Javier Ballester

Modelling the Temporal Evolution of a Reduced Combustion Chemical System With an Artificial Neural Network

The present work introduces a way of embedding a combustion chemical system in a neural network, in such a way that it can be used, with considerable CPU time and RAM memory savings, in fluid-flow-simulation codes. The system is composed of four neural networks, with three of them simulating the evolution of the reactive species and one providing density and temperature as a function of composition. The performance in terms of accuracy of the networks is assessed by comparison with the results of the direct integration of the thermochemical system for a large number of random samples. Error measurements are reported, and sample evolutions of the chemical system with both methods are compared. It can be summarized that the results of this exercise are satisfactory, and the CPU-time and memory savings encouraging.

Combustion characteristics of heavy oil-water emulsions

The combustion of heavy oil and its emulsions with water was investigated in experiments on a semi-industrial scale. Two comparisons between heavy oil and oil-water emulsion flames are presented that, due to the different initial conditions of the spray, provide complementary information. Reported results include spatial distributions in the flame of temperature and species concentrations (O2, CO, UHC, NOx) as well as gaseous and solid emissions in the flue gases. The measurements inside the emulsion flame display a remarkable improvement in the combustion process with respect to that of the neat oil with poor atomization; differences are much less important if a fine spray is achieved with the heavy oil. Solid emissions are significantly reduced in the emulsion tests and the morphology of the particle samples demonstrates the fragmentation of the drops and/or the coke particles initially formed. The flame temperatures are reduced by ∼65 K. The heat absorbed by the water injected in the emulsion and enhanced radiative heat transfer due to the higher particle number density could explain this difference. The spatial distribution of NOx indicates that a significant reduction is obtained in the final part of the flame; this may be attributed to a decrease in the rate of thermal-NO formation as a consequence of lower gas temperatures. No measurable difference in NOx concentration is found in the inner core of the flames.

Particulate matter formation and emission in the combustion of different pulverized biomass fuels

ABSTRACT Particle formation and emission in the combustion of four (orujillo, eucalyptus, oak and chestnut tree) pulverized biomass fuels have been studied. The fuels have been burned in an entrained flow reactor, under controlled and realistic conditions. In all the cases, the final emission distributions contained at least two modes, one of them peaking at 30–200 nm, and the other being in the supermicron range. Alkali sulphates and chlorides account for most of the mass of the fine particles in all cases, while coarse particles essentially retain the inorganic matter properties of the original fuel. K2SO4 is experimentally found to start nucleation over 900°C for orujillo, while KCl is not observed at this temperature. Condensation of KCl on these nuclei is observed in a sample taken at 560°C and in a greater amount at 360°C. The same formation schema was found for the other biomass fuels; the much lower ash content of the latter is thought to cause a “delay” in the onset of these nucleation/condensation steps along the gas cooling process. In spite of these differences, the global similarities observed among the studied fuels confirm the generality of the fine particle formation process in biomass combustion.

Formation and Emission of Submicron Particles in Pulverized Olive Residue (Orujillo) Combustion

The particle formation and emission in the combustion of pulverized olive residue (orujillo) is studied in this work. The fuel has been burned in controlled combustion conditions in an entrained flow reactor. A bimodal distribution with mode peaks at 155 nm and 110 μm is found for fly ashes. Coarse particles have been characterized by laser diffractometry and SEM, while fines have been analyzed by low-pressure impaction, DMA, SEM, and X-ray diffraction. Particles with Dp < 1 μm are composed of only K2SO4 and KCl in the same mass proportion, and possibly K3PO4 (less than 7% in mass). The use of a new particle sampling probe and a TEM allowed a detailed study of the formation of these particles when flue gases cool down. K2SO4 is experimentally found to start nucleation over 900°C, while KCl is not observed at this temperature. Condensation of KCl on these nuclei is observed in a sample taken at 560°C. These “formation steps” are in good agreement with both theoretical calculations by other authors and a simple equilibrium schema shown here.

A comparative study of different methods for the sampling of high temperature combustion aerosols

Three different probes, respectively based on aerodynamic quenching, N2 dilution, and thermophoresis, were used for particle sampling in biomass postcombustion gases from a laboratory reactor at different temperatures (from 1300 to 560°C). The different artifacts caused by the use of each probe were identified and compared. While the classical dilution sampling resulted in the formation (inside the probe) of particles comparable in size and composition to those already existing in the sampled gases, the aerodynamic quenching particle sampling (AQPS) probe always caused the nucleation of vapors into very small (15–20 nm) particles. The difference is attributed to the much higher cooling rate (∼ 108 K/s) at the entrance of the probe. The preliminary results of the thermophoretic probe also reveal significant deviations between the collected sample and the actual particle population in the sampled gas. In general, the results shown here highlight the difficulties in obtaining representative samples in this type of systems, as well as the existence of a variety of possible sampling artifacts, whose interpretation is not straightforward. The AQPS probe is found to provide better results than the other methods considered, and its use might enable the study of both the condensed and vapor phase in hot gases containing important amounts of condensable inorganic vapours.

Investigation of low-NOx strategies for natural gas combustion

Abstract Different NOx control techniques have been investigated and implemented in a natural gas burner at two different scales. Both fuel- and air-staging strategies can be implemented in a single burner, by properly adjusting some of the available settings. Experiments were performed on a semi-industrial scale (thermal input 0.3–0.33 MW) in a laboratory furnace. The study included a thorough parametric analysis to identify the optimum operating conditions for minimum NOx emission. In addition, some detailed in-flame measurements provided valuable information for the understanding of the relevant phenomena. In a second stage, a prototype of the burner was tested in a boiler (6.5–17 MW(t)) to check the validity of the conclusions obtained on the smaller scale. The tests demonstrated that the new burner can reduce NOx emissions by > 70% with respect to the values obtained with the burner originally installed in the same boiler. Comparison between the measurements performed on both scales also provided valuable information regarding the effects of scaling on the performance of low-NOx burners. A scaling law intermediate between the constant-velocity and residence-time criteria was selected in one of the test series, leading to a remarkable similarity of the results obtained on the two scales, in terms of both NOx emissions and qualitative burner performance.

Experimental Study of the Influence of Atomization Characteristics on the Combustion of Heavy Oil

Abstract The influence of the atomization characteristics on the combustion of heavy oil is investigated. Combustion tests are conducted in an experimental furnace with a thermal input of 0.33 MW. The characteristics of the spray produced by a simplex pressure-swirl nozzle are known in detail from a previous investigation in an atomization rig. Two cases are selected and tested under combusting conditions, while keeping constant the remaining experimental parameters. Reported results include extensive measurements of (he spatial distribution of the velocity field, temperalure and species (O2, CO, CO2, UHC and NOx inside the flame, as well as the emission of pollutants in the flue gases. The differences in the spray lead to changes in the fuel vapor distribution inside the flame. The consequences are important variations of the flame characteriscics and of the emission of gaseous and solid pollutants.

Low-cost thermal Σ-Δ air flow sensor

This paper describes the design and optimization of a hot-wire air flowmeter. A low-cost design of the packaging allows good thermal contact with the airflow, as well as good thermal isolation between the hot and cold points. It is a compact solution which allows easy PCB mounting and adaptation to standard-size small air pipes. The design has been optimized for low-cost applications. The sensor is read out by a thermal sigma-delta modulator. The dynamic range of this modulator has been extended by adding a constant power offset to its output. The fractal nature of the modulator response at low-clock frequencies is also experimentally shown.This paper describes the design and optimization of a hot-wire air flowmeter. A low-cost design of the packaging allows good thermal contact with the airflow, as well as good thermal isolation between the hot and cold points. It is a compact solution which allows easy PCB mounting and adaptation to standard-size small air pipes. The design has been optimized for low-cost applications. The sensor is read out by a thermal sigma-delta modulator. The dynamic range of this modulator has been extended by adding a constant power offset to its output. The fractal nature of the modulator response at low-clock frequencies is also experimentally shown.

Low-cost thermal s-d air flow sensor

This paper describes the design and optimization of a hot-wire air flowmeter. A low-cost design of the packaging allows good thermal contact with the airflow, as well as good thermal isolation between the hot and cold points. It is a compact solution which allows easy PCB mounting and adaptation to standard-size small air pipes. The design has been optimized for low-cost applications. The sensor is read out by a thermal sigma-delta modulator. The dynamic range of this modulator has been extended by adding a constant power offset to its output. The fractal nature of the modulator response at low-clock frequencies is also experimentally shown.This paper describes the design and optimization of a hot-wire air flowmeter. A low-cost design of the packaging allows good thermal contact with the airflow, as well as good thermal isolation between the hot and cold points. It is a compact solution which allows easy PCB mounting and adaptation to standard-size small air pipes. The design has been optimized for low-cost applications. The sensor is read out by a thermal sigma-delta modulator. The dynamic range of this modulator has been extended by adding a constant power offset to its output. The fractal nature of the modulator response at low-clock frequencies is also experimentally shown.

Use of a Berner Low-Pressure Impactor at Low Inlet Pressures. Application to the Study of Aerosols and Vapors at High Temperature

The behavior of a Berner-type low-pressure impactor operated at low inlet pressures (down to 0.04 bar) has been studied both theoretically and experimentally. The stage cut diameters have been found to decrease with inlet pressure, the general behavior of the impactor being similar to that of the atmospheric case down to ∼0.1 bar. At this inlet pressure, particles in the range 3–100 nm are classified along six different impactor stages, which offer the possibility of studying, e.g., freshly nucleated aerosols. The impactor so operated has been applied to the study of aerosol formation in biomass combustion, connected to the exit of a probe used in a previous work. The results presented in this article confirm and complement the conclusions of that work regarding submicron particle formation (i.e., nucleation of alkali sulfate occurs first and is followed by chloride condensation at lower temperatures) and also illustrate the possibilities of the impactor operated at low pressures. Finally, the relevance of diffusion on the collection efficiency of the successive stages is briefly discussed.