Generation of hydrogen from biogas with inherent carbon dioxide sequestration via a hybrid chemical looping – steam iron process

Notorious for its substantial contribution to anthropogenic climate change, the power industry has been implied to provide long and short term solutions for its emissions of the greenhouse gas CO2. Possible approaches include more effective energy conversion and consumption, utilization of less carbon intensive fuels or shifting to renewable energy sources. Still, most analysts agree that fossil fuels will be dominating the energy market in the near future, and as a consequence novel concepts for combustion of fossil fuels without emissions of CO2 to the atmosphere such as Carbon Capture and Storage have attracted great interest. An innovative technology for CO2 capture is Chemical Looping (CL) combustion, in which a solid oxygen carrier material performs the task of transporting oxygen between two reactors. Flexible in terms of fuel types (solid, gas) the chemical looping concept provides easily sequestrable CO2, potentially at low cost and without energy penalty. A wide range of designs are possible, depending on the targeted end-product. One much desired product considered as a potential alternative to fuels in transport industry is hydrogen, production of which is far from clean and simple. In this study, hydrogen production via the steam-iron reaction in in a process configured in similar fashion as chemical looping combustion is examined. The hybrid chemical-looping-steam-iron system proposed in the study consists of three fluidized bed reactors, a fuel reactor, a steam reactor and an air reactor. Selected on its ability to achieve multiple oxidation states and suitable thermodynamic properties, iron oxide is transported through the interconnected fluidized beds in a cyclic manner. In the fuel reactor, fuels such methane, syngas and biogas from gasifier are oxidized with oxygen provided by Fe2O3, producing CO2, H2O and FeO. In the steam reactor, FeO is oxidized to Fe3O4 with steam, producing H2. Finally, in the air reactor Fe3O4 is oxidized to Fe2O3 with air producing heat to sustain the endothermic reactions in the other reactors. The potential of the process was examined by thermodynamically modeling with Aspen Plus software, while an experimental approach was used to examine if the proposed approach would be practically feasible. Methane, syngas and gasifier output simulated biogas fuels were tested with the aim of examining the fuel composition effect on the overall efficiencies. Between analyzed fuels, syngas indicated the highest net hydrogen efficiencies, with values as high as 86.1% for a stoichiometric steam feed ratio, to 76.14% for an excess steam ratio. Meanwhile biogas system, due to its low heating values and steam presence in the fuel composition displayed the lowest net efficiencies with 68.1 and 42.5% for stoichiometric and excess steam feed ratios. The experiments were conducted in a batch fluidized bed reactor at a temperature range between 700 to 950oC. Different fuels and oxygen carrier materials were examined. The results indicated that operation with synthetic iron oxide particles supported on MgAl2O4 and syngas as fuel was feasible, while the use of waste materials and natural minerals as oxygen carrier and simulated biogas a fuel were much more challenging.