Modellierung der Verbrennung von Braunkohle in einer Zykloidfeuerung unter Berücksichtigung der Schadstoffemissionen CO, NOX und SO2 (English shop)

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For heating plants in the capacity range of 1 to 20 MWth, cycloid combustion can be employed by municipalities and industry. The adiabatic combustion chamber, designed as a cyclone furnace with dry ash removal, is fired with Lusatian dried lignite. Co-firing of mechanically dewatered sewage sludge, wood chips and paper residues is possible at combustion chamber temperatures of max. 950 °C. Using a combustion air mixture consisting of 40 wt.-% fresh air and 60 wt.-% recirculated flue gas, flameless combustion is established.

Within the scope of the present work, an overall combustion chamber model was developed as a tool for optimisations, for supporting combustion chamber design and for assessing the possible use of various fuels. Application in the operational control and instrumentation systems of commercial plants is likewise possible owing to the comparatively short computing times. In the field of research projects, the overall combustion chamber model can be used for in-depth evaluations and parameter studies.

As a reactor engineering configuration, the combustion chamber was divided into a cascade of stirred tanks consisting of six ideal continuous stirred-tank reactors. Using the stirred-tank cascade model, the formal-kinetic sub-models for single-particle burnout (shrinking-particle approach) and for the formation and reduction of the relevant gas components were mathematically linked. The specially developed overall combustion chamber model was programmed in MATLAB. For each stirred-tank reactor, the concentrations of O2, CO, CO2, HCN, NO and SO2, the carbon conversion, the mean particle size for six particle size classes as well as the mean gas residence times can be calculated in the axial one-dimensional direction, taking into account the process parameters of the combustion chamber.

Extensive experimental investigations were carried out on a 0.5 MWth pilot plant to determine gas concentration profiles and to establish the residence time behaviour. Various load cases were considered and the composition of the combustion air was varied. The experimentally determined data were used, on the one hand, for comparative calculations with the combustion chamber model and, on the other hand, for determining the residence time behaviour of the real combustion chamber.
For the single-particle burnout model, the reaction kinetic parameters frequency factor, activation energy and reaction order were determined for six particle size classes in a thermogravimetric analysis system. The particle samples were fed into the furnace of the thermobalance preheated to reaction temperature, so that heating rates close to those of the process were realised.

The comparison of the results of the simulation calculations with the experimental values showed very good agreement. The application of the overall combustion chamber model was demonstrated using the example of the 0.5 MWth pilot plant. With the aid of parameter studies on essential influencing variables, the combustion process was analysed and optimised with regard to the reduction of the gas components CO, NO and SO2.