In this study, a pyrolysis biomass cookstove with separate combustion and pyrolysis chambers (two-chamber stove) is investigated and compared to a widely-used char producing cookstove design (top-lit updraft, TLUD). The influence of pyrolysis fuel type (pellets of hardwood, corn stover, or switchgrass) on CO, NO, CO2 and particulate emissions, and the time dependence of particulate size distribution are quantified. Water boiling tests are conducted in a hood with pine wood as the combustion fuel for the two-chamber stove. Thermal and modified combustion efficiencies, and char yields and elemental compositions are reported. The sensitivity to fuel choice is far lower in the two-chamber stove than in the TLUD, thus making the two-chamber stove design well suited to challenging waste biomass fuels. The NO emission factors are positively related to the nitrogen content of biomass pellets, whereas the particulate emission factor (measured only for the two-chamber stove) follows an order of hardwood < switchgrass <= corn stover (i.e. woody biomass < herbaceous biomass). By mass, 70-80% of the particulates are smaller than 0.25 mu m. This size range is the dominant fraction at all times during the water boiling test.