FI108809B - Process for reducing nitric oxide emissions formed by combustion in fluidized bed - Google Patents

Description

108809

The present invention relates to a process for reducing nitrogen oxide (NOx) emissions from fluidized bed combustion according to claim 5 of claim 1.

Fluidized bed refers to a mixture of particulate bedding material and gas, wherein the bedding material is fluidized by introducing gas into the bed material from the bottom up at a suitable rate. In power generation, fluidized bed technology is mainly used to burn solid fuels such as coal, peat and wood chips. Fuel is supplied to the bed material at the bottom of the fluidized bed 15 at the bottom of the furnace furnace. The combustion air required by the fuel is fed upwards from the bottom of the furnace, where it also acts as a fluidizing gas for the bed. If a fluidized bed boiler is used for phasing the combustion air supply, primary air is used as the fluidizing gas and the remainder of the combustion air 20 is supplied from a higher level to the bed or above the bed in one or more different stages.

Fluid bed combustion is suitable for a variety of fuels, and * · * · ·; ··: can also be used to burn low quality fuels: ***: 25. The sulfur contained in the flue gases can be effectively removed by feeding lime to the amount of bed material. Other good features of fluidized bed combustion include:

• · good combustion efficiency, good usability and '... · efficient heat transfer between the bed and furnace walls. In addition, the nitrogen oxide emissions of Lei pulverized combustion are low due to the low combustion temperature.

Nitrogen oxide emissions are mainly formed in two different ways when burning nitrogen containing fuels. In another mechanism, nitrogen oxides are formed from molecular nitrogen accompanying combustion air and fuel which oxidizes at high temperature to nitrogen oxides. Nitric oxides produced by this mechanism are referred to as thermal NOx because they require a high temperature to form. In fluidized bed combustion, the combustion temperature is low, about 800-900 ° C, which results in little formation of thermal nitrogen oxides. In another mechanism for the formation of nitrogen oxides, the organic nitrogen contained in the fuel is oxidized to nitrogen oxides, which is referred to as fuel-10ne-ΝΟχ. The nitrogen oxides formed in fluidized bed combustion are mainly produced by this mechanism.

Ways to reduce nitrogen oxide emissions can be divided into combustion and secondary processes. Combustion techniques, such as lowering the combustion temperature, prevent the formation of nitrogen oxides or convert the nitrogen oxides formed in previous combustion steps into other compounds. Nitrogen oxides formed by secondary processes are in turn removed from the flue gas. 20 dogs. For example, the injection of nitrogen oxide reducing compounds into flue gases is a secondary method.

One combustion technique to reduce nitrogen oxide emissions **. sex is a so-called reburning technique in which nitrogen oxides are degenerated into molecular molecules in several zones by supplying fuel and combustion air by refining. The first zone burns most of the ** "fuel. The remainder of the fuel is fed to a second, so-called reburning zone, and burned low. ··. In an oxygen-containing environment, the hydrocarbon radicals formed when the additional fuel is burned convert The reburning reaction described above is most effective in a low oxygen environment and at a high temperature of 3 108809. In a third step, combustion air is added to complete the combustion of the still unburned fuel and convert the hydrocarbon molecules (HCN) formed in the reburning reaction to molecular nitrogen (N2). ) The amount of fuel burned in Reburning-5 is about 10-20% of the total heat input to the process Reburning technology is used to reduce nitrogen oxide emissions, for example in low-NO boilers of power plants The x-burners have achieved a reduction of 50-80% in nitrogen oxides in the flue gases at baseline. For example, U.S. Patent No. 5,139,755 describes a reburning technique in which compounds reducing nitrogen oxides from flue gases are introduced into the process in addition to the reburning fuel.

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In FI 102411, a secondary air supply is formed in the furnace of a fluidized bed boiler by directing four vortices over the fluidized bed. Swirls are designed to mix fuel, combustion air, and combustion gas as efficiently as possible so that the combustion temperature of the furnace is constant and low nitrogen oxides are formed.

·· Additionally, the centrifugal force caused by the eddies prevents the transport of fuel particles rising from the fluidized bed to the boiler superheater by separating them into the furnace walls.

In WO 93/18341, a secondary-air swirl is formed in the furnace of a fluidized bed boiler above the fluidized bed. The combustion gas sulfur-binding sorbent is also fed to the secondary air, which also contributes; decomposition of nitric oxide formed in fluidized bed mole- * ♦ *. · ·. Coulomb nitrogen and oxygen.

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EP 082673 discloses a fluidized bed reactor in which secondary air is supplied tangentially to the furnace above the fluidized bed, whereby a turbulent vortex is formed in the furnace. The vortex increases the reaction speed of the reactor, 5 decreases the reaction temperature and increases the heat generation in the reactor.

In US 4,867,079 a vortex is directed to the furnace of a fluidized bed boiler by directing a secondary air supply.

10 Due to the centrifugal force caused by the vortex, the particles among the gases emerging from the fluidized bed move to the walls of the furnace and fall back into the fluidized bed. In addition, the vortex enhances combustion in the furnace.

It is an object of the present invention to provide a novel method for the production of nitrogen oxides from fluidized bed combustion. reburning technology can be used to reduce bond emissions.

The invention is based on providing reburning conditions for a fluidized bed boiler on a full scale scale. The invention provides combustion air in the fluidized bed boiler furnace by: • re-phasing and redirecting zones of fuel and combustion air supply | These vary so that a reburning reaction can occur. In conventional fluidized bed combustion, the aim is to mix fuel and combustion air as efficiently and uniformly as possible at each air supply level in order to keep the combustion temperature and nitrogen oxide content of the flue gases low.

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

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The invention provides considerable advantages.

By using the process of the invention, nitrogen oxide emissions from fluidized bed combustion can be significantly reduced at their present levels. Although much of the fluidized bed boilers currently in use fall below the current NOx emission limits, future emission limits will have to be tightened, requiring the introduction of new methods or devices to reduce emissions. The invention is inexpensive compared to other nitrogen oxide emission reduction solutions which would achieve the same level of nitrogen oxide emissions as the solution according to the invention. The air supply levels required by the invention 15 are already in use in many Lei fluidized bed boilers, which generally do not require major modifications to the boilers. It is usually sufficient that the combustion air supplied to the furnace is merely phased and redirected. It is not necessary to use a separate rebur-Ning fuel in the rat-20 slip according to the invention, since the pyrolysis gas released when the fuel is gasified can be used for reburning »···! as a fuel. In addition, the solution of the invention is applicable to fuels of * * different types and grades, such as conventional fluidized bed combustion.

The invention will now be illustrated with reference to the accompanying drawings.

FIG. 1 is a schematic cross-sectional view of a fire bed of a fluidized bed boiler.

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Figure 2 is a schematic cross-sectional view of the fluidized bed boiler shown in Figure 1 at the secondary air supply.

In Figure 1, primary air 1, which is also used as fluidizing air for the bed material, is fed upwards from the bottom 12 of the furnace 11. The amount of primary air 1 is about 30% of the total air supply to the boiler. The fuel is fed to the bed material, whereupon it dries and gasses in the upper part 5 of the bed 2. The pyrolysis gases formed by the gassing of the fuel partially react with the oxygen contained in the primary air 1 and rise in the furnace 11. The non-gaseous portion of the fuel burns both in bed 2 and as particle size decreases above bed 2. The combustion of non-directional gas in bed-15 and proper reaction of the pyrolysis gases with oxygen is ensured by controlling the flow of primary air 1.

Above the fluidized bed 2, secondary air 3, *. 20 ka is approximately 50% of the total air supply to the boiler • · · «··. in the order of. In flame · · · zone 6 on the secondary air supply 3, part of the pyrolysis ·· lysis gases rising from bed 2, non-gaseous part of the fuel supplied to the bed, i.e. coke and 25 carbon monoxide produced in coke combustion reactions, is burned. The combustion temperature in flame zone 6 is about 1100-1400 ° C, which is significantly higher than in a conventional fluidized bed boiler. The upward portion of the pyrolysis gases from the central portion j '... · of the bed 2 is only heated by:: ·· the flame zone 6 to form highly reactive hydrocarbon radicals. The secondary jets of air supply 3, for example tangentially directed to the furnace 11, form a vortex 4 containing combustion gases 6 and hydrocarbon radicals formed from pyrolysis gases in the furnace 11 so that the oxygen contained in the secondary air 3 is not entrained. The orientation of the air passageways of the secondary air supply 3 is illustrated in more detail in Figure 2. In the middle of the vortex 4 there is a low-oxygen zone containing hydrocarbon radicals 5 and at the edges a high-oxygen zone. There is a reburning reaction at their boundary zone where the nitrogen oxides contained in the flue gases are converted to hydrogen cyanide (HCN). In the solution of the invention, the supply of separate reburning fuel to the boiler furnace tu-10 liquor 11 is replaced by forming a low-oxygen rich hydrocarbon radical zone in the vortex 4.

Above the secondary air supply 3, tertiary air 7 is fed into the furnace 11, which converts the cyanive-15 contained in the vortex 4 to molecular nitrogen (N2). The amount of tertiary air 7 is about 20% of the total amount of air needed for combustion. By changing the feed volume of the tertiary air 7, the conversion of the cyanogen hydrogen to the molecular nitrogen can be controlled. The direction of the jets of the tertiary air supply 7 is illustrated in Fig. · 20, in more detail in Figure 3. The residence time between the secondary gas 3 and the tertiary air supply 7 must be long enough for the reburning reaction to occur. .j tertiary air supply 7.

After the tertiary air supply 7, any remaining fuel in the flue gases is exhausted by air jets 8 directed to the upper space.

In Figure 2, most of the secondary air 3 is supplied to the furnace 30 11 from the main air supply 9, located in the center of the wall. whereby two upwardly rotating wheels are formed in the furnace

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The formation of vortices 4 may, if necessary, be enhanced by additional air feeds 10 placed in the corners of the furnace 11.

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The invention may also have embodiments other than those described above.

Vortexes 4 may be formed by secondary air supply 3 by directing only one or more of the two shown in the example. Similarly, the secondary air feeds 9, 10 shown in Fig. 2 may also be positioned and directed differently from the foregoing; For example, the air supply points may be located only in the corners or in the middle of the walls of the furnace 11, thereby directing the air supply tangentially to the furnace 11 to form a vortex 4. The furnace 11 is provided with a vortex 4 containing reburning conditions. 3, a separate reburning fuel is also fed to enhance the reburning reaction. The proportions of combustion air supplied to different combustion zones depend on the total amount of air supplied to the boiler, e.g. load of the boiler, whereby the distribution of air between the various zones 20 may differ significantly from the above values. Under partial load conditions, the amount of primary air 1 is kept nearly the same as that of a full load, and the amounts of secondary 3 and tertiary air 7 are reduced, so that the solution of the invention can be used for up to about 70% partial load * · · 25. In addition to fluidized bed boilers for power generation plants, the method of the invention is also applicable to other combustion applications utilizing fluidized bed technology.

Claims (5)

A method for reducing nitric oxide emissions when combusted in a fluid bed in a fireplace (11), wherein method 5. primary air (1) is measured from the hearth (12) of the fireplace (11) to obtain the fluidized bed (2). bed material to float, - fuel is fed into the fluidized bed (2), - fuel is gasified into pyrolysis gas and un-gasified fuel is combusted in the fluidized bed (2) by means of the primary air fed into the bed (2), - secondary air (3, 9, 10) are fed into the fireplace (11) at a distance above the fluidized bed (2), and - tertiary air (7) is fed into the fireplace (11) above the secondary air feed (3, 9, 10), Characterized in that a portion of the pyrolysis gas is combusted in a pre-firing zone (6) disposed in the vicinity of the secondary air supply (3, 9, 10), in the combustion zone (6) a portion of the pyrolysis gas is heated to form a hydrocarbon radical, * · • ·. ** · - in the fireplace (11), a stream of secondary air *, 25 (3, 9, 10) is formed by tangentially directing at least »» IM ... one a vortex (4 ) containing combustion • I * * gases, the center of which of the vortex exhibits an oxygen • «In poor zone containing ample hydrocarbon radicals and whose edges exhibit an oxygen-rich zone, 12 1 08809 nitrogen oxides in the combustion gases is converted to hydrogen cyanide in the vortex (4) in the boundary zone of the oxygen-poor and oxygen-rich zone, and - tertiary air (7) is directed to the vortex (4) to convert hydrogen cyanide to molecular nitrogen. 2. A method according to claim 1, characterized in that combustion air is conducted to an overhead air space (8) arranged at a distance above the tertiary air supply (7) for combustion of the fuel remaining in the combustion gases after the tertiary air supply (7). Method according to claim 1, characterized | by generating in the fireplace (11) a secondary air feed (3, 9, 10) by tangentially directing two swirls (4) containing flue gases. 15 Method according to claim 1, characterized in that separate reburning fuel is fed into the fireplace I: V (11). « 5. A method according to claim 1, characterized in that the temperature of a fire zone (6) arranged in the vicinity of the input level of the secondary air (3) is raised to 1100 - 1400 ° C. i · ·> · »* *« · • * »· * I» I I »t · *» t t »25