The CFD Modeling of NOx Emission,HiTAC and Heat Transfer in an Industrial Boiler

NOx emission, heat transfer, and high temperature air combustion (HiTAC) in a boiler of Mobin Petrochemical Complex, Iran was numerically studied. The comparison between the measured values and the CFD predicted results showed good agreement, which implied that the adopted combustion and NOx formation models are suitable for correctly predicting characteristics of the heat recovery steam generation (HRSG) boiler. The predicted results show that NOx emission within the boiler depends highly on temperature, as well as oxygen concentration. Moreover, the influence of the equivalence ratio at a fixed air mass flow rate on the flame temperature and NOx formation has been investigated.     

CFD study on influence of fuel temperature on NOx emission in a HiTAC furnace

The influence of the fuel temperature on NOx formation was investigated numerically. For this purpose CFD modeling of NOx emission in an experimental furnace equipped with high temperature air combustion (HiTAC) system was studied. The comparison between the predicted results and measured values have shown good agreement, which implies that the adopted combustion and NOx formation models are suitable for predicting the characteristics of the flow, combustion, heat transfer, and NOx emissions in the HiTAC chamber. Moreover the predicted results show that increase of the fuel temperature results in a higher fluid velocity, better fuel jet mixing with the combustion air, smaller flame and lower NOx emission.     

NO mechanisms of syngas MILD combustion diluted with N2, CO2, and H2O

The NO mechanism under the moderate or intense low-oxygen dilution (MILD) combustion of syngas has not been systematically examined. This paper investigates the NO mechanism in the syngas MILD regime under the dilution of N₂, CO₂, and H₂O through counterflow combustion simulation. The syngas reaction mechanism and the counterflow combustion simulation are comprehensively validated under different CO/H₂ ratios and strain rates. The effects of oxygen volume fraction, CO/H₂ ratio, pressure, strain rate, and dilution atmosphere are systematically investigated. For all the MILD cases, the contribution of the prompt and NO-reburning routes to the overall NO emission is less than 0.1% due to the lack of CH₄ in fuel. At atmospheric pressure, the thermal route only accounts for less than 20% of the total NO emission because of the low reaction temperature. Moreover, at atmospheric pressure, the contribution of the NNH route to NO emission is always larger than 55% in the N₂ atmosphere. The N₂O-intermediate route is enhanced in CO₂ and H₂O atmospheres due to the increased third-body effects of CO₂ and H₂O through the reaction N₂ + O (+M) ? N₂O (+M). Especially in the H₂O atmosphere, the N₂O-intermediate route contributes to 60% NO at most. NO production is reduced with increasing CO/H₂ ratio or pressure, mainly due to decreased NO formation from the NNH route. Importantly, a high reaction temperature and low NO emission are simultaneously achieved at high pressure. To minimize NO emission, the reactions should be operated at high values of CO/H₂ ratios (i.e., >4) and pressures (e.g., P > 10 atm), low oxygen volume fractions (e.g., X_O2 < 15%), and using H₂O as a diluent. This study provides a new fundamental understanding of the NO mechanism of syngas MILD combustion in N₂, CO₂, and H₂O atmospheres.     

Study on the NOx emissions mechanism of an HICE under high load

A hydrogen fueled internal combustion engine (HICE) CFD simulation model consisting of detailed chemical reaction mechanisms was built using CONVERGE to study the NOx formation mechanisms. The Simulation Results are consistent with the experiments we had reported. The Simulation results show that the temperature inside the flame front and the OH concentration in the flame front increased with the fuel-air equivalence ratio. NO, as the major component of NOx, was generated abundantly during the rapid combustion period with the temperature rising and decreased after the rapid combustion period to a stable amount when the temperature dropped below 2200 K in cylinder. NO was generated mainly through three route named as thermal NO, NNH and N₂O. The Thermal NO path contributed a large proportion of the total NO emissions and the contribution increased with the fuel-air equivalence. NNH and N₂O routecontributed 24.2% of the total NO emissions when the fuel-air equivalence was 0.6, but contributed ?23.9% when the fuel-air equivalence was 1.0.     

H2 – CH4 blending fuels combustion using a cyclonic burner on colorless distributed combustion

H₂ – CH₄ mixture fuels can be promising for reducing carbon-based emissions. However, because of higher pollutant emission (such as NO_X) problems during hydrogen combustion, a new combustion method can be favorable. Colorless distributed combustion (CDC) is emerging here. CDC enables ultra-low pollutant emissions along with reduced flame instabilities, combustion noise, improved combustion efficiency, etc. Considering those benefits, methane and the hydrogen-enriched methane (60% CH₄ – 40% H₂, 50% CH₄ – 50% H₂, 40% CH₄ – 60% H₂) fuels have been consumed using a cyclonic burner providing more residence time at an equivalence ratio of 0.83 under distributed regime. For the modelings, Reynolds Stress Model (RSM) turbulence model, the assumed-shape with β-function Probability Density Function combustion model, and P-1 radiation model have been selected. To seek CDC, the oxygen concentration in the oxidizer was reduced with N₂ or CO₂ diluent from 21% O₂ to 13% O₂ at an interval of 2%. The air and the fuel temperatures were kept constant at 300 K. Besides, for seeking high-temperature air combustion (HiTAC) conditions the oxidizer temperature was changed to 600 K to simulate flue gas recirculation. The results showed that the temperature distributions changed to be more uniform considerably with a decrease in oxygen concentration for all cases. CDC also provided a considerable decrease in NO_X and a favorable reduction in CO at a certain oxygen concentration. It has been concluded that CO₂ as the diluent was more effective for reducing temperature levels and NO_X levels due to its greater heat capacity.     

Progress on the studies about NOx emission in PFI-H2ICE

Owing to its brilliant combustion performance and cleanest combustion products, hydrogen has been widely considered as one best alternative fuel for internal combustion engines. However, in the cylinder of hydrogen internal combustion engines, high combustion temperature and oxygen enrichment make NOx is still one but the only combustion pollutant. Therefore, it is particularly important to control NOx emission for hydrogen fuelled engines. Since PFI-H₂ICE (port-fuel-injection hydrogen internal combustion engine) is the normal type of hydrogen fuelled engines, the present article will focus on the studies about NOx emission in PFI-H₂ICE researches. First, the present article reviews the mechanism of NOx generation in PFI-H₂ICE; upon chemical kinetics, the generation of NOx will be summarized and discussed into three major paths which including thermal NO path, NNH–NO path and N₂O–NO path. Then, the researches on the control methods of NOx for PFI-H₂ICE in recent years will be systematically reviewed, the influencing factors to reduce NOx emission will be summarized in some aspects which including combustion component control strategy, injection control strategy, ignition control strategy and engine compression ratio control strategy. To the PFI-H₂ICE operated at lean fuel conditions (like equivalence ratio is less than 0.5) or rich fuel conditions (like equivalence ratio is higher than 1), the technologies and the strategies of EGR (exhaust gas re-circulation) will be reviewed and discussed. It is hoped this literature review would enable researchers to systematically understand the progress of NOx emissions research in PFI-H₂ICE and explore further research directions.     

The decreasing effect of ammonia enrichment on the combustion emission of hydrogen,methane, and propane fuels

In the present study, non-premixed combustion and NOx emission of H₂, NH₃, C₃H₈, and CH₄ fuels have been studied in a combustion test unit under lean mixture conditions (λ = 4) at 8.6 kW thermal capacity. Furthermore, the combustion and NOx emission of the H₂, C₃H₈, and CH₄ fuels have been investigated for various NH₃ enrichment ratios (5, 10, 20, and 50%) and excess air coefficients (λ = 1.1, 2, 3, and 4) at the same thermal capacity. The obtained results have been compared for each fuel. Numerical simulation results show that H₂ emits intense energy through the reaction zone despite the lowest fuel consumption in mass, among others, due to its high calorific value. Therefore, it has a higher flame temperature than others. At the same time, C₃H₈ has the lowest flame temperature. Besides, NH₃ has the shortest flame length among others, while C₃H₈ has the most extended flame form. The highest level of NOx is released from the NH₃ flame in the combustion chamber, while the lowest NOx is released from the CH₄. However, the lowest NOx emission at the combustion chamber exit is obtained in NH₃ combustion, while the highest NOx emission is obtained with H₂ combustion. It results from the shortest flame length of NH₃, short residence time, and backward NOx reduction to N₂ for NH₃. As for H₂, high flame temperature and relatively long flame, and high residence time of the products trigger NOx formation and keep the NOx level high. On the other hand, excess air coefficient from 1.1 to 2 increases NOx for H₂, CH₄, and NH₃ due to their large flame diameters, unlike propane. Then, NOx emission levels decrease sharply as the excess air coefficient increases to 4 for each fuel. NH₃ fuel also emits minimum NOx in other excess air coefficients at the exit, while H₂ emits too much emission. With NH₃ enrichment, the NOx emissions of H₂, CH₄, and C₃H₈ fuels at the combustion chamber exit decrease gradually almost every excess air coefficient apart from λ = 1.1. As a general conclusion, like renewable fuels, H₂ appears to be a source of pollution in terms of NOx emissions in combustion applications. In contrast, NH₃ appears to be a relatively modest fuel with a low NOx level. In addition, the high amount of NOx emission released from H₂ and other fuels during the combustion can be remarkably reduced by NH₃ enrichment with an excess air combustion.     

射流对高温空气燃烧过程中NO_x生成的影响

总结了燃料燃烧过程中NOx的生成机理和各种影响因素,并结合高温空气燃烧（High Temperature Air Combustion-HiTAC）的特点和射流的基本原理,研究了燃料和空气射流的卷吸作用对该燃烧方式NOx生成量的影响。为选择合理的设计与运行参数,实现该燃烧方式的超低NOx排放和高效节能,也为更好地在我国推广和应用这一先进技术提供理论基础。     

Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging

Low NOx burner and air staged combustion are widely applied to control NOx emission in coal-fired power plants. The gas-solid two-phase flow, pulverized coal combustion and NOx emission characteristics of a single low NOx swirl burner in an existing coal-fired boiler was numerically simulated to analyze the mechanisms of flame stability and in-flame NOx reduction. And the detailed NOx formation and reduction model under fuel rich conditions was employed to optimize NOx emissions for the low NOx burner with air staged combustion of different burner stoichiometric ratios. The results show that the specially-designed swirl burner structures including the pulverized coal concentrator, flame stabilizing ring and baffle plate create an ignition region of high gas temperature, proper oxygen concentration and high pulverized coal concentration near the annular recirculation zone at the burner outlet for flame stability. At the same time, the annular recirculation zone is generated between the primary and secondary air jets to promote the rapid ignition and combustion of pulverized coal particles to consume oxygen, and then a reducing region is formed as fuel-rich environment to contribute to in-flame NO_X reduction. Moreover, the NOx concentration at the outlet of the combustion chamber is greatly reduced when the deep air staged combustion with the burner stoichiometric ratio of 0.75 is adopted, and the CO concentration at the outlet of the combustion chamber can be maintained simultaneously at a low level through the over-fired air injection of high velocity to enhance the mixing of the fresh air with the flue gas, which can provide the optimal solution for lower NOx emission in the existing coal-fired boilers.     

Investigation of flame characteristics using various design parameters in a pulverized coal burner for oxy‐fuel retrofitting

In oxy‐coal combustion for carbon capture and storage, oxygen and recirculated CO₂ are used as oxidizers instead of air to produce CO₂‐rich flue gas. Owing to differences between the physical and chemical properties of CO₂ and N₂, the development of a burner and boiler system based on fundamental understanding of the flame type, heat transfer, and NOx emission is required. In this study, computational fluid dynamic analysis incorporating comprehensive coal conversion models was performed to investigate the combustion characteristics of a 30 MWth tangential vane swirl pulverized coal burner. Various burner design parameters were evaluated, including the influence of the burner geometry on the swirl strength, direct O₂ injection, and O₂ concentrations in the primary and secondary oxidizers. The flame characteristics were sensitive to the oxygen concentration in the primary oxidizer. The performance of direct O₂ injection around the primary oxidizer with low O₂ concentration was dependent on the mixing of the fuel and oxidizer. The predictions showed that swirl number adjustment and careful direct oxygen injection design are essential for retrofitting air‐firing pulverized coal burners as oxy‐firing burners.     

Nitrogen oxides emission from a circulating fluidized bed combustor

Experiments were carried out in a pilot-scale circulating fluidized bed (CFB) coal combustor to investigate the mechanism of N₂O formation, nitrogen oxides (including NO_x and N₂O) emission and the effect of temperature, excess air ratio, recirculation ratio, etc. The concentrations of nitrous oxide and nitric oxide were measured along the height of the CFB furnace. N₂O concentration increased with height, and in the exit of the combustor N₂O reached the highest level. NO_x, however, decreased with height, showing the inverse trend compared with N₂O. The N₂O emission decreased sharply with the rise of temperature at the bottom of the combustor; at the same time, the NO_x concentration increased.     

Numerical calculation on combustion process and NO transformation behavior in a coal-fired boiler blended ammonia: Effects of the injection position and blending ratio

Ammonia (NH₃), as a potential carbon-free alternative fuel, can be blended into coal-fired boiler to achieve significant pollution reduction and carbon reduction, but there are concerns about high NOx emissions due to high nitrogen content. According to the characteristics of coal/NH₃ co-combustion, a dual-fuel co-combustion model with strong adaptability and high accuracy was established in this study through Chemkin software to study the influence of different injection positions and blending ratios on combustion characteristics and NOx generation process. Then, the co-combustion model was applied to the three-dimensional CFD calculation process of a 330 MWe front-fired boiler, and the combustion characteristics, NOx distribution and reaction process were calculated when cal. 20% NH₃ was blended in the primary air. The results show that when cal. 20% NH₃ is blended, the change of NO content mainly occurs in ignition zone and flame zone, and the transformation behavior of N in NH₃ is optimized to a 15-step elementary reaction; The temperature distribution in the furnace is similar, and the average temperature at the furnace outlet decreases from 1033 °C to 988 °C, while NH₃ have a preferential combustion reaction with air than coal, resulting in a decrease in the burnout rate of coal; The NOx concentration at the furnace outlet decreases from 355 mg/Nm³ to 281 mg/Nm³, which is 20.85% lower than that under the pure coal burning condition, and the variation range of O₂ concentration and unburned NH₃ concentration is small.     

Unreacted-core model applied on the NOx emission of coke combustion

Despite numerous experimental studies of the NOx emission of coke combustion, there is no good computational model for NOx emission of the coke combustion. In this study, experimental studies of exhaust emissions of coke combustion were conducted by using micro-sinter equipment. Unreacted-core model was applied to kinetic analysis. The calculated NOx content curve described the measured data well, and the peak and integral total NOx difference values between the calculated and experimental data were 2.7 ppm and 0.15 mL, respectively, which illustrated successful application of unreacted-core model for the NOx emission of coke combustion. The equation also illustrated that particle size of coke breeze and O₂ concentration had a great influence on the NOx emission of coke combustion.     

Separate and combined effects of hydrogen and nitrogen additions on diesel engine combustion

Shortage of non-renewable energies, increase in fossil fuel prices and stricter emissions regulations due to high NOx and soot emissions emitted from combustion of heavy diesel fuels by compression ignition engines, has led consumers to use renewable, cleaner and cheap fuels. An investigation has been computationally carried out to explore the influences of hydrogen and nitrogen addition on engine performance such as indicated power and indicated specific energy consumption and amounts of pollutant emissions like NOx, soot, and CO in an HSDI (High-Speed Direct Injection) diesel engine. Optimized sub-models, such as turbulence model, spray model, combustion model and emissions models have selected for the main CFD code. Meanwhile, HF (Homogeneity Factor) has been employed for analysing in-cylinder air-fuel mixing quality under various addition conditions. After validations with experimental data of diesel combustion with a single addition of 4% hydrogen and combined addition of 6% hydrogen + 6% nitrogen, investigations have conducted for modeling mixing and combustion processes with additions of hydrogen and nitrogen by ranges of 2–8% (v/v). Results showed that a single addition of H₂ increased NOx and decreased CO and soot and improved ISEC and IP. In the case of nitrogen addition, NOx decreased, both CO and soot emission increased and ISEC and IP considerably ruined compared with NDC operation. Based on the results obtained for simultaneous addition of N₂ (8% of v/v) and H₂ (8% of v/v), NOx and soot emissions decreased by 11.5% and 42.5% respectively, and ISEC and IP improved 25.7% and 13%, respectively. But amount of CO emissions had an increase of 52% should be paid necessary attention as a main disadvantage.     

Nitrous oxide formation and emission in selective non-catalytic reduction process

Pulverized coal-fired boilers are not nitrous oxide sources because of high temperature combustion. But selective non-catalytic reduction may produce N₂O by NO reduction reactions. Chemical kinetics calculation and experimental research were used to find out the mechanism between N₂O and N-agent species, N-agent/NO nitrogen stoichiometric ratio (NSR), reaction temperature, reaction time, etc. The results show that N₂O emission decreases with increasing reaction temperature and NSR decreases when reaction time is enough. N₂O concentration first increases then decreases as SNCR reactions keep on occuring. Ammonia SNCR tests indicated that N₂O emission was 0–7 μmol/mol. About 8.7% of NO was transformed to N₂O, and N₂O emission was 27.8 μmol/mol at urea-SNCR test. Urea-SNCR is likely to bring N₂O emission problem. Translated from Proceedings of the CSEE, 2005, 25(13): 91–95 [译自: 中国电机工程学报]     

Combustion of nanoaluminum at elevated pressure and temperature behind reflected shock waves

This study presents experimental measurements on the combustion of nanoaluminum particles behind reflected shock waves in a shock tube. These experiments were performed at elevated pressures (4-32 atm) and temperatures (1200-2100 K) in the oxidizers oxygen and carbon dioxide, with nitrogen also present. The light emission from the reacting particles was monitored. For all cases, a brief period of intense light emission was observed soon after exposure to the reflected shock conditions. The time scales of this emission event are quantified by the 10-90% integrated emission intensity method to yield a reaction time for this rapid exothermic process. The duration of the emission is found to be 50-500 μs for the conditions tested here. Reaction times in 50% O₂ and 50% N₂ were shown to decrease significantly with ambient temperature, with Arrhenius-type exponentials fitting reasonably well to the observed experimental data. The reaction times were also dependent on pressure, with the timescales decreasing by 1.6-4 times as the pressure was increased from 8 to 32 atm over the range of temperatures in the experiments. In 50% CO₂ and 50% N₂, the reaction occurs in two sequential stages, with more of the emission at earlier times under higher-temperature conditions. Particle temperatures were also measured. During the bright emission event, the temperature rises above the ambient and then cools to near the ambient as the emission event ends. The peak temperature of the particle varied with ambient temperature, pressure, and oxidizer, with high ambient temperatures (2000 K), high pressures (32 atm), and high oxygen mole fractions (50%) giving the highest particle temperatures (∼3500 K). Conversely, 50% CO₂ atmospheres produced particle temperatures just slightly above the ambient. The spectral output of the light emission was shown to be dominated by broadband emission. At high temperatures and pressures in oxygen, weak emission from the AlO B-X transition was observed.     

Experimental research on a blended hydrogen-fuel engine

An experimental study on the performance of a single cylinder engine fueled with hydrogen/gas fule blends was carried out. The performance of engine with different fuel components under the load characteristics of the engine was analyzed. The experimental results showed that with the increase of hydrogen blending ratio, the combustion speed was accelerated, and the maximum torque and maximum pressure in the cylinder were increased; The maximum torque of blended fuel with 40% CO₂ was 68.3% of that without CO₂; The maximum pressure in cylinder of blended fuel with 40% H₂ was 1.6 times higher than that without hydrogen; When the proportion of hydrogen was more than 30%, the torque decreased; When the mixture was blended with 30% N₂, the engine torque reached the maximum at the hydrogen ratio of 15%; With the increase of hydrogen blending ratio, the emission of CO increased and the emission of HC and NOx decreased; When the hydrogen blending ratio remained unchanged, the CO emission was the largest at medium load, the HC emission was the largest at small load, and the NOx emission was the largest at high load; When the mixture was blended with 15% H₂, with the increase of the proportion of nitrogen, emission of CO decreased, emissions of HC and NOx increased. The research of this paper provided an experimental basis for the design and development of gas fuel engines.     

NOx emission from high-temperature air/methane counterflow diffusion flame

The objectives of the present study are to measure NO_x emission of counterflow diffusion flame, to compare the findings with numerical results, and finally to demonstrate efficacious effect of high-temperature air with low concentration of oxygen on NO_x emission. Recently, high-temperature air with low concentration of oxygen is used for various industrial furnaces, resulting high efficiency and low emission of pollutants. Since high-temperature air increases NO_x emission and air with low concentration of oxygen decreases it, these effects are competitive. Measurement and computation were conducted to clarify these two effects by use of counterflow diffusion flame. Since it is difficult to employ very high temperature over 1100 K in a laboratory-scale apparatus, a quantitative agreement between experimental and numerical results was confirmed first, and then a numerical approach was used to obtain a larger effect of low oxygen to reduce NO_x emission. In the experiments, the methane concentration is changed from 10 to 30 vol% diluted by nitrogen, oxygen from 10 to 21 vol%, and air temperature from room temperature to 1100 K. The total amount of NO_x sufficiently agreed between experimental and numerical results, although NO and NO₂ could not be separated. By the numerical method, it was found that NO_x emission from the counterflow diffusion flame of high-temperature low-oxygen air of 1500 K and 5% oxygen is comparable with that of room-temperature air of 21% oxygen.     

高温贫氧燃烧过程中NOx排放的特点

对高温贫氧燃烧过程中NOx的排放特点，以及燃烧过程中影响NOx生成的各主要因素，如预热空气中的含氧量，预热空气温度，预热空气和燃料的流动状态及混合方式以及燃料的化学成分等进行了研究和分析。并在此基础上提出了今后研究工作的方向和重点。图8表2参l0