Thermochemical Conversion Behaviours and Reaction Mechanisms of Cattle Manure Under an O2/H2O Atmosphere

Gasification is a route for cattle manure resource utilisation and emission mitigation, but its mass-change mechanism under an O2/H2O atmosphere remains unclear. This work aims to reveal the staged apparent mass-gain behaviour and its underlying coupling mechanism during cattle manure thermochemical conversion under O2/H2O atmospheres, so as to provide support for biomass gasification process optimisation and industrial circulating fluidised-bed gasifier parameter regulation. Thermogravimetric (TG) experiments were conducted under five O2:H2O mass ratios (1:4, 1:2, 1:1, 2:1 and 4:1), set with reference to the typical gas–steam ratio of circulating fluidised beds, combined with four heating rates (5, 10, 15 and 20 °C min−1), and the mass-change behaviour was interpreted using multiple characterisation techniques. The strongest peak appeared at 634 °C at an O2:H2O mass ratio of 1:2 and a heating rate of 10 °C min−1, with a maximum mass-gain rate of 11.0% min−1. The main mass-loss peaks occurred between 242 and 291 °C and were associated with organic-structure cracking and volatile release. The medium-temperature (297–457 °C) mass gain was attributed to oxidative adsorption on active char surfaces and transient oxygen-containing intermediates, whereas the high-temperature (521–864 °C) response was linked to ash mineral restructuring and char–mineral interfacial reactions.