Energy & Fuels 2008, 22, 3005–3015
3005
Properties of Particles in the Fly Ash of a Biofuel-Fired Circulating Fluidized Bed (CFB) Boiler Linda S. Johansson,*,† Bo Leckner,‡ Claes Tullin,† Lars-Erik Åmand,‡ and Kent Davidsson‡ Department of Energy Technology, SP Technical Research Institute of Sweden, Box 857, SE-501 15 Borås, Sweden, and DiVision of Energy ConVersion, Department of Energy and EnVironment, Chalmers UniVersity of Technology, SE-412 96 Go¨teborg, Sweden ReceiVed April 18, 2008. ReVised Manuscript ReceiVed June 4, 2008
A quench/dilution probe was designed and used for sampling of particles and vapors in the flue gas upstream of the convective path in a circulating fluidized bed boiler. Downstream of the convective path, flue gas was sampled by a heated probe. The sampled gas was lead through low-pressure impactors for analysis of the mass size distribution of particles. The particles were analyzed for Cl, S, K, Na, Ca, Mg, P, Al, Fe, Ba, Mn, and Si. A series of tests was carried out to study minimization of problems originating from the alkali content in the fuel, including the effect of additives to the fuel, change of bed material, and co-combustion with sewage sludge. Particle concentration and composition were compared to deposit growth and composition on a deposit probe, which represents a superheater. Alkali chloride in the particles was compared to online measurements of gaseous alkali chlorides. The continuous mass deposition on the heat-transfer surface in the convective path was illustrated by calculation of the reduction of heat transfer by means of the temperature drop of the flue gas. Fine particles (1 µm) particles increased. These test cases also showed the lowest deposit growth on the superheater probe. The particles deposited in the convective path were mostly coarse ones. The most favorable case with respect to low deposits was cocombustion with sewage sludge.
Introduction The alkali and chlorine contents may be high in biofuel, and this may cause operation and maintenance problems by deposit formation and subsequent corrosion on heat-transfer surfaces in boilers, especially on superheater tubes. Prevention of deposit formation is urgent from an operation and maintenance point of view. To decrease the formation of corrosive deposits, sulfur can be added to convert alkali chloride into alkali sulfate,1 or kaolin can be added to form aluminum-silicate compounds, both more high-melting compounds.2 Sewage sludge, containing both sulfur and aluminum-silicate compounds, also has a positive effect on deposit formation.3 Besides problems related to deposit formation, sintering of bed material is also common during biomass combustion. Sometimes other bed materials than silica sand are used as a bed material to prevent sintering.4,5 In such cases, deposit formation may be affected as a side effect. To apply the correct measures to prevent deposit formation, an understanding of the formation mechanisms is crucial. It is then important to know if particles or gases form the deposits * To whom correspondence should be addressed. Telephone: +46-10516-5501. Fax: +46-33-13-1979. E-mail:
[email protected]. † SP Technical Research Institute of Sweden. ‡ Chalmers University of Technology. (1) Brostro¨m, M.; Kassman, H.; Helgesson, A.; Berg, M.; Andersson, C.; Backman, R.; Nordin, A. Fuel Process. Technol. 2007, 88, 1171–1177. (2) Steenari, B.-M.; Lindqvist, O. Biomass Bioenergy 1998, 14, 67–76. (3) Åmand, L.-E.; Leckner, B.; Eskilsson, D.; Tullin, C. Fuel 2006, 85, 1313–1322. ¨ hman, M.; Nordin, A.; Bostro¨m, D.; Hedman, H.; Eklund, (4) Brus, E.; O A. Energy Fuels 2004, 18, 1187–1193. ¨ hman, M.; Bostro¨m, D.; Eriksson, M.; Nordin, A. (5) Geyter De, S.; O Energy Fuels 2007, 21, 2663–2668.
by inertia impaction of particles, diffusion (particles or gas), thermophoresis (particles), condensation (gas), or chemical reaction (particles or gas).6,7 Coarse particles (>1 µm), originating from nonvolatilized compounds, form deposits by impaction. The other particle mechanisms are related to deposit formation by fine (
