pyrolysis, hydrothermal carbonization has also been employed to
biomass upgrading and it was reported that the biochar produced
from hydrothermal carbonization at 250 C have similar compositions
to that of lignite [9]. The difference between pyrolytic biochar
and hydrothermal biochar reveals that the biomass underwent a
less decomposition/carbonization degree under pyrolysis conditions
compared to hydrothermal carbonization at same temperature
[19].
Fuel ratio is a ratio of fixed carbon against volatile matter and is
a characteristic value representing the property of a solid fuel and
classifying coal rank according to ASTM 388. As shown in Table 2,
the volatile matter contents in biochars decrease and fixed carbon
contents increase with the increasing pyrolysis temperature.
Therefore, fuel ratios of the biochars increase with the increasing
temperature, especially at the temperatures higher than 275 C.
The increased fuel ratios imply elevated combustion efficiencies
and reduced pollutant emissions during biochars combustion compared
to raw biomass combustion. Despite increased HHVs, the
energy yields decrease with the increasing temperature due to
the significantly decreased mass yields of the biochars. In comparison
to non-woody biomass CF, woody PW has higher mass and
energy yields under identical pyrolysis conditions. For CF, there
is a significant decrease for energy yield when the temperature
increases from 275 to 300 C. While in the case of PW, the remarkable
decrease is observed when the temperature increases from
300 to 330 C.
To determine ash-related problems during biochar combustion,
the fate of major ash forming metals in raw biomass were
investigated during pyrolysis upgrading. The retention rate is
defined as percentage of the mass content in the biochar relative
to those contained in parent biomass and Fig. 4 presents the
retention rates of major ash forming metals at different temperatures.
Within tested temperatures, all major ash forming metals
are totally remained in resultant biochars. As for the fate of metals
during biomass pyrolysis, only several papers are available in
the literature [20]. For example, it was reported that K and Na
started to release around 400 C, and Ca and Mg around 600 C
for pine sawdust pyrolysis in a wire-mesh reactor [20]. The
results obtained in the present study are well consistent with
previous reports and the 100% retention rates possibly are related
to the low volatilization rate due to the low heating rate and low
pyrolysis temperature. It is worthy to note that metal retention
rate s are different under pyrolysis and hydrothermal carbonization
conditions, and most of alkali and alkaline earth metals
(AAEMs) and high fraction of heavy metals are removed in
hydrothermal biochars [19].
The slagging and fouling inclination during biochar combustion
are indicated as the values calculated by the follows equations
[21]:
Slagging index ðSIÞ ¼ ðB=AÞ S% ð1Þ
Fouling index ðFIÞ ¼ ðB=AÞ ðNa2O þ K2OÞ ð2Þ
where B/A = (Fe2O3 + CaO+MgO+ Na2O + K2O)/(SiO2 + Al2O3 + TiO2);
S is the percent of the sulfur in dry fuel sample.
The 100% retention rates of major ash forming metals and comparable
sulfur contents suggest that more serious slagging and
fouling issues are expected during biochar combustion compared
to raw biomass combustion. The calculated slagging and fouling
indices according to Eqs. (1) and (2) also confirm that the slagging
and fouling inclinations increase with increasing pyrolysis temperature
and the biochars have increased SI and FI values than raw
biomass generally (major metal oxides content in the ash and SI
and FI values of raw biomass and the biochars are shown in Supplementary Material). Considering lower ash content in PW
derived biochars together with higher energy yields and lower
slagging and fouling inclinations, woody biomass are more suitable
for biochar production than agricultural biomass.