Organic micro-pollutants from waste incineration
Bergström, J., Öberg, T.
Miljökonsulterna, Report MKS-86/26, 1986..

Abstract
Environmental Consultants at Studsvik have carried out measurements on waste incineration at Bäckelundsverket, Borlänge and evaluated results from published measurements conducted in Sweden and abroad. The assignment was commissioned by the National Energy Administration within the framework of the investigation on energy from waste (ENA investigation).

The goal of the assignment is to provide the ENA investigation with a basis for evaluating the influence of the operational conditions on the release of organic compounds and nitrogen oxides from waste incineration.

The release of nitrogen oxides from waste incineration is usually of the same order as the release from wood fuel-fired boilers. The release is usually 1.5 kg per ton of waste, reckoned as NO₂. This is equivalent to 0.15 g/MJ of fuel energy. The measurements taken in Borlänge show that an optimization of incineration by reducing the 0₂ content in the flue gas and raising the energy density in the furnace can reduce the release of nitrogen oxides by half.

Waste incineration in all Swedish plants is combined with energy production. The resulting energy is used for district heating in most cases. The systems for flue gas condensation, which are now being introduced into Sweden, aim to increase energy recovery without increasing fuel consumption. Flue gas condensation can in certain cases be combined with flue gas cleaning but does not yield a lower release than dry flue gas cleaning systems.

A very efficient oxidation of the organic substances in waste takes place during waste incineration in Swedish furnaces. The particulate size of waste varies and there is a big difference in composition. In spite of this, usually less than 2% of the organic substance in the waste leaves the furnace with slag and fly ash. More than 99.5% of amount of burnt waste oxidizes into carbon dioxide and water. The quantity of non-oxidized gases which leaves the furnace along with the flue gas mainly consists of carbon monoxide and the same type of light hydrocarbons that occur with all incineration. Waste incineration is notable, in comparison with wood fuel firing, for its essentially higher chlorine content. About 75% of the chlorine in the fuel occurs as hydrochloric acid in flue gas. Less than one pro mille of the chlorine produces chlorinated aromatics. The reactions, which form these organic micro-compounds, are generally unknown.

Polychlorinated dibenzodioxins (PCDD) and dibenzofurans (PCDF) comprise a small part of the chlorinated aromatics that are produced. In spite of this fact, they attract a large amount of interest. Of a total of more than 200 possible PCDDs and PCDFs, Sweden has chosen to describe the emission by a weighed mean value of 12 isomers, which are considered to be especially toxic. Calculated according to a model developed by Eadon, the TCDD equivalents that are released with the flue gasses amount to between 1 and 50 ng per m³ of flue gas. Taking into consideration the uncertainty of the method of measurement, the differences are very small and do not allow for the influence of individual, or combinations of parameters which control production in incineration. The incineration conditions have a greater influence on many of the other organic miro-compounds, such as, polycyclic aromatic hydrocarbons (PAH). PCDD and PCDF have a high boiling point, and therefore bind to dust particles and are separated with the fly ash. The release with the flue gases is considered to comprise half of the quantity that is produced in the oven, however, large differences are reported from the different plants. The quantity of TCDD equivalents that form during incineration and the quantity supplied by the waste are still very uncertain.

Three factors can be considered when limiting the production of chlorinated aromatics during waste incineration. These are:

1. The chlorine content during incineration
2. Combustion efficiency
3. Energy density in the furnace

In addition to changes in the quantity of chlorine in the fuel, the practical measures that are possible in new and old furnaces are compatible with those that have been suggested in order to increase energy utilization and to minimize the released PAH. The measures to reduce the production of chlorinated aromatics are the same for incineration in grate fired and fluidized bed combustion.

The homogeneous composition of the fuel, the particle size and heating value are recognized factors, which are of great importance to the possibilities of carrying out good combustion in FBC-boilers. The same type of requirements influences grate firing. The importance of the fact that the fuel, furnace construction and operational parameters are suited to each other is demonstrated very clearly by the measurements performed during the ENA investigation.

Waste contains much more chlorine than is required to produce the quantities of chlorinated aromatics that arise during incineration. The measures for reducing the production of chlorinated aromatics should be focused on eliminating reactive forms of chlorine. In order to achieve this, extensive research and product development are required.

In spite of this, it is already possible to influence the fuel, and by air heating, raise the energy density in the furnace so that the operating conditions lead to better combustion efficiency will result in smaller production of chlorinated aromatics. However, there are as yet no methods for waste incineration that totally exclude the production of chlorinated aromatics, which can be applied on a large scale during the 1980's.

As far as is known, flue gas cleaning is not used for reducing the quantity of pollutants in incineration. Flue gas cleaning prevents the spread of certain pollutants with the flue gases. It is generally accepted that the pollutants are less harmful in solid or liquid products. Flue gas cleaning, involving only dry waste products, and cleaning equipment that condenses the moisture in the flue gases, can both be dimensioned to the same separation degree by the pollutants. Flue gas cleaning combined with condensation of the flue gases can be motivated by the considerably increased energy recovery, which is possible. The energy recovery can be increased as much as 25% without increasing the emission from the waste incineration. Release with the flue gas during the incineration of Swedish municipal waste with both dry and wet cleaning systems can be calculated as shown in the table, provided that a fabric filter is included in the system. The emissions refer to the standard value when the best incineration and flue gas cleaning technique is used. The emission levels cannot be translated to the general limit values for a separate plants.

The emission levels with flue gas during the incineration of Swedish municipal waste, using the best available technique

	Quantity in flue gas m3 sdg 10% CO2	Release g/ton waste
Dust	10 mg	50
HCl	40 mg	200
HF	1 mg	5
SO2 as sulphur	100 mg	500
NOx as NO2	200 mg	1000
Hg	80 µg	0.4
Cd	20 µg	0.1
Pb	300 µg	1.5
Zn	1000 µg	5
PAH	<0.1 µg	<0.5 x 10-3
TCDD-equivalents (Eadon)	0.1 ng	0.5 x 10-6

First of all, investment in research is required in order to determine how the results given in the table can be reached. The measures must be chosen so that the result is reached with low cost rise in waste treatment and energy production. The report concludes with a survey of the most important R+D investments.