Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23C—METHODS OR APPARATUS FOR COMBUSTION USING FLUID FUEL OR SOLID FUEL SUSPENDED IN  A CARRIER GAS OR AIR 
  • F23C6/00—Combustion apparatus characterised by the combination of two or more combustion chambers or combustion zones, e.g. for staged combustion
  • F23C6/04—Combustion apparatus characterised by the combination of two or more combustion chambers or combustion zones, e.g. for staged combustion in series connection
  • F23C6/045—Combustion apparatus characterised by the combination of two or more combustion chambers or combustion zones, e.g. for staged combustion in series connection with staged combustion in a single enclosure
  • F23C6/047—Combustion apparatus characterised by the combination of two or more combustion chambers or combustion zones, e.g. for staged combustion in series connection with staged combustion in a single enclosure with fuel supply in stages
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F22—STEAM GENERATION
  • F22B—METHODS OF STEAM GENERATION; STEAM BOILERS
  • F22B31/00—Modifications of boiler construction, or of tube systems, dependent on installation of combustion apparatus; Arrangements of dispositions of combustion apparatus
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23L—SUPPLYING AIR OR NON-COMBUSTIBLE LIQUIDS OR GASES TO COMBUSTION APPARATUS IN GENERAL ; VALVES OR DAMPERS SPECIALLY ADAPTED FOR CONTROLLING AIR SUPPLY OR DRAUGHT IN COMBUSTION APPARATUS; INDUCING DRAUGHT IN COMBUSTION APPARATUS; TOPS FOR CHIMNEYS OR VENTILATING SHAFTS; TERMINALS FOR FLUES
  • F23L7/00—Supplying non-combustible liquids or gases, other than air, to the fire, e.g. oxygen, steam
  • F23L7/002—Supplying water

Definitions

• the present invention concerns a method in accordance with the preamble of claim 1 for operating a power plant.
• the method aims at reducing nitrogen oxide emission of a power plant while improving the efficiency of fuel combustion.
• the combustion process of a power plant using solid fuels is controlled by regulating the feed of air.
• the solids contained in the flue gases formed in connection with the combustion process are at least partially separated.
• Said solids are at least essentially formed by ash containing unburned carbon.
• the ash is conducted to an afterburning process for burning that carbon.
• the techniques for removing nitrogen oxides from flue gases are not as straight-forward as those intended for removing sulphur oxides. This is mainly due to the fact that a large part of the nitrogen oxides formed is comprised of NO, i.e. nitrogen oxide, which is not subjected to an acid reaction in contact with water and which cannot be removed by alkaline treatment or neutralized as can the oxides of sulphur.
• nitrogen oxides are, of this reason, removed from flue gases by reacting them with urea, ammonia, or equivalent industrial bulk chemicals.
• the oxidizing and reducing chemicals together form nitrogen and water.
• the reaction rate has been increased by, for instance, electron beam activation and catalytic surface reactions.
• Many of the preferred methods for removing nitrogen oxides are not very well suited for use together with the necessary sulphur removal methods.
• Burners known as Low-NOx burners which primarily have been developed in Japan, achive a considerable reduction of the amount of nitrogen oxides formed by arranging laminar air feed and by preventing the formation of the hottest part by employing the "reburning" technique.
• These burners are one sensible way of dealing with the above-mentioned problem, but they are in themselves expensive and sometimes, because of the additional installation space needed, difficult to adapt to existing power plants.
• FIGS 1 and 2 of the attached drawings depict the relationship between the concentration of nitrogen oxides and fuel losses (figure 1) and the relationship between fuel losses and the air ratio (figure 2) , respectively.
• the figures show that fuel losses rapidly grow when the nitrogen oxide emissions are reduced by regulating the excess air ratio.
• the best thermal efficiency is reached when the excess air ratio is as low as possible, i.e. when the amount of air is equivalent to the amount of fuel and, at the same time, the combustion of the fuel is complete.
• fly ash is obtained with 30 to 35 per cent of unburned carbon.
• this fly ash is fractioned by screening or air classification, the most finely divided and the coarsest fractions of the carbon contained in fly ash can be recovered, the carbon being sintered together with the ash. Fractioning or classification do not, to any large extent, improve the situation because the carbon-free ash of the middle fractions only amounts to about 20 to 25 per cent of the total amount of ash.
• Fractioning would help in burning the loose carbon particles of the finely divided solids, but it would not help dealing with the carbon which is agglomerated among the ash.
• a combustion method including fractioning of the fly ash would greatly increase the circulation load in the boiler and the mechanical wear.
• fly ash At the same time as the value of fly ash has been steadily increasing, it has become more and more difficult to dump it at dumping grounds. The increased value is a result of the utilization studies carried out in order to find new uses for fly ash.
• the biggest use for fly ash is in the cement and concrete industry. For these applications it is highly desirable that the fly ash used be almost completely free of carbon, because carbon spoils the effect of many organic additives, it increases the creep of hardened concrete and causes colour changes.
• the present invention aims at eliminating the drawbacks of the prior art while providing an entirely novel method for operating a power plant.
• Our invention is based on the following ideas: Combustion of solid fuels, such as coal, is carried out by using a very small excess of air in order to minimize nitrogen oxide emissions. Preferably the excess air ratio ranges from 1.02 to 1.15.
• Combustion ash containing up to 30 % unburned carbon is conducted to an auxiliary combustion process, wherein a boiler is used having an effect which in relation to the effect of the main boiler has been selected according to the ash content of the fuel.
• the effect of the auxiliary boiler is at the most equal to the ash content of the coal in order to attain complete combustion of all the carbonaceous ash.
• the second combustion process will provide at least substantially carbon-free fly ash suitable for many uses. By circulating the flue gases from the second combustion process to the main boiler it is, at the same time, possible to reduce fuel losses of the process and to improve the efficiency thereof.
• the method according to the invention is mainly characterized by what is stated in the characterizing part of claim 1.
• excess air denotes an amount of air exceeding the amount theoretically needed for the complete combustion of the fuel.
• an excess air ratio of 1.02 to 1.15 indicates that the amount of air is 2 to 15 per cent larger than that theoretically needed.
• Staged air secondary air
• the total excess air ratio in that space may be considerably greater than in the primary combustion zone.
• the invention can be carried out in connection with all the different methods, wherein fuel or air is fed in stages into the different parts or at different levels of the boiler.
• the invention can be applied to methods using special burners which are fed in stages with air and/or fuel, such that the nitrogen oxide reduction is achieved by operational means.
• the Finnish Patent Specification No. 81970 concerns the removal of sulphur oxides and the afterburning of unburned coal contained in fly ash.
• the sulphur compounds are catalytically oxidized to sulphur trioxide.
• Said patent does not deal with the technology for removing nitrogen oxides nor with the use of low excess air ratios.
• the effect of the afterburning boiler is according to the invention less than 10 %, preferably in the range from about 3 to 8 %, of the effect of the main boiler.
• the flue gases may be conducted directly to the main boiler.
• the fire resistance of the materials used ash separation cyclones
• the benefit to be obtained by the use of a heat transfer system using water or steam circulation resides in the fact that the auxiliary boiler can now be regulated by the air ratio without there being any - risk of the ash getting sintered.
• the flue gases are fed as staged air for these burners in order to achieve really low nitrogen oxide concentrations in the flue gases.
• the method in itself without Low-Nox -burners reduces the content of nitrogen oxides to the same level as the best Low-Nox burners.
• the best results of the afterburning are achieved by feeding small amounts of water or steam to the afterburning boiler to improve the gasification of coal by the water gas reaction.
• this known method has turned out to be an easily controllable way of regulating the combustion temperature.
• the amounts of nitrogen oxides in the flue gases from the afterburning boiler are very small because the combustion is carried out at low temperatures.
• the added small amount of water or steam further decreases the amounts of nitrogen oxides. This method is commonly used in gas turbines, where it decreases the amounts of nitrogen oxides from 161 mg/MJ to 80 mg/MJ.
• auxiliary boiler is thus preferably but not necessarily a fluidized bed boiler or a circulating bed boiler, which is a variant of the fluidized bed.
• the combustion temperature should be less than 900 °C, preferably in the range from 800 to 840 °C, to avoid that sulphur which already has been bound is released in connection with the afterburning and to avoid sintrering of the ash.
• the invention provides considerable benefits. As mentioned above, it is desired at the power plant to use a low rate of excess air and at the same time to produce ash without carbon. In some cases it is still necessary to circulate flue gases for use as diluent gases in the burner. By means of the method in accordance with the invention, it is possible to attain these goals in a novel and extremely simple way.
• Figure 1 depicts the nitrogen oxide concentration (mg/MJ) in the flue gases as a function of the fuel losses of the boiler.
• Figure 2 shpws the fuel losses (%) as a function of the feed air.
• Figure 3 shows the process scheme of a nitrogen oxide removal system suitable for a normal power plant.
• reference numeral 1 stands for a coal-burning boiler which is fed with fuel, air and gas.
• burners of the Low-Nox -type in the boiler. It may, however, be equipped with normal burners as well.
• the amount of air feed is 2 to 15 times larger than that theoretically needed.
• the main combustion is typically carried out at almost adiabatic temperatures ranging from 1200 to 1600 °C.
• the flue gases stemming from the boiler 1 are conducted to a filter 2 for removal of the solids.
• the solids are separated from the flue gases, the flue gases being conducted to additional cleaning, if necessary.
• the solids are conducted to a afterburner 3 where the ash is burned to remove as much of the carbon as possible.
• the residual carbon content is normally about 0.1 to 0.5 %.
• the afterburner is fed with air and, for regulating the combustion process, with water or steam. There is a large excess air ratio used in the afterburning, for instance a 1.2 to 2.0 fold excess or air.
• the combustion temperature is kept below 900 °C, preferably it is kept in the range from 800 to 840 °C.
• the afterburner preferably comprises a fluidized bed furnace or a circulating fluidized bed furnace.
• the flue gases from the secondary burner 3 are circulated for use as diluent gases in the boiler 1 in order to reduce the amount of nitrogen oxides.
• the flue gases may be conducted straight to the burners of the main combustion process or they may be combined with the air feed of the main combustion process.
• the flue gases may first be conducted through a separation device 4 which is depicted in the drawing as a cyclone.
• the separation device may be comprised of any separation device known per se which is capable of removing at least the largest part of the fly ash particles from the flue gases in order to reduce the circulation load.
• the separated material is discarded. It is not necessary to remove the finest portion of the ash because said portion still contains some of the unburned carbon.
• a part of the circulating and generated fly ash works as a catalyst promoting the reaction between nitrogen oxides and carbon monoxide.
• a power plant having a thermal effect of 580 MW was run with Colombian coal which had been pulverized to a fineness of 95 % ⁇ 0.2 mm.
• the excess air ratio of the air. fed into the boiler amounted to 1.08.
• a nitrogen oxide content of 200 mg N0 2 /MJ in the flue gas was then attained.
• the residual carbon content of the fly ash was 30 %.
• the fly ash was combusted in an afterburning boiler by using water injection and an excess air ratio of 2.
• the residual amount of carbon of the ash was about 0.5 % C, whereas the amount of carbon-free ash was about 14 % of the dry coal feed.
• the pure ash which essentially does not contain carbon is worth about 35 FIM/t at the power plant. At the deposition site the cost of ash containing more than 5 % carbon is about 10 FIM/t.
• the additional value obtained from the ash can be calculated as follows:
• the savings of the future fees for emissions of nitrogen oxides are about 40 MFIM/a if the NOx amounts are reduced from 500 MJ to 200 MJ.

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• Engineering & Computer Science (AREA)
• Chemical & Material Sciences (AREA)
• Combustion & Propulsion (AREA)
• Mechanical Engineering (AREA)
• General Engineering & Computer Science (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)

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• 1991
  • 1991-04-30 FI FI912091A patent/FI89741C/fi not_active IP Right Cessation
• 1992
  • 1992-04-30 WO PCT/FI1992/000135 patent/WO1992019912A1/en not_active Application Discontinuation
  • 1992-04-30 EP EP92909201A patent/EP0583286A1/en not_active Withdrawn

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