Exergoeconomic analysis of hydrogen production from biomass gasification

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 3 7 ( 2 0 1 2 ) 1 6 4 0 2 e1 6 4 1 1

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Exergoeconomic analysis of hydrogen production from biomass gasification Yildiz Kalinci a, Arif Hepbasli b,*, Ibrahim Dincer c a

Department of Technical Programs, Izmir Vocational High School, Dokuz Eylul University, Education Campus Buca, Izmir, Turkey Department of Energy Systems Engineering, Faculty of Engineering, Yas¸ar University, 35100 Bornova, Izmir, Turkey c Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ont. L1H 7K4, Canada b

article info

abstract

Article history:

In this study, we investigate biomass-based hydrogen production through exergy and

Received 13 October 2011

exergoeconomic analyses and evaluate all components and associated streams using an

Received in revised form

exergy, cost, energy and mass (EXCEM) method. Then, we define the hydrogen unit cost and

20 February 2012

examine how key system parameters affect the unit hydrogen cost. Also, we present a case

Accepted 28 February 2012

study of the gasification process with a circulating fluidized bed gasifier (CFBG) for hydrogen

Available online 29 March 2012

production using the actual data taken from the literature. We first calculate energy and exergy values of all streams associated with the system, exergy efficiencies of all equipment,

Keywords:

and determine the costs of equipment along with their thermodynamic loss rates and ratio of

Hydrogen production

thermodynamic loss rate to capital cost. Furthermore, we evaluate the main system

Circulating fluidized bed gasifier

components, consisting of gasifier and PSA, from the exergoeconomic point of view. More-

Energy

over, we investigate the effects of various parameters on unit hydrogen cost, such as unit

Exergy

biomass and unit power costs and hydrogen content of the syngas before PSA equipment and

Efficiency exergoeconomics

PSA hydrogen recovery. The results show that the CFBG system, which has energy and exergy efficiencies of 55.11% and 35.74%, respectively, generates unit hydrogen costs between 5.37 $/kg and 1.59 $/kg, according to the internal and external parameters considered. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Energy demand has increased drastically and will continue increasing even further in the future, due to increasing population of the world, advancing technology and expectations of high living standard. At present, most of the energy comes from fossil sources, e.g., oil, natural gas and coal, which cause major environmental concerns. Also, due to a decrease in fossil fuel reserves and political disorders in the Middle East, prices are continuously fluctuating. So, exploring alternative energy sources is vital for better environment and

sustainability. Production of hydrogen energy from renewable sources appears to be a potential option in this regard. At present, hydrogen is mostly produced from natural gas through steam methane reforming. There is a set of renewable energy based methods to generate hydrogen energy. Biomass systems are among such publications. Biomass appears to be the fourth largest source of energy in the world, accounting for about 15% of the world’s primary energy consumption and about 38% of the primary energy consumption in the developing countries [1]. There are two main routes for biomass-based hydrogen production,

* Corresponding author. Tel.: þ966 4672911; fax: þ966 4672636. E-mail addresses: [email protected], [email protected] (A. Hepbasli). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.02.173

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 3 7 ( 2 0 1 2 ) 1 6 4 0 2 e1 6 4 1 1

namely thermo-chemical and bio-chemical. Thermo-chemical methods are commonly used as gasification, pyrolysis and supercritical water gasification. Various studies have been reported about hydrogen production from biomass gasification in the literature. Some of them focused on system parameters to increase hydrogen content of syngas. Hanaoka et al. [2] investigated effect of the reaction parameters, such as [Ca]/[C], reaction pressure and reaction temperature on H2 yield and conversion to gas in the steam gasification of woody biomass using CaO as a CO2 sorbent. At a [Ca]/[C] of 2, the maximum yield of H2 was obtained. Mahishi and Goswami [3] used a thermodynamic equilibrium model to predict the chemical composition of the products of biomass gasification (for wood). The effects of temperature, pressure, steam biomass ratio and equivalence ratio on the equilibrium hydrogen yield were studied using Stanjan (v 3.93L) software. The optimum conditions for hydrogen production were found to be 726.85  C, 3, 0.1 and 54% for the gasification temperature, steam biomass ratio, equivalence ratio and the efficiency, respectively. Nikoo and Mahinpey [4] developed a model for the gasification of biomass in an atmospheric fluidized bed gasifier using the Aspen Plus simulator. The simulation results for the product gas composition and carbon conversion efficiency versus temperature, equivalence ratio, steam biomass ratio and biomass average particle size were compared with the experimental results. In their study, temperatures varied from 700 to 900  C. Biomass feed rate, air and steam rate were obtained to be 0.445e0.512 kg/h, 0.5e0.7 Nm3/h and 0e1.8 kg/h, respectively. Some of the studies put importance on hydrogen production and electricity generation as Koroneos et al. [5]. They presented the environmental feasibility and efficiency of producing hydrogen from biomass via two processes. Biomass gasification followed by reforming of the syngas was compared to gasification followed by electricity generation and electrolysis. Using supercritical water in gasification is preferred, especially for high moisture biomass utilization. Yong et al. [6] presented a study about potential of hydrogen from oil palm biomass as a source of renewable energy worldwide through gasification reaction in supercritical water. The energy efficiencies of the gasification reaction were calculated to be 72.91% and 46.54%, while the maximum pure hydrogen production efficiencies were found to be 57.96% and 34.93% with and without heat recovery, respectively. Also, supercritical water gasification and biomass gasification have theoretically and experimentally been studied by Guo et al. [7] since 1997. The high energy conversion efficiency was achieved as the process avoids the drying step. Their operating conditions covered the temperatures between 649.85 and 799.85  C, pressures between 17.5 and 30 MPa and residence times from 9 to 46 s. Regarding hydrogen production from biomass gasification more details can be found in the authors’ another study in Ref [8]. In the analysis of such systems, energy analysis has generally been a common tool for analysis. The first law of thermodynamics embodies energy analysis, which identifies only externally energy wastes and losses. The potential improvements for the effective use of resources cannot be considered by energy as the second law of thermodynamics does it. In this regard, exergy comes out of the second law to

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be a powerful and effective tool for (i) designing and analyzing energy systems by combining the conservation of mass and energy principles with the second law of thermodynamics, (ii) furthering the goal of more efficient energy resource use by assessing meaningful efficiencies and enabling the locations, types and true magnitudes of wastes and losses to be determined, (iii) revealing whether or not, and by how much, it is possible to design more efficient energy systems by reducing the inefficiencies in existing systems, (iv) addressing the impact on the environment of energy resource utilization, (v) helping to achieve sustainable development [9]. For designing and assessing various thermal systems, these two energy and exergy based analysis tools are highly important, but not enough. Energy systems also need to be investigated from the economics point of view. In the literature, there are a number of economic analysis methods, which can be used to evaluate the economic performance of thermal systems. These include life cycle cost (LCC) method, net benefits (net present worth), payback method, benefit-to-cost (or savings-to-investment) ratio method, internal rate-ofreturn method, overall rate-of-return method, and analytic hierarchy process (AHP) [10]. Also, various exergoeconomic analysis methodologies were categorized by Tsatsaronis [11] into four main types depending on which of the following forms the basis of the technique: (i) exergy-economic cost accounting, (ii) exergyeconomic calculus analysis (iii) exergy-economic similarity number, and (iv) product/cost efficiency diagrams. During the past two decades, exergoeconomic analysis has been applied to various thermal systems [e.g., [11e16]]. Another exergoeconomic method is the EXCEM method. The basic concept of synthesizing subdisciplines into the EXCEM analysis methodology was first proposed in 1982 as mass, energy, availability and dollars (MEAD). Availability was used in this sense as a synonym for exergy, and dollars was used to mean cost. MEAD evolved into EXCEM over time [9]. The main objective of the EXCEM method is to investigate capital costs and thermodynamic losses for devices in thermal and power systems. The methodology provides a comprehensive assessment by accounting for the quantities exergy, cost, energy and mass [10,17]. Also, a parameter, which is calculated as the ratio of thermodynamic loss rate to capital cost, is defined to compare equipments of a system or systems. Rosen and Dincer [18] applied the EXCEM method to a coal fired electrical generating station. The devices in modern coal fired electrical generating stations appeared to conform approximately to a particular value of the thermodynamic loss rate to capital cost ratio (based on exergy loss), which reflected an ‘‘appropriate’’ trade-off between exergy losses and capital costs. Also, Rosen and Dincer [19] investigated capital costs and thermodynamic losses for devices in modern coal-fired, oil-fired and nuclear electrical generating stations in another study. The development of a code for EXCEM analysis was enhanced by using Aspen Plus as reported in Ref. [9]. Ozgener et al. [10] undertook a parametric study on the exergoeconomic assessment of a vertical ground-coupled (geothermal) heat pump system. An exergoeconomic model of a vertical ground- source heat pump residential heating system was given in their study using the EXCEM method.

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They also investigated how varying reference temperatures affect the exergoeconomic analysis of the system. A correlation between the ratio of thermodynamic loss rate to capital cost and reference state temperature was developed. Using the EXCEM method, exergoeconomic analyses of a hybrid copperechlorine cycle driven by geothermal energy for hydrogen production were conducted by other investigators [20,21]. They investigated and illustrated the relations between thermodynamic losses and capital costs. The results indicated that hydrogen cost is closely and directly related to the plant capacity and also exergy efficiency. The present study aims at contributing to the area of biomass-based hydrogen production through exergy and exergoeconomic analyses. While there are some studies about exergoeconomic analysis, the number of studies on EXCEM or exergoeconomic analyses of hydrogen production from biomass gasification systems is very small in the open literature, to the best of the authors’ knowledge. In this regard, the main objectives of this study are: (i) to discuss the importance of biomass-based syngas and hydrogen production, (ii) to investigate all components and streams using the EXCEM method, (iii) to define hydrogen unit cost, and (iv) to examine how key system parameters affect the unit hydrogen cost.

2.

follows the sulphur removal and wateregas-shift reactors. After the wateregas-shift, the gas is quenched to near the ambient temperature whereby water is condensed and removed. The gaseous components are lead to PSA equipment that separates the gaseous species into hydrogen and others, using a separation calculation block. The pressure swing adsorber is assumed to give a high enough purity, 99.99%, at a hydrogen recovery rate in the high 70% range. The waste heat taken from heat exchangers in the system is used to generate steam. The steam is generated at 50 bar and is first expanded down to 30 bar through a primary turbine for power generation. The steam flow is then split and a part of it goes to the gasifier and the rest is condensed through a secondary turbine for additional power generation. Further details on system operation are available in Ref. [22]. We modified the system using HX2 in producing steam line, so we modified the numbers of all streams accordingly. A flow diagram of the integrated process is depicted in Fig. 1.

3.

The analysis consists of two steps, namely energy and exergy and exergoeconomic analyses. In the first one, energy and exergy analyses are applied to all streams to calculate energy and exergy efficiencies of the main components of the system. Energy and exergy correlations can be taken from the previous studies of the authors [8,23,24]. In the second one, the system is exergoeconomically analyzed using the EXCEM method. The EXCEM analysis of systems and processes used was developed by Rosen and Dincer [9,18,19], as given below.

System description

The process includes a steam/oxygen blown gasifier. Due to the process size range, a pressurized (30 bar) CFBG is used. Before gasification, wood biomass needed for the gasification is milled into smaller particles (5 mm). The wood is then dried in a rotary dryer at 120  C. Oxygen is taken from air separate unit (ASU) equipment. Syngas, which is produced at 30 bar and 850  C, passes through a tar cracker for both thermal tar decomposition as well as a catalytic tar decomposition and

28 4

6

Analysis

3.1.

The general balance equations

A general balance for a quantity in a system may be written as

27

25

16

14

12

10

8

17 Hydrogen

1. Oxygen HX1

2. Biomass 3. Steam Gasifier

Tar Cracker

HX2

9

13

11 HTS

De-Sulpherization

HX5

HX4

HX3 7

MTS

18

15 PSA

LTS 24

5. Oxygen

23

26

Water PSA Off-Gas

29 Dryer 22

31

21

19. Air

30

32

Off-Gas Combustion Wet Biomass

33

CT

HX6 20

Gas Line Steam Line

HPT

Fig. 1 e A block diagram for hydrogen production from CFBG (Modified from Ref. [22]).

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 3 7 ( 2 0 1 2 ) 1 6 4 0 2 e1 6 4 1 1

Input þ Generation  Output  Consumption ¼ Accumulation

(1)

Here, input and output refer, respectively, to quantities entering and exiting through system boundaries, generation and consumption refer respectively to quantities produced and consumed within the system and accumulation refers to build-up (either positive or negative) of the quantity within the system. Differential and integral forms of the general balance equation may be written. The terms in Eq. (1) are written as rates, in the differential form: Input rate þ Generation rate  Output rate

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where Kin , Kout and Ka represent, respectively, the cost associated with all inputs, outputs and accumulations for the system. Kgen stands for the appropriate capital and other costs associated with the creation and maintenance of a system. Keq þ KC; M ¼ Kgen

(10)

Here, Keq and KC; M mean equipment costs and all other creation and maintenance costs. The Kgen term in a differential cost balance represents the total cost generation levelized over the operating life of the system. The “amount of cost generated” term in an integral cost balance represents the portion of the total cost generation occurring in the time interval under consideration.

 Consumption rate ¼ Accumulation rate

(2)

3.4.

(3)

Two types of thermodynamic losses are considered in the present paper, namely energy and exergy losses. Energy losses can be identified directly from the energy balances in Eqs. (5) and (7). For convenience, the energy loss rate for a system is denoted in the present analysis as L_ en (loss rate based on energy). As there is only one loss term, the “waste energy output rate”, which is denoted by E_ out; W in Eq. (7):

and as amounts, in the integral form: Amount input þ Amount generated  Amount output  Amount consumed ¼ Amount accumulated

The differential balance describes what is happening in a system at a given instant of time, and the integral balance describes what happens in a system between two instants of time. Differential balances are usually applied to continuous processes and integral balances to batch processes. For steady-state processes, the accumulation rate term in the differential balance is zero.

3.2.

Thermodynamic balance equations

As well known, mass and energy due to the conservation law are conserved during the process. Mass and energy balances are subject to first law of thermodynamic (neglecting nuclear reactions), can be neither generated nor consumed. Exergy is consumed during the process due to irreversibility and exergy consumption is proportional to entropy creation. Consequently, the general balance equation (Eq. (1)) can be written in the rate form for mass, energy and exergy as follows: _ out ¼ m _a _ in  m m

(4)

E_ in  E_ out ¼ E_ a

(5)

_ in  Ex _ out  L_ ex ¼ Ex _ a Ex

(6)

The output terms in Eqs. (5) and (6) can be separated into product and wastes as follows: E_ out ¼ E_ out; P þ E_ out; W

(7)

and

3.3.

L_ en ¼ E_ out; W

_ con þ Ex _ out; W L_ ex ¼ Ex

The general balance equation (Eq. (1)) can be written for cost as Kin þ Kgen  Kout ¼ Ka

(9)

(12)

where con means consumption. The capital cost is defined here using the cost balances in Eqs. (9) and (10),and is denoted by K. K ¼ Capital cost of equipment

(13)

For a thermal system operating normally in a continuous steady-state steady-flow process mode, the accumulation terms in Eqs. (1)e(6) and (9) are zero. Hence all losses are associated with the already discussed terms L_ en and L_ ex . The energy and exergy loss rates can be obtained through the following equations: L_ en ¼

X

Energy flow rates 

X in

X

Energy flow rates

(14)

Exergy flow rates

(15)

P

in

(8)

Economic balance equations

(11)

Exergy losses are determined from the exergy balances given in Eqs. (6) and (8). There are two types of exergy losses: the “waste exergy output” in Eq. (8), which represents the loss associated with exergy that is emitted from the system, and the “exergy consumption” in Eq. (6), which represents the internal exergy loss due to process irreversibility. These two exergy losses sum to the total exergy loss. Hence, the loss rate based on exergy, L_ ex , is defined as

L_ ex ¼

_ out ¼ Ex _ out; P þ Ex _ out; W Ex

Ratio of thermodynamic loss rate to capital cost

Exergy flow rates 

X P

where the summations are overall input streams and all product output streams. A parameter, R_ is defined as the ratio of thermodynamic loss rate L_ to capital cost K as follows: L_ R_ ¼ K

(16)

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The value of R_ generally depends on whether it is based on energy loss rate (in which case it is denoted (R_ en ), or exergy loss rate (R_ ex ), as follows: L_ en R_ en ¼ K

(17)

and L_ ex R_ ex ¼ K

(18)

In addition, the component costs are taken from the literature in this study. Due to different capacities and different dates, purchased equipment costs are modified according to Bejan [25].

4.

Results and discussion

In the present study, a comprehensive thermodynamic analysis of a CFBG system for hydrogen production is investigated and discussed via exergoeconomic parameters and thermodynamic loss ratios. In the analysis, the reference environment temperature (T0 ) and pressure (P0 ) values are taken as 298.15 K and 1 atm (100 kPa), respectively. All processes are treated as steady state and steady-flow with negligible potential and kinetic energy changes. Also, the system boundary is taken only came from the outside streams. In other words, the produced steam and electric energy in the system are used in the same system, so that these are not included in the main streams. As known, mass is preserved in systems. So, waste flow rates can be calculated from mass flow rates of reactants and products using Eq. (4). In the system, 8.5 kg/s dry biomass is gasified via steam/O2, then, the tar is fired with O2 again to increase the quantity of syngas. Total consumed O2 mass flow rate is 4.95 kg/s, which is produced from the ASU equipment. Also, a total of 16.731 kg/s steam is produced in the same processes. The part of the mass flow rates is separated as ash and H2S, 0.001 and 0.065 kg/s, respectively. Also, to increase hydrogen content of the syngas, it passes from shift reactions with high temperature shift (HTS), middle temperature shift (MTS), low temperature shift (LTS) equipments and the hydrogen content becomes 54.2 (v/v%). Before the PSA equipment, the temperature is taken to be nearly the environmental temperature, so the rest H2O with 1.969 kg/s condenses and separates. The produced hydrogen is 0.521 kg/s, namely it is 0.83% of the total input mass flow rates. Also, the outgoing steam from the condensing turbine is not considered as waste in the present study and the mass flow rates for the CFBG system are given in Fig. 2. Using Eqs. (5) and (7), an overall rate balance for energy in the CFBG system is made and given in Fig. 3. As clearly shown in this figure, energy is conserved. An important part of the input energy rates comes from biomass as 89.42%. The produced hydrogen and steam from the condensing turbine cover 35.95% and 19.17% of the input energy rate, respectively. The CFBG system has an energy loss of 44.88%, which involves energy rates of the condensed water, off-gas and other thermal losses in equipments. Also, the power is produced as 2.14 MWe and 11.61 MWe via the high pressure turbine and

Fig. 2 e A percentage breakdown of mass flow rates in the CFBG system.

condensing turbine. But this power is not enough for these systems due to high power consumptions of ASU and PSA, which are taken to be 0.5 kWh/Nm3 O2 or H2 from Ref. [26], and 6.49 MWe power has to be bought from the grid. Energy efficiency of the overall system is calculated to be 55.11%. To identify the real performance of thermal systems, exergy analysis is used as a beneficial tool. Eqs. (6) and (8) give main correlations of exergy rates. To calculate the exergy rates of all streams, correlations can be obtained in more detail from the previous studies of the authors Refs. [8,23,24]. Also, we use the engineering equation solver (EES) and some tables from Ref. [25] to define the physical and chemical properties of all streams. The calculated exergy rates of all streams are given in Table 1 and Fig. 4. As given in the above table data and the figure, exergy is not conserved. In this study, the condensing water and off gas are considered as waste flows. Exergy losses mean the waste flow plus exergy destructions in components due to irreversibility. While total exergy rate is 183.305 MW in the inlet, biomass has the most exergy rate as 174.872 MW. As products, steam and H2 have 1.11% and 34.6% of input exergy rates,

Fig. 3 e Energy rates of the CFBG system.

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Table 1 e Flow data for the CFBG system. Stream

Stream

number 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 30 31 32 33

O2 Biomass Steam Syngas O2 Syngas Syngas Syngas Syngas Syngas Syngas Syngas Syngas Syngas Syngas Cond. H2O H2 Off-gas Air Off-gas Off-gas Off-gas Water Water Water Water Water Water Water Water Water Water Water _ HPT W _ CT W _ ASU W _ PSA W _ GRID W

_ m

T

P

h0

s0

h

s

_ T Ex

E_

(kg/s)

(K)

(bar)

(kJ/kg)

(kJ/kgK)

(kJ/kg)

(kJ/kgK)

(MW)

(MW)

2.504 8.5 2.908 13.911 2.448 16.359 16.359 16.294 16.294 16.294 16.294 16.294 16.294 16.294 14.325 1.969 0.521 13.804 32.304 46.108 46.108 48.709 16.731 16.731 16.731 16.731 16.731 16.731 16.731 16.731 16.731 13.823 13.823

298.15 393 714.20 1123.20 363.00 1573.15 623.15 623.15 619.85 773.15 529.70 598.15 457.10 488.20 298.20 298.15 298.15 298.15 309.600 873.2 773.15 557.7 298.15 300 428.32 478.3 537.1 537.1 617 750.15 682.6 682.6 318.9

30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 1 30 1 1 1 1 1 1 50 50 50 50 50 50 50 30 30 0.1

0.979 174.872 3.563 131.464 0.958 130.640 109.530 107.936 107.886 106.603 102.674 101.841 100.044 100.018 97.720 0.000 63.468 31.620 0.006 16.260 12.743 7.720 0.000 0.096 1.652 2.910 5.994 6.030 19.100 22.180 19.810 16.370 2.041 2.14 11.61 9.73 10.51 6.49

0.00 155.52 9.67 142.74 0.15 149.92 120.28 120.26 120.17 115.20 107.89 105.34 101.18 100.34 91.30 0.21 62.52 28.78 10.02 32.93 27.41 23.50 1.75 1.97 11.00 14.67 21.95 22.02 51.05 56.55 54.41 44.95 33.34 2.14 11.61 9.73 10.51 6.49

0.000

6.407

0.000

5.530

104.890 -9577.100 0.000 -9220.900 -9220.900 -9256.700 -9256.700 -9240.300 -9240.300 -9202.400 -9202.400 -9204.500 -8366.800 104.890 0.000 -8674.300 298.600 -3284.400 -3284.400 -3955.400 104.890 104.890 104.890 104.890 104.890 104.890 104.890 104.890 104.890 104.890 104.890

0.367 6.791 6.407 7.068 7.068 7.065 7.065 7.685 7.685 7.909 7.909 7.996 8.327 0.367 65.340 6.053 5.699 6.445 6.445 6.309 0.367 0.367 0.367 0.367 0.367 0.367 0.367 0.367 0.367 0.367 0.367

3323.640 -7346.700 59.728 -6109.200 -7920.900 -7952.800 -7958.600 -7952.800 -8401.400 -8396.300 -8651.500 -8649.800 -8366.800 104.890 0.000 -8674.300 310.100 -2570.300 -2690.000 -3472.900 104.890 117.820 657.400 876.530 1312.000 1316.000 3051.100 3380.000 3252.000 3252.000 2412.000

7.053 9.899 5.712 10.947 9.200 9.206 9.197 9.325 8.629 8.678 8.192 8.212 6.985 0.367 51.250 6.053 5.737 7.848 7.702 7.609 0.367 0.392 1.889 2.372 3.213 3.220 6.420 6.905 6.953 6.953 7.611

respectively. H2 is 298.15 K and 30 bar after the PSA equipment. It has physical and chemical exergy rates of 2.188 MW and 61.28 MW, respectively. Also, exergy destruction rate and exergy efficiency are calculated to be 110.076 MW and 35.74%, respectively.

Fig. 4 e Exergy rates of the CFBG system.

The EXCEM method considers two main subjects, which are energy and exergy loss rates and capital cost of equipments. Using the data in Table 1 and Eqs. (11), (12), (14) and (15), energy and exergy loss rates are calculated for main components of the CFBG system and given in Table 2. As can be seen in Table 2, while the most energy losses belong to the gasifier and PSA equipments as 22.45 and 10.51 MW, respectively, the most exergy losses are due to the gasifier, combustion chamber and PSA components with 47.95, 15.36 and 13.14 MW, respectively. Off-gas and H2 are considered as product in the PSA equipment due to using thermal energy of the off-gas to dry biomass. So, energy and exergy efficiencies are high in the component as 89.67% and 87.85%, respectively. When H2 is considered as product only, the efficiencies decrease to 61.4% and 58.64%, respectively. Also, in this study, the capital costs of equipments are taken from Hulteberg and Karlsson [22]. To calculate the capital cost of combustion chamber and PSA equipments, Refs. [25,26] are used. Also, we can calculate ratios of thermodynamic loss rate to capital cost using Eqs. (16)e(18). The ratios give a loss rate for 1 $ capital cost. So, the ratio should be

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Table 2 e Capital cost and thermodynamic loss data for main devices. Components

L_ en

R_ en

L_ ex

R_ ex

h

3

(10 $)

(MW)

(W/$)

(MW)

(W/$)

(%)

(%)

43.74 12.93

22.45 0.02

0.51 0.00

47.95 1.594

1.10 0.12

86.41 99.9

73.27 98.54

2.05 2.05 2.05 57.00 0.05

4.97 2.55 0.84 10.51 5.87

2.42 1.24 0.41 0.18 117.24

1.283 0.833 0.026 13.14 15.36

0.63 0.41 0.01 0.23 306.77

95.86 97.64 99.17 89.67 84.87

98.81 99.18 99.97 87.85 51.41

5.47 0.02 0.14 0.16 0.22 8.89 12.47 10.56 78.95

0.61 0.02 0.03 0.46 0.21 0.02 0 0

0.11 1.00 0.21 2.82 0.94 0.00 0.00 0.00

8.04 0.016 0.85 1.426 0.73 0.437 0.23 2.72

1.47 0.80 5.97 8.75 3.25 0.05 0.02 0.26

97.94 77.77 99.58 88.86 100 99.63 100 100

61.9 71.4 78.37 70 68.2 87.57 90.29 81.02

236.76

48.56

0.21

94.64

0.40

55.11

35.74

K 6

Gasifier+ Cyclone De-Sulpherization (filters, scrubbers, guard beds) HTS MTS LTS PSA Combustion Chamber HX1 HX2 HX3 HX4 HX5 HX6 HPT CT Others Equipments CFBG System 1 US$¼1.42 TL (October 2010)

as small as possible. As can be seen in Table 2, the maximum R_ en and R_ ex values belong to the combustion chamber as 117.24 (W/$) and 306.77 (W/$), respectively. The statistical data for thermodynamic loss rate to capital cost rate for main components are also calculated and given in Table 3. In the CFBG system, R_ means the set of 15 values, R_ 1 ; R_ 2 ; R_ 3 ::::R_ 15 , which are a measure of the center of the set as 8.47 and 21.99 W/$, based on energy and exergy losses, _ of the set is respectively. The standard deviation, SDðRÞ a measure of the absolute variation in the set of R_ values about _ The coefficient of variation CVðRÞ _ is the standard the mean R. deviation as a percentage of the mean, 355.24% and 358.46% for energy and exergy losses, respectively. Also, hydrogen production cost can be calculated using stream costs. In this study, the Kgen term in a differential cost balance represents the total cost generation levelized over the operating life of the system. The annual operating and maintenance costs and capital investment costs are taken from Ref. [22]. Then, the unit power, water and biomass costs are modified according to Refs. [27-29] as 31.38 $/GJ, 4.59 $/t and 35.42 $/t, respectively. Calculation of hourly levelized capital costs can be found in more detail in Refs. [14,25,30]. In

this study, hourly levelized costs are illustrated in Fig. 5. According to this figure, the cost generation rate is 4157.52 $/h, which contains total of equipment and operating and maintenance cost rates. Also, the capital investment cost rate of ASU is considered in the equipment costs and the utilized power is reflected to O2 cost rate. The produced hydrogen is 1875.6 kg/h and unit hydrogen cost is calculated as 5.37 $/kg using these data, indicating a very high cost. We can also define, which parameter affects the cost mostly. First, we consider costs of external streams as unit electric cost and unit biomass cost. Figs. 6 and 7 illustrate the effect of unit electric cost and unit biomass cost to unit hydrogen cost. The costs change depending on countries. We take them for Turkey as defined above. While the unit power cost changes from 0.04 to 0.15 $/kWh, the unit hydrogen cost increases to 1.802 $/kg. The effect is considered to be very

Table 3 e Statistical data for thermodynamic loss rate to capital cost rate for main components. Parameter Minimum R_ min (W/$) Maximum R_ max (W/$) Mean R_ (W/$) _ Standard deviation SDðRÞ

_ (%) Coefficient of variation CVðRÞ

Based on energy loss

Based on exergy loss

0 117.24 8.47 30.10 355.24

0.01 306.77 21.99 78.82 358.46

Fig. 5 e Cost generation rates of the CFBG system.

Unit Hydrogen Cost ($/kg)

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The shifted gas stream must contain the least 70 (v/v%) hydrogen before it can be economically purified in the PSA unit. The results obtained from the present study are in a reasonably well good agreement with those given in the literature [31e33].

7 6 5 4 3 2 1 0

5.

Conclusions

The present study investigates a biomass-based hydrogen production through exergy and exergoeconomic analyses using the EXCEM method. The main purposes are to discuss the performance of all components economically, to define hydrogen unit cost and to examine how key system parameters affect the unit hydrogen cost. The following main concluding remarks are drawn from the present study:

Unit Power Cost ($/kWh) Fig. 6 e Variation of hydrogen production cost with unit power cost.

Unit Hydrogen Cost ($/kg)

16409

8 7 6 5 4 3 2 1 0

Unit Biomass Cost ($/t) Fig. 7 e Variation of hydrogen production cost with unit biomass cost.

small. It changes based on the process. In the present study, 13.78% of input cost rates comes from electrical energy. Therefore, it has a low effect as expected. Another parameter is the unit biomass cost. While the unit biomass cost ranges from 20 to 100 $/t, the unit hydrogen cost increases to 2.47 $/kg. Second, we consider the internal parameters e.g., hydrogen content of syngas before the PSA equipment and PSA hydrogen recovery. While the hydrogen content changes from 50% to 80%, the PSA hydrogen recovery varies from 60% to 80%. The accretions decrease the unit hydrogen cost from 7.49 $/kg to 1.59 $/kg. The minimum unit hydrogen cost is taken to be 80 (%v/v) at H2 content of syngas and 80 (%) at the PSA hydrogen recovery, as can be seen in Fig. 8.

 Energy and exergy losses (including both wastes and destructions) are pointed out for main devices. The most energy loss rates belong to the gasifier, the PSA and the combustion chamber units. The same components have the most exergy loss rates. In other words, the operation conditions of the equipment need to be improved.  When the thermodynamic loss rates to the capital cost ratio are investigated for the main components, the maximum R_ values belong to the combustion chamber based on energy and energy analysis results.  Hydrogen cost rate and unit hydrogen cost are defined. The unit hydrogen cost from biomass is higher than that from fossil sources. To decrease the cost, effects of some parameters such as unit costs of biomass and power are examined. Also, internal parameters, e.g., hydrogen content of the syngas before PSA equipment and PSA hydrogen recovery, are considered. The results indicated that the hydrogen content of the syngas and PSA hydrogen recovery should be at least 70 (v/v %) and 70% due to the economic considerations, respectively.  The systems can be designed for multi products, e.g., electrical power to the grid, heat, process steam and hydrogen to reduce the unit hydrogen cost. The future work will focus on life cycle assessment of hydrogen production from biomass systems.

Acknowledgement Hydrogen Content of Syngas (%)

90

PSA Hydrogen Recovery 80

80

70 70 60 60

50 40

50

Unit Hydrogen Cost ($/kg)

Fig. 8 e Variation of H2 content of syngas and PSA hydrogen recovery with unit hydrogen cost.

PSA Hydrogen Recovery (%)

Hydrogen Content of Syngas (%)

90

The authors gratefully acknowledge the support provided by their universities and the Natural Sciences and Engineering Research Council.

Nomenclature

CV E_ _ Ex h K

coefficient of variation, % energy rate, MW exergy rate, MW specific enthalpy, kJ/kg capital cost of equipments, $

16410 L_ _ m P R_ R_ s SD T _ W

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 3 7 ( 2 0 1 2 ) 1 6 4 0 2 e1 6 4 1 1

loss rate, MW mass flow rate, kg/s pressure, kPa or bar the ratio of thermodynamic loss rate to capital cost, W/$ arithmetic mean, W/$ entropy, kJ/kg K standard deviation temperature,  C or K work rate, MW

Greek letters h energy or first law efficiency, % ε exergy or second law efficiency, %

Subscripts a accumulation ASU air separation unit C creation con consumption CT condensing turbine en energy eq equipment ex exergy gen generation GRID grid HPT high pressure turbine in inlet M maintenance max maximum min minimum out outlet P product PSA pressure swing adsorption W waste 0 reference index

Superscripts Over dot quantity per unit time T total

Abbreviations AHP analytic hierarchy process ASU air separation unit CFBG circulating fluidized bed gasifier CHP combined heat and power CT condensing turbine EES engineering equation solver EXCEM exergy, cost, energy, mass HPT high pressure turbine HX heat exchanger HTS high temperature shift LCC life cycle cost LTS low temperature shift MEAD mass, energy, availability and dollars MTS middle temperature shift PSA pressure swing adsorption

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