Method and furnace for incinerating organic matter, such as sludge, from the treatment of industrial or agricultural waste or wastewater

Abstract

The present invention relates to a method for incinerating organic matter from the treatment of wastewater, industrial or agricultural waste, such as sewage sludge, in a fluidized-bed incinerator. The incinerator comprises a housing, in the lower part of which is a bed of particles, preferably a sand bed, forming a fluidization zone Zf. The organic matter is introduced into the fluidization zone Zf as fuel, while air is injected into the sand bed from a wind box through a fluidization dome located at the top of the housing, the air passing through channels provided in the fluidization dome. The incinerator is configured to treat a nominal value Vn of the volume Vmo of the organic matter to be treated. The method comprises a step of adjusting the volume of the fluidization zone Zf according to the volume Vmo of the organic matter to be treated. In this step, when the volume Vmo of the organic matter to be treated is less than the nominal value Vn, the volume of the fluidization zone is reduced from an initial volume Vfi to a reduced volume Vfr, and the air flow rate is reduced by sealing the air channels so that only the channels leading to the thus reduced fluidization zone Zfr are active.

Description

[0001] The present invention relates to a furnace for incinerating organic matter such as sludge from the treatment of industrial or agricultural waste or wastewater, and more particularly to a fluidized sand bed furnace, and a method for incinerating organic matter such as sludge.

[0002] A fluidized-bed incinerator comprises a housing, in the lower part of which a granular bed, preferably a sand bed, is located. This forms the fluidization zone, into which the sludge is injected as fuel. Below the fluidized bed is a wind box, which receives, preferably heated, fluidizing air via radial inlets. The upper part of the wind box supports the fluidization dome. This fluidization dome is a crucial component of the furnace because the fluidizing air is distributed through it via channels arranged through the dome, for example, those equipped with air blowing nozzles.

[0003] The operating principle of a fluidized bed furnace is to suspend pre-calibrated particles, preferably sand, in a fluid that may or may not be preheated. The physics and thermodynamics of fluidized beds are extensively described in reference books such as "Fluidisation Engineering" by Daizo Kunni and Octave Levenspiel, "Fluidisation" by Max Leva, or Nichols. The fluidization dome separates the fluid manifold from the fluidized particles. It allows for good distribution of the fluid through the air blowing nozzles or fluidizing tuyeres that form the channels to the fluidized area.

[0004] The fluidized sand bed is heated to incineration temperature (750-850°C) and constitutes an extremely turbulent medium in which heat exchange reaches very high transfer coefficients. Due to the turbulence of the sand, the organic matter, usually dewatered sludge, which is generally fed into the base of the bed at one or more points, is decomposed very quickly, evaporation occurs instantaneously, and partial combustion of the organic matter occurs together with the fluidizing air as an oxidant.

[0005] Typically, a dense fluidized bed with or without bed overflow is used, which has a stable, expanded bed height (taking into account the loss of sand due to abrasion). This is because the mineral matter of the sludge has a very small particle size compared to the particles of the sand in the bed, so that all the mineral ash is pneumatically driven into the upper region of the furnace and then into the discharge duct for the fumes (fly ash).

[0006] The incineration method implemented in this type of furnace thus allows the destruction of organic matter trapped in mineral and / or liquid matrices, such as sludge from sewage treatment plants or from the treatment of industrial or agricultural waste or wastewater. This is the most widely used method for achieving this destruction while meeting regulatory standards.

[0007] The sludge burned in this type of furnace is generally in a paste-like state and still contains a lot of water at the outlet of traditional dewatering systems. The water content in the sludge is generally 55% to 85% of the raw product. In the furnace, a redox reaction is carried out by reacting organic matter as fuel with an expanding fluid (in most cases air) as an oxidant.

[0008] Since the treated sludge generally still contains a lot of water, the highly cooled bed and furnace, the sludge's organic matter alone, which has a high calorific value, may not be sufficient to maintain the furnace at 850°C, which is its minimum operating temperature.

[0009] Thus, depending on the quality of the sludge, the furnace may require the injection of auxiliary fuel (natural gas, biogas or fuel oil or charcoal or any type of organic residue with a calorific value higher than 15000 kJ / kg of organic matter) to maintain the temperature required by regulations, which is 850°C in France.

[0010] However, when the moisture content of the organic matter is low, the only heat input to the organic matter is through the sludge, and the use of this auxiliary fuel during incineration can be avoided.

[0011] The applicant has also proposed an improved method for recovering energy from the fumes in order to obtain a more dehydrated sludge by partial evaporation of the water fraction and to achieve a balance between water and organic matter with a minimized need for auxiliary fuel.

[0012] However, in order to start the furnace, an auxiliary fuel is generally always used, such as gas or fuel oil. This fuel injection is carried out in the bed, preferably in the lower part of the bed, above the fluidization grid, above the dome, in the separating fluid and in the first third of the bed.

[0013] However, it is not currently possible to completely eliminate the need for consumption of auxiliary fuel.One of the reasons for this drawback is that the amount of organic matter, such as sludge, to be treated by incineration is often very small compared to the nominal value used to size the furnace.

[0014] In practice, incineration plant construction projects must meet specifications that generally require the incineration of a given sludge yield at the start of operation and a significantly higher yield in the future (e.g., 10 or 20 years) to account for projected growth. Between these two scenarios, the sludge flow rate may double, while the incinerator has a maximum flexibility of 1 to 1.6 for the range of sizes it chooses.

[0015] Often, during the commissioning phase, the sludge flow rate is significantly less than the target volume required by the furnace design. Therefore, to begin operations, it may be necessary to use a furnace with dimensions corresponding to the commissioning flow rate. Then, as operations progress toward the target sludge flow rate, it may be desirable to replace the commissioning furnace with a furnace whose dimensions are adapted to handle the target sludge flow rate. This solution is ruled out due to the costs involved.

[0016] Therefore, furnaces are usually designed to operate in the future and they can operate in two modes when put into operation:

[0017] - Either the sludge production for a given period (usually a week) is processed over a period of several days to reach furnace operating conditions.

[0018] - Either distribute the sludge production during a week for example to treat the sludge continuously, which means operating at a lower hourly rate and generally less than the minimum flow rate that the furnace accepts in order to have optimal efficiency and / or to remain self-heating.

[0019] In both cases, auxiliary fuel is used as a supplement:

[0020] - Either to keep the furnace hot during the non-incineration phase (called maintenance phase). Maintaining the furnace hot is achieved at around 800°C + / - 50°C in the sand bed, which also allows to keep the equipment downstream of the furnace hot and limits the amplitude of thermal cycles that can damage the equipment.

[0021] - Either during incineration, to compensate for the energy loss caused by the reduced flow rate of the sludge and thus the organic matter, which is an inherent energy source, and also by the greater heat loss in the fumes. In fact, the air flow rate is higher than the stoichiometric ratio for sludge combustion.

[0022] This stoichiometric ratio corresponds to the air flow rate strictly necessary to ensure complete combustion (or oxidation) of the fuel.In practice, any excess air generates an energy loss since it is an additional mass to be heated.

[0023] One solution may consist in reducing the amount of air, so as to be sufficient for the amount of sludge and to achieve a stoichiometric ratio close to 1.

[0024] However, there is a lower limit to the amount of air that allows the particles (here, particularly sand) to fluidize. This air is injected into the furnace at the base of the sand bed. Below this minimum flow rate, the sand does not fluidize well, and sludge injected in the form of columns with a diameter of 50 mm to 250 mm (especially around 100 mm) breaks into small pieces and no longer operates. For a given furnace dimension, a variation of 60% in air flow rate between the maximum and minimum values corresponds to a variation of 60% in sludge flow rate (with a few percent error due to varying excess air). In other words, the sludge flow rate can vary from 1 to 1.6. This furnace design range, with a maximum operating dimension corresponding to the furnace's operating capacity for a nominal sludge production (denoted by X), allows operation at a reduced capacity of up to 60%, i.e., at a sludge flow rate of up to 60% less than the furnace's nominal capacity, while maintaining an air-to-sludge ratio that is favorable for good combustion and good fluidization.

[0025] If the flow rate is too low, combustion of the volatile substances in the sand bed is also not ensured. The temperature of the sand decreases and is no longer uniform, which can create dead zones with the risk of sand accumulation that stops the entire fluidization of the bed.

[0026] It is also possible to reduce the particle size of the particles, for example by changing the matrix of the fluidized bed, and thereby allow a reduction in the air flow rate required for fluidization. Such a solution exists, but it is still difficult to implement and may be insufficient: even within the same furnace dimensions, the capacity range could potentially increase from 1 to 1.6 to 1 to 2.0.

[0027] The main limitation of such solutions emerges when the extreme production rates (starting and future) cannot be scaled to a single furnace dimension.

[0028] In order to overcome these drawbacks, the present invention therefore proposes to be able to vary the fluidizing air flow rate according to the volume of sludge to be treated, without affecting the fluidization quality of the sand bed, and this can be done temporarily if necessary. In other words, the invention consists in providing a furnace whose dimensions, defined by future production targets (generally operating at a low rate of its dimensions), are defined at a fluidizing dome of, for example, 2.93 m, said dimensions being able to be temporarily varied to furnace dimensions of smaller dimensions, for example, to a diameter of 2.34 m for the commissioning of the furnace.

[0029] Thus by combining the minimum operating rate of the furnace with its reduced volume and air flow rate with the maximum rate of the furnace with its initial volume and flow rate, the maximum capacity range of a furnace of given dimensions of 1 to 1.6 is extended to a range of 1 to 3 (for furnaces of small dimensions) and 1 to 2 (for furnaces of large dimensions) for sludge flow rate.

[0030] To this end, an object of the present invention is a method for incinerating organic matter in a fluidized bed incinerator, the organic matter consisting of organic matter trapped in a mineral and / or liquid matrix, in particular from the treatment of wastewater, industrial or agricultural waste, such as sewage sludge, in particular sewage treatment plant sludge,

[0031] The furnace comprises a housing in the lower part of which there is a bed of particles, preferably a bed of sand, constituting a fluidization zone Zf,

[0032] The organic matter is introduced into the fluidization zone Zf as fuel, while air is injected into the sand bed from a wind box through a fluidization dome located on top of the box as an oxidant.

[0033] The air passes through channels provided in the fluidization dome,

[0034] The furnace is constructed to treat a nominal value Vn of a volume Vmo of organic matter to be treated,

[0035] The method comprises a step of adjusting the volume of the fluidization zone Zf and the air flow rate advantageously entering the fluidization zone Zf according to the volume Vmo of the organic matter to be treated, in which step, when the volume Vmo of the organic matter to be treated is less than the nominal value Vn, the volume of the fluidization zone is reduced from the initial volume Vfi to the reduced volume Vfr, and the air flow rate is reduced by sealing the air passages so as to render active only the passages leading to the thus reduced fluidization zone Zfr.

[0036] Advantageously, the method according to the invention, by reducing the volume of the fluidization zone Zf and by limiting the channels of the fluidization dome to those acting in this reduced fluidization zone Zfr, makes it possible to define a reduced fluidization zone volume Vfr, which allows an incineration treatment that is as optimized as possible with respect to the volume Vmo of the organic matter to be treated, which is suitable for start-up conditions with small organic matter flow rates.

[0037] Thus, when the volume of the sludge to be treated is less than the nominal value of the volume of the sludge to be treated and the lower limit of fluidization is reached with the furnace of dimension X, the volume is reduced, which corresponds to the middle size of the furnace or fluidization zone of dimension X-1, and the air channels are sealed so that only the channels leading to this reduced fluidization zone are active.

[0038] Advantageously, this adjustment of the volume of the fluidizing zone is also a preferably temporary volume reduction. Thus, the method then comprises a supplementary volume adjustment step in which, once the volume Vmo of the organic matter to be treated approaches or reaches the nominal value Vn, the initial volume Vfi of the fluidizing zone, suitable for treating the nominal volume Vn of the organic matter to be treated, is restored, and the initial air flow rate into the zone is also restored, advantageously by releasing the previously sealed air passages of the fluidizing dome. In this way, the incineration method can be implemented while limiting the consumption of fuel and, therefore, auxiliary fuel.

[0039] This method can advantageously be applied to existing operating furnaces and also to future projects (furnaces under construction or to be constructed). Thus, an insert is installed in the form of a kit, said insert having a shape complementary to that of the portion of the furnace housing that defines the fluidizing zone Zf and extending from the fluidizing dome against said portion of the housing at a height less than or equal to the height of the fluidizing zone.

[0040] Thus, at least the volume of the fluidizing zone Zf is reduced by arranging means for reducing the volume of the fluidizing zone Zf, said means consisting of an insert in the form of a kit having a shape and a height complementary to the portion of the outer shell of the furnace defining the fluidizing zone Zf, extending from the fluidizing dome against said portion of the outer shell and defining an internal volume corresponding to the volume of the reduced fluidizing zone.

[0041] According to an alternative embodiment of the method, the volume of the furnace casing above the fluidization zone can be reduced by installing inserts throughout the entire height of the furnace. Advantageously, reducing the volume of the casing above the fluidization zone improves ash discharge and limits heat losses. This alternative further improves energy and economic efficiency.

[0042] For the purposes of the present invention, the sleeve is a cylindrical element whose shape corresponds to the cylindrical shape of the fluidizing zone. However, the fluidizing zone of a furnace generally has a frustoconical shape, and the sleeve is also frustoconical.

[0043] According to this embodiment, the thickness of the wall of the sheath may be constant or varying, in particular progressive, defining an internal diameter of said sheath corresponding to the reduced diameter of the new reduced fluidization zone Zfr defined by the sheath.

[0044] The sleeve is advantageously mounted along the wall of the fluidizing zone in the furnace and extends from the fluidizing dome, covering the air passages provided through the fluidizing dome at its periphery and terminating below the thickness of the sleeve, which reduces the air intake into the reduced fluidizing zone. The sleeve also constitutes a sealing means.

[0045] Thus, advantageously, the reduced fluidizing air volume flow rate can be expressed according to the following formula, which corresponds to the same formula for the nominal case and takes into account the flow rate of the organic matter to be treated:

[0046] Φv'=Φb' x S x MV x Coef1 x Coef2, where

[0047] Φv': Fluidizing air flow rate in kg / h

[0048] Φb': original sludge flow rate in kg / h (original sludge is water and dry matter, and dry matter is mineral matter and organic matter).

[0049] S: The proportion of dry matter in organic matter in %.

[0050] MV: The proportion of organic matter in the dry matter in %

[0051] Coef1: The stoichiometric ratio corresponding to the amount of air in kg / h for burning 1 kg of organic matter, which is 5 to 10 depending on the type of organic matter

[0052] Coef2: Excess air desired to ensure complete combustion of organic matter, 1.01 to 1.4.

[0053] "Raw sludge", also called "wet sludge", refers to a mixture of dry matter and water that is input. The dryness of these raw sludges is preferably 18 to 35%, generally about 22 to 26%.

[0054] According to a further more preferred embodiment, the housing has a frustoconical shape at an angle of 0° to 45° relative to the vertical in the fluidization zone, and the insert or sleeve preferably has a frustoconical shape:

[0055] - The outer diameter Dext of the cone frustum of the shell is equal to the height of the added cone:

[0056] Dext=D1, which can vary over height H, where

[0057] D1: inner diameter of the fluidized area of the housing designed for future production;

[0058] H: Height of the region on which particle fluidization dynamics develop

[0059] - Its inner diameter Dint is equal to the outer diameter of the added frustum minus twice the thickness A of the sleeve

[0060] Dint=Dext-A×2, where A is the thickness of the insert added at the periphery of the fluidizing zone in order to reduce its diameter, said thickness being constant or variable, in particular gradual, and advantageously ranging from 0.15 to 0.7 m, preferably from 0.25 to 0.35 m,

[0061] - the height H of the frustum is chosen such that: H=B+C,

[0062] wherein B corresponds to the height of the particle bed at rest, B is between 0.3 m and 1.5 m, preferably equal to about 1 m, and

[0063] C corresponds to the dynamics, ie the desired turbulence in the sand bed of 0 to 2 m, preferably 0.3 to 0.5 m. This turbulence or dynamics corresponds to the projection height of the particles.

[0064] The reduction of the air flow rate is advantageously achieved by sealing the air channels situated at the periphery of the fluidising dome and terminating below the means for reducing the volume, the means for reducing the volume advantageously serving as the means for sealing the air channels.

[0065] Thus, for the purpose of maintaining the fluidization of the particles, the volume Vfr of the fluidized zone is preferably reduced by an annular volume of material resistant to temperature and wear added at its periphery. This annular volume of walls inserted against the wall of the housing constitutes an insert or sleeve.

[0066] It is also contemplated to install inserts in the fluidizing zone above the fluidizing dome but not along the walls of the fluidizing zone to reduce the volume of the fluidizing zone and to reduce the air flow rate by sealing a portion of the air passage.

[0067] An example of a material that can withstand abrasion and high temperatures is refractory concrete or refractory brick.

[0068] The volume flow rate Φv′ of the fluidizing air with the fluidizing zone of reduced volume thus decreases in proportion to the initial air flow rate Φv and the thickness of the cylindrical element according to the following formula:

[0069] Φv'=Φvx((D1-2A) / D1) 2 ,in,

[0070] D1: The inner diameter of the shell at the base of the fluidized area designed for future production, in meters

[0071] A: The thickness of the element added to the periphery of the fluidizing area in order to reduce the diameter of the fluidizing area, which is generally 0.2m to 0.7m, preferably 0.25m to 0.35m.

[0072] An object of the present invention is also a furnace for incinerating organic matter consisting of organic matter trapped in a mineral and / or liquid matrix, in particular from the treatment of wastewater, industrial or agricultural waste, such as sludge, in particular sewage treatment plant sludge, said furnace comprising a housing in the lower part of which there is a bed of fluidized particles, preferably sand, and comprising, at least from the bottom to the top:

[0073] a wind box, the upper part of which supports the fluidization dome and which has channels through which the air coming from the wind box is distributed in the fluidization zone Zf corresponding to the particle bed,

[0074] at least one organic matter supply device and at least one supplemental fuel injection device, arranged to supply the fluidizing zone,

[0075] - above the fluidization zone Zf there is an expansion and afterburning zone, on top of which the crown of the housing rests, at which crown there is a discharge duct for the combustion products,

[0076] The furnace is configured to treat a nominal value Vn of a volume Vmo of organic matter to be treated, and

[0077] The furnace is characterized in that it comprises means for reducing the volume of the fluidization zone from an initial volume Vfi to a reduced volume Vfr, and means for sealing the air channels, said means for sealing the air channels being configured to reduce the input air flow rate by activating only the channels leading to the reduced fluidization zone Zfr when the volume Vmo of the organic matter to be treated is less than the nominal value Vn of the volume of the organic matter.

[0078] Thus, when the volume Vmo of the organic matter to be treated is less than the nominal value Vn (typically 30% to 80% of Vn), the furnace can be operated in an optimized manner, since the volume of the fluidized area is reduced from the initial volume Vfi to the reduced volume Vfr, and the injected air flow rate is reduced by sealing off a portion of the air passage so as to only enable the passage to the thus reduced fluidized area. The furnace according to the invention allows the implementation of the incineration method described above.

[0079] Very advantageously, the reduction device and the sealing device are also removable so that when the volume Vmo of the organic matter to be treated approaches or is equal to the nominal value Vn, they can be removed and the initial volume Vfi and the number of air channels in the fluidizing zone can be restored.

[0080] Thereby, the volume of the fluidizing zone and the volume of air blown into it can be reduced in order to treat in an optimized manner a volume Vmo of organic matter less than the nominal value Vn, while maintaining the dimensions of the furnace for future use.

[0081] Advantageously, the means for reducing the volume of the fluidizing zone and the means for sealing the air channel are identical.

[0082] According to a preferred embodiment, the reduction and sealing means consist of an insert mounted in the fluidizing zone of the furnace in the form of a sleeve having a shape complementary to that of the casing that defines the fluidizing zone and extending from the fluidizing dome against said portion of the casing, forming an additional wall that doubles the wall of the fluidizing zone and seals the air channels provided in the dome at its periphery.

[0083] If the housing area is cylindrical, the sleeve can also be cylindrical. Preferably, if the fluidizing area has a frustoconical shape, the reduction and sealing device consists of a frustoconical sleeve that is resistant to wear and temperature. It is embedded in the existing frustoconical area up to the base of the fluidizing grid. This reduces the cross-sectional area at the fluidizing grid and, at the same velocity, advantageously reduces the fluidizing air flow rate.

[0084] The thickness of the insert, for example a cylindrical or frustoconical insert, is proportional to the desired reduction in air flow rate for burning sludge or other waste materials. Preferably, its thickness corresponds to the thickness of the ring of fluidizing nozzles located around the periphery of the fluidizing dome. In practice, the dome can be composed of a series of concentric rings each carrying, for example, fluidizing nozzles and air channels for blowers. The thickness of the insert is thus selected to define the reduced volume Vfr of the fluidizing region and the reduced cross-sectional area of the fluidizing dome.

[0085] The thickness of the jacket, which may be constant or variable, preferably progressive, is thus advantageously chosen to define a reduced volume Zfr of the fluidization zone and a reduced cross-sectional area of the fluidization dome, the thickness of the jacket being proportional to the reduction in the air flow rate desired for burning sludge or other waste material.

[0086] The volume Vfr of the fluidizing zone is thus preferably reduced by adding an annular volume of material resistant to wear and temperature around the fluidizing zone, for example constituting an insert or sleeve of a wall inserted against the wall of the housing.

[0087] According to one embodiment of the furnace according to the invention, the volume reduction device extends over the entire height of the furnace, outside the fluidization zone. This also limits the volume of the furnace casing. This reduction in the volume of the casing above the fluidization zone can improve energy and economic efficiency, in particular by improving ash emissions and limiting heat losses.

[0088] According to one embodiment, the reduction and sealing device is inserted into an existing furnace, optimizing its operation with respect to a nominal amount of organic matter to be treated that cannot yet be reached.

[0089] Alternatively, the reduction and sealing means are provided during construction of the furnace to allow for operational enablement.

[0090] Advantageously, the reducing and sealing means extend along the wall of the fluidizing zone and, through its thickness, reduce the diameter of said zone while sealing the channels provided in and above the fluidizing dome. These reducing and sealing means can be placed on existing furnaces, but can also be deployed on newly constructed furnaces to optimize fuel consumption from start-up.

[0091] They are easily added by constructing the basic parts (additional walls added to the fluidizing zone Zf) after final construction or during initial construction. They can therefore be disassembled just as easily to restore the overall volume required by the furnace for the nominal flow rate of organic matter (especially sludge) for which it is designed.

[0092] The material used to realize this wall can be of similar quality to the refractory inner wall of the furnace. This is, for example, refractory bricks which generally contain at least 20% alumina (preferably 42 to 45% alumina) to resist the wear exerted by the moving sand. They are usually connected with caulking cement.

[0093] Alternatively, the additional wall can be constructed from refractory concrete, particularly one containing 30 to 35% by weight of aluminum oxide, based on the total weight of the concrete. In this case, the concrete can be cast or molded against the inner wall of the fluidization zone. Such materials allow the reduction devices to be used for approximately one to two years in operational furnace conditions, but are generally weaker than the materials forming the furnace itself, which can contribute to the destruction of these reduction devices when restoring the original volume Vfi. The material used for the reduction and sealing devices, which is weaker than the materials forming the furnace itself, can preferably be refractory concrete with an aluminum oxide content of 30 to 35% by weight, based on the total weight of the concrete, while the concrete used to form the furnace itself preferably contains 40 to 42% aluminum oxide. This allows the inserts forming the wall to be constructed from a more cost-effective material.

[0094] The furnace according to the invention is advantageously realized with the same materials and production tools as are used to construct the furnace interior.

[0095] These materials are generally refractory bricks and / or refractory concrete, or a combination of the two. The reducing and sealing means can be pre-fabricated in the factory before installation in the furnace, or they can be manufactured in situ, i.e. in the furnace, by assembling bricks, projecting or moulding concrete in the furnace to form additional walls that make up the kit.

[0096] The kit consists of an additional wall inserted on the wall of the fluidizing area and can be made of refractory bricks or concrete, the technology for placing and removing the refractory material used to manufacture and remove this additional temporary wall being the same as that used to carry out maintenance work or dismantle refractory walls, i.e. by loosening the bricks or cutting them into small, easily transportable pieces for discharge.

[0097] The present invention will now be described in more detail with reference to the accompanying drawings, which show:

[0098] Figure 1 : A longitudinal cross-sectional view of an incinerator according to the present invention in a nominal operating mode;

[0099] Figure 2 : Figure 1 Longitudinal section of a furnace in operational mode.

[0100] As in Figure 1 As can be seen in FIG, a furnace 1 for incinerating organic matter, such as sludge, from the treatment of industrial or agricultural waste or wastewater has a housing 11 , usually made of refractory concrete, in the lower part of which there is a fluidized particle bed 2 , preferably a sand bed.

[0101] The furnace also comprises a wind box 3 which receives fluidizing air, preferably already heated, through radial inlets 31. The upper part of the wind box 3 supports a fluidizing dome 4 which constitutes an important part of the furnace 1. In fact, it is through this dome 4 that the fluidizing air coming from the wind box 3 is distributed by means of channels provided in the dome 4 and through which the air coming from the wind box 3 is distributed in the portion of the housing called the fluidizing zone Zf 5 corresponding to the position of the sand bed 2.

[0102] In this fluidization zone Zf 5, there is at least one sludge feed 6 and at least one supplementary fuel injection device 7. In some furnaces, the sludge feed is implemented above the fluidization zone. A feed device 10 is also provided for introducing sand into the furnace. Above the fluidization zone 5, there is an expansion and post-combustion zone 8, on top of which rests the crown of the housing 11, on which a discharge duct (not shown) for the combustion products is located.

[0103] The furnace 1 is configured to treat a nominal value Vn of volume Vmo of sludge to be treated.

[0104] At the fluidization zone Zf 5 , the wall of the furnace 1 has the shape of a frustoconical body, the largest upper base of which is situated on the side of the shell 11 and the smallest lower base of which is situated on the side of the fluidization dome 4 .

[0105] The fluidization zone 5 thus defines a volume situated above the fluidization dome 4 , in which volume the sand bed 2 is present, which fluidization zone is provided with blowers 51 .

[0106] Thus, a displacement of the sand bed 2 is achieved which is only possible towards the top of the furnace 1, under the action of the heated air coming from the wind boxes 3 and passing through the fluidizing dome 4. The rising speed is related to the injected air flow rate and the lower cross-sectional area of the frustum.

[0107] The rising speed determined for nominal treatment values is typically 0.75 m / s for good fluidization of the sand and 1.2 m / s to avoid excessive sand flying, preferably a speed of 0.9 m / s.

[0108] The furnace 1 is therefore dimensioned for a sludge handling nominal value Vn. However, often, during the commissioning phase of a furnace 1 of this type, the sludge volume is typically much smaller than the desired target nominal value for which the furnace 1 was designed.

[0109] In order to ensure that the furnace 1 operates in an optimized manner even during the commissioning phase, i.e. when the volume of sludge to be treated Vmo is less than the nominal value Vn of the volume of sludge to be treated, the volume of the fluidization zone zf 5 is reduced from the initial volume Vfi to the reduced volume Vfr and the air channels are sealed so that only the channels leading to the reduced fluidization zone 5 ' are active.

[0110] For this purpose, means for reducing the volume of the fluidizing zone Zf 5 and means for sealing the air passages are placed in the fluidizing zone Zf 5 and are designed to activate only the passage to the reduced fluidizing zone 5 ′.

[0111] As in Figure 2 As can be seen in the drawing, the portion of the housing 11 defining the fluidizing zone Zf 5 has supported therethrough an insert in the form of a frustoconical sleeve 9. Said sleeve extends from the fluidizing dome 4 to a height H2 less than or equal to the height H1 of the frustoconical portion of the housing 11.

[0112] This sleeve 9 has an outer diameter Dext corresponding to the inner diameter D1 of the fluidizing zone 5, which varies according to the height H2 of the insert 9, and a smaller inner diameter Dint corresponding to the diameter Dext minus twice the thickness A of the insert 9. The volume of the resulting fluidizing zone Zfr 5' is thereby reduced.

[0113] Since this volume Vfr is reduced, the volume of the sand is also reduced. The air volume flow rate is then limited by sealing the air passages with an insert 9 located partially on the fluidizing dome 4. The frustoconical effect also allows the frustoconical element to be loaded through the frustoconical portion of the housing. The cross-sectional area at the fluidizing dome 4 is reduced, and at the same air velocity, the fluidizing air flow rate is reduced.

[0114] The thickness A of the frustoconical insert 9 is proportional to the reduction in the air flow rate desired for the combustion of the organic matter to be treated (sludge and / or other waste material).

[0115] The fluidizing air volume flow rate of the fluidizing zone 5' having a reduced volume Vfr is therefore reduced in proportion to the initial flow rate according to the following formula:

[0116] Φv'=Φv x ((D1-2A) / D1) 2 ,in,

[0117] D1: Average inner diameter at the base of the fluidized area of the housing designed for future production

[0118] A: The thickness of the element added to the periphery of the fluidizing area in order to reduce the diameter of the fluidizing area, which is generally 0.2 to 0.7, preferably 0.25 to 0.350 m.

[0119] To determine the required air flow rate when the volume of sludge to be treated is less than the nominal value, the formula is:

[0120] Φv'=Φb' x S x MV x Coef1 x Coef2, where

[0121] Φv': Fluidizing air flow rate in kg / h

[0122] Φb': original sludge flow rate in kg / h

[0123] S: the proportion of dry matter in the sludge (original) in %

[0124] MV: The proportion of organic matter in the dry matter in %

[0125] Coef1: The stoichiometric ratio corresponding to the amount of air in kg / h for burning 1 kg of organic matter, which is 5 to 10 depending on the type of organic matter

[0126] Coef2: Excess air desired to ensure complete combustion of organic matter, 1.01 to 1.4.

[0127] Thus, according to one embodiment of the method, a furnace is realized whose fluidization zone has a diameter of 2.83 m (dimension X) and is configured to process at least 963 kgMS / h. Such a furnace is operated with a preheated fluidizing air flow rate of, for example, 550° C., ranging from 6700 Nm3 / h (minimum blowing air velocity for fluidization of 0.75 m / s), corresponding to a sludge dry matter flow rate of, for example, 963 kgMS / h, to 10750 Nm3 / h (maximum fluidization velocity of 1.2 m / s), corresponding to a flow rate of 1550 kgMS / h. The oxygen content is the same in both cases.

[0128] The commissioning of the furnace is characterized by the fact that for flow rates much lower than the minimum flow rate of 600 kgMS / h, a fluidizing air flow rate of 4250 Nm3 / h would be sufficient, and therefore a furnace of smaller dimensions (dimension X-1) would be required. If this flow rate were distributed over seven days of the week and applied to a furnace of the original dimensions (X), the fluidization velocity for this type of constructed furnace would then be 0.47 m / s, which is therefore too low in terms of turbulence in the bed.

[0129] Two solutions are possible. In the first case, the sludge is treated 3 to 4 days a week at the minimum flow rate for which the furnace is designed (963 kgMS / h), with a gas consumption of 19 kg / h during the sludge incineration period. On the other hand, the furnace temperature is then maintained for the rest of the week, which corresponds to a high natural gas consumption of approximately 40 kg / h.

[0130] In the second scenario, the sludge was treated seven days a week at a high air flow rate of 6700 Nm³ / h to meet the minimum fluidizing air velocity of 0.75 m / s to achieve the required turbulence in the sand bed. The excess air caused losses and significantly increased the consumption of auxiliary fuel, so that the consumption went from 19 kg / h (as in the previous scenario, over three to four days of incineration) to 46 kg / h of natural gas over seven days to compensate for the losses. The operator chose this solution to protect the equipment downstream of the furnace.

[0131] The method of the present invention is then implemented, reducing the dimensions of the fluidizing zone to a diameter of 2.23 m by adding, for example, a 300 mm thick insert at a height of 1.3 m. The air volume is then adapted to the sludge volume while maintaining good turbulence in the furnace. The flow rate can be reduced to 4200 Nm³ / h to meet the minimum turbulence velocity of 0.75 m / s in this new, reduced fluidizing zone. Excess air is reduced, and natural gas consumption is advantageously reduced from 46 kg / h to 18 kg / h.

[0132] The process according to the invention not only allows operational savings of approximately 100,000 euros per year, but also advantageously limits CO 2 emissions (616 t of CO 2 per year are no longer released into the atmosphere).

[0133] In certain waste treatment plants already in place, the implementation of an incinerator according to the invention allows annual savings of 50,000 to 200,000 euros in natural gas used as fuel, for an estimated investment of 30,000 to 70,000 euros. The invention thus offers significant economic advantages while being easy to implement, both in existing and new facilities.

Claims

1. A method for incinerating organic matter in a fluidized bed incinerator, said organic matter consisting of organic matter trapped in a mineral and / or liquid matrix, The furnace comprises a casing, in the lower part of which there is a bed of particles constituting a fluidization zone Zf, into which the organic matter is introduced as fuel, while air is injected into the bed of particles as oxidant from a wind box through a fluidization dome situated at the top of the box, the air passing through channels provided in the fluidization dome, The furnace is configured to treat a nominal value Vn of a volume Vmo of organic matter to be treated, It is characterized by: The method includes the step of adjusting the volume of the fluidizing zone Zf and the air flow rate entering the fluidizing zone Zf according to the volume Vmo of the organic matter to be treated, in which step, when the volume Vmo of the organic matter to be treated is less than the nominal value Vn, the volume of the fluidizing zone is reduced from the initial volume Vfi to the reduced volume Vfr, and the air flow rate is reduced by sealing the air passage so that only the passage leading to the thus reduced fluidizing zone Zfr is activated.

2. The method according to claim 1, characterized in that Adjusting the volume of the fluidizing zone is a temporary volume reduction.

3. The method according to claim 2, characterized in that The method comprises a supplementary volume adjustment step in which, once the volume Vmo of the organic matter to be treated approaches or reaches the nominal value Vn, the initial volume Vfi of the fluidization zone and the initial air flow rate suitable for treating the nominal volume Vn of the organic matter to be treated are restored.

4. The method according to any one of claims 1 to 3, characterized in that An insert is mounted in the form of a kit (9) having a shape complementary to that portion of the furnace casing (11) defining the fluidizing zone Zf (5) and extending from the fluidizing dome (4) against said portion of the casing (11) at a height (H) less than or equal to the height (H1) of the fluidizing zone (5).

5. The method according to any one of claims 1 to 3, characterized in that By installing inserts extending over the entire height of the furnace, the volume of the furnace casing above the fluidization zone is also reduced.

6. The method according to claim 4, characterized in that The housing (11) has a frusto-conical shape at an angle of 0° to 45° relative to the vertical direction in the fluidization zone (5).

7. The method according to claim 4, characterized in that The sleeve (9) has a frustoconical shape and: - Its external diameter Dext is equal to the diameter of the frustum of said housing (11) at the height of the added frustum: Dext=D1, which can vary over the height H, wherein D1 is the inner diameter of the fluidization zone of the housing designed for future production, and H is the height of the zone over which particle fluidization dynamics develop, - its inner diameter Dint is equal to the outer diameter of the added frustum minus twice the thickness A of the said set, Dint=Dext-A×2, wherein A is the thickness of the element added around the fluidized area to reduce the diameter of the fluidized area, and the thickness is 0.15 to 0.7 m. The height H of the truncated cone is chosen such that: H=B+C, where B corresponds to the height of the particle bed at rest, B being 0.3 to 1.5 m, and C corresponds to the dynamics, ie the turbulence expected in the particle bed, being 0 to 2 m.

8. The method according to claim 1, characterized in that The organic matter originates from the treatment of wastewater, industrial or agricultural waste.

9. The method according to claim 1, characterized in that The organic matter is sludge.

10. The method according to claim 1, characterized in that The organic matter is sewage treatment plant sludge.

11. The method according to claim 1, wherein The particle bed is a sand bed.

12. The method according to claim 7, characterized in that The thickness A of the elements added around the fluidization zone is between 0.25 m and 0.35 m.

13. The method according to claim 7, characterized in that The height B of the particle bed at rest is equal to about 1 m.

14. The method according to claim 7, wherein: The desired turbulence C in the particle bed is 0.3 m to 0.5 m.

15. A furnace (1) for incinerating organic matter, said furnace comprising a housing in the lower part of which there is a bed of fluidized particles (2), said furnace further comprising, at least from the bottom to the top: a wind box (3), the upper part of which supports a fluidization dome (4), the wind box having a channel through which the air coming from the wind box (3) is distributed in a fluidization zone Zf (5) corresponding to the particle bed (2), at least one organic matter supply device (6) and at least one supplementary fuel injection device (7), arranged to supply said fluidizing zone, Above the fluidization zone Zf (5) there is an expansion and afterburning zone (8), the dome of the housing (11) is located on top of the expansion and afterburning zone, at which dome there is an exhaust duct for combustion products, the furnace (1) is constructed to treat a nominal value Vn of the volume Vmo of the organic matter to be treated, characterised in that the furnace (1) comprises means for reducing the volume of the fluidization zone Zf (5) from an initial volume Vfi to a reduced volume Vfr, and means for sealing the air channel, the means for sealing the air channel being constructed to reduce the input air flow rate by activating only the channel to the reduced fluidization zone Zfr (5') when the volume Vmo of the organic matter to be treated is less than the nominal value Vn.

16. The furnace (1) according to claim 15, characterized in that The means for reducing the volume Vf of the fluidization zone Zf and the means for sealing the air channel are identical.

17. Oven (1) according to one of claims 15 and 16, characterized in that The reducing and sealing means consist of an insert mounted in the fluidizing zone Zf in the form of a kit (9) having a shape complementary to that portion of the housing (11) defining the fluidizing zone Zf (5) and extending from the fluidizing dome (4) against said portion of the housing (11).

18. The furnace (1) according to claim 17, characterized in that The thickness A of the insert (9) is selected to define the volume Vfr of the reduced fluidization zone and the reduced cross-sectional area of the fluidization dome, the thickness A of the insert (9) being proportional to the reduction in the air flow rate desired for burning the organic matter, the desired air flow rate being defined according to the following formula: Φv'=Φb'×S×MV×Coef1×Coef2, where Φv': fluidizing air flow rate in kg / h, Φb': organic matter flow rate in kg / h, S: proportion of dry matter in the organic matter (original) in %, MV: the proportion of organic matter in the dry matter in %, Coef1: stoichiometric ratio corresponding to the amount of air in kg / h for burning 1 kg of organic matter, said stoichiometric ratio being 5 to 10 depending on the type of organic matter, Coef2: The excess air required to ensure complete combustion of the organic matter is 1.01 to 1.4, The air flow rate is also defined by the following formula: Φv'=Φv×((D1-2A) / D1) 2 ,in, D1: average internal diameter at the base of the fluidization zone (5) of the housing designed for future production, Φv: nominal fluidizing air flow rate in kg / h, A: The thickness of the insert (9) added to the periphery of the fluidizing zone (5) in order to reduce the inner diameter of the fluidizing zone (5), which is generally 0.15m to 0.7m.

19. Oven (1) according to one of claims 15 and 16, characterized in that The reducing and sealing device is inserted into an existing furnace.

20. The furnace (1) according to one of claims 15 and 16, characterized in that The reducing and sealing means are provided during construction of the furnace (1).

21. The furnace (1) according to claim 17, characterized in that The insert (9) consists of an additional wall made of refractory material which is inserted into the wall of the fluidizing zone (5).

22. The furnace according to claim 21, characterized in that The materials used are refractory bricks and concrete.

23. Oven (1) according to one of claims 15 and 16, characterized in that The reducing and sealing device can be disassembled.

24. Oven (1) according to one of claims 15 and 16, characterized in that The means for reducing the volume extend beyond the fluidisation zone over the entire height of the furnace.

25. The furnace (1) according to claim 15, characterized in that The organic matter comes from the treatment of wastewater, industrial or agricultural waste.

26. The furnace (1) according to claim 15, characterized in that The organic matter is sludge.

27. The furnace (1) according to claim 15, characterized in that The fluidized particle bed is a sand bed.

28. The furnace (1) according to claim 18, characterized in that The thickness of the insert (9) added around the periphery of the fluidization zone (5) is 0.25m to 0.35m.

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