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A systematic survey of chemical kinetic mechanisms (both simplified and global) shows an absence of a robust kinetic mechanism that can accurately predict characteristics of syngas-air nonpremixed flames. In this work, a three-step global kinetics mechanism is proposed for predicting the extinction strain rate (ag) of syngas-air nonpremixed flames. The available experimental ag data-set from the Tsuji type burner was used to evaluate the performance of six syngas kinetics models (four simplified and two global). For this, 2D-planar computations were performed to obtain ag values using four CO/H2 ratios (1, 3, 5, and 29). The results showed that all the simplified kinetics mechanisms overpredicted the ag values in the range of 16–224% for the four CO/H2 ratios. In the case of the global kinetics models namely, CFFR and WO, except for CO/H2 ratio of 29, CFFR underpredicted ag values by 22–55%. WO underpredicted ag by 10–44% for CO/H2 = 1 and overpredicted ag by 19–58% for CO/H2 = 3 and 5. For CO/H2 = 29, WO overpredicted ag values by 151–205%. For CO/H2 = 1, WO underpredicted ag due to two reasons, (1) double accounting of CO hydroxyl oxidation path and, (2) suppression of H2 oxidation pathways in four-step global JL mechanism, a source for the construction of WO mechanism. From the overall perceptive, WO has performed better compared to other mechanisms. Hence, WO was chosen for further optimization using experimental ag data as a target. Sensitivity analysis in terms of ag and maximum flame temperature (TF) was performed for WO using two CO/H2 ratios (1 and 5). It was found that for CO/H2 = 1 and 5, ag is most sensitive to Rxn1 (H2 + 0.5O2↔ H2O) and Rxn2 (CO + 0.5O2 → CO2). Results from sensitivity analysis, 0D (PSR) computations, and 2D computations have shown that the improper quantification of CO and H2 oxidation pathways leads to inaccurate ag prediction by the WO model. Based on these new insights, a modified three-step WO syngas kinetics model (WO-Opt) is proposed for predicting the extinction strain rate of syngas-air nonpremixed flames. The WO-Opt model was validated against ag experimental data. Compared to the WO model, ag predictions obtained from the modified WO-Opt model were in good agreement with the syngas-air nonpremixed flames experiments. [ABSTRACT FROM AUTHOR]

In this paper the phenomenon of flame jump vis-a-vis steady propagation in biomass packed beds in counter-current mode is discussed. By analyzing the fuel flux and propagation rate data from experiments with a range of oxidizers, namely, air, O2-N2, O2-CO2, and O2-steam mixtures, parameter regimes of steady propagation, and flame jump are identified. A theoretical basis for this classification is developed by analyzing the thermo-chemical conversion of single particles subject to flow and thermal conditions in a packed bed. The ratio of the ignition ( t i g ) to devolatilization ( t v ) times is shown to emerge as the controlling parameter in determining the flame propagation regimes. It is found from the theoretical analysis that steady propagation occurs for t v / t i g < 2 and transition to flame jump occurs if t v / t i g > 2. Operational zones of a packed bed biomass system is mapped using the predicted ratio of t v / t i g as a function of volatiles-based equivalence ratio ( ϕ v ). Implications of these results to practical ligno-cellulosic biomass combustion and gasification systems, especially using oxygen-steam mixtures for hydrogen generation, are brought out. [ABSTRACT FROM AUTHOR]

In this paper, experimental and theoretical investigations on the stability of coal-air flames in high-speed jets stabilized recirculating flow are presented. The current work reports the operational limits of high-ash coal-air flames stabilized by large velocity differentials. Experiments are performed to investigate the effects of particle size distribution, flow rate of primary and high-speed jets, and feed rate of coal on the stability of recirculating coal-air flames. Temperature measurements of the primary reaction zone are used to assess the stability of the reactor under various operating conditions. Experiments are carried out with two different particle size distributions, namely: (1) the fine ( < 355 μ m), and (2) the coarse ( < 700 μ m) distributions. Investigations are carried out with a range of primary jet velocities (4 m/s to 9 m/s), high-speed jets mass flow rates (1.24 × 10 − 3 to 4.94 × 10 − 3 kg/s), and feed rates (1.11 × 10 − 3 to 2.1 × 10 − 3 kg/s). A nondimensional ignition index ( τ) which is a ratio of the particle flow time ( t f ) to particle ignition time ( t i g ) is shown to indicate the stability of recirculating coal-air flames. Temperature measurements indicate that the operating conditions with τ>0.8 τ > 0.6 are stable whereas those with τ < 0.6 lead to reactor quenching. Force balance on particles with numerically resolved velocity profiles is used to estimate the particle flow time. Particle ignition times are quantified by the extended unified ignition-devolatilization model. Results indicate a strong influence of primary jet velocity on the stability of the flame. Only a marginal increase in stability can be attributed to the high-speed jets. Particle size distribution, along with the feed rate plays a crucial role in determining the stability of recirculating coal-air flames. Temperatures with the fine distribution exceed ash-fusion temperature causing slag deposition in the reactor. Model-based inferences on the increase in stability of recirculating coal-air flames with preheated coal are presented. [ABSTRACT FROM AUTHOR]