CA1116472A - Process for the efficient conversion of water-containing organic materials as fuels into energy - Google Patents

Abstract

PROCESS FOR THE EFFICIENT CONVERSION OF WATER-CONTAINING ORGANIC MATERIALS AS FUELS INTO ENERGY
ABSTRACT OF THE DISCLOSURE
A process is provided for the efficient conversion of water-containing organic materials such as bark, peat and sludge as fuels into energy, including drying the materials to convert them into fuels and then combusting the fuels in order to recover as much energy as possible at the lowest possible cost; which comprises heating the material in a steam vessel while enveloping the material in steam at superatmospheric pressure, heating the steam by heat exchange with steam at a higher pressure and higher temperature than the steam in the vessel;mechanically dewatering the material;
and then drying the material to convert the material into a fuel, converting water driven from the material into steam, and forming excess steam in the dryer, of which steam all or part is recycled and condensed directly on the material in the steam vessel; and then combusting the dry material in finely divided form in a steam boiler, utilizing the high pressure steam that is generated in a turbine, which in turn operates a generator, converting the energy content of the steam into electric energy; and utilizing part of the turbine steam for heat exchange with the dryer system.

Description

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SPECIFICATION
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State of the Art:
Within large industrial plants, for instance, cellulose pulp and paper mills, and large municipal sewage plants, disposal 5 of the waste sludge can be a serious problem. Landfill, pond and dump sites pose a threat to the environment. Sludge recovery processes, for instance, bacterial decomposition and chemical treatment such as liming, are costly, and the costs cannot always be recovered, since the mar~et for the products is small. Systems for 10 dewatering and drying sludge therefore have been developed in order to convert the sludge into a fuel and then burn it.
Swedish patent No. 71 1~295-5 describes a process for dewatering sludge filter cakes. According to this process, steam is blown onto the filter cake, reducing the viscosity, and applying a ~acuum tc remove the water. However, the economics of this process are poor. Characteristic of the sludge handling systems which have been developed i~ a high ope~ating cost, in spite of the recovery of -the high combustion heat of the sludge as fuel. Further-more7 the ef~iciency and reliabil~ of these ~ystems is unsatisfactory.
Bark is today to a great extent ILsed as a fuel primarily for environmental reasons, since the dumping of bark is forbidden in many places. The efficiency of b~rk combustion plants is usLlally low, and especially in the winter one has to add and burn oil, in o~der to keep the bark burning in the boUer. If the solids content of the bark is below 30%, which is often the case, the bark will not burn without ~b ~ cled fuel, such ~s gas or oil. Tn many cases, the bark is de~watered mechanically; however, dewaterillg usually does not yield a higher than about 40~?C solids content. The bark can be dried, using flue gases from a special boiler, in which part of the bark is burned, 5 or using flue gases from the steam boiler, in which all the bark is burned. However, in a plant of the first type, about one-third of the bark has to be burned in order to dry the remaining two-thirds of -~the bark. The dried bark h~s a solids content of about 45'Yc, as compared to a solids content of about 30% before the drying. This 10 means that one loses about 15% of the heat value of the bark which could have been utilized in the steam boiler, if it had been possible to burn the bark with a solids content of 30~C without operating problems. A plant of the second type is economical only at an increase of the solids content of at most 10 Yc~ and is used mainly to 15 obtain more uniform operating conditions. If one seeks to increa~e the solids content b~ more than 10% by means of flue gases, the total efficiency decreases too much.
Burning of bark usually is carried out on grates-,so-called sloping grates, or in cyclone furnaces. Both types of device are - 20 relatively expensive, and also labor-intensive, when compared9 for instance, with suspension fir ing. As is known, the wetter the bark, the lower the efficiency and the larger and more expensive the boiler. The effective heat value of the bark varies with the wood species. For spruce bark, the effective heat value is about 25 19 MJ/kg of dry bark. Spruce bark of 40~C solids content has an ,,~

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ffeclive heat ~alue of about 14. 5 MJ/kg of dry bark. Taking into consideration the decrease in efficiency of the boiler as the solids content increases of the material which is to be burned, one finds that the total degree of efficiency in today's bark combustion- that 5 is with a bark solids conten~ of about 40~C~ is only insignificantly more than 50~c.
Peat is today used only to a small extent as a fuel. The technique that has been available is inadequate to meet present-day requirements for continuous operation, high operating reliability 10 and efficiency. Peat milled by cutting during the summer months has at best a solids content of between 40 and 50%. During rainy periods, the peat is wetter and its heat value rapidly decreases as the solids content decreases.
In order to obtain a uniform production of peat o~er the 15 entire 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 to mechanically dewater peat to an about 50~C solids content. Peat as existing in the peat moss has a varying solids content and composition. Normally, raw peat has a solids 20 content of between 10 and 20~c, and cannot easily be dewatered mechanically to more than 35% solids content, if it is not treated in some way.
In wet coaling, the peat is heat-treated at a concentration of 5 to 10%7 by pumping the suspension through a heat-exchanger 25 in which it is heated into a wet coaling reactor, in which direct 3~'7~
eating ~ith steam is carried out for a longer time. The h eatment is usually carr ied out l)atchwise over a time of one to two hours at from 150 to 200C.
British patent No. 183,180 and Swedish patents Nos. 40,679 5 and 46, 995 describe methods and devices for heating the peat sus-pension with steam. Ouring the heat treatment, between one and two tons o~ steam per ton of solids is used, and the steam is con-densed in the peat suspension or on a heating sl:Lrface, depending on which type of equipment is used. During the long wet-coaling 10 period, the amount of solids is reduced by at east 5 to 10~C by oxidation of the organic material to carbon dioxide and water. After the wet-coaling, the suspension is heat-exchanged with the new peat suspension to be treated, and which is thereby preheated.
This preheating canbe carried out inanapparatus which is described in Swedish patent No. 46, 386.
After wet-coaling~ the peat is dewatered mechanically with presses to an at best 50~C solids content. A greater part of the press-water so obtained is reused for dilution of the peat suspension;
the rest of the press-water has to be dumped. The rnoist press-20 cakes of hal dry peat so obtained are used as fuel, although thethermal value of the peat is low, because of the low- solids content.
A further increase in the solids content of the peat is possible only by drying. There is no drying method which is economically a~vantageous today. While peat briquettes with a 25 solids content of about 90~c are manufactured, such briquettes annot be used industrially as a fuel because of the high price, although they can be used in households.
The drying method~ that are used often make use of flue gases from the combustion of the fuel as a dlrying medium. Ho~-ever, the process is not economic when one tries to increase the solids content of the peat by more than 10~. If the peat is dried in several stages, a somewhatbetter degree of efficiency is obtained.
Tn the so-called Bojner dryer, which is described in 10 Swedish patent No. 9837, the materiai first is dried in air with moist air or steam as a heating medium, and then in flue gases from a combustion oven for the fuel.
U. S . patent No . 2, 014 , 764 descr ibes an apparatus in which steam is used as the drying medium. In this apparatus, 15 however, the peat is first dried in air, with hot water as the 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. This process is very complicated, and reduces the heat consumption during drying only by about 30~
There are also other special drying methods for peat, such as for example, drying in molten metal, which is described in British patent No. 183, 180.
The combustion of peat is carried out in different types of boilers. Burning of coarser particles such as peat press-cakes is 25 carried out on some sort of ~ate. Usually, air from the combustion . , .

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, blown through the ~urnace in order` to dry the pea-~ in the boilcr before it is set on fire. Smaller and above all dry peat particles are burned in powdered form. The powder is blo~Jn often together with the gases of combustion into the boiler.
The burning of moist fuel 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 con-sumed to evaporate the water content of the moist fuel results in a lower degree of efficiency for the boiler. In order to recover a given amount of energy, more fuel and a greater and a thereby more expensive boiler are needed than if dry fuel could have been used.

DESCRIPTION O:F THE INVENTION:
The present invention solves the problem of improving the recovery of energy by increasing efficiency in the drying and combustion of solid fuels obtained from water-containing organic materials such as bark, peat and similar substances.
Broadly speaking the present invention provides a process for the efficient conversion of water-containing organic material as a fuel into heat and electric energy, including drying the material to convert it into a fuel and then combusting the fuel to recover electric energy at low cost;
which comprises heating the material in a steam vessel iII which the material is enveloped in steam at superatmospheric pressure that is heated by heat exchange with steam derived from combustion of the fuel at a higher pressure and higher temperature than the steam in the vessel; recycling water driven from the material as steam and forming excess steam in the vessel at least in part to the material in the vessel and condensing it directly thereon;
mechanically separating water from the material, and then drying ; the material to convert the material into a fuel; combusting the dry fuel in particulate form, thereby generating steam at high 7.;~

~ ssure; uti]izin~ the high pressure steam to operate a turbine;
which in turn operates a ~enerator; converting the energy content of the steam into electric energy; and recycling at least part of the steam from -the turbine for heat exchange with the steam in the steam vessel.
The process in accordance with the present invention frees the water-containing material from solid impurities such as stone and metal and then converts it into a fuel by mechanically dewatering it in one or more stages, then drying it, if desired, after disintegrating it into coarse- and/or finely divided form, and then combusting the dry fuel in a thermal electricity-generating power plant. Before the final mechanical dewatering, it is directly heated with steam from the drying plant. The drying medium is steam at superatmospheric pressure, and steam generated in the -thermal electricity-generating power plant is used to operate one or more turbines, and also for heat exchange with steam in -the drying plant.
ADVANTAGES:
The process has several advantages. Most important, it is possib~e to recover more efficiently than hefore the energy which is present in water-containing organic materials such as, for instance, bark, peat and sludge. By converting the material into a fuel in stages, using energy for successively raising the temperature in each stage, and optimizing operation of each stage from an energy point of view, the losses can be minimized so that the total efficiency will be surprisingly high. Further-more, the process of the invention makes it possible to treat the organic material in a simple and operationally reliable way, so that it is possible -to build a drying and combustion plant which can be run continuously without disturbing interruptions.
In addition, the materials can be recovered in an environmenta]ly advantageous way.
The process of the invention is applicable to any type , '''-_7_ watc~r-contc~ ing flammable organic materia]s. E~amples of such materials are bark, wood shives, pieces, sp]inters, shavinys and other wood rejects from saw mills and cellulose pulp mills; peat; sludges from municipal as well as industrial plants; and household waste (selected garbage).
DESCRIPTION OF THE DRAWINGS:
Figure 1 shows how the effeetive heat value as ordinate varies with deereased solids content of the peat as abscissa~
Figure 2 shows a plant, in which the process of the invention is used in the drying of peat having a solids content of 10% and then combustion;
Figure 3 shows a plant, in whieh the process of the invention is used in the drying of peat having a solids eontent of 25% and then eombustion;
Figure 4 shows a plant in a cellulose pulp mill in which the process of the invention is applied to bark; and Figure 5 shows a plant in which the proeess of the ; invention is used for the drying and combustion of municipal sewage sludge.
DETAILED DESCRIPTION OF TIIE INVENTION
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If the organic material contains solid impurities, such as stones and metal pieces, -these are removed by any known techniques before the material is -trea-ted in accordance with the invention. Furthermore, it is sometimes necessary to reduce the particle size of the material before treatmen-t 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 into small pieees.

For peat, the effective heat value is about 20 MJ/kg - 30 of dry peat. Figure 1 shows in graph form how the heat value - decreases as the solids content deereases. Assuming that about 3 MJ are required in a boiler to produce 1 kg of s-team, one ean theoretically obtain about 6.7 kgs of steam per kg of dry peat.

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ll the peat has a solids content of 50%, the effective lleat value as can be seen from Fl ure l is about 8.5 MJ/kg. Taking into consideration a decreased degree of combustion efficiency, one can tllen produce 5.1 kgs of steam per kg of dry peat. If it is to be economically worthwhile to dry peat to a solids content of 50%, no more than 1.6 kgs of steam per kg of dry peat can be consumed, which is not possible with present drying methods.
With the available pretreatment technique, that is, wet coaling, peat with a solids content of 50% can be obtained for a steam consumption of between l and 2 kgs/kg of dry peat. There thus remains a net production of between 3 and 4 kgs per kg of ary peat. Depending on how economical the drying is, 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 energy recovery is obtainable using modern techniques, if peat is used as fuel.
The incoming organic material is dewatered mechanically in one or more stages. The number of dewatering stages is dependent upon the solids content of the organic materia]. If the solids content is relatively high, for ins-tance, 20% and more, it can be increased satisfactorily in only one dewatering stage. This is for instance the case of peat that in part has been dewatered and , ~,~

iried during conversion from peat moss. In most cases, it is however, necessary to use two or more dewatering stages.
The dewatering process is cal~ried out using known apparatus, such as, for instance, a hy~raulic press, a screw press, 5 a decanting centrifuge, a belt screen press, or a roller press.
Before the last mechanical dewatering stage, the organic material is heated with steam by directly condensin!g steam on the organic material. This direct condensation of steam is carried out either at atmospheric pressure or at superatmospheric pressure.
10 The desigl~ of the apparatus used for this pretreatment stage is dependent upon whether atmospheric pressure or superatmospheric pressure is used.
The organic material at the start of this pretreatment stage should have a solids content of at least 10~C. In this stage, 15 the solids content of the organic material is temporarily decreased, since steam condenses in the material. The temperature of the organic materia~ during this pretreatment stage usually lies within the range from about 40 to about 150C7 and the residence time varies from several minutes to one hour, depending upon the 20 temperature and the m~terial that is to be handled.
The steam used for direct condensation on the material is obtained as surplus steam from a steam dryer later in the process, that is, a dryer in which the transport medium and the drying medium are the same, namely, steam of a superatm~spheric pressure. The 25 steam dryer is described in more detail below.

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Between the pret~eatment stage using a direct condensatio~
of steam on the material and the final drying stage, the material is subjected to a 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 same. Whether the material should be disintegrated or not in this stage of the treatment is determined ~rtly by the sort of material that is to be handled, for instance~ peat, bark or sludge, and partly by the sort of dewatering eqt1ipment that is used, and to a certain extent, also, the design of the steam dryer. The material can be ground or ~isintegrated in any suitable apparatus, such as for instance a hammer mill, pin mill, or a ball mill. Suitable de~Tices for feeding the material into the drying and/or treatment equipment are rotary vane feeders, screw feeders, screw presses and the like.
The design of the steam dryer is not critical. In common with other steam dryers, the drying steam circulating system should be a closed recirculating system, so that a superatmospheric steam pressure is maintained. The superatmospheric steam pressure must be at least 1 MPa (10 bar).
When the organic material is bark or peat, the dryer is so const~ucted that heat is transferrei~ from the steam to the material - by means of convection.
When the organic material consists of sludge, the steam ~ryer is so constructed that tl e greatest part of the heat is tr~nsferred by means of conduction.
In the first case, the drying system contairls a fan, which drives the steam and the material around in the system, a heat 5 exchanger and a cyclone. In the heat exchanger, the drying and the transport medium, that is, the carrier steam, is supplied with all heat necessary for the drying of the material by heat exchange in indirect contact of steam at higher pressure and higher temperature with carrier stea~n.
The heating steam is taken from a turbine. The turbine in its turn is fed with steam rom a steam boiler, in which steam is produced by combustion of the dried material, and, if desired9 additional fuel, for example, oil or gas. In the cyclone, the dried material is separated from the carrier steam, which continues its 15 circula~ion, and in part is transferred to the pretreatment stage, as has been described earlier.
In the bottom of the cyslone there is a device, which dis-charges the dried material to atmospheric pressure. This device can consist of a rotary vane feeder or a screw feeder. The drying of 20 the material is carried out during the transport of the material through the drying system. The water in the material thus is con-verted to steam in the course of transport. As a consequence of this, excess steam is formed in the steam dryer, which means that one can continuously withdraw a certain amount of steam from the 25 drying system. The drying system may also include a fluidized bed, in which thc material is kept for a certain time before the mate-rial, because of the decreased weight caused by the drying, is entrained in the steam and carried further, to the remaining part of the drying system. When drying sludge, a so-called contact dryer is used, 5 in which the heat is transferred by means of conduction. Also in that case the indirect heating steam is obtained from a turbine.
After drying and discharge from the drying system, the material is transported, usually by means of a fan, to the furnace of a ~team boiler for combustion. As transport or carrier media, 10 air and/or flue gases are used. In the course of this t~ansport, the material is further dried by means of so~called flash evapora~
tion.
When the material is fed to the steam boiler for combustion, its solids content should exceed 90~Yc. Furthermore, the particle 15 size of the material during combustion should be less than 3 mm, ~referably less than 1 mm.
If the particle size exceeds this, the material should be subjected to a grinding or a disintegration after the discharge from the drying equipment, and before the combustlon. This can for 20 instance be done in a so-called Kramer mill.
These two requirements 9 that is, a solids content exceeding 90~c, and a particle size less than 3 mm, make it possible to carry out the combustion so completely that both the amount and the dust content in the flue gases will be low, which means a small and thereby 25 inexpensive steam boUer. Tn addition, the steam boiler can be simple - and reliable in operation.

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The steam generated in the steam boiler by combustion of the dried material is, as is earlier descrlbed, led to a turbine or, if required, to several turbines. The pressure and the tempera-ture of the steam upon arrival at the turbine or turbines are very high, for instance, 11. 5 MPa (115 bar), and 530C .
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 about 1 to 2 MPa (10 to 20 bar), a part of the steam is discharged for use as an indirect heating medium for the drying and carrier 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 can be recovered in a distant heat exchanger in the preparation of hot water for local heating purposes . If the remaining steam is to be used as process steam, for instance, within a cellulose pulp mUl7 - its pressure should preferably correspond to the pressure used in the drying equipment, that is, from 0. 3 to 0. 6 MPa (3 to 6 bar).
The invention is illustrated by the following Examples, which represent preferred embodiments of the invention:

In the system shown in the flow sheet of Figure 2, the process of the invention is applied to the drying of raw peat and following combustion of the peatO
The peat suspens ion is introduced via line la at a solids content of 10'7C, and is preheated in a heat exchanger 2 provided with scrapers that move against the heating surface and keep it clean, and that also establish good mi~ing of the peat suspension. The suspension is preheated to about 50C in the heat exchanger 2, with 10 press water at a temperature of about 65C recycled from the dewater-ing stages 3 and 9. In the first dewatering stage, the peat suspension is carried via line lb ~rom the heat exchanger 2 to the dewatering press 3, where it is dewatered at a maximum pressure of about

2. 0 MPa (20 bar), in the course of which the solids content of th~
15 peat suspension is increased to about 35%. The pressed-out water is collected by ~avity below the lpress 3 in the hopper 4, and led via the conduit 5 to the common discharge conduit 6, which recycles it to the heat exchanger 2.
In the second dewatering stage, the dewatered peat is carrîed 20 to a pressure vessel 8 by means of a feed screw 7. The vessel 8 is provided with carr iers. In the vessel 8 the peat is treated with saturated steam via line 18, at a pressure of 0. 5 MPa (5 bar), during a transit time of thirty minutes. The steam~treated peat then passes into a screw press 9, also functioning as feeding screw, where the peat 25 is dewatered to a solids content of about 50~C? and then fed into the cylindrical circulating dryer 10 of the type of U.S.patents Nos. 3,80~3,093 patented April30, 1974, and4,043,049 patented August 237 l977 to Hedstrom. The water pressed out in the screw press 9 is collected from below in a hopper ll, and carried via the conduit 12 to the common 5 discharge line 6.
In the dryer 10, superheated steam is circulated at a pressure of 0. 5 MPa (5 bar), the same pressure as in the pressure vessel 8. The steam serves both as a drying medium and as a carrier for the peat particles. The peat particles pass the fan 13 lO where they are finely disintegrated and then transported in the fan-propelled ~tream through the dryer 10 to the cyclone l4. The steam, which is substantially saturated, is separated from the peat in the cyclolle, and recirculated to the dryer lO via a super-heater 15, inwhich heat is indirectly transferred to the steam.
15 Heat for the superheater 15 is taken from excess steam from the turbine 16 of the plant which is condensed at a pressure of 1. 5 ~Pa (15 bar). The discharge steam is transported through the steam conduit 17 to the superheater 15. Excess steam rom the drying of the peat is separated and led through the conduit 18 to the pressure 20 vessel 8.
After steam drying, the peat has reached a solids content of 80~C. The peat passes via the feed screw 19 to atmospheric pressure and is transported pneumatically through the conduit 20 to the boiler 21. During this travel the peat will dry further, partly 25 because of the pressure drop, and partly due to transport in conduit 20. 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, and is then blown as a dust into t~le boiler 21.
At the entrance to the furnace, the solids content of the peat is about 98~c. In the boiler 21, working with a closed-feed water 5 system, superheated steam is generated at a pressw~e of 11. 5 MPa (115 bar) and a temperature of 530C. The steam is led via the line 23 to one or more twrl~ines 16 connected to a generator ~4 for the production of electx ic power . From the turbine 16, a part of the steam is withdrawn and led via line lt.' to superheater 15, for 10 superheating the steam used as drying medium in the dryer 10.
Residual steam is led from the turbine 16 at a temperature of 105C
through the conduit 25 and condensed in the heat condenser 26.
Other conventional discharges from the turbine to the boiler, between the superheaters etc., are not shown. Condensate feed water is 15 carried back to the boiler 21 from the heat condenser 26 via the conduit ~7, and from the superheater 15 via the line 28.
In Table I below, a comparison is made between the energy recovered according to the process of the invention and the energy recovered according to the previously known process for the 20 preparation of peat press cakes using wet coaling. The fîgures relating to the previously know p3 ocess have been obtained rom a process in which peat having a solids content of 8 Yc in counter-current flow was prehated in a heat exchanger in which part of the heat was taken from already treated, that is wet coaled, peat. The 25 rest of the energy required for the preheating was added in the form Z

of fresh steam Erom the boiler. After the preheating, the peat was fed to a wet coaling reactor, and held there 1. 5 hours at a temperature o 190C and a steam pressure 13 bar. The steam during the 1. 5 hour long treatment was added in the form of fresh 5 steam.
After the wet coaling stage the peat was transported in countercurrent flow with recelltly introduced peat, and then sub-jected to pressing in a plate filter press to a solids content of 49~c-The press cakes so obtained were then burned in a boiler.

TABLE I
Pr ocess of Wet coaling Energy balance the invention process Theoretical energy content of raw peat 200 200 for 10 kgs of dry peat/sec, MW
Net heat need, in preheater MW - 20 Net heat need, in wet coaling ~W - 29 Net heat need, in steam dryer MW 17 Heat value of the peat at introduction 19. 5 15 . 5 in the boiler MJ/kgs of dry peat Boiler degree of efficiency, ~c 86 80 E~oiler energy, ~W 168 124 Boiler energ~ in the form of 54 33 electricity MW
~oiler energy, in the form of 97 42 distant heat MW
As is evident from the above Table, the process of the in-vention as compared to the so-called wet coaling process gives (1~8 11224) , that is, 35~c, more total energy, and the increase in the form of recovered electric energy is (54 333) 100, that is, 63~c.

.~7 Figure 3 shows another system in flow sheet form of a plant f~r drying and combustion of peat in accordanGe with the invention.
In this case, the peat is drained and treated while in the 5 peat moss, to a solids content of 25'~c. The costs for the transport of the peat from the peat moss to the plant in the Figure are correspondingly decreased.
Peat is thus fed via line 29 at a solids content of 25~C to a steam-treating vessel 30. In this vessel the peat is treated at a 10 temperature of ~5 C and at substantLall~atmospheric pressure (1.15 bar~ with steam added via the conduit 31. From the vessel 30 the peat is transported via the rotary vane feeder 32 to a roller dewatering press 33, in which the peat is dewatered to 37~c solids content. The water that is pressed out is removed through the 15 conduit 34.
The peat web from the roller press is disintegrated illto small particles b~ means of a disintegrator 35 and fed to a dryer 36 through which superheated steam (150C) is transported by means of a fan 3~, and serves as a carrier for a fluidized bed of peat particles.
20 The superheated steam used in the dryer 36 is led from the super-heater 38 via the line 3B. In the fluidized bed in the dryer 36, the peat particles are dried. When they are dry enough, and thereby also light enough, they a~e drawn off with the steam out of the fluldized bed to a cyclone 40.

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In the cyclone 40 there is a coarse separation, so that steam is withdrawn from the upper part of the dryer 36, and peat is withdrawn from the lower part. The steam and any entrained peat particles are transferred via line 40a to a multicyclone array 41, 5 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 line 40b from the bottom of the cyclone 40, where it is mixed with the rest of the peat. The peat is discharged via the rotary vane feeder 43 to a conduit 44, which îs connected with the combustion boiler 45.
10 When the peat is sluicecl out of the cyclone 40 it has a solids content of 75~c-Flue gases axe removed from the boiler 45 and carriedvia line 46 to a fan 4rl~ which propels the flue gases in line 44 to the feed from the boiler 45, thus drawing peat into the boiler 45, where 15 combustion takes place. The flue gases serve as a carrier for the peat, and also further dry the peat, so that dur ing this h ansport the solids content of the peat is increased from 75 7c to 92~c. In the boiler superheated steam at high pressure (11. 5 MPa) is generated, and carried through the line 48 to one or more turbines 49 connected 20 to a generator 50 for the production of electric power. From the turbine 4g a part of the steam is removed at a pressure 1 MPa, and ~ransported through the line 51 to the superheater 38 for superheating ! steam used in dryer 36 as the drying medium steam. The dryirlg medium steam has a pressure of 0.115 MPa (1.15 bar)7 and is 25 recirculated by the fan 3~ through the dryer 36, the cyclone 40, the ' ~0 t7~

multicyclone array 41, the superheater 38 and the conduit 39. Part of the steam from the cyclone 40 is withdrawn and sent via line 31 to the incoming peat in the vessel 30, as has been previously described.
The steam removed from the turbine 49 is condensed ';o water in the 5 superheater 38 and the water condensate is recirculated to the boiler 45 through the conduit 52. Excess steam from the turbine 49 is carried via the line 53 for other uses.
This application of the process of the invention gives the same high energy recovery from the peat described in Example 1, 10 that is, an increase in the energy recovery Of 35~3~c, as compared to the wet coaling process.

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_XAMP LE 3 In the system shown in the flow sheet of Figure 4, the process of the invention is applied to bark in a cellulose pulp mill.
S~ruce bark (in which the coarsest bark pieces have been S crushed in a mill~ not shown in the Figure) is transporterl via line 54 to a hydraulic press 55. The bark has a solids content of 30~C before dewatering in the press, and is dewatered to a solids content of 36 Yc. Then the bark is carried ~ia feed screw 56 to a pressure vessel 57. Steam at a pressure of 0. 4 MPa (4 bar) iæ
added via the line 58 onto the bark in the,pressure vessel, so that the bark is heated to 140C while the steam is condensed. The residencetime for the bark in the pressure vessel 57 is three 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~c and fed into a recirculating superheated steam system 64. The bark pieces fall from the screw feeder 61 down to a mill 62 on the bottom of the dryer 63, where they are ground to such fine particles that the carrier steam added through ' 20 the line 64 can entrain them ar~ carry them away. The carrier steam and the finely divided bark thereafter pass through superheater 65, in which e~cess steam from the turbine 6G transported via lines 67 and 68 condenses at a pressure of 1. 6 MPa (16 bar). Then the carrier steam and the bark pass via conduit 70, and propelled by fan 69,, to a cyclone 71.

.s~

In this cyclone the dried bark is separated from the steam. The bark is discharged bymeans of a rotary vane feeder 72 out of the cyclone 71, and carried via line 73 to a line 74. At the end of the line 74 is a fan 75, by mealls of which the finely-divided bark (particle size less 5 than 0. 4 mm) together with a part of the combustion gas and any other gases are blown tangentially into the furnace 76.
At the enl~rance to the ~urnace, the bark powder has a solids content of 90~c- The steam separated in the cyclone 71 is recycled via the line 64 to the dryer 63, and introduced into the 10 same close to the disintegration apparatus, that is,the mill 62.
From the recirculation conduit 64 part of the steam is removed through the line 77 to a steam reformer 78, and part through the line 58 to the pressure vessel 57, as previously described.
In the steam reformer 78 steam is fed into the bottom of the same, 15 on one side of the heat exchanger in the steam reformer. Impurities present in the steam (such as inert gases, turpentine, acids, etc.) are purged from the top. These gaLses are led through the line 79 to the fan 75 which carries them urther in~o the boiler for combustion.
The condensate from the steam is removed from the steam 20 reformer 78 via line 80 to the plant for evaporation of digestion liquor of the mill, and the evaporation residue is then burned in the soda boiler. To the line 80 press water from the screw feeder 61 is fed through the line 81, and from the hydraulic press 55 through the line 82.

1J'I ~ ,r"

On the other side of the heat exchanger in the steam reformer 78, feed water obtained from the mill and added throu~h the lines 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 5 mill for use there.
The steam condensed in the superheater 65 is carried throu~h the line 85 to line 83, where it is mixed with the feed water that is introduced into the boiler 76. During combustion of the dried bark in the boiler 76, superheated steam at high pressure is generated, 10 which stea~ is carried through the line 88 to one or more turbines 66 connected to a generator 89 for production of elect~ic power.
From the turbines steam is transported through the line 67 at a pressure of 1. 6 ~a. This steam is divided into two streams.
One part of the steam is ~ arried through the line 68 to the superheater 15 65, for indirect transfer of heat to the carrier steam in the drying system, and the other part of the steam is withdrawn via line 90 for use elsewhere in the plant. The steam remaining in the turbine 66, that is ? after dischar ge and reforming to electr ic energy, is carr ied at a pressure of 0.4 MPa through the line 87 to the line 86, which 20 is in connection with the plant.
In order to evaluate the importance of the pretre~trnent of the bark in the pressure vessel 57, two tests were made in addition to the one described above. In one test, the steam treatment in the E~essure vessel 57 was excluded. In the other test, the bark was 25 ~eated in the pressure vessel 57 with steam at a temperature of z 105C, instead of the previously stated steam temperature of 140C.

The solids content of the bark after passing the screw feeder 61 was measured, and the following results were obtained:

TABLE II

No addition of Addition of Addition of steam steam in the 105 C steam of 1 40C in the pressure vessel in the pressure pressure vessel 5~ vessel 5~ 57 -Solids content in 10 % after t}le press 36.3 35.8 36.0 Solids cont~nt in ~c after steam treatment ~ 32. 5 31. 4 15 Solids content in % after the screw feeder 61 38.5 43.0 4q.7 As is evident from Table ri the steam pretreatment of the bark in accordance with the present invention gives a solids content 20 of the bark after the second pressing that is substantially higher than if the addition of steam is left out. Even if the value of the steam added is deducted from the process of the invention in a calculation of the energy balance, the pretreatment stage gives a better result.
If the process according to the in~ention relating to drying 25 and burning of bark described above is compared with the con~entional h~ndling of bark, that is, hy mechanically dewatering of the bark by means of pressing to a solids content of 40'~c, followed by combustion in a boller provided with a sloping grate, one finds that the price of the steam produced accordillg to the in~eIItion is 35~c lower than the price of steam in the conventional handling of bark. This also is true in spite of the fact that the equipment shown in Figure 4 com-bined with the steam boiler leads to a higher investment cost, and 5 also to some increase in the costs of operation, as compared to conventional handling. The lower cost of production per ton of steam is dependent on the fact that considerably more steam is obtained from the same amount of bark, as compared with previously known techniques. Fllrthermore, the process of the invention enables one to 10 make the steam boiler itself more simply, and thereby more cheaply, - and also more reliably, than for instance a steam boiler with sloping grate. By an improved combustion of the barl~ the amount of dust has been decreased from about ~80 mg/normal m3 (Nm~) of flue gas in a steam boiler with sloping gratetoabout 40 mg/normal ~.a3 (Nm3) of 15 flue gas in the process according to the invention. A further ad-vantage with the system in accordance with Figure 4 is, that the discharge of oxygen-consuming substances will be low, because of the evaporation of condensate from the steam reformer 78, the press water from the hydraulic press 55, and the press water from 20 the screw feeder 61.

EX~PL~ 4 ___ In the system shown in the flow sheet of_~ re 5, the process according to the invention is used in a plant for drying and combustion of raw municipal sewage sludge.
The sludge comes via line 91 from an activated sludge plant, has a solids content of 4~c, and is dewatered in a conventional press 92 to a solids content of 10~/C. The press 92 can be replaced with for instance a decanting centrifuge. The dewatered sludge is transferred to a container 93, in which the sludge is heated to 80C by means of direct condensation of steam, which is obtained from the dryer 94. From the dryer the steam is transported through the lines 95 and 96. During condensation of the steam in the sludge, ill-s ~elling gases are li~erated, which are collected at the top of the container 93, and led via line 97 to the steam boiler 98, in which the gases are combusted together ~ ~ith dr ied sludge and oil. The sludge is then transferred to a belt screen press 997 and dewatered to a solids content of 37~c- The water that is pressed out of the band screen press 99 together with water pressed out of the press 92 is recirculated via line 100 back to the activated sludge plant.
After the final mechanical dewatering, the sludge is t~ansferred to the dryer 94. The dryer comprises a pressure vessel with three axially arranged transport screws, which are constructed so tha$ they clean each other during rotation, and serve as pressure-tight sluices in the feeding and discharge of the sludge to and from the dryer 94.

z Heat is supplied indiI ectly to the dryer 94 by withdrawing steam of a pressure of 0. 9 MPa (9 bar) frorn 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 the conduit 103 5 is carried by means of the conduit 104 to the mantel of the pressure vessel, where the steam is condensed. The pressure of the steam in the drying apparatus 94 where the sludge is kept is 0. 2 MPa (2 bar).
In this type of dryer, which is called a contact dryer, it is 10 essential that good heat conduction between the screws and the sludge is obtainedO
The sludge has a solids content of 90~C when discharged from the dryer 94. The sludge is discharged in finely divided form, as a powder, and is blown by fan 105 through the line 106 to a cyclone 15 107. In the cyclone the powdered sludge is separated and ~ansported further through the line 108 to the furnace of the steam boiler 98.
The sludge is combusted together with oil in the steam boiler 98, whereupon superheated steam at high pressure is generated, and transported through the line 109 to one or more turbines 102, con-20 nected to a generator 110 f or pr oduction c electr ic power .
As has been earlier stated, steam is withdrawn from theturbine 102 and led as indirect heating steam to the dryer 94 via lines 103 and 104. The steam remaining in the turbine after discharge and reforming to electric energy passes via the line 111 to the heat 25 condenser 112, where it is condensed. The condensate is transferred .'.t~ "7~

back to the steam boiler 98 via line 113 as feed water.
The condensate from the dryer 94 is carried via the line 114 to the line 113, ~or further transfer as feed water to the steam boiler 98. Part of the steam recovered in the dryer 94, as has been earlier 5 described, is transferred by means of the conduits 95 and 96 to the pretreatment vessel 93.
The rest of the steam recovered in the dryer 94 is carried through the line 115 to the activated sludge plant (not shown in the Figure). In this plant, which contains basins, among other things, 10 the waste water comes into eontact 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 thereby one also obtains an elevation of the temperature of the water in the basins .
As is evident from the above description, oil is added to 15 the boiler and burned together with the dried sludge. Since the solids content of the sludge is ~rery low in the beginning, and due to the physical st~ucture of the sludge, it is not possible to dry the sludge and then during combustion obtain enough energy for the whole sludge treatment operation; one must always add energy, and this is usually 20 in the form of oil. The handllng of sludge thus always adds expense.
Depending on the type oi sludge and the solids content of the sludge, between 0. 5 and 1. 0 kg of oil/kg of dry sludge are required for disposing of the sludge in conventional drying and combustion processes.
The cost for conventional handling of sludge, for instance, compr ising 25 dewatering the sludge in a decantering centrifuge and drying and ~9 .3i.~t.:~

combustion of the same in a multistage oven and depositing the ash, amounts to about $125. OO/ton 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 decreased 25~7c. Furthermore, 5 the process results in the prevention of odor discharges~ and the problem with uncombusted dust is decreased.

~0

Claims (13)

Having regard to the foregoing disclosure, the following is claimed as inventive and patentable embodiments thereof:

1. A process for the efficient conversion of water-containing organic material as a fuel into heat and electric energy, including dry-ing the material to convert it into a fuel and then combusting the fuel to recover electric energy at low cost; which comprises heating the material in a steam vessel in which the material is enveloped in steam at superatmospheric pressure that is heated by heat exchange with steam derived from combustion of the fuel at a higher pressure and higher temperature than the steam in the vessel; recycling water driven from the material as steam and forming excess steam in the vessel at least in part to the material in the vessel and condensing it directly thereon; mechanically separating water from the material, and then drying the material to convert the material into a fuel;
combusting the dry fuel in particulate form, thereby generating steam at high pressure; utilizing the high pressure steam to operate a turbine; which in turn operates a generator; converting the energy content of the steam into electric energy; and recycling at least part of the steam from the turbine for heat exchange with the steam in the steam vessel.

2. A process according to claim 1 in which the solids content of the organic material during heating with steam is increased to at least 10% by weight.

3. A process according to claim 1 in which the organic material is reduced to finely divided form after the separation of water and before combusting.

4. A process according to claim 1 in which the organic material is dried sufficiently to form a fuel having a solids content of at least 90% by weight.

5. A process according to claim 1 in which the dry fuel before the combustion is reduced to a particle size of less than 3 mm in diameter.

6. A process according to claim 1 in which the organic material in the steam vessel is heated by convection, and is selected from the group consisting of bark and peat.

7. A process according to claim 1 in which the organic material in the steam vessel is sludge, and is heated by conduction.

8. A process according to claim 1 in which the organic material is selected from the group consisting of bark, peat and sludge.

9. A process according to claim 1 in which the water is separated by application of mechanical compression to the material.

10. A process according to claim 1 in which the water is separated by application of centrifugal force.

11. A process according to claim 1 which comprises mechanically separating water from the material before heating the material in a steam vessel.

12. A process according to claim 11 in which the water is separated by application of mechanical compression to the material.

13. A process according to claim 11 in which the water is separated by application of centrifugal force.