GB2226122A - Reducing nitrogen oxide formation during combustion - Google Patents

Description

1.

C c A method of combustion for reducing. the:formation of nitrog n oxides durinc g combustion and an apparatus for applying the method,& The invention relates to a method of combustion for reducing the formation of nitrogen oxides during combustion, wherein air required for the combustion of a fuel is introduced in at least two steps, the air being introduced understoichiometrically in the first step, preferably with an air coefficient ranging from 0. 80 to 0. 95, and a gas or gas mixture substantially free from elementary oxygen is mixed with the air to be introduced into the first step.

The invention is also concerned with an apparatus for applying the method comprising means for introducing air into a furnace, means for introducing fuel into the furnace, and means for mixing a gas or gas mixture containing less oxygen than air with the air to be introduced into the first understoichiometric combustion step before the air is introduced into the furnace.

All combustion processes produce nitrogen oxides when the nitrogen of both air and fuel combines with oxygen to form oxides of different kinds. In the reducing flame, NO, is derived mainly from the nitrogen of the fuel through rapid formation, that is, so called prompt NO, is obtained. At high temperatures, mostly nitrogen oxide (NO) is obtained. When the temperature drops, NO is easily converted into the other nitrogen oxides in the presence of oxygen, mainly into nitrogen dioxide (N02). The formation of nitrogen oxides occurs at a rapid reaction rate as soon as the required chemical equilibrium conditions are met, i.e., mainly at high temperature and in the presence of oxygen. If the equilibrium k 0 2 conditions are altered after the formation of nitrogen oxides so as to cause decomposition of the nitrogen oxides, the reaction rate of the decomposing process is very slow, the decomposing requiring mainly time, catalysts or additional chemicals. From the environmental point of view, nitrogen oxides are highly disadvantageous. They are formed abundantly in industrial processes as well as in power plants and other boiler works, and one of the most important objectives of environmental protection is to reduce NOx emissions to the atmosphere.

In an attempt to reduce NOX emissions, nitrogen oxides are converted into another form in various ways. Such techniques include various reduction methods based on the use of catalysts, and the use of absorbing agents for simultaneous absorption of sulphur and nitrogen oxides In various ways. These methods involve various problems difficult to solve, such as the high price and difficult availability of precious metals used as catalysts and the poor absorption properties of the absorbing agents. Moreover, it is often difficult to dimension the apparatus when applying absorption methods due to variation in boiler capacities and other such factors.

Technically, it is more advantageous to try to prevent the formation of nitrogen oxides during the combustion step instead of removing them. For this purpose, a variety of low NO, burners have been developed, and attempts have been made to carry out the combustion in a pressurized space, in addition to which supply of air Into the boiler has been carried out in stages before the superheaters. Contrary to expectations, these methods, however, have not providdd any particularly good results, because in prac- 1 1 3 Q tice the operation of the methods has been prevented or substantially deteriorated by such factors as variation in the formation conditions of nitrogen oxides, reaction kinetics, operational conditions of boilers and variation occurring therein. Furthermore, removal of nitrogen oxides has been attempted by means of circulation bed furnaces operating at very low temperatures (about 800"C), that is, at conditions disadvantageous for the formation of NO.. This, however, has deteriorated the efficiency of the furnaces as well as their ability to burn different kinds of fuel -as it has been necessary to drop the temperature as low as near the minimum temperature required for continuously maintaining the combustion process. The methods described above are widely known and therefore will not be described more closely (Finnish Ministry of Trade and Industry/Energy Department D:140, Helsinki 1987).

DE Offenlegungsschrift 30 40 830 discloses a method in which completely combusted cooled flue gas obtained from a gas flue after the boiler is mixed with the air to be introduced into the understoichiometric first combustion zone in order to reduce the amount of nitrogen oxides. Even though this method helps to prevent the formation of nitrogen oxides to some extent, it does not enable sufficient control of the amount of nitrogen oxides. In addition, the recycling of the flue gas increases the mass flow of gas flowing through the boiler, thus requiring a somewhat larger combustion space and larger conduits everywhere in the boiler.

The NO content is usually low within the reducing area, depending on the reducing effect of hydrogen (H2) and carbon monoxide (C02). These substances decompose the possibly formed No roughly ac- k_ 0 4 cording to the following reactions:

NO + CO -> 1/2 N2 + C02 NO + H2 -> 1/2 N2 + H20 In combustion carried out understoichiometrically in a manner known per se, the NO concentration can, in principle, be kept on a low level. Problems occur only when the conditions become reducing or when very high temperatures occur, that is, over 15000C. The problems result even from a minor excess of air, which under furnace conditions causes rapid formation of NO, or from very high temperatures (over 1500"C) at which H2 and CO cannot any more prevent the formation of NO due to their reduced reducing potential. In prior art apparatuses such situations occur particularly in the primary flame but also in connection with the introduction of secondary and tertiary air. One of the most important reasons for the formation of NO in the primary flame of prior art apparatuses is that the heterogeneous flame contains, e.g., oil drops or carbon particles and, as a consequence, there occurs high concentration gradients of oxygen and burning gases as well as high temperature gradients. Thereby it is always possible that minor temperature peaks occur locally at phase boundaries, for instance, if the amount of oxygen at such a point is stoichiometric or slightly overstoichiometric. In a typical combustion apparatus, the temperature may rise instantaneously and locally up to about 2000C. As a result, the local NO concentration rises rapidly up to about 3500 ppm (prompt NOx). The NO so formed will not decompose to any greater degree under boiler conditions. Accordingly, it is obvious that even minor locally and instantaneously occurring temperature peaks increase rapidly the average NO value of exhaust gas, which should remain on a concentration

C.

G level of about 100 ppm.

The object of the present invention is to provide a method by means of which the formation of NOx during a reducing combustion step, usually in so called primary combustion, particularly in the f lame can be minimized and in which conditions prerequisite for the formation of NOx are prevented without any complicated apparatuses. Removal of NOx after combustion is not required. The method is characterized in that a gas or gas mixture containing reducing agents, such as H2 and CO, is mixed with the air to be introduced into the first step, that the oxygen content of the gas mixture introduced into the first step is preferably 12-19%, and that the oxygen content and reducing potential of the air mixture to be introduced are adjusted so that the nitrogen oxide concentration of flue gas from combustion carried out at the adiabatic combustion temperature of the fuel used, corresponding to the supplied oxygen content and reducing potential, is no more than a predetermined concentration value.

The basic idea of the invention is that air is introduced into the combustion process in such a manner that the NOX formation in the reducing part of the furnace, particularly in the difficultly controllable flame, remains on a sufficiently low level at all temperatures and oxygen/fuel ratios possibly occurring during this combustion step. This is achieved by carrying out the combustion under reducing conditions by using a gas or a gas mixture having an oxygen content lower than that of ordinary air and containing reducing agents. By means of the method according to the invention, the concentration of nitrogen oxides can be controlled so that the equilibrium concentration of the nitrogen oxides in 6 0 the flue gas, in practice, also the maximum concentration, remains all the time at a very low value.

A further object of the invention is to provide an apparatus for applying the method. The apparatus is characterized in that the mixing means comprise at least one gas flue for passing part of the f lue gas from the first combustion step into the air to be introduced into the first step to be mixed with it.

The basic idea of the apparatus of the invention is that the reducing gas or gas mixture, that is, non-oxygen-containing or low-oxygen-content gas containing reducing agents, and air are mixed thoroughly with each other and introduced at least into the boiler zone in which fuel and air are normally poorly mixed with each other so that local temperature peaks are likely to occur. Typically, this zone is the reducing combustion zone of the boiler, mainly the flame.

The invention will be described in greater detail in the attached drawings, wherein Figure 1 illustrates the interdependence of the temperature and the air coefficient (ratio of oxygen to the amount of theoretical oxygen required for the combustion irrespective of the other components present in the gas mixture, such as inert components and reducing agents) in a prior art application with respect to a predetermined NO concentration level when the combustion is carried out normally with air, and the interdependence of the adiabatic temperature and the air coefficient in a typical oil combustion process carried out normally with air and with a mixture of air and gas consisting of completely combusted flue gas from a boiler, the mixture having an oxygen content of 17% (see DE Patent Application 3 040 830);

Figure 2 illustrates by way of example the 7 a interdependence of the maximal NO amount obtained in the combustion of pure methane (CH4) and the air coefficient when the gas maintaining the combustion is air, a mixture of completely combusted flue gas and air, as disclosed in Figure 1, or a mixture of cooled gas recycled from the reducing combustion and air; and Figure 3 is a schematic view of an apparatus for applying the method of the invention.

In Figure 1, the curve A-B shows by way of example the adiabatic combustion temperature of a widely used oil type as a function of the air coefficient when the combustion is carried out normally with air. The curve C-D illustrates by way of example the adiabatic combustion temperature of the same oil type as a function of the air coefficient when the combustion is carried out with air diluted with completely combusted flue gas, the oxygen content of the mixture being 17%. The curve E-F shows by way of example the pairs of temperature and air coefficient values corresponding to the NO concentration 100 ppm when the combustion is carried out normally with air. Above the curve the NO concentration is more than 100 ppm. Very high temperatures (over 1500"C) in particular are important for the invention. As the local temperatures in the hottest portions of the flame may rise very close to the adiabatic temperature, it is to be seen from the figure that the NO concentration of 100 ppm (point G) can be achieved with an air coefficient as low as 0. 82 when the combustion is carried out normally with air. When using air diluted with completely combusted flue gas, the concentration of 100 ppm is achieved with an air coef f ici ent of 0.93 (point H). At worst, the maximum NO concentratúon within the reducing area is about 2700 ppm when 8 0 the combustion is carried out normally with air and only 800 ppm when the coqoustion is carried out with diluted air as disclosed In the example. The firstmentioned value is represented by point I and the last-mentioned by point J in Figure 1.

It has now been found unexpectedly that the NO formation can be prevented within the reducing area of the burner, particularly in the flame, when oxygen required by the combustion is introduced understoichiometrically and at an uniform oxygen content of less than 21% by mixing a gas or gas mixture containing considerable amounts of reducing agents with the combustion air. In this way the combustion temperature, particularly that of the flame, can be decreased, simultaneously increasing reducing potential so that abundant formation of NOX is no more possible, not even locally or instantaneously. This preferably takes place so that the local temperature peak of the flame does not exceed about 1500"C and the oxygen concentration is reduced by means of cooled flue gas recycled from the reducing combustion step, typically containing hydrogen (H2) and carbon monoxide (CO). The formation of NOx is efficiently prevented both by the temperature drop and the increase in the reducing potential.

In Figure 2, the curve K-L illustrates the maximum NO concentration obtained in the combustion of pure methane (CH4) when the combustion is carried out normally with air and the burning takes place adiabatically. The curve M-N illustrates the maximum NO concentration when completely combusted flue gas has been mixed with the combustion air. The curve 0N, in turn, illustrates an example where the oxygen content of the combustion air has been decreased by adding to it properly cooled flue gas from the re- 9 ducing step, operated with the same air coefficient, in an amount of 24% on the volume flow of the primary air, whereby reducing agents H2 and CO will also be recycled to a considerable degree. It clearly appears from the f igure that the recycling of reducing gases decreases greatly the NO concentration while the temperature is decreased and the reducing potential increased. For instance, the maximum decrease in NO concentration with the air coefficient 0.80 is as great as 97% with the air coef f icient 0. 80 as compared with combustion with air, that is, the concentration drops from 0.048 mol NO/kg CH4 (point P in Figure 2) to 0. 0012 mol NO/kg CH4 (point Q in Figure 2) and about 73% on the value obtained when completely combusted f lue gas is mixed with the combustion air (corresponding to the ratio between Q and R). When the method of the invention is applied, the oxygen concentration as well as the amount and reduction capability of the reducing agents, that is, their reducing potential, can be adjusted in a desired manner according to the used fuel and other combustion conditions.

It has been unexpectedly found that the maximum efficacy of the method of the present invention, that is, the greatest decrease in NO formation as compared with the prior art, is to be obtained with air coefficients ranging from 0.80 to 0.95. It is likewise unexpected that the maximum decrease in NO concentration occurs within the air coefficient range conventionally applied in typical prior art primary combustion in power plant boilers. Accordingly, the method of the present invention decisively decreases NO, úormatlon In the flame of the burner, that is, the formtIon of prompt NO,, which has been most difficult if not Impossible to prevent in prior art appar-

0 atuses. When comparing the curves M-N and O-N of Figure 2, it appears that when a gas or gas mixture containing reducing agents is mixed with air according to the invention, the air coefficient 0.95 still gives a NOX concentration (point S) which is about 92-W lower than that obtained by combustion with ordinary air (point T) and 40% lower than the value obtained by adding completely combusted flue gas (corresponding to the ratio between points U and S). Furthermore, the use of completely combusted flue gas increases the amount of gas used, which requires a greater boiler and greater gas flues, whereas in the method of the invention the increased amount of gas and the greater space requirement concern only that part of the boiler in which the reducing combustion takes place. As further appears from Figure 2, the curves M-N and O-N join when the air coefficient is 1, which is due to the fact that f lue gas f rom f uel burned with stoichiometric ratio cannot any more contain reducing agents to any greater degree. This, however, is unimportant for the end result in combustion processes carried out with lower air coefficients. Essential in the invention is that local overheating is prevented in an understoichiometric combustion so that nitrogen oxides will not be formed.

Figure 3 shows schematically an apparatus f or applying the method of the inventiqn. The apparatus comprises a burner such as a boiler 1 with a furnace 2. Fuel is introduced into the furnace 2 by means of one or more feeding devices 3. Oxygen-containing gas mixture required for the combustion is introduced into the same part of the furnace 2 through a conduit 4 belonging to air supply means. Air is supplied into the cc?nduit 4 through a conduit 5, while reducing c gas, that is, at least substantially non-oxygen-containing gas mixture containing substantial amounts of reducing agents, mainly H2 and CO, is supplied through a conduit 6 belonging to mixing means via a blower 7 and a gas mixer 8. The gas to be mixed is preferably flue gas derived from the furnace 2, in which reducing combustion takes place. The flue gas, which contains reducing agents, is cooled by means of coolers 9 and 10 and its amount is controlled by means of a valve 11. The reducing flue gas is mixed in the mixer 8 with the air to be introduced into the furnace. A major portion of the flue gas produced during the reducing combustion step is passed into subsequent combustion steps, shown schematically with a single combustion step 12. During the subsequent combustion steps, additional air is introduced into the boiler by means of a valve 13 through a conduit 14, whereby the fuel will be combusted as completely as possible. At this stage the flue gas can be cooled by means of a heat exchanger 15 and thereafter by means of coolers 16, whereaf ter it is passed through a blower 17 into a gas f lue 18. Depending on the required amount of reducing agents, f inal cooled f lue gas can be mixed with the air to be introduced into the furnace 2 during the first step in a manner known per se through a conduit 19 in addition to the f lue gas from the reducing step introduced through the conduit 6, the amount of the f inal f lue gas being adjusted by a valve 20. In this way, both the oxygen content of the mixture of air and gas to be introduced into the furnace and the concentration of the reducing agents in it can be adjusted according to the combustion conditions and fuel used. If there exists a danger that the f lame or a portion of it becomes too hot at the beginning of the reducing com- 12 OD bustion step 12 with resultant excessive formation of nitrogen oxide, the flame temperature during this step can also be decreased by feeding reducing. gas through a valve 21 and a conduit 22 at the beginning of the reducing combustion step 12. Heat losses from the burner can b decreased by insulating the combustion chambers by insulations 23 and 24, shown schematically in the figure.

It Is to be understood that some of the devices described above can be combined into a single entity to obtain a structurally more advantageous solution. For instance, the parts 2, 10, 12, 15 and 16 can be easily combined.

It is essential in the apparatus of the invention that the air and reducing gas are mixed properly before their introduction into the reducing part of the furnace and that the temperature of the flame or a flame portion is decreased only to such an extent as is required for preventing the formation of NOwithout any risk of the combustion process being interrupted. In the present method the mix ratio of air and fuel is determined, e.g., by the thermal value of the fuel used, the minimum temperature required for maintaining combustion, the chemical analysis of the gas, the desired NOX level, the dimensions of the heat surfaces of the boiler, the degree of cooling (temperature) of the recycled gas, and the positions of the gas introduction steps. As a consequence, this ratio may vary widely; typically, the amount of gas is 10 to 70% on the amount of air supplied.

It is obvious that when the same combustion efficiency is to be obtained, the mass volume of the gas used In the apparatus of the invention is greater as compared with prior art apparatuses, though mainly -1 13 19M only in the reducing part of the boiler. However, the dimensions of the boiler will not change to any greater degree because the recycling preferably takes place only during the understoichiometric combustion step(s) and because increase in the flow volume of gas is for a major part compensated for by change in -'the gas density caused by the temperature drop. It is also obvious that, theoretically, the recycling of gas does not reduce the efficiency of the boiler; varying heat losses, however, may result in slightly reduced efficiency. In view of the advantages obtained, this drawback is not of any greater importance.

The invention has the advantage that the apparatus can be constructed by means of well-known inexpensive constructions and no separate expensive means for removing NOx are needed because the formation of NOx has been prevented sufficiently. Further, the method of the invention is easy to realize and very easy to control when its principles are applied to apparatuses and control systems known per se. It is likewise possible to control the high local formation of NO, in the hottest, spotlike portions of the flame of the burner because the formation of NOX is so restricted that its concentration cannot exceed a set limit value. The NO concentration of the flue gas emission to the surrounding is, of course, dependent on the operational properties and structure of the oxidizing part of the boiler.

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Claims (12)

1. CLAIMS 1. A method for reducing the formation of nitrogen oxides during

   combustion, wherein air required for the combustion of a fuel is introduced in at least two steps, and wherein the air is introduced understoichiometrically in the first step, and a gas or gas mixture containing a reducing agent and substantially free from elementary oxygen is mixed with the air to be introduced in the first step. and the oxygen content and reducing potential of the air mixture to be introduced are adjusted so that the nitrogen oxide concentration of flue gas from combustion carried out at the adiabatic combustion temperature of the fuel, corresponding to the supplied oxygen content and reducing potential, is no more than a predetermined concentration value.
2. 2. A method according to claim 1, wherein the gas mixture to be introduced is obtained by mixing flue gas formed after the adiabatic combustion step with air.
3. 3. A method according to claim 2, wherein the flue gas is cooled before being mixed with air.
4. 4. A method according to claim 2 or claim 3, wherein the said combustion step is the first combustion step.
5. 5. A method according to any preceding claim, wherein' the air introduced in the first step has an air coefficient of 0.80 to 0.95.
6. 6. A method according to any preceding claim, wherein the reducing agent is H2 and CO.
7. 7. A method according to any preceding claim, wherein the oxygen content of the gas mixture is 12 to 19%.
8. 8. A method according to claim 1, substantially as herein described.
9. 9. Apparatus. suitable for applying a method according to any preceding claim, comprising means for introducing air into a furnace; means for introducing fuel into the furnace; and means for mixing a gas or gas mixture 1 n -is- 8 containing less oxygen than air with the air to be introduced into the. first understoichiometric combustion step before the air is introduced into the furnace, wherein the mixing means comprises at least one gas flue for passing part of the flue gas from the first combustion step into the air to be introduced into the first step to be mixed with it.
10. 10. Apparatus according to claim 9, wherein the mixing means comprises means for cooling the flue gas before it is mixed with the air.
11. 11. Apparatus according to claim 9, substantially as herein described with reference to Figure 3 of the accompanying drawings.
12. 12. A method according to any of claims 1 to 8, when conducted in apparatus according to any of claims 9 to 11.

    lpubils,Imi11990 atThePatent=ce,State House, 66171 Holborn.IondonWC1R4TP. Further copies maybe obtainedfrom The Patent Office. hu Villt.inIpc techninues ltd. St Marv Crav. Kent. Con. 1187