NO173457B - PROCEDURE AND DEVICE FOR BLACK LIFE TREATMENT USING DOUBLE AIR SUPPLY - Google Patents

Description

This invention relates to a method for

treatment of a concentrated, aqueous and carbonaceous black liquor material and alkali metal-sulfur compounds to form a combustible gas and a sulphide-rich melt comprising: a) that a gasifier boiler is kept at a pressure of 1-50 atmospheres, during which there is a relatively

shallow pool of molten salt on the bottom of the boiler, which boiler is equipped with an edge outlet, as the boiler also has:

i) a black liquor drying zone in its upper part,

ii) a gasification zone for black liquor-dry

substance located below the drying zone, and

iii) a sulfur reduction zone with salt melt

including the pond of molten salt

b) that concentrated, aqueous black liquor containing carbonaceous material is introduced into the upper part of the drying zone

and alkali metal sulfur compounds,

c) that water is driven out of the concentrated, aqueous black liquor in the drying zone by direct contact between it

aqueous black liquor and the hot gas rising upwards from the gasification zone forming dried black liquor substance which falls into the gasification zone, and a cooled, flammable gas containing water vapor, whereby the flammable gas is at a lower temperature than the melting point of entrained droplets of molten salt,

which causes the droplets to solidify, and

a device for treating concentrated aqueous black liquor into a combustible gas and a sulphide-rich melt comprising a closed, vertical, elongated gasifier boiler with

(i) a black liquor drying zone in the upper part of the boiler,

(ii) a black liquor solids gasification zone below the drying zone, and (iii) a salt melt sulfur reduction zone below the gasification zone;

a sump area in the bottom of the boiler to accommodate a pond of alkali metal salt melt, the salt melt pond comprising the salt melt sulfur reduction zone;

inlet devices located in the upper part of the boiler for supplying the black liquor to the drying zone;

outlet devices located in the lower part of the boiler above the sump area which serve to limit the height of the molten salt pond and to remove overflow of molten salt from the sump area.

In the production of pulp and paper using sodium-based sulphate and sulphide processes, digesting wood with aqueous alkaline solutions leads to the formation of a by-product known as leachate or black liquor, hereinafter called black liquor. In order to achieve economy in the overall grinding process, this by-product cannot be discharged as a waste material, but must instead be converted into usable products. In particular, it is desired to regenerate sodium sulphide, which can be used to reconstitute active solutions for the pulp digestion step in the process.

In addition, it is desirable to use the black liquor as an energy source.

The most used method for treating black liquor makes use of the Tomlinson recovery furnace (also cold Tomlinson recovery boiler). In this plant, concentrated black liquor is burned in the top of a specially designed boiler to produce steam; a molten salt product generally called "smelt", containing sodium carbonate and sodium sulphide, and non-combustible flue gas which, after appropriate purification, is discharged into the atmosphere. The process using the Tomlinson boiler has served the pulp and paper industry for approx. 5D years, but still has serious weaknesses. The large volume of flue gas is difficult to clean and can pose an environmental problem, all recovered energy is in the form of steam which has limited use, explosions can occur if the boiler tubes leak and bring water into contact with the smelt, and the reduction of sulfur compounds to sulphide is incomplete.

Various methods containing alternatives to, or improvements to, the Tomlinson furnace have been used or proposed to convert black liquor into usable products.

U.S. Patent No. 1,808,773 describes a method using a black liquor recovery furnace with two pre-firing zones. In the first high-temperature combustion zone, black liquor injected into the furnace is dehydrated and substantially completely burned. In the second zone located between the first zone and the bottom of the furnace, a further amount of black liquor is injected into the furnace together with sodium sulfate. In this second zone, water is removed from the black liquor by evaporation. Partial combustion of the black liquor leads to the formation at the bottom of the furnace of a solid molten layer of sponge coal mixed with alkali residues from black liquor and added sodium sulfate. Reducing conditions maintained at the bottom of the furnace lead to the reduction of sulphate to sulphide. The molten salts seep down through the layer of sponge coal and leave the furnace through a bottom outlet. Although this method provides an alternative to using the Tomlinson recovery boiler, the need for two separate combustion zones requires a complex apparatus. The lack of heat recovery also leads to a loss of the heating value of black liquor. Although there is conversion of sodium sulphate to sodium sulphide and burning of the black liquor, the percentage of unconverted sulphate is relatively high and ranges from 8 - 12%.

U.S. patent l\lr. 1,931,536 describes a method for controlling the combustion zone for both sprayed black liquor and black liquor powder in a melting furnace. An inert gas is introduced into the melting furnace at or near the point of entry of the sprayed black liquor or dried black liquor powder. The inert gas inhibits the combustion of the volatile components in the black liquor and makes it possible to place the sprayed concentrated black liquor or dried black liquor powder in the melting furnace at a certain distance before combustion of the organic and carbon-containing content of the black liquor occurs in a relatively deep layer in the melting furnace . This method represents an improvement over a conventional Tomlinson recovery boiler, but has the same basic limitations; the black liquor undergoes complete combustion and gives a large volume of impure flue gas and only steam is produced.

U.S. patent 2,056,266 describes the use of a combined smelter and boiler, very similar to the Tomlinson boiler, for the recovery of alkali metal values from black liquor and the utilization of the heat content thereof. Dried black liquor solids are taken to a fixed fuel bed zone where they are burned in a reduced atmosphere with the result that partially burned gases rise from the fuel bed. These partially burned gases are then completely burned by introducing a stream of air into a combustion zone above the bed. The combustion zone contains cooking tubes for the production of steam. Flue gases formed in the combustion zone are allowed to rise, and an inert gas is blown down onto the fuel bed to prevent the capture of solids in the gases rising from the fuel bed and in a separate separation line between the zones. Burnt alkali values are emptied from the bottom of the layer. This process requires conversion of black liquor to black liquor solids before introduction into the fuel bed zone. In addition, the necessary equipment to carry out the process is complicated and requires separate devices for drying black liquor.

U.S. patent no. 2,182,428 describes a method for drying wastewater by spraying the liquid to be vaporized on the surface of a heat transfer medium such as oil, tar, pitch, asphalt or wax. The heat transfer medium is inert and no combustion or reduction reactions take place, only the lyes are evaporated without any useful product being recovered from the evaporated lyes.

U.S. patent nos. 3,639,111 and 3,718,446 describe

a method for producing a clear-burning fuel by high-temperature distillation and pyrolysis of an organic material such as kraft black liquor. In order to achieve the required cracking temperatures in the pyrolysis zone, a controlled amount of an oxygen-containing gas (up to approx. 15% of what is required for complete combustion) is introduced during the cracking operation. Because the oxygen-containing gas, pyrolyzing black liquor and product gases flow together through the system and the product gas exits at full reaction temperature without giving off heat to incoming material,

the process thermally inefficient. The requirements for both indirect heat exchange and direct combustion further lead to the need for relatively large, complicated equipment.

U.S. patent no. 3,916,617 describes the use of a molten metal to form Btu gas from the gasification and partial oxidation of a carbonaceous material. The carbonaceous material is held in the molten salt zone to provide the desired reducing atmosphere when air is introduced into this molten salt zone for partial combustion of the carbonaceous material. When air and black liquor are introduced into a molten salt reaction zone, the necessary heat to evaporate water in the black liquor must be supplied by combustion reactions. This leads to the need for a high air/black liquor ratio and the production of low quality gas (typically less than 70 Btu/scf). Consequently, the method according to this patent is primarily applicable for the gasification of coke and other relatively dry carbonaceous materials.

U.S. patent no. 4,441,959 describes a method for extracting heat and chemical values from pulp effluents using a fluidized bed reaction chamber. A concentrated pulp liquor is combusted with air in a fluidized bed comprising several inert solid particulate materials, at least one of which has a finer particle size than another. After combustion, the particulate materials with finer particle size are processed in an external fluidized bed heat exchanger to recover heat and to separate the finer particles from gaseous and solid products formed in the combustion. The solid products are then subjected to treatment in a molten salt reduction, which leads to the formation of a melt containing sodium sulphide and other salts. The gaseous products mainly consist of a non-combustible flue gas whose heat content is used to produce steam. The resulting cooled flue gas can be released into the atmosphere after appropriate cleaning. Although this method recovers some of the heat and chemical values from mass effluents, the solid combustion products are not reduced in the fluidized beds. Therefore, a separate molten salt reduction is required, which makes the process even more complicated.

None of the previously available methods can therefore be seen as able to easily and effectively extract essentially all the energy and the chemical contents of the black liquor as very valuable products.

Although they cannot be considered part of the state of the art, the present inventor and his associates have previously proposed other processes for gasification of black liquor.

Thus, it has been proposed that dried black liquor solids can be gasified in a molten salt pond. In such a process, combustible off-gas is formed and has a degree of reduction of the sulfur content in the black liquor solids until sulphide is obtained. However, it is first necessary to dry the black liquor into black liquor solids which are required as feed to the molten salt pond. This increases the complex nature and costs of the process.

In the U.S. patent application 06/667,937 of 2 November 1984, the present inventor has proposed a method for extracting energy and chemical content in an aqueous black liquor by using a reactor containing a drying zone located above a gasification zone. The reactor contains a layer of porous phase carbonaceous material in the gasification zone. An oxygen-containing gas is introduced into the gasification zone in a sub-stoichiometric amount which provides partial combustion and gasification reactions sufficient to maintain the temperature of the upper surface of the layer of solid carbonaceous material in the gasification zone in the range of about 870 to 1200°C and to form a hot combustible gas rising from the gasification zone. A concentrated black liquor containing alkali-metal-oxy-sulfur compounds is introduced into the drying zone, and the water in this is evaporated by contact with the hot gases rising from the gasification zone.

In the drying zone, a product gas with a reduced temperature is formed

and dry black liquor solids falling on the surface of the bed in the gasification zone. The dried black liquor solids are converted in the hot combustible gas that rises from it

the gasification zone, and alkali metal salts, which melt and penetrate the layer. The product gases are taken out from an upper part of the drying zone. A melt in which the sulfur content is at least approx. 80% in the form of alkali metal sulphide is taken out from the lower part of the gasification zone. Despite the advantageous features of this invention in accelerating the gasification and sulfur reduction reactions, the reactions that take place are inefficient due to a relatively poor contact between the air and solid carbon. The operating characteristics are also uncertain as the layer of solid carbonaceous material can change height with small changes in the operating conditions.

In the U.S. patent application 06/669,498 of 8 February 1985, the present inventor has described the gasification of aqueous black liquor using molten salt ponds. An oxygen-containing gas is introduced below the surface of the molten salt pond, consisting of an alkali metal carbonate and an alkali metal sulphide within a closed gasification boiler at a sufficient rate to produce a high degree of turbulence in the molten salt pond. Black liquor in the form of a coarse spray is introduced into the rising hot gases above the pond. Thereby, water is evaporated from the aqueous black liquor into the hot gases, which gives a product gas with a reduced temperature and dried black liquor solids that fall on the surface of the pond and are dispersed in it. The dried black liquor solids are converted in the pond into a hot flammable gas that rises from the pond, and into alkali metal salts that mix with the salts present in the pond. A stream of product gas with a dry basic heating value (HHV) of at least approx. 90 Btu/scf is taken out from the gasifier boiler together with a molten salt product where the sulfur content is at least approx. 90% in the form of alkali metal sulphide. Although the method according to this invention is applicable to achieve the desired results of producing a flammable gas and a molten salt product in which alkali metal sulfide predominates, the process is beset with certain problems. Corrosion and destruction of the material content generally accompanies the use of turbulent ponds of molten salts. Capture of molten salts can also occur in the gases rising from the dam. This may require limiting the gas velocity through the dam. It has also been found that some of the carbonaceous material in the black liquor is gasified before the particles reach the pond. Consequently, only a portion of the carbon-containing material enters the pond. If all the air required for gasification of the black liquor is supplied to the pond below its surface, the conditions in the pond may be too strongly oxidizing for effective reduction of the sulfur compounds to occur.

The present invention is a continuation-in-part

of the U.S. application 06/699,498 and constitutes an improvement over the invention shown therein and described in U.S. Pat. patent application 06/667,937. It retains the advantageous features of the basic black liquor gasification processes described in these two concurrent applications, while addressing the above-mentioned problems. It also provides the significant advantage that operation is made possible at a very high carburisation rate for a given carburettor size.

According to the invention, the initially mentioned method is characterized by the fact that a first part of an oxygen-containing gas is introduced into the gas space in the gasification zone, which space is located below the drying zone immediately above the pond of molten salt for partial oxidation and gasification of part of the carbonaceous material in the dried black liquor dry substance falling through the zone to form a hot, combustible gas, that the second portion of the oxygen-containing gas is introduced below the surface of the pond of molten salt in sufficient quantity to effect gasification of substantially all carbonaceous material entering the pond from the gasification zone, but in an amount insufficient to provide oxidizing conditions in the pond, whereby the formed gas rises upwards from the pond, and whereby the total amount of the first and second part oxygen-containing gas is between 25 and 55% of the amount of oxygen-containing gas required for complete combustion of the black liquor feed and make up the total amount the oxygen-containing gas that is fed to the gasifier boiler, that the cooled, combustible gas with a higher heating value of at least 3,350 kJ/m<3> (on a dry basis) is driven out from an upper part of the drying zone, and that from the edge drain in the reduction zone with molten salt is driven out out a melt, in which the sulfur content is predominantly in the form of alkali metal sulphide. The oxygen-containing gas is preferably air.

It is a key feature of the invention that the oxygen-containing gas, preferably air, is led controllably both directly to the body of the molten salt pond (the sulfur reduction zone) and also over the surface of the pond in the lower part of the gas space (the gasification zone). The distribution of air supplies is controlled so that only that amount of air is introduced into the sulfur-reduction ion zone of the molten salt pond, which is required to ensure complete destruction of the carbonaceous material actually entering the molten salt pond. This typically represents approx. 30 - 70% of the total amount of added air in the gasification boiler. The rest of the air is led to the gasification zone immediately below the pond. Thus, approx. 30 - 70% of the total amount of air supplied to the gasification boiler is supplied to the gasification zone. As the purpose of the present invention is to gasify the black liquor and form a combustible gas, as well as to recover sulfur values, the total amount of air supplied to the gasifier boiler will typically be approx. 25 - 55% of the amount required to complete the combustion of the supplied black liquor. The total air supplied to the carburettor will preferably be preheated to a temperature in the range 120 - 450 °C, preferably 150 - 400 °C.

The combustible gas, whose combustible components are mainly hydrogen and carbon monoxide, and which is produced in the gasification zone, can, after suitable purification, be used in a gas turbine to utilize the energy value of the black liquor supply to the maximum extent. The alkali metal sulphide produced in the molten salt pond can be recovered as an aqueous solution and recycled to the papermaking process as green liquor.

The device according to the present invention is characterized by a first set of gas inlet devices located in the gasification zone for supplying an oxygen-containing gas to the gas space of the gasification zone immediately above the salt melt pond;

a second set of gas inlet devices located in the sump area of the boiler to feed oxygen-containing gas directly into the salt melt pond, and

outlet devices placed in the top part of the boiler and connected to the upper part of the drying zone to remove a flammable gas from the boiler.

Fig. 1 is a schematic flowchart illustrating

a preferred embodiment of the method of the present invention.

Fig. 2 is a partial elevational section of the gasification boiler

and associated cooling tank according to the present invention. These are used to carry out the method according to the present invention.

Black liquor, often produced in a wood digestion operation as part of the papermaking process, contains combustible organic metareals, alkali metal sulphide and alkali metal hydroxide, as well as various alkali metal oxysulfur compounds. These oxysulphur compounds will typically be the sulphate, thiosulphate or sulphide of sodium. Economic considerations in the papermaking process require that essentially all of the combustible material be removed, the alkali metal and sulfur values recovered from the black liquor, and the sulfur oxide compounds transferred into alkali metal sulfide for return to the process without oxidizing the alkali metal sulfide that was initially present.

In the implementation of the present invention, a concentrated aqueous black liquor is introduced which contains at least approx. 45% by weight solids and has a higher heating value (HHV) of at least 3440 kJ/kg in the drying zone, typically as a coarse spray.

The drying zone provides direct contact heat exchange between the falling wires of black liquor and the gas flow that rises from the gasification zone. Water evaporates from the concentrated black liquor due to heat absorption by convection from the rising gas stream. Heat is also supplied to the droplets by radiation from high-temperature components. Gas temperatures in this drying zone vary from approx. 500°C to approx. 800°C near the top of the zone, and from approx. 800°C to approx. 1,000°C near the bottom.

Dried black liquor solids are formed in the drying zone which

a result of the aqueous black liquor being contacted in the drying zone with gases rising from the gasification zone. In the black liquor solids gasification zone, a significant part of the carbonaceous material in the dried particles that fall from the drying zone is transferred to gas. The gas-producing reactions entail pyrolysis with the release of volatile hydrocarbons, gasification of carbon with carbon dioxide and steam with the formation of carbon monoxide and hydrogen, and combustion of both gas and solid-phase components by reaction with the oxygen present in the injected oxygen-containing gas in this zone.

As a consequence of the partial combustion zones, gas is released which gives a higher temperature in this zone in the range of 900 - 1200°C. As the solid particles only prereact in the relatively short time they fall through the gasification zone, they are only partially gasified. Typically carburized 25 . 75% of the organic carbon present in the black liquor that enters the gasification boiler in the gasification zone. A similar or higher percentage of the organic carbon is also converted into gaseous components in this zone. The unreacted carbonaceous material, together with most of the inorganic material from the original black liquor feed, falls out of this zone into the sulfur reduction zone. The sulfur reduction zone comprises a pond of molten alkali metal salts in a shallow sump at the bottom of the gasification boiler.

Mainly all sulfur compounds present

in the falling particles are reduced to sulphide in the molten salt sulfur reduction zone, or retained as sulphides if they are already present in this form. Air is introduced into this zone in sufficient quantity to provide the necessary oxygen to gasify all introduced carbonaceous material,

but considerably less than the amount required to complete the combustion. Typically, the required amount of oxygen is in the range 25 - 55% of what is required for the complete combustion of carbon to carbon dioxide, hydrogen to water and sulfur compounds to sulphate form. By only bringing in this amount of oxygen, the conditions in the smelting pond are greatly reduced, and at least approx. 90% of the sulfur compounds in the product melt will be in the form of sulphide.

The present invention provides several significant advantages. As only part of the carbonaceous materials in the black liquor feed enter the molten salt pond and only part of the air is introduced into the pond and reacts with it, the pond can be relatively shallow - typically 30 - 120 cm deep. The use of shallow dam reduces the amount of expensive melt-resistant refractory material that must be used to line the carburettor. Another advantage of this invention is an increase in carburetor capacity. In a pond-type gasifier, the gas velocity is normally limited by the problems of severe entrainment and turbulence when the surface velocity of gas leaving the pond exceeds 60 - 90 cam per second. This in turn limits the capacity of a carburettor of a given diameter. In the present invention, only part of the gas passes through the dam. If thus e.g. half of the gas passes through the pond at a surface velocity of 75 cm per second and the other half is formed in the gasification zone, the total gas will enter the drying zone at an ouerflate velocity of approx. 150 cm per sec., which is perfectly acceptable for gas/solid or gas/liquid reactors in which gas is the continuous phase. Consequently, the capacity of a carburetor of given diameter is increased by introducing this improvement in the invention covered by U.S. Pat. patent application 06/699,498.

As described in the two concurrent pending applications,

heat loss from the reaction zone must be minimized. Thus, operation at a higher temperature is preferred, as this reduces the size of the gasifier boilers that are required, thereby reducing heat loss and costs.

A typical plant which uses the molten salt gasifier boiler and method according to the present invention will now be described with reference to the drawing.

In fig. 1 in the drawing shows a molten salt gasifier boiler used with a gas turbine-combined circulation system which constitutes a preferred embodiment of the present invention. Wood waste from any suitable source is introduced through a pipe 2 and a valve 4 into a sluice hopper 6. From there the wood waste passes through a valve 8 to a second sluice hopper 10. The sluice hoppers are operated with compressed gas in the usual manner used for solids fed into a receiver under pressure. From the sluice funnel 10, the wood waste goes through a pipe 12 and a supply valve 14 to a pipe 16 with a valve through which compressed air flows, the wood waste is transported by the compressed air and is blown together with a subsurface molten salt pond 18 into a sump at the bottom of a molten salt gasification boiler 20.

Air is also blown over the surface of the forge through a

pipe 21 with valve. The amount of air that is introduced into the smelter through the pipe 16 is typically 30 - 70% of the total air supply to the gasification boiler. The amount of air supplied over the melting surface through pipe 21 makes up the remainder

30 - 10% of the total.

Molten salt gasification boiler 20, which uniquely enables the execution of the method according to the present invention, shall be described in great detail in connection with fig. 2.

Referring again to fig. 1, black liquor is sprayed

from a papermaking process into the boiler 20 near the top of the boiler through a pipe 22 and a nozzle 24. The sprayed aqueous black liquor typically has a concentration of approx. 45-75% solids. The gaseous product from the boiler 20 exits through a pipe 26 to a heat removal system 28, which can

contain a steam generator, and then through a pipe 30 to an absorber 32. Absorbent is introduced into the absorber 32 through a pipe 34. Absorbent can be an alkaline paper mill liquor or a common absorbent such as ethanolamine solution.

This absorbent is used to remove H^S and other unwanted constituents from the gas. The absorber system used is preferably selected for the selective absorption of H^S in the presence of CO, as the product gas contains a significant amount (10 - 15%) of CO,. Spent absorbent leaves the absorber 32 through a pipe 36. Partially cleaned gasifiers from the absorber 32 are led through a pipe 38 to a flue gas scrubber 40 for further purification. l/ann is fed into the flue gas scrubber 40 through pipe 42

and exits through a pipe 44. Washed gases exit through a pipe 46 to a gas turbine combustor 48. The flue gas scrubber 40 is shown as a typical device for removing very fine particles of soluble salts. Other devices such as cloth filters can also be used to remove the fine particles.

Air is led to the combustor 48 through a pipe 50, a compressor 52 and a pipe 53. Air from compressor 52 is also led through a pipe 54 to a compressor 56 and then to a compressed air line 58. This feeds pipes 16 and 21, which lead air in and respectively above the molten salt pond. Hot, clean combustion gases leave the burner 48 through a pipe 59 and are led to a gas turbine 60, which drives a generator 62 and compressor 52. Expansion gases from the gas turbine 60 are led through a pipe 64 to a waste heat boiler 66 into which water is introduced through a line 68 for conversion to steam. The produced steam in the waste heat boiler 66 exits through a pipe 70 to a steam turbine 72 which drives a generator 74. Process steam is supplied from steam turbine 72

through a pipe 75. Exhaust gases from the waste boiler 66 exit through a pipe 76 to a factory pipe 77 for discharge into the atmosphere.

Overflow melt from the boiler 20 flows through a

pipe 78 to a cooling tank 80. Water is introduced into the cooling tank 80 through a pipe 82. The aqueous solution that comes from cooling the melt is removed from the cooling tank 80 through a pipe 84, a pump 86 and a pipe 88. Part of the removed aqueous solution is returned to the cooling tank 80 through a

pipe 90 and serves to break up the falling stream of melt as it exits pipe 78. Another portion of the solution is conveyed to pipe 88 through a pipe 92 to a green liquor storage tank 94. A pipe 96 carries the green liquor from the storage

the tank 94 to a corresponding point in the papermaking process, e.g. the causticisation stage in the sulphate (power) process plant.

Fig. 2 describes the molten salt gasification boiler according to the present invention and the associated cooling tank and their operation in more detail. A gasifier boiler 100 contains a black liquor drying zone 110 which is located in the upper part of the boiler, a black liquor solids gasification zone 104 which is located below the drying zone, and a sulfur reduction ion zone 106 which includes a salt melt pond 108 which is located in a sump at the bottom of the boiler. The boiler 100

is shown consisting of an external metal-containing wall shell 110 which is lined with an insulating refractory material 112 which can withstand the temperature and the environment in the boiler 100. Insulating refractory material 112 is available in sufficient thickness to minimize to a practical extent the heat losses from the interior of the boiler 100. The salt melt pond 108 is further located in a melt-resistant refractory lining 113 which goes up at least partly through the gasification zone 104.

A black liquor 114 to be treated is introduced (from a source not shown) through a pipe 116 to a pump 118. From the pump 118, the black liquor is introduced into the boiler 100 through a spray system 120 which blows in the concentrated aqueous black liquor as a coarse spray through several spray nozzles 122

in an upper part of the drying zone 102.

There is a gas supply system for the boiler 100 which contains an inlet pipe 124 for an oxygen-containing gas (typically air) which leads to a compressor 126 driven by a motor 128. Advantageously, the temperature of the gas is increased by compressing the oxygen-containing gas. Alternatively, a gas heater may be used to raise the temperature of an oxygen-containing gas to the desired level. The pressurized oxygen-containing gas leaves the compressor 126 through a pipe 130 to an air flow distribution control system 132 containing one or more distribution valves. These serve to selectively control the flow of compressed air through a pipe 134 directly to a reduction zone 106 (salt melt pond 108) and through a pipe 136 to the gasification zone above the salt melt pond. Pipes 134 and 136 generally constitute a circumscribing device and gas injection openings for the respective compressed air supplies to the reduction zone 106

and gasification zone 104.

According to a key feature of this invention, the distribution of the air supply is controlled so that only the necessary amount of air to ensure the destruction of the carbonaceous materials that actually enter the melt pond is introduced into the salt melt reduction zone 106.

This is typically 30 - 70% of the total amount of air supplied to the boiler 100. The rest of the air supply to the boiler is led to the gasification zone 104 immediately above the pond.

When a flammable gas is produced, the total air supply

delivery to the boiler typically 25 - 55% of the required amount for complete combustion of the total materials added. As noted in FIG. 1, other materials such as wood waste can be supplied using compressed air directly to the salt smelting pond.

An overflow melt outlet 138 leads to a closed cooling tank 140. During normal operation, the salt melt

product 141 from the pond 108 through melt outlet 138 in the cooling tank 140. Water or another suitable salt solution such as recycled green liquor is introduced into the cooling tank 140 through a pipe 142. Water serves to shake and quench the melt that enters the cooling tank and form a pond of base liquor 144 which contains reduced chemical salts from the black liquor. The green liquor is taken out through a pipe 146, typically for return to a digestion process. Part of the green liquor product can be recycled to a pipe 142 as an aid to breaking up the smelt 141. During the cooling of the smelt 141, a hot gas is produced, which mainly comprises water vapour, which is taken out from the cooling tank 140 through a pipe 148.

A small part of the gas produced in the gasifier boiler 100 can flow through the melt outlet 138 together with the exhausted melt, and this gas is also taken out from the cooling tank 140 through the pipe 148. The total gas flow that is taken out from the cooling tank 140 through the pipe 148 is preferably set to the main product gas stream by leading the gas in pipe 148 to a corresponding point in the product gas cooling and cleaning system, such as in pipe 30 in fig. 1, or in the gas stream before its last cooling stage.

Back to the boiler 100, the lean product gas is removed from the boiler through a gas outlet pipe 150 which is located at the upper end of the boiler above the drying zone. As indicated in fig. l, the product gas exiting the boiler 100 may pass through a heat recovery system and then through an absorber to remove H2S and other unwanted constituents from the gas. The heat removal system 28 in fig. 1 may contain a steam generator, supply water heater or other heat-exchanger devices. The last step in the heat removal is typically carried out by heat exchange with cooling water and leads to the condensation of water vapor which forms liquid water. This condensed water is preferably returned to the cooling tank 140 through a pipe 142.

As stated in the U.S. patent application 06/667,937, it may be advantageous to equip the gasifier boiler 100 with a burner unit to produce a flow of hot gas in the boiler 100 to preheat it before starting operation and possibly to provide an additional source of heat during operation. As shown in fig. 1, an acid gas absorbing device 32 may also be equipped to provide contact between an absorbent and the product gas to remove harmful acid gases such as H2S and the like from the product gas, so that it can be made suitable for use as fuel for a

gas turbine or other purposes.

With regard to the operation of heaters, steam generators, condensers and absorbers which are previously known, it is not necessary to discuss these associated components used with the black liquor gasification plant in further detail.

During operation of the process, it is desirable that a relatively constant temperature is maintained in the gasification zone 104, e.g. 1000°C in the part of the zone which is close to the upper surface of the salt melt pond 108. This can be achieved by adjusting the air/black liquor ratio up or down to raise or lower the temperature as needed to maintain the desired value. If other parameters such as black liquor composition, air for superheating and heat loss are not varied,

this form of operation will lead to the production of product gas with a relatively constant composition and heating value. The product gas calorific value can be increased if desired by introducing

a high-quality fuel such as oil or petroleum coke i

the gasification zone; increase au the temperature of the supplied air; or reduce heat loss by e.g. add insulation.

A gaseous fuel such as natural gas or volatile hydrocarbons can therefore be added directly to the product gas to increase its heating value.

Salt melt product 141 which flows out of the boiler IDO

until the cooling tank 140 is dissolved in water and forms green liquor.

It is an advantage to operate the cooling tank at the same pressure

as the carburettor to avoid the need for a pressure control valve operating on molten salt. The green liquor, which contains dissolved sodium sulphide, can be returned to the digestion process or used for other purposes.

The gas rising from the gasification zone 104 contains CO, H^, H^O» CO^» CH^, and if air is used IM^

plus various trace components and contaminants, and is at a temperature in the range of approx. 870 to 1200°C.

Two pollutants of particular interest are H^S which originates

from sulfur in the black liquor supply, and fine particles of sodium salts such as sodium carbonate and sodium sulphide, formed by evaporation and reaction processes. When the gas then passes through the drying zone 102, it is cooled to a temperature in the range of approx.

350 to 850 C depending on its temperature into the drying zone, the water content of the black liquor and related factors. Preferably, the gas is cooled to a temperature at which the particles of sodium salts are solid, which is below approx. 790°C for typical salt mixtures.

As pointed out above, an oxygen-containing gas is controllably introduced into the gasification zone 104 and the reduction zone 106

of the boiler 100 to effect partial oxidation of the carbon-containing materials in the black liquor, at the necessary high temperature and produce the desired products. The oxygen-containing gas is suitably preferably air; if desired, oxygen-enriched air or pure oxygen can be used. Although pure oxygen can be used in the process of this invention, it is less preferable than air or oxygen-enriched air because the high cost of oxygen and the necessity of locating an oxygen plant close to the black liquor gasification system. In general, the upward velocity of the gas leaving the gasification zone should not exceed approx. 6 m/sec. and should preferably be in the range 0.6 to 4.5 m/sec.

The pressure in the gasification boiler 100 bar lies within a °"adS ^ approx. 1 tU 50 = id.t pressure above .tmo.f»,

pressure is particularly desired. Preferably a tap on

about. 3 to 30 atmospheres are used. The use of pressure above atmospheric pressure is desirable for a number of reasons. The safety of the process is increased at pressure above atmospheric pressure because explosions can occur when mixing melt and water in the cooling process because the smelting is inhibited at increased pressure. The volume of product gas and, consequently, the size of the necessary equipment to extract

before the process is reduced by a factor as large as approx. 20:1

when pressure above atmospheric pressure is used. This reduces both costs and heat loss. In addition, salt evaporation is reduced, which makes extensive cleaning of the gas that is formed in the process unnecessary. The removal of the water-phase contamination such as hydrogen sulphide from the product gas using absorption or adsorption processes becomes easier with increased pressure.

Another advantage of running the process under pressure is increased thermal efficiency of the process due to partial recovery of thermal melting energy, which is made possible by the increase in boiling point of the cooling tank solution as the pressure is increased.

Another advantage is that the product gas is available at the necessary pressure for use in subsequent operations, such as at the inlet to a gas turbine.

The temperatures in the gasification zone 104 near it

the upper surface of the salt melt pond 108 is kept in the area approx. 870 - 1200 °C, and preferably in the range approx. 900 - 1070°C. It should be noted that the gasification zone is not operated at a completely uniform temperature. The highest temperature in this zone is normally near the surface of the salt melt pond where entrained oxygen reacts with carbonaceous material. The temperature near the top of the gasification zone decreases as the gas approaches the drying zone.

The high-temperature gases rise from the gasification zone, cool to a temperature of approx. 350 to 850°C during passage through the drying zone. The cooling effect is a further advantage of this invention because it causes droplets of solidified metal that could be trapped in the rising gas to solidify before leaving the reactor. The resulting particles do not adhere to or corrode armor transfer surfaces or other equipment in the product gas treatment system. Temperatures in the salt melt pond reduction zone can be somewhat lower than in the gasification zone due to the endothermic sulfur reduction ion reactions that occur in the reduction zone. However, the temperature in the reduction zone must be kept at a sufficiently high level to ensure that solidification of salts does not occur, and the reduction reactions can proceed at one high rate. An area of approx. 860 - 1100°C is applicable,

and the preferred area is approx. 870 - 1050°C for the salt melt pond reduction zone.

It is important that the heat is retained in the gasification and reduction zones. Otherwise, heat loss will require a higher air-to-black liquor feed ratio to maintain the temperature. As the air-to-black liquor ratio is increased, more complete combustion occurs, especially in strongly exothermic reactions to 00^ and h^O from CO and H^. This compensates for heat loss, but reduces the heat value in the product gas. It is somewhat less important that heat loss be minimized from the drying zone because heat loss from this zone primarily works to reduce the temperature, but not the heating value of the product gas. Heat loss from all three zones is reduced by using insulation material 112. All ordinary insulation can be used for this purpose. For example insulating blankets, castable refractory materials, bricks, fiberglass and tiles are suitable. Materials that are in contact with molten salt at high temperature and salt vapors must be resistant to attack by these substances. Very clean fusion-cast aluminum blocks are e.g. found to be quite effective for use as a melt-resistant refractory lining 113.

The control of heat loss is an important feature of the present invention and stands in stark contrast to the practice of using a Tomlinson boiler or an equivalent thereof, where the heat generated by burning black liquor is used to convert water into steam in boiler tubes in the reactor. Instead of removing the heat in this way to produce a combustible gas product with the desired higher heating value, it has been found essential to

prevent heat from being lost. In particular, you want to have a higher heating value (HHV/) for the product gas of at least approx. 90 Btu/scf, it is necessary to design the system so that the total heat loss from the gasification and reduction zones is less than approx. 1392 j/g added black liquor and preferably less than 1160 j/g.

In order to limit the heat loss from these zones by radiation up into the colder cooling zone, it is desirable that the cross-sectional area of the boiler at the top of the gasification zone be limited.

For example, a cross-sectional area less than approx. 0.044 m 2/kg/hr black liquor supply limit the radiation losses to less than approx. 1160 j/g black liquor for typical operating conditions. As some heat loss by conduction through the walls and bottom of the boiler can also be expected, a smaller cross-section is normally required

flat than approx. 0.039 m /kg/hr black liquor supply. Thus, a technical unit for haying 100 tonnes per day added black liquor require a cross-sectional area at the top of the gasification zone less than 6.20 m 2, or an internal diameter less than approx. 2.7 m for a circular cross-section.

Even smaller cross-sectional areas are preferred (e.g. smaller

than approx. 0.03 m 2 /kg/hr and can easily be achieved with acceptable gas velocities when operating at higher pressures. Reduction of the cross-sectional area necessarily leads to an increase in gas velocity in the carburettor if other conditions are not changed. In order to thus avoid excessive speeds during operation with a cross-sectional area in the preferred area, it is desirable to

operate the carburettor at a higher pressure.

The heat loss or heat removal referred to

in the discussion above only refers to heat leaving the carburettor and reduction zones by radiation upwards or conduction into or through the walls and ^if therefore controllable by suitable plant construction. In addition, it is important that the dried black liquor almost completely enters the gasification zone, so that the heat will not be consumed to evaporate water and that the air supply to both lower zones can be preheated to minimize the necessary

v/arme to raise its temperature. Certain heat losses are unavoidable, however, and set an upper limit of approx. 75%

on the calorific value of the black liquor which can be converted into product gas calorific value. The inevitable heat losses result in sensible heat in the product gas and the product melt and the heat value of sulphide in the smelt. In order to achieve the desired gasification of the black liquor in the method according to the present invention, aqueous black liquor is introduced into a drying zone 102 of the boiler 100

in such a way that a correct black liquor surface is obtained in direct contact with the rising flow of hot gas and a suitable contact time. The black liquor can be injected into the boiler and form falling droplets which are dried by the gases rising from the gasification zone, with the water evaporated from the black liquor before the black liquor leaves the drying zone. Spray droplets can also hit boiler inner walls in the drying zone where they become suspended and dried and form deposits of carbon-containing materials and salts which then fall from the walls into the gasification and reduction zones. However, it is not desirable to introduce the black liquor as such a fine spray that the spray droplets or the resulting dried, finely divided black liquor solids are captured in the hot gases rising through the gasification boiler. The coarseness of the spray is adjusted so that appropriate drying with minimal pick-up occurs.

The gas produced as a result of the gasification of the black liquor solids has a higher dry matter heating value of at least 90 Btu/scf which is primarily due to the presence of CO, H^, CH^. As the product gas rises through the black liquor drying zone, its water vapor content increases and its temperature decreases as a result of evaporation of water from the black liquor.

In addition, an increase in water vapor causes the water gas displacement reaction as follows:

This leads to a change in gas composition, like this

that the gas leaving the top of the drying zone contains less CO and more H_ than that leaving the gasification zone.

However, this heating value has not changed significantly

by the reaction.

Gas leaving the drying zone can be treated in a number of ways. Preferably, the sensible heat is used to produce steam in a steam generator or other heating service. For most purposes it is desirable to remove water vapour, fine salt particles and H^S from the gas before it is used. These steps can be performed in common equipment such as a condenser to remove water vapor, absorption contactors using alkaline solutions to absorb H^S, and flue gas scrubbers or factory filters to remove particulate matter. Water, salt and sulfur extracted in such steps can be returned to the pulp mill or the gasification process. In some cases, it may be desirable to clean the product gas when it leaves the gasifier without further cooling, so that the sensible heat and compression energy in the gas and in the water vapor can be used in a gas turbine or other energy conversion system.

As pointed out, the exhausted melt 141 flows from the boiler 100 through the pipe 138 to the cooling tank 140, where it is dissolved in water by gasification pressure. The melt will solidify and block the flow path if it is allowed to cool below approx. 7B0°C while in contact with the flow nozzle. It is therefore desirable to let part of the high-temperature gas from the gasification zone flow through the melt discharge line to help maintain a high temperature in this line. This gas will flow into the cooling tank 140 from where it can be discharged to the product gas system at a point downstream of the gasifier. Other devices may be used to maintain a clear path for the melt flow including auxiliary burners and mechanical breaking systems.

The following examples are illustrative of the invention.

EXAMPLES

The basic process chemistry in salt melt gasification of concentrated aqueous black liquor was previously demonstrated by the present inventor in a series of laboratory-scale experiments. These were performed a 15 cm ID laboratory-scale carburettor installed with an electric furnace that could be operated to minimize heat loss through the walls. Product gas with higher heating values (HHV, dry basis) was from approx. 120

to 140 Btu/scf depending on the black liquor composition and other variables. Recovered sulfur from the smelter was generally over 90% in the form of sodium sulphide. The effects of pressure on the basic chemistry were also previously demonstrated by experimental programs.

To further demonstrate the technical potential of the molten salt-black liquor gasification process, a multi-purpose salt smelting test facility (MSTF) was modified to provide a black liquor gasifier chain capable of demonstrating the present process at a pilot plant level. The modification produced a three-zone gasification kettle consisting of an aqueous black liquor drying zone, a gasification zone for black liquor solids and a reduction zone for molten salt sulphur. The MSTF used consists of a boiler of approx. 84 cm. internal diameter with approx. 424 cm. internal height. The lower 244cm section is lined with fused cast aluminum oxide bricks approx. 15 cm thick, which are baked with approx. 1.25 cm tall aluminum oxide castable refractory material. These materials are very resistant to attack by the high-temperature molten salt, but are not effective as thermal insulation. To reduce the heat losses from the gasification and reduction zones, 0.32 cm. thick layer of mineral fiber insulating paper installed on the outside of the metal boiler, however, a more effective thicker layer could be used without the permissible temperature in the metal boiler being exceeded.

In previous tests and analytical studies on black-air gasification, the present inventor had demonstrated that a key requirement to produce a combustible gas with a higher HHV than 100 Btu/scf and greater smear reduction than 90% reduction to sulfide required that the heat loss from the combined gasification - and reduction zones should preferably be smaller than approx. 278 j/g feed for a typical black liquor mixture. As the original purpose of the M5TF-

boiler was to try chemical waste with a complete combustion, the unit to maximize yield was designed so that a very large heat loss rate through the walls was allowed (about 6.3 x IO<-8> to 8.4 x IO<-8> j/h). Consequently, due to the original high heat loss design of the MSTF boiler, the main objectives of the black liquor gasification program at a pilot plant level were limited to demonstrating the operation of the equipment on a relatively large scale and predicting its performance based on laboratory scale experiments and analytical studies.

Two key structural modifications were made in the MSTF boiler according to the present invention. The melt outlet port was located 193 cm above the bottom of the boiler and was plugged with a ceramic plug and covered with a flange. A new melt overflow spout was designed and fabricated for trial operation and installed 35 cm above the boiler bottom. By lowering the melt outlet port, the melt stock was reduced and a relatively shallow pond was obtained.

In addition to the four existing nozzles that were

used for air injection into the salt smelting pond, six new nozzles were arranged at a height of 50 cm above the boiler bottom so that part of the injected air could be injected above the smelting pond. These newly arranged nozzles were distributed around the perimeter of the boiler and pointed down and inwards at a 45° angle, so that the air was directed towards the surface of the salt smelting pond. Balancing orifices were used at each nozzle to provide a

even air distribution to the individual air openings. Changes were also made to the black liquor injection system to increase and maintain black liquor flow.

The total batch time was approx. 46 hours of operation from the beginning of the black liquor supply to the system was closed and involved 14 samples. About. 8,620 kg of black liquor was gasified, but the black liquor flow was not continuous throughout the batch.

The carburettor was started by first setting the air flows to the nominal values for full load conditions, for example for a nominal surface gas velocity of five fps at 980°C. The total air distribution to the carburettor boiler was first set so that you got approx. 40% of the air to the six upper nozzles (above the smelter) and 60% of the air to the 4 bottom nozzles (in the smelter). However, this ratio was reversed for trials 10 through 14. The top six nozzles received preheated air, the bottom four nozzles received room temperature air. A temporary natural gas burner was installed on the boiler head to preheat the unit. The carburettor was preheated to 930 - 980°C before the charge. Table 1 shows an analysis of the black liquor used in the experiments.

Analysis of the test results showed that the product gas had a maximum HHV dry of 52.3 Btu/scf during steady-state operation and a maximum reduction of sulfur in the smelt of 67.4%. Due to the construction of the MSTF boiler, it was not possible to increase these values significantly below the rate by changes that would enable operation at a lower air/fuel ratio, such as by providing the boiler with additional insulation or by increasing the black liquor feed rate. Experiments 10 and 13 (see Table 2) are typical examples of the effect of the MSTF in the construction of this invention. For comparison, the results from a previous experiment called experiment A with a different configuration are included in the table. During this previous experiment, all the air was passed below the surface of a deep salt melt pond.

A comparison of trials 13 and A, which took place at approximately the same air/black liquor ratio, shows that the reduction of the melt pond depth from 193 to 35 cm. had no detrimental effect on the heating value of the product gas. The sulfur reduction ion effect is seen to be significantly higher in both experiments 10 and 13 than in experiment A. This can be attributed to the air supply device for experiments 10 and 13, whereby only 37% of the air is passed through the melt pond while the rest is injected into the gasification zone. This device does that

also possible to supply more total air (and therefore more black liquor)

to the gasifier during trials 10 and 13 without excessive heating of melt droplets. As a result, the unit could be operated at a lower air/black liquor ratio during Experiment 1

and give a gas that has a higher calorific value than is possible with the construction used for experiment A.

Trial 10 represents the maximum steady-state operability of the MSTF in its finished form with respect to throughput, product gas calorific value and sulfur reduction. The throughput is limited by the permissible gas velocity and could be increased by increasing the operating pressure, or to a lesser extent, by operating at a lower air/black liquor supply ratio. The heating value and sulfur reduction efficiency of the product gas can also be increased by operating at a lower air/black liquor ratio; however, this form of operation would cause the plant temperature to drop, unless the heat loss per kg. supply is reduced. This could be achieved either by reducing the total heat loss (eg by using additional insulation), or by increasing the allowable feed rate (eg by increasing the pressure).

At achievable conditions in the MSTF boiler, the results show that operation with a significant portion (30 - 70%) of the air injected above the molten metal pond leads to more effective sulfur reduction than operation with 100% of the air injected below the pond surface, and allows also operation with a higher gas production rate. The results thus show that a very shallow melting pond (nominal depth approx. 35 cm.) is just as effective for black air gasification as a deep pond (l93 cm.)

The present trials compared to previous ones

experiments at both laboratory scale and pilot plant scale show that the increase of air to black liquor ratio leads to an increase in both product gas HHV and molten sulfur reduction efficiency. The data show that a sulfur reduction efficiency will be achieved when the air/black liquor ratio is reduced to the point where the HHV of the gas exceeds approx. 60 Btu/scf. This enables projection of the pilot-plant-scale results which

shows that technical facilities will operate and produce gas with an HHV above 100 Btu/scf and melt in which the sulfur content is above 20% in the form of sulphide.

It will be known that power mass production process

is approx. 100 years old. Because the chemicals used in the caustic-lye mixture for treating cellulose raw material are too expensive to throw away, from the beginning of the power process, many attempts have been made to recover these caustic materials by flat recovery of heat by burning organic caustic material dissolved from wood. The Tomlinson cooker was introduced for approx. 50 years ago to carry out this desired recovery. Because of the aforementioned disadvantages of the Tomlinson boiler, many modifications and substitutes for it have been proposed. The present method avoids the disadvantages of the previously proposed methods in that it uses the same concentrated black liquor supply without the need to pre-dry, oxidise, hydrolyse or otherwise prepare the supply. The present process also produces a melt essentially identical to that produced by the Tomlinson digester. Due to the above-mentioned advantageous features as well as its use of a boiler in one component, the present method can be easily incorporated into existing pump mill systems and replace or supplement Tomlinson boilers.

It will be seen that various modifications by using long-known knowledge in the black liquor recovery field can be made to the construction of the boiler and the operation of the process according to this invention without going beyond the inventive idea. E.g. the boiler can be designed with a smaller diameter in the drying zone than in the gasification and reduction zones to reduce thermal radiation from these latter zones. Other gasification kettle shapes can also be used in place of the elongated vertical walls of constant diameter shown. Furthermore, the black liquor supply can be broken up by a rotary disc atomizer, steam atomizer, or flow distribution system instead of the spray nozzles illustrated.

Claims (14)

1. Process for treating a concentrated, aqueous and carbonaceous black liquor material and alkali metal-sulphur compounds to form a combustible gas and a sulphide-rich melt comprising: a) that a gasifier boiler (100) is maintained at a pressure of 1-50 atmospheres, during which the in the boiler there is a relatively shallow pool (108) of molten salt at the bottom of the boiler, which vessel is equipped with an edge outlet (138), the boiler also having: i) a black liquor drying zone (102) in its upper part, ii) a gasification zone ( 104) for black liquor-dry substance located below the drying zone, and iii) a sulfur reduction zone (106) with molten salt including the pond (108) of molten salt b) that in the upper part of the drying zone (102) concentrated, aqueous black liquor (114) containing carbonaceous material and alkali metal-sulphur compounds, c) that water is driven out of the concentrated, water-containing black liquor (114) in the drying zone (102) by direct contact between the water-containing black liquor and the hot gas that rises pover from the gasification zone (104) forming a dried black liquor substance which falls into the gasification zone (104), and a cooled, combustible gas containing water vapor, whereby the combustible gas is at a lower temperature than the melting point of entrained droplets of molten salt, which causes that the droplets solidify, characterized by) that a first part of an oxygen-containing gas is introduced into the gas space in the gasification zone (104), which space is located below the drying zone (102) immediately above the pool of molten salt for partial oxidation and gasification of part of the carbonaceous material in the dried black liquor dry substance falling through the zone to form a hot, combustible gas, e) that the second part of the oxygen-containing gas is introduced below the surface of the pond (108) of molten salt in a quantity sufficient to effect gasification of substantially all carbonaceous material which enters the pond from the gasification zone (104), but in insufficient quantity to provide oxidase ionic conditions in the pond, whereby the formed gas rises upwards from the pond, and whereby the total quantity of the first and the second part oxygen-containing gas constitutes between 2 5 and 55% of the quantity of oxygen-containing gas required for complete combustion of the black liquor feed and constitutes the total quantity of oxygen-containing gas which is fed to the gasifier boiler (100), f) that the cooled, combustible gas with a higher heating value of at least 3-350 kJ/m<3> (on a dry basis) is driven out from an upper part of the drying zone (102), and g) that a melt is taken from the edge drain (138) in the reduction zone (106) with molten salt, in which the sulfur content is predominantly in the form of alkali metal sulphide. 2. Method according to claim 1, characterized in that the first and second parts of the oxygen-containing gas make up 30 to 70% and 70 to 30%, respectively, of the total amount of oxygen-containing gas that is fed to the boiler. 3. Method according to claim 2, characterized in that the oxygen-containing gas is air. 4. Method according to claim 1, characterized by the gasifier boiler (100) being kept at a pressure of 3-30 atmospheres, and that the concentrated, aqueous black liquor which is fed to the boiler (100) contains at least 45% by weight of dry matter and has a higher calorific value of at least 3440 kJ/kg . 5. Method according to claim 1, characterized in that the hot gas leaving the gasification zone (104) is at a temperature of 870-1200°C, and that the cooled, combustible gas leaving the drying zone (102) is at a temperature of 350-850°C. 6. Method according to claim 1, characterized in that the temperature in the reduction zone (106) is 860-1100°C. 7. Method according to claim 1, characterized in that the total heat loss from the gasification (104) and reduction zone (106) is less than 1390 kJ/kg black liquor supply to the gasifier boiler (100). 8. Device for treating concentrated, aqueous black liquor into a combustible gas and a sulphide-rich melt, comprising a closed, vertical, elongated gasifier boiler (100) with (i) a black liquor drying zone (102) in the upper part of the boiler, (ii) a black liquor solids gasification zone (104) below the drying zone, and (iii) a salt melt sulfur reduction zone (106) below the gasification zone (104); a sump area in the bottom of the boiler to accommodate a pond (68) of molten alkali metal salt, the molten salt pond comprising the molten salt sulfur reduction zone (106); inlet devices (122) located in the upper part of the boiler for supplying the black liquor to the drying zone (102); outlet devices (138) located in the lower part of the boiler (100) above the sump area which serve to limit the height of the molten salt pond (108) and to remove overflow of molten salt from the sump area; characterized by a first set of gas inlet devices (136) located in the gasification zone (104) for supplying an oxygen-containing gas to the gas space of the gasification zone (104) immediately above the salt melt pond (108); a second set of gas inlet devices (134) located in the sump area of the boiler to feed oxygen-containing gas directly into the salt melt pond (168), and outlet devices (150) located in the top part of the boiler and which are connected to the upper part of the drying zone (102) to remove a flammable gas from the boiler (100). 9. Device according to claim 8, characterized in that it further contains devices (132) for controllable distribution of the supplied oxygen-containing gas to the first (136) and second (134) sets of inlet devices for oxygen-containing gas. 10. Device according to claim 9, characterized in that at least one air compressor (126) provides an oxygen-containing gas under pressure as supply to the first (136) and second (134) sets of gas inlet devices. 11. Device according to claim 8, characterized in that the outlet device (138) from the sump area is connected at its other end to a cooling tank (140) which receives the melt overflow from the boiler's sump area. 12. Device according to claim 11, characterized in that the cooling tank (140) contains an inlet pipe (142) for the supply of a water source to cool the tank and further contains gas outlet devices (148) for removing gases from the cooling tank (140). 13. Device according to claim 8, characterized in that the black liquor inlet device (122) contains a spray system for injecting concentrated, aqueous black liquor as a coarse spray into the upper part of the drying zone (102). 14. Device according to claim 8, characterized in that the boiler has an external metal-containing wall shell (110) lined with an insulating, refractory material (112) to withstand the temperature and the environment in the boiler (100) and to minimize heat loss from it.