good and poor combustors was adopted the range of 10 – 500 ng I-TEQ/kg ash /Dykes et al. 1997/.

On wood a few studies are available:

Burning of natural wood in different wood combustion systems concentrations of 0.23 – 1.12 ng I-TEQ/kg in bottom ash and 117 – 272 in filter ash has been reported / Oehme & Müller 1995/. The same study reported concentrations of 22 ng TEQ/kg bottom ash and 722 - 7620 ng TEQ/kg in filter ash after burning a mixture of PCP-treated and untreated wood.

In a Swiss study /Wunderli et al 1996/ of natural wood incineration in installations of from 20 kW to 1.8 MW fly ash collected in cyclones and bottom ash contained only low levels of dioxins (0.6 - 8.5 ng I-TEQ/kg) and lower than bio compost. Fly ashes from waste wood incineration had much higher dioxin content of 700 - 21,000 ng I-TEQ/kg. If the combustion process has been efficient, bottom ashes were as low in dioxin concentration as ashes from clean wood incineration. Otherwise the concentration could be as high as for fly ashes.

At a German test facility for industrial wood combustion burning contaminated wood the dioxin content in filter dust and bottom ash ranged from 30 to 23,300 µg I-TEQ/ton dust and 30 - 3,300 g I-TEQ/ton ash, respectively /Landesumweltamt Nordrhein-Westfalen 1997/.

In a UK study of a boiler burning treated wood (however, not PCP-treated) was reported grate ash concentrations of 584 - 1090 ng TEQ/kg and grit ash levels of 891-1070 ng TEQ/kg /Dykes et al. 1997/.

For a stoker burning wood slightly contaminated by PUR, soot collected from the inside of the stack was reported to contain 0.2 g I-TEQ/kg /Bremmer et al. 1994/.

For larger plants the interval of <0.001 - 1.3 ng I-TEQ/kg dry matter results in an annual dioxin content of <0.001 - 0.03 g I-TEQ/year.

The total of the two intervals for annual dioxin content is considerably lower than the estimate used in SFA 2000 /Hansen, 2000/, where the dioxin quantity was added up to 0.04 - 36 g I-TEQ/year based on international values.

As ash from farm boilers often is spread on soil the dioxin content to soil could be up to 0.005 – 0.09 g I-TEQ/year, as ash from farmboilers correspond to 12,800 tons of the 13,700 tons of ash from smaller plants.

3.4 Summary

The assessments and estimates related to formation and turnover of dioxins by energy production activities in Denmark by the end of the nineties and is presented in section 3.1 to 3.3 are summarised in table 3.4.

Table 3.4
Summary of formation and turnover of dioxins by energy production activities in Denmark