Background In a recovery boiler there are normally two types of coating, which cause reduction of the capacity of the heat absorbing surfaces and clogging. One of the coatings is caused by the so called"physical carry-over". It is just physical matter, organic and inorganic, which is carried along by the ascending flue gases from the lower part of the boiler.

A phenomena, which increases carry-over of physical matter depends on that non-combusted liquor droplets are carried along. The liquor is supplied to the boiler via black liquor sprayers equipped with nozzles. The liquor droplets, which are formed in the nozzle are first subjected to an intense heating, which evaporates all water. Thereupon the exterior of the drop is heated and the drop begins to pyrolyse. This takes place very swiftly but the interior parts of the drop are cooler and the exterior surface will have time to pyrolyse so rapidly that all oxidative gases, including water, will be kept away. This means that the exterior surface will carbonize and radicals that have been formed are recombinated with polymerisation as a result. The external surface becomes almost tight but elastic. The interior of the drop will continue to be pyrolysed, but the pyrolytic gases can not escape. An overpressure is created, which makes the black liquor drop swell up as a popcorn.

This makes it especially sensitive to be carried along at high gas speeds and explains why also noncombusted black liquor drops are transported to the upper parts of the boiler.

When this physical carry-over reaches the upper part of the boiler the narrow sections will start to get clogged. After some time this clogging has become so heavy that the pressure drop begins to increase in the boiler and finally the flue gas fans will not manage to keep the draught, and the boiler must be closed down for cleaning.

The other type of coating is caused by the chemical dust, which is formed in the upper regions of the boiler when gaseous sulphur and sodium compounds are combined and condensate. This dust is often named"fume"and is a dust which is comparatively easy to sweep as long as the chemical composition is the right and the melting point of the dust exceeds the temperature of the surrounding gas. The factor mainly influencing-the chemical composition of the dust is the amount of evaporated sulphur and sodium, which in turn are controlled by the temperature in the lower part of the combustion chamber. A high temperature gives a big evaporation of alkali resulting in big volumes of dust and a high portion of carbonate in the dust, which increases the pH-value of the dust. Normally the chloride discharge from the bed will also increase with increasing temperature. A concentration of chlorides in the dust will lower the melting point of the dust and make it"stick"and make it more difficult to sweep dust also at lower gas temperatures in the upper region of the boiler. If the temperature in the lower combustion chamber is too low, i. e. if the boiler is driven too cold, the result will be a high sulphur discharge in relation to the sodium discharge, which lowers the pH- value of the dust. This to gives raise to a sticky dust, which can be difficult to sweep away at too high surrounding temperatures in the upper part of the boiler.

Normally air registers are used above the black liquor sprayers in a recovery boiler in order to obtain final combustion, so called overfire air registers.

The overfire air is fed into the boiler under high pressure through especially designed air ports. The air is accelerated in the air port in that the static pressure is transformed to an impulse which increases the linear speed of the air. Inside the boiler the air is slowed down in that the impulse is transferred to gas present in the path and it will flow with an upward movement and in that other gas being sucked into the jet and admixed with the air. The fuel which is sucked in will oxidize very rapidly as the concentration of oxygen is high. When the impulse from the air port has been reduced the oxygen molecule will move at about the same speed as other molecules in the gas. The transport of oxygen up to the fuel is now controlled by diffusion, convection and chemical reaction (through the temperature effects). The speed of combustion will be very strongly dependent of the concentration of oxygen and fuel.

PURPOSE AND MOST ESSENTIAL FEATURES OF THE INVENTION A purpose of the present invention is to provide a method for minimizing in a recovery boiler the risk for coatings and clogging of heat absorbing surfaces situated above the black liquor sprayers, and this has been achieved mainly therein that the oxygen concentration in the recovery boiler is raised at at least one level above said black liquor sprayers for the purpose of increasing the combustion speed for the pyrolytic gases and thereby reducing the gas temperature prior to its entrance in the superheater area.

Other purposes and features according to the present invention are defined in the sub claims.

BRIEF DESCRIPTION OF DRAWING Hereinafter the invention will be further described with reference to a recovery boiler, schematically illustrated in the accompanying drawing.

A recovery boiler 1 of the type used in connection to the method according to the invention is illustrated schematically in the drawing figure.

The recovery boiler 1 incorporates in a conventional manner a lower combustion chamber 2 and an upper combustion chamber 3 in which final combustion takes place. At the level between these lower and upper combustion chambers 2,3 a number of black liquor sprayers 4 are provided in the boiler wall for supply of black liquor, which shall be burned. In the area above the black liquor sprayers 4 on at least one level is provided supply means 5 for staged supply of final, oxygen enriched combustion air, in the embodiment shown arranged in three burning-off registers, situated between the black liquor sprayers and the superheater part 6 of the boiler.

The same effect can also be obtained in eliminating parts of not wanted matters, or bulk matters, such as nitrogen from the normal combustion air, e. g. with aid of membrane technology.

Furthermore it is of course possible to add pure oxygen for obtaining the desired increase of the oxygen concentration.

By increasing the oxygen concentration in the combustion air at the level for the means 5 in the boiler the combustion speed will increase very rapidly and the final combustion of pyrolytic gases will take place more rapidly than at ordinary air supply. A more rapid final combustion raises the temperature in this zone, which results in that the boiler walls will absorb more heat.

This means that the gas temperature in to the superheater part 6 of the boiler will fall and thereby gets a lower gas temperature profile throughout the entire superheater package, through the set of tubes and the economiser. As earlier mentioned, it is important that in this region the surrounding gas temperature is lower than the melting point of the dust for avoiding sticky dust. This method, where a an increased oxygen concentration increases bulk diffusion and convection of oxygen up to fuel, which increases the combustion speed, thus makes it easier to keep the boiler clean.

An increase of the temperature in this zone, will also cause that the popcorn-like black liquor droplets, which have been carried along, will burst more rapidly and the escaping pyrolytic gases are burned more rapidly in the oxygen enriched environment. After the pyrolysis there is also left a smaller volume of the dry substance of the black liquor droplet, than what is the case at a common supply of air, where about 25% of the dry substance of the black liquor droplet is still left. This means that this droplet will get a higher density and therefore the particle/the droplet will easier return to the lower region of the boiler for being finally burned, instead of being carried along to the superheater surfaces and to cause clogging problems.

PRACTICAL TESTS VERIFYING THE INVENTION An ordinary recovery boiler was furnished with the possibility of supplying oxygen to the overair system of the boiler. Then oxygen was charged to the air channel under control and in especially designed nozzles for obtaining a good stirring before the oxygen enriched air is supplied to the boiler via air ports (5).

The physical carry-over was measured by especially installed carry-over meters in the upper part of the boiler. The gas temperature profile was measured with installed IR pyrometers before the superheater, and with protected thermoelements before the set of tubes and after the economiser.

The following table shows the results from a test where the oxygen content in the overair was increased with steps of 1% from ordinary air under otherwise constant conditions.

In the test is shown only levels with 1 and 2% of oxygen addition. However it is evident that such oxygen addition will give a positive effect at additions over 0% of oxygen and up to about 10%, i. e. from about 21% of ordinary air to about 30% air oxygen, whereas additions above this level will not lead to improved effect. From an economic point of view the effect however should be less interesting in the upper part of this area, as no further positive effect is achieved but rather negative effects in form of forming of nitrogen oxides. Ordinary O2 content 02 content Return to air increase 1% increase 1% ordinary air Visual obs-L: contin. L: some No diff. L: cont. with ervations With sparks sparks with comp. with sparks of carry-pauses 1% increase over to R: some R: few with in tertiary R: few super-sparks register sparks heater Carry-over Rear: 60% Rear: 30% Rear: 30% Rear: 60% meter R: 40% R: 10% R: 25% R: 50% Temp. at 976 964 964 978 noose level cl Note: The visual observation was made through the inspection covers of the boiler on the left and the right hand side just above noose level.

The carry-over meters were positioned at the rear wall of the boiler and at the right hand side wall at noose level.

The physical measuring principle was based on the number of sparks measured.

This test shows clearly that an increased oxygen content in the overair system of the boiler will contribute to a more total combustion of swollen black liquor particles and thereby a reduced carry-over.

The following test gives an example of how the gas temperature profile through superheater, set of tubes and economiser is influenced at an increased oxygen content in the overair during otherwise substantially constant conditions. Test 1 Test 2 Test 3 ~ Load Liquor, MW 130 130 130 ~ Air(%) Primary 40 41 41 Secondary 40 40 39 Overair 20 19 19 Oxygenaddition 0 0 0, 1 Overairkg/s Gastemperature Before superheater 1000 996 945 Before set of tubes 530 530 520 Aftereconomiser 188 192 192 After electro filter 183 182 184 02 after economiser 2.7 2. 9 2. 7 It can clearly be seen how heavily dependent the combustion speed is upon the oxygen concentration.

The combustion speed is also very strongly dependent of the fuel concentration. By increasing the fuel concentration in the gases, which arrive to this final combustion zone it is possible to increase the combustion speed still more and thereby is achieved still lower gas temperatures into the superheater section.

In earlier Swedish patent application no: 9703769-1, is described a method of increasing the fuel concentration in the gases which arrive from the lower part of the combustion chamber into this final combustion zone. By combining such a method with the present invention is achieved a positive effect which is more than additive to the combustion speed and which further contributes to the capacity and accessibility of the boiler.

The invention is not limited to the embodiment illustrated and described in connection thereto but modifications and variants are possible within the scope of the following claims.