Exploration of KCl deposition dynamics for the formation of coarse and fine layer deposits

In biomass and waste combustion, the release of KCl into the gas phase results in its deposition on heat exchanger surfaces, enhancing their stickiness and accelerating deposit buildup from incoming ash particles. Despite this, the mechanisms governing KCl deposition remain inadequately explored. KCl has been observed to form either a fine, powdery layer through thermophoresis of nucleated KCl particles or a crystal-like, coarse layer through heterogeneous condensation of KCl vapor. However, the quantitative assessment of coarse and fine layer formation has never been performed. In this study, an entrained flow reactor and mechanistic model were employed to analyze KCl deposition behavior, examining the effect of probe temperature and exposure time. Quantification of both the coarse and fine deposit layers was accomplished. Probe temperature was found to have no influence on the deposition in the range from 300 °C – 595 °C. For coarse layer formation, the critical factor was the development of a fine layer upon which the crystal-like structure of the coarse layer grows. Lower probe temperatures accelerate initial layer formation, initiating the coarse section sooner. Additionally, it slightly increases nucleation and condensation, however, nucleation is increased stronger resulting in a lower ratio of coarse to fine deposit. The quantification of the coarse mass, in conjunction with the simulation, provides a first estimation of the critical mass required to initiate heterogeneous condensation on a steel tube. In this study, a critical mass of 1.26e-3 g/cm2 was determined.