This study is the first to investigate the effect of combined Lewis and Bronsted acid catalysts on the hydrothermal carbonization of fir wood samples; here, hydrothermal carbonization of fir wood-with and without catalyst-was performed. In non-catalytic runs, the effects of temperature and residence time on hydrochar yields were investigated; temperature significantly affected hydrochar yields, whereas residence time had very little effect. A gradual increase in temperature resulted in a decrease in hydrochar yields while increasing the carbon content of hydrochars. At all tested temperatures, the use of a catalyst led to a decrease in hydrochar yields. The highest heating value of 29.12 MJ kg(-1) was obtained at the highest temperature (275 degrees C) and the longest residence time (24 h). The use of catalysts slightly decreased the heating values. The hydrochars were mainly in the class of lignite coal; hydrochar obtained at 275 degrees C and a residence time of 12 h-either with or without catalysts-was classified as bituminous coal. Irregular carbon sphere formation was observed at all temperatures tested in the catalytic runs; however, no carbon spheres were observed in the non-catalytic runs. XRD patterns of hydrochars from the non-catalytic runs were similar for temperatures of 225, 250 and 275 degrees C; the peak observed at 2 theta of 22 degrees broadened after HTC processing. In the catalytic runs, two new peaks at 2 theta of 38 degrees and 49 degrees were observed, in addition to broadened peaks (2 theta = 22 degrees). The use of catalysts led to the formation of the secondary char.