ABSTRACT
Iron containing minerals play a central role in the initiation of deposits in the burner region of pulverized coal fired boilers. The stickiness of the depositing particles depends on the mineral association of the iron and the flame fate of this mineral. Of the transformations of siderite (iron carbonate) little is known. According to thermodynamics the flame product is molten, however fragmentation in the flame is expected. The fragments are expected to be too light for deposition. Quantification of the fragmentation was the first objective in the study reported here. The tests were performed in a laboratory burner with typical European import coals containing siderite. Particles were analyzed for siderite via CCSEM (Computer Controled Scanning Electron Microscopy). With this method not only siderite but also iron oxide and iron hydrate are classified as siderite. For this reason "siderite" is used as a collective name for these three types of coal minerals. Visual inspection of the particles revealed the morphological flame-changes. It was tried to measure the actual siderite (iron carbonate) content of some of the coals with TGA-MS (ThermoGraphic Analysis) coupled with Mass Spectrometry. The mass spectrometer measures on-line the amount of carbon dioxide released. The method proved to be quantitative. The following was found for the coals in the tests. Fragmentation did not occur. The fate of the "siderite" in the various coals was found to be coal-dependent because of the variation in composition of the "siderite". The "siderite" in (Australian) Hunter Valley was mostly iron carbonate and these particles melted in the flame but were not sticky. In Colombian El Cerrejon coal the "siderite" did not melt. The vast majority of the "siderite" in this coal is not iron carbonate but magnetic Fe2O3. The "siderite" in Scotts Branch (eastern US) did not melt. It most likely is iron hydrate (FeOOH). "Siderite" from the Belgian Behringen mine is iron carbonate. The flame products are very sticky. This stickiness might be due to the fact many of the siderite particles contain an alumino silicate which might serve as a flux. It is concluded that the amount of iron carbonate is a measure for the "melting" fraction of coal-"siderite". TGA-MS appears the appropriate method to measure this.
