Modelling in-duct mercury removal by activated carbon injection in incinerator flue gas

In the present work a model for the in-duct mercury capture in incinerator flue gas by powdered activated carbon injection is proposed. Material balances on mercury in both gaseous and adsorbed phases are carried out along the duct length and inside the activated carbon particles, taking into account mass transfer resistance and adsorption kinetics. The set of the coupled partial differential equations is transformed by means of an orthogonal collocation technique and integrated using a Runge-Kutta method with adaptive stepsize control. The model has been applied to an activated carbon sorbent of practical interest, whose parameters have been evaluated from available literature data. The values and range of the operating variables have been chosen in order to simulate typical incinerators operating conditions. Results of simulations indicate that large sorbent loadings in the duct are needed to obtain high mercury removal efficiencies, due to the short residence time. As a consequence very low utilization of the sorbent is achieved in any case. In order to minimize the sorbent feed rate it is particularly advisable to lower the operating temperature as much as possible. Improvements in the mercury capture performance can be obtained also by increasing the in-duct particles residence time and by decreasing the sorbent particles size.