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Hydrogen from Biomass
Technologies for the generation of hydrogen from biomass are not commercially available so far. Dependent on the process they are at different stages of research and development.
Experts differentiate between the following methods for the generation of hydrogen: conversion of firm biomass (e.g. pellets from cultivation, residues consisting of biomass), fermentation of biomass like liquid-manure and biological generation of hydrogen.
The charming thing about generating H2 directly from biomass is that the generation of hydrogen is effected directly from the renewable energy source without taking the detour of converting the energy contained in the biomass to electric power (needed for electrolysis). By doing this a high system efficiency with a positive general balance is made possible.
The process of water vapour gasification of biomass generates a gas mixture which consists of
- 0% hydrogen
- 20% carbon monoxide
- 10% carbon dioxide
- just under 5% methane
- 45% nitrogen
When using pure oxygen or only water vapour the product gas contains no nitrogen.
In this process due to the heat the organic substances decompose into coke, condensate and gases before the gasification itself takes place. This process is called thermal decomposition or pyrolysis. Because of the oxygen present in the reactor the intermediate products are not reformed but there is a partial oxidation instead.
In a second phase of the shift-reaction the carbon monoxide together with water vapour is converted into hydrogen and carbon dioxide. After that the gas mixture is dissociated in a pressure-swing-absorption process into pure hydrogen and residual gas.
Fermentation of Biomass
Biogas can be generated by anaerobe methane fermentation when biomass or liquid manure contains a high percentage of moisture. Biogas contains a high percentage of carbon monoxide and methane. Even though this gas mixture contains very little hydrogen it can be used as fuel in advanced high-temperature fuel cells (MCFC). Because of the high process temperatures (~ 650°C) the reforming of the methane takes place directly at the electrode. Before it can be used in membrane fuel cells (PEM) the gas has to be converted into hydrogen in a reformer.
Biological Hydrogen Production
There are different biological processes in which hydrogen is set free or is produced as an intermediate product. In principle two different types of processes can be distinguished: The photosynthesis which requires light and the fermentation which takes place in darkness. Hydrogen is produced by algae in the first case and by micro-organisms in the latter case.
These methods of generating hydrogen are still in the development stage but they are a complementing option for a future hydrogen economy.
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