Background Information

The plant has three Babcock and Wilcox (B&W) design boilers, each rated at 528 MWe capacity. All units are functionally identical, each having a single furnace. The total physical height of the furnaces is in excess of 100 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 55 feet deep and 60 feet wide, hence the total furnace presents a total surface area of over 25,000 square feet for heat transfer. All burners are all mounted at the front and rear of the lower furnace. All boilers on site are fueled with 100% western coal. This type of coal is well known for its low sulphur qualities and low cost/BTU as well as the difficulty of removing the combustion residue. The problems associated with the tenacity of the deposits are further aggravated by the highly reflective nature of the deposit, which is extremely efficient at inhibiting heat transfer (4 times more so than asbestos).

Previous Cleaning System Design

Due to the historical use of western fuel as the coal for these units, each furnace was previously equipped with over 30 conventional water lance type wall blower cleaning sootblowers, each of which facilitates a maximum of approximately 350 square feet of cleaning. Hence a total area of 8,000 square feet, or about 32% 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. This would hence imply that the unit average heat flux is around 28 kBTU/sq.ft./hr. In addition to this, there were severe problems relating to overheating in the convection pass and air preheaters, 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 units, 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.

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 50% more than previously, or 405,000 kBTU/hr, or 118,000 kW, or 118 MW. Only a portion of this is realized overall, due to the furnace being a component within the total boiler system. This particular site realized gains in the range of 20-25 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).
Other tangible benefits at this site include: vast savings in conventional sootblower maintenance ($200k p.a.); a reduction in the operations of other convection pass sootblowers (due to a reduction in furnace exit gas temperature of about 100 degrees Fahrenheit) and associated maintenance and operational costs.
Overall, the customer was able for the first time in operational history (since mid 1980s) to reach the unit goal of 528 MWe 24 hours a day for 7 days a week, and proceeded to install this technology at the earliest opportunity on every boiler within the site and others outside. The thermal NOX output level of the boilers has been reduced by about 15% since installation of the water cannon system.