WO2005003449A1

WO2005003449A1 - Method of utilising unreacted carbon from black-liquor gasification

Info

Publication number: WO2005003449A1
Application number: PCT/FI2004/050100
Authority: WO
Inventors: Paterson Mckeough
Original assignee: VTT Technical Research Centre of Finland Ltd
Current assignee: VTT Technical Research Centre of Finland Ltd
Priority date: 2003-07-04
Filing date: 2004-06-23
Publication date: 2005-01-13
Anticipated expiration: 2006-01-04
Also published as: FI20035119A0; FI20035119L; FI115848B

Abstract

The subject of the invention is a method for utilising the unreacted carbon from processes for gasifying the spent liquor (black liquor) of alkaline pulping processes. In the new method, the final separation of the unreacted carbon from the soluble salts derived from black liquor does not take place until after the causticising stage. The carbon and limestone are separated and washed together and the limestone-carbon mixture is fed to the calcination reactor where the said carbon functions as a fuel. With this method, carbon replaces, to a significant extent, the normal fuel used in the calcination process. The method differs from other alternatives for utilising unreacted carbon in that its application requires virtually no additional equipment and has no negative effects on the water balance of the pulp mill.

Description

METHOD OF UTILISING UNREACTED CARBON FROM BLACK-LIQUOR GASIFICATION

Description

Technical field

The method is related to black-liquor gasification technology. Black liquor is the spent liquor formed when cellulose is produced by alkaline pulping. In addition to the soluble spent pulping chemicals, about half of the organic material of the raw material dissolves into the black liquor during pulping.

Prior art

Gasification is a known method for processing black liquor (e.g. Komulainen, et al, NTT Research Notes 1542, VTT Technical Research Centre of Finland, 1994). In the gasification process, fuel gas is produced from the organic material of black liquor and the inorganic compounds are simultaneously converted into such forms that, after the causticising treatment, they can be re- utilised as pulping chemicals.

In respect of the temperature of the black-liquor gasification process, two operating regions exist:

• high temperature; > about 900°C; inorganic salts melt • low temperature; < about 700°C; inorganic salts remain in the solid state.

Especially when the gasification temperature is low, one of the greatest technical challenges is to get the gasification reactions to proceed to completion. Gasification is comprised of two stages: (1) pyrolysis; i.e thermal degradation and (2) the gasification of the carbon in the pyrolysis char through reactions with CO₂ and H O. At low gasification temperature the latter stage is relatively slow and, as a result, a small part of the carbon often remains ungasified. It has been suggested, for example, that the unreacted carbon could be used as a fuel in the power boiler of the pulp mill.

Following gasification, it is possible to separate the unreacted carbon from the inorganic salts after the salts have been dissolved in water. The solution is known as green liquor. For example, the insoluble carbon can be separated from green liquor by filtration. The separated carbon needs still to be washed well with water if the objective is to use it in the power boiler of the pulp mill or in any other furnace which has not been specifically designed for fuels with high contents of alkaline compounds. The required efficient washing process is comprised of several wash-filtration stages. A final additional dewatering stage may be required to reduce the moisture content of the carbon material prior to combustion. In addition to the fact that such a separation-wash process is a relatively expensive operation, it has also negative impacts on the water balance of the pulp mill. The washing process requires relatively clean water and produces a wash effluent that needs to be treated. Recovery of the salts in the wash effluent would be preferred but adding the effluent to the chemical recovery cycle could increase the evaporation requirement.

Description of the invention

The subject of the invention is a new method for utilising the unreacted carbon as a fuel in the calcination reactor. In the traditional recovery process, the calcination reactor is a rotary drum kiln known as the lime kiln. The most essential feature of the invention is that the separation and washing of the carbon are carried out in connection with the separation and washing of the limestone (CaCO₃). In this way the need for additional equipment is avoided and the water balance of the pulp mill is not significantly altered.

The greatest advantages of the new method are:

(1) unreacted carbon replaces a significant part of the normal fuel used in the calcination process and

(2) the method deviates from other methods of utilising unreacted carbon in that its application requires virtually no additional equipment and has no negative effects on the water balance of the pulp mill.

The first advantage is especially significant when the fuel in question is a purchased fossil fuel - as is normally the case. In the chemical recovery process based on gasification, the causticising requirement and the consequent fuel requirement for calcination are generally greater than those of the traditional recovery process. If, in addition, only fossil fuel would be used in the calcination reactor, the application of black-liquor gasification would lead to an increase in fossil-derived CO₂ emissions from the calcination process. Although this increase would, in any case, be clearly smaller than the reduction in CO emissions that the application of black-liquor gasification would otherwise make possible, replacement of fossil fuel with the unreacted carbon would be obviously beneficial from the point of view of the overall net emissions of CO₂.

In its most simple form, the new method is realised in such a way that the unreacted carbon is transported through the causticising stage with the green/white liquor - that is, in the same way that the insoluble particles of calcium salts are normally transported through the causticising stage. The unreacted carbon and the limestone (CaCO₃) are separated together from the white liquor and are washed together. This embodiment is described in more detail with the help of the example shown in the attached diagram. Black liquor (stream 7) is gasified in stage 1. Particles are possibly removed from the product gas (stream 8) in stage 2. The product gas (stream 9) is then led to the cooling/cleaning train. In addition to the product gas, a mixture comprising inorganic compounds and possible unreacted carbon is formed during gasification. The inorganic salts can be in the form of either solid material or molten liquid depending on the gasification temperature. The salts- carbon mixture is removed from the gasification line either from the gasification stage itself (stream 10), from the separation stage (stream 11) or from both stages, after which the mixture is led to a combined dissolving and lime-slaking stage 3. To this stage is added the aqueous solvent (such as weak wash, stream 12) and, from the lime cycle, the amount of burnt lime (CaO, stream 13) required for causticisation. In process stage 3, the soluble alkali salts dissolve, the lime is slaked and the causticising reaction begins. The stream exiting stage 3 (stream 14) is a slurry, which is made up of partly causticised green liquor, of particles comprised of calcium compounds and of carbon particles. After the causticising stage 4, the slurry (stream 15) has been altered in the following ways: (1) the liquor is so-called white liquor and (2) the particles of calcium compounds are, in the main, comprised of limestone (CaCO₃). In stage 5 the limestone particles and the carbon particles are separated together from the white liquor and washed together. In the typical case, where the amount of unreacted carbon is significantly smaller than the amount of calcium compounds, the presence of carbon would hardly affect the performance of the equipment. From stage 5 white liquor (stream 16) is sent to the pulping stage and a wet limestone-carbon mixture (stream 17) is sent to the calcination stage 6. In the calcination reactor, burnt lime (CaO, virta 13) is produced from limestone, CaCO₃. A significant part of the heat necessary for process stage 6 is obtained from the unreacted carbon which burns in the oxidising conditions present in the reactor. The remainder of the heat comes from the externally supplied support fuel. A favourable situation for the new method is one where the unreacted carbon represents a significant part of the energy requirement of the calcination stage but does not exceed it. If the requirement is exceeded, either some carbon has to be removed from the lime cycle or heat has to be recovered from the flue gases of the calcination reactor. Both measures would increase costs. However, it seems that the amount of unreacted carbon will usually be suitable. For example, 10 % of the carbon present in black liquor would usually be sufficient to cover the full energy requirement of the traditional lime kiln. On the other hand, (1) the objective of gasification processes is to keep the amount of unreacted carbon clearly below 10 % and (2), as already mentioned, in the chemical recovery process based on gasification, the causticising requirement and the consequent fuel requirement for calcination are generally greater than those of the traditional recovery process.

The rate of the calcination reaction is possibly increased as a result of the fact that a part of the fuel is evenly distributed throughout the lime mud. A speeding-up of the reaction would increase the capacity of the calcination reactor. This could be a very valuable benefit when applying black- liquor gasification to an existing pulp mill. Namely, the already-mentioned larger causticising requirement, resulting from the application of black-liquor gasification, would not, in that case, necessarily lead to a need for a larger calcination reactor.

In the new method, the insoluble material formed in the dissolving step differs considerably from the green-liquor dregs of the traditional process because the former material contains considerably more unreacted carbon. In the new method, the dregs can not be purged as such from the recovery cycle because the carbon has to be utilised. It should be noted that the amount of those compounds that comprise the traditional green-liquor dregs material is considerably smaller for a low- temperature gasification process than for the traditional high-temperature combustion process. From the lime-slaking stage, grits can be removed in the traditional way because the carbon particles are finely sized - as are most of the particles comprised of calcium compounds. In the new method, a part of the traditional green-liquor dregs material will probably accumulate in the lime cycle. As a result, it might be necessary to purge some material from the lime cycle in order to keep the amount of impurities at an acceptable level.

Other embodiments and variations are described in the following. A variation of the embodiment presented in the accompanying diagram is one in which the dissolving and lime-slaking steps are kept separate.

The new method can be realised in such a way that, in deviation from the process in the attached diagram, the main part of the unreacted carbon is not transported through the causticising stage with the green liquor. Carbon is separated from the green liquor after the dissolving stage - for example, in a clarifier. This necessitates that the dissolving and lime-slaking stages are separate. The separated carbon is not washed. It is re-added to the white liquor-limestone slurry before the separation and the washing of the solids. After that, the process is realised in the same manner as in the attached diagram.

The new method can be applied in conjunction with any type of calcination reactor. In the traditional process the calcination reactor is normally a rotary drum kiln known as the lime kiln. Fluidised-bed furnaces have also been applied. When the recovery process is based on black-liquor gasification, possibilities exist to integrate the calcination process with the gasification process. For example according to Finnish Patent FI 81140, the calcination reactor can be located in the gas-handling train after the gasifier and can be utilised to simultaneously recover H₂S. The new method presented herein can be applied in connection with this type of calcination process.

The embodiments and variations presented above are examples that elucidate the invention without restricting it in any way.

Claims

1. A method for utilising the unreacted carbon from processes for gasifying the spent liquor (black liquor) of alkaline pulping processes, wherein (1) the final separation of the unreacted carbon from the soluble salts derived from black liquor does not take place until after the causticising stage, (2) the separation and washing of the carbon are carried out so that the carbon and the limestone are separated and washed together and (3) the washed mixture of limestone and carbon is fed to the calcination reactor where the carbon functions as a fuel.

2. A method as in claim 1, wherein the unreacted carbon is transported through the causticising stage with the green/white liquor.

3. A method as in claim 1, wherein (1) unreacted carbon is separated from the green liquor after the dissolving stage and (2) the unwashed carbon so separated is re-added to the slurry of white liquor and limestone before the final separation and washing stages.

4. A method as in claim 2, wherein the dissolving stage and the lime-slaking stage are combined.

5. A method as in claim 1, wherein the said calcination reactor is a traditional lime kiln.