NOx Distribution Research Articles

Effect of oxy-fuel combustion with different O2/CO2 fractions on the pulverized coal combustion mechanism and heat transfer characteristics in rotary kilns: A numerical simulation study

Oxy-fuel combustion combined with carbon capture and storage is the most effective and direct technology to achieve carbon neutrality in cement industry. In this work, numerical simulation was employed to investigate the effects of air combustion and oxy-fuel combustion with different O2/CO2 fractions on the airflow field, temperature field, pulverized coal combustion mechanism, heat transfer characteristics and NOx distribution in rotary kiln. The results show that the airflow field under different conditions has no significant change. When transitioning from the air condition to the oxy-fuel condition, the ignition point of pulverized coal extends backward by approximately 0.2 m. The maximum temperature decreases from 2306 K to 2052 K, and the temperature becomes more uniform. As the oxygen concentration in the primary air increases, both the maximum and average temperatures gradually rise, and the flame length shortens from 25.4 m to 24.16 m. In the rotary kiln, the locations of char oxidation and gasification reactions differ, but these reaction locations remain consistent under different conditions. Under oxy-fuel conditions, the oxidation rate and oxidation proportion of char decrease, while the gasification rate and gasification proportion of char increase, resulting in a shortening of the burnout distance of coal from 34.5 m to 31.3 m. With increasing oxygen concentration in the primary air, the rates of char oxidation and gasification gradually rise, further shortening the burnout distance. The decrease in temperature at the front end of the rotary kiln under oxy-fuel conditions reduces the total heat transfer in the burning zone by 15 %. When the oxygen content of the primary air is 70 %, it ensures stable calcination of clinker. Additionally, under oxy-fuel conditions, the NOx generation is significantly reduced. With the increase of oxygen concentration in the primary air, the NOx concentration at the outlet decreases slightly from 451 ppm to 406 ppm, due to the increase in kiln temperature that favors the reduction of fuel NOx.

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.

Investigation on co-disposal technology of sludge and municipal solid waste based on numerical simulation

Rapidly growing municipal sludge (sludge) with a low calorific value and high nitrogen content leads to high treatment difficulties. This study uses numerical simulations to simulate the steady-state combustion process of waste and sludge (40% moisture content) blending in a 750 t/d waste incinerator, to analyze the effect of sludge blending ratio on the combustion characteristics and to optimize the selective non-catalytic reduction (SNCR) spray gun structure. It was found that 13% is a suitable maximum sludge blending rate for the incinerator, satisfying the 3 T principle. Combining denitrification efficiency, NOx distribution uniformity, and modification feasibility, five spray guns are a more suitable SNCR solution with a denitrification efficiency of 45.40%. The results provide support and reference for the subsequent optimization of the working conditions of the waste incinerator and the optimal design of the incinerator.

Modelling of the gas-turbine colorless distributed combustion: An application to hydrogen enriched – kerosene fuel

This study examines hydrogen-enriched kerosene combustion under distributed regime in a gas turbine combustion chamber. With hydrogen enrichment, it is aimed at increasing combustion performance of those fuels. However, in this circumstance, it is obvious to increase the flame temperature with taking place hydrogen enrichment. Thus colorless distributed combustion (CDC), which is one of the advanced combustion techniques, can be suggested to control flame temperature with slowing down the reaction rate, resulting in ultra-low NOX emissions and more uniform temperature distribution with a broadened flame. For this purpose, the hydrogen-enriched kerosene fuels were examined by modeling a CFD code using the eddy dissipation concept, the radiation model (P-1) and the turbulence model (standard k-ε). In this way, the thermal fields and the NOX distributions have been obtained. The results showed that hydrogen enrichment increased the flame temperatures from about 2490 K to 2605 K at air-combustion conditions until 30% H2, resulting in the NOX levels predicted increased in the combustor. With reducing oxygen percentage the flame started to be the broadened one. The flame temperatures decreased, for instance, from about 2605 K to 2230 K at 15% O2 for the 30% H2 containing fuel. As a result of this, the NOX levels reduced from about 30 ppm to the values lower than 1 ppm in the combustor. It is concluded that increments in temperature and NOX levels with hydrogen can be suppressed with distributed regime, which enables that gas turbines can be operated at wider flammability limits with hydrogen enrichment.

Temperature and NOx distribution characteristics of coal particles under high-temperature and low-oxygen environments simulating MILD oxy-coal combustion conditions

A diffusion-flamelet-based Hencken burner was used to produce moderate and intense low-oxygen dilution oxy-coal combustion (MILD-OCC) of coal particles under O2/CO2 environments with the coflow temperatures of 1000–1400 K and oxygen concentrations of 2.86–8.75%. The flame temperature distributions were measured by a R-type thermocouple coupled with temperature continuous recorder. The NOx was captured by a self-made air extraction sampling device and measured by UV spectrophotometer. Three influencing factors including fuel-air ratio of methane, secondary air and particle size of pulverized coal on temperature and NOx distributions were experimentally researched. It is found that increasing the fuel-air ratio and the volume of secondary air leads to lower peak temperature, more uniform temperature distribution, lower NOx and stronger NO-reburning effect in flame. To a certain extent, decreasing the particle size of pulverized coal causes lower ignition delay, higher release rate of volatiles and little change in NOx emissions, further reducing the particle size of pulverized coal results in higher NOx in flame. Moreover, the mathematical criterion of MILD-OCC state is deduced based on criterion of MILD coal combustion. The research has important reference value for in-depth understanding of MILD-OCC regime and the generation mechanism of fuel NOx.

Numerical investigation on combustion and NOx formation characteristics under deep-staging conditions within a cyclone barrel

Numerical investigation on combustion and NOx distribution characteristics of coal within a cyclone barrel under deep-staging conditions was conducted. Char conversion model and NOx formation model with refined NO heterogeneous reduction mechanism and oxidation rate of HCN suitable for cyclone barrel were implemented and validated with experimental data. The maximum deviations between the predicted and experimental values of NO and CO concentrations are 12.1% and 18.6% respectively under various stoichiometric ratios. Based on the refined models, the NO distribution in cyclone barrel under deep-staging condition is obtained. In the cyclone barrel, the concentration of NOx is higher in the primary-air-controlled and near-wall zones, which hinders further NOx reduction. The appropriately delayed secondary air supply can avoid the centralized supply of O2 and form a reducing atmosphere region in the primary-air-controlled zone, which greatly reduces the initial formation of NOx. In addition, the central zone of the cyclone barrel is the core region for NOx reduction. The area of the central zone with a strong reducing atmosphere can be significantly enlarged by increasing the velocity of secondary air. The NOx formation in cyclone barrel drops by 26.5% under deep-staging condition when the secondary air velocity increases from 20 m/s to 50 m/s. Finally, the oxygen-enriched and deep-staging technology is applied to the cyclone barrel. An ultra-low NOx formation in cyclone barrel is achieved. When the O2 concentration of secondary air increases from 0.21 to 0.40, the NOx formation drops by 64.1% under deep-staging condition. Moreover, the elevated flue gas temperature with O2 enriched in cyclone barrel makes it more suitable for the safe operation of slag tapping cyclone barrel.

CFD Simulation of Changes in NOx Distribution according to an Urban Renewal Project

CFD Simulation of Changes in NOx Distribution according to an Urban Renewal Project

An Experimental Study on the Characteristics of NOx Distributions at the SNCR Inlets of a Large-Scale CFB Boiler

The unknown NOx distributions inside large-scale CFB (circulating fluidized bed) boilers have always hindered the economy of the SNCR (selective non-catalytic reduction) process. In this study, field tests were carried out on a typical 300 MW CFB boiler, where multi-level 316 L-made probe and Ecom-J2KN/Testo 350 analyzers were used to perform detailed two-dimensional distributions of flue gas composition at SNCR inlets for the first time. The penetration depth inside the horizontal flue pass was up to 7 m. The NOx distributions were analyzed in detail combining with the auxiliary test in the dilute phase zone. Key results show that the average O2 concentrations in #A and #C regions were 6.52% and 0.95%, respectively. The vertical NOx distributions of #A and #C SNCR inlets were similar, showing a trend of first increasing and then decreasing with peak value all appeared at 5 m depth, while the NOx distribution of #B SNCR inlet was basically increasing. Some local areas with extremely high NOx concentration (over 2000 mg/m3) were observed near the inclined edge of SNCR inlets, which has never been reported before. Based on this, the optimization of urea injections was conducted, which could save 15.7% of the urea solution consumption while ensuring ultra-low emission of NOx.

Optimization of NOx emission control based on high sulfur coal combustion

Combined with the operation of an 800MW supercritical boiler burning high sulfur coal, in order to control the emission of nitrogen oxides, the optimization adjustment of ammonia injection and NOx emission reduction based on combustion adjustment is proposed. According to the test of the inlet and outlet of denitration system under 800 MW condition, the parameters of NOx, O2, NH3, wind speed and so on at the inlet and outlet of denitration system are tested, and the ammonia injection manual valve is adjusted according to the test results. After adjustment, the NOx distribution at the outlet of denitration system is uniform, and the ammonia injection quantity is reduced. After adjustment, the flow field under 800 MW and 500 MW conditions is tested, and the flow field distribution under the two conditions is uniform. The adjustment test has achieved the expected effect.

Effects of wall cooling with microchannels on swirl combustor performance

Microchannel cooling provides a potential idea for high-temperature rise combustor cooling design. The performance of the swirl combustor with a rectangular microchannel embedded inside its walls are experimentally investigated for the coolant Reynolds numbers ranging from 0 to 600. Furthermore, an analytical model of the interactions between microchannel cooling heat transfer and the combustion is established. The corresponding Reynolds average numerical simulation is performed and the realisable k-epsilon turbulence mode where the eddy-dissipation concept is adopted for the turbulence chemistry interactions with a 14-species 19-step methane/air reaction mechanism. The results show that the heat exchange capacity of the microchannel coolant, flame temperature, and NOX distributions are all piecewise functions of the coolant Reynolds number that flows through the microchannels. The wall temperature can quickly decrease by 55% at low Reynolds numbers while the outlet flame temperature decreases gently. The combustion reaction regions determined by the CH* chemiluminescence images and gained from the simulation maintain a V-shape under the same equivalent ratio regardless of the inlet Reynolds number, indicating the microchannel cooling has little effect on the main combustion zone. The experiments show the NOx emissions have fallen by more than 35% while the combustion efficiencies are maintained above 98%. Those results indicate the microchannel wall cooling can greatly reduce the wall temperature at low coolant Reynolds numbers to protect the combustor components and effectively reduce the NOx emissions, as the same time have a negligible impact on the main combustion zone. The simulation results match well with experiment for the structure of the swirling flame and the effects of microchannel cooling on the flame temperature, and the temperature distribution tendency is consistent with theoretical analysis.

High-resolution simulation of local traffic-related NOx dispersion and distribution in a complex urban terrain

Urban air pollution features large spatial and temporal variations due to the high heterogeneity in emissions and ventilation conditions, which render the pollutant distributions in complex urban terrains difficult to measure. Current urban air pollution models are not able to simulate pollutant dispersion and distribution at a low computational cost and high resolution. To address this limitation, we have developed the urban terrain air pollution (UTAP) dispersion model to investigate, at a spatial resolution of 5 m and a temporal resolution of 1 h, the distribution of the local traffic-related NOx concentration at the pedestrian level in a 1 × 1 km2 area in Baoding, Hebei, China. The UTAP model was shown to be capable of capturing the local pollution variations in a complex urban terrain at a low computational cost. We found that the local traffic-related NOx concentration along or near major roads (10–200 μg m−3) was 1–2 orders of magnitude higher than that in places far from roads (0.1–10 μg m−3). Considering the background pollution, the NO and NO2 concentrations exhibited similar patterns with higher concentrations in street canyons and lower concentrations away from streets, while the O3 concentration exhibited the opposite behavior. Sixty percent of the NOx concentration likely stemmed from local traffic when the background pollution level was low. Both the background wind speed and direction substantially impacted the overall pollution level and concentration variations, with a low wind speed and direction perpendicular to the axes of most streets identified as unfavorable pollutant dispersion conditions. Our results revealed a large variability in the local traffic-related air pollutant concentration at the pedestrian level in the complex urban terrain, indicating that high-resolution computationally efficient models such as the UTAP model are required to accurately estimate the pollutant exposure of urban residents.

Research on ammonia injection optimized adjustment test of selective catalytic reduction denitration device for 600MW coal-fired unit

After the ultra-low emission reform of the coal-fired power plant, the ammonia slip at the outlet of the Selective Catalytic Reduction(SCR) denitrification device increases, resulting in abnormal air preheater resistance, which affects the economic and safety of the unit. The optimized adjustment test was carried out on the ammonia injection system of the SCR denitration device for 600 MW coal-fired unit. Under the premise of meeting the ultra-low emission of the unit, the NOx and ammonia escape at the outlet of the denitration device are controlled by adjusting the ammonia injection manual door to reduce the NOx distribution deviation of the outlet. The test shows that the standard deviation of the NO concentration distribution at the outlet of the reactor A reduced from 27.72% before adjustment to 12.53% after adjustment. The standard deviation of the NO concentration distribution at the outlet side B decreased from 55.78% before adjustment to 14.52% after adjustment. The uniformity of the NOx distribution in the outlet zone of the reactor is significantly improved, thereby effectively controlling the escape of ammonia.

Effect of Slot Wall Jet on Combustion Process in a 660 MW Opposed Wall Fired Pulverized Coal Boiler

Abstract Numerical investigations of an anti-corrosion design and the combustion process (original conditions and optimal conditions) were conducted for a 660 MW opposed wall fired boiler. In order to solve high-temperature corrosion of the side wall, a scheme was proposed: slotting in the side wall and introducing air (closing-to-wall air) from the secondary air. The effect of anti-corrosion was disclosed in detail by varying the structures of slotting, gas velocities from nozzles and jet inclination angles. The temperature and NOx distribution in the furnace at optimized conditions were compared with those at the original operating conditions. Simulation results showed that the structures of the slot and gas velocities from the nozzles had a marked effect on anti-corrosion of the side wall. When the gas velocity was 4 m/s, an inclination angle of the gas velocity was not conducive to anti-corrosion of the side wall. When the gas velocity increased at the middle and bottom of the side wall, the anti-corrosion effect increased significantly. When the optimal scheme was adopted, the corrosion area of the side wall decreased obviously, but the furnace temperature and the NOx emission increased slightly. The detailed results of this work promote a full understanding of closing-to-wall air and could help to reduce the corrosive area in pulverized-coal furnaces or boilers.

Numerical study on the effect of separated over-fire air ratio on combustion characteristics and NOx emission in a 1000 MW supercritical CO2 boiler

As an advanced power generation system, supercritical CO2 (S-CO2) coal fired power plant has advantages of high electric efficient and compact system layout. In this paper, the coal combustion characteristics and NOx formation in coal-fired S-CO2 boiler has been explored by numerical simulation. The boiler wall temperature distributions were determined according to the 1/8 partial flow strategy for S-CO2 coal-fired power plant. The effect of SOFA ratio on the gas flow field, the gas temperature, the species concentration including NOx distribution was investigated. The results show that the SOFA ratio has a great influence on the coal combustion process and NOx formation. With the increase of SOFA ratio, the O2 concentration decreases in the main coal burning zone, a higher CO concentration can be found. A lower gas temperature zone in the main burning zone and a lower NOx emission near the furnace outlet can also be observed with a higher SOFA ratio. The results presented promote a better understanding of the NOx formation characteristics and would be benefit for the NOx control in coal-fired S-CO2 boiler.

Combustion and emission characteristics of premixed CNG/H2/CO/CO2 blending synthetic gas flames in a combustor with variable geometric swirl number

This paper presents the results of experimental studies conducted on premixed CNG/H2/CO/CO2 blending synthetic gas flames in a laboratory scale combustor, with particular emphasis on effects of swirl number (from 0.2 to 1.6, at intervals of 0.2) and equivalence ratio (0.6, 0.8 and 1.0) on combustion and emission characteristics of such mixtures. Furthermore, effects of swirl number on lean blowout and flashback limits of studied mixtures were also investigated. Irrespective of the swirl number and equivalence ratio, H2/CO ratios of tested gas mixtures were kept constant (1.0), and mixtures of H2/CO/CNG/CO2 which were diluted with different amount of CO2 (0–20%, at intervals of 5%) were derived. Considering broadest operating range, mixture of 20%CNG/30%H2/30%CO/20%CO2 was determined as base fuel to be tested. Combustion and emission behavior of tested gas mixtures were assessed by examining axial and radial temperature, NOx, CO and CO2 distributions. Besides, swirl number and equivalence ratio effects on flame structure were evaluated by investigating instantaneous flame images (with the same focal length and exposure time). Results of this study showed that effects of swirl number on flame characteristics (temperature and pollutant emission distributions) is not monotonous, and equivalence ratio dependent flame behavior alters differently at different swirl numbers.

A technical method to improve NOx/NH3 mixing ratio in SCR system and its engineering applications

A technical method for diagnosing the distribution of NOx flux within the cross-section area in front of ammonia injection grid (AIG) was proposed for guiding the valve-tuning of AIG branch-pipes, in order to optimize the NOx/NH3 mixing ratio in the selective catalytic reduction (SCR) system of power plant. The weight coefficient of each branch-pipe in AIG system can be quantitatively determined with regard to the distribution of NOx flux in the corresponding sub-zone of the cross-section area. The control strategy of the valves for different AIG branch-pipes can be achieved for guiding the NH3 injection and improving the NOx/NH3 mixing ratio within the whole cross-section area in front of AIG. The technology has been applied on one side of the SCR system flue-gas tunnels (normally two tunnels for the SCR system called as A-side and B-side) of a 660 MW plant for more than one year. The ammonia consumption rate of the SCR system was reduced about 12.62% and the uniformity of outlet NOx distribution was estimated to be greatly improved by about 79.01% with regard to the standard deviation. The rising rate of the flue gas resistance of the air preheater was slown down by 39.18% compared to that of the other flue-gas tunnel of SCR system. This implied that the formation of the sticky ammonium bisulfate (ABS) on air preheater was significantly inhibited through the application of this technology.

Numerical and Experimental Study on the Effect of Over Fire Air on NOx Distribution in Furnace

In this paper, a numerical investigation and experimental study was used to research the effect of a power plant 600MW supercritical four walls tangentially fired boiler furnace over fire air opening size on the inside furnace NOx concentration distribution and the results coincide. There are four cases in all. The influence and formation of NOx that was produced by pulverized coal furnace during combustion processes were analyzed. The research was proved that the over fire air has great effect on the concentration distribution of NOx in the furnance.

Single-cylinder engine evaluation of a multi-component diesel surrogate fuel at a part-load operating condition with conventional combustion

Simple, yet fully-representative surrogate fuels are needed to model market Diesel fuels. The surrogates must closely mimic market Diesel fuel properties and match the engine combustion and emissions behavior. To this end, the combustion and emissions performance of a market Diesel fuel and a four-component surrogate fuel were investigated using a single-cylinder, light-duty Diesel engine with contemporary combustion and fuel injection technology. The surrogate fuel, which was developed in previous work, was composed of normal-hexadecane/2,2,4,4,6,8,8-heptamethylnonane/decahydronaphthalene/1-methylnaphthalene with a volume fraction formulation of 0.37/0.33/0.18/0.12. Fuel test results showed the physical, chemical and combustion properties of the market Diesel fuel were closely matched by the surrogate. The fuels were evaluated by engine tests conducted at 1500 r/min and 9 bar IMEP. At this operating condition, the Diesel combustion process demonstrated low-temperature heat release, premixed and diffusion combustion regions. Test results from EGR and combustion phasing sweeps showed the engine combustion behavior, exhaust CO, HC, NOx, smoke and particle size distributions from the market Diesel fuel were very closely matched by the surrogate fuel. It can be concluded that under the engine conditions evaluated by this work, the four-component surrogate fuel accurately represents the market Diesel fuel. Thus, for conventional Diesel combustion conditions, the surrogate should prove useful for future applications such as kinetic mechanism development, fuel spray and combustion simulation, and further experimental investigations.

Residential cohort study on mortality and hospitalization in Viggiano and Grumento Nova Municipalities in the framework of HIA in Val d'Agri (Basilicata Region, Southern Italy)

to evaluate the associations among the emissions produced by "Centro olio Val d'Agri" (COVA), with mortality and hospitalization of residents in the Viggiano and Grumento Nova Municipalities, located in Val d'Agri (Basilicata Region, Southern Italy). residential cohort study. Lagrangians dispersion models to estimate the level of exposure at the address of residence to NOX concentrations as tracers of COVA emissions. Based on the tertile of NOX distribution, individual exposure was classified and a Cox model analysis was performed (hazard ratio, HR, trend with relative 95%CI). The association among exposure to NOX and the cohort mortality/hospitalization was evaluated considering age, socioeconomic status, and distance from the high traffic density road. The cohort included 6,795 residents (73,270 person-years) in the period 2000-2014. causes of mortality and hospitalization due to cardio-respiratory diseases, recognised as associated to air pollution, with medium-short latency induction period, consistent with the period of operation at the COVA. increasing trends were observed on three exposure classes for mortality due to circulatory system diseases (HR trend: 1.19; 95%CI 1.02-1.39), stronger considering women (HR trend: 1.19; 95%CI 1.02-1.39). From hospitalizations results, an increased risk emerges for respiratory diseases (HR trend: 1.12; 95%CI 1.01-1.25) and, for women, for diseases of the circulatory system (HR trend: 1.19; 95%CI 1.03-1.38), for ischemic diseases (HR trend: 1.33; 95%CI 1.02-1.74) and respiratory diseases (HR trend: 1.22; 95%CI 1.03-1.46). the excesses of mortality and hospitalization emerged in areas most exposed to pollutants of industrial origin are relevant for preventive actions. It is recommended to define and implement a surveillance system for the entire resident population based on indicators of environmental pollution and related health outcomes on the basis of the scientific literature and the results achieved by the present study.

Optimization study on combustion in a 1000-MW ultra-supercritical double-tangential-circle boiler

The combustion process of one 1000-MW ultra-supercritical double-tangential-circle boiler was numerically studied and the three-dimensional full-size structure of the boiler was full considered. The influences of primary and over-fire air velocity as well as the jet structure on NOx generation characteristics were examined. In addition, the NOx generation characteristics of the improved burner structure were compared with those of the original one. Numerical results show that there exist two inverse elliptical flow fields and temperature fields. Moreover, the NOx generation and distribution characteristics are related to the temperature field to a certain extent. For different burner jet structures and arrangements, NOx distribution curves of the horizontal cross section are all W-shaped, but the NOx generation and distribution performance are correspondingly different, while the NOx emission changes are unobvious for different design schemes of the boiler burner. When we arrange one layer of auxiliary air from the burner undersurface, the flow area of the primary coal powder jet is enlarged by 100% and the coal feeding is increased by 20%. As a result, the temperature around the burner zone rises significantly. However, when two layers of auxiliary air are adopted, the combustion characteristic is promoted and NOx generation increases slightly. Based on the arrangement of the burner in the ultra-supercritical boiler, NOx generation does not vary obviously at different boiler loads. To achieve a better scheme, numerical and experimental studies are both performed in this study, and identical results are obtained. The current results may provide a theoretical basis for burner design improvement.