POLJOPRIVREDNI FAKULTET
NOVI SAD
KINETIC COMBUSTION OF
WHEAT STRAW BALES
Prof. dr Todor Janić
Dr Saša Igić, doc.
Prof. dr Miladin Brkić
AIM OF THE RESEARCH
 The aim of this experimental research work
is aimed at determining the fullest
functional dependencies, ie. correlation
equations which represent the
mathematical modeling of the interaction
between multiple variable factors on the
combustion process baled wheat straw in
furnaces with stationary flat grate.
ELEMENTARY CHEMICAL ANALYSIS OF
WHEAT STRAW
The elemental composition of wheat straw:
C+H+O+N+S+A+W=1
TECHNICAL ANALYSIS OF WHEAT STRAW
 Wheat straw in terms of technical analysis
has the following characteristics:
burns with a large amount of volatile matter (from
57,2 to more than 80%);
in composed almost no sulfur;
burns with little ash (5 to 8%), which is problematic
because of solubility at temperatures below od
800oC.
ignition temperature is about 220oC;
low heat value of straw is in the range of about 14
MJ/kg, with straw moisture content of 14%;
moisture content in straw is changeable, and
proper storage conditions is in the range of 1015%.
INFLUENTIAL FACTORS ON THE
COMBUSTION PROCESS
FACTORS OF INFLUENCE ON THE BURNING RATE
The characteristics and concentration of reactants;
Pressure;
Temperature;
Influence of catalyst and others.
INFLUENTIAL FACTORS ON THE COMBUSTION
RATE IN THE FIREBOX
Types of furnaces;
Combustion technology;
Physico-chemical characteristics of the fuel;
PHASE OF WHEAT STRAW COMBUSTION
Stages of combustion are:
Drying - endothermic process where fuel is
receiving heat for evaporation of moisture.
Distillation - gasification of fuel when the fuel
vaporize all volatile components - volatiles.
Combustion - combustion process of solids is
performed in the third phase on the grid.
Dogorevanje - the continuation of the third
stage, but on a smaller scale.
Combustion stages overlap and their length is
variable on what influenced by many factors (type
of fuel, their caracteristic, etc.).
THERMAL DESTRUCTION OF WHEAT
STRAW
Decrease in mass
Increase in
temperature
Temperatura (oC)
Masa (mg)
Drying of fuels from 80-90oC
Hidden
decomposition from 90-200oC
Devolatisation
intense from 270-370oC
Reduction in
burning rate from 370-550oC
Significant
reduction in burning
rate - over 550oC
Time (min)
MATERIAL USED IN RESEARCH
 The material used in this paper are selected
bales of wheat straw.
 Straw that was used in the study was
collected from the same field in Kać.
 Average cross-section of straw bales was due
to the chosen strainer and amounted to about
0,37 x0,50 m, while in order to get as similar
length bales (approximately 0,70 m) made
their selection on the field.
 On the field was allocated 300 bales.
INFLUENCING FACTORS ON
COMBUSTION OF WHEAT STRAW





Work plan are considered two factors: the density of straw
bales and excess air ratio in the combustion of straw.
The first factor was investigated at three levels, and second
in four levels.
As the levels of the factors of bale density are: low, normal
and high-density with initial weight of the bale of 5, 7 and 9
kg.
Coefficient of excess air factor selected size are: 1,8; 2,3;
2,8 i 3,3.
This work has the dimension of multiple factor experimental
research. In this way enabled the simultaneous application of
mathematical statistics to examine the field dependence of
the kinetics of the combustion process resulting influence of
selected factors, their levels, and their mutual influence
(interaction).
1 - electric motor, 2 - fan 3 - air
schedulers, 4 - damper, 5 - tube for
air intake below the grille, 6 damper, 7 - Upper air intake pipe
above the tank, 8 and 9 - metering
orifice 10 - grid , 11 - scales, 12 - air
supply pipe above the tank, 13 acquisition device, 14 - flue gas
analyzer, 15 - computer, 16 - probe
for the analysis of combustion
products, 17 - damper, 18 - thermal
couple for measuring tempetatura
heated water , 19 - chimney, 20 - hot
water boiler (40 kW), 21 - "U" tube
for measuring pressure, 22 - port to
measure pressure in the boiler, 23 thermal probe, 24 - pipe for heated
water, 25-thermal couple for
measuring the temperature of the
water coming out of the boiler, 26 heat accumulator, 27 - water pump,
28 - overflow pipe, 29 - bowl for
regulating the temperature of
incoming water into the boiler, 30 pipe for water supply from hydrants,
31 - door ashtrays
CHART OF MEASURING
Ambijent air
Straw
THE MEASUREMENT RESULTS
V_2,3_I
M_1,8_I
M_3,3_I
S_2,8_II
10,00
V_2,3_II
M_1,8_II
M_3,3_II
S_2,8_I
V_2,8_I
M_2,3_I
S_1,8_I
S_3,3_I
V_2,8_II
M_2,3_II
S_1,8_II
S_3,3_II
V_3,3_I
M_2,8_I
S_2,3_I
V_1,8_I
V_3,3_II
M_2,8_II
S_2,3_II
V_1,8_II
9,00
Masa
bal e (kg)
( kg)
of bales
Mass
8,00
7,00
6,00
5,00
4,00
3,00
2,00
1,00
0,00
1
246 491 736 981 1226 1471 1716 1961 2206 2451 2696 2941 3186 3431 3676 3921
Time
Vr
eme (s)
( s)
MATHEMATICAL MODEL WHICH
DESCRIBES MASS REDUCTION BALE
WHEAT STRAW COMBUSTION IN THE
PROCESS OF THEIR FIREBOX WITH FLAT
IMMOVABLE GRATE

m   5,53 0,272  1,286   e

0, 0016
6
MATHEMATICAL MODELS DESCRIBING
THE MASS REDUCTION OF WHEAT
STRAW BALES EACH OF COMPACTION
OBSERVED IN THE PROCESS OF THEIR
COMBUSTION IN THE FIREBOX WITH
FLAT IMMOVABLE GRATE

m M  7 ,89311  1 ,37254     e

0 ,011039
6


mY  10 ,37717  1 ,55648     e
0 ,0034305

m B  12 ,82155  1 ,30865     e
0 ,0014672
6

6
CONCLUSIONS
 The modifications of the boiler, it is possible to directly monitor
"decline" mass burning bales of straw.
 Analysis of variance confirmed the significance of the density bales of
wheat straw and the excess air coefficient of combustion
 Defined mathematical models that describing the kinetics
combustion of baled wheat straw on a flat immovable grate.
of
 Obtained depending on the intensity of a comprehensive mathematical
model is significant. This, among other conclusions can be drawn
based on the value of the correlation coefficient is R = 0,8882.
 It also defines special forms of mathematical models, which are
available for an average weight loss of straw bale for each bale density
level.
 Further work in this area should be directed towards determining the
intensity and direction of temperature gradient and the mass of straw
bales, coupled with an increase in their porosity during combustion,
and the impact of the way (up-down ratios) of air penetration , as layer
of straw bales, and exhaust combustion products.