COMBUSTION CHEMISTRY
CHEMICAL REACTION AND THE RATE OF REACTION General chemical reaction αa + βb = γc + δd A and B are substracts and C and are products, α, β, γ and δ are stoichiometric coefficients. The chemical reaction is performed in two directions: forward, with the rate r f = k f [A] α [B] β backward, with the rate r b = k b [C] γ [D] δ
ORDER OF REACTION (in gas phase) 0 order da/dt = - k f, n = 0 I order (A = B + C) II order (A + B = C) da/dt = - k f [A], n = 1 da/dt = - k f [A][B], n = 2 III order (A + B + M = CD + M) da/dt = - k f [A] [B] [M], n = 3
REACTANTS Stable molecules: O 2, H 2, N 2,... Excited molecules: O 2*, CO 2*, N 2*,... Radicals: O, H, N, C, OH, HO 2, CH, CH 3, C 2 H 5,...
RADICALS Radicals are free atoms and fragments of molecules with a non paired number of electrons or two non-paired electrons. Concentration of radicals is small, in comparison to concentration of stable molecules: 10-5 -10-12 mol/m 3 however their role is important in every type of reactions because they are very reactive and very energetic.
SOURCES OF RADICALS 1. Reactions of dissociation: Thermal dissociation O 2 + M O + O + M Photodissociation NO 2 + νh NO + O 2. Reactions of branching: H + O 2 O + OH O + H 2 H + OH
RATE OF CHEMICAL REACTION: influence of concentration I order (A B + C) II order (A+B C+D) dc A /dt = -k f C A dc A /dt = -k f C A C B III- order (A+B+M CD+M) dc A /dt = - k f C A C B C M k f rate constant, C A concentration of A [mol/m 3 ]
RATE OF CHEMICAL REACTION: the influence of pressure Because the concentration C A is defined in mol/m 3, hence the rate of reaction r is proportional to the density (kg/m 3 ). Because the equation of state for gas: p: ρ = prt, the reaction rate depends on pressure and the order of reaction n : dc A /dt - p n
RATE OF CHEMICAL REACTION: the influence of temperature The Arrhenius law (k rate constant) k = k o e -E/(RT) k o preexponential coefficient E activation energy (J/mol) T absolute temperature (K) R universal gas constant (J/(mol*K))
ELEMENTARY REACTIONS Elementary reactions are single acts between reagents, e.g.: H + O 2 O + OH These are mainly radical reactions, like above. The order of the reaction corresponds to the number of moles of subtracts in the reaction.
SUMMARY REACTIONS The summary reaction describes of what happens between the entry of the reactor and its outlet, i.e. it is a balance equation, e.g: CH 4 + 2O 2 CO 2 + 2H 2 O The rate of this reaction is written: dc A /dt = -k C Aα C B β where: α i β are experimental coefficients depending on the conditions of the reaction. In fact summary chemical reactions are composed of many elementary reactions (radical).
TYPES OF RADICAL REACTIONS 1. Branching reactions (radicals R): R + A R 1 + R 2 2. Propagation reactions: R 1 + A R 2 + B 3. Recombination reactions: R + A + M AB + M
THE THIRD BODY MEANING IN THE RECOMBINATION REACTION Three-molecular recombination reactions: A + B + M C + M M the third body (O 2, N 2, H 2, H 2 O,...) absorbing energy of conversion of a compound. Example: H + O 2 + M HO 2 + M H + OH + M H 2 O + M
CHEMICAL MECHANISMS OF COMBUSTION
H2+O2 SYSTEM: initiation of oxidation The system H 2 +O 2 is important because its chemical reactions are present in chemical mechanisms of burning of all fossil fuels. H + O 2 O + OH O + H 2 H + OH OH + H 2 H + H 2 O H + O 2 + M HO 2 + M branching branching propagation recombination
H 2 +O 2 SYSTEM: recombination Recombination of radicals is very important because most of energy is produced during recombination. Recombination takes place in the after-flame zone, they finish the combustion process. H + H + M H 2 + M H + OH + M H 2 O + M OH + HO 2 + M H 2 O + O 2 + M Recombination reaction in the after-flame zone
OXIDATION OF CO CO is an important compound in flames because: it is a half product of carboceneous fuels burning, rate of oxidation of CO is slow, oxidation of CO produces much of energy. In flame CO is oxidized almost only in the reaction: CO + OH CO 2 + H Conclusion: oxidation of CO requires water in flame. Additionally CO is oxidised in the three-molecular reaction: CO + O + M CO 2 + M which is slow and has a marginal meaning for the rate of CO burning.
HYDROCARBONS OXIDATION 1. Mechanisms of hydrocarbons oxidation are very important because fossil fuels are a composition of different hydrocarbons (even coal). 2. Oxidation of hydrocarbons has a hierarchic structure: higher hydrocarbons oxidation mechanism involves the mechanisms of lower hydrocarbons. RH hydrocarbon, R radical, H hydrogen atom.
MECHANISMS OF HYDROCARBONS OXIDATION Hydrocarbons oxidation obeys very complicated chemical mechanisms. Generally, the two chemical mechanisms of oxidation of hydrocarbons can be distinguished: low-temperature (cold flames, Diesel engines), high-temperature (flames).
HIGH-TEMPERATURE OXIDATION OF HYDROCARBONS RH + H R + H 2 RH + OH R + H 2 O RH + O R + OH RH CHO(C n-1 H m-1, C n-2 H m-2... C 2 H 4, CH 4...) CHO, CO Radicals attack of hydrocarbons RH: Further destruction of C n H m and formation of CHO and CO (CO, CHO) + O 2 (CO 2, H 2 O) Oxidation of CHO and CO
KINETIC SCHEME OF METHANE OXIDATION The number of elementary reaction of hydrocarbon oxidation depends on the hydrocarbon structure and can range from a few hundreds to 1-2 thousands.
KINETIC SCHEME OF THERMAL NOx FORMATION Thermal NO are formed by oxidation of N 2 in air. O + N 2 NO + N N + O 2 NO + O O 2 + M O + O + M N 2 molecules are attacked by O atoms. Resulted N atoms quickly react with oxygen O 2. Oxygen atoms O could be produced in the dissociation reaction. In flame the additional source of NO is the reaction with hydroxyl radical: COMBUSTION OH + N NO AND + FUELS H