Boiler efficiency calculations are done using ASME PTC 4.1 methods, as shown in Q6.19. When a combination of fuels is fired, the calculations can be involved. The results from a program developed are shown in Fig. 3.14. They show the performance of a boiler firing two different fuels at the same time. Based on the exit gas temperature and measured or predicted oxygen for the flue gas mixture, one can simulate the excess air and obtain the performance with individual fuels first and then obtain the combined effect on air and gas flows, flue gas analysis, combustion temperatures, heat losses, and efficiency.