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Combust ion Character istics of Moist Wood
Kenji AMAG AI Masahiro SAITO Goro OGIWARA Chun Jung KIM and Masataka ARAI
Dept. Mech. Sys. Engineering, School of Engineering, Gunma University
1-5-1 Tenjin-sho, Kiryu, Gunma, 376-8515 Japan
Tel.: +81-27 7-30-1522, Fax.: +81-277-30-1521
E-mail: [email protected]
ABSTRACT
Combu stion characteristics of water-contain ed balsa were
experimentally studied as the fundamental investigation of a thermal
recycling of urban dusts. Urb an dusts usually contain plastics,
vegetables and woo d materials. An d also, these dusts contain water in
their moisture compo sitions and on their wet surfaces. Then, dry and
wet balsa pieces were chosen as examples of wet urban dusts, and
bunted in an electric furnace. Mass reductions before ignition and
during volatile and char combustion periods were measured by an
electric micro-balance. Als o, flam e temp eratu re duri ng volatile
combustion period was recorded. Ignition delay and the period of
volatile combustion were greatly affected by the water content in a test
piece of balsa. Ignition of the wet balsa was onset before the dry-up
of all water content. Wh en the water content was less than 50%,
ignition delay and volatile combustion periods were elongated but the
total combustion period including char co mbustion was shghtly changed.
Ignition dd ay was strongly elongated by the water content of over 50%.
However, other characteristic periods exce pt the ignition delay were
slightly changed w ith increasing over 50% of water content.
INTRODUCTION
The waste materials forme d during industrial produ ction process and
urban living, have been considered as the negative productions in the
urban society. Harm less treatments o f industrial wastes and urba n
dusts should be, then, developed to keep healthy circumstance of the
enviromnent. Material and therma l recyclin g treatments mig ht be the
main solutions of this problem. Bu m-u p treatments in incinerators [1-
3] are considered as the most effective way o f the harmless treatments o f
wastes and dusts, and also considered as the thermal recycling treatment
of th e waste materials.
Waste materials have va rious compo nents such as woo d, plastic, oil,
paper, vegetable, and they contain ed water. Then, thermal recycling
systems for waste materials h ave been de velope d for individual waste
material. The comb ustion techno logies of bioma ss [4-6] have been
developed as the thermal recycling treatment of waste materials.
Waste plastics, rubbers and used tires have been considered to be
thermally recy cled to fuel throu gh gasificatio n [7-10].
The waste materials, which are abandoned as garbage from a hom e
kitchen, b ecome also increasing with the de velopm ent of urban life.
Vegetables and other remains of meals, plastics, woo ds and papers are
the main part of these urba n dusts. Uncertain characteristics o f
ignition behavior, comb ustion heat and burn ing rate of urban dusts are
the main reasons of the difficulties of the effective treatment in the
thermal recycling. More fundamental problems are owed to the luck
of technical information about the c ombu stion characteristics of each
componen t of urban dusts and fundamental research works are intended
to clarify them. Pyro lysis and comb ustion of cellulose [11-13], paper
combustion [14] and w ood combu stion [15-18] had been studied on this
purpose.
Wood and other biomass materials with high moisture are the maha
components of urban dust. The dry-up and ignition processes of wood
and biomass are considered as the key processes of thermal recycling
throu gh an urban dust incinerator. Then, to clarify the combu stion
characteristics of high mo isture ur ban dusts, combu stion characteristics
of wood with high moisture were investigated in this study. Balsa
wood with high w a t e r c o n t e n t was selected as a test material, and
combustion b ehavior affected by moisture were studied.
EXPERIMENTAL APPARATUS AND TEST PIECE
Com bustion characteristics of wate.r contained balsa wo od were
studied in a small electric furnace shown in Fig.l . A test furnace was
set on a bench and co uld move fro m left to right at an instance of
experim ent start. The furnace had 100m m diameter and 250nm~
height in its inside sizes. It had an entrance gate for test piece and an
observation win dow for a video camera. Temperature of the furnace
was controlled and monitored by a thermocouple. A small block of
test piece was put on the wire net holder. An electric microbalance
located below the furnace ben ch co uld measure w eight change o f tile test
piece during a combustion test and this w eight change w as recorded.
According the following reasons, a small cubic block of balsa wood
was selected as a samp le of the water co ntained test piece.
(1) Waste materials such as urban dust have usually contained wood
material as their main com ponents
(2) Balsa is a relatively homog eneo us woo d material.
(3) W a t e r c o n t e n t
in a balsa piece can be controlled easily.
(4) Cubic shape test piece is considered as a typical example of urban
dusts o f small chips.
A result o f industrial fuel analysis o f tested balsa is listed in Table 1.
It shows that balsa had about 75% of volatile matter and a tew percents
of moisture. Then, balsa was dried-up before experiment and added
t •
Proceedings of2000 International Joint Power Generation Conference
Miami Beach, Florida, July 23-26, 2000
IJPGC2000-15071
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¢ ~ q r t a ~ t A ' 2 4
Shutter
Recoder
Experimental set-up
Video recorder
Fig. 1
Recoder
Tablel Industrial fuel analysis for balsa
Volat i le matter 74.2 %
F ixed ca rbon 18 .0 %
Ash 1.2 %
Mois tu re 6 . 6 %
Bulk dens i ty 0 . 21 g /em 3
water to obtain a water-con tained balm. The water content Cw[% in
the test piece was defined as follows.
Cw = Mw Mw
..... × 100 = x 100 (1 )
Mo Mb + M w
Where, .~4 is mass of dry balsa and M~ is mass o f water adding to the
test piece, Btdk density of balsa was 0.21g/era3. It mean s that a
cubic test piece of O.3g mass had ll. 3m m length in height and 0.5g mass
had 13.4mm.
OBSERVATION OF FLAME
The fundamental com bustion behavior of the wood had the volatile
combustion stage with visible flame and char combustion stage.
Figure 2 sho ws the illustration of the balsa combu stion observed in the
furnace of 650 K. There was some preheating duration between the
start of experime nt and ignition (a). At the instance of ignition (b),
there was a small visible flame near the surface of test piece. After
short period elapsed from ignition (c), flame was developed by an
increased emission of volatile matter from the test piece. The flame
size took the max imum (d), and decreased (e). At the final stage of the
combu stion, char comb ustion (f) was observed. It was characterized
by a bright red fight emission from fixed carbon combustion in the test
piece. The ash stage was clearly noticed by the end of light emission.
These illustrations were the fu ndamen tal combustio n characteristics and
they were co mmon ly observed on the dry balsa and water-contained wet
balsa.
To monitor the flame temperature during the volatile combustion
stage, Pt-PtRh13% thermocouple wa s set at twice height position above
the test piece. This thermo couple wa s also illustrated in Fig.2. The
mass reduction of the test piece during the combustion stage and change
of flame temperature were typica lly illustrated in Fig. 3. Usually, mass
reduction was started in the preheating duration and rapid reduction w as
observe d in the volatile com bustio n stage. Since, the thenn ocoup le
was set at the fixed position, the flame temperature monitored by it
showed the maximum when the top of the flame was touched to the
thermo couple in the middle stage in volatile combustion.
Ignition limit of volatile combustion was checked using the test
pieces of dry balsa. Figure 4 shows the ignition limit temperature of
dry balsa. In the lower temperature condition than 450K, only the
char combu stion wa s observed. As increasing the temperature,
volatile combustio n becam e observed i f the test piece had a sufficient
initial mass (larger than 0.05g). It means that the ignition with visible
flame was greatly influenced by the absolute mass of the volatile matter
emitted from the test piece. Fro m this study, the cubic shaped test
pieces of 0.3g, 0.5g and 1.0g was selected for the combustion test
concer ning the effect of wate r content. An d also, the ambient furnace
temperature o f 650 K was selected as a test temperature where a visible
flame of volatile matter was constantly observed in the balsa
combustion.
EFFECT OF WATER CONTENT ON COMBUSTION PROCESS
The combustion process of 0,3 g dry balsa in the furnace of 650 K
Thermocor
(a)
Preheat
)le
( b ) ( c ) ( d ) ( e ) ( f )
Ignition Volatile .combustion Char combustion
Fig.2 Illustrations o f comb ustion process
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~ m
O
m ~
~ , T
Fig.3
( a )
a.,
P r e h e a l
E l a p s e d t i m e t
Models o f mass reduction and flame temperature
10 2 ~ ' '
O 3
(,3
10 ~
10
10 ~
10 ~
10 ~
- 4
10350
Volat i le comb.
x I 0 0 0 0 0
x ~ 0 0 0 0 0
x / 0 0 0 0 0
~ ~ 8 8 8 8 8
x / 0 0 0 0 0
. \ 8 8 8 8 8
x ~A 0 0 0 0
** ~
x ~ 0 0 0 0
C h a r c ; m b . x x V
Ignit ion l imit
4 0 450 500 5 0 6 0 650
Ambient temperature
T a
°C
700
Fig.4 Ignition limit of dry balsa
was shown in Fig. 5. The solid symbols used in the combustion period
indicated the volatile combustion stage, and open symbols were char
combustion. The ignition was indicated at the beginn ing of the plots
of solid symbols• The ignition delay of this condition was very short
and reduction of mass was smoothly processing with volatile
combustion.
Figure 6 shows the combustion process of C ,=5 0% . The net mass
of the balsa was 0.3g. It w as the same w eigh t w ith the test piece used
in the dry balsa experimen t but the total mass of the test piece w as 0.6g.
Since, this test piece contained water, evapor ation of water con tent was
colmnenced first and the temperature decreased once before the ignition.
Taking into account of the thermocouple being set at the twice height
locatiou, the test piece in pre-heating stage was considered to be
enveloped by low temperature w ater vapor. An d ignition was
occurred in the mixture of water vapor and volatile components. The
mass reduction before the ignition was observed and ignition delay was
significantly longer than the case of dry balsa.
t -
O
O
O
o9
03
1.5
1
0.5
Z
| , i | l l i
C w = 0 % M b = O . 3 g T a = 6 50 ° C
• • W i th
f lame
• o Without f lame
2 0 0 0
1500
1000
500
o
k .
L- -
o.
E
E
LL
0 0
0 30 60 90 120 150 180
Fig.5
E l a p s e d t i m e t [ s ]
Ma ss reduct ion and flam e temperature
in dry balsa combustion
[ o
- [
1.5
1 • , 0 , , Q • • t , • , , - , •
i C w = 5 0 % M b = 0 . 3 g T a = 6 50 ° C
0
i
i
, • •
With F lam e
._o
- I
' A ,, o With out Fla me
I
• , f
0 30 60
2000
1500
I 0 0 0
I I
90 120 150
500
i ,---JO
180
oo
m
E
E
u .
Fig.6
E l a p s e d t i m e t [ s ]
Mas s reduction and flame temperature in
50% w ater contained balsa combustion
The overall combustion phenomena observed in dry balsa and 50%
water contained balsa were almost same. However, the combustion
characteristics of 70% w ater contained balsa had somew hat different
comb ustion characteristics as shown in Fig.7. It was characterized by
the delayed ignition and the delayed temperature rise. In this case, the
temperature rise was. delaye d from the ignition. It mean t that a flame
forme d initially aroun d the test piece had low flam e temperature and the
flame temperature took the maximum when the mass reduction was
processe d to the final stage of volatile combustion. The period of
volatile co mbustio n increased with increasing the water content from 0%
to 70%.
Figure 8 shows the maximum temperature monitored by the
therrnocouple. The furnace temperatur e was 650K. There were
two examples of the test pieces in the figure. Both the results of 0.3g
and 0.5g test pieces showed that the flame temperature was not
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.o
- -
0 30 60 9 0 120 150 180
i . . . . . . . . . . . . . . . . . o 1 o o E
Cw=70O/o,, Mb = O.3g ra= 650 C t
i • • With Flame
i
~. 1.5
, , , o W i t h o u t F l a m e t 1 5 0 0
~
,,
~
, ~
'
1000 ~
E
i o o E :
LL
E l a p s e d t i m e t [ s ]
Fig.7 Mass reduction and flame temperature i n
70% w ater contained balsa combustion
P
G t .
E
(1)
E
t~
v,
Q .
Fig.8
2000
15od
1000
i - i i
T. =650 °C
o Mb = 0 .3 g
,, Mb = 0.5 g
Z~
' 2 ' o ' 4 ' o ' 6 ' o ' d o ' 1 o o
Water content Cw [ % ]
The maximu m flame temperature at volatile com bustion stage
influenced by the water content and w as also independent from the size
of test piece. As shown in the previous figures, the elapsed time when
the temperature took the maximum was delayed with an increase of the
water contents. Then, it was considere d that the max imum flame
temperature of water contained test piece appeared after the all of the
water content was evaporated, and fro m this reason, the maxim um
temperature was not chang ed with the water content. In other words,
the expansion of the volatile com bustio n period w as caused by the slow
and low temperature comb ustion of the early stage of volatile
combustion, The later stage of volatile comb ustion was not affected
by the water content.
t - -
.9
- i
O
o9
o9
Fig. 9
B a l s a M o = 6 . 3 g T . = ~ S 0 ° 6
o o o zx z t~ W ithout f lame
,,
• ° - z H With
f lame
N Water content
o
0 % 5 0 %
n 1 0 % z 7 0 %
\ \
100 200 3
Elapsed tim e t Is]
Mass reductio ns and starts o f volatile comb ustion
of various water contents balsa pieces (0.3g)
¢..
. 9
O
o9
o9
¢0
Fig,10
n * ~ = i . . . . . I ~ . . . . - ' - l w
Mb 1 .0g Ta= '650cC
• ' ~ Wate r con ten t
~ . o 0 % 4 7 0 %
~ o 30 % o 8 0 %
~ ~ ~ 50 °/°
. ~ %,, o ot~ , I o. w ithout f lam e
i • • H
with
f l ame
250
O O
Elapsed time t [s]
Mass reductions an d starts of volatile combustion
of various w ater contents balsa pieces (1.0g)
EFFECT OF WATER CONTENT ON MASS REDUCTION
The ignition delay and combustion, phen ome na just after ignition
were greatly affected by the w a te r v ap o r evapora ted from th e test piece,
Figure 9 shows the results of mass reduction o f the test pieces of
different water contents. The initial mass of the test pieces were
different ow ing the different contents of water, however, the pieces had
the same net mass of the balsa (0.3g), The ignition delay and the mass
reduction at the ignition were corresponding the start of volatile
combustion indicated by the solid symbols.
The ignition delay time increased with an increase of the water
content. The mass of the test
p i e c e a t
ignition was not consisted to the
net mass of the balsa. It mea nt that the test piece was ignited before
the dry-up. Since, the test piece bad a cubic shape, the com er of the
cubic was dried-up at first. Then, both of the volatile matter and water
vapor wer e emitted in the dry-u p duration. The combustio n
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characteristics of the test piece were character ized by the auto-ignition
and combustion of the mixture that had the water vapor in it .
These behaviors of mass reduction were also observed by the test
pieces having 1.0g of net balsa. The experimental results wer e show n
in FigA0. From the mass reductions shown in both figures, it was
concluded that the mass reduction rates of before and after ignition were
not obviously different with each other. Since the mass emission was
the results of the balance o f heat transfer from the surroundings, it meant
that this heat transfer was not affected by flame. In other word ,
emission rate of the water and volatile matters were controlled by the
factors inside the te st piece•
I G N I T I O N A N D C O M B U ST I O N CHARACT E RI ST I CS
Mass reduction s of test pieces at ignition w ere su mmarized in Fig. 11.
Mass of the test piece at ignition stage became smaller with an increase
of water contents. Howeve r the test piece having water content of less
than 50% shows the little reduction of mass at ignition instance. It
shows that the test pieces o f these conditions could have an envelope
flame before dry-up. At early stage of volatile combustio n, the
envelope flame contained a water v apor emitted fro m the test piece, then,
the flanle temperature did not increased at this stage. Whe n the water
content was exceeded over 50% o f the total mass o f test piece, water
contained volatile mixture was hardly ignited. As the result, ignition
was delayed until concentration of water vapor was reduced, then, the
mass reduction at the ignition point was obvious. The ignition was
taken place before the main part of water being dried-up from the test
piece, then, the mass at the ignition wa s still larger than the net ma ss o f
balsa.
Ignition delay, start o f char comb ustion and over-all combustion time
were summed up in Fig. 12. All of the characteristic times show n
here, increased with an increase of the water content in a test piece.
However, the increasing ratios we re much different betwee n low water
content and high water content test pieces. Wh en the water content
was lower than 50%, ignition delay and all the combustion period were
slightly changed from those o f dry balsa, however the combustion period
of the volatile matter was expanded. It means the volatile comb ustion
with water vapor processed slowly with an increase of water content.
Ignition delay was strongly elongated by the water content of over 50%.
However, other characteristic periods ex cept the ignition delay wer e
slightly changed with increasing over 50% of water content
1 5
7
¢.-
._o
¢..
~
0 . 5
{O
Fig . 11
I I I I I I I I
Ta=650oc o M b=0.1 g
zx 0.3g
n 1.0g
I I I I t I I I I
0 20 4 0 60 80 100
W ater content £~, [%]
Mass reduc tions at the starts o f volatile comb ustion
~ 2 2
> ~ 0 0
- - ~ E ~
~ 8 ~
3
200
10(
Fig. 12
i
Balsa
. . . . . .
Ta=650 °C Mb=0 .3 g
Ov er-all combustion time R
Start of
char combustion / o °° /
5 0
Water content Cw [%1
Ignition delay and combustion periods
of water contained balsa pieces
100
CONCL USI ONS
Water contained balsa were selected as a typical example of the
urban dusts. Ignitio n and combustio n characteristics investigated here
led to the fo llowing results.
(1) The envelope-type visible flame around a test piece was only
observed when the furnace temperature was higher than 450K and
test piece had a sufficient mass (abo ut 0.05g).
(2) The maxim um flame temperatu re appeared at late stage of volatile
combustion where the w ater contents in the test piece seemed to be
dried-up,
(3) Whe n the water content was lowe r than 50%, ignition delay and all
the combustion period were slightly changed from those of dry
balsa, however period of volatile combustion was expanded.
(4) Wh en the water content was higher than 50%, ignition was
extremely delayed but volatile and char combustion periods was
slightly change d with wa ter content.
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6 Copyright (C) 2000 by ASME