AU706722B2

AU706722B2 - Method to obtain energy from brown coal

Info

Publication number: AU706722B2
Application number: AU50821/96A
Authority: AU
Inventors: Friedrich Dr Klauke; Klaus-Dieter Dr Tigges
Original assignee: LENTJES KRAFTWERKSTECHNIK; Babcock Lentjes Kraftwerkstechnik GmbH
Current assignee: Babcock Lentjes Kraftwerkstechnik GmbH
Priority date: 1995-05-20
Filing date: 1996-04-23
Publication date: 1999-06-24
Anticipated expiration: 2016-04-23
Also published as: PL314328A1; HRP960224A2; CN1159541A; AU5082196A; YU29196A; GR960100137A; TR199600401A2; BA96106A; GR1003187B; PL183995B1; DE19518574A1

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Applicant: BABCOCK LENTJES KRAFTWERKSTECHNIK GmbH Invention Title: METHOD TO OBTAIN ENERGY FROM BROWN COAL The following statement is a full description of this invention, including the best method of performing it known to me/us: 2 Method to obtain energy from brown coal The invention concerns a method to obtain energy from brown coal wherein the brown coal is dried in a fluidised bed or a revolving dryer at low temperatures.

Such a method is known as the preliminary stage for a subsequent gasification of the dried brown coal in a fluidised bed (BWK 41 (1989), pages 231-234). Since heat from a lower temperature level is used for the drying, this type of drying is favourable from the point of view of energy utilisation. On the other hand, however, the apparatus for the drying is expensive, and the brown coal used has a relatively large grain size of up to 6 mm. It is difficult to fully burn such a course coal, while, in addition, the course grain promotes the formation of NOx.

Furthermore to dry crude brown coal, which in the raw state may have a water content of up to 65 during the grinding in, for example, a fan mill or a fan hammer mill is also known. In this case the mill aspirates hot *flue gases from the connected furnace with a temperature of approx. 900 0 C. This method of drying by grinding delivers "a ground product with a small grain size which is favourable for the combustion, and, in addition, a .o 25 sufficiently large volume of recirculated gas is available for the heat transfer into the flame and radiation chamber.

Nevertheless, for this type of drying heat is used from a high temperature level, consequently reducing the thermal efficiency of the plant. Furthermore, several mills are 30 assigned to the fire chamber of the known furnace, each of these mills supplying fuel to one group of burners. As for reasons of reserve at least one mill is on standby, not all burners of a plane can be supplied with fuel.

Consequently, each burner plane is unevenly charged.

The object of the invention is to produce a method to obtain energy from brown coal wherein the Sfavourable efficiency of drying the brown coal at a lower H!\R~ould\Keep\Speci\5821.96.doc 4/21/99 3 temperature is made use of and wherein the dried brown coal is combusted in such a manner that the increased cost of the apparatus to dry the brown coal is compensated for and the combustion process is carried out in full and with a low NOx content.

This objective is achieved by the claimed invention which provides a method to obtain energy from brown coal, wherein brown coal is dried in a fluidised bed or a revolving dryer at low temperatures, characterised in: that the dried brown coal is injected through burners directly or after an intermediate storage indirectly into the fire chamber of a gas generator without further comminution, that the dried brown coal is combusted in the burners sub-stoichiometrically with an air ratio of n 0.8, that burn-out air is injected through nozzles into the fire chamber and/or into a radiation chamber connected to it, that by virtue of a symmetrical charging of the burners a uniform distribution of the temperature throughout the cross-section of the fire chamber is .achieved and that by arranging the burners and/or the nozzles for the burn-out air in the fire chamber and/or radiation chamber a uniform temperature distribution in a vertical direction of the combustion chamber is achieved Advantageous configurations of the invention are subjects of the sub-claims. The method according to the 30 invention and its advantages are explained below.

Crude brown coal with a water content of 45 to is adequately comminuted to, for example, a maximum grain 'size of 6 mm. Following this the comminuted crude brown coal is introduced to a fluidised bed dryer for the purpose of drying at low temperatures between 50 and 150 0 to which dryer steam is introduced as drying and fluidising medium. In the fluidised bed dryer the crude brown coal is H:\RBould\Keep\spec\082196-doc 4/21/99 4 heated to approx. 110 0 C, due to which the water content is reduced to approx. 12 Instead of the fluidised bed dryer a revolving dryer can also be used, in which case, for instance, the low-pressure steam from a steam turbine of a power station is used as heating medium.

The dried, relatively coarse brown coal is combusted in the combustion chamber of a steam generator without further post-treatment dispensing with expensive mills. The combustion chamber encompasses a fire chamber which becomes a radiation chamber. To the radiation chamber a flue gas exhaust is joined, in which downstream connected heating surfaces are provided. The walls of the combustion chamber accommodate the burners.

The dried brown coal is introduced directly to the burners with the aid of a carrier gas. It is also feasible to store first the dried brown coal in an intermediate storage, from which the burners are supplied with the fuel.

The burners are constructed so that in the burner port the brown coal in the coal/carrier gas mixture is .distributed very evenly and without stratification.

Burners satisfying this task are described in the German e patent application 44 07 198.1 of the applicant. These burners are constructed as round burners which are 25 surrounded by an annular air channel. On their inlet side the burners have a displacement and rifling body and in the burner port a stabilising ring, which is provided with inwardly directed segments. The segments slow down temporarily the brown coal particles, so that the velocity 30 of the backfiring of the components of the brown coal which are to be gasified will be particularly greater than their flow velocity. Therefore the brown coal will ignite on the burner port in a controlled manner even in case of a low proportion of oxygen, enabling these burners to operate with oxygen deficiency.

The combustion air introduced to the burners is l in a sub-stoichiometric quantity. The term air ratio is H:\RBould\Keep\Speci\50821-96.doc 4/21/99 5 the ratio of the actual air mass to the theoretical air mass which is necessary for complete stoichiometric combustion of the coal. Due to the special construction of the burners the air ratio can be reduced to n 0.8 without causing problems regarding the CO. In the case of such a low air number the maximum combustion temperature is lowered and little NOx is formed. The remaining combustion air is injected into the combustion chamber through the burners as burn-out air.

As the dried brown coal is introduced to the burners without subsequent comminution, the arrangement of the burners in the walls of the combustion chamber has no spatial constraints and can be chosen freely. The burners are symmetrically arranged in one cross-section plane and are aligned so that the fuel jets exit perpendicularly to the wall of the combustion chamber in the direction of the internal space of the fire chamber. In this manner the uncombusted gases as well as the sticky or liquid ash components which occur during the combustion of brown coal, will be kept as far away as possible from the wall of the combustion chamber. Despite the sub-stoichiometric ratios at the burners' port damages due to corrosion to the walls will be avoided.

The burners can be also aligned so that a 25 rotating flame is produced by the exiting fuel jets, whereby the sticky or liquid ash components will reach the walls of the combustion chamber. A melt combustion will occur, wherein the temperatures in the fire chamber are above the melting point of the brown coal ash. As much of 30 ash as possible is removed in liquid form, due to which the flue gas carries fewer ash particles, which could lead to a wear of the heating surfaces of the steam generator.

The burners provided in a horizontal plane are supplied with fuel symmetrically. In this manner an even temperature distribution is achieved throughout the crosssection of the fire chamber. In the vertical direction the S burners and the nozzles for the supply of the burn-out air H:\RBould\Keep\Speci\50821-96.doc 4/21/99

M

6 are provided by considering a reliable burning out of the brown coal. In addition, peak temperatures should be avoided, so that in the fire chamber and the connected radiation chamber a uniform temperature distribution is achieved in the vertical direction also. This temperature management is possible due to the fact that when arranging the burners neither the operation of the mills nor space requirement for supply lines from and to the mills need to be considered.

When comparing it with the known drying by griding of brown coal, in the case of the described drying method with the subsequent introduction of the dried brown coal into the combustion, the steam-containing vapours introduced into the combustion with the brown coal, are absent. A smaller amount of heat transfer medium is available to the heating surfaces of the steam generator.

For this reason the volume of the combustion chamber and consequently of the entire gas generator can be reduced.

In the radiation chamber bulkhead walls can be provided, which will increase the available heating surface .and the overall volume of the entire steam generator can be further reduced.

e• p o• H:\RBould\Keep\Speci\5O821.96.doe 4/21/99

Claims

1. A method to obtain energy from brown coal, wherein the brown coal is dried in a fluidised bed or a revolving dryer at low temperatures, characterised in: that the dried brown coal is injected through burners directly or after an intermediate storage indirectly into the fire chamber of a gas generator without further comminution, that the dried brown coal is combusted in the burners sub-stoichiometrically with an air ratio of n 0.8, that burn-out air is injected through nozzles into the fire chamber and/or into a radiation chamber connected to it, that by virtue of a symmetrical charging of the burners a uniform distribution of the temperature throughout the cross-section of the fire chamber is achieved and that by arranging the burners and/or the nozzles for the burn-out air in the fire chamber and/or radiation chamber a uniform temperature distribution in a vertical direction of the combustion chamber is achieved.

2. A method according to claim 1, characterised in that the burner is aligned so that the sticky or liquid components which occur during the combustion of brown coal will be kept as far away as possible from the wall of the 30 fire chamber.

3. A method according to claim 1 or 2, characterised in that the heating surface available in the radir.tion S' chamber will be increased by providing bulkhead walls. H:\RBould\Keep\speci\50821-96.doc 4/21/99

8- 4. A method according to any one of claims 1 to 3, characterised in that the temperatures are maintained above the melting point of the ash so that as much ash as possible is removed from the flue gases to reduce wear. Dated this 21st day of April 1999 BABCOCK LENTJES KEAFTWERKSTECHNIK GmnbH By their Patent Attorneys GRIFFITH HACK Fellows Institute of Patent and Trade Mark Attorneys of Australia H:\RBould\Keep\Speci\50821..96.doc 4/21/99