Hi Parvin,

Calcination reactions usually take place at or above the thermal decomposition temperature (for decomposition and volatilization reactions) or the transition temperature (for phase transitions). This temperature is usually defined as the temperature at which the standard Gibbs free energy for a particular calcination reaction is equal to zero. For example, in limestone calcination, a decomposition process, the chemical reaction is

CaCO3 → CaO + CO2(g)

The standard Gibbs free energy of reaction is approximated as ΔG°r = 177,100 − 158 T (J/mol).[3] The standard free energy of reaction is zero in this case when the temperature, T, is equal to 1121 K, or 848 °C.

Examples of chemical decomposition reactions common in calcination processes, and their respective thermal decomposition temperatures include:

CaCO3 → CaO + CO2; 848 °C

Calcination of limestone using charcoal fires to produce quicklime has been practiced since antiquity by cultures all over the world. The temperature at which limestone yields calcium oxide is usually given as 825 °C, but stating an absolute threshold is misleading. Calcium carbonate exists in equilibrium with calcium oxide and carbon dioxide at any temperature. At each temperature there is a partial pressure of carbon dioxide that is in equilibrium with calcium carbonate. At room temperature the equilibrium overwhelmingly favors calcium carbonate, because the equilibrium CO2 pressure is only a tiny fraction of the partial CO2 pressure in air, which is about 0.035 kPa.

At temperatures above 550 °C the equilibrium CO2 pressure begins to exceed the CO2 pressure in air. So above 550 °C, calcium carbonate begins to outgas CO2 into air. However, in a charcoal fired kiln, the concentration of CO2 will be much higher than it is in air. Indeed if all the oxygen in the kiln is consumed in the fire, then the partial pressure of CO2 in the kiln can be as high as 20 kPa.[36]

The table shows that this equilibrium pressure is not achieved until the temperature is nearly 800 °C. For the outgassing of CO2 from calcium carbonate to happen at an economically useful rate, the equilibrium pressure must significantly exceed the ambient pressure of CO2. And for it to happen rapidly, the equilibrium pressure must exceed total atmospheric pressure of 101 kPa, which happens at 898 °C.

Equilibrium pressure of CO2 over CaCO3 (P) vs. temperature (T).[37]

P (kPa) 0.055 0.13 0.31 1.80 5.9 9.3 14 24 34 51 72 80 91 101 179 901 3961

T (°C) 550 587 605 680 727 748 777 800 830 852 871 881 891 898 937 1082 1241

But, three Au/TiO2 catalysts, with the same Au loading and with different particle sizes, were prepared by the deposition–precipitation method followed by calcination at three different temperatures, 473, 573, and 873 K. The mean diameters of Au particles were 2.4, 2.5, and 10.6 nm, respectively. On all the samples the CO adsorption and different CO–O2 interactions were examined by FTIR at 90 K and at room temperature. The higher catalytic activity on CO oxidation found for the samples calcined at 473 and 573 K is related to the higher concentration of step sites over the Au surfaces and to a higher concentration of step sites at the borderline with the support. At 90 K, CO and molecular oxygen are competitively adsorbed on step sites. By CO pre-adsorption on hydrated catalysts, the reaction with O2 gives CO2 already at 90 K, while by oxygen pre-adsorption the reaction is completely inhibited, unless moisture is present in the gas phase.

Hence Parvin, the Au/TiO2 catalyst seems to have the desired effect on the reduction of calcination temperature. Check it out and save energy!

Cheers,

Frank

Hi Kenneth,

I did not look up the cost of the Au/TiO2 catalyst. The Au and in fact the TiO2 to a somewhat lesser extent needed to produce the catalyst will evidently be pretty expensive, not speaking about the production of the catalyst itself.

Question is whether the catalyst requires regeneration at regular time intervals or not? I honestly don't have a clue. So yep, the cost is essential for application. But it works, and can save a lot of energy, and that has to be thrown on the cost balance as well. I am forgetting about the external costs of CO2 emission reduction as well here.

Interesting to check this out as well. The potential of catalytically reducing Gibbs free energy of reaction in calcination is enormous. There are thousands of cement plants globally. Each mm of sea level rise reduction due to reducing CO2 emissions is worth its weight in gold in the near future ;-)

Cheers,

Frank

Dear Frank, Thanks so much for your great information. That is what I exactly looking for...I appreciate your help.

Hi Parvin,  Done with pleasure. Cheers, Frank.

Yes Kenneth,

Human induced emissions of CO2 are still growing. Apparently the BRIC countries show a steady increase in CO2 emissions in the first half of 2014 according to an EEA report. Hence, assuming that we could drastically reduce CO2 emissions from calcination, I agree that would probably just have the same effect on the global carbon dioxide mixing ratio as a droplet of water on a red hot plate. Countries which show the highest economic growth are still those which use fossil fuels, 'a volonté', Especially China, now also fueled with a big contract for natural gas and oil straight from Russia with a pipeline through the Chinese Xinjiang province, which has a common border with Russia.  I am a bit like a soldier in a lost war. Rather die trying, than watch and start crying.

Thanks,

Frank

Hi Frank,

Now, my question is: how will these two solids ( CaCO3 and Au/TiO2) react with each other? I mean, How would I be able to have Au/TiO2 catalyst on the surface of CaCO3 crystal structure in order to have itself active?

Thanks for any advice you could provide me.

Hi Parvin,

A feasible approach (according to me) is based on the suggestion made by Kenneth Towe. Make a very fine airborne suspension of the CaCO3 carrying substance. It is based a bit on the principle of some energy plants burning wood or coal in Fluidized Bed Combustion (FBC). An example for calcination in fluidized bed. Is given below.

FBC calcination units are perfect for small production or small sized stone. These units produce up to 120 tpd. These systems can be fired on gas or coal. Efficiency is slightly below other types of kiln systems. Metso is one company able to build this type of plant turnkey. A system could come with feed and product material handling and storage system, a feed preheater, a fluid bed calciner, waste heat recovery, a fluid bed product cooler and a dust collection system. Controls are from a single control area.

The main benefits of fluid bed calciner are:

Less moving parts coming in contact with the product

Able to calcine much finer material than other calciners

Years of experience

Control systems for entire facility

Capable of installation anywhere in world

True understanding of process conditions

In house engineering for custom fit applications

AND

The possibility to implement an immobilized catalyst in the FBC reactor, in our case the Au/TiO2 catalyst, with the catalytic active area evidently optimized in function of reducing the recator temperature and hence energy consumption per MT CaO produced.

The application of suspension or fluid bed calciners has been in use well over 60 years old. Prescreened 4 mm x 1 mm limestone is fed to the surge hopper. The hopper distributes the stone into a four compartment preheater through which hot gases at 950 degrees C drawn from the kiln.This preheater is a spouted bed type, which quickly quenches the exhaust gases from 950 degrees C to 580 degrees C, thereby circumventing the critical transition temperature. Any buildup occurs on the feed particles and not on the wails or ducts of the processing equipment.

A controlled percentage of the kiln exhaust gases bypass the preheater and mix with the preheater exhaust gases and then pass to a recuperator. The recuperator is a combustion air to gas heat exchanger, where the combustion air is preheated. Fuel is injected into the bed of the calciner zone by means of a number of injectors to raise the temperature to that required for calcination. For coal firing, a pre screened coal of 1 mm x 4 mm is injected along with the limestone into the calciner freeboard. The product is cooled in a fluid bed cooler, either as an indirect water tube cooler or as a direct fluid bed cooler with exhaust gases going to a dust collector.

The fluid bed calcining system of the above mentioned design is available in 40- 200 TPD units. Fuel consumption is 1240 kcal/MT. Natural gas, oil or coal can be used as the fuel. The power consumption is relatively high (because of the pressure drop through the fluid bed) at 90 kW -h/MT.

So Parvin, That's just how I see a possibility to go ahead to demonstrate catalytic calcination at the small research plant scale.

Cheers,

Frank

Dear Frank, I really appreciate your comprehensive answer. Thank you.

Could it help to heat the CaCO3 in an oven with reduced pressure. It would not take much energy to reduce the pressure to e.g. 0.1 bar!

Just ran into this (about 2 yrs late). :)

How about adding acid to decompose CaCO3. You will release CO2 though you won't end up with CaO but CaCl2 (in the case of adding HCl) but depending on your goal that might be fine. 

Sorry to answer this late to the last three comments.

- Yes Henrik, a pressure reduction would be pretty beneficial to increase the overall efficiency of the FBC process. The gain in efficiency should be evaluated.

- And Yes Ammasi, there are a lot of techniques which can be tested with an FBC process approach, including the ones you cited.

- Hi Robin. I am not sure whether CaCl2 is useful when you want to produce cement. Is CaO not the product you want, to produce cement? Calcium chloride is commonly encountered as a hydrated solid with generic formula CaCl2(H2O)x, where x = 0, 1, 2, 4, and 6. These compounds are mainly used for deicing and dust control. Because the anhydrous salt is hygroscopic, it is used as a desiccant.

Cheers and a happy New 2021!

Frank

On TGA analysis of calcium carbonate minerals (e.g. calcite) containing iron: https://www.researchgate.net/post/How_would_the_addition_of_metallic_iron_to_a_sample_of_CaCO3_effect_the_TGA_and_DSC_curves_of_CaCO3