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CCHHEEMMIICCAALL EENNGGIINNEEEERRIINNGG TTRRAANNSSAACCTTIIOONNSS

VOL. 45, 2015

A publication of

The Italian Association

of Chemical Engineering

www.aidic.it/cet
Guest Editors: Petar Sabev Varbanov, Jiří Jaromír Klemeš, Sharifah Rafidah Wan Alwi, Jun Yow Yong, Xia Liu

ISBN 978-88-95608-36-5; ISSN 2283-9216 DOI: 10.3303/CET1545103

Please cite this article as: Aziz M., Oda T., Kashiwagi T., 2015, Clean hydrogen production from low rank coal: novel

integration of drying, gasification, chemical looping, and hydrogenation, Chemical Engineering Transactions, 45, 613-618

DOI:10.3303/CET1545103

613

Clean Hydrogen Production from Low Rank Coal: Novel

Integration of Drying, Gasification, Chemical Looping, and

Hydrogenation

Muhammad Aziz*, Takuya Oda, Takao Kashiwagi

International Research Center for Advanced Energy Systems for Sustainability, Solutions Research Laboratory, Tokyo

Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan

maziz@ssr.titech.ac.jp

State-of-the-art integrated processes for hydrogen production from low rank coal (LRC) consisting of

drying, gasification, chemical looping, and hydrogenation is proposed based on enhanced process

integration (EPI). This paper focuses mainly on the performance of drying module, especially related to

energy efficiency during drying. EPI, which is a combination of exergy recovery and process integration, in

developed in order to minimize the total exergy destruction throughout the integrated processes. Each

process module is designed initially based on exergy recovery to maximize the recirculated heat amount in

each process module. Furthermore, the developed process modules are integrated based on process

integration, hence the exergy destruction throughout the integrated processes can be minimized. LRC is

initially dried in drying module employing exergy recovery and utilizing steam as drying medium. Hot dried-

LRC is fed directly to gasification module for conversion producing syngas. The produced syngas is then

flowing to chemical looping module to produce hydrogen, CO2 and power. Finally, the produced hydrogen

is then stored in liquid organic hydrogen carrier (toluene-methylcyclohexane). On the other hand, the

separated CO2 can be sequestered to achieve a clean coal conversion. From process calculation, the

proposed drying module shows very high energy efficiency, which is about one sixth to which is required in

drying with conventional heat recovery.

1. Introduction

Hydrogen is considered as a potential secondary energy source due to some advantages of cleanliness,

high efficiency and high variety of production and utilization technologies. Hence, hydrogen is expected to

have an important role in the future energy system. Unfortunately, at present, the utilization of hydrogen is

still far from its expectation due to some challenging problems covering production, transportation, storage,

and energy conversion (Orhan et al., 2012).

Hydrogen is currently produced mainly from natural gas, oil reforming, coal gasification, and water

electrolysis (Dincer, 2012). In addition, among the fossil energy resources, low rank coal (LRC) is believed

to have relatively large reserve, almost the half of the total worldwide coal reserve. LRC has some

advantages including low ash content, possibility for open-cut mining and low production cost.

Unfortunately, LRC also has disadvantages especially on its low calorific value, high moisture content,

high risk of spontaneous combustion, and larger CO2 emission in its utilization (Aziz et al., 2011). The

moisture content of just-mined LRC is ranging from 40 to 65 wt% on wet basis (wb). Therefore, drying

becomes a compulsory process to solve these disadvantages. Unfortunately, LRC drying is a very energy

intensive process. In addition, recently, the trend to convert and decarbonize LRC to secondary clean

energy resources, including hydrogen, increases significantly.

Under ambient pressure, although the energy density of hydrogen by weight is high (33 kWh kg
-1

), its

energy density by volume at ambient state is very low (3 Wh l
-1

). This leads to some difficulties in its

storage and transportation. A liquid organic hydrogen carrier (LOHC) is one of the promising hydrogen

storage because of high safety, storage capacity, long storage time, and reversibility (Jiang et al., 2014). In

614

LOHC, hydrogen is covalently bound to LOHC through hydrogenation. Furthermore, the bound hydrogen

can be released from LOHC through dehydrogenation. Among available LOHCs, toluene (C7H8)-

methylcyclohexane (C7H14, MCH) is considered to be well-established and promising because it is

relatively cheap, stable, and easy to store/transport. In addition, a successful demonstration test has been

achieved by Chiyoda Corporation in Japan (Okada and Shimura, 2013).

In addition, in currently available conversion processes, the conversion of LRC to hydrogen has relatively

low total energy efficiency due to limitation in their energy conservation and unintegrated multiple

processes. For example, an integrated system including coal hydrogasification, power generation, and

electrolysis to produce synthetic natural gas has been proposed by Minutillo and Perma (2010).

Unfortunately, their proposed processes were designed based on conventional process integration

technology, hence still large amount of exergy destruction is generated. Furthermore, a huge amount of

CO2 will be emitted leading to some environmental problems.

Research and development of innovative method and design of the well-integrated processes are urgently

demanded. This paper discusses and proposes a novel integration of hydrogen production from LRC,

including drying, gasification, chemical looping, combined cycle, and hydrogenation based on enhanced

process integration (EPI) to achieve high total energy efficiency. Furthermore, a detailed evaluation of

drying module related to energy efficiency is performed.

2. Proposed integrated processes

To minimize the total exergy destruction in the overall integrated processes, EPI has been proposed and

evaluated in several applications processes including algae drying (Aziz et al., 2014), empty fruit bunch-

based power generation (Aziz et al., 2015) and algae-based power generation (Aziz, 2015). EPI mainly

consists of two core technologies: exergy recovery and process integration. Different to conventional

process integration, in EPI, process intensification in term of energy efficiency is performed initially in each

single process through exergy recovery to recover thoroughly the energy involved in that process. Exergy

recovery can be achieved through exergy elevation and effective heat coupling (Liu et al., 2012).

Unfortunately, although each single process has been intensified through exergy recovery, at the last,

there is still energy that cannot be recovered due to some limitations in heat exchange, etc. To minimize

this unrecoverable energy, the idea of process integration is adopted. Hence, the total exergy destruction

throughout the integrated processes can be minimized leading to higher energy efficiency.

Figure 1 shows the proposed integrated-processes of LRC conversion to hydrogen including drying,

gasification, chemical looping, combined cycle, and hydrogenation. Solid and dotted lines represent

material/heat and electricity. The as-mined LRC is fed initially to drying module to remove its moisture, as

well as increasing its calorific value. Next, the dried LRC is converted to syngas through gasification which

is also involving air and steam. The produced syngas is flowing to chemical looping module for co-

production into hydrogen, CO2, and heat. The produced hydrogen is then flowing to hydrogenation module

to be covalently bound to toluene producing MCH. On the other hand, the separated CO2 is sequestered

leading to clean energy conversion. In addition, the produced heat from chemical looping module is utilized

to produce electricity through combined cycle module. The produced electricity is distributed to the

involved integrated-processes and the excess power could be sold to the grid as an additional economic

benefit.

Figure 1: Basic schematic diagram of the proposed integrated-processes of LRC conversion to hydrogen.

615

2.1 Drying and gasification modules

Figure 2: Process flow diagrams: (a) drying module, (b) gasification module

Figure 2(a) shows the proposed drying module which is developed based on EPI. Drying is a moisture

removal which is influenced strongly by its surrounding conditions leading to the concept of equilibrium

moisture content. Superheated stem is adopted as drying medium due to its advantages, especially

related to the possibility for high energy recovery.

A fluidized bed with immersed heat exchanger is selected as the dryer due to its advantages on good

particle mixing, uniform temperature distribution, and rapid moisture and heat transfer (Aziz et al., 2012).

To realize the exergy recovery in drying, the evaporated steam is compressed to certain pressure

depending on the performance of heat coupling afterward. As its pressure increases, the exergy rate of

steam increases leading to the possibility of self-heat exchange. The exhausted steam from fluidized bed

dryer is split to recirculated steam (for fluidization) and compressed steam. The compressed steam from

dryer which is mixed with the superheated recirculated-water is compressed and utilized as heat source for

subsequent drying. This leads to self-heat exchange inside the drying module.

Furthermore, Figure 2(b) shows the proposed gasification module in where dual circulating fluidized beds

of gasifier is adopted due to its higher carbon conversion efficiency and conversion rate. Hot dried LRC is

fed directly to the first gasification reactor where the pyrolysis and gasification take place. The unreacted

char leaves together with inert material (such as silica sand) to the second gasification reactor for

combustion. Furthermore, the inert material is recirculated to the first reactor transporting the heat for

pyrolysis and gasification. Next, the produced syngas is filtered by particulate remover eliminating the ash

or slag. A ceramic particulate removal is considered as the best candidate due to its removal capability

under high temperature condition.

2.2 Chemical looping, combined cycle, and hydrogenation modules

Figure 3 shows the process flow diagram for integrated chemical looping, combined cycle, and

hydrogenation. Chemical looping of produced syngas from gasification is adopted in this study converting

syngas to hydrogen and CO2, and also producing the heat. In chemical looping, iron-based metal oxide is

utilized as an oxygen carrier transferring the oxygen from combustion to the fuel because it does not

involve catalytically and it has high conversion of syngas. Therefore, there is no direct contact between

oxygen (air) and fuel. The advantages of chemical looping include high purity of separate CO2 leading to

clean and efficient energy conversion.

In chemical looping, there are three main fluidized bed type reactors involved: reducer, oxidizer, and

combustor. In reducer, the produced syngas is utilized directly as fluidizing gas and the following reactions

occur:

232
CO 2FeOCOOFe  H= -2.8 kJ mol

-1
(1)

2
CO FeCOFeO  H= -11 kJ mol

-1
 (2)

OH FeOHOFe
22

 2
32

H= 38.4 kJ mol
-1
 (3)

OH FeHFeO
22

 H= 30.2 kJ mol
-1
 (4)

616

(a) (b)

Figure 3: Process flow diagrams: (a) chemical looping and combine cycle modules, (b) hydrogenation

module

As the product of reduction reaction, CO2, together with steam, is produced which is further cooled down

for separation. The remainders are going to oxidizer to produce high purity of hydrogen. Following

reactions are considered take place in the oxidizer:

22
H FeOOHFe  H= -30.2 kJ mol

-1
(5)

22
H OFeOH3FeO 

43
H= -60.6 kJ mol

-1
 (6)

The produced hydrogen together with steam are exhausted from oxidizer and cooled down to achieve high

purity of hydrogen. The separated hydrogen is then flowing to hydrogenation module for storage. In

addition, in combustor, the following reaction occurs:

3243
6 OFeOO4Fe

2
 H= -471.6 kJ mol

-1
(7)

As combustion is very exothermic reaction, its produced heat is utilized to cover the heat required in

reducer and the excess heat is used to generate steam in combined cycle module generating electricity.

The separated hydrogen is reacted with toluene producing MCH. The following reaction occurs in

hydrogenation module:

147
HCHHC 

287
3 H= -205 kJ mol

-1
(8)

3. Process analysis for drying module

The current study focuses mainly on the evaluation of the proposed drying module developed based on

EPI in term of energy efficiency. To perform this analysis including material and energy balances, process

modelling and calculation are conducted using SimSci Pro/II (Schneider Electric Software, LLC.). Related

to LRC drying, the correlation between the vapour pressure, p/po, to the equilibrium moisture content, Meq,

can be approximated as follows (Chen et al., 2001):

   























58.1

47.0

100
27353.2exp1

eq
b

o
M

M
T

p (9)

where Tb is mean temperature of the fluidized bed during drying (K). Furthermore, the minimum fluidization

velocity, Umf, for LRC particles are approximated as fluidization velocity for coarse particle with

consideration of correction factor for wet coal particles. It can be presented as follows:

  















 















58.1

2
42

1
2 10413.7

1065.5182.17.280494.07.28
M

dArU
pssmf

 (10)

where, s, Ar, s, dp, and M are dynamic viscosity of steam (Pa s), Archimedes number (-), steam density

(kg m
-3

), average diameter of LRC particle (m), and moisture content (wt% wb).

Table 1 shows the detailed drying conditions. The flow rate of LRC is 100 t h
-1

. The heat exchangers are

considered as counter current, and the temperature approach during heat exchange is fixed at 10 K. The

617

ambient temperature and pressure are 25 °C and 101.25 kPa. In this study, the effect of target moisture

content to the total required energy input is calculated and compared to one with conventional heat

recovery. Three target moisture contents are evaluated: 5, 10 and 15 wt% wb.

Table 1: Drying and gasification conditions

Component Value

Drying conditions

Initial moisture content (wt% wb)

Mean particle diameter (mm)

Bulk density (kg m
-3

)

Particle density (kg m
-3

)

Sphericity (-)

1.5

900

1,470

0.6

LRC ultimate analysis (wt% db)

Carbon

Hydrogen

Nitrogen

Oxygen

Sulphur

Ash

64.06

4.54

0.26

24.5

0.25

6.39

4. Results and discussion

The performance of developed drying module is evaluated in term of energy consumption for each target

moisture content. In addition, the comparison to the drying which employs a conventional heat recovery is

performed. Figure 4(a) shows the total energy consumption of the proposed drying module compared to

the conventional one for each target moisture content. The proposed drying module shows significant

lower energy consumption up to 1/6 to which required in drying with conventional heat recovery.

Figure 4: Simulation results: (a) total energy input required for drying in different target moisture contents,

(b) temperature-enthalpy diagram of the developed drying module (target moisture content of 10 wt% wb)

Table 2: Detailed simulation results of LRC drying for each target moisture content

Component
Target moisture content

5 (wt% wb) 10 (wt% wb) 15 (wt% wb)

Recirculated water (t h
-1

) 5 20 70

Outlet pressure (kPa) 326 283 199

Compressor work (MW) 4.509 4.896 6.293

Blower work (MW) 0.65 0.615 0.606

Total required work (MW) 5.159 5.511 6.569

Preheater #1 duty (MW) 0.294 0.527 0.899

Dryer duty (MW) 2.061 3.236 2.813

Superheater duty (MW) 0.111 0.14 0.209

Preheater #2 duty (MW) 1.918 1.47 5.596

Dried LRC temperature (ºC) 101.1 61.94 66.55

618

In addition, different to conventional heat recovery-based drying, in the developed drying module the total

required energy decreases for lower target moisture content. Lower target moisture content means a

larger flow rate of evaporated water from LRC. As the result, more effective heat coupling and recovery,

especially in dryer and preheater #1, can be achieved due to better material balance. Drying to lower

moisture content can minimize the compression work which is the largest duty in the drying module. On

the other hand, drying to higher target moisture content leads to less amount of evaporated. Hence, the

heat required for drying is provided by the sensible heat of the compressed steam and recirculated steam.

As the result, the compressor works excessively and acts as a heater rather than its main purpose which is

elevating the exergy rate of steam for heat coupling.

For each target moisture content, there is an optimum amount of recirculated water used as fluidizing gas

to achieve the minimum total required energy. Water recirculation leads to better heat recovery and

coupling. A higher target moisture content demands larger flow rate of recirculated water to cover the

material balance and also recover the heat of the dried LRC.

Figure 4(b) shows the temperature-enthalpy diagram in the proposed drying module in case of target

moisture content of 10 wt% wb. Solid and dotted line represent hot and cold streams. It is clear that almost

the heat can be recovered effectively leading to small exergy destruction (hot and cold streams are almost

parallel to each other). Generally, the largest amount of heat exchange occurs in the fluidized bed dryer

5. Conclusions

Integrated processes to convert LRC to hydrogen covering drying, gasification, chemical looping,

combined cycle, and hydrogenation has been proposed and discussed. EPI consisting of exergy recovery

and process integration is adopted to achieve high total energy efficiency of the integrated processes. In

addition, detailed evaluation on proposed drying module has been conducted and significantly high energy

efficiency can be achieved. Furthermore, it is believed that the proposed integrated processes developed

based on EPI can achieve high total energy efficiency.

Acknowledgment

This research was financially supported in part by Mitsubishi Corporation, Tokyo, Japan.

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