Enhanced CO2 capture using organic acid structure modified waste eggshell derived CaO sorbent
Graphical Abstract
Introduction
The atmospheric concentration of carbon dioxide (CO2) has risen sharply due to the industrial revolution, which has raised serious issues such as global warming, climate change and acid rain [1]. Reduction in CO2 emissions can be achieved by focusing on these options such as improved energy efficiency, utilisation of low carbon fuels and also by CO2 capture and storage [2], [3]. For decades to come, fossil fuel will remain a major source of energy production as renewable energy remains limited and costlier and the emissions are expected to rise as per directions from International Energy Agency (IEA). It will therefore be necessary to capture and store CO2 to reduce the global emissions which is one of the sustainable development goals (SDGs) [4]. Various techniques were employed from direct capturing through membrane techniques. Membranes remove pollutants such as dyes, CO2 capture and can be used for other applications such as SOx and NOx capturing and removal for gas cleaning systems [5], [6], [7]. Currently, commercially operated CO2 capture technology includes using amines solutions such as monoethanolamine (MEA) or methyl diethanolamine (MDEA) [8]. The two major disadvantages of this process such as high energy penalty and solvent degradation leads to the development of degradation products which are possibly harmful to humans and the ecosystem [9], [10].
Calcium looping cycle (CaL) is a second-generation CO2 capture technology consisting of two chambers i.e., carbonator where CO2 adsorption occurs at 650 °C while calciner regenerates CaO at operating at 900 °C [11]. The basic reaction is presented in reaction (R1).
The sharp deterioration of the sorbent potential over many cycles due to sintering and attrition are the key obstacles for the industrial implementation of this process [12]. Utilisation of cheap and renewable materials to synthesise effective CaO sorbents is one of the viable alternatives to facilitate calcium looping technology competitiveness [13], [14], [15]. Several techniques are available which can reduce these problems such as reducing the CaO particle size, improving the porosity of CaO, enhancing reactivity using surface modified CaO or using inert phases such as MgO, Al2O3 and TiO2 for better CO2 capture performance [16]. The other sources of calcium such as limestone, dolomite and huntite are also showed a good competitiveness in this process [17], [18], [19], [20]. Other than these, some of the waste derived calcium sources includes cockle shell, clamshell, eggshells, oyster shell, shellfish shell are also promising sources [21], [22], [23] along with carbide slag, lime mud, phosphogypsum [24], [25], [26]. In most foodstuffs, egg and chicken meat are used abundantly and around 5150 million eggs were produced every year and it increase exponentially [27]. Hatchery generates eggshell in many tons of waste every day and retains large disposal costs worldwide. Typical treatment methods for solid hatchery waste include landfilling [28], composting [29] and incineration [30]. All these methods are costly and rip the chicken meat industry millions of dollars yearly, so the better utilisation methods are requisite [31]. Hence the consumption of eggshell waste for capturing CO2 reduces the solid waste and the CO2 emissions is an economical route. The CaO also shows sintering and decayed easily hence required some modifications over the surface to enhance the surface morphology and crystallographic properties. This work aims to produce an economical sorbent for calcium looping cycle, along with minimising the deleterious environmental impacts. Hence, it could aid in cleaning CO2 emissions and waste simultaneously.
In this study, the waste eggshell was utilised to synthesise the CaO sorbent. The CaO was treated using various organic acids to modify its surface structure to enhance the CO2 adsorption for multiple cycles. The surface modified sorbent was characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM), elemental dispersive spectroscopy (EDS), X-ray fluorescence (XRF), BET surface area and thermogravimetric analysis (TGA). The sorbent was then testing for various cycles and analyse the capacity of CO2 uptake in the fixed bed TGA system and comparative analysis for various acid treated sorbent is reported.
Section snippets
Sorbent preparation
The sample preparation scheme is presented in Fig. 1. CaO was prepared (Rout I) from waste eggshell, collected from household waste locally. The eggshell was washed with deionized water to remove impurities. The sample was grinded and sieved to remove large particles using 200 µm mesh. Calcination of samples were performed by using a tube furnace (SK2-3-12TPA2) at 850 °C with N2 (99.8%) atmosphere for 2 h. In Route II the grinded sample of eggshell was treated via six organic acids for surface
Physicochemical properties
The diffraction peaks of eggshell and calcined eggshell (CaO) are presented in Fig. 3. All the identified phases associated with the XRD pattern of both samples are also presented in Table 1. In case of fresh eggshell, the major peak represented hexagonal phase CaCO3 (PDF# 05-0586) main peak at 29.4° with hkl (104), whereas the calcined eggshell (CaO) showed cubic phase CaO (PDF# 37-1497) main peak at 37.5° with hkl of (200). These identified structure and peaks are in good agreement with
Conclusion
This work aimed to recycle eggshells as an environmentally sustainable alternative to conventional sorbents for CO2 adsorption. Further characterisations were carried out to check its morphological feasibility for CO2 uptake application. The fresh eggshell displayed CO2 uptake capacity of 0.57 g CO2 per g sorbent for the 1st cycle (CaO conversion 82.5%) whereas the CO2 uptake was 0.25 g CO2/g sorbent (CaO conversion 29.2%) The average sorption decay of eggshell 19.75 mg CO2/cycle. Using the
CRediT authorship contribution statement
Azra Nawar, Majid Ali and Asif Hussain Khoja: Experimentation and data collection, Writing - original draft. Majid Ali, Adeel Waqas and Mariam Mahmood: Conceptualization, Methodology, Writing - original draft, Supervision. Azra Nawar, Asif Hussain Khoja and Mustafa Anwar: Data validation, Reviewing and editing references. Asif Hussain Khoja and Adeel Waqas: Editing and proofing of manuscript.
Declaration of Competing Interest
There is no conflict of interest.
Acknowledgements
Authors are grateful to National University of Sciences and Technology (NUST), Islamabad for providing the facilities. Authors also thank to Mr. Qamar-Ud-Din (Lab Technologist) USPCAS-E NUST for his technical assistance in this work.
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