13.3.1 Feedstocks for fermentative hydrogen production

It is well founded that carbohydrates are the main source of hydrogen during fermentative processes and therefore wastes/wastewater or agricultural residues, rich in carbohydrates, can be considered as potential hydrogen feedstocks (Kapdan and Kargi, 2006). The main criteria for substrate selection are: availability, cost, carbohydrate content and biodegradability. Glucose, sucrose and to a lesser extent starch and cellulose are the fermentation substrates mostly studied in the laboratory (Mizuno et al., 2000; Ueno et al., 2001; Fang et al., 2004). They have been used as model substrates for research purposes due to their easy biodegradability and because they can be present in different carbohydrate-rich wastewaters and agricultural wastes. However, synthetic carbohydrates are expensive raw materials for a pilot or full-scale hydrogen production process and therefore the use of zero-cost, rich in carbohydrates wastes, seems to be ideal in real hydrogen production applications.

Rice, winery, noodle, sugar, and molasses manufacturing, olive pulp and cheese whey are among actual wastewaters that have been studied for hydrogen production at a laboratory scale (Table 13.4). In addition, hydrogen could be produced using as feedstocks complex solid wastes, such as wastes from kitchen, food processing, mixed wastes, and municipal wastes containing along with carbohydrates, proteins and fats. In the later case, the hydrogen conversion efficiencies are low, due to the complex structure of the wastes. In general the hydrogen yield from wastes rich in carbohydrates is higher than those rich in proteins and fats.

Moreover, the rich in sugars energy crops, sugar beet, sugar cane and sweet sorghum, as well as the rich in starch energy grains, corn and wheat are among the most suitable substrates for hydrogen production (Table 13.5). However, the potential to produce hydrogen from the residues remaining after harvesting and processing of these starch or sugar crops, that cannot be further exploited in the food industry chain, is more likely to yield a solution with far better overall prospects for economic and environmental sustainability (Lynd et al., 2005). Hydrogen generated from such feedstocks can be characterized as ‘second — generation hydrogen’ since its production is not competitive to food production, but rather a side product of the food production industry. The agricultural residues contain carbohydrate polymers such as cellulose, hemicellulose and lignin, and thus a pre-treatment process (mechanical, chemical or enzymatic) is always necessary, for solubilization of cellulose and hemicellulose to simple sugars, which could easily be degraded by hydrogen-producing bacteria. Rice and wheat straws, corn stover, wheat bran are some of the lignocellulosic feedstocks used for hydrogen generation (Table 13.5). Figure 13.3 presents different potential feedstocks for hydrogen production.