BOILER WITH A SHAKING GRATE

Abstract: The shaking grate (14) merges at the outlet end thereof into a boundary wall (14') of a fuel shaft (19) for receiving the fuel bed discharged from the shaking grate, such that said boundary wall moves together with the shaking grate. The fuel shaft connects at the lower end thereof to a perforated plate (20) for the support of a fluidized ash bed (55), arranged below the shaking grate.

Description: BOILER WITH A SHAKING GRATE The invention relates to a boiler with a shaking grate. In prior art boilers with a shaking grate, all combustion takes place on the shaking grate and the final combustion has been finished when the bed on the shaking grate reaches the end portion thereof. Then, a rather complicated control of the combustion process is required; it follows that the prior art boilers with a shaking grate are suitable for large heating systems only. The purpose of the invention is to provide a boiler with a shaking grate for small heating systems, i.e. heating systems of a power which is of a maximum ' rate of 10 MW, which can be operated with dry or humid fuels of different types which are rich or poor in ash, within a relatively wide fuel spectrum, the combustion efficiency attained being higher than that attained with conventional grate systems, while the hazardous environment effects are reduced, the control of the combustion process being simpler than in conventional boilers with a shaking grate. It should be possible to operate the boiler of the invention in the first place with coal and in the second place with low-grade fuels (biofuels), such as peat, chips, and bark as well as pellets and briquettes made of different raw materials. In order to achieve the purpose mentioned above the boiler of the invention has obtained the charac¬ teristics appearing from claim 1. It is previously known to have a boiler operating with a fluidized bed, but this provides some drawbacks which have not to be taken into account when the fluidized bed is provided in combination with a shaking grate and a fuel shaft in the manner proposed according to the invention. It may not be out of place to mention these drawbacks herein. 1. A large amount of flue dust which necessitates qualified dust separation. 2. Control of the bed temperature is difficult. 3. The load flexibility is low. 4. The grain size. of the fuel must be accurately adjusted. 5. The bed material, usually sand, must be changed at intervals. 6. The supply of the fuel requires accurate dosage. 7. Fuels having a large proportion of volatile substances easily provide a too intense combustion above the bed. 8. Cooling tubes in the bed interfere with the combustion and are exposed to heavy wear. 9. Normal bed thicknesses require large fan power for effecting the fl uidi zation. 10. The final combustion of coal requires a high air excess and a deep bed. It is also previously known per se to arrange shaft combustion, the shaft being stationary arranged as in case of shaft furnaces and magazine furnaces. In that case the fuel falls down by gravity to the combustion zone, and such stationary shafts have not earlier been combined with a fluidized bed. The invention and the advantages achieved by the invention will be explained in more detail below by description of an embodiment with reference to the accompanying drawings in which FIG. 1 is a vertical sectional view of a boiler of the invention, and FIG. 2 is an enlarged portion of the vertical sectional view of FIG. 1. Referring to FIG. 1 , the boiler of the invention shown therein comprises a heat insulating boiler cover¬ ing 10 with a bottom brick lining I I. A combustion com¬ partment 12 is defined by water tubes 13 interconnected by welding to form panels, and in the lower portion of the combustion compartment a water-cooled shaking grate 14 is provided, which is connected to a shaker 15 and slopes downwards from a fuel hopper 16. The shaking grate 14 is reciprocated intermi tently as has been indicated by a double arrow 17 in order that there will be formed on the grate a fuel bed 18 which is supplied from the fuel hopper 16 and is fed towards the lower end of the shaking grate while the fuel bed is being levelled. During shaking, fine fractions of the fuel forming the fuel bed 18 partly will fall down through the shaking grate. The shaking grate 14 connects at the lower end thereof to a fuel shaft 19 which is defined partly by the wall tubes 13 and partly by a depending portion 14* of the shaking grate 14. The fuel shaft 19 opens above a perforated sheet or lower grate element 20. Below this grate element there is an air box 21 which is connected by a conduit 22 to a fan 23 for the supply of primary air. At the transition between the fuel shaft 19 and the perforated sheet 20 there is provided a reciprocating ram piston 24 which is connected to an operating cylinder 25 and the purpose of which is to forward the material from the lower end of the fuel shaft 19 onto the perforated sheet 20. The space between the shaking grate 14 and the perforated sheet 20 is available through an ash door 26, and also a slag dis¬ charge tube 27 with a closure valve 28 extends from the perforated sheet. Said space is also provided with an inspection door 29. Also a conduit 30 is connected to the fan 23, said conduit extending to a box 21 for primary air. A number of nozzles extend from this box, viz. nozzles 32 opening in the fuel shaft 19, and nozzles 33 located above the shaking grate 14 to open in the fuel bed 18 located on said grate. The combustion compartment 12 is connected at the top thereof to a flame tube 34 which extends through a convection system 35 of a conventional design, which is made up of a number of water tubes which also communicate with the wall tubes 13 of the combustion compartment. The flame tube 34 connects to an upper header 36 in order that flue gas escaping from the combustion compartment 12 through the flame tube 34 shall be reversed against an end wall which is not cooled, for reignitioπ of non-combusted gas components, if any, and shall pass down through the convection system to a lower header 37 arranged below said system, and then pass again through the convection system up¬ wards to an outlet compartment 38. In the lower header 37 there is provided a conical boundary wall 39, and in the bottom of said header there are provided a number of nozzles 40 opening into the combustion compartment 12. These nozzles are connected to a box 41 for secondary air, which in turn receives secondary air from the fan 23 through a conduit 42. For details of this arrangement reference is made to FIG. 2. It will be seen from said figure that the nozzles 40 form an injector 43 and that the header 37 at the loweir end thereof communicates with this injector through an opening 44. The function of this arrangement will be described in more detail below. Reference is made again to FIG. 1. A multiple cyclon separator comprising a number of separate cyclon separators 45, which can be distributed over about 1/3 of the circumference of the convection system, is connected to the outlet compartment 38. The tangential inlet 46 of each cyclon separator communicates with the outlet compartment 38, while the bottom outlet thereof is connected to a dust bin 48 and the central tube 5 49 thereof is connected via a fan 50 to a chimney 51. The dust bin 48 is connected at the lower end thereof via a cell wheel 52 to a conduit 53 which opens above the perforated sheet 20 at 54. The main combustion of the fuel in the bed 18 on 1^ the shaking grate 14 will take place in the shaft 19 from which substantially pure coal will be fed onto the perforated sheet 20 by means of the ram piston 24. By means of the air supplied from the fan 23 through the conduit 22 to the air box 21 there is obtained a U fluidized bed 55 on top of the perforated sheet 20, and coal and combustible slag will be finally combusted in this fluidized bed, which means that hot flue gas together with air will pass from the fluidized bed 55 upwards through the shaking grate 14 and the fuel bed 0 18 located on said grate, to the combustion compartment 12. The fuel supplied from the fuel hopper 16 thus will be dried and evaporated in the fuel bed 18, fine particles of the fuel simultaneously falling down through the grate 14 to be combusted in the fluidized bed 55. 5 A large amount of excess air should be supplied to the fluidized bed 55 from the air box 21, and due to the fact that a flow of flue gas and air at high temperature passes through the bed 18, drying and evaporation will be accelerated. Also non-combusted flue dust from the 0 multiple cyclon separator 45 is supplied to the fluidized bed 55 via the dust bin 48, the cell wheel 52 and the conduit 53 at the opening thereof in the fluidized bed 55 in order that this uncombusted flue dust shall be finally combusted in the bed 55. Ash 5 which is accumulated in the fluidized bed 55 is utilized as fluidizing medium in this bed. Dust is prevented from accompanying the flue gas and the primary air to the combustion compartment 12, because such dust will be caught by the shaking grate 14 and the fuel bed 18 located on said grate. This means that lower demands may be put on the multiple cyclon separator 45. The final combustion in the fluidized bed 55 will proceed for a /ery long time, 6 to 8.hours. Such dust as is precipitated in the lower header 37 and to a major portion comprises combustible dust will be mixed by injection with the secondary air supplied to the combustion compartment through the nozzles 40, said dust being aspirated into the injector through the openings 44. Thus, the dust will be combusted in the combustion compartment 12 and also in the flame tube 34. The secondary air provides some cyclon effect on the combustion compartment 12, pro¬ viding an efficient mixture of air and flue gas. All commercial fuels can be used in the boiler of the invention, a particular advantage being that fuels in the form of large pieces need not be comminuted. However, also fine grain fuels such as coal dust, milled peat, shavings and the like are well suited for combustion in the boiler of the invention. Fuels which are rich in ash are mastered due to the fact that the ash is enriched in the fluidized bed 55. Since the ash is produced in a pulverulent condition, it can easily be fed out through the slag discharge tube 27. Clinker that has been produced, if any, and similar material can be removed manually at the ash door 26. However, - with fuels tending to cause sintering of ash, the ash can be granulated by injecting steam into the primary air. The capacity ot the boiler principally can be controlled by adjusting the thickness of the fuel bed 18 to a selected level for the fuel used, said level then being kept constant by means of weighing and by controlling the flow of primary air from the air box 21 to the shaking grate 14. The flow of secondary air to the boiler is adjusted in dependence on the oxygen content measured in the flue gas discharged from the boiler. The ash space defined partly by water-cooled tube walls, absorbs an adjusted energy portion from the panel , because the water-cooled grate system and the ash space at high load absorb a major portion of the heat content of the fuel. It follows a lower combustion temperature than in conventional boiler systems. This means a new method of controlling the combustion temperature within certain limits according to actual requirements considering the properties of the ash, optimal combustion of gas components in the combustion compartment and residue coal in the fluidized bed at the same time being obtained. The boiler of the invention satisfies all the requirements that can be set up today as far as large plants are concerned. By the compact vertical construc¬ tion the specific boiler cost will be considerably lower than in case of other existing boiler systems. The elongate combustion compartment 12 with the terminating vertical flame tube 34 accounts for fuels of large flame length, and the vertical location of the convec¬ tion system minimizes the risks of clogging of the tube passages. Since part of the fuel is used for direct pre- heating of tne primary air in the fluidized ash bed 55 the need of preheated air is eliminated. The combination of a shaking grate and a fluidized ash bed with feedback of flue dust via the secondary air system and the ash bed is a guarantee that the losses in the form of ash and slag will be considerably lower than in traditional grate combustion. It would be possible to increase the efficiency by 5 to 10 % for small and average size plants due to the fact that the amount of non-combusted material in gas, slag and dust will be minimized. Since a high combustion temperature can be reached in the combustion compartment and there is also obtained a long combustion period, the discharge of non-combusted hydrocarbons will be reduced, and the mixing of flue gas with the fuel bed 18 also may be expected to provide low values of the NOX emission. The discharge of sulphur can be reduced by the addition of lime to the fuel supplied, for binding sulphur contained in the fuel.

Claims: CLAIMS 1. Boiler with shaking grate, c h a r a c - t e r i z e d in that the shaking grate (14) at the outlet end thereof merges into a boundary wall (14') of a fuel shaft (19) for receiving the fuel bed discharged from the shaking grate such that the boundary wall is moving together with the shaking grate, and that the shaft at the lower end thereof connects to a perforated plate (20) located below the shaking grate, for the support of a fluidized ash bed (55). 2. Boiler as claimed in claim 1, c h a r a c ¬ t e r i z e d in that secondary air nozzles (40) are connected to a combustion compartment (12) arranged above the shaking grate (14),. for the supply of dust obtained from the flue gas by low grade separation. 3. Boiler as claimed in claim 1, c h a r a c ¬ t e r i z e d in that the combustion compartment (12) is elongate in the upward direction from the shaking grate (14) and connects to a substantially vertical flame tube (34) in a convection system (35). 4. Boiler as claimed in claims 2 and 3, c h a r a c t e r i z e d in that the convection system (35) comprises upper and lower headers (36, 37) and that a dust outlet (44) of the lower header is connected to the secondary air nozzles (40) for addition of separated dust to the secondary air. 5. Boiler as claimed in claim 1, c h a r a c - t e r i z e d in that an inlet (54) for dust separated from the flue gas by high grade separation is arranged above the ash bed (55) on the perforated plate (20).

Inventors: FRILUND EYVIND, FRILUND, EYVIND
Applicants: HB CONSULT RAODGIVANDE ING AB, HB-CONSULT RAODGIVANDE INGENJOERER AB
Application year: 1984
Main classification: F23B1
IPC codes: F23C10, F23L1, F23L9, F24H1