I'd need to determine Moisture, Volatiles and Fixed carbon for activated carbons by using a thermogravimetric analyzer (TGA). I found different programmes by literature which slightly differ one another. Just wondering which is the most suitable one to follow.
I report the type of programme I was planning to use below:
25-105 degC, 25 degC/min, N2
105 degC, 10 min, N2
105-900 degC, 25 degC/min, N2
900 degC, (10 min), N2
900 degC, (15 min), air
Is there any steps I could omit/should add/change?
Any feedback is more than welcome!
Thanks,
Antonio
Hi,
I think You could omit isothermal steps, just keep going with heating the sample up to 900C in inert atmosphere, It might be worthwhile to reduce the heating rate from 25 to 15 degC/min in the first stage.
Next step could be isothermal combustion of carbon in Air as You wrote or
just change gas to Air and reduce heating rate to 2 degC/min
In my opinion, the baseline will be more stable without isothermal steps
I Attach publications, where authors used such a program:
1) RT - 900C - 15degC/min, in Argon 50 ml/min: ( Moisture, Volatiles)
2) 900C - 1000C - 2 degC/min, in synthetic air 50ml/min (Fixed carbon, ashes)
(see Fig 2, and 3 - TG, DTA and mass spectra for CO2, samples:quick coke and anthracite)
GOOD LUCK
The point is that I have got quite a few samples and I'm trying to minimize the duration of the programme...however, I'm mostly interested in the determination of the volatiles..I might focus on that step only...
I suggest that you decrease the heating rate to 10 C°/min so that you can get better results and by decreasing the heating rate you can see the degradation stages more clearly.
the heat rate you are going to use totally depends on the TGA. If the TGA is the typical ceramic furnace one, then 25 C/min is way to high specially if you have an isothermal steps in between. The reason is not related to the sample (if you have a sample size of ~5 mg). It is more related to the mass of the surrounding ceramic in the furnace which makes what TGA program thinks the temperature is deviate from what the temperature actually is. If you are using an IR furnace (as is in Q-5000), then the heat rate has minimal effect even if the sample size is big (~ 30 mg) given the sample is spread evenly on the pan.
You would need to have an isothermal step after 100 for sure to get ride of moisture, and depending on how much moisture you've got, of course, the dwelling time would be different. To my experience, for pre-dried coal, I had to keep the the sample at 100 C for 15 min to reach to a steady sample weight (pre-dried black coal is quiet dry, the humidity it has is just a result of being kept in lab air). The same is more or less true for the last combustion step. Although combustion is relatively much faster, but you have much more material to burn which prolongs the combustion step.
My samples are activated carbons which easily tend to adsorb moisture from air. So, in order to determine moisture, should I perform the first two steps of the programme (25-100 say 10 degC/min and 100 degC say for 10 min) under N2 or air atmosphere?
Thermogravimetry can be employed to perform the analysis of moisture, volatiles, fixed carbon and ash (proximate analysis) in biomass samples. For moisture content determination, the samples is heated from room temperature up to 110 °C, with the hold time of 15 min and for volatiles determination, the temperature was raised up to 600 °C and an isothermal of 30 min is applied, both steps under inert atmosphere (130 mL min-1). Once a mass loss plateau is established, the furnace environment is switched from inert to air atmosphere (100 mL min-1) for 30 min. In both steps, the same heating rate is maintained (50°C min-1) and a sample mass of 10 mg is employed. This temperature programming was adopted based on the standard method (ASTM) E1131 after the proper modifications.
Moisture, volatiles, and ash performed simultaneously is fast; however, the data is of low repeatability. The repeatability is hampered because if the room humidity changes between the time the first sample is started and the last sample is finished then this change affects every following measurement. This measurement drift is apparent in a climate with humidity variation like we have here in the Southeastern US. Because of our climate we cannot reproduce any simultaneous TGA proximate analysis.
Separate moisture, volatiles, and ash tests have served us well. Moisture is tested in an oven so all samples can be tested simultaneously to avoid the humidity change issue. In addition to avoiding the change in humidity issue, drying outside the TGA puts a larger amount of dry material in the crucible thus improving the accuracy of the ash test for material with low ash contents.
Before any TGA tests the crucibles with the samples are dried at 110°C for 15 minutes and allowed to cool. The sample robot removes the pan and places it right back before doing the experiment. This action shifts the sample and starts the weighing over so as to avoid many issues concerning loss of accuracy.
The volatiles and ash test use temperature profile for ASTM D-3174 for ash and ASTM D-3175 for volatiles. Each test is run on separate dry samples. An implicit part of the temperature profile which not written in the method is the cooling curve. The cooling curve must be included in a TGA proximate method for proper results as this duplicates the action of allowing an oven to cool before withdrawing a sample for weighing.
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