AU659320B2 - Construction of the combustion chambers of a boiler - Google Patents

Description

659320 Our Ref: 448279 P/00/011 Regulation 3:2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT 0*G A 9 4, 4 4 0V 4, (4 444b40 4,

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4* 9* (4 4 o (4 4,4 9* 4 Applicant(s): 4 4 Address for Service: Takuma Co., Ltd 3-23, Dojima Hama 1-chome KITA-KU Osaka

JAPAN

DAVIES COLLISON CAVE Patent Trade Mark Attorneys Level 10, 10 Barrack Street SYDNEY NSW 2000 Invention Title: Construction of the combustion chambers of a boiler The following statement is a full description of this invention, including" the best method of performing it known to me:- 5020 0792k/lfg The present invention relates to an improvement of combustion chambers of a boiler with fire grates enabling combustion of solid fuels such as bagasses, wooden barks, etc., and coals.

In recent years, the industrial sectors have actively used as a boiler fuel biomasses or coals in pursuance of diversification in energy supply sources and non-dependency on oil.

For example, in sugar factories, power used in the whole facility have recourse to bagasse, after the juice has been extracted from sugar cane, utilized as a boiler fuel.

Biomasses such as wooden chips, barks, husks, etc., are widely used as alternative boiler fuels, use of which also serves as a waste disposal by combustion as well as recovery of heat energy.

Ste"nd Such use of solid fuels such as biomasses, or coals tends to spur the industrial sectors on to non-dependency on oil, making a great contribution to the solution of energy problems.

Said biomass fuels have characteristic features as follows: i. high volatile matter, and a low carbon content (fixed carbon/volatile matter 0.2-0.3); 2. a low ash content 3. irregular shapes (particle diameters); it- 4. low sulphur content, with resultant reduction of air pollution; a comparatively large amount of moisture contained (typically 30-60%); 6. high bulk (typically a density of: 150-200 kg/m3).

It is known that these biomass fuels have a range of calorific values, which greatly vary with moisture-content, generally from 2000 to 3000 kcal/kg, and properties of combustion that they are well ignitable compared with bituminous coals which burn in the hCte and that their burning speed is high. A boiler in which solid matters such as said bagasses I I 1 0792k/ifg -2or coals are used as fuels generally includes travelling fire grate fixed fire grates, or dumping fire grates provided-i-r~ the bottom of comLustion chambers.

Figs. 9 and 10 show one example of a combustion chamber portion of a boiler having a travelling fire grate 18 having substantially the same width as the width W of the combustion chamber 17 and substantially the same length as the depth L of the combustion chamber 17.

The travelling fire grate 18 is designed to move from the backward portion of the combustion chamber 17 to the forward portion(the so-called reverse-move system), and its driving speed is determined so as to complete combustion of the fuels when they arrive at the forward portion of the combustion chamber 17.

Additionally, 70% 80% of an air needed for combustion is supplied as primary air through the travelling fire grate 18 to the interior of the combustion chamber 17, and the rest is supplied as secondary air from nozzles 20 disposed between water-tubes 19 of the forward and backward walls of the combustion chamber 17 into the combustion chamber 17.

The numeral 21 designates an air box for the travelling fire grate, 22 an air box for the secondary air and 23 a fuel spout.

With said boiler as shown in Figs. 9 and 10, when bagasses or coals are spread to burn, fuels having large sizes tend to drop to the fire grate 18 to burn thereon, and fuels having small sizes are subjected to the so-called combustion in suspension in the combustion chamber 17.

Conventional boilers of this type are designed such that es aforementioned, the area of of the fire grate 18 is substantially the same as the lateral sectional area of the combustion chamber 17.

Basically, however, the area of said fire grate 18 should be determined in accordance with the amount of fuel to be burnt on the fire grate 18, and the area and volume of the combustion chamber 17 should be determined in accordance with the amount of fuel to be burnt in suspension. For example, it is known that 30-70% in weight of the whole 4:T 1.C _Tl 0~ fE

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4 0792k/if'g 3 bagasses fuels as supplied may reach the fire grate 18, and therefore, it is possible to calculate the necessary area of the fire grate 18 from said value.

Also, said combustion chamber 17 should have an ideal configuration to be selected in association with the sectional area determined by the velocity at which combustion gas goes upward.

However, with the recent progress of extracting techniques for sugar cane and the like, the bagasses which are left behind after extracting tend to contain much more fine particles than large sized particles. Consequently, in boilers using such bagasses as fuels, the amount of the bagasses subjected to combustion in suspension is greater than that of the bagasses subjected to combustion on the fire grate.

In other words, when calculating the size and construction of the combustion chamber, the area of the fire grate should be reduced while the combustion chamber, particularly its transverse cross sectional area, should be S. enlarged. In addition, a way for supplying the secondary air should be improved to promote combustion of the suspension °o particles.

In recent boiler facilities, the power output of a unit of boiler has gradually increased in order to improve the economy and efficiency in operation of the boiler facilities as well as to facilitate maintenance.

For example, with a natural circulating mid-and-low pressure boiler with two drums, as required by general established methods of design, the width W of the combustion chamber is increased to secure the necessary volume of combustion chamber in accordance with the increase in the amount of evaporation.

Such a method permits the volume of a steam chamber in an attached steam drum and the heating surface for absorption of heat to increase in proportion with the width W of the combustion chamber, thereby to ensure that a well-balanced condition of boiler design as a whole will be provided.

li~l IIC---- 0792k/lfg -4- In contrast thereto, since the depth L of the combustion chamber is chiefly determined by how far the fuels are thrown' there is not so much latitude as in the determination of the width W of the combustion chamber, and although affected by the type of the fuels used, a depth L of 6-7 meters is usually adopted in accordance with the boiler output. Figs. 11 to 13 schematically illustrate the shapes of the cross section of the combustion chambers of the conventional boilers of various types with small to large capacity, and the arrows indicate supply nozzles for secondary air.

The supply of secondary air which is most important for acceleration of said combustion in suspension is carried out by nozzles for secondary air provided on the front and rear walls, and side walls of the combustion chamber 17, as shown o in Figs. 11 to 13.

If the cross section area, or above all the width W is o 0 :o enlarged in accordance with increase in the amount of 0*90 boiler output, there arises a problem that there may be produced an area, as shown by the dotted line in Fig. 13, in 9400 the central portion of the combustion chamber, in which an insufficient amount of secondary air is supplied.

The combustion chamber and the fire grate are conventionally designed to have substantially the identical dimension in width with the result that the width of the fire grate will be increased in accordance with the increase of the output of boiler, thereby raising a problem that the upper limit of the boiler output may be dependent on the restrictions encountered in the production of the driving mechanism of the fire grates.

The reasonable design for the ordinary travelling fire qrate requires that the economically optimum width of the fire grate be about 6 meters. Thus, a boiler with the combustion chamber having a greater width W than aforemtnioned is provided with two separate driving shafts on both sides each acting as a mechanically independent mechanism, which mechanism employs a system that both fire grates 18a, 18b are driven by drives 24a, 24b through speed 0%T Oi reducers 25a, 25b mounted on the opposed ends of the shaft as shown in Figs. 14 and However, there lies a problem in said system as shown in Figs. 14 and 15 that since two separated fire grates 18a, 18b are merely arranged side by side with each other it not only requires high cost of manufacturing equipment but also causes the operati 'n of the fire grates to be difficult and maintenance troublesome.

The restrictions in production of the fire grates are substantially moderate with regard to the longitudinal dimension of the fire grate as compared with the width, and a fire grate with a length L, extending between chain wheel being as much as 8 meters is now in practical use.

As aforementioned, when the boiler has an increase capacity and the width W is as long as 13 meters, many portions undesirable from the view point of the mechanical 15 construction may exist, thereby making the production of the fire grates difficult, even if the system is employed where the fire grates are driven by the separate driving shafts.

1t Assuming that the driving of fire grates is possible, there still are difficulties that the cost of equipment for fire grates will be rendered remarkably high and the operation and maintenance of the fire grates will be troublesome.

In accordance with the present invention there is provided a combustion chamber construction for a boiler for burning solid fuel on travelling fire grates as well as in suspension, the combustion chamber constructions, comprising: S 25 a combustion chamber; p:\wpdocs\amd\448279.tak\dh -6a partition wail assembly provided in the combustion chamber and dividing the combustion chamber into separated chamber zones; and travelling fire grates provided in each of the chamber zones for moving fuel on the fire grates away from the partition wall assembly; with the partition wall assembly comprising a plurality of air nozzles for supplying air into each of the chamber zones on either side of the partition wall assembly.

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The partition wall is preferably formed of spaced arranged water tubes for absorbing radiant heat in the combustion chamber without constituting any hindrance to the passage of combustion gas, the tubes having the circulation of boiler water there within.

Preferably, the partition wall assembly includes a lower portion having downwardly diverging lower face portions.

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In addition, the fire grates preferably are driven by i a p:\wpdocs\anmd\448279.takA\dh 0792k/lfg -7 1 I ;1 O separate corresponding drives to move towards the respective forward walls of the combustion chambers to complete the combustion of fuels on the fire grates when the fuels have travelled to the forward walls of the combustion chambers.

The fuels are spread from the high point of the forward walls of the respective combustion chambers to the partition walls of the centr.e of the whole combustion chamber. The moisture in the fuel of relatively fine particles is promptly evaporated since the combustion chamber is at a high temperature and then, said fuels are supplied together with a secondary air disposed at the central partition walls or in the vicinity of the fuel supply spouts, start burning in the suspended state and then completes its combustion.

On the other hand, with referonce to the fuels having comparatively large particle diameters, the moisture contained in such fuels is partially evaporated by the combustion gas within the furnace while the fuels are spread.

However, because of their large mass, said fuels fail to become suspended even in the presence the rising combustion gases, only to fall down to the fire grates.

The particles, wlhn dropped on the fire grates, receive the radiant het from the combustion chambers so as to evaporate any remaining moisture, and start burning by the supply of a primary air from under the fire grates at the time when the ignition temperature is reached.

Tne travelling speed of each of the fire grates is selected in such a manner th<t' all of the fuels accumulated on the fire grates have fully burnt themselves when they reach the end of the fire grates.

As described above, in accordance with the particle sizes of the fuels, both the combustion in suspension and combustion on the fire grates take place simultaneously in a single combustion chamber. Further in the combustion chamber of the present invention, the secondary air is supplied uniformly in the widthrqise and depth wise directions in the chambers by the nozzles for te secondary air secured to the inclined surfaces of the lower portions 0792k/lfg -8of central partition walls so that a considerable promotion of combustion in suspension in the combustion chambers can be achieved.

A non-limiting embodiment of the present invention will be described in detail in which: Fig. 1 is a schematic longitudinal sectional view of a boiler provided with combustion chambers in accordance with the present invention.

Fig. 2 is a schematic cross sectional view taken along the lin A-A of Fig. 1.

Fig. 3 is a schematic cross sectional view taken along the line B-B of Fig. 2.

Fig. 4 is a perspective view showing part of the intermediate partition wall.

Fig. 5 is a view showing the mounted state of nozzle 1 1 Fig. 6 is a view showing a jetting state of the secondary combustion air from the nozzle 11.

Fig. 7 is a view sbh ,ing the relationship between the width of the fire grates and the depth L of the combustion chamber.

Fig. 8 is a view showing the other example of the relationship between the width of the fire grates and the depth L of the combustion chamber.

Fig. 9 is a schematic view of a boiler combustion chamber provided with a conventienal type of fir grates and intended for use of biomass fuels.

Fig. 10 is a schematic sectional view taken along the line B-B of Fig. 9.

Fig. 11 is a view showing the plane shape of a conventional boiler of small capacity.

Fig. 12 is a view showing the plane shape of a conventional boiler of middle capacity.

Fig. 13 is a view showing the plane shape of a conventional boiler of large capacity.

Fig. 14 is a schematic longitudinal sectional view of a boiler of large capacity with a plurality of conventional fire grates arranged in parallel with each other.

Fig. 15 is schematic sectional view taken along the L 1 0792k/lfg line B-B of Fig. 14.

In the drawings, the numeral 1 designates a furnace wall, 2 a combustion chamber, 2a a forward wall of each divisional combustion chamber, 3 a boiler proper, 4a, 4b travelling fire grates, 5 an air box for fire grate, 6a, 6b a fuel spout opening, 7 a air preheater, 8 a primary forced draft unit, 9 a secondary forced draft unit, 10 a secondary air box, 11 an air blowing nozzle, 12 a water-tube on the combustion chamber wall, 13 a partition wall of combustion chamber, 14a, 14b secondary air supply holes for spreading fuel, and 16a, 16b drives.

The bagasses are used as a fuel in this water-tube boiler.

The furnace wall 1 is a water cooling wall in the form of. a fin-clad water-tube or bare water-tube, having a large number of water-tubes 12 arranged longitudinally at a given pitch on the inner wall thereof.

°o The combustion chamber 2 is formed by enclosing with the furnace walls 1 and the partition walls 13 a space above o.o; the travelling fire grate 4, in which combustion chamber the fuel burns in the suspended state.

The partition wall 13 is positioned in the centre of said whole combustion chamber 2 and comprises bate water-tubes arranged and tied with each other as shown in ,Fig. 4.

That is, in Fig. 4, the numeral 13 designates a partition wall in the combustion chamber, 13a water-tubes, and 13b metallic pieces for binding the water-tubes, and as a apparent from Fig. 4, said partition walls 13 comprise the water tubes spaced at a pitch about 1.5 times as large as the diameter of the water-tube, so that the partition walls are by no means a hindrance to the passage of combustion gas.

As shown in Fig. 2, each of said partition walls 13 of the combustion chamber is inclined at its lower portion toward both the forward walls 2a, 2b of the combustion chambers and the inclination is set at an angle a which is more than the angle of repose of a spread fuel.

The inclination angle a is set in such a manner that

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~1 I, L 0792k/lfg 10 *l 99 9~l *149 *9 94 9 a 9 0 999, a o t 1 fuels dropped to the inclined wall surface may slide down along the inclined wall surface. Specifically, the inclined portion is at an angle a of 50 80° relative to the horizontal plane.

The air box 10 for secondary air is disposed at said inclined wall surface, and the secondary air may be supplied from said air box 10 for secondary air to the nozzle 11.

The nozzles 11 are spaced and positioned between the adjacent water-tubes 13a at the inclined portions of the partition walls 13 so as to be connected with the air boxes for secondary air as shown in Fig. That is, each of said nozzles 11 is adapted to jet out the secondary air to uniformly spread onto the travelling fire grates the fuel particles or ashes of a large mass which have dropped on the inclined portions of the walls 13 and then to provide the suspending fuel particles and volatile matters with the secondary air for promotion of combustion in suspension.

Referring to Figs. 1 and 2, any air necessary for combustion is supplied from the forced-draft unit 8, via the air preheater 7 and an air duct 15, to the air box 5 for the fire grate and the air box 10 for the secondary air. That is, part of the combustion air is supplied from the air box 5 for the fire grate to the travelling fire grates 4a, 4b for use as a primary air, and the remainder of the combustion air, after further pressurization by a second forced-draft unit 9, is supplied to the air box 10 for the secondary air to discharge through the nozzles 11 into the combustion chambers 2 for use as the secondary air.

Fuels (bagasses) supplied from the fuel spouts 6a, 6b are blown out in the combustion chamber 2 by means of the secondary air nozzles 14a, 14b disposed near the4flued spout 6, and part of the fuel falls on the travelling fire grates 4a, 4b, where after the fuel fallen onto the fire grates 4a, 4b burn thereon by the supply of the primary air which is conveyed from the air box 5 for fire grate to the travelling fire grates 4a, 4b, the fuel reduced to ash falling off the travelling fire grates 4a, 4b to be discharged. The 0792k/lfg 11 0 9a 0 00 0 0o *000 *0o 00 00 0 0000 0 0* 0'1 0rv

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remaining fuel is burned in the suspended state within the combustion chamber 2 by supply of the secondary air which has been jetted out through the nozzles 11 into the combustion chamber 2.

At this time a part of the fuel supplied into the combustion chamber 2 fall onto the inclined surface portion of the partitionlwall 13 to slide down the inclined surface so that they may uniformly accumulate on the travelling fire grates 4a, 4b in the absence of preventive means. But, according to the present invention, the fuels will be blown O by the secondary air which has been discharged out of the nozzles 11 to the combustion chamber 2, with larger particles of the fuels being spread again on the travelling fire grates 4a, 4b in a uniform manner, while fine particles will be suspended in the air again.

As shown in Fig. 6, the nozzles 11 for supplying the secondary air are provided at the partition walls 13 positioned in the centre of the whole combustion chamber.

Accordingly, the secondary air can cover each one half of the cross sectional area of the whole combustion chamber.

As a result, even the central part of the whole combustion chamber 2 is uniformly provided with the secondary air from the nozzles 11, which, together with secondary air from the fuel supply unit, fully comes into contact with the suspending fuels particles and volatile matters.

This may result in further acceleration of combustion in suspension, thus leading to a complete combustion of the suspending fuels and volatile matters, so that soot and dust contained in the components of exhaust gas will be decreased, thereby making possible prevention of CO contamination.

In accordance with the preferred embodiment of the present invention, the cross sectional area of the lower part of the combustion chamber 2 is reduced by inclining the lower side of said intermediate partition wall 13 in order to reduce the areas of the travelling fire grates 4a, 4b.

This may increase the thickness of fuel layers or ash layers on the travelling fire grates 4a, 4b to prevent air from ;0792k/ifg 12 0*00 0r 0 00, o blowing through. Accordingly, the efficiency of combustion may be improved and the travelling fire grates 4a, 4b may be protected against a radiant heat radiated from the combustion chamber 2, to ensure that the travelling fire grates 4a, 4b enhanced by the reduction of the area of the travelling grates 4a, 4b may further effectively prevent damage to the fire grates 4a, 4b by overheating.

Furthermore, as shown in Fig. 6, both fire grates 4a, 4b are driven so as to move cumbustible materials from the central partition wall 13 to the forward walls of the opposite combustion chambers.

That is, with a conventional boiler as shown in Fig.

the width of the furnace of one of the fire grates disposed in the right to left side of the combustion chamber is dependent upon the width W of the combustion chamber.

However, in accordance with the present invention, as shown in Fig. 7, the width of the furnace of the fire grate can be substantially identical to the depth L of the combustion chamber 2, and the length of the fire grate can be greatly reduced by employing the nozzles 11 provided on the partition wall 13 serving to feed 'the secondary air for further advancement of combustion in suspension.

If, in planning the travelling fire grates 4a, 4b, any problems arise from too large a dimension of the depth L of the combustion chamber 2, the fire grates 4a, 4b can be divided into the front part and the rear part, i.e. 4al, 4a2, 4bl, 4b2, respectively so as to be driven by the separate drives 16al, 16a2, 16bl, 16b2, thus making it possible to produce a fire grate for a boiler of larger capacity with the result that it has become possible to manufacture a boiler of great capacity employing fire grates which has ever been considered impossible due to some restrictions in manufacturing. In this connection, it will be noted that the present invention has realized a boiler of a great capacity with an enhanced safety in combustion on the fire grate and improved efficiency of combustion in suspension.

As described above, since the boiler combusti)n chamber 0792k/lfg 13 in accordance with the present invention is arranged such that the partition wall serving as a radiant heat conducting surface is included in the intermediate portion of the whole combustion chamber, the radiant heat transfer effect may be increased by the flaming action within the combustion chamber.

The lower side of the water cooling wall constituting the partition wall is inclined to the side of the furnace positioned in the lower portion of the combustion chamber so that the cross sectional area of the combustion chamber will be decreased gradually, and a group of the nozzles for jetting out secondary air to the combustion chamber are located between the water-tubes on the inclined portion of the partition wall, so that the area of the fire grate may be reduced, and fuel particles or ashes fallen to the inclined portions of the furnace walls may be blown off to fall uniformly onto the fire grates. As a result, the fuel layers and ash layers on the fire grates tend to become thicker to such a degree that an uniform draft through the whole area of grate is permitted, which, therefore, may invite increase of the efficiency in combustion, protecting So. the fire grates against the radiant heat from the combustion chamber and preventing the fire grates from suffering damage S by overheating. In addition, since the area of the fire ee a o grate can be reduced, the cooling effect of the fire grate can be increased to ensure that the travelling fire grate will be free from damage due to overheating.

Furthermore, because the secondary air jets on the lower part of the intermediate partition wall are provided near the central portion of the combustion chamber in such a manner that the secondary air may completely cover one-half of the range of the cross sectional area of the whole combustion chamber, the combustion in suspension may be S' considerably accelerated to enable complete combustion of the suspending fuel particles and volatile matters in the combustion chamber. This may result in decrease of the amount of soot and dust existing in the component of exhaust gas, thus avoiding generation of CO.

t it S 0792k/Ifg 14 Additionally, in accordance with the present invention, both fire grates are designed to move from the central partition wall of the forward wall, and therefore, the driving shaft can be made to have substantially the same length as the depth L of the combustion chamber, and also can be selected in terms of the mechanical strength and economy. Thus, a stoker firing boiler with a capacity of 300-ton evaporation per hour, which has been considered impossible to manufacture, will really be producible.

As aforementioned, the instant present invention makes it possible to realize a boiler of a great capacity equipped with fire grates, ensuring the safety and easiness in operation and rationalization of combustion in suspension as well as a good practical utility.

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Claims (7)

1. A combustion chamber construction for a boiler for burning solid fuel on travelling fire grates as well as in suspension, the combustion chamber constructions, comprising: a combustion chamber; a partition wall assembly provided in the combustion chamber and dividing the combustion chamber into separated chamber zones; and travelling fire grates provided in each of the chamber zones for moving fuel on the fire grates away from the partition wall assembly; with the partition wall assembly comprising a plurality of air nozzles for supplying air into each of the chamber zones on either side of the partition wall assembly.

2. The combustion chamber construction of claim 1 wherein the partition wall assembly comprises a portion which allows the passage of gas between adjacent combustion chamber zones.

3. The combustion chamber construction of claim 1 or 2 wherein the partition wall assembly includes a lower portion having downwardly diverging lower face portions.

4. A combustion chamber construction wherein the downwardly diverging lower face portions are inclined at an angle more than the angle of repose of a spread fuel. A combustion chamber construction of claim 3 or 4, wherein the inclined portion 20 is at an angle of 50° 80° relative to the horizontal plane. i

6. The combustion chamber construction of any one of the preceding claims wherein fuel is directed into the combustion chamber zones towards the partition wall assembly.

7. The combustion chamber construction of any one of the preceding claims wherein fuel is directed into the combustion chamber zones from outer walls opposing the if 25 partition wall assembly. I p:\wpdocs\amd\448279.tak\dh C 0 I:

16- 8. The combustion chamber construction of any one of the preceding claims wherein the or each partition wall assembly comprises water tubes. 9. The combustion chamber construction of claim 7 wherein a portion of the partition wall assembly has wall portions between adjacent water tubes and said air nozzles are in said walls. A combustion chamber construction, substantially as herein described with reference to Figures 1 to 8. DATED this 28th day of February, 1995 TAKUMA CO. LTD. By Its Patent Attorneys DAVIES COLLISON CAVE 'toe *44aa4 4toc1 4,, r Cir t p:\wpdocs\amd\448279.t~\dh 0792k/if g ABSTRACT A combustion chamber construction has a combustion chamber divided into a plurality of combustion chamber zones by at least one partition wall Air jets (11) are provided in the partition wall (13) to ensure complete combustion in all of each combustion chamber zone. I C 14 tt t¢i