Analysis of-heat-flow-in-downdraft-gasifier
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Analysis of-heat-flow-in-downdraft-gasifier
- 1. Heat Transfer Analysis of Downdraft Biomass Gasifier Guide Dr.Subrata Kumar(Asst. Professor) Department of Mechanical Engineering IIT Patna Vishwajeet Kumar Anant Arya 1301ME45 1301ME07
- 2. Background and Motivation Biomass can be used for substituting the fossil fuel usage to counter energy crisis and the environmental problem. Thermal efficiency of downdraft gasifier is relatively low but gives relatively less contaminated syngas. The high exit temperature of produced gas produced from downdraft gasifier can be reduced to increase its efficiency.
- 3. Problem Statement To increase efficiency and to reduce heat loss of downdraft gasifier by heat analysis.
- 4. Approach Assumptions:- a. The porous bed is assumed to be isotropic, where solid and gas is considered to be in thermal equilibrium. b. Within each zone, the particle size, bed porosity and temperature are assumed to be uniform. c. Heat of the pyrolysis reactions is taken to be negligible. d. The heat transfer from one zone to another is due to conduction, convection, and radiation. e. Fluid velocity is assumed to be constant.
- 5. The energy equation for gas phase in pyrolysis zone is 𝜌𝑐 𝑝 𝜕𝑇 𝜕𝑡 + 𝜌𝑢𝑐 𝑝 𝜕𝑇 𝜕𝑧 = 𝐾 𝜕2 𝑇 𝜕𝑧2 + 𝐻𝑤 + 𝑄 𝑑𝑟𝑦 − ℎ 𝑝 𝑛 𝑝 𝐴 𝑝(𝑇 − 𝑇𝑠) Discretization 𝑇𝑝 = 𝑇𝑃 ° + ∆𝑡𝐾𝐴 ∆𝑥𝜌𝑐∆𝑉 𝑇𝐸 ° − 2𝑇𝑃 ° + 𝑇 𝑊 ° + 𝐻𝑤∆𝑡 𝜌𝑐 − 𝑢𝐴 ∆𝑉 𝑇 𝐸 ° +𝑇 𝑃 ° 2 − 𝑇 𝑊 ° +𝑇 𝑃 ° 2 − ℎ 𝑝 𝑛 𝑝 𝐴 𝑝(𝑇𝑃 ° − 𝑇𝑠 𝑜 ) Equation for combustion is 𝜌𝑐 𝑝 𝜕𝑇 𝜕𝑡 + 𝜌𝑢𝑐 𝑝 𝜕𝑇 𝜕𝑧 + 𝑄 𝑐𝑜𝑚 = 𝐾 𝜕2 𝑦 𝜕𝑧2 + 𝐻𝑤 − ℎ 𝑝 𝑛 𝑝 𝐴 𝑝 𝑇 − 𝑇𝑠 Discretization equation 𝑇𝑝 = 𝑇𝑃 ° + ∆𝑡𝐾𝐴 ∆𝑥𝜌𝑐∆𝑉 𝑇𝐸 ° − 2𝑇𝑃 ° + 𝑇 𝑊 ° + (𝐻𝑤 − 𝑄 𝑐𝑜𝑚)∆𝑡 𝜌𝑐 − 𝑢𝐴 ∆𝑉 𝑇𝐸 ° + 𝑇𝑃 ° 2 − 𝑇 𝑊 ° + 𝑇𝑃 ° 2 − ℎ 𝑝 𝑛 𝑝 𝐴 𝑝(𝑇𝑃 ° − 𝑇𝑆 ° ) Equation for gasification is 𝜌𝑐 𝑝 𝜕𝑇 𝜕𝑡 + 𝜌𝑢𝑐 𝑝 𝜕𝑇 𝜕𝑧 = 𝐾 𝜕2 𝑦 𝜕𝑧2 − ℎ 𝑝 𝑛 𝑝 𝐴 𝑝 𝑇 − 𝑇𝑠 Discretization equation 𝑇𝑝 = 𝑇𝑃 ° + ∆𝑡𝐾𝐴 ∆𝑥𝜌𝑐∆𝑉 𝑇𝐸 ° − 2𝑇𝑃 ° + 𝑇 𝑊 ° − 𝑢𝐴 ∆𝑉 𝑇𝐸 ° + 𝑇𝑃 ° 2 − 𝑇 𝑊 ° + 𝑇𝑃 ° 2 − ℎ 𝑝 𝑛 𝑝 𝐴 𝑝(𝑇𝑃 ° − 𝑇𝑆 ° ) Heat equation for solid phase for all zones is given by 𝜌𝑠 𝑐 𝑝𝑠 𝜕𝑇𝑠 𝜕𝑡 = 𝐾𝑠 𝜕2 𝑇𝑠 𝜕𝑧2 − 𝜕𝑞 𝑟 𝜕𝑧 + ℎ 𝑝 𝑛 𝑝 𝐴 𝑝 𝑇 − 𝑇𝑠 Discretization equation 𝑇𝑃𝑠 = 𝑇𝑃𝑠 ° + (𝐾𝑠−𝐾 𝑟)𝐴∆𝑡 2∆𝑥𝜌𝑐∆𝑉 𝑇𝐸𝑠 ° − 2𝑇𝑃𝑠 ° + 𝑇 𝑊𝑠 ° + ℎ 𝑝 𝑛 𝑝 𝐴 𝑝∆𝑡 𝜌𝑐 (𝑇𝑃 ° − 𝑇𝑆 ° ) Analytical Structure (Kitipong Jaojaruek et. al (2014)).
- 6. Heat equation is discretized using finite volume method implementing explicit scheme. The discretized equation is solved using MATLAB and plotted the graph. The theoretical temperature is compared with experimental values taken from literature (Kitipong Jaojaruek et. al (2014)).
- 7. Heat Loss Calculation Temperature profile is obtained solving the heat equation of each zone. A best fitted curve is obtained using all the data points. These curves give temperature as function of distance (z). T = 100z +313 (0<z<0.06) T = 6468𝑧2.044 + 298.4 (0.06<z<0.14) T = 8717z – 805.3 (0.14<z<0.2) T = -1.495× 104 𝑧3.974 + 962.9 (0.2<z<0.4) Heat Loss without Insulation 𝑄𝑙𝑜𝑠𝑠 = ℎ 𝑎𝑖𝑟 𝐴 𝑇 − 𝑇𝑜 𝑑𝑄 = ℎ 𝑎𝑖𝑟2𝜋𝑟𝑑𝑧 𝑇 𝑧 − 𝑇𝑜 𝑁𝑒𝑡 ℎ𝑒𝑎𝑡 𝑙𝑜𝑠𝑠 (𝑄𝑙𝑜𝑠𝑠) = 2𝜋𝑟ℎ 𝑎𝑖𝑟 ( 0 𝐿 𝑇 𝑧 𝑑𝑧 − 0 𝐿 𝑇𝑜 𝑑𝑧) Heat Loss with Insulation 𝐻𝑒𝑎𝑡 𝑙𝑜𝑠𝑠 = 𝑇 𝑧 − 𝑇° (ln 𝑟2/𝑟1 2𝜋𝐾𝑖𝑛𝑠 𝑑𝑥 + 1 2𝜋ℎ 𝑎𝑖𝑟 𝑟2 𝑑𝑥 ) 𝑁𝑒𝑡 ℎ𝑒𝑎𝑡 𝑙𝑜𝑠𝑠 = 𝑄𝑙𝑜𝑠𝑠 𝑤𝑖𝑡ℎ𝑜𝑢𝑡 𝑖𝑛𝑠𝑢𝑙𝑎𝑡𝑖𝑜𝑛 (ln 𝑟2/𝑟1 2𝜋𝐾𝑖𝑛𝑠 + 1 2𝜋ℎ 𝑎𝑖𝑟 𝑟2 )ℎ 𝑎𝑖𝑟2𝜋𝑟
- 8. Calculation of Optimum Thickness of Insulation It has been observed that radius of gasifier is always greater than critical radius. So, any insulation wrapped around the gasifier will always decrease heat loss to surrounding. But after certain value of radius of insulation reduction in heat loss is not significant.
- 9. Result and Findings Initial wrapped insulation around gasifier drastically reduces heat loss to surrounding. But further insulation does not reduce heat loss significantly but instead cost of insulation increases. From figure 3.4 we can observe that after certain radius of insulation (0.14-0.15m) reduction in heat loss is not significant.
- 10. Conclusion • The model predicted gives temperature profile similar to experimental with error less than 10%. • . Heat loss without insulation is found to be 4.425KW for a 11KW downdraft gasifier which is quite high. Heat loss is calculated using different insulating material. It is found that nearly 80% reduction in heat loss can be achieved using appropriate insulating material. • The optimum thickness of insulation is the range of 0.14-0.15m.Furter insulation increases the cost but does not reduce heat loss significantly and is uneconomical.
- 11. Scope of future work • Uniform velocity flow field has been assumed. Navier stokes equation can be solved for more accurate results. • Economic thickness of insulation can be calculated by analyzing radius of thermal insulation with respect to cost. • Working on preheating of feedstock by producer gas to reduce exit temperature of producer gas. • The effect of particle size on temperature profile and pressure drop can be studied.
- 12. Reference • 1. Avdesh Kumar Sharma, “Modeling fluid and heat transport in the reactive, porous bed • of downdraft (biomass) gasifier”, International Journal of Heat and Fluid Flow 28, pp. 1518–1530, 2007 • 2. Gajanan N Shelke, “Experimental Studies on Thermal Behavior of Downdraft Gasifier”, Proceedings of the World Congress on Engineering, Vol.2, 2014 • 3. Combustion Reaction:- http://www.netl.doe.gov/research/coal/energy- systems/gasification/gasifipedia/gasification-chemistry. • 4. Reduction: http://www.energy.kth.se/compedu/webcompedu/WebHelp/media%5CPrint%5C S4B10C1_A4.pdf