The agglomeration behaviour of lochiel coal during fluidised bed gasification

Abstract

Fluidised bed gasification (FBG) is a promising process for utilising high-sodium and highsulphur coals, but agglomeration and defluidisation are potential problems. Experiments have been conducted to investigate these phenomena, to assist in elucidating the mechanisms at play and to develop operating solutions. It was found that agglomeration and defluidisation of Lochiel coal occur, within the 4-hour timeframe of these experiments, only when bed temperature exceeds 850°C and the air/fuel ratio is above 2.5. Agglomerates are composed mainly of silicate material, in combination with Na, Ca, Mg, Al and Fe in varying amounts. It is surmised that non-crystalline phases in agglomerates are sodium disilicate, with inclusions of Ca, Fe, Mg and Al that are expected to lower the mixture melting point, causing FBG operating problems when the total inorganic content of the bed rises above approximately 88wt% and with temperatures exceeding 850°C. It is expected that defluidisation can be avoided if operation is maintained outside of these conditions.