Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C11/00—Regeneration of pulp liquors or effluent waste waters
  • D21C11/10—Concentrating spent liquor by evaporation
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C11/00—Regeneration of pulp liquors or effluent waste waters
  • D21C11/0064—Aspects concerning the production and the treatment of green and white liquors, e.g. causticizing green liquor
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C11/00—Regeneration of pulp liquors or effluent waste waters
  • D21C11/12—Combustion of pulp liquors
  • D21C11/125—Decomposition of the pulp liquors in reducing atmosphere or in the absence of oxidants, i.e. gasification or pyrolysis
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P40/00—Technologies relating to the processing of minerals
  • Y02P40/40—Production or processing of lime, e.g. limestone regeneration of lime in pulp and sugar mills

Definitions

• 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.
• a spent liquor commonly termed black liquor
• black liquor 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.
• Tomlinson process has for a long time been the commercially dominant method for effecting this recovery of energy and chemicals.
• 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) .
• CO carbon monoxide
• C0 2 carbon dioxide
• CH 4 methane
• H 2 hydrogen
• H 2 S hydrogen sulphide
• 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.
• 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.
• 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.
• the partial pressure of H 2 0(g) in the equilibrium reaction 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
• a black liquor 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 ⁇ quizd water decreases.
• the partial pressures of the gases in reaction (b) behave in relation to each other in accordance with
• 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.
• a decreased proportion of H 2 S in the gas leads to a decreased con ⁇ sumption of lime in the subsequent causticization stage.
• 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.
• the consumption of quicklime (CaO) in the causticization 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.
• 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:
• 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.
• 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.
• 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.
• 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. )
• 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
• 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 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.
• auxiliary fuel for example in the form of oil or recirculated pyrolysis gas.
• 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.
• 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.

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Citations (2)

• 1996
  • 1996-03-22 SE SE9601099A patent/SE9601099L/sv not_active Application Discontinuation
• 1997
  • 1997-03-19 EP EP97914715A patent/EP0888476A1/en not_active Withdrawn
  • 1997-03-19 BR BR9708319A patent/BR9708319A/pt not_active Application Discontinuation
  • 1997-03-19 WO PCT/SE1997/000452 patent/WO1997036043A1/en not_active Application Discontinuation
  • 1997-03-19 JP JP9534312A patent/JP2000507318A/ja active Pending
  • 1997-03-19 CA CA002247815A patent/CA2247815A1/en not_active Abandoned

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