FI125314B - Process for reducing nitric oxide emissions and corrosion in a BFB boiler and a BFB boiler - Google Patents

Description

Method for reducing nitrogen oxide emissions and corrosion in a fluidized bed boiler and a fluidized bed boiler

The invention relates to a method according to the preamble of claim 1 for reducing nitrogen oxide emissions and corrosion in a fluidized bed boiler burning biofuel or waste. The invention also relates to a fluidized bed boiler according to claim 4.

Fluidized bed boilers are commonly used to generate energy from various biofuels (peat, wood, industrial waste sludge, etc.) and oil-based waste (eg plastic waste). This type of fuel is characterized by a high volatile matter content in the dry matter of the fuel.

In fluidized bed combustion, nitrogen oxides (NOx) are produced mainly as a result of the oxidation of organic nitrogen in the fuel. Nitrogen oxide emission limits for fluidized bed boilers will be significantly tightened in the coming years. The amount of nitrogen oxides can be reduced e.g. combustion techniques, injection of ammonia into the furnace and catalytic purification of the flue gases. Combustion techniques prevent the formation of nitrogen oxides or convert the nitrogen oxides formed in previous combustion steps to other compounds. The present invention relates to a combustion process and a related apparatus construction.

At the bottom of the furnace in a fluidized bed boiler is a bubbling fluidized bed or fluidized bed consisting of a finely divided non-combustible material, such as sand, which is fluidized by supplying fluidizing gas from below. The fluidizing gas may consist exclusively of primary air or may be a mixture of primary air and an inert gas such as flue gas. The fluidization gas flow rate is set such that the particles of the fluidized bed do not escape with the gas flow to the top of the boiler, but remain at the bottom of the furnace to form a continuous fluidized bed which effectively mixes bed material and fuel.

The combustion air required for the combustion of the fuel is generally supplied in stages and in several portions to the furnace of the boiler such that a portion of the combustion air consists of primary air or fluidized air blown with the fluidizing gas, a portion of the secondary air supplied above the fluidized bed and the rest of the combustion air.

The combustion chamber of the fluidized bed boiler can be divided in height into three successive combustion zones based on the combustion air supplied to the various zones. The fluidized bed and the space above it below the secondary air nozzles form the first combustion zone (I), which supplies the primary air closest to the fluidized bed supplied to the fluidized bed, in addition to a small amount of primary air as a supply air as fuel and ice. The second combustion zone (II) is located between the secondary air supply plane and the tertiary air supply plane. The second combustion zone (II) is supplied with secondary air mainly through the secondary air nozzles located at the bottom of the zone. If the furnace has load burners, a small amount of secondary air is supplied as cooling air to the load burners. The third combustion zone (III) begins at the tertiary air supply plane and is supplied with combustion air through the tertiary air nozzles.

The fuel is supplied to the fluidized bed by means of carrier air, which serves e.g. prevents clogging of fuel feed hoses. In and above the fluidized bed, the fuel particles dry, the volatile material is released, i.e. pyrolysis, and the remaining carbon residue is burned. Dehydration and pyrolysis are very rapid events. The volatiles released in the pyrolysis are mainly methane CH4 and carbon monoxide CO as well as ammonia NH3 and hydrogen cyanide HCN. The volatiles rise up in the furnace and burn when they reach the oxygen-rich area. In a fluidized bed boiler with a stepped air supply, the combustion of volatile substances is mainly effected by secondary air and partly by tertiary air. The combustion of the carbon particle in the fuel particle is effected by primary, secondary and tertiary air.

With the help of a phased air supply it is possible to reduce the formation of nitrogen oxides in a fluidized bed boiler. In the presence of oxygen, NH3 and HCN react with NO to form nitric oxide. By phasing the air supply into the furnace of the fluidized bed boiler, reducing sub-free areas are formed. In these regions, the NH3 and HCN formed from the fuel are reduced to the molecular nitrogen according to reaction equations (1) and (2):

In addition, the oxides of nitrogen are reduced by an internal rebuming reaction in which the radicals formed in pyrolysis are involved in the reduction of nitrogen oxides. An example of such a reaction is described in Reaction Equation (3) wherein the hydrocarbon radical is -CHp

Generally, the reducing regions are formed by controlling the amount of primary and secondary air. The furnace is kept subtotal to oxygen until the tertiary air supply, thereby maximizing the dwell time available for reactions (1) and (2) and minimizing the amount of NH and HCN prior to the tertiary air level. Depending on the combustion temperature, the total air coefficient SRtot, optimum for NOx emissions, is slightly below 1. Depending on the combustion temperature, the air needed to burn off volatile matter and carbon residue is introduced into the furnace as tertiary air. The NH3 and HCN remaining in the flue gases after oxidation with tertiary air are oxidized to nitrogen oxides. With these traditional phased air supply measures, nitrogen oxide emissions can be reduced by about 30% compared to non-phased air supply. Fine and light fuels are problematic because some of the fuel particles do not reach the fluidized bed but are trapped with the fluidizing gas and secondary air in the upper parts of the furnace. Thus, it is almost impossible for such fuels to provide controlled combustion conditions in the furnace that are favorable for the reduction of nitrogen oxides. WO-A-2006084954 A1 describes a solution which seeks to reduce nitrogen oxide emissions from a fluidized bed boiler using a stepped air supply such that a portion of the primary air is fed to the furnace in the same direction as the fuel itself when the fuel is supplied. This part of the primary air is called volatile combustion air in the publication. Thus, substantially all of the fuel fed to the furnace is forced onto the surface of the fluidized bed, causing the fuel to mix with the fluidized bed and dry rapidly. Pyrolysis following drying and combustion of volatiles released in pyrolysis also occur almost immediately after the fuel is mixed with the fluidized bed. Due to the rapid mixing of fuel and oxygen, most of the volatiles released from the fuel can be burned above the fluidized bed before the secondary air feed. Combustion of volatiles results in high temperatures, which maximizes the formation of carbon monoxide radicals in the fuel and contributes to the reduction of released nitrogen oxides.

In accordance with WO 2006084954 A1, the amount of volatile material fed into the fuel feed is adjusted so that the volatile materials released from the respective fuel in pyrolysis are burned with respect to the volatiles under submetric conditions. The air coefficient for volatile substances, i.e., SRVi in the primary air zone, is then as high as possible, however less than 1, preferably between 0.75 and 0.97 and most preferably between 0.90 and 0.95. The total air coefficient or SRtot at the same furnace height ranges from 0.50 to 0.80, preferably 0.65. The solution does not change the amount of air supplied to the furnace or the total air factor, but modifies the distribution of air in the boiler to maximize the air volatility of the fuel in the furnace as low as possible and as long as possible before the secondary air supply level.

The stoichiometric ratio SR indicates how much air is available for combustion compared to the theoretical (stoichiometric) air needed for complete fuel combustion. In the case of underfree combustion the air coefficient SR is less than 1 and in the case of overfree combustion the air coefficient SR is above 1.

In an effort to reduce nitrogen oxide emissions by combustion techniques in existing boilers, the temperature of the flue gas should not generally increase from current values at the boiler furnace spout. If the temperature of the flue gas becomes too high in connection with the combustion engineering changes or if the temperature distribution of the flue gas is very uneven, the heat transfer surfaces of the second draft of the boiler will become dirty, especially when burning fuels containing high alkali metals. On the other hand, high flue gas temperatures can cause severe corrosion of the superheater area with chlorine-containing fuels.

Fluidized bed boilers are typically manufactured for various firebox stresses. According to one classification, low fire load is about 90-110 kW / m3, medium fire load is about 110-130 kW / m3 and high fire load is about 130-150 kW / m3. In high-stove boilers, the combustion temperature is high, whereby the fluidized bed temperatures can easily exceed the allowable temperatures for ash softening. Typically, the temperature of the fluidized bed should be maintained within the range of 800 to 900 ° C. If the temperature of the bed rises well above 900 ° C and the fuel contains a lot of alkali metals, this may result in the agglomeration of the ash particles in the fluidized bed.

The object of the invention is to provide a combustion process and a fluidized bed boiler which can reduce nitrogen oxide emissions in the combustion of biofuel or waste, while preventing corrosion of the furnace and superheater area.

The method according to the invention is characterized in what is stated in the characterizing part of claim 1.

Correspondingly, the sandwich fluidized bed boiler of the invention is characterized in what is set forth in the characterizing part of claim 4.

The invention relates to a process for reducing nitrogen oxide emissions and corrosion in a fluidized bed boiler burning biofuel and / or waste. In this method, the combustion air required for the combustion of the boiler is supplied in a stepwise manner such that at least two sub-free zones (I) and (II) are formed, one of which is to reduce nitrogen oxide formation, and at least one over-free zone (III), where the combustion is completed. The combustion zones comprise a first combustion zone (I) which starts at the height of the primary air supply nozzles and extends above the fluidized bed to below the height of the secondary air nozzles; a second combustion zone (II) starting at the height of the secondary air nozzles and extending below the height of the tertiary air nozzles; and a third combustion zone (III) beginning at the elevation of the tertiary air nozzles.

In the method according to the invention, the length of the first combustion zone (I) is optimized by positioning the secondary air nozzles at a height hs above the surface of the bubbling fluidized bed calculated from the formula:

where: hs = height of secondary air plane above top surface of bubbling fluidized bed [m]

Qf = full boiler fuel capacity [MW] A = boiler cross section [m2] qf = full boiler firing load below the beak [kW / m3] LHVref = lower calorific value of fuel [MJ / kg]

Pref = fuel density [kg / m3] qf, ref = furnace load [kW / m3] fa, ref = air volatile fuel requirement [kg air / kg fuel] hef = residence time (NTP in normal mode, T = 0 ° C, p = 1) atm) [s] SRvoi, ref = air coefficient for volatile fuel in the first combustion zone (I), where in the formula some variables are furnace-specific variables, some variables are reference values and thus constants, and some variables are design point-specific refractive values by.

Some of the variables used in the calculation, such as the full fuel efficiency of the boiler Qf, the cross-sectional area A of the boiler, and the full furnace load of the boiler below the beak qf, are specific to the furnace.

The reference values, which are the lower calorific value of the fuel LHVref, the density of the fuel pref, the fire load qf, ref and the air requirement fa, ref of the fuel volatiles are empirically chosen constants.

The residence time tref and air coefficient SRvoi, ref in relation to the volatile fuel in the first combustion zone (I) are the reference values per design point. The design point to be used at any given time will be selected on the basis of a furnace-specific consideration. Typically, in a high-stove boiler, the residence time tref is shorter and the volatile air coefficient SRVo, ref is lower than in a low-stove boiler.

Preferably, the design point-specific reference values tref and SRvoi, ref are selected based on the boiler firing stress such that tref = 6 s and SRvoi, ref = 0.7 are used in the high furnace stress boiler and tref = 7 s and SRvoi, ref = 0.9 are used. .

According to one embodiment of the invention, a boiler with a qf > 130 kW / m3 is considered a high-firing boiler and a boiler with qf < 130 kW / m3 is a low-firing boiler. If necessary, the limit of 130 kW / m3 may be departed from, for example, when the fuel mainly used in the boiler is particularly dry or extremely wet.

The invention also relates to a fluidized bed boiler comprising a furnace having a fluidized bed at its lower end fed with primary air through the primary air nozzles below the fluidized bed and fuel through the fuel inlet openings on the upper side of the fuel inlet opening. The furnace also has a plurality of secondary air nozzles disposed at a second height above the surface of the fluidized bed and a plurality of tertiary air nozzles disposed at a third height above the surface of the fluidized bed. Thus, a first combustion zone (I) is formed in the furnace furnace furnace furnace, comprising the fluidized bed and the volume above it up to the height of the secondary air nozzles, the second combustion zone (II) extending from the height of the secondary air nozzles and (III) beginning at the level of the tertiary air nozzles. Said first combustion zone (I) and second combustion zone (II) are arranged to operate as sub-detectors such that the total mesh coefficient SRtot is less than 1 in each zone.

In the fluidized bed boiler according to the invention, the secondary air nozzles are located at a certain height above the top surface of the bubbling fluidized bed calculated by the following formula:

where: hs = height of secondary air plane above top surface of bubbling fluidized bed [m]

Qf = full boiler fuel capacity [MW] A = boiler cross section [m2] qf = full boiler firing load below the beak [kW / m3] LHVref = lower calorific value of fuel [MJ / kg]

Pref = fuel density [kg / m3] qf, ref = furnace load [kW / m3] fa, ref = air volatile fuel requirement [kg air / kg fuel] W = residence time (NTP in normal mode, T = 0 ° C, p = 1) atm) [s] SRo, ref = air coefficient for volatile fuel in the first combustion zone (I), where in the formula some variables are firebox specific variables, some of the variables are reference values and thus constants, and some of the variables are design point specific refs, based on fireside stress.

If combustion is to be technically minimized by nitrogen oxide emissions while minimizing the temperature of the flue gas at the furnace nozzle, the fluidized bed boiler should always be operated so that the air coefficient of volatile matter in the first combustion zone (I) is 0.9 to 1.0. Raising SRv to close to 1.0 by supplying to the first combustion zone (I) the theoretical amount of oxygen required for combustion of the volatiles will result in an increase in the combustion temperature at the top of the fluidized bed. Thus, increased heat radiation also raises fluidized bed temperatures typically from about 20 ° C to about 50 ° C. If the fluidized bed boiler is made with high furnace stress and if the fuel is particularly dry (water content less than 45% by weight), the temperature of the fluidized bed may rise above the allowable temperature limits.

One way to lower the fluidized bed temperature of a fluidized bed boiler is to reduce the masonry surface of the boiler in the area between the primary air level and the secondary air level. In presently typical fluidized bed boilers, the lower part of the furnace is masonry about 2.5 to 5 meters above the surface of the bubbling fluidized bed. The purpose of masonry is to protect the water pipes of the boiler from corrosion and contamination, but at the same time it also raises the temperatures of the upper part of the fluidized bed, because the masonry prevents radiation heat transfer to the water pipes surrounding the furnace.

In the solution according to the invention, it is recommended, in particular for boilers with a high fire load, to move to a masonry height of 1.8 to 2.4 meters from the surface of the fluidized bed, preferably 1.8 to 2.0 meters from the surface of the fluidized bed.

Even after this, if the fluidized bed temperatures were too high, the nitrogen oxides can be reduced by keeping the volatile air coefficient SRvoi low in the first combustion zone (I), e.g. close to 0.7, and short residence time in the first combustion zone (I). The short residence time is achieved by placing the secondary air supply plane relatively close to the surface of the fluidized bed above which the optimum distance can be calculated using the mathematical formula set forth in the claims. The second combustion zone (II) following the first combustion zone (I) may then have a longer delay, whereby the combustion of the fuel volatiles and the nitrogen oxide minimizing reactions can be completed in the second combustion zone (II).

The invention comprises two alternative ways of reducing nitrogen oxides in the flue gases of a fluidized bed boiler. The method used is primarily based on the strain on the boiler. When the combustion load of the fluidized bed boiler is high -5 (e.g. above 130 kW / m) and especially when burning dry fuel (e.g. humidity less than 45% by weight), the first combustion zone (I) according to the invention does not utilize the optimum air volatility 0.9 - 1.0 but operates in the lower SRVi range of 0.65 - 0.75. In contrast, when the furnace strain is low (e.g., less than 130 kW / m3), the first combustion zone (I) utilizes an optimum SRvO range of 0.9 to 1.0 for nitrogen oxide reduction. The air coefficient SRv of the volatiles in the first combustion zone (I) can be increased, for example as described in WO 2006084954 A1, by supplying to the first combustion zone (I) with the supply of fuel a desired amount of combustion air preferably from a secondary air register 165-130 ° C. . WO 2006084954 A1 defines air coefficients for different combustion zones, but does not take a position on its overall air distribution waste for design criteria and optimum equipment placement. Nor does the publication address the limitations of the combustion process, such as high furnace strain. Therefore, the air coefficients for two different methods as defined in WO 2006084954 A1 do not contain sufficiently detailed basic design information.

In the fluidized bed boiler according to the invention, the fuel can be used e.g. peat, wood, industrial sludges, chlorinated plastic waste and mixtures thereof. The optimum air coefficient for volatile fuel, ie SRVi in the first combustion zone (I) depends on the fuel used. For example, the optimal air coefficient for peat is SRvoi = 1.0 and for wood SRvoi = 0.9.

Also, the fuel feed at the openings distance from the surface of the bubbling fluidized bed is important. When the first combustion zone (I) operates in the range SRVo = 0.9 to 1.0 and the fuel is supplied with volatile fuel air, the fuel inlets should not be too close to the bubbling fluidized bed surface and the vertical fuel feed angle should be too steep to avoid the risk of the volatile combustion air supplied enters particles from the fluidized bed that are carried with the flue gas to the boiler outlet.

Preferably, the fuel inlets are arranged at a height of 2.0 to 2.6 meters above the surface of the fluidized bed in a low-firing strainer and 1.8 to 2.0 meters above the surface of the fluidized-bed in a high-firing strainer. The vertical feed angle of the fuel horns, herein referred to as the normal angle between the fuel horn and the furnace wall, may be from about 25 ° to about 35 ° in a low firing boiler and from about 35 ° to about 40 ° in a high firing furnace.

There are generally two or three fuel inlet hoses and fuel inlets on one or both side walls of the fluidized bed boiler, at the same height. The inlet hoses are generally disposed perpendicular to the wall in question, with the fuel flow exiting the fuel inlet in the normal direction of the wall. There is a risk that the fuel supply openings in the outermost, i.e. closest to the front or rear wall of the furnace may spread the fuel to the front or rear wall of the furnace. This danger can be avoided by turning the outermost fuel feed horns horizontally so that the fuel discharged from the feed openings is 5 - 15 ° away from the boiler side wall closest to the feed opening. This angle is referred to as the horizontal feed angle of the fuel horn. Controlling the fuel flow away from the adjacent wall is particularly important when burning chlorine- or sulfur-containing fuel.

In a preferred embodiment of the invention, a side air nozzle is provided between the fuel horn on the first wall of the furnace and an adjacent second wall through which an air stream can be blown to prevent the fuel released from the fuel hose from adhering to the second wall.

In a preferred embodiment of the invention, the secondary air nozzles and / or tertiary air nozzles are arranged in two opposed rows on two opposite walls of the furnace and each row has small air jet blowing nozzles and a medium air jet blowing nozzles. The nozzles are arranged to alternate such that each other nozzle is a small air jet blowing nozzle and each other nozzle is a medium or large air jet blowing nozzle. In addition, the nozzles are interleaved so that a medium or large air jet nozzle is provided on the opposite wall opposite each of the small air jet blowing nozzles. At least one of the outermost nozzles of each row is a large air jet blowing nozzle, and at least one of the nozzles located in the center portion of each row is a medium air jet blowing nozzle. The most basic air jets are used to 'wipe' adjacent walls and create an oxygen-rich area near the walls perpendicular to these air nozzle walls. Small air jets have poor penetration, but are designed to wipe the air nozzle wall and create an oxygen rich area near the air nozzle wall.

When a chlorine- and / or sulfur-rich fuel is to be burned in a fluidized bed boiler, the fuel and combustion air supply are preferably arranged in a manner that effectively prevents fouling and corrosion of the furnace walls adjacent to the fuel supply. In this case, the boiler preferably utilizes all three of the above solutions: the outermost feed horns are rotated 5-15 ° away from the fuel sidewall adjacent side wall, the side air nozzles are disposed between the outermost feed horns and the sidewall and the secondary and / or tertiary such that conditions preventing corrosion and contamination of the furnace walls are formed.

The reduction of nitrogen oxides by two-stage underfree combustion followed by the use of tertiary air in the third combustion zone will result in underfree conditions in the vicinity of the furnace walls. High levels of chlorine and / or sulfur in the fuel will result in severe corrosion of the evaporator walls. The process according to the invention can reduce the amount of nitrogen oxides in the flue gas while maintaining the corrosion of the superficial surfaces of both the furnace and the second draft within the permissible limits.

Preferably, the tertiary air nozzles are located 2 to 4 meters below the furnace beak.

At present, the primary air supplied to the fluidized bed below it is evenly distributed over the entire fluidized bed. Particularly, coarse wood fuel flies from the fuel feed horn to the fluidized bed in a very punctured manner, resulting in high fuel and low air areas and high air and low fuel areas. This results in uneven combustion and high nitrogen oxide emissions. Preferably, primary air could be divided into a fluidized bed so that more fuel than average would be supplied to the fuel-rich areas and less primary air would be supplied to the fuel-poor areas. However, the unevenness of the primary air supply should not cause the fluid to float unevenly.

The invention will now be described with reference to the examples set forth in the accompanying drawings, but the embodiments thereof are not to be construed as narrowly limiting.

Figure 1 is a schematic perspective view of a fluidized bed boiler.

Figure 2 is a schematic perspective view of the bottom of a fluidized bed boiler.

Fig. 3 is a schematic sectional front view of the furnace of a fluidized bed boiler designed for high furnace stress.

Fig. 4 is a schematic sectional front view of the furnace of a fluidized bed boiler designed for low firing stress.

Figure 5 is a reduced sectional view of the furnace in cross-sectional elevation of the fuel feed hoses.

Fig. 6 is a side view of air jets fed into the furnace.

Figure 7 is a schematic sectional view of the furnace from the height of the air jets.

Fig. 1 schematically shows a fluidized bed boiler and its furnace 1. The lower part of the furnace 1 is provided with a masonry 8 which protects the water vapor circuit pipes (not shown) on the walls A, B of the furnace 1 from overheating. The fuel is supplied to the fire chamber 1 through the fuel supply openings 5 on its two opposite walls A. The air required for combustion of the fuel is supplied through the primary air nozzles (not shown) at the bottom of the furnace and through the secondary air nozzles 6 and tertiary nozzles (not shown) on the walls B of the furnace 1. At the top of the furnace 1 there is a beak 9 for reducing the flow section of the boiler, after which the flow of flue gases turns through the superheaters (not shown) in the top of the boiler to the flue gas duct 13. Fig. 1 also shows starters 10 near the fuel feed height and B.

Fig. 2 is an enlarged view of a portion of the lower part of the furnace 1 of the fluidized bed boiler. The fuel supply openings 5 are arranged on the wall A of the furnace. Between each fuel supply opening 5 and the closest wall B there is a side air nozzle 12 mounted on the wall A, from which air can be blown between the fuel flow and the side wall B. The side air nozzles 12 are used to prevent corrosion and contamination of the side walls B, particularly when the fuel is high in sulfur or chlorine.

Figures 3 and 4 show, in principle, a front view of the furnace 1 of a fluidized bed boiler. Figure 3 is a high-pressure fluidized bed fluidized bed boiler and Figure 4 is a low-fluidity fluidized bed boiler. Note that the patterns are for principle only and are not intended to represent a fluidized bed boiler on the right scale.

The lower part of the furnace 1 has a fluidized bed 2 consisting of a fluidized bed material, to which, through nozzles 3 arranged on the bottom of the furnace 1, a fluidizing gas is introduced which causes the fluidized bed material to float and bubble. The fluidizing gas may be exclusively air or may be a mixture of air and recycle gas. The height hn of the fluidized bed is generally about 800 mm, i.e. the distance between the top surface of the bubbling fluidized bed 2 and the height of the primary air nozzles 3 is about 800 mm.

The fuel is supplied above the surface of the fluidized bed 2 by means of fuel supply means 4, 5 comprising an inclined downwardly directed fuel supply channel 4 which terminates in the inlet 5 of the furnace wall A. The fuel is also generally supplied with a small amount of carrier air. 4 Clogging. The fuel inlet openings 5 are located at an optimum height from the upper surface of the fluidized bed 2, which optimum height hF can be determined based on the combustion stress of the fluidized bed boiler and some other variables.

Most commonly, the distance hF between the fuel feed openings 5 and the upper surface of the fluidized bed 2 is in the range of 1.8 to 2.0 meters for high-strain boilers. A sharp angle α is formed between the fuel supply horn 4 and the normal N of the furnace wall A, which in the high furnace stress boiler is about 35 ° to 40 °. This angle is called the vertical feed angle α. The masonry 8 extends from the surface of the fluidized bed to a height Iir, which is a height of 1.8 to 2.0 meters in a high furnace strainer.

Secondary air is supplied to the furnace 1 from the secondary air nozzles 6 located at a height hs above the fuel feed level 6. The distance hs of the secondary air nozzles 6 from the upper surface of the fluidized bed 2 can be optimized based on the fluid bed boiler stress.

The area between the height of the primary air nozzles 3 and the secondary air nozzles 6, which includes the fluidized bed 2 and its volume up to below the secondary nozzles 6, forms the first combustion zone (I). Fuel is supplied to the first combustion zone (I) through the apertures 5 with a small amount of carrier air, and through the primary air nozzles 3 a fluidizing gas, which may be for example air or a mixture of air and flue gas.

The tertiary air is supplied above the secondary air nozzles 6 via tertiary air nozzles 7 arranged in the upper part of the furnace 1. Also, the distance hr of the tertiary air nozzles 7 from the upper surface of the fluidized bed 2 can be optimized based on firebox stress and some other variables. The tertiary air nozzles 7 are generally located 2 to 4 meters below the furnace nose 9.

The region between the secondary air nozzles 6 and the tertiary air nozzles 7, which starts at the height level hs of the secondary air nozzles 6 and ends below the tertiary air nozzles 7, forms a second combustion zone (II). The second combustion zone (II) is supplied via secondary air nozzles 6 with secondary air, which is mixed with an upstream flue gas stream from the first combustion zone (I), which also contains fuel unburned gases and particles which continue to burn in the second zone.

Fuel combustion continues in the third combustion zone (III) beginning at a height hr of the tertiary air nozzles 7. The first combustion zone (I) and the second combustion zone (II) are sub-free with the third combustion zone being free, i.e. the total air coefficient SRtot in the third combustion zone (III) is greater than 1.

The fine fuel fed to the fluidized bed 2 dries immediately upon contact with the hot fluidized bed material and substantially pyrolyses. During pyrolysis, the volatiles released from the fuel burn on the surface of the fluidized bed 2 and above the surface with the help of primary air. The combustion of the volatiles occurs in the first combustion zone (I) with respect to the air coefficient SRV1i of the volatiles under sub-electrochemical conditions. The higher the air coefficient SRv for volatiles, the faster the volatiles burn, generating high local temperatures and generating the maximum amount of hydrocarbon radicals needed to reduce nitrogen oxides formed from the fuel.

The reduction of nitrogen oxides formed from fuel to molecular nitrogen is carried out in two main steps. In the first sub-free combustion zone (I), a large portion of the fuel volatiles and some of the carbon residue are burned. This occurs under the air volumetric coefficient SRVi of the fuel volatiles under submetric conditions, which results in high hydrocarbon radicals. The primary air required for this step is introduced into the furnace in the embodiment of Figure 3 (high furnace stress boiler) mainly through the primary air nozzles 3 as fluidizing gas. A small amount of air enters the first combustion zone (I) during the supply of fuel through the supply horns 4 and as cooling air for the starter burner.

In the second combustion zone (II), the combustion air from the secondary air nozzles 6 is supplied to the furnace in such a way that the subchrommetric combustion conditions are maintained, so that the total air coefficient SRtot in the second combustion zone (II) is in the range 0.75-0.85.

Figure 4 shows a low-fired fluidized bed boiler. In this embodiment, primary air is supplied to the first combustion zone (I) in two main stages: on the one hand, the fluidized air through the primary air nozzles 3, and, on the other hand, the combustion air of the volatiles in the fuel supply. Preferably, the air supplied to the first combustion zone (I) in connection with the fuel supply is taken from the secondary air register, thereby reducing the amount of air supplied to the second combustion zone (II). Higher air supply to the first combustion zone (I) causes the temperatures in the first combustion zone (I) to be higher than the high furnace stress boiler shown in Fig. 3. When air is supplied to the furnace 1 along with the fuel supply, the fuel is ignited rapidly and most of the volatiles in the fuel are burned before the second combustion zone (II).

In a low-firing fluidized bed boiler, the distance between the fuel feed openings 5 and the upper surface of the fluidized bed 2 is preferably 2.0 to 2.6 meters and the height hR of the masonry 8 is preferably 2.5 to 4.0 meters. Although the refractory masonry 8 is shown in Figures 3 and 4 to extend approximately to the height of the fuel feed openings 5, this need not necessarily be the case, but the height hR of the masonry 8 may be different from the fuel feed height hi. The vertical feed angle α formed between the fuel supply horn 4 and the normal N of the wall A of the furnace 1 in the low furnace stress boiler is approximately 25 ° to 35 °. When air is supplied to the fluidized bed for combustion of volatile materials, the higher fuel feed height h and the shallower feed angle α ensure that no particles escape from the fluidized bed 2 when the air / fuel mixture hits the upper surface of the bubbling fluidized bed 2.

The optimal height level hs of the secondary air nozzles 6 is calculated based on the furnace strain and some other parameters according to the above formula.

In the low furnace strain boiler, the air coefficient SRo for fuel volatiles in the first combustion zone (I) is in the range of 0.9 to 1.00, preferably about 0.95. The total air coefficient SRtot in the second combustion zone (II) is about 0.8, or substantially the same order of magnitude as the high furnace stress boiler. In the third combustion zone (III), the total air coefficient SRtot is about 1.15, regardless of whether it is a low-fired boiler or a high-fired boiler.

The solution of the invention seeks to optimize the combustion of fuel volatiles in the first two sub-free combustion zones (I) and (II) of the furnace. In the low-stove boiler, the first combustion zone (I) is longer and operates at a higher volatile air coefficient SRVi than the corresponding zone (I) in the high-stove boiler. The second combustion zone (II) is shorter in a low furnace strainer than in a high furnace strainer, but the total air coefficient SRtot of the second combustion zone (II) in each boiler is substantially the same, about 0.8. In practice, this means that in the low-strain boiler, the first combustion zone (I) operates in the optimum range for reducing nitrogen oxides SR 50 = 0.95 - 1.00, whereby the majority of the volatiles in the fuel are already burned in the first combustion zone (I). The temperature of the first combustion zone (I) has to be limited in a high furnace stress boiler, which is why it is sought to maintain the air coefficient of volatile substances in the range SR 50 = 0.65 - 0.75. In order to complete the nitrogen oxide reduction reactions before the third free combustion zone (III), which is overexpressed, the length (hr - hs) of the second combustion zone (II) is increased relative to the corresponding length in the low furnace stress boiler.

Fig. 5 is a cross-sectional view of the furnace 1 of the fluidized bed boiler close to the fuel feed height. The first two walls A of the furnace each have three fuel supply horns 4, 4a, 4b. Each fuel supply pipe 4, 4a, 4b terminates in the fuel supply port 5, from which the fuel flow F is discharged into the furnace. The central fuel supply pipes 4 are arranged, viewed from above, perpendicular to the wall A, whereby the fuel flow F leaves the supply openings 5 in the direction of the normal N of the wall A. The fuel supply pipes 4a and 4b closest to the side wall B are mounted at an angle N deviating from normal N with respect to the first wall A. This so-called the horizontal feed angle β is 5 to 15 °, preferably about 10 °. The purpose of the angle β is to deflect the fuel flows F from the fuel supply pipes 4a, 4b closest to the side wall B of the furnace, away from the adjacent wall B, in order to reduce contamination and / or corrosion of the side walls B. This is especially important when burning fuel that is high in chlorine and / or sulfur.

In addition to the obliquely mounted fuel feed horns 4a, 4b, the furnace of Figure 5 is provided, as is known from Figure 2, with side air nozzles 12, each disposed between the outermost fuel feed horn 4a, 4b and the side wall B. The side air nozzles 12 are arranged to blow air parallel to the side wall B near the fuel feed height, thereby preventing the flow of fuel discharged from the fuel supply horn 4a, 4b to the side wall B. The side air nozzles 12 can also be directed obliquely downwardly. This solution is particularly advantageous when a high-chlorine and / or sulfur-containing fuel is burned in a fluidized bed boiler.

A third way to reduce fouling of furnace walls is to adjust the motion of the jets of secondary and / or tertiary air nozzles 6, 7 so that the airflows from the nozzles closest to the fuel supply wall A Figures 6 and 7 illustrate this principle.

Fig. 6 is a side view of the air jets S, L blown from the secondary or tertiary air nozzles on the two opposite walls B1 and B2 of the furnace 1, i.e. in the direction of wall A. Fig. 7 is a top plan view of corresponding air jets S, M, L.

The two opposite walls Bi and 1¾ of the furnace 1 each have eight air nozzles mounted on one row R 1, R 2, some of which are small air jet S blowing nozzles 6s, some of which are large air jet S blowing nozzles 6m. Different rates of motion of the air jets S, M and L from the various nozzles 6s, 6m, 6l are achieved by techniques known per se, for example by means of valves (not shown). The air jets from the nozzles at different positions have different momentum with each other such that the low jet air jets S alternate with the higher jet M, L. The opposite wall of the furnace has either a small motion air stream S or one of the larger motion air jets M or L opposite the small motion air jets S. The air jets discharged from the air nozzles Bi, B2 effectively wipe the surface of both the nozzle walls Bi, B2 and adjacent walls A. The upper large air jets L create an oxygen rich region near the walls A perpendicular to the nozzle walls Bi and B2. The low penetration of the small air jets S creates an oxygen rich region near the nozzle walls Bi, B2. The velocity S of small air jets is preferably less than 15 m / s.

For burning fuels that are difficult to corrode and contaminate, the horizontal incline of the extreme fuel horns by angle β and the side air blowers 12 as well as the interleaving air jets shown in Figures 6 and 7 to create an oxygen rich zone B,

The invention will now be described with reference to the examples set forth in Tables 1 and 2. The tables show the total air coefficients SRtot for the high-fired boiler (Table 1) and the low-fired boiler (Table 2) in stages using peat or wood as fuel. The total air coefficient SRtot increases in the height of the furnace as more air is introduced into the furnace.

Table 1. Total air coefficients SRtot in a fluidized bed boiler with high strain.

Air is supplied to the first combustion zone (I) of the furnace mainly with the fluidizing gas in the form of fluidized air and in connection with the supply of fuel as carrier air. The small amount of air used to cool the starter burners has little effect on the total air factor SRtot of the first combustion zone (I).

The addition of secondary air at the beginning of the second combustion zone (II) and the addition of tertiary air at the beginning of the third combustion zone (III) clearly increase the total air ratio SRt.

In a high furnace strain boiler, the fuel inlets are preferably located about 1.8 to 2.0 meters above the surface of the bubbling fluidized bed and the vertical feed angle of the fuel inlet hoses is 35 ° to 40 °.

The air distribution according to Table 1 is suitable for use in boilers with high furnace stress and when the fuel is particularly dry (less than 45% by weight of water in the fuel). In this air distribution, the first combustion zone (I) has an IL-factor for fuel volatiles, i.e., SRVi, in the range of 0.65 to 0.75, with low combustion temperatures in the lower part of the furnace.

Table 2 illustrates the distribution of air in a low-strain bed fluidized-bed boiler in which additional air taken from the secondary air register for fuel combustion in the first combustion zone (I) is fed to the boiler furnace in the fuel supply. The volatility of the volatile substances is determined to be in the optimum range of 0.9 to 1.0 for the reduction of nitrogen oxides.

Table 2. Total air coefficients SRtot in a low-fired fluidized bed boiler.

In this case, the volatile combustion air supplied to the first combustion zone (I) of the furnace in connection with the fuel supply clearly increases the total air factor. However, after the secondary air supply, the total air coefficient is at the same level as in Table 1. Thus, the total air input to the fluidized bed boiler is the same as in Table 1, but the air distribution is different when some of the secondary air in Table 1 is supplied to the first combustion zone (I). fuel supply.

In a low-stove boiler, the fuel inlets are preferably located at about 2.0 to 2.6 meters above the surface of the bubbling fluidized bed and the vertical feed angle of the fuel inlets is 25 ° to 35 °. A sufficient distance of the inlet openings from the fluidized bed and a sufficiently gentle feed angle ensure that volatile combustion air does not entrain particles from the fluidized bed.

Preferably, the volatile combustion air supplied to the furnace in connection with the fuel supply is taken from the secondary air, the temperature of the supply air being between 165 and 330 ° C. Sufficiently high volatile combustion air temperature keeps the fuel supply horn dry and clean and does not require separate "fluidization air" for horn cleaning. The volumetric combustion air flow rate in the fuel inlet hoses is optimally at 10-15 m / s. This low enough velocity also prevents the volatile substances from aggressively colliding the combustion air with the fluidized bed and, as a result, the sand escaping.

The tertiary air nozzles are preferably disposed about 2-4 meters below the furnace beak.

The use of a low-stove boiler method to reduce nitrogen oxides is recommended wherever possible.

Many variations of the invention are possible within the scope defined by the following claims.

Claims (15)

A method for reducing nitric oxide emissions and corrosion in a bubbling fluidized bed boiler which burns biofuel and / or waste, the air needed for fuel combustion being fed into the boiler (1) incrementally to generate at least two zones ( I), (II) with under air, for which one of the purposes is to reduce the generation of nitric oxide, and at least one zone (III) with over-air, where the combustion is completed, which combustion zones comprise: - a first combustion zone (I) beginning at the height level of primary air feeder nozzles (3) and extending above the fluidized bed (2), ending below the height level (hs) of secondary air nozzles (6); a second combustion zone (II) which begins at the height level (hs) of the secondary air nozzles (6) and ends below the height level (hT) of the tertiary air nozzles (7); and - a third combustion zone (III), beginning at the height level (hT) of the tertiary air nozzles (7); characterized in that the length of the first combustion zone (I) is optimized by placing the secondary air nozzles (6) at a given height (hs) over the upper surface of the bubbling fluidized bed (2), the height (hs) being calculated according to the formula: <img img-format = "tif" img-content = "drawing" file = "FI125314BC00341.tif" id = "icf0003" /> in which: hs = the height of the secondary air level from the upper surface of the bubbling fluidized bed [m] Qf = boiler full fuel power [MW] A = boiler cross-sectional area [m2] qf = boiler full fire pressure under the nose [kW / m3] LHVref = fuel reading MJ / kg] pref = fuel density [kg / m3] qf, ref = fireplace stress [kW / m3] fa, ref = air demand for the volatiles of the fuel [kg air / kg fuel] W = residence time (normally NTP) [s ] SRvoi, ref = air coefficient for the volatiles of the fuel in the first combustion zone (I), in which formula some of the variables are fireplace-specific variables, some of the variables are reference values and thus constant, and some of the variables are measured point specific reference values, which are selected based on the boiler fireplace strain. Method according to claim 1, characterized in that the measurement point-specific reference values hit and SRvoi, ref are selected on the basis of the boiler fireplace stress so that in the boiler with high fireplace stress, whose fireplace stress qf is above 130 kW / m3, the values tref = 6 s and , ref = 0.7, and in the boiler with low fireplace stress, whose fireplace stress qf is below 130 kW / m3, the values tref = 7 S and SRv0l, ref = 0.9 are used. Method according to Claim 1 or 2, characterized in that in the boiler with a high fireproof stress, the air coefficient SRvoi of the volatiles in the first combustion zone (I) is kept in the range 0.65 - 0.75, preferably approximately at the value 0.7. by regulating the supply of primary air. Method according to Claim 1 or 2, characterized in that the air coefficient SRvoi of the volatile substances (I) in the first combustion zone (I) is maintained in the low combustion zone boiler in the range 0.9 - 1.0, preferably in the range 0.95-1. In addition to primary air in the first combustion zone (I), in addition to the primary air, supply additional air intended for combustion of the volatile substances of the fuel, which additional air is fed into the fireplace in connection with the supply of fuel and is preferably taken from the secondary air register. A bubbling fluidized bed boiler suitable for combustion of biofuel and / or waste, comprising a fireplace (1) having - a fluidized bed (2) in which primary air is fed through primary air nozzles (3) located under fluidized bed and fuel through fuel feed. openings (5) in the walls of the fireplace (1), which fuel feed openings (5) are placed at a first height (hF) above the surface of the fluidized bed (2), - a number of secondary air nozzles (6) placed at a second height (hs) above a number of tertiary air nozzles (7) placed at a third height (hT) above the surface of the fluidized bed (2), forming the fireplace (1) of the bubbling fluidized bed boiler - a first combustion zone (I) , comprising a fluidized bed (2) and above it a volume up to the level below the height level (hs) of the secondary air nozzles (6), - a second combustion zone (II) extending from that of the secondary air nozzles (6) altitude level (hs) up to the level below the altitude level (hT) of the tertiary air nozzles (7), - a third combustion zone (III), starting at the height level (hT) of the tertiary air nozzles (7), which first combustion zone (I) and second combustion zone (I) II) arranged to operate with lower air, so that the total air coefficient SRtot is below 1 in each zone, characterized in that the secondary air nozzles (6) are placed at a given height (hs) from the upper surface of the bubbling fluidized bed (2), which height is calculated using the following formula: <img img-format = "tif" img-content = "drawing" file = "FI125314BC00361.tif" id = "icf0004" /> in which: hs = height of secondary air level from bubbling fluidized bed top surface [m] Qf = boiler full fuel power [MW] A = boiler cross-sectional area [m2] qf = boiler full fireplace stress under nose [kW / m3] LHVref = fuel heat M / kg] pref = fuel density [kg / m3] cf ref = fireplace stress [kW / m3] fa, ref = air demand for the volatile substances of the fuel [kg air / kg fuel] W = residence time (normally NTP) [s] SRVoi , ref = air coefficient of the volatiles of the fuel in the first combustion zone (I), in which formula some of the variables are fireplace-specific variables, some of the variables are reference values and thus constant, and some of the variables are the metric-specific reference values. selected on the basis of the boiler fireplace strain. Bubbling fluidized bed boiler according to claim 5, characterized in that the height hs of the secondary air is calculated using the values hit = 6 s and SRvoi, ref = 0.7, for a boiler with a high fireplace load, whose fireplace load is above 130 kW / m3 . 7. Bubbling fluidized bed boiler according to claim 5, characterized in that the height hs of the secondary air is calculated using the values hit = 7 s and SRvoi, ref = 0.9, for a boiler with a low fireplace load, whose fireplace load is below 130 kW / m3 . Bubbling fluidized bed boiler according to any of claims 5-7, characterized in that the lower part of the fireplace (1) is provided with a mumming (8) which extends to a height of 1.8 - 2.4 meters from the fluidized bed ( 2) surface, preferably a height of 1.8 - 2.0 meters from the surface of the fluidized bed (2). 9. A bubbling fluidized bed boiler with a low fireplace load according to claim 7, characterized in that the fuel feed openings (5) are arranged at a height (hF) of 2.0 - 2.6 meters from the surface of the fluidized bed (2) and the vertical feed angle (4) of the fuel hopper (4). a) is 25 ° - 35 °. Bubbling fluidized bed boiler with a high fireplace stress according to claim 6, characterized in that the fuel feed openings (5) are arranged at a height (hF) of 1.8 - 2.0 meters from the surface of the fluidized bed (2) and the vertical feed angle (4) of the fuel hopper (4). a) is 35 ° - 40 °. Bubbling fluidized bed boiler according to any one of claims 5-10, characterized in that the fuel feed hoppers (4, 4a, 4b) are placed adjacent to the first wall (A) of the fireplace and that the fuel feed hoppers (4a, 4b) are located closest to the second wall of the fireplace. (B) perpendicular to the first wall (A) is turned in the horizontal direction away from the affected second wall (B) at an angle corresponding to the horizontal feed angle (β), the horizontal feed angle (β) being 5-15 °, preferably about 10 ° . Bubbling fluidized bed boiler according to any of claims 5-11, characterized in that a side air nozzle (12) is mounted between the fuel feed hopper (4a, 4b) located on the first wall (A) of the fireplace and the nearest wall (B). can blow an air stream into the fireplace (1) to prevent the fuel released from the fuel feed hopper (4a, 4b) from adhering to the nearest second wall (B). Bubbling fluidized bed boiler according to any one of claims 5-12, characterized in that - the secondary air nozzles (6) and / or the tertiary air nozzles (7) are arranged in two opposite rows (Ri, R2) on two opposite walls (Bi, B2) of the fireplace ( 1), - in the respective row (R 1, R 2) are nozzles (6 s) which blow a small air jet (S), nozzles (6 M) which blow a medium air jet (M), and nozzles (6 L), which blow a large air jet (L), which nozzles are arranged to alternate so that every other nozzle is a nozzle (6s) blowing a small air jet (S) and every other nozzle is a nozzle (6M, 6L) blowing a medium (M) or a large ( L) air jet, - the nozzles have been arranged in turn so that opposite the respective nozzle (6s) blowing a small air jet (S) there is on the opposite wall (Bi, B2) a nozzle (6m, 6l) blowing a medium size (M) or a large (L) air jet - among the outermost nozzles in the respective row (R1, R2) is at least one nozzle (6L) blowing a large air jet (L) and h among the nozzles in the center region of the respective row (R1, R2) is at least one nozzle (6M) blowing a medium-sized jet of air (M). Bubbling fluidized bed boiler according to claim 5, characterized in that in a manner as described in claim 11, it comprises inwardly inclined outer fuel feed hoppers (4a, 4b), in a manner as described in claim 12, side air nozzles (12) and in a manner as described in claim 13 located supply nozzles (6, 7) for secondary and / or tertiary air, the boiler being particularly well suited for combustion of a fuel containing chlorine and / or sulfur. Bubbling fluidized bed boiler according to any one of claims 5-14, characterized in that the tertiary air nozzles (7) are placed 2-4 meters below the nose (9) of the fireplace (1).