14.3 Characteristics of Hydrothermal Treatment Product
14.3.1 Physical and Chemical Characteristics of Product
Two typical MSW characteristics are being used and shown in Table14.3. It can be seen that both types had low density, high variation in their values, relatively high moisture content and high ash content which would reduce the heating value of MSWs. It also should be noted that both raw MSW visually had high plastic content, which would increase the chlorine content in the MSWs.
After hydrothermal treatment in various conditions as shown in Table14.4, the product characteristics were showing significant increase of density up to four times of that of MSW in dry basis, shown in Table14.5. Noting these results, it is predicted that 75–80 % waste volume reduction by the hydrothermal treatment can be achieved. It should be underlined that contrary to raw MSW characteristics, the products showed high repeatability and low variation, which means that the product is more uniform compared to raw MSW.
particle moisture content
water
addition hydrothermal
treatment
Fig. 14.10 Visualization of particle breakage due to hydrothermal treatment
Table 14.3 Raw MSW characteristics
Density(g/cm)a Moisture content (wt%)a Combustible content (wt%)a
Ash content (wt%)a
MSW-1 0.15±0.02 33.0±10.1 50.0±6.5 17.0±6.0
MSW-2 0.15±0.02 33.2±8.9 54.5±8.9 12.3±9.1
aWet basis
Table 14.4 Varied experimental operating conditions
Pressure (MPa) 2.0 2.0 2.4 2.4 2.4
Temperature (C) 215 210 225 225 225
MSW mass (kg) 705 670 665 640 599
Holding period (min) 30 90 30 90 90
Experiment ID A11 A21 B11 B21 B22
It can be observed that higher reaction temperature and longer holding period would produce denser products. This phenomenon should be attributed to the similar reason as shown in Fig.14.10, where the hydrothermal process produced smaller particle size and at the same time reduced the product bulk void. As a result, higher bulk density of the product can be achieved.
In the term of composition, all product samples exhibited similar combustible and ash content but with higher moisture content. The high moisture content from hydrothermally treated product reduced the confidence whether they can be used as solid fuel, but one should analyze their heating values, which will be presented afterwards.
Subsequent experiments have been conducted in Tokyo Tech, focusing in hydrothermal treatment on basic material such as paper to represent cellulose, and kimchi (traditional vegetable pickles in Korea) to represent the food waste.
Hydrothermal treatment breaks the physical and chemical structure in the mate- rials such as cellulose, hemicellulose, and lignin [35–39], and these biomasses were broken down into smaller and simpler molecules.
Table14.6 shows the property of raw samples of kimchi and paper and their products after the hydrothermal treatment which were produced at 180, 200, and Table 14.5 Product characteristics
Experiment ID Density (g/cm3)b
Moisture content (wt%)a
Combustible content (wt%)a
Ash content (wt%)a
A11 0.61 43.7 46.1 10.2
A21 0.70 43.9 46.0 10.1
B11 0.71 43.4 44.6 12.0
B21 0.75 39.4 45.8 14.8
B22 0.48 55.9 36.3 7.8
aWet basis
bDry basis
Table 14.6 Effect of the hydrothermal treatment on the properties of products
Paper Kimchi
Raw @180C @200C @220C Raw @180C @200C @220C
Moisture (%) 2.3 4.0 4.2 6.5 92.4 93.8 93.2 93.3
Proximate analysis(wt%)(dry basis)
Volatile matter 87.0 76.2 58.4 56.6 67.1 60.3 60.9 57.8
Fixed carbon 5.3 14.1 27.0 29.2 22.6 29.8 29.7 31.0
Ash 7.7 9.7 14.6 14.2 10.3 10.0 9.4 11.3
Ultimate analysis(wt%) ((dry basis))
C 40.3 45.0 54.5 54.8 33.6 34.4 35.8 37.0
H 5.6 5.4 5.0 4.8 5.3 4.6 4.6 4.5
N 0.2 0.1 0.4 0.2 3.5 3.2 3.2 3.0
O 46.2 39.8 25.5 26.0 47.3 47.8 47.0 44.1
220C. It is shown that paper and kimchi had high volatile matter content (87.0 % and 67.1 %) and oxygen content (46.2 % and 47.3 %) like other biomass. Similar to that of MSW treatment, with the increase of the hydrothermal reaction tem- perature, the volatile matter and oxygen content decreased while the fixed carbon content increased. Comparing between paper and kimchi, the hydrothermal products of paper showed more significant changes than that of kimchi, suggesting that cellulosic materials are easier to degrade compared to typical food waste.
Considering the microphysical structure of the two materials, Figs.14.11and 14.12show the SEM microphotographs of the paper and kimchi before and after Fig. 14.11 SEM
microphotographs of kimchi treatment.araw kimchi bproduct at 200Ccproduct at 220C
the hydrothermal treatment. These SEM microphotographs reveal the changes between the raw materials and the upgraded solid products, showing disruption of physical structures and formation of individual grains in the products. Apparently the hydrothermal treatment breaks the structure of the paper and kimchi and converts them into smaller, uniform particle products.
The hydrothermal treatment apparently has disrupted the structure of kimchi and paper that contains cellulose, hemicelluloses, and lignin like woody biomass, the structure of biomass was decomposed and disrupted to become small molec- ular and particle size as shown in Fig.14.13. Moreover, colloidal bond between Fig. 14.12 SEM
microphotographs of the paper treatment.araw paper bproduct at 200Ccproduct at 220C
water and material in the sample was broken to separate moisture. The results of changing of physical structure were one of significant causes to improve the dewatering and drying performance of hydrothermal products. Removal of moisture contents in MSW is a major target of the pretreatment, and the moisture content of MSW has a strong influence on the characteristics and treatment method of MSW [40–43].