Bed Agglomeration Characteristics of Biomass Fuels Using Blast-Furnace Slag as Bed Material

Abstract

Agglomeration of bed material may cause severe operating problems during fluidized bed combustion. The attack or coating layers that are formed on the bed particles during combustion play an important role in the agglomeration process. To reduce bed agglomeration tendencies, alternative bed materials may be used. In this paper, bed agglomeration characteristics during the combustion of biomass fuels using a relatively new bed material (iron blast-furnace slag) as well as ordinary quartz sand were determined. Controlled agglomeration tests lasting 40 h, using five representative biomass fuels (bark, olive residue, peat, straw, and reed canary grass) were conducted in a bench-scale fluidized bed. The bed materials and agglomerates were analyzed using SEM/EDS and X-ray diffraction. Chemical equilibrium calculations were performed to interpret the experimental findings. The results showed that blast-furnace slag had a lower tendency to agglomerate than quartz sand for most of the fuels. The quartz particles showed an inner attack layer more often than did the blast-furnace slag. The blast-furnace slag had a lower tendency to react with elements from the fuel. The outer coating layer had similar characteristics and thickness for both bed materials when the same fuel was combusted. However, the inner attack layer thickness was larger for quartz particles. SEM/EDS analyses of the agglomerates showed that the inner Ca-K-silicate-rich attack layer was responsible for the agglomeration of quartz sand. The composition of blast-furnace slag agglomerate was similar to the outer coating layer. Chemical equilibrium calculations showed that the original composition of the blast-furnace slag was close to the equilibrium composition, and hence there was no major driving force for reactions between that bed material and K and Ca from the fuel. The homogeneous silica-rich attack layer (with a low melting temperature) was not formed to the same extent for blast-furnace slag, thus explaining the lower bed agglomeration tendency.

Co-Combustion of Animal Waste in a Commercial Waste-to-Energy BFB Boiler

“…Therefore, it was important to determine whether their original composition could have affected the compounds bonding to their surface during combustion. The results, however, showed no indications of differences between the particles and, in contrast to previous findings, no signs could be found of alkali silicate melt on the inner surface of the silica sand particles [6,19,20].…”

Evolution of fuel-N in gas phase during biomass pyrolysis

“…For circulating fluidized bed, silicacontaining sand is usually used as the bed material, and potassium in biomass can react with silica to form a low temperature eutectic during combustion, which will damage the operation [49][50][51][52][53][54][55][56][57][58][59][60][61][62] and some possible ways have been used to solve the alkali metal related problems [63][64][65][66][67][68][69][70][71][72][73][74][75][76][77].…”

Bed agglomeration characteristics of rice straw combustion in a vortexing fluidized-bed combustor

“…The methods mentioned above can only prolong the defluidization time but still cannot avoid agglomeration and defluidization. Some previous studies have shown that biomass co-combustion with peat or coal can be an inexpensive measure for preventing bed agglomeration (Arvelakis and Frandsen, 2007;Brus et al, 2004;Gogebakan et al, 2009;Lundholm et al, 2005). The main mechanism for preventing the formation of viscous coatings is the retention of alkali by chemisorptions or physical adsorption by minerals and ash constituents in coals (Lundholm et al, 2005).…”