Background Information

The installation is a 350 Mwe Unit. The total physical height of the furnaces is in excess of 116 ft from the top of the couton slopes to the elevation of the nose arch which typically marks the highest point in effective heat transfer, and the point where the convection pass (superheater, reheater etc) starts. The furnace is approximately 42 feet deep and 50 feet wide, hence the total furnace presents a total surface area of over 21,300 square feet for heat transfer.

Previous Cleaning System Design

The furnace was previously equipped with over 60 conventional wall blower cleaning sootblowers, each of which facilitates approximately 50 square feet of cleaning and 14 DP water lances, each of which facilitates approximately 360 square feet of cleaning. Hence a total area of 8,040 square feet, or about 38% of the available area, was effectively cleaned. As a rough rule, typical heat flux (quantity of energy transferred per unit time per unit area) in an effectively cleaned area is in excess of 45 kBTU/sq.ft./hr., whereas in an uncleaned area (due to the deposit inhibiting heat transfer) less than 20 kBTU/sq.ft./hr. Further, the cleaning effect of these conventional cleaning devices was weak or non-existent on many types of deposit. This would hence imply that the unit average heat flux is around 29.5 kBTU/sq.ft./hr. even assuming the conventional cleaning devices were effective and there was no overlap of cleaning areas. In addition to this, there were severe problems relating to overheating in the convection pass, and clogging of the ash removal systems.

Water Cannon Cleaning System Design

Following the installation of the complete Clyde Bergemann Model WLB90 water cannon system, using a total of 4 the utility was able to clean in excess of 90% of the available surface area. This new cleaning balance permits the following result: 90% of furnace at an average 45 kBTU/sq.ft./hr. and 10% at 25 kBTU/sq.ft./hr.. Hence the average boiler heat flux is now at 43 kBTU/sq.ft./hr. The actual recorded results with operation of previous cleaning devices and water cannons in parallel resulted in an average measured heat flux in excess of 50 kBTU/sq.ft/hr.

Implications

This improvement in the average heat flux translates directly into increase in boiler efficiency (heat rate) and/or unit load capacity. Doing the numbers, the furnace is absorbing over 43% more than previously, or 458,000 kBTU/hr, or 134,000 kW, or 134 MW. Only a portion of this is realized overall, due to the furnace being a component within the total boiler system. This particular site may realize gains in the range of 10-20 MW after all other system limitations are observed. Alternatively, this additional transfer can be used to produce the same MWh using proportionally less fuel (an increase in efficiency).