The progressive depletion of fossil fuels along with the need to mitigate global climate change associated to their use, have stimulated an increasing interest in alternative resources for energy generation and chemicals production (Baggio et al., 2008; Guo et al., 2015). Biomass is a promising, clean source of renewable energy and valuable chemicals primarily due to its abundance, wide distribution, and CO2 neutrality (Kanaujia et al., 2014; Ma et al., 2015). Biomass comes from a large number of different sources, and in a wide variety of forms. In particular, agricultural residues and food processing wastes from agroindustry represent an important source of biomass having widespread availability, especially in countries where economies are considerably based on activities related to the agroindustry (White et al., 2011; Bonelli et al., 2012; Long et al., 2013). Use of wastes or by-products generated from industrial processing as biomass feedstock is recommended in order to avoid competition with biomass derived from energy crops for the soil, an issue that has engendered controversy.
Thermochemical processing is a recognized route for conversion of biomass feedstock. Pyrolysis, i.e., thermal degradation of biomass in an oxygen-depleted atmosphere, is a key thermochemical process which enables conversion of biomass into value-added products mainly directed to energy applications (Gonzalez et al., 2008; Cukierman et al., 2012; Shi et al., 2012). They are often lumped into three groups: permanent gases, a pyrolytic liquid (bio- oil/tar), and a carbon enriched solid product (char or bio-char). Biomass pyrolysis has been extensively investigated to optimize the process for the production of bio-oil or charcoal and for the implications in the companion thermochemical processes of gasification and combustion (Di Blasi et al., 2010). Biomass pyrolysis is a complex process. The process kinetics as well as yields and properties of the pyrolysis products depend on various parameters, such us the origin and physico-chemical characteristics of the biomass, and the experimental conditions used (Shi et al., 2012; Guizani et al., 2014).
Besides, the advantage of using agricultural by-products or wastes as raw materials for the manufacture of activated carbons (ACs) has been highlighted. Activated carbons in powder or granular forms are used worldwide in several different applications, which include domestic and industrial uses, such as gas storage and delivery, removal of liquid and gaseous pollutants, purification and separation of gases, metal recovery, in catalysis, as a catalyst or catalytic support, and also in biomedical applications, among others (Kwiatkowski et al., 2011; Mezohegyi et al., 2012). The increasing demand of this adsorbent, primarily due to its growing use in applications pertaining to environmental pollution, has promoted the necessity to develop activated carbons from cheaper and readily available raw materials in order to favor their sustainable large scale production (Valente Nabais et al., 2008; 2013; Nabarlatz et al., 2012; Cukierman, 2013; El Sayed et al., 2014). Wood and coconut shell have been mostly used as conventional lignocellulosic precursors for the production of activated carbons. Agricultural solid wastes also investigated for the production of low cost effective ACs include olive wastes (Baccar et al., 2012), cherry stones (Duran Valle et al., 2005), shells from peach stone, almond and walnut (Girgis et al., 2007; Gonzalez et al., 2009); sugarcane bagasse (Blanco Castro et al., 2000; Bonelli et al., 2012), corncob (El Sayed et al., 2014), orange peels (Fernandez et al., 2014), rose stems (Cifuentes et al., 2013), among many others. Conversion of these wastes into ACs adds economic value, helps reduce the cost of waste disposal, and provides a potentially less expensive alternative to the existing commercial samples (Fernandez et al., 2014). Thermochemical processes are also used for biomass conversion into activated carbons. The two main processes applied for the development of activated carbons are the so-called physical or thermal activation, and chemical activation. Briefly, physical activation generally involves pyrolysis of the precursor, subsequently followed by gasification with an oxidizing agent, often steam or CO2, under strictly controlled conditions. On the other hand, chemical activation involves impregnation of the precursor frequently with a Lewis acid, such as zinc chloride or phosphoric acid, followed by pyrolysis of the impregnated precursor. Washing of the so-obtained product to eliminate the remaining impregnating agent leads to complete porous structure development. Advantages and drawbacks of these processes have been reported in detail elsewhere (Marsh and Rodriguez- Reinoso, 2006). Properties of the resulting activated carbons are highly dependent on the precursor, the activation process, and process parameters used (Basso et al., 2002;
Cukierman, 2013). Therefore, research on the production of ACs from different precursors and processes is relevant to the global community (Valente Nabais et al., 2013).
In this context, the aim of the present chapter is to examine the feasibility of converting a native, abundant agro-industrial biomass - yerba mate twigs (YMT) - into useful, value- added products by applying thermochemical processes. YMT arise from industrial processing of whole branches (leaves and twigs) from a native evergreen tree Ilex paraguariensis Saint Hilaire, belonging to the Aquifoliaceae family, for the manufacture of yerba mate. It is a widespread product massively consumed in Southern Latin America countries - Argentina, Brazil, Paraguay, and Uruguay - to prepare a popular herbal tea-like beverage appreciated for its flavor, and increasingly recognized worldwide for its nutritional and medicinal value, being included in several food codes (Anesini et al., 2006). Argentina is the largest producer of yerba mate worldwide with a yearly average production of around 260 x 103 ton, which is in part exported, followed by Brasil and Paraguay. It is mostly produced in the provinces of Misiones and Corrientes, Argentina (Canitrot et al., 2011). The per capita domestic consumption is estimated in 5-6 kg per year. Yerba mate is a strategic crop in the country due to its relevance related to regional development and workforce occupation. Industrial processing of the commercial product consists of a series of sequential stages that include heat treatment at relatively high temperatures in order to inactive enzymes and avoid enzymatic degradation, drying, grinding, and seasoning (Heck et al., 2008; Scipioni et al., 2010). The final product usually contains more than 65% dried leaves and less than 35% twigs, since the latter provides an unpleasantly bitter taste to the infusion. Therefore, huge quantities of unused twigs are discarded, thus constituting an attractive biomass resource for their conversion into valuable products. An advantage of YMT as biomass feedstock is their low moisture content (< 5 - 6 wt%) because of the thermal processing involved in the manufacture of the commercial product. This characteristic avoids the need of a first dryingconditioning stage often required for the thermochemical conversion of a wide variety of biomasses mostly characterized by high moisture contents (Cukierman et al., 1999: Mohan, 2006).
Pyrolysis of YMT is firstly investigated focusing on the kinetic characterization of the process in order to obtain fundamental information for the proper design of full-scale pyrolyzers. Likewise, yields, physicochemical characteristics, and higher heating value of the three kinds of pyrolysis products, comprising bio-char, bio-oil, and gases, are determined with emphasis on the effect of the temperature. Furthermore, conversion of the yerba mate twigs into activated carbon by the chemical activation process using phosphoric acid solution as activating agent is also examined. In this sense, it must be stressed that although a great deal of research has been published on the subject of producing activated carbons from several different biomasses, use of yerba mate twigs as raw material for this purpose has been unexplored. Potential applications of the different resulting products are analyzed and discussed in terms of their main characteristics.