METHOD AND FURNACE FOR INCINERATING ORGANIC MATTER DERIVED FROM THE TREATMENT OF INDUSTRIAL OR AGRICULTURAL WASTE OR WASTEWATER, SUCH AS SLUDGE

Abstract

A method for incinerating organic matter derived from the treatment of wastewater, or of industrial or agricultural waste, such as sludge and notably treatment plant sludge, is in a fluidized-bed incineration furnace, the furnace including a chamber in the lower part of which there is a bed of particles, preferentially sand, constituting a fluidization zone, in which fluidization zone the organic matter is introduced as fuel whilst air is injected as oxidizer into the bed of sand from a wind box through a fluidization dome surmounting the box. The air passes through passages made in the fluidization dome, and the furnace is configured to treat a nominal value of volume of organic matter to be treated. The method includes a step of adjusting the volume of the fluidization zone as a function of the volume of organic matter to be treated in which, when the volume of organic matter to be treated is lower than the nominal value, the volume of the fluidization zone is reduced from an initial volume to a reduced volume, and the incoming air flow is reduced by closing air passages so only the passages opening into the thus reduced fluidization zone are left active.

Claims

1. A method for incinerating, organic matter trapped in a mineral and/or liquid matrix issued from the treatment of waste water, industrial or agricultural waste, in a fluidised bed incineration furnace, the method comprising: the furnace comprising an enclosure in a lower part of which there is a bed of particles, comprising a fluidisation zone, introducing the organic matter as fuel into the fluidisation zone while injecting air as an oxidant into the bed from a wind box through a fluidisation dome on top of the box, the air passing through passages provided in the fluidisation dome; treating a nominal value of volume of the organic matter to be treated in the furnace; adjusting the volume of the fluidisation zone and a flow rate of the air coming into the fluidisation zone as a function of the volume of the organic matter to be treated, in which, when the volume of the organic matter to be treated is lower than the nominal value, reducing the volume of the fluidisation zone from an initial volume to a reduced volume, and reducing the air flow rate by sealing the air passages so as to leave active only passages opening into the thus reduced fluidisation zone.

2. The method according to claim 1, wherein the adjustment of the volume of the fluidisation zone is a temporary volume reduction.

3. The method according to claim 2, wherein it comprises a complementary volume adjustment step in which, as soon as the volume of the organic matter to be treated approaches or reaches the nominal value, the initial volume of the fluidisation zone suitable for treating the organic matter nominal volume to be treated and the initial air flow rate are restored.

4. The method according to claim 1, further comprising installing an insert in the form of a sleeve of a shape complementary to the portion of the enclosure of the furnace delimiting the fluidisation zone and extending against the portion of the enclosure from the fluidisation dome over a height lower than or equal to a height of the fluidisation zone.

5. The method according to claim 1, further comprising reducing the volume of the furnace enclosure above the fluidisation zone by installing an insert extending over an entire height of the furnace.

6. The method according to claim 4, wherein the enclosure at the fluidisation zone has a cone frustum shape between 0° and 45° with respect to a vertical, the sleeve having a frustoconical shape with; (a) its external diameter Dext equal to a diameter of the cone frustum of the enclosure over a height of the added cone frustum:
Dext=D1 which can vary over a height H D1 being an internal diameter of the fluidisation zone of the enclosure designed for future production and H the height of the zone over which the fluidisation of the particles develops dynamically; (b) its internal diameter Dint equal to an external diameter of the added cone frustum minus two times a thickness A of the sleeve Dint=Dext−A×2, with A being a thickness of the element added at the periphery of the fluidisation zone to reduce the diameter of the fluidisation zone, between 0.15 and 0.7 m; and (c) the height of the cone frustum H being chosen such that: H=B+C, with B corresponding to the height of the particle bed at rest, B being between 0.3 m and 1.5 m, and C corresponding to the dynamics, that is a desired turbulence in the bed between 0 and 2 m.

7. A furnace for incinerating organic matter from treatment of waste water, industrial or agricultural waste, the furnace comprising an enclosure in the lower part of which there is a fluidised particle bed, the furnace further comprising, at least from bottom to top: a wind box, the upper part of which supports a fluidisation dome, having passages through which the air coming from the wind box is distributed into a fluidisation zone which corresponds to the particle bed, at least one organic matter feed and at least a supplemental fuel injector being provided to feed the fluidisation zone, above the fluidisation zone there is an expansion and post-combustion zone, on top of which lies an upper vault of the enclosure at which there is a discharge pipe for combustion products, the furnace being configured for the treatment of a nominal value of volume of the organic matter to be treated, wherein the furnace includes reducing the volume of the fluidisation zone from an initial volume to a reduced volume and a seal operably sealing the air passages, configured to reduce an incoming air flow rate by leaving active only the passages opening into the reduced fluidisation zone, when the volume of the organic matter to be treated is lower than the nominal value.

8. The furnace according to claim 7, wherein the seal operably reducing the volume of the fluidisation zone and sealing the air passages are one and the same part.

9. The furnace according to claim 7, wherein the seal comprises an insert installed in the fluidisation zone, in the form of a sleeve with a shape complementary to a portion of the enclosure delimiting the fluidisation zone and extending against the portion of the enclosure from the fluidisation dome.

10. The furnace according to claim 8, wherein a thickness A of the insert is chosen to define the volume of the reduced fluidisation zone and a reduced cross-sectional area of the fluidisation dome, the thickness A of the insert being proportional to a reduction in the air flow rate desired to combust the organic matter, the desired air flow rate being defined according to the following formula:
Φv′=Φb′×S×MV×Coef1×Coef2 where: Φv′: a fluidisation air flow rate in kg/h Φb′: an organic matter flow rate in kg/h S: a fraction of dry matter in the organic matter (raw) in % MV: a fraction of the organic matter in the dry fraction in % Coef1: a stoichiometric ratio corresponding to an amount of the air in kg/h to combust 1 kg of the organic matter, ranging from 5 to 10 depending on the type of the organic matter Coef2: excess air desired to ensure complete combustion of the organic matter between 1.01 and 1.4, this air flow rate being also defined by the following formula:
Φv′=Φv×((D1−2A)/D1).sup.2 with D1: an average internal diameter at a base of the fluidisation zone of the enclosure designed for future production, Φv: a nominal fluidisation air flow rate in kg/h and A: the thickness of the insert added at a periphery of the fluidisation zone to reduce the internal diameter of the fluidisation zone between 0.15 m and 0.7 m.

11. The furnace according to claim 7, wherein the seal is inserted in an existing furnace.

12. The furnace according to claim 7, wherein the seal is provided during the construction of the furnace.

13. The furnace according to claim 7, further comprising an insert comprising an additional wall, inserted to a wall of the fluidisation zone, made of refractory materials.

14. The furnace according to claim 13, wherein the materials are bricks of refractory material, or concrete.

15. The furnace according to claim 11, wherein the seal can be dismantled.

16. The furnace according to claim 7, wherein the volume reduction extends beyond the fluidisation zone over an entire height of the furnace.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0056] The invention will now be described in more detail with reference to the figures where:

[0057] FIG. 1 is a longitudinal cross-section view of a incineration furnace according to the invention in nominal operating mode; and

[0058] FIG. 2 is a longitudinal cross-sectional view of the furnace of FIG. 1 in start-up operating mode.

DETAILED DESCRIPTION

[0059] As can be seen in FIG. 1, a furnace 1 for incinerating organic matter from the treatment of industrial, agricultural wastes or waste water, such as sludge, has an enclosure 11, typically made of refractory concrete, in the lower part of which is a fluidised particle bed, preferably a sand bed 2. The furnace further comprises a wind box 3 which receives fluidisation air, which preferably has been heated, through a radial inlet 31. The upper part of the wind box 3 supports a fluidisation dome 4 which constitutes an essential part of the furnace 1. Actually, it is through this dome 4 that the fluidisation air coming from the wind box 3 is distributed, by means of passages arranged in the dome 4, and through which the air coming from the wind box 3 is distributed into the part of the enclosure known as the fluidisation zone Zf 5 which corresponds to the location of the sand bed 2.

[0060] In this fluidisation zone Zf 5 there is at least one sludge feed 6 and at least one supplemental fuel injection 7. In some furnaces, this sludge feed is carried out above the fluidisation zone. A feed 10 is also provided for introducing sand into the furnace. Above the fluidisation zone 5 there is the expansion and post-combustion zone 8, on top of which lies the upper vault of the enclosure 11, at which there is a discharge pipe for the combustion products (not represented).

[0061] This furnace 1 is configured for the treatment of a nominal value Vn of volume Vmo of sludge to be treated. At the fluidisation zone Zf 5, the wall of the furnace 1 has the shape of a cone frustrum, the largest upper base of which is on the side of the enclosure 11 and the smallest lower base of which is on the side of the fluidisation dome 4. The fluidisation zone 5 thus delimits a volume in which the sand bed 2 is located above the fluidisation dome 4, arranged with blowing ejectors 51.

[0062] As a result, under the effect of the heated air coming from the wind box 3 and passing through the fluidisation dome 4, a displacement of the sand bed 2 is achieved which can only go towards the top of the furnace 1. The ascensional velocity is related to the injected air flow rate and the lower section of the cone frustum. The ascensional velocity determined for the nominal treatment value is typically between 0.75 m/s for good fluidisation of the sand and 1.2 m/s to avoid too much sand flying away, preferably a velocity of 0.9 m/s.

[0063] The furnace 1 is therefore dimensioned for a sludge treatment nominal value Vn. However, the volume of sludge in the production start-up phase of this type of furnace 1 is recurrently much lower than the desired targeted nominal volume for which the construction of furnace 1 was designed.

[0064] In order to operate the furnace 1 optimally even during the production start-up phase, that is when the volume Vmo of sludge to be treated is lower than the nominal value Vn of the volume of sludge to be treated, the volume of the fluidisation zone zf 5 is reduced from an initial volume Vfi to a reduced volume Vfr, and the air passages are sealed so as to leave active only the passages opening into the reduced fluidisation zone 5′. In order to implement this, means for reducing the volume of said zone 5 and means for sealing the air passages, configured so as to leave active only the passages opening into the reduced fluidisation zone Zfr 5′, are placed in the fluidisation zone Zf 5.

[0065] As can be seen in FIG. 2, the portion of enclosure 11 delimiting the fluidisation zone Zf 5 has an insert supported by it in the form of a frustoconical sleeve 9. This sleeve extends from the fluidisation dome 4 to a height H2 lower than or equal to the height H1 of the frustoconical portion of the enclosure 11. This sleeve 9 has an external diameter Dext which varies as a function of the height H2 of the insert 9, corresponding to the internal diameter D1 of the fluidisation zone 5, and a smaller internal diameter Dint corresponding to the diameter Dext minus twice the value of the thickness A of the insert 9. As a result, the volume of the thus created fluidisation zone Zfr 5′ is reduced.

[0066] As this volume Vfr is reduced, the volume of sand is also reduced. The air volume flow rate is limited by the air passages being sealed by the insert 9 which rests partly on the fluidisation dome 4. The frustoconical effect also allows a load recovery of the frustoconical element by the frustoconical part of the enclosure. The section at the fluidisation dome 4 is reduced and at the same air velocity the fluidisation air flow rate is reduced. The thickness A of the frustoconical insert 9 is proportional to the reduction in the air flow rate desired to combust the organic matter to be treated (sludge and/or other waste).

[0067] The fluidisation air volume flow rate in the fluidisation zone 5′ of reduced volume Vfr is therefore reduced in proportion to the initial flow rate according to the following formula:

Φv′=Φv×((D1−2A)/D1).sup.2 where

[0068] D1: average internal diameter at the base of the fluidisation zone of the enclosure designed for future production

[0069] A: thickness of the element added at the periphery of the fluidisation zone to reduce the diameter of the fluidisation zone and between 0.2 and 0.7 and preferably between 0.25 m and 0.350 m.

[0070] To determine what air flow rate is required with a volume of sludge to be treated lower than the nominal value, the formula is:

Φv′=Φb′×S×MV×Coef1×Coef2, where

Φv′: fluidisation air flow rate in kg/h
Φb′: raw sludge flow in kg/h
S: fraction of dry matter in sludge (raw) in %
MV: fraction of organic matter in the dry fraction in %
Coef1: stoichiometric ratio corresponding to the amount of air in kg/h to combust 1 kg of organic matter, ranging from 5 to 10 depending on the type of organic matter
Coef2: excess air desired to ensure complete combustion of the organic matter between 1.01 and 1.4.

[0071] Thus, according to an exemplary embodiment of the method, a furnace is made whose fluidisation zone diameter is 2.83 m (size X) and which is constructed to treat at least 963 kgMS/h. Such a furnace is operated with a preheated fluidisation air flow rate, for example at 550° C., between 6700 Nm.sup.3/h (minimum blown air velocity for fluidisation ˜0.75 m/s) corresponding to a sludge dry matter flow rate, for example 963 kgMS/h and 10,750 Nm.sup.3/h (maximum fluidisation velocity ˜1.2 m/s) corresponding to a flow rate of 1550 kgMS/h. The oxygen content in both cases is identical.

[0072] The furnace start-up is characterised by a flow rate well below the minimum rate of 600 kgMS/h for which a fluidisation air flow rate of 4250 Nm.sup.3/h would be sufficient and therefore a smaller furnace (size X-1) would be required. If this flow rate is distributed 7 days a week and applied to the original size (X) furnace, then the fluidisation velocity for this type of already constructed furnace would be 0.47 m/s and is therefore too low in terms of turbulence in the bed.

[0073] Two solutions are possible. In the first case, the sludge is treated over 3 to 4 days per week at the minimum flow rate of the furnace for which it was constructed (963 kgMS/h) with a consumption of 19 kg/h of gas required during the sludge incineration. On the other hand, the furnace will then be maintained at temperature for the rest of the week, which corresponds to a significant consumption in the order of 40 kg/h of natural gas.

[0074] In a second case, the sludge is treated 7 days a week with an air flow rate of 6,700 Nm3/h in order to respect the minimum fluidisation air velocity of 0.75 m/s to obtain the necessary turbulence in the sand bed. The excess air generates losses and strongly increases the consumption of auxiliary fuel, increasing from a consumption of 19 kg/h (as in the previous case over the 3 to 4 days of incineration) to 46 kg/h of natural gas over the 7 days, to compensate for the losses. This solution is chosen by the operator to preserve the equipment downstream of the furnace.

[0075] The method of the invention is then implemented, and the dimension of the fluidisation zone is reduced to a diameter of 2.23 m by adding a 300 mm thick insert over a height of 1.3 m, for example. The amount of air is then adapted to the amount of sludge while maintaining good turbulence in the furnace. The flow rate can be reduced to 4200 Nm.sup.3/h to meet the minimum turbulence velocity of 0.75 m/s in this new reduced fluidisation zone. The excess air is reduced, and advantageously the natural gas consumption from 46 kg/h to 18 kg/h.

[0076] The method of the invention not only allows operating savings in the order of 100,000 €/year but also advantageously limits CO.sub.2 emissions (616 t CO.sub.2/year are no longer released into the atmosphere). In certain waste treatment plants already in place, the implementation of an incineration furnace according to the invention allows a gain of natural gas used as fuel of 50 to 200 k€uros per year for an estimated investment of 30 to 70 k€uros. The invention thus provides an interesting economic advantage while being simple to implement in both existing and new installations.