Effect of heating rate on the secondary reaction in low-rank coals pyrolysis with the real-time evolution analysis of in-situ tar

Performing online evolution analysis of tar in actual pyrolysis process is a major challenge. In this work, the effects of heating rate (HR) on evolution curves of in-situ tar for low-rank coals pyrolysis were investigated in a novel laboratory bench. The escape law of volatiles was obtained, the influence of HR on secondary reactions was analyzed, the association between products and molecular structures was constructed, and pyrolysis mechanism was deduced. Aliphatic hydrocarbons (except dienes), phenols, and oxygenated compounds have only one peak, while dienes and aromatics have multiple peaks throughout the pyrolysis process. The first peak is attributed to coal primary pyrolysis. At increased HRs, the second peak for 1∼2ring aromatics is from the cracking of primary volatiles, and that for 3∼4ring aromatics from coal continued cracking. The proportion of phenols and oxygenated compounds decreases, aromatics increases, and aliphatic hydrocarbons varies for different coals. Generally, HR increases the yield of primary volatiles by enhancing coal primary pyrolysis, elevates the proportion of light aromatics by promoting secondary cracking and aromatization of primary volatiles, reduces char yield by inhibiting condensation reactions throughout the pyrolysis process. The similar macromolecular structures of different coals make the evolution curve versus HRs follow a common law.