•Catalytic and non-catalytic co-gasification process using dolomite were carried out.•The catalytic and non-catalytic binary and ternary blend reactivities were examined.•Gaseous products released during the process were evaluated by TGA-MS.•Synergistic/antagonistic effects of catalytic and non-catalytic blends were studied.•Morphology of remaining residues on the gasification process were analysed by SEM. The dolomite effect on the co-gasification process for binary and ternary blends of coal, petcoke and olive pomace was studied by thermogravimetric analysis (TGA) coupled with mass spectrometry (MS). Additionally, gas emissions, the synergistic effect and the residue morphology obtained were also evaluated. Ternary blends showed lower weight loss and higher reactivity than binary ones during gasification. Weight loss in the binary and ternary blends containing dolomite was lower than in the parent ones. Regarding gas emissions, the highest H2/CO ratio and the lowest sulphur and nitrogen compound emissions were obtained for the binary blend of petcoke and olive pomace containing dolomite. On the other hand, the highest synergistic effect was observed in the binary blend containing dolomite. To sum up, the best results were obtained when dolomite was in the blend in which the higher H2/CO ratio, synergistic effect and reactivity and lower pollutant emissions were observed. Finally, the residue obtained from the co-gasification process could be used as an adsorbent for removing pollutant gases.