燃煤烟气中Cr_(2)O_(3)生成及其在CaO的吸附研究

The formation of Cr_(2)O_(3) in coal-fired flue gas and its adsorption by CaO

煤燃烧过程中会释放多种痕量重金属,其中铬元素因其剧毒性而易对人体及生态系统造成危害。铬作为5种重点污染防控的重金属元素之一,研究其在烟气氛围下的形态转化规律和在CaO的吸附特性则对其进一步减排具有重要意义。利用密度泛函理论(DFT)对烟气中Cr、CrO与O_(2)均相生成Cr_(2)O_(3)的反应机理,和Cr_(2)O_(3)在CaO(001)表面的吸附过程进行研究。结果表明:Cr和CrO均相生成Cr_(2)O_(3)的最大反应能垒分别是12.048 Kcal/mol和12.270 Kcal/mol,该反应在烟气中均易发生。在500 K~2000 K温度下,Cr与O_(2)反应生成CrO、CrO_(2)的过程中,正反应速率处于较高水平,正向反应进行完全。而CrO到CrO_(2)存在2个过渡态,其中TS2为速控步骤,反应进行较慢,正向进行不完全。CrO与CrO_(2)生成Cr_(2)O_(3)是物理吸附过程,不需要克服能垒即可自发进行,所以Cr_(2)O_(3)是燃煤烟气中主要的存在形式。Cr反应生成Cr_(2)O_(3)的平衡常数受温度影响较大,但始终大于105,可认为是单向反应。CrO反应生成Cr_(2)O_(3)的平衡常数也明显受到温度的影响,在低温阶段平衡常数较大,反应物转化率高,当温度超过1700 K,平衡常数很小,反应进行不完全。因此可知,Cr向Cr_(2)O_(3)转化的路径较易发生,是燃煤烟气中Cr_(2)O_(3)生成的主要途径。同时,研究发现Cr_(2)O_(3)在CaO(001)表面吸附后,Cr与吸附底物表面O形成了Cr-O共价键,为化学吸附。因此,利用钙基吸附剂对烟气中铬的固化具有可行性。

A variety of trace heavy metals are released during coal combustion.Chromium as one of the five heavy metals for pollution control is easily harmful to human body and ecosystem.It is significant to study the morphology transformation of chromium and its adsorption by CaO in flue gas for further emission reduction.The reaction mechanism of Cr,CrO and O_(2) in the flue gas and the adsorption process of Cr_(2)O_(3) on CaO(001)surface were analyzed by density functional theory(DFT).The energy barrier for Cr generated Cr_(2)O_(3) is 12.048 kcal/mol.The energy barrier for CrO generated Cr_(2)O_(3) is 12.270 kcal/mol.Both processes are easy to achieve in flue gas.The forward reaction rate is at a higher level in the process of CrO or CrO_(2) produced from Cr and O_(2)reaction under 500 K-2000 K,the positive reaction is complete.However,there are two transition states from CrO to CrO_(2).TS2 is a rate-determining step,the reaction is slow and the forward reaction is incomplete.The reaction of Cr_(2)O_(3) from CrO and CrO_(2) is a physical adsorption process,which can be carried out spontaneously.Therefore,Cr_(2)O_(3) is the main existing form of coal-fired flue gas.The equilibrium constants of the reaction Cr to Cr_(2)O_(3) are influenced by temperature,but it is always higher than 105.It can be considered as one-way reaction.The equilibrium constant of the reaction CrO to Cr_(2)O_(3) is also obviously influenced by the temperature.At the low temperature stage,the equilibrium constants are larger and the conversion rate of reactants is higher,but the equilibrium constant is small when the temperature exceeds 1700 K,the reaction is incomplete.Therefore,the pathway of Cr conversion to Cr_(2)O_(3) can be carried out thoroughly in the coal-fired flue gas.This process is considered to be the main way to produce Cr_(2)O_(3).At the same time,the Cr-O covalent bond is formed on the surface of the substrate when Cr_(2)O_(3) is adsorbed on CaO(001)surface.The process is a chemical reaction.It is easy to realize the solidification of chromium in flue gas by calcium-based adsorbent.