Supplementary Material
Proximate analysis
As In agreement with Crombie et al. (2013) the ash content of the biochar samples
was influenced greatly by feedstock (P<0.0001)with higher concentrations found for SP (12.3
– 22.9%) compared to WP (1.2 – 2.8) as well as increase with peak temperature (P<0.0001).
Fixed C and volatile matter concentrations were expressed on a dry ash free basis to negate
the influence of ash content. It can be clearly seen that as pyrolysis temperature is increased
the fraction of volatiles expelled from the biomass is increased due to the decomposition of
the biomass components (R2=0.87, P<0.0001). This in turn results in a higher proportion of
fixed C remaining in the biochar sample as well as substantially lower volatile material.
Feedstock appeared to have no influence (P>0.05) on the biochar fixed C% produced at
350oC however as the temperature was increased to 650oC there is a significantly (P<0.0001)
larger increase in the fixed C content for WP biochar compared to SP potentially identifying
further interactions of ash content with the final biochar product. Problems with proximate
analysis of biochar with increased levels of ash content have previously been reported in
literature showing potentially inflated values for fixed C due to volatilization of ash species
such as phosphorous and magnesium above 500oC (Darvell et al., 2005; Sonoyama et al.,
2006). As the volatile matter concentration measured by proximate analysis is directly related
to the fixed C% they exhibit the same strong correlation with temperature but negative rather
than positive. Volatile matter has been shown to be beneficial as well as detrimental in soil
with no acceptable fraction yet in practice. By expressing the concentrations of fixed C and
volatile matter on a feedstock C% basis the influence of production conditions on the
proportion of this C can be demonstrated as fixed C and volatile matter yields in Table 2. The
yields of fixed C and volatile matter both followed the same trends as their corresponding
concentrations in biochar. Carrier gas flow rate and residence time were both found to have
no significant effect (P>0.05) on the biochar concentrations and yields of fixed C and volatile
matter at high and low temperatures.
Ultimate analysis
An increase in pyrolysis temperature resulted in an increase in C content and
decreasing H and O concentrations for both types of feedstock and all residence times and
carrier gas flow rates. Similar to the trends seen for fixed C and volatile matter, there was a
strong correlation (R2=-0.921, P<0.0001) between total C% and volatile matter resulting from
the elevated release of volatiles with increasing temperature resulting in a higher proportion
of C in the final char product. The elimination of O and H during pyrolysis can be credited to
the scission of weak alky-aryl either bonds as well as the attraction of forming increasingly
stable compounds (Demirbas, 2004; Mohan et al., 2006). As seen with fixed C there appeared
to be a significant influence of feedstock on the total C% with the high ash SP biochar
containing a lower amount of C to WP biochar at corresponding temperatures. The
concentration of oxygen has long been calculated based on the subtraction of C, H, N and ash
from 100%. Due to the inclusion of ash% in this calculation the variation in O appears to be
balanced by the lower C for high ash chars producing similar O:C for biochar at
corresponding temperatures with no significant influence of feedstock (P<0.05). Unlike
biochar total C, the C yield (Feedstock C basis) was seen to be significantly influenced by the
residence time (P=0.035) and carrier gas flow rate (P=0.001) in addition to temperature
(P<0.0001) and feedstock (P=0.018). The increased pyrolysis temperature resulted in a
decreasing C yield due to the coinciding decrease in char yield with rising pyrolysis
temperature. Although overall the effect of residence time, carrier gas flow rate and feedstock
were deemed to be significant on analysing the data at different pyrolysis temperatures it was
found that only feedstock selection and carrier gas flow rate were significant at 350oC
whereas at 650oC feedstock, residence time and carrier gas flow rate all had no significant
effect on the C yield. This demonstrates that as pyrolysis temperature was increased so did
the influence of temperature on the final C yield negating the effect that feed selection,
residence time and carrier gas flow rate might have at lower temperatures.
The utilisation of elemental ratios has been previously used to give an indication of
the long term stability of biochar through the progression of deoxygenation (O:C ratio) and
indication of aromaticity (H:C)(Spokas, 2010; IBI Guidelines, 2012; Schmidt et al., 2012;
Enders et al., 2012; Crombie et al., 2013). The elemental ratio can also be used to construct a
Van Krevelen diagram providing a visual representation of the age and origin of hydrocarbon
materials such as coal and petroleum but are more widely being applied to biochar to monitor
the evolving composition with temperature (Hammes et al., 2006). The distinct separation of
low and high temperature biochars can be seen in Fig. 3 by comparing the H:C and O:C
ratios. The overall spread in the sample range diminishes at higher temperatures further
demonstrating the dominating influence of temperature at 650oC. Guidelines have been
suggested by Spokas (Spokas, 2010), IBI Guidelines (IBI Guidelines, 2012) and European
guidelines (Schmidt et al., 2012) for the classification of biochar samples into different
categories of stability. Based on these regions of stability it can be seen that the chars
produced at 650oC can be viewed as extremely stable whereas only one 350oC char falls
within the require guidelines set by Schmidt et al. (2012).
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