BURNING RATE OF WOOD2 Introduction Continuous burning takes place in solid materials if the thermal energy feedback to surface of the materials emits volatiles which causes the burning to continues. Mean mass reduction for the burning material is a function of the external flux, heat losses on the surface and heat flux from the burning frame. Effective gasification of the material defines the thermal energy that is needed to produce 1kg of the volatiles at give temperatureTvover a given period of time where energy balance approach is used. Equation 1 shows the expression of the mass loss rate for the system(Peacock, Davis & Babrauskas, 2011). The objective of this lab exercise is to investigate the mass loss of pieces of wood that have different areas and dimensions. The results are then plotted to visualizes the effect. Theory One-dimension heating equation for the mass burning rate is given as shown below; ˙m¿=m As . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Equation1 Where˙m¿is the mass burning rate per unit area, andAssurface are of the sample set in the horizontal orientation… A mass loss graph for would have a general trend as shown in the figure 1 below. Figure1: Typical mass loss for a given piece of wood.
BURNING RATE OF WOOD3 The steady massloss rate can be estimated by use of the formula shown below; ˙m¿=˙qnet Lv . . . . . . . . . . . . .. . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Equation2 Where˙qnetis net heat flux to the surface of the burning material andLvis the heat of gasification. The above equation can be rewritten as shown below; ˙m¿=qe ¿+qft ¿−ϵσTs 4 Lv . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Equation 3 WhereTsis the surface temperature andqfl ¿is the heat flux of the flame … For a non- charring material, the surface temperature would be fixed pyrosis temperature. If the heat flux of the flameqfl ¿and surface temperatureTsare assumed to be constant, equation 3 can be rearranged as follows; ˙m¿=qe ¿ Lv +qft ¿−ϵσTs 4 Lv . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Equation4 If equation 4 is plotted,1 Lv would be the slope andqft ¿−ϵσTs 4 Lv would be the y intercept. Methodology a.Apparatus 1.Cone heaters 2.Spark igniter 3.Load cell 4.Timer
BURNING RATE OF WOOD4 b.Procedure A new record file was generated on the computer. The cone heater temperature was set to 650°C. the˙qewas measured using the radiometer. The sample had their mass and dimension measured and recorded on the table. The tray under the cone heater was first crossed and the load cell set to zero. After the shutter was opened, ignition was started and immediately recording was done on the computer. Using the computer, the mass history of the wood is recorded. The experiment was repeated now after the temperatures were set to 850°at a step interval of50°C. Results The mass and dimensions are as follows; Table1: The samples` dimension, mass and het flux SampleLengthWidth Thicknes s As(mm2 )As(m2) 174731816096 0.01609 6 275731816278 0.01627 8 375741816464 0.01646 4 474741715984 0.01598 4 574721815912 0.01591 2 677761817212 0.01721 2 Graphs as follows;
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BURNING RATE OF WOOD5 02468101214161820 0 10 20 30 40 50 f(x) = − 1.78600968201115 x + 37.3418597111701 R² = 0.995114821607419 mass change against time for sample 1 Time (min) change in mass Figure2:mass change against time for sample 1 024681012141618 0 5 10 15 20 25 30 35 40 45 f(x) = − 2.17099836763617 x + 39.5693434718101 R² = 0.987174542692138 mass change against time for sample 2 Time (min) change in mass Figure3:mass change against time for sample2
BURNING RATE OF WOOD6 02468101214 0 10 20 30 40 50 f(x) = − 2.47038333591027 x + 42.3046354650672 R² = 0.995779137237269 mass change against time for sample 3 Time (min) change in mass Figure4:mass change against time for sample3 02468101214 0 10 20 30 40 f(x) = − 2.76323596586128 x + 37.9564623151821 R² = 0.996159804744432 mass change against time for sample 5 Time (min) change in mass Figure5:mass change against time for sample5
BURNING RATE OF WOOD7 02468101214 0 5 10 15 20 25 30 35 40 45 f(x) = − 2.59709066176101 x + 37.9306618751341 R² = 0.996649598243078 mass change against time for sample 4 Time (min) change in mass Figure6:mass change against time for sample4 02468101214 0 20 40 60 80 100 120 f(x) = − 3.02261515729506 x + 43.3550446609193 R² = 0.733882499214585 mass change against time for sample 6 Time (min) change in mass Figure7:mass change against time for sample 6 The gradients for the mass against time plots for samples 1 through 6 are-1.786,- 2.171, -2.4704 , - 2.597 , -2.7632, and -3.022 respectively. Below is a comparison graph for all the sample
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BURNING RATE OF WOOD8 Figure8: Comparison graph for all samples ( plotted using MATLAB) Below is m* against time 02468101214161820 0 0.0005 0.001 0.0015 0.002 0.0025 0.003 f(x) = − 0.0001109598460494 x + 0.00231994655263233 R² = 0.995114821607419 ma ss cha ng e a g ai nst ti me for sa mpl e1 Time(min) m*(g) Figure9:mass change against time for sample 1 024681012141618 0 0.0005 0.001 0.0015 0.002 0.0025 0.003 f(x) = − 0.000133370092618023 x + 0.00243084798327866 R² = 0.987174542692138 ma ss cha ng e ag a i nst ti me for sa mpl e2 Time(min) m(g) Figure10:mass change against time for sample 2
BURNING RATE OF WOOD9 02468101214 0 0.0005 0.001 0.0015 0.002 0.0025 0.003 f(x) = − 0.000150047578711751 x + 0.00256952353407842 R² = 0.995779137237269 ma ss cha ng e a g ai nst ti me for sa mpl e3 Time(min) m*(g) Figure11:mass change against time for sample 3 02468101214 0 0.0005 0.001 0.0015 0.002 0.0025 0.003 f(x) = − 0.000173657363364837 x + 0.00238539858692698 R² = 0.996159804744432 ma ss cha ng e a g ai nst ti me for sa mpl e5 Time(min) m*(g) Figure12:mass change against time for sample 5 02468101214 0 0.0005 0.001 0.0015 0.002 0.0025 0.003 f(x) = − 0.00016248064700707 x + 0.00237303940660248 R² = 0.996649598243078 ma ss cha ng e a g ai nst ti me for sa mple4 Time(min) m*(g) Figure13:mass change against time for sample 4
BURNING RATE OF WOOD10 02468101214 0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 f(x) = − 0.000175610920131017 x + 0.00251888476998137 R² = 0.733882499214585 mass ch an g e ag ain st ti me fo r samp le6 Time(min) Axis Title Figure14:mass change against time for sample 6 a.gradients for the m* Thenew gradientsfor the m* against time plots for samples 1 through 6 are- 0.00032479, -0.0001, -0.0002, -0.0002, -0.0002, and -0.0002 b.Lv Since the gradient =1 Lv The Lv is =gradient−1 =(0.00032479)−1 = 3.079 MJkg−1 And =1 0.0002 = 5,000 Lv is3.079 MJkg−1for all samples. The intercept isqft ¿−ϵσTs 4 Lv
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BURNING RATE OF WOOD11 c.Theintercept Theinterceptfor the m* against time plots for samples 1 through 6 are0.0023, 0.0024, 0.0026, 0.0024, 0.0024, and 0.0025. d.qfl ¿ Assuming Ts = 500 k andϵ=σ=0.001,qfl ¿can calculated as follows; 1.Sample 1 0.0023 =qft ¿−(0.001)∗(0.001)∗5004 3.079MJkg−1 qfl ¿=69582 2.Sample 2 0.0024=qft ¿−(0.001)∗(0.001)∗5004 3.079MJkg−1 qfl ¿=6 9890 3.Sample 3 0.0026 =qft ¿−(0.001)∗(0.001)∗5004 3.079MJkg−1 qfl ¿=70,505 4.Sample 4 0.0024 =qft ¿−(0.001)∗(0.001)∗5004 3.079MJkg−1 qfl ¿=6 9890
BURNING RATE OF WOOD12 5.Sample 5 0.0024 =qft ¿−(0.001)∗(0.001)∗5004 3.079MJkg−1 qfl ¿=6 9890 6.Sample 6 0.0025 =qft ¿−(0.001)∗(0.001)∗5004 3.079MJkg−1 qfl ¿=70198 Conclusion The lab exercise was successful since the mass loss rate was determined for various sizes of burning materials. The gradients obtained from the m* against time gave the value of Lv andqfl ¿. Errors were also present mainly because of using few decimal places. Also, using non calibrated tools may have contributed to a significant amount of errors in the final results. Furthermore, it was also assumed that the pieces of wood burnt steadily which was not the case. This for sure may contributed to errors in great way. It was found that, the heat pyrolysis obtained was3.079MJkg−1. even when the char occurs on the surface of the wood, burning continues.Assuming Ts = 500 k and ϵ=σ=0.001, it was possible to contain values ofqfl ¿which were 69582, 6 9890, 70,505, 6 9890, 6 9890 and 70198. In the further studies, emissivity of the char surface can be included for analysis.
BURNING RATE OF WOOD13 References National Bio-Energy Convention-15 on Bio-Energy for Rural Energisation, In Maheshwari, R. C., In Chaturvedi, P., & Bio-Energy Society of India. (2017).Bio-energy for rural energisation: Proceedings of the National Bio-Energy Convention-15 on bio-energy for rural energisation. New Delhi: Concept Publishing Company. Peacock, R. D., Davis, S., & Babrauskas, V. (2011).Data for room fire model comparisons. Gaithersburg, MD: National Institute of Standards and Technology.