Clean combustion of wood?

Clean combustion of wood?

Citation for published version (APA):

Khan, A. M. H. R., & Verhaart, P. (1989). Clean combustion of wood? Eindhoven University of Technology.

Document status and date: Published: 01/01/1989

Document Version:

Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers)

Please check the document version of this publication:

• A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website.

• The final author version and the galley proof are versions of the publication after peer review.

• The final published version features the final layout of the paper including the volume, issue and page numbers.

Link to publication

General rights

Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain

• You may freely distribute the URL identifying the publication in the public portal.

If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, please follow below link for the End User Agreement:

www.tue.nl/taverne

Take down policy

If you believe that this document breaches copyright please contact us at:

openaccess@tue.nl

providing details and we will investigate your claim.

CLEAN COMBUSTION OF WOOD

? by

Dr.

A.M.

Hasan

R.

Khan M &

Ir. P. Ver-haart

The Woodburning Stove Group Eindhoven University of Technology,

Eindhoven, The Netherlands

·, Permanent address: Institute of Fuel Research and Development, Bangladesh Council of Scientific and

Industrial Research. Mirpur Road, Dhaka - 1205 Bangladesh

CONTENTS

ACKNOWLEDGEMENTS

ABSI"RACT

1 . INTRODUCTION 1.1 General

1.2 Toxicity of Carbon Monoxide Concentration 1.3 Pyrolysis of wood

2. EXPERIMENTS

2.1 Downdraft Stoves 2.2 Procedure

3. RESULTS AND DISCUSSION

3.1 The General Behaviour of The Downdraft Stove

3.2 Effect of Wood Block Sizes

3.3 Effect of Fuel Charging Interval 3.4 Effect of Wood Species

3.5 Effect of Secondary Air By-pass 3.6 Effect of Power Output

3.7 Calculation of the Carbon Monoxide Concentration in an Enclosed Space

4. CONCLUSIONS

5. REFERENCES

page nr. II II I 1 1 3 5 7 7 9 12 12 16 19 23 26 28 32 35 36 I

ACKNOWLEDGEMENTS

The work reported in this study was made possible by a fellowship granted by the Commission of the European Communities, Brussels to Dr. A.M. Hasan R. Khan and arranged by the Science and

Technology Division, Ministry of Education, Government of the People's Republic of Bangladesh. The work was supervised by Dr. K. Krishna Prasad of the faculty of Physics, at the Eindhoven University of Technology. For this investigation, the laboratory facilities of the Woodburning Stove Group were used. Dr. Hasan was assisted by Ernst Schutte and Niek Verhoeven in the

ABSTRACT

In the present investigation, systematic experiments have been carried out in which the influence of some operational variables viz. woodblock size, fuel charging intervals, wood species, secondary air by-pass and power output on the quality of combustion, were studied. It has been observed that the

volume/surface area of woodblocks and the fuel charging intervals play an important role in maintaining clean combustion of wood. Woodblocks with a volume/surface area from 0.37 to 0.49 cm produce a reasonable low constant CO/C02 ratio which on the average is 0.13%. As the volume/surface area exceeds the above mentioned limits, the CO/C02 ratio increases sharply and at a volume/surface area of 0.53 cm this ratio is 1.23%. It has also been observed that with proper adjustment of charging intervals at a power output of 7.3 kW, the CO/C02 ratio is 0.006%, which is even lower than in gas burning appliances. The secondary air by-pass has no influence on the combustion. Wood species have much influence on combustion behaviour. The excess air factor for burning Oak and Beech is much lower than for White Fir. The

lowest excess air factor ever recorded in this investigation is 1.38 for Beech. Thus it may be concluded that, with correct size of woodblocks and proper adjustment of charging intervals, clean combustion of wood is possible for long periods.

Application of this principle in a domestic and large scale cookstove will result in a healthier kitchen environment without the need for a tall chimney to let the combustion products from the kitchen to the outside atmosphere. The clean hot gases from the downdraft stove can also be used in direct heating of food, as in bread ovens, where much fuel can be saved as compared to traditional ovens with direct or indirect heating.

1.

INTRODUCTION

1.1 General

From the dawn of civilization man has known the use of fire. Since then, fire-wood and other traditional fuels are used for cooking purposes. About half of the world's population, living in predominantly rural areas of the developing countries use

traditional fuels for cooking and other purposes. The most common device used for this purpose is the three stone stove. Available information indicates huge amounts of fire-wood are being

consumed in the developing countries for cooking purposes. It is generally believed that this is due to inefficient cooking

devices used. Apart from low efficiency, these stoves emit smoke which affects the health of the users and make the kitchen dirty.

With the growth of population in the developing countries the regeneration rate of traditional sources of fuel is gradually falling behind the consumption rate. A fuel supply crisis has appeared on the horizon and in some countries already has assumed alarming dimensions. Agricultural residues, fallen leaves,

cow-dung, etc. which should be used to increase the fertility of the soil for cash crops production. now are being used as cooking fuels. Even the bark of large trees, by the side of the road, is taken off for cooking fuel. Most developing countries have a forest area much below the sustainable level. This is causing a change in the eco-system leading to erosion and changes in the climate. To check this crisis, attempts are being made the world-over to improve the efficiency of cooking devices, to

increase the regeneration rate of the traditional sources by more planting, including fast growing species and to make use of

alternative sources of energy such as solar energy.

For the past 8 - 10 years scientists and technologists all-over the world realized this alarming crisis of traditional fuels.

Since then they have been working on renewable sources of energy and developed series of improved biomass stoves suited to a particular country according to local food habits and type of fuels used. These stoves save 50-80% of fuel when compared with the traditional ones. The improvement was made by proper

dimensioning of combustion chambers to provide maximum heat transfer to the utensils. In many cases provisions were made to recover waste heat. As a matter of fact, the main aim was maximum utilization of combustion energy and very little attention was paid to the combustion products.

Woodburning stoves always produce carbon monoxide (Sulilatu. 1985) and its release with other combustion products in a kitchen or other enclosed space will increase the concentration of carbon monoxide. Depending on stove, kitchen volume, and air exchange

rate, carbon monoxide concentrations can reach such a level that it will affect the health of the users.

It was also stated by Smith (Smith, 1984) that incomplete

combustion of biomass in cookstoves posed a major threat to the respiratory system of users due to the release of large

quantities of irritants. toxins and carcinogens in the kitchen environment.

Wood, a product of photosynthesis is a complex chemical substance (Krishna Prasad et al., 1984). For all practical purposes wood consists of carbon, oxygen and hydrogen. Thus the pollution caused by wood combustion is more manageable because it does not contain sulphur or other toxic elements and its ash content is rather small. In general, combustion products of wood are carbon dioxide (C02 ) , water vapour (H20) carbon monoxide (CO),

particulates and polycyclic organic matter (POM). The last three are considered hazardous pollutants with respect to human health.

During the past 4-5 years, the Woodburning Stove Group at the Eindhoven University of Technology has been working on clean combustion of wood. This is another approach for improving qualities of woodburning stoves. It is assumed that if the fluegas is odourless, the only remaining pollutant is CO. The

yardstick for measurement of clean combustion is the CO/C0₂ ratio in the combustion gases. In a reasonably clean combustion gas, this ratio must be less than 0.5% and should not have any odour. The latest development by the Woodburning Stove Group is the so-called downdraft stove, which can exhibit clean combustion for extended periods even at a high power output (7.3 Kw). The

principle of operation of the stove, in contrast to conventional designs, is that the flow of air is in the same direction as the combustion gases and fuel. Possible application of this principle in the near future, may do away with tall chimneys in domestic and large scale cooking stoves, which are very difficult to maintain as well as very costly. Also direct application of hot clean combustion gases in heating food-stuffs even in bread ovens, is envisaged when much fuel can be saved in comparison to conventional ovens with indirect heating.

The present design, the so-called downdraft stove is very sensitive to operational variables. In this investigation systematic experiments have been carried out to relate stove design, size of wood blocks, charging intervals, effect of secondary air-by pass, wood species and power output.

l

1.2 Toxicity of Carbon Monoxide

While designing a cookstove, importance should be given to the combustion quality. The products of incomplete combustion are carbon monoxide, soot and polycyclic organic matter. Sulilatu

---~c:;.--, (Sulilatu, 1985) clearly explained in his article, the effect of carbon monoxide in the atmosphere as a function of the exposure time for various conditions of labour.TCarbon monoxide is a colourless, odourless and highly poisonous gas. It has a strong affinity to hemoglobin (Hb) in the blood which carries oxygen to body tissues. Carbon monoxide deprives the tissues of the

necessary supply of oxygen. The following reactions show how oxygen and carbon monoxide react with hemoglobin.

L

CO + Hb ---~coHb ( 1. 2)

However, binding force of CO to Hb, is about 300 times that of 02 to Hb.

When breathing polluted air, CO as well as 02 are bound to the hemoglobin according to the equilibrium reaction:

( 1.3)

The effect of the carbon monoxide concentration in the atmosphere as a function of the exposure time for various conditions of

is shown in figure 1.1. co(%) atmosphere

t

015 0.1 0.05 0

b

0 0 l I

I

I

I

I

I

I

~--'

_'

---

--- ~-~---\

--·-\

\ \ poisoning

\

--\ . , maxi mum allowable

'.._..__ . (MAC)

--

---resting ₆₀ moving 30 20 cone 120 60 40

working ..,. exposure time (min)

Figure 1.1: Effect of carbon monoxide concentration in the atmosphere as a function of exposure time for various conditions of labour.

1.3 Pyrolysis of Wood

Wood consists of hemicellulose, cellulose and lignin. The

combustion of wood is a very complex process. However, combustion of wood follows three distinct ways (Prasad, 1985):

(i) pyrolysis resulting in the liberation of volatiles and the formation of char;

(ii) burning of the char; and (iii) burning of the volatiles.

Pyrolysis of wood begins around 250°C and very actively around 325°C. Hemicellulose decomposes between 200 - 260°C. It is

followed by the decomposition of cellulose between 240 - 350°C. Lignin breaks down finally at a temperature of 280°C or more. The entire pyrolysis process ends at about temperature of around 500°C.

:.

0

-

-Gaseous phase combustion

diffusion flame, mostly

Flame

turbulent - a 'free' fire

..---_o ____

!~~ooo·c

(probably

~

12oo·c)

Char

(

""""'-

- -

-Simultaneous heat & mass transfer

with chemical reaction; surface

combustion - a slow process

_s_Qq_~r ~aoo

_{• c}

Problem same as in zone A

Pyrolytic

but with sources/sinks(?) due

_

zone

to pyrolytic reactions

~

B

200 dtT -'500 • C

:I: - - -

-Heat conduction in a medium

with a moving boundary

;

migration of moisture & gases;

uncertain properties

Virgin

wood

A

-

T

~200

•

C

Chemical analysis of different wood species showed that they are almost similar in composition. Therefore the burning

characteristics are more or less assumed similar. Figure 1.2 represents the burning process of wood (Williams, 1974, and Brame

&

King, 1967).

According to the figure, the burning process starts as follows (Bussmann, 1988). When wood is heated, it first loses its moisture, and at about 350°C it starts releasing volatiles consisting mainly of lower hydrocarbons, carbon monoxide, tar

(viscous organic liquids) etc .. These volatiles burn with the well known characteristic bright yellow flames. The residue left

is mainly charcoal. It burns at about 800°C with a faint blue flame. After the burning of charcoal, the residue left is ash, which mainly consists of Si02 . These processes occur

simultaneously in a piece of wood at different locations.

2. EXPERIMENTS

2.1 Downdraft Stoves

There were two types of stoves used in these investigations. (i) Downdraft stove-I as shown in figure 2.1.

(ii) Dowmdraft stove-II as shown in figure 2.2.

0120 <>-'----C>

c

L() /jl'

j

,.

---

~

A

E

190 0 130 500

~

I;'.

!

.

~ :~·1

~

~

~

, r---'"---'

~

'~: " 4 '.1 " ~

f::

~

. I fl (,?\ C•"'~ ~-

--~

Ii

'· t· ~ l:: ~ i.:i

~

N ) ~ : :~ J.' 'I

I

]

t'

I

Figure 2.1: Downdraft stove-I (dimensions in mm).

....,

""" :

-·

_-.

~

·

-

~

A: bowl; B: grate; C: main body with chimney; D: insulation; E: Secondary air by-pass.

The first stove was burnt away after performing only part of the experiments due to high power outputs, so the rest of the

experiments was done with the second one. Both stoves produced fluegases which contain very small percentages of carbon

monoxide. As the specifications of the two stoves are different, the first one is capable of a power output of 2.6 - 4.4, and the second one between 5.4 - 7.3 kW.

0105

c

0 120

§1

__

J0

1

___

J

A

E

~""'

C> 0 130 < J c - - - C > 245 Figure 2.2: Downdraft stove-II (dimensions in mm).

I

I

IQ

10 0

A: bowl; B: grate; C: main body with chimney: D: insulation; E: Secondary air by-pass.

Principle of combustion

When wood is heated, the volatiles are released and the charcoal is left on the grate. For proper combustion of the volatiles and the charcoal in a classical stove, air enters the stove by

natural draught in two different directions. (i) Primary air enters from under the grate and moves in upward direction through the fuelbed and helps the burning of the charcoal. (ii) Secondary air enters along with the fuel through the feedhole and helps the burning of the volatiles.

In the present design of the downdraft stove, there is no

distinction between primary and secondary air. All the air passes through the fuelbed by natural draught. It acts both as primary and secondary air. In the present design there is a provision for a secondary air by-pass.

2.2 Procedure

During each experiment, the fuel consumption rate and CO, C02 and

02 contents of flue gases were recorded. Figure 2.3, represents

the experimental set-up. The entire downdraft stove was made with mild steel sheet. It has three parts: stove body, bowl and

chimney. The dimensions are mentioned in Figures 2.1 and 2.2. At the bottom of the bowl there is a grate. The grate was made by welding 8 metal strips at lOmm intervals at the bottom of the bowl. The distance between the last strip to the wall, where the provision was made for secondary air by-pass, was 20 mm. The entire stove was covered with insulating material, Al203 fibre.

It was then placed on an electronic balance, with a sensitivity of lg. The desired number of wood blocks for an experiment was placed on a tray attached to the balance. The bowl was placed in the mouth of the stove. The calibration of the appratus was done with pure Nitrogen gas for 4 minutes, calibration gas for 6 min. (02 = 7.34%; CO= 0.47%; C02 = 6.04%;

N

2 = 86.15%) and with air for 12 min. (02 = 20.94%; C02 = 0.03%;

N

2 = 79.03%).

3 12 4 5 2a

~

~

==

~,-

-

8

---,1======1

7 6

Figure 2.3: Flow sheet for combustion experiments of wood in the downdraft stove.

1: downdraft stove: 2a: mass balance: 2b: tray with wood

blocks: 3: soot filter; ~: cooling chamber; 5: micro

filter; 6: pump; 7: CO detector meter; 8:. C02 detector

meter; 9: 02 detector meter; 10: data-logger; ll:computer;

12: disk

All the data were recorded by a data logger attached to a computer. A few wood blocks soaked with kerosene were put into the bowl of the stove. It was then ignited. When the fire in the

bowl reached the steady state after burning for 5-8 minutes, and

latter blowing with a piece of hardboard, the computer was started for recording the fuel consumption and CO, C02 and 02

content of the flue gases. The duration of each experiment was 60

minutes. The charging of fresh wood blocks was done manually and

the charging rate was monitored by a stop watch. The flue gases were sampled by a copper tube 20cm below the top of the chimney. Fluegas is subsequently passed through a soot filter, moisture

trap, micro-filter and finally to the CO, C02 and 02 meters. Before starting each experiment, the moisture trap was filled with ice and after each experiment the water deposited in the

trap was removed. Also after each experiment the filter paper, in the micro-filter was replaced. The data collected by the computer is later processed with personal computer with LOTUS 123.

3. RESULTS AND DISCUSSION

3.1 The General Behaviour of The Downdraft Stove

The general behaviour of the downdraft stove is illustrated by analysing one typical experiment in detail. The experiment chosen is SL. Nr. 3 from table V. The variables studied are stated

below.

Concentration of CO and C02 in the flue gases

In the experiments. besides the absence of odour, the

concentration of CO is used as an indicator of the combustion quality. Figure 3.1 represents the concentration of carbon

monoxide in the flue gases as a function of time.

0. 10 20 0.08 15 0 0.06 (.) ':!?. 0 10 _j 0 _0.04

>

71

5 0.02 0.00 ₀ 0 10 20 30 40 50 60 TIME (MIN) (\J 0

u

':!?. 0 _J 0

>

Figure 3.1: Concentration of CO and C02 in the flue gases as a

function of time.

The figure clearly shows that the concentration of carbon

monoxide in the flue gases is very small and remains practically 12

constant during the course of the experiment. The average concentration of carbon monoxide in the flue gases is 0.003%

(Vol.). However the figure shows 2 large peaks. This is probably due to poking the fuelbed during the experiment. The

concentration of carbon dioxide varies from 8 - 10% (Vol.). The average concentration is 9.2% (Vol.).

Concentration of oxygen and CO/C02 ratio in the flue gases.

Figure 3.2 represents the concentration of oxygen and CO/C02 ratio in the flue gases. The figure shows that the concentration of oxygen varies from 9 - 12% and the average oxygen content in the flue gases is 11.3% (Vol.). The CO/C02 ratio was almost constant during the experiment and its average value is 0.03%, which is negligible in comparison with both traditional and

improved woodburning stoves.

(\J 0 20: ci'2 10 _J 0

>

5

10.80

J

0.60 0.40 0.20 0 L...!...~L.!_~~----'-"_L.'.---'---"---'-'--'--'---'--'--~'----~~-~ 0.00 0 10 20 30 40 50 60 TIME (MIN) ~ C\J 0

u

... 0

u

Figure 3.2: Concentration of 02 and CO/C02 ratio in the flue gases as

a fW'lction of time.

Excess air factor

The excess air factor is defined as the ratio of the total amount of air (VT) involved in the burning process and the

stoichiometric amount of air (Vst), which is the theoretical amount of air to burn lkg of dry wood. It is denoted by:

A.

=

(3.1)

Figure 3.3 represents the excess air factor in the flue gases. The horizontal line through the experimental data indicates the average value, which is 2.33.

5 ;---~~~~~~~~~~~~~~~~~~~~~~~~~~~~-er 0 1-(.) < LI.. er

<

(/) (/) lJJ (.) >< lJJ I 4.5

~

i

I 4 j I I I 3.5

1

I i 3 I

1

2.5 I

J,v

,J .

I 0

/:

(

1 :I\

I~

i/

~

20 40 TME!MINI

Figure 3.~: Excess air factor in the flue gases as a function of time.

Fuel consumption

The fuel consumption rate is represented in figure 3.4. The

average charging rate of the fuel was 333.3 mg/s, and the average combustion rate recorded by the computer is 334.6 mg/s. The

figure shows that the fuel consumption rate during the experiment is constant.

14

'.

\)

1000

/

800

/

£>

0 w

2

600 :i ({)

z

0

u

₄₀₀ _J

w

:i LL 200 0 10 20 30 40 TIME (MIN)

Figure 3.4: Fuel consumed as a function of time.

Power output

The general equation for the average power output is:

p

=

av

Where:

=average power output of the fire (kW).

50

(3.2)

sum of the individual wood charges during the experiment (kg).

B =net calorific value of wood (kJ/kg). For White Fir this value is 16600 kJ/kg in case the moisture content is 11%. tT =total duration of the experiment (s).

The average power output of this experiment was 5.6 kW.

Qualitative tests of the flue gases

60

During the course of the experiment some qualitative tests of the flue gases were made.

i) Smell

ii) Inflamability

iii) Presence of dust particles iv) Accumulation of charcoal

The smell was determined by sniffing the fluegases from time to time. No smell was detected.

The inflamability wa~ tested by holding a flame just above the chimney. In this experiment, the flue gases did not catch fire. The presence of dust particels in the flue gases and the

accumulation of charcoal were determined by observation. This experiment showed neither dust particles in the flue gases nor accumulation of charcoal.

The downdraft stove, in its present design, is very sensitive to its operational variables. To ascertain these, a systematic study of various variables, viz effect of woodblock sizes, fuel

charging intervals, wood species, secondary air by-pass and power output was carried out. The results in tables I and II were

obtained by using downdraft stove-I and the rest of the tables contain the results obtained by using downdraft stove-II.

3.2 Effect of Wood Block Size

To verify the effect of different woodblock sizes, a series of experiments has been carried out, as listed in table I. To

maintain constant power output, as the different woodblocks have different weight, various charging intervals and various numbers of woodblocks of particular size were selected. Figure 3.5

represents the CO/C02 ratio as a function of woodblock sizes. It

clearly indicates that with the increase of volume/surface area of the woodblocks, the CO/C02 ratio increases. For volume/surface

areas of woodblocks from 0.37-0.49 cm, the CO/C02 ratio is very

low and almost constant. As the volume/surface area increases to above 0.49 cm, the CO/C02 ratio increases very sharply. To

explain this behaviour, the fire penetration rates for the

different woodblock sizes were computed by applying equation 3.3 (Bussmann, 1988) and listed in table I and shown in figure 3.5.

The fire penetration rate is the velocity at which the char boundary advances into the virgin wood.

w

=

w

v

A v p

v

av A

=

Penetration rate (nun/s)

=

Volume/surface area (mm)

=

Volatile fraction

(3.3)

It clearly shows that with an increase of the volume/surface area, the penetration rate increases. This behaviour was also noticed in the case of an open fire with different woodblock sizes (Bussmann, 1988). This also implies an increase in release

l'·! 0

u

' -0

u

~.SC . _·::J/)70

I

_! ! _' ' I

?'

:

l

l

%6) I ~

oc.

~ I / I

~r050

I

+ 0.50 + + 0.040 <J 0 000 _0.030 0.30 0.40 0.50 _0.60

WOOD BLOCK SIZE (V0LLJrv1E1_{SUR.=-ACE AREA}

" ' - , § ~ .,

r-<

c

z

0

f--<

[[

f--w

z

w

Q_

Figure 3.5: CO/C02 ratio in the flue gases as a function of the wood

...

00

TABLE I: EFFECT OF VOODBLOCK SIZES

Vood

sp~cies:

Vhite Fir

Secondary air by-pass: half open

fola11e/

Charging

Po11er

Average flae gas composition

*

Sl. sarface

rate

oatpat

/Jr.

area( c11) {blocks/s) {kV}

co

C02

02

{fol. 7.) {fol.7.} {fol.'!.}

1

0.367

2/45

3.74

0.016

11.49

8.99

2

0.393

1/30

3.83

0.014

10.99

9.66

3

0.423

2/75

4.38

0.006

6.42

13.79

4

0.485

1/45

3.79

0.019

11.29

9.21

5

0.532

1/60

3.87

0.032

9.21

11.34

6

0.589

1/120

3.00

0.086

7.52

13.15

0.393

3/60

7

0.485

2/60

2.65

0.084

10.55

9.90

0.589

1/60

Smell in the flue gases

=

S

No smell in the flue gases

=

X

Inf lamability of the flue gases

=

I

Presence of dust particles

in

the flue gases

=

D

- Absence of dust particles in the flue gases

=

A

Accumulation of charcoal-bed on the grate

=

C

No accumulation of charcoal-bed on the grate

=

N

CO/C02

('!.)

0.136

0.128

0.088

0.175

0.364

1.228

0.751

Penetration

rate

le11arks

1

{mm/s)

0.039

X, A, N

0.043

X, A, N

0.044

X, A, N

0.046

X, D, N

0.052

X, A, N

0.059

S,

A, N

-

X, A, N

rate of volatiles, which requires more air to obtain clean combustion, which is apparently not available (see CO/C02 curve in figure 3.5)

3.3 Effect of Fuel Charging Interval

Fuel charging intervals in a woodburning stove are an important factor. Most users prefer longer intervals. The flue gas

composition also changes with changing fuel c'harging intervals. A large number of experiments has been carried out to ascertain this influence. The first set of experiments was carried out with downdraft stove-I and the results are listed in table II. The other set of experiments was done with downdraft stove-II and the results are listed in table III. Figure 3.6 represents the effect of varying charging intervals on the CO/C02 ratio of the flue gases. These experiments were carried out by using downdraft stove-I. The CO/C02 ratios of three experiments are plotted in the same figure. The size of the woodblocks used is 20*20*36,6 mm. The power output of the stove was kept constant while

changing the charging interval. It is clear from the figure that, with the increase of charging int.erval, the CO/C02 ratio

increases sharply. 6

f

v

I

_I

:

I

I I I 0 ~ 6 N 0

u

' -0

u

0 6

0 ""'-'~~=-<:~~~~-L.o ... ~=-Co...,,i::~ci.,.."'"""""'=>l::t:::::::..~-=~~~~"""'8:~

0 10 20 30 40 50 60

51150

2160

1130

Figure 3.6: CO/C02 ratio in the flue gases as a function of time for

different charging intervals (wood block size

=

I'.)

0

TABLE II:EffECT Of FUEL CIAIGING INTERVALS

Vood species: Vhite Fir

Secondary air by-pass: half open

Charging

folue/

fo•er

Average

/lse

ga~ co•po~ition

SL.

rate

~•rface

ostpst

Kr.

(block~/~)

area(c•}

(kY}

Cl

Cl2

12

Cl/Cl2

(fol.'!.} (fol.

i.)

(fol.

i.)

fl.)

1

1/30

0.393

3.83

0.014

10.99

9.66

0.129

2

2/60

0.393

3.97

0.029

9.37

11.20

0.346

3

5/150

0.393

3.44

0.068

5.54

14.71

1.117

4

3/75

0.393

3.87

0.038

7.78

12.33

0.445

5

1/60

0.532

3.87

0.032

9.21

11.34

0.364

6

2/150

0.532

2.97

0.043

8.62

11.89

0.534

*

Smell in the flue gases

₌

s

No smell in the flue fases

₌

x

Inflamability of the lue gases

₌

I

Presence of dust particles in the flue gases

= D

Absence of dust particles in the flue gases

=

A

Accumulation of charcoal

-

bed on the grate

₌

c

No accumulation of charcoal-bed on the grate

₌

N

le11ark~ #

x,

_{A' N}

X,

A, N

X, D,

c

x,

A,

c

x,

A' N

x,

A' N

TABLE III:EPPECT OP FUEL CB!IGING INTElV!LS

Vood species: Vhite Fir

Secondary air by-pass: closed

Volume/surface area (c•): 0.367

Charging lo•er Average flae ga& co•po&ition

*

SL.

rate oat pat

Kr. {block&/&)

(kl/}

Cl Cl2

12

1

2

3

4

5

{Vol.'!.) (Vol.'!.} (Vol.'!.)

2/30

5.57

0.003

9.18

11.30

4/60

5.49

0.016

7.47

13.07

8/120

5.61

0.024

8.78

11.83

16/240

5.92

0.035

9.64

10.70

20/240

7.03

0.013

10.78

9.37

Smell in the flue gases

No smell in the flue gases

Inf lamability of the flue gases

Presence of dust particles in the flue gases

Absence of dust particles in the flue gases

Accumulation of charcoal-bed on the grate

No accumulation of charcoal-bed on the grate

Cl/Cl2

(1.)

0.030

0

.

266

0.415

0.624

0.151

le11arb

*

X,

A, N

X,

A, N

x,

A, N

x,

A, N

X, I, D

, N

=

s

=

x

= I

=

D

= A =

c

= N

To verify this finding some more experiments were carried out with the downdraft stove-II using smaller woodblocks of size 20*20*27,5 mm. Figure 3.7 again represents the CO/C02 ratio's of

the flue gases as a function of time. The same phenomena are noticed in this case. But one thing is very striking here. With the increase of the charging intervals, the peak height and width of CO/C02 ratio's increased very sharply. It has been observed during the experiment that, at the end of each charging interval, the fuel bed became partially exhausted and some empty space appeared on the grate. This means that very little fuel is available, in which case there is very little C02 , resulting in an increase of the CO/C02 ratio. But the absolute quatities of CO are still at an acceptable level ( range: 0 - 0.7 Vol%). After subsequent charging of fresh wood, again the CO/C02 ratio fell down. To verify this, one experiment has been carried out with 20 woodblocks/240 seconds. The CO/C02 ratio's of this experiment are plotted at the top of figure 3.7. It was observed that at the end of each charging interval, no empty space appeared on the grate and the CO/C02 ratio's remained very low.

9 0 9 ~ ₀ ~ 9 C\J 0

u

81120 '-0 0

u

9 4/60 0 9 2130 0 0 10 20 30 40 50 60 TIME (MIN)

Figure 3.7: CO/C02 ratio in the flue gases as a function of time for

different charging intervals (wood block size =

20M20M27.5

mm).

It was also noticed that, during the experiment due to high power output, sometimes flames appeared at the top of the chimney. At that time the flue gas collection point in the chimney was about 20 cm from the top (see figure 2.3), and took in unburnt gas. So the CO/C02 peaks which appear at the top of the figure (20

blocks/240 s) are not indicative of complete combustion. We conclude that it is possible to bring down the CO/C02 ratio's by

proper adjustment of the charging intervals.

3.4 Effect of Wood Species

Burning characteristics of wood depend on the chemical

composition and some physical properties of the wood species.

To determine the combustion characteristics of wood species, three experiments have been carried out using three wood species viz. White Fir, Oak and Beech. The results are listed in table

IV. It has not been entirely possible to use the same size of

0.06 ,...,

,-,

+

Vo!% CC

i

I L.V I b, Vol% C02

I

I + 0 Vo!% 02 _I r 6 ₁₅ 0.04 0 0

u

~ 0 10 _I 0

>

6 0.02 + 0 0 ₅ + 0.00 0

White f

1r Oak

Beech

WOOD SPECIES

Figure 3.8: Concentration of

co.

C02 and 02 in the flue gases as a function of wood species.

C'~ r;

0

'o~

0

>

(\! 0 ~ 0 _J 0

>

wood blocks in the three experiments because of non availability of proper block size in Oak and Beech. But the nearest possible sizes were used. The aimed charging rate for Oak and Beech could not be maintained because of too high power output. While using Oak and Beech in the experiment, the entire apparatus became red

hot. Figure 3.8 represents the concentrations of CO, C02 and 02

in the flue gases as a function of wood species. It is clearly seen from the figure that the concentrations of C02 are 14.6% and 15.6% for Oak and Beech respectively. In case of White Fir, the concentration is 7.4%. Similarly the concentration of oxygen, in case of Oak and Beech, is much lower than that for White fir. Figure 3.9 represents the CO/C02 ratio and excess air factor of the flue gases as a function of wood species.

~ 2... (\) 0 () ' 0 () 0.40 t=.. I _,

Q Excess air factpr

I

i i CO!C02 I I

t

0.30 _I I + + D

r

0.20 2 D _D 0.10 +

-

1 0.00 ~---~---~---' 0 Oak WOOD SPECIES

Beech

,;

E

1

u

<t i.L

a:

<[ if) en w ()

x

w

Figure 3.9: CO/C02 ratio and excess air factor in the flue gases as a

function of wood species.

The CO/C02 ratio's show not much difference among the wood species. But the excess air factor is much lower in case of Oak and Beech. The lowest excess air factor recorded in this

investigation for Beech is 1.38.

TABLE IV:EFFECT OF VOOD SPECIES

Secondary air by-pass: half open

Vo lame/

Charging

Po11er

Average flue gas composition

Excess

Sl.

food

surface

rate

output

air

lemarks

u

Kr.

species

area(cm}

{blocks/s) {kY}

co

C02

02

CO/C02 factor

(Yol.

7.)

(Vol.

7.)

(Yol.

i.)

('!.)

1

Vhite Fir

0.367

2/30

5.58

0.017

7.44

13.27

0.257

2.87

X, D, N

*

2

Oak

0.295

2/30 1/30

5.68

0.013

14.58

5.47

0.083

1.46

X,

I, D,

*

3

Beech

0.295

2/30 1/30

6.42

0.052

15.57

5.09

0

.

269

1.38

x,

_I

_'

D

,

*

Due to high ppwer output, the aimed charging rate could not b

e

maintained and consequently the

charging rate was lowered

**

Smell in the flue gases

No smell in the flue gases

Inflamability of the flue gases

Presence of dust particles in the flue gases

Absence of dust particles in the flue gases

Accumulation of charcoal_bed on the grate

No accumulation of charcoal bed on the grate

=

s

=

x

=

I

=

D

=

A

=

c

=

N

N

N

3.5 Effect of Secondary Air By-pass

In the downdraft stove, the flow of air, combustion gases and fuel are all in the same direction. In the present design, there is a provision for secondary air in the shape of a by-pass

(section 2.2). To determine its influence on the quality of combustion, some experiments have been carried out which are

listed in table V. Figure 3.10 represents the concentrations of CO, C02 and 02 in the flue gases as a function of secondary air

by-pass setting (open, half open and closed).

0.020

+

Vol% CO I 2C, ,6 _{Vol% C02} I 0 Vo1°10 02 _I +

~

0.015

1 '.':· I 0 I 0 0

u

0 ':!?.

_0.010

-l 10 0 _J _{+ 6} 0 6

>

0.005

5 +

0

0

OPEN

HALF OPEN

CLOSED

SECONDARY AIR BY-PASS

Figure 3.10: Concentration of CO, C02 and 02 in the flue gases as a function of secondary air by-pass setting.

The figure shows that there is not much difference among the concentrations of C02 and 02 for different by-pass settings.

However, the concentration of CO in the case of a closed

secondary air by-pass is 0.003%, which is little lower than that of the other two settings. Figure 3.11 represents the CO/C02

ratio and excess air factor of the flue gases as a function of different secondary air by-pass settings.

26

-~· ,,c 0 ,-~·

>

~ 0

u

::< 0 I

0

>

TABLE V: EFFECT OF SECONDAIY All BY-PASS

Vood species: Yhite Fir

Charging rate {block/s): 2/30

Volume per surface area (c•): 0.367

Seco~dary

lo•er

Average flae

ga~ co•po~ition

SL.

*

air

oat pat

Nr. by-pa~~

{kY}

co

C02

02

{fol.'!.} {Vol.

i.)

(Vol.

i.)

1

open

5.44

0.009

8.30

12.32

2

half open

5.58

0.016

7.44

13.27

3

closed

5.57

0.003

9.18

11.30

- Smell in the flue gases

No smell in the flue gases

Inf lamability of the flue gases

Presence of dust particles in the flue gases

Absence of dust particles in the flue gases

Accumulation of charcoal

-

bed on the grate

No accumulation of charcoal-bed on the grate

CO/C02

(7.)

0.120

0.257

0.030

Exce~~

air

factor

2.57

2.87

2.33

=

s

=

x

=

I

=

D

=

A

=

c

=

N

le11ark~

*

X, D, N

X,

D, N

X, D, N

C\J 0

u

..._ 0

u

O.SO

+

COIC02 /::,. Excess i'l'' facl( r 0.40 0.30 0.20 + 0.10 6 + + I i ~ ~ ~ 3 I

t

J

0.00 ~---~---~---~ (!

OPEN

H

A

LF OPE

f''\J

C

LO

S

ED

SECOr'-JDARY AIR BY-PASS

Figure 3.11: CO/C02 and excess air factor in the flue gases as a function of the secondary air by-pass setting.

It is clearly observed from the figure that the CO/C02 ratio is much lower, and the excess air factor a little lower when the secondary air by-pass is closed. The results obtained by variation of secondary air by-pass, are not clearly

understandable. So it might be concluded that the secondary air by-pass in this stove has no influence on the combustion of wood.

3.6 Effect of Power Output

According to the definition of the power output, (equation 3.2), it can be easily changed either by changing the quantity per charge or the charging interval. More commonly the C02 content in

the flue gases is used as a scale in power output variation. Power output is another important factor in woodburning devices.

To determine its influence on the quality of combustion, some experiments have been carried out (see table VI.) The power

28

TABLE VI: EFFRCT OF POVRR OUTPUT

Vood species: Vhitc Fir

Volu•c per surface area (cm): 0.367

Secondary air by-passs: closed

Po1Jer Charging

Average flue gas composition

Sl. output rate

Kr.

{kV}

{block/s}

co

C02

02

CO/C02

{Yol.

7.) {Yol. 7.) {Yol. 7.)

(7.)

1

5.57

2/30

0.003

9.18

11.30

0.030

*

2

6.73

3/30

0.007

12.93

7.85

0.054

*

3

7.32

4/30

0.006

11.58

8.31

0.054

-··

*

Excess

air

lemarks

n

factor

2.33

X, A, N

1.

73

x,

A, N

1.85

X, I, D, N

Due to high power output, the aimed charging rate could not be maintained

and

consequently the charging rate was lowered.

**

Smell in the flue gases

No smell in the flue gases

Inf lamability of the flue gases

Presence of dust particles in the flue gases

Absence of dust particles in the flue gases

Accumulation of charcoal-bed on the grate

No accumulation of charcoal-bed on the grate

=

s

=

x

=

I

=

D

=

A

=

c

= N

output was varied from 5.6 - 7.3 kW. It has not been possible to bring the power output below 5.6 kW or above 7.3 kW with this particular size of woodblocks and charging rate because the power output will be too low and too high respectively and impossible to control. Figure 3.12 represents the effect of power output on the carbon monoxide, carbon dioxide and oxygen content of the flue gases as a function of time.

0.015

+

Vol%

c

o

(\j !:::, _{Vol% C02} ~ 3 ·:-:' 0.010 0 Vol% 02 0

u

~ 0 _J 0

>

9

0.00

5

7 0 POWER OUTPUT (kW)

Figure 3.12: Concentration of CO, C02 and 02 in the flue gases as a

function of power output.

With the increase of power output both the concentration of CO and C02 increase. But the concentration of CO is very small. Even at the highest power output of 7.3 kW (CO= 0.006%). The

concentration of oxygen in the flue gases decreases with the increase in power output, which is obvious. According to Bussmann (Bussmann et al., 1985), the C02 increases proportionally when the power output increases by change of mass weight. But when a change in time interval was used for this purpose, similar behaviour was not observed. The first observation is also in agreement with this investigation. Figure 3.13 represents the effect of power output on the CO/C02 ratio and excess air factor

30 (\J G ~ o· ...J 0

>

of the flue gases. With the increase of power output, the CO/C02

ratio slightly increased. It is very interesting to note that at a power output of 5.6 kW, the CO/C02 ratio was 0.030%, but even

at the highest power output of 7.3 kW it was 0.054%. The difference in CO/C02 ratio is still very low. In most of the

other woodburning devices, with the increase of power output, this ratio increases sharphly. The excess air factor also

decreases with an increase of the power output. The lowest value is 1.73.

0.30

+

COIC02 3.00

!:::,. Excess arr factor

2.40 0.20 ~ ~ 1.80 (\J 0 (j ---.. 0 _1.20 (j 0.10

~__+--t

0.60 0.00 _0.00 4 5 6 7 8 9 POWER OUTPUT (kW)

c::

0 f-(j <! LL g <! (./) lf) w (j

x

w

Figure 3. 13: CO/C02 ratio and excess air factor in the flue gases as a

function of different power outputs.

Figure 3.14 shows the CO/C02 ratio's in the flue gases of various

burning devices viz. wood, gas, kerosene, coal etc. along with the downdraft stove, as a function of the power output. It is clearly shown that the downdraft stove has very low CO/C02

ratio's even at a power output of 7.3 kW, which is even much lower than the figure allowed for gas burners.

10 ~, ~~~~~~~~~~~~~~~~~~~~~~~~~~~. i 0 f-<t

a

(\! 0

u

' -0

u

5 / I - - · - - - ·- ·- ·- · -· - -·- -·-·-·-

--

·

"'

/

/

/ / / / / ;.f. POWER OUTPUT (kW) ./ / / / o/·

Figure 3.14: CO/C02 ratio in the flue gases as a function of the power

output, for various burning devices.

--4- = Downdraft stove; = Cas appliances;

= Kerosene burners; - =Anthracite burners;

=Domestic space heaters (P max):

. =Domestic space heaters (P ma.x/2);

Woodburning cookstoves, shielded fire type

~- = Combustion chamber diameter = 20cm;

-o- = Combustion chamber diameter = 25cm;

-0- = Combustion chamber diameter = 30cm.

3.7 Calculation of the Carbon Monoxide Concentration in an enclosed Space

In order to determine the pollution caused by carbon monoxide, while firing the downdraft stove in an enclosed space, the

following formula has been used to calculate the CO concentration in the atmosphere as a function of time (de Vries et al .. 1973).

c

=

Q

K (1 - e-Rt) co

V R

100

Where: C

=

CO concentration in the atmosphere

Qco

=

CO emission

32

(3.4)

(%)

K

₌

Inhomogeneity factor

(-)

-1

R

₌

Air exchange rate (h )

v

₌

Space volume (mo)

t

₌

time (h)

The inhomogeneity factor K varies between 0.4 - 1. K

=

0 means a complete exhaustion of combustion products, whereas K

=

1, means homogeneous mixing of combustion products with air but in

practice this never happens. However K has been taken 0.4 (Sulilatu, 1985). The values of Rand V are taken as 2 and 40m3 respectively. The experiment taken as an example for calculation of the CO concentration in the kitchen atmosphere, is the same one which was taken for explaining the general behaviour of the downdraft stove. For determining the CO concentration in the atmosphere some calculation was done and the results listed in table VI I.

Stove Powe.r Consumed

aJ2

aJ

type OUtp.Lt IDOod

x

x

g/h •'"'/h ppm

(kW)

kg/h /h

Down- _-4

draft 5.58 1.22 9.18 0.003 0.465 4.13.10 10

metal 4 0.84 9. 1 0.36 41 0.036 893

Table VII: Concentration of CO in the kitchen atmosphere

The main aim of this section is to determine the pollution of the downdraft stove by its CO emission and compare it with the

results of a metal stove with relatively good combustion

characteristics. Sulilatu (1985) determined the concentration of CO in the atmosphere for a metal stove. Figure 3.15 represents the build-up of the CO concentration as a function of time. The figure clearly shows that in case of the metal stove, CO

concentration of 0.008% and 0.016% are reached in about 15 and 60 minutes respectively. Within about 40 minutes it exceeds the MAC value. In the case of the downdraft stove, the CO concentrations

w 0:: w I o_ U) 0 2

I-<

0

u

C1.Ci2r-=.+ ! r ; 1 . 0 - ·r\L .U ··-· 1 I

I

i / /

\

\

/ / / /

\

\ ./ ·"-...

.

,., ,... ! v.,..., -(:.(}:; '--:/ _ __:._~~=========- - - -- - - -- -:J 20 E.x::.cs~.r=~ -· c ·~· • : 0 ·._·.-. ,_

Figure 3.15: Build-up of carbon monoxide concentration in the kitchen atmosphere as a function of time for two types of stoves. 1: Downdraft stove; 2: Metal stove.

MAC: Maximum Allowable Concentration

after 15 and 60 minutes are 0.00008 and 0.0001% respectively. It is interesting to note that during the entire period of operation of the stove, CO concentrations remain far below the toxic

concentration zone of the kitchen atmosphere.

Sulilatu stated that according to the National Air Quality

Standards for the USA, the maximum allowable concentration of CO for an exposure time of 1 hour, is 35 ppm . In the above

mentioned calculations for the downdraft stove, the CO concentration only reaches 10 ppm in 1 hour.

4. CONCLUSIONS

(i) The downdraft stove in its present design, is very

sensitive to its operational variables.

(ii) Woodblock size and charging intervals play an important role in the clean combustion of wood.

(iii) With particular woodblock sizes and proper adjustment of the fuel charging intervals, it is even possible to produce clean combustion for longer periods at a power output of 7.3 kW (CO/C02

=

0.006%) in a small device.

(iv) Computations show that the CO emission of the downdraft stove in an enclosed space (10ppm), remains well below the maximum allowable concentration of 35ppm per hour exposure time according to the National Air Quality Standard for the USA.

(v) When comparing various burning devices emission wise, the

performance of the downdraft stove is even better than gas appliances.

(vi) As the downdraft stove generates heat at a high

temperature. the construction material must be a good quality ceramic. Some insulating material should also be used to minimize the heat losses.

(vii) The principle of downdraft burning has enormous

applications both for industrial and domestic use. The clean hot gases produced can be directly used in the

heating of food, such as in bread ovens. In that case much fuel can be saved as compared to traditional ovens with direct and indirect heating.

This design may also reduce the chimney lengths of the presently used domestic and large scale cooking stoves, which will make them cheaper and easier to maintain.

5. REFERENCES

Bussmann, P .. Krishna Prasad, K .. and Sulilatu, F. (1985). On the testing of woodburning cookstoves, Proceedings of the 3rd Conference on Energy from Biomass, W. Palz, J. Coombs. and D.O. Hall (eds.). Elsevier, London, United Kingdom.

Bussmann, P. (1988). Woodstoves, theory and applications in developing countries. Thesis for obtaining a Ph.D degree at the Eindhoven University of Technology, Eindhoven, The Netherlands.

Krishna Prasad, K., Sangen, E .. and Visser, P. (1985).

Woodburning Cookstoves. In Advances in Heat Transfer, vol. 17. pp. 159 - 317. Academic Press Inc.

Smith, K.R. et al. (1984). Carbon Monoxide and Particles from Cooking Stoves. Presented at the 3rd International Conference on Indoor Air Quality and Climate. Stockholm.

Sulilatu, W.F. (1985). Danger Signals to Human Health. In From

Design to Cooking : Some Studies on Cookstoves. C.E.

Krist-Spit and D.J. van der Heeden (eds.). Woodburning Stove Group, Eindhoven University of Technology, Eindhoven, The Netherlands.

Vries, H. de and Bartholomeus, P.H.J. (1973). Gas en Milieuhygiene, Gas 93.