The High Heat of Fast Pyrolysis for Large Particles

Abstract

The heat FOR pyrolysis, h _p , (including sensible heats required to raise the reactants to pyrolysis temperature; the heat OF pyrolysis, Δh_p; and the heat to raise the products to the surface temperature) is an important value required for the science and engineering of biomass thermal conversion.

A novel “water tracer” method of measurement of the heat FOR pyrolysis has been tested and the results are presented here. The method is based on measuring the time required for pyrolysis of a dry particle and an identical particle with high moisture content. The difference in time is attributed to the heat required to vaporize the water at the temperature of pyrolysis from which one can calculate the heat required for pyrolysis.

A dry biomass particle (birch dowel) was heated in the reducing flame of a Meeker burner while being slowly rotated around its axis and the time for pyrolysis was measured. The time for pyrolysis of an identical particle with high moisture content was also measured. The flame intensity is kept constant by monitoring its heat transfer to a thermocouple. The heat FOR pyrolysis can then be calculated from the difference in time according to

h_b = h_w m_w t_b /[m_b (t_w,b - t_b)]

Birch dowels with nominal diameters of 0.95, 1.27 and 1.90 cm X 2 cm long were pyrolysed in the flame of a Meeker burner maintained at a constant heat transfer rate of 28.1 watts/cm² with a thermocouple (reading 1000°C). The time for pyrolysis varied from 53 to 355 seconds and was reproducible within 1–3 sec. The heat for pyrolysis of the dowel was then calculated to be 3472, 3280, 2909 J/g respectively. (These values are about 15% of the heat of combustion.) The charcoal yields were 3.1%, 7.8% and 10.7% reflecting higher yields in larger particles.