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

1. Problem description

A key factor to model the chemical looping gasification process is to predict the volatile release of the pellets. For this purpose, all steps leading to the gas release have to be described. The process starts with feeding the pellets into the fluidized fuel reactor. During motion of pellets in the fluidized bed, 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. These changes in particle size are crucial for the further steps as well as for the fluid mechanics. 

Simultaneously to the mechanical effects, the actual gasification process is occurring. The main part of the heat for this process is generated during the oxidation of the oxygen carrier. The heat is then transferred by conduction during impact as well as by convection and radiation to the pellets. Due to the heat flux, drying and devolatilization processes occur. During drying the moisture from solid fuel is converted into vapor. The dried solid is decomposed into char and volatiles including tar. Both processes are leading to an internal overpressure resulting in primary fragmentation and the gas release, which we want to describe. 

2. Methodology

The first step is to model the mechanical behavior. The approach is to fist measure basic mechanical properties like stiffness, strength, friction, etc. Subsequently, complex stress conditions are simulated via the discrete element method (DEM). In this project, the simulation framework MUSEN, developed under supervision of Prof. Dosta, is used. Based on the micromechanical simulations, correlations for the system simulation of subproject A are developed.

Secondly, the effects due to the heat transfer are considered. For this purpose, the mechanical model is extended by variables for temperature and pressure.  From measured data of the heat transfer for the different impact scenarios, a heat model can be implemented and calibrated. To consider drying and devolatilization, models for the reaction kinetics of these processes are included. In the final step, the discrete element simulation is coupled with a CFD simulation to model the gas release. Based on this model different scenarios are analyzed to derive correlations for the system simulations in subproject A.

Project funding

We gratefully acknowledge for the financial support the German Research Foundation (DFG) (Germany).

Project number DO 2026/5-1.

Contact Data

Kolja Jarolin
Kolja Jarolin
Research Assistant
Phone: +49 40 42878 4068
Email: kolja.jarolin(at)tuhh.de