Coal boiler and coal boiler combustion method

Disclosed is a coal boiler that makes it possible to reduce the height of the boiler and shorten the period of construction. The coal boiler includes a first furnace in which a combustion gas generated by burning coal and air ascends; a second furnace in which the combustion gas supplied from the first furnace flows downward; and a heat recovery area in which the combustion gas supplied from the second furnace flows upward.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a coal boiler and to a coal boiler combustion method.

2. Description of the Related Art

Boilers burn fuel to generate heat and generate steam through the use of the generated heat. Further, the boilers use the generated steam to drive a steam turbine and generate electrical power. However, boilers generating an electrical power of 500 MW or more have a 50 m or taller furnace and require a long construction period. An inverted boiler described, for instance, in JP-A-2003-314805 (claims and FIG. 1) and a transverse boiler described, for instance, in Japanese Patent No. 3652988 were invented to solve the above-mentioned problem. When such an inverted boiler or transverse boiler was used, the flow of a combustion gas was directed downward or sideways, respectively.

As regards a small-size boiler, a three-pass boiler is disclosed in a nonpatent document entitled “Steam, Its Generation and Use” (Babcock & Wilcox, 39th Edition, page 13-2). This three-pass boiler operates so that a combustion gas ejected from a burner sequentially flows upward, downward, and upward, and is discharged to the outside.

Meanwhile, if unburned carbon and NOx discharged from a boiler are to be reduced, it is important that the combustion time be increased. Such being the case, it was necessary, as described in JP-A-2002-81610, to increase the height of a furnace of a two-pass boiler in which the combustion gas ejected from a burner sequentially flows upward and downward and is discharged to the outside.

SUMMARY OF THE INVENTION

In the boiler described in JP-A-2003-314805, the fuel and air ejected from a burner descend and burn. When temperature rises due to combustion, the flame ascends due to buoyancy. However, a high-concentration unburned gas descends while a low-concentration combustion gas ascends. As a result, the amount of unburned carbon increases, thereby making the roof gas temperature unduly high.

As regards the boiler described in Japanese Patent No. 3652988, a roof was difficult to design because the combustion gas flows transversely and a high-temperature gas gathers at the roof due to buoyancy. When combustion is taken into consideration, it is preferred that the flame ascend at the beginning of combustion as in the case of a two-pass boiler.

In a three-pass boiler in which a pendant heat exchanger is installed at a place where combustion gas ascends as described in the nonpatent document entitled “Steam, Its Generation and Use,” a high-temperature gas, which has once ascended, flows into the heat exchanger. Such a high-temperature gas flow into the heat exchanger may shorten the useful life of the heat exchanger or block a flow path with ash. Therefore, it was necessary to maintain a low combustion gas temperature within a furnace. Thus, the two-pass boiler and three-pass boiler were not effectively used. In addition, NOx and unburned carbon increased in amount because fuel combustion terminated in a furnace in which a burner was installed.

An object of the present invention is to provide a coal boiler and coal boiler combustion method that make it possible to reduce the height of the boiler and shorten the period of construction.

The present invention includes a first furnace in which a combustion gas generated by burning coal and air ascends; a second furnace in which the combustion gas supplied from the first furnace flows downward; and a heat recovery area in which the combustion gas supplied from the second furnace flows upward.

The present invention provides a coal boiler and coal boiler combustion method that make it possible to reduce the height of the boiler and shorten the period of construction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The coal boiler and coal boiler combustion method according to the present invention will now be described with reference to the accompanying drawings.

First Embodiment

FIG. 1is a side view illustrating a boiler according to a first embodiment of the present invention.FIG. 2is a view of the boiler taken along line A-A inFIG. 1. The boiler200includes a first furnace26, a furnace joint27, a second furnace28, and a heat recovery area29. The boiler is housed in a building that is composed of an iron frame20.FIG. 1shows only the iron frame for the outer circumference of the building. In reality, however, many iron frames are employed to provide increased building strength.

The flue gas ejected from the heat recovery area29of the boiler200is discharged through a DeNOx device18for NOx removal, an air heater (e.g., an air heater19for heating air with flue gas), and an induced draft fan (e.g., IDF25). In general, an electric precipitator, a desulfurization system (DeSOx system), a gas/gas heater, a chimney, and the like are installed downstream of the fan. The necessity of installing these devices is determined in accordance, for instance, with the type of fuel and design temperature.

The first furnace26, the furnace joint27, the second furnace28, and the heat recovery area29are hung from a plurality of hanging wires21connected to the iron frame20. Since a boiler wall expands due to heat, this configuration is employed to prevent the boiler and iron frame from being stressed.

Preheated air23b, which is heated by the air heater19, passes through a duct and is introduced into wind boxes3a,3b. Each wind box3a,3bis used to uniformly distribute air to many burners1and after-air ports (AAPs)2. When pulverized coal is to be used as the fuel for the burners1, the coal stored in a coal silo is pulverized with a coal pulverizer, and the resulting pulverized coal is supplied to the burners1. When oil is to be used as the fuel for the burners1, on the other hand, the oil is supplied from an oil tank to the burners1through a fuel pipe. For example, biomass, gas, or coke can also be supplied as the fuel for the boiler.

The first furnace26is composed by a front wall5a, a side wall5b, a rear wall5c, and a roof wall7. A water wall tube provided for these walls may be either spiral or vertical. The front wall5aand rear wall5cof the first furnace26are both provided with three-stage burners1and one-stage after-air ports2. Six rows each of burners1and after-air ports2are arranged.

The fuel and oxidant are introduced from the burners1. The subsequent explanation assumes that coal and air are to be burned. When the coal and air are supplied from the burners1and burned, a burner jet6ais formed in the first furnace26. Twenty to fifty burners1are installed to provide improved combustion quality. In an example shown inFIG. 1, two-stage after-air ports (AAPs) are installed. While the amount of air supplied from the burners1is rendered smaller than the amount of air required for complete combustion, the AAPs2supply additional air in the form of an AAP jet6bto make up a shortfall and reduce the amount of NOx ejection from the boiler. The first furnace26uses the burner jet6aand AAP jet6bto generate a combustion gas6c.

The combustion gas6cgenerated by the first furnace26passes through the furnace joint27in which screen tubes8,9are installed, and flows to the second furnace28. The screen tubes8are used as members for maintaining furnace strength. A plurality of screen tubes8are positioned in parallel with the roof wall7so as to block a flow path of the combustion gas6c. The two-pass boiler was designed so that the temperature of a gas passing through a screen tube is lower than the melting point of ash. This design was employed to prevent ash from adhering to the screen tube. Since the present embodiment divides the furnace into two, the first furnace26is not taller than the furnace of the two-pass boiler. Therefore, the gas temperature prevailing around the screen tubes8is higher than the melting point of ash. The boiler is designed so that the temperature of the screen tubes8does not exceed the upper-limit temperature of an employed material even when such conditions exist. It is preferred, for example, that a heat-resistant material be employed to resist high temperature or that low-temperature water be supplied to the screen tubes8for cooling purposes. Further, since ash is likely to adhere to the screen tubes8, the spacing intervals between the screen tubes are increased. Spacing the screen tubes at intervals, for instance, of 1 m or longer reduces the possibility of the intervals between the screen tubes being blocked by ash.

Next, the combustion gas6cpasses through the screen tubes9and flows to the second furnace28. The second furnace28is enclosed by a front wall12a, a side wall12b, a rear wall12c, and the roof wall7. These walls are made of a water wall tube that permits water or steam to flow. The water wall tube may be oriented either vertically or spirally. However, the thermal load on the second furnace28is relatively uniform as compared to the thermal load on the first furnace26. Therefore, orienting the water wall tube vertically simplifies the furnace structure.

The combustion rate of the combustion gas6cin the second furnace28can be adjusted by varying the air flow rate distribution by the burners1and AAPs2. Decreasing the rate of mixture provided by the AAPs2can achieve NOx reduction. Using the furnace of a two-pass boiler for slow combustion increases the amount of unburned carbon such as CO. However, the combustion gas6cin the three-pass boiler according to the present embodiment ascends in the first furnace26, descends in the second furnace28, and ascends in the heat recovery area29. Therefore, there are two bends. The two bends mix the combustion gas discharged from the heat recovery area29to reduce the amount of unburned carbon. Further, this feature can be effectively used to conduct an operation with a reduced amount of air. In other words, it makes it possible to perform an operation at a low outlet oxygen concentration. As a result, the efficiency of a plant can be enhanced.

Superheaters10,11are mounted on the roof wall7of the second furnace28. Since the combustion gas temperature of the second furnace28is moderately high, the second furnace28is suitable for the installation of the superheaters10,11. In the second furnace28, the combustion gas6cflows downward. Since combustion has progressed in the second furnace28, the combustion gas temperature and concentration do not significantly vary. Thus, the second furnace28is insusceptible to buoyancy. Subsequently, the combustion gas6fflows to the heat recovery area. When the ash attached to the second furnace28and heat recovery area29is removed, it falls. Therefore, a device (ash hopper13) for collecting and storing the ash is required. It is preferred that the ash hopper13be angled to avoid ash accumulation.

The heat recovery area29is enclosed by a front cage wall14, a rear cage wall16, and a side cage wall17. Further, the heat recovery area29is provided with a heat exchanger that includes an economizer32, a reheater33, and a superheater34. This heat exchanger is formed by bending a tube. The present embodiment relates to a reheating cycle that uses main steam and reheated steam for a steam turbine.

Parallel dampers30,31are used to adjust the temperatures of the main steam and reheated steam. The combustion gas6fis divided into combustion gases6dand6e. The ratio between the two combustion gases6d,6eis adjusted by the parallel dampers30,31. The associated two flow paths are separated by a partition15that is provided inside the heat recovery area29. When, for instance, the reheated steam temperature is to be raised, the opening of the parallel dampers30should be increased to raise the flow rate of the combustion gas6d.

The upstream combustion gas temperature is higher than the downstream combustion gas temperature. More specifically, the temperature of the gas passing through the reheater33and superheater34on the upstream side is high, whereas the temperature of the gas passing through the economizer32on the downstream side is low. Heat recovery from a low-temperature combustion gas can be effectively achieved by raising the combustion gas flow rate for heat transfer coefficient enhancement. Thus, heat transfer tubes of the economizer32are spaced at narrow intervals. As regards the boiler according to the present embodiment, the heat transfer tubes of a heat exchanger positioned downstream (placed at an upper position) are spaced at relatively narrow intervals, whereas the transfer tubes of a heat exchanger positioned upstream (placed at a lower position) are spaced at relatively wide intervals. The reverse is the case with a two-pass boiler. Therefore, when the ash attached to a heat exchanger is removed, for instance, with a soot blower, the removed ash falls into a heat exchanger having transfer tubes spaced at wide intervals. This prevents the combustion gas flow path from being blocked, thereby providing enhanced boiler reliability.

As described above, the two-pass boiler has only one furnace, whereas the furnace of the three-pass boiler according to the present embodiment is divided into two. When the height of a furnace is decreased by dividing the furnace into two, it is possible to reduce the necessity of performing high-place work with a crane or the like and lifting a heavy item against gravity. Further, as the lower structure of a furnace is integral with the upper structure, the lower structure cannot be assembled until the upper structure is assembled. Therefore, dividing the furnace into two doubles the work speed. As described above, dividing the furnace into two makes it possible to reduce the height of the boiler (furnace) and shorten the period of construction.

When the furnace capacity is increased, the combustion time can be reduced while avoiding a cost increase. This makes it possible to reduce the NOx concentration and decrease the amounts of CO and UBC (unburned carbon in ash). When the furnace capacity of a two-pass boiler is increased, the furnace height increases. However, the three-pass boiler according to the present invention makes it possible to decrease the furnace height while minimizing the combustion time for the combustion gas.

Second Embodiment

FIG. 3is a side view illustrating a boiler according to a second embodiment of the present invention. The second embodiment is structured to decrease the amounts of NOx and CO to a greater extent than the first embodiment.

A NOx generation mechanism can be roughly divided into two types. One of them generates fuel NOx from nitrogen in fuel. The other generates thermal NOx by allowing nitrogen in the air to oxidize. Referring toFIG. 3, the amount of fuel NOx is decreased by staged combustion. The amount of thermal NOx can be reduced by lowering the combustion gas temperature. For such purposes, the amount of air ejection from the AAPs and the rate of such ejection are important.

In the present embodiment, many AAPs37are provided for the second furnace28in addition to the AAPs2for the first furnace26. The flow rates and ejection rates of such AAPs2and AAPs37are regulated to control the amounts of NOx and unburned carbon. If, for instance, the air ejected from the AAPs rapidly mixes with the combustion gas, a local gas temperature rise occurs to increase the amount of thermal NOx.

FIG. 4shows an example of combustion gas temperature control. The upper diagram inFIG. 4shows combustion gas temperature changes within a furnace. The horizontal axis of this diagram indicates a location in a combustion gas flow path that connects the first furnace26, the furnace joint27, and the second furnace28, whereas the vertical axis indicates temperature. The lower diagram inFIG. 4shows NOx amount changes in the combustion gas in a furnace. The horizontal axis of this diagram indicates the same as the counterpart in the upper diagram whereas the vertical axis indicates the amount of NOx.

When the combustion gas temperature exceeds 1800 K, the amount of thermal NOx tends to increase sharply as shown inFIG. 4(graph X inFIG. 4). More specifically, when an AAP positioned at the most upstream end supplies air to the combustion gas ejected from a burner1, the amount of air ejection from the AAP is controlled so that combustion takes place at a temperature of not higher than 1800 K (graph Y inFIG. 4). It should be noted that the “AAP positioned at the most upstream end” is the AAP positioned at the most upstream end within the combustion gas flow path connecting the first furnace26, the furnace joint27, and the second furnace28. Therefore, the “AAP positioned at the most upstream end” inFIG. 3is an AAP2that is provided for the first furnace26.

When the combustion gas temperature falls below 1500 K, the rate at which coal particles of pulverized coal and solid particles of unburned carbon (e.g., soot) generated in a combustion process burn decreases. To reduce the amount of unburned carbon while minimizing the amount of NOx generation, it is therefore preferred that combustion take place at a temperature between 1500 K and 1800 K. As described above, the increase in NOx concentration can be minimized by controlling the amount of air ejection from the “AAP positioned at the most upstream end” in the above-mentioned manner.

The boiler shown inFIG. 3adjusts the flow rate of air to be supplied from many AAPs37that are provided for the second furnace28. However, if the amount of air supplied from the AAPs2provided for the first furnace26is small, the combustion gas6ccontains a large amount of fuel rich gas. Therefore, the first furnace26and furnace joint27may corrode. To reduce the degree of corrosion, it is preferred that the oxygen concentration of the combustion gas6cbe adjusted to approximately 0.5% after subjecting the air ejected from the AAPs2to mixture. It is also preferred that the upper structure of the first furnace26and the furnace joint27be made of a corrosion-resistant material.

To further reduce the amount of NOx, it is preferred that ammonia, urea, or other NOx reducing agent be supplied from a port or ports38of the second furnace28. This approach is referred to as a noncatalytic NOx reduction method. Further, the amount of NOx can be reduced by supplying methane or other combustible gas from the port or ports38for reburning purposes.

Third Embodiment

FIG. 5is a side view illustrating a boiler according a third embodiment of the present invention. The third embodiment will be described mainly with reference to a structure for reducing the amount of ash adhesion.

Ash mainly adheres to the roof wall7of the first furnace26and to an area close to the furnace joint27. If the ash adheres to the roof wall7of the first furnace26, it is preferred that an AAP2bbe oriented toward the roof wall7for ejection and cooling purposes.FIG. 5indicates that the AAP2bis mounted on the upper part of the front wall5aof the first furnace26.

If the ash adheres to the screen tubes8, it can be dropped with a water sprayer (e.g., water cannon39).FIG. 5indicates that the water cannon39is mounted on the front wall5aof the first furnace and positioned between the AAPs2and AAP2b. The ash attached to the bottom of the screen tubes8,9is likely to accumulate on the bottom of the furnace joint27. An AAP2cis therefore added to seal the lower part of the furnace joint27for the purpose of avoiding such ash accumulation. Alternatively, an ash removal device (e.g., soot blower40) may be mounted on a side wall of the furnace joint27. As regards the boiler according to the present embodiment, the furnace joint27should be provided with many ash removal devices (e.g., soot blowers40). Ash removal devices (e.g., wall blowers44) should also be installed to remove the ash attached to the roof wall7. The wall blowers44are mounted on the roof wall7.

The combustion gas that has flowed to the second furnace28passes through the pendant superheaters10,11. Ash removal devices (e.g., soot blowers40) are installed to remove the ash attached to the pendant superheaters10,11. The bottom of the second furnace28is inclined to avoid the accumulation of the ash that falls from the pendant superheaters10,11. The heat recovery area29is also provided with many ash removal devices (e.g., soot blowers).

Fourth Embodiment

FIG. 6is a side view illustrating a boiler according to a fourth embodiment of the present invention. The fourth embodiment differs from the other embodiments in that the former uses a different method of adjusting the temperature of steam to be generated by the boiler. The boiler shown inFIG. 6differs from the one shown inFIG. 1in that the pendant superheater11is positioned downstream of the second furnace28, and that the heat recovery area29does not have a partition.

Referring toFIG. 6, part of the flue gas discharged from the air heater19is returned to the second furnace28by a gas recirculation fan41aand used to adjust the amount of heat absorption by the water wall tube. In most cases, the flue gas temperature roughly ranges from 100° C. to 150° C. The flue gas is used to adjust the steam temperature of a reheated steam system. When, for instance, the rate of flue gas flow to the second furnace28is increased, the combustion gas temperature of the second furnace28decreases. This reduces the amount of heat transfer by the pendant superheater11, which mainly provides radiant heat transfer. In the present embodiment, the pendant superheater11and reheater33decrease the amount of heat transfer for the same reason. The superheater34and economizer32mainly provide convective heat transfer. Therefore, when the flue gas is supplied to increase the combustion gas flow rate of the heat recovery area29, the combustion gas flow velocity increases to increase the amount of heat transfer by the superheater34and economizer32.

An alternative is to connect the downstream end of the economizer32to the second furnace28with a gas flow path and supply the combustion gas to the second furnace28through a gas recirculation fan41b. The use of this alternative makes it possible to decrease the gas temperature of the second furnace28and avoid ash adhesion. Particularly, the gas temperature of the rear wall12cof the second furnace can be decreased to inhibit ash adhesion. The gas temperature of the second furnace28should be approximately 350° C.

In the present embodiment, the depth of the second furnace28is smaller than in the first embodiment. This design is not essential to a configuration that includes the gas recirculation fan41b. In such a configuration, it is likely that ash may adhere to the rear wall12cof the second furnace. Therefore, many ash removal devices (e.g., soot blowers42) are mounted on the wall surface. Further, an AAP2dcan be mounted on the roof of the second furnace28to avoid ash adhesion to the rear wall. Although the structure for minimizing the amount of ash adhesion to the rear wall is described here, the same method can be applied to the front wall and side wall.

Another alternative is to connect the downstream end of the economizer32to the first furnace26with a gas flow path and return low-temperature flue gas to the first furnace26with a gas recirculation fan41c. When the low-temperature flue gas returns, it can be used to cool the roof wall7.

Fifth Embodiment

FIG. 7is a side view illustrating a boiler according to a fifth embodiment of the present invention. The boiler shown inFIG. 7differs from the one shown inFIG. 1in that the rear wall12cof the second furnace28is integral with the front wall of the heat recovery area29. The use of this structure reduces the number of required members. In this case, however, it is well to remember that thermal expansion occurs to generate stress when the side wall12bof the second furnace, the rear wall12cof the second furnace, and the cage wall17of the heat recovery area29are welded together. To avoid such a problem, it is necessary to design the boiler so that the temperatures of water and steam passing through the above sections are uniform wherever possible.

Sixth Embodiment

FIG. 8is a side view illustrating a boiler according to a sixth embodiment of the present invention. In the sixth embodiment, a joint member (joint43) that resists high-temperature gas is employed as the joint between the second furnace28and heat recovery area29. Further, a ground-supported, free-standing heat recovery area29is employed instead of a pendant type. The free-standing type reduces the construction period and cost because it can be constructed more easily than the pendant type. The two-pass boiler could not use the free-standing type because the heat recovery area29was mounted on a high part of a furnace. The present embodiment can use the free-standing type because the heat recovery area29is positioned close to the ground.

Since the temperature of the combustion gas6fpassing through the joint43is approximately 1000° C., it is necessary that the joint43resist such a high temperature. In addition, since the upper structure of the second furnace28is fixed, the second furnace28expands downward when the temperature of its material rises. Meanwhile, since the lower structure of the heat recovery area29is fixed, the heat recovery area29expands upward when the temperature of its material rises. Thus, the joint43needs to absorb both of these expansions. The expansions can be absorbed by using a bellows that is shown inFIG. 9or by using a slide that is shown inFIG. 10.

Seventh Embodiment

FIG. 11is a side view illustrating a boiler according to a seventh embodiment of the present invention.FIG. 12shows a steam flow path of the boiler shown inFIG. 11. A water supply pump is used so that a condenser supplies water to the economizer32. The economizer32warms the water and supplies it to the bottom105of a first furnace water wall. Since the furnace shown inFIG. 11has a spiral wall, the water wall tubes are laid around the outer circumference of the furnace and connected to the upper structure. Further, the water wall tubes are positioned vertically on the tops106a,106bof the first furnace water wall. The use of the vertical water wall tubes simplifies the structure. When the tops106a,106bof the first furnace water wall have a spiral structure, the water wall tubes provide uniform heat absorption. Rear wall water tubes branch into the screen tubes8and the top106cof the first furnace water wall and connect to the upper structure. When the tops of the first furnace are reached, the water is supplied to a mixing header107, which mixes water and steam of each tube, for the purpose of making the water and steam temperatures uniform.

The water and steam from the mixing header107flow downward along the front wall12a, side wall12b, and rear wall12cof the second furnace. When a gas-liquid two-phase downward flow occurs, evaporation may slow down because a liquid phase rapidly falls by gravity. Therefore, ribbed tubes or other tubes exhibiting high heat transfer efficiency should be used to accelerate the mixture within the tubes.

Next, the steam flows to a mixing header108and then to a water-steam separator109, which separates the water and steam. The boiler should be designed so that the water almost evaporates when the bottom of the second furnace is reached. Construction is easy because the heavy mixing header108and the water-steam separator109can be installed at a low place slightly above the ground. The water separated by the water-steam separator109returns to a water supply line through a water storage tank and a boiler circulation pump (BCP). If the water-steam separator109is not installed, the water stays at the bottom so that there is no steam flow in some tubes. Although the figure indicates that two mixing headers and one water-steam separator are installed, the number of such units should be adjusted as needed.

Next, the steam separated by the water-steam separator109is distributed to the heat recovery area cage walls14,16,17and partition15. Since the steam ascends, the length of the tubing between the mixing header and the above walls is reduced.

Next, the steam is supplied to the roof wall7. If the steam temperature is unbalanced, a mixing header should be installed before the roof wall7.

Next, the steam is superheated by the superheater34and further superheated by the pendant superheater10. A device (sprayer) for supplying a low-temperature fluid should be installed before and after the superheater34and pendant superheater10in order to adjust the temperature of the steam. The steam discharged from the pendant superheater10is supplied to a high-pressure turbine.

After being used in the high-pressure turbine, the steam is returned to the boiler water wall tubes as reheated steam. The returned reheated steam is then supplied to an intermediate-pressure turbine through the reheater33and pendant superheater11. The use of the above route makes it possible to decrease the length of tubing and reduce the degree of steam/metal temperature unbalance.

Eighth Embodiment

FIG. 13shows a steam flow path according to an eighth embodiment of the present invention. The eighth embodiment differs from the seventh embodiment in the sequence of water/steam flow. As is the case with the seventh embodiment, the water discharged from the economizer32enters the bottom105of the first furnace water wall. Subsequently, the water enters the mixing header107through the tops106a,106b,106cof the first furnace water wall and the screen tubes8,9. The water-steam mixture then descends along the heat recovery area cage walls14,16,17and partition15. Since the heat load on the cage walls is lower than that on the second furnace28, a metal temperature rise and other problems are not likely to occur when a downward flow occurs. The water-steam mixture that has descended along the heat recovery area cage walls14,16,17and partition15is collected by the mixing header108and separated into water and steam by the water-steam separator109. The separated steam descends along the front wall12a, side wall12b, and rear wall12cof the second furnace and is supplied to the roof wall7. The steam supplied to the roof wall7then flows along the same path as indicated inFIG. 12.

Ninth Embodiment

FIG. 14shows a steam flow path according to a ninth embodiment of the present invention. The ninth embodiment differs from the seventh and eighth embodiments in the sequence of water/steam flow. As is the case with the seventh embodiment, the water discharged from the economizer32enters the bottom105of the first furnace water wall. Subsequently, the water enters the mixing header107through the tops106a,106b,106cof the first furnace water wall and the screen tubes8,9. The water-steam mixture then descends along the front wall12aand rear wall12cof the second furnace. Next, the water-steam mixture passes through the mixing header108and water-steam separator109, then only the steam reaches the side wall12bof the second furnace and ascends. In this case, a large amount of heat is transferred along the front wall12aand rear wall12cof the second furnace. Therefore, the present embodiment is suitable for a situation where great thermal expansion occurs.

Tenth Embodiment

FIG. 15shows a steam flow path according to a tenth embodiment of the present invention. The water discharged from the economizer32is supplied by distributing it to the bottom105of the first furnace water wall, the front wall12aof the second furnace, the side wall12bof the second furnace, and the rear wall12cof the second furnace. The water and steam generated from these heat transfer surfaces are mixed by the mixing header107and separated into water and steam by the water-steam separator109. The steam flows to the cage walls14,16,17via the roof wall7. The use of this method simplifies the boiler structure because the use of water/steam tubing is minimized.

It should be noted that the mass flow rate of each water wall tube decreases because the furnace connected to the economizer32is large in size. In this case, DNB (Departure from Nucleate Boiling) may occur to significantly raise the metal temperature. The boiler should be designed in consideration of such DNB. Further, even when the mass flow rate decreases, a change from liquid phase to gas phase quickly occurs in a water wall tube that transfers a large amount of heat. This decreases the amount of pressure loss. Consequently, the flow rate increases to decrease the amount of temperature rise. This advantage can be effectively used to enlarge a flow velocity design rage without sacrificing reliability. In addition, the pressure loss of a furnace can be reduced.

Eleventh Embodiment

FIG. 16is a side view illustrating a boiler according to an eleventh embodiment of the present invention. In the eleventh embodiment, the burners are mounted on the right- and left-hand walls instead of the front and rear walls. The use of this configuration provides a simplified device layout because nothing is installed between the rear wall of the first furnace and the front wall of the second surface. Further, the boiler shown inFIG. 16does not have the front wall of the second furnace. The front wall of the second furnace is substituted by the rear wall5cof the first furnace. The use of this configuration reduces the amount of materials.

For the boiler shown inFIG. 16, a wind box duct48is installed to connect the air heater19to a wind box3. The wind box duct48is mounted on the side walls of the second furnace28and heat recovery area29. Since the wind box duct48is relatively long in a horizontal direction, it should be used as an inspection passage. Further, since the furnaces are not tall, devices are installed at a low place slightly above the ground. This makes it easy to inspect the devices.

The present embodiment assumes that a mill45, which pulverizes coal, a coal silo46, which stores coal, and fuel pipes47, which convey coal, are also included. Placing the coal silo46inside a building provides increased ease of maintenance. When, in this instance, the coal silo height is substantially equal to the furnace height, construction can be accomplished with ease because the ceiling height of the building can be uniform.

Twelfth Embodiment

FIG. 17is a side view illustrating a boiler according to a twelfth embodiment of the present invention. The twelfth embodiment assumes that the furnace joint27has no screen tubes. If there are screen tubes, ash adhesion, wear, corrosion, or other problem is likely to occur. Therefore, the water wall tube provided for the rear wall5cof the first furnace26is connected to a rear wall header51of the first furnace so as to discharge the steam to the outside. Further, the water wall tube provided for the front wall12aof the second furnace is connected to a front wall header52of the second furnace so as to discharge the steam to the outside. A joint upper beam49is furnished to hang the rear wall5cof the first furnace26and the front wall12aof the second furnace28. The joint upper beam49is connected to the iron frame20. In addition, a joint lower beam50is also installed between the rear wall5cof the first furnace and the front wall12aof the second furnace to hang the rear wall5cof the first furnace and the front wall12aof the second furnace.

FIG. 18is an enlarged view illustrating the joint upper beam49and joint lower beam50. A joint hanger structure53connects the joint lower beam50to the joint upper beam49, thereby supporting the load on the rear wall5cof the first furnace and the front wall12aof the second furnace.

FIG. 19Ais an enlarged view illustrating the heat recovery area29,FIG. 19Bis a view of the boiler taken along line B-B inFIG. 19A. Water or steam flows in the heat transfer tubes70(70a,70b), whereas the combustion gases6d,6eflow outside the heat transfer tubes70. The heat transfer tubes70heat the water or steam by effecting heat exchange between the combustion gases and the water or steam. The heat transfer tubes70, which are positioned adjacent to each other, are placed at a predetermined interval71(71a,71b) from each other so that the combustion gases6d,6eflow along the outer surfaces of the heat transfer tubes70. This interval71means the distance between the outer surfaces of the heat transfer tubes70.

A heat transfer tube70athat is shown in the upper half ofFIG. 19Bis positioned downstream of the heat recovery area29, whereas a heat transfer tube70bthat is shown in the lower half ofFIG. 19Bis positioned upstream of the heat recovery area29. The combustion gases6d,6ethat flow in the heat recovery area29flow upward in FIGS.19A and19B. The interval71bof the heat transfer tube70b, which is positioned on the upstream side, is longer than the interval71aof the heat transfer tube70a, which is positioned on the downstream side. Therefore, when the ash attached to the outer surface of the heat transfer tube70ais removed with a soot blower, the ash readily passes through the interval71bof the upstream heat transfer tube70b, thereby making it possible to inhibit the combustion gas flow path from being blocked.

The description of the heat transfer tube interval, which has been set forth with reference toFIG. 19B, can also be applied to the heat transfer tubes of the economizer32, reheater33, and superheater34. More specifically, the positional relationship between the heat transfer tubes for the economizer32conforms to the positional relationship depicted inFIG. 19B. The same also holds true for the reheater33and superheater34.

Further, even when the economizer32positioned downstream of the heat recovery area29and the reheater33or superheater34positioned upstream of the heat recovery area29are compared, their heat transfer tubes conform to the positional relationship depicted inFIG. 19B.

The present invention is applicable to a boiler that shortens the construction period and reduces the amounts of NOx and CO.