Cristóbal Cortés

Bio: Cristóbal Cortés is an academic researcher from University of Zaragoza. The author has contributed to research in topics: Heat transfer & Thermal conduction. The author has an hindex of 17, co-authored 33 publications receiving 1179 citations.

Modeling the gas and particle flow inside cyclone separators

Abstract: This paper reviews the models developed for the flow field inside inverse-flow cyclone separators. In a first part, traditional algebraic models and their foundations are summarized in a unified manner, including the formulae for tangential velocity and pressure drop. The immediate application to the prediction of collection efficiency is also reviewed. The approach is the classical, treating first the dilute limit (clean-gas correlations), and afterwards correcting for “mass loading” effects. Although all these methods have had a remarkable success, more advanced ideas are needed to model cyclones. This is put forward by exploring the work done on the so-called “natural” length of the cyclone, that has led to the discovery of instability and secondary flows. The resort to computational fluid dynamics (CFD) in this case is difficult, however, due to the very nature of the flow structure. A closing section on the subject reviews past and recent CFD simulations of cyclones, both single- and two-phase, steady and unsteady, aiming at delineating the state-of-the-art, present limitations and perspectives of this field of research.

Numerical investigation of NOx emissions from a tangentially-fired utility boiler under conventional and overfire air operation

Abstract: A CFD investigation has been carried out about the performance of a 600 MW e tangentially coal-fired boiler, focusing on the reduction of NO x attainable by using overfire air. To this purpose, a comprehensive combination of NO x chemistry models has been used, coupled with the numerical simulation of fluid and particle flow, solid fuel combustion and heat and mass transfer. Predicted values of gas temperature and species concentration have been adopted to validate the model against actual measurements from the full-scale boiler, under conventional and overfire air arrangements. A reasonable agreement has been attained in most cases. Additionally, modelling sensitivity has been evaluated against variations in some fuel-dependent parameters hard to measure or estimate (devolatilisation rates, nitrogen content in volatiles and char, reburning rates). As a result, an analysis tool is available to study the response of this kind of boilers to a variety of coal feedstock and combustion conditions, in a feasible and economic manner.

Monitoring and prediction of fouling in coal-fired utility boilers using neural networks

Abstract: This paper describes a systematic approach to predict ash deposits in coal-fired boilers by means of artificial neural networks. The approach is of a “grey box” nature, decomposing the problem into logical parts, and avoiding the use of sophisticated data. Although it is relative to the specific fuel and equipment, the prediction is very detailed and can be used on-line; it accounts not only for the deposition rate, but also for short-term cleaning occurrences, thus simulating a complex and chaotic time-evolution. The model is developed with the aid of a case-study, that of the fouling of a furnace, as detected by heat flux meters. Provided that an adequate amount of heat transfer measurements can be gathered, the procedure can be used to simulate the evolution of boiler heat absorption under realistic conditions of deposition. Applications include obviously new possibilities for automatic control of the equipment, as well as the optimization of operating set points to maximize thermal efficiency, such as the sequence and operation of on-load cleaning devices. It is thought that the method developed would be applicable to other instances of fouling or equipment degradation exhibiting similar behavior, specially with respect to on-line corrective measures.

Optimization of boiler cold-end and integration with the steam cycle in supercritical units

Abstract: In order to gain an extra increment of efficiency to compensate for capital costs, one of the main issues in the design of advanced supercritical power plants is the reduction of boiler exit gas temperature below typical values of conventional, subcritical units. Currently, the use of heat exchange surfaces made of plastic has become feasible, thereby avoiding corrosion and fouling problems derived from cold-end acid condensate. In this manner, flue gas temperature can be reduced down to typically 90 °C, which obviously leads to an increase of boiler efficiency. Besides, there is an additional energy available for heating the main condensate flow of the power cycle. If modification of air–gas rotary heaters is also considered, a manifold of possibilities opens up for plant optimization and integration of components. The objective of this paper is to analyze this class of schemes for increasing power output and net efficiency of a reference supercritical plant. A complete simulation of the steam cycle is assembled using Aspen Plus and different plant configurations are examined under reduced exit gas temperatures. Several uses of flue gas energy are considered, taking into account limits of temperature and realistic efficiencies of heat exchangers. Mass flow rates, point of extraction of condensate, pressures and temperatures are selected heuristically to optimize performance. Finally, required exchange areas are estimated, and a cost analysis is carried out in order to economically assess the new configurations and estimate the additional profit for the plant.

Numerical study of co-firing coal and Cynara cardunculus in a 350 MWe utility boiler

Abstract: In this work, a CFD numerical study of co-firing coal and cynara in a 350 MWe utility boiler is presented. The most influent operational factors related to the biomass feeding conditions such as biomass mean particle size, level of substitution of coal by biomass and feeding location in the furnace, are analyzed, determining their influence in the combustion process. Validation of the simulations is performed using measurements gathered at the plant. Results from the study show interesting conclusions for their implementation in the power plant, suggesting recommendable limits in the maximum biomass substitution level and particle size in order to keep a reasonable boiler efficiency, and pointing out the outstanding influence of the biomass injection location discussing thermal and fluid-dynamic implications and the possibility of introducing retrofitted or specific biomass burners.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Modeling of biomass gasification in fluidized bed

Abstract: Modeling of biomass gasification in bubbling and circulating fluidized bed (FB) gasifiers is reviewed. Approaches applied for reactor modeling, from black-box models to computational fluid-dynamic models, are described. Special attention is paid to comprehensive fluidization models, where semi-empirical correlations are used to simplify the fluid-dynamics. The conversion of single fuel particles, char, and gas is examined in detail. The most relevant phenomena to be considered in modeling of FB biomass gasifiers are outlined, and the need for further investigation is identified. An updated survey of published mathematical reactor models for biomass and waste gasification in FB is presented. The overall conclusion is that most of the FB biomass gasification models fit reasonably well experiments selected for validation, despite the various formulations and input data. However, there are few measurements available for comparison with detailed model results. Also, validation of models with data from full-scale FB biomass gasification units remains to be done.

An overview of the behaviour of biomass during combustion: Part I. Phase-mineral transformations of organic and inorganic matter

Abstract: An extended overview of the phase-mineral transformations of organic and inorganic matter that occur during biomass combustion was conducted. Some general considerations and particularly problems associated with the composition of biomass and biomass ash (BA) and behaviour of biomass during burning were discussed initially. Then, reference peer-reviewed data plus own investigations were used to organise and describe systematically the above topics. It was demonstrated that the phase composition of BA is polycomponent, heterogeneous and variable and includes: (1) mostly inorganic matter (IM) composed of non-crystalline (amorphous) and crystalline to semi-crystalline (mineral) constituents; (2) subordinately organic matter (OM) consisting of char and organic minerals; and (3) some fluid matter associated with both IM and OM. Approximately 291 phases or minerals were identified in BA. These species have primary, secondary or tertiary origin in the combustion residue and they are generated from natural (authigenic and detrital) and technogenic phases or minerals originally present in biomass. Afterwards, common issues related to the composition, occurrence, transformation and origin of common constituents in biomass and BA such as: (1) OM, namely cellulose, hemicellulose, lignin, char and other organic phases plus organic minerals; and (2) IM such as silicates, oxides and hydroxides, phosphates, sulphates (plus sulphides, sulphosalts, sulphites and thiosulphates), carbonates (plus bicarbonates), chlorides (plus chlorites and chlorates), nitrates, glass, amorphous (non-glass) material and other inorganic phases; were described and compared to coal ash. As a final point, a systematization of physico-chemical transformations during biomass combustion is given. It was found that the original OM and IM in biomass during combustion transform: (1) initially to devolatilization of OM and burning of combustible gases and char with formation of intermediate and less stable oxalates, nitrates, chlorides, hydroxides, carbonates, sulphates and inorganic amorphous (non-glass) material; (2) subsequently to more stable silicates, phosphates and oxides; (3) then to melting accompanied by dissolution of the refractory minerals; with increasing combustion temperatures in the system; and (4) followed by crystallisation of melt and formation of glass accompanied by some salt condensation and hydroxylation, hydration and carbonation of newly formed phases during cooling of BA. Finally, some post-combustion transformations of the newly formed minerals and phases to stable during weathering species among silicates, hydroxides, phosphates, sulphates, carbonates, chlorides and nitrates also occur due to their hydration, hydroxylation and carbonation by moisture and CO 2 in the air through storage of BA. Certain major associations related to the occurrence, content and origin of elements and phases were identified in the BA system and they include: (1) Si–Al–Fe–Na–Ti (mostly glass, silicates and oxyhydroxides); (2) Ca–Mg–Mn (commonly carbonates, oxyhydroxides, glass, silicates and some phosphates and sulphates); and (3) K–P–S–Cl (normally phosphates, sulphates, chlorides, glass and some silicates and carbonates). These associations were applied for classification of BAs to four types and six sub-types. It was found that such systematic relationships have a key importance in both fundamental and applied aspects related to innovative and sustainable processing of biomass and BA. The ash formation mechanisms and ash fusion behaviour, as well as some indications of potential technological problems and environmental risks during combustion of biomass types and sub-types and application of their BAs will be described in Part II of the present work.

Modeling the gas and particle flow inside cyclone separators

Abstract: This paper reviews the models developed for the flow field inside inverse-flow cyclone separators. In a first part, traditional algebraic models and their foundations are summarized in a unified manner, including the formulae for tangential velocity and pressure drop. The immediate application to the prediction of collection efficiency is also reviewed. The approach is the classical, treating first the dilute limit (clean-gas correlations), and afterwards correcting for “mass loading” effects. Although all these methods have had a remarkable success, more advanced ideas are needed to model cyclones. This is put forward by exploring the work done on the so-called “natural” length of the cyclone, that has led to the discovery of instability and secondary flows. The resort to computational fluid dynamics (CFD) in this case is difficult, however, due to the very nature of the flow structure. A closing section on the subject reviews past and recent CFD simulations of cyclones, both single- and two-phase, steady and unsteady, aiming at delineating the state-of-the-art, present limitations and perspectives of this field of research.

The effect of cyclone inlet dimensions on the flow pattern and performance

Abstract: The effect of the cyclone inlet dimensions on the performance and flow field pattern has been investigated computationally using the Reynolds stress turbulence model (RSM) for five cyclone separators. The results show that, the maximum tangential velocity in the cyclone decreases with increasing the cyclone inlet dimensions. No acceleration occurs in the cyclone space (the maximum tangential velocity is nearly constant throughout the cyclone). Increasing the cyclone inlet dimensions decreases the pressure drop. The cyclone cut-off diameter increases with increasing cyclone inlet dimension (consequently, the cyclone overall efficiency decreases due to weakness of the vortex strength). The effect of changing the inlet width is more significant than the inlet height especially for the cut-off diameter. The optimum ratio of inlet width to inlet height b/a is from 0.5 to 0.7.