Innovative additives for chemical desulphurisation in biogas processes: A comparative study on iron compound products

•The removal efficiency increased to 37–51% in dosing stage one.•A higher dosing resulted in higher removal efficiency (61–77%).•FeCl2 showed the fastest response and the highest removal efficiency.•FeOOH types showed a high removal efficiency throughout the experiment.•An depot effect was measured after stopping the supplementation of FeOOH.