Biomass has been considered as an alternative fuel through reburning in utility boilers because of its vast availability and renewable nature. The biochar contribution in biomass reburning technology and the transformation mechanism of its nitrogen foundational groups at different oxygen contents were investigated in this study. Results demonstrate that the biochar contribution rate increased first and then decreased as the oxygen content increased. The peak value appeared at an oxygen content of 1.5%. As the oxygen content increases from 0 to 6%, the pyridine N (N-6) content gradually decreased from 47.73% to 36.86%, and the contents of pyridine N (N-5), quaternary N (N-Q), and nitrogen oxides N (N-X) increased by 2.67%, 2.88%, and 5.33%, respectively, in the sewage sludge char. In the peanut-shell char, the N-6 content decreased from 42.54% to 35.89%, the N-Q content increased from 19.11% to 24.59%. N-5 content has the minimum value, while N-X content has the maximum value at the oxygen content of 3%. (C) 2018 Elsevier Ltd. All rights reserved.