Multi-scale analysis and optimization of chemical looping gasification of biomass: Macro-scale simulation

Timo Dymala, 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 A: Modelling of fluid dynamics and chemical reactions

1. Problem description

Essential for a sophisticated model of the gasification of biomass inside a plant is the simulation of the hydrodynamics to evaluate the performance of the process. Characteristic for fluidized beds are the complex hydrodynamics as a result of intensive interactions between fluid and particles. Therefore, the computational fluid dynamics (CFD) are needed to allow the numerical simulation of the fluid dynamics. For the investigation of the particle dynamics during the process two main approaches are suitable: The Euler-Euler and the Euler-Lagrange approach. The Euler-Euler approach, or two-fluid model (TFM), assumes the fluid as well as the particle flows as fluids, whereas in the Euler-Lagrange model a discrete particle model (DPM) is used. This results in a multiple higher accuracy of the Euler-Lagrange model but also enormously high computational costs. Therefore, it is currently nearly impossible to accurately simulate industrial sized applications with reasonable computational time.

2. Methodology

A modification of the Euler-Lagrange approach results in the Multi Phase - Particle in Cell (MP-PIC) method. The fluid is described by the Navier-Stokes equations and a defined number of particles with the same properties are represented by so-called parcels, which are modelled by using Lagrangian computational particles. This facilitates the implementation of various effects like particle size distributions as well as shrinkage effects and reduces the computational costs compared to the Euler-Lagrange approach while increasing the accuracy of the simulation compared to the Euler-Euler approach. This is expected to allow the simulation of industrial plants with reasonable computational time.

The first step of this subproject is to model the cold plant behavior and to validate the results with the experimental data measured at the Southeast University. For this purpose, the commercial software Barracuda VR® is used and suitable drag models for the different particles species have to be chosen.

In the second step the behavior of the hot plant including the chemical reactions and volatile release as well as breakage and shrinkage of biomass pellets based on experimental data and correlations of subproject B has to modelled. This can be achieved by the implementation of user-defined functions (UDF) using the open source software OpenFOAM®.

The aim of the study is to optimize the CLG process and detect suitable operation conditions, e.g. for the gasification temperature, ratio of bed materials, syngas composition and the OC lifespan.

Project funding

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

Project number HE 4526/21-1. Project start: May 2018.

Contact Data

Timo Dymala
Timo Dymala
research assistant
phone: +49 40 428 78 22 06
email: timo.dymala@tuhh.de