Tough-to-remove deposits can form on the surfaces of recovery boiler (RB) tubes, even when the flue gas temperatures are too low to melt the dust.  Deposits formed from fume particles can begin to sinter and harden around 300^o C, and the rate of sintering increases significantly with temperature.  The sintering tendency is affected by the dust’s composition and physical properties, and the RB operating conditions. 

The composition of the fume particles changes as they move through the upper furnace, reacting with sulphur oxides. However, beyond the superheater region, the composition does not change very much (as the SO₂ levels are low) and the composition of the fume that deposits on the boiler bank tubes is the same as the ESP dust. Research has shown that the chloride content of the dust has a direct effect on sintering [1]. At Cl levels of less than 2 mole % (Cl/(Na+K)), the dust does not sinter well.  The effect of K becomes significant when the Cl content is more than 2 mole %.  The combination of high Cl and high K increases the sintering rate significantly. This is to be expected, as NaCl and KCl have a high vapour pressure relative to the other components of the dust, and they also decrease the first melting point temperature (FMT).  Rapid sintering takes place when the FMT is lowered. The FMT of RB dust without any Cl is very high (780^o C). However, this is greatly reduced (<600^o C) if a small amount of Cl is present. The FMT of dusts containing Cl will be affected mainly by K and, to a lower extent, by carbonate. Of the dust’s physical properties, an increase in un-compacted bulk density results in reduced sintering. This means that light dusts sinter more, while more dense dust (i.e., from the ash hopper) sinter less. The dust composition and particle size can be affected by the boiler’s operating conditions.  High solids firing, and increasing the firing load, result in hot beds and a low SO₂ concentration in the flue gas. The light dust will contain more carbonate and Cl, and sinter more readily. 

SO₂ at 1.0% concentration has been found to have a major impact, greatly increase the rate of sintering [2]. Since high levels of SO₂ are not usually found in RB in the superheater section and beyond, SO₂ is not usually a factor in the sintering of deposits. However, during upset conditions (i.e., a cold char bed) when the SO₂ concentration is increased for a short time, sintering conditions are favoured.

 1.             The sintering tendency of recovery boiler precipitator dust, Duhamel, M., et al, 2002 Tappi Fall Conf. & Trade Fair

2.             Effect of gas composition on fume sintering rates, Lien, S.J., et al, 2004 Int. Chem. Rec. Conf.