Chemical looping reforming and gasification
The syngas is an important intermediate for generation of such diverse products as electricity, chemicals, hydrogen, and liquid fuels.
As shown in Fig 1b, the diagram can be divided into four different sections based on the following four key reactions: The sections identified in Fig 1b provide the information on metal oxide materials that can be selected as potential oxygen carriers for desired chemical looping applications.
In addition to the relative redox potentials of metal oxide materials illustrated in Fig 1b, the development of desired oxygen carriers for chemical looping applications requires to consider such properties as oxygen carrying capacity, redox reactivity, reaction kinetics, recyclability, attrition resistance, heat carrying capacity, melting point, and production cost.
[1][6][7][8][9][10][11][12][13][14] The CLR and CLG processes can be configured based on the types of carbonaceous feedstocks given and desired products to be produced.
The state-of-art SMR system places the tubular catalytic reactors in a furnace, in which fuel gas is burned to provide the required heat.
In the SMR with chemical looping combustion (CLC-SMR) concepts shown in Fig 2,[15][16] the syngas production is carried out by the SMR in a tubular catalytic reactor while the chemical looping combustion system is used to provide the heat for the catalytic reaction.
Depending on which chemical looping reactor is used to provide the SMR reaction heat, two CLC-SMR schemes can be configured.
In either scheme, the combustion of metal oxide by air in the chemical looping system provides the heat source that sustains the endothermic SMR reactions.
The NiO-based oxygen carriers exhibit excellent reactivity, as shown by the high conversion of natural gas.
The moving bed reactor that does not have the effects of back mixing of the metal oxide particles is another gas-solid contact configuration for CLR/CLG operation.
[18] This reactor system developed by Ohio State University is characterized by a co-current gas-solid moving bed reducer as given in Fig 4.
[18][19] The CLR moving bed process applied to the methane to syngas (MTS) reactions has the flexibility of co-feeding CO2 as a feedstock with such gaseous fuels as natural gas, shale gas, and reducing tail gases, yielding a CO2 negative process system.
[20][21][22][23][24] The CLR-MTS system can yield a higher energy efficiency and cost benefits over the conventional syngas technologies.
The light organic compounds may reduce the purity of the syngas, while the tars may accumulate in downstream pipelines and instruments.
A carbon stripper may be needed at the solid outlet of the fluidized bed reducer to allow the unconverted char to be separated from the oxygen carriers.
In a similar operating scheme to the CLR - MTS system given in Fig 4, chemical looping gasification (CLG) of solid fuels carried out in a co-current moving bed reducer to partially oxidize solid fuels into syngas can reach an appropriate H2/CO ratio for downstream processing.
The moving bed prevents the channeling or bypassing of the volatiles and chars, thereby maximizing the conversion of the solid fuel.
[1] The selective oxidation based chemical looping processes can be used to produce directly in one step value-added products beyond syngas.
An example is the chemical looping selective oxidation process developed by DuPont for producing maleic anhydride from butane.
Its commercial operation, however, was hampered in part by the inadequacies in the chemical and mechanical viability of the oxygen carrier VPO and its associated effects on the reaction kinetics of the particles.
