4.2.1 Source and Preparation of Feedstock
The waste streams used in this study were classified into the group described as rejects and
were obtained from two different fibre recycling mills in South Africa. Two of the streams
used in this study were sourced from the Mpact Springs mill and the third from Mpact
Felixton mill. Rejects are produced as a sludge from the manufacturing of paper and contain
significant amounts of water (>50 %). Each waste sample underwent a drying process in a
tunnel greenhouse for a period of five days before being milled down to a particle size of 2
mm using a Retsch SM 100 cutting mill. In order to improve packing density inside the fixed
bed reactor, the milled feedstock was pelletized. This was done by rehydrating the dried
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size of 6 mm. The pellets were subsequently dried at 60 °C until no further mass loss of the
pelletized material occurred. By pelletizing, an increase in packing density by a factor of two
was achieved.
4.2.2 Physico-Chemical Characterisation
The moisture content of the as received samples was determined in accordance with TAPPI
T264 om -88 standard procedure. A thermogravimetric analyser TGA/DCS 1 Star Systems
Mettler Toledo was used for proximate analysis. As observed in other studies [6,24], the
fibres obtained from the paper recycling mills contained calcium carbonate (filler). As a
result, the proximate analysis of the pelletized recycling residues and char samples was
conducted by making a modification to the standard ASTM E1131 testing method as
suggested by Ridout et al. [24]. An additional step was added to the standard ASTM E1131
where the sample was held at 650 °C for 5 minutes to drive off all the organic volatiles while
avoiding the decomposition of CaCO3 which occurred from 700 °C (Eq1). After this, the sample was heated to 900 °C and held there for an additional 5 minutes to ensure the full
decomposition of CaCO3 occurred before the combustion of the fixed carbon. As CaCO3 is an inorganic component unlikely to produce any fuel product, in the proximate analysis
results (waste stream and char products) the percentage of CaCO3 was included in the ash content.
𝐶𝑎𝐶𝑂3→ 𝐶𝑂2+ 𝐶𝑎𝑂 Eq 1
Ultimate analysis of the raw material was determined using an Elementar Microcrube
ELIII. This method estimates organic carbon based on the CO2 produced by carbon combustion. As combustion occurred at temperatures above 700 °C, the decomposition of
CaCO3 into CO2 was unavoidable. A correction to the organic carbon content had to be made. This was achieved by estimating the CO2 produced from the inorganic source by using
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the weight percentage of CO2 produced from CaCO3 decomposition as measured by the TGA during the relevant step of proximate analysis.
HHVs of the feedstock samples and pyrolysis products were determined in
accordance with the ASTM standard D5865-11a, using a Cal2K Eco Calorimeter, which was
calibrated using benzoic acid. Fourier transform infrared (FTIR) spectroscopy was
performed on the raw materials using a Nicolet iS10 spectrometer operating in ATR mode
with a diamond crystal. ThermoScientific OMNIC software was used for collection of data.
This enabled the identification of different functional groups present in the sample. Inorganic
composition of waste streams was determined via X-ray fluorescence (XRF) analysis using
an AXIOS PANalytical.
Thermal behaviour investigation was carried out using a thermogravimetric analyser
TGA/DCS 1 Star Systems Mettler Toledo. Experiments were carried out using 20 mg of
sample from a temperature 30 °C to 900 °C with a heating rate of 10 °C min-1. Nitrogen was used as the inert purged gas at a flow rate of 80 ml min-1.
4.2.3 Pyrolysis Experiments
Bench Scale Experiments
Pyrolysis experiments were carried out using a fixed bed reactor depicted in Figure 4-1. The
pyrolysis setup consisted of four distinct sections, 1) Pyrolysis oven used to provide heat of
reaction, 2) 1m reaction tube made from quartz along with a quartz sample boat that housed
12g of pelletized waste material, 3) A stepwise condensation train that consisted of 5
condensers and 4) Nitrogen gas feeding system. Technical grade nitrogen was fed at a flow
rate of 0.5 L min-1. Before each experimental run, the reactor was checked for leaks using a vacuum pump and subsequently purged with nitrogen for 10 minutes to maintain an oxygen
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brown viscous liquid/wax was collected, hereafter referred to as tarry phase. Condensers 2 -
3 (C2 and C3) were cooled using dry ice and were used to collect the aqueous phase. In
condensers 4 and 5 (C4 and C5), silica beads were placed to adsorb the aerosols remaining in
the gas stream.
Figure 4-1: Bench scale pyrolysis reactor
Pyrolysis experiments were carried out at 3 distinct temperatures (300 °C, 425 °C and
500 °C) chosen based on the characteristic steps of conversion of the three samples
(Section Thermal Behaviour). The experiments were carried out at a heating rate of 25
°C/min and once the desired temperature was achieved, held there for an additional 60
minutes. Once the experiment was completed, the reactor was cooled to 90 °C before
dismantling took place. All experiments were conducted in triplicate to ensure
reproducibility of results and resulted in a standard deviation of char and tarry phase product
66 Product Yields
The yields of pyrolysis products were calculated according to Equations 2 – 7 with Ychar standing for yield of char produced, Ytarry standing for the tarry phase produced, Yaqueous for
the aqueous (watery) phase and Yaerosols for aerosols, mresidue and mC1 to mC5 represents the mass of product collected in the sample boat (residue) and at certain points in the
condensation system. The water content of the aqueous phases produced from all three
streams was determined in accordance with ASTM E203 standard, using a Metrohm 701
Titrino Karl-Fischer Titrator, hydranal composite 5 titrant (Sigma –Aldrich).
Ychar(wt.%) = mresidue msample× 100 Eq 4-2 Ytarry(wt.%) = mC1 msample× 100 Eq 4-3 Yaqueous(wt.%) = m C2+C3 msample × 100 Eq 4-4 Yaerosols(wt.%) = m C4+C5 msample × 100 Eq 4-5
Yoil (wt.%)= Ytarry+ Yaqueous+ Yaerosols Eq
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Ygas (wt.%)= 100 - (Ytarry+ Yaqueous + Ychar + Yaerosols) Eq
4-7
4.2.4 Energy Conversion Assessment
The energy conversion (EC) of the waste feedstock into pyrolysis products (char and tarry
phase) was determined by Equation 8, mi and HHVi representing the mass and HHV of the respective pyrolysis products and msample and HHVsample representing the mass and HHV of the particular waste stream under investigation with ECi representing the energy conversion of chosen product. ECi(%)= mi×HHVi msample×HHVsample ×100 Eq 4-8