SE470516B - Purifying gas from the gasification of black liquor - Google Patents

Abstract

The present invention relates to a method for purifying gas which is produced during black liquor gasification, preferably in a gasification reactor of the CHEMREC <IMAGE> type, wherein the major part of the alkali is separated off from the gas stream in a special separating device 4, after which the purified crude gas is supplied to an H2S washing- out device 7 and finally conveyed to an alkali washing-out device 11 before its alkali content is sufficiently low for it to be able to used as fuel in a gas turbine, with the alkali washing-out device 11 being supplied with a steam condensate 12 or with water which has been demineralized in some other way, and with the liquid obtained from the alkali washingout device being supplied to the lime sludge wash 10 and, where appropriate, also to a washing stage which is located upstream of the said alkali washing device. <IMAGE>

Description

'I .DW v- \ (,, \ N be impractical in that the water balance in the chemical recovery system is adversely affected. The supply of fresh water to the chemical recovery system is normally limited to the water which must be applied to the mesafilter for washing. of the mesa from the pulp mill's causticizing plant.

Solution A prerequisite for being able to get down to alkali contents below 1 ppm is that the gas is washed with steam condensate or otherwise demineralized water. Such washing is advantageously done in a bottom or nozzle column. For leaching H25 from the gas, so-called weak liquor from the pulp mill's mesate wash has proven to be a very effective washing liquid due to its relatively high alkalinity. In order to optimize the water balance in the pulp mill's chemical recycling system, it is also an advantage if the weak liquor from the mesate wash can be added to the gasification plant to provide the liquid additive required for the dissolution of produced chemical melt into so-called green liquor. By first using the weak liquor for efficient leaching of hydrogen sulphide from the gas, it is in this way given an extra and for the recycling system extra advantageous function.

The main object of the present invention is to economize the water use in the pulp mill by using spent washing liquid from the alkali cleaning step as washing liquid to the mesa filter for the preparation of the weak liquor, which then goes to hydrogen sulphide washing and green liquor preparation within the gasification plant. The small amount of alkali absorbed in the washing liquid from the alkali wash does not in any way interfere with the function of the mesafilter or the quality of the “weak liquor” produced there. The addition point for the fresh water addition to the chemical recovery system is moved by the invention from the 'mesafilter' to the alkali wash step in the black liquor gasification system.

This measure can thus be taken without any increased need for fresh additive water to the chemical recovery system M) ..§ ':. ». -.__ * \ (J 1 (LN and without any other adverse effects on the chemical recovery system in general, while achieving the purpose of achieving high-grade leaching of alkali from the fuel gas).

Brief description of the figures The invention will be explained in more detail below with the aid of figures, in which: figure 1 shows a schematic arrangement of how a gasification plant according to a preferred embodiment is constructed. figure 2 shows an alternative embodiment and figure 3 shows specific parts of the alternative embodiment.

DETAILED DESCRIPTION OF FIGURE Figure 1 shows a reactor 1 to which black liquor 2 and air or oxygen 3 are fed. Inside the reactor, the liquor is gasified sub-stoichiometrically, at a temperature suitably between 800 ° C and 1200 ° C. In a preferred case, the reactor 1 is also pressurized at least in excess of 3 bar. After the reactor 1 there is a gas cooling and green liquor preparation device 4, from which raw gas 5 and green liquor 6 are extracted.

The raw gas 5 is first fed to an H25 (sulfur sulphide) washing device 7 from which H28-purified gas 8 is extracted at the top. As the washing liquid in this HZS washing device 7, weak liquor 9 is used, which is extracted from the mesate wash 10. At the bottom, the weak liquor enriched with hydrogen sulphide, which is recycled to the cooling and green liquor preparation device 4, is extracted from the H2S washing device.

The hydrogen sulfide-purified gas is then introduced into an alkali scrubber 11, for example consisting of a scrubber. The washing liquid 12 for the alkali leaching consists of a vapor condensed "___- J CD (__: 1" à (__f \ Jb or other demineralized water. The purified fuel gas 13 is extracted at the top and passed to a fired gas turbine or other combustion process. the weakly alkaline liquid formed by the water 12 together with the washed out alkali.This weak liquor 14 is added to the mesate wash 10 for washing the mesafilter.

In the preferred case, a portion 15 (this line is thus not necessary for carrying out the invention) of this weak liquor 14 is recycled to further increase the efficiency of the alkali scrubber 11.

Figures 2 and 3 show an alternative embodiment according to the invention, which is based on the use of static mixer elements. Fig. 2 shows three such units 7, 11B, 11 connected in series, the washing liquid being carried countercurrently to the gas. Just as in the preferred embodiment, a demineralized water 12 is supplied to the last alkali wash 11, from which the final purified gas 13 is extracted. Furthermore, there is the similarity that washing liquid 14 after the alkali wash is fed to the mesate wash. In contrast to the preferred embodiment described above, it is shown that a part of the washing liquid from the alkalith wash 11 can be led to the H28 stage 7, that the alkali wash here consists of two separate units 11, 11B (which are described in detail in Fig. 3) and is shown that a pH meter / regulator 16 can be combined with an inlet line 9 for alkaline liquid, preferably weak liquor from the mesate wash, in order to optimize the H25 washing step 7.

Fig. 3 thus shows that each alkali washing stage 11 consists of static mixer elements 18 to which the gas 8 is fed and with very short contact time is intensively mixed with scrubber liquid 12 (the short contact time suppresses undesired CO 2 absorption), which liquid is passed countercurrently from a gas downstream 11 to a gas upstream scrubber unit 11B. Each scrubber unit 11 is connected to a separating part / chamber 19 into which gas and the liquid mixer are introduced in such a way that an efficient separation between gas and liquid is obtained, after which the gas is led away via a pipe unit 20 and the liquid than m). ax is collected in a container unit 21 before being passed on. A heat exchanger 22 can advantageously be arranged between two scrubber units 11. This last-described embodiment can of course also be used in other processes than the preferred one, in which case the washing liquid is thus not fed to the mesate wash.

The invention is not limited by the above shown but can be varied within the scope of the appended claims. Thus, for example, instead of a scrubber device, two such can be used.

The water supplied to the scrubber 12 can advantageously be obtained from the factory's feed water treatment plant or the steam system condensate tank. Instead of an integrated reactor 1 and gas cooling device 4 as schematically shown, an integrated unit can of course be used. Alternative washing equipment for efficient contact between gas and liquid can be a venturi scrubber, some form of bottom-filled column, filler body column, nozzle scrubber or a so-called static mixer. Furthermore, it is obvious to the person skilled in the art that the invention can be practiced in connection with gasification processes other than the preferred CHEMRECR method, for example in connection with fluidized bed.

Claims (5)

fi fïf Ef-C * z-.f (J L; PATENTKRAV f A method for purifying gas produced in black liquor gasification, preferably in a CHEMREC® gasification reactor, wherein the main part of alkali is separated from the gas stream in a first separation device (4), after which the purified raw gas is fed to an H2S scrubber (7 ) and finally fed to an alkali scrubber (11) before it has a sufficiently low alkali content to be used as a fuel in a gas turbine characterized in that the alkali scrubber (11) is fed to a steam condensate (13) or otherwise demineralized water, and that the liquid (14) taken from the alkali washing device (11) is supplied to the mesate wash (10) and possibly also a slightly upstream washing stage than said alkali washing device (11). 2. A method of purification according to claim 1, characterized in that said washing liquid (14) is only supplied to the mesate wash (10) and that all fresh water / condensate (12) to the plant is supplied to the alkali washing device (11). 3. A method according to claim 1 or 2, characterized in that the weak liquor (9) obtained from the mesate wash (10) is returned to the H 5 washing device (7). 2 4. A method according to claim 1, characterized in that the washing liquid (14) withdrawn from the alkali washing device (11) is partially recirculated (15) before use in a first washing step of several inside the alkali washing device (11). * N Fx * -.] 'IJ r% l J. i. F \ 5. A method according to claim 1, characterized in that the washing liquid is passed in countercurrent in relation to the gas from one (11; 11B) to another washing unit (11B; 7).