Biochar as the effective adsorbent to combustion gaseous pollutants: Preparation, activation, functionalization and the adsorption mechanisms

The massive combustion utilization of fossil fuel in human industrial activities, such as power plants, waste incineration, and kiln combustion for cement production, would emit serious gaseous pollutants (SO₂, NO_x, VOC, and mercury), aerosols and CO₂. There is a growing interest in using novel solid sorbents, i.e., activated carbon (AC), zeolites, carbon nanotube, carbon molecular sieve, and MOFs (metal-organic frameworks), for their ability to capture gaseous pollutants from combustion flue gas through adsorption processes. However, these emerging alternatives are generally expensive, limiting large-scale industrial utilization. Biochar, as a stable carbon-rich solid by-product from biomass thermal treatment, is not only capable of replacing coal as fuel in power plants but also widely reported to be an effective and cheap sorbent for removing the gaseous pollutants in flue gas, including SO₂, NO_X, Hg, CO₂ and VOC, due to its high porosity and specific surface area and surface functional groups. In this review, the physical activation, chemical activation, and novel modification methods including microwave, ultrasonic, plasma, ball-milling, and molten salts were introduced as their optimization to the porous properties and active surface functional groups for biochar sorbents. The functionalized treatments including metal, ammonia/amines, and halogen modification on activated biochar were reviewed to observe the further improved adsorption performance of biochar, for possible engineering application. The abundant amounts of the oxygenic functional group increase the number of active sites onto which NH₃ or Hg can be adsorbed, resulting in higher NO and Hg removal efficiencies. Oxygenated anchoring sites are also effective intermediate stage to introduce the nitrogen functional groups, which are generally more effective than the porous texture for acidic SO₂ and CO₂ adsorption, especially at adsorption temperature higher than ∼100 °C. The redox reactions of metal catalyst in biochar and the improved adsorption ability of NH₃ and Hg mainly determine the removal performance of biochar for NO_x and Hg⁰. The halogen addition to form C-halogen groups can transform Hg⁰ into mercury halide retained on the biochar. The practical removal performance of various gaseous pollutants is affected by the adsorption conditions, such as adsorption temperature, humidity and impurities concentrations in simulated flue gas, selectivity, synergistic adsorption of typical gases, and regeneration capacity. The adsorption isotherm models and the adsorption kinetic models are helpful for predicting the adsorption amount and controlling mechanism and calculating the energy of adsorption to indicate the strength and potential of adsorption and desorption. Finally, the review presents the research gaps on biochar adsorption mechanisms, industrial application and evaluation of economy and energy-saving analysis.