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Energy and exergy analyses of an integrated CCHP system with biomass air gasification
Applied Energy 142 (2015) 317–327
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Energy and exergy analyses of an integrated CCHP system with biomass air gasification Jiang-Jiang Wang a,b,⇑, Kun Yang a, Zi-Long Xu a, Chao Fu a a b
School of Energy, Power and Mechanical Engineering, North China Electric Power University, Baoding, Hebei Province 071003, China Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China
h i g h l i g h t s Propose a biomass-gasification CCHP system. A heat pipe heat exchanger is used to recover waste heat from product gas. Present the energy and exergy analyses of the biomass CCHP system. Analyze the annual off-design performances.
a r t i c l e
i n f o
Article history: Received 8 August 2014 Received in revised form 29 December 2014 Accepted 31 December 2014
Keywords: Combined cooling heating and power (CCHP) system Biomass air gasification Energy analysis Exergy analysis
a b s t r a c t Biomass-fueled combined cooling, heating, and power (CCHP) system is a sustainable distributed energy system to reduce fossil energy consumption and carbon dioxide emission. This study proposes a biomass CCHP system that contains a biomass gasifier, a heat pipe heat exchanger for recovering waste heat from product gas, an internal combustion engine to produce electricity, an absorption chiller/heater for cooling and heating, and a heat exchanger to produce domestic hot water. Operational flows are presented in three work conditions: summer, winter, and the transitional seasons. Energy and exergy analyses are conducted for different operational flows. The case demonstrated that the energy efficiencies in the three work conditions are 50.00%, 37.77%, and 36.95%, whereas the exergy efficiencies are 6.23%, 12.51%, and 13.79%, respectively. Destruction analyses of energy and exergy indicate that the largest destruction occurs in the gasification system, which accounts for more than 70% of the total energy and exergy losses. Annual performance shows that the proposed biomass-fueled CCHP system reduces biomass consumption by 4% compared with the non-use of a heat recovery system for high-temperature product gas. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Distributed combined heat and power (CHP) and combined cooling heating and power (CCHP) systems are considered worldwide as an energy-saving and environment-friendly energy system for buildings [1]. Renewable energy resources are sustainable alternatives to natural gas in driving traditional CHP/CCHP systems [2]. Among renewable energy resources, biomass is one of the most attractive options in CHP and CCHP systems to produce continuous power and simultaneously reduce carbon dioxide (CO2) emissions. Furthermore, distributed energy system also relieves the
⇑ Corresponding author at: School of Energy, Power and Mechanical Engineering, North China Electric Power University, Baoding, Hebei Province 071003, China. Tel.: +86 312 7522443; fax: +86 312 7522440. E-mail address: [email protected] (J.-J. Wang). http://dx.doi.org/10.1016/j.apenergy.2014.12.085 0306-2619/Ó 2015 Elsevier Ltd. All rights reserved.
transportation problem due to biomass feedstock and production distribution [3]. Various studies of biomass-based CHP/CCHP systems summarize the state of the art technology and near future perspectives [2,4–6]. The primary conversion technologies developed in biomass-fueled CHP/CCHP systems include combustion, gasification, pyrolysis, biochemical, and chemical processes [2]. Based on hot water, steam, gaseous, or liquid products, the secondary conversion technologies (e.g., steam turbine [7], organic Rankine cycle (ORC) [8], gas turbine [7], internal combustion engine (ICE) [3,9], and others) harness the products to produce power, heat, and even cooling. Different integrated forms of biomass conversion and utilization were proposed and researched. Biomass-fueled CHP systems have been studied over many years [3,10–12]. Mertzis et al. [3] studied the performance of a micro biomass CHP system and reported the effect of different types of biomass feedstock, gasification parameters, and engine
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Nomenclature CCHP CHP CO2 COP HHV HPHE HT ICE LHV LT MGT ORC PHE SOFC
combined cooling heating and power combined heat and power carbon dioxide coefficient of performance higher heating value heat pipe heat exchanger high temperature internal combustion engine lower heating value low temperature micro gas turbine organic Rankine cycle plate heat exchanger solid oxide fuel cells
Symbols c COP ex E _ Ex f h HHV LHV m P Q R s T
specific heat at a constant pressure (kJ kg1 K1) coefficient of performance specific exergy (kJ mol1 or kJ kg1) electricity (kW) exergy (kW) load factor specific enthalpy (kJ mol1 or kJ kg1) higher heating value (MJ kg1 or MJ Nm3) lower heating value (MJ kg1 or MJ Nm3) mass flow rate (kg s1) pressure (Pa) heat (kW) universal gas constant specific entropy (kJ mol1 K1 or kJ kg1 K1) temperature (K)
intake mixtures on long-term operation and energy output. Fryda et al. [10] investigated three micro biomass CHP configurations integrated with solid oxide fuel cells (SOFC) and micro gas turbine (MGT), and they presented an exergy analysis to optimize these configurations. Bang-Møller et al. [11] analyzed the exergy destruction of a biomass steam gasification, SOFC, and MGT hybrid plant and optimized its configuration. Huang et al. [12] compared the techno-economic performances of biomass gasification- and ORC-based CHP systems, and they emphasized that both systems are not economically viable in generating power to supply present needs. Al-Sulaiman et al. [13] reported that the energy and exergy analyses of a biomass CCHP system using an ORC and concluded that there is a significant improvement when CCHP is used in place of only electrical generation. Recently, the research has been extended from CHP to CCHP systems. Biomass combustion is the most common utilization method in CCHP systems in almost all studies. P. Ahmadi et al. designed a biomass CCHP system integrated with ORC [14], and they proposed a thermoeconomic multi-objective optimization model to improve its multi-performances, including energy, exergy and environmental impact [15]. Lian et al. [16] evaluated the thermoeconomic potentials of four different biomass CCHP configurations and concluded that exergy destruction is most extensive in the furnace, amounting to nearly 60% of net exergy loss. Compared with biomass combustion, gasification is more efficient at producing electricity and heat [17]. Puig-Arnavat et al. [17] developed a simple but rigorous CCHP model for designing, optimizing and simulating small–medium scale plants, including a realistic biomass gasification model, and achieved approximately 10% primary energy saving. Huang et al. [18] modeled and simulated a small
w x z
a g
water stoichiometric coefficient (kmol s1) percentage mass in biomass (%) concentration (%v) efficiency
Superscripts ch chemical ki kinetic ph physical po potential Subscripts A ash Air air b biomass c cooling C carbon e electricity gas product gas h heating H hydrogen I index for the thermodynamic state point N nitrogen O oxygen S sulfur tar tar uc unconverted carbon wb wet biomass 0 standard reference state
scale CCHP system integrated by a biomass downdraft gasifier, an ICE, and an absorption chiller, and they concluded that this system would be beneficial to the building in case of appropriate load characteristic. The literature survey shows that very few studies focused on the energy and exergy analyses of CCHP systems based on biomass gasification. The specific objective of the present work is to propose a biomass gasification CCHP system integrated with a heat pipe heat exchanger (HPHE). The HPHE recovers the waste heat from the high-temperature (HT) product gas for effective energy-utilizations corresponding to different operating conditions. The comprehensive energy and exergy analyses are performed to system performances of different operational flows. Exergy destructions throughout the system are determined in order to understand the source and determine the locations of irreversibilities. Section 2 presents the energy flows and operating conditions of the integrated biomass-fueled CCHP system. Section 3 analyzes the thermodynamic performance of the system. Section 4 demonstrates energy and exergy performances in a building case. Section 5 summarizes the conclusions of this study.
2. CCHP system integrated with biomass air gasification 2.1. Plant concept The concept of a CCHP system integrated with biomass air gasification is depicted in Fig. 1. The system is divided into two subsystems: the biomass air gasification and the CCHP subsystems. Biomass is converted into product gas in the gasification subsystem,
J.-J. Wang et al. / Applied Energy 142 (2015) 317–327 Biomass air gasification system Biomass Air
Product gas
Gasifier
Purifier
HT generator, and the hot water in the low-temperature (LT) generator. The chiller/heater can produce chilled water for space cooling, hot water for space heating, and domestic hot water.
Cooler
2.3. System operation Heat Building cooling, heating and power system
Product gas
319
Gas engine
Two-stage LiBr-H2O Absorption Chiller/Heater
Heat exchanger
Power
Cool
Heat
2.3.1. Operating conditions during summer During summer, the V3, V4, V5, V6, V8, and V9 valves are closed, whereas the others are open in Fig. 2. The flow is shown in Fig. 3. The heat recovered through the HPHE is transmitted to the LT generator of the absorption chiller. 2.3.2. Operating conditions during winter During winter, the V3, V4, V7, V8, and V9 valves are open, whereas the other valves are closed in Fig. 2, which is simplified to Fig. 4. The heat recovered through the HPHE is used to preheat hot water for space heating.
Domestic hot water
Fig. 1. Plant concept of integrated CCHP system with biomass air gasification.
and the product gas is used as fuel to produce power, cooling, and heating in the CCHP subsystem. After undergoing air gasification, the product gas consists of product gas, tar, steam, and dust. Its temperature is approximately 573–673 K or higher. The HT product gas is purified, cooled, and then sent to the prime mover. During the cooling process, the waste heat is removed by cooling water. In the CCHP subsystem, the ICE combusts the product gas to generate power. The jacket water from the ICE is then used to produce domestic hot water through the heat exchanger, and the exhausted gas is sent to a two-stage Libr-H2O absorption chiller/heater to produce cooling in summer and heating in winter, respectively. The product gas serves as a supplement when the exhausted gas is insufficient to meet the cooling/heating demand.
2.3.3. Operating conditions during the transitional season The integrated CCHP system produces electricity and domestic hot water during the transitional seasons. The V5 and V6 valves are open, whereas the other valves are closed in Fig. 2. The flow is shown in Fig. 5. The heat recovered through the HPHE is used to preheat domestic hot water. 3. Thermodynamic analysis 3.1. Energy analysis The higher heating value (HHV) of biomass can be predicted by the following correlation [19]:
HHV b ¼ 0:3491zC þ 1:1783zH 0:1034zO 0:0151zN þ 0:1005zS 0:0211zA
ð1Þ
The lower heating value (LHV) is calculated as [20]: 2.2. Integrated design Waste heat is emitted in the gasification subsystem, and this waste heat can be used in the cooling/heating subsystem. Consequently, the waste heat during gasification is integrated into the CCHP system, as shown in Fig. 2. The CCHP system integrated with biomass air gasification mainly includes the following components: gasifier, HPHE, product gas conditioning subsystem (i.e., cyclone, spray scrubber), Roots blower, gas storage tank, ICE, two-stage Libr-H2O absorption chiller/heater, plate heat exchanger (PHE), and hot water tank. The biomass with air is gasified in the gasifier, and then the HT product gas is cooled by the HPHE. The HT biomass tar in the product gas is condensed below 473 K, and it easily combines with water and carbon granules to pollute the equipment. Consequently, the HT gas is to be cooled above 473 K. The cooled product gas is then purified in the cyclone and further cooled in the spray scrubber, and the tar is separated. The clean product gas is stored in a tank as fuel. The waste heat recovery system of the HPHE is integrated with the CCHP system. To maximize the use of the waste heat, three integration forms can be utilized to produce the required heat during different seasonal conditions (see Section 2.3). The HPHE is used for the following reasons: (1) changing the heat transfer area to adjust the temperature of the heat pipe easily avoids the corrosion interval of mixed gas, and (2) adjusting the structure and shape of the heat pipe relieves abrasion and ash clogging. The two-stage Libr-H2O absorption chiller/heater can utilize heat at various temperatures. The heater is driven by three types of energy: the exhausted gas from the ICE, the product gas in the
LHV b ¼ HHV b 21:978zH
ð2Þ
where HHVb and LHVb are the HHV and LHV, respectively, (MJ kg1), and zC, zH, zO, zN, zS, and zA are the percentages of carbon, hydrogen, oxygen, nitrogen, sulfur, and ash in the raw biomass, respectively, (%). The proximate and ultimate analysis of biomass in this study is shown in Table 1 [21]. The biomass energy into the gasifier is expressed to
Q 1 ¼ m1 LHV b
ð3Þ
and the energy in the gasification system is balanced with
Q 7 ¼ ggasification Q 1 ¼ Q 8 þ Q 9
ð4Þ
where Q1 is the biomass heat (kW), m1 is the biomass flow rate (kg s1), Q7 is the heat of product gas (kW), ggasification is the gasification efficiency, Q8 and Q9 are the heat of product gas to the ICE and absorption chiller/heater, respectively, (kW). To describe clearly and simplify the symbol expression, the following symbols are used in Eqs. (3)–(22): m is the mass flow rate (kg s1), c is the specific heat at a constant pressure (kJ kg1 K1), T is the temperature (K), and the subscript denotes the state point. The energy balances of the HPHE are expressed as
Q 56 ¼ c5 m5 ðT 5 T 6 Þ
ð5Þ
Q 1615 ¼ c15 m15 ðT 16 T 15 Þ Q 1615 ¼ ghepipe Q 56
ð6Þ ð7Þ
where Q5–6 and Q16–15 are the released heat from HT product gas and the absorbed heat of LT water in the HPHE, respectively, (kW), and ghe-pipe is the HPHE efficiency.
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Product gas conditioning Biomass
Heat Pipe Heat Exchanger
1 5
Cyclone
Spray scrubber
6 15
16
V1
V2
Roots Blower
Gas Storage Tank
Gasifier Air
V5
V3
V4 V6
2
Product gas
Cool water
7
Exhausted gas 23
24
V7 13
11
14
Product gas Air 3
Two-stage LiBr-H2O Absorption Chiller/Heater HT Generator
8
V14
Exhausted 10 gas
Gas ICE
22
9 Jacket water 17 19 Plate Heat Exchanger
Condenser
LT Generator
Air 4
18
Cooling water to cooling tower
V17 HT Solution Heat Exchanger 20 V12
V13
LT Solution Heat Exchanger
V15
25 V16 Evaporator
Absorber Hot water tank V9 26
V8 V10
21 Cooling water from cooling tower
Domestic 12 hot water 13 14 V11 Chilled/hot water of air Electricity condition system Hot water Air Electricity Product gas
Chilled water 13
Chilled water Cooling water
14
Cool water Exhausted gas
Fig. 2. Flowchart of the integrated CCHP system with biomass air gasification.
The input energy and output electricity of the ICE are, respectively expressed as
Q 8 ¼ m8 LHV prodcutgas E12 ¼ ge Q 8
ð8Þ ð9Þ
where E12 is the generated electricity of the ICE (kW), and ge is the generation efficiency of the ICE. The energy balances of the PHE can be expressed as
Q 1718 ¼ c17 m17 ðT 17 T 18 Þ
ð10Þ
Q 2019 ¼ c20 m20 ðT 20 T 19 Þ
ð11Þ
Q 2019 ¼ gheplate Q 1718
ð12Þ
where Q17–18 and Q20–19 are the released heat from HT water and the absorbed heat of LT water in the PHE, respectively, (kW), and ghe-plate is the PHE efficiency. The integration of the absorption chiller/heater varies during different seasonal conditions, so the energy balance can be expressed as follows: During summer:
Q 9 þ Q 1011 þ Q 1516 ¼
Q 1314 Q 2524 þ COP c COP h
ð13Þ
During winter:
Q 9 þ Q 1011 ¼
Q 1416 þ Q 2524 COP h
ð14Þ
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1 Biomass
5
Gasifier
Air
2 23 13
6
Water
15
14 Chilled water
9
8
Two-stage LiBr-H2O Absorption Chiller/Heater
Exhausted gas
Gas
25
Air
ICE
10
Air
Table 1 Fuel properties of solid biomass [21].
7 Product gas
16
24
11
4
Gas conditioning
3
17
Tank 20 18 22
21 Cooling water
26
19
12
Domestic hot water
Water
Electricity
Type of fuel
Straw
Proximate analysis (% wet fuel): Volatile Moisture Ash Fixed carbon
69.70 10.30 4.20 15.80
Ultimate analysis (% dry fuel): Carbon Hydrogen Oxygen Nitrogen Sulfur Chlorine Ash HHV, MJ/kg LHV, MJ/kg b
45.80 5.96 40.00 0.45 0.16 0.03 7.60 18.72 17.41 1.156
Fig. 3. Summer operation condition.
1 Biomass Air
2
5
Gasifier
13
15
23 Water 14 Hot water 11 Exhausted gas
Gas conditioning
Table 2 Specific enthalpy, entropy, and standard chemical exergy values of some materials at 298 K and 1 atm [23,24].
7 Product gas
16
24
9
8
Two-stage LiBr-H2O Absorption Chiller/Heater
Gas
Air
ICE
10 25
Air
4
6
Component
h0 (kJ kmol1)
s0 (kJ kmol1 K1)
exch (kJ kmol1)
CO CO2 H2 CH4 N2 O2
137150 394374 0 74850 0 0
197.543 213.685 130.574 186.16 191.61 205.033
275100 19870 236100 831650 668 3970
3
17
Tank
2000
20 18 26
19
12
Domestic hot water
Water
Electricity
Cooling Heating
load (kW)
1600
Fig. 4. Winter operation condition.
1200 800 400 0
Air
2
5
6
Gasifier 15
Gas conditioning
Load (kW)
1
1 Biomass
7 Product gas
16
2001
3001
4001
5001
6001
7001
8001
Power 400 0
24 11 Exhausted gas
9
8
Two-stage LiBr-H2O Absorption Chiller/Heater
Gas ICE
10 25
Air
Air 3
1001
2001
3001
4001
5001
6001
7001
8001
7001
8001
Domestic hot water
400 0 1
1001
2001
17
3001
4001
5001
6001
Fig. 6. The hourly loads of cooling, heating, power and domestic hot water.
20 18 26
19
12
Domestic hot water
Water
Electricity
in winter (kW), Q25–24 is the absorbed heat of domestic hot water from absorption chiller/heater (kW), COPc and COPh are the cooling coefficient of performance (COP) and heating COP, respectively, and
Q 9 ¼ m9 LHV prodcut
Fig. 5. Transitional season operation condition.
During the transitional seasons:
Q 9 þ Q 1011
1 800
time (h)
Tank
Q ¼ 2524 COP h
Load (kW)
23
4
1001
800
ð15Þ
where Q10–11 is the released heat of exhausted gas from the ICE (kW), Q13–14 is the released cooling of chilled water (kW), Q14–16 is the absorbed heat of hot water from absorption chiller/heater
gas
ð16Þ
Q 1011 ¼ c10 m10 ðT 10 T 11 Þ Q 1314 ¼ c13 m13 ðT 13 T 14 Þ
ð17Þ ð18Þ
Q 1416 ¼ c13 m13 ðT 14 T 16 Þ
ð19Þ
Q 2524 ¼ c24 m24 ðT 25 T 24 Þ
ð20Þ
The energy of the hot water tank is balanced as
Q 2524 þ Q 2019 ¼ Q 2623
ð21Þ
Q 2623 ¼ c26 m26 ðT 26 T 23 Þ
ð22Þ
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Table 3 Ultimate analysis of the product gas [21]. Biomass
CO (%)
CO2 (%)
H2 (%)
CH4 (%)
CmHn (%)
N2 (%)
O2 (%)
LHV (kJ Nm3)
Straw
17.60
14.00
8.50
1.360
0.10
56.74
1.70
3663.5
Fig. 7. Sankey diagram of the energy (top) and exergy (bottom) flows in summer condition.
3.2. Exergy analysis The total exergy of the material stream flow is defined as
_ ¼ Ex _ ph þ Ex _ ch þ Ex _ ki þ Ex _ po Ex
ð23Þ
_ Ex _ ph ; Ex _ ch ; Ex _ ki , and Ex _ po are the total, physical, chemical, where Ex; kinetic, and potential exergy rates of the material stream (kW),
respectively. The kinetic and potential exergies can generally be neglected. The physical exergy of biomass is neglected, and the exergy of solid biomass fuel is equal to its chemical exergy obtained from its LHV as follows [22]:
exb ¼ bLHV b
ð24Þ
J.-J. Wang et al. / Applied Energy 142 (2015) 317–327 Table 4 Energy and exergy efficiencies of the biomass CCHP system.
a
323
4. Results and analysis
Component
Energy efficiency
Exergy efficiency
Gasifier HPHE Gas conditioning ICE Absorption chiller/heater PHE
47.59 94.09 94.15 35.00 137.00/93.00/93.00a 98.97
34.19 35.70/18.02/2.71a 98.54 39.22 9.62/14.70/13.23a 30.44
Biomass CCHP system
50.00/37.77/36.95a
9.32/12.51/13.79a
Summer/winter/transitional season.
4.1. Building loads This section analyzes the proposed biomass CCHP system for a hypothetical travel hotel located near a national park. The hourly loads are simulated in the software DeST [25], as shown in Fig. 6. The annual peak loads of cooling, heating, power, and domestic hot water are 1804, 595, 446, and 323 kW, respectively. 4.2. Properties of biomass and product gas The properties of biomass and product gas are shown in Tables 1 and 3, respectively [21].
120% 100%
destruction, (%)
Energy
80%
4.3. Energy and exergy analysis
Exergy
60% 40% 20% 0% - 20% gasificer
gas conditioning
ICE
HPHE
PHE
absorption unit
Energy
104.83%
5.23%
4.03%
0.34%
0.14%
-14.56%
Exergy
70.19%
0.51%
6.74%
0.84%
0.75%
20.96%
Fig. 8. Energy/exergy destruction distribution as a percentage of the total energy/ exergy loss (2585 kW/5603 kW) in summer condition.
where exb is the biomass exergy (kJ kg1), and the multiplication factor b is calculated as [22]
b¼
4.3.1. Performance in summer A Sankey diagram of the energy and exergy flows in summer is presented in Fig. 7. From the energy flows, it can be observed that the domestic hot water produced by the PHE is sufficient to meet the demand, and the absorption chiller/heater does not necessarily produce hot water at the peak electricity load. In addition, it is apparent that the heat loss during gasification is the largest, and this loss reaches approximately 52.41% of bioenergy. Only 8.62% of the biomass feedstock is converted to net electricity, 34.89% of it is used for producing chilled water for space cooling, and 6.48% of it is used for producing hot water. The total energy efficiency in summer is approximately 50.00%. Compared with the energy flows, the exergy flows of chilled water is obviously different. The energy efficiency of the absorp-
1:044 þ 0:0160ðzH =zC Þ 0:3493ðzO =zC Þð1 þ 0:0531ðzH =zC ÞÞ þ 0:0493ðzN =zC Þ for zO =zC 6 2:0 1 0:4124ðzO =zC Þ
The physical exergy of the pure material stream is defined with respect to a restricted dead state, which is characterized by the reference temperature (T0 = 298 K) and pressure (P0 = 1 atm). The specific physical exergy is calculated from the following generalized equation:
exph i ¼ ðhi h0 Þ T 0 ðsi s0 Þ
ð25Þ
where i is the state point, 0 is the state point at the exergy reference environment, and h and s represent the specific enthalpy and entropy (kJ mol1 or kJ kg1), respectively. The values of the specific enthalpy and entropy of some gases are shown in Table 2 [23,24]. The differences are determined as
hi h0 ¼ si s0 ¼
Z
Z
Ti
cp dT
ð26Þ
cp ln Pi dT R T P0
ð27Þ
T0 Ti
T0
where cp is the specific heat capacity in kJ kmol1 K1 at a constant is the universal gas constant (8.314 kJ kmol1 K1). pressure, and R Chemical exergy is considered when useful work can be extracted through the chemical reaction at the reference temperature and pressure. The specific chemical exergy of an ideal gas mixture can be evaluated as [24]
exch i ¼ RT 0
X X nj ln nj þ nj exch j j
ð28Þ
j
where nj is the mole fraction, and exch j is the standard chemical exergy of the j-th component. The standard chemical exergy values of selected components are shown in Table 2 [23,24].
tion chiller is approximately 137%, while its exergy efficiency is only 9.62%. The exergy loss of the gasifier is larger than the energy loss, and this loss reaches approximately 65.81% of bioenergy. Only 7.46% of biomass exergy is converted to net electricity, 1.54% of it is used for producing chilled water, and 0.30% of it is used for producing hot water. The total exergy efficiency is only 9.32%. The energy and exergy efficiencies of different components are listed in Table 4. Then, the energy and exergy destructions of components are discussed, which are shown in Fig. 8. The largest energy destruction occurs in the gasifier, and it results in a loss corresponding to 52.41% of the energy input and 104.83% of the total energy loss in the plant (because the energy efficiency of the absorption chiller is larger than 100%). Approximately 5.85% of the energy input and 5.23% of the total energy loss are destroyed in the gas conditioning subsystem. The energy destruction of the ICE is approximately 65.00% of the energy input, whereas it is only approximately 4.03% of the total energy loss because the waste heat from its exhaust gas and jacket water is recovered in later processes. The energy destruction of the two heat exchangers is moderate, which is less than 6% of the energy input and approximately 0.35% of the total energy loss. Because its COP is larger than 1.0, the absorption chiller does not destroy its energy input; moreover, it compensates for 14.56% of the total energy loss. The two largest exergy destructions occur in the gasifier and the absorption chiller. Approximately 65.81% of the exergy input and 70.19% of the total plant exergy loss leave the gasifier in the form of ash and char. The exergy destruction of the absorption chiller
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Fig. 9. Sankey diagram of the energy (top) and exergy (bottom) flows in winter condition. 100% Energy
80%
Exergy
destruction, (%)
corresponds to 90.38% of the plant exergy input and 20.96% of the total exergy loss in the plant. The exergy destruction in the ICE is moderate in that it is slightly more than 60% of the plant exergy input and almost 7% of the total exergy loss in the plant. The exergy destructions of the heat exchangers and gas conditioning subsystem are less than 1% of the total exergy loss. Of the total exergy loss (approximately 5603 kW), approximately 70.19% is lost in the gasifier, 20.96% in the absorption chiller, and 6.74% in the ICE. These results emphasize the importance of the gasification component.
60% 40% 20% 0% -20%
4.3.2. Performance in winter The Sankey diagrams in winter are shown in Fig. 9. The flows in winter are similar to those in summer. However, the biomass feedstock is less than in summer condition. 37.77% of biomass is converted to energy outputs (i.e., 12.24% for electricity, 16.33% for
gasificer
absorption unit
ICE
gas conditioning
HPHE
PHE
Energy
84.23%
6.55%
4.59%
4.20%
0.27%
0.15%
Exergy
75.22%
11.68%
10.25%
0.55%
1.15%
1.14%
Fig. 10. Energy/exergy destruction distribution as a percentage of the total energy/ exergy loss (2267 kW/3684 kW) in winter condition.
J.-J. Wang et al. / Applied Energy 142 (2015) 317–327
325
Fig. 11. Sankey diagram of the energy (top) and exergy (bottom) flows in transitional season condition.
space heating and 9.20% for hot water). However, only 12.51% of biomass is converted to exergy outputs (i.e., 10.59% for electricity, 1.50% for space heating, and 0.43% for hot water). Compared with the component’s efficiencies in Table 4, the total energy efficiency in winter is less than that in summer, whereas the total exergy efficiency in winter is larger than that in summer. The exergy efficiency of the HPHE in winter (18.02%) is less than that in summer because the temperature of the cold fluid side is higher in summer than in winter. The exergy efficiency of the absorption chiller increases from 9.62% in summer to 14.70% in winter because of the different products. Fig. 10 shows the energy/exergy destruction distribution of each component in winter. Of the total energy loss (approximately 2267 kW), the largest energy destruction occurs in the gasifier,
which results in a loss corresponding to 52.41% of the energy input and 84.23% of the total energy loss. The energy destructions of the absorption chiller/heater, the ICE, and gas conditioning are moderate and correspond to slightly more than 5%. The energy destruction of both heat exchangers is less than 0.30% of the total energy loss. The order of exergy loss of the components in winter is the same as that in summer. However, the exergy loss of the absorption chiller/heater increases, whereas other components decrease. The largest exergy loss occurs in the gasifier and is higher 5.03% in winter than in summer. The exergy loss of the chiller/heater decreases from 20.96% to 11.68% in winter. The exergy loss of the ICE increases by 3.51% in winter. The exergy losses of the heat exchangers and gas conditioning slightly increase.
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100% 80% 60% 40% 20% 0%
Case A Case B
30
20
10
0
-20% gasificer
absorption unit
Energy
83.13%
6.82%
5.45%
4.15%
0.27%
0.18%
Exergy
76.34%
7.81%
12.51%
0.56%
1.40%
1.39%
ICE
gas conditioning
PHE
HPHE
Fig. 12. Energy/exergy destruction distribution as a percentage of the total energy/ exergy loss (1911 kW/3020 kW) in transitional season condition.
2500
10
Exergy efficiency (%)
destruction, (%)
Exergy
Energy efficiency (%)
Energy
5
Biomass consumption (Mg)
0 summer
Case A Case B
1500
Transitional season
Annual
Fig. 14. Energy and exergy efficiencies of two biomass CCHP cases.
1000
4.3.4. Annual performance To show the annual energy and exergy off-design performance, the variation in the electricity generation efficiency with the load factor is expressed as [26]
500 0 120
Surplus heat ratio (%)
winter
2000
100
8 1:24f e > > > < 0:12f e þ 0:28 ge ¼ > 0:04f > e þ 0:32 > : 0:35
80 60 40 20 0 summer
winter
Transitional season
Annual
Fig. 13. Biomass consumptions and surplus heat output of two biomass CCHP cases.
0:00 < f e 6 0:25 0:25 < f e 6 0:50 0:50 < f e 6 0:75
ð29Þ
0:75 < f e 6 1:00
and the cooling COP of the absorption chiller/heater is fitted to the following expression based on the experimental data: 5
4
3
2
COPc ¼ 2:4645f c 9:9251f c þ 15:6980f c 12:3560f c þ 4:9003f c þ 0:5917 4.3.3. Performance in transitional season The Sankey diagrams in the transitional seasons are shown in Fig. 11. The heat recovered by the HPHE is used to preheat the domestic water. The domestic hot water produced by the absorption chiller and the PHE is larger than the demand in the designed conditions. Here, in the case where the surplus hot water is included in the useful products, the energy efficiency is approximately 36.95%. 14.72% of biomass is converted to net electricity and 22.22% is used for producing hot water. In the case of excluding the surplus heat, the energy efficiency is only 25.79%. Therefore, the surplus heat must be used or stored when the ICE operates at the peak load. From exergy analysis, only 12.74% of the total exergy of biomass is used for generating electricity and 1.03% is converted to produce hot water. Compared between the efficiencies in Table 4, the exergy efficiency of the HPHE is only 2.71% because of the large difference in fluid temperature. The total exergy efficiency in the transitional seasons, 13.79%, is slightly larger than that in winter, but the energy efficiency in the transitional seasons is less than that in winter. The energy and exergy destructions are shown in Fig. 12, which are similar to those in winter, as shown in Fig. 10. By contrast, the ICE has the second greatest contribution to the total exergy loss, and the chiller/heater is the third.
ð30Þ
where fe and fc are the electricity and cooling load factors, respectively. To indicate the contribution of the HPHE to the energy and exergy efficiencies, the two following biomass CCHP cases are compared: Case A: the biomass CCHP system with a HPHE. Case B: the biomass CCHP system without a HPHE. The annual biomass consumption and efficiencies are calculated, as shown in Figs. 13 and 14, respectively. The energy efficiency of case A is the highest during summer at approximately 37%, which is less than the energy efficiency in the design condition. The lowest energy efficiency occurs during the transitional seasons at 16%, and the annual energy efficiency is approximately 28%. The improvements in energy efficiency of case A compared with case B during winter and summer are the largest at more than 5%. The improvement during the transitional seasons is only 0.14%, and the annual improvement is approximately 4%. The exergy efficiencies during the different seasons are almost equivalent, slightly more than 9%. The improvements in the exergy efficiency of case A compared with case B are the same as those in energy efficiency.
J.-J. Wang et al. / Applied Energy 142 (2015) 317–327
Fig. 14 and Table 4 show that the annual performance is much less than the performance in the design conditions. Analysis of the input and output of CCHP system shows that the production of surplus domestic hot water is accompanied by electricity generation. In Fig. 13, the surplus heat during the transitional season is the largest, which is 105% more than the demand. The surplus heat during summer is the least, approximately 16%, and the annual surplus heat is 35% more than the energy demand. If the surplus heat is fully used by other users, the annual energy efficiency reaches 38% (i.e., 10% more than that of case A), and the exergy efficiency also improves by approximately 1%. The improvement in the energy and exergy efficiencies of case A compared with case B reaches 5% and 4%, respectively. Therefore, integrating heat storage in the system is necessary to improve efficiency.
5. Conclusions This study designed a CCHP system based on biomass air gasification. To recover waste heat from the HT product gas, a HPHE is integrated to produce the required heat in different work conditions. The operation flows and modes are presented, and the energy and exergy performances are analyzed. The following conclusions are obtained: A large fraction of the losses, mainly in the form of heat loss, occurs at the gasifier, which has an energy efficiency of 47.59% and accounts for more than 80% of the total energy loss. The heat exchangers and the absorption chiller/heater have an energy efficiency of above 90% and an exergy efficiency of below 36%. This is due to the large temperature difference between the fluids on the two sides. The gasifier reactor causes the greatest exergy destruction, which is more than 70% of the total exergy loss. Almost 14% of the total exergy loss in the plant occurs in the absorption chiller/heater, 10% in the ICE, and less than 3% in the heat exchangers and gas conditioning system. The energy efficiency of the proposed biomass CCHP system is at its highest during summer (37%) and is at its lowest during the transitional seasons (16%) because of the increase in the surplus heat output, and the annual energy efficiency is approximately 28%. The exergy efficiencies during the different seasons are almost equivalent and are slightly more than 9%. Using heat storage in the system is necessary to optimize the CCHP configuration and improve its annual performance. The proposed HPHE reduces biomass consumption by 4%. The exergy efficiency in summer is largest and it reaches approximately 35.70%, which the recovered heat by the HPHE is sent to the LT generator to produce chilled water. The integration of the HPHE improves the annual energy efficiency by 5% and the annual exergy efficiency by 4%. Acknowledgements This research has been supported by National Natural Science Foundation of China (51406054), and China Postdoctoral Science Foundation (2014M550842).
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Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Energy and exergy analyses of an integrated CCHP system with biomass air gasification Jiang-Jiang Wang a,b,⇑, Kun Yang a, Zi-Long Xu a, Chao Fu a a b
School of Energy, Power and Mechanical Engineering, North China Electric Power University, Baoding, Hebei Province 071003, China Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China
h i g h l i g h t s Propose a biomass-gasification CCHP system. A heat pipe heat exchanger is used to recover waste heat from product gas. Present the energy and exergy analyses of the biomass CCHP system. Analyze the annual off-design performances.
a r t i c l e
i n f o
Article history: Received 8 August 2014 Received in revised form 29 December 2014 Accepted 31 December 2014
Keywords: Combined cooling heating and power (CCHP) system Biomass air gasification Energy analysis Exergy analysis
a b s t r a c t Biomass-fueled combined cooling, heating, and power (CCHP) system is a sustainable distributed energy system to reduce fossil energy consumption and carbon dioxide emission. This study proposes a biomass CCHP system that contains a biomass gasifier, a heat pipe heat exchanger for recovering waste heat from product gas, an internal combustion engine to produce electricity, an absorption chiller/heater for cooling and heating, and a heat exchanger to produce domestic hot water. Operational flows are presented in three work conditions: summer, winter, and the transitional seasons. Energy and exergy analyses are conducted for different operational flows. The case demonstrated that the energy efficiencies in the three work conditions are 50.00%, 37.77%, and 36.95%, whereas the exergy efficiencies are 6.23%, 12.51%, and 13.79%, respectively. Destruction analyses of energy and exergy indicate that the largest destruction occurs in the gasification system, which accounts for more than 70% of the total energy and exergy losses. Annual performance shows that the proposed biomass-fueled CCHP system reduces biomass consumption by 4% compared with the non-use of a heat recovery system for high-temperature product gas. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Distributed combined heat and power (CHP) and combined cooling heating and power (CCHP) systems are considered worldwide as an energy-saving and environment-friendly energy system for buildings [1]. Renewable energy resources are sustainable alternatives to natural gas in driving traditional CHP/CCHP systems [2]. Among renewable energy resources, biomass is one of the most attractive options in CHP and CCHP systems to produce continuous power and simultaneously reduce carbon dioxide (CO2) emissions. Furthermore, distributed energy system also relieves the
⇑ Corresponding author at: School of Energy, Power and Mechanical Engineering, North China Electric Power University, Baoding, Hebei Province 071003, China. Tel.: +86 312 7522443; fax: +86 312 7522440. E-mail address: [email protected] (J.-J. Wang). http://dx.doi.org/10.1016/j.apenergy.2014.12.085 0306-2619/Ó 2015 Elsevier Ltd. All rights reserved.
transportation problem due to biomass feedstock and production distribution [3]. Various studies of biomass-based CHP/CCHP systems summarize the state of the art technology and near future perspectives [2,4–6]. The primary conversion technologies developed in biomass-fueled CHP/CCHP systems include combustion, gasification, pyrolysis, biochemical, and chemical processes [2]. Based on hot water, steam, gaseous, or liquid products, the secondary conversion technologies (e.g., steam turbine [7], organic Rankine cycle (ORC) [8], gas turbine [7], internal combustion engine (ICE) [3,9], and others) harness the products to produce power, heat, and even cooling. Different integrated forms of biomass conversion and utilization were proposed and researched. Biomass-fueled CHP systems have been studied over many years [3,10–12]. Mertzis et al. [3] studied the performance of a micro biomass CHP system and reported the effect of different types of biomass feedstock, gasification parameters, and engine
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Nomenclature CCHP CHP CO2 COP HHV HPHE HT ICE LHV LT MGT ORC PHE SOFC
combined cooling heating and power combined heat and power carbon dioxide coefficient of performance higher heating value heat pipe heat exchanger high temperature internal combustion engine lower heating value low temperature micro gas turbine organic Rankine cycle plate heat exchanger solid oxide fuel cells
Symbols c COP ex E _ Ex f h HHV LHV m P Q R s T
specific heat at a constant pressure (kJ kg1 K1) coefficient of performance specific exergy (kJ mol1 or kJ kg1) electricity (kW) exergy (kW) load factor specific enthalpy (kJ mol1 or kJ kg1) higher heating value (MJ kg1 or MJ Nm3) lower heating value (MJ kg1 or MJ Nm3) mass flow rate (kg s1) pressure (Pa) heat (kW) universal gas constant specific entropy (kJ mol1 K1 or kJ kg1 K1) temperature (K)
intake mixtures on long-term operation and energy output. Fryda et al. [10] investigated three micro biomass CHP configurations integrated with solid oxide fuel cells (SOFC) and micro gas turbine (MGT), and they presented an exergy analysis to optimize these configurations. Bang-Møller et al. [11] analyzed the exergy destruction of a biomass steam gasification, SOFC, and MGT hybrid plant and optimized its configuration. Huang et al. [12] compared the techno-economic performances of biomass gasification- and ORC-based CHP systems, and they emphasized that both systems are not economically viable in generating power to supply present needs. Al-Sulaiman et al. [13] reported that the energy and exergy analyses of a biomass CCHP system using an ORC and concluded that there is a significant improvement when CCHP is used in place of only electrical generation. Recently, the research has been extended from CHP to CCHP systems. Biomass combustion is the most common utilization method in CCHP systems in almost all studies. P. Ahmadi et al. designed a biomass CCHP system integrated with ORC [14], and they proposed a thermoeconomic multi-objective optimization model to improve its multi-performances, including energy, exergy and environmental impact [15]. Lian et al. [16] evaluated the thermoeconomic potentials of four different biomass CCHP configurations and concluded that exergy destruction is most extensive in the furnace, amounting to nearly 60% of net exergy loss. Compared with biomass combustion, gasification is more efficient at producing electricity and heat [17]. Puig-Arnavat et al. [17] developed a simple but rigorous CCHP model for designing, optimizing and simulating small–medium scale plants, including a realistic biomass gasification model, and achieved approximately 10% primary energy saving. Huang et al. [18] modeled and simulated a small
w x z
a g
water stoichiometric coefficient (kmol s1) percentage mass in biomass (%) concentration (%v) efficiency
Superscripts ch chemical ki kinetic ph physical po potential Subscripts A ash Air air b biomass c cooling C carbon e electricity gas product gas h heating H hydrogen I index for the thermodynamic state point N nitrogen O oxygen S sulfur tar tar uc unconverted carbon wb wet biomass 0 standard reference state
scale CCHP system integrated by a biomass downdraft gasifier, an ICE, and an absorption chiller, and they concluded that this system would be beneficial to the building in case of appropriate load characteristic. The literature survey shows that very few studies focused on the energy and exergy analyses of CCHP systems based on biomass gasification. The specific objective of the present work is to propose a biomass gasification CCHP system integrated with a heat pipe heat exchanger (HPHE). The HPHE recovers the waste heat from the high-temperature (HT) product gas for effective energy-utilizations corresponding to different operating conditions. The comprehensive energy and exergy analyses are performed to system performances of different operational flows. Exergy destructions throughout the system are determined in order to understand the source and determine the locations of irreversibilities. Section 2 presents the energy flows and operating conditions of the integrated biomass-fueled CCHP system. Section 3 analyzes the thermodynamic performance of the system. Section 4 demonstrates energy and exergy performances in a building case. Section 5 summarizes the conclusions of this study.
2. CCHP system integrated with biomass air gasification 2.1. Plant concept The concept of a CCHP system integrated with biomass air gasification is depicted in Fig. 1. The system is divided into two subsystems: the biomass air gasification and the CCHP subsystems. Biomass is converted into product gas in the gasification subsystem,
J.-J. Wang et al. / Applied Energy 142 (2015) 317–327 Biomass air gasification system Biomass Air
Product gas
Gasifier
Purifier
HT generator, and the hot water in the low-temperature (LT) generator. The chiller/heater can produce chilled water for space cooling, hot water for space heating, and domestic hot water.
Cooler
2.3. System operation Heat Building cooling, heating and power system
Product gas
319
Gas engine
Two-stage LiBr-H2O Absorption Chiller/Heater
Heat exchanger
Power
Cool
Heat
2.3.1. Operating conditions during summer During summer, the V3, V4, V5, V6, V8, and V9 valves are closed, whereas the others are open in Fig. 2. The flow is shown in Fig. 3. The heat recovered through the HPHE is transmitted to the LT generator of the absorption chiller. 2.3.2. Operating conditions during winter During winter, the V3, V4, V7, V8, and V9 valves are open, whereas the other valves are closed in Fig. 2, which is simplified to Fig. 4. The heat recovered through the HPHE is used to preheat hot water for space heating.
Domestic hot water
Fig. 1. Plant concept of integrated CCHP system with biomass air gasification.
and the product gas is used as fuel to produce power, cooling, and heating in the CCHP subsystem. After undergoing air gasification, the product gas consists of product gas, tar, steam, and dust. Its temperature is approximately 573–673 K or higher. The HT product gas is purified, cooled, and then sent to the prime mover. During the cooling process, the waste heat is removed by cooling water. In the CCHP subsystem, the ICE combusts the product gas to generate power. The jacket water from the ICE is then used to produce domestic hot water through the heat exchanger, and the exhausted gas is sent to a two-stage Libr-H2O absorption chiller/heater to produce cooling in summer and heating in winter, respectively. The product gas serves as a supplement when the exhausted gas is insufficient to meet the cooling/heating demand.
2.3.3. Operating conditions during the transitional season The integrated CCHP system produces electricity and domestic hot water during the transitional seasons. The V5 and V6 valves are open, whereas the other valves are closed in Fig. 2. The flow is shown in Fig. 5. The heat recovered through the HPHE is used to preheat domestic hot water. 3. Thermodynamic analysis 3.1. Energy analysis The higher heating value (HHV) of biomass can be predicted by the following correlation [19]:
HHV b ¼ 0:3491zC þ 1:1783zH 0:1034zO 0:0151zN þ 0:1005zS 0:0211zA
ð1Þ
The lower heating value (LHV) is calculated as [20]: 2.2. Integrated design Waste heat is emitted in the gasification subsystem, and this waste heat can be used in the cooling/heating subsystem. Consequently, the waste heat during gasification is integrated into the CCHP system, as shown in Fig. 2. The CCHP system integrated with biomass air gasification mainly includes the following components: gasifier, HPHE, product gas conditioning subsystem (i.e., cyclone, spray scrubber), Roots blower, gas storage tank, ICE, two-stage Libr-H2O absorption chiller/heater, plate heat exchanger (PHE), and hot water tank. The biomass with air is gasified in the gasifier, and then the HT product gas is cooled by the HPHE. The HT biomass tar in the product gas is condensed below 473 K, and it easily combines with water and carbon granules to pollute the equipment. Consequently, the HT gas is to be cooled above 473 K. The cooled product gas is then purified in the cyclone and further cooled in the spray scrubber, and the tar is separated. The clean product gas is stored in a tank as fuel. The waste heat recovery system of the HPHE is integrated with the CCHP system. To maximize the use of the waste heat, three integration forms can be utilized to produce the required heat during different seasonal conditions (see Section 2.3). The HPHE is used for the following reasons: (1) changing the heat transfer area to adjust the temperature of the heat pipe easily avoids the corrosion interval of mixed gas, and (2) adjusting the structure and shape of the heat pipe relieves abrasion and ash clogging. The two-stage Libr-H2O absorption chiller/heater can utilize heat at various temperatures. The heater is driven by three types of energy: the exhausted gas from the ICE, the product gas in the
LHV b ¼ HHV b 21:978zH
ð2Þ
where HHVb and LHVb are the HHV and LHV, respectively, (MJ kg1), and zC, zH, zO, zN, zS, and zA are the percentages of carbon, hydrogen, oxygen, nitrogen, sulfur, and ash in the raw biomass, respectively, (%). The proximate and ultimate analysis of biomass in this study is shown in Table 1 [21]. The biomass energy into the gasifier is expressed to
Q 1 ¼ m1 LHV b
ð3Þ
and the energy in the gasification system is balanced with
Q 7 ¼ ggasification Q 1 ¼ Q 8 þ Q 9
ð4Þ
where Q1 is the biomass heat (kW), m1 is the biomass flow rate (kg s1), Q7 is the heat of product gas (kW), ggasification is the gasification efficiency, Q8 and Q9 are the heat of product gas to the ICE and absorption chiller/heater, respectively, (kW). To describe clearly and simplify the symbol expression, the following symbols are used in Eqs. (3)–(22): m is the mass flow rate (kg s1), c is the specific heat at a constant pressure (kJ kg1 K1), T is the temperature (K), and the subscript denotes the state point. The energy balances of the HPHE are expressed as
Q 56 ¼ c5 m5 ðT 5 T 6 Þ
ð5Þ
Q 1615 ¼ c15 m15 ðT 16 T 15 Þ Q 1615 ¼ ghepipe Q 56
ð6Þ ð7Þ
where Q5–6 and Q16–15 are the released heat from HT product gas and the absorbed heat of LT water in the HPHE, respectively, (kW), and ghe-pipe is the HPHE efficiency.
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Product gas conditioning Biomass
Heat Pipe Heat Exchanger
1 5
Cyclone
Spray scrubber
6 15
16
V1
V2
Roots Blower
Gas Storage Tank
Gasifier Air
V5
V3
V4 V6
2
Product gas
Cool water
7
Exhausted gas 23
24
V7 13
11
14
Product gas Air 3
Two-stage LiBr-H2O Absorption Chiller/Heater HT Generator
8
V14
Exhausted 10 gas
Gas ICE
22
9 Jacket water 17 19 Plate Heat Exchanger
Condenser
LT Generator
Air 4
18
Cooling water to cooling tower
V17 HT Solution Heat Exchanger 20 V12
V13
LT Solution Heat Exchanger
V15
25 V16 Evaporator
Absorber Hot water tank V9 26
V8 V10
21 Cooling water from cooling tower
Domestic 12 hot water 13 14 V11 Chilled/hot water of air Electricity condition system Hot water Air Electricity Product gas
Chilled water 13
Chilled water Cooling water
14
Cool water Exhausted gas
Fig. 2. Flowchart of the integrated CCHP system with biomass air gasification.
The input energy and output electricity of the ICE are, respectively expressed as
Q 8 ¼ m8 LHV prodcutgas E12 ¼ ge Q 8
ð8Þ ð9Þ
where E12 is the generated electricity of the ICE (kW), and ge is the generation efficiency of the ICE. The energy balances of the PHE can be expressed as
Q 1718 ¼ c17 m17 ðT 17 T 18 Þ
ð10Þ
Q 2019 ¼ c20 m20 ðT 20 T 19 Þ
ð11Þ
Q 2019 ¼ gheplate Q 1718
ð12Þ
where Q17–18 and Q20–19 are the released heat from HT water and the absorbed heat of LT water in the PHE, respectively, (kW), and ghe-plate is the PHE efficiency. The integration of the absorption chiller/heater varies during different seasonal conditions, so the energy balance can be expressed as follows: During summer:
Q 9 þ Q 1011 þ Q 1516 ¼
Q 1314 Q 2524 þ COP c COP h
ð13Þ
During winter:
Q 9 þ Q 1011 ¼
Q 1416 þ Q 2524 COP h
ð14Þ
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1 Biomass
5
Gasifier
Air
2 23 13
6
Water
15
14 Chilled water
9
8
Two-stage LiBr-H2O Absorption Chiller/Heater
Exhausted gas
Gas
25
Air
ICE
10
Air
Table 1 Fuel properties of solid biomass [21].
7 Product gas
16
24
11
4
Gas conditioning
3
17
Tank 20 18 22
21 Cooling water
26
19
12
Domestic hot water
Water
Electricity
Type of fuel
Straw
Proximate analysis (% wet fuel): Volatile Moisture Ash Fixed carbon
69.70 10.30 4.20 15.80
Ultimate analysis (% dry fuel): Carbon Hydrogen Oxygen Nitrogen Sulfur Chlorine Ash HHV, MJ/kg LHV, MJ/kg b
45.80 5.96 40.00 0.45 0.16 0.03 7.60 18.72 17.41 1.156
Fig. 3. Summer operation condition.
1 Biomass Air
2
5
Gasifier
13
15
23 Water 14 Hot water 11 Exhausted gas
Gas conditioning
Table 2 Specific enthalpy, entropy, and standard chemical exergy values of some materials at 298 K and 1 atm [23,24].
7 Product gas
16
24
9
8
Two-stage LiBr-H2O Absorption Chiller/Heater
Gas
Air
ICE
10 25
Air
4
6
Component
h0 (kJ kmol1)
s0 (kJ kmol1 K1)
exch (kJ kmol1)
CO CO2 H2 CH4 N2 O2
137150 394374 0 74850 0 0
197.543 213.685 130.574 186.16 191.61 205.033
275100 19870 236100 831650 668 3970
3
17
Tank
2000
20 18 26
19
12
Domestic hot water
Water
Electricity
Cooling Heating
load (kW)
1600
Fig. 4. Winter operation condition.
1200 800 400 0
Air
2
5
6
Gasifier 15
Gas conditioning
Load (kW)
1
1 Biomass
7 Product gas
16
2001
3001
4001
5001
6001
7001
8001
Power 400 0
24 11 Exhausted gas
9
8
Two-stage LiBr-H2O Absorption Chiller/Heater
Gas ICE
10 25
Air
Air 3
1001
2001
3001
4001
5001
6001
7001
8001
7001
8001
Domestic hot water
400 0 1
1001
2001
17
3001
4001
5001
6001
Fig. 6. The hourly loads of cooling, heating, power and domestic hot water.
20 18 26
19
12
Domestic hot water
Water
Electricity
in winter (kW), Q25–24 is the absorbed heat of domestic hot water from absorption chiller/heater (kW), COPc and COPh are the cooling coefficient of performance (COP) and heating COP, respectively, and
Q 9 ¼ m9 LHV prodcut
Fig. 5. Transitional season operation condition.
During the transitional seasons:
Q 9 þ Q 1011
1 800
time (h)
Tank
Q ¼ 2524 COP h
Load (kW)
23
4
1001
800
ð15Þ
where Q10–11 is the released heat of exhausted gas from the ICE (kW), Q13–14 is the released cooling of chilled water (kW), Q14–16 is the absorbed heat of hot water from absorption chiller/heater
gas
ð16Þ
Q 1011 ¼ c10 m10 ðT 10 T 11 Þ Q 1314 ¼ c13 m13 ðT 13 T 14 Þ
ð17Þ ð18Þ
Q 1416 ¼ c13 m13 ðT 14 T 16 Þ
ð19Þ
Q 2524 ¼ c24 m24 ðT 25 T 24 Þ
ð20Þ
The energy of the hot water tank is balanced as
Q 2524 þ Q 2019 ¼ Q 2623
ð21Þ
Q 2623 ¼ c26 m26 ðT 26 T 23 Þ
ð22Þ
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J.-J. Wang et al. / Applied Energy 142 (2015) 317–327
Table 3 Ultimate analysis of the product gas [21]. Biomass
CO (%)
CO2 (%)
H2 (%)
CH4 (%)
CmHn (%)
N2 (%)
O2 (%)
LHV (kJ Nm3)
Straw
17.60
14.00
8.50
1.360
0.10
56.74
1.70
3663.5
Fig. 7. Sankey diagram of the energy (top) and exergy (bottom) flows in summer condition.
3.2. Exergy analysis The total exergy of the material stream flow is defined as
_ ¼ Ex _ ph þ Ex _ ch þ Ex _ ki þ Ex _ po Ex
ð23Þ
_ Ex _ ph ; Ex _ ch ; Ex _ ki , and Ex _ po are the total, physical, chemical, where Ex; kinetic, and potential exergy rates of the material stream (kW),
respectively. The kinetic and potential exergies can generally be neglected. The physical exergy of biomass is neglected, and the exergy of solid biomass fuel is equal to its chemical exergy obtained from its LHV as follows [22]:
exb ¼ bLHV b
ð24Þ
J.-J. Wang et al. / Applied Energy 142 (2015) 317–327 Table 4 Energy and exergy efficiencies of the biomass CCHP system.
a
323
4. Results and analysis
Component
Energy efficiency
Exergy efficiency
Gasifier HPHE Gas conditioning ICE Absorption chiller/heater PHE
47.59 94.09 94.15 35.00 137.00/93.00/93.00a 98.97
34.19 35.70/18.02/2.71a 98.54 39.22 9.62/14.70/13.23a 30.44
Biomass CCHP system
50.00/37.77/36.95a
9.32/12.51/13.79a
Summer/winter/transitional season.
4.1. Building loads This section analyzes the proposed biomass CCHP system for a hypothetical travel hotel located near a national park. The hourly loads are simulated in the software DeST [25], as shown in Fig. 6. The annual peak loads of cooling, heating, power, and domestic hot water are 1804, 595, 446, and 323 kW, respectively. 4.2. Properties of biomass and product gas The properties of biomass and product gas are shown in Tables 1 and 3, respectively [21].
120% 100%
destruction, (%)
Energy
80%
4.3. Energy and exergy analysis
Exergy
60% 40% 20% 0% - 20% gasificer
gas conditioning
ICE
HPHE
PHE
absorption unit
Energy
104.83%
5.23%
4.03%
0.34%
0.14%
-14.56%
Exergy
70.19%
0.51%
6.74%
0.84%
0.75%
20.96%
Fig. 8. Energy/exergy destruction distribution as a percentage of the total energy/ exergy loss (2585 kW/5603 kW) in summer condition.
where exb is the biomass exergy (kJ kg1), and the multiplication factor b is calculated as [22]
b¼
4.3.1. Performance in summer A Sankey diagram of the energy and exergy flows in summer is presented in Fig. 7. From the energy flows, it can be observed that the domestic hot water produced by the PHE is sufficient to meet the demand, and the absorption chiller/heater does not necessarily produce hot water at the peak electricity load. In addition, it is apparent that the heat loss during gasification is the largest, and this loss reaches approximately 52.41% of bioenergy. Only 8.62% of the biomass feedstock is converted to net electricity, 34.89% of it is used for producing chilled water for space cooling, and 6.48% of it is used for producing hot water. The total energy efficiency in summer is approximately 50.00%. Compared with the energy flows, the exergy flows of chilled water is obviously different. The energy efficiency of the absorp-
1:044 þ 0:0160ðzH =zC Þ 0:3493ðzO =zC Þð1 þ 0:0531ðzH =zC ÞÞ þ 0:0493ðzN =zC Þ for zO =zC 6 2:0 1 0:4124ðzO =zC Þ
The physical exergy of the pure material stream is defined with respect to a restricted dead state, which is characterized by the reference temperature (T0 = 298 K) and pressure (P0 = 1 atm). The specific physical exergy is calculated from the following generalized equation:
exph i ¼ ðhi h0 Þ T 0 ðsi s0 Þ
ð25Þ
where i is the state point, 0 is the state point at the exergy reference environment, and h and s represent the specific enthalpy and entropy (kJ mol1 or kJ kg1), respectively. The values of the specific enthalpy and entropy of some gases are shown in Table 2 [23,24]. The differences are determined as
hi h0 ¼ si s0 ¼
Z
Z
Ti
cp dT
ð26Þ
cp ln Pi dT R T P0
ð27Þ
T0 Ti
T0
where cp is the specific heat capacity in kJ kmol1 K1 at a constant is the universal gas constant (8.314 kJ kmol1 K1). pressure, and R Chemical exergy is considered when useful work can be extracted through the chemical reaction at the reference temperature and pressure. The specific chemical exergy of an ideal gas mixture can be evaluated as [24]
exch i ¼ RT 0
X X nj ln nj þ nj exch j j
ð28Þ
j
where nj is the mole fraction, and exch j is the standard chemical exergy of the j-th component. The standard chemical exergy values of selected components are shown in Table 2 [23,24].
tion chiller is approximately 137%, while its exergy efficiency is only 9.62%. The exergy loss of the gasifier is larger than the energy loss, and this loss reaches approximately 65.81% of bioenergy. Only 7.46% of biomass exergy is converted to net electricity, 1.54% of it is used for producing chilled water, and 0.30% of it is used for producing hot water. The total exergy efficiency is only 9.32%. The energy and exergy efficiencies of different components are listed in Table 4. Then, the energy and exergy destructions of components are discussed, which are shown in Fig. 8. The largest energy destruction occurs in the gasifier, and it results in a loss corresponding to 52.41% of the energy input and 104.83% of the total energy loss in the plant (because the energy efficiency of the absorption chiller is larger than 100%). Approximately 5.85% of the energy input and 5.23% of the total energy loss are destroyed in the gas conditioning subsystem. The energy destruction of the ICE is approximately 65.00% of the energy input, whereas it is only approximately 4.03% of the total energy loss because the waste heat from its exhaust gas and jacket water is recovered in later processes. The energy destruction of the two heat exchangers is moderate, which is less than 6% of the energy input and approximately 0.35% of the total energy loss. Because its COP is larger than 1.0, the absorption chiller does not destroy its energy input; moreover, it compensates for 14.56% of the total energy loss. The two largest exergy destructions occur in the gasifier and the absorption chiller. Approximately 65.81% of the exergy input and 70.19% of the total plant exergy loss leave the gasifier in the form of ash and char. The exergy destruction of the absorption chiller
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Fig. 9. Sankey diagram of the energy (top) and exergy (bottom) flows in winter condition. 100% Energy
80%
Exergy
destruction, (%)
corresponds to 90.38% of the plant exergy input and 20.96% of the total exergy loss in the plant. The exergy destruction in the ICE is moderate in that it is slightly more than 60% of the plant exergy input and almost 7% of the total exergy loss in the plant. The exergy destructions of the heat exchangers and gas conditioning subsystem are less than 1% of the total exergy loss. Of the total exergy loss (approximately 5603 kW), approximately 70.19% is lost in the gasifier, 20.96% in the absorption chiller, and 6.74% in the ICE. These results emphasize the importance of the gasification component.
60% 40% 20% 0% -20%
4.3.2. Performance in winter The Sankey diagrams in winter are shown in Fig. 9. The flows in winter are similar to those in summer. However, the biomass feedstock is less than in summer condition. 37.77% of biomass is converted to energy outputs (i.e., 12.24% for electricity, 16.33% for
gasificer
absorption unit
ICE
gas conditioning
HPHE
PHE
Energy
84.23%
6.55%
4.59%
4.20%
0.27%
0.15%
Exergy
75.22%
11.68%
10.25%
0.55%
1.15%
1.14%
Fig. 10. Energy/exergy destruction distribution as a percentage of the total energy/ exergy loss (2267 kW/3684 kW) in winter condition.
J.-J. Wang et al. / Applied Energy 142 (2015) 317–327
325
Fig. 11. Sankey diagram of the energy (top) and exergy (bottom) flows in transitional season condition.
space heating and 9.20% for hot water). However, only 12.51% of biomass is converted to exergy outputs (i.e., 10.59% for electricity, 1.50% for space heating, and 0.43% for hot water). Compared with the component’s efficiencies in Table 4, the total energy efficiency in winter is less than that in summer, whereas the total exergy efficiency in winter is larger than that in summer. The exergy efficiency of the HPHE in winter (18.02%) is less than that in summer because the temperature of the cold fluid side is higher in summer than in winter. The exergy efficiency of the absorption chiller increases from 9.62% in summer to 14.70% in winter because of the different products. Fig. 10 shows the energy/exergy destruction distribution of each component in winter. Of the total energy loss (approximately 2267 kW), the largest energy destruction occurs in the gasifier,
which results in a loss corresponding to 52.41% of the energy input and 84.23% of the total energy loss. The energy destructions of the absorption chiller/heater, the ICE, and gas conditioning are moderate and correspond to slightly more than 5%. The energy destruction of both heat exchangers is less than 0.30% of the total energy loss. The order of exergy loss of the components in winter is the same as that in summer. However, the exergy loss of the absorption chiller/heater increases, whereas other components decrease. The largest exergy loss occurs in the gasifier and is higher 5.03% in winter than in summer. The exergy loss of the chiller/heater decreases from 20.96% to 11.68% in winter. The exergy loss of the ICE increases by 3.51% in winter. The exergy losses of the heat exchangers and gas conditioning slightly increase.
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100% 80% 60% 40% 20% 0%
Case A Case B
30
20
10
0
-20% gasificer
absorption unit
Energy
83.13%
6.82%
5.45%
4.15%
0.27%
0.18%
Exergy
76.34%
7.81%
12.51%
0.56%
1.40%
1.39%
ICE
gas conditioning
PHE
HPHE
Fig. 12. Energy/exergy destruction distribution as a percentage of the total energy/ exergy loss (1911 kW/3020 kW) in transitional season condition.
2500
10
Exergy efficiency (%)
destruction, (%)
Exergy
Energy efficiency (%)
Energy
5
Biomass consumption (Mg)
0 summer
Case A Case B
1500
Transitional season
Annual
Fig. 14. Energy and exergy efficiencies of two biomass CCHP cases.
1000
4.3.4. Annual performance To show the annual energy and exergy off-design performance, the variation in the electricity generation efficiency with the load factor is expressed as [26]
500 0 120
Surplus heat ratio (%)
winter
2000
100
8 1:24f e > > > < 0:12f e þ 0:28 ge ¼ > 0:04f > e þ 0:32 > : 0:35
80 60 40 20 0 summer
winter
Transitional season
Annual
Fig. 13. Biomass consumptions and surplus heat output of two biomass CCHP cases.
0:00 < f e 6 0:25 0:25 < f e 6 0:50 0:50 < f e 6 0:75
ð29Þ
0:75 < f e 6 1:00
and the cooling COP of the absorption chiller/heater is fitted to the following expression based on the experimental data: 5
4
3
2
COPc ¼ 2:4645f c 9:9251f c þ 15:6980f c 12:3560f c þ 4:9003f c þ 0:5917 4.3.3. Performance in transitional season The Sankey diagrams in the transitional seasons are shown in Fig. 11. The heat recovered by the HPHE is used to preheat the domestic water. The domestic hot water produced by the absorption chiller and the PHE is larger than the demand in the designed conditions. Here, in the case where the surplus hot water is included in the useful products, the energy efficiency is approximately 36.95%. 14.72% of biomass is converted to net electricity and 22.22% is used for producing hot water. In the case of excluding the surplus heat, the energy efficiency is only 25.79%. Therefore, the surplus heat must be used or stored when the ICE operates at the peak load. From exergy analysis, only 12.74% of the total exergy of biomass is used for generating electricity and 1.03% is converted to produce hot water. Compared between the efficiencies in Table 4, the exergy efficiency of the HPHE is only 2.71% because of the large difference in fluid temperature. The total exergy efficiency in the transitional seasons, 13.79%, is slightly larger than that in winter, but the energy efficiency in the transitional seasons is less than that in winter. The energy and exergy destructions are shown in Fig. 12, which are similar to those in winter, as shown in Fig. 10. By contrast, the ICE has the second greatest contribution to the total exergy loss, and the chiller/heater is the third.
ð30Þ
where fe and fc are the electricity and cooling load factors, respectively. To indicate the contribution of the HPHE to the energy and exergy efficiencies, the two following biomass CCHP cases are compared: Case A: the biomass CCHP system with a HPHE. Case B: the biomass CCHP system without a HPHE. The annual biomass consumption and efficiencies are calculated, as shown in Figs. 13 and 14, respectively. The energy efficiency of case A is the highest during summer at approximately 37%, which is less than the energy efficiency in the design condition. The lowest energy efficiency occurs during the transitional seasons at 16%, and the annual energy efficiency is approximately 28%. The improvements in energy efficiency of case A compared with case B during winter and summer are the largest at more than 5%. The improvement during the transitional seasons is only 0.14%, and the annual improvement is approximately 4%. The exergy efficiencies during the different seasons are almost equivalent, slightly more than 9%. The improvements in the exergy efficiency of case A compared with case B are the same as those in energy efficiency.
J.-J. Wang et al. / Applied Energy 142 (2015) 317–327
Fig. 14 and Table 4 show that the annual performance is much less than the performance in the design conditions. Analysis of the input and output of CCHP system shows that the production of surplus domestic hot water is accompanied by electricity generation. In Fig. 13, the surplus heat during the transitional season is the largest, which is 105% more than the demand. The surplus heat during summer is the least, approximately 16%, and the annual surplus heat is 35% more than the energy demand. If the surplus heat is fully used by other users, the annual energy efficiency reaches 38% (i.e., 10% more than that of case A), and the exergy efficiency also improves by approximately 1%. The improvement in the energy and exergy efficiencies of case A compared with case B reaches 5% and 4%, respectively. Therefore, integrating heat storage in the system is necessary to improve efficiency.
5. Conclusions This study designed a CCHP system based on biomass air gasification. To recover waste heat from the HT product gas, a HPHE is integrated to produce the required heat in different work conditions. The operation flows and modes are presented, and the energy and exergy performances are analyzed. The following conclusions are obtained: A large fraction of the losses, mainly in the form of heat loss, occurs at the gasifier, which has an energy efficiency of 47.59% and accounts for more than 80% of the total energy loss. The heat exchangers and the absorption chiller/heater have an energy efficiency of above 90% and an exergy efficiency of below 36%. This is due to the large temperature difference between the fluids on the two sides. The gasifier reactor causes the greatest exergy destruction, which is more than 70% of the total exergy loss. Almost 14% of the total exergy loss in the plant occurs in the absorption chiller/heater, 10% in the ICE, and less than 3% in the heat exchangers and gas conditioning system. The energy efficiency of the proposed biomass CCHP system is at its highest during summer (37%) and is at its lowest during the transitional seasons (16%) because of the increase in the surplus heat output, and the annual energy efficiency is approximately 28%. The exergy efficiencies during the different seasons are almost equivalent and are slightly more than 9%. Using heat storage in the system is necessary to optimize the CCHP configuration and improve its annual performance. The proposed HPHE reduces biomass consumption by 4%. The exergy efficiency in summer is largest and it reaches approximately 35.70%, which the recovered heat by the HPHE is sent to the LT generator to produce chilled water. The integration of the HPHE improves the annual energy efficiency by 5% and the annual exergy efficiency by 4%. Acknowledgements This research has been supported by National Natural Science Foundation of China (51406054), and China Postdoctoral Science Foundation (2014M550842).
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