It is known since long ago to combust totally or partially organic material dissolved or finely sus¬ pended in water by means of molecular oxygen or gases containing molecular oxygen, such as air, for example, under pressure and at elevated temperature, which de¬ pending on the degree of combustion and the nature of the organic substance should be in the range of 180 to 340 C. The process is suitably carried out cont nu¬ ously, and the combustion can be performed both in concurrent flow or in counter-current flow with an almost complete conversion of the molecular oxygen. When using air in the combustion of e.g. lignocellulose- containing biologic substance, such as wood, peat, bagasse etc. , or waste liquors obtained by acid or alkaline pulp digesting of biologic substance, the escaping combustion gases seldom contain more than 0.2 % of molecular oxygen. If, nevertheless, an almost complete combustion of the organic material shall be obtained, the combustion temperature usually must ex- ceed 300°C, e.g. be held between 300 and 340°C.

Due to the continuously declining content of oxygen during the progress of the combustion process, there occur parallelly hereto a deterioration of the complete oxidation of the organic material into carbon dioxide and water. Hereto contributes also during the combustion process, especially in the combustion of lignocellulosic material , the formation of compounds insensitive against oxidation and constituted mainly by acids of low molecular

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weight, such as acetic acid, proprionic acid, or salts thereof, and this phenomenon is independent thereof, whether the combustion takes place in concurrent or in counter-current flow.

In the wet combustion process the incoming liquid may lose volatile combustible material by expellation to¬ gether with the effluent mixture of steam and gas formed by the combustion, irrespective whether the combusti on takes place in counter-current and concurrent flow. Volatile products can be present firstly in the incoming liquid and secondly be formed during the combustion. Due to the low concentration of molecular oxygen in leaving flue gas in the final stage of the combustion, there exists the risk that the volatile products remain un¬ affected, and either remain in the solution as e.g. salts, or follow with the generated steam. However, experiments have shown that molecular oxygen in excess and high concentration as in air, for example, or higher, from 20 to 50 % , facilitates the combustion of the com¬ pounds insensitive against oxidation, which when they result from combustion of biologic substance or products thereof, normally consist of fatty acids having low mole¬ cular weight, such as in the first place acetic acid.

In that case where the incoming liquid is alkaline and the formed acids bound as salts, generally the same problem exists, viz. to decompose the acids into carbon dioxide and. water, as in that case that the acids are uncombined.

A great advantage with the combustion in alkaline solution is, however, that the generated steam is free from acidness, which facilitates use thereof for heating and power generati ng purposes and simplifies selection of suitable construction material for these purposes.

It is further known from experience that the com¬ bustion of lignocellulosic biologic substances or product thereof can be performed under relatively moderate temper ture conditions, between 180 and.300 C, if the released heat volume is restricted to between 75 and 90 % of the calorific value of the organic material , but that higher temperatures are required to release the last 5 to 10 % of the calorific value, and in that case where this organ material is consti tuted by acids of low molecular weight, the combustion temperature must substantially exceed 300°C.

From experiments made with wet combustion of alkalin waste liquor from digestion of wood by means of pure sodium hydroxide solution and from the results to be re- ferred to more below, it becomes evident that use of a surplus and high concentration of oxygen gas in the final s.tage of the combustion process facilitates the decom¬ position of the compunds i nsensi tive agai st oxidation into carbon dioxide and water. There was combusted, for example, a waste liquor obtained by digestion of pine-wood by means of 220 g NaOH and 2 g of anthraquinone per kilogram wood calculate as bone dry substance at a temperature of 170 C into a pulp yield of 47.8 % . The waste iiquor has a dry solids content of 14.7 % with a calorific value of 3,762 Cal per kg and contained 24,6 % of Na _? 0 calculated as bone dry substance. In the combustion of this waste liquor in an autoclave while using air with an initial pressure of 3,800 kPa at 20°C, resulting in a partial pressure of the oxygen gas amounting to 800 kPa at 20 C, 83 % of the calorific value of the waste liquor were set free at a temperature of 275 C, the partial pressure of the oxyge gas thereunder falling to 400 kPa. Thereupon, the tem¬ perature was raised to 300°C, whereby additional heat was

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released so that, calculated on the original waste liquor, 90 % or the calori fi c value has been released. Then the partial pressure of the oxygen gas had dropped to 250 kPa. In a similar experiment, the combustion was started with air having the same partial pressure of the oxygen gas of 800 kPa as in the preceding experiment. Hereby 89 % of the calorific value of the waste liquor was re¬ leased when a temperature of 275°C had been reached. Now pure oxygen gas was supplied and the temperature was raised to 300°C, when altogether 96 % of the calorific value of the waste liquor was . rel eased . Then the partial pressure of ^' the oxygen gas was 500 kPa calculated at 20°C and re¬ presents a twice as great surplus of oxygen gas in the final stage as in the previous experiment, in which the total combustion process until a final temperature of 300 C was carried out with that quantity of molecular - oxygen which was present in the air supplied from the out¬ set. To reach a high degree of combustion in the com¬ bustion of l gnocellulosic biologic substance without re¬ sorting to extraordinary conditions of temperature sub¬ stantially exceeding 300 C, such as e.g. to 340 C, the combustion in the final stage must be effected with a great surplus of molecular oxygen, and in order at the same time to limit the consumption of oxygen for all the organic material present in the waste liquor, the com¬ bustion must be realized in two separate steps, in the first step the combustion of the incoming liquid con-" taining organic substance bei ng _. carri ed to such degree of combustion that between 75 and 95 % of the calorific value is released, which can be done with small excesses or molecular oxygen, whereupon in the second step re¬ maining organic substance is combusted with a great sur-

plus of molecular oxygen in such a manner that steam and gas effluent from this second step can be fed to the firs step with a content of oxygen gas adjusted so that the combustion in this step can be effected to the aforesa i d degree of 75-95 % . If desired, an extra addition of molecular oxygen may be supplied to the effluent steam and gas in order that the stated combustion degree shall be reached. The gas containing the molecular oxygen and incoming into the second step must be saturized with steam of 300 C in .order to avoid cooling of the liquid in the second step and thereby staying at a too. low com¬ bustion, temperature.

Assuming that 10 % , for example, of the combustion heat of the earlier mentioned v/aste liquor is preserved after the first step and that air is used for the com¬ bustion, the surplus of molecular oxygen in the second step becomes about 10 times greater than the theorethic requirement, if the whol e quanti ty 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 O , or other indifferent gases having a higher content of molecular oxygen than air. Considering solely the reaction mechanism, it is advantageous to use pure oxygen gas, as has been verified also, but for reasons of security it appears not to be appropriate 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 pre¬ pared depending on the local conditions either by mixing together air and oxygen gas at atmospheric pressure and thereupon compressing the gas mixture, or by mixing com¬ pressed air with oxygen air under pressure, e.g. by vaporization of liquid molecular oxygen.

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The combustion gases leaving the wet combustion plant can also be recirculated under almost the same pressure that prevails in the combustion apparatus after this latter having been released, for instance under pressure, from generated carbon dioxide and possible excess of in¬ different gases, e.g. nitrogen, 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 in the realization here suggested of the process, 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 consists of a relatively small quantity owing to the evaporation of incoming aqueous solution which occurred by escape of vapour during the combustion.

When e.g. waste liquor containing 18 % of dry substance whereof 81 % is organic material , from a pure soda pro- cess is combusted, the waste liquor must be diluted with water in order to render possible that the generated heat can be converted totally into vapour. Furthermore, water must be added for removal of the soda formed in the combustion. In this case, 10-12 % only of the - water quantity entering into the first step will be supplied to the second step and oxidized there finally. This is done most effectively in a counter-current flow in e.g. a tower filled with annular elements which in¬ crease the surface of contact between the liquid and the moleculary oxygen containing gas, which facilitates the diffusion of the gas into the liquid and thereby accel¬ erates the combustion reaction. The filling material of the tower may be made of materials which in a catalytic manner stimulate oxidation, such as, for example, nickel or chromium, vanadium, titanium etc. containing alloyed steel, or the tower filling material may be coated with

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active material , e.g. platinum or nickel etc. 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 finished oxidation and deduction is separated or precipitated and, if desired, reactivated and recirculated. What types of catalysts which shall be sel ected depends mai nly thereon whether the combustion of the incoming organic material is to be performed in acid, neutral or alkaline environment.

In order to exemplify, how the process may be carried out, reference is made to the following example and the accompanying drawing. The drawing figure is a flow sheet indicating the essential equipment parts of a plant for carrying out combustion of black liquor from the produc¬ tion of kraft pulp by means of sulphur-free sodium hyrd- oxide solution and recovery of soda. Since the waste ^• liquor is alkaline, no problems arise regarding purifica¬ tion of the water vapour leaving the process. Otherwise i.e. when non-combusted vol ati 1 e compounds are formed and follow with the water vapour, e.g. free acetic acid, such acid must be removed in one or other way either directly from the water vapour or from the condensate formed therefrom.

Referring now to the drawing black liquor from a pulp production of 20 t/h consisting of e.g. 127,560 kg of water and 28 000 kg of dissolved solids, of which 81 % is organic substance, which is fed into vessel 1 through pipe 2. Simultaneously 47,823 kg of steam condensate of 40°C and 6,092 kg of warm water of 151°C are fed into a storage vessel 1 through pipes 3 and 4, respectively and furthermore, 13 425 kg of steam of 100°C through pipe 24, so that altogether 194,900 kg of diluted black liquor of 80°C is present in the vessel 1. The black liquor solution holding a temperature of 80°C is pumped by means

of the pump 5 through pipe 6 to preheater 7, into which at the same time 29,026 kg of steam of 5 atmospheres absolute pressure is introduced through pipe 8, which departs from steam generator 9, which latter imparts to the solution fed from the preheater 7 to high-pressure pump 10 at a temperature of 151 C for further transport through pipe 11 to reactor vessel 12, which stands under a pressure of steam and gas of 149 atmospheres above atmospheric*, and which constitutes the first combustion stage in which 90 % of the combustion heat of the liquor

3 is assumed to be set free. Simul aneously, 113,000 m of air compressed to 150 atmospheres absolute pressure is supplied to the reactor from compressor 13. Of this

3 quantity of air 50,000 m is fed through pipe 14 to scrubber 15, within which the air in a counter-current flow meets an aqueous solution of 310 C coming from cyclone 16 and supplied through pipe 17 to the top of ^* the scrubber and recycled from the base part thereof by means of pump 29 into the reactor vessel 12., In the scrubber the air is saturated with steam and preheated to about 300°C and supplied via pipe 18 to the base section of reactor 19 for final oxidation, while at.the same time from the cyclone 16 about 50,000 kg of solution containing soda and Na-salts and having a temperature of 310 C 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 of 310°C, which departs

3 from the top of the reactor 19 together with 50,000 m of air containing about 2.5 % C0 _? , ^' and the escaping gas, sinc it is saturated with steam of 310°C carries along, in addi

' tion 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 holding the temperature of 310 C. The mixture of steam and gas from the reactor 19 is in-

3 troduced through the pipe 11 together with 63,000 m 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 of 310°C and 156,250 kg of gas under a steam- gas pressure of 149 atmospheres above atmospheric, i .e. 0.92 kg of gas per kg of steam. Theoretically, a working pressure of* 124 atmospheres above atmospheric should be sufficient, but in -order to ensure reliability in opera¬ tion, some predetermined over-pressure must exist, and 149 atmospheres above atmospheric should guaranteee that difficulties due to fall of temperature in the reactor will not arise as a consequence of escape of a steam-gas mixture 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 30,000 kg ^' of water and 4,770 kg of soda, of which about 10-15 % may consist of sodium acetate, are recycled to the pulp cooking equipment with the causticized liquor. The with¬ drawn soda solution is brought to expand to atmospheric pressure in a cyclone 23, whereunder 13,425 kg of steam escape through pipe 24 to the vessel 1. From the cyclone the soda solution of 100 C leaves through pipe 28 and is diluted at the same time with 13,425 kg of water of 40°C 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 25 C is fed to tank 26. From the tank 26 the warm soda solu- tion is conveyed through pipe 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 full pressure of 149 atmospheres above atmospheric for generation of steam of 34 atmospheres above atmospheric from feed water of 151 C. Hereby the steam and gas of 310°C are cooled down to 249°C, while at the same time 134,355 kg of saturated steam under a pressure of 34 atmospheres above atmospheric leave steam boiler 31 through pipe 32. Steam, condensate and gas of 249 C 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 of 151 C. The steam is derived from the steam generator 9 through pipe 36 in 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 disponible quantity of steam of 4 atmospheres above atmospheric will amount to 12,624 kg.

From residual condensate, steam and gas still stand¬ ing under the pressure of 149 atmospheres above atmospheri warm water of 151° is produced by causing steam condensate of 20 C to exchange heat with condensate and gas derived from the heat exchanger 33 and conveyed through pipe 37 to a second heat exchanger 38 for production of warm feed water. Condensate and gas leaving the heat exchanger 38 are collected in a pressure vessel 40, within which they have a temperature of 40 C and stand under a gas pressure of 149 atmospheres above atmospheric. 182,137 kg of condensate of 20 C are conveyed from vessel 41 by pump 42 through pipe 39 to the heat exchanger 38, where the condensate is heated to 151 C and conveyed further to a column 45 via pipe 44 and relieved from dissolved carbon dioxide and other gases before the feed water of 150 C 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 420°C and conveyed further to a reaction turbine 51 , which delivers 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 conducte 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 gen¬ erator producing 27,000 kW. The superheaters 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 liquor according to- the preceding example for production of steam and re¬ covery of the chemicals, the heat content of the steam re- presents 92 % of the calorific value of the dry sub¬ stance content 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 uncondensabl e gas producing a minor surplus of power, the calories- in the steam re¬ present 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 additional heat which is supplied, corresponds to 0.095 Swedish Crowns per kWh , if the price for heavy oil is assumed to be Swedish Crowns 1 ,000 per ton .