FI115848B - A process for utilizing unreacted carbon in black liquor gasification - Google Patents

Description

1115848

METHOD FOR UTILIZING CARBON LEAVES IN THE GASEOUS

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Explanation

Technical field

The method relates to black liquor gasification technology. Black liquor is a waste liquor that is produced when cellulose is made by an alkaline cooking process. In addition to the cooking chemicals used, about half of the organic material containing the raw material has been dissolved in the black liquor.

15 Previous Knowledge

Gasification is a well-known method of utilizing black liquor (e.g., Komulainen, et al., VTT Releases 1542, State Technical Research Center, 1994). The gasification process produces fuel gas from the black liquor organic matter, while at the same time bringing the inorganic compounds 20 into forms so that they can be reused as cooking chemicals after causticization treatment.

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There are two operating ranges for the black liquor gasification temperature:; \ * · High temperature; > 900 ° C; inorganic salts melting * * * · ',; 25 · low temperature; < 700 ° C; the inorganic salts remain solid.

Especially when the gasification temperature is low, one of the biggest technical challenges is to complete the gasification reaction. The gasification consists of two steps: (1) pyrolysis, i.e. thermal digestion, and (2) carbon contained in the carbon residue ("coke") formed by pyrolysis * · * ·! ' Gasification by reactions with CO 2 and H 2 O. At low temperatures, the latter phase is relatively slow, with many times a small fraction of the carbon is degassed. It has been suggested * ... * mm that unreacted carbon can be used as fuel in a pulp mill power boiler.

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It is possible to separate unreacted carbon from inorganic salts after they have been dissolved in water. The solution is called green liquor. Insoluble carbon can be separated from the green carbon by e.g. filtration. The separated carbon should still be thoroughly washed with water if it is to be used in a pulp mill boiler or other conventional furnace not specifically designed for high alkali fuels. The necessary effective washing process consists of several washing-filtration steps. A separate dewatering step may be required at the end to reduce the water content of the carbonaceous material for combustion. In addition to being a relatively expensive operation, such a carbon separation-scrubbing process also has negative effects on the pulp mill's water balance. The washing process requires relatively clean water 10 and produces a washing solution for treatment. Recovery of salts contained in the washing solution would be desirable, but adding the washing solution to the chemical cycle may increase the need for evaporation.

15 Description of the Invention

The novel process object of the invention utilizes unreacted carbon as fuel for the calcination reactor. In the conventional recovery process, the calcination reactor is usually a rotary drum furnace, or so-called. lime kiln. The essential feature of the invention is that the separation and washing of the carbon takes place in connection with the separation and washing of the lime sludge (CaCOa). This avoids additional equipment and the water balance of the pulp mill is not substantially altered.

The major advantages of this new process are: • 25 (1) unreacted carbon to a significant extent substitutes 0 and 1 1 r i I t of the conventional fuel for the calcination process.

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',, (2) the process differs from other unreacted coal recovery options in that it'; 'The application does not require any additional equipment and does not adversely affect the water balance of the pulp mill.' · 1 30.

1 The first benefit is particularly significant when the fuel in question is a fossil fuel purchase: ''; fuel - as usual. In the case of the black liquor gasification recovery process, the 115848 causticisation requirements and the resulting calcination reactor fuel requirements are generally higher than in the conventional recovery process. In addition, if only fossil fuel were used in the calcination reactor, the application of black liquor gasification would lead to an increase in the CO 2 emissions at the calcination process. In any event, even if this increase were clearly less than 5 compared to the reduction in CC2 emissions that black liquor gasification would otherwise produce, replacing fossil fuel with unreacted coal would be apparently beneficial to overall CCFr emissions.

In its simplest form, the new process is implemented so that the unreacted carbon passes through the causticization step among the green / white liquor - i.e. in the same way as the insoluble lime particles pass through the causticizing step. The unreacted carbon and the lime (CaCO3) are separated from the white liquor and washed together. This embodiment is illustrated in more detail by the example shown in the attached figure. The black liquor (stream 7) is gasified in step 1. Optionally, particles are separated from the gas (stream 8) in step 2. Thereafter, the gas 15 (stream 9) is led to a cooling / purification line. In gasification, in addition to gas, a mixture of inorganic compounds and possibly unreacted carbon is formed. The inorganic salts may exist as either a solid or a liquid melt, depending on the gasification temperature. The said salt-carbon mixture is removed from the gasification line either from the gasification step 1 (stream 10), the separation step 2 (stream 11), or both, after which the said mixture is led to a combined leaching and lime slagging step 3. To this step. thus, in addition to the solvent (such as lime white liquor, stream 12), also the causticizing agent is added. amount of calcined limestone (CaO, stream 13) from the lime cycle. In process step 3, the alkali salts dissolve, the burnt lime goes out and the causticization reaction begins. Current leaving phase 3,. (stream 14) is a slurry consisting of partially causticized green liquor, limestone and '25 carbon particles. After the causticization step 4, the slurry (stream 15) has changed in the following ways: (1) the lye is a white liquor and (2) the limestone particles are mainly composed of limestone (Ca-CO3). In step 5, the solid limestone and carbon particles are separated from the white liquor and washed together in one. From step 3, white liquor (stream 16) goes to the pulp cooking stage and wet lime »* '·. * vi-carbon blend (stream 17) for calcining step 6. Limestone burned in calcination reactor: · (CaO, stream 13) is produced from limestone, CaCO3. A significant portion of the heat required in process step 6 is> 1 * 4,115,848 by combustion of unreacted carbon under these oxidizing conditions. The rest comes from external support fuel.

The novel process works advantageously when the amount of unreacted carbon represents a significant portion of the energy requirement of the calcining step, but does not exceed that need. If the demand is exceeded, either the carbon must be decalcified or the heat recovered from the flue gas of the calcination reactor. Both measures would increase costs. However, it seems that the amount of unreacted carbon will usually be suitably foggy. For example, 10% of black-backed carbon would often be sufficient to cover the total energy requirement of a conventional lime kiln. By-10, (1) the gasification process aims to keep the amount of unreacted carbon well below 10%, and (2) as already mentioned, the black liquor gasification recovery process generally requires higher causticisation and associated calcination reactor fuel requirements than conventional recovery.

15 The calcination reaction may be accelerated by the fact that some of the fuel is evenly distributed among the lime mud. Accelerating the reaction would result in an increase in the capacity of the calcination reactor. This can be quite valuable when applying black liquor gasification to an existing pulp mill. Namely, the already mentioned higher causticization need due to the application of gasification would not necessarily require a larger calcining reactor.

. The new process does not produce the same soda precipitate upon leaching as. , in the traditional process because the precipitate contains significantly more unreacted carbon.

Ϊ1 \ 'Thus, in the new method, the precipitate in question cannot be removed as such because \ ,! 25 the coal it contains must be recovered. It should be noted that the traditional

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The amount of authorized compounds is expected to be lower in the low temperature gasification process than in the conventional high temperature gasification process. The lime slime • * * .. can still be removed from the leaching step. lime sand because the carbon particles are very lean - like most lime particles. In the new process, some of the insoluble compounds belonging to the traditional soda-30 precipitate are expected to accumulate in the lime cycle. As a consequence, it may be necessary to remove some material from the lime cycle in order to keep the amount of impurities in the material sufficiently low.

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The following describes some other implementation options or variations.

Another variation of the process shown in the figure is one in which the leaching and lime quenching steps are separate.

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The new process can be implemented so that, unlike the figure below, most of the unreacted carbon does not pass through the causticization step. The carbon is separated from the green liquor after the leaching step - for example by means of a clarifier. This requires that the leaching and lime quenching steps be separate. The separated carbon is not washed but added back to the white liquor-lime-10 mixture prior to the filtration and washing steps of the lime. After that, the implementation is the same as in the picture below.

The novel process can be applied to any calcination reactor. In the conventional recovery process, the calcination reactor is usually a rotary drum furnace, or so-called. honey-15 sauna. Fluidized bed furnaces have also been applied. When the recovery process is based on black liquor gasification, there is an opportunity to integrate the calcination process into the gasification process. For example, according to Finnish patent FI81140, the calcination reactor can be located in a gas line downstream of the gasifier, whereby H2S can be recovered at the same time. Thus, the novel process disclosed herein can be applied to such a calcining process as well.

The above applications and modifications are examples which illustrate the invention without limiting it in any way. 1 * »• I ·

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Claims (5)

A process for utilizing koi that do not react in gasification of liquor (so-called black liquor) from alkaline cellulose processes, characterized (1) that the unreacted carbon is finally separated from soluble salts from the black liquor only after the causticization step, (2) and washed so that koi and mesa are separated and washed together, and (3) by feeding the washed mesa-carbon mixture into a calcination reactor, where the carbon acts as a fuel. Process according to claim 1, characterized in that the non-reacting carbon passes the causticization step among the green / white liquor. The process according to claim 1, characterized in (1) that the non-reacting carbon is separated from the green liquor after the dissolution step and (2) that this unwashed cow is returned to the white liquor mesa mixture before the final separation and washing steps. Method according to claim 2, characterized in that the steps for dissolution and lime quenching have been combined. Process according to Claim 1, characterized in that said calcination reactor is a traditional lime kiln.