Evaluation, comparison and validation of deposition criteria for numerical simulation of slagging
Highlights
► Coal ash deposition. ► Sticking probability. ► CFD-simulation. ► Deposition rate measurement.
Introduction
All over Europe there is an increasing dependence on imported coal in the energy sector. Recent figures show that the origins of imported coal have changed significantly during the last years. Due to the high deviations in coal quality and ash properties, slagging and fouling is a severe operational problem. The capability of a power plant to handle also low quality coals is an important factor for competitiveness. Slagging itself affects not only the heat transfer in the boiler, but also leads to mechanical damages and failures of the water/steam cycle [1]. Temperature tilts in the boiler can be also an indicator for slagging and fouling [2]. Under oxy-fuel conditions slagging and fouling is expected to be even more problematic due to higher combustion temperatures [3], [4]. The co-firing of biomass or other fuels [5], [6] also affect the deposition of ash and the formation of an insulating layer on the heat exchangers. These operational problems decrease the overall efficiency, the performance and the availability of the power plant [1].
On the numerical side a lot of effort was done to model the deposition behavior (e.g. [7], [8], [9]). The driving forces for such depositions are inertial impaction, thermophoresis and diffusion. While inertial impaction is dominant for large particles (>1 μm), the others play a significant role for smaller particles (<1 μm). Although, these mechanisms can be calculated in CFD simulations and the equations are well known, the available models for the simulation of all the driving forces is far from a satisfactory level [10]. It is state-of-the-art to focus only on inertial impaction. For instance, a 1D approach to model both inertial impaction and condensation has been presented by [10], which has been extended towards 2D simulations [11]. Moreover, 2D simulations have been carried out covering, sticking probability based on temperature, as well as on contact angle and on surface energy [12]. The deposition model applied in such approaches is based on ash fusion investigations where the ash fusion temperature is used for determining a sticking probability. 3D simulations of slagging and fouling have been performed with the ash deposition post-processor described in [7], where a critical viscosity and a visco-elastic model have been used. However, since inertial impaction is the most dominant effect for the large fly ash particles these models can only be as accurate as their data base for the derivation of the sticking probability. For the modeling of inertial impaction it is important to know the sticking probability of the particles and the impingement rate on the surface. The sticking probability describes how likely a particle can adhere to the surface. According to Walsh et al. [13] the deposition rate is based on three different mechanisms, (1) the deposition of the sticky particles on the surface, (2) the deposition of solid particles on the sticky surface, as well as (3) the erosion of the deposits by solid particles. For this approach it is common to derive the sticking probabilities for each fuel separately. General approaches are lacking. Besides the well-known slagging indices [14], [15], it is necessary to have continuous and temperature-sensitive parameters which can be used for CFD-calculations.
It is also state-of-the-art to evaluate the sticking probability based on the viscosity, which is empirical in nature and based on the ash composition. Many modeling approaches are based on this method (e.g. [16], [17]). This sticking probability makes use of a reference viscosity. However, the definition of this reference viscosity is discussed very controversial in literature. It can be considered as a numerical fitting parameter and its value ranges in literature within 8–108 Pa s [13], [18]. Unfortunately, this fact makes the viscosity based sticking probability strongly sensitive towards this reference viscosity and thus inaccurate. More recent simulations consider visco-elastic models of the ash, which take into account, that a cooling particle has a higher viscosity at its shell and a lower viscosity in the particle core [7]. Other models are based on thermodynamic equilibrium calculations, where the melted fraction of the deposition layer is considered and a deposit thickness is calculated by the heat transfer properties [8].
Thus, it is the aim of this study to investigate and compare different approaches to derive these sticking probabilities. The sticking probabilities are compared in this publication for two different coals and applied in a simple CFD Model to compare these results with experimental observations.
Section snippets
Coal characterization
Two different coals were investigated, namely El Cerrejon and Pittsburgh No. 8. The composition of these coals can be found in Table 1. For the proximate and ultimate analysis humidity [19], ash [20] and volatiles [21] were measured in a muffle furnace. The fixed carbon was calculated by mass difference. The oxygen content was calculated by mass difference from the C, H, N, S (vario macro, Elementar Analysensysteme GmbH) and proximate analysis. A bomb calorimeter (Parr Instrument GmbH) has been
Derivation of sticking probabilities
In this section the approaches for the different sticking probabilities are described in terms of their theoretical and experimental derivation, as well as their equations. The sticking probabilities are based either on empirical correlations for the viscosity, or experimental data from ash fusion tests and TGA/DTA measurements, or thermodynamic equilibrium calculations. In order to obtain deposition criteria from ash fusion tests two different approaches were used. One is based on the ash
Deposition rate measurement
According to Bryers [1] the deposition rate is difficult to measure continuously. In practical there are only two different approaches for this kind of measurement in large scale facilities. One is to evaluate the heat flux through the deposition probe, where shedding and deposit thickness can be evaluated qualitatively. On the other hand, changes in temperature, composition and time can only be considered by multiple probes and different test intervals [1] and by means of strain gauges. In a
Numerical calculations
Numerical calculations have been carried out for the entrained flow reactor, described in Section 2.3. A mesh of 130,000 cells was created, where also the probe was integrated, in order to model deposition in the reactor. First, a base case without deposition has been calculated until the simulation converged with a reactor temperature of 1300 °C. The air inlet temperature was set to ambient conditions. The boundary and operating conditions for the simulations were chosen according to the
Summary and conclusion
Several sticking probabilities have been derived for two different kinds of coals. The viscosity criteria do not differ significantly for the two coals, since both are rich in SiO2, Fe2O3 and Al2O3. The ash fusion criteria for the Pittsburgh No. 8 are not reasonable, since the numerical application would not lead to any deposition on the probe. The criteria based on FactSage and on TGA/DTA measurements seem to bring the most promising results, since they take the melted fraction in the ash
Acknowledgments
The authors would like to thank the support of the IFK and EnBW for the cooperation in the frame of KW21. The research joint-venture is funded by the ‘‘Bayerisches Staatsministerium für Wissenschaft, Forschung und Kunst’’, the ‘‘Bayerisches Staats-ministerium für Wirtschaft, Infrastruktur, Verkehr und Technolo-gie‘‘, the ‘‘Ministerium für Wissenschaft, Forschung und Kunst Baden Württemberg‘‘. The work was carried out in the frame of the sub-project BY4DE.
The authors also gratefully acknowledge
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