Cristóbal Cortés

Ash fouling in coal-fired utility boilers. Monitoring and optimization of on-load cleaning

Even though considerable advances have been made in the fields of boiler design and coal characterization, ash deposition on heat transfer surfaces continues to be a significant problem in existing conventional utility boilers. A cost effective way to deal with this difficulty is the continuous monitoring of fouling tendencies. These techniques have become a widespread practice in coal-fired power stations as a tool for operation optimization. In spite of that, little information has been given about design criteria and case-study experiences. In this paper, the fundamentals, concepts and application principles are reviewed, attempting a systematic analysis of current developments and open questions. The main operational measure related to ash fouling is on-load cleaning by mechanical means. Based on a fouling monitoring system, improvement of sootblowing procedures can lead to significant savings, although a rigorous approach is not a trivial issue. The paper reviews the state of the art, as well as several stages in cleaning optimization. Finally, an attempt is made to formulate possible perspectives.

Gas-particle flow inside cyclone diplegs with pneumatic extraction

This paper presents a detailed flow analysis inside cyclone diplegs equipped with bottom gas extraction. Up to now, little experimental work has been done related with these devices, specially used for hot gas cleaning applications, such as pressurized fluidized bed combustion (PFBC). Tests varying inlet solid loading and gas extraction flow rate were carried out in a one-fifth cold-flow model of a PFBC cyclone. Velocity and pressure measurements along cyclone dipleg have been carried out. Results from experimental measurements reveal that the swirling flow penetrates inside the dipleg, deeper than can be anticipated by literature predictions on conventional cyclone designs. This effect is caused by the high inlet velocities of this kind of cyclones and the gas suction at dipleg bottom. Dipleg pressure coefficient is shown to be a function of the inlet solid loading and the tangential to axial velocity ratio. Although for standard cyclone designs it has been claimed that underneath the vortex end there is a stagnant region where solid recirculation and reentrainment can occur, it has been experimentally verified that even with a small percent of gas extracted at the bottom a substantial tangential velocity component is induced at dipleg bottom. This velocity assures solids conveyance to the extraction device. As solid loading is increased, an upward movement of the vortex end is detected from wall pressure measurements. The solid loading causes an important decrease of the vortex energy and, consequently, a weakening of the dipleg tangential velocity. A new method for measuring the vortex penetration in dipleg is presented.

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.

Reflections on lumped models of unsteady heat conduction in simple bodies

Abstract This paper re-examines the venerable lumped model of unsteady heat conduction by means of a detailed study of the exact temperature distributions in bodies of elementary geometry (i.e., large slab, long cylinder and sphere). The space-mean temperature is used as a vehicle for demonstrating that the lumped calculation directly follows as a particular case from the infinite series solution of the general distributed model. In this manner, several methods to find a limit Biot number can be established as simpler alternatives to the traditional procedure. Additionally, the discussion offers a different perspective of this classical subject of heat conduction theory, gaining more insight on the limiting behavior of unsteady temperature distributions.

Effect of the Solid Loading on a PFBC Cyclone with Pneumatic Extraction of Solids

This paper presents the effects of solid loading on the performance of a cyclone with pneumatic extraction of solids. The cyclone is a non-conventional design, especially used for hot-gas cleaning applications such as pressurized fluidized bed combustors (PFBC). A scaled-down cold-flow model was employed for the research. Experiments were conducted at 9–14 m/s inlet gas velocities, inlet solid loadings ranging from 30 to 230 g/kg gas, and bottom gas extraction percentages from 0.3 to 1.5%. Experimental results of pressure drop resistance coefficients and collection efficiency were compared with literature predictions. At PFBC operating conditions, cyclone geometry and solid concentration are the main parameters influencing cyclone pressure drop and collection efficiency. The vortex penetration in dipleg causes lower pressure drop values and higher collection efficiencies than predicted. These parameters can be suitably predicted for PFBC cyclones by introducing a modified penetration length in Muschelknautzs model [1]. For the present cyclone design, a new correlation of pressure drop, including the influence of solid loading, is proposed. A new method for detecting cyclone fouling, not previously addressed, is also presented, based on the evolution of the pressure drop resistance coefficient. An enhanced separation efficiency has been found, related to collection efficiency, which is especially important for particle sizes below 10 μm revealing agglomeration effects.

Comparison of Different URANS Schemes for the Simulation of Complex Swirling Flows

We perform numerical calculations of the unsteady, Reynolds-averaged Navier-Stokes (URANS) equations to simulate isothermal single-phase flow in the geometry of a pulverized-solids burner, with double air intake and swirl, at large Reynolds numbers. Two simulations are run with different turbulence closures, viz., the standard k–ϵ and Reynolds stresses models. Computations are validated concerning grid density and placement of boundaries. Results describe an almost periodic flow that exhibits very convincing time-dependent, coherent structures. We analyze it, as well as the differences arising from the nature of the turbulence model, which is an important issue given the cost involved.

Transient Thermal Network Modeling Applied to Multiscale Systems. Part II: Application to an Electronic Control Unit of an Automobile

This paper applies the methodology of transient thermal network modelling (TTNM) introduced in Part I to the heat transfer analysis of an electronic control unit (ECU) located in the engine enclosure of a motorcar. The complexity of the geometry, the diverse heat transfer mechanisms involved and the duration of the operating cycle prevent the use of both simple, lumped models and detailed numerical simulations. The TTNM methodology relies instead in steady, approximate heat transfer correlations and a division of the system into the largest possible isothermal elements, based on the analysis of characteristic time and length scales. The dynamic heat balance of each element is then written down, conforming the TTNM of the system, which is numerically integrated with an adequate time step. The practical aspects of the TTNM methodology (design stage) are finally demonstrated; in this particular case-study, the model reveals a very high risk of damage of electronic components due to the radiative heat load received from the exhaust pipe of the engine. A design modification consisting of a radiative shield is proposed and model-tested, achieving an appropriate reduction of heat flux and temperatures, and thus an adequate protection of critical components.

Transient Thermal Network Modeling Applied to Multiscale Systems. Part I: Definition and Validation

This paper formulates the methodology of transient thermal network modeling (TTNM) for the study of unsteady heat transfer in systems where the presence of multiple length and time scales prevents the analysis by means of current computational or experimental techniques. The TTNM is based on reduced order models (ROMs) and it is established under the essential premise that a transient heat transfer process can be modeled by its division in a succession of stationary states and the division of the geometry in isothermal elements, according to the characteristic time and length scales obtained by scale analysis. The methodology is subsequently validated with canonical examples and considerations are given for the application to practical problems.

Prediction of Flow Instabilities in an Atmospheric Low Swirl Burner Using URANS Models

Swirl-induced phenomena are used in gas turbine burners as a mechanism to stabilize the flame. The formation of coherent structures under turbulent swirling conditions plays a fundamental role in the stabilization and needs to be completely understood also in the absence of combustion. In this work, numerical calculations of the unsteady, Reynolds-averaged Navier-Stokes (URANS) equations for isothermal flow in an unconfined annular low swirl burner (50 kW) are reported. The standard k-ϵ and Reynolds stress models are used to run computational cases at a Reynolds number of 12,000 and two swirl numbers (S L = 0.57 and S H = 0.64). The numerical method is validated with the experiments reported by Legrand et al. [27]. Numerical results agree well with experiments for mean flow, temporal pressure measurements, and transient coherent structures. 2-D proper orthogonal decomposition (POD), 3-D iso-surfaces and advanced, vortex-related visualization methods are used to document the latter.

Rapid computation of the exit temperature of hot combustion gases flowing inside chimneys

Abstract This paper addresses the practical calculation of the thermal performance of industrial chimneys by means of recent results in forced convection heat transfer. Aside from its use in the estimation of static draft, the magnitude of the mean bulk temperature of combustion gases at the chimney exit is a key ingredient for the estimation of levels of air pollution in the vicinity of the stack. In addition, and depending on fuel and process characteristics, the wall temperature should be high enough to avoid acid condensation on the inner lining. The temperature decay of the combustion gases is a result of heat transfer processes involving both internal forced convection and external heat transfer to the surroundings. The coupling of these mechanisms renders highly difficult its accurate simulation. Possibly due to this fact, the analysis of the problem is scarcely treated in the energy- and environmental-related literature. Thermal design and performance estimations of chimneys are therefore based on engineering rules of thumb dictated by experience. The aim of this paper is to introduce rigorous heat transfer results into the subject, but from a framework accessible to design engineers. To this end, a 1-D lumped model of this kind of situations is used [1] . Although largely simplified, the model provided adequate estimates of the mean bulk temperatures when compared with those computed with a 2-D distributed model [2] , thus satisfying the requirements of simplicity and accuracy for the present purpose. The paper describes the design criteria for thermal calculation of industrial chimneys, the application of the 1-D lumped model through the use of standard heat transfer correlations, and several examples. The sequence of calculations is easy to accomplish and it is explained in such a way that it can be directly employed by engineers engaged in the thermal design of tall chimneys. Comparison with experimental data measured at the stack of a large coal-fired power station is also discussed.