A comparative study of natural gas and biogas combustion in a swirling flow gas turbine combustor

Abstract

In this study, the non-premixed combustion of a traditional fuel- natural gas, and an alternative fuel- biogas, in a swirl-stabilized gas turbine combustor are simulated. The combustion results are analyzed and compared to evaluate the viability of the alternative fuel, biogas, for use in industrial gas turbine combustors. A comprehensive and exhaustive literature review on topics relating the current work is carried out. Two benchmark experimental cases of swirl-stabilized non-reacting and reacting flows are simulated in 3D and validated against the experiments to select the proper numerical, physical and combustion modeling of such complex flows. A swirling gas turbine combustor is designed to carry out non-premixed combustion of the fuels, using a well-known and recognized combustor design methodology and empirical equations. Investigating the existing literatures, the suitable compositions and stoichiometric air-fuel ratio of the gases are determined. Unlike the combustion works in existing literature, the outer annulus region (between the liner and casing) is considered in the computational domain to obtain more realistic results on the flow physics and chemical reactions during combustion. As the swirling flow is 3D in nature, a full 3D grid is generated to address complex flow physics and turbulent-chemistry interactions. Afterward, the combustion of both gases is numerically simulated, and the combustion performance is evaluated based on the design objectives: combustion efficiency, pollutant CO and NOx emission, Merit Function, and temperature uniformity of the exhaust gases at the combustor exit (Pattern Factor). The effects of two design parameters, namely: swirl number and fuel injector radius, in achieving best performance in design objectives are examined. It was found that, typically, a combination of higher fuel injector radius (or lower fuel velocity) and higher swirl number (2.0 in current study) produces best performance in achieving the design objectives. The swirling flow should be dominant over the incoming fuel flow to facilitate better and finer mixing of air and fuel, which typically contributes to a better combustion efficiency, pattern factor, and low pollutant emission. It is important to point out that, the empirical swirl number (0.9), achieved through an empirical formulation, does not provide the best performance in any of the design objective for both gases. Lastly, the comparison of the combustion performances of both gases revealed that, despite possessing much lower methane and hence lower heating value (LHV), biogas of a specific composition demonstrates an equal combustion performance to natural gas, although at the expense of higher pollutant emission. Therefore, biogas can potentially be utilized as an alternative fuel in industrial gas turbine combustors and methods for reducing pollutant emission can be devised.