in-hee hwang, hiroya aoyama

7/23/2019 In-Hee Hwang, Hiroya Aoyama

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Recovery of solid fuel from municipal solid waste by hydrothermal treatment

using subcritical water

In-Hee Hwang ⇑, Hiroya Aoyama, Toshihiko Matsuto, Tatsuhiro Nakagishi, Takayuki Matsuo

Laboratory of Solid Waste Disposal Engineering, Faculty of Engineering, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo 060 8628, Japan

a r t i c l e i n f o

Article history:

Received 24 March 2011

Accepted 4 October 2011

Available online 21 November 2011

Keywords:

Hydrothermal treatment using subcritical

water (HTSW)

Municipal solid waste (MSW)

Char

Heating value

Cl removal

a b s t r a c t

Hydrothermal treatments using subcritical water (HTSW) such as that at 234 C and 3 MPa (LT condition)

and 295 C and 8 MPa (HT condition) were investigated to recover solid fuel from municipal solid waste

(MSW). Printing paper, dog food (DF), wooden chopsticks, and mixed plastic film and sheets of polyeth-

ylene, polypropylene, and polystyrene were prepared as model MSW components, in which polyvinyl-

chloride (PVC) powder and sodium chloride were used to simulate Cl sources.

While more than 75% of carbon in paper, DF, and wood was recovered as char under both LT and HT

conditions, plastics did not degrade under either LT or HT conditions. The heating value (HV) of obtained

char was 13,886–27,544 kJ/kg and was comparable to that of brown coal and lignite. Higher formation of

fixed carbon and greater oxygen dissociation during HTSW were thought to improve the HV of char.

Cl atoms added as PVC powder and sodium chloride to raw material remained in char after HTSW.

However, most Cl originating from PVC was found to converse into soluble Cl compounds during HTSW

under the HT condition and could be removed by washing.

From these results, the merit of HTSW as a method of recovering solid fuel from MSW is considered to

produce char with minimal carbon loss without a drying process prior to HTSW. In addition, Cl originat-

ing from PVC decomposes into soluble Cl compound under the HT condition. The combination of HTSW

under the HT condition and char washing might improve the quality of char as alternative fuel.

2011 Elsevier Ltd. All rights reserved.

1. Introduction

Hydrothermal treatment using subcritical water (HTSW) has

been widely employed for the solubilization, extraction, and lique-

faction of target materials. Subcritical water has hydrolytic and

pyrolytic reaction characteristics, which result from a decrease in

the dielectric constant and increase in the ion product of water

at temperatures and pressures below and near 374 C and

22.1 MPa (Kang et al., 2001; Brunner, 2009).

There have been several research works on HTSW for municipal

and industrial solid-waste treatment. Most researchers have fo-

cused on solubilization and extraction to recover valuable organic

compounds such as glucose and organic acids ( Kang et al., 2001;

Goto et al., 2004; Yoshida and Tavakoli, 2004; Ren et al., 2006;

Watchararuji et al., 2008; Lamoolphak et al., 2008). Some research-

ers have dealt with hydrothermal treatment as pretreatment prior

to fermentation, gasification, composting, and other processes

(Eley et al., 1996; Sawayama et al., 1997; Kato and Matsumura,

2003; Papadimitriou et al., 2008). Only few researchers have fo-

cused on recovering solid fuel from municipal solid waste

(MSW). Nouguchi and Inoue (2007) performed HTSW to recover

char from waste at 150–350 C but it was limited to model food

waste.

Recovery of solid fuel from MSW using HTSW has several

advantages: the effect of carbonization (or pyrolysis) can be

achieved under the relatively low temperature below 300 C, com-

pared with the temperature of 400–600 C for MSW carbonization

performed at little or no oxygen condition; moisture removal is not

necessary for wet MSW unlike MSW carbonization; and con-

versely, the moisture included in MSW can be used as a heating

medium, stream, to decompose MSW.

A 37 t/day-scale HTSW plant has been operating in Shiraoi-Cho,

Hokkaido, since April 2009 to treat combustible wastes including

food waste collected from residential and commercial areas for so-

lid-fuel conversion. Batch-type HTSW is performed using three

autoclaves and saturated steam at 234 C and 3 MPa. The total

operating time per batch is 4–6 h including the time for waste in-

put and product discharge (1–1.5 h). Obtained solid product, char,

is pelletized with shredded wooden and plastic wastes and used as

alternative fuel for the boiler of a paper-manufacturing plant.

However, for better understanding the process, further information

is needed on the effects of the temperature and pressure of HTSW

the holding time in reactor, the physical composition of MSW, and

0956-053X/$ - see front matter 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.wasman.2011.10.006

⇑ Corresponding author. Tel./fax: +81 11 706 6828.

E-mail address: [email protected] (In-Hee Hwang).

Waste Management 32 (2012) 410–416

Contents lists available at SciVerse ScienceDirect

Waste Management

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / w a s m a n

http://dx.doi.org/10.1016/j.wasman.2011.10.006

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the presence of Cl compounds in MSW on the yield and quality of

char.

In this work, we performed HTSW to recover solid fuel from

MSW. Paper, food, wood, and plastics were prepared as the main

components of MSW and polyvinyl chloride (PVC) and NaCl were

used as sources of Cl in MSW. Decomposition characteristics of

each MSW component and yield and composition of obtained char

were investigated under two conditions of HTSW. Moreover, theheating value (HV) and Cl removal of solid product were evaluated

in terms of fuel utilization.

2. Material and methods

2.1. Experimental apparatus and procedure

Fig. 1 shows a laboratory-scale experimental apparatus for

HTSW. A 50 g of raw material and 150 ml of distilled water were

put in a batch type of autoclave reactor and the reactor was then

sealed and purged with nitrogen gas through valve A. After nitro-

gen gas purging, valves A and B were closed. The speed of stirrer

was set up 150 rpm. Temperature and pressure were set at

234 C and 3 MPa or 295 C and 8 MPa. The former is the same

operating condition as for the hydrothermal treatment plant in

Shiraoi-Cho, Hokkaido and is referred to as the LT condition. The

latter is the upper limits of temperature and pressure for the reac-

tor and is referred to as the HT condition. The temperature and

pressure within the reactor were monitored by sensors and signal-

ized by a data logger connected with a computer system. As pre-

sented in Fig. 2, nitrogen gas in the reactor was discharged by

opening valve A at 75 C and the reactor was filled with steam in

the temperature range of 75–115 C (A). The reactor was heated

to a set LT or HT condition (B) and maintained for 5 min after

reaching set condition (C). Afterward, the heater was turned off

and the reactor was decompressed by opening valve B. Some mix-

ture of vapor and gas product in the reactor was discharged to con-

densing bottles at the same time and then the reactor was cooleddown (D).

Solid, liquid, and gas products were collected when the reactor

had cooled to room temperature. Nitrogen gas was injected

through valve A to discharge remaining gas in the reactor into a

gas sampling bag completely. The volume of non-condensable

gas was measured with a dry-gas meter and the entire volume

was collected in a 10 L Tedlar bag to analyze the gas composition.

Solid product, char, was collected first. Remaining solid and liquid

mixture was collected by rinsing with distilled water and then fil-

tered with a 1 lm pore size filter paper. Solid trapped by the filter

paper was collected and weighted as char. Char was dried at 105 C

for 12 h and then weighed and pulverized to a grain size of 500 lm.

Filtrate was collected as liquid product. Liquid in the gas cooling

and scrubbing bottles was collected and then filtered with a

1 lm pore size filter paper too. All filtrate was collected as liquid

product and was provided for measuring the total carbon

concentration.

2.2. Analytical method

Raw material and char were provided for determination of ash

and volatile matter (VM) based on ASTM D3172-89. Fixed carbon

(FC) was est imated according to the mass balance

(FC = 100 VM ash). C, H, and N contents were measured with

an elemental analyzer (CHN Corder MT-6, Yanaco Co.). S and Cl

contents were measured employing combustion and gas-absorp-

tion methods. The sulfate concentration of the absorption solution

was measured by ion chromatography (DX 500 series, Dionex Co.).

The chloride ion concentration was measured employing the mer-

curic thiocyanate method and an absorption spectrophotometer

(U-1101, Hitachi Co., Tokyo, Japan) at 460 nm.Total carbon and chloride ion concentration of liquid product

were measured employing a total organic carbon analyzer (TOC-

V CPH, Shimadzu Co., Kyoto, Japan) and the mercuric thiocyanate

method and an absorption spectrophotometer at 460 nm respec-

tively. CO2, CO, and CH4 concentrations of non-condensable gas

were measured by a GC using thermal conductivity detector (Type

164, Hitachi Co., column type: WG-100, flow rate of He: 33 ml/

min; detector temperature: 50 C).

HV of char was measured using a bomb calorimeter (CA-4PJ,

Shimazu Ltd.). Washing test of char was carried out to investigate

Cl removal. A 10 g of char was shaken with 100 ml of distilled water

at 150 rpm for 20 min and the mixture was filtered using a 1lm fil-

Autoclavereactor

Gasscrubbing

bottles

Gas bag

Stirrer

Computer system Vapor and gascondensing bottles

Gas meter

Pressure sensor Temperature sensor

Valve BValve A

Gasdehydrating

bottle

Reactorcontroller

N2 gas

Fig. 1. Laboratory-scale apparatus for HTSW.

Holding time (min)

0 30 60 90 120 150

T e m p e r a t u r e ( o C )

0

50

100

150

200

250

300

350

P r e s s u r e ( M P a )

0

2

4

6

8

10

THT

TLT

PHT

PLT

B A C D

Fig. 2. Variations in temperature and pressure within the reactor under LT and HT

conditions.

I.-H. Hwang et al./ Waste Management 32 (2012) 410–416 411



ter paper. This procedure was repeated up to four times to recover

the entire soluble fraction. The concentration of released chloride

ionin distilledwater was measured employingthe mercuric thiocy-

anate method and an absorption spectrophotometer at 460 nm.

2.3. Raw material and Cl source

Shredded printing paper (20 20 mm), dog food (DF) as foodwaste, shredded wooden chopsticks (<5 mm), and mixed plastic

film and sheets (<20 mm; PE:PP:PS = 5:3:2) were prepared as

raw materials. According to the ingredients table offered by the

manufacturer, DF was composed of 50% carbohydrates, 25% pro-

tein, 11% fat, 6% fiber, and 9% ash on a dry basis. Table 1 presents

the composition of the raw material.

To investigate the behavior of Cl compounds during HTSW, NaCl

and PVC powders were added to paper and wood respectively such

that Cl was 2% of the total dry weight of the raw material.

3. Results and discussion

3.1. Carbon balance of product

The reliability of the experiment was checked using the carbon

balance between raw materials and products obtained by HTSW.

Table 2 shows the carbon recovery ratio of product (CR ) when

the total carbon of raw material is normalized to one and the car-

bon distribution ratio of char (Cc), liquid (Cl), and gas (Cg) when the

total carbon recovered as product is normalized to one respec-

tively. More than 88% of carbon in raw material was recovered as

products, which demonstrates the reliability of the experiment.

For paper, DF, and wood, as shown by Cc, more than 75(=0.88 X

0.85 X 100)% of carbon in raw material was recovered in char under

both LT and HT conditions, which was considered a merit in solid-

fuel recovery. Such a high value of Cc was due to the remaining VM,

which was usually much more degraded during carbonization at

400–600 C. The Cc for the LT condition was about 5–8% higherthan that for the HT condition for the same input material. The

sum of Cl and Cg was less than 19% of CR . Generated gas is com-

posed of carbon dioxide and carbon monoxide (CO2 >> CO). Zhang

et al. (2011) noted that carbon dioxide and carbon monoxide were

primarily formed as a result of decarboxylation of organic fraction

during HTSW. Liquid product was not provided for the qualitative

analysis in this work but it was expected to contain organic com-

pounds such as alcohols, ethers, aldehydes, phenols, carboxylic

acids, etc. according to previous research (Qian et al., 2010; Zhang

et al., 2011).

On the other hand, Cc for plastics is 100% because plastics do not

decompose under either LT or HT.

3.2. Variations in char yield and composition depending on rawmaterial and HTSW condition

Table 3 shows the yield and composition of char derived from

each raw material depending on the LT and HT conditions. Char

yield was determined from the weight ratio of char to input raw

material on a dry basis. For the same raw material excepting plas-

tics, a higher char yield was obtained under the LT condition.

The largest difference in the char yield between LT and HT con-

ditions was observed for char derived from paper. The main com-

ponent of paper is fiber such as hemicellulose and cellulose, which

have been reported to decompose rapidly in the temperature range

of 200–400

C under the pyrolytic condition (Sørum et al., 2001;Völker and Rieckmann, 2002; Myung et al., 2004; Shen and Gu,

2009). The greater degradation of hemicellulose and cellulose

seems to drastically decrease the char yield.

Compared with the char derived from paper, the difference in

the char yield between the LT and HT conditions was not signifi-

cant for DF and wood. Wood, another fibrous biomass in this work,

has a relatively high yield compared with paper under the HT con-

dition. Wooden chopsticks were made of broadleaf trees such as

silver birch, which usually contains from 18% to 24% lignin (Mura-

ta, 2004). Considering that lignin starts to decompose at tempera-

tures over 330 C under a pyrolytic condition (Myung et al., 2004),

the presence of lignin may explain why the char yield of wood is

higher than that of paper under the HT condition.

The yields of char derived from paper noticeably deviated under

the LT condition (Table 3). This was probably due to difference in

heating rates by fluctuation in the rising and falling conditions of

temperature during HTSW. For this reason, the difference in the

holding time of raw material in the reactor was checked. As shown

in Fig. 2, the holding time indicated the entire elapsed time during

HTSW but the extent of decomposition of raw material might be

related to the holding time at higher temperature from stage B to

stage D. Fig. 3 shows the relationship between the yield of VM + FC

of char and the holding time of raw material at temperature over

220 C. The yield of VM + FC of char means the weight of VM + FC

of recovered char when the weight of raw material is normalized

to one. As the holding time increased under the LT condition, the

yield of VM + FC of char derived from paper decreased. This indi-

cates that the variation in the holding time leads to the deviation

of the char yield of paper. The relationship between the yield andholding time at temperatures exceeding 200 and 210 C was also

observed, but their correlation was not strong compared with that

for temperatures exceeding 220 C.

On the other hand, no effect of holding time was observed for

DF and wood char because the deviation of the holding time was

small under both LT and HT conditions. Finally, the mixed plastics

were expected to degrade under the high pressure of the HT con-

dition but no weight loss of plastics was observed as shown by

the yield in Table 3. The plastics appeared to melt partially through

hydrothermal treatment and to harden again during cooling of the

reactor.

3.3. Evaluation of the char composition and heating value

Several chars, which were selected according to the type of raw

material and HTSW condition, were provided for measurements of

HVs. The results are presented in Table 4. HVs of char (HVchar) de-

rived from paper, DF, and wood were 13,886–26,000, 24,627–

27,145, and 22,134–27,544 kJ/kg respectively, which are compara-

ble to HVs of lignite and brown coal. Char yield decreased under

the HT condition (Table 3) but HVchar was higher under the HT con-

dition (Table 4). To clarify the relationship between char composi-

tion and HVchar the compositions and HVs recovered from 1 kg of

raw material were calculated by multiplying the values char com-

positions and of HVchar by char yield respectively. These results are

compared in Fig. 4.

Although a small amount of char was obtained from 1 kg of raw

material under the HT condition (Fig. 4b), the recovered energy per1 kg of raw material did not differ between LT and HT conditions

Table 1

Composition of raw material.

Ash FC VM C H N S Cl

Paper 0.05 0.06 0.89 0.38 0.06 ND ND 0.002

DF 0.08 0.14 0.79 0.45 0.07 0.04 0.004 0.022

Wood 0 0.09 0.91 0.47 0.06 ND ND 0.001

Plastics 0 0.01 0.99 0.86 0.13 0.06 ND ND

Dry basis.Unit: weight ratio ().

412 I.-H. Hwang et al. / Waste Management 32 (2012) 410–416



(Fig. 4a). The increase in FC content was likely to improve the HV of

char obtained under the HT condition. At the same time, the qual-

ity of VM might vary and affect the HV of char too.

To prove the variation of VM quality depending on LT and HTconditions, the HVs of VM were estimated. Six chars obtained from

the different raw materials under LT and HT conditions were

heated for 7 min at 900 C to remove VM and were then taken to

measure the HV of FC (HVFC). As listed in Table 3, HVFC ranged from

29,203 to 34,084 kJ/kg, with there being no large difference be-

tween LT and HT conditions. The average value of HVFC was

32,120 ± 1680 kJ/kg, which is near the heat of combustion of car-

bon (32,800 kJ/C-kg). Assuming that HVFC was equal to 32,800 kJ/

kg, HVVM was estimated using an Eq. 1:

HV VM ðkJ =kg VM Þ ¼HV char FC HV FC

VM ð1Þ

Unlike HVFC, HVVM varied from 7375 to 26,936 kJ/kg depending

on raw materials and HTSW conditions (Table 4). To clarify the rea-

son for the change in HVVM, the elementary composition of VM wasestimated as described in Fig. 5 and the results are presented in Ta-

ble 4. Variation of carbon and oxygen content was obvious be-

tween LT and HT condition.

Fig. 6 shows the molar ratios of C/H and O/H of VM. At the HT

condition, the drop in oxygen content was obvious for char derived

from paper whereas the drop in carbon content was obvious for

char derived from wood. In regard to char derived from DF, the var-

iation of the molar ratios of C/H and O/H of VM was not significant.

Comparing these results with HVVM in Table 4, oxygen dissociation

seems to contribute to the rise in HVVM at HT condition. High oxy-

gen dissociation might be related to the formation of carbon mon-

oxide and carbon dioxide by decarboxlyation reaction during

HTSW. As shown in Table 2, the Cg was highest for HTSW of paper

under the HT condition might be one of good evidence for the high-est oxygen drop in char derived from paper at HT condition.

Table 2

Carbon balance of product depending on raw material and HTSW condition.

Carbon recovery ratio a Carbon distribution ratio b

CR Cc Cl Cg

Paper LT (n = 8) 0.99 ± 0.03 0.89 ± 0.02 0.05 ± 0.01 0.06 ± 0.02

HT ( n = 8) 1.00 ± 0.06 0.81 ± 0.03 0.08 ± 0.02 0.12 ± 0.02

DF LT (n = 4) 0.93 ± 0.03 0.92 ± 0.03 0.05 ± 0.02 0.03 ± 0.01

HT ( n = 2) 0.88 ± 0.03 0.85 ± 0.02 0.07 ± 0.01 0.09 ± 0.01

Wood LT (n = 8) 0.89 ± 0.02 0.89 ± 0.01 0.09 ± 0.01 0.03 ± 0.01

HT ( n = 6) 0.92 ± 0.01 0.84 ± 0.01 0.08 ± 0.01 0.07 ± 0.00

Plastics LT (n = 2) 1.01 ± 0.01 1.00 ± 0.00 0.00 ± 0.00 0.00 ± 0.00

HT ( n = 2) 1.01 ± 0.01 1.00 ± 0.00 0.00 ± 0.00 0.00 ± 0.00

Unit: weight ratio (–).a when the total carbon of raw material is normalized to 1.b when the total carbon of recovered as product is normalized to 1.

Table 3

Yield and composition of char depending on raw material and HTSW condition.

Yield Compositiona

Ash FC VM C H N S Ob

Paper LT (n = 8) 0.66 ± 0.07 0.08 ± 0.01 0.25 ± 0.06 0.67 ± 0.06 0.49 ± 0.03 0.05 ± 0.00 0.00 ± 0.00 0.01 ± 0.00 0.37 ± 0.03

HT (n = 8) 0.47 ± 0.02 0.09 ± 0.02 0.42 ± 0.01 0.49 ± 0.02 0.63 ± 0.02 0.05 ± 0.00 0.00 ± 0.00 0.02 ± 0.00 0.22 ± 0.02DF LT (n = 4) 0.66 ± 0.03 0.13 ± 0.03 0.25 ± 0.01 0.62 ± 0.02 0.56 ± 0.01 0.06 ± 0.00 0.05 ± 0.00 0.03 ± 0.00 0.18 ± 0.02

HT (n = 2) 0.55 ± 0.00 0.13 ± 0.01 0.33 ± 0.02 0.53 ± 0.03 0.58 ± 0.01 0.05 ± 0.00 0.05 ± 0.00 0.03 ± 0.00 0.15 ± 0.00

Wood LT (n = 8) 0.59 ± 0.03 0.01 ± 0.02 0.36 ± 0.04 0.63 ± 0.02 0.61 ± 0.03 0.05 ± 0.00 0.00 ± 0.00 0.01 ± 0.01 0.32 ± 0.01

HT (n = 6) 0.51 ± 0.02 0.02 ± 0.00 0.53 ± 0.01 0.44 ± 0.00 0.69 ± 0.01 0.05 ± 0.00 0.00 ± 0.00 0.01 ± 0.01 0.24 ± 0.01

Plast ics LT (n = 2) 0.98 ± 0.01 0.00 ± 0.01 0.01 ± 0.00 0.99 ± 0.00 0.85 ± 0.00 0.12 ± 0.00 0.00 ± 0.00 0.01 ± 0.01 0.01 ± 0.01

HT (n = 2) 1.00 ± 0.01 0.00 ± 0.01 0.01 ± 0.00 0.99 ± 0.00 0.84 ± 0.01 0.11 ± 0.01 0.00 ± 0.00 0.01 ± 0.00 0.03 ± 0.02

Unit: weight ratio ().

LT: HTSW at 234 C and 3 MPa.

HT: HTSW at 294 C and 8 MPa.a based on dry weight.b calculated by mass balance.

Holding time over 220oC (min)

0 20 40 60 80 100

Y i e l d o f V M + F C

o f c h a r w h e n

t h e w e i g h t o f r a w

m a t e r i a l i s 1 ( - )

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Paper-LT

Paper-HT

DF-LT

DF-HT

Wood-LT

Wood-HT

Plastics-LT

Plastics-HTP-L8

P-L5

P-L1

P-L2P-L7

P-L3

P-L4

P-L6

D-H1D-H2

P-H3

P-H5

P-H2

P-H4

P-H1

W-L2

W-L4

W-L1W-L3

D-L1

D-L2

W-H2

W-H1

W-H3

Fig. 3. Variation of VM + FC of char obtained from HTSW of different raw material

with holding time over 220 C (P: paper, D: DF, W: wood, L: LT condition, H: HT

condition).




3.4. Cl content of char and Cl removal by washing

Table 5 shows the Cl recovery ratio (ClR ) when the total Cl

added to raw material is normalized to one and the Cl distribution

ratio of char (Clc) and liquid + gas (Cll+g) when total Cl recovered as

products is normalized to one respectively. After HTSW under LT

and HT conditions, more than 83% of input Cl was recovered inproducts and more than 85% of recovered Cl was found in char.

When PVC is pyrolyzed in the range of 200–300 C, dehydro-

chlorination emits HCl gas, which dissolves in water (Mikataet al., 1996; Takeshita et al., 2004). As HTSW was performed over

234 C in this work, a considerable quantity of Cl atoms in PVC

was expected to shift to liquid products. However, more than

85% of Cl originating from PVC remained in char regardless of the

LT or HT condition (Table 5). Cl added in the form of NaCl also re-

mained in the char owing to its high melting point (Table 5).

To examine the feature of Cl compounds remaining in char, char

washing was performed. Fig. 7 shows the soluble and insoluble Cl

contents in char. In the case of PVC, a greater amount of soluble Cl

was observed in the char obtained under the HT condition (Fig. 7a).

This indicates that dehydrochlorination advanced more and Cl

atoms remained in char in the form of a soluble compound under

the HT condition. It is possible that PVC once decomposes into HCl,

which is dissolved in moisture at the surface and in the pores of char particles. Another possibility is that Cl atoms chemically

Table 4

HVs of char, FC, and VM and composition of VM.

IDa HVChar HVFC HVVMb Composition of VM c ()

(kJ/kg-char) (kJ/kg-FC) (kJ/kg-VM) C H N S O

Paper LT P-L1 18,290 32,110 16,464 0.36 0.07 0.00 0.02 0.54

P-L2 15,791 12,188 0.36 0.08 0.00 0.01 0.56

P-L3 13,886 7375 0.35 0.08 0.00 0.01 0.56

Avg ± st dev 15,989 ± 2209 12,009 ± 4547 0.36 ± 0.01 0.08 ± 0.01 0.00 ± 0.00 0.01 ± 0.01 0.55 ± 0.01

HT P-H1 26,000 32,210 24,097 0.42 0.10 0.00 0.02 0.46

P-H2 23,929 22,755 0.41 0.10 0.00 0.04 0.45

P-H3 21,267 16,467 0.43 0.10 0.00 0.04 0.42

Avg ± st dev 23,732 ± 2373 21,106 ± 4073 0.42 ± 0.01 0.10 ± 0.00 0.00 ± 0.00 0.03 ± 0.01 0.44 ± 0.02

DF LT D-L1 24,627 29,203 24,911 0.47 0.09 0.08 0.04 0.33

D-L2 27,145 31,325 0.50 0.09 0.08 0.04 0.29

Avg ± st dev 25,886 ± 1780 28,118 ± 4535 0.49 ± 0.02 0.09 ± 0.00 0.08 ± 0.00 0.04 ± 0.00 0.31 ± 0.03

HT D-H1 25,114 31,738 26,936 0.45 0.10 0.10 0.06 0.29

D-H2 26,985 29,698 0.48 0.10 0.10 0.06 0.27

Avg ± st dev 26,050 ± 1323 28,317 ± 1953 0.47 ± 0.02 0.10 ± 0.00 0.10 ± 0.00 0.06 ± 0.00 0.28 ± 0.01

Wood LT W-L1 24,853 33,378 20,288 0.40 0.08 0.00 0.00 0.52

W-L2 22,134 18,895 0.39 0.08 0.00 0.03 0.51

Avg ± st dev 23,494 ± 1923 19,592 ± 985 0.40 ± 0.01 0.08 ± 0.00 0.00 ± 0.00 0.02 ± 0.02 0.52 ± 0.01

HT W-H1 26,036 34,084 19,066 0.32 0.10 0.00 0.03 0.56

W-H2 27,544 22,714 0.33 0.10 0.00 0.00 0.57

Avg ± st dev 26,790 ± 1066 20,890 ± 2580 0.33 ± 0.01 0.10 ± 0.00 0.00 ± 0.00 0.02 ± 0.02 0.57 ± 0.01

a

Refer Fig. 3.b estimated using equation 1 assuming that HVFC is 32,800 kJ/kg.c when the VM weight is normalized to 1.

040008000120001600020000 0.0 0.2 0.4 0.6 0.8 1.0

FC

VM

Ash

Recovered FC, VM, and ash as char

(kg/kg-raw material)

(b)

Paper

DF

Wood

Recovered energy as char

(kJ/kg-raw material)

(a)

P-L1P-L2P-L3

P-H1P-H2P-H3

D-L1D-L2

D-H1D-H2

W-L1W-L2

W-H1W-H2

Fig. 4. Recovered energy and composition as char from 1 kg raw material (P: paper, D: DF, W: wood, L: LT condition, H: HT condition).

C H O AshCHAR

C H

FCFC

VM

NS

ONS

Fig. 5. Estimation of elementary composition of VM composition.




combine with alkali metals included in char during HTSW. PVC was

added as 2% Cl (0.56 Cl-eq/kg) for raw material. However, the con-

tents of alkali metals such as K, Na, Mg, and Ca were 1.47 and 0.05

Cl-eq/kg for paper and wood respectively. Thus, the latter seems

slim for char derived from wood. On the other hand, Cl added in

the form of NaCl was largely removable by washing char produced

under both LT and HT conditions as shown in Fig. 7b.

Soluble Cl of char derived from paper + PVC was obviously high-

er than that of char derived from wood + PVC under the LT condi-

tion (Fig. 7a). Considering that the yield of VM + FC decreased with

increasing holding time over 220 C (Fig. 3), the same correlation

might be observed between the formation of soluble Cl and theholding time over 220 C. Accordingly, a longer holding time for

PVC over 220 C led to the progression of dehydrochlorination,

enhancing the generation of soluble Cl compounds in char that

can be removed by washing.

4. Conclusions

HTSW at 234 C and 3 MPa (LT) and 295 C and 8 MPa (HT) was

investigated as a method of recovering solid fuel from MSW. More

than 75% of carbon in raw material was recovered as char by HTSW

under LT and HT conditions, and the char had an HV comparable to

O/H molar ratio (-)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

C / H

m o l a r r a t i o

( - )

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Paper

Paper-LT

Paper-HT

DF

DF-LT

DF-HT

Wood

Wood-LT

Wood-HT

D-L2 D-L1

D-H2

D-H1

P-H3

P-H2

P-H1

W-L1

W-L2

P-L1

P-L3P-L2

W-H2

W-H1

Fig. 6. Molar ratios of C/H and O/H of VM (P: paper, D: DF, W: wood, L: LT condition,

H: HT condition).

Table 5

Cl balance of product depending on the Cl source and HTSW condition.

IDa Cl recovery ratio b Cl distribution ratio c

ClR Clc Cll+g

Paper + PVC LT P-L3 1.01 0.98 0.02

P-L4 0.89 0.98 0.02

P-L5 1.00 1.00 0.00

P-L6 0.89 0.98 0.02

Avg ± stdev 0.95 ± 0.07 0.98 ± 0.01 0.02 ± 0.01

HT P-H4 0.98 0.87 0.13

P-H5 1.07 0.98 0.02

Avg ± stdev 1.02 ± 0.06 0.92 ± 0.08 0.08 ± 0.08

Wood + PVC LT W-L3 0.93 0.97 0.03

W-L4 0.98 0.99 0.01

Avg ± stdev 0.95 ± 0.04 0.98 ± 0.01 0.02 ± 0.01

HT W-H2 0.83 0.94 0.06

W-H3 1.01 0.85 0.15

Avg ± stdev 0.92 ± 0.13 0.89 ± 0.07 0.11 ± 0.07

Paper + NaCl LT P-L7 1.02 0.97 0.03

P-L8 1.04 0.98 0.02

Avg ± stdev 1.03 ± 0.01 0.97 ± 0.01 0.03 ± 0.01

HT P-H2 0.97 0.93 0.07

P-H3 0.97 0.95 0.05

Avg ± stdev 0.97 ± 0.00 0.94 ± 0.01 0.06 ± 0.01

a Refer Fig. 3b

when the amount of input Cl is 1.c when the amount of recovered Cl is 1.

0.0 0.2 0.4 0.6 0.8 1.0 1.2

(a) Addition of PVC

0.0 0.2 0.4 0.6 0.8 1.0 1.2

(b) Addition of NaCl

Cl content (mg-Cl/g-char)

Paper-LT

Paper-HT

P-L3

P-L4

P-L5

P-L6

P-H4

P-H5

Cl content (mg-Cl/g-char)

Wood-LT

W-L3

W-L4

Wood-HT

W-H2

W-H3

Paper-LTP-L7

P-L8

Paper-HTP-H2

P-H3

Fig. 7. Soluble and insoluble Cl contents of char ( soluble Cl, insoluble Cl).




that of brown coal and lignite. A higher concentration of carbon as

FC and greater oxygen dissociation of VM during HTSW are

thought to improve the HV of char. These reaction characteristics

were obvious for char derived from paper and wood. Considering

that paper occupies 30–50% of household waste, such reaction

characteristics are important to the recovery of qualified fuel from

MSW during hydrothermal treatment.

Plastics did not degrade under LT and HT conditions in thiswork. If a large amount of plastic is mixed in the input waste,

the quality of char may be uneven owing to the partial melting

of plastics during HTSW under conditions considered in this work.

Most Cl originating from PVC and NaCl remained in char after

HTSW. However, PVC substantially degraded into soluble Cl com-

pounds that could be removed by washing under the HT condition.

Considering the presence of salt of food waste and PVC in MSW the

combination of HTSW under the HT condition and char washing

might be a proper method in case a strict standard of Cl content

of char is required to use it as alternative fuel.

From these results, the merits of HTSW as a method of recover-

ing solid fuel from MSW are considered to require no drying pro-

cess prior to HTSW, to produce char with minimal carbon loss,

and to decompose PVC into soluble Cl compounds removable by

washing.

Acknowledgements

The authors would like to acknowledge financial support from

Kubota Co. Ltd. and the Support Office for Female Researchers at

Hokkaido University (FResHU).

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in-hee hwang, hiroya aoyama

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