SOx emissions from combustion processes can be reduced by the application of in-flame methods, including sulfur self-retention (SSR). Novel indexes for fossil fuels and biomass are proposed for the evaluation of the SSR process. The application of biomass as an additive to avoid SOx emissions from an industrial-scale boiler is presented, and the possible mechanism of the SSR process is discussed. The cocombustion of fossil fuels with biomass containing a high amount of CaO-ash reduces the amount of limestone required for the deSO(x) process.