Theoretical modeling of laminar and turbulent dispersed multiphase flow in a solar reactor for hydrogen production by cracking of methane

 

Summary of the MSc Thesis by Antonio Rubino, presented on February, 25, 2013

Supervisors:
Prof. Dr. D.J.E.M. Roekaerts,  Dr. M.K. Stöllinger

Solar thermal decarbonization of methane has been proposed as a midterm step towards a renewable energy scenario in which hydrogen plays a key role. Thermo-chemical conversion of solar energy can be also regarded as an efficient long-term storage option for solar energy. The feasibility of solar thermal cracking technology of methane has been demonstrated by several research groups, in particular, by A. Steinfeld and co-workers  [1-3]. However, further research is needed in order to increase thermo-chemical efficiency and to enable scale-up of the proposed processes.

In this work, the formulation of theoretical predictive models for multiphase reactive flows is presented, with the aim to provide tools to overcome the bottlenecks of available and future technologies, in the field of solar thermal cracking of natural gas. The solar reactor proposed by Maag et al. [4] is taken as reference for the theoretical modeling. This reactor is characterized by a flow of methane mixed with argon laden with carbon particles and exposed to concentrated solar radiative energy. The carbon particles are necessary since they act as radiant absorbers and provide surface needed to enhance the cracking reaction of methane. As the reaction progresses, hydrogen is produced together with solid carbon. The solid carbon produced is deposited on the particle surface, resulting in particle growth.

Both Euler-Euler and Euler-Lagrange theoretical modeling approaches of a laminar multiphase flow solar reactor are described. An implementation of the Euler-Lagrange approach was carried out using the commercial software ANSYS-Fluent together with in-house user defined functions routines. The implementation of the Euler-Lagrange approach was validated and showed good agreement with experimental results from literature.

Next, with a view on scale-up of current technologies to conditions involving turbulence, a statistical flow simulation model of solar thermal cracking in turbulent flow is presented, based on the work made in the field of pulverized coal combustion by M.K. Stöllinger et al. [5,6]. The proposed approach employs a model equation for the joint velocity-composition probability density function (PDF) that is solved by a hybrid Finite-Volume/Monte Carlo algorithm. In the proposed PDF approach, mean quantities, such as mean hydrogen concentration, are obtained as averages over multiple samples of the flow, represented in a Monte Carlo simulation. This approach provides the mean chemical source terms in exact form, in the evolution equation for the gas mass density function and thus, the effect of the non-linear reaction rates is taken into account without approximation. Due to the analogy made with the work of pulverized combustion [5], the approach considered in this study can be implemented in the TU-Delft in-house code PDFD, which has already been successfully used in Ref. [6].

References

[1] D. Hirsch and A. Steinfeld (2004). Solar hydrogen production by thermal decomposition of natural gas using a vortex-flow reactor. International Journal of Hydrogen Energy, 29(1):47 – 55

[2] G. Maag, (2006). Monte Carlo radiative heat transfer analysis of a CH4  flow laden with carbon particles. Master Thesis, ETH Zürich

[3] A. Steinfeld (2005). Solar thermochemical production of hydrogen. A review. Solar Energy, 78(5):603 – 615

[4] G. Maag, G. Zanganeh and A. Steinfeld, (2009). Solar thermal cracking of methane in a particleflow reactor for the co-production of hydrogen and carbon. International Journal of Hydrogen Energy , 34(18):7676 – 7685.

[5] Michael Stöllinger, Bertrand Naud, Dirk RoekaertsNijso Beishuizen, Stefan Heinz, PDF modeling and simulations of pulverized coal combustion – Part 1: Theory and modeling, Combustion and Flame, 160(2) (2013) 384-395

[6] Michael Stöllinger, Bertrand Naud, Dirk RoekaertsNijso Beishuizen, Stefan Heinz, PDF modeling and simulations of pulverized coal combustion – Part 2: Application, Combustion and Flame, 160(2) (2013) 396-410

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