Catalytic Upgrading Of Coal Gaseous Tar Over USY Zeolites

A certain amount of tar product is produced during coal pyrolysis, including light compounds such as benzene, toluene, ethyl benzene, xylene, and naphthalene(BTEXN), and heavy components such as poly aromatic hydrocarbons(PAHs), and so on. Light aromatics such BTEXN are basic high-value-added organic industrial materials which can be widely used in the syntheses of pesticides, medicines, and dyestuffs. However, in coal tar products, the contents of light compounds are rather low while the pitch content up to 55%. So far, the heavy components such as pitch, which is mainly containing PAHs, have not been efficiently used as high value-added products. Even as important industrial materials, these PAHs can release to the surroundings in the processes of production, transportation, and storage without proper processing and utilization, thus result in the environment pollution, ecosystem destruction, and even human’s health endangerment. Therefore, it is a good idea and best way to achieve the extensive cracking of PAHs then obtain much more light products such as BTEXN for the efficient use of coal tar product.The influence of three ultra-stable Y-type(USY) zeolites with different acidities on the yields of aromatic hydrocarbons such as BTEXN in coal gaseous tar from coal pyrolysis, especially PAHs, had been investigated online by Py-GC/MS. According to the properties of USY zeolites and coal samples, catalytic effects of USY zeolites on coal gaseous tar were analyzed, and a reasonable catalytic mechanism had been proposed. What’s more, different coal samples were extracted in pyridine to separate low molecular compounds from macromolecular network structures. Then the influence of low molecular compounds on the catalytic upgrading of coal gaseous tar was also studied. The main conclusions are shown as following:(1) USY zeolites show excellent catalytic activity for the conversion of condensed aromatics to light aromatics. BTEXN yields in AJL long flame coal and HX coking coal gaseous tar increase to 3.8 wt% and 4.0 wt% after catalytic cracking with USY1, which is 1.0 wt% and 1.5 wt% for raw coal pyrolysis, respectively. More importantly, the amounts of 3–4 ringed condensed aromatics in two coal gaseous tars decrease 23.7% and 32.8%, respectively.(2) Catalyst acidity and coal property are the main factors that influence the catalytic activity of USY zeolites. The weak Br?nsted acid sites of USY zeolites show better ability for BTEXN formation in AJL long flame coal gaseous tar, and strong Br?nsted acid sites of USY zeolites is favorable to that of HX coking coal gaseous tar.(3) Low molecular compounds are much favorable for the formation of BTEXN in the process of gaseous tar upgrading. At the temperature of 600 °C, the existence of low molecular compounds results in 2 wt%, 1.9 wt%, and 0.8 wt% BTEXN yield increments during the catalytic upgrading of AWXM, AWWCW, and AWHX coal gaseous tar, respectively.(4) In the process of gaseous tar upgrading, a certain amount of H-containing radicals are coupled by volatile products. These H donors may come from the pyrolysis of low molecular compounds. Thus, both the catalytic cracking ability of USY zeolites and the H donating ability of low molecular compounds are key factors that achieve the upgrading of coal gaseous tar.