Char Reactivity

Char Macropore Structure

The pore structure of coal and char particles is an interconnected network of micropores (<100 nanometers) and macropores (>1 micrometer). Nearly all of the internal surface area is associated with the micropores, while most of the porosity is attributed to the macropores.

We have developed a procedure to quantify the internal macropore structure of char particles. This allows for a determination of the effects of pyrolysis process conditions on pore structure, and enables us to determine the effects of macropore structure on char reactivity.

Our studies have revealed that higher pyrolysis heating rates lead to char particles with

• larger macroporosities,
• larger macropore surface areas, and
• larger size.

The following images are cross sections of char particles pyrolyzed in nitrogen at 3 different heating rates. These polished cross sections of the chars were analyzed with digital image processing to determine internal surface area and other properties.

0.1 °C/sec 1.0 °C/sec 10.0 °C/sec

Mechanism of Char Combustion

Our studies of char combustion in oxygen revealed the following:

• At low combustion temperatures, chars produced at different pyrolysis heating rates exhibit the same reactivity.
  
• At high combustion temperatures, however, chars produced at higher rates exhibit much higher reactivity and ignite more easily.

What causes these differences in char reactivity ?

Different degrees of utilization of the micropore surface area.

At low combustion temperatures (450 °C):

• There are no diffusional limitations in the micropores. As a result, the entire micropore surface area (indicated in the images below by the gray color) is available for reaction.
  
  
  
  
  
  
• Since all chars exhibit the same reactivity, we conclude that pyrolysis heating rates do not affect the micropore structure of produced chars.

At higher combustion temperatures (650 °C):

• Diffusional limitations in the micropores become important and reaction takes place in a narrow zone just below the surface of the macropores. Therefore, only a small fraction of the total micropore surface area is utilized for reaction.
  
  
  
  
  
  
• If d is the thickness of the reaction zone and S_mg is the macropore surface area, the observed rate of reaction in the regime of diffusional limitations is proportional to

  d x S_mg
  
  

• Thus, chars with large macropore surface area will exhibit higher reactivity. Here, the char produced at a pyrolysis heating rate of 10 °C/s will react faster than the char produced at 1 °C/s.
  
  

©1996 Sam Perkins and Kyriacos Zygourakis
Rice University