EP0593665B1 - Incineration method, particularly for spent graphite - Google Patents

Abstract

A method for incinerating a combustible material (1), wherein, prior to the combustion process (13), the material is divided (2) into particles, the particulate material is added (3) to an aqueous medium such that the proportion of solid material to liquid is no greater than 35 % wt. and the suspension is foamed and pressurized by inserting an oxidant gas (10) and surfactant additives (9) into a mixer (8) for the suspension.

Description

The present invention relates to a process for preparing for its incineration graphite, in particular irradiated graphite and more generally a combustible material, such as waste, with a limited production of fumes, in order to avoid a significant and costly treatment of these.

One of the most widely used methods of degrading waste is incineration. Environmental protection however imposes increasingly severe constraints with regard to discharges into the atmosphere, which leads to large smoke treatment installations for retaining undesirable and / or polluting elements.

It is therefore useful to produce a minimum of fumes so as to reduce the volumes to be treated at the outlet of the incinerator.

It is also useful to avoid the production of harmful gaseous components (for example nitrogen oxides) during incineration and one of the ways to limit this production is to limit the combustion temperature.

Finally, it is advantageous to be able to carry out complete combustion and to have rapid reaction kinetics.

For this purpose, document US-A-38 48 548 which describes a process for preparing explosive solid waste to be incinerated, consisting of dividing the waste and then forming an aqueous suspension of this waste which is introduced into a rotary incinerator according to combustion. particular with addition of oxidizer. Most of the results obtained by this process come from the rotary incinerator, the proper functioning of which guarantees minimal pollution. The effects of the invention are, however, independent of the operating conditions of equipment.

In order to achieve these objectives, the invention proposes a process for the preparation prior to an incineration of a combustible material.

• division du matériau en particules,
• incorporation du matériau divisé dans un milieu aqueux additionné de produits moussants (tensio-actifs) à raison d'un maximum de 35 % à 45% en poids de solide selon la nature du solide considéré,
• élévation en pression de la suspension ainsi obtenue,
• et moussage de la suspension sous pression réalisé par incorporation d'un gaz comburant sous pression.

Thus, in accordance with the invention, the process comprises, before combustion, the continuous carrying out of the following steps:

• division of the material into particles,
• incorporation of the divided material in an aqueous medium supplemented with foaming products (surfactants) at a rate of a maximum of 35% to 45% by weight of solid depending on the nature of the solid considered,
• pressure rise of the suspension thus obtained,
• and foaming the suspension under pressure by incorporating an oxidizing gas under pressure.

The oxidizing gas can be oxygen or enriched air and the pressure of the foam obtained will be of the order of 2 to 10 bars, depending on either the opening of a control valve located before the combustion furnace, i.e. the pressure in the furnace and the pressure drop between the pump outlet and the combustion furnace.

The maximum of the solid fraction (from 35 to 45% by weight of the suspension) will depend on the apparent density of the pulverized solid and on its lower calorific value. For example, since graphite has a high calorific value and an apparent density, the solid fraction will be of the order of 35%. For lower criteria, the fraction may be higher without damaging the pumpable nature of the product or leading to too hot combustion.

Other characteristics and advantages of the invention will emerge from the description of an embodiment given below by way of example, applied to the incineration of graphite and in particular irradiated graphite.

Reference will be made to the appended drawing which represents by a single figure the diagram of the process implemented. work by the invention.

There is currently a need to incinerate used and irradiated graphite from the dismantling of nuclear reactors. This graphite comes in two geometric forms, in bricks and in tubes. The nuclear quality of the material is such that it contains practically only pure carbon; the impurities are in very low concentration. In order to reduce the volume of this graphite to be stored as waste, it has been proposed to crush and incinerate it. It has indeed been shown that the combustion is all the more complete the smaller the diameter of the particles, the longer the residence time in the focus of these particles and the greater the gas-solid speed is also important.

To meet these conditions, a cyclone type firebox has been chosen, although this firebox is not a perfect solution to all the problems encountered during this combustion, in particular by the transport of the solid divided into very fine particles, and of on the other hand due to the generation of toxic nitrogen oxides.

Furthermore, given the fact that the material to be incinerated contains radioactive particles, and that a fraction of these particles will be entrained by the combustion fumes, it is completely essential to treat these fumes with the outlet from the incinerator, and the treatment process will be all the more economical as the volume of smoke will be as small as possible.

The answer to these constraints, according to the invention, lies in the preparation of the fuel.

The diagram shown in the drawing illustrates a processing and incineration chain for a crude product 1, which in this case is graphite. The first step in processing this product consists of grinding 2. Grinding can consist of several stages using several types of apparatus so as to obtain an output powdery product with an appropriate particle size (for example 80% of the particles whose diameter is less than 200 μ). This grinding can be carried out in a humid environment to avoid the release of dust and reduce the risk of explosion which results therefrom, and also to obtain a fluid which is transportable by means of pumps.

The powdered graphite obtained at the outlet of the mills 2 is added with water in a mixer 3. With the aid of a pump 4, the continuous circulation of the product coming from the mixer 3 is ensured in the direction for example of a concentration 5 (acting by measuring viscosity for example) to arrive at a mixer 6 for adjusting this concentration by adding water. The product leaving this mixer 6 will have a content at most equal to 35% of solid particles. This is a relatively charged aqueous suspension which is very easily pumpable. This suspension is using a pump 7 introduced into a mixer 8 intended to produce a three-phase liquid-solid-gas mixture. To do this, is introduced into the mixer, in addition to the suspension to be transformed into foam, surfactant additives 9 and a swelling gas 10. In the present case the gas will be oxygen.

Thanks to a pressure control valve 11 downstream of the pump or to the pressure drop in the circuit, it is possible to obtain a pressurized foam, for example between 2 and 10 bars. The pressurization of the foam has a double advantage: firstly it becomes self-propelling by partial expansion in the supply lines 12 of a combustion hearth 13, and secondly the relaxation of this foam in the hearth promotes his injection.

The hearth 13 is a cyclone hearth known in itself which has the advantages mentioned above with regard to the residence time of the particles and the relative gas-solid speed, in order to achieve complete combustion of the product. Oxygen 14 is introduced into the hearth 13 as combustion gas. It should be noted that the combustion may include a post-combustion stage. The ashes are collected in an ashtray 15 while the fumes 16 are led into a water cooler 17 which makes it possible to condense the water vapor produced by combustion. This cooler will include a demister that retains water droplets. The fumes thus freed from the aqueous effluents, pass through a heater 18 which places them at a temperature above the dew point to avoid any condensation which could be harmful to the operation of the very high efficiency filters 19 through which the fumes pass. These filters make it possible to trap the solid residues 20 while the gaseous phase of the fumes is transmitted to the atmosphere by an extraction fan and a chimney 22.

In order to illustrate the advantage of preparing graphite in the form of a loaded foam with an aqueous base (three-phase mixture of water, oxygen, graphite), the following elements will be given for comparison between an incineration carried out using a technology known (for example in a fluidized bed) and an incineration by cyclone hearth with the fuel introduced in the form of foam.

This table shows a 60% reduction in the mass flow rate of the fumes produced by the combustion of graphite in a cyclone hearth compared to those in a fluidized bed. In addition, the fumes leaving the cyclone hearth contain almost 50% water. Thus, the fumes to be treated before discharge into the atmosphere are 78% less in mass during an incineration in a cyclone hearth than during an incineration in a fluidized bed. It is therefore understandable that the filtration and purification installations are much less important in one technique than in the other.

From this table we also understand the advantage of proceeding, on the one hand, to the formation of a foam from oxygen (although this oxygen represents only less than 4% of the oxygen necessary for the combustion of the product ) and secondly and above all, to be used as an oxidizer for pure oxygen: this avoids the production of nitrogen oxides.

The use of an aqueous foam greatly facilitates the control of the incineration hearth since the production of water vapors tends to lower the temperature and the regulation of the solid load of the foam makes it possible to achieve simple regulation of the operation of the oven, or even its extinction in an emergency if the solid fuel is completely removed from the foam.

Finally, it will be added that it is possible to include a relatively small fraction of polluted combustible liquid (for example a used or contaminated oil) in the suspension before expansion.

Claims (2)

1. A method of preparing a combustible material (1) with a view to its incineration, characterized in that it comprises before combustion (13) the continuous performance of the following steps:

   . dividing (2) the material into particles,

   . incorporating (3) the divided material into an aqueous medium at a maximum rate of 35% to 45% by weight of solid relative to the liquid,

   . increasing the pressure of the suspension thereby obtained, and

   . foaming the suspension under pressure, by incorporating an oxidizing gas (10) and surface-active additives (9) in a mixer (8) of the suspension.
2. A method according to claim 1, characterized in that the oxidizer gas is oxygen.

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