Hydrogen production from fossil fuels with carbon dioxide capture, using chemical-looping technologies
Carbon capture and storage have been receiving increasing interest lately, mainly as an option to reduce CO2 emissions from the power sector. The concept could be adapted for production of H2 as well, which would provide a carbon free energy carrier that could be used for example as transportation fuel. In this doctoral thesis, the option to use chemical-looping technologies to produce H2 from fossil fuels with CO2 capture is explored.
In chemical-looping combustion, direct contact between fuel and combustion air is avoided. Instead, a solid oxygen carrier performs the task of bringing oxygen from the air to the fuel. Thus, the resulting CO2 and H2O are not diluted with N2, and pure CO2 can easily be recovered by cooling and condensation. The heat of reaction is the same as for ordinary combustion. Chemical-looping reforming uses the same basic principles as chemical-looping combustion, but operates at understoichiometric conditions. Therefore chemical-looping reforming can be said to be a process for partial oxidation of hydrocarbon fuel into H2 and CO, where chemical looping is used as a source of undiluted oxygen.
In the theoretical part of this work, the technical feasibility to use chemical-looping technologies for production of H2 with CO2 capture has been examined. Two main ideas have been explored. Steam reforming integrated with chemical-looping combustion means that chemical-looping combustion is used for CO2 capture and as heat source for generation of H2 via the endothermic steam reforming reaction. Chemical-looping autothermal reforming utilizes understoichiometric conditions and the possibility to add some H2O to the fuel, in order to have both partial oxidation and steam reforming reactions occurring in the same reactor vessel. This way, a thermo neutral process is obtained. It is found that both options have potential to provide substantial advantages compared to conventional technologies. Thermodynamic modelling shows that reformer efficiencies of 80% or higher, including CO2 capture and CO2 compression, seem to be obtainable with both processes.
In the experimental part, chemical-looping reforming and chemical-looping combustion has been demonstrated in two different circulating-fluidized bed reactors. In total, over 200 hours of experiments have been recorded. Natural gas was used as fuel, and four different NiO-based materials have been used as oxygen carrier. Process conditions have varied from almost complete combustion into CO2 and H2O, to almost stoichiometric partial oxidation into CO and H2. The reactor temperature has been between 800 and 950ºC. The experiments worked very well, but occasionally limited formation of solid carbon could occur in the reactor. Adding 25-30% H2O or CO2 to the natural gas reduced this tendency significantly.
In addition to this, a few unconventional materials that could be used as oxygen carrier in chemical-looping applications have been examined by reduction with CH4 in a fixed-bed reactor at 900ºC. Some potentially useful materials were identified, such as Lax Sr1-xFeO3─δ perovskites and mixtures of Fe2O3 and NiO supported on MgAl2O4.
carbon capture and storage