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.

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

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.

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.

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.

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 LiOH⋅H₂O, 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%.

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 sorbent^17,18,19. 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 CO₂^20,21.

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.

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).

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.

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%.

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.

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 CaSO₄^25,26. Furthermore, the sulphate forms a surface layer on the sorbent particles and prevents the diffusion of CO₂ through the pores of the sorbent itself.