How was the chars prepared? There might be tars in the pores.

Take a look at the degassing protocol (pressure, temperature, time). Can You see any yellowish-brown stuff on the walls of the sample tube after degassing?

Please add an isotherm, it makes it much easier to answer the question.

What do You mean with this sentence "Volume of adsorption is higher for low relative pressure values and decreases as the relative pressure increases". Does it mean a fast rise in adsorption at low pressures (micropores) and then an almost flat portion? If so it is completely normal.

I am Attaching the File, For your reference

Henrik Romar , The char Preparation was carried out with nitrogen purging.

Adsorption is decreasing continuously till the last point. as attached above.

I have seen the same type of isotherms for biochars, in all cases they have been contaminated with tars.

Are the biochars produced using a protective carrier gas like nitrogen or are they just carbonized without an inert gas flow? In the later case tars will accumulate in the chars. All isotherms I have seen are obtained with chars produced in the later way.

yes sir, its all with nitrogen gas purging only., but i had closed the lids of the crucible during experiments.

I think You get tars into the chars if You have an almost closed system. Try to put a small amount of the chars in a beaker and add some 5 to 10 ml organic like isopropanol or even acetone. Mix for 30 minutes. If You see the liquid getting yellow to brown You have tars.

Ok.. I will check., if dats the case than what is the solution for this. Degassing and than using for analysis?

Tars are almost impossible to remove by degassing, the risk is that they will condensate inside the instrument and then You will have really big problems.

I have redone the BET analysis, I have got better adsorption isotherm. But the volume of gas adsorbed is less. I am trying to study the blending of coal and biomass kinetics. So small blending of biomass is giving a better results i.e. higher BET surface area. as biomass Increases Surface area reduced almost 10 times. Is it possible??

Result file trail 2

I think You have a BET surface something 15-20 m2/g (first point *4-4.5 gives about the surface). This surface area is slightly over the one obtained from virgin biomass.

There is still some problems, the desorb line should join to ads line at a P0/P around 0.4

I think the second value is closer to the right one than the one calculated from the first graph.

Still take a look at the transform plot, Where does it cut the y-axis? Should not be negative.

What is the C value for the measurement? It is not allowed to be negative, but if it is so, try to use a lower P/P0 ration for the calculation, this is often the case for carbons.

This says one big thing: degassing is a serious problem in your analysis. Both the adsorption and desorption isotherms should close (converge) after the analysis. If the desorption isotherm remains hanging it means that more gas than you put in came out at that particular pressure. My experience with carbonaceous materials is that they trap helium that is used for determining the free space (volume and surface area of the sample holder) and this interferes with the final sample analysis.

Usually there are two consecutive analysis:

(1) Free space analysis using He to determine the internal surface area of the sample tube;

(2) Sample analysis using N2 gas to determine the BET surface area of the sample, which includes the surface area of the sample tube.

Then the instrument automatically subtracts the free space from the second analysis. That is why sometimes you obtain negative surface area.

The way to go round this is to:

(1) Externally degas carbon materials at about 250-350oC for extended periods (even 2 weeks) to desorb the oils and open pores. Ensure that the temperature is not too high to alter the material.

(2) Analyze the free space or the empty sample tube on its own.

(3) Physically enter the free space values into the analysis method and override the free space analysis during sample analysis, which prevents the introduction of He in the sample.

When more gas than you put in comes out of the sample, it means that the material might be highly microporous and it traps He. By blocking the pores, the N2 has nowhere to go during analysis.

• True story: 15 years ago, I analyzed an activated carbon sample many times for over a month and the BET specific surface area was always less than 1 m2.g-1, but after using the separate free space analysis, the surface area come to over 1,300 m2.g-1 as expected.
• Believe me, carbonaceous materials need special attention.
• Please follow this link: http://www.micromeritics.com/Repository/Files/apnote104.pdf

The SSA for chars can increase by 300% just by prolonging the degassing time from 4 to 16 hours. And as I and Dr Aluha said , tars (oils) are a real problem. If they stay in the sample they affect the results, if You degass them out they might condense in the lines and disrupt the instrument.

What is the real value for the BET? If we need 2 weeks of degassing to get "tar-free" chars, is this the value needed for commercial chars? I dont think any commercial producer have time and space to degas the production for 2 weeks. BET measured without proper degassing gives much lower values. The whole thing is a question of what possible customers need.

Dr. Romar asks, "What is the real value for the BET?" My view is that it all depends on the intended application of the carbon material. Some carbonaceous materials include graphite, coke, coal of different types like peat, anthracite, etc, and they all have different industrial applications. It is therefore up to one to make the call and decide whether the surface area is worth waiting for during a 2+ week analysis.

• Some years ago, my clients used to wait for even a month because they were using anthracite to generate electricity and for them the oils and the tar were not an issue because they will be vapourised before the coal becomes red hot, when the true surface area of the material is revealed.

NB: The best way is to have a side sample preparation system with multiple ports where the materials can be heated without holding the BET analysis equipment. See this:

http://www.micromeritics.com/Product-Showcase/Sample-Preparation-Systems/Sample-Prep-Systems-Physisorption-Italian-Tran.aspx?no_redirect=true

Dr. Romar., Thxs for your time. I have attached the BET graph, My Y intercept is negative and also C value.

BET Surface Area: 20.3128 ± 0.4331 m²/g

Slope: 0.216256 ± 0.004521 g/cm³ STP

Y-Intercept: -0.001948 ± 0.000661 g/cm³ STP

C: -109.993779

This is the data i got

Sir, James Aluha , Thanks a lot for your time and suggestions., hope u understood that i am using ASAP 2020 instrument. Which has auto degassing attached., Ratio of cold free space to warm free space is appr equal to 3 or more than it which is reasonable.

If You inspect the graph carefully You will find two different regions., one from 0.05 to about 0.18, the other from 0.2 upward. Use less points for the plot, I suggest a maximum of 0.18.

This is not according to the BET theory but, You are dealing with carbon and they are so special. This is an advice I got from a representative from Micrometitics.