Recycling and energy recovery of waste plastic are major categories of modern solid waste management systems. Since the lower heating values of plastics are around 30 MJ/kg, equivalent to traditional solid fuels, such as petroleum coke, coal and charcoal, waste plastics are considered as potential fossil fuels alternatives in various industrial sectors. Already nowadays, energy recovery of waste plastic is typically conducted by combustion in incinerators or cement kilns. However, due to various types of waste plastics, their thermochemical behaviour has been rarely investigated. Even more, the thermochemical behaviour of waste plastics under oxy-combustion conditions has even rarely been studied. In this study non-isothermal thermogravimetric analyser was used to study the thermochemical behaviour of polyurethane plastic waste under seven different atmospheric conditions. Polyurethane decomposition kinetic constants have been estimated by the simultaneous evaluations of seven weight loss curves measured for the heating rate of 20 K/min and a final temperature of 1,073 K. The obtained results showed that the combustion of polyurethane is mainly composed of two stages, and the higher oxygen concentration slightly influenced the first stage but highly accelerates the second one, ascribing to a higher mean weight loss rate and an increasing activation energy. Replacing carbon dioxide with nitrogen slightly influences the first stage while positively influences the second stage, expressing in lower peak temperatures in differential thermogravimetry curves and higher peak values in differential scanning calorimetry curves.