A New Method for Direct Determination of Char Yield during Solid Fuel Pyrolysis in Drop-Tube Furnace at High Temperature and Its Comparison with Ash Tracer Method

A drop-tube furnace with a novel double-tube configuration was successfully developed to directly determine char yields during the pyrolysis of a wide range of solid fuels (mallee wood; mallee leaf; rice husk; biosolid; and subbituminous, bituminous, and anthracite coal) at a gas temperature of 1573 K. The char yield from pyrolysis of mallee wood and mallee leaf is <5%, similar to 13% for rice husk, similar to 16% for biosolid, similar to 45% for subbituminous and bituminous coal, and similar to 75% for anthracite coal. The retentions of Na, K, Mg, and Ca in biomass chars are <50%. About 35% of Na and K and similar to 66-85% of P and refractory species in biosolid are retained in the char. In contrast, the retentions of major inorganic species in coal chars are >85%. This study shows using total ash as ash tracer results in 45-220% overestimation of char yields for biomass fuels and 13-27% for coals due to partial evaporation of ash. Similarly, selecting Na and K results in overestimation of biomass char yields by at least 2.5 times and selecting P leads to overestimation of biomass char yields by at least 80% because of substantial release of these species during pyrolysis. Similarly, selecting Mg, Ca, Al, Fe, Ti, or Si as tracer also results in inaccurate estimation of char yields due to partial release of these elements during pyrolysis. It is noted that for Si, which is often used as a tracer, the overestimation of char yields is 9-16% for coals but can be substantial (17-50%) for the case of biomass samples because of the substantial Si release during the pyrolysis of biomass (especially mallee wood with similar to 32% of Si released). Clearly, for the solid fuels studied, no single element can be reliably used as tracer for calculating char yield during pyrolysis at high temperature. The new experimental method developed in this study fills this critical gap and enables direct determination of char yield during solid fuel pyrolysis in drop-tube furnace at high temperature.