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Biogas is a renewable fuel predominantly composed of carbon dioxide (CO₂) and methane (CH₄) in varying proportions. The effects of the varying CO₂ proportion need to be clarified for the development of engines. The laminar burning velocity and the burned gas Markstein length of premixed CH₄/CO₂/Air were measured with CO₂ concentration ranging from 0.3 to 0.7 dilution ratios. The equivalence ratio was varied from 0.8 to 1.2, the initial pressure was set at 0.5 MPa, and the temperature was set to 298 K. The experiment was performed using a high-pressure constant volume combustion chamber. One-dimensional simulation of the flames was conducted using GRI-Mech 3.0. The results showed a reduction in the laminar burning velocity of CH₄/CO₂/Air mixtures with an increase in CO₂ dilution ratio. A non-monotonic relationship was discovered between measured Markstein length and CO₂ dilution ratio with different equivalence ratios. It was found that an increase in the CO₂ dilution increased the response of the flames to stretch. For the lean and stoichiometric flames, the Markstein length was nearly constant with CO₂ dilution of 0–0.5 and decreased with CO₂ dilution of 0.7, suggesting an increase in susceptibility of the flame to the intrinsic flame instability. This was found to be mainly due to an increase in the Zel’dovich number and a decrease in the effective Lewis number with CO₂ dilution. The Markstein length of the rich flame increased with CO₂ dilution as it was more sensitive to CO₂ dilution. Thermo-diffusive effects and pure stretch effects had similar influences on the burning velocity of the rich flames with an increase in stretch rate.

Ammonia is a promising sustainable fuel, however, its low reactivity creates challenges in industrial applications. In this study, ammonia/methane mixtures were considered for premixed and non-premixed counterflow flames. The extinction stretch rate was measured over a wide range of ammonia/methane mixing ratios and compared to 1D numerical results from four different mechanisms. Additionally, for counterflow premixed twin flames, quantitative analysis based on the comparison of experimental and numerical FWHM of OH and NO profiles was performed. Results showed that in premixed flames, all the mechanisms investigated were inadequate for predicting the extinction stretch rate, specifically for lean flames. In non-premixed flames, Okafor’s mechanism was accurately predicting the extinction stretch rate. For the FWHM analysis, the numerical mechanisms overpredicted both OH and NO apparition in the flame, except for Tian’s mechanism which underpredicted OH apparition. GRI Mech 3.0 performed well for small quantities of ammonia but failed to reproduce the pure ammonia case. Okafor’s and UCSD mechanisms gave better predictions of experimental trends, though overestimated both OH and NO apparition. In the FWHM analysis of both OH and NO, the lower R2 values in the lean region suggest that the lean region should be the focus for mechanism improvement.

形態: カラー図版あり, Physical characteristics: Original contains color illustrations, 資料番号: PA1710035000

Ammonia is a promising hydrogen-energy carrier as well as a carbon-free fuel. However, turbulent burning behavior of ammonia flame had yet to be sufficiently studied. In this work, laminar and turbulent burning velocities of ammonia/oxygen/nitrogen flames were investigated under the condition of oxygen enrichment. The turbulent burning velocity of ammonia/oxygen/nitrogen mixtures was found to increase with increasing turbulence intensity. The ratio of the turbulent burning velocity to stretched laminar burning velocity, Utr/UN, increased with the turbulence Karlovitz number. However, because of the diffusional–thermal instability effect, given the same turbulent Karlovitz numbers, Utr/UN in ammonia-lean cases is larger than in ammonia-rich cases. These findings indicate that consideration of the effects of diffusional–thermal instability and of the turbulence is important for the prediction of turbulent flame propagation velocity in ammonia combustion fields.

第53回燃焼シンポジウム（2015年11月16日-18日. つくば国際会議場), つくば市, 茨城, The Fifty Third Symposium (Japanese) on Combustion (November 16-18, 2015. Tsukuba International Congress Center) Tsukuba, Ibaraki, Japan, 形態: 図版あり, Physical characteristics: Original contains illustrations, 資料番号: AC1600112000

Application of ammonia as fuel is a potential candidate to achieve carbon neutrality. As laminar burning velocity of ammonia is slow, hydrogen addition is also considered to improve combustion characteristics with no carbon emission. In this study, we experimentally investigated product gas characteristics of strain stabilized ammonia/hydrogen/air premixed laminar flames under atmospheric pressure for various equivalence ratios. In a lean condition, large amount of N2O production was observed. To clarify N2O production mechanisms, numerical simulations were conducted using a reaction mechanism developed by Gotama et al. In the Gotama reaction mechanism, major N2O production path was NH+NO=N2O+H and major N2O consumption paths were N2O+H=N2+OH and N2O(+M)=N2+O(+M). It was clarified that a decrease in N2O consumption via N2O(+M)=N2+O(+M) increases N2O emission for lean and strained conditions.

Product gas characteristics of ammonia/hydrogen/air laminar premixed flames stabilized in stagnation flows were experimentally and numerically studied. Although the maximum value of NO mole fraction increased compared with pure ammonia/air flames, the trade-off relationship between NO and unburnt NH3 were also observed. In addition, N2O production for very lean conditions were observed. To clarify the N2O production mechanism in detail, numerical simulation modelling was employed using CHEMKIN software. Sensitivity analysis suggested that the reactions of (R58) NH+NO = N2O+H, (R105) N2O+H = N2+OH, and (R106) N2O(+M) = N2+O(+M) play an important role in N2O production. Product gas characteristics of N2O were numerically investigated for various stagnation plane temperatures and the equivalence ratios. The reaction rate of R106 decreases for low stagnation plane temperature cases and small equivalence ratios. It was considered that the decrease in the reaction rate of R106 decreased the reduction rate of N2O to N2, and thus large amount of N2O were detected. Also, the N2O amount decreased when the stagnation plane temperatures were sufficiently high. This also suggested that the N2O production may be restricted by a decrease in the heat loss in an ammonia-fueled combustor.

Ammonia is a promising hydrogen energy vector and a carbon-free fuel; hence the use of ammonia-hydrocarbon fuel blends can be viewed as an intermediate step towards a hydrogen economy. The characterization of methane-ammonia emissions is essential for designing combustors for a broader range of fuels while fulfilling strict NOx emission requirements and global warming targets. The product gas trends of laminar premixed ammonia-methane flames at atmospheric pressure were studied for 0.1 to 0.6 ammonia heat ratios at the operable range of equivalence ratios. Gases including NO, N2O, NO2, HCN, CO and NH3 were measured using the dual dilution gas method and compared against numerical predictions. Experimental results showed the highest NO emissions at approximately 8,000 ppm for the 0.3 and 0.4 ammonia heat ratios, reducing twofold at the extreme heat ratio conditions. The optimal condition for reducing NOx emissions while maintaining low unburnt NH3 was found to occur at a 1.20 equivalence ratio for higher ammonia ratios, moving incrementally closer towards 1.35 as the methane ratio was increased. These results can aid a further reaction model analysis due to the availability of stain stabilised stagnation flame models in numerical software.