Combustion in Radiant Tube Heaters

Abstract

The Process and Energy department of the Mechanical, Maritime and Materials faculty of TU Delft and the Dutch company Petrogas Gas-Systems B.V. are working together on the commissioning of a small 50 kW (th) Indirectly Heated Bubbling Fluidised Bed Steam Reformer (IHBFBSR) which will be used to gasify the energy crop Miscanthus. However, the new feature is that this fluidised bed will be heated indirectly using radiant tube heaters installed in the reactor. These heaters are made by assembling radiant tubes with self-recuperative burners fired using Dutch Natural Gas. All tubes and burners have been manufactured by the German company WS Warmeprozesstechnik GmbH. This thesis consists of the study of two such distinct burner-tube assemblies, both of different heating capacity. The smaller capacity is of the C80 burner assembled with C100 tube while the larger is of the C100 burner assembled with C150 tube. The heat transfer and fluid dynamics inside both the tubes have been numerically modelled using CFD techniques. The models of the burners have been done using the commercially available software ANSYS Fluent version 18.2.
The main objective of this thesis has been to analyse the heat transfer from both the assemblies and calculation of their respective efficiency. To this end, the temperature, velocity, species and turbulence fields have been calculated for inside both the tubes. Also, the total heat output from the radiant tubes has been calculated. However, the mechanism of heat transfer from the radiant tube to the fluidised bed was not known at the time of these calculations. Hence, appropriate boundary conditions have been used to simulate the outer environment. The total heat transfer from the combusting flow to the inner surface of the radiant tube has been calculated. This total heat input has been equated to the heat transfer from the outer surface of the tube to the fluidised bed, assuming steady state condition. The variation of all these parameters has been studied with air factor and preheat temperature. The air factor has been varied from 1.0 to 1.5 for both tubes, by reducing the fuel inlet keeping the air inlet constant. The air preheat temperature has been varied from 300 C to 700 C for C80-C100 assembly and from 300 C to 800 C for C100-C150 assembly. This has been done to calculate an optimum operating condition for both the burner-tube assemblies in terms of maximising heat transfer and radiant tube efficiency and minimising fuel wastage. It has been found that operating at lean condition of 20% excess air by mass at highest air preheat temperature would be the optimum operating condition.
The last part of this thesis is the analysis of the robustness of calculations. Three grid sizes have been chosen other than the main grid and all the parameters have been checked for variations, if any. The variation is found to be within scientifically acceptable limits. Hence, it is concluded that the calculations are robust at this level.