Heat integration and optimization of hydrogen production for a 1 kW low-temperature proton exchange membrane fuel cell

doi:10.1016/j.ces.2014.10.036

Chemical Engineering Science

Volume 123, 17 February 2015, Pages 81-91

https://doi.org/10.1016/j.ces.2014.10.036 Get rights and content

Highlights

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A hydrogen production system via MSR was designed for a 1 kW LT-PEMFC.
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The optimized system heat efficiency (86.1%) was obtained using the pinch method.
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The maximum energy recovery was obtained in the hydrogen production process.
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The system efficiency was evaluated with the sensitivity analysis.
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The device insulation and catalyst performance possess the system efficiency.

Abstract

Heat integration and process optimization for the hydrogen production units of 1 kW low-temperature proton exchange membrane fuel cells (LT-PEMFCs) were studied. Hydrogen is produced from methane in a process consisting of a steam methane reforming reaction (SMR), a high-temperature water–gas shift reaction (HWGS), a low-temperature water–gas shift reaction (LWGS), and a preferential oxidation reaction (PROX). Using commercial Aspen Plus^® software, the process was designed and simulated with the objective of energy recovery. Furthermore, the unit operation parameters were discussed based on the mass/heat balances. Pinch analysis was applied for heat integration and process optimization to improve the energy recovery level of the hydrogen production system. By integrating system heating, a maximum energy recovery network (MER) and also a relaxed network were developed. The efficiencies of the hydrogen production systems designed with these MER and relaxed networks were 84.3% and 80.1%, respectively. Compared with the MER design, the relaxed network greatly simplifies the hydrogen production process and reduces the exchanger number from 18 to 9. The penalty in terms of power for relaxation is 120 W. Sensitivity analysis shows that heat loss and the steam-to-carbon (S/C) ratio are the primary factors influencing the system efficiency, i.e., a 5% heat loss may result in a 7% efficiency drop, and increasing the S/C ratio from 2.5 to 3.0 may result in a 2.5% efficiency drop. The conversion of methane ( $X_{{CH}_{4}}$ ) and the relative deviation from chemical equilibrium for the WGS reactions (both HWGS and LWGS) are secondary factors: efficiency drops 3.5% at $X_{{CH}_{4}}$ =0.8 and at 0.9 of the WGS equilibrium, compared with equilibrium conditions. Finally, the minimum pinch temperature difference (ΔT_min) and reaction temperatures have little impact on system efficiency. Therefore, the device insulation and catalyst performance are very important for system performance.

Introduction

Proton exchange membrane fuel cells (PEMFCs) are commonly used commercial fuel cells which convert hydrogen to electricity and heat with a significantly high degree of energy efficiency. The net electrical efficiency of a PEMFC can reach 40–58% (based on the lower heating value (LHV)), which is much higher than the limit of the Carnot cycle (Arsalis et al., 2011a). Today, PEMFCs are used in many wide-ranging applications such as power stations, portable devices, electric vehicles, space shuttles, and combined-heat-and-power (CHP) systems (Holladay et al., 2009, Men et al., 2008).

Currently, the steam methane reforming (SMR) reaction is the primary method for obtaining hydrogen and is used in numerous PEMFC processes (Balasubramanian et al., 1999, De Jong et al., 2009, Di Bona et al., 2011, Kamarudin et al., 2004, Stutz and Poulikakos, 2005). Even though it is arguable whether PEMFCs offer better energy efficiency than when starting from fossil fuels, the benefits of PEMFCs are clear when using distributed methane sources such as small oil fields or biomethane. Methane is stored and transported much more easily than pure hydrogen, particularly for mobile fuel cell applications (Arsalis et al., 2011b, Hubert et al., 2006, Kamarudin et al., 2004, Seo et al., 2006).

The hydrogen production process for PEMFCs includes the series of reactions listed in Table 1. The reforming reaction is strongly endothermic and consumes the most heat energy in the process. The subsequent reactions—the water–gas shift reactions (WGS), preferential oxidation reaction (PROX), and combustion of the depleted fuel—are exothermic, and the heats of these reactions can be used as SMR heating sources. If the system heat is unbalanced, a supplementary methane fuel supply is needed. Because the system is composed of many streams with different temperatures, the proper organization of heat exchangers and subunits can greatly improve system efficiency and reduce supplementary utility use.

The practice of heat integration is widely applied in chemical and petroleum processes (Kemp, 2007, Linnhoff et al., 1983, Linnhoff and Hindmarsh, 1983, March, 1998). Pinch analysis is a useful tool for analyzing the heating network and achieving more efficient heating strategies. To our knowledge, for low-temperature proton exchange membrane fuel cells (LT-PEMFCs), most research has been focused on the membrane, stack performance, unit design, or CHP system efficiency, but very few studies have been concerned with the heat integration and process optimization of the hydrogen production process. Pasdag et al. (2012) reported an analysis of the possible heat integration of a fuel processor coupled with a high-temperature (HT)/LT-PEMFC. The electrical efficiency increased about 5.5% when a condensing burner technology was adopted. Godat and Marechal (2003) also optimized a fuel cell system by using process integration techniques. The fuel cell and fuel processing subsystems were contained, and four operating conditions (i.e., steam-to-carbon (S/C) ratio, SMR temperature, PEMFC temperature, and fuel utilization) were optimized for higher efficiency. The overall efficiency was raised from 35% for the reference system to 49% in the optimized design. However, the heat exchange networks for the integrated system and process flow diagrams were not involved in their study.

In this work, the hydrogen production system for a 1 kW LT-PEMFC system was studied. Using commercial Aspen Plus^® process simulation software, the hydrogen production process and operating parameters were simulated and optimized. Pinch analysis was applied to integrate the heating networks of the hydrogen production system. More practically, a heat exchange network was designed based on maximum energy recovery (MER) and relaxed networks. Sensitivity analyses that considered the minimum pinch temperature difference (ΔT_min), the relative deviation from chemical equilibrium for both SMR and WGS, the S/C ratio, the reaction temperatures, and heat losses were also investigated. Our aim was to achieve higher system efficiency and balance the investment costs. This work can further guide reformer design and structure optimization in subsequent studies.

Section snippets

Process models

Commercial process simulation software (Aspen Plus^®, AspenTech, Burlington, MA, USA) was used for the optimization of operations and the calculations of mass and heat balances. A schematic flow diagram for a 1 kW LT-PEMFC system is shown in Fig. 1, and the corresponding abbreviations are defined in Table 2. The model is based on a zero-dimensional approach with steady and isothermal operation (Tanim et al., 2013). All the working fluids are calculated using the Peng–Robinson equation of state

Integration method

Pinch analysis is a widely used process integration method in the petroleum and chemical industries, as well as metallurgy, papermaking, food, beverages, and many other fields (Godat and Marechal, 2003). It was proposed by Linnhoff and his team as a heat exchange network design and optimization method in the late 1970s, with the aim of maintaining a four-dimensional balance of energy savings, productivity improvements, reduced investments, and environmental protection (Kemp, 2007, March, 1998).

Process designs

The designed hydrogen production processes according to the MER and relaxed heat network optimizations are shown in Fig. 8. The values in black indicated on the heat exchangers represent the inlet and outlet temperatures of the streams, while the red values represent the heat duties of the different heat exchangers. Obviously, process (a) is much more complex than (b). Streams in Fig. 8a flow through 18 heat exchangers which will lead to complex links and higher heat losses. In contrast,

Conclusions

A hydrogen production system for a 1 kW LT-PEMFC was designed and discussed in this paper. The system is composed of a series of units, including a steam methane reformer, water–gas shift reactors (both HWGS and LWGS), a preferential oxidation reactor, water evaporator, a fuel/anodic off-gas burner, and many heat exchangers. The operating conditions were optimized: for the SMR, a temperature, pressure, and S/C ratio of 1000 K, 1 atm, and 3.0, respectively; for the HWGS, the temperature and

Notation

PEMFC

proton exchange membrane fuel cell

LT-PEMFC

low temperature PEMFC

HT-PEMFC

high temperature PEMFC

SMR

steam methane reforming

CHP

combined-heat-and-power

WGS

water–gas shift

HWGS

high-temperature water–gas shift

LWGS

low-temperature water–gas shift

PROX

preferential oxidation

X_{{CH}_{4}}

conversion of methane

S/C ratio

mole ratio of steam to methane

ΔT

heat transfer temperature difference

ΔT_min

minimum pinch temperature difference

T–H

temperature–enthalpy diagram method

heat capacity flow rate

the specific heat capacity

Acknowledgments

This work is sponsored by the National 863 Program of China (Grant no. 2012AA053402) and the Strategic Program of Sichuan Province (Grant no. 12XXCP0096). The authors would like to acknowledge China Chengda Engineering Co., Ltd. for its software support in this work.

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Heat integration and optimization of hydrogen production for a 1 kW low-temperature proton exchange membrane fuel cell

Highlights

Abstract

Introduction

Section snippets

Process models

Integration method

Process designs

Conclusions

Notation

Acknowledgments

Energy

Int. J. Hydrog. Energy

Chem. Eng. Sci.

J. Clean. Prod.

Chem. Eng. Sci.

Int. J. Hydrog. Energy

Int. J. Hydrog. Energy

Energy

J. Power Sources

Appl. Catal. A:Gen.

J. Power Sources

Catal. Today

J. Power Sources

Chem. Eng. J.

Chem. Eng. Sci.