Formation of blue deposits in kraft recovery boilers

Fireside deposits in recovery boilers are typically white, red, pink, grey, black, and occasionally yellow, depending on where they are in the boiler, the mechanisms by which they are formed and the environment to which they are exposed. While rare, blue deposits have been occasionally reported, and some were “bluer” than others. This study systematically examines the cause of the blue coloration of deposits in recovery boilers. The results show that in order for a deposit to become blue, it must a) contain sodium carbonate (Na2CO3), b) contain a small amount of manganese (Mn), c) be molten or partially molten, and d) have been exposed to an oxidizing atmosphere. Since deposits always contain Na2CO3 and Mn, these requirements suggest that blue deposits can form only in the superheater region of the recovery boiler when oxidizing conditions prevail. Blue coloration is thus more likely to be observed in boilers operating at a reduced firing load with a high excess O2 target. BACKGROUND Fireside deposits form massively on heat transfer tube surfaces in kraft recovery boilers due to the high ash content of the fuel (black liquor) coupled with the low melting temperature of the ash [1]. Deposits can be different colors at different locations in the boiler due to the different mechanisms through which they form and the different gas environment to which they are exposed. In the lower superheater near the bullnose region, deposits are formed mainly by inertial impaction of carryover, entrained molten/partially molten smelt and unburned char particles in the flue gas. They are usually red or pink as shown in Figure 1, and often speckled with black char particles. In the upper superheater region upstream of the generating bank, deposits are essentially a mixture of better-burned carryover and condensed alkali salts (fume), and so they are usually pinkish, greyish or silver-white. In the generating bank and economizer regions, deposits become whiter and whiter toward the economizer outlet as they contain increasingly more fume and less carryover. Figure 1. Pink and white deposits at different locations in a recovery boiler Regardless of where and how they form, deposits consist of mainly sodium sulfate (Na2SO4) and sodium carbonate (Na2CO3), with a small amount of sodium chloride (NaCl), sodium sulfide (Na2S) and potassium salts (i.e. K2SO4, K2CO3, KCl, etc.) [1]. Since all of these compounds are white, what makes smelt and carryover deposits red or pink is not well understood. In their study of the Na2S-Na2SO4 phase diagram by reducing Na2SO4 to Na2S in hydrogen (H2) at constant temperatures, Tran and Barham [2] observed that the resulting Na2S-Na2SO4 melt was always red. However, when the gas stream was switched from a strong reducing H2 gas to an inert neutral nitrogen (N2) gas, the melt color turned quickly from red to white, even though the melt composition was unchanged. They postulated that the red coloration of the melt is a result of the presence of sodium sulphoxylate (Na2SO2) an intermediate reduced