Bioresource Technology 154 (2014) 201–208
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Effect of secondary gas injection on the peanut shell combustion and its pollutant emissions in a vortexing fluidized bed combustor Feng Duan a, Chien-Song Chyang b,⇑, Yuan-Jie Wang b, Jim Tso c a
School of Energy and Environment, Anhui University of Technology, Maanshan 243002, Anhui Province, PR China Department of Chemical Engineering, Chung Yuan Christian University, Chungli 32023, Taiwan, ROC c R&D Center for Environmental Technology, Chung Yuan Christian University, Chungli 32023, Taiwan, ROC b
h i g h l i g h t s Effect of secondary gas injection on peanut shell combustion in a VFBC is studied. High combustion efficiency is achievable for burning crushed peanut shell in VFBC. The secondary gas flow rate impacts the combustion behavior of VFBC significantly. The pollutant emissions meet EPA’s regulation of Taiwan in this study.
a r t i c l e
i n f o
Article history: Received 21 August 2013 Received in revised form 29 November 2013 Accepted 30 November 2013 Available online 16 December 2013 Keywords: Peanut shell VFBC Vortexing effect Combustion
a b s t r a c t Peanut shell is a common agricultural waste in Asia, and its high calorific value is suitable to be used as a fuel. In this study, a vortexing fluidized bed combustor (VFBC) with silica sand as the bed material was used for peanut shell combustion. There was no indication of bed agglomeration during combustions for as long as 12 h. The temperatures and gas concentrations were measured along the axial direction at various operating conditions, including excess oxygen ratio and secondary gas flow rate. Results show that CO emission decreases with rising excess oxygen ratio and secondary gas flow rate, while NOx emissions show a reverse trend. To meet the minimum CO and NOx emission standards of Taiwan EPA, excess oxygen ratio ranging from 40% to 55% and secondary gas flow rate ranging from 1.56 to 2 Nm3/min are found optimal for crushed peanut shell combustion in a VFBC. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The annual global production of peanut cultivation is roughly 31 million tons. According to the statistics of Food and Agriculture Organization of The United Nations (FAOSTAT) 2010, Asian peanut crop yield makes up about 75% of the world production. China, Vietnam, and Indonesia are the leading peanut cultivation countries (Kim and Dale, 2004). Peanut shell is a common agricultural waste from peanut cultivation; about 7.44 million tons are produced annually around the world. In Taiwan, about 16,500 tons of peanut shell is used yearly as the raw material for battery, activated carbon and compost etc. However, peanut shell has great potential to be converted into energy (Ahmad et al., 2012; Chen et al., 2008; Sivakumar and Krishna, 2010). The peanut-producing countries can enjoy the environmental and economic benefits from the utilization of peanut shell as a source of renewable energy. In the past few years, the potential of agricultural residues in fluidized bed combustion (FBC) has been explored by numerous ⇑ Corresponding author. Tel.: +886 3 2654119; fax: +886 3 4636242. E-mail address: [email protected] (C.-S. Chyang). 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.11.093
studies (Chyang et al., 2012b; Madhiyanon et al., 2011). Arromdee and Kuprianov (2012) studied the burning characteristics of peanut shell in a conical fluidized-bed combustor. However, to our knowledge, there are only a few studies dealing with peanut shell as a fuel. In the field of biomass energy, fluidized bed combustion has several advantages over other conventional combustion methods such as space heating stove combustion (Koyuncu and Pinar, 2007) or fixed bed combustion (Porteiro et al., 2010; Zhang et al., 2009). The intense turbulence of inert bed material in fluidized bed expedites the heat transfer to the fuels that are fed into the combustor. It enhances fuel flexibility by allowing a variety of low-grade fuels to burn effectively with high combustion efficiency and low air pollutant emissions (Janvijitsakul and Kuprianov, 2008; Sirisomboon et al., 2010). Kuprianov and Arromdee (2013) compared the thermal and combustion reactivity of peanut and tamarind shells in a conical fluidized-bed combustor. Combustion was performed at excess air ratios ranging from 20% to 80%. In the case of 60 kg/h peanut shell input rate, excess air of 40% was found to be an optimal value to ensure high combustion efficiency and rather low
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emissions of CO and NO. The axial temperature and gas concentration profiles inside the combustor exhibited sensible effects of fuel properties and excess air ratio on combustion and emission performance. Vortexing fluidized bed combustor (VFBC) has been used extensively in coal and biomass combustion for many years (Chyang et al., 2012a; Duan et al., 2013a,b), nonetheless, few have focused on its application in peanut shell combustion. Due to smaller particle sizes and lighter weight of biomass, the combustion fraction in the freeboard zone is higher compared to fossil fuels. Splitting the input air into primary and secondary air can improve combustion efficiency. In a conventional fluidized bed combustor for biomass combustion, some fine fuel particles such as ash and unburned char are carried with the flue gas out of the combustor. On the other hand, in a vortexing fluidized bed combustion system, secondary gas is injected into the freeboard tangentially, which intensifies the mixing of air with volatile and char, thus boost the gas–gas and gas–solid combustion reactions. Furthermore, vortexing can help to capture these fine particles, and bring them back to the bed zone. This increases the residence time of unreacted particles in the combustor, resulting in higher combustion efficiency (Chyang et al., 2012b; Duan et al., 2013a). The operating parameters such as excess air ratio and gas flow rate are in close correlation with each other. In previous studies, some focused on the effect of the excess air ratio on combustion characteristics (Rao and Reddy, 2008). However, increasing excess air would also increase the flow rate of the secondary gas, and the experimental results are influenced by the physical effect caused by secondary gas and chemical effect caused by excess oxygen. It is difficult to distinguish which factor plays a more significant role in the combustion of biomass in a VFBC, therefore, the effect of excess oxygen ratio was observed at a fixed secondary gas flow rate. The effect of the secondary gas flow rate is also investigated at a fixed excess oxygen ratio. The stoichiometric oxygen ratio in the bubbling bed can be adjusted by controlling the primary air flow rate, which also changes the degree of fluidization in the bed. To avoid changing the degree of fluidization, we mix flue gas with primary air to maintain a constant primary gas flow rate with various stoichiometric oxygen ratios. Flue gas recirculation (FGR) has been used in this study due to its widely acceptance as an efficient NOx emission reducer in biomass combustion (Duan et al., 2013b; Qian et al., 2009), coal (Hayashi et al., 2002; Hu et al., 2003), municipal solid waste (MSW) (Liuzzo et al., 2007), and sludge (Sänger et al., 2001). The bed material agglomeration is a serious problem in biomass combustion. The interaction between silica sand and alkaline materials in the ash is known to cause the agglomeration (Shimizu et al., 2006), and different kinds of bed materials have been tested in conventional fluidized bed combustion (FBC). It was found that combustion temperature is a key factor in avoiding agglomeration in biomass FBC. In this study, silica sand is used as bed material and agglomeration was not observed due to our appropriate bed temperature control.
2. Methods
Table 1 Proximate and ultimate analyses of peanut shell. Fuel Proximate analysis Moisture Volatile Fixed carbon Ash
a b
Unit a
Peanut shell
wt.% 5.73 70.46 19.6 4.16
Ultimate analysisa C H O N S
wt.%
Heating value HHVa LHVb
kJ/kg
Bulk density
kg/m3
46.42 6.61 41.77 0.50 0.54 17,355 14,875 168.8
Dry basis. Wet basis.
2.2. VFBC test facility Fig. 1A shows the process flow diagram of the vortexing fluidized bed combustion system in this study which was introduced in previous paper (Duan et al., 2013b). Fig. 1B shows the configuration of the combustion chamber with a cross section of 0.8 0.4 m2, a freeboard with an inner diameter of 750 mm, and 4.6 m in total height is fabricated of SS41 steel. Refractory bricks and ceramic fibers are used for thermal insulation. The oxygen concentration of flue gas is continuously monitored by a Novatech oxygen analyzer 1632 (with ±1% precision). FGR combustion mode is used in this study. The primary gas is composed of first air and recirculated flue gas (FGR), and is injected into the combustor through a windbox. The first air is supplied by a 11.2 kW Root’s blower, and FGR is pumped by a 5.6 kW turbo blower. Four equally spaced secondary gas injection nozzles of 30 mm in diameter are installed tangentially at the level of 2.05 m above the gas distributor. The secondary gas is preheated up to about 200 °C with exhaust heat from the combustor before entering the freeboard. To investigate the effects of secondary gas flow rate and excess oxygen ratio on the peanut shell combustion, the secondary gas is formed with a mixture of air from a blower and pure nitrogen from high-pressured nitrogen cylinders. Two diesel burners are used to heat up the combustor during the start up. Secondary air is injected from the beginning. First air was injected into the combustor occasionally to disturb the sand. Wood blocks and pulverized coal are added into the combustor when bed temperature reaches 400 °C. Then the flow rate of first air was increased slightly. The feeding material is injected into the combustor via a screw feeder when bed temperature reaches 500 °C and the flow rate of first air is increased gradually. The burners and pulverized coal feeding were stopped once the bed temperature reaches up to 700 °C, which takes between 12 and 16 h.
2.1. Fuel and bed material
2.3. Data acquisition
Silica sand (99.5% SiO2) is used as the inert bed materials in this study, with a mean diameter of 0.57 mm, and a particle density of 2600 kg/m3. The crushed peanut shell with a bulk density of 168.8 kg/m3 and a mean diameter of 0.704 mm were used as the fuel with a feeding rate of 36.5 kg/h. Table 1 shows the proximate and ultimate analyses of crushed peanut shell and the lower heating value is 14,875 kJ/kg.
Fig. 1B also presents the position of the thermocouples and gas sampling probes. In this figure, thermocouples (K-type) are positioned at 0.45, 1.15, 1.55, 2.05, 2.55, 2.80, 3.00, and 4.50 m above the air distributor, respectively. The components of the flue gas, such as CO, CO2, O2, and NOx are analyzed by Anapol EU5000 gas analyzers. The measurement accuracy of the gas analyzer with O2, CO, CO2 and NOx are 0.4%, 6%, 0.5%, and 1%, respectively. The
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(A)
1
1
2
2
10 22
3
5
8
9
11
5 8 7:
24
7
6
3*
14
19 20
15
4 2
2
2
7
2
12
P
P
P
P
23 P
16
P
P
P
1. Hopper 2. Screw Feeder 3. Air Lock 4. Nitrogen System 5. Burner 6. Incinerator
18
13
8
21
7. Roots Blower 8. Orifice Meter 9. Preheater 10. Reheater 11. FGR Blower 12. Oxygen-Nitrogen System
17 13. Quench Tower 14. Quench Pump 15. Baghouse 16. Scrubber 17. Scrubber Pump 18. "Plate and Frame" Heat Exchanger
19. Induced Draft Fan 20. Cooling Tower 21. Cooling Tower Pump 22. Air Jacket 23. Air Jacket Blower 24. Stack
(B)
Fig. 1. Vortexing fluidized bed combustion system. (A) Process flow diagram of the vortexing fluidized bed combustion system. (B) Schematic diagram of the vortexing fluidized bed combustor.
emissions values reported in this study are all calibrated based on 11% residual oxygen on a dry basis. To interpret the combustion characteristics of crushed peanut shell in a VFBC, it is necessary to build a model of combustion fraction by calculating the consumed oxygen. As seen in Fig. 1B, the location of z = 1.5 m is defined as the boundary between the combustion chamber (bed and splashing zone) and freeboard zone.The combustion fraction can be calculated by the following equation as:
Yi ¼
C O2 ;in Q in C O2 ;out Q out 100% ðQ 1st 0:21 þ Q 2nd 0:21 þ Q FGR C O2 ;ID Þ Q FG C O2 ;OL
ð1Þ
where, C O2 ;in : inlet oxygen concentration to each zone (%); Q in : inlet gas flow rate in each zone, Nm3/min; C O2 ;out : outlet oxygen concen-
tration in each zone, (%); Q out : outlet gas flow rate in each zone, Nm3/min; Q 1st : flow rate of first air (Nm3/min); Q 2nd : flow rate of secondary air (Nm3/min); Q FGR : flow rate of FGR (Nm3/min); C O2 ;ID : oxygen concentration of FGR at the outlet of ID fan, (%); Q FG : flow rate of flue gas from the combustor (Nm3/min); C O2 ;OL : oxygen concentration of flue gas at the outlet of combustor, (%). In this study, the combustion efficiency, gc, is calculated using heat-loss method (Kuprianov et al., 2011), and can be written as:
gc ¼ 100% ðLC þ LCO þ LCH4 Þ
ð2Þ
where LC, LCO, and LCH4 , represent the heat loss due to unburned carbon (%), heat loss due to incomplete combustion of CO (%), and heat loss due to incomplete combustion of hydrocarbon, CxHy, rep-
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resented by CH4 (%), respectively. Because there is no bottom ash removed from the combustor, the heat loss due to unburned carbon, LC (%), can be determined by:
LC ¼
Cc W F C A 100C DHc;C c
where, Q1st: flow rate of first air, Nm3/min; QFGR: flow rate of recirculated flue gas, Nm3/min; QPRG: flow rate of primary gas, Nm3/min; C O2 ;ID : concentration of oxygen detected at the outlet of induce fan, %; Sb: stoichiometric oxygen ratio, %.
ð3Þ
W F LHVðWBÞ
3. Results and discussion where, WF: fuel feeding rate, kg/h; CA: ash content in fuel, wt.%; Cc: carbon content in fly ash, wt.%; LHV(WB): wet-basis lower heating value of fuel, kJ/kg; DHc;C : heating value of carbon, 39,750 kJ/kg.
LCO ¼
NFG ½CO DHc;CO W F LHVðWBÞ
ð4Þ
where NFG: dry flue gas flow rate at the outlet of combustor, calculated with wet-basis ultimate analysis of feeding material and given operating condition, kg mol/h; [CO]: CO concentration detected at the outlet of combustor, %; DHc;C : heating value of CO, 282854 kJ/ kg mol;
LCH4 ¼
NFG ½CH4 DHc;CH4 W F LHVðWBÞ
ð5Þ
where [CH4]: CH4 concentration detected at the outlet of combustor, %; DHc;CH4 : lower heating value of CH4, 801,936 kJ/kg mol. Combustion loss resulting from the fly ash is the largest portion in the total combustion loss. Fly ash is collected at the baghouse, quench tower, and heat exchanger. The carbon content in the fly ash is analyzed with a Pyris 1 TGA (thermo-gravimetric analyzer). The fly ash was weighted first, and the original weight was recorded as X1. Then it was heated from 25 °C to 105 °C at a heating rate of 40 °C/min, and held at 105 °C for 1 h, and the dehydrated weight X2 was recorded. Then, the operating temperature increased from 105 °C to 850 °C at a heating rate of 40 °C/min, and held at 850 °C for 2 h. The final weight X3 was obtained and recorded. According to the Eq. (6), the carbon content of fly ash (dry base), Cc, was calculated:
C c ðwt:%Þ ¼
X2 X3 100 X2
ð6Þ
2.4. Operating conditions The working conditions for the experiments are given in Table 2. The oxygen concentration data of the flue gas at the outlet of induced draft fan were transmitted to the primary gas control system for adjusting the mixing ratio of the first air and FGR to maintain the oxygen content in the primary gas at a constant value. The flow rate of FGR can be calculated by the following equations:
Q 1st þ Q FGR ¼ Q PRG
ð7Þ
Q 1st 21% þ Q FGR C O2 ;ID ¼ Sb Stoichiometric oxygen
ð8Þ
Table 2 Operating conditions. Parameters
Symbol
Unit
Value
Feeding rate Excess oxygen ratio Stoichiometric oxygen ratio in the combustion chamber Mean fuel particle size Total primary air flow rate Secondary gas flow rate Additional nitrogen flow rate of secondary gas
Wfuel Eo Sb
kg/h % %
36.5 40, 50, 55, 60 90
dp, fuel Qpri Qsec Qn2
mm Nm3/min Nm3/min Nm3/min
0.704 3 1.56, 2, 2.31, 2.56 1.44, 1, 0.69, 0.44
3.1. Excess oxygen ratio For fluidized bed combustion of biomass, adding more oxygen can achieve high efficient combustion. However, Piao et al. (2000) reported that too much excess oxygen decreases the combustion temperature, which counters the effort in reducing pollutant emissions. In this test, the effect of the excess oxygen ratio on combustion behavior and pollutant emissions is investigated. The stoichiometric oxygen ratio in the bed zone is 90%, and the primary and secondary gas flow rates are 3 and 2 Nm3/min, respectively. Fig. 2A shows the temperature distribution in the VFBC at different excess oxygen ratios. The bed temperature is lower than that in the other parts of the VFBC which can be attributed to the lower combustion fraction in the bubbling bed. At fixed primary and secondary gas flow rate, excess oxygen is introduced into the freeboard zone through secondary gas. This increases the combustion fraction of the freeboard, and decreases the combustion fraction of the combustion chamber. The combustion fraction of the combustion chamber decreases from 40% to 33.5% when the excess oxygen ratio increases from 40% to 60%. The bed temperatures changed little at different excess oxygen ratios; however, the temperatures increase for higher excess oxygen ratio in the splashing and freeboard zones. The local temperatures within the VFBC are affected by the amount of heat lost through the wall and released (generated) from fuel combustion. Because of the smaller sizes, lighter weight and high volatile content of crushed peanut shell, it tends to combust in the splashing zone and freeboard zone. In Fig. 2A, maximum temperatures are observed at z = 2.8 m, despite the cooling effect by secondary gas injection at z = 2.05 m. It can be attributed to the vigorous combustion of combustible matter with oxygen added from secondary gas. At a higher region above the freeboard, the temperature decreases with the distance from the distributor. It can be attributed to the decrease of exhausted combustible material at a higher level of freeboard. Meanwhile, freeboard temperature and outlet temperature increase with higher excess oxygen ratio, which can be attributed to the fact the higher oxygen content in the secondary gas intensifies the combustion of unburned carbon and volatiles. Fig. 2B shows the effect of excess oxygen ratio on the combustion efficiency. The combustion efficiency increases with the excess oxygen ratio. At higher excess oxygen ratios, the combustion temperatures at all measuring points increase, which favors complete combustion. The result obtained in this study agrees with the observation of Sirisomboon et al. (2010), Rao and Reddy (2011), Arromdee and Kuprianov (2012), and Kuprianov et al. (2010). Fig. 3A shows the effect of excess oxygen ratio on CO and NOx emissions. Higher excess oxygen ratio can decrease CO emission which is a function of operating variables, such as excess oxygen ratio, and combustion temperature (Permchart and Kouprianov, 2004). As fuel is fed into the combustor, the fuel particles get heated up rapidly and most volatiles are released due to the higher heat and mass transfer rates in fluidization. Crushed peanut shell as biomass has higher volatile content (Arromdee and Kuprianov, 2012), thus a high percentage of volatile and some fine char are carried up to the freeboard zone, and the combustion efficiency is sensitive to freeboard temperature (Chyang et al., 2008; Duan et al., 2013c). NOx emissions increase with excess oxygen ratio.
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Pollutant emissions (ppm) @11%O2
800
o
Temperature ( C)
120
(A)
850
750
Eo= 60% Eo= 55% Eo= 50% Eo= 40%
Sb=90% 3
700
Qpri=3 Nm /min Qsec=2 Nm 3/min
CO emission NOx emissions
100
S =90% b
80
3 Q =3 Nm /min pri 3 Q =2 Nm /min sec
60
40
20
(A)
650 0
1
2
3
4
0 38
5
Distance above the distributor, z (m) 100.0
44
46
48
50
52
54
56
58
60
62
10000
Combustion efficiency 8000
Sb=90%
CO concentration (ppm)
Combustion efficiency (%)
42
Excess oxygen ratio, Eo (%)
(B)
99.5
99.0
40
3
QPRI=3 Nm /min Qsec=2 Nm 3/min
98.5
98.0
97.5
Eo= 40% Eo= 50% Eo= 55% Eo= 60%
6000
3
QPRI =3 Nm /min
4000
3
QSEC =2 Nm /min Eo=50% Sb=90%
2000
(B)
97.0 40
45
50
55
60
0 0
1
2
3
Fig. 2. Effect of the excess oxygen ratio on the combustion behavior. (A) Temperature distribution within the VFBC at different excess oxygen ratios. (B) Effect of the excess oxygen ratio on the combustion efficiency.
5
400
Eo=60% Eo=55% Eo=50% Eo=40%
NOx concentration (ppm)
350
Similar tendencies were observed by Arromdee and Kuprianov (2012), Kuprianov et al. (2006) and Madhiyanon et al. (2010) during the combustion of peanut shell, rice husk and sugar cane bagasse in conical fluidized-bed combustor or swirling fluidizedbed combustor. This is primarily due to the fact NOx reduction reaction with CO is reduced at high excess oxygen ratio and low CO emission, resulting in higher NOx emissions. Besides, heightened freeboard temperature at higher excess oxygen ratio also accelerates the NOx emissions. As seen in Fig. 2A, the freeboard temperature increases with the excess oxygen ratio. This improves the combustion of the volatile and un-burned char in the freeboard zone, and decreases the CO emission at the outlet of the VFBC. Fig. 3B and C show the CO and NOx concentrations within the combustor at different excess oxygen ratio, respectively. In all the tests, the axial CO and NOx concentration profiles show the maximum values at approximately the same locations above the distributor, z = 2.05 m, and the maximum value decreases with excess oxygen ratio. The lower part of the combustor can be assumed to be the CO formation zone due to the lower stoichiometric oxygen ratio. In the upper part of the combustor, CO is oxidized with excess oxygen directly and results in lower CO concentration. The maximum NOx concentrations appear at same locations as the maximum CO concentrations. This can be attributed to the fact the higher temperature favors the combustion of nitrogenous species released with volatile matter and the oxidation of nitrogen retained in the char (Permchart and Kouprianov, 2004).
4
Distance above the distributor, z (m)
Excess oxygen ratio (%)
300
3
QPRI =3 Nm /min
250
3
QSEC =2 Nm /min Eo=50%
200
Sb=90%
150
100
(C)
50 0
1
2
3
4
5
Distance above the distributor, z (m) Fig. 3. Effect of the excess oxygen ratio on the pollutant emissions. (A) Effect of the excess oxygen ratio on the CO and NOx emissions. (B) CO concentration distribution within the combustor at various excess oxygen ratio. (C) NOx concentration distribution within the combustor at various excess oxygen ratio.
3.2. Vortexing effect Fig. 4A shows the temperature distribution profiles with various secondary gas flow rates, where the stoichiometric oxygen ratio in the bed zone is 90%, the excess oxygen ratio is 50%, and the primary gas flow rate is 3 Nm3/min. Nitrogen was added with the flow
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rates of 0, 0.44, 0.75, and 1 Nm3/min, and the final flow rates of secondary gas become 1.56, 2, 2.31, and 2.56 Nm3/min, respectively. In addition, from Fig. 4A, the bed and freeboard temperatures are impacted differently by the increasing secondary gas flow rate. Below the secondary gas injection points (z = 2.05 m), the bed and splashing zone temperatures increase with the secondary gas flow rate. It can be attributed to the fine particles and unburned char carried back to these zones. Increasing secondary gas flow rate strengthens the vortexing intensity, thus more particles are
captured. This increases the particle residence time and its combustion fraction (from 31% to 40%) in the bed and splashing zones. The freeboard and outlet temperatures change slightly with the increasing secondary gas flow rates. Besides, the temperature slope between z = 1.45 m and z = 2.8 m decreases with increasing secondary gas flow rates. The temperature slope becomes negative at 2.31 and 2.56 Nm3/min secondary gas flow rate, which can be attributed to the lower combustion fraction in the freeboard zone and the cooling effect by the secondary air. Combustion fraction of
900
(A)
o
Temperature ( C)
850
800
750
Secondary gas flow rate 3 1.56 Nm /min 3 2 Nm /min 3 2.31 Nm /min 3 2.56 Nm /min
Sb=90% Qpri=3 Nm3/min Eo=50%
700
650 0
1
2
3
4
5
Distance above the distributor (m) 900 800
(B)
o
Temperature ( C)
700
Sb=90%
600
Qpri=3 Nm3/min Eo=50%
500
Secondary gas flow rate 3 1.56 Nm /min 3 2 Nm /min 3 2.31 Nm /min 3 2.56 Nm /min
400 300 200 100 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
s/D 100.0
1300
Fly ash elutriated Combustion efficiency
Fly ash elutriated (g/hr)
1100
99.5
1000
99.0
900 800 700
Sb=90%
98.5
Qpri=3 Nm 3/min Eo=50%
98.0
600
Combustion efficiency (%)
1200
(C)
97.5 500 400 1.4
97.0 1.6
1.8
2.0
2.2
2.4
2.6
3
Secondary gas flowrate (Nm /min) Fig. 4. Effect of the secondary gas flow rate on the combustion behavior. (A) Temperature distribution within the combustor at various secondary gas. (B) Radial temperature distribution within the combustor at various secondary gas flow rates (z = 2.03 m). (c) Effect of the secondary gas flow rate on the fly ash yield and combustion efficiency.
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the freeboard decreases from 69% to 60% as the secondary gas flow rate goes from 1.56 to 2.56 Nm3/min. Fig. 4B shows the radial temperature distribution with various secondary gas flow rates. The temperature probe point is located at z = 2.03 m. The inner diameter of freeboard is 0.75 m, the distances between the detective point and combustor wall(s) are 0.1, 0.2, 0.3, 0.375, 0.45, 0.55, and 0.65 m, respectively. The lowest temperature appears near the wall, which is very close to the
Pollutant emissions (ppm) @11%O2
120
CO emission NOx emissions
100
80
Sb=90% Qpri=3 Nm3/min Eo=50%
60
40
(A)
20
0 1.4
1.6
1.8
2.0
2.2
2.4
2.6
3
Secondary air flowrate (Nm /min) 10000
6000
Secondary gas flow rate 3 1.56 Nm /min 3 2 Nm /min 3 2.31 Nm /min 3 2.56 Nm /min
4000
QPRI =3 Nm /min
CO concentration (ppm)
8000
3
Eo=50% Sb=90%
2000
(B)
0 0
1
2
3
4
5
Distance above the distributor, z (m) 300
Secondary gas flow rate 3 1.56 Nm /min 3 2 Nm /min 3 2.31 Nm /min 3 2.56 Nm /min
NOx concentration (ppm)
250
200
temperature of injected secondary gas. The highest temperature appears at the center of the freeboard; it is due to the different flow characteristics between the center and the wall. The fine particles and unburned char are thrown to the combustor wall by centrifugal force by the vortex and drop back to the bed zone. Therefore, the overall temperatures increase with the secondary gas flow rate in this figure. Fig. 4C shows the effect of the secondary gas flow rate on the elutriated fly ash and combustion efficiency. In this test, the fly ash was collected at the baghouse, quench tower, and heat exchanger. The elutriated fly ash decreases significantly with secondary gas flow rate. The carbon content in the fly ash was analyzed by a TGA, and the results show that the carbon content of fly ash also decreases significantly with the secondary gas flow rate. This is in agreement with the results obtained by Okasha (2007). It can be attributed to the intensified combustion caused by the vortexing effect which increased with the secondary gas flow rate. In this figure, the maximum combustion efficiency for peanut shell appears at a secondary gas flow rate of 2.56 Nm3/h. Fig. 5A shows the effect of the secondary gas flow rate on the pollutant (CO and NOx). The CO emission goes from 46 to 40 ppm, while NOx emission goes from 91 to 107 ppm with increasing secondary gas flow rate, which can also be attributed to the vortexing effect. At a higher secondary gas flow rate, more fine particles and unburned char are brought back to bed zone for re-burning, resulting in lower CO emission. The reduction in CO level can also be ascribed to high turbulence created by vortex, which results in better combustion of volatile and unburned char. Despite NOx emission can be reduced by the reduction reaction with unburned char (Chaiklangmuang et al., 2002; Okazaki and Ando, 1997), the higher bed and splashing temperatures enhance the formation of NOx predecessors such as HCN, and NH3. In addition, lower CO concentration reduces the chance of NOx reduction reaction with CO. The CO emission in this study complies with the minimum emission requirement (within 100 ppm) of Taiwan EPA regulations, and NOx emission also complies with the minimum emission requirement (220 ppm). Considering the combustion efficiency and air pollutant emissions, 1.56–2 Nm3/h secondary gas flow rates are optimal for burning peanut shell in this study. Fig. 5B and C show the CO and NOx concentrations profiles within the combustor at various secondary gas flow rates, respectively. In Fig. 5B, at the location of 2.03 m above the distributor, the CO concentration values increase with secondary gas flow rate. At the highest secondary gas flow rate (2.56 Nm3/min), the CO concentration reaches the peak value. This can be attributed to the fact that more un-burned particles are carried back to the splashing zone. The stoichiometric oxygen ratio in the bed is 90%; therefore, as more particles carried back to the bed and splashing zones, the CO concentration goes higher. In Fig. 5C, at the location of 2.03 m above the distributor, NOx concentrations increase with the secondary gas flow rate which is due to the fact the higher secondary
3
QPRI =3 Nm /min Eo=50%
Table 3 Elemental analysis of ash.
Sb=90%
150
100
(C)
50 0
1
2
3
4
5
Distance above the distributor, z (m) Fig. 5. Effect of the secondary gas flow rate on the pollutant emissions. (A) Effect of the secondary gas flow rate on CO and NOx emissions. (B) CO concentration distribution within the combustor at various secondary gas flow rate. (C) NOx concentration distribution within the combustor at various secondary gas flow rate.
Element
Units
Bottom ash
Fly ash
Na Mg Al Si P S K Ca Ti Fe
ppm % % % % ppm % % ppm %
7273 1.504 4.616 21.07 0.793 ND 7.086 3.016 2867 4.084
9408 8.591 2.606 7.072 10.20 ND 10.65 9.393 1392 3.973
ND: Not detected.
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gas flow rate increases the residence time of un-burned char and small particles resulting in high temperature as shown in Fig. 4A. 3.3. Bed material agglomeration The elemental analyses of the fly ash and bottom ash are given in Table 3. The fly ash was collected from the baghouse. The ash deposited on the distributor after a 12 h combustion test was scooped up as the bottom ash. From Table 3, the concentrations of alkali and alkaline-earth metals of fly ash are higher than those of the bottom ash. Owing to the accumulation of ash in the bed, the amount of alkali and alkaline-earth metals remained in the bed is increased with operating time. Meanwhile, there is no sign of bed agglomeration from 12 h combustion tests due to the lower operating temperatures in the bed. 4. Conclusion CO emission decreases with the excess oxygen ratio and the secondary gas flow rate, while NOx emissions show an inverse trend. The temperature profiles within the VFBC show a generally similar trend under various operating conditions. Higher secondary gas flow rate favors complete combustion of crushed peanut shell due to the stronger vortexing effect, resulting in higher combustion efficiency. To meet both minimum CO and NOx emission requirements of Taiwan EPA regulations, excess oxygen ratios ranging from 40% to 55% and the secondary gas flow rates ranging from 1.56 to 2 Nm3/min are optimal for crushed peanut shell combustion in the VFBC. References Ahmad, M., Lee, S.S., Dou, X., Mohan, D., Sung, J.-K., Yang, J.E., Ok, Y.S., 2012. Effects of pyrolysis temperature on soybean stover- and peanut shell-derived biochar properties and TCE adsorption in water. Bioresour. Technol. 118, 536–544. Arromdee, P., Kuprianov, V.I., 2012. Combustion of peanut shells in a cone-shaped bubbling fluidized-bed combustor using alumina as the bed material. Appl. Energy 97, 470–482. Chaiklangmuang, S., Jones, J.M., Pourkashanian, M., Williams, A., 2002. Conversion of volatile-nitrogen and char-nitrogen to NO during combustion. Fuel 81, 2363– 2369. Chen, H., Bin, L., Yang, H., Yang, G., Zhang, S., 2008. Experimental investigation of biomass gasification in a fluidized bed reactor. Energy Fuels 22, 3493–3498. Chyang, C., Duan, F., Lin, S., Tso, J., 2012a. A study on fluidized bed combustion characteristics of corncob in three different combustion modes. Bioresour. Technol. 116, 184–189. Chyang, C.S., Duan, F., Lin, S.M., Tso, J., 2012b. A study on fluidized bed combustion characteristics of corncob in three different combustion modes. Bioresour. Technol. 116, 184–189. Chyang, C.S., Qian, F.P., Lin, Y.C., Yang, S.H., 2008. NO and N2O emission characteristics from a pilot scale vortexing fluidized bed combustor firing different fuels. Energy Fuels 22, 1004–1011. Duan, F., Chyang, C.S., Chin, Y.C., Tso, J., 2013a. Pollutant emission characteristics of rice husk combustion in a vortexing fluidized bed incinerator. J. Environ. Sci. 25, 335–339. Duan, F., Chyang, C.S., Lin, C.W., Tso, J., 2013b. Experimental study on rice husk combustion in a vortexing fluidized-bed with flue gas recirculation (FGR). Bioresour. Technol. 134, 204–211. Duan, F., Liu, J., Chyang, C.-S., Hu, C.-H., Tso, J., 2013c. Combustion behavior and pollutant emission characteristics of RDF (refuse derived fuel) and sawdust in a vortexing fluidized bed combustor. Energy 57, 421–426.
Hayashi, J.i., Hirama, T., Okawa, R., Taniguchi, M., Hosoda, H., Morishita, K., Li, C.Z., Chiba, T., 2002. Kinetic relationship between NO/N2O reduction and O2 consumption during flue-gas recycling coal combustion in a bubbling fluidized-bed. Fuel 81, 1179–1188. Hu, Y.Q., Kobayashi, N., Hasatani, M., 2003. Effects of coal properties on recycledNOx reduction in coal combustion with O2/recycled flue gas. Energy Convers. Manage. 44, 2331–2340. Janvijitsakul, K., Kuprianov, V.I., 2008. Major gaseous and PAH emissions from a fluidized-bed combustor firing rice husk with high combustion efficiency. Fuel Process. Technol. 89, 777–787. Kim, S., Dale, B.E., 2004. Global potential bioethanol production from wasted crops and crop residues. Biomass Bioenergy 26, 361–375. Koyuncu, T., Pinar, Y., 2007. The emissions from a space-heating biomass stove. Biomass Bioenergy 31, 73–79. Kuprianov, V.I., Arromdee, P., 2013. Combustion of peanut and tamarind shells in a conical fluidized-bed combustor: A comparative study. Bioresour. Technol. 140, 199–210. Kuprianov, V.I., Janvijitsakul, K., Permchart, W., 2006. Co-firing of sugar cane bagasse with rice husk in a conical fluidized-bed combustor. Fuel 85, 434–442. Kuprianov, V.I., Kaewklum, R., Chakritthakul, S., 2011. Effects of operating conditions and fuel properties on emission performance and combustion efficiency of a swirling fluidized-bed combustor fired with a biomass fuel. Energy 36, 2038–2048. Kuprianov, V.I., Kaewklum, R., Sirisomboon, K., Arromdee, P., Chakritthakul, S., 2010. Combustion and emission characteristics of a swirling fluidized-bed combustor burning moisturized rice husk. Appl. Energy 87, 2899–2906. Liuzzo, G., Verdone, N., Bravi, M., 2007. The benefits of flue gas recirculation in waste incineration. Waste Manage. 27, 106–116. Madhiyanon, T., Sathitruangsak, P., Soponronnarit, S., 2011. Co-firing characteristics of rice husk and coal in a cyclonic fluidized-bed combustor (W-FBC) under controlled bed temperatures. Fuel 90, 2103–2112. Madhiyanon, T., Sathitruangsak, P., Soponronnarit, S., 2010. Combustion characteristics of rice-husk in a short-combustion-chamber fluidized-bed combustor (SFBC). Appl. Therm. Eng. 30, 347–353. Okasha, F., 2007. Staged combustion of rice straw in a fluidized bed. Exp. Therm. Fluid Sci. 32, 52–59. Okazaki, K., Ando, T., 1997. NOx reduction mechanism in coal combustion with recycled CO2. Energy 22, 207–215. Permchart, W., Kouprianov, V.I., 2004. Emission performance and combustion efficiency of a conical fluidized-bed combustor firing various biomass fuels. Bioresour. Technol. 92, 83–91. Piao, G., Aono, S., Kondoh, M., Yamazaki, R., Mori, S., 2000. Combustion test of refuse derived fuel in a fluidized bed. Waste Manage. 20, 443–447. Porteiro, J., Patino, D., Collazo, J., Granada, E., Moran, J., Miguez, J.L., 2010. Experimental analysis of the ignition front propagation of several biomass fuels in a fixed-bed combustor. Fuel 89, 26–35. Qian, F.P., Chyang, C.S., Chiou, J.B., Tso, J., 2009. Effect of flue gas recirculation (FGR) on NOx emission in a pilot-scale vortexing fluidized-bed combustor. Energy Fuels 25, 5639–5646. Rao, K.V.N.S., Reddy, G.V., 2011. Comparison of the combustion characteristics of rice husk, sawdust, and groundnut shells in a bubbling fluidized bed. Energy Sources Part A 33, 2181–2193. Rao, K.V.N.S., Reddy, G.V., 2008. Effect of secondary air injection on the combustion efficiency of sawdust in a fluidized bed combustor. Braz. J. Chem. Eng. 25, 129– 141. Sänger, M., Werther, J., Ogada, T., 2001. NOx and N2O emission characteristics from fluidised bed combustion of semi-dried municipal sewage sludge. Fuel 80, 167– 177. Shimizu, T., Han, J., Choi, S., Kim, L., Kim, H., 2006. Fluidized-bed combustion characteristics of cedar pellets by using an alternative bed material. Energy Fuels 20, 2737–2742. Sirisomboon, K., Kuprianov, V.I., Arromdee, P., 2010. Effects of design features on combustion efficiency and emission performance of a biomass-fuelled fluidized-bed combustor. Chem. Eng. Process. 49, 270–277. Sivakumar, K., Krishna, M.N., 2010. Performance analysis of downdraft gasifier for agriwaste biomass materials. Indian J. Sci. Technol. 3, 58–60. Zhang, H., Xiao, R., Huang, H., Xiao, G., 2009. Comparison of non-catalytic and catalytic fast pyrolysis of corncob in a fluidized bed reactor. Bioresour. Technol. 100, 1428–1434.
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Effect of secondary gas injection on the peanut shell combustion and its pollutant emissions in a vortexing fluidized bed combustor Feng Duan a, Chien-Song Chyang b,⇑, Yuan-Jie Wang b, Jim Tso c a
School of Energy and Environment, Anhui University of Technology, Maanshan 243002, Anhui Province, PR China Department of Chemical Engineering, Chung Yuan Christian University, Chungli 32023, Taiwan, ROC c R&D Center for Environmental Technology, Chung Yuan Christian University, Chungli 32023, Taiwan, ROC b
h i g h l i g h t s Effect of secondary gas injection on peanut shell combustion in a VFBC is studied. High combustion efficiency is achievable for burning crushed peanut shell in VFBC. The secondary gas flow rate impacts the combustion behavior of VFBC significantly. The pollutant emissions meet EPA’s regulation of Taiwan in this study.
a r t i c l e
i n f o
Article history: Received 21 August 2013 Received in revised form 29 November 2013 Accepted 30 November 2013 Available online 16 December 2013 Keywords: Peanut shell VFBC Vortexing effect Combustion
a b s t r a c t Peanut shell is a common agricultural waste in Asia, and its high calorific value is suitable to be used as a fuel. In this study, a vortexing fluidized bed combustor (VFBC) with silica sand as the bed material was used for peanut shell combustion. There was no indication of bed agglomeration during combustions for as long as 12 h. The temperatures and gas concentrations were measured along the axial direction at various operating conditions, including excess oxygen ratio and secondary gas flow rate. Results show that CO emission decreases with rising excess oxygen ratio and secondary gas flow rate, while NOx emissions show a reverse trend. To meet the minimum CO and NOx emission standards of Taiwan EPA, excess oxygen ratio ranging from 40% to 55% and secondary gas flow rate ranging from 1.56 to 2 Nm3/min are found optimal for crushed peanut shell combustion in a VFBC. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The annual global production of peanut cultivation is roughly 31 million tons. According to the statistics of Food and Agriculture Organization of The United Nations (FAOSTAT) 2010, Asian peanut crop yield makes up about 75% of the world production. China, Vietnam, and Indonesia are the leading peanut cultivation countries (Kim and Dale, 2004). Peanut shell is a common agricultural waste from peanut cultivation; about 7.44 million tons are produced annually around the world. In Taiwan, about 16,500 tons of peanut shell is used yearly as the raw material for battery, activated carbon and compost etc. However, peanut shell has great potential to be converted into energy (Ahmad et al., 2012; Chen et al., 2008; Sivakumar and Krishna, 2010). The peanut-producing countries can enjoy the environmental and economic benefits from the utilization of peanut shell as a source of renewable energy. In the past few years, the potential of agricultural residues in fluidized bed combustion (FBC) has been explored by numerous ⇑ Corresponding author. Tel.: +886 3 2654119; fax: +886 3 4636242. E-mail address: [email protected] (C.-S. Chyang). 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.11.093
studies (Chyang et al., 2012b; Madhiyanon et al., 2011). Arromdee and Kuprianov (2012) studied the burning characteristics of peanut shell in a conical fluidized-bed combustor. However, to our knowledge, there are only a few studies dealing with peanut shell as a fuel. In the field of biomass energy, fluidized bed combustion has several advantages over other conventional combustion methods such as space heating stove combustion (Koyuncu and Pinar, 2007) or fixed bed combustion (Porteiro et al., 2010; Zhang et al., 2009). The intense turbulence of inert bed material in fluidized bed expedites the heat transfer to the fuels that are fed into the combustor. It enhances fuel flexibility by allowing a variety of low-grade fuels to burn effectively with high combustion efficiency and low air pollutant emissions (Janvijitsakul and Kuprianov, 2008; Sirisomboon et al., 2010). Kuprianov and Arromdee (2013) compared the thermal and combustion reactivity of peanut and tamarind shells in a conical fluidized-bed combustor. Combustion was performed at excess air ratios ranging from 20% to 80%. In the case of 60 kg/h peanut shell input rate, excess air of 40% was found to be an optimal value to ensure high combustion efficiency and rather low
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emissions of CO and NO. The axial temperature and gas concentration profiles inside the combustor exhibited sensible effects of fuel properties and excess air ratio on combustion and emission performance. Vortexing fluidized bed combustor (VFBC) has been used extensively in coal and biomass combustion for many years (Chyang et al., 2012a; Duan et al., 2013a,b), nonetheless, few have focused on its application in peanut shell combustion. Due to smaller particle sizes and lighter weight of biomass, the combustion fraction in the freeboard zone is higher compared to fossil fuels. Splitting the input air into primary and secondary air can improve combustion efficiency. In a conventional fluidized bed combustor for biomass combustion, some fine fuel particles such as ash and unburned char are carried with the flue gas out of the combustor. On the other hand, in a vortexing fluidized bed combustion system, secondary gas is injected into the freeboard tangentially, which intensifies the mixing of air with volatile and char, thus boost the gas–gas and gas–solid combustion reactions. Furthermore, vortexing can help to capture these fine particles, and bring them back to the bed zone. This increases the residence time of unreacted particles in the combustor, resulting in higher combustion efficiency (Chyang et al., 2012b; Duan et al., 2013a). The operating parameters such as excess air ratio and gas flow rate are in close correlation with each other. In previous studies, some focused on the effect of the excess air ratio on combustion characteristics (Rao and Reddy, 2008). However, increasing excess air would also increase the flow rate of the secondary gas, and the experimental results are influenced by the physical effect caused by secondary gas and chemical effect caused by excess oxygen. It is difficult to distinguish which factor plays a more significant role in the combustion of biomass in a VFBC, therefore, the effect of excess oxygen ratio was observed at a fixed secondary gas flow rate. The effect of the secondary gas flow rate is also investigated at a fixed excess oxygen ratio. The stoichiometric oxygen ratio in the bubbling bed can be adjusted by controlling the primary air flow rate, which also changes the degree of fluidization in the bed. To avoid changing the degree of fluidization, we mix flue gas with primary air to maintain a constant primary gas flow rate with various stoichiometric oxygen ratios. Flue gas recirculation (FGR) has been used in this study due to its widely acceptance as an efficient NOx emission reducer in biomass combustion (Duan et al., 2013b; Qian et al., 2009), coal (Hayashi et al., 2002; Hu et al., 2003), municipal solid waste (MSW) (Liuzzo et al., 2007), and sludge (Sänger et al., 2001). The bed material agglomeration is a serious problem in biomass combustion. The interaction between silica sand and alkaline materials in the ash is known to cause the agglomeration (Shimizu et al., 2006), and different kinds of bed materials have been tested in conventional fluidized bed combustion (FBC). It was found that combustion temperature is a key factor in avoiding agglomeration in biomass FBC. In this study, silica sand is used as bed material and agglomeration was not observed due to our appropriate bed temperature control.
2. Methods
Table 1 Proximate and ultimate analyses of peanut shell. Fuel Proximate analysis Moisture Volatile Fixed carbon Ash
a b
Unit a
Peanut shell
wt.% 5.73 70.46 19.6 4.16
Ultimate analysisa C H O N S
wt.%
Heating value HHVa LHVb
kJ/kg
Bulk density
kg/m3
46.42 6.61 41.77 0.50 0.54 17,355 14,875 168.8
Dry basis. Wet basis.
2.2. VFBC test facility Fig. 1A shows the process flow diagram of the vortexing fluidized bed combustion system in this study which was introduced in previous paper (Duan et al., 2013b). Fig. 1B shows the configuration of the combustion chamber with a cross section of 0.8 0.4 m2, a freeboard with an inner diameter of 750 mm, and 4.6 m in total height is fabricated of SS41 steel. Refractory bricks and ceramic fibers are used for thermal insulation. The oxygen concentration of flue gas is continuously monitored by a Novatech oxygen analyzer 1632 (with ±1% precision). FGR combustion mode is used in this study. The primary gas is composed of first air and recirculated flue gas (FGR), and is injected into the combustor through a windbox. The first air is supplied by a 11.2 kW Root’s blower, and FGR is pumped by a 5.6 kW turbo blower. Four equally spaced secondary gas injection nozzles of 30 mm in diameter are installed tangentially at the level of 2.05 m above the gas distributor. The secondary gas is preheated up to about 200 °C with exhaust heat from the combustor before entering the freeboard. To investigate the effects of secondary gas flow rate and excess oxygen ratio on the peanut shell combustion, the secondary gas is formed with a mixture of air from a blower and pure nitrogen from high-pressured nitrogen cylinders. Two diesel burners are used to heat up the combustor during the start up. Secondary air is injected from the beginning. First air was injected into the combustor occasionally to disturb the sand. Wood blocks and pulverized coal are added into the combustor when bed temperature reaches 400 °C. Then the flow rate of first air was increased slightly. The feeding material is injected into the combustor via a screw feeder when bed temperature reaches 500 °C and the flow rate of first air is increased gradually. The burners and pulverized coal feeding were stopped once the bed temperature reaches up to 700 °C, which takes between 12 and 16 h.
2.1. Fuel and bed material
2.3. Data acquisition
Silica sand (99.5% SiO2) is used as the inert bed materials in this study, with a mean diameter of 0.57 mm, and a particle density of 2600 kg/m3. The crushed peanut shell with a bulk density of 168.8 kg/m3 and a mean diameter of 0.704 mm were used as the fuel with a feeding rate of 36.5 kg/h. Table 1 shows the proximate and ultimate analyses of crushed peanut shell and the lower heating value is 14,875 kJ/kg.
Fig. 1B also presents the position of the thermocouples and gas sampling probes. In this figure, thermocouples (K-type) are positioned at 0.45, 1.15, 1.55, 2.05, 2.55, 2.80, 3.00, and 4.50 m above the air distributor, respectively. The components of the flue gas, such as CO, CO2, O2, and NOx are analyzed by Anapol EU5000 gas analyzers. The measurement accuracy of the gas analyzer with O2, CO, CO2 and NOx are 0.4%, 6%, 0.5%, and 1%, respectively. The
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(A)
1
1
2
2
10 22
3
5
8
9
11
5 8 7:
24
7
6
3*
14
19 20
15
4 2
2
2
7
2
12
P
P
P
P
23 P
16
P
P
P
1. Hopper 2. Screw Feeder 3. Air Lock 4. Nitrogen System 5. Burner 6. Incinerator
18
13
8
21
7. Roots Blower 8. Orifice Meter 9. Preheater 10. Reheater 11. FGR Blower 12. Oxygen-Nitrogen System
17 13. Quench Tower 14. Quench Pump 15. Baghouse 16. Scrubber 17. Scrubber Pump 18. "Plate and Frame" Heat Exchanger
19. Induced Draft Fan 20. Cooling Tower 21. Cooling Tower Pump 22. Air Jacket 23. Air Jacket Blower 24. Stack
(B)
Fig. 1. Vortexing fluidized bed combustion system. (A) Process flow diagram of the vortexing fluidized bed combustion system. (B) Schematic diagram of the vortexing fluidized bed combustor.
emissions values reported in this study are all calibrated based on 11% residual oxygen on a dry basis. To interpret the combustion characteristics of crushed peanut shell in a VFBC, it is necessary to build a model of combustion fraction by calculating the consumed oxygen. As seen in Fig. 1B, the location of z = 1.5 m is defined as the boundary between the combustion chamber (bed and splashing zone) and freeboard zone.The combustion fraction can be calculated by the following equation as:
Yi ¼
C O2 ;in Q in C O2 ;out Q out 100% ðQ 1st 0:21 þ Q 2nd 0:21 þ Q FGR C O2 ;ID Þ Q FG C O2 ;OL
ð1Þ
where, C O2 ;in : inlet oxygen concentration to each zone (%); Q in : inlet gas flow rate in each zone, Nm3/min; C O2 ;out : outlet oxygen concen-
tration in each zone, (%); Q out : outlet gas flow rate in each zone, Nm3/min; Q 1st : flow rate of first air (Nm3/min); Q 2nd : flow rate of secondary air (Nm3/min); Q FGR : flow rate of FGR (Nm3/min); C O2 ;ID : oxygen concentration of FGR at the outlet of ID fan, (%); Q FG : flow rate of flue gas from the combustor (Nm3/min); C O2 ;OL : oxygen concentration of flue gas at the outlet of combustor, (%). In this study, the combustion efficiency, gc, is calculated using heat-loss method (Kuprianov et al., 2011), and can be written as:
gc ¼ 100% ðLC þ LCO þ LCH4 Þ
ð2Þ
where LC, LCO, and LCH4 , represent the heat loss due to unburned carbon (%), heat loss due to incomplete combustion of CO (%), and heat loss due to incomplete combustion of hydrocarbon, CxHy, rep-
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resented by CH4 (%), respectively. Because there is no bottom ash removed from the combustor, the heat loss due to unburned carbon, LC (%), can be determined by:
LC ¼
Cc W F C A 100C DHc;C c
where, Q1st: flow rate of first air, Nm3/min; QFGR: flow rate of recirculated flue gas, Nm3/min; QPRG: flow rate of primary gas, Nm3/min; C O2 ;ID : concentration of oxygen detected at the outlet of induce fan, %; Sb: stoichiometric oxygen ratio, %.
ð3Þ
W F LHVðWBÞ
3. Results and discussion where, WF: fuel feeding rate, kg/h; CA: ash content in fuel, wt.%; Cc: carbon content in fly ash, wt.%; LHV(WB): wet-basis lower heating value of fuel, kJ/kg; DHc;C : heating value of carbon, 39,750 kJ/kg.
LCO ¼
NFG ½CO DHc;CO W F LHVðWBÞ
ð4Þ
where NFG: dry flue gas flow rate at the outlet of combustor, calculated with wet-basis ultimate analysis of feeding material and given operating condition, kg mol/h; [CO]: CO concentration detected at the outlet of combustor, %; DHc;C : heating value of CO, 282854 kJ/ kg mol;
LCH4 ¼
NFG ½CH4 DHc;CH4 W F LHVðWBÞ
ð5Þ
where [CH4]: CH4 concentration detected at the outlet of combustor, %; DHc;CH4 : lower heating value of CH4, 801,936 kJ/kg mol. Combustion loss resulting from the fly ash is the largest portion in the total combustion loss. Fly ash is collected at the baghouse, quench tower, and heat exchanger. The carbon content in the fly ash is analyzed with a Pyris 1 TGA (thermo-gravimetric analyzer). The fly ash was weighted first, and the original weight was recorded as X1. Then it was heated from 25 °C to 105 °C at a heating rate of 40 °C/min, and held at 105 °C for 1 h, and the dehydrated weight X2 was recorded. Then, the operating temperature increased from 105 °C to 850 °C at a heating rate of 40 °C/min, and held at 850 °C for 2 h. The final weight X3 was obtained and recorded. According to the Eq. (6), the carbon content of fly ash (dry base), Cc, was calculated:
C c ðwt:%Þ ¼
X2 X3 100 X2
ð6Þ
2.4. Operating conditions The working conditions for the experiments are given in Table 2. The oxygen concentration data of the flue gas at the outlet of induced draft fan were transmitted to the primary gas control system for adjusting the mixing ratio of the first air and FGR to maintain the oxygen content in the primary gas at a constant value. The flow rate of FGR can be calculated by the following equations:
Q 1st þ Q FGR ¼ Q PRG
ð7Þ
Q 1st 21% þ Q FGR C O2 ;ID ¼ Sb Stoichiometric oxygen
ð8Þ
Table 2 Operating conditions. Parameters
Symbol
Unit
Value
Feeding rate Excess oxygen ratio Stoichiometric oxygen ratio in the combustion chamber Mean fuel particle size Total primary air flow rate Secondary gas flow rate Additional nitrogen flow rate of secondary gas
Wfuel Eo Sb
kg/h % %
36.5 40, 50, 55, 60 90
dp, fuel Qpri Qsec Qn2
mm Nm3/min Nm3/min Nm3/min
0.704 3 1.56, 2, 2.31, 2.56 1.44, 1, 0.69, 0.44
3.1. Excess oxygen ratio For fluidized bed combustion of biomass, adding more oxygen can achieve high efficient combustion. However, Piao et al. (2000) reported that too much excess oxygen decreases the combustion temperature, which counters the effort in reducing pollutant emissions. In this test, the effect of the excess oxygen ratio on combustion behavior and pollutant emissions is investigated. The stoichiometric oxygen ratio in the bed zone is 90%, and the primary and secondary gas flow rates are 3 and 2 Nm3/min, respectively. Fig. 2A shows the temperature distribution in the VFBC at different excess oxygen ratios. The bed temperature is lower than that in the other parts of the VFBC which can be attributed to the lower combustion fraction in the bubbling bed. At fixed primary and secondary gas flow rate, excess oxygen is introduced into the freeboard zone through secondary gas. This increases the combustion fraction of the freeboard, and decreases the combustion fraction of the combustion chamber. The combustion fraction of the combustion chamber decreases from 40% to 33.5% when the excess oxygen ratio increases from 40% to 60%. The bed temperatures changed little at different excess oxygen ratios; however, the temperatures increase for higher excess oxygen ratio in the splashing and freeboard zones. The local temperatures within the VFBC are affected by the amount of heat lost through the wall and released (generated) from fuel combustion. Because of the smaller sizes, lighter weight and high volatile content of crushed peanut shell, it tends to combust in the splashing zone and freeboard zone. In Fig. 2A, maximum temperatures are observed at z = 2.8 m, despite the cooling effect by secondary gas injection at z = 2.05 m. It can be attributed to the vigorous combustion of combustible matter with oxygen added from secondary gas. At a higher region above the freeboard, the temperature decreases with the distance from the distributor. It can be attributed to the decrease of exhausted combustible material at a higher level of freeboard. Meanwhile, freeboard temperature and outlet temperature increase with higher excess oxygen ratio, which can be attributed to the fact the higher oxygen content in the secondary gas intensifies the combustion of unburned carbon and volatiles. Fig. 2B shows the effect of excess oxygen ratio on the combustion efficiency. The combustion efficiency increases with the excess oxygen ratio. At higher excess oxygen ratios, the combustion temperatures at all measuring points increase, which favors complete combustion. The result obtained in this study agrees with the observation of Sirisomboon et al. (2010), Rao and Reddy (2011), Arromdee and Kuprianov (2012), and Kuprianov et al. (2010). Fig. 3A shows the effect of excess oxygen ratio on CO and NOx emissions. Higher excess oxygen ratio can decrease CO emission which is a function of operating variables, such as excess oxygen ratio, and combustion temperature (Permchart and Kouprianov, 2004). As fuel is fed into the combustor, the fuel particles get heated up rapidly and most volatiles are released due to the higher heat and mass transfer rates in fluidization. Crushed peanut shell as biomass has higher volatile content (Arromdee and Kuprianov, 2012), thus a high percentage of volatile and some fine char are carried up to the freeboard zone, and the combustion efficiency is sensitive to freeboard temperature (Chyang et al., 2008; Duan et al., 2013c). NOx emissions increase with excess oxygen ratio.
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Pollutant emissions (ppm) @11%O2
800
o
Temperature ( C)
120
(A)
850
750
Eo= 60% Eo= 55% Eo= 50% Eo= 40%
Sb=90% 3
700
Qpri=3 Nm /min Qsec=2 Nm 3/min
CO emission NOx emissions
100
S =90% b
80
3 Q =3 Nm /min pri 3 Q =2 Nm /min sec
60
40
20
(A)
650 0
1
2
3
4
0 38
5
Distance above the distributor, z (m) 100.0
44
46
48
50
52
54
56
58
60
62
10000
Combustion efficiency 8000
Sb=90%
CO concentration (ppm)
Combustion efficiency (%)
42
Excess oxygen ratio, Eo (%)
(B)
99.5
99.0
40
3
QPRI=3 Nm /min Qsec=2 Nm 3/min
98.5
98.0
97.5
Eo= 40% Eo= 50% Eo= 55% Eo= 60%
6000
3
QPRI =3 Nm /min
4000
3
QSEC =2 Nm /min Eo=50% Sb=90%
2000
(B)
97.0 40
45
50
55
60
0 0
1
2
3
Fig. 2. Effect of the excess oxygen ratio on the combustion behavior. (A) Temperature distribution within the VFBC at different excess oxygen ratios. (B) Effect of the excess oxygen ratio on the combustion efficiency.
5
400
Eo=60% Eo=55% Eo=50% Eo=40%
NOx concentration (ppm)
350
Similar tendencies were observed by Arromdee and Kuprianov (2012), Kuprianov et al. (2006) and Madhiyanon et al. (2010) during the combustion of peanut shell, rice husk and sugar cane bagasse in conical fluidized-bed combustor or swirling fluidizedbed combustor. This is primarily due to the fact NOx reduction reaction with CO is reduced at high excess oxygen ratio and low CO emission, resulting in higher NOx emissions. Besides, heightened freeboard temperature at higher excess oxygen ratio also accelerates the NOx emissions. As seen in Fig. 2A, the freeboard temperature increases with the excess oxygen ratio. This improves the combustion of the volatile and un-burned char in the freeboard zone, and decreases the CO emission at the outlet of the VFBC. Fig. 3B and C show the CO and NOx concentrations within the combustor at different excess oxygen ratio, respectively. In all the tests, the axial CO and NOx concentration profiles show the maximum values at approximately the same locations above the distributor, z = 2.05 m, and the maximum value decreases with excess oxygen ratio. The lower part of the combustor can be assumed to be the CO formation zone due to the lower stoichiometric oxygen ratio. In the upper part of the combustor, CO is oxidized with excess oxygen directly and results in lower CO concentration. The maximum NOx concentrations appear at same locations as the maximum CO concentrations. This can be attributed to the fact the higher temperature favors the combustion of nitrogenous species released with volatile matter and the oxidation of nitrogen retained in the char (Permchart and Kouprianov, 2004).
4
Distance above the distributor, z (m)
Excess oxygen ratio (%)
300
3
QPRI =3 Nm /min
250
3
QSEC =2 Nm /min Eo=50%
200
Sb=90%
150
100
(C)
50 0
1
2
3
4
5
Distance above the distributor, z (m) Fig. 3. Effect of the excess oxygen ratio on the pollutant emissions. (A) Effect of the excess oxygen ratio on the CO and NOx emissions. (B) CO concentration distribution within the combustor at various excess oxygen ratio. (C) NOx concentration distribution within the combustor at various excess oxygen ratio.
3.2. Vortexing effect Fig. 4A shows the temperature distribution profiles with various secondary gas flow rates, where the stoichiometric oxygen ratio in the bed zone is 90%, the excess oxygen ratio is 50%, and the primary gas flow rate is 3 Nm3/min. Nitrogen was added with the flow
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rates of 0, 0.44, 0.75, and 1 Nm3/min, and the final flow rates of secondary gas become 1.56, 2, 2.31, and 2.56 Nm3/min, respectively. In addition, from Fig. 4A, the bed and freeboard temperatures are impacted differently by the increasing secondary gas flow rate. Below the secondary gas injection points (z = 2.05 m), the bed and splashing zone temperatures increase with the secondary gas flow rate. It can be attributed to the fine particles and unburned char carried back to these zones. Increasing secondary gas flow rate strengthens the vortexing intensity, thus more particles are
captured. This increases the particle residence time and its combustion fraction (from 31% to 40%) in the bed and splashing zones. The freeboard and outlet temperatures change slightly with the increasing secondary gas flow rates. Besides, the temperature slope between z = 1.45 m and z = 2.8 m decreases with increasing secondary gas flow rates. The temperature slope becomes negative at 2.31 and 2.56 Nm3/min secondary gas flow rate, which can be attributed to the lower combustion fraction in the freeboard zone and the cooling effect by the secondary air. Combustion fraction of
900
(A)
o
Temperature ( C)
850
800
750
Secondary gas flow rate 3 1.56 Nm /min 3 2 Nm /min 3 2.31 Nm /min 3 2.56 Nm /min
Sb=90% Qpri=3 Nm3/min Eo=50%
700
650 0
1
2
3
4
5
Distance above the distributor (m) 900 800
(B)
o
Temperature ( C)
700
Sb=90%
600
Qpri=3 Nm3/min Eo=50%
500
Secondary gas flow rate 3 1.56 Nm /min 3 2 Nm /min 3 2.31 Nm /min 3 2.56 Nm /min
400 300 200 100 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
s/D 100.0
1300
Fly ash elutriated Combustion efficiency
Fly ash elutriated (g/hr)
1100
99.5
1000
99.0
900 800 700
Sb=90%
98.5
Qpri=3 Nm 3/min Eo=50%
98.0
600
Combustion efficiency (%)
1200
(C)
97.5 500 400 1.4
97.0 1.6
1.8
2.0
2.2
2.4
2.6
3
Secondary gas flowrate (Nm /min) Fig. 4. Effect of the secondary gas flow rate on the combustion behavior. (A) Temperature distribution within the combustor at various secondary gas. (B) Radial temperature distribution within the combustor at various secondary gas flow rates (z = 2.03 m). (c) Effect of the secondary gas flow rate on the fly ash yield and combustion efficiency.
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the freeboard decreases from 69% to 60% as the secondary gas flow rate goes from 1.56 to 2.56 Nm3/min. Fig. 4B shows the radial temperature distribution with various secondary gas flow rates. The temperature probe point is located at z = 2.03 m. The inner diameter of freeboard is 0.75 m, the distances between the detective point and combustor wall(s) are 0.1, 0.2, 0.3, 0.375, 0.45, 0.55, and 0.65 m, respectively. The lowest temperature appears near the wall, which is very close to the
Pollutant emissions (ppm) @11%O2
120
CO emission NOx emissions
100
80
Sb=90% Qpri=3 Nm3/min Eo=50%
60
40
(A)
20
0 1.4
1.6
1.8
2.0
2.2
2.4
2.6
3
Secondary air flowrate (Nm /min) 10000
6000
Secondary gas flow rate 3 1.56 Nm /min 3 2 Nm /min 3 2.31 Nm /min 3 2.56 Nm /min
4000
QPRI =3 Nm /min
CO concentration (ppm)
8000
3
Eo=50% Sb=90%
2000
(B)
0 0
1
2
3
4
5
Distance above the distributor, z (m) 300
Secondary gas flow rate 3 1.56 Nm /min 3 2 Nm /min 3 2.31 Nm /min 3 2.56 Nm /min
NOx concentration (ppm)
250
200
temperature of injected secondary gas. The highest temperature appears at the center of the freeboard; it is due to the different flow characteristics between the center and the wall. The fine particles and unburned char are thrown to the combustor wall by centrifugal force by the vortex and drop back to the bed zone. Therefore, the overall temperatures increase with the secondary gas flow rate in this figure. Fig. 4C shows the effect of the secondary gas flow rate on the elutriated fly ash and combustion efficiency. In this test, the fly ash was collected at the baghouse, quench tower, and heat exchanger. The elutriated fly ash decreases significantly with secondary gas flow rate. The carbon content in the fly ash was analyzed by a TGA, and the results show that the carbon content of fly ash also decreases significantly with the secondary gas flow rate. This is in agreement with the results obtained by Okasha (2007). It can be attributed to the intensified combustion caused by the vortexing effect which increased with the secondary gas flow rate. In this figure, the maximum combustion efficiency for peanut shell appears at a secondary gas flow rate of 2.56 Nm3/h. Fig. 5A shows the effect of the secondary gas flow rate on the pollutant (CO and NOx). The CO emission goes from 46 to 40 ppm, while NOx emission goes from 91 to 107 ppm with increasing secondary gas flow rate, which can also be attributed to the vortexing effect. At a higher secondary gas flow rate, more fine particles and unburned char are brought back to bed zone for re-burning, resulting in lower CO emission. The reduction in CO level can also be ascribed to high turbulence created by vortex, which results in better combustion of volatile and unburned char. Despite NOx emission can be reduced by the reduction reaction with unburned char (Chaiklangmuang et al., 2002; Okazaki and Ando, 1997), the higher bed and splashing temperatures enhance the formation of NOx predecessors such as HCN, and NH3. In addition, lower CO concentration reduces the chance of NOx reduction reaction with CO. The CO emission in this study complies with the minimum emission requirement (within 100 ppm) of Taiwan EPA regulations, and NOx emission also complies with the minimum emission requirement (220 ppm). Considering the combustion efficiency and air pollutant emissions, 1.56–2 Nm3/h secondary gas flow rates are optimal for burning peanut shell in this study. Fig. 5B and C show the CO and NOx concentrations profiles within the combustor at various secondary gas flow rates, respectively. In Fig. 5B, at the location of 2.03 m above the distributor, the CO concentration values increase with secondary gas flow rate. At the highest secondary gas flow rate (2.56 Nm3/min), the CO concentration reaches the peak value. This can be attributed to the fact that more un-burned particles are carried back to the splashing zone. The stoichiometric oxygen ratio in the bed is 90%; therefore, as more particles carried back to the bed and splashing zones, the CO concentration goes higher. In Fig. 5C, at the location of 2.03 m above the distributor, NOx concentrations increase with the secondary gas flow rate which is due to the fact the higher secondary
3
QPRI =3 Nm /min Eo=50%
Table 3 Elemental analysis of ash.
Sb=90%
150
100
(C)
50 0
1
2
3
4
5
Distance above the distributor, z (m) Fig. 5. Effect of the secondary gas flow rate on the pollutant emissions. (A) Effect of the secondary gas flow rate on CO and NOx emissions. (B) CO concentration distribution within the combustor at various secondary gas flow rate. (C) NOx concentration distribution within the combustor at various secondary gas flow rate.
Element
Units
Bottom ash
Fly ash
Na Mg Al Si P S K Ca Ti Fe
ppm % % % % ppm % % ppm %
7273 1.504 4.616 21.07 0.793 ND 7.086 3.016 2867 4.084
9408 8.591 2.606 7.072 10.20 ND 10.65 9.393 1392 3.973
ND: Not detected.
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gas flow rate increases the residence time of un-burned char and small particles resulting in high temperature as shown in Fig. 4A. 3.3. Bed material agglomeration The elemental analyses of the fly ash and bottom ash are given in Table 3. The fly ash was collected from the baghouse. The ash deposited on the distributor after a 12 h combustion test was scooped up as the bottom ash. From Table 3, the concentrations of alkali and alkaline-earth metals of fly ash are higher than those of the bottom ash. Owing to the accumulation of ash in the bed, the amount of alkali and alkaline-earth metals remained in the bed is increased with operating time. Meanwhile, there is no sign of bed agglomeration from 12 h combustion tests due to the lower operating temperatures in the bed. 4. Conclusion CO emission decreases with the excess oxygen ratio and the secondary gas flow rate, while NOx emissions show an inverse trend. The temperature profiles within the VFBC show a generally similar trend under various operating conditions. Higher secondary gas flow rate favors complete combustion of crushed peanut shell due to the stronger vortexing effect, resulting in higher combustion efficiency. To meet both minimum CO and NOx emission requirements of Taiwan EPA regulations, excess oxygen ratios ranging from 40% to 55% and the secondary gas flow rates ranging from 1.56 to 2 Nm3/min are optimal for crushed peanut shell combustion in the VFBC. References Ahmad, M., Lee, S.S., Dou, X., Mohan, D., Sung, J.-K., Yang, J.E., Ok, Y.S., 2012. Effects of pyrolysis temperature on soybean stover- and peanut shell-derived biochar properties and TCE adsorption in water. Bioresour. Technol. 118, 536–544. Arromdee, P., Kuprianov, V.I., 2012. Combustion of peanut shells in a cone-shaped bubbling fluidized-bed combustor using alumina as the bed material. Appl. Energy 97, 470–482. Chaiklangmuang, S., Jones, J.M., Pourkashanian, M., Williams, A., 2002. Conversion of volatile-nitrogen and char-nitrogen to NO during combustion. Fuel 81, 2363– 2369. Chen, H., Bin, L., Yang, H., Yang, G., Zhang, S., 2008. Experimental investigation of biomass gasification in a fluidized bed reactor. Energy Fuels 22, 3493–3498. Chyang, C., Duan, F., Lin, S., Tso, J., 2012a. A study on fluidized bed combustion characteristics of corncob in three different combustion modes. Bioresour. Technol. 116, 184–189. Chyang, C.S., Duan, F., Lin, S.M., Tso, J., 2012b. A study on fluidized bed combustion characteristics of corncob in three different combustion modes. Bioresour. Technol. 116, 184–189. Chyang, C.S., Qian, F.P., Lin, Y.C., Yang, S.H., 2008. NO and N2O emission characteristics from a pilot scale vortexing fluidized bed combustor firing different fuels. Energy Fuels 22, 1004–1011. Duan, F., Chyang, C.S., Chin, Y.C., Tso, J., 2013a. Pollutant emission characteristics of rice husk combustion in a vortexing fluidized bed incinerator. J. Environ. Sci. 25, 335–339. Duan, F., Chyang, C.S., Lin, C.W., Tso, J., 2013b. Experimental study on rice husk combustion in a vortexing fluidized-bed with flue gas recirculation (FGR). Bioresour. Technol. 134, 204–211. Duan, F., Liu, J., Chyang, C.-S., Hu, C.-H., Tso, J., 2013c. Combustion behavior and pollutant emission characteristics of RDF (refuse derived fuel) and sawdust in a vortexing fluidized bed combustor. Energy 57, 421–426.
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