EP0835410B1

EP0835410B1 - A method and an apparatus for drying and combusting water-containing matter

Info

Publication number: EP0835410B1
Application number: EP96921913A
Authority: EP
Inventors: Christian Bisgaard; Niels Simonsen
Original assignee: Kruger AS
Current assignee: Kruger AS
Priority date: 1995-06-26
Filing date: 1996-06-26
Publication date: 1999-12-15
Anticipated expiration: 2016-06-26
Also published as: EP0835410A1; AU6298996A; WO1997001731A1; DE69605672D1; DK0835410T3

Description

The invention relates to a method for the drying and combustion of water-containing material containing combustible components, such as dehydrated waste water sludge, according to the preamble of claim 1 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 inconveniences 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.
GB 2190178 teaches a system for drying sludge. In this system the sludge is fed through a preheater and led into a dryer where it is heated by means of superheated steam.

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 the preheater, whereby the water evaporates, and is superheated,
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 temperature 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 introduction 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 exchanger, 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-containing 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 preheating 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 discharged 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	%	4.6
S	%	1.4
O	%	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 (a 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 added	No water 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:
Air_in:
Amount, dry	kg/h	1846	2148
x-air	kg/kg	0.028	0.028
Temperature	°C	600	600
Gas_out:
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
Air_out:
x-air	kg/h	0.412	0.004
Temperature	°C	450	450
Evaporated water	kg/h	754	0
Gas_in:
x-gas	kg/kg	0.473	0.473
Temperature	°C	575	575
Gas_out:
x-gas	kg/kg	0.155	0.473
Temperature	°C	60	366
Condensed water:	kg/h	606	0
Heat Exchanger 2:
Air_in:
x-air	kg/kg	0.028	0.028
Temperature	°C	30	30
Air_out:
Temperature	°C	600	600
Gas_in:
Temperature	°C	850	850
Gas_out:
Temperature	°C	575	575
Dryer:
Waste water
sludge_in:
Dry matter	kg/h	260	303
DM	%	19.6	22.8
Temperature	°C	10	10
Waste water
sludge_out:
DM	%	26.1	26.1
Temperature	°C	95	95
Air_in:
x-air	kg/kg	0.412	0.004
Temperature	°C	450	450
Air_out:
x-air	kg/kg	0.591	0.085
Temperature	°C	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 added	No water 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	°C	30	30
Gas_out:
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
Air_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	°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	°C	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	°C	700	700
Air_out:
x-air	kg/kg	0.608	0.212
Temperature	°C	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 (a 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 added	No water 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_in: Amount, dry	kg/h	1507	2167
x-air	kg/kg	0.028	0.028
Temperature	°C	30	30
Gas_out:
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	°C	20	20
Air_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	°C	95	95
Air_out:
x-air	kg/kg	0.480	0.253
Temperature	°C	78	68
Condensed water:	kg/h	978	0
Heat Exchanger 2:
Air_in:
x-air	kg/kg	0.609	0.004
Temperature	°C	85	85
Air_out:
Temperature	°C	650	650
Gas_in:
Temperature	°C	850	850
Gas_out:
Temperature	°C	127	530
Heat Exchanger 3:
Waste water
sludge_in:
Temperature	°C	10	10
Waste water
sludge_out:
Temperature	°C	85	85
Gas_in:
x-gas	kg/kg	0.315	0.315
Temperature	°C	127	530
Gas_out:
x-gas	kg/kg	0.250	0.315
Temperature	°C	68	362
Condensed water:	kg/h	101	0
Dryer:
Waste water
sludge_in:
Dry matter	kg/h	212	305
DM	%	16.0	23.0
Temperature	°C	85	85
Waste water
sludge_out:
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

A method for the drying and combustion 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),

(2) drying of the material introduced into the dryer (2) by the introduction of atmospheric air as drying air into the dryer (2),

(3) introduction of the dried material into a furnace (5) and combustion of the material therein,

(4) introduction into the furnace (5) of at least a portion of the drying air after the dryer (2) 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 (5),

characterized in the

(6) addition of water to the atmospheric air in the preheater (2), whereby the water evaporates, and is superheated,
and

(7) reduction of the water content in the drying air following the dryer (2) in a condenser (3) prior to the introduction as combustion air into the furnace (5).
A method according to claim 1, characterized in that at least a portion of the drying air is preheated in a heat exchanger (4) after the dryer (2) and condenser (3) and prior to its introduction into the furnace (5).
A method according to claim 1 or 2, characterized in that at least a portion of the drying air is further preheated in a heat exchanger (4) where the heating medium is flue gas from the furnace, after the evaporation of the water and prior to its introduction into the dryer (2).
A method according to claims 1-3, characterized in that, after removal from the dryer (2), at least a portion of the drying air is utilized as heating medium in a heat exchanger (4) for preheating the drying air and evaporating the water added.
A method according to claims 1-4, characterized in that at least a portion of the water-containing material is preheated in a heat exchanger (4) prior to its introduction into the dryer.
A method according to claims 1-5, characterized 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 (2).
A method according to claims 1-6, characterized in that the water added to the drying air is vapour or water-containing drying air discharged after the dryer (2).
A method according to claims 1-7, characterized 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 (2) relative to the temperature, water content and volume flow of the water-containing material will produce a dried material which is exactly auto-combustible.
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, characterized 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 (3) is provided in connection with a conduit connecting the dryer to the furnace.
A plant according to claim 9, wherein at least a portion of the drying air is used as combustion air, characterized in comprising means (4) for preheating the drying air after it has left the dryer (2) and the condenser (3) and prior to introduction of this air into the furnace.