Waste-derived biochar for CO₂ sequestration

Carbon dioxide (CO₂) emissions caused by the extensive energy consumption have been regarded as one of main cause of global warming. Therefore, it is crucial to develop efficient and cost-effective technologies for CO₂ capture. Various separation processes are now under R&D developments. Among them, CO₂ adsorption has been proposed to be the most efficient technology in separating CO₂ from flue gas. However, how to cut down the cost in the process of CO₂ adsorption should be a key issue if the large-scale applications are carried out. In this chapter, low-cost biochars from waste valorization are investigated in terms of adsorbent synthesis, modification methods, CO₂ adsorption capacities, reusability, and selectivity. The cheap resources of wastes include unburned carbons in fly ash, agricultural wastes (bagasses, olive stone, almond shells, coffee grounds, and so on), organic sludges, plastic wastes, and wood wastes. In order to increase the CO₂ adsorption performance, further surface modifications (amine functionalization, metal doping, amoxidation, KOH activation) have also been made onto the waste-derived biochars after the carbonization process. These biochars with unique properties were proposed for the CO₂ capture at low temperatures (25-75°C) and high temperatures (75-150°C). The abundant waste-derived biochars used as CO₂ adsorbents are not only cost-effective but also lessened the environmental concerns (landfills burden and CO₂ emissions).