A METHOD AND AN APPARATUS FOR DRYING AND COMBUSTING WATER-CONTAINING MATTER

Abstract: The invention relates to a method for the drying and combustion of water-containing material containing combustible components, such as dehydrated waste water sludge, and wherein a water-containing material is introduced into a direct dryer, drying of the material introduced into the dryer by introduction of atmospheric air as drying air into the dryer, introduction of the dried material into a furnace and combustion of the material therein, introduction into the furnace of at least a portion of the drying air after the dryer as combustion air, preheating of the drying air intended for the drying of the material by utilization of the heat contained in the hot flue gas discharged from the furnace, while water is supplied to the atmospheric air in connection with the preheating whereby the water is evaporated, and wherein the water content in the drying air is reduced after the dryer in a condenser prior to its introduction as combustion air into the furnace. Moreover, the invention relates to a plant for carrying out this method. In such method it is possible to dry e.g. waste water sludge with a high water content to a degree of dryness that allows burning without auxiliary fuelling. Hereby improved operational economy is obtained.

Description: A method and an apparatus for drying and combusting water- containing matter The invention relates to a method for the drying and com- bustion of water-containing material containing combustible components, such as dehydrated waste water sludge, and wherein the following steps are carried out: (1) introduction of water-containing material into a direct dryer, (2) drying of the material introduced into the dryer by the introduction of atmospheric air as drying air into the dryer, (3) introduction of .the dried material into a furnace and combustion of the material therein, (4) introduction into the furnace of at least a portion of the drying air after the dryer as combustion air, (5) preheating of the drying air intended for the drying of the material, by utilization of the heat contained in the hot flue gas discharged from the furnace. For instance in the biological purification of waste water, a residual product consisting of water and organic matter is produced. The organic matter possesses a sufficiently high heating value which makes it attractive to use it as a fuel with a view to exploiting the energy bound therein. Prior to using the organic matter as fuel, however, it should be dried to a degree of dryness that allows it to burn. This may be done by mechanical dehydration of the sludge, for instance by means of centrifuges, strain belt presses or chamber filter presses, in order to subsequently introduce this dehydrated sludge into a dryer, e.g. a direct dryer, where the dehydrated sludge is dried by the introduction of drying air into the dryer. The drying air is usually heated either in a heat exchanger which utilizes hot flue gas as a heating medium, or by means of direct heating. The introduction of the drying air into the furnace as combustion air following drying means that smelling incon¬ veniences are avoided since the foul-smelling substances released from the sludge to the drying air are burnt off. If the drying air is not burnt off in the furnace, it is to be purified in a separate purification plant. In order to avoid these odour inconveniences associated with the flue gas, the furnace temperature should be maintained at about 800-900°C. It is difficult with mechanical dehydration of sludge from state-of-the-art purification plants with nitrogen and phosphorus removal to obtain a high dry matter content. With centrifuges and strain belt presses it is typically possible to obtain 17-18% of dry matter by dehydration of biological/chemical sludge. With chamber filter presses it is typically possible to obtain 22-25% of dry matter by sludge dehydration. It is impossible to dehydrate the sludge at very high pressures to increase the dry matter contents since the sludge forms a filter skin under high pressures which is impervious to water. Moreover, in case of high pressures there is a risk of blowing the cells in the sludge thereby releasing substances whose recycling to the purification plant is undesirable. In the thus known methods for the treatment of sludge, it is difficult to obtain a sufficiently high degree of sludge dryness to allow it to burn without auxiliary fuelling, e.g. fossile fuels. The exercise of the known sludge treatment methods thus involves considerable costs for auxiliary fuelling. It is thus the object of the present invention to provide a method wherein it is possible to incinerate sludge without auxiliary fuelling, said sludge being introduced into the dryer with an increased water content compared to the known methods. This is obtained with a method of the type described in the introductory part for the treatment of water-containing material which contains combustible components, such as dehydrated waste water sludge, and wherein the following steps are carried out: (6) addition of water to the atmospheric air in connec¬ tion with the preheating, whereby the water eva- porates, and (7) reduction of the water content in the drying air following the dryer in a condenser prior to its introduction as combustion air into the furnace. The addition of water to the drying air provides a more effective heat transfer in the heat exchanger, and the enthalpy of the drying air is increased. The enthalpy increase is obtained simultaneously with a reduced tempera¬ ture increase in the drying air. The increased enthalpy of the drying air enables a more effective drying of the sludge introduced into the dryer, and the reduced temperature ensures that the drying may be effected at a reduced risk of pyrolysis of the sludge in connection with the drying. Following discharge of the drying air from the dryer and prior to utilization of this air as combustion air, the water content of the drying air is reduced since this water content would otherwise presuppose a large amount of energy to obtain the required high furnace temperature. Thus, since the method according to the invention eliminates the need for auxiliary fuelling to the same extent by the burning of the sludge, an overall improved operational economy is obtained. Advantageously at least a portion, preferably all of the drying air/combustion air is preheated in a heat exchanger following discharge from the dryer and prior to its introduc¬ tion into the furnace. This enables the introduction of material having a higher water content than usual, since the combustion air has a higher heat content which serves to evaporate the water. The means for preheating the drying air may be divided into several heat exchanges. Examples of such means include a first heat exchanger, wherein the air is evaporated to a certain extent, and wherein the water added is evaporated following which the air is conveyed to a second heat ex¬ changer, where the air is preheated to the temperature required for the drying. Hereby the flue gas may be used as heating medium in both heat exchangers since the discharge side for the heating medium in the one heat exchanger is in communication with the supply side in the second heat exchanger. Moreover, it is an option that only the one heat exchanger is in contact with the flue gas, since the drying air still contains much heat following discharge from the dryer, and therefore it may be used as heating medium in the first heat exchanger. Moreover it is possible to preheat the dehydrated material prior to its introduction into the dryer. Preheating of the material reduces the amount of thermal energy required from the drying air to evaporate the requisite amount of water. The supply of water to the atmospheric air is advantageously controlled in accordance with a desired temperature of the drying air supplied to the dryer. This control is utilized for regulating the drying process whereby the dried material becomes exactly auto-combustible. The addition of water to the drying air may be effected in the form of a separate addition of water, in the form of a condensate from the flue gas in instances where the latter is conveyed through a condensing heat exchanger, or in the form of a condensate of the drying air discharged after the dryer. Moreover it is possible to utilize vapour- or water-contai¬ ning drying air after the dryer as water supply to the drying air. Thus, the drying air is recycled. Moreover the invention relates to a plant for carrying out the method described above and comprising means for prehea¬ ting the drying air, a direct dryer for drying material, a combustion furnace for the combustion of dried material, a conduit for connecting the preheating means to the dryer, and a conduit for conveying dried material from the dryer to the furnace. The plant is characterised in comprising a device for the introduction of water into the drying air, said device being so arranged that the preheating of the drying air in the preheating means causes the formation of hot vapour-containing air for introduction into the dryer, and in having a condenser in connection with a conduit that connects the dryer to the furnace. Advantageously such plant may comprise means for preheating the drying air after the latter has left the dryer and prior to its introduction into the furnace. Since a condenser is used for the condensing of the water content in the drying air/the combustion air, such condenser may conveniently be coupled to a district heating plant whereby the energy released in the condensing may be utilized for heating purposes. The invention will now be described in further detail with reference to the drawings, wherein Figure 1 shows a first plant for exercising the method according to the invention, Figure 2 shows a second plant for exercising the method according to the invention, and Figure 3 shows a third plant for exercising the method according to the invention. Figure 1 is a schematical view of a plant wherein drying air/combustion air is introduced into a heat exchanger 1 where the air is heated and the water evaporated prior to the air being introduced into a direct dryer 2, and into which the dehydrated waste water sludge is also introduced. Following drying of the dehydrated waste water sludge, the air which now contains water is conveyed to a condenser 3 where a part of the water contained in the air is condensed by the introduction of water to cool the air. After the condenser, the air is introduced into a heat exchanger 4 where it is heated prior to its introduction into a furnace 5. Moreover, the dehydrated and dried waste water sludge from the dryer 2 is introduced into the furnace. The waste water sludge is incinerated off in the furnace 5 and hereby residual products in the form of ashes are produced which are discharged from the furnace, and flue gas which is also dis¬ charged from the furnace and conveyed in part to the second heat exchanger 4 and then to the first heat exchanger 1. The flue gas is subsequently discharged. In the first heat exchanger means for the supply of water to the drying air are provided. Condensing operation of the flue gas side of the heat exchanger allows the use of a portion of the condensate as water supply to the drying air side. It is possible to recycle a portion of the air after the dryer 2 to the first heat exchanger since the air still contains a substantial amount of water after the dryer. Figure 2 illustrates a plant which differs from the plant shown in figure 1 in that no heat exchanger is provided after the condenser, and wherein the heat exchanger before the dryer is divided into two heat exchangers 1,4. This division into two heat exchangers is particularly advantageous in case of condensing operation of the latter heat exchanger since condensing operation makes substantial demands to the material. The second heat exchanger 4 may thus be less resistant to corrosion that the first 1. Figure 3 illustrates a plant where air is introduced into a first heat exchanger 1 with water supply for subsequent transfer to a second heat exchanger 4. From the second heat exchanger the air is conveyed to the dryer 2, and from the dryer the air which now contains water is recycled through the first heat exchanger 1 where a portion thereof may optionally be condensed, and from here to a condenser where further vapour in the air is condensed prior to introduction of the air into the furnace 5. The dehydrated waste water sludge is first introduced through a third heat exchanger 6 prior to being introduced into the dryer 2 and then into the furnace 5. The flue gas from the furnace is first used as a heating medium in the second heat exchanger 4 and then as a heating medium in the third heat exchanger 6. Like in connection with the plants shown in Figures 1 and 2, the option is available of utilizing the condensate from the condensed operation as water supply to the drying air in connection with the first heat exchanger 1. In connection with the three plants shown in Figures 1 through 3, the option is also available of introducing auxiliary fuel into the furnace. This is necessary in connection with the starting up of such plant but not necessary provided the degree of dryness of the waste water sludge introduced into the furnace is sufficient for it to be burnt without auxiliary fuelling which will be the normal situation. In the following, examples of treatment of the waste water sludge in the three plants are shown schematically in Figures 1 through 3. Comparative data are shown for drying and combustion without and drying and combustion with water supply to the drying air, respectively. CONDITIONS Waste water sludge of the following composition of the combustible portion of the waste water sludge (water- and ash-free sample) is treated: C % 56.6 H % 7.4 N N % % : 4.6 S % 1.4 0 % 30.0 The combustible portion of the dry matter is 65%. The remaining portion 35% is ashes. This yields the following thermal values for the water- and ash-free sample: Upper heating value: 24,400 kJ/kg Lower heating value: 22,800 kJ/kg The following operating conditions remain unchanged in all examples: T flue gas, furnace: 850°C Lambda (excess air count) : 1.4 x represents the water contents of the respective gas (kg water/kg dry gas) . EXAMPLE 1 fa plant corresponding to Figure 1) In order to burn without auxiliary fuelling the waste water sludge introduced into the furnace must have a dry matter content of 26.1%. Water No water added added Dehydrated waste water sludge: kg/h 1327 1327 Combustible dry matter kg/h 169 197 Ashes kg/h 91 106 Water kg/h 1067 1024 Furnace: Airin* Amount, dry kg/h 1846 2148 x-air kg/kg 0.028 0.028 Temperature β C 600 600 Gasout * Amount, dry kg/h 1904 2215 x-gas kg/kg 0.473 0.473 Heat Exchanger 1: Air in : x-air kg/kg 0.004 0.004 Temperature ■ C 20 20 Alr out : x-air kg/h 0.412 0.004 Temperature °C 450 450 Evaporated water kg/h 754 0 Gasin* x-gas kg/kg 0.473 0.473 Temperature 575 575 Gas out * x-gas kg/kg 0.155 0.473 Temperature β C 60 366 Condensed water: kg/h 606 0 Heat Exchanger 2 : Air in : 10 Temperature °C 30 30 Air out * Temperature β C 600 600 Gas in - Temperature 850 850 15 Gas out * Temperature 575 575 Dryer: 20 Waste water sludge in : Dry matter kg/h 260 303 DM % 19.6 22.8 Temperature • c 10 10 25 Waste water sludge out : DM % 26.1 26.1 Temperature • c 95 95 30 Air jn : x-air kg/kg 0.412 0.004 Temperature °C 450 450 35. Airout * x-air kg/kg 0.591 0.085 Temperature 95 95 Condenser: Necessary cooling: kw 773 128 Comparison of the data relating to the example without addition of water to the drying air with the example with addition of water will show that in order to obtain the dry matter content of 26.1% necessary for combustion without addition of water, the initial waste water sludge material must have a dry matter content of 22.8%. With addition of water, a waste water sludge material with a dry matter content of 19.6% may be introduced into the dryer to obtain the requisite 26.1% of dry matter. Thus, in the given operational conditions for the method according to the invention, a dry matter content is allowed which, relative to the total amount of waste water sludge, is 3.2 percent points less that in the method known today. This corresponds to a reduction of the requirement to the dry matter content in the waste water sludge of 14.0% compared to the known method. EXAMPLE 2 (a plant corresponding to Figure 2) In order to burn without auxiliary fuelling the waste water sludge introduced into the furnace must have a dry matter content of 37.4%. Water No water added added Dehydrated waste water sludge: kg/h 1327 1327 Combustible dry matter kg/h 172 208 Ashes kg/h 93 112 Water kg/h 1062 1007 Furnace : Air in : Amount, dry kg/h 1884 2270 x-air kg/kg 0.028 0.028 Temperature β C 30 30 Gasout * Amount, dry kg/h 1943 2341 x-gas kg/kg 0.315 0.315 Heat Exchanger 1 + 2: Air in : x-air kg/kg 0.004 0.004 Temperature • c 20 20 Alr out : x-air kg/h 0.278 0.004 Temperature • c 700 700 Evaporated water kg/h 515 0 Gas in : x-gas kg/kg 0.315 0.315 Temperature °C 850 850 Gas out : x-gas kg/kg 0.155 0.315 Temperature β C 60 463 Condensed water: kg/h 311 0 Waste water sludge dryer: Waste water sludge in : Dry matter kg/h 265 320 DM % 20.0 24.1 Temperature 10 10 Waste water sludge out : DM % 37 . 4 37 . 4 Temperature ° C 95 95 Air in : x-air kg/kg 0. 278 0. 004 Temperature β C 700 700 Airout' Temperature 95 95 Condenser: Necessary cooling: kw 811 340 Comparison of the data relating to the example without addition of water to the drying air with the example with addition of water will show that in order to obtain the dry matter content of 37.4% necessary for combustion without addition of water, the initial waste water sludge material must have a dry matter content of 24.1%. With addition of water, a waste water sludge material with a dry matter content of 20.0% may be introduced into the dryer to obtain the requisite 37.4% of dry matter. Thus, in the given operational conditions for the method according to the invention, a dry matter content is allowed which, relative to the total amount of waste water sludge, is 4.1 percent points less that in the method known today. This corresponds to a reduction of the requirement to the dry matter content in the waste water sludge of 17.0% compared to the known method. EXAMPLE 3 fa plant corresponding to Figure 3) In order to burn without auxiliary fuelling, the waste water sludge introduced into the furnace must have a dry matter content of 37.4%. Water No water added added Dehydrated waste water sludge: kg/h 1327 1327 Combustible dry matter kg/h 138 198 Ashes kg/h 74 107 Water kg/h 1115 1022 Furnace: Air jn : Amount, dry kg/h 1507 2167 x-air kg/kg 0.028 0.028 Temperature • C 30 30 Gasout* Amount, dry kg/h 1554 2234 x-gas kg/kg 0.315 0.315 Heat Exchanger 1: Fresh air Air in : x-air kg/kg 0.004 0.004 Temperature β C 20 20 Alr out : x-air kg/h 0.609 0.004 Temperature °C 85 85 Evaporated water kg/h 912 0 Drying air Air in : x-air kg/kg 1.129 0.253 Temperature β C 95 95 Alr out : x-air kg/kg 0.480 0.253 Temperature β C 78 68 Condensed water: kg/h 978 0 Heat Exchanger 2 : Air in : Temperature • C 85 85 Air out ' Temperature 650 650 Gas in : Temperature • C 850 850 Gas out ' Temperature β C 127 530 Heat Exchanger 3: Waste water sludge in : Temperature 10 10 Waste water sludge out : Temperature 85 85 Gas i-n : x-gas kg/kg 0.315 0.315 Temperature ° C 127 530 Gas out * x-gas kg/kg 0.250 0.315 Temperature β C 68 362 Condensed water: kg/h 101 Dryer: Waste water sludge jn : Dry matter kg/h 212 305 DM % 16.0 23.0 Temperature • c 85 85 Waste water sludge^: DM % 37.4 37.4 Temperature • c 95 95 Air in : x-air kg/kg 0.609 0.004 Temperature • c 650 650 Air.out' x-air kg/kg 1.129 0.253 Temperature • c 95 95 Condenser: Necessary cooling: kW 1205 387 Comparison of the data relating to the example without addition of water to the drying air with the example with addition of water will show that in order to obtain the dry matter content of 37.4% necessary for combustion without addition of water, the initial waste water sludge material must have a dry matter content of 23.0%. With addition of water, a waste water sludge material with a dry matter content of 16.0% may be introduced into the dryer to obtain the requisite 37.4% of dry matter. Thus, in the given operational conditions for the method according to the invention, a dry matter content is allowed which, relative to the total amount of waste water sludge, is 7.0 percent points less that in the method known today. This corresponds to a reduction of the requirement to the dry matter content in the waste water sludge of 30.4% compared to the known method. Reference numerals: 1 Heat exchanger 1 2 Dryer 3 Condenser 4 Heat exchanger 2 5 Furnace 6 Heat exchanger 3

Claims: l a i m s 1. A method for the drying and combustion of water- containing material containing combustible components, such 5 as dehydrated waste water sludge, and wherein the following steps are carried out: (1) introduction of water-containing material into a direct dryer, 10 (2) drying of the material introduced into the dryer by the introduction of atmospheric air as drying air into the dryer, (3) introduction of the dried material into a furnace and combustion of the material therein, 15 (4) introduction into the furnace of at least a portion of the drying air after the dryer as combustion air, (5) preheating of the drying air intended for the drying of the material, by utilization of the heat 20 contained in the hot flue gas discharged from the furnace, c h a r a c t e r i z e d in the 25 (6) addition of water to the atmospheric air in connec¬ tion with the preheating, whereby the water eva¬ porates. and 30 (7) reduction of the water content in the drying air following the dryer in a condenser prior to the introduction as combustion air into the furnace. 35. 2. A method according to claim 1, c h a r a c t e r - i z e d in that at least a portion of the drying air is preheated in a heat exchanger after the dryer and condenser and prior to its introduction into the furnace. 3. A method according to claim l or 2, c h a r a c ¬ t e r i z e d in that at least a portion of the drying air is further preheated in a heat exchanger where the heating medium is flue gas from the furnace, after the evaporation of the water and prior to its introduction into the dryer. 4. A method according to claims 1-3, c h a r a c t e ¬ r i z e d in that, after removal from the dryer, at least a portion of the drying air is utilized as heating medium in a heat exchanger for pre.heating the drying air and evaporating the water added. 5. A method according to claims 1-4, c h a r a c t e ¬ r i z e d in that at least a portion of the water-containing material is preheated in a heat exchanger prior to its introduction into the dryer. 6. A method according to claims 1-5, c h a r a c t e ¬ r i z e d in that the water added to the drying air is a condensate of the flue gas or a condensate of the drying air discharged after the dryer. 7. A method according to claims 1-6, c h a r a c t e ¬ r i z e d in that the water added to the drying air is vapour or water-containing drying air discharged after the dryer. 8. A method according to claims 1-7, c h a r a c t e ¬ r i z e d in that water is added to the drying air in an amount determined by the fact that the temperature and volume flow of drying air through the dryer relative to the tempera¬ ture, water content and volume flow of the water-containing material will produce a dried material which is exactly auto- combustible. 9. A plant for- exercising the method according to claims 1-8 and comprising means (1) for preheating drying air, a dryer (2) for drying material to be burnt, a furnace (5) for burning dried material, a conduit for connecting the preheating means to the dryer and a conduit for conveying dried material from the dryer to the furnace, c h a r a c - t e r i z e d in comprising a device for the introduction of water into the drying air, where said device is so arranged that the preheating of the drying air in the preheating means causes the formation of air containing superheated steam for introduction into the dryer, and in that a condenser is provided in connection with a conduit connecting the dryer to the furnace. 10. A plant according to claim 9, wherein at leaεt a portion of the drying -air is used as combustion air, c h a - r a c t e r i z e d in comprising means for preheating the drying air after it has left the dryer and the condenser and prior to introduction of this air into the furnace.

Inventors: BISGAARD CHRISTIAN, SIMONSEN NIELS, BISGAARD, CHRISTIAN, SIMONSEN, NIELS
Applicants: BISGAARD CHRISTIAN, KRUEGER AS I, SIMONSEN NIELS, BISGAARD, CHRISTIAN, KRUEGER A/S, SIMONSEN, NIELS
Application year: 1996
Main classification: F23G5
IPC codes: F23G5, F23G7