IE48941B1

IE48941B1 - A process for drying and combustion of water-containing solid fuels

Info

Publication number: IE48941B1
Application number: IE1768/79A
Authority: IE
Original assignee: Modo Chemetics Ag
Priority date: 1978-10-10
Filing date: 1979-09-18
Publication date: 1985-06-26
Also published as: FI793144A; JPS6027895B2; SE419974C; CA1116472A; BR7906502A; FI70919C; GB2036787A; NO151504B; JPS5553619A; IE791768L; SE7810558L; GB2036787B; NO151504C; SE419974B; FI70919B; DE2940164C2; DE2940164A1; NO793237L

Abstract

A process for the drying and combustion of solid fuels from water- containing organic materials, such as peat, bark and the like, with improved energy utilisation which comprises (a) mechanically dewatering the material in at least one stage, (b) prior to the final dewatering stage, preheating the material by treatment with steam derived from drying step (c), (c) Drying the material in a drying plant by directly heating it with steam at superatmospheric pressure and recycling at least a part of the steam to preheating step (b), (d) combusting the dried material in a power/heating plant and generating high pressure steam therein, the high pressure steam so generated being used to drive one or more energy-producing turbines and at least a part of the steam, which issues from the turbine(s) at a temperature and pressure higher than that of the steam used in drying step (c), being used as an indirect heating medium for the production of the steam used in drying step (c).

Description

The present invention relates to a process to improve the recovery of energy in the drying and combustion of solid fuels from water-containing organic materials. Examples of such materials are bark, shavings and other wood rejects from saw mills and cellulose pulp mills, peat sludges from communal as well as industrial outlets and household waste (sorted garbage).

Within larger industries, for instance, cellulose pulp and paper industries, and larger communities, great amounts of sludge often are a problem. Depositing these on waste tips can threaten the environment. Other processes of recover ing the sludge, for instance rotting and liming will involve great costs which,as regards rotting, cannot be recovered since the interest in rotted sludge as a soil improving agent is small. Systems for dewatering and drying sludge therefore have been developed in order to burn the sludge. Published Swedish patent specification No. 71 13295—5 describes a process for dewatering filter cakes of sludge. According to this process, steam is blown onto the filter cake so that the water in the filter cake gets less viscous and therefore is easier to remove, for instance by suction. Apparently the operating economy of this process is bad. Characterising the systems for sludge handling which have been presented up till now is that they display high operating costs in spite of the recovery of the high combustion heat - 3 48841 of the sludge. The efficiency and reliability of these systems in operation is furthermore not satisfactory.

Bark is today used to a great extent as fuel, e.g. for environmental reasons in Sweden since the depositing of bark is forbidden at many places in Sweden. The efficiency of bark combustion plants is, in most cases, low, as shown by the fact that, especially in winter time one has to add and burn oil to keep the bark burning in the boiler. If the solids content of the bark is under 30%, which is often the case, the bark will not burn without supporting(additional) fuel. Percentages in this specification are on a w/w basis, in many cases, the bark is dewatered mechanically, however, it is unusual that in this way one can reach higher solids contents than about 40%. It is also known to dry the bark. This is carried out using flue gases from a special boiler, in which part of the bark is combusted or using flue gases from the steam boiler, in which all bark is burned. Measurements from a plant of the first type show that about 1/3 of the bark was burned separately in order to dry the remaining 2/3 of the bark. The dried bark then had a solids content of about 45% as compared to a solids content of about 30% before drying. If one looks to the value of the amount of bark originally available, this process means that one loses about 15% of the heat value, which could have been utilised in the steam boiler, if it had been possible to burn the bark with a solids content of 30% without operational problems.

A drying of the second type is economically motivated to increase the solids content by at most 10% and is used mainly to obtain more even operating conditions. If one increases the solids content by more than 10% by means of flue gases, the total efficiency will decrease too much.

Burning of bark usually is carried out on fire grates— so called sloping fire grates (rosts). Combustion in cyclone furnaces also occurs. Both these devices are relatively expensive and also work-consuming, when compared for instance, with suspension firing. It is known that the coefficient of efficiency will be lower and the boiler larger and more expensive, the wetter the bark is. The effective heat value of the bark varies with the wood species and is, for spruce bark, about 19 MJ/kg of dry bark. Spruce bark of 40% solids con10 tent has an effective heat value of about 14.5 MJ/kg of dry bark. Taking into consideration how the efficiency of the boiler decreases with the solids content of the material which is to be burned, one finds that the total degree of efficiency in today's bark combustion—that is with a bark solids content of about 40%—only significantly exceeds 50%.

Peat is today used only to a small extent as a source of energy. The technique that has been available corresponds poorly with today' s requirements for continuous operation, high operation security and economy. Peat, milled by cutting, is produced during the summer months with a solids content of between 40 to 50% at the best. During rainy periods, the peat will be wetter and its heat value decreases rapidly when the solids content is decreased. In order to obtain an even production of peat during the whole year, different ways of treating the peat have been developed. What has been most interesting is the so-called wet coaling, which makes it possible mechanically to dewater peat to about 50% solids content. Peat, as existing in the peat moss, has varying solids content and composition. Normally this raw peat has a solids content of between 10 and 20% and can not be easily - 5 dewatered mechanically to more than 35% solids content, if it is not treated in any way. Wet coaling means that the peat is heat-treated at a concentration of 5 to 10%. Thereby the suspension is pumped through a heat-exchanger, in which it is heated into a wet coaling reactor, in which direct heating with steam is carried out for a longer time.

The treatment is usually carried out batchwise for 1—2 hours at 150—200°C. U.K. Patent Specification No. 183180 and the Swedish patent specifications 40 679 and 46 995 describe methods and devices for heating the peat suspension by means of steam. During the heat treatment, between 1 and 2 tons of steam per ton of dry substance is used and the steam is condensed in the peat suspension or on a heating surface, depending on which type of equipment is used. During the long wet-coaling period, the amount of dry substance is reduced by at least between 5 and 10% by oxidation of the organic material to carbon dioxide and water. After the wetcoaling the suspension is heat-exchanged with the new peat suspension to be treated and which is thereby preheated.

This preheating can be carried out in apparatus of the type described in Swedish patent specification no. 46 386. After wet-coaling the peat is dewatered mechanically with presses to 50% solids content at best. A greater part of the presswater so obtained is reused for dilution of the peat suspension, whereas the rest of the press-water has to be discharged. The moist press-cakes of half-dry peat so obtained are used as fuel, even if the thermal value of the peat is low because of the low solids content. To further increase the solids content of the peat is possible only by drying. There is no drying method, which is economically advantageous today. It is true that peat briquettes, with a solids content of about 90%, are manufactured, but this fuel cannot be used industrially because of its high price, but is used domestically. The methods of drying that are used often make use of flue gases from the combustion of the fuel as drying medium.

This means that the solids content of the peat cannot be raised by more than 10% for thermal economical reasons. If the peat is dried in several stages, a somewhat better degree of efficiency is obtained. In the so-called Bojner-dryer, which is described in the Swedish patent specification No. 9837, the material first is dried in air with moist air or steam as heating medium and then in flue gases from a combustion oven for the fuel. U.S. Patent specification no. 014 764 describes apparatus in which steam is used as drying medium. In this apparatus, however, the peat is first dried in air with hot water as heating medium, which water has been obtained by scrubbing the moist air coming from the steam-heated drying stage, where the peat is finally dried. It has been stated that this process, in spite of being very complicated, reduces the heat consumption during the drying by only about 30%. There are also other special drying methods for peat, such as for example, drying in molten metal, which is described in the U.K. Patent Specification No. 183180.

The combustion of peat is carried in different types of boilers. Burning of coarser particles, such as peat press-cakes, is carried out on a grate. Usually, air from the combustion is blown through the rost in order to dry the peat in the boiler before it is set on fire. Smaller, and above all, dry peat particles are burned in powderous form.

The powder is blown, often together with the gases of combus48941 - 7 tion, into the boiler. The burning of most fuels requires greater excess air than dry fuel, which gives a greater amount of flue gases and a lower combustion- and flue gas temperature. This, together with the fact that often a great part of the energy content is consumed to evaporate the water content of the moist fuel, results in a lower degree of efficiency for the boiler. In order to recover a certain amount of energy thus more fuel is needed and a greater and a thereby more expensive boiler than if dry fuel could have been used. For peat, the effective heat value is about 20 MJ/kg of dry peat. Figure 1 of the accompanying drawings shows how the heat value decreases when the moisture quotient increases. Assuming that about 3 MT are required in a boiler to produce 1 kg of steam, one can theoretically obtain about 6.7 kgs of steam per kg of dry peat. If the peat has a solids content of 50%, the effective heat value as can be seen from figure 1 is about 8.5 MJ/kg. Taking into consideration a decreased degree of combustion efficiency, one can then produce 5.1 kgs of steam per kg of dry peat. If it is to be economically worth while to dry peat with a solids content of 50%, no more than 1.6 kgs of steam per kg of dry peat must be consumed, which is not possible with the drying methods presented up till now. With the available pretreatment technique, that is wet coaling, peat with a solids content of 50% can be obtained using a steam consumption of between 1 and 2 kgs/kg of dry peat. It thus requires a net production of between 3 and 4 kgs per kg of dry peat. Depending on how well the drying is arranged from thermal economical point of view, the total degree of efficiency thus will be between 3 . 100/6.7 = 45% and 4 . 100/6.7 = 60%. This simple balance shows what is obtained as far as concerns 8941 energy recovery with today's technique, if peat is used as fuel.

The present invention is directed to the problem of improving the recovery of energy during the drying and com5 bustion of solid fuels frora water-containing organic materials such as bark, peat and similar substances.

The present invention provides a process for the drying and combustion of solid fuels from water-containing organic materials, such as peat and bark with improved energy utilisa10 tion which comprises (a) mechanically dewatering the material in at least one stage, (b) prior to the final dewatering stage, preheating the material by treatment with steam derived from drying step (c), (c) drying the material in a drying plant by directly heating it with steam at superatmospheric pressure and recycling at least a part of the steam to preheating step (b), (d) combusting the dried material in a power/heating plant and generating high pressure steam therein, the high pressure steam so generated being used to drive one or more energy-producing turbines and at least a part of the steam, which issues from the turbine (s) at a temperature and pressure higher than that of the steam used in the drying step (c), being used as an indirect heating medium, for the production of the steam used in drying step (c). - 9 The process of the invention has several advantages.

The most important advantage is that it has been possible to recover in a much more effective way than before the energy which is present in water-containing organic material such as for instance bark, peat and sludge. By working up the fuel in stages, using energy of successively increasing temperature and optimizing, from the energy point of view, each stage, the losses can be minimized so that the total degree of efficiency will be surprisingly high. Furthermore the process of the invention makes it possible for the organic material to be handled in a simple and operationally secure way, meaning that it is possible to build a drying and combustion plant which can be run continuously. In addition, the materials can be recovered in an advantageous way from the environmental point of view. The advantages of the process of the invention are shown more directly in the Examples, in which applications of the process are described more in detail and illustrated with the help of the accompanying drawings.

The process of the invention is described first generally and then specifically for the preferred water-containing organic materials in the Examples.

Figure 1 shows how the effective heat value varies with increased moisture quotient of the peat.

Figure 2 shews diagrammatically a plant, in which the process of the invention is used for the drying and combustion of peat of solids content 10%.

Figure 3 shows diagrammatically a plant, in which the process of the invention is used for the drying and combustion of peat of solids content of 25%. - 10 Figure 4 shows diagrammatically a plant in cellulose pulp mill, in which the process of the invention is applied to bark.

Figure 5 shows diagrammatically a plant for the drying 5 and combustion of unrotted communal sewage sludge on which the process of the invention is used.

If the organic material contains solid impurities such as stone and metal, these are removed by known techniques before the material is treated in accordance with the inven10 tion. Furthermore, it is sometimes necessary to decrease the particle size of some of the material before treatment in accordance with the invention. When the organic material consists of bark, it is, for instance as a rule necessary to disintegrate the coarsest and biggest bark pieces.

The incoming organic material is normally first dewatered mechanically in one or more stages. The number of dewatering stages depends on the solids content of the incoming organic material. If the solids content is relatively high, for instance, 20% and more, it can be satisfactory with only one dewatering stage. This is, for instance, the case when peat or peat moss that has been partly dewatered and dried is used. In most cases two or more dewatering stages are needed. The dewatering process is carried out with known apparatus such as, for instance, a hydraulic press, a screw press, a decanting centrifuge, a band screen press or a roller press. Before the last mechanical dewatering stage, the organic material is heated with steam by directly condensing steam on the organic material. This direct condensation of steam is carried out either at atmospheric pressure or at super-atmospheric pressure. The design of the apparatus 48841 - 11 used foi this pretreatment stage depends on whether atmospheric pressure or superatmospheric pressure is used. The organic material at this pretreatment stage, will normally have a solids content of at least 10%. In this stage, the solids content of the organic material is temporarily decreased since steam condenses in the material. The temperature of the organic material during this pretreatment stage is usually 40—150°C and the residence time varies from some minutes to 1 hour depending on the temperature and the material that is to be handled. The steam to be used for direct condensation on the material is obtained as surplus steam from a steam dryer used later in the process, that is a dryer in which the transport medium and the drying medium are the same, namely steam of a pressure exceeding atmospheric pressure.

The steam dryer is described in more detail below. Between the pretreatment stage with direct condensation of steam on the material and the final drying stage, the material is subjected for a final mechanical dewatering in which the previously mentioned dewatering apparatus is used. Before the material is subjected to the drying treatment in the steam dryer, it is sometimes necessary to grind or disintegrate the material. Whether the material should be disintegrated or not in this stage of the treatment is determined partly by which sort of material that is to be handled, for instance peat, bark or sludge and partly by which sort of dewatering equipment that is used and to a certain part also of the design of the steam dryer. The material can be ground or disintegrated in any suitable apparatus such as, for instance, a hammer mill, pin mill or a ball mill. Suitable devices for feeding the material into the drying and/or treatment equipment are rotary vane feeders, screw feeders. - 12 screw presses and the like. As regards the design of the steam dryer, it can vary considerably. Whichever dryer is used, it is important that the drying system shall be closed, so that a superatmospheric pressure is maintained. The magni5 tude of the superatmospheric pressure can vary but is normally at least 1 MPa (10 bar). When the organic material consists of bark or peat, the dryer is preferably constructed so that heat is transferred from the steam to the material by convection. When the organic material consists of sludge, the dryer is preferably constructed so that the heat is transferred mainly by conduction. In the first case, the drying system contains a fan, which dries the steam and the material around that system, a heat exchanger and a cyclone. In the heat exchanger, the drying and the transport medium, that is the supporting steam, is supplied with all heat necessary for the drying of the material by contacting steam of higher pressure and higher temperature indirectly with the supporting steam. The heating steam is taken from a turbine. The turbine, in its turn, is fed with steam from a steam boiler, in which steam is produced by combustion of the dried material and, if desired, supporting fuel, for example oil.

In the cyclone the dried material is separated from the supporting steam, which continues its circulation and is partly transferred to the pretreatment stage as has been described earlier.

In the bottom of the cyclone there is a device, which discharges dried material. This device can consist of a rotary vane feeder or a screw feeder. As far as the drying of the material is concerned, it is carried out during the transport of the material through the drying system, that is - 13 48841 the water in the material successively is transferred to steam during transport. As a consequence of this, excess steam is formed in the dryer, which means that one can continuously withdraw a certain amount of steam from the drying system. The drying system may also include a fluidized bed, in which the material is kept for a certain time before the material, because of the decreased weight caused by the drying, is caught by the steam and carried further to the remaining part of the drying system. When drying sludge, a so-called contact dryer is used, in which the heat is transferred by conduction. Also in that case, the indirect heating steam is obtained from a turbine.

After drying and discharging the material from the drying system, it is transported usually by means of a fan to the furnace of a steam boiler for combustion. As transport medium, air and/or flue gases can be used. During this transport, the material is further dried by means of so-called freeze drying (flashing). When the material is fed to the steam boiler for combustion its solids content should exceed 90%. Furthermore, the particle size of the material at the combustion should preferably be less than 3 mms, preferably less than 1 mm. If the particle size exceeds this measure, the material can be subjected to grinding or disintegration after discharge from the drying equipment and before the combustion. This can for instance be done in a so-called Krdmer-mill. These two requirements, that is a solids content exceeding 90% and a particle size less than 3 mms, helps to ensure that the combustion will be as complete as possible, and there is a low dust content in the flue gases which will be of reduced volume which means a small and - 14 therefore cheap steam boiler can be used. In addition, the steam boiler will be simple and secure in operation. The steam generated in the steam boiler by combustion of the dried material is, as described above, led to a turbine, or if necessary to several turbines. The pressure and the temperature of the steam on arrival at the turbine or turbines is very high, for instance 11.5 MPa (115 bar), and 53O°C. A considerable amount of this energy is transformed to electricity by means of one or more generators. When the pressure of the steam has been decreased to 1—2 MPa (10—20 bar), a part of the steam is discharged for use as indirect heating medium for the drying and transport steam, as has been earlier described. The steam of lower pressure remaining in the turbine can be used for several useful purposes, For instance, the energy could be recovered in a distant heat exchanger by the preparation of hot water for local heating purposes. If the remaining steam is to be used as process steam, for instance within the cellulose industry, its pressure should preferably correspond to the pressure used in the drying equipment, that is 0.3—0.6 MPa (3—6 bar).

The invention is now illustrated by the following Examples.

EXAMPLE 1 Figure 2 shows a plant, in which the process of the invention is applied to the drying of raw peat and subsequent combustion of the peat.

The peat suspension 1, which has a solids content of %, is preheated in a heat exchanger 2 provided with scrapers The scrapers keep the heating surface clean and they also establish good mixing of the peat suspension. The suspension - 15 ο is preheated to about 50 C in the heat exchanger 2 with press water having a temperature of about 65°C and obtained from the following two dewatering stages. The first dewatering of the peat suspension is carried out in a dewatering press 3 at a highest pressure of about 2.0 MPa (20 bar,, where the solids content of the peat suspension is increased to 35%.

The pressed-out water is collected in a hopper 4 and led through conduit 5 to a common discharge conduit 6. The dewatered peat is carried to a pressure vessel 8 by means of a screw 7, which vessel is provided with carriers. In vessel 8, the peat is treated with saturated steam at a pressure of 0.5 MPa (5 bar) for 30 minutes. By means of a screw press 9, also functioning as feeding screw, the peat is again dewatered to a solids content of 50% and simultaneously fed into a dryer 10. The water pressed-out in the screw press is collected in a hopper 11 and carried through a conduit 12 to the common discharge conduit 6. In dryer 10, superheated steam at a pressure of 0.5 MPa (5 bar) is circulating, this pressure being the same as in pressure vessel 8. The steam is both drying medium and transport medium for the peat. The peat particles finely disintegrated in fan 13 are transported by means of the fan in a steam drying channel to a cyclone 14. The steam, which is substantially saturated, is separated from the peat and recirculated via a superheater 15, in which heat is indirectly transferred to the steam. The superheating heat is taken from steam, which condenses at a pressure of 1.5 MPa (15 bar) and consists of excess steam from a turbine 16 in the plant. The discharge steam is transported through a steam conduit 17 to the superheater 15. Excess steam formed in the drying of the peat is separated and led through a joining conduit 48S41 \ to the pressure vessel 8.

' After the steam drying, the peat has reached a solids content of 80%. The peat is discharged by means of a transport screw 19 out to atmospheric pressure and transported pneumatically through a conduit 20 to a boiler 21. During this discharge and transport, the peat will dry further partly because of the pressure drop and partly because of convection in the transport line. Before the peat is burned, it is ground in a special type of hammer mill with fixed hammers (Krdmer-mill) 22 together with circulating flue gases and is then blown as a dust into the boiler. At the entrance to the fire place, the solids content of the peat is about 98%.In the boiler 21, working with a closed feed water system, superheated steam is generated with a pressure of 11.5 MPa (115 bar) and a temperature of 530°C. The steam is led through a line 23 to one or more turbines 16 connected to a generator 24 for the production of electric power. From the turbine 16, a part of the steam is withdrawn and transported to the previously mentioned superheater 15 for superheating the steam used as drying medium. Residual steam is led from the turbine 16 at a temperature of 105°C through tha conduit 25 and condensed in a distant heat condenser 26. Other discharges from the turbine, which are utilized in known way in the boiler, between the superhea25 ters etc., are not shown in Figure 2. Condensed feed water is carried back to the boiler from the distant heat condenser 26 through the conduit 27 and from the superheater 15 through line 28.

In the table below, a comparison is made between the energy recovered according to the process of the invention - 17 and the energy recovered according to the previously known process for the preparation of peat press cakes using wet coaling. The figures relating to the previously known process have been obtained from a process, in which peat having a solids content of 8% in counterstream was preheated in a heat exchanger, in which the required heat was partly recycled, that is wet coaled peat. The rest of the energy required for the preheating was added in the form of fresh steam from the boiler. After the preheating, the peat was fed to a wet coaling reactor, in which the temperature was 190°C and the steam pressure 13 bar. The steam during the 1.5 hour long treatment was added in the form of fresh steam. After the wet coaling stage, the peat was transported in counter stream with recently introduced peat and then subjected to pressing in a plate filter press so that a solids content of 49% was obtained. The press cakes so obtained were then burned in a boiler. - 18 Table 1 Effect balance Process of the invention Wet coaling process Theoretical effect content of raw peat for 10 kgs of 200 200 dry peat/sec, MJ sec~l Net heat need, in preheater MJ sec 20 at wet coaling MJ sec 29 in steam dryer MJ sec 17 Heat value of the peat at the intro- duction in the boiler, Mj/kgs of dry peat 19.5 15.5 Boiler degree of efficiency, 5 ϋ86 80 -1 Boiler effect. MJ sec 168 124 tl tl I in the form of electricity MJ _ £ sec 54 33 II in the form of dis- tant heat MJ sec“^ 97 42 As is evident from the above table the process of the invention as compared to the so-called wet coaling process means that one obtains (168—124) 100 a 124 that is 35% more energy totally and that the increase in form of recovered electric energy is (54—33) 100 that is 63%. - 19 48841 EXAMPIE 2 Figure 3 shows another plant for drying and combustion of peat in accordance with the invention.

In this case the peat is drained and treated in the peat moss so that a solids content of 25% is obtained. In this way, the costs are decreased for the transport of the peat from the peat moss to the plant in the figure. Peat 29 with a solids content of 25% is fed to a vessel 30. In this vessel, the peat is treated at substantially atmospheric pressure (1.15 bar) with steam added through the conduit 31. The peat then reaches a temperature of 95°C. From vessel 30, the peat is transported by discharging apparatus 32 to a roller press 33, in which the peat is dewatered to 37% solids content. The water pressed out is removed through conduit 34. The peat web from the roller press 33 is disintegrated by means of a disintegrator 35 into small particles, which are fed to a drying apparatus 36 containing a fluidized bed through which superheated steam (150°C) is transported by means of a fan 37. The superheated steam is led from a superheater 38 through a line 39. In the fluidized bed of the drying apparatus 36, the peat particles are dried and when they are dry enough (and also light enough), they are pulled off the steam out of the fluidized bed to a cyclone 40. In this cyclone, there is a coarse separation so that steam leaves the upper part of the apparatus and peat the lower part. The steam and remaining peat particles are transferred to a multicyclone aggregate 41, in which a final separation of peat and steam is carried out. This part of the dried peat is carried through the line 42 to the bottom part of the cyclone 40, where it is mixed with the rest of the peat and is discharged by means of a rotary - 20 vane feeder 43 to a conduit 44, which is in connection with a combustion boiler 45. When the peat is sluiced out of the cyclone 40, it has a solids content of 75%. Flue gases are removed from boiler 45 and carried in a line 46 to a fan 47, which transports the flue gases further on so that the peat is pulled into the boiler, where combustion takes place During this pneumatic transport, the solids content of the peat is increased from 75% to 92%. In the boiler, superheated steam of high pressure (11.5 MPa) is generated which is carried through line 48 to one or more turbines 49 connec ted to a generator 50 for the production of electric power. From the turbine 49, a part of the steam having the pressure 1 MPa is removed and transported through line 51 to the previously mentioned superheater 38 for superheating of the drying medium steam. The drying medium steam, which, by means of the fan 37, is carried in a cycle through the drying apparatus 36, the cyclone 40, the multicyclone aggregate 41, the superheater 38 and the conduit 39, has a pressure of 0.115 MPa (1.15 bar). The steam in this cycle is removed and carried through line 31 to the incoming peat in the vessel 30 as has been previously described. The steam removed from the turbine 49 is condensed to water in the superheater 38 and the condensed water is transferred back to the boiler through the conduit 52. Residual steam from the turbine 49 is carried through line 53 for suitable consumption.

This application of the process of the invention gives the same high energy recovery from the peat, which has been described in Example 1, that is an increase in the energy recovery of 35% as compared to the wetcoaling process. - 21 EXAMPLE 3 Figure 4 shows a plant in a cellulose pulp mill in which the process of the invention is applied to bark.

Spruce bark 54, in which the coarsest bark pieces have been crushed in a mill (not shown in the figure) is transported to a hydraulic press 55. At the entrance to the press, the bark has a solids content of 30% and is dewatered in the press to a solids content of 36%. Then the bark is carried by means of a transport screw 56 to a pressure vessel 57. In the pressure vessel, steam is condensed, which is added through the line 58 onto the bark at a pressure of 0.4 MPa (4 bar). This heats the bark to 140°C. The residence time for the bark in the pressure vessel 57 is 3 minutes. The bark is discharged by means of a rotary vane feeder 60 to a screw feeder 61. In this screw feeder, the bark is dewatered to a solids content of 47.7% simultaneously as the bark is fed into a closed drying system in which superheated steam is circulating. The bark pieces fall from the screw feeder 61 down to a mill 62 on the bottom of a dryer 63 and is ground to such fine particles that the transport steam, which is added through line 64, can transport them away. The transport steam and the finely divided bark thereafter pass a superheater 65, in which excess steam from a turbine 66, that has been transported through lines 67 and 68, condenses at a pressure of 1.6 MPa (16 bar). Then the transport steam and the bark is carried through a conduit 70 by means of a fan 69 to a cyclone 71. In this cyclone, the dried bark is separated from the steam. The bark is discharged by means of a rotary vane feeder 72 out of the cyclone and carried - 22 through the line 73 to a further line 74. At the end of the line 74 is a fan 75 by means of which the finely divided bark (particles size less than 0.4 mm) together with a part of the combustion gas and some other gases are blown tangentially into a furnace 76. At the entrance, to the fire place, the bark powder has a solids content of 90%. The steam separated in the cyclone 71 is recycled through the line 64 to the dryer 63 and introduced into the disintegration apparatus, that is mill 62. From the recirculation conduit 64, steam is removed partly through line 77 to a steam reformer 78 and partly through line 58 to the pressure vessel 57 as has been earlier described. Steam is fed into the bottom of steam reformer 78, that is on one side of the heat exchanger, that is arranged in the steam reformer. Impurities present in the steam such as inert gases, turpentines and acids are purged from the top. These gases are led through line 7-9 to the fan 75, which carries them further into the boiler for combustion. The condensate from the steam is removed from the steam reformer 78 through line 80 to the plant for evaporation of digestion liquor of the mill and the evaporation residue is then burned in the soda boiler. Press water from the screw feeder 61 is fed through the line 81 to line 80 and from the hydraulic press 55 through line 82. On the other side of the heat exchanger in the steam reformer 78, feed water obtained from the mill and added through line 83 and 84 is circulating. The feed water evaporates at a pressure of 0.4 MPa (4 bar) and the steam is carried through the line 86 to the mill for use there. The steam condensed in superheater 65 is carried through line 85 and mixed in line 83 with the feed water that is introduced into boiler 76. 48841 - 23 During combustion of the dried bark in the boiler 76, superheated steam of high pressure is generated, which steam is carried through line 88 to one or more turbines 66 connected to a generator 89 for production of electric power.

From the turbine, steam is transported through lines 67 having a pressure of 1.6 MPa. This steam is divided into two streams. One part of the steam is carried through line 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 carried through the line 90 for use in the mill. The steam remaining in the turbine 66, that is after discharge and reforming to electric energy, is carried at a pressure of 0.4 MPa through line 87 to line 86, which is in connection with the mill.

In order to investigate the importance of the pretreatment of the bark in pressure vessel 57, two tests were made in addition to the one described above. In one test, the steam treatment in the pressure vessel 57 was excluded. In the other test, the bark was treated in pressure vessel 57 with steam at a temperature of 105°C instead of 140°C. The solids content of the bark after passing screw feeder 61 was measured and the following results were obtained. 489 4 1 - 24 Table 2 No addition of steam in the pressure vessel 57 Addition of 105°C steam in the pressure vessel 57 Addition of steam of 140°C in the pressure vessel 57 Solids content in % after the press 55 36.3 35.8 36.0 Solids content in % after steam treatment 32.5 31.4 Solids content in % after the screw feeder 61 38.5 43.0 47.7 As is evident from table 2, pretreatment of the bark in accordance with the present invention, in which the bark is directly heated with steam, results in the solids con5 tent of the bark after the second pressing being substantially higher than if the addition of steam is omitted.

Even if the value of steam added is deducted from the process of the invention in a calculation of energy balance, the pretreatment stage still leads to a gain.

If the process according to the invention is compared with the conventional handling of bark, that is by mechanically dewatering the bark by pressing to a solids content of 40% and followed by combustion in a boiler with a sloping grate, one finds that the price of the steam produced according to the invention is 35% lower than the price of steam needed for conventional handling of bark. This also is true in spite of the fact that the equipment shown in Figure 4 combined with the steam boiler leads to a higher - 25 investment cost and also to some increase in the cost of operation as compared to conventional handling. The lower cost of production per ton of steam depends on the fact that considerably more steam is obtained from the same amount of bark as compared with previously known techniques.

Furthermore, the process of the invention enables one to make the steam boiler itself more simple and thereby cheaper and also more reliable than, for instance, a steam boiler with sloping grate. By an improved combustion of the bark, the amount of dust have been decreased from about 180 mg/ 3 normal m (Nm ) of flue gas in a steam boiler with sloping 3 3 grate to about 40 mg/normal m (Nm ) of flue gas in the process according to the invention. A further advantage of closing the system, in accordance with Figure 4, is that the discharge of oxygen consuming substance will be low because of the evaporation of, as well as the condensate from, steam reformer 78 as the press water from hydraulic press 55 and the press water from screw feeder 61.

EXAMPLE 4 Figure 5 shows a plant for drying and combustion of unrotted communal sewage sludge in which the process according to the invention is used.

The sludge 91 comes from an activated sludge plant and has a solids content of 4% and is dewatered in a conventional press 92 to a solids content of 10%. The press 92 can be replaced with, for instance, a decanting centrifuge. The predewatered sludge is transferred to a container 93, in which the sludge is heated to 80°C by means of direct condensation of steam, which is obtained from a subsequent drying apparatus 94. From the drying 8941 - 26 device, the steam is transported through lines 95 and 96. During condensation of the steam in the sludge, ill-smelling gases are liberated, which are collected at the top of the container 93 and transferred through the line 97 to (not shown in the figure) the steam boiler 98, in which the gases are coiribusted together with dried sludge and oil. The sludge is then transferred to a belt screen press 99 and dewatered to a solids content of 37%. The water that is pressed out of band screen press 99 is carried, together with water pressed out of press 92, through line 100 back to the activated sludge plant. After the final mechanical dewatering, the sludge is transferred to the previously mentioned drying apparatus 94. The drying apparatus comprises a pressure vessel with three axially arranged trans15 port screws, which are constructed so that they are cleaning each other during rotation and serve as pressure tight sluices during the feeding and discharging of the sludge to and from drying apparatus 94. Heat is supplied indirectly to drying apparatus 94 by withdrawing steam of a pressure of 0.9 MPa (9 bar) from the turbine 102 and carrying it through the line 103 to the hollow transport screws, in which the steam is condensed. Part of the steam in line 103 is carried by means of conduit 104 to the jacket of the pressure vessel, where the steam is condensed. The pressure of the steam in drying apparatus 94, that is where the sludge is kept, is 0.2 MPa (2 bar). In this type of drying equipment, which is called a contact dryer, it is essential that good heat contact between the screws and the sludge is obtained. The sludge has a solids content of 90% when discharged from drying apparatus 94. The sludge is discharged in finely divided form as a powdered sludge and - 27 is transferred by means of a fan 105 through line 106 to a cyclone 107. In the cyclone the powdered sludge is separated and transported further through the line 108 to the furnace of steam boiler 98. The sludge is combusted together with oil in steam boiler 98 in which superheated steam of high pressure is generated. This superheated steam is transferred through line 109 to one or more turbines 102 connected to a generator 110 for production of electric power. As has been earlier stated, steam is withdrawn from turbine 102 and led as indirect heating steam to the drying apparatus 94 through lines 103 and 104. The steam remaining in the turbine after discharge and reforming to electric energy is transferred through line 111 to the distant heat condensor 112, where it condenses. The condensate is transferred back to the steam boiler 98 through line 113 as feed water. The condensate from the drying apparatus 94 is carried through the line 114 to line 113 for further transfer as feed water to the steam boiler. Part of the steam recovered in the drying apparatus 94, as has been earlier described, is transferred by means of conduits 95 and 96 to the pretreatment container 93. The rest of the steam recovered in drying apparatus 94 is carried through line 115 to the active sludge plant (not shown in the figure). In this plant, which contains basins, amongst other things, the waste water comes into contact with activated sludge and air. In order to obtain a high growth velocity of sludge in the basins, the air is heated with the recovered steam and this increases the temperature of the water in the basins.

As is evident from the above description, oil is then 489 41 - 28 added to the boiler and burned together with the dried sludge. Since the solids content of the sludge is very low in the beginning and, depending on the physical structure of the sludge, it is not possible to dry the sludge and during combustion, obtain enough energy for the whole sludge treatment operation, one must always introduce energy externally e.g. in the form of oil. The handling of sludge is costly. Depending on the type of sludge and the solids content of the sludge, 0.5—1.0 kg of oil/kg of dry sludge are required to handle sludge in conventional drying and combustion processes. If one examines the cost for conventional handling of sludge, for instance, comprising dewatering the sludge in a decanting centrifuge and drying and burning the sludge in a multistage oven and depositing the ash, one finds that it amounts to about 500 Swedish Crowns/1000 kg of dry sludge. If the sludge is treated in the way shown in Figure 5, that is, in accordance with the process of the invention, this cost can be reduced by 25%. Furthermore, our process results in the prevention of odour dis20 charges and the problem with uncombusted dust is also reduced.

Claims

1. A process for the drying and combustion of solid fuels from water-containing organic materials, with improved energy utilisation which comprises, (a) mechanically dewatering the material in at least one stage, (b) prior to the final dewatering stage, preheating the material by treatment with steam derived from drying step (c), (c) drying the material in a drying plant by directly heating it with steam at superatmospheric pressure and recycling at least a part of the steam to preheating step (b), (d) combusting the dried material in a power/heating plant and generating high pressure steam therein, the high pressure steam so generated being used to drive one or more energy-producing turbines and at least a part of the steam, which issues from the turbine (s), at a temperature and pressure higher than that of the steam used in drying step (c), being used as an indirect heating medium for the production of the steam used in drying step (c).

2. A process according to claim 1, in which the solids content of the organic material subjected to the direct heating with steam exceeds 10% w/v.

3. A process according to claim 1 or 2 in which the organic material is finely disintegrated after the drying. - 30

4. A process according to any one of the preceding claims in which the organic material is dried to an extent such that its solids content, when subjected to combustion exceeds 90% w/v.

5. 5. A process according to any one of the preceding claims in vzhich before the combustion, the organic material is disintegrated so that the average particle size is below 3 mm.

6. A process according to any one of the preceding claims in which before the combustion, the organic material is dis10 integrated so that the average particle size is below 1 mm.

7. A process according to any one of the preceding claims in which the organic material is bark or peat and the heat in the drying plant is transferred by means of convection.

8. A process according to any one of claims 1-6, in which 15 the organic material consists of sludge and the heat in the drying plant substantially is transferred by conduction.

9. A process according to any one of the preceding claims in which the organic material is disintegrated after the final mechanical dewatering. 20

10. A process according to claim 9 in which after the mechanical dewatering, the matetial is disintegrated into coarse and/or finely divided particles.

11. A process substantially as hereinbefore described with reference to any one of Examples 1-4.