Study On Effect Of Ash And Active Element Modification On Performance Of Iron-based Oxygen Carrier In Chemical Looping Combustion Of Coal

The coal ash after coal conversion will be deposited on oxygen carrier in the coal chemical looping combustion(CLC)system.Therefore,it is of great significance to study the interaction between derived ash and oxygen carrier for developing CLC systems.In this research,iron-based oxygen carrier was taken as the research object.Through the combustion experiment of the oxygen carrier directly mixed with coal ash and in situ gasification chemical looping combustion(iG-CLC)of coal,the influence of the type of coal ash,combustion temperature and fuel composition on the reactivity and cycle stability of iron-based oxygen carrier was investigated.The action mechanism,migration and permeation process of coal ash on the oxygen carrier were studied.Distribution characteristic of S from coal was preliminarily explored in reduction stage.Considering the effective catalytic components in coal ash,this research investigated the effects of Na,K and Ca on the reactivity,thermal stability,mechanical strength and life of iron-based oxygen carriers,and the types,addition amount and working mechanism of modifiers were systematically studied.The main contents include:(1)In a lab-scale batch fluidized bed(LBFB)CLC system,taking natural hematite as oxygen carrier,the influence of the type of coal ash,reaction temperature and fuel composition on reactivity and thermal stability of oxygen carrier was studied,and pore development and ash infiltration of oxygen carrier was investigated.The results showed that the reaction between hematite and fuel can be promoted by the ash with high CaSO4 content,but the carbon deposition on the surface of oxygen carrier was aggravated by ash.After repeated cycles,CaSO4 decomposition weakened the promoting effect of coal ash.The decomposition process of CaSO4 would be accelerated with the increase of temperature and H2O.Pores of hematite expand inward to increase the specific surface area.The coal ash permeated into the pores and blocks the pores.At the same time,the high Si and A1 ash also caused slagging on the surface of oxygen carrier,which was disadvantageous for reduction.(2)To be close to the actual iG-CLC,combustion characteristics of different rank coal and the effect of deashing on fuel conversion were studied in a LBFB and thermogravimetric analysis(TGA)using raw coal and deashing coal as fuel.The migration of elements in coal ash to oxygen carrier was explored.The results showed that low rank coal had higher reactivity and shorter reduction time.The combustion efficiency of low rank coal was lower than that of high rank coal due to higher volatile content and lower density,thus CO2 concentration in the product was lower.The composition and content of coal ash had a great influence on the activity of oxygen carriers.After combustion of high Ca coal,reactivity of oxygen carrier was significantly improved,and reduction time was shortened significantly.Si had a negative effect on the reactivity.(3)The reaction between CO and Ca modified Fe2O3/Al2O3 was predicted based on the shrinking core model,and the activation energy was calculated.Effects of Ca modification on the combustion efficiency and circulation stability of oxygen carriers were investigated by the redox operation in a LBFB.The results showed that Ca could replace the Fe in Fe2O3,and reduce the formation energy of surface oxygen vacancy to improve the reactivity obviously.CaSO4 had a stronger promoting effect than CaCO3,but its thermal decomposition made the reactivity of oxygen carrier decreased rapidly after the previous cycles,while the oxygen carrier with CaCO3 had a better stability.The CaAl2O4 and CaAl4O7 formed after the Ca content of oxygen carrier exceeded 5%,which resulted in slagging and sintering,and it was independent of the type of modifier.(4)Similar to Ca,alkali metal elements could significantly improve the reactivity of iron-based oxygen carriers.Performance of Na and K modified Fe2O3/Al2O3 were studied by TGA and LBFB.The results showed that the type and amount of alkali metals had a great influence on the crystal structure and reduction rate of oxygen carriers.Na and K had strong catalytic effect on the conversion of Fe2O3 to Fe3O4 and Fe3O4 to FeO.When the content of K was more than 3%,the structure of oxygen carrier was fragile and easy to agglomerate,and high Na capacity formed low melting point solid solution and led to slagging.Addition of both K and Na could inhibit the negative effect of the other side to reduce agglomeration and slagging and prolong the lifetime of the oxygen carriers.(5)Based on the Fe2O3/Al2O3 oxygen carrier,effects of reaction temperature and Fe2O3 load on combustion products and S element distribution were studied in a batch fixed bed reactor.The results showed that the release of carbon gas increased with increase of reaction temperature and decreased with increase of Fe2O3 loading.Proportion of CO2 in the product increased first and then decreased with the increase of reaction temperature,and reached the highest at 850?.The enrichment degree of S in the oxygen carrier increased with increase of temperature and Fe2O3 loading at 2 h,and the distribution proportion of S in the oxygen carrier decreased significantly with increase of temperature at 5.5 h,indicating that S in the oxygen carrier was rereleased into the gas phase at the later stage of reaction.