VALORIZATION OF WASTES FROM INDUSTRIAL PROCESSING OF AN AGRICULTURAL PRODUCT VIA THERMOCHEMICAL CONVERSION PROCESSES

P. BONELLI; A. L. CUKIERMAN

Agricultural Wastes: Characteristics, Types and Management

Nova Science Publishers, Inc

Lugar: Hauppauge; Año: 2015; p. 141 - 167

Large amounts of wastes arising from industrialprocessing of agricultural products constitute alternative renewablebioresources potentially attractive for bioenergy generation and/or for the manufactureof other useful products. Their conversion additionally contributes to reduceenvironmental pollution. The present chapter examines thermochemical conversionof the wastes generated from industrialization of an agricultural product intobiofuels and/or products potentially applicable for environmental remediation.The selected wastes arise from industrial processing of whole branches (leavesand twigs) from a native evergreen tree Ilexparaguariensis, belonging to the Aquifoliaceae family, for the manufactureof yerba mate. It is a widespread product massively consumed in Southern LatinAmerica countries to prepare a popular herbal tea?like beverage. The commercialfinal product generally contains less than ~ 35% twigs, since they provide anunpleasantly bitter taste to the infusion, and therefore huge quantities ofunused twigs emerge as a by-product. Kinetics for the pyrolysis of the twigs ischaracterized by non-isothermal thermogravimetric analysis from roomtemperature up to 900 ºC to obtain information for the proper design offull?scale pyrolyzers. A deactivation model which assumes an overallfirst-order process and considers the physicochemical changes taking place inthe biomass with the pyrolysis course through variations of the reaction rateconstant with the temperature and solid conversion enables a properrepresentation of the experimental data over the whole temperature range, withestimated energy activation values between 49 and 137 kJ mol-1.Likewise, yield and characteristics of the three kinds of pyrolysis products,comprising bio-char, bio-oil, and gases, are examined from experimentsconducted in a bench-scale fixed-bed installation at temperatures in the range 400? 700 °C. Gas yield increases with increasing temperature, attaining 43% at 700°C, while the biochar yield decreases from 30% to 20% with temperature rise.Yield of the bio-oil attains a maximum (53%) at 500 °C, likely arising from thecompetition between primary formation of volatiles, at relatively low temperatures,and secondary degradation of the condensable vapors at the higher temperatures.All the pyrolysis products could be used in energy applications. The obtainedbiochars with higher heating value (HHV) of 23 ? 24 MJ kg-1 havepotential as environmentally friendly solid biofuel and could be employed forthe manufacture of briquettes mainly for domestic use. Accounting for theirhigh stability, as judged from the molar O:C ratio, another possibleapplication could be incorporation of the biochars into the soil for thestorage of atmospheric carbon. In turn, the bio-oils show organic fractionswith HHV between 28 and 33 MJ kg-1. Density values of the as-produced liquids (~1 kg dm-3)are rather higher than those for conventional hydrocarbon fuels due to theirhigher contents of oxygen and water. The crude bio-oils could be directlyburntor subjected to further upgrading to attain characteristics similar to those offuel-oil. Pyrolysis of the twigs yields low to medium heating value-gases (5 ?11 MJ m-3), mostly composed by CO2, CO, CH4and H2. Gas composition depends on the temperature, even though CO2isthe major generated species, followed by CO. Proportion of CO2decreases with temperature, particularly at 700 ºC, accompanied by enhancementsin the HHV of the gaseous mixtures, as a consequence of compositionalvariations, attaining a maximum value of 11 MJm-3. They mightcontribute to the energy sustainability of the process. Besides, phosphoricacid activation of the yerba mate twigs at pre-established moderate conditionsleads to good quality activated carbons with well-developed porous structurescharacterized by textural parameters (BET surface area of ~ 1000 m2g-1; total pore volume of 1cm3 g-1)comparable to those of commercially available samples.