Synergistic effects in rapid co-pyrolysis of semi-coke and coal at high temperature

•Rapid co-pyrolysis of semi-coke and coal was studied at high temperature.•The synergies on pore properties and fuel-N conversion were analyzed.•The non-additive interaction on BET surface areas of mixtures were observed.•The correlations between NO and factors were assessed by grey relational analysis.•The high temperature and co-pyrolysis are helpful to reduce char-N and NOx yields.