Simulation of gas-solid combustion characteristics in a 1000 MW CFB boiler for supercritical CO2 cycle

Supercritical CO₂ (S-CO₂) power cycle has become one of the most efficient and low-pollution cycle schemes to improve thermal power generation efficiency and reduce energy consumption around the world. In this study, the 3D physical model of a 1000 MW S-CO₂ circulating fluidized bed (CFB) boiler with annular furnace is established to simulate the gas-solid combustion process based on the MP-PIC method under the Eulerian-Lagrangian framework. By comparing with the conventional water steam CFB boiler, the S-CO₂ CFB boiler has a smooth and stable gas-solid flow pattern with good uniformity of the particle concentration and velocity distribution, indicating that the annular structure and the layout of the heating surfaces is conducive to the gas-solid flow uniformity. The gas-solid phase temperature distributes uniformly basically without sudden rise or sudden drop, and the temperature difference between the solid phase and the gas phase is not large, which reflects the good combustion uniformity of the S-CO₂ CFB boiler. Compared with 300 MW and 600 MW S-CO₂ CFB boilers, the 1000 MW one shows a higher carbon conversion rate, lower desulphurization effect, and lower nitrogen removal performance with the CO, NO, and SO₂ outlet concentration of 0.002%, 5.8 mg/m³, and 125 mg/m³, respectively.     

Coupled simulation and deep-learning prediction of combustion and heat transfer processes in supercritical CO2 CFB boiler

The supercritical CO₂ (S-CO₂) power cycle has a wide application prospect in coal-fired power generation field because it’s highly effective, compactly structured, and flexible of operation. To observe more accurate heat transfer and coal combustion characteristics in the circulating fluidized bed (CFB) with the distinctive S-CO₂ boundary, a 3D computational fluid dynamics (CFD) simulation of the furnace-side combustion process treated by the multiphase particle-in-cell (MP-PIC) method was conducted in a 600 MW S-CO₂ CFB boiler coupled with the heat transfer process on working fluid side based on the polynomial fitting calculation model. Furthermore, a novel method to predict simulation results via Radial Base Function (RBF) neural network was proposed to simplify the computational process, enhance the sample data fusion, and improve the prediction accuracy. Results show that staggered high-temperature fluid and high heat flux was a major concern in S-CO₂ heating surface arrangement. The temperature rise of wall heaters was less than the conventional steam CFB, and the heat flux of spiral and vertical heat transfer tubes decreased along the tube. The predicted temperature distribution of tubes and cold walls was in a good agreement with the coupling simulation results, whose accuracy can meet the engineering requirements.     

Study on scale-up characteristics of oxy-fuel combustion in circulating fluidized bed boiler by 3D CFD simulation

The oxy-fuel combustion CFB technology as a promising carbon capture technologies needs to study the scale-up process for the commercial diffusion. Numerical simulation would be a rational tool to investigate the gas-solid flow and oxy-fuel combustion process before constructing an expensive and complicated industry-scale plant. A three-dimensional (3D) CFD simulation according to the Eulerian-Lagrangian approach was applied to simulate the hydrodynamics of gas-solid flow and oxy-fuel combustion process in lab-scale, pilot-scale and industry-scale CFB boiler (from 0.1 MW_th to 330 MW_e). Results present the differences of the boiler configuration, the gas-solid flow and the oxy-fuel combustion characteristics between lab-scale, pilot-scale and industry-scale CFB boilers. The cross-section thermal load gradually decreased, while the cross-section area increased with the thermal inputs increased. In the lab-scale and pilot-scale oxy-fuel CFB, the particle velocity field was more uniform than that in the industry-scale CFB. The carbon conversion ratio increased with an increase in the thermal input. The emission of CO, NO and SO₂ in the industry-scale oxy-fuel CFB boilers was lower than those in the lab-scale and pilot-scale. A larger oxy-fuel combustion power plant is beneficial to carbon capture and low pollutant emission. The results would be beneficial to the design and operation of industry-scale oxy-fuel CFB.     

Combustion of agro-waste with coal in a fluidized bed

In this study, a review of the studies done on the co-combustion of some agro-waste in a bubbling fluidized bed combustor (BFBC) having an inside diameter of 102 mm and a height of 900 mm is given. The agro-waste used to investigate the co-combustion characteristics were peach and apricot stones produced as a waste from the fruit juice industry, and olive cake produced as a waste from the olive oil industry. These are typical wastes for a Mediterranean country. A lignite coal was used for co-combustion. On-line concentrations of O₂, CO, CO₂, SO₂, NO _x and total hydrocarbons (C _m H _n ) were measured in the flue gas during combustion experiments. Variations of emissions of various pollutants were studied by changing the operating parameters (excess air ratio, fluidization velocity and fuel feed rate). Temperature distribution along the bed was measured with thermocouples. Emissions were also monitored from the exhaust. Various combinations of coal and biomass mixtures were tested. During the combustion tests, it was observed that the volatile matter from the biomass quickly volatilizes and mostly burns in the freeboard. The temperature profiles along the bed and the freeboard also confirmed this phenomenon. It was found that as the volatile matter of the biomass increases, combustion takes place more in the freeboard region. Better combustion conditions occur at higher excess air ratios. The results showed that co-combustion with these three proposed biomasses lowers the SO₂ and NO _x emissions considerably. CO and hydrocarbon emissions are lower at the higher excess air ratios.     

Analysis of heat transfer in the furnace of the P-67 boiler P-67 furnace and improvement of its design

The results of experimental study of heat transfer in the furnace of the P-67 boiler (under the Russian trademark) burning Kansk-Achinsk coal are presented. Means of improving the design of the furnace device are proposed.Notation N energy unit power, MW - _fur furnace air excess coefficient - r gas recirculation degree - q_in incident radiation flux density, kW/m² - a spacing between the combustion chamber walls, m - b_b burner width, m - n number of the burner row, starting from below - and inclination angles of the burners located in front of and behind the combustion chamber axis - d diameter of the conventional circumference, tangential to which the burners are directed, m - q_ch heat-stress of the radiant heat absorbing surface of the active combustion zone, MW/m² - furnace-mean thermal efficiency of deflecting walls Krasnoyarsk Institute of Non-Ferrous Metals, Siberian Branch of the All-Union Heat Engineering Institute, Krasnoyarsk. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 64, No. 3, pp. 275–278, March, 1993.     

An overview of air emission intensities and environmental performance of grey cement manufacturing in Canada

Air emissions generated in grey cement manufacturing originate primarily from the combustion of fossil fuels required to heat the kiln and the chemical reaction of raw materials in the pyroprocessing phase. Given that the kiln system is enclosed, air emissions generated, discharge from a single point source kiln stack. Unlike other industries, the point source kiln stack enables the cement sector to accurately monitor and record total air emissions. The largest contributors to air emissions from grey cement manufacturing are carbon dioxide (CO₂), oxides of nitrogen (NO _x ), sulphur dioxide (SO₂) and dust/particulate matter (PM). In Canada, grey cement manufacturing facilities are required to annually report these emissions through the National Pollutant Release Inventory (NPRI). Since CO₂, NO _x , SO₂ and PM are the largest contributors to air emissions, and Canadian grey cement facilities are required to report these emissions, combining NPRI data with annual grey cement production data allows for the development of intensity-based environmental performance indicators. Based on data provided by NPRI, in combination with industry production, we can better understand the environmental performance of Canada’s grey cement manufacturing. On the global stage, intensity-based performance measures provide a useful tool for comparison and demonstrate a strong environmental performance for grey cement production in Canada. As an energy intensive and trade exposed (EITE) grey cement manufacturing is vulnerable to unbalanced environmental policy, which may ultimately result in leakage of production and air emissions to developing countries.     

Prediction and real-time monitoring techniques for corrosion characterisation in furnaces

Abstract

Combustion modifications to minimise NO_x emissions have led to the existence of reducing conditions in furnaces. As regulations demand lower NO_x levels, it is possible (to a degree) to continue to address these requirements with increased levels of combustion air staging. However, in most practical situations, a number of adverse impacts prevent the application of deep combustion air staging. One of the more important limitations is the increased corrosion that can occur on wall tubes exposed to fuel rich combustion environments. Current boiler corrosion monitoring techniques rely on ultrasonic tube wall thickness measurements typically conducted over 12 to 24 month intervals during scheduled outages. Corrosion coupons are also sometimes used; typically require considerable exposure time to provide meaningful data. The major drawback of these methods is that corrosion information is obtained after the damage has been done. Management of boiler waterwall loss and system optimisation therefore requires a real-time indication of corrosion rate in susceptible regions of the furnace. This paper describes the results of a program of laboratory trials and field investigations and considers the use of an on-line technology in combination with innovative applications, also modelling and precision metrology to better manage waterwall loss in fossil fuelled boilers while minimising NO_x emissions.     

The influence of flue gas recirculation on oxy-oil combustion in an existing furnace

Oxy-fuel combustion has received increasing attention as one of the major solutions for CO₂ capture. This study aimed to obtain a better understanding on the oxy-oil firing process and to gain experience handling flue gas recirculation. A 300 kW_th multi-fuel combustion test furnace at National Cheng-Kung University in Tainan, Taiwan was chosen as the experimental facility. This experimental study had successfully converted a conventional air-fired furnace to operate on oxy-fuel firing with various operating conditions. Oxygen-enriched combustion and oxy-fuel combustion tests were conducted. The effects of oxygen enrichment, flue gas recirculation, fuel type, and thermal loading on the operational characteristics of the furnace such as temperature distribution, pressure variation, and emission characteristics were examined and discussed. Several concerns about a conventional air-fired furnace adapted for oxy-fuel combustion were scrutinized, including optimization of flue gas recirculation for better combustion efficiency, higher SO₂ concentration in the flue gas under oxy-fuel operation, and air leakage coming from negative pressure operation. These findings provided valuable data to improve the performance of oxy-fuel combustion and to allow better conceptual designs in future development.     

Heat exchange with different schemes for introducing the heat-exchange agent into a furnace

The effect of the scheme for introducing the heat-exchange agent into a furnace for heat exchange in a selective nonisothermal gaseous medium is investigated taking into account the heat loss due to convection depending on the emissivity of the metal and the furnace lining.Notation T temperature - Er specific resulting radiative flux - q_c convective heat flux density - q_Me overall density of the resulting heat flux to the metal - emissivity - a_g0 absorbing capability of the gas relative to the radiation of the lining - a_g4 same for the metal - a _go /a+b+c absorbing ability of a layer of gas a+b+c... relative to the radiation of the lining - a_gj/ikmnp absorbing ability of the gas layer i+k + m + n + p with radiation from layer j - A emissivity of the walls - l _ef effective length of a ray path - coefficient of convective heat transfer - k heat-exchange coefficient from the internal surface of the lining with temperature T_o in the medium with temperature T_m - B fuel expenditure - Q_H ^P heat of combustion - v volume of the combustion fuel products per 1 m³ of gas - c_g specific heat capacity of the gas - h overall height of the channel - x instantaneous height coordinate in the channel The indices for the temperatures and heat fluxes are as follows: 0, lining; 4, metal; 1, 2, and 3, gas zones; m, media.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 4, pp. 692–698, October, 1980.     

浅谈锅炉烟气余热回收技术及其工程应用

随着能源价格日益高涨,空气污染日渐严重,降低能耗,减少污染物排放已成为重要议题。而现在工业锅炉的使用中普遍存在着热量利用率低下,排放烟气余热温度过高,以及烟气内污染环境气体含量过高等问题。因此,阐述了锅炉烟气余热回收技术和其优缺点,并详细介绍了热管技术和冷凝式锅炉的工程应用方案以及与其他系统的联合方案,然后以实际工程为例分析了锅炉烟气余热回收的效果,最后得出当前余热回收技术仍存在很大的问题,还有很长的路要走的结论。     

Aspen Plus-based simulation of a cement calciner and optimization analysis of air pollutants emission

The cement industry is a typical high energy consumption and heavy pollution industry, in which amounts of CO₂, NO, NO₂, and SO₂ discharge from the pre-calciner kiln system and cause severe greenhouse and acid rain effects. Meanwhile, reasonable division of the combustion environment in the calciner is the main method to control the formation of pollutant gases. In this article, a calciner process model in Aspen Plus is proposed based on the combustion mechanism analysis of the Dual Combustion and Denitration calciner (DD-calciner) and verified by industrial data. Then, for a concrete DD-calciner, the article studies the effects of the flow rate of coal and tertiary air on flue gas compositions and effects of the staging combustion technology on the NO _x , SO₂, and CO concentrations in the flue gas. Through comparing the model results with the relevant environmental standards, the optimization analysis for staging combustion parameters of the calciner is done, and the result shows that when the proportion of tertiary air entering the pyrolysis and combustion zone is controlled within the range of 57–65.52% (0.89 < α < 1.004), all the gas pollutants emit within accepted standards simultaneously. The calciner process model outlined in this article describes the key processes of the physical and chemical reactions in the calciner. It can be used to study the key operation and design parameters which influence the flue gas constituents, so as to provide data support for determining the pollutant emission reduction plan of the cement industries with a view to reduce air pollutant emission.     

Study of radiation heat transfer and the temperature state in the combustion chambers of small-size gas-turbine engines (GTEs)

The experimental data on the radiation flux surface density distribution in the combusion chamber of a small-size gas-turbine engine are presented. Experiments are made at elevated pressures and temperatures of the stagnated flow at the chamber inlet. Satisfactory agreement between theory and experiment is obtained.Notation q_s.r. semispherical self-radiation flux - E^* surface radiation flux density measured by the radiometers - E_s.r. surface density of the self-radiation flux of the flame - P _ch ^* stagnated flow density at the combustion chamber inlet - T _ch ^* stagnated flow temperature at the combustion chamber - air excess coefficient - reduced velocity coefficient at the combustion chamber inlet - l beam path length in the chamber. Indices - s.r. self-radiation; ch, chamber - l local - ^* stagnation - 1) first cross section of the chamber - 2) second cross section Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 64, No. 3, pp.271–274, March, 1993.     

Investigation of non-uniform characteristics in a 300 MWth circulating fluidized bed with different coal feeding modes

Circulating fluidized bed (CFB) has been practised in many engineering fields, but the non-uniform characteristics deteriorate combustion performance and system control. In this study, the improvement of external-loop and in-furnace non-uniformity of a 300 MW_th industrial-scale CFB with multiple cyclones by a dual-side coal feeding mode was numerically quantified. The results show that the pressure is non-uniformly distributed among three external loops, where the pressure in the middle loop seal is lower than that in the corner loop seals by 4.5 %. The pressure gradient positively correlates with the solid holdup. As compared with the traditional single-side coal feeding mode, the dual-side coal feeding mode: (i) promotes the final mixing degree by 11.15 %; (ii) extends the residence time of coal particles by 10.3 %; (iii) reduces the magnitude and fluctuation of the horizontal solid flux by 28.5 % and 77%, respectively; (iv) narrows the temperature range and reduces the mean temperature by 7 °C; (v) enhances the combustion of coal particles by consuming more O₂; (vi) decreases the concentrations of SO₂ and NO by 3 % and 5 %, respectively. The present study shows the superiority of the dual-side coal feeding mode to the traditional single-side coal feeding mode in the optimization of CFBs in practical industrial processes.     

Near-Zero Air Pollutant Emission Technologies and Applications for Clean Coal-Fired Power

Coal is the dominant energy source in China, and coal-fired power accounts for about half of coal consumption. However, air pollutant emissions from coal-fired power plants cause severe ecological and environmental problems. This paper focuses on near-zero emission technologies and applications for clean coal-fired power. The long-term operation states of near-zero emission units were evaluated, and synergistic and special mercury (Hg) control technologies were researched. The results show that the principle technical route of near-zero emission, which was applied to 101 of China’s coal-fired units, has good adaptability to coal properties. The emission concentrations of particulate matter (PM), SO₂, and NO_x were below the emission limits of gas-fired power plants and the compliance rates of the hourly average emission concentrations reaching near-zero emission in long-term operation exceeded 99%. With the application of near-zero emission technologies, the generating costs increased by about 0.01 CNY∙(kW∙h)^–1. However, the total emissions of air pollutants decreased by about 90%, resulting in effective improvement of the ambient air quality. Furthermore, while the Hg emission concentrations of the near-zero emission units ranged from 0.51 to 2.89 μg∙m⁻³, after the modified fly ash (MFA) special Hg removal system was applied, Hg emission concentration reached as low as 0.29 μg∙m⁻³. The operating cost of this system was only 10%–15% of the cost of mainstream Hg removal technology using activated carbon injection. Based on experimental studies carried out in a 50 000 m³∙h⁻¹ coal-fired flue gas pollutant control pilot platform, the interaction relationships of multi-pollutant removal were obtained and solutions were developed for emissions reaching different limits. A combined demonstration application for clean coal-fired power, with the new “1123” eco-friendly emission limits of 1, 10, 20 mg∙m⁻³, and 3 μg∙m⁻³, respectively, for PM, SO₂, NO_x, and Hg from near-zero emission coal-fired power were put forward and realized, providing engineering and technical support for the national enhanced pollution emission standards.     

Reduction of the NOx generation through enhancing heat transfer in a furnace

The possibility of reducing the NO_x concentration through enhancing heat transfer in a furnace is demonstrated. A method for approximate calculation of the reduction of the NO_x concentration, with an intermediate radiator placed in the flame, is proposed.Notation T_max maximal temperature of the flame, K - T_rad temperature of the radiator surface, K - C_NO, C_{O 2}, C_{N 2} concentrations of nitrogen oxides and oxygen in combustion products, and of molecular nitrogen, wt.% - R universal gas constant, kJ/(kmol·K) - T temperature in the reaction zone, K - H gas enthalpy, kJ/m³.₀, Stefan-Boltzmann constant, W/(m²·K⁴) - _f emittance of the furnace medium - F running area of the radiating heating surface, m² - heart efficiency of the screens - heat retentivity - F_w surface area of the furnace walls, m² - V_gC_g mean total heat capacity of the combustion products, K - X_max relative location of the temperature maximum in the course of the flame burnt-out expressed in fractions of a full length of the flame (furnace) - T_f gas temperature at the furnace exit, K - Bo Boltzmann number - Q₁ and Q₂ heat absorption of the heating surfaces from the flame without and with a radiator, kJ/m³ - Q_f usable heat release in the furnace, kJ/m³ - H₁ and H₂ gas enthalpies at the exit from the furnace without and with a radiator, kJ/m³ - M parameter accounting for the temperature distribution along the furnace height - C₀ emittance of the black body, W/(m²·K⁴) - T_W temperature of the heat-absorbing surface, K - ₁ and ₂ thermal emission coefficients of the radiator and of the heat-absorbing surface - A₂ absorptivity of the heat-absorbing surface - B fuel flow rate, m²/sec Moscow State Textile Academy. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 64, No. 3, pp. 337–340, March, 1993     

Investigations on the Failure of Economizer Tubes in a High-Pressure Boiler

This article describes the results of an investigation concerning the failure of economizer tubes of a high-pressure boiler in a dual-purpose power/water cogeneration plant. The failure was observed in the form of rupturing of one tube and a macrohole or pinhole in another tube. The boiler had an operating period of 116,123 h since its inception. For approximately the first 100,000 h, the fuel for the boiler was crude oil, which was replaced by Bunker C oil. The boiler tube is fabricated from carbon steel SA 210A1. The location of the failure was determined by on-site visual inspection of the boiler. Detailed macro- and microexaminations of inner and outer scales on the tube were begun to determine the cause of the rupture. The composition of the fire- and waterside scale and ash deposited on the outer surface of the tubes was analyzed by energy-dispersive x-ray (EDX) technique. The reduction percentage of wall thickness of the tube facing inside and outside the furnace was calculated. The cause of the failure of the economizer tube appears to be H₂SO₄ dew-point corrosion. The relatively low temperature of feedwater lowered the tube metal temperature and promoted the condensation of H₂SO₄. The external deposits on the tubes, as a result of bunker oil firing, further helped to lower the tube metal temperature, thus promoting H₂SO₄ condensation over the deposit and subsequent corrosion of the tube wall. Recommendations are given to prevent/minimize such failures.     

Investigation of parameters of an instrument for measuring high-temperature heat fluxes

Results of an experimental investigation of an instrument for measuring high-temperature heat fluxes with a density up to 800 kW/m² are presented.Notation q_inc, Q_inc incident heat flux density - A, , B, , x dimensionless parameters - T_a adiabatic combustion temperature of fuel - H_i instantaneous height of furnace chamber - H full height of furnace chamber - T_f temperature of gases at the furnace outlet - T_f increment in the temperature of gases at the furnace outlet - a_f emissivity of furnace - T_f temperature of flame - ₀ Stefan—Boltzmann constant - T temperature on the heat pipe body - Q supplied power - E_rad sensitivity of sensor - D hourly rate of fuel supplied to the boiler furnace - time Academic Scientific Complex A. V. Luikov Institute of Heat and Mass Transfer of the Academy of Sciences of Belarus, Minsk. I. I. Polzunov Central Boiler and Turbine Institute, St. Petersburg. Scientific and Technical Center Flame-Complex, Ust'-Ilimsk. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 64, No. 3, pp. 324–329, March, 1993     

Free convection of a heat generating fluid in a hemispherical closed volume

From a theoretical analysis and results of previous studies, a model of the free convection of a heat-generating fluid in a hemispherical closed volume with a completely isothermal boundary is developed. By applying analytical estimates, we establish that at values of modified Rayleigh numbers Ra₁ > 10¹³, the flow in the entire volume is turbulent, which substantially simplifies the problem. Integrated relations for heat emission through the upper and lower parts of the boundary of the hemispherical capacity are obtained as a result. Results of numerical calculations have shown that the majority of heat is removed through the spherical surface downwards. The ratio of the heat flux through the upper horizontal boundary to heat flux through the lower boundary decreases from 0.5 to 0.3 in the range of modified Rayleigh numbers Ra_I from 10⁹ to 10¹⁷.     

Effect of boundary conditions on the heat transfer law for a turbulent boundary layer

The heat transfer at an impermeable plate has been experimentally investigated under various boundary conditions. The conservativeness of the heat transfer law St₀=f(Re _T ^** ) is demonstrated for a monotonic increase of temperature and heat flux along the surface.     

The formation and phase stability of lead magnesium niobate in the presence of a molten flux

The formation of perovskite Pb(Mg_1/3Nb_2/3)O₃ (PMN) by the molten salt synthesis method using sulphate flux has been investigated as a function of calcination temperature and mole ratio between Li₂SO₄ and Na₂SO₄. A 97% perovskite phase was synthesized at 750° C for 30 min with 0.635Li₂SO₄-0.365Na₂SO₄ flux without any sub-products affecting the formation reaction of the PMN phase. The percentage of the perovskite phase was influenced by changes in the Li₂SO₄/Na₂SO₄ mole ratio at a given temperature. The pyrochlore phase present was identified as Pb₃Nb₄O₁₃ (P₃N₂) when 0.635Li₂SO₄-0.365Na₂SO₄ flux was used. The results for other lead-based ferroelectrics are also discussed.