Pyrolysis of various biomass residues and char utilization for the production of activated carbons
Introduction
Among the technologies concerning clean energy generation, the energy exploitation of biomass is an interesting challenge since it is clean (zero net CO2 emission), it is unlimited, and it minimizes the disposal problems associated with the generation of agricultural by-products. Moreover, biomass exploitation allows the possibility of generating added value products such as chemicals or activated carbons (ACs) which means an attractive economic and technological solution.
It has been widely reported the feasibility of using agricultural by-products as renewable source of energy by means of pyrolysis and gasification processes [1], [2], [3], [4], [5]. The thermal degradation characteristics of lignocellulosic materials are profoundly influenced by their chemical composition (cellulose, hemicellulose and lignin) [6], and TG and DTG curves provide semiquantitative understanding of the thermal degradation processes occurring during thermochemical conversion under various atmospheres. Thus, the comparative thermogravimetric study of different materials can provide useful information about the differences associated to their lignocellulosic composition. On the other hand, three phases are produced when biomass is subjected to pyrolysis processes: char, liquid and gas. The distribution of these phases is a function of operating parameters (mainly temperature and heating rate). The most interesting one from the energetic point of view is the gas (mainly composed by H2, CO, CO2, CH4, etc.) which has a Higher Heating Value (HHV) high enough to be used for the total energy requirements of a biomass waste pyrolysis plant [7] and might also be employed in internal combustion engines, gas turbines and other operating devices [8]. The liquid phase generated (known as tar) is the oil mainly composed of oxygen-containing structures (derivatives of phenol, dihydroxybenzenes, guaiacol, syringol, vanilin, veratrol, furan, acids) [9] and can be used directly as fuel or added to petroleum refinery feedstock [10], although its use involves some nuisances such as high water content (that are detrimental to ignition), presence of corrosive organic compounds, etc., and may also be an important source of chemicals. The char is a solid carbonaceous residue with a high content in fixed carbon (>75%), which can be used directly as fuel, as briquettes [4], [5] or as precursor for activated carbons production [2], [5]. ACs are porous materials that are able to adsorb certain amounts of compounds in the liquid and gaseous phases; this property makes these solids very interesting to be used in many industrial applications. The porosity of activated carbons is conditioned, among other factors, by the carbonaceous precursor, the activation method used and the operating parameters. There is a huge amount of research on the production of ACs from agricultural by-products by physical [11], [12], [13], [14] or chemical activation [15]. Most of these works study the influence of operating parameters on the activation process, but the research focussed on the influence of the lignocellulosic composition of the raw material on a concrete activation process is scarce. The investigation concerning this field is very interesting because in the industrial production of ACs around 50% of the precursors used by the manufacturers are lignocellulosic [16].
The aim of this work is to study the exploitation of several biomass residues carrying out the following specific objectives:
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To conduct thermogravimetric analysis (under inert atmosphere) on the four biomass residues employed, estimating the relationship between the devolatilization curves and the lignocellulosic composition of the materials.
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To study the pyrolysis process in terms of the energy exploitation of the three phases generated, determining the HHV of the gas, tar and char.
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To produce activated carbons from the char obtained from each material, by steam activation under common activation conditions, evaluating the influence of the parent material composition on the textural characteristics of the carbons produced.
Section snippets
Materials
The materials employed in this study were walnut shell, almond tree pruning, almond shell and olive stone, which are extensively generated in Europe. These precursors will be named as WS, ATP, AS and OS, respectively. They were all obtained from local manufacturers and shredded to 1–2 mm. The ultimate analysis was made using a LECO CHNS (EA 1108) analyzer and the proximate analysis of the four materials was made according to standard methods [17], [18], [19]. The HHV of the materials was
Discussion of results
The ultimate and proximate analyses of these raw materials are shown in Table 1. It can be seen that these materials have a high content of volatile matter and low content of ash, nitrogen and sulfur, which is interesting with respect to their applications in gasification and pyrolysis processes.
Conclusions
Several biomass materials were subjected to pyrolysis process and the energetic exploitation of the phases generated was studied. Then the effectiveness of using the chars in the preparation and characterization of effective adsorbents was analysed. It was found that all these materials are a prospective starting material for the preparation of high quality activated carbons. However, the rate of thermal decomposition of the parent material (which is related to its lignocellulosic composition)
Acknowledgments
The authors express their gratitude to the “Junta de Extremadura-Consejería de Economía, Comercio e Innovación” for the financial support through projects 2PRO4B016/PRI07A088 and 2PR01A034. S. Román and G. Martínez thank the “Junta de Extremadura” for their research grants.
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