The performance of some ZnO-based regenerable sorbents in hot coal gas desulfurization long-term tests using graphite as a pore-modifier additive

In a previous study it was shown that the overall reactivity in thermobalance tests and the efficiency at breakthrough in 2-cycle tests in a fixed bed reactor of a sorbent used for hot coal gas desulfurization increased substantially by the addition of a 5 wt % concentration of graphite during the preparation of sorbent ZT(0.8:1). In this paper, the study of the graphite effect as a pore forming additive is extended to other types of sorbents whose performance is investigated through the sulfidation breakthrough curves obtained in 20-cycle tests in a fixed bed reactor. Experiments show that the efficiency of the studied sorbents, ZFC, ZFT, and ZT, a zinc oxide, and a zinc ferrite doped with TiO2 or CuO, respectively, are dramatically increased by the addition of a 5 wt % graphite concentration as compared with the efficiency shown by the same sorbents prepared without graphite. The graphite effect is also evidenced by the achievement of a substantial delay of spalling appearance, a detrimental process consisting in the appearance and development of a crack network in the extrudate, decreasing the mechanical strength of the sorbent progressively down to excessively low values, after enough number of successive sulfidation-regeneration cycles. A first study trying to recover the reactivity of a sorbent, that became inactive after 20-cycles operation in a fixed bed reactor, by graphite addition is also included.