Valorisation of biomass combustion ash in preparation and application of activated carbon for CO₂ adsorption

Abstract

Carbon capture and storage is a proposed pathway for mitigation of CO2 emissions, suggesting bioenergy with carbon capture and storage to be a net-negative power generation technology. Combustion of sustainable biomass produces a waste residue, biomass combustion bottom ash, that is commonly landfilled. However, valorisation of biomass combustion bottom ash to prepare solid carbonaceous adsorbents can address the issues of both waste management as well as CO2 emissions simultaneously. Therefore, in this thesis, extraction, activation (both physical and chemical) of BA-derived carbon followed by pelletisation and chemical modification of the adsorbents (accompanied by extensive characterisation of the different samples) have been investigated. These campaigns were conducted using appropriate design of experiments techniques to elucidate the main effects and any potential interactions of the plethora of studied parameters. It was found that chemical activation facilitated a nearly four-fold increase in CO2 uptake (1.29 mmol/g at 1 bar and 50 °C) compared to the virgin extracted carbon (0.34 mmol/g), whilst physical activation allowed the uptake to double (0.69 mmol/g). However, the physically activated sample presented a high working capacity (96%) in addition to other promising (e.g. environmental, monetary) features. Therefore, it was selected for chemical modification and pelletisation studies. The conducted modification facilitated a ~35% increase in adsorption capacity, whilst investigation of different pelletisation routes revealed crucial differences between the approaches. For instance, the pelletised-thenactivated (P-A) sample adsorbed 40% more CO2 than its activated-then-pelletised counterpart. The latter pellet presented a crush strength of 0.794 N/mm, whilst the PA carbon had inferior mechanical properties (below measurement threshold). As such, this thesis concludes valorisation of BA via extraction and preparation of carbonaceous sorbents to be feasible. Future work may now focus on development of the pelletisation technique (with industrial-grade equipment) as well as investigations of sorption properties from a synthetic flue gas stream.