Evolution of the total mass of refractory organics and of the carbon-rich gas they release upon thermal processing at the organics line as a function of time in our fiducial disc setup (α = 10⁻³, M_∗ = 1 M_⊙). The blue line represents a standard model that excludes the thermal decomposition of refractory organics (e.g. Mah et al. 2023), where the carbon-rich gas released by refractory organics remains inside the organics line. Meanwhile, the red line shows the results from our model, including the thermal decomposition of refractory organics into gaseous C₂H₂. We see that the reservoir of carbon-rich gas originating from the thermal processing of refractory organics remains in the disc in higher abundance for a longer time when the irreversible decomposition process is included.