Converting the biomass into porous carbons involves carbonization (pyrolysis) and activation that can be of physical and/or chemical types. Physical-activation is a two-step process: the raw material is carbonized in the absence of O₂ at temperatures between 400–850°C, followed by activation of the resulting char with oxidant/gasifying gases like steam, air, N₂, O₂, NH₃, CO₂ or a mixture of these gases, at temperatures around 600–1000°C (Marsh and Rodríguez-Reinoso, 2006; González-García, 2018). The activation with CO₂ produces porous carbons with narrow micropore size distribution, providing optimum pore size for CO₂/CH₄ separation, while steam activation generates carbons with wider pore size distribution and smaller micropore volume (Marsh and Rodríguez-Reinoso, 2006). Nevertheless, in the physical-activation there is a poorer control of the porosity. The reactions during physical-activation can form surface oxygen functional groups, while activation with NH₃ adds N-containing groups on carbon surface; however, this is usually coupled with other gas to provide more porosity (Tan et al., 2017). The high temperatures usually used in physical-activation represent an energetic disadvantage.

Chemical-activation can be a one-step or two-step method since the impregnation with the activating chemical (dehydrating agents and/or oxidants) can be made directly in the biomass or in the resulting char from the first step of carbonization. After impregnation, the mixture of precursor and activating agent is heated under inert atmosphere at temperatures between 400–800°C (Marsh and Rodríguez-Reinoso, 2006; González-García, 2018). As physical-activation, chemical-activation provides porosity development and functional groups at carbon surface. Acids, alkalis, and salts like H₃PO₄, H₂SO₄, ZnCl₂, K₂CO₃, NaOH, and KOH are usually used in chemical-activations of biomass precursors (Yahya et al., 2015; González-García, 2018).

Chemical-activation usually provides biomass-derived porous carbons with high surface areas and a good control of the porosity, but washing the produced carbon to remove the residual activating agent present in the carbon matrix turns the process into a time- and energy-consuming one, and environmentally less friendly.

When producing biomass-derived porous carbons, it is possible to tune their properties by an appropriate choice of the precursor and activation conditions. Nonetheless, the resulting material can be further tailored, specifically the surface chemistry properties to increase the CO₂ uptake capacity. Modification treatments can be envisaged to add and/or increase relevant functional groups on the surface of the carbon to enhance CO₂ retention. Increasing the basicity of the carbon is the most efficient way to improve the adsorption efficiency toward CO₂ uptake, namely through the removal of acidic functional groups from carbon surface and/or introduction of nitrogen groups that provide basic sites able to attract the acidic CO₂ (Adelodun et al., 2015; Rashidi and Yusup, 2016). Depending on the biomass precursor and if an acid is used as activating agent, several oxygen functional groups with strong or mild acidity might be present on the carbon surface. To remove these oxygenated groups, heat treatments under inert (N₂, Ar, He) or H₂ atmosphere are usually performed, but this requires very high temperatures (800–1000°C) (Shafeeyan et al., 2010).

Heat treatment with ammonia can be employed at temperatures between 200 and 1000°C (Rashidi and Yusup, 2016). Since carbon surface is typically non-reactive to NH₃, pre-oxidation of the surface is required prior to amination. After the treatment, the carbon surface is usually enriched with nitrogen functionalities like −NH₂−, −CN, pyridinic, pyrrolic, and quaternary N₂ (Shen and Fan, 2013). Nitrogen-rich biomass precursors, such as chitosan (Fujiki and Yogo, 2016), crab and prawn shell (Chen et al., 2015; Gao et al., 2016), and protein enriched biowaste (Huang et al., 2015; Shi et al., 2019), can directly provide porous carbons enriched with nitrogen functionalities.

Metal impregnation has been also employed to increase the carbon CO₂ uptake capacity and selectivity, although this strategy is not usual as nitrogen-doping. Metal oxides of alkaline-earth and transition elements can provide catalytic active sites able to interact with CO₂ although the presence of ultra microporosity remains an important feature.

Table 1 presents the most recent works (2020–2021) dealing with biomass derived-porous carbons for CO₂ uptake. The biomass precursor, activation conditions, textural properties and CO₂ uptake capacity are shown. It should be highlighted that most of the studies use powder carbons, and static CO₂ adsorption measurements are performed volumetrically using gas sorption apparatus or through thermogravimetric analysers. Pure CO₂ or synthetic gas mixtures under controlled conditions are typically used. Dynamic CO₂ adsorption breakthrough tests and real gas samples are scarcely studied for the biomass-based adsorbents, although these conditions are closer to industrial applications. Also, dynamic adsorption/desorption/regeneration cycles allow a more efficient utilization of the adsorbent capacity; however, most of the published studies do not consider this important feature. Considering the lack of knowledge about the performance of biowastes derived porous carbons in continuous flow systems and their regeneration, it is of utmost importance that studies move forward to reflect realistic applications.

The selection of a biogas upgrading technology depends mainly on plant location, investment availability, biogas composition, bio-CH₄ aim quality, and plant productivity. Adsorption-based processes like Pressure-Swing Adsorption (PSA) is one of the most established know-how used in gas separation/purification/capture applications (Riboldi and Bolland, 2017). Its compacted set of fixed-bed adsorption columns, operating with pressure modulation in a cyclic mode, use the adsorbent(s) to selectively adsorb and desorb the undesired gases like CO₂ in biogas (Esteves, 2005; Esteves and Mota, 2007; Augelletti et al., 2017; Canevesi et al., 2018). The selective adsorption occurs due to different equilibrium capacities of the species (equilibrium adsorbent) or distinct gas uptake rates (kinetic adsorbent) in the adsorbent’s surface. Activated carbons have demonstrated to perform effectively in relevant operating conditions, surpassing zeolites when CO₂ partial pressure overpasses a certain threshold (ca. 1.7 bar) (Riboldi and Bolland, 2017). This means that what really matters is not the total CO₂ adsorption capacity of the solid adsorbent at a given pressure, but the pressure difference necessary to be applied between adsorption and desorption to obtain a satisfactory gas separation in PSA (adsorption isotherms). Carbons have higher adsorption capacity than zeolites at high pressure, but zeolites are regenerated by vacuum desorption with very small pressure variation. Carbon materials have the advantages of high thermal/chemical/mechanical/moisture stabilities, electrical and heat conductivities and reasonable cost. Their affinities for CO₂ can be improved with functionalization (Lee and Park, 2015). Achieve higher CO₂ selectivities requires the development of better adsorbents that, additionally, need to be easy regenerable for proper application in a PSA system that, otherwise, would require reduction in partial pressures or increase in the operating temperature. Recently, MOFs appeared as potential highly selective materials for gas separation/capture (Ferreira et al., 2019; Ribeiro et al., 2019), although they still need further research related with their cost, particle shaping, and stability (structural, mechanical, moisture, aging, etc.).

Upon this scenario, the use of porous biocarbons opens a double opportunity, in the sense that biomass turns to be a useful resource that can contribute, thereafter, to capture CO₂ and separate it from biogas, aiming to upgrade it to bio-CH₄ (Álvarez-Gutiérrez et al., 2014). This pathway closes the loop regarding carbon neutrality and contributes significantly to the field of renewable energy. In the last 5 years, when crossing PSA, biogas upgrading and activated carbon topics in Web of Science (WOS) Core Collection, one gets only 16 publications. Adding CO₂ capture topic to this search, only nine articles are found. From those papers, only four of them are about biomass-derived porous carbons applied to PSA. Table 2 depicts those publications. The lack of research on bioadsorbents followed by their effective transfer to adsorption-based processes applied to biogas upgrading is still noticed in a sector so relevant to answer to the renewable energy and environmental concerns of the 21^st century.

Despite the good potential demonstrated by porous carbons developed from cherry stones, pine wood pellets, silver fir sawdust and modo bamboo, the few studies published in the last 5 years emphasizes the need to gather more data on biomass-based activated carbons for biogas upgrading under operational conditions similar to real cases. Besides exhibiting high selectivity and optimal uptake capacity for CO₂, one of the features to be considered is to obtain porous carbons with the desired particle size to be directly packed in fixed bed columns. This avoids high pressure drops in columns that turn the process unfeasible. Some of these biomass derived adsorbents are obtained as powders and therefore their shaping into pellets, granules or spheres is another challenge for the near future.