NO151504B - PROCEDURE FOR THE DRYING AND BURNING OF Aqueous, Solid Fuels. - Google Patents

Description

The present invention relates to a method for improving the energy recovery by drying and burning solid fuels from water-containing, organic materials. Examples of such materials are bark, shavings and other wood waste from sawmills and cellulose pulp factories, peat, sludge from both municipal and industrial drains and household waste (sorted rubbish).

In large-scale industry, such as e.g. the cellulose pulp and paper industry, and in larger municipalities the large amounts of sludge are often a problem. Disposal on landfills can be a threat to the environment. Other methods for handling the sludge, e.g. rotting and calcification, entail large costs, which, e.g. as far as rotting is concerned, cannot be covered, as there is little interest in rotted sludge as a soil conditioner. Systems have therefore been developed for dewatering and drying sludge with a view to

burn this. Swedish patent document no. 378 186 describes a method for dewatering filter cakes from sludge. In this method, the filter cake is inflated with water vapour, so that the water contained in the filter cake becomes less viscous and thus easier to remove, e.g. in vacuum. It is obvious that the operating economy of this method is poor. It is characteristic of the systems proposed so far for sludge treatment that they exhibit high operating costs, despite the fact that the sludge's high heat of combustion is utilized. The efficiency and reliability of these systems are also not satisfactory.

As far as bark is concerned, it is used to a large extent today

as fuel, primarily for environmental reasons, as the disposal of bark is prohibited in many places. The efficiency of bark incinerators is in most cases poor, which is proven by the fact that, especially in winter, you have to add and burn oil to

keep the bark burning in the cauldron. If the dry matter content of the bark

is less than 30%, which is often the case, the bark does not burn without additional fuel. In many places the bark is drained mechanically. , However, it is unusual to achieve a greater dry matter content than approx. 40%

in this way. It also happens that the bark is dried. This happens with the help of flue gases from a special boiler where part of the bark is burnt, or with the help of flue gases from the steam boiler, where all the bark is burnt. Measurements carried out in a facility of the first type show that approx. 1/3 of the bark is burned separately to dry the remaining 2/3 of the bark. The dried bark then had a dry matter content of approx. 4 5% against approx. 30% before drying. If you consider the value of the originally available amount of bark, this method means that you lose approx. 15% of the heat value, which would have been able to be utilized in the steam boiler, if it had been possible to burn the bark with a 30% dry matter content without operational problems. A drying according to the second type of method is economically motivated by an increase in the dry matter content by no more than 10% and is primarily used to achieve smoother operating conditions. If the dry matter content is increased by more than 10% with the help of flue gases, the overall efficiency is reduced far too much.

Burning of bark usually takes place on a grate, namely on a so-called inclined grate. Burning in cyclone furnaces also occurs. Both of these devices are relatively expensive and also labor-intensive, compared to e.g. powder firing. As is well known, the more moist the bark is, the lower the efficiency and the larger and more expensive the boiler. The bark's effective heating value varies with the type of wood and is for spruce bark approx. 19 MJ/kg dry bark. Spruce bark with 40% dry matter content has an effective heating value of approx. 14.5 MJ/kg dry bark.

Considering how the boiler's efficiency decreases with the dry matter content of the material to be burned, it is found that the total efficiency with today's bark burning, i.e. with a bark dry matter content of approx. 40%, only slightly exceeding 50%.

Today, peat is only used to a small extent as an energy source. The technique that has been shown to be applicable does not agree well

with today's requirements for continuous operation, great operational reliability and good economy. Mill turf is produced in the summer months with a dry matter content of between 40 and 50% at best. 1 rainy periods, the peat becomes wetter, and its value drops rapidly when dry matter

the content decreases. In order to achieve consistent production of peat throughout the year, different treatment techniques have been developed for the peat. The technique that has been most relevant is the so-called wet charring, which makes it possible to dewater the peat mechanically to approx. 50% dry matter content. Peat found in the bog has varying dry matter content and composition. Normally, this raw peat has a dry matter content of between 10 and 20% and is difficult to dewater mechanically to more than 35% dry matter content, if it is not treated in some way. Wet charring means that the peat is heat-treated at a concentration of from 5 to 10%. The suspension is pumped through heat exchangers where it is heated, and into the wet carbonization reactor, where a longer period of direct heating with steam is carried out. The treatment is usually carried out in batches over 1 - 2 hours at 150 - 200°C. British Patent Document 183 180 and Swedish Patent Documents 40 679 and 46 995 describe methods and devices for heating the peat suspension with steam. During the heat treatment, between 1 and 2 tonnes of steam are used per tonnes of dry matter, and the steam condenses in the peat suspension or on a heating surface, depending on the type of equipment used. During the long wet charring period, the dry matter content is reduced by at least 5 - 10% by oxidation of the organic material to carbon dioxide and water. After the wet charring, the suspension is heat-exchanged with the new peat suspension to be treated, which is thereby preheated. This pre-heating can be carried out in a device described in Swedish patent document 46 386. After the wet charring, the peat is mechanically dewatered with presses to a dry matter content which in the best case reaches 50%. A larger proportion of the obtained pressed water is used again for diluting the peat suspension, while the rest of the pressed water must be discharged. The thus obtained moist pressed cakes of semi-dry peat are used as fuel, even though the heating value of the peat is low due to the low dry matter content. Further increasing the peat's dry matter content is only possible by drying. No drying method that is economically advantageous exists today. Admittedly, peat briquettes are produced with a dry matter content of approx. 90%, but this fuel cannot be used industrially due to its high price. It is preferably used as household fuel. The drying methods used often use flue gases from the combustion of the fuel as a drying medium. This means that the dry matter content of the peat cannot be increased by more than 10% for reasons of heat economy. If the peat is dried in several stages, a somewhat better efficiency is achieved. In the so-called Bojner dryer, which is described in Swedish patent document 9837, the material is first dried in air with moist air or steam as the heating medium and then in flue gases from an incinerator for the fuel. US. patent document 2 014 764 shows an apparatus where steam is used as a drying medium.

In this apparatus, however, the peat is first dried in air with hot water as the heating medium, which hot water is obtained by scrubbing the moist air coming from the steam-heated drying stage, where the peat is completely dried. It has been stated that this method, despite being very complicated, reduces heat consumption during drying by only approx. 30%. There are also other special drying methods for peat, such as e.g. drying in molten metal, which is described in British patent document 183 180.

The peat is burned in different types of boilers.

Burning of coarser particles, such as peat cake, takes place on a grate of one type or another. Vanliavis, combustion air is blown through the grate in order to dry the peat in the boiler before it is ignited. Smaller and above all dry peat particles are burned in powder form. The powder is often blown into the boiler together with the combustion air. Burning moist fuel requires a greater air surplus than dry fuel. fuel, which gives a larger amount of flue gas and a lower combustion and flue gas temperature. This, together with the fact that an often large proportion of the energy content goes into evaporating the moist fuel's water content, results in a reduced efficiency for the boiler. In order to extract a certain amount of energy, more fuel and a larger and thus more expensive boiler is required than if dry fuel could be used. For peat, the effective heat value approx.

20 MJ/kg dry peat. In fig. 1 it is shown how the heating value

decreases when the moisture quotient of the peat increases. If one assumes that a boiler requires approx. 3M MJ to form one kg of steam, one can. theoretically get out approx. 6.7 kg of steam per kg dry peat. If the peat has a dry matter f content of 50%, as can be seen from fig. 1, the effective heating value approx. 8.5 MJ/kg. If a reduced is taken into account

combustion efficiency, one can then produce 5.1 kg of steam per kg dry peat. For it to be profitable to dry peat with a dry matter content of 50%, no more than approx. 1.6 kg of steam per kg of dry peat, which has not been possible with them until now

suggested drying methods. By means of a known pre-treatment technique, i.e. wet charring, peat with a dry matter content of 50% can be obtained with a steam consumption of between 1 and 2 kg/kg of dry peat. This leaves a net<p>roduction of steam of between 3 and 4

kg per kg dry peat. Depending on how well the drying method is adapted in terms of heat economy, the total efficiency is thus between 3. 100/6.7 = 45% and 4 . 100/6.7 = 60%. This simple balance sheet shows what can be achieved in terms of energy extraction with today's technology, if peat is used as fuel.

The present invention provides a solution to the problem of improving energy recovery by drying and burning solid fuels from water-containing, organic materials, such as bark, peat and the like.

In accordance with this, the invention provides a method by which the water-containing material is freed from solid contaminants, such as stone and metal, mechanically dewatered in one or more stages, optionally coarse and/or finely divided, dried and burned in a power/heating plant and before the final mechanical dewatering is directly heated with steam from the drying device, where the drying medium consists of water vapor of superatmospheric pressure. The method is distinguished by the fact that steam generated in the power/heating plant, after being passed through one or more turbines, is used as an indirect heating medium for the steam in the drying device.

The method has several advantages. The most important advantage is that it becomes possible in a much more efficient way than before to take care of the energy found in water-containing organic materials, such as e.g. bark, peat and mud. By working up the fuel step by step using energy with successively increasing temperature and by energy-wise optimizing each individual step, losses can be minimized, so that the overall efficiency is high. Furthermore, the method according to the invention means that the organic material can be handled in a simple and reliable way, i.e. it makes it possible to build a drying and combustion device which can be run continuously without problem-causing interruptions. In addition, the materials can be taken care of in an environmentally beneficial way. The advantages of the method according to the invention appear in more detail from the design examples, where embodiments of the method are described in more detail and illustrated with the help of figures.

The method according to the invention is initially described more generally and then specified for the preferred aqueous organic materials in exemplary embodiments, with reference to the drawings, where

fig. 1 shows how the effective heating value changes at

peat with increasing moisture quotient,

fig. 2 shows a plant where the method according to the invention is used for drying and burning peat with a dry matter content of 10%,

fig. 3 shows a plant where the method according to the invention is used for drying and burning peat with a dry matter content of 25%,

fig. 4 shows a plant in a cellulose pulp factory, where the method according to the invention is used for bark, and

fig. 5 shows a plant for drying and burning non-rotten municipal sewage sludge using the method according to the invention.

If the organic material contains solid contaminants, such as stone and metal, these are removed in a known manner, before the material is treated in accordance with the invention. Furthermore, it is sometimes necessary to reduce the size of a certain proportion of the material before the treatment according to the invention is carried out. When the organic material consists of bark, it is e.g. as a rule, it is necessary to cut up the coarsest and largest pieces of bark.

The incoming organic material is mechanically dewatered

in one or more steps. The number of dewatering stages depends on the dry matter content of the incoming organic material. If the dry matter content is relatively high, e.g. 20% and more, a single dewatering step may be sufficient. This is e.g. the case when peat, which to a certain extent has already been dewatered and dried on the peat bog, is treated according to the invention. In the vast majority of cases, however, two or more dewatering steps are necessary. Dewatering is carried out using known equipment, such as e.g. one hydraulic press, screw press, decanting centrifuge, sieve belt press or roller press.

Before the last mechanical dewatering step, the organic material is heated with steam, with the steam condensing directly on the organic material. This direct condensation of steam is carried out either at atmospheric pressure or at overpressure. The design of the equipment used in this pretreatment step is adapted to the pressure to be used. The organic material must have a dry matter content of at least 10% when introduced into this pre-treatment step. In this step, the dry matter content of the organic material is temporarily lowered, as steam condenses in the material. The temperature of the organic material during this treatment step is usually in the range 40 - 150°C, and the residence time varies from a few minutes to 1 hour, depending on the temperature and the material being treated. The steam used for direct condensation on the material is obtained as surplus steam from a steam dryer used later in the process, i.e. a dryer where the transport medium and the drying medium are the same, namely water vapor at a pressure that exceeds atmospheric pressure. The steam dryer will be described in more detail below. Between the pretreatment step with direct condensation of steam on the material and the drying step itself, the material is subjected to a final mechanical dewatering, where the previously mentioned dewatering equipment is used. Before the material is subjected to drying treatment in the steam dryer, it is sometimes necessary to grind or disintegrate it. Whether or not the material is to be disintegrated at this stage of the treatment is partly determined by which material is being treated, e.g. peat, bark or sludge, and partly by which dewatering equipment is used, and partly even by the design of the steam dryer. The material can be ground or disintegrated in any suitable a<p>parature, such as e.g. a hammer mill, pin mill or ball mill. Suitable devices for feeding the material into drying and/or treatment devices are cell feeders, screws, screw presses and the like. The design of the steam dryer can vary greatly. Common to all dryers used, however, is that the drying system

must be closed so that an overpressure can be maintained. The size of the over-pressure can vary and can reach at least 1 MPa (10 bar). When the organic material consists of bark or peat, the drying device is designed so that the heat is transferred from the steam to the material by convection. When the organic material consists of sludge, the drying device is designed so that the heat for

the largest part is transferred by heat conduction. In the first case, the drying system contains a fan that drives the steam and material around the system.

The peculiarity of the method according to the invention is that the drying and transport medium (i.e. the carrier steam) for drying the material is heated indirectly in a heat exchanger with steam that has been generated in a steam boiler connected to the combustion plant where the dried material is burned, which steam is first been passed through one or more turbines for the production of electrical power. In a cyclone connected to the heat exchanger, the dried material is separated from the carrier steam, which continues its cycle<p> and is partly transferred to the pretreatment step,

as this has been explained previously. At the bottom of the cyclone there is a device that feeds the dried material out at atmospheric pressure. This device can consist of a sluice feeder or screw feeder. As far as the drying of the material is concerned, it takes place during the material's transport through the drying system, i.e. the water in the material successively changes to steam during transport. As a result, excess steam is formed in the dryer, which means that you can continuously take a certain amount of steam out of the drying system. The drying system can also include a fluidized layer, where the material stays

itself for a certain time before the material, due to reduced weight, caused by the drying, is carried away by the steam into the other part of the drying system. When drying sludge, a so-called contact dryer is used, where the heat is transferred by wire. In that case too, the indirect heating steam is obtained from a turbine.

After drying and discharging the material from the drying system, it is usually transported by means of a fan to a steam boiler for combustion. Air and/or flue gases are used as the transport medium. During this transport, the material is further dried by so-called freeze drying. When the material is fed into the steam boiler for combustion, its dry matter content must exceed 90%. Furthermore, the material's particle size during combustion must be less than 3 mm, preferably 1 mm. If the particle size exceeds this value, the material must, after discharge from the drying device and before incineration, be subjected to grinding or disintegration. This can be done e.g. in a so-called Kramer mill. These two specifications, i.e. a dry matter content higher than 90% and a particle size lower than 3 mm, lead both to the combustion being as complete as possible, with the resulting low dry matter content in the flue gases, and to the amount of flue gas being low and the flue gas temperature high, which means a small and therefore affordable steam boiler. In addition, the steam boiler is simple and reliable. The steam produced in the steam boiler through the combustion of the dried material is fed, as previously described, to a turbine or, if necessary, to several turbines. The pressure and temperature of the steam on arrival at the turbine or turbines is very high, e.g. 11.5 MPa (115 bar) and 530°C respectively. A significant proportion of this energy is converted into electricity with the help of one or more generators. When the steam pressure is reduced to approx. 1-2 MPa (10 - 20 bar), a part is tapped off for use as an indirect heating medium for the drying and transport steam, which is described above. The lower pressure steam remaining in the turbine can be used for several different useful purposes. For example, you can take care of the energy in a district heat exchanger when producing hot water for local heating purposes. 1 the case where the residual steam is to be used as process steam, e.g. in the cellulose industry, the pressure of the process steam should appropriately correspond to the pressure used in the drying device, i.e. 0.3 -

0.6 MPa (3-6 bar).

The invention is illustrated in the following examples.

Example 1

Fig. 2 shows a plant where the method according to the invention is used for drying raw peat and subsequent burning of the peat.

The peat suspension 1, which has a dry matter content of 10%, is preheated in a heat exchanger 2 equipped with scrapers. The scrapers-e ensure that the heating surface is kept clean, and they also ensure good mixing of the peat suspension. The suspension is preheated to approx. 50°C in the heat exchanger 2 using pressurized water of a temperature approx. 65°C obtained from two subsequent dewatering steps. The first dewatering of the peat suspension is carried out in a dewatering press 3 at a maximum pressure of approx. 2.0 MPa (20 bar). Thereby, the dry matter content of the peat suspension is increased to 35%. The squeezed water is collected in the pocket 4 and led through the pipeline 5 to the common drain

pipeline 6. The dewatered peat is then fed by means of a screw'

7 to a pressure vessel 8 provided with an entrainer, in which the peat is treated with saturated steam at a pressure of 0.5 MPa (5 bar) for 30 minutes. With the aid of a screw press 9, which is also a feeding screw, the peat is dewatered again to a dry matter content of 50% and simultaneously fed into a drying device 10. The water squeezed out in the screw press 9 is collected in a pocket 11 and led via a pipe line 12 to the common drain pipe 6. In the drying device 10, superheated steam circulates at a pressure of 0.5 MPa (5 bar),

i.e. the same pressure as that which prevails in the pressure vessel 8. The steam is both a drying medium and a transport medium for the peat, and with the help of a fan 13 the finely divided peat particles in the fan are transported in a current drying channel to a cyclone 14. The largely saturated steam is separated from the peat and is recycled via a superheater 15, in which heat is indirectly transferred to the steam. The superheating heat is taken from steam that condenses at a pressure of 1.5 MPa (15 bar) and is made up of extraction steam from the plant's turbine 16. The extraction steam is transported through the steam pipeline 17 to the superheater 15. Excess steam formed during the drying of the peat is separated and is led through connecting pipeline 18 to the pressure vessel 8.

When dried in steam, the peat has reached a dry matter content of 80%. The peat is fed by means of a transport screw 19 out to atmospheric pressure and transported pneumatically through the pipeline 20 to the boiler 21. During the discharge and transport, the peat dries further, partly as a result of the pressure reduction and partly as a result of convection in the transport pipeline. Before the peat is burned, it is ground in a special type of hammer mill with fixed hammers (Kramer mill) 22 together with circulating flue gases, after which it is blown as dust into the boiler. As the peat is fed to the burning site, its dry matter content is approx. 98%. In the boiler 21, which works with a closed feed water system, superheated steam is produced with a pressure on

11.5 MPa (115 bar) and a temperature of 530°C. The steam is passed through the pipeline 23 to one or more turbines 16, which are connected to a generator 24 for the production of electrical power. From the turbine 16, a portion of steam is tapped off and transported to the above-mentioned superheater 15 for superheating the drying medium vapour. The remaining part of the steam is led from the turbine 16 at 105°C <q>through the pipeline 25 and condensed in a district heating condenser 26. Other

drains from the turbine, which are conventionally utilized in the boiler, intermediate heaters, superheaters, etc., are omitted. Condensed feedwater is fed back to the boiler, from the district heating condenser 26 through the pipeline 27 and from the superheater 15 through the pipeline 28.

In the table below, a comparison of the extracted energy has been made between the method according to the invention and the previously known method for producing peat press cakes, where wet charring is used. The numerical values are, as far as the previously known method is concerned, obtained from a process where peat with a dry matter content of 8% was preheated in countercurrent in heat exchangers, where the heat was partly taken from already treated, i.e. wet charred peat. The rest of the energy required for preheating was supplied in the form of fresh steam from the boiler. After preheating, the peat was taken to a wet carbonization reactor, where the temperature was 190°C and the steam pressure 13 bar. During the 1.5-hour treatment, the steam was added in the form of fresh steam. After the wet charring step, the peat was fed in countercurrent to the newly introduced peat and then subjected to pressing in a plate filter press until a dry matter content of 49%.

The resulting press cakes were then burned in a boiler.

As can be seen from the above table, the method according to the invention, compared to the so-called wet charring process, means that ^^124^^ i.e. 35% more energy is extracted from the peat, and that the increase in the form of extracted electrical energy is (< 54-33>)<100> ^ 63%>

Example 2

Fig. 3 shows another plant for drying and burning peat according to the invention.

In this case, the peat on the peat bog is drained and treated, so that a dry matter content of 25% is achieved. As a result, the costs for transporting the peat from the peat bog to the facility shown in the figure are reduced. The peat 29 is thus fed with a dry substance f content of 25% to a vessel 30. In this vessel, the peat is treated at a pressure close to atmospheric pressure (1.5 bar) with steam supplied through the pipeline 31. The peat then attains a temperature of 95° C. From the vessel 30, the peat is fed by means of a feeder 32 to a roller press 33, in which the peat is dewatered to 37% dry matter content. The squeezed water is carried away through the pipeline 34. The peat web from the roller press is torn up into small pieces by means of a disintegrator 35 particles, which are fed to a drying device 36 containing a fluidized layer, through which superheated steam (150°C) is fed by means of a fan 37. The superheated steam is fed from the superheater 38 through the pipeline 39. In the fluidized layer in the drying device 36, drying the peat particles, and when they are sufficiently dry, and thereby also sufficiently light, they are transported with the steam out of the fluidized layer and into a cyclone 40. In this cyclone, a rough separation takes place, as steam is separated above and the peat flows out below. The steam with remaining peat particles is fed to a multicyclone unit 41 where a final separation of peat and steam takes place. This portion of the dried peat is fed through a reed pipe 42 to the bottom section of the cyclone 40, where it is mixed with the rest of the peat and, by means of a sluice feeder 43, discharged to a pipe 44 which is connected to the combustion boiler 45. When the peat is taken out of the cyclone 40, it has a dry matter content of 75%. Flue gases are taken out from the boiler 45

and is led in a pipeline 46 to a fan 47 which transports the flue gases further, so that the peat is torn down and led into the boiler, where combustion takes place. During this pneumatic transport, the dry matter content of the peat is increased from 75% to 92%. In the boiler is produced

superheated steam of high pressure (11.5 MPa) which is passed through the pipeline 48 to one or more turbines 49, which are connected to a generator 50 for the production of electrical power. From the turbine 49, some steam of pressure 1 MPa is tapped off and transported

ert through the pipeline 51 to the above-mentioned superheater 38 for superheating the desiccant vapour. The desiccant vapor, which by means of the fan 37 is led in a circuit through the drying device 36, the cyclone 40, the multi-cyclone unit 41, the superheater 38 and the pipeline 39, has a pressure of o.115 MPa (1.15 bar). Part of the steam in this NOK flow is taken out and led through the pipeline 31

to the peat flowing into the vessel 30, as previously described.

The steam extracted from the turbine 49 is condensed to water in the superheater 38, and the condensed water is fed back to the boiler through the pipeline 52. Remaining steam from the turbine 49 is fed through the pipeline 53 for use as desired and needed.

This embodiment of the method according to the invention leads to the same high extraction of energy from the peat as that indicated in example 1, i.e. to an increase of the energy extraction by 35%, compared to the wet charring process.

Example 3

In fig. 4 shows a facility in a cellulose pulp factory, where the method according to the invention is used for bark.

Spruce bark 54, whose coarsest pieces of bark have been disintegrated

in a mill (not shown in the figure), transported to a hydraulic press 55. When entering the press, the bark has a dry matter content of 30%, and it is dewatered in this to a dry matter content of 36%. The bark is then guided using a transport screw 56

into a pressure vessel 57. In the pressure vessel, steam, which is supplied through the pipeline 58, is condensed on the bark at a pressure of 0.4 MPa

(4 bars). The bark is thereby heated to 14 0°C. The residence time for the bark in the pressure vessel 57 is 3 minutes. The bark is fed out using a cell feeder 60 to a screw feeder 61. In this screw feeder, the bark is dewatered to a dry matter content of 47.7%, while the bark is fed into a closed drying system where superheated steam circulates. The pieces of bark fall from the screw feeder 61 down to a grinder 62

at the bottom of the dryer 6 3 and is ground into such fine particles that the transport steam, which is supplied through the pipeline 64, is able to

to transport them. The transport steam and the finely divided bark then pass a superheater 65, in which the discharge steam from the turbine 66, which is passed through the pipes 67 and 68, condenses at a pressure of 1.6 MPa (16 bar). The transport steam and the bark are then led by means of a fan 69 through a pipeline 70 to a cyclone 71. In this cyclone, the dried bark is separated from the steam. The bark is fed by means of a cell feeder 72 out of the cyclone and led through pipeline 73 to a further pipeline 74.1 at the end of the pipeline 74 a fan 75 is arranged with the help of which the finely divided bark (of particle size less than 0.4 mm) together with part of the combustion air and part of other gases are blown tangentially into the combustion chamber 76. When entering the combustion chamber, bark powder has a dry matter content of 90%. The steam separated in the cyclone 71 is recycled through the pipeline 64 to the dryer 63 and introduced into it by the comminution device, i.e. at the grinder 62. From the recirculation pipeline 64, steam is drained, partly through the pipeline 77 to a steam converter 78 and partly through the pipeline 58 to the pressure vessel 57, which previously stated.

In the steam converter 78, the steam is fed into the bottom, i.e. on one side of the heat exchanger arranged in the steam converter, and impurities found in the steam, such as inert gases, turpentine, acids etc., are vented out at the top. These gases are led through the pipeline 79 to the fan 75, which leads them further into the boiler for combustion. The condensate from the steam is led from the steam converter 78 through the pipeline 80 to the factory's boiler evaporation plant, and the evaporation residue is then incinerated in the soda boiler. Pressurized water from the screw feeder 61 is also fed to pipeline 80 through pipeline 81 and from the hydraulic press 55 through the pipeline

82. On the other side of the heat exchanger in the steam converter 78, feed water supplied from the factory circulates through lines 83 and 84. The feed water is evaporated at a pressure of 0.4 MPa (4 bar), and the steam is carried through the pipeline 86 to the factory for use. The steam condensed in the superheater 65 is passed through the pipeline 85 and is mixed in the pipeline 83 with the feed water introduced into the boiler 76. By burning the dried bark in the boiler 76, superheated steam of high pressure is produced, which is passed through the pipeline 88 to one or more turbines 66 , which is connected to a generator 89 for the production of electrical power. From the turbine, steam of 1.6 MPa is transported through the pipeline 67. This steam is split into two streams, with part of the steam being led through the pipeline 68 to the superheater 65 for indirect transfer of heat to the transport steam in the drying system and the other part of the steam is passed through the pipeline 90 to be used in the factory. The steam remaining in the turbine 66, i.e. that which is left after draining and conversion to electrical energy, is led at a pressure of 0.4 MPa through the pipeline 87 to the pipeline 86, which is connected to the factory.

To be able to assess the significance of the pre-treatment

of the bark in pressure vessel 57, two trials were carried out

in addition to the above. In one experiment, the steam treatment in the pressure vessel 57 was omitted. In the second experiment, the bark in the pressure vessel 57 was treated with steam at a temperature of 105°C, in contrast to the previously stated steam temperature of 140°C. The dry matter content of the bark after the screw feeder 61 was measured. The following results were obtained..

As can be seen from table 2, the pre-treatment of the bark according to the invention, i.e. the direct heating of the bark with steam, leads to the dry matter content of the bark after the second pressing being significantly higher than if the dam<p>supply is omitted. Even if the value of added steam is deducted when calculating the energy balance for the method according to the invention, the pretreatment leads to a positive result.

If the above-described method according to the invention with regard to drying and burning bark is compared with conventional treatment of bark, dys. mechanical dewatering of the bark by pressing to a dry matter content of 40% and a subsequent combustion in a boiler equipped with a slanting grate, then "one finds that the price of the steam produced according to the invention is 35% lower than the price now of steam obtained by conventional treatment of the bark. This applies despite the fact that the equipment around the steam boiler shown in Fig. 4 entails increased investment costs and even a certain increase in operating costs, compared to conventional treatment. The lower production costs per ton of steam are due to the fact that significantly more steam is obtained from the same quantity of bark, compared to the previously known methods.Furthermore, the method of progress according to the invention leads to the fact that the dam<p>boiler itself can be made simpler and thereby cheaper and more reliable than, for example, a steam boiler with a slanted grate. Through an improved combustion of the bark, the amount of dust has been able to be reduced from approx. 180 mg/normal m (Nm 3) flue gas when using an inclined grate steam boiler to approx. 40 mg/normal m 3 (Nm 3) flue gas using the method according to the invention. A further advantage of the system according to fig. 4 consists in the fact that the emission of oxygen-consuming material becomes low by evaporation of both the condensate from the steam converter 78 and the pressurized water from the hydraulic press 55 and the pressurized water from the screw feeder 61.

Example 4

In fig. 5 shows a plant for drying and incinerating non-rotten municipal sewage sludge where the process according to the invention is used.

The sludge 91 comes from an activated sludge plant with a dry matter content of 4% and is dewatered in a conventional press 92 to a dry matter content of 10%. The press 92 can be replaced, for example, with a decanter centrifuge. The previously dewatered sludge is taken to a container 93, where the sludge is heated to 30°C by direct condensation of steam obtained from a subsequent drying device 94. From the drying device, the steam is transported through pipelines 95 and 96. During the condensation of the steam in the sludge, il -smelling gases, which are collected at the top of the container 9 3 and are led through the pipeline 97 to the steam boiler 98 (not shown in the figure), where the gases are burned together with dried sludge and other oil. The sludge is then fed to a sieve belt press 99 and dewatered to a dry matter content of 37%. The water squeezed out in the sieve belt press 99 is fed together with water squeezed out in the press 92 through the pipeline 100 back to the activated sludge plant. After the final mechanical dewatering, the sludge is taken to the previously mentioned drying device 94. The drying device consists of a pressure vessel with three axially arranged transport screws which are designed so that they both clean each other by rotation and serve as pressure-tight sluices during the input and output of the sludge. The heat is supplied indirectly to the drying device 94

in that steam of a pressure of 0.9 MPa (9 bar) is drained from the turbine 102 and led through the pipeline 103 to the hollow transport screws, where the steam is condensed. Part of the steam in the pipeline 103 is led via the pipeline 104 to the pressure vessel's mantle, where the steam is condensed. The pressure of the steam in the drying device 94, i.e. where the sludge is located, is 0.2 MPa (2 bar). With this type of drying device, which is referred to as a contact dryer, it is essential that good heat conduction is achieved between screws and sludge. When the sludge is discharged from the drying device 94, it has a solids content of 90%. The sludge is discharged in finely divided form, such as in powder form, and is carried by means of a fan 105 through a pipeline 106 to a cyclone 107. In the cyclone, the pulverulent sludge is separated, after which it is transported through the pipeline 108 to the combustion chamber in the steam boiler 98. The sludge is burned together with oil in the steam boiler 98, whereby superheated, high-pressure steam is formed, which is passed through the pipeline 109 to one or more turbines 102, which are connected to a generator 110 for the production of electrical power. indirect heating medium to the drying device 94 through the pipelines 103 and 104. The steam that remains in the turbine after draining and conversion to electrical energy is led through pipelines ning 110 to the district heating condenser 112, where it condenses. The condensate is fed back to the steam boiler 98 as feed water through the pipeline 113. The condensate from the drying device 94 is fed through the pipeline 114 to the pipeline 113 for further transport as feed water to the steam boiler. Part of the steam extracted in the drying device 94 is led as previously indicated via the pipelines 95 and 96 to the pre-treatment container 93. The rest of the steam extracted in the drying device 94 is led through the pipeline 115 to the activated sludge plant not shown in the figure. In this facility, which, among other things, consists of pool beds, the waste water comes into contact with activated sludge and air.

To achieve a high growth rate of sludge in the pools becomes

the air heated with the extracted steam, and thereby also obtained

an increase in the temperature of the water in the pools.-

As can be seen from what has been previously stated, is added

oil to the boiler together with the dried sludge. When the sludge dry-

substance content to begin with is very low and, furthermore, the

depending on the physical nature of the sludge, it is not possible to dry the sludge and then generate enough energy during combustion to be sufficient for the entire sludge treatment. Energy must always be supplied from outside, usually in the form of oil. slut-

action thus always involves costs. Depending on the sludge

type and the solids content of the sludge is required there between 0.5 and 1.0

kg oil/kg dry sludge for treatment of the sludge by conventional drying and incineration processes. If one studies the cost

nuisance with conventional sludge treatment, e.g. consisting of dewatering the sludge in a decanter centrifuge and drying and burning the same in a floor oven as well as depositing the ash, it is found that they run up to approx. 500 Skr/ton of dry sludge. If the sludge is treated in the manner shown in fig. 5, i.e. in accordance with the method according to the invention, it has been shown that these

costs can be reduced by 25%. In addition, the described method prevents odor emissions and reduces the problems with unburned dust.

Claims (7)

Procedure for drying and burning solid fuels from water-containing, organic materials, such as bark, peat and the like to improve energy recovery, whereby the water-containing material is freed from solid contaminants, such as stone and metal, mechanically dewatered in one or more stages, optionally coarsely and/or finely chopped, dried and is burned in a power/heating plant and, before the final mechanical dewatering, is directly heated with steam from the drying device, where the drying medium consists of water vapor from superatmospheric pressure, characterized in that steam formed in the power/heating plant, after to have been led through one or more turbines, are used as indirect heating medium for the steam in the drying device. 2. Method according to claim 1, characterized in that the dry matter content of the organic material is brought to exceed 10% by direct heating with steam. 3. Method according to claim 1 or 2, characterized in that the organic material is disintegrated after the final mechanical dewatering. 4. Method according to claims 1-3 characterized in that the organic material is dried to such an extent that the dry matter content during combustion exceeds 90%. 5. Method according to claims 1-4 characterized in that the organic material is disintegrated before combustion so that the particle size is less than 3 mm, preferably 1 mm. 6. Method according to claims 1-5 characterized in that the heat in the drying device - is transferred by convection when the organic material consists of bark or peat. 7. Method according to claims 1-5 characterized in that the heat in the drying process is mainly transferred by heat conduction when it reaches the organic matter ale is made up of sludge..