ASH FORMATION DURING FLUIDIZED-BED INCINERATION OF PAPER-MILL WASTE SLUDGE

Ash formation during industrial-scale bubbling fluidized bed (BFB) and circulating fluidized-bed (CFB) combustion of bark, waste wood and pa per mill sludges has been studied. The principal ash formation mechani sm was the sintering of 0.2-4 mu m paper filler mineral particles into larger, porous ash agglomerates From 10 to 200 mu m in size which dom inated the fly sh total mass and specific surface area. Fly ash total mass concentration at the electrostatic precipitator inlet conditions varied from 4.5 g N(-1)m(-3) at BFB to 8.3 g N(-1)m(-3) at CFB while t he respective total surface area concentrations as determined with nit rogen absorption were 32 and 83 m(2) N(-1)m(-3). During bark combustio n in the BFB, we observed an ultrafine mode at 0.05 mu m consisting of spherical particles and their chain-type agglomerates. Ultrafine mode mass and surface area concentrations were 3 and 0.1 m(2) N(-1)m(-3), respectively, as determined with the electrical low-pressure impactor and with the differential mobility analyzer. Ultrafine particles were formed via nucleation of volatilized ash species followed by particle growth via collision and coalescence and via vapor condensation. Durin g waste wood combustion in the CFB, ultrafine particle concentration w as significantly lower than during bark combustion in BFB, indicating reduced ash volatilization during CFB combustion. When co-firing sludg e with bark in the BFB end with waste wood in the CFB ultrafine partic le concentration was Further reduced. Results from continuous aerosol measurements by the electrical low-pressure impactor (ELPI), by the sc anning differential mobility analyzer (SMPS) and by the tapered elemen t oscillating microbalance (TEOM) as the function of BFB process condi tions suggest that co-combustion of sludge reduces significantly the u ltrafine particle formation rate. No significant enrichment of alkali and trace metals in the ultrafine particles was observed. (C) 1998 Els evier Science Ltd. All rights reserved.