Journal of Scientific & Industrial Research
440 Vol. 67, June 2008, pp. 440-444
J SCI IND RES VOL 67 JUNE 2008
Numerical study of heat loss from boiler using different ratios of fibre-toshell from palm oil wastes Mohamed Harimi1*, S M Sapuan2, M M H Megat Ahmad2 and Fuad Abas2 1
Centre of Minerals and Materials, School of Engineering and Information Technology, Universiti Malaysia Sabah, 88999 Kota Kinabalu, Sabah, Malaysia 2
Department of Mechanical and Manufacturing Engineering, Universiti Putra Malaysia, 43400 Serdang, Selangor Darul Ehsan, Malaysia Received 06 August 2007; revised 04 April 2008; accepted 10 April 2008
This study presents effect of excess air and fibre-to-shell (F/S) ratio on heat losses. Five heat losses are computed based on ASME standard (heat loss) and STANJAN code (flue gases). Heat loss due to dry flue gas is major heat loss in boiler and has been found statistically affected by the amount of excess air and F/S ratio, whereas other heat losses are somehow negligible, except heat loss due to moisture and hydrogen in fuel, which is mainly related to variation of F/S ratio. Boiler efficiency may reach 85% practically if global optimisation based on excess air and F/S ratios are consideredM with respect to combustion efficiency (low CO) and thermal efficiency (less heat losses). Keywords: Dry flue gas, Excess air, Heat losses, Palm oil wastes, Ratio of fibre-to-shell
Introduction Most of the palm oil mills in Malaysia use fibre and shell (F/S) as fuel for boiler, but ratio or proper mixing of these two wastes are not considered. Husain et al1 found that boiler efficiency (73%) affects effective utilization of steam in palm oil mill, due to nonhomogeneity of fuel, improper blending of F/S, high percentage of moisture and incomplete combustion. The proportional weights of F/S used to feed a boiler in seven palm oil mills in Perak state of Malaysia (Table 1) show really inconsistency of F/S ratio. Palm oil mills are continuing using only F/S (65:35) as fuel for grid-connected power generation and CHP2. Mahlia et al3 highlighted that composition of fuel in boiler is changing and therefore excess air (EA) used to combust F/S has to be identified, in order to get an acceptable emission limit. Generally, if fuel composition of fuel is not varying too much then the best excess air can be obtained and maintained. But if the fuel composition is changing, then excess air should be changed. Mahlia et al3 used F/S (70:30) with 30% excess air without indicating the reason of choosing such percentage of EA. Leong4 has suggested F/S of 60:40 *Author for correspondence E-mail:
[email protected]
Table 1—Proportion of fibre and shell used in the palm oil mills of Perak1 Mill #
Proportional weight, % Fibre
Shell
Calorific value MJ.kg-1
1 64 36 14.310 2 60 40 14.621 3 67 33 14.077 4 70 30 13.844 5 60 40 14.621 6 50 50 15.399 7 50 50 15.399 HHV (Fibre)=11.512 MJ.kg-1; HHV (Shell)=19.285 MJ.kg-1; Moisture = shell 10%, fibre 40%
and 70:30 with an optimal air fuel ratio of 12/1 to 16/1 respectively. Air fuel ratio of 12/1 corresponds to percentage EA > 120%, whereas 16/1 corresponds to percentage EA > 150%. Such range may be acceptable for reducing black smoke but it will also reduce boiler efficiency. This study presents effects of amount of EA and F/S ratio on heat losses due to dry flue gas, moisture in fuel, incomplete combustion, water from combustion of hydrogen (H2) in fuel, and finally moisture in combustion air.
HARIMI et al: EFFECT OF EXCESS AIR AND FIBRE TO SHELL RATIO ON HEAT LOSSES
Methodology Chemical composition of fibre and shell (% by mass), taken from a palm oil mill in Malaysia, is as follows: C, 30.68, 47.16; H, 3.90, 5.67; S, 0.20, 0.18; N, 0.91, 0.54; O, 23.86, 33.57; H2O (moisture), 35.00, 10.00; and ash, 5.45, 2.88%; and higher heating value, 11.69, 18.05 MJ/kg. Fixed data for heat losses, including average ambient air temperature (Ta), average relative humidity of air (RH) and stack flue gas are 28°C5-6, 80%5-6, and flue gases (STANJAN Code)17 respectively. Based on the data of RH (80%), amount of water in air at 28°C is computed using Eq. (1) or using Psychrometric chart based on a barometric pressure of 1.01325 bar7. ω=0.622 Ps / (P – Ps)
…(1)
where ω, amount of water vapour per amount of dry air (kg water/kg air); P, total pressure of water vapour and air (1.01325 bar); Ps, partial pressure of water vapour in mixture (bar). Total pressure P includes Ps and partial pressure of dry air (Pa), as P = Ps + Pa
…(2)
Amount of fraction of RH is related by φ = Ps /Pg
…(3)
where φ, fraction of RH; Pg, saturation pressure of water vapour at Ta (bar). Combining Eqs (1) and (3) ω = (0.622 φ . Pg) / (P – φ .Ps)
…(4)
From steam table at Ta = 28°C => Pg = 0.03778 bar, and with φ = RH /100 amounting to 0.8 and including the total pressure of 1.01325 bar, when all those data are substituted into Eq. (4) gives ω = 0.01912 kg vapour/kg dry air. Computation of heat losses is based on ASME standard and limited to heat losses due to dry flue gas, moisture in the fuel, incomplete combustion, water from combustion of hydrogen in fuel, and finally moisture in combustion air.
441
Heat Losses based on ASME Standard
There are two methods (direct method and indirect method) of analysing boiler efficiency. In this study, indirect method is used based on ASME standard PTC 4.18. Heat Loss due to Dry Flue Gas
There is an energy loss associated with N2, which enters boiler as a constituent of combustion air and leaves boiler at a higher temperature. Additionally, gaseous combustion products leave boiler at an elevated temperature, thus energy is lost from the system. This loss in a correctly operated system can be calculated as HLg = mg . Cpg . (Tg – Ta)
…(5)
where HLg, heat loss due to dry flue gas (MJ/kg); mg, mass of dry flue gas per kg of fuel as-fired; Cpg, average specific heat capacity of flue gas (0.001 MJ/kg °C between 20-200 °C); Ta and Tg, average inlet air and exit flue gas temperatures respectively (°C). Heat Loss due to Moisture in Fuel
Moisture entering boiler with fuel leaves as superheated vapour. This moisture loss is made up of sensible heat to bring moisture to boiling point [Cpw .(100 – Ta)], e superheat required to bring this steam to temperature of exhaust gas [Cpv .(Tg – 100)], and latent heat of evaporation (hfg). All the three forms of losses are grouped as HLmf =
mc f 100
[
⋅ Cp w ⋅ (100 − Ta ) + Cp v ⋅ (Tg − 100 ) + h fg
]
…(6) where HLmf, heat loss due to moisture in fuel, MJ/kg; mcf, moisture content of fuel, % as-fired; hfg, latent heat of evaporation of water (2.26 MJ/kg at 100°C); Cpw, average specific heat capacity of water (0.0042 MJ/kg°C at 100°C); Cpv, average specific heat capacity of water vapour (MJ/kg°C). Heat Loss due to Incomplete Combustion
Product formed by incomplete combustion could be mixed with oxygen and burned again with further release of energy. Such products include carbon monoxide (CO), H2 and various hydrocarbons and are generally only found in flue gases. CO is the only gas whose concentration can be determined conveniently in a power plant test9. The expression of heat loss due to incomplete combustion has been proposed by Ganapathy10 and Kumar & Sah11 as
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J SCI IND RES VOL 67 JUNE 2008
Table 2—Percentagea of heat losses due to various parameters F/S Parameter ratio
Excess air, % 0
20
40
60
80
100
120
150
0:100
HLg 4.38 5.18 6.00 6.81 7.63 8.45 9.26 10.49 HIC 4.6718 1.6041 0.4408 0.1119 0.0288 0.0077 0.0022 0.0003 HLa 0.1566 0.1879 0.2193 0.2506 0.2819 0.3132 0.3445 0.3915 HCL 15.4768 13.2476 12.9278 13.4457 14.2113 15.0397 15.8842 17.1577 20:80 HLg 4.40 5.21 6.03 6.85 7.67 8.49 9.32 10.55 HIC 4.3031 1.3972 0.3705 0.0928 0.0238 0.0064 0.0018 0.0003 HLa 0.1575 0.1890 0.2205 0.2520 0.2835 0.3150 0.3464 0.3937 HCL 16.0639 14.0009 13.8225 14.3966 15.1810 16.0178 16.8679 18.1489 40:60 HLg 4.43 5.24 6.06 6.89 7.72 8.54 9.37 10.61 HIC 3.9014 1.1833 0.3017 0.0746 0.0191 0.0051 0.0014 0.0002 HLa 0.1584 0.1900 0.2217 0.2534 0.2850 0.3167 0.3484 0.3959 HCL 16.7711 14.9003 14.8720 15.5016 16.3044 17.1496 18.0055 19.2939 50:50 HLg 4.44 5.26 6.08 6.91 7.74 8.57 9.40 10.64 HIC 3.6874 1.0744 0.2682 0.0659 0.0168 0.0045 0.0013 0.0002 HLa 0.1588 0.1906 0.2223 0.2541 0.2859 0.3176 0.3494 0.3970 HCL 17.1810 15.4176 15.4675 16.1243 16.9362 17.7854 18.6441 19.9362 60:40 HLg 4.45 5.27 6.10 6.93 7.76 8.59 9.43 10.68 HIC 3.4648 0.9652 0.2358 0.0575 0.0147 0.0039 0.0011 0.0002 HLa 0.1593 0.1911 0.2230 0.2548 0.2867 0.3185 0.3504 0.3981 HCL 17.6371 15.9895 16.1188 16.8023 17.6227 18.4761 19.3377 20.6337 80:20 HLg 4.48 5.31 6.14 6.97 7.81 8.65 9.48 10.74 HIC 3.1682 0.6811 0.1591 0.0383 0.0098 0.0026 0.0007 0.0001 HLa 0.1602 0.1922 0.2242 0.2563 0.2883 0.3203 0.3524 0.4004 HCL 18.9592 17.0773 17.8030 18.6330 19.4884 20.3508 21.1651 22.4689 100:0 HLg 4.51 5.34 6.18 7.02 7.86 8.70 9.54 10.81 HIC 2.4805 0.5404 0.1205 0.0287 0.0073 0.0020 0.0006 0.0001 HLa 0.1611 0.1933 0.2256 0.2578 0.2900 0.3222 0.3544 0.4028 HCL 20.0924 19.0147 19.4648 20.2455 21.0980 21.9670 22.8401 24.1518 a Percentage of heat losses are computed with respect to higher heating value of F/S (fibre to shell ratio) HLg, computed based on Eq. (5); HIC, computed based on Eq. (7); HLa, computed based on Eq. (9); HCL, computed based on Eq. (10)
y2 HIC = HHVCO ⋅ mC ⋅ y1 + y 2
…(7)
where HIC, heat loss due to incomplete combustion (MJ/ kg); HHVCO , higher heating value of CO, typically HHVCO =10.143 MJ/kg; mC, carbon in fuel, % as-fired; y1 and y2 are volume fraction of CO2 and CO respectively in flue gas. Heat Loss due to Water from Combustion of Hydrogen in Fuel
Combustion of hydrogen causes a heat loss because water is formed in combustion product. This water is converted to steam in boiler and carries away heat, particularly because of its latent heat content. Heat loss is calculated as
HLH =
9 ⋅ mH 2 ⋅ Cp w ⋅ (100 − Ta ) + Cp v ⋅ (T g − 100 ) + h fg 100
[
]
…(8) where, HLH, heat loss due to water from combustion of hydrogen in fuel (MJ/kg); mH2, hydrogen content in fuel, % as-fired. Heat Loss due to Moisture in Combustion Air
Vapor as humidity in incoming air is superheated as it passes through boiler. Water vapor mass that air contains can be obtained from Psychrometric charts, and for this loss Eq. (9) is used. HLa = ω . ma . Cpv . (Tg – Ta)
…(9)
where HLa, heat loss due to moisture in combustion air (MJ/kg); ω, mass of water vapor per kg of combustion
HARIMI et al: EFFECT OF EXCESS AIR AND FIBRE TO SHELL RATIO ON HEAT LOSSES
air; ma, mass of combustion air per kg of fuel as-fired; Cpv, average specific heat capacity of water vapor (0.002 MJ/kg°C between 20-200°C). Due to some difficulties in estimating some losses such as heat loss due to carbon-in-ash, heat loss due to sensible heat in fly ash, bottom ash are not included though, most of biomass has a low ash content, and represents a small energy loss if dumped hot12-13. Heat loss due to radiation and convection is not included, and taken as a maximum value of 2% from energy content of solid fuel14-15. Sum of five losses can be grouped as total heat loss due to combustion (HCL) as …(10)
HCL = HLa + HLg + HLmf + HLH + HIC
Results and Discussion Application of ASME method, PTC 4.1 is mainly based on analysis of flue gases and to any type of fuel used and does not require measurement of the boiler16. Table 3—Percentageb of heat loss due to moisture and hydrogen in the fuel F/S HLmf, %
0:100 1.48
20:80 2.38
40:60 3.43
50:50 4.03
60:40 80:20 100:0 4.68 6.16 7.97
HLH, %
2.80
2.82
2.85
2.87
2.88
2.91
2.97
b
Percentage of heat losses are computed with respect to higher heating value of F/S (fibre to shell ratio) HLmf, computed based on Eq. (6); HLH, computed based on Eq. (8)
Thus, validity of results obtained using ASME, PTC4.1 standard heat losses are valid with the validity of results obtained from STANJAN software17 flue gases, and also related to properties of air, which were taken as an average value5,6 of temperature (28°C) and humidity (80%). Heat losses due to combustion, flue gas, incomplete combustion and moisture in air have been computed (Table 2). Variation of heat losses due to moisture and hydrogen in the fuel related to given change of F/S ratio but not to the variation of excess air (Table 3). Now in order to study the effect of excess air and F/S ratio for each single heat loss, a statistical analysis is carried out, where ANOVA 2-way is used to assess heat losses due to total heat loss, dry flue gases, and incomplete combustion (Table 4). For cases of other losses, statistical t-test is used because they are not related to the variation of excess air. Total heat loss, heat loss due to dry flue gases, and heat loss due to moisture in air are found statistically highly affected by excess air, and fuel mixture (F/S ratio). As for the case of heat loss due to incomplete combustion, it is highly affected by variation of excess air but not too much affected by F/S ratio. For heat loss due to moisture and hydrogen in the fuel, t-tests computed are found to be 5.1323 and 4.9054 respectively. From statistical table, critical t-test (degree of freedom, df = 6) is lower than t-tests computed; therefore, heat loss due to moisture and hydrogen is more
Table 4—Statistical analysisc of heat losses Heat losses HLg
HIC
HLa
HCL
443
Analysis of variance SS
Excess air, %
F/S ratio
Error
219.2827
0.2718
0.01972
MS
31.3261
0.04530
0.000469
Fcomp
66726.1157
96.4798
-
SS
79.9579
1.1713
2.9250
MS
11.4226
0.1952
0.06964
Fcomp
164.0176
2.8031
-
SS
0.3229
0.000332
0.000026
MS
0.04612
0.000055
6.22 10-7
Fcomp
74177.98
88.93
-
SS
121.1882
239.7505
3.4661
MS
17.3126
39.9584
0.08253
Fcomp 209.7857 484.1970 c Degree of freedom: excess air, 7; F/S ratio, 6; error, 42; total, 55 HLg, heat loss due dry flue gas based on Eq. (5) and Table 2; HIC, heat loss due incomplete combustion based on Eq. (7) and Table 2; HLa, heat loss due moisture in air based on Eq. (9) and Table 2; HCL, heat loss due combustion based on Eq. (10) and Table 2
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J SCI IND RES VOL 67 JUNE 2008
affected by variations of F/S ratio. Boiler efficiency (100, total heat losses) varies from 76% to a maximum value of 87% (Table 2). Practically, boiler efficiency for modern waste fuel-fired boilers used for biomassbased power plant varies from 80 to 85%20-22.
7
8
9
Conclusions Statistical method, ANOVA 2-way, was used and found that heat loss due to dry flue gases and moisture in air are statistically highly affected by change in excess air, and fuel mixture (fibre and shell). Heat loss due to incomplete combustion was found highly affected by variation in excess air, but not too much affected by F/S ratio, and it is negligible above 40% excess air. The t-test confirmed that heat loss due to moisture and hydrogen in fuel was found more affected by variations of F/S ratio. Maximum heat loss due to moisture in air was found to be 0.4% from total heat loss. Boiler efficiency was found to vary from 76% to a maximum of 87%, and improvement can be achieved if global optimisation based on excess air and F/S ratio are considered with respect to combustion efficiency (low CO), and thermal efficiency (less heat losses).
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13 14
15
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