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The heat of pyrolysis of beech and spruce wood was investigated by means of a differential scanning calorimeter. Wide variations were found for the heat of the primary pyrolysis process, depending on the initial sample weight and on the conditions used in the measurements. However, reporting the heat of the primary pyrolysis process versus the final char yield resulted in a linear correlation. This strong dependency of the heat of wood pyrolysis on the final char yield, that is in turn highly sensitive to the experimental conditions, can explain the uncertainty of the data for the heat of wood pyrolysis reported in the literature. A possible explanation for the variability of the heat of wood pyrolysis depending on the final char yield seems to be an exothermic primary char formation process competing with an endothermic volatile formation process.

A possible cause of the wide differences present in the literature values for the calorimetric measurements of the pyrolysis heat of biomass was analyzed. The apparent specific heat of char from pyrolysis of beech wood was investigated by means of a differential scanning calorimeter (DSC). The objective was to find out the influence of the use of a lid on the crucible on the experimental results. In a first experimental series, the crucible containing the sample and the empty reference crucible were closed by a pierced lid. In a second experimental series, the crucible containing the sample and the reference crucible were used without a lid. Results of experiments within the same series showed excellent agreement, but the results of the first series were completely different with respect to those of the second series. A possible explanation for these differences are the different emissivities of the crucible material (aluminium) and char. During experiments without a lid a different radiative heat exchange occurs between the two crucibles and the opposite wall of the DSC measurement cell. In order to confirm the effect of heat radiation inside the DSC measuring cell, additional experiments with aluminium oxide and graphite were carried out.