From feed coal to fly ash: how transient oil co-firing reshapes hazardous-element budgets, sulphur balance and leaching behaviour of power-plant ashes

Feed coals, fly ashes and bottom ashes were collected simultaneously from seven pulverised-fuel units of a large Brazilian thermoelectric complex and examined by combined petrographic, mineralogical and geochemical methods in order to trace the links between the inorganic constituents of the fuel and those of the resulting combustion residues. Two of the units were sampled while their boilers were being started up with co-fired fuel and diesel oil, which provided an opportunity to assess how oil co-firing alters the residues. The feed coals proved remarkably uniform in mineral matter and bulk ash chemistry across the seven units, and the crystalline assemblage of the ashes, dominated by quartz, mullite and maghemite within a large amorphous fraction, was consistent with thermal transformation of that mineral matter. The fly ashes generated during oil co-firing were distinguished by markedly higher contents of unburnt carbon and retained sulphur, which reflect less complete combustion and lower furnace temperatures during start-up. These same ashes were enriched in several moderately volatile trace elements, with arsenic and mercury increasing by more than an order of magnitude. Principal component analysis of the trace-element budget resolved the normally fired ashes, the co-fired ashes and the bottom ashes as discrete populations, and the loadings ordered the elements along a volatility and chalcophile affinity gradient. In water-based leaching tests the co-fired fly ashes produced acidic conditions, in contrast to the alkaline leachates of the normally fired fly ashes, a difference governed by the balance between sulphate and the alkaline-earth oxides. Cationic elements were consequently more mobile from the acidic co-fired ashes, whereas elements released as oxy-anions remained more mobile from the alkaline ashes despite comparable or higher total contents. The work shows that a transient operational practice such as oil co-firing during start-up can measurably change the carbon content, sulphur retention, trace-element budget and leaching response of the ash produced. The dataset is reinterpreted here with an environmental focus, considering leaching from open coal stockpiles held for months on site, the small but element-rich fraction of fly ash that escapes capture and handling and reaches the atmosphere despite extensive reuse in cement, and the large bottom-ash deposits that remain incompletely utilised around the station.