Investigation on desulfurization performance and pore structure of sorbents containing zinc ferrite

Regenerable and durable sorbents for high-temperature sulfur removal are desired to achieve advanced power generation using coal gas fuel. Different precipitation methods using ammonia and urea were studied in this work to prepare zinc ferrite-silica composite particles. Two sorbents containing those powders were extruded and were evaluated in desulfurization performance and durability during cycles of repeated desulfurization at pressurized simulated coal gas conditions. Both sorbents could reduced sulfur compounds to less than 1 ppm during consecutive desulfurization cycles operated at 723 K and 0.98 MPa. The sorbent prepared by ammonia precipitation exhibited higher performance at initial sulfidation but declined in sulfidation kinetics after 20 cycles of tests. The urea-precipitated sorbent maintained its performance during the same desulfurization cycles. Although the residual sulfur was gradually increased for both sorbents, the sorbents maintained over 75% of their initial sulfur capacity at the end of the test. Pore distribution obtained by mercury porosimetry showed significant loss of mesopores of the ammonia-precipitated sample during the cycles, while the urea-precipitated sorbent maintained its mesopore structure. The primary particles of zinc ferrite in the fresh and spent sorbents were observed using FE-TEM. Primary particles of zinc ferrite in the ammonia-precipitated powder were much smaller (10-20 nm) than those of the urea-precipitated powder (100 nm diameter). Rapid decline in the ammonia-precipitated sorbent was due to the sintering of the fine primary particles. The latter kept their size after cyclic desulfurization. The durability of the sorbent was strongly affected by the primary particle size as determined by the preparation method and sorbent preparation is an important factor to enhance durability of the sorbent.