CFD investigation of NOx reduction with a flue-gas internal recirculation burner in a mid-sized boiler

The demand for reduction of nitrogen oxide (NO_x) emissions from industrial facilities continuously increases, and considerable efforts have been exerted to achieve this goal. In this work, we propose a novel flue-gas internal recirculation (FIR) burner emphasizing the function of FIR to accomplish single-digit NO_x emissions from a mid-/large-sized combustion system. In the new design, a FIR passage is installed inside the conventional non-FIR burner to draw back the flue gas from the combustion chamber and release it into the chamber as a mixture of air and flue gas. The effectiveness of FIR burner is evaluated by employing extensive computational fluid dynamics (CFD) simulations with an enhanced reaction rate model. The existing eddy dissipation model for reaction rate, including the turbulence-chemical interaction, is improved by introducing a position-dependent scaling factor, which is validated by comparison with temperature profiles in experiments. CFD predictions show that a small amount of flue gas returned to the burner still significantly alters the flow structure and temperature distribution. Accordingly, NO_x emission is dramatically diminished (82.83 and 9.7 ppm in the non-FIR and FIR systems, respectively) using the FIR burner. These observations confirm that the new FIR burner effectively accomplishes ultra-low NO_x emissions in field-scale combustion systems. In addition, the fundamentals of NO_x reduction by the FIR burner are thoroughly examined in the present study. The findings will provide essential knowledge in designing other ultra-low NO_x burners.

     

Numerical investigation on NOX formation in oxygen-CH4 MILD combustion with air leakage

In this paper, we focus on formation of NO_X under oxy-fuel MILD combustion using JHC (jet-in hot co-flow) burner. The effect of the different oxygen dilution ratio in the co-flow is analyzed for MILD combustion formation and also for characterizing NO_X emission. RANS (Reynolds averaged Navier Stokes) with the modified k-ε equation and EDC (eddy dissipation concept) model were applied for 2D-axisymmetric computational domain using the Ansys Fluent. It is found that the effect of the oxygen concentration rather than the composition of dilution gas is more significant for the formation of the MILD combustion and as the oxygen concentration of the dilution gas increases under MILD combustion conditions, the region with D_a < 1 expressed as MILD regime tends to be decreased. The effect of a diluted oxygen concentration in the oxy-fuel combustion on NO_x emission under MILD condition is found to be much greater than the amount of air leakage into co-flow, and NO emissions are almost unaffected by the increase of N₂ concentration in case of MILD combustion condition within practical air leak ranges.

     

Effects of gas and particle emissions on wall radiative heat flux in oxy-fuel combustion

Oxy-fuel combustion exhibits combustion and heat transfer characteristics different from air-fuel combustion due to high concentrations of CO₂ and H₂O. This study evaluated the effect of gas and particle emissions on radiative heat transfer in oxy-fuel combustion of coal. For a hexahedral furnace, prescribed gas compositions based on combustion calculation were used to simplify the combustion reactions. The values of radiative heat fluxes (q_rad) were compared for different combustion modes, flue gas recirculation (FGR) methods, particle concentrations, furnace sizes and O₂ concentrations in the oxidizer. The radiation was calculated by the discrete ordinate method with gaseous emission predicted by the weighted sum of gray gases models (WSGGMs). The results showed that employing an optimized WSGGM is essential for the accurate prediction of q_rad in oxy-fuel combustion for gaseous fuels. The conventional WSGGM showed large errors for larger furnace volumes or under dry FGR conditions. With higher particle concentrations such as in pulverized coal combustion, however, q_rad was dominated by emission of particles. The effect of gas emissivity was not critical in the furnace with a mean beam length of 8.3m. Oxy-fuel combustion with wet FGR had higher q_rad than dry FGR. The O₂ concentration in the oxidizer was a key parameter for oxy-fuel combustion since increasing its value linearly increased q_rad.     

Performance analyses of oxy-fuel power generation systems including CO2 capture: comparison of two cycles using different recirculation fluids

With increasing concerns on global warming, reduction of CO₂ emission has become a hot issue and studies of CO₂ capture and storage (CCS) technology in power plant applications are in progress. Oxy-fuel combustion is one of the several available technologies that intend to capture CO₂. Since the combustion gas consists mainly of CO₂ and H₂O in oxy-fuel combustion systems, it is easy to separate CO₂ from the flue gas using a simple mechanical method instead of complex chemical processes. There have been suggested a couple of power cycles using different recirculation fluids for combustion dilution purpose. This study aimed to investigate the influence of CO₂ capture on the performance of two promising oxy-fuel combustion power cycles adopting H₂O and CO₂ as the recirculation fluid. Optimal integration between the carbon capture process and the power cycle was examined and the influences of carbon capture on the entire system performance were compared for the two cycles.     

Experimental study of honeycomb regenerator system for oxy-fuel combustion

An experimental study of a honeycomb regenerator for oxy-fuel combustion has been performed. A laboratory-scale test rig is set up and various experimental parameters such as cell density and length of the honeycomb regenerator, switching time, and bypass effect are investigated. The typical temperature trend of the heating and regenerating process is obtained for the oxy-fuel combustion. The regenerative characteristics for various combinations of these parameters are shown. It is found that part of the exhaust gas should be bypassed to use waste heat more efficiently and to optimize the efficiency of the honeycomb regenerator system for oxy-fuel combustion.     

方箱炉旋流燃烧技术研究与应用

旋流燃烧可以提高火焰速度,从而强化传热、提高燃烧效率,采用多个同旋向喷嘴形成旋流燃烧器,或者通过喷口不同方向的倾斜排列形成两排燃烧器,可以实现很好的方箱炉旋流速度场。通过冷态流场数值模拟和试验分析,验证了旋流燃烧能加强炉子流场的均匀性,热态试验和工业应用结果表明,炉膛下部燃烧和传热得到强化,烟气出口烟温下降,方箱炉热效率显著提高。     

A new flame-stabilization technology for lean mixtures

The development of a low-pollution burner is important for saving energy and preserving the environment. A low-pollution burner can be produced by lean-mixture combustion and general combustion technology. The flammable limit of premixed flame is narrower than that of diffusion flame. Producing a lean mixture of fuel results in an effective combustion condition, which in turn produces high load and low pollution. In this study, it was found that the influx of Q₂ had an effect on extending the lean flammable limits and flame stabilization in a doubled jet burner. And the flame, consisting of small eddies, can be stabilized by the nozzle neck phenomena.     

Application of DFB diode laser sensor to reacting flow (I)- estimation and application to laminar flames -

Diode laser sensor for measuring gas temperature and species concentration in combustion chamber was developed using 2.0 μm distributed teed back lasers. To evaluate the measurement sensitivity of diode laser sensor system, CO₂ survey spectra near 2.0 μm were measured and compared with the calculated one. This diode laser absorption sensor was applied to measure gas temperatures in a premixed flat flame of CH₄-air mixture. Experimental results were in good agreement with the values by an R-type thermocouple within 6.12%. In addition, successful demonstration of measurement of gas temperature and species concentration in a soot flame showed the promising possibility of diode laser absorption sensors for practical combustion system with non-intrusive method.     

Thermal characteristic evaluation of functionally graded composites for PSZ/metal

The functionally graded material (FGM) is the new concept for a heat resisting material. FGM consists of ceramics on one side and metal on the other. A composition and microstructure of an intermediate layer change continuously from ceramics to metal at the micron level. This study is carried out to analyze the thermal shock characteristics of functionally graded PSZ/metal composites. Heat-resistant property was evaluated by gas burner heating test using C₂H₂/O₂ combustion flame. The ceramic surface was heated with burner flame and the bottom surface cooled with water flow. Also, the composition profile and the thickness of the graded layer were varied to study the thermomechanical response. Furthermore, this study carried out the thermal stress analysis to investigate the thermal characteristics by the finite element method. Acoustic emission (AE) monitoring was performed to detect the microfracture process in a thermal shock test.     

Combustion and emission characteristics of HCNG in a constant volume chamber

Finding an alternative fuel and reducing environmental pollution are the main goals for future internal combustion engines. Hydrogenmethane (HCNG) is now considered an alternative fuel due to its low emission and high burning rate. An experimental study was carried out to obtain fundamental data for the combustion and emission characteristics of pre-mixed hydrogen and methane in a constant volume chamber (CVC) with various fractions of hydrogen-methane blends. A pre-mixed chamber was used to obtain a good mixture of these gases. Exhaust emissions were measured using a Horiba exhaust gas analyzer for various fractions of hydrogen-methane blends. The results showed that the rapid combustion duration was shortened, and the rate of heat release elevated as the hydrogen fraction in the fuel blend was increased. Moreover, the maximum mean gas temperature and the maximum rate of pressure rise also increased. These phenomena were attributed to the burning velocity, which increased exponentially with the increased hydrogen fraction in the fuel blend. Exhaust HC and CO₂ concentrations decreased, while NO_X emission increased with an increase in the hydrogen fraction in the fuel blend. Our results could facilitate the application of hydrogen and methane as a fuel in the current fossil hydrocarbon-based economy and the strict emission regulations in internal combustion engines.     

Effect of various gas compositions on gas interchangeability and combustion characteristics for domestic appliances

In this study, an investigation into the gas interchangeability and combustion characteristics of various gas compositions for domestic appliances was performed. In order to suggest the appropriateness of gas interchangeability using the specific gravity (SG) and the Wobbe index (WI) values, combustion characteristics included incomplete combustion and flame lifting were measured and observed for the upper and lower limits using the gas-oven as a domestic partial-premixed type appliance and the condensing boiler as a domestic premixed type appliance. The flame was stable, and the CO and NO_x concentrations increased when the WI and SG values increased. Specifically, the behavior of the flame lifting changed between WI values of 52.0 MJ/Nm³ and 53.0 MJ/Nm³. The CO and NO_x concentrations of the reference gas were approximately 173 ppm and 74 ppm for the gas-oven and 175 ppm and 35 ppm for the boiler, respectively. Consequently, a WI value of 53.0 MJ/Nm³ can be considered the flame lifting limit as the lower limit for gas interchangeability. For the upper limit of gas interchangeability, the CO and NO_x concentrations varied significantly with the type of burner.     

Effect of water induction on the performance and exhaust emissions in a diesel engine (II)

This study was to investigate the effects of water induction through the air intake system on the characteristics of combustion and exhaust emissions in an IDI diesel engine. The fuel injection timing was also controlled to investigate a method for the simultaneous reduction of smoke and NOx when water was injected into the combustion chamber. The formation of NOx was significantly suppressed by decreasing the gas peak temperature during the initial combustion process because the water played a role as a heat sink during evaporating in the combustion chamber, while the smoke was slightly increased with increased water amount. Also, NOx emission was significantly decreased with increase in water amount. A simultaneous reduction in smoke and NOx emissions was obtained when water was injected into the combustion chamber by retarding more 2°CA of the fuel injection timing than without water injection.     

Performance and emission characteristics of a low heat rejection spark ignited engine fuelled with E20

In internal combustion engines, the concept of low heat rejection (LHR) using thermal barrier coating on the surface of combustion chamber is gaining attention. Thermal barrier coating reduces the heat transfer to the cooling system, protects engine components from peak heat flux and fluctuating temperature produced during combustion and improves the performance of the engine. Information in the literature is plentiful for LHR diesel engine and only few studies exist on LHR spark ignited engine. The application of thermal barrier coating in spark ignited engine is limited by pre-ignition and knocking due to elevated combustion chamber temperature. A spark ignited engine with moderate insulation on the combustion chamber and higher octane fuel can overcome this difficulty. The objective of the present experimental study is to quantify the changes in performance and emission characteristics brought by partial thermal insulation on the combustion chamber of a four stroke spark ignited engine fueled with E20 blend. Partial thermal insulation was created by coating 0.3 mm thick Alumina (Al₂O₃) on the cylinder head, inlet and exhaust valves. The changes are quantified with respect to unmodified engine fueled with gasoline. The combustion parameters such as flame development and rapid burn duration are also estimated and compared. The results indicate that partially insulated SI engine when fueled with E20 improves performance and reduces emission. A maximum of 48% reduction in THC and 50% reduction in CO emission at part load was achieved.

     

Chemical,thermal and dilution effects of carbon dioxide in oxy-fuel combustion of wood in a fixed bed

Experimental and numerical modeling was performed on eucalyptus wood combustion under oxy-fuel conditions using a fixed bed reactor in order to isolate the role of various carbon dioxide effects on the burning rate. Wood combustion was investigated under four different mixtures of O₂ and Ar/CO₂/N₂: 21 % O₂/79 % N₂; 21 % O₂/22.5 % CO₂/56.5 % Ar; 40 % O₂/60 % CO₂; and 40 % O₂/47 % CO₂/13 % Ar. The first three mixtures were designed to have the same peak temperatures in order to isolate chemical and dilution effects of CO₂. This was achieved by substituting some percentage of CO₂ with Ar in O₂/CO₂ mixture while maintaining a constant concentration of O₂. The fourth mixture was meant to isolate the thermal effect of CO₂. The results were obtained from both the experimental rig and numerical simulation for a fixed bed configuration. Wood combustion in the fixed bed was modeled using Lagrange-Euler method, where gas-phase was calculated using computational fluid dynamics (CFD), that is Euler phase, while solid-phase was tracked in Lagrange phase using discrete element method (DEM). The results show that ignition time in CO₂ environment decreases gradually as O₂ concentration is increased. On the other hand, burning rate and flame front speed increase as O₂ concentration is increased. It was established that dilution effect is the most influential parameter on the burning rate of wood combustion in an oxy-fuel system.

     

A CFD study on thermo-acoustic instability of methane/air flames in gas turbine combustor

Thermo-acoustic instability of methane/air flames in an industrial gas-turbine combustor is numerically investigated adopting CFD analysis. The combustor has 37 EV burners through which methane and air are mixed and then injected into the chamber. First, steady fuel/air mixing and flow characteristics established by the burner are investigated by numerical analysis with single burner. And then, based on information on the flow data, the burners are modeled numerically via equivalent swirlers, which facilitates the numerical analysis with the whole combustion system including the chamber and numerous burners. Finally, reactive flow fields within the chamber are investigated numerically by unsteady analysis and thereby, spontaneous instability is simulated. Based on the numerical results, scaling analysis is conducted to find out the instability mechanism in the combustor and the passive control method to suppress the instability is proposed and verified numerically.     

Investigation of two-phase flow mixing mechanism of a swirl burner using an electrostatic sensor array system

A novel electrostatic sensor array was designed to measure particle concentration downstream of a swirl burner. The fundamental mechanism and the primary constituent elements of the sensor array were described. The root-mean-square magnitude of the measured electrostatic voltage was determined as an indication of the particle concentration. The accuracy of the electrostatic sensor array was calibrated by the optical fluctuation method. Local particle concentrations at different cross-sections of the measuring chamber were measured to investigate the diffusion characteristic of the pulverized coal particles. Electrostatic sensor array showed its ability in the field measurement in this work. The measurements indicated that the velocity of the inner secondary air had a significant effect on the diffusion of the pulverized coal particles. The particles concentrated in the center of the cross-section after leaving the burner. With the development of the gas–solid two-phase flow, the particles distributed like a ring shape. The radius of the particle ring increased with the increase of the velocity of the inner secondary air. But the effect of the velocity of outer secondary air on the radius of the particle ring is very slight. The maximum radius occurred when the velocity of inner secondary air was 21 m/s, which was favorable for stable combustion.     

中心给粉旋流燃烧器气固两相流动的数值模拟

电厂采用的煤粉燃烧技术应达到稳燃、低污染、防结渣及防高温腐蚀的要求。中国电厂燃用煤的煤质偏差,煤种多变。在燃用这些煤的时候,锅炉的稳燃能力较低。针对这些问题,提出中心给粉旋流煤粉燃烧技术。由于燃烧器的气固流动特性对燃烧器的性能有很大的影响,利用可实现的k-ε和Lagrangian随机轨道模型对中心给粉旋流燃烧器的气固两相流动进行数值模拟,并将计算结果和三维相位多普勒测速技术(Phase-Doppler anemometry,PDA)试验结果进行详细比较,计算值和试验值速度分布的趋势基本相同。计算和试验结果表明,在轴向方向产生了回流区,切向速度分布出现典型的Rankine涡结构,中心线附近区域的径向速度小。当颗粒的轴向速度衰减为0之后,颗粒的运动方向发生偏转,开始向后上方运动。颗粒迂回型运动轨迹延长了煤粉在回流区中的停留时间。     

Parametric simulation and performance analysis of a solar gas turbine power plant from thermodynamic and exergy perspectives

Using solar energy in gas turbine cycles is a new method that can improves the efficiency of gas turbines. Placing a solar receiver before a combustion chamber can raise the temperature of the air coming into the chamber and reduce the consumption of fuel in the chamber. The system that combines a solar energy receiver with a gas turbine cycle is technically called a “solar gas turbine”. The goal of this paper is the parametric simulation and performance analysis of a gas turbine cycle equipped with a solar receiver from thermodynamic and exergy aspects of view. The selected parameters in this study, include the pressure ratio of compressor, the temperature of gases at the turbine inlet and the direct normal irradiance. The obtained results indicate that the fuel consumption of this combined system is reduced by using a solar receiver and the temperature of gases entering the combustion chamber increased. The reduction of consumed fuel, in turn, reduces the rate of exergy destruction in the combustion chamber. Another important point is that the solar receiver itself has the least amount of exergy destruction. The net power generated by a solar gas turbine cycle is 10 % higher than that produced by a simple gas turbine cycle. Also, the studies show that the electrical efficiency of a solar gas turbine cycle is about 41 % higher than the simple gas turbine cycle.

     

Development of an air fuel control system for a domestic wood pellet boiler

Wood pellets are a kind of solid biomass energy and a renewable energy source. Made by compressing sawdust, wood pellets have a higher energy density than split firewood and wood chips. In 2007, the new and renewable energy (NRE) portion was 2.4% with respect to total primary energy in Korea. The Korean government wants to increase the new and renewable energy (NRE) portion up to 6.1% by 2020 [1]. To achieve this target, the government has been establishing some policies, such as incentive policy, NRE mandatory use for public building and renewable portfolio standard (RPS) and so on. To supply wood pellets as fuel for the combustion chamber of a wood pellet boiler, most domestic wood pellet boilers put a constant volume by using an auger type fuel feed system. In an auger system as fuel feeding, there is the possibility of changing energy input due to the different density of wood pellets even in a constant volume flow rate of wood pellets. If fuel input rate is changed without any correction of air flow rate for combustion, the condition of combustion in a wood pellet boiler can be deteriorated. We have developed an air-fuel control system for a domestic wood pellet boiler by using flue gas oxygen concentration measurement and a PID controller. To measure O₂ concentration of flue gas, a wide band O₂ sensor was adopted. We changed fuel input from 100% to 50% by artificial manipulation to confirm the control system. The O₂ concentration in flue gas can be controlled to be 8.5% ± 1% without significant change of CO and NO_x concentration.