Hydrogen-enriched Syngas Production By Lignite Chemical Looping Gasification With Composite Oxygen Carriers Of Phosphogypsum And Steel Slag

Fossil fuel combustion emits a large amount of carbon dioxide,which intensifies global warming and poses a serious threat to environment.Chemical looping gasification（CLG）is a clean and efficient carbon energy conversion pathway using oxygen carriers（OCs）to control carbon emissions.In CLG process,fuel can be converted efficiently,and the high value-added syngas,mainly CO and H₂,is produced.The performance of OCs is critical in the CLG,and it is developing to prepare composite OCs with multifunctional and high stability.However,complex preparation methods and expensive additives have limited their wide application in the industry.In the absence of resources,industrial wastes as resources to prepare OCs have been a promising strategy.As by-products from industrial production,substantial phosphogypsum（PG）and steel slag（SS）have not been suitably deposited,causing adverse impact on the environment.Landfill is main disposal methods with low utilization ratio.Previous works investigated the lignite CLG process of PG as well as the migration and transformation process of its elements,then,loading metals to modify it.These results prove that PG is a good oxygen carrier.In this work,PG and SS were used to prepare OCs for hydrogen-enriched syngas production through CLG technology.Multi-solid waste collaborative treatment could replace single solid waste modification to make full use of the characteristics of different solid waste,improve the solid waste utilization ratio,and make solid waste treatment more practical.The implications of this concept are to reduce the cost of preparation of OCs and provide an economical way to treat solid waste that can be disposed of sustainably.The reaction process and the effects of reaction conditions in CLG process were studied based on the Fact Sage calculation and verified in fixed bed experiments.Based on thermodynamics and dynamics analysis,the reaction mechanism of the CLG process with SS-PG OCs was explored,the following conclusions are drawn:（1）Based on calculational and experimental results,adding SS improved the higher hydrogen concentration in syngas and fixed more sulfur in solid.Higher temperature can improve the CLG process;however,over temperature adversely affects hydrogen production.Higher lignite concentrate and water vapor concentration improve hydrogen production.In fixed bed,the most economic optimal conditions for producing hydrogen-enriched syngas（found to be 72.51%）were determined as a reaction temperature of around 1023 K,lignite/OCs mass ratio of 1,50～67 wt%of SS/OCs mass ratio,and water vapor/OCs mass ratio of 0.6.（2）The reaction mechanism was investigated and following conclusions have been obtained:The formation of Ca₂Fe₂O₅ by PS and SS obtains O atom from water vapor,resulting in higher oxygen carrying capacity of OCs and producing hydrogen simultaneously.Comparing with Ca SO₄,Ca₂Fe₂O₅ has more distinct pores and offer more lattice oxygen to improve the hydrogen concentration in syngas.SS promotes the fixation of gaseous sulfur due to the formation of Fe S.The gasification reaction process belongs to two-dimensional nucleation and nuclei growth model,and the control step is the internal diffusion in the ash layer.（3）The CLG process in different reactors was analyzed and following conclusions have been obtained:Comparing with fixed bed,the reaction rate is the faster and the H₂ concentration in syngas is lower in fluidized bed.Syngas with 49.66%H₂ concentration can be obtained within 5 min in fluidized bed with boiling state.In conclusion,SS-PG OCs are the new era for hydrogen-enriched syngas production in CLG.