Background Information

The plant has three Combustion Engineering (CE) boilers, each rated at 750 MWe capacity. All units are functionally identical, each having a double or twin furnace, in which the center water wall heat transfer surface is shared by each furnace half. The total physical height of the furnaces is in excess of 120 feet 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. Each furnace half is approximately 42 feet deep and 42 feet wide, hence the total furnace presents a total surface area of over 40,000 square feet for heat transfer.
The furnace employs a tangentially fired burner technology, the burners are all mounted near the corners of each furnace half, and angled to facilitate a stirring of the fireball to achieve proper combustion of the injected coal. All boilers on site are fueled with lignite, a fuel which has inherent cleaning issues, both in terms of the absolute volume of combustion residue (frequently blocking the free passage of deposits from the unit) and occasionally the tenacity of the deposit (against which the conventional wall blower technology was found to be ineffective).

Previous Cleaning System Design

Each furnace was previously equipped with over 200 conventional wall blower cleaning sootblowers, each of which facilitates approximately 50 square feet of cleaning. Hence a total area of 10,000 square feet, or about 25% 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 26 kBTU/sq.ft./hr. even assuming the conventional cleaning devices were effective. 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 initially 4, then later 8 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 60% more than previously, or 680,000 kBTU/hr, or 200,000 kW, or 200 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 30-35 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.); reduction of compressor operation for cleaning media ($150k p.a. energy costs); 200-250 degree Fahrenheit reduction in furnace exit gas temperatures.

Overall, the customer invested in the total system and enjoyed a payback period of less than one week. The units are currently operating in excess of 800 MW. The thermal NOX output level of the boilers has been reduced by about 30% since installation of the water cannon system.