Raw materials for fermentative hydrogen production

Transcription

1 Raw materials for fermentative hydrogen production Krzysztof Urbaniec, Robert Grabarczyk CERED Centre of Excellence, Warsaw University of Technology, Jachowicza 2/4, Plock, Poland, fax: Abstract The basics of thermophilic hydrogen fermentation and photofermentation are outlined. Various types of biomass which can be used as raw materials for hydrogen fermentation are named and the methods of biomass pretreatment are briefly presented. Hydrogen yield from processing of four specific raw materials is estimated. Introduction The European Commission's 1997 White Paper on renewable energy sources sets out the objective of increasing the share of renewable energies to 12% of gross inland energy consumption by 2010 [1]. Amongst renewable energy sources, the biggest contribution comes from biomass. It can be converted to energy carriers by thermal (combustion, pyrolysis and gasification) or biological (fermentation, digestion) processes [2, 3]. An interesting and promising method of biomass utilisation is hydrogen production by the fermentation process. Hydrogen fermentation is the topic of Integrated Project HYVOLUTION, financed under the Energy Priority of the 6 th Framework Programme of the EU [4]. The work is coordinated by Wageningen UR Agrotechnology and Food Innovations, The Netherlands, and the project consortium is composed of 21 partners representing 11 European countries plus Republic of South Africa. The project is aimed at developing a blueprint for a complete hydrogen production plant employing two-stage hydrogen fermentation. Hydrogen fermentation The fermentation based biomass conversion to hydrogen builds on anaerobic digestion of carbohydrate-rich substances under influence of bacteria [5]. As a matter of fact, the well known fermentation process converting organic substrates to methane also involves production of hydrogen as an intermediate product but under natural conditions this hydrogen is promptly consumed by methanogenic bacteria. Microorganisms known to produce hydrogen include: strict and facultative anaerobic bacteria which convert fermentable sugars to organic acids, H 2 and CO 2 ; the highest rates of hydrogen yield were obtained with certain species of thermophilic bacteria [6, 7], photofermentative bacteria which use light energy for complete oxidation of substrates to H 2 and CO 2. Consequently, the hydrogen production by thermophilic fermentation and photofermentation can be considered. Thermophilic fermentation consists in converting sugars or polysaccharides to hydrogen, carbon dioxide and organic acids. The fermentation can be carried out continuously or in batch mode. The typical chemical reactions of thermophilic fermentation of glucose, xylose and sucrose are presented below.

2 C 6 H 12 O 6 + 2H 2 O 4H 2 + 2CH 3 COOH + 2CO 2 3C 5 H 10 O 5 + 5H 2 O 10H 2 + 5CH 3 COOH + 5CO 2 C 12 H 22 O H 2 O 8H 2 + 4CH 3 COOH + 4CO 2 The maximum theoretical yields of hydrogen from glucose, xylose and sucrose, expressed in moles of hydrogen per 1 mole of substrate are 4, 3,3 and 8, respectively. Photofermentation consists in converting organic acids to hydrogen and carbon dioxide. Assuming that acetic acid is the substrate, the photofermentation can be represented by the reaction: CH 3 COOH + 2H 2 O 4H 2 + 2CO 2 The maximum theoretical yield of hydrogen is 4 moles per 1 mole of acetic acid. During photofermentation the photoheterotrophic microorganisms absorb and use the electromagnetic radiation as an energy source [7]. The advantage of photofermentation is the production of hydrogen-rich gas containing very little carbon dioxide (10-20% vol) due to the alkalinity of the fermentation broth. After simple upgrading, the gas can be supplied to a fuel cell. As the organic acids produced in thermophilic fermentation can be used as substrates for photofermentation, both processes can be coupled as schematically shown in Figure 1. Compared to the attainable hydrogen yield of thermophilic fermentation, the yield of the two-stage process can be higher by a factor of up to almost 3 making it possible to obtain from biomass about 70% of theoretically available hydrogen. H 2 CO 2 Gas upgrading H 2 + CO 2 H 2 + CO 2 light Biomass Biomass pretreatment Thermophilic fermentation Photofermentation nonfermentable fraction nonfermentables Figure 1. Scheme of two-stage fermentative hydrogen production The main stages of the fermentative hydrogen production process are following [8]: Biomass pretreatment to give fermentable feedstock and nonfermentables, Thermophilic fermentation in which fermentable feedstock is converted to hydrogen gas and organic acids,

3 Photofermentation in which organic acids are converted to hydrogen gas, Upgrading of hydrogen gas to meet product specification, Separation and treatment of non-fermentables. Thermophilic fermentation is rather well understood but the knowledge of photofermentation is still far from complete. From the engineering point of view it is difficult to design a photofermenter ensuring effective supply of light energy needed for achieving a satisfactory hydrogen yield. Preliminary economic estimates indicate that industrial-scale applications of the two-stage hydrogen production process can be economically viable. Potential raw materials for hydrogen fermentation With regard to the chemical composition there are three different kinds of biomass, which can be used as raw materials for thermophilic fermentation [7]: Sugar containing biomass (i.e. sugar beet, sugar cane, sweet sorghum), Starchy biomass (i.e. potato, cereals), Lignocellulosic biomass (i.e. grass, wood, straw). Apart from various kinds of crops, waste or byproducts from biomass processing industries (beet pulp, molasses, potato peels, apple pulp, wheat bran, brewers grain etc.) can be used for hydrogen production. Characteristic data including chemical composition of four selected kinds of biomass are given in Tab. 1 [9, 10, 11]. Table 1. Data of selected kinds of biomass Component, % of dry mass Raw material Sugar beet Potato Pressed beet pulp Wheat straw Sucrose Starch Cellulose ~4 ~ Hemicellulose ~ Lignin ~ Average yield, t fresh biomass/ha Water content in fresh biomass, % , The substrates to thermophilic fermentation are simple sugars (glucose, xylose, arabinose etc.) and therefore each biomass type requires a proper method of pretreatment in order to obtain a water solution of the simple sugars. The method of

4 biomass pretreatment depends mainly on the type and initial form of raw material. In case of sugar containing biomass, the pretreatment step consists in an extraction process of raw juice which can be directly supplied to the thermophilic fermenter. The pretreatment of starchy and lignocellulosic biomass is more complicated due to the hydrolysis process, in which polysaccharides are converted to simple sugars. An important problem is the removal of lignin before the fermentation unit. In contrast to cellulose and hemicellulose, lignin is not converted to simple sugars and moreover it may hamper the growth of microorganisms used to hydrogen production [7]. Nonfermentable fraction which contains lignin can be utilized by combustion or gasification to generate energy. It is also known that pure lignin is a good raw material for the production of chemicals and other products, e.g.: Biopolymer additives (antioxidant, UV stabiliser, colouring agent), Plasticizers in cement, Precursors for phenol-based chemicals, Binder for briquette, fertilizers and feed, Surfactants, Emulsifiers, Carbon fibres. These wide directions of use of lignin have resulted in an increasing interest in the research on new or modified methods of lignocellulosic biomass pretreatment, in which lignin is separated before hydrolysis. For example in an organosolv process, ethanol is added to lignocellulosic material formed of chips [12]. It causes chemical breakdown of lignin and solubilization of lignin components. Then dissolved lignin is recovered via distillation with almost 99% efficiency. The obtained results are shown in Table 2. Table 2. Theoretical and real yields of simple sugars and lignin Theoretical/real yield, kg/t fresh biomass Sucrose Sugar beet ,2 Glucose - Potato - 211,1 198,4 Xylose - - Lignin - - Raw material Pressed beet pulp 10 9,6 55, ,9 81,8 10 9,9 Wheat straw - 322,8 290,5 235,8 212,2 149,4 147,9 On the basis of data given in Table 1 and information from literature, the theoretical and real yields of fermentable simple sugars and lignin were calculated. Calculations were carried out under the following assumptions: 96% efficiency of raw juice extraction process in beet sugar factory, 94% efficiency of potato starch hydrolysis,

5 90% efficiency of lignocellulose saccharification with 99% efficiency of lignin recovery. Hydrogen yield of the fermentation process At the present stage of development of hydrogen fermentation it is known that the real efficiency of two-stage process described in Section 2 above is up to 70%. The data shown in Tab. 2 were used to estimate the theoretical and real hydrogen yields of two-stage process. The results are shown in Fig 2. For crops the real yields were also converted to kilograms of hydrogen per hectare of growing area and the results are shown in Figure theoretical yield 70% conversion 46,9 kg H 2 /t fresh biomass , ,5 18,5 18,9 13, sugar beet potato pressed beet pulp wheat straw Figure 2. Theoretical and real hydrogen yield of two-stage fermentation process , ,5 600 kg H 2 /ha ,4 0 sugar beet potato wheat straw Figure 3. Real hydrogen yields from crops

6 Conclusions Due to the low moisture content - about 17% - the highest yield of hydrogen per kilogram of biomass is obtained from wheat straw. The yields from sugar beet, potato and beet pulp are considerably lower. On the one hand, a high moisture of raw material increases the costs of biomass transport, but on the other hand it facilitates hydrogen production process by reducing fresh water requirement. Another advantage of wheat straw is a high content of lignin, which after separation process becomes a valuable co-product. When the yield is expressed in kilograms of hydrogen per hectare of growing area, the highest yield is obtained from potato. However, the pretreatment of potato is more expensive than the pretreatment of sugar beet. Acknowledgement Support of the European Commission under contract HYVOLUTION is gratefully acknowledged. References [1] Energy for the future: Renewable sources of energy. White paper for a community strategy and action plan. COM(97)599. [2] Claassen PAM, van Lier JB, Lopez Contreras AM, van Niel EWJ, Sijtsma L, Stams AJM, de Vries SS, Weusthuis SA. Utilisation of biomass for the supply of energy carriers. Applied Microbiology and Biotechnology, 1999; 52: [3] Hofbauer H. Conversion technologies: gasification overview. Paper presented at 15 th European Biomass Conference and Exhibition, Berlin, [4] Claassen PAM, de Vrije T. Non-thermal production of pure hydrogen from biomass: HYVOLUTION. International Journal of Hydrogen Energy, 2007; 41: [5] Claassen PAM, de Vrije T, Grabarczyk R, Urbaniec K. Development of fermentation based process for biomass conversion to hydrogen gas. Paper presented at PRES Conference, Prague, [6] van Niel EWJ, Budde MAW, de Haas GG, van der Wal FJ, Claassen PAM, Stams AJM. Distinctive properties of high hydrogen producing extreme thermophiles, Caldicellulosiruptor saccharolyticus and Thermotoga elfii. International Journal of Hydrogen Energy, 2002; 27: [7] Reith JH, Wijffels RH, Barten H. Bio-methane & Bio-hydrogen. The Hague: Smiet offset, [8] Wukovits W, Friedl A, Markowski M, Urbaniec K, Ljunggren M, Schumacher M, Zacchi G, Modigell M. Identification of a suitable process scheme for the non-thermal production of biohydrogen. Paper presented at PRES Conference, Ischia, [9] van der Poel PW, Schiweck H, Schwartz T. Sugar technology. Berlin: Bartens, 1998.

7 [10] Urbaniec K, Grabarczyk R. Selected kinds of biomass as raw materials for hydrogen fermentation. Inzynieria Systemow Bioagrotechnicznych, 2007; 15:11-17 (in Polish). [11] Jorgensen H, Vibe-Pedersen J, Larsen J, Felby C. Liquefaction of lignocellulose at high-solids concentrations. Biotechnology and Bioengineering, 2007; 96: [12] Mabee WE, Greeg DJ, Arato C, Berlin A, Bura R, Gilkes N, Mirochnik O, Pan X, Pye EK, Saddler JN. Updates on softwood-to-ethanol process development. Applied Biochemistry and Biotechnology, 2006; 129: