- Email: [email protected]
Modeling of Bubbling Fluidized Bed Combustor Based on Biomass &Co-firing
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 4 (2017) 1615–1625
www.materialstoday.com/proceedings
5th International Conference on Materials Processing and Characterization (ICMPC 2016)
Modeling of Bubbling Fluidized Bed Combustor Based on Biomass & Co-firing Mohit Gabaa*, Hemant Kumarb, S.K.Mohapatrac a
Departmentof Mechanical Engineering, Asra College of Engg. & Technology, Sangrur148001 (India) b Departmentof Mechanical Engineering, Punjabi University, Patiala 147002 (India) c Departmentof Mechanical Engineering, Thapar University, Patiala 147004 (India)
Abstract As the rising demand of the fossil fuelsand their limited resources, the use of biomass fuels as energy resource in the thermal power plants is increasing popularly. The combustion of the biomass fuel with conventional combustion techniques is not suitable but the fluidized bed combustion (FBC) is one of the most promising energy conversion options available for the combustion of such low density fuel. The scope of this paper is to investigate the suitability of mathematical model of fluidized bed combustion based upon the bio-mass fuel &co-firing of bio-mass fuel with coal to predict and simulate the performance of 15 TPH plant situated at Ladhar paper mill, Nakodar and 75 TPH plant situated at Satia Paper Industry, ShriMuktsar Sahib, Punjab (India). The Ladhar paper mill runs at 100% bio-mass fuel &Satia paper industry plant runs on 85% husk with 15% coal to 100% husk. The model produces the relevant input/output parameter of bed such as fluidized velocity of air, combustion efficiency of boiler, the exit value of CO2 to the atmosphere etc. by the change in the supply of excess air to the boiler. © 2017 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the conference committee members of the 5th International conference on Materials Processing and Characterization (ICMPC 2016) Keywords:Fluidized bed combustion;rice husk;coal; modeling;co-firing
1. Introduction Use of energy in these days is largely dependent on fossils fuels. However, In future these fossil fuels would make the development very difficult to keep going. During combustion there is rapid release of polluting products from the fossil fuels and these polluting products have effective role in changingthe composition and behavior of our atmosphere
.*Corresponding author. Tel.:+91-987-616-2828. E-mail address:[email protected]
2214-7853© 2017 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the conference committee members of the 5th International conference on Materials Processing and Characterization.
1616
Mohit Gaba et al/ Materials Today: Proceedings 4 (2017) 1615–1625
By generating electricity from the fossil fuels, there remains a significant amount of the carbon dioxide emissions. Day by day demand for electricity is growing very fast which results in the increase in the use of fossil fuels and increase the carbon dioxide emissions and some other pollutants. So there is a need of some other fuels that can be used in place of the fossil fuels. To overcome this problem biomass or agriculture wastage, a non-conventional fuel, is used and it is ensured that the use of non-conventional fuel minimize the environmental degradation and also minimize the stress associated with mining, processing and transporting of conventional fuel. There are large quantities of agricultural residues which can be used for the production of energy. All of them are not economically suitable to make use as a fuel because of large investments required for collection, transportation and storage. However there are some biomass residues concentrated at specific location, where demand for energy also exists.They include rice husk at the rice mills, rice straw in the fields and bagasse at the sugar mills [1]. Utilization of biomass as an energy resource has various environmental advantages such as reduction in the emissions of toxic gases i.e. SOx, NOx. It is due to the low contents of sulphur and nitrogen present in the biomass fuels. Another benefit is to reduction in the amount of solid waste and utilization of agricultural or agro industrial residues and minimization of waste disposal [2]. Rice is cultivated in more than 100 countries in the world. China is at first place in the cultivation of rice and India is on second place. India is an agriculture country and production of biomass is plentiful. So these different biomasses can be used to fulfill the increased demand of the electricity. Punjab produces about 11% of country’s rice and 2% of world’s rice. Fluidized bed combustion system is proved to be a very effective technique for the combustion of the such low grade fuels due to various advantages like high efficiency of combustion, ability to burn low grade fuel, pollution control, easier ash removal, less excess air, high reliability, reduced maintenance and high efficiency of power generation. Nomenclature 2
Cross sectional area of the bed (cm ) At Ab Cross sectional area occupied by the bubbles (cm2) Db Diameter of bubble (cm) Dg Diffusivity of gas (cm2/s) Dp Diameter of rice husk particles (cm) Exair Fraction of Excess air FME Actual air feed rate to the combustor (g-mol/sec) F0 Total carbon feed rate (g/s) F1 Carry over rate, g/s F2 Fly ash (Air heater and ESP hopper) F3 Total carbon in carry over rate (g/s) F4 Actual carbon in fly ash (Air heater and ESP hopper) F5 Carbon lost due to unburnt rice husk (g/s) K Effective reaction rate constant (cm/s) Greek symbols Density of rice husk particle 𝜌p Density of gas 𝜌g Viscosity of gas 𝜇g 𝜖mf Voidage in the bed at minimum fluidization 𝜖b Volume fraction of bubbles in the bed
Pav Average pressure in the combustor (atm) R Gas constant Absolute bed temperature (K) Tb Uo Superficial gas velocity (cm/s) Umf Minimum fluidization velocity/ Ubr Single bubble velocity (cm/s) Ub Average absolute velocity of crowd of bubbles (cm/s) Z Bed height (cm) Ub Velocity of gas through bubble phase (cm/s) Wc Bed carbon load (g) XC Carbon weight percentage XH Hydrogen weight percentage XN Nitrogen weight percentage XW Moisture weight percentage XS Sulfur weight percentage SIL Satia Industry Limited LPM Ladhar Paper Mill g Acceleration due to gravity (cm/s2) Kg Mass transfer coefficient (cm/s)
Mohit Gaba et al/ Materials Today: Proceedings 4 (2017) 1615–1625
1617
1.1. Plant Detail 1.2. 1.1.1 Description of 15TPH F.B.C. Boiler at L.P.M. Nakodar The Ladhar paper mill is a mediumscale mill producing paper from the recycling of the waste paper obtainedfrom the market. The boiler is installed for the generation of steam which is used in the paper machine for drying of the paper. The mill is situated on the Nakodar-Moga road in the Ladhar village & around 5 Km away from the Nakodar. The mill is producing paper from 40 GSM to 120 GSM. It has only one paper machine. 1.1.2. Description of 75TPH FBC Boiler at S.I.L. Muktsar Satia paper mill was established in year 1994 with purpose of using the agro industrial biomass product specially the rice husk. In these days the name of Satia paper mill is changed to Satia Industries Ltd. The plant was set up for the production of papers and main product of the plant is writing and printing paper. Area of ShriMuktsar Sahib is very beneficial for the growth of rice and cotton. The fuel availability is plentiful for the combustion.S.I.L. Muktsar has three units out of these two are of 5MW capacity and one is of 12.5 MWcapacity. The plant uses the Co-firing of coal & rice husk from the range of 95% husk + 5% coal to 85% husk + 15%coal depending upon the load. 2. Literature Review A dynamic model of a bubbling fluidized bed boiler. Air flue gas model and a model for water steam circuit were described. In air flue gas model combustions processes in furnace were explained and in water steam model describes the heat transfer from the hot flue gases to water and steam, separation of the steam model describes the heat transfer from the hot flue gases to water and steam, separation of thesteam in drum section and super heating of the steam. By taking account of fuel quality parameters a versatile furnace model was prepared for the simulation of the effect of moisture, particle size and heat value [3].The validation of the model describe the static and dynamics gains very well of the process. Combustion efficiency was not dependent on the fluidization velocity and excess air but it would minor dependent on the temperature. With the increase in fluidization velocity the carbon mono oxide emission increases and it also increases with increase in temperature. Sulphur dioxide and nitrogen oxides had less emission than carbon mono oxide emission [4]. The major differences between the two correlations were the shape factor and the minimum fluidization void age should depend only on the shape factor regardless of the diameter of the particle [5]. The equation obtained from the two correlations was simple, gives great accuracy and also expressed in a convenient graphical form for a rapid estimation of the minimum fluidization velocity. The minimum fluidization velocity increases with the increase in the mass fraction of rice husk particles in the bed [6] and it was also found that the rice husk particles gives a lower fluctuation energy, while the sand particles had a higher kinetic energy of fluctuations[7]. The mathematical modelling of catalytic fluidized bed reactor-I. Development of the model for gas-solid fluidized bed catalytic reactors had been done. The three phase theory of fluidization had been developed by considering the fluid bed to be consisted of a number of equivalent stages in series. In three phase theory the bubble phase, cloud-wake phase and emulsion phase were described. Within each stage exchange of gas taken place between these three phases. Model parameters were expressed in terms of Model parameters were expressed in terms of readily obtainable properties of bed. The validity of the multistage three-phase model was confirmed by the experimental data on reactant conversions and concentration profiles, and concluded that this model is useful for design and scale-up of fluid-bed reactors. The three phase theory of fluidization had been developed by considering the fluid bed to be consisted of a number of equivalent stages in series [8], in three phase theory the bubble phase, cloud-wake phase and emulsion phase were described. For shrinkage particles, the single film theory of char combustion had been considered by reaction of carbon with oxygen to produce carbon dioxide. The conversion of oxygen in the fluidized bed combustion boiler plant decreases with the increase in theexcess air. It was feasible to burn the rice husk in a fluidized bed reactor successfully and normally attained the high combustion efficiency [9]. Bubbling velocity of husk was same as that of char and mixture and also observed that the fluidization behaviour of husk char was little better than that of char but not as good as that of ash. Fluidization of mixed husk, ash and char showed the poor movement of individual particles at low air velocities [10]. Production
1618
Mohit Gaba et al/ Materials Today: Proceedings 4 (2017) 1615–1625
of sulphur and nitrogen dioxide was reduced by using the low bed temperature up to 8000 C. A solid population model was used to calculate the bed carbon load and carbon utilization efficiency. It was found that with the increase in the depth of bed the concentration of oxygen decreases and percentage of carbon dioxide increases. The problem found was the defluidization of bed exists which causes to stop the operation of the bed due to the agglomeration and the agglomeration was due to potassium, magnesium and sodium present in the rice husk [11].The particle mixing, bubble formation, formation of slugs etc. affect the heat and mass transfer processes directly. The commercial software fluent is used to solve the multiphase problem of bubbling gas-solid fluidization under vacuum conditions. It is observed that under vacuum conditions the amount of supply of air is reduced and the velocity distribution is also varied [12].The experimental data gathered from a semi industrial fluidized bed gasifier and a numerical model built in FLUENT was compared. Results showed good agreement although slight deviation was shown especially on CO and H2. This was mainly due to kinetics were taken and there for may differ greatly from one source to another. Also devolatilization was assumed instantaneous and no particular attention was paid to char conversion [13]. 3. Formulation of the model The oxygen that is present in the bubble cannot take part in combustion. The bubble phase oxygen is transported across the bubble cloud interface into the more carbon rich cloud-wake phase, where the combustion reaction takes places. Unreacted oxygen in the cloud-wake is further transported to very carbon rich emulsion phase. Unreacted oxygen in the cloud-wake is further transported to very carbon rich emulsion phase. Most of the oxygen is consumed in the emulsion phase. The net result of these events is that the observed combustion rate depends upon the transfer coefficients and chemical reaction rate constant, table1. Oxygen balance around stage ‘n’ for bubble phase |oxygen in by convection| - | oxygen out by convection| - |oxygen transfer to cloud-wake| = 0 Oxygen balance around stage ‘n’ for cloud-wake phase |oxygen in by convection| + |oxygen coming from bubble phase by transfer| - |oxygen going out from cloud-wake to emulsion by transfer| -| oxygen out from cloud-wake by convection|+ |oxygen consumed in cloud-wake phase| =0 Oxygen balance around stage ‘n’ for emulsion phase |oxygen in by convection| - |oxygen out by convection| + |oxygen transfer from cloud-wake phase| = |oxygen consumed by combustion reaction| Table 1. Hydrodynamic parameters for fluidized Bed combustion Parameter Theoretical or empirical correlation 1.Single bubble velocity (cm/s) 2. Average absolute velocity of crowd of bubbles (cm/s) 3. Superficial gas velocity through the bed (cm/s)
4. Minimum fluidization velocity (cm/s) 5. Gas viscosity (g/cm-s) 6. Gas density(g/cm3) 7. Diffusivity of Gas Dg (cm2/s) 8. Average bubble diameter (cm) 9. Gas velocity through bubble phase (cm/s)
ubr=0.711�𝑔𝐷𝑏 ub=U0-Umƒ+ubr U0 =
Reddy et al.[8]
𝐹𝑀𝐸.𝑅.𝑇𝑏 𝑃𝑎𝑣𝐴𝑡
where FME=FMTH(1+Exair) 𝑋𝐶/12+𝑋𝐻/4+𝑋𝑆/32−𝑋𝑂/32 FMTH=WRiceHusk (1-XW)[ ] Umƒ = (
µ
𝑑𝑝 𝜌𝑔
)[{(33.7)2+
µ =1.4 (10-5 ) (Tb )1/2 ρg=353.2(10)-3/Tb 5.14(Tb)1.5/100000
0.21 0.0408𝑑𝑝 𝜌𝑔 �𝜌𝑠−𝜌𝑔 �𝑔 0.5 } -33.7] µ2𝑔
𝐴
Db=0.54(U0-Umƒ)0.4�𝑍 + 4� 𝑡 �.g-0.2 Ub=
𝑁𝐷
𝑈0 −𝑈𝑚ƒ
1+ƒ𝑐𝑤 η𝑚ƒ
10.Gas velocity through cloudwake phase (cm/s)
Ucw=
11. Cross sectional area of bed occupied by bubble (cm2) 12. Bubble to cloud wake gas interchange coefficient (s-1)
Ab= εbAt
𝑈0 −𝑈𝑚ƒ
1+ƒ𝑐𝑤 η𝑚ƒ
(Kbc)c=4.3�
ƒcwε mf
𝑈𝑚ƒ 𝐷𝑏
Reference Reddy et al.[8] Reddy et al.[8]
1/2
𝐷𝑔 𝑔1/4
�+5.85�
5/4
𝐷𝑏
�
Wen and Yu [14] Bird et al.[15] Bird et al.[15] Bird et al.[15] Stubington et al.[16] EL-Halwagi and ELRifai[17] EL-Halwagi and ELRifai[17] EL-Halwagi and ELRifai[17] Kunii and Levenspiel [18]
Mohit Gaba et al/ Materials Today: Proceedings 4 (2017) 1615–1625
1619
4. Results & Discussions The input data required for the Exit gas model is taken from Satia Industry Ltd., Shri Muktsar Sahib & Ladhar Paper Mill, Nakodar. The S.I.L. uses the co-firing of the Rice Husk with Coal in range of 85% Rice Husk + 15% Coal to the 100% Rice Husk. But at the time of study the plant uses 90% Rice Husk + 10% Coal. The L.P.M. uses the 100% Rice husk as fuel for the Boiler (At the time of Study). The average diameter of the rice husk particle is found to be 0.438 cm. The Ultimate & Proximate analysis of rice husk & Coal for the S.I.L. is given in Table 2 & for L.P.M. is give in Table 3. The model has been used to predict the outlet gas composition & variation of average oxygen concentration.The fraction of cloud wake has been used as an parameter for the value of fcw = 0.3 the predicted exit gas composition along with the plant data are given in the table 4. Table 2. Proximate & Ultimate Analysis of S.I.L. Shri Muktsar Sahib Components Moisture Ash Volatile matter Fixed carbon
Proximate Analysis Rice Husk 11.84% 16.27% 56.44% 15.45%
Coal 2% 9.25% 38.63% 50.12%
Components Carbon Hydrogen Sulphur Oxygen Nitrogen
Table 3. Proximate & Ultimate Analysis of L.P.M., Nakodar Proximate Analysis
Ultimate Analysis Rice Husk 35.5% 5.85% 0.68% 41.12% 0.58%
Coal 66.8% 4.2% 0.8% 25.2% 2.6%
Ultimate Analysis
Components
Rice Husk
Components
Rice Husk
Moisture
11.84%
Carbon
38.9
Ash
16.27%
Hydrogen
5.1
Volatile matter
56.44%
Sulphur
0.12
Fixed carbon
15.45%
Oxygen
37.9
Nitrogen
2.17
Table 4. Comparison of model prediction with real plant date at fcw = 0.3
O2
Real (%age) S.I.L. 6-9%
L.P.M. 6-8 %
Simulated (%) S.I.L. 7.12%
L.P.M. 7.6%
CO2
10-14%
14-16 %
13.87%
13%
N2
76-79%
77-79%
79.41%
78.6%
%age in Flue Gases
Components Percentage by Volume in Exit gas
20
CO2 %age (Model Result) SIL
10 O2 %age (Model Result) SIL 0 0.1
0.2
0.3
0.4
Excess Air
0.5
0.6
CO2 %age (Model Result) LPM
Fig. 1. Variation of the %age in flue gases versus excess air
From the table 4 it is clear that the model prediction are agree with plant data therefore the model is valid. Fig.1.shows the %age of CO2 & O2 in flue gas with fraction of excess air for SIL & LPM. The amount of combustibles available in the bed remains same due to which the %age of CO2 decreases with the increase in excess
Mohit Gaba et al/ Materials Today: Proceedings 4 (2017) 1615–1625
1620
Superficial Velocity (cm/s)
air. However the %age of O2 increases with the increase in excess air.Fig. 2.shows the variation of superficial velocity versus excess air for S.I.L. & L.P.M. The superficial velocity is dependent on actual feed rate of the air & the actual feed rate directly depends on the fraction of excess air. So an increase in the fraction of excess air causes the increase in superficial velocity in the fluidized bed combustor. Fig. 3.shows the variation between minimum fluidization velocity & fraction of Excess air for S.I.L. & L.P.M. Minimum fluidization velocity dependent on viscosity of fluidizing gas & the viscosity fluidizing gas is affected with the variation in temperature. With the increase in fraction of excess air there is the variation in temperature of fluidized bed which causes slight variation in viscosity of fluidizing gas. As minimum fluidization velocity is inversely proportional to the viscosity of fluidizing gas so it decreases slightly with increase in fraction of excess air.
150 100 50
S.I.L.
0
L.P.M 0.1
0.2
0.3
0.4
0.5
0.6
Excess- Air
Minimum Fluidization Velocity (Cm/s)
Fig. 2. Variation of Superficial Velocity versus Excess Air
40 35
SIL
30
LPM 0.1
0.2
0.3
0.4
0.5
0.6
Excess Air Fig.3. Variation of Minimum Fluidization Velocity versus Excess Air
Velocity of gas through cloud wake face (cm/s)
Fig.4.shows the variation of velocity of gas through cloud wake face versus excess air. The velocity of cloud wake face goes on increasing with the increase in excess air. Due to the increase in cloud wake face the combustion efficiency increases which also increases the bed temperature. 150 100 50
S.I.L
0 0.1
0.2
0.3
0.4
0.5
0.6
Excess Air
Fig. 4. Variation of Velocity gas through Cloud Wake Face versus fraction of Excess-air
L.P.M
Bubble Diameter
Mohit Gaba et al/ Materials Today: Proceedings 4 (2017) 1615–1625
1621
30 20 10
S.I.L.
0
L.P.M 0.1
0.2
0.3
0.4
0.5
0.6
Excess- Air Fig. 5. Variation of Bubble Diameter versus fraction of Excess-air
Overall Reaction Rate
Fig.5.shows the variation of bubble diameter with the fraction of excess air. This is due to the superficial gas velocity increases as fraction of excess air increases, leading to the gradual increase of Bubble diameter. The increase in bubble diameter causes a decrease in gas interchange co-efficient, consequently the oxygen conversion also decreases. Fig.6.shows the overall reaction rate with fraction of excess air. It increases with increase in fraction of excess air. This is due to increase in the bed temperature that causes the overall reaction rate to increase with the fraction of excess air. 30 20 10
S.I.L.
0
L.P.M 0.1
0.2
0.3
0.4
0.5
0.6
Excess- Air Fig. 6. Variation of Overall reaction rate versus Excess- air
%age inFlue gases
The fuel input to the plant has different %age of moisture in it & the maximum allowable value of moisture in plant is 12.5%. In usual days the moisture present in the rice husk particle is 7-10%. But in rainy days the %age of moisture in the fuel is quite high. The effect of moisture on oxygen concentration & Carbon dioxide emitted from the FBC is shown in Fig.7.the %age of CO2 increases with increase in moisture contents & %age of O2 decreases with increase in the moisture contents in the fuel particles. The increase in moisture contents in the fuel causes the bed temperature to decrease in the plant. The bed temperature decreases because more heat is needed to convert moisture from fuel into vapors form. Since the phase change of liquid take place so the latent heat required to change the phase is more which causes bed temperature to decrease. Due to lower bed temperature the complete carbon conversion occurs & causing to increase CO2 as mentioned in Fig.7.the value of carbon monoxide increases &NOx decreases with the increase in moisture in fuel. The value of excess air is taken constant 0.2 in this study. 20 CO2 %age (S.I.L)
10
O2 %age (S.I.L.)
0 7.5
8.5
9.5
10.5
11.5
12.5
%age of Moisture contents Fig. 7. Variation of percentage of gases in flue gas versus moisture
CO2 %age (L.P.M) O2 %age (L.P.M)
Mohit Gaba et al/ Materials Today: Proceedings 4 (2017) 1615–1625
Superficial Velocity (cm/sec)
1622
150 100 50
S.I.L.
0
L.P.M 7.5
8.5
9.5
10.5
11.5
12.5
%age moisture content Fig. 8. Variation of Superficial Velocity versus Moisture
Velocity through cloud wake face (cm/sec)
Fig. 8.shows the variation between superficial velocity & moisture in fuel increases. This is due to the temperature decreases in the bed because of the latent heat carried away by flue gases increases. Since the superficial velocity depends upon the bed temperature. So the superficial velocity decreases with increase in moisture %age in the fuel.Fig. 9.shows the variation of velocity of gas through cloud wake face versus excess air. The velocity of cloud wake face goes on decreases with the increase in moisture contents. Due to the decrease in cloud wake face the combustion efficiency decreases which also decreases the bed temperature.Fig. 10.shows the variation of bubble diameter with moisture contents. The bubble diameter will decrease with the increase in the moisture contents. Fig. 11.shows the overall reaction rate in combustion process. It decreases with increase in %age of moisture contents of the fuel. This happens as the bed temperature decreases with increase in moisture contents in the fuel. The bed temperature is directly affecting the reaction rate in the fluidizing bed. Fig. 12 andFig. 13.shows the variation of elutriation rate constant and carry over rate with the variation of excess air respectively. This is due to the fact that due to increase in the excess air the temperature increases which increase the overall reaction rate. 100 50
S.P.M.
0
L.P.M. 7.5
8.5
9.5
10.5
11.5
12.5
%age moisture contents
Diameter of Bubble
Fig. 9. Variation ofVelocity gas through Cloud Wake Face versus fraction of Moisture
26 24 22
S.P.M.
20
L.P.M. 7.5
8.5
9.5
10.5
11.5
%age moisture contents Fig. 10. Variation of Bubble diameter versus moisture
12.5
Overall reaction rate
Mohit Gaba et al/ Materials Today: Proceedings 4 (2017) 1615–1625
1623
30 20 10
S.I.L.
0
L.P.M. 7.5
8.5
9.5
10.5
11.5
12.5
%age moisture contents Fig. 11. Variation of Overall reaction rate versus moisture
Elutration rate constant
As the fraction of excess air increases the bed temperature increases because more oxygen is available to burn the combustibles present in the bed. The combustibles present in the fluidized bed burn in an efficient manner in the bed with more available air. As the fraction of excess air increases the fine generated due to combustion of fly ash increases. The increase is probably due to higher overall reaction rate.Fig. 14.shows the variation of carbon utilization efficiency versus excess air. Carbon utilization efficiency decreases as the fraction of excess air is increased. As the fraction of excess air increases the superficial velocity also increases which will decrease the carbon utilization efficiency. 0.01 0.005
S.I.L.
0
L.P.M. 0.1
0.2
0.3
0.4
0.5
0.6
Excess air
Carry over rate
Fig. 12. Variation of Elutriation rate constant versus excess air
1000 500
S.I.L
0 0.1
0.2
0.3
0.4
0.5
0.6
L.P.M
Excess air
Carbon utilization efficiency (%age)
Fig.13. Variation of Carry over rate versus excess air
100 95 90 85
S.I.L. 0.1
0.2
0.3
0.4
0.5
Excess air Fig. 14. Variation of Carbon utilization efficiency versus Excess air
0.6
L.P.M.
1624
Mohit Gaba et al/ Materials Today: Proceedings 4 (2017) 1615–1625
5. Conclusions 1. The Mathematical modeling presented here predicts exit gas composition from 75 TPH FBC boiler at Satia Industry Ltd. ShriMuktsar Sahib which uses 90% rice husk with 10% coal as the fuel for the boiler and 15 TPH FBC boiler at Ladhar Paper Mill, Nakodar which uses the 100% rice husk as the fuel for the boiler. With increase in fraction of excess air from 0.1 to 0.6 in both the plants, the percentage of oxygen in the flue gas increases from 7.3% to 10.2 %in case of Satia Industry Ltd. and 6.7% to 9.2% in case of Ladhar Paper Mill. The percentage of carbon dioxide decreases from 15.7% to 12.1% in case of Satia Industry Ltd. and 13.6% to 11.2% in case of Ladhar Paper Mill. 2. With the increase in excess air from .1 to .6 there is increase in the Superficial velocity from 67.02 to 97.49cm/s in case of Satia Industry Ltd. and 90.27 to 131.30cm/s in case of Ladhar Paper Mill, Minimum fluidization velocity decreases from 38 to 35.4 cm/s in case of Satia Industry Ltd. and 34.8 to 32.9cm/s in case of Ladhar Paper Mill, Velocity through cloud wake face increase from 57.82 to 84.23 cm/s in case of Satia Industry Ltd. and 78.59 to 114.46 cm/s in case of Ladhar Paper Mill, Bubble diameter increases from 15.83 to 20.9 cm in case of Satia Industry Ltd. and 13.49 to 17.23 cm in case of Ladhar Paper Mill, Overall reaction rate increases from 16.26 to 18.7 in case of Satia Industry Ltd. and 18.66 to 20.98 in case of Ladhar Paper Mill, Carbon Utilization efficiency decreases from 95 to 90.4 percent in case of Satia Industry Ltd. and 96.1 to 91 percent in case of Ladhar Paper Mill. 3. With increase in the percentage of moisture in the fuel from 7.5 to 12.5 there is decrease in percentage of oxygen from 6.9 to 6.3 in case of Satia Industry Ltd. and 7.4 to 6.7 in case of Ladhar Paper Mill in the flue gas. The percentage of carbon dioxide increase from 11.7 to 14.9 in case of Satia Industry Ltd. and 10.5 to 13 in case of Ladhar Paper Mill in the flue gas. 4. With increase in percentage of moisture in fuel from 7.5 to 12.5 there is decrease in the Superficial velocity from 74.19 to 70.89 cm/s in case of Satia Industry Ltd. and 103.05 to 98.48 cm/s in case of Ladhar Paper Mill, Velocity through cloud wake face decreases from 64.84 to 61.96 cm/s in case of Satia Paper Mill and 90.7 to 86.08 cm/s in case of Ladhar Paper Mill, Bubble diameter decreases from 24.5 to 23.27 cm in case of Satia Industry Ltd. and 22.8 to 21.61cm in case of Ladhar Paper Mill, Overall reaction rate constant decreases from 18.54 to 16 in case of Satia Industry Ltd. and 20.9 to 17.9 in case of Ladhar Paper Mill. 5. Acknowledgement Authors would like to express their thanks to Er. Deepak Sharma (Sr. Engg. S.I.L. Muktsar) &Er. DeewanGarg (G.M. Boiler Section L.P.M. Nakodar) for their valuable guidance and technical support for conducting their research work at their plants. References [1] E. Natrajan, A. Nordin, A.N. Rao, Overview of combustion & gasification of rice husk in fluidized bed reactor. Biomass Bioenerg 14
(1998) 533-546. K. Gorkem, E. Morali, Mehmet, S. Nevin,Mathematical modeling of a bubbling fluidized bed combustor cofiried with lignite and biomass. Combust Sci Technol (2010)600-612. [3] K. Tuomas, M. Yrjo, Dynamic model of bubbling fluidized bed boiler (2005), 140-148. [4] F. Preto, E. J. Anthony, D.L. Desari, F.D. Friedrich, Combustion trails of rice hulls in a pilot-scale fluidized bed,Proceedings of internationalconference on fluidized bed combustor (1987)1123-1127. [5] S.C. Bhattacharya,W. Wu, Fluidized bed combustion of rice husk for disposal and energy recovery. In: Proceeding of energy from biomass and wastes-XII (1988), 591-601. [6] S. Qiaoqun, L. Huilen, L. Wentie,H. Yurong, Y. Lidan, D. Gidaspow,Simulation & experiment of segregating/mixing of rich husk-sand mixture in a bubbling fluidized bed (2005)1739-1748. [7] S. Van Loo, J. Koppejan (Eds.), The handbook of biomass combustion and co-firing, Earthscan (2008) London. [8]H. Kumar, S.K. Mohapatra, R.I. Singh, Three-dimensional CFD modelling of a fluidised bed combustor fuelled by biomass and coal. Mater Res Innov, 19, 118-124, 2015. [2]
Mohit Gaba et al/ Materials Today: Proceedings 4 (2017) 1615–1625 [9]
1625
L. Armesto, A. Bahilloa, K. Veijonenb, A. Cabanillasa, J. Oteroa, Combustion behaviour of rice husk in a bubbling fluidized bed. Biomass Bioenerg 23 (2002)171-79. [10] R. Sen, D. N. Gosh, Fluidization and combustion characteristics of rice husk. Indian Chemical Engineer 34 (1992) 1-4. [11] G.V. Reddy, S.K. Mohapatra, A mathematical model for exit gas composition in a 10MW fluidized bed coal combustion power plant. Energy Conversation Mgmt. 35(1994)1049-1060. [12] A. Kumar,Numerical study of hydrodynamics under vacuum condition in bubbling fluidized bed, International journal of advanced material manufacturing and characterisation 1(2012). [13] H. Kumar, S.K. Mohapatra, R.I. Singh, Study of a 30 MW bubbling fluidized bed combustor based on co-firing biomass and coal.Sadhana Acad P Eng S40 (2015)1283–1299. [14] C.Y. Wen, Y.H. Yu, A generalized method for predicting the minimum fluidization velocity (1966) 610-612. [15] R.B. Bird, W.E. Stewart, F.N. Lightfoot, Transport Phenomenon. Wiley New York (1960). [16] J.F. Stubington, D. Barrt, G. Lowry, Bubble size measurements and correlation in a fluidized bed at high temperature. Chem. Eng. Res. Des. 62 (1984) 173-178. [17] EL-Halwagi Mahmoud M. and EL-Rifai Mahmoud,Mathematical modeling of catalytic fluidized bed reactors-I. The multistage three-phase model. Chemical Engineering Science, (1988) vol. 43, No. 9, 2477-2486. [18] D. Kunii, O. Levenspiel, Fluidization Engineering.2nd Eds, Butterworth-Heinemann, Boston, U.S.A. (1991).
ScienceDirect Materials Today: Proceedings 4 (2017) 1615–1625
www.materialstoday.com/proceedings
5th International Conference on Materials Processing and Characterization (ICMPC 2016)
Modeling of Bubbling Fluidized Bed Combustor Based on Biomass & Co-firing Mohit Gabaa*, Hemant Kumarb, S.K.Mohapatrac a
Departmentof Mechanical Engineering, Asra College of Engg. & Technology, Sangrur148001 (India) b Departmentof Mechanical Engineering, Punjabi University, Patiala 147002 (India) c Departmentof Mechanical Engineering, Thapar University, Patiala 147004 (India)
Abstract As the rising demand of the fossil fuelsand their limited resources, the use of biomass fuels as energy resource in the thermal power plants is increasing popularly. The combustion of the biomass fuel with conventional combustion techniques is not suitable but the fluidized bed combustion (FBC) is one of the most promising energy conversion options available for the combustion of such low density fuel. The scope of this paper is to investigate the suitability of mathematical model of fluidized bed combustion based upon the bio-mass fuel &co-firing of bio-mass fuel with coal to predict and simulate the performance of 15 TPH plant situated at Ladhar paper mill, Nakodar and 75 TPH plant situated at Satia Paper Industry, ShriMuktsar Sahib, Punjab (India). The Ladhar paper mill runs at 100% bio-mass fuel &Satia paper industry plant runs on 85% husk with 15% coal to 100% husk. The model produces the relevant input/output parameter of bed such as fluidized velocity of air, combustion efficiency of boiler, the exit value of CO2 to the atmosphere etc. by the change in the supply of excess air to the boiler. © 2017 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the conference committee members of the 5th International conference on Materials Processing and Characterization (ICMPC 2016) Keywords:Fluidized bed combustion;rice husk;coal; modeling;co-firing
1. Introduction Use of energy in these days is largely dependent on fossils fuels. However, In future these fossil fuels would make the development very difficult to keep going. During combustion there is rapid release of polluting products from the fossil fuels and these polluting products have effective role in changingthe composition and behavior of our atmosphere
.*Corresponding author. Tel.:+91-987-616-2828. E-mail address:[email protected]
2214-7853© 2017 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the conference committee members of the 5th International conference on Materials Processing and Characterization.
1616
Mohit Gaba et al/ Materials Today: Proceedings 4 (2017) 1615–1625
By generating electricity from the fossil fuels, there remains a significant amount of the carbon dioxide emissions. Day by day demand for electricity is growing very fast which results in the increase in the use of fossil fuels and increase the carbon dioxide emissions and some other pollutants. So there is a need of some other fuels that can be used in place of the fossil fuels. To overcome this problem biomass or agriculture wastage, a non-conventional fuel, is used and it is ensured that the use of non-conventional fuel minimize the environmental degradation and also minimize the stress associated with mining, processing and transporting of conventional fuel. There are large quantities of agricultural residues which can be used for the production of energy. All of them are not economically suitable to make use as a fuel because of large investments required for collection, transportation and storage. However there are some biomass residues concentrated at specific location, where demand for energy also exists.They include rice husk at the rice mills, rice straw in the fields and bagasse at the sugar mills [1]. Utilization of biomass as an energy resource has various environmental advantages such as reduction in the emissions of toxic gases i.e. SOx, NOx. It is due to the low contents of sulphur and nitrogen present in the biomass fuels. Another benefit is to reduction in the amount of solid waste and utilization of agricultural or agro industrial residues and minimization of waste disposal [2]. Rice is cultivated in more than 100 countries in the world. China is at first place in the cultivation of rice and India is on second place. India is an agriculture country and production of biomass is plentiful. So these different biomasses can be used to fulfill the increased demand of the electricity. Punjab produces about 11% of country’s rice and 2% of world’s rice. Fluidized bed combustion system is proved to be a very effective technique for the combustion of the such low grade fuels due to various advantages like high efficiency of combustion, ability to burn low grade fuel, pollution control, easier ash removal, less excess air, high reliability, reduced maintenance and high efficiency of power generation. Nomenclature 2
Cross sectional area of the bed (cm ) At Ab Cross sectional area occupied by the bubbles (cm2) Db Diameter of bubble (cm) Dg Diffusivity of gas (cm2/s) Dp Diameter of rice husk particles (cm) Exair Fraction of Excess air FME Actual air feed rate to the combustor (g-mol/sec) F0 Total carbon feed rate (g/s) F1 Carry over rate, g/s F2 Fly ash (Air heater and ESP hopper) F3 Total carbon in carry over rate (g/s) F4 Actual carbon in fly ash (Air heater and ESP hopper) F5 Carbon lost due to unburnt rice husk (g/s) K Effective reaction rate constant (cm/s) Greek symbols Density of rice husk particle 𝜌p Density of gas 𝜌g Viscosity of gas 𝜇g 𝜖mf Voidage in the bed at minimum fluidization 𝜖b Volume fraction of bubbles in the bed
Pav Average pressure in the combustor (atm) R Gas constant Absolute bed temperature (K) Tb Uo Superficial gas velocity (cm/s) Umf Minimum fluidization velocity/ Ubr Single bubble velocity (cm/s) Ub Average absolute velocity of crowd of bubbles (cm/s) Z Bed height (cm) Ub Velocity of gas through bubble phase (cm/s) Wc Bed carbon load (g) XC Carbon weight percentage XH Hydrogen weight percentage XN Nitrogen weight percentage XW Moisture weight percentage XS Sulfur weight percentage SIL Satia Industry Limited LPM Ladhar Paper Mill g Acceleration due to gravity (cm/s2) Kg Mass transfer coefficient (cm/s)
Mohit Gaba et al/ Materials Today: Proceedings 4 (2017) 1615–1625
1617
1.1. Plant Detail 1.2. 1.1.1 Description of 15TPH F.B.C. Boiler at L.P.M. Nakodar The Ladhar paper mill is a mediumscale mill producing paper from the recycling of the waste paper obtainedfrom the market. The boiler is installed for the generation of steam which is used in the paper machine for drying of the paper. The mill is situated on the Nakodar-Moga road in the Ladhar village & around 5 Km away from the Nakodar. The mill is producing paper from 40 GSM to 120 GSM. It has only one paper machine. 1.1.2. Description of 75TPH FBC Boiler at S.I.L. Muktsar Satia paper mill was established in year 1994 with purpose of using the agro industrial biomass product specially the rice husk. In these days the name of Satia paper mill is changed to Satia Industries Ltd. The plant was set up for the production of papers and main product of the plant is writing and printing paper. Area of ShriMuktsar Sahib is very beneficial for the growth of rice and cotton. The fuel availability is plentiful for the combustion.S.I.L. Muktsar has three units out of these two are of 5MW capacity and one is of 12.5 MWcapacity. The plant uses the Co-firing of coal & rice husk from the range of 95% husk + 5% coal to 85% husk + 15%coal depending upon the load. 2. Literature Review A dynamic model of a bubbling fluidized bed boiler. Air flue gas model and a model for water steam circuit were described. In air flue gas model combustions processes in furnace were explained and in water steam model describes the heat transfer from the hot flue gases to water and steam, separation of the steam model describes the heat transfer from the hot flue gases to water and steam, separation of thesteam in drum section and super heating of the steam. By taking account of fuel quality parameters a versatile furnace model was prepared for the simulation of the effect of moisture, particle size and heat value [3].The validation of the model describe the static and dynamics gains very well of the process. Combustion efficiency was not dependent on the fluidization velocity and excess air but it would minor dependent on the temperature. With the increase in fluidization velocity the carbon mono oxide emission increases and it also increases with increase in temperature. Sulphur dioxide and nitrogen oxides had less emission than carbon mono oxide emission [4]. The major differences between the two correlations were the shape factor and the minimum fluidization void age should depend only on the shape factor regardless of the diameter of the particle [5]. The equation obtained from the two correlations was simple, gives great accuracy and also expressed in a convenient graphical form for a rapid estimation of the minimum fluidization velocity. The minimum fluidization velocity increases with the increase in the mass fraction of rice husk particles in the bed [6] and it was also found that the rice husk particles gives a lower fluctuation energy, while the sand particles had a higher kinetic energy of fluctuations[7]. The mathematical modelling of catalytic fluidized bed reactor-I. Development of the model for gas-solid fluidized bed catalytic reactors had been done. The three phase theory of fluidization had been developed by considering the fluid bed to be consisted of a number of equivalent stages in series. In three phase theory the bubble phase, cloud-wake phase and emulsion phase were described. Within each stage exchange of gas taken place between these three phases. Model parameters were expressed in terms of Model parameters were expressed in terms of readily obtainable properties of bed. The validity of the multistage three-phase model was confirmed by the experimental data on reactant conversions and concentration profiles, and concluded that this model is useful for design and scale-up of fluid-bed reactors. The three phase theory of fluidization had been developed by considering the fluid bed to be consisted of a number of equivalent stages in series [8], in three phase theory the bubble phase, cloud-wake phase and emulsion phase were described. For shrinkage particles, the single film theory of char combustion had been considered by reaction of carbon with oxygen to produce carbon dioxide. The conversion of oxygen in the fluidized bed combustion boiler plant decreases with the increase in theexcess air. It was feasible to burn the rice husk in a fluidized bed reactor successfully and normally attained the high combustion efficiency [9]. Bubbling velocity of husk was same as that of char and mixture and also observed that the fluidization behaviour of husk char was little better than that of char but not as good as that of ash. Fluidization of mixed husk, ash and char showed the poor movement of individual particles at low air velocities [10]. Production
1618
Mohit Gaba et al/ Materials Today: Proceedings 4 (2017) 1615–1625
of sulphur and nitrogen dioxide was reduced by using the low bed temperature up to 8000 C. A solid population model was used to calculate the bed carbon load and carbon utilization efficiency. It was found that with the increase in the depth of bed the concentration of oxygen decreases and percentage of carbon dioxide increases. The problem found was the defluidization of bed exists which causes to stop the operation of the bed due to the agglomeration and the agglomeration was due to potassium, magnesium and sodium present in the rice husk [11].The particle mixing, bubble formation, formation of slugs etc. affect the heat and mass transfer processes directly. The commercial software fluent is used to solve the multiphase problem of bubbling gas-solid fluidization under vacuum conditions. It is observed that under vacuum conditions the amount of supply of air is reduced and the velocity distribution is also varied [12].The experimental data gathered from a semi industrial fluidized bed gasifier and a numerical model built in FLUENT was compared. Results showed good agreement although slight deviation was shown especially on CO and H2. This was mainly due to kinetics were taken and there for may differ greatly from one source to another. Also devolatilization was assumed instantaneous and no particular attention was paid to char conversion [13]. 3. Formulation of the model The oxygen that is present in the bubble cannot take part in combustion. The bubble phase oxygen is transported across the bubble cloud interface into the more carbon rich cloud-wake phase, where the combustion reaction takes places. Unreacted oxygen in the cloud-wake is further transported to very carbon rich emulsion phase. Unreacted oxygen in the cloud-wake is further transported to very carbon rich emulsion phase. Most of the oxygen is consumed in the emulsion phase. The net result of these events is that the observed combustion rate depends upon the transfer coefficients and chemical reaction rate constant, table1. Oxygen balance around stage ‘n’ for bubble phase |oxygen in by convection| - | oxygen out by convection| - |oxygen transfer to cloud-wake| = 0 Oxygen balance around stage ‘n’ for cloud-wake phase |oxygen in by convection| + |oxygen coming from bubble phase by transfer| - |oxygen going out from cloud-wake to emulsion by transfer| -| oxygen out from cloud-wake by convection|+ |oxygen consumed in cloud-wake phase| =0 Oxygen balance around stage ‘n’ for emulsion phase |oxygen in by convection| - |oxygen out by convection| + |oxygen transfer from cloud-wake phase| = |oxygen consumed by combustion reaction| Table 1. Hydrodynamic parameters for fluidized Bed combustion Parameter Theoretical or empirical correlation 1.Single bubble velocity (cm/s) 2. Average absolute velocity of crowd of bubbles (cm/s) 3. Superficial gas velocity through the bed (cm/s)
4. Minimum fluidization velocity (cm/s) 5. Gas viscosity (g/cm-s) 6. Gas density(g/cm3) 7. Diffusivity of Gas Dg (cm2/s) 8. Average bubble diameter (cm) 9. Gas velocity through bubble phase (cm/s)
ubr=0.711�𝑔𝐷𝑏 ub=U0-Umƒ+ubr U0 =
Reddy et al.[8]
𝐹𝑀𝐸.𝑅.𝑇𝑏 𝑃𝑎𝑣𝐴𝑡
where FME=FMTH(1+Exair) 𝑋𝐶/12+𝑋𝐻/4+𝑋𝑆/32−𝑋𝑂/32 FMTH=WRiceHusk (1-XW)[ ] Umƒ = (
µ
𝑑𝑝 𝜌𝑔
)[{(33.7)2+
µ =1.4 (10-5 ) (Tb )1/2 ρg=353.2(10)-3/Tb 5.14(Tb)1.5/100000
0.21 0.0408𝑑𝑝 𝜌𝑔 �𝜌𝑠−𝜌𝑔 �𝑔 0.5 } -33.7] µ2𝑔
𝐴
Db=0.54(U0-Umƒ)0.4�𝑍 + 4� 𝑡 �.g-0.2 Ub=
𝑁𝐷
𝑈0 −𝑈𝑚ƒ
1+ƒ𝑐𝑤 η𝑚ƒ
10.Gas velocity through cloudwake phase (cm/s)
Ucw=
11. Cross sectional area of bed occupied by bubble (cm2) 12. Bubble to cloud wake gas interchange coefficient (s-1)
Ab= εbAt
𝑈0 −𝑈𝑚ƒ
1+ƒ𝑐𝑤 η𝑚ƒ
(Kbc)c=4.3�
ƒcwε mf
𝑈𝑚ƒ 𝐷𝑏
Reference Reddy et al.[8] Reddy et al.[8]
1/2
𝐷𝑔 𝑔1/4
�+5.85�
5/4
𝐷𝑏
�
Wen and Yu [14] Bird et al.[15] Bird et al.[15] Bird et al.[15] Stubington et al.[16] EL-Halwagi and ELRifai[17] EL-Halwagi and ELRifai[17] EL-Halwagi and ELRifai[17] Kunii and Levenspiel [18]
Mohit Gaba et al/ Materials Today: Proceedings 4 (2017) 1615–1625
1619
4. Results & Discussions The input data required for the Exit gas model is taken from Satia Industry Ltd., Shri Muktsar Sahib & Ladhar Paper Mill, Nakodar. The S.I.L. uses the co-firing of the Rice Husk with Coal in range of 85% Rice Husk + 15% Coal to the 100% Rice Husk. But at the time of study the plant uses 90% Rice Husk + 10% Coal. The L.P.M. uses the 100% Rice husk as fuel for the Boiler (At the time of Study). The average diameter of the rice husk particle is found to be 0.438 cm. The Ultimate & Proximate analysis of rice husk & Coal for the S.I.L. is given in Table 2 & for L.P.M. is give in Table 3. The model has been used to predict the outlet gas composition & variation of average oxygen concentration.The fraction of cloud wake has been used as an parameter for the value of fcw = 0.3 the predicted exit gas composition along with the plant data are given in the table 4. Table 2. Proximate & Ultimate Analysis of S.I.L. Shri Muktsar Sahib Components Moisture Ash Volatile matter Fixed carbon
Proximate Analysis Rice Husk 11.84% 16.27% 56.44% 15.45%
Coal 2% 9.25% 38.63% 50.12%
Components Carbon Hydrogen Sulphur Oxygen Nitrogen
Table 3. Proximate & Ultimate Analysis of L.P.M., Nakodar Proximate Analysis
Ultimate Analysis Rice Husk 35.5% 5.85% 0.68% 41.12% 0.58%
Coal 66.8% 4.2% 0.8% 25.2% 2.6%
Ultimate Analysis
Components
Rice Husk
Components
Rice Husk
Moisture
11.84%
Carbon
38.9
Ash
16.27%
Hydrogen
5.1
Volatile matter
56.44%
Sulphur
0.12
Fixed carbon
15.45%
Oxygen
37.9
Nitrogen
2.17
Table 4. Comparison of model prediction with real plant date at fcw = 0.3
O2
Real (%age) S.I.L. 6-9%
L.P.M. 6-8 %
Simulated (%) S.I.L. 7.12%
L.P.M. 7.6%
CO2
10-14%
14-16 %
13.87%
13%
N2
76-79%
77-79%
79.41%
78.6%
%age in Flue Gases
Components Percentage by Volume in Exit gas
20
CO2 %age (Model Result) SIL
10 O2 %age (Model Result) SIL 0 0.1
0.2
0.3
0.4
Excess Air
0.5
0.6
CO2 %age (Model Result) LPM
Fig. 1. Variation of the %age in flue gases versus excess air
From the table 4 it is clear that the model prediction are agree with plant data therefore the model is valid. Fig.1.shows the %age of CO2 & O2 in flue gas with fraction of excess air for SIL & LPM. The amount of combustibles available in the bed remains same due to which the %age of CO2 decreases with the increase in excess
Mohit Gaba et al/ Materials Today: Proceedings 4 (2017) 1615–1625
1620
Superficial Velocity (cm/s)
air. However the %age of O2 increases with the increase in excess air.Fig. 2.shows the variation of superficial velocity versus excess air for S.I.L. & L.P.M. The superficial velocity is dependent on actual feed rate of the air & the actual feed rate directly depends on the fraction of excess air. So an increase in the fraction of excess air causes the increase in superficial velocity in the fluidized bed combustor. Fig. 3.shows the variation between minimum fluidization velocity & fraction of Excess air for S.I.L. & L.P.M. Minimum fluidization velocity dependent on viscosity of fluidizing gas & the viscosity fluidizing gas is affected with the variation in temperature. With the increase in fraction of excess air there is the variation in temperature of fluidized bed which causes slight variation in viscosity of fluidizing gas. As minimum fluidization velocity is inversely proportional to the viscosity of fluidizing gas so it decreases slightly with increase in fraction of excess air.
150 100 50
S.I.L.
0
L.P.M 0.1
0.2
0.3
0.4
0.5
0.6
Excess- Air
Minimum Fluidization Velocity (Cm/s)
Fig. 2. Variation of Superficial Velocity versus Excess Air
40 35
SIL
30
LPM 0.1
0.2
0.3
0.4
0.5
0.6
Excess Air Fig.3. Variation of Minimum Fluidization Velocity versus Excess Air
Velocity of gas through cloud wake face (cm/s)
Fig.4.shows the variation of velocity of gas through cloud wake face versus excess air. The velocity of cloud wake face goes on increasing with the increase in excess air. Due to the increase in cloud wake face the combustion efficiency increases which also increases the bed temperature. 150 100 50
S.I.L
0 0.1
0.2
0.3
0.4
0.5
0.6
Excess Air
Fig. 4. Variation of Velocity gas through Cloud Wake Face versus fraction of Excess-air
L.P.M
Bubble Diameter
Mohit Gaba et al/ Materials Today: Proceedings 4 (2017) 1615–1625
1621
30 20 10
S.I.L.
0
L.P.M 0.1
0.2
0.3
0.4
0.5
0.6
Excess- Air Fig. 5. Variation of Bubble Diameter versus fraction of Excess-air
Overall Reaction Rate
Fig.5.shows the variation of bubble diameter with the fraction of excess air. This is due to the superficial gas velocity increases as fraction of excess air increases, leading to the gradual increase of Bubble diameter. The increase in bubble diameter causes a decrease in gas interchange co-efficient, consequently the oxygen conversion also decreases. Fig.6.shows the overall reaction rate with fraction of excess air. It increases with increase in fraction of excess air. This is due to increase in the bed temperature that causes the overall reaction rate to increase with the fraction of excess air. 30 20 10
S.I.L.
0
L.P.M 0.1
0.2
0.3
0.4
0.5
0.6
Excess- Air Fig. 6. Variation of Overall reaction rate versus Excess- air
%age inFlue gases
The fuel input to the plant has different %age of moisture in it & the maximum allowable value of moisture in plant is 12.5%. In usual days the moisture present in the rice husk particle is 7-10%. But in rainy days the %age of moisture in the fuel is quite high. The effect of moisture on oxygen concentration & Carbon dioxide emitted from the FBC is shown in Fig.7.the %age of CO2 increases with increase in moisture contents & %age of O2 decreases with increase in the moisture contents in the fuel particles. The increase in moisture contents in the fuel causes the bed temperature to decrease in the plant. The bed temperature decreases because more heat is needed to convert moisture from fuel into vapors form. Since the phase change of liquid take place so the latent heat required to change the phase is more which causes bed temperature to decrease. Due to lower bed temperature the complete carbon conversion occurs & causing to increase CO2 as mentioned in Fig.7.the value of carbon monoxide increases &NOx decreases with the increase in moisture in fuel. The value of excess air is taken constant 0.2 in this study. 20 CO2 %age (S.I.L)
10
O2 %age (S.I.L.)
0 7.5
8.5
9.5
10.5
11.5
12.5
%age of Moisture contents Fig. 7. Variation of percentage of gases in flue gas versus moisture
CO2 %age (L.P.M) O2 %age (L.P.M)
Mohit Gaba et al/ Materials Today: Proceedings 4 (2017) 1615–1625
Superficial Velocity (cm/sec)
1622
150 100 50
S.I.L.
0
L.P.M 7.5
8.5
9.5
10.5
11.5
12.5
%age moisture content Fig. 8. Variation of Superficial Velocity versus Moisture
Velocity through cloud wake face (cm/sec)
Fig. 8.shows the variation between superficial velocity & moisture in fuel increases. This is due to the temperature decreases in the bed because of the latent heat carried away by flue gases increases. Since the superficial velocity depends upon the bed temperature. So the superficial velocity decreases with increase in moisture %age in the fuel.Fig. 9.shows the variation of velocity of gas through cloud wake face versus excess air. The velocity of cloud wake face goes on decreases with the increase in moisture contents. Due to the decrease in cloud wake face the combustion efficiency decreases which also decreases the bed temperature.Fig. 10.shows the variation of bubble diameter with moisture contents. The bubble diameter will decrease with the increase in the moisture contents. Fig. 11.shows the overall reaction rate in combustion process. It decreases with increase in %age of moisture contents of the fuel. This happens as the bed temperature decreases with increase in moisture contents in the fuel. The bed temperature is directly affecting the reaction rate in the fluidizing bed. Fig. 12 andFig. 13.shows the variation of elutriation rate constant and carry over rate with the variation of excess air respectively. This is due to the fact that due to increase in the excess air the temperature increases which increase the overall reaction rate. 100 50
S.P.M.
0
L.P.M. 7.5
8.5
9.5
10.5
11.5
12.5
%age moisture contents
Diameter of Bubble
Fig. 9. Variation ofVelocity gas through Cloud Wake Face versus fraction of Moisture
26 24 22
S.P.M.
20
L.P.M. 7.5
8.5
9.5
10.5
11.5
%age moisture contents Fig. 10. Variation of Bubble diameter versus moisture
12.5
Overall reaction rate
Mohit Gaba et al/ Materials Today: Proceedings 4 (2017) 1615–1625
1623
30 20 10
S.I.L.
0
L.P.M. 7.5
8.5
9.5
10.5
11.5
12.5
%age moisture contents Fig. 11. Variation of Overall reaction rate versus moisture
Elutration rate constant
As the fraction of excess air increases the bed temperature increases because more oxygen is available to burn the combustibles present in the bed. The combustibles present in the fluidized bed burn in an efficient manner in the bed with more available air. As the fraction of excess air increases the fine generated due to combustion of fly ash increases. The increase is probably due to higher overall reaction rate.Fig. 14.shows the variation of carbon utilization efficiency versus excess air. Carbon utilization efficiency decreases as the fraction of excess air is increased. As the fraction of excess air increases the superficial velocity also increases which will decrease the carbon utilization efficiency. 0.01 0.005
S.I.L.
0
L.P.M. 0.1
0.2
0.3
0.4
0.5
0.6
Excess air
Carry over rate
Fig. 12. Variation of Elutriation rate constant versus excess air
1000 500
S.I.L
0 0.1
0.2
0.3
0.4
0.5
0.6
L.P.M
Excess air
Carbon utilization efficiency (%age)
Fig.13. Variation of Carry over rate versus excess air
100 95 90 85
S.I.L. 0.1
0.2
0.3
0.4
0.5
Excess air Fig. 14. Variation of Carbon utilization efficiency versus Excess air
0.6
L.P.M.
1624
Mohit Gaba et al/ Materials Today: Proceedings 4 (2017) 1615–1625
5. Conclusions 1. The Mathematical modeling presented here predicts exit gas composition from 75 TPH FBC boiler at Satia Industry Ltd. ShriMuktsar Sahib which uses 90% rice husk with 10% coal as the fuel for the boiler and 15 TPH FBC boiler at Ladhar Paper Mill, Nakodar which uses the 100% rice husk as the fuel for the boiler. With increase in fraction of excess air from 0.1 to 0.6 in both the plants, the percentage of oxygen in the flue gas increases from 7.3% to 10.2 %in case of Satia Industry Ltd. and 6.7% to 9.2% in case of Ladhar Paper Mill. The percentage of carbon dioxide decreases from 15.7% to 12.1% in case of Satia Industry Ltd. and 13.6% to 11.2% in case of Ladhar Paper Mill. 2. With the increase in excess air from .1 to .6 there is increase in the Superficial velocity from 67.02 to 97.49cm/s in case of Satia Industry Ltd. and 90.27 to 131.30cm/s in case of Ladhar Paper Mill, Minimum fluidization velocity decreases from 38 to 35.4 cm/s in case of Satia Industry Ltd. and 34.8 to 32.9cm/s in case of Ladhar Paper Mill, Velocity through cloud wake face increase from 57.82 to 84.23 cm/s in case of Satia Industry Ltd. and 78.59 to 114.46 cm/s in case of Ladhar Paper Mill, Bubble diameter increases from 15.83 to 20.9 cm in case of Satia Industry Ltd. and 13.49 to 17.23 cm in case of Ladhar Paper Mill, Overall reaction rate increases from 16.26 to 18.7 in case of Satia Industry Ltd. and 18.66 to 20.98 in case of Ladhar Paper Mill, Carbon Utilization efficiency decreases from 95 to 90.4 percent in case of Satia Industry Ltd. and 96.1 to 91 percent in case of Ladhar Paper Mill. 3. With increase in the percentage of moisture in the fuel from 7.5 to 12.5 there is decrease in percentage of oxygen from 6.9 to 6.3 in case of Satia Industry Ltd. and 7.4 to 6.7 in case of Ladhar Paper Mill in the flue gas. The percentage of carbon dioxide increase from 11.7 to 14.9 in case of Satia Industry Ltd. and 10.5 to 13 in case of Ladhar Paper Mill in the flue gas. 4. With increase in percentage of moisture in fuel from 7.5 to 12.5 there is decrease in the Superficial velocity from 74.19 to 70.89 cm/s in case of Satia Industry Ltd. and 103.05 to 98.48 cm/s in case of Ladhar Paper Mill, Velocity through cloud wake face decreases from 64.84 to 61.96 cm/s in case of Satia Paper Mill and 90.7 to 86.08 cm/s in case of Ladhar Paper Mill, Bubble diameter decreases from 24.5 to 23.27 cm in case of Satia Industry Ltd. and 22.8 to 21.61cm in case of Ladhar Paper Mill, Overall reaction rate constant decreases from 18.54 to 16 in case of Satia Industry Ltd. and 20.9 to 17.9 in case of Ladhar Paper Mill. 5. Acknowledgement Authors would like to express their thanks to Er. Deepak Sharma (Sr. Engg. S.I.L. Muktsar) &Er. DeewanGarg (G.M. Boiler Section L.P.M. Nakodar) for their valuable guidance and technical support for conducting their research work at their plants. References [1] E. Natrajan, A. Nordin, A.N. Rao, Overview of combustion & gasification of rice husk in fluidized bed reactor. Biomass Bioenerg 14
(1998) 533-546. K. Gorkem, E. Morali, Mehmet, S. Nevin,Mathematical modeling of a bubbling fluidized bed combustor cofiried with lignite and biomass. Combust Sci Technol (2010)600-612. [3] K. Tuomas, M. Yrjo, Dynamic model of bubbling fluidized bed boiler (2005), 140-148. [4] F. Preto, E. J. Anthony, D.L. Desari, F.D. Friedrich, Combustion trails of rice hulls in a pilot-scale fluidized bed,Proceedings of internationalconference on fluidized bed combustor (1987)1123-1127. [5] S.C. Bhattacharya,W. Wu, Fluidized bed combustion of rice husk for disposal and energy recovery. In: Proceeding of energy from biomass and wastes-XII (1988), 591-601. [6] S. Qiaoqun, L. Huilen, L. Wentie,H. Yurong, Y. Lidan, D. Gidaspow,Simulation & experiment of segregating/mixing of rich husk-sand mixture in a bubbling fluidized bed (2005)1739-1748. [7] S. Van Loo, J. Koppejan (Eds.), The handbook of biomass combustion and co-firing, Earthscan (2008) London. [8]H. Kumar, S.K. Mohapatra, R.I. Singh, Three-dimensional CFD modelling of a fluidised bed combustor fuelled by biomass and coal. Mater Res Innov, 19, 118-124, 2015. [2]
Mohit Gaba et al/ Materials Today: Proceedings 4 (2017) 1615–1625 [9]
1625
L. Armesto, A. Bahilloa, K. Veijonenb, A. Cabanillasa, J. Oteroa, Combustion behaviour of rice husk in a bubbling fluidized bed. Biomass Bioenerg 23 (2002)171-79. [10] R. Sen, D. N. Gosh, Fluidization and combustion characteristics of rice husk. Indian Chemical Engineer 34 (1992) 1-4. [11] G.V. Reddy, S.K. Mohapatra, A mathematical model for exit gas composition in a 10MW fluidized bed coal combustion power plant. Energy Conversation Mgmt. 35(1994)1049-1060. [12] A. Kumar,Numerical study of hydrodynamics under vacuum condition in bubbling fluidized bed, International journal of advanced material manufacturing and characterisation 1(2012). [13] H. Kumar, S.K. Mohapatra, R.I. Singh, Study of a 30 MW bubbling fluidized bed combustor based on co-firing biomass and coal.Sadhana Acad P Eng S40 (2015)1283–1299. [14] C.Y. Wen, Y.H. Yu, A generalized method for predicting the minimum fluidization velocity (1966) 610-612. [15] R.B. Bird, W.E. Stewart, F.N. Lightfoot, Transport Phenomenon. Wiley New York (1960). [16] J.F. Stubington, D. Barrt, G. Lowry, Bubble size measurements and correlation in a fluidized bed at high temperature. Chem. Eng. Res. Des. 62 (1984) 173-178. [17] EL-Halwagi Mahmoud M. and EL-Rifai Mahmoud,Mathematical modeling of catalytic fluidized bed reactors-I. The multistage three-phase model. Chemical Engineering Science, (1988) vol. 43, No. 9, 2477-2486. [18] D. Kunii, O. Levenspiel, Fluidization Engineering.2nd Eds, Butterworth-Heinemann, Boston, U.S.A. (1991).