EP0363812A2 - Method and installation for generating steam, especially in heat power plants - Google Patents

Abstract

The fluidised bed technique is used in generating steam in heat power plants in order to avoid the aftertreatment of flue gases for the purpose of reducing toxic constituents. In this process, the coal is burned at relatively low temperatures of around 850 DEG C, none or scarcely any thermal nitrogen oxide being formed in the fluidised bed of coal and lime, and SO2 which forms being converted to calcium sulphate. However, due to the low temperatures there is a higher emission of CO, especially in part-load operation. In order to reduce the latter, heat is additionally supplied to the free space above the fluidised bed, with the result that it is possible to achieve the purpose of influencing the temperature of the free space independently of that of the fluidised bed, and substantially to increase the conversion of CO to CO2. A reduction in the NOx values can further be achieved by restricting the quantity of air to be supplied to the fluidised bed below the stoichiometrically necessary additional supply of air to the free space while simultaneously supplying heat. <IMAGE>

Description

The invention relates to a method for generating steam, in particular in thermal power stations, by means of combustion of solid fuels, in particular coal, by a fluidized bed process in which the combustion takes place in a fluidized bed comprising the fuels at - relatively low - temperatures up to approx. 900 ° C., whereby the gases above the fluidized bed are additionally supplied with heat, and a system for generating steam by burning solid fuels in a stationary fluidized bed boiler, a space for the generated heating gases being formed in the boiler above a fluidized bed, generally made of coal and lime. In the case of coal as a solid fuel, that is to say a sulfur-containing material, SO₂ which forms is converted to calcium sulfate by means of the lime.

The fluidized bed technology is used in practice, in particular, in thermal power stations with the aim of avoiding the aftertreatment of flue gases to reduce pollutants by regularly burning the solid fuels at low temperatures of around 850 ° C. This ensures that, on the one hand, no or hardly any thermal nitrogen oxide is formed and, on the other hand, SO₂ on limestone which forms during the combustion process of coal is optimally converted to calcium sulfate. However, the disadvantage of burning at low temperatures is that toxic carbon monoxide emissions are higher than with conventional rust or coal dust furnaces.

As a result of the fluidized bed, the atmospheric, stationary fluidized bed boiler which is customary in practice forms a chemical reactor in a steam boiler. By dissipating heat from the fluidized bed to the heating surfaces, the most important parameter, the temperature of the fluidized bed, cannot be changed independently of the boiler output. On the other hand, the temperature in the fluidized bed cannot be raised above 900 ° C, because then the ash softening sets in, which brings the fluidized bed combustion to a standstill. To avoid these disadvantages, a circulating fluidized bed has been developed in which the fluidized bed reactor, the steam boiler and the ash cooler are separate apparatuses. This technology is considerably more expensive to purchase and can therefore only be used economically for larger steam boilers (over 30 t / h).

The lack of this general state of the art in practice is that the existing atmospheric, stationary fluidized bed boilers no longer meet the stricter requirements of immission control, especially with regard to the carbon monoxide value in the flue gas of at most 250 mg / m³, especially in the part-load range, so that new stationary fluidized bed boilers for combustion coal can no longer be built economically, particularly in the area of small capacities.

The problem of post-combustion of pollutants is known in a wide variety of areas.

For example, a method and a plant for generating steam of the type described at the outset (DE-PS 32 18 724) aim to substantially reduce the amount of unburned fuel particles emerging from the fluidized bed in order to reduce the load on the downstream filter. In other words, solid particles should be combusted. This combustion takes place in the area of a drawn-in flow cross-section of the flue gas flue, that is to say in a zone which is largely dependent on Fluid bed is removed and in which there has already been a significant reduction in the temperature of the exhaust gases after exiting the fluid bed. In order to burn such solid particles, a considerable increase in the temperature and, above all, a narrowing of the cross section for the gas train is necessary in order to send the solid particles through the flame and to be able to burn them. For this purpose, an average temperature of substantially more than 900 ° C. must prevail in the afterburner area in order to be able to carry out the combustion of the solid particles completely. This temperature is usually between 1200 and 1600 ° C and the particles have to stay at this temperature for some time. This requires a relatively large space. Following this afterburning, the temperatures drop due to heat dissipation on the heating surfaces. This process aims at the combustion of solid particles, which necessitates constructive measures such as narrowing the cross-section of the boiler and a longer, uncooled residence zone behind the flame zone in order to achieve sufficient burnout. The disadvantage is that thermal nitrogen oxide is formed by the necessary high temperatures and excess air.

Furthermore, a method is known (JP 59-81409) in which unburned constituents, that is to say both gaseous and solid, are to be burned in the free space above a fluidized bed by supplying gas in flame form. For this purpose, on the one hand a dry distillation of coal is carried out and the resulting gas is fed to the burner nozzle via a flap. A coal storage container serves as a further heating gas source, from which exhaust gas is fed to another burner nozzle via a flap. The aim of this process is to lower the fluidized bed temperature to 700 ° in order in any case to generate thermal nitrogen oxides, that is to say additional nitrogen oxides to those which occur when the coal decays thermally during combustion to avoid the result of the nitrogen compounds present in this. These nitrogen compounds, which are already present in the coal, should not be confused with the thermal nitrogen oxides that are formed during combustion at high temperatures due to the combination of the atmospheric nitrogen with the atmospheric oxygen and which may have to be eliminated by secondary processes.

Furthermore, a two-stage furnace with a first stage in the form of a pyrolysis or smoldering chamber and a second stage in the form of a post-combustion chamber is known (DE-A1 35 03 603), in which the latter is designed as a cyclone. Otherwise the ash would not be separated. The aim of the two-stage incineration is to avoid the evaporation of heavy metals, such as those that occur in particular during waste disposal. The aim of this is to ensure that non-combustible, environmentally harmful substances such as heavy metals can be brought into the gas phase and released into the environment with the exhaust gas or have to be separated again by means of complex other processes. With the two-stage process, such dangers do not arise from the low temperature in the first combustion chamber, and these substances can be excreted in the ashes. The low air supply therefore only serves to keep the temperatures in the smoldering chamber low. In other words, the reduced air supply for temperature control is used here, with the aim of avoiding excessive gas pressure of the heavy metals. Due to the low air supply in the first combustion stage and the - targeted - low temperatures, there are high unburned proportions, so that in the second combustion phase not only the air required for combustion has to be supplied, but also a particularly good mixing of the air with the combustion air Gases must occur, which is why the arrangement of a technically complex cyclone is expressly required.

The invention is therefore based on the object to improve the fluidized bed technology and these realizing systems, that is to say steam generation using atmospheric, stationary fluidized bed boilers, in such a way that they meet the requirements for optimum air purity with regard to their emission behavior.

This object is achieved in the method described at the outset in that the additional heat is supplied in the free space above the fluidized bed, the amount of air to be fed to the fluidized bed is kept below the stoichiometrically required amount of air while maintaining the combustion temperature, and the air supply to the free space is increased while the system is in operation The solution is that the free space is assigned at least one heat source, for example a burner, for additional, targeted energy supply and a device for supplying additional air into the combustion gases located in the free space. These measures can, viewed generally, optimize the conditions in the fluidized bed with regard to the temperatures and the exhaust gas components (the latter with regard to environmental pollution). It should be noted that the fluidized bed temperature must not be raised above approx. 900 ° to avoid sintering the ash (with the result of lump formation) and also to keep the nitrogen oxide formation within limits. Sufficient oxidation of CO to CO₂ could take place at temperatures above 900 ° in the space above the fluidized bed, but only with the aforementioned disadvantage of lump formation. According to the invention, however, the free space temperature can be made independent of the operation of the fluidized bed temperature by the supply of heat to the free space and can be influenced in a targeted manner. By increasing the free space temperature, the reaction rate when converting the carbon monoxide with oxygen to carbon dioxide can be increased considerably. As experiments have shown, this achieves a considerably low level Lower levels of carbon monoxide in the exhaust gas and the temperature of the fluidized bed can be kept at low values.

The free space is a zone of the boiler which belongs to the cross section of the combustion chamber caused by the fluidized bed, i.e. is not a constriction in the exhaust gas flue gas duct, but a space in which the fuel particles are caused to do so due to a reduction in the gas velocity - fall back into the fluidized bed due to gravity. How much free space ultimately depends on the design conditions.

As a result of throttling the amount of air to be supplied to the fluidized bed below the stoichiometrically required and increasing the air supply to the free space while simultaneously supplying heat to the latter, there is a particularly expedient possibility of reducing the NO _x values, especially with regard to those prescribed by the new guidelines Value of less than 500 mg / m³ flue gas. One uses the reducing effect of the carbon monoxide on the nitrogen oxides, whereby initially the CO formation is deliberately increased by throttling the air supply to the fluidized bed, so that due to the equilibrium conditions there is a greater reduction of the nitrogen oxides to elemental nitrogen by the carbon monoxide and then the excess carbon monoxide the additional air at the heat sources in the open space is converted to carbon dioxide. In contrast to the procedure known from DE-A1-3503603, the air supply to the fluidized bed is not throttled in the invention for the purpose of reducing the fluidized bed temperature. Rather, the fluidized bed should always be kept within the economically sensible limits, i.e. at approximately 850 °, in order to remain just below the sintering limit and to be able to keep the size of the system to a minimum in the design of the system. The temperature in the fluidized bed should explicitly not be reduced, because such a drop in temperature would reduce the reaction rate in the conversion of CO and NO _x to CO₂ and N₂.

In this way it is possible that the atmospheric, stationary fluidized bed boilers for all performance classes can comply with the legally prescribed values for CO of less than 250 mg / m³ flue gas and for NO _x of less than 500 mg / m³ at full load and part load .

The physical features of the system can also be realized very advantageously by retrofitting existing boiler systems.

In order to avoid the cooling of the flue gases, in a preferred embodiment of the project the heat can be applied directly above the fluidized bed.

The lowering of the temperature caused by the substoichiometric air supply can advantageously be compensated for by an increased supply of coal to the fluidized bed. As a result, there is no loss of energy because the energy potential of the carbon that is not used in the fluidized bed is recovered in the oxidation of the CO in the free space. However, this shift in energy conversion from the fluidized bed to the free space is only in the order of 1 - 3%.

To explain the invention in more detail, an exemplary embodiment of a steam generation system is shown in the schematic drawing.

One can see on average a fluidized bed boiler 1 of conventional construction with a fluidized bed of coal and lime arranged in the lower part, in which SO₂ forming during the combustion process is converted to calcium sulfate and is withdrawn with the ash marked by an arrow. Here air becomes air over an air fuhreinrichting 6 supplied controlled. Above the fluidized bed there is a free space 3 for the rising combustion gases, as well as the usual steam generation system, which need not be described in more detail here. Two burners 4 each, which are operated with natural gas, and two additional air supply devices 5 open into the free space. The burners 4 serve to increase the free space temperature; any other heat sources, for example electrical ones, can of course also be used in their place. With their help, the free space temperature is increased to values above 900 ° C and an effective conversion of CO in the flue gas to harmless CO₂ is achieved. By reducing the air supply via the air supply device 6, the CO formation is initially increased by the accumulation in the fluidized bed during combustion Convert nitrogen oxides to elemental nitrogen, while the excess CO is then converted to CO₂ by supplying air via the additional air supply devices 5 and supplying heat via the heat source 4.

It is of course possible to use only one heat source 4 and two air nozzles 5 instead of each, the resulting pollutant values then being somewhat higher, such as the following test evaluation of a system of conventional arrangement, ie without a burner and additional air supply, and with one and each can be taken from two burners:

Claims (4)

1. Process for generating steam, in particular in thermal power stations, by means of combustion of solid fuels, in particular coal, by a fluidized bed process in which combustion takes place in a fluidized bed comprising the fuels at low temperatures up to approximately 900 ° C., the gases above the fluidized bed additional heat is supplied, characterized in that the additional heat supply takes place in the free space above the fluidized bed, the amount of air to be supplied to the fluidized bed is kept below the stoichiometrically required amount of air while maintaining the combustion temperature, and the air supply to the free space is increased. 2. The method according to claim 1, characterized in that the heat is applied directly above the fluidized bed. 3. The method according to claim 1 or 2, characterized in that the fuel supply to the fluidized bed to compensate for a temperature drop with substoichiometric air supply is increased. 4. Plant for steam generation by burning solid fuels in a stationary fluidized bed boiler, wherein in the boiler above a fluidized bed, usually made of coal and lime, a free space for the generated heating gases is formed, characterized in that the free space (3) at least each a heat source (4), for example a burner, for additional, targeted energy supply and a device (5) for supplying additional air into the combustion gases located in the free space.

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