The influence of volatiles to carrier gas ratio on gas and tar yields during fluidized bed pyrolysis tests

The formation of tar and light hydrocarbons during pyrolysis of pruning waste pellets in a fluidized bed (FB) reactor was experimentally studied as a function of temperature (700-900 °C), gas residence time (0.8-2.0 s) and volatiles to carrier gas ratio (13/87-42/58 vol%). The latter is defined as the ratio between the volume flowrates of volatiles released from the fuel particle at a given instant and that of the carrier gas used for the experiment at the same instant, and it quantifies the concentration of volatiles in the bulk gas. Experiments were conducted to analyze separately the effect of each of these three parameters on the yield of the different pyrolysis products. Special attention was paid to study the influence of the concentration of volatiles in the gas (controlled by changing the carrier gas/fuel ratio), since this parameter has been usually neglected in lab-scale kinetics studies up to now. The yields of the main light gases CO, CO2, H2, C1-C3 hydrocarbons as well as 41 tar compounds, grouped in five families, are reported. The increase in the reactor temperature strongly influenced the composition of the gas, reducing the yield of light hydrocarbons, and aliphatic and heteroatomic tars, and increasing the yield of CO, H2, benzene and polyaromatic hydrocarbons (PAH). The variation of the gas residence time between 0.8 and 2.0 s showed only slight influence on the yields of pyrolysis products. Remarkably, the increase in the concentration of volatiles from 13 to 27 (vol%) was seen to significantly affect the composition of the tar mixture, leading to higher yield of PAH while the yields of monoaromatic hydrocarbons decreased. This result highlights the significance of the concentration of volatiles during the study of the evolution of pyrolysis products.