Separating the Loop in Chemical Looping Reformation: How a Two-Stage Reformer and Decoupling Alleviates Process Challenges.

Chemical looping reformation (CLR) is a process that converts a carbonaceous feedstock (refuse, biomass, coal, or natural gas) into syngas using a metal-oxide oxygen carrier. The traditional concept of CLR uses moving or fluidized beds to fully convert carbon to syngas in a pair of reactors for the oxidation and reduction. Sensible heat from the oxidation of the metal-oxide provides heat to sustain the reforming reactions. Steam can be added to the reformer as a source of additional oxygen. While CLR promises to provide a pathway to sustainable syngas, complexities with the process and physical limitations of the metal-oxide oxygen carrier have hindered its commercialization.

A decoupled approach to CLR may provide a solution. Rather than relying on the sensible heat from oxidizing the metal carrier, electric heaters provide the energy to break down biomass in the pyrolysis zone of the reactor. Steam added to the reactor aids in the conversion of long carbon chains to shorter, volatile species around 750°C. Not all the carbon is converted and the addition of an oxygen carrier, in this case Fe₂O₃, supports additional conversion. Unreacted feedstock, now mostly unconverted carbon and ash, discharges from the reactor mixed with Fe₃O₄ and FeO. Iron species are separated from the solids product and are sent to a separate, uncoupled oxidation reactor, hence open-loop. Gaseous products continue to the 1200°C gasification zone further reacting with steam to produce syngas. Syngas further undergoes water-gas shift and cleanup to produce commodity chemicals.

A simulation study using Aspen plus examined the effect of changing the operational parameters of the process focusing on minimizing the electrical load per unit mass of hydrogen produced. Refuse derived fuel (RDF) is the assumed feedstock. Parameters varied include steam and Fe₂O₃ feed rates as well as temperature in the pyrolysis zone of the reactor. Other benefits of decoupling the process are examined such as:

1. The catalytic effect of iron during pyrolysis
2. Improved heat transfer due to iron particle disbursement
3. Reduction of sulfur species in the syngas (e.g. H₂S) with the formation of iron-sulfides
4. Simplified material handling