Simulation and Thermal Analysis of the Effect of Fuel Size on

Energy & Fuels 2007, 21, 1895-1900

1895

Simulation and Thermal Analysis of the Effect of Fuel Size on Combustion in an Industrial Biomass Furnace K. S. Shanmukharadhya* Department of Mechanical Engineering, Bannari Amman Institute of Technology, Sathyamangalam 638 401, India ReceiVed June 17, 2006. ReVised Manuscript ReceiVed February 21, 2007

In stoker-firing furnaces, possible conditions created by adverse characteristics are excess fuel size and rapid changes in fuel moisture. The excess fuel size may lead to reduced suspension burning and grate piling, while the moisture content of the fuel leads to fuel slugging to the furnace, piling and clinkering, and heat damage to grates. Bagasse is one such biomass fuel, a renewable energy source used in stoker-fired furnaces in the sugar industry for power generation. The fuel has various particles sizes. The particle sizes have their own influence on the combustion in the furnace. This paper explains the simulation and experimental work conducted on the combustion of bagasse on a grate and the thermal decomposition of different size bagasse particles with sophisticated tests. The experimental results obtained validate the simulation work carried out on bagasse combustion. The distribution of temperature fields and their locations inside the furnace are well predicted.

Introduction Sugar cane bagasse is a residual fuel from the sugar industry. Because of the depletion of fossil fuels and global warming, biomass fuels are becoming very real alternative to the fossil fuels. In addition to its use in the production of sugar and molasses, sugarcane grows faster and produces more biomass than most energy crops. In the past the sugar industry was dependent on on sugar alone for its commercial viability, but now it can now opt for an integrated production facility to produce power and ethanol. New sugar mills have shown interest in first implementing the cogeneration and then beginning sugar production. Raw bagasse coming out of the mill with different particle sizes and moisture has its own influence on the efficiency of the boiler. Bagasse which is very wet cannot ignite and causes stability problems in the furnace. Mounds of wet bagasse can accumulate on the grate when fuel moisture increases. During these times, the mounds are broken manually with crowbars. Wet bagasse has particle sizes that vary from a few microns to a few centimeters. The sizes of these particles have their own influence on the combustion. The demoisturizaton and thermal degradation of these particles are very important for combustion. To understand the influence of these particles, the furnace was modeled with the three-dimensional CFD package FLUENT by incorporation of various submodels. Bagasse with different particle sizes is tested by TG-DTA and differential scanning calorimetry (DSC) analysis. The gas temperatures were recorded at key points using a thermocouple. The species produced were measured with a flue gas analyzer. Both the experiment and the simulation clearly display various temperature fields within the furnace. Computer simulation works for the bagasse combustion with steady-state conditions that are well predicted by previous reasearchers.1-10 The bagasse contains 83.01% volatile matter, * E-mail: [email protected]. (1) Luo, M.; Stanmore, B. R. J Inst. Energy 1994, 67, 128-135.

4.2% ash, and 12.7% fixed carbon on dry basis.2-6 The thermal decomposition of bagasse has significant influence on the combustion. The various sizes of the bagasse particles have different densities and drying rates. Past researchers have found that this thermal degradation in bagasse is caused by either removal a bound form of moisture or evaporation of light volatile matter.4-6 The rate of demoisturization and devolatilization are higher for bagasse at higher heating rates.7-8 For TG analysis, the sample size of the bagasse should be as small as possible without losing the inhomogeneities inherently present in biomass material.9 The influence of moisture leading to unstable regimes in a bagasse-fired furnace caused by the periodical deposit of bagasse on the grate and burn out on the grate of the furnace with reasonable agreement with measurement has been investigated.10 Excess air is a key operating variable and plays a vital role in achieving higher combustion efficiency.11 The influences of the boiler operating parameters such as excess air and boiler load on emissions in a bagassefired furnace have been studies and evaluated.12 The test furnace used for this research has tangential over fire air jets to aid (2) Aimen, S.; Stubington, J. F. Biomass Bioenergy 1993, 5 (2), 113120. (3) Rodriguez, R.; Magne, P.; Deglise X. J. Anal. Appl. Pyrolysis 1987, 12, 301-318. (4) Nassar, M. M.; Ashour, E.A.; Wahid, S. S. J. Appl. Polym. Sci. 1987, 61, 885-890. (5) Nassar, M. M. Wood Fiber Sci. 1985, 17 (3), 226-273. (6) Roque-Diaz, P; Shemet, V. Z.; Lavrenko, V. A.; Khristich, V. A. Thermochem. Acta 1985, 93, 349-352. (7) Ravinderan, K.; Ganesh, A.; Khilar K. C. Fuel 1997, 76, 802-811. (8) Williams, P. T.; Besler, S. Renewable Energy 1996, 7 (3), 233250. (9) Antal, M. J.; Varhegyi, G. Ind. Eng. Chem. Res. 1995, 34, 703717. (10) Woodfield, P. Prediction of unstable regimes in the operation of bagasse fired furnaces. In Second International Conference on CFD in the Minerals and Process Industries; CSIRO, Melbourne, Australia, 1999; pp. 299-304. (11) Kuprinov, V. I.; Janvijitsakul, K.; Permachart, W. Fuel 2006, 85, 434-442. (12) Teixeira, F. N.; Lora, E. S. Biomass Bioenergy 2004, 26, 571-577

10.1021/ef060278y CCC: $37.00 © 2007 American Chemical Society Published on Web 04/20/2007

1896 Energy & Fuels, Vol. 21, No. 4, 2007

Figure 1. Images of the bagasse particles used for analysis.

burning of the suspension. There are not many research works related to furnaces of this type. This work focuses mainly on the simulation of the combustion of bagasse in a bagasse-fired furnace to locate the various temperature fields at various locations. Experimental work is also carried out to validate the temperature fields at the key point of interest. Thermal analysis of different-sized bagasse particles is performed to determine how particles behave in the temperature fields. The temperature of the flue gas was measured using a thermocouple with a digital indicator. The moisture content of the bagasse was measured using a moisture analyzer. Experimental Section The furnace used for the test was a spreader stoker furnace with tangential over fire air system and traveling grate. Air and bagasse are the two main input parameters for the furnace. There are two forced draft fans supplying the air for combustion. Nearly 65% of the combustion air is supplied through the grate, 30% through the tangential over fire windows, and the rest 5% through the distributors below the bagasse spreaders. Before it enters the combustion chamber, the air is heated by the flue gas at air preheater. The test boiler operates at a nominal power of 20 MW. There are five bagasse distributors placed evenly, horizontal on the front wall of the furnace. The furnace consists of four tangential air ducts and five distribution air ducts at the bottom of the spreader. A specially built k-type thermocouple (3 m length) was used to record the temperatures with a digital indicator. The fuel moisture was tested with a moisture analyzer in the laboratory. The boiler operating parameters were noted regularly during the test period. The measurements were made at the grate level, as well as at some key points on the tangential duct level and near the neck of the furnace. The concentrations of oxygen and other flue gas species are measured with a flue gas analyzer. The test boiler operates from 60 to 110% maximum capacity rating (MCR). The MCR depends on the crushing capacity of the plant. The percentage of oxygen present in the flue gas was around 5.6%. The specific energy of the bagasse considered is 19 554 kg/kg daf. The higher heating value is 2272 kcal/kg. Particles Used for Analysis. Bagasse particles are available in different sizes from 100 µm to a few centimeters in length. Bagasse consists of three components namely, pith, fiber, and rind mixed in different proportions. There is a considerable difference in shapes and sizes of the three components. The rather regular shape of the spongy pith particles with a near-unity length/width ratio can be approximated by a spherical shape. The particle size distribution of the powder is generally described by sieve analysis.13 For the present analysis, the particles are sieved by using standard sieves. The sieved particles used for analysis are shown in Figure 1. Figure (13) Rasul, M. G.; Rudolph, V.; Carsky M. Fuel 1999, 78, 905-910.

Shanmukharadhya

Figure 2. Picture showing the falling of larger particles in front of the spreader.

2 shows the larger particles falling in front of the spreaders. The photograph was taken during the normal operating condition of the furnace. Particles with sizes of 250-1003 µm normally burn in suspension because of their smaller size. For medium-sized particles, combustion takes place at the rear sloping back of the furnace as they are carried by the distributor air jets. The scanning electron microscope images taken for the bagasse particles are shown in Figure 3. The images are taken to understand the structure of the surface of the particles considered for analysis. There are numerous minute holes observed on the entire surface of the particle. When larger particles are observed, the particles seemed to be cylindrical in shape. But for smaller particles, the shape is irregular and varies from particle to particle. These variations are the result of the crushing of the sugar cane. Thermal Analysis. Since bagasse has various moisture levels and different particle sizes, the influence of these factors on the temperature distribution inside the furnace is a significant factor. In this regard, a few samples of bagasse with different particle sizes were tested by TG-DTA and DSC analysis. The TG-DTA analysis carried out for three different-sized particles with sizes of 250, 710, and 1003 µm and for large particles. In Figure 4, there is a weight loss of 7.512% between 30 and 130 °C, and from 130 to 250 °C, it is thermally stable. The degradation is 29% between 250 and 350 °C, and it is 30% between 350 and 400 °C. The degradation reduces to 17% between 400 and 590 °C, and from 590 °Cand up, it is only residue. The thermogram was analyzed to obtain the drying and pyrolysis characteristics and the thermal degradation rates. As seen from TG analysis in Figure 4, there are three distinct zones or regions during the bagasse pyrolysis. The first zone is from ambient temperature to onset of active pyrolysis. Initially, in this zone, the weight loss of the material occurs because of the demoisturization, which starts around 30 °C and peaks at 61 °C. The total weight loss in this stage is 7.512% of the wet sample. Thereafter, the thermogram manifests an almost horizontal line with slight loss of weight which is followed by an onset of degradation of the material as shown by first visible loss of weight after the horizontal line. Past researchers have found that this degradation in bagasse is caused by either the removal of the bound form of moisture or the evaporation of light volatile matter.4-6 The second zone represents the major decomposition of the material and is considered to occur between the initial temperature of 250 °C and the final temperature of 350 °C; it is also considered to be the active or first pyrolysis zone with a weight loss of around 58%. This step manifests the intense loss of weight at rapid rates and the release of different types of volatiles. The third zone represents temperature above the final temperature of active pyrolysis zone (350 °C) and is considered to be passive pyrolysis. In this zone, the amount and rate of mass loss is lower and slower than those in the active pyrolysis zone. In Figure 4, DTA line indicates the endothermic reaction up to 61 °C and later degradation progresses up to 411 °C during which rapid devolatilization takes place. The maximum heat liberated by the particle is 516 °C.

Effect of Fuel Size on Combustion

Energy & Fuels, Vol. 21, No. 4, 2007 1897

Figure 3. Scanning electron microscope images of the bagasse particle.

Figure 4. TG-DTA analysis of a 250 µm bagasse particle.

Figure 5. TG-DTA analysis of a 710 µm bagasse particle.

Bagasse is considered to be composed of of various constituents, which decompose at different temperature regions. At a temperature of less than 100 °C, the biomass losses mainly moisture; between 100 and 250 °C, the extractives start decomposing. Between 250 and 350 °C, predominantly, the hemicellulose decomposes, and between 350 and 516 °C, cellulose and lignin decomposition occurs. At temperatures above 516 °C, mainly lignin decomposes and contributes to char formation. The major degradation of bagasse occurs between 300 and 500 °C and leads to rapid reduction of char yield between these temperatures.14 (14) Katyal, S.; Thambimuthu, K.; Valix, M. Renewable Energy 2003, 28, 713-725.

Figure 6. TG-DTA analysis of larger bagasse particles.

Figure 7. Differential scanning calorimetry results for bagasse particles with a size of 250 µm.

The differential scanning calorimetry results obtained for three samples are illustrated in Figures 7-9. The analyses are carried out in an atmosphere of air at a rate of 100 °C/min. It is observed that there is an increase in the peak temperatures attained by the particles with an increase in size during exothermic reaction: 502 °C for the 250 µm particle, 512 °C for 710 µm particle, and 556 °C for larger particles. Because these particles are tested an the atmosphere of air, the exothermic reaction starts from the datum line. In nitrogen atmosphere, the particles take time to liberate energy, and there will be two peaks in the endothermic and exothermic directions. The difference in activation energy between


Shanmukharadhya The turbulent kinetic energy, k, and its rate of dissipation, , are obtained from the following transport equations ∂ ∂ ∂ (Fkui) ) (Fk) + ∂t ∂xi ∂xj

[( ) ] µ+

µt ∂k + Gk + Gb - F - YM + σk ∂xj Sk (1)

and ∂ ∂ ∂ (F) + (Fui) ) ∂t ∂xi ∂xj

[( ) ] µ+

µt ∂ + C1 (Gk + C3Gb) σ ∂xj k C2F

Figure 8. Differential scanning calorimetry results for bagasse particles with a size of 710 µm.

In these equations, Gk represents the generation of turbulent kinetic energy due to the mean velocity gradients. Gb is the generation of turbulent kinetic energy due to buoyancy, and YM represents the contribution of the fluctuating dilatation in compressible turbulence to the overall dissipation rate. C1, C2 , and C3 are constants. σk and σ are the turbulent Prandtl numbers for k and , respectively. Sk and S are user-defined source terms. Particle devolatilization is modeled with a single-step Arrhenius scheme15-16 such as rv ) fvmpAv exp

Figure 9. Differential scanning calorimetry result for larger bagasse particles. Table 1. The DSC Analysis Results of Bagasse Carried under a Nitrogen Atmosphere details

250 µm

1003 µm

large particles

raw bagasse

integral normalized onset peak end endo-mW sample wt rate of heat

-460.71 mJ -334.57 J/g 29.80 °C 90.64 °C 140.91 °C -6 mW 1.3770 mg 50 °C/min

-566.66 mJ -329.83 J/g 31.38 °C 97.25 °C 145.99 °C -7 Mw 1.7120 mg 50 °C/min



the 250 and 710 µ particles is 67.2 kJ. The activation energy for the larger particle is 86.4 kJ. This may be the result of the size of the sample taken for test, which is smaller than the 250 and 710 µm particles; otherwise, it would have taken value higher than that for the 710 µm particle. Computational Modeling. The furnace was modeled using the three-dimensional computational fluid dynamics package FLUENT. The segregated implicit solver was used to solve the transport equations. The turbulence was modeled by the standard k - model. Radiation was modeled by the discrete ordinate method. PrePDF was developed by consideration of species C, H, O, S, N, CO2, and H2O. The bagasse particles were tracked using the Lagrangen method, and combustion particles were assumed for bagasse particles. The simplest reaction scheme is the flame sheet, and the “mixed-is-burned” approximation is used. Combustion is modeled by non-premixed approach, where bagasse and the oxidizer enter the reaction zone in distinct streams.

2 + S (2) k

( ) Ev RTp

(3)

where rv is the volatile production rate, fv is the fraction of volatiles in the mass of the fuel particle, mp is the particle mass, Av and Ev are the rate constants, R is the universal gas constant, and Tp is the temperature of the particle. Devolatilization is modeled using the single-step Arrhenius form rate. The volatile material is divided into combustibles and pyrolysis moisture. The kinetic parameters for combustibles are Apyr ) 2.66 × 104 s-1 and the activation energy Epyr ) 66.1 kJ/mol. For the pyrolysis moisture, Apyr ) 2.66 × 104 s-1, and the activation energy Epyr ) 40.1 kJ/mol. The ultimate yield for combustibles is 75.75 and 16% for pyrolysis moisture.17 Char oxidation during combustion is modeled by the kinetic relationship presented in eq 418

( )

Eox n P RT op

rox ) Aox exp -

(4)

where Aox, Eox, and n are rate parameters and Pop is the partial pressure of oxygen at the surface of the particle. A value of 0.5 is assumed for n

Computational Results and Discussions Figure 10 shows the temperature distribution at various spreader planes. The intensity of temperature is less at the center because of the tangential over fire air. The temperature distribution is higher at the furnace walls. The particles burning at the front end above the spreaders are also indicated in the prediction. The maximum temperature attained is around 1400 K near the neck of the furnace, which was measured by the instrument and was also well predicted by the simulation model. Figure 11 shows the velocity of the particles. The bagasse particles coming from the spreader are influenced by the under grate air and also from the tangential over fire air. The smaller (15) Truelove, J. S.; Jamaluddin, A. S. Combust. Flame 1986, 64, 369. (16) Ubhayshankar, S. K.; Sticker, D. B.; Von Rosenberg, C. W.; Gannon, R. E. In The 16th International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA. 1976; p 427. (17) Stubington, J. F.; Aiman, S. Energy Fuels 1994, 8, 194-203. (18) Mitchell, R.E. In The 22nd International Symposium on Combustion; The Combustion Institute: Pittsburg, PA, 1988; p 69.

Effect of Fuel Size on Combustion

Figure 10. Contours of total temperature at different spreaders in kelvin.

Energy & Fuels, Vol. 21, No. 4, 2007 1899

Figure 13. Velocity and distribution of particles from the spreader at (m/s).

Figure 14. Temperature measurements on the grate at different air flows. Figure 11. Velocity of the particles inside the furnace (m/s).

Figure 15. Temperature measurements at the front and rear side of the grate. Figure 12. Flow pattern of the particles inside the furnace (K).

particles rise and burn in suspension. The medium particles reach the sloping back and burn at the rear end of the furnace, while the large particles fall on the grate. It is observed that there will be recirculation of the particles at the front water wall inside the furnace. Figure 11 shows the flow pattern inside the furnace. Most of the smaller particles move upward along with the updraft wind and burn in suspension. Figure 12 shows the temperature distribution and flow pattern inside the furnace. The particles coming from the spreaders are immediately influenced by the tangential over fire air, which results in pushing the particles in the upward direction. The air

coming from these tangential ducts helps in the rapid mixing of the fuel, and the fuel burns in suspension The front side water wall above the spreaders is heated by the particles that rise above the spreader and move in the upward direction. The blue color represents the tangential air inlets. Figure 13 shows the velocity distribution of particles. The bagasse particle coming through silos enters the combustion chamber thought the bagasse spreader. The particles are aided by the jets of distributor air provided at the bottom of the spreader. The particles then travel with the same velocity as the distributor air. They are subjected to under grate air and gravitational forces while traveling. The small particles rise along with the under grate air. The medium particles reach the


rear of the furnace, while larger particles settle on the grate. From the observation, it is clear that the velocity is diluted from the center to the rear of the furnace. Particle velocity reduces with the time while traveling on the grate. Figure 14 shows the temperature measured along the grate at different air flow rates. The measured values had same pattern of temperature distribution from front to rear side of the furnace. Figure 15 shows the measured temperature from the front and rear side of the grate. Conclusion Modern day stoker furnaces operating with bagasse as fuel are being researched for design improvement. Bagasse may become a sustainable biomass energy for the future power generation. But its high moisture content and varying particle sizes have significant influence on the combustion. Thermal analysis conducted for the various particle sizes reveals that they behave differently. Analysis made under nitrogen atmosphere for differential calorimetric analysis is a valid support for the behavior of the particles during combustion in a furnace. The calculations and experiment clearly shows that much combustion activity occurs over the rear half of the test furnace. The fuel moisture does significantly affect the size of the preignition zone and hence furnace stability. Fuel moisture plays a very important role in the initiation of instability in bagassefired furnaces. Actual observations of furnaces suggest that

Shanmukharadhya

sudden changes in bagasse moisture which arise from problems with mill operation appear to have a great effect on the furnace behavior. Tangential air flow rates have an effect on the size of recirculation zone by forming an imaginary circle of flame inside the furnace to increase more heat transfer to the water walls. The maximum temperature occurs at the neck of the furnace. It was observed that an increase of the under grate air flow can significantly increase the delay to ignition. Perhaps this may be attributed to the effect that the flow rate of under grate air has upon the rate of deposition of fuel on the furnace grate. Also the bagasse and air flow rates through spreaders are found to have some influence on the ignition delay and the location on the furnace grate where the large particles come to rest. Finally, it is clear from the results that the pyrolysis of bagasse plays an important role in prediction of the thermal fields and ultimately stability of the bagasse furnace. This influence is particularly significant in the predicted delay to ignition of the fuel Acknowledgment. The author extends sincere thanks to Dr. K. Subramanian, Professor, Polymer Chemistry, Department of Biotechnology, Bannari Amman Institute of Technology, for useful discussions related to the interpretation of the thermograms of the bagasse samples. EF060278Y

Simulation and Thermal Analysis of the Effect of Fuel Size on

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