Tertiary Air Research Articles

Numerical simulation study on the influences of the secondary-tertiary air proportion on the airflow mixing effects and pulverized coal combustion characteristics in a 300-MW down-fired boiler

In this paper, based on the MIMSC (multi-injection and multi-stage combustion) technology, new burner arrangement and air distribution parameter settings were proposed for a 300-MW subcritical down-fired boiler originally using MBEL (Mitsui Babcock Energy Limited) combustion technology. Numerical simulations were conducted to study the influences of the secondary-tertiary air proportion on the airflow mixing effect in the furnace, the ignition and pulverized coal combustion characteristics. The airflow mixing effect in the furnace is characterized by the size of the dimensionless vertical velocity decay area (Vda) and the fluctuation of the maximum vertical velocity decay curve (Vdc). During the research, the sum of the secondary and tertiary air rate remained constant, and the secondary air rate was set to 30.54%, 33.54%, 36.54%, 39.54%, and 42.54%. It was found that under the condition of using new type burner, with the secondary air rate increased from 30.54% to 42.54%, the ignition distance of pulverized coal decreased from 1.30m to 0.84m, the dimensionless penetration depth decreased from 1.46 to 1.27, and the NOx emission and carbon in the fly ash decreased first and then increased. The variation of Vdc showed the following regulation. At the secondary air rate of 30.54% to 33.54%, increasing the secondary air rate decreased the fluctuation amplitude. While at the secondary air rate of 39.54% to 42.54%, increasing the secondary air rate increased the fluctuation amplitude. Only at the secondary air rate of 36.54%, Vdc in the tertiary air mixing area was smooth. When Vdc was smooth, Vda was small at 0.043, the carbon in fly ash at the furnace outlet was the lowest at 4.19%, and the NOx emission was low at 675.9 mg/m3 at 6% O2. An optimal secondary air rate of 36.54% is recommended. For the subsequent design of burner structure and parameters, it is suggested that Vdc in the tertiary air mixing area should be smooth.

Optimization design of the key structural parameters of an aggregate drying pulverized coal burner

ABSTRACTIn order to reduce the costs of drying aggregate, an efficient coal-fired aggregate pulverized coal burner was optimized. A transient numerical simulation of the internal flow field of pulverized coal burner was carried out by means of computational fluid dynamics (CFD) to obtain the experimental data needed for structural optimization. Taking the straight length, radius, primary air inlet diameter, front cone angle, rear end cone angle, tertiary air inlet diameter, and secondary air inlet size as the optimization variables, and the production amount of CO was taken as the optimal target. The Box-Behnken Design response surface was established to optimize seven structural parameters through grouping. The results showed that the first structural parameter optimization demonstrated that the production amount of CO was decreased 34%, the second structural parameter optimization demonstrated that the production amount of CO was decreased by 58%.

Analysis of Influence of Adjusting Air Distribution Mode on Boiler Combustion under Different Loads

In view of the main problems faced by power plants, many scholars in China have focused on the problem of distribution adjustment of secondary air. This paper also conducted research on the adjustment of secondary air distribution. In this paper, a 200MW four-corner tangential boiler of a power plant is taken as the research object, and the overall size of the boiler and the modification of the damper are clarified. The boiler is tested under different loads and different working conditions. Through the inverse balance calculation of boiler thermal efficiency on the SCR inlet NOX analysis, the combination of numerical simulation and test method provides an important reference for the four-corner tangential boiler optimization and variable load operation in the future. At present, the domestic research on the numerical simulation of boiler combustion is mainly to change the different secondary air distribution methods. In this paper, the analysis of the effects of different number of tertiary air nozzles on the combustion of boilers under different loads is studied under the condition of equal secondary air.

Energy and Exergy Analysis of Clinker Cooler in the Cement Industry

Grate coolers which represent one of the pyro-processing units are extensively used in cement industries. The essential function of grate coolers is a waste heat recovery from hot clinker which leaves the rotary kiln. This paper presents an analysis of energy and exergy based on the clinker temperature profile. It focuses on the distributions of temperature, energy and exergy for cooling air and hot clinker along the grate cooler in cement industries. Consequently, this distribution allows predicting the temperatures of secondary air, tertiary air, and exhaust air, which will lead to estimation of waste heat recovery from the clinker cooler system.

Experimental investigation and prediction of heat transfer in a swirling fluidized-bed combustor

In this experimental study, the heat transfer coefficient in a twin-cyclonic swirling fluidized-bed combustor (T-FBC) with a conical shape bed was investigated to verify the effects of combustor design feature and operating parameters. The combustor was operated at a ratio of secondary and tertiary air to primary air ranging from 0 to 0.5 at primary air velocity (up) near minimum swirling fluidization condition (ums), to about 3ums by using silica sand with particle size diameters (dp) of 300–500, 600–710, and 710–1000 µm as bed material, for swirl number of an annular spiral air distributor of 2.76, and 2.98. The local heat transfer coefficients were measured in radial positions at the levels of 60, 125, 223, and 288 cm above the primary air distributor and the axial profiles of average heat transfer coefficient were characterized. The experimental results revealed that the radial profile of heat transfer coefficient was maximum at the center of the combustor for most of the test trials. Within the range of the tests, the heat transfer coefficient noticeably increased with the increase of the primary air flow rate and swirl number of the air distributor, and reached the almost constant value at 2.5–3ums. With design features of this combustor, the heat transfer coefficient in the bed region was comparable to the value in freeboard region though about 5–12% higher. Based on the test results, semi-empirical models for prediction of average Nusselt numbers were developed in the dense bed and freeboard zones for the different swirl numbers of 2.76 and 2.98, and the proposed models were in close agreement with the experimental data within ±20% error for both regions.

Studying the Aerodynamics of the TPP-210A Boiler Furnace When It Is Shifted to Operate with Dry-Ash Removal and Vortex Fuel Combustion

To reduce the amount of nitrogen oxide emissions and achieve more reliable operation of the TPP-210A boiler, a process arrangement for firing Grade TR Kuznetsk coal that involves using straight-flow burners and shifting the boiler from slag-tap to dry-ash removal is developed. Owing to a large burner downward slope angle and special arrangement of burners and nozzles, four large vertical vortices rotating in opposite directions are produced in the furnace lower part, as a result of which the combustion products dwell for a longer period of time in the burning zone and more complete fuel combustion is achieved. For verifying the operability and efficiency of the proposed combustion arrangement, investigations on a boiler furnace physical model were carried out using a technique for visualizing fuel jets and secondary and tertiary overfire air jets. The fuel jet temperature boundaries in the course of jet propagation in the furnace model are also determined. The study results have shown that staged fuel combustion will be set up with using the proposed arrangement of burners and nozzles. In addition, large vertical vortices produced in the furnace lower part will help to achieve more efficient use of the dry bottom hopper heating surfaces, due to which lower coal combustion product temperature in the furnace upper part and smaller content of combustible products in fly ash will be obtained. Owing to low values of air excess factor at the pulverized coal burner outlet and gradual admission of air into the vortex zone through a few nozzles with intense inner recirculation of combustion products to the jet initial section, staged combustion of pulverized coal and low nitrogen oxide emissions will be secured. Owing to expansion of fuel jets, a rapid growth of mass in the fuel jet is achieved, which is obtained both due to ejection of the jet itself and due to forced admission of hot fuel gases from other jets. Investigations carried out on the physical model have confirmed that the proposed combustion arrangement features high efficiency and that a low content of nitrogen oxides in flue gases is obtained.

The influence of air-stage method on flameless combustion of coal gasification fly ash with coal self-preheating technology

As the by-product of circulating fluidized bed (CFB) coal gasification, coal gasification fly ash (CGFA) contains low volatile matter (lower than 6 wt%) and high ash matter (higher than 40 wt%), which cannot be burnt steadily and effectively with conventional combustion technologies. However, the annual output of CGFA is enormous in China and the available energy is considerable. Therefore it is necessary to achieve a highly efficient and clean combustion of CGFA. As a novel combustion technology, coal self-preheating combustion has proven its potential to utilize semi-coke, anthracite and other flammable solid fuels, which can also be applied for CGFA. In this paper, flameless mode of CGFA was achieved in a 30 kW coal self-preheating combustion test rig and the influence of air-stage method on image luminosity, temperature profiles and NOX emissions was studied. The results show that stable flameless combustion of CGFA can be easily established, and in the combustion zone, the image luminosity is uniform and no obvious flame fronts can be observed. Compared with flame mode, the combustion zone is enlarged in flameless mode and the temperature profile is more uniform. The effect of the tertiary air nozzle position on temperature profiles is greater than that of the secondary air equivalence ratio. And with the increase of λ2, the temperatures of zone close to the preheated fuel nozzle decrease. The oxygen concentration along the axis of the down-fired combustor is uniform with the peak value less than 6 vol% for all cases. The exit NOX emissions can be lowered down a lot for flameless combustion, and the general emissions are less than 100 mg/Nm3 (@ 6% O2) with the lowest value even less than 50 mg/Nm3 (@ 6% O2).

CFD analysis of a swirl stabilized coal combustion burner

Anthropogenic Greenhouse Gases (GHG) emissions have increased since the pre-industrial era, driven by the economic and population growth. In the last decades, significant efforts have been made to develop cleaner and more efficient technologies able to control GHG emissions. In the field of combustion burners, this commitment strongly relies on the development of new and detailed numerical models, which allow a deep investigation of complex burner performance and a significant reduction of full-scale experimental costs. The knowledge previously acquired on the TEA-C coal burner CFD study is here expanded in the framework of the Be4GreenS project, targeting to the development of a new generation of burners. The new implemented model accounts for the detailed chemical and kinetic characterization of the pulverized coal together with the exact inner volume geometry of the experimental combustion chamber and the actual extension of the heat exchanging and refractory surfaces. In addition, the secondary and tertiary air registers are here numerically investigated. The outcome from the CFD analysis is validated against the experimental data. The NOx formation analysis is also provided.

Experimental study on combustion, flame and NOX emission of pulverized coal preheated by a preheating burner

Experimental researches on combustion characteristics of pulverized Ningdong bitumite preheated by a preheating burner were carried out in a 0.2 MW coal preheating combustion test rig, and the effects of preheating temperature, secondary air equivalence ratio and the positions of tertiary air nozzles on combustion, flame and NOX emissions of preheated fuels were studied. The results showed that the 0.2 MW coal preheating combustion test rig can operate stably and the combustion efficiency can be higher than 97% while the NOX emissions are lower than 100 mg/Nm3. With the increase of preheating temperature, NOX emissions increase, and the combustion efficiency does not change obviously. With the increase of secondary air equivalence ratio, the combustion efficiency increases, and NOX emissions increase as well. With the increase of the distance between the tertiary air nozzle and the preheated fuel nozzle, NOX emissions decrease, and the combustion efficiency decreases as well. The flame characteristic is not sensitive to preheating temperature, but closely related to the air distribution of down-fired combustor. With reasonable air supply positions, flameless combustion can be realized.

Experimental study on NOx emissions of pulverized bituminous coal combustion preheated by a circulating fluidized bed

This study investigated nitrogen oxide (NOx) emissions of pulverized coal combustion preheated by a circulating fluidized bed (CFB). During the test process, high-temperature fuel preheated in a CFB was burned in a down-fired combustor (DFC). The effect of air distribution on NOx emissions was studied in the DFC, including three types of secondary air nozzle structures, five secondary air ratios, and three tertiary air position arrangements. Under stable conditions, the conversion ratio of fuel-nitrogen to N2 in the CFB was 41.4%, which resulted in lower NOx emissions in the platform. In this study, secondary air could be injected into the combustor at the top (annular) or through the side wall (circular) of the DFC, both with high combustion efficiency. This means that the secondary air is completely separated from the burner, and burner structure is greatly simplified. NOx emissions from secondary air nozzle structures of center, annular, and circular ports were 565.66, 345.45, and 220.38 mg/Nm3 (@6% O2) respectively. NOx emissions initially decreased then increased with increases in secondary air ratio with the annular nozzle structure. NOx emissions could be further inhibited by rationally arranging tertiary air positions.

Experimental research on combustion characteristics of coal gasification fly ash in a combustion chamber with a self-preheating burner

Coal gasification fly ash (CGFA) is the by-product of coal gasification in a circulating fluidized bed (CFB) and is characterized by its low volatile-content, low carbon-content, and high ash-content, which can be used as a secondary fuel. In China, the annual output of CGFA is enormous; therefore, the realization of a highly efficient CGFA combustion method would be beneficial to achieve better overall coal utilization. Nevertheless, it is challenging to burn CGFA steadily with conventional technologies because of its low volatile content. However, the coal preheating combustion technique has been proven to be an effective method to burn semi-coke and anthracite, both of which are poorly flammable, and could also be applied to CGFA. We have used an advanced preheating combustion technique with a self-preheating burner and carried out experimental investigations into CGFA combustion in a 0.4-MW CGFA preheated combustion test rig. The results show that the preheated combustion system ran smoothly and the CGFA combustion efficiency reached 98.6% with NOX emissions of 155 mg/m3 (6% O2). Therefore, the highly efficient and clean combustion of CGFA was realized, and the combustion of poorly flammable fuels using a self-preheating burner was shown to be feasible. The effects of the primary air ratio, secondary air ratio, and the position of the tertiary air inlet on the combustion characteristics and NOX emissions of CGFA were also studied.

Mathematical model and numerical solutions for the coupled gas–solid heat transfer process in moving packed beds

A theoretical study was performed into the coupled gas–solid heat transfer process in a moving cooling packed clinker bed. A mathematical model for the clinker cooling process including effects from clinker movement is proposed. The aim is to predict through numerical methods the temperature distribution in the clinker layer and determine the optimal operating conditions. The equations and boundary conditions are discretized using the Taylor series expansion. The Jacobi iteration algorithm is employed to solve the difference equations to obtain temperature distributions for the clinker cooling process. The results are validated using industrial data. The relative errors are 7.0% and 1.1% for secondary and tertiary air temperatures, respectively. An analysis of the temperature difference distribution between clinker and cooling air confirmed the need to apply thermal non-equilibrium conditions in packed bed modeling. The different clinker speeds and thicknesses are also calculated. The results show that, with the same thickness of clinker layer, the most effective speed of the clinker is 0.008 m/s and ensures clinker cooling and heat recovery requirements. With the same clinker mass flow rate, operating with a thicker clinker layer can improve heat recovery and decrease the clinker outlet temperature; both can be used as a guiding framework in real cement production.

Numerical simulation of the complex thermal processes in a vortexing precalciner

To reveal the coupling process of coal combustion and calcium carbonate (CaCO3) decomposition, an actual vortexing precalciner was numerically simulated. Precalciners are used to pre-decompose the raw meal (mainly CaCO3) in a suspended condition. The flow pattern of a mixed stream of tertiary air and flue gas was displayed, the coal combustion characteristics in the precalciner were analyzed, and the coupling mechanism of the coal combustion and raw-meal decomposition was explored. The predicted burn-off rate of the pulverized coal stream was 98%, and the decomposition rate of the CaCO3 was 93%, in accordance with actual engineering data. The results show that the interaction of the transverse rotating tertiary air and the vertically spouting flue gas causes a spirally rising gas-flow field. The pulverized coal stream starts to burn when injected into the prechamber, and burns rapidly when rising into the cone section and the column section above the prechamber, forming a violent combustion region where the CaCO3 decomposes rapidly. Then, when the pulverized-coal and raw-meal streams spirally rise along the precalciner, the char burning rate reduces gradually and the CaCO3 decomposition rate decreases until the coal-combustion reaction and the CaCO3 decomposition end. The results obtained in this study not only provide important theoretical guidance for optimizing the precalciner’s operation parameters, but also lay a foundation for further research about this type of precalciner, e.g., the staged and refuse-derived fuel combustion processes, to save energy and reduce emissions.

Effects of tertiary air damper opening on flow, combustion and hopper near-wall temperature of a 600 MWe down-fired boiler with improved multiple-injection multiple-staging technology

In consideration of increasing the tertiary air damper opening of a 600 MWe down-fired boiler with prior multiple-injection multiple-staging technology facilitated the coal burnout, while largely increasing the NOx emissions. Additionally, the flame kernel was greatly moved downwards, thus causing significant temperature variations in the hopper near-wall region and the water wall in the lower furnace was vulnerable to overheating. This work concentrated on the comprehensively improved multiple-injection multiple-staging technology, both 1:20 scale cold aerodynamic tests and industrial experiments were conducted to examine the effects of tertiary air damper opening on flow, combustion, NOx emissions and especially the hopper near-wall temperatures. The aerodynamic tests indicated that, on increasing the tertiary air damper opening from 40 to 70%, all the flow fields exhibit good symmetry. The tertiary air flows downwards along the hopper near-wall region, with a maximum near-wall dimensionless vertical velocity significantly increasing from 0.48 to 0.66, and accordingly, the dimensionless depth of downward airflow increases from 0.744 to 0.846. The industrial experimental results showed that, upon introducing more tertiary air, the ignition distance of fuel-rich coal/air flow shortens from 1.25 to 0.87 m. The coal burnout is enhanced, carbon in fly ash drops from 6.90 to 6.15%, while the NOx emissions slightly increase from 593 to 641 mg/m3 at 6% O2. On reducing the measuring height of hopper near-wall temperature from 9.1 to 3.3 m, the average heating rate increases from 0.44 to 0.63 °C/mm. The increased tertiary air damper opening presents an increasingly obvious cooling effect on the hopper near-wall region, with the temperature reductions around 50 °C, which is conductive to protect the water wall in the lower furnace from overheating.

Influence of calcination process on the formation of selected air pollutants

The subject of the study is to analyze the phenomena of thermal flow in the precalcinator chamber of the exchanger's furnace tower including the combustion of coal dust and decarbonisation of raw lime powder. During the research were provided development of a mathematical model of particulate solid fuels combustion, calcining the raw material, NO x and CO x formation. Moreover conducting the number for the current and the upgraded design of the precalcinator and analysis of the results. In this study, a mathematical model based on Euler's method to describe the motion of the gas phase and the Lagrange method to describe the motion of particles [1-4]. In the calculations there were assumed fractional particles raw material and fuel, and the following processes: flow of exhaust gases from the rotary kiln through the precalcinator chamber, heat exchange between the particles of raw material and exhaust gases, the additional fuel combustion in the precalcinator, the process of raw material calcination, transformation of gaseous substances, effect of the additional (tertiary) air delivery on the processes in the chamber.

Factors affecting the downward flame depth in a 600 MW down-fired boiler incorporating multiple-injection and multiple-staging technology

Considering the excessively deep flame depth existing in a 600 MW down-fired boiler incorporating multiple-injection and multiple-staging technology, 1:20 scale aerodynamic tests were conducted to incrementally improve factors responsible for the deep flame depth. These trials demonstrated that, in all cases, the downward velocity near the wing walls decayed more rapidly than that near the furnace center. Increasing the mass ratio of pulverized coal in fuel-rich flow to that in fuel-lean flow and reducing the secondary air ratio while simultaneously increasing the tertiary air ratio was found to increase the penetration depth. Increasing the distance between adjacent burners, optimally lowering the fuel-rich and fuel-lean flow velocities, and reducing the secondary air velocity all decreased the penetration depth. The comprehensively improved downward airflow depth and air flux into the furnace hopper were reduced by 6.3% and 14.9%, respectively. Cold-state airflow tracing tests were performed in an actual boiler, the improved downward airflow depth reduced apparently, which was consistent with the results from modeling tests. Industrial-scale hot-state experiments determined improved hopper near-wall temperatures of 700–800 °C near the furnace center (values that were lower than those of 800–900 °C near the wing walls). These values were approximately 450 °C less than the prior temperatures at the same location, indicating the flame penetration depth was greatly reduced.

Modification of the inlet to the tertiary air duct in the cement kiln installation

Abstract Rotary kiln installation forms a very complex system, as it consists of various components which affect cement production. However, some problems with particle settling are encountered during operation of tertiary air installation. This paper reports on the results of a study into gas-particle flow in a tertiary air duct installation. This flow was calculated using Euler method for air motion and Lagrange method for particle motion. The results in this paper demonstrate that study focus on the tertiary air installation is a practical measure without the analysis of other processes in the rotary kiln. A solution to this problem offers several alternatives of modifying the inlet to the tertiary air duct. As a result of numerical calculations, we demonstrate the influence of geometry of a rotary kiln modification on the number of large particles transported in the tertiary air duct. The results indicate that in order to reduce large particles, rotary kiln head geometry needs to be modified, and a particle settler should be installed at its outlet.

Gas/particle two-phase flow characteristics of a down-fired 350 MWe supercritical utility boiler at different tertiary air ratios

To investigate the influence of the air distribution in the down-fired boiler which adopts the multi-injection and multi-stage combustion technology, a 1:20 small-scale model of a down-fired pulverized-coal 35 MWe supercritical utility boiler was set up to study the gas–solid two-phase flow characteristics at different tertiary air ratios by using PDA (particle dynamics anemometer) measurement. Gas and particles move downward near the front/rear wall and toward the furnace center area. A U shape flow field formed in the half furnace, the volume flow flux of the particles exhibited a peak beneath the primary air nozzles, the value of the peak decreased as the air flow injected downward In the area of the tertiary air nozzles, the horizontal/vertical fluctuation velocity for gas/solid phases near the furnace hopper wall increased rapidly, the airflow mixed here intensely. As the tertiary air ratio increases, the maximum volume flux increased obviously, and was conducive for the pulverized coal to ignite. Maximum gas-phase vertical velocity and maximum solid-phase particles volume flux moved further away from the hopper. This feature reduced the possibility of the occurrence of both flushing and slagging of the hopper.

Quantitative Assessment of Flame Stability Through Image Processing and Spectral Analysis

This paper experimentally investigates two generalized methods, i.e., a simple universal index and oscillation frequency, for the quantitative assessment of flame stability at fossil-fuel-fired furnaces. The index is proposed to assess the stability of flame in terms of its color, geometry, and luminance. It is designed by combining up to seven characteristic parameters extracted from flame images. The oscillation frequency is derived from the spectral analysis of flame radiation signals. The measurements involved in these two methods do not require prior knowledge about fuel property, burner type, and other operation conditions. They can therefore be easily applied to flame stability assessment without costly and complex adaption. Experiments were carried out on a 9-MW heavy-oil-fired combustion test rig over a wide range of combustion conditions including variations in swirl vane position of the tertiary air, swirl vane position of the secondary air, and the ratio of the primary air to the total air. The impact of these burner parameters on the stability of heavy oil flames is investigated by using the index and oscillation frequency proposed. The experimental results obtained demonstrate the effectiveness of the methods and the importance of maintaining a stable flame for reduced NO x emissions. It is envisaged that such methods can be easily transferred to existing flame closed-circuit television systems and flame failure detectors in power stations for flame stability monitoring.

Integration of oxygen membranes for oxygen production in cement plants

The present paper describes the integration of oxygen membranes in cement plants both from an energy, exergy and economic point of view. Different configurations for oxygen enrichment of the tertiary air for combustion in the pre-calciner and full oxy-fuel combustion in both pre-calciner and kiln are examined. The economic figures of merit are compared with those from a standard cryogenic plant. Both oxygen enriched air and full oxy-fuel cases allow for an increase in clinker production, use of alternative fuels as well as on-site electricity production. In addition, the full oxy-fuel cases generate a concentrated CO2 source that can be used for enhanced oil recovery, in combination with biomass gasification and electrolysis for synthesis gas production, or possibly sequestered. The cases with oxygen enriched air provide very promising economic figures of merit with discounted payback periods slightly higher than one year. The full oxy-fuel cases have a discounted payback period of approximately 2.3 years assuming a CO2 selling price of 35 US$/ton. The sensitivity analysis of full oxy-fuel cases clearly shows that for the discounted payback period, the most sensitive parameters are the CO2 price and the clinker selling price.