Utility Boiler Furnaces Research Articles

Prediction of H2S corrosion depth for water-cooled wall in lower furnace of utility boiler

The presence of H2S near the water-cooled wall of utility boiler can lead to accelerated corrosion wastage on the furnace wall. While the existing evaluation of annual corrosion wastage caused by H2S is not suitable for the boiler that operated in a wide load range. In this study, a novel H2S corrosion model is developed to predict the metal wastage of water-cooled walls in the lower furnace. Three key independent variables including metal temperature, H2S concentration and corrosion time are considered simultaneously. Based on the experimental corrosion data of pure Fe and boiler steel SA213-T2, the prediction model is represented by a mathematical equation derived from the regression analysis. The validation of this equation is conducted by comparing the predicted and lab-scale corrosion depths, and a reasonable agreement is obtained. The single-factor analyses indicate that the corrosion depth of SA213-T2 almost increases linearly with the increase of H2S concentration, and the growth rate of corrosion depth could raise as the metal temperature increases. Under the extreme condition that temperature as 773 K and H2S as 800 ppm, the predicted annual corrosion depth of SA213-T2 is 1129 μm. In practical terms, the annual corrosion wastage should be calculated by summing the sub-wastages formed during different periods of a year. The predictive model proposed in this study could play an indispensable role in the sub-wastage calculation and benefit the estimation of the annual corrosion wastage.

Temperature-field reconstruction algorithm based on reflected sigmoidal radial basis function and QR decomposition

Information on temperature-field distribution can reflect the combustion state within utility boiler furnaces, which is conducive for the formulation of control strategies to ensure safe boiler operation; therefore, it is significant for the realization of automatic boiler control. Ultrasonic thermometry is a noncontact acoustic pyrometry technique that utilizes the time-of-flight of ultrasonic waves on limited wave propagation paths to continuously reconstruct distributed temperature fields, and thus overcomes the shortcomings of traditional physical temperature measurement methods. To address the issue of low reconstruction accuracy of the temperature-field edges caused by traditional acoustic pyrometry algorithms and realize high-accuracy reconstruction of two-dimensional temperature fields, this study proposes a temperature-field reconstruction algorithm based on a combination of the reflected sigmoidal radial basis function and QR decomposition (referred to as the RSQR algorithm). The results of simulation experiments indicate that the accuracy of the reconstruction results obtained using the proposed algorithm increase by 2.03% on an average compared to the commonly used temperature-field reconstruction algorithms, with an increase of 1.88% in the reconstruction accuracy of the temperature-field edges.

Numerical modeling of in-furnace sulfur removal by sorbent injection during pulverized lignite combustion

Results of the study on SO2 reduction in a utility boiler furnace by means of furnace sorbent injection are presented in this paper with analysis of major influential parameters. The Ca-based sorbent injection process in pulverized lignite fired boiler furnace with tangentially arranged burners is simulated. In simulations sorbent particles are distributed among the burner tiers, where they are injected together with coal, and also through sorbent injection ports located above the burners. The sorbent reactions model was adapted to be efficiently implemented in the code for CFD simulations of complex processes considering both the calculation time and the results accuracy. The sorbent particles reaction model was simplified with several assumptions to allow for faster calculations and significantly reduce simulation time without loss in calculation precision during the particle tracking in boiler furnace. Two phase gas-particle flow is modeled, with coal and sorbent particles reactions and interactions with gaseous phase. Test-cases based on fuels with different composition and combustion organization were simulated in details, and results showed that significant increase in reduction of SO2 at furnace exit could be achieved by proper sorbent injection. The sorbent injection locations were analyzed with special care to enable maximum SO2 capture in the case-study furnace under investigated conditions. Most of the test-cases with low SO2 capture had one or more of the following problems: intensive particle sintering, low local temperatures (leading to low calcination rates), or bad particles distribution. Significant SO2 retention was possible when the process was organized in such a way that particles were exposed to optimal temperature range, and injected in the furnace zones with high SO2 concentration simultaneously. It was shown that better results can be achieved by injection of sorbent through multiple burner tiers, with SO2 emission reduction efficiency around 60% at the furnace exit in several well optimized test-cases.

Numerical investigation of the nonlinear flow characteristics in an ultra-supercritical utility boiler furnace

The temperature deviation is an inherent issue in large-scale tangentially fired pulverized-coal boilers. This article investigates numerically the forming mechanism of gas velocity and temperature deviations from the perspective of nonlinear flow characteristics in an ultra-supercritical utility boiler. Firstly, the cold state simulations of two-dimensional and three-dimensional models of furnace are performed. With the increase of Reynolds number from burner nozzles, the temperature and velocity distributions in furnace show the trend of symmetrical, slightly asymmetrical, and completely asymmetrical. The generated tangential radius of flow becomes larger and declining. It is concluded that the nonlinear flow characteristics is the key issue for the velocity and temperature deviations in the tangentially fired pulverized-coal boiler furnace. Secondly, the full combustion simulation is carried out under BMCR (Boiler Maximum Continuous Rating) conditions and validated by experimental measurement. It is also found that the centre of tangent circle becomes larger along the height of the boiler furnace and deflects though the burner structure and boundary conditions are absolutely symmetrical. The temperature and velocity distributions are asymmetrical under BMCR conditions. Due to the nonlinear flow, it inevitably brings the residual swirling flow at the upper furnace zone which is one origin of temperature deviation of the gas after the furnace exit.

Plasma assisted power coal combustion in the furnace of utility boiler: Numerical modeling and full-scale test

This work presents modern plasma technology for solid fuel ignition and combustion. It promotes more effective and environmentally friendly low-rank coal incineration. To implement this technology at coal fired thermal power plants plasma-fuel systems (PFSs) were developed. PFS is a pulverized coal burner equipped with arc plasmatron producing high temperature air stream of 4000–6000°C. Basis of technology PFS is plasma thermo-chemical preparation of coal for burning. It consists in plasma heating of air–coal mixture up to temperature of coal volatiles release and char carbon partial gasification. In PFS air–coal mixture is deficient in oxygen therefore carbon is oxidized mainly to carbon monoxide. As a result, at the PFS exit a highly reactive mixture is formed of combustible gases and partially burned char particles, together with products of combustion, while the temperature of the mixture is around 1050°C. Further mixing with air promotes intensive ignition and combustion of the prepared in the PFS fuel.PFS have been tested for boilers start up and pulverized coal flame stabilization at 30 power boilers of 75–950t/h steam productivity. The boilers were equipped with different types of pulverized coal burners: direct flow, muffle and swirl burners. At tests of the PFS power coals of all ranks (lignite, bituminous, anthracite and their mixtures) were incinerated. Volatile content of them was in range of 4–50%, ash varied from 15% to 48% and heat of combustion was from 6690 to 25100kJ/kg.To show advantages of the plasma technology of coal combustion before conventional one numerical investigation of plasma ignition, gasification and thermo-chemical preparation of air–coal mixture for incineration in a power boiler was fulfilled. The numerical modeling was performed for low-rank bituminous coal of 40% ash content incinerated at a boiler of 420t/h steam productivity. Both analysis of the numerical modeling and experience of PFS industrial use showed ecological efficiency of the plasma technology. When plasmatrons operate in the regime of plasma stabilization of pulverized coal flame, NOX emission is reduced twice and amount of unburned carbon is reduced four times. The PFSs reduce the temperature at the exit of the furnace. Tests of the PFS for the boiler BKZ-420 of Almaty TPP-2 cold startup confirmed the possibility of high-ash Ekibastuz coal ignition in the cold furnace without heating of primary air.

Refined Weighted Sum of Gray Gases Model for Air-Fuel Combustion and Its Impacts

Radiation is the principal mode of heat transfer in utility boiler furnaces. Models for radiative properties play a vital role in reliable simulations of utility boilers and simulation-based design and optimization. The weighted sum of gray gases model (WSGGM) is one of the most widely used models in computational fluid dynamics (CFD) simulation of air-fuel combustion processes. It represents a reasonable compromise between an oversimplified gray gas model and a comprehensive approach addressing high-resolution dependency of radiative properties and intensity upon wavelength. The WSGGM coefficients evaluated by Smith et al. for several partial pressures of CO2 and H2O vapor are often used for gas temperatures up to 2400 K, which is supplemented by the coefficient values presented by Coppalle and Vervisch for higher temperatures until 3000 K. This paper refines the air-fuel WSGGM in terms of accuracy, completeness, and implementation and demonstrates the use and impacts of the refined model in CFD simulati...

Numerical Simulation on Improving NO<sub>x</sub> Reduction Efficiency of SNCR by Regulating the 3-D Temperature Field in a Furnace

The optimum temperature within the reagent injection zone is between 900 and 1150°C for the NOX reduction by SNCR (selective non-catalytic reduction) in coal-fired utility boiler furnaces. As the load and the fuel property changes, the temperature within the reagent injection zone will bias from the optimum range, which will reduces significantly the de-NOX efficiency, and consequently the applicability of SNCR technology. An idea to improve the NOX reduction efficiency of SNCR by regulating the 3-D temperature field in a furnace is proposed in this paper. In order to study the new method, Computational fluid dynamics (CFD) model of a 200 MW multi-fuel tangentially fired boiler have been developed using Fluent 6.3.26 to investigate the three-fuel combustion system of coal, blast furnace gas (BFG), and coke oven gas (COG) with an eddy-dissipation model for simulating the gas-phase combustion, and to examine the NOX reduction by SNCR using urea-water solution. The current CFD models have been validated by the experimental data obtained from the boiler for case study. The results show that, with the improved coal and air feed method, average residence time of coal particles increases 0.3s, burnout degree of pulverized coal increases 2%, the average temperature at the furnace nose decreases 61K from 1496K to 1435K, the NO emission at the exit (without SNCR) decreases 58 ppm from 528 to 470 ppm, the SNCR NO removal efficiency increases 10% from 36.1 to 46.1%. The numerical simulation results show that this combustion adjustment method based on 3-D temperature field reconstruction measuring system in a 200 MW multi-fuel tangentially fired utility boiler co-firing pulverized coal with BFG and COG is timely and effective to maintain the temperature of reagent injection zone at optimum temperature range and high NOX removal efficiency of SNCR.

Numerical Prediction of Pulverized Coal Flame in Utility Boiler Furnaces

In optimization of a utility boiler furnace operation, special attention is given to the flame geometry and position. As an illustration of possibilities for application of mathematical prediction and numerical experiment in efficient optimization of the flame, the paper presents selected results of simulations of processes in pulverized coal tangentially fired furnace of Kostolac-B 350 MW electric boiler unit. To analyze the furnace working under different conditions, a differential 3D mathematical model of two-phase turbulent reactive flow with heat and mass transfer and corresponding computer code have been developed. Using the model and the code, previously carefully verified and validated against field measurements, an extensive numerical study has been performed to investigate the dependence of the furnace flame characteristics on different operating conditions, including distribution of the coal, air flow rates, and particle size classes over the burner tiers, as well as the quality and grinding fi...

A numerical study of pulverized coal ignition by means of plasma torches in air–coal dust mixture ducts of utility boiler furnaces

Paper presents selected results of numerical simulation of processes in air–coal dust mixture duct of pulverized coal utility boiler furnace with plasma-system for pulverized coal ignition and combustion stabilization. Application of the system in utility boiler furnaces promises to achieve important savings compared with the use of heavy oil burners. Plasma torches are built in air–coal dust mixture ducts between coal mills and burners. Calculations have been performed for one of rectangular air–coal dust mixture ducts with two opposite plasma torches, used in 210MWe utility boiler firing pulverized Serbian lignite. The simulations are based on a three-dimensional mathematical model of mass, momentum and heat transfer in reacting turbulent gas-particle flow, specially developed for the purpose. Characteristics of processes in the duct are analyzed in the paper, with respect to the numerical results. The plasma-system thermal effect is discussed as well, regarding corresponding savings of liquid fuel. It has been emphasized that numerical simulation of the processes can be applied in optimization of pulverized coal ignition and combustion stabilization and enables efficient and cost-effective scaling-up procedure from laboratory to industrial scale.

D206 SIMULATION ON AN OPTIMAL COMBUSTION CONTROL STRATEGY FOR 3-D TEMPERATURE DISTRIBUTIONS IN PC-FIRED UTILITY BOILER FURNACES

The 3-D temperature distribution in a utility boiler furnace is essential for the safe, economic and clean operation of PC-fired furnaces with multi-burner system. In the past, due to the lack of applicable techniques to monitor the 3-D temperature distributions in furnaces, it is difficult to improve the control quality for the combustion processes. The authors is now establishing a technique to visualize 3-D temperature distributions in PC-fired furnace, which makes it possible to study a new combustion control strategy. In this strategy, the whole height of the furnace is divided into several parts, and the average temperatures, the bias magnitudes of the flame centers in the two axis of the horizontal plane in every part is extracted from the visualization results of the 3-D temperature distributions. A linear model is set up to relate the features of the temperature distributions with the input of the combustion processes, such as the flow rates of fuel and air fed into the furnaces through all the burners. The genetic algorithm is adopted to find the optimal combination of the whole input parameters which ensures to form an optimal 3-D temperature field in the furnace desired for the operation of boiler. Simulation results show that the strategy can soon find the factors causing the temperature distribution apart from the optimal state and give correct adjusting suggestions.

Characteristics of Pulverized-Coal Combustion in CO2-Recovery Power Plant Applied O2/CO2 Combustion.

The system of oxygen/recycled flue gas combustion for the removal of carbon dioxide from pulverized-coal fired power plants, the combustion characteristics of pulverized coal, and the trial designing of a 1 000 MWe pulverized-coal power plant were studied. The flame propagation speed of pulverized-coal cloud was measured in a microgravity combustion chamber at O 2 /CO 2 , O 2 /N 2 and O 2 /Ar atmosphere. NO x and SO 2 emissions from the system were investigated in the industrial scale combustion test facilities. Based on these results and the previous works, the trial designing of a 1 000 MWe pulverized--coal fired power plant applying this combustion system was also studied. As for the heat absorption performance of the boiler furnace, a 3 dimensional numerical analysis was applied to a large utility boiler furnace. It was compared with another promising system CO 2 -recovery, pulverized-coal power plant combined with amine absorption process.

Characteristics of pulverized-coal combustion in the system of oxygen/recycled flue gas combustion

Abstract Concerning to the system of oxygen/recycled flue gas combustion for the removal of carbon dioxide from pulverized-coal firing power plants, combustion characteristics of pulverized coal and the heat absorption performance of boiler furnace were studied. To determine the ignition characteristics in the CO2-rich atmosphere, the flame propagation speed of pulverized-coal cloud was measured in a microgravity combustion chamber. The results revealed that the flame propagation speed in an O 2 CO 2 atmosphere was markedly low compared with that in O 2 N 2 and O 2 Ar , and that it was improved by increasing the O2 concentration. NOx and SO2 emissions from the system were investigated in the industrial-scale combustion test facilities. NOx emission was not so largely decreased by staging as air-blown combustion, while a considerable part of sulfur was absorbed in ash in the system. As for the heat absorption performance of the boiler furnace, a 3-dimensional numerical analysis was applied to a large utility boiler furnace. It was found that a furnace designed for normal air-blown combustion was just large enough for the oxygen/recycled flue gas combustion with a realizable ratio of flue gas recycling.

Diffusion of a Jet Injected Perpendicularly into Uniform Cross Flow. 4th Report, In the Case of Coaxial Swirling Jets.

Velocity vectors and concentration profiles of a coaxial swirling jet have been measured using flow visualization and image processing. It has been found that for X/D<2, where D is nozzle diameter and X is distance from the nozzle, an inner nonswirling jet and an outer swirling jet flow separately with a slight inclination caused by the cross flow, but for X/D>2 they rapidly mix with each other and start to incline. The diffusion width of a coaxial jet is nearly equal to that of a simple jet, but the inclination of a coaxial jet is smaller. These results can be applied to the design of burners or air ports in utility boiler furnaces.

Diffusion of a jet injected perpendicularly into uniform cross flow. 3rd report, Influence of swirl intensity on diffusion of a jet.

Velocity vectors and concentration profiles of a swirling jet have been measured by flow visualization and its image processing. At visualization a water flow model is used and a single jet is injected to simulate diffusion of a burner or an airport jet in large utility boiler furnaces. The conclusions show that (1) because of a increase of swirl intensity, a jet spreads widely near the throat and decreases its penetration depth into uniform flow, and that (2) the uniform flow is accelerated where the direction of the uniform flow velocity vector coincides with the direction of the swirling jet rotating velocity component.

Ash formation during pulverized fuel combustion: 3. The structure and strength of boiler deposits

The chemical and physical structure, and the strength of ash deposits that were formed from a number of pulverized coals in a pilot scale combustor have been compared under conditions similar to those in a utility boiler furnace. Samples were collected on cooled targets in the combustor furnace, and their in situ strength was measured by ‘sootblowing’ with an air lance. Deposits were formed in two distinct layers. The initial deposit was of small (< 5 μm) discrete particles, with a higher proportion of iron to aluminosilicate particles than in the coal ash. This formed a thin (< 1 mm) layer on cooled substrates. Subsequent deposition of larger particles formed a thicker sintered layer in which calcium had combined with aluminosilicates to form a crystalline phase, and the iron and phosphorus were concentrated into a separate vitreous phase. With some coals, partial melting occurred at the outer surface and a dense slag deposit was formed. Iron, phosphorus and calcium all contribute to the tendency of the outer surface of a deposit to form slag, but it is the bonding strength of the inner layer which determines the difficulty of cleaning deposits by sootblowing. This is related mainly to the concentration of the vitreous phase, which is rich in iron and phosphorus.

Prediction of Three-Dimensional Flows in Utility Boiler Furnaces and Comparison with Experiments

Abstract The turbulent flow field in large utility boiler furnaces is strongly affected hy heat release from combustion and by the heat extracted from the furnace through the boiler tube walls. To describe and predict these effects with reasonable engineering accuracy, a mathematical model termed COMpac has been developed at the University of Stuttgart. The series of codes is based on a finite difference solution of the turbulence-modelled Navier-Stokes equations in axisymmetric and in fully three-dimensonal coordinates. Radiative heat transfer is presently predicted using flux method approximatians. A simplified mechanism of pulverized coal combustion is described in a Eulerian framework for both the gaseous and particulate phase. In the present work, validation studies are reported and discussed. The combustion model has been tested for single enclosed flames where detailed measurements were available. Prediction results for three-dimensional furnace flows are compared with velocity data from an isother...