RECOVERY BOILER CHEMICAL PRINCIPLES

摘要

The complicated chemistry of the recovery boiler has become much clearer in the past few years. Interesting further research is being done, and more information is expected to be available, particularly concerning the release of sulphur and sodium contained in black liquor. The most important process in a recovery boiler, the reduction of sulphur compounds into molten sulphide, is mainly achieved by means of the char in the black liquor. To ensure an efficient reduction, the char content of the smelt bed must be maintained at a sufficient level. The sulphur and sodium compounds in the flue gases are formed as the hydrogen sulphide and the sodium vapours released in the lower furnace react with each other. The reactions proceed in different ways depending on the ratio of sulphur to sodium in the combustion gases. The S/Na_2 ratio in the flue gas is high at low furnace temperatures or high sulphidities of the black liquor. In cases like this, there will be a surplus of sulphur dioxide, which may turn the dust into acidic sulphate, with resulting fouling and corroding effects. The S/Na_2 ratio in the furnace gases is low at high furnace temperatures or low sulphidities of the black liquor. Under these conditions the sodium compounds of the combustion gases bind virtually all the sulphur dioxide. Then, acidic sulphates will not be formed, and the sulphur dioxide emission with the flue gas will be completely eliminated. The pH-value of dust is a sensitive indicator for the sulphur/sodium chemistry of the flue gases. The dust samples should be taken fresh, directly from the flue gas before the electrostatic precipitators. High pH (>10) indicates that the S/Na_2 ratio in the flue gases is less than unity. Low pH (<7), again, indicates that this ratio is high, with an increased tendency for formation of acidic sulphates. Foreign elements potassium and chlorine are enriched into gaseous form in the lower furnace, mainly as alkali chlorides. If the S/Na_2 ratio of the combustion gases is sufficiently high, the alkali chlorides transform at a later stage into hydrochloric acid and alkaline sulphate. The presence of potassium and chlorine in the dust -especially in combination - lowers the dust melting temperature range, which results in increased sticking and fouling tendencies.