The influence of swirl burner structure on the gas/particle flow characteristics

Improvements were made to a low-NO_x axial swirl burner (LNASB), aimed at mitigating slagging in a 600-MWe boiler burning bituminous coal. The new design is referred to as improved low-NO_x axial swirl burner (ILNASB). This paper describes investigations of the influence of swirl burner structure on the gas/particle flow characteristics using a three-dimensional particle-dynamics anemometer. In comparing results from both ILNASB and LNASB, a central recirculation zone is seen to form in the region x/d = 0.1–0.3 within the ILNASB. This zone had shifted from the region between primary and secondary air in LNASB to a region between inner and outer secondary air. In the vicinity of the burner outlet, particle volume flux is reduced significantly in the central recirculation zone. In contrast, this flux is high near the central axis in ILNASB, thus concentrating a great fraction of pulverized coal near the central axis. Form the study, the gas/particle flow characteristics of the ILNASB show that the improved burner has the ability to ease slagging and reduce NO_x emissions.     

切向叶片角度对中心给粉旋流燃烧器气固流动特性影响

利用三维激光多普勒测速仪,在气固两相实验台上,对不同切向叶片角度下中心给粉旋流燃烧器出口区域的气固流动特性进行了研究,获得了三种切向叶片角度下的三维平均速度和颗粒体积浓度的分布。结果显示,随着切向叶片角度的减小,轴向、径向和切向平均速度峰值都增加,回流区尺寸和旋流数也增大。同时,在x/d=0.1～0.7截面,随着切向叶片角度的减小,在燃烧器中心区域的颗粒体积流量峰值增加。     

Measurement of gas species,temperatures, char burnout,and wall heat fluxes in a 200-MWe lignite-fired boiler at different loads

We measured various operational parameters of a 200-MW_e, wall-fired, lignite utility boiler under different loads. The parameters measured were gas temperature, gas species concentration, char burnout, component release rates (C, H and N), furnace temperature, heat flux, and boiler efficiency. Cold air experiments of a single burner were conducted in the laboratory. A double swirl flow pulverized-coal burner has two ring recirculation zones that start in the secondary air region of the burner. With increasing secondary air flow, the air flow axial velocity increases, the maximum values for the radial velocity, tangential velocity, and turbulence intensity all increase, and there are slight increases in the air flow swirl intensity and the recirculation zone size. With increasing load gas, the temperature and CO concentration in the central region of burner decrease, while O₂ concentration, NO_x concentration, char burnout, and component release rates of C, H, and N increase. Pulverized-coal ignites farther into the burner, in the secondary air region. Gas temperature, O₂ concentration, NO_x concentration, char burnout and component release rates of C, H, and N all increase. Furthermore, CO concentration varies slightly and pulverized-coal ignites closer. In the side wall region, gas temperature, O₂ concentration, and NO_x concentration all increase, but CO concentration varies only slightly. In the bottom row burner region the furnace temperature and heat flux increase appreciably, but the increase become more obvious in the middle and top row burner regions and in the burnout region. Compared with a 120-MW_e load, the mean NO_x emission at the air preheater exits for 190-MW_e load increases from 589.5 mg/m³ (O₂ = 6%) to 794.6 mg/m³ (O₂ = 6%), and the boiler efficiency increases from 90.73% to 92.45%.     

Influence of mass air flow ratio on gas-particle flow characteristics of a swirl burner in a 29 MW pulverized coal boiler

In a gas/particle two-phase test facility, a three-component particle-dynamics anemometer was used to measure the characteristics of gas/particle two-phase flows in a 29 megawatt (MW) pulverized coal industrial boiler equipped with a new type of swirling pulverized coal burner. The distributions of three-dimensional gas/particle velocity, particle volume flux, and particle size distribution were measured under different working conditions. The mean axial velocity and the particle volume flux in the central region of the burner outlet were found to be negative. This indicated that a central recirculation zone was formed in the center of the burner. In the central recirculation zone, the absolute value of the mean axial velocity and the particle volume flux increased when the external secondary air volume increased. The size of the central reflux zone remained stable when the air volume ratio changed. Along the direction of the jet, the peak value formed by the tertiary air gradually moved toward the center of the burner. This tertiary air was mixed with the peak value formed by the air in the adiabatic combustion chamber after the cross-section of x/d = 0.7. Large particles were concentrated near the wall area, and the particle size in the recirculation zone was small.     

Gas/particle flow and combustion characteristics and NOx emissions of a new swirl coal burner

Due to the limits of reserves and price for the high rank coal, the low rank coal has been employed as fuel for power generation in China and will be eventually employed in the world. To burn low rank coal, centrally fuel-rich swirl coal combustion burner has been studied in Harbin Institute of Technology. This paper reviews and analyzes the major research results. The work has included both experiments and numerical simulation. The experiments were conducted using small-scale single-phase experimental equipment, a gas/particle two-phase test facility and 200- and 300-MW_e wall-fired utility boilers. For the burner, the primary air and glass beads partially penetrate the central recirculation zone and are then deflected radially. At the center of the central recirculation zone, there is high particle volume flux and large particle size. For the burners the local mean CO concentrations, gas temperatures and temperature gradient are higher, and the mean concentrations of O₂ and NO_x in the jet flow direction in the burner region are lower. Moreover, the mean O₂ concentration is higher and the gas temperature and mean CO concentration are lower in the side wall region. Centrally fuel-rich burners have been successfully used in 200- and 300-MWe wall-fired pulverized coal utility boilers.     

Influence of primary air ratio on flow and combustion characteristics and NOx emissions of a new swirl coal burner

Cold airflow experiments on a small-scale burner model, as well as in situ experiments on a centrally fuel-rich swirl coal combustion burner were conducted. Measurements were taken from within a 300 MW_e wall-fired pulverized-coal utility boiler installed with eight of centrally fuel-rich swirl coal combustion burners in the bottom row of the furnace during experiments. Various primary air ratios, flow characteristics, gas temperature and gas species concentrations in the burner region were measured. The results of these analyses show that with decreasing primary air ratio, the swirl intensity of air, divergence angles and maximum length and diameter of the central recirculation zone all increased, and the turbulence intensity of the jet flow peaked but decayed quickly. In the burner nozzle region, gas temperature, temperature gradient and CO concentration increased with decreasing primary air ratio, while O₂ and NO_x concentration decreased. Different primary air ratios, the gas temperatures and gas species concentrations in the side-wall region varied slightly.     

Mixing of confined coaxial flows

The paper presents experimental results on the mixing process in a coaxial jet mixer in two mixing regimes. In the first mixing regime, a recirculation zone develops just behind a nozzle near mixer walls, while in the second regime a jet is mixed with the co-flow without developing a recirculation zone. In the both regimes, the mixing process is studied at Re_d = 10 000. Behind the nozzle over the range 0.1 < x/D < 9.1, a velocity field in mixer cross-sections is measured by a one-component laser Doppler velocity meter and a scalar field is detected by the laser image fluorescence (LIF) method. A transverse autocorrelation function, integral length scales and probability density functions (PDF) are calculated using instantaneous distributions of a scalar and its fluctuations. It is shown that the scalar field acquires a homogeneous state faster than the velocity one. A quasi-uniform scalar distribution over the mixer cross-section is completed at the distance x/D = 5.1 in the first mixing regime, while this distribution has not been yet attained in the second. Analysis of the turbulent statistical moments and the autocorrelation function reveals how unsteady vortex structures exert a dramatic influence on the mixing. When the recirculation zone has developed, long-period antiphase oscillations exist near the mixer walls.     

Experimental investigation on the heat transfer of an impinging inverse diffusion flame

This paper presents the results of an experimental study on the heat transfer characteristics of an inverse diffusion flame (IDF) impinging vertically upwards on a horizontal copper plate. The IDF burner used in the experiment has a central air jet surrounded circumferentially by 12 outer fuel jets. The heat flux at the stagnation point and the radial distribution of heat flux were measured with a heat flux sensor. The effects of Reynolds number, overall equivalence ratio, and nozzle-to-plate distance on the heat flux were investigated. The area-averaged heat flux and the heat transfer efficiency were calculated from the radial heat flux within a radial distance of 50 mm from the stagnation point of the flame, for air jet Reynolds number (Re_air) of 2000, 2500 and 3000, for overall equivalence ratios (Φ) of 0.8–1.8, at normalized nozzle-to-plate distances (H/d_IDF) between 4 and 10. Similar experiments were carried out on a circular premixed impinging flame for comparison.It was found that, for the impinging IDF, for Φ of 1.2 or higher, the area-averaged heat flux increased as the Re_air or Φ was increased while the heat transfer efficiency decreased when these two parameters increased. Thus for the IDF, the maximum heat transfer efficiency occurred at Re_air = 2000 and Φ = 1.2. At lower Φ, the heat transfer efficiency could increase when Φ was decreased. For the range of H/d_IDF investigated, there was certain variation in the heat transfer efficiency with H/d_IDF. The heat transfer efficiency of the premixed flame has a peak value at Φ = 1.0 at H/d_P = 2 and decreases at higher Φ and higher H/d_P. The IDF could have comparable or even higher heat transfer efficiency than a premixed flame.     

The influence of distance between adjacent rings on the gas/particle flow characteristics of a conical rings concentrator

A phase Doppler anemometer system was used to study gas-particle flow characteristics of a conical rings concentrator for a centrally fuel-rich burner. The influences of distance between adjacent rings on the distribution of mean axial velocity, particle volume flux, particle number concentration, rich/lean air ratio, concentration ratio and resistance coefficient were obtained. In each cross-section, within the radius range from 0 to 35 mm, the particle axial volume flux for four distances was always far larger than at other radial positions. When x/D was larger than 1.5 and L was larger than 48 mm there was only a slight influence of distance on the concentration ratio. The resistance coefficient decreased with increasing distance and the larger the distance was, the more slowly the resistance coefficient decayed. In the five cross-sections from x/D = 0.1 to 1.5, R_CR for the four distances were always greater than 2. This indicates that the centrally fuel-rich burner with a conical rings concentrator for four distances could achieve a stable flame. Although the length of the primary air duct is long enough and the primary air fan total head is limited, it is best to choose the larger distance.     

Velocity field and turbulence effects on heat transfer characteristics from surfaces with V-shaped ribs

The flow field of smooth surfaces and surfaces with V-shaped ribs (V-SR) was studied experimentally with a Laser-Doppler Anemometry (LDA) system. In addition, heat transfer characteristics were experimentally investigated. Heat transfer results from these surfaces under impingement of a circular jet array (5 × 3) using an infrared thermal imaging technique are presented.The velocity profiles were measured at Reynolds number of 10,000 and at H/d equal to 3 and 12. For each H/d position, profiles were collected from x/d = 0 to 6 axial locations. The heat transfer data were obtained at Reynolds numbers equal to 2000, 6000, and 10,000. Along the target plate, different boundary layer profiles were obtained for smooth and V-SR plates at H/d = 3 and 12. Positions of maximum radial and axial velocities and turbulence intensities have been determined for smooth and V-SR plates. For low jet-to-plate spacings, the production of turbulence kinetic energy is higher for the V-SR surfaces as compared to smooth surfaces. For H/d = 3, the radial velocities are higher for the V-SR surfaces as compared to smooth surfaces but for H/d = 12, the radial velocities are not nearly changed all x/d locations. The heat transfer results have also been compared with those of a smooth surface under the same flow conditions to determine the enhancement in the heat transfer coefficient from x/d = 0 to 3 locations. In these locations, the Nusselt numbers are higher for the V-SR surfaces as compared to smooth surfaces. The locations of the peaks and the minima are influenced by cross flow velocities which in turn depend on jet-to-plate spacing and V-SR arrangements. For all results, the Nusselt numbers at the stagnation points decrease with increase in H/d.     

Heat transfer characteristics of three interacting methane/air flame jets impinging on a flat surface

An experimental study has been conducted for three interacting methane/air flame jets (arranged in a triangular configuration) impinging normally on a flat surface. Surface heat flux distributions have been determined for various dimensionless inter-jet spacings (S/d = 3, 4, 6 and 7.58) and separation distances between the exit plane of the burners and the target plate (H/d = 2, 2.6, 5 and 7). All experiments were conducted for stoichiometric mixture at a Reynolds number of 800. The surface heat flux distributions were intimately related to flame shapes. For small inter-jet spacings and small separation distances, flames were deflected outward from the centroid of the triangular arrangement due to strong interaction between the jets. The heating was quite non-uniform at very large inter-jet spacings. Zones of low heat flux were obtained when the tip of inner reaction zones were intercepted by the plate (H/d = 2). There were sharp peaks in the heat flux distribution when the tips of the inner reaction zones just touched the impingement surface (H/d = 2.6). Heat flux distribution was non-uniform at small separation distances (H/d = 2 and 2.6). For the system of flame jets under consideration, the optimum configuration, considering the magnitude of the average heat flux and the uniformity in the heat flux distribution, was corresponding to H/d = 5 and S/d = 3.     

Cold flow characteristics of a novel bluff body hydrogen burner

The cold flow characteristics of a novel partial premixed bluff body (PPBB) low NO_x burner, capable of operating with hydrogen as well as methane-hydrogen blends, were investigated numerically. The PPBB burner features a frustum shaped conical bluff body generating a flame stabilizing recirculation zone. Fuel is partially premixed via jets in an accelerating cross-flow. Steady-state and transient non-reacting simulations using five different turbulence models, i.e. standard k-ε, realizable k-ε, shear stress transport (SST) k-ω, stress-blended eddy simulation (SBES) and large eddy simulation (LES), were conducted. The simulations were validated against particle image velocimetry (PIV) measurements of an unconfined non-reacting flow. All turbulent models were able to predict the recirculation zone length in good agreement with the experimental data. However, only scale resolving simulations could reproduce velocity magnitudes with sufficient accuracy. Time averaged and instantaneous results from the scale resolving simulation were analysed in order to investigate flow characteristics that are special about the PPBB burner design and of relevance for the combustion process. Two different burner configurations were studied and their effects on the flow field were examined. The recirculation zone volume as well as the entrainment into the wall jet around the bluff body were found to correlate with the elevation of the bluff body relative to the burner throat. Both of these parameters are expected to have a strong impact on the overall NO_x emission, since the near burner region is typically one of the main contributors to the NO_x formation.     

Experimental study on velocity characteristics of recirculation zone in humid air non-premixed flame

To examine the effect of the flow field within the recirculation zone on flame structure, the characteristic velocity fields of methane/humid air flame in non-premixed combustion behind a disc bluff-body burner were experimentally studied by particle image velocimeter (PIV).The results show that two stagnation points exist on the centerline in the recirculation zone flame. However, the distance of the two stagnation points in humid air combustion shortens, and the minimal dimensionless velocity increases compared with the conventional nonhumid air combustion. In addition, the positional curves of the minimal velocities can be partitioned into three phases representing three different flame patterns. The analysis of axial minimal velocities on the centerline and their positions under different co-flow air velocity conditions reveals that fuel-to-air velocity ratio is the crucial parameter that governs humid air combustion flame characteristics. __________ Translated from Journal of Shanghai Jiaotong University, 2007, 41(3): 357–360, 365 [译自: 上海交通大学学报]     

Combustion behavior in a dual-staging vortex rice husk combustor with snail entry

The combustion characteristics of rice husk fuel in a dual-staging vortex-combustor (DSVC) are experimentally investigated. In the present work, the vortex flow is created by using a snail entrance mounted at the bottom of the combustor. The temperature distributions at selected locations inside the combustor, the flue gas emissions (CO, CO₂, O₂, NO_x), and the combustion/thermal efficiency are monitored. Measurements are made at a constant rice husk feed rate of 0.25 kg/min with various excess airs (37%, 56%, 74% and 92%) and different secondary air injection fractions (λ = 0.0, 0.15 and 0.2), respectively. The combustion chamber is 1800 mm high and 300 mm in diameter (D) with a centered exhausted pipe while the middle chamber of the combustor is set to 0.5D. The smaller section at the middle chamber is introduced to split the chamber to be dual-staging chamber where a large central toroidal recirculation zone induced by swirl flow through the small section is generated in the top chamber. The experimental results reveal that the highest temperature inside the combustor is about 1000 °C whereas both the thermal and the combustion efficiency are 41.6% and 99.8% for 74% excess air without the secondary air injection (λ = 0.0). In addition, the emissions are CO₂ = 8.1%, O₂ = 9.3%, CO = 352 ppm, NO_x = 294 ppm and small amount of fly ash. Therefore, the DSVC shows an excellent performance, low emissions, high stabilization and ease of operation in firing the rice husk.     

EINOx scaling in a non-premixed turbulent hydrogen jet with swirled coaxial air

The effect of swirl flow on pollutant emission (nitrous oxide) was studied in a non-premixed turbulent hydrogen jet with coaxial air. A swirl vane was equipped in a coaxial air feeding line and the angle of the swirl vane was varied from 30 to 90 degrees. Under a fixed global equivalence ratio of φ_G = 0.5, fuel jet air velocity and coaxial air velocity were varied in an attached flame region as u_F = 85.7–160.2 m/s and u_A = 7.4–14.4 m/s. In the present study, two mixing variables of coaxial air and swirl flow were considered: the flame residence time and global strain rate. The objective of the current study was to analyze the flame length behavior, and the characteristics of nitrous oxide emissions under a swirl flow conditions, and to suggest a new parameter for EINOx (the emission index of nitrous oxide) scaling. From the experimental results, EINOx decreased with the swirl vane angle and increased with the flame length (L). We found the scaling variables for the flame length and EINOx using the effective diameter (d_F,eff) in a far-field concept. Normalized flame length (L divided by d_F,eff) fitted well with the theoretical expectations. EINOx increased in proportion to the flame residence time (∼τ_R^1/2.8) and the global strain rate (∼S_G^1/2.8).     

Experimental‐ and numerical‐simulation research on the inner–secondary‐air ratio in a 600‐MWe down‐fired boiler

Using a phase Doppler‐anemometer measurement system, the cold gas/particle‐airflow behavior in a 1:40 scale‐model furnace was assessed to study the influences of adjusting the inner–secondary‐air ratio in a 600‐MW_e multi‐injection and multistaging down‐fired boiler. Numerical simulations were also conducted to verify the results of the modeling trials and to provide heat‐state information. The results demonstrate that reducing the inner–secondary‐air ratio from 19.66% to 7.66% gradually enhances the downward velocity decay of the gas/particle airflow, while the inner secondary‐air downward‐entraining effect on the fuel‐rich flow is weakened. Lowering the inner–secondary‐air ratio greatly inhibits the decay of the near burner–particle volume flux. In addition, the fuel rich–flow ignition distance is reduced, from 1.02 to 0.87 m. A lower inner–secondary‐air ratio is harmful to restrain early NO_x formation. Reducing the ratio also causes the fuel‐rich flow to turn upwards ahead, while the penetration depth of this flow gradually decreases and the maximum temperature in the hopper region falls from 1900 to 1800 K. On the basis of these data, an optimal inner–secondary‐air ratio of 13.66% is recommended.     

Performance evaluation of producer gas burner for industrial application

The purpose of this study is to evaluate the performance of premixed type industrial burner with producer gas, in terms of emission, axial and radial flame temperature. The developed burner was a concentric tube type where the air supplied through a central tube, which is surrounded by another one. It having 150 kWth capacity and tested on open core downdraft type gasifier. The burner consists of swirl vane for mixing the air and producer gas, mixing tube and bluff body for flame stabilization. Swirl angle and bluff body diameter was kept constant throughout the study. Burner was evaluated with open core downdraft type gasifier. In order to understand the qualitative differences in the temperature profile and emissions during combustion of producer gas, an experiment was conducted on three-flow rate and air-fuel ratio. Study shows low NO_x and CO emission at 125 Nm³ h⁻¹ as compared to that of 75 and 100 Nm³ h⁻¹. Maximum flame temperature (753 °C) was recorded at of 10 cm axial and 10 mm radial distance.     

Nozzle effect on heat transfer and CO emission of impinging premixed flames

Experiments were carried out to investigate the heat transfer and CO emission characteristics of a premixed LPG/air circular flame jet impinging upwards normal to a flat plate. The effects of nozzle diameter and nozzle arrangement on the heat transfer and CO emission under different fuel/air mixture flow rates (Q_mix), equivalence ratios (Ф) and normalized nozzle-to-plate distances (H/d) were examined. For the effect of nozzle diameter, burners of nozzle diameters of d = 7.9, 9 and 10 mm were used, and for the effect of nozzle arrangement, a twin-nozzle burner and a triple-nozzle burner, each with a cross-sectional area equal to that of the 9 mm diameter burner, were investigated under different normalized jet-to-jet spacing, S/d, of 3, 5 and 7. The heat transfer rate and CO emission index (EICO) are enhanced significantly with the decrease in the nozzle diameter for the single-nozzle flames. For the twin- and triple-nozzle flames, when the other operational conditions including Q_mix, Ф and H/d are invariant, the moderate S/d of 5 gives the highest heat transfer rate, whereas the EICO increases with increasing S/d. Comparison of the flames from all the burners shows that the highest heat transfer rate and EICO are obtained on the single-nozzle burner with the smallest nozzle diameter while the lowest heat transfer rate and EICO are obtained on the triple-nozzle burner with the smallest S/d.     

Investigations of AB5-type hydrogen storage materials with enhanced hydrogen storage capacity

The present study deals with investigations on synthesis, characterization and hydrogenation behavior of the MmNi₅-type hydrogen storage alloys Mm_0.9Ca_0.1Ni_4.9−xFe_xAl_0.1 (x = 0, 0.1, 0.2 and 0.3). All the alloys are synthesized by radio frequency induction melting following the composite pellet route. The X-ray diffraction pattern of dehydrogenated alloy without iron, detects peaks corresponding to calcium hydride, which are absent in the XRD pattern of the alloy with iron. The hydrogenation behavior is monitored by means of activation curves, absorption-desorption pressure-composition isotherms, hysteresis factors and desorption kinetic curves. The substitution of Iron at the place of nickel in the alloys Mm_0.9Ca_0.1Ni_4.9−xFe_xAl_0.1 (x = 0, 0.1, 0.2 and 0.3) gives an increase in the hydrogen storage capacity as 1.82, 1.90, 2.2 and 1.95 wt% corresponding to x = 0, 0.1, 0.2 and 0.3 respectively. The correlation between structural characteristics and hydrogenation behavior is described and discussed.