Decomposition of pyrite during pulverized coal combustion

Abstract

The decomposition of pyrite in air, 2% oxygen in nitrogen and nitrogen atmospheres, has been studied at temperatures ranging from 300 to 1100 °C. Pyrite was found to lose sulphur from 300 °C upwards, to form pyrrhotite and haematite. In both 2% O 2 and N 2, magnetite forms above 800 °C in preference to haematite. In 2% O 2, an atmosphere which closely resembles boiler conditions, the maximum amount of haematite was found in the range 800–900 °C. Drop-tube experiments, using finely ground pyrite and pyrrhotite as starting materials, have shown that liquid iron sulphides form from 1000 °C upwards. This enables a deposit to adhere well to a metal substrate surface. Microstructural analysis has shown that these deposits consist of deformed spherical agglomerates with a porous internal structure. Burner impeller deposits, rich in iron, taken from a pulverized coal-fired utility boiler have been examined. By comparing the microstructures of the deposits with those from the drop-tube experiments, it has been possible to propound a mechanism for their formation. Preferential deposition of liquid iron sulphides followed by in situ oxidation, is believed to be the mechanism by which these deposits form.