Cohce_Thermo Analysis of H2 Production From Gasification

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Thermodynamic analysis of hydrogen production from

biomass gasification

M.K. Cohce*, I. Dincer, M.A. Rosen

Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ontario,

Canada L1H 7K4

a r t i c l e i n f o

Article history:

Received 19 June 2009

Received in revised form

26 August 2009

Accepted 31 August 2009

Available online 7 October 2009

Keywords:

Biomass

Gasification

Hydrogen

Thermodynamics

EnergyExergy

Efficiency

Oil palm shell

SMR

a b s t r a c t

An investigation is reported of the thermodynamic performance of the gasification process

followed by the steam-methane reforming (SMR) and shift reactions for producing

hydrogen from oil palm shell, one of the most common biomass resources. Energy and

exergy efficiencies are determined for each component in this system. A process simula-

tion tool is used for assessing the indirectly heated Battelle Columbus Laboratory (BCL)

gasifier, which is included with the decomposition reactor to produce syngas for producing

hydrogen. A simplified model is presented here for biomass gasification based on chemical

equilibrium considerations, with the Gibbs free energy minimization approach. The

gasifier with the decomposition reactor is observed to be one of the most critical compo-

nents of a biomass gasification system, and is modeled to control the produced syngas

yield. Also various thermodynamic efficiencies, namely energy, exergy and cold gas effi-

ciencies are evaluated which may be useful for the design, optimization and modification

of hydrogen production and other related processes.Crown Copyright ª 2009 Published by Elsevier Ltd on behalf of Professor T. Nejat Veziroglu. All

rights reserved.

1. Introduction

Biomass, a relatively large energy source globally which

includes wood, municipal solid wastes and agricultural resi-dues, is being investigated in various countries as a potentially

significant renewable resource. Biomass is derived from solar

energy. Biomass is relatively clean compared to other sources

of energy, as it releases no net CO2 emissions when carefully

managed since CO2 is fixed by photosynthesis during biomass

growth and is released during utilization. This form of energy

can be converted to gaseous fuel through thermochemical

gasification [1]. Such a fuel can be used for various tasks,

including producing hydrogen, which can be used cleanly and

efficiently as a fuel in combustion engines and fuel cells.

Hydrogen is likely to be an important energy carrier in thefuture. Presently, it can be produced by the steam reforming of

natural gas, coal gasification and water electrolysis among

other processes. However these current processes are not

sustainable because they use fossil fuels or electricity from

non-renewable resources. Hydrogen production can be made

more sustainable if it is produced from sustainable energy

resources. In this regard, alternative thermochemical

* Corresponding author.E-mail addresses: [email protected] (M.K. Cohce), [email protected] (I. Dincer), [email protected] (M.A. Rosen).

A v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / h e

0360-3199/$ – see front matter Crown Copyrightª 2009 Published by Elsevier Ltd on behalf of Professor T. Nejat Veziroglu. All rights reserved.

doi:10.1016/j.ijhydene.2009.08.066

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 5 ( 2 0 1 0 ) 4 9 7 0 – 4 9 8 0

mailto:[email protected]



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(pyrolysis and gasification) and biological (biophotolysis,

water-gas shift reaction and fermentation) processes are

practical and can be more sustainable than present processes

[2,3]. Overviews of technologies for hydrogen production from

biomass have been reported [4–8]. Many researchers are

focusing their research on the gasifier portion of this process,

as gasification appears to be more favorable for hydrogen

production than pyrolysis [9].The gasifier is the most important component in any

biomass gasification system [10]. Gasification, which is char-

acterized by partial oxidation, is utilized in numerous clean

energy processes including hydrogen production via biomass

gasification. Currently, 80–85% of the world’s total hydrogen

production is derived from natural gas via steam-methane

reforming (SMR) [11]. Much research has been reported on the

production of hydrogen by SMR [12–15], with many studies

concentrating on analysing the reforming reactor [16]. Gasifier

modelling and simulation, using programs such as Aspen

Plus, have been ongoing [17–19]. Some researchers predict

biomass gasification in supercritical water to be a promising

technology for hydrogen production for it allows the utiliza-tion of wet biomass [20,21]. Recent research has focused on

determining energy and exergy efficiencies [22–25] and

improving understanding and data availability with experi-

mental studies. In this study, the gasifier, the most significant

part of the system, is analyzed in detail.

Exergy analysis is a tool for understanding and improving

efficiency [4], and is used throughout this investigation in

addition to energy analysis. Extensive exergy analyses have

been reported using devices, technologies and systems, in

a variety of fields. Some examples include exergy assessments

of power plants for transportation and power generation,

chemical and metallurgical processing facilities and building

systems [26,27]. Exergy-based assessments and comparisonshave also been performed of such energy technologies as

hydrogen production [28–33], electricity generation with fuel

cells and other technologies, for instance, coal-fired and

nuclear [34,35], and larger energy systems [36,37].

The aim of the present work is to investigate hydrogen

production by thermochemical biomass gasification using

energy and exergy methods, and to evaluate the potential of

hydrogen production from biomass. A parametric analysis of

factors influencing the thermodynamic efficiency of biomass

gasification is carried out. Energy and exergy assessments are

performed and the effects are determined on both system

efficiency and hydrogen yield of varying parameters of the

fractioned syngas. The system considered has two parts, andboth include syngas production as an input for producing

hydrogen in the hydrogen plant. The manner is assessed by

which hydrogen production is changed by fractioning the

syngas streams and by importing methane from external

sources. The focus of this research is on the gasifier since it

requires the most improvement in terms of energy and exergy

utilization. The significance of the present work is its focus on

green energy sources and increasing the efficiency of systems

for hydrogen production. It is anticipated that the results will

assist efforts to optimize and enhance the environmental

performance of energy systems. The use of computer simu-

lation yields a better understanding of the overall system, and

is a particularly useful tool in support of design.

2. Thermodynamic evaluation

A thermodynamic evaluation of a complex system requires

consideration of its components and their characteristics,

chemical reactions and thermal losses. Recently, biomass

gasification in indirectly heated steam gasifiers has received

much attention for the conversion of biomass to combustiblegas [38,39]. In our simulation, we consider the energy effi-

ciency of the gasification reaction as the total energy of the

desired products divided by the total energy of the process

inputs [28]. For this analysis, the products are taken to be

a mixture of H2O, N2, H2, CO2, CH4, CO, NH3 and H2S. Char is

assumed to consist of solid carbon (C) and tar is not taken into

account in the simulation.

Simulations are preformed with the Aspen Plus simulation

software, which is utilized in a wide range of industrial

applications [18]. A Fortran subroutine is applied to control

process yields. In Aspen Plus, streams represent mass or

energy flows. Energy streams may be defined as either work or

heat streams, of which the latter also contain temperatureinformation to avoid infeasible heat transfer. Mass streams

are divided by Aspen Plus into three categories: mixed, solid,

and non-conventional (for substances like biomass). Mixed

streams contain mixtures of components, which can be in

gaseous, liquid and solid phases. The solid phase component

in this simulation is solid carbon (C). Thermodynamic prop-

erties are defined in the Aspen Plus libraries for chemical

components. Components present in the mixed and solid

stream classes may participate in phase and chemical equi-

librium, andare automaticallyflashedby Aspen Plus at stream

temperature and pressure. Non-conventional components are

defined in Aspen Plus by supplying standard enthalpy of

formation and the elementary composition (ultimate andproximate analyses) of the components may also be defined.

Biomassis characterized in thismannerhere. Although Aspen

Plus calculates enthalpies and entropies for conventional

components, ambient temperature and pressure, which are

required in evaluations of exergy, are not readily available in

the result output. A property termed availability by Aspen Plus

is calculated for conventional components, but this does not

include chemical exergy. Forthese reasons, the EESprogram is

used to calculate total exergy (physical and chemical) for each

stream in this simulation.

The following simplifying assumptions are made in the

simulation:

Char only contains solid carbon and ash, and there is no tar

yield.

The process occurs at steady state and isothermally, and

residence time is not considered. Also catalysis is not

used.

The ZNO-bed and the pressure swing adsorption (PSA)

system are not included in the energy and exergy

calculations.

All gases behave ideally.

Air is considered on a volume basis as 79% nitrogen and 21%

oxygen.

A heat stream is used as a heat carrier in Aspen Plus instead

of sand.

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 5 ( 2 0 1 0 ) 4 9 7 0 – 4 9 8 0 4971



2.1. Balances

Mass and enthalpy values are evaluated with Aspen Plus. For

a general steady-state process, we can write mass and energy

balances, respectively, as

Xi

_mi ¼

Xo

_mo (1)

Xi

_Ein ¼X

o

_Eout (2)

An overall exergy balance can be written for a steady-state

process as follows:

X_Exi

in

¼X

_Ex j

out

þX

_Ex

dest(3)

where

X_Exi

in

¼ _Exair þ _Exdrybio þ _Exbiomoist þ _Exst þ _Exmeth (4)

X _

Ex j

out¼_

Exprodg þ_

Exunconcarb þ_

Exst þ_

Exexh þ_

W turb (5)

These values are available in Table 1

Both physical and chemical exergy inlet and outlet values

are determined for the Gasification, Combustion, SMR, HTS

and LTS product gases, are used to assess exergy destructions.

Some components possess only physical exergy. The specific

flow exergy associated with a specified state is expressible by

the sum of specific physical and specific chemical exergy:

exprodg ¼ exph þ exch (6)

The physical exergy defined as

exph ¼ ðh À hoÞ À T0ðs À soÞ (7)

and the chemical exergy contribution can be calculated for an

ideal gas mixture as follows:

exch ¼X

i

xi

Àexch

i À RT0 ln xi

Á(8)

Here, xi is the mole fraction and exchi the standard chemical

exergy of component i. Standard chemical exergy values used

here are taken from model 2 in Szargut et al. [26].

The entropy balance for a steady-flow reacting system can

be written asX _Q j

_T j

þX

_misi ÀX

_moso þ _Sgen ¼ 0 (9)

The exergy destroyed due to irreversibility can be expressed

as follows:

_Exdest ¼ T0_Sgen (10)

The physical exergy of biomass is zero when it is entering

the system at temperature T0 and pressure P0. Thermody-

namic properties are needed for the calculation of the chem-

ical exergy of biomass. Since such properties for oil palm shell

biomass are not available, a correlation factor for solid

biomass (for O/C < 2) is used based on statistical correlationsdeveloped by Szargut et al. [29]:

b¼1:044 þ 0:0160H=C À 0:3493O=Cð1 þ 0:0531H=CÞ þ 0:0493N=C

1 À 0:4124O=C

(11)

The specific chemical exergy for biomass can then be

determined as

ExchbioðsolidÞ ¼ bLHVbio (12)

Although the magnitude of the physical exergy of biomass

is small, it is calculated in this simulation after the drying

process. The calculation is performed using the heatcapacity of dry biomass which is described by Gronli and

Melaaen [37]:

C pðbioÞ ¼ 1:5 þ 10À3T (13)

where C p(bio) is the heat capacity and T is the temperature of

the biomass.

The change in specific entropy in Eq. (7) can be written for

biomass as

Ds ¼

Z T

T0

C p;i

TdT (14)

Equation (14) can be used to calculate the physical exergy

with Eq. (7) at the specified temperature.The heat capacity of solid carbon is determined using

Aspen Plus property data and substituted into Eq. (7) to find

entropy values, which are used for the thermal exergy calcu-

lation in Eq. (7).

2.2. Energy, cold gas and exergy efficiencies for BCL

gasification

Tables 2 and 3 show the properties and gas composition of the

main streams and the overall system performance based on

the simulation results. The latter can be expressed with the

cold gas efficiency. This measure is the ratio of the chemical

energy of the produced gas to that of the biomass feed energy

Table 1 – Exergy results for the overall hydrogen plant forCase 1.

Exergy flowrate (kJ/h)

Percentage of totalexergy inlet

Inputs

Wet biomass 2,397,523 81.0Water 8,469 3.0

Air 3,432 1.0

Methane (CH4) 527,500 16.0

Purchased electricity 0 0.0

Total 2,936,924 100.0

Outputs and destructions

Hydrogen 653,052 22.3

Electricity 46,800 1.5

Exhaust 303,573 10.3

Water 49,850 1.7

Total output 1,053,265 35.9

Exergy destruction 1,883,658 64.1

Total output

and destruction

2,936,923 100.0




content. The cold gas energy efficiency of the BCL hydrogen

plant is determined as

hcg ¼_mprodg LHVprodg

_mdrybioLHVdrybio(15)

Hydrogen is the desired product in this simulation, so theproduction efficiency can be described as the system effi-

ciency in general. Energy efficiency h and exergy efficiency j

values are often evaluated for steady-state processes, and can

be written here as follows:

hsys ¼_Eprodg þ _W turb

_Eair þ _Edrybio þ _Ebiomoist þ _Est þ _Emeth

(16)

jsys ¼_Exprodg þ _W turb

_Exair þ _Exdrybio þ _Exbiomoist þ _Exst þ _Exmeth

(17)

where _Eprodg is the rate of product energy output, _Exprodg is the

rate of product exergy output and_

W turb is the turbine workrate. Also the energy efficiency of component i may be written

as follows:

hi ¼ 1 Àð _EoutÞi

ð _EinÞi

(18)

where ð _EoutÞi and ð _EinÞi are the energy output and input rates

for component i. Similarly, the exergy efficiency for compo-

nent i may be written as

ji ¼ 1 Àð _ExdestÞi

ð _ExinÞi

(19)

where ð _ExdestÞi and ð _ExinÞi respectively are the exergy destruc-

tion rate and the exergy input rate for component i.

The steam–biomass ratio (STBR) can be expressed as

STBR ¼_mst

_

mdrybio

(20)

Also, the ratio of exergy destruction xdest for a component

can be evaluated by dividing its exergy destruction by the total

exergy provided to the system. Here, we can write

xdest ¼ð _ExdestÞi

ðP

_ExiÞin

(21)

where ð _ExdestÞi is the exergy destruction for each component

and ðP

_ExiÞin is the exergy flow of all input material streams.

3. Case study

Heating and drying are endothermic processes that require

a source of heat. Heat can be supplied by an external source

via indirect heating. In gasification, indirect heating allows us

to have indirectly heated gasification. More often, a small

amount of air or oxygen, typically not more than 25% of the

stoichiometric requirement for complete combustion of the

fuel, is input for the purpose of partial oxidation, which

releases sufficient heat for drying and pyrolysis as well as for

the subsequent endothermic chemical reactions. These

include the carbon–oxygen, boudovard, carbon–water and

hydrogenation reactions [35].

The system simulated in this paper includes the gasifica-

tion plant and the hydrogen plant. Related energy efficiencyand economic analyses have been performed [38]. In this

investigation, the system analyzed by those authors is modi-

fied and energy and exergy efficiencies are evaluated for

varied conditions (e.g., water streams and the amount of

entered biomass).

In this study, we focus on the gasifier. The R-GIBBS block

reactor uses single-phase chemical equilibrium, or simulta-

neous phase and chemical equilibrium, by minimizing the

Gibbs free energy, subject to atom balance constraints. This

block reactor is useful when the temperature and pressure are

known and the reaction stoichiometry is unknown. The latter

reactor and the decomposed (RYIELD) reactor combined have

been used to model the BCL low-pressure indirectly heated

Table 2 – Properties and composition of the main streams in the H 2 plant.

Quantity Stream

COMP Outlet SMR outlet HTS Outlet LTS outlet PSA inlet Off-gas

T (C) 43.5 850 518 241 43 40

P (bar) 31 28 27 26.5 25.2 1

Flow rate (kg/h) 52.06 121.23 121.23 120.90 70.695 69.70

Dry gas composition (% vol)

H2O 0.0 49.0 38.0 34.0 0.0 0.0

H2 46.0 28.0 40.0 43.0 65.60 26.58

CO 30.0 15.0 4.00 1.0.0 0.40 0.30

CO2 14.0 6.0 17.0 21.0 32.15 72.32

CH4 10.0 0.01 1.00 1.0 1.85 0.8

Table 3 – Simulation results for cases 1 and 2.

Quantity Case 1 Case 2

Biomass flow rate (wet) (kg/h) 166.67 166.67

Biomass flow rate (dry) (kg/h) 88.40 88.40

Steam input to gasifier (kg/h) 33.17 33.17

Syngas fraction to SMR-COMB 0.196 0.0

CH4 input to SMR-COMB (kmol/h) 0.0 0.475

Hydrogen production rate (kg/h) 5.53 6.91

Steam-biomass ratio (STBR) 0.38 0.38

Cold gas efficiency, hcg 0.30 0.40

System energy efficiency, hsys 0.24 0.27

System exergy efficiency, jsys

0.22 0.25




gasifier. The feedstock used for this analysis is oil palm shell,

delivered at 50 wt% moisture; the ultimate and the proximate

analyses for the feed used in this study are given in Table 4.The plant capacity is designed to be 2000 drytonne/day(83.3 t/

h), and the lower heating value (LHV) of the dry biomass is

22.14 MJ/kg.

In this paper we divide the system into two parts. The first

is the gasification plant in Fig. 1, which produces syngas for

the second part of the system, which is the hydrogen plant. In

this assessment, we consider two cases that have been

designed for producing hydrogen via gasification. In the first

case, 19.6% of the produced syngas and the total amount of

off-gas are fed to the steam-methane reformer combustion

(SMR-COMB) in order to supply heat for the SMR in the

hydrogen plant in Fig. 1. Also, in the first case energy and

exergy analyses for the entire system are carried out. In thesecond case, instead of feeding the SMR-COMB with frac-

tioned syngas, it is fed by just enough methane gas (CH4) from

external sources to enhance the hydrogen yield at the end of

the simulation.

As a case study, biomass and flue gas are mixed in the dry

reactor in order to evaporate the water, and dry the wood

from 50% to 5.7% moisture content. After the drying process

is completed in the stoichiometric reactors (RSTOIC) in Fig. 1,

the gas passes through the decomposition (RYIELD) reactor.

Normally the BCL device is an indirectly heated gasifier

consisting of two main reactors [38]: the gasifier and the

combustor. However, in this simulation the BCL unit contains

three main reactors which are the decomposer, the gasifierand the combustor. First the biomass is decomposed in the

RYIELD reactor (this reactor is used in the Aspen plus simu-

lation); this reactor simulates the decomposition of the feed

at low temperature (394 K, 1 atm). In this step, biomass is

converted into its constituent components including carbon

(C), hydrogen (H2), oxygen (O2), sulphur (S), nitrogen (N2) and

ash, by specifying the yield distribution according to the

biomass ultimate analysis. These components enter the

Gibbs reactor (at 1 atm and 1162 K) to produce syngas using

steam (at 923 K and 1 atm), as seen in Table 5. The heat of

combustion of the actual indirectly heated gasifier system is

transferred to the gasifier by recirculating hot inert material,

usually sand.

In this simulation, however, it is a designed heat stream

using just enoughheat to supply thegasifier heat demand and

the gas cleaning section (see Fig. 1). At the same time

combustion occurs in a third reactor, which is fed with

methane gas (CH4) from an external supply and char gener-

ated by the gasifier. There are two combustors which operate

at different temperatures; the first combustor runs at

a temperature of 1255 K, while the second runs at 1355 K with15% excess air.

Fig. 1 shows that the syngas enters the scrubber which is

designed for syngas cleaning. During this process some of the

toxic gas is cleaned and water in the syngas is condensed.

After entering the scrubber the syngas passes to the separator

(SP1), from which part goes to the SMR-COMB. Note that in

this study, the first case has the fraction of the product

stream at 19.6% but the fraction of the product stream is 0%

for the second case. After, the syngas passes through the five-

stage compressor system, which has polytropic efficiency of

79% for each compressor stage and a mechanical efficiency of

95%. The syngas is cooled, the preferred method being air

cooling as it avoids excess pressure losses. After thecompression and cooling processes, the syngas pressure

increases from 1 to 31 bar while the temperature increases

by 43 K. Before reaching the ZnO-Bed, the syngas is heated

to 653 K because the ZnO-Bed cannot function at a lower

temperature [40].

After sulphur cleaning in the ZnO-Bed, the syngas

undergoes three main reactions: steam-methane reforming

(for which the main reaction is CH4 þ H2O$CO þ 3H2), high-

temperature shift (for which the main reaction is

CO þ H2O$CO2 þ H2), and low-temperature shift. The water-

gas shift reaction is usually performed in two stages in

commercial processes: a high-temperature shift (HTS) in the

range of 643–693 K and a low-temperature shift (LTS) in therange of 473–523 K [38]. The sulphur-free syngas mixes with

the steam from the superheater in mixer 1 to drive SMR. The

reforming condition is fixed at 1123 K and 28 bar because

methane conversion decreases at high pressure. After the

syngas enters the water heat boiler (WHB3), it cools to 677 K

before entering the high-temperature shift reactor (HTS).

While reducing carbon monoxide in the shift reactor, the

hydrogen yield increases by almost 7.5%. As shown in Table 2,

after the HTS, the syngas passes through a heat exchanger

(HE6) and superheater, where its temperature reduces to

473 K. The final treatment for the syngas before pressure

swing adsorption (PSA) is the low-temperature shift reactor

(LTS), where the carbon monoxide is converted and hydrogencontent increased. The outlet of the LTS has the highest

hydrogen flow rate (6.51 kg/h here).

The PSA unit purifies the syngas by separating the

hydrogen from the other components in the shifted gas

stream, mainly CO2 and unreacted CO, CH4 and other

hydrocarbons. Based on studies and data from industrial

gas producers, the shifted gas stream must contain at least

70 mol% hydrogen before it can be economically purified in

the PSA unit. For the present analysis, the concentration of

hydrogen in the shifted stream prior to the PSA unit is

between 60 and 65 mol%. Therefore, part of the PSA unit

hydrogen product stream is recycled back into the PSA

feed. For a 70 mol% hydrogen PSA feed, a hydrogen

Table 4 – Proximate and ultimate analyses and other datafor oil palm shell [41].

Proximate analysis (wt% dry basis)

Volatile matter 73.74

Fixed carbon 18.37

Ash 2.21

Ultimate analysis (wt% dry basis)

C 53.78

H 7.20

O 36.30

N 0.00

S 0.51

Moisture content (wt %) 5.73

Average particle size (mm) 0.25–0.75

Molecular formula CH1.61O0.51

Lower heating value (MJ/kg) 22.14




recovery rate of 85% is typical with a product purity of

99.9% by volume [40].

For the SMR-COMB unit, air enters at 298 K and 1 atm and

passes through two heat exchangers (HE2 and HE4). The

temperature of the air rises to 1060 K, while concurrently the

off-gas from the PSA and the fractioned syngas passes

through heat exchanger 5 and heat exchanger 3 at 720 K and

1 atm, and enters the SMR-COMB to supply heat for SMR (Case

1). In addition heat exchanger 3 and WHB3 produce the steam

for the turbine which produces electricity.

The effects of fractioned syngas on the hydrogen produc-

tion yield are shown in Fig. 2. That figure shows that the

steam-methane reformer combustion (SMR-COMB) is

supplied with methane gas at a rate of 0.76 kg/h CH4. The

equivalent thermal contribution of this gas is the same as the

19.6% of the fractioned syngas in Case 1.

4. Results and discussion

We now report the results of the energy and exergy analyses,

including the energy and exergy efficiencies and exergy

destructions for each component. The results are reported for

Case 1 in Table 1. It is demonstrated that the inlet and outlet

exergy flows for the hydrogen plant are mainly attributable to

the energy andexergyinlet with the biomass and the methane

gas. The produced electricity also contributes to the system

products and efficiency, on energy and exergy bases. Further,the exergy losses are observed to be due to emissions and

internal consumptions associated with chemical reactions,

particularly those related to combustion and gasification.

Note that inlet exergy values are evaluated for fuels on an LHV

basis.

Some key results from the simulations for the two cases

are presented and compared in Table 3. In this simulation at

1162 K and 1 atm, for the low-pressure indirectly heated

gasifier (R-GIBBS) with the decomposition reactor (RYIELD),

the maximum hydrogen production efficiency for the first

case is found to be 24% while exergy efficiency is 22%, as

determined using Eqs. (16) and (17). These values are relatively

low because of losses for the gasifier, combustor and SMR. For

the second case, performance improvements are noted, as the

energy efficiency is found to be 27% and the exergy efficiency

25%. The efficiency increase occurs because importing

methane gas from external sources increases the hydrogen

production rate from 5.53 kg/h to 6.91 kg/h after the PSA unit.

The energy and exergy efficiency values for Cases 1 and 2

differ because of the improvement applied to the system in

Case 2. These differences are not attributable to uncertainties

associated with computer simulation. The improvements in

Case 2, as previously noted, include non-fractioned syngaswhich passes sequentially through the SMR, HTS and LTS

units in order to increase the hydrogen yield in the reforming

and shift reactions. In Case 1 which used fractioned syngas,

19.6% of the gaseous product cannot pass the SMR, HTS and

LTS processes so less hydrogen is produced, which affects the

overall system energy and exergy efficiencies. Also, the cold

gas efficiencies differ because the second case has a higher

productivity rate than the first.

The products from this process are hydrogen and elec-

tricity, but other energy streams also exit. There are two

sources of flue gas: the char combustor and the second

combustor (SMR-COMB). Together, their energy contents

account for about 4% of the energy in the dried biomass.The simulated hydrogen production flow rates for Cases 1

and 2 are shown in Fig. 3. It is seen there that the hydrogen

production is affected by fractioned and unfractioned syngas

through thecomponent split 1 (point 13 in Fig. 1). It is observed

for the first case that the hydrogen production rate is lower

than for the second case, in which the SMR-COMB unit is fed

with fractioned syngas. At the end of the simulation, there-

fore, the fractioned syngas is prevented from passing through

the SMR, HTS and LTS processes, which are where the

hydrogen yield is increased. In the second case, the unfrac-

tioned syngas passes through of the SMR, HTS and LTS units.

The energy andexergyefficiencies of the main components

involving chemical reactions are shown in Fig. 4. The energy

Table 5 – Conditions at the gasifier outlet.

Quantity Value

Gasifier outlet temperature (C) 890

Combustor 1 outlet temperature (C) 982

Gasifier outlet composition (kg/h)

H2O 38.23

H2 3.52CO 31.87

CH4 5.93

CO2 22.98

NH3 0.08

H2S 0.08

N2 0.70

C (solid) 18.02

Ash 0.77

SMR-COMB

32

40 54

SMR

Q2

CH4

HE1

21

22

AIR from HE2Split syngas

and off gasfrom HE3

Fig. 2 – Aspen Plus simulation flow diagram of a process for

steam-methane reforming with heat supplied by

combustion of methane gas (CH4

) from external source.




efficiency of the gasifier is approximately 72% while the cor-

responding exergy efficiency is 66% based on Eqs. (18) and (19).

Normally the energy efficiency at these conditions may be

expected to be around 80%; the value here is lower since the

system assessed has unconverted solid carbon as char and

catalysis is not used to promote the gasifier reactions. In

addition, the fuels are over-oxidized in the gasifier in order to

attain the required gasification temperature [41], and this

process may reduce the gasifier efficiency. The gasifier exergy

efficiency is lower than the energy efficiency, mainly due to

chemical reactions and oxidization. Both combustion reactors

operate with high energy and low exergy efficiencies. Thelatter are associated primarily with internal irreversibilities.

For the steam-methane reformer, the energy efficiency is

found to be 83% and the exergy efficiency 77%. These values

are consistent with those reported in the literature [34,42].

Note that in the HTS and LTS units, the shift reactions occur

but there is no combustion. Therefore internal exergy

destructions are very low, leading to high exergy efficiencies

for these devices. It is observed that significant heat is trans-

ferred to water to produce steam in heat exchangers, boilers

and economizers. The results suggest that the low-pressure

indirectly heated gasifier requires improvements in terms of

energy recovery.

For this system, with its feed rate of 4000 kg per day of wet

biomass to the gasification process, the hydrogen production

energy efficiency is 24% (see Table 3). It is determined with the

simulation that 132.7 kg hydrogen can be produced from

4000 kg biomass with an energy rate of 4.5 MW. For the second

case, also with a feed rate of 4000 kg per day of wet biomass tothe gasification process, the hydrogen production energy

efficiency is improved to 27% (see Table 3). Also, it can be

found that 165 kg hydrogen is produced from 4000 kg biomass

with an equivalent thermal input 5.6 MW.

It can be seen in Fig. 5 that the reactors with the highest

exergy destruction rates based on Eq. (21) are the gasifier

Fig. 3 – Simulated hydrogen production rates for Cases 1 and 2.

Fig. 4 – Energy and exergy efficiencies for main system components for Case 1.




(R-GIBBS), in which 34% of the total exergy inlet is destroyed.

This observation implies that the gasifier is an important

component for efficiency system improvement, especially

since the biomass can be gas, solid and liquid. The combustion

reactors are also responsible for large exergy destructions,

mainly due to irreversibilities associated with the combustion

reactions. These exergy losses mainly relate to chemical

exergy destructions, andfor both combustion units 1 and 2 are

around 11%. It is also interesting that the dry reactor has

a high exergy destruction rate, which is approximately 4.5%

due to the 50% moisture content of the inlet biomass. Thus,

the heat demand is high for this process, resulting in high

exergy destruction rates.

Exergy destruction ratios (on a percentage basis) for Case 1

are shown in Fig. 5 for the system components and in Fig. 6 for

components with lower exergy losses. The exergy destruction

ratio is the ratio of the exergy destruction for a component to

the overall system exergy destruction. The exergy destruction

ratio in Fig. 5 shows that the HE2 and WHB units are each

responsible for about 1% of the exergy destruction rate, and

the SMR, HTS and LTS reactors, which do not include

combustion, also have very low exergy destruction rates.

There are two cases of note in this assessment. The steam-

methane reforming (SMR) heat demand is supplied by

fractioned syngas in one case, and by external methane gas

(CH4) supply in the other. Assessments are required to

understand better how hydrogen production is affected by

changing this parameter, with the aim of investigating the

feasibility of producing hydrogen from biomass and better

understanding the potential of biomass as a renewable energy

source. As pointed out earlier, detailed energy and exergy

assessments were performed for just one system (Case 1). For

Case 2, the improvement of the system performance was

gauged by altering one parameter and observing the impact

on hydrogen production and system overall energy and exergy

efficiencies. The energy and exergy efficiencies and exergy

destruction ratios for the main system components for Case 2

are similar to those for Case 1, so performance figures are not

presented for Case 2. Also, the exergy destruction ratios for

auxiliary components for Cases 1 and 2 are similar. However,

improvements are observed for Cases 1 and 2 in hydrogen

production rate and overall system energy and exergy

efficiencies.

On a broader scale, the results of this study support the

contention by many that biomass may contribute to a future

hydrogen economy. Although biomass has the advantage of

being renewable if managed properly, it has challenges; large

quantities of biomass need to be grown and transported to

Fig. 5 – Exergy destruction ratios for main components for Case 1.

Fig. 6 – Exergy destruction ratio for auxiliary components for Case 1.




produce a small amount of hydrogen. Transportation

concerns may be alleviated by using pyrolysis of biomass to

produce bio-oil, as opposed to direct gasification. Based on the

costs and availability of hydrogen production processes, it is

likely that hydrogen will be produced by steam-methane

reforming or coal gasification during a transition to

a hydrogen economy. Future advances in water-splitting

processes may allow them to replace fossil fuel processes ascleaner, long-term energy solutions. Many predictions of how

a hydrogen economy will unfold have been published. For

instance, a roadmap was created that provides an overview of

a possible evolution of hydrogen production technologies in

the future [43]. The timing of each step in this evolution

towards a hydrogen economy depends on how quickly tech-

nology advances and other factors.

5. Conclusions

The energy and exergy analyses performed of biomass-based

hydrogen production have yielded energy and exergy effi-

ciencies and an understanding of the impact on performance

of several parameters. The feasibility of producing hydrogen

from biomass and a better understanding the potential of

biomass as a renewable energy source have been attained by

considering two methods: 1) the heat required for steam-

methane reforming is supplied by fractioned syngas, and 2)

the SMR-COMB reactor is provided with externally supplied

methane gas. Oil palm shellis the biomass considered. For the

direct gasification reaction, a BCL-type low-temperature

indirectly heated steam gasifier is examined. The thermody-

namic assessments for the two cases demonstrate that the

processes have low efficiencies. The simulation confirms for

the system that the second case considered, which indicates

performance improvements, has higher energy and exergy

efficiencies than the first case.

Acknowledgments

The authors acknowledge the support provided by the Ontario

Research Excellence Fund and the Natural Sciences and

Engineering Research Council of Canada.

Nomenclature

C p specific heat, kJ/kg K_E energy flow rate, kJ/h_Ex exergy flow rate, kJ/h

ex specific exergy, kJ/kg

h specific enthalpy, kJ/kg

LHV lower heating value, MJ/kg

mi inlet mass, kg

mo outlet mass, kg

Po reference-environment pressure, kPa

Q heat, kJ

R universal gas constant, kJ/kmol K

S entropy, kJ/K

STBR steam–biomass ratio

T temperature, K

T0 reference-environment temperature, K_W work rate, kJ/h

x exergy ratio

Greek symbols

j exergy efficiency, %h energy efficiency, %

b correlation factor, %

Subscripts

bio biomass

biomoist biomass moisture

cg cold gas

dest destroyed

drybio dry biomass

en energy

gen generated

i, j index for components

in input

meth methane gas (CH4)out output

st steam

sys system

turb turbine

prodg produced gas

unconcarb unconverted carbon

Supercripts

ch chemical

ph physical

Acronyms

COMB combustion

COMP compressor

COOL cooling

EC economizer

HE heat exchanger

HTS high-temperature shift

LTS low-temperature shift

PSA pressure swing adsorption

WHB waste heat boiler

r e f e r e n c e s

[1] Kruse A, Gawlik A. Biomass conversion in water at 330–410 C and 30–50 MPa: identification of key compounds forindication different chemical reaction pathways. IndustrialEngineering and Chemistry Research 2003;42(2):267–79.

[2] Ni M, Leung DYC, Leung MKH, Sumathy K. An overview of hydrogen production from biomass. Fuel Processing Technology 2006;87:461–72.

[3] Ntaikou I, Gavala HN, Kornaros M, Lyberatos G. Hydrogenproduction from sugars and sweet sorghum biomass using Ruminococcus albus. International Journal of Hydrogen Energy2008;33:1153–63.

[4] Dincer I, Rosen MA. Exergy: energy, environment andsustainable development. Oxford, UK: Elsevier; 2007.

[5] Kelly-Yong TL, Lee KT, Mohamed AR, Bhatia S. Potential of hydrogen from oil palm biomass as a source of renewable

energy worldwide. Energy Policy 2007;35:5692–701.




[6] Midilli A, Dincer I. Key strategies of hydrogen energy systemsfor sustainability. International Journal of Hydrogen Energy2007;32(5):511–24.

[7] Milne TA, Elam CC, Evans RJ. Hydrogen from biomass: stateof the art and research challenges. Report No IEA/H2/TR-02/001. Golden, CO, USA: National Renewable EnergyLaboratory; 2002.

[8] Luyben WL. Chemical reactor design and control. Hoboken,

New Jersey: Wiley; 2007.[9] Mahishi MR, Goswami DY. Thermodynamic optimization of

biomass gasifier for hydrogen production. International Journal of Hydrogen Energy 2007;32:3831–40.

[10] Mahishi MR, Sadrameli MS, Vijayaraghavan S, Goswami DY.A novel approach to enhance the hydrogen yield of biomassgasification using CO2 sorbent. Journal of Engineering for GasTurbines and Power 2008;130. paper 011501–1.

[11] Rosen MA. Thermodynamic investigation of hydrogenproduction by steam–methane reforming. International

Journal of Hydrogen Energy 1999;16:207–17.[12] Simpson AP, Lutz AE. Exergy analysis of hydrogen

production via steam methane reforming. International Journal of Hydrogen Energy 2007;32:4811–20.

[13] Toonssen R, Woudstra N, Verkooijen AHM. Exergy analysis

of hydrogen production plants based on biomassgasification. International Journal of Hydrogen Energy 2008;33:4074–82.

[14] Lutz AE,Bradshaw RW,Keller JO,Witmer DE.Thermodynamicanalysis of hydrogen production by steam reforming.International Journal of Hydrogen Energy 2003;28:159–67.

[15] Handbook of heterogeneous catalysis. Weinheim, PA: VCHVerlagsgesellschaft Mbh; 1997.

[16] Kotas TJ. The exergy method of thermal plant analysis.Malabar, Florida: Kreiger; 1995.

[17] Robinson PJ, Luyben WL. Simple dynamic gasifier model thatruns in Aspen dynamics. Industrial Engineering andChemistry Research 2008;47:7784–92.

[18] Nikoo MB, Mahinpey N. Simulation of biomass gasification influidized bed reactor using ASPEN PLUS. Biomass and

Bioenergy 2008;32:1245–54.[19] Lu Y, Guo L, Zhang X, Yan Q. Thermodynamic modeling and

analysis of biomass gasification for hydrogen production insupercritical water. Chemical Engineering Journal 2007;131:233–44.

[20] Lei Y, Feng X, Min S. Parameters optimization of hydrogenproduction from glucose gasified in supercritical water byequivalent cumulative exergy analysis. Applied ThermalEngineering 2007;27:2324–31.

[21] Feng W, Van der Kooi HJ, Arons JS. Biomass conversions insubcritical and supercritical water: driving force, phaseequilibria, and thermodynamic analysis. ChemicalEngineering and Processing 2004;43:1459–67.

[22] Penninger JML, Rep M. Reforming of aqueous wood pyrolysiscondensate in supercritical water. International Journal of

Hydrogen Energy 2006;31:1597–606.[23] Penninger JML, Maass GJJ, Rep M. Compressed hydrogen-rich

fuel gas (CHFG) from wet biomass by reforming in supercritical water. International Journal of Hydrogen Energy 2007;32:1472–6.

[24] Antal Jr MJ, Allen SG, Schulman D, Xu X, Divilio RJ. Biomassgasification in supercritical water. Industrial Engineering andChemistry Research 2000;39:4044–53.

[25] Simbeck DR. Hydrogen costs with CO2 capture. Presented at7th international conference on greenhouse gas controltechnologies (GHGT-7). Vancouver, British Columbia,Canada; September 6–10, 2004.

[26] Szargut J, Morris DR, Steward FR. Exergy analysis of thermal,chemical and metallurgical processes. New York:Hemisphere; 1988.

[27] Edgerton RH. Available energy and environmental

economics. Toronto: D.C. Heath; 1982.[28] Ptasinski KJ, Prins MJ, Pierik A. Exergetic evaluation of

biomass gasification. Energy 2007;32:568–74.[29] Szargut J. Exergy method: technical and ecological

applications. Boston: WIT Press; 2005.[30] Berthiaume R, Bouchard C, Rosen MA. Exergetic evaluation

of the renewability of a biofuel. Exergy International Journal2001;1:256–68.

[31] Turpeinen E, Raudaskoski R, Pongracz E, Keiski RL.Thermodynamic analysis of conversion of alternative hydrocarbon-based feedstocks to hydrogen.International Journal of Hydrogen Energy 2003;33:6635–43.

[32] Moran MJ. Availability analysis: a guide to efficient energyuse. New York: American Society of Mechanical Engineers;

1989.[33] Rosen MA, Scott DS. Comparative efficiency assessments for

a range of hydrogen production processes. International Journal of Hydrogen Energy 1998;23:653–9.

[34] Rosen MA, Scott DS. A thermodynamic investigation of thepotential for cogeneration for fuel cells. International Journalof Hydrogen Energy 1988;13:775–82.

[35] Rosen MA. Thermodynamic comparison of coal-fired andnuclear electrical generating stations. Transactions of theCanadian Society for Mechanical Engineering 2000;24(1B):273–83.

[36] Higman C, Burgt VM. Gasification. Amsterdam: Elsevier; 2003.[37] Lover KA. Biomass gasification integration in recuperative

gas turbine cycles and recuperative fuel cell integrated gasturbine cycles. MSc thesis, Department of Energy and

Process Engineering, Norwegian University of Science andTechnology, Norway; 2007.

[38] Corradetti A, Desideri U. Should biomass be used forpower generation or hydrogen production? ASME Journalof Engineering for Gas Turbines and Power 2007;129:629–34.

[39] Li XT, Grace JR, Lim CJ, Watkinson AP, Chen HP, Kim JR.Biomass gasification in a circulating fluidized bed. BiomassBioenergy 2004;26:171–93.

[40] Spath P, Aden A, Eggeman T, Ringer M, Wallace B, Jechura J.Biomass to hydrogen production detailed design andeconomics utilizing the BCL indirectly heated gasifier. ReportNo. TP-510–37408. National Renewable Energy Laboratory;2005.

[41] Babu SP. Biomass gasification for hydrogen production –

process description and research needs. Des Plaines, Illinois:Gas Technology Institute; 2002.

[42] Rosen MA. Evaluation of energy utilization efficiency inCanada using energy and exergy analyses. EnergyInternational Journal 1992;17(4):339–50.

[43] Miller GQ, Penner DW. Possibilities and challenges fora transition to a hydrogen economy. Chemical Engineering Transactions 2004;4:5–18.


Cohce_Thermo Analysis of H2 Production From Gasification

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