Abstract Carbon emissions from underground coal fires (UCF) have become an emerging research topic and their role in global climate warming has been widely debated. Currently, one big uncertainty for assessing UCF's carbon emission is the hypothesized carbon emission factors (EF) from the complete combustion of coal, while the EF of smoldering combustion of coal in the context of UCF is still unknown yet. In this work, a 1/20 scale laboratory experimental framework was proposed to characterize transient carbon emissions and quantify EFCO2 and EFCO. Effects of fire depth, ventilation area (aperture size), and coal rank on carbon emissions were explored with the extrapolation to the full-scale UCF. Results showed that total carbon emissions increase with the carbon content of coal. Volatile content is an important factor impacting the burning behavior and gas emission. Stable EFCO2 and EFCO of UCF, independent of the fire depth and aperture size, were estimated as 2006 ± 36 g kg−1 and 345 ± 132 g kg−1, respectively; its combustion efficiency was 85% ± 3%. The extrapolation of experimental data estimates the CO2 emission of coal fires in China and the USA as 2.34 × 107–4.61 × 107 t yr−1, which accounted for 0.4% - 0.9% of total CO2 emissions in the world in 2016.

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