Volatile fatty acids (VFAs) are inevitable intermediates of biogas production during the anaerobic digestion of organic matter. The excessive accumulation of VFAs leads to a pH drop and the strong inhibition of methanogenesis. On the other hand, VFAs are useful commodities with different applications, and their fermentative production may compete with traditional production methods based on oil derivatives. The fermentation methods have commonalities with the biorefinery concept. The present review considers the methods of VFA fermentative production together with competitive simultaneous biogas and hydrogen production. Methods of the enhanced production of volatile fatty acids are presented, showing the option of integrated processes of product removal and energy production from the obtained biogas. On the basis of the present review, the following conclusion can be drawn. Volatile fatty acids (formic, acetic, propionic, and butyric ones) are useful commodities with various applications. That is why their targeted production with their desired production rate may shift the aims of the anaerobic digestion toward volatile fatty acids instead of biogas release. On the other hand, VFA production combined with biogas release can make the overall process self-consistent, with energy production sufficient to maintain the target processes using biogas for heating the digestor. The maintenance of optimum VFA concentrations can be accomplished by simultaneous VFA removal from the fermentation broth, thus integrating the product recovery with the maintenance of optimum operation conditions in the digester. The substrate preparation and the operating conditions (organic loading rate and hydraulic retention time) are of crucial importance for the successful fermentation process.

Hydrogen is a prospective energy carrier because there are practically no gaseous emissions of greenhouse gases in the atmosphere during its use as a fuel. The great benefit of hydrogen being a practically inexhaustible carbon-free fuel makes it an attractive alternative to fossil fuels. I.e., there is a circular process of energy recovery and use. Another big advantage of hydrogen as a fuel is its high energy content per unit mass compared to fossil fuels. Nowadays, hydrogen is broadly used as fuel in transport, including fuel cell applications, as a raw material in industry, and as an energy carrier for energy storage. The mass exploitation of hydrogen in energy production and industry poses some important challenges. First, there is a high price for its production compared to the price of most fossil fuels. Next, the adopted traditional methods for hydrogen production, like water splitting by electrolysis and methane reforming, lead to the additional charging of the atmosphere with carbon dioxide, which is a greenhouse gas. This fact prompts the use of renewable energy sources for electrolytic hydrogen production, like solar and wind energy, hydropower, etc. An important step in reducing the price of hydrogen as a fuel is the optimal design of supply chains for its production, distribution, and use. Another group of challenges hindering broad hydrogen utilization are storage and safety. We discuss some of the obstacles to broad hydrogen application and argue that they should be overcome by new production and storage technologies. The present review summarizes the new achievements in hydrogen application, production, and storage. The approach of optimization of supply chains for hydrogen production and distribution is considered, too.

While there is a universal trend to replace fossil fuels at least partially, renewable fuels seem to impose new solutions. Hydrogen sulfide, typical for closed water ponds such as the Black Sea, seems to offer one namely, a new sulfide-driven fuel cell providing for exchange of OH⁻ anions across the membrane by use of hydrogen sulfide in natural marine water. When tested in batch and continuous operation modes, this solution showed that the initial sulfide concentration needed to achieve results of practical value was within 200 to 300 mg dm⁻³. The predominating final products of the energy production process were sulfite and sulfate ions. Very low overpotentials and mass transfer resistances were observed. The mass balance and the electrochemical parameters showed about 30% efficiency in sulfate ions as the final product. Efforts should be made to enhance sulfide to sulfate conversion. The observed current and power density were comparable and even better than some of the results so far reported for similar systems. Three types of ion exchange membranes were tested. Comparison of their ion conductivity to literature data shows good performance. At higher initial sulfide concentrations polysulfides and thio-compounds were formed with considerably low current yield.

Biogas, Biodiesel and Bioethanol as Multifunctional Renewable Fuels and Raw Materials | InTechOpen, Published on: 2017-01-25. Authors: Venko Beschkov

Glucose, alcohol stillage and glycerol were used as substrates for bio-hydrogen production by the newly isolated strain Clostridium beijerinckii 6A1 under batch conditions. High molar yields of hydrogen from the studied organic substrates were observed. There was a neat difference in the metabolic pathways of substrate digestion when hexose-based substrate or glycerol were used. The products of glycerol digestion showed that a pathway with no formic acid formation as intermediate was probable. In this case, considerable concentrations of acetic and propionic acid (up to 6 g dm⁻³) and small amounts of butanol were observed after 48 h. When glucose or hexose-based substrates were used, considerable amounts of formic acid (up to 6 g dm⁻³), i.e., the pathway proposed for Clostridia mixed cultures, were appropriate for the observed process of hydrogen release. For these substrates, considerable amounts of propionic acid in concentrations up to 1 g dm⁻³ were observed. That is why the pathway proposed for mixed cultures seemed more appropriate for our experiments carried out with hexose-based substrates. When hexoses were used, substrate digestion stopped the formation of acetic acid, propionic acid and ethanol. Probably, these intermediates are inhibitors to the further digestion to other products.