WO1995008022A1

WO1995008022A1 - Continuous process and apparatus for the exploitation of energy contained in cellulose production process wastes

Info

Publication number: WO1995008022A1
Application number: PCT/FI1994/000402
Authority: WO
Inventors: Kari Saviharju; Allan Johansson; Kai SIPILÄ
Original assignee: Valtion Teknillinen Tutkimuskeskus
Priority date: 1993-09-14
Filing date: 1994-09-13
Publication date: 1995-03-23
Also published as: FI934028A0; FI934028A

Abstract

The invention relates to an improved continuous process for the exploitation of cellulose production process wastes. The process comprises the following operations: a) the said process waste is fed into a pyrolysis reactor and is pyrolyzed therein by heating the process waste with a residence time of 0.1-100 s at a temperature of 300-700 °C into a pyrolysis product which comprises a liquid fuel, a gaseous fuel and a solid residue, b) the pyrolysis product is fed into a separator, wherein the solid residue is separated from the liquid and gaseous fuels, c) the liquid and gaseous fuels are recovered, and d) the solid residue is fed into a combustion vessel, wherein it is burned to generate heat. The invention also relates to an apparatus for carrying out such a process.

Description

Continuous process and apparatus for the exploitation of energy contained in cellulose production process wastes

The invention relates to a continuous process for the exploita¬ tion of energy contained in cellulose production process wastes. The invention also relates to a continuous-working ap¬ paratus for the exploitation of energy contained in cellulose production process wastes, the apparatus comprising a heat- generating combustion vessel.

In a forest-industry cellulose mill, less than 50 % of the wood is used for raw material, the rest is converted into energy in the bark vessel and in the combustion and recovery vessel for the cooking chemicals (liquor or soda recovery boiler) . Most of the energy is released in the combustion and recovery vessel, which is discussed below from the viewpoint of a sulphate mill (i.e. recovery vessel = soda recovery boiler) , but the inven¬ tion is by no means limited to a sulphate cellulose mill. The recovery vessel constitutes the most important link in the recovery of cooking chemicals, the soda recovery boiler is one of the most expensive components of a cellulose mill and is often a factor limiting the increasing of mill capacity.

The combustion and recovery vessel for the cooking chemicals of a cellulose mill in general operates so that the residue from cellulose cooking, e.g. the black liquor, is fed into a vessel in which the organic matter is burned to produce heat, and the remaining inorganic salts are regenerated for reuse. The capac¬ ity of a mill in operation is often limited by the black liquor combustion capacity of the combustion and recovery vessel, i.e. the high quantity of organic matter.

A second problem associated with the recovery of cellulose mill cooking chemicals is the sulphur problem. A third disadvantage connected with the combustion and recovery of bark and cooking chemicals is that, even though a cellulose mill is in general more than self-sufficient in its energy balance, the quality distribution of the fuels is such that purchased fuels must be resorted to; on the other hand, if the combustible process wastes of the cellulose mill could be con¬ verted and/or classified so that their quality distribution would be improved, purchase costs of insufficient fuels would be saved and electricity could be produced more efficiently. It is also conceivable that cellulose mill process wastes would be converted so as to produce from them more expensive fuels, which could be sold.

The problems involved in the combustion and recovery of bark and cooking chemicals also include the corrosive damage caused to the vessels by the chlorine present in process wastes. If the chlorine could be removed before the burning of the process wastes in the vessels, the useful life of the vessels would be lengthened and mill cycles could be better closed.

The object of the present invention is to eliminate the above disadvantages and to improve the prior-art processes for the exploitation of energy contained in cellulose production proc¬ ess wastes. The process according to the invention is thus mainly characterized in that it comprises the following oper¬ ations: a) the above-mentioned process waste is fed into a pyrolysis reactor and is pyrolyzed therein by heating the process waste with a residence time of 0.1-100 s at a temperature of 300- 700 °C into a pyrolysis product which comprises a liquid fuel, a gaseous fuel, and a solid residue, b) the pyrolysis product is fed into a separator, wherein the solid residue is separated from the liquid and gaseous fuels, c) the liquid and gaseous fuels are recovered, and d) the solid residue is fed into a combustion vessel, wherein it is burned to produce heat. It has thus been realized that the above-mentioned disadvantages can be eliminated by first pyrolyzing the cellulose production process wastes either in part or in full, by recovering the produced liquid and gaseous fuels, and by feeding only the solid waste from pyrolysis to the said combustion and recovery vessel.

According to one preferred embodiment, cooking chemicals of the cellulose production process are recovered from the said com¬ bustion vessel, i.e. the combustion vessel serves as a recovery boiler in cellulose production. In this case, the fact that the proportion of organic waste is considerably smaller will facil¬ itate the recovery of the said chemicals and increase the ca¬ pacity of the said vessel.

Dried process waste should preferably be fed into the pyrolysis reactor. In this case it is preferable to exploit the combus¬ tion vessel combustion gases in the drying of the process waste. This can be arranged, for example, by means of heat exchange equipment or the like.

The pyrolysis reactor used in the process is a flash pyrolysis reactor, in which the residence time of process waste is within the range 0.1-100 s, while its operating temperature is rela¬ tively low, i.e. 300-700 °C. The most advantageous pyrolysis reactor is a reactor operating according to the fluidized-bed principle. A preferred residence time is 0.1-10 s and the most preferred is 0.1-1 s. The most preferred operating temperature is 400-500 °C.

The pyrolysis produces a liquid fuel, a gaseous fuel, and a solid residue. Their separation is best carried out by means of a centrifugal separator, such as a cyclone-type separator, arranged in conjunction with the pyrolysis reactor. The pyrol¬ ysis product arrives in the separator as a hot and strong stream, the solid residue sinking to the separator bottom and the liquid and gaseous products continuing their flow.

When a fluidized-bed pyrolyzer is used, also the fluidized-bed material is separated from the liquid and gaseous fuels.

In the process it is preferable to take the heat required by the pyrolysis reactor from the combustion vessel, for example from the combustion gases or melt, preferably from the combus¬ tion gases.

It is preferable to heat the possible fluidized-bed material and/or the solid residue coming from the pyrolysis reactor before its being fed into the combustion vessel. The combustion vessel combustion gases are well suited for this heating, in which case the fluidized-bed material and/or the solid residue batch are, according to a preferred embodiment, heated by means of the combustion gases emerging from the combustion vessel by mixing it/them with the combustion gases and by thereafter separating it/them from the combustion gases. If the pyrolysis reactor operates according to the fluidized-bed principle, the solid residue is separated from the combustion gases and, in addition, large particles of solid residue by means of a sieve, whereafter the solid residue is fed into the combustion vessel and the fluidized-bed material is fed back into the pyrolysis reactor, the combustion gases continuing their travel, for example, to heat exchange, drying operations, or purification.

Most preferably the fluidized-bed pyrolyzer obtains all or a substantial proportion of its heat from the combustion vessel combustion gases. The sieve by means of which the solid residue is separated from the flue gases and the fluidized-bed material is preferably of the fluidized-bed type or the transport type, in which an upward flowing combustion gas raises the lighter fluidized-bed material, while the heavier solid material falls downward. Further on, the fluidized-bed material can be sepa¬ rated from the combustion gas by directing it from the said sieve to, for example, a cyclone, wherefrom the fluidized-bed material is directed to the fluidized bed of the pyrolysis reactor.

When the solid residue of the pyrolysis product has been sepa¬ rated from the liquid and gaseous fuels, the latter components can, according to one embodiment of the invention, be separated from each other. This can be done, for example, by means of condensation. The liquid fuel, i.e. pyrolysis oil, can be re¬ covered and be sold or used, for example, for producing elec¬ tricity for the mill. The obtained gaseous fuel can in part or in full be fed back, as fluidization gas, into the pyrolysis reactor, but it may also be fed as fuel into the lime sludge reburning kiln. A large proportion of the sulphur contained in the process wastes will also be present in the gaseous fuel, in which case sulphur recovery can be arranged, for example, after the lime sludge reburning kiln or in some other manner.

Heat can be recovered from the combustion vessel combustion gases by means of heat exchange. According to one embodiment, the combustion gases are used for heating both the fluidized- bed material and the solid residue, for drying the process wastes to be fed in, and for generating heat.

The invention also relates to a continuous-working apparatus for the exploitation of energy contained in cellulose produc¬ tion process wastes. Such an apparatus comprises, in addition to a heat-producing combustion vessel: a) a pyrolysis reactor provided with a feed inlet for process waste, in which pyrolysis reactor the residence time is 0.1- 100 s and the operating temperature 300-700 °C and the process waste is pyrolyzed into liquid and gaseous fuels and solid residue, b) a separator for separating the solid residue from the liquid and gaseous fuels, c) a system for the recovery of the liquid and gaseous fuels, and d) conveying means for feeding the solid residue from the sepa¬ rator to the combustion vessel.

The combustion vessel of the continuous-working apparatus ac¬ cording to the invention is preferably equipped with recovery equipment for the cellulose production process cooking chemi¬ cals or the like, in which case it at the same time serves as a recovery vessel. One such important combustion and recovery vessel is the soda recovery boiler of a sulphate mill.

The apparatus may also comprise a dryer, arranged at a point before the feed inlet, for drying the process waste before pyrolysis. It is preferable if such a dryer is operated by means of combustion gases from the combustion vessel of the ap¬ paratus.

As was mentioned above in connection with the discussion of the process, the most preferred pyrolysis reactor is a fluidized- bed reactor. Preferred residence times are 0.1-10 s and 0.1- 1 s, and a preferred operating temperature is 400-500 °C.

According to one embodiment of the invention, there is provided in conjunction with the pyrolysis reactor a separator for sepa¬ rating the solid residue from the liquid and gaseous fuels. In the case of a fluidized-bed reactor, the separator also sepa¬ rates the fluidized-bed material such as sand from the liquid and gaseous fuels. The cyclone separator is a typical such separator.

Conveying means are arranged between the separator arranged in conjunction with the pyrolysis reactor and the combustion ves- sel. It is preferable to exploit the heat of combustion vessel combustion gases for pre-heating the fluidized-bed material to be returned to the pyrolyzer and/or the solid residue to be fed into the combustion vessel. In this case the apparatus com¬ prises a heat exchanger which is coupled to the said conveyor means for any possible fluidized-bed material and/or the solid material. According to one embodiment, the heat exchanger has space for mixing the fluidized-bed material and/or the solid residue with the combustion gases and space for their subse¬ quent separation.

If the pyrolysis reactor operates according to the fluidized- bed principle, there will emerge from the pyrolysis reactor not only the three-component pyrolysis product but also the gas which has transported it and the fluidized-bed material, which may be, for example, sand. When the solid residue is separated from the liquid and gaseous fuels, also the said fluidized-bed material will travel with it. When the solid residue is there¬ after fed into the combustion vessel, it is preferable to ar¬ range in conjunction with the feed conveyor means a sieve by means of which the fluidized-bed material is separated from the solid residue. When combustion gases are used for heating the solid residue, it is the function of the sieve to separate from the solid residue both the fluidized-bed material and the com¬ bustion gases coming from the combustion vessel. In this case there is arranged in conjunction with the sieve conduits to the fluidized bed of the pyrolysis reactor, to the combustion ves¬ sel, and to combustion gas removal. According to one embodi¬ ment, the heat exchanger for heating the solid residue by means of combustion gases at the same time functions as the said sieve in such a manner that the combustion gases flowing upward in its space carry with them the lighter fluidized-bed mate¬ rial, the heavier solid residue travelling to the bottom and from there to the combustion vessel. In this case there is preferably a cyclone separator coupled to the heat exchanger/- sieve to separate the fluidized-bed material from the combus¬ tion gases.

The apparatus according to the invention preferably comprises a condenser for separating the liquid fuel from the gaseous fuel. If it is desired to use the gaseous fuel as the fluidization gas in the pyrolysis reactor, the apparatus according to the invention comprises a gaseous-fuel conduit which extends from the condenser to the fluidized bed of the pyrolysis reactor. A heat exchanger may be arranged in conjunction with the combus¬ tion vessel to recover heat from the combustion vessel combus¬ tion gases.

The invention also relates to the use of the process and ap¬ paratus described above in the cellulose production process. As was pointed out at the beginning, the advantages of the inven¬ tion are largely associated specifically with the exploitation of cellulose process wastes.

It has been stated above that all the process waste travels via the pyrolysis reactor. The invention may also be implemented in such a manner that only a portion of the materials flow of the mill, such as sulphate soap, is directed to the pyrolysis reac¬ tor, and the rest, such as black liquor, either in part or in full, is directed to the recovery vessel. In this case the combustion gas stream will constitute only a portion of the en¬ tire combustion gas stream of the recovery vessel. Process wastes potentially suitable for fuel in the invention include:

- black liquor

- sulphite cook waste liquors

- sulphate soap

- other raw material waste, if its contents of substances det¬ rimental to the cooking chemicals cycle are low

- wood, grass biomass, or the like - in general any solid or liquid fuel which forms a carboniza¬ tion residue.

The basic parts of the invention are the flash pyrolysis reac¬ tor with recovery and the heat exchanger/sieve for particles. The flash pyrolyzer according to the invention operates at a low temperature and produces a large amount of an oily product, and alkalis are hardly removed from the carbonization residue.

The pyrolysis reactor may also be, for example, coupled to the fluidized-bed vessel. By using the heat exchanger/sieve de¬ scribed above, the invention can be applied to a grate and dust combustion furnace for solid fuels and also to a fluidized-bed vessel. In this case the fuel of the pyrolysis reactor may be, for example:

- coal

- brown coal

- tars

- wood waste, wood

- heavy fuel oil, bitumen, oil refining residue

- oil sand

An example is presented below, its only purpose being to il¬ lustrate the invention.

Example

The main features of the process and apparatus are shown in

Figure 1, wherein the numerals have the following meanings:

1. recovery vessel (soda recovery boiler, liquor recovery boiler)

2. mound in soda recovery boiler

3. melt to solvent

4. large particles, formed by coke and salts, from heat exchanger/sieve 6

5. combustion gas stream to heat exchanger/sieve 6 6. heat exchanger/sieve (of the fluidized-bed type or a so- called wind sieve)

7. separator cyclone

8. cooled combustion gases to convection surfaces

9. solids stream

10. stream divider and "back flow"

11. return stream to beginning of heat exchanger/sieve

12. heated particle stream to pyrolysis reactor

13. "back flow"

14. fuel feed into pyrolysis reactor

15. flash pyrolysis reactor (of the fluidized-bed type)

16. separator (cyclone)

17. oil and gas stream to oil separator

18. oil separator (condenser)

19. pyrolysis oil

20. uncondensed pyrolysis gases

21. fluidization gas to pyrolysis reactor

22. blower

23. bed

24. cooled particles + carbonization residue to heat exchanger/sieve

25. recovery of combustion gas heat (convection surface)

26. recovery medium, e.g. water/steam

27. combustion gases to dryer

28. dryer

29. dried fuel to pyrolysis reactor feed

30. fuel feed to dryer

31. cold combustion gas to dryer

The operating principle of the process is as follows: Cellulose mill waste fuel (black liquor, sulphate soap, waste liquor or other "pure" wood residue or other thermally decomposing fuel) is directed, either dried (dryer 28, undried fuel 30, dried fuel 29) or undried, into the pyrolysis reactor 15 via feeding point 14. The pyrolysis reactor 15 is preferably of the prior- known flash type, which typically has a short residence time, conventionally 0.5 s, at a low temperature of 350-700 °C, most preferably 400-500 °C. In this case a considerably large pro¬ portion of the fuel is converted into oil and gaseous products, and coke is formed in an amount less than 50 % of the organic portion of the fuel. With certain fuels, such as sulphate soap, coke is formed in an amount of only about 10 %. The alkalis of the fuel are almost completely left in the carbonization resi¬ due (carbonization residue = coke + salts) . The oil and gases emerge from the pyrolysis reactor in gaseous form, and solids are separated from them in a separator 16, for example a cyc¬ lone. The pyrolysis reactor is preferably a fluidized-bed reac¬ tor, on which Figure 1 is based. A second alternative is to implement the pyrolysis reactor by heating by means of stream 12 either the walls of the reactor 15 or the gas 21, and by returning the cooled gases together with the separated solids 24 to the heat exchanger/sieve 6. The oil is separated from the uncondensed pyrolysis gases in a separator 18, which is of a prior known construction, e.g. a scrubber, in which separated and cooled oil circulates. The quality of the oil 19 may be af¬ fected by adjustment of the separation temperature. The uncon¬ densed gases 20 are directed, for example, to the lime sludge reburning kiln or the recovery vessel (soda recovery boiler) or some other boiler, furnace or scrubber, in which gases which have passed oil separation can be recovered or be used for energy by burning. A portion of the gases can be used as fluid- ization gases (21) in the pyrolysis reactor; of course, for ex¬ ample soda recovery boiler gases or other process gases (e.g. odorous gases) of the mill may also be used as fluidization gases. The separated pyrolysis oil (19) can further be washed free of alkalis, or it may be refined by known methods to im¬ prove its properties.

In the separator 16, the solids 24 separated from the gas and oil, the solids comprising the particle stream 12 and the car- bonization residue formed in the pyrolysis, are directed to the heat exchanger/sieve 6. In the sieve, coarse particles 4 are separated from the particle stream, and the fine particles are heated in order to be returned to the pyrolysis reactor as a source of heat 12.

The heat exchanger/sieve 6 is preferably of the fluidized-bed type or the transport type. In this case the operational idea of the apparatus is such that, when the velocity of the upward flowing gas in the conduit 6 is adjusted upward, the coarse particles cannot follow the stream but fall from the bottom of the sieve via the lock (back-flow effect) to the bottom of the recovery vessel 1, where the coke oxidizes and the chemicals are recovered from the carbonization residue. The fines which follow the gas stream will heat up until they are separated in the separator 7, which is preferably a centrifugal separator (cyclone) , and are returned 9 via the divider 10 either to the lower section of the sieve or to the pyrolysis reactor 15.

The divider 10 is preferably of the fluidized-bed type, but it may also be based on control of the flow apertures by means of, for example, flaps. The lock 13 may be based merely on small size as compared with the flow aperture 5, or it may be of the fluidized-bed type (so-called knee) or some other known struc¬ ture. The gas stream 5 may be such that the salts present as an impurity therein are in solid form, in which case the heat ex¬ changer/sieve 6 is of a conventional construction, or the tem¬ perature is higher than the melting point of the salts, in which case the mixing temperature of the streams 5 and 11 is maintained below the softening point by regulating the mass flows, i.e. in the case of black liquor approx. 700 °C, in which case the salts will deposit on the surface of the solid particles, thereby cleansing the gas of alkalis and ultimately falling together with the coarse particles in the stream 4 into the recovery vessel. When the operating temperature of the bed 2 is sufficiently low and the properties of the salts of the pyrolyzing fuel are good (slight vaporization) , the salt con¬ tent 5 will be low and the operating temperature 5 may be above the softening point of the salts, regardless of the operating principle of the heat exchanger/sieve. The cooled recovery vessel combustion gases 8 are cooled on conventional heat de¬ livery surfaces 25 to the outlet temperature of the combustion gases, or if the fuel is dried before the pyrolysis, to the temperature required by the dryer 28. The dryer may be, for example, of the fluidized-bed type. The cooled combustion gases 31 are purified by conventional methods, for example by using an electro-filter and/or a scrubber.

The limitation of the soda recovery boiler capacity can thus be eliminated by equipping it with a so-called flash pyrolysis reactor, which converts, under the effect of heat, the black liquor fed into it into a liquid fuel, i.e. pyrolysis oil, a solid carbonization residue, and residual, i.e. pyrolysis gases. In this case only about 40 % of the organic matter of the black liquor is left in the carbonization residue in addi¬ tion to the salts of the black liquor. When only this carbon¬ ization residue is introduced into the soda recovery boiler, the load of organic matter introduced into it will drop to below 50 % of the conventional load, and thus the capacity of the recovery boiler will increase significantly (advantage 1) .

In rapid pyrolysis (flash pyrolysis) , a significant proportion (up to one-half) of the sulphur of the black liquor leaves together with the pyrolysis oil and the pyrolysis gases. The pyrolysis gases may be directed further, for example, as fuel to the lime sludge reburning kiln, in which case purchased fuel for the lime sludge reburning kiln will not be necessary (ad¬ vantage 2) ; this will improve the energy balance of the mill, which will be more than self-sufficient with respect to energy. Since only one-half of the sulphur of the black liquor is left in the carbonization residue to be oxidized in the recovery vessel, this provides an opportunity for controlling the sul¬ phur balance of the mill (advantage 3) , for example so that the pyrolysis gases are directed to the lime sludge reburning kiln, which will be followed by sulphur recovery. Furthermore, the possibilities for control of the cooking will be improved, since sulphur in proportion to sodium can be fed into the cook in stages (advantage 4) . If more sulphur is released in the pyrolysis than is advantageous for the cooking, a portion of the sulphur can be returned to the recovery vessel, in which it is bound, for example into Na₂S.

The oil produced in flash pyrolysis has only a small alkali content when the pyrolysis temperature is low, below 700 °C, in which case many options will be open for the use of the oil produced, either without refining or with further refining. The use may take place within the mill, for example in a gas tur¬ bine, a diesel engine or a boiler, in which case the electrici¬ ty production of the mill will become significantly more effi¬ cient owing to the increase in the build-up in electricity production (advantage 5) . Furthermore, the pyrolysis oil may be sold to outside the mill as a bio-oil advantageously prepared from a renewable raw material (advantage 6) ; this improves the control of the energy balance of the mill and further the ad¬ vantage obtained from the raw material, as compared with merely replacing the energy input of the lime sludge reburning kiln with the mill's own fuel (advantage 7) . If fuel is sold to outside the mill, it may be washed clean of alkalis (Na + K) by prior-known methods. The removal of alkalis from the oil would be clearly easier than if the question were of a gasification product, i.e. if the black liquor were converted by pressurized gasification into fuel for a gas turbine or a diesel engine. By means of the invention it is also possible to reduce corrosion and soiling of the recovery vessel, since most of the chlorine present in the liquor is released in the pyrolysis, and so it will not travel into the recovery vessel together with the carbonization residue (advantage 8) .

Claims

1. A continuous process for the exploitation of energy con¬ tained in cellulose production process wastes, characterized in that it comprises the following operations: a) the said process waste is fed into a pyrolysis reactor and is pyrolyzed therein by heating the process waste with a resi¬ dence time of 0.1 - 100 s at a temperature of 300-700 °C into a pyrolysis product which comprises a liquid fuel, a gaseous fuel and a solid residue, b) the pyrolysis product is fed into a separator, wherein the solid residue is separated from the liquid and gaseous fuels, c) the liquid and gaseous fuels are recovered, and d) the solid residue is fed into a combustion vessel, wherein it is burned to generate heat.

2. A process according to Claim 1, characterized in that bark chemicals and cooking chemicals of the cellulose production process are recovered from the combustion vessel.

3. A process according to Claim 1 or 2, characterized in that dried process waste is fed into the pyrolysis reactor.

4. A process according to Claim 3, characterized in that proc¬ ess waste dried by means of combustion vessel combustion gases is fed into the pyrolysis reactor.

5. A process according to any of the above claims, charac¬ terized in that the pyrolysis is carried out using a fluidized- bed reactor.

6. A process according to any of the above claims, charac¬ terized in that a residence time of 0.1-10 s, preferably 0.1- 1 s, is used in the pyrolysis.

7. A process according to any of the above claims, charac- terized in that a temperature of 400-500 °C is used in the pyrolysis.

8. A process according to any of the above claims, charac¬ terized in that the fluidized-bed material and/or the solid residue are/is separated from the liquid and gaseous fuels by using a cyclone-type separator.

9. A process according to any of Claims 5-8, characterized in that, if the pyrolysis operates according to the fluidized-bed principle, before being fed back into the pyrolysis reactor, the fluidized-bed material is heated by means of combustion gases coming from the combustion reactor.

10. A process according to any of the above claims, charac¬ terized in that before being fed into the combustion vessel the solid residue is heated by means of combustion gases coming from this vessel.

11. A process according to Claim 9 or 10, characterized in that the solid residue is heated by means of combustion gases from the combustion vessel, by mixing it with the combustion gases and by separating it from the combustion gases.

12. A process according to any of the above claims, charac¬ terized in that, if the pyrolysis reactor operates according to the fluidized-bed principle, the solid residue is separated from the combustion gases and additionally from the fluidized- bed material by means of a sieve, whereafter the solid residue is fed into the combustion vessel and the fluidized-bed mate¬ rial is returned to the pyrolysis reactor.

13. A process according to Claim 12, characterized in that a sieve is used which is of the fluidized-bed type or the trans¬ port type, in which an upward flowing combustion gas raises the lighter fluidized-bed material while the heavier solid material falls down.

14. A process according to Claim 13, characterized in that the fluidized-bed material is separated from the combustion gas by directing it from the sieve to a cyclone, wherefrom the fluid¬ ized-bed material is directed to the fluidized bed of the py¬ rolysis reactor.

15. A process according to any of the above claims, charac¬ terized in that the liquid fuel and the gaseous fuel are sepa¬ rated from each other by means of, for example, condensation.

16. A process according to Claim 15, characterized in that at least a portion of the separated gaseous fuel is returned as fluidization gas to the pyrolysis reactor.

17. A process according to any of the above claims, charac¬ terized in that heat is recovered from the combustion vessel combustion gases by means of heat exchange.

18. A continuous-working apparatus for the exploitation of energy contained in cellulose production process wastes, the apparatus comprising a heat-generating combustion vessel (1) , characterized in that it also comprises: a) a pyrolysis reactor (15) provided with a process waste feed inlet (14), which reactor has a residence time of 0.1-100 s and an operating temperature of 300-700 °C, and in which the proc¬ ess waste is pyrolyzed into liquid and gaseous fuels (17) and a solid residue (4) , b) a separator (16) for separating the solid residue (24, 4) from the liquid and gaseous fuels (17) , c) a system for the recovery of the liquid and gaseous fuels, and d) conveyor means (24, 13) for feeding the solid residue (24, 4) from the separator (16) into the combustion vessel (1) .

19. An apparatus according to Claim 18, characterized in that the combustion vessel (1) is provided with means for the recov¬ ery of bark chemicals and cooking chemicals of the cellulose production process, in which case it also serves as a recovery vessel.

20. An apparatus according to Claim 19, characterized in that the combustion vessel is the soda recovery boiler of a sulphate mill.

21. An apparatus according to Claim 18, 19 or 20, characterized in that it comprises a dryer (28) , arranged at a point before the feed inlet (14) , for the drying of the process waste (30) .

22. An apparatus according to Claim 21, characterized in that the dryer (28) is a dryer operating by means of the combustion gases (27) of the combustion vessel (1) .

23. An apparatus according to any of Claims 18-22, charac¬ terized in that the pyrolysis reactor (15) is a fluidized-bed reactor.

24. An apparatus according to any of Claims 18-23, charac¬ terized in that the residence time in the pyrolysis reactor (15) is 0.1-10 s, preferably 0.1-1 s.

25. An apparatus according to any of Claims 18-24, charac¬ terized in that the operating temperature of the pyrolysis apparatus is 400-500 °C.

26. An apparatus according to any of Claims 18-25, charac¬ terized in that the separator (16) for separating the solid residue and, the pyrolysis reactor (15) being a fluidized-bed reactor, also the fluidized-bed material (24, 4) from the li¬ quid and gaseous fuels (17) is a cyclone separator.

27. An apparatus according to any of Claims 23-26, charac¬ terized in that it comprises a heat exchanger (6) which is coupled to the conveyor means (24, 12) for the fluidized-bed material (24, 4) and to the combustion vessel combustion gas outlet (5) , for heating, by means of combustion vessel (1) com¬ bustion gases (5) , the fluidized-bed material to be returned to the pyrolysis reactor (15) .

28. An apparatus according to any of Claims 18-27, charac¬ terized in that it comprises a heat exchanger (6) which is coupled to the conveyor means (24, 13) for the solid residue (24, 4) and to the combustion vessel combustion gas outlet (5), for heating, by means of combustion vessel (1) combustion gases (5) , the residue to be fed into the combustion vessel (1) .

29. An apparatus according to Claim 28, characterized in that the heat exchanger (6) has spaces for the mixing and separating of the solid residue and, if the pyrolysis reactor (15) is a fluidized-bed reactor, also the fluidized-bed material (24) with the combustion gases (5) .

30. An apparatus according to any of Claims 23-29, charac¬ terized in that it comprises, if the pyrolysis reactor operates according to the fluidized-bed principle, a sieve (6) by means of which the fluidized-bed material and selectively the combus¬ tion gases coming from the combustion vessel (1) are separated from the solid residue, in which case from the sieve there is arranged a conduit (12) to the fluidized bed (23) of the pyrol¬ ysis reactor (15) , to the lower section (4) of the combustion vessel for the removal (8) of selected combustion gases.

31. An apparatus according to Claim 29 or 30, characterized in that the heat exchanger (6) also functions as a sieve (6) .

32. An apparatus according to any of Claims 23-31, charac¬ terized in that it comprises a cyclone separator (7) coupled to the heat exchanger (6) and/or sieve for separating the fluidized-bed material from the combustion gases (8) coming from the combustion vessel (1) and a conduit (12) for returning the fluidized-bed material to the pyrolysis reactor (15) .

33. An apparatus according to any of Claims 18-32, charac¬ terized in that it comprises a condenser (18) for separating the liquid fuel (19) from the gaseous fuel (20, 21).

34. An apparatus according to Claim 23 and 33, characterized in that it comprises a gaseous fuel conduit (21, 22) which extends from the condenser (18) to the fluidized-bed (23) of the pyrol¬ ysis reactor (15) .

35. An apparatus according to any of Claims 18-34, charac¬ terized in that it comprises, in conjunction with the combus¬ tion vessel (1) , a heat exchanger (25, 26) for the recovery of heat from the combustion gases (8) of the combustion vessel (1).

36. The use of the process according to any of Claims 1-17 or of the apparatus according to any of Claims 18-35 in the cel¬ lulose production process.