15071 Combustion - Moist Woods

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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

1 Copyright (C) 2000 by ASME



¢ ~ 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




~ 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


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




.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


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




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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1. S.Kim, D .Shin and S.Choi, Com parativ e Evaluation of Mmficipal

Solid Waste Incinerator Designs by Flow Simulation , Comb. &

Flame, 106, 1996, pp241-251.

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the Burning Bed of a W aste Incinerator' , J. of Inst. o f Energy, 71,

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Solid Wastes in Mala ysia , J. ofins t, of Energy, 71, 1998, pp55-58.

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Characteristics o f Self-Desulfurkzation an d Self-Denitrification in

Biobriquette Combustion , 27th Int. Symp. on Combustion, 1998,

i •




pp2973-2979.

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Automotive Tire

Waste ,

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Combustion of Cellulouse under Conditions of Rapid Heating ,

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Properties of Pure and Fire-Retarded Cellulose , Comb. & Flame,

84, 19 91, pp121-140.

14. T.Kashiwagi and H.Nambu, G lobal Kinetic Constants for Thermal

Oxidative Degradation o f a Cellulosic Paper , Com b. & Flame, 88,

1992, pp345-368.

15. F.Y.Hshieh and G.N .Richards, Th e Effect of Preheating of Wood

on Ignition Temperature of Wood Char , Com b. & Flame, 80, 1989,

pp395-398.

16. F.YHshieh and G.N.Richards, Infrared Investigation of the

Oxygen Chemissorption of Wood , Comb. & Flame, 84, 1991,

pp423-426.

17. C.K.Ndiema, Thermogravimetric Analysis of Wood-Base Biomass

Materials ,

J. of Inst. of Energy, 71, 19 98, pp59-61.

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Characteristics of Balsa Ch ips (I), Combustion Behavior and M ass

Reduction Rate (in Japanese) , J. the Japan Inst. o f Energy, 77-10,

1998, pp1019-1027.


15071 Combustion - Moist Woods

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