The amount of air leaking into the boiler envelope is difficult to estimate. Traditional methods of measuring oxygen at the furnace exit and economizer exit do not account for all types of air leakage. By using molar calculations and total airflow measurement, a good approximation of the total air in-leakage rates of a boiler can be quickly determined using station instruments.
As a furnace ages, air leakage from the small gaps that form between the tube penetrations, casing, sootblower openings, observation doors, and dead air spaces begins to contribute to the total air supplied to the furnace. However, when leakage occurs between the furnace exit and the boiler exit, this air does not contribute to the total air required for proper combustion. It simply dilutes the gas flow and artificially raises the excess oxygen indication.
Most large generating companies use oxygen probes at the economizer outlet for monitoring the furnace condition. A 3% reading at the economizer could represent 1% at the furnace exit if leakage is present in the penthouse or convection pass. This leakage can be measured by taking an in-furnace oxygen reading at the furnace exit and then comparing it to the readings at the economizer outlet. However, this cannot account for any air leakage that does contribute to the total air requirement or if combustion is delayed. If secondary combustion (combustion downstream of the furnace exit) is present, the oxygen reading will actually drop before it begins to increase due to leakage. In this case, actual leakage would be higher than measured. In addition, air that infiltrates upstream of the furnace exit contributes to the total air requirement.
This article provides an overview of how to use molar calculations and measurement of total airflow into the furnace to approximate total leakage rates between the furnace inlet and boiler outlet (from the burners to the economizer oxygen probes). If a reliable airflow indication is available, it can be compared to the total air requirement for a given fuel. As leakage increases, the percentage of the total air requirement that comes from the intentionally supplied air will decrease, assuming a constant oxygen trim at the economizer outlet.
In our earlier article, “Boiler Optimization Increases Fuel Flexibility” (POWER’ s June 2008 issue), we focused on how burning spot market fuels can reduce fuel costs but can also introduce unexpected operational problems throughout the boiler island. This article includes a case study that provides results and data collected by Orlando Utilities Commission’s (OUC) Stanton Energy Center related to its online air in-leakage detection efforts. OUC’s Stanton Energy Center optimized its Unit 2 combustion system and improved operations and maintenance practices as part of a project to increase the unit’s fuel flexibility without degrading reliability or heat rate.
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