Hydrogen gas has successfully been produced from biomass by hydrothermal gasification. Various alkaline additives have been found to increase the hydrogen yield. However, studies have not yet been done on pollutants that would be produced in the hydrothermal gasification with real biomass waste containing hetero-atoms such as S. N, and P elements. Studies have also yet to be done on finding ways to suppress the formation of pollutants. For this purpose, L-cysteine containing hetero-atoms S and N, and O-phospho-DL-serine with P. were selected as pure test samples. The objective was to determine the optimum conditions for the suppression of the pollutants produced in the hydrothermal gasifications, while effectively generating hydrogen. Phosphate ion was found in the liquid phase after the gasification of O-phospho-DL-serine with and without additive at 400 degrees C. Phosphorus compounds were not detected in the gas phase. When a large quantity of Ca(OH)(2) was added, phosphorus compounds were precipitated in the solid phase. Hydrogen gas yields were increased and other gases were suppressed by using the additive. These results were same as those of chicken manure. The test samples were used to determine the optimum conditions of 400 degrees C, 21 MPa, and the Ca(OH)(2) additive. NH(4)(+) was mainly produced in liquid phase by using the model sample of L-cysteine. With the addition of Ca(OH)(2), the yields of SO(2) and H(2)S are significantly decreased. NO(2)(-) and NO(3)(-) were detected at trace levels. The main gas produced was hydrogen and the generation of CO(2) gas was efficiently suppressed. These optimum conditions were then applied to real biowastes, cow dung. Hydrogen production increased and was found to be mainly in the gas phase, while there was negligible generation of pollutants, with these were in the gas phase. H(2)S were detected at trace levels. NO was not recorded, and production of other gases were suppressed by using the additive. Nitrogen in the sample was trapped as NH(4)(+) in the liquid phase. (C) 2011 Elsevier Ltd. All rights reserved.