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Thermodynamic performance assessment of CCHP system driven by different composition gas
Applied Energy 136 (2014) 599–610
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Thermodynamic performance assessment of CCHP system driven by different composition gas Penghui Gao a,c,⇑, Wangliang Li c, Yongpan Cheng c, YenWah Tong b,c, Yanjun Dai d, Ruzhu Wang d a
School of Architecture and Civil Engineering, China University of Mining and Technology, Xuzhou 221116, China Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576, Singapore c Environmental Research Institute, National University of Singapore, Singapore 117576, Singapore d School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China b
h i g h l i g h t s Gas components of anaerobic digestion gas and thermal cracking gas were analyzed. Thermodynamic model of CCHP driven by different compositions gas was proposed. Performance of CCHP was studied under the design and off-design conditions. Different composition gas had evident effect on the performance of CCHP.
a r t i c l e
i n f o
Article history: Received 2 July 2014 Received in revised form 2 September 2014 Accepted 22 September 2014
Keywords: CCHP (Combined Cooling Heating and Power) Gas Thermodynamic analysis
a b s t r a c t In order to facilitate sustainable solutions in megacities, waste-to-energy study of municipal solid waste handling is conducted including anaerobic digestion and thermal cracking, together with power generation technologies and the utilization of residual waste heat. In this paper, performance of CCHP (Combined Cooling Heating and Power) system, which is driven by different composition gases that the main components are CH4, H2, CO, SH2, C2H4, etc. is evaluated. The related parameters of the system, such as PER (primary energy ratio) of CCHP, exergy efficiency of CCHP gex, the efficiency of gas engine g, and COP (Coefficient of Performance) of absorption refrigeration, were analyzed in different compression ratio of gas engine and under off-design conditions. The results indicated that the different composition gases had evident effect on the performance of CCHP. For the waste handling and waste-to-energy in metropolis, the gas input to CCHP system form anaerobic digestion and thermal cracking (pyrolysis) should be determined by considering all kinds of factors, such as cost of gas, efficiency of gas engine, cooling and heat load. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Great consumption of fossil fuels, global warming, environment deterioration and municipal waste increment have attracted many researchers to find more scientific and efficient methods of energy conservation, recycling of waste, and reducing greenhouse gas emissions as well as pollutants. For the municipal solid wastes, there are two methods to recycle which are anaerobic digestion and thermal cracking. Anaerobic digestion is a series of biological processes in which microorganisms break down biodegradable ⇑ Corresponding author at: School of Architecture and Civil Engineering, China University of Mining and Technology, Xuzhou 221116, China. Tel.: +86 (516)83882193; fax: +86 (516)83885478. E-mail addresses: [email protected], [email protected] (P. Gao). http://dx.doi.org/10.1016/j.apenergy.2014.09.070 0306-2619/Ó 2014 Elsevier Ltd. All rights reserved.
material in the absence of oxygen and one of the end products is biogas, which can be combusted to generate electricity and heat [1,2]. For the digestion process, temperature, pH, buffering capacity and fatty acid concentrations are main influence factors. Thermal cracking, that biomass and/or solid wastes are converted to syngas containing hydrogen, carbon monoxide, and methane, is the another method and provides an attractive alternative process to generate energy. A number of researchers have performed experimental studies on thermal cracking [3–5]. For the thermal cracking, operation temperature and raw material are important factors. However, gas compositions of anaerobic digestion and thermal cracking are different and change with the operational conditions. In order to facilitate the understanding, design and implementation of sustainable solutions for megacities, it should
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Nomenclature COP h q n H V
g V0 M d N C FR m 0 e r t
c I R
q
coefficient enthalpy (kJ/kg) heat capacity (kJ/kg) solution concentration heat value (kJ/N m3) volume fraction (N m3/N m3 dry gas) efficiency gas consumption (N m3/s) molecular moisture content (kg/N m3 dry gas) output work of gas engine (W) volume specific heat at constant pressure (kJ/(N m3 K) flow ratio mass flow rate (kg/s) ideal state effectiveness of heat exchanger volume of component temperature (°C) compression ratio flue gas enthalpy (kJ/N m3 dry gas) gas constant density (kg/N m3)
be made clear the effect of gas compositions on the performance of CCHP system from anaerobic digestion and/or thermal cracking. In recent years, the development of polygeneration systems has received an increasing attention in the area of small scale power systems for different applications ranging from residential to utilities [6,7]. CCHP, being a typical polygeneration system, is a distributed generation technology that reduces the use of electricity from the grid by using low emission fuels such as natural gas, anaerobic digestion gas and thermal cracking gas. For the realization of the energy cascade utilization, the overall fuel energy utilization of CCHP systems can be as much as 70–90%, which is higher than that of conventional independent energy supply systems [8]. Moreover, CCHP systems can improve the stability and power quality considerably by making up the peak-loads; for micro-grid with distributed energy systems, CCHP systems can meet the various energy demands from diverse users by adjusting operating modes. Many researchers have studied CCHP systems from different aspects such as operation model [9–12], optimization [13–16], feasibility analysis [17–20] and evaluation [21–24]. For the operation of CCHP systems, Mago et al. [9] analyzed the performance of CCHP and CHP systems following the electric load and dioxide emissions for different climate conditions. Fumo and Chamra [10] carried out the analysis of CCHP systems according to the source primary energy consumption. Wang et al. [11] studied the performance of a new CCHP system with a transcriptical CO2 refrigeration cycle, which was driven by solar energy. The results indicated that increasing turbine inlet pressure and ejector inlet temperature could lower the efficiency of the system. Full chain energy performance was applied to a CCHP system by Li et al. [12], and the study presented that complementation utilization of solar energy and fossil fuel was energy efficient. For the optimization of CCHP systems, Hu and Cho [13] proposed a stochastic multi-objective optimization model to optimize the CCHP operation strategy for different climate conditions based on operational cost, primary energy consumption and carbon dioxide emissions. The potential of thermoeconomics for analysis of CHCP applications in buildings was explored by Cardona and Piacentino [14]. Kong et al. [15] investigated the problem of energy
Subscripts A absorber C condenser E evaporator G generator s isentropic process e electric f flue gas h heating c cooling p constant pressure v constant volume g gas a air l low value u off-design condition o design condition ex exergy RO2 three atomic gas ther gas combustion temperature WS weak solution SS strong solution
management and optimal operation of cogeneration system for micro-combined CCHP system. Ren and Gao [16] used a mixedinteger linear programming model to evaluate the distributed energy resources systems and the result illustrated that gas engine was the most popular distributed energy technology from the economic point of view. In regard to the feasibility of CCHP, Chua et al. [17] evaluated the potential of integrating renewable energy technological resources for tri-generation applications in an island. A simple yet effective economic dispatch strategy with goal to use distributed generation to minimize the cost was presented by Robert et al. [18] and the result proved that dispatch strategy was effective in reducing total energy costs. Costa et al. [19] analyzed the economics of trigeneration in a kraft pulp mill and the results supported the feasibility of the technology in the framework of national energy-saving policy. In literature [20], process flexibility and feasibility characteristics of a trigeneration system was analyzed and evaluated from a new perspective. For the evaluation of CCHP systems, Maraver et al. [21] reviewed the technologies involved in CCHP plants based on solid biomass combustion and evaluated the performance in comparison to standalone conventional systems. The performance of the different operation strategies evaluated was compared based on primary energy consumption, operation cost and carbon dioxide emissions by Mago et al. [22]. Li et al. [23] used a static calculation methodology for evaluating the primary energy consumption for CCHP system. Ge et al. [24] carried out the evaluation of a tri-generation system with simulation and experiment. However, effect of gas compositions on the performance of system was not analyzed in the former literatures. From the recycling of municipal waste and power generation, we need to know the performance of CCHP when it is driven by different components of gas, so we can choose appropriate gas components or mix anaerobic digestion and thermal cracking gas to get higher efficiency of CCHP and to obtain more economic interests. The objective of this work is to make clear the performance of CCHP system driven by different components gas. We analyzed the gas components of anaerobic digestion gas and thermal
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cracking gas in different conditions, proposed the thermodynamic model of CCHP and simulated its performance of the system driven by different compositions gas. PER (primary energy ratio) and exergy efficiency of CCHP were employed to indicate the energy utilization conditions. Output work of gas engine, refrigerating capacity and COP of absorption refrigerating system were analyzed and discussed. It is of great beneficial to understanding the operation of CCHP driven by gas from anaerobic digestion and/or thermal cracking of municipal waste.
2. Gas into the CCHP For the municipal waste, the gas could be obtained according to different operation conditions by two methods, one is anaerobic digestion and the other is thermal cracking. 2.1. Anaerobic digestion Anaerobic digestion can do with all kinds of feedstock including industrial and municipal waste waters, agricultural, food industry wastes and plant residues, and convert them to gas in which methane is main component. The digestion, as originally developed, is sealed to eliminate and to trap the methane produced. The process is controlled by the addition rate of raw sludge. Further improvements in control are obtained by maintaining the temperature at 35 °C and providing as much mixing as is possible from a technical and economic point of view. Many such units are in operation in many places of the world in the past.
Now, Multi-stage systems are used to improve the stability of the process, which attempt to separate the hydrolysis/acidification processes from the acetogenesis/methanogenesis processes, as these do not share the same optimum environmental conditions [25]. With the development of anaerobic digestion, it has been a viable alternative to landfill for category three wastes which are food industry wastes, domestic wastes and some abattoir wastes. It plays an important role in the recycling of waste and environmental protection. We have tested the product components of anaerobic digestion for the different feedstock. Fig. 1 is the experiment unit which can control digestion temperature and carry out different feedstock digestion. For the digestion is affected by temperature, concentration, pH, anaerobic bacteria, etc. [2], we chose that the digestion temperature is 35 °C and raw materials are straw, guano, kitchen waste and sludge respectively. The gas composition for these materials is shown in Table 1. The columnar maps of different composition are presented in Fig. 2. From Fig. 2, we can clearly obtain the volume content of CH4, H2, CO2, H2S and N2 for different raw material digestions. Because the biogas would send to engine to generate power and heat, we mainly analyze the volume content of combustible gas. Average volume content value of CH4 is about 56% and H2 is about 3% for the straw digestion; average volume content value of CH4 is about 53% and H2 is about 2% for the guano digestion; average volume content value of CH4 is about 55% and H2 is about 4% for the kitchen waste digestion; average volume content value of CH4 is about 44% and H2 is about 3% for the sludge digestion. 2.2. Thermal cracking Pyrolysis is one method of thermal cracking, which is a very complex process of interdependent reactions. In this study, gas components of three biomasses from wood, coconut shell and straw varying from 400 °C to 900 °C were tested. Prior to pyrolysis, wood is simply cut into chips of 5 mm in thickness; coconut shells are broken into small fragments with a hammer; straw wisps is cut into fragments of about 8–10 cm length. The moisture content of the raw biomass is determined after 10 h drying in an oven at 105 °C. The major components of gases are shown in Table 2, in which the gas components of three biomasses are similar to the outcome of literature [26]. The gas composition for three raw materials are shown in Fig. 3, which clearly presents the volume content of CO, CO2, H2, CH4, and
Fig. 1. Experiment unit of anaerobic digestionss.
Table 1 The main gas composition for different raw material (vol%). Raw material
Operation conditions
CH4
H2
CO2
H2S
N2
Straw
pH = 6.8, pH = 6.9, pH = 7.1, pH = 7.3,
t = 35 °C t = 35 °C t = 35 °C t = 35 °C
61.3 56.1 55.2 57.3
2.1 3.0 1.1 1.3
22.3 25.3 25.0 26.2
3.3 2.0 1.2 4.0
5.3 9.1 7.1 6.0
Guano
pH = 6.9, pH = 7.1, PH = 7.2, pH = 7.3,
t = 35 °C t = 35 °C t = 35 °C t = 35 °C
57.4 53.2 49.3 51.4
3.1 2.2 3.2 2.1
21.1 23.0 22.1 23.3
2.0 1.0 2.0 3.1
3.1 6.9 5.0 4.0
Kitchen waste
pH = 6.7, pH = 6.9, pH = 7.0, pH = 7.3,
t = 35 °C t = 35 °C t = 35 °C t = 35 °C
53.1 51.3 55.4 56.3
4.0 3.0 2.0 4.0
19.2 21.3 23.3 22.2
2.0 1.0 3.0 2.0
3.9 8.1 4.1 5.0
Sludge
PH = 6.7, pH = 6.9, pH = 7.0, pH = 7.3,
t = 35 °C t = 35 °C t = 35 °C t = 35 °C
43.2 41.3 45.2 46.4
3.1 4.0 2.1 3.1
18.3 21.3 22.3 23.0
2.0 4.2 3.0 3.1
6.1 7.1 5.1 7.0
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Fig. 2. Volume content of CH4, H2, CO2, H2S and N2 of gas for different raw material.
C2Hx from thermal cracking temperature 500–800 °C. For the wood, volume content of CO and H2 increase with the thermal cracking temperature, but the volume content of CO2 decreases. CH4 content gets to maximum which is 16.5% at the 600 °C. Thermal cracking gases of coconut shell and straw have similar trend with that of wood. CH4 content of coconut shell’s gas gets to maximum that is 17.2% at the 700 °C and straw’s maximum is 15.3%. As a whole, the volume content of combustible gas including CO, H2, CH4 and C2Hx can get to maximum value at temperature 800 °C that the total volume content reaches about 90%.
3. System structure and description Fig. 4 illustrates a schematic diagram of the CCHP system driven by the thermal cracking gas and/or the anaerobic digestion gas, which can produce power, heating and refrigeration simultaneously. The proposed system consists of a thermal cracking unit, an anaerobic digestion unit, one gas distribute unit, one gas engine, one heat exchanger, one heat recovery boiler, one absorption refrigeration unit, one hot water storage tank and other related components. As can be seen from Fig. 4, the gas from thermal cracking unit and/or anaerobic digestion unit flows into gas engine through gas distribute unit in which the gas composition can be adjusted according to need of gas engine or absorption refrigeration unit. The biogas is combusted and releases much heat, generates power in gas engine. Flue gas, which is from gas engine, flows into heat
Table 2 Major components of gases (vol%). T (°C) 400
500
600
700
800
900
CO
A B C
34.3 30.9 –
39.3 34.8 34.8
42.1 38.3 37.5
43.9 40.1 41.0
49.8 44.1 48.3
53.5 – 52.8
CO2
A B C
51.6 53.3 –
36.2 42.0 40.8
22.8 28.4 30.2
16.3 17.6 15.8
8.9 9.6 8.6
5.0 – 4.3
H2
A B C
1.3 1.0 –
7.6 5.2 7.3
10.8 12.2 12.6
15.5 18.5 19.0
20.8 23.3 23.1
25.3 – 24.3
CH4
A B C
9.1 10.0 –
12.5 12.9 11.9
16.2 16.3 13.1
16.0 17.2 15.1
14.1 15.5 13.3
12.0 – 11.8
C2Hx
A B C
3.2 4.2 –
3.2 4.3 5.0
7.1 3.8 5.3
7.1 6.3 8.6
5.6 5.1 6.3
3.9 – 5.2
A – wood, B – coconut shell, C – straw.
recovery boiler. From schematic diagram of the system, one part of heat from heat recovery boiler is used to drive the absorption refrigeration unit, in which a lithium bromide-water mixture is applied as working fluid to absorb the heat from exhaust gas, to provide cooling load for users; the other part of heat from heat recovery boiler and from refrigeration unit is used for heating.
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Fig. 3. Volume content of CO, CO2, H2, CH4, and C2Hx of gas in different thermal cracking temperature.
Monitoring and control system can collect the signals from different spots and give the orders to units, such as thermal cracking unit, anaerobic digestion unit, gas distribute unit, gas engine and the absorption refrigeration unit according to the need of users. 4. Mathematical model Modeling of CCHP driven by gas and performance assessment are presented in this section. The related data used in this model is given in Table 3. The absorption refrigeration is one single-effect absorption system which is similar to the approach used by ASHRAE [27]. In this study, some assumptions are employed as follows: (1) Gas is completely combusted and chemical reaction reaches equilibrium state. (2) Flow temperature at the inlet/outlet of every unit is constant. (3) Radiation heat transfer in every unit is neglected. (4) The whole system reaches steady state. (5) Heat loss of every unit is not considered. 4.1. Thermal cracking/anaerobic digestion gas and flue gas
V RO2 ¼ V CO2 þ V SO2 ¼ 0:01ðCO2 þ CO þ
ð1Þ
X
mC m Hn þ H2 SÞ 3
ð2Þ 3
where V RO2 is volume content of three atomic gases (N m /N m dry gas). Steam:
h i Xn V 0H2 O ¼ 0:01 H2 þ H2 S þ C m Hn þ 120ðdg þ V 0 da Þ 2
ð3Þ
where V 0H2 O is the steam volume content in theoretical flue gas (N m3/N m3 dry gas), dg is moisture content of gas (kg/N m3 dry gas), da is moisture content of air (kg/N m3 dry air). Nitrogen:
V 0N2 ¼ 0:79V 0 þ 0:01N2
ð4Þ
where V 0N2 is nitrogen volume fraction in theoretical flue gas (N m3/ N m3 dry gas), V0 is theoretical flue gas volume (N m3/N m3 dry gas), which can be obtained by
(
Low heat value of gas can be calculated as
H l ¼ H 1 r 1 þ H 2 r 2 þ . . . þ Hn r n
where H is heat value of gas (kJ/N m3), H1, H2 . . . Hn are heat value of combustible components (kJ/N m3), r1, r2 . . . rn are volume content of components. Volume content of combustion products can be obtained as follows. Three atomic gases (including CO2, H2S, SO2, etc.):
V 0 ¼ 0:209 Hl ; Hl < 10500 kJ=N m3 1000 0:26 V 0 ¼ 1000 Hl 0:25; Hl > 10500 kJ=N m3
ð5Þ
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heat output
hot water storage tank
cold output
heat from condenser
absorption refrigeration unit
feed water
monitoring and control system heat recovery boiler
heat exchanger 1
heat exchanger 2 flue gas dust collector
flue gas gas engine
electric generator
gas distribution unit solid waste CO, H2,CH4 anaerobic digestion unit
thermal cracking unit solid waste
material flow
signal feedback control network
Fig. 4. Schematic diagram of the CCHP based on waste to energy.
Total volume of theoretical flue gas is:
Table 3 Input data CCHP system under design conditions.
Gas engine
Absorption refrigeration
Heat recovery boiler
V 0f
¼ V RO2 þ V 0H2 O þ V 0N2
ð6Þ
Parameters
Value
Rated power output Efficiency of power generation Exhaust gas temperature Rated compression ratio
1.0 MW 36.5% 510 °C 8
Overall heat transfer coefficient of desorber Overall heat transfer coefficient of condenser Overall heat transfer coefficient of evaporator Overall heat transfer coefficient of absorber Heat transfer efficiency Effectiveness of solution heat exchanger Inlet/outlet temperature of frozen water Rated refrigerating capacity Concentration of strong solution in design condition Concentration of weak solution in design condition Flow ratio Rated refrigerating coefficient (COP)
70 kW/K 80 kW/K 95 kW/K
4.2. Gas engine
75 kW/K 80% 70% 12–7 °C 2800 kW 58.2%
The operation process of gas engine consists of two isometric processes and two isentropic processes. In order to obtain the performance of CCHP, we consider that the gas engine operates in ideal process. So, the efficiency of gas engine is
Exhaust gas temperature of boiler Heat transfer efficiency Heat recovery rate
125 °C 0.83 0.73
55.6% 22.38 0.71
Gas combustion temperature in the engine is
tther ¼
Hl V RO2 C RO2 þ V H2 O C H2 O þ V N2 C N2
ð7Þ
where C RO2 , C H2 O , C N2 are the average pressure volume specific heat of three atomic gases, steam and nitrogen respectively.
g¼1
T1 1 ¼ 1 k1 T2 c
ð8Þ
¼ vv 12 is compression ratio, T1, T2 are the beginning compres-
where c sion temperature and completed compression temperature (K), k is adiabatic exponent. Completed compression temperature can be expressed by
T 2 ¼ T 1 cðk1Þ
ð9Þ
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For the gas, adiabatic exponent k is determined by certain gas composition. The theoretical flue gas enthalpy is
I0f ¼ V RO2 C RO2 t f þ V 0N2 C N2 tf þ V 0H2 O C H2 O tf
V RO2 C RO2 V 0f
þ
V 0N2 C N2 V 0f
þ
V 0H2 O C H2 O V 0f
M¼
ri Mi
ð25Þ ð26Þ
qE qG
ð27Þ
ð12Þ
R0 8314 ¼ M M
4.4. Assessment of the CCHP system
ð13Þ
The relation between specific heat at constant pressure and specific heat at constant volume is
C 0p C 0v ¼ qf R
ð14Þ
where qf is density of flue gas. Using formula (11) and (14), adiabatic exponent k can be obtained. When the compression ratio c is constant, efficiency of gas engine g is obtained by formula (8). The exhaust temperature T4 of gas engine is determined by
T4 ¼
FR 1 FR
ð24Þ
Then, the COP of the absorption refrigeration system is
COP ¼
where Mi is the molecular weight for each component. Average gas constant of flue gas is
R¼
h10 ¼ h9 þ ðh11 h12 Þ
qSHE ¼ ðh11 h12 Þ ðFR 1Þ ¼ ðh10 h9 Þ FR ð11Þ
Equivalent molecular weight of flue gas is
X
T 12 ¼ eSHE T 9 þ ð1 eSHE Þ T 11
ð10Þ
where tf is gas combustion temperature in the gas engine (tther). Then, the specific heat at constant pressure of flue gas is
C g;p ¼
The energy balance and heat equilibrium of the solution heat exchanger are
T1 T3 T1 T3 T3 ¼ ¼ T2 T 1 ck1 ck1
ð15Þ
where T3 can be deemed as the gas combustion temperature in the gas engine which is tther. 4.3. Absorption refrigeration Fig. 5 illustrates the single-effect absorption refrigeration system (ARS). It includes a generator, an absorber, a condenser, an evaporator, a pump, two expansion valves, one solution heat exchanger. Mass balance in the generator can be written as follows:
mws ¼ mss þ mH2 O
ð16Þ
mws nws ¼ mss nss
ð17Þ
Performance assessment of the CCHP system is very important. The index to evaluate the performance includes PER (primary energy ratio) and exergy efficiency. (1) Primary energy ratio Based on discussing the performance of CCHP system by different composition gas, we can determine which type gas should be used to drive CCHP system according to need of user and its economic benefits. In order to indicate the performance of CCHP system, parameter of PER (primary energy ratio) is used, which is
PER ¼
We þ Qh þ Qc Gg H l
ð28Þ
where We is total engine electric power (kW), Qh is total heating power (kW), Qc is total refrigerating capacity (kW), Gg is gas consumption (N m3/s), Hl is the heat value of gas (kJ/N m3). The parameters of We, Qh and Qc can be determined, which are
W e ¼ C v ðT 3 T 2 Þ g
ð29Þ
Q c ¼ qE
ð30Þ
Q h ¼ qC þ qh
ð31Þ
In formula (31), qh is the heat used by hot water through the direct flue gas. (2) Exergy efficiency
mss ¼
nws mH2 O nss nws
ð18Þ
Exergy efficiency is the ratio of the output exergy to the input exergy of the CCHP system, which can be written to
mws ¼
nss mH2 O nss nws
ð19Þ
gex ¼
Exe þ Exh þ Exc Exf
ð32Þ
The heat capacities of the main components can be calculated as follows:
where gex is the exergy efficiency, Exe, Exh Exc and Exf are the exergy of electricity, heating, cooling and gas respectively,
q E ¼ h6 h5
ð20Þ
Exe ¼ W e
ð33Þ
q C ¼ h3 h4
ð21Þ
T0 Exh ¼ Q h 1 Th
ð34Þ
qG ¼ h2 þ FR h1 ðFR 1Þ h10
ð22Þ
T0 1 Tc
ð35Þ
where qE is determined by user need.
where qG is determined by waste heat from the gas engine, qG ¼ C g;p ðT 4 T 04 Þ, T 04 is the returned flue gas temperature.
qA ¼ FR h8 ðFR 1Þ h13 h7 ss where FR ¼ mmHwsO ¼ nssnn . ws 2
ð23Þ
Exc ¼ Q c
where T0 is the ambient temperature, Tc and Th are the cold water temperature and the heat water temperature respectively (in this case study, Tc and Th are assumed to be constant, which are respectively 278 K and 353 K).
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qC
3
1
2 genertator
condenser 4
10 11 solution heat exchanger
expansion valve 9 solution pump 5
12 expansion valve
8 evaporator
6
7
absorber 13 qA
qE
Fig. 5. The schematic of single-effect absorption refrigeration system.
The exergy of gas can be written as
Exf ¼
V 0f
Hl
ð36Þ
where V 0f is the gas consumption.
5. Results and discussion In order to facilitate the understanding, design and implementation of sustainable solutions for megacities, we analyzed the performance of CCHP system driven by different composition gas from anaerobic digestion and/or thermal cracking. The gas compositions from anaerobic digestion and thermal cracking (pyrolysis) are different. Considering the complexity and diversity of gas components, we chose three different type gases and discussed their effects on the CCHP system according to gas components of anaerobic digestion and pyrolysis (Tables 1 and 2, Figs. 2 and 3). The different composition gases into the CCHP are shown in Table 4, which gives the volume content of main combustible gas components. The adiabatic exponent, low heat value and combustion temperature of three different gases are shown in Table 5. For CCHP system, gas combusts in the gas engine and generates the power, and the waste heat of exhaust flue gas is collected to drive the absorption refrigeration system and heating system. So, the operation state of gas engine is very important for the performance of whole CCHP system. Hence, the related performance is analyzed in different conditions based on the engine operation in which the compression ratio c is main factor. Fig. 6 shows the COP of absorption refrigeration and the efficiency of gas engine g changing with compression ratio of gas engine. As seen in Fig. 6, as the compression ratio increases from 7.0 to 12.0, the efficiency of gas engine increases and COP of absorption refrigeration decreases from 0.77 to 0.51. The reason is that for the certain gas, the higher compression ratio will make g increase according to formula (8). For the gas engine, when inlet temperature and the combustion temperature are determined, the bigger compression ratio means that the exhaust gas temperature of engine declines according to formula (15). So, the little temperature of flue gas makes refrigerating capacity reduce, and the COP of absorption refrigeration decreases for this.
Table 4 Different combustible compositions gas (vol%).
Anaerobic digestion gas (pH = 6.9, t = 35 °C) Thermal cracking gas/pyrolysis (coconut shell, T = 700 °C) Mixed gas (anaerobic digestion and thermal cracking)
CH4
H2
CO
C2Hx
56.1 17.2
3.0 18.5
– 40.1
– 6.3
45.2
8.1
10.0
2.3
Table 5 Thermal properties of three different gases.
Anaerobic digestion gas Thermal cracking gas (pyrolysis) Mixed gas (anaerobic digestion and thermal cracking)
k (adiabatic exponent)
Hl (kJ/ N m3)
tther (°C)
1.475 1.231 1.420
20,684 17,521 18,287
2203 2086 2175
Fig. 6. Effect of compression ratio on COP and g.
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Fig. 7. Effect of compression ratio on exhaust gas temperature and output work of engine. Fig. 10. The change of PER and g with compression ratio.
Fig. 8. Effect of compression ratio on efficiency of engine and exhaust gas temperature.
As the compression ratio is constant, the change line of gas engine efficiency, which has bigger adiabatic exponent k, will have the bigger value. From Table 5, the adiabatic exponent of anaerobic digestion gas is the largest and the adiabatic exponent of thermal cracking gas is the smallest. So, the line g of anaerobic digestion gas is in higher place. And then, the COP line is on the contrary with the line of g.
Fig. 7 shows that the relation between exhaust gas temperature, output work of engine and the compression ratio of engine. From Fig. 7, exhaust gas temperature of engine decreases with the pressure, which changes from 1200 K to 800 K, for the certain gas whose compositions are constant; it can be explained by the formula (15). At the same time, the output work of gas engine will increase with the compression ratio of engine from 6500 kJ/N m3 gas to 9300 kJ/N m3 gas. For the certain compression ratio, the adiabatic exponent of anaerobic digestion gas is the largest which is 1.475 and the adiabatic exponent of thermal cracking gas is the smallest from Table 5 which is 1.231. The combustion temperature in the gas engine (tther) is like this. So, the anaerobic digestion gas has the highest combustion temperature and lowest exhaust temperature, and the output work of anaerobic digestion gas is the largest and thermal cracking has the smallest value. The effect of compression ratio on the efficiency of engine and exhaust gas temperature of engine is shown in Fig. 8. From Fig. 8, we can find that exhaust gas temperature of engine decreases with the compression ratio from 1185 K to 750 K, but the efficiency of gas engine increases from 0.41 to 0.56. The reason of exhaust gas temperature of engine decreasing can be explained by formula (15) and the engine efficiency’s increasing can be explained by formula (8). Fig. 9 gives the relation among the heat collected to absorption refrigeration, the heat to hot water and the compression ratio of engine. As shown in Fig. 9, we can find that when the exhaust
Fig. 9. Effect of compression ratio on heat collected to absorption refrigeration and heat to hot water.
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Fig. 11. The change of gex and g with compression ratio.
Fig. 12. Effect of output work ratio of gas engine on g and exhaust gas temperature of engine.
gas temperature declines with the increasing of compression ratio, the heat input to absorption refrigeration reduces from 1390 kJ/ N m3 gas to 485 kJ/N m3 gas and the heat to hot water reduces from 5485 kJ/N m3 gas to 1980 kJ/N m3 gas. It is easily understood because the exhaust gas temperature decreasing means the quality of waste heat to be decreasing.
The change of PER of CCHP system and the efficiency of engine g with compression ratio are shown in Fig. 10. With the compression ratio increasing from 7.0 to 12.0, the efficiency of engine g increases from 0.41 to 0.56, but the PER of CCHP decreases from 0.96 to 0.62. In order to explain this, it is necessary to analyze the change of output work of engine, heat collected to absorption refrigeration and heat input to heating hot water. As a whole, the total energy used by CCHP is reduced, and the PER presents the downward trend. Fig. 11 shows the change of exergy efficiency of CCHP gex and efficiency of engine g with compression ratio. As the compression ratio increasing, the exergy efficiency of CCHP gex decreases from 0.78 to 0.23 and the efficiency of engine g increases from 0.25 to 0.56. The reason is that the exhaust gas temperature of engine decreases with the compression ratio increasing, and the exergy of heating and cooling are reduced. So, the exergy efficiency of CCHP shows the decline. The above analysis is performed under the design conditions of CCHP system. Nevertheless, the CCHP system often operates under off-design conditions. In order to indicate the performance of CCHP system under the off-design conditions, we study the operation characteristic of the system under off-design state. The main analysis is based on the gas engine’s off-design operations. When the design conditions of gas engine is considered as a standard of comparison, the exhaust temperature ratio, efficiency of power generation ratio of gas engine are determined according to literature [28],
T g;4 ¼ T g;u;4 =T g;o;4 ¼ 0:53 þ 0:38N þ 0:09N2
ð37Þ
g ¼ g=go ¼ 0:13 þ 2:47N 1:6N2
ð38Þ
where Tg,4 is exhaust temperature of gas engine (K), g is efficiency of engine, N is output work of gas engine (W). ‘‘–’’ denotes the ratio. Effect of output work ratio N/N0 of gas engine on efficiency of engine g and exhaust gas temperature of engine are shown in Fig. 12. From Fig. 12, efficiency of gas engine g increases at first and then declines. The reason is that for the efficiency of power generation g, the operation state of gas engine is usually best under the design conditions and is not well under off-design conditions. Hence, near the design conditions, the efficiency of power generation g will be largest which is about 0.52. With the increase of output work ratio N/N0, the exhaust gas temperature of gas engine increases from 600 K to 1340 K. Fig. 13 shows the effect of output work ratio of gas engine on absorption refrigeration and heating hot water. From Fig. 13, we can find heat input to absorption refrigeration and heat input to heating hot water both increase with the increase of output work
Fig. 13. Effect of output work ratio of gas engine on heat collected to absorption refrigeration and heating hot water.
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Fig. 14. Effect of output work ratio of gas engine on PER and g.
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quantity of heat input to heating hot water is more than the input to absorption refrigeration system. Fig. 14 shows the effect of output work ratio of gas engine on PER and efficiency of engine g. We can find that efficiency of engine first increase from 0.24 to 0.52 and then decline from 0.52 to 0.27, the PER of the CCHP system increases from 0.18 to 1.0. For the PER, with the output work of gas engine increasing when N/N0 is increasing, the efficiency of engine g is reduced and exhaust gas temperature of engine is increasing from Fig. 12 and more waste heat will be used to drive the absorption refrigeration system and heating system from Fig. 13, hence, the PER of the CCHP system will increase. The effect of output work ratio of gas engine on exergy efficiency of CCHP gex and efficiency of engine g shows in Fig. 15. From Fig. 15, exergy efficiency of CCHP gex increases from 0.05 to 0.38 with output work ratio of gas engine increasing. The variation is similar with Fig. 14. As the output work ratio of gas engine increasing, the exhaust gas temperature increases and the exergy of heating and cooling increase. So, the exergy efficiency of CCHP gex shows rising trend, whose largest value is about 0.38. Fig. 16 shows the effect of gas temperature on refrigeration capacity and COP. From Fig. 16, we find that the COP of absorption refrigeration driven by anaerobic digestion gas is minimal and its refrigeration capacity is also minimal that the average value is about 350 kJ/m3 gas in the same conditions. COP of absorption refrigeration driven by thermal cracking gas and refrigeration capacity are maximal values that the average value is about 0.67. The reason is that exhaust temperature of engine driven by anaerobic digestion gas is minimal and by thermal cracking gas is maximal. In the same conditions, the higher temperature flue gas can provide more energy to absorption refrigeration and make COP be higher. 6. Conclusions
Fig. 15. Effect of output work ratio of gas engine on gex and g.
ratio N/N0, which change from 250 kJ/m3 gas to 1580 kJ/m3 gas and from 985 kJ/m3 gas to 6000 kJ/m3 gas respectively. The reason is that the exhaust temperature of gas engine increases and the heat input to absorption refrigeration will increase. For the exhaust heat of absorption refrigeration can be used by heating system, the
In order to evaluate the effect of different composition gases, which are anaerobic digestion, thermal cracking and the mixture respectively, on the performance of CCHP system, we analyzed the composition of gases produced from different raw materials, and presented the calculation model according to combustion characteristic of gas and thermodynamic process of CCHP system. The related parameters, such as PER of CCHP system, exergy efficiency of CCHP system gex, the efficiency of gas engine g, COP of absorption refrigeration, output work of engine, exhaust gas temperature of engine, heat input to absorption refrigeration and hot water production, were analyzed in different compression ratio
Fig. 16. Effect of exhaust gas temperature on refrigeration capacity and COP under off-design conditions.
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of engine and under off-design conditions. The main conclusions drawn from this study are summarized as follows: (1) For the main combustible gas from anaerobic digestion and thermal cracking process, the volume content of CH4 in anaerobic digestion gas is more than in thermal cracking gas, and the volume content of H2 in thermal cracking gas exceeds anaerobic digestion gas usually. (2) In the same compression ratio of gas engine, more work from the anaerobic digestion gas can be extracted, but the exhaust temperature of gas engine is lower and the heat input the absorption refrigeration is less, compared with the thermal cracking gas. The reason is that they have different heat value (heat value of anaerobic digestion gas is about 20,684 kJ/N m3, heat value of thermal cracking gas is about 17,521 kJ/N m3) and different gas combustion temperature (anaerobic digestion gas is about 2203 °C and thermal cracking gas is about 2086 °C). The anaerobic digestion gas can provide more work in the same compression ratio of gas engine. (3) The variation of PER of CCHP system, exergy efficiency of CCHP gex and the efficiency of engine g with the change of compression ratio are different. When the efficiency of engine g increases, PER of CCHP and exergy efficiency of CCHP gex decline. The variation of absorption refrigeration COP and the efficiency of engine g with the change of compression ratio are also different. When the efficiency of engine g increases, the exhaust temperature of gas engine will decline, and then the heat input the absorption refrigeration will reduce, leading to the decrease of COP. (4) Under off-design conditions, PER of CCHP system and exergy efficiency of CCHP system gex will increase with the output work of gas engine increasing. As a whole, the performance of CCHP system should be determined by considering all kinds of factors, such as cost of gas, efficiency of gas engine, cooling and heat load.
Acknowledgements The work was supported by the National Research Foundation (NRF), Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) program, Jiangsu Overseas Research & Training Program for University Prominent Young & Middle-aged Teachers and Presidents of China and Fundamental Research Funds for the Central Universities (No. 2014QNA74). References [1] Kelleher BP, Leahy JJ, Henihan AM, O’Dwyer TF, Sutton D, Leahy MJ. Advances in poultry litter disposal technology – a review. Bioresour Technol 2000;83:27–36. [2] Chen Ye, Cheng Jay J, Creamer Kurt S. Inhibition of anaerobic digestion process: a review. Bioresour Technol 2008;99:4044–64.
[3] Midilli A, Dogru M, Howarth CR, Ling MJ, Ayhan T. Combustible gas production from sewage sludge with a downdraft gasifier. Energy Convers Manage 2001;42:157–72. [4] Zhang Q, Wua Y, Dor L, Yang W, Blasiak W. A thermodynamic analysis of solid waste thermal cracking in the plasma thermal cracking melting process. Appl Energy 2013;112:405–13. [5] Gao N, Li J, Qi B, Li A, Duan Y, Wang Z. Thermal analysis and products distribution of dried sewage sludge pyrolysis. J Anal Appl Pyrol 2014;105:43–8. [6] Suamir IN, Tassou SA. Performance evaluation of integrated trigeneration and CO2 refrigeration systems. Appl Therm Eng 2013;50:1487–95. [7] Mancarella P, Chicco G. Assessment of the greenhouse gas emissions from cogeneration and trigeneration systems. Part II: analysis techniques and application cases. Energy 2008;33:418–30. [8] Wu DW, Wang RZ. Combined cooling, heating and power: a review. Prog Energy Combust Sci 2006;32(5–6):459–95. [9] Mago PJ, Fumo N, Chamra LM. Performance analysis of CCHP and CHP systems operating following the thermal and electric load. Int J Energy Res 2009;33:852–64. [10] Fumo N, Chamra LM. Analysis of combined cooling, heating, and power systems based on source primary energy consumption. Appl Energy 2010;87:2023–30. [11] Wang Jiangfeng, Zhao Pan, Niu Xiaoqiang, Dai Yiping. Parametric analysis of a new combined cooling, heating and power system with transcritical CO2 driven by solar energy. Appl Energy 2012;94:58–64. [12] Li Sheng, Sui Jun, Jin Hongguang, Zheng Jianjiao. Full chain energy performance for a combined cooling, heating and power system running with methanol and solar energy. Appl Energy 2013;112:673–81. [13] Hu Mengqi, Cho Heejin. A probability constrained multi-objective optimization model for CCHP system operation decision support. Appl Energy 2014;116:230–42. [14] Cardona E, Piacentino A. Optimal design of CHCP plants in the civil sector by thermoeconomics. Appl Energy 2007;84:729–48. [15] Kong XQ, Wang RZ, Li Y, Huang XH. Optimal operation of a micro-combined cooling, heating and power system driven by a gas engine. Energy Convers Manage 2009;50:530–8. [16] Ren Hongbo, Gao Weijun. A MILP model for integrated plan and evaluation of distributed energy systems. Appl Energy 2010;87:1001–14. [17] Chua KJ, Yang WM, Er SS, Ho CA. Sustainable energy systems for a remote island community. Appl Energy 2014;113:1752–63. [18] Flores Robert J, Shaffer Brendan P, Brouwer Jacob. Dynamic distributed generation dispatch strategy for lowering the cost of building energy. Appl Energy 2014;123:196–208. [19] Costa A, Paris J, Towers M, Browne T. Economics of trigeneration in a kraft pulp mill for enhanced energy efficiency and reduced GHG emissions. Energy 2007;32:474–81. [20] Lai SM, Hui CW. Feasibility and flexibility for a trigeneration system. Energy 2009;34:1693–704. [21] Maraver Daniel, Sin Ana, Royo Javier, Sebastian Fernando. Assessment of CCHP systems based on biomass combustion for small-scale applications through a review of the technology and analysis of energy efficiency parameters. Appl Energy 2013;102:1303–13. [22] Mago PJ, Chamra LM. Analysis and optimization of CCHP systems based on energy, economical, and environmental considerations. Energy Build 2009;41:1099–106. [23] Li H, Fu L, Geng K, Jiang Y. Energy utilization evaluation of CCHP systems. Energy Build 2006;38:253–7. [24] Ge YT, Tassou SA, Chaer I, Suguartha N. Performance evaluation of a trigeneration system with simulation and experiment. Appl Energy 2009;86:2317–26. [25] Liu DW, Liu DP, Zeng RJ, Angelidaki I. Hydrogen and methane production from household solid waste in the two-stage fermentation process. Water Res 2006;40:2230–6. [26] Fagbemi L, Khezami L, Capart R. Pyrolysis products from different biomasses: application to the thermal cracking of tar. Appl Energy 2001;69:293–306. [27] American Society of Heating Refrigerating and Air-Conditioning Engineers Inc.. Chapter 1-thermodynamics and refrigeration cycles. In: ASHRAE Handbook: Fundamentals, Inch Pound Edition. Atlanta, USA: ASHRAE; 2009. [28] XiaoHong HE, Ruixian CAI. Typical off-design performances of internal combustion engine and its CHP system. J Eng Thermophys 2008;29:191–4.
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Thermodynamic performance assessment of CCHP system driven by different composition gas Penghui Gao a,c,⇑, Wangliang Li c, Yongpan Cheng c, YenWah Tong b,c, Yanjun Dai d, Ruzhu Wang d a
School of Architecture and Civil Engineering, China University of Mining and Technology, Xuzhou 221116, China Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576, Singapore c Environmental Research Institute, National University of Singapore, Singapore 117576, Singapore d School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China b
h i g h l i g h t s Gas components of anaerobic digestion gas and thermal cracking gas were analyzed. Thermodynamic model of CCHP driven by different compositions gas was proposed. Performance of CCHP was studied under the design and off-design conditions. Different composition gas had evident effect on the performance of CCHP.
a r t i c l e
i n f o
Article history: Received 2 July 2014 Received in revised form 2 September 2014 Accepted 22 September 2014
Keywords: CCHP (Combined Cooling Heating and Power) Gas Thermodynamic analysis
a b s t r a c t In order to facilitate sustainable solutions in megacities, waste-to-energy study of municipal solid waste handling is conducted including anaerobic digestion and thermal cracking, together with power generation technologies and the utilization of residual waste heat. In this paper, performance of CCHP (Combined Cooling Heating and Power) system, which is driven by different composition gases that the main components are CH4, H2, CO, SH2, C2H4, etc. is evaluated. The related parameters of the system, such as PER (primary energy ratio) of CCHP, exergy efficiency of CCHP gex, the efficiency of gas engine g, and COP (Coefficient of Performance) of absorption refrigeration, were analyzed in different compression ratio of gas engine and under off-design conditions. The results indicated that the different composition gases had evident effect on the performance of CCHP. For the waste handling and waste-to-energy in metropolis, the gas input to CCHP system form anaerobic digestion and thermal cracking (pyrolysis) should be determined by considering all kinds of factors, such as cost of gas, efficiency of gas engine, cooling and heat load. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Great consumption of fossil fuels, global warming, environment deterioration and municipal waste increment have attracted many researchers to find more scientific and efficient methods of energy conservation, recycling of waste, and reducing greenhouse gas emissions as well as pollutants. For the municipal solid wastes, there are two methods to recycle which are anaerobic digestion and thermal cracking. Anaerobic digestion is a series of biological processes in which microorganisms break down biodegradable ⇑ Corresponding author at: School of Architecture and Civil Engineering, China University of Mining and Technology, Xuzhou 221116, China. Tel.: +86 (516)83882193; fax: +86 (516)83885478. E-mail addresses: [email protected], [email protected] (P. Gao). http://dx.doi.org/10.1016/j.apenergy.2014.09.070 0306-2619/Ó 2014 Elsevier Ltd. All rights reserved.
material in the absence of oxygen and one of the end products is biogas, which can be combusted to generate electricity and heat [1,2]. For the digestion process, temperature, pH, buffering capacity and fatty acid concentrations are main influence factors. Thermal cracking, that biomass and/or solid wastes are converted to syngas containing hydrogen, carbon monoxide, and methane, is the another method and provides an attractive alternative process to generate energy. A number of researchers have performed experimental studies on thermal cracking [3–5]. For the thermal cracking, operation temperature and raw material are important factors. However, gas compositions of anaerobic digestion and thermal cracking are different and change with the operational conditions. In order to facilitate the understanding, design and implementation of sustainable solutions for megacities, it should
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Nomenclature COP h q n H V
g V0 M d N C FR m 0 e r t
c I R
q
coefficient enthalpy (kJ/kg) heat capacity (kJ/kg) solution concentration heat value (kJ/N m3) volume fraction (N m3/N m3 dry gas) efficiency gas consumption (N m3/s) molecular moisture content (kg/N m3 dry gas) output work of gas engine (W) volume specific heat at constant pressure (kJ/(N m3 K) flow ratio mass flow rate (kg/s) ideal state effectiveness of heat exchanger volume of component temperature (°C) compression ratio flue gas enthalpy (kJ/N m3 dry gas) gas constant density (kg/N m3)
be made clear the effect of gas compositions on the performance of CCHP system from anaerobic digestion and/or thermal cracking. In recent years, the development of polygeneration systems has received an increasing attention in the area of small scale power systems for different applications ranging from residential to utilities [6,7]. CCHP, being a typical polygeneration system, is a distributed generation technology that reduces the use of electricity from the grid by using low emission fuels such as natural gas, anaerobic digestion gas and thermal cracking gas. For the realization of the energy cascade utilization, the overall fuel energy utilization of CCHP systems can be as much as 70–90%, which is higher than that of conventional independent energy supply systems [8]. Moreover, CCHP systems can improve the stability and power quality considerably by making up the peak-loads; for micro-grid with distributed energy systems, CCHP systems can meet the various energy demands from diverse users by adjusting operating modes. Many researchers have studied CCHP systems from different aspects such as operation model [9–12], optimization [13–16], feasibility analysis [17–20] and evaluation [21–24]. For the operation of CCHP systems, Mago et al. [9] analyzed the performance of CCHP and CHP systems following the electric load and dioxide emissions for different climate conditions. Fumo and Chamra [10] carried out the analysis of CCHP systems according to the source primary energy consumption. Wang et al. [11] studied the performance of a new CCHP system with a transcriptical CO2 refrigeration cycle, which was driven by solar energy. The results indicated that increasing turbine inlet pressure and ejector inlet temperature could lower the efficiency of the system. Full chain energy performance was applied to a CCHP system by Li et al. [12], and the study presented that complementation utilization of solar energy and fossil fuel was energy efficient. For the optimization of CCHP systems, Hu and Cho [13] proposed a stochastic multi-objective optimization model to optimize the CCHP operation strategy for different climate conditions based on operational cost, primary energy consumption and carbon dioxide emissions. The potential of thermoeconomics for analysis of CHCP applications in buildings was explored by Cardona and Piacentino [14]. Kong et al. [15] investigated the problem of energy
Subscripts A absorber C condenser E evaporator G generator s isentropic process e electric f flue gas h heating c cooling p constant pressure v constant volume g gas a air l low value u off-design condition o design condition ex exergy RO2 three atomic gas ther gas combustion temperature WS weak solution SS strong solution
management and optimal operation of cogeneration system for micro-combined CCHP system. Ren and Gao [16] used a mixedinteger linear programming model to evaluate the distributed energy resources systems and the result illustrated that gas engine was the most popular distributed energy technology from the economic point of view. In regard to the feasibility of CCHP, Chua et al. [17] evaluated the potential of integrating renewable energy technological resources for tri-generation applications in an island. A simple yet effective economic dispatch strategy with goal to use distributed generation to minimize the cost was presented by Robert et al. [18] and the result proved that dispatch strategy was effective in reducing total energy costs. Costa et al. [19] analyzed the economics of trigeneration in a kraft pulp mill and the results supported the feasibility of the technology in the framework of national energy-saving policy. In literature [20], process flexibility and feasibility characteristics of a trigeneration system was analyzed and evaluated from a new perspective. For the evaluation of CCHP systems, Maraver et al. [21] reviewed the technologies involved in CCHP plants based on solid biomass combustion and evaluated the performance in comparison to standalone conventional systems. The performance of the different operation strategies evaluated was compared based on primary energy consumption, operation cost and carbon dioxide emissions by Mago et al. [22]. Li et al. [23] used a static calculation methodology for evaluating the primary energy consumption for CCHP system. Ge et al. [24] carried out the evaluation of a tri-generation system with simulation and experiment. However, effect of gas compositions on the performance of system was not analyzed in the former literatures. From the recycling of municipal waste and power generation, we need to know the performance of CCHP when it is driven by different components of gas, so we can choose appropriate gas components or mix anaerobic digestion and thermal cracking gas to get higher efficiency of CCHP and to obtain more economic interests. The objective of this work is to make clear the performance of CCHP system driven by different components gas. We analyzed the gas components of anaerobic digestion gas and thermal
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cracking gas in different conditions, proposed the thermodynamic model of CCHP and simulated its performance of the system driven by different compositions gas. PER (primary energy ratio) and exergy efficiency of CCHP were employed to indicate the energy utilization conditions. Output work of gas engine, refrigerating capacity and COP of absorption refrigerating system were analyzed and discussed. It is of great beneficial to understanding the operation of CCHP driven by gas from anaerobic digestion and/or thermal cracking of municipal waste.
2. Gas into the CCHP For the municipal waste, the gas could be obtained according to different operation conditions by two methods, one is anaerobic digestion and the other is thermal cracking. 2.1. Anaerobic digestion Anaerobic digestion can do with all kinds of feedstock including industrial and municipal waste waters, agricultural, food industry wastes and plant residues, and convert them to gas in which methane is main component. The digestion, as originally developed, is sealed to eliminate and to trap the methane produced. The process is controlled by the addition rate of raw sludge. Further improvements in control are obtained by maintaining the temperature at 35 °C and providing as much mixing as is possible from a technical and economic point of view. Many such units are in operation in many places of the world in the past.
Now, Multi-stage systems are used to improve the stability of the process, which attempt to separate the hydrolysis/acidification processes from the acetogenesis/methanogenesis processes, as these do not share the same optimum environmental conditions [25]. With the development of anaerobic digestion, it has been a viable alternative to landfill for category three wastes which are food industry wastes, domestic wastes and some abattoir wastes. It plays an important role in the recycling of waste and environmental protection. We have tested the product components of anaerobic digestion for the different feedstock. Fig. 1 is the experiment unit which can control digestion temperature and carry out different feedstock digestion. For the digestion is affected by temperature, concentration, pH, anaerobic bacteria, etc. [2], we chose that the digestion temperature is 35 °C and raw materials are straw, guano, kitchen waste and sludge respectively. The gas composition for these materials is shown in Table 1. The columnar maps of different composition are presented in Fig. 2. From Fig. 2, we can clearly obtain the volume content of CH4, H2, CO2, H2S and N2 for different raw material digestions. Because the biogas would send to engine to generate power and heat, we mainly analyze the volume content of combustible gas. Average volume content value of CH4 is about 56% and H2 is about 3% for the straw digestion; average volume content value of CH4 is about 53% and H2 is about 2% for the guano digestion; average volume content value of CH4 is about 55% and H2 is about 4% for the kitchen waste digestion; average volume content value of CH4 is about 44% and H2 is about 3% for the sludge digestion. 2.2. Thermal cracking Pyrolysis is one method of thermal cracking, which is a very complex process of interdependent reactions. In this study, gas components of three biomasses from wood, coconut shell and straw varying from 400 °C to 900 °C were tested. Prior to pyrolysis, wood is simply cut into chips of 5 mm in thickness; coconut shells are broken into small fragments with a hammer; straw wisps is cut into fragments of about 8–10 cm length. The moisture content of the raw biomass is determined after 10 h drying in an oven at 105 °C. The major components of gases are shown in Table 2, in which the gas components of three biomasses are similar to the outcome of literature [26]. The gas composition for three raw materials are shown in Fig. 3, which clearly presents the volume content of CO, CO2, H2, CH4, and
Fig. 1. Experiment unit of anaerobic digestionss.
Table 1 The main gas composition for different raw material (vol%). Raw material
Operation conditions
CH4
H2
CO2
H2S
N2
Straw
pH = 6.8, pH = 6.9, pH = 7.1, pH = 7.3,
t = 35 °C t = 35 °C t = 35 °C t = 35 °C
61.3 56.1 55.2 57.3
2.1 3.0 1.1 1.3
22.3 25.3 25.0 26.2
3.3 2.0 1.2 4.0
5.3 9.1 7.1 6.0
Guano
pH = 6.9, pH = 7.1, PH = 7.2, pH = 7.3,
t = 35 °C t = 35 °C t = 35 °C t = 35 °C
57.4 53.2 49.3 51.4
3.1 2.2 3.2 2.1
21.1 23.0 22.1 23.3
2.0 1.0 2.0 3.1
3.1 6.9 5.0 4.0
Kitchen waste
pH = 6.7, pH = 6.9, pH = 7.0, pH = 7.3,
t = 35 °C t = 35 °C t = 35 °C t = 35 °C
53.1 51.3 55.4 56.3
4.0 3.0 2.0 4.0
19.2 21.3 23.3 22.2
2.0 1.0 3.0 2.0
3.9 8.1 4.1 5.0
Sludge
PH = 6.7, pH = 6.9, pH = 7.0, pH = 7.3,
t = 35 °C t = 35 °C t = 35 °C t = 35 °C
43.2 41.3 45.2 46.4
3.1 4.0 2.1 3.1
18.3 21.3 22.3 23.0
2.0 4.2 3.0 3.1
6.1 7.1 5.1 7.0
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Fig. 2. Volume content of CH4, H2, CO2, H2S and N2 of gas for different raw material.
C2Hx from thermal cracking temperature 500–800 °C. For the wood, volume content of CO and H2 increase with the thermal cracking temperature, but the volume content of CO2 decreases. CH4 content gets to maximum which is 16.5% at the 600 °C. Thermal cracking gases of coconut shell and straw have similar trend with that of wood. CH4 content of coconut shell’s gas gets to maximum that is 17.2% at the 700 °C and straw’s maximum is 15.3%. As a whole, the volume content of combustible gas including CO, H2, CH4 and C2Hx can get to maximum value at temperature 800 °C that the total volume content reaches about 90%.
3. System structure and description Fig. 4 illustrates a schematic diagram of the CCHP system driven by the thermal cracking gas and/or the anaerobic digestion gas, which can produce power, heating and refrigeration simultaneously. The proposed system consists of a thermal cracking unit, an anaerobic digestion unit, one gas distribute unit, one gas engine, one heat exchanger, one heat recovery boiler, one absorption refrigeration unit, one hot water storage tank and other related components. As can be seen from Fig. 4, the gas from thermal cracking unit and/or anaerobic digestion unit flows into gas engine through gas distribute unit in which the gas composition can be adjusted according to need of gas engine or absorption refrigeration unit. The biogas is combusted and releases much heat, generates power in gas engine. Flue gas, which is from gas engine, flows into heat
Table 2 Major components of gases (vol%). T (°C) 400
500
600
700
800
900
CO
A B C
34.3 30.9 –
39.3 34.8 34.8
42.1 38.3 37.5
43.9 40.1 41.0
49.8 44.1 48.3
53.5 – 52.8
CO2
A B C
51.6 53.3 –
36.2 42.0 40.8
22.8 28.4 30.2
16.3 17.6 15.8
8.9 9.6 8.6
5.0 – 4.3
H2
A B C
1.3 1.0 –
7.6 5.2 7.3
10.8 12.2 12.6
15.5 18.5 19.0
20.8 23.3 23.1
25.3 – 24.3
CH4
A B C
9.1 10.0 –
12.5 12.9 11.9
16.2 16.3 13.1
16.0 17.2 15.1
14.1 15.5 13.3
12.0 – 11.8
C2Hx
A B C
3.2 4.2 –
3.2 4.3 5.0
7.1 3.8 5.3
7.1 6.3 8.6
5.6 5.1 6.3
3.9 – 5.2
A – wood, B – coconut shell, C – straw.
recovery boiler. From schematic diagram of the system, one part of heat from heat recovery boiler is used to drive the absorption refrigeration unit, in which a lithium bromide-water mixture is applied as working fluid to absorb the heat from exhaust gas, to provide cooling load for users; the other part of heat from heat recovery boiler and from refrigeration unit is used for heating.
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Fig. 3. Volume content of CO, CO2, H2, CH4, and C2Hx of gas in different thermal cracking temperature.
Monitoring and control system can collect the signals from different spots and give the orders to units, such as thermal cracking unit, anaerobic digestion unit, gas distribute unit, gas engine and the absorption refrigeration unit according to the need of users. 4. Mathematical model Modeling of CCHP driven by gas and performance assessment are presented in this section. The related data used in this model is given in Table 3. The absorption refrigeration is one single-effect absorption system which is similar to the approach used by ASHRAE [27]. In this study, some assumptions are employed as follows: (1) Gas is completely combusted and chemical reaction reaches equilibrium state. (2) Flow temperature at the inlet/outlet of every unit is constant. (3) Radiation heat transfer in every unit is neglected. (4) The whole system reaches steady state. (5) Heat loss of every unit is not considered. 4.1. Thermal cracking/anaerobic digestion gas and flue gas
V RO2 ¼ V CO2 þ V SO2 ¼ 0:01ðCO2 þ CO þ
ð1Þ
X
mC m Hn þ H2 SÞ 3
ð2Þ 3
where V RO2 is volume content of three atomic gases (N m /N m dry gas). Steam:
h i Xn V 0H2 O ¼ 0:01 H2 þ H2 S þ C m Hn þ 120ðdg þ V 0 da Þ 2
ð3Þ
where V 0H2 O is the steam volume content in theoretical flue gas (N m3/N m3 dry gas), dg is moisture content of gas (kg/N m3 dry gas), da is moisture content of air (kg/N m3 dry air). Nitrogen:
V 0N2 ¼ 0:79V 0 þ 0:01N2
ð4Þ
where V 0N2 is nitrogen volume fraction in theoretical flue gas (N m3/ N m3 dry gas), V0 is theoretical flue gas volume (N m3/N m3 dry gas), which can be obtained by
(
Low heat value of gas can be calculated as
H l ¼ H 1 r 1 þ H 2 r 2 þ . . . þ Hn r n
where H is heat value of gas (kJ/N m3), H1, H2 . . . Hn are heat value of combustible components (kJ/N m3), r1, r2 . . . rn are volume content of components. Volume content of combustion products can be obtained as follows. Three atomic gases (including CO2, H2S, SO2, etc.):
V 0 ¼ 0:209 Hl ; Hl < 10500 kJ=N m3 1000 0:26 V 0 ¼ 1000 Hl 0:25; Hl > 10500 kJ=N m3
ð5Þ
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heat output
hot water storage tank
cold output
heat from condenser
absorption refrigeration unit
feed water
monitoring and control system heat recovery boiler
heat exchanger 1
heat exchanger 2 flue gas dust collector
flue gas gas engine
electric generator
gas distribution unit solid waste CO, H2,CH4 anaerobic digestion unit
thermal cracking unit solid waste
material flow
signal feedback control network
Fig. 4. Schematic diagram of the CCHP based on waste to energy.
Total volume of theoretical flue gas is:
Table 3 Input data CCHP system under design conditions.
Gas engine
Absorption refrigeration
Heat recovery boiler
V 0f
¼ V RO2 þ V 0H2 O þ V 0N2
ð6Þ
Parameters
Value
Rated power output Efficiency of power generation Exhaust gas temperature Rated compression ratio
1.0 MW 36.5% 510 °C 8
Overall heat transfer coefficient of desorber Overall heat transfer coefficient of condenser Overall heat transfer coefficient of evaporator Overall heat transfer coefficient of absorber Heat transfer efficiency Effectiveness of solution heat exchanger Inlet/outlet temperature of frozen water Rated refrigerating capacity Concentration of strong solution in design condition Concentration of weak solution in design condition Flow ratio Rated refrigerating coefficient (COP)
70 kW/K 80 kW/K 95 kW/K
4.2. Gas engine
75 kW/K 80% 70% 12–7 °C 2800 kW 58.2%
The operation process of gas engine consists of two isometric processes and two isentropic processes. In order to obtain the performance of CCHP, we consider that the gas engine operates in ideal process. So, the efficiency of gas engine is
Exhaust gas temperature of boiler Heat transfer efficiency Heat recovery rate
125 °C 0.83 0.73
55.6% 22.38 0.71
Gas combustion temperature in the engine is
tther ¼
Hl V RO2 C RO2 þ V H2 O C H2 O þ V N2 C N2
ð7Þ
where C RO2 , C H2 O , C N2 are the average pressure volume specific heat of three atomic gases, steam and nitrogen respectively.
g¼1
T1 1 ¼ 1 k1 T2 c
ð8Þ
¼ vv 12 is compression ratio, T1, T2 are the beginning compres-
where c sion temperature and completed compression temperature (K), k is adiabatic exponent. Completed compression temperature can be expressed by
T 2 ¼ T 1 cðk1Þ
ð9Þ
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For the gas, adiabatic exponent k is determined by certain gas composition. The theoretical flue gas enthalpy is
I0f ¼ V RO2 C RO2 t f þ V 0N2 C N2 tf þ V 0H2 O C H2 O tf
V RO2 C RO2 V 0f
þ
V 0N2 C N2 V 0f
þ
V 0H2 O C H2 O V 0f
M¼
ri Mi
ð25Þ ð26Þ
qE qG
ð27Þ
ð12Þ
R0 8314 ¼ M M
4.4. Assessment of the CCHP system
ð13Þ
The relation between specific heat at constant pressure and specific heat at constant volume is
C 0p C 0v ¼ qf R
ð14Þ
where qf is density of flue gas. Using formula (11) and (14), adiabatic exponent k can be obtained. When the compression ratio c is constant, efficiency of gas engine g is obtained by formula (8). The exhaust temperature T4 of gas engine is determined by
T4 ¼
FR 1 FR
ð24Þ
Then, the COP of the absorption refrigeration system is
COP ¼
where Mi is the molecular weight for each component. Average gas constant of flue gas is
R¼
h10 ¼ h9 þ ðh11 h12 Þ
qSHE ¼ ðh11 h12 Þ ðFR 1Þ ¼ ðh10 h9 Þ FR ð11Þ
Equivalent molecular weight of flue gas is
X
T 12 ¼ eSHE T 9 þ ð1 eSHE Þ T 11
ð10Þ
where tf is gas combustion temperature in the gas engine (tther). Then, the specific heat at constant pressure of flue gas is
C g;p ¼
The energy balance and heat equilibrium of the solution heat exchanger are
T1 T3 T1 T3 T3 ¼ ¼ T2 T 1 ck1 ck1
ð15Þ
where T3 can be deemed as the gas combustion temperature in the gas engine which is tther. 4.3. Absorption refrigeration Fig. 5 illustrates the single-effect absorption refrigeration system (ARS). It includes a generator, an absorber, a condenser, an evaporator, a pump, two expansion valves, one solution heat exchanger. Mass balance in the generator can be written as follows:
mws ¼ mss þ mH2 O
ð16Þ
mws nws ¼ mss nss
ð17Þ
Performance assessment of the CCHP system is very important. The index to evaluate the performance includes PER (primary energy ratio) and exergy efficiency. (1) Primary energy ratio Based on discussing the performance of CCHP system by different composition gas, we can determine which type gas should be used to drive CCHP system according to need of user and its economic benefits. In order to indicate the performance of CCHP system, parameter of PER (primary energy ratio) is used, which is
PER ¼
We þ Qh þ Qc Gg H l
ð28Þ
where We is total engine electric power (kW), Qh is total heating power (kW), Qc is total refrigerating capacity (kW), Gg is gas consumption (N m3/s), Hl is the heat value of gas (kJ/N m3). The parameters of We, Qh and Qc can be determined, which are
W e ¼ C v ðT 3 T 2 Þ g
ð29Þ
Q c ¼ qE
ð30Þ
Q h ¼ qC þ qh
ð31Þ
In formula (31), qh is the heat used by hot water through the direct flue gas. (2) Exergy efficiency
mss ¼
nws mH2 O nss nws
ð18Þ
Exergy efficiency is the ratio of the output exergy to the input exergy of the CCHP system, which can be written to
mws ¼
nss mH2 O nss nws
ð19Þ
gex ¼
Exe þ Exh þ Exc Exf
ð32Þ
The heat capacities of the main components can be calculated as follows:
where gex is the exergy efficiency, Exe, Exh Exc and Exf are the exergy of electricity, heating, cooling and gas respectively,
q E ¼ h6 h5
ð20Þ
Exe ¼ W e
ð33Þ
q C ¼ h3 h4
ð21Þ
T0 Exh ¼ Q h 1 Th
ð34Þ
qG ¼ h2 þ FR h1 ðFR 1Þ h10
ð22Þ
T0 1 Tc
ð35Þ
where qE is determined by user need.
where qG is determined by waste heat from the gas engine, qG ¼ C g;p ðT 4 T 04 Þ, T 04 is the returned flue gas temperature.
qA ¼ FR h8 ðFR 1Þ h13 h7 ss where FR ¼ mmHwsO ¼ nssnn . ws 2
ð23Þ
Exc ¼ Q c
where T0 is the ambient temperature, Tc and Th are the cold water temperature and the heat water temperature respectively (in this case study, Tc and Th are assumed to be constant, which are respectively 278 K and 353 K).
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qC
3
1
2 genertator
condenser 4
10 11 solution heat exchanger
expansion valve 9 solution pump 5
12 expansion valve
8 evaporator
6
7
absorber 13 qA
qE
Fig. 5. The schematic of single-effect absorption refrigeration system.
The exergy of gas can be written as
Exf ¼
V 0f
Hl
ð36Þ
where V 0f is the gas consumption.
5. Results and discussion In order to facilitate the understanding, design and implementation of sustainable solutions for megacities, we analyzed the performance of CCHP system driven by different composition gas from anaerobic digestion and/or thermal cracking. The gas compositions from anaerobic digestion and thermal cracking (pyrolysis) are different. Considering the complexity and diversity of gas components, we chose three different type gases and discussed their effects on the CCHP system according to gas components of anaerobic digestion and pyrolysis (Tables 1 and 2, Figs. 2 and 3). The different composition gases into the CCHP are shown in Table 4, which gives the volume content of main combustible gas components. The adiabatic exponent, low heat value and combustion temperature of three different gases are shown in Table 5. For CCHP system, gas combusts in the gas engine and generates the power, and the waste heat of exhaust flue gas is collected to drive the absorption refrigeration system and heating system. So, the operation state of gas engine is very important for the performance of whole CCHP system. Hence, the related performance is analyzed in different conditions based on the engine operation in which the compression ratio c is main factor. Fig. 6 shows the COP of absorption refrigeration and the efficiency of gas engine g changing with compression ratio of gas engine. As seen in Fig. 6, as the compression ratio increases from 7.0 to 12.0, the efficiency of gas engine increases and COP of absorption refrigeration decreases from 0.77 to 0.51. The reason is that for the certain gas, the higher compression ratio will make g increase according to formula (8). For the gas engine, when inlet temperature and the combustion temperature are determined, the bigger compression ratio means that the exhaust gas temperature of engine declines according to formula (15). So, the little temperature of flue gas makes refrigerating capacity reduce, and the COP of absorption refrigeration decreases for this.
Table 4 Different combustible compositions gas (vol%).
Anaerobic digestion gas (pH = 6.9, t = 35 °C) Thermal cracking gas/pyrolysis (coconut shell, T = 700 °C) Mixed gas (anaerobic digestion and thermal cracking)
CH4
H2
CO
C2Hx
56.1 17.2
3.0 18.5
– 40.1
– 6.3
45.2
8.1
10.0
2.3
Table 5 Thermal properties of three different gases.
Anaerobic digestion gas Thermal cracking gas (pyrolysis) Mixed gas (anaerobic digestion and thermal cracking)
k (adiabatic exponent)
Hl (kJ/ N m3)
tther (°C)
1.475 1.231 1.420
20,684 17,521 18,287
2203 2086 2175
Fig. 6. Effect of compression ratio on COP and g.
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Fig. 7. Effect of compression ratio on exhaust gas temperature and output work of engine. Fig. 10. The change of PER and g with compression ratio.
Fig. 8. Effect of compression ratio on efficiency of engine and exhaust gas temperature.
As the compression ratio is constant, the change line of gas engine efficiency, which has bigger adiabatic exponent k, will have the bigger value. From Table 5, the adiabatic exponent of anaerobic digestion gas is the largest and the adiabatic exponent of thermal cracking gas is the smallest. So, the line g of anaerobic digestion gas is in higher place. And then, the COP line is on the contrary with the line of g.
Fig. 7 shows that the relation between exhaust gas temperature, output work of engine and the compression ratio of engine. From Fig. 7, exhaust gas temperature of engine decreases with the pressure, which changes from 1200 K to 800 K, for the certain gas whose compositions are constant; it can be explained by the formula (15). At the same time, the output work of gas engine will increase with the compression ratio of engine from 6500 kJ/N m3 gas to 9300 kJ/N m3 gas. For the certain compression ratio, the adiabatic exponent of anaerobic digestion gas is the largest which is 1.475 and the adiabatic exponent of thermal cracking gas is the smallest from Table 5 which is 1.231. The combustion temperature in the gas engine (tther) is like this. So, the anaerobic digestion gas has the highest combustion temperature and lowest exhaust temperature, and the output work of anaerobic digestion gas is the largest and thermal cracking has the smallest value. The effect of compression ratio on the efficiency of engine and exhaust gas temperature of engine is shown in Fig. 8. From Fig. 8, we can find that exhaust gas temperature of engine decreases with the compression ratio from 1185 K to 750 K, but the efficiency of gas engine increases from 0.41 to 0.56. The reason of exhaust gas temperature of engine decreasing can be explained by formula (15) and the engine efficiency’s increasing can be explained by formula (8). Fig. 9 gives the relation among the heat collected to absorption refrigeration, the heat to hot water and the compression ratio of engine. As shown in Fig. 9, we can find that when the exhaust
Fig. 9. Effect of compression ratio on heat collected to absorption refrigeration and heat to hot water.
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Fig. 11. The change of gex and g with compression ratio.
Fig. 12. Effect of output work ratio of gas engine on g and exhaust gas temperature of engine.
gas temperature declines with the increasing of compression ratio, the heat input to absorption refrigeration reduces from 1390 kJ/ N m3 gas to 485 kJ/N m3 gas and the heat to hot water reduces from 5485 kJ/N m3 gas to 1980 kJ/N m3 gas. It is easily understood because the exhaust gas temperature decreasing means the quality of waste heat to be decreasing.
The change of PER of CCHP system and the efficiency of engine g with compression ratio are shown in Fig. 10. With the compression ratio increasing from 7.0 to 12.0, the efficiency of engine g increases from 0.41 to 0.56, but the PER of CCHP decreases from 0.96 to 0.62. In order to explain this, it is necessary to analyze the change of output work of engine, heat collected to absorption refrigeration and heat input to heating hot water. As a whole, the total energy used by CCHP is reduced, and the PER presents the downward trend. Fig. 11 shows the change of exergy efficiency of CCHP gex and efficiency of engine g with compression ratio. As the compression ratio increasing, the exergy efficiency of CCHP gex decreases from 0.78 to 0.23 and the efficiency of engine g increases from 0.25 to 0.56. The reason is that the exhaust gas temperature of engine decreases with the compression ratio increasing, and the exergy of heating and cooling are reduced. So, the exergy efficiency of CCHP shows the decline. The above analysis is performed under the design conditions of CCHP system. Nevertheless, the CCHP system often operates under off-design conditions. In order to indicate the performance of CCHP system under the off-design conditions, we study the operation characteristic of the system under off-design state. The main analysis is based on the gas engine’s off-design operations. When the design conditions of gas engine is considered as a standard of comparison, the exhaust temperature ratio, efficiency of power generation ratio of gas engine are determined according to literature [28],
T g;4 ¼ T g;u;4 =T g;o;4 ¼ 0:53 þ 0:38N þ 0:09N2
ð37Þ
g ¼ g=go ¼ 0:13 þ 2:47N 1:6N2
ð38Þ
where Tg,4 is exhaust temperature of gas engine (K), g is efficiency of engine, N is output work of gas engine (W). ‘‘–’’ denotes the ratio. Effect of output work ratio N/N0 of gas engine on efficiency of engine g and exhaust gas temperature of engine are shown in Fig. 12. From Fig. 12, efficiency of gas engine g increases at first and then declines. The reason is that for the efficiency of power generation g, the operation state of gas engine is usually best under the design conditions and is not well under off-design conditions. Hence, near the design conditions, the efficiency of power generation g will be largest which is about 0.52. With the increase of output work ratio N/N0, the exhaust gas temperature of gas engine increases from 600 K to 1340 K. Fig. 13 shows the effect of output work ratio of gas engine on absorption refrigeration and heating hot water. From Fig. 13, we can find heat input to absorption refrigeration and heat input to heating hot water both increase with the increase of output work
Fig. 13. Effect of output work ratio of gas engine on heat collected to absorption refrigeration and heating hot water.
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Fig. 14. Effect of output work ratio of gas engine on PER and g.
609
quantity of heat input to heating hot water is more than the input to absorption refrigeration system. Fig. 14 shows the effect of output work ratio of gas engine on PER and efficiency of engine g. We can find that efficiency of engine first increase from 0.24 to 0.52 and then decline from 0.52 to 0.27, the PER of the CCHP system increases from 0.18 to 1.0. For the PER, with the output work of gas engine increasing when N/N0 is increasing, the efficiency of engine g is reduced and exhaust gas temperature of engine is increasing from Fig. 12 and more waste heat will be used to drive the absorption refrigeration system and heating system from Fig. 13, hence, the PER of the CCHP system will increase. The effect of output work ratio of gas engine on exergy efficiency of CCHP gex and efficiency of engine g shows in Fig. 15. From Fig. 15, exergy efficiency of CCHP gex increases from 0.05 to 0.38 with output work ratio of gas engine increasing. The variation is similar with Fig. 14. As the output work ratio of gas engine increasing, the exhaust gas temperature increases and the exergy of heating and cooling increase. So, the exergy efficiency of CCHP gex shows rising trend, whose largest value is about 0.38. Fig. 16 shows the effect of gas temperature on refrigeration capacity and COP. From Fig. 16, we find that the COP of absorption refrigeration driven by anaerobic digestion gas is minimal and its refrigeration capacity is also minimal that the average value is about 350 kJ/m3 gas in the same conditions. COP of absorption refrigeration driven by thermal cracking gas and refrigeration capacity are maximal values that the average value is about 0.67. The reason is that exhaust temperature of engine driven by anaerobic digestion gas is minimal and by thermal cracking gas is maximal. In the same conditions, the higher temperature flue gas can provide more energy to absorption refrigeration and make COP be higher. 6. Conclusions
Fig. 15. Effect of output work ratio of gas engine on gex and g.
ratio N/N0, which change from 250 kJ/m3 gas to 1580 kJ/m3 gas and from 985 kJ/m3 gas to 6000 kJ/m3 gas respectively. The reason is that the exhaust temperature of gas engine increases and the heat input to absorption refrigeration will increase. For the exhaust heat of absorption refrigeration can be used by heating system, the
In order to evaluate the effect of different composition gases, which are anaerobic digestion, thermal cracking and the mixture respectively, on the performance of CCHP system, we analyzed the composition of gases produced from different raw materials, and presented the calculation model according to combustion characteristic of gas and thermodynamic process of CCHP system. The related parameters, such as PER of CCHP system, exergy efficiency of CCHP system gex, the efficiency of gas engine g, COP of absorption refrigeration, output work of engine, exhaust gas temperature of engine, heat input to absorption refrigeration and hot water production, were analyzed in different compression ratio
Fig. 16. Effect of exhaust gas temperature on refrigeration capacity and COP under off-design conditions.
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of engine and under off-design conditions. The main conclusions drawn from this study are summarized as follows: (1) For the main combustible gas from anaerobic digestion and thermal cracking process, the volume content of CH4 in anaerobic digestion gas is more than in thermal cracking gas, and the volume content of H2 in thermal cracking gas exceeds anaerobic digestion gas usually. (2) In the same compression ratio of gas engine, more work from the anaerobic digestion gas can be extracted, but the exhaust temperature of gas engine is lower and the heat input the absorption refrigeration is less, compared with the thermal cracking gas. The reason is that they have different heat value (heat value of anaerobic digestion gas is about 20,684 kJ/N m3, heat value of thermal cracking gas is about 17,521 kJ/N m3) and different gas combustion temperature (anaerobic digestion gas is about 2203 °C and thermal cracking gas is about 2086 °C). The anaerobic digestion gas can provide more work in the same compression ratio of gas engine. (3) The variation of PER of CCHP system, exergy efficiency of CCHP gex and the efficiency of engine g with the change of compression ratio are different. When the efficiency of engine g increases, PER of CCHP and exergy efficiency of CCHP gex decline. The variation of absorption refrigeration COP and the efficiency of engine g with the change of compression ratio are also different. When the efficiency of engine g increases, the exhaust temperature of gas engine will decline, and then the heat input the absorption refrigeration will reduce, leading to the decrease of COP. (4) Under off-design conditions, PER of CCHP system and exergy efficiency of CCHP system gex will increase with the output work of gas engine increasing. As a whole, the performance of CCHP system should be determined by considering all kinds of factors, such as cost of gas, efficiency of gas engine, cooling and heat load.
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