Study On PAHs Formation In Coal Direct Chemical Looping Combustion Of Modified CaSO₄/Bentonite Oxygen Carriers

To control the generation of polycyclic aromatic hydrocarbons?PAHs?in coal direct chemical looping combustion?CLC?and restrain the appearance of carbon deposition phenomenon on the surface of oxygen carriers,the mechanical mixing method to prepare three oxygen carriers which are CaSO₄/bentonite oxyen carrier,CaSO₄-K2C03/bentonite oxyen carrier and CaSO₄-Fe₂O₃/bentonite oxygen carrier was adopted to react with coals in a simulated chemical looping combustion fuel reactor.Gas chromatography was employed for qualitative and quantitative detecting for PAHs in flue gas.It was to explore the effect of modified oxygen carriers on the formation of PAHs and find a method to reduce carbon deposition.In this paper,three kinds of oxygen carriers were physically analyzed by SEM and XRD at first;Secondly,the influence of coal structure on the formation of PAHs was discussed in coal direct CLC of CaSO₄/bentonite oxygen carrier;Finally,the emission characteristics of PAHs of three oxygen carriers in CLC were described at different coal types?anthracite,bituminous coal 1,bituminous coal 2,bituminous coal 3?,temperatures?800?,850?,900?,950?,1000??and oxygen carriers/coal ratios?2.4,2.8,3.2,3.6,4.0?.The content differences of PAHs formation of modified and unmodified oxygen carriers reacting with coals were compared but analyzed the reason.The results show that:?1?Compared with SEM images,it can be seen that the surface of modified oxygen carriers became rough and the surface area was increased.XRD patterns shows that the reduction product of CaSO₄ was CaO in CLC,it was not detected CaO and no side reaction basically taking place.?2?In the coal direct CLC of CaSO₄/bentonite oxygen carrier,with the carbon content increasing in coal,the total amount of PAHs formation presented a trend of increasing first and then decreasing,the largest is around 75%.With the increase of n?H?/n?C?,the total amount of PAHs formation increased first and then decreased,the maximum is near n?H?/n?C?=0.76.In the effect of n?O?/n?C?on the formation of PAHs,the total amount of PAHs increased rapidly and then decreased with the increase of n?O?/n?C?,the maximum is near n?O?/n?C?=0.17.With the volatile increasing,the total amount of PAHs formation was also increased,which was in the order as anthracite<bituminous coal 1<bituminous coal 2<bituminous coal 3.?3?The amount of PAHs produced by the reaction of modified oxygen carriers with different coals in CLC was all decreased.The distribution trend of PAHs formation of three oxygen carriers reacting with four coals was the same,followed by anthracite<bituminous coal 1<bituminous coal 2<bituminous coal 3.CaSO₄/bentonite oxygen carrier and CaSO₄-K₂CO₃/bentonite oxygen carrier are suitable for high-volatile bituminous coal in CLC.?4?The amount of PAHs produced by the reaction of two modified oxygen carriers with bituminous coal 3 in CLC was basically decended at each temperature.With the temperature increasing,the total amount of PAHs produced by the reaction of three oxygen carriers at different temperatures in CLC showed a trend of decreasing first and then increasing rapidly and then decreasing again,the optimum reaction temperature of oxygen carriers reacting with high volatile bituminous coal 3 is 900?.?5?Both of the modified oxygen carriers reacting with bituminous coal 3 generated less PAHs than unmodified oxygen carrier at different oxygen carrier/coal ratios.With the oxygen carrier/coal ratios increasing constantly,the tendency of PAHs formation of three oxygen carriers reacting with bituminous coal 3 increased first and then decreased,the optimum ratio of oxygen carriers reacting with high volatile and high calorific value coal?such as bituminous coal 3?is 2.4.Above of all,the modified oxygen carriers can effectively reduce the amount of PAHs formation and control the generation of carbon deposition.The effect of oxygen carrier with adding K₂CO₃ is better.