Hydrogen Production by Integration of Fluidized Bed Heat Exchangers in Steam Reforming

Doktorsavhandling, 2020

The possibility to integrate fluidized beds as heat sources for steam reforming in industrial hydrogen production plants is examined in this work. The proposed processes include traditional catalytic steam reforming of natural gas (SMR), shift reactors and purification steps; the main difference is the reformer furnace. The conventional SMR plant includes a gas-fired furnace where the heat necessary for the endothermic steam reforming reaction is generated from combustion of gaseous fuels and transferred to the steam reforming tubes mainly by radiation. The alternative which is investigated in this work is to immerse the reformer tubes in fluidized bed heat exchangers (FBHE) with the potential of improved heat transfer to the reformer tubes. Experiments at laboratory and semi-industrial scale have been conducted with different bed materials of relevant particle sizes where it has been observed that high bed-to-tube heat transfer coefficients (>500 W/(m2K)) can be obtained in the targeted FBHE units. In the experiments different fluidization gases, bed temperatures, gas velocities and two different tube orientations were evaluated to obtain a better overall picture of the bed-to-tube heat transfer.

The first of the proposed processes consists of a single fluidized bed heat exchanger where the fuel is converted in the bed; this is a novel process facilitated by the use of oxygen carrier particles as bed materials. The process is estimated to reduce the supplementary fuel consumption of natural gas which thereby causes the reduction of CO2 emissions from the process by approximately 12% and the levelized hydrogen production cost is approximately 7% lower in comparison with the conventional SMR plant. Lab-scale experiments were performed where inert silica sand was compared with two oxygen carriers using methane and PSA off-gas as fuel. The experiments showed that the fuel was converted in the dense bed even at moderate furnace temperatures (i.e., 600-800˚C) and the use of oxygen carrier increased the fuel conversion in the bed.

The second proposed process is based on integration of SMR with chemical-looping combustion where an oxygen carrier is circulated between two interconnected fluidized bed reactors. In this process the flue gas stream obtained from the fuel reactor is not diluted with N2 resulting in that the CO2 produced can easily be captured. The supplementary fuel consumption increases only slightly compared to the first proposed process. The process presents a hydrogen production efficiency which is 8% lower than the conventional process at a levelized production cost which is around 1% lower than for the conventional process despite that the suggested process includes CO2 capture and CO2 compression.

The third proposed process is also based on chemical-looping combustion but uses biomass instead of natural gas as supplementary fuel, which enables the possibility to achieve significant net negative emissions. The estimated net CO2 emissions corresponds to a reduction by 142% compared to the conventional SMR plant at a levelized hydrogen production cost which is 1.4% higher.

The experimental campaigns on bed-to-tube heat transfer and oxygen carrier aided combustion support the claim that fluidized bed heat exchangers are suitable for use in the proposed SMR application. In the techno-economic assessment it is observed that the proposed processes display high thermal efficiency and the possibility to achieve significant reductions in CO2 emissions in industrial hydrogen production. All three proposed processes are considered as interesting for large-scale plants which could provide several environmental benefits and this work could be used as a support for industrial implementation.