Thermochemical Processing of Wood from Invasive Arboreal Species for Sustainable Bioenergy Generation and Activated Carbons Production

CUKIERMAN, A.L.; NUNELL, G.V.; FERNANDEZ, M.E.; DE CELIS, J.P.; KIM, M.R.; GUREVICH MESSINA, L.; BONELLI, P.R.

Invasive Species: Threats, Ecological Impact and Control Methods

Nova Publishers

Año: 2012; p. 1 - 46

Thermochemical conversion of wood sawdust from two invasive arboreal species (Prosopis ruscifolia and Parkinsonia aculeata) into valuable products is explored to provide novel possible means of spread control attempting to their use as alternative, easily renewable bio-resources for bioenergy generation and activated carbons production. Rapid pyrolysis of both wood species is examined to assess the feasibility of their conversion into energy products. Yields of the three kinds of pyrolysis products, comprising bio-oil, non-condensable gas, and bio-char, are determined from experiments performed in a bench-scale installation at 500 °C. The wood species noticeably affects yields and distributions of the pyrolysis products. Main physicochemical characteristics and higher heating value (HHV) of the products generated from Prosopis ruscifolia, leading to higher bio-oil yield (~42%) than Parkinsonia aculeata, are determined. All the products show potentialities for energy applications. The bio-oil, further upgraded, could be employed as a liquid fuel with characteristics similar to those of fuel-oil, whereas the low heating value-gas evolved with pyrolysis course, mostly composed by CO2, CO, CH4 and H2, might contribute to energy sustainability of the process. In turn, the bio-char (HHV≈ 27 MJ/kg) has potential as environmentally friendly solid bio-fuel. Kinetic measurements for the pyrolysis of the two wood species are also carried out by non-isothermal thermogravimetric analysis from room temperature up to 500 °C. An overall first-order decomposition model enables to properly represent kinetic data for both species over the whole temperature range. Differences in pyrolytic reactivity are reflected in the estimated kinetic parameters. On the other hand, use of wood sawdust from the two species as precursor of activated carbons targeted at wastewater treatment is investigated. For this purpose, the phosphoric acid activation process is applied at pre-established moderate conditions (acid/precursor weight ratio of 2, 450 ºC, 0.5 h). The activation atmosphere involved in the thermal treatment stage of the process is in situ modified to examine its influence on the development of porous structures, surface chemistry, and adsorptive behaviour of the resulting activated carbons. Either a self-generated atmosphere, flowing air or nitrogen is used. The activation atmosphere and the wood species have a marked influence on the characteristics of the derived carbons. Those developed from Prosopis ruscifolia wood in the self-generated atmosphere show higher BET surface area and total pore volume (2281 m2/g, 1.7 cm3/g) than the ones obtained under flowing air (1638 m2/g and 1.3 cm3/g), although the latter possess a higher total content of surface acidic/polar oxygen groups (2.2 meq/g) than the former (1.5 meq/g). Textural characteristics of P. ruscifolia-based activated carbons are superior to those for the carbons developed from P. aculeata wood in the self-generated atmosphere (968 m2/g, 0.70 cm3/g) and flowing N2 (1103 m2/g, 0.75 cm3/g). These samples have total contents of surface acidic/polar oxygen functionalities of 1.9 meq/g and 1.7 meq/g, respectively. The ability of the activated carbons in removing representative toxic metal ions, oxo-anions, and organic compounds is examined from batch assays using dilute aqueous solutions as models of wastewater. Adsorption isotherms of Cu[II] ion onto P. ruscifolia-based activated carbons are determined and properly described by the Langmuir model. Maximum sorption capacity (Xm) for the air-derived carbons (Xm= 0.44 mmol/g) almost duplicates the value for those obtained in the self-generated atmosphere (Xm= 0.24 mmol/g), pointing to a predominant effect of the surface acidic functionalities on metal sequestering behaviour. Adsorption isotherms of salicylic acid onto the air-derived carbons are also ascertained, and satisfactorily fitted to the Langmuir model. On the other hand, effectiveness of the P. aculeata-based activated carbons in removing individual nitrate ion, phenol, and methylene blue at pre-established equilibrium conditions is comparatively investigated. High removal levels are attained for the organics (~ 90% and 100% for phenol and methylene blue, respectively), but nitrate removal is low (≤ 40%). Removal effectiveness of these carbons is inversely related to water solubility of the solutes and is slightly affected by the activation atmosphere. Accordingly, effective activated carbons of potential applicability in tertiary wastewater treatment may be developed from the two invasive species, as suitable, alternative precursors, by phosphoric acid activation at moderate conditions.