Diego Alvarez - Academia.edu (original) (raw)

Papers by Diego Alvarez

17th International Conference on Fluidized Bed Combustion, 2003

ABSTRACT Increasing atmospheric concentration of CO2 and concern over its effect on climate is a ... more ABSTRACT Increasing atmospheric concentration of CO2 and concern over its effect on climate is a powerful driving force for the development of new advanced energy cycles incorporating CO2 capture. This paper investigates the feasibility of CO2 capture using the carbonation reaction of CaO “in situ” in a fluidised bed combustor, where natural gas or petroleum coke (or any other fuel with low ash content) is being burned. The sorbent can be partially regenerated for CO2 capture by combustion of part of the fuel with O2 /CO2 in a separate FBC. The thermodynamic limits in the proposed cycles, in terms of CO2 capture efficiencies, are examined along with the limits imposed by the rapid decay in the sorbent activity during repeated carbonation/calcination cycles, which will be exacerbated by the presence of S. Despite these limitations, it is shown that operating windows exist where it is possible to integrate fuel combustion, CO2 and SO2 capture in a single dual reactor facility. The decay in activity in the sorbent appears to be the major practical limitation to this concept, but this can be compensated for by using a relatively large supply of fresh sorbent, which appears to be acceptable considering the low cost of limestone. Also, a novel concept to reactivate the spent sorbent using sonic energy is outlined here as an alternative to reduce the use of fresh limestone.

2006 International Conference of the IEEE Engineering in Medicine and Biology Society, 2006

Wearable accelerometry provides easily portable systems that supply real-time data adequate for g... more Wearable accelerometry provides easily portable systems that supply real-time data adequate for gait analysis. When they do not provide direct measurement of a spatio-temporal parameter of interest, such as step length, it has to be estimated with a mathematical model from indirect sensor measurements. In this work we are concerned with the accelerometry-based estimation of the step length in straight line human walking. We compare five step length estimators. Measurements were taken from a group of four adult men, adding up a total of 800 m per individual of walking data. Also modifications to these estimators are proposed, based on biomechanical considerations. Results show that this modifications lead to improvements of interest over previous methods.

2012 IEEE International Instrumentation and Measurement Technology Conference Proceedings, 2012

We present a waist-worn personal navigation system based on inertial measurement units. The devic... more We present a waist-worn personal navigation system based on inertial measurement units. The device makes use of the human bipedal pattern to reduce position error. We describe improved algorithms, based on detailed description of the heel strike biomechanics and its translation to accelerations of the body waits, to estimate the periods of zero velocity, the step length, and the heading

Sensors, 2013

Hyperspectral imaging and analysis approaches offer accurate detection and quantification of fluo... more Hyperspectral imaging and analysis approaches offer accurate detection and quantification of fluorescently-labeled proteins and cells in highly autofluorescent tissues. However, selecting optimum acquisition settings for hyperspectral imaging is often a daunting task. In this study, we compared two hyperspectral systems-a widefield system with acoustic optical tunable filter (AOTF) and charge coupled device (CCD) camera, and a confocal system with diffraction gratings and photomultiplier tube (PMT) array. We measured the effects of system parameters on hyperspectral image quality and linear unmixing results. Parameters that were assessed for the confocal system included pinhole diameter, laser power, PMT gain and for the widefield system included arc lamp intensity, and camera gain. The signal-to-noise ratio (SNR) and the root-mean-square error (RMS error) were measured to assess system performance. Photobleaching dynamics were studied. Finally, theoretical sensitivity studies were performed to estimate the incremental

Sensors, 2012

In this paper we propose an approach for the estimation of the slope of the walking surface durin... more In this paper we propose an approach for the estimation of the slope of the walking surface during normal walking using a body-worn sensor composed of a biaxial accelerometer and a uniaxial gyroscope attached to the shank. It builds upon a state of the art technique that was successfully used to estimate the walking velocity from walking stride data, but did not work when used to estimate the slope of the walking surface. As claimed by the authors, the reason was that it did not take into account the actual inclination of the shank of the stance leg at the beginning of the stride (mid stance). In this paper, inspired by the biomechanical characteristics of human walking, we propose to solve this issue by using the accelerometer as a tilt sensor, assuming that at mid stance it is only measuring the gravity acceleration. Results from a set of experiments involving several users walking at different inclinations on a treadmill confirm the feasibility of our approach. A statistical analysis of slope estimations shows in first instance that the technique is capable of distinguishing the different slopes of the walking surface for every subject. It reports a global RMS error (per-unit difference between actual and estimated inclination of the walking surface for each stride identified in the experiments) of 0.05 and this can be reduced to 0.03 with subject-specific calibration and post processing procedures by means of averaging techniques.

Probabilistic Engineering Mechanics, 2010

Monte Carlo simulation is a general and robust method for structural reliability analysis, affect... more Monte Carlo simulation is a general and robust method for structural reliability analysis, affected by the serious efficiency problem consisting in the need of computing the limit state function a very large number of times. In order to reduce this computational effort the use of several kinds of solver surrogates has been proposed in the recent past. Proposals include the Response Surface Method (RSM), Neural Networks (NN), Support Vector Machines (SVM) and several other methods developed in the burgeoning field of Statistical Learning (SL). Many of these techniques can be employed either for function approximation (regression approach) or for pattern recognition (classification approach). This paper concerns the use of these devices for discriminating samples into safe and failure classes using the classification approach, because it constitutes the core of Monte Carlo simulation as applied to reliability analysis as such. Due to the flexibility of most SL methods, a critical step in their use is the generation of the learning population, as it affects the generalization capacity of the surrogate. To this end it is first demonstrated that the optimal population from the information viewpoint lies around in the vicinity of the limit state function. Next, an optimization method assuring a small as well as highly informative learning population is proposed on this basis. It consists in generating a small initial quasi-random population using Sobol sequence for triggering a Particle Swarm Optimization (PSO) performed over an iteration-dependent cost function defined in terms of the limit state function. The method is evaluated using SVM classifiers, but it can be readily applied also to other statistical classification techniques because the distinctive feature of the SVM, i.e. the margin band, is not actively used in the algorithm. The results show that the method yields results for the probability of failure that are in very close agreement with Monte Carlo simulation performed on the original limit state function and requiring a small number of learning samples.

Fuel Processing Technology, 2001

Ž This study reports on the reactivity of chars obtained at 10008C and 13008C within the range . ... more Ž This study reports on the reactivity of chars obtained at 10008C and 13008C within the range . of temperatures reached by coal particles in the near-burner zone of pulverised fuel boilers from three different coals. The coals were selected according to petrographic criteria: two of them are high volatile bituminous coals differing in maceral composition and the third one is a vitrinite-rich low volatile bituminous coal.

Fuel, 1997

This paper shows how the most relevant morphological parameters of chars can be accurately measur... more This paper shows how the most relevant morphological parameters of chars can be accurately measured with the aid of standard image analysis techniques. The descriptions of char particles thus obtained allow a more precise application of most of the classification schemes in use to date, or the production of useful data for the modelling of p.f. combustion. The application of these techniques is illustrated with a simplified scheme for the classification of char structures, with emphasis on the discrimination of the main structural types considered in current classification systems. Data obtained from the application of the proposed techniques to char samples from three coals of different rank and maceral composition clearly reflect the differences that could be expected from the plastic properties of the parent coals. The accuracy and objectivity of the measurements by these techniques make them a valuable tool for improving the repeatability and reproducibility of char petrographic analyses.

Energy & Fuels, 2005

The use of carbonation/calcination cycles of CaO/CaCO 3 is emerging as a viable technique for the... more The use of carbonation/calcination cycles of CaO/CaCO 3 is emerging as a viable technique for the capture of CO 2 generated in the combustion of coals for power generation. Specifically, the choice of natural limestones as CO 2 carriers is an attractive option because they are cheap and abundant materials, although previous studies indicate that the reactivity of the calcines toward CO 2 rapidly drops with cycling. This paper reports on the effects of the internal morphology of CaO particles on their capability of absorbing CO 2 . Calcines of a natural limestone with different initial textures were repeatedly submitted to carbonation/calcination conditions (up to 100 cycles). The textural evolution, as well as the carbonation conversion, of the calcined and recarbonated samples was followed along the experiments. In addition to the known mechanisms of deactivation due to grain growth and limited diffusion of CO 2 through the product layer, we have found that pore closure is also taking place in our samples, together with an overall shrinkage of the particle. All of these factors play a role in limiting the maximum carbonation conversions to around 10% after just 100 cycles.

Energy & Fuels, 2003

The use of calcines of natural limestones as CO 2 regenerable sorbents is investigated in this wo... more The use of calcines of natural limestones as CO 2 regenerable sorbents is investigated in this work by studying the decay of the maximum carbonation conversion during many carbonation/ calcination cycles. New experimental information is complemented with a compilation of previously published data on this subject. The observed conversion limits in the reaction of CO 2 with lime are interpreted in terms of a certain loss in the porosity associated with small pores and a certain increase in the porosity associated with large pores. In the carbonation part of every cycle, the CaCO 3 fills up all the available porosity made up of small pores plus a small fraction of the large voids, limited by the thickness of the product layer that marks the onset of the slow carbonation rate. A simple model based on textural changes, observed by scanning electron microscopy, fits equally well all the data from this work and from other authors. The two model parameters are consistent with known mechanism occurring during calcination and carbonation.

Fuel, 2003

The phase-mineral and chemical composition of feed coals and their fly ashes (FAs) produced in fo... more The phase-mineral and chemical composition of feed coals and their fly ashes (FAs) produced in four large Spanish thermo-electric power stations was characterized as a basis for multicomponent FA utilization. The feed fuels used are bituminous coals, semi-anthracites and anthracites with high detrital mineral abundance and mixed carbonate and sulphide–sulphate authigenic mineral tendency. Their mineral composition includes quartz, kaolinite, illite–muscovite, pyrite, chlorite, plagioclase, K-feldspar, gypsum, siderite, calcite, dolomite, marcasite, montmorillonite, jarosite, and ankerite. The FAs studied have aluminosilicate composition with higher concentrations of alkaline and alkaline-earth oxides than Fe oxide. Elements such as Ag, As, Ba, Cr, Cs, Li, P, Sb, Sc, Sn, Sr, Ti, V, Zn, and Zr are relatively enriched in these FAs in comparison with the respective mean values for bituminous coal ashes worldwide. The FAs consist basically of aluminosilicate glass, to a lesser extent of mineral matter (with high silicate abundance and dominant oxide tendency) and moderate char occurrence. The phase-mineral composition (in decreasing order of significance) of these FAs is normally glass, mullite, quartz, char, kaolinite–metakaolinite, hematite, cristobalite, plagioclase, K-feldspar, melilite, anhydrite, wollastonite, magnetite and corundum plus 42 important accessory minerals or phases. A scheme of conventional separation procedures was applied to recover sequentially six initial and potentially useful and/or hazardous products from FAs, namely: (1) a ceramic cenosphere concentrate; (2) a water-soluble salt concentrate; (3) a magnetic concentrate; (4) a char concentrate; (5) a heavy concentrate; and finally (6) an improved FA residue.

Journal of Structural Engineering, 2003

The assessment of the reliability of structural systems is increasingly being estimated with rega... more The assessment of the reliability of structural systems is increasingly being estimated with regard to the spatial fluctuation of the mechanical properties as well as loads. This leads to a detailed probabilistic modeling known as stochastic finite elements ͑SFE͒. In this paper an approach that departs from the main stream of methods for the reliability analysis of SFE models is proposed. The difference lies in that the reliability problem is treated as a classification task and not as the computation of an integral. To this purpose use is made of a kernel method for classification, which is the object of intensive research in pattern recognition, image analysis, and other fields. A greedy sequential procedure requiring a minimal number of limit state evaluations is developed. The algorithm is based on the key concept of support vectors, which guarantee that only the points closest to the decision rule need to be evaluated. The numerical examples show that this algorithm allows obtaining a highly accurate approximation of the failure probability of SFE models with a minimal number of calls of the finite element solver and also a fast computation.

Industrial & Engineering Chemistry Research, 2005

Calcium oxide can be an effective CO 2 sorbent at high temperatures. When coupled with a calcinat... more Calcium oxide can be an effective CO 2 sorbent at high temperatures. When coupled with a calcination step to produce pure CO 2 , the carbonation reaction is the basis for several hightemperature separation systems of CO 2 . The formation of a product layer of CaCO 3 is known to mark a sudden change in the reaction regime, from a very fast CO 2 uptake to very slow carbonation rates. The critical thickness of this product layer of CaCO 3 has been measured in this work on real sorbent materials, using different limestone precursors and submitting them to many repeated carbonation calcination cycles (up to 100). Mercury porosimetry curves of the calcines and their carbonated counterparts have been obtained and their differences interpreted with a simple pore model, from which the thickness of the product layer is derived. An average value of 49 nm ((19% standard deviation) has been obtained, which is quite insensitive to the type of limestone and to the texture of the calcine as long as the model is fulfilled. The implications of this value on our understanding of the sorbent performance in these CO 2 -capture systems are discussed.

Aiche Journal, 2004

Experiments in a pilot-scale fluidized-bed reactor have been carried out to investigate the carbo... more Experiments in a pilot-scale fluidized-bed reactor have been carried out to investigate the carbonation reaction of CaO, as a potential method for CO2 capture from combustion flue gases at high-temperatures. Results show that CO2 capture efficiencies are very high, while there is a sufficient fraction of CaO in the bed reacting in the fast reaction regime. The total capture capacity of the bed decays with the number of carbonation-calcination cycles. The experimental CO2 concentration profiles measured inside the bed during the fast reaction period are interpreted with the KL fluid bed model, by supplying information on sorbent deactivation from laboratory tests. It is concluded that a fluidized bed of CaO can be a suitable reactor to achieve very effective CO2 capture efficiencies from a combustion flue gas. © 2004 American Institute of Chemical Engineers AIChE J, 50:1614–1622, 2004

Industrial & Engineering Chemistry Research, 2005

Calcium oxide can be an effective CO 2 sorbent at high temperatures. When coupled with a calcinat... more Calcium oxide can be an effective CO 2 sorbent at high temperatures. When coupled with a calcination step to produce pure CO 2 , the carbonation reaction is the basis for several hightemperature separation systems of CO 2 . The formation of a product layer of CaCO 3 is known to mark a sudden change in the reaction regime, from a very fast CO 2 uptake to very slow carbonation rates. The critical thickness of this product layer of CaCO 3 has been measured in this work on real sorbent materials, using different limestone precursors and submitting them to many repeated carbonation calcination cycles (up to 100). Mercury porosimetry curves of the calcines and their carbonated counterparts have been obtained and their differences interpreted with a simple pore model, from which the thickness of the product layer is derived. An average value of 49 nm ((19% standard deviation) has been obtained, which is quite insensitive to the type of limestone and to the texture of the calcine as long as the model is fulfilled. The implications of this value on our understanding of the sorbent performance in these CO 2 -capture systems are discussed.

Gait & Posture, 2010

In this paper we report on a novel algorithm for the real-time detection and timing of initial (I... more In this paper we report on a novel algorithm for the real-time detection and timing of initial (IC) and final contact (FC) gait events. We process the vertical and antero-posterior accelerations registered at the lower trunk (L3 vertebra). The algorithm is based on a set of heuristic rules extracted from a set of 1719 steps. An independent experiment was conducted to compare the results of our algorithms with those obtained from a Digimax force platform. The results show small deviations from times of occurrence of events recorded from the platform (13 AE 35 ms for IC and 9 AE 54 ms for FC). Results for the FC timing are especially relevant in this field, as no previous work has addressed its temporal location through the processing of lower trunk accelerations. The delay in the real-time detection of the IC is 117 AE 39 ms and 34 AE 72 ms for the FC, improving previously reported results for real-time detection of events from lower trunk accelerations. ß

Fuel, 1997

This paper shows how the most relevant morphological parameters of chars can be accurately measur... more This paper shows how the most relevant morphological parameters of chars can be accurately measured with the aid of standard image analysis techniques. The descriptions of char particles thus obtained allow a more precise application of most of the classification schemes in use to date, or the production of useful data for the modelling of p.f. combustion. The application of these techniques is illustrated with a simplified scheme for the classification of char structures, with emphasis on the discrimination of the main structural types considered in current classification systems. Data obtained from the application of the proposed techniques to char samples from three coals of different rank and maceral composition clearly reflect the differences that could be expected from the plastic properties of the parent coals. The accuracy and objectivity of the measurements by these techniques make them a valuable tool for improving the repeatability and reproducibility of char petrographic analyses.

Energy & Fuels, 2005

The use of carbonation/calcination cycles of CaO/CaCO 3 is emerging as a viable technique for the... more The use of carbonation/calcination cycles of CaO/CaCO 3 is emerging as a viable technique for the capture of CO 2 generated in the combustion of coals for power generation. Specifically, the choice of natural limestones as CO 2 carriers is an attractive option because they are cheap and abundant materials, although previous studies indicate that the reactivity of the calcines toward CO 2 rapidly drops with cycling. This paper reports on the effects of the internal morphology of CaO particles on their capability of absorbing CO 2 . Calcines of a natural limestone with different initial textures were repeatedly submitted to carbonation/calcination conditions (up to 100 cycles). The textural evolution, as well as the carbonation conversion, of the calcined and recarbonated samples was followed along the experiments. In addition to the known mechanisms of deactivation due to grain growth and limited diffusion of CO 2 through the product layer, we have found that pore closure is also taking place in our samples, together with an overall shrinkage of the particle. All of these factors play a role in limiting the maximum carbonation conversions to around 10% after just 100 cycles.

Energy & Fuels, 2003

This study shows the utility of optical microscopy to trace the origin of unburned carbon in fly ... more This study shows the utility of optical microscopy to trace the origin of unburned carbon in fly ashes from coal blends. Coal blends currently burned in four power groups in Spain have been investigated. Mill samples were separated into size fractions, and petrographic ...

17th International Conference on Fluidized Bed Combustion, 2003

ABSTRACT Increasing atmospheric concentration of CO2 and concern over its effect on climate is a ... more ABSTRACT Increasing atmospheric concentration of CO2 and concern over its effect on climate is a powerful driving force for the development of new advanced energy cycles incorporating CO2 capture. This paper investigates the feasibility of CO2 capture using the carbonation reaction of CaO “in situ” in a fluidised bed combustor, where natural gas or petroleum coke (or any other fuel with low ash content) is being burned. The sorbent can be partially regenerated for CO2 capture by combustion of part of the fuel with O2 /CO2 in a separate FBC. The thermodynamic limits in the proposed cycles, in terms of CO2 capture efficiencies, are examined along with the limits imposed by the rapid decay in the sorbent activity during repeated carbonation/calcination cycles, which will be exacerbated by the presence of S. Despite these limitations, it is shown that operating windows exist where it is possible to integrate fuel combustion, CO2 and SO2 capture in a single dual reactor facility. The decay in activity in the sorbent appears to be the major practical limitation to this concept, but this can be compensated for by using a relatively large supply of fresh sorbent, which appears to be acceptable considering the low cost of limestone. Also, a novel concept to reactivate the spent sorbent using sonic energy is outlined here as an alternative to reduce the use of fresh limestone.

2006 International Conference of the IEEE Engineering in Medicine and Biology Society, 2006

Wearable accelerometry provides easily portable systems that supply real-time data adequate for g... more Wearable accelerometry provides easily portable systems that supply real-time data adequate for gait analysis. When they do not provide direct measurement of a spatio-temporal parameter of interest, such as step length, it has to be estimated with a mathematical model from indirect sensor measurements. In this work we are concerned with the accelerometry-based estimation of the step length in straight line human walking. We compare five step length estimators. Measurements were taken from a group of four adult men, adding up a total of 800 m per individual of walking data. Also modifications to these estimators are proposed, based on biomechanical considerations. Results show that this modifications lead to improvements of interest over previous methods.

2012 IEEE International Instrumentation and Measurement Technology Conference Proceedings, 2012

We present a waist-worn personal navigation system based on inertial measurement units. The devic... more We present a waist-worn personal navigation system based on inertial measurement units. The device makes use of the human bipedal pattern to reduce position error. We describe improved algorithms, based on detailed description of the heel strike biomechanics and its translation to accelerations of the body waits, to estimate the periods of zero velocity, the step length, and the heading

Sensors, 2013

Hyperspectral imaging and analysis approaches offer accurate detection and quantification of fluo... more Hyperspectral imaging and analysis approaches offer accurate detection and quantification of fluorescently-labeled proteins and cells in highly autofluorescent tissues. However, selecting optimum acquisition settings for hyperspectral imaging is often a daunting task. In this study, we compared two hyperspectral systems-a widefield system with acoustic optical tunable filter (AOTF) and charge coupled device (CCD) camera, and a confocal system with diffraction gratings and photomultiplier tube (PMT) array. We measured the effects of system parameters on hyperspectral image quality and linear unmixing results. Parameters that were assessed for the confocal system included pinhole diameter, laser power, PMT gain and for the widefield system included arc lamp intensity, and camera gain. The signal-to-noise ratio (SNR) and the root-mean-square error (RMS error) were measured to assess system performance. Photobleaching dynamics were studied. Finally, theoretical sensitivity studies were performed to estimate the incremental

Sensors, 2012

In this paper we propose an approach for the estimation of the slope of the walking surface durin... more In this paper we propose an approach for the estimation of the slope of the walking surface during normal walking using a body-worn sensor composed of a biaxial accelerometer and a uniaxial gyroscope attached to the shank. It builds upon a state of the art technique that was successfully used to estimate the walking velocity from walking stride data, but did not work when used to estimate the slope of the walking surface. As claimed by the authors, the reason was that it did not take into account the actual inclination of the shank of the stance leg at the beginning of the stride (mid stance). In this paper, inspired by the biomechanical characteristics of human walking, we propose to solve this issue by using the accelerometer as a tilt sensor, assuming that at mid stance it is only measuring the gravity acceleration. Results from a set of experiments involving several users walking at different inclinations on a treadmill confirm the feasibility of our approach. A statistical analysis of slope estimations shows in first instance that the technique is capable of distinguishing the different slopes of the walking surface for every subject. It reports a global RMS error (per-unit difference between actual and estimated inclination of the walking surface for each stride identified in the experiments) of 0.05 and this can be reduced to 0.03 with subject-specific calibration and post processing procedures by means of averaging techniques.

Probabilistic Engineering Mechanics, 2010

Monte Carlo simulation is a general and robust method for structural reliability analysis, affect... more Monte Carlo simulation is a general and robust method for structural reliability analysis, affected by the serious efficiency problem consisting in the need of computing the limit state function a very large number of times. In order to reduce this computational effort the use of several kinds of solver surrogates has been proposed in the recent past. Proposals include the Response Surface Method (RSM), Neural Networks (NN), Support Vector Machines (SVM) and several other methods developed in the burgeoning field of Statistical Learning (SL). Many of these techniques can be employed either for function approximation (regression approach) or for pattern recognition (classification approach). This paper concerns the use of these devices for discriminating samples into safe and failure classes using the classification approach, because it constitutes the core of Monte Carlo simulation as applied to reliability analysis as such. Due to the flexibility of most SL methods, a critical step in their use is the generation of the learning population, as it affects the generalization capacity of the surrogate. To this end it is first demonstrated that the optimal population from the information viewpoint lies around in the vicinity of the limit state function. Next, an optimization method assuring a small as well as highly informative learning population is proposed on this basis. It consists in generating a small initial quasi-random population using Sobol sequence for triggering a Particle Swarm Optimization (PSO) performed over an iteration-dependent cost function defined in terms of the limit state function. The method is evaluated using SVM classifiers, but it can be readily applied also to other statistical classification techniques because the distinctive feature of the SVM, i.e. the margin band, is not actively used in the algorithm. The results show that the method yields results for the probability of failure that are in very close agreement with Monte Carlo simulation performed on the original limit state function and requiring a small number of learning samples.

Fuel Processing Technology, 2001

Ž This study reports on the reactivity of chars obtained at 10008C and 13008C within the range . ... more Ž This study reports on the reactivity of chars obtained at 10008C and 13008C within the range . of temperatures reached by coal particles in the near-burner zone of pulverised fuel boilers from three different coals. The coals were selected according to petrographic criteria: two of them are high volatile bituminous coals differing in maceral composition and the third one is a vitrinite-rich low volatile bituminous coal.

Fuel, 1997

This paper shows how the most relevant morphological parameters of chars can be accurately measur... more This paper shows how the most relevant morphological parameters of chars can be accurately measured with the aid of standard image analysis techniques. The descriptions of char particles thus obtained allow a more precise application of most of the classification schemes in use to date, or the production of useful data for the modelling of p.f. combustion. The application of these techniques is illustrated with a simplified scheme for the classification of char structures, with emphasis on the discrimination of the main structural types considered in current classification systems. Data obtained from the application of the proposed techniques to char samples from three coals of different rank and maceral composition clearly reflect the differences that could be expected from the plastic properties of the parent coals. The accuracy and objectivity of the measurements by these techniques make them a valuable tool for improving the repeatability and reproducibility of char petrographic analyses.

Energy & Fuels, 2005

The use of carbonation/calcination cycles of CaO/CaCO 3 is emerging as a viable technique for the... more The use of carbonation/calcination cycles of CaO/CaCO 3 is emerging as a viable technique for the capture of CO 2 generated in the combustion of coals for power generation. Specifically, the choice of natural limestones as CO 2 carriers is an attractive option because they are cheap and abundant materials, although previous studies indicate that the reactivity of the calcines toward CO 2 rapidly drops with cycling. This paper reports on the effects of the internal morphology of CaO particles on their capability of absorbing CO 2 . Calcines of a natural limestone with different initial textures were repeatedly submitted to carbonation/calcination conditions (up to 100 cycles). The textural evolution, as well as the carbonation conversion, of the calcined and recarbonated samples was followed along the experiments. In addition to the known mechanisms of deactivation due to grain growth and limited diffusion of CO 2 through the product layer, we have found that pore closure is also taking place in our samples, together with an overall shrinkage of the particle. All of these factors play a role in limiting the maximum carbonation conversions to around 10% after just 100 cycles.

Energy & Fuels, 2003

The use of calcines of natural limestones as CO 2 regenerable sorbents is investigated in this wo... more The use of calcines of natural limestones as CO 2 regenerable sorbents is investigated in this work by studying the decay of the maximum carbonation conversion during many carbonation/ calcination cycles. New experimental information is complemented with a compilation of previously published data on this subject. The observed conversion limits in the reaction of CO 2 with lime are interpreted in terms of a certain loss in the porosity associated with small pores and a certain increase in the porosity associated with large pores. In the carbonation part of every cycle, the CaCO 3 fills up all the available porosity made up of small pores plus a small fraction of the large voids, limited by the thickness of the product layer that marks the onset of the slow carbonation rate. A simple model based on textural changes, observed by scanning electron microscopy, fits equally well all the data from this work and from other authors. The two model parameters are consistent with known mechanism occurring during calcination and carbonation.

Fuel, 2003

The phase-mineral and chemical composition of feed coals and their fly ashes (FAs) produced in fo... more The phase-mineral and chemical composition of feed coals and their fly ashes (FAs) produced in four large Spanish thermo-electric power stations was characterized as a basis for multicomponent FA utilization. The feed fuels used are bituminous coals, semi-anthracites and anthracites with high detrital mineral abundance and mixed carbonate and sulphide–sulphate authigenic mineral tendency. Their mineral composition includes quartz, kaolinite, illite–muscovite, pyrite, chlorite, plagioclase, K-feldspar, gypsum, siderite, calcite, dolomite, marcasite, montmorillonite, jarosite, and ankerite. The FAs studied have aluminosilicate composition with higher concentrations of alkaline and alkaline-earth oxides than Fe oxide. Elements such as Ag, As, Ba, Cr, Cs, Li, P, Sb, Sc, Sn, Sr, Ti, V, Zn, and Zr are relatively enriched in these FAs in comparison with the respective mean values for bituminous coal ashes worldwide. The FAs consist basically of aluminosilicate glass, to a lesser extent of mineral matter (with high silicate abundance and dominant oxide tendency) and moderate char occurrence. The phase-mineral composition (in decreasing order of significance) of these FAs is normally glass, mullite, quartz, char, kaolinite–metakaolinite, hematite, cristobalite, plagioclase, K-feldspar, melilite, anhydrite, wollastonite, magnetite and corundum plus 42 important accessory minerals or phases. A scheme of conventional separation procedures was applied to recover sequentially six initial and potentially useful and/or hazardous products from FAs, namely: (1) a ceramic cenosphere concentrate; (2) a water-soluble salt concentrate; (3) a magnetic concentrate; (4) a char concentrate; (5) a heavy concentrate; and finally (6) an improved FA residue.

Journal of Structural Engineering, 2003

The assessment of the reliability of structural systems is increasingly being estimated with rega... more The assessment of the reliability of structural systems is increasingly being estimated with regard to the spatial fluctuation of the mechanical properties as well as loads. This leads to a detailed probabilistic modeling known as stochastic finite elements ͑SFE͒. In this paper an approach that departs from the main stream of methods for the reliability analysis of SFE models is proposed. The difference lies in that the reliability problem is treated as a classification task and not as the computation of an integral. To this purpose use is made of a kernel method for classification, which is the object of intensive research in pattern recognition, image analysis, and other fields. A greedy sequential procedure requiring a minimal number of limit state evaluations is developed. The algorithm is based on the key concept of support vectors, which guarantee that only the points closest to the decision rule need to be evaluated. The numerical examples show that this algorithm allows obtaining a highly accurate approximation of the failure probability of SFE models with a minimal number of calls of the finite element solver and also a fast computation.

Industrial & Engineering Chemistry Research, 2005

Calcium oxide can be an effective CO 2 sorbent at high temperatures. When coupled with a calcinat... more Calcium oxide can be an effective CO 2 sorbent at high temperatures. When coupled with a calcination step to produce pure CO 2 , the carbonation reaction is the basis for several hightemperature separation systems of CO 2 . The formation of a product layer of CaCO 3 is known to mark a sudden change in the reaction regime, from a very fast CO 2 uptake to very slow carbonation rates. The critical thickness of this product layer of CaCO 3 has been measured in this work on real sorbent materials, using different limestone precursors and submitting them to many repeated carbonation calcination cycles (up to 100). Mercury porosimetry curves of the calcines and their carbonated counterparts have been obtained and their differences interpreted with a simple pore model, from which the thickness of the product layer is derived. An average value of 49 nm ((19% standard deviation) has been obtained, which is quite insensitive to the type of limestone and to the texture of the calcine as long as the model is fulfilled. The implications of this value on our understanding of the sorbent performance in these CO 2 -capture systems are discussed.

Aiche Journal, 2004

Experiments in a pilot-scale fluidized-bed reactor have been carried out to investigate the carbo... more Experiments in a pilot-scale fluidized-bed reactor have been carried out to investigate the carbonation reaction of CaO, as a potential method for CO2 capture from combustion flue gases at high-temperatures. Results show that CO2 capture efficiencies are very high, while there is a sufficient fraction of CaO in the bed reacting in the fast reaction regime. The total capture capacity of the bed decays with the number of carbonation-calcination cycles. The experimental CO2 concentration profiles measured inside the bed during the fast reaction period are interpreted with the KL fluid bed model, by supplying information on sorbent deactivation from laboratory tests. It is concluded that a fluidized bed of CaO can be a suitable reactor to achieve very effective CO2 capture efficiencies from a combustion flue gas. © 2004 American Institute of Chemical Engineers AIChE J, 50:1614–1622, 2004

Industrial & Engineering Chemistry Research, 2005

Calcium oxide can be an effective CO 2 sorbent at high temperatures. When coupled with a calcinat... more Calcium oxide can be an effective CO 2 sorbent at high temperatures. When coupled with a calcination step to produce pure CO 2 , the carbonation reaction is the basis for several hightemperature separation systems of CO 2 . The formation of a product layer of CaCO 3 is known to mark a sudden change in the reaction regime, from a very fast CO 2 uptake to very slow carbonation rates. The critical thickness of this product layer of CaCO 3 has been measured in this work on real sorbent materials, using different limestone precursors and submitting them to many repeated carbonation calcination cycles (up to 100). Mercury porosimetry curves of the calcines and their carbonated counterparts have been obtained and their differences interpreted with a simple pore model, from which the thickness of the product layer is derived. An average value of 49 nm ((19% standard deviation) has been obtained, which is quite insensitive to the type of limestone and to the texture of the calcine as long as the model is fulfilled. The implications of this value on our understanding of the sorbent performance in these CO 2 -capture systems are discussed.

Gait & Posture, 2010

In this paper we report on a novel algorithm for the real-time detection and timing of initial (I... more In this paper we report on a novel algorithm for the real-time detection and timing of initial (IC) and final contact (FC) gait events. We process the vertical and antero-posterior accelerations registered at the lower trunk (L3 vertebra). The algorithm is based on a set of heuristic rules extracted from a set of 1719 steps. An independent experiment was conducted to compare the results of our algorithms with those obtained from a Digimax force platform. The results show small deviations from times of occurrence of events recorded from the platform (13 AE 35 ms for IC and 9 AE 54 ms for FC). Results for the FC timing are especially relevant in this field, as no previous work has addressed its temporal location through the processing of lower trunk accelerations. The delay in the real-time detection of the IC is 117 AE 39 ms and 34 AE 72 ms for the FC, improving previously reported results for real-time detection of events from lower trunk accelerations. ß

Fuel, 1997

This paper shows how the most relevant morphological parameters of chars can be accurately measur... more This paper shows how the most relevant morphological parameters of chars can be accurately measured with the aid of standard image analysis techniques. The descriptions of char particles thus obtained allow a more precise application of most of the classification schemes in use to date, or the production of useful data for the modelling of p.f. combustion. The application of these techniques is illustrated with a simplified scheme for the classification of char structures, with emphasis on the discrimination of the main structural types considered in current classification systems. Data obtained from the application of the proposed techniques to char samples from three coals of different rank and maceral composition clearly reflect the differences that could be expected from the plastic properties of the parent coals. The accuracy and objectivity of the measurements by these techniques make them a valuable tool for improving the repeatability and reproducibility of char petrographic analyses.

Energy & Fuels, 2005

The use of carbonation/calcination cycles of CaO/CaCO 3 is emerging as a viable technique for the... more The use of carbonation/calcination cycles of CaO/CaCO 3 is emerging as a viable technique for the capture of CO 2 generated in the combustion of coals for power generation. Specifically, the choice of natural limestones as CO 2 carriers is an attractive option because they are cheap and abundant materials, although previous studies indicate that the reactivity of the calcines toward CO 2 rapidly drops with cycling. This paper reports on the effects of the internal morphology of CaO particles on their capability of absorbing CO 2 . Calcines of a natural limestone with different initial textures were repeatedly submitted to carbonation/calcination conditions (up to 100 cycles). The textural evolution, as well as the carbonation conversion, of the calcined and recarbonated samples was followed along the experiments. In addition to the known mechanisms of deactivation due to grain growth and limited diffusion of CO 2 through the product layer, we have found that pore closure is also taking place in our samples, together with an overall shrinkage of the particle. All of these factors play a role in limiting the maximum carbonation conversions to around 10% after just 100 cycles.

Energy & Fuels, 2003

This study shows the utility of optical microscopy to trace the origin of unburned carbon in fly ... more This study shows the utility of optical microscopy to trace the origin of unburned carbon in fly ashes from coal blends. Coal blends currently burned in four power groups in Spain have been investigated. Mill samples were separated into size fractions, and petrographic ...