Electric Field Effect on Combustion of Pelletized Biomass in Swirling Flow
Abstract
The Doctoral Thesis examines the control of the swirling flame flow dynamics with an external static electric field by firing the gaseous products of thermal decomposition of pelletized straw, woody biomass, and peat with the aim of more efficient heat production with a decrease of flue gas emissions. The intensification of the downward vortex in the electric field has been determined, ensuring improved mixing of the air vortex with the biomass thermal decomposition gas flow, intensifying the convective mass transfer towards the heating surfaces, and increasing the amount of heat energy produced in the biomass thermochemical conversion process.
ResearchGate has not been able to resolve any citations for this publication.
With the aim to control and improve the thermo-chemical conversion of straw pellets, the experimental investigations of the DC electric field effect on the combustion dynamics and heat energy production were made. The electric field effect on the gasification/combustion characteristics was studied using three different positions of the positively charged electrode in flame. First, the electrode was positioned coaxially downstream the flame flow. Next, the electrode was positioned coaxially upstream the flame flow and, finally, the electrode was positioned across the downstream flow. The bias voltage of the electrode varied in the range from 0.6 up to 1.8 kV, while the ion current in flame was limited to 5 mA. The results of experimental investigations show that the DC electric field intensifies the thermal decomposition of straw pellets and enhances mixing of volatiles with air causing changes in combustion dynamics and heat energy production, which depend on position and the bias voltage of the electrode. The increase in the average volume fraction of CO2 (by 6 %) and the decrease in the mass fraction of unburned volatiles in the products (CO by 60 % and H2 by 73 %) for the upstream field configuration of the electrode and the ion current 0.5–1.8 mA indicate more complete combustion of volatiles.
The main goal of the present study is to promote a more effective use of agriculture residues (straw) as an alternative renewable fuel for cleaner energy production with reduced greenhouse gas emissions. With the aim to improve the main combustion characteristics at thermo-chemical conversion of wheat straw, complex experimental study and mathematical modelling of the processes developing when co-firing wheat straw pellets with a gaseous fuel were carried out. The effect of co-firing on the main gasification and combustion characteristics was studied experimentally by varying the propane supply and additional heat input into the pilot device, along with the estimation of the effect of co-firing on the thermal decomposition of wheat straw pellets, on the formation, ignition and combustion of volatiles .
A mathematical model has been developed using the environment of the Matlab (2D modelling) and MATLAB package ”pdepe”(1D modelling) considering the variations in supplying heat energy and combustible volatiles into the bottom of the combustor. Dominant exothermal chemical reactions were used to evaluate the effect of co-firing on the main combustion characteristics and composition of the productsand. The results prove that the additional heat from the propane flame makes it possible to control the thermal decomposition of straw pellets, the formation, ignition and combustion of volatiles and the development of combustion dynamics, thus completing the combustion of biomass and leading to cleaner heat energy production.
The aim of this study was to provide more effective use of straw for energy production by co-firing wheat straw pellets with solid fuels (wood, peat pellets) under additional electric control of the combustion characteristics at thermo-chemical conversion of fuel mixtures. Effects of the DC electric field on the main combustion characteristics were studied experimentally using a fixed-bed experimental setup with a heat output up to 4 kW. An axisymmetric electric field was applied to the flame base between the positively charged electrode and the grounded wall of the combustion chamber. The experimental study includes local measurements of the composition of the gasification gas, flame temperature, heat output, combustion efficiency and of the composition of the flue gas considering the variation of the bias voltage of the electrode. A mathematical model of the field-induced thermo-chemical conversion of combustible volatiles has been built using MATLAB. The results confirm that the electric field-induced processes of heat and mass transfer allow to control and improve the main combustion characteristics thus enhancing the fuel burnout and increasing the heat output from the device up to 14% and the produced heat per mass of burned solid fuel up to 7%.
This research investigates heat transfer phenomena on a plate used with impinging electric field flames; i.e., flames burning in the presence of an electric field. Electric field effects on flames have been investigated in different applications but not when the flames are impinging on nearby surfaces. Challenges to measurement methods when an electric field is applied in the system have limited the understanding of changes to the temperature distributions and species concentrations caused by the field. This study uses an infrared forward looking infrared (FLIR) camera with Schlieren visualization to examine the heat flux from flames over an impinging plate with different electric fields applied. In particular, we study the electric field effects on flames when those flames transfer heat to a nearby plate, and then how that transfer can be controlled using the electric field. The results show that electric fields affect substantially the heat flux distribution through the ion-driven wind, particularly when the plate location is just above the flame tip.
Experimental investigations of the DC electric field effect on thermal decomposition of biomass, formation of the axial flow of volatiles (CO, H2, CxHy), mixing of volatiles with swirling airflow at low swirl intensity (S ≈ 0.2-0.35), their ignition and on formation of combustion dynamics are carried out with the aim to understand the mechanism of electric field influence on biomass gasification, combustion of volatiles and heat energy production. The DC electric field effect on combustion dynamics was studied by varying the positive bias voltage of the central electrode from 0.6 kV to 3 kV, whereas the ion current was limited to 2 mA. The results of experimental investigations confirm the field-enhanced biomass gasification with enhanced release of volatiles and the development of endothermic processes at the primary stage of thermochemical conversion of biomass determining the field-enhanced heat energy consumption with the correlating decrease of the flame temperature and heat energy production at this stage of flame formation. Further, the field-enhanced radial expansion of the flame reaction zone correlates with a more complete combustion of volatiles increasing the combustion efficiency by 3% and decreasing the mass fraction of CO, H2 and CxHy in the products, whereas by 10% increases the average volume fraction of CO2 and the heat energy production downstream the combustor increases by 5-10%
Two-dimensional axisymmetric simulations for counterflow non-premixed methane-air flames were undertaken as an attempt to reproduce the experimentally observed electro-hydrodynamic effect, also known as the ionic wind effect, on flames. Incompressible fluid dynamic solver was implemented with a skeletal chemical kinetic mechanism and transport property evaluations. The simulation successfully reproduced the key characteristics of the flames subjected to DC bias voltages at different intensity and polarity. Most notably, the simulation predicted the flame positions and showed good qualitative agreement with experimental data for the current–voltage curve. The flame response to the electric field with positive and negative polarity exhibited qualitatively different characteristics. In the negative polarity of the configuration considered, a non-monotonic variation of the current with the voltage was observed, along with the existence of an unstable regime at an intermediate voltage level. With positive polarity, a typical monotonic current–voltage curve was obtained. This behavior was attributed to the asymmetry in the distribution of the positive and negative ions resulting from ionization processes. The present study demonstrated that the mathematical and computational models for the ion chemistry, transport, and fluid dynamics were able to describe the key processes responsible for the flame-electric field interaction.
Decomposition modeling of biomass often uses commercially available xylan as model compound representing hemicelluloses, not taking in account the heterogeneous nature of that group of carbohydrates. In this study, the thermal decomposition behavior of seven different hemicelluloses (P-glucan, arabinogalactan, arabinoxylan, galactomannan, glucomannan, xyloglucan, and xylan) were investigated in inert atmosphere using (i) thermogravimetric analysis coupled to Fourier transform infrared spectroscopy, (ii) differential scanning calorimetry, and (iii) pyrolysis-gas chromatography/mass spectroscopy. Results on decomposition characteristics (mass loss rate, reaction heat and evolving gas composition) were compared and summarized for the different hemicelluloses and for comparison also crystalline cellulose was included in the study. The mass loss rate characteristics differed between the polysaccharides, with cellulose and glucan-based hemicelluloses as the thermally most stable and xylan as the least stable sample. The heat flow during slow heating in nitrogen flow showed a much more exothermal decomposition of xylan compared with the other hemicelluloses. The composition of off-gases during heating showed large differences between the samples. During decomposition of xylan high levels of CO2 and lower levels of other components were formed, whereas also CO, methanol, methane, furfural, 5-hydroxymethylfurfural and anhydrosugars were formed in substantial amounts from the other polysaccharides. The formation of anhydrosugar was correlated to the monosaccharide composition of the polysaccharide chain. The results from the current study contribute to new knowledge concerning thermochemical behavior of different hemicelluloses.
Pyrolysis and combustion of pine sawdust have been investigated by using thermogravimetric analyzer coupled with Fourier transform infrared spectrometry (TG–FTIR) analysis in this paper. Pyrolysis–gas chromatography and mass spectrometry (Py–GC/MS) analysis was employed to characterize subsequently the structure and composition of evolving gas in pine sawdust pyrolysis process. TG results showed that both pyrolysis and combustion of pine sawdust presented three weight loss stages, respectively. The apparent activation energy of pyrolysis reaction is 108.18 kJ mol−1 in temperature of 239–394 °C, while under combustion process which is 128.43 kJ mol−1 and 98.338 kJ mol−1 in 226–329 °C and 349–486 °C, respectively. The evolving gaseous products during the pyrolysis and combustion infrared spectrums such as H2O, CH4, CO, CO2, phenol and alkane were found. Py–GC/MS results indicated that the main compounds of pine sawdust thermal decomposition were small molar gases, acetaldehyde, acetic acid, anhydride with formic and acetic anhydride. And possible formation pathways for main pyrolysis products were tentatively presented.
The application of an external electric field is known to improve flame stability significantly. Until now, few studies have proposed modelling approaches for combustion in the presence of an externally applied voltage. In these numerical studies, the negative ions are overlooked, and only the displacement of positive ions and electrons under the effect of a direct electric field was examined. In the present paper, a simplified mathematical model including negative ions is proposed based on a kinetic mechanism featuring 39 ionic reactions and 5 charged species. This mechanism is first evaluated by comparison of a monodimensional premixed flame with the available experimental data. Then it is used to analyse the stabilisation mechanism of a diffusion lifted flame in the presence of direct or alternating electric fields. It was concluded that the role of negative ions is crucial, and they are not to be neglected. Moreover, the simulations have shown that the magnitude of the flame stabilisation improvement depends, mainly, on the intensity and polarity of the applied voltage. If the applied voltage is alternating, its frequency is also found to influence the extent of the flame stabilisation improvement.
To effectively produce clean heat energy from biomass, microwave (mw) pre-processing of its different types - pelletized wood (spruce), herbaceous biomass (reed canary grass) and their mixture (50:50) - was carried out at the 2.45 GHz frequency with different durations of biomass exposure to high-frequency oscillations. To estimate the mw pre-processing effect on the structure, composition and fuel characteristics of biomass, its thermogravimetric (TG), infrared spectroscopy (FTIR) measurements and elemental analysis were made. The pre-processing is shown to enhance the release of moisture and low-calorific volatiles and the partial destruction of biomass constituents (hemicelluloses, cellulose), promoting variations in the elemental composition and heating values of biomass. The field-enhanced variations of biomass characteristics and their influence on its gasification and combustion were studied using an integrated system of a biomass gasifier and a combustor with swirl-enhanced stabilization of the flame reaction zone. The results show that the mw pre-processing of biomass pellets provides a faster weight loss at the gasification, and, therefore, faster ignition and combustion of the activated pellets along with increased output of heat energy at their burnout
There has been extensive work to show how electric fields can influence combustion. However, many different set ups are used. This work shows how different set ups produce different field strengths and that the field is not always uniformly distributed. The field strength is modelled using Ansys Maxwell. The type of material used is discussed and the set up of apparatus. It is recommended to use parallel plates for experimentation. Parallel plates produce the most uniform field this allow's it's influence to be directly investigated and related to the field strength. © 2011 Timothy J. C. Dolmansley et al.
The main goal of the present study is to obtain a more effective utilization of wheat straw for energy production during a co-combustion with peat. For this purpose, an electric field was applied to the flame produced by the combustion of volatiles. This work combines experimental study and mathematical modelling of the processes developing during the co-combustion of straw pellets with peat pellets. The main gasification/combustion characteristics, the heat output from the device and the composition of the flue gas were analysed by varying separately the bias voltage of the axial electrode and the straw mass load in the mixture with intention to assess the electric field impact. A mathematical model of the electric field impact on the main combustion characteristics (flow velocity, flame temperature and composition) of co-combustion have been built within the MATLAB environment. The numerical simulation was performed for two dominant second order combustion reactions of CO and H2 with account for the electric field effect on the displacement of equilibrium during the thermal decomposition of the biomass.
The main goal of the present study is to assure a more effective use of CO2 neutral fuel (wheat straw) for cleaner energy production with reduced greenhouse carbon emissions by partially replacing a fossil fuel (crashed coal) with a renewable one. This work combines experimental study and mathematical modelling of the processes developing during the co-combustion of straw pellets and crashed coal, aimed at assessment of the influence of the elemental composition and heating values of the mixture components on the main gasification/combustion characteristics, heat output from the device and on the composition of the flue gas products. The experimental study of the development of main gasification/combustion characteristics involves a complex DTG and DTA analysis of straw pellets and crashed coal and an estimation of the main steps of their thermal decomposition, combustion of volatiles, char formation and burnout, thus providing a complex kinetic study of the mixture weight loss rates and of the formation of combustible volatiles (CO, H2) at different stages of thermal decomposition of the solid fuel mixtures. A mathematical model for the combustion of volatiles (CO, H2) downstream the combustor has been built using the MATLAB package, with an account of the CO:H2 molar ratio variations at the inlet of the combustor and the development of exothermic reactions for the H2 and CO combustion dependent on the changes in straw mass load in the mixture of solid fuels.
With the aim to provide control of biomass thermal decomposition and combustion of volatiles, the external electric field effect on the wood biomass gasification/combustion characteristics during biomass thermo-chemical conversion at different stages of the swirling flame formation and heat energy production was experimentally studied.The DC electric field effect on combustion dynamics was studied using a single electrode configuration by inserting an electrode through the biomass layer to the base of flame reaction zone. An experimental study of the electric field effect on the development of two-stage process of biomass thermal decomposition and combustion of volatiles was performed by providing complex measurements of field-induced variations of biomass weight loss determining the formation of the axial flow of volatiles, kinetics of ignition and combustion of volatiles and formation of swirling flow structure. Results of experimental study and analysis of the electric field effect on combustion dynamics have shown that the field-enhanced thermal decomposition and the swirl-induced upstream flow formation with swirl-enhanced mixing of flame compounds are the primary factors determining the above mentioned effect on the combustion dynamics and heat energy production, promoting a faster ignition and a more complete combustion of volatiles. In addition, the field-induced interrelated processes of radial heat and mass transfer (ion wind effect) in the flame reaction zone can be used to control the formation of the swirling flow structure and heat energy production.
The present work is done to investigate the combustion and emission behavior of wheat straw pellets of different size and shape in a high temperature air flow. Prior to the combustion experiments, thermogravimetric analysis of the pellet was carried out to understand the thermo-chemical characteristics of the wheat straw pellet. A sequential method was used to determine the kinetic parameters. A combustor using a preheated air is used to investigate the effect of different parameters such as: pellet size, starting temperature and primary air flow velocity on combustion and emission characteristics of biomass pellets. The surface temperatures as well as the time dependent mass loss rates are recorded to study the ignition behavior of pellets. The results showed that the increasing in starting temperature, and air flow velocity and decreasing in pellet size leads to a reduction in combustion time and increasing in the total combustion rate. The results also showed that, the concentration of CO formed increases as the heating time increases and the formation of CO2 increased with increasing the starting air temperature and air flow velocity inside the combustion chamber. It was found that the combustion efficiency ranged from 99% to100% and the combustion efficiency reached 100% in some experiments due to complete combustion.
The main goal of this research is to promote a more efficient use of wheat straw for cleaner energy production by co-firing straw pellets with solid and gaseous fuels (wood pellets, propane) and to assess the impact of the mixture composition on the gasification/combustion characteristics and heat energy production. The effect of straw co-firing on the gasification/combustion characteristics was studied using a pilot device combining a biomass gasifier and a combustor, with swirl-enhanced stabilization and electric control of the swirling flow dynamics. The effect of straw co-combustion with wood and its co-firing with propane on the development of gasification/combustion characteristics was estimated from complex measurements of biomass thermal decomposition, swirling flow velocity, flame temperature, composition profiles and heat output from the device. The results prove that different elemental composition and calorific values of the mixture components and the variations of the heat input by the propane flame allow to control the thermal decomposition of straw pellets at their co-combustion with wood and co-firing with propane, whereas the formation of flow dynamics is responsible for the ignition and combustion of volatiles (CO, H2) and for the composition of the products. The obtained results show that electric field effects on the flow dynamics can be used to control the thermal decomposition of straw pellets and thermochemical conversion of volatiles, depending on the bias voltage of the axially inserted electrode.
A methane–air Bunsen flame was tested under dc electric field forcing with different ring anode sizes and locations. The anodes were placed within the flame to far outside the flame, both axially and radially. The focus of the work is to understand the limit of electrode placement and its effect on the flame under a dc field. A stoichiometric flame with voltages up to 9.1 kV was tested. The electrical current and flame height were measured as a function of the bias voltage and anode location. Plasma density measurements of the unmodified flame at various axial locations were used to corroborate that anode placement at regions of high electron density caused the largest reductions in flame height. The results showed the anodes closest to the burner at 35 mm caused the largest reduction (24%) in flame height. The effect of the electric field on flame height decreased as the anode moved farther downstream of the flame or radially outward. For the same voltage, larger currents were also observed for anodes close to the burner, whereas anodes placed far outside of the flame had minimal effects on current and, consequently, on the flame height. These differences are due to the variable electron density at the anode, which limits the net current collected and the strength of the field.
In air staged gasification and advanced carbonization processes, oxidative pyrolysis occurs in downdraft continuous fixed bed reactors. An oxidation zone separates the virgin fuel from the resulting char and propagates upward. Here, the oxidation zone was stabilized at a fixed elevation in a 20 cm I.D. fixed bed reactor using wood chips or wood pellets. In controlled continuous operating mode, we investigated the impact of air flux and bed bulk density on the behavior of the oxidation zone in terms of wood consumption, and yields of char, gas and tars. An air:wood mass ratio of 0.7 was measured and in our operating conditions, and was not sensitive to air mass flux and bed density. With oxidative pyrolysis, yields of organic condensates were lower than with allothermal pyrolysis, whereas the production of pyrolysis water and permanent gases increased. Finally, the oxidation zone was shown to be flat and horizontal in a wood pellet bed but inclined in a wood chip bed.
Combustion is a key process of energy utilization in fluidized beds. To achieve higher efficiency and lower pollution, a fundamental understanding of the complex combustion behavior is required. Mathematical modeling plays important roles in the exploration of the complex process and the prediction of combustion performance. Up to now, many models of the combustion of solid fuel particles have been presented. This study gives a detailed review of the proposed combustion models for common solid fuels, i.e., coal, biomass, and solid waste. The combustion models for coal and biomass have matured. However, models for solid waste have seldom been studied and need to be studied further. Considering the complex compositions of biomass and solid waste, a systematic model library based on many works should be proposed for them in the future. In addition, the transformation behavior of hazardous substance (F, Cl, and heavy metals) in solid waste should also be considered in combustion models in the future. Moreover, advanced measuring methods, such as laser measurements, should be used in future works to better understand the reaction mechanism during combustion and improve the accuracies of the models. An overall and accurate model library for the combustion of various type of solid fuels is expected to be established in the future, which will be helpful in the design, adjustment, and operation of combustion systems.
Fast pyrolysis is a promising thermochemical technology that breaks down renewable and abundant lignocellulosic biomass into a primary liquid product (bio-oil) in seconds. The bio-oil can then be potentially catalytically upgraded into transportation fuels and multiple commodity chemicals. Hemicellulose is one of the three major components of lignocellulosic biomass and is characterized as a group of cell wall polysaccharides that are neither cellulose nor pectin. The composition and structural features of hemicellulose (mixture of different heterogeneous polysaccharides) and different specific hemicellulose polysaccharides are reviewed. Particular focus is then given to reviewing the status of hemicellulose pyrolysis in terms of experimental investigations, reaction mechanisms, and kinetic modeling. For each aspect, recent results, challenges, and future prospects are addressed.
Experimental study and mathematical modelling were aimed to provide electric control of biomass thermochemical conversion and analysis of the DC electric and electromagnetic effects on combustion dynamics to obtain a cleaner and a more effective heat energy production. Mathematical modelling of the formation of flame velocity and temperature profiles was performed considering the Lorentz force effect on the flame. The results of numerical simulation show that increasing the electrodynamic Lorentz force parameter Pe leads to the increase of flame vorticity enhancing thus the fuel mixing with the air and to the correlating decrease of the flame temperature and reaction rates. Experimental study and analysis of the DC field effect on development of the swirling flow dynamics shows that the electric field-induced ionic wind disturbs the formation of the swirling flow velocity field by enhancing the upstream swirling flow formation and mixing of the axial flow of combustible volatiles with an upstream air swirl. The field-enhanced mixing of the axial flow of volatiles with an air leads to improvement of combustion conditions and to an increase in combustion efficiency giving a more complete combustion of volatiles by increasing the produced heat energy at thermo-chemical conversion of biomass.
An experimental study of the electric field effect on combustion dynamics at thermo-chemical conversion of biomass pellets was carried out with the aim to determine the DC field effect on the processes of biomass gasification, combustion of volatiles, formation of swirling flame structure, efficiency of heat energy production and composition of polluting emissions. The effect of DC field-enhanced mass transfer of flame species on the formation of swirling flame structure, determining variations of the combustion characteristics, heat energy production and composition of polluting emissions was studied by varying bias voltage and polarity of the axially inserted electrode. The effect of electric field on the flame characteristics has been explained considering the ion wind effects on the interrelated processes of heat/mass transfer determining the formation of the recirculation zone and development of combustion dynamics.
Carbon gasification reactions form the basis of several important industrial processes. This Chapter considers the fundamental science of gasification of carbons including cokes and chars. The distinction is made between chemical and diffusional control of reaction rate and the influences of porosity in the carbon. Rates of gasification, referred to as reactivity parameters, are a function of reacting gas, temperature, pressure, impurity content and structure of the carbon. The use of active and reactive surface areas to normalise gasification rates as a basis for comparison is explained. Mechanisms of reaction of carbon with atomic and molecular oxygen, carbon dioxide, steam, oxides of nitrogen and hydrogen are reviewed and compared. Catalysis of oxidation reactions is assessed in terms of mechanisms involving oxygen-transfer stages and topographical changes causing pitting and channelling. Inhibition of gasification is introduced. The difficulty in using the concept of reactivity in a rigorous comparative way is discussed.
An experimental study was conducted by applying a DC electric field to the swirling flame of hydrocarbons with the aim to provide electric control of the gasification/combustion characteristics of biomass (wood pellets). An experimental study of the DC electric field effect on the biomass gasification/combustion characteristics was carried out by varying the bias voltage and polarity of the axially inserted electrode in the range ±0.9 to ±2.7 kV, whereas the ion current was limited to 2 mA. The field effect on biomass gasification was estimated by measuring the field-induced variations of the biomass mass loss rate. The electric field effect on combustion dynamics at thermo chemical conversion of biomass was estimated from complex measurements of the flame velocity, temperature and composition profiles and from calorimetric measurements of cooling water flow. The measurements of the biomass mass loss rate confirm the field-enhanced thermal decomposition of biomass (up to 12-16 %) with field-enhanced mixing of the flame compounds, as well as the improvement of combustion conditions for flaming combustion of volatiles and the radial expansion of the flame reaction zone. The field-enhanced thermal decomposition of biomass and flame homogenization results in increase of the average value of the CO2 volume fraction in the products by 4-10 % with a correlating decrease of the air excess by 2-6 % in the products as well as in increase of the average temperature values by 2-6 %, whereas the produced heat energy at field-enhanced thermo-chemical conversion of biomass increases by 3-5 % indicating a more complete combustion of volatiles and a more effective heat energy production.
Development of the swirling flame dynamics at thermo chemical conversion of biomass (wood pellets) has been investigated experimentally at a low swirl number of the swirling flame flow (S < 0.6) different rates of biomass thermal decomposition and swirling air supply with the aim to estimate key factors determining the formation of the flame dynamics and local variations of the flame temperature composition and processes of heat energy production for such type of swirling flame flow. The experimental study is carried out using a small-scale pilot device with an integrated wood biomass gasifier and combustor. The limited primary axial air supply below the layer of wood pellets (a < 0.5) is used to support biomass gasification and the secondary swirling air at air excess supply (a > 1) is used to provide mixing of the axial flow of volatiles with an air swirl and to support the combustion of volatiles with the formation of a downstream reaction zone. It is shown that the development of combustion dynamics is influenced by the competitive processes of endothermic thermal decomposition of biomass and exothermal combustion of volatiles. The enhanced thermal decomposition of biomass by increasing the primary air supply and the axial flow of volatiles results in an intensive heat energy consumption with a correlating increase of the air excess ratio at the bottom of the combustor and in ignition delay. This determines the incomplete combustion of the volatiles at the primary stage of flame formation and restricts the development of combustion dynamics. The combustion conditions can be improved by decreasing the secondary air supply and air excess ratio at the bottom of the combustor which leads to enhanced ignition with a faster and more complete combustion of volatiles.
Dc electric fields of only a few thousands volts can be used to stabilize methane-air flames with a minimum of electric power dissipation, about 0.01% of the combustion power being controlled. Both the lean blowoff composition and the maximum blowoff velocity are markedly affected. The mechanism for this phenomenon is discussed in terms of an effect of the electric field on the boundary velocity gradient. The electric field applies an electric force on the chemi-ions in the flame, producing an electric wind which forces the flame closer to the burner rim both reducing the dead space and decreasing the boundary velocity gradient.
Electric field effects (EFE) on combustion characteristics, heat energy production and composition of polluting emissions have been investigated experimentally for different types of fuels (natural gas, biomass) providing experimental study of the EFE in a district heating boiler (DKVR) and complex modelling experiments in a small-scale pilot device. The DC field-induced variations of the produced heat energy, efficiency of heat energy production, flame characteristics and the composition of polluting emissions have been studied for a positively biased axially inserted electrode and negatively biased (grounded) heat surfaces by varying the applied DC voltage, net current and consumed electric field power. Experiments in the district heating boiler have shown that electrodynamic control of the heat production and combustion characteristics depends on the applied field voltage, power and on the flame region, where the top of the axially inserted electrode is located. The most pronounced EFE was observed when the top of the electrode was placed in the primary mixing zone intensifying the mixing of the flame compounds and thereby completing combustion. The mechanism of the electric field effects on the combustion characteristics is discussed with reference to the analysis of electric field effects on the flame characteristics observed in modelling experiments.
The understanding of soot formation in combustion processes and the optimization of practical combustion systems require in situ measurement techniques that can provide important characteristics, such as particle concentrations and sizes, under a variety of conditions. Of equal importance are techniques suitable for characterizing soot particles produced from incomplete combustion and emitted into the environment. Additionally, the production of engineered nanoparticles, such as carbon blacks, may benefit from techniques that allow for online monitoring of these processes.
In order to study the effects of electric field intensity and distribution on flame propagation speed, the paper investigated the flame propagation of premixed CH4/O2/N2 mixtures under direct-current (DC) electric fields at an excess air ratio of 1.4. Different high-voltage electrodes were chosen in the experiment in two aspects: electrodes size (the outer-diameter of high-voltage electrodes) and electrodes structure. Results show that the flame propagation speed increases significantly with the voltage applied to the high-voltage electrodes. Secondly, the electric field intensity can significantly affect the flame propagation, and it can be observed that the mean flame propagation speed increases linearly with the mean electric field intensity under fixed high-voltage electrodes. In addition, electric field distribution is another important influence factor on the flame propagation. Specifically, the effect of the electric field on the flame propagation speed increases apparently with the uniformity coefficient, so a uniform electric field works better than an uneven one for promoting the flame propagation speed. In fact, with the smallest uniformity coefficient of 0.21, the electric field under the 60 mm-diameter mesh electrodes has the most uniform electric field, while the effect of that on the flame propagation is the strongest, with the highest mean flame propagation speed of 1.25 m/s at an applied voltage of −10 kV. The results substantiate the importance of the electric field intensity and distribution on promoting the flame propagation speed of lean combustion.
Xylan-based hemicellulose sample is tested in TG-MS under He, 7% O2, 20% O2 and 60% O2, in order to underpin the understanding of thermo-degradation mechanism of hemicellulose and biomass. The mass loss history recorded by TG can be divided into two main stages: (1) low-temperature stage with the peak located at around 265°C associated with thermal cracking of hemicellulose, and (2) high-temperature stage with the peak enhanced and shifted to lower temperatures by oxygen concentration ascribed to char combustion. A number of prominently evolved ions identified by MS can be designated to acetone, acetic acid, furfural, water, CO, CO2 and so on. The releasing profile of smaller fragments (water, CO and CO2) follows the pattern of DTG curve under different oxygen concentrations (especially for that in the high temperature stage). A three-step consecutive kinetic model employing "n-order reaction function" is proposed and achieved good fit for the experimental mass loss data of thermo-oxidation of hemicellulose.
Copyright © 2015 Elsevier Ltd. All rights reserved.
Using an improved detailed soot model and based on a reduced diesel surrogate fuel chemical reaction mechanism of n-heptane/toluene, a computational validation study is performed on the veracity and effectiveness of the detailed soot model, which can predict the process of soot formation and oxidation in an optical engine and a single- cylinder diesel engine, respectively. In this detailed soot model, the effects of soot precursors including isomers of acetylene and PAHs on soot formation are considered. The result shows that the simulated in-cylinder combustion pressure, heat release curve and ignition timing are in excellent agreement with the experimental values. And the simulated 2-dimensional transient distributions of soot volume fraction are in good agreement with those obtained by the two-color method. The simulated change trend of soot emission value is in consistent with the experimental data. So the improved detailed soot model can achieve better prediction. Meanwhile, the changes of particle number concentration and average size in diesel engine obtained by the improved detailed soot model are studied. The result shows that the value of particle diameters increases dramatically at the start of the combustion and tends to get stable afterwards.
Plasma sheath theory is applied to understand the plasma behavior in electric field modified flames. This paper presents a set of 1D plasma sheath equations with approximated analytical solutions to calculate the sheath thickness for given applied voltages and plasma properties. The results show that the anode sheath is ten of microns thick, less than 1 V, and largely independent of the applied voltage. The cathode sheath grows with the applied voltage to centimeters thick. The limited extent of the anode and cathode sheaths, which limits the reach of the electric field, in part explains the different flame behaviors reported in the literature. The ionic wind body force is also calculated based on ion energy losses due to collisions. The sheath analysis provides a possible explanation for reported flame behavior under a DC field modified such as saturation current and diode-like behavior.
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 CO2 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.
Simulations of ion and electron transport in flames routinely adopt plasma fluid models, which require transport coefficients to compute the mass flux of charged species. In this work, the mobility and diffusion coefficient of thermal electrons in atmospheric premixed methane/air flames are calculated and analyzed. The electron mobility is highest in the unburnt region, decreasing more than threefold across the flame due to mixture composition effects related to the presence of water vapor. Mobility is found to be largely independent of equivalence ratio and approximately equal to 0.4 m2 V−1 s−1 in the reaction zone and burnt region. The methodology and results presented enable accurate and computationally inexpensive calculations of transport properties of thermal electrons for use in numerical simulations of charged species transport in flames.
Previous studies of large hydrocarbons in fuel-rich hydrocarbon flames have shown that the hydrogen content of intermediate large polycyclic aromatic hydrocarbons (PAH), which is correlated with their molecular structure, plays an important role in their growth pathways. For this reason it was investigated whether n-butane as fuel which is rich in hydrogen would have influence on the formation of H-rich PAHs. Because large, positive PAH ions rather closely simulate the fate of their uncharged counterparts, the method of sampling flame ions was chosen for this study. A low-pressure premixed n-butane-oxygen flame (C/O = 0.60, p = 2.66 kPa, unburned gas velocity, vu = 50 cm/s) was analyzed for large ions using molecular beam sampling combined with a time-of-flight mass spectrometer equipped with a reflectron. In contrast to flames of acetylene or benzene a large number of aliphatic protonated ions is formed in the first part of the oxidation zone. These ions contain as many as about 12 to 18 carbon atoms. In the early stages their average H/C is > 1, and some aliphatic ions are nearly saturated. Besides the prominent C13H9+ and C19H11+ which belong to the so-called ‘standard PAH’ there is a great number of larger hydrogen-rich PAH+ in the oxidation zone which are interpreted as PAH+ with aliphatic side groups and pericondensed PAH with open structures. The largest PAH+ have about 50 carbon atoms. At the end of the oxidation zone (the region of maximum temperature) the C-distributions of pericondensed PAH have their maxima with the formulae of standard structures found in acetylene and benzene flames. However, a large fraction of the intermediate, hydrogen-rich PAH has decomposed upstream of this point. PAH+ concentrations increase again slightly in the burned gas where their C-distributions indicate growth by addition of acetylene.
The status of our understanding of the-mechanisms of ion formation in flames is summarized, including not only a review of the literature but also some new proposals on cumulative and chemi-ionization. The high observed concentrations of ions in the combustion zone cannot be accounted for by thermal ionization of impurities, of equilibrium species existing at the flame temperature, or of non-equilibrium species. Ionization of carbon particles may account for the ions in high temperature-rich hydrocarbon flames, but experimental evidence is poor. This explanation is unsatisfactory for flames in lean mixtures. Ionization via translational energy or a simple extension of the high-energy tail of the Boltzmann curve is too slow. The most probable mechanism except for high temperature flames where thermal ionization predominates is cumulative excitation by processes such as CO(1II)+C2(1σ2+)→CO(1σ+)+C2++e− or chemi-ionization by processes such as CO(3II)+C2(1σ2+)→CO(1σ+)+C2++e−The most critical problem in this field is the actual identification of the ion.
The objective of this study is to conduct laboratory experiments on low-swirl injectors (LSIs) to obtain the basic information for adapting LSI to burn H{sub 2} and diluted H{sub 2} fuels that will be utilized in the gas turbines of the integrated gasification combined cycle coal power plants. The LSI is a novel ultralow emission dry-low NOx combustion method that has been developed for gas turbines operating on natural gas. It is being developed for fuel-flexible turbines burning a variety of hydrocarbon fuels, biomass gases, and refinery gases. The adaptation of the LSI to accept H{sub 2} flames is guided by an analytical expression derived from the flow field characteristics and the turbulent flame speed correlation. The evaluation of the operating regimes of nine LSI configurations for H{sub 2} shows an optimum swirl number of 0.51, which is slightly lower than the swirl number of 0.54 for the hydrocarbon LSI. Using particle image velocimetry (PIV), the flow fields of 32 premixed H{sub 2}-air and H{sub 2}-N{sub 2}-air flames were measured. The turbulent flame speeds deduced from PIV show a linear correlation with turbulence intensity. The correlation constant for H{sub 2} is 3.1 and is higher than the 2.14 value for hydrocarbons. The analysis of velocity profiles confirms that the near field flow features of the H{sub 2} flames are self-similar. These results demonstrate that the basic LSI mechanism is not affected by the differences in the properties of H{sub 2} and hydrocarbon flames and support the feasibility of the LSI concept for hydrogen fueled gas turbines.
The theory of the ionic wind is developed for flame ions travelling towards electrodes of various configurations so that entrainment as well as main stream gas velocities can be predicted. It is shown that, by confining entrainment to specified regions, large flow velocities can be induced at the flame itself, where they can be used to modify a variety of combustion processes. Theoretical maximum values of the flow parameters are calculated for several configurations likely to be of practical use and these are compared with results of experiment. The experiments are designed to test the general theory and to determine to what extent the theoretically deduced maxima are altered by inevitable practical complications such as entrainment of hot gas, deposition of soot and other specks on the electrodes and convergence of lines of force on to individual strands of gauze-electrodes. The potentialities of varying parameters such as geometry, temperature, pressure and composition as well as superimposing magnetic fields are also examined. A variety of practical examples is considered in the light of this theory. Experiments confirm that confined entrainment can be used to aerate diffusion flames in an accurately controllable manner without risking flash-back or requiring an air supply, metering and mixing systems. Similarly, it is demonstrated that combustion intensity can be increased by field-induced recirculation of hot products, thereby minimizing random turbulence and heat losses to the large obstacles usually employed for this purpose.
It is shown how Langmuir probes can be used in flames to obtain not only the positive ion concentration but electron concentrations and electron temperatures. An internal method is presented for checking the results by comparing the wall potential calculated from the above three quantities with the observed wall potential. Satisfactory agreement between these two values in hydrocarbon air/or oxygen flames from 1.5 to 760 mm Hg increases the confidence in the use of Langmuir probes to obtain plasma profile properties in flames. The accuracy of positive ion and electron collision cross sections appears to be the major factor limiting the accuracy of the probe.
Detailed results are presented for a number of flames. The positive ion concentration always exceeds the electron concentration indicating the formation of negative ions. Electron temperatures exceed the gas temperature and do not decay as rapidly as might be expected.
A mass spectrometric technique for obtaining ion profiles of good spatial resolution is outlined and detailed profiles are presented for an acetylene-oxygen flame at 2.5 mm Hg. The ion CHO+ peaks ahead of C3H3+ which precedes H2O+. Many other ions are observed to peak at about the same position in the flame as C3H3+. There are still problems, however, with respect to interpreting the results in terms of the first ion produced from neutral species and the sequence of ion molecule reactions which follows.
The detailed negative ion chemistry is discussed for two premixed methaneoxygen flames of fuel-lean and fuel-rich compostion burning at atmospheric pressure. As in the preceding paper on positive ions (Part I), a complete family of concentration profiles for natural, negative flame ions has been measured mass spectrometrically below 100 amu for each flame, sampled along the axis through the flame front. The ion chemistry is explained with reference to published data on neutral concentration profiles, rate constants for ion-molecule reactions, and thermochemical values when available. Total positive and negative ion profiles show the relative location of the negative ions. The negative ions initiate upstream as O2− by electron attachment to O2 in both flames. A group of profiles peaking in the reaction zone arises by charge-transfer and proton-transfer reactions with a variety of netural intermediates where OH− and O−, being strong bases, play a major role; in the fuel-lean flame the ion pairs and are exceptional, however. All profiles decrease rapidly near the downstream edge of the reaction zone by collisional detachment processes and associative detachment with hydrogen, hydroxyl, and oxygen radicals. In the fuel-lean flame only, several oxygenated ionic species persist through the burnt gas. The outstanding difference in the fuel-rich flame is the presence of carbonaceous anions CnHx−(n ⩾ 2, x = 0–3). Proton abstraction from stable acetylenic compounds yields broad single-peaked profiles. In contrast, carbonaceous radicals give rise to sharp double-peaked profiles where the downstream peaks appear to involve nondissociative electron-attachment processes. Compared with the positive ion discussion (Part I), these negative ion studies provide both corroborative and additional information about the underlying neutral chemistry of combustion.
The maxima limiting all practical effects of the movement of flame ions in electric fields are shown to depend on the current densities available. The theory of the electric field and space charge distributions inside and outside the flame in developed, checked experimentally, and used to deduce such maxima. Two factors are identified as limiting current densities; the rate of ion generation per unit flame area and the space charge-induced breakdown at the electrodes. The latter is shown to be ultimately limiting and the theory is used to calculate numerical values for all practical maxima. The former is limiting only in some flat laminar flames parallel to electrodes, but it leads to a method of measuring rates of ion generation in flames. The method is developed experimentally on the basra of the theory and applied to a series of hydrocarbon/air and hydrogen/hydrocarbon/air flames. As an example of its use, the results are applied to calculations of activation energies and orders of the ion-forming process.