**1. Introduction**

Growing population and economy race are the two main concerns regarding the increase in emissions of greenhouse gases (GHG) and, subsequently, the accelerating climate change and global warming. Carbon dioxide is the first responsible for these unwanted effects [1]. Nowadays, reducing CO2 emissions is one of the most challenging issues facing humanity [2]. Global climate reports show that the average world temperature is steadily increasing with respect to 20th century data [3]. Besides, worldwide researchers recognize the need for a sustainable development and more equity, including poverty eradication, and provide an ethical foundation for limiting the effects of climate changes. The rate of atmospheric accumulation of GHGs and the capacity to address its mitigation and adaptation differ for each nation. Often, many of the countries most vulnerable to climate change contribute little to GHG emissions.

Intergovernmental panel on climate change (IPCC) recently reported on the impacts of global warming of 1.5 ◦C above pre-industrial levels and the related global greenhouse gas emission pathways [4]. The attention is on the decision to adopt the Paris Agreement [5,6].

Comprehensive and sustainable strategies in response to climate change should consider the co-benefits, adverse side-effects, and risks inherent to both adaptation and mitigation options. European policy options emerge in the European Union (EU) Emission Trading System (ETS) that involve the promotion of investment on clean and low carbon technologies in power and heat generation, energy-intensive industry sectors, materials, and chemicals' production. The legislative framework of the EU Emission Trading System (ETS) achieved the EU's 2030 emission reduction targets [7] and contributed to the 2015 Paris Agreement. Now, the goals include keeping the rise in temperature below 2 ◦C, that is, CO2 below 450 ppm [8].

Practical ways to meet these goals could be planting trees, caring the existing forests, rebuilding of soils, and developing the biomass-to-energy chain with coupled with carbon capture and storage.

Carbon dioxide capture/separation onto solid matrices is broadly studied for reducing greenhouse gas discharge [9]. Also, biofuels and bio-methane are strategic for fuel transition to sustainability, or as a reagen<sup>t</sup> in the steam and dry reforming and catalytic processes. A paradigm is biogas production by anaerobic digestion of organic matter. The mixture mainly consists of 40 vol% to 75 vol% methane and 15 vol% to 60 vol% carbon dioxide [10,11].

Biogas cleaning and upgrading to bio-methane require to remove water, foam, dust H2S, and trace components, as well as CO2 [11] and bio-methane production, in Europe is spreading progressively [12]. In Italy, such technology struggles to spread with only five operating plants and a total upgrading capacity of 500 Nm<sup>3</sup>/h. The delay is mainly owing to the imprecise legislative references regarding grid injection techniques. As a mean to overcome these limits, an effort was made toward the development of implementing rules and guidelines for accessing the incentives.

From a purely technological point of view, several techniques are available for commercial use, at laboratory and industrial scale [13–18]. CO2 capture technologies, like absorption and adsorption, are proliferating as commonly used capture techniques worldwide, as proven by the several patent applications and published articles [19]. Adsorbent media include activated carbon, alumina, metallic oxides, and zeolites. The regeneration of the adsorbent is carried out by temperature or pressure swing adsorption (TSA or PSA) [20]. For TSA, solid adsorbents with a lower heat capacity are claimed for reducing the energy required for regeneration [21–23].

The most important property of an adsorbent is its CO2 adsorption capacity, which depends strongly on the pore structure, the surface area, and the type of functionalization. Besides, the capacity depends on the partial pressure of CO2, temperature, and humidity [24,25]. Absorption proved to not be economical for treating flue gas streams with CO2 partial pressures lower than 15 vol% [26]. Chemical absorption has relatively high selectivity, but also high energy consumption for regeneration, chemicals' make-up, and high environmental impact [27,28]. Taking into account the potential role of porous carbon materials for CO2 capture, the authors propose the production of these materials starting from renewable sources, mainly residual biomasses.

Hydrothermal carbonization (HTC) is an alternative synthesis method to produce precursors for high value-added renewable carbon materials from residual lignocellulosic biomasses [29]. This thermochemical conversion occurs in hot compressed water, at a relatively low temperature under autogenous pressure [30]. HTC optimization was studied considering the re-use of the liquid phases in the process itself [31,32], the online monitoring of the carbonization time-course [33,34], and the recovery of valuable platform chemicals from the liquid [35,36].

Hydrochar attracts the attention for several potential applications, mainly in support of the energy production chain [37,38]. Nevertheless, owing to its coal-like structure, hydrochar could be favorably transformed into porous activated carbon with tunable morphologies and porosities, which is hard to achieve using traditional pyrolytic methods, with a surface area up to 3000 m2g−<sup>1</sup> and pore volume in the range of 0.5–1.5 cm3g−<sup>1</sup> [39,40].

The focus of this paper is on the separation of CO2 for the upgrading of biogas to bio-methane by PSA onto HTC from silver fir sawdust, a waste product available in Central Europe and *Abies* species available worldwide [41].

#### **2. Materials and Methods**

#### *2.1. Synthesis of Carbon Porous Sorbent*
