WFL Publisher Science and Technology Meri-Rastilantie 3 B, FI-00980 Helsinki, Finland e-mail:
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Journal of Food, Agriculture & Environment Vol.8 (3&4): 968-972. 2010
www.world-food.net
Continuous fermentative hydrogen production under various process conditions Nima Nasirian 1*, Morteza Almassi 1, Saeid Minaei 1 and Renatus Widmann 1
2
Department of Agricultural Mechanization, Science and Research Branch, Islamic Azad University, Simon Bolivar Blvd, Ashrafi Esfehani highway, Tehran, Iran. 2 Department of Environmental Engineering: Waste and Water, University of Duisburg-Essen, Universitätsstr. 15, 45141 Essen, Germany. *e-mail:
[email protected]
Received 30 June 2010, accepted 7 November 2010.
Abstract Hydrogen is a clean energy carrier which has a great potential for an alternative fuel. To produce hydrogen by fermentation of biomass, a continuous process using a non-sterile substrate with a readily available mixed microflora is desirable. This work investigates a continuous procedure at pH 5.2 and 37°C, using heat-treated digested sewage sludge of a wastewater treatment plant and continuous hydrogen production from waste sugar at various stirring speeds and hydraulic retention times (HRT). For continuous biohydrogen production the experimental set-up of three 5.5-L working volume continuously stirred tank reactors (CSTR) having three stirring speed levels (240, 135, 80 rpm) were constructed. At the optimum operation condition, seven HRTs at different organic loading rates (OLR) of 3-11 kg DOC/m3.d (Dissolved Organic Carbon) were examined. Results indicated that the stirring speed of 135 rpm had a beneficial effect on H2 fermentation. The best performance was obtained in 135 rpm and 8 h of HRT. The gas amount varied with different OLRs, but could be stabilized on a high level as well as the hydrogen concentration in the biogas was 62-64%. No methane was detected in the HRTs less than 16 h of operation. The most stable result achieved with reactor at 135 rpm. In addition, specific rate of hydrogen production reached its highest value at 2.13 L H2/Lmedia.d. Key words: Biohydrogen, dark fermentation, stirring, hydraulic retention time, CSTR.
Introduction Hydrogen, an entirely carbon-free fuel with a high combustion enthalpy (141.9 kJ/g) is considered a feasible alternative to fossil fuels, with the technology for hydrogen as a transport fuel already well established. Hydrogen is currently produced in large amounts by the chemical industry, e.g. by steam reforming of fossil fuels. For the hydrogen economy, hydrogen must be produced sustainably, for example, from water through electrolysis powered by renewable energy, photosynthetically, or by gasification or pyrolysis of biomass. It should also be possible to develop a cost-effective and reliable technology to produce hydrogen directly from renewable biomass or organic waste products by anaerobic fermentation 1. Fermentation reactions can be operated at mesophilic (25–40°C), thermophilic (40–65°C), extreme thermophilic (65–80°C) or hyperthermophilic (>80°C) temperatures 2. Bacteria known to produce hydrogen include species of Enterobacter, Bacillus and Clostridium, either in pure or mixed cultures 2. The production of hydrogen occurs in two of the four steps of anaerobic degradation (hydrolysis and acetogenesis). In the absence of methanogenic bacteria, a production of biohydrogen can be achieved by degradation of organic matter to volatile fatty acids (VFA) as liquid compounds and CO2 and H2 as gaseous compounds. Acetate fermentation processes are well understood with a maximum yield of 4 mol H2/mol glucose equivalent 3 and butyrate fermentation processes with a yield of 2 mol H2/mol glucose equivalent. The following equations (1) and (2) simply describe the reactions involved in fermentative hydrogen 968
production from organic substances. Since anaerobic bacteria do not require light energy, the decomposition of substrates is incomplete and organic acids remain. Although main metabolites are acetate and butyrate, butyrate is more dominant because of its lower Gibbs free energy value and involved enzyme activity 5, 6. C6H12O6 + 2H2O → 2CH3COOH + 4H2 + 2CO2 ∆G = –184.2 kJ
(1)
C6H12O6 → C3H7COOH + 2H2 + 2CO2 ∆G = –257.1 kJ
(2)
In mixed cultures, this reaction often occurs as a combined production 3, modified: 4C6H12O6 + 2H2O → 2CH3COOH + 3C3H7COOH + 10H2 + 8CO2 (3) However, there are some limitations in fermentative H2 production, such as: 1) only 10–20% of the energy potential in substrates is recoverable as H2 due to the limited metabolic energy derived from H2 fermentation 7; 2) satisfactory stability for fermentative H2 production has not yet been obtained 1. To overcome these problems, much researches have focused on especially about operating parameters such as pH, hydraulic retention time (HRT), gas partial pressure, stirring 4 carbon source, etc . 8-10. Partial pressure or H2 in the liquid phase is one of the key factors affecting fermentative H2 production. Generally, it is known
Journal of Food, Agriculture & Environment, Vol.8 (3&4), July-October 2010
