Secondary Air Flow Rate Research Articles

Mitigation of thermoacoustic instability utilizing steady air injection near the flame anchoring zone

The objective of this work is to investigate the effectiveness of steady air injection near the flame anchoring zone in suppressing thermoacoustic instabilities driven by flame–vortex interaction mechanism. We perform a systematic experimental study which involves using two different configurations of air injection in an atmospheric pressure backward-facing step combustor. The first configuration utilizes a row of micro-diameter holes allowing for air injection in the cross-stream direction just upstream of the step. The second configuration utilizes an array of micro-diameter holes located on the face of the step, allowing for air injection in the streamwise direction. The effects of each of these configurations are analyzed to determine which one is more effective in suppressing thermoacoustic instabilities at different operating conditions. The tests are conducted while varying the equivalence ratio and the inlet temperature. The secondary air temperature is always the same as the inlet temperature. We used pure propane or propane/hydrogen mixtures as fuels. Combustion dynamics are explored through simultaneous pressure and heat release-rate measurements, and high-speed video images. When the equivalence ratio of the reactant mixture is high, it causes the flame to flashback towards the inlet channel. When air is injected in the cross-stream direction, the flame anchors slightly upstream of the step, which suppresses the instability. When air is injected in the streamwise direction near the edge of step, thermoacoustic instability could be eliminated at an optimum secondary air flow rate, which depends on the operating conditions. When effective, the streamwise air injection prevents the shedding of an unsteady vortex, thus eliminating the flame–vortex interaction mechanism and resulting in a compact, stable flame to form near the step.

Investigation of co-combustion of coal and olive cake in a bubbling fluidized bed with secondary air injection

In this study, a bubbling fluidized bed of 102 mm inside diameter and 900 mm height was used to burn olive cake and coal mixtures. Tunçbilek lignite coal was used together with olive cake for the co-combustion tests. Combustion performances and emission characteristics of olive cake and coal mixtures were investigated. Various co-combustion tests of coal with olive cake were conducted with mixing ratios of 25%, 50%, and 75% of olive cake by weight in the mixture. Operational parameters (excess air ratio, secondary air injection) were changed and variation of pollutant concentrations and combustion efficiency with these operational parameters were studied. The results were compared with that of the combustion of olive cake and coal. Flue gas concentrations of O 2, CO, SO 2, NO x , and total hydrocarbons (C m H n ) were measured during combustion tests. For the setup used in this study, the optimum operating conditions with respect to NO x and SO 2 emissions were found to be 1.35 for excess air ratio, and 30 L/min for secondary air flowrate for the combustion of 75 wt% olive cake and 25 wt% coal mixture. The highest combustion efficiency of 99.8% was obtained with an excess air ratio of 1.7, secondary air flow rate of 40 L/min for the combustion of 25 wt% olive cake and 75 wt% coal mixture.

Emissions and wall temperatures for lean prevaporized premixed gas turbine combustor

Experimental studies are carried out for investigating emission and wall temperature for traditional gas turbine combustor converted to lean premixed prevaporized (LPP) combustor. Vortex chamber, air preheating system, flat flame burner and inlet temperature control system are designed. Vortex chamber was maintained at the main air inlet port for controlling secondary air flow rate and wall temperature. Kerosene/air mixture temperature at exit from burner and entering combustion chamber was kept constant at 650 K for all runs. Special considerations were given for measuring NO X , UHC, CO, local A/F ratio, flame temperature, exhaust gases temperature and wall temperature. For swirl and non swirl cases, secondary air ratio and primary zone air/fuel ratio were varied. The different operating parameters affecting flame temperature through it is affecting on local A/F ratio which is the main parameter for controlling flame temperature, emissions and walls temperatures. Flat flame burner and vortex chamber are useful tools for reducing emission and controlling walls temperatures. The inner liner wall temperatures are more affected by primary zone equivalence ratio while the outer liner wall temperatures are more affected by secondary air flow rate. Semi empirical correlations for NO X , UHC and CO concentrations, exhaust gases temperature and maximum inner liner wall temperature are carried out. Good agreement between the measured and the calculated results are obtained. The present results are useful for further development of the traditional gas turbine combustor converted to LPP combustor.

Effect of interaction of operation parameters on elutriation behavior in a vortexing fluidized bed

A systematic investigation on elutriation behavior conducted in a vortexing fluidized bed (VFB) cold model is studied. The effects of various parameters on the elutriation are investigated from the data obtained by using 'response surface methodology' (RSM) to determine the relationship between elutriation rate and operating conditions. The results show that all the interactions among primary air flow rate, secondary air flow rate, fine particle size, secondary air inlet diameter, and imaginary circle diameter had significant influences on elutriation behavior in the VFB. The modified regression models of the 'specific elutriation rate constant' (K*) were also submitted in this study.

Co-combustion of coal and biomass in a circulating fluidized bed combustor

In this research, co-combustion of coal and rice husk was studied in a circulating fluidized bed combustor (CFBC). The effects of mixed fuel ratios, primary air and secondary air flow rates on temperature and gas concentration profiles along riser (0.1 m inside diameter and 3.0 m height) were studied. The average particle size of coal from Maetah used in this work was 1,128 mm and bed material was sand. The range of primary air flow rates was 480–920 l/min corresponding to Ug of 1.0–2.0 m/s for coal feed rate at 5.8 kg/h. The recirculation rate through L-valve was 100 kg/hr. It was found that the temperatures along the riser were rather steady at about 800–1,000 degrees Celsius. The introduction of secondary air improved combustion and temperature gradient at the bottom of the riser, particularly at a primary air flow rate below 1.5 m/s. Blending of coal with biomass, rice husk, did improve the combustion efficiency of coal itself even at low concentration of rice husk of 3.5 wt%. In addition, the presence of rice husk in the feed stocks reduced the emission of both NOx and SO2.

Monitoring pulverised coal flames

A series of burner diagnostic tests using infrared (IR), microphone and acoustic emission (AE) sensors were conducted on a 150 kW pf burner rig. These experiments systematically varied the burner swirl number and the secondary airflow rate over a significant range for two different coals so that both satisfactory and 'poor' combustion conditions were obtained. The signals from the sensors were analysed by using advanced signal processing techniques to reveal a number of features which in turn were then compared with the nitrogen oxides (NOx), carbon monoxide (CO) and oxygen (O2) through a correlation coefficient analysis. It was concluded that these sensors can be used for the prediction of these emissions and will be particularly attractive for multiple burner installations where the pollutant emissions are often discharged through a common manifold, so that individual burner performance is often not known and cannot be optimised.

Modeling circulating fluidized bed biomass gasifiers. Results from a pseudo-rigorous 1-dimensional model for stationary state

Results from a 1-dimensional and semirigorous model for atmospheric and circulating fluidized bed biomass gasifiers (CFBBGs), presented in the (previous) paper by Corella and Sanz [J. Corella, A. Sanz, Modeling circulating fluidized bed biomass gasifiers. A pseudo-rigorous model for stationary state. Fuel Process. Technol. 86 (2005) 1021–1053], are shown here. Process variables predicted by the model are gas composition (H 2, CO, CO 2, CH 4, C 2H n , H 2O and O 2 contents), gas yield, tar content in the flue gas and char concentration in the solids. Both axial profiles in the riser and values at the gasifier exit are calculated from the model and are shown here for some selected sets of process variables. Variables analyzed in depth are: total air flow (used as equivalence ratio, ER), percentage of secondary air flow, height (location) of the secondary air flow, biomass moisture and biomass flow rate, expressed as the biomass weight hourly space velocity in the gasifier. All the results from the model agree both with known published data and with some tests made to check the model.

Performance of Flammability and NOx Emission in Premixed Oil Burner Using Ultrasonic Atomization.

In the combustion of low vapor pressure liquid fuel, it is necessary for instantaneous ignition to preheat the liquid fuel evaporator at any required time. For example, in the kerosene vaporizing type boiler for home use, electric power is consumed in the preheating kerosene vaporization to supply hot water, even on standby. To avoid the preheating process by electric power during the standby period, the technology that can vaporize liquid fuel instantaneously needs to be developed. From this viewpoint, the application of the ultrasonic atomization was proposed to ignite liquid fuel without any heating process. Similar trials had already been done but no stabilized combustion was accomplished by ultrasonic atomizing, using an MHz frequency.In this work, a kerosene pre-mixed burner equipped with a 1.7-MHz ultrasonic oscillator was manufactured and the characteristics of the atomization of kerosene, stable combustion region and NOx emission on this ultrasonic atomization burner are examined. The relatively quick ignition with a few seconds delay was realized. The atomization of kerosene was obtained at the rate of 0.75 g/min, which is proportional to the time and the average kerosene droplet size, 3.1 μm, produced by 1.7 MHz ultrasonic oscillation. 0.54 kW load was obtained by using this combustor. Stable combustion was obtained around 0.7 in the equivalence ratio. The stable combustion was defined as [CO]/[CO2] x emission (O2 0% base) is accomplished in the stable combustion region as it can provide the balance between both primary and secondary air flow rates.

The characteristics of pulverized coal combustion in the two stage cyclone combustor

Numerical investigations on air staging and fuel staging were carried out with a newly designed coaxial cyclone combustor, which uses the method of two stage coal combustion composed of pre-combustor and main combustor. The pre-combustor with a high air/fuel ratio is designed to supply gas at high temperature to the main combustor. To avoid local high temperature region in this process, secondary air is injected in the downstream. Together with the burned gas supplied from the pre-combustor and the preheated air directly injected into main combustor, coals supplied through the main burner react rapidly at a low air/fuel ratio. Strong swirling motion of cyclone combustor keeps the wall temperature high, which makes slagging combustion possible. Alaska, US coal is used for calculations. Predictions were made for various coal flow rates in the main combustor for fuel staging and for the various flow rate of secondary air in the pre-combustor for air staging. In-scattering angles are also chosen as a variable to increase residence times of coal particles. Temperature fields and particle trajectories for various conditions are described. Predicted temperature variations at the wall of the combustor are compared with corresponding experimental data and show a similar trend. The in-scattering angle of 20° is recommended to increase the combustion efficiency in the main chamber.

Pyrolysis and Combustion of Refuse-Derived Fuels in a Spouting−Moving Bed Reactor

A reactor with two reaction zones (pyrolyzer and combustor) is developed, which is proposed for the thermal disposal of solid wastes, especially RDFs (refuse-derived fuels). A model RDF was used to evaluate the performance of the reactor. The RDF is continuously fed to the spouting−moving bed pyrolyzer in which the RDF partially combusts to supply heat for pyrolysis of the RDF at low temperature, then the pyrolysis product burns with secondary air at high temperature in the upper combustor. A special connector cascades the two parts to a vertical configuration, where secondary air is introduced. The effect of operating parameters, such as spouting air and auxiliary air, as well as secondary air flow rate on the pyrolyzer temperature, pyrolysis gas composition at different positions, combustor temperature, and emissions of CO and NOx in flue gas, are studied.

Incineration of Paper Sludge in a Prototype Vortexing Fluidized Bed Combustor

ABSTRACT All experiments were carried out in a prototype vortexing fluidized bed combustor (VFBC). The dimension of the combustion chamber is 0.7 x 1.4 x 2 m, and the freeboard section is 1 m i.d. and 4 m in height. Paper sludge was used as the feeding material. Two types of coal particles were employed as the supplementary fuel. In order to understand the characteristics of the VFBC system for paper sludge incineration, the effect of various operating parameters, such as the primary airflow, excess air ratio, and secondary airflow rates, on temperature distribution, ash elutriation, combustion efficiency, and pollutant emissions were investigated.

A Study of Convective Heat Transfer in a Model Rotor–Stator Disk Cavity

In this study, the fluid (air) temperature field and the convective heat flux distribution on the rotor disk surface were measured and computed in a model rotor–stator disk cavity. Both mainstream flow and secondary air flow were provided. The radial distribution of convective heat transfer coefficient on the rotor disk surface, which was calculated as the ratio of the local heat flux and the local temperature difference across the thermal boundary layer on the disk, is also reported. In the experiments, the disk rotational Reynolds number, Reϕ, ranged from 4.65×105 to 8.6×105, and the nondimensional secondary air flow rate, cw, ranged from 1504 to 7520. The secondary air was supplied at the cavity hub. All experiments were carried out at the same mainstream air flow rate, Rem=5.0×105. The cavity fluid temperature distribution was measured by traversing miniature thermocouples, and the rotor disk surface temperature and heat flux were measured by a quasi-steady thermochromic liquid crystal technique in conjunction with resistance temperature detectors embedded in the disk. The measurements are compared with predictions from the commercial CFD code Fluent. The Fluent simulations were performed in the rotationally symmetric mode using a two-zone description of the flow field and the RNG k-ε model of turbulence. The convective heat transfer coefficient distribution on the rotor disk surface exhibited the influence of the source region and the core region of air flow in the cavity. In the source region, which is radially inboard, the convective heat transfer was dominated by the secondary air flow rate. In the core region, which is radially outboard, the heat transfer was dominated by the rotational motion of the fluid relative to the rotor disk. An empirical correlation for the local Nusselt number on the rotor disk surface is suggested for the core region.

Hydrodynamic behaviour of solid transport for a closed loop circulating fluidized bed with secondary air injection

The aim of this work was to study the mechanism of solid circulation in a Circulating Fluidized Bed pilot as a function of secondary air flow rate A rectangular column of 7 m height equipped with a U type siphon was used for this purpose The results obtained showed that the solid circulating phenomenon depends on different limiting steps like feeding step (dense bed), siphon circulating capacity and suspension saturation capacity.

ACTIVE CONTROL OF PERFORMANCE AND NOx EMISSIONS OF STAGED GASEOUS FUEL BURNER

Closed loop feedback control is implemented in gaseous fuel staged burner to maintain low NO„ emission. The experimental set-up is a swirl-stabilized, diffusion flame, staged combustion gaseous fuel burner. The emissions of NO and carbon dioxide are monitored in real time and controlled as a function of burner load. The control input to the system is the . primary and secondary air flow rate. The NO,, emissions and CO2 concentrations are measured through two analyzers near the burner exit plane. This information is conveyed to a computer control system which invokes an optimization algorithm to maintain the low NO„ emissions in addition to high CO2 concentrations which, for fixed overall air/fuel ratio, entails achievement of high combustion efficiency. The success of the control methodology encourages its application on practical systems.

Nitric Oxide Formation in an Iron Oxide Pellet Rotary Kiln Furnace

ABSTRACT A one-dimensional numerical model was developed to simulate the effects of heat and mass transfer on the formation of oxides of nitrogen (NOx) in a rotary kiln furnace for iron oxide pellet induration. The modeled kiln has a length-to-diameter ratio of approximately seven. The principal mechanism of heat transfer is radiation from the flame, which was described by the net radiation method. The well known Zeldovich mechanism was used to predict thermal NOx generation. Temperature fluctuations in the vicinity of the flame were estimated with a clipped Gaussian probability density function. The thermal energy and mass balance model equations were solved numerically. The model is capable of predicting temperature profiles and NOx production rates in agreement with observed plant performance. The model was used to explore the effects of process changes on the total NOx formation in the kiln. It was concluded that the gas temperature as well as the partial pressure of oxygen in the process gases controls the rate of NOx formation. Lowering the temperature of the kiln gases by increasing the secondary air flow rates requires simultaneously decreasing the pellet production rate in order to maintain the pellet temperatures needed for blast furnace conditions.

A hybrid, knowledge-based system as a process control ‘tool’ for improved energy efficiency in alumina calcining furnaces

This paper describes the development and implementation of a hybrid SCADA (Supervisory Control and Data Acquisition) system for the air suspension ‘flash’ alumina calciner at Aluminium de Grece St. Nicholas plant. The system includes two additional modules, an ‘expert system’ based on 10 years of commissioning and operating experience and a dynamic model continuously predicting the quality of the product. It is implemented on an industrial PC under an industrial control software environment and interfaces to the furnace through a programmable logic controller (PLC) system. The system guides the furnace operator to reduce sensible heat losses by operating with a marginal excess air flow rate, to take advantage of the higher energy efficiency at higher production rates and, at the same time, to avoid potentially dangerous operating conditions leading to loss of production and consequential energy losses. It also performs the traditional SCADA functions (data acquisition, maintaining history files for important parameters and display of important operating parameters in both a mimic process diagram and a series of graphs.) In addition to these tasks a simplified dynamic model gives the operator a prediction of the current value of the main product quality parameter, which can only be measured with laboratory equipment. Preliminary results have shown that reduction of secondary air flow rate and reduced downtime achieved by the operator under the guidance of the system can reduce energy consumption by as much as 10%. A similar, or even greater, improvement in energy efficiency can be expected by implementing a similar system to cement calcination kilns based on the same technology.

Hydrodynamics of a circulating fluidized bed operated with different secondary air injection devices

A cold model of a circulating fluidized bed combustor having a 0.120 m i.d. and 5.75 m high riser, was operated by splitting the total gas flow rate in a primary air stream fed through the bottom distributor and a secondary air stream laterally injected at a fixed level along the riser. Two different feeding devices were used: in one case the injection of secondary air stream occurred over the whole circumference of the riser through a slot; in the other the injection occurred through four opposite ports. Solids used were 89 μm Ballotini. Overall superficial gas velocity was fixed at 6 m/s. Solids mass flux was alternatively equal to 35 and 55 kg/(s m 2). The ratio between secondary and primary air flow rates was kept at a value of 1. An isokinetic probe was used to measure radial profiles of upward and downward solids mass fluxes and those of tracer gas concentration. A stream of CO 2 was mixed with the secondary air stream before the injection and used as tracer gas. For each set of operating conditions, axial profile of voidage and radial profiles of solids mass flux and CO 2 concentration were measured along the riser. Results confirm that lateral air stream remarkably affects the hydrodynamics in the regions below and above the gas inlet. They also show that the mechanism of interaction between the secondary air stream and the gas-solids suspension rising from the primary zone completely changed when the injection device was changed. Solids and tracer gas distribution along the riser were noticeably different.

95/06307 Effect of secondary superficial air velocity on the elutriation rate of fine particles in a cold-model two-stage swirl-flow fluidized-bed combustor

The elutriation rate of fine particles were measured in a cold model Two-Stage Swirl-Flow Fluidized Bed Combustor(TSSF-FBC). The range of secondary air flow rate was varied from 0.1 m/s to 0.35 m/s while the primary air flow rate was kept to a constant value of 1 m/s. It was found that the elutriation rate of fine particles slightly increased with the secondary superficial air velocity. When the secondary superficial air velocity exceeded over 0.3 m/s, however, the elutriation rate rapidly increased. The elutriation rate of fine particles was strongly dependent upon the swirl intensity. It was exponentially decreased as the swirl intensity increased. The elutriation rate constant increased with the secondary superficial air velocity, and the critical particle diameters were in range of . Under the experimental conditions, the elutriation rate constant of fine particles can be correlated with secondary excess air velocity() and as follows;

Combustion-Driven Oscillation in a Furnace with Multispud-Type Gas Burners. 4th Report. The Effects of Position of Secondary Air Guide Sleeve and Openness of Secondary Air Guide Vane on Combustion Oscillation Condition.

In the preceeding papers, "the resonance factor", which predicts the degree of combustion oscillation, was proposed, and applied to both the furnace with a single burner and that with multiple burners. It was found that the combustion oscillation occurred in the region of low primary and secondary air flow rates in both furnaces and that the region of combustion oscillation was affected by the openness of secondary air vane and the burner arrangement in a furnace under a fixed flame holder condition. If the operational conditions of the practical burners are not within this region, the furnace will not oscillate at all. This study's objective is to clarify the effects of the position of the secondary air guide sleeve and the openness of the secondary air guide vane on the combustion oscillation region in a multiburner furnace, since the combustion oscillation condition varies with position and strength of the heat release zone is a furnace and thus is governed by the configurations of flame holder which is composed of an impeller, air guide sleeves and air guide vanes. Futhermore, a procedure of burner operation while avoiding combustion oscillation is demonstrated.

Combustion-Driven Oscillation in a Furnace with Multispud-Type Gas Burners. 3rd Report, The Effects of Burner Arrangements on Combustion Oscillation in a Multiburner Furnace.

Combustion-driven oscillation is excited in the burning zone of a furnace, and its magnitude is determined by the position and heat release strength of the heat sources. In previous papers, it was shown that "the resonance factor" is useful in detecting the degree of combustion oscillation in a furnace. In this paper, by applying this method to multiburner furnaces, the effects of burner arrangements, two-stage combustion rate, excess air ratio and gas mixing ratio on the occurence of oscillation are examined. Consequently, the following results are obtained. (1) The oscillation condition is characterized by the fuel flow rate and primary and secondary air flow rates in the burner throat. (2) The low-positioned burner has stronger ability to excite the depth mode oscillation of the gas column in a furnace than the middle and high-positioned ones. (3) The effects of the two-stage combustion rate and excess air ratio on combustion oscillation are found to be represented only by the primary and secondary air flow rates. (4) The exhaust gas mixing combustion method suppresses the combustion oscillation.