(133d) Porosity and Surface Characteristics of High Alkali Content, Rapid Pyrolysis Biomass Chars

Authors: 
Frederick, J., National Renewable Energy Laboratory
Nimlos, M. R., National Renewable Energy Laboratory


Biomass chars produced by very rapid pyrolysis often exhibit high surface areas and are effective activated carbons. These high surface area chars become more reactive when gasified with water vapor or CO2.

In the work reported here, small particles of a biomass-derived, high alkali metal content industrial residue from chemical delignification of wood were pyrolyzed rapidly. The development of porosity, specific surface area, and pore characteristics was examined using conventional characterization methods.

Pyrolysis of this and similar biomass materials proceeded in a manner that is uniquely different from higher carbon content fuels such as medium to higher rank coals. These materials exhibited a much greater tendency to become plastic and flow as they were heated to pyrolysis temperatures. This behavior results in very limited surface area development at lower extents of pyrolysis, below about 55% conversion of biomass carbon to volatile matter. The pores we observed in chars from pyrolysis below about 55% carbon conversion were mesopores in the 140 to 250 micron equivalent diameter range. Specific surface areas of these chars obtained by N2 adsorption were low, not exceeding 10 m3/g. These results seemed to be independent of pyrolysis temperature, although data was limited to carbon conversions above 50% for the two higher pyrolysis temperatures (1000C and 1100C).

Once a critical extent of pyrolysis, corresponding to between 56 and 62% conversion of carbon to volatiles, was achieved, we observed a very rapid development of micropores, uniformly about 15 microns in diameter. The chars exhibit the characteristics of Type V N2 adsorption isotherms, with hysteresis clearly evident. Surface area development proceeded rapidly and continuously with carbon conversion above 60%, reaching a high of over 300 m2/g at 80% carbon conversion. Data at higher carbon conversions was available only at pyrolysis temperatures of 1000C and 1100C.