Article pubs.acs.org/EF
Effect of H2/CO Composition on Extinction Strain Rates of Counterflow Syngas Flames A. B. Sahu* and R. V. Ravikrishna Combustion and Spray Laboratory, Department of Mechanical Engineering, Indian Institute of Science, Bangalore, Karnataka 560012, India ABSTRACT: This work presents a detailed experimental and numerical investigation of the effect of H2/CO composition on extinction characteristics of premixed and nonpremixed syngas flames. Experimental measurements of local and global extinction strain rates in counterflow diffusion flames have been reported at atmospheric pressure for six different compositions of syngas fuel. The concentration of H2 was varied from 5 to 20% with a 3% increment, and correspondingly, CO was decreased from 35 to 20% in steps of 3%. Particle imaging velocimetry has been used to determine the local extinction strain rates. Local extinction strain rates increased with an increase in the H2/CO ratio in both nonpremixed and premixed flames. The predicted extinction strain rates for both nonpremixed and premixed counterflow flames using five different mechanisms available in the literature were compared with measurements. The Davis H2/CO and Ranzi H2/CO mechanisms predicted extinction strain rates within 10% of experimental values irrespective of the H2/CO ratio. In the nonpremixed case, the C1 mechanism by Li et al., GRI 3.0, and the Ranzi H2/CO mechanism predicted extinction strain rates well for low H2/CO ratios (from 5:35 to 14:26) but deviated from experiments for higher H2/CO values (17:23 and 20:20). In addition to kinetics, preferential diffusion effects were found to affect the reaction zone significantly and create distinct localized reaction zone structures in nonpremixed flames, which could contribute to discrepancies in extinction predictions. for a fundamental understanding of the flame structure via numerical simulations of laminar and turbulent combustion. Extinction strain rate is one such property because almost all practical flames are strained either axially, tangentially, or both. They provide an insight into the flow time scales and residence time scales of the reactants, which become comparable to the overall reaction rate in a combustion system under limiting conditions. A few experimental and numerical studies have been carried out previously investigating the kinetics of syngas premixed flames.4−8 Examination of ignition characteristics of CO/H2 against heated air using a counterflow diffusion flame configuration by Fotache et al.9 identified three distinct ignition regimes as a function of H2 concentration. Park et al.10 numerically investigated the chemical effects of CO2 dilution on extinction characteristics of syngas diffusion flames. Further numerical studies on extinction of H2/CO syngas diffusion flames have been carried out by Shih et al.11 using a statistically narrowband radiation model to study the effects of dilution of N2, CO2, and H2O and the effect of pressure and the composition of syngas. Park et al.12,13 studied the preferential diffusion effects and the effect of mixture composition and radiative heat losses in H2/CO diffusion flames diluted with CO2. The effect of pressure, preheat temperature, and CO2 dilution on laminar flame speed of H2/CO/CO2 mixtures has been experimentally investigated by Natarajan et al.,14,15 followed by assessment of kinetic mechanisms by comparing their predictions with measurements. Studies of NOx kinetics
1. INTRODUCTION Currently, 80% of the energy utilized in power and transport sectors is still extracted from fossil fuels.1 With fast-depleting fossil fuel resources, identifying potential fuels derived from renewable resources assumes importance. Gasification is a process that has been identified as a promising technique that can produce synthesis gas or syngas from a wide range of input materials. The feedstocks used for gasification could be coal, petroleum coke, biomass, and biodegradable wastes including wood pellets/chips, solid wastes, agricultural and industrial wastes, discarded corn seeds, and other crop residues. Syngas is a clean gaseous fuel that is free from particulates and corrosive ash elements such as chlorides and potassium because of the high-temperature nature of the gasification process. However, the syngas produced via gasification has a low volumetric energy content (about 5−7 MJ/m3), typically about 5−20% H2 and 20−30% CO and about 50% of N2 by volume. It has been recognized as a viable and an attractive energy source for stationary power generation with integrated gasification combined cycle (IGCC) technology.2 The composition of syngas depends heavily on the feedstock used for gasification.3 In addition to the energy content, properties such as flammability limits, flame structures, kinetics, and emissions vary with changes in syngas composition because of radically different properties of H2 and CO. Therefore, there is a need to focus on studies concerning the above-mentioned properties of syngas for compositions that are relevant, typically low-calorificvalue compositions produced as a result of gasification. In this paper, six compositions of syngas (described in a later section) have been chosen for extinction studies in nonpremixed flames, which represent the overall variety of syngas fuels available quite well. An accurate knowledge of fundamental flame properties is very important from a practical standpoint as well © XXXX American Chemical Society
Received: March 13, 2015 Revised: May 29, 2015
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DOI: 10.1021/acs.energyfuels.5b00539 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels have also been carried out in counterflow syngas flames identifying key NO formation pathways and assessing the efficiency of diluents in reducing NOx emissions.16−22 The effects of composition on extinction limits of syngas has been numerically studied in both premixed23,24 and nonpremixed flames; however, to date there are very few experimental measurements of local and global extinction strain rates of syngas flames.23 Modern stationary gas turbines are operated in a premixed or partially premixed mode where the reactants are usually mixed just prior to injection into the combustion chamber, thereby resulting in significant degrees of unmixedness in real scenarios. This leads to local areas of premixed flames and local areas of nonpremixed flames inside the combustor. Moreover, nonpremixed flames are also observed in swirl-stabilized combustor, so nonpremixed flames are observed in a wide range of practical flames. Therefore, studies on laminar and turbulent flame speeds, ignition delays, and extinction strain rates carried out to determine ignition characteristics and assess kinetics in premixed flames should also be complemented with extinction strain rate studies in nonpremixed flames. This work reports experimentally measured local and global extinction strain rates for six different compositions syngas in a laminar counterflow diffusion flame. The effect of burner L/D on global extinction strain rate in a counterflow burner has also been reported. Detailed assessment of chemical kinetics in terms of predicting extinction strain rates for premixed and nonpremixed flames has been conducted for different syngas compositions. In addition to the above, the following sections also present sensitivity analyses of reaction rates, species concentrations, and diffusivities, revealing key reactions and species affecting the extinction predictions.
Figure 1. Mean axial velocity profiles and turbulence intensity profiles 3 mm downstream of the nozzle exit for an unopposed air jet at Re = 933, 5100, 7127, 9330. measurements was found to be around 5%. Axial velocity profiles along the center line necessary for determining local strain rates were obtained using the PIV technique. A schematic of the experimental setup is shown in Figure 2. The second harmonic of a dual-pulsed (Δt = 30 μs) Nd:YAG laser (Litron Lasers) was used to generate a 532 nm beam that was then converted into a 50 mm × 2 mm parallel sheet using a collimator in combination with sheet optics. The Mie scattering signal from the seeded flow was imaged using a LaVision ImagerProX CCD camera (oriented at right angles to the incident laser sheet) that was fitted with a Nikon Rayfact PF10545MF-UV lens and a narrow band pass filter (centered at 532 nm, bandwidth of 10 nm) to avoid interferences from flame luminosity. A field of view of 22 mm × 22 mm was attained, providing a spatial resolution of 0.01 mm/ pixel. Approximately 200 images were recorded at a frequency of 10 Hz, which were then processed using the LaVision DaVis software to obtain the velocity field. The minimum interrogation window size used was 64 pixels × 64 pixels, corresponding to a physical area of 0.64 × 0.64 mm2. Interrogation windows were created with a 50% overlap, providing a vector density resolution of about 3 vectors/mm. An interpulse time gap of 30 μs corresponds to a nominal particle displacement of 0.15 mm. The air jet was seeded uniformly with DEHS (di-ethyl-hexyl-sebacat) droplets (
