Regelung von Biomassefeuerungen durch Messung von Luftzahl und Kohlenmonoxidkonzentration

Dissertation von Jan Bischof
Universität Stuttgart, 2016

The study examines the potential of new carbon monoxide sensors for controlling automatically feeded small-scale biomass firing systems.

This work presents a novel control concept for these biomass firing systems based on simultaneous measurements of oxygen and carbon monoxide. The new control system is able to optimize the excess air ratio of the combustion process in real-time in order to minimize the resulting emissions. Unlike previous approaches, there is no need for a direct estimation of a local gradient of the CO-O2-characteristic. Instead, the new approach is globally estimating the CO-O2-characteristic. A Kalman filter is used to determine the parameters of a previously selected base function.

In addition to the detailed description of the new concept, this work contains an extensive validation of the control concept for typical disturbances, such as changing fuel moisture contents, sensor drifts, air leakage and changing load demands. The experiment for typical load cycles demonstrates that the predetermined set points of the oxygen set point for different loads is no longer necessary. The emission-optimal oxygen concentration is determined by the new CO-O2-controller in real time during the combustion. This leads to a strong decrease of the resulting emissions.

The results of experiments with false air and artificial sensor drift show, that a shift of the CO-O2-characteristic can be detected and compensated by the new control system. This means for practical applications that for example the negative influence of a drifting lambda probe can be reduced. A comparison to measurements without the new control system demonstrates that the new controller leads to a considerable improvement of the burnout quality. Additionally, this work shows that the new controller is able to adapt the oxygen working point according to the current fuel moisture content.

In general, the controller is characterized by a high robustness and is able to adapt the oxygen concentration within a few minutes.

The new control system is a valuable improvement of existing control strategies and this work demonstrates that CO-O2-control strategies are a promising alternative to classical oxygen based concepts.

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