LS Kraftwerkstechnik
Heat-transfer from horizontal tube bundles into fluidized beds with Geldart A lignite particles
(2014)
This work investigates the heat transfer mechanism in fluidized beds equipped with horizontal heat exchanger surfaces such as single tubes and especially tube bundles.
Previous works were performed for varying operating conditions and solids' material properties, leading to a heterogeneous and therefore not applicable spectrum of derived correlations for the influence of tube diameter and tube bundle geometry on heat transfer.
Based on a wide range validation of the correlation for the influence of single tube diameter on heat transfer coefficient, tube bundles with different tube diameters and horizontal and vertical spacings and alignments were investigated. Experiments were carried out using electrical heat transfer probes in a 0.15 m2 fluidized bed test facility at ambient conditions. Geldart A lignite and Geldart D solids were used in order to distinguish between predominant particle-convective and gas-convective heat transfer mechanisms.
The tube bundle reduction factor on heat transfer coefficient was derived as a key parameter based on dimensionless geometric numbers. This factor predicts the reduction of the heat transfer coefficient compared to that of the single tube due to the particle and gas flow disturbances caused by the immersed tubes.
Tube diameter as well as tube bundle geometry were successfully implemented into useful correlations for predicting tube bundles' heat transfer of gas–solid fluidized bed systems operating with fine particles.
The German Energiewende is resulting in high grid load changes caused by renewable energies. Therefore flexibility of power plants is getting more and more important. Future CCS power plants are usually equipped with more components than conventional power plants, resulting in a more complex and inert reaction on changes in power output. Additionally, due to the change in price structures and higher fixed and operational costs for CCS power plants, it is harder for them to be economically efficient. This study will show different options to increase the flexibility of CCS power plants and evaluate their benefits.
The challenges in maintaining a stable oxy-coal flame have been reported in the open literature. In case of pulverized coal combustion, narrower flammable regimes arise not only of differences in the feed gas composition, but also due to changes in the flow dynamics because of reduced volume flow rate through the burners to match similar temperatures and radiative heat transfer within the furnace. The present work focuses on the study of flame structure and stability of pre-dried lignite fired by a staged feed-gas burner. Experimental runs were carried out in a 0.40 MWth laboratory facility, with the objective of investigating the effects of swirl strength and feed gas distribution in the burner registers. To assess the experimental data, a numerical study of the non-reacting flow in the near burner region is performed. Predicted flow pattern and flame images show that the primary fuel jet penetrates the internal recirculation zone, forming an annular doughnut reverse flow region for both firing conditions and resembling a type-1 flame pattern. Temperature measurements indicate that the feed gas distribution among the burner registers control the flame temperature in parallel with the swirl number in both air-fired and oxy-fired environments. The oxy-fuel cases exhibite markedly reduced temperatures on the flame axis. This difference in the core arises not just from the more pronounced penetration depth of the primary jet, but also due to reaction mechanisms including pyrolysis and gasification reactions at the higher concentrations of CO2 and water vapor. Flame stabilization is shown to be strongly dependent on an appropriate distribution of the feed gas and strength of swirling flow in the burner front. The results correlated in terms of swirl number, secondary/tertiary flow ratio and secondary/primary axial momentum point to similarities in the stability limits between air-firing and oxy-firing.
The present study investigates the fireside corrosion behavior of selected superheater materials, namely: T24, P92,
VM12-SHC, A800HT, and 7RE10 under oxy-coal combustion atmospheres. Data on mass change, scale thickness,
metal loss, surface morphology and micro-structural characteristics of corrosion products were obtained. The alloy
specimens were analyzed by SEM-EDX, light microscopy and X-Ray diffraction techniques. The results after
2000 hours of exposure at a metal surface temperature of 600 °C indicate that metal wastage increased with
decreasing Cr-content under oxy-coal conditions.
Numerical simulations using the Two Fluid Model are performed for gas-solid bubbling fluidized beds. The possibility to detect disturbances in the fluidization process is investigated by common analysis of pressure fluctuations and variation of the computational domain. Additionally the influences of different particle sizes and superficial gas velocities are studied. The results are compared with bubble properties obtained from a digital image analysis technique