Torrefaction of Woody Waste for Use as Biofuel - Energy & Fuels (ACS

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Torrefaction of woody waste for use as biofuel Corinna Maria Grottola, Paola Giudicianni, Jean-Bernard Michel, and Raffaele Ragucci Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01136 • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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Energy & Fuels

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Torrefaction of woody waste for use as biofuel

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C. M. Grottola1, P. Giudicianni1, J. B. Michel2, R. Ragucci1

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1. Istituto di Ricerche sulla Combustione-CNR, P.le Tecchio 80, 80125, Naples, Italy

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2. HEIG-VD – Haute Ecole d’Ingenierie et de Gestion du Canton de Vaud– Switzerland

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*Corresponding author: Corinna Maria Grottola

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Email address: [email protected]

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Postal address: P.le Tecchio 80, 80125, Naples (Italy)

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Phone: +39 081 768 2245

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Abstract

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Biomass for energy production has been extensively studied in the recent years. To overcome

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some constraints imposed by the chemical-physical properties of the biomass, several

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pretreatments have been proposed. Torrefaction is one of the most interesting pretreatments

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because torrefied biomass holds a wide range of advantages over raw biomass. The

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devolatilization of water and some oxygenated compounds influences the increase in the

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calorific value on both a mass and volumetric basis. The increase in the density reduces the

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transportation costs. Moreover, the decreased moisture content increases the resistance of

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biomass to biological degradation, thus facilitating its storage for long periods.

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Under torrefaction conditions, approximately 10-40 wt% of the initial biomass is converted

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into volatile matter including liquid and non-condensable combustible gases.1,2 The energy

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efficiency of the process could greatly benefit the exploitation of the energy content of these

1 ACS Paragon Plus Environment

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products. Recent studies and technological solutions have demonstrated the possibility to

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realize polygeneration systems that integrate torrefaction/pyrolysis to a combustion process

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with the aim of obtaining torrefied material/biochar and/or energy from biomass. Some

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examples include Pyreg, Pyreg-Aactor GT3, TorPlant, and Top Process.4 The identification of

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the main volatiles produced under torrefaction regime is useful for the optimization of the

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operating conditions of the integrated system. The integrated process raises some concerns

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when biomass from phytoremediation and wood from demolition and construction activities

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are used as feedstock because they could contain potential toxic elements (PTEs). During the

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torrefaction treatment, the fate of PTEs should be controlled in order to avoid their release in

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the gas phase and to evaluate the extent of their concentration in the torrefied biomass.

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The present work aims at studying torrefaction as an eco-sustainable process for the

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combined production of a solid biofuel with improved characteristics with respect to the

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starting material and a combustible vapor phase, embedded in the gas carrier flow, to be

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directly burned for energy recovery. Herein, torrefaction tests on Populus nigra L. branches

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from phytoremediation, and demolition wood were conducted at three temperatures, 250, 270

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and 300 °C, at a holding time of 15 min. The energetic content of torrefied materials was

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determined. At the same time, the fate of the heavy metals (Cd, Pb, and Zn) in the raw

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biomass at different torrefaction temperatures was studied, and their mobility in the torrefied

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biomass was investigated and compared to the mobility in the raw biomass.

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Keywords: biomass, phytoremediation, woody waste, torrefaction, heavy metals, acetic acid

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Introduction

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Energy consumption is increasing progressively with the rapid population growth and

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economic development. A great interest is oriented to the research of new renewable energy

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sources and technologies to not only cope with the growing demand for energy but also to

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facilitate a reduction in greenhouse gases emissions (GHGs). Biomass conversion processes

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are viewed as a viable option even though it could be advantageous to consider a number of

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biomass pretreatments in order to fit various chemical and physical characteristics of biomass

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to the existing combustion technologies.

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Torrefaction was proposed as a pretreatment process for improving inherent biomass

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characteristics such as increasing energy density and facilitating storage and handling

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systems1 through the improvement of the grindability, the reduction in the moisture content,

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the decrease in microbial degradation, and the sanitization of pest-affected plants.5 The partial

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decomposition of biomass at low torrefaction temperature generates condensable and non-

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condensable products and a solid residue rich in carbon, which is referred to as torrefied

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material; this material can be utilized as high-quality fuel in different applications including

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cofiring in power plants, entrained flow gasification, and small-scale combustion facilities.6

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Extensive literature is available on the effect of the two main operating variables, i.e., the

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final torrefaction temperature and the solid residence time. It was observed that temperature,

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in the range 250-300 °C, affects the physicochemical characteristics and energy properties of

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the solid product more than residence time does.1,7-9 A residence time ranging from few

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minutes to one hour was typically used in torrefaction tests, and a residence time longer than

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approximately 30 min had only negligible effects.1,7-9

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However, the environmental sustainability of the torrefaction of lignocellulosic waste such as

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woody waste and plants grown on contaminated soils must be addressed due to their high


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content of potentially toxic elements (PTEs)10,11 that may affect the quality of the gas product

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and the solid residue, depending on the process temperature.

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To our knowledge, few studies have addressed this issue. The torrefaction of demolition

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wood was studied by Edo et al.12 The elemental trace metal analysis suggested that most of

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the trace metals detected in the raw material remained in the chars at the torrefaction

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temperature (220 °C) used in the work. Bert et al.13 found that up to 290 °C heavy metals

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contained in the biomass were retained in the solid matrix. Nevertheless, other authors14,15

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showed that when some heavy metals are present in the form of chlorides, the devolatilization

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temperature was greatly reduced, mainly under anoxic conditions. No investigation has been

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conducted on the mobility of heavy metals retained in the torrefied biomass.

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In this work, a comprehensive approach is proposed for simultaneously studying the

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improvement of the energetic characteristics of two kinds of contaminated woody wastes and

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the environmental aspects related to the presence of contaminants. Contaminations from

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different sources were considered: Populus nigra L. branches (PN-B) containing heavy

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metals translocated from the soil to the plant organs during phytoremediation and demolition

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wood (DW) rich in heavy metals derived from operational activities during the construction

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and the disposal of woody shipping crates. The aim of the present work was twofold:

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studying the effect of the torrefaction temperature on the energetic properties of the torrefied

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biomass; and evaluating the environmental impact of the process by monitoring the release of

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heavy metals in the vapor phase as well as their mobility in the torrefied materials.

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Torrefaction tests were conducted under oxygen-limited conditions at a constant heating rate

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(10 °C/min) and at three final temperatures ranging from light to severe torrefaction regime16,

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namely, 250, 270, and 300° C, with a residence time of 15 min. The product yields were

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determined, and the organic and inorganic fractions of the solid products were characterized.

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The mobility of PTEs in the torrefied materials was also investigated. Finally, condensable


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volatiles from torrefaction tests were collected separately and analyzed by gas

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chromatography (GC) in order to understand their potential utilization.

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2. EXPERIMENTAL MATERIALS AND METHODS

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2.1 Torrefaction system

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The experimental tests were performed in a small-scale reactor "SOLO furnace", available at

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HEIG-VD, Switzerland. The cross-section of the furnace is shown in Figure 1. The reactor is

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divided into two connected and concentric cylindrical zones separated by a perforated plate:

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the internal one is the torrefaction chamber (diameter 20 cm and height 40 cm), whereas the

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external cylinder is for gas recirculation (diameter 50 cm and height 80 cm).

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A cylindrical steel container (diameter 15 cm and height 20 cm) with a perforated plate at the

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bottom is positioned in the internal section of the reactor and is used to accommodate the

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feedstock (100 g for each test run) packed in an aluminum paper (15 cm × 5 cm) and closed

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with a metal ring. The lid is provided with a hole that allows the passage of the gas outlet line

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and the thermocouple. In the external section, the recirculation of the exhausted gases

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produced during the torrefaction test occurs. The recirculation is provided through a fan

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located at the bottom of the external section under a perforated grid (frequency = 30 Hz). No

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Nitrogen flux was used during the experiments. The fan located at the bottom of the reactor,

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and the extractor placed on the up-section, are both regulated with the aim to reduce the

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oxygen content in the reaction environment.

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The temperature of the sample and the reaction environment was monitored constantly

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through six K-type thermocouples sketched in Figure 1, connected to a Keysight

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(Agilent/HP) 34970A Data Acquisition / Data Logger Switch Unit variable drive. The heat

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flux of the heating coil was used as an adjustable variable in a proportional-integral-


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derivative (PID) controller to produce a nominal heating rate equal to 10 °C/min during the

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tests. The temperature of the sample (TC2) was used as set point temperature. Due to the

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thermal inertia of the system, the actual heating rate was 5.6 °C/min, up to 220 °C. At higher

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temperatures, the heating rate increased, probably due to the exothermic decomposition of

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hemicellulose.17 In any case, this increase was reproducible in all the tests and did not affect

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the average heating rate that remained equal to 5.6 °C/min. In all the tests, a maximum

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overshoot of 2 °C was observed.

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The volatiles produced in the reaction unit entered the condensation device, which consists of

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two Pyrex condensers in series where condensable volatiles cooled and condensed. At the

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condenser’s outlet, a Pyrex flask was allocated for the collection of the liquid products. The

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non-condensing phase was fed to the analytical system for online characterization (Horiba

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Mexa 7170D). After the process, the aluminum container was quenched by immersing it

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rapidly in a glass beaker with 5 liters of water at a temperature of 10 °C. The water cooled the

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container, and any contact between the torrefied material and the water was avoided.


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50 cm

20 cm

GasGas

TC4 TC5

TC6

40 cm

Heating Coils

TC3 TC3

Biomass

TC2

80 cm

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Motor Motor

133 134 135 136

Figure 1. Cross Section of the “SOLO” furnace At the end of the test, the sample was immersed in a metal vessel containing 10 °C water.

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The quenched sample was heated in the furnace for 24 h at 105 °C before final weight

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measurement in order to remove the water. Solid yields were determined gravimetrically with

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respect to the fed sample.

140 141

2.2. Material characterization

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2.2.1 Solid materials

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PN-B were collected during phytoremediation tests conducted in Litorale Domitio - Agro

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Aversano NIPS (South Italy, Campania region) in the framework of the European LIFE

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Project ECOREMED (LIFE11/ENV/IT/275 – ECOREMED, 2016), whereas DW was


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obtained from the disposal of shipping crates. The material was ground, and the sieved

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fraction, in the 400–600 µm size range, was recovered for the torrefaction tests. The samples

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were oven dried at 105 °C for 24 h and kept in the desiccator before the characterization

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analyses and torrefaction tests. The moisture content of feedstock and torrefied biomass was

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measured with a thermobalance (Sartorius Moisture Analyzer - Model MA35) according to

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the ISO18134-3 procedure. The CHONS content was measured using the elemental analyzer

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Analyseur Flash 2000 (Thermo Scientific) according to the ISO 16948:2015 procedure.

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Carbolite AFF 1100 furnace was used for the determination of ash and volatile content

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according to the ISO 1171/18123:2015 and ISO 18123:2015 procedures, respectively. Fixed

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carbon was calculated as the amount required to complete the mass balance. The calorific

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value was determined using a bomb calorimeter (Oxygen Combustion Vessel 1108 - Parr

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Instrument Company) according to EN14918. Ash composition was determined by dissolving

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the biomass samples via microwave-assisted acid digestion based on US-EPA Methods 3051

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and 3052. The digested samples were then analyzed by inductively coupled plasma mass

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spectrometry (ICP/MS) using an Agilent 7500CE instrument. The results were reported in

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terms of content of the inorganic species and ion recovery in the torrefied biomass. The first

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is defined as mass of ion per mass of char and is used to calculate the ion recovery by

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multiplying it by torrefied yield and then dividing by the mass of ions in the raw biomass.

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The energy yields of the torrefied materials were calculated on dry basis by equation (1),

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where “t” stands for torrefied material and “f” for feedstock.

=

∗ ∗ 100 1


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Metal mobility was determined through a leaching test on biomass and corresponding

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torrefied materials using water and an EDTA-NH4 solution, as reported in Gonsalvesh et al.18

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The amount of heavy metals in the leachate was estimated based on the PTEs recovered in

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the torrefied biomass. The ion release was the ratio between the amount of PTEs released in

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the leachate and the amount of PTEs in the torrefied material.

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2.2.2 Liquid product

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For the identification and quantification of the main condensable species, liquids obtained

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from torrefaction tests at 250, 270, and 300 °C were filtered with 0.20-µm microfilters

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(Millex-FG). Chemical analysis was performed by a gas chromatograph coupled with a flame

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ionization detector (Agilent Technologies 7820A GC System) and a DB-1701 capillary

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column (60 m × 0.25 mm i.d., 0.25-mm film thickness). Helium (99.9999%) was used as

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carrier gas with a constant flow of 1.0 mL/min. The oven temperature was programmed from

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318 (4 min) to 508 K at a heating rate of 3 K/min and held at 508 K for 30 min. The injector

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and the FID were kept at 523 K and 573 K, respectively. A sample volume of 1 µL (4.5 wt%

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of pyrolysis liquid in acetone) was injected.

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The identification of the main compounds (acetic acid, hydroxyacetone, furfural, 5-methyl

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furfural, and 5-hydroxy methyl furfural) is based on the match with the retention times of the

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corresponding standards (Sigma Aldrich 319910) analyzed by GC/FID under the same

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conditions. The identified compounds were quantified by the internal standard method, using

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fluoranthene as an internal standard. A calibration curve was prepared by the injection of four

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standard solutions. The concentration range was determined by successive approximations

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until it became relatively narrow and encompassed the quantified value. Injections of the

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liquid samples were made in duplicate, and the maximum relative error observed was ±5% of

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the average values.

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3. RESULTS AND DISCUSSION

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The results of the chemical characterization of PN-B and DW samples are reported in Tables

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1 and 2. It should be noted that even though the origin of the waste was different, the results

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of the elemental analysis were comparable except for those of the nitrogen content. The

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biomass from phytoremediation was richer in ash than the DW was. The higher nitrogen

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content of DW compared to that of PN-B could be attributed to adhesives used in the

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production of timber goods (such as particle boards) that ended up in the DW wood waste

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stream.19 The proximate analysis highlighted a comparatively higher content of volatiles in

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DW and a higher ash content in PN-B, whereas the fixed carbon content was comparable.

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The heavy metals present in both samples were Cd, Cu, Pb, and Zn, with the last two being

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the most abundant, as shown in Table 2. C

H

N

O

wt % daf DW PN-B 202 203 204 205

47.7 (0.3) 47.1 (0.2)

6.1 (0.2) 5.9 (0)

HHV MJ/kg

2.1 (0.4) 0.7 (0.1)

42.6 (0.1) 19.2 (0.3) 41.5 (0.1) 19.0 (0.3)

Table 1. Feedstock characterization: elemental analysis and HHV. The relative error of three replicates is reported in brackets.

moisture

volatiles

wt % as received DW 1.1 (0.1) PN-B 7.0 (0.4)

fixed carbon

ash

Cd

wt % db 80.2 (0.8) 77.0 (0.4)

Cu

Pb

Zn

mg/kg

18.3 (0.7) 1.5 (0) 0.1 (6) 6.4 (6) 30.6 (5.5) 142.4 (6.2) 18.2 (0.4) 4.8 (0) 2.2 (4) 8.2 (1.7) 60.3 (4.8) 50 (8.9)

206 207 208 209

Table 2. Feedstock characterization: proximate analysis and heavy metal content. The relative error of three replicates is reported in brackets.

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The torrefied biomass yields are shown in Figure 2. As expected, for both the PN-B and DW,

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the mass yield decreased with the torrefaction temperature. Despite the similar results

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obtained from the elemental analysis and the higher volatile content of DW, at each


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temperature, the mass loss was higher for PN-B than that for DW. The mass yield of the

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torrefied biomass varied between 74.6 and 64.0 wt% for PN-B and between 87.4 and 80.5

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wt% for DW. Basu et al.20 reported a yield of 78 wt% for poplar wood torrefied at 250 °C,

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whereas Kim et al.21 obtained solid yields between 92 and 60 wt% for yellow poplar torrefied

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in the temperature range 240-280 °C. The results on DW mixed with RDF (refuse-derived

218

fuel) are available in the temperature range 220-270 °C12,22 and show yields varying in the

219

range of 94 and 84 wt%. According to the previous findings, in the torrefaction regime,

220

hemicellulose is the main component undergoing devolatilization.23 The higher solid yield

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observed for DW in this study could be explained by the lower content of hemicellulose in

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the raw sample. Nevertheless, the results of DW suffer heavily from the inhomogeneity of

223

this type of woody refuse, and as a consequence, this makes it impossible to draw firm

224

conclusions for these materials.

225 226 227 228

Figure 2. Torrefied biomass yields at T = 250, 270, and 300 °C.

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The evolution of the gas composition during the torrefaction of PN-B at 300 °C, reported in

230

Figure 3a, and the main liquid compounds identified at 300 °C (Figure 3b) confirm that the

231

hemicellulosic fraction of the PN-B sample is decomposed, producing mainly CO2 from the


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decomposition of side chains (acetyl groups and carboxylic groups), CO, from the carbonyl

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end groups left after dehydration of the side chain groups, and condensable compounds.

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Among the detected liquid compounds, acetic acid is the most abundant, followed by acetol

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and furan derivatives. However, the torrefaction liquids typically are greatly diluted in

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water.24

237 238 239 240

Figure 3. Gas species (panel a) and liquid compounds concentration (panel b) obtained from PN-B torrefied at T = 300 °C.

241 242

3.2 Torrefied biomass characterization

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The results of characterization of torrefied DW and PN-B obtained at different temperatures

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are reported in Tables 3 and 4.

C

H

N

O

wt % daf DW 250 DW 270 DW 300 PN-B 250 PN-B 270 PN-B 300

50 (1.0) 53 (0.5) 54.4 (0.8) 51.4 (0.9) 51.8 (0.6) 54.2 (0.5)

5.8 (0.2) 5.8 (0.1) 5.8 (0.2) 5.4 (0.1) 5.2 (0.1) 5.1 (0.1)

2.4 (0.3) 3.0 (0.6) 3.4 (0.2) 1.0 (0.1) 0.8 (0.1) 0.9 (0.1)

39.0 (1.3) 36.0 (0.5) 34.5 (0.7) 38.0 (0.6) 37.3 (0.9) 34.0 (0.6)

MJ/kg

Energy yield %

19.3 (0.7) 21.5 (0.2) 22.5 (0.1) 20.8 (0.9) 21.5 (0.7) 23.3 (0.3)

88.1 95.6 94.3 78.1 75.8 75.0

HHV

245


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Energy & Fuels

Table 3. Elemental analysis and energy properties of torrefied DW and PB-N obtained at 250 °C, 270 °C, and 300 °C. The relative error of three replicates is reported in brackets.

248 moisture

volatiles

wt % as received DW 250 DW 270 DW 300 PN-B 250 PN-B 270 PN-B 300

1.3 (0.5) 1.5 (0.1) 1.3 (0.1) 1.2 (0.7) 1.5 (0.5) 1.6 (0.2)

fixed carbon

ash

Cd

wt % db 71.8 (6) 71.9 (0.8) 70.3 (0.6) 69.1 (0.1) 67.9 (2.8) 62.5 (0.1)

25.4 (5.0) 25.9 (1.2) 27.9 (0.6) 25.7 (0.5) 27.2 (3.2) 31.7 (0.2)

Cu

Pb

Zn

mg/kg 2.7 (1) 2.1 (0.5) 1.8 (0) 5.1 (0.1) 4.9 (2.8) 5.8 (0.1)

0.1 (19) 0.2 (4) 0.2 (2) 2.5 (3.4) 2.6 (1.8) 3.2 (3.5)

8.3 (5) 8.5 (26) 8.9 (7) 9.6 (0.3) 9.7 (4.3) 11.4 (5)

51.7 (10) 185 (7) 52.9 (17) 188 (15) 54.1 (71) 195.4 (7) 70.4 (10) 61 (0.6) 72.3 (3.2) 65.4 (4) 89 (4.6) 73.2 (0.1)

249 250 251

Table 4. Proximate analysis and heavy metal content of torrefied DW and PB-N obtained at 250 °C, 270 °C, and 300 °C. The relative error of three replicates is reported in brackets.

252 253

With increasing torrefaction temperature, for both the feedstocks, an increase in the amounts

254

of elemental carbon and a decrease in the elemental oxygen and hydrogen amounts were

255

observed, in agreement with the literature1. This result is due to the breaking of the weak C–

256

O and C–H bonds in the hemicellulose matrix responsible for the release of volatile species

257

and permanent gases (mainly CO and CO2)25 that are rich in oxygen and hydrogen, thus

258

causing the deoxygenation of the torrefied biomass. The thermal behavior of elemental

259

nitrogen was different in the two feedstocks, revealing a different chemical nature of the N-

260

compounds in PN-B and DW. The nitrogen content always increased with the torrefaction

261

temperature for the DW sample, whereas for the PN-B sample, the trend was not evident. The

262

O/C and H/C ratios, represented in the Van Krevelen diagram in Figure 4 for both the PN-B

263

and DW torrefied samples, are considered important parameters to characterize solid biofuel

264

composition with respect to coal. Typical H/C and O/C values for torrefied biomass are in the

265

range of 1-1.5 and 0.4-0.65, respectively.26 An increase in the torrefaction temperature

266

reduced both H/C and O/C ratios to values that are within the typical ranges observed for


Energy & Fuels

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other torrefied biomasses, even though they were still high in comparison with the

268

characteristic values of coal. At 300 °C, it was observed that the O/C and H/C ratios were

269

greatly decreased to values close to the lignite coal range.27 The lowest torrefaction

270

temperature significantly affected the O/C ratio for the PN-B sample, indicating that

271

significant devolatilization of oxygenated compounds occurred even at low temperature, in

272

agreement with the observed weight loss (Figure 2). At the highest torrefaction temperature,

273

DW and PN-B were characterized by comparable O/C ratios. At each temperature, the H/C

274

ratio, similar in both the feedstocks, was always lower for torrefied PN-B, denoting the

275

devolatilization of a greater amount of compounds containing saturated C-H bonds as well as

276

bound water.

1.6

DW

DW 250

PN-B

1.4 Atomic H/C ratio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 19

DW 270 DW 300

PN-B 250

1.2 PN-B 270 PN-B 300

1.0

0.8 0.3

277 278

0.4

0.5 0.6 Atomic O/C ratio

0.7

0.8

Figure 4. Van Krevelen diagram for untreated and torrefied PN-B and DW.

279 280

Table 4 shows that, as expected, the volatile content of both torrefied feedstocks decreased

281

with temperature, whereas the fixed carbon content increased. According to the higher mass

282

loss observed for PN-B than for DW, the fixed carbon content was higher and the volatile

283

content was lower for the corresponding material torrefied at 270 °C and 300 °C. The fixed

284

carbon content of torrefied DW and in particular the PN-B sample increased greatly and was


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285

comparable to that of coal.1 The HHVs of torrefied solids were remarkably improved at

286

higher torrefaction temperatures and were always slightly higher for PN-B than those for DW

287

across the whole temperature range. However, it should be noted that the energy yield was

288

always lower for PN-B due to the higher devolatilization. Moreover, in the case of DW, the

289

energy yield had a non-monotonous trend with the temperature, showing a maximum at 270

290

°C. In contrast, the energy yield decreased with temperature for PN-B. As temperature

291

increased from 250 °C to 270 °C, the char yields decreased for both PN and DW.

292

Nevertheless, in the case of PN-B, the mass loss is greater than that for DW. In contrast, the

293

decrease in the O/C ratio in the char is smaller for PN-B than that for DW. This result implies

294

that DW released a lower amount of vapors (condensable and permanent gases) with greater

295

oxygen content. It is likely that mainly H2O was produced and released and that most of the

296

energy-containing volatiles were still in the torrefied material.28 As a consequence, with

297

increasing temperature, the increase in the char calorific value is greater than the mass loss,

298

thus determining the increase in the energy yield.

299

The concentration and the ion recovery of the detected heavy metals, namely, Cd, Pb, Cu, and

300

Zn, for the torrefied materials are reported in Tables 4 and 5, respectively. The concentration

301

increased with the torrefaction temperature for both feedstocks. The ion recovery for all

302

torrefied materials is very close to 1, and thus, it can be inferred that the condensable and gas

303

phases evolved from the torrefaction tests were essentially free of heavy metals.

Ion Recovery Cd

Cu

Pb

Zn

gr/gr

DW 250

0.99 (1.5)

0.98 (4.9)

1.12 (9.7)

1.06 (6.8)

DW 270

1.00 (1.5)

1.15 (5)

0.95 (1.75)

0.97 (3)

DW 300

1.15 (6)

1.1 (6)

1.12 (5)

1.00 (4)

PN-B 250

0.99 (3)

0.97 (4.9)

1.05 (2.8)

0.99 (3)


Energy & Fuels

PN-B 270

1.01 (1.5)

1.07 (5)

1.02 (1.75)

1.00 (3)

PN-B 300

1.02 (7)

1.04 (6)

1.03 (5)

1.06 (7)

304 305

Table 5. Ion recovery of torrefied DW and PB-N obtained at 250 °C, 270 °C, and 300 °C. The relative error of three replicates is reported in brackets.

306

To investigate the effect of torrefaction on the mobility of the heavy metals retained in the

307

torrefied PN-B samples, two leaching tests were performed, in water and in an EDTA-NH4

308

solution. A higher ion release in water was observed for Zn, followed by Cu, Cd, and Pb, and

309

their mobility decreased with increasing torrefaction temperature. This result could be related

310

to the increase in the hydrophobic character of the torrefied biomass with the torrefaction

311

temperature.1 Leaching with EDTA-NH4 was more severe, and all metals were released from

312

the raw materials. Temperature did not have any effect on the PTE mobility up to 300 °C,

313

where part of the metals retained in the char are immobilized in the solid matrix even in more

314

severe leaching conditions. It is likely that in acid conditions, the acid groups, associated with

315

lignin, hemicellulose, and extractives, were easily removed together with the associated

316

inorganic elements.29

317 Water

a) 1.00

PN-B

PN-B 250

PN-B 270

PN-B 300

1.00

ion release g/g

0.60 0.40 0.20

0.80 0.60 0.40 0.20

0.00

0.00 Cd

318 319

EDTA-NH4

b)

0.80 ion release g/g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 19

Pb

Zn

Cu

Cd

Pb

Zn

Cu

Figure 5. Ion release of heavy metals in water (a) and an EDTA-NH4 leaching solution (b).

320 321

Conclusion


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Energy & Fuels

322

The torrefaction of woody waste (demolition wood and biomass from soil phytoremediation)

323

was studied with the aim of evaluating the energetic properties of the torrefied material and

324

the fate of heavy metals during the pretreatment. It was found that with increasing

325

torrefaction temperature the energy properties of both torrefied biomasses were improved. In

326

particular, the study revealed that demolition wood has a high potential in terms of its energy

327

content as well as energy yield. Some concerns arise for the high nitrogen content of DW

328

compared to that of PN-B both in the raw and torrefied materials. For both feedstocks, PTEs

329

were retained in the torrefied biomass up to 300 °C, allowing the production of a heavy

330

metal-free vapor-phase fuel. The higher the torrefaction temperature was, the lower the PTE

331

release by water leaching of torrefied material, thus increasing the safety of the material

332

storage in open areas.

333 334

Acknowledgements:

335

This article is based upon work from COST Action SMARTCATs (CM1404), supported by

336

COST (European Cooperation in Science and Technology, http://www.cost.eu).

337

This work was supported by the European Commission (Project LIFE11/ENV/IT/275-

338

Ecoremed) and the Accordo di Programma CNR-MSE 2013-2014 under the contract

339

‘‘Bioenergia Efficiente”.

340

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Torrefaction of Woody Waste for Use as Biofuel - Energy & Fuels (ACS

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