Effects of Porosity on Carbon Reactivity in NO and O₂

Abstract

A set of carbons has been made by pyrolyzing cellulose that has been oxidized in air for different amounts of time. The porosity of the carbons was characterized by N 2 adsorption at 77 K, smallangle X-ray scattering (SAXS), and temperature-programmed desorption (TPD) of surface complexes formed during steady-state gasification in 5% NO gas. O 2 and NO reactivities were determined for the carbons. It was found that the reactivity in O 2 correlated well with N 2 surface area, suggesting that all of the surface was active. However, no correlation was found between NO reactivity and BET surface area, which suggests that not all of the surface was active. NO reactivity correlated well with both the ratio of CO 2 :CO produced during the TPD experiments and surface fractal dimensions from the SAXS. The CO 2 from the TPD experiments was due to a surface interaction between desorbed CO and nondesorbed C-O complexes. The TPD CO:CO 2 ratio correlated well with fractal dimension, and high fractal dimensions were associated with the production of CO 2 . Therefore, fractal dimensions are a good indicator of the mass transfer properties of the pore systems. The fact that NO reactivity correlated well with surface fractal dimension suggests that mass transfer effects determine reactivity. The carbon with the highest reactivity was a mass fractal, which is associated with an open pore structure.

Packed bed combustion of non-uniformly sized char particles

“…This is the only parameter in the model which was varied to agree with the data. The internal surface area a I assumed corresponds to the value of 400 m 2 /g given by Cetin et al (2004) and is similar to values reported by others for chars produced under fairly slow heating ( Della Rocca et al, 1999;Janse et al, 1998;Mermoud et al, 2006;Ruiz Machado and Hall, 1998;Struis et al, 2002). Since a I and A CO 2 are multiplied together both in the reaction rate and in the definition of the Thiele modulus, it is their product which determines the overall rate rather than their individual values.…”

Waste-derived activated carbons for control of nitrogen oxides

“…For example, Illan-Gomez et al (1993) reported that no correlation was found between the NO reduction activity and carbon structure, and that activated carbons with different surface areas were all effective for NO reduction. The same conclusion was reported by other researchers, such as Guo et al (2001), Neathery et al (1997) and Ruiz Machado and Hall (1998). Neathery et al (1997) found that the activated carbon with the lowest total pore volume and an intermediate surface area had the highest NO uptake capacity.…”

Waste-derived activated carbons for control of nitrogen oxides

“…However, Neathery et al (1997) observed that the NO adsorption capacity of the studied activated carbons was not directly related to their surface areas and micropore volumes. The same conclusion was derived by Guo et al (2001) and Ruiz Machado and Hall (1998). Other overriding factors that may influence NO adsorption over the carbon surface area are the size of the molecule being adsorbed such that the pollutant cannot enter the micropores of the carbon and the surface chemistry of the carbon which may not be conducive to nitrogen oxide adsorption (Mohan and Pittman, 2006).…”

“…There is also disagreement in the literature regarding the influence of surface area and porosity on the rate of nitrogen oxide reduction on a carbon surface. Several authors (Guo et al, 2001;Ruiz Machado and Hall, 1998) have considered that the rate of NO adsorption is proportional to the internal surface area of the char. However, Neathery et al (1997) observed that the NO adsorption capacity of the studied activated carbons was not directly related to their surface areas and micropore volumes.…”