Valorization of Wastes from Industrial Processing of an Agricultural Product via Thermochemical Conversion Processes

P.R. BONELLI; A.L. CUKIERMAN

Agricultural Wastes: Characteristics, Types and Management

Nova Science Publisher

Año: 2015; p. 141 - 168

Large amounts of wastes arising from industrial processing of agricultural products constitute alternative renewable bioresources potentially attractive for bioenergy generation and/or for the manufacture of other useful products. Their conversion additionally contributes to reduce environmental pollution. The present chapter examines thermochemical conversion of the wastes generated from industrialization of an agricultural product into biofuels and/or products potentially applicable for environmental remediation. The selected wastes arise from industrial processing of whole branches (leaves and twigs) from a native evergreen tree Ilex paraguariensis, belonging to the Aquifoliaceae family, for the manufacture of yerba mate. It is a widespread product massively consumed in Southern Latin America countries to prepare a popular herbal tea?like beverage. The commercial final product generally contains less than ~ 35% twigs, since they provide an unpleasantly bitter taste to the infusion, and therefore huge quantities of unused twigs emerge as a by-product. Kinetics for the pyrolysis of the twigs is characterized by non-isothermal thermogravimetric analysis from room temperature up to 900 ºC to obtain information for the proper design of full?scale pyrolyzers. A deactivation model which assumes an overall first-order process and considers the physicochemical changes taking place in the biomass with the pyrolysis course through variations of the reaction rate constant with the temperature and solid conversion enables a proper representation of the experimental data over the whole temperature range, with estimated 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 experiments conducted 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 the competition 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 obtained biochars with higher heating value (HHV) of 23 ? 24 MJ kg-1 have potential as environmentally friendly solid biofuel and could be employed for the manufacture of briquettes mainly for domestic use. Accounting for their high stability, as judged from the molar O:C ratio, another possible application could be incorporation of the biochars into the soil for the storage of atmospheric carbon. In turn, the bio-oils show organic fractions with 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 their higher contents of oxygen and water. The crude bio-oils could be directly burnt or subjected to further upgrading to attain characteristics similar to those of fuel-oil. Pyrolysis of the twigs yields low to medium heating value-gases (5 ? 11 MJ m-3), mostly composed by CO2, CO, CH4 and H2. Gas composition depends on the temperature, even though CO2 is the major generated species, followed by CO. Proportion of CO2 decreases with temperature, particularly at 700 ºC, accompanied by enhancements in the HHV of the gaseous mixtures, as a consequence of compositional variations, attaining a maximum value of 11 MJ m-3. They might contribute to the energy sustainability of the process. Besides, phosphoric acid activation of the yerba mate twigs at pre-established moderate conditions leads to good quality activated carbons with well-developed porous structures characterized by textural parameters (BET surface area of ~ 1000 m2 g-1; total pore volume of 1cm3 g-1) comparable to those of commercially available samples.