Study On The Improvement Of Ca-based Sorbent In Direct Hydrogen Production From Carbonaceous Energy

A large amount of CO₂ is produced in direct combustion of carbonaceous energy. Oxyful combustion,CO₂ capture from flue gas,and direct hydrogen production from carbonaceous energy are all the important approaches for reducing CO₂ emissions.In direct hydrogen production from carbonaceous energy,CO₂ sorbent is necessary.CaO was widely used as CO₂ sorbent because its low price and high CO₂ capture capacity. CaO sorbents obtained from naturally limestone and dolomite are usually microporous in nature.Their reactivity and cyclic performance are always poor.It is necessary to investigate the methods for improving CaO reactivity and cyclic performance.This paper investigated several methods for improving CaO reactivity using a pressurized thermogravimetric analyzer(TherMax 500).The applicability of the methods in improving CaO cycle performance was also conducted.The main research contents and results are listed as follows:(1)The effect of steam on CaO carbonation performance was investigated.The results revealed that steam could improve CaO carbonation performance significantly even if Ca(OH)₂ was not produced.The improvement of steam on CaO carbonation performance should not be attributed to the enhancement of steam on the physical structure of CaO but be attributed to steam catalysis in CaO carbonation.(2)The kinetics of CaO carbonation under low steam partial pressure was investigated. According to the carbonation kinetics of CaO,a steam catalytic mechanism was proposed.(3)The improvement of MgO on CaO carbonation performance was intimately related to steam.MgO could improve CaO carbonation performance significantly when steam was added.However,MgO did not improve CaO carbonation performance markedly without steam addition.The improvements of different kinds of MgO additives on CaO carbonation performance were different from each other significantly.The differences were not related to particle size and surface area of MgO but intimately related to the surface basicity of MgO.(4)Na₂SO₄ could improve steam catalysis in CaO carbonation significantly.Steam could also enhance the improvement of Na₂SO₄ on CaO carbonation significantly. There were obvious synergistic effect between Na₂SO₄ and steam.There were also marked synergistic effects between steam and the other alkali metallic inorganic salts such as K₂SO₄,Rb₂SO₄,KCl,and Na₂CO₃.However,Cs₂SO₄ and NaCl improved steam catalysis slightly.The differences of synergistic effects between different alkali metallic inorganic salts and steam were possibly related to their different steam adsorption capacities.(5)The effects of steam,MgO+steam,or K₂SO₄+steam on CaO carbonation performance were investigated.The results revealed that CaO cyclic performance with only steam addition was better than that without steam addition.Moreover,CaO cyclic performance with simultaneous steam and MgO or K₂SO₄ addition was better than that with only steam addition.A new model was proposed which was used to predicting the relationship between CaO cyclic performance and cyclic number.The model could fit not only the experimental results but also the data obtained from the literatures.