WO1992018690A1

WO1992018690A1 - Method of recovering energy from waste liquors from pulp processes

Info

Publication number: WO1992018690A1
Application number: PCT/FI1992/000092
Authority: WO
Inventors: Samuli Nikkanen
Original assignee: A. Ahlstrom Corporation
Priority date: 1991-04-09
Filing date: 1992-03-31
Publication date: 1992-10-29
Also published as: JPH06506506A; FI911688A0; FI934399A0; FI934399A; PT100356A; FI911688A

Abstract

Method of recovering energy from waste liquors from pulp processes, wherein the waste liquor is burned in a waste liquor recovery boiler and energy is recovered from the flue gases, which are formed by producing saturated or partially superheated steam in the recovery boiler. According to the invention, a portion of the waste liquor is pressure heat treated to release gases and/or treated in a liquid phase in a pressurized process for converting the waste liquor into a fraction containing an oleaginous fuel and a fraction containing inorganic components. The waste liquor is thereby treated for instance in a hydrogenation process. The waste liquor from the pressure heat treatment and/or the waste liquor fraction containing inorganic components is burned in the recovery boiler. The released gases and/or the fraction containing oleaginous fuel is burned in a separate superheating boiler, in which the steam produced in the recovery boiler is superheated.

Description

METHOD OF RECOVERING ENERGY FROM WASTE LIQUORS FROM PUL PROCESSES

The present invention relates to a method of recoverin energy from waste liquors from pulp processes. The inventio relates especially to a method wherein waste liquor i burned in a waste liquor recovery boiler, e.g. in a sod recovery boiler, and wherein heat is recovered from th resulting flue gases by producing saturated and/or partiall superheated steam in the recovery boiler.

When combusting waste liquors in pulp processes, the ai is to separate the organic and the inorganic parts of the dry substance of the waste liquor from each other. The heat from the organic part of the dry substance is recovered and the largest possible amount of steam is produced by means of this heat. Pulping chemicals are recovered from the inorganic part of the dry substance in such a form that they can, in subsequent stages of processing, be converted into a suitable form to be reused in the cooking process .

The soda recovery boiler has, until now, proved to be superior for the recovery of heat and chemicals from waste liquors. The waste liquor is sprayed in the form of small drops into the boiler. In the hot combustion chamber water vapor, volatile parts of the dry substance, and eventually gasifiable parts of the dry substance evaporate from the drops. The gases inflame, thereby delivering heat to the heat surfaces disposed in the boiler and are discharged from the upper end of the boiler. The ash from the waste liquor drops, i.e. the inorganic substances of the waste liquor, accumulate on the bottom of the boiler, from which they are removed and through various stages of processing are conveyed back, to the cooking process. The flue gases from the soda recovery boiler contain a great deal of ash, e.g. sodium sulfate, a portion of which flows along with the flue gases upwards in the boiler in the form of fine dust or molten drops . The salts contained in the ash melt at a relatively low temperature and become, when melting, easily adhesive and corrosive. The deposits formed by the molten ash cause the risk of clogging of the flue gas channels and, furthermore, cause corrosion and erosion of the heat surfaces of the boiler. The risk of clogging and corrosion increases considerably the number of the shutdowns for inspection and maintenance.

A high temperature of the boiler tubes speeds up the formation of deposits and thereby the corrosion of the heat surfaces. Thus, the deposits affect particularly the heat surfaces for the superheated steam. Usually the corrosion of the materials is reduced by controlling the temperature of the superheater surfaces.

In such places in the superheater, in which the temperature tends to rise especially high or in which there is a great: deal of liquid-phase chemicals, in other words, where the corrosion and the erosion is a problem, special-alloy steels have to be be used. However, even the special-alloy steels have their maximum operating temperatures, above which they behave in the same way as the cheaper pressure vessel steels. This temperature is substantially lower in soda recovery boilers than in, for instance, oil-fired

,. boilers.

The principal way of avoiding corrosion is, anyhow at the present time, to choose a sufficiently low temperature and pressure for the produced steam, whereby the detrimental effects of the molten salt decrease. This means that the steam cannot be superheated to as high a temperature as desired for the production of electric power in steam turbine plants. In a steam power plant, the higher the pressure and the temperature of the steam can be raised in the boiler the higher the overall electrical efficiency, i.e. the ratio between the net production of electric power and the consumption of process heat. There is a need to raise the overall electrical efficiency of the soda recovery boilers nearer to that^' of the conventional coal-fired power plants,, i.e., the pressure and the temperature of the steam produced by the soda recovery boilers should be raised to as high a level as possible.

In other industrial boilers, a conventional steam pressure/temperature is, e.g., 130 bar/535 °C. In the soda recovery boilers, the pressure and the temperature have to be regulated in accordance with the strength of the available pressure vessel steels. In the superheater of the soda recovery boilers, ferritic heat resistant steels and austenitic steels reach a longer working life in the hottest part only when the surface temperature of the tubes does not exceed 550 - 600 °C. The temperature of the superheated steam in the soda recovery boilers is therefore usually not allowed to rise to 500 °C. At a pressure of 60 - 90 bar, a temperature of 450 - 480 °C is usually considered to be the maximum temperature.

Attempts have been made to reach higher temperatures than mentioned above, by controlling the combustion process in the boiler. The incoming secondary and tertiary air of the

«. boiler have been controlled in order to achieve as even a combustion process as possible, in which no great temperature variations in the flue gas flow occur. The purpose has been in this way to eliminate sudden and, as regards corrosion, dangerously high temperature peaks in the superheater section, in which case it would be possible to increase the mean temperature in the superheater. In this way, an increase of a few degrees can possibly be achieved in the superheater section temperature. Attempts have also been made to achieve higher steam temperatures without the risk of corrosion by reducing the fouling of the superheater surfaces. It is possible to some extent, for instance by appropriate feeding of the air, to reduce the amount of molten, inorganic material which is carried by the flue gas flow upwards to the super¬ heater section and which fouls the surfaces of the super¬ heater. On the other hand, the formation of deposits can also be reduced by continuos sweeping.

Attempts have also been made to reduce the fouling of the surfaces and the clogging of the flue gas channels by dimensioning the convection section sufficiently large and increasing the distance between the superheater surfaces. Larger clearences facilitate the sweeping and cleaning of the surfaces. These arrangements increase, however, the size of the boiler and are thus, as regards the building costs, unfavourable.

A soda recovery boiler in which the size of the superheater has had to be increased is, as regards the heat transfer and evaporation efficiency, inferior to a corresponding coal-fired boiler. The tendency of the superheater surfaces to foul multiplies the number of heat surfaces needed in comparison with a boiler, the flue gases of which contain only small amounts or no ash at all.

The suggested improvements mentioned above have not proved to guarantee a continuos production of steam having a temperature of above 500 °C. The corrosion of the surfaces of the superheater proceeds, in spite of the alterations, at an uneconomically fast rate. So far, no such method is being used in the pulp mills, by means of which steam of the same high quality could be produced in soda recovery boiler plants as in other, conventional power boiler plants.

The object of the present invention is to provide a method wich is better than the methods mentioned above for producing high-quality superheated steam in a process for the recovery of chemicals from the pulp cooking.

The object of the invention is in particular to provide a method better than the above mentioned for producing electric energy in a waste liquor burning process.

Thus, the object of the invention is to provide a method by means of which it is possible to significantly reduce the corrosion of the superheater in a waste liquor burning process, whereby a saving in material costs is achieved, but above all the utilization factor of the pulp mill is improved, i.e., the number of shutdowns for service is reduced.

The above mentioned objects of the invention are realized by a method, characterized in that

- at least a portion of the waste liquor is treated, before it is conveyed to the recovery boiler, by a pressure heating process, in order to form a gaseous fuel containing organic components, and/or by a pressurized liquid-phase process in order to form a fraction containing an oleaginous fuel and a fraction containing inorganic components;

- the waste liquor from the pressure heating process and/or the fraction containing inorganic components formed by said pressurized liquid-phase process are burned in the recovery boiler, and

- the gas formed during said pressure heating process and/or the fraction containing an oleaginous fuel formed by said pressurized liquid-phase process are burned in the superheating boiler, in which the steam produced in the recovery boiler is superheated.

The invention provides a method for generating steam in a substantially conventional recovery boiler and superheating the steam in a separate superheating boiler by burning gas or oleaginous fuel which is clean of impurities, in- organic compounds, causing problems at higher temperatures in the recovery boiler.

A separate superheating boiler is known as such. The improvement according to the present invention is that the same fuel which is used in the recovery boiler, i.e. the waste liquor, may be processed to provide the fuel for the superheating boiler. The waste liquor is pressure heated to give fuel gas or treated by a pressurized liquid-phase process to give an oleaginous fuel. The recovery process can be performed without auxiliary fuel, such as natural gas or mineral oil.

The pressure heating pro.cess or the thermal treatment of the waste liquor can be arranged to take place between some of the evaporation stages or immediately before the burning. In the pressure heating, the temperature of the waste liquor is raised to a temperature higher than the cooking temperature, preferably to 170 - 350 °C, in order to split its macromolecular lignine fractions. The heating can be effected by any economically suitable method: direct or indirect steam heating, or some other heating, such as electrical heating. As the source of heat for the pressure heating, for instance bled steam from a steam turbine plant can be used. The treatment time is 10 to 30 min, typically about 20 min in order to generate a gas that contains a significant amount of organic sulfur compounds. In the pressure heating of black liquor, generally about 1 to 3 weight-% of the dry solids contained therein will be released as a gas containing dimethylsulfide (DMS).

The gas released during the pressure heating can be used as such as fuel in the superheating boiler. The gas released can be further converted to give methane and hydrogen sulfide, which both could be used separately or together as fuel in the superheating boiler. A gas achieved from black liquor by pressure heating or a subsequent converting to methane and hydrogen sulfide does not contain the inorganic components of black liquor. The thermal energy of the gas corresponds to that of the pure components. The gas can be burnt as natural gas. If preferred, only the methane may be burnt in the superheating boiler and the hydrogen sulfide may be used to increase the sulfidity of the recovered cooking chemicals.

The total amount of dry solids to be burned in the recovery boiler is decreased if a portion of the organic material is separated in a gaseous form in the pressure heating stage and burned separately. This leads to a decreased load of the recovery boiler or an increase in the capacity of the recovery boiler.

By means of the pressure heating, the viscosity of the waste liquor is reduced, which improves the properties of the waste liquor as regards handling and evaporating and transferring it from one stage to another. Thus, the pressure heating renders' it possible to evaporate the waste liquor to a very high dry solids content, and for that reason the effective heat value of the waste liquor rises and consequently also the efficiency of the electric power production of the turbine power plant increases.

The waste liquor substantially maintains its thermal value after the pressure heating and can, when evaporated to its final dry-solids content, be introduced into the recovery boiler for burning.

The pressurized liquid-phase process for the fractionation of the waste liquor is preferably a hydrogenation process, in which a portion of the waste liquor is fractionated into an oil fraction and an inorganic fraction containing the major portion of the inorganic components of the waste liquor before it is conveyed to the recovery boiler. The fractionation is carried out by means of a high pressure and a high temperature under reducing conditions, for instance in the presence of hydrogen or carbon monoxide. he inorganic fraction formed in this way by the hydrogena tion process is burned in the waste liquor recovery boile and the oil fraction is burned in the superheating boiler, in which the steam produced in the recovery boiler i superheated.

In a preferred embodiment of the invention, about 10 - 35 % of the waste liquor is conveyed to the hydrogenation process for fractionation. The main part, about 65 - 90 % of the waste liquor is conveyed to the recovery boiler without being fractionated. The waste liquor is preferably concentrated to a dry solids content of about 15 - 80 %, most preferably to 30 - 50 % prior to the hydrogenatior. process. The waste liquor may also be pressure-heated prior to the hydrogenation process.

In the superheating boiler, superheated steam of > 100 bar and > 520 °C can be produced by the method according to the invention. In the recovery boiler, saturated or partially superheated steam, for instance steam of < 350 °C can be produced, which is finally superheated in the superheating boiler. The superheating boiler can, if so desired, be pressurized, in which case the space required for it is significantly smaller than that of a conventional boiler. The flue gases from the recovery boiler and the superheating boiler can be combined and can, for instance, be used for preheating the air required for the combustion process. The flue gases from the superheating boiler can, for instance, be conveyed to the lower part of the recovery boiler.

The total efficiency of the boiler plant depends, as previosly stated, on the pressure and the temperature of the superheated steam, being the higher the higher the values in question are. Taking into consideration the overall economy, it is advantageous to produce as much electric power as possible with the turbine, whose power production efficiency increases when the pressure and the temperature increase.

Thus, the spirit of the invention is to produce in a con- ventional waste liquor recovery boiler, for instance in a recovery boiler resembling the present-day soda recovery boiler, steam of lower quality and to superheat this steam in a separate superheating boiler. The superheating is brought about by flue gases which do not contain appreciable amounts of the corroding impurities present in the flue gases from the soda recovery boiler. The fuel used by the separate superheating boiler is made by a pressure heating or hydrogenation process from the waste liquor, the main part of which is burned in^" the recovery boiler. Additional fuel, mineral oil or the like, is not essentially needed in the separate superheating, unless it is profitable to use some, owing to the dimensioning or for other reasons.

The waste liguor from a pulp cooking, e.g. black liquor, is concentrated by evaporation before it is burned in the waste liquor recovery boiler. According to the latest technique available, it is possible to concentrate the black liquor up to a dry solids content of 80 %, in which case it is a relatively good fuel. The viscosity of the black liquor has until now imposed a limit to the concentration. By means of a pressure heating process which has been developed, it is possible to reduce the viscosity of the black liquor by heating it under pressure, in which case the evaporation can be carried out up to a very high dry solids content, facilitating thus the burning of the black liquor in a recovery boiler.

The waste liquor can be conveyed from various stages of the evaporation to the hydrogenation according to the present invention, prior to or after pressure heating.

The hydrogenation process is known per se from the Finnish patent specification FI 70057. In the hydrogenation process, the black liquor is subjected to a treatment by a reducin agent, such as CO or H2, at an elevated pressure an temperature. The treatment results in the formation of a oleous phase, which is insoluble in the aqueous phase an which can easily be separated by decantation. In the process, most of the organic materials are converted into oil, while a small part of them gasifies. The amount of organic material remaining in the aqueous phase is relatively small.

In the hydrogenation process, a portion of the waste liquor coming directly from the digester or via the evaporation units or the pressure heater is thus brought into contact with carbon monoxide, hydrogen or some other reducing agent at an elevated pressure and temperature, until substantially all of the organic matter has been converted into liquid or gaseous products. The liquefied part forms a liquid phase, which is insoluble in the aqueous phase, from which it can easily be separated. A pretreatment of the waste liquor is not necessary in the process but can be performed. The hydrogenation is carried out at a pressure of 50 - 200 bar, preferably 100 - 200 bar and a temperature of 200 - 350 °C. In practice all kinds of waste liquor from the pulp industry can be treated by hydrogenation. The reactions can be carried out in reactors of any kind in which a liquid can be treated at an elevated pressure and temperature. The hydrogenation can be performed in one stage or, if desired, in two or a plurality of stages.

From waste liquor is thus by hydrogenation produced, by means of a high pressure and a high temperature under reducing conditions, an oleous fraction resembling mineral oil which does not contain appreciable amounts of the inorganic components of the waste liquor and the heat value of which is of the same order as that of mineral oil. It is combusted in the same way as heavy fuel oil. In the hydrogenation process, the heat value of the fraction containing the inorganic compounds of the waste liquor decreases somewhat as does also its combustability. Therefore, preferably only a portion, preferably less than 35 %, of the waste liquor coming from the digester is hydrogenated, in which case the amount of the untreated waste liquor is still large enough for combusting the difficult to burn, inorganic fraction remaining after the hydrogenation, for the recovery of the inorganic salts in it. The inorganic fraction contains inter alia the following sodium salts: NaHS, NaHC03, aliphatic sodium salts, such as sodium acetate and formate, and smaller amounts of other organic salts, which can not be directly returned to the pulp cooking.

The oleaginous fraction of the black liquor obtained by hydrogenation can at least partially be used to replace mineral oil required for the pulping process, in which case the self-suffiency of the pulping process as regards fuel increases. Now it has, surprisingly, been discovered that energy can be recovered from waste liquor from a pulping process at an overall efficiency better than ever before by superheating the steam obtained from a conventional waste liquor recovery boiler in a superheating boiler, in which an oil fraction made from waste liquor by hydrogenation is burned.

The limiting value of the amount of waste liquor which is to be hydrogenated can be calculated from the effective heat value of the inorganic fraction obtained by hydrogena¬ tion. Preferably, the inorganic fraction is burned in the recovery boiler by means of the waste liquor without any auxiliary fuel.

It is thus a distinguishing feature of the present inven¬ tion that the waste liquor is pressure heated for releasing organic gases or fractionated into two parts, one of which is oil and the other waste liquor containing the inorganic components of the waste liquor. The gases or oleous fraction thus received are burned separately in a superheating boiler, in which case the design and the materials of the superheater can be chosen as in any oil-fired boiler without having to consider the harmful effects of the inorganic, molten salts, i.e:

- the superheating surfaces can be spaced much closer to each other than in a soda recovery boiler, because they do not foul to the same extent as in the soda recovery boiler; very little or no cleaning at all of the superheating surfaces is needed, because the superheating is effected by clean flue gases;

- the materials for the superheating and other surfaces can be chosen merely on the basis of their heat strength, without having to consider the corrosion risk;

- if it seems desirable from an economical point of view, gases from the pressure heating of waste liquor or oil made from the waste liquor and conventional mineral oil can easily be burned at the same time in the separate superheating boiler, or even mixed together.

The soda recovery boiler or a corresponding waste liquor recovery boiler is still needed for burning the major portion of the waste liquor or black liquor stream and for recovering chemicals. The soda recovery boiler functions, however, in the embodiment according to the invention principally as a vaporizing boiler, in which only a partial superheating is effected at the most. In the vaporizing boiler, the surface temperature of the materials does not rise as high as in the superheating section, wherefore the risk of corrosion is smaller. The superheater possibly disposed in the soda recovery boiler will be small compared with a traditional boiler, as the main part of the superheating is effected in a separate boiler in a considerably hotter environment. The heat surfaces of the superheating boiler are more efficiently used than in a traditional soda recovery boiler, which means that a smaller amount of tubes are needed in a smaller boiler, in other words: a saving of capital costs is achieved.

The flue gases from the separate superheating boiler can be conducted into the flue gases from the soda recovery boiler at some suitable point, in which case their heat content is recovered more efficiently. Their heat can be recovered for instance in the preheating of air as in conventional power plant boilers. In a traditional soda recovery boiler, the flue gases are too impure to be conducted to the air preheater. By using flue gases for the preheating of air, back-pressure and bled steam can be saved for other purposes.

Depending on the required steam and power of the mill, the superheating can be adjusted to some extent, but the steam turbines have an optimum operation point, from which it is not advisable to deviate too much.

The pressure and temperature of the separate superheating boiler can be chosen relatively freely so as to suit the steam system of the mill. There is then no need to build separate pipe systems for many different pressures. The same turbine is available for the steam of the whole mill.

The separate superheating boiler and the waste ^" liquor recovery boiler can be built in such a way that their pressure frames are closely connected to each other, for instance by some kind of precombustion chamber construction.

By means of the invention, the following additional advantages can be achieved:

- by increasing the pressure and the operating tempera- ture of the steam recovered from the waste liquor recovery process by means of the superheating boiler, the overall electrical efficiency of the plant is improved, i.e. more power is generated by the heat recovered in the steam; - the design of the recovery boiler will be simpler, sinc the superheater can be omitted from the upper part of th combustion chamber and the boiler can for that reason b built in the shape of a tower, in which the height of th economizer determines the height of the boiler;

- special-alloy steels are either not needed at all with clean flue gases or at least they are required less extensively than in conventional soda recovery boilers, even when operating at higher temperatures; - when burning gases or the oil fraction, substantially no detrimental, melting salt compounds are formed, and for that reason there is significantly less corrosion, even if the boiler is operated at a considerably higher temperature level than at present; - the gases or the oil fraction can be used as starting fuel in the waste liquor recovery boiler, in which case no purchased oil is required for the burning of the waste liquor.

The invention will now be described more in detail with reference to the accompanying drawing, in which one embodiment of the process for recovery of energy from waste liquor according to the invention is illustrated schematicall .

The unit for recovery of energy from waste liquor shown in the figure comprises evaporator sections 10 and 12, a pressure heating unit 14, a hydrogenation unit 16, a waste liquor recovery boiler 18, a heat recovery section 20 connected to it, a separate superheating boiler 22, an air preheater 24, a steam turbine 26 and a generator 28.

In the method according to the invention, the pulp coming from the digester house is transferred to the washing department 30, in which the pulp is cleaned and separated from the waste liquor. The waste liquor from the washing, which usually contains about 15 - 20 % dry matter, is led through a line 32 to the first evaporator section 10, in which the waste liquor is concentrated to a dry solids content of about 45 %. From the first evaporator section the waste liquor is led to the pressure heating unit 14 through a line 34.

Gases released during the pressure heating are led through a conduit 35 from the pressure heating unit 14 to the superheating boiler 22.

From the pressure heating, the waste liquor is led through a line 36 to the evaporator section 12, in which the waste liquor is finally evaporated to a dry solids content of about 80 %. From the final evaporation, the waste liquor is led through a line 38 to be sprayed by nozzles 40 into the recovery boiler 18.

Combustion air is supplied to the recovery boiler through a line 42 from the air preheater 24. Through the char bed 44 accumulated on the bottom of the recovery boiler, the molten, inorganic matter flows through an outlet 46 to a dissolving vat, not shown in the figure.

In the recovery boiler 18 and the heat recovery section 20, heat is recovered by means of vaporazing surfaces 48 and a water preheater 50. The flue gases are discharged from the boiler through a line 52. The steam can, if desired, be partially heated in the recovery boiler by means of superheating surfaces 51.

To the hydrogenation stage 16 according to the invention can be supplied unevaporated waste liquor through a line 54 or pre-evaporated waste liquor through a line 56 or pre-evaporated and pressure heated waste liquor through a line 58 or, finally, evaporated waste liquor through a line 60. The oil fraction formed in the hydrogenation stage is led through a line 62 to the superheating boiler 22, and the residual, inorganic fraction is led through a line 64 to be combined, at a desired point, for instance before the final evaporation, with the waste liquor strea which is to be introduced into the recovery boiler. Th small amount of ga-s which is formed in the hydrogenatio stage is removed through a line 66.

Combustion air is supplied to the superheating boile trough a line 68 from the air preheater. Superheatin surfaces 72 are disposed in the superheating boiler fo the superheating of the steam coming from the recover boiler through steam pipes 74. The superheated steam i led through a line 76 to the steam turbine plant, in whic electric power is generated by the turbine 26 and th generator 28.

The flue gases are passed from the superheating boile directly to the upper part of the recovery boiler o through a line 78 to the lower part of the recovery boiler. A portion of the flue gases are passed through a line 80 to the air preheater 24.

In the embodiment of the invention described above both gaseous and oleaginous fuel for the superheating boiler are provided for by both pressure heating and hydrogenation of the waste liquor. It is of course possible and may even be more preferable to only use one of the waste liquor treatment processes, either the pressure heat treatment providing a gaseous fuel or the pressure liquid-phase treatment providing an oleaginous fuel.

One of the principal starting points when using the pressure liquid-phase treatment is that an appropriate amount of waste liquor, such as black liquor, is fractionated to oil and an inorganic waste liquor residue. By an appropriate amount is meant an amount of waste liquor by the oil fraction of which the desired separate superheating can be accomplished and the inorganic residue fraction of which, when combined with the waste liquor, can be burned in the recovery boiler essentially without auxiliary fuel in connection with the burning of waste liquor.

By means of the recovery boiler and superheating boiler system, the measures for restricting the pressure and the temperature of the soda recovery boiler are not needed. Thus, the method according to the invention significantly improves the recovery of energy from waste liquor.

The economical profit from the method according to the invention results especially from the big electric power portion in comparison with the steam portion which can be achieved by it. In present-day integrated paper and pulp mills, the steam demand has decreased. Therefore, it is not any more profitable to recover the energy received from the burning of waste liquor in the form of surplus steam but the energy should be recovered in the form of electric power. By means of the system according to the invention, the electric power yield achieved in the paper and pulp mills is significantly higher than in the system used at present.

EXAMPLE

In the examples below are compared the steam values, the net electric power and the overall electrical efficiency of a traditional soda recovery boiler and two soda recovery boiler-superheating boiler combinations according to the invention, operating in a pulp mill, in which - the black liquor flow is 18,5 DS/s

- the bled steam (12bar) demand is 15,4 kg/s and

- the low pressure steam (4,5 bar) demand is 49,6 kg/s

Example case I: A traditional soda recovery boiler which together with a bark-fired boiler produces high pressure steam for turbines, from which further bled steam and low pressure steam for the sulphate pulp process is obtained. In addition to the high pressure and bleeder turbine electric power is also generated by a condensing turbine. The values of the produced high pressure steam are: 48 °C, 85 bar.

Example case II: A soda recovery boiler and a super heatin boiler which burns oil obtained from a hydrogenatio reactor. The steam is already partially superheated in th soda recovery boiler, but the final superheating is, however, effected in the superheating boiler. A bark-fire boiler additionally produces high pressure steam. Th turbine plant does not include a condensing turbine. Th oil -consumption of the lime sludge reburning kiln is substantially met by the oil and gas obtained from the hydrogenation reactor.

The values of the produced high pressure steam are: 540 °C, 100 bar.

Example case III: A soda recovery boiler and a superheating boiler which uses the main part of the oil from the hydrogenation reactor. A condensing turbine is needed.

Possible surplus hydrogenation oil is used together with the gas from the hydrogenation in the lime reburning kiln.

The values of the produced high pressure steam are: 540 °C, 100 bar:

The results are shown in the table below. In example case

II, the demand of purchased oil for the lime reburning kiln is dispensed with to such an extent that some gas from the hydrogenation will be left over. In example case

III, the electric power production has increased considerably and the overall electrical efficiency has risen from 0,239 to 0,257. Additionally, in example case III, the demand of purchased oil for the lime reburning kiln has decreased.

The invention is not intended to be limited to the embodiment illustrated and described above but can be modified and varied within the scope and spirit of the invention as defined by the following claims.

Claims

1. Method of recovering energy from waste liquors fro pulp processes, in which method waste liquor is burned i a waste liquor recovery boiler and heat is recovered fro the resulting flue gases by producing saturated and/o partially superheated steam, characterized in that

- at least a portion of the waste liquor is treate before it is conveyed to the recovery boiler by a pressure heating process, in order to form a gaseous fuel containing organic components, and/or by a pressurized liquid-phase process in order to form a fraction containing an oleaginous fuel and a fraction containing inorganic components;

- the waste liquor from the pressure heating process and/or the fractiom containing inorganic components formed by said pressurized liquid-phase process are burned in the recovery boiler, and - the gas formed during said pressure heating process and/or the fraction containing an oleaginous fuel formed by said pressurized liquid-phase process are burned in the superheating boiler, in which the steam produced in the recovery boiler is superheated.

2. Method according to claim 1, characterized in that 10 - 35 % of the waste liquor is fractionated by a hydrogenation process by means of a high pressure and temperature under reducing conditions, in the presence of e.g. hydrogen or carbon monoxide.

3. Method according to claim 2, characterized in that waste liquor having a dry solids content of 15 - 80 %, preferably 30 - 50 % is supplied to the hydrogenation process.

4. Method according to claim 1, characterized in that the waste liquor is pressure heated in order to release about 1 - 3 % weight-% of the dry solids contained therein as gas containing dimethylsulfide and burning said gas in th superheating boiler.

5. Method according to claim 1, characterized in that the waste liquor is pressure heated and the gas released thereby converted to give methane and hydrogen sulfide.

6. Method according to claim 5, characterized in that the methane is burnt in the superheating boiler.

7. Method according to claim 1, characterized in that superheated steam of > 100 bar and > 520 °C is produced in the superheating boiler.

8. Method according to claim 1, characterized in that partially superheated steam of < 350 °C, which is finally superheated in the superheating boiler, is produced in the recovery boiler.

9. Method according to claim 1, characterized in that the superheating boiler is pressurized.

10. Method according to claim 1, characterized in that the flue gases from the superheating boiler are combined with the flue gases from the recovery boiler.

11. Method according to claim 1, characterized in that the flue gases from the superheating boiler are used to preheat the air which is to be supplied to the recovery boiler.

12. Method according to claim 1, characterized in that the flue gases from the superheating boiler are led to the lower part of the recovery boiler.