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High return temperatures are a frequent issue leading to inefficiencies in district heating networks. The causes for high return temperatures usually lie on the secondary side, within the building heating system. However, the district heating operator will in most cases only have access to primary side data through the heat meter. This makes it difficult for the operator to identify and remedy these causes. This contribution uses coupled building and system simulations to investigate issues leading to high return temperatures. The resulting synthetic data replace inaccessible secondary side data for the training of supervised classification algorithms allowing these issues to be diagnosed based on temperature and flow measurements in district heating substations. These classification algorithms are tested with three cases differing in the assumed availability of secondary side data. Fault detection and diagnosis can be performed with primary data only, with a modest degree of accuracy. Temperature measurements on the secondary side of the substation are shown to considerably improve the quality of predictions, from 78% to 96% classification accuracy.

Externally-fired gas turbines (EFGT) are currently being investigated for co-generation from biomass, because of their ability to deal with low-grade fuels without the complexity of gasification. Main drawbacks of the technology are related to the high thermal stresses experienced by the heat exchanger. The present work proposes a computational fluid dynamics (CFD) analysis of a grate-fired furnace installed in a EFGT cycle, with the purpose to provide a tool for detecting the most critical regions in the furnace. The model is complemented with a process simulation of the entire EFGT cycle. Different approaches for treating the fuel bed and their impact on the CFD analysis are discussed and validated through the availability of in-flame measurements of temperature and chemical species. Predictions indicate the need for a detailed fluid dynamic characterization of the grate region, which was found to largely impact the furnace flow and thermo-chemical fields

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