Seite - 49 - in Advanced Chemical Kinetics

This decomposition causes the concentration of OH• to grow very quickly. The importance of Reaction (24) is clearly seen in Figure 4d for mole fraction, where the concentration of H2O2 decreases rapidly during HCCI combustion as OH radicals are being formed, increas- ing the temperature of the reacting mixture and setting in motion as effective chain branch- ing sequence. As a result, the fuel is very rapidly consumed by reacting with this sudden source of OH•, and the temperature increases very rapidly, due to the production of signifi- cant amounts of water by reaction of ‘RH + OH ⇒ R• + H2O’, further accelerating the rate of H2O2 decomposition. All of these events occurring together create an autoignition event. The Reaction (24) sequence proceeds until the temperature has increased sufficiently that the high-temperature chain branching sequence take over, controlled by H• + O2 ⇒ O• + OH• which dominates the remainder of the overall HCCI combustion process. The decomposition of H2O2 (Reaction (24)) ‘triggers’ ignition in HCCI combustion. This reaction has a critical temperature for ignition that is also a function of the pressure of the reactive system. H2O2 decomposition can be written, ignoring for the moment all other reactions of H2O2, by the simple differential equation d [ H 2   O 2 ] _______dt   =  −  k 5 [ H 2   O 2 ] [M] (1) where M is the molar concentration and k5 is the rate of Reaction (24). This equation can be rearranged to define a characteristic decomposition time (α). α  =  [ H 2   O 2 ] / (d [ H 2   O 2 ] / dt)   =  1 / ( k 5 [M] ) (2) The rate expression for this reaction is k 5   =  1.2 × 10 17 × exp  (−  45500 / RT) (3) so the characteristic decomposition time α becomes Figure 5. Schematic of H2O2 reaction loop [24]. Autoignition and Chemical-Kinetic Mechanisms of Homogeneous Charge Compression Ignition... http://dx.doi.org/10.5772/intechopen.70541 49