AU594867B2

AU594867B2 - Method of burning, in particular, salt-containing brown coal

Info

Publication number: AU594867B2
Application number: AU13506/88A
Authority: AU
Inventors: Rudi Derksen; Hans-Georg Jarmuzewski; Karl-Heinz Maintok; Rainer Dr. Nottenkamper
Original assignee: Deutsche Babcock Werke AG
Current assignee: Deutsche Babcock Werke AG
Priority date: 1987-04-15
Filing date: 1988-03-23
Publication date: 1990-03-15
Anticipated expiration: 2008-03-23
Also published as: EP0286845A1; DD268758A5; DE3712801C2; ATE64190T1; DE3863115D1; AU1350688A; DE3712801A1; EP0286845B1

Abstract

In particular saline brown coal with a high alkali content is burned with air in a fluidised bed (4). The resulting combustion gas is post-combusted and cooled in a cooled free space (7) above the fluidised bed (4). Via a recycling blower (15) and flue gas pipes (16), cold flue gas is blown into the fluidised bed (4) and the free space (7) in such a manner that, by virtue of the heat transfer to the cold flue gas and the cooling of the walls of the free space (7), a temperature of 780 degrees C maximum is maintained in the fluidised bed (4) and on the gas-side end of the free space (7), a wall temperature below the fusing temperature of the ash is maintained, and in the free space a post-combustion zone with a temperature of 850 degrees C maximum is maintained. <IMAGE>

Description

AUSTRALIA PATENTS ACT 1952 COMPLETE SPECIFICATION 94867 Form

(ORIGINAL)

FOR OFFICE USE Short Title: Int. Cl: Application Number: Lodged: Complete Specification-Lodged: Accepted: Lapsed: Published: p 4 ft ftp ft 9* ftp ft *4 ft ft oft ,~ft ft S ft Priority: Related Art: TO BE COMPLETED BY APPLICANT Name of Applicant: ft ft ft.

4 ft DEUTSCHE BABCOCK WERKE

AKTIENGESELLSCHAFT

1 Address of Applicant: DUISBURGER STR. 375 D-4200 OBERHAUSEN 1

GERMANY

Actual Inventor: oft ftft ft 4 ft ft ft ft ft ft Address for Service: CLEMENT HACK CO., 601 St. Kilda Road, Melbourne, Victoria 3004, Austral-Ia.

Complete Specification for the invention entitled; METHOD OF BURNING, IN PARTICULAR, SALT-CONTAINING BROWN COAL The following statement ii a full description of this invention including the best method of performing it known to me:i/ 14 T t_ L- t 2- The present invention relates to the burning of brown coal, in particular of brown coal containing salts with an increased content of alkalis, in accordance with the preamble to Patent Claim 1.

Biown coals of normal composition may be burned in pulverized-fuel combustion chambers and fluidized-bed combustion chambers of steam generation plants. However, if the ash from the brown coal has an unfavorable ratio of so-called basic components such as CaO and MgO to so-called acidic components such as SiO and Al 0 in the presence of alkalis, then the brown coals become all the more difficult to burn as the ratio of bases/acids and the content of j alkalis increase (BiUNKOHLE 1976, No. 6, pp. 206 217).

These types of brown coal can then only be burned with the i ruse of specifically-constructed fire-boxes or else they are considered to be unsuitable for the firing of steam boilers.

The special construction of the fire-box for pulverized-fuel fired boilers which come under consideration requires, amongst other things, a low flame temperature which is obtained by the recycling of cold flue gases.

The present invention seeks to develop a method for the burning of brown coal which, with the utilization of normal brown coal, allows the most favorable combustion S" temperature to be established in the fluidized bed and in 'f the free gas space (freeboard or disengaging space) and which is also suitable for use with such types of salt-containing brown coals which can either not be used at all for the firing of conventional steam boilers or else only under restricted conditions.

too $t i/ r .i r A- tk/ 1 v -t~a J7 r -2A Accordingly the present invention provides a method for the burning of brown coal, in particular of brown coal S_3 containing salts with an increased content of alkalis, for the production of steam, for which purpose the coal is burned with air in a fluidized bed and the combustion gas derived therefrom is subjected to after-burning in a cooled free gas space above the fluidized bed and thereafter cooled, wherein cold waste gas is blown into the fluidized bed and the free gas space so that, because of the heat absorbed by the cold waste gas and the cooling of the tube walls of the fluidized bed and the free gas space, a maximum temperature of 780 degrees Celsius is established in the fluidized bed and at the gas-side end of the free gas space, and a wall temperature below the softening temperature of the fly ash is maintained, as well as post-combustion zone with a maximum temperature of 850 degrees Celsius in the free gas space.

Compared with pulverized-fuel firing, fluidized-bd E tfiring allows for the use of a lower combustion temperature.

SIn the absence of heat-concentration surfaces in the

S

t c r: fluidized bed and/or in the free gas space above the bed, the cold flue gas serves as the heat-transfer medium for taking up the heat of I tr a t 0 44 3 combustion from the fluidized bed and the free gas space. In the case of firing with normal brown coal, this signifies that, because of the flue gas recirculation, there is adaptation to the currently most economical combustion temperature, under conditions dictated by the specific nature of the coal, in the fluidized bed and in the free gas space. When salt-containing brown coal with an increased alkali content is being used, then such a low temperature can be maintained at which only a partial volatilization of the alkalis will take place. The maintenance of lower wall temperatures in the free gas space Sprevents the accretion on the tube wall of deposits which are very difficult to remove. Provision is made in the post- ,t combustion zone for the complete burning of the gaseous components of the coal issuing from the fluidized bed, along I F, with any entrained solid particles of coal, at a positively- S adjustable higher temperature. With the features described in Patent Claim 2, the effect is chieved that the heating surfaces with higher wall temperatures on which salt-containing accretions are deposited, depending upon the particular coal SZQ being used, are located in a second flue-gas compartment. The salt-containing products which are derived from the cleaningoff of the heating surfaces are thus not returned into the fluidized bed, so that any build-up of salts is avoided. The features described in Patent Claim 3 serve for the maintenance of a sufficient quantity of material in the bed for the operation of the fluidized bed, without increasing the alkali content of the fluidized bed.

An example of embodiment of the invention will now be discussed in greater detail with reference to the drawing which illustrates the method for implementation of the invention.

A steam generator which serves for the burning of brown coal is configured as a multi-compartment boiler and has a first flue-gas compartment 1, a second flue-gas compartment 2 and a third flue-gas compartment 3, which are separated from one another by gas-tight tubs walls which have, water and steam flowidg through them. The first flue-gas compartment l serves S. as the combustion chamber and accommodates the fluidized bed 4 in its lower portion. The fluidized bed 4 is connected to an !I 4 external ash-treatment device 5. A constriction 6 can be formed by appropriate bends in the walls above the fluidized bed 4 and this gives rise to additional mixing of the flue gases. The internal space of the first flu -gas compartment 1 is kept free from heat-concentration surfaces and represents the freeboard Sor disengaging space 7. The second 2 and third 3 flue-gas compartments are connected together by way of a boiler funnel 8 which has a discharge device 9 located at its lower end.

Located inside the second 2 and third 3 flue-gas compartments there are heating surfaces 10, 11 which are connected as a superheater or economizer. Soot blowers are provided for cleaning-off the heating surfaces 10, 11.

In the fluidized bed 4 of the steam generator which has been described, brown coal whi-h preferably contains salts is S*1* burned to yield an ash which contains more than 2 of alkalis.

This coal is fed by way of the coal supply device 12 into the fluidized bed 4 where it is burned with the aid of air. The air is forced into the fluidized bed 4 through an air pipeline by means of a blower fan 13. It is preferable to introduce the I?0 total amount of air required for complete combustion into the fluidized bed 4. Alternatively, provision can be made for a portion of the air to be introduced as secondary and tertiary air into the free gas space 7 above the fluidized bed 4. This partitioning of the air supply should be provided especially if 25' a brown coal with a high content of nitrogen-containing substances is being burned.

Cold flue gas at the temperature of the waste gas is drawn off from the cold end of the steam generator and is recirculated into the fluidized bed 4' and the free gas space 7 above the bed through the waste gihs pipeline 16 by means of the recirculation blower fan 15. A pressure-increasing blower fan 17 is provided for equilibration of the pressure differences within the fluidized bed 4. The cold waste gas partly absorbs the heat of combustion generated in the fluidized bed 4 and in the post-combustion zone within the free gas space 7. The amount and the distribution of the cold waste gas is determined in such a manner that a maximum temperature of 780 degrees Celsius is established in j(he fluidized bed 4 and before the w I r~iiii-; i i l a.rrC~ first heating surface 10 in the second flue-gas compartment 2.

In the post-combustion zone in which the particles of coal entrained in the gas from the fluidized bed 4 and the combustible gases are subjected to after-burning, the temperature has a value of approximately 850 degrees Celsius.

Because of the medium flowing through the tube wall of the free gas space 7, a wall temperature is established which is approximately 400 degrees Celsius below the softening temperature of the fly ash. Because of the low temperature of the fluidized j l'Q bed, the effect is achieved that a large portion of the alkalis S in the fluidized bed 4 is not volatilized and the alkalichlorides and -sulphates of the coal ash are bound, for It S I. example, to lime which is supplied to the fluidized bed 4 9, tC separately or at the same time as the pulverized coal.

Continuous operation for longer periods of time with saltcontaining brown coal leads to an increase in concentration of the salt content and to the formation of similar-sized granules ,t in the fluidized bed, which has an inhibiting effect on the 9,T fluidization. For this reason it is necessary to remove ash from the bed continuously. A portion of the ash from the fluidized bed 4 is removed by way of an ash pipeline 18, cooled and supplied to the ash treatment device 5. The ash treatment device 5 is furnished with a supply line 19 and mixing Or *t equipment 20. By treatment of the ash with a solvent in the ash :1 treatment device 5, the greater part of the ash or alkali is dissolved and removed by way of the discharge pipeline 21. In this manner, the bed material is reactivated for binding of the i sulphates and chlorides. The reactivated ash is fed back into the fluidized bed 4 to maintain the salt-free condition of the bed material. The return of the ash can be effected directly or by way of the coal supply device ii. A portion of the treated ash can be removed from the system by way of a discharge pipeline 22.

K

Claims

1. Method for the burning of brown coal, in particular of brown coal containing salts with an increased content of alkalis, for the production of steam, for which purpose the coal is burned with air in a fluidized bed and the combustion gas derived therefrom is subjected to after-burning in a cooled free gas space above the fluidized bed and thereafter cooled, wherein cold waste gas is blown into the fluidized bed and the free gas space so that, because of the heat absorbed by the cold waste W gas and the cooling of the tube walls of the fluidized bed and the free gas space, a maximum temperature of 780 degrees Celsius is established in the fluidized bed and at the gas-side end of the free gas space, and a wall temperature below the softening temperature of the fly a~h is maintained, as well as post-combustion zone with a maximum temperature of 850 degrees Celsius in the free gas space. I I

2. Method according to Claim 1, wherein the combustion gas after leaving the free gas space is cooled in one or more adjoining flue-gas compartments with heating surfaces located Q. e inside them, the wall temperatures of which are above the softening temperature of the fly ash.

3. Method according to Claim 1 or Claim 2, wherein a portion of the ash is removed from the fluidizd bed and the alkalis contained therein are removed from bhe ash by washing, a portion of the washed ash being returned to the fluidized bed. DATED THIS 23RD AY OF MARCH 1988 DUETSCHE BABCOCK WERKE AKTIENGESELLSCHAFT By its Patent Attorneys: CLEMENT HACK CO. Fellows Institute of Patentl Attorneys of Australia.