Dedicated incineration of carcases and parts of carcases Description of animal carcases and parts of animal carcases

Carcases containing up to 70 % moisture and up to 5 % incombustible solids have a heating value of approximately 5815 kJ/kg [29, US Environmental Protection Agency, 1997]. Other figures, based on a limited experience of large-scale carcase incineration quote calorific values in the order of 10000 - 12000 kJ/kg for whole carcases, 11000 - 13000 kJ/kg for quartered meat

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RHC/EIPPCB/SA_BREF_FINAL Version November 2003 83 [6, EA, 1997] and 12000 – 15000 kJ/kg for SRM, comprising skulls, intestines and vertebral columns [248, Sorlini G., 2002].

Animal carcases are incinerated in fixed hearth incinerators in the UK. Other combustion technologies which have been reported to be suitable include pulsed hearth, rotary kiln and semi-pyrolitic incinerators [65, EA, 1996, 144, Det Norske Veritas, 2001]. Promising trials using bubbling fluidised bed incinerators to dispose of crushed animal carcases have been reported. [200, Widell S., 2001]. The incinerators are described below. BFB incinerators are described under 2.2.8.2, as the technique is more widely used for incinerating animal meal.

Fixed hearth incinerator

A fixed hearth incinerator operates as follows. A ram loader pushes the carcases into a primary chamber where they are over-fired with primary air and/or with burners, depending on whether combustion is self-sustaining. Proper mixing of the material on the hearth can be difficult and requires careful adjustment of the feed and ash removal rates. Achieving constant burnout is difficult. The skill and training of the operator are particularly important.

A secondary chamber with an injection of supplementary fuel and secondary air is essential.

Semi-pyrolitic incinerators

This technology is described more a control method than a specific configuration of incineration. A primary chamber operates at less than the stoichiometric air requirement for complete combustion and a secondary chamber operates under excess air conditions. The material is dried, heated and pyrolised in the primary chamber, releasing moisture and volatile components. The gas is driven off and then burned in the secondary chamber which is supported by a supplementary fuel burner.

This method of combustion is reported to ensure a controlled burn with both relatively low releases of VOCs and CO. Also, the low combustion airflow results in a low entrainment of particulate borne pollutants.

Stepped hearth

Stepped hearth incinerators comprise a series of concrete steps, typically three, with embedded air channels. The materials are moved from step to step by a series of rams. The first step is a drying stage, with sub-stoichiometric oxygen conditions, during which most volatile compounds are released and burned above the grate in the combustion chamber. The remaining, less volatile material is pushed onto the next step, where the main combustion takes place. The third step is the burnout stage, before the ash is discharged into a final ash burnout chamber, which incorporates air injection and agitation. Material can take eight hours to pass through the hearths and a further eight hours in the burnout chamber. This depends, to an extent, on the feed rate, which will also determine the supplementary fuel requirements.

The steps between the hearths provide good agitation as the waste tumbles down the step, however, this also produces surges of unincinerated material, so good secondary combustion and residence time is important.

Pulsed hearth incinerator

Pulsed hearth incinerators use the pulsed movement of one or more refractory hearths to move the waste and ash through the incinerator. The hearths, which are stepped at each side to form a

“U” shape, are suspended from four external supports. The smooth hearth can handle difficult wastes without the risk of jamming and there are no moving mechanical parts exposed to burning material or hot gases. There may, however, be problems achieving the good burnout of solid wastes. [65, EA, 1996].

Rotary kiln incinerator

Incineration in a rotary kiln is normally a two stage process, which takes place in a primary combustion chamber and a secondary combustion chamber. The kiln is a cylindrical shell lined

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84 Version November 2003 RHC/EIPPCB/SA_BREF_FINAL with a refractory substance. It is inclined downwards from the feed end and rotates slowly about

its cylindrical axis. The rotation moves the waste through the kiln with a tumbling action, thus exposing fresh surfaces to heat and oxygen. Structures may be added within the kiln, to aid turbulence and to slow the passage of liquid wastes. The residence time of material incinerated in the kiln can be changed by adjusting its rotational speed.

Rotary kilns can operate at very high temperatures. Careful attention needs to be paid to the rotating kiln and the endplates, to prevent the leakage of gases and unburnt waste. The tumbling of waste may generate fine particles.

Commissioning

Commissioning tests carried out for new plants and existing installations planning to incinerate a different fuel from that for which it is authorised for or would normally incinerate, enable checks to be made about whether the desired results are achieved.

Delivery, storage and handling

Unloading, storage and handling can be done in totally enclosed buildings and equipment. There may be a risk of theft of meat which is unfit for human consumption, so security need to be applied.

Charging the incinerator

For batch processes, carcases are generally fed into the incinerator intermittently, by front loader vehicles, ram-feed or manually. Opening of the doors for loading can allow considerable ingress of cold air which may upset combustion conditions and increase emissions. Fans capable of responding to changes in furnace pressure during charging, to avoid the escape of fumes or excess airflows may, therefore, be used. Large drops in temperature, e.g. during charging of batch incinerators, can be avoided by using charging systems which incorporate airlocks. Continuously operating incinerators are generally fed from enclosed handling and sometimes pretreatment and charging systems. Control of air and consequently combustion is easier with continuously operating systems.

The incineration process

Residence time in the furnace has to be long enough to ensure good burnout, as measured by the total organic carbon content and it needs to be controllable. The supply of air to different combustion zones also has to be controllable. Minimum conditions of 850 ºC combustion gas temperature, with a gas residence time of 2 seconds are specified in ABP Regulation 1774/2002/EC [287, EC, 2002], for animal carcases, which are excluded from the scope of WID Council Directive 2000/76/EC [195, EC, 2000]. WID Council Directive 2000/76/EC sets similar conditions, for all other animal by-products, including parts of carcases, although it also allows the competent authority to lay down different conditions, provided the requirements of that Directive are met.

With most furnace designs, the minimisation of primary air will both minimise NOx production as well as the velocities which lead to the entrainment of particles. An adequate distribution of air and fuel on the bed will prevent the formation of hot zones and thereby reduce the volatilisation of material, which could otherwise lead to the formation of heavy metal oxides and alkali metal salts in the fly ash. Combustion zones may be separate chambers or, as in the case of BFB incinerators, they may simply be areas within the same chamber where primary and secondary air are introduced.

Water cooling of grates may be an alternative to providing excess air in grates to control metal temperatures. This can also improve primary air control and hence combustion.

Ash handling and storage

Enclosed handling systems avoiding the use of brushes or compressed air minimise dust emissions and, therefore, assist compliance with both occupational health and environmental controls.

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RHC/EIPPCB/SA_BREF_FINAL Version November 2003 85 Cleaning

One rotary kiln incinerator operating continuously and its enclosed equipment upstream of the rotary kiln, i.e. the storage, handling, grinding and charging mechanisms, is cleaned by feeding wood chips through the system periodically, usually before maintenance and then incinerating them in the incinerator. This incinerator is dedicated to the destruction of SRM comprising cattle heads and vertebral columns.