Shrinkage Characteristic And Kinetics Of Large-sized Briquettes During Pyrolysis Process

Aiming at the general problem that both for the production of foundry cokemade by anthracite pulverized coal and furnace coke made by tamped coke inthe coke oven, namely the shrinkage problem for large-size briquette andtamped coke in the pyrolysis process, by using a mass and volume integratedmeasuring device, the influence of material particle size, molding size, heatingrate and temperature on the shrinkage of briquette were being invistigated; theshrinkage process of coal, binder and briquette were being studied and theshrinkage mechanization were also being preliminary analyzed, according tothe mass loss and volume shrinkage regulation of large size briquette (volumeequals103cm3) under constant carbonized temperatures (450℃、550℃、650℃and850℃), the briquette shrinkage kinetic model was also established and theshrinkage kinetic parameters were revised by means of sensitivity analysis.Also the volume shrinkage calculation method of tamped coal cakes (13280mm×500mm×4300mm) was also obtained on the basis of briquetteshrinkage kinetic model. The main results and conclusions were as follows:1. At the carbonization temperature of850℃and with the increase of rawmaterials particle size (0.15-0.425mm,0.425-1mm,1-1.4mm,1.4-2mm, 2-2.36mm), the carbonization reaction time of briquette (with the size of100mm) will be shorted from120to90min, the final volume will be increasedfrom0.7435V0to0.8231V0, and the initial shrinkage rate of briquette will beincreased from6.481×10-3to9.135×10-3%min separately.2. At the carbonization temperature of850℃and with the increase ofbriquette size (25mm,50mm,75mm,100mm,125mm), the carbonizationreaction time of briquette (with the particle size of1-1.4mm)will be increasedfrom60to110min, the final volume will be increased from0.6968V0to0.7785V0, and the initial shrinkage rate of briquette will be decreased from21.954×10-3to6.589×10-3%min separately.3. With the increase of heating rate (2℃/min,3℃/min,4℃/min), the finalvolume of briquette (with the size of100mm and the particle size of1-1.4mm)will be increased from0.7646V0to0.6733V0. The increase of heating ratewill move the briquette shrinkage curve and first shrinkage rate peak to thehigher temperature area. The difference of briquette volume is large in the hightemperature area(600℃～900℃), but is minute in the low temperaturearea(200℃～600℃).4. With the increase of carbonization temperature (450℃,550℃,650℃,850℃), the final volume of briquette (with the size of100mm and the particlesize of1-1.4mm)will be decreased from0.8751V0to0.7668V0, thecarbonization reaction time will be decreased from120to92min, and theinitial shrinkage rate of briquette will be increased from2.222×10-3to5.25× 10-3%min separately.5. In the carbonization process of briquette, dehydration and degassingreaction reacted before450℃(accounting at51.63%and29.07%separately),and polycondensation reaction reacted in the temperature period of450℃to550℃also has an influence on shrinkage (accounting at18.52%), andhydrogen evolution reaction reacted after850℃has little effect on shrinkage(accounting at18.52%).6. The shrinkage process of large-sized briquette could be divided into threestages: The the desiccation shrinkage stage (room temperature~350℃), thedecomposition shrinkage stage (350℃~650℃), and the condensationpolymerization stage (650℃~950℃). The volume of briquette in thedesiccation shrinkage stage changes little, and takes physical reaction mainly.The volume of briquette in the decomposition shrinkage stage changesenormous, and crystallite parameters increased. The volume of briquette in thecondensation polymerization stage has no marked shrinkage, and the crystalliteparameter faincreased. The volume of briquette in the condensationpolymerization stage has no significant shrinkage.7. There were three main reasons causing the shrinkage of large-sizebriquette in the pyrolysis procress:①the thermal decomposition andcondensation reaction of anthracite coal particle;②the dissociation andpolycondensation reaction of coal based binder(active components);③theco-carbonization between anthracite coal particle and coal based binder in the interface. The shrinkage of coal particle is the main reason causing the briquette shrinkage. The cementation and polycondensation between coal particles that caused by coal based binder was also has an influence on the briquette shrinkage. The reaction between coal based binder and coal particles in the contact interface has a promotion on the shrinkage of briquette, but is not the main reason in the whole process.8. The briquette shrinkage kinetic model was represented as follows: dVt/dt=V0[(1-x) dβ/dt-A0e-E/RT(1-x)nβ]. There was a positive correlation between the volume changing parameter P and the density of briquette, and the volume changing parameter increased with the reaction; the shrinkage reaction could be divided into three stages; with the reaction went on, the reaction order and reaction rate constant decreased, the pre-exponential factor increased and the activation energy decreased firstly and increased then.9. The shrinkage of the tamped coal cakes from the large-scale chamber was counted in the calculated unit whose thickness was100mm. The shrinkage velocity of the tamped coal cakes in carbonization chamber was obtained by weighted calculating the shrinkage parameters of each volume unit at different temperatures. The tamped coal cakes shrinkage kinetic model is dV’t/dt=1/5×∑15dV’ti/dt, and shrinkage velocity of each calculated unit when the reaction time tis dV’ti/dt=V’0i[(1-xi)dβi/dt-ki(1-xi)nβi].