An Experimental Study on Combustion Instability and NOX Emission Characteristics of H2/CO/CH4 Syngas in a Gas Turbine Combustor

Abstract

The effects of the H2/CO/CH4 syngas compositions and the N2/CO2/steam dilution ratios are investigated in a partially-premixed gas turbine model combustor, which can prevent flashback problem even in high hydrogen content to understand the combustion characteristics of new fuel for IGCC and SNG plants. The temperatures of flame and combustor, NOx/CO emissions, combustion instability, flame stability, and flame structures are examined. Cause and effects of each combustion characteristics were determined from the relationship between them. For example, in order to identify the detailed flame structure during combustion instability, OH*-chemiluminescence images were taken at the rate of 12500 frame/sec using the high-speed intensified ICCD camera and/or planar OH* distribution in flame was obtained using phase-locked OH*-PLIF setup. First, the fuel composition effects of H2/CO/CH4 syngas (0~100%, span: 12.5% by LHV) on trends of each combustion performance were investigated by using ternary diagram. A lot of NOx is emitted at high H2 composition due to high flame temperature and high CO composition due to long residence time. This feature could be also explained by flame structure and combustor temperature. H2/CH4 syngas flame generated high combustion instability at the frequency of 750 Hz, 1000 Hz and 1500 Hz correspond to 3L, 4L and 6L mode which varies according to the fuel composition. This self-excited high multi-mode combustion instability characteristics have been studied by investigating combustion properties, flame structure, Rayleigh indices, proper orthogonal decomposition and characteristic time scales from the images of high-speed chemiluminescence and OH*-PLIF. The flames have different shapes and generate different combustion instability frequency at near 4L or 3L mode with their harmonics. The combustion oscillation frequency which is non-linear for fuel composition was appeared to be linearly proportional to Tad and SL. Phase-synchronized OH*-PLIF images suggested clues of an important combustion instability driving mechanism including the periodic alternation of flame attachment/detachment and coupling of vortex with everlasting heat release at the outer recirculation zone due to high reactivity of high hydrogen fuels. These images also used for the precise calculation of 2-D flame length by obtaining centroid of heat release intensity when performing time-lag analysis. The Rayleigh index results notified the dependence of the location and intensity of combustion instability driving/damping on the fuel composition and instability mode. For higher H2 containing flame (4th mode), driving and damping is occurred in narrow region but with high heat release density and the frequency of Rayleigh index was doubled. Otherwise in case of 3rd mode Rayleigh index shape was similar with pressure fluctuation due to the characteristics of multi-mode combustion instability with high superposition of higher harmonics. Analysis of proper orthogonal decomposition from high-speed OH* images showed the distinct coherent structures and large roll-up of flame are responsible for generating flame oscillations for each mode. High cross-correlation was found between proper orthogonal modes of 4th mode indicating convection of these coherent structures. When conducting time-lag analysis for syngas in a partially-premixed gas turbine model combustor, the significance of skewness time induced by wave distortion and importance of careful inspection on Lflame using 2-D OH*-PLIF is verified by showing the improvement in prediction accuracy. Next, the effect of the fuel-side dilution of N2, CO2 and steam on the combustion characteristic of syngas has been studied. This fuel-side dilution reduced flame temperature, combustor temperature and consequently NOx but significantly increased CO emission due to incomplete combustion at low flame temperature. From the NOx results of each diluent, it can be obtained that the dilution of syngas with nonflammable gas decreases NOx emissions, and the amount of NOx reduction per unit power is logarithmically related to only the diluents heat capacity which is the product of mass flow rate of the diluent and constant pressure heat capacity. This relationship between NOx reduction and diluent heat capacity is verified by inducing analytic solutions with some appropriate assumptions. Finally, based on the combustion results of H2/CO/CH4 syngas with N2/CO2/steam dilution, the combustion tests were performed for the commercial fuels: coal-derived syngas and SNG. The results of coal gas of which H2/CO ratio is 1/2 showed that N2 dilution is appeared to be negative in view of flame stability and CO emission but can be operable with enough stability margins and to be very positive in view of NOx emission. Combustion test of C0, C1, C3 and C5 SNGs of which H2 content is 0%, 1%, 3% and 5% but Weber index is constant was performed. Combustion characteristics of temperature and NOx/CO emission were almost identical for all SNGs but combustion instabilities of C0 and C1 were slightly differed from that C3 and C5 in frequency as well as amplitude. This feature also closely visualized by examining the high-speed unsteady flame behaviors. Even though the impact on combustion instability is not so significant for SNG which contains over 1% H2, this 1% where the change of combustion starts can be provided as the quality standard in SNG considering the existence of various types of natural gas firing gas turbines.