Plugging problems in kraft recovery boilers (RB) are caused by a combination of factors. These are: the particle quantity, particle composition (e.g., stickiness), recovery boiler operating conditions and sootblowing efficiency. The concentration of soluble elements such as Cl and K increases in the recovery cycle when mills reduce water consumption and liquor losses, and/or recycle bleaching effluent. Cl and K lower the melting point and decrease the sticky temperature of the deposits formed on the tube surfaces. Deposits include carryover (0.01-3 mm), fume (0.1-1 ?m) and intermediate size particle [1].
Carryover particles are partially oxidized smelt or partially burnt black liquor (BL) droplets, and they deposit mostly on the superheater tubes. Fume particles form by condensation of the vapours of Na/K compounds, and mostly deposit on the generating bank and economizer tubes. The quantity of carryover particles increases when the firing load is increased, when the proportion of smaller liquor droplets increases (i.e., from firing low viscosity liquors), and when the flue gas velocity is high. The quantity of fume particles is determined by the vapourization rate of Na/K compounds from the char bed, and the rate of Na/K release during liquor pyrolysis. Therefore, operating the boiler with a hot bed (i.e., high solids firing) will generate more fumes in the upper furnace. High solids firing will also produce larger droplets, lower the quantity of particles and reduce fouling by carryover.
The particles formed in the RB have to be sticky to adhere to the tube surfaces. The stickiness of the particles depends on their liquid content when they impact the tube surface. The liquid content depends on the particle composition and temperature. It has been shown that particles with a liquid content of over 15% are sticky. The sticky temperature can be estimated if the particle’s composition is known [2]. However, this is not usually possible, as the composition changes continuously when carryover particles are being formed and deposited. Laboratory and field tests indicate that the Cl content of carryover particles is around 30% of the Cl in the feed BL, and the K content is about 80% that of the feed BL [3]. Two factors appear to be responsible for the depletion of Cl in the carryover with time: one is the vapourization of Na/KCl, and the other is the sulphation of Na/KCl by SO2 in the flue gas, that releases HCl.
2Na/KCl +SO2+O2+H2OàK2/Na2SO4+2HCl?
Higher bed temperature leads to greater depletion of Cl and K, higher sticky temperatures and reduced deposition of carryover. However, greater depletion of Cl and K in carryover leads to greater enrichment of these elements in the fume. Field studies have shown that there is a linear correlation between the K and Cl contents of the as-fired BL, the smelt, the carryover deposits and the ESP dust [3]. This linear correlation is not expected to hold if there is excess sulphur in the flue gas. Simple approximation can be used to estimate the composition and sticky temperature of carryover in a RB. This approximation is based on the above- mentioned linear relationship. Chemical analysis of the as-fired BL and ESP dust is all that is required for the calculation of the sticky temperature. First of all, Cl and K enrichment factors are calculated for the ESP dust. The enrichment factor for Cl can then be plotted versus that of K. The linear graph is extrapolated to estimate the Cl and K content in the carryover, using a Cl enrichment factor of about 0.4 and a K enrichment factor of about 0.88 [3]. The sticky temperature can then be predicted for a range of carryover, using available graphical data [2].
1. Tran, H.N. et al, Fouling of tube surfaces in kraft recovery boilers, 40th anniversary, Int. Rec. boilers conf., Porvoo,
2. Tran, H.N. et al, The sticky temperature of recovery boiler fireside deposits, Pulp & Paper Canada, 103:9, P.29-33 (2002).
3. Khalaj, A. et al, Composition of carry over particles in recovery boilers, Chem. Rec. Conf. Charleston, SC, USA, June 6-10 (2004).
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