Primary Air Velocity Research Articles

Identification of the Optimal Operating Parameters of Locally Available Coal for Use in Pakistani Industries

A parametric study intended to determine optimum burning parameters of locally available coal from mines in Darra Adam Khel is being reported. The coal samples were tested in a cyclone combustor using the pulverized coal combustion technique. Effect of mesh size, primary and secondary air velocity at various air fuel ratios was investigated to quantify the combustion performance by measuring the furnace temperature. Mesh size 200 was found to be readily combustible within the operating ranges, while tight control of the operating parameters was required to combust coal at mesh size 150. Optimum ranges of primary air and Air-Fuel Ratio are suggested for effective combustion. The study will help in effective utilization of indigenous coal reserves of Pakistan.

Characterization of gas–solid fluidization in fluidized beds with different particle size distributions by analyzing pressure fluctuations in wind caps

For the operation of a fluidized bed, solid particle size distribution is related to fluid dynamic behaviors, heat transfer efficiency, combustion efficiency, and desulfurization performance. Measurement of pressure fluctuations has a great advantage for detecting fluidization and combustion conditions, due to its flexible adaption to any operating conditions. In this study, the measurement and analysis of pressure fluctuations in the inlet of a wind cap of fluidized beds were carried out, to investigate the effects of particle size distribution on the gas–solid fluidization behaviors. Pressure fluctuations in the inlet of central wind cap were measured at different primary air velocities in cold circulating fluidized beds with different particle size distributions. In order to obtain the corresponding characteristic parameters, pressure fluctuations were analyzed by the methods including standard deviation, power spectral density estimation, wavelet analysis, and homogeneous index. Based on these characteristic parameters, the effects of particle size distribution on the gas–solid fluidization behaviors in fluidized beds were systematically analyzed. Variations of characteristic parameters of pressure fluctuations were found to be related to the variations of the gas–solid fluidization behaviors, which were caused by changes in particle size distribution or particle sizes. The results of this study indicate that the proposed method is practical, and the research achievements can be used as a theoretical basis for the condition monitoring of particle size distribution or the detection of agglomeration in fluidized beds based on pressure fluctuations in wind caps.

Influence of offset angle of mid-secondary air nozzles on gas-particle flow characteristics in a furnace.

A three-component particle-dynamics anemometer was used to measure the characteristics of two-phase gas-particle flows in a primary air nozzle using a gas/particle two-phase test facility. The jet trajectory of primary air velocities, Reynolds stress and particle concentration profiles were obtained. Based on this, the advantages and disadvantages of increasing the offset angle were analyzed to optimize performance. The results show that gas-particle slippage is not noticeable in the furnace, even though the mid-secondary air offset angle changes. Within the jet's free developing zone, the location of the peak concentration does not change with the mid-secondary air offset angles. Properly enlarging the offset angle of mid-secondary air nozzles will delay the mixing of the secondary air into the primary-air-and-coal mixture and realize horizontal-staged combustion in a furnace. It also can enhance the stability of airflow and form a better blended jet tail, meanwhile reducing the vortex at the back-fire side and avoiding a short circuiting of the airflow. It is favorable for increasing combustion efficiency and preventing slagging and high-temperature corrosion.

Optimization study on combustion in a 1000-MW ultra-supercritical double-tangential-circle boiler

The combustion process of one 1000-MW ultra-supercritical double-tangential-circle boiler was numerically studied and the three-dimensional full-size structure of the boiler was full considered. The influences of primary and over-fire air velocity as well as the jet structure on NOx generation characteristics were examined. In addition, the NOx generation characteristics of the improved burner structure were compared with those of the original one. Numerical results show that there exist two inverse elliptical flow fields and temperature fields. Moreover, the NOx generation and distribution characteristics are related to the temperature field to a certain extent. For different burner jet structures and arrangements, NOx distribution curves of the horizontal cross section are all W-shaped, but the NOx generation and distribution performance are correspondingly different, while the NOx emission changes are unobvious for different design schemes of the boiler burner. When we arrange one layer of auxiliary air from the burner undersurface, the flow area of the primary coal powder jet is enlarged by 100% and the coal feeding is increased by 20%. As a result, the temperature around the burner zone rises significantly. However, when two layers of auxiliary air are adopted, the combustion characteristic is promoted and NOx generation increases slightly. Based on the arrangement of the burner in the ultra-supercritical boiler, NOx generation does not vary obviously at different boiler loads. To achieve a better scheme, numerical and experimental studies are both performed in this study, and identical results are obtained. The current results may provide a theoretical basis for burner design improvement.

Combustion Characteristics of a Two-Dimensional Twin Cavity Trapped Vortex Combustor

Trapped vortex combustor (TVC) is a relatively new concept, having potential application in gas turbine engines. In this work, an attempt has been made to characterize the 2D twin cavity TVC experimentally in terms of its visible flame length, pollutant emission level, and exit temperature profile. Besides this, numerical results are also discussed to explain certain intricacies in flow and flame characteristics. Experimental results reveal that visible flame length value is sensitive to mainstream Reynolds number (Rems), primary (cavity) air velocity (Vp), and cavity equivalence ratio (Φc). For a particular Rems and Φc, an increase in Vp results in longer flame length; whereas, flame length gets shortened at higher mainstream Reynolds number cases. Numerical studies indicate that shortening of flame length at higher Rems cases is caused due to quenching of flame at the shear layer by the incoming flow. An attempt has been made to correlate flame length data with the operating parameters and Damkohler number (Da); Da takes care of flame quenching effects. Moreover, it is also brought out that the emission profile at the combustor exit is dependent on primary air velocity, mainstream Reynolds number, and cavity equivalence ratio. Emission studies indicate that higher primary air velocity cases make the carbon monoxide (CO) and unburned hydrocarbon (UHC) emission levels to lower values. Reduction in emission level is caused mainly due to the flame merging effects. Besides this, the influence of cavity flame merging on the exit temperature profile uniformity is also brought out. This study reveals that merging of cavity flames is essential for the optimized operation of a 2D trapped vortex combustor.

Numerical study of reacting flow characteristics of a 2D twin cavity trapped vortex combustor

In the present work, reacting flow characteristics of a 2D trapped vortex combustor (TVC) have been investigated numerically. Turbulent flow prevailing in the combustor is modelled using the two equation shear stress transport (SST) k-ω model and the turbulence–chemistry interactions are modelled using the eddy dissipation concept (EDC) model. Validation study reveals that the data generated by numerical model for reacting flow cases matches reasonably well with the experimental data. Simulation results indicate that for a particular operating condition, the flow structure within the cavity for reacting flow cases is significantly different from non-reacting flow cases. Besides this, under reacting flow condition, the vortex core location shifts with variation in operating condition. This study also reveals significant differences in the velocity gradient at the shear layer between reacting and non-reacting flow conditions. Furthermore, the turbulent kinetic energy at the cavity zone increases for the reacting flow condition, which is attributed to the volume expansion associated with the combustion processes. Also, temperature contours at locations downstream of the trailing edge indicate that both cavity flames are merged together for higher primary air velocity cases, and this is essential for efficient performance of TVC.

Effect of the Primary Air Velocity on Ignition Characteristics of Bias Pulverized Coal Jets

Experiments were conducted under a hot condition of bituminous coal ignition in a 250 kW pilot-scale bias combustion simulator. The effects of the primary air velocity (PAV) on the ignition characteristics of bias pulverized coal jets in a reducing atmosphere were investigated to develop a better new burner for experimental bituminous coal. Multiple means of measurement and sampling were used for the axial and radial temperatures, flame spectrum, flue gas components, and residual solid inside the furnace. The standoff distance changed nonmonotonically between axial distances of 90 and 330 mm with increasing PAV and was shortest for a PAV of 16 m/s. The radiation heat transfer from the hot environment had more effect on the ignition than the convection heat transfer from high-temperature flue gas in the initial stage, while the convection heat transfer from the high-temperature flue gas played a greater role in the subsequent combustion of the char. At PAVs of 13 and 16 m/s, there was volatile and char hom...

Investigating ventilation system performance in large space building: A nozzle primary air supply with secondary airflow-relay system

In large space buildings, nozzle is usually used for ventilation and air conditioning. A secondary airflow-relay system is proposed for situations that building span is larger than the primary jet flow range. A field measurement was conducted to investigate the performance of primary airflow nozzle and secondary jet fan system; a computational fluid dynamics model was built up to research the secondary airflow velocity and relative location of primary and secondary terminal on the ventilation performance. The performance of different computational fluid dynamics turbulence models in predicting the primary jet trajectory path was evaluated by comparing predicted result with field measurements. The temperature field predicted by computational fluid dynamics method was also validated by experimentally measured vertical temperature distribution data in certain measurement points. Various indices including Air Diffusion Performance Index, percentage dissatisfied, and coefficient of variation of temperature and velocity at breathing level were used to quantitatively evaluate the ventilation performance of different configurations. It is found that re-normalization group k-ϵ turbulence model gives the best overall prediction on the primary air nozzle jet trajectory path and field temperature distribution. In a certain large space building utilizing secondary airflow system, the relative location of primary and secondary equipment and air velocity should be properly specified in order to improve the ventilation performance of a large space building. The secondary airflow-relay may potentially increase the cooling energy consumption as indicated by computational fluid dynamics result.

Experimental study on ignition characteristics of pulverized coal under high-temperature oxygen condition

The high-temperature oxygen ignition technology of pulverized coal, which can replace the oil gun and achieve oil-free pulverized coal ignition by mixing the high- temperature oxygen and the pulverized coal stream directly, was proposed and a relevant ignition experimental system was built. The ignition characteristics of pulverized coal under high-temperature oxygen condition were investigated: the ignition process was described and analyzed, the influence of relevant parameters on the pulverized coal stream ignition were obtained and analyzed. The results showed: when the oxygen heating temperature is over 750 °C, the pulverized coal stream could be ignited successfully by high-temperature oxygen; increasing the pulverized coal concentration, primary air temperature and oxygen volume flow rate or decreasing the primary air velocity is helpful for the ignition and combustion of the pulverized coal stream.

Detection of gas-solid flow faults of a circulating fluidized bed using pressure fluctuations in wind caps

Wind cap partial blockages and agglomeration are two of the most common gas-solid flow faults that occur under the actual operations of circulating fluidized bed boilers. Using the method of measuring pressure fluctuations, for the characterization of fluid dynamics in fluidized beds, has a great advantage, due to its flexible adaptation to any operating conditions to monitor fluidization. This paper presents research into the use of measuring and analyzing pressure fluctuations in wind caps, for the analysis of the gas-solid fluidization characteristics in a fluidized bed with wind cap partial blockages or agglomeration fault. Partial blockages in a wind cap near feeding side and partial blockages in another wind cap near recycling side as well as agglomeration of different extents were simulated in a cold circulating fluidized bed. Pressure fluctuations in the inlets of several wind caps were measured at different primary air velocities under different fault conditions. They were then analyzed with the methods of statistical average, standard deviation, wavelet analysis and homogeneous index. Based on the calculated characteristic parameters, the effects of gas-solid flow faults on the gas-solid fluidization characteristics were analyzed. Results showed that variations of characteristic parameters of pressure fluctuations were related to variations of the gas-solid flow condition, which were caused by wind cap partial blockages or agglomerations. It is shown that the proposed method is practical.

Experimental investigation on the performance characteristics of a 2D trapped vortex combustor

The performance characteristics of a 2D trapped vortex combustor (TVC) are investigated experimentally in terms of the exhaust gas emission level, combustion efficiency, and the exit temperature uniformity. The present study reveals that the EICO and EIUHC emission levels are sensitive to mainstream Reynolds number ( Rems), cavity equivalence ratio ( Øc), and primary air velocity ( Vp). Besides this, mainstream premixing also has significant impact on the emitted pollutant level. Moreover, the NOx emission level can be reduced to very low levels (<2.6 ppm), which will be difficult to achieve in conventional combustors. Combustion efficiency calculated from these emission results indicates that it is influenced by cavity equivalence ratio and primary air velocity. For particular Rems, Vp, and Øc, combustion efficiency closer to 99% could be achieved for the merged flame cases, especially at higher primary air velocity. On the other hand, for a particular value of Vp, an increase in Rems tends to quench the cavity flame leading to reduction in combustion efficiency. This study also indicates that flame merging helps in efficient utilization of fuel and thus leading to better combustion efficiency. Besides this, the conditions for achieving lower pattern factor ( PF) are also brought out from exit temperature measurements. Pattern factor as low as 0.1 could be achieved particularly at higher Vp cases. It can be noted that this value is lower than that of the PF value of swirl combustors (0.2–0.4). Based on the present investigations, it can be concluded that TVC is a viable technology to be considered for future gas turbine application.

Combustion noise characteristics of an experimental 2D trapped vortex combustor

The objective of present investigation is to understand the effects of various operating parameters on the combustion noise characteristics of a 2D trapped vortex combustor (TVC). Spectral analysis of the microphone signal indicates that the power spectral density (PSD) magnitude increases with primary air velocity (Vp) for both mainstream Reynolds number (Rems) 20 000 and 40 000 cases considered in this study. Combustion noise measurements reveal that the spectral amplitude of the combustor increases with cavity equivalence ratio (ϕc). Moreover, for a particular Vp case, the PSD magnitude is observed to be higher for Rems=20000 as compared to that of Rems=40000 case due to the quenching of mainstream flame in the latter case. For a particular mainstream Reynolds number, the noise level increases with the momentum flux ratio between cavity and mainstream flow. Analysis of results reveals that the noise level varies between 80 dB for non-merged flame and 100 dB for merged flame cases. An attempt has also been made to correlate the overall sound pressure level (OASPL) with the non-dimensional operating variables.

An experimental study of heat pipe heat exchanger for cooling and dehumidification in an indirect evaporative way

A study of energy retrofit by combined heat pipe heat exchanger with cold condensate was carried out and results are presented in this paper. The parameters of study including inlet water temperature, water flow rate, primary air velocity, and exhaust air velocity on systems performance are experimentally investigated. Experiment results indicate that for a small condensate flow rate, both the latent heat of exhaust air and sensible heat of condensate can simultaneously be utilized for cooling and dehumidifying primary air, which is expected to be efficient for energy retrofit in central air conditioning systems. Practical application: Cold condensate from Heating, Ventilating, and Air-Conditioning (HVAC) plants are usually discharged or drained off. Reuse of cold condensate not only reduces wastage of water, it also fully utilizes energy and improves overall efficiency of HVAC plants. The installation and capital cost for heat pipe heat exchanger are reasonable. The physical space required is also affordable. Its practical value is useful.

Experimental Research on High Temperature Corrosion Prevention Technology for Water Wall of Opposed Wall Fired Ultra Supercritical Boiler

In order to solve the problem of high temperature corrosion to water wall of opposed wall fired ultra supercritical boiler, comprehensive study is carried out by combining theoretical analysis and experimental research, obtaining the variation of flue gas composition before and after introduction of near wall wind and effect of primary air velocity, central air opening, swirl intensity of outer secondary air and inner secondary air opening on composition of flue gas in near wall region. The results show that 1) the introduction of near wall wind significantly improves reducing atmosphere in near wall region; 2) among the four parameters listed above, swirl intensity of outer secondary air and inner secondary air opening have greater influence on composition of near wall flue gas; 3) under the comprehensive effect of swirl intensity of outer secondary air and inner secondary air opening, the volume fraction of CO in near wall flue gas can be controlled below 1500ppm, which effectively disturbs the condition for high temperature corrosion.

Flow Measurement of Primary Air Containing Pulverized Coal with Simple Venturi Tube

The flow measurement of primary air is very important for thermal power plants to maintain the safe state of operation. For direct firing pulverizing system, the measuring is essentially affected by pulverized coal particles. Abrasion and occlusion would bring great trouble to the measurement. Many measuring methods are invalid under that circumstance. Lots of efforts have been made to find appropriate ways to measure the flow of primary air containing pulverized coal. Venturi tube had been successfully used to measure the flow of air. If a venturi tube is used to measure the flow velocity of primary air, the differential pressure goes higher with the increase of the concentration of pulverized coal in the primary air. The neglect of the affection of pulverized coal on the measuring would result in remarkable deviation. With the aids of coal flow rate, a method of using venturi tube to measure the velocity of primary air containing pulverized coal was established. The affection of pulverized coal on flow measuring of primary air cannot be ignored.

Integrating multi-objective optimization with computational fluid dynamics to optimize boiler combustion process of a coal fired power plant

The dominant role of electricity generation and environment consideration have placed strong requirements on coal fired power plants, requiring them to improve boiler combustion efficiency and decrease carbon emission. Although neural network based optimization strategies are often applied to improve the coal fired power plant boiler efficiency, they are limited by some combustion related problems such as slagging. Slagging can seriously influence heat transfer rate and decrease the boiler efficiency. In addition, it is difficult to measure slag build-up. The lack of measurement for slagging can restrict conventional neural network based coal fired boiler optimization, because no data can be used to train the neural network. This paper proposes a novel method of integrating non-dominated sorting genetic algorithm (NSGA II) based multi-objective optimization with computational fluid dynamics (CFD) to decrease or even avoid slagging inside a coal fired boiler furnace and improve boiler combustion efficiency. Compared with conventional neural network based boiler optimization methods, the method developed in the work can control and optimize the fields of flue gas properties such as temperature field inside a boiler by adjusting the temperature and velocity of primary and secondary air in coal fired power plant boiler control systems. The temperature in the vicinity of water wall tubes of a boiler can be maintained within the ash melting temperature limit. The incoming ash particles cannot melt and bond to surface of heat transfer equipment of a boiler. So the trend of slagging inside furnace is controlled. Furthermore, the optimized boiler combustion can keep higher heat transfer efficiency than that of the non-optimized boiler combustion. The software is developed to realize the proposed method and obtain the encouraging results through combining ANSYS 14.5, ANSYS Fluent 14.5 and CORBA C++.

The Study of Waste-to-Energy Plant Combustion Characteristics Based on Computational Fluid Dynamics

The combustion process for using municipal solid waste as a fuel within a waste to energy plant calls for a detailed understanding of the following phenomena. Firstly, this process depends on many input parameters such as proximate and ultimate analyses, the season of the year, primary and secondary inlet air velocities and, secondly, on output parameters such as the temperatures or mass fraction of the combustible products. The variability and mutual dependence of these parameters can be difficult to manage in practice. Another problem is how these parameters can be tuned to achieving optimal combustible conditions with minimal pollutant emissions, during the plant-design phase. In order to meet these goals, a waste-to-energy plant with bed combustion was investigated by using computational fluid-dynamics approach.

Influence of Boiler Combustion Adjustment on NO<sub>x</sub> Emission and Boiler Efficiency

A tangentially fired boiler with twin furnace is tested to analyze the effect of such factors as primary air velocity,air distribution mode, over fired air,oxygen content on NOx and boiler efficiency during boiler combustion adjustment,providing reference for boiler operation optimization and reducing NOx emissions.

Study on Velocity Variation of Primary Air in Power Plants

Uniform velocity of primary air is very important in the operation of utility boilers. Regulation of the resistance of each pipe was done without pulverized coal to achieve equal flow velocity. The mass flow rate of pulverized coal and the length difference of pipes would lead to velocity variation of primary air. By the research of primary air flow and the regulation process, model of the velocity variation was built to calculate the velocity of each pipe and their difference. The arrangement of pipes and the operation parameters were taken into consideration. With the experimental data, calculation of velocity under different states was made. The velocity difference of different pipes was estimated. The length difference between pipes and the variation of the mass flow rate of pulverized coal play the most important role that affects the velocity of primary air.

Investigations on Lean Coal Combustion for Different Primary Air Velocities in a Lateral-Ignition Tiny-Oil Burner

AbstractAn experimental study was conducted to show the influence of the primary air velocity on lean coal combustion in a lateral-ignition tiny-oil burner. Using the lateral ignition scheme, a lean-coal ignition experiment was conducted at a coal feed rate of 3.5 t/h and primary air velocities of 15, 20, 25, and 30 m/s. During firing up with three oil guns in the presence of coal, the gas temperature along the center line increased along the flow of primary air. The gas temperatures on the side of the oil guns (r=+116.5 mm) were higher than those on the opposite side (r=−116.5 mm). As the primary air velocity increased, the gas temperature along the center line at the same measurement point decreased while that on the side with the oil guns rose. However, the gas temperature on the opposite side changed only a little. O2 concentrations at the exit of the burner were 1–3%. The burner resistance resulting from the flow and combustion of pulverized coal was 1,300–1,800 Pa.