Besides the advantage of no required separation and crushing of MSW in hydrothermal treatment, environmental advantage of organic chlorine reduction is also observed as additional features of the process.
The raw MSW characteristics in dry basis (db) can be observed in Table14.9. It can be seen that although the heating value of MSW was similar to that of low- grade sub-bituminous rank coal, the chlorine content of MSW was not fulfilling the requirements of low-chlorine solid fuel. MSW-1 has a chlorine content of 1.27 %, or about 12,700 ppm, of which 9,700 ppm was organic or water-insoluble chlorine. MSW-2 exhibited a higher chlorine content of 1.63 %, or 16,300 ppm, with 13,800 ppm organic chlorine.
The total chlorine content of the products obtained from MSW-1 (A11 to B21, see Table14.4) should range from 1.02 to 1.52 % (1.27±0.25 %), while those from MSW-2 (B22 to C11, see Table14.4) should have a chlorine content range of 1.24 to 2.02 % (1.63±0.39 %). The chlorine content results in Fig14.22show that the chlorine content for all products was in the expected range, suggesting that little or no chlorine was vented out of the system.
As shown in Fig.14.22, the organic chlorine contents of the products were generally reduced. The case of B21 showed a reduction down to 0.16 % (1,600 ppm), approximately an 83 % reduction of chlorine, even though the treatment temperature of 225C at 90 min was lower than conventional dechlo- rination temperature processes. Since the total chlorine content of the product remained at the same level as raw MSW, consequently the water-soluble, inor- ganic chlorine contents of the products were increased up to 1.04 %. This phe- nomenon suggested that in the case of MSW hydrothermal process, the organic Table 14.9 Raw MSW analysis (dry basis)
Organic chlorine (wt%)
Inorganic chlorine (wt%)
Total
chlorine (wt%)
Higher heating value (MJ/kg)
MSW-1 0.97±0.33 0.47±0.06 1.27±0.25 18.0±2.4
MSW-2 1.38±0.32 0.37±0.08 1.63±0.39 21.9±4.4
chlorine was converted into inorganic chlorine in the lower temperatures compared to conventional dechlorination process.
It can be seen from Fig.14.22that after hydrothermal treatment, the organic chlorine contents of the products were low compared to that of raw MSW. The increase of the reaction temperature (for example, A11 compared with B11, see Table14.4) and the holding period (for example, B11 compared with B21, see Table14.4) would further reduce the organic chlorine content.
The water-soluble inorganic chlorine content in the product can be easily removed by water-leaching and dewatering process. It was shown that one-time leaching can effectively reduce 80 % of the water-soluble chlorines [56]. In the case of B21, 80 % reduction equals to 0.83 % inorganic chlorine, reducing total residual chlorine in the product to 0.37 % (3,700 ppm). Therefore, it is suggested Table 14.10 Energy requirement of hydrothermal and conventional pelletizing
Process Hydrothermal
treatment
Conventional pelletizing
MSW heating energy 0.5 0.25
(MJ/kg MSW)
Steam generation energy 0.3 0
(MJ/kg MSW)
Drying energy 0 0.67
(MJ/kg MSW)
Mechanical energy 0.03 0.43
(MJ/kg MSW)
Total required energy 0.8 1.35
(MJ/kg MSW)
0.0 0.5 1.0 1.5 2.0
MSW-1 A11 A21 B11 B21 MSW-2 B22d B31d C11d
%weight
Water-insoluble Chlorine Water-soluble Chlorine
Fig. 14.22 Organic to inorganic chlorine conversion in MSW under various processing conditions
that a combination of the hydrothermal process and multiple water-washing pro- cesses could further reduce the chlorine content of hydrothermally treated product from MSW.
The chlorine contents of the raw MSW (Raw MSW), the product after the hydrothermal treatment before the washing process (Unwashed Product) and the product after the hydrothermal treatment followed by the washing process (Washed Product) are presented in Fig.14.23, where the total chlorine content in the raw MSW is taken as 100 %. The unwashed product exhibited low organic chlorine content because it was converted into inorganic chlorine during the hydrothermal process.
In the term of energy consumption, hydrothermal treatment also showed superior performance compared to conventional waste treatment system, also known as RDF (Refuse Derived Fuel) system. According to Caputo and Pelagagge [57], the power requirements for a hammer mill as a particle shredder are the largest, and usually reach about 360 kJ/kg MSW. Total power requirements for other equipment such as a densifier, pelletizer, magnetic separator, and shredder usually reach about 72 kJ/kg input material. The energy required for drying, however, is usually significant for the overall process, especially as it is obligatory to dry the MSW before inputting it into the shredder or mill. Sikka [58] reported an energy requirement as low as 324 kJ/kg MSW. In Perry [59], the energy requirements for a plate dryer type, to dry foodstuff, was about 420 kJ/kg, while for a turbo-tray dryer it was about 560 kJ/kg.
Those drying numbers are considered to be low, since theoretically the energy required to dry the MSW should be at least equal to the latent heat of vaporization of the water inside the MSW, that is, about 2257 kJ/kg water [60]. Using the typical
33%
87%
3%
67%
13%
13%
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Raw MSW Unwashed Product Washed Product
Chlorine Content (%)
Inorganic Chlorine Organic Chlorine
Fig. 14.23 Chlorine contents of raw MSW and the hydrothermal products before and after the washing process
moisture content of MSW, which is about 30 %, the energy required to dry the MSW should be approximately 677 kJ/kg MSW.
The hydrothermal treatment does not necessarily need a drying system since the particle size reduction is performed in wet conditions inside the reactor. It does, however, need steam generation energy to supply steam to the reactor. Table14.10 compares the difference between the characteristics and energy requirements of a conventional pelletizing process to produce RDF and the developed hydrothermal treatment. The final product temperature is assumed to be 110C for the con- ventional pelletizing process, and 215C for the hydrothermal treatment.
It can be seen that both systems have their own advantages and disadvantages, since the product appearances for both systems were also different; pulp for hydrothermal treatment and pellet for conventional pelletizing. From the table, it can be summarized that the total specific energy requirement for conventional RDF production is about 1.35 MJ/kg MSW, and hydrothermal treatment exhibits a slightly lower energy requirement of about 0.8 MJ/kg MSW. It then can be expected that the total required energy for hydrothermal treatment will be about 40 % less, compared to that of conventional RDF production.
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Sewage Sludge Treatment
by Hydrothermal Process for Producing Solid Fuel
Kunio Yoshikawa and Pandji Prawisudha
Abstract Sludge treatment and disposal is one of the focus points in the waste treatment technology research due to the fact that sewage sludge is a form of pollution. One of the promising methods of sewage sludge treatment, considering its maximum sludge reduction and short processing time, is incineration. However, the usage of sewage sludge as fuel in incinerator is hindered by its high water content and high nitrogen content. A hydrothermal process for producing solid fuel from sewage sludge is developed by using saturated steam at 160–200C and about 60 min holding time. It was shown that the product has improved dehyd- rability, but at the same time exhibiting higher solubility in the water, resulting in slightly lower calorific value due to the loss of dissolved solids, which have significant calorific value; therefore, an optimum operating condition is required to improve the dehydrability of sludge without reducing its solid content and calorific value. In the term of sewage odor, it was shown that after the hydrothermal treatment the sulfur-containing compound concentration was decreased, the same with the total odor intensity in the solid product. On the other hand, the odor intensity in the liquid and gaseous products were increased, suggesting the transfer of these compounds to the liquid and gaseous parts. During the combustion experiment, it was shown that the hydrothermally treated sludge emits lower NO emission compared to the raw sludge, promoting its possibility to be used in incinerator as direct or co-fuel without resulting in secondary pollution.
K. Yoshikawa (&)
Department of Environmental Science and Technology, Tokyo Institute of Technology, Tokyo, Japan
e-mail: yoshikawa.k.aa@m.titech.ac.jp P. Prawisudha
Department of Mechanical Engineering, Bandung Institute of Technology, Bandung, Indonesia
e-mail: pandji@termo.pauir.itb.ac.id
F. Jin (ed.),Application of Hydrothermal Reactions to Biomass Conversion, Green Chemistry and Sustainable Technology, DOI: 10.1007/978-3-642-54458-3_15, Springer-Verlag Berlin Heidelberg 2014
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