A study of nitrogen conversion and polycyclic aromatic hydrocarbon (PAH) emissions during hydrochar-lignite co-pyrolysis

Nitrogen conversion and polycyclic aromatic hydrocarbon (PAH) formation during rapid pyrolysis of hydrochar, lignite and hydrochar-lignite blends have been investigated within a temperature range of 600-900 °C. The results showed that in comparison to lignite, a higher percentage of hydrochar nitrogen was retained in the char, and less NH3 and HCN were formed during pyrolysis. During pyrolysis of the individual hydrochar and lignite components, yields of NH3 and HCN reached a maximum at 800 °C and then decreased with increasing temperature. Addition of hydrochar to the lignite increased yields of total HCN and NH3 at low pyrolysis temperatures (⩽700 °C), but suppressed their formation at high temperatures (⩾800 °C). Synergistic interactions in hydrochar-lignite blends significantly decreased the total nitrogen percentage in the char, and promoted the conversion into N2 at temperatures ⩾800 °C. These synergistic interactions increased with (but were not linearly proportional to) increasing temperatures and hydrochar ratios in the blends. With regard to PAH emissions, relatively less high-ring PAHs were present in tars from pyrolysis of hydrochar-lignite blends than in tars from pyrolysis of lignite alone. These findings suggest that co-processing of hydrochar-lignite blends for energy production may have the additional benefit of reducing emissions of nitrogen pollutants and PAHs.