In tsa hdac main study, the gas temperatures in the kiln were lower than the set temperature of 400 °C when treating all ash samples. The highest temperature, 386 °C, was reached in experiments with ash A, while in experiments with ashes B and C gas temperatures reached 352 and 366 °C, respectively. The lower temperatures recorded in experiments with ash A during the main study compared to the screening study could be due to increases in its moisture content as the ash exhibited hygroscopic properties. Overall, the lower gas temperatures likely depended on the ash heat transfer capacity as wall temperatures were the same in all experiments. Stable treatment temperatures were reached approximately 40 min into the 120-min treatment time (Fig. S2, Supplementary information). Ash B differed in behavior from ashes A and C, as geological time tended to stick to the kiln wall while A and C samples were coarser and tumbled more. This difference may have been due to ash B’s higher chlorine content, as alkali chlorides tend to be sticky and bind together.