Page - 48 - in Advanced Chemical Kinetics

H2O2 + M + 216kJ ⇒ OH + OH + M Re.(12) OH + CH2O ⇒ HCO + H2O + 122kJ Re.(13) HCO + O2 ⇒ CO + HO2 +138kJ Re.(14) HO2 + HO2 ⇒ H2O2 + O2 + 168kJ Re.(15) These four reactions, which are also called ‘H2O2 loop reactions’, show similar activation ener- gies and reaction rates. The rate constants of Reactions from (13) to (15) are significantly larger than that of Reaction (12), and their activation energies are very small. These suggest that Reactions from (12) to (15) compose a reaction loop in which the rate-determining process is Reaction (12), as schematically shown in Figure 5. On the assumption that 100% of OH, HCO and HO2 generated by ‘H2O2 loop reactions’ are consumed by the succeeding reactions, the overall reaction is to be Reaction (16). 2CH2O + O2 ⇒ 2H2O + 2CO + 472kJ Re.(16) This is a reaction to release a considerable amount of heat from CH2O without consuming H2O2. In addition, following reactions support ‘H2O2 loop reactions’ by supplying the key species of formaldehyde, formyl (HCO) and OH. C2H3 + O2 ⇒ CH2O + HCO + 359kJ Re.(17) CH2CHO + O2 ⇒ CH2O + CO + OH + 212kJ Re.(18) CH3 + O ⇒ CH2O + H + 293kJ Re.(19) The sub intermediate-temperature mechanism, Reactions from (17) to (20), also participates in the process. H + O2 + M ⇒ HO2 + M + 202kJ Re.(20) H2O2 + OH ⇒ H2O + HO2 + 130kJ Re.(21) HO2 + OH ⇒ H2O + O2 + 297kJ Re.(22) HO2 + CH2O ⇒ H2O2 + HCO - 7kJ Re.(23) Reaction (20) enhances ‘H2O2 loop reactions’ by supplying HO2 using H generated mainly by Reaction (19). In the real process, Reaction (23) and Reactions from (17) to (19) contribute to additional H2O2. 3.3. High-temperature reactions The heat-release rate by intermediate-temperature reactions grows steadily, until at about 1000 K, four important events occur. The H2O2, which has been relatively stable due to the strength of its O–O bond and the correspondingly large value of the activation energy of its decomposition reaction, begins to decompose at ever-increasing rates by following reaction. H2O2 + M ⇒ OH• + OH• + M chain branching Re.(24) Advanced Chemical Kinetics48