Mechanisms of Deposit formation in the Grate-kiln Process

Abstract: Fly ash particles from the combustion of coal together with disintegrated particles arising from iron-ore pellets result in accumulation of deposits on the refractory linings of the grate-kiln induration machine during the iron-ore pelletizing process. Deposit formation gives rise to decreased production efficiency, unscheduled shutdowns, high cleaning costs, and equipment failures. The deposits amass in the high-temperature regions of the induration furnace thus disturbing the flow of gas and pellets. Therefore, to tackle the above-mentioned issues, an understanding of deposit formation mechanism is of crucial importance. In contrast to pulverized-fuel-fired boilers, a grate-kiln process is characterized by a longer residence time, a highly oxidizing atmosphere, the presence of recirculating alkalis and disintegrated iron-ore pellet dust in the process gas. Given the foregoing, ash deposition phenomena in the grate-kiln process are much more complex compared to ordinary pulverized-fuel-fired boilers. This study was conducted with the objective of addressing the effect of disintegrated iron-ore pellet dust on deposit formation and the mechanisms behind deposition (slagging) in the grate-kiln process.Because of climate change and other pollution concerns, there is a desire to reduce on the combustion of carbon-intensive fuels. Moreover, as previously mentioned, the ash material from coal together with the disintegrated pellet dust cause deposition and slagging in rotary grate kilns, which can inflict severe production losses. For these reasons, it is of interest to study the potentials of co-firing coal with alternate fuels such as woody biomass. While the combustion of such woody biomass can be carbon-neutral, their effects upon deposition and slagging during co-firing with coal in a grate-kiln is unknown. Therefore, co-firing coal and woody biomass (softwood bark) was also thermodynamically investigated with particular emphasis on the effect of alkali recirculation upon deposition (slagging).To initiate elucidation of deposit formation during the iron-ore palletization process, a comprehensive set of experiments was conducted in a 0.4 MW pilot-scale pulverized-coal-fired furnace (ECF) where three different scenarios were considered as follows; Case1 (reference case): a high-rank bituminous coal was combusted without the presence of disintegrated iron-ore pellet dust. Case2: Natural gas was combusted together with simultaneous addition of hematite dust to the gas stream. Case 3: Coal was combusted together with the addition of hematite dust simulating the situation in the large-scale setup. Particles and short-term deposits were sampled from 3 positions of different temperature via a water-cooled rapid dilution sampling probe. Several characterization methods coupled with thermochemical equilibrium calculations (TECs) and viscosity estimations were employed to shed light on deposit formation and the mechanisms behind it in rotary kilns of iron-ore pelletizing plants. The most extensive interaction between hematite dust and coal-ash particles was observed in the coarse mode where a significant number of coal ash globules were abundantly found attached to the surface of the hematite particles. The morphology of the sharp-edged hematite dust particles was changed to smooth-edged round particles suggesting that hematite dust particles must have interacted with the surrounding aluminosilicate glassy phase. Consequently, the Fe content of the aforementioned glassy phase experienced a considerable increase. The short-term deposits during coal combustion (Case1) were highly porous in contrast to the high degree of sintering observed in the experiments with hematite addition (Case3). The incorporation of Fe into the aluminosilicate glassy phase (liquid fraction) decreased the viscosity and resulted in the formation of stronger (heavily sintered) deposits. The results suggested that hematite dust slagging tendency was independent of temperature, within the studied temperature-range (approximately 1100-1500 ºC), and required an auxiliary phase-provided by coal-ash- to form tenacious particles and cause slagging. In light of the experimental observations and TECs, a scheme of slag formation during the iron-ore pelletizing process was proposed.The TECs carried out for the woody biomass/coal blends indicated that that woody biomass is likely to increase the fraction of molten slag and exacerbate slagging when the molar ratio of Si/Ca > 2ii(equivalent to addition of more than 30 wt.% woody biomass, i.e. bark). However, when the abovementioned molar ratio is less than 2 (equivalent to addition of less than 10 wt.% bark), the fraction of molten slag increases by about 10 wt.% which does not seem to promote slagging extensively and can serve as a reasonable experimental blend.Overall, this work forms part of a wider study which aims at deepening the understanding of ash transformation phenomena during the large-scale pelletizing process. The findings from the current work are necessary to pave the path towards achieving the aforementioned goal.