Porous biochar material for carbon capture and wastewater treatment

Abstract

As a derivative of biomass energy, biochar materials have been widely used in soil improvement, soil remediation, water pollution treatment, carbon fixation and other fields. The biochar generated by the pyrolysis and carbonisation of biomass has a high and stable carbon content, reducing the carbon emissions from biomass combustion. In addition, bio-oil and syngas produced during biomass pyrolysis are also potential fuels that can replace fossil energy in the future. Therefore, biochar is considered an eco-friendly material due to its many advantages.

Firstly, based on literature research, the thesis investigates the CO₂ adsorption performance of various biochar materials under different activation conditions. CO₂ adsorption studies were mainly carried out with bamboo charcoal, charcoal and coconut shell charcoal as the main adsorbents. In addition, porous structure generating was carried out under specific conditions of physical and chemical activation to enable the biochar to increase its adsorption activity. Meanwhile, there are no uniform standards regarding the CO₂ adsorption activity of biochar, and the work also uses three different assays for comparing the CO₂ capture capacity of biochar. Additionally, it was concluded that specific chemical activation is helpful for the improvement of the CO₂ adsorption capacity of lignocellulosic biochar, where the 15 % CO₂ adsorption capacity (25°C, 1 bar) of bamboo biochar and wood pellet biochar increased from 0.59 mmol g^-1 and 0.28 mmol g^-1 to 0.88 mmol g^-1 and 0.66 mmol g^-1, respectively, under the activation condition of an equal mass of KOH and 550°C activation for the one-hour duration.

Secondly, after studying the mechanism and properties of carbon dioxide adsorption by biochar, charcoal was used to carry out 15% carbon dioxide adsorption, while the effect of water vapour during the adsorption process was explored. As biochar is used as a carbon dioxide adsorbent, environmental influences have to be considered. Therefore, the presence of large amounts of water vapour in the flue gas of a plant is one of the factors affecting the adsorption activity and stability of biochar that is worth discussing. As the results obtained from the experiments, the capture ability of biochar decreased as CO₂ concentration decreased from 100% to 15%. The relative humidity had a negative impact on CO₂ capture over biochar. The CO₂ capture capability of biochar materials treated by steam decreased by 31.38% to 62.89% as the relative humidity rose from 8.8% to 87.9%. Furthermore, the lifetime of biochar samples at various relative humidity shows that increased relative humidity had a negative impact on CO₂ adsorption due to water molecules occupying active sites.

In addition, it is also necessary to try using biochar for direct carbon capture after it has been verified that it can adsorb carbon dioxide. Direct air capture is an emerging technology that has specific social development prospects. In this work, KOH-activated bamboo biochar shows a CO₂ capture capacity of 3.49 mmol g^-1 (25°C, 1bar), and it is used as an adsorbent to perform direct air capture using a fixed-bed reactor with an ambient CO₂ capture capacity of up to 51.74 μmol g^-1. Furthermore, the effect of relative humidity on CO₂ adsorption by biochar was investigated and exhibited promising stability under 2.7% relative humidity. However, the biochar activity for CO₂ capture is reduced to 63.88% under 67.1% humidity after 50 cycles.

Eventually, the application of porous biochar for phosphorus-containing wastewater treatment has also been investigated to study biochar's sustainability. In this study, the irrigation of dirty water phosphorus treatment by biochar samples derived from digestate of anaerobic digestion was investigated. The biochars were further activated by steam activation with different duration time, and KOH activation with different introducing ratios, the textural properties of biochars was optimised after activation from the aspect of biochar characterisation. Notably, the biochar demonstrates a remarkable adsorption effect of phosphorus, with an adsorption efficiency of 8.99 mg g^-1. Besides, the effect of biochar dosage on phosphorus removal, adsorption kinetics, and thermodynamic isotherms are studied. According to the adsorption kinetics, the adsorption of phosphorus from dirty water fits the Elovich equation (R² = 0.95). Furthermore, the thermodynamic isotherm results illustrate the process of phosphorus removal by biochar is endothermic (ΔH0 =17.93 kJ mol^-1) and spontaneous (ΔS=96.24 J mol^-1 K^-1). Therefore, this work suggests a promising solution to phosphorus-related environmental challenges in industry and agriculture.

Date of Award	Jul 2023
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	Chunfei Wu (Supervisor), Bo Xiao (Supervisor) & Peter Robertson (Supervisor)