K. R. Rout¹ J. Meyer² H. A. Jakobsen¹

¹Chemical Engineering Department, Norwegian University of Science and Technology, Norway

²Institute for Energy Technology (IFE), Oslo, Norway

Corresponding author’s e-mail address: rout@nt.ntnu.no

Keywords:Hydrogen production, Sorption-enhanced reforming, fluidized bed reactor, two-fluid model,

Hydrogen is considered as the next generation energy carrier that can be used as an environmentally friendly  fuel in high efficiency energy systems, mainly in the energy production and transport sectors  [1, 2]. Steam methane reforming (SMR) is a heterogeneous catalyzed process where methane and steam react over a nickel-based catalyst at high temperature to produce synthesis gas, a mixture of H₂, CO and CO₂. SMR is followed by the so called water gas shift reaction (WGS) run usually in two steps over two different catalysts, where CO reacts with more steam to produce H₂ and CO₂. The main reactions in the conventional SMR/WGS process are represented by the equations (I), (II) and (III) [3], and the kinetics of the SMR reaction are described by Xu and Froment [4].

 (I) CH₄ (g)+ H₂O (g) = CO (g) + 3H₂(g)

 (II) CO (g) + H₂O (g) = CO₂ (g) + H₂ (g)

 (III) CH₄ (g) + 2H₂O (g) = CO₂ (g) + 4 H₂ (g)

Combining simultaneous hydrogen production from the SMR process and capture of CO₂  can be achieved in a so-called sorption-enhanced steam methane reforming process (SE-SMR) [5]. According to Le Chatelier’s principle, if CO₂ is removed in the SMR process as soon as it is formed, the reforming- and water-gas shift (WGS) reactions proceed beyond the conventional thermodynamic limitations, and consequently, more methane is converted to hydrogen and reaction temperature can be reduced. This improved conversion scheme at lower temperature has the potential to show significant investment and energy savings due to process simplification and intensification [2, 5]. In the SE-SMR process, CO₂ is usually removed by a high temperature CaO-based sorbent in a carbonation reaction. The CO₂capture process can be written as:

CO₂ (g) + CaO (s) = CaCO₃(s)

Thus, by adding the SMR, WGS and carbonation reactions, the overall SE-SMR reaction becomes [6]:

CH₄ + 2H₂O + CaO = 4H₂ + CaCO₃

The SE-SMR operates best in a circulating fluidized bed reactor, because this process requires continuous regeneration of CaO-based sorbent. This study is based on a reactor concept using a Dual Bubbling Fluidized Bed reactor system (DBFB) [7] with circulating solids, at near atmospheric pressure. The solids (sorbent and catalyst) are circulated between a reformer and a regenerator. The main advantages of this system are that it can be run continuously, good temperature uniformity and an efficient heat exchange can be obtained, and solids can be purged and refilled continuously.

The kinetics of the carbonation and SE-SMR reactions have been studied using a thermo-gravimetric bubbling bed reactor set-up to take into account the gas-solid hydrodynamics and obtain kinetic data for relevant conditions closer to the conditions of the DBFB system. In this study, a bubbling bed reactor of 4.27 cm inner diameter which carries up to 300 g of solid material hangs on a high resolution balance. The whole reactor is inserted in a furnace, which can heat up to 1200^oC. The temperature along the axial direction of the reactor is measured by a thermocouple inserted into the reactor. Limitations on the scale  of  the  setup  allowed  fluidization  velocities  between  0.04  and  0.06  m/s,  i.e  roughly between  3  and  7  times  the  actual  minimum  fluidization  velocities  in  the  different  test conditions.  A static bed height of 6.7 cm has been used in the experiments. 

In order to bring the SE-SMR technology forward to up-scaling and commercialization, knowledge regarding stable operation of a DBFB reactor system suited to the SE-SMR process in steady state continuous operating conditions is needed. Hence, a multi-fluid Eulerian model has been derived from binary kinetic theory of granular flows, free path theory and an empirical friction theory. The effects of inter- and inner-particle collisions, particle translational motions and particle-particle friction are included.

The numerical model implementation is based upon the Finite Volume Method with a staggered grid arrangement. The exchange of solids (solid flux) between the reactor units of the DBFB is implemented through additional mass source/sink terms in the continuity equations of the two phases.

Institute for Energy Technology is currently testing the SE-SMR technology in a small DBFB pilot producing 10 Nm³/h H₂ from upgraded biogas at the HyNor Lillestrøm R&D hydrogen station, Norway. The developed model is validated with data produced from the pilot plant. The present paper focuses on the description of the coupling between flow phenomena and chemical kinetics, and mass- and heat transfer processes when operating the DBFB reactor system. During operation of the DBFB pilot, key parameters such as solids inventory, ratio of sorbent to catalyst as well as the solid circulation rate have been optimized to obtain a high H₂-yield. The outcomes of such simulations represent important results enabling a reliable validation of the process and further up-scaling.

References:

1.         Rout, K.R., A Study of Sorption Enhanced Steam Methane Reforming Process. Doctoral Thesis.Norwegian University of Science and Technology, Norway, 2012., 2012.

2.         Balasubramanian, B., et al., Hydrogen from methane in a single-step process. Chemical Engineering Science, 1999. 54(15-16): p. 3543-3552.

3.         Xu, J.G. and G.F. Froment, Methane Steam Reforming, Methanation and Water-Gas Shift .1. Intrinsic Kinetics. Aiche Journal, 1989. 35(1): p. 88-96.

4.         Xu, J.G. and G.F. Froment, Methane Steam Reforming .2. Diffusional Limitations and Reactor Simulation. Aiche Journal, 1989. 35(1): p. 97-103.

5.         Hufton, J.R., S. Mayorga, and S. Sircar, Sorption-enhanced reaction process for hydrogen production. Aiche Journal, 1999. 45(2): p. 248-256.

6.         Rout, K.R., et al., A numerical study of multicomponent mass diffusion and convection in porous pellets for the sorption-enhanced steam methane reforming and desorption processes. Chemical      Engineering Science, 2011. 66(18): p. 4111-4126.

7.         Meyer, J., et al., Techno-economical study of the Zero Emission Gas power concept. 10th International Conference on Greenhouse Gas Control Technologies, 2011. 4: p. 1949-1956.