Cassava rhizome, a bulky residue from cassava plantation, was reformed using a modular downdraft gasifier in order to produce a high temperature fuel gas with low tar content. Effects of biomass particle size between 5 and 15 mm, inlet air flow rate of 1.98-3.06 m(3)/h and additional 5% Ni/char catalyst for secondary syngas cleanup were investigated. The air flow rate had integrated effects on product yield and composition; higher air flow rate resulted in higher gas yield with less tar and char. The conversion to H-2 and CO increased with the biomass particle size, whereas the conversion to CH4 and CO2 decreased. The optimal operating conditions without catalyst addition were obtained with air flow rate of 2.5 m(3)/h and particle size of 10 mm where carbon and hydrogen conversions were 94.02% and 65.92%, respectively. In the presence of catalyst, the yield of CO2 was reduced whereas the CO increased. The carbon and hydrogen conversions were 92.87% and 55.69% with air flow rate of 1.98 m(3)/h and particle size of 10 mm, respectively. At this condition, tar formation was relatively low with gas heating value of 4.45 MJ/m(3) and H-2/CO of 1.22. Obviously, prepared Ni/char catalyst enhanced condensable tar reforming to smaller gases resulting in increased gas heating value and cold gas efficiency with superior synthesis gas yield. The produced gas contained quality combustible gases, which can be readily used in heat and power applications. (C) 2015 Elsevier B.V. All rights reserved.