This work presents a thermodynamic analysis of fire-tube boilers operating with natural gas, aiming to improve system efficiency and reduce pollutant gas emissions. The research is based on numerical simulations of heat and mass transfer phenomena occurring inside the boilers, seeking to enhance an existing model in EES (Engineering Equation Solver). Thus, the work includes an evaluation of the influence of flame profiles (parabolic, uniform, and exponential) during combustion and how these affect the thermal performance of the boiler. Additionally, the study addresses heat losses in turning chambers, which are responsible for redirecting combustion gases along the path inside the boiler and directly affect the system s thermal efficiency. A relevant aspect of the work is the application of the model to assess the impact of adding hydrogen to natural gas as a fuel. This mixture has the potential to reduce CO2 emissions, as well as increase the temperature of the combustion gases and consequently the boiler s energy efficiency. The results showed that the flame profile has a very small impact on both energy and exergy efficiency, with a maximum temperature difference of 33 C between different profiles. However, adding hydrogen to the natural gas increased gas temperatures in all passes and raised the average energy efficiency of the boiler from 77.71 per cent to 80.36 per cent. The modeling of the turning chambers revealed that thermal losses contributed to a reduction in average efficiency of up to 12 per cent.