KINETIC COMBUSTION OF WHEAT STRAW BALES

POLJOPRIVREDNI FAKULTETNOVI SAD

Prof. dr Todor JanićDr 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 10-15%.

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;

INFLUENTIAL FACTORS ON THE COMBUSTION PROCESS

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

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)

Decrease in mass

Increase in temperature

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

Straw

Ambijent air

THE MEASUREMENT RESULTS

0,00

1,00

2,00

3,00

4,00

5,00

6,00

7,00

8,00

9,00

10,00

1 246 491 736 981 1226 1471 1716 1961 2206 2451 2696 2941 3186 3431 3676 3921

Vreme (s)

Mas

a ba

le

(kg

)

V_2,3_I V_2,3_II V_2,8_I V_2,8_II V_3,3_I V_3,3_II

M_1,8_I M_1,8_II M_2,3_I M_2,3_II M_2,8_I M_2,8_II

M_3,3_I M_3,3_II S_1,8_I S_1,8_II S_2,3_I S_2,3_IIS_2,8_II S_2,8_I S_3,3_I S_3,3_II V_1,8_I V_1,8_II

Time (s)

MATHEMATICAL MODEL WHICH DESCRIBES MASS REDUCTION BALE WHEAT STRAW COMBUSTION IN THE PROCESS OF THEIR FIREBOX WITH

FLAT IMMOVABLE GRATE

60016,0

286,1272,053,5

em

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

6011039,0

M e37254,189311,7m

60034305,0

Y e55648,137717,10m

60014672,0

B e30865,182155,12m

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 of combustion of baled wheat straw on a flat immovable grate.

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.