1.1

Adsorption technology

It has previously been shown that several adsorbents such as activated carbon, metal-organic frameworks, and polymers^9,10 can be used for post-combustion CO₂ capture. These adsorbents include activated carbon, which has been reported to have promising structural properties, such as high adsorption capacity, selectivity, hydrophobicity, good thermal and chemical stability, and good reusability. Additionally, activated carbon has low production costs due to the availability of precursors. Researchers are looking to overcome the shortcomings of CCS technology, such as high energy consumption, solvent vaporisation, corrosion and degradation of amine solutions, by employing the usage of adsorption technology, which indicates great potential for improvement. The low heat of regeneration is due to the weak forces responsible for adsorption. Another advantage that adsorption technology has for CCS is the basicity of some adsorbents, which provides a good surface for the adhesion of weak acidic CO₂. These adsorbent surfaces can also be modified to increase their basicity. According to Reference¹⁰, activated carbon is superior to alternative physical and chemical procedures for wastewater treatment due to its inherent limitations, such as the creation of toxic by-products, high cost, and intensive energy requirements of other techniques. However, commercially available activated carbons are still considered a premium product. This is a result of the use of coal, a non-renewable and expensive source of energy, in pollution control applications. The development of cheaper and renewable sources for activated carbons, primarily derived from industrial and agricultural byproducts, has sparked new studies in recent times.

1.2

Environmental challenges

Our country and the rest of the globe are facing major human and environmental challenges due to the continued production of carbon dioxide being released into the atmosphere. Waste agricultural commodities such as cocoa pods, which pollute the environment can be converted into usable products for the oil and gas industry. Hence, there is a need to develop an optimum adsorbent for the storage of carbon capture for industrial deployment in process systems. This will entail carbonising the agricultural wastes as precursors for the production of activated carbon. The adsorption possibility of the activated carbon is defined by the adsorption rate of CO₂ from the developed activated carbon.

This paper presents the production of activated carbon to resolve CO₂ capture by using cocoa pod waste, which was activated with phosphoric acid (H₃PO₄) and potassium hydroxide (KOH) in ratios of 1:1, 1:2, 1:3 and 2:1, 3:1 at temperatures of 100°C, 200°C and 300°C, respectively. This was also evaluated in post-combustion conditions. This study devised a novel way to ameliorate emissions of CO₂ from industrial source points of carbon emission in order to safeguard the environment.

2.1

Materials

Experiments on the generation of Activated Carbon (AC) from cocoa pods and the evaluation of its CO₂ adsorption capacity at post combustion conditions were conducted in this research. These tests used the following materials and equipment:

2.2

Preparation of precursor

Figure 1 shows cocoa pods obtained from local vendors at Eku market in Ethiope East Local Government Area of Delta State, Nigeria. They were washed using distilled water to remove dust and debris. The washed samples were then dried under sunlight for 3 days until the mass of samples became constant. This was done to remove surface moisture. The dried cocoa pod samples were then crushed to smaller sizes using a blender in order to increase their surface area for the carbonization process. To improve the carbonisation surface area, the dried cocoa pod samples were crushed in a blender to smaller sizes.

Figure 1.

Cocoa pod obtained from vendor.

2.3

Carbonisation of precursor

An electric furnace as shown in Figure 2 was used to carbonize 126 pieces of crushed cocoa pod waste for 1 hour and 30 minutes to allow for the removal of volatile materials and ash in order to increase the carbon content of the fixed carbon. In addition, the carbonization of the samples allowed CO₂ to penetrate the sample pores. Additionally, the carbonisation of the samples allowed CO₂ to penetrate the sample pores. The carbonised samples (CS) shown in Figures 3 and 4 were allowed to cool and further crushed to increase surface area after the carbonization process was completed. To obtain finer samples for activation, the samples were filtered again. CS was used to represent the carbonized materials.

Figure 2.

Electric furnace.

Figure 3.

Carbonized sample of cocoa pod at 600 μm.

Figure 4.

Carbonized Sample of cocoa pod at 325 μm.

2.4

Activation of precursor

It was shown that activating five grams of CS with H₃PO₄ and KOH in the ratios of 1:1, 1:2, 1:3, as well as 2:1 and 3:1 at 100°C, 200°C, and 300°C for both 325 μm and 600 μm (CS: KOH) was the most efficient way to activate the CS. As indicated in Figure 2, the mixture was agitated until both components were homogeneous before being allowed to react for 72 hours at room temperature. Activated samples are in Figures 3 and 4. Subsequently, distilled water was used to wash the ASC until the pH was balanced (6-7). presented. After that, distilled water was used to wash AS until the pH was balanced (6-7). Once the mass change became consistent, washed CS was placed in a desiccator and dried at 200°C. This is illustrated in Figures 5 and 6.

Figure 5.

Homogenous samples of carbonized activating agent for 600 μm.

Figure 6.

Homogenous sample of carbonised activating agent for 325 μm.

2.5

Experimental design procedure

The experimental design for the CO₂ adsorption loop used in this work is similar to that detailed by Aimikhe & Eyankware¹¹ in terms of design details as illustrated in Figure 7 and testing techniques used.

Figure 7.

Schematics of experimental loop (Source: Aimikhe & Eyankware¹¹).

An airtight apparatus was sealed using a vacuum pump (VP) to ensure that no gases were present before the adsorption process began. After that, the Dosing valve (DV) and Bleeding valve (BV) were closed, and the Inlet valve (IV) was opened to allow CO₂ gas to pass. It was then determined that the specific quantity of AC (AC-1.08 g) needed to fill half the volume of the Reactor (V2) had been measured using a weighing scale.

Staging Manifold (V1) was then filled with CO₂ gas from the CO₂ gas cylinder (CGC) while the DV and BV were closed to allow the pressure in V1 to build up to a certain pressure of interest (P1-5.0, 10.0, 15.0, 20.0 and 25.0, psi). As a result, the IV was swiftly shut down to keep the pressure in V1 constant, and the Staging Manifold was closed for 15 minutes to check for leaks using the Pressure Gauge to detect any noteworthy pressure drop (PD). Adsorption was initiated after 30 minutes of equilibration in which CO₂ gas flowed through the DV and into a water bath (WB) maintained at a constant temperature, allowing the adsorbents to come into contact with the CO₂ gas.

3.1

Iodine value of AC for 325 μm and 600 μm

The iodine value of activated carbon was used to determine its capacity to absorb a radioactive isotope. The iodine values for activated carbon of sizes 325 μm and 600 μm are shown in Tables 1 and 2, respectively. Table 1 shows that the iodine value (mg of iodine adsorbed/g in activated carbon) indicates that the initial volume of iodine solution of sodium thiosulfate for size 325 μm was 100 mL throughout the experiment, while the final volume of iodine solution for ratio 1:1 at 100°C, 200°C, and 300°C was 34.9 mL, 39.0 mL, and 43.2 mL, respectively. Table 1. Iodine values for 325 μm sample of activated carbon before adsorption. At temperature ranges of 100°C, 200°C, and 300°C, the final volume of the iodine solution was 39.8 mL, 40.8 mL, and 41.3 mL for the ratio of 1:2, and 37.4 mL, 37.8 mL, and 39.0 mL for the ratio of 1:3.

Table 1.

Iodine values for 325 μm sample of activated carbon before adsorption.

Table 2.

Iodine value for 600 μm sample activated carbon before adsorption.

Table 2 shows the iodine value for 600 μm. The iodine solution of sodium thiosulfate as the first sample had an initial volume of 100 mL, while the end volume of iodine was 38.2 mL, 40.9 mL and 41.7 mL for the ratio 1:1 at 100°C, 200°C and 300°C, respectively. There were 39.9 mL of iodine solution in a ratio of 1:2 at 100°C, 41.3 mL in a ratio of 1:3 at 200°C, and a final volume of 43.9 mL in a ratio of 1:3 at 300°C. It was observed that iodine solution containing sodium thiosulfate increased as the temperature rose. However, the iodine levels decreased with the rise in temperature. Researchers^2-15 indicated that the qualities of activated carbon were evaluated based on the iodine number. The iodine number adsorption is a simple and quick way to determine the adsorption quality of an activated carbon sample.

3.2

CO₂ Adsorption of activated carbon of 325 μm and 600 μm

Using the highest iodine values from both samples, researchers were able to determine the CO₂ adsorption capacity of each adsorbent, regardless of the adsorption temperature. This finding shows that a 45-minute retention time increases CO₂ gas pressure, which in turn increases CO₂ adsorption capacity for each adsorbent. In Table 3, using H₃PO₄, it was observed that the adsorption capacities of size 325 μm for AC at final pressures of 21.0 psi and 4.2 psi were found to be 1.2204 mmol/g and 0.0901 mmol/g, respectively at a temperature of 26°C, implying that the adsorption capacity decreases with decreasing initial pressure. A similar trend was observed for a temperature of 40°C, as depicted in Figure 8. The same situation was observed in Table 4 when KOH was used for temperatures of 26°C and 40°C.

Figure 8.

Adsorption capacity for 325 μm at 26°C and 40°C using H₃PO₄.

Table 3.

CO2 adsorption capacity for 325 μm sample activated carbon (AC) using H3PO4.

Table 4.

CO2 adsorption capacity for 325 μm sample activated carbon (AC) using KOH.

In Table 5, when H₃PO₄ was used, it was observed that the cumulative adsorption capacities were 0.8196 mmol/g and 0.0787 mmol/g at ultimate pressures of 22.4 psi and 4.3 psi, respectively, at temperature 26°C. The same trend was seen for a temperature of 40°C, as shown in Figure 9. The working principle is that CO₂ molecules can enter the pore spaces in adsorbents due to the large impact forces they use to penetrate the porous structure, resulting in a high adsorption capacity. A similar situation was observed in Table 6 when KOH was used at temperatures of 26°C and 40°C. Figure 10 shows the CO₂ adsorption capacity of activated carbon for sizes 325 μm and 600 μm at 26°C using H₃PO₄.

Figure 9.

Adsorption capacity for 600 μm at 26°C and 40°C using H₃PO₄.

Figure 10.

CO₂ adsorption capacity of activated carbon with particle sizes of 325 μm and 600 μm at 26°C using H₃PO₄.

Table 5.

CO2 adsorption capacity of 600 μm sample activated carbon (AC) using H3PO4.

Table 6.

CO2 adsorption capacity of 600 μm sample activated carbon (AC) using KOH.

It was also observed that adsorption capacity decreased as the temperature of adsorption increased, as seen in Tables 3-6. When compared with AC of size 600 μm, AC of size 325 μm showed the highest capacity of 3.1845 mmol/g when heated at 26°C, while AC of size 325 μm showed the highest capacity of 2.0494 mmol/g when heated at 40°C. This is a significant difference. According to literature, this trend can be related to the fact that the porous structure of adsorbents collapses at higher temperatures, which reduces the adsorption capacity of the adsorbent^11-13.

In Table 6, at 26°C and 40°C, the activated samples showed less adsorption capacity than AS of 325 μm in Table 4, despite being impregnated with potassium hydroxide for 72 hours, as revealed by the study’s results. The adsorption capacity of a porous material is a function of its porosity, therefore activating agents may have blocked pores that could be used for CO2 absorption, thereby limiting the substance’s storage capacity. This can be explained by its larger porosity, which results in more activated carbon porosity, as shown in Figure 11, as opposed to AC of size 600 μm, which has more restricted access to its pore spaces and hence lower CO₂ adsorption capacity. Additionally, the highest CO₂ adsorption capacity was found for AC of 325 μm at 26°C and pressure of 25 psi, with a value of 1.5362 mmol/g (activated carbon of 325 μm had a value of 1.1230 mmol/g at 40°C and pressure of 25 psi as the second-best performing adsorbent); this is because activated carbon of 325 μm 26°C has an adsorption capacity. The findings of this study demonstrate that 600 is too small for CO₂ adsorbents because it clogs the pore spaces and reduces CO₂ absorption capacity under post-combustion conditions.

Figure 11.

Effect on particle sizes on CO₂ adsorption capacity at 40°C using H₃PO₄.

It was shown that carbon activated with H₃PO₄, KOH and commercial AC at 26°C and 40°C had adsorption capacities of 5, 10, 15.0, 20.0 and 25.0 psi at these temperatures as shown in Tables 3-6.

3.3

Comparison of CO₂ adsorption of commercial AC (CAC) and laboratory prepared AC (size 325 μm)

CO₂ adsorption capabilities at 26°C were studied between CAC and AC and indicated that AC had a much higher adsorption capacity at P1 pressure of 15 psi than CAC (1.0311 mmol/g), as depicted in Table 7. As Reference⁶ noted, AC produced from coconut shell outperformed a commercial AC in CO₂ adsorption at post-combustion circumstances; this is consistent with the findings of this study. Using cocoa pod waste as a precursor, laboratory-prepared AC has an enhanced capacity for CO₂ adsorption because of its low micro-porosity; this is due to the fibrous and cellulose-based material that is used to make AC.

Table 7.

CO2 adsorption capacity for commercial AC (CAC).

Figures 12 and 13 depict the CO₂ adsorption capacity (CAC) versus initial pressure (P1) for H₃PO₄ and KOH activating agents (formulated) and commercial AC. As seen, the CAC of the formulated exhibits similarity. Similar observations were reported elsewhere^16-17.

Figure 12.

Comparison of CO₂ adsorption capacity of H₃PO₄ and KOH activating agent of size 325 μm at 26°C.

Figure 13.

Comparison of CO₂ adsorption capacity of commercial activated carbon (CAC) and laboratory prepared AC of size 325 μm at 26°C using KOH activating agent.

3.4

Comparison of AC with other prepared AC in literature

For the sake of comparison, the CO₂ adsorption capability of the activated carbon (AC) generated from agricultural waste at 15 psi and 26°C under similar post-combustion conditions is shown in Table 8. The AC produced in this study was the most promising precursor for adsorption under post-combustion circumstances compared to other works.

Table 8.

Comparison of AC synthesized by other authors for CO2 capture and storage.

4.1

Recommendation

The following recommendations are proposed in the study: