METHOD FOR OPERATING A COMBUSTION PROCESS

Abstract: The invention concerns a method for operating a power plant. According to the present method, the combustion process of the power plant is regulated by controlling the air feed. Subsequently, the solids of the flue gases formed, which at least substantially consist of uncombusted carbon, are at least partially separated from the flue gases. The ash is conducted to afterburning for burning the carbon of the ash. According to the invention, air is fed into the combustion process of the main boiler (1) at an excess air ratio of about 1.02 to 1.15, preferably 1.05 to 1.12, and the ash of the combusted solid fuel is subsequently burned in a separate boiler (3) having an effect, which in relation to the effect of the main boiler (1), is at the most essentially equal to the proportion of ash in the solid fuel. With the aid of the invention it is possible at the same time to reduce nitrogen oxide emissions of the power plant and to improve the combustion efficiency of the fuel.

Description: METHOD FOR OPERATING A COMBUSTION PROCESS. The present invention concerns a method in accordance with the preamble of claim 1 for operating a power plant. In particular, the method aims at reducing nitrogen oxide emission of a power plant while improving the efficiency of fuel combustion. According to a method of the present kind, 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 tightening emission standards pose new challanges for power plant technology. An essential part of the new technology will, in connection with the use of fossile fuels, be devoted to purification of flue gases and to increasing the efficiency of the power plant. The methods for removing sulphur oxides contained in flue gases have already reached a certain technical level, and a stable situation has been attained in the competition between the different methods. However, the final technical solutions have probably not been developed yet; there are considerable drawbacks associated with the present sulphur removal technology. Thus, some of the processes generate large amounts of gypsym for which there is no established use. Therefore, gypsym forms a new waste problem. Some of the processes produce elemental sulphur for which there is not enough demand, either. 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. According to the prior art, 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. It is known that excess air and high temperature of the flame increase the formation of nitrogen oxides as does a long retention time (> 0,01 s) in a high temperature flame. On the other hand, it is also known in the art that steam reduces the formation of nitrogen oxides. The same applies to the presence of carbon monoxide. The last-mentioned reaction requires the presence of a catalyst. If a power plant is operated without a sufficient amount of excess air, the amount of unburned fuel is strongly increased, i.e. fuel losses grow. This concerns in particular coal and other solid fuels. Figures 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. On the other hand, it should also be noticed that, theoretically, 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. When typical American coal is used, reduction of the excess air ratio from 1.25 to 1.05 will increase thermal efficiency by 2.1 per cent, provided that coal combustion is complete. Figure 2 shows that an excess air ratio of 1.25 gives 0.4 per cent of unburned carbon. This and the flue gas loss (180 °C) caused by the excess air together lead to a 2.6 per cent drop in efficiency compared to the theoretical thermal efficiency. If the price of coal is, for instance, 240 FIM per ton, in case of a typical σoal- burning power plant with a thermal effect of 600 MW, the calculated annual losses will amount to 4.3 million FIM. If, again at a normal power plant, the excess air ratio is lowered from 1.25 to 1.05, the amount of unburned carbon increases to about 4.5 per cent, which equals 7.6 million FIM annually. In this way fly ash is obtained with 30 to 35 per cent of unburned carbon. When 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. Thus, a combustion method including fractioning of the fly ash would greatly increase the circulation load in the boiler and the mechanical wear. Increasing the degree of fineness of the carbon is one way of improving the carbon content of the ash and raising the quality of the ash. However, grinding consumes a lot of electric energy, which is used for raising the thermal efficiency. Furthermore, all coal qualities are not easy to grind. Coal which easily can be finely ground is hard and brittle, that is, it contains only small amounts of volatile substances and it is less combustible that the tougher qualities. Another reason why grinding of the fly ash for facilitating the removal of carbon also is not preferred is that grinding crushes the spherical particles "genespheres" of the fly ash, which are its most valuable part. 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. As shown above, prior art technology fails to meet the above- mentioned requirements concerning low fuel losses, low nitrogen oxide concentrations and a suitable air ratio. 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. In particular, the method according to the invention is mainly characterized by what is stated in the characterizing part of claim 1. Within the scope of the present invention, the term excess air denotes an amount of air exceeding the amount theoretically needed for the complete combustion of the fuel. Thus, 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. In this context, using a small excess air ratio during combustion means that said conditions prevail in the combustion zone of the boiler. Staged air (secondary air) may be fed into the space above the burner plates. The total excess air ratio in that space may be considerably greater than in the primary combustion zone. In other words, within the scope of the present application, 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. Similarly, 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. Typically, 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. It is not necessary to equipe the afterburning boiler with heat recovery; the flue gases may be conducted directly to the main boiler. As far as the fire resistance of the materials used (ash separation cyclones) is concerned and, in particlar, as far as the controllability of the process is concerned, it is preferred to carry out the -heat recovery of the auxiliary boiler by water or steam circulation for transferring heat from the auxiliary boiler to the main boiler or for other use, such as process heat or for the district heating system. 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. In the preferred embodiment of the invention employing Low- Nox burners in the boiler, the flue gases are fed as staged air for these burners in order to achieve really low nitrogen oxide concentrations in the flue gases. However, the method in itself without Low-Nox -burners reduces the content of nitrogen oxides to the same level as the best Low-Nox burners. According to the present method, 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. In our experiments, 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. It is known in the art that the amount of carbon monoxide is rapidly increased when the excess air ratio is lowered from 1.05 to 1.00. Carbon monoxide reacts well with nitrogen oxide in the presence of a suitable catalyst. Of the commercially available catalysts, the rhodium catalyst provided by General Electric may be mentioned. The surface of fly ash works in several respects as a catalyst. This property of the fly ash may be used at the power plant. However, the catalytic activity of fly ash is considerably lower that the activities of nobel metal surfaces. Thus, large amounts of fly ash are required when the catalytic properties of the fly ash is to be used. This concerns mainly a reaction wherein carbon monoxide reacts with nitrogen oxide according to the reaction equation 2 CO + 2 NO > C0 2 + N 2 . Large operational amounts of fly ash are, according to the invention, obtained by circulating large amounts of fly ash in the gas stream. By catalysing the reaction between carbon monoxide and nitrogen oxide during combustion it is possible to find an optimal excess air ratio which will decrease the concentrations of nitrogen oxides and carbon monoxide in the flue gases close to 0 already when the flue gases are formed. This excess air ratio is obviously close to the range from 1.05 to 1.03. Lower amounts of air will cause the formation of hydrogen cyanide and other most undesirable pyrolysis gases. Irrespective of the combustion process, an even temperature of the combustion chamber is of great importance as regards the final result and the regulation of the process. This applies as well to the removal efficiency, the combustion efficiency as to other aspects of the process. In this respect, different fluidized bed boilers are thermodynamically close to optimal boilers. The 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. If a parallel process for removing sulphur is being used, 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. The invention will be next studied in more detail with the aid of the attached drawings. 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. In figure 3, reference numeral 1 stands for a coal-burning boiler which is fed with fuel, air and gas. As mentioned above, it is preferred to use 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 filter used may be of any known kind, such as an electrofilter (= eletrostatic filter) , a sand filter, a fluidized bed filter, a cyclone, a ceramic filter or a combination thereof, or any other filter which is operational at high or normal temperatures. In filter 2 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. If desired, 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. Example: 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. At the indicated effect level the amount of coal feed was 24.84 kg/s. 4.97 kg/s ash was obtained containing 1.74 % coal. Annually, this unburned coal amounts to 1.74 x 3600 x 8000 / 1000 = 50,112 t/a x 250 FIM/t = 12.5 MFIM/a 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: (4.97 - 1.74) X 3600/1000 X 8000 X (35 + 10) = 4.2 MFIM/a The economical value of the process is according to this calculation 16.7 MFIM annually, which corresponds to 9.4 % of the price of the fuel bought to the power plant. At the same time, considerable reductions of amounts of nitrogen oxides are obtained. According to the latest proposition by the Ministry of Environment (Proposition NO. 59/91) 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.

Claims: Claims : 1. A method for reducing nitrogen oxide emissions of a power plant using solid fuel and for improving the efficiency of the solid fuel combustion, comprising - controlling the combustion process by regulating the air feed, c h a r a c t e r i z e d by - feeding air at an excess air ratio of from about 1,02 to about 1,15 into the combustion process of the main boiler (1) and - afterburning the ash of the combusted solid fuel in a separate boiler (3) , whose effect in relation to the effect of the main boiler (1) is at the most equal to the proportion of ash in the solid fuel. 2. The method as claimed in claim 1, wherein air is fed to the combustion (1) at an excess air ratio of about 1.05 to 1.12 and the boiler (3) used in the afterburning has an effect which is less than about 10 %, preferably from 5 % to 8 %, of the effect of the main boiler. 3. The method as claimed in claim 1, wherein an excess air ratio ranging from 1.02 to 1.15 is maintained in the combustion zone of the boiler, a second part of the boiler being fed with staged air during the combustion process. 4. The method as claimed in claim 3, wherein the staged air is fed into the space above the combustion planes. 5. The method as claimed in any of claims 1 to 4, wherein the flue gas obtained from the afterburning (3) of the ash is conducted to the burners of the main combustion to be used as diluent gas for reducing the nitrogen oxide content. 6. The method as claimed in claim 5, wherein the flue gas obtained from the afterburning (3) is combined as such with the input air of the main combustion (1) . 7. The method as claimed in claim 5 or 6, wherein the heat released is not separately recovered in connection with the afterburning process. 8. The method as claimed in any of claims 1 to 4, wherein heat is transferred from the auxiliary boiler by means of water or steam circulation for the desired use. 9. The method as claimed in claim 8, wherein the heat is transferred to the main boiler, it is used as process heat or it is transferred to a district heating network. 10. The method as claimed in any of the previous claims, wherein steam or water mist is fed to the afterburning boiler (3) for controlling the combustion temperature. 11. The method as claimed in any of the previous claims, wherein the afterburning (3) is carried out at a temperature below 900 °C, preferably at a temparature in the range from 800 to 850 °C.

Inventors: AHONEN HEIKKI, AHONEN, HEIKKI
Applicants: HJA ENG OY, HJA-ENGINEERING OY
Application year: 1992
Main classification: F23L7
IPC codes: F22B31, F23C6, F23L7