Energy, exergy, environmental and economic analysis of an agricultural waste-to-energy integrated multigeneration thermal power plant

Energy Conversion and Management 171 (2018) 222–240

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Energy, exergy, environmental and economic analysis of an agricultural waste-to-energy integrated multigeneration thermal power plant

T

⁎

O.J. Ogorure, C.O.C. Oko, E.O. Diemuodeke , K. Owebor Department of Mechanical Engineering, University of Port Harcourt, Port Harcourt, Nigeria

A R T I C LE I N FO

A B S T R A C T

Keywords: Absorption refrigeration cycle Biomass-to-syngas conversion Cogeneration power plant Combined power cycle Solid oxide fuel cell Organic Rankine cycle

This paper presents the energy, exergy, environmental and economic analysis of a proposed agricultural wasteto-energy integrated multi-generation power plant. The proposed plant will use agro-wastes composed of selected crops and animals wastes from a farm in Rivers state, Nigeria (latitude 4.44°N, longitude 7.1°E). Combined anaerobic digestion and gasification was used in converting the agro-wastes to synthetic gas, and subsequently converted to electrical energy and refrigeration in an integrated multi-generation plant composed of solid oxide fuel cell stack, gas turbine, steam turbine, organic Rankine and absorption refrigeration cycles. The lower heating value of the syngas is estimated as 23.47 MJ/kg; the proposed plant had a net power of 5.226 MW, energy and exergy efficiencies of 63.62 and 58.46%, respectively. The highest exergy destruction rate was in the combustion chamber with a contribution of 15% to the overall exergy destructed. Life cycle cost of $3.753 million, breakeven point of 7.5 years and unit energy cost of $0.0109 per kWh were obtained. Environmental analysis showed specific emission of CO2 of 141.2 kg/MWh and sustainability exponent of 3.65, with exit flue gas stack temperature of 60.4 °C, respectively. The paper introduced energo-environmental sustainability and energo-economic sustainability exponents to holistically assess the sustainability of the proposed plants.

1. Introduction Sustainable, clean and affordable power supply is key to the success of any organization whose goal is to impact positively on the society and the environment by reducing pollution from fossil fuel resources, utilizing readily available and cheap renewable resources from agricultural waste [16,37]. According to Charles [13], a stable power generation and supply from agricultural waste is possible in Nigeria. With appropriate technology and policies, clean and sufficient power can be made available to meet the energy challenges of the Nigeria [33]. Papapostolou et al. [39] showed that waste-to-energy generation schemes are an attractive option, producing energy with low pollutant emissions, low cost and elimination of waste accumulation. Also exploiting biomass from agricultural residues (livestock manure and crop wastes) exhibits beneficial features as there is no interference with the production of primary food and is available at low cost. Biomass can be converted into energy through gasification and anaerobic digestion to syngas (which comprises hydrogen, carbon dioxide and methane) and biogas (methane) that can be used as fuel in power plants [6]. Gasification is a thermo-chemical process whereas anaerobic digestion is a bio-chemical process; both of which are the most likely cost effective

⁎

Corresponding author. E-mail address: [email protected] (E.O. Diemuodeke).

https://doi.org/10.1016/j.enconman.2018.05.093 Received 27 March 2018; Received in revised form 14 May 2018; Accepted 26 May 2018 0196-8904/ © 2018 Elsevier Ltd. All rights reserved.

conversion processes of biomass. Solomie et al. [47] performed an economic analysis of anaerobic digestion of a biogas plant situated in a farm using net present value (NPV) and internal rates of return (IRR) concepts; they concluded that the conversion process is economically viable. Hailong et al. [20] investigated the technology that combines anaerobic digestion and biomass gasification to produce bio-methane with the hydrogen gas from the syngas used in upgrading the biogas from anaerobic digestion. Shahida and Mohd [44] analysed the potential of power generation from biogas obtained from palm oil waste (palm oil mill effluent). They confirmed its technical and economic viability, citing possible investment sizes of such biogas production plant. Rade and Zoran [41] considered piggery wastes for generating biogas fuel for a cogeneration plant of 3 MW. Equilibrium models to predict the amount of gasification product in a downdraft gasifier have been developed by Zainal et al. [52], Jarungthammachote and Dutta [23], Athari et al. [4] and Allesina et al. [2]. Predicted values from their results compared reasonably with experimental data for different biomass. Spyridon and Gerrit [48] developed a theoretical model of anaerobic digestion in order to predict the amount of biogas in animal waste slurries. According to Dincer and Ratlamwala [17], the concept of multi generation system is aimed at recovering thermal energy in waste heat

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Nomenclature h P − R cp Q̇ X ż f st s T ṁ Ẇ η n a g

AC air compressor ARCOND absorption refrigeration condenser CC combustion chamber EVP evaporator EX expansion valve FC fuel compressor GEN absorption refrigeration generator HP-HRSG high pressure heat recovery steam generator HP-ST high pressure steam turbine HROVG heat recovery organic vapour generator HX preheater INV inverter LP-HRSG low pressure heat recovery steam generator LP-ST low pressure steam turbine MT micro turbine OFCOND organic fluid condenser P feed water pump PP preprocessor POME palm oil mill effluent SOFC solid oxide fuel cell SReg solution regenerator

specific enthalpy, kJ/kg pressure, kPa gas constant, kJ/kmol K isobaric specific heat capacity, kJ/kg K heat, kW concentration of LiBr molar flow rate of fuel, kmol/s fuel steam specific entropy, kJ/kg K temperature, °C or K mass flow rate, kg/s power, kW efficiency number of moles, kmol air flue gases

Abbreviations ABS

absorber

highest irreversibilities of 31.4 and 27.9%, respectively. Maximum thermal efficiency of 60.6% was obtained at a compression ratio of 4.0. Chan et al. [11] presented a hybrid SOFC-GT power plant with internal reforming, fueled by biogas. They obtained an energy efficiency of 60%, and also showed that high operating pressure improves the systems efficiency, while increase in specific fuel consumption decreases the plant output. Absorption refrigeration systems (ARS) utilize low grade, low cost thermal energy available to produce cooling. In commercial systems, two commonly used refrigerant-absorbent pairs are Water-Lithium Bromide (LiBr-H2O) and Ammonia-Water (NH3-H2O). LiBr-H2O, with water as the refrigerant and lithium bromide as the absorbent, is suitable for moderate temperature applications (above 5 °C), such as air conditioning·NH3-H2O is for low temperature applications (below 5 °C) with ammonia as the refrigerant and water as absorbent[17]. Shahata et al. [45] and Al-Tahaineh et al. [3] studied vapour absorption refrigeration using the First and Second law of thermodynamics, with results that showed maximum coefficient of performance (COP) and exergetic efficiency at low values of absorber and generator temperatures. Bhargav et al. [10] and Zadeh and Narvid [53] analysed a single effect absorption chiller for air conditioning purpose, with LiBr-H2O as the working fluid. From their results, high exergy loss was recorded in the generator of the system. According to Oyedepo et al. [38] and Memon et al. [30] reduced flue gas temperature reduces the fuel harmful emissions to the environment and is beneficial in increasing the system efficiency, thereby increasing the sustainability of the plant with regards to fuel resources. Saidur et al. [42] identified a reverse relation between a power plant’s sustainability index and environmental impact with respect to exergy efficiency, as sustainability increases the environmental impact decreases with increased efficiency. While many studies have been devoted to the study of multigeneration power plants, study on combined anaerobic digestion and gasification to convert agricultural wastes to electrical energy and refrigeration in an integrated multi-generation plant comprised of solid oxide fuel cell stack, gas turbine, steam turbine, organic Rankine and absorption refrigeration cycles has not been fully established in terms of energy, exergy, environmental and socio-economic analyses. Furthermore, study on holistic sustainability (combined energo-economic, energo-environment and energo-social sustainability exponents)

in order to improve the efficiency and economy of the system. Solid oxide fuel cell (SOFC) has been used for power generation, in hybrid systems, combined heat and power generation and for combined cooling, heating and power generation; with operating temperature of 500 (°C) to 1000 (°C) and power of 2 kW to several megawatts [7]. SOFC also produce significant quantity of waste heat due to its high operating temperature, and off-fuel, making it possible for it to be coupled with a gas turbine or an organic Rankine cycle (ORC) or a steam turbine as a bottoming cycle for total efficiency improvement [31]. Siefert and Litster [43] presented an exergy and economic analysis of an integrated biogas plant with an SOFC and gas turbine (GT) power unit. Their results showed the economic viability of an SOFC-GT power plant fueled by biogas. Harfei [21] and Ebrahimi and Moradpoor [18] considered an SOFC, micro-gas turbine and ORC for power generation. Their results show that an increase in thermal efficiency is achievable in micro-scale power generation. Sreeramulu and Deepak [49] carried out a comparative analysis of an SOFC-GT combined power cycle fueled with natural gas and diesel, respectively. High exergy destruction was recorded in the SOFC and combustion chamber of the plant. Fahad et al. [19] studied the energy performance of a tri-generation plant with an SOFC, ORC, a heat exchanger for process heat and a single effect absorption refrigeration system (ARS) for cooling. Their result show 22% gain in efficiency with trigeneration when compared with only the SOFC and ORC only. The use of trigeneration systems ensures higher exergy efficiency and lower emissions [1,14,32]. Bellomare and Rokni [9] considered an integrated gasification SOFC-GT plant fueled with syngas produced from the gasification of biomass. An optimum efficiency of 52% was obtained. Pirkandi et al. [40] carried out a performance analysis of a SOFC combined heat and power plant and obtained an overall efficiency of 73%. The thermoeconomic analysis of four different configurations of natural gas and biogas fed SOFC was carried out by Mehr et al. [29]. Their results showed a maximum thermal efficiency at current density of 6000 A/m2, with an optimum anode recycling ratio of 0.25–0.3. Thermal efficiency of the biogas fed SOFC was higher than that of natural gas. Haseli et al. [22] examined the performance of high temperature SOFC combined with an air preheater and a dual-pressure gas turbine. Their results indicated that an increase in the turbine inlet temperature resulted in the decrease of the thermal efficiency of the cycle and increased net power output. Also the combustion chamber and the SOFC had the 223

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• Kinetic and potential energy losses are neglected; • The system operates under steady state conditions; • The fuel and air are modeled as ideal gases; • Combustion process in the combustion chamber is complete; • Air and fuel entering the system are at standard environmental conditions; • Thermal equilibrium is maintained in the SOFC and temperature of the products at the exit of both the anode and cathode are equal; • Adiabatic conditions are maintained in each component; • Composition of air at the compressor inlet is 79% N and 21% O ; • Components of the agricultural waste consist of only

of multi-generation plant is currently insufficient. Therefore, this study covers the utilization of biogas and syngas potentials from agricultural wastes in a commercial farm in Rivers state, Nigeria (latitude 4.44°N, longitude 7.1°E), to drive a multi-generation power, heat and refrigeration plant. Only the biomass energy content, energy and exergy analysis, economic, environmental and the holistic sustainability assessments of the integrated plant are considered in this study. 2. Problem formulation and solution methods

2

The proposed integrated multi-generation agricultural waste to energy power, heat and refrigeration plant in this work is made up of a combined anaerobic digestion and gasification unit (CAdGU) with a SOFC-GT power plant; a steam turbine cycle (STC), ORC power plant and an ARC system as shown in the block diagram in Fig. 1. The ARC is used for the purpose of intercooling of ambient air for the gas power cycle and cooling of the condensers of the STC and ORC, respectively. The proposed plant may seem too complex to completely have all its components same time on board, but a theoretical plausibility of developing the proposed plants is here demonstrated.

2

C , H , O, NandS .

2.3. Biomass-to-syngas analysis The conversion of biomass from available crop and animal waste through anaerobic digestion and gasification is presented in this section. Only the crop waste is gasified due to the high amount of moisture in the animal waste, which results in lower energy content of the syngas produced [6]. 2.3.1. Agricultural waste assessment The amount of waste generated per crop and farm animal resource ṁ cr and ṁ fa (kg/yr) is evaluated as

2.1. System description The plant is fueled by syngas produced from biomass via a two-step process, the first one being the anaerobic digestion of animal manure, and the gasification of crop waste. Air and fuel are compressed in the compressors (AC and FC) and passed through preheaters (HX1 and HX2) before entering the SOFC to produce electricity. Hot air and unused fuel from the SOFC are fed into the combustion chamber (CC) of a gas turbine cycle (GTC), which utilizes the combustion products to drive a gas turbine (GT) to produce electricity, as shown in Fig. 2. Exhaust flue gases from the turbine are passed through a dual pressure Heat Recovery Steam Generator (HP-HRSG and LP-HRSG), which produces steam to drive a dual pressure steam turbine (ST) for power generation. Flue gases exiting the LP-HRSG enter a refrigerant generator (RG) and generates refrigerant vapour for the absorption refrigeration cycle (ARC); before entering the heat recovery organic vapour generator (HROVG), providing vapour to drive a micro-turbine (MT) for electric power generation. A T-s diagram and enthalpy concentration diagram are shown in Figs. 3 and 4 respectively.

ṁ cr = Acul ṁ cr − waste

(1a)

ṁ fa = 365Nfa ṁ fa − res

(1b)

where Acul (hec) , ṁ cr − waste (kg/hecyr) , ṁ fa − res (kg/day) , Nfa (−) are the cultivated land area of crop, mass of plant waste per hectare per annum, average daily mass of waste per farm animal and population of farm animals, respectively. 2.3.2. Anaerobic digestion model The modified Buswell and Mueller chemical reaction for anaerobic digestion of biomass is employed to obtain the fraction of biomethane (CH4 ) in the biomass and is given as [48]

Ca Hb Oc Nd Se + n1 H2 O → n2 CH4 + n3 CO2 + n4 NH3 + n5 H2 S

(2a)

where ni (mol) are the amount of water required and respective products of the digestion process, i = 1, 2, 3, 4, 5.

2.2. Assumptions

b c 3d e n1 = ⎛a− − + + ⎞ 4 2⎠ ⎝ 4 2

(2b)

The following assumptions are made for the system modeling [8,25]:

a b c 3d e n2 = ⎛ + − − − ⎞ 8 4 8 4⎠ ⎝2

(2c)

Feedstock

CAdGU

Fuel

SOFC-GT

Electric energy

Flue gas Electric energy

STC Flue gas

Cooled Air

ARC AR

Ambient Air

Flue gas ORC

Electric energy

Exhaust gas Fig. 1. Block diagram of integrated multi-generation agricultural waste to energy plant. 224

Energy Conversion and Management 171 (2018) 222–240

O.J. Ogorure et al.

BIOMASS

ANAEROBIC DIGESTER

BIOGAS

BIOGAS CLEANUP

GASIFIER

SYNGAS

SYNGAS CLEANUP

PP SYNGAS

12

eel-Gen3

18 SOFC

3

CC

11

2

1 FC

CATHOD

9

47 exit STC

PHP

elel-Gen2

el-Gen1

AC

22 2 Cooling air

SCOND

LPHRSG

INV

6

21

20

25

13

8

7

HX2

19

HPHRSG

4

ANODE

HX1

LP-ST

HP-ST

PLP

24

GT

23

10 5 Cooling air exit

14 43 39

29

ARCOND

GEN

EV2 31

32

EVP

28

HROVG

34 EV1 35

Ambient air

OFCOND

37 MT

16

el-Gen4

ABS

45 4

44 4

41 40

36

27 ARP

26

46 4

ORC

SReg

ARC

42

15

33

30

38 OFP

Cooling air

Exhaust flue gases

Fig. 2. Plant configuration of the integrated agricultural waste-to-energy power plant.

a b c 3d e n3 = ⎛ − + + + ⎞ 4 8 4⎠ ⎝2 8

(2d)

n4 = d

(2e)

n5 = e

(2f)

where CHx Oy Nz is the chemical formula of the dry and ash free biomass; n H2 , nCO , nCO2 , n H2 , n H2 O , nCH4 (mol/molbio) are the specific molar amount of the constituents of the syngas; w (mol/molbio) is the specific molar amount of the biomass moisture; m (mol/molbio) is the specific molar amount of air; −

The fraction of CH4 in biomass, x CH4 (−), is given as

x CH4 =

n2 ∑ ni

w=

−

Mbio × MC

MH2 O (1−0.01MC )

(4b)

m = ϕ (1 + 0.25x −0.5y )

(4c)

(3) −

Mbio (g/mol) is the molecular mass of biomass; MC (%) is the total moisture content; and ϕ (−) is the equivalence ratio. To estimate the specific molar amount of the components of the syngas, an algorithm presented in Allesina et al. [2] is adopted as follows: first calculate the equation constant of the water-gas shift, K1, and methane formation reaction, K2 , respectively;

2.3.3. Gasification process modeling A downdraft gasifier is represented in this model [2,52]. The general gasification equation is given as:

CHx Oy Nz +wH2 O + m (O2 + 3.76N2) → n H2 H2 + nCO CO + nCO2 CO2 z + n H2 O H2 O + nCH4 CH4 + ( + 3.76m) N2 2 (4a)

K1: CO + H2 O ↔ CO2 + H2 225

(4d)

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O.J. Ogorure et al.

Fig. 3. T-s diagram of power cycles of the integrated multi-generation plant.

K2: C + 2H2 ↔ CH4

(4e)

LHVfuel =

K1and K2 are evaluated as

K1 = exp K2 =

4276 − 3.961 T

∑ γi LHVi

where γi (−) is the mass fraction of the i th gas component. (4f) 2.4. Thermodynamic analysis of the power plant

7082.842 7.467 × 10−3T 2.167 × 10−6T 2 0.702 −6.567ln(T ) + − + T 2 6 2T 2 (4g) + 32.541

A thermodynamic analysis of the integrated power plant composed of energy and exergy characteristics of the components, and the plant as a whole is presented in this section. The energy balance of a control volume based on the first law of thermodynamics, [12]:

A chemical balance for the component of the syngas is obtained from the generic gasification equation and the two equilibrium constant equations for water-gas and methane formation to solve for the specific molar amount of the syngas components:

̇ + Win ̇ + Qin

∑ ṁ ⎛h + ⎜

in

Carbon: nCO + nCO2 + nCH4 = 1 Hydrogen: n H2 + n H2 O + 2nCH4 = 0.5x + w

Oxygen: nCO + 2nCO2 + n H2 O = w + 2m + y

(6)

(5a)

+

(5d)

K2 =

nCH4 . ntot n H2 2

(5e)

⎝

⎟

v2 + gz ⎞ 2 ⎠out ⎟

(7)

where E ̇ (kJ) ; is the total energy; Ẇ and Q̇ (kW) , h (kJ/kg), v (m/s), g (m/ s2) and z (m) are the power, heat transfer rate, specific enthalpy, velocity, acceleration due to gravity and elevation of the working fluid, respectively; the indices “in”, and “out” refer to the inlet and exit, respectively. The exergy balance equation of a control volume process with steady flow condition is given as [34]

(5c)

nCO2 ·n H2 nCO ·n H2 O

⎜

out

(5b)

K1 =

∑ ṁ ⎛h +

⎝

v2 ̇ + Wout ̇ + gz ⎞ = Qout 2 ⎠in

where ntot is the total number of gaseous moles in the gasifier reactor. Eqs. (5a)–(5e) are evaluated by an energy balance equation of the enthalpy of reactants and products of the biomass gasification equation, and solution method from Zainal et al. [52] was adopted in obtaining the values of the unknowns. The low heating value of the gas from both anaerobic digestion and gasification of the waste, LHVfuel (kJ / kg ) , is given as

Eẋ in−Eẋ out −Eẋ D = ΔEẋ

(8)

or

T

̇ = ΔEẋ ∑ ⎛1− T0 ⎞ Qk̇ −W −̇ Po V −To Sgen ⎜

⎟

⎝

k⎠

(8a)

where Eẋ in and Eẋ out (kW) represents the exergy rate entering and 226

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O.J. Ogorure et al.

Fig. 4. Enthalpy concentration diagram of absorption refrigeration cycle.

exiting the system, respectively; ΔEẋ is the stream exergy rate; Qk̇ (kW) is the rate of heat transfer at the relevant boundary of the control vȯ ∂ and lume at temperature Tk (K); Ẇ (kW) is the rate of work transfer; Ex, ̇ To Sgen represents the rate of exergy destroyed associated with the system. Furthermore, exergy of a stream, Eẋ , is divided into physical exergy, Eẋ ph , chemical exergy, Eẋ ch , kinetic exergy, Eẋ ke , and potential exergy, Eẋ pe .

Eẋ = Eẋ ph + Eẋ ch + Eẋ ke + Eẋ pe

̇ , IPP + Q̇ AREV Wnet ηII , IPP =

ηII , k =

γD, k =

(8b)

(1 −

To TAREV

)

Eẋ ch

Eẋ Product Eẋ Fuel

(9b)

(9c)

Eẋ Dk ∑ Eẋ D, k

(9d)

where k represents the k-th components of the integrated power plant.

where Eẋ ke andEẋ pe ≈ 0 from the stated assumptions;

Eẋ = Eẋ ph + Eẋ ch

(8c)

Eẋ ph = ṁ k {(hk−ho)−To (sk−so)}

(8d)

2.4.1. Energy and exergy characteristics of components of the IPP 2.4.1.1. SOFC analysis. The fuel cell model in this work is based on a tubular design, fueled by syngas. The following chemical reactions are considered for the thermodynamic analysis of the SOFC stack:

(8e)

z:̇H2 +

−

Eẋ ch = ṁ k {x i Ex ch, i + Rk T0 ∑ (x i lnx i )}

1 O2 → H2 O (Electrochemical) 2

(10a)

−

where hk , ho (kJ/kg) , sk , so (kJ/kgK) , Ex ch, i (kJ/kg) and x i (−) are the specific enthalpy, and entropy of the stream and environment, respectively; standard chemical exergy of the i-th component of the stream (fuel, air, refrigerant), and fraction of the i-th component of the stream, respectively. The first and second law efficiencies of the integrated power plant, second law efficiency of plant components and exergy destruction ratio γD, k (−) are given as[16]

̇ , IPP + Q̇ AREV ⎞ Wnet ηI , IPP = ⎜⎛ ⎟ ṁ f LHVf ⎠ ⎝

x ̇: CH4 + H2 O → CO + 3H2 (Reforming)

(10b)

y ̇: CO + H2 O → CO2 + H2 (Shifting)

(10c)

With the initial molar concentration of the respective gases known, molar flow rate of the gases taking part in the electrochemical(z )̇ , reforming (x ̇) and shifting (y )̇ reactions, are determined using molar balance of the reacting species. Fuel and air utilization factors and equilibrium constant values of the reforming and shifting reactions are determined as in Chan et al. [11] and Ebrahimi and Moradpoor [18]. The reversible open circuit voltage of the fuel cell, Vrev (V ) , is calculated by Nernst equation [18]:

(9a) 227

Energy Conversion and Management 171 (2018) 222–240

O.J. Ogorure et al. −

Vrev

where Qel̇ , Qṙ , Qṡ (kW) are the heat generated from the electrochemical process, reformining and shifting reactions, respectively; and are evaluated as:

1

RTfc ⎛ PH2 PO22 ⎞ −ΔG ° ln ⎜ = + ⎟ nF nF ⎜ PH2 O ⎟ ⎠ ⎝

(10d)

where PH2 , PO2 , and PH2 O (kPa) represent the partial pressures of hydrogen, oxygen and steam, respectively; −ΔG ° (kJ) is the change in standard Gibbs energy of steam due to oxidation of hydrogen; n (−) is the number of electrons released in the reaction; F (C )is Faraday con− stant; R (kJ/kmolK) is universal gas constant; and Tfc (K) is the fuel cell operating temperature. Due to irreversibilities resulting from activation, concentration and ohmic over potential in the fuel cell, the reversible open circuit voltage is not achievable and these factors must be accounted for to obtain the net voltage. The net voltage is obtained as

Vnet = Vrev−ΔVohm−ΔVact −ΔVconc

ΔVohm = i ∑

RT ⎛ ⎛ i ⎞⎛ i ⎞⎞ ln ⎜ ⎜1− ⎟ ⎜1− ⎟⎟ nF ⎝ ⎝ iL, an ⎠ ⎝ iL, ca ⎠ ⎠

Qṡ = y ̇ (hCO2 + h H2−hCO−h H2 O )

(10q)

(10r)

−o −o −o wheres H2 O , s H2 and sO2

(kJ/kmolK) are the standard entropy of water, hydrogen and oxygen, respectively.

(10e)

• Fuel and air compressors: ẆkC =

ṁ k cpk ΔTkC ηC

Eẋ D, kC = ẆkC + Eẋ in−Eẋ out

(11a) (11b)

where kC = FC , AC for fuel and air compressors, respectively; k = f , a for fuel and air, respectively; ΔTkC (K) is the isentropic temperature difference across the compressors; cpk (kJ/kg K) is the isobaric specific heat capacity of the working fluid; and ṁ k (kg/s) is the mass flow rate; Eẋ D, kC , Eẋ in, Eẋ out (kW) are the exergydestroyed , exergy in, and exergy out of the compressors, respectively.

(10f)

• Fuel and air Preheater:

(10g)

where Aj (Ωm), lj (m) and Bj (K) are ohmic loss constant parameters that depends on the component materials of the SOFC, j represents anode cathode electrode and interconnections. The concentration loss ΔVconc (V ) , which is as a result of mass transfer limitations of the gases into and through the porous electrodes is given as[18]

ΔVconc =

(10p)

1

where i , i 0, an and i 0, ca (mA/cm2) are the fuel cell current density, current densities at the anode and cathode, respectively. The ohmic loss is due to the resistance by the electrodes, electrolyte and interconnections to the conduction of electrons and ions, is given as [25]:

⎛ ⎛ Bj ⎞ ⎞ ⎜Aj l j exp ⎜ T ⎟ ⎟ ⎝ fc ⎠ ⎠ ⎝

Qṙ = x ̇ (hCO + 3h H2−h H2 O−hCH4 )

⎛ −o ⎛ PH2 PO2 ⎞ ⎞ −o 1 −o 2 ΔSel = z ̇ ⎜ ⎛s H2 O−s H2 − sO2 ⎞ + Rln ⎜ ⎟⎟ 2 ⎠ ⎜ PH2 O ⎟ ⎟ ⎜⎝ ⎝ ⎠⎠ ⎝

−

ΔVact

(10o)

ΔSel (kW/K) is the entropy growth in the SOFC given as

where ΔVohm , ΔVact , ΔVconc are the ohmic, activation and concentration losses respectively. The activation loss, ΔVact (V ) , results from the slow rate of reaction on the electrode surface and can be evaluated using the Butler–Volmer equation [25]:

2RTFC ⎛ i ⎞ −1 ⎛ −1 ⎛ i ⎞ ⎞ = ⎜ ⎟ + sinh ⎜ ⎟⎟ ⎜sinh nF ⎝ ⎝ 2i 0, an ⎠ ⎝ 2i 0, ca ⎠ ⎠

Qel̇ = I (ΔVohm + ΔVact + ΔVconc ) + Tfc ΔSel

Q̇HX , k = ṁ k cp, k

(12a)

ΔTHX , k = ṁ g cp, g ΔTHX , g

(12a)

Eẋ D, HX , k = (Eẋ in, k−Eẋ out , k )+(Eẋ in, j−Eẋ out , j )

(12b)

where Q̇HX , k and Eẋ D, HX , k (kW) are the rate of heat transfer and exergy destroyed in the air/fuel preheaters, respectively; ṁ g (kg/s) and cp, g (kJ/kgK) is the mass flow rate and isobaric specific heat capacity of the flue gases; ΔTHX , g and ΔTHX , k (K) is the temperature differences on the flue gases side and air/fuel sides, respectively.

• Combustion Chamber:

(10h)

(mA/cm2)

are the limiting current densities at the where iL, an and iL, ca anode and cathode, respectively. Power generated in the SOFC is calculated as

Energy and exergy balance in the combustion chamber is given as [15]

̇ WSOFC , DC = IVnet

(10i)

̇ = ṁ g c p T9 + ṁ f LHV (1−ηCC )=ṁ a c p T8 + ṁ f LHV QCC g a

(13a)

Ẇloss = IVloss

(10j)

ṁ g = ṁ a + ṁ f

(13b)

̇ ̇ WSOFC , AC = ηinv WSOFC , DC

(10k)

Eẋ D, CC = Eẋ 8 + Eẋ 4−Eẋ 9

(13c)

(10l)

̇ and Eẋ D, CC (kW) are the efficiency of combustion, where ηCC (−) , QCC heat of combustion and exergy destroyed rate in the CC, respectively.

I = iAact Ncell

̇ ̇ ̇ whereWSOFC , DC , Wloss , WSOFC , AC (kW) are the DC power, power loss and AC power respectively, ηinv (−) is the inverter efficiency; Aact (m2) is activation area; and Ncell (–) is the number of cells.

ηSOFC = μf

• Gas turbine

nFVnet −

−Δh f

(10m)

(14a)

̇ Eẋ D, GT = Eẋ 9−Eẋ 10−WGT

(14b)

̇ and Eẋ D, GT (kW) are the GT rate of work transfer and exergy where WGT destroyed, respectively. The firstandsecondlaw efficiencies of SOFC-GTU are given as

ηSOFC (−) is the SOFC efficiency, and LHVfuel (kJ/kg) is the lower heating value of fuel . ̇ (kW) is given as [11] and Net heat generated in the SOFC stack, Qsofc [25] ̇ = Qel̇ −Qṙ −Qṡ Qsofc

̇ = ηGT ṁ g (h9−h10s ) = ηGT ṁ g cp, g (T9−T10s ) WGT

ηI , SOFC , GT =

(10n) 228

̇ + W −Ẇ −Ẇ ̇ WSOFC GT A, C FC ṁ f LHV

(15a)

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ηII , SOFC, GT =

̇ + W −Ẇ −Ẇ ̇ WSOFC GT A, C FC Eẋ ch, f

2.4.1.2. Absorption refrigeration cycle. The energy and exergy analysis for the ARC components are given as ([53;24])

(15b)

• Generator:

• Steam turbine cycle

̇ QGEN = ṁ g cp g (T14−T15) = ṁ ss h33 + ṁ r h29−ṁ ws h28

(22a)

Eẋ D, GEN = Eẋ 14 + Eẋ 28−Eẋ 15−Eẋ 29−Eẋ 33

(22b)

The energy and exergy balance for the steam turbine cycle are given as:

• Heat recovery steam generators (HP-HRSG and LP-HRSG):

• Regenerator:

̇ QkHRSG = ṁ ks Δhks = ṁ g cp, g ΔTg

(16a)

̇ QSReg = ṁ ss (h33−h34 ) = ṁ ws (h28−h27)

(23a)

Eẋ D, kHRSG = ΔEẋ g + ΔEẋ ks

(16b)

Eẋ D, SReg = Eẋ 27 + Eẋ 33−Eẋ 28−Eẋ 34

(23b)

̇ and Eẋ D, kHRSG (kW) are the mass where k = HP , LP ; ṁ ks (kg/s) , QkHRSG flow rate of high pressure and low pressure steam, heat transfer rate and exergy destroyed in the steam generators, respectively.

• Absorption refrigeration pump:

• Steam turbines (HP-ST and LP-ST): ṁ s T21 = ṁ LP T20 + ṁ HP T19

(17a)

ẆHP, ST = ṁ HP (h18−h19)

(17b)

Eẋ D, HPST = Eẋ 18−Eẋ 19−ẆHP, ST

(17c)

ẆLP, ST = ṁ LP (h21−h22)

(17d)

Eẋ D, LPST = Eẋ 21−Eẋ 22−ẆLP, ST

(17e)

(18a)

Eẋ D, SCOND = Eẋ 22 + Eẋ 46−Eẋ 23−Eẋ 47

(18b)

Q̇ ABS = ṁ r h32 + ṁ ss h39−ṁ ws h26

(25a)

Eẋ D, ABS = Eẋ 32 + Eẋ 39 + Eẋ 42−Eẋ 26−Eẋ 49

(25b)

Q̇ ARCON = ṁ r (h29−h30)

(26a)

Eẋ D, ARCON = Eẋ 29 + Eẋ 42−Eẋ 43−Eẋ 30

(26b)

• Feed water pumps (FWP

HP andFWPLP ) :

ṁ (h −h ) Ẇ FWPHP = HP 25 24 ηp

(19a)

Eẋ D, FWPHP = Ẇ FWPHP + Eẋ 24−Eẋ 25

(19b)

ṁ (h −h ) Ẇ FWPLP = LP 24 23 ηp

(19c)

Eẋ D, FWPLP = Ẇ FWPLP + Eẋ 24−Eẋ 23

(19d)

(20a)

Q̇ A, STC = Q̇HP, HRSG + Q̇LP, HRSG

(20b)

(27a)

Eẋ D, EVP = Eẋ 31 + Eẋ 41−Eẋ 32−Eẋ 42

(27b)

ṁ ΔP Ẇ Fk = a k ηFan ρa

(28a)

Eẋ D, Fk = Ẇ Fk + Eẋ in, k−Eẋ out , k ; k = {1, 2}

(28b)

where ṁ r , ṁ ws and ṁ ss (kg/s) are the mass flow rate of refrigerant, weak ̇ , Q̇ ABS , Q̇ ARCON , ̇ , QSReg solution and strong solution, respectively; QGEN and Q̇EVP (kW) are the heat transfer rate in the generator, solution regenerator, absorber, condenser and evaporator, respectively; and Ẇ ARP , andẆ Fk (kW) are the AR pump and fan work transfer rate. Concentrations of the weak and strong solution mixtures of LiBr-H2O in the absorber, regenerator and generator are evaluated as [26]

Ẇ FWPHP , Ẇ FWPLP , Eẋ D, FWPHP and Eẋ D, FWPLP (kW) are the rate of work transfer in the high and low pressure feed water pumps, and their exergy destruction rate, respectively. ̇ , STC and Q̇ A, STC (kW) , The net power generated and heat added, Wnet respectively, in the STC are given as ̇ , STC = ẆHP, ST + ẆLP, ST −Ẇ FWPHP−Ẇ FWPLP Wnet

Q̇EVP = ṁ r (h31−h32)

• Fan

̇ ṁ s (kg/s) , QSCOND and Eẋ D, SCOND (kW) are the mass flow rate of steam, heat interaction, and exergy destroyed in the SCOND, respectively.

Xws =

1.125tABS−tEVP + 49.04 0.47tABS + 134.65

(29a)

Xss =

1.125tGEN −tARCON + 49.04 0.47tGEN + 134.65

(29b)

where Xws and Xss (−) are the concentration of LiBr in the weak and strong solution mixtures of LiBr/H2O, respectively. The coefficient of performance, COP (−) of the ARC is evaluated as

COP =

First and second law efficiencies of STC are given as

ηII , STU

(24b)

• Evaporator:

̇ QSCOND = ṁ s (h22−h23) = ṁ a cp, a (T47−T46)

̇ , STC Wnet = Eẋ 12−Eẋ 14

Eẋ D, ARP = Ẇ ARP + Eẋ 26−Eẋ 27

• Absorption refrigeration condenser:

• Steam condenser:

ηI , STU

(24a)

• Absorber:

where ẆHP, ST , ẆLP, ST , Eẋ D, HPST and Eẋ D, LPST (kW) are the rate of work transfer and exergy destruction rate in the high pressure and low pressure steam turbines, respectively.

̇ , STC Wnet = Q̇ A, STC

ṁ (h −h ) Ẇ ARP = ws 27 26 ηP

(21a)

̇ QEVP ̇ + Ẇ ARP QGEN

ECOP =

(21b) 229

(

(30a) T

−Q̇EVP 1− T o

(

T ̇ QGEN 1− T o

14

31

)

) + Ẇ

ARP

(30b)

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2.4.1.3. Organic Rankine cycle. Energy and exergy balance of the ORC are given as

Table 1 Purchase equipment cost of plant components [14,38,25,46,5,43]. System component

• Heat recovery organic vapour generator: Q̇HROVG = ṁ w (h36−h39) = ṁ g cp, g (T15−T16)

(31a)

Eẋ D, HROVG = Eẋ 15 + Eẋ 39−Eẋ 16−Eẋ 36

(31b)

•

Anaerobic digester Gasifier Air and fuel compressors Air and fuel heat exchangers SOFC Inverter

Micro-turbine:

ẆMT = ṁ w (h36−h37)

(32a)

Eẋ D, MT = Eẋ 36 + Eẋ 37−ẆMT

(32b)

• Organic fluid condenser: ̇ QOFCOND = ṁ w (h37−h38)

(33a)

Eẋ D, OFCOND = Eẋ 37 + Eẋ 45−Eẋ 38−Eẋ 46

(33b)

•

Combustion chamber Gas turbine, microturbine Heat recovery steam generator Steam turbine

Organic fluid pump:

ṁ (h −h ) ̇ WOFP = w 39 38 ηp

(34a)

̇ Eẋ D, OFP = WOFP + Eẋ 38−Eẋ 39

(34b)

Condenser

The net power, first and second law efficiencies of the ORC are given as

̇ , ORC = ẆMT −WOFP ̇ − ∑ ẆFi Wnet

ηI , ORC

̇ , ORC Wnet = Q̇HROVG

(35a)

̇ , ORC Wnet Eẋ 15−Eẋ 16

i (1 + i)n (1 + i)n−1

ZHX 1, HX 2 = 130

( ) AHX 0.093

0.78

ZSOFC = Aact Ncell (2.96TFC−1907) Zinv = 105 ⎛ ⎝

0.7 ̇ WSOFC , DC ⎞ 500 ⎠

⎛ 46.08ṁ ⎞ a ZCC = ⎜ ⎟ [1 + exp (0.018T9−26.4)] ⎜ 0.995 − P9 ⎟ P8 ⎠ ⎝ 479.34ṁ

wf ⎞ ln ZGT , MT = ⎛ ⎝ 0.92 − ηGT ⎠

( ) [1 + exp (0.036T−54.4)] Pi Pe

ZHP − HRSG, LP − HRSG = 4745

(

i

hs log(Ti − Te )

)

0.8

+ 11820ṁ s + 658ṁ f 1.2

̇ )0.71 ZHP − ST , LP − ST = 6000(WST ZCOND, COND1, COND2 = 1773ṁ wf

AGEN 0.78 0.093 AAB 0.78 ZAB = 130 0.093 0.78 A ZEVP = 130 EVP 0.093 P 0.68 ZEV1,EV2 = 37 i Pe

ZGEN = 130

(

)

( ) ( )

()

( )⎞ ⎟ ( ) ⎟⎠ 1+d

(36d)

where d is the inflation rate.

UCOE =

ALCC 365Es

Es = 24ẆIPP

(36e) (36f)

(36a)

BEP =

LCC Q AP × UECMC

Q AP = 365Es (36b)

(36g) (36h)

where Q AP (kWh/yr) is the annual energy production; UECMC ($/kWh) is the cost of the conventional electricity supply; Es (kWh/day) is the daily energy demand; and Ẇ plant (kW) is the plant capacity.

where i is the interest rate; n (yr) is the total operating period or system life. Parameters of economic merit which include the life cycle cost LCC ($) , annualized life cycle cost ALCC ($/yr) , unit cost of energy, UCOE ($/kWh) and breakeven point, BEP (yr) , are given as[35]:

2.5.1. Exergoeconomic analysis This section presents a formulation of cost balance equations showing the sum of cost rates associated with all exiting streams equals the sum of cost rates of all entering streams, plus appropriate charges due to capital investment and operating and maintenance cost for each component of the plant. Each plant component is treated as a control volume. Mathematically, this can be expressed generally as [8]

z

LCC =

71.1ṁ wf r ln(rp) 0.9 − ηc p

⎛ 1− 1 + i ALCC = LCC ⎜ n ⎜ 1+d ⎝ 1+i

where φ (−) is the maintenance cost factor; N (h) is the operating hours per year; CRF (−) is the capital recovery factor:

CRF =

ZAC, FC =

where P,i Pe (kPa) andṁ wf (kg/s) are the inlet and exit pressures and mass flow rate of the working fluid, respectively.

Purchase equipment cost equations, Zk ($) of each component of the multi-generation plant are shown in Table 1. The purchase equipment cost expressed as cost per unit of time Zk̇ ($/s) for the kth component is given as [46]

φCRFZk 3600N

)

ZGf = 2.9 × 106 (3.6ṁ waste )0.7

ZHRVG = 1010(AHRVG )0.78

Expansion valves

2.5. Economic analysis

Zk̇ =

τṁ waste 0.75 21, 000

0.71 ZP1, P 2, P3 = 3540Ẇ P

(35b)

̇ (kW) are the heat interactions in the heat where Q̇HROVG , and QOFCOND recovery organic vapour generator and heat removed in the organic ̇ and Wnet ̇ , ORC (kW) are the power fluid condenser, respectively; ẆMT , WOFP of the micro-turbine, organic fluid pump and net power generated in the ORC, respectively.

(

Heat recovery vapour generator Generator Absorber

(35c)

ZAD = 350, 000

Pump

Evaporator

ηII , ORC =

Purchase equipment cost function ($)

∑ Cq; q ∊ {1, 2, 3} ≡ {AD&Gsf , SOFC, GTU , STU , ORC, ARS} q=1

(36c) where Cq ($) is the cost of the plant components, q ; 230

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Table 2 (continued)

Table 2 Operating parameters for the integrated multi-generation plant. Agricultural waste data for Rivers Songhai (Projected to 2025) a

Total (kg/day)

Activation Area

Aact

Ncell μa μf

m2 – – –

0.0834

Number of cells Air utilization factor Fuel utilization factor Inverter efficiency

ηinv

%

95.00

2000 0.15 0.85

Farm animal

Population

Daily waste per head

Cattle Pigs Poultry

2000 10,000 50,000

16.5 4.5 0.045

33,000 45,000 2500

Crops

Cultivated area (hec)

Yearly waste per hectare

Total (ton/yr)

j

A (Ωm)

B (K)

l (m)

Maize stover Rice husk POME Cassava peel

12.56 12.56 12.56 12.56

7.4 12 2.5 4.2

92.944 150.720 31.400 52.752

Cathode Anode Electrolyte Interconnector

0.0000811 0.0000298 0.0000294 0.0012

600 −1392 10,350 4690

0.0022 0.001 0.0004 0.000085

Ohmic loss parameters

Ultimate analysis of waste resourcesa,b,c,d

∑ Cė ,k + Ċw,k = ∑ Ci̇ ,k + Cq̇ ,k + Zk̇ Resource

C(%)

H(%)

O(%)

N(%)

S(%)

MC(%)

Maize stover Rice husk POME Cassava peel Crop waste Cattle Pigs Poultry Animal waste

44.18 37.55 51.38 52.7 46.45 50.5 36.45 45.82 44.26

5.52 4.61 4.37 7.1 5.40 7.1 4.89 5.85 5.95

37.69 37.67 41.5 37.9 38.69 32.8 37.89 27.38 32.69

0.53 0.46 0.35 1.2 0.64 7.3 4.52 5.16 5.30

0.1 0.012 0 0.015 0.03 2.3 0.88 0.45 1.21

5 9.88 2.4 1.8 8.79 – 15.37 15.34 10.23

e

Parameter e

Gas turbine cycle

Inlet air temperature Pressure ratio

Steam turbine cycle

HP-HRSG pressure LP-HRSG pressure Steam condenser pressure ORC-evaporator pressure

Organic Rankine cycle (R134a) Absorption refrigeration

ORC-condenser pressure AR-Generator temperature AR-Absorber temperature AR-Evaporator temperature AR-Condenser temperature AR-Reg effectiveness

Symbol

Units

Value

T1 rp PHP − HRSG PLP − HRSG PScon PORCevp

°C kPa kPa kPa kPa kPa

10.00 11.50

PORCcon TARgen

kPa °C

465.00 88.00

TARAbs TAREvp

°C °C

40.00 5.00

TARCond

°C

35.00

εRG

%

80.00

(37a)

Or in terms of exergy

∑ (ce Eẋ e )k + c w,k Ẇk = ∑ (ci Eẋ i )k + cq,k Eẋ q,k + Zk̇ e

i

(37b)

where Cė , Ci̇ , Cq̇ , Ċw ($/s) are cost rate associated with the exiting, entering streams, stream of power and heat transfer respectively; ce, ci, c w, cq ($/kJ) are average cost per unit exergy of the exiting stream, entering stream, power and heat transfer of the k-th component. Eẋ e, Eẋ i, Ẇk , Eẋ q (kW) are the exergy rate of exiting and, entering stream, power and heat transfer of the k-th component; A system of linear equations is obtained from the cost balance of each component and solved with auxiliary equations formulated using the specific exergy costing approach (SPECO) which involves the fuel (F) and product (P) principle [27]. The cost rates associated with exergy destruction(CḊ , k ($)) , average cost per unit fuel exergy (cF , k ($/kJ)) and product exergy (cP, k ($/kJ) ) and the exergoeconomic factor (fk ) of the k-th component can be determined as presented by Bejan and others [8]:

Integrated power, heating and refrigeration units Plant unit

i

14500.00 4000.00 21.00 1500.00

CḊ , k = cF , k Eẋ D, k

(37c)

CṖ , k CḞ , k Zk̇ , cP, k = ,f = Eẋ P, k k Eẋ F , k Zk̇ + CḊ , k

cF , k =

(37d)

where Eẋ F , k (kW) and Eẋ P, k (kW) are the unit cost exergy of fuel and product respectively.

Design conditions

2.6. Environmental analysis Item

Symbol

Units

Value

Ambient temperaturef Ambient pressure Compressor isentropic efficiency

To Po ηComp

°C kPa %

27 101.325 0.85

Turbine isentropic efficiency Pump isentropic efficiency

ηt ηp

% %

0.85 0.90

The environmental impact of the power plant and emissions of CO, NOx, and CO2 from the combustion chamber due to the reaction of unused fuel and air exiting the SOFC are presented in this section. The mass flow rate of CO2, ṁ CO2 (kg/s) and specific amount of CO2 emission per MWh of energy output, seCO2 (kg/MWh) are given as [38] Exhaust flow rates in terms of mass of CO and NOx are given as [28]

Economic datag Interest rate Operating hours per annum Maintenance factor System lifespan Income per capitai Energy per capitaj Population doubling timesk Energy doubling times Electricity tariffm

ir N φ n Ipc Enpc PDT EDT –

−

hrs −

yr $/person kWh/person yr yr $/kWh

0.12 8760 1.10 20 2177 141 25 22 0.17

ṁ CO =

ṁ NOx =

TFC PFC i

°C kPa

A/m2

P70.5 τ

7800 T9

)

ΔP 0.5 P7

( )

1.5 × 105τ 0.5exp( P70.05

(38b) −7100 T9

ΔP 0.5 P7

( )

) (38c)

ΔP

where τ (s) , P7 (kPa) , P (−) , T9 (K) are the residence time, combustion 7 chamber inlet pressure, non-dimensional pressure drop in the combustion chamber and combustion temperature. Fuel harmful emission factor, fef (–), is defined as

SOFCg,h Operating temperature Operating pressure Current density

1.79 × 105exp(

850.00 11.5 5000

fef =

231

ṁ CO + ṁ CO2 + ṁ NOx ṁ g

(38d)

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Table 3 Comparison of gasification constituents of wood.

Table 5 Ultimate analysis and energy content of wastes generated from the farm.

Gasification constituents

Present model

Athari et al. [4]

Resource

C (%)

H (%)

O (%)

N (%)

S (%)

MC (%)

CH4 (%) H2 (%) CO (%) CO2 (%) N2 (%)

0.69 19.29 18.87 13.62 47.13

0.68 18.01 18.77 13.84 48.70

Animal waste Crop waste

44.26 46.45

5.95 5.40

32.69 38.69

5.30 0.64

1.21 0.03

10.6 8.79

Composition of products of anaerobic digestion

Table 4 Validation of SOFC model. Parameters

Pirkandi et al. [40]

Present work

Error (%)

Cell voltage (V) Net power (kW) Electrical efficiency (%)

0.587 944.4 49.0

0.5592 949.9 47.936

4.74 0.58 2.17

To ; 0 < gT , ecf < 1 T16

Percentage by mass (%)

CH4 CO2 NH3 H2 S

30.61 78.17 6.67 1.28

26.18 66.85 5.88 1.10

0.72 2.02 70.38

CH4 H2 CO

0.18 0.49 17.21

Energy content

Animal waste Crop waste Total

Lower heating value of waste, LHVi (MJ/kg)

Lower heating value of fuel, LHVfuel (MJ/kg)

73.34 25.02 61.04

26.69 4.40 23.46

environment sustainability and is given as

(38e)

fT , eif = 1−gT , ecf ; 0 < fT , eif < 1

Mass (kg)

Composition of syngas from gasification

The enviro-thermal conservation factor, gT , ecf (−) , which is desired to be as high as possible for the environmental thermal state to be conserved; and the enviro-thermal impact factor, fT , eif (−) , which is expected to be low for the integrated multigeneration plant in order to reduce environmental thermal pollution and also reduce exergy destruction due to flue gases exiting the stack to the environment at state 16, are given as

gT , ecf =

Element

SEEEnvr = f (sfc, ηcc , fef , ExDf , gT , ecf , Δhg, stack )

(38f)

(39b)

̇ , IPP ηcc × gT , ecf × Eẋ in × Wnet ̇ ṁ f × cp, g × Tstack × ExD × fT , eif × fef

2.7. Sustainability analysis

SEEEnvr =

The sustainability index, SI (−) , which assesses the plant sustainability as regard to fuel resource is expressed as[38]

where sfc (kg/kJ) , ExDf (−), Δhg, stack (kJ/kg) are the specific fuel consumption, exergy destruction factor and specific enthalpy of flue gases exiting the stack. SEEEcon seeks to assess the plant with respect to energy conversion and the economy. It measures the efficiency of the processes of energy conversion and their economic impact on the society relative to costs, benefits and socio-economic sustainability. It is a function of specific energy earnings, sEnE ($/kWh) , which is the unit cost of energy generated; energy cost benefit ratio, EnCBR (−) , and is desired to be as low as possible; economic cost benefit ratio, EcCBR (−) , expected to be as low as possible; and energy population ratio, EnPopGR (−) , desired to be as high as possible. These parameters are given as

1 1−ηII , IPP

(39a)

The energo-environmental sustainability exponent, SEEEnvr (−) , and energo-economic sustainability exponent, SEEEcon (−) , are two parameters that seeks to assess the sustainability of a power plant with respect to the environment and the economy, respectively. SEEEnvr ascertains the fuel consumption and process effectiveness with respect to fuel resource (biofuel), technology and physical environment. High values denote better resource (fuel), technology (process) and ambient

550

45000 Crop waste

500

Animal waste

40000

Crop waste, mcr (ton/yr)

450

35000

400 350

30000

300

25000

250

20000

crop waste= 21.731t - 43500 R² = 0.9853

200 150

15000

Animal waste = 2178.92t - 4.37x106 R² = 0.9857

100 50 2010

2015

2020

10000 5000 2025

Year, t(yr) Fig. 5. Crop waste, ṁ cr (ton/ yr ) and animal waste, ṁ fa (ton/ yr ), projections for the farm. 232

Animal waste, mfa (ton/yr)

SI =

(39c)

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O.J. Ogorure et al.

doubling times, respectively.

Table 6 Performance characteristics of multi-generation power plant.

SEEEcon =

EnPopGR Icp × EDT × AR ≡ EcCBR × EnCBR Enpc × PDT × UCOE × ALCC

Unit

Parameter

Symbol

Unit

Value

SOFC-Gas turbine cycle unit Steam turbine cycle unit

Net work

̇ WSOFCGT

MW

4.030

Net work

̇ WSTU ̇ WCHP

MW

1.154

Finally the integrated multi-generation power plant exponent, SEIPP (−) is given as

MW −

5.184

SEIPP = SEEEnvr × SEEEcon

Q̇EVP ̇ WORC ̇ , IPP Wnet

MW

0.9389

MW

0.0325

MW

5.216

Overall thermal efficiency Overall exergy efficiency

ηI , IPP

%

63.62

ηII , IPP

ECOPARC

% −

58.46

Exergetic coefficient of performance Stack temperature

T16

°C

63.40

Combined cycle

Net work

Absorption refrigeration cycle unit

Coefficient of performance Cooling load

Organic Rankine unit

Net work

Integrated power plant

Overall output

COPARC

Ipc sEnE = Enpc

(39j)

0.7803

3. Results and discussion The results obtained for key parameters of the integrated multigeneration power plant modeled in the preceding section and parametric analysis are presented in this section.

0.3584

3.1. Plants specifications The input data specifications for the agricultural waste driven integrated multi-generation plant are tabulated in Table 2. Source of data: (a) Spyridon and Gerrit [48], (b) Allesina et al. [2], (c) Suryadi et al. [51], (d) Basu [6], (e) Stewart [50], (f) Oko and Ogoloma [36], (g) Khani et al. [25], (h) Ebrahimi and Moradpoor [18], (i) [54], (j) [55], (k) [56], (l) [57].

(39d)

whereIpc ($/person) andEnpc (kWh/person) are the income per capita and energy consumption per capita.

EnCBR =

UCOE sEnE

(39e)

EcCBR =

ALCC ARIPP

(39f)

3.2. Model validation Validation of the gasification model was performed using wood with biomass composition of 50% carbon, 6% hydrogen, 44% oxygen, 20% moisture content and gasification temperature of 800 °C from Athari et al. [4]. The result of the syngas constituents is presented in Table 3 and shows the agreement in both works. Model validation of the SOFC analysis was computed with input data from Pirkandi et al. [40], with fuel composition of 97% methane, 1.5% nitrogen and 1.5% carbon dioxide. Results of key performance parameters shown in Table 4 indicate the accuracy of the model used in this work with a maximum relative error of 4.74%. The relative error

where ARIPP ($/yr) the annual revenue is generated from the integrated multigeneration plant and is given as

̇ , IPP × N ARIPP = Tarrif × Wnet

(39g)

EDT PDT

(39h)

EnPopGR =

(39i)

whereEDT and PDT (yr) arethe energy demand and population

Table 7 Thermodynamic characteristics of respective components of the integrated power plant. Components

Heat or work rate Qi̇ orWi̇ (kW)

Exergy input, Eẋ in (kW)

Exergy destroyed, Eẋ D (kW)

Exergy efficiency, ηII (%)

Irreversibility ratio, yD (%)

Fuel Compressor Air Compressor Fuel Preheater Air Preheater SOFC Combustion Chamber Gas Turbine HP HRSG LPHRSG HP Pump LP Pump HP Steam Turbine LP Steam Turbine STU Condenser AR-Generator AR-Absorber AR-Condenser AR-Evaporator AR-Regenerator AR-Pump AR-Fan1 AR-Fan2 HROVG ORC-Turbine ORC-Pump ORC-Condenser

236.70 3462.00 238.30 1965.00 520.50 7988.00 7208.00 2464.00 1108.00 5.86 11.13 216.60 954.20 2473.00 1203.00 1142.00 1001.00 938.90 284.90 0.01 10.57 13.14 923.60 56.27 3.42 870.75

236.70 3462.00 501.00 456.00 986.00 11607.00 7433.00 1488.00 599.10 11.13 5.86 248.10 1168.60 300.09 440.40 8670.83 67.27 73.22 58.25 1.83 10.31 13.14 183.71 66.00 3.42 8916.00

21.70 265.94 377.00 109.00 438.10 543.00 225.00 398.17 55.90 1.00 0.53 31.50 214.40 205.09 405.10 71.71 28.21 46.11 32.10 0.00 0.81 1.14 110.01 9.73 0.32 24.80

90.83 92.32 24.75 76.10 55.57 95.32 96.97 73.24 90.67 91.02 91.02 87.30 81.65 31.66 8.02 99.17 58.06 37.03 44.90 100.00 92.17 91.32 40.12 85.26 90.64 99.72

0.60 7.35 10.42 3.01 12.11 15.02 6.22 11.01 1.55 0.03 0.01 0.87 5.93 5.67 11.20 1.98 0.78 1.27 0.89 0.00 0.02 0.03 3.04 0.27 0.01 0.69

SOFC-GT 46.29

STC 75.97

ARC 35.84

ORC 28.77

Parameter 2nd law efficiency (%)

233

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HROVG 3%

Air Compressor 7% Others 9%

AR-Generator 11%

Fuel Preheater 11%

STU Condenser 6%

Air Preheater 3%

SOFC 12%

LP Steam Turbine 6%

HP HRSG 11%

Combustion Chamber 15%

Gas Turbine 6%

Fig. 7. Percentage distribution of plant units equipment cost on capital investment cost.

Fig. 6. Percentage distribution of exergy destruction rate in the power plant.

between the results of this work and Pirkandi et al. [40] may be attributed to the different computational algorithms adopted in computing the cell voltage.

3.5. Results of economic analysis The capital investment cost, operation, and maintenance cost of the integrated multi-generation power plant were obtained from the economic analysis and are presented in Table 8. Capital investment cost of $3.753 million, with breakeven point of 7.5 years, and unit energy cost of $ 0.0109per kWh were obtained for the integrated multi-generation power plant with absorption refrigerating system. The unit energy cost obtained is well below the 0.131$/kWh cost of energy from the national grid, which indicates economic competitiveness of the proposed power plant. The unit cost of energy (UCOE) is low as compared to the current cost of electricity from fossil fuel ($0.0757–$ 0.1809/kWh ) in the locality where the commercial farm is situated. This can be attributed to the free source of energy (crop and animal waste) compared to fossil

3.3. Energy assessment of biomass The lower heating value of syngas obtained from the combined anaerobic and gasification analysis of the selected biomass in Table 1 was 23.466 MW which meets the combustion requirement for fuel [6]. Estimated amount of waste of 106.8 tonnes per day, generated from selected crop and animal wastes are shown in Fig. 5. Composition of fuel generated from the combined anaerobic digeation and gasification unit (CAdGU) is shown in Table 5. 3.4. Thermodynamic performance of the co-generation power plant

Table 9 Exergoeconomic analysis of the integrated power plant.

The net power generated by the co-generation power plant using the operating conditions in Table 6 was 5.216 MW with energy end exergy efficiencies of 63.62% and 58.46% respectively. The energy efficiency obtained fairly agreed with energy efficiency of 60% presented in Chan et al. [11]. The deviation can be attributed to the level of integration in the current analysis. The thermodynamic characteristics for the various power plant components are presented in Table 7. Fig. 6 presents the percentage contribution of plant components to total exergy destruction rate. The highest exergy destruction rate occurred in the combustion chamber, with an irreversibility ratio of 15%. The highest exergy destruction in the combustor is expected since high temperature processes are more susceptible to high entropy generation due to irreversibilities. The second highest exergy destruction was in the SOFC, followed by absorption generator.

Components

Fuel Compressor Air Compressor Fuel Preheater Air Preheater Inverter SOFC Combustion Chamber Gas Turbine HP HRSG LPHRSG HP Pump LP Pump HP Steam Turbine LP Steam Turbine STU Condenser AR-Generator AR-Absorber AR-Condenser AR-Evaporator AR-Regenerator AR-Pump AR-Fan1 AR-Fan2 ORC-HRVG ORC-Turbine ORC-Pump ORC-Condenser Expansion Valve1 Expansion Valve2 Overall

Table 8 Economic analysis of the integrated power plant with absorption refrigerating system. Parameter

Symbol

Units

Value

Combined anaerobic digestion and gasification unit Power plant equipment cost Engineering and contingency cost Life cycle cost Daily energy production

CAdGU

M$

0.6528

ZIPP ZEQ LCC Q̇DP

M$ M$ M$ kWh/day

2.6980 0.4047 3.7555 125,763

Q̇ AP ALCC UCOE BEP

kWh/yr

45,900,000

$/kWh $/kWh yr

0.4990 0.0109 7.5

Annual energy production Annualized life cycle cost Unit cost of electricity Breakeven point

234

Exergoeconomic parameters

Zk (M$ )

Zk̇ × 10−3 ($/ s )

CḊ , k × 10−3 ($/ s )

fk (%)

0.0165 0.4091 0.0126 0.0702 0.1066 0.2611 0.2066 0.2256 0.1318 0.1383 0.0143 0.0118 0.1844 0.7380 0.0022 0.0301 0.0301 0.0011 0.0301 0.0165 0.0001 0.0001 0.0002 0.0301 0.0197 0.0063 0.0044 0.0001 0.0002 2.6980

0.0769 1.9100 0.0589 0.3276 0.4978 1.2190 0.0204 1.0530 3.6320 3.9780 0.0669 0.0551 0.8610 3.4460 0.0101 0.1405 0.1405 0.0052 0.1405 0.0771 0.0006 0.0005 0.0007 0.1405 0.0922 0.0006 0.0204 0.0006 0.0006 17.9732

0.0203 0.2485 0.2882 0.0833 0.0411 0.2734 0.3389 0.1720 0.3044 0.0427 5.0010 2.6305 3.1973 23.0909 22.0882 0.3097 0.0000 0.2606 3.2592 0.3771 0.0000 0.0114 0.0161 0.0841 4.7619 0.1566 5.8478 0.1914 0.0002 73.0968

79.1262 88.4866 16.9660 79.7215 92.3687 81.6795 5.6727 85.9583 92.2670 98.9371 1.3195 2.0524 21.2160 12.9857 0.0456 31.2084 99.9998 1.9745 4.1328 16.9647 100.0000 3.9477 4.1817 62.5549 1.8996 0.3997 0.3473 0.3195 72.3073 19.7355

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and high value of enviro-thermal conservation factor were also obtained. High sustainability index and sustainability exponent values obtained can be attributed to the integration of different power cycles of the power plant.

Table 10 Environmental analysis from power generation with inlet air cooling of the integrated multi-generation power plant. Parameter

Symbol

Unit

Value

Specific emission ofCO2 Mass flow rate ofCO2 Mass flow rate ofCO

seCO2 ṁ CO2 ṁ CO

kg/MWh kg/s kg/s

141.20 0.2036

Mass flow rate ofNOx

ṁ NOx

Fuel harmful emission factor Enviro-thermal impact factor

fef fT , eif

kg/s − −

2.01 × 10−5 0.01853 0.1006

3.6. Simulation of key parameters

4.07 × 10−7

Enviro-thermal conservation factor

gT , ecf

−

0.8994

Sustainability index Energo-environmental sustainability exponent Energo-economic sustainability exponent Sustainability exponent

SI SEEEnvr SEEEcon SE

− − − −

2.43 3.37 1.08 3.65

Parametric simulation was performed on key performance parameters of the power plant as shown in Figs. 8–18. The effect of gasification temperature on the low heating value and percentage of syngas products is shown in Fig. 8. The low heating value of syngas decreased as the gasification temperature increased. This can be attributed to the negative dependence of the water-gas shift constants on the operating temperature. Composition of CH4 is the least and decreases, while CO and H2 attain a maximum before subsequent decrease. This agrees with similar trends in the work of Zainal et al. [52]. Fig. 9 shows that an increase in the current density increases the net power of the SOFC-GT unit to a maximum before decreasing. This is attributed to decline fuel flow rate as current density is increasing, resulting to reduction of electrochemical reaction to occur. Although this favours the gas turbine power output, but the decline of the SOFC power output outweighs the increase power output of the gas turbine, which is responsible for the decrease SOFC-GT output power as the current density is increasing after the maximum power output. The net voltage of the cell decreases and the losses (Ohmic, concentration and activation) increases with increasing current density which agrees with Khani et al. [25]. This may be attributed to the significant increase in voltage losses over the increase in reversible cell voltage with increasing current density. The variation of net power, energy and exergy efficiencies of the power plant with pressure ratio is simulated in Fig. 10. It was observed that an increase in pressure ratio resulted to an increase in the net power, energy and exergy efficiencies to a maximum before decreasing. This may be attributed to the increase in compressor power consumption and SOFC power output with increasing compression ratio. However, the compressor power consumption has an exponential relationship with the compression ratio whereas the SOFC power output has a linear relationship with the compression ratio; therefore, the net power output increased to a maximum until decreasing. Fig. 11 shows the effect of air temperature at inlet to air compressor on the net power and specific fuel consumption of the power plant. It can be observed that reducing the compressor inlet air temperature

fuel that requires huge cost of extraction and refining. The percentage distribution of the integrated plant units equipment cost is presented in Fig. 7. The solid oxide fuel cell/gas turbine cycle equipment has the highest cost, followed by the steam turbine cycle and the combined anaerobic digestion and gasification unit, respectively. It is expected that the SOFC-GT unit should have the highest cost because it combines the cost of SOFC sub-unit and the gas turbine sub-unit. Results of the exergoeconomic analysis based on energy, exergy and economic analysis are presented in Table 9. Low values of the exergoeconomic parameter, fk , indicate high cost of exergy destruction in the component, with the implication of improvement in the component exergy efficiency. The overall exergoeconomic parameter was 19.74%, which implies 80.26% of the total cost is associated with exergy destruction. An increase in the capital cost of equipment may improve the exergoeconomic performance of the integrated power plant[8]. The specific emission of CO2 in kg per MWh, fuel harmful emission factor, enviro-thermal factors and sustainability parameters of the integrated power plant are shown in Table 10. Specific emission of CO2 of 141.20kg/MWh was obtained for the power plant, which is 65.2% decrease in carbon footprint compare to a typical natural gas fueled turbine power plant in Nigeria that emits 406.18 kg/MWh of CO2 [38]. The specific emission obtained meets the Intergovernmental Panel on Climate Change CO2 emissions factor. Low values of the fuel harmful emission and enviro-thermal impact factor,

40 LHV of Syngas

Low heating value of syngas, LHV(MJ/kg)

8

CH4

CO

CO2

H2

7

35 30

6

25

5 20 4 15

3

10

2

5

1 0 500

600

700

800

900

1000

1100

1200

1300

Percentage composition of syngas components (%)

9

0 1400

Gasification temperature T(oC) Fig. 8. Effect of gasification temperature on lower heating value of syngas and percentage composition of syngas components of combined crop waste. 235

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Net Voltage, V(Volts)

1

V_net(V)

V_act(V)

V_ohm(V)

W_sofc/gt(MW)

4

V_conc(V)

0.8

3

0.6 2

0.4 0.2 0 2000

3000

4000

5000

6000

7000

8000

9000

10000

SOFC-GT Power, Wnet,SOFCGT (MW)

1.2

1 11000

Current density, i(A/m2) Fig. 9. Effect of current density on SOFC/GT power, net voltage and voltage losses.

68

64

6.0

60 5.6 56 5.2

52 Net power(MW)

Exergy efficiency

Thermal efficiency

4.8

Thermal and exergy efficienies,

Net Power output, Wnet,IPP (MW)

(%)

6.4

48 3

6

9 Compression ratio, rP is P6/P5(-)

12

15

Fig. 10. Effect of pressure ratio, rP, on the net power output, exergy and energy efficiencies of the integrated power plant.

5.2

Net power, Wnet,IPP (MW)

0.305 5.1 0.3 0.295

5

0.29 4.9 0.285 Net power 4.8 0.94

0.96

0.98

Specific fuel consumption 1.00

1.02

1.04

Specific fuel consumption, sfc (kg/MWh)

0.31

0.28

Air compressor inlet temperature ratio, T5 /To(-) Fig. 11. Variation of net power and specific fuel consumption with air compressor inlet temperature ratio, rt = T5/To.

resulted in increased net power output, and reduced specific fuel consumption. This is expected as low compressor inlet temperature implies less compressor power consumption and reduction in specific fuel

consumption of gas turbine plant. Thermal and exergy efficiencies of the solid oxide fuel cell/gas turbine cycle, combined cycle (SOFC-GT and STC) and the integrated cycle 236

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85

74 69

SOFC-GT cycle, ηII

Integrated cycle ηII

SOFC-GT, ηI

Integrated cycle ηI

Combined cycle, ηII

Combined cycle, ηI 75

59

65

54 55

49

Thermal efficiency, ηI(%)

Exergy efficiency, ηII (%)

64

44 45 39 34 4.8

4.9 5.0 5.1 5.2 Gas turbine /air compressor inlet temperature ratio, T9/T5(-)

5.3

35

Fig. 12. Effect of gas turbine/air compressor inlet temperature ratio, T9/T5, on thermal and exergy efficiencies for integrated, combined and SOFC-GT cycles.

towards the utilization of agricultural wastes for power generation will favour the rapid investment in the power sector by utilizing agrowastes-to-energy technology proposed in this work.

(SOFC-GT, STC, ARC and ORC) increases with increasing temperature ratio (gas turbine inlet temperature to air compressor inlet temperature) as shown in Fig. 12. It can be observed that with increased temperature ratio (or gas turbine inlet temperature), both thermal and exergy efficiencies increases and are higher in the integrated cycle and lower in the solid oxide fuel cell/gas turbine cycle [30]. This shows one of the advantages of the integrated cycle over simple plant configurations.

3.8. Results of environmental and sustainability analysis The environmental impact in terms of specific emissions of CO2 and fuel harmful emission factor are simulated in Fig. 14 with varying compression ratio. Specific emissions of CO2 and fuel harmful emission factor were observed to increase with increased compression ratio. Compared to the work of Haseli [22], with specific emissions of CO2 range 300–400 kg/MWh , Fig. 14 shows a positive environmental impact of the integrated power plant, as the CO2 emissions range between 131.8 and 161.2kg/MWh . The effect of adiabatic flame temperature variation (expressed as a ratio of the ambient temperature) on the mass flow rates of CO and NOx

3.7. Economic characteristics of the integrated multi-generation plant Fig. 13 shows the effect of varying interest rate on the unit cost of energy of the integrated plant. The figure shows that the interest rate has a strong effect on the economic competitiveness of the proposed power plant. It shows that investment on the plant at interest rate below the current interest rate of 12% will make the plant more economically attractive. The implication is that favourable government’s fiscal policy

Unit cost of energy, UCOE ($/kWh)

0.014

0.012

0.01

0.008

0.006

0.004 0.05

0.06

0.07

0.08

0.09

0.1

0.11

0.12

0.13

Interest rate, ir ( - ) Fig. 13. Effect of interest rate on unit energy cost of the integrated multigeneration power plant. 237

0.14

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160 fuel harmful emission factor specific emission of CO2

0.022

152

0.02

144

0.018

136

0.016

3

6

9 Compression ratio, rP(-) is P6/P5

12

15

Specific emission of CO2, seCO2 (kg/MWh)

Fuel harmful emission factor, fef(-)

0.024

128

Fig. 14. Variation of specific emission of CO2 and fuel harmful emission factor with compression ratio, rp.

6.00E-07

1.27E-04

5.00E-07

1.07E-04

mass flowrate of Nox

8.69E-05 4.00E-07

6.69E-05 4.69E-05

3.00E-07 2.69E-05 2.00E-07 4.82

Mass flowrate of Nox, mNOx(kg/s)

Mass flowrate of CO, mCO(kg/s)

mass flowrate CO

6.90E-06 4.88

4.94 5.00 5.06 Temperature ratio, Tgas turbine inlet /To(-)

5.12

Fig. 15. Mass flow rates of CO and NOx, as functions of adiabatic flame temperature ratio.

64 Sustainability exponent, SE* Energo-environmental sustainability exponent, SE-env

3.00

62

Exergy efficiency

2.50

60

2.00

58

1.50

56

1.00 0.8494

Exergy efficiency, ηII(%)

Sustainability exponent, SE(-), Energoenvironmental sustainability exponent, SEEEnvr(-)

3.50

54 0.8742

0.899

0.9238

Enviro-thermal conservation factor, gT,ecf (-) Fig. 16. Variation of sustainability exponent, energo-environmental sustainability exponent, and exergy efficiency with enviro-thermal conservation factor, gT , ecf = To/Tstack

exit,

of the integrated power plant. 238

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Sustainability index, SI(-)

2.70

Sustainability Index, SI Energo-economic sustainability exponent, SEecon 1.084

2.55

1.082

2.40

2.25 0.8494

0.8742

0.8990

0.9238

Energo-economic sustainability exponent, SEEEcon(-)

1.086

1.080

Enviro-thermal conservation factor, gT,ecf(-) Fig. 17. Variation of sustainability index, and energo-economic sustainability exponent with enviro-thermal conservation factor, gT , ecf = To/Tstack exit, of the integrated power plant.

is presented in Fig. 15. Increase in the mass flow rates of both CO and NOx can be observed as the adiabatic flame temperature ratio increases. This is expected as high temperature substantially favours the formation of NOx. Relationship between the enviro-thermal conservation factor, sustainability parameters and exergy efficiency of the integrated power plant is presented in Figs. 16–18. Increase in exergy efficiency with increase in the enviro-thermal conservation factor can be observed in Fig. 16. This is expected because increase in enviro-thermal conversion factor is an indication of better utilization of fuel, hence the better performance indicators observed in Fig. 16. The sustainability exponent and energo-environmental sustainability exponent increases with the enviro-thermal conservation factor, while negligible changes in the sustainability index and energo-economic sustainability exponent are observed in Fig. 17. This indicates that reduced stack temperature favour high exergy efficiency and high sustainability with negligible economic effect [42]. Fig. 18 shows the variation of sustainability exponent and sustainability index with exergy efficiency.

Increased exergy efficiency resulted in subsequent increase in the sustainability parameters. Higher values of the sustainability exponent were obtained compared to the sustainability index due to the expression of the sustainability exponent as a per capita parameter.

4. Conclusion In this study, the performance of a proposed multi-generation power/ cooling plant was analysed based on first and second laws of thermodynamics. Economic analysis of the plant was also presented. Thermodynamic and economic models formulated were implemented in the engineering equation solver (EES), with parametric simulation in Microsoft Excel environment. The major conclusions drawn from the study are that the net power generated from the biomass driven power plant is enough to meet the power requirements of the farm with an excess of 4.826 MW , available for export to a mini-grid or the national grid to argument the huge power supply deficit in the locality under consideration. Secondly, the largest contributions to the total system exergy destruction

3.50

Sustainability exponent, SE (-)

Sustainability Index, SI

Sustainability exponent, SE*

3.00

2.50

2.00

1.50

1.00 56.23

58.41

60.59

62.77

Exergy efficiency, ηII,IPP(%) Fig. 18. Variation of sustainability exponent and sustainability index with exergy efficiency of the integrated power plant. 239

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rate, in order, are in the fuel cell, combustion chamber and gas turbine and absorption refrigeration generator. Results obtained indicated the successful integration of several thermodynamic cycles to achieve higher power output, efficiencies, low cost of energy and low environmental impact from the utilization of fuel from biomass. However, the proposed plant may seem too complex to completely have all its components same time on board, but a theoretical plausibility of developing the proposed plants is here demonstrated. This study has shown that the agro-industries have the potentials for providing affordable and clean energy in the developing nations, especially in the Sub-Saharan Africa countries, where electrification rate is relatively poor. It is here suggested, therefore, while the policymakers consider this study to navigate through the complex decision making regarding the energy challenges in the global south, attention should also be given to the development of appropriate agro-business energy model fashioned in a manner to improve the economic competiveness of agro-industries, nearby cottage industries and households.

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