Hydrogen is an energy carrier with a great variety of use and one of the most important feedstock for chemical industry. One possible path for continuous and renewable production of hydrogen is the conversion of biomass into hydrogen. Within this work a promising gasification process for hydrogen production from biomass was developed further, investigated experimentally and optimized.
The gasification process involves three fluidized bed reactors, which include a gasifier and a carbonator connected in series with regard to the gas flow and a regenerator. In the gasifier, the biomass is gasified under steam atmosphere in a temperature range of 800–850 °C. The necessary heat for the endothermic gasification reactions is provided by the circulating flow of CaO transferred from the regenerator to the gasifier. Subsequently, the gasifier product gas is fed into the carbonator. The carbonator is operating in a temperature range from 600 °C to 700 °C. In this reactor, in-situ adsorption of carbon dioxide takes place by the CaO-sorbent through the carbonation reaction. Due to CO2 adsorption, the equilibrium of the water-gas shift reaction is shifted to the hydrogen product and the H2 content is increased. The third reactor, the regenerator, is operating in a temperature range above 900 °C where the calcination reaction and the combustion of residual char from the gasifier and additional fuel takes place.
In the first part of this work an experimental characterization of the gasification process is presented. The experimental results show that due to the high temperature in the gasifier and the CaO bed material a product gas with a content of none condensable hydrocarbons (methane, ethane,..) below 8 vol.-%dry and a tar content of 4.3 g/m3STP is produced. In the subsequent carbonator, the hydrogen content of the product gas is increased up to 85,4 vol.-%dry.
Based on the experimental results, a basic design of the triple fluidized bed system used for this process is developed and hydrodynamic studies are carried out in a scaled cold model. The preliminary aim of the cold model investigation is to study the feasibility in terms of long term steady operation of the triple fluidized bed system as well as to study the hydrodynamic interaction of different process parameters. Results prove the feasibility of the proposed triple fluidized bed system and operating windows for the operation of the real facility could be derived.
By means of process simulation the entire gasification process is modelled and optimized regarding the hydrogen yield. The highest hydrogen yield achieved within this simulation is 0.79 m3STP/kgfuel,waf. The potential of the steam gasification with subsequent CO2 capture could be shown by the results obtained and their comparison with literature.
Results shown within this thesis prove that the biomass-to-hydrogen process can be simplified by producing a product gas with high hydrogen and low hydrocarbon content via steam gasification with subsequent CO2 capture.