Luis I. Díez

Combustion and heat transfer monitoring in large utility boilers

As a result of the quick and vast development of instrumentation and software capabilities, the optimization and control of complex energy systems can presently take advantage of highly sophisticated engineering techniques, such as CFD calculations and correlation algorithms based on artificial intelligence concepts. However, the most advanced numerical prediction still relies on strong simplifications of the exact transport equations. Likewise, the output of a neural network, or any other refined data-processing device, is actually based in a long record of observed past responses. Therefore, the implementation of modern diagnosis tools generally requires a great amount of experimental data, in order to achieve an adequate validation of the method. Consequently, a sort of paradox results, since the validation data cannot be less accurate or complete than the predictions sought. To remedy this situation, there are several alternatives. In opposition to laboratory work or well-instrumented pilot plants, the information obtained in the full scale installation offers the advantages of realism and low cost. This paper presents the case-study of a large, pulverized-coal fired utility boiler, discussing both the evaluation of customary measurements and the adoption of supplementary instruments. The generic outcome is that it is possible to significantly improve the knowledge on combustion and heat transfer performance within a reasonable cost. Based on the experience and results, a general methodology is outlined to cope with this kind of analysis.

Ash Fouling Under Co-Firing in a Pulverized Fuel Combustion Rig

Co-firing of coal and biomass in existing coal-fired power stations is a cost-effective method to reduce CO2 emissions in energy generation. Nevertheless, the introduction of biomass has to be carefully considered since it could significantly modify combustion and heat transfer phenomena and enhance fouling and corrosion inside the boiler. This paper investigates the effect of substituting a fraction of coal by biomass on the heat transfer and ash deposition rates, by performing pilot tests under different operating conditions in a pulverized fuel combustion rig. Fouling rates have been characterized by means of air-cooled deposition probes installed at one tube bank, reproducing the performance of a large-scale superheater. Heat transfer has been simulated coupling thermal radiation models with semi-empirical approaches for the tube bank behaviour. Ash samples compiled from the wind- and the lee-side of the probe has been collected and analysed by SEM (Scanning Electron Microscopy). Low-to-moderate fouling rates have been typically observed for the tested coal and coal + biomass blends, but with somewhat potassium enrichment at the lee-side deposits when biomass is introduced. As a matter of fact, sootblowing manoeuvres in utility boilers should not be affected when co-firing the tested fuels. Furthermore, chlorine-induced corrosion on heat transfer surfaces is not expected to be significant since the concentration of chlorine in the sampled deposits has been always found to be negligible.Copyright

On the Calculation of Coated Fins

Coated fins constitute a new concept in heat transfer enhancement. This type of fin is made from a primary material (the substrate) that usually possesses a low-to-moderate thermal conductivity. To augment the transfer of heat from the primary material to a surrounding fluid, a viable avenue is to coat the substrate with a thin layer of a high conductivity material (the coating). Undoubtedly, the formal model for a two-material fin is complicated because it involves a conjugate system of two heat conduction equations in two space variables. As a simpler alternative, Campo (2001) proposed a simplified quasi one-dimensional model that engages an ordinary differential equation with embedded spatial means of the thermal conductivities of the substrate and the coating. The objective of the present study is to extend the statistically-based ideas for a one material fin to two-material fins of variable thickness. To this end, a system of two heat conduction equations, coupled with the applicable boundary conditions, is solved with the Finite Element Method (FEM). The adequacy of the approximate algebraic route for the estimation of fin efficiencies is tested against the numerically-determined fin efficiencies supplied by the FEM.Copyright