Study on biogas upgrading characteristics and reaction mechanism of low energy consumption 2-Amino-2-methylpropanol (AMP)/piperazine (PZ)/H2O mixed amines

•AMP/PZ = 1/1 can achieve 100% CH4 purity and 0.598 mol CO2/mol amine CO2 uptake.•AMP/PZ = 1/1 heat of desorption was 10.67% lower than 30 wt% MEA.•All mixed amines have near 0 methane loss.•Sterically hindering effect can effectively reduce thermal stability of carbamates.•Piperazine can effectively improve the biogas upgrading effect.