## 6 Importance of Biohydrogen and Biomethane  
Application of biohydrogen and biomethane, or the mixture of these, biohydthane, has become an increasing interest for the industries as alternative renewable energy. Currently, an increment in energy demand and continuous usage of fossil fuels is vulnerable by the concerns of global warming due to the increase of carbon dioxide (CO\({}_{2}\)) released in the climate [110]. Hydrogen is a high presence in nature, contrasted with fossil fuel [111]. When burning the biohydrogen, water is produced as a by-product, which left the hydrogen, which has higher calorific value due to its higher energy value [112]. This high energy (heating) value (142 kJ g\({}^{-1}\)) makes biohydrogen applicable for combustion engines. Pure biohydrogen can produce electricity in fuel cells. These criteria make hydrogen the most environmentally friendly and an ideal alternative to fossil fuels [113]. For the future energy economy, hydrogen has become a key energy trajectory [114].  
Attentions have been focused on the fuel cell efficiency and technology for hydrogen storage for transport applications to meet commercial viability, by having a clean environment and reducing the pollution [115]. In general, hydrogen is applied in ammonia production [116; 117], petroleum refining [118; 119] and metal refining (tungsten, copper and lead) [120; 121]. Hydrogen is highly used for ammonia synthetization, hazardous waste hydrogenation, desulphurization (e.g., hydrodesulfurization and hydrogenation reactions) and refining, food preparation, chemical plants, rocket fuel and high-temperature industrial furnace fuel [122]. In ammonia production, with 500 billion cubic meters (Bm\({}^{3}\)) of hydrogen, 250 Bm\({}^{3}\) of hydrogen is consumed for ammonia production, 65 Bm\({}^{3}\) of other chemical products production and 185 Bm\({}^{3}\) of petrochemistry production [122; 123]. [PERSON] reported a significant hydrogen application on cooking food, hydrogen-powered industries, electricity generation, jet planes, fuel for automobiles, hydrogen village and not to forget the domestic requirements [124].  
Production of biohydrogen from organic waste is followed by the production of organic acids, which become the source of substrate for methane production [125]. Biomethane has the potential to reduce fossil fuels demanding, for example, coal, oil and natural gas that provided power. In order to improve energy yields from other biofuel production processes (e.g., biohydrogen, bioethanol and biodiesel), biomethane production can be applied together. Digestion technology implementation at municipal, industrial as well as agricultural industries will allow effective distribution and decentralized energy generation [7]. Biomethane also can be produced from bioethanol production industries for electricity or fuel usage. Production of biomethane via anaerobic digestion will produce clean fuel, especially from renewable feedstocks. Instead of producing energy from fossil fuels, biomethane can also act as a source of energy that can reduce the environmental impacts (i.e., global warming and acid rain) [126]. Applications of pure methane in appliances, industries, vehicles and power generation are increasing every year. However, different states of purity can also be applied especially in energy conversion and transportation compared to electricity.