Experimental Investigation And Numerical Simulation Of Non Premixed Syngas Flame With Dual-swirler Structure

Coal, as China's most dominant energy source within several decades, is burnt along with severe environment pollution, drawing more and more attention around the world. The reasearch and adoption of clean coal power generation technology is essential for sustainable development of our country. Among those technologies, Integrated Gasification Combined Cycle, or IGCC, is considered the most promising method for efficient use of energy with low pollution in the near future. Gas turbines for power generation in IGCC system is fueled with synthesis gas, or syngas, produced by coal gasification, instead of traditional natural gas. Syngas is consisted of hydrogen, carbon monoxide, as well as hydrocarbon compounds, at different content. Due to its relatively lower LHV (Lower Heat Value), wide range of variations of its component, and its necessity of NOx reduction by dilution or other methods, gas turbine fueled with syngas has to meet requirements of high effiency, low emission, high stability, and good tolerance of fule variety. To solve those problems listed above during designing process of combustion chamber of syngas gas turbines, characteristics of syngas combustion need to be well studied and understood.For this purpose, an experimental test rig consisted of syngas sprayer with dual-swirler structure and model combustor was first designed and manufactured. Typical syngas at mid-LHV with different components was simulated with four independent fuel control system. Pure methane, hydrogen, and carbon monoxide was adopted for reference. A planar laser induced fluorescence (PLIF) system was developed in order to measure the concentration of OH radicals during combustion process inside model combustor. Results of OH-PLIF measurement was then used to provide valuable information regarding flame front structure, size of main reaction zone, flame shape, and combustion stability, et al. Temperature both within model combustor and at exhaust section were measured by thermal couples, while pollutant concentration was monitored by gas analyzer. On the other hand, large eddy simulation (LES), with proper chosen sub-grid scale models based on experimental data from PIV and thermal couple measurements, was used here as the numerical method.Studies show that CH₄ and CO flames exhibit W-shaped distribution of flame front near sprayer exit at same equivalence ratios, while H2 and syngas flame fronts exhibit M-shaped distribution, and deficiency of OH radicals is found near the centerline of model combustor. Among these four different fuels, syngas flame shows least spray angle, when H₂ flame starts to burn earliest and burns most uniformly. CO flame has longest fuel penetration depth in flame root regions, and CH₄ flame demonstrates narrowest main reaction zone. Both CO and syngas burn along with largest zone of highest temperature inside combustor.Then, two different syngas with mid- and low-LHV are chosen for study of combustion characteristics under double-swirled condition inside model combustor. Results show that syngas flames demonstrate crescent shapes at relatively low equivalence ratio. When equivalence ratio increases, mid-LHV syngas flame shows M-shaped flame root, while low-LHV syngas flame begins to burn in a lifted way. Meanwhile, large areas of chemical reaction appear downstream of the combustor. With the increase of equivalence ratio, highest recirculation velocity increases, causing the flame root compression, the increase of exhaust temperature, and decrease of CO emission. At same equivalence ratio, the highest temperature inside combustor of syngas with low-LHV is much lower, and the size of high temperature region is much smaller, when compared to mid-LHV counterpart. Thus, syngas with low-LHV helps to reduce NO_x formation, increase CO formation, and decrease combustion efficiency.Furthermore, combustion characteristics of syngas composed with different CO/H₂ ratios are compared. Experimental and numerical results show that syngas flame main shuttle structure is not influenced by CO/H₂ ratio within the range of 0.2 to 4.0. With the increase of CO/H₂ ratio, axial location of the start of the chemical reaction moves upstream, and main reaction zones is enlarged and spread toward centerline. The spray angle of flame front both inside and outside decreases, meaning flame root begins to shrink. The radial distribution of OH radicals tends to be more uniform, and highest temperature inside combustor increases. The region of high temperature enlarges with the increase of CO/H₂ ratio, thus exhaust temperature also increases. The CO emission increases relatively fast at first, but more slowly with the CO/H₂ ratio increased from 0.2 to 4.0.Finally, nitrogen is used as dilution for syngas fuel, as much as 40% of volume fraction, studying the effect of diluents on combustion characteristics. Research shows that with the increase of N2 fraction in syngas fuel, flame front is gradually stretched, and flame front near spray exit changes from M-shaped to W-shaped distribution. The region without chemical reaction extends, and main reaction zones also spread, with the increase of diluents. Inner flame front tends to contract toward centerline, when moving downstream at the same time. Outer flame angle increases, when flame front moves upstream. The highest temperature declines quickly with the existence of N2, and the area of high temperature is smaller at higher level of diluents, leading to less NOx but more CO emissions. Also with the increase of N2 diluents, inner recirculation zone tends to shrink, when the location of outer recirculation zone moves downstream. At higher level of N2 content in syngas fuel, the intensity of chemical reaction decreases, and flame exhibits local extinction and re-ignition, indicating more flame instabilities.