Development and analysis of a novel biomass-based integrated system for multigeneration with hydrogen production

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 5 1 1 e3 5 2 6

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Development and analysis of a novel biomassbased integrated system for multigeneration with hydrogen production Farid Safari*, Ibrahim Dincer Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ontario L1H 7K4, Canada

article info

abstract

Article history:

Energy and exergy analyses of an integrated system based on anaerobic digestion (AD) of

Received 13 November 2018

sewage sludge from wastewater treatment plant (WWTP) for multi-generation are inves-

Received in revised form

tigated in this study. The multigeneration system is operated by the biogas produced from

1 December 2018

digestion process. The useful outputs of this system are power, freshwater, heat, and

Accepted 13 December 2018

hydrogen while there are some heat recoveries within the system for improving efficiency.

Available online 11 January 2019

An open-air Brayton cycle, as well as organic Rankine cycle (ORC) with R-245fa as working fluid, are employed for power generation. Also, desalination is performed using the waste

Keywords:

heat of power generation unit through a parallel/cross multi-effect desalination (MED)

Biomass

system for water purification. Moreover, a proton exchange membrane (PEM) electrolyzer

Digestion

is used for electrochemical hydrogen production option in the case of excess electricity

Desalination

generation. The heating process is performed via the rejected heat of the ORC's working

Hydrogen

fluid. The production rates for products including the power, freshwater, hydrogen, and

Energy

hot water are obtained as 1102 kW, 0.94 kg/s, 0.347 kg/h, and 1.82 kg/s, respectively, in the

Exergy

base case conditions. Besides, the overall energy and exergy efficiencies of 63.6% and 40% are obtained for the developed system, respectively. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Increasing trend of global temperature as a result of humancaused carbon emission gives rise to environmental instability and climatic change. On the other hand, world population and in turn, energy demand are being increased. Therefore, fossil fuel reserves are no longer reliable sources for a sustainable and environmentally benign production of energy carriers [1,2]. Research and development on providing efficient and viable solutions for production, storage, and

consumption of energy are carried out growingly worldwide. Alternative energy sources for fossil fuels are renewables such as geothermal, biomass, wind, solar and tidal energy [3]. Nevertheless, the useful output of the renewable energy systems is recommended to be in a way that covers the needs of customers as most as possible. Therefore, multigeneration systems have gain importance and are developed for enhancing the productivity of systems and giving a holistic service according to the 3S concept presented by Dincer et al. [4]. The products of such systems can be used instantaneously or stored via storage technologies. Integrating the energy

* Corresponding author. E-mail addresses: [email protected] (F. Safari), [email protected] (I. Dincer). https://doi.org/10.1016/j.ijhydene.2018.12.101 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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streams within a system in a practical and meaningful way is another approach for minimizing the loss and maximizing the efficiency [5]. Among renewable resources, biomass as an abundant resource which stores the energy from the sun can be converted into value added bio-products [6]. However, among different kinds of biomass, wasted biomass (bio-waste) which has no major competition with food chain or other utilizations is regarded as promising source for biofuel and chemicals production [7]. Wet sludge, which is regarded as the bio-waste of urban life, has attracted a lot of attention for production of different kinds of products in the recent years [8]. The conversion of sludge can be performed through either thermochemical or biochemical conversion [9]. The biochemical conversion of biomass is a process of biomass decomposition by microorganisms. AD is a biochemical route for gas production from biomass which is being developed to be used in different upstream technologies for a variety of applications depending on consumer demands [10]. The mesophilic AD takes place in temperatures in the range of 30e40
C and the biogas yield depends mostly on the biomass composition and also reactor condition [11]. However, in addition to the improving biogas yield and quality, utilization of the produced biogas is an important issue for the development of biorenewable energy systems. Produced biogas from digestion can be directly used as a fuel in gas engines (GEs), gas turbines (GTs) or fuel cells (molten carbonate and solid oxide fuel cells), or can be upgraded for producing chemicals [12]. Using GTs in power generation has some features such as the relatively higher power to capital cost ratio, reliability, and fuel flexibility along with short running and start-up time [13]. Besides, although fuel cells result in higher electric efficiency, due to the sensible catalyst layer inside the fuel cells, highly efficient purification of biogas is required [14]. The combusted gas of the gas turbine can be further used for generation of heat, stream or other products depending on the thermodynamic viability and the need of the customer. To date, the dominant utilization of biogas is for combined heat and power (CHP) production. Wu et al. [15], demonstrated a comparison between different pathways of biogas utilization. CHP system had lower efficiency than biogas upgrading while CHP was more environmentally friendly. Integration and multi-generation within the utilization of biogas were recommended for improving the efficiency. Sung et al. [16], investigated the thermo-economic analysis of biogas-fueled gas turbine with an ORC using nheptane as a working fluid for heat recovery. The effect of different qualities of biogas depending on the CH4 molar fraction on the system performance was investigated. Higher molar fractions of methane indicated bigger utilization factors. Sevinchan et al. [17] performed a study on energy and exergy analyses of an integrated system based on anaerobic digestion of chicken manure and maize sludge for multigeneration. The highest exergy destruction among system components was associated to the combustion chamber with the share of 65%, respectively. However, development of a modern system with desalination and hydrogen production options to supply freshwater for household consumption and hydrogen for modern Fuel cell electric vehicles (FCEVs) is promising [18,19]. Demir and Dincer [20] proposed an

integrated energy system for freshwater and electricity production based on solar energy. 3.36 kg/s of freshwater was produced in a flash distillation unit running by a steam Rankine cycle. Ishaq et al. [21] developed an integrated trigeneration system for electricity, hydrogen and fresh water production using waste heat from a glass melting furnace. Energy and exergy efficiency of 47.7% and 37.9% were reported for the overall system, respectively. El-Emam and Dincer [22] developed a new system based on solar energy for freshwater and hydrogen production where 1.25 kg/h of hydrogen and 90 kg/s of freshwater were produced. However, the integration of parallel/cross MED and PEM electrolyzer with a biogas-based power generation system has not been studied yet. This study aims to investigate a novel integration of AD process with power, heat, freshwater, and hydrogen production with a focus on the combustion of biogas. The significance of this study is to develop a novel system and provide a comprehensive thermodynamic analysis for this biomassbased multigeneration system overall along with each of the system components. A biogas combustion model is also incorporated into analysis methodology. The main objectives of the current study are listed as follows:  Develop an integrated system based on the AD of sludge for power generation, heating, desalination, and hydrogen production.  Perform energy and exergy analyses of the system components and overall system.  Investigate the effect of biogas quality and quantity as well as some other operating parameters on the system outputs and system's overall energy and exergy efficiencies.

System description The multigeneration system proposed in this study is based on the AD of sewage sludge as a bio-waste coming from a WWTP. Sewage sludge has a high portion of water and a low portion of dry matter (almost 5 wt%) which is the main source for biogas production. The digestion process results in two main products; biogas and digestate. Although the composition of biogas is mainly related to the dry matter composition of sludge, it is mainly comprised of CO2 and CH4 and usually contains negligible amounts of other gases [11]. Hence, for the base case condition in this study, the biogas is considered as the mixture of CH4 with the molar fraction of 60% and CO2 with the molar fraction of 40% [13]. The remaining digestate in the digester can be stored for further utilization. Utilization of the digestate can be in the form of soil amendment for agriculture or for gas production through gasification. Produced biogas is used as a fuel for power generation in an open Brayton cycle. Air in the atmospheric pressure and the temperature of 25
C is first compressed by a certain compression ratio in the compressor and meets the fuel in the combustion chamber for high-temperature gas production. The ratio between air and fuel is determined according to the operating parameters of the system. The exhaust gases with the temperature of 1200 K leave the combustion chamber and expand for power generation in a gas turbine. Next, the expanded

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generation system is developed and designed for power generation, heating, desalination, and hydrogen production.

exhaust gases are further used for steam generation in an ORC as a secondary power generation unit where the pressurized R-245fa with the pressure of 2.5 MPa is superheated at Heat exchanger 1(HX1), for expansion in the expander to 100 kPa. The R-245fa is then saturated in the Heat exchanger 4 (HX4) where heat is transferred for hot water production. The exhaust gases deliver some heat to the condensed water for production of saturated steam for freshwater production in MED parallel unit. MED-parallel produces freshwater from saline seawater based on multi-effect thermal evaporation. The saturated steam rejects the latent heat to the feed water for vaporization and freshwater production. The inputs of the desalination unit are sea water and the heat from the saturated steam. The outputs are cooling water, freshwater, and saline brine. A small portion of the electricity generated (2%) in Brayton cycle is dedicated to electrochemical water decomposition in PEM electrolyzer. In addition, the water needed for hydrogen production can be supplied from the 80
C hot water produced in HX4. Hydrogen Produced by electrolyzer can be stored or used directly for some applications. Oxygen can also be stored for applications such as oxy-fuel combustion or desulfurization in the case that the digested gas has a significant amount of H2S. The H2S content in biogas should be less than 200 parts per million (ppm) to ensure a long life for the power and heat generators [23]. As seen in Fig. 1, the sewage sludge-based multi-

3

Analysis and assessment Thermodynamic assessment of the integrated system is performed for evaluation of both energetic and exergetic performances of the system based on the first and second laws of thermodynamics. Calculations were made by EES software in order to evaluate both energy and exergy efficiencies for system components as well as exergy destructions. The following general assumptions are made for analysis of the system.  The system operates at steady state and steady flow conditions.  The air considered here contains 77.48% nitrogen, 20.59% oxygen, 0.019% water vapor and 0.0003% carbon dioxide [24].  The pressure drops in the pipes and heat exchange processes are neglected. Pressure drop in the combustion chamber is considered to be 3%.  The ambient temperature and pressure are 298.15 K and 1.013 bar.  The exhaust gases leave the combustion chamber at 1200 K.

4

Gasometer

Biogas

Sludge from WWTP Digester

Combuson Chamber

Digestate

2

1

6

16

7

Turbine Compressor

17

G

Gas Turbine

Inlet Air 5

HX 1 HX 2

10

20

Cooling 21 water

13

Sea water

12

18 G

Mul Effect Desalinaon

HX3 11

8

9

19

Expander

24

Brine

15

HX 4

14 22

Pump

R245fa ORC

HW

25

26

Hot water

23

27

PEM Electrolizer

29 28

O2 H2

Fresh water

Fig. 1 e Schematic diagram of the biomass-based multigeneration system.

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 The air, biogas and combustion products are considered as ideal gases.  The heat losses in the turbines and the compressors are considered to be a negligible while for combustion chamber is considered as 2% of the lower heating value (LHV) of the fuel.  The digestion temperature ðTDig Þ, is 35
C.  The seawater temperature ðTSea Þ, is 30
C.

General thermodynamic analysis For each component of the system, the energy balance equations based on the first law of thermodynamics are written accordingly. The general equation for energy balance can be expressed as below [25]. X   X   dECV _ CV þ ¼ Q_ CV  W m_ i hi þ V2i þ gZi  m_ o ho þ V2o þ gZo dt o i (1) _ are heat transfer and work crossing the where Q_ andW boundaries of the system and its components. Based on the exergetic analysis of energy systems, the _ Þ, can be generally exergy destruction of each component ðEx d written as follows: _ d¼ Ex

X

_ Q Ex

X

_ Wþ Ex

X

m_ i exi 

X

m_ o exo

(2)

_ Q corresponds to here, Ex_ W shows the energy of work and Ex the heat transfer within the boundaries as   T0 ExQ;i ¼ Qi 1  Ts;i

(3)

where T0 , is the temperature of the state which the system is completely in equilibrium with the environment and Ts;i , is the temperature of the source corresponding to the heat transfer. However, the total exergy of each stream ðexi Þ is comprised of both physical and chemical exergies. exi ¼ exph;i þ exch;i

(4)

The physical exergy regarding the difference of the stream's temperature and reference environment temperature is defined as exph ¼

X ½ðhi  h0 Þ  T0 ðsi  s0 Þ

(5)

i

The chemical exergy is considered in chemical reactions when the composition of the streams changes after the reaction and can be calculated by [24]: exch ¼

X

m*i  mi;o



(6)

i

where ni ; m*i ; mi;o are the molar flow rate of the substance (based on the equilibrium point of the view), restricted state potential and potential at ultimate dead state, respectively. For gas mixtures the following equation can be re-written as exch;gas mixture ¼

X

xi ex0ch;i þ RT0

X

xi lnðxi Þ

(7)

where xi is the mole fraction of gas “i” in the gas mixture. Moreover, according to Song et al. [26] the definition for

specific chemical exergy of the organic matter (OM), which in this study can be considered as the specific chemical exergy of sludge dry matter ðexch;sludge;dm Þ; and the specific chemical exergy of digestate dry matter ðexch;digestate;dm Þ; is given by exch;OM ¼ 362:0083  C þ 1101:841  H  86:218  O þ 2:418  N þ 196:701  S  21:1  A (8) where C, H, O, N, S and A correspond to the percentages of carbon, hydrogen, oxygen, nitrogen, sulfur and ash weight percentages in the organic matter, sludge or digestate, respectively. Table 1 presents the exergy balance equation and exergy efficiency definition for main system components considering the input and output energy streams of each component as a control volume. However, not necessarily all input and output are considered in the exergy efficiency definition. If one stream is not considered as a useful product, it can be neglected in exergy efficiency definition.

Digestion unit The sewage sludge AD is a biochemical conversion of biomass organic fraction using microorganisms as digestion agents. Mesophilic digestion usually occurs at the temperatures between 30 and 40
C [10]. The sludge mass flow rate of 12.06 kg/s and biogas flow rate of 0.212 kg/s are considered in accordance with ref [27]. The input streams of the digester are sewage sludge from WWTP and heat from warm water which is used for maintaining the digester temperature at 35
C. Heat load depends on the ambient temperature and is supplied by the exhaust gases of power generation unit via heat exchanger 2 (HX2). The products (outlet streams) are digestate and biogas. As mentioned, here, the biogas is considered to be methane and carbon dioxide with the molar fractions of 60% and 40%, respectively, due to the negligible portions of other gas compounds in the biogas. The digested sludge as digestate can be either used as fertilizer for agriculture or gasified for syngas and hydrogen production [28]. The digestate of sludge usually contains significant amount of alkali and alkaline earth metals which promises its utilization for soil amendment. Moreover, the hydrothermal gasification is reported to be an efficient technology for production of hydrogen rich gas and also porous structured biochar for industrial utilizations [28,29]. For energy and exergy analyses of the digester, the sludge and digestate are considered as two-part streams containing water and dry sludge so that exergy content of sludge and digestate can be obtained through the summation of exergy content of each portion. Biogas also can be considered as a gas mixture. Exergy flow rates for sludge, digestate, and biogas can be defined accordingly as follows:    _ sludge ¼ m_ sluge;d exph;sludge;dm þ exch;sludge;dm Ex   þm_ sludge;w exph;sludge;w þ exch;sludge;w    _ Digestate ¼ m_ digestate;dm exph;digestate;dm þ exch;digestate;dm Ex   þm_ digestate;w exph;digestate;w þ exch;digestate;w

(9)

(10)

Table 1 e Exergy balances and exergy efficiency definitions for the system components Component

Digester

Exergy destruction

T _ _ 1 ex1 þ Q_ Dig 1  0 Ex d;Dig ¼ m TDig

Exergy efficiency

!

 m_ 2 ex2  m_ 3 ex3

hex;digester ¼

m_ 2 ex2;dm þ m_ 3 ex3 T0 m_ 1 ex1 þ Q_ Dig 1  TDig

_ _ AC  m_ 6 ex6 _ 5 ex5 þ W Ex d;AC ¼ m

hex;AC ¼

Combustion chamber

_ _ 4 ex4 þ m_ 6 ex6  m_ 7 ex7 Ex d;cc ¼ m

hex;cc ¼

Gas turbine

_ _ GT  m_ 8 ex8 _ 7 ex7  W Ex d;GT ¼ m

HX 1

_ _ 8 ex8 þ m_ 12 ex12  m_ 13 ex13  m_ 9 ex9 Ex d;HX1 ¼ m

hex;HX1 ¼

m_ 13 ex13  m_ 12 ex12 m_ 8 ex8  m_ 9 ex9

HX 2

_ _ 9 ex9 þ m_ 16 ex16  m_ 17 ex17  m_ 10 ex10 Ex d;HX2 ¼ m

hex;HX2 ¼

m_ 17 ex17  m_ 16 ex16 m_ 9 ex9  m_ 10 ex10

HX 3

_ _ 10 ex10 þ m_ 19 ex19  m_ 11 ex11  m_ 18 ex18 Ex d;HX3 ¼ m

hex;HX3 ¼

m_ 18 ex18  m_ 19 ex19 m_ 10 ex10  m_ 11 ex11

HX 4

_ _ 14 ex14 þ m_ 24 ex24  m_ 15 ex15  m_ 25 ex25 Ex d;HX4 ¼ m

hex;HX4 ¼

MED

  T0 _ _ _ 1   m_ 21 ex21  m_ 22 ex22  m_ 23 ex23 ¼ m ex þ Q Ex 20 20 d;MED MED TS

ORC expander

_ _ Exp _ 13 ex13  m_ 14 ex14  W Ex d;Exp ¼ m

ORC pump

_ _ P;ORC  m_ 12 ex12 _ 15 ex15 þ W Ex d;P;ORC ¼ m

hex;P;ORC ¼

PEM electrolyzer

_ _ Elec  m_ 28 ex28  m_ 29 ex29 _ 27 ex27 þ W Ex d;Elec ¼ m

hex;Elec ¼

m_ 6 ex6  m_ 5 ex5 _ AC W m_ 7 ex7  m_ 6 ex6 m_ 4 ex4

hex;GT ¼

hex;MED ¼

hex;Exp ¼

_ GT W _ m7 ex7  m_ 8 ex8

m_ 25 ex25  m_ 24 ex24 m_ 14 ex14  m_ 15 ex15 m_ 23 ex23  m_ 20 ex20   T0 Q_ MED 1  TS _ Exp W m_ 13 ex13  m_ 14 ex14

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Air compressor

!

m_ 12 ex12  m_ 15 ex15 _ P;ORC W

m_ 28 ex28 _ WElec þ m_ 27 ex27

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Table 2 e Chemical composition of the organic matters [6].

Table 3 e Operating parameters for the open air Bryton cycle.

Element

Component

C H N S O Ash

Sludge (wt.%, dm)

Digestate (wt.%, dm)

37 4.5 19 3.3 0.65 30

35 2.5 12 1.1 0.4 40

  _ biogas ¼ m_ biogas exch;biogas þ exph;biogas Ex

Compressor Combustion Chamber Gas Turbine

(11)

Moreover, energy and exergy efficiencies of the digester can be defined as 

hen; Dig ¼ 

   m_ biogas  LHVbiogas þ m_ digestate;dm  LHVdigestate;dm    m_ sludge;dm  LHVsludge;dm þ m_ sludge;w  hsludge;w þ Q_ Dig

hex;Dig

Pin ¼ 1.013 bar, Tin, C ¼ 25
C, rAC ¼ 10, hAC ¼ 0.83 DPcc ¼ 3%, hcc ¼ 0.98 Tin, C ¼ 1200 K, hGT ¼ 0.86

Note that the energy efficiency of the Brayton cycle can be defined as the ratio of the net generated power within the cycle to the thermal energy content of biogas entering the combustion chamber. In addition, exergy efficiency can be calculated as the ratio of the net power output to the exergy rate of the biogas entering the combustion chamber. These definitions can given as follows: hen ¼

_ net; Brayton W m_ Biogas  LHVBiogas

(15)

hex ¼

_ net; Brayton W m_ Biogas  exBiogas

(16)

(12) _ biogas þ Ex _ digestate;dm Ex  ¼ _ _ Exsludge þ Q Dig 1  T0

Input and accessory information

(13)

TDig

where Q_ Digester , can be calculated through the temperature difference between the digestion temperature ðTDigester Þ and the ambient temperature ðT0 Þ as follows:   Q_ Dig ¼ m_ sludge $CP;Sludge $ TDig  T0

Air compressor The air entering the compressor is assumed to behave as an ideal gas. The isentropic efficiency of the air compressor is defined below.

(14)

Since water content of sludge is usually more than 95%, the specific heat capacity ðCP;Sludge Þ, is considered to be equal to that of water [30]. The elemental composition of the sludge and digestate in this study are indicated in Table 2, in accordance with ref. [6]. Moreover, the lower heating value (LHV) of sludge and digestate are reported as 18 and 14.5 MJ/kg, respectively.

Open type air Brayton cycle An open-type air standard Brayton cycle consisting of a compressor, a combustion chamber, and a gas turbine is used for power generation from the biogas obtained from the AD of sewage sludge. Mostly used turbine for biogas applications are micro gas turbines (MGTs) while combustion gas turbines can also be used. Turbines range in size from 30 kW (microturbines) to 250 MW (large industrial units). The air mass flow rate is determined by the molar fuel to air ratio (l), considering the quality and quantity of biogas and the outlet temperature of the combustion chamber. The assumptions and operating conditions for Brayton cycle are mentioned in Table 3.

hAC ¼

h6s  h5

(17)

h6  h5

Using the isentropic efficiency and according to the pressure ratio, the outlet temperature of the air compressor can be defined as [31]. 0

0

11

ga 1 B CC 1 B B ga CC T6 ¼ T5  B @1 þ h @rAC  1AA AC

(18)

Using the outlet temperature, the work needed for air compression can be calculated as follows: _ AC ¼ m_ a CP;a ðT6  T5 Þ W

(19)

where CP;a in our analysis is considered as temperature variable function as follows:       3:8371T 9:4537T2 3:8371T3 þ  4 7 10 10 10 10   3:8371T4 þ 1014

CP;a ¼ 1:04841 

(20)

The change in the specific entropy can be written as [24]:





 P6 P6 P6 s6s  s5 ¼ XN2 s0 ðT6s Þ  s0 ðT5 Þ  Rln þ XO2 s0 ðT6s Þ  s0 ðT5 Þ  Rln þ XCO2 s0 ðT6s Þ  s0 ðT5 Þ  Rln P5 N2 P5 O2 P5 CO2

 P 6 þ XH2 O s0 ðT6s Þ  s0 ðT5 Þ  Rln ¼0 P5 H2O

(21)

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To evaluate the specific entropy of gases in various temperatures, the following equation is applied

P7 ¼ 1  DPcc P6

c d s0 ¼ Sþ þ alnT þ by  y2 þ y2 b 2

Gas turbine

(22)

where y ¼ 103 T and other coefficients can be found in Ref. [24]. Also, to evaluate enthalpy values of different gases in different temperatures, the corresponding equations are as follows:   0 b d h ¼ 103  Hþ þ ay þ y2  c1 þ y3 2 3

(23)

Combustion chamber Biogas from digestion unit is combusted in combustion chamber using the compressed air. The molar fuel to air ratio (lÞ, is defined as

l¼

n_F n_P ; 1þl¼ n_a n_a

(24)

where, n_P is the molar flow rate of products ðn_f þ n_a Þ, n_f is the molar flow rate of biogas, and n_a is the moles of air. The chemical equation for the combustion of biogas consisting of methane and carbon dioxide on per mole of air basis could be written as follows [24]:   l xCH4 CH4 þ xCO2 CO2 þ ½0:7748N2 þ 0:2059O2 þ 0:0003CO2    þ 0:019H2 O/ 1 þ l YN2 N2 þ YO2 O2 þ YCO2 CO2 þ YH2 O H2 O (25) hence, the mole fractions for the exhaust gas constituents are , as follows: YN2 ¼ 0:7748 1þl 0:2059  1:2l ; 1þl 0:0003 þ l ; YCO2 ¼ 1þl 0:019 þ 1:2l : YH2 O ¼ 1þl

YO2 ¼

Heat loss from the combustion chamber is considered to be 2% of the lower heating value of the fuel. Energy balance equation for combustion chamber can be expressed Eq. (26) as follows:   0:02lLHV þ ha þ lhbiogas þ 1 þ l hp ¼ 0



0:7748DhN2 þ 0:2059DhO2 þ 0:0003DhCO2 þ 0019DhH2 O  l¼ hbiogas  0:02LHV   1:2lhO2 þ hCO2 þ 1:2lhH2 O

½T6

½T7 

(27) The lower heating value of biogas combustion at 25
C and 1 atm is calculated as follows: LHV ¼ HP  Hr ¼

X

0

Np hf ;p 

X

0

Nr hf ;r

(28)

Also, for considering pressure drop in the combustion chamber, the following equation is used.

(29)

For a gas turbine, using the outlet temperature of the combustion chamber, the outlet temperature of the gas turbine can be determined according to the turbine isentropic efficiency and the inlet and outlet pressures as following [31]: 2

13  ggg1 g P 7 A5 T8 ¼ T7  41  hGT @1  P8

0

(30)

Using the outlet temperature, the work generated by the gas turbine can be calculated as follows: _ GT ¼ m_ G CP;g ðT7  T8 Þ W

(31)

where CP;G in our analysis considered as temperature variable function as CP;a ðTi Þ ¼ 1:048 

        3:8Ti 9:45Ti 2 5:49Ti 3 7:92Ti 4 þ  þ 4 7 10 14 10 10 10 10 (32)

ORC system R-245fa is considered as working fluid in this study for secondary power generation which is running gas turbine's exhaust gases. R-245fa is widely used as working fluid in ORCs and it is previously shown that is suitable and efficient for heat recovery from the exhaust gases of biogas combustion [32]. As shown in Fig. 1, thermal energy enters the system through HX1 for superheating R245fa under the pressure of 2.5 MPae160
C from state 14 to 15. Organic fluid is then expanded through the expander to the pressure of 100 kPa and is condensed while rejecting heat through HX4 to the water in the ambient temperature for producing hot water for the residential sector. The isentropic efficiency of expander and pump are assumed to be 0.9 and 0.85, respectively. Moreover, energy and exergy efficiencies of ORC can be written as Eqs. 33 and 34, respectively. Energy efficiency of ORC is the net power output over the amount of heat passed to the organic fluid in HX1 while exergy efficiencies are the net power output over the exergy input of ORC in HX1.

(26)

hence with some calculations, l can be determined through the following equation.

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hen;ORC ¼

_ net; ORC W m_ ORC ðh13  h12 Þ

(33)

hex;ORC ¼

_ net; ORC W m_ ORC ðex13  ex12 Þ

(34)

PEM electrolyzer Electricity can be used for hydrogen production when excessive power is available. Electrochemical hydrogen production via water electrolysis is studied within the past decades. Proton exchange membrane (PEM) electrolysis is more common in the recent year because of easier maintenance and more environmentally friendliness. Besides, it is more efficient to be coupled with the intermittent system. The reaction

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regarding the hydrogen production in PEM electrolyzer can be written as [33]. 1 (35) Overall reaction : H2 O þ DH/2H2 þ O2 2 Anodic : H2 O/2Hþ þ 2e

(36)

Cathodic : 2Hþ þ 2e/ H2

(37)

Also, the amount of energy required for the reaction can be written in the following form:

Table 4 e Parameters considered for analysis of PEM electrolyzer [34]. Parameter

Value

PO2 (kPa), PH2 (kPa) TPEM (
C) Eact;a ðkJ=molÞ Eact;c ðkJ=molÞ la lc D (mmÞ

101.3 80 76 18 14 10 100 1.7  105

ref

DG ¼ DH þ TDS

(38)

where DG is Gibb's free energy and TDS is the thermal energy requirement. Here, the theoretical energy required for electrochemical decomposition of water is considered as the total energy. However, this amount can be reduced by using the catalyst layer in the PEM electrolyzer. The molar flow rate of the produced H2 can be calculated by Eq. (39) as following [34]: n_H2 ;o ¼

J 2F

(39)

where F is Faraday constant and J is current density. The electrical energy input for electrolyzer is associated with the current density by the following formula. Eelec ¼ Qelec ¼ JV

(40)

where V is the overpotential of the cell and is comprised of the following terms [34]. V ¼ V0 þ Vohm þ Vact;a þ Vact;c

(41)

here, V0 as the reversible potential associated with the reactants and products difference in terms of free energy which can be calculated via the Nernst equation as V0 ¼ 1:229  8:5  104 ðTPEM  298Þ

(42)

Ja (A/m2)

4.6  103

ref

Jc (A/m2) F (C/mol)

ZD RPEM ¼ 0

96,486

dx s½lðxÞ

(45)

Based on the Ohm's law, the following equation can be written for the ohmic overpotential [33]. Vohm ¼ JRPEM

(46)

The activation overpotential, Vact;a , results from the difference of net current with its equilibrium, and also an electron transfer reaction, must be differentiated from the concentration of the oxidized and reduced species and can be calculated as Vact;a ¼

  RT J sinh1 ; F 2J0;i

i ¼ a; c

(47)

here, J0 is the exchange current density, an important parameter in calculating the activation overpotential. It determines the electrode's potential in the electrochemical reaction. The higher J, the higher electrode reactivity, and consequently, the lower overpotential. The exchange current density for electrolysis can be written as follows [33]:

where TPEM is the temperature of electrolyzer. Also, Vohm ; Vact;a ; Vact;c , are the ohmic overpotential of the electrolyte, anode activation overpotential and the cathode activation overpotential, respectively. Ohmic overpotential is the result of the resistance against the hydrogen ions transportation within the membrance. The ionic resistance, on the other hand, is associated with the level of humidification, thickness, and temperature of the membrane [33]. The local ionic conductivity, sðxÞ, of the proton exchange membrane can be written as

The parameters used for the PEM electrolyzer design are presented in Table 4. Moreover, the definitions for energy and exergy efficiencies of PEM electrolyzer based on the useful output and inputs of the unit are written as



 1 1  sPEM ½lðxÞ ¼ ½0:5139lðxÞ  0:326 exp 1268 303 T

hen;Electrolizer ¼

m_ H2  LHVH2   _ Elec þ m_ H O;in  hH O;in W 2 2

(49)

hex;Electrolizer ¼

m_ H2  exH2   _ Elec þ m_ H O;in  exH O;in W 2 2

(50)

(43)

where x is the distance in the membrane measured from the cathode-membrane interface and lðxÞ, is the water content at a location x in the membrane. The value of lðxÞ, can be calculated in the term of the water content at the membrane electrode edges and be written as lðxÞ ¼

la  lc x þ lc D

(44)

where D is the membrane thickness, and la and lc are the content of the water at the interfaces of the anode-membrane and the cathode membrane, respectively. Hence, the total ohmic resistance can be calculated as

  Eact;i ref ; J0;i ¼ Ji exp  RT

i ¼ a; c

(48)

_ Elec Þ, is considered here, the electricity input of electrolyzer ðW to be 2% of the net power output of Brayton cycle.

MED system The parallel/cross MED system can operate at low temperatures which is flexible with start-up process. Also, Contamination can be avoided due to the higher pressure of vapor side

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Fig. 2 e Schematic diagram of the parallel/cross MED system.

than that of seawater side [35]. In this work, a parallel/cross flow MED system working in the optimal condition determined by Ref. [36] is considered. As seen in Fig. 2, in the parallel/cross MED configuration, feed enters each stage, typically in equal amount while the brine is passed from each stage to the next one to be rejected from the last stage [35,36]. The saturated steam at 72
C is considered to be provided as a heat source for desalination via the exhaust heat of the power generation units. The boiling point elevation (BPE) is also set as 0.6.  The steady state processes take place.  The distillate is salt-free.  There are negligible heat losses according to the insulated walls.  The temperature difference is same (3.44
C) between all effects.  The boiling point elevation (BPE) is 0.6.  The seawater is at 30
C, brine top temperature is 63.46, and the salinity for seawater and brine are 42,000 and 70,000 ppm, respectively [35]. The mass, salt, and energy balance equations for each stage can be written as follows: m_ fi þ m_ Bi1 ¼ m_ fwi þ m_ Bi

hen;overall;II ¼

hex;overall;II ¼

(51)

 hen ¼

m_ fw  hfw Q_ MED

 (54)



hex ¼

 m_ fw  exfw  Q_ MED 1  T0

(55)

TS

where m_ fw ; hfw , and exfw , are the mass flow rate, specific enthalpy and the specific exergy of produced freshwater, respectively. In the efficiency definitions, freshwater is regarded as desalination's useful output and the latent heat from saturated steam are considered as the input.

Overall system efficiencies In the developed integrated system, the net power output of Brayton cycle and ORC, the rate of the heat consumed for purification of seawater, and the rate of the thermal energy content of the produced hydrogen and hot water are considered as useful outputs. For the case of inputs, the main input of the system is sewage sludge from WWTP. Considering the above-mentioned explanations, the energy and exergy efficiencies of the biomass-based multigeneration system can be defined by

  _ Total þ Q_ MED þ m_ 28  LHVH þ m_ 26 ðh26  h24 Þ þ m_ digestate;dm  LHVdigestate W 2     m_ sludge;dm  LHVsludge þ m_ sludge;w  hsludge;w

(56)

   _ digestate;dm _ Total þ m_ 28  exH þ Q_ MED 1  T0 þ m_ 26 ðex26  ex23 Þ þ Ex W 2 TS

(57)

_ Sludge Ex

XBi1 m_ Bi1 ¼ Xfi m_ fi þ XBi1 m_ Bi1

(52)

  m_ fwi1 hi1 ¼ m_ fwi hi þ m_ f CP;f Ti  Tf

(53)

The detailed modeling of MED system used in this study may be found elsewhere [36]. The energy and exergy efficiencies of the parallel/cross MED can be expressed as follows:

_ Total ¼ W _ net; where. W

Brayton

_ net; þW

ORC

_ elec W

Results and discussion For the base case conditions discussed in this study, by conducting thermodynamic analysis for each state point of the system using EES software, thermodynamic properties

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Table 5 e Process data for the system state points. State No 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

State composition

_ (kg/s) m

T (
C)

P (kPa)

h (kJ/kg)

s (kJ/kg.K)

ex ðkJ=kgÞ

Sludge Digestate Biogas (60% CO2, 40% CH4) Biogas (60% CO2, 40% CH4) Air Air Exhaust gases Exhaust gases Exhaust gases Exhaust gases Exhaust gases R-245fa R-245fa R-245fa R-245fa water water water water Sea water (4.2 wt % salt) cooling water (4.2 wt % salt) Saline water (7 wt % salt) Fresh water water water water water Hydrogen Oxygen

12.06 11.85 0.21 0.21 3.11 3.11 3.32 3.32 3.32 3.32 3.32 1.82 1.82 1.82 1.82 2 2 0.2163 0.2163 10.38 7.48 1.95 0.944 1.812 1.812 1.794 0.018 0.000096 0.0179

25 35 35 35 25 359.1 927 429.5 300 166.8 25 15.84 160 73.14 14.86 25 85.23 72.02 72.02 30 39.44 56.16 56.16 25 80 80 80 80 80

101.3 101.3 101.3 101.3 101.3 101.3 98.29 98.29 98.29 98.29 98.29 2500 2500 100 100 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3

104.8 146.7 357.6 357.6 298.6 633.9 1278 716.5 579.2 441.9 298.6 221.2 531.4 469.9 219.1 104.8 356.9 2630 301.5 98.71 156 214.4 235.2 104.8 335 335 335 791.4 50.68

0.367 0.505 7.282 7.282 5.696 5.791 6.526 6.585 6.369 6.097 5.704 1069 1902 1.922 1.068 0.367 1.137 7.724 0.979 0.344 0.532 0.7015 0.783 0.367 1.075 1.075 1.075 67.2 6.563

1308.37 941.34 18,570.75 18,570.75 0 306.9 733.2 153.8 79.86 23.65 2.60 431 493.4 425.9 429.6 0 22.6 332.2 14.08 0.14 1.3 6.2 6.432 0 19.04 19.04 69.04 118,045 128.26

including Temperature (T), pressure (P), specific enthalpy (h), specific entropy (s) and specific exergy (ex) are reported in Table 5. The reference condition for each state is considered as the temperature of 25
C and the pressure of 1 atm, respectively. Noting that for the components where reactions take place such as digester, combustion chamber and electrolyzer, both specific chemical and physical exergies are included for exergy values of the inputs and outputs of the units. The overall energy and exergy efficiencies of the integrated system in the base case conditions is obtained as 63.6% and 40%, respectively. For the case of digestion unit, the energy and exergy efficiencies are 78% and 57.3%, respectively. The high energy and exergy efficiencies of the digester are due to the biological conversion which doesn't require much energy except the low amount of heat required for maintaining the digestion temperature. For the Brayton cycle, the energy and exergy efficiencies are obtained as 26.8% and 25.5%, respectively, and for the ORC the energy and exergy efficiencies of 19.2% and 48.5%, are obtained, respectively. Also, in the base case study analyses, 1002 kW and 100 kW of power is generated in Brayton cycle and ORC, respectively. Besides, the mass flow rate of hydrogen and hot water (water in 80
C) produced are obtained as 0.347 kg/h and 1.832 kg/s, respectively. Furthermore, the freshwater production rate in the base case condition obtained as 0.94 kg/s in the MED unit while the energy and exergy efficiencies of MED unit are obtained as 44% and 6%, respectively. The power generation is performed in a Brayton cycle as the primary and an ORC as the secondary unit. In the open air

Brayton cycle, air goes through a compression process with a slight increase in entropy with compression ratio of 10, followed by a combustion process with the air to fuel ratio of 14.67, where its pressure drops by about 3% and reaches the temperature of 1200 K. The exhaust gases expand in the gas turbine where the entropy increases up to 6.585 kJ/kg.K, and temperature goes to 429.5
C in state 8. However, the R-245fa ORC works in the much lower temperatures and entropies compared to that of Brayton cycle. Therefore, due to the reasonable change in specific volume of organic fluids within these ranges of temperatures, ORCs are recommended for waste heat recovery from low-grade heat resources. Besides, the organic fluid is remained superheated after the expansion in ORC expander because of the dry fluid properties of R-245fa. The exergy destruction rates, as well as energy transfer rates of system components in the base case condition, are listed in Table 6. As seen, the highest rate of exergy destruction is observed in the combustion chamber with the value of 2456 kW where an extreme entropy generation occurs as a result of biogas combustion. Changes in the fuel quality and the fuel composition, exhaust gas temperature and also the air compression ratio affect the exergy destruction rate in the combustion chamber which will be investigated through parametric studies. Moreover, exergy is destructed in the air compressor and gas turbine by the rates of 58.49 and 116.7 kW, respectively. The other major sources of exergy destruction are the digester and heat exchangers. The high exergy destruction in digester is due to the high water content of the digestate leaving the digester. The destructed exergy rate in the ORC pump, electrolyzer, and ORC expander are

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 5 1 1 e3 5 2 6

Table 6 e Thermodynamic analysis data of the integrated system components. Component Digester Air compressor Combustion chamber Gas turbine HX 1 HX 2 HX 3 HX 4 MED ORC expander ORC pump PEM electrolyzer

Exergy destruction Power or heat rate (kW) transfer rate (kW) 725 116.8 2456 58.5 141.4 49.3 188.2 309.6 118 10 0.54 7.65

504.1 1071 3670 2073 521.6 504.1 501.5 421.6 501.5 103.5 3.47 20.04

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can be associated with the biochemical process of digestion with the low rates of heat and work transferred through the boundaries of the digester. On the other hand, MED unit shows the lowest exergy efficiency due to the multi-effect heat losses within the system and considerable heat transfer within the system which causes a significant entropy generation and low exergy efficiency of 6%. For the parametric study, the variation of some affecting parameters of an integrated system including biogas mass flow rate, biogas quality, TIT, and air compression ratio in compressor on the energy and exergy efficiencies as well as production rates of some useful outputs are investigated for a better understanding of energetic and exergetic performances of the system. Besides, the effect of air compression ratio on the useful outputs and the effect of seawater temperature on the MED process is assessed.

Effect of biogas quality

Fig. 3 e Energy and exergy efficiencies for main units of the integrated system.

significantly lower than that of others. Furthermore, energy and exergy efficiencies of the main components of the integrated system are indicated and compared in Fig. 3. As seen, the digestion process has the highest energy and exergy efficiencies with the values of 78.1% and 57.2%, respectively. This

Sludge-based biogas in this study is comprised of CO2 and CH4. The variation of molar fractions of CH4 which is reversely proportional with the fraction of CO2 affects the heating value of the biogas and also the specific chemical exergy of biogas which affects the energy and exergy performances of power generation in biogas fueled Brayton, respectively. Fig. 4a, indicates the variation of specific chemical exergy and lower heating value of biogas with the molar fraction of methane. As seen, the LHV of biogas increases from 9766 to 29,659 kJ/kg, when the molar fraction of methane increases from 0.4 to 0.8 in biogas. Moreover, the specific chemical exergy also increases from 342.5 to 667.6 kJ/kg by the same increment in methane molar fraction in biogas. Fig. 4b, indicates the effect of methane molar fraction in the biogas on the characteristics of the combustion process. As seen, assuming the turbine inlet temperature (TIT) of 1200 K, and the compression ratio of 10, when the molar fraction of CH4 varies from 0.4 to 0.8, the air to fuel mass ratio (AF) increases from 8.26 to 24.92. Also, the mass flow rate of the exhaust gases increases from 1.96 to 5.49 kg/s. In the case of exhaust gas composition, the molar fraction of methane, gives rise to the change in exhaust gas

Fig. 4 e Effect of CH4 molar fraction in biogas on (a) LHV and specific chemical exergy of biogas, and (b) air to fuel ratio and exhaust gas mass flow rate in combustion.

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composition. This is important both thermodynamically, which is related to the system's efficiencies, and environmentally, regarding the emitting gases such as CO2. As seen in Fig. 5, as molar fraction of CH4 in biogas increases, all of the mass flow rates of the exhaust gas stream increase. However, nitrogen and oxygen increase with a more drastic trend. Here, the CO2 mass flow rate is the case of a concern which is associated with the global warming potential and imposes an environmental cost. As shown, with an increase of methane mole fraction from 0.5 to 0.9 in biogas as a fuel, CO2 mass flow rate increases from 0.488 to 0.781 kg/s. Considering the CO2 equivalent cost (18$/t) in accordance with the Ontario cap and trade program in Canada [37], this increment costs about 166,321$ annually. In this study, biogas combustion can be considered as the core of the integrated system. The thermal energy for producing most of the system outputs is supplied from the combusted gas. Thus, the biogas as a fuel has the high importance in our analysis. Fig. 6, indicates the effect of the CH4 molar fraction in biogas on the energy and exergy efficiencies of Brayton cycle, ORC, and overall system. As seen, the overall energy and exergy efficiencies of the system show an increment from 55% to 76% and from 37% to 45%, respectively. However, although the mass flow rate of the combusted gas and the work output of gas turbine increase as a result of higher CH4 molar fraction (Fig. 4), an increase in the LHV and specific chemical exergy of biogas (Fig. 3) leads to a decrease in the energy and exergy efficiencies of the Brayton cycle. Besides, since the mass flow rate of ORC doesn't affect cycle efficiencies, the energy and exergy efficiencies of ORC remain constant. Biogas quality has also some effects on the main system outputs. Fig. 7, illustrates the effect of the CH4 molar fraction on some useful outputs of the system. As seen, as CH4 molar fraction rises from 0.4 to 0.8, the net power generation rates of Brayton cycle increase drastically while the ORC power generation rate increases slightly. Also, the hydrogen production show an increase from 0.22 to 0.54 kg/h while the freshwater production rate increases from 0.17 to 2.18 kg/s, and the hot water production rate increases from hot water production rates increase from 1 to 3 kg/s.

Fig. 5 e Variation of exhaust gas composition with biogas quality.

Fig. 6 e Effect of CH4 molar fraction on the Brayton, ORC, and overall efficiencies.

Fig. 7 e Effect of CH4 molar fraction on the main outputs of the system.

Effect of biogas quantity The biogas flow rate, which is associated with the digestion process and the composition of the feedstock used for biogas production, affects the energetic and exergetic performances of the system. Some methods for pretreatment of sludge before the digestion can enhance the biogas yield. Codigestion is also recommended as another promising method for enhancing the digestion process and increasing the biogas yield [8]. In this study, the effect of biogas quantity is investigated on energy and exergy efficiencies as well as main system outputs. As seen in Fig. 8a, when the sludge mass flow rate as input remains constant, an increase in the mass flow rate of biogas from 0.15 to 0.3 kg/s, leads to an increase in the digester energy and exergy efficiencies from 76% to 81% and from 55% to 60%, respectively. However, the overall energy and exergy efficiency decrease slightly from 65% to 61.7% and from 42.6% to 36.2%, respectively. Since the sludge input is constant, the more biogas production in the digester, the less digestate mass flow rate. Thus, it can be inferred that the effect of reduction in digestate production rate on the overall system is higher than the effect of biogas production rate on overall efficiencies when digestate is counted as system's

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 5 1 1 e3 5 2 6

3523

Fig. 8 e Effect of biogas mass flow rate on (a) the digester and overall energy and exergy efficiencies (b) main outputs of the system.

useful output. Biogas production rate has some effects on the main outputs of the system. Fig. 8b, demonstrates the effect of biogas mass flow rate on the power generation, hydrogen production, freshwater production, and hot water production rates. As seen, when biogas mass flow rate rises up from 0.15 to 0. l3 kg/s, the power generation rate in Brayton cycle and ORC rise up from 709 to 1418 kW and from 70.76 to 141.5 kW, respectively. Also, the more biogas means the more mass flow rate of organic fluid in the ORC and thus, an increase in hot water production rate from 1.3 to 2.6 kg/s. The freshwater production also increases from 0.39 to 1.73 kg/s as a result of the more motive steam production in HX3 for desalination unit. Hydrogen production also increases from 0.25 to 0.48 kg/ s as a result of higher electricity delivery to the electrolyzer from Bryton cycle.

Effect of TIT TIT is an important factor affecting the performance of biogas-fueled Brayton cycle. However, the upper band of TIT for small-scale gas turbines is usually set as about 1127
C because of the limited size gas turbine which doesn't allow

Fig. 9 e Effect of the TIT on the energy and exergy efficiencies of Brayton cycle, and exergy destruction of combustion chamber.

proper cooling. Fig. 9, indicates the effect of TIT of energy and exergy efficiencies of Brayton cycle as well as exergy destruction rate in the combustion chamber. As shown, as a result of an increment in TIT from 727
C to 1127
C, both energy and exergy efficiency show an increase from 22.8% to 28% and from 22.6% to 26.6%, respectively. Moreover, the exergy destruction rate of combustion chamber decreases from 2535 to 2391 kW by increasing the TIT. The increment of efficiencies is associated with the higher power generation rate in gas turbine when the work consumed by air compressor remains the same. On the other hand, although the exhaust mass flow rate decreases by increasing the TIT, as seen in Fig. 10, the specific heat transfer to ORC in HX1 plays a major role and hence, an increase in TIT from 727 to 1127
C, results in a slight increase in a slight increase in the net power generation in Brayton cycle and ORC. As a result, as ORC mass flow rate increases, the hot water production rate increases from 0.27 to 2.62 kg/s. Besides, the rate of hydrogen production which is directly correlated with the Brayton cycle's power output shows an increase from 0.31 to 0.36 kg/h. However, the freshwater production rate shows a reverse trend. As shown, due to the

Fig. 10 e Effect of the TIT on the main system outputs.

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Fig. 11 e Effect of air compression ratio on the main system outputs.

lower exhaust gas mass flow rate, motive steam production decreases consequently and in turn, freshwater production rate shows a decrease from 2.01 to 0.44 kg/s.

Effect of air compression ratio Compression ratio is another operating parameter of the multigeneration system which affects the system performance. An increase in the air compression ratio provides air at the higher pressure and temperature for combustion process which in turn, causes an increase in AF and also an increase in power generation. Moreover, Fig. 11, demonstrates the effect of pressure ratio on the system outputs. As shown, the net power generation in Brayton cycle increases from 595.7 to 1079 kW by an increase in compression ratio from 3 to 15. Moreover, Hydrogen production rate, which is directly proportional to the electricity generated in Brayton cycle, shows an increase from 0.21 to 0.37 kg/h. Nevertheless, since higher pressure ratio means lower turbine outlet pressure when TIT is fixed, the heat transfer to ORC and R-245fa mass flow rate decrease and hence, net power output in ORC and hot water production rate show a decrease from 224.4 to 61.4 kW and from 4.11 to 1.13 kg/s, respectively. Besides, the freshwater production rate shows a decrease from 0.55 to 1.2 kg/s, due to the higher mass flow rate of exhaust gases in higher compression ratios which leads to a higher motive steam flow rate in MED unit.

Effect of seawater temperature Fig. 12 illustrates the effect of seawater temperature on the freshwater production rate and the performance ratio (PR) of MED system as the ratio of the freshwater mass flow rate to the motive steam mass flow rate. As shown, the freshwater production shows an increase from 0.71 to 1.12 kg/s when the seawater temperature increases from 15 to 40
C. The higher seawater temperatures mean higher feed water temperature and in turn, higher thermal energy for evaporation of water in MED stages which leads to the higher freshwater production rate. As the other parameters of the integrated system remain constant, the motive steam mass flow rate is constant and hence, the performance ratio consequently increases from 3.2 to 5.

Fig. 12 e Effect of seawater temperature on the freshwater production and performance ratio of MED system.

Conclusions A novel biomass-based integrated system for power generation, heating, hydrogen production and desalination is developed and analyzed in this study. Energy and exergy analyses are performed for all units of the system including digester, Brayton cycle, ORC, MED, and electrolyzer. The overall energy and exergy efficiencies of the multi-generation system are obtained as 63% and 40%, respectively, whereas the PR of MED system is calculated as 4.25. The higher molar fraction of CH4 in biogas, the higher overall energy and exergy efficiencies and the higher production rates of hydrogen and other useful outputs. Moreover, the highest exergy destruction rate among the system components is associated with the combustion chamber with the value of 2546 kW which can be reduced though an increase in TIT, air compression ratio, and CH4 molar fraction in biogas.

Nomenclature AD AF BPE C CP CHP d Dig E · Ex ex F g G GE GT GWP H h HX

Anaerobic digestion Air to fuel mass ratio Boiling point elevation Carbon Specific heat Combined heat and power Diameter (m) Digester Energy (kJ) Exergy rate (kW) Specific exergy (kJ/kg) Faraday constant (C/mol) Gravitational acceleration (m/s2) Gibbs free energy (J) Gas engine Gas turbine Global warming potential Hydrogen, enthalpy Specific enthalpy (kJ/kg) Heat exchanger

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 5 1 1 e3 5 2 6

J J0 Jref i L LHV _ m MED MGT , n N O ORC P PEM PR Q_ r R S s T TIT V _ W WWTP x X Y Z

Current density (A/m2) Exchange current density (A/m2) Pre-exponential factor (A/m2) Length (m) Lower heating value (kJ/kg) Mass flow rate (kg/s or kg/h) Multi-effect desalination Micro gas turbine Molar flow rate (mol/s) Nitrogen, mass (mol) Oxygen Organic Rankine cycle Pressure (kPa) Proton exchange membrane Performance ratio Heat rate (kW) Compression ratio Gas constant (kJ/kgK), resistance (U) Sulfur, entropy Specific entropy (kJ/kgK) Temperature (
C or K) Turbine inlet temperature Overpotential (V) Power generation rate (kW) Wastewater treatment plant Molar fraction Salinity (ppm) Molar fraction Elevation (m)

Greek letters l Molar fuel to air ratio water content at the anode-membrane interface la (U1) water content at the cathode-membrane interface lc (U1) water content at the location x in the membrane l(x) (U1) h Efficiency g Specific heat ratio r Density (kg/m3) m Chemical potential D Difference s Electric conductivity (s/m) Local ionic conductivity (s/m) s (x) Subscripts a Air, anode AC Air compressor act Activation B Brine c Cathode C Compressor cc Combustion chamber ch Chemical CV Control volume d Destruction dm Dry matter elec Electrolysis en Energy

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ex Exergy exp Expander f Feed water, formation F Fuel fw Freshwater g Combustion gases hw Hot water GT Gas turbine i Inlet, State number is Isentropic net Net power ohm Ohmic o Outlet ORC Organic Rankine cycle p Product P Pump ph Physical PEM Proton exchange membrane Q Heat r Reactant S Steam w Water, work 0 Dead state, reversible 1,2, …,29 State points Superscripts * Hypothetical term 0 Standard state value

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