High-density Endothermic Fuel: Cracking And Combustion Characteristics Study

Thermal and catalytic cracking of JP-10,including conversion,product distribution,mechanism and kinetic parameters,were investigated at atmospheric pressure.The effects of six kinds of additives on the combustion characteristics of JP-10 and some initiators on the cracking of n-heptane,a model endothermic hydrocarbon fuel,were examined.The purpose of these researches is to promote cracking and combustion properties of the hydrocarbon fuels that can be suitable for use in hypersonic engines by using soluble additives to replace heterogeneous catalysts.The main products of thermal cracking of JP-10 are ethylene,propene,butene, butadiene,cyclopentadiene,benzene,toluene,1-vinyl-cyclopentene,C₅ and C₆ isomers in the temperature range of 460℃～650℃.JP-10 started to crack at 460℃with the residence time of 15.23 s.By calculating the selectivities of the products, C₁～C₄,cyclopentadiene,benzene,1-vinyl-cyclopentene and toluene are thought to be the initial products.A primary mechanism containing all the possible initiation steps, including unimolecular monoradical and diradicals has been developed,which can be used to explain the experimental results.The decomposition of JP-10 was accompanied by the formation of coke.The mass fraction of coke during thermal cracking of JP- 10 increased exponentially with temperature,and also was related with the residence time.Coke can be inhibited distinctly by shortening the residence time.The thermal decomposition kinetics of JP-10 were calculated at three different flow rates.The pre-exponential factor and activation energy at each flow rate were very close.The average values were 5.06×10¹²s^-1,244.41 kJ/mol,respectively.To overcome the shortcomings of powdery zeolite catalysts,Catalytic ZSM-5 membranes were synthesized using stainless steel grids as supports.In the presence of the catalytic ZSM-5 membrane,the cracking conversion of JP-10 reached to be 15.24%and 42.09%at 400℃and 500℃,respectively.The main products were C₁～C₄,cyclopentadiene,benzene and toluene.Ethylene was no longer the main component of C₁～C₄ here.Presumably because of the presence of catalytic ZSM-5 membrane,the pathways of the reaction were changed.The first step for cracking was the formation of a carbonium,followed by theβ-scission.BHT was found to be effective in promoting the cracking of n-heptane between 500 and 650℃.The total yield of the gas products at 550℃could be 80%higher than that calculated by neglecting the interaction between BHT and n-heptane when the mass fraction of BHT was 3.4%.It was demonstrated that the accelerating effect originated mainly from the scission of the bond between the tert-butyl carbon atom and methyl radical.The effect of six kinds of additives was investigated on the combustion characteristics of JP-10 in a bomb calorimeter under oxygen-poor conditions.It was found that the additives such as esters,amines,peroxides,azo compounds and alkyl aluminums had almost no influence on the combustion characteristics of JP-10.On the other hand,the addition of a small amount of the organometallic compound RM could effectively improve the combustion performance of JP-10.With the oxygen pressure of 0.6 MPa and the content of RM of 2.7μmol·g^-1,the combustion efficiency of JP-10 was enhanced from 92.7%to 99.0%,in addition to an increase of ca.10%in the burning rate and a reduction of 75-85%in the combustion residues.