Capture of harmful gaseous KCl with coal fly ash under conditions typical for fluidized beds

During co-combustion of coal and biomass, potassium (K) from the biomass will be released as KCl to the gas phase. This KCl causes corrosion on refractories, water walls and superheaters and these issues often limit the share of biomass to be co-fired. One way of avoiding harmful KCl could be to absorb it with an additive or with ash from the fuel mix during intelligent fuel-mix firing. One example is the use of kaolin to absorb potassium as shown in the reaction below:

 Al₂O₃∙2SiO₂(s) + 2KCl(g) + H₂O(g) => K₂O∙Al₂O₃∙2SiO₂(s) + 2HCl(g)

 In this study coal ash originating from a bench-scale circulating fluidized bed (CFB) has been studied for its ability to capture gaseous KCl.

 The experiments were done in a laboratory reactor by introducing KCl to the heated reactor, whereas the salt started to evaporate and was led through a sample holder with a coal ash bed. The ash bed was kept in place by a metallic net. The carrier gas consisted of 15% water and air and tests were done at 800, 850, 900 and 950°C.

 After each experiment, the weight of the remaining KCl was determined by dissolving it in water in the sample holder and analyzing the solution for chlorine ions by means of ion chromatography. The difference between the introduced KCl and the remaining Cl-ions give the KCl gaseous concentration during each test when the gas flow and temperature are known. The solid sample (absorbent) was weighed and analyzed for K (wet chemical analysis). A part of the sample was casted in epoxy, cut and polished and further studied with a SEM/EDX (elemental mapping, imaging, etc.).

 In this paper, we present the KCl capture ability of coal fly ash produced in a fluidized bed. It was shown that the kaolin-type minerals in fly ash were as effective as kaolin to react with potassium, while cyclone ash was not as effective. It was also shown that a reaction temperature increase from 900 to 950°C, significantly increased the potassium capture. The temperature effect was speculated to be connected to the phase change of kaolin above 900°C.