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• Radical concentrations of co-pyrolysis are lower than calculated values. • Mass loss of co-pyrolysis is lower than the calculated values at ~ 380–600 °C. • Combination of S· and V· is the main interaction and mainly occur at ~ 380–600 °C. • Yields of coke, tar, and gas increase while water decreases for co-pyrolysis. • Secondary reaction of volatiles increased for co-pyrolysis. In this study, interactions between free radicals during co-pyrolysis of lignite and biomass were explored. Changes in free radical concentrations were investigated using electron spin resonance (ESR). Additionally, the interactions between free radicals during co-pyrolysis were analyzed based on free radical concentration, thermogravimetric (TG) behavior, and changes in yields and properties of pyrolysis products in a fixed-bed reactor. The results of ESR and TG analyses indicate that, during co-pyrolysis, the combination of radicals, especially that of stable and volatile free radicals, leads to a decrease in both radical concentration and mass loss. The main temperature range for the interaction of free radicals was ~ 380–600 °C. The co-pyrolysis results from the fixed-bed reactor show that the yields of products such as gas, tar, and char are higher than their calculated values, except for water, which confirms the existence of synergy between the coal and biomass. As the resistance of the volatile free radicals escaping from the bed increases, the secondary reactions of volatiles increase during co-pyrolysis, which leads to changes in the composition and properties of gas and tar. [ABSTRACT FROM AUTHOR]