Low NOx Combustion Research Articles

Efficient and low-NOx combustion in a grate-fired boiler by feeding biomass non-uniformly along grate width: An integrated modeling study with experimental validation

The distribution of air flows and fuel feeds are two crucial operational parameters determining the high-efficiency combustion and low-nitrogen emissions in biomass-fired grate boilers. However, current optimizations primarily focus on air distribution, with limited attention given to fuel feeds optimization. In this study, a three-dimensional integrated model was developed in FLUENT platform for simulating a grate-fired boiler with four feed inlets, in which the DPM model was adopted to track particle information in detail, and the Ergun model was used to account for the flow resistance generated by the thick packed bed, implemented in the form of a UDF code. The reliability of the model was validated through comprehensive on-site experimental data. It was found that key parameters such as temperature and ignition front of char are non-uniformly distributed along the width of the four grates. Specifically, a significant lag in volatiles release and char oxidation occurs near the water-cooled wall on both sides. On this basis, the optimization strategies on improving efficient and low-NOx combustion were proposed by redistributing the biomass fuel from near the sidewalls to the middle sections, and the optimal non-uniform feeding mode along the grate width was found. Results demonstrate that when the mass ratio of fuel between the middle and side grates is 0.30 to 0.20, yielding a maximum overall char burnout ratio of 84.74%. Additionally, the concentration of NO decreases from 163.2 mg/m3 of uniform feeding mode to 149.4 mg/m3 (at 11% O2). These predictions will provide reliable theoretical guidance for enhancing boiler efficiency and reducing NOx emissions in grate-fired boilers.

Nonpremixed Approaches for Fuel Flexible, Low NOx Combustors in High-Efficiency Gas Turbines

Abstract Lean, premixed combustor designs have almost completely replaced non-premixed combustors for industrial gas turbine applications where NOx emissions are regulated. Nonetheless, these designs have also introduced turndown and fuel flexibility constraints and made combustion instabilities and flashback more problematic. Future gas turbine applications will require combustors to accommodate a range of alternative fuels, provide operational flexibility, and compete for services with a host of new technologies, including energy storage and fuel cells. The purpose of this paper is to propose nonpremixed, multi-stage designs for the next generation of high turndown, high fuel flexibility, low NOx combustion designs – referred to here as a Nonpremixed, Rich, Relaxation, Lean (NRRL) combustor. The key concept we explore is non-premixed combustion, followed by additional fuel mixing to locally fuel-rich conditions, a relaxation stage, and then a lean stage. This non-premixed approach can handle essentially any fuel composition, including pure hydrogen, liquid fuels, pure methane, pure ammonia, and any combination in between while breaking the NOx-CO tradeoff and reducing combustion instability risk. This paper provides chemical reactor network modeling calculations to identify key kinetic processes and time scales required for such a concept. This concept has completely inverted sensitivities from lean, premixed systems which prefer short residence times, low pressures, and low temperatures to minimize NO formation. This concept prefers long residence times, high pressures, and high temperatures, indicating a very different set of design trades for part load and off-design performance.

Effect of the coal-spreading air on airflow and low-NOx combustion within a 75 t/h coal-fired grate furnace

Coal-fired grate furnaces equipped with coal throwers, which conventionally own a low-NOx combustion framework consisting of (i) the partitioned primary air for the horizontal air staging on the grate and (ii) the coal-spreading air and secondary air for the vertical air staging in the freeboard, actually suffer from poor burnout and apparently high NOx emissions at habitual operations. While comprehensive investigations on the in-furnace airflow, coal combustion, and NOx formation, which are devoted to first explaining and then improving these problems, are absent up to now. In order to unveil the airflow, combustion, and NOx emission characteristics within a 75 t/h coal-fired reverse grate furnace, evaluate its coal-spreading air impact on the low-NOx combustion performance, and determine an appropriate coal-spreading air ratio for improving the furnace performance, both industrial-scale tests and numerical simulations were performed at the coal-spreading air ratios of 2 %, 4 %, 6 %, and 8 %, respectively. The flow-field deflection and asymmetric combustion appeared in the upper furnace, with the upward gas deflecting towards the rear-wall side and the front-half furnace part dominated by a weak recirculation zone without effective combustion. With increasing the coal-spreading air, problems of the flow-field deflection and asymmetric combustion aggravated. The combustion intensity weakened in the layer combustion zone while displayed an increase-to-decrease trend in the suspension combustion zone. In terms of indexes related to the furnace performance, burnout rate decreased continuously, and NOx emissions initially raised but then decreased. A comprehensive consideration of the coal-spreading requirement via air, flow-field symmetry, low-NOx combustion, and burnout suggests that a 2 % coal-spreading air ratio should be preferred, where NOx emissions and burnout loss were gained at 249 mg/m3 (6 % O2) and 5.75 %, respectively. It reduced NOx emissions by 49 % while raised slightly burnout loss, as compared with the corresponding levels of 486 mg/m3 and 5.4 % at the habitual case. These results not only negate the air-staging function of the coal-spreading air but also highlight the novelty of this work.

Experimental study on combustion characteristics of a 40 MW pulverized coal boiler based on a new low NOx burner with preheating function

In this study, a novel low NOx burner is proposed and tested on a 40 MW pilot pulverized coal combustion system, aiming to experimentally investigate the impact of preheating temperature, air ratio distribution, and thermal power on furnace combustion characteristics. The experimental results demonstrate that the pulverized coal could be preheated stably above 700 °C in the designed burner. Furthermore, an increase in preheating temperature from 500 °C to 750 °C corresponds to a proportional rise in the total volume fraction of combustible pyrolysis gases released from pulverized coal, ranging from 18.34 % to 22.85 %. Consequently, the release and combustion of pyrolysis gases effectively improve the combustion characteristics within the boiler furnace and lead to reduced NOx emissions at the furnace outlet. When the thermal power is increased from 22.5 MW to 37.5 MW, the NOx emissions at the furnace output range from 46.8 mg/Nm3 to 98.3 mg/Nm3 (at 9 % O2). Under a specific thermal power of 25MWth, the boiler furnace output NOx emissions are reduced to 47.9 mg/Nm3 (@9 % O2) without any additional NOx reduction equipment. These findings offer valuable insights for advancing cleaner and more efficient low-NOx pulverized coal combustion technology.

Furnace-height FGR location impact of a cascade-arch-firing configuration in ameliorating low-NOx combustion and eliminating hopper overheating for a large-scale arch-fired furnace

This study analyses an alternative to the existing multiple-injection and multiple-staging combustion technique (MIMSCT) of large-scale arch-fired furnaces. The alternative approach is a cascade-arch-firing, low-NOx, and high-efficiency configuration (CLHC) with flue gas recirculation (FGR). The research is necessary as typical 600 MWe large-scale arch-fired furnaces produce high concentrations of NOx and have a high risk of overheating in their hoppers when firing low-volatile coals. Three furnace-height FGR locations were considered in descending order (i.e., FGR through (i) primary burners to mitigate fuel-NO (FGR-PB), (ii) tertiary air to restrain thermal-NO (FGR-TA), and (iii) auxiliary burners to reduce NO (FGR-AB)). Industrial-scale measurements and simulations were conducted in the existing MIMSCT furnace (which was used to verify simulations and compare with the subsequent CLHC furnace results), followed by numerical investigations of the CLHC furnace at the three FGR-locations. The FGR’s NO inhibition in the upstream zone decreased in the following order: FGR-PB > FGR-TA > FGR-AB. Initial combustion and NO generation in the reburning stage was most intense in the FGR-TA configuration (producing the highest NO amounts) whereas the FGR-AB configuration contributed the lowest amounts. As the FGR locations descended, burnout loss dropped a little, while NOx production was initially elevated and then descended. The FGR-PB was optimal among the three options, with a low NOx output of 604 mg/m3 (O2 = 6 %) and combustible level in fly ash of about 5 %. Compared to the MIMSCT, the CLHC not only enhanced the low-NOx environment and shortened the flame penetration but also improved combustion intensity and extended the overfire air penetration. This sharply reduced NOx without affecting burnout nor lowering apparent hopper temperatures. The above results demonstrate that a staged FGR consisting of primary FGR-PB and supplementary FGR-AB, is a promising solution to lower NOx while retaining high furnace burnout levels.

Effects of the addition of arch-supplied secondary air on the performance of the Foster Wheeler down-fired boiler: Air/particle flow, combustion and NOx emissions

This study proposes a low NOx combustion method for Foster Wheeler type down-fired boilers. The effectiveness of this technique was validated through laboratory-scale gas-particle two-phase experiments and industrial trials. The flow field characteristics within FW-type boiler furnaces equipped with arch-supplied secondary air injection nozzles were investigated. Furthermore, the application of this technique to a 660 MW subcritical FW-type boiler was examined, focusing on NOx emissions and combustion characteristics. Results demonstrate that optimizing the damper openings for F-layer and arch-supplied secondary air can significantly reduce NOx emissions by over 33 %. Additionally, increasing the mass flow rate of arch-supplied secondary air aids in reducing unburned carbon in fly ash and inducing a downward shift in the flame center within the furnace.

The Structural Design of and Experimental Research on a Coke Oven Gas Burner

A novel low-NOx burner was proposed in this study to achieve the stable and clean combustion of low- and medium-calorific-value gas and promote energy sustainability, and the influence of the gas pipe structure on the burner’s characteristics was studied with coke oven gas as a fuel. A 40 kW burner test bench was established to conduct cold-state experiments to investigate the influences of the gas pipe structure on the aerodynamic characteristics of the burner. We performed numerical simulations on both a 40 kW burner and a 14 MW prototype burner to investigate the thermal performance of the burners and their impact on low NOx emissions. The experimental results showed that increasing the deflection angle of the gas pipe nozzle direction relative to the circumferential tangent direction, the high-velocity zone and the high-concentration zone of the flow field move towards the central axis. Increasing the bending angle of gas pipe nozzle direction relative to the axis direction caused the high-velocity zone and the high-concentration zone to move upstream direction of the jet. The simulation reveals that the NO concentration at the exit cross-section of the combustion chamber of the 14 MW prototype burner is 17.00 mg/m3 (with 3.5% oxygen content). A recommended design structure of the burner was proposed, with a deflection angle of 0°and a bending angle of 0° for the No. 3 gas pipe, and a deflection angle of 15° and a bending angle of 30° for the No. 4 gas pipe.

Novel ultra-low NOx coal combustion technologies based on local microenvironment targeted regulation. Part 1. Selective oxygenation

Developing a novel high-efficiency coal combustion technology with ultra-low NOx emission is urgently needed to sustain the good ecological environment. Here, the targeted regulation of local microenvironment around fuel-N, such as functional groups, radicals, molecular configurations, and reaction atmosphere, is realized by the selective oxidation. The molecular configurations, including pore networks and microcrystalline structure in coal, are well characterized through synchrotron radiation-induced SAXS (small angle X-ray scattering) and WAXS (wide angle X-ray scattering) simultaneously. Furthermore, by combining density functional theory (DFT) and experiments, the effects of the local microenvironment on the nitrogen transformation and NO evolution during the thermal conversion are focused on. The results indicate that for the PPA oxidation, the H radicals attack the adjacent carbon to pyrrole/pyridine nitrogen, promoting the conversion of fuel-N to HCN. On the other hand, for the H2O2 oxidation, disrupting the π bond electron cloud by the CO and C = O on the ortho carbon of pyrrole/pyridine dominates the NH3 generation. Additionally, the increased La (average graphene layer extent), a3 (average interlayer spacing), σ3 (standard deviation of interlayer spacing) and σ1 (standard deviation of the first-neighbor distribution) induce massive smaller pores, promoting the generation of abundant reaction defects inside the particles. Importantly, the intensified adsorption on abundant active sites lead to the decreased HCN and increased NH3 evolution, which is adverse for the interaction between homogeneous and heterogeneous NO reduction. Interestingly, the PAA selective oxidation can reduce NO emission by 31.72 % - 34.30 % during the air combustion, which is far better than the H2O2 oxidation. Overall, the attack of free radicals on nitrogen-containing heterocycles promotes the conversion of fuel-N to HCN, the adsorption of which on char surfaces can further enhance the heterogeneous reduction in a lean oxygen atmosphere. The work here provides a novel route for developing high-efficiency and low-NOx combustion technologies.

Synergistic effects of mechanochemical activation and selective O-alkylation on the chemical structure and gaseous products evolution during coal flash pyrolysis

Mechanochemical activation offers significant advantages in enhancing the pore structure and active sites of materials, while selective O-alkylation boosts both the homogeneous and heterogeneous reactivity of coal. In this work, mechanochemical activation and selective O-alkylation are synergistically applied to modify pulverized coal. The evolution of the chemical structure was thoroughly characterized using ultimate analysis, Fourier-transform infrared spectroscopy (FTIR), and nuclear magnetic resonance (NMR). Pyrolysis gas chromatography-mass spectrometry (PY-GC-MS) was utilized to investigate the release of crucial gaseous products during the pyrolysis. Additionally, this study focuses on the synergistic effects of mechanochemistry and selective O-alkylation on the chemical structure and pyrolysis characteristics. The results reveal that alkylation boosts the formation of C-O bonds and aliphatic carbon while reducing the presence of aromatic carbon. Moreover, by disrupting intermolecular forces, alkylation induces the fragmentation of expansive aromatic layers into smaller aromatic fragments, thereby generating additional structural defects. Furthermore, the electron transfer process facilitates the conversion of aromatic carbon into aliphatic forms through ring-opening reactions. On the other hand, incorporating alkyl functional groups and initiating ring-opening reactions of aromatic carbons lead to a rise in aliphatic hydrocarbons during the pyrolysis of alkylated coal. However, intensified mechanochemical action hinders the production of aliphatic hydrocarbons. Additionally, during alkylation, the creation of esters and ethers with elevated thermal stability delays the release of oxygen-containing gas products. The findings propose a new pretreatment method for coal utilization, promoting the development of new low-NOx combustion technology.

Numerical studies of a water-cooled premixed burner for low NOx combustion of natural gas

Natural gas, known for its high combustion efficiency and low CO2 emissions, is experiencing a significant increase in both market share and application range. Despite these advantages, there are also challenges emerged during the combustion process, including occurrence of high flame temperature, difficulty in controlling nitrogen oxide emissions, combustion instability, and a tendency to backfire. To address these issues and achieve clean and stable burning, a novel 14 MW water-cooled pre-mixed burner has been developed. In-depth simulations are conducted to assess the impact of various structural and operating parameters on combustion and pollutant emission characteristics. Results indicate that the combustion temperature follows a “W" shape with height of the low cooling component while its width governs combustion stability. Gas inlet spacing influences the flame center position and anti-backfire performance. The flare angle of gas channels determines the minimum ignition energy of the mixed gas fuel which affects ignition point and flame center position. Although large height of high cooling component contributes to low NOx emissions at the gas outlet, the resultant low temperature is undesirable for combustion stability. Further examinations of operating parameters through simulations reveal that combustion temperature initially decreases and then increases with excess air coefficient, while a significant decrease is noted with lower operating load. The premixed water-cooled technology could achieve low-NOx emissions to 2.89 mg·Nm−3 with the combustion temperature controlled at 1642K. This study provides valuable data for the design of low-NOx gas burners, serving as a reference for achieving clean and stable combustion of gas fuel.

Effects of H2 enrichment on combustion characteristics in inverse swirl diffusion flames of H2-doped jet fuel using OH, CH2O, fuel and temperature imaging

Hydrogen (H2) and jet fuel co-firing is regarded as a promising transition strategy of decarbonization in aviation industry. Considering the simplicity and safety during highly hydrogen enrichment conditions, the inverse swirl diffusion (ISD) combustion holds research value. Given the current lack of understanding for the flame properties of H2-enriched pre-evaporative jet fuel ISD co-combustion, in this work, the simultaneous planar laser-induced fluorescence (PLIF) of hydroxyl (OH), formaldehyde (CH2O), and fuel, as well as the non-linear excitation regime two-line atomic fluorescence (NTLAF) thermography imaging, are used to study the effects of hydrogen enrichment on ISD flame intricate structure and temperature field. The results show the positive effects of H2 enrichment. The ISD flames display a blue hue and the increase in doping hydrogen helps the flames stabilize under global-lean conditions with almost no CO emission and low CO2 emission at the chamber exit. And the H2 enrichment contributes to inhibiting the CH2O formation upstream, reducing the generation of isolated reaction islands and promoting the strength of the flame wings, favoring the suppression of flame instability. In contrast, when at low H2 enrichment rate, the shift of the heat release zone and high-temperature zone, and the enlargement of the cold zone, are found in flame upstream. The analysis of the probability distribution functions (PDF) of temperatures confirms the effective effects of hydrogen on inhibiting the local flame fluctuations. Although the increase in the overall temperature level due to the hydrogen addition would lead to a promotion in NOx generation, the discussion on the structural analysis and flame propagation stability of ISD flames demonstrates the significance of H2 enrichment for the achievement of leaner low-NOx combustion of ultra-H2-enriched jet fuel in the future.

Retrofit and application of pulverized coal burners with LNG and oxygen ignition in utility boiler under ultra-low load operation

An oxygen-rich and low NOx burner integrated with liquefied natural gas (LNG) was proposed to address unstable combustion and high NOx emissions from a 330 MW subcritical boiler under ultra-low load operation in China. To assess the effectiveness of the retrofit, Chemkin and Fluent softwares were utilized to construct a new NOx model and calculate NOx generation, based on the combustion of pulverized coal gas and LNG. Further, an eddy dissipation concept (EDC) model, which can reflect detailed chemical reactions, was applied to calculate gas-phase reactions in the furnace. The results showed that when performing the deep peak shaving after the retrofit, the combustion in the furnace was stable under 50% or more load, and NOx emission level at the furnace outlet was lower than 350 mg/m3 (6% O2 content, dry basis). Under 25% load, the oxygen-rich burner integrated with LNG was applied, and the pulverized coal flow entered the furnace in a state of high-intensity combustion, which effectively promoted the stability of combustion in the furnace. The reductive combustion state with reductive free radicals generated by LNG decomposition inhibited NOx formation. Consequently, NOx emissions from the furnace outlet decreased from 380 mg/m3 to 316 mg/m3.

Combustion Diagnostics and Emissions Measurements of a Novel Low NOx Burner for Industrial Gas Turbine Operated With CO2 Diluted Methane/Air Mixtures

Abstract While the employment of Exhaust Gas Recirculation (EGR) is a well-established technique in Internal Combustion Engines to limit NOx emissions, its adoption in Gas Turbine engines has not yet found a practical application due to its expensive and complex installation that doesn't justify the emissions reduction when compared to already established DLN combustion technologies. EGR becomes an interesting option in GT engines considering the possibility of increasing the CO2 content of the exhaust gases to improve the efficiency of Carbon Capture and Storage (CCS) units. However, the decrease in oxygen content of the combustion air is extremely challenging in terms of combustion stability and therefore of engine operability. In the present work, a low NOx burner was studied at ambient pressure in a reactive single burner test rig. The burner was fed with methane and characterized in terms of emissions and stability limits at different operating conditions. In addition, the flame position and shape were studied through OH* chemiluminescence imaging together with the flow field thanks to PIV measurements. The effects of CO2 addition on the flame were then investigated at different EGR increasing levels, highlighting the impact of the oxygen content on the combustion reaction intensity. Variations in emissions and burner stability limits in terms of maximum sustainable CO2 content were also studied, to detail the burner operating window. Data have been thoroughly analyzed to gather information on the burner behavior to support the design of new technical solutions capable of ensuring both proper flame stability and low CO and NOx emissions.

Power augmentation and Ramp-Up rate improvement through compressed air Injection: A dry low NOx combustor CFD analysis

Gas turbines play a key role in accelerating the transition towards more environmentally friendly power generation. This role includes backup of renewable generation that is intermittent, providing grid inertia as well as other ancillary services for grid stability. For quick backup power, the ramp-up rate of gas turbines can be improved through air injection at the back of the compressor, facilitated by integrating compressed air energy storage. Published works have mostly focused on low-fidelity engine system analysis of air injection overall effects. No study has focused on the detailed combustor performance presented in this study. The work shows the impact of air injection on the emissions, thermoacoustic stability and liner wall durability. These yardsticks in assessing the operability of the combustor have also been used for air power augmentation and ramp-up analysis. ANSYS software was used in the computational fluid dynamics (CFD) analysis of the three-dimensional dry low NOx combustor. Low-order models were used for the thermoacoustic stability and durability analysis. For the power augmentation study, the NO and CO emissions produced at 15 % air injection are below the maximum values of the combustor in design operations. Also, the stability and durability were within limits. The ramp-up investigation indicates up to 10 % air injection is allowed and the emissions are similarly acceptable. However, the thermoacoustic analysis shows a potential for combustion instabilities at high frequencies above 1800 Hz. Generally, there was no unusual wall liner durability in these two studies. When benchmarked against previous engine-level analysis, the ramp-up rate can be potentially improved by 54 % if the small concern on thermoacoustic instability is resolved.

Perhitungan Beban Emisi Particulate Matter berdasarkan Data CEMS dari PLTU Batu Bara Milik PT PLN (Persero)

In the Coal-Fired Power Plant (CFPP) operation, various pollutants will be generated, including Particulate Matter (PM), which can be monitored with the Continuous Emission Monitoring System (CEMS). The parameters monitored include PM concentration, volumetric flow rate, % O2, and the operation hours and/or CEMS monitoring hours. Emission load calculation by utilizing CEMS data is a Tier 3 calculation method that uses real operation data from a CFPP unit. Information related to PM emission load from CFPP can be considered for corporate decision making in developing emission control strategies. This research utilizes secondary data from January 2021 to July 2023 period, from 52 PT PLN (Persero)’s CFPP units with various boiler, installed capacity, and types of Air Pollution Control Devices specifications. The largest average/month emission load value is 139,632 kg PM/month for CFPP unit that uses PC boiler, have a capacity of > 600 MW, and use low NOx burners and ESP as its APCD; while the smallest average/month emission load value is 525 kg PM/month for CFPP unit that uses PC boiler, have a capacity of 101-300 MW, and use ESP as its APCD. If the average emission load/month from January 2021 to July 2023 is compared with the maximum emission load/month with a PM quality standard of 100 mg/Nm3, all CFPP units do not exceed it and are generally ready to face the tightening of PM quality standard up to 75 mg/Nm3.

Simulation of Heat and Mass Transfer in a Moving Bed Part-Fluidized Boiler

Abstract Moving bed part-fluidized boiler is a new type of furnace. The new combustion method in the furnace has attracted a lot of attention and shown attractive prospects. Two-dimensional computational fluid dynamic (CFD) simulations were performed for a 116 MW moving bed part-fluidized boiler to investigate the different combustion patterns of coal particles of different particle sizes inside the furnace chamber. A low-NOX combustion method based on the combination of laminar combustion and fluidized combustion is proposed. By comparing the effects of different air distributions on the fluidization state of coal particles, the air distribution values required for optimal fluidized combustion were obtained. The temperature field and pollutant distribution in the furnace chamber for the conventional combustion method and the new combustion method were also simulated. The results show that the combustion technology combining laminar combustion and fluidization of a moving bed part-fluidized boiler can significantly improve the combustion rate and reduce the NOX concentration at the furnace exit. When the secondary air speed is up to 15 m/s, the coal particles smaller than 5 mm are fully fluidized and burned in the whole furnace chamber. The coal particles larger than 5 mm are burned on the bed. The pollutant emission of the boiler can reach the best condition. The new type of boiler can reach a super clean emission in which the NOX emission value is below 47 mg/m3, and the SO2 emission value is reduced to 0.15 mg/m3.

Effect of air staging on low-NOx combustion performance of regenerative heating furnace for oil shale retort

Effect of air staging on low-NOx combustion performance of regenerative heating furnace for oil shale retort

Structure and nitric oxide formation in laminar diffusion flames of ammonia–hydrogen and air

In this work, we present an investigation of the structure and formation of nitrogen oxides (NOx) in an axisymmetric laminar diffusion flame in which an ammonia–hydrogen fuel stream is surrounded by co-flowing air. Green ammonia, produced using renewable energy, is a promising carbon-free fuel for replacing hydrocarbon fuels over a diverse range of combustion applications. A key challenge for ammonia combustion is to understand the kinetics of nitrogen oxide formation for designing low-NOx combustion systems. In this experimental study, ammonia fuel is blended with hydrogen to increase fuel reactivity, with a fuel composition range of 15%–100% hydrogen. NOx emissions and fuel slip are measured by downstream sampling of NO, NO2, N2O and NH3 with a Fourier Transform Infrared (FTIR) gas analyzer. Flame structure is visualized by planar laser-induced fluorescence (LIF) for NO, NH, and OH radicals, as well as OH* by filtered chemiluminescence. Species concentration profiles are compared to predictions from 2-dimensional axisymmetric numerical models of the laminar flame using recent kinetic mechanisms developed for ammonia combustion. No measurable ammonia slip was recorded for any fuel composition, in agreement with model predictions. Good agreement between predictions and measurements was obtained for exhaust nitrogen oxide concentrations and for in-flame radical profiles, although larger variation and deviations are observed for predictions of NO2 and N2O than for NO. Numerical predictions for spatial distribution of flame radicals and flame liftoff height also showed good agreement with LIF and chemiluminescence measurements. For high fuel hydrogen content, the measured NH and NO profiles are shifted inward toward the fuel outlet, away from the peak temperature contour. Reaction path analysis is used to illustrate the important contributions to NO creation and destruction, including the differing role of key reaction pathways with varying fuel hydrogen content.

Microscopic pyrolysis mechanisms of superfine pulverized coal based on TG-FTIR-MS and ReaxFF MD study

Current studies on coal pyrolysis characteristics mainly focus on macroscopic aspects. Here, the structural properties of coal residues were well characterized based on HRTEM, XPS, and 13C NMR etc., and the macromolecule models were constructed. Combining the small molecule compositions of extracts and the macromolecule models of residues, the multi-component models of raw coals with different particle sizes were successfully constructed. Thus, the microscopic pyrolysis mechanisms of raw coals were obtained by combining TG-MS-FTIR and ReaxFF MD analysis. More importantly, the evolution of chemical bonds and carbon-containing products was discussed. The effect of particle size on microscopic pyrolysis characteristics was focused on. The results show that for smaller particles, the abundant alkylated aromatic and aromatic bridgehead carbon enhances the pyrolysis process. During the primary thermal depolymerization, the conversion of C100+ to C14–C40 and C41–C100 and the generation of pyrolysis gases collectively govern the thermochemical process. For the secondary thermal depolymerization, the release of massive pyrolysis gases is dominant, and most oxygen-containing intermediate groups transform into CO and CO2. In the thermal polymerization stage, the C100+ mainly evolves into C41–C100 and C14–C40, accompanied by the evolution of substantial H2. The microscopic pyrolysis mechanisms shed light on further developing new low-NOx combustion technologies.

Lowering further NOx emissions and improving the hopper's overheating environment in a low-NOx down-fired furnace by a staged arch-firing framework with a primary-burner flue gas recirculation

For the still high NOx emissions and a high hopper's thermal fatigue risk in a 600-MWe low-NOx down-fired furnace, a staged arch-firing framework (SAF) is developed as a solution to replace its original deep-air-staging low-NOx art. SAF strategies in resolving these problems include (i) lowering the primary-burner coal/air fluxes and positioning hopper air to shrink the flame penetration and strengthen further air staging and (ii) additionally introducing the primary-burner flue gas recirculation (FGR) and fuel reburning. For a SAF down-fired furnace first using the primary-burner FGR as firing the hard-to-burn anthracite, affirming its availability and evaluating the primary-burner FGR ratio effect on the potentially improved low-NOx combustion and hopper's overheating environment are crucial to the furnace's environmental and safe operations. Accordingly, detailed real-furnace measurements plus simulation are first performed in the original furnace and thereafter, the retrofitted furnace with SAF is evaluated under FGR ratios of 0%, 10%, 12.5%, 15%, and 17.5%. In comparison to the original art, SAF develops a shortened but more effective flame penetration, improves combustion intensity, lengthens the overfire air penetration, and inhibits fuel-NO. The fuel-lean mixture roles first combustion behaviors and then fuel reburning on NO reduction in the fuel-reburning zone. Increasing the FGR ratio drops NOx emissions while weakens burnout. In balancing NOx emissions and burnout, a shallow FGR 12.5% presents the optimal low-NOx merit with NOx emissions lowered to about 600 mg/m3 (6% O2) plus a high burnout. In replacing the original art by the SAF, a further 33.3% NOx reduction plus a high burnout is obtained and the hopper's thermal fatigue risk is eliminated due to its sharp temperature reduction by 400 K.