Hydrothermal carbonization of water hyacinth was experimentally conducted in the range of 30 min to 24 h at 240 degrees C, chemical and structural properties of hydrochar products were investigated. Oxygen/carbon ratio, and hydrogen/carbon ratio in all hydrochar products were 0.19-0.45, and 0.94 to 1.51. Higher heating value of hydrochar products was 16.83 MJ/kg to 20.63 MJ/kg. Residence time had little effect on the chemical properties of hydrochar samples after 4 h. When the residence time was more than 4 h, hydrochar products exhibited almost the same pyrolysis behavior under TGA (thermogravimetric analysis). Hydrochar developed better structural characterization as time increased. The formation of microspheres on the surface of hydrochar was discussed, and transformation of cellulose and hemicellulose should be the reaction for microsphere production. (C) 2013 Elsevier Ltd. All rights reserved.