Ammonia (NH₃) is a high potential zero-carbon fuel and renewable hydrogen carrier. However, its combustion properties are less favourable than hydrocarbons, with challenges such as lower flame speeds, longer ignition delay times, and a higher tendency for NOₓ emissions. As a result, ammonia is typically blended with conventional fuels like methane, kerosene, and diesel for use in gas turbines or dual-fuel compression ignition (CI) engines, particularly during the early stages of implementation. This study focuses on two main objectives: (a) developing a semi-detailed NH₃/CH₄/H₂/n-C₁₂H₂₆ chemical mechanism involving 242 species and 1769 reactions, including sub-mechanisms for nitrogen oxides (NOₓ) and soot formation, to analyse the combustion and emission behaviour of ammonia-fuel blends in engines; and (b) applying this mechanism to study the combustion properties of a simplified micro-gas turbine using a steady flamelet model. The validation of the mechanism was done using experimental and numerical data from the literature. The mechanism was integrated with CFD solver utilizing the steady flamelet approach to analyse the combustion characteristics of an 11 kW (thermal) micro-gas turbine combustor. Realizable \(k\space - \) \(\varepsilon\) turbulence model was used in this work. Time averaged combustor exit temperature and emissions like NOx, CO and unburnt fuel fractions were estimated. Though overall equivalence ratio was maintained to be lean, a rich-lean dual combustion regime was created with the help of dilution holes to control NOx emissions. The effect of system pressure on emission characteristics was also investigated.