Effects on Boiler Efficiency Standards and Computed Coal Flow Given Variable Ambient Oxygen and Humidity

Abstract

This paper examines the sensitivities of ambient conditions on the most basic understanding of fossil-fired power plants: boiler efficiency and associated computed fuel flow. Conditions studied were ambient oxygen in the combustion air, and the air’s water content. This research was conducted at the 610 MWe Boardman Coal Plant operated by Portland General Electric. Burning Powder River Basin coal, it has been tested numerous times by the authors including several 4 to 6 month long projects used to throughly understand the system. This experience, coupled with many other such projects, has suggested that ambient oxygen and humidity — normally taken as constants or simply ignored in common analyses — may have significant influence on boiler efficiency. There are, of course, any number of inputs which might affect a computed efficiency, and associated computed fuel flow. This work separates data making up efficiency into water-side data and system stoichiometric data. We argue that fundamental understanding of fossil-fired systems begins with system stoichiometrics, and, for this work, ambient oxygen and humidity. Demonstrated is that depletion of ambient oxygen can cause a very high error in boiler efficiency (>1% Δη) if unrecognized. In addition, given intrinsic complexity of combustion air systems, uncertainty in the amount of water in combustion air can well contribute to error. Further, this work demonstrates a technique whereby plant fuel flow can be verified based on ambient conditions. This paper demonstrates the very real advantage of using high accuracy ambient instrumentation — for oxygen and relative humidity — whose measurements may not have dramatic affects when used at a well run plant such as Boardman, but whose use at plants not well monitored will easily justify their employment. Taking such ambient measurements a step further, this paper demonstrates a method whereby ambient humidity can be used to verify a coal-fired plant’s fuel flow.