PROCESS FOR COMBUSTING SULPHITE LIQUOR CONTAINING MAGNESIUM OR AMMONIUM, AND A BOILER FOR COMBUSTING SULPHITE LIQUOR IN ACCORDANCE WITH THE PROCESS

Abstract: The invention relates to a proccess for combusting sulphite liquor, containing magnesium or ammonium and sulphite in some form, in a boiler of the two-chamber type (1, 3). The process is characterized in that the combustion in the first chamber (1) takes place with an understoichiometric quantity of oxygen so that a reducing environment arises, without the addition of reducing substances, in the flue gases, and in that oxygen is then added to the flue gases in the second chamber (3) at a time of 1-2 seconds after the combustion in the first chamber (1) and at a temperature of at most 1,250 DEG C in the flue gases, so that further organic material is combusted without any appreciable formation of NOx and So3. The invention also includes a boiler for implementing the process, which boiler, in contrast to conventional MgO boilers, is provided with supply openings (10) at one or more levels (11, 12) in the second chamber (3) in the bipartite boiler.

Description: TITLE : Process for combusting sulphite liquor containing magnesium or ammonium, and a boiler for combusting sulphite liquor in accordance with the process. TECHNICAL FIELD: The present invention relates to a process for combusting sulphite liquor which has been separated off after cooking cellulose pulp in accordance with the sulphite method, and to a combustion boiler for imple¬ menting the process according to the invention. STATE OF THE ART: When wood chips are cooked in order to obtain a cellulose pulp in accordance with the so-called sulphite method, an acidic cooking liquid is used which contains magnesium sulphite or ammonium sulphite or other salts of sulphurous acidity. Once the cooking is complete, the sulphite liquor is separated off from the cellulose pulp and is normally evaporated and combusted to extract heat and to recover chemicals such as magnesium oxide, MgO, and sulphur dioxide (S0 2 ) . Usually, the sulphite liquor is combusted in a com¬ bustion device which is colloquially termed a MgO boiler, since MgO is extracted in particle form from the sulphite liquor. These MgO boilers consist of two chambers which are usually separated by only one wall and which are designed as one unit, with the addition of liquor and the actual combustion taking place in the one chamber, which, by way of its lower part, communi¬ cates with the second chamber. The flue gases are therefore pressed downwards in the actual combustion chamber and turn upwards in the second chamber, whose outlet leads from the upper part of the chamber into a so-called superheater for extracting the heat content in the gases. In the first chamber, where the combustion takes place, the liquor is injected at a certain level and the combustion air is supplied both at the same level and, frequently, at one or more levels below that at which the liquor is sprayed in, i.e. somewhat later in the direction of flow of the combustion gases. The walls of these chambers are clad with pipes to which water is supplied for cooling the flue gases and generating steam. This steam is collected in the upper part of the plant, in the so- called dome, and is superheated in the superheater to which heat is supplied from the flue gases when the latter leave the second chamber. The superheated steam is used as desired. Downstream of the superheater, the flue gases are cooled in several stages and then leave the device at a temperature in the region of 150°C. Subsequently, the dust in the flue gases, which mainly consists of MgO, is separated off, after which the S0 2 gas is absorbed in an aqueous solution, 1 which is then combined with the magnesium oxide which was previously separated off and used as new cooking liquid. Magnesium oxide and sulphur dioxide, which are intended to be recovered, and a large quantity of heat are produced during the combustion, in addition to carbon dioxide, water, etc., from the organic part of the liquor. Due to its high melting point, the magnesium oxide comes to be present in solid form as dust particles which accompany the smoke while the sulphur dioxide is present in gaseous form. Thus, no smelt phase in the form of drops, as is found in the recovery of sulphate liquor, occurs in association with sulphite liquor combustion. In addition to the abovementioned chemicals, nitrogen oxides are also formed during the combustion. These oxides originate, on the one hand, from nitrogen-containing material in the liquor and, on the other, from the nitrogen in the combustion air. Combustion in a MgO boiler takes place at relatively low temperature, approximately 1,300-1,400°c, and it can be assumed that the majority of the nitrogen in the nitrogen oxides originates from the sulphite liquor. THE PROBLEM: In recent years, ever stricter requirements have been introduced as regards discharges of nitrogen oxides into the atmosphere. It is well known that these oxides contribute to acidification and to other unfavourable effects on the natural environment. Nevertheless, the quantity of nitrogen oxides which is emitted from the MgO boilers of the forest industry is low as compared with that which is derived from traffic. Normally, the contents are within the range 60-130 mg of N0 2 /mJ (calculated as actual calorific value in a reducing environment) , but even these low contents have to be decreased substantially in the future. Factors which can exert an influence on the formation of nitrogen oxides originating from the air, so-called thermal N0 X , are exposure time, flame temperature and oxygen content, while those affecting the formation of nitrogen oxides originating from the fuel, so-called fuel NO x , are, in addition to the quantity of nitrogen in the fuel, how the nitrogen is bound and the speed at which heating occurs. As a result of experience from conventional kraft boilers based on coal, oil and gas, it is known that understoichiometric ratios as regards oxygen supplied to the combustion zone, in combination with a final combustion, in order to obtain maximum evolution of energy, in a so-called surplus air register located immediately above or downstream of the combustion zone, result in a lower emission of NO x , mainly due to the fact that the formation of so-called thermal N0 X is suppressed. This technique, in combination with so-called flue gas recirculation, has also been discussed as a possibility for MgO boilers having a certain type of combustion arrangement in which air is supplied directly downstream of the point at which sulphite is injected, in the same way as surplus air is used for kraft boilers. This is described in more detail by Michael Bobik in the issue of TAPPI JOURNAL for January 1993, pp. 128-130. In order to, reduce the NO x from the MgO boiler still further, it has been proposed that the technique of thermal or selective non-catalytic reduction "SNCR" (selective non-catalytic reduction) (see the above article, pp. 130-131) be utilized, which technique was developed for kraft boilers, by means of supplying a reducing substance in the form of natural gas, ammonia or urea, in order to reduce nitrogen oxides which have been formed. When natural gas is used, large quantities of non-combusted residual products are formed which need to be finally combusted by adding additional air. In view of the relatively low specific NO x contents in the flue gases from MgO boilers, the costs occasioned by all the so-called SNCR methods for investment in equipment for preparing, metering and injecting reducing agents are relatively high, while the operational costs involved in purchasing reducing agents are also high. THE SOLUTION: However, by means of the present invention, a method has been developed for combusting sulphite liquor, containing magnesium or ammonium and sulphite in some form, in a boiler of the two-chamber type, in order to obtain flue gases which have a low content of nitrogen oxides, without high costs for equipment and reducing agents, which process is characterized in that the combustion in the first chamber takes place with an understoichiometric quantity of oxygen, so that a reducing environment arises, without the addition of reducing substances, in the flue gases, and in that oxygen is added to the flue gases in the second chamber at a time of 1-2 seconds after the combustion in the first chamber and at a temperature of at most 1,250°C in the flue gases, so that further organic material is combusted without any appreciable formation of NO x and S0 3 . According to the invention, it is expedient for the oxygen, which is added in the form of air, to be added to the flue gases when these gases are at a temperature of l,100°C, preferably 1,000°C. The addition of oxygen to the second chamber can take place at one or more levels. According to the invention, a suitable quantity of oxygen to be added to the second chamber is approxi¬ mately 2.5-40%, preferably 5-15%, of the total oxygen supply. According to the invention, it is expedient for the proportion of nitrogen oxides in the flue gases to be regulated with the aid of a control system which automatically regulates the supply of air to the various addition points. The invention also includes a boiler for combusting sulphite liquor in accordance with the process, which boiler includes two chambers which are in communication with each other, one of which chambers is provided with the device for injecting liquor and air, which boiler is characterized in that the second chamber is provided with devices for supplying additional air to the combustion gases. It is furthermore expedient, according to the invention, for the devices for supplying air to the second chamber to be located at such a distance from the combustion zone that the combustion gases, during normal operation, require 1-2 seconds after the com¬ bustion in order to pass by them. According to the invention, the devices for supplying air to the second chamber can be located at one or more levels. According to the invention, the MgO boiler expediently includes a regulating system for automatically con- trolling the different supply streams of the combustion air. DESCRIPTION OF THE FIGURES: The invention will be described in more detail below with reference to the drawings, in which Fig. 1 shows, diagrammatically and partly in section, a combustion boiler according to the invention, and in which Fig. 2 shows the decrease in the N0 X content as a consequence of adding oxygen in accordance with the invention. DETAILED DESCRIPTION: Fig. 1 shows an MgO boiler in accordance with the present invention. For the most part, it is a conven- tional boiler which contains two chambers. The first chamber 1 is separated by the partition wall 2 from the second chamber 3. The first chamber 1 is provided with devices for combusting sulphite liquor, which devices consist of liquor nozzles 4 for injecting liquor and primary air ports 5 for supplying primary air, which is the first part of the air to be added during the combustion. Fig. 1 shows an embodiment in which the liquor nozzle is located in the centre of the primary air port, resulting in the formation of a device termed a burner 6. Directly downstream of, and in association with, the so-called main combustion zone 7 there can be so-called surplus air ports 8 for the further addition of air, so-called secondary air. The dwell time of the flue gases, calculated on the basis of a stoichiometric flue gas flow and the mean temperature of the flue gas at maximum continuous load in the abovementioned main combustion zone, which begins at the top of the first chamber and finishes at level 9, corresponding to the level of the location of any surplus air ports which may be present, is approximately 0.5 seconds, if we assume a stoichiometric quantity of flue gas at maximum continuous load, calculated at the mean temperature of the flue gas. In accordance with the invention, the last part of the air, so-called distance air, is supplied through so- called distance air ports 10 which are situated at a long distance downstream of the combustion zone, which distance corresponds to a flue gas dwell time from the combustion zone to the distance air ports of 1-2 seconds or more (calculated from the end of the main combustion zone) . The air which is supplied through these distance air ports meets the flue gas, which, at that point, has an average temperature in the flow cross-section which is less than approximately 1,250°C, preferably less than 1,100°C, for example 1,000°C. As is evident from the figure, the distance air ports can be located at one or more levels 11 and 12. While the proportion of air which is supplied as distance air most expediently constitutes 5-15% of the total air which is added, limits which are as wide as 2.5-40% can also be used. In this way, a reducing atmosphere is created in the region downstream of the main combustion zone and up to the point at which distance air is added. However, it is very important that the supply of air in this final round is adjusted so that all, or at least as much as possible, of the organic material is combusted, so that as much heat as possible can be extracted. On the other hand, the quantity of oxygen must be kept at such a low level that no, or very little, N0 X is formed. Of great assistance in this respect is the fact that, at this level, the combustion gases have been cooled down from the high temperature to a temperature at which NO x is not so readily formed. Another very important factor in the process is that the gas contains large quantities of S0 2 , which it is the intention to recover in this oxidation form and which must not be oxidized to S0 3 . Too large a quantity of oxygen in the final addition stage can produce such an oxidation, particularly if the temperature is too low. If, however, the temperature is kept at a sufficiently high level during this addition of oxygen, very little formation of S0 3 takes place; however, at a later stage, when the gases have been cooled down still further, S0 3 will be formed if oxygen is present in any appreciable quantity. Oxidation of S0 2 to S0 3 is an equilibrium reaction which goes in the direction of S0 3 at lower temperatures. Fig. 1 also shows that the inner side of the walls in the boiler is clad with water pipes 13 which provide the steam dome 14 with steam. This steam in the steam dome 14 is subsequently superheated in the superheater 15, whence it is removed and used in an expedient manner. The flue gases are subsequently cooled, in a conventional manner, in several stages, while flowing downwards, and leave the lower part of the rear shaft at a temperature in the region of 150°C. Water and air are used to cool the flue gases in these heat exchangers. Apart from the devices for supplying air to the second chamber 3, the construction of the plant is conventional and is not, therefore, described in more detail here. The effect of the process according to the invention is shown in Fig. 2. This figure indicates the quantity of air which is added to the second chamber as a percentage of the total quantity of air, and the scale on the left-hand ordinate indicates the content of N0 X in parts/million (ppm) , while that on the right-hand ordinate indicates the percentage NO x reduction. The experiments were carried out using the above described plant, having a steam capacity of 75 t/h with liquor firing. The experiments were carried out with proportions of air being added to the second chamber of 0%, 10%, 12%, 15% and 18%. It was found that the content of NO x in the flue gases fell from approximately 96 ppm, when all the air was added to chamber 1, to somewhat over 60 ppm when somewhat less than 18% of the total quantity of air was added to chamber 2. This signifies a reduction in N0 X of approximately 35%, which demonstrates the very powerful technological effect of the invention. That which distinguishes the process according to the invention is that the time for reducing the NO x molecules which are formed in the combustion zone has been greatly extended and that the final addition of air takes place at a temperature which is less than 1,250°C, when the risk of the nitrogen being reoxidized to nitrogen oxide is very low. Naturally, the process can also be employed when the main combustion zone is equipped with types of combustion devices other than those which have been described here. The invention is not restricted to the embodiment which has been demonstrated, and can be varied in various ways within the scope of the patent claims.

Claims: PATENT CLAIMS 1. Process for combusting sulphite liquor, con- taining magnesium or ammonium and sulphite in some form, in a boiler of the two-chamber type, c h a r a c t e r i z e d i n that the combustion in the first chamber takes place with an understoichiometric quantity of oxygen, so that a reducing environment arises, without the addition of reducing substances, in the flue gases, and in that oxygen is added to the flue gases in the second chamber at a time of 1-2 seconds after the combustion in the first chamber and at a temperature of at most 1,250°C in the flue gases, so that further organic material is combusted without any appreciable formation of N0 X and S0 3 . 2. Process according to Claim 1, c h a r a c t e r i z e d i n that oxygen in the form of air is added at a temperature in the flue gases of 1,100°C, preferably 1,000°C. 3. Process according to either of Claims 1 or 2, c h a r a c t e r i z e d i n that oxygen is added at one or more levels in the second chamber. 4. Process according to any one of Claims 1-3, c h a r a c t e r i z e d i n that the quantity of oxygen which is added to the second chamber constitutes 2.5-40%, preferably 5-15%, of the total supply of oxygen. 5. Process according to any one of Claims 1-4, c h a r a c t e r i z e d i n that the proportion of nitrogen oxides in the flue gases is regulated with the aid of a control system which automatically regulates the supply of air to the different levels. 6. Boiler for combusting sulphite liquor in accor¬ dance with any one of Claims 1-5, containing two chambers (1, 3) , which are in communication with each other, one (1) of which chambers is provided with devices (4, 5, 6) for injecting liquor and air, c h a r a c t e r i z e d i n that the second chamber (3) is provided with devices (10) for supplying additional air to the combustion gases. 7. Boiler according to Claim 6, c h a r a c t e r i z e d i n that the devices (10) for supplying air to the second chamber (3) are located at such a distance from the combustion zone that the combustion gases, during normal operation, require 1-2 seconds after the combustion in order to pass by them. 8. Boiler according to Claim 1 or 7, c h a r a c t e r i z e d i n that the devices (10) for supplying air to the second chamber 'are located at one or more levels (11, 12) . 9. Boiler according to any one of Claims 6-8, c h a r a c t e r i z e d i n that it contains a regulating system for automatically controlling the different supply streams of the combustion air.

Inventors: BODESTIG LEIF, OLAUSSON LARS, BODESTIG, LEIF, OLAUSSON, LARS
Applicants: BODESTIG LEIF, KVAERNER PULPING TECH, OLAUSSON LARS, BODESTIG, LEIF, KVAERNER PULPING TECHNOLOGIES AB, OLAUSSON, LARS
Application year: 1995
Main classification: D21C11
IPC codes: D21C11, F23G7