Advanced Membrane Reactors in Energy Systems A Carbon-Free Conversion of Fossil Fuels

The purpose of this project is to develop hydrogen and CO2 selective membranes to allow combination of natural gas reforming with H2 or CO2 separation in separation enhanced reactors, i.e. membrane reactors, for carbon-free hydrogen production or electricity generation. To achieve this, the project comprises three distinct tasks: system and reactor analysis, membrane materials research and catalysis. The results of the system and reactor analysis demonstrate that doubling the target permeation (derived from H2-selective WGS-MR, Water-Gas-Shift-Membrane Reactor) of CO2 selective WGS-MR already leads to a competitive advanced membrane reactor configuration. Moreover, the high CO2 purity as well as the flexibility with respect to syngas conversion and CO2 separation in combination with the reduced hydrogen loss due to combustion with traces of oxygen, are important advantages for implementation of the CO2-selective WGS-MR. Materials science has led to the following findings. Structure, composition and decomposition pathway of Mg-Al Hydrotalcite have been determined. The Mg/Al ratio is found to be 1.8, the net formula is Mg0.64Al0.36(OH)2(CO3)0.18·1.0 H2O. At about 250C the carbonate-water layer disappears and with this the presupposed transport path for CO2 through a dense hydrotalcite membrane. Porous hydrotalcite based membranes where the transport and selectivity are based on affinity of CO2 to the micro pore walls are now the focus of R&D at ECN. Coatings based on nano particulate dispersions or sol gel synthesized particles and sensitizing microporous supports are the three routes to achieve our goal. The catalysis work revealed that specific nobel metal and FeCr based WGS catalysts prove to be stable for at least 200 h, under experimental conditions without sulphur compounds. Introduction A sustainable use of fossil fuels in the future will undoubtedly make use of concepts, where the energy content of the fossil fuel is first transferred to hydrogen, followed by the conversion to the desired energy form. The driving force for these concepts is the possibility of capturing CO2 elegantly, while using the favorable thermodynamics to increase the efficiencies of fossil fuel conversion. We, ECN and TU-Delft, have identified membrane reactors as a game changing technology for highly efficient conversion of fossil fuels to carbon free energy carriers. The purpose of this project is to develop hydrogen and CO2 membranes to allow combination of natural gas reforming with H2 or CO2 separation in separation enhanced reactors, i.e. membrane reactors, for carbon-free hydrogen production or electricity generation. These devices offer multiple advantages, such as eliminating the requirement of water gas shift reactors with associated costs reductions; offering higher conversion efficiencies at lower temperatures; and decreasing primary energy use for CO2 separation/capture associated with electricity generation. Background The steam reforming and the water gas shift equilibriums are key reactions for the production of hydrogen from fossil fuels: CH4 + H2O D CO + 3H2 (1) CO + H2O D CO2 + H2 (2) By removing either CO2 or hydrogen from the reaction mixture, the equilibrium can be shifted to the product side. Effectively, this can lower the reaction temperature and improve the purity of the product. In conventional, hydrogen production from natural gas, the steam-reforming step is followed by two water gas shift (WGS) reactors. When separating either CO2 or hydrogen inside the reforming reactor, both the WGS steps can be eliminated. This implies that separation-enhanced techniques can also lead to investment costs reductions. Hydrogen or CO2 separation is a flexible technique that can be used in hydrogen production from natural gas, but also can replace the WGS section of an IGCC or Biomass gasification plant. These techniques are especially suited for CO2 capture, because the production of pure hydrogen and CO2 streams is intrinsic to separation-enhanced reactors. The combination of separation and reaction, as foreseen in membrane rectors, offers higher conversion of the reforming reactions at lower temperatures due to the removal of hydrogen or CO2 from these equilibrium reactions, as shown in equations 1 and 2. For instance, in case natural gas reforming for carbon free hydrogen production, the use of membrane reactors will result in significantly lower operation temperatures (400 500C) and higher efficiencies 85 90 instead of 75%.[1] In fact membrane reactors allow for low-irreversibility production and conversion of hydrogen to another energy form with integrated CO2 capture. Membrane reformers/reactors can be integrated in power generation systems but also in central heating devices. Our assessment studies clearly showed that in a more integrated approach of electricity production and CO2 capture, using high-temperature membrane reactors will result in a substantially lower primary energy use for the CO2 separation/capture [2]. Besides that, the low operation temperature of the membrane reactor creates possibilities for so-called chemical recuperation, compensating part of the CO2 capture efficiency penalty. The tasks defined within this project are: Task 1 Task 2 Task 3-a Task 3-b Task 4 Task 5 System analysis and thermodynamic evaluations Hydrogen membrane research and development Hydrotalcite CO2 membranes research and development Ionic liquids CO2 membranes research and development Catalyst screening Reactor modeling and design Executed by ECN Executed by TUD Executed by ECN Executed by TUD Executed by ECN Executed by ECN Tasks 1, 4 and 5 pertain to both the hydrogen and carbon dioxide membrane cases. Results System and reactor analysis: Two advanced membrane reactor configurations were assessed wit respect to implementation in an Integrated Gasification Combined Cycle (IGCC) with precombustion CO2 capture. The advanced membrane reactors comprise a H2and CO2selective Water-Gas-Shift Membrane Reactor (WGS-MR), in which the separation enhances the equilibrium limited water-gas-shift reaction. Dry-fed coal gasification was selected for these detailed system assessments. The assessments were performed with AspenPlus combined with the in-house developed membrane model [3], as well as ‘Exercom’ that facilitates second law analysis. Sensitivity analysis identified the most important variables that govern the specific electric loss, when a H2-selective WGS-MR is implemented in an IGCC. These are the steam/CO-ratio at the inlet of the pre-WGS reactor and the pressure of the N2 sweep stream. Figure 1 displays that the steam consumption should be minimised since steam is applied to generate electricity in the bottoming cycle, whereas the membrane sweep pressure should be equal to the inlet pressure of the gas turbine combustion chamber, being 23 bars. 0.8 1.0 1.2 1.4 1.6 1.8