Methanization of Lemna Minor for the Purpose of obtaining Hydrogen
Abstract
The article discusses the theoretical possibilities of using biomass produced on open water bodies by growing the Lemna minor plant for hydrogen production. The process of obtaining hydrogen is solved primarily in two stages, including the methanization of biomass and the subsequent transformation of methane into hydrogen and carbon dioxide. It deals with current technologies of methane oxidation to form hydrogen using water vapor, such as WGSR, SMR, MPS and PSMR technologies. It indicates various methods of purifying hydrogen from unwanted elements with their subsequent applicability in small local hydrogen production plants in decentralized hydrogen economy.
Keywords
Download Options
Introduction
Globally increasing requirements for obtaining hydrogen using technologies that do not use fossil resources for their operation nor for the production of hydrogen, are increasing with the rising prices of fossil sources and gradual reduction of the number of their abundant deposits. The production of green hydrogen by the means of the electrolytic decomposition of water using renewable sources of energy currently faces obstacles with the lack of renewable sources of electric energy. One of the ways to decrease the dependence on fossil sources during the hydrogen production process is the use of organic biomass, the processing of which produces the so-called blue hydrogen. However, the use of biomass also encounters problems with land, when large volumes of arable land intended for growing food are occupied at the expense of the cultivation of biomass that can be used for energy. This problem is solved by growing biomass on water bodies. The advantage of producing biomass on open water surfaces is the reduction of arable land required for the cultivation of energy recoverable biomass. Another advantage is the increase of yields due to the faster growth of aquatic biomass compared to its terrestrial equivalent. Lemna minor is one of the smallest and fastest-growing flowering plants on the earth. It´s an extremely reduced floating freshwater plant. The growth rate of Lemna minor in the wastewater in an open uncontrolled environment is at the level of 29 g∙m-2 ∙day1 , as it is dry biomass, which corresponds to 104 t∙ha-1 ∙year-1 . During experiments with new types of fast-growing biomass, such as sun hemp (Crotalaria juncea L.), yields were achieved only at the level of 11 t∙ha-1 ∙year-1 . [2] In the case of fast-growing trees such as willow, the yield can be in the range of 16 – 17 t∙ha-1 ∙year-1 and in the case of poplar in the range of 18 – 16 t∙ha1 ∙year-1 . [3] The use of fast-growing herbs such as bamboo can represent of the possibilities of fast and efficient biomass cultivation for energy purposes, but even in this case there are limitations caused by bamboo's requirements for growth conditions. In ideal conditions for growth in an uncontrolled environment, the bamboo reached a maximum yield of 47 t∙ha1 ∙year-1 . [4]
Conclusion
Current state of hydrogen production from renewable sources is not currently sufficient for the full competitiveness of green or blue hydrogen against grey hydrogen. The possibilities of producing blue hydrogen using fast-growing biomass appear to be promising in the near future. As proven by the research at the beginning of this article, fast-growing aquatic plants, primarily Lemna family, represent non-negligible source of biomass for the production of the blue hydrogen. Their yields significantly exceed those of terrestrial crops grown for energy purposes. However, the technology of biomass valorisation during the process of hydrogen production from water biomass must take place in less conventional ways due to the high proportion of oxygen in the feed. The consequence of this high proportion is an unacceptable proportion of carbon dioxide during thermal processing of biomass. The possibilities of valorising biomass through its transformation into methane with the use of methanogenic bacteria represent the possibility of increasing the yield of hydrogen compared to thermal decomposition. The subsequent processing of methane using conventional treatment methods, such as SMR and WGSR or new MPS and PSMR methods represents a good basis for improving hydrogen yield. After the subsequent treatment of hydrogen necessary to increase its purity, either by using membrane purification or the PSA process, it is possible to obtain more competitive blue hydrogen compared to grey hydrogen. The processes of hydrogen production from aquatic biomass could enable its decentralization into smaller local enterprises and thereby improve the availability of this fuel for ordinary users.