NOx reduction mechanism in coal combustion with recycled CO2
Abstract
Coal combustion with is one of several promising new technologies associated with mitigating the CO2 rise in the atmosphere. In coal combustion with recycled CO2, the amount of NOx exhausted from the system is reduced to less than one third of that with combustion in air. This result is associated with decreased conversion of fuel-N to NOx and reduction of recycled NOx in the flame zone. The effects of CO2 concentration, reduction of recycled NOx, and interaction between fuel-N and recycled NOx on the decrease of the final NOx exhausted from the coal-combustion system with recycled CO2 have been separated for appropriate NOx-reduction mechanisms.
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Cited by (244)
Effect of secondary air on NO emission in a 440 t/h circulating fluidized bed boiler based on CPFD method
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NO<inf>x</inf> and N<inf>2</inf>O emissions from Ca-rich fuel conversion in oxyfuel circulating fluidized bed combustion
2023, Thermal Science and Engineering ProgressA part of Estonia’s power generation is based on oil shale combustion in thermal power plants, resulting in high CO2 and pollutant emissions. The presented paper is a part of long-term experimental work to investigate the combustion of Ca-rich oil shale for air and oxyfuel combustion environments. The experiments were performed in a 60 kWth circulating fluidized bed (CFB) test facility under air, O2/CO2, and O2 with the recycled flue gas (RFG) modes of inlet O2 as vol.% ranging from 21% to 52%, and with RFG of 50% and 87%. The influence of different combustion atmospheres, excess oxygen volumetric ratios in the primary oxidizer (λPr), and dense bed temperatures (TDB), on the total NOx and N2O formations, have been studied in detail.
The results show that specific concentrations of NOx (per MJ) were reduced by 14% in O2/CO2 mode, and 22% in O2/RFG compared to air combustion. N2O emissions were increased in OXY21 mode and reduced significantly from 20 mg/MJ to 4 mg/MJ with increasing inlet O2% to 52% under O2/CO2. NOx and N2O emissions were the lowest of all combustion experiments at high inlet O2% with RFG application (OXY87 + RFG). Under all tested atmospheres, NOx emissions were increasing with increasing excess oxygen in the primary oxidizer. N2O enhanced with excess oxygen, accounted for the homogenous gas-phase reaction of volatile-N and the heterogenous reaction of char-N. NOx emissions at low operating TDB were decreasing with increasing bed temperatures and increased at higher TDB > 770 °C in air combustion and TDB > 850 °C in OXY30. N2O emissions were slightly decreased at higher bed temperatures under air and O2/CO2 modes. Combustion efficiency enhanced at higher inlet O2% resulting in lower CO concentrations with reduced fuel burnout, and increased NOx and N2O emissions.
Overall, NOx and N2O formations were more affected by the condition of operating parameters under both air and oxyfuel combustion experiments, and the obtained results are giving confidence that oxyfuel technology does not influence the release of nitrogen emissions from oil shale combustion. To lower the cost of CO2 impurities removal, the optimal operating conditions during oil shale CFB combustion can be confirmed after considering the discussed parameters. However, further CO2 purification resulting from oil shale oxyfuel flue gas stream is possibly required.
Nitrogen oxides (NOx) generated from fossil fuel combustion are the main sources of air pollutants and photochemical smog. Moderate and Intense Low-oxygen Dilution (MILD) combustion has been regarded as a novel combustion technology which can significantly improve combustion efficiency and reduce pollutant emissions. The natural gas MILD combustion in the International Flame Research Foundation (IFRF) semi-industrial test furnace were simulated by using standard k-ε model and Eddy Dissipation Concept (EDC) model with GRI-Mech 2.11 mechanism. The predicted results of flue gas velocity, temperature and species volume fractions were compared with the experimental data to validate better performance of the present model. The relative contributions of various NO generation and reduction routes to total NOx emissions were quantitatively determined to obtain the dominant NO formation pathway under MILD combustion conditions. The NO in the preheated oxidizer and the in-furnace recycled flue gas may have an important effect on the NOx generation and reduction during the combustion process. The effects of the initial NO volume fraction on the key reactions of individual NOx generation and reduction route were studied by adjusting the NO volume fraction in the oxidizer. The results showed that the net NOx emission and CO emission in the IFRF furnace were as low as 13.6 mg/m3 and 3.7 mg/m3 referring to 15% O2 volume fraction, respectively, which were much less than the NOx and CO emission thresholds (40 mg/m3 for NO and 67 mg/m3 for CO) for natural gas MILD combustion. Moreover, the thermal route and N2O-intermediate route played the dominant roles in NOx generation and their contributions to the total NOx emission in the exhaust gas were 48% and 34% for IFRF experiment case, respectively. Although the kinetic rates of NO-reburning reactions in the furnace continued to increase, the net emission index of NO (EINO) in the exhaust gas increased by 27% due to the enhanced thermal route when the initial NO volume fraction in the oxidizer increased from 0 ppm to 500 ppm.
Nitrogen evolution, NO<inf>X</inf> formation and reduction in pressurized oxy coal combustion
2022, Renewable and Sustainable Energy ReviewsOxy-combustion is one of the most prominent solutions for reducing CO2 emissions from coal-fired power plants using carbon-capture-and-utilization technology. However, when compared to air combustion at atmospheric pressure, oxy-combustion is very expensive, owing to the significant efficiency penalties associated with air separation, flue gas recirculation (FGR), treatment, compression, and storage or transit of CO2. In comparison, pressurized oxy-combustion (POC) is more efficient as it recovers a significant amount of heat energy from the flue gas moistures. Nevertheless, CO2 derived from pressurized oxy coal combustion has impurities, e.g., acid gases (NOX and SOX) that can corrode the plant equipment, transport lines as well as deteriorating effect on the environment. Fortunately, in pressurized combustion systems, both NOX and SOX can be scrubbed by a single-column direct contact cooler (DCC), but this requires a minimum ratio of NOX to SOX at the inlet to be efficiently removed. Therefore, NOx is one of the important hindering parameters in commercializing the pressurized oxy-combustion. Although NOx evolution during oxy-coal combustion has been explored extensively at 1 atm, higher pressure studies are rare. Much still needs to be done to better understand the NOX mechanism and the effects of different parameters on NOX emissions under these conditions. This paper reviews the published literature on nitrogen evolution, NOX formation and reduction in pressurized oxy-coal combustion. At higher pressures, the NOX from fuel-bound nitrogen is generated through volatiles, tar and char, all of which are discussed. Where literature is not available, the effect of pressure on NOx evolution in different stages of coal combustion is predicted through CHEMKIN simulation. Homogeneous and heterogeneous pathways of NOX formation and their destruction in pressurized oxy-coal combustion are evaluated. Additionally, the effect of pressure on a few mature and commercialized NOx abatement methods is explored. In the last, the future perspective and recommendation are given. This review will aid in the provision of basic knowledge about NOx evolution and control in pressurized fuel combustion, as well as the identification of new research areas to pursue.
A reactive molecular dynamics study of HCN oxidation during pressurized oxy-fuel combustion
2021, Fuel Processing TechnologyIn the present work, HCN oxidation during pressurized oxy-fuel combustion is studied using reactive molecular dynamics (ReaxFF MD) simulations. The effects of CO2, pressure, and O2 concentration on HCN oxidation are investigated. Under fuel-rich conditions, CO2 reduces the overall oxidation rate of HCN due to the lower diffusion coefficient of O2 in O2/CO2 environment. On the other hand, CO2 increases the amount of OH radicals through the reaction of CO2 + H → CO + OH at high temperatures. Although high pressure reduces the diffusion coefficient of O2, the oxidation rate of HCN still increases with the increase in pressure due to the higher partial pressure of O2. Moreover, NO emissions decrease with the increase in pressure because high pressure promotes the conversion of NO to N2. Furthermore, CO2 reduces the amount of CN, which is an important intermediate for the reduction of NO at high temperatures. Due to this reason, a positive effect of CO2 on the emissions of NO is observed. Additionally, the oxidation rate of HCN increases with the increase in the concentration of O2. Unlike fuel-rich conditions, CO2 exhibits an inhibitory effect on the emissions of NO under high concentration of O2.
Theoretical and experimental investigation on the effect of CO on N migration and conversion during air-staged coal combustion
2021, Journal of the Energy InstituteAir-staging combustion is the widely-used low-nitrogen combustion technology. In this process of NOx reduction, the role played by CO resulted from the air-staged combustion cannot be ignored. In this study, quantum chemistry method is used to investigate the effects of the CO on the NOx generation/reduction and on the char surface. Theoretical calculations indicate that CO is involved in nitrogen fixation during the migration and transformation of N. During the oxidation of N-containing char, CO increases the binding energy of N of pyridine with adjacent C atoms, which is not conducive to the N precipitation. During reduction of NOx by char, CO decreases the desorption energy barrier of N2 and promotes NO heterogeneous reduction. On the other hand, CO promotes the re-embedding of N in generated NO into the char to form char-N. XPS experiments are carried out for the coal char samples after the air-staged combustion. The experimental results show that high CO concentration is conducive to retention N. When the excess air coefficient (α) is 0.8, the increase of N-6 content is 7.2% larger than that of α = 0.9. This work discovers the nitrogen fixation of CO, enriches and expands the understanding of the role of CO.