Integrated Pyrolysis of Waste on Cement Plants: Influence of Operating Conditions on Yields and Product Properties

Pyrolysis of waste integrated into cement plants can potentially supply energy to the cement processes through char and light gases while producing pyrolysis oil, which can be used on-site or externally as a marine fuel or refinery input for further upgrading. This study investigated the feasibility of applying waste pyrolysis in cement plants through laboratory (0.35 kg/h) experiments. It is the objective to quantify how and to what extent expected variations in the operating conditions of the reactor and feedstock properties might influence product yields and pyrolysis oil quality. The influence of condition changes relevant for cement plant pyrolysis such as feedstock (two wastes, two wood and plastic mixtures, and pure wood), pyrolysis temperature (500–640 °C), gas residence time (2–5 s), air ingress (λ = 0.08), and bed material (sand, cement raw meal (CRM), and calcined CRM) have been investigated. The plastic-rich waste 1 (approximately 73 wt % plastic, 19 wt % biomass, and 8 wt % ash) exhibited a maximum oil yield of 49 ± 4 wt % at 550 °C with a gas residence time of 5 s. On the other hand, the more biomass-rich waste 2 (approximately 39 wt % plastic, 46 wt % biomass, and 15 wt % ash) required a slightly higher temperature (570 °C) to achieve its optimal oil yield (44 ± 1 wt %), possibly due to the presence of plastic fractions that are more resistant to thermal degradation. The oils derived from the two wastes exhibited similar compositions and heating values; however, waste 2 required a higher pyrolysis temperature. In addition, the oil obtained from waste 2 was more waxy in nature and displayed a more favorable flash point (81 °C) compared to that of the oil derived from waste 1 (47 °C). The freeboard gas residence time (2 and 5 s) had a minimal impact on oil yields and quality. However, using calcined CRM as a bed material led to small reductions in oil yield at 550 °C, compared to a sand bed (from 49 ± 4 to 41 wt %), but also a decrease in dry oxygen content for the oil (from 13 ± 2 to 8 wt %) and a small bed uptake of CO2. Air addition (λ = 0.08) to the bed or freeboard decreased the oil yield (from 44 to 24 and 31 wt %, respectively) and increased gaseous products yields (mainly CO and CO2). The oil was similar in terms of elemental composition and dry oxygen content but showed a considerable decrease in total acidity (total acid number (TAN) fell from 80 to 25 mg KOH/g). The results indicate that fluidized bed pyrolysis of the waste is technically feasible. The quality of the resulting liquid was enhanced using calcined raw meal as the bed material, the addition of a small amount of air, and the application of pyrolysis temperatures in the range of 550–570 °C.