Secondary Air Research Articles

Experimental study on gas uniformity at the inlets of six cyclones in a CFB with multi-tracer gas method

To investigate the non-uniform distribution of different gases passing through the parallel cyclones, experiments were conducted on a circulating fluidized bed (CFB) equipped with six asymmetrical cyclones. A multi-tracer gas method was used, with CO, O2, and CO2 chosen to represent gases with different properties in the flue gas at the inlets of the cyclones. The uniformity of multi-gas distribution was evaluated by measuring the concentration deviations of each tracer gas passing through individual cyclones. The results indicate that the concentrations of multi-tracer gases are higher in the middle cyclone among the three, which are located on the tracer gas injection side during the test of single-side secondary air (SA) tracing. The maximum concentration deviation of tracer gases is for CO2, while the minimum is for CO. At the three cyclone inlets on the opposite side, the tracer gas with higher density exhibits a more uniform distribution, and the gas uniformity decreases as the density decreases. The effects of superficial velocity, SA ratio, bed inventory, and tracer gas injection region on the uniformity of gas distribution were studied. The results show that superficial velocity and SA ratio primarily affect the uniformity of higher density gases, while bed inventory has a greater influence on lower density gases. The gas distributions are most non-uniform, especially for CO2, when the tracer gas injection region is near the rear wall closer to the induced draft fan during the test of regional SA tracing.

Revealing flow structures in horizontal pipe and biomass combustor using computational fluid dynamics simulation

AbstractComputational fluid dynamics (CFD) is a powerful tool to provide information on detailed turbulent flow in unit processes. For that reason, this study intends to reveal the flow structures in the horizontal pipe and biomass combustor. The simulation was aided by ANSYS Fluent employing standard ‐ model. The results show that a greater Reynolds number generates more turbulence. The pressure drop inside the pipe is also found steeper for small pipe diameters following Fanning's correlation. The fully developed flow for the laminar regime is found in locations where the ratio of entrance length to pipe diameter complies with Hagen–Poiseuille's rule. The sucking phenomenon in jet flow is also similar to the working principle of ejector. For the biomass combustor, the average combustion temperature is 356–696°C, and the maximum flame temperature is 1587–1697°C. Subsequently, air initially flows through the burner area and then moves to the outlet when enters the combustor chamber. Not so for particle flow, the particle experiences sedimentation in the burner area and then falls as it enters the combustor chamber. This study also convinces that secondary air supply can produce more circulating effects in the combustor.

Effect of the coal-spreading air on airflow and low-NOx combustion within a 75 t/h coal-fired grate furnace

Coal-fired grate furnaces equipped with coal throwers, which conventionally own a low-NOx combustion framework consisting of (i) the partitioned primary air for the horizontal air staging on the grate and (ii) the coal-spreading air and secondary air for the vertical air staging in the freeboard, actually suffer from poor burnout and apparently high NOx emissions at habitual operations. While comprehensive investigations on the in-furnace airflow, coal combustion, and NOx formation, which are devoted to first explaining and then improving these problems, are absent up to now. In order to unveil the airflow, combustion, and NOx emission characteristics within a 75 t/h coal-fired reverse grate furnace, evaluate its coal-spreading air impact on the low-NOx combustion performance, and determine an appropriate coal-spreading air ratio for improving the furnace performance, both industrial-scale tests and numerical simulations were performed at the coal-spreading air ratios of 2 %, 4 %, 6 %, and 8 %, respectively. The flow-field deflection and asymmetric combustion appeared in the upper furnace, with the upward gas deflecting towards the rear-wall side and the front-half furnace part dominated by a weak recirculation zone without effective combustion. With increasing the coal-spreading air, problems of the flow-field deflection and asymmetric combustion aggravated. The combustion intensity weakened in the layer combustion zone while displayed an increase-to-decrease trend in the suspension combustion zone. In terms of indexes related to the furnace performance, burnout rate decreased continuously, and NOx emissions initially raised but then decreased. A comprehensive consideration of the coal-spreading requirement via air, flow-field symmetry, low-NOx combustion, and burnout suggests that a 2 % coal-spreading air ratio should be preferred, where NOx emissions and burnout loss were gained at 249 mg/m3 (6 % O2) and 5.75 %, respectively. It reduced NOx emissions by 49 % while raised slightly burnout loss, as compared with the corresponding levels of 486 mg/m3 and 5.4 % at the habitual case. These results not only negate the air-staging function of the coal-spreading air but also highlight the novelty of this work.

Identifying optimal location for control of thermoacoustic instability through statistical analysis of saddle point trajectories.

We propose a framework of Lagrangian Coherent Structures (LCSs) to enable passive open-loop control of tonal sound generated during thermoacoustic instability. Experiments were performed in a laboratory-scale bluff-body stabilized turbulent combustor in the state of thermoacoustic instability. We use dynamic mode decomposition on the flow-field to identify dynamical regions where the acoustic frequency is dominant. We find that the separating shear layer from the backward-facing step of the combustor envelops a cylindrical vortex in the outer recirculation zone, which eventually impinges on the top wall of the combustor during thermoacoustic instability. We track the saddle points in this shear layer emerging from the backward-facing step over several acoustic cycles. A passive control strategy is then developed by injecting a steady stream of secondary air targeting the identified optimal location where the saddle points spend a majority of their time in a statistical sense. After implementing the control action, the resultant flow-field is also analyzed using LCS to understand the key differences in flow dynamics. We find that the shear layer emerging from the dump plane is deflected in a direction almost parallel to the axis of the combustor after the control action. This deflection, in turn, prevents the shear layer from enveloping the vortex and impinging on the combustor walls, resulting in a drastic reduction in the amplitude of the sound produced.

Sealing efficiency performance of four different rim seals in turbine cavity

In modern aero-engine, the turbine rim seals whose purpose is to reduce the ingress are located in a complex flow region between the mainstream and secondary air flows. The sealing air discharged from the compressor of engine is used to purge turbine cavity. In order to examine the effects of ingress through four distinct rim seals, a one-stage test rig was used in the experiments presented in this work. At a Reynolds number of 5.16 × 105 in the mainstream and (0.85–2.13) × 106 in the rotational, the radial pressure distribution on the stator was determined. To assess flow characteristics and sealing efficacy, measurements of the swirl ratio and CO2 concentration were made. The reliability of turbine rim-seals is determined by the locations of the hook and teeth, as well as their effect on hot gas ingestion. To evaluate the performance of four types of seals: a datum double-rim seal and three derivatives with different clearances, the data are employed. Due to the inlet air position, the swirl ratio exhibited a pronounced inflection for all the rim seals. The sealing efficiency can be decreased by putting the hook closer to the rim or by taking the teeth out. Static pressure measurements in the turbine cavity indicate that the seal position has a significant impact. The analysis results revealed that the existence of slot vortex is beneficial to improving efficiency.

Numerical Simulation Study of Combustion under Different Excess Air Factors in a Flow Pulverized Coal Burner

The basic national condition that is dominated by coal will not alter in the foreseeable future. Coal-fired boiler is the main equipment for coal utilization, and cyclone burner is a practical type of burner. There is a cyclone formation, a primary air duct inside the center air duct, and a secondary air duct. Introducing a small stream of pulverized coal gas or oil mist stream or gas directly into the reflux zone in the center duct ignites first a stable combustion and a small fluctuation of ignition pressure. In this paper, the variation of furnace temperature for cyclone pulverized coal burner corresponding to different excess air factors and the composition of gases such as O2, CO, CO2, and NOX produced by combustion were investigated using fluent software. A single cyclone pulverized coal burner from an actual coal-fired boiler is used, and a combustion zone applicable to the study of a single pulverized coal burner is established to study the actual operation of a single pulverized coal burner at different excess air coefficients. The findings indicate that the ignition position of pulverized coal combustion advances with decreasing α (Excess Air Factors); however, the length of the produced high-temperature flame gets shorter. As the value of α decreases, the burnout in the furnace decreases and the CO emission concentration increases, with a maximum CO mole fraction of 0.38% at α = 1.2 and a maximum CO mole fraction of 3.13% at the axial position when α decreases to 0.8. The furnace’s concentration of NOX, the NOX emission level decreases significantly with decreasing α. The NOX mole mass increases gradually with increasing α, and in the bottom portion of the primary combustion zone, more NOX is produced. The concentration of NOX in the chamber changes significantly after α exceeds 1.0, and the NOX at the outlet surges from 417.25 ppm to 801.07 ppm, which is attributed to the increase in the average temperature of the chamber, which promotes the generation of thermophilic NOX. The distribution pattern of O2 mole fraction along the furnace height cross-section at different excess air factors is basically the same, with a maximum at the burner inlet and a gradual decrease in the O2 content as it enters the combustion chamber to react with the pulverized coal in a combustion reaction. The value of α = 0.8 when the air supply is obviously insufficient, the fuel cannot be fully combusted, and only a small amount of CO2 is produced.

An integrated model for flexible simulation of biomass combustion in a travelling grate-fired boiler

Grate-fired boiler is of considerable interest for burning biomass owing to its reduced sensitivity to variations in fuel composition and size. However, grate-fired technology exhibits unsatisfactory performance in terms of combustion efficiency, contaminant emissions, and flame stability. CFD modelling can provide reasonable optimizations to overcome these drawbacks and ultimately achieve clean and efficient energy production. Conventional approach simulates the fuel-bed and freeboard combustion separately using an in- and over-bed coupling procedure, where data of the gases leaving from the packed-bed top and the radiative heat flux emitted by the high-temperature flame onto the bed cannot be interacted in real-time. This may cause distinct deviations in the calculations. In this paper, a three-dimensional (3D) full-scale integrated model is developed for the simulation of grate-fired boiler, in which the intensive combustion of both the freeboard and fuel-bed is solved in one scheme using the Eulerian-Lagrangian method. The gas phase is considered as a continuous medium and calculated by the Eulerian model, while the solid particles are considered as a discrete phase and tracked via the Lagrangian method. The reliability of the model is well verified through various field measurements and observations. Results shows that the key parameters are non-uniformly distributed along the grate width, i.e., the aerodynamic and combustion environment is poor at the vicinity of water-cooled walls compared with that at the furnace center. This results in a significant lag in volatile gases release and char oxidation near the water-cooled wall on both sides, which finally causes obvious differences in the distribution of temperature and species. For instance, the peak of CO concentration at the center is 12.6 % at 3.3m from the feed entrance, while that near the water-cooled wall is 8.6 % around 3.5m. The char burnout ratio in the bottom ash can be accurately calculated by this model, with a negligible relative error between the simulation of 81.97 % and the measurement of 82.50 %. It also provides a novel method for the calculation of fly ash particles by using size-grouped and non-spherical particles. Small-size particles in the vicinity of the feed inlet, as well as a part of the particles burning at the end of the grate are entrained into the freeboard by the primary air supplied beneath the grate and the high-momentum secondary air on the bed top. This model can provide valuable theoretical guidance for optimizing the refined distribution of fuel and air in grate-fired power plants and mitigating irregular deposition (corrosion) on heat exchangers and water-cooled walls caused by fly ash.

Performance prediction of co-rotating disk cavity with finned vortex reducer based on machine learning

Machine learning method was applied for rapid and precise prediction of flow and heat transfer characteristics within co-rotating disk cavity with a finned vortex reducer. A new data preprocessing scheme was developed to reduce modeling cost. By using this scheme, classical radial basis function neural network (RBFNN) shows better prediction performance compared with deconvolutional neural network. Furthermore, RBFNN has a simpler topological structure and fewer hyperparameters. By testing, the relative root mean square error for total pressure, total temperature, and swirl ratio is 0.51 %, 0.32 %, and 1.99 %, and corresponding coefficient of determination reaches 99.68 %, 96.55 %, and 99.16 %. The effects of input parameters on the outputs of RBFNN were analyzed, and a sensitivity analysis was conducted. Within the investigated parameter range, the total pressure loss increases as dimensionless mass flow rate, rotational Reynolds number, and radial position of the fin increases. The total temperature drop increases with the increase of dimensionless mass flow rate and rotational Reynolds number, but the decrease of radial position of the fin. Additionally, an increase in dimensionless mass flow rate or a decrease in rotational Reynolds number causes the increase of swirl ratio. This research provides an efficient modeling method for components in secondary air system in gas turbines.

Effect of momentum flow ratio on entrainment of a confined coaxial jet in a co-flow

Iron ore pellet production processes produce large amounts of CO2 emissions when burning fossil fuels. One example is the grate-kiln process, where normally a coal flame is used to indurate pellets. To mitigate the emissions, coal used in rotary kilns can be replaced with for example hydrogen. However, the fuel is mixed with secondary process air, and replacing a solid fuel with a gaseous one changes the mixing characteristics. This demands different means of injecting the fuel. A coaxial jet can be used to control the mixing of fuel and secondary air, as well as the flow field. The aim is to control the mixing of hydrogen and secondary air to achieve a hydrogen flame that is similar to the reference coal flame. This study numerically investigates different coaxial jet configurations. Steady-state simulations of a simplified model of the real kiln are performed using a Reynolds Stress turbulence model. Results show that decreasing the momentum flow ratio between the outer and inner jet, M jet , to a certain value delays the spread of the hydrogen jet and thus gives a longer jet. Further decreasing this ratio gives an even longer jet, but has the side effect of producing recirculation of hydrogen.

AERODYNAMIC IMPACT OF SECONDARY AIR INJECTION FLOWRATES ON THE MAIN ANNULUS FLOW FIELD FOR A TRANSONIC HIGH-PRESSURE TURBINE STAGE

Abstract In this study, the influence of secondary air blowing on the main annulus flow field of a transonic high-pressure turbine (HPT) was investigated experimentally and numerically. The experimental setup was a single-stage, unshrouded turbine with a blading consistent with modern HPTs. The entire testing campaign was conducted in a full annular, rotating, continuous turbine test rig at the Japan Aerospace Exploration Agency (JAXA), which realized the most faithful matching unrivalled of both the primary and secondary stream similarity parameters to reality. An elaborate secondary air system implemented in the hardware enabled it to mimic all the dominant coolant/purge streams representative of advanced hot sections: the full coverage film-cooling on stator and rotor airfoils, disk wheelspace purge air blown forward and aft of the rotor, and coolant injection through the over-tip casing. Detailed three-dimensional flow field measurements for various secondary air flowrates were performed by traversing a pneumatic thermometric combination probe downstream of the rotor. A complete set of numerical simulations was run concurrently with the testing to determine how well they captured the flow physics, particularly the interaction of the ejected secondary streams with the primary flow. The JAXA in-house code, UPACS, and its best practice settings, based on past verification and validation efforts, were employed and not intentionally tuned to better fit the data.

The influence of air distribution ratio on the fluctuation characteristics and stability of horizontal jet diesel non-premixed flame under extreme sub-atmospheric pressures

The burning problem in the plateau environment attracts more and more people’s attention, and the plateau adaptability and flame stability of burner are the focus of recent research. The main purpose of this paper is to reveal the horizontal jet spray flame fluctuation characteristics and stability change rule under extremely low pressure (0.05 MPa) at a high altitude (about 5500 m) inhabited by human beings. Flame morphology and temperature properties were investigated in a low-pressure chamber with different secondary air ratios (23–53 %). It is found that the normalization of the flame trajectory equation is not affected by the ratio of secondary air. The temperature in the flame recirculation zone can be increased by 110 K as the proportion of secondary air increases to 45 %, but the maximum temperature is still lower than 1500 K. A continuous flame image processing method was implemented to define and analyze the flame fluctuation region. The results show that the dimensionless fluctuation parameter γ can predict the flame stability in advance than β. When the secondary air ratio is 40 %, the flame has the largest upward fluctuation; and when it exceeds 45 %, the flame stability becomes worse. This work is the first to obtain the results of the study on flame morphology and stability of horizontal jet spray flame distribution ratio under extremely low atmospheric pressure, which provides a theoretical scientific basis for the design of plateau burner, flame control, and flame stability improvement.

CFD modeling and industry application of a self-preheating pulverized coal burner of high coal concentration and enhanced combustion stability under ultra-low load

Deep participation in peak regulation is a fundamental issue in flexible peak regulation. The ability to operate coal-fired power units under ultra-low load plays a crucial role in China’s pursuit of its energy goals. This study focuses on the performance analysis of a self-preheating pulverized coal burner with high coal concentration under ultra-low load conditions through simulations and experiments. A numerical comparison between the performance of a traditional swirl burner and a self-preheating burner is conducted using a 5 MW combustion test furnace. The results demonstrate that the self-preheating burner exhibits superior combustion stability and can maintain stable combustion even at an ultra-low load of 15 %. Additionally, the air distribution of the self-preheating burner under ultra-low load conditions is investigated numerically. The results reveal that under 15 % load, primary air rate of 12.0 % and internal secondary air rate of 54.2 % enable the self-preheating burner to achieve stable combustion performance with a char burnout rate of 98.6 %. Furthermore, in the absence of separated over fire air (SOFA) conditions, the NOx emission level ranges from 150 to 165 mg/m3. Based on numerical results, the self-preheating burner is implemented in an industrial setting, specifically a 29 MW coal-fired industrial boiler demonstration project. The results indicate that the self-preheating burner can achieve stable combustion at approximately 13 % load, with an average combustion efficiency of 93.24 %. The research validates the excellent performance of the designed self-preheating burner under ultra-low load conditions, which contributes to addressing the challenge of deep participation in peak regulation.

A view on the performance comparison between adiabatic and internally-cooled dehumidification processes using liquid desiccant

Humidity control is emphasized in various fields. However, the effectiveness comparison between adiabatic dehumidifier (AD) and internally-cooled dehumidifier (ICD) was only conducted in limited cases, without comprehensive understanding of different cooling mediums and working conditions. In this study, the component effectiveness of these two dehumidifiers is parametrically compared. When chilled water serves as cooling medium, AD has sensible effectiveness of 0.65 and latent effectiveness of 0.84 under baseline case, while those of ICD are 0.52 and 0.85 respectively. AD performs better under higher sensible loads, and ICD is more capable of dealing with more moisture transfer. Low water temperature, low air-desiccant ratio and adequately low water-desiccant ratio also highlight the superiority of AD. When secondary air serves as cooling medium, AD always has higher effectiveness than ICD in the studied range, and the superiority is more significant at lower temperature and humidity of secondary air. Sensible effectiveness of 0.44 and latent effectiveness of 0.78 for AD are obtained under baseline case, while those of ICD are 0.07 and 0.74 respectively. ICD has better adaptivity than AD at a circumstance with great variation of operation parameters. The results contribute to enlightening insights for further design of moisture capture components.

COMPARATIVE ANALYSIS OF ENERGY EFFICIENCY OF THERMAL OPERATION OF ROTARY KILNS WITH DIFFERENT HEATING SYSTEMS

Analysis of the design features of known heating systems for large rotary kilns and modern methods of influencing the formation of the flame and the distribution of the temperature profile is presented. It has been established that most of the known methods of forming the flame and temperature profile of the working space of rotary kilns are based on methods of influencing air flows, in particular secondary air, the share of which in the total volume of combustion air is 70–100 %. On the basis of previous studies and observations, it is proposed to form a flame using additional sideways gas jets from the burner. Examples of modernization of heating systems of existing industrial rotary kilns for firing various materials, including ferronickel ore, fireclay, and lime, are presented. After installing burners with controlled flame parameters and changing the design of the combustion air supply system in the working space of the kilns, an optimal temperature distribution along their length was obtained. It was determined that the required temperature at about half the length of the kilns is almost constant, without significant fluctuations, differing at the beginning and end of the firing zone by 30–70 °C. The decrease in the temperature of the exhaust gases from the kilns after the modernization of the heating systems indicates an intensification of heat exchange in the workspace, which leads to a decrease in specific fuel consumption by 7–15 %, as well as an improvement in the quality of the final material. A comparative analysis of the thermal efficiency of operating rotary kilns depending on the design features of the heating system is presented. Bibl. 24, Fig. 4.

Effects of secondary air on the emission characteristics of ammonia–hydrogen co-firing flames with LES-FGM method

Ammonia, being a carbon-free fuel, is garnering increasing attention in the combustion community. However, its application is still impeded by its limited stability range and significantly NOx emissions. This study delves into the impact of primary and overall equivalence ratios (ϕpri and ϕovr) on emissions production in ammonia–hydrogen co-firing flames, utilizing a two-stage swirling model gas turbine combustor. The emissions concentration is measured using Fourier Transform Infrared Spectroscopy (FTIR). The physical and chemical processes influencing emissions production are comprehensively analyzed through a combination of Large Eddy Simulation (LES) and Flamelet-Generated Manifold (FGM) method. The reliability of the simulation is validated by the experimental data, showing a good capability in predicting the combustion. Results indicate that the two-stage combustion strategy exhibits significantly controlling effects on NOx and unburned NH3 emission. The NO emission exhibits a non-monotonic variation with ϕpri, reaching its lowest value at ϕpri≈ 1.2. Further analysis shows that ϕpri plays a crucial role in determining the temperature and OH concentration in the primary combustion zone, thereby influencing NOx production in this area. When the primary zone is operated at lean condition, the reduction on NO in secondary zone is primarily attributed to dilution effect by the secondary air. In contrast, when ϕpri> 1, the concentrations of the main emissions in secondary zone are dominated by chemical effects. In this scenario, unburned NH3 and H2 from the primary zone are consumed, resulting in a substantial production of NO in the secondary combustion zone. Additionally, with an increase in ϕpri, the secondary NO production also increases. The ϕovr exhibits two considerable effects. On one hand, an increase in ϕovr results in a higher local equivalence ratio in the secondary zone, thereby promoting local NOx production. Simultaneously, the elevated flame temperature enhances the consumption of N2O. On the other hand, the rise in ϕovr results in a reduction in vortex scale and residence time in the secondary combustion zone, leading to a decrease in local NO production.

Effects of the addition of arch-supplied secondary air on the performance of the Foster Wheeler down-fired boiler: Air/particle flow, combustion and NOx emissions

This study proposes a low NOx combustion method for Foster Wheeler type down-fired boilers. The effectiveness of this technique was validated through laboratory-scale gas-particle two-phase experiments and industrial trials. The flow field characteristics within FW-type boiler furnaces equipped with arch-supplied secondary air injection nozzles were investigated. Furthermore, the application of this technique to a 660 MW subcritical FW-type boiler was examined, focusing on NOx emissions and combustion characteristics. Results demonstrate that optimizing the damper openings for F-layer and arch-supplied secondary air can significantly reduce NOx emissions by over 33 %. Additionally, increasing the mass flow rate of arch-supplied secondary air aids in reducing unburned carbon in fly ash and inducing a downward shift in the flame center within the furnace.

Computational boundary improvement and performance optimization of the flue gas waste heat cascade utilization system of the ultra-supercritical 1000 MW generator set

Abstract There is no uniform performance test rule and standard calculation method for efficiency improvement of a system that utilizes waste heat from flue gas and it is mainly carried out through turbine parameters, without considering boiler system performance. Given the above problem, in this paper, the steam turbine, boiler and waste heat from the flue gas step-up utilization system are calculated by establishing a thermal model of ultra-supercritical secondary reheating 1000 MW unit. At the same time, unit design data and field performance test results are used for verification. Analysis comparing simulation outcomes with experimental data revealed that the discrepancies in the heat consumption rate per unit, the efficiency of the boiler, and the rate of coal consumption for power generation peaked at a marginal variance of 0.2%. The optimum operating parameters of the system for low-grade flue gas recovery in a cascading manner are obtained through variable operating condition calculations. The optimum value of high-pressure bypass feed-water flow rate is 52 kg/s at 970 MW operating condition. At the same time, the difference between high-pressure bypass effluent and high plus effluent temperatures should be controlled to be close to 0°C, and the optimal value of low-pressure bypass condensate flow rate is 53 kg/s. As the flow of flue gas in the bypass increases, the efficiency of heat exchange in the bypass economizer also increases, resulting in greater absorption of heat in the utilization apparatus for flue gas waste heat, leading to a more effective energy-saving outcome. However, it is necessary to regulate the temperature of both the primary and secondary air to levels that exceed the minimum requirements for the safe functioning of the coal mill.

Effects of activated fuel and staged secondary air distributions on purification, combustion and NOx emission characteristics of pulverized coal with purification-combustion technology

Under the strategic objectives of carbon peaking and carbon neutrality, clean and efficient coal combustion was important in China. As a novel combustion technology, purification-combustion technology had good research prospects, and high-temperature reduction unit (HTRU) played a crucial role in fuel activation, subsequent combustion and NOx emissions based on the design of this technology. Despite previous research had proved the pivotal role of fuel and combustion air distributions in combustor in influencing the entire thermochemical process, there was the lack of research on their effects in HTRU employing purification-combustion technology. To realize deep NOx emission reduction, this study focused on influences of activated fuel and staged secondary air distributions in HTRU on purification, combustion and NOx emission characteristics in a 30 kW purification-combustion test rig. With the exception of carbon microcrystalline structure, the majority of reductive char properties exhibited superior performance when activated fuel was introduced tangentially, as opposed to axially. Nevertheless, the relationship of carbon microcrystalline structure was regulated by staged secondary air ratio (c12). Properly increasing c12 improved reductive char properties. Additionally, axial introduction of activated fuel more consistently yielded ultra-low levels of NOx emissions; however, this often occurred at the expense of combustion efficiency (η). Properly increasing c12 reduced NOx emissions while increasing η; nevertheless, the benefits derived from c12 were limited. By adjusting introduction direction and c12 (axial, 0.33), minimum NOx emissions decreased to 39.10 mg/m3 (@6 % O2) while maintaining high η of 99.39 %.

V-shaped effect of primary and secondary airflow momentum ratio on performance of a 660 MW tangentially fired lignite boiler: Identification and verification

To explore how the primary and secondary air ratios affect the overall performance of lignite-fired power generation boilers, a 660 MW wall-tangentially fired lignite boiler was chosen as research subject. 17 combustion scenarios were designed and then numerically simulated through a validated model, and the in-furnace coal combustion behavior, heat transfer process, and NOx conversion characteristics under various conditions were analyzed in detail. Results show that, except for the monotonous increase in NOx emission, the overall boiler performance follows a V-shaped trend with the continuous increase in primary air ratio (PAR). Initially, as PAR is slightly increased, the in-furnace combustion temperature, heat transfer intensity and coal utilization rate notably decrease, followed by a subsequent increase in these parameters as PAR is significantly increased. The separated over fire air (SOFA) ratio doesn’t alter this V-shaped pattern; however, a higher SOFA ratio does reduce the critical PAR at which the overall boiler performance is most compromised. This is attributed to changes in the momentum ratio between primary and secondary airflow caused by varying PAR. Boiler performance deteriorate significantly when their momentum ratios are nearly equal, and a higher SOFA ratio makes it easier. These observations are confirmed under different thermal load conditions, where the close proximity of primary and secondary air momentum is avoided by reducing SOFA ratio. Based on these findings, a substantial increase in PAR should be avoided in practice, if necessary, lowering the SOFA ratio helps mitigate significant deterioration in boiler performance.

Redesigning the Element Profile of Air Preheater in Coal-fired power plants for Emission Reduction and Efficiency Improvement

PT PLN Nusantara Power Unit Pembangkit Tanjung Awar-Awar (UPTA) is a state-owned company that supplies electricity to areas of Java and Bali. Using coal as the main fuel, UPTA currently has two mills that each has capacity of 350 MW in coal-fired mode. Since the last few years, UPTA has been focusing on bringing efficiency into its overall operations, maximizing efforts for an improved output, modernizing machinery, and reducing emissions. After conducting routine inspection and examinations, UPTA decided to improve the air preheating element, so-called air preheater (APH), as an enabling condition to heat both the primary and secondary air to a temperature required for effective combustion in a boiler. To seek for an optimal APH, a redesign project was initiated to primarily change the element profile from a distorted wavy flow path to a linear flow path. The profile surface was coated with an added "enamel" layer, which prevents ash from adhering readily. Our measurements and calculations were based upon the data collected in October 2022 (pre- implementation) and within the periods of February to June 2023 (post-implementation). As a result, the average power consumption was reduced: from over 1740 kWh per month to become around 1377 - 1535 kWh per month, or 11% - 21%, for the induced draft fan (ID Fan) and from over 1900 kWh per month to become around 1450 - 1542 kWh per month, or 19% - 29%, for the primary air fan (PA Fan).