Reduction of chlorine deposition in FB boilers with aluminium-containing additives

Chlorine-containing fuels tend to form corrosive deposits on the heat transfer surfaces of boiler furnaces. High risk chlorine compounds are of type MCl, where M is Na or K. Experiments were performed with an electrically stabilised 20 kW bubbling bed reactor to determine the concentrations of Cl compounds in deposits and fine fly ash. The fuels of interest were fir bark and agricultural waste, fir bark and plastic waste, and forest residue. Deposits were collected on superheater tube simulators with controlled surface temperatures. Kaolin (hydrous 40:60 wt% Al2O3∗ SiO2) sharply decreased the tendency of Cl to bind in the deposits at high metal temperatures. Minimum kaolin dosage needed to decrease the Cl concentration in deposits collected at 500°C to the same level as in the main fuel (fir bark) was 30–50 wt% of the mass flow of the fuel ash. Kaolin did not prevent Cl from depositing at probe temperature of 400°C, where high-temperature corrosion is not a problem. The presence of other elements in the kaolin (e.g. Fe and Ca) increased the required dosage.