WO1989006334A1

WO1989006334A1 - A method of comubustion for the reduction of the formation of nitrogen oxides in a combustion process, and an apparatus for applying the method

Info

Publication number: WO1989006334A1
Application number: PCT/FI1988/000203
Authority: WO
Inventors: Jouko Laine
Original assignee: Oy Tampella Ab
Priority date: 1988-01-04
Filing date: 1988-12-29
Publication date: 1989-07-13
Also published as: FI880002A0; FI87013C; FI880002A; FI87013B

Abstract

A method of combustion and an apparatus for the reduction of the formation of nitrogen oxides in a combustion process. In the method the air required for the burning is introduced into a furnace (2) in at least two stages so that in the first stage the coefficient of air ranges preferably from 0.90 to 0.97 while the combustion is carried out under understoichiometric conditions, and in at least one stage so that the total coefficient of air exceeds the value one. According to the method the gases formed in the first combustion stage are cooled to a lower temperature while introducing more air among the gases at the earliest in connection with the cooling. The introduction of air is carried out so that the total coefficent of air at each particular point within the furnace (2) is at the most such as would be required by the temperature of the gases at this particular point for a concentration of nitrogen monoxide not exceeding a predetermined value to be formed in the gases. The apparatus comprises a superheater (7) which effects cooling in the furnace (2) of the boiler (1) after the first combustion stage and through which the gases formed in the first stage are forced to flow. More air for the final combustion is introduced at the superheater (7) and/or after it.

Description

A method of combustion for the reduction of the forma¬ tion of nitrogen oxides in a combustion process, and an apparatus for applying the method

The invention relates to a method of combustion for the reduction of the formation of nitrogen oxides in a combustion process, wherein the combustion is carried out by introducing air required for the burn¬ ing of fuel into a furnace in at least two stages so that in the first stage the introduction of air is carried out understoichiometrically in relation to the fuel, preferably with a coefficient of air ranging from 0.90 to 0.97, and in at least one stage overstoi- chiometrically so that the total coefficient of air exceeds the value one. The invention is also concerned with an apparatus for applying the method, which ap¬ paratus comprises at least first and second air supply means for introducing air into the furnace of a boiler in at least two stages. Combustion processes of different types always produce nitrogen oxides when nitrogen contained in the air and fuel reacts with oxygen to form various com¬ pounds. At high temperatures, nearly solely nitrogen oxide (NO) is formed, which is easily converted into other nitrogen oxides, usually into nitrogen dioxide (NO2), in the presence of oxygen when the temperature drops. Nitrogen oxides are hazardous to the environ¬ ment. Great amounts of them are formed in industrial processes and in power plants and other boilers, and one of the most important objects of environmental protection is to reduce the emission of nitrogen oxides into the atmosphere.

Various attempts have been made to reduce the amount of nitrogen oxides by converting them into an- other form. Such methods include various reduction techniques based on the use of catalysts, and the use of various absorption agents for simultaneous absorp¬ tion of sulphur and nitrogen oxides in various ways. The use of the different methods generally involves various problems which are difficult to solve, such as the high price and poor availability of useful noble metals acting as catalysts and the poor absorption properties of the absorption agents. It is also diffi¬ cult to dimension the apparatuses when applying these methods due to, e.g., variation in the efficiency of the boilers and other such factors.

Instead of removing nitrogen oxides, it is technically easier to try to prevent their formation already at the combustion stage. For this purpose, various low nitrogen oxide combustion means have been developed; attempts have been made to carry out the combustion under pressurized conditions; and air has been introduced into the boiler in a stepwise manner before the superheaters. However, these methods have not provided any particularly good results because the various ways nitrogen oxides can be formed, the reac¬ tion kinetics, the operating conditions of boilers, and variations therein, have prevented the realization of an effective method. Further, in an attempt to re- move nitrogen oxides, circulation bed furnaces operat¬ ing at low temperatures have been used, whereby condi¬ tions for the formation of nitrogen oxides have been disadvantageous. This, however, has resulted in a low coefficient of efficiency and the ability of the fur- naces to burn fuels of various kinds has deteriorated. The methods described above are commonly known, where¬ fore they will not be described more closely herein (see Finnish Ministry of Trade and Industry/Publica¬ tion series of the Energy Department, D:140 Helsinki 1987). The object of the present invention is to pro¬ vide a method by means of which the formation of nit¬ rogen oxides at the combustion stage can be reduced to a minimum and in which the formation of oxides can be constantly controlled without any expensive catalyst solutions difficult to control and the like solutions used in the methods described above. The invention is characterized in that the gases formed in the combus¬ tion in the first stage are cooled from a temperature obtained during the combustion, preferably as high as possible, to a lower temperature, and that more air is introduced among the gases at the earliest during the cooling of them so that substantially within the entire furnace the amount of air introduced among the gases is at each particular point at the most such as would be required for the formation of a predetermined concentration of nitrogen oxides at the temperature which the gases have at this particular point.

An essential idea of the invention is that the introduction of air into the combustion process is controlled so that the formation of nitrogen oxides at a temperature at different points in the furnace and with a corresponding ratio of air to fuel, remains be¬ low a sufficiently low level, that is, below a pre- determined concentration level, which is achieved by carrying out the combustion in a stepwise manner so that in the first stage air is introduced under- stoichiometrically while allowing the temperature to rise so as to improve the coefficient of efficiency to the highest possible value, whereafter the formed gases are cooled to a temperature such that the con¬ centration level of nitrogen oxides at this particular temperature remains sufficiently low when air is introduced in the second stage so that the total coef- ficient of air increases to a value at least 1. In this way, as much heat energy as possible is generated in the first combustion stage, and this heat energy can be transferred via a cooler to be further util¬ ized, e.g., by using the first superheater as a cool- ing means, after which the remaining heat energy can be utilized by introducing more air in connection with the superheater or after it by means of which the final combustion and further cooling of the flue gases will be effected. By means of the method according to the invention, it is possible to control the concen¬ tration of nitrogen oxides so that the balance concen¬ tration of nitrogen oxides contained in the flue gases, i.e., the maximum concentration, remains con¬ stantly below a preset limit value while the boiler operates with a coefficient of efficiency as high as possible.

A further object of the invention is to provide an apparatus for applying the method, the apparatus being characterized in that it comprises a cooler, preferably a superheater which is mounted in the fur¬ nace of the boiler after the first air supply means in the direction of flow of the gases contained in -the furnace, so that the gases formed in the first under- stoichiometric combustion stage are forced to flow through the cooler, and that the second air supply means are arranged to introduce more air into the fur¬ nace at the earliest at the cooler in the direction of flow of the gases.

An essential idea of the apparatus according to the invention is that the first superheater of the boiler is positioned in the furnace after the first combustion stage but nevertheless no further than in the second combustion stage so that the superheater separates the space for the combustion stage to be carried out under understoichiometric conditions from the rest of the furnace space, whereby the flue gases flow through the superheater, being simultaneously cooled according to the invention to a desired tem¬ perature, and more air is introduced into the super- heater or after it, whereby the flue gases are at the desired temperature after the introduction of air. Since the cooling effect of the superheater can be go¬ verned in a controlled manner, the temperature of the gases in the superheater or after it can be adjusted by introducing more air, and thereby it is possible to keep the temperature of the boiler at a value suitable for the kinetics of the formation of nitrogen oxides by adjusting the coefficient of air so that the forma¬ tion of nitrogen oxides can be kept on the allowed level or even below it.

The invention will be described in more detail in the attached drawings, wherein

Figure 1 illustrates the dependance between the temperature occurring in the method according to the invention and the coefficient of air in relation to one level of nitrogen oxides, and schematically the dependence between the coefficient of air and the tem¬ perature when applying the method according to the in¬ vention; and Figure 2 illustrates schematically an apparatus applied in the method according to the invention.

In Figure 1, the curve A illustrates schema¬ tically the influence of the dependance between the temperature in the furnace and the mole ratio between fuel and air, i.e. the coefficient of air, on the for¬ mation of nitrogen monoxide at a NO level of 100 ppm. When burning fuels with varying coefficients of air below 1, it has been found that the formation of nit¬ rogen monoxide is insignificant irrespective of the temperature and the coefficient of air. Accordingly, the maximum concentration of nitrogen monoxide in flue gases with a coefficient of air of 0.99, for instance, and within the temperature range from 700 to 1,500°C, is about 65 ppm, and correspondingly with a coeffi- cient of air of 0.95 only 10 ppm. The low concentra¬ tion of nitrogen monoxide is due to the fact that as there is only little oxygen present, carbon monoxide and hydrogen are formed in the combustion, and the nitrogen oxide possibly occurring is reduced by them through reactions approximately such as the following. NO + CO -> 1/2 N2 + C02 NO + H2 -> 1/2 N2 + H2O Thereby the concentration of nitrogen monoxide in the combustion carried out understoichiometrically in a manner known per se can be kept at a low value, and one faces with problems only when more air is intro¬ duced into the boiler for completing the combustion process. This is because at a high temperature even a small excess of air results in rapid formation of nitrogen monoxide in an amount hazardous to the envi¬ ronment? for instance, at a temperature of 1,500°C and with a coefficient of air of 1.01, the maximum con¬ centration of nitrogen monoxide is about 400 ppm and with a coefficient of air of 1.05 about 800 ppm. However, it has been found that the concentra¬ tion of nitrogen monoxide in a combustion process carried out at a low temperature is very low irrespec¬ tive of the coefficient of air (ranging from 0.9 to 1.3) to be used in a practical case. At a temperature of 950°C, for instance, the maximum concentration of nitrogen monoxide is about 120 ppm, which requires a high coefficient of air; at 700°C, the corresponding nitrogen monoxide value is as low as 10 ppm.

It has been unexpectedly found that these prin- ciples can be connected in one and the same combustion means in such a manner that the air required for the combustion is fed in at least two stages so that in the first combustion stage the coefficient of air is below 1, preferably from 0.90 to 0.97, whereby the amount of nitrogen monoxide formed at a high tempera¬ ture, typically 1,400°C, is insignificant. Thereafter the gases are cooled to a low temperature, preferably below 1050°C with the concentration of 100 ppm as a limit value, whereby more air is introduced among the gases either after the cooling or simultaneously with the cooling process, the formation of nitrogen monox¬ ides remaining insignificant all the time. By intro¬ ducing more air among the gases so that the total coefficient of air will be above 1.1, and the final temperature after completed combustion no more than 950°C, the entire combustion process can be carried out in a stepwise manner so that the formation of nitrogen monoxide constantly remains on a low level, that is, below the allowed level of nitrogen monoxide concentration. Figure 1 shows schematically by way of example the curve A representing the nitrogen monoxide concentration of 100 ppm as a function of the tempera¬ ture and the coefficient of air, and the different combustion stages of the method according to the in- vention in such a manner that the nitrogen monoxide concentration in the gases always remains below the value indicated by the curve A.

Figure 2 shows schematically an apparatus suited for applying the method according to the inven- tion, comprising a combustion means such as a boiler 1 or the like comprising a furnace 2. Fuel 3 is intro¬ duced into the furnace 2 by means of one or more feed¬ ing means 4, and primary air 5 is fed into the same portion of the furnace 2 by means of first air supply means 6. When the ratio of the primary air to the ture of the gases after the superheater is preferably below 1050°C when a concentration of 100 ppm is used as a limit value. These combustion stages are illus¬ trated in Figure 1, in which the line B-C represents understoichiometric combustion, the line C-D cooling without introduction of air, and the line C-E cooling by introducing secondary air 8. In cases where sec¬ ondary air 8 is introduced only after the superheater 7, the temperature and coefficient of air curve typi- cally ascends along the line D-E. After the addition of secondary air 8, tertiary air 10 is added to the gases by means of third air supply means 11, whereby the total coefficient of air preferably rises above the value 1.1 while the final temperature of the re- suiting flue gases drops as indicated by the line E-F so that it is not more than 950°C after the addition of tertiary air. Thereafter the flue gases are passed in a known manner into superheaters 12, represented schematically in Figure 2 by a single superheater, and further into a flue gas duct 13 in which their tem¬ perature drops as indicated by the line F-G.

Secondary air and tertiary air can, of course, be introduced at more than two points, or air can be introduced in connection with the cooling or after it at a single point, whereby the introduction of air is adjusted so that the principle represented by the tem¬ perature/coefficient of air nomogram B-F in Figure 1 will be followed.

One advantage to be obtained by means of the invention is that the apparatus can be constructed by means of inexpensive construction technical alterna¬ tives known from the prior art and no expensive cata¬ lysts or pressure chambers are required. Furthermore, the method according to the invention is easy to real- ize and can be controlled in a simple manner by apply- 8 amount of air theoretically required for complete com¬ bustion is below one, that is, the coefficient of air is preferably in the range from 0.90 to 0.97, there is a constant small undersupply of oxygen in that portion of the furnace with a resultant abundant formation of carbon monoxide and hydrogen. These substances prevent the formation of nitrogen monoxide, or if nitrogen monoxide should be formed, it is immediately reduced to nitrogens by these substances. Since the burning in this portion of the furnace 2 is nearly adiabatic, the temperature rises to a very high value, above 1,400°C, when carbon or oil is used. Gases formed during com¬ bustion in this portion of the furnace 2 are passed on into a cooler, preferably a superheater 7 of conven- tional construction, in which the temperature of the gases drops considerably due to the cooling effect of the superheater 7. In connection with or after the su¬ perheater 7, secondary air 8 is introduced among the gases by means of second air supply means 9 so that the total coefficient of air will be at least one. If secondary air 8 is not introduced until after the superheater 7, the temperature of the gases in the furnace may drop to a value as low as 650°C, whereby the sulphur dioxide contained in the flue gases is possibly at least partially reduced to hydrogen sul¬ phide. In view of the essential idea of the invention, it is, however, of importance that the temperature of the gases drops to such a low value that the formation of nitrogen monoxide is nevertheless insignificant in a later combustion stage, i.e., when the coefficient of air rises to an overstoichiometric value. The addi¬ tion of secondary air 8 may also be carried out in connection with the superheater 7, whereby the amount of secondary air is adjusted so that when the coeffi- cient of air is one or slightly higher, the tempera- ing the principles of the method and the apparatus de¬ signed accordingly by measures and means connected with the operation of a conventional boiler. In addi¬ tion, the desired maximum value of nitrogen monoxide 5 can be estimated in advance, and the combustion pro¬ cess can be effectively modified so that a prede¬ termined limit value will not be exceeded. The final temperature of flue gases as such does not have any decisive importance if only the temperature/coeffi-

10 ient of air situation corresponds to the desired con¬ centration of nitrogen monoxide or to a value below chat. When the temperature is about 950°C or slightly lower, an advantage is obtained in that the removal of sulphur oxides from flue gases is thereafter easy to

15 effect by prior art methods. Besides a separate cooler, the cooling can be effected by some other method, such as a thermodynamic process.

Claims

Claims :

1. A method of combustion for the reduction of the formation of nitrogen oxides in a combustion pro- cess, wherein the combustion is carried out by intro¬ ducing air required for the burning of fuel into a furnace (2) in at least two stages so that in the first stage the introduction of air is carried out understoichiometrically in relation to the fuel, pre- ferably with a coefficient of air ranging from 0.90 to 0.97, and in at least one stage overstoichiometrically so that the total coefficient of air exceeds the value one, c h a r a c t e r i z e d in that the gases formed in the combustion in the first stage are cooled from a temperature obtained during the combustion, preferably as high as possible, to a lower tempera¬ ture, and that more air is introduced among the gases at the earliest during the cooling of them so that substantially within the entire furnace (2) the amount of air introduced among the gases is at each par¬ ticular point at the most such as would be required for the formation of a predetermined concentration of nitrogen oxides at the temperature which the gases have at this particular point.

2. A method according to claim 1, c h a r a c¬ t e r i z e d in that more air is introduced among the gases overstoichiometrically in at least one stage, so that the total coefficient of air is at least 1.1, and that thereafter the temperature of the formed gases is not more than 950°C.

3. A method according to claim 2, c h a r a c¬ t e r i z e d in that air is introduced in at least three stages so that in the second stage air is intro¬ duced substantially simultaneously with the cooling of the gases so that the total coefficient of air is at the most one, and that the remaining air is introduced after the cooling of the gases.

4. A method according to claim 2, c h a r a c¬ t e r i z e d in that air is introduced in two stages so that in the second stage the introduction of air is carried out substantially simultaneously with the cooling of the gases.

5. A method according to any of the preceding claims, c h a r a c t e r i z e d in that the intro- duction of air into the furnace in the first stage is effected so that the temperature of the gases is as close to the adiabatic burning temperature thereof as possible.

6. An apparatus for applying a method according to claim 1, comprising at least first and second air supply means (6, 9) for introducing air into a furnace (2) in a boiler (1), c h a r a c t e r i z e d in that it comprises a cooler, preferably a superheater (7) which is mounted in the furnace (2) of the boiler (1) after the first air supply means (6) in the direc¬ tion of flow of the gases contained in the furnace, so that the gases formed in the first understoichiometric combustion stage are forced to flow through the cool¬ er, and that the second air supply means (9) are ar- ranged to introduce more air into the furnace (2) at the earliest at the cooler (7) in the direction of flow of the gases.