Chemical looping combustion (CLC) is seen as a highly promising technology supporting the transition towards low-carbon societies. In this paper, a systematic assessment of the coal-based SNG (synthetic natural gas) and power cogeneration system with CLC was presented to justify the environmental unsustainability of such process. The results shows that lifetime of the oxygen carrier (OC) is the key element to determine the sustainability of decarbonized polygeneration system and the benefits from integrating CLC are significantly highlighted unless the lifetime exceeds 3000 h since the global warming impact (GWI) is almost independent. Besides, higher environmental disadvantages by using Cu-based OC (i.e. lowest energy output and highest global warming potential (GWP) by manufacturing copper oxides) as against Fe-based and Ni-based cases at the similar operating conditions. And similar environmental impacts by comparing with traditional cogeneration system that achieves a GWI of 45.03 kg/MW h increment, In addition, the effects of key parameters (e.g. steam to carbon ratio (O/C), recycling unreacted gas (Ru), etc.) on system performance and the carbon emissions allocation of products in terms of this polygeneration system have been discussed in detail. (C) 2019 Elsevier Ltd. All rights reserved.