The co-combustion of fossil fuels with CO2-neutral fuels is an attractive way both to decrease CO2 emissions in energy production and to use fuel synergies which decrease each other’s undesirable properties. This paper presents a new approach to understand and predict the chlorine deposition tendency in the co-combustion of coal with biomass and RDF. This novel approach combines the results from deposit analysis with flue gas emission measurement and advanced fuel characterization methods. The experiments were carried out in a 0.1 MW circulating fluidized bed reactor. Two different types of bituminous coal (South African and Polish) were co-fired with RDF, demolition wood and bark. The traditional way to predict risk for chlorine deposition, the fuel S/Cl molar ratio, and the safe limit molar ratio > 4 for biofuels were shown to be inadequate. The mineral kaolinite in coal ash was found to be able to capture alkalis and, in most cases, more effectively than sulphur compounds. The alkali capture capability of coal sulphur is quickly consumed due to reactions with calcium compounds. Furthermore, the ability of SO2 to sulfate alkali chlorides were found to be weaker than presented in the literature. Thus in many cases it is only kaolinite that keeps Cl away from the deposit. New index to predict chlorine deposition tendency were introduced: the reactive (Al+Si)/fuel Cl molar ratio. The results showed a good correlation between the chlorine concentration in the deposit and the new index. The reactive (Al+Si)/fuel Cl molar ratio higher than 8–10 was found to prevent chlorine to deposit. The new approach for better understanding and preventing of chlorine deposition promotes the co-combustion of coal with biomass and RDF by introducing new synergy benefits.

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