Combustion performance of various syngas compositions in swirl combustor

Abstract

The challenge of using syngas in combustion system is the composition variability and low calorific value. Syngas mainly consists of H2 and CO and other sub component such as N2, CO2 and H2O. High H2-enriched syngas would result in high NOx production for some combustion cases. Whereas high CO concentration is posed with stability issues. The presence of sub-component as a diluent improves the emission characteristic but slows down the chemical reaction rate and calorific values. The variability in syngas strongly depends on the type of gasification technique, feedstock and oxidation agent. The present study therefore aims to investigate the combustion performance of different configuration in composition of syngas using premixed swirl mode technique. Various simulated syngases of CO and H2-dominant syngas or CO-rich and H2-rich syngas were used as fuels to evaluate the performance of emissions, diluent effects, lean blowout limit and flame structure. Further investigation on combustion of syngas was fundamentally conducted using numerical approach in which a comparative study on flame structure and reaction zone species were evaluated between those syngas fuels. Measurement by gas analyser was used to evaluate the performance of combustion emission and direct photography was used to analyse the flame appearance. Lean blowout test was performed by gradually reducing the fuel flowrate until flame blowout occur. For numerical method, two different combustion models namely flamelet generated manifold (FGM) and chemical equilibrium (CE) models were implemented to predict the combustion characteristic of syngas and the result obtained was then validated with experimental results. The results indicate that high CO-rich syngas shows evidently less NOx and CO emissions as compared to the other dominant CO fuel. Higher fraction of CO2 dilution results in reduction of NOx emissions, with pronounced impact on fuel-rich cases. There was minimal effect on CO emissions with increased dilution of CO2. The lean blowout limit test shows that higher CO content results in blowout at higher equivalence ratio. Addition of hydrocarbon fuel such as CH4 or hydrogen extends the blowout limit as the flammability limit is stretched to ultra-lean region. Dilution of unreactive CO2 in syngases results in higher lean blowout limit. Higher fraction of H2 in syngas produces both lower NOx emission and lean blowout limits. The optimum characteristic of high H2-rich syngas is also validated by numerical approach using FGM method. The numerical computation found that the increasing content of H2 in syngas results in lower flame temperature, subsequently leading to reduced flame height and lower NO emissions.