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air premixed flames
Fuel 254 (2019) 115693
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Full Length Article
Experimental and kinetic modelling investigation on NO, CO and NH3 emissions from NH3/CH4/air premixed flames
T
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C. Filipe Ramosa, Rodolfo C. Rochaa, Pedro M.R. Oliveiraa, Mário Costaa, , Xue-Song Baib a b
IDMEC, Mechanical Engineering Department, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal Division of Fluid Mechanics, Lund University, P.O. Box 118, S-22100 Lund, Sweden
A R T I C LE I N FO
A B S T R A C T
Keywords: Ammonia Methane Emissions Experimental Kinetic simulations
Ammonia (NH3) is regarded as one of the most viable alternatives to produce carbon-free energy. However, its low flammability and high NOx emissions associated with its combustion inhibit the implementation of pure NH3 as a fuel. Dual-fuel approaches with NH3 and more reactive fuels have been proposed; however, information on pollutant emissions is still scarce. The present work focuses on quantifying the gaseous pollutant emissions from the combustion of mixtures of NH3 and CH4 as a function of the equivalence ratio and amount of NH3 in the fuel mixture. A premixed laminar burner was fired with various mixtures of NH3/CH4 in air with equivalence ratios of 0.8, 0.9 and 1, and a fixed thermal input of 300 W. The NH3 molar fraction in the fuel mixture was varied from 0 (pure CH4) up to 0.7. Gas temperatures along the burner axis and emissions of NOx, CO and NH3 were measured for all conditions. The results were compared with kinetic simulations using recent chemical kinetic mechanism models. The experimental results showed that the NOx concentration initially increases as the fraction of NH3 in the fuel mixture rises up to 0.5, decreasing afterwards. Furthermore, NOx emissions decrease as the equivalence ratio is reduced towards fuel-lean conditions. CO and NH3 emissions are fairly low indicating complete combustion of CH4 and NH3. The chemical kinetic mechanisms considered showed fair agreement with the experimental trends, with the NOx and CO emissions being over-predicted for most conditions.
1. Introduction The global energy demand is expected to keep growing in the next decades and liquid fossil fuels are expected to be the major source of energy [1]. Concerns about global warming and increasingly severe regulations aimed at reaching the Paris Agreement goals motivate research in fast and cost-effective decarbonisation. Therefore, carbon-free fuels have gathered interest for their potential of supplying energy devoid of CO2 emissions. Ammonia (NH3) has been considered as a possible replacement of fossil fuels due to its carbon-free composition, energy density [2] and for being safe to store, especially when compared to hydrogen. Although its current production is mainly dependent on steam reforming of natural gas, carbon capture and storage (CCS) solutions can be implemented for achieving carbon-neutral synthesis of NH3 [3]. Furthermore, new processes of producing NH3 from renewable energy sources are in development [4,5], thus ensuring the complete absence of carbon in its synthesis process. The long-lasting use of NH3 as a refrigerant and component in the fertilizer industry has resulted in a robust infrastructure for handling, storage and transportation [2]. Therefore, the possible implementation
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of NH3 as a primary energy source has a unique economic feasibility compared to other carbon-free alternatives, when the analysis is performed on a distribution-storage-handling basis for long storage periods [6]. Research has shown that NH3 can be employed as an energy resource in a wide range of devices, including fuel cells [7], engines [8] and gas turbines [9–11]. However, NH3 combustion generates very high NOx emissions. One of the first documented real-word applications of NH3 as energy source was in Belgian motor-buses, in 1945, in which a fuel mixture of NH3 and coal gas was used to power these vehicles for more than a year [12]. This successful implementation spawned some fundamental studies throughout the 1960’s [13–15]. Starkman and Samuelson [13] investigated the properties of the reactions taking place in a research engine when firing NH3, noting that the flame propagation of NH3 was 30% slower than that of iso-octane. Verkamp et al. [14] experimentally studied pure NH3 flames in a flat-flame burner, concluding that the range of equivalence ratios that allowed for ignition was much narrower. Pratt [15] examined NH3 combustion in gas turbines, observing that the main difficulty when burning NH3 is the slow chemical reaction between this substance and air, aggravated by the much less effective
Corresponding author at: Mechanical Engineering Department, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal. E-mail address: [email protected] (M. Costa).
https://doi.org/10.1016/j.fuel.2019.115693 Received 6 March 2019; Received in revised form 7 June 2019; Accepted 20 June 2019 Available online 26 June 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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pressure (4 kPa) CH4/NH3/O2/Ar flames for a range of NH3 fractions in the fuel mixture (0–1) in a McKeena burner. The NOx patterns showed a constant increase with NH3 addition up to 1. Okafor et al. [23] developed a novel mechanism for NH3/CH4 mixtures based on those from Tian et al. [22] and GRI-mech 3.0 [24], validating it against flame speed data from a cylindrical constant volume vessel, for CH4/NH3/air flames at atmospheric conditions and molar fractions of NH3 in the fuel mixture from 0 to 0.3. Recently, the UC San Diego Combustion group [25] updated its sub-mechanism of nitrogen-related species with reactions and rates from recent works, which, when integrated to their full mechanism, allows it to simulate CH4/NH3 fuel mixtures. Fuel mixtures of NH3 and CH4 appear to have satisfactory flammability conditions, and the understanding of the reactions involved in their combustion constitutes a middle-step towards the implementation of NH3 as a viable energy resource. Given that NOx emissions are the major concern inhibiting a wide acceptance of NH3 as fuel, the existing experimental research is neither sufficient for a thorough understanding of the processes behind their formation, nor for an extensive validation of the chemical kinetic mechanisms. Therefore, the main aim of this work is to contribute with quantitative data on how different variables influence the emissions of NOx, CO and NH3, when burning fuel mixtures of NH3 and CH4. Furthermore, the experimental measurements are compared with kinetic simulations to evaluate the performance of the current chemical kinetic mechanisms. In addition, rate of production and sensitivity analyses were performed for the kinetic models used, allowing for the understanding of the reactions that most contribute to the formation or consumption of NO.
mixing of the constituents, comparing to more common fuels. More recently, Hayakawa et al. [16] investigated the laminar burning velocity of pure NH3 premixed flames. The maximum flame speed was found to be ∼7 cm/s for an equivalence ratio of 1.1 and an initial pressure of 0.1 MPa, which is rather low when compared to conventional fuels such as CH4 (∼35 cm/s). This adds to the findings of low flammability of pure NH3 encountered earlier [13–15]. To circumvent these difficulties, flammability enhancements have been studied through mixtures of NH3 and other fuels [8–11,17–19]. Reiter and Kong [8] investigated the combustion and emission characteristics of a compression-ignition engine using blends of NH3 and diesel as fuel. The most favourable fuel efficiency was found for ratios around 1:1 of NH3 to diesel. However, not only the CO and unburned hydrocarbon emissions were higher than they would be for independent diesel use, but also the NOx emissions were significantly high, especially when NH3 amounted for more than 40% of the total fuel energy. Additionally, although the overall NH3 conversion was approximately 100%, NH3 could still be found at the exhaust in values between 1000 and 3000 ppm. Valera-Medina et al. [11] studied NH3/H2 premixed flames in a swirl combustor. The results showed that the flame velocity achieved was similar to CH4, with the drawback of narrow operational conditions due to the high diffusivity of H2. Furthermore, the excess production of OH and O radicals contributed for high NOx emissions of up to 104 ppm, but NH3 emissions were negligible. Konnov et al. [17] investigated NH3-doped CH4 flames stabilized on a perforated plate burner at atmospheric pressure, with the aim of understanding fuelbound NOx emissions. While employing fuel mixtures with 0.5 vol% of NH3, they concluded that NOx emissions peaked around stoichiometric conditions, decreasing for lower and higher equivalence ratios. Similar results were reported by Valera-Medina et al. [18], who investigated NH3/CH4 blends in an atmospheric-pressure swirl burner. Mixtures with more than 66% of NH3 in the fuel mixture volume yielded maximum NOx emissions for an equivalence ratio of 0.9. The decrease in emissions was more pronounced for higher equivalence ratios than for lower ones. A similar pattern was found for the same fuel mixtures in gas turbines [9]. Kurata et al. [10] conducted an experimental research with pure NH3 and NH3/CH4 blends in a gas turbine to evaluate the influence of the NH3 content in the fuel mixture. Pure NH3 combustion proved to have the lowest NOx emissions, with less than 1000 ppm. However, it was at the cost of fuel-rich combustion zones in the turbine, resulting in NH3 emissions of more than 1500 ppm. Fuel mixtures of NH3 and CH4 reduced the NH3 considerably to less than 30 ppm, although the NOx emissions increased overall. The increase of the NH3 amount in the NH3/CH4 blends leads to the growth of NOx emissions, until NH3 reaches about 60% of the energy in the fuel mixture. Afterwards, emissions drop. In terms of chemical kinetics, several studies have tried to describe the behaviour of NH3 and its mixtures as fuels. One of the first investigations on the sub-mechanisms involved in the formation of NO in hydrocarbon combustion was conducted by Dagaut et al. [20], who studied the oxidation of HCN, one of the most important pathways in the formation of pollutants in NH3/CH4 combustion. These authors presented a novel detailed kinetic mechanism based on previous works, containing a pathway to NH3 and CH4 oxidation. Xiao et al. [19] adapted the commonly used Konnov mechanism [21], adjusting Arrhenius parameters in the reactions in order to fit ignition delay times from the literature; these authors also conducted one dimensional, freely propagating premixed laminar flame simulations with several NH3/CH4 mixtures for different equivalence ratios. NO emissions were found to increase as the quantity of NH3 in the fuel mixture grew, thus achieving a similar pattern to that found in [10]. As for the influence of equivalence ratio on NO emissions, a maximum was found for near stoichiometry, while emissions for fuel-lean and fuel-rich conditions were lower [9,17,18]. Tian et al. [22] improved a previously developed mechanism by updating the reactions related to the CH4/NH3 chemistry, validating it against gas species concentration data for low
2. Experimental Fig. 1 shows a schematic of the experimental setup used in this work. This premixed laminar flame burner has been featured in a previous study from the present research group involving NH3 [26]. It consists of an inverted stainless-steel cone, fixed on a mount, with a hollow interior. It is actively cooled around the boundaries covering the flame position, resourcing to tap water. The interior is filled with small alumina spheres and several transversal stainless-steel meshes in order to achieve flow uniformity in the cross-section. A quartz tube sits on top of the base, allowing for flame development and visual observation. The quartz tube has a length of 50 cm, an external diameter of 4.5 cm and an internal diameter of 3.3 cm, and is insulated with ceramic wool to reduce heat losses. On the top of the quartz tube rests a stainless-steel chimney. NH3 (99.98% purity) and CH4 (99.995% purity), from gas cylinders, are fed to the intake of the burner, premixed with air from a compressor. The flow rates of NH3, CH4 and air are measured with digital flowmeters. Temperature measurements were taken with fine wire (27 μm) thermocouples of platinum/platinum: 13% rhodium. The thermocouple was deployed within a probe placed on a vertical slider, which could traverse the centreline of the quartz tube. The probe was comprised of two 350 μm wires of the same composition as that of the hot junction, which ran inside an alumina sheath and supported the thermocouple at the exposed end. A compensated wire connects the probe to an amplifying board, which is in turn connected to a computer. Temperature measurements along the vertical centreline were taken at distances from the flame between 2.5 cm and 40 cm. The uncertainty due to radiation heat transfer was estimated to be less than 5% by considering the heat transfer by convection and radiation between the thermocouple bead and the surroundings [26]. For each test condition, measurements were repeated at least three times – each measurement was sampled at 100 Hz during 30 s. Reproducibility of the data was, on average, within 5% of the mean value. Combustion products were sampled from the centreline of the tube, at 30 cm from the flame, using a stainless steel, water-cooled probe. The inner diameter of the probe through which the gases were evacuated was of 1.3 mm. The gas composition analysis was performed using a 2
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Fig. 1. Schematic of the experimental set-up.
3. Kinetic modelling
chemiluminescent analyser (Horiba, model PG-250) for measurements of NOx concentrations, a non-dispersive infrared analyser (Horiba, model CMA-331A) for measurements of CO and CO2 concentrations, and a magnetic pressure analyser (Horiba, model CMA-331A) for measurements of O2 concentrations. For NOx concentration measurements, nitrogen (99.9% purity) from a gas cylinder was added to the sampled gas for dilution, in a proportion of 2.6:1 in volume, in order to keep values to the scale of the analyser. As in the temperature measurements, for each test condition, measurements were repeated at least three times – each measurement was sampled at 100 Hz during 10 min. Reproducibility of the data was, on average, within 5% of the mean value for the concentrations of NOx, CO2, and O2, and within 15% for the concentrations of CO. Finally, Gastec detector tubes were employed to measure the emissions of NH3. These tubes have a minimum detection limit of 2 ppm, while allowing for quantitative assessments starting at 10 ppm. Again, for each test condition, measurements were repeated at least three times, without NH3 being detected for any of the conditions examined in this work. According to the manufacturer, the uncertainty of the present detector is within 5% of the measured value. The fuel thermal input of the NH3/CH4 fuel mixtures was fixed throughout the experiments at a value of 300 W. The equivalence ratios employed were 0.8, 0.9 and 1. It should be stressed that for equivalence ratios below 0.8 the flame was unstable. The NH3 mole fraction in the fuel mixture, defined as xNH3 = VNH3/(VNH3 + VCH4 ) , was varied from 0 to the achievable maximum in 0.1 increments, for each of the equivalence ratios. For the equivalence ratios of 0.8, 0.9 and 1, the maximum achievable xNH3 were 0.5, 0.6 and 0.7, respectively. Beyond these values, the flame was unstable and blow-off occurred. Photographs of the flames with different percentages of NH3 in the fuel mixture were taken in order to assess the colour-shift that occurs as the presence of NH3 in the fuel mixture grows. The photographs were taken at an angle of about 30° from the vertical centreline, with an SLR camera (Canon EOS 700D), with a focal distance of 50 mm, aperture of ƒ/5, shutter speed of 1/20 s and ISO of 6400.
The kinetic simulations were performed with Cantera [27], a code library for chemical kinetics calculations, using Python language. The mechanisms developed by Okafor et al. [23], Dagaut et al. [20] and the UC San Diego Combustion group [25] were chosen for the calculations, due to their different sub-mechanisms and their successful convergence for all studied equivalence ratios. For additional information on existing chemical mechanisms, the reader is referred to a very recent study [28], where the present group evaluated the performance of 10 chemical mechanisms available in the literature. The numerical methodology considered in this work is based on the research by Barbas et al. [26], which proved to be successful for the same burner employed here. The flame region, from the base of the burner up to the flame front, is modelled as an adiabatic, burner-stabilized one-dimensional premixed laminar flame, defining mesh points along the burner vertical axis. A first adiabatic simulation is done for a flame domain of 4 cm up from the burner metallic grid, generating a temperature profile. From this profile, temperatures are considered only up to a certain point. Barbas et al. [20] arbitrated this point at 1.5 cm, deemed enough to define the flame region for the thermal input used in that experiment. For the present work, the thermal input is much lower, holding the flame much closer to the burner grid, therefore demanding a better definition of the flame front and the flame region. For this purpose, the calculated adiabatic flame temperatures are then considered up to the end of the predicted flame region, a point numerically calculated in Cantera [27] as:
z = z ign + δ
(1)
where z ign is the ignition point, defined as the point where the derivative of the temperature in the flame length, dT/dz, is maximum, while δ is the flame thickness, defined by the following equation:
δ=
Tb − Tu max
3
( ) dT dz
(2)
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where Tb is the temperature of the burned gases and Tu the temperature of the unburned gases. Thereafter, the predicted temperature profile is added to the one taken from the experiments, up to the end of the burner length (40 cm). However, the experimental temperatures are used starting at 4 cm, a point for which the residence times are higher than 40 ms for all conditions. Amato et al. [29] considers it the minimum residence time for which a real burner behaves as an adiabatic flame, a criterion used by Barbas et al. [26] to define the first measured temperature, in their case, 10 cm. The combined temperature profile is then used as input by the software in another simulation of a geometrically equivalent burner-stabilized one-dimensional premixed laminar flame, generating chemical species concentration profiles along the burner axis. Finally, sensitivity and rate of production analysis were performed for NO, for equivalence ratios of 0.8 and 1, and for xNH3= 0.2 and the maximum NH3 content in the fuel mixture for each flame. Due to their different behaviour and full convergence for all cases, only the mechanisms of Okafor et al. [23] and the UC San Diego Combustion group [25] were chosen. Another sensitivity analysis was done for CO under stoichiometric conditions, for xNH3= 0.5, at the point of maximum CO consumption, in order to understand what influences its concentration in the exhaust gas.
4. Results and discussion Fig. 2 shows photographs of the flames for various amounts of NH3 in the fuel mixture – the photographs were taken without the insulation around the quartz tube. The colour for pure CH4 (Fig. 2a) is not entirely blue, because of the incandescence of the steel mesh immediately below the flame. Nevertheless, a blue aura can still be seen in the periphery. As the amount of NH3 in the fuel mixture increases (cf. Fig. 2a to e), the colour of the flame turns progressively whiter. Although reflections can be observed on the inside of the quartz tube, the flames are reasonably flat and remain stable above the burner. Fig. 3 shows on-axis temperature measurements for various NH3/ CH4 fuel mixtures with equivalence ratios of 0.8, 0.9 and 1. It is observed that temperature decreases along the axial distance, which is an expected behaviour due to heat losses. As the amount of NH3 in the fuel mixture increases, the temperature profiles increase as well, regardless of the equivalence ratio. It is worth mentioning that adiabatic flame simulations retrieved temperature behaviours similar to the experimental results, presenting higher temperatures for higher NH3 contents
a)
b)
Fig. 3. Temperature measurements for NH3/CH4 fuel mixtures. a) ɸ = 0.8; b) ɸ = 0.9; c) ɸ = 1.
c)
d)
e)
Fig. 2. Still photographs of the flames for various amounts of NH3 in the fuel mixture, for a thermal input of 300 W and ɸ = 1. a) Pure CH4, b) xNH3 = 0.2, c) xNH3 = 0.4, d) xNH3 = 0.6, e) xNH3 = 0.7. 4
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Fig. 4. NOx measurements and predictions for NH3/CH4 fuel mixtures. a) ɸ = 0.8; b) ɸ = 0.9; c) ɸ = 1.
Fig. 5. CO measurements and predictions for NH3/CH4 fuel mixtures. a) ɸ = 0.8; b) ɸ = 0.9; c) ɸ = 1.
in the fuel mixture. Fig. 4 shows measured and predicted NOx emissions as a function of the amount of NH3 in the fuel mixture for equivalence ratios of 0.8, 0.9 and 1. The presence of NOx in the exhaust gases is significant. The measured NOx increases as the amount of NH3 in the fuel mixture rises up to about 0.5. The values reach a maximum at that point and start decreasing afterwards. This pattern is similar for the three equivalence ratios and is in accordance with the literature [10,19]. The maximum measured NOx values are around 2800, 3600 and 4300 ppm, corrected for 6% of O2, for the equivalence ratios of 0.8, 0.9 and 1, respectively. The predictions do not reproduce well the experimental trends, in particular for ɸ = 0.8 and 0.9. As the fraction of NH3 in the fuel mixture increases, the predicted NOx emissions also increase continuously, but do not attain a maximum like it is observed in the experimental data. Note that the mechanism of Dagaut et al. [20] did not converge for ɸ = 0.8 and xNH3 = 0.5 so that this result was not included in the respective graph. Overall, the mechanism of Okafor et al. [23] predicts the highest NOx emissions, followed by that of Dagaut et al. [20] and, lastly, by the San Diego mechanism [25]. As for the effect of the equivalence ratio, it can be observed that, as ɸ decreases from stoichiometry to fuel-lean conditions, the experimental overall NOx emission decreases. Despite the small range of
equivalence ratios studied here, this trend is in agreement with existing literature [18,19]. However, the simulations predicted an opposite trend – as the equivalence ratio increases, the predicted NOx emissions decrease. For this reason, the discrepancy between the measured and predicted results is larger for the lean mixtures, whereas for the stoichiometric mixture, the numerical results are rather close to the experimental counterparts. Fig. 5 shows measured and predicted CO emissions as a function of the amount of NH3 in the fuel mixture for equivalence ratios of 0.8, 0.9 and 1. The average CO concentrations are below 60 ppm, and no discernible differences are observed between stoichiometry and the fuellean conditions. Consequently, the CH4 oxidation is almost complete regardless of the equivalence ratio. The numerical results for the lean conditions are in accordance with the experimental counterparts, since the predicted CO emissions are negligible. Furthermore, there are no discernible differences among the mechanisms. However, for the stoichiometric mixtures, the three mechanisms over-predict the CO emissions by almost two orders of magnitude. The San Diego mechanism [25] predicts the lowest CO emissions for all conditions examined, while the other two mechanisms switch places for the highest predictions according to the fraction of NH3 in the fuel mixture. 5
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Fig. 6. Rate of NO production (mol/ cm3 s ) for ɸ = 0.8. (a) xNH3 = 0.2 , mechanism of San Diego [25], (b) xNH3 = 0.2 , mechanism of Okafor et al. [23], (c) xNH3 = 0.5, mechanism of San Diego [25], (d) xNH3 = 0.5, mechanism of Okafor et al. [23].
generating HCN and HNCO. In fact, since the San Diego mechanism [25] does not contain carbon-nitrogen interaction reactions, no influence of these can be anticipated. For the stoichiometric condition (ɸ = 1) and high NH3 content in the fuel mixture ( xNH3 = 0.7 ), the formation of NO exhibits similar patterns when compared to the lean condition. In the present case, however, the HNO decomposition in the presence of a third body (M) increases its relative contribution. For the consumption of NO, the patterns are also similar; however, the reactions of NO with N and NH radicals increase their importance, when compared with the lean condition and lower NH3 content in the fuel mixture. Figs. 8 and 9 show the normalized NO sensitivity on the flame front for the 15 most important reactions in the mechanisms of San Diego [25] and Okafor et al. [23] for ɸ = 0.8 and 1, respectively. For the lean condition (ɸ = 0.8) and low NH3 content in the fuel mixture ( xNH3 = 0.2 ), the mechanisms show that NO formation is very sensitive to the reaction H + O2 ⟺ O + OH, related to the hydrogen oxidation in the flame, a very exothermic reaction that influences the temperature of the flame. Other reactions that enhance NO formation are those related to the oxidation of NH3, reactions between N, NH and NH2 radicals and O2, OH and O, as well as a reaction involved in the formation of CO, HCO (+M) ⟺ CO + H (+M). On the NO mitigation side, the radicals N, NH and NH2, when reacting with NO, present the most important contribution. It suggests that the abundance of oxygen might be directly related to NO formation in the flame. Other reactions related to the combustion of CH4, such as CH3 + H (+M) ⟺ CH4 (+M), seem to contribute to the mitigation of NO, along with the reaction H + O2 (+M) ⟺ HO2 (+M) (M is H2O in the mechanism of Okafor et al. [23]), related to flame stabilization. For the lean condition (ɸ = 0.8) and high NH3 content in the fuel mixture ( xNH3 = 0.5), both mechanisms present very similar results when compared with the condition with low NH3 content, but the relative importance of the reaction H + O2 ⟺ O + OH for NO formation
As for the NH3 emissions, the presence of NH3 in the exhaust gases was less than 10 ppm regardless of the condition examined, revealing the complete oxidation of the NH3. The predictions yielded similar patterns, as the predicted NH3 values at the exhaust were less than 1 ppm for all conditions. Figs. 6 and 7 show the rate of NO production for the 15 most important reactions in the mechanisms of San Diego [25] and Okafor et al. [23] at ɸ = 0.8 and 1, respectively. For the lean condition (ɸ = 0.8) and low NH3 content in the fuel mixture ( xNH3 = 0.2 ), it can be seen that NO production occurs mostly through the HNO consumption, as well as the reactions between N and NH with oxygenated radicals (O and OH) and O2. Consumption of NO2 and N2O through their reactions with O is also ranked in the San Diego mechanism [25]. NO consumption takes place mainly through its reaction with NH/NH2 radicals, as well as with CH2 (for the mechanism of Okafor et al. [23] only) and N radicals, with the CH2 being produced during the CH4 oxidation. For the lean condition (ɸ = 0.8) and high NH3 content in the fuel mixture ( xNH3 = 0.5), NO formation paths are very similar, with HNO decomposition occurring through its reaction with OH and H radicals, and also through reactions between N and NH with O, OH and O2. In this case, the reaction H + NO (+M) ⟺ HNO (+M), which implies the decomposition of HNO in the presence of a third body (M), increases in importance. In addition, NO consumption is mainly driven by its reaction with N, NH and NH2 radicals, generating N2, H2O, NO2 and N2O and intermediate radicals. The differences between the mechanisms are mainly due to the relative importance of each reaction. For the stoichiometric condition (ɸ = 1) and low NH3 content in the fuel mixture ( xNH3 = 0.2 ), the NO formation is due to the same set of reactions seen under the lean condition, with enhanced contribution from the reaction H + HNO ⟺ H2 + NO. The NO consumption also comes from its reaction with nitrogen-containing radicals. The mechanism of Okafor et al. [23], contrarily to the mechanism of San Diego [25], counts with the effect of CH and CH2 in the NO consumption, 6
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Fig. 7. Rate of NO production (mol/ cm3 s ) for ɸ = 1. (a) xNH3 = 0.2 , mechanism of San Diego [25], (b) xNH3 = 0.2 , mechanism of Okafor et al. [23], (c) xNH3 = 0.7 , mechanism of San Diego [25], (d) xNH3 = 0.7 , mechanism of Okafor et al. [23].
Fig. 8. Normalized NO sensitivity on the flame front for ɸ = 0.8. (a) xNH3 = 0.2 , mechanism of San Diego [25], (b) xNH3 = 0.2 , mechanism of Okafor et al. [23], (c) xNH3 = 0.5, mechanism of San Diego [25], (d) xNH3 = 0.5, mechanism of Okafor et al. [23]. 7
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Fig. 9. Normalized NO sensitivity on the flame front for ɸ = 1. (a) xNH3 = 0.2 , mechanism of San Diego [25], (b) xNH3 = 0.2 , mechanism of Okafor et al. [23], (c) xNH3 = 0.7 , mechanism of San Diego [25], (d) xNH3 = 0.7 , mechanism of Okafor et al. [23].
Fig. 10. Normalized CO sensitivity on the maximum CO consumption point for ɸ = 1. (a) xNH3 = 0.5, mechanism of San Diego [25], (b) xNH3 = 0.5, mechanism of Okafor et al. [23].
reactions related to the NH3 oxidation and the H2/O2 sub-mechanism. For this condition, there is an important contribution to NO from the decomposition of HNO with a third body (M). NO consumption, at this point, is very significative, thus leading to higher sensitivity of reactions between NO and N, NH and NH2, especially with the first, which is the most important. The reactions of HO2 formation also influence NO mitigation, as well as CH3 + H (+M) ⟺ CH4 (+M), but less significatively than for flames with low NH3 content. In brief, it can be concluded that both mechanisms present slight differences in terms of reactions’ rate of production and sensitivity. The HNO pathway seems to be the most important for the production of NO, while the NO consumption is mainly driven by its reaction with N, NH and NH2 radicals. On the sensitivity side, it can be concluded that the reactions involved in the H2/O2 sub-mechanism, which regulates the amount of O, H and OH radicals as well as the temperatures, are those that most affect the process of NO formation and consumption. The
increases, as that of the reaction between NO and NH for NO mitigation. The results for the mechanism of Okafor et al. [23] also point to a lower sensitivity to the reaction between CH4 and OH, expected due to the smaller amount of that component in the fuel mixture. For the stoichiometric condition (ɸ = 1) and low NH3 content in the fuel mixture ( xNH3 = 0.2), the NO formation is most sensitive to a similar set of reactions when compared to the lean condition. In the present case, the reaction HCO (+M) ⟺ CO + H (+M) (M is H2O [23]) loses its relative importance, as well as the CH4 reaction with OH. The NO consumption also presents similar patterns; however, the reaction between N and NO becomes the most important one for the mechanism of San Diego [25] in this case. For both mechanisms, the results also show that the reaction CH3 + H (+M) ⟺ CH4 (+M) increases its importance for NO mitigation. For the stoichiometric condition (ɸ = 1) and high NH3 content in the fuel mixture ( xNH3 = 0.7), the NO formation is mainly sensitive to 8
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(FCT), through IDMEC, under LAETA, project UID/EMS/50022/2013. The authors would like to express appreciation to the COST Action CM1404 “Chemistry of Smart Energy Carriers and Technologies (SMARTCATS)”.
reactions related to the oxidation of CH4 are least important for both rates of production and sensitivity, and seem to have a mitigating effect on NO formation. Fig. 10 shows the normalized CO sensitivity at the maximum CO consumption point to the 15 most important reactions in the mechanisms of San Diego [25] and Okafor et al. [23] for ɸ = 1. As expected, the bulk of the contribution to CO formation and mitigation is due to reactions related to the CH4 oxidation, as well as those of the H2/O2 sub-mechanism, namely H + OH (+M) ⟺ H2O (+M) and H + O2 (+M) ⟺ HO2 (+M) (M is H2O in the mechanism of Okafor et al. [23]), which are related to the consumption of oxygenated radicals. However, reactions related to the NO formation, especially for the mechanism of Okafor et al. [23], have a positive effect on CO formation, while reactions of NO consumption, or related to it, present a negative effect. In conclusion, it appears that the addition of NH3 to the fuel mixture influences the consumption of CO; however, the direct relation between the NH3 oxidation and the high levels of CO emissions cannot be established solely by the sensitivity analysis. Since NO is mostly formed before the bulk of the CO consumption, it can be speculated that the NO formation would remove oxygen that could convert CO into CO2. Unfortunately, this was not corroborated by the experiments, which indicate very low CO levels at stoichiometry.
References [1] IEA, World Energy Outlook, 2017. [2] Zamfirescu C, Dincer I. Using ammonia as a sustainable fuel. J Power Sources 2008;185:459–65. [3] Haugen HA, Eldrup NH, Fatnes AM, Leren E. Commercial capture and transport of CO2 from production of ammonia. Energy Procedia 2017;114:6133–40. [4] Lan R, Alkhazmi KA, Amar IA, Tao S. Synthesis of ammonia directly from wet air at intermediate temperature. Appl Catal B 2014;152–153:212–7. [5] Sánchez A, Martín M. Optimal renewable production of ammonia from water and air. J Cleaner Prod 2018;178:325–42. [6] Bartels JR. A feasibility study of implementing an ammonia economy MSc Thesis Iowa State University; 2008. [7] Afif A, Radenahmad N, Cheok Q, Shams S, Kim JH, Azad AK. Ammonia-fed fuel cells: a comprehensive review. Renew Sustain Energy Rev 2016;60:822–35. [8] Reiter AJ, Kong SC. Combustion and emissions characteristics of compression-ignition engine using dual ammonia-diesel fuel. Fuel 2011;90:87–97. [9] Valera-Medina A, Marsh R, Runyon J, Pugh D, Beasley P, Hughes T, et al. Ammonia–methane combustion in tangential swirl burners for gas turbine power generation. Appl Energy 2017;185:1362–71. [10] Kurata O, Iki N, Matsunuma T, Inoue T, Tsujimura T, Furutani H, et al. Performances and emission characteristics of NH3-air and NH3-CH4-air combustion gas-turbine power generations. Proc Combust Inst 2017;36:3351–9. [11] Valera-Medina A, Pugh DG, Marsh P, Bulat G, Bowen P. Preliminary study on lean premixed combustion of ammonia-hydrogen for swirling gas turbine combustors. Int J Hydrogen Energy 2017;42:24495–503. [12] Kroch E. Ammonia – a fuel for motor buses. J Inst Pet 1945;31:213–23. [13] Starkman ES, Samuelsen GS. Flame-propagation rates in ammonia-air combustion at high pressure. Proc Combust Inst 1967;11:1037–45. [14] Verkamp FJ, Hardin MC, Williams JR. Ammonia combustion properties and performance in gas-turbine burners. Proc Combust Inst 1967;11:985–92. [15] Pratt DT. Performance of ammonia-fired gas-turbine combustors, Technical Report No. 9, Contract DA-04-200-AMC-791(x) (1967), https://apps.dtic.mil/dtic/tr/fulltext/u2/657585.pdf. [16] Hayakawa A, Goto T, Mimoto R, Arakawa Y, Kudo T, Kobayashi H. Laminar burning velocity and Markstein length of ammonia/air premixed flames at various pressures. Fuel 2015;159:98–106. [17] Konnov AA, Dyakov IV, De Ruyck J. Probe sampling measurements of NO in CH4 + O2 + N2 flames doped with NH3. Combust Sci Technol 2006;178:1143–64. [18] Valera-Medina A, Morris S, Runyon J, Pugh DG, Marsh R, Beasley P, et al. Ammonia, methane and hydrogen for gas turbines. Energy Procedia 2015;75:118–23. [19] Xiao H, Valera-Medina A, Bowen PJ. Study on premixed combustion characteristics of co-firing ammonia/methane fuels. Energy 2017;140:125–35. [20] Dagaut P, Glarborg P, Alzueta MU. The oxidation of hydrogen cyanide and related chemistry. Prog Energy Combust Sci 2008;34:1–46. [21] Konnov AA. Implementation of the NCN pathway of prompt-NO formation in the detailed reaction mechanism. Combust Flame 2009;156:2093–105. [22] Tian Z, Li Y, Zhang L, Glarborg P, Qi F. An experimental and kinetic modeling study of premixed NH3/CH4/O2/Ar flames at low pressure. Combust Flame 2009;156:1413–26. [23] Okafor EC, Naito Y, Colson S, Ichikawa A, Kudo T, Hayakawa A, et al. Experimental and numerical study of the laminar burning velocity of CH4-NH3-air premixed flames. Combust Flame 2018;187:185–98. [24] Smith GP, Golden DM, Frenklach M, Moriarty NW, Eiteneer B, Goldenberg M, et al., http://www.me.berkeley.edu/gri_mech/. [25] Chemical-Kinetic Mechanisms for Combustion Applications: San Diego Mechanism, Mechanical and Aerospace Engineering, University of California, San Diego, http:// combustion.ucsd.edu. [26] Barbas M, Costa M, Vranckx S, Fernandes RX. Experimental and chemical kinetic study of CO and NO formation in oxy-methane premixed laminar flames doped with NH3. Combust Flame 2015;162:1294–303. [27] Goodwin DG, Moffat HK, Speth RL. Cantera: An object-oriented software toolkit for chemical kinetics, thermodynamics, and transport processes, http://cantera.org. [28] da Rocha RC, Costa M, Bai X-S. Chemical kinetic modelling of ammonia/hydrogen/ air ignition, premixed flame propagation and NO emission. Fuel 2019;246:24–33. [29] Amato A, Hudak B, D’Souza P, D’Carlo P, Noble D, Scarborough D, et al. Measurements and analysis of CO and O2 emissions in CH4/CO2/O2 flames. Proc Combust Inst 2011;33:3399–405.
5. Conclusions Laminar premixed NH3/CH4/air flames were investigated in a laboratory-scale laminar flame burner, for three equivalence ratios and various fractions of NH3 in the fuel mixture. Gas temperatures along the burner axis and emissions of NOx, CO and NH3 were measured for all conditions. In parallel, three literature chemical kinetic mechanisms were used to predict the experimental data. In addition, sensitivity and rate of production analysis were carried out in order to understand the reaction pathways involved in the formation and consumption of NO and CO. The main conclusions of this study are as follows. 1) NOx is produced in high quantities regardless of the NH3 present in the fuel mixture, reaching the highest values for xNH3 = 0.5. In addition, NOx emissions decrease as the equivalence ratio is reduced towards fuel-lean conditions. 2) CO and NH3 emissions are quite low for all conditions, indicating almost full combustion of CH4 and NH3. 3) The chemical kinetic mechanisms considered showed fair agreement with the experimental trends, with the NOx and CO emissions being over-predicted for most conditions. 4) The rate of production analysis points to the HNO pathway in the formation of NO, and to its consumption through N, NH and NH2 radicals. NO formation is highly sensitive to the H2/O2 chemistry, which determines the formation of O, OH and H radicals. This behaviour is common in the three chemical mechanisms considered. 5) CO formation and consumption are sensitive to the NH3 oxidation reactions, suggesting that the consumption of oxygen in NO formation might be responsible for the over-prediction of the CO concentration in the exhaust gases. Experiments do not corroborate these findings. This calls for further revision of the chemical kinetic mechanisms for NH3 co-firing with hydrocarbon fuels. Acknowledgements This work was supported by Fundação para a Ciência e a Tecnologia
9
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Full Length Article
Experimental and kinetic modelling investigation on NO, CO and NH3 emissions from NH3/CH4/air premixed flames
T
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C. Filipe Ramosa, Rodolfo C. Rochaa, Pedro M.R. Oliveiraa, Mário Costaa, , Xue-Song Baib a b
IDMEC, Mechanical Engineering Department, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal Division of Fluid Mechanics, Lund University, P.O. Box 118, S-22100 Lund, Sweden
A R T I C LE I N FO
A B S T R A C T
Keywords: Ammonia Methane Emissions Experimental Kinetic simulations
Ammonia (NH3) is regarded as one of the most viable alternatives to produce carbon-free energy. However, its low flammability and high NOx emissions associated with its combustion inhibit the implementation of pure NH3 as a fuel. Dual-fuel approaches with NH3 and more reactive fuels have been proposed; however, information on pollutant emissions is still scarce. The present work focuses on quantifying the gaseous pollutant emissions from the combustion of mixtures of NH3 and CH4 as a function of the equivalence ratio and amount of NH3 in the fuel mixture. A premixed laminar burner was fired with various mixtures of NH3/CH4 in air with equivalence ratios of 0.8, 0.9 and 1, and a fixed thermal input of 300 W. The NH3 molar fraction in the fuel mixture was varied from 0 (pure CH4) up to 0.7. Gas temperatures along the burner axis and emissions of NOx, CO and NH3 were measured for all conditions. The results were compared with kinetic simulations using recent chemical kinetic mechanism models. The experimental results showed that the NOx concentration initially increases as the fraction of NH3 in the fuel mixture rises up to 0.5, decreasing afterwards. Furthermore, NOx emissions decrease as the equivalence ratio is reduced towards fuel-lean conditions. CO and NH3 emissions are fairly low indicating complete combustion of CH4 and NH3. The chemical kinetic mechanisms considered showed fair agreement with the experimental trends, with the NOx and CO emissions being over-predicted for most conditions.
1. Introduction The global energy demand is expected to keep growing in the next decades and liquid fossil fuels are expected to be the major source of energy [1]. Concerns about global warming and increasingly severe regulations aimed at reaching the Paris Agreement goals motivate research in fast and cost-effective decarbonisation. Therefore, carbon-free fuels have gathered interest for their potential of supplying energy devoid of CO2 emissions. Ammonia (NH3) has been considered as a possible replacement of fossil fuels due to its carbon-free composition, energy density [2] and for being safe to store, especially when compared to hydrogen. Although its current production is mainly dependent on steam reforming of natural gas, carbon capture and storage (CCS) solutions can be implemented for achieving carbon-neutral synthesis of NH3 [3]. Furthermore, new processes of producing NH3 from renewable energy sources are in development [4,5], thus ensuring the complete absence of carbon in its synthesis process. The long-lasting use of NH3 as a refrigerant and component in the fertilizer industry has resulted in a robust infrastructure for handling, storage and transportation [2]. Therefore, the possible implementation
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of NH3 as a primary energy source has a unique economic feasibility compared to other carbon-free alternatives, when the analysis is performed on a distribution-storage-handling basis for long storage periods [6]. Research has shown that NH3 can be employed as an energy resource in a wide range of devices, including fuel cells [7], engines [8] and gas turbines [9–11]. However, NH3 combustion generates very high NOx emissions. One of the first documented real-word applications of NH3 as energy source was in Belgian motor-buses, in 1945, in which a fuel mixture of NH3 and coal gas was used to power these vehicles for more than a year [12]. This successful implementation spawned some fundamental studies throughout the 1960’s [13–15]. Starkman and Samuelson [13] investigated the properties of the reactions taking place in a research engine when firing NH3, noting that the flame propagation of NH3 was 30% slower than that of iso-octane. Verkamp et al. [14] experimentally studied pure NH3 flames in a flat-flame burner, concluding that the range of equivalence ratios that allowed for ignition was much narrower. Pratt [15] examined NH3 combustion in gas turbines, observing that the main difficulty when burning NH3 is the slow chemical reaction between this substance and air, aggravated by the much less effective
Corresponding author at: Mechanical Engineering Department, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal. E-mail address: [email protected] (M. Costa).
https://doi.org/10.1016/j.fuel.2019.115693 Received 6 March 2019; Received in revised form 7 June 2019; Accepted 20 June 2019 Available online 26 June 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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pressure (4 kPa) CH4/NH3/O2/Ar flames for a range of NH3 fractions in the fuel mixture (0–1) in a McKeena burner. The NOx patterns showed a constant increase with NH3 addition up to 1. Okafor et al. [23] developed a novel mechanism for NH3/CH4 mixtures based on those from Tian et al. [22] and GRI-mech 3.0 [24], validating it against flame speed data from a cylindrical constant volume vessel, for CH4/NH3/air flames at atmospheric conditions and molar fractions of NH3 in the fuel mixture from 0 to 0.3. Recently, the UC San Diego Combustion group [25] updated its sub-mechanism of nitrogen-related species with reactions and rates from recent works, which, when integrated to their full mechanism, allows it to simulate CH4/NH3 fuel mixtures. Fuel mixtures of NH3 and CH4 appear to have satisfactory flammability conditions, and the understanding of the reactions involved in their combustion constitutes a middle-step towards the implementation of NH3 as a viable energy resource. Given that NOx emissions are the major concern inhibiting a wide acceptance of NH3 as fuel, the existing experimental research is neither sufficient for a thorough understanding of the processes behind their formation, nor for an extensive validation of the chemical kinetic mechanisms. Therefore, the main aim of this work is to contribute with quantitative data on how different variables influence the emissions of NOx, CO and NH3, when burning fuel mixtures of NH3 and CH4. Furthermore, the experimental measurements are compared with kinetic simulations to evaluate the performance of the current chemical kinetic mechanisms. In addition, rate of production and sensitivity analyses were performed for the kinetic models used, allowing for the understanding of the reactions that most contribute to the formation or consumption of NO.
mixing of the constituents, comparing to more common fuels. More recently, Hayakawa et al. [16] investigated the laminar burning velocity of pure NH3 premixed flames. The maximum flame speed was found to be ∼7 cm/s for an equivalence ratio of 1.1 and an initial pressure of 0.1 MPa, which is rather low when compared to conventional fuels such as CH4 (∼35 cm/s). This adds to the findings of low flammability of pure NH3 encountered earlier [13–15]. To circumvent these difficulties, flammability enhancements have been studied through mixtures of NH3 and other fuels [8–11,17–19]. Reiter and Kong [8] investigated the combustion and emission characteristics of a compression-ignition engine using blends of NH3 and diesel as fuel. The most favourable fuel efficiency was found for ratios around 1:1 of NH3 to diesel. However, not only the CO and unburned hydrocarbon emissions were higher than they would be for independent diesel use, but also the NOx emissions were significantly high, especially when NH3 amounted for more than 40% of the total fuel energy. Additionally, although the overall NH3 conversion was approximately 100%, NH3 could still be found at the exhaust in values between 1000 and 3000 ppm. Valera-Medina et al. [11] studied NH3/H2 premixed flames in a swirl combustor. The results showed that the flame velocity achieved was similar to CH4, with the drawback of narrow operational conditions due to the high diffusivity of H2. Furthermore, the excess production of OH and O radicals contributed for high NOx emissions of up to 104 ppm, but NH3 emissions were negligible. Konnov et al. [17] investigated NH3-doped CH4 flames stabilized on a perforated plate burner at atmospheric pressure, with the aim of understanding fuelbound NOx emissions. While employing fuel mixtures with 0.5 vol% of NH3, they concluded that NOx emissions peaked around stoichiometric conditions, decreasing for lower and higher equivalence ratios. Similar results were reported by Valera-Medina et al. [18], who investigated NH3/CH4 blends in an atmospheric-pressure swirl burner. Mixtures with more than 66% of NH3 in the fuel mixture volume yielded maximum NOx emissions for an equivalence ratio of 0.9. The decrease in emissions was more pronounced for higher equivalence ratios than for lower ones. A similar pattern was found for the same fuel mixtures in gas turbines [9]. Kurata et al. [10] conducted an experimental research with pure NH3 and NH3/CH4 blends in a gas turbine to evaluate the influence of the NH3 content in the fuel mixture. Pure NH3 combustion proved to have the lowest NOx emissions, with less than 1000 ppm. However, it was at the cost of fuel-rich combustion zones in the turbine, resulting in NH3 emissions of more than 1500 ppm. Fuel mixtures of NH3 and CH4 reduced the NH3 considerably to less than 30 ppm, although the NOx emissions increased overall. The increase of the NH3 amount in the NH3/CH4 blends leads to the growth of NOx emissions, until NH3 reaches about 60% of the energy in the fuel mixture. Afterwards, emissions drop. In terms of chemical kinetics, several studies have tried to describe the behaviour of NH3 and its mixtures as fuels. One of the first investigations on the sub-mechanisms involved in the formation of NO in hydrocarbon combustion was conducted by Dagaut et al. [20], who studied the oxidation of HCN, one of the most important pathways in the formation of pollutants in NH3/CH4 combustion. These authors presented a novel detailed kinetic mechanism based on previous works, containing a pathway to NH3 and CH4 oxidation. Xiao et al. [19] adapted the commonly used Konnov mechanism [21], adjusting Arrhenius parameters in the reactions in order to fit ignition delay times from the literature; these authors also conducted one dimensional, freely propagating premixed laminar flame simulations with several NH3/CH4 mixtures for different equivalence ratios. NO emissions were found to increase as the quantity of NH3 in the fuel mixture grew, thus achieving a similar pattern to that found in [10]. As for the influence of equivalence ratio on NO emissions, a maximum was found for near stoichiometry, while emissions for fuel-lean and fuel-rich conditions were lower [9,17,18]. Tian et al. [22] improved a previously developed mechanism by updating the reactions related to the CH4/NH3 chemistry, validating it against gas species concentration data for low
2. Experimental Fig. 1 shows a schematic of the experimental setup used in this work. This premixed laminar flame burner has been featured in a previous study from the present research group involving NH3 [26]. It consists of an inverted stainless-steel cone, fixed on a mount, with a hollow interior. It is actively cooled around the boundaries covering the flame position, resourcing to tap water. The interior is filled with small alumina spheres and several transversal stainless-steel meshes in order to achieve flow uniformity in the cross-section. A quartz tube sits on top of the base, allowing for flame development and visual observation. The quartz tube has a length of 50 cm, an external diameter of 4.5 cm and an internal diameter of 3.3 cm, and is insulated with ceramic wool to reduce heat losses. On the top of the quartz tube rests a stainless-steel chimney. NH3 (99.98% purity) and CH4 (99.995% purity), from gas cylinders, are fed to the intake of the burner, premixed with air from a compressor. The flow rates of NH3, CH4 and air are measured with digital flowmeters. Temperature measurements were taken with fine wire (27 μm) thermocouples of platinum/platinum: 13% rhodium. The thermocouple was deployed within a probe placed on a vertical slider, which could traverse the centreline of the quartz tube. The probe was comprised of two 350 μm wires of the same composition as that of the hot junction, which ran inside an alumina sheath and supported the thermocouple at the exposed end. A compensated wire connects the probe to an amplifying board, which is in turn connected to a computer. Temperature measurements along the vertical centreline were taken at distances from the flame between 2.5 cm and 40 cm. The uncertainty due to radiation heat transfer was estimated to be less than 5% by considering the heat transfer by convection and radiation between the thermocouple bead and the surroundings [26]. For each test condition, measurements were repeated at least three times – each measurement was sampled at 100 Hz during 30 s. Reproducibility of the data was, on average, within 5% of the mean value. Combustion products were sampled from the centreline of the tube, at 30 cm from the flame, using a stainless steel, water-cooled probe. The inner diameter of the probe through which the gases were evacuated was of 1.3 mm. The gas composition analysis was performed using a 2
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Fig. 1. Schematic of the experimental set-up.
3. Kinetic modelling
chemiluminescent analyser (Horiba, model PG-250) for measurements of NOx concentrations, a non-dispersive infrared analyser (Horiba, model CMA-331A) for measurements of CO and CO2 concentrations, and a magnetic pressure analyser (Horiba, model CMA-331A) for measurements of O2 concentrations. For NOx concentration measurements, nitrogen (99.9% purity) from a gas cylinder was added to the sampled gas for dilution, in a proportion of 2.6:1 in volume, in order to keep values to the scale of the analyser. As in the temperature measurements, for each test condition, measurements were repeated at least three times – each measurement was sampled at 100 Hz during 10 min. Reproducibility of the data was, on average, within 5% of the mean value for the concentrations of NOx, CO2, and O2, and within 15% for the concentrations of CO. Finally, Gastec detector tubes were employed to measure the emissions of NH3. These tubes have a minimum detection limit of 2 ppm, while allowing for quantitative assessments starting at 10 ppm. Again, for each test condition, measurements were repeated at least three times, without NH3 being detected for any of the conditions examined in this work. According to the manufacturer, the uncertainty of the present detector is within 5% of the measured value. The fuel thermal input of the NH3/CH4 fuel mixtures was fixed throughout the experiments at a value of 300 W. The equivalence ratios employed were 0.8, 0.9 and 1. It should be stressed that for equivalence ratios below 0.8 the flame was unstable. The NH3 mole fraction in the fuel mixture, defined as xNH3 = VNH3/(VNH3 + VCH4 ) , was varied from 0 to the achievable maximum in 0.1 increments, for each of the equivalence ratios. For the equivalence ratios of 0.8, 0.9 and 1, the maximum achievable xNH3 were 0.5, 0.6 and 0.7, respectively. Beyond these values, the flame was unstable and blow-off occurred. Photographs of the flames with different percentages of NH3 in the fuel mixture were taken in order to assess the colour-shift that occurs as the presence of NH3 in the fuel mixture grows. The photographs were taken at an angle of about 30° from the vertical centreline, with an SLR camera (Canon EOS 700D), with a focal distance of 50 mm, aperture of ƒ/5, shutter speed of 1/20 s and ISO of 6400.
The kinetic simulations were performed with Cantera [27], a code library for chemical kinetics calculations, using Python language. The mechanisms developed by Okafor et al. [23], Dagaut et al. [20] and the UC San Diego Combustion group [25] were chosen for the calculations, due to their different sub-mechanisms and their successful convergence for all studied equivalence ratios. For additional information on existing chemical mechanisms, the reader is referred to a very recent study [28], where the present group evaluated the performance of 10 chemical mechanisms available in the literature. The numerical methodology considered in this work is based on the research by Barbas et al. [26], which proved to be successful for the same burner employed here. The flame region, from the base of the burner up to the flame front, is modelled as an adiabatic, burner-stabilized one-dimensional premixed laminar flame, defining mesh points along the burner vertical axis. A first adiabatic simulation is done for a flame domain of 4 cm up from the burner metallic grid, generating a temperature profile. From this profile, temperatures are considered only up to a certain point. Barbas et al. [20] arbitrated this point at 1.5 cm, deemed enough to define the flame region for the thermal input used in that experiment. For the present work, the thermal input is much lower, holding the flame much closer to the burner grid, therefore demanding a better definition of the flame front and the flame region. For this purpose, the calculated adiabatic flame temperatures are then considered up to the end of the predicted flame region, a point numerically calculated in Cantera [27] as:
z = z ign + δ
(1)
where z ign is the ignition point, defined as the point where the derivative of the temperature in the flame length, dT/dz, is maximum, while δ is the flame thickness, defined by the following equation:
δ=
Tb − Tu max
3
( ) dT dz
(2)
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where Tb is the temperature of the burned gases and Tu the temperature of the unburned gases. Thereafter, the predicted temperature profile is added to the one taken from the experiments, up to the end of the burner length (40 cm). However, the experimental temperatures are used starting at 4 cm, a point for which the residence times are higher than 40 ms for all conditions. Amato et al. [29] considers it the minimum residence time for which a real burner behaves as an adiabatic flame, a criterion used by Barbas et al. [26] to define the first measured temperature, in their case, 10 cm. The combined temperature profile is then used as input by the software in another simulation of a geometrically equivalent burner-stabilized one-dimensional premixed laminar flame, generating chemical species concentration profiles along the burner axis. Finally, sensitivity and rate of production analysis were performed for NO, for equivalence ratios of 0.8 and 1, and for xNH3= 0.2 and the maximum NH3 content in the fuel mixture for each flame. Due to their different behaviour and full convergence for all cases, only the mechanisms of Okafor et al. [23] and the UC San Diego Combustion group [25] were chosen. Another sensitivity analysis was done for CO under stoichiometric conditions, for xNH3= 0.5, at the point of maximum CO consumption, in order to understand what influences its concentration in the exhaust gas.
4. Results and discussion Fig. 2 shows photographs of the flames for various amounts of NH3 in the fuel mixture – the photographs were taken without the insulation around the quartz tube. The colour for pure CH4 (Fig. 2a) is not entirely blue, because of the incandescence of the steel mesh immediately below the flame. Nevertheless, a blue aura can still be seen in the periphery. As the amount of NH3 in the fuel mixture increases (cf. Fig. 2a to e), the colour of the flame turns progressively whiter. Although reflections can be observed on the inside of the quartz tube, the flames are reasonably flat and remain stable above the burner. Fig. 3 shows on-axis temperature measurements for various NH3/ CH4 fuel mixtures with equivalence ratios of 0.8, 0.9 and 1. It is observed that temperature decreases along the axial distance, which is an expected behaviour due to heat losses. As the amount of NH3 in the fuel mixture increases, the temperature profiles increase as well, regardless of the equivalence ratio. It is worth mentioning that adiabatic flame simulations retrieved temperature behaviours similar to the experimental results, presenting higher temperatures for higher NH3 contents
a)
b)
Fig. 3. Temperature measurements for NH3/CH4 fuel mixtures. a) ɸ = 0.8; b) ɸ = 0.9; c) ɸ = 1.
c)
d)
e)
Fig. 2. Still photographs of the flames for various amounts of NH3 in the fuel mixture, for a thermal input of 300 W and ɸ = 1. a) Pure CH4, b) xNH3 = 0.2, c) xNH3 = 0.4, d) xNH3 = 0.6, e) xNH3 = 0.7. 4
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Fig. 4. NOx measurements and predictions for NH3/CH4 fuel mixtures. a) ɸ = 0.8; b) ɸ = 0.9; c) ɸ = 1.
Fig. 5. CO measurements and predictions for NH3/CH4 fuel mixtures. a) ɸ = 0.8; b) ɸ = 0.9; c) ɸ = 1.
in the fuel mixture. Fig. 4 shows measured and predicted NOx emissions as a function of the amount of NH3 in the fuel mixture for equivalence ratios of 0.8, 0.9 and 1. The presence of NOx in the exhaust gases is significant. The measured NOx increases as the amount of NH3 in the fuel mixture rises up to about 0.5. The values reach a maximum at that point and start decreasing afterwards. This pattern is similar for the three equivalence ratios and is in accordance with the literature [10,19]. The maximum measured NOx values are around 2800, 3600 and 4300 ppm, corrected for 6% of O2, for the equivalence ratios of 0.8, 0.9 and 1, respectively. The predictions do not reproduce well the experimental trends, in particular for ɸ = 0.8 and 0.9. As the fraction of NH3 in the fuel mixture increases, the predicted NOx emissions also increase continuously, but do not attain a maximum like it is observed in the experimental data. Note that the mechanism of Dagaut et al. [20] did not converge for ɸ = 0.8 and xNH3 = 0.5 so that this result was not included in the respective graph. Overall, the mechanism of Okafor et al. [23] predicts the highest NOx emissions, followed by that of Dagaut et al. [20] and, lastly, by the San Diego mechanism [25]. As for the effect of the equivalence ratio, it can be observed that, as ɸ decreases from stoichiometry to fuel-lean conditions, the experimental overall NOx emission decreases. Despite the small range of
equivalence ratios studied here, this trend is in agreement with existing literature [18,19]. However, the simulations predicted an opposite trend – as the equivalence ratio increases, the predicted NOx emissions decrease. For this reason, the discrepancy between the measured and predicted results is larger for the lean mixtures, whereas for the stoichiometric mixture, the numerical results are rather close to the experimental counterparts. Fig. 5 shows measured and predicted CO emissions as a function of the amount of NH3 in the fuel mixture for equivalence ratios of 0.8, 0.9 and 1. The average CO concentrations are below 60 ppm, and no discernible differences are observed between stoichiometry and the fuellean conditions. Consequently, the CH4 oxidation is almost complete regardless of the equivalence ratio. The numerical results for the lean conditions are in accordance with the experimental counterparts, since the predicted CO emissions are negligible. Furthermore, there are no discernible differences among the mechanisms. However, for the stoichiometric mixtures, the three mechanisms over-predict the CO emissions by almost two orders of magnitude. The San Diego mechanism [25] predicts the lowest CO emissions for all conditions examined, while the other two mechanisms switch places for the highest predictions according to the fraction of NH3 in the fuel mixture. 5
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Fig. 6. Rate of NO production (mol/ cm3 s ) for ɸ = 0.8. (a) xNH3 = 0.2 , mechanism of San Diego [25], (b) xNH3 = 0.2 , mechanism of Okafor et al. [23], (c) xNH3 = 0.5, mechanism of San Diego [25], (d) xNH3 = 0.5, mechanism of Okafor et al. [23].
generating HCN and HNCO. In fact, since the San Diego mechanism [25] does not contain carbon-nitrogen interaction reactions, no influence of these can be anticipated. For the stoichiometric condition (ɸ = 1) and high NH3 content in the fuel mixture ( xNH3 = 0.7 ), the formation of NO exhibits similar patterns when compared to the lean condition. In the present case, however, the HNO decomposition in the presence of a third body (M) increases its relative contribution. For the consumption of NO, the patterns are also similar; however, the reactions of NO with N and NH radicals increase their importance, when compared with the lean condition and lower NH3 content in the fuel mixture. Figs. 8 and 9 show the normalized NO sensitivity on the flame front for the 15 most important reactions in the mechanisms of San Diego [25] and Okafor et al. [23] for ɸ = 0.8 and 1, respectively. For the lean condition (ɸ = 0.8) and low NH3 content in the fuel mixture ( xNH3 = 0.2 ), the mechanisms show that NO formation is very sensitive to the reaction H + O2 ⟺ O + OH, related to the hydrogen oxidation in the flame, a very exothermic reaction that influences the temperature of the flame. Other reactions that enhance NO formation are those related to the oxidation of NH3, reactions between N, NH and NH2 radicals and O2, OH and O, as well as a reaction involved in the formation of CO, HCO (+M) ⟺ CO + H (+M). On the NO mitigation side, the radicals N, NH and NH2, when reacting with NO, present the most important contribution. It suggests that the abundance of oxygen might be directly related to NO formation in the flame. Other reactions related to the combustion of CH4, such as CH3 + H (+M) ⟺ CH4 (+M), seem to contribute to the mitigation of NO, along with the reaction H + O2 (+M) ⟺ HO2 (+M) (M is H2O in the mechanism of Okafor et al. [23]), related to flame stabilization. For the lean condition (ɸ = 0.8) and high NH3 content in the fuel mixture ( xNH3 = 0.5), both mechanisms present very similar results when compared with the condition with low NH3 content, but the relative importance of the reaction H + O2 ⟺ O + OH for NO formation
As for the NH3 emissions, the presence of NH3 in the exhaust gases was less than 10 ppm regardless of the condition examined, revealing the complete oxidation of the NH3. The predictions yielded similar patterns, as the predicted NH3 values at the exhaust were less than 1 ppm for all conditions. Figs. 6 and 7 show the rate of NO production for the 15 most important reactions in the mechanisms of San Diego [25] and Okafor et al. [23] at ɸ = 0.8 and 1, respectively. For the lean condition (ɸ = 0.8) and low NH3 content in the fuel mixture ( xNH3 = 0.2 ), it can be seen that NO production occurs mostly through the HNO consumption, as well as the reactions between N and NH with oxygenated radicals (O and OH) and O2. Consumption of NO2 and N2O through their reactions with O is also ranked in the San Diego mechanism [25]. NO consumption takes place mainly through its reaction with NH/NH2 radicals, as well as with CH2 (for the mechanism of Okafor et al. [23] only) and N radicals, with the CH2 being produced during the CH4 oxidation. For the lean condition (ɸ = 0.8) and high NH3 content in the fuel mixture ( xNH3 = 0.5), NO formation paths are very similar, with HNO decomposition occurring through its reaction with OH and H radicals, and also through reactions between N and NH with O, OH and O2. In this case, the reaction H + NO (+M) ⟺ HNO (+M), which implies the decomposition of HNO in the presence of a third body (M), increases in importance. In addition, NO consumption is mainly driven by its reaction with N, NH and NH2 radicals, generating N2, H2O, NO2 and N2O and intermediate radicals. The differences between the mechanisms are mainly due to the relative importance of each reaction. For the stoichiometric condition (ɸ = 1) and low NH3 content in the fuel mixture ( xNH3 = 0.2 ), the NO formation is due to the same set of reactions seen under the lean condition, with enhanced contribution from the reaction H + HNO ⟺ H2 + NO. The NO consumption also comes from its reaction with nitrogen-containing radicals. The mechanism of Okafor et al. [23], contrarily to the mechanism of San Diego [25], counts with the effect of CH and CH2 in the NO consumption, 6
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Fig. 7. Rate of NO production (mol/ cm3 s ) for ɸ = 1. (a) xNH3 = 0.2 , mechanism of San Diego [25], (b) xNH3 = 0.2 , mechanism of Okafor et al. [23], (c) xNH3 = 0.7 , mechanism of San Diego [25], (d) xNH3 = 0.7 , mechanism of Okafor et al. [23].
Fig. 8. Normalized NO sensitivity on the flame front for ɸ = 0.8. (a) xNH3 = 0.2 , mechanism of San Diego [25], (b) xNH3 = 0.2 , mechanism of Okafor et al. [23], (c) xNH3 = 0.5, mechanism of San Diego [25], (d) xNH3 = 0.5, mechanism of Okafor et al. [23]. 7
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Fig. 9. Normalized NO sensitivity on the flame front for ɸ = 1. (a) xNH3 = 0.2 , mechanism of San Diego [25], (b) xNH3 = 0.2 , mechanism of Okafor et al. [23], (c) xNH3 = 0.7 , mechanism of San Diego [25], (d) xNH3 = 0.7 , mechanism of Okafor et al. [23].
Fig. 10. Normalized CO sensitivity on the maximum CO consumption point for ɸ = 1. (a) xNH3 = 0.5, mechanism of San Diego [25], (b) xNH3 = 0.5, mechanism of Okafor et al. [23].
reactions related to the NH3 oxidation and the H2/O2 sub-mechanism. For this condition, there is an important contribution to NO from the decomposition of HNO with a third body (M). NO consumption, at this point, is very significative, thus leading to higher sensitivity of reactions between NO and N, NH and NH2, especially with the first, which is the most important. The reactions of HO2 formation also influence NO mitigation, as well as CH3 + H (+M) ⟺ CH4 (+M), but less significatively than for flames with low NH3 content. In brief, it can be concluded that both mechanisms present slight differences in terms of reactions’ rate of production and sensitivity. The HNO pathway seems to be the most important for the production of NO, while the NO consumption is mainly driven by its reaction with N, NH and NH2 radicals. On the sensitivity side, it can be concluded that the reactions involved in the H2/O2 sub-mechanism, which regulates the amount of O, H and OH radicals as well as the temperatures, are those that most affect the process of NO formation and consumption. The
increases, as that of the reaction between NO and NH for NO mitigation. The results for the mechanism of Okafor et al. [23] also point to a lower sensitivity to the reaction between CH4 and OH, expected due to the smaller amount of that component in the fuel mixture. For the stoichiometric condition (ɸ = 1) and low NH3 content in the fuel mixture ( xNH3 = 0.2), the NO formation is most sensitive to a similar set of reactions when compared to the lean condition. In the present case, the reaction HCO (+M) ⟺ CO + H (+M) (M is H2O [23]) loses its relative importance, as well as the CH4 reaction with OH. The NO consumption also presents similar patterns; however, the reaction between N and NO becomes the most important one for the mechanism of San Diego [25] in this case. For both mechanisms, the results also show that the reaction CH3 + H (+M) ⟺ CH4 (+M) increases its importance for NO mitigation. For the stoichiometric condition (ɸ = 1) and high NH3 content in the fuel mixture ( xNH3 = 0.7), the NO formation is mainly sensitive to 8
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(FCT), through IDMEC, under LAETA, project UID/EMS/50022/2013. The authors would like to express appreciation to the COST Action CM1404 “Chemistry of Smart Energy Carriers and Technologies (SMARTCATS)”.
reactions related to the oxidation of CH4 are least important for both rates of production and sensitivity, and seem to have a mitigating effect on NO formation. Fig. 10 shows the normalized CO sensitivity at the maximum CO consumption point to the 15 most important reactions in the mechanisms of San Diego [25] and Okafor et al. [23] for ɸ = 1. As expected, the bulk of the contribution to CO formation and mitigation is due to reactions related to the CH4 oxidation, as well as those of the H2/O2 sub-mechanism, namely H + OH (+M) ⟺ H2O (+M) and H + O2 (+M) ⟺ HO2 (+M) (M is H2O in the mechanism of Okafor et al. [23]), which are related to the consumption of oxygenated radicals. However, reactions related to the NO formation, especially for the mechanism of Okafor et al. [23], have a positive effect on CO formation, while reactions of NO consumption, or related to it, present a negative effect. In conclusion, it appears that the addition of NH3 to the fuel mixture influences the consumption of CO; however, the direct relation between the NH3 oxidation and the high levels of CO emissions cannot be established solely by the sensitivity analysis. Since NO is mostly formed before the bulk of the CO consumption, it can be speculated that the NO formation would remove oxygen that could convert CO into CO2. Unfortunately, this was not corroborated by the experiments, which indicate very low CO levels at stoichiometry.
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5. Conclusions Laminar premixed NH3/CH4/air flames were investigated in a laboratory-scale laminar flame burner, for three equivalence ratios and various fractions of NH3 in the fuel mixture. Gas temperatures along the burner axis and emissions of NOx, CO and NH3 were measured for all conditions. In parallel, three literature chemical kinetic mechanisms were used to predict the experimental data. In addition, sensitivity and rate of production analysis were carried out in order to understand the reaction pathways involved in the formation and consumption of NO and CO. The main conclusions of this study are as follows. 1) NOx is produced in high quantities regardless of the NH3 present in the fuel mixture, reaching the highest values for xNH3 = 0.5. In addition, NOx emissions decrease as the equivalence ratio is reduced towards fuel-lean conditions. 2) CO and NH3 emissions are quite low for all conditions, indicating almost full combustion of CH4 and NH3. 3) The chemical kinetic mechanisms considered showed fair agreement with the experimental trends, with the NOx and CO emissions being over-predicted for most conditions. 4) The rate of production analysis points to the HNO pathway in the formation of NO, and to its consumption through N, NH and NH2 radicals. NO formation is highly sensitive to the H2/O2 chemistry, which determines the formation of O, OH and H radicals. This behaviour is common in the three chemical mechanisms considered. 5) CO formation and consumption are sensitive to the NH3 oxidation reactions, suggesting that the consumption of oxygen in NO formation might be responsible for the over-prediction of the CO concentration in the exhaust gases. Experiments do not corroborate these findings. This calls for further revision of the chemical kinetic mechanisms for NH3 co-firing with hydrocarbon fuels. Acknowledgements This work was supported by Fundação para a Ciência e a Tecnologia
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