Comparative combustion properties of raw and washed Minto coals

The combustion performance of raw Minto coal (run of mine) and three levels of cleaned (washed) Minto coal was compared with that of a commercially available thermal bituminous coal from Western Canada. Tests were conducted in a pilot-scale pulverized coal-fired research boiler located at the Energy Research Laboratories (ERL) in Ottawa. The three cleaned coal samples were obtained from the CANMET Coal Research Laboratories coal cleaning facility in Devon, Alberta. Chemical, physical, petrographic and thermogravimetric analyses were performed on samples of the coals as they were received in Ottawa. Each of the coals flowed readily through ERL pilot-scale conveying and coal-handling system. The coals, which ground readily to produce a consistent size product from the pulverizer, ignited easily to produce stable flames at greater than 97% combustion efficiency. The highest combustion efficiencies (>99%) were obtained with the more finely ground coal feeds. The washed minto coal, containing the highest ash content, gave the lowest combustible content in the fly ash and the highest combustion efficiency. The ash from each of the Minto coal samples was quite fluid and slagged readily in the refractory-lined furnace bottom, whereas the ash deposits on the water-walls and superheater surfaces were light and powdery and did not constitute a fouling problem. At a 29% reduction (highest) in the ash content of the Minto raw coal the sulphur content was reduced from 6.88 to 5.31%. This washed coal sample burned in the pilot-scale utility boiler to produce a flue gas containing 2.9 g S02/MJ of fuel input, compared with 4.2 g S02/MJ for the raw coal. If the relationship between the per cent ash and sulphur contents removal is sustained, it is estimated that a 90% reduction in the ash content would result in a 72% reduction in the sulphur content. The resulting coal product, assuming no detrimental changes, should then burn to produce a flue gas containing about 1.3 g S02/MJ of fuel input.