WO1990003837A1

WO1990003837A1 - Method for binding of the sulfur compounds formed in a pulverized-coal boiler

Info

Publication number: WO1990003837A1
Application number: PCT/FI1989/000197
Authority: WO
Inventors: Jouko Niemi; Pertti Heinonen
Original assignee: Oy Finnpulva Ab
Priority date: 1988-10-14
Filing date: 1989-10-13
Publication date: 1990-04-19
Also published as: FI81503C; FI81503B; FI884744A0; AU4347489A; FI884744A

Abstract

The present invention concerns a method for binding of the sulfur compounds formed in a pulverized-coal boiler by means of a pulverulent reagent, such as limestone. The method is characterized in that pre-crushed fossil solid fuel and pre-crushed reagent are ground by means of a counter-jet mill which employs compressed air as the operating gas into ultrafine particles and passed directly into the boiler by means of an adjustable after-pressure of the counter-jet mill, the gas portion in the fuel/gas mixture being used as the primary air for the combustion process.

Description

Method for binding of the sulfur compounds formed in a pulverized-coal boiler

The present invention concerns a method for binding of the sulfur compounds formed in a pulverized-coal boiler by means of a pulverulent reagent, such as limestone.

In prior art, the fuel for a pulverized-coal boiler has been ground usually in large ball mills, which require an abundance of space and whose costs of investment and operation are also highly unfavourable. Attempts have been made to lower the contents of sulfur dioxide in the flue gases of the boiler by into the combustion chamber of the boiler feeding calcium oxide, calcium carbonate, or some other alkaline compound. If the sulfur contained in the fuel is bound to a reagent, such as calcium car¬ bonate, calcium-magnesium carbonate, or a corresponding oxide, said reagent is introduced as a powder whose particle size is mainly within the range of 0.05 to 1 mm. The reagent is introduced as a dry powder or as a suspen¬ sion, usually as a separate jet either directly into the boiler or into the flue-gas reaction chamber placed after the boiler. Pulverulent oxide and/or carbonate is fed into the combustion chamber of the boiler in accordance with the sulfur content of the fuel so that alkali metals and/or alkaline-earth metals are present, in the mole ratio in accordance with the reaction formula, at least as a quantity corresponding to the quantity of sulfur, advantageously, however, higher than the quantity that is needed for the reaction. Since the particle size of the reagent varies and includes an abundance of coarse, large particles, the reagent must be used as an abundant excess quantity in relation to the amount of sulfur dioxide gas formed in the combustion chamber in order that an adequate result of removal of sulfur could be obtained. In such a case, out of the excess quantity of the reagent, mainly lime, considerable amounts of waste material are produced, such as gypsum and only partly reacted reagent.

The object of the present invention is to eliminate these drawbacks. This takes place so that pre-crushed fossil solid fuel and pre-crushed reagent are ground by means of a counter-jet mill which employs compressed air as the operating gas into ultrafine particles and passed directly into the boiler by means of an adjustable after-pressure of the counter-jet mill, the gas portion in the fuel/gas mixture being used as the primary air for the combustion process.

In a counter-jet mill, the material to be ground is accelerated as a gas suspension into a grinding chamber through at least two acceleration nozzles, which are directed so that the material-gas jets rushing out of the nozzles collide against each other in the middle of the grinding chamber, whereby the grinding takes place. As the counter-jet mill it is advantageously possible to use a pressure-chamber grinder unit described in the Finnish Patent Application No. 86-1961 from which the mechanical after-grinder has been removed. The construction of the grinder housing may be, e.g., of the type described in the Finnish Patent No. 74,222.

In the method in accordance with the invention, as the fuel it is possible to use, for example, coal, brown coal, or equivalent.

By using the counter-jet grinder for grinding both the fuel and the reagent, usually limestone or dehydrated lime, and for feeding them into the boiler, a highly advantageous and readily adjustable combustion process is obtained for the boiler, at the same time as the sulfur- binding capacity of the process is improved significantly. The particle size of the fuel dust and limestone dust fed into the boiler is below 30 μm to the extent of 90 per cent. Owing to the large specific surface of the fuel dust and the limestone dust, the chemical reactions in the combustion take place extremely rapidly and completely. This applies in particular to the reaction of combustion of carbon and sulfur, to the decomposition of calcium carbonate to its reaction products (CO2 and CaO), and further, when the temperature is lowered to the range of 1050 to 750°C, to 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 operate almost with a stoichiometric quantity of limestone, because the operation takes place near the thermodynamic equilibrium.

In this way, by means of the method of the invention, the efficiency of the boiler can be improved, because the combustion takes place more efficiently than in prior art and because, owing to the reduced amount of limestone to be fed into the boiler, the requirement of energy consumed for burning of the lime is lowered at the same time as the amount of waste material produced becomes smaller.

Owing to the efficient grinding effect of the counter-jet grinder, an abundance of fresh active surface area is formed on the reagent particles, improving the reactivity of the reagent to such an extent that, for the sulfur- binding reaction it is also possible to make efficient use of limestone of inferior quality, such as dolomite, whose content of CaCθ3 is lower. Owing to the large specific surface formed on jet grinding, the MgO formed on burning of dolomite is reactive and also has time to react with the sulfur compounds.

Further characteristics of the invention come out from the accompanying patent claims 1 to 10.

The method of the invention can be carried out so that the fuel and the reagent are ground either in the same counter-jet grinder or in separate counter-jet grinders. If two counter-jet grinders are employed, the capacity of the unit that grinds the fuel must be considerably higher than the capacity of the unit that grinds the reagent, usually limestone. To reach a correct mixing ratio, it is necessary to carry out an analysis that determines the sulfur content in the fuel and the contents of the sulfur-binding compounds contained in the limestone sufficiently frequently. On the basis of the analysis results, it is possible to regulate the grinding conditions of the counter-jet mills rapidly and accurately so that the relative feed quantities of fuel and limestone cor¬ respond to an optimal situation. When the equipment has separate feed and grinding units for the fuel and for the reagent, regulation of the equipment for an optimal mixing ratio can be controlled readily. A second advantage that is obtained from the use of two counter-jet grinders is that the reactive fuel dust and the reagent, such as limestone dust, can be introduced into the boiler either as fully separate gas jets or, by combining the outlet ducts of the counter-jet grinders, as a common gas mixture in which the mixing ratio of the fuel dust and the reagent dust is optimal.

The simplest equipment is obtained if the fuel and the reagent are ground and fed into the boiler by means of a common counter-jet grinder, in which case the fuel and the reagent are fed at a correct mixing ratio, based on analysis results, into the feed funnel of the counter-jet grinder.

If it is desired to avoid any contact between the reagent and the fuel before the grinding, the counter-jet grinder may be provided with two feeder devices, a large one for the fuel and a small one for the reagent. The large feeder feeds the fuel advantageously trough two accelera¬ tion nozzles directed at the centre point of the grinding chamber in the counter-jet grinder, and the small feeder feeds the reagent through two other acceleration pipes, which are directed at the same point.

If the equipment comprises separate feeders for fuel and for reagent, the ratio of these two components can be adjusted easily by regulating the operating-gas pressures and/or the speeds of rotation of the screw feeders in said feeder units.

Owing to the large specific surface obtained by the fuel and by the reagent in the counter-jet grinder, their reactivity is increased to such an extent that, to bind the sulfur contained in the fuel, it is possible to operate almost with a stoichiometric quantity of reagent.

The distribution of the particle size of the fuel dust and of the reagent dust can be regulated easily and rapidly by changing the grinding conditions in the counter-jet grinder or grinders, such as the operating-gas pressure, the gas-solids ratio, etc.

Since the quantity of compressed air required in the grinding process can be varied within wide limits, in particular the amount of the high-pressure operating gas, which flows along with the fuel dust into the boiler and which acts as the primary air in the combustion process, can be adjusted readily to a level advantageous in view of the burning of the coal dust. In this way, the after- pressure present in the grinding chamber of the counter- jet grinder can be utilized for carrying the ground reagent and fuel from the counter-jet mill into the boiler, at the same time as the gas portion in the mixture is utilized for burning of the coal dust.

Claims

WHAT IS CLAIMED IS:

1. Method for binding of the sulfur compounds formed in a pulverized-coal boiler by means of a pulverulent reagent, such as limestone, c h a r a c t e r i z e d in that pre-crushed fossil solid fuel and pre-crushed reagent are ground by means of a counter-jet mill which employs compressed air as the operating gas into ultrafine par- tides and passed directly into the boiler by means of an adjustable after-pressure of the counter-jet mill, the gas portion in the fuel/gas mixture being used as the primary air for the combustion process.

2. Method as claimed in claim 1, c h a r a c t e r ¬ i z e d in that the fossil fuel and the reagent are ground in separate counter-jet grinders.

3. Method as claimed in claim 2, c h a r a c t e r - i z e d in that the outlet pipes of both of the counter- jet grinders are passed into the boiler as separate ducts.

4. Method as claimed in claim 2, c h a r a c t e r ¬ i z e in that the outlet pipes of both of the counter- jet grinders are combined to make a common nozzle duct ending in the boiler.

5. Method as claimed in claim 1, c h a r a c t e r ¬ i z e d in that the fossil fuel and the reagent are ground in the same counter-jet mill.

6. Method as claimed in claim 5, c h a r a c t e r ¬ i z e d in that the fuel and the reagent are fed into the grinding chamber in the counter-jet grinder as separate solids/gas jets which collide against each other, each material through a feeder unit of its own.

7. Method as claimed in claim 5, c h a r a c t e r ¬ i z e d in that the fuel and the reagent are fed at the desired mixing ratio into the feeder unit of the counter- jet grinder.

8. Method as claimed in any of the claims 1 to 6, c h a r a c t e r i z e d in that the ratio between the fuel and the reagent to be fed into the boiler is regulated by adjusting the operating-gas pressures and/or the speeds of rotation of the screw feeders in the counter-jet grinder or grinders.

9. Method as claimed in any of the claims 1 to 8, c h a r a c t e r i z e d in that the fuel and the reagent are fed into the boiler at an almost stoichiometric ratio to the amount of sulfur contained in the fuel.

10. Method as claimed in claim 9, c h a r a c t e r ¬ i z e d in that the distribution of the particle size both in the fuel dust and in the reagent dust is regulated by changing the grinding conditions.