Clean Incineration fact or fiction?

Unfortunately negative opinions still exist among the German public on waste to energy.

Waste incinerators have quickly evolved into waste to energy plants but emissions have always been the centre of a heated debate. How do modern incineration plants now compare with other thermal energy generation processes?

By Peter Quicker and Johann Hee

Unfortunately negative opinions still exist among the German public on waste to energy. For example: "Waste incineration is an ancient technology, the installations are badly polluting the environment and threatening public health" - opinions like these are still common among the general public in Germany, despite the fact that great technological advances have been made and that modern Waste to Energy (WtE) plants now have minimal environmental impacts.

This attitude is partly due to mistakes made in the past. In the early stage of the ecology movement in the 1970s and 1980s operators, and the incineration lobby in Germany at the time, did not respect the justified concerns and demands of the public for environmentally friendly waste treatment installations. On the contrary, they tried to appease the critics and denied the existence of ecological problems coming from the incineration plants. The public opposition formed at that time has not yet grasped the technological improvements that have been made, and this can usually be observed when new projects are discussed.

Historical overview

By the time of the birth of Christ, the first descriptions of waste incineration were recorded in Kidron valley next to Jerusalem, but details about the 'technological approach' or the performance were not passed on.

The development of modern waste incineration technology started in the 1870s in England. The first attempts carried out in Paddington were not successful. This was because the waste did not burnout sufficiently and produced heavy smoke emissions at low combustion temperatures, caused by the low calorific value of the waste at the time.

It is obvious that the waste is dominated by ashes and dust with no or very low energy content. Ash fractions up to 50% and calorific values below 5 MJ/kg (today about 10 MJ/kg) were the rule. In England the conditions were a little more advantageous: As a result of hard coal being widely used for domestic heating, the residual waste contained a great deal of carbonaceous material (unburnt coal), which enhanced its heating value. This fact is often quoted as the reason why the development of waste incineration started in England.

The first reliable waste incineration facilities to operate long-term were established in Nottingham and Manchester. The plants were operated at temperatures between 700°C and 900°C with a good burnout, and produced a hard glassy slag.

The new technology was a success story, and by end of the 19th century more than 200 incineration plants were in operation in England and the technology had spread to the European continent and all over the whole world. The first plant on the continent was built and put into operation between 1893 and 1896 in Hamburg. Japan followed in 1897. The first installation of the U.S., in New York started operation in 1885.

The arrival of energy recovery

At the end of the 19th century waste incineration transformed into waste to energy. In 1896 the waste incinerator in Oldham started to deliver steam to a powerhouse generating electricity in the neighbourhood. In London, Shoreditch, the first waste incinerator with its own power generation started operation in 1897. In the early days of waste incineration it was also already common to use the ashes and slag from the incineration process for building purposes, e.g. to produce paving stones or as a substitute for sand.

Despite this success, criticism also arose in the early years: Neighbours complained about nuisance and negative health impacts due to nasty smells and dust. This resulted in the closing of some incinerators.

Others were closed during the First World War, because of the reduced calorific values which prevented a sound incineration of the waste without auxiliary fuels.

Increasing amounts of waste and reduced landfill space once more moved the focus back to incineration. Whereas the facilities in Europe were planned and built more centrally, with capacities of 50,000 to 100,000 tonnes per year, in Japan and the U.S. many small incinerators were operated directly where the waste originated - large residential or public buildings.

In the 1970s around 200,000 small scale batch incinerators were operated in Japan; in New York 1963 approximately 17,000 waste furnaces were counted. Of course, all these small scale installations were not equipped with adequate abatement technologies for treating the pollutants in the flue gas.

Emission control

The initial efforts to reduce emissions made at the larger centralised incineration plants, focused on the capture of dust by installing of particle filters. For example, in the incineration plant Hamburg Borsigstraße, which started operation in1927, an electrostatic precipitator was installed.

The next step in pollutant reduction was the abatement of acidic gases, like HCl or SO2. This was initially realised by installing scrubbers and later also by dry sorption systems on the basis of lime, or nowadays sodium bicarbonate.

Then the collection of heavy metals and toxic organic compounds on carbon carriers, and selective reduction technologies for nitrogen oxides completed the standard abatement systems currently found in modern waste to energy plants at the end of the 20th century.

The development of technologies for emission control has historically been driven by legislation. The first respective German regulation was the 'Technical Instructions on Air Quality Control' (Technische Anleitung zur Reinhaltung der Luft, TA Luft) was introduced in 1964, which has been amended several times and is still valid today.

This regulation covers all industrial facilities with potential environmental impacts from air emissions. A special regulation for the incineration and co-incineration of waste, the 17th Federal Immission Control Ordinance, came into force in 1990.

Steadily reductions of emission limits, followed by improvements to technologies were the drivers for constant progress in waste management installations. Figure 1 shows the development of emission limits in Germany. Limits for dust, HCl, HF and CO were introduced in 1974. In the 1980s also limit values for SOx, NOx, heavy metals and organic compounds were set. The limits have continuously been tightened over the last 40 years. This process will continue in the future.

Figure 2: Development of selected emission limits for waste incineration in Germany (O2-reference 11%; emissions of PCDD/F and heavy metals are not considered, because of the very low values, e.g. 0,1 ng TEQ/m³N for PCDD/F)

In a period of only 14 years the annual emitted load of dioxins and furans from waste incineration could be reduced drastically from some 100 grams to a value below 1 gram per year in 2004.

Emission limits for different thermal treatment options

Today there are various possibilities for the thermal treatment of residual waste available in Germany. Besides specialised mono-incineration facilities for household waste, hazardous waste, sewage sludge or waste wood, also the co-incineration of (pre-processed) waste in cement kilns or in large coal power plants is common. In earlier years, emission limits for co-incineration were less stringent than for waste to energy.

This situation was changed by the amendment of the 17th Federal Immission Control Ordinance in 2003. Since then, a relative equality between the treatment methods has been reached.

In power plants, the maximum proportion of waste that can be co-incinerated is limited to 25% of the fuel used. In the case of higher ratios, the emission limits for waste to energy (mono-incineration) plants have to be applied. The waste proportion for cement kilns is only restricted, when hazardous waste is utilised. Fractions higher than 40% require the application of the mono-incineration limits.

A comparison of the emission limits shows that cement kilns and waste to energy plants are now treated more equally, while the limits for power plants are still noticeably higher. According to thermal capacity, basic fuel and plant configuration, the emission limits permitted especially for nitrogen and sulphur oxides can vary in a wide range.

However, also for cement kilns many exceptions can be made via the local authorities, when the higher emissions of pollutants are not caused by the incinerated waste but by the raw materials used for clinker production.

It must be emphasised that especially for toxic compounds, such as mercury or dioxins, the emission limits for co-incineration are identical to those for waste to energy.

The State of Emission Control Today

The more or less harmonised emission values for waste to energy and co-incineration suggests similar emissions from both thermal waste treatment methods. When comparing the mean annual emission values for waste to energy plants, co-incinerating power plants and cement kilns based on pollutants for dust emissions, waste to energy plants have by far the lowest concentrations - below 10 mg/m³N.The average value for dust emissions of 45 participating plants (more than 60% of the entire German WtE facilities) amounts less than 1 mg/m³N; the emission value of the 'worst' facility did not exceed 3 mg/m³N.

Due to the size of the facilities the emitted loads of the treatment methods are significantly different. Power plants emit by far the highest exhaust gas volume streams and thereby also the highest mass streams of dust - up to a maximum of about 560 tonnes per year.. The highest value of all the considered WtE facilities amounts 3.5 tonnes per year.

A comparison of the emitted nitrogen oxides gives a slightly different picture. Cement kilns show the highest NOx concentrations in the off-gas, due to the extremely high operation temperature of 2000°C, which induces the formation of thermal NOx.

Nevertheless, once again the highest loads were found to be being emitted from power plants due to their extremely high waste gas volumes.

Waste to energy facilities also demonstrated by far the lowest emission concentration and load for sulphur dioxides.

Power plants and cement kilns have a similar concentration level, but the load of the power plants is much higher, again due to the extremely high flue gas streams.

In the case of the mercury concentrations, power plants and waste to energy plants operate on a similar concentration level. The main reason for the low values for power plants should be the 'dilution' of the pollutant concentrations, because of the limited waste fraction in the fuel - maximum 25%; the rest is coal.

Another factor may be the consequent receiving controls in power plants, to keep the introduced pollutant amounts low. The loads show the same pattern as in the case of the other pollutants, emission loads from power plants are far the highest.


Even in Germany, waste to energy plants still do not have a good reputation with the broader public. The reasons for this impression are the mistakes made in the past. In fact, waste to energy plants are a mature, reliable technology.

All of the legal requirements are safely met and the emissions generally lay significantly below the permitted limits for waste to energy facilities.

The comparison of emission levels from different options for thermal waste treatment shows that waste to energy facilities are emitting particularly low pollutant concentrations and loads, compared to co-incineration in cement kilns and in power plants. This is also due to the high standard of flue gas cleaning in Waste to energy plants.

The comparison of the three incineration technologies discussed - waste to energy, cement kilns and power stations - by means of the annual emitted loads of each single pollutant, shows that a significant reduction of emissions has to focus primarily on power plants, and it can be seen that there is very little scope for waste to energy plants to make further reductions.

Prof. Dr. - Ing. Peter Quicker and Johann Hee are from the Unit of Technology of Fuels, at RWTH Aachen University, Germany.