US20080163804A1

US20080163804A1 - Method of and arrangement for co-combustion of biomasses and/or organic wastes as secondary fuel in a coal dust firing system

Info

Publication number: US20080163804A1
Application number: US11/937,775
Authority: US
Inventors: Rolf Hauk
Original assignee: RWE Power AG
Current assignee: RWE Power AG
Priority date: 2006-11-10
Filing date: 2007-11-09
Publication date: 2008-07-10
Also published as: CA2610111A1; DE102006053337A1; DE102006053337B4; EP1921375A2

Abstract

The invention concerns a method of and an arrangement for co-combustion of biomasses and/or organic wastes as secondary fuel in a coal dust-fired boiler of a vapor generator. The secondary fuel is dried directly or indirectly with the flue gas of the boiler. The vapors produced in the drying operation are fed to the combustion chamber of the boiler as final combustion air and/or secondary air.

Description

The invention concerns a method of and arrangement for co-combustion of biomasses and/or organic wastes as secondary fuel in a coal dust firing system.
Such a method is known for example from DE 196 49 986 A1. The method described in DE 196 49 986 provides that sludge is introduced in the form of a suspension tangentially into a cyclone combustion chamber and dried and degassed on a spiral path with a heat transfer medium. The fuel gas produced in that way, the vapors and the degassed fine sludge dust are fed to the combustion chamber and/or the flue gas suction return system and/or an injection burner. The degassed coarse sludge dust is separated by centrifugal action from the cyclone combustion chamber and distributed to the carbon mill and/or the carbon burner line.
That method is comparatively complicated and expensive in terms of energy technology, especially as the cyclone combustion chamber is operated by means of an oil or gas burner.
DE 101 56 616 A1 discloses a method of co-combustion of sludge in a fluidised bed boiler with a stationary or slightly expanding fluidised bed, wherein mechanically dewatered, pumpable sludge is fed on to the fluidised bed in the fluidised bed combustion chamber of the fluidised bed boiler immediately above the fluidised bed at a plurality of locations over the periphery of the fluidised bed combustion chamber, by means of injection lances, and wherein the introduction is effected in each case by means of compressed air so that scattering of the sludge over the fluidised bed is effected. In the method described in DE 101 56 616 the sludge introduced into the fluidised bed is also carried by the fluidised bed, mixed with the fluidised bed material and finally undergoes joint combustion therewith.
Direct introduction of the sludge into the firing chamber of a boiler without previous treatment in that way, as is described by way of example in DE 196 49 986, is only possible in the case of a fluidised bed firing system.
Organic materials which are to be introduced into a dust-fired boiler require preliminary drying depending on their respective moisture content. That applies in any case to mechanically dewatered sludges.
Biomasses and waste which have a comparatively high water content are being increasingly used in coal-fired power stations with dust firing.
DE 196 49 986 A1 already presupposes that the direct distribution of moist biomasses to coal dust mills is known. In fact moist biomasses and waste are being increasingly distributed directly to the coal mills. In the case of a direct feed to the coal mills, only a limited amount can be used as the drying reserves of the mills are limited and in most cases the coal mills are also not suitable for comminuting moist, fibrous substances. Depending on the biomasses used, for example when dealing with moist wood, a drying operation is necessary prior to a fine comminuting step. A certain grain size of the biomasses to be used is not to be exceeded because of a possible adverse effect on the dust firing system.
Previous methods of co-combustion of biomasses and other organic wastes in coal dust firing systems therefore provide, as is also described in DE 196 49 986, for drying with natural gas or petroleum. That involves comparatively high energy costs.
It is known from the drying of brown coal prior to the combustion thereof that the energy required for the drier is to be branched out of the water vapor circuit of the power station process. That is linked to a comparatively high level of apparatus complication and expenditure, which is not in any way related to the energy yield which can be attained when drying biomasses and other organic waste.
At any event post-treatment of the drier vapors and fumes is complicated and expensive. Under some circumstances when condensation of the drier vapors and fumes takes place effluent reprocessing is required.
Therefore the object of the invention is to provide a method of and an arrangement for co-combustion of biomasses and/or organic wastes in a coal dust firing system, with which the aforementioned problems are avoided.
To attain the object of the invention there is provided a method of co-combustion of biomasses and/or organic wastes as secondary fuel in a boiler of a vapor generator, which is fired with coal in grain and/or dust form as fuel, wherein the secondary fuel is dried directly or indirectly with the flue gas of the boiler, and wherein the vapors from the drying of the secondary fuel are added to the combustion air fed to the boiler.
An advantage of that operating procedure is that condensation of the vapors with downstream-disposed effluent reprocessing and possibly concomitant odor annoyance are not required.
In a preferred variant of the invention in which the combustion air is added to the boiler in a stepped configuration over a plurality of levels it is provided that the vapors are at least partially added in the combustion air level which is last downstream in the flow direction of the flue gas.
To state that in different terms, the method according to the invention involves co-combustion of moist biomasses and organic wastes as secondary fuel in a coal dust firing system, wherein the secondary fuel is pre-dried, the energy required for the drying operation is coupled directly out of the flue gas flow and the vapors produced in the drying operation—primarily water vapor—are added again to the flue gas still in the boiler upstream in the direction of flow of the flue gas before the ancillary heating surfaces. The dried secondary fuel can be allocated to the coal mills and introduced with the primary air into the boiler.
Such coal mills can be for example in the form of known roller bowl-type mills in which the energy of the flue gas is used for pre-drying the crushing material and the flue gas is used for transporting that material.
Desirably the addition of the vapors to the combustion air is effected in dependence on the demand for output power of the drier for the secondary fuel in such a way that optionally a partial amount of the vapors is added to the next combustion air level which is disposed upstream in relation to the flue gas. Desirably the amount of air required for the drier for the secondary fuel corresponds to the design amount for the last combustion air level in the downstream direction. If the demand for output power of the drier for the secondary fuel is higher than the air required in the last downstream combustion air level, for example in the form of final combustion air, cascade-like distribution of the vapors can be effected in a direction towards the next combustion air levels disposed in the upstream direction.
In a preferred embodiment of the method according to the invention the energy required for the drier for the secondary fuel is coupled out of the flue gas flow downstream of an air preheater.
Desirably in that case a partial amount of the air flow is branched off downstream of the air preheater for drying the secondary fuel. Another partial amount of the air flow can be fed to the boiler downstream of the air preheater as primary and/or secondary air.
The primary air which is branched off downstream of the air preheater is desirably fed to the boiler by way of at least one coal mill. There the energy thereof is used for drying the primary fuel.
In a preferred variant of the method according to the invention final combustion air is added to the combustion air level which is last in the downstream direction. The vapors of the drier can be completely supplied as the final combustion air. In the case involving a stepped combustion air feed without the addition of final combustion air, the vapors can be fed to the boiler as secondary air.
Preferably direct drying is effected in the air flow in the drier for the secondary fuel.
As an alternative thereto it is possible that indirect drying is effected in the drier for the secondary fuel.
Compression and removal of dust from the vapors can be provided downstream of the drier. In that case compression serves to compensate for the pressure losses of the drier.
The object of the present invention is further attained by an arrangement for co-combustion of biomasses and/or organic wastes as secondary fuel in a coal dust firing system, comprising at least one drier for the secondary fuel, wherein the drier is connected downstream of an air preheater for conditioning of combustion air and/or mill air and wherein the vapors outlet of the drier is connected to the final combustion air feed and/or the secondary air feed of the boiler.

The invention is described hereinafter by means of three embodiments by way of example illustrated in the drawings in which:

FIG. 1 shows an arrangement of a dust-fired boiler with air preheater and drier for secondary fuel in accordance with a first embodiment of the invention,

FIG. 2 shows a diagrammatic view of a method implementation modified in comparison to the FIG. 1 embodiment in accordance with a second embodiment of the invention, and

FIG. 3 shows a diagrammatic view of a third embodiment of the invention.

The embodiment illustrated in the drawings has a dust-fired boiler 1 which is operated for example with hard coal as primary fuel. The coal with which the boiler 1 is operated is ultimately not critical to the invention, it can equally be fired with brown coal.
Of the boiler 1, only the combustion chamber is illustrated, the upper region of the boiler with the ancillary heating surfaces not being shown for the sake of simplicity in the flow chart. The boiler 1 is in the form of a boiler with stepped combustion air feed. The individual combustion air levels are denoted by references 2 through 6, wherein the lowermost combustion air level 2 represents the infiltrated air feed to the combustion chamber, the following combustion air level 3 represents the primary air feed, the combustion air level 4 disposed downstream thereof represents the secondary air feed, the combustion air level 5 in adjoining relationship downstream in relation to the flow gas represents the upper air feed and the last downstream combustion air level 6 represents the final combustion air level. Reference 7 denotes the flue gas flow out of the combustion chamber or the boiler 1.
As can be seen in particular from FIG. 1 the flue gas flow is passed by way of an air preheater 8 in the form of a rotary air preheater—referred to hereinafter as the preheater. The preheater 8 is connected in known manner to a fresh air feed 9. The fresh air is heated in the preheater 8 directly or indirectly with flue gas from for example 35° C. to about 350° C. At about 350° C. the heated air flows through the drier 10 in which biomasses in the form of for example moist wood chips or the like are dried in direct contact with the heated air.
The vapors outlet 11 of the drier 10 is directly connected to the combustion chamber of the boiler 1. In the described embodiment 100% of the vapors is fed to the boiler 1 as final combustion air 6. The dried material withdrawn from the drier is allocated to the coal mills (not shown) which are arranged in the primary air feed 3 to the combustion chamber.
Alternatively the biomasses can be ground downstream of the drier 10 in a separate mill and after the grinding operation injected separately or jointly with the coal into the boiler.
In accordance with a further variant it can be provided that the biomass is crushed prior to the drying operation in a separate mill. Finally the biomasses can already be present in comminuted form so that additional grinding is unnecessary.
Whether separate grinding of the biomasses is required optionally prior to or after the drying operation is predominantly a question of the physical properties of the biomass in regard to grindability thereof.
When injecting the biomasses jointly with the coal it is appropriate for it to be allocated to the coal mills in which intensive thorough mixing of biomass and coal and possibly also post-crushing of the biomass are achieved.
Downstream of the preheater 8 and upstream of the drier 10 heated air is distributed in known manner by way of the combustion air levels of the combustion chamber of the boiler 1, wherein the primary air feed 3 is passed in known manner by way of one or more coal mills which are not illustrated here.
Regulation provides that, with an increased demand for output power of the drier 10, a partial amount of the vapors is added cascade-like to the upper air 5 and possibly also the secondary air 4.
By way of the final combustion air feed 6 and/or the upper air feed 5 and/or the secondary air feed 4, the vapors pass out of the drier 10 into the flue gas flow 7 and finally into the flue gas outlet 12. To compensate for any pressure losses in the drier 10, a vapors compressor 13 is connected downstream of the drier 10. A filter can also be connected downstream of the vapors compressor 13, if required, such a filter is not shown in the simplified view of the operating procedure of the method.
The embodiment illustrated in FIG. 2 approximately corresponds to that of FIG. 1, with the difference that the drier 10 is in the form of an indirect drier and has two outlets 11 a, 11 b, the outlet 11 a serving as a vapors outlet and the outlet 11 b in contrast passing the cooled heat exchange medium of the drier 10. The outlets 11 a and 11 b are brought together again upstream of the vapors compressor 13.
In other respects identical parts are denoted by the same references in the various Figures.
That also applies to the embodiment which is illustrated in FIG. 3 and which differs from the preceding embodiments insofar as the drier 10 is in the form of an indirect drier which however has a separate circuit 15 for the heat exchange medium. That heat exchange circuit 15 is closed in itself. Provided therein is a compressor 16 in order to produce the circulatory flow of the heat exchange medium.
Provided for heating the heat exchange medium is a heat exchanger 17, the outlet 11 b of which is combined downstream of the drier 10 with the vapors outlet 11 a of the drier 10.
In the embodiment described with reference to FIG. 2 by way of example the drier 10 could be in the form of a fluidised bed drier with inwardly disposed heating surfaces.
In the embodiment described with reference to FIG. 3 it would be possible to provide for example vapor, nitrogen or flue gas as the heat exchange medium. The cooled air from the outlet 11 b of the heat exchanger 17 is added together with the vapors to the final combustion air or introduced into the combustion chamber of the boiler 1 as a final combustion air feed.

LIST OF REFERENCES

1 boiler
2 infiltrated air feed to the combustion chamber
3 primary air feed to the combustion chamber
4 secondary air feed to the combustion chamber
5 upper air feed to the combustion chamber
6 final combustion air feed to the combustion chamber
7 flue gas flow
8 air preheater
9 fresh air feed to the air preheater
10 drier
11 vapor outlet
11 a vapors outlet
11 b drier outlet/heat exchanger outlet
12 flue gas outlet
13 vapors compressor
14 secondary fuel flow
15 heat exchanger circuit
16 compressor
17 heat exchanger

Claims

1. A method of co-combustion of biomasses and/or organic wastes as secondary fuel in a boiler of a vapor generator, which is fired with coal in grain and/or dust form as primary fuel, wherein the secondary fuel is dried directly or indirectly with the flue gas of the boiler, and wherein the vapors from the drying of the secondary fuel are added to the combustion air fed to the boiler.

2. A method as set forth in claim 1 wherein in which the combustion air is added to the boiler in a stepped configuration over a plurality of levels characterised in that the vapors are at least partially added in the combustion air level which is last downstream in the flow direction of the flue gas.

3. A method as set forth in claim 1 characterised in that the addition of the fuel to the combustion air is regulated in dependence on the demand for output power of the drier for the secondary fuel in such a way that optionally a partial amount of the vapors is also added to the next combustion air level which is disposed upstream in relation to the flue gas.

4. A method as set forth in claim 2 characterised in that the amount of air required for the drier for the secondary fuel corresponds to the design amount for the last combustion air level in the downstream direction.

5. A method as set forth in claim 1 characterised in that the energy required for the drier for the secondary fuel is coupled out of the flue gas flow downstream of an air preheater.

6. A method as set forth in claim 5 characterised in that a partial amount of the air flow is branched off downstream of the air preheater for drying the secondary fuel.

7. A method as set forth in claim 5 characterised in that a partial amount of the air flow is fed to the boiler downstream of the air preheater as primary and secondary air.

8. A method as set forth in claim 7 characterised in that the primary air which is branched off downstream of the air preheater is fed to the boiler by way of at least one coal mill.

9. A method as set forth in claim 2 characterised in that final combustion air is added to the combustion air level which is last in the downstream direction.

10. A method as set forth in claim 1 characterised in that direct drying is effected in the air flow in the drier for the secondary fuel.

11. A method as set forth in claim 1 characterised in that indirect drying is effected in the drier for the secondary fuel.

12. An arrangement for co-combustion of biomasses and/or organic wastes as secondary fuel in a coal dust firing system, comprising at least one drier for the secondary fuel, wherein the drier is connected downstream of an air preheater for conditioning of combustion air and/or mill air and wherein the vapors outlet of the drier is connected to the final combustion air feed and/or the secondary air feed of the boiler.