Post combustion CO2 capture with calcium and lithium hydroxide (2023)

Calcium and Lithium hydroxide performance

CO2 uptake of Ca(OH)2 was tested in the range between ambient temperature and 300°C. Results are reported in Fig.3 which shows carbonation capacity (molCO2/kg sorbent) in dry conditions for both pellets and powder. Ca(OH)2 pellets were tested for two space velocity (SV): 31,200 and 15,600h−1. Looking at all data, the highest carbonation efficiency is observed in the range of temperatures from 80 to 100°C. For temperature higher than 100°C, the carbonation capacity decreases because of the loss of sorbent humidity. As mentioned above, in fact, soda lime contains almost 10–15% of water which promotes the carbonation reaction. Indeed, where the temperature makes possible the water evaporation (> 100°C), the carbonation reaction is not enhanced.

Soda lime carbonation capacity as a function of the temperature.

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At the investigated temperatures, main carbonation reaction involves directly Ca(OH)2 according following Eq.(2).

$${\text{Ca}}\left( {{\text{OH}}} \right)_{{2}} + {\text{ CO}}_{{2}} = {\text{ CaCO}}_{{3}} + {\text{ H}}_{{2}} {\text{O}}$$


It takes place until 350–400°C; at higher temperatures, instead, dehydration of Ca(OH)2 to CaO can occur15. Some papers indicate that the optimal temperature of Ca(OH)2 carbonation is almost at 200°C16. Carbonation capacity trend as a function of the temperature, reported in Fig.3, confirms this statement.

The carbonation capacity of calcium hydroxide is dependent on Space Velocity. The variation of this parameter was obtained by varying the height of the reactor filling (10 and 20cm for 15,600 and 31,200h−1, respectively). By grouping the data with the same space velocity and same granulometry (pellets), it should be stated that an increase in space velocity determines an average reduction in carbonation capacity of almost 25% (Fig.4a). Therefore, a longer contact time favors the CO2 capture capacity.

Influence of space velocity (a) and granulometry (b) on soda lime carbonation capacity.

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For stating the influence of sorbent granulometry, data at space velocity of 31,200h−1 were grouped according sorbent granulometry, pellets and dust (Fig.4b). Fine granulometry involves an increment of carbonation capacity of almost 40% compared to pellets. Main reason is the increased surface area of dust which is involved in the carbonation reactions. It has to be noted that the fine granulometry corresponds to the best absolute result obtained with soda lime (almost 3.6 molCO2/kg sorbent corresponding to 35% of maximum possible carbon uptake by the tested sorbent mass).

Figure5 shows carbonation capacity measured with LiOH anydrous and monohydrate, as a function of the temperature. Data are referred to a space velocity of 32,100h−1. When increasing temperature, carbonation capacity of LiOH decreases: the highest capacity of 4 molCO2/kg sorbent is measured at the ambient temperature. At T = 120°C, the carbonation capacity drops to 1.5mol CO2/kg sorbent. The increasing of temperature, in fact, does not promote the formation of monohydrate hydroxide and then carbonation rate decreases 17.

Carbonation capacity of LiOH anhydrous and monohydrate as a function of the temperature.

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Main reactions (Eqs.2 and 3) occurring for CO2 absorption involve the production of the intermediate hydroxide monohydrate; then, it reacts with CO2 to form lithium carbonate.

$${\text{2LiOH}}\left( {\text{s}} \right) \, + {\text{ 2H}}_{{2}} {\text{O}}\left( {\text{g}} \right) \, = {\text{ 2LiOH}}\cdot{\text{H}}_{{2}} {\text{O}}\left( {\text{s}} \right)$$


$${\text{2LiOH}}\cdot{\text{H}}_{{2}} {\text{O}}\left( {\text{s}} \right) \, + {\text{ CO}}_{{2}} \left( {\text{g}} \right) \, = {\text{ LiCO}}_{{3}} \left( {\text{s}} \right) \, + {\text{ H}}_{{2}} {\text{O}}\left( {\text{g}} \right)$$


Therefore, the efficiency of CO2 absorption is strictly influenced by water adsorption rate on the LiOH surface. As consequence, the percentage carbonation capacity measured for anydrous LiOH is almost 13% of maximum CO2 mass which is possible to capture, by considering the stoichiometry. Moreover, it was observed that, despite the presence of water, the carbonation capacity of LiOH monohydrate is always lower than 0.5mol CO2/kg sorbent. This is caused by the too fine granulometry of the available sorbent; in fact, since CO2 absorption reactions lead to the water formation in addiction to that already present in the LiOHH2O, the granules agglomerate forming macro-granules characterized by a reduced surface area. Better performance than the two pure sorbents were obtained by mixing the two hydroxides (50% v/50% v). In this way the new mixed sorbent was characterized by a mixed granulometry (pellets and powder) and also by a reduced water content compared with the pure lithium monohydrate hydroxide. The carbonation capacity of mixed sorbent is greater than 4mol CO2 / kg sorbent, reaching the best performance with lithium sorbent (26% of CO2 captured compared with the maximum allowed).

SEM/EDS analysis

Some samples of soda lime exposed to CO2 gas stream were also analyzed with SEM to obtain elemental composition. Figure6 shows the elemental composition of soda lime exposed to a gaseous mixture containing CO2 at 20%v in nitrogen, at the reactor temperature of 120°C and with a Space Velocity of 31,200h−1. The graph shows the compositions for the fresh (not exposed) sample and for 2 carbonated samples: original particle size (pellets) and fine particle size (powder). The data correspond to 20 scans average for each sample analyzed. SEM analysis confirms that the fine grain size shows a greater carbonation capacity. In fact, the average percentage concentration of C measured in the exposed soda lime powders is 21%; the soda lime of original grain size, instead, has a C content of approximately 16,7%.

Elemental analysis of fresh and carbonated soda lime by SEM/EDS.

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Considering that the fresh sample has an initial carbon content of 13,6%, the C trapped is 7,3% for the powders and 3,1% for the pellets. Starting from these percentages, the carbonation capacities were estimated in the two samples of soda lime, considering that the number of moles of CO2 trapped by the sorbent coincides with the number of carbon atoms. The results are compared with carbonation capacity estimated by online CO2 measurement downstream of the reactor (left side of Fig.6). The greater exposure of the fine particle size to the gaseous stream allows to obtain a good agreement between the direct measurement of the carbonation capacity and the indirect measurement by means of elemental SEM/EDS analysis (difference of 4%). When analyzing the pellets, the difference is greater (+ 40% of the direct measurement compared to the SEM).

Influence of the moisture on the carbonation capacity of Ca(OH)2

Solid sorbents capable of capturing CO2 in gaseous effluents show the best performance in the presence of moisture. Water has a catalytic effect in the process of CO2 absorption by hydroxides12. The steam, in fact, performs a catalytic action for the diffusion of CO2 through the upper carbonated layer of the sorbent17,18,19. Furthermore, it has been shown that in high temperature applications, the steam can induce the widening of the average size of the absorbent pores, enhancing the capture of CO220,21.

It was demonstrated that the reaction mechanism is dependent by alkalinity of adsorbed water on Ca(OH)2 surface 22. Equations from (4) to (9) describe the possible reactions in presence of water.

$${\text{Ca}}\left( {{\text{OH}}} \right)_{{{2}({\text{s}})}} = {\text{ Ca}}^{ + + }_{{({\text{aq}})}} + {\text{ 2OH}}^{ - }_{{({\text{aq}})}}$$


$${\text{CO}}_{{{2}({\text{aq}})}} + {\text{ H}}_{{2}} {\text{O}}_{{({\text{aq}})}} = {\text{ H}}_{{2}} {\text{CO}}_{{{3}({\text{aq}})}}$$


$${\text{H}}_{{2}} {\text{CO}}_{{{3}({\text{aq}})}} = {\text{ H}}^{ + }_{{({\text{aq}})}} + {\text{ HCO}}_{{3(aq)}}^{ - }$$


$${\text{H}}_{{2}} {\text{CO}}_{{{3}({\text{aq}})}} = {\text{ H}}^{ + }_{{({\text{aq}})}} + {\text{ CO}}_{{3(aq)}}^{2 - }$$


$${\text{CO}}_{{{2}({\text{aq}})}} + {\text{ OH}}^{ - }_{{({\text{aq}})}} = {\text{ HCO}}_{{3(aq)}}^{ - }$$


$${\text{Ca}}^{2 + }_{{({\text{aq}})}} + {\text{ CO}}_{{3(aq)}}^{2 - }= {\text{ CaCO}}_{{{3}({\text{s}})}}$$


For higher water alkalinity, carbonate ion is predominant and carbonation reaction (9) is favored; otherwise, bicarbonate ion is predominant (reactions (6) and (8)) and dissolved in water layer. Reactions of adsorption and hydration of CO2 and the formation of carbonate ion are very fast, whereas the dissolution of Ca(OH)2 may be slow, depending on the adsorbed humidity.

In order to study the effect of moisture on CO2 capture, wet conditions were realized in small-scale reactor by using a NafionTM tube humidifier, positioned upstream of the reactor inlet. In particular, it was possible to achieve 2 and 5% as humidity (H). It has to be noted that the addition of moisture in gas stream adds also the advantage to simulate chemical gas composition closer to those of an internal combustion engine exhaust (water concentration between 5–10% v).

Each condition was examined with the Ca(OH)2 sorbent both in pellet and in powder, in the temperature range between ambient temperature and 150°C. The Space Velocity used for these tests was 31,200h−1.

Figure7 reports data of wet and dry carbonation capacity as a function of temperature. By increasing the water content, an improvement of carbonation capacity is clearly visible only for pellets. Soda lime powder shows a higher CO2 uptake only in correspondence of few temperatures (80 and 120°C).

Carbonation capacity of soda lime in wet and dry conditions as a function of the temperature.

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In order to deeply investigate the influence of humidity, the box plot of Fig.8 summarizes the average carbonation capacity measured in dry and wet conditions (H = 2% v and 5% v). The data were grouped according to the granulometry of the sorbent. It is evident that in the investigated experimental conditions, the humidification of the gas stream introduced appreciable benefits for the pellets. In this case, in fact, the average carbonation capacity goes from 2,3 molCO2/ kg sorbent to 3.9 molCO2/ kg sorbent. For the powder size, on the other hand, the improvement in the carbonation capacity for the humidification of the gaseous current is low due to agglomeration phenomena of the sorbent with a consequent decrease in the surface area.

Average carbonation capacity of soda lime in wet and dry conditions.

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These results are comparable with literature. Recently23, investigated the CO2 capture by using a fluidized bed of calcium hydroxide, previously humidified. They analyze the capacity of CO2 uptake at ambient temperature, atmospheric pressure and 1%v CO2 inlet concentration. When increasing the relative humidity from 24 to 100%, carbonation capacity doubled up to almost 0,3 molCO2/kg sorbent. A similar research carried out by24, showed that if Ca-based sorbent is 8h pre-hydrated, carbonation capacity is almost 6 molCO2/kg sorbent corresponding to almost 10 times higher than dry value.

Influence of chemical composition of gas stream on CO2 capture

The interference of other gas on CO2 capture was studied. The attention was focused on some compounds normally present in the exhaust flue gas of internal combustion engine. To this aim, carbonation capacity of soda lime was monitored by using the following gas mixtures:

Mix 1: CO2 (10-20%v) in nitrogen

Mix 2: CO2 (10%v), CO (0,5%v), C3H8 (330 ppmv), NO (1000 ppmv) in nitrogen

Mix 3: CO2 (6%v), SO2 (400 ppmv) in nitrogen

Mix 2 has a chemical composition close to that of the engine exhaust, whereas mix 3 was used to analyze the interference of sulphur on the carbonation capacity of soda lime.

Figure9 reports average carbonation capacity of Ca(OH)2 as a function of chemical composition of gas mixture. Data are grouped according sorbent granulometry (pellet or powder). It should be noted a lower CO2 uptake in presence of other gaseous compounds compared with the binary mixture of CO2 and nitrogen. Compounds such as NO, CO, C3H8 and SO2 reduce carbonation capacity more than 60%.

Ca(OH)2 carbonation capacity with gas mixtures of different chemical composition.

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SO2 interference was deeply investigated varying the reactor temperature and moisture content in gas stream (Fig.10). Each graph reports carbonation capacity with and without SO2 as a function of the temperature. Moisture content and granulometry are fixed.

Carbonation capacity of Ca(OH)2 w and w/o SO2.

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The efficiency reduction in CO2 capture increases as the humidity of the gas stream increases. In dry conditions, differences in the carbonation capacity due to the presence of SO2 is almost 15% for pellets and 29% for powder. The greatest difference in carbonation capacity is, however, measured at a water vapor concentration of 5%v. In this case reduction of carbonation capacity drops of almost 50% compared with data measured in absence of SO2 (data available only for Ca(OH)2 in pellets). Many literature studies have shown that the carbonation efficiency with calcium-based sorbents is significantly reduced by the presence of SO2, due to the irreversible reaction between Ca and SO2 to form CaSO425,26. Furthermore, the sulphate forms a surface layer on the sorbent particles and prevents the diffusion of CO2 through the pores of the sorbent itself.


How much CO2 does calcium hydroxide absorb? ›

Its calculated absorption capacity of 3.05 g of CO2 [g of Ca(OH)2]−1 L–1 was 3-fold more than that of 1% Ca(OH)2 suspension solution, which confirmed Ca(OH)2-saturated aqueous solution as the most efficient absorbent for CO2 in a Ca(OH)2 aqueous solution system.

What happens when calcium hydroxide reacts with CO2? ›

Carbon dioxide reacts with limewater (a solution of calcium hydroxide, Ca(OH) 2), to form a white precipitate (appears milky) of calcium carbonate, CaCO 3. Adding more carbon dioxide results in the precipitate dissolving to form a colourless solution of calcium hydrogencarbonate.

When CO2 is passed through the solution of calcium hydroxide which precipitate is formed? ›

When carbon dioxide gas is passed through lime water or calcium hydroxide, double displacement takes place and a white precipitate of calcium carbonate is formed.

Does calcium hydroxide absorb CO2? ›

Calcium Hydroxide (Ca(OH)2) has many environmental applications. It is, in fact, used for flue gas treatment to reduce the emission of acidic gases (HCl, SOx, and NOx) ant it is also an effective solvent to absorb CO2.

How does lithium hydroxide remove CO2? ›

The absorption of carbon dioxide is accomplished in a chemical reaction using a sorbent known as lithium hydroxide (LiOH). This method relies on the exothermic reaction of lithium hydroxide with carbon dioxide gas to create lithium carbonate (Li2CO3) solid and water (H2O).

Why calcium hydroxide is not used to absorb carbon dioxide? ›

absorption of CO2 using solid calcium hydroxide alone is not efficient because CO2 must be dissolved in water before it can react with calcium hydroxide.

What type of reaction is carbon dioxide and calcium hydroxide? ›

(D) Thermal decomposition. Hint: In the above question, it is asked about which reaction is carried out when carbon dioxide reacts with calcium hydroxide to form calcium carbonate and water. Here, the product formed is salt and water and hence, it is an example of reaction where salt and water are formed.

Is reaction of calcium hydroxide with CO2 exothermic or endothermic? ›

The reaction is highly exothermic in essence, an exothermic process describing a process or reaction that releases energy from the system to its surroundings. When calcium hydroxide is treated with carbon dioxide, the calcium carbonate and water is formed, then carbon dioxide is released.

Why calcium hydroxide reacts slowly with carbon dioxide in air? ›

Assertion (A) : Calcium hydroxide reacts slowly with the CO_(2) in air to form a thin layer of CaCO_(3) on the walls. Reason (R) : Decomposition reactions require energy in the form of heat, light or electricity for breaking down the reactants.

What happens when CO2 is passed in excess through lime? ›

When excess CO2 is added, it reacts with calcium carbonate( CaCO3) and water (H2O) to form calcium bicarbonate Ca(HCO3)2 and since it is soluble in water, it dissolves in it making the solution clear again. The chemical reaction is as follows: CaCO3 + CO2 + H2O ---------> Ca(HCO3)2.

When CO2 is passed through lime water it turns milky? ›

Why does limewater turn milky when carbon dioxide gas is passed through it? Due to the formation of calcium carbonate.

Does calcium hydroxide react with carbon dioxide to produce salt and water? ›

In the given reaction, calcium hydroxide, which is a base, reacts with carbon dioxide to give salt (calcium carbonate) and water.

What absorbs CO2 the best? ›

1) Trees and Forests

Plants remove carbon dioxide from the air naturally, and trees are especially good at storing CO2 removed from the atmosphere by photosynthesis.

What absorbs CO2 fastest? ›

Bamboo: THE solution against greenhouse gases

Indeed, thee bamboo absorbs 5 times more greenhouse gases and produces 35% more oxygen than an equivalent volume of trees! It has a very important CO2 retention capacity since one hectare of bamboo grove can capture up to 60 tons of CO2 each year.

What absorbs the most CO2? ›

A carbon sink absorbs carbon dioxide from the atmosphere. The ocean, soil and forests are the world's largest carbon sinks.

How much CO2 does calcium oxide absorb? ›

Theoretically, 1 g of CaO can capture 786 mg of CO2. In addition, this sorbent could be applied at room temperature and atmospheric pressure. The reaction of calcium oxide (CaO) with CO2 at high temperatures has great significance.

How much CO2 can NaOH absorb? ›

As per molar calculations, 30 g of NaOH should absorb 16.5 g of CO2. When 5% CO2 is used with 30 g of NaOH pellets, the CO2 concentration in the outlet gas was reduced by 15- fold.

How much CO2 can soda lime absorb? ›

Soda lime is the most common absorber, and at most can absorb 23 L of CO2 per 100 g of absorbent.

What absorbs most of the CO2? ›

A carbon sink absorbs carbon dioxide from the atmosphere. The ocean, soil and forests are the world's largest carbon sinks. A carbon source releases carbon dioxide into the atmosphere. Examples of carbon sources include the burning of fossil fuels like gas, coal and oil, deforestation and volcanic eruptions.

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