Full-Scale Investigations of Initial Deposits Formation in a Cement Plant Co-Fired With Coal and SRF

This work investigates the initial (short-term) deposit formation in a cement calciner co-fired with coal and SRF (Solid Recovered Fuel). The main objective was to evaluate and compare the tendencies of deposit formation (i.e. material deposition rate and composition) at different locations (heights) in the calciner system, during different operation conditions. A steel probe was used to collect initial deposits from four different sampling locations: i) in the kiln riser (two sampling locations), ii) in the mid calciner, and, iii) at the outlet from the last preheater cyclone (C5). When inserted into the calciner, deposits start to form on the probe. After an exposure time of the probe between 1 and 20 min, the collected material is recovered and weighed. The collected samples were characterized by SEM-EDX, ICP-OES respectively XRD. Plant operation data, measured gas concentrations, e.g. KCl(g) and SO2(g), as well as measured temperatures support the evaluation of deposit formation. Furthermore, the composition of the short-term deposits is compared with that of long-term (mature) deposits which have previously been collected from the site, thus giving further insight into how deposits initially form and mature.
The deposit rate measurements revealed that a lot of deposit is formed in a short time (within the first minute) due to inertial impaction. The average deposit formation rate is around 500 kg/(m².h) for 1 min exposure time, decreasing to < 100 kg/(m2.h) for exposure times ≥ 4 min (due to spontaneous shedding and erosion). These deposition rates are several orders of magnitude higher than typical deposition rates reported for e.g. coal and biomass combustion systems, which is ascribed to the very high particle loads in cement calciners. XRD-analyses identified five phases present in the samples: CaO, CaCO₃, Ca(OH)₂, KCl, and SiO₂. The degree of calcination of the samples clearly increases from
the C5 outlet and mid calciner to the kiln riser, which can be seen from the XRD-pattern showing varying contents of CaO and CaCO₃ depending on sampling location. The deposit samples show little variation in the content of K, Cl and S, except from a slight enrichment in KCl in the samples from the C5 outlet, as compared to the samples from the lower measuring locations. The content of K₂O is generally in the range [2.1-5.2 wt.%], and Cl is in the range [1.4-2.7 wt.%]. The content of SO₃ is [~1 wt.%]. The composition of the short-term deposits differs significantly from mature deposits. The short-term deposit formation is dominated by impaction, and, the composition is influenced mainly by the conditions in the hot “bulk” gas: the composition of the short-term deposits is similar to the composition of the entrained particles; while the build-up of mature deposits is influenced greatly by surface temperatures and long-term fluctuations; i.e. in zones with locally lower temperatures, “cold” surfaces/walls facilitate heterogeneous condensation of KCl.