Defluidization caused by potassium salts in a laboratory bubbling fluidized bed

Many studies have investigated agglomeration of bed material in fluidized bed boilers during combustion of biomass fuels.  However, most studies, whether conducted in industrial or lab.-​scale, have used biomass or biomass ash.  In contrast, we have focused on the use of pure chem. compds. to shed more light on agglomeration reactions and mechanisms.  Also, by identifying the bed tolerance of a certain compd., plant operators may, based on fuel and ash analyses, in the future be able to adjust the bed material discharge and make up to prevent forced shut-​downs due to sudden bed defluidization.  So called difficult fuels, i. e. agrofuels, can also be understood as opportunity fuels due to their relative low prices.  They often contain significant amts. of reactive potassium, which may leave the bed as evapd. KCl, or with the fly ash in some solid form (e.g. K₂CO₃, K₂SO₄) or it may alternatively react with the bed particles producing a product layer of K-​silicates which may eventually cause agglomeration.  An elec. heated lab. scale bubbling fluidized bed reactor has been constructed primarily to study bed agglomeration.  The reactor is equipped with six thermocouples and two pressure gauges.  The pressure measurements are used for detecting any pressure changes over the bed as a sign of agglomeration.  In the expts. quartz sand was used as bed material and KCl, K₂CO₃ or K₂SO₄ was fed in small batches into the reactor at certain time intervals.  Tests were done between 750-​900°C.  In this study the fluidizing media was air, however, it is also possible to introduce a synthetic flue gas (over- or sub-​stoichiometric)​.  Once agglomeration occurred the expt. was finished and the reactor was dismantled to remove the bed and take samples for further analyses.  Bed samples were also taken for wet chem. analyses of the potassium amt. remaining, since some of it may evap. during the tests.  In this paper we present agglomeration results from these tests.  At 800°C, for example, the results showed that defluidization occurred when the KCl fraction in the bed reached 0.16 wt.-​%, while the corresponding no. was 1.1 wt.-​% for K₂CO₃.