Packed-bed pyrolysis of some leguminosae PODS – exothermicity magnitude and products

Packed bed pyrolysis is carried out for a heating temperature of 575 K of pellets made by ten Leguminosae pods (peas, broad beans, cranberry and cannellini beans, peanuts, broom, tamarind, silver wattle, acacia, and carob). Except for some tree pods (carob, silver wattle, acacia), remarkable exothermicity is displayed with temperature overshoots approximately between 50–130 K. Based on thermogravimetric curves, it can be stated that high alkali contents (low peak rate temperatures) promote exothermic decomposition. Instead, negative factors are high contents of low molecular weight carbohydrates to the detriment of cellulose (low peak rates). The yields of charred solid are in the range 36-62 wt%. Carbon dioxide is the most abundant gaseous product, followed by smaller yields of carbon monoxide. The pyrolysis liquids mainly consist of acetic acid, hydroxy propanone, methyl alcohol, furfural, furfuryl alcohol and, in some cases, toluene. In general, the pyrolysis characteristics are quantitatively similar for pods belonging to the same botanical family, with a few exceptions caused by peculiar chemical compositions such as in the case of carob pods.