Boiler Cleaning Technologies Sootblowers are used to clean boiler ash and slag deposits. The cleaning media used in sootblowers may be saturated steam, superheated steam, compressed air or water. In most cases, superheated steam has become the preferred cleaning media because it has a greater cleaning potential due partially to teh higher sonic velocity through the nozzles and it has less erosion than saturated steam. On larger boilers, compressed air is oftern used as the cleaning media. Water may also be used as a cleaning medium either alone or in combination with steam or air. For certain high temperature ranges in which the plastic is i s plastic or where the the deposi de positt strongly strongly sinters sinters to th the e tube (when burning some Western low sulfur coals), neither air nor steam is effective. In these cases, water is required as a cleaning medium to remove the the deposi de posit. t. Boiler cleaning technology has developed significantly in the last several decades, especially in the furnace cleaning area. The Th e traditional tradi tional method method of o f furnace furnace cleaning is th the e retractable retractab le wall blower, a concept which exists in its present form from the 1940s. This device is inserted into the furnace wall through an opening. The blower tube rotates while a jet of steam or air is sprayed on the furnace wall to clean off deposits. A blower typically can cover an eight to ten foot diameter area, or about fifty square feet.Wall blowers are often not adequate for clean boilers that are burning PRB, lignite, or other coals that form furnace deposits as tests show . Reasons for this are suggested to be the impact angle of the cleaning media upon the slag as well as the relatively low mass flow of the cleaning media. Water Lances
Hence water lances were developed to clean larger areas and use a water jet as the cleaning medium. This device is a modification of the retractable sootblower. It sprays a jet of water back on to the wall it is i s mounted mounted on in a spiral sp iral pattern as it it is inserted int i nto o the furnace. furnace. Water lances cover about a 20-foot diameter area and clean a spiral shaped area where the water impingement area traverses the tube water wall. Thus, a 500 Mw size boiler may be outfitted with 40 or more in an attempt to keep the furnace clean. They have small nozzle area (typically 2/16” with 1/32” satellite nozz nozzles diameter) di ameter) and require high purity water. Lances can be bent if they are hit by falling slag while inserted into the boiler. While generally effective on most fuels, water lances have been limited by several several factors:
Their relatively high installed cost per unit area cleaned (typically in the range of $100 per square foot). As a result of this, their implementation is usually piecemeal, only addressing a small proportion of the ideal cleaned area. Power plant operators had to compromise on less devices than would be ideal, and achieve less than optimal cleaning capacity, limiting unit performance. The low incident angle of impact of the water on the wall, ß, dictates the use of high intensity water jets, as noted by the use of very small nozzles. This high velocity is required in order to compensate for the shallow impact angle of the water on the wall. Typically “back rake” angles of 15 or 20 degrees are used on water lances, meaning that the water impacts the boiler wall at 75 of 70 degrees, respectively, off a normal or direct wall impingement. This low angle on incidence is responsible for multiple cooling impacts per cleaning cycle. The relatively small nozzles used on them (1/8” to 3/16”) are prone to blockage. The lance then fails, melting or deforming in the furnace, due to lack of cooling when inserted. Water Cannons
As a major development from water lances originating in the 1980s, water cannons spray a targeted and precisely controlled jet of water from an opening in the furnace wall to the opposite wall. Robotic control mechanisms are used to achieve pinpoint accuracy in positioning and cleaning. See the image below for an illustration of this process. In many installations a cannon can clean from the nose arch to the bottom slope tubes and from one side to the other. Thus 4 cannons, one on each wall, can clean an entire furnace. More complex geometries, for example wing walls, center division walls A concentrated water jet, which is produced by a special nozzle, crosses the boiler inside and impacts on the slagged wall surface. The cleaning effect is based on the fact that the impacting water which penetrates the topmost layer and expands into steam. In this way the slag is broken up and removed from the surface. Water cannons offer a number of advantages over conventional wall blowers and water lances. A selection of the most obvious are given here: The large number of wall blowers and/or water lances requires an extraordinarily high maintenance expense. The use of steam, air, or high purity water as cleaning agents results in high operating costs. Water cannons can
use service water with a clear reduction in cost. The water cannons also allow areas to be cleaned that cannot be outfitted with wall blowers and water lances. Division walls, nose arches, and lower slope tubes are examples of areas routinely cleaned by water cannons. Dramatic cleaning area coverage: In a typical furnace the area cleaned can often be more than doubled. This increase in cleaning has several positive effects: Reducing furnace exit gas temperature (FEGT) by increasing the amount of heat absorbed in the furnace. Hence unit efficiency is improved. Load can be increased as FEGT reductions permit an increase in coal flow (given adequate materials and air handling capacity). Improving the distribution of heat transfer throughout the furnace. As water lances and wall blowers have capability to clean only a small section of the wall, the heat transfer must be ‘forced’ through this small area. By being able to clean a larger area, the cleaning ‘load’ for these areas can be reduced and more evenly distributed over other, previously uncleaned, areas. An additional benefit is the reduction in thermal NOx production. Reductions of 100 F in FEGT are common and NOx reductions of 10% or more have been documented.
Slagging and Fouling Factors Influencing Slagging and Fouling Boiler Cleaning Technologies Mechanism of Water Cleaning Mechanism of Thermal Tube Impact
