FIELD OF THE INVENTION

The invention relates to a method for combus- tion of sludge as de ^' fined in the preamble of claim 1 and to an apparatus for combustion of sludge as defined in the preamble of claim 18.

BACKGROUND OF THE INVENTION

The closest prior art in terms of this invention is represented by the drying methods based on fluidized ^' bed dryers using hot water or steam as the heat source in the drying, and combustion methods based on fluidized bed combustion, the installations based on these being supplied by several manufacturers .

It is further known to dry wet sludge by drying the wet . sludge in a fluidized bed dryer in which hot water or steam is conveyed into the tubes of a tube heat exchanger fitted in the fluidized bed thereof or to the jacket that surrounds the fluidized bed. It is also known to combust sludge in a fluidized bed reactor that may further partially or entirely■ produce ^' the hot water or steam that is necessary for the dry- ing.

The above-mentioned known sludge combustion methods have for example the following problems. All hot water or steam operated dryers require a pressurized hot water or steam system which significantly i ^ creases the manufacturing costs of the sludge combustion installation. The hot water or steam ^' operated dryers require a pressurized hot water or steam system, the use of which requires that the user be qualified as provided in the Pressure Equipment Act. In the hot water or steam heated sludge combustion installations, a separate heat recovery part disposed after the combustion reactor is necessary for cooling the flue gas, which significantly increases the cost of manufacture of the sludge combustion installation. The heat exchanger of the circulating mass dryer must be designed and implemented as a pressure vessel, which makes the dryer weighty and expensive. When hot water or steam is used as the heat source of the dryer, the heat exchanger temperature difference becomes small, practically only 30 - 40 °C, and consequently the thermal power intensity in the heat exchanger becomes low, being only of the order of 2 - 3 kW/m ² . This makes the dryer, as well as the sludge combustion installation,, costly. As the height of the tubes in the heat exchanger must be limited to 6 meters, for in^ stance due to maintainability, this together with the low heat exchange power intensity results in a large specific tube number, i.e. tube number/drying power, and specific cross-sectional surface of the riser, i.e. cross-sectional surface/drying power. This in turn results in a high specific internal . consumption of the sludge combustion installation, i.e. internal consumption/drying power.

OBJECTIVE OF THE INVENTION

The objective of the invention is to provide a solution by which the above-mentioned problems of the known sludge combustion and sludge drying technology, of which the greatest ones include the high apparatus investment and operating costs, can be substan- tially reduced.

The objective of the invention is to disclose a novel method and apparatus for treating wet sludge at low costs and in an easy-to-use and efficient manner .

SUMMARY OF THE INVENTION The method and apparatus according to the invention are characterized by what is presented in the claims .

The invention is based on a method for com- bustion of sludges. According to the invention, the wet sludge is dried in a circulating mass dryer, the dried sludge is combusted in a circulating mass reactor, and flue gas is conveyed from the circulating mass reactor to the circulating mass dryer as a heat exchange materi- al in order to transfer heat to the sludge to be dried. Preferably, at least a part of the flue gas formed in the circulating mass reactor is conveyed to the circulating mass dryer.

In addition, the invention is based on an appa- ratus for combustion of sludges. According to the invention, the apparatus includes a circulating mass dryer for drying wet sludge, first feeding means for feeding wet sludge to the circulating mass dryer, a circulating mass reactor for combusting dried sludge, second feeding means for conveying dried sludge from the circulating mass dryer to the circulating mass reactor, and flue gas circulating means for conveying flue gas from the circulating mass reactor to the circulating mass dryer as a heat exchange material in order to transfer heat to the sludge to be. dried.

In this connection, sludge refers to any sludge-type raw material formed by a liquid and a solid material.

In one embodiment, the flue gas is arranged to have a temperature of 500 - 900 °C, preferably a temperature of 500 - 700 °C. Preferably, the flue gas is cooled after the circulating mass reactor. In one embodiment, the temperature of the flue gas is adjusted by means of a heat exchanger. In one embodiment, the sepa- rated flue gas is conveyed from the circulating mass reactor to the heat exchanger, e.g. in order to cool flue gas by means of combustion _. air, wherein the temperature of the flue gas is adjusted to be 500 - 700 °C. In one embodiment, the flue gas is conveyed to a separator, e.g. a cyclonic separator, at a temperature of 500 - 700 °C. In one embodiment, the cooled flue gas is conveyed to the circulating mass dryer after the heat exchanger or after the separator, such as the cyclonic separator.

In one embodiment, a circulating mass dryer including two adjacent circulating mass systems in heat exchange communication to one another is used, wherein the first circulating mass system is a sludge drying side and the second circulating mass system is a heat releasing side, and wet sludge is fed to the first circulating mass' system and flue gas to the second circu- lating mass system. In one embodiment, fluidization material, wet sludge and gas are provided to the fluidization chamber of the first circulating mass system, wet sludge is fluidized, i.e. carried in a fluidized state, upwards together with fluidization material and gas while being dried in at least one elongated first riser, the mixture formed by solid fluidization material and dried sludge is returned by means of a first set of cir^ culation channels to the fluidization chamber and dried sludge including fluidization material is discharged by means of dried sludge discharging means from the dryer. The first set of circulation channels may include the upper part of the riser on the drying side, a separator for separating gas from solid material and a solid material return channel. The separated gas may be circulated as a circulation gas back to the circulating mass dryer or alternatively it may be conveyed to a condenser or to the circulating mass reactor. From the condenser, the uncondensed gas may be conveyed to the circulating mass reactor or to the circulating mass dryer. In one embodi- ment, flue gas is fed to the second circulating mass system, flue gas is fluidized upwards together with flu- idization material in at least one elongated second riser and fluidization material is circulated by means of a second set of circulation channels. The second set of circulation channels may include a separator for sepa- rating flue gas from solid material and a solid material return channel. In this connection, the riser may be any type and shape of a tubular channel, a tube or the like for the conveying of material compositions upwards in the dryer in a closed space.

In one embodiment, a circulating mass dryer de- fined in the patent application filed by the same applicant on the same day as the present application or any embodiment of said circulating mass dryer is used.

In one embodiment, the temperature at the lower part of the second riser in the circulating mass dryer is adjusted by controlling the fluidization material flow that passes through the return channel of the second set of circulation channels.

In one embodiment, dried sludge including flu- idization material is conveyed from the circulating mass dryer to the circulating mass reactor.

In one embodiment, dried sludge including fluidization material is conveyed from the circulating mass dryer to the circulating mass reactor as the setpoint control of the pressure difference of the drying side of the dryer.

In one embodiment, a circulating mass reactor is used, wherein dried sludge including fluidization material is fed from the circulating mass dryer to the lower combustion chamber of the circulating mass reactor, fluidization material and gas are provided to the lower combustion chamber of the circulating mass reactor including a fluidization chamber, dried sludge is conveyed upwards with fluidization material and gas along a flow channel to an upper combustion chamber, the dried sludge is combusted in the device assembly formed by the lower combustion chamber, the flow channel and the upper combustion chamber, the fluidization material is separated from flue gases after the upper combustion chamber, e.g. by means of a separator, and the fluidization material is circulated through a set of return channels to the lower combustion chamber, preferably to the fluidization chamber thereof. In one embodiment, the set of return channels includes a cooled set of return channels. In one embodiment, the set of return channels in- eludes an uncooled set of return channels. In one embodiment, the set of return channels includes a cooled and an uncooled set of return channels, wherein fluidization material is circulated to the lower combustion chamber through the cooled and/or the uncooled set of return channels in a desired ratio. In one embodiment, fluidization material or at least a part thereof is cooled by means of a heat exchanger before conveying it. back to the lower combustion chamber through a return channel. Preferably, the dried sludge is substantially completely combusted in the device assembly formed by the lower combustion chamber, the flow channel and the upper combustion chamber before discharging the flue gases from the upper combustion chamber.

In one embodiment, a circulating mass reactor defined in application PCT/FI2012/050057 or any embodiment thereof is used.

In one embodiment, the temperature of the circulating mass reactor is adjusted by controlling the circulating mass flow in the heat exchanger fitted in the set of return channels in the circulation of fluidization material.

In one embodiment, at least a part of the heat contained by flue gas formed in the circulating mass reactor is transferred for use in the circulating mass re- actor, e.g. by means of fluidization material. The heat may be recovered from flue gas e.g. by means of the heat exchanger and transferred to fluidization material. In one embodiment, the heat is recovered from flue gas in connection with the cooling of flue gas before conveying it to the circulating mass dryer, e.g. by means of the heat exchanger.

In one embodiment, the volume fraction of fluidization material in the upper combustion chamber of the circulating mass reactor, is provided between 0.005 and 0.05.

In one embodiment, the horizontal speed component of gas in the lower combustion chamber of the circulating mass reactor is provided between 0.5 and .7.0 m/s.

In one embodiment, the vertical speed component of gas in the flow channel of the circulating mass reactor is provided between 3 and 20 m/s, preferably between 3 and 15 m/s .

In one embodiment, preferably regenerated and hot fluidization material is conveyed from the circulating mass reactor to the circulating mass dryer, preferably to the drying side of the circulating mass dryer to complement the heat quantity necessary for drying. In one embodiment, fluidization material is conveyed from the circulating mass reactor to the circulating mass dryer as the setpoint control of the temperature of the fluidized bed of the drying side of the dryer.

Preferably, the invention is specifically based on drying sludge by conveying flue gas of the combustion reactor to the circulating mass system on the heat releasing side of the dryer and fluidization material of the combustion reactor to the circulating mass system- on the heat receiving side of the dryer and on the combustion of dried sludge in said combustion reactor based on the circulating mass technology.

In the circulating mass dryers of the prior art, problems have basically been caused by the use of hot water or steam as the heat source instead of flue gas of the combustion reactor and/or by fluidization material being formed by dried sludge.

The method according to the invention sub- stantially reduces the problems of the known sludge combustion methods. In the method according to the invention, the thermal energy of flue gas in the combustion reactor is utilized by directly conveying the flue gas to the drying of the circulating mass dryer as the heat releasing material. This arrangement provides significant advantages. The heat exchange of the dryer may be carried out in a non-pressurized structure, which makes the heat exchange solution substantially lighter and cheaper relative to a pressurized heat exchanger. The average heat exchange temperature difference may be increased to be 4 - 5 fold relative to a water or steam heated dryer, so that the heat delivery surface required for the same heat exchange power is only about 25% of the heat delivery surface in a water or steam heated dryer of the same efficiency. As the blowing power of the dryer is proportional to the heat delivery surface of the dryer, the blowing power of a flue gas heated dryer needs to be only about 25% of the blowing power in a water or steam heated dryer of the same efficiency. The blowing power means herein that, as the thermal power of the dryer is proportional to the heat delivery surface, the tube number with a given tube size is proportional to the heat delivery surface, and as the volume flow of the blower in the dryer is proportional to the tube number, the volume flow, and thus the blowing power, of the blower is proportional to the heat delivery surface. The heat recovery part after the combustion reactor is significantly reduced and in many cases be- comes even unnecessary, which significantly reduces the costs of manufacturing the sludge combustion installation.

In addition, as the sludge combustion method according to the invention uses the fluidization mate- rial of the combustion reactor as the fluidization material that is most suitably conveyed to the dryer as the setpoint control of the temperature of the fluid- ized bed of the dryer and fluidization material covered by sludge or a mixture of sludge and fluidization material is conveyed from the dryer to the combustion reactor as the setpoint control of the pressure difference of the heat receiving side, i.e. the drying side, of the dryer, many significant advantages are provided. If the thermal energy of flue gas after the combustion reactor only covers a part of the heat requirement of the dryer, the rest of the heat requirement of the dryer may be brought to the dryer in the method according to the invention by conveying hot fluidization material for example at a temperature of about 800°C to the dryer from the combustion reactor. This further reduces the heat exchange heat delivery surface and the internal consumption of the dryer. The growth and agglomeration of particles are prevented in the dryer because the fluidization material covered by sludge that is conveyed from the dryer to the combustion reactor is regenerated in the combustion reactor to its original size before returning it to the dryer.

Before, the use of hot gas as the heat source of dryers has been considered problematic. To convey gas after the combustion reactor to the heat exchanger of the dryer may cause structural problems in differ ^¬ ent parts of the process due to the different thermal expansion. To provide high power intensity, the gas after the combustion reactor should more preferably be conveyed to the dryer at a temperature of above 500°C, in which case the surface temperature of the heat ex- changer in the dryer may temporarily and locally rise too much for the durability of the structure. The gas after the combustion reactor contains a great deal of ash that causes a need to sweep the heat delivery surfaces, which is why the packing density of the heat delivery surface becomes small and sweeping equipment is needed. The problems presented above have been solved in the sludge combustion method according to the invention by circulating a powdery material, e.g. sand or other suitable fluidization material of a particle size most suitably between 0.1 and 0.5 mm, on the heat releasing gas side of the dryer. The fluidization material that circulates on the heat releasing gas side of the dryer continuously sweeps the heat releasing side so as to keep it clean, and the packing density of the heat delivery surface can be high. In addition, the maximum temperature on the heat releasing side of the dryer may be limited to a desired value by controlling the mass flow of fluidization material irrespective of the temperature of the gas supplied. Another advantage is that the fluidization mass flow increases the thermal transmittance coefficient on the gas side of the dryer.

As a main rule, sludges are defined as waste, so the sludge combustion flue gas must be kept at a temperature of at least 850°C for at least 2 seconds. On the other hand, the rise of temperature substantially higher than required causes problems in the form of melting of ash and shortening of the service life of the structures. Thus, the control of the tem ^¬ perature in the combustion reactor is substantially important. For this reason, the temperature of the flue gas in the combustion reactor is maintained at the setpoint in the method according to the invention, e.g. by conveying a part of the fluidization material flow that circulates in the combustion reactor to the heat exchanger fitted in the return channel of the combustion reactor, e.g. an intermediate circulation cooler, in which the temperature of the fluidization material drops. As the fluidization material cooled in the intermediate circulation cooler is returned to the substantially thermally insulated combustion chamber, it provides adiabatic cooling, by virtue of which the temperature of the flue gas in the combustion reactor is maintained at the setpoint irrespective of the changes i the fluidization material exchange between the dryer and the combustion reactor.

The invention provides reduction of the manufacturing and operating costs in sludge combustion installations for less than a half of the manufacturing and operating costs in a sludge combustion installation of the same efficiency that uses water or steam as the heat source of the dryer.

The method according to the invention may be applied in the treatment of any kind of sludge, e.g. in the treatment of sludges from waste water purification plants.

LIST OF FIGURES

Fig. 1 shows one process according to the in- vention.

DETAILED DESCRIPTION OF THE INVENTION

Below, the invention will be described by detailed exemplary embodiments with reference to the ac- companying figure.

Example 1

Fig. 1 shows one process with proper devices according to the invention. The wet sludge (104) is fed from a silo (1) by a first feeding means (2) to the fluidization chamber (5) of a circulating mass dryer (100) in which the wet sludge (104) is mixed with fluidization material in a fluidized bed. Steam that is formed in the dryer (100) and that contains, to some extent, uncondensed gases, such as air, and organic compounds is used as fluidization gas in a preferred embodiment and is circulated from the central tube (9) of the cyclonic sep- arator (8) in the dryer through a secondary cyclone (24) by a circulating gas blower (3) as circulating gas (105) ^' to the gas box (4) of the dryer, from which the circulating gas is distributed by means of a grate (18) to the fluidized bed of the fluidization chamber (5) . From the fluidization chamber (5), the mixture of wet sludge, circulating gas and fluidization material rises through the elongated riser (6) on the drying side of the dryer, which has a common heat exchange surface (121) with the elongated riser (12) on the heat releasing side, to the upper part (7) of the riser on the drying side, and is tangentially conveyed to the cyclonic separator (8). The circulating gas is separated from the mixture of fluidization particles and sludge in the cyclonic separator (8) and the cir- culating gas is conveyed from the cyclonic separator (8) through the central tube (9) to the secondary cy ^¬ clone (24) and the mixture of fluidization particles and sludge gravitationally passes through a return channel (10) back to the fluidization chamber (5). Steam generated in the dryer may be conveyed from the secondary cyclone (24) to a condenser (21) by separating the flow to the condenser (21) from the circulat ^¬ ing gas flow to the dryer (100) after the circulation blower (3) and before the gas box (4) of the dryer. Most of the steam is condensed in the condenser (21) and the condensate that is generated is removed from the condenser. The dust separated in the secondary cyclone (24) is conveyed to the feed tube (25) of a circulating mass reactor (101) . The uncondensed gas is conveyed from the condenser (21) by a gas blower (22) to the flow channel (33) of the reactor (101) in which the hydrocarbon compounds of the uncondensed gas are oxidized into odorless oxides.

The circulating mass dryer (100) includes two adjacent circulating mass systems in heat exchange communication to one another, wherein the first circulating mass system is the wet sludge (104) drying side as described above and the second circulating mass system is the heat releasing side to which flue gas (103) is fed from the circulating mass reactor (101) as the heat releasing . heat exchange material to release heat through the common heat exchange surface (121) to the sludge (104) to be dried. In a preferred embodiment, the flue gas is provided to a temperature of 500 -- 700 °C before feeding it to the dryer.

The dried sludge (106) together with fluidization material on the drying side of the dryer are conveyed from the fluidization chamber (5) most suita ^¬ bly through a discharge tube (19) fitted on the bottom thereof to the conveyor (20) of the dryer which car- ries the sludge and fluidization particles to the feed tube (25) of the reactor (101) under control of the setpoint control of the pressure difference (DPC1) of the drying side of the dryer. The temperature in the fluidization chamber (5) on the drying side of the dryer is maintained at the setpoint by conveying re ^¬ generated and preferably hot fluidization material (102) from the reactor (101) through a connecting tube (26) to the fluidized bed (5) of the dryer by control ^¬ ling the fluidization material flow of the connecting tube (26) by an actuator (27) as the setpoint control of the temperature (TC2) of the fluidized bed (5). The heat quantity that is necessary for the drying is complemented by means of the regenerated fluidization material (102) .

The dried sludge and fluidization particles 5 (106) are directed through the feed tube (25) to the. lower combustion chamber (32) of the reactor (101) including a fluidization chamber, to which combustion air is also conveyed through the grate (31) of the reactor. Additional fluidization material may be fed to

10 the lower combustion chamber (32). The sludge (106) is oxidized in the lower (32) and upper (34) combustion chamber, while the fluidization particles are regenerated. Coarse solid material accumulated in the lower combustion chamber (32) of the reactor is discharged

15 by a discharge tube (42) and a conveyor (43). under control of the pressure difference (DPC2) of the lower combustion chamber (32) . It is preferred that the feed tube (25) and the flow channel (33) are disposed at

^'' opposite ends of the lower combustion chamber (32), so

20 that a significant horizontal speed component of gas is generated in the lower combustion chamber (32), the ratio between combustion air and sludge is constantly changed and the sludge and combustion air are efficiently mixed.

25 From the combustion chamber (32), the sludge, gas and fluidization particles rise through the flow channel (33) to the upper combustion chamber (34) .· In the flow channel (33) fitted at the end of the lower combustion chamber (32), the vertical speed of gas is -30 much -higher than in the lower fluidization chamber (32) and preferably 3 - 20 m/s, most preferably 3 - 15 m/s. From the upper combustion chamber (34), the flue gas and fluidization material are conveyed to a separating cyclone (35) in which the flue gas and fluidi-

35 zation particles are separated and the fluidization particles gravitationally drop to a return channel (37) and the flue gas exits the separating cyclone

(35) through a central tube (36) . It is preferred in the circulating mass reactor (101) according to the invention that the combustion chambers (32,34) and the flow channel (33) that connects them are substantially thermally insulated.

Fitted after the return channel (37) are return channels (38) and (40), wherein the return channel (40) includes heat exchange surfaces (39) and the return channel (38) is uncooled. Through the return channels (38) and (40), fluidization material is returned to the lower combustion chamber (32) . The fluidization material flow that passes through the heat exchanger (39) is controlled by an actuator (41) fit- ted in the lower part of the return channel (40) and controlled according to the setpoint of the temperature (TC3) of the upper combustion chamber (34) . Preferably, fluidization material is cooled by means of the heat exchanger (39) . The part of the fluidization material not directed to the return channel (40) is directed to the lower combustion chamber (32) of the reactor through the return channel (38) as the set- point control of the pressure difference (DPC3) of the flow channel (33).

The flue gas separated from the central tube

(36) of the separating cyclone (35) in the reactor is conveyed to a LUVO heat exchanger (29) . In the heat exchanger (29), flue gas is cooled to a temperature of 500 - 700 °C by the combustion air conveyed by means of an air blower (28) to the heat exchanger (29) . From the heat exchanger (29) , the heated combustion air is conveyed as combustion air to the air box (30) of the circulating mass reactor (101) and distributed to the reactor through the grate (31) . The cooled flue gas (103) is conveyed from the LUVO heat exchanger (29) through a hot cyclone (23) to the flue gas box (11) of the dryer. (100) and further through a grate (17) to the lower part of the riser (12) on the heat releasing side of the dryer where fluidization material is present. The flue gas (103) and fluidization particles rise upwards along the riser (12) to a cyclonic separator (13) in which the flue gas and fluidization particles are separated. The flue gas exits through the central tube (14) of the cyclonic separator (13) and the separated fluidization particles . are passed to a return channel (15) in which the fluidization particles are kept in a packed state. From the return channel (15), fluidization particles are conveyed to the lower part of the riser (12) by means of an actuator (16) as the setpoint control of the temperature (TCI) of the riser (12) . The flue gas discharged from the dryer (100) may be treated by means of a flue gas filter (44) and conveyed by a flue gas blower (45) to a chimney (46).

In an alternative embodiment, dry sludge may be combusted in a traditional fluidized bed or circulating mass boiler. In an alternative embodiment, the steam generated in the dryer may be conveyed to combustion without condensation. In an alternative embodiment, the risers (6,12) of the dryer and the separat- ing heat exchange surface (121) are formed by a tube heat exchanger, wherein the first riser (6) is formed by the insides of the tubes and the second riser (12) by the jacket of the tube heat exchanger. The method and apparatus according to the invention are applicable as different embodiments for use in the treatment of the most different kinds of sludges.

The invention is not limited merely to the above-described examples; instead, many variations are possible within the scope of the inventive idea defined by the claims.