Dissertations / Theses on the topic 'Low-NOx burner'

Current and future pollutant enussion legislation calls for decreased NOx emissions from combustion systems. A review of techniques used for NOx abatement led to the choice of combustor redesign to be the most cost effective method available. This led to the design, construction and development of a combustion system that utilised a Coanda ejector to generate recirculation of the exiting high temperature combustion products to mix with the air supply. Cooling of the burner was integrated into the design through the use of the air and fuel supplies. Computational fluid dynamics was used to model and aid development of the design. The model was used to predict NOx and CO emissions and the fuel-air mixing pattern. This, along with an analysis of experimental results and observations led to an understanding of the burner operation with respect to pollutant emissions and stability. NOx emissions from the Coanda burner were found to be lowest when using a 0.2 mm Coanda gap width, resulting in 16 ppm NOx being emitted at an air to fuel ratio of 1.5. However, the use ofa 0.2 mm Coanda gap width required an air supply pressure of up to 4 bar. The use of a 0.5 mm Coanda gap width enabled burner operation at lower air supply pressures. The resulting NOx emissions were measured as 23 ppm at an air to fuel ratio of 1.I, with a corresponding exit gas temperature of 2200 K. Flue gas recirculation quantity, flame stability, flame stabiliser shape and operational limits proved to be inter-linked in the reduction of NOx emissions. It was found that fuel-air mixing was controlled by the entrainment properties of the Coanda ejector and the flame stabiliser. The average oxygen concentration entering the combustion chamber when using a 0.2 mm and 0.5 mm Coanda gap width was 13.7 % and 16.6 %, respectively. Due to the position of the fuel injector, a fuel rich region formed behind the flame stabiliser. With a suitable flame stabiliser geometry and the use of 'fingers', low NOx combustion and flame stability was achieved near stoichiometric conditions. It was shown that the design of the burner enabled very low pollutant emissions near stoichiometric conditions, resulting in high exit gas temperatures. Conceivable applications of this type of burner could lie in small and intermediate furnaces where low NOx emissions are required. Additionally, very high temperature applications, such as glass furnaces could benefit in both cost and pollutant emissions from such a burner.

Nitrogen oxides emitted to the atmosphere can cause health problems for humans and environmental problems such as acid rain and global warming. The main part of the world energy consumption involves combustion; hence nitrogen oxide abatement in combustion is an important research field. Formation and reduction of NO_x in combustion and the current regulations on NO_x emissions are reviewed.

A novel low NO_x swirl stabilized gas burner concept, the Swirl Burner, has been studied experimentally, theoretically and numerically. Flame stabilization, rapid air and fuel mixing and internal flue gas recirculation are provided by a strongly swirling flow generated in this patented burner concept. NO_x emissions have been measured below 25 and 45 ppmv dry corrected to 3% O₂ in the flue gases using methane and propane as fuel respectively.

Studying the effect of varying geometrical parameters on the emissions of NO_x, fuel and air supply pressure and flame stability, have resulted in an optimized burner design. The optimized Swirl Burner has successfully been scaled from a 200 kW burner down to a 20 kW burner and up to a 370 kW burner, using a constant velocity scaling criteria which is the most commonly used scaling criteria for industrial burners. Experiments with the scaled burners have revealed that the fuel to air momentum should be preserved while scaling the burner. The 200 kW and the 370 kW burners were operated stable with the boiler to burner diameter (confinement) ratio in the range 5.3-6.7. The 20 kW burner, which was operated in an un-cooled and a water-cooled combustion chamber with confinement ratio of 8.1, was found to have a narrower range of stable operation with regards to thermal throughput. High post-flame heat extraction, which is enhanced by increased confinement ratio and combustion chamber cooling, reduces the emissions of NO_x, but might cause flame instabilities.

NO_x emissions measured from the three Swirl Burners scale well with NO_x scaling correlations based on flame volume as a leading-order parameter for NO_x formation (Weber, 1996). The correlations consider the effect of heat extraction on flame volume and emissions of NO_x. These correlations indicate that the heat extraction from the 20 kW burner is increasing with increasing thermal throughput. The 200 kW and the 370 kW burners were, from the correlations, found to operate with constant heat extraction.

Flame volume and shape are studied by non-intrusive measurements of OH radicals with the 20 kW burner using laser induced fluorescence. The measurements show that the flame volume is reduced with increasing thermal throughput. Measurements of NO_x from this burner also show a reduction with increasing thermal throughput. These results support the theoretical considerations of the flame volume as being the leading-order parameter for NO_x formation.

An evaluation of turbulence models and combustion models suitable for studying the Swirl Burner by computational fluid dynamics has been carried out. For this evaluation, a 2D computational model of the 20 kW burner has been used. For closure of the Reynolds Averaged Navier-Stokes equations for turbulent flow, three models have been evaluated.

These are the standard k-ε model, the RNG k-ε model and the Reynolds Stress model.

Also for modelling of combustion, three models have been evaluated, namely the Eddy Dissipation model, the Equilibrium PDF model and the Flamelet PDF model. For studying the Swirl Burner, a combination of the Reynolds Stress model and the Flamelet PDF model were found to be most suitable for modelling of turbulence and combustion respectively.

Computational results with the 20 kW burner indicate that flue gases are recirculated into a central toroidal recirculation zone downstream the burner exit. The computations are further compared with the OH concentrations measured with laser induced fluorescence.

In today’s industrial world, there are high demands on the environmental aspects.

Siemens Industrial Turbomachinery AB (SIT AB) is a company that is keen about the environment, and therefore spends a lot of effort in developing combustion processes in order to reduce NOx (nitrogen oxides) emissions on their engine products. They are also researching in optional fuels, which are more environment-friendly.

In order to provide lower emissions the SIT designed a water rig to study the flow dynamics in a DLE (Dry Low Emission) burner.

An analyze program (GUI horizontal) was developed with new functions and the existing functions were improved. The program’s function was to evaluate different experimental tests of the flow dynamics in the 3rd generation DLE burners, of the SGT-800 gas turbine engine.

The aim was to ensure repeatability to enhance reliability, of the experimental test results for further comparison, for upcoming projects concerning future DLE burners.

When repeatability was achieved, implementations of different geometrical modifications were performed in the 3rd generation DLE burner.

The reason of the geometrical alterations was to look over if better fuel air mixture could be obtained and accordingly (thus) to reduce hotspots in the burner and in that case reduce NOx emissions.

Coal firing power stations represent the second largest source of global NOx emissions. The current practice of predicting likely exit NOx levels from multi-burner furnaces on the basis of single burner test rig data has been proven inadequate. Therefore, to further improve current NOx reduction technologies and assist in the assessment of NOx levels in new and retrofit plant cases, an improved understanding of the impact of burner interactions is required. The aim of this research is two-fold: firstly, to experimentally investigate isothermal flow interactions in multi-burner arrays for different swirl directions and burner pitches in order to gain a better understanding of burner interaction effects within multi-burner furnaces. Secondly, to carry out numerical modelling in order to determine turbulence models which give the best agreement to experimental data. Experimental investigations were carried out using flow visualisation for qualitative and 3D laser Doppler anemometry for quantitative measurements. Numerical modelling was performed using the computational fluid dynamics software, Fluent, to compare performance between k-ε, k- ω and RSM turbulence models. Experimental investigation showed that the recirculation zone of the chequerboard configuration is more sensitive to the change in pitch than that of the columnar configuration. Further, it was found that the smaller pitch is more sensitive to change in configuration than the wider pitch. The analysis of fluctuating components, u’, v’ and w’ showed that the burner flow is highly anisotropic at burner exit. Numerical investigation showed that the k-ω turbulence model consistently performed below the other two models. The statistical comparison between k-ε and RSM turbulence models revealed that, for prediction of the swirl velocity profiles, the RSM model overall performed better than the k-ε turbulence model. The visual and statistical analyses of turbulent kinetic energy profiles also showed that the RSM turbulence model provides a closer match to the experimental data than the k-ε turbulence model.

Low swirl burners (LSBs) have gained popularity in heating and gas power generation industries, in part due to their proven capacity for reducing the production of NOx, which in addition to reacting to form smog and acid rain, plays a central role in the formation of the tropospheric ozone layer. With lean operating conditions, LSBs are susceptible to combustion instability, which can result in flame extinction or equipment failure. Extensive work has been performed to understand the nature of LSB combustion, but scaling trends between laboratory- and industrial-sized burners have not been established. Using hydrogen addition as the primary method of flame stabilization, the current work presents results for a 2.54 cm LSB to investigate potential effects of burner outlet diameter on the nature of flame stability, with focus on flashback and lean blowout conditions. In the lean regime, the onset of instability and flame extinction have been shown to occur at similar equivalence ratios for both the 2.54 cm and a 3.81 cm LSB and depend on the resolution of equivalence ratios incremented. Investigations into flame structures are also performed. Discussion begins with a derivation for properties in a multicomponent gas mixture used to determine the Reynolds number (Re) to develop a condition for turbulent intensity similarity in differently-sized LSBs. Based on this requirement, operating conditions are chosen such that the global Reynolds number for the 2.54 cm LSB is within 2% of the Re for the 3.81 cm burner. With similarity obtained, flame structure investigations focus on flame front curvature and flame surface density (FSD). As flame structure results of the current 2.54 cm LSB work are compared to results for the 3.81 cm LSB, no apparent relationship is shown to exist between burner diameter and the distribution of flame surface density. However, burner diameter is shown to have a definite effect on the flame front curvature. In corresponding flow conditions, a decrease in burner diameter results a broader distribution of curvature and an increased average curvature, signifying that compared to the larger 3.81 cm LSB, the flame front of the smaller burner contains tighter, smaller scale wrinkling.

A burner is very important device in process furnaces that significantly affect the production of emissions during the combustion process. One of the key things in development of the modern low-NOX burners is the evaluation of flow field downstream of an axial blade swirler inside the burner. The computational fluid dynamics (CFD) is often used to predict the attributes of the flow. Predicted values should be validated with measurement. It is the reason why the velocity fields for several choosen swirlers were measured. The hot wire anemometry was choosen and the dual-sensor probe was used during the measurement. The data can be then used for CFD validation. This thesis describes procedure of measurement set-up. The experimental facility was designed according to the anemometry method. The new probe traversing system was designed, which provides desired accuracy. Five different swirlers were measured. Large data set, need for customized post-processing and control over calculation procedures lead to new software design. For each swirler the velocity profiles were gathered and the swirl numbers calculated. That final data were transferred in to graphical format. Uncertainty of measured data was calculated. Results show counter-rotating flow in some areas closed to the swirler. Some drawbacks of current measurement set-up are discussed. Based on the thesis reader can obtain the information and knowledge for consequent measurements of swirl burners velocity profiles.

The main aim of the work was the investigation of the effect of operational parameters of the combustion process (combustion air excess, primary fuel ratio) and burner constructional parameters (the pitch angle of secondary nozzles, tangential orientation of secondary nozzles towards the axis of the burner) on the formation of NOx and CO, flue gas temperature, the shape, dimensions and stability of the flame, in-flame temperatures in the horizontal symmetry plane of the combustion chamber and the amount of heat extracted from the hot combustion gases in the combustion chamber’s shell. Experimental activities were carried out in the laboratory of the Institute of Process and Environmental Engineering, which is focused on burners testing. The combustion tests were performed with the experimental low-NOx type burner, namely the two-gas-staged burner. Mathematical model developed based on the experimental data describes the dependency of NOx on the operating parameters of the combustion process and burner constructional parameters. The model shows that increasing air excess and increasing angle of tangential orientation of the secondary nozzles reduce the formation of NOx. The temperature peaks in the horizontal symmetry plane of the combustion chamber decreases with increasing combustion air excess. The thermal load to the combustion chamber’s wall along the length of the flame was evaluated for selected settings. It was validated that the thermal efficiency of is reduced when higher air excess is used.

There is no dispute that combustion by-products like sulphur dioxide, SO₂ and nitrogen oxides, NO_x, can cause environmental damage. New, tougher legislation on gas emission has started to push fundamental research work (like this project) on understanding particle/fluid dynamics in combustion and related systems to the fore front. The knowledge gained will not only offer immediate help in the control and abatement of gas emission, but also the data obtained will complement the available empirical (industrial) knowledge of roping behaviour which will be valuable in developing new numerical models and/or verifying existing ones. A test facility delivering up to 40 m/s in a 4 inch glass test section was designed, fabricated, assembled and tested. This includes swirl generators for generating swirl of 0.2 to 1.35 theoretical swirl numbers. The facility also includes a particle feed section, cyclone separator for recovering the particles and a dual pulsed Nd:YaG laser, related optics and other equipment for use in future Particle Image Velocimetry (PIV) research. LDA and Pitot-static measurements verified that the test section was capable of delivering the planned/design velocity measurement range of 0-40 m/s. PIV experiments were done for particle jet density of 95 kg/m_-3 to 198 kg/m_-3 and the results obtained on particle jet dispersion were in good agreement with previous work showing that the centre line velocity showed less fluctuation and that jets that are less dense disperse more than the denser ones.

The present thesis is concerned with methods for determination and modeling of characteristic parameters of combustion of gaseous fuels. The focus is stressed on formation of nitrogen oxides and heat transfer from hot flue gases into combustion chamber’s walls. Experimental work, which is focused on testing of two burners with suppressed formation of nitrogen oxides, is an important part of the thesis. Its aim is to obtain data that is necessary for further processing and modeling. The work presents two methods that may be used in modeling of characteristic combustion parameters, namely the method based on statistical processing of data and the method based on computational fluid dynamics. The approaches are applied to two devices (burner with two-staged fuel supply, burner with two-staged air supply) with the objective to analyze their parameters. First approach covers detailed planning of burner test prior to its own carrying out (definition of the goal of experiment, choice of input factors and response, experimental plan) and subsequent statistical processing of experimental data. On the contrary, CFD approach offers simulations as an alternative option to traditional experimental methods. The simulation of combustion includes building of computational grid, setup of boundary conditions, turbulence model, heat transfer model and chemical kinetics. Results of simulations are compared with experimental measured data.

A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Engineering. Johannesburg 2016
Stringent worldwide emissions legislation, the drive to lower carbon emissions, together with the ever increasing demand to preserve the environment has led to a considerable demand for cleaner and more efficient coal combustion technologies. A primary technology for the reduction of emissions of oxides of nitrogen (NOx) is the installation of low NOx coal combustion burners. Extensive research into various burner characteristics and, in particular, the aerodynamic characteristics required to improve combustion performance of low NOx coal burners has been extensively undertaken and is ongoing. In this work the aerodynamic behaviour of a full-scale, aerodynamically staged, single low-NOx coal burner was numerically investigated. The objective of the study was to develop a single low NOx burner CFD model in Ansys Fluent, to better characterize and understand the flame shape in terms of the temperature profile achieved. CFD serve as an additional tool to assist with plant optimization, design proposals and occurrence investigations. To have confidence in the single burner coal combustion CFD model, the results of the model were compared to data obtained from an existing operational low NOx burner on site during a pre-defined load condition. To further improve on the theoretical CFD combustion model, drop tube furnace (DTF) experiments have been done to calculate the single rate Arrhenius kinetic parameters (pre-exponential factor and activation energy) for coal devolatilization and char combustion of the specific South African coal used. The combustion CFD simulations showed with a lower than design air flow through the burner, a reduced amount of swirl was achieved. This reduced amount of swirl produces a jet like flame and influences the way in which the combustion species are brought together. Under these operating conditions the flame distance from the burner mouth was predicted to be 1.2 (m). A very promising result was obtained through CFD and compared well with the in-flame temperature measurement obtained through the burner centre-line of approximately 1.4 (m). In an attempt to improve the aerodynamic profile of the burner under the same operating conditions the swirl angle on the tertiary air (TA) inlet was increased. The increased swirl on the TA inlet of the burner showed an improvement on the aerodynamic profile and had a significant impact on the temperature distribution within the flame. The increased swirl resulted in an improved flame distance of approximately 0.5 (m) from the burner mouth. The effect of increased swirl on the temperature profile of the flame displayed the aerodynamic dependence of the low NOx burner on combustion performance.
MT2017

碩士
國立中央大學
機械工程研究所碩士在職專班
99
This thesis studies experimentally a low NOX hydrogen burner, adopting the design of the weak swirl-jet burner (WSJB) proposed by Bédat & Cheng (1995) with new modifications using a rapid mixing mechanism and the hydrogen doping lean premixed turbulent methane combustion technology previously developed by our laboratory led by professor Shy. The weak jet-swirl generator is equipped at the exit of the burner, composed of four small circumferential jets inclined at 20°, capable of generating a uniform diverging flow field in the downstream of the burner exit, so that the original Bunsen-type jet flame can be stabilized in a form of the bowl-shape flame with very low NOX emissions. Furthermore, the abovementioned new modifications are used to make the burner more compact and to develop a pure hydrogen burner by gradually increasing the amount of doping hydrogen. Specifically, the flame stable operation domain of the burner is studied using different volume ratios of lean CH4/H2 mixtures and finally to pure H2 mixtures. Turbulent burning velocities (ST) and [NOX] emissions are then measured by a high speed particle image velocimetry (PIV) and the gas analyzer, respectively. The result shows that by adding hydrogen the lean flammability limit of methane can be effectively extended and values of ST at any given values of equivalence ratios (?) and root-mean-square turbulent fluctuating velocities (u'') can be increased. All measured [NOX] values for CH4/H2 flame are found to be lower than 5 ppm, while for pure H2 flames at ? = 0.4 no measurable [NOX] can be found. This study also applies chemiluminescence measurements to analyze the flame characteristics, using appropriate lenses with different wavelengths. The Abel inversion is then applied to analyze these flame appropriate chemiluminescence data which should be related to various. The selected wavelengths for chemiluminescence species include hydroxyl (OH*) between 305 nm and 315 nm, hydrocarbon-based (CH*) ranging from 425 nm to 445 nm, and dual-carbon (C2*) from 465 nm to 475nm or from 505 nm to 520 nm. As a final remark, these results should be of help to the future development of carbon-free hydrogen combustion technology with extremely low NOX emissions.

碩士
國立中央大學
機械工程研究所碩士在職專班
99
The basic design concept of weak-swirl-jet burner (WSJB) is coming from Bédat & Cheng (1995). To settle a weak jet swirl generator in advance position, produce a stable diverging flow field at the lower reaches and a bowl-shaped premixed turbulent flame. Then, this generator is made by 4 tangential weak air jets at 20 degree_inclination. Syngas, a mixture of mainly carbon monoxide (CO) and hydrogen (H2), is an important fuel in Integrated Gasification Combined Cycles. The most of previous studies in the feature of syngas combustion are limited in the laminar flame regime, and it’s scanty of studies to focus on turbulence combustion of syngas. However, utilizing the fast mixing apparatus and the lean turbulent combustion technique developed in our laboratory, we investigated the turbulent burning velocities (ST) and the exhaust emissions of the premixed lean syngas flames in the WSJB at two equivalence ratios of equivalence rate = 0.5 and equivalence rate = 0.7. The syngas used is the product from fluidized bed, with 65% CO and 35% H2. We identified the stable operation regime of the burner. Using particle image velocimetry and a gas analyzer, we measured the turbulent burning velocities and the emissions of NOx and CO under different equivalence ratios, mixture flow rates (characterized by the jet Reynolds number Rej ), and swirl strength (characterized by the swirl number S). The main results from our experiments include: (1) On the (Rej , S) plane, the stable operation region for equivalence rate equivalence rate = 0.7 is broader than that for equivalence rate = 0.5; (2) Increasing the concentration of H2 extends the flammable limits of the syngas and increases the ST ; (3) Both ST and turbulent intensity increase with the increase of Rej ; (4) Turbulent burning velocities normalized by the laminar burning velocity, ST/SL, of the bowl-shaped flames in the WSJB are higher than the turbulent spherical flames under the same conditions; and (5) Over the range of Rej and S explored, the NOx emission is always below 10 ppm, with the minimum below 5 ppm for equivalence rate = 0.5. Our experiment demonstrated that the WSJB is superb burner in terms of reducing NOx emission. Moreover, our study improves to understanding the turbulent flame of syngas feature and practical apply limitation. That benefits the development of Integrated Gasification Combined Cycles (IGCC).

碩士
國立中央大學
機械工程研究所
93
This study applies heat-recirculation technology to develop a low NOx burner. A commercial numerical program (CFD-RC) is used as a design tool to model chemical reacting flow using propane/air mixtures in a Swiss-roll burner (SRB). Measurements of the temperature distributions, heat-recirculation rate, and concentrations of exhaust gas in the SRB are carried. We employ 2 K-type and 9 R-type thermocouples to measure temperature distributions of the SRB and estimate the heat-recirculation rate (HR=Qr×100%/(Qi +Qr)), where Qr is the recirculation heat per unit time and Qi is the chemical heat of the fuel per unit time. The experimental result shows, φ =0.5 and the Reynolds number (Re) ranging from 60 to 880, values of the temperature distribution in SRB increase as Re increases. The maximum temperature occurs in the center of combustor, proving that C3H8 /air premixed flames can be stabilized in the combustion zone. We analyze the effect of Re on HR, for which HR increases as Re increases. As Re increases from 60 to 880, the percentage of HR increases from 11% to 25%. As Re increases, the wall temperature increases and more reactants with ambient temperature enter into the SRB. Therefore, the temperature gradient between the mixture and the walls is enhanced with increasing Re. This promotes the convective heat transfer and thus increasing HR. Measurements of emissions show that NOx concentrations are less 10 ppm at φ =0.5 for Re = 60~880. On the other hand, the concentrations of CO increase as Re increases. This is probably because CO has insufficient time to react to CO2 due to large Re, or the backward reaction step for CO2 to CO occurs during high temperature in the combustion zone. For numerical simulations, only 2-D simulations are carried out to compare with experimental result for Re = 630 at φ =0.5. The comparison shows that the temperature distribution between numerical and experimental results has similar trend, but there are quantitative differences due to 2-D numerical simulation, the unstructured grid model, and the overly simplified chemical reaction steps. Finally, as the goal of the present work, we combine a series of Bismuth Telluride thermoelectric (TE) materials with the aforementioned SRB to successfully establish a small TE power generator. The result indicates that only two TE chips, each with 3×3 cm2, can easily produce more than 1.6 watts for lighting several small bulbs constantly.

New regulations like the Clean Air Interstate Rule (CAIR) will pose greater challenges for Coal fired power plants with regards to pollution reduction. These new regulations plan to impose stricter limits on NOX reduction. The current regulations by themselves already require cleanup technology; newer regulations will require development of new and economical technologies. Using a blend of traditional fuels & biomass is a promising technology to reduce NOX emissions. Experiments conducted previously at the Coal and Biomass energy lab at Texas A&M reported that dairy biomass can be an effective Reburn fuel with NOX reduction of up to 95%; however little work has been done to model such a process with Feedlot Biomass as a blend with the main burner fuel. The present work concerns with development of a zero dimensional for a low NOx burner (LNB) model in order to predict NOX emissions while firing a blend of Coal and dairy biomass. Two models were developed. Model I assumes that the main burner fuel is completely oxidized to CO,CO2,H20 and fuel bound nitrogen is released as HCN, NH3, N2; these partially burnt product mixes with tertiary air, undergoes chemical reactions specified by kinetics and burns to complete combustion. Model II assumes that the main burner solid fuel along with primary and secondary air mixes gradually with recirculated gases, burn partially and the products from the main burner include partially burnt solid particles and fuel bound nitrogen partially converted to N2, HCN and NH3. These products mix gradually with tertiary air, undergo further oxidation-reduction reactions in order to complete the combustion. The results are based on model I. Results from the model were compared with experimental findings to validate it. Results from the model recommend the following conditions for optimal reduction of NOx: Equivalence Ratio should be above 0.95; mixing time should be below 100ms. Based on Model I, results indicate that increasing percentage of dairy biomass in the blend increases the NOx formation due to the assumption that fuel N compounds ( HCN, NH3) do not undergo oxidation in the main burner zone. Thus it is suggested that model II must be adopted in the future work.

碩士
國立中央大學
能源工程研究所
95
In 1995, Bédat & Cheng (B & C) proposed a low swirl burner which can enhance flame stability, reduce NOx formation and increase thermal efficiency. In this study we apply the concept of B & C to design and make a low swirl jet burner (LSJB) for measurements of turbulent burning velocities (ST) and investigation of the effect of hydrogen addition for the first time. Lean methane doping with 0 ~ 30% hydrogen by volume as a fuel is used. Quantitative measurements of velocity fields including both average velocities (U) and r.m.s turbulent intensities (u'') from the nozzle exit to the stabilized bowl-shape flame and above are obtained using high-speed particle imaging velocimetry (PIV). The value of ST is chosen as the value of U just at the bottom position of the stabilized bowl-shape flame. Identification of the stable operation ranges between flashback and blowoff limits of the LSJB is also made over a range of a swirling number (S) defined as the ratio of the axial flux of the angular momentum divided by the radius of the burner exit to the axial flux of the linear momentum, and the jet Reynolds number (Rej). We apply the wavelet transform (WT) to analyze spatiotemporal scales of the LSJB using these PIV time sequent data for the first time. [NOx] and [CO] in the products are measured by the gas analyzer. Thus, the knowledge of hydrogen addition on lean premixed combustion can be learned. The results show that the stable combustion ranges of S for the LSJB becomes narrower with increasing Rej. PIV measurements indicate that at the same lowermost point of the bowl-shaped flame, U of nonreacting cold flow is twice more in magnitude than that of reacting hot flow with about the same u''. Probability density functions and energy spectra of reacting hot flows indicate that the present flow is not isotropic and this result differs with that found by B & C. The spatial characteristic length scales, approximate to the Taylor length scale, determined from the axial and radial velocity data along the nozzle axis using WT are found to be approximately the same as the thicknesses of a mean progress variable (flame brush thickness), about 0.8 cm. Temporal characteristic scales are also identified by the WT analysis and both values are roughly the same in both axial and radial directions. The measured values of ST and u'' are found to increase with increasing Rej, at least in the range of Rej = 4,700 ~ 11,000, where errors of values of ST and u'' at a fixed Rej cannot be neglected especially when S approaching blowoff limits. Moreover, by adding a small amount of hydrogen [CO] can be significantly reduced with only a slightly increase of [NOx]. As the hydrogen addition increases from 0% to 30%, values of ST can be increased 30% more the increases of u'' smaller than 10%, and [NOx] and [CO] vary from 0 to 1.7 ppm and from 5,500 to 2,500 ppm, respectively. Hence, it is concluded that the present LSJB cannot be as a benchmark device for accurate measurements of ST, but it is indeed an excellent low-NOx burner which can be need in many practical applications such as gas turbines for electricity generation.

The low NOx burner (LNB) is the most cost effective technology used in coal-fired power plants to reduce NOx. Conventional (unstaged) burners use primary air for transporting particles and swirling secondary air to create recirculation of hot gases. LNB uses staged air (dividing total air into primary, secondary and tertiary air) to control fuel bound nitrogen from mixing early and oxidizing to NOx; it can also limit thermal NOx by reducing peak flame temperatures. Previous research at Texas A&M University (TAMU) demonstrated that cofiring coal with feedlot biomass (FB) in conventional burners produced lower or similar levels of NOx but increased CO. The present research deals with i) construction of a small scale 29.31 kW (100,000 BTU/hr) LNB facility, ii) evaluation of firing Wyoming (WYO) coal as the base case coal and cofiring WYO and dairy biomass (DB) blends, and iii) evaluating the effects of staging on NOx and CO. Ultimate and Proximate analysis revealed that WYO and low ash, partially composted, dairy biomass (LA-PC-DB-SepS) had the following heat values and empirical formulas: CH0.6992N0.0122O0.1822S0.00217 and CH_1.2554N_0.0470O_0.3965S_0.00457. The WYO contained 3.10 kg of Ash/GJ, 15.66 kg of VM/GJ, 0.36 kg of N/GJ, and 6.21 kg of O/GJ while LA-PC-DB-SepS contained 11.57 kg of Ash/GJ, 36.50 kg of VM/GJ, 1.50 kg of N/GJ, and 14.48 kg of O/GJ. The construction of a LNB nozzle capable of providing primary, swirled secondary and swirled tertiary air for staging was completed. The reactor provides a maximum residence time of 1.8 seconds under hot flow conditions. WYO and DB were blended on a mass basis for the following blends: 95:5, 90:10, 85:15, and 80:20. Results from firing pure WYO showed that air staging caused a slight decrease of NOx in lean regions (equivalence ratio, greater than or equal to 1.0) but an increase of CO in rich regions (=1.2). For unstaged combustion, cofiring resulted in most fuel blends showing similar NOx emissions to WYO. Staged cofiring resulted in a 12% NOx increase in rich regions while producing similar to slightly lower amounts of NOx in lean regions. One conclusion is that there exists a strong inverse relationship between NOx and CO emissions.

碩士
國立中央大學
機械工程研究所
94
This thesis studies experimentally to design a hydrogen burner that can be used in burning the anodic offgas (H2) ejected from the fuel cell system. We first use lean premixed methane/air mixtures as a fuel to test the performance of the round burner and to find its optimum operation conditions. Thus, pure H2 premixed turbulent combustion experiments can be safely conducted. Four tangential small jets inclined at 20o are equally positioned around the bottom of the nozzle of the burner for generating weak jet-swirl flow field. So a diverging flow field in the downstream of the burner’s nozzle can be formed. Such diverging flow field not only can stretch the original parabolic Bunsen flame(without swirl) to the bowl-like flat flame but also it can stabilize the flame front. Focuses are on some the effects that may influence the limit of the stable operation (flashback and blowoff limits), including solidity ratio (SR = 36% and 64%) of perforated plates, the nozzle diameter (Dj = 15 mm and 25 mm), nozzle length (Lj = 1~3 Dj), the exit rim with 45o tapered or without, and the swirl number (S ≡ the ratio of the azimuthal to the axial momentum flux). Emissions from different thermal inputs are also measured. The bowl like turbulent flame front images obtained via the high-speed (5000 frames/s) laser tomography are processed and averaged to extract the mean reaction variable ( ), where c = 0 and c = 1 are reactants and products, respectively. Furthermore, we apply high-speed particle image velocimetry to measure the time evolution of corresponding instantaneous velocity fields, so that we can obtain root-mean-square turbulent intensities (u’) and mean velocities () of the whole flow field and thus turbulent burning velocities (ST) as a function of u’ may be estimated. Results show that SR affect values of u’ for which u’ increases with increasing SR. The optimal stable combustion range is found when a nozzle with Dj = 25 mm, Lj = 50 mm hvaing 45o tapered rim is used. At �� = 0.9, emissions of [NOx] are found to be very small, all smaller than 13 ppm (corrected to 15% O2). The higher the mean volume flow rate of the nozzle (Qj) ranging from 53 to 75 L/min, the more [NOx] emissions from 2 ppm to 13 ppm. Concerning the hydrogen combustion, we have tested and found the flashback and the blow off limits of the burner when it is operated at �� = 0.3 ~ 0.6 corresponding to H2 fuel flow rate Qf = 10 ~ 20 L/min; no measurable [NOx] emissions are found. About ST measurements, we apply the method proposed by Bédat & Cheng (1995) by choosing = 0.05 for the estimate of values ST and u’. We found that this method in estimating ST has great errors and uncertainties. Finally, it is concluded that this weak-swirl lean premixed jet burner has the advantage of very low [NOx] emissions, but it’s estimated ST data need to view with great cautions.

碩士
國立交通大學
機械工程系
88
This study investigated the combustion characteristics of the coal- fired utility boiler with low NOx burners. The optimal operational conditions are analyzed in associated with the Taguchi methods. The effects of different parameters for the NOx emission are compared. Results show that the variation of NOx emission is strongly influenced by the secondary vane and the average of NOx emission is strongly influenced by the secondary air. The effects of coal feeder between 6.25 to 6.80 Tons/hr on the combustion characteristics are also investigated. For the case of 6.80 Tons/hr, the boiler efficiency is decreased due to a low air ratio. By increasing the excess air, the boiler efficiency increases and the unburned carbon decreases. However, the NOx emission is obviously increasing.

碩士
國立中央大學
機械工程研究所
96
This thesis measures fractal properties and turbulent burning velocities (ST/SL) of lean premixed methane/air weak swirl jet flames using laser tomography and particle image velocimetry (PIV). Two weak swirl jet burners (WSJB) of 25 mm and 50 mm diameters are applied, which are similar to the previous design by Bédat & Cheng (1995) for providing stabilized flames above. The instantaneous images of weak swirl jet flames were recorded by a high-speed, high-resolution CMOS camera (5,000 frames/s, 512 ? 512 pixels). After binarization, the flame front images were analyzed using the stepping-caliper method to obtain the fractal dimensions (D3) and inner and outer cutoff length scales (?I and ?o). In this study, the dimensionless turbulent intensities (u′/SL) of turbulence can be controlled from u′/SL = 2.6 to u′/SL = 20.4 with corresponding turbulent Reynolds number (ReT = u?LI/ν) ranging from 400 to 7000 which are much larger than previous studies, where u′, SL, LI and ? are the r.m.s. turbulent intensity, laminar burning velocity, integral length scale of turbulence, and kinematic viscosity of reactants, respectively. It is found that values of D3 are only 2.22 independent of u′/SL. This result differs drastically with most of previous studies, such as I.C. engines, Bunsen flames, and V-shape flames, that reported values of D3 ≈ 2.33 when u′/SL > 3. However, D3 ≈ 2.22 is in support of a previous study using Bunsen-type flames at smaller values of ReT (<500) by Gülder et al. (2000). As values of u′/SL increase from 2.6 to 20.4, values of ?i and ?o decrease from 1.5 mm and 12 mm to 1.0 mm and 10 mm. Finally, these fractal parameters obtained at high ReT cannot predict ST/SL correctly using available fractal area closure model, indicating a need for further improvement of existing fractal models.