FI81503C - Process for bonding sulfur compounds formed in a coal powder boiler - Google Patents

Description

81503

The present invention relates to a process for the binding of sulfur compounds formed in a coal dust boiler with a powdered reagent such as limestone. The present invention relates to a process for binding sulfur compounds formed in a coal dust boiler with a powdered reagent such as limestone.

In the past, coal boiler fuel has usually been ground in large ball mills, which are space consuming and, in addition, very disadvantageous in terms of both investment and operating costs. Efforts have been made to reduce the sulfur dioxide content of the boiler flue gases by feeding calcium oxide, calcium carbonate or some other alkaline compound to the boiler combustion chamber. In the event that the sulfur in the fuel is bound to a reagent such as calcium carbonate, calcium magnesium carbonate or a similar oxide, that reagent is fed as a powder having a particle size mainly in the range of 0.05 to 1 mm. The reagent is fed as a dry powder or slurry, usually as a separate jet, either directly into the boiler or into the flue gas reaction chamber following the boiler. The powdered oxide and / or carbonate is fed to the boiler combustion chamber according to the sulfur content of the fuel so that the alkali and / or alkaline earth metals are present in a molar ratio of at least the same amount as sulfur, but preferably more than the amount required for the reaction. Because the particle size of the reagent varies and contains a large amount of coarse, large particles must be used in excess of the amount of sulfur dioxide gas generated in the combustion chamber to achieve a sufficient desulfurization result. In this case, considerable amounts of waste material such as gypsum and only partially reacted reagent are generated from the excess of the reagent, mainly lime.

It is an object of the present invention to obviate these disadvantages. This is done by grinding the fossil pre-crushed 81503 2 solid fuel and the pre-crushed reagent into ultrafine particles by means of a counter-jet mill using compressed air as working gas and directing it to the boiler by adjustable back-pressure of the counter-jet mill, using the fuel / gas mixture gas portion

In the counter-jet mill, the material to be ground is accelerated as a gas suspension through at least two acceleration nozzles oriented in the grinding chamber in such a way that the material-gas jets rushing from the nozzles collide with each other in the middle of the grinding chamber. As the counter-jet mill, the pressure chamber refiner unit disclosed in Finnish patent application 86 1961 can be advantageously used, from which the mechanical trace of the refiner has been removed. For example, the refiner housing may be of the type described in Finnish patent publication 74222.

In the method according to the invention, for example, coal, lignite or the like can be used as fuel.

By using a counter-jet grinder to grind and feed both fuel and reagent, usually limestone or quicklime, to the boiler, a very advantageous and easily controllable combustion event in the boiler is obtained, while the sulfur-binding capacity of the process is significantly enhanced. The particle size of the fuel dust and limestone dust fed to the boiler is ninety percent less than 30 μm. Due to the large specific surface area of fuel dust and limestone dust, the chemical reactions in the combustion chamber take place very quickly and completely. This applies in particular to the combustion reaction of carbon and sulfur, the decomposition of calcium carbonate into its reaction products (into CO2 and CaO) and further to a reaction in the range of 1050 to 750 ° C with the reaction of sulfur dioxide.

With CaO to sulfide and / or sulfate. In order to bind the sulfur contained in the fuel, it is possible to work with an almost stoic 3,850,503 meter of limestone, because it operates close to the thermodynamic equilibrium state.

The method according to the invention can thus improve the efficiency of the boiler, because the combustion takes place more efficiently than before and due to the smaller amount of limestone fed to the boiler, the energy requirement for burning lime is reduced while less waste is generated. Due to the efficient grinding effect of the counter-jet mill, a large amount of fresh active surface area is formed in the reagent particles, improving the reactivity of the reagent to the extent that low quality limestone such as dolomite with a lower CaCO 3 content can also be effectively used for the sulfur scavenging reaction. Due to the large specific surface area generated by jet grinding, the MgO generated in dolomite combustion is reactive and also has time to react with sulfur compounds.

Other features of the invention appear from the appended claims 1-10.

The method according to the invention can be carried out in such a way that:. the fuel and reagent are ground either in the same counter-jet mill or in different back-jet mills.

In the case of using two counter-jet grinders, the fuel grinding unit must be significantly larger in capacity than the reagent, usually a limestone grinding unit. In order to obtain the correct mixture ratio, an analysis to determine the sulfur content of the fuel and the content of sulfur-binding compounds in the limestone must be carried out frequently enough. Based on the results of the analysis, the grinding conditions of the counter-jet mills can be adjusted quickly and accurately so that the mutual feed rates of fuel and limestone correspond to the optimum situation. When the equipment has separate 4 81503 feed and grinding units for fuel and reagent, the adjustment of the equipment to the optimal mixture ratio can be easily controlled. Another advantage of using two counter-jet mills is that reactive fuel dust and a reagent such as limestone dust can be fed to the boiler either as completely separate gas jets or by combining the exhaust jet exhaust pipes as a common gas mixture where the fuel and reagent dust mixture ratio is optimal.

The simplest apparatus is obtained if the fuel and reagent are ground and fed to the boiler by a common counter-jet refiner, whereby the fuel and reagent are fed to the hopper of the counter-jet refiner in the correct mixture ratio based on the analysis results.

If it is desired to avoid that the reagent and the fuel come into contact with each other before grinding, two feeders can be provided in the counter-jet refiner, a large one for the fuel and a small one for the reagent. The large feeder preferably feeds the fuel through two accelerator nozzles directed to the center of the refining chamber of the counter-jet refiner, and the small feeder feeds the reagent to two other, co-located. through the accelerator tube.

In the event that the apparatus includes a different feed unit for fuel and reagent, the ratio between the two components can be easily adjusted by adjusting the working gas pressures of that feed unit and / or the speed of the feed screw.

Due to the large specific surface area achieved by the fuel and reagent in the counter-jet refiner, their reactivity increases so much that it is possible to operate with an almost stoichiometric amount of reagent to bind the sulfur contained in the fuel.

Il 81503 5

The particle size distribution of fuel dust and reagent dust can be easily and quickly adjusted by changing the grinding conditions of the counter-jet refiner or refiners, such as working gas pressure, gas-solids ratio, etc.

Since the amount of compressed air required in the grinding process can be varied within wide limits, the amount of high pressure working gas flowing into the boiler with fuel dust and acting as the primary air of the combustion process can be easily adjusted to a level favorable for burning coal dust. The back pressure in the grinding chamber of the counter-jet refiner can thus be utilized to transport the ground reagent and fuel from the counter-jet mill to the boiler, while the gas portion of the mixture is used to burn coal dust.

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Claims (10)

A process for bonding sulfur compounds arising in a coal-fired boiler with a powdery reagent, such as limestone, characterized in that fossil-crushed solid fuel and crushed reagent are ground by a counter-jet mill using compressed air as working gas, to ultrafine particles and directly to the boiler by means of the adjustable residual pressure of the counter-jet mill, whereby the fuel / gas mixture's gas proportion is used as primary air for the combustion process. 2. A process according to claim 1, characterized in that the fossil fuel and the reagent are milled in separate counter-jet mills. 3. A method according to claim 2, characterized in that both outlet pipes of the counter-radiator mills are conducted as separate channels to the boiler. 81503 4. A method according to claim 2, characterized in that both outlet pipes of the counter-jet mills are connected to a common nozzle channel opening in the boiler. Process according to Claim 1, characterized in that fossil fuel and reagent are ground in the same counter-mill. 6. A method according to claim 5, characterized in that fuel and reagent are measured in the grinding chamber of the counter-jet mill as separate, particulate / gas jets against one another via a separate feed unit. 7. A method according to claim 5, characterized in that fuel and reagents are measured in a desired mixing ratio in the feed unit of the counter-jet mill. 8. A method according to any one of claims 1 to 6, characterized in that the ratio of fuel to reagent to be measured in the boiler is controlled by controlling the working pressure of the feed units of the counter-grinder or mills and / or the rotational speed of the feed screws. 9. A process according to any one of claims 1 to 8, characterized in that fuel and reagent are measured in the boiler in an almost stoichiometric ratio to the amount of sulfur in the fuel. Process according to Claim 9, characterized in that both the particle size distribution of the fuel powder and the reagent powder is controlled by changing the milling conditions.