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Previous work addressed the effects of CO2 rich atmospheres typical of oxy-combustion during the early stages of fast coal heat up and pyrolysis. A laminar drop tube reactor (DTR) was used for the experiments, reaching the temperature of 1573 K and heating rate of 104 - 105 K/s. It was shown that the presence of CO2 affects severely the reactivity of the coal chars. In the present work the same experimental equipment has been used to investigate fast pyrolysis of walnut shells, a lignin rich biomass sample, in nitrogen and carbon dioxide atmospheres. Char samples collected at different residence time along the reactor have been characterized by ultimate and elemental analysis and by SEM. Their combustion reactivity has then been measured by non-isothermal thermogravimetric analysis in air. The analyses show that at short residence times pyrolysis was not completed, however, it is more advanced in CO2 than in N2 atmospheres. Increasing the residence time in the rector pyrolysis is progressively completed, the O/C and H/C content of the samples decreases following a typical Van Krevelen pattern and differences between N2 and CO2 samples level off. The derivative thermogravimetric (DTG) curves of the char combustion show the existence of multiple peaks, which can be attributed to residues of the different ligno-celluliosic components originally present in the raw biomass. A kinetic model of char combustion has therefore been proposed which includes multiple parallel reactions. Notably, early combustion peaks progressively fade along with the particles residence time in the reactor, confirming the gradual completion of pyrolysis. The work is complemented by a companion paper which investigates the kinetics of pyrolysis and the effect of carbon dioxide under the low heating rate conditions typical of TGA and fixed bed reactor, showing minor effects of the gaseous atmosphere on the chars properties.