Fractal desulfurization kinetics of high-sulfur coal

International Joumal of Mining Science and Technology 22(2012)111-114燃到Contents lists available at Sciverse Science Direct氵容International Journal of Mining Science and TechnologyELSEVIERjournalhomepagewww.elsevier.com/locate/ijmstfractal desulfurization kinetics of high-sulfur coalXu Longjun**, Peng Tiefeng a b, Zhang Dingyue Zhang Fukaia.State Key Laboatory of Coai Mine Disaster Dynamos and Control Chongqing University, Chongqing 40004, chinaSChool of Chemical Engineering University of Queensland, Brisbane, QLD 4072, AustraliaARTICLE INFOABSTRACTThe pore structure characteristics of high-sulfur coal from Wansheng in Chongqing have been studiedved 11 May 2011a nitrogen adsorption method(BET) The effects of grinding and prerm 4 June 2011of coal have been investigated. Organic sulfur in coal pretreated with nitric acidAvailable online 9 March 2012was desulfurized by using propylene-glycol-KOH (PG-KOH) Fractal kinetic properties of these twodesulfurization procedures were investigated by using fractal geometric theory. the results show thatoth the specific surface area and pore volume increased with the decrease in particle diameter. TheKeywords:spore surface of coal had fractal characteristics: the fractal dimension was 2. 48. The sulfur contentDesulfurization kineticsdecreased with the decrease in particle diameter by grinding. After pretreatment with nitric acid, thedesulfurization ratio(DFR)of inorganic sulfur increased to over 99% and the DFR of total sulfur to over70%, The desulfurization procedure of inorganic sulfur had fractal kinetic characteristicsctive fracNirric acidtal dimension was 294. The organic sulfur desulfurization procedure by PG-KOH was also tallied withPropylene-plycol-KOHractal kinetic properties: the reactive fractal dimension was 2.57. The effect of temperature on the desul-furcation ratio of organic sulfur can be described with an arrhenius empirical equation. the rateconstant, pre-exponential factor and the activation energy of the reaction increased with the decreasein particle diameterD 2012 Published by Elsevier B V. on behalf of China University of Mining Technology1. Introductiokinetic equation and tuned out to be a physical extraction process[5] Based on desulfurization by using propylene glycol-KOH to getThe coal industry could make an important contribution to sus- rid of organic sulfur, the effects of particle diameter on desulfurizetainable development of the energy resources of some countries. tion ratio and its kinetics were researched in this papHowever high-sulfur coal is not suitable for direct use because itcould lead to serious environmental pollution. Coal desulfurization 2 Samples and methodsprior to usage is a pre-process in order to achieve clean fuel whichcould be used in industry and reduce environmental impacts such 2.1. Preparation of coal samplesas acid rain. Therefore. it is essential to reduce sulfur and mineralmatter(ash) from coal prior to its use in many applications. InOur coal was obtained Igeneral, organic sulfur is fixed in the chemical structure of coal first crushed and groundand then screened. The average particlediam-and often much more difficult to remove than inorganic sulfur. eters(d) of the samples were 0.28, 0.15, 0.108, 0.0855 and 0.054Leaching is one of the mild methods for desulfurization of coal16] The characteristic parameters of coal are listed in Table 1Particle diameter is a basic and significant physical parameterThe chemical reagents were propylene-glycol (PG), potassiumfor coal, which has an important effect on other parameters such hydroxide, MgO Na2 CO3, BaCI, and nitric adid; all were analyticalas specific surface area and geometric shape. On the other hantreagents.its physical structure has a great effect on the desulfurization ratioThe experimental equipment consisted of a Rapid Infrared Coal(DFR)of organic sulfur, so it is useful to study the effect of particle Analyzer 5E-MACIlL, a muffle furnace and an analytical instrumentdiameter on the DFR from a fundamental aspect (1-4).for measuring specific surface areas and pore diameters(MicromIn a kinetic study. Cui and Xu investigated the effect of reaction critics ASAP2010, USA)time on the dfr when the reaction time was less then 90 min thedesulfurization process conformed to an apparent first-order 2.2. Experimental method and determination of DFR中国煤化工or.T:+862365106873h)of coal were removedocquedu. cn(L Xu),YHCN MHGsamples were placed in5-2686/s-see front matter 0 2012 Published by Elsevier B.V. on behalf of China University of Mining Technology112Technolog22(2012)1-114TableTable 2haracteristic parameters of coaL.Specihc surface area of various particle diameters(mig).npls d(mm) Man(8) Aa(s) S,(%) Spa(a) S(3) d(mm) Ar A, A, Ar A2 A,/A, A, A,Raw coal 0.28094325010050099004007926589600340.380283.14sture content, Ae ash content S, total sulfur content spa inorganisulfur content. and S, organic sulfur contentTable 3amL glass beaker and then a 1: 4 nitric acid solution was Pore volume of various particle diameters(10/g).The ratio of coal mass and solution volume was 1: 80(g/mL).123423212.22, Desulfurization of organic sulfurExperimental coal samples in this procedure were from coaldesulfurized with nitric acid, Coal samples were placed into a651.571944250 mL reactor. Then the PG(PG: H20=1: 1)-KOH(20%)solution005430950.8602781722556312874159(VpG: VKOH-1: 1)was added at 180C. The reaction time of desul-furization was 90 min the solution was then filtered by suction.214-2007 and GB/T 215-1996 (both are national standards in Chaiacteristic parameters of coal reated with nitric acidChina)were applied to determine the total sulfur content and suh-Sampld (mm) Mod A S Sps Sfur forms content, respectively the calculation of DFR followed theprotocol by Xu et al. [2].Coal pre-treated by nitric acid 0.280 2.16 17.6 1.24 0.D2 1.223. Results and discussion00863201660930020900543401640.740.023. 1. Pore structure characteristics and surface fractal dimensions ofsurface dimension of coal. The logarithm of Eq. (1)can be trans-formed to Eq(2).The methods applied in our research in measuring the surface ofcoal and its pore structure were the usual gas adsorption, intrusion InA=(D-3)Inr+cnercury porosimetry. density method and electron microscopyBy fitting with Eq (2). the fractal surface dimension(D)is 2.48We selected the BET method using gas adsorption. The pore diam- and its correlation coefficient 0.91. It implies that the micro-poreeter of coal was divided into three types: micro-pores(<8 nm), surface of coal has fractal character isticstransitional pores(8-100 nm)and macropores(100 nm). Specificsurface areas of the various particle diameters of coal are listed inTable 2.A,,A2 and A3 are, respectively. the specific surface area of 3.2. Effect of grinding on sulfur content of coalmacropores, transitional pores and micro-pores and Ar is the totalThe total sulfur content of coal can be regarded as the sum ofTable 2 shows that the total specific surface area increases with inorganic and organic sulfur. Table 1 shows that the sulfur contenta decrease in particle diameter. It conforms to the characteristics of is closely related to particle diameter. The larger the particles material. The specific surface area of the micro-porous was diameter, the higher the sulfur content. This is consistent withmore than 70% of the total specific surface area, which suggests anfndings of yenormous growth of micro-pores in the coaL.decrease and the degree of mineral release increases [10]. Inor-Pore volumes(Barrett-Joyner-Halenda model, BJH) are pre- ganic sulfur in coal is mainly in the form of pyrite, which can besented in Table 3, where Vi V2 and V, are respectively. the pore a monomer or in massive form. With a decrease in particle diamevolume of macropores, transitional pores and micro-pores and v ter. the proportion of monomer increases. Pyrite has high densityis the total pore volume of coal From Table 3. it can be concluded and poor grindability. Therefore, the sulfur content decreases withthat pore volume increases with the decrease in particle diameter. a decrease in particle diameter.a trend similar to that of specific surface area.After fitting the data from Table 1. the relationship betweenThe surface area of most material is fractal within the range of atal sulfur content and particle diameter of raw coal wasmolecular scale. Kaiser and Gethner applied Small Angie Neutroncatering(SANS)to investigate micro-pore structures and thoughtthat coal may have fractal characteristics (7). There are a number of (t/%)=3.44(d/mm)+3.36ays to obtain fractal surface dimensions, e.g gas adsorption, with a correlation coefficient of 0.95.detrusion mercury method and small angle X-ray scattering(SAXS)the surface of a particle is fractal, its surface area is correlated3.3.FLimeWire afwith radius [ 8:中国煤化工alfur using nitric acidCompCNMHGare presented in Table 4.inorganic sulfur contenthere A is surface area. R the particle radius and d the surface frac- of coal acrearkedly atter pre-treatment with nitric acid. Ittal dimension We opted for gas adsorption to obtain the fractal also shows that all removal ratios of inorganic sulfur for coal sam-L Xu et al/International Joumal of Mining Sdence and Technology 22(2012)111-114Table 5Table 6DFR by using nitric acid at room temperature.Effect of particle diameter on DFR of organic sulfur.0860054d(mm)0108DFR(s)74.3DFR()2623045.1ples are larger than 99%, while those of organic sulfur ranged onlyfrom3.17-8.86‰The total DFRs, by using nitric acid, are shown in Table 5Farin and Avnir thought that diffusion resultedence between experiment features and reaction properti8e5E--om mmfractal dimension is considered a controlling geometic[11]. Thus, Eq (1)was re-written as:SooR1-3where s is the effective surface area of a reaction, and de thetive surface reaction dimension.Temperature℃)The initial reaction velocity at the surface is directlytional to the effective surface area; hence the relationshipof organic sulfur with various particle diameters at differentDFR and particle diameter as follows:Table 7Rate constants for various particle diameters at different temperatures (10 minnn=a+(DR-3)In02800.15001080086005where n is the DFR, d the particle diameter, mm; d the standard4.635.22rticle diameter (1 mm and aa constantWe fitted the data from Table 5. with the following result:n=-0.3979-00618In s with a correlation coefficient of3674850.921. The reaction process has fractal kinetic characteristics, withIt was believed that desulfurization of coal was a chemical reac- Table 8tion on the fractal surface. This conclusion agrees with the idea by kinetic parameters of desulfurization for different particle diameters.Farin and Avnir who obtained the reaction dimensions of 31 types0054of material (or material constitute)and the surface fractal dimen2.7784.506sions of 25 different types of material by using Eqs. (1).(3)and(4) ko(min")[11]. Meantime, the reaction dimension of desulfurization of coal Cormorreiation coefficient 0.993099809810999w sg of ce of micro-pores. This situation was similar to the 3.5. Desulfurization reaction mechanism of organic sulfur using F04)was larger than its fractal surface dimension(2. 48). Thisests that the desulfurization reactions may selectively occurdissolving of Madagascar quartz in a 3.66 mol/L HF solution 111. KOHThe reaction dimension(Da) and the surface fractal dimension ofthe dissolving process for Madagascar quartz were 2.14 and 2.0-Desulfurization of organic sulfur using coal (each 5.0 g) with2. 15. respectively.various particle diameters was carried out The DFR of organic sulfur is shown in Fig. 13.4. Fractal desulfurization kinetics of organic sulfur using PG-KOHAccording to Ref [5]. this desulfurization process was inity with the characteristic of apparent first-orderA sample of 5.0 g coal(pre-treated with nitric acid )was put into equation at 90 min, Took the five temperaturesa 250 mL reactor: optimal experimental parameters were used for first-order kinetics equation, the reaction rate constants (k) at dif-desulfurization 17]. We then determined the sulfur content and ferent temperatures can been calculated. the results were in Tablealculated the dFR of organic sulfur. The results are presented in 7Ka The data from Table 6 were fitted with Eq (5). with the result: formed to the arrhenius formule the reaction rate constant con-The effect of temperature onn=-1554-04255h(6) k-kexpIF RTThe correlation coefficient was 0.986. This indicated that this where ko is the pre-exponential factor(min-. E, the activated en-process was in conformity with the characteristics of fractal kinet- ergy UAics and the fractal reactive dimension was 2.57. this reaction a linea中国煤化工Te7. The activationdimension(2.57)was also larger than the fractal surface dimension energy,coefficients are preof coal(2.48), suggesting that the desulfurization reactions of sentedCNMHGorganic sulfur in coal may selectively occur on the surface ofFrom Table 8, it is seen that particle dmicro-pores (11].n the kinetic parameters of the desulfurization reaction. The reac-114L Xu et al international joumal of Mining Sience and Technology 22(2012)111-114tion rate constant, pre-exponential factor and the activation energynius empirical formula. the rate constant theall increased with the decrease in particle diameter, The activationtial factor and the activation energy of the desenergy of most chemical reactions is 50-250 kJ/mol. This suggestsreaction increased with the decrease in partithat the coal desulfurization reaction is largely a physical extracDesulfurization of coal is largely a physical4. ConclusionsReferences(1)Micro-pores of coal have rich growth. Micro-pore surfaces ofcoal have fractal characteristics, with a fractal dimension of Xem long uf oul byi l brent d choi coad creveas no on oganie-s fur(2)The sulfur content decreases with the decrease in particle [2] Xu Longjun, Zou Dejun. Cheng Yujuan. Study on removal of arganic sulfur fromdiameter, owing to the differences in density and grindabi(3)When nitric acid was used to pre-treated coal, the dfr ofEnviron prot chem ind 2002: 22(5)249-53 (in chines对ominorganic sulfur was over 99% and the total DFR of sulfur 4] Zhao Wel, Zhu Hong Yan xuehai. tudy on organic sulfur removal from coal byver 70% Our desulfurization process was in conformityTechnol 2003: 31(5): 390-4 in Chinewith the characteristics of fractal kinetics and the reactive [ 5] cui Caixi, Xu Losm of removing organic sulfur in coal by 1fractal dimension was 2.94. The removal reaction of inor-08:31(355-8[ in chinesanic sulfur may selectively occur on the surface of[6] Xu Longiun, Qu Ge, Zhou Zhengguo. A process for recovery leaching ofstrontium from strontium waste residues using ammonia chioride.(4)For coal pre-treated with nitric acid, the organic sulfur [71gs of the 1983 international conference ondesulfurization process by using PG-KOH had fractal kineticroperties and the reactive fractal dimension was 2.57. The[8] Fairbridge C, Ng SH. Palmer AD Fractal analysis of gas adsorption on SyncrudeDFR of organic sulfur, 26. 2-51.4%, was affected by particle 9)uan. Distribution of sulfur with coal padiameter and the reaction temperature. the removal reac- [10] Song Lei, He Fayu, Sun Chuanyao, Sun Zhijian. Effects of grtion of organic sulfur may selectively occur on the surfaceof desulfurization reaction was conformed to be an Arrhe. [11 Fann 2007: 1: 30.4 [in Chinese.of micropores. The effect of temperature on the rate constantAvnir D. Reaction fractal surtace. J Phys Chem 1987: 91: 5517,中国煤化工CNMHG