Article pubs.acs.org/EF
Experimental Investigations of the Lean Blowout Limit of Different Syngas Mixtures in an Atmospheric, Premixed, Variable-Swirl Burner Parisa Sayad,* Alessandro Schönborn, and Jens Klingmann Department of Energy Sciences, Lund University, 22100 Lund, Sweden ABSTRACT: An atmospheric, variable-swirl combustor was used to study the influence of syngas composition on the lean blowout (LBO) limit under various flow conditions created at different swirl numbers. The fuels used in the experiments consisted of generic mixtures of CO, H2, and CH4. The results were compared to those for CH4 at the same swirl numbers. The effect of dilution on the LBO limit was studied by adding N2 to the syngas mixture. The swirl number was varied by changing the ratio of axial/tangential flow through the combustor inlet and was determined using laser Doppler anemometry (LDA). A perfectly stirred reactor (PSR) model was used to test whether the experimental results could be explained by changes in chemical kinetics. The experiments showed that increasing the swirl number reduced the LBO equivalence ratio for a given fuel composition. At a certain swirl number, increasing the H2/CO molar ratio of a binary mixture decreased the LBO equivalence ratio significantly. The addition of CH4 to a binary mixture shifted the LBO limit to higher equivalence ratios. N2 dilution increased the LBO equivalence ratio of the syngas mixture; however, the impact was relatively small. The PSR model was able to predict the effect of adding CH4 reasonably well, but it underestimated the effect of the H2/CO molar ratio on the LBO limit. parameters, such as the inlet temperature and mass flow. For this reason, it is crucial to determine the LBO limits of the combustor when varying any of these parameters. The current literature describes two theoretical approaches for the characterization of LBO.4−6 The first considers the ratio of chemical and flow times. It is based on the assumption that LBO occurs under conditions where the residence time is not sufficient for the chemical reactions to take place. The chemical reaction time scale can be estimated from the laminar flame speed and the thermal diffusivity of the mixture,6,7 and the residence time is defined on the basis of flow characteristics. The second approach considers the ratio of flame and flow speed. It is based on the assumption of flamelet-like combustion behavior, where LBO occurs when the flow speed exceeds the flame speed at critical locations within the combustor.8 The diffusivity, heat content, and kinetic characteristics of the fuels,8 as well as the combustor flow field, influence the prediction of LBO in both approaches. It is important to note that the reaction mechanisms and, thus, the combustion behavior of blended fuels, such as syngas, may be completely different from their separate components and cannot be estimated by an arithmetic mean based on proportions. Several studies have been carried out regarding the effect of different fuel components on its combustion properties. Lee et al.9 investigated the combustion performance of a model industrial gas turbine using CO/H2 blends diluted with N2, CO2, or steam. They reported that reliable operation and sufficiently low emissions can be achieved using diluted CO/H2 syngas mixtures in the combustor. Starkey et al.,10 Schefer et al.,11 and Griebel et al.12 studied the LBO of H2-enriched
1. INTRODUCTION The depletion of fossil fuels and environmental concerns about CO2 emissions have led to a growing interest in the use of alternative fuels, such as synthesis gas (syngas), in gas turbine combustors. Syngas can be obtained from renewable sources, such as biomass, or from traditional fuels, such as coal or heavy oil.1 Dependent upon the gasification process and the feedstock used, syngas may contain varying amounts of CO, H2, and CH4, as well as CO2, N2, H2O, and small amounts of higher hydrocarbons.1−3 Because of the variation in composition, such fuels may vary significantly from natural gas in terms of their chemical and physical properties. Therefore, firing syngas in gas turbine combustors may cause problems, such as blowout, flashback, dynamic instability, and autoignition. Because lowemission combustion systems are optimized for operation within a narrow fuel specification, developing fuel-flexible combustors requires extensive investigations of the combustion behavior of unconventional fuels. It is imperative that gas turbines fueled with such gases have similar, reliable operation and low pollutant emissions to their natural-gas-fired counterparts. This study was carried out to investigate the impact of syngas composition on the lean blowout (LBO) limit at different swirl numbers using an atmospheric, premixed, variable-swirl burner. Blowout is usually avoided using a pilot flame at richer conditions than the main flame. The disadvantage of pilot flames is their higher combustion temperature, which is responsible for the increased formation of nitrogen oxides (NOx). It is thus desirable to use as little pilot flame as possible while preventing blowout through accurate knowledge of the blowout limits of the combustor. The kinetic rates of chemical reactions and flame propagation speeds vary widely with fuel composition, and these affect the LBO limits of the combustor. The amount of pilot flame required to avoid LBO depends upon the fuel composition, the design of the combustor (mainly through the fluid flow field), and operational © 2013 American Chemical Society
Received: November 12, 2012 Revised: April 5, 2013 Published: April 5, 2013 2783
dx.doi.org/10.1021/ef301825t | Energy Fuels 2013, 27, 2783−2793
Energy & Fuels
Article
velocities, reducing the turbulence intensity shifted the LBO limit to lower values. One of the practical flow-related parameters that can be used to characterize the flow field in swirl-stabilized combustors is the swirl number (S). The swirl number is defined as the ratio of the axial flux of tangential momentum to the axial flux of axial momentum. Varying the swirl number can have substantial effects on the flow field in the combustor. Johnson et al.22 studied the flow fields and emissions of high-swirl (S = 0.73) and low-swirl (S = 0.5) injectors in lean premixed gas turbines. They reported that the LBO limit of the low-swirl injector was lower than that of the high-swirl injector and that the LBO equivalence ratio was relatively insensitive to the bulk velocity in the injector with the low swirl number. In another experimental study, Littlejohn et al.23 investigated the LBO limits, flow field, and emissions of different syngas compositions using a low-swirl injector. Their measurements were conducted at a constant swirl number of 0.51. The present work examines the effect of syngas composition on LBO limits in an atmospheric swirl combustor at several swirl numbers. In contrast to previous works, a series of swirl numbers were investigated and the influence of the swirl number on blowout behavior was studied over the transition from “low” to “high” swirl flow. The fuels studied contained varying amounts of CO, H2, and CH4. The effect of dilution on the LBO limit of syngas was also investigated by adding N2 to a blend of CO, H2, and CH4. To allow for a comparison with a more conventional fuel, the blowout experiments were further conducted for CH4. Experiments were performed at four different swirl numbers, corresponding to four different flow fields, for each fuel mixture. The swirl numbers ranged from low-swirl conditions (S = 0.03) to high-swirl conditions (S = 0.60) with a recirculation zone caused by vortex breakdown. The swirl number was varied by changing the ratio of the axial to tangential flow through the combustor inlet and was determined using laser Doppler anemometry (LDA). It was concluded that fuel composition and swirl number had strong effects on the LBO equivalence ratio. Varying swirl number may be used as a means of controlling the LBO equivalence ratio for a given fuel composition.
mixtures in a swirl-stabilized combustor and reported that the addition of hydrogen extended the LBO limit of CH 4 significantly. They suggested that the addition of H2 increased the OH radical concentration and the global reaction rate of the mixtures, leading to higher flame speeds and lower LBO equivalence ratios. It has also been reported that mixtures containing H2 have a higher resistance to the strain rate, which was attributed to high molecular mobility and the high diffusivity of H2, which improves the resistance of the flame to the effects of fluid mechanics.13 Various investigations on the combustion properties of H2/ CO blends have shown that the overall reactivity of CO/O2 mixtures is greatly accelerated in the presence of H2 because of the kinetic effect of the addition of H2 on the rate of the CO oxidation reaction (CO + OH → CO2 + H).14−16 In another study, Page et al.17 investigated the LBO limits of various H2/ CO and natural gas/H2 mixtures. They reported that the LBO equivalence ratio of a H2/CO mixture decreased linearly as its H2 content increased. Noble et al.8 studied the effect of syngas composition on the LBO limit using different blends of CO, H2, and CH4 and reported that the LBO limit is a very strong function of the H2 content of the fuel mixture. In another study, Zhang et al.18 presented the LBO limits of various CO/H2/ CH4 mixtures and tried to correlate the measured LBO limits with the adiabatic flame temperature, laminar flame speed, Lewis number, and chemical time. Apart from the marked influence of H2 on the reactivity of a mixture, CO/CH4-coupled chemistry may also affect the combustion behavior of a syngas mixture. Vagelopoulos and Egolfopoulos16 investigated the laminar flame speed and extinction strain rates for various mixtures of CO/CH4. They concluded that the addition of CH4 to a CO/O2 mixture accelerated the CO oxidation reaction because of an increase in the H radical concentration, resulting in higher extinction rates and laminar flame speeds. Their findings are consistent with the results by Scholte and Vaags15 and Chao et al.19 With regard to the effect of diluents, it has been suggested in different studies that diluents can affect the combustion properties of the mixture by changing the specific heat capacity and adiabatic flame temperature.8 It has further been suggested that CO2 dilution may also affect the combustion behavior by changing the chemical kinetic rates and radiative heat transfer.20 As noted above, fuel composition is not the only factor that influences the LBO limit of a combustor. The LBO limit may vary significantly for a specific fuel mixture because of changes in the flow field. Therefore, it may be necessary to study the lean operability of the combustor under various flow conditions. A number of experimental studies have been performed to address the effects of bulk flow velocity, total mass flow rate, and turbulence intensity on the LBO limit for different fuel mixtures. The effect of bulk flow velocities on the LBO of different mixtures has been investigated by Littlejohn and Cheng,21 who found that, regardless of the fuel mixture, the LBO limit remained relatively constant for bulk velocities below a certain value. In the work by Schefer et al.11 and Griebel et al.,12 it was reported that increasing the total mass flow rate increased the LBO equivalence ratio for a specific fuel mixture as a result of higher strain rates in the flame. The influence of turbulence intensity on the LBO limit was also investigated by Griebel et al. in the same work.12 At low bulk velocities (up to 50 m/s), they observed no difference in the critical equivalence ratio when varying the turbulence intensity, but at higher bulk
2. EXPERIMENTAL APPROACH 2.1. Burner. The apparatus used was a variable-swirl burner with a circular cross-section, as illustrated in Figure 1. The combustion chamber consisted of a quartz tube, 63 mm in diameter and 350 mm in length, with a wall thickness of 3.4 mm. Air and fuel entered the burner through a single premixing tube, 15 mm in diameter and 30 mm in length. A swirl mixer, used to combine axial and tangential air flows, was located directly upstream of this premixing tube. The swirl mixer allowed for the introduction of air into the premixing tube in both axial and tangential directions. The flow entering the swirl mixer in the axial direction was premixed with fuel 250 mm upstream of the inlet tube and passed through a fine wire mesh. The flow entering the inlet tube in the tangential direction passed through four channels that were 3 mm wide and 10 mm high. The swirl number in the combustion chamber was varied by changing the proportions of the axial and tangential flows into the premixing tube. Axial and tangential flows were metered and preheated individually, using two laminar-flow, differential-pressure mass-flow meters (Alicat MCR250) and feedback-controlled air heaters (Sylvania Sureheat Jet). The fuel components were metered using laminar-flow, differential-pressure mass-flow meters (Alicat MCR50). The exhaust from the main combustion chamber was discharged into a forceventilated extractor hood. A schematic of the burner cross-section and details of the geometry are shown in Figure 2. 2784
dx.doi.org/10.1021/ef301825t | Energy Fuels 2013, 27, 2783−2793
Energy & Fuels
Article
system was a two-component, optical-fiber-based LDA system (Dantec) operated in backscatter mode. The light source consisted of an argon ion laser (Spectra Physics) operated between 1 and 2 W power output (all lines). Beam expansion (1.94 times) was used before the front lens (310 mm focal length) to reduce the size of the measuring volume to approximately 50 μm in diameter and 500 μm in length. Signal processing was performed using burst spectrum analyzers (Dantec). During the LDA measurements, a length of 100 mm of the quartz liner was replaced with a stainless-steel liner of a similar diameter but with two flat windows located on opposite sides to allow for the laser light to enter and exit the combustion chamber with reduced light refraction losses. The overall length of the liner was 350 mm, as in the combustion experiments. The cross-sectional geometry of this altered section is shown in Figure 3.
Figure 1. Schematic of the variable-swirl burner.
Figure 3. Cross-section of the liner used during the LDA measurements. To measure the velocity as close as possible to the dump plane of the burner, the probe was tilted by 7°. Using a tilted probe had no impact on the tangential velocities, but the axial velocities should ideally be corrected for the probe angle and radial velocity component at each measuring position. However, because of the limited optical access of the liner preventing the radial velocities from being measured, the radial velocities were measured for the highest and lowest swirl cases with the liner removed and, as expected, were an order of magnitude smaller than the axial and tangential velocities. Because the probe tilting angle was also very small, the effect of the radial velocity component on the axial velocity was neglected, which introduced an additional relative error of 2.5% to the axial velocity measurements. In addition, a transit-time velocity bias correction was made to the measured velocities, but the difference was
