NOx Destruction Research Articles

A chemical kinetic analysis: Influence of post-flame chemistry, combustion pressure, premixing degree (fully premixed to non-premixed), and secondary air supply on NOx emissions from NH3/CH4-air combustion

Numerous studies assessed the viability of ammonia blends for the wide-scale application of ammonia in carbon-free energy systems. NOx emissions from ammonia combustion are the primary concern constraining the adoption of NH3 as a fuel. The present study explores the effect of post-flame chemistry, combustion pressure, reactant-mixing (premixing degree), and secondary air supply on NOx emissions from NH3/CH4 swirl flame. Furthermore, an extensive chemical kinetic analysis is carried out using the rate of production and sensitivity analyses to study NOx formation and destruction with the above-discussed techniques. The present study adopts the NH3/CH4 reaction model developed by Okafor et al. and a PSR-PFR chemical reactor network for predicting the NOx formation chemistry in NH3/CH4-air combustion. The analyses are conducted for the values of ammonia heat fraction in the fuel mixture (ENH3) = 0.1–0.3 and equivalence ratio (ϕ) = 0.6–1.4. The present work suggests that providing a post-flame zone is advantageous in reducing both NOx and NH3 emissions, especially for rich NH3/CH4 flames. Increasing the combustion pressure to achieve low NOx without significant ammonia slip appears to be a viable method for NH3/CH4 flames with ϕ < 1.2. Adopting a non-premixed injection mode is a feasible low NOx combustion technique for 0.9 < ϕ < 1.2, especially for higher ammonia fractions in the fuel mixture. An excess air supply of 10% reduces the NOx emissions for fuel-lean conditions (ϕ < 0.9) with a negligible rise in unburnt NH3 emissions under atmospheric conditions.

Numerical optimization of the influence of multiple deep air-staged combustion on the NOx emission in an opposed firing utility boiler using lean coal

Multiple deep air-staged combustion is a promising technology to significantly reduce NOx emission in the opposed firing utility boilers using low-volatile coal. The gas-solid two-phase flow, pulverized coal combustion and NOx emission characteristics of an existing 600 MW coal-fired boiler were numerically simulated to evaluate the influence of excess air coefficient in primary combustion zone (αM), variational air distribution modes and multiple deep air-staged combustion on the NOx formation and destruction process in the furnace. The detailed NOx formation and reduction models were proposed to consider the reduction reaction between hydrocarbon and NO under fuel rich conditions and well validated by the experimental results in the laboratory and field tests. The results show that the αM has an important influence on flue gas temperature distribution and forms a reducing atmosphere in the primary combustion zone to significantly reduce NOx concentration. The NOx concentration at the furnace outlet is greatly decreased when the deep air-staged combustion is adopted with αM of 0.75, and the CO concentration maintains within a lower level. Compared with the balanced air distribution, the pagoda and inverse pagoda air distributions are found to be ineffective to enhance NOx reduction performance under deep air-staged combustion. However, the pagoda air distribution effectively reduces NOx emission under middle air-staged combustion condition. The NOx emission is further reduced by adopting the multiple air-staged combustion due to the higher CO concentration formed in the burnout zone. The results are helpful to the design and operation optimization of the opposed firing utility boilers using lean coal.

Theoretical Study on the Reactions Originating from Solid Char(N): Radical Preference and Possible Surface N2 Formation Reactions

In this work, density functional theory calculations were performed to study the reactions originating from char(N). This study first provides a clear indication of the radical preference order for char(N). The approaching oxygen inhibits the formation of the N–N bond by attracting electron density from the surface nitrogen. On the contrary, CO is beneficial considering its large partial of the electronic distribution. The structure sensitive energies resulting from the electron difference and steric hindrance give us the indication that NCH3, HNC, NH, and NCO are very important in the reduction of char(N) to N2. Furthermore, our calculations suggest that the exothermic surface desorption by about −233.9 kJ/mol is the dominant reaction for N2 formation. It is expected to be competitive with surface migration. The results will provide reliable data required to predict NOx destruction, which will help in design of the low NOx burner to enable the expanded use of coal in China.

Photochemical evolution of continental air masses and their influence on ozone formation over the South China Sea

To investigate photochemical ozone (O3) pollution over the South China Sea (SCS), an intensive sampling campaign was conducted from August to November simultaneously at a continental site (Tung Chung, TC) and a marine site (Wan Shan Island, WSI). It was found that when continental air masses intruded the SCS, O3 episodes often occurred subsequently. To discover the causes, a photochemical trajectory model (PTM) coupled with the near-explicit Master Chemical Mechanism (MCM) was adopted, and the photochemical processes of air masses during the transport from TC to WSI were investigated. The simulated O3 and its precursors (i.e. NOx and VOCs) showed a reasonably good agreement with the observations at both TC and WSI, indicating that the PTM was capable of simulating O3 formation for air masses traveling from TC to WSI. The modeling results revealed that during the transport of air masses from TC to WSI, both VOC and NOx decreased in the morning while O3 increased significantly, mainly due to rapid chemical reactions with elevated radicals over the SCS. The elevated radicals over the SCS were attributable to the fact that higher NOx at TC consumed more radicals, whereas the concentration of radicals increased from TC to WSI because of NOx dilution and destruction. Subsequently, the photochemical cycling of radicals accelerated, leading to high O3 mixing ratios over the SCS.Furthermore, based on the source profiles of the emission inventory used, the contributions of six sources, i.e. gasoline vehicle exhaust, diesel vehicle exhaust, gasoline evaporation and LPG usage, solvent usage, biomass and coal burning, and biogenic emissions, to maritime O3 formation were evaluated. The results suggested that gasoline vehicles exhaust and solvent usage largely contributed the O3 formation over the SCS (about 5.2 and 3.8 ppbv, respectively). This is the first time that the contribution of continental VOC sources to the maritime O3 formation was quantified.

Design and numerical analysis of a 3 kWe flameless microturbine combustor for hydrogen fuel

In this work, a new 3 kWe flameless combustor for hydrogen fuel is designed and analyzed using CFD simulation. The strategy of the design is to provide a large volumetric combustion for hydrogen fuel without significant rise of the temperature. The combustor initial dimensions and specification were obtained from practical design procedures, and then optimized using CFD simulations. A three-dimensional model for the designed combustor is constructed to further analysis of flameless hydrogen combustion and consideration that leads to disappearance of flame-front and flameless combustion. The key design parameters including aerodynamic, temperature at walls and flame, NOX, pressure drop, combustion efficiency for the hydrogen flame is analyzed in the designed combustor. To well demonstrate the combustor, the NOX and entropy destruction and finally energy conversion efficiency, and overall operability in the microturbine cycle of hydrogen flameless combustor is compared with a 3 kWe design counterpart for natural gas. The findings demonstrate that hydrogen flameless combustion is superior to derive the microturbines with significantly lower NOX, and improvements in energy efficiency, and cycle overall efficiency with low wall temperatures guaranteeing the long-term operation of combustor and microturbine parts.

NH3 as a Transport Fuel in Internal Combustion Engines: A Technical Review

Ammonia (NH3) is an excellent hydrogen (H2) carrier that is easy to bulk manufacture, handle, transport, and use. NH3 is itself combustible and could potentially become a clean transport fuel for direct use in internal combustion engines (ICEs). This technical review examines the current state of knowledge of NH3 as a fuel in ICEs on its own or in mixtures with other fuels. A particular case of interest is to partially dissociate NH3 in situ to produce an NH3/H2 mixture before injection into the engine cylinders. A key element of the present innovation, the presence of H2 is expected to allow easy control and enhanced performance of NH3 combustion. The key thermochemical properties of NH3 are collected and compared to those of conventional and alternative fuels. The basic combustion characteristics and properties of NH3 and its mixtures with H2 are summarized, providing a theoretical basis for evaluating NH3 combustion in ICEs. The combustion chemistry and kinetics of NH3 combustion and mechanisms of NOx formation and destruction are also discussed. The potential applications of NH3 in conventional ICEs and advanced homogenous charge compression ignition (HCCI) engines are analyzed.

Theoretical Study of the Ozone Production and NOx Molecules Processing in N2/O2 Pulsed Discharge

ABSTRACTEnvironmental decontamination is a major challenge due to the rapid growth of industrial and technological development, requiring an important consumption of fossil energies. Nowadays, a new way to treat polluting molecules based on the use of nonequilibrium reactive plasmas, is under development. These plasmas are generated by electrical discharges at atmospheric pressure, for neutralizing or transforming the toxic oxides. In this respect, the goal of the present work is to analyze the possibilities of treatment of NOx molecules and ozone generation by pulsed discharge. The study was carried out by using a one-dimensional model based on the parallel resistor network concept, consisting in dividing the discharge volume into plasma elements which are connected in no way but through their contribution to the total resistance of the plasma. The model calculations indicate the effect of gas heating and inhomogeneous preionization on the ozone production and NOx destruction.

NOx Emissions from Regenerator of Calcium Looping Process

The calcium looping (CaL) process usually employs a dual-fluidized bed (DFB) solid circulating unit. In the regenerator (calcinator or calciner), decomposition of CaCO3 proceeds. To supply the heat of decomposition, oxyfuel combustion of coal is conducted. However, since coal contains nitrogen, the NOx formation occurs during oxyfuel combustion. Because of the fact that NOx formation and destruction during combustion of solid fuels in a fluidized bed is a complex process, a predictive approach of NOx emissions has not yet been sufficiently recognized, especially during oxyfuel combustion conditions in the CaL systems. The paper introduces a regression-based method for the prediction of NOx emissions from a CaL DFB experimental unit. Effects of fuel type, excess oxygen feed, and NO addition to primary or secondary feed gas on NOx emissions in the regenerator were evaluated. The presented way constitutes a straightforward method to run a complementary technique in relation to other methods of data handling,...

Research of Formation and Destruction of NOx During Combustion of Low-Grade Coal in CHP

The emissions of harmful substances such as sulphur and nitrogen oxides from thermal power plants are major source of adverse impact on the environment. In this regard, it remains be urgent the deeper studying of the solid fuel combustion processes. In this paper, two global schemes of formation and destruction of harmful nitrogen compounds NOx studied during combustion of low-grade Kazakh coal in a real industrial boiler BKZ-75.

NOX reduction in a 40 t/h biomass fired grate boiler using internal flue gas recirculation technology

A decoupled numerical modelling method is developed in this study to simulate the whole combustion process of biomass in a grate firing boiler, which includes the thermochemical conversion of biomass in the fuel-bed and gaseous combustion in the freeboard. With the aid of this modelling method, the objective of this study is to explore the NOX reduction mechanism as well as to investigate the potential of internal flue gas recirculation technology (IFGRT) on the combustion process and emissions formation in a 40 t/h biomass-fired grate boiler. Computational fluid dynamics (CFD) modelling results show that IFGRT can be realized in the grate boiler by establishing intense flue gas recirculation within the boiler, which allows for lower peak combustion temperatures and smaller flame kernel sizes, while improving the overall average gas temperature. Consequently, NOX emission can be reduced mainly via the thermal formation route in comparison with the conventional combustion case. More specifically, the parallel over-fired air (OFA) burner configuration is suggested for implementation to produce even lower NOX emission compared to the staggered OFA burner configuration. To further understand the reasons behind the NOX reduction, NOX formation and destruction mechanisms are also examined through reduced order modelling (ROM) with the help of detailed reaction chemistry. It is revealed that flue gas recirculation inhibits NOX formation from thermal, NNH and N2O routes. Although NOX destruction rate through reburning is suppressed, the net NOX production rate is found to be decreased under the condition of IFGRT. Moreover, as the flue gas recirculation ratio increases, final NOX emission shows a decreasing trend.

Coal-nitrogen release and NOx evolution in the oxidant-staged combustion of coal

Air-staged combustion is one of the most sophisticated and effective technologies for reducing NOx emissions. To better understand NOx formation and destruction in air-staged combustion, measurements of the major gas species and NOx profiles along the axial distance of a furnace were obtained in a 20 kW down-fired pilot-scale combustion facility. The furnace of this facility has a representative configuration of a full-scale boiler with a staged combustion system in terms of residence times and temperature characteristics. The combustion air is divided into primary air, secondary air, and burnout air, which can be heated to 80 °C, 200 °C, and 200 °C, respectively by electric heaters before entering the furnace. Under the staged case, a usual NOx trend was obtained by the measurements: coal nitrogen can be rapidly converted to NOx and then reaches a peak concentration in the primary combustion zone, followed by a significant reduction in NOx in the reduction zone. The greater the staged level is, the earlier the reduction stage starts. For the deepest air-staged case with ratio of burnout air to total air of 0.42, the NOx profile curve does not exhibit the features of the NOx peak and a pronounced reduction, as the NOx values are always at very low levels. There exists an NO reducing saturation phenomenon in an overlong reduction zone. For providing data associated with the oxygen-enriched combustion technology that will be available in the near future, low-level oxygen-enriched combustions of coal (21–30%O2) have been performed to assess their NOx emission characteristics compared to the air-staged configuration. There is no essential difference in the mechanism of NO formation and destruction between the staged combustion with oxygen enrichment and the air-staged combustion. With the increase of the staged degree, the impact of O2 concentration promotion on the rise of NOx emission gradually decreases. The NOx rises again at the point where burnout oxidizer addition is caused by the oxidation of some unknown nitrogen-containing intermediates prevailing in the oxygen-poor reduction zone.

Detailed multi-dimensional study on NOx formation and destruction mechanisms in dimethyl ether/air diffusion flame under the moderate or intense low-oxygen dilution (MILD) condition

In this paper, the moderate or intense low-oxygen dilution (MILD) technology was employed to reduce NOx emission index of the DME flame, whose NOx emission was expected high due to its higher adiabatic flame temperature. Experiments and modeling were conducted to study NOx emission mechanism of the DME-MILD jet diffusion flame. Various co-flow temperatures (Tco∗) and oxygen concentrations (XO2∗) were involved to investigate dependence of NOx emission mechanism on hot dilution level. NOx formation pathway in all subzones was clarified, employing the reaction path and sensitivity analyses. The results showed that for the DME-MILD jet flame, NOx was generated mainly through the NNH and N2O pathways, which was rather different from the traditional DME jet diffusion flame. EINOx increased gradually with the increment in XO2∗ or Tco∗. A strong NONO2 conversion process and ultra-clean performance were verified for the DME-MILD jet flame.

Study of pollutant emissions and dynamics of non-premixed turbulent oxygen enriched flames from a swirl burner

The paper investigates the dynamics of methane – oxygen enriched air turbulent non-premixed swirled flames and their emission characteristics. The burner configuration consists of two concentric tubes with a swirler placed in the annular part supplying the oxidant. Fuel injection is radial through holes in the central tube at the burner exit. This allows enhancing air-fuel mixing and eases flame stabilisation. Fuel is injected into the highest turbulence level regions where the local mixing is enhanced, and generating a shift from a non-premixed flame towards a partially premixed one. The idea of oxygen enrichment of the air is related to the augmentation of CO2 concentration in the flue gases to improve the CO2 capture efficiency by membranes. Stereo-Particle Image Velocimetry is used to analyse the velocity flow fields. The measurements are performed for oxygen concentrations ranging from 21% to 30%, with swirl numbers from 0.8 to 1.4 and global equivalence ratio from 0.8 to 1. Results show that combustion noticeably affects the swirling motion and that tangential velocities rapidly decrease along the axial axis. The recirculation mass flow ratio dramatically increases compared to non-reactive cases. Increasing global equivalence ratios increases the recirculation mass flow ratio contrary to oxygen enrichment effects. The exhaust gas compositions are measured using gas analysers. It is observed that NOx emissions decrease when the global equivalence ratio increases. It suggests that CO2 in the recirculated burned gases could be responsible for NOx destruction at relatively high oxygen enrichment rates. Globally, the results show that, by combining enhanced mixing and moderate oxygen enrichment of the air, the developed and characterized new burner configuration has a good potential to be used to stabilize non-premixed fuel and oxidant streams as partially premixed low emission flames with an increased concentration of CO2 in the flue gases.

Assessing the impact of primary measures for NOx reduction on the thermal power plant steam boiler

The European normatives prescribe content of 200 mg/Nm3 NOx for pulverized coal combusting power plants. In order to reduce content of NOx in Serbian thermal power plant (TPP) ꞌKostolac B’ it's necessary to implement particular measures until 2016. The mathematical model of lignite combustion in the steam boiler furnace is defined and applied to analyze the possibility of implementing certain primary measures for reducing nitrogen oxides and their effects on the steam boiler operation. This model includes processes in the coal-fired furnace and defines radiating reactive two-phase turbulent flow. The model of turbulent flow also contains sub-model of fuel and thermal NOx formation and destruction. This complex mathematical model is related to thermal and aerodynamic calculations of the steam boiler within a unified calculation system in order to analyze the steam boiler overall work. This system provides calculations with a number of influential parameters. The steam boiler calculations for unit 1 (350 MWe) of TPP ꞌKostolac Bꞌ are implemented for existing and modified combustion system in order to achieve effective, reliable and ecological facility work. The paper presents the influence analysis of large number of parameters on the steam boiler operation with an accepted concept of primary measures. Presented system of calculations is verified against measurements in TPP ꞌKostolac Bꞌ.

NOx emissions and burnout characteristics of bituminous coal, lignite, and their blends in a pulverized coal-fired furnace

For coal-fired power plants, the application of low-rank coals like brown coal and lignite steadily has been a great issue due to the shortage and rising prices of high-rank coal. In this research, therefore, co-firing experiments with low-rank coal, lignite, were performed in a 15kWth pulverized coal-fired furnace, and combustion and emission characteristics of bituminous, lignite, and their blends were investigated. Influences of staged-air injection, primary zone air ratio, coal types, and blending ratios of bituminous coal and lignite on NOx emission and particle burnout were evaluated. For the bituminous coal, NOx emissions decreased slightly with a decreasing primary zone air ratio and increasing staged-air injector level. At a given value of the primary zone air ratio and staged-air injector level, NOx emissions were lower for coal with lower nitrogen content. However, in the case of the lignite, the NOx concentration was the highest, because of maintenance of a constant thermal output. For a multi-staged combustion, increasing the distance between the staged-air injectors has a positive effect on NOx emission but a negative effect on particle burnout. Furthermore, in co-combustion with bituminous coal and lignite, the blends with a 10% addition of lignite show a minimum value for NOx concentration and a similar temperature distribution to the bituminous coal case. This is because of the higher devolatilization rate of lignite and the formation of a strong reducing environment at high temperatures from the reaction between oxygen and the combustible gas species. Such gas species were released from lignite devolatilization in the primary combustion zone that is conducive to NOx destruction.

UV, visible and near-infrared lights induced NOx destruction activity of (Yb,Er)-NaYF4/C-TiO2 composite

Titanium dioxide (TiO2) is a well-known photocatalyst for environmental cleaning and energy conversion. However, it can only be excited by ultraviolet light for photocatalysis due to its wide band gap (3.2 eV). In this paper, we present a novel (Yb,Er)-NaYF4/C-TiO2 composite which can be perfectly induced not only by ultraviolet light but also weak visible and near infrared lights, owing to the increased carbon doping contents and optimal energy transfer between up-conversion phosphor and C doped TiO2 compared with that of solely C-TiO2. Consequently, the (Yb,Er)-NaYF4/C-TiO2 composite can present the outstanding continuous NOx gas destruction ability under the irradiation of ultraviolet, weak visible and infrared lights much superior to pure C-TiO2, P25 titania and even that of (Yb,Er)-NaYF4/N-TiO2 composite, due to the nice synergetic effect of (Yb,Er)-NaYF4 and C-TiO2, indicating a promising potential in the photocatalyst application with high efficiency of ultraviolet, visible and infrared lights induced photocatalysis simultaneously.

The visible-light driven photocatalytic destruction of NOx using mesoporous TiO2 spheres synthesized via a “water-controlled release process”

Mesoporous anatase TiO2 spheres with tunable sizes ranging from 400 nm to 3 μm were synthesized using an original so-called "water-controlled solvothemal release process". In this method, the well-known esterification reaction between ethanol and acetic acid was creatively employed to generate water gradually during a solvothermal process. Thereafter, the slowly released water molecules functioned as nucleation centers for completing the hydrolysis of titanium tetraisopropoxide to produce homogenous mesoporous TiO2 spheres. In reality, these samples consisted of densely packed nanoparticles that formed spherical secondary particles with interparticle pores. Research has demonstrated that the diameter of the TiO2 spheres can be easily tuned by controlling the concentration of the Ti source in the starting solution. Regardless of their diameter, all of these TiO2 spheres exhibited a high specific surface area (above 150 m(2) g(-1)) originating largely from the contribution of mesopores. On the merits of their porous structure and related high specific surface area, the mesoporous TiO2 spheres showed a higher photocatalytic activity than P25 for the oxidative photo-destruction of NOx gas.

Measurement of nitrogen evolution in a staged oxy-combustion coal flame

Oxy-combustion is a technology attracting a significant amount of attention because it enables CO2 capture from the products of coal combustion. Understanding the potential for NOX reduction is a key factor in the economic analysis and design of oxy-combustion systems. In order to better understand NOX formation and destruction in oxy-combustion, measurements of major gas species and nitrogen containing species, NH3 and HCN, were obtained in a down-fired, premixed, staged, laboratory reactor. The near-burner region was fuel-rich and reducing followed by the addition of burnout oxidizer in order to simulate the staged combustion process of a full-scale boiler. In the vicinity of the burner, gas species measurements were obtained at a resolution of 25mm. The data show that nitrogen from the coal is rapidly converted to NOX in the first 50–100mm of the reactor followed by a reduction of NOX in the fuel-rich, near-burner region. NOX increases at the point where burnout oxidizer is added and then remains constant. A slight increase in NOX which is potentially the result of the thermal mechanism in air combustion does not occur in oxy-combustion. In the fuel-rich, near-burner region, stoichiometric ratio (S.R.)=0.75, oxy-fuel combustion produces more hydrocarbons, NH3, and HCN suggesting reburning reactions are more prevalent. Air combustion can be made to produce a similar NOX profile to oxy-combustion by lowering the S.R. (deeper staging) of the fuel-rich region. The minimum NOX achievable is approximately the same for air and oxy-combustion, but the oxy-combustion minimum NOX occurs at a higher S.R. in the fuel-rich zone which allows oxy-combustion to achieve higher carbon conversion for the same NOX reduction. The higher concentrations of NH3 and HCN seen at a given fuel-rich zone equivalence ratio also suggest recycled NOX may be reduced more efficiently in oxy-combustion than would be expected for air combustion.

Abatement of CF&lt;sub&gt;4&lt;/sub&gt; and Associated Gas (NOx) with Combined Microwave Plasma and TiO&lt;sub&gt;2&lt;/sub&gt; Catalyst

The atmospheric-pressure microwave plasma can remove perfluorocompounds (PFCs) efficiently, which have long lifetime and serious global warming implications. However, associated gases such as NOx are produced in the abatement process of PFCs. In order to destroy PFCs and NOx pollutants, TiO2 catalyst is introduced into the abatement system. The effective TiO2 catalysts are prepared by a sol-gel method with the ratio of tetrabutyl titanate, ethanol, H2O and acetic acid = 1: 7: 1: 0.4. The abatement of CF4 and associated gas (NOx) is carried out with combined microwave plasma and TiO2 catalyst. The results show that the combined microwave plasma and catalyst abatement system can be successfully used to remove CF4 and NOx. The destruction and removal efficiency of CF4 and NOx were higher with the combined catalyst and plasma process than that with catalyst or plasma only.

Photocatalytic Air Purification

Titanium dioxide represents an effective photocatalyst for destruction of volatile organic compounds (VOC), NOx and SO2 in indoor and outdoor air. The mechanism of selected pollutants removal as well as the dependence of reaction rate on some key influencing factors are discussed. Recent papers and patents considering photocatalysts preparation and immobilization techniques including type of supports, surface pretreatment procedure are reviewed. Photocatalytic reactors are designed in order to provide strictly controlled conditions for photocatalytic air purification. Some reactors can be utilized in commercial applications as a part of HVAC systems (Heating, Ventilation and Air Conditioning) but most of them are used in laboratories to measure the activity of different types of photocatalysts applied for gas streams treatment. Examples of photoreactors used for gas phase treatment are presented.