PROCESS FOR EXTRACTING CHEMICALS AND ENERGY FROM CELLULOSE SPENT LIQUOR

Abstract: Process for extracting chemicals and energy from black liquor which is obtained during the production of paper pulp by means of the chemical digestion of fibre raw material, in which the black liquor is gasified in a reactor at a pressure exceeding 5 bar, with the products formed principally consisting of CO, CO2, CH4, H2 and H2S in gas form and Na2CO3, NaOH and Na2S in the form of drops of smelt, and the resulting mixture of gas and smelt is cooled by means of direct contact with a water-containing liquid, in association with which the smelt drops are separated off and dissolved with the formation of green liquor, after which the gas is separated off for energy extraction. In the process, the said black liquor is supplied to the reactor at a dry substance content which exceeds 80 %. A major portion of the green liquor which is formed is causticized with an optimized, low consumption of lime.

Description: Title: Process for extracting chemicals and energy from cellulose spent liquor. TECHNICAL FIELD The present invention relates to a process for extracting chemicals and energy from black liquor which is obtained during the production of paper pulp by means of the chemical digestion of fibre raw material. STATE OF THE ART AND PROBLEM When paper pulp is being produced by the sulphate method, a spent liquor, commonly termed black liquor, is obtained which contains organic material and the residual chemicals which have been obtained during the cooking of the fibre raw material. This black liquor is generally evaporated and conveyed to a separate process for extracting the energy content of the organic material and recovering the cooking chemicals as so-called green liquor. The so-called Tomlinson process has for a long time been the commercially dominant method for effecting this recovery of energy and chemicals. However, a disadvantage of this process, which is now very old, is that it requires combustion ovens which are very large and complicated both technically and with regard to their operation. Swedish Patent SE 448 173 describes a more recent process which, apart from substantial simplifi¬ cation of the requisite process equipment also achieves an improved extraction of both energy and chemicals. This process is based on a pyrolysis reaction in which the black liquor, in the understoichio etric presence of oxygen, is gasified in a reactor, with an energy- rich gas being formed which principally comprises carbon monoxide (CO) , carbon dioxide (C0 2 ) , methane (CH 4 ), hydrogen (H 2 ) and hydrogen sulphide (H 2 S) as well as inorganic chemicals in the form of small drops of smelt, principally comprising sodium carbonate (Na 2 C0 3 ) , sodium hydroxide (NaOH) and sodium sulphide (Na 2 S) . The resulting mixture of gas and smelt drops is rapidly cooled, m a first stage, by direct contact with a cooling liquid consisting of water and green liquor, which latter is formed when the molten chemicals and the hydrogen sulphide are dissolved in the cooling liquid. The gas is subsequently washed, in a second stage, in a gas wash of the scrubber type. The gas is then used as a fuel for generating steam and/or elec¬ trical power, preferably employing a gas turbine. The physical calorific value of the gas can also be utilized when the gas is cooled from the gasification temperature to the saturation temperature for aqueous steam at selected pressure. For example, at the saturation temperature of 200°C, corresponding to 30 bar, steam having a pressure of 3-8 bar can be generated when the green liquor is cooled and when the gas is cooled and its water content is condensed downstream of the gas wash tower. However, this process also shows room for improvement, despite the fact that it is considerably simpler and more flexible than the To linson process. When black liquor is gasified by the known technique, the sulphur content of the black liquor is converted, as has been mentioned above, on the one hand to Na 2 S, which, together with Na 2 C0 3 and NaOH, forms smelt which dissolves in the cooling liquid, and, on the other hand, to H 2 S, which leaves the reactor in the gas phase. The distribution of sulphur between Na 2 S and H 2 S is heavily dependent on the gasification pressure. During atmospheric gasification at approx. 950°C, approx. 90% of the sulphur in the black liquor is con¬ verted to Na 2 S in the smelt and only approx. 10% forms H 2 S. If the gasification pressure is increased to approx. 25 bar, which is an expedient pressure if the gas which is produced is to be combusted in a gas turbine, only approx. 40-50% of the sulphur in the black liquor is converted, at the same temperature, to Na 2 S in the smelt, which dissolves to form green liquor, while 50-60% forms H 2 S in the gas phase. As a rule, the sulphur which ends up in the gas phase in the form of H 2 S has to be returned to the green liquor. When H 2 S is absorbed in green liquor, some of the alkali (carbonate) which is present in the green liquor is required for the reaction Na 2 C0 3 + H 2 S <r- NaHC0 3 + NaHS (a) It is a disadvantage that formed Na 2 C0 3 is converted to NaHC0 3 when H 2 S is absorbed. Conventionally, Na 2 C0 3 and NaHC0 3 in green liquor have to be causticized to NaOH with slaked lime in a subsequent causticization stage. In this connection, twice as much lime is needed for causticizing NaHC0 3 as for causticizing Na 2 C0 3 . Reaction (a) above thus leads to an increased requirement for lime in the causticization and consequently also to an increased load on the lime sludge reburning procedure, in which CaC0 3 which has been formed during the causticization is burnt to CaO, which is reused. In accordance with the above reasoning, an increased pressure in the gasification reactor gives rise to an increased formation of H 2 S in the gas phase, something which leads to conversion of carbonate to bicarbonate when the gas is absorbed in the green liquor, leading in turn to an increased requirement for lime in the causticization. By contrast, an increase in the reactor tem¬ perature gives rise to a decrease in the formation of H 2 S in the gas phase. Other types of processes for gasifying black liquor have also been disclosed. A characteristic shared by these processes is that the black liquor is introduced as an aqueous suspension having dry sub¬ stance contents which are typically about 75% at most. SOLUTION AND ADVANTAGES The present invention is a further development of the concept presented in SE 448 173 and effectively eliminates a disadvantage associated with this known technique. The idea of the method which has been devised is to bring about the possibility of producing green liquor by the understoichiometric gasification of black liquor in a reactor having an elevated pressure, with sulphur which is present in the black liquor as far as possible forming the reaction product Na 2 S in the smelt and with formation of H 2 S in the gas phase being suppressed. This is brought about by a process according to Patent Claim 1, As the dry substance content of the black liquor is increased to a value exceeding 80%, preferably exceeding 90% and even more preferably exceeding 95%, the partial pressure of H 2 0(g) in the equilibrium reaction Na 2 S(l) + H 2 0(g) + C0 2 (g) <- Na 2 C0 3 (l) + H 2 S(g) (b) is decreased. In the most preferred embodiment, a black liquor is used which has a dry substance content of 100 or almost 100%. Carbon dioxide is formed when carbon monoxide is combusted, with oxygen or oxygen-containing gas which is introduced into the reactor, for the purpose of vaporizing the water in the black liquor which has been introduced into the reactor. Consequently, a decreased quantity of introduced water results in less carbon monoxide having to be combusted to carbon dioxide for the purpose of vaporizing water, for which reason the partial pressure of carbon dioxide in the gas phase also decreases when the quantity of intro¬ duced water decreases. The partial pressures of the gases in reaction (b) behave in relation to each other in accordance with This implies that a decreased partial pressure of H 2 0(g) and C0 2 (g) due to a decreased quantity of water being introduced together with the black liquor leads to a decreased partial pressure of H 2 S(g) in the gas. Sulphur which is present in the black liquor instead finishes up, to a greater extent, as the reaction product Na 2 S in the smelt. The total pressure in the reactor is maintained at the same high level by means of an increased partial pressure of H 2 (g) and CO(g) . It is preferred for sulphur which is present in the black liquor, at a gasification temperature of 950°C, to be caused to partition between formed smelt and gas in a ratio exceeding 1.3:1 (molar ratio), preferably exceeding 2.5:1 and even more preferably exceeding 3.5:1. According to previous reasoning, a decreased proportion of H 2 S in the gas leads to a decreased con¬ sumption of lime in the subsequent causticization stage. In this context, it is preferred, according to the invention, for the consumption of quicklime (CaO) in the causticization to be decreased by at least 3% for each 5% increase in the dry substance content of the black liquor, preferably at least 5% and even more preferably at least 8%, at a pressure exceeding 10 bar and a temperature of about 950°C in the gasification reactor. It is also preferred for the consumption of quicklime (CaO) in the causticization to be less than 100 kg/m 3 of green liquor, preferably to be less than 90 kg/m 3 and even more preferably to be less than 85 kg/m 3 , at a pressure exceeding 10 bar and a tem¬ perature of about 950°C in the gasification reactor. These consumption figures apply on condition that no carbon dioxide is absorbed in the green liquor. Thus, carbon dioxide absorption also leads to increased consumption of lime since each mole of absorbed carbon dioxide consumes two mol of NaOH in accordance with the reaction: 2 NaOH + C0 2 → Na 2 C0 3 + H 2 0 (c) Another advantage of the invention is, there¬ fore, that the production of carbon dioxide, in conformity with previous reasoning, decreases as the quantity of water supplied to the reactor decreases. It is indeed known, through R. Backman et al., Basic studies on black-liquor pyrolysis and char gasification, Bioresource Technology 46 (1993), 153-158 to gasify a dry black liquor. The paper describes experiments in which dry black liquor has been gasified, with the addition of aqueous steam, in a flu dized bed. The pressure-dependent partitioning of sulphur between gas and smelt s also touched upon. However, the authors have not drawn any conclusions regarding the advantages of gasifying a dry black liquor but, on the contrary, mention that it is possible (and evidently regarded as being desirable) to achieve almost complete conversion of sulphur to gas form when aqueous steam is present. The authors do not deal either with the problem regarding bicarbonate formation when H 2 S is absorbed in green liquor and make no mention whatsoever of the subsequent causticization and the lime consumption which is required m this connection. According to the invention, oxygen or an oxygen-containing gas is supplied to the reactor, with the quantity of supplied 0 2 being less than 300 m 3 N/ton of dry substance, preferably being less than 280 m 3 N/ton and even more preferably being less than 260 m 3 N/ton, at a pressure exceeding 10 bar and at a temperature of about 950°C m the gasification reactor and also a sto chiometric oxygen factor exceeding 0.3. The black liquor, which, m the most preferred embodiment, has been brought to 100 or almost 100% dry substance content by, for example, spray drying, is expediently supplied m finely divided form to the brick-lmed reactor with the aid of a pneumatic feeding system. Apart from the abovementioned positive effect on the lime consumption, another advantage of the process according to the invention is that a higher proportion of recovered energy in the form of fuel energy can be utilized in both gas turbine and steam cycle, with a higher yield of electricity being achieved. Another advantage is that the extracted gas has a higher calorific value, which places less demand on the combustion chamber of the gas turbine. Another great advantage is that the process according to the invention results in lower gas volumes, so that a more compact and cheaper apparatus can be employed. EXAMPLE The table below shows how sulphur which is present in the black liquor partitions between gas (H 2 S and a relatively small quantity of COS) and smelt (Na 2 S) in association with differing dry substance contents in the black liquor which has been introduced into the reactor. The table also indicates the decrease in the requirement for quicklime (%) , apart from the effect of any absorption of carbon dioxide in the green liquor, in the subsequent causticization stage due to each 5% increment in the dry substance content from 75%. (One mol of CaO is consumed for each mol of H 2 S. ) In addition, the table indicates the quantity of air which is required for the gasification. Conditions which apply in all cases are: Black liquor flow (dry substance) 15 ton/h Sulphur content in the black liquor 5.5% by weight Temperature 950°C Pressure 25 bar Carbon conversion virtually complete Dry substance content in the 75% 80% 85% 90% 95% 100 % black liquor S in the gas phase (mol/ g 0.83 0.72 0.60 0.48 0.35 0.27 of dry substance) S in the smelt (mol/kg of 0.88 1.00 1.12 1.24 1.36 1.45 dry substance) percentage fraction of S in 0.51 0.58 0.65 0.72 0.79 0.84 the smelt Decrease in lime (%) 0 4.4 5.0 5.0 5.5 4.5 Air (m 3 N/h) x 10 "3 21.5 20.5 19.6 18.9 18.3 17.8 The table accordingly shows that an increase in the dry substance content of the black liquor from 75% to 100% leads to a 65% increase in the fraction of sulphur which finishes up in the smelt. At the same time, the consumption of air decreases by 17%, with the stoichiometric oxygen factor exceeding 0.3, that is exceeding 30% of the theoretical consumption for complete combustion. Figures 1 and 2 show diagrams of how the fuel energy in the gas and the calorific value of the gas vary with dry substance contents between 75 and 100% under the conditions which pertain in the example. As can be seen, an increase in the dry substance content has a positive effect on these parameters. The values given in Figures 3 and 4 are in agreement with those given in the above table except that Figure 3 shows the fraction of the sulphur in the gas instead of in the smelt. The embodiment in accordance with the above description is a preferred embodiment. However, the invention is not limited to this description and can be varied within the scope of the patent claims. The gasification temperature in the reactor can be 800 - 1100°C, preferably 850 - 1050°C and more preferably 900 - 1000°C, and the system pressure expediently exceeds 10 bar absolute pressure, preferably exceeds 20 bar and even more preferably exceeds 23 bar. The brick-lined reactor can be provided with built-in cooling loops, thereby enabling the wall temperature to be regulated in such a way that a protective layer of solidified smelt is formed on the brick wall. During the gasification, it can be advantageous to use an auxiliary fuel, for example in the form of oil or recirculated pyrolysis gas. Naturally, the concept of the invention can also be applied to chemical recovery in processes which use other types of spent liquors, for example chlorine- free bleaching department spent liquors, spent liquors from the production of semi-chemical pulp (for example CTMP) or spent liquors from a pulp process which is based on potassium as the base rather than sodium.

Claims: PATENT CLAIMS 1. Process for extracting chemicals and energy from black liquor which is obtained during the production of paper pulp by means of the chemical digestion of fibre raw material, in which the black liquor is gasified in a reactor at a pressure exceeding 5 bar, with the products formed principally consisting of CO, C0 2 , CH , H 2 and H 2 S in gas form and Na 2 C0 3 , NaOH and Na 2 S in the form of drops of smelt, and the resulting mixture of gas and smelt is cooled by means of direct contact with a water-containing liquid, in association with which the smelt drops are separated off and dissolved with the formation of green liquor, after which the gas is separated off for energy extraction, c h a r a c t e r i z e d i n that the said black liquor is supplied to the reactor at a dry substance content exceeding 80% and in that a major portion of the green liquor which is formed is causticized with an optimized, low consumption of lime. 2. Process according to Patent Claim 1, c h a r a c t e r i z e d i n that the said black liquor is supplied to the reactor at a dry substance content exceeding 90%, preferably exceeding 95%. 3. Process according to Patent claim 1, c h a r a c t e r i z e d i n that the said black liquor is supplied to the reactor at a dry substance content of 100% or almost 100%. 4. Process according to any one of the preceding patent claims, c h a r a c t e r i z e d i n that sulphur which is present in the black liquor partitions between formed smelt and gas in a ratio exceeding 1.3:1 (molar ratio), preferably exceeding 2.5:1 and even more preferably exceeding 3.5:1. 5. Process according to any one of the preceding patent claims, c h a r a c t e r i z e d i n that the reactor temperature is utilized for the purpose of guiding the minimization of the lime consumption in the causticiza¬ tion, in association with which the reactor temperature exceeds 800°C, preferably exceeds 850°C and even more preferably exceeds 900°C. 6. Process according to any one of the preceding patent claims, c h a r a c t e r i z e d i n that the consumption of quicklime (CaO) in the causticization is decreased by at least 3% for each 5% increase in the dry substance content of the black liquor, preferably at least 5% and even more preferably at least 8%, at a pressure exceed¬ ing 10 bar and at a temperature of about 950°C in the gasification reactor. 7. Process according to any one of the preceding patent claims, c h a r a c t e r i z e d i n that the consumption of quicklime (CaO) in the causticization, apart from the effect of any absorption of carbon dioxide m the green liquor, is less than 100 kg/m 3 of green liquor, is preferably less than 90 kg/m 3 and even more preferably less than 85 kg/m 3 , at a pressure exceeding 10 bar and at a temperature of about 950°C in the gasification reactor. 8. Process according to any one of the preceding patent claims, c h a r a c t e r i z e d i n that oxygen or oxygen- containmg gas s supplied to the reactor, with the quantity of supplied 0 2 being less than 300 m J N/ton of dry substance, preferably being less than 280 πv ton and even more preferably being less than 260 πv ton, at a pressure exceeding 10 bar and at a temperature of about 950°C in the gasification reactor and also a stoichiometric oxygen factor exceeding 0.3. 9. Process according to any one of the preceding patent claims, c h a r a c t e r i z e d i n that the system pressure exceeds 10 bar absolute pressure, preferably exceeds 20 bar and even more preferably exceeds 23 bar.

Inventors: KIGNELL JAN-ERIK, KIGNELL, JAN-ERIK
Applicants: KIGNELL JAN ERIK, KVAERNER PULPING TECH, KIGNELL, JAN-ERIK, KVAERNER PULPING AB
Application year: 1997
Main classification: D21C11
IPC codes: D21C11