CA1197729A - Method for wet combustion of organic material - Google Patents
Method for wet combustion of organic materialInfo
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- CA1197729A CA1197729A CA000405572A CA405572A CA1197729A CA 1197729 A CA1197729 A CA 1197729A CA 000405572 A CA000405572 A CA 000405572A CA 405572 A CA405572 A CA 405572A CA 1197729 A CA1197729 A CA 1197729A
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Abstract
ABSTRACT OF THE DISCLOSURE
A method of combusting an aqueous solution contain-ing organic material, according to the wet combustion process.
The organic material is oxidized by introducing molecular oxygen under elevated temperature and correspondingly elevated pressure in two steps. In the first step, the organic mate-rial is oxidized so as to release an amount ranging between 75% and 95% of the total heat of combustion of the organic material. In the second step, the residual organic material is finally oxidized by the introduction of an excess amount of molecular oxygen sufficient to impart to the gaseous ef-fluent generated during the second step a content of molecu-lar oxygen sufficient to achieve the oxidation of the organic material of the first step.
A method of combusting an aqueous solution contain-ing organic material, according to the wet combustion process.
The organic material is oxidized by introducing molecular oxygen under elevated temperature and correspondingly elevated pressure in two steps. In the first step, the organic mate-rial is oxidized so as to release an amount ranging between 75% and 95% of the total heat of combustion of the organic material. In the second step, the residual organic material is finally oxidized by the introduction of an excess amount of molecular oxygen sufficient to impart to the gaseous ef-fluent generated during the second step a content of molecu-lar oxygen sufficient to achieve the oxidation of the organic material of the first step.
Description
'7'7~
The presen-t invention rela-tes to a method for wet combustion of organic material.
More particularly, the invention relates to a method o~ directly wet combusting an aqueous solution con-tain-ing dissolved or finely dispersed organic material by molecu-lar oxygen at elevatecl pressure and at high temperature.
It is known to totally or partially combust organic material dissolved or finely suspended in water by molecular oxygen or gases containing molecular oxygen, such as air, under pressure and at elevated temperature, which temperature depending on the degree of combustion and the type of the organic substance, should be in the range of 180 to 340C.
The process is suitably carried out continuously, and the combustion can be effected using both concurrent flow and counter-current flow with an almost complete use of the mole-cular oxygen. When using air in the combustion of e.y. ligno-cellulose-containing biological substances, such as wood, peat, and bagasse, or waste liquors obtained by the acid or alkaline pulp digestion of biological substances, the exhaust combustion gases seldom contain more tharl 0.2% of molecular oxygen. If, nevertheless, an almost complete com-bustion of the organic material is to be obtained, -the combus-tion temperature usually must exceed 300C, e.g. be between 300 and 340C.
Due to the continuously reclining content of oxyyen during the combustion process, -there occurs a corresponding reduction of the complete oxidation of the organic material into carbon dixoide and water. This contributes during the combustion process, especially in the combustion of lignocel-lulosic ma-terial, to the formation of difficult oxidizable compounds which are mainly low molecular weiyht acids, such as acetic acid, proprionic acid, or salts thereof. This ~' t7';~
is independent of whether the combustion takes place in con-current or counter-curren-t flow.
In the wet combustion process, the incoming liquid may lose volatile combustible material by expulsion with the exhaust mixture of steam and gas formed by the combustion, irrespective whether the combustion takes place in counter-current and concurrent flow. Volatile products can be present in the incoming liquid and also be formed during the combus-tion. Due to the low concentration of molecular oxygen in exhaust flue gas in -the final stage of the combustion, there is the risk that the volatile produc-ts remain unoxidized and either remain in the solution as e.g. salts, or are en-trained in the s-team generated.
However, experiments have shown that excess molecu~
lar oxygen at high concentrations, such as in air, of from 20 to 50%, facilitates the combustion of the difficult oxi-dizable compounds which, when they result from combustion of biological substances or products thereof, normally consist of low molecular weight fatty acids~ primarily acetic acid.
Where the incoming liquid is alkaline and -the formed acids are bound in the form of salts, generally the same problem exists, viz. the decomposition of the acids into carbon dioxide and water, as in the case of free acids.
A great advantage with the combustion in alkaline solution is, however, tha-t the generated steam is free from acidity, which facilitates its use for heating and power generating purposes and simplifies -the selec-tion of suitable construc-tion material for these purposes.
It is further known from experience tha-t the combus-tion of lignocellulosic biological substances or products thereof can be effected under relatively moderate ternperature conditions, between 180 and 300C when the generated heat p~
is restricted to between 75 and 90~ of the calorific value of the organic material, but that higher te~peratures are required to release the last 5 to 10% of the calorific value, and where this organic material is cons-ti.tuted by low molecu-lar weight acids, the combustion -temperature must substantial-ly exceed 300C. From experiments with wet combustion of alkaline waste liquor from digestion of wood using pure sodium hydroxide solution, and from the resul.ts to be referred to below, it becomes evident that use of a surplus and high concentration of oxygen gas in the final stage of the combus-tion process facilitates the decomposition of the difficultly oxidizable compounds into carbon dioxide and water.
There was combusted, for example, a waste liquor obtained by digestion of pine-wood using 220 g NaOH and 2 g of anthraquinone per kilogram wood calculated as bone dry substance at a temperature of 170C into a pulp yield of 47.8%. The waste liquor has a dry solids content of 14.7o with a calorific value of 3,762 Cal per kg and contained 24.6% of Na2O calculated as bone dry substanceO In -the com-bustion of this waste liquor in an autoclave while using air with an initial pressure of 3,800 kPa at 20C, resulting in a partial pressure of the oxygen gas of 800 kPa at 20 C, 83% of the calorific value of the waste liquor were released at a temperature of 275C, the partial pressure of the oxygen gas thereunder falling to 400 kPa. Thereupon, the temperature was raised to 300C, whereby additional heat was released so that, calculated on the original waste liquor, 90% or the calorific value was released~ Then the partial pressure of the oxygen gas dropp^ed to 250 kPa.
In a similar experiment, the combustion was started with air having the same par-tial pressure of the oxygen gas of 800 kPa as in the preceding experimen-t~ Then, 8~% of the calorific value of the waste liquor was released when ~ ~''J'7'~
a temperature of 275C had been reached. Pure oxygen gas was then supplied and the temperature was raised to 300C, when altogether 96% of the calorific value of the waste liquor was released. Then the partial pressure of the oxygen gas was 500 kPa calculated at 20C and represents double surplus of oxygen gas in the final stage as in the previous experi-ment, in which the total combustion process until a final temperature of 300C was carried out with that quantity of molecular oxygen which was present in the air initially sup-plied.
To reach a high degree of combustion in the combus-tion of the lignocellulosic biological substance withou-t resorting to extraordinary conditions of temperature subs~an-tially exceeding 300C, such as e.g. to 340C, the combustion in the final stage must be effected with a great surplus of molecular oxygen, and so as at the same time to limit the consumption of oxygen for all the organic material present in the waste liquor~ the combustion must be effected in two separate steps. In the first step the combustion of the incoming liquid containing organic substance is carried to such degree that between 75 and 95% of the calorific value is released, which can be effected with small excesses or molecular oxygen. In the second step the remaining organic substance is combusted with a great surplus of molecular oxygen in such a manner than steam and gas effluent from this second step can be fed to the first step with a content of oxygen gas adjusted so that the combustion in this step can be effected to the aforesaid degree of 75 to 95%. If desired, an extra addition of molecular oxygen may be supplied to the effluent steam and gas so that the stated combustion degree is reached. The yas containing the molecular oxygen incoming into the second step must be satura-ted with steam at 300C in order to avoid cooling of the liquid in the second step and thereby staying at a too low combustion temperature.
~ _ ~1~'7'~
Assuming that 10%, for example, of the combustion heat of the earlier mentioned wasteliquor is preserved after the first step and that air is used for the combustion, the surplus of molecular oxygen in the second step becomes about times greater than the theoretical requirement, if the whole quantity of air necessary for the combustion of the incoming organic substance is supplied to the second step.
Instead of air, it is, to advantage, possible to use air enriched with oxygen gas, e.g. with between 20 and 50% of 2~ or other non-reactive gases having a higher content of molecular oxygen than air. Considering solely the reaction mechanism, it is advantageous to use pure oxygen gas. However for safety reasons, it appears to be inappropriate to operate with higher oxygen contents than 30-50% of the gas entering into the combustion zone.
The compressed air enriched with oxygen can be prepared depending on local conditions eithex by mixing toge-ther air and oxygen gas at atmospheric pressure and there-upon compressing the gas mixture, or by mixing compressed air with oxygen under pressure, e.g. by vaporization of liquid molecular oxygen.
The combustion gases leaving the wet combustion plant can also be recirculated under almost the same pressure that prevails in the combustion apparatus after it has been cleared, for instance under pressure, of generated carbon dioxide and possible excess of non-reactive gases, e.g. nitro-gen, and thereupon having been supplied with an adequate quantity of oxygen gas under pressure.
Usually, the wet combustion process is carried out in concurrent flow, but it may in many cases be more suitable to carry out the combustion in the second step in a counter-current flow, which then also takes into account that the liquid fed into the second step is of a relatively "7;~3 small quantity due to -the evaporation of incoming aqueous solution which occurred due to escape of vapour during the combustion.
When e.g. waste liquor containing 18~ of dry sub-stance, of which 81~ is organic material, from a pure soda process is combusted, the waste liquor must be diluted with water so that the generated heat can be totally converted into vapour. Furthermore, water must be added for removal of the soda formed in the combustion. In this case, only 10-12% of the quantity of water entering the first step will be supplied to the second step and finally oxidized. This is done most effectively in a counter-current flow in, e.g.
a tower filled with annular elements, which increase the surface of contact between the liquid and the molecular oxygen ; containing gas, which facilitates the diffusion of the gas ~; into the liquid and thereby accelerates the combustion reac-tion. The filling material of the tower may be a material which in a catalytic manner stimulate oxidation, such as, for example, nickel, or chromium, vanadium and titanium con-taining alloyed steel, or the tower filling material may be coated with active material, e.g. platinum or nickel, precipitated on ceramic materials. It is also possible to utilize heterogeneous catalysts in the form of powder, e.g.
copper chromite, finely divided platinum which is added to the incoming liquid and after completion of the oxidation is separated or precipitated and, if desired, reactivated and recirculated. The types of catalysts depend mainly whe-ther the combustion of the incoming organic material is to be performed in an acid, neutral or alkaline environment.
According to the present invention, therefore, there is provided in the wet combustion method of combusting an aqueous solution containing organic material in which ~ ~5~'~1'7~ ~
the organic material is oxidized by introducing into the solution molecular oxygen material at a temperature ranging between 180C and 340C and a correspondingly elevated pres-sure, the improvement comprising: a) a first oxidizing step, in which the organic material in said aqueous solution is oxidized so as to release an amount of the heat of combustion ranging between 75% and 9S% thereof; b) a second oxidizing step, in which the residual organic material is finally oxidi-zed by introducing surplus molecular oxygen in an amount sufficient to impart to the gaseous effluents generated by said second step a content of molecular oxygen sufficient to achieve the oxidation of the first step; and c) a feeding step, in which said gaseous effluents are supplied to said first step. Suitably, the organic material comprises cellu-lose-containing biologic material and in which low molecular acids formed in the first step and resistant to oxidation therein are oxidized in said second step. Desirably, the molecular oxygen comprises a stream of compressed air. Pre-ferably, the oxidation of the organic material in the first step is carried out in concurrent flow with the stream of compressed air and in which the oxidation in step two is carried out in counter-current flow to -the flow of said stream of com-pressed air.
In one embodiment of the present invention, the flue gases cooled by said gaseous effluents are liberated from carbon dioxide and excess of non-condensible gases formed during the combustion process, and in which the residual non~condensi-ble gases are recirculated together with molecular oxygen to the second oxidation step.
In order to exemplify how the process may be carried out, reference is made to the following example and the accom-panying drawing, in which the single figure is a flow sheet indicating the essential equipment parts of a plant for carry-7~7~
ing out combustion oE black liquor from the production of kra~t pulp by means of sulphur-free sodium hydroxide solu-tion and recovery of soda. Since the waste liquor is alka-line, no problems arise regarding purification of the water vapour leaving the process~ Otherwise, when non-combusted volatile compounds are formed and pass with the water vapour, e.g. free acetic acid, such acid must be removed either directly from the water vapour or from the condensate formed thereform.
Referring now to the Figure, black liquor from a pulp production of 20 t/h consisting of e.g. 127,560 kg of water and 2B,000 kg of dissolved solids, of which 81%
is organic substance, is fed into vessel 1 through pipe 2.
Simultaneously, 47,823 kg of steam condensate at 40C and 6,092 kg of warm water at 151C are fed into a storage vessel 1 through pipes 3 and 4, respectively and furthermore, 13,425 kg of steam at 100C through pipe 24, so that altogether 194,900 kg of diluted black liquor at 80C is present in the vessel 1. The black liquor solution at a temperature of 80C is pumped by a pump 5 through pipe 6 into preheater 7, into which at the same time 29,026 kg of steam at 5 atmos-pheres absolute pressure is introduced through pipe 8, from steam generator 9, which steam passesthe solution from the preheater 7 to a high-pressure pump 10 at a temperature of 151C for further transport through pipe 11 to reactor vessel 12, which is under a pressure of s-team and gas of 149 atmospheres above atmospheric and which consti-tutes the first combustion stage in which 90% of the combus-tion heat of the liquor is considered to be set free. Simul-taneously, 113,000 m3 of air compressed to 150 atmospheres absolute pressure is supplied to the reactor from compressor 13. Of this quantity, o~ air, 50,000 m3 is fed through pipe '7';'~'~
14 to scrubber 15, within which the air in counter-current flow meets an aqueous solution at 310C coming from cyclone 16 and supplied through pipe 17 to the top of the scrubber and recycled from the bottom thereof by means of pump 29 into the reactor vessel 12. In the scrubber the air is saturated with steam and preheated to about 300C and suppli-ed via pipe 18 to the bottom section of reactor 19 for final oxidation. At the same time from the cyclone 16, about 50,000 kg of solution containing soda and Na-salts and having a temperature of 310C is fed to the top of the reactor 19 through pipe 20. In the final oxidation step 6,400,000 Cals are produced which generate about 20,000 kg of steam at 310C, which passes from the top of the reactor 19 together with 50,000 m3 of air containing about 2~5% CO2, and the escaping gas, since it is saturated with steam of 310C, carries along, in addition to the 20,000 kg of steam generated in the reactor 19, also about 29,000 kg of steam which the air has taken up in the scrubber 15, when being preheated by direct contact with the water having the temperature of 310C. The mi~ture of steam and gas from the reactor 19 is introduced through the pipe 11 together with 63,000 m3 of air coming from the compressor 13 via pipe 21 into the reactor vessel 12, enough of molecular oxygen thus being supplied to this reactor vessel 12 for combustion of 90% of the organic substance contained in the black liquor. At the same time, there escape from the top of the reactor vessel 12 via the cyclone~ 16,170,000 kg of steam at 310C and 156,250 kg of gas under a stearn-gas pressure of 149 atmospheres above atmospheric, i.e. 0.92 kg of gas per kg of stearn. Theoretically, a wor3cing pressure of 124 atmospheres above atrnospheric should be sufficient, but in order to ensure reliability in operation, sorne predetermined over-pressure rnust exist, and 149 atmospheres above atmospheric should guarantee that difficulties due to a fall of te}nperature in the reactor will not g 3'7'i'~
arise as a consequence of escape of a steam-gas mix-ture too rich in steam. The residual burn-out portion of the black liquor is derived from the reactor 19 through pipe 22. This residual portion amounting to 20,000 kg of water and 4,770 kg of soda, of which about 10~15% may consis~ o~ sodium acetate, are recycled to the pulp cooking equipment with the causticized liquor. The withdrawn soda solu-tion is expanded to atmospheric pressure in a cyclone 23, where 13,425 kg of steam escape through pipe 24 to the vessel 1. From the cyclone the soda solution of 100C leaves through pipe 28 and is diluted at the same time with 13,425 kg of water at 40C through pipe 25, whereby the soda solution regains a volume of 30,000 kg of water containing 4,770 kg of soda, and which while having a temperature of about 25C is fed to tank 26. From the tank 26, the warm soda solution is conveyed through plpe 27 to become causticized for further feed to a digester.
The steam and gas escaping from the reactor 12 via the cyclone 16 and consisting of 170,000 kg of steam and 156,250 kg of combustion gases are introduced into a heat exchanger 30 and cooled down under a full pressure of 149 atmospheres above atmospheric for generation of steam at 34 atmospheres above atmospheric from feed water of 151&. The steam and gas of 310C are thus cooled down to 249&, while at the same time 134,355 kq of saturated steam under a pressure of 34 atmospheres above atmospheric leave~ steam boiler 31 through pipe 32. Steam, condensate and gas of 249C from the heat exchanger 30 are fed to heat exchanger 33 through the pipe line 34 for generation of steam of 4 atmospheres above atmospheric from water at 151C. The steam ` 30 is obtained from the steam generator 9 through pipe 36 in a quantity of 41,690 kg, of which 29,026 kg are supplied to the preheater 7 via the line 8, and in this way the quan-11 1~3'~"7~
tity of steam of 4 atmospheres above atmospheric wil] amount to 12,624 kg.
From residual condensate, steam and gas are still under the pressure of 149 atmospheres above atmospheric, warm water at 151C is produced by causing steam condensate at 20C to exchange heat with condensate and gas from the heat exchanger 33 and conveyed through pipe 37 to a second heat exchanger 38 for production of warm feed water. Conden-sate and gas leaving t~e heat exchanger 38 are collected in a pressure vessel 40, within which they have a temperature of 40 C and are under a gas pressureof 149 atmospheres above atmospheric. 182,137 kg of condensate of 20C are conveyed from vessel 41 by pump 42 through pipe 39 to the heat exchan-yer 38, where the condensate is heated to 151C and conveyed further to a column 45 via pipe 44 and relieved from dissolved carbon dioxide and other gases before the feed water of 150C is supplied to the steam generator 9 and steam boiler 31 by pump 46 via pipe 49. A surplus of warm water of 151 C
amounting to 6,092 kg is conveyed through pipe 4 to the vessel 1 for dilution of the black liquor. 134~355 kg of steam subjected to a pressure of 34 atmospheres above atmospheric is fed from the boiler 31 via pipe 32 to superheater 50, where the steam is superheated to 420C and conveyed further to a reaction turbine 51, which de]ivers 15,700 kW at a back pressure of 11 atmospheres above atmospheric. Back pressure steam is drawn off from a steam accumulator 52. From the gas and condensate streaming to the pressure vessel 40 the latter is conducted through pipe 55 to a water turbine 56 driving an electric generator which delivers 480 kW. The gas still under pressure is passed through pipe 57 via a superheater 58 to an expansion machine 59 which drives an electric generator producing 27,000 kW. The superheaters ~ 11 --50 and 58 are heated by hot flue gases from the furnace 60, the quantity of heat consumed thereby corresponding to 2.7 tons of oil per hour.
In the wet combustion of black liyuor according to the preceding example for production of steam and recovery of the chemicals, the heat content of the s-team represents 92% of the calorific value of the dry substance conten-t of the liquor. For comparison may be mentioned that according to a corresponding manner of calculation for a plant with soda furnace the result is about 56%~
When considering the additional heat required for superheating the steam and non-condensable gas producing a minor surplus of power, the calories in the steam represent 74.3% of the calories in the liquor and the additional fuel, and then all power needed for operation of pumps, auxiliary machines and compressors has been produced also. The addi-tional heat which is supplied, corresponds to 0.095 Swedish Crowns per kWh, if the price for heavy oil is assumed to be Swedish Crowns l,000 per ton.
The presen-t invention rela-tes to a method for wet combustion of organic material.
More particularly, the invention relates to a method o~ directly wet combusting an aqueous solution con-tain-ing dissolved or finely dispersed organic material by molecu-lar oxygen at elevatecl pressure and at high temperature.
It is known to totally or partially combust organic material dissolved or finely suspended in water by molecular oxygen or gases containing molecular oxygen, such as air, under pressure and at elevated temperature, which temperature depending on the degree of combustion and the type of the organic substance, should be in the range of 180 to 340C.
The process is suitably carried out continuously, and the combustion can be effected using both concurrent flow and counter-current flow with an almost complete use of the mole-cular oxygen. When using air in the combustion of e.y. ligno-cellulose-containing biological substances, such as wood, peat, and bagasse, or waste liquors obtained by the acid or alkaline pulp digestion of biological substances, the exhaust combustion gases seldom contain more tharl 0.2% of molecular oxygen. If, nevertheless, an almost complete com-bustion of the organic material is to be obtained, -the combus-tion temperature usually must exceed 300C, e.g. be between 300 and 340C.
Due to the continuously reclining content of oxyyen during the combustion process, -there occurs a corresponding reduction of the complete oxidation of the organic material into carbon dixoide and water. This contributes during the combustion process, especially in the combustion of lignocel-lulosic ma-terial, to the formation of difficult oxidizable compounds which are mainly low molecular weiyht acids, such as acetic acid, proprionic acid, or salts thereof. This ~' t7';~
is independent of whether the combustion takes place in con-current or counter-curren-t flow.
In the wet combustion process, the incoming liquid may lose volatile combustible material by expulsion with the exhaust mixture of steam and gas formed by the combustion, irrespective whether the combustion takes place in counter-current and concurrent flow. Volatile products can be present in the incoming liquid and also be formed during the combus-tion. Due to the low concentration of molecular oxygen in exhaust flue gas in -the final stage of the combustion, there is the risk that the volatile produc-ts remain unoxidized and either remain in the solution as e.g. salts, or are en-trained in the s-team generated.
However, experiments have shown that excess molecu~
lar oxygen at high concentrations, such as in air, of from 20 to 50%, facilitates the combustion of the difficult oxi-dizable compounds which, when they result from combustion of biological substances or products thereof, normally consist of low molecular weight fatty acids~ primarily acetic acid.
Where the incoming liquid is alkaline and -the formed acids are bound in the form of salts, generally the same problem exists, viz. the decomposition of the acids into carbon dioxide and water, as in the case of free acids.
A great advantage with the combustion in alkaline solution is, however, tha-t the generated steam is free from acidity, which facilitates its use for heating and power generating purposes and simplifies -the selec-tion of suitable construc-tion material for these purposes.
It is further known from experience tha-t the combus-tion of lignocellulosic biological substances or products thereof can be effected under relatively moderate ternperature conditions, between 180 and 300C when the generated heat p~
is restricted to between 75 and 90~ of the calorific value of the organic material, but that higher te~peratures are required to release the last 5 to 10% of the calorific value, and where this organic material is cons-ti.tuted by low molecu-lar weight acids, the combustion -temperature must substantial-ly exceed 300C. From experiments with wet combustion of alkaline waste liquor from digestion of wood using pure sodium hydroxide solution, and from the resul.ts to be referred to below, it becomes evident that use of a surplus and high concentration of oxygen gas in the final stage of the combus-tion process facilitates the decomposition of the difficultly oxidizable compounds into carbon dioxide and water.
There was combusted, for example, a waste liquor obtained by digestion of pine-wood using 220 g NaOH and 2 g of anthraquinone per kilogram wood calculated as bone dry substance at a temperature of 170C into a pulp yield of 47.8%. The waste liquor has a dry solids content of 14.7o with a calorific value of 3,762 Cal per kg and contained 24.6% of Na2O calculated as bone dry substanceO In -the com-bustion of this waste liquor in an autoclave while using air with an initial pressure of 3,800 kPa at 20C, resulting in a partial pressure of the oxygen gas of 800 kPa at 20 C, 83% of the calorific value of the waste liquor were released at a temperature of 275C, the partial pressure of the oxygen gas thereunder falling to 400 kPa. Thereupon, the temperature was raised to 300C, whereby additional heat was released so that, calculated on the original waste liquor, 90% or the calorific value was released~ Then the partial pressure of the oxygen gas dropp^ed to 250 kPa.
In a similar experiment, the combustion was started with air having the same par-tial pressure of the oxygen gas of 800 kPa as in the preceding experimen-t~ Then, 8~% of the calorific value of the waste liquor was released when ~ ~''J'7'~
a temperature of 275C had been reached. Pure oxygen gas was then supplied and the temperature was raised to 300C, when altogether 96% of the calorific value of the waste liquor was released. Then the partial pressure of the oxygen gas was 500 kPa calculated at 20C and represents double surplus of oxygen gas in the final stage as in the previous experi-ment, in which the total combustion process until a final temperature of 300C was carried out with that quantity of molecular oxygen which was present in the air initially sup-plied.
To reach a high degree of combustion in the combus-tion of the lignocellulosic biological substance withou-t resorting to extraordinary conditions of temperature subs~an-tially exceeding 300C, such as e.g. to 340C, the combustion in the final stage must be effected with a great surplus of molecular oxygen, and so as at the same time to limit the consumption of oxygen for all the organic material present in the waste liquor~ the combustion must be effected in two separate steps. In the first step the combustion of the incoming liquid containing organic substance is carried to such degree that between 75 and 95% of the calorific value is released, which can be effected with small excesses or molecular oxygen. In the second step the remaining organic substance is combusted with a great surplus of molecular oxygen in such a manner than steam and gas effluent from this second step can be fed to the first step with a content of oxygen gas adjusted so that the combustion in this step can be effected to the aforesaid degree of 75 to 95%. If desired, an extra addition of molecular oxygen may be supplied to the effluent steam and gas so that the stated combustion degree is reached. The yas containing the molecular oxygen incoming into the second step must be satura-ted with steam at 300C in order to avoid cooling of the liquid in the second step and thereby staying at a too low combustion temperature.
~ _ ~1~'7'~
Assuming that 10%, for example, of the combustion heat of the earlier mentioned wasteliquor is preserved after the first step and that air is used for the combustion, the surplus of molecular oxygen in the second step becomes about times greater than the theoretical requirement, if the whole quantity of air necessary for the combustion of the incoming organic substance is supplied to the second step.
Instead of air, it is, to advantage, possible to use air enriched with oxygen gas, e.g. with between 20 and 50% of 2~ or other non-reactive gases having a higher content of molecular oxygen than air. Considering solely the reaction mechanism, it is advantageous to use pure oxygen gas. However for safety reasons, it appears to be inappropriate to operate with higher oxygen contents than 30-50% of the gas entering into the combustion zone.
The compressed air enriched with oxygen can be prepared depending on local conditions eithex by mixing toge-ther air and oxygen gas at atmospheric pressure and there-upon compressing the gas mixture, or by mixing compressed air with oxygen under pressure, e.g. by vaporization of liquid molecular oxygen.
The combustion gases leaving the wet combustion plant can also be recirculated under almost the same pressure that prevails in the combustion apparatus after it has been cleared, for instance under pressure, of generated carbon dioxide and possible excess of non-reactive gases, e.g. nitro-gen, and thereupon having been supplied with an adequate quantity of oxygen gas under pressure.
Usually, the wet combustion process is carried out in concurrent flow, but it may in many cases be more suitable to carry out the combustion in the second step in a counter-current flow, which then also takes into account that the liquid fed into the second step is of a relatively "7;~3 small quantity due to -the evaporation of incoming aqueous solution which occurred due to escape of vapour during the combustion.
When e.g. waste liquor containing 18~ of dry sub-stance, of which 81~ is organic material, from a pure soda process is combusted, the waste liquor must be diluted with water so that the generated heat can be totally converted into vapour. Furthermore, water must be added for removal of the soda formed in the combustion. In this case, only 10-12% of the quantity of water entering the first step will be supplied to the second step and finally oxidized. This is done most effectively in a counter-current flow in, e.g.
a tower filled with annular elements, which increase the surface of contact between the liquid and the molecular oxygen ; containing gas, which facilitates the diffusion of the gas ~; into the liquid and thereby accelerates the combustion reac-tion. The filling material of the tower may be a material which in a catalytic manner stimulate oxidation, such as, for example, nickel, or chromium, vanadium and titanium con-taining alloyed steel, or the tower filling material may be coated with active material, e.g. platinum or nickel, precipitated on ceramic materials. It is also possible to utilize heterogeneous catalysts in the form of powder, e.g.
copper chromite, finely divided platinum which is added to the incoming liquid and after completion of the oxidation is separated or precipitated and, if desired, reactivated and recirculated. The types of catalysts depend mainly whe-ther the combustion of the incoming organic material is to be performed in an acid, neutral or alkaline environment.
According to the present invention, therefore, there is provided in the wet combustion method of combusting an aqueous solution containing organic material in which ~ ~5~'~1'7~ ~
the organic material is oxidized by introducing into the solution molecular oxygen material at a temperature ranging between 180C and 340C and a correspondingly elevated pres-sure, the improvement comprising: a) a first oxidizing step, in which the organic material in said aqueous solution is oxidized so as to release an amount of the heat of combustion ranging between 75% and 9S% thereof; b) a second oxidizing step, in which the residual organic material is finally oxidi-zed by introducing surplus molecular oxygen in an amount sufficient to impart to the gaseous effluents generated by said second step a content of molecular oxygen sufficient to achieve the oxidation of the first step; and c) a feeding step, in which said gaseous effluents are supplied to said first step. Suitably, the organic material comprises cellu-lose-containing biologic material and in which low molecular acids formed in the first step and resistant to oxidation therein are oxidized in said second step. Desirably, the molecular oxygen comprises a stream of compressed air. Pre-ferably, the oxidation of the organic material in the first step is carried out in concurrent flow with the stream of compressed air and in which the oxidation in step two is carried out in counter-current flow to -the flow of said stream of com-pressed air.
In one embodiment of the present invention, the flue gases cooled by said gaseous effluents are liberated from carbon dioxide and excess of non-condensible gases formed during the combustion process, and in which the residual non~condensi-ble gases are recirculated together with molecular oxygen to the second oxidation step.
In order to exemplify how the process may be carried out, reference is made to the following example and the accom-panying drawing, in which the single figure is a flow sheet indicating the essential equipment parts of a plant for carry-7~7~
ing out combustion oE black liquor from the production of kra~t pulp by means of sulphur-free sodium hydroxide solu-tion and recovery of soda. Since the waste liquor is alka-line, no problems arise regarding purification of the water vapour leaving the process~ Otherwise, when non-combusted volatile compounds are formed and pass with the water vapour, e.g. free acetic acid, such acid must be removed either directly from the water vapour or from the condensate formed thereform.
Referring now to the Figure, black liquor from a pulp production of 20 t/h consisting of e.g. 127,560 kg of water and 2B,000 kg of dissolved solids, of which 81%
is organic substance, is fed into vessel 1 through pipe 2.
Simultaneously, 47,823 kg of steam condensate at 40C and 6,092 kg of warm water at 151C are fed into a storage vessel 1 through pipes 3 and 4, respectively and furthermore, 13,425 kg of steam at 100C through pipe 24, so that altogether 194,900 kg of diluted black liquor at 80C is present in the vessel 1. The black liquor solution at a temperature of 80C is pumped by a pump 5 through pipe 6 into preheater 7, into which at the same time 29,026 kg of steam at 5 atmos-pheres absolute pressure is introduced through pipe 8, from steam generator 9, which steam passesthe solution from the preheater 7 to a high-pressure pump 10 at a temperature of 151C for further transport through pipe 11 to reactor vessel 12, which is under a pressure of s-team and gas of 149 atmospheres above atmospheric and which consti-tutes the first combustion stage in which 90% of the combus-tion heat of the liquor is considered to be set free. Simul-taneously, 113,000 m3 of air compressed to 150 atmospheres absolute pressure is supplied to the reactor from compressor 13. Of this quantity, o~ air, 50,000 m3 is fed through pipe '7';'~'~
14 to scrubber 15, within which the air in counter-current flow meets an aqueous solution at 310C coming from cyclone 16 and supplied through pipe 17 to the top of the scrubber and recycled from the bottom thereof by means of pump 29 into the reactor vessel 12. In the scrubber the air is saturated with steam and preheated to about 300C and suppli-ed via pipe 18 to the bottom section of reactor 19 for final oxidation. At the same time from the cyclone 16, about 50,000 kg of solution containing soda and Na-salts and having a temperature of 310C is fed to the top of the reactor 19 through pipe 20. In the final oxidation step 6,400,000 Cals are produced which generate about 20,000 kg of steam at 310C, which passes from the top of the reactor 19 together with 50,000 m3 of air containing about 2~5% CO2, and the escaping gas, since it is saturated with steam of 310C, carries along, in addition to the 20,000 kg of steam generated in the reactor 19, also about 29,000 kg of steam which the air has taken up in the scrubber 15, when being preheated by direct contact with the water having the temperature of 310C. The mi~ture of steam and gas from the reactor 19 is introduced through the pipe 11 together with 63,000 m3 of air coming from the compressor 13 via pipe 21 into the reactor vessel 12, enough of molecular oxygen thus being supplied to this reactor vessel 12 for combustion of 90% of the organic substance contained in the black liquor. At the same time, there escape from the top of the reactor vessel 12 via the cyclone~ 16,170,000 kg of steam at 310C and 156,250 kg of gas under a stearn-gas pressure of 149 atmospheres above atmospheric, i.e. 0.92 kg of gas per kg of stearn. Theoretically, a wor3cing pressure of 124 atmospheres above atrnospheric should be sufficient, but in order to ensure reliability in operation, sorne predetermined over-pressure rnust exist, and 149 atmospheres above atmospheric should guarantee that difficulties due to a fall of te}nperature in the reactor will not g 3'7'i'~
arise as a consequence of escape of a steam-gas mix-ture too rich in steam. The residual burn-out portion of the black liquor is derived from the reactor 19 through pipe 22. This residual portion amounting to 20,000 kg of water and 4,770 kg of soda, of which about 10~15% may consis~ o~ sodium acetate, are recycled to the pulp cooking equipment with the causticized liquor. The withdrawn soda solu-tion is expanded to atmospheric pressure in a cyclone 23, where 13,425 kg of steam escape through pipe 24 to the vessel 1. From the cyclone the soda solution of 100C leaves through pipe 28 and is diluted at the same time with 13,425 kg of water at 40C through pipe 25, whereby the soda solution regains a volume of 30,000 kg of water containing 4,770 kg of soda, and which while having a temperature of about 25C is fed to tank 26. From the tank 26, the warm soda solution is conveyed through plpe 27 to become causticized for further feed to a digester.
The steam and gas escaping from the reactor 12 via the cyclone 16 and consisting of 170,000 kg of steam and 156,250 kg of combustion gases are introduced into a heat exchanger 30 and cooled down under a full pressure of 149 atmospheres above atmospheric for generation of steam at 34 atmospheres above atmospheric from feed water of 151&. The steam and gas of 310C are thus cooled down to 249&, while at the same time 134,355 kq of saturated steam under a pressure of 34 atmospheres above atmospheric leave~ steam boiler 31 through pipe 32. Steam, condensate and gas of 249C from the heat exchanger 30 are fed to heat exchanger 33 through the pipe line 34 for generation of steam of 4 atmospheres above atmospheric from water at 151C. The steam ` 30 is obtained from the steam generator 9 through pipe 36 in a quantity of 41,690 kg, of which 29,026 kg are supplied to the preheater 7 via the line 8, and in this way the quan-11 1~3'~"7~
tity of steam of 4 atmospheres above atmospheric wil] amount to 12,624 kg.
From residual condensate, steam and gas are still under the pressure of 149 atmospheres above atmospheric, warm water at 151C is produced by causing steam condensate at 20C to exchange heat with condensate and gas from the heat exchanger 33 and conveyed through pipe 37 to a second heat exchanger 38 for production of warm feed water. Conden-sate and gas leaving t~e heat exchanger 38 are collected in a pressure vessel 40, within which they have a temperature of 40 C and are under a gas pressureof 149 atmospheres above atmospheric. 182,137 kg of condensate of 20C are conveyed from vessel 41 by pump 42 through pipe 39 to the heat exchan-yer 38, where the condensate is heated to 151C and conveyed further to a column 45 via pipe 44 and relieved from dissolved carbon dioxide and other gases before the feed water of 150C is supplied to the steam generator 9 and steam boiler 31 by pump 46 via pipe 49. A surplus of warm water of 151 C
amounting to 6,092 kg is conveyed through pipe 4 to the vessel 1 for dilution of the black liquor. 134~355 kg of steam subjected to a pressure of 34 atmospheres above atmospheric is fed from the boiler 31 via pipe 32 to superheater 50, where the steam is superheated to 420C and conveyed further to a reaction turbine 51, which de]ivers 15,700 kW at a back pressure of 11 atmospheres above atmospheric. Back pressure steam is drawn off from a steam accumulator 52. From the gas and condensate streaming to the pressure vessel 40 the latter is conducted through pipe 55 to a water turbine 56 driving an electric generator which delivers 480 kW. The gas still under pressure is passed through pipe 57 via a superheater 58 to an expansion machine 59 which drives an electric generator producing 27,000 kW. The superheaters ~ 11 --50 and 58 are heated by hot flue gases from the furnace 60, the quantity of heat consumed thereby corresponding to 2.7 tons of oil per hour.
In the wet combustion of black liyuor according to the preceding example for production of steam and recovery of the chemicals, the heat content of the s-team represents 92% of the calorific value of the dry substance conten-t of the liquor. For comparison may be mentioned that according to a corresponding manner of calculation for a plant with soda furnace the result is about 56%~
When considering the additional heat required for superheating the steam and non-condensable gas producing a minor surplus of power, the calories in the steam represent 74.3% of the calories in the liquor and the additional fuel, and then all power needed for operation of pumps, auxiliary machines and compressors has been produced also. The addi-tional heat which is supplied, corresponds to 0.095 Swedish Crowns per kWh, if the price for heavy oil is assumed to be Swedish Crowns l,000 per ton.
Claims (7)
EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS
FOLLOWS:
1. In the wet combustion method of combusting an aqueous solution containing organic material in which the organic material is oxidized by introducing into the solution molecular oxygen at a temperature ranging between 18°C and 340°C and a correspondingly elevated pressure, the improvement comprising: a) a first oxidizing step, in which the organic material in said aqueous solution is oxidi-zed so as to release an amount of the heat of combustion ranging between 75% and 95% thereof; b) a second oxidizing step, in which the residual organic material is finally oxi-dized by introducing surplus molecular oxygen in an amount sufficient to impart to the gaseous effluents generated by said second step a content of molecular oxygen sufficient to achieve the oxidation of the first step; and c) a feeding step, in which said gaseous effluents are supplied to said first step.
2. A method according to claim 1, in which the organic material comprises cellulose-containing biologic material and in which low molecular acids formed in the first step and resistant to oxidation therein are oxidized in said second step.
3. A method according to claim 1, in which the molecular oxygen comprises a stream of compressed air.
4. A method according to claim 3, in which the oxidation of the organic material in the first step is carried out in concurrent flow with the stream of compressed air and in which the oxidation in step two is carried out in counter-current flow to the flow of said stream of compressed air.
5. A method according to claim 1 or 2, in which the final oxidation step is carried out in the presence of an oxidation catalyst.
6. A method according to claim 1 or 2, in which flue gases cooled by said gaseous effluents are liberated from carbon dioxide and excess of non-condensible gases formed during the combustion process and in which the residual non-condensible gases are recirculated together with molecular oxygen to the second oxidation step.
7. A method according to claim 3, in which said stream of compressed air is conducted at least partially in counter-current flow with the hot liquid flow from the first oxidation step.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000405572A CA1197729A (en) | 1982-06-21 | 1982-06-21 | Method for wet combustion of organic material |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000405572A CA1197729A (en) | 1982-06-21 | 1982-06-21 | Method for wet combustion of organic material |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1197729A true CA1197729A (en) | 1985-12-10 |
Family
ID=4123053
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000405572A Expired CA1197729A (en) | 1982-06-21 | 1982-06-21 | Method for wet combustion of organic material |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1197729A (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2014185957A1 (en) * | 2013-05-14 | 2014-11-20 | Ciris Energy, Inc. | Treatment of carbonaceous feedstocks |
| US10323193B2 (en) | 2015-03-12 | 2019-06-18 | Ciris Energy, Inc. | Discriminate mass transfer in a wet oxidation system |
-
1982
- 1982-06-21 CA CA000405572A patent/CA1197729A/en not_active Expired
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2014185957A1 (en) * | 2013-05-14 | 2014-11-20 | Ciris Energy, Inc. | Treatment of carbonaceous feedstocks |
| CN105431403A (en) * | 2013-05-14 | 2016-03-23 | 克里斯能量有限公司 | Treatment of carbonaceous feedstocks |
| US10323193B2 (en) | 2015-03-12 | 2019-06-18 | Ciris Energy, Inc. | Discriminate mass transfer in a wet oxidation system |
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