Patent Grant
8770155
The present invention relates to sootblowers used to clean industrial boilers and, more particularly, relates to a sootblower equipped with a nozzle having deep reaching jets and edge cleaning jets.
Industrial boilers, such as oil-fired, coal-fired and trash-fired boilers in power plants used for electricity generation and waste incineration, as well as boilers used in paper manufacturing, oil refining, steel and aluminum smelting and other industrial enterprises, are huge structures that generate tons of ash while operating at very high combustion temperatures. These boilers are generally characterized by an enormous open furnace in a lower section of the boiler housed within walls constructed from heat exchanger tubes that carry pressurized water, which is heated by the furnace. An ash collection and disposal section is typically located below the furnace, which collects and removes the ash for disposal, typically using a hopper to collect the ash and a conveyor or rail car to transport it away for disposal. In case of pulp and paper black liquor recovery boilers, the products of the combustion in the furnace are directed to a green liquor tank to recover the inorganic cooking chemicals used in the pulping process.
A superheater section is typically located directly above the furnace, which includes a number of panels, also called platens or pendants, constructed from heat exchanger tubes that hang from the boiler roof, suspended above the combustion zone within the furnace. The superheater platens typically contain superheated steam that is heated by the furnace gas before the steam is transported to steam-driven equipment located outside the boiler, such as steam turbines or wood pulp cookers. The superheater is exposed to very high temperatures in the boiler, such as about 2800 degrees Fahrenheit [about 1500 degrees Celsius], because it is positioned directly above the combustion zone for the purpose of exchanging the heat generated by the furnace into the steam carried by the platens. The boiler also includes a number of other heat exchangers that are not located directly above the furnace, and for this reason operate at lower temperatures, such as about 1000-1500 degrees Fahrenheit [about 500-750 degrees Celsius]. These boiler sections may be referred to as a convection zone typically including one or more pre-heaters, re-heaters, superheaters, and economizers.
There is a high demand for thermal energy produced by these large industrial boilers, and they exhibit a high cost associated with shutting down and subsequently bringing the boilers back up to operating temperatures. For these reasons, the boilers preferably run continuously for long periods of time, such as months, between shut down periods. This means that large amounts of ash, which is continuously generated by the boiler, must be removed while the boiler remains in operation. Further, fly ash tends to adhere and solidify into slag that accumulates on high-temperature interior boiler structures, including the furnace walls, the superheater platens, and the other heat exchangers of the boiler. If the slag is not effectively removed while the boiler remains in operation, it can accumulate to such an extent that it significantly reduces the heat transfer capability of the boiler, which reduces the thermal output and economic value of the boiler. In addition, large unchecked accumulations of slag can cause huge chunks of slag to break loose, particularly from the platens, which fall through the boiler and can cause catastrophic damage and failure of the boiler.
The slag accumulation problem in many conventional boilers has been exacerbated in recent years by increasingly stringent air quality standards, which have mandated a change to coal with a lower sulphur content. This low-sulphur coal has a higher ash content and produces more tenacious slag deposits that accumulate more quickly and are more difficult to remove, particularly from the superheater platens. To combat this problem, the industry has developed increasingly sophisticated boiler cleaning equipment that operates continually while the boiler remains in operation. In particular, water cannons can be periodically used to clean the boiler walls in the open furnace section, and steam, water, air, and multi-media sootblowers can be used to clean the heat exchangers. These sootblowers generally include lance tubes that are inserted into the boiler adjacent to the heat exchangers and operate like large pressure washers to clean the heat exchangers with steam, water, air or multi-media blasts while the boiler remains in operation.
Fireside deposit accumulation in both power and recovery boilers not only reduces the boiler thermal efficiency, but can also lead to costly unscheduled shutdown due to the plugging of the gas passages. Although full plugging of the gas passages in power boilers can be considered a rare case, localized plugging can significantly accelerate the gas velocity and increase the risk of tube erosion.
Generally, sootblowers are configured with balanced jets to minimize the torque imposed on the sootblower lance. A first type of conventional sootblower has perpendicular nozzles with jets directed at opposing right angles to the major axis of the sootblower. Sootblowers with perpendicular nozzles work well at removing thin slag deposits and deposits inset from the leading edges of the platens but are less effective at removing thick slag deposits on the leading edges. An alternative type of conventional sootblower has lead-lag nozzles with jets directed at opposing acute angles to the major axis of the sootblower. Sootblowers with lead-lag nozzles work well at removing thick deposits on the leading edges of the platens but are less effective at removing thin deposits and slag deposits inset from the leading edges. At present, there is a need for a sootblower that successfully removes thick slag deposits on the leading edges of the platens, thin deposits on the leading edges, as well as slag deposits inset from the leading edges of the platens.
The present invention meets the needs described above in a sootblower having a nozzle that includes one or more deep reaching jets aligned with its respective platen bank to clean slag deposits inset from the leading edge of the platen bank. The nozzle also includes one or more edge cleaning jets substantially angled with respect to the platen bank for cleaning the leading edges of the platen bank. For most applications, the major axis of the sootblower is perpendicular to the major axis of its respective platen bank, resulting in a sootblower with a nozzle having angled and perpendicular jets, referred to as angled-perpendicular nozzles.
The jet sizes are selected to balance the opposing components of force perpendicular to the major axis of the sootblower to avoid the imposition of torque on the sootblower lance. As a result, the angled jet size increases as the angle increases from perpendicular to the major axis of the sootblower. The desired jet angle is also a function of the distance between adjacent platens to be cleaned, resulting in a range of jet angles and jet sizes appropriate for different boiler configurations and, potentially, different location within a boiler. Sootblowers with different lengths and diameters can be configured with the angled-perpendicular nozzles on new equipment and retrofit bases.
The present invention may be embodied as improvements to water sootblowers, steam sootblowers, air sootblowers and multi-media sootblowers, such as those described in U.S. Pat. Nos. 6,892,679 and 7,367,079, which are incorporated herein by reference. Because sootblowers are typically installed as permanent equipment in power plants, the invention may be deployed as an angled-perpendicular nozzle for a sootblower, a retrofit angled-perpendicular nozzle for an existing sootblower, a sootblower with an angled-perpendicular nozzle, and as a power plant boiler having one or more sootblowers with angled-perpendicular nozzles installed as new or retrofit equipment.
Brittle break-up and debonding are the two most important deposit removal mechanisms by sootblower jets. Brittle break-up occurs when the stress exerted by the fluid stream emitted by the sootblower jet on the deposit S(jet) is powerful enough to fracture the deposit and/or to enlarge the existing cracks around the jet/deposit impact point. The deposit is detached from the boiler tube when the propagation of the crack reaches the deposit/boiler tube interface and the crack is enlarged by the act of circumferential tensile stress and the shear stress developed by the fluid stream emitted by the sootblower jet. This mechanism can only take place if S(jet) exceeds the deposit tensile strength S(tensile).
Debonding is a deposit removal mechanism that relies on weak deposit adhesion strength S(adhesion) at the interface between the deposit and the tube (platen) surface. To remove a deposit with debonding, the S(jet) has to be greater than the S(adhesion). A deposit with high tensile strength S(tensile) can be dislodged from the tube, even with a relatively weak sootblower jet force, providing that the fluid stream can overcome the S(adhesion).
The brittle break-up deposit removal criteria for thin layer of deposit strongly attached to a boiler tube is:
While, for a thick layer of deposit, the deposit removal criteria is as follows
where:
P(jet)=Sootblower jet stagnation pressure at the jet/deposit impact point
v=Deposit Poisson's ratio
S(tensile)=Deposit tensile strength
The fluid stream power required to break a brittle deposit increases with the thickness of the deposit. In other words, it is more difficult to remove thick deposits than thin deposits with the brittle break-up mechanism. For a typical slag deposit having a Poisson's ratio of v=0.2, the removal criteria for thin layer, equation (1), becomes P>1.33 S(tensile) and the removal criteria for thick layer, equation (2), reduces to P>3.33 S(tensile). In this case, the fluid stream power required to remove a thick deposit with a Poisson's ratio v=0.2 is two and a half times higher than that required for a thin deposit. In addition, for a thick deposit, the tensile stress created by the sootblower fluid stream drops quickly from the region where the fluid stream impacts the deposit. As a result, the crack created by the fluid stream may not be able to penetrate deep into deposit/boiler tube interface. Hence, only a small portion of the deposit may be removed by the sootblower.
Unlike brittle break-up, it is easier to remove thick deposits than thin deposits by debonding. Analysis of stresses at the interface between the deposit and tube shows that removal criteria for debonding may be represented as follows:
where:
P(jet)=Sootblower jet stagnation pressure at the jet/deposit impact point
Ψ=A coefficient which depends on deposit shape and interface area
Ψ≈1 for deposit that covers half of the tube circumference
S(adhesion)=Deposit adhesion strength
D(tube)=Tube diameter
h(deposit)=Deposit thickness as shown in
As seen in equation (3), h(deposit) is located in the denominator of the equation. Hence, the thicker the deposit, the easier it is to remove by debonding. This principle can also be understood by evaluating the torque exerted by the fluid stream on thick versus thin deposits. The torque experienced by the deposit is proportional to the magnitude of the fluid stream force times the moment arm of the force, which makes thick deposits easier to remove by debonding due to the larger moment arm created by the thickness of the deposit. The conclusion is that brittle break-up mechanism is generally more effective in removing thin and small deposits, while debonding is generally more effective in removing thick and large deposits.
Plugging in the convection section of a recovery boiler generally starts from the deposit accumulation on the leading edges at the entrance of a tube bank. These deposits are typically responsible for the plugging of a recover boiler, especially in the superheater section. Nevertheless, conventional sootblowers with perpendicular nozzles generally consist of two 180° opposing nozzles directed in alignment with the platen bank, which is typically perpendicular to the major axis of the sootblower (i.e., the direction of lance insertion and retraction). Because of this nozzle arrangement, conventional sootblowers are only configured to remove the leading-edge deposits with the brittle break-up mechanism. The fluid stream emitted by the perpendicular jet, which exerts a force parallel to the gas flow, aligned with the platen bank, and perpendicular to the deposit, hits the deposit and pushes it against the leading edge of the tube. Hence, there is no significant toque or shear force produced by the perpendicular jet to promote the debonding removal mechanism. Since the deposits accumulated on the leading edge of a tube bank are generally fast-growing and thick, the brittle break-up mechanism is ineffective in removing the deposits. This shortcoming of sootblowers with perpendicular jets has been confirmed by many boiler inspections carried out using high temperature infrared cameras.
In regions where the deposit temperature is above 662° F. (350° C.), the deposit adhesion strength S(adhesion) is generally significantly smaller than the deposit tensile strength S(tensile). This suggests that it would be more effective to remove deposits in the superheater or hot-side of the generating bank with debonding rather than brittle break-up. Some sootblowers, mainly for coal fired boiler applications, are designed with a lead-lag nozzle to promote the debonding removal mechanism. Although the lead-lag nozzle arrangement may be effective in removing deposits that are accumulating on the leading edge of the tube, lead-lag nozzles are not effective in removing thin deposits and may fail to penetrate deep down into the tube bank passage where the deposits are inset from the leading edges of the platens. This is especially true for recovery boilers that have tight platen spacing, typically 10 inches (24.5 cm) between platens. In this case, the deposit located deep inside the tube bank may accumulate and plug the banks inset from the leading edges of the platens.
The new angled-perpendicular nozzle equips the sootblower with a perpendicular jet to remove thin leading-edge deposits with brittle break-up and to also reach deposits inset from the leading edges of the platens, along with an angled jet for removing thick deposits on the leading edges of the platens through debonding. As shown in
In the embodiments show in
The angled-perpendicular nozzle 10 is located at the end of a lance tube 60 that communicates a pressurized fluid 64, which may be steam for the lance sootblower shown in
Although
Referring to
Since the two jets have different angles of attack, the resultant forces have to be balanced in the opposing perpendicular directions to prevent the imposition of torque on the sootblower lance. In order to balance the jet force, the angled jet 12 is designed with a larger throat diameter than the straight jet 16 counterpart or by manipulating the shape factor (β) to equalize the perpendicular component of force imparted by the angled jet F(1×) with the opposing perpendicular component of force imparted by the perpendicular jet F(2×):
F(1×)=F(1)cos δ=βF(2×) (4)
where β is a shape factor, which depends on the nozzle configuration, such as the distance between the two nozzles, lance diameter, nozzle size, etc. In practice, β approaches one for design purposes as the lance diameter increases. The nozzle angle (δ) should be designed to create maximum debonding effects on the leading edge deposits 34A. The smaller the distance between the upstream and downstream tube banks (d as shown in
As a specific example, if it is determined for a certain area in a boiler that the debonding force F(y) is 155 lbf, the F(jet) is 200 lbf, and β (the shape factor) is assumed to be 1, the jet angle (δ) can be calculated as follows
For this example, the angled jet may be designed with a throat diameter of 1.25 inches (3.175 cm). The throat diameter of the perpendicular jet can then be sized accordingly to balance the forces in opposing perpendicular directions, i.e., 1 inch (2.54 cm):
D(2)=√{square root over (1.252 cos(50.8°))}≈1″
For a lance tube with diameter less than 4 inches (101.6 cm), the distance between the jets is typically set to 6 times the straight nozzle throat diameter (Jet Spacing Distance), i.e., 6 inches (15.24 cm) to prevent the generation of strong turbulence between the jets, which is an undesired phenomenon that may adversely affect the cleaning performance of the sootblower:
Jet Spacing Distance=6(1.0″)=6 inches(15.24 cm)
In practice, the nozzle angle can be as small as about 30° and as large as about 80°, but field testing indicates that about 50° appears to be the optimal angle for most conditions.
The principles of the present invention can be readily extended to sootblowers having nozzles with more than two jets. As a first example,
F(1×)+F(3×)=β(F2×)
F1 cos δ+F(3×)=β(F2×)
As a second example,
F(1×)+F(2×)=β[F(3×)+F(4×)]
F(1)cos δ1+F(2)cos δ2=β[F(3×)+F(4×)]
It will be appreciated that the specific jet configurations shown above are representative but not exclusive examples of embodiments of the invention, and that the jets can be sized, angled and located in other combinations. It should also be apparent that the need to balance the resulting forces increases with the length (i.e., moment arm) of the sootblower. As a result, very short sootblowers may be somewhat unbalanced, whereas the very long sootblowers should be very closely balanced.
This application claims priority to commonly-owned copending U.S. Provisional Patent Application Ser. No. 61/150,491 entitled “L-Nozzle” filed Feb. 6, 2009, which is incorporated herein by reference.
| Number | Name | Date | Kind |
|---|---|---|---|
| 3216044 | Chappell | Nov 1965 | A |
| 4407237 | Merritt, Jr. | Oct 1983 | A |
| 4503811 | Hammond | Mar 1985 | A |
| 4565324 | Rebula et al. | Jan 1986 | A |
| 4566406 | Appleman | Jan 1986 | A |
| 5423483 | Schwade | Jun 1995 | A |
| 5505163 | Jameel | Apr 1996 | A |
| 5778831 | Jameel | Jul 1998 | A |
| 6764030 | Habib et al. | Jul 2004 | B2 |
| 6892679 | Jameel et al. | May 2005 | B2 |
| 7028926 | Habib et al. | Apr 2006 | B2 |
| 7360508 | Habib et al. | Apr 2008 | B2 |
| 20020121563 | Habib et al. | Sep 2002 | A1 |
| 20040222324 | Habib et al. | Nov 2004 | A1 |
| 20080250598 | Holden et al. | Oct 2008 | A1 |
| Number | Date | Country |
|---|---|---|
| 888 737 | Sep 1953 | DE |
| Number | Date | Country | |
|---|---|---|---|
| 20100199930 A1 | Aug 2010 | US |
| Number | Date | Country | |
|---|---|---|---|
| 61150491 | Feb 2009 | US |
