Dioxin - Occurrence and Decrease in Combustion and Drying Processes

8/13/2019 Dioxin - Occurrence and Decrease in Combustion and Drying Processes

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Dioxin - Occurrence and Decrease in Combustion and Drying Processes

Source: Horst Feistel - Certified Engineer & Consultant, Volkach, Germany

(dated 17/11/2003)

Discussion on dioxin contamination in animal feed is currently focused on minimising or avoidingdioxin in thermal production processes. The following description gives an overview about dioxinitself, its generation and some ideas how to reduce the dioxin content in products (animal feed).

Part I

What are dioxins?

In scientific terms dioxin is a name for unsaturated compounds of carbon (C), oxygen (O), chlorine (CI),bromium (Br), hydrogen (H) and compositions with other chemical and toxicological substances. Theyare generally also described as polychlorinated dibenzofuranes (PCDF) or as polychlorinated dibenzones(PCDD). Dioxins always occur as complex mixtures with changing composition.

How do dioxins occur?

(a) Dioxins are formed in combustion operations from carbon compounds and organic or inorganicchlorine or bromium compounds. They are formed fastest at temperatures of about 250 to 300C andare thermally stable up to a temperature o approx. 700C. At higher temperatures incompletelydestroyed chlorinated hydrocarbons the precursors can recombine from the gaseous phase in thetemperature range of about 600 to 800C. Studies conducted at the Karlsruhe Research Centre(Germany) have shown that th occurrence of dioxins increases in line with rising moisture levels in the

fuel. (b) Studies in Canada, Austria and Germany confirmed the occurrence of dioxins in the abovetemperature range, especially in waste heat use (steam/water boiler, heat exchanger) of a combustionplant. Electric filters located downstream of combustion, operated in the temperature range of 250 to300C, were also identified as a source of dioxin occurrence.(c) Dioxins are also formed when cellulose raw materials are bleached with chlorine and caustic soda, forinstance in paper production.(d) Dioxins are also found in the waste air or foundries and metallurgical processes.(e) Further sources of dioxin are composting and forest fires.

To summarise, therefore, it can be ascertained that dioxin formation is not unusual, but it is undesirable.To put it simply, dioxins occur when fuels with a carbon content (oil, coal, wood, waste etc.) are burnt inconjunction with chlorine, or bromium, and this is reinforced by the presence of moisture andfavourable temperatures ofbout 250 to 300C. Other substances, such as for example carbon monoxide(CO), carbon dioxide (CO2), sulphur dioxide (S02),nitrous oxides (N0x), polycyclic aromatic hydrocarbons(PAH), hydrocarbon (CxHy), hydrochloride (HCI), formaldehyde (CH2O), nitrous amines (CxHyNzOn),heavy metals an other also occur within the above combustion parameters, or are found in particularwhen biosubstances and/or waste timber are incinerated.


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How can dioxins be reduced?

The objective in this section is to reduce dioxins, not to avoid them. The outlay required for dioxinreduction is considerably lower than that for dioxin avoidance. The lawgiver provides assistance her withpertinent technical guidelines and regulations specifying maximum boundary values and the location at

which samples are to be drawn. There are basically two options for minimising the dioxin content at thesampling point.

1. Lowering dioxin generation at the point of occurrence for example firing as a primary measure.2. Reducing the dioxin content in the sampling point for example flue gas by precipitation as asecondary measure.

If a product, e.g. animal feed, is dried with waste heat after combustion the entrainment of dioxins inthe product must be avoided, e.g by a suitable drying process.

Considering the basic occurrence process, the formation of dioxin during combustion can be restrictedsubstantially by appropriate selection of fuel an low oxygen surplus in the incineration. This influencethe firing technology to be used, or conversely the firing technology available limits the bandwidth ofpossible fuels and the nature of the combustion management. The thermal stability of dioxins clearlyprescribes combustion temperatures of over 700C. A temperature of 800 to 850C has provedfavourable, since in this range only minimal quantities of other substances such as e.g. Nitrous oxides(NOx) and carbon monoxides (CO) occur too. If the waste gases of combustion are channelled forthermal use like steam production, hot water, drying etc., the temperature is reduced. They than passthrough the critical temperature range of 250 to 300C. It has been shown in experiments and in theoperation of large scale firing plants that secondary dioxin generation can be kept within limits if thistemperature range is passes through quickly. This is done in practice, for instance, by injecting coldwater, spray cooling. If fast cooling down is not possible or desirable, dioxin can be precipitated withinthe framework of secondary measures. On the one hand a catalyser in which the PCDD/PCDF oxidise

(OXCAT) can be used. The dust must be removed from the flue gases before they enter the catalyser. Onthe other hand activated carbon absorbs dioxins, among other substances. It is common practice toinject activated carbon into the gas flow with downstream precipitation in a fabric filter. Relatively largeactivated carbon filters are used mainly in large-scale firing technology. In many cases however, the useof secondary measures, in other words additional apparatus technology, is not possible or cost-effective.The drying process must be examined thoroughly with a view to preventing dioxin contamination of theproduct, e.g. Animal feed, when drying heat resulting from previous combustion is used.

What is drying?

Drying is the separation of liquid from a solids-liquid system (damp material), be evaporating or

volatilising the liquid. The final product is the dry matter. To this end the liquid, e.g. Water is passed invapour form to a carrier medium, generally air, until saturation occurs. If the saturated air (wastevapours) is replaced by unsaturated air, water vapour can be absorbed again. The operation is repeateduntil the desired degree of drying is reached. The driving force for diffusing the water vapour into the airis the pressure drop between the vapour pressure of the water and the partial pressure of the air. Thispressure drop can be influenced by changing temperatures and changing pressure in the dryingchamber. In simplified terms, hot air absorbs more water than cold air, and a under-pressure in thedrying chamber increases the rate of evaporation. The drying speed remains constant as long as a


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surface water evaporates. After this the speed of drying drops due to the slower speed of diffusion ofthe vapour from the interior of the dry matter (capillary action, pores).

Drying process:

Drying takes place at overpressure, normal pressure or low pressure. The temperature in the dryingchamber is equal to, higher than or below the ambient temperature. The required thermal energy canbe passes to the damp material by convection, contact, radiation or high frequency. In the simplest casethe damp material is exposed to the ambient air. In order to improve efficiency, drying can be carriedout at normal pressure higher temperature. In this case the vapour pressure of the water is increasedby raising the temperature an thus the aforementioned pressure drop is magnified. If the drying processnow takes place in a vacuum, the drying temperature can be reduced, since the boiling temperature ofthe water sinks as the pressure drops. This process is suitable for protecting the product against excesstemperatures. Drying at under-temperature and vacuum in the ranges minus 15C to minus 50C iscalled freeze-drying. In the vacuum the frozen dampness is translated directly into the steam condition(sublimation).

Part II

Part II gives an overview about the thermal processes and technology and more information about theoptimal design of drying plants.

In brief:

How does the heat enter the drying process?

The necessary heat is generated in special heat generators, or waste heat from the combustion plants isused. Special heat generators like infrared radiators, high frequency radiators and others are relatively

costly facilities.

Optimal design of new or existing plants:

Special attention should be paid to the product, as well as to the above drying variants. The speed ofdrying, the drying temperature, pressure sensitivity.

Applications in complete feeds

Since a large share of the above pollutants occur especially when bio materials are burned, thegeneration of pollutants can be minimised by optimising the burn-out and observing the temperature

boundaries... that is why an accredited and authorised laboratory or institute is frequently charged withproduct controls. These facilities are subject to surveillance, are impartial, and carry out the necessarymeasurements correctly. The results are recognised by authorities and courts. Please order the Part II(charge free) to get the final information how to optimise existing or new animal feed productionprocesses.

Please order the Part II (charge free) to get the final information how to optimise existing or new animal


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feed production processes.

Your questions are welcome, please contact the author via Email:

Horst Feistel

(certified engineer & consultant)Prof.-Jaecklein-Str. 8D-97332 VolkachGermanyTel: +49 (0)9381 2186Fax: +49 (0)9381 2183Email: [email protected]
mailto:[email protected]:[email protected]

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Dioxin - Occurrence and Decrease in Combustion and Drying Processes