In Korea, municipal solid waste (MSW) treatment is conducted by converting wastes into energy resources using the mechanical-biological treatment (MBT). The small size MSW to be separated from raw MSW by mechanical treatment (MT) is generally treated by biological treatment that consists of high composition of food residue and paper and so forth. In this research, the hydrothermal treatment was applied to treat the surrogate MT residue composed of paper and/or kimchi. It was shown that the hydrothermal treatment increased the calorific value of the surrogate MT residue due to increasing fixed carbon content and decreasing oxygen content and enhanced the dehydration and drying performances of kimchi. Comparing the results of paper and kimchi samples, the calorific value of the treated product from paper was increased more effectively due to its high content of cellulose. Furthermore, the change of the calorific value before and after the hydrothermal treatment of the mixture of paper and kimchi can be well predicted by this change of paper and kimchi only. The hydrothermal treatment can be expected to effectively convert high moisture MT residue into a uniform solid fuel.
In recent years, the global issue in the energy field is that with the combination of increasing energy consumption and the steady depletion of fossil fuel reserves, coal can only be used to last 122 years on the basis of the 2008 production rate. This, together with the global environmental issues of the appropriate treatment of increasing municipal solid waste (MSW) has prompted a global research to develop alternative energy resources as well as to reduce CO2 emissions by using renewable energy from biomass and waste [
However, in the MBT system, the BT stage has common problems requiring long treatment time, more than 1 week to 1 month, with unpleasant smells [
The hydrothermal treatment is one of the thermochemical processes, treating waste in high-temperature and high-pressure water media to upgrade the material in a short time [
MSW differs in quality and quantity depending on the policy and culture of the nation. The composition of MSW should be different according to seasons and sectors and affected by custom, living style, and so forth as well as regulation of the country. Separated MSW by MT in Korea was usually consisted of high food residue (40–50%), paper (30–40%), plastic, and so forth. Therefore, it has high moisture content of about 50–60% since the food residue affects the moisture content and major properties of MSW. Especially, Korean food residue has had high moisture and high salinity to affect the treatment solution. Its property should provide a negative product to treat MT residue in BT stage. If we want to utilize the MT residue as a solid fuel together with RDF the MT residue should be dehydrated, dried, upgraded, and compacted.
In this research, a mild reaction condition of subcritical water (180°C <
The hydrothermal treatment employed in this research is utilized using high-temperature water to treat the raw material. Solid wastes are fed into the reactor, and then, about 200°C, 2 MPa saturated steam is supplied into the reactor for about 30 minutes and the blades are installed inside the reactor to mix the wastes for about 30–90 minutes. Then steam inside of the reactor will be discharged, condensed, and treated to be utilized as the boiler feed water again. The product is powder-like substance, and the moisture content is higher than the raw material, but it shows much improved dehydration and drying performances than the raw material. The hydrothermal reactions include the hydration, hydrous pyrolysis and decarboxylation, and so on, and the reaction temperature affects the properties of hydrothermal products [
The hydrothermal treatment experiments were performed using the 500 mL autoclave facility as shown in Figure
Schematic diagram of the autoclave facility.
The MT residue composition is based on the one obtained from Mokpo city, middle sized city in Republic of Korea. It is consisted of food residue (40–50%), paper (30–40%), plastic, wood, rubber, and others in negligible amount. In detail, the food residue is composed of various compositions such as vegetables (72%), fruits (15%), cereals, meat, and fish.
In this experiment, the food residue and paper contents were chosen as two parameters used, which are the highest ratio in the composition of the MT residue. Japanese newspaper and Korean kimchi in various compositions were used in substitute of paper and food residues, which were manually prepared by blending after the crushing process. In order to investigate the effect of main components in paper and kimchi, cellulose sample (
The dehydration performance of raw sample and hydrothermal products was determined using a centrifugal separator with variable speed from 2,000 to 14,000 rpm. The natural drying tests to evaluate the moisture content reduction of the raw materials and the hydrothermally treated products after the centrifuge dehydration were conducted in the room temperature.
The ultimate analysis of the raw samples and solid products were carried out using the PerkinElmer made 2400 Series II CHN organic elemental analyzer. The proximate analyses were conducted using the SHIMADZU D-50 simultaneous TGA/DTA analyzer. The calorific values were measured using the bomb calorimetric method according to the JIS M-8814. SEM microphotographs were taken by JSM-6610LA analytical scanning electronic microscope after drying the solid products. The biomass composition measurement of raw paper and kimchi was entrusted to Nihon Hakko Shiryo Company. The biomass composition of cellulose, hemicelluloses, and lignin was defined in (
Table
Biomass composition of paper and kimchi.
Biomass composition (wt. %), (d.b) | Paper | Kimchi | Calorific value (MJ/kg), (d.b) |
---|---|---|---|
Cellulose | 57.26 | 16.78 | 16.5 |
Hemicellulose | 6.95 | 0.51 | 16.7 [ |
Lignin | 12.28 | 4.64 | 20.4 |
Others and Ash | 23.51 | 78.07 | — |
d.b: dry basis.
Table
Proximate and ultimate analysis results of paper and kimchi and their products.
Paper | Kimchi | |||||||
Raw material | Treated at 180°C | Treated at 200°C | Treated at 220°C | Raw material | Treated at 180°C | Treated at 200°C | Treated at 220°C | |
Moisture (a.r) | 2.3 | 4.0 | 4.2 | 6.5 | 92.4 | 93.8 | 93.2 | 93.3 |
Proximate analysis (d.b) | ||||||||
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. %) (d.a.f.) | ||||||||
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 | 53.8 | 49.5 | 40.1 | 40.2 | 57.6 | 57.8 | 56.4 | 55.4 |
Composition of biomass (wt. %), (d.b) | ||||||||
Cellulose | 57.3 | 41.0 | 41.3 | 36.2 | 16.8 | 11.5 | 11.1 | 9.5 |
Hemicellulose | 7.0 | 0.8 | 0.8 | 0.5 | 0.5 | 0.1 | 0.1 | 0.0 |
Lignin | 12.3 | 13.6 | 14.1 | 14.7 | 4.6 | 4.9 | 4.9 | 5.0 |
Other and ash | 23.5 | 44.6 | 43.9 | 48.6 | 78.1 | 83.5 | 84.0 | 85.5 |
Weight loss (wt. %), (d.b) | 9.5 | 16.3 | 20.6 | 5.6 | 8.4 | 11.2 | ||
Calorific value (MJ/kg) (d.b) | 15.5 | 16.9 | 21.4 | 21.7 | 14.7 | 14.7 | 16.0 | 15.8 |
a.r: as-received, d.b: dry basis, d.a.f: dry ash free.
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 [
Figure
Dehydration performance of raw kimchi and its hydrothermal products.
The hydrothermal products had better dehydration performance compared with raw kimchi, and the moisture content could be reduced down to 24.2% when treated at 220°C, with the centrifugal separator speed of 14,000 rpm. In the case of the hydrothermal treatment at 180°C, the color of the product was changed but the result of the dehydration performance was similar to raw kimchi.
Figure
Natural drying performance of raw kimchi and, the dehydrated residue of its hydrothermal products.
The results clearly show that the hydrothermal treatment can improve the dehydration and drying performances of kimchi, which should lead in the reduction of energy requirement for the moisture removal from kimchi.
Hydrothermal treatment breaks the physical and chemical structure in the materials such as cellulose, hemicelluloses, and lignin [
Table
The paper and kimchi had high volatile matter content (87.0% and 67.1%) and oxygen content (53.8% and 57.6%) like other biomass. With the increase of the hydrothermal reaction temperature, the volatile matter and oxygen content decreased while the fixed carbon content increased, which were caused by the hydrolysis reaction.
Proximate and ultimate analysis of the paper and its hydrothermal products showed more significant change than the kimchi. The volatile matter of the paper decreased from 87.0% to 76.2%, 59.7%, and 56.9% at the reaction temperature of 180°C, 200°C, and 220°C, respectively.
Figure
Effect of the hydrothermal treatment on the calorific value of products.
As shown in Table
The hydrothermal treatment changes the properties of products like the coalification process. Figure
Coalification band of the paper and the kimchi at different hydrothermal reaction temperatures in comparison with cellulose, lignin and coal.
Paper and kimchi are known to have high atomic H/C ratios and atomic O/C ratios similar to other biomass [
Figure
SEM microphotographs of the raw materials and their hydrothermal products; (a) paper and (b) kimchi.
If we can predict the effect of the hydrothermal treatment for the mixture of materials based on the hydrothermal behavior of individual material in the MT residue, it becomes much easier to evaluate the effectiveness of the hydrothermal treatment. Figures
Change of the calorific value of the hydrothermal products by changing the mixture ratio of the paper andkimchi (hydrothermal reaction temperature = 200°C).
Change of the calorific value of the hydrothermal products by changing the mixture ratio of the paper and kimchi (hydrothermal reaction temperature = 220°C).
The blending ratios of paper to kimchi were from 100% : 0% to 0% : 100%. In these figures, the measured calorific values of the hydrothermal products of each mixture and the predicted calorific value of the hydrothermal products by the linear interpolation of the calorific value of the hydrothermal products of the paper and kimchi showed a good agreement. From these figures, we can see that the calorific value of the hydrothermal products of the mixture of the paper and kimchi can be well predicted based on the individual hydrothermal behavior, and the fuel upgrading by the hydrothermal treatment becomes more significant by increasing the amount of the paper.
In this research, the hydrothermal treatment was conducted for the paper, the kimchi and their mixture surrogating the Korean MT residue to demonstrate the improvement of dehydration and drying performances as well as fuel upgrading with the reaction temperature of 180°C, 200°C and 220°C, with the reaction time of 30 minutes. The dehydration and natural drying performances of the kimchi were significantly improved by the hydrothermal treatment. SEM microphotography showed that the physical structure of fibers of the paper and kimchi were broken into smaller and simpler molecules by the hydrothermal treatment.
In the case of paper, the volatile matter decreased from 87.0% to 58.4% and the fixed carbon increased from 5.3% to 27.0% at the reaction temperature of 200°C. As a result, the calorific value also increased from 14.7 MJ/kg to 21.7 MJ/kg at the reaction temperature of 200°C. On the other hand, this fuel upgrading behavior of the kimchi was rather weak due to its low cellulose content. As for the mixture of the paper and kimchi, the fuel upgrading behavior by the hydrothermal treatment was well predicted by the individual fuel upgrading behavior of the paper and kimchi.
These results demonstrated the effectiveness of the hydrothermal treatment of the MT residue for fuel upgrading.