Combustion reactivity and morphological change in coal chars: Effect of pyrolysis temperature, heating rate and pressure

Changes in the combustion reactivity of sets of chars from five coals have been determined as a function of increasing devolatilisation temperature, heating rate and pressure. Chars were prepared in wire-mesh pyrolysis reactors, where evolving volatiles are rapidly removed from the vicinity of the heating coal. Samples were heated at rates between 1 and 5000 K s(-1) to temperatures up to 1500 degrees C and H-2-pressures up to 150 bar. Combustion reactivities of the chars have been determined by iso-thermal thermogravimetric methods. For rapidly heated chars, reactivity was found related to temperature by: In (R(max)) = a - b * T. Char reactivities were found to increase with increasing heating rates up to 1000 K s(-1), but to level off between 1000 and 5000 K s(-1). At 700 degrees C, reactivities of hydropyrolysis chars were found to go through a minimum at around 40 bar, whilst at 850 degrees C the upturn of char reactivity took place at lower pyrolysis pressure. Taken together, our results suggest that char reactivity is related to tar release and that an upper limit exists to the enhancement of char porosity and reactivity, determined by patterns - and limits - of tar release from coal particles.