Kinetic model on coke oven gas with steam reforming

J. Cent, South Univ TechnoL. (2008)15: 127-131DOI:10.1007/s11771008-0025-82 springerKinetic model on coke oven gas with steam reformingZHANG Jia-yuan(张家元), ZHoU Jie-min(周孑民), YAN Hong-jie(闫红杰)(School of Energy Science and Engineering, Central South University, Changsha 410083, ChinaAbstract: The effects of factors such as the molar ratio of H,o to CH(n(H, O)(CH,), methane conversion temperature and timeon methane conversion rate were investigated to build kinetic model for reforming of coke-oven gas with steam. The results ofexperiments show that the optimal conditions for methane conversion are that the molar ratio of H20 to CH varies from 1.1 to 1.3and the conversion temperature varies from 1 223 to 1 273 K. The methane conversion rate is more than 95% when the molar ratio ofHyO to CH is 1.2, the conversion temperature is above 1 223 K and the conversion time is longer than 0. 75 s. Kinetic model ofmethane conversion was proposed. All results demonstrate that the calculated values by the kinetic model accord with theexperimental data well, and the error is less than 1.5%Key words: coke oven gas; steam reforming: kinetic model; conversion rate1 IntroductionOutlet ofsynthesis gasThe coke oven gas is a by-product from cokingplants during the production of blast furnace coke.Besides CO and H2, these product gases always containFlowmetlarge amount of hydrocarbons like methane, benzene andnaphthalene. The actuality of application technique aboutthe coke oven gas behind productivity leads to that alarge amount of coke oven gas cannot be utilizedTesto 360 gassufficiently, resulting in environmental pollution andresource wasting. With the development of the cokeindustry, the issue is acute increasingly-2.Reformingcoke oven gas into synthesis gas, which can be used asResistance wiremetallurgical reduction gas or raw material for theproduction of chemicals, such as methanol or ammonith regard to the useSteam preheaterof the product gases, hydrocarbons should beconverted!l6l. Steam-reforming is effective to reformFlowmeter Flowmetercoke-oven gas, but the kinetic behavior needs furtherFig 1 Scheme of experimental systemIn this paper, to develop a process of reformingcoke-oven gas, the conversion of methane in the Table I Compositions of raw coke oven gas for experimentspresence of H20 was studied, and the kinetic model ofCH conversion was proposed.2 Experimental58174601.6025205.903.20The scheme of experimental system is shown ina tubular electric flow reactor made of quartz wasFig 1. The experiments were made in a laboratory-scaled used to heat and maintain the furnace at the requiredapparatus at 1.4 X 10 Pa and 773-1 323 K. The temperature. The alumina balls were put at the bottom ofcompositions of the coke oven gas for the experiments the fumace to keep the evenly-distributed flow field.are listed in Table 1Temperature of the fumace can be controlled in the rangen item: Project(291054)supported by Postdoctoral Fund of ChinaReceived date: 2007-06-ll; Accepted date: 2007-08-27Corresponding author: ZHANG Jia-yuan, PhD, Associate professor; Tel: +86-731-8876H中国煤化工CNMHGJ. Cent. South Univ. Technol. (2008)15: 127-131of 773-1 323 K. Real temperatures were measured bylatinum- rhodium thermocouple that can be shifted in asmall quartz tube with 5 mm in diameter. During theexperiments, the coke oven gas from storage flowed intethe reactor at the fixed speed of 18 m/h. Water wasinjected into an electrothermal evaporator by a highprecision syringe pump to from steam of about 573 K, gand then the steam was blown into the furnace with cokein proportion. The desired molar ratio was— Conversion rate of CH.t90djusted by controlling the temperatures in theMolar fraction ofsaturator&). The flow rate and composition of the cokeoven gas were kept constant during the experiments, thevolume of the reactor(namely conversionwaschanged through adjusting the depth of then(H,O)n(CH)composition of product was analyzed and recorded byFig 4 Effect of molar ratio of H2O to CH ratio on conversiongas analysis recorder.rate and molar fraction of H+Co at 1 223 K for 0.6 s3 Results and discussionFig 2 shows that the methane conversion increaseswith decreasing temperature gradually. It can be seenDuring the experiments, the factors including the from Fig 3 when conversion time is longer than 0.6 s andmolar ratio of H20 to CH4, conversion temperature and the temperature is above 1 223 K, the conversiontime in the reactor on methane conversion rate were reaction approaches equilibrium state and the methaneinvestigated. The conversion temperature varied from conversion rate approaches the maximum value773 to 1 323 K, and the molar ratio of H,o to CH4 variesFig 4 shows that the methane conversion ratein the range of 0.8-1. 4. Conversion time was changed by increases with decreasing the molar ratio of H,0 to CH4adjusting the depth of outlet. The results of experiments in a rang of 0.8-1.4, the methane conversion rate isare shown in Figs 2-4above 87%. When the molar ratio of H2o to CH4 is in arange of 1. 4-1.6, the methane conversion ratio decreasesas increasing the molar ratio of H2o to CH4. It is knownthat the gas conversion rate depends on its conversiontime, viz., the contact time among reactant moleculesWhen the flow rate of coke oven gas is fixed, thevelocity of the admixture is faster as the molar ratio ofH,O to CH4 increases, so the conversion time of theadmixture in reactor is reduced, resulting in less methane0.3040.50.60.708091.0To sum up, the optimal conditions for methaneconversion are that the molar ratio of H2o to CH4 variesFig 2 Effect of temperature and conversion time on conversion in a range of 1. 1-1.3 and the conversion temperaturerate of CH: 1-823K; 2-923 K; 3-1023K 4-1 123 K; varies in a range of 1 223-1 273 K. The methane5-1173K;61223K;71273Kconversion rate is more than 95% when the molar ratioof H,0 to CH, ratio is 1. 2, the conversion temperature isabove 1 223 K and the conversion time is longer thanEquilibrium0.75s.4 Model of kinetic equationIntegral conversion equationThe experimental reactor can approximatelyconsidered as a ideal piston integrate reactor. Reactant中国煤化工 emetic flow rate8009001000110012001300and isiHof outletgas IsCNMH Gle reactor lol. theFig3 Conversion rate of CH, at different temperatures for 0.6s scheme of integral reactor is shown in Fig.5J Cent. South Univ. Technol. (2008)15: 127-131conversion indexesThe original concentrations of CH4, CO2 and H2Oare set as co(cH4), co co2), co(H2O).If the conversion amount of CH at a certain timex,+dxis Y, then the concentration of CH4, H,0 and COz arerespectively:t (CH4)=co(CH4)YFig 5 Scheme of integrate reactorC(H2OFco(H2O)-mrc(CO2)=co(CO2)-nrThe original concentration of reactant A is assumedas cAo, and a flows into reactor at the flow rate of u, the where m and n are the coefficients of H, 0 and cO2conversion rate of a is xa before entering micro bulk dv, respectively, mr+nr-Y, namely, mtml, m and n can beand