Pyrolysis of black liquor for phenols and impact of its inherent alkali

This study is devoted to investigating the potential of producing phenols via pyrolysis of lignin in black liquor (BL) by a fixed bed reactor at 350–550 °C. The chemicals in the pyrolysis liquid of both black liquor solid (BLS) and purified lignin (PL) from BL are mainly phenols including 4-methoxyphenol, 3,4-dimethoxyphenol, 2-methoxy-4-alkylphenols and alkylphenols. For pyrolysis of BLS, high temperature facilitated the breakage of methoxyl group(s). The highest liquid yield was 27 wt.% at 450 °C, and the content of phenols free of -OCH3 reached 55 wt.% in addition to a small amount of other phenols. The pyrolysis of PL with and without addition of alkali additives of NaOH and Na2CO3 at 450 °C verified that the alkali could promote the formation of phenols. The strong alkali of NaOH greatly promoted the production of phenols free of -OCH3, whereas the effect of Na2CO3 in this aspect was much less. Catalyzed by its self-contained alkalis, the pyrolysis of BLS had high contents of alkylphenols and phenol in the tar (organics). This shows the prospective for production of phenols from pyrolyzing black liquor, while the heating value of the pyrolysis residue was above 6300 kJ/kg to further support the cascade utilization of BLS.