Online Performance Monitoring of Discrete Legs in a Convective Heat Exchanger of a Coal Fired Power Plant Boiler

摘要

The convective pass of a coal fired power plant boiler consists of several heat exchanger stages. Knowledge about the real-time performance of these heat exchangers is valuable for informing operational practices, preparing maintenance strategies and performing root cause analysis. For the specific boiler being studied, the steam in each stage is split into four legs that run in parallel across the boiler width. These legs alternate between the sides and across the centre of the boiler to disperse the effects of an uneven heat profile along the cross section of the flue gas flow path. Previous work has shown that the typical measurement setup in these boilers allows the performance parameters of each complete heat exchanger stage to be determined online using one-dimensional mass and energy balance equations [1][2]. However, flow and temperature maldistribution occurring in steam headers and the flue gas flow path leads to a non-uniform distribution of the performance of individual legs. We present and compare two methods of determining the performance of the individual heat exchanger legs online from process measurements. The first method employs soft sensors based on a thermo-fluid process model that not only accounts for the mass and energy balance, but also solves for momentum conservation. This allows the prediction of the flow distribution in common headers and ducts. The Flownex one-dimensional thermo-fluid network software is used for the modelling effort. The second method is based on the reconstruction-based contribution method which compares measured data to plant performance signatures that allows identification of deterioration of individual heat exchanger legs. These signatures are originally generated with the aid of the process model developed above for different fault scenarios. A case study of a final superheater stage is presented to assess the benefits and limitations of each method. These methods enable the performance of individual legs to be monitored online from process measurements and therefore provide higher resolution results within a heat exchanger stage.