Multi-scale analysis and optimization of chemical looping gasification of biomass: Micro-scale simulation
Kolja Jarolin, M.Sc.
Motivation
It is a global consensus to increase the amount of renewable energies due to the limited amounts of fossil fuels and the reduction of greenhouse gas emissions. To mitigate the anthropogenic climate change, which is partly due to the combustion of fossil fuels like coal, the EU countries have committed to achieve at least a 27 % share of renewable energies in the EU’s final energy consumption by 2030. One major energy source to obtain this goal is biomass. Especially, by using agricultural crop residues like straws the emission of greenhouse gases as well as the dependence on fossil fuels can be reduced.
System description
A difficulty in the utilization of biomass like crop straws is a low energy specific content compared to fossil fuels. In order to increase the volume specific energy content, to improve fuel properties and to simplify handling and storage, biomass can be pelletized.
By converting the pellets via gasification one can further increase the efficiency of the process und reduce the emission of pollutants. Additionally, gasification results in a higher flexibility since the produced syngas can be used in a wide range of applications. However, the resulting syngas heating value is relatively low due to the dilution effect of large amounts of nitrogen in the combustible species. Therefore, the principal of chemical looping is applied to the process. The principle of chemical looping is based on the use of oxygen carrier materials (primarily metal oxides) undergoing oxidation-reduction cycles. A well-accepted approach to realize a chemical looping process is to use two fluidized reactors, a fuel reactor (FR) and an air reactor (AR), connected by solid transportation lines. Between these two reactors, the oxygen carrier (OC) is transported which supplies lattice oxygen to the fuel reactor.
A common problem of the chemical looping gasification (CLG) process is the high amount of volatiles and tar escaping the fuel reactor. Therefore, the present project examines a novel CLG process with a two-stage fuel reactor. The lower first stage will act as the gasification reactor, the upper stage as reforming reactor. The two-stage design increases the complexity but is expected to significantly improve the efficiency.
To prove the effectiveness and to improve the understanding of the internal processes, the CLG process of biomass pellets is studied experimentally and by detailed simulations. The experiments will be carried out on pilot scale at the Southeast University in Nanjing, China. At the Hamburg University of Technology the process will be investigated by simulations on multiple scales.
In subproject A the system is simulated and analyzed on plant scale including the mixing, fluid dynamics and chemical reactions within the plant.
In subproject B the micro scale is examined by simulating the devolatilization, breakage and gas release of a single pellet.
Subproject B: Modelling biomass pellet gasification
Problem description
A key factor to optimize the chemical looping gasification process is to predict the conversion efficiency. One crucial step for this goal is to predict the breakage and attrition of the pellets. The conversion of the pellets starts with feeding the pellets into the fluidized fuel reactor. Heat is then transferred to the pellets by conduction, convection, and radiation due to its contact with the hot bed material. The heat flux leads to drying, devolatilization, and finally to the gasification. During drying the moisture from solid fuel is converted into vapor. The dried solid is decomposed into char and volatiles including tar. For typical biomass pellets, these steps occur on a time scale of a minute and the pellet loses around 85% of its mass. In the final step, the char is converted by reacting with the gasifying agent, e.g. steam or carbon dioxide. This process is much slower than the devolatilization. During the motion of the pellets in the fluidized fuel reactor, different mechanical stresses are acting on them. They are caused by the impacts with other pellets, with bed material or with apparatus walls, and lead to attrition and breakage. While the raw pellet is nearly unaffected, the devolatilization process is drastically degrading the mechanical properties of the pellets. Hence, attrition and breakage are significantly increased for the devolatilized pellet. The produced fines have a high change to be exhausted unconverted.
Methodology
To model breakage and attrition of the pellets, the discrete element method (DEM) is applied. In this project, the simulation framework MUSEN, developed under the supervision of Prof. Dosta, is used. The mechanical properties of the pellets before and after devolatilization are measured and the discrete element model is calibrated against these measurements. Afterward, different loading scenarios and different pellet size can be simulated. Based on the simulations different scenarios are analyzed to derive correlations for the system simulations in subproject A
Project publications
Jarolin K., Dosta M. (2020). Linearization-based methods for the calibration of bonded-particle model. Comput. Part. Mech. https://doi.org/10.1007/s40571-020-00348-z
Project funding
We gratefully acknowledge for the financial support the German Research Foundation (DFG) (Germany).
Project number DO 2026/5-1.
