Research On The Polycyclic Aromatic Hydrocarbons In Coal Tar Pitch Extracts And Fume

Polycyclic aromatic hydrocarbons (PAHs) can be formed during theutilization of coal tar pitch (CTP). Researchers at home and abroad have payedmuch attention on the PAHs’ emission, transportation and control due to itspotential harm to the human-being and the environment.The analysis anddetermination of the organic components in CTP make an important significancein CTP efficient utilization. The heating process plays a key role in theutilization of CTP. This study puts the key on the chemical components of pitchfume and the organic solvents soluble fraction from CTP and the emissionproperty of PAHs in CTP, in order to reduce or avoid the harm of CTP to theenvironment．6kinds of CTPs were selected and sequentially extracted with hexane,benzene using Soxhlet extraction. All the extracts were analyzed with GC-MS,the chemical components and the compounds distribution characteristic inextracts from6CTPs were studied. The analysis method is established toquantify PAHs by gas chromatography with FID to evaluate the toxicity of CTP. In order to evaluate the harm of pitch fume, the laboratory pitch fumesgeneration system was built up by reconstructing a low temperaturecarbonization furnace. The pitch fumes obtained at180℃were qualitativelyanalysed by GC-MS to identify the chemical constitution and the compoundsdistribution characteristic. Then, the effects of time, temperature and chemicalcomponents of pitch on the pitch fume emission were studied using TGA. Mainresearch results in this paper are obtained as follows．1. Fractional extraction rate of different CTPs show that the two-extractionwith different solvents can make most free-state components dissolve and departfrom the residues. The total extraction rate of hexane, benzene decreased withthe increasing of carbon content, C/H, soft point. PAHs of4-5rings make a greatcontribution to the extraction rate of hexane, while PAHs of5-6rings play animportant role in the extraction rate of benzene.2. The step-by-step extracts mainly consist of PAHs andheteroatom-containing compounds. In hexane or benzene extracts, PAHs has thehighest relative abundance (about90%).4-5rings PAHs without alkyl aredominant components in hexane extracts, while5-6rings PAHs without alkylare dominant components in benzene extracts. The species of PAHs in extractsfrom the raw CTPs increase as rings increase，the peak point at5rings, thendecrease. Alkyl substitutes which mainly consist of1-2alkyls in two-extractionsoluble fractions are mainly3-5rings. The bigger the number of aromatic ringwas, the smaller the species of alkyl substitutes. Among16priority PAHs selected by the US Environmental Protection Agency (EPA), PAHs likenaphthalene, acenaphthene, fluorene phenanthrene, anthracene which are lowrelative content are mainly detected in hexane extracts. PAHs likebenzo[b]fluoranthene, benzo[a]pyrene, Indeno[1,2,3-cd]pyrene,benzo[g,h,i]perylene which are high relative content are mainly detected inbenzene extracts. The relative content of fluoranthene, pyrene,benzo[a]anthracene, chrysene are equal between hexane extracts and benzeneextracts. Almost half hexane extracts or benzene extracts are carcinogenic PAHs,while the carcinogenic PAHs in raw pitch are about1/3.3. With the rising of softening point, the amount of phenanthrene,fluoranthene, pyrene decrease, but the amount of benzo[b]fluoranthene,benzo[a]pyrene, Indeno[1,2,3-cd]pyrene, benzo[g,h,i]perylene increase, the peakis at the softening point of100℃, then reduce; With the rising of the extractionrate, the amount of phenanthrene, fluoranthene, pyrene increase, but the amountof benzo[b] fluoranthene, benzo[a]pyrene, Indeno[1,2,3-cd]pyrene,benzo[g,h,i]perylene increase, the peak is at the extraction rate of63.99%, thenreduce. Due to existence of strong carcinogenic PAHs, such asbenzo[b]fluoranthene, benzo[a]pyrene, indeno[1,2,3-cd]pyrene, benzo[g,h,i]perylene, the toxicity of high temperature CTP is much stronger than middletemperature CTP.4. The pitch fume mainly consists of PAHs and heteroatom-containingcompounds. PAHs without alkyl are dominant components in fumes,2-4rings especially. Alkyl substitutes which mainly consist of1-2alkyl substitutes infumes are mainly2-3rings. The species of PAHs in fumes increase as ringsincrease，the peak is3rings, then decrease. The relative content of2-3ringsPAHs and species in fumes are much more than in raw pitch. Among16priorityPAHs selected by EPA, the relative content of naphthalene, acenaphthene,fluorene, phenanthrene, fluoranthene, pyrene is higher than the others. Thecontent of carcinogenic PAHs is over50%in pitch fume.5. During pyrolysis, the major weight loss of CTP is the volatilization andpyrolysis of benzene soluble fraction in CTP. Under non-high temperatureenvironment conditions, the major weight loss of CTP is the volatilization ofhexane soluble fraction. Pitch fume emission increases with the increasing oftemperature and time. But time only plays a key role in the early stage, and itseffect decreases gradually by the extension of time. When the temperature risesto180℃from160℃, there are more volatile from raw pitch.