Thailand is an agriculture-based country. It produces large amounts of open burned agricultural residues. A strategy to use them as biofuel all year round is to enhance their fuel properties by coupling blending and thermochemical pretreatment. In this study, the pyrolytic behaviour of major residues (napier grass , rice straw, cassava stalks and corn cob) exposed to a high torrefaction temperature (300°C) was investigated for various blending ratios, i.e. 100:0, 50:50 and 70:30. The release of chlorine was quantified for each biomass blend, including, a new fouling risk index ratio. Raw biomasses and untreated biomass blends were found to be less suitable as biofuel than torrefied biomasses. The ratio K2O:SiO2, indicator of fouling risk during combustion, was found to be low for all torrefied blends. The HHV:Cl ratio, indicator of combustion quality, indicated that NG mixed with RS (50:50 proportion) is the most promising blend. Significant synergetic effects were observed for biomasses mixed before torrefaction