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Reduced chemical kinetic mechanisms for NOx emission prediction in biomass combustion
Abstract
Because of the complex composition of biomass, the chemical mechanism contains many different species and therefore a large number of reactions. Although biomass gas-phase combustion is fairly well researched and understood, the proposed mechanisms are still complex and need very long computational time and powerful hardware resources. A reduction of the mechanism for biomass volatile oxidation has therefore been performed to avoid these difficulties. The selected detailed mechanism in this study contains 81 species and 703 elementary reactions. Necessity analysis is used to determine which species and reactions are of less importance for the predictability of the final result and, hence, can be discarded. For validation, numerical results using the derived reduced mechanism are compared with the results obtained with the original detailed mechanism. The reduced mechanism contains much fewer reactions and chemical species, that is, 35 species and 198 reactions, corresponding to 72% reduction in the number of reactions and, therefore, improving the computational time considerably. Yet, the model based on the reduced mechanism predicts correctly concentrations of NOx and CO that are essentially identical to those of the complete mechanism in the range of reaction conditions of interest, especially for the medium-temperature range. The reduced mechanism failed to predict the concentrations in the high- and low-temperature range. Therefore, two more reduced mechanisms are also proposed for the high- and low-temperature range with 26 and 52 species, respectively. The modeling conditions are selected in a way to mimic values in the range of temperature 700–1400°C, excess air ratio 0.8–3.3, and four different residence times: 1, 0.1, 0.01, and 0.001 s, since these variables are the main affecting parameters on NOx emission.
... Previous mechanistic studies on the combustion of biomassbiofuel model compounds in a well-controlled closed system have investigated detailed nitrogen chemistry in the gas phase, suggesting that NO x and HONO are formed from chain reactions involving the oxidation of the precursors NH 3 and HCN, which are produced via the devolatilization and pyrolysis of amines and proteins in biomass-biofuel (Houshfar et al., 2012;Lucassen et al., 2011). When the combustion conditions favor the oxidation of NH 3 and HCN, NO is first formed and the chain reactions control the cycling of reactive nitrogen species (NO, NO 2 , and HONO). ...
... Detailed and mechanistic nitrogen chemistry for the chemical relationship between NO x and HONO in the combustion environment has been discussed in earlier works (Chai and Goldsmith, 2017;Shrestha et al., 2018;Skreiberg et al., 2004). In addition, Houshfar et al. (2012) performed biomass combustion J. Chai et al.: Isotopes of NO x , HONO, and pNO − 3 from lab biomass burning kinetic modeling with a reduced mechanism via sensitivity analysis. From these works, we extract major pathway Reactions (R1)-(R11) that are likely responsible for fast gasphase interconversion between NO x and HONO within the combustion system. ...
... Although our studied fuels are more complicated in composition than a model system involving no more than a few starting species, results from the above studies provide fundamental underpinnings for biomass combustion. Also note that heterogeneous chemistry after these species were emitted was not considered here as the residence time of the fresh plume in our study was ∼ 5 s, which is of the same magnitude as that predicted in the nitrogen flow analysis (Houshfar et al., 2012). Kinetic isotope effects (KIEs) of these reactions have not been characterized, so only a semiquantitative prediction is presented here. ...
New techniques have recently been developed and applied to capture reactive
nitrogen species, including nitrogen oxides (NOx=NO+NO2),
nitrous acid (HONO), nitric acid (HNO3), and particulate nitrate
(pNO3-), for accurate measurement of their isotopic composition.
Here, we report – for the first time – the isotopic composition of HONO
from biomass burning (BB) emissions collected during the Fire Influence on
Regional to Global Environments Experiment (FIREX, later evolved into
FIREX-AQ) at the Missoula Fire Science Laboratory in
the fall of 2016. We used our newly developed annular denuder system (ADS),
which was verified to completely capture HONO associated with BB in
comparison with four other high-time-resolution concentration measurement
techniques, including mist chamber–ion chromatography (MC–IC), open-path
Fourier transform infrared spectroscopy (OP-FTIR), cavity-enhanced
spectroscopy (CES), and proton-transfer-reaction time-of-flight mass
spectrometry (PTR-ToF).
In 20 “stack” fires (direct emission within ∼5 s of
production by the fire) that burned various biomass materials from the
western US, δ15N–NOx ranges from −4.3 ‰ to +7.0 ‰, falling near the
middle of the range reported in previous work. The first measurements of
δ15N–HONO and δ18O–HONO in biomass burning smoke
reveal a range of −5.3 ‰ to +5.8 ‰
and +5.2 ‰ to +15.2 ‰,
respectively. Both HONO and NOx are sourced from N in the biomass fuel,
and δ15N–HONO and δ15N–NOx are strongly
correlated (R2=0.89, p<0.001), suggesting HONO is directly
formed via subsequent chain reactions of NOx emitted from biomass
combustion. Only 5 of 20 pNO3- samples had a sufficient amount
for isotopic analysis and showed δ15N and δ18O of
pNO3- ranging from −10.6 ‰ to −7.4 ‰ and +11.5 ‰ to
+14.8 ‰, respectively.
Our δ15N of NOx, HONO, and pNO3- ranges can
serve as important biomass burning source signatures, useful for
constraining emissions of these species in environmental applications. The
δ18O of HONO and NO3- obtained here verify that our method
is capable of determining the oxygen isotopic composition in BB plumes. The
δ18O values for both of these species reflect laboratory conditions
(i.e., a lack of photochemistry) and would be expected to track with the
influence of different oxidation pathways in real environments. The methods
used in this study will be further applied in future field studies to
quantitatively track reactive nitrogen cycling in fresh and aged western US
wildfire plumes.
... Eq. (7) 510 Previous mechanistic studies on combustion of biomass/biofuel model compounds in a 511 well controlled closed system have investigated detailed nitrogen chemistry in the gas 512 phase, suggesting NO x and HONO are formed from chain reactions involving oxidation 513 of precursors NH 3 and HCN, which are produced via devolatilization and pyrolysis of 514 amines and proteins in biomass/biofuel (Houshfar et al., 2012;Lucassen et al., 2011). 515 ...
... works (Chai and Goldsmith, 2017;Shrestha et al., 2018;Skreiberg et al., 2004). In 520 addition, Houshfar et al. (2012) performed biomass combustion kinetic modeling with 521 reduced mechanism via sensitivity analysis. From these works, we extract major 522 pathways (R1-R11) that are likely responsible for fast gas-phase inter-conversion 523 ...
... predicted in the nitrogen flow analysis (Houshfar et al., 2012). Kinetic isotope effects 548 ...
New techniques have recently been developed to capture reactive nitrogen species for accurate measurement of their isotopic composition. Reactive nitrogen species play important roles in atmospheric oxidation capacity (hydroxyl radical and ozone formation) and may have impacts on air quality and climate. Tracking reactive nitrogen species and their chemistry in the atmosphere based upon concentration alone is challenging. Isotopic analysis provides a potential tool for tracking the sources and chemistry of species such as nitrogen oxides (NOx = NO + NO2), nitrous acid (HONO), nitric acid (HNO3) and particulate nitrate (NO3−(p)). Here we study direct biomass burning (BB) emissions during the
Fire Influence on Regional to Global Environments Experiment (FIREX, later evolved into FIREX-AQ) laboratory experiments at the Missoula Fire Laboratory in the fall of 2016.
An annular denuder system (ADS) developed to efficiently collect HONO for isotopic composition analysis was deployed to the Fire Lab study. Concentrations of HONO recovered from the ADS collection agree well with mean concentrations averaged over each fire measured by 4 other high time resolution techniques, including mist chamber/ion chromatography (MC/IC), open-path Fourier transform infrared spectroscopy (OP-FTIR), cavity enhanced spectroscopy (CES), proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF). The concentration validation ensures complete collection of BB emitted HONO, of which the isotopic composition is preserved during the collection process. In addition, the isotopic composition of NOx and NO3−(p) from direct BB emissions were also characterized.
In 20 stack fires (direct emission within ~ 5 seconds of production by the fire) that burned various biomass materials, δ¹⁵N-NOx ranges from −4.3 ‰ to +7.0 ‰, falling near the middle of the range reported in previous work. The first measurements of δ¹⁵N-HONO and δ¹⁸O-HONO in biomass burning smoke reveal a range of −5.3 – +5.8 ‰ and +5.2 – +15.2 ‰ respectively. Both HONO and NOx are sourced from N in the biomass fuel and δ¹⁵N-HONO and δ¹⁵N-NOx are strongly correlated (R² = 0.89, p x and HONO are connected via formation pathways.
Our δ¹⁵N of NOx, HONO and NO3−(p) ranges can serve as important biomass burning source signatures, useful for constraining direct emissions of these species in environmental applications. The δ¹⁸O of HONO and NO3− obtained here verify our method is capable of determining oxygen isotopic composition in BB plumes. The δ¹⁸O for both species in this study reflect the laboratory conditions (i.e. a lack of photochemistry), and would be expected to track with the influence of ozone (O3), photochemistry and nighttime chemistry in real environments. The methods used in this study will be further applied in future field studies to quantitatively track reactive nitrogen cycling in fresh and aged Western US wildfire plumes.
... Very few skeletal mechanisms for modelling of nitrogen chemistry during combustion of solids can be found in the open literatures. Houshfar et al. [20] developed three skeletal mechanisms with 26, 35, and 52 species respectively, based on a detailed kinetic model consisting of 81 species, by employing a reaction flow and sensitivity analysis [21]. It was found that under high-temperature oxidation conditions, all three skeletal mechanisms gave reasonable results, apart from predictions of some minor species. ...
... The relatively high nitrogen content released from the solid fuel makes it relevant to waste incineration as well. The same condition has also been used in previous reduction works [20,22]. Since a relative large cutoff limit was applied, the skeletal mechanism produced directly from the necessity analysis may not preserve all main pathways and species listed in Table 1. ...
Emission of nitrogen oxides (NOx) is a major challenge for combustion of solid fuels. Strategies for emission control can be developed from computational fluid dynamics (CFD) simulation. This, furthermore, requires a computational efficient kinetic model that is able to capture both formation and destruction of NOx in a wide range of conditions. In this work, three skeletal mechanisms with varying degrees of reduction were developed based on a detailed kinetics model proposed recently (148 species and 2764 reactions). By preserving all major reaction pathways of NO formation, the most comprehensive skeletal mechanism Li45 (45 species and 788 reactions) behaved very similar compared to the base mechanism with regard to the prediction of NO. The more compact skeletal mechanism Li37 (37 species and 303 reactions) was generated specifically for the conditions relevant to large scale industrial combustion of solid fuels. The Li37 mechanism is capable of predicting NO formation as well as simulating common measures of NOx reduction such as the staged combustion and selective non-catalytic reduction (SNCR). Without the consideration of SNCR, the smallest skeletal mechanism Li32 (32 species and 255 reactions) still maintained a good predictability over broad temperature and excess air ratio ranges. Compared to the base mechanism, the skeletal mechanisms achieved over 70% reduction in species. Furthermore, the computational cost was lowered to a large extent, particularly with Li37 and Li32. This makes the developed skeletal mechanisms very suitable to be implemented in CFD simulations.
... Increasingly stricter emission regulations and costly secondary measures motivates the use and optimisation of primary measures, such as staged air combustion. Previous work has investigated the NOx reduction potential by staged air combustion, both through detailed chemical kinetics modelling in ideal reactors [1,2], CFD simulations [3,4] and experimental work on biomass [5,6,7,8,9,10] and waste [11,12,13,14]. ...
... When comparing with earlier modelling results using an ideal plug flow reactor (Fig. 1, and [1,2], the main changes that can be seen are significant quantitative differences, which are directly attributed to chemical kinetics changes, while qualitatively similar results are found. ...
NOx emissions from biomass and Municipal Solid Waste (MSW) combustion is a great concern. Technologies for reducing NOx emissions based on flue gas cleaning using e.g. Selective Non-Catalytic Reduction (SNCR) or Selective Catalytic Reduction (SCR) are well known. However, limiting NOx emissions by primary measures is an attractive opportunity, avoiding or limiting the use of costly secondary measures. Many factors influence the NOx emission level from biomass and MSW combustion plants, like fuel-N content and speciation, and operating parameters as air distribution and staging, flue gas recirculation, temperature and residence time. In this work a recent detailed chemical kinetics mechanism is used to investigate the NOx reduction potential by staged air combustion at different operational conditions, using two ideal plug flow reactors connected in series. This enables revealing a kinetics limited reduction potential, as a guideline for further detailed studies at targeted conditions using stochastic reactor networks or Computational Fluid Dynamics (CFD). The results show, for demolition wood as fuel, a large NOx reduction potential, as high as above 95% total fixed nitrogen reduction at theoretically optimum conditions. Through reaction path analyses the main reaction pathways and reactions are identified. Comparison with experimental results from grate combustion units is made. Finally, a comparison with earlier and less extensive detailed chemical kinetics mechanisms is made, to check for significant differences and assess the reasons for those. The final goal would be to arrive at computationally effective reduced chemical kinetics mechanisms that could be applied in CFD simulations of real biomass and MSW combustion plants, to arrive at improved operational conditions or improved design.
... It is fair to say that the need to mitigate these emissions is one of the main driving forces of contemporary engine design [2]. The issue acquires increased significance as oxy-combustion [3] and oxygenated biofuels [4] emerge as contemporary technologies. Engine designers have been struggling in particular with the existence of oxides of nitrogen in the exhaust gases at concentrations substantially larger than the equilibrium concentrations at the relatively low temperature of the exhaust gases. ...
... The fundamentals were reviewed in the classical work of Miller and Bowman [6], where several mechanisms of oxide formation were identified: thermal NO (Zeldovich), prompt NO (Fenimore), formation of NO 2 and N 2 O. Bozzelli and Dean [7] later proposed an additional mechanism of NOx formation in which NNH played an important role. Nitrogen chemistry has been incorporated in detailed or simplified mechanisms of chemical kinetics (e.g., [4,8,9]) and high-quality work has addressed the accurate determination of pertinent reaction rate constants (e.g., [10]). The relative importance of the NOx formation mechanisms (and, in particular, the relative contribution of thermal and prompt NO) varies according to flame configuration and it has been studied in anything varying from edge flames [11], jet flames [12], counterflow flames [13,14] and a series of reactor configurations [15][16][17]. ...
The Computational Singular Perturbation (CSP) algorithm is employed in order to determine how -dilution influences ignition delay and chemical paths that generate NO during isochoric homogenous lean /air autoignition. Regarding the ignition delay, it is shown that -dilution enhances reactivity, mainly due to the increased OH production throughout the explosive stage via reaction . With regard to NO generation, the relative importance of thermal and chemical effects are examined and it is concluded that both are important. The thermal effects result in a lower temperature at the end of the explosive stage, while the most notable chemical effect is the lower level of O after this stage, mainly due to the effect of -dilution on the equilibrium of the reaction . The depletion of O, together with the thermal effect, causes a substantial decrease in final NO generation.
... These assumptions lead to the well-stirred or perfectly stirred reactor model (PSR). The focus is therefore on kinetic modeling, often limited to the gas phase kinetics of biomass conversion [21,22]. A more detailed treatment of biomass pyrolysis is proposed by Lee et al. [23]. ...
In this paper, a partially stirred stochastic reactor model is presented as an alternative for the modeling of biomass pyrolysis and gasification. Instead of solving transport equations in all spatial dimensions as in CFD simulations, the description of state variables and mixing processes is based on a probability density function, making this approach computationally efficient. The virtual stochastic particles, an ensemble of flow elements consisting of porous solid biomass particles and surrounding gas, mimic the turbulent exchange of heat and mass in practical systems without the computationally expensive resolution of spatial dimensions. Each stochastic particle includes solid phase, pore gas and bulk gas interaction. The reactor model is coupled with a chemical mechanism for both surface and gas phase reactions. A Monte Carlo algorithm with operator splitting is employed to obtain the numerical solution. Modeling an entrained flow gasification reactor demonstrates the applicability of the model for biomass fast pyrolysis and gasification. The results are compared with published experiments and detailed CFD simulations. The stochastic reactor model is able to predict all major species in the product gas composition very well for only a fraction of the computational time as needed for comprehensive CFD.
... For the reduction, a number of species such as NO x and CO are defined as the target species of the combustion products and temperature is regarded as the important variable. This method is effective, since the modeling aim is to study emissions from combustion processes [Løvås et al. 2004; Houshfar et al. 2012]. Temperature, excess air ratio and residence time are the important parameters for the NO x formation. ...
Agricultural and prescribed burning activities emit large amounts of trace gases and aerosols on regional to global scales. We present a compilation of emission factors (EFs) and emission ratios from the eastern portion of the Fire Influence on Regional to Global Environments and Air Quality (FIREX‐AQ) campaign in 2019 in the United States, which sampled burning of crop residues and other prescribed fire fuels. FIREX‐AQ provided comprehensive chemical characterization of 53 crop residue and 22 prescribed fires. Crop residues burned at different modified combustion efficiencies (MCE), with corn residue burning at higher MCE than other fuel types. Prescribed fires burned at lower MCE (<0.90) which is typical, while grasslands burned at lower MCE (0.90) than normally observed due to moist, green, growing season fuels. Most non‐methane volatile organic compounds (NMVOCs) were significantly anticorrelated with MCE except for ethanol and NMVOCs that were measured with less certainty. We identified 23 species where crop residue fires differed by more than 50% from prescribed fires at the same MCE. Crop residue EFs were greater for species related to agricultural chemical use and fuel composition as well as oxygenated NMVOCs possibly due to the presence of metals such as potassium. Prescribed EFs were greater for monoterpenes (5×). FIREX‐AQ crop residue average EFs generally agreed with the previous agricultural fire study in the US but had large disagreements with global compilations. FIREX‐AQ observations show the importance of regionally‐specific and fuel‐specific EFs as first steps to reduce uncertainty in modeling the air quality impacts of fire emissions.
Ammonia (NH3) has been suggested as a fuel to attain zero carbon emissions. However, dealing with ammonia needs careful studies to reveal its limits as a suitable and promising fuel for broad applications within large power requirements. Chemical reaction mechanisms, widely employed in the modeling of these applications, are still under development. Therefore, this review is aimed to shed light on the current mechanisms available in the literature, highlighting modeling parameters that directly affect reaction rates which in turn govern the performance of each reaction mechanism. The key findings denote that most of the reaction mechanisms have poor performance when predicting combustion characteristics of ammonia flames such as laminar flame speed, ignition delay time, and nitrogen oxide emissions (NOx). In addition, none of the mechanisms have been optimised efficiently to predict properly experimental measurements for all these combustion characteristics. For example, Duynslaegher's mechanism perfectly predicted the laminar flame speed at lean and stoichiometric conditions, while Nakamura's reaction mechanism worked properly at rich conditions for the estimation of laminar flame speed. Although the aforementioned mechanisms achieved good estimation in terms of laminar flame speed, they showed poor performance against NO mole fractions. Similarly, Glarborg's (2018) mechanism properly estimated NO mole fractions at lean and stoichiometric flames while Wang's mechanism performed well in rich conditions for such emissions. Other examples are presented in this manuscript. Finally, the prediction performance of the assessed mechanisms varies based on operating conditions, mixing ratios, and equivalence ratios. Most mechanisms dealing with blended NH3 combinations gave good predictions when the concentration of hydrogen was low, while deteriorating with increasing hydrogen concentrations; a result of the shift in reactions that require more research.
Ammonia (NH3) has been receiving the attention of researchers as an alternative promising green fuel to replace fossil sources for energy production. However, the high NOx emissions are one of the drawbacks and restrictions of using NH3 on a broad scale. The current study investigates NO production/consumption for a 70/30 (vol%) NH3/H2 mixture using kinetic reaction mechanism concepts to shed light on the essential reaction routes that promote/inhibit NO formation. Sixty-seven kinetic reaction mechanisms from the literature have been investigated and compared with recently reported measurements at a wide range of equivalence ratios (ϕ) (0.6–1.4), atmospheric pressure and temperature conditions. Both numerical simulations and experimental measurements used the same combustion reactor configuration (premixed stabilized stagnation flame). To highlight the best kinetic model for the predicting of the NO experimental measurements of NO, a symmetric mean absolute percentage error (SMAPE) has been determined as a preliminary estimation by comparing both numerical and experimental measurements. The results found that the kinetic reaction mechanism of Glarborg showed an accurate prediction with a minor error percentage of 2% at all lean and stoichiometric conditions. Meanwhile, the kinetic model of Wang accurately predicted the experimental data with 0% error at ϕ = 1.2 and underestimated the mole fraction of NO at 1.4 ϕ with an error of 10%. The sensitivity analysis and rate of production/consumption of NO mole fractions analysis have also been implemented to highlight the most important reactions that promote/inhibit NO formation. At lean and stoichiometric conditions, Glarborg kinetic model shows that the kinetic reactions of HNO + H ⇌ NO + H2, HNO + O ⇌ NO + OH, and NH + O ⇌ NO + H are the most important reaction routes with considerable effect on NO formation for 70/30 (vol%) NH3/H2 mixture. In contrast, the reactions of NH2 + NO ⇌ N2 + H2O, NH2 + NO ⇌ NNH + OH, NH + NO ⇌ N2O + H, and N + NO ⇌ N2 + O significantly consume NO to N2, NNH, and N2O. Further, Wang’s mechanism illustrated the dominant effect of each HNO + H ⇌ NO + H2, N + OH ⇌ NO + H, NH + O ⇌ NO + H in NO formation and NH + NO ⇌ N2O + H, NH2 + NO ⇌ NNH + OH, and NH2 + NO ⇌ N2 + H2O in the consumption of NO mole fractions.
To achieve a reduced chemical model for comprehensive prediction of ammonia/hydrogen/methane mixture combustion, a detailed chemical mechanism with 128 species and 957 reactions was first assembled using models from literature. Directed relation graph with error propagation (DRGEP) with sensitivity analysis reduction method was then used to obtain compact reaction models. The studied reduction conditions cover ɸ = 0.5–2.0, temperature 1000–2000 K, and pressure 0.1–5 MPa. Finally, two reduced models have been obtained: 28 species and 213 reactions for ammonia/hydrogen and 51 species and 420 reactions for ammonia/hydrogen/methane. Ignition delay times and laminar burning velocities for single component and fuel mixtures predicted using the detailed and reduced mechanisms were compared with available experiments. Results showed that both detailed and reduced mechanisms performed fairly well for ignition delays, while over-predicted laminar burning velocity at fuel-rich conditions for single ammonia fuel and mixtures. The 51 species reduced mechanism was also tested in non-premixed coflow hydrogen/methane jet flames, while 1%–50% mole ammonia were added to the fuel stream. Modelling results showed that this 51-species mechanism was suitable for CFD modelling, and the speedup factor was over 5 when using the reduced mechanism with different codes. The flame structure, as well as NO and NO2 formation was studied. High NO concentrations were found in high-temperature region near the stoichiometric zone, while NO2 was dominant in the lean flame zone. Reaction flux analysis was performed to better understand NH3 oxidation and NOx emissions at low- and high-temperature conditions.
This study investigates the characteristics of fuel NOx formation resulting from the combustion of producer gas derived from biomass gasification using different feedstocks. Common industrial burners are optimized for using natural gas or coal-derived syngas. With the increasing demand in using biomass for power generation, it is important to develop burners that can mitigate fuel NOx emissions due to the combustion of ammonia, which is the major nitrogen-containing species in biomass-derived gas. In this study, the combustion process inside the burner is modeled using computational fluid dynamics (CFD) with detailed chemistry. A reduced mechanism (36 species and 198 reactions) is developed from GRI 3.0 in order to reduce the computation time. Combustion simulations are performed for producer gas arising from different feedstocks such as wood gas, wood + 13% DDGS (dried distiller grain soluble) gas and wood + 40% DDGS gas and also at different air equivalence ratios ranging from 1.2 to 2.5. The predicted NOx, emissions are compared with the experimental data and good levels of agreement are obtained. It is found out that NOx is very sensitive to the ammonia content in the producer gas. Results show that although NO-NO2 interchanges are the most prominent reactions involving NO, the major NO producing reactions are the oxidation of NH and N at slightly fuel rich conditions and high temperature. Further analysis of results is conducted to determine the conditions favorable for NOx reduction. The results indicate that NOx can be reduced by designing combustion conditions which have fuel rich zones in most of the regions. The results of this study can be used to design low NOx burners for combustion of gas mixtures derived from gasification of biomass. One suggestion to reduce NOx is to produce a diverging flame using a bluff body in the flame region such that NO generated upstream will pass through the fuel rich flame and be reduced.
We present in this paper simplified chemical mechanisms for gas phase biomass combustion based on automatic reduction of detailed and comprehensive kinetics. The reduction method that has been employed is a combined reaction flow and sensitivity analysis well known to combustion, resulting in a necessity index ranking all chemical species for automatic reduction. The objective is to obtain more compact chemical models, so called skeletal mechanisms, for implementation into CFD in order to reduce computational time. In the current work the physical system used for the development and validation of the chemical models is that of a tubular reactor, or plug flow reactor, with operating conditions typically found in biomass reactors. Focus has been on gas phase reactions only, and the fuel composition is based on experimental values from biomass and coal gasification. Emphasis has been on the reliability of the simplified models and the correct prediction of important emission parameters such as NOX and important intermediate species. The original chemical model, consisting of several sub models for important reaction paths known in biomass combustion, contained 81 species and 1401 reactions. This was successfully reduced down to 36 species providing a compact and reliable chemical model for implementation into CFD. The model still contains the reaction paths of C2 species, allowing for more realistic fuel gas compositions. The model has been experimentally validated for a wide range of temperatures including low temperature chemistry and reducing conditions for NOX. The computational time saved using the simplified models was significant with over 80% reduction in CPU time.
Flue gas recirculation (FGR) is a conventional means of reducing NOx emissions that involves lowering the peak flame temperature and reducing the oxygen concentration in the combustion region. Staged air combustion is also an effective means of NOx reduction, especially in biomass combustion. This article reports results on NOx emissions in a set of experiments combining FGR and staged air combustion in a grate-fired laboratory-scale reactor. Two different compositions of the recirculated flue gas were used: CO2 and CO2 + NO. The CO2 concentration varied between 0–8 vol % of the total inlet flow rate and the NO concentration varied between 0 and 64 ppm. Two different FGR locations were also tested: above and below the grate. The results are compared with a reference experiment performed without FGR. The NOx reduction level from staged air combustion at the optimal primary excess air ratio is 70%, while employing FGR can reduce the NOx emissions by an additional 5%–10%. The optimal primary excess air ratio range is 0.9–1. However, FGR more effectively reduces NOx when employed outside of the optimum primary excess air ratio range, i.e., excess air ratios higher than 1 and less than 0.9. The experiments with FGR located above the grate exhibit higher reduction potential, while FGR located below the grate produces decreased reduction. The recycled-NO conversion factor, which gives a measure of maximal FGR efficiency, at the maximum point, is nearly 100% when FGR is applied below the grate and is 85%–100% in the case of recirculation above the grate.
An experimental and modeling study of 11 premixed NH3/CH4/O2/Ar flames at low pressure (4.0 kPa) with the same equivalence ratio of 1.0 is reported. Combustion intermediates and products are identified using tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry. Mole fraction profiles of the flame species including reactants, intermediates and products are determined by scanning burner position at some selected photon energies near ionization thresholds. Temperature profiles are measured by a Pt/Pt–13%Rh thermocouple. A comprehensive kinetic mechanism has been proposed. On the basis of the new observations, some intermediates are introduced. The flames with different mole ratios (R) of NH3/CH4 (R0.0, R0.1, R0.5, R0.9 and R1.0) are modeled using an updated detailed reaction mechanism for oxidation of CH4/NH3 mixtures. With R increasing, the reaction zone is widened, and the mole fractions of H2O, NO and N2 increase while those of H2, CO, CO2 and NO2 have reverse tendencies. The structural features by the modeling results are in good agreement with experimental measurements. Sensitivity and flow rate analyses have been performed to determine the main reaction pathways of CH4 and NH3 oxidation and their mutual interaction.
Combustion is an old technology, which at present provides about 90% of our worldwide energy support. Combustion research in the past used fluid mechanics with global heat release by chemical reactions described with thermodynamics, assuming infinitely fast reactions. This approach was useful for stationary combustion processes, but it is not sufficient for transient processes like ignition and quenching or for pollutant formation. Yet pollutant formation during combustion of fossil fuels is a central topic and will continue to be so in the future. This book provides a detailed and rigorous treatment of the coupling of chemical reactions and fluid flow. Also, combustion-specific topics of chemistry and fluid mechanics are considered and tools described for the simulation of combustion processes. The fourth edition has been restructured: Mathematical Formulae and derivations as well as the space-consuming reaction mechanisms have been replaced from the text to appendix. A new chapter discusses the impact of combustion processes on the earth's atmosphere, the chapter on auto-ignition is extended to combustion in Otto- and Diesel-engines, and the chapters o. © Springer-Verlag Berlin Heidelberg 1996, 1999, 2001, and 2006.
For modeling the formation of nitrogen oxides in combustion via both the prompt-NO and the fuel-NO mechanisms, as well as for modeling the reduction of nitrogen oxides via reburning, a good knowledge of the kinetics of oxidation of hydrogen cyanide (HCN) is required. The formation routes to HCN and the thermochemistry of HCN-related species are reviewed. The available kinetic data for the oxidation of HCN are presented and a comprehensive detailed chemical kinetic reaction mechanism for the oxidation of HCN is proposed and discussed.
The oxidation of NHâ during oxy-fuel combustion of methane, i.e., at high [COâ], has been studied in a flow reactor. The experiments covered stoichiometries ranging from fuel rich to very fuel lean and temperatures from 973 to 1773 K. The results have been interpreted in terms of an updated detailed chemical kinetic model. A high COâ level enhanced formation of NO under reducing conditions while it inhibited NO under stoichiometric and lean conditions. The detailed chemical kinetic model captured fairly well all the experimental trends. According to the present study, the enhanced CO concentrations and alteration in the amount and partitioning of O/H radicals, rather than direct reactions between N-radicals and COâ, are responsible for the effect of a high COâ concentration on ammonia conversion. When COâ is present as a bulk gas, formation of NO is facilitated by the increased OH/H ratio. Besides, the high CO levels enhance HNCO formation through NHâ+CO. However, reactions NHâ+ O to form HNO and NHâ+H to form NH are inhibited due to the reduced concentration of O and H radicals. Instead reactions of NHâ with species from the hydrocarbon/methylamine pool preserve reactive nitrogen as reduced species. These reactions reduce the NHâ availability to form NO by other pathways like via HNO or NH and increase the probability of forming Nâ instead of NO. (author)
A level of importance (LOI) selection parameter is employed in order to identify species with general low importance to the overall accuracy of a chemical model. This enables elimination of the minor reaction paths in which these species are involved. The generation of such skeletal mechanisms is performed automatically in a pre-processing step ranking species according to their level of importance. This selection criterion is a combined parameter based on a time scale and sensitivity analysis, identifying both short lived species and species with respect to which the observable of interest has low sensitivity. In this work a careful element flux analysis demonstrates that such species do not interact in major reaction paths. Employing the LOI procedure replaces the previous method of identifying redundant species through a two step procedure involving a reaction flow analysis followed by a sensitivity analysis. The flux analysis is performed using DARS {sup copyright}, a digital analysis tool modelling reactive systems. Simplified chemical models are generated based on a detailed ethylene mechanism involving 111 species and 784 reactions (1566 forward and backward reactions) proposed by Wang et al. Eliminating species from detailed mechanisms introduces errors in the predicted combustion parameters. In the present work these errors are systematically studied for a wide range of conditions, including temperature, pressure and mixtures. Results show that the accuracy of simplified models is particularly lowered when the initial temperatures are close to the transition between low- and high-temperature chemistry. A speed-up factor of 5 is observed when using a simplified model containing only 27% of the original species and 19% of the original reactions. (author)
A detailed chemical kinetic mechanism for the simulation of the gas-phase combustion and pyrolysis of biomass-derived fuels was compiled by assembling selected reaction subsets from existing mechanisms (parents). The mechanism, here referred to as ''AaA,'' includes reaction subsets for the oxidation of hydrogen (H), carbon monoxide (CO), light hydrocarbons (C and C), and methanol (CHOH). The mechanism also takes into account reaction subsets of nitrogen pollutants, including the reactions relevant to staged combustion, reburning, and selective noncatalytic reduction (SNCR). The AaA mechanism was validated against suitable experimental data from the literature. Overall, the AaA mechanism gave more accurate predictions than three other mechanisms of reference, although the reference mechanisms performed better occasionally. The predictions from AaA were also found to be consistent with the predictions of its parent mechanisms within most of their range of validity, thus transferring the validity of the parents to the inheriting mechanism (AaA). In parametric studies the AaA mechanism predicted that the effect of methanol on combustion and pollutants is often similar to that of light hydrocarbons, but it also showed that there are important exceptions, thus suggesting that methanol should be taken into account when simulating biomass combustion. To our knowledge, the AaA mechanism is currently the only mechanism that accounts for the chemistry of methanol and nitrogen relevant to the gas-phase combustion and pyrolysis of biomass-derived fuels. (author)
One of the remaining issues in our understanding of nitrogen chemistry in combustion is the chemistry of NNH. This species is known as a key intermediate in Thermal DeNOx, where NH3 is used as a reducing agent for selective non-catalytic reduction of NO. In addition, NNH has been proposed to facilitate formation of NO from thermal fixation of molecular nitrogen through the so-called NNH mechanism. The importance of NNH for formation and reduction of NO depends on its thermal stability and its major consumption channels. In the present work, we study reactions on the NNH+O, NNH+O2, and NH2+O2 potential energy surfaces using methods previously developed by Miller, Klippenstein, Harding, and their co-workers. Their impact on Thermal DeNOx and the NNH mechanism for NO formation is investigated in detail.
Chemical mechanisms have been employed in hydrocarbon combustion as a means of understanding the underlying phenomenology of the combustion process in terms of the elementary reactions of individual species. This chapter provides an introduction to most of the mathematical methods that have been used for the construction, investigation, and reduction of combustion mechanisms. The use of algebraic manipulation in techniques, such as the quasi-steady-state approximation (QSSA) and lumping, make the production of a reduced mechanism essential and make subsequent calculations as simple as possible. Computational singular perturbation (CSP) is an alternative to the rate-of-production and sensitivity methods for mechanism reduction and provides an automatic selection of the important reactions as well as time-scale analysis. The simplest and most widely used technique involving the separation of time scales is the QSSA; however, a possible limitation is that it may not provide the minimum low-order system. Chemical lumping can prove very useful in areas, such as the combustion of hydrocarbon mixtures or soot formation. Several programs are available for the investigation and reduction of combustion mechanisms, including MECHMOD, a code for the automatic modification of CHEMKIN format combustion mechanisms, and KINALC, which is an almost automatic program for the investigation and reduction of gas-phase reaction mechanisms. KINALC is a postprocessor to CHEMKIN-based simulation packages SENKIN, PREMIX, OPPDIF, RUN1DL, PSR, SHOCK and EQLIB. Because models in combustion are expected to cover a wide range of conditions, it is natural to expect that a different approach might be used for different cases.
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In solid fuel flames, reburn-type reactions are often important for the concentrations of NOx in the near-burner region. To be able to model the nitrogen chemistry in these flames, it is necessary to have an adequate model for volatile/NO interactions. Simple models consisting of global steps or based on partial-equilibrium assumptions have limited predictive capabilities. Reburning models based on systematic reduction of a detailed chemical kinetic model offer a high accuracy but rely on input estimates of combustion intermediates, including free radicals. In the present work, an analytically reduced nitrogen scheme is combined with simplified correlations for estimation of O/H and hydrocarbon radicals. Correlations are derived for volatile compositions representative of solid fuels ranging from bituminous coal to biomass, for temperatures of 1200−2000 K and excess air ratios in the range of 0.6 ≤ λ ≤ 2.0. The combined model is tested against reference calculations with a comprehensive mechanism. The results indicate that the approximations in the simplified hydrocarbon radical scheme are satisfactory. However, when this scheme is combined with the semi-empirical correlations for the O/H radicals, the modeling predictions for the radicals become less accurate. Despite these deviations, the combined model provides a satisfactory prediction of NO under reburning conditions over the range of fuels, temperatures, and stoichiometries tested.
The potential for reduction of nitrogen oxides in gas turbine combustors was studied by detailed chemical kinetic modeling under ideal flow conditions. The investigation focused on turbines burning biomass-derived gasification gas from an air-blown integrated gasification combined cycle plant. The aim was to give detailed information about the parameters that favor reduction of NOx emissions, providing a solid background for designing an air-staged, low-NOx gas turbine. The potential and limitations of the detailed chemical kinetic modeling as a predictive tool for simulating the process were discussed. Instantaneous, delayed, and back-streamed air/fuel mixing models were tested to study the effect of mixing on the emissions. Predictions showed that the nitrogen chemistry was mainly affected by temperature and pressure: low temperatures of about 900-1000 degrees C and high pressures of about 10-20 bar favored fuel nitrogen conversion to N-2. At atmospheric pressure, an increase in the number of air addition stages increased the conversion to N-2, but at higher pressure the reduction was more efficient with three-stage addition than with either one- or six-stage addition. The conversion efficiency of NH3 to N-2 increased with the inlet NH3 concentration, but the final NOx emission calculated in ppm(v) increased as well. NOx emission often was higher when HCN replaced ammonia in the gasification gas. The main paths for fuel-NH3 conversion to NOx and N-2 were predicted to occur via intermediate formation of amino radicals (NHi). Another important conversion path to N-x was shown to proceed via a H2NO intermediate. Models accounting for delayed mixing led to more realistic predictions, showing the effect of CH4 in the gasification on increased NOx emission by means of its CHi radicals.
Nitrogen release is a little known aspect of pyrolysis of biomass. In this study on thermally thick samples of three biomass residues with high N-content, the NOx precursors NH3 and HCN were measured with a Fourier transform infrared (FTIR) analyzer at different heating rates (low and high) and temperatures (400−900 °C). The feedstocks investigated have been given scarce or no attention. At a high heating rate, (1) NH3 is the main N-compound with increasing yield with increasing temperature until reaching a plateau at 825−900 °C at a conversion level of 31−38%; (2) HCN release is increasing sharply with temperature to reach a conversion of 9−18%; (3) the (HCN + NH3) conversion levels of all samples are close; (4) N-selectivity is affected by temperature and particle size; (5) release patterns and thermal behaviors of N and C are different and influence of fuel properties (intrinsic and physical) may be inferred; (6) the intricate structure of biomass indicates that decomposition paths may include (N-compounds + non-N-compounds) reactions. At a low heating rate, (1) NH3 is the main N-compound; (2) HCN and NH3 release are significantly different for the various fuels (7.9−19.2%) and fuel properties (intrinsic and physical) might be of importance; (3) the release pattern of N is affected by fuel properties.
A detailed chemical kinetic model for oxidation of C2H4 in the intermediate temperature range and high pressure has been developed and validated experimentally. New ab initio calculations and RRKM analysis of the important C2H3+O2 reaction was used to obtain rate coefficients over a wide range of conditions (0.003–100bar, 200–3000K). The results indicate that at 60 bar and medium temperatures vinyl peroxide, rather than CH2O and HCO, is the dominant product. The experiments, involving C2H4/O2 mixtures diluted in N2, were carried out in a high pressure flow reactor at 600–900K and 60 bar, varying the reaction stoichiometry from very lean to fuel-rich conditions. Model predictions are generally satisfactory. The governing reaction mechanisms are outlined based on calculations with the kinetic model. Under the investigated conditions the oxidation pathways for C2H4 are more complex than those prevailing at higher temperatures and lower pressures. The major differences are the importance of the hydroxyethyl (CH2CH2OH) and 2-hydroperoxyethyl (CH2CH2OOH) radicals, formed from addition of OH and HO2 to C2H4, and vinyl peroxide, formed from C2H3+O2. Hydroxyethyl is oxidized through the peroxide HOCH2CH2OO (lean conditions) or through ethenol (low O2 concentration), while 2-hydroperoxyethyl is converted through oxirane.
The formation of benzene in a series of fuel-rich premixed reactant systems with a constant carbon-to-oxygen ratio of approximately 0.6 is investigated experimentally in a laminar flow reactor at temperatures between 1073 and 1823 K and at atmospheric pressure. The experimental data are compared to model results using a chemical kinetic mechanism based on the work of Pope and Miller. Modifications to their mechanism include changes in the reaction subsets of C2H2, C3H4, and 1,3-C4H6. The experimental data show that benzene formation may exhibit two distinct peaks as a function of the reaction temperature. A high-temperature peak is observed between 1500 and 1600 K, and it appears with a similar magnitude of concentration for all sets of reactants. A low-temperature peak is observed between 1200 and 1300 K for reactant sets CH4/C2H2, CH4/C3H4, and CH4/1,3-C4H6. The low-temperature peak is comparable in magnitude for the CH4/C2H2 and CH4/1,3-C4H6 Mixtures, while it is 5 times larger in the CH4/C3H4 system. In general, there is good agreement between modeling and experimental results. However, some improvements are needed, in particular in the acetylene chemistry and the initiation kinetics for 1,3-butadiene.
The pyrolysis of thermally thick (approximately 75g) biomass residues samples (i.e. brewer spent grains, fibreboard and coffee beans waste) has been investigated in an in-house designed and fabricated macro-TGA both by rapid sample introduction at reactor temperatures from 600 to 900°C and by applying a constant heating rate of 10K/min. The composition of the product gas is determined by simultaneous online use of a micro-GC and a FTIR analyser. The product yields (liquid, char and gas) and the gas composition show a clear dependence on temperature and heating rate. The main gas products are CO2, CO, CH4, H2, C2H2, C2H6 and C2H4. The results show that a rise in temperature leads to increasing gas yields and decreasing liquid and char yields. Lower heating rates favour liquid and char yields. The release patterns of the gaseous species are also greatly affected by the temperature history of the sample.
The reduction of nitric oxide by reaction with C1 and C2 hydrocarbons under reducing conditions in a flow reactor has been analyzed in terms of a detailed chemical kinetic model. The experimental data were partly adopted from previous work and partly obtained in the present study; they cover the temperature range 800–1500 K and the reburn fuels CH4, C2H2, C2H4, C2H6, and natural gas. Modeling predictions indicate that, under the conditions investigated, HCCO + NO and CH3 + NO are the reactions most important in reducing NO. The HCCO + NO reaction is the dominant reaction when using natural gas or C2 hydrocarbons as reburn fuels. This reaction leads partly to HCNO, which is recycled to NO, and partly to HCN, which is converted to N2 or NO. When methane or natural gas are used as reburn fuel, the CH3 + NO reaction contributes significantly to remove NO. Modeling predictions are in reasonably good agreement with the experimental observations for the fuels investigated, even though the NO reduction potential is overpredicted for methane and underpredicted for ethane. Modeling predictions for NO are very sensitive to the formation of HCCO and to the product branching ratio for the HCCO + NO reaction. Furthermore, the present analysis indicates that more work is needed on critical steps in the hydrocarbon oxidation scheme.
The Chemkin general-purpose chemical kinetics package uses a data base that contains polynomial fits to specific heats, standard state enthalpies, and standard state entropies. The fourteen coefficient fits are in the same form as used in the NASA Complex Chemical Equilibrium Program (Gordon and McBride, 1971). This report represents a compilation of the data that is currently in use at Sandia National Laboratories. 16 refs.
An automated procedure has been previously developed to generate simplified skeletal reaction mechanisms for the combustion of n-heptane/air mixtures at equivalence ratios between 0.5 and 2.0 and different pressures. The algorithm is based on a Computational Singular Perturbation (CSP)-generated database of importance indices computed from homogeneous n-heptane/air ignition solutions. In this paper, we examine the accuracy of these simplified mechanisms when they are used for modeling laminar n-heptane/air premixed flames. The objective is to evaluate the accuracy of the simplified models when transport processes lead to local mixture compositions that are not necessarily part of the comprehensive homogeneous ignition databases. The detailed mechanism was developed by Curran et al. and involves 560 species and 2538 reactions. The smallest skeletal mechanism considered consists of 66 species and 326 reactions. We show that these skeletal mechanisms yield good agreement with the detailed model for premixed n-heptane flames, over a wide range of equivalence ratios and pressures, for global flame properties. They also exhibit good accuracy in predicting certain elements of internal flame structure, especially the profiles of temperature and major chemical species. On the other hand, we find larger errors in the concentrations of many minor/radical species, particularly in the region where low-temperature chemistry plays a significant role. We also observe that the low-temperature chemistry of n-heptane can play an important role at very lean or very rich mixtures, reaching these limits first at high pressure. This has implications to numerical simulations of non-premixed flames where these lean and rich regions occur naturally.
In pulverised coal flames, the most important volatile nitrogen component forming NOx, is HCN. To be able to model the nitrogen chemistry in coal flames it is necessary to have an adequate model for HCN oxidation. The present work was concerned with developing a model for HCN/NH3/NO conversion based on systematic reduction of a detailed chemical kinetic model. Models of different complexity were developed and tested under conditions similar to those in a pulverised coal flame. Comparisons of the models were made for ideal chemical reactors simulations (plug flow reactor and well-stirred reactor). Provided that the CO/H2 chemistry was described adequately, the reduced HCN/NH3/NO model compared very well with the detailed model over a wide range of stoichiometries. Decoupling of the HCN chemistry from the CO/H2 chemistry resulted in over-prediction of the HCN oxidation rate under fuel rich conditions, but had negligible effect on the CO/H2 chemistry. Comparison with simplified HCN models from the literature revealed significant differences, indicating that these models should be used cautiously in modelling volatile nitrogen conversion.
The potential for reduction of nitrogen oxides in gas turbine combustors was studied by detailed chemical kinetic modeling under ideal flow conditions. The investigation focused on turbines burning biomass-derived gasification gas from an air-blown integrated gasification combined cycle plant. The aim was to give detailed information about the parameters that favor reduction of NOx emissions, providing a solid background for designing an air-staged, low-NOx gas turbine. The potential and limitations of the detailed chemical kinetic modeling as a predictive tool for simulating the process were discussed. Instantaneous, delayed, and back-streamed air/fuel mixing models were tested to study the effect of mixing on the emissions. Predictions showed that the nitrogen chemistry was mainly affected by temperature and pressure: low temperatures of about 900−1000 °C and high pressures of about 10−20 bar favored fuel nitrogen conversion to N2. At atmospheric pressure, an increase in the number of air addition stages increased the conversion to N2, but at higher pressure the reduction was more efficient with three-stage addition than with either one- or six-stage addition. The conversion efficiency of NH3 to N2 increased with the inlet NH3 concentration, but the final NOx emission calculated in ppmv increased as well. NOx emission often was higher when HCN replaced ammonia in the gasification gas. The main paths for fuel−NH3 conversion to NOx and N2 were predicted to occur via intermediate formation of amino radicals (NHi). Another important conversion path to N2 was shown to proceed via a H2NO intermediate. Models accounting for delayed mixing led to more realistic predictions, showing the effect of CH4 in the gasification on increased NOx emission by means of its CHi radicals.
In part 1 of the present work (10.1021/ef900752a), experimental data and computational fluid dynamics (CFD) modeling predictions for velocity field, temperatures, and major species were compared for a 50 kW axisymmetric, non-swirling natural gas fired combustion setup, constructed to simulate the conditions in the freeboard of a grate-fired boiler. Here, in part 2, the ability of CFD to predict volatile N oxidation to NO and N2 is evaluated. Trace amounts of ammonia were added to the natural gas, and local measurements of NH3 and NO in the reactor were compared to modeling predictions. Different modeling approaches, including global schemes and analytically reduced mechanisms, were tested in the CFD calculations. In addition, the simplified schemes were compared to reference calculations with a detailed mechanism under isothermal plug flow reactor conditions. While none of the global ammonia schemes was able to provide satisfactory predictions over a wider range of conditions, an analytically reduced nitrogen scheme generally provided a satisfactory agreement with the detailed mechanism. Application of the selected schemes in a CFD analysis showed that both the standard Fluent postprocessing approach with the De Soete global scheme and the combination of a skeletal combustion mechanism with the analytically reduced N scheme provided a reasonable agreement with the experimental data. Most of the tested ammonia oxidation schemes were able to qualitatively predict the trends in NO formation going from one operational case to the other, but the main combustion solution on which the ammonia oxidation was based proved to have a large impact on the quantitative NO prediction.
This work deals with the reburn chemistry in oxy-fuel combustion of methane. It was studied experimentally in an atmospheric-pressure flow reactor under diluted conditions in N2 and CO2, respectively. The experiments covered temperatures from 1173 to 1773 K and stoichiometries ranging from fuel-rich to fuel-lean. The results showed similar NO reduction efficiencies in N2 and CO2 under reducing conditions, while under stoichiometric and fuel-lean conditions, the NO reduction obtained in CO2 was higher than that obtained in N2. The temperature at which CO exhibited its maximum concentration coincided with a sharp increase in NO reduction. This temperature was higher in the experiments with CO2 compared to those in N2. The results were interpreted in terms of an updated detailed chemical kinetic model. The effect of CO2 is to increase the OH/H ratio, lower the overall concentration of the O/H radical pool, and increase the availability of CO. Under reducing conditions, the different ways that CO2 interacts with the nitrogen chemistry largely cancel out and the overall impact is small. Under stoichiometric conditions, the NO reduction is enhanced in CO2 because of the suppression of the O/H radical pool, in particular of atomic oxygen.
The oxidation of methane in an atmospheric-pressure flow reactor has been studied experimentally under highly diluted conditions in N2 and CO2, respectively. The stoichiometry was varied from fuel-lean to fuel-rich, and the temperatures covered the range 1200–1800 K. The results were interpreted in terms of a detailed chemical kinetic mechanism for hydrocarbon oxidation. On the basis of results of the present study, it can be expected that oxy-fuel combustion will lead to strongly increased CO concentrations in the near-burner region. The CO2 present will compete with O2 for atomic hydrogen and lead to formation of CO through the reaction CO2 + H CO + OH. Reactions of CO2 with hydrocarbon radicals may also contribute to CO formation. The most important steps are those of singlet and triplet CH2 with CO2, while other radicals such as CH3 and CH are less important for consuming CO2. The high local CO levels may have implications for near-burner corrosion and slagging, but increased problems with CO emission in oxy-fuel combustion are not anticipated.
In solid fuel flames, NOx is largely formed from the oxidation of volatile nitrogen compounds such as HCN and NH3. To be able to model the nitrogen chemistry in these flames, it is necessary to have an adequate model for volatile-N oxidation. Simple global models for oxidation of HCN and NH3 from the literature should be used cautiously, since their predictive capabilities are limited, particularly under reducing conditions. Models for HCN/NH3/NO conversion based on the systematic reduction of a detailed chemical kinetic model offer high accuracy but rely on input estimates of combustion intermediates, including free radicals. In the present work, simple, semiempirical expressions are presented for estimation of H, O, and OH radicals. Correlations are derived for volatile compositions representative of solid fuels ranging from bituminous coal to biomass, for temperatures of 1200−2000 K, and excess air ratios in the range 0.6 ≤ λ ≤ 2.0. The radical estimation tool is combined with the analytically reduced N-scheme of Pedersen et al. [Combust. Sci. Technol. 1998, 131, 193−223], and the combined model is tested against reference calculations with a comprehensive mechanism. For excess air ratios of λ ≥ 0.8 and temperatures of 1400 K and above, the prediction of NO formation from both HCN and NH3 is very good for volatile compositions representing all tested fuels. For lower values of λ, the predictions are good for biomass and lignite, while they become less accurate for the sub-bituminous and bituminous coals, especially at lower temperatures. The semiempirical correlations for estimating radical concentrations may also be useful in combination with models for other trace species, such as sulfur oxides, organic species, etc.
Since biomass is the only carbon-based renewable fuel, its application becomes more and more important for climate protection. Among the thermochemical conversion technologies (i.e., combustion, gasification, and pyrolysis), combustion is the only proven technology for heat and power production. Biomass combustion systems are available in the size range from a few kW up to more than 100 MW. The efficiency for heat production is considerably high and heat from biomass is economically feasible. Commercial power production is based on steam cycles. The specific cost and efficiency of steam plants is interesting at large scale applications. Hence co-combustion of biomass with coal is promising, as it combines high efficiency with reasonable transport distances for the biomass. However, biomass combustion is related to significant pollutant formation and hence needs to be improved. To develop measures for emission reduction, the specific fuel properties need to be considered. It is shown that pollutant formation occurs due to two reasons: (1) Incomplete combustion can lead to high emissions of unburnt pollutants such as CO, soot, and PAH. Although improvements to reduce these emissions have been achieved by optimized furnace design including modeling, there is still a relevant potential of further optimization. (2) Pollutants such as NOX and particles are formed as a result of fuel constituents such as N, K, Cl, Ca, Na, Mg, P, and S. Hence biomass furnaces exhibit relatively high emissions of NOX and submicron particles. Air staging and fuel staging have been developed as primary measures for NOX reduction that offer a potential of 50% to 80% reduction. Primary measures for particle reduction are not yet safely known. However, a new approach with extensively reduced primary air is presented that may lead to new furnace designs with reduced particle emissions. Furthermore, assisting efforts for optimized plant operation are needed to guarantee low emissions and high efficiency under real-world conditions.
A new model for describing the fuel-N oxidation to NO and N2 in biomass combustion is presented. The formulation is based on the assumption that in biomass combustion the fuel-N is released as ammonia. The model consists of two formal reactions describing the oxidation of volatilized fuel-N: NH3 + O2 = NO + H2O + 1/2H2, and NH3 + NO = N2 + H2O + 1/2H2. The rate expressions were extracted from perfectly stirred reactor simulations using a comprehensive mechanism. The rate of NH3 oxidation to NO was determined by adding the net rate of all reactions involving NH3. The rate was determined at conditions where the formation of NO was dominating. The rate of the reaction between NH3 and NO was obtained by adding the net reaction rate of all reactions involving N2. The following rate expressions were obtained: r1 = 1.21 × 108T2e-8000/T[NH3][O2]0.5[H2]0.5, and r2 = 8.73 × 1017T-1e-8000/T[NH3][NO]. The rates are given in mole·cm-3·s-1, the temperature in K and the concentrations in mole·cm-3. The model is developed for use in CFD modeling of full-scale combustion devises. It describes the fuel-N chemistry well in flame-like conditions. In flue gas it predicts faster conversion than expected by a comprehensive mechanism.
The thermal decomposition of nitromethane under highly diluted conditions in shock tubes has been analyzed in terms of a detailed chemical kinetic model. The experimental data were adopted from Glänzer and Troe, Hsu and Lin, and Zhang and Bauer, respectively; they cover the temperature range 1000–1400 K and pressures from 0.5 to 6.0 bar. Based on these results, rate constants for the reactions CH3NO2(+M) ⇌ CH3 + NO2(+M) (R1) and CH3 + NO2 ⇌ CH3O + NO (R14) have been re-examined.The high and low pressure limits for reaction (R1) determined by Glänzer and Troe have been shown to be consistent with more recent shock tube data, provided a center broadening parameter is introduced to describe the fall-off behavior. Our reinterpretation of the shock tube results of Glänzer and Troe together with room temperature measurements indicate that the rate constant for (R14) decreases slightly with temperature, as k14 = 4.0 · 1013T−0.2 cm3mol−1s−1. At high temperatures and atmospheric pressure this reaction is more than an order of magnitude faster than recombination of CH3 and NO2 to form nitromethane. Based on the available data for the forward and reverse rate of reaction (R1) a value of 66.7 ± 2.0 cal/(mol K) for the entropy S0,298 of CH3NO2 is estimated. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 591–602, 1999
A detailed chemical kinetic model for homogeneous combustion of the light hydrocarbon fuels CH4 and C2H6 in the intermediate temperature range roughly 500–1100 K, and pressures up to 100 bar has been developed and validated experimentally. Rate constants have been obtained from critical evaluation of data for individual elementary reactions reported in the literature with particular emphasis on the conditions relevant to the present work. The experiments, involving CH4/O2 and CH4/C2H6/O2 mixtures diluted in N2, have been carried out in a high-pressure flow reactor at 600–900 K, 50–100 bar, and reaction stoichiometries ranging from very lean to fuel-rich conditions. Model predictions are generally satisfactory. The governing reaction mechanisms are outlined based on calculations with the kinetic model. Finally, the mechanism was extended with a number of reactions important at high temperature and tested against data from shock tubes, laminar flames, and flow reactors. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 778–807, 2008
This paper presents results from lean CO/H2/O2/NOx oxidation experiments conducted at 20–100 bar and 600–900 K. The experiments were carried out in a new high-pressure laminar flow reactor designed to conduct well-defined experimental investigations of homogeneous gas phase chemistry at pressures and temperatures up to 100 bar and 925 K. The results have been interpreted in terms of an updated detailed chemical kinetic model, designed to operate also at high pressures. The model, describing H2/O2, CO/CO2, and NOx chemistry, is developed from a critical review of data for individual elementary reactions, with supplementary rate constants determined from ab initio CBS-QB3 calculations. New or updated rate constants are proposed for important reactions, including OH + HO2 ⇋ H2O + O2, CO + OH ⇋ [HOCO] ⇋ CO2 + H, HOCO + OH ⇋ CO + H2O2, NO2 + H2 ⇋ HNO2 + H, NO2 + HO2 ⇋ HONO/HNO2 + O2, and HNO2(+M) ⇋ HONO(+M). Further validation of the model performance is obtained through comparisons with flow reactor experiments from the literature on the chemical systems H2/O2, H2/O2/NO2, and CO/H2O/O2 at 780–1100 K and 1–10 bar. Moreover, introduction of the reaction CO + H2O2 → HOCO + OH into the model yields an improved prediction, but no final resolution, to the recently debated syngas ignition delay problem compared to previous kinetic models. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 454–480, 2008
The oxidation of methanol in a flow reactor has been studied experimentally under diluted, fuel-lean conditions at 650–1350 K, over a wide range of O2 concentrations (1%–16%), and with and without the presence of nitric oxide. The reaction is initiated above 900 K, with the oxidation rate decreasing slightly with the increasing O2 concentration. Addition of NO results in a mutually promoted oxidation of CH3OH and NO in the 750–1100 K range. The experimental results are interpreted in terms of a revised chemical kinetic model. Owing to the high sensitivity of the mutual sensitization of CH3OH and NO oxidation to the partitioning of CH3O and CH2OH, the CH3OH + OH branching fraction could be estimated as = 0.10 ± 0.05 at 990 K. Combined with low-temperature measurements, this value implies a branching fraction that is largely independent of temperature. It is in good agreement with recent theoretical estimates, but considerably lower than values employed in previous modeling studies. Modeling predictions with the present chemical kinetic model is in quantitative agreement with experimental results below 1100 K, but at higher temperatures and high O2 concentration the model underpredicts the oxidation rate. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 423–441, 2008
A four step mechanism for combustion of methane in perfectly stirred reactors with special emphasis on formation and destruction of hydrocarbon radicals has been developed using steady state and partial equilibrium assumptions for minor species. The reduced mechanism has been extended to include the nitrogen chemistry with NO and HCN as independent reactive species. The reduced nitrogen scheme includes thermal NO and prompt NO formation, as well as the NO to HCN recycle reactions, and conversion of HCN to NO and N2.We have tested the reduced mechanism by comparing perfectly stirred reactor calculations performed with full and reduced chemistry over a wide range of stoichiometries, temperatures and residence times. The reduced model generally provides a good description of the methane oxidation process as well as formation and destruction of nitrogen oxides. However, at low temperatures or very fuel-rich conditions reduced model predictions deteriorate, partly due to neglection of the C2-chemistry and partly because the OH partial equilibrium assumption becomes less accurate.
Bark pellets have been pyrolyzed in a fluidized bed reactor at temperatures between 700 and 1000 °C. Identified nitrogen-containing species were hydrogen cyanide (HCN), ammonia (NH3), and isocyanic acid (HNCO). Quantification of HCN and to some extent of NH3 was unreliable at 700 and 800 °C due to low concentrations. HNCO could not be quantified with any accuracy at any temperature for bark, due to the low concentrations found. Since most of the nitrogen in biomass is bound in proteins, various protein-rich model compounds were pyrolyzed with the aim of finding features that are protein-specific, making conclusions regarding the model compounds applicable for biomass fuels in general. The model compounds used were a whey protein isolate, soya beans, yellow peas, and shea nut meal. The split between HCN and NH3 depends on the compound and temperature. It was found that the HCN/NH3 ratio is very sensitive to temperature and increases with increasing temperature for all compounds, including bark. Comparing the ratio for the different compounds at a fixed temperature, the ratio was found to decrease with decreasing release of volatile nitrogen. The temperature dependence implies that heating rate and thereby particle size affect the split between HCN and NH3. For whey, soya beans, and yellow peas, HNCO was also quantified. It is suggested that most HCN and HNCO are produced from cracking of cyclic amides formed as primary pyrolysis products. The dependence of the HNCO/HCN ratio on the compound is fairly small, but the temperature dependence of the ratio is substantial, decreasing with increasing temperature. The release of nitrogen-containing species does not seem to be greatly affected by the other constituents of the fuel, and proteins appear to be suitable model compounds for the nitrogen in biomass.
The fuel staging for NOx reduction in biomass combustion in fixed bed systems was analyzed. Wood chips and chipboards were used to study the influences of the main process parameters such as stoichiometric ratio, temperatures, residence time, and the fuel properties on the conversion of fuel nitrogen. The nitrogen conversion during air and fuel staging were investigated using a furnace model based on ideal flow patterns as perfectly stirred reactors and plug flow reactors. Results showed that fuel staging was suitable for the combustion of nonpulverized wood fuels in fixed bed combustion, and the temperature range for reduction of nitrogen species by homogeneous reactions was lower than for air staging.
The oxidation of NH3 under fuel-rich conditions and moderate temperatures has been studied in terms of a chemical kinetic model over a wide range of conditions, based on the measurements of Hasegawa and Sato. Their experiments covered the fuels hydrogen (0 to 80 vol%), carbon monoxide (0 to 95 vol%), and methane (0 to 1.5 vol%), stoichiometries ranging from slightly lean to very fuel rich, temperatures from 300 to 1330 K, and NO levels from 0 to 2500 ppm. A detailed reaction mechanism has been established, based on earlier work on ammonia oxidation in flames and on selective noncatalytic reduction of NO by NH3. The kinetic model reproduces the experimental trends qualitatively over the full range of conditions covered, and often the predictions are in quantitative agreement with the observations. Using reaction path analysis and sensitivity studies, the major reaction paths have been identified. The comparatively low temperatures in the present study, as well as the presence of NO, promote the reaction path NH3→NH2→N2 (directly or via NNH), rather than the sequence NH3→NH2→NH→N important in flames. The major conversion of fuel-N species to N2 occurs by reaction of amine radicals with NO, in particular NH2+NO. In the presence of CH4, NO is partly converted to cyanides by reaction with CH3. The mechanism is recommended for modeling the reduction of NO by primary measures in the combustion of biomass, since it has been validated under conditions resembling the conversion of early nitrogenous volatile species in a staged combustion process. It is also appropriate for studies of NO formation in the combustion of gas from gasifying coal.
Experimental and detailed chemical kinetic modeling work has been performed to investigate the role of hydrocarbon oxidation in NO-NO2 conversion. An atmospheric pressure., quartz flow reactor was used to examine the dependence of NO oxidation to NO2 by hydrocarbon type, reaction temperature, and residence time. The five hydrocarbons examined were methane, ethylene, ethane, propene, and propane. In the experiment, probe measurement of the species concentrations was performed in the flow reactor using a mixture of NO(20 ppm)/air/hydrocarbon(50 ppm) at residence times from 0.16 to 1.46 s and temperatures from 600 to 1100 K. In the chemical kinetic calculation, the time evolution of NO, NO2, hydrocarbons, and reaction intermediates were evaluated for a series of the hydrocarbons and the temperatures. The chemical mechanism consisted of 639 reversible reactions and 126 species.Experimental results indicate that, in general, ethylene and propane effectively oxidize NO to NO2 while methane is less effective. The calculation indicates the important chemical kinetic features that control NO-NO2 conversion for each hydrocarbon type. The dependence of NO-NO2 conversion with hydrocarbon type and temperature is qualitatively reproduced by the calculation. The calculation indicates that all five hydrocarbons oxidize NO to NO2 predominantly through NO+HO2 ahNO2+OH and that the contribution of oxidation by RO2 and HORO2 is minor. Highest effectiveness comes from hydrocarbons that produce reactive radicals (i.e., OH, O atom) that promote hydrocarbon oxidation and lead to additional HO2 production. On the other hand, if hydrocarbons produce radicals, such as methyl and allyl, which resist oxidation by O2, then these radicals tend to reduce NO2 to NO. Experimental results show that the effectiveness of hydrocarbons varies appreciably with temperature and only within the low-temperature range. Propane shows the greatest NO-NO2 conversion for the lowest temperatures. This ability is primarily due to the hydroperoxy-propyl plus O2 reactions as indicated by the sensitivity analysis results.
The CH4/O2/NOx system is investigated in a laboratory-scale high pressure laminar flow reactor with the purpose of elucidating the sensitizing effects of NOx on CH4 oxidation at high pressures and medium temperatures. Experiments are conducted at 100, 50, and 20 bar, 600–900 K, and stoichiometric ratios ranging from highly reducing to oxidizing conditions. The experimental results are interpreted in terms of a detailed kinetic model drawn from previous work of the authors, including an updated reaction subset for the direct interactions of NOx and C1–2 hydrocarbon species relevant to the investigated conditions. The results reveal a significant decrease in the initiation temperature upon addition of NOx. A similar effect is observed with increasing pressure. The sensitizing effect of NOx is related to the hydrocarbon chain-propagating NO/NO2 cycle operated by NO2+CH3⇋NO+CH3O and NO+CH3OO⇋NO2+CH3O as well as the formation of chain-initiating OH radicals from interactions between NO/NO2 and the H/O radical pool. At low temperatures, reactions between NO/NO2 and CH3O/CH2O also gain importance. The results indicate a considerable intermediate formation of nitromethane (CH3NO2) as a characteristic high-pressure phenomenon. The formation of CH3NO2 represents an inactivation of NOx, which may result in a temporary reduction of the overall hydrocarbon conversion rate.
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