Tangentially-fired Boiler Research Articles

Effects of structural parameters of a novel burner scheme with annularly combined multiple airflows on performance of a 660 MW tangentially fired boiler

Based on a previously validated 660 MW tangentially-fired boiler model, effects of equivalent ring diameter (ERD) and ring centroid offset distance (RCOD) of the proposed burner with annularly combined multiple airflows (ACMA) on boiler performance are investigated. Results show that both ERD and RCOD have great influences on ACMA boiler performance. Small ERD enhances the mutual entrainments of combined airflows, but the interweave of combined airflows hinders flue gas to get into their interior. Although large ERD weakens airflows’ mutual entrainments, flue gas can enter the interior of combined airflows to avoid the large pressure difference from the fireside to the backfire sides, so airflow deflection is alleviated. For coal combustion process, small ERD only promotes the intense combustion inside each ACMA burner, while increased ERD makes the intense coal combustion process more continuous at the burner gaps. The increased RCOD adversely affects both the aerodynamics and coal combustion performance, resulting in large airflow deflections and deteriorated coal combustion process. For the investigated boiler, ERD of 3.6 m and RCOD of 5.0 m are recommended. These findings deepen the understanding of ACMA burners and, together with future experimental research results, can provide guidance for its application in practical boilers.

Numerical optimization of combustion and NOX emission in a retrofitted 600MWe tangentially-fired boiler using lignite

In order to meet the ultralow emission standard of NOx, the combustion system of a 600 MWe tangentially-fired lignite boiler was retrofitted using air-staged combustion technology. The retrofit scheme is described in detail. Combustion and NOx emission characteristics of the original and retrofitted boiler were numerically studied and compared. Optimization simulations for retrofitted boiler were carried out under different over-fired air ratios. The moisture content in lignite was specially considered in simulations by simplified coal compositions method. The results show that the ignition of lignite particles is delayed due to the high moisture content. After retrofit, the temperature level in furnace rises owing to the reduction of total excess air coefficient in furnace, and the high temperature region moves upward due to the introduction of separated over-fire air. The NOx emission is reduced by approximately 40% through retrofitting, which proves that the low-NOx retrofit for the lignite-fired boiler is successful. With over-fired air ratio increasing from 15% to 25%, the NOx emission for retrofitted design decreases from 394 mg/Nm3 to 317 mg/Nm3 (at 6% O2). OFA ratio of 20% is the most economical case, and OFA ratio of 25% is the most environmentally friendly case. Taking NOx emission and boiler thermal efficiency into account, the practical operating condition with OFA ratio of 20% is optimal for retrofitted boiler. The results can be beneficial for the retrofit and practical operation of similar lignite-fired boilers.

Low load performance of tangentially-fired boiler with annularly combined multiple airflows

A novel burner arrangement scheme with annularly combined multiple airflows (ACMA) was previously proposed for wall-tangentially fired boiler (WTFB), and its superiority over traditional WTFB had been proved. However, coal-fired boilers are often required to play the role of peak-shaving. To this, low load performances of ACMA boiler were investigated in this work to test whether ACMA can still work well under low boiler thermal load (BTL) conditions. Results show that ACMA boiler performance is still better than that of traditional WTFB under low BTL condition, with smaller airflow deflection, improved coal combustion behavior and alleviated thermal deviation on suspended heating surfaces. Besides, NOx emission characteristic of ACMA boiler under low BTL is more desirable. Furthermore, the location of non-service burners has significant effects on coal combustion and heat transfer processes under 440 MW load conditions, and closing the upper burner group is the optimal choice in terms of the comprehensive boiler performance. These findings deepen the understanding of ACMA boiler and provide theoretical and practical guidance in its application in power plants.

A novel burner arrangement scheme with annularly combined multiple airflows for wall-tangentially fired pulverized coal boiler

To improve the performance of tangentially-fired pulverized boiler, a new burner arrangement scheme with annularly combined multiple airflows is proposed and its performance is thoroughly investigated with a previously validated 660 MW lignite boiler model. Results show that compared with conventional wall-tangentially fired boiler, the in-furnace aerodynamic field under the proposed burner arrangement scheme is obviously improved, embodying as reduced airflow deflections, increased furnace fullness and alleviated phenomenon of airflow scouring furnace walls. Besides, coal combustion behavior is also improved, resulting in increased combustion temperature and enhanced heat transfer process in the main burners’ region. Furthermore, the reduced flue gas swirling intensity mitigates the deviations of flue gas velocity and temperature in the upper furnace region, which finally lowers the unevenness of heat absorption on the suspended heating surfaces. With the improvement of the abovementioned aspects, the safety and economy of tangentially-fired boilers can be largely increased. Considering the increasingly enlarged boiler capacity and the widespread use of low-rank coal, the proposed burner arrangement scheme can be of great use in retrofitting the existing boilers and designing new ones.

Detailed assessment of mesh sensitivity for CFD simulation of coal combustion in a tangential-firing boiler

Computational fluid dynamics (CFD) has become an essential tool for optimizing the design and diagnosing the operation of a boiler. However, the validity of the results depends on the degree of numerical diffusion as well as the reliability of the submodels. This study aims to assess mesh sensitivity in the reacting two-phase flow of pulverized coal in a common tangential-firing boiler. Three mesh versions were constructed for the boiler with the number of cells ranging between 1.2 million and 5.4 million, corresponding to 0.0114 -0.0022 m3 per cell in the burner zone. The velocity distribution was found to be highly sensitive compared to temperature, heat flux, and NO concentration. By contrast, the use of key performance parameters such as total wall heat absorption, exit NOx concentration, and carbon conversion, was not appropriate criteria for the mesh sensitivity test. These parameters were determined by integration over the entire surface or volume, which made them sensitive to the overall reaction stoichiometry instead of the mesh fineness. It suggests that the use of a coarse mesh could be acceptable in evaluating the key performance parameters influenced by major operation variables, such as air distribution and fuel properties. However, sufficient mesh fineness is necessary for studies requiring accurate prediction of detailed flow patterns such as the evaluation of burner tilting/yawing or ash deposition on the wall.

Sensitivity analysis of several operational parameters on gas temperature deviation in a tangential firing boiler

In this work, an integrated numerical model combining combustion and fluid heating was established for a 660 MW coal-fired boiler. To validate the accuracy of the developed model, related experiment was also conducted, and shows that the relative errors (REs) of several predicted parameters on both flue-gas side and water-steam side of the boiler are less than 7 %, which meets the requirement of analysis. The results indicate that the effect of SOFA tilt angle on gas temperature deviation (GTD) is greatest, and that of SOFA’s proportion follows; while GTD presents a smallest sensitivity to the concentration of O2. This is because O2 concentration mainly affects the average value of the gas temperature, but barely changes its distribution. By contrast, altering SOFA’s tilt angle and its proportion directly varies the shape, location, and area of the high-temperature and low-temperature regions at the entrance profile of the horizontal crossover pass, so these two factors play a more important role in the variation of GTD.

Integration of continuous restricted Boltzmann machine and SVR in NOx emissions prediction of a tangential firing boiler

The NOx emissions prediction modeling of a coal-fired boiler is a complicated problem because of its highly nonlinear and strongly correlated multi-variables. To address this problem, this paper presents a novel deep structure using continuous restricted Boltzmann machine (CRBM) with support vector regression (SVR). A new steady-state identification method is first established by applying kernel principal component analysis (KPCA). Subsequently the stationary samples are subjected to the stacked CRBM network for extracting implied features. The combination of the optimal features and target NOx values are further utilized for training SVR to establish the regression pzmart of the CRBM-SVR prediction model. Additionally, particle swarm optimization (PSO) is applied for optimizing hyper-parameters of SVR, and weights of CRBMs are fine-tuned with gradient descent on account of prediction errors. Compared with the existing state of the art, the proposed structure achieves a good performance of 4.83 mg/m3, 3.55 mg/m3 and 0.963 of RMSE, MAE and R2.

Optimization of Air Distribution to Reduce NOx Emission and Unburned Carbon for the Retrofit of a 500 MWe Tangential-Firing Coal Boiler

The use of separated overfire air (SOFA) has become a standard technique of air staging for NOx reduction in the coal-fired boiler and can also be applied to existing boilers by retrofit. This study was to optimize the air distribution for the proposed SOFA installation in a 500 MWe tangential-firing boiler that has 20 identical units in Korea. Using computational fluid dynamics (CFD) incorporating advanced coal combustion submodels, the reference case was established in good agreement with the design data, and different flow ratios of burner secondary air, close-coupled OFA (CCOFA), and SOFA were evaluated. Increasing the total OFA ratio effectively suppressed NO formation within the burner zone but had a negative impact on the boiler performance. With moderate air staging, NO reduction became active between the CCOFA and SOFA levels and, therefore, the OFA distribution could be optimized for the overall boiler performance. For total OFA ratios of 25% and 30% with respective burner zone stoichiometric ratios of 0.847 and 0.791, increasing the SOFA ratio to 15% and 20%, respectively, was ideal for decreasing the unburned carbon release and ash slagging as well as NO emission. Too high or low SOFA ratios rapidly increased the unburned carbon because of inefficient mixing between the strong air jets and char particles. Based on these ideal cases, the actual air distribution can be adjusted depending on the coal properties such as the ash slagging propensity.

Numerical investigation of combustion optimization in a tangential firing boiler considering steam tube overheating

A comprehensive numerical study of the combustion optimization of a 660-MW tangentially fired boiler under the constraint of steam tube overheating is presented. To obtain the chemical products and the heat flux at the heating surfaces, a three-dimensional (3D) combustion model was established using computational fluid dynamics (CFD) software. In addition, a 1D + 1D thermal hydraulic model was developed using MATLAB to describe the heating process inside the tubes. Detailed mapping methods were also provided to connect the mesh systems in CFD and MATLAB. As a result, the tube wall temperature can be calculated by considering the coupled heat transfer on the gas and steam sides. The simulation results reveal that under full load conditions, the upward tilting of the separated over-fire air (SOFA) burners can reduce the formation of CO and NOx, which helps achieve higher boiler efficiency. However, when the SOFA tilt angle increases from +20° to +30°, 0.16% of the outer surfaces of the final re-heater would exceed the allowable temperature of the TP347H material; additionally, the improvement in the boiler efficiency and decrease in NOx formation are much less. Thus, considering the unit’s safe operation, the optimal SOFA tilt angle for combustion optimization should be +20°.

Uncertainty Analysis for the CH4 Emission Factor of Thermal Power Plant by Monte Carlo Simulation

Thermal power plants are a large source of greenhouse gas emissions among energy industry facilities. Emission factors for methane and nitrous oxide depend on combustion technologies and operating conditions and vary significantly with individual thermal power plants. Due to this variability, use of average emission factors for these gases will introduce relatively large uncertainties. This study determined the CH4 emission factors of thermal power plants currently in operation in Korea by conducting field investigations according to fuel type and type of combustion technique. Through use of the Monte Carlo simulation, the uncertainty range for the CH4 emission factor was determined. The estimation showed, at the 95% confidence level, that the uncertainty range for CH4 emission factor from a tangential firing boiler using bituminous coal was −46.6% to +145.2%. The range for the opposed wall-firing boiler was −25.3% to +70.9%. The range for the tangential firing boiler using fuel oil was −39.0% to 93.5%, that from the opposed wall-firing boiler was −47.7% to +201.1%, and that from the internal combustion engine boiler was −38.7% to +106.1%. Finally, the uncertainty range for the CH4 emission factor from the combined cycle boiler using LNG was −90% to +326%.

Thermal resistance by slagging and its relationship with ash properties for six coal blends in a commercial coal-fired boiler

Evaluating ash slagging can alleviate the operational problems and improve the efficiency of coal-fired boilers. This paper presents methods to quantify the extent of slagging in a 500 MWe tangential firing boiler by determining the thermal resistance and relating it to ash properties. To determine the thermal resistance by slagging, the operational data of the boiler were extracted for six coal-blend cases and analyzed using a one-dimensional process model. Although the conventional slagging indices based on ash composition have generated conflicting results, new indices based on thermo-mechanical analysis (TMA) of the coal-blend samples provided reasonable agreement with the operational data. In particular, the second peak of the TMA associated with the growth rate was correlated with the thermal resistance of the superheaters, and the third peak representing the slag melting was inversely correlated with that of the furnace wall. It was also found from the numerical modeling of the slag flow on the furnace wall that the critical viscosity temperature at 25 Pa·s can be positively related with the thermal resistance, and consequently, with the slagging propensity of specific coal ashes.

PERFORMANCE OF A TANGENTIALLY FIRED BOILER IN A THERMAL POWER PLANT

The most effective method for producing intense turbulence is by the impingement of one flame on another. This action is secured through the use of burners located in each of the four corners of the furnace. The burner nozzles are so directed that the streams of coal and air are projected along a line tangent to a small circle, lying in a horizontal plane, at the centre of the furnace. A scrubbing action is present which assures contact between the combustible and oxygen, thus promoting rapid combustion and reducing carbon loss. The experimental results of this study can be summarized as the use of tangentially fired boiler which increase splendid air-fuel mixture and pulverized coal is entrained in Primary Air (PA), and in-addition to that the Secondary Air (SA) is also projected towards the mill, finally a rotating “FIRE BALL” is formed. The present work analysis is carried out on Tangentially Fired Boiler by considering heat input through four burners. The heat output is determined by considering various losses from the boiler that is loss due to dry gas, loss due to moisture, loss due to H2 for combustion, heat loss due to combustile and refuse and losses due to radiation etc. In addition heat absorbed by economizer, boilers, superheater and reheater are studied, thus finally the net heat absorbed and thermal efficiency of tangentially fired boiler are determined. Graphs are drawn to evaluate the various effects of heat input on output parameters in the boiler. Sankey diagrams are drawn to show the performance in the boiler. The values calculated are put and shown as tables in Appendix.

Aerodynamics of burner jet in a tangentially-fired boiler: A CFD modelling and experiment

• Aerodynamics of burner jet in a tangentially-fired furnace is significant. • Visualisation of dye injection, acid/alkali modelling is used in physical model. • An empirical model of the concentration distribution in the boiler was generated. • Flow fields of the boiler for a wide range of inlet flow rates were observed.

Gas-particle turbulent flow of foursquare tangential jets in a simplified pulverized-coal firing boiler

An experimental cold-model of a simplified tangential firing boiler was established to investigate the mesoscale turbulent flow behaviors, including gas vortex structures, particle motions and interactions between two phases. A modified PIV technology, employing two pairs of lasers and cameras, was applied to measure the velocity and velocity gradient of turbulent flow in foursquare tangential jets alternatively. At a given initial gas velocity and particle mass loading, the interaction between gas and particles was studied at three different particle sizes. It was found that two main coherent vortex structures, circular eddy and hairpin eddy, distributed mainly in low speed area and heavy impingement area, respectively. The characteristics of particle motion in foursquare tangential jets correlated with gas turbulence dissipation, particle size, particle concentration and particle density. Small particles were easily entrained by gas vortex, so that they consumed more turbulence energy and attenuated the gas turbulence intensity. On the contrary, large particles had more inertia and led to heavier impingement in the chamber center, resulting in particle random distribution and complex momentum transfer between gas and particles. Moreover, large particles stretched the coherent vortex to be narrow and long, while small particles pulled down the vortices rotation intensity.

Effects of reburning mechanically-activated micronized coal on reduction of NOx: Computational study of a real-scale tangentially-fired boiler

The paper presents some results of numerical simulations of the effects of reburning mechanically-activated micronized coal on reduction of NOx and unburnt carbon residual in a tangentially fired boiler of 500 t/h steam production. The conventional RANS approach was used to compute two-phase (reactive dispersed particles in gaseous medium) multi-component system, with some modifications related to particle heat transfer and their reactions. The comprehensive model was verified in simulation of the same boiler with conventional firing, as well as in an experimental pilot-scale combustor using micronized, non-activated and mechanically-activated coal. The comparison with the standard dust-coal firing showed that reburning reduces NOx by 37.5%, but leads to an increase in heat loss due to unburnt carbon residual by 61%, primarily due to the imposed suboptimal stoichiometric ratio. Switching to reburning micronized coal reduced NOx by 49% while heat loss was still substantial, 42% higher than in the conventional firing. Using mechanically-activated micronized coal of the same granulation brought only marginal further improvement in NOx reduction (to 50%), but to a remarkable decrease in heat loss, only 3.3% higher than in the case without reburning. The simulations were performed for a set of plausible operating parameters (stoichiometric ratios, flow rates of reburn coal, flue gas recirculation, overfire air), which all need to be optimized to achieve maximum effects. Nevertheless, the simulations demonstrated that reburning of activated micronized coal is a feasible option to achieve significant NOx reduction without being penalized with excessive unburnt fuel/heat residual.

Oxy-fuel Combustion in a 600 MW Gaseous Fuel Tangentially Fired Boiler

Tangentially fired furnaces are vortex-combustion burners preferably used for oxy-fuel combustion in order to minimize the CO2 emission and control the boiler temperature. The objective of the present study is to investigate numerically the combustion and emission characteristics of oxy-fuel combustion of turbulent reacting flows in a three-dimensional model furnace of a 600 MW tangentially fired boiler. Numerical calculations of the flow field and thermal fields as well as the species concentrations for the oxy-fuel combustion in the furnace of a gaseous fuel tangentially fired boiler were conducted. Three cases of oxy-fuel combustion were investigated. These were 73%, 79%, and 65% of volumetric inlet CO2 while the remaining inlet volume was O2. The results of these three oxy-fuel cases are compared to the air-fired combustion. In the present model, the radiation heat transfer was solved by the weighted sum gray gases approach, whereas the chemical reaction model was facilitated by a simple lumped reacti...

Numerical and experimental study on biased tube temperature problem in tangential firing boiler

In this study, a numerical and experimental investigation on the flow field in a 500-MW unit of a tangentially fired boiler has been performed to understand the effect of residual swirl on the temperature distribution and inconsistency of flow velocity in the crossover pass. The k–kL–ω turbulence model was used in this study to analyse the possible formation of transition flow in the tangential firing boiler. Furthermore, 1D and 3D hybrid modelling were used for the simulation of the convective heat transfer in the boiler tube assembly. The commercially available code ANSYS Fluent and COMSOL were used for the studies, respectively. The numerical results for the velocity profile and residual swirl in the boiler were validated using the experimental results from the cold air velocity test, in which the flow velocity was directly measured in the full-scale boiler. The computational results agreed well with the experimental findings. Furthermore, from a comparison of the simulation and experiment, it was found that the biased gas flow induces non-uniform tube temperatures at the crossover pass area of the tangential firing boiler. These results aid in the detailed study of the residual swirl effects on the temperature or flue gas at various locations of the boiler. This study indicates that a non-uniform tube temperature at the crossover pass is an inherent problem resulting from residual swirl in the tangential firing boiler. Therefore, the pulverised coal burner should be fine-tuned to avoid tube rupture due to local overheating.

Numerical Study on Combustion and Heat Transfer Properties Under Oxy-fuel Condition in a 600MW Utility Boiler

Oxy-fuel combustion is recognized as one of the most promising CO2 capture technologies. Under oxy-fuel combustion, the concentrations of CO2 and H2O are much higher, result in different temperature profiles and heat transfer properties. In this study, a modified computational fluid dynamics (CFD) model-was developed for a 600 MW tangentially-fired boiler. Different oxygen volume fractions and two flue gas recycle modes (dry recycle and wet recycle) were numerically investigated, focusing on the properties of combustion and heat transfer. The results indicate the peak temperature in the burner zone decreases in oxy-fuel combustion, because of the high heat capacity and chemical reactivity of CO2. Compared with air-fired combustion, similar total heat transfer rate in furnace can be achieved at 27.0% and 28.3% of oxygen volume fraction for wet and dry recycle. These findings indicate retrofitting an existing boiler with oxy-fuel combustion technology is entirely feasible.

Investigation of aerodynamics of a recessed rectangular slot-burner used in tangentially-fired furnaces by varying jet velocity ratio in the presence of cross-flow

The power generation industry throughout the world stands to gain significantly from process improvements and optimization which can potentially lead to cleaner production of cost effective electricity. The efficient operation of lignite-based tangentially-fired combustion systems, as commonly used in Victoria, Australia, depends on critical issues such as ignition and combustion of the fuel, which are largely controlled by burner aerodynamics. The geometry of the burner and the ratio of velocities between the primary and secondary jets play an important role in achieving stable combustion, high burnout of fuel, low production of pollutants and control of fouling. Slot-burners are vertically aligned stacks of rectangular nozzles delivering primary fuel and secondary air jets, and are commonly used in tangentially-fired boilers. To obtain a better understanding of the overall combustion process, it is important to understand the aerodynamics of jet development from these burners. This paper reports an experimental investigation into the aerodynamics of a recessed rectangular slot-burner of the type used in the Yallourn stage-2 tangentially-fired furnace. The 1:15 scale model burner consisted of a central (primary) rectangular fuel nozzle with two secondary rectangular air jets positioned above and below it. The burner jets were at an angle of 60° to the wall and were diffused and mixed in a recess before entering into the measurement vessel. A cross-flow jet was attached to the wall of the vessel to simulate the recirculation similar to tangentially-fired furnaces. Experiments for both single and two-phase flow were performed. For single-phase flow, the air flow was seeded with a fine mist of sugar particles (mean diameter 1μm) introduced into the primary, secondary and cross-flow ducts. For two-phase flow, solid glass spheres (mean diameter 66μm, density 2450kg/m3) were used as the representative of the coal particles and were introduced only at the centre of the primary duct from a bubbling fluidized bed. Experiments were conducted using a primary to cross-flow jet velocity ratio (φ) of 1.0 and secondary to primary jet velocity ratios (ϕ) of 1.0 and 3.0 as found in real furnaces. Laser Doppler Anemometry (LDA) was used to measure mean and turbulent velocity components in the near field and downstream regions of the jets. The flow pattern inside the recess was very complex and greatly influenced the flow where it entered the measurement vessel. For single-phase flow and ϕ=1.0, the jets diverged completely from the geometric axis of the burner and attached to the wall. The degree of deflection of the lower secondary jet was slightly more than the primary jet. For ϕ=3.0, after exiting from the nozzle, the primary jet diffused outwards and mixed with the higher momentum secondary jets in the recess and came out with the secondary jets. For two-phase flow and ϕ=1.0, in the mixing region between the primary and cross-flow jet, the gas-phase velocities were higher than the particle-phase. Further downstream the peak velocities of the particle-phase slightly deviated and occurred farther from the wall. For ϕ=3.0, downstream of the nozzle there was a tendency of separation for the gas-phase near the wall while the particle-phase was attached to the wall.

석탄화력발전소 보일러의 수냉벽튜브 부식 메카니즘에 대한 실증적 고찰

In this study, boiler tube thickness measurement and numerical analysis were conducted for standard 500MW coal-fired power plant in order to research the mechanism of tangential-fired boiler corrosion empirically. The most dominant corrosion mechanism of tangential-fired boiler waterwall was corrosion by sulfur contained in the unburned carbon. And the secondary mechanism was gas corrosion at localized reducing atmosphere. It is required to decrease the air-stage combustion operation in order to mitigate the waterwall tube corrosion. Also stringent coal pulverization quality control and reinforcing work for corrosion susceptible area such as anti-corrosion coatings is required