Experimental Study On Gasification Of Sludge Based On Modified Calcium-Based Sorbents

In recent years,with the rapid growth of sewage sludge and the limitations of traditional sludge treatment methods,the treatment and disposal of sludge efficiently and environmental friendly has become a growing concern.Sludge produced by sewage treatment plants contains a large number of organic substances.Sludge gasification can achieve the decrement of sludge,produce combustible syngas,and reduce the emissions of heavy metals,which has good prospects of development.Sludge gasification process unavoidably suffers from the problem of unpleasant tar and CO₂ formed within the process.Tar will not only reduce the gasification efficiency,but also affect the operation of gasification equipment.CO₂ is a typical greenhouse gas,which also reduces the calorific value of syngas.Sludge gasification based on calcium-based sorbents is an efficient way to produce hydrogen-rich syngas.CaO can absorb CO₂ at high temperature,and the removal of CO₂ during gasification is conducive to the positive process of water-gas shift reaction and methane reforming reaction,thereby increasing the production of H₂.However,CaO usually suffers from sintering and agglomeration at high temperature,and the effect of CaO catalytic tar cracking is very limited.Therefore,it is meaningful to improve the CO₂ capture capacity,catalytic gasification and tar cracking performances of calcium-based sorbents.First of all,two kinds of calcium-based sorbents are prepared by wet mixing method and sol-gel method respectively,using Al₂O₃ as the support.The effect of calcium based sorbent prepared by different methods on the steam gasification of sludge was studied.The CO₂ capture ability and cyclic stability of calcium-based sorbents prepared by different methods were investigated.The results show that the increase of gasification temperature and the addition of calcium-based adsorbent both improve the concentration and yield of H₂ in syngas and the cold gas efficiency of gasification,which is beneficial to the production of hydrogen-rich syngas by sludge gasification.The H₂ concentration,H₂ yield and the cold gas efficiency of gasification are higher when sorbent prepared by sol-gel method is applied.The calcium based sorbent prepared by sol-gel method shows higher CO₂ capture ability and cyclic stability.Three calcium-based sorbents were prepared by sol-gel method using Al,La and Zr as supports respectively.The effects of three calcium-based adsorbents on sludge steam gasification at different temperatures were studied.The CO₂ capture capacity and cyclic stability of the three calcium-based sorbents were investigated.The results show that the three metals of Al,La and Zr have no obviously catalytic effect on sludge steam gasification.Among them,The H₂ concentration,H₂ yield and the cold gas efficiency of gasification are highest when Al modified calcium-based sorbent is applied,and Al supported calcium-based sorbent exhibits highest CO₂ capture capacity and cyclic stability,after 10 cycles?carbonation at 700^oC for 35 min,calcination at 850 ^oC for 5 min?,the carbonation conversion stabilized at about 68%.On the basis of Al₂O₃ as support,a series of calcium-based sorbents were prepared by sol-gel method,using Co,Mg and Ce as catalytic active ingredients respectively.The effects of calcium-based sorbents modified by different catalytic active ingredients on sludge steam gasification were studied.The results show that Mg and Ce have no obvious catalytic effect on sludge steam gasification for hydrogen-rich syngas,and the addition of Co in calcium-based sorbents promote tar cracking and methane reforming,and significantly increases H₂ yield and concentration of syngas and cold gas efficiency of sludge gasification.On the basis of Al₂O₃ as support,the effects of different content of Co in Calcium-based sorbent on sludge steam gasification and the CO₂ capture capacity and cyclic stability of calcium-based sorbents were also studied.The results show that increasing the amount of Co can promote tar cracking and methane reforming,which is conducive to the production of hydrogen-rich gases.Compared with pure CaO,H₂ yield increases by 102%and the concentration of the H₂ reaches to 85%in syngas when sorbents with 15%Co content is applied at 650?.Modified calcium-based sorbents with Al₂O₃ as support and Co as catalytic active ingredient exhibits excellent CO₂ cyclic capture performance,and its performance decreases with the increase of Co Content in Calcium-based sorbent.Among them,the carbonation conversion of the sorbent with 10%Co content stabilized at about 70%in 30 cycles?carbonation at 700^oC for 35 min,calcination at 850 ^oC for 5 min?.