By heating a "green" coal, porosity first increases, as many gaseous volatile matters are released, then the porosity decreases, as high temperature allows the reorganisation of the structure into a more compact one. You can have a look to the attached paper about heat-treatment of anthracites (especially Fig. 5). Does it answer your question ?

Alain

Thank you very much for your answer. we have found that with the increase of the temperature in the  range 1000-1600℃, the porosity and BET surface area of coal char decrease, while the crystalline structure is more ordered. However, these results conflict with decrease of the real density. Therefore, I wonder if the decrease of  real density  associate with changes of composition, for example, the discharge of oxygen atoms?

We are also studying the electrical conductivity of carbon materials,your research will bring us great help. Thank you again.

I still don't understand this decrease of real density. Can you give me the range of values ? If it was due to the release of volatile, such decrease would be possible only in case of closed porosity. If the porosity is open (as I expect), and if you, just like me, find that the carbon ordering increases, then the density should increase as well.

Alain

The real density was usually tested with the particle size less than 74 μm, but in order to compare, the real density of other particle size of coal tar were also tested,as well as the petroleum coke. These datas are shown in attachment. From these datas, the real density decrease of coal char may be not caused by closed porosity.

The order of magnitude of these data is correct. The first point of the blue triangle series is high compared to the others, which might be considered as constant. Too high density values are sometimes observed in activated carbons because of non-negligible He adsorption. This happens in materials having a rather high surface area. It is the case of your material at 1000°C ? If such phenomenon occurs, pycnometry at higher temperature should decrease adsorption, and therefore decrease the value of true density. You should check that, i.e., measuring the density at 30 and 50°C for example: the calculated value of density should decrease.

Alain

 Li Fachuang,

I think This density loss must be associated with the %Ash of Coal Char, if the Coal Char has less Ash Content as compared to its parental Anthracite coal then its density must be reduced from the coal. ( in Coal char Ash% will be lesser and Carbon % will be greater than Anthracite coal)

Please check the density of Anthracite Coal and Coal Char (dry Volatile free based) for the clarification.

Regards.

Muhammad Imran

S. É. Pashchenko

Thank you very much. But I  didn't understand the picture  because of language problems. The air and CO2 are the evolution of gases?  The values of horizontal axis are the temperature?