Solid yields from all sixteen experiments for the partial factorial design (Table 3.3), in- cluding repeats, are given in Table 4.6. Experiment numbers 13, 14, and 15 were carried out to compare the solid yield of product at three separate water loading with the tem- perature and residence time remaining constant. The average solid yield of product was highest for 90% water loading, which is aligned with literature reports. The model was narrowed down to focus on 90% water loading only.
TABLE4.6: Solid yield (dry basis) obtained from the experiments carried out
Experiment Temperature (◦C) Time (minute) Water (%) Solid Yield (%) (dry basis)
3 240 20 90 66.7
6 240 30 90 75.2
9 240 40 90 66.1
12A 275 20 90 78.2
12B 275 20 90 73.8
13A 275 30 70 60.9
13B 275 30 70 56.5
14 275 30 80 55.8
15A 275 30 90 56.5
15B 275 30 90 63.2
18A 275 40 90 60.2
18B 275 40 90 55.7
21 300 20 90 50.5
24A 300 30 90 59.1
24B 300 30 90 56.9
27 300 40 90 56.1
Proximate analyses were carried out on the Municipal Solid Organic Waste (MSOW) ob- tained from DNCC to obtain moisture content, ash content, combustible material content,
and higher heating value (HHV). The results of the proximate analyses are shown in Ta- ble 4.4. The solid yield, and HHV of the biocoal product are depicted in Figure 4.1 and Figure 4.2, respectively.
(a) (b)
(c)
66.7
78.2
50.5
0 20 40 60 80
240 275 300
Solid Yield %
Temperature /°C
20 minutes
75.2
56.5 59.1
0 20 40 60 80
240 275 300
Solid Yield %
Temperature /°C
30 minutes
66.1
55.7 56.1
0 20 40 60 80
240 275 300
Solid Yield %
Temperature /°C
40 minutes
FIGURE4.1: Solid yield percentage (dry basis) of HTC experiments at 90%
water loading
From the ash analysis of each biocoal product, and the EDS data, dry ash-free elemental compositions of the biocoal products were determined using equation 4.1 and are given in Table 4.7.
dry ash-free element %= element % from EDS
(C + N + S + O) % ×100 (4.1)
(b)
(c)
12.4 11.5
9.6
0 5 10 15
240 275 300
HHV / MJ Kg-1
Temperature/ °C
20 minutes
11.6 11.0
9.7
0 5 10 15
240 275 300
HHV / MJ Kg-1
Temperature/ °C
30 minutes
11.6 10.8
14.9
0 5 10 15
240 275 300
HHV / MJ Kg-1
Temperature/ °C
40 minutes
FIGURE4.2: High heating value of the biocoal products at 90% water loading
TABLE 4.7: Elemental composition of biocoal product and parent MSOW sample (dry ash-free basis)
Experiment Condition C (%) N (%) S (%) O (%)
3 240,20,90% 64.23 5.56 0.15 30.06
6 240,30,90% 63.10 7.17 0.47 29.27
9 240,40,90% 59.78 6.11 0.91 33.19
12A 275,20,90% 65.15 5.40 0.11 29.33
13A 275,30,70% 58.66 7.40 0.52 33.42
14 275,30,80% 53.25 7.42 - 39.33
15A 275,30,90% 62.58 6.98 0.92 29.52
18B 275,40,90% 58.49 6.41 - 35.09
21 300,20,90% 46.59 4.01 1.21 48.19
24A 300,30,90% 51.63 4.45 0.12 43.80
27 300,40,90% 51.74 5.06 0.05 43.15
N/A Parent MSOW, no HTC 54.48 9.55 0.17 35.81
FIGURE4.3: EDS image of biocoal sample from experiment 12A
FIGURE4.4: EDS image of parent MSOW
Result of experiments in literature revealed that the ash and fixed carbon percentage in initial MSOW sample nearly doubles in the biocoal or hydrochar and volatile matter per- centage nearly halves due to the volatile matter leaving the solid [91]. From the ash content, measurements of the biocoal and the proximate analysis of the parent sample, the
TABLE4.8: Proximate analyses of biocoal products
Experiment Condition Ash (%) Fixed C (%) Moisture (%) Volatiles (%)
3 240,20,90% 60.7 10.8 3.0 25.5
6 240,30,90% 65.7 9.6 4.1 20.7
9 240,40,90% 63.6 10.9 2.7 22.8
12A 275,20,90% 66.9 9.2 0.8 23.1
13A 275,30,70% 63.6 11.8 0.9 23.6
14 275,30,80% 66.6 12.9 2.7 14.8
15A 275,30,90% 69.4 12.8 1.7 16.1
15B 275,30,90% 70.9 11.4 2.9 14.8
18B 275,40,90% 68.7 12.9 1.5 16.8
21 300,20,90% 74.0 14.3 0.8 10.9
24A 300,30,90% 75.5 12.2 3.3 9.0
27 300,40,90% 31.6 12.8 1.4 54.1
volatile contents and and fixed carbons were estimated.
The average fixed carbon percentage of the parent MSOW sample was found to be 7.2%
as mentioned in Table 4.4. This amount of fixed carbon is also present in the biocoal product. By considering that the balance between parent sample and solid product yield to be the volatile matter, the fixed carbon percentage in each biocoal experiment can be calculated using the solid yield percentage by equation 4.2:
Fixed carbon % in biocoal= fixed carbon in parent MSOW
solid yield % of HTC experiments (4.2)
The moisture content of each of the HTC solid sample is analyzed using a moisture an- alyzer (Model: RADWAG MA 110.R., Poland). The amount of volatile matter can be calculated by subtracting the moisture content from the balance between 100 - solid yield
%, as shown by the equation below:
volatile matter % in biocoal=100−solid yield %−moisture % (4.3)
Figure 4.5 shows ash and combustible organic contents of biocoal obtained at different temperatures and at different residence times. It can be observed from Figure 4.5(c) that at 300◦C, 40 minutes and 90% water, highest amount of combustible organics were obtained.
(a) (b)
(c)
60.7 66.9 74.0
39.3 33.1 26.0
0%
50%
100%
240 275 300
Composion / %
Temperature/ °C
20 minutes
Ash Combusble Organics
65.7 69.4 75.5
34.3 30.6 24.5
0%
50%
100%
240 275 300
Composion / %
Temperature/ °C
30 minutes
Ash Combusble Organics
63.6 68.7
31.6
36.4 31.3
68.4
0%
50%
100%
240 275 300
Composion / %
Temperature/ °C
40 minutes
Ash Combusble Organics
FIGURE4.5: Composition of solids obtained via HTC at 90% water loading
This is most likely an outlier due to the high inhomogeniety of MSOW; this particular sample likely to contained very small amount of ash.
From Figure 4.1(a), as well as Table 4.6, it can be observed that the highest amount of solid yield was found for experiment 12A (275◦C, 20 mins, and 90% water); the literature reported solid yield percentage is 72% [151], which is less than the solid yield found for experiment 12A. This is due to the high ash content in the MSOW of DNCC; the amount of ash in the biocoal in literature is reported to be between 22.8 - 46.0 % [91, 100]. In contrast, the ash content of the biocoal produced in this study ranged from 60.7 - 75.5%, with an outlier at 31.6%. This could be due to presence of high amounts of construction sand in the MSOW of Bangladesh.
However, the dry ash-free carbon content of 65.2% for experiment 12A (275◦C, 20 mins, and 90% water) matched the biocoal mass percentage (derived from biomass) of around
63% [151]. Heilmannet al. reported that the carbon content of biocoal derived via HTC of algae (Dunaliella salina) was 66.3 % [152].
At higher temperatures, solid yield decreases because hydrothermal liquefaction and hy- drothermal gasification takes place [90, 92, 102, 152]. At longer residence times, the biocoal yield is also reported to decrease [153]. These are consistent with the findings of this study. The reported biocoal or hydrochar is said to have a calorific value (HHV) between 15-20 MJ/kg [89, 91, 92]. The calculated calorific value (HHV) for the biocoal produced in this study ranged from 9.56 - 14.92 MJ/kg.
Table 4.7, shows that experiment 12A (275◦C, 20 mins, and 90% water) had the highest carbon content. Figure 4.2(a) shows that experiment 12A has a HHV higher (11.5 MJ/kg) than the parent MSOW sample (10.90 MJ/kg). These observations are consistent with literature. Yoshikawa and Bergeet al. reported that the HHV and carbon content of the biocoal should be higher than the parent MSOW fraction [91, 100].