Method for controlling a chemical recovery boiler and a chemical recovery boiler

A method for controlling a chemical recovery boiler that is configured to burn black liquor or brown liquor to produce smelt, wherein the smelt forms a char bed in a furnace of the chemical recovery boiler and is let out from the furnace through at least two smelt spouts, the method comprising:

• imaging an area of the char bed to obtain an image of the area, wherein the area of the char bed consists of areas close to the at least two smelt spouts;
• determining a measured temperature of the areas close to the at least two smelt spouts using the image of the area of the char bed, wherein the measured temperature of the areas close to the at least two smelt spouts represents an average temperature of the areas close to the at least two smelt spouts;
• determining whether the measured temperature of the areas close to the at least two smelt spouts is less than a target temperature;
• and
• controlling the chemical recovery boiler such that the average temperature of the areas close to the at least two smelt spouts is higher than the target temperature,
• wherein:
• the average temperature of the areas close to the at least two smelt spouts is an average of measured temperatures at points q, each point q being located further away from a center O of the furnace than from a corresponding one of the at least two smelt spouts in a horizontal cross section of the furnace, and
• the target temperature is a melting temperature of the smelt calculated using the concentrations of sodium carbonate, sodium sulfide, and sodium sulfate in the smelt.