GENERATION OF ACTIVATED CARBON FROM SPENT COFFEE GROUNDS: PROCESS OPTIMIZATION, KINETICS AND CO2 CAPTURE
Date
2022-09-21
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Degree Level
Doctoral
Abstract
Carbon dioxide capture technology is gaining popularity owing to the environmental concerns and deterioration of the climatic conditions related to atmospheric CO2 emission. Of the candidate, lignocellulose biomass samples have been considered a promising source for producing carbon-based adsorbents that can be utilized to capture recalcitrant CO2 from the post-combustion capture facility. Recently, the removal of CO2 using activated carbon (AC) has gained immense attention owing to its environmentally friendly characteristics and low cost for synthesis. Conversely, with the increasing population, the demand for coffee consumption is also accelerating. The generation of coffee residues owing to the increased consumption would increase and demand effective utilization and management. The research primarily focused on utilizing the waste generated from the coffee industries, namely spent coffee grounds (SCG) and coffee husk (CH), to evaluate their potential in synthesizing carbon-based adsorbents for CO2 removal under the post-combustion capture scenario and propose a cost-effective AC production strategy from lignocellulose-based biomass. This study focused on investigating the influence of different thermochemical conversion techniques on the physicochemical properties of biochar, followed by assessing the impact of activation parameters and the effect of deep eutectic solvent (DES) on the physicochemical characteristics of AC and the removal of CO2. Moreover, a techno-economic analysis was performed to assess the economic feasibility of biomass conversion technology used to synthesize AC. Overall, this study is divided into five research objectives with multiple sub-objectives.
The first phase of this research conducted a parametric study of the conventional torrefaction technique. In this regard, SCG and CH were used as the lignocellulose-based precursor. The impact of torrefaction parameter on the physicochemical transformation of biomass was examined by varying each parameter (torrefaction temperature and reaction time) independently. The influence of torrefaction conditions on the precursors' physicochemical changes and structural transformations were analyzed using diverse analytical techniques. This is followed by evaluating the candidacy of the corresponding torrefied biomass samples towards CO2 capture performance. The physicochemical transformation of torrefied biomass samples mainly synthesized in severe torrefaction conditions (300 ℃ and 1 h) demonstrated their candidacy for CO2 removal. The equilibrium CO2 adsorption capacities of SCG-300-1 and CH-300-1 were 0.38 and 0.23 mmol/g, respectively, at 25 ℃ of column temperature and in the presence of 30 vol% CO2 in N2. Comparatively, the torrefied biomass sample derived from SCG displayed superior CO2 capture performance under a similar capture scenario than CH derived torrefied biomass sample owing to the textural properties and surface chemistry developed during the torrefaction process.
The findings from phase one raised numerous research questions. For instance, is it necessary to study the thermal treatment of biomass at a temperature higher than 300 ℃ like undergoing slow pyrolysis to elevate the removal of tar from the carbon matrix and accelerate the volatilization reactions to generate biochar with enhanced textural characteristics and improved surface chemistry for superior CO2 capture performance? Furthermore, evaluating the kinetic and thermodynamic parameters before performing a high-temperature thermal treatment (slow-pyrolysis) is essential to obtain necessary information regarding the feedstock and the process parameters. Hence, phase two implemented the kinetic and thermodynamic study of the slow pyrolysis technique using SCG and CH. In this regard, the kinetic data of SCG and CH during slow pyrolysis were obtained to fit the thermogravimetric data taken using the TGA-DTG analyzer. The kinetic parameters were estimated using different iso-conversational methods followed by estimating thermodynamic parameters. In this regard, the conversion technique using SCG showed higher activity and would be efficient in terms of energy requirement as the requirement of activation energy for SCG was low (90.4-141.7 kJmol-1) compared to CH (96.0-162.1 kJmol-1). Also, SCG can be identified as a superior lignocellulose-based precursor for further valorization than CH in terms of physicochemical properties.
Appropriate utilization of biochar derived from slow pyrolysis of SCG could improve the overall economics of the post-combustion CO2 capture facility. In the third phase, the impact of slow pyrolysis process parameters was assessed on biochar yield and specific surface area (SBET). Secondly, this research aimed to elucidate the correlation of pyrolysis temperature as a critical parameter with textural properties, surface composition, aromatic structure, and CO2 mitigation efficiency of SCG-derived biochar samples. The results demonstrated that biochar yield reduced with the rising pyrolysis temperature, but biochar's textural characteristics, surface functionalities and aromatic structure were positively correlated with the increasing temperature conditions. Correspondingly, the impact of pyrolysis process parameters on biochar yield complemented the findings of the second phase of this study. In this study, SCG-600 showed the highest equilibrium CO2 uptake of 2.8 mmol/g in 30 vol% of CO2 (in N2) and 30 ℃ (column temperature). It was evident that the well-developed textural characteristics and porosity, availability of basic surface functional groups, and aromatic structure of SCG-600 influenced the CO2 removal performance owing to the enhanced acid-base interactions and the presence of Vander Waals force of attraction.
Treating the promising biochar sample derived from SCG at a higher temperature in the presence of a suitable activating agent becomes necessary to improve the physicochemical properties of the carbon-based adsorbent for improved CO2 capture performance. Therefore, the fourth phase's main objective was to optimize the two-stage CO2 activation process of SCG-600 using the Box-Behnken design (BBD) method. The impact of activation parameters (activation temperature, holding time, and CO2 gas flow rate) were investigated on surface area (SBET) and AC yield. In addition, the influence of thermal pre-treatment techniques (torrefaction and slow-pyrolysis) for the two-stage physical activation technique were evaluated and compared in terms of textural characteristics and CO2 adsorption performance. Further, the impact of tailoring the surface functionalities of the pristine AC sample was evaluated using deep eutectic solvent (DES) and compared with pristine AC (AC-CO2) through complementary analytical techniques. CO2 breakthrough experiments were performed under different ranges of temperatures and CO2 concentration in N2 to investigate the influence on adsorption performance and selectivity. The estimated highest equilibrium CO2 uptake for two sets of ACs (pristine and DES-treated) were 4.34 mmol/g and 5.5 mmol/g, respectively, at 25 ℃ and 15 vol % of CO2 in N2. The DES-treated AC displayed a superior adsorption capacity, selectivity, and regeneration ability due to a well-developed porous structure, morphology, and availability of a wide variety of desired functional groups that facilitated the CO2 capture process under a simulated post-combustion scenario.
A comparative techno-economic assessment and sensitivity analysis were performed using different AC production scenarios from SCG. The economic viability of the AC production technique using SCG as the potential precursor was assessed based on the discounted cash flow analysis (DCFA) technique. The minimum selling price (MSP) of AC samples derived from different scenarios evaluated were US $0.15/kg, $ 0.21/kg, and $ 0.28/kg, respectively. Comparatively, the MSP of DES functionalized AC was lower than commercial AC (US $0.45/kg). A positive net rate of return (NRR) for all the production scenarios indicates that AC production using dried SCG is profitable from an economic point of view. The sensitivity analysis demonstrates that the feedstock cost and utility cost influence the MSP of AC production.
Description
Keywords
Spent coffee grounds, Biomass, Activated carbon, Post-combustion CO2 capture, Adsorption
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Chemical and Biological Engineering
Program
Chemical Engineering