Ramon Voncken, PhD
It is widely considered that hydrogen will play an increasingly important role in future energy systems. On an industrial scale, more than 80% of the hydrogen is currently produced by steam reforming (SR) of natural gas/methane carried out in multi-tubular fixed-bed reactors. The reaction system consists of two main reactions, generally carried out on Ni-based catalysts. The first reaction (1) is the reforming of methane to CO and H2 with steam, the second reaction (2) is the water gas shift (WGS) reaction to convert the CO to further H2.
(1) CH4 + H2O ↔ CO + 3H2 ΔH°298 K = +206 kJ/mol
(2) CO + H2O ↔ CO2 + H2 ΔH°298 K = - 41 kJ/mol
The reforming reaction is an equilibrium limited endothermic reaction system requiring high temperature for complete fuel conversion.
Conventional hydrogen production plants consist of at least four different stages to produce pure hydrogen; two reforming reactors (high and low temperature), water gas shift reactor and a purification system such as pressure swing adsorption (PSA). Next to the economical drawbacks of this production method, such as the high energy penalties due to endothermic character of the system and the large number of process stages required, there is a large environmental impact by this process due to the anthropogenic CO2 which is released (reaction 2). The ClingCO2 project aims to provide a solution to the drawbacks of conventional systems via the fundamental understanding and quantitative description of the interplay of transport phenomena and chemical reactions in a novel integrated reactor concept with a high degree of process intensification; the membrane assisted chemical looping reforming reactor (MA-CLR, Fig. 1).
The MA-CLR consists of an air reactor and a fuel reactor. In the air reactor, metal particles (e.g. Ni) are oxidized exothermically by exposing them to air. The depleted air (mainly N2) and metal-oxide particles are then separated and they are introduced to a fluidized bed which contains hydrogen-selective membranes. In the fuel reactor, methane and steam are converted into syngas. The syngas reacts with the oxygen carrier, which also acts as a catalyst for both SMR and WGS reactions. The extraction of hydrogen from the reactor via the membranes tilts the chemical equilibrium towards the hydrogen side, i.e. resulting in a higher production of hydrogen. The exiting gas stream contains CO2 and steam, which can easily be separated, after which the CO2 can be stored (Carbon Capture & Storage (CCS)). The oxygen carrier is subsequently transported to the air reactor where it is oxidized with air again and the hot regenerated material is ready to start a new cycle.
This part of the ClingCO2 project started in August 2013 and will mainly focus on the detailed modeling of the MA-CLR reactor system, to obtain fundamental understanding of the operation of chemical looping reforming reactors and fluidized beds with immersed membranes. One of the models that will be used to simulate the fluidized bed and its membranes is a Discrete Particle Model (DPM) (see Fig. 2), combined with an immersed boundary technique, coupled with mass and energy balances. The modeled results will also be compared with experimental results.
This research is supported by the Dutch Technology Foundation STW, which is part of the Netherlands Organization for Scientific Research (NWO), and which is partly funded by the Ministry of Economic Affairs.