• Surface Area
    And Pore Size Distribution

Background to Surface Area and Porosity Analysis

Biochar and other carbonaceous materials obtained after thermochemical modification processes like pyrolysis, torrefaction, hydrothermal carbonization or gasification typically develop a honey-comb porous configuration. This very complex network of pores allows these carbonaceous materials to be potentially used to produce: soil amendment; activated carbon; carbon molecular sieves; catalysts during the production of advanced biofuels; or as media for upgrading the production of biogas.

However, not all pyrolysis processes result in a competitive porous material. This is because there are numerous variables that can impact the porosity of the sample. These include: the type of feedstock, the pretreatment, process conditions, the presence of ash or condensates clogging the pores, and the selected activation method. Therefore, a sample obtained after pyrolysis-like processes needs to be analysed to determine its surface area and pore size distribution in order to reveal the true porous nature of the sample and its most suitable application.

We determine the surface area and pore size distribution of carbonaceous samples using a Quantachrome NOVA-e Series 2200e analyser which has been designed to satisfy the procedures outlined in EBC (2012-2022) 'European Biochar Certificate - Guidelines for a Sustainable Production of Biochar.' European Biochar Foundation (EBC), Arbaz, Switzerland. Version 10.1 from 10th Jan 2022.

Surface Area Analysis methods

The surface area of the samples is determined by the Brunauer-Emmett-Teller (BET) method which is the most well-known technique for accurate determination of the surface area of carbonous materials, together with a micropore BET Assistant AI tool that helps us to improve the accuracy of the analysis.

We also determine the pore size distribution of the samples by the Quenched Solid Density Functional Theory (QSDFT) which is a method that, coupled with computer simulations, provides an accurate description of micro- and mesoporous carbons based on the sorption and phase behaviour of fluids in narrow pores on a molecular level. Furthermore, we provide an interpretation of the sample isotherm for supporting the data obtained from the surface area and pore size distribution analysis.

In order to carry out this analysis, as outlined in EBC (2012-2022), we need to mill the sample to a particle size below 3.15 mm and dry it overnight at 40 °C, followed by a degassing procedure carried out under vacuum for 2 hours at 150 °C. After the samples have been degassed, we start the analysis by measuring the sample pore volume at relative pressures from 0.05 to 0.35 distributed along 5 points. Nitrogen is used as the adsorbate and liquid nitrogen as the media for keeping the thermal equilibrium of the experiment.

If the analytical package also includes the pore size distribution of the sample, the pore volume is measured at relative pressures from 0.05 to 0.99 distributed along 20 points for producing a standard isotherm or along 40 points for producing a high-resolution isotherm.

Surface Area and Porosity Analysis at Celignis

The analysis can also be done using CO2 as the absorbate for getting an even higher resolution description of the smallest micropores (less than 1.5nm) which are commonly present in carbonaceous materials, such as biochar or activated carbons.

At Celignis we provide highly comprehensive reports for the surface-area and porosity analysis of samples. These reports are provided online, on the Celignis Database, as soon as we obtain the results, as well as in detailed Excel and pdf reports that are provided once the order has been completed. The Celignis Database includes interactive charts, like the examples provided on this page, as well as detailed summary statistics. The final reports also incldue these results along with interpretations of the data, personally written by trained members of the Celignis team.

Analysis Packages for Surface Area and Porosity