Investigation of polycyclic aromatic hydrocarbons from coal gasification

ournal of Environmental Sciences Vol 17, No 1, pp 141-145, 2005Article ll:10010742(2005)01-0141-05CLC number: x701 Document code: AInvestigation of polycyclic aromatic hydrocarbons from coal gasificationZHOU Hong-cang, JIN Bao-sheng, ZHONG Zhao-ping, HUANG Ya-jJi, XIAO Rui, LI Da-jiKey Laboratory of Clean Coal Power Generation and Combustion Technology of Ministry of Education, Thermoenergy Engineering Research InstituteSoutheastUniversity,Nanjing210096,China.E-mail:zhouhongcang@163.com)Abstract: The hazardous organic pollutants generated from coal gasification, such as polycyclic aromatic hydrocarbons PAHs), are highlymutagenic and carcinogenic. More researchers have paid particular attention to them. Using air and steam as gasification medium, theexperiments of three kinds of coals were carried out in a bench-scale atmospheric fluidized bed gasifier, the contents of the 16 PAHsspecified by US EPA in raw coal, slag, bag house coke, cyclone coke and gas were measured by HPLC to study the contents of PAHs inraw coal and the effects of the inherent characters of coals on the formation and release of paHs in coal gasification the experimentaresults showed that the distributions of PaHs in the gasified products are similar to raw coals and the total- PAHs content in coal gasificationis higher than in raw coal( except Coal C). The total-PAHs contents increase and then decrease with the rise of fixed carbon and sulfur ofcoal while there has an opposite variation when volatile matters content increase. The quantities of PAHs reduce with the increase of ascontent or the drop of heating value during coal gasificationKeywords: polycyclic aromatic hydrocarbons( PAHs); coal gasification: fluidized bed; coal rankanthracene( BaA), chrysene( Chr), benzo(b)fluorantheneIntroduction(B F), benzo(k)fluoranthene( BkF ) benzo(a) pyreneCoal is generally regarded as being composed of two (BaP), indeno(1, 2, 3,cd)pyrene(In(1, 2, 3-cd)P)complementary structures. The dominant component is a dibenzo( a, h)anthracene( DbA), and benzo( ghi ) perylenemacromolecular, insoluble, three-dimensional network ( BghiP)composed of condensed aromatic and hydroaromatic unitsAt present, PAHs have been widely researched in coalconnected by ether or thioether linkages and short alkyl utilization processes including coal combustion and coalbridges. The second structure is a molecular phase of pyrolysis, however, PAHs contained in raw coals andcompounds, which are typically soluble in organic solvents. generated from coal gasification have not been seriouslyThis molecular phase contains varying distribution of aliphatic considered as a source of these materials to be released duringhydrocarbons, polycyclic aromatic and hydroaromatic the utilization of coal( Liu, 2000; 2001a; 2001b; 2001chydrocarbons, hydroxylated polycyclic aromatic compounds, Li, 2003; Lu, 2002; Astral, 1999; Sarma,2000).Theand heterocyclic compounds( Liu, 2000)relation between the contents of PAHs in different coals andCoal direct combustion not only has a low utilizing the elemental compositions of coals, and the effects of theefficiency, but also produces a large numbers of pollutants inherent characters of coals on the formation and emission ofsuch as particulate, CO,, SO2, NOx, polycyclic aromatic PAHs during coal combustion, have been studied (Li, 2003;hydrocarbons( PAHs ) PCDD/PCDFs, and so on. Coal Liu, 2001c; Zhao, 2000). This study was carried out in agasification is a clean coal technology that presents good bench-scale atmospheric fluidized bed for coal gasificationprospects for coal use, mainly for producing electricity with a and investigated the contents and distribution of PAHs in rawhigh coal conversion efficiency and low environmental impact coand release of PAHOcampoa, 2003). However, there is a problem about the gasification. The results obtained from the experimentsation of PAHs in coal gasification or coal direct warrant the need for seeking better technologies for controllingombustionPAHs during coal gasification in the futurePAHs are harmful to the environment and the health of1 Experimentalpeople due to their high degree of mutagenicity ancarcinogenicity when they enter the human bodies through the 1.1 Materialsbreathing, eating, and drinking of human being( ZhouThree different rank coals were used in the fluidized bed2003a: 2003b; Jin, 2004). It is said that about 75%asifier in this study. The properties of them are given90% cancers of human being are mainly caused by PAHs Table IZhang, 2002). So the US EPA has prioritized 16 PAH 1.2 Apparatusompounds as hazardous air pollutants; these compounds areFig. I shows a schematic of the fluidized bed system forarena中国煤化工 system was composed of(AcP), fluorene( Flu), phenanthrene( PhA), anthracene startCNMHGstem, steam generation(AnT), fluoranthene( FluA), pyrene( Pyr), benzo(a)eeding system, fluidized bed gasifierFoundation item: The Key Project of State Basic Study of Ministry of Science Technology of China( No. G19990221053): the Doctoral Foundation of Ministry of Educationof China( No. 20030286005), and Shanghai Tongji Gao Tingyao Environmental Science Technology Development Foundation142ZHOU Hong-cang et alVol, 17Table 1 Proximate and ultimate analysis of three different rank coalsUltimate analProximate analysis, sNMJ/kgCoal ACoal B70.40860.6312.622.7230.57measurement and control system, and so on. The fluidized heat exchanger placed in the start-up bumer and entered thebed gasifier was made of 4 mm thick, stainless steel cylinder gasifier to fluidize the bed material and supply the reagentsof 100 mm diameter and 4. 4 m height. The coal feeding for coal gasification after mixing with the steam generatedsystem consisted of a frame, a hopper and a drive system from the steam boiler. At the top of the gasifier, the primaryconsisting of an electric motor and a speed controller. The cyclone allowed the recovery of entrained particles.Thecold air from the blast blower was divided into two path- temperature probes and pressure gauges were placed along theways, one supplied the oxygen for the combustion of diesel oil height of gasifier and gas pipes. There was a samplingin the start-up bumer and entered the interlayer to enhance position for gas downstream of the secondary cyclonethe temperature of gasifier, the other was preheated by theCooling air outPreheated air outGas outk- water in12Ash out}[0}[9ter outSlag out , DistilledWater outFig. I The schematic of the fluidized bed gasifier for coal gasificationblast blower; 2. diesel oil tank; 3. start-up burner; 4tank:5. water pump: 6, steam boiler: 7.steaeater:8.fluidized bed gasifier; 9, eletric motor; 10. speed controller: 11. screw feeder: 12. coal feed hopper; 13. primarycyclone; 14. secondary cyclone: 15, removal tar tower: 16. bag house; 17. I D,fan1. 3 Proceduresystem was shut down after the temperature dropped below theEach run was started with the filling of the bed of quartz safe temperaturesand up to the required height. The screw feeder was turned 1.4 Sampling and analysis methodon and the minimum fluidizing air flow rate required toThe samples of PAHs were collected from raw coalfluidize the bed particles in the gasifier was supplied through slag, bag house coke, cyclone coke, and gas. The samplingthe diesel oil start-up burmer. The start-up period was train of PAHs for the gas is shown in Fig.2. The gasnecessary to preheat the bed up to the required temperature containing PAHs was sampled using a stainless samplingbefore commencement of fuel feeding. When the bedtemperature reached 600C, coal was added into the fluidizedbed gasifier by an auger, coal feed rate was adjusted to alloy011121314a certain excess air in order to achieve complete combustionof coal. When the bed temperature in gasifier reached thetemperature of coal gasification, the air, steam and coal flowout Nz ingas outrates were varied to give the desired equivalence ratio. Thebed was fluidized at this condition for an hour to ensure thatsteady state condition was reached in the fluidized bed. Thepressure drops and temperatures wereregistered every 10 min. After the bed temperatureTH中国煤化mv,CNMHGerstabilized, gas samples were collected for analysis. When alltube; 7. XAD.ampling and data recording was completed, coal feedingshwork;9, thermometer: 10. absorption bottle: 11dichloromethane: 12. silica gel: 13. flow meter; 14system was stopped. The whole coal gasification fluidized bedn pumpInvestigation of polycyclic aromatic hydrocarbons from coal gasification143probe and passed through the glass fiber filter, which HPLC system coupled to a Waters 2695 Seperations Moduleremoved the fine particles that escaped from the bag housea Waters 2475 Multi A Fluorescence Detector, and a WatersThen it was passed through a cooling tube which condensed 2996 PDA Detector. Injections of I ul samples were madethe light molecular weight PAHs into the liquid phase and the onto a C18 column( length 250 mm, 4.6 mm i. d)remaining PAHs was captured using an adsorption tube containing 5 um particles for PAHs. The mobile phaseked with 5g XAD-2 resin and an absorption bottle filled flowing at 1. 2 ml/min, was programmed to hold, followed bywith dichloromethane. All samples containing PAHs were an 11 min ramp to a composition of 100% of acetonitrilerefrigerated at 4C before extractionand finally held a mixture of 60: 40 water and acetonitrile forAll samples containing PAHs were first extracted for 8 h 7 min. The excitation and emission wavelength ofby Soxhlet extraction and condensed to 1 ml by Kuderna- fluorescence detector were 224 nm and 330 nm, respectivelyDanish(K-D)concen-trator, respectively. Then, the The wavelength of pDa detector was 254 nm.Theconcentrated solution was made to pass through a purifying concentrations of the 16 PAHs recommended by US EPA wereube packed with activated silica gel. The purifying tube was determined from naphthalene to indeno(1, 2, 3-cd )pyrenethen eluted to obtain PAHs in the purifying solution. The HPLC was calibrated with a diluted standard solution ofFurther, the purified solution was condensed to 1 ml by using 16 PAH compounds( PAH Mixture-610M from Suplecoa KD concentrator and a gentle stream of pure nitrogen. recommended by US EPA. The recovery efficiencies of 16Finally, the samples were put into brown vials and stored at PAH compounds ranged from 60% to 117%, and the method4℃ for HPLC analysisdetection limit( MDL)of them ranged from 1.92 pg to 233The samples were analyzed by the Waters AlliancpgTable 2 The contents of PAHs in raw coals, gas, bag house coke, and cyclone coke, pg/gBag house cokePAHscoal aCoal CCoal ACoal ACoal Ccoal ACoal cMDLMDL0.490.950.09MDLCal BMDLMDL5.04MDIMDIMDI< MDLMDL MDL 0. 600.020.32MDIFluMDL 0.24 MDL0.200.08MDLMDL MDL0.78008AnT0.520.720.270.710.370.210.13MDL2.772,080.590.290.300.090,0y0.500.15030.MDL MDL MDL 0. 060.150.810.03MDLMDL0.890.58MDIMDL Coal C > Coal AOcampoa A, Arenasb E, Chejnea F ef al., 2003, An experimental study ongasification of Colombian coal in fluidised bed[J].Fuel, 82:161-164Sarma V P. Ronald S W, Alan W S, 2000. An investigation on polycyclicJ. Joumal of Hazardous Materials,B74: 91-107Xia Y Q, Wang C J, Meng Q X et al., 1998. The simulation of the formation of0.75polycyclic aromatic hydrocarbons [Jedimentologica Siniea, 16(2):1-4Zhang D Y, 2002. Simultaneous determination of 15 PAHs in indoor air byrse phase high performance liquid chromatography D]. West China0.25University of Medical SciencesZhao Z B, Liu K L, Xie W ef al., 2000. Soluble polycyclic aromatichydrocarbons in raw coal[ J]. Journal of Hazardous Materials, B73: 77-85Coal ACoal CZhou H C, Jin B S, Zhong Z P et al., 2003a. Generation and control of中国煤化工 in coal-fired power plantsg value onCNMHG3 Conclusionsomatic hydrocarbons in coal-fired fluidized bed[ J]. Boiler Technology,34(5):22-26The contents of the 16 PAHs specified by US EPA inraw coal, slag, bag house coke, cyclone coke and gas wereReceived for review April 9, 2004. Accepted June 30, 2004)