Mercury retrieval from flue gas by monolithic adsorbents based on sulfurized sepiolite

The current sulfur-impregnated activated carbons are extremely costly sorbents in mercury control technologies. In this regard, the present study offers a cheaper alternative by using a natural silicate, sepiolite, as sulfur substrate. Sepiolite is a fibrous hydrated magnesium silicate clay that is very cheap (around $100/t of the 0.6-0.8-mm fraction) and abundant with unique textural and structural characteristics: S(BET), 150-350 m(2) g(-1); pore volume, 0.3-0.8 cm(3) g(-1); density, similar to 0.8 g cm(-3). Samples of 10% S supported on monolithic sepiolite have been used to retain 90 ppm mercury in air at 320 K and ambient pressure. The experimentation has been effected in a dynamic system at fluid velocities in the 4.7-18.8 cm s(-1) range. A comparative study of the performance of three different monoliths against a fixed-bed reactor of powders of 0.026-cm size is made. Under the work conditions, the process is governed by mass transfer, which on fresh solids fits the expression Sh = 0.075 Re(4/3) for powders and Sh = 0.766 (Re Sc d L(-1))(0.483) for monoliths. In the work, the importance of the adsorbents texture and, consequently, of the monoliths preparative method is demonstrated: the capacity per mass unity of the samples is better when impregnation is the last preparation step because porosity is better preserved and sulfur is more uniformly distributed.