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Furnace wall cleaning is a requirement in any coal fired boiler. Depending upon the property of the ash (inorganics), designers adopt different methods for cleaning the deposits on the furnace walls. There are many types of deposit-cleaning devices used for boiler furnaces firing coal. Steam wall blowers, water lancers, and water cannons, also called HydroJet, are used to remove furnace deposit or slag.
Steam wall blowers
Large numbers of steam wall blowers are provided in the furnace walls to make sure the deposits are cleaned effectively. If the furnace wall ash deposits are expected to be loosely adhered, designers go for a short travel retractable-type soot blower called a wall blower which directs steam on the furnace wall to remove the deposits. This consists of a poppet valve, feed tube, lance tube, and a drive mechanism for inserting the soot blower tube and rotating it in the fully inserted position. The lance tube has a nozzle near the tip, which during non-operating condition is located outside of the boiler and within a sleeve to protect it from the furnace radiated heat. When it is activated, the lance tube is advanced to where the nozzle is about 40 mm from the face of the boiler tubes. The operation is synchronized in such a way that steam flows through the nozzle when it comes to the blowing position. The cleaning radius is normally around 3 meters and is a function of steam pressure, the type of nozzle, the nature of the deposit, and the boiler surface.
Steam acts as a good cleaning medium when the deposit is loosely adhered as it needs only penetration and expansion to give the required impact energy to dislodge the deposit from the wall. Some designers use air instead of steam, but this has been largely discontinued.
When the ash deposit is mainly a dense slag layer, mechanical impact energy alone may not be sufficient to clean effectively. A water lance removes slag by water jet and precisely controlling the pattern and progression velocity of the water. The water first absorbs the heat from the slag making it friable in nature and then the wall is cleaned by the steam produced during heat absorption from slag. The operation is more like the steam wall blower except that the water flow is delayed until the nozzle is about 50 cm from the rest position.
The water cannon directs a high pressure jet of water through a side furnace wall opening to clean surfaces on the opposite wall of the furnace. Here the nozzle is at the leading tip of the lance tube and the lance tube is articulated by a two-axis drive system. The water cannon offers several advantages that make it an attractive cleaning option. The ±45 deg in both the horizontal and vertical directions, single water cannon can provide the cleaning coverage equivalent to many water lances, depending on the size and layout of the furnace.
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About the Author
Dr V T Sathyanathan is a boiler consultant with 35 years of experience in various areas of high pressure boiler trouble shooting. He holds a PhD in coal combustion in boilers.
High Ash Coals - A challenge to Power Plants - Optimisation of combustion in high ash coal fired boilers is of special interest due to the organic and inorganic mix up and the large amount of variation in the organics. One such experience with Indian low reactive coal in a tangential fired steam generator of 670 T/hr capacity is given here.
Coal Formation Theories - Coal may be defined as a compact stratified mass of plant debris which has been modified chemically and physically by natural agencies, interspersed with smaller amounts of inorganic matter. In situ and Drift are the two major theories of coal formation.