It has long been known in the art to employ pulverized solid fuel nozzle tips in firing systems of the type that are utilized in pulverized solid fuel-fired furnaces. A typical pulverized solid fuel nozzle tip comprises inner and outer shells disposed coaxially in spaced relationship to define a first flow passageway within the inner shell through which a pulverized fuel and air mixture passes into a furnace, and a second flow passageway between the inner shell and the outer shell through which air passes into the furnace. Typically, one or more splitter plates are disposed within the inner shell parallel to the axis of the nozzle tip to divide the flow passageway within the inner shell into multiple subpassages. Oftentimes nozzle tips are configured so as to be tiltable upward or downward in order to direct the fuel-air mixture discharging into the furnace.
Examples of pulverized solid fuel nozzle tips can be found in U.S. Pat. No. 2,895,435 entitled “Tilting Nozzle For Fuel Burner”, which issued on Jul. 21, 1959 and which is assigned to the same assignee as the present patent application; U.S. Pat. No. 4,274,343 entitled “Low Load Coal Nozzle”, which issued on Jun. 23, 1981 and which is assigned to the same assignee as the present patent application; U.S. Pat. No. 4,356,975 entitled “Nozzle Tip For Pulverized Coal Burner”, which issued on Nov. 2, 1982 and which is assigned to the same assignee as the present patent application; U.S. Pat. No. 4,434,727 entitled “Method For Low Load Operation Of A Coal-Fired Furnace”, which issued on Mar. 6, 1984 and which is assigned to the same assignee as the present patent application; U.S. Pat. No. 4,520,739 entitled “Nozzle Tip For Pulverized Coal Burner”, which issued on Jun. 4, 1985 and which is assigned to the same assignee as the present patent application; U.S. Pat. No. 4,634,054 entitled “Split Nozzle Tip For Pulverized Coal Burner”, which issued on Jan. 6, 1987 and which is assigned to the same assignee as the present patent application; U.S. Pat. No. 5,315,939 entitled “Integrated Low NOx Tangential Firing System”, which issued on May 31, 1994 and which is assigned to the same assignee as the present patent application; and U.S. Pat. No. 6,089,171 entitled “Minimum Recirculation Flame Control (MRFC) Pulverized Solid Fuel Nozzle Tip”, which issued on Jul. 18, 2000 and which is assigned to the same assignee as the present patent application.
A common material composition for pulverized solid fuel nozzle tips is stainless steel. Typically, a stainless steel used in such a nozzle tip is one with a relatively high temperature rating. While stainless steel has several desirable material properties, including ease of effort in incorporating it into the finished product, toughness, durability, high temperature strength, and ductility, certain material properties of conventional pulverized solid fuel nozzle tips comprised of stainless steel often force operators of pulverized solid fuel combustion facilities to operate their facilities in a less than optimal economic manner to avoid exceeding the physical limits of conventional pulverized solid fuel nozzle tips.
Two such limiting material properties are the ability of a stainless steel pulverized solid fuel nozzle tip to maintain its structural integrity at a high temperature (i.e., the maximum operating temperature) and the wear resistance of the pulverized solid fuel nozzle tip. A common maximum operating temperature for a stainless steel pulverized solid fuel nozzle tip is about 2100 degrees Fahrenheit (2100° F.), though it is not uncommon that the actual operating temperature of the pulverized solid fuel combustion facility can reach or exceed 2500 degrees Fahrenheit (2500° F.). Although there exist design and operating approaches which are configured to prevent exposure of the pulverized solid fuel nozzle tip to the actual pulverized solid fuel combustion facility operating temperature such as, for example, providing cooling air within or around the pulverized solid fuel nozzle tip, there is still some risk that the pulverized solid fuel nozzle tip may nonetheless be exposed to temperatures above the recommended maximum operating temperature in spite of the use of such design and operating approaches. For example, in the event that cooling air, which would normally protect a pulverized solid fuel nozzle tip, is, in fact, not supplied, or is only inadequately supplied, the pulverized solid fuel nozzle tip may be exposed to temperatures greater than its recommended maximum operating temperature. Excess exposure to temperatures beyond a recommended maximum operating temperature may cause a stainless steel pulverized solid fuel nozzle tip to fail during operation, causing negative economic impact.
The relatively modest wear resistance properties of the stainless steel in a stainless steel pulverized solid fuel nozzle tip may so compromise the pulverized solid fuel nozzle tip that the pulverized solid fuel nozzle tip fails between regularly scheduled maintenance outages, thus leading to the necessity of replacing the pulverized solid fuel nozzle tip at an unscheduled, economically disadvantageous time. While the wear resistance of a stainless steel pulverized solid fuel nozzle tip may be enhanced by measures such as, for example, coating the leading edges of the splitter plates of the pulverized solid fuel nozzle tip with a wear resistant material, such measures add to the manufacturing complexity and the weight of the thus treated pulverized solid fuel nozzle tip, thus detrimentally adding to the costs of the pulverized solid fuel nozzle tip.
In addition to those typical characteristics of a stainless steel pulverized solid fuel nozzle tip which may lead to unplanned operational failure, there are other characteristics of a stainless steel pulverized solid fuel nozzle tip which detract from the desirability of such pulverized solid fuel nozzle tips. For example, depending upon the pulverized solid fuel combustion facility and the type of pulverized solid fuel being combusted, a stainless steel pulverized solid fuel nozzle tip may experience slag build up attributable, in part, to the tendency of slag to bond to the surface of stainless steels. If the slag build up continues, the pulverized solid fuel nozzle tip may ultimately be completely blocked to through flow of the pulverized solid fuel.
One solution to the deficiencies of stainless steel pulverized solid fuel nozzle tips discussed above is found in U.S. Pat. No. 6,439,136 entitled “Pulverized Solid Fuel Nozzle Tip With Ceramic Component”, which issued on Aug. 27, 2002 and which is assigned to the same assignee as the present patent application, the contents of which are incorporated herein in their entirety. U.S. Pat. No. 6,439,136 provides a pulverized solid fuel nozzle tip having a single shell comprised of a ceramic material such as, for example, silicon nitride, siliconized silicon carbide (having a silicon content of between about twenty percent (20%) to sixty percent (60%) by weight, mullite bonded silicon carbide alumina composite, and alumina zirconia composites.
The single shell of the ceramic nozzle tip is of a unitary construction, i.e., is formed as a single ceramic piece. It has been found that during normal operating conditions this single shell is subject to cracking due to thermal expansion and contraction, i.e., thermal stresses. As will be appreciated, such a failure results in an economic loss for those utilizing the nozzle tip. Accordingly, a need exists for a ceramic pulverized solid fuel nozzle tip that remedies the deficiencies of the above-described ceramic pulverized solid fuel nozzle tip.