Algal biomass is rich in carbohydrates which can be utilized as a promising source of substrate for dark fermentation. It becomes more significant when biomass is produced by capturing atmospheric greenhouse gas, CO2. In the present study, clean energy was generated in the form of biohydrogen utilizing algal biomass. Biohydrogen production was carried out by thermophilic dark fermentation using mixed culture. The culture of Chlorella sorokiniana was cultivated in helical airlift photobioreactor at 30 degrees C under continuous light intensity of 120 mu mol m(-2) s(-1) provided by white fluorescent lamps. Biomass reached to stationary phase on 9th day giving maximum dry cell weight of 2.9 kg m(-3). Maximum carbohydrate and protein content observed was 145 g kg(-1) and 140 g kg(-1), respectively. Maximum volumetric productivity of 334 g dm(-3) d(-1) was observed. Algal biomass was subjected to various physical and chemical pre-treatments processes for the improvement of hydrogen production. It was observed that the pretreatment with 200 dm(3) m(-3) HCl-heat was most suitable pretreatment method producing cumulative hydrogen of 1.93 m(3) m(-3) and hydrogen yield of 958 dm(3) kg(-1) volatile suspended solid or 2.68 mol mol(-1) of hexose. Growth kinetics parameters such as mu(max) and K-s were estimated to be 0.44 h(-1) and 120 g m(-3), respectively. The relationship between biomass and hydrogen production was simulated by the Luedeking-Piret model showing that H-2 production is growth associated. The study thus showed the potential of algal biomass as substrate for biological hydrogen production. (C) 2013 Elsevier Ltd. All rights reserved.