Energy crisis has become a serious global problem, and Proton exchange membrane fuel cell (PEMFC) has played an important role in the solution of the energy crisis. The hydrogen production process by dimethyl ether steam reforming for PEMFC was studied. The yield of H 2 and energy efficiency of system with different mass ratios (0.3:0.7, 0.35:0.65, 0.4:0.6) of dimethyl ether (DME) and steam were analyzed, and both of yield of H 2 and energy efficiency of system increased with the increase of the mass ratio of DME and steam. The energy efficiency of hydrogen production system using reactor as heat source and hydrogen production system using engine exhaust gas as heat source is compared, and energy efficiency of using reactor as heat source (57.96117%, 63.89651%, 69.0002%) is higher than that using engine exhaust gas as heat source (54.4913%, 60.11311%, 66.25342%).
An increase in energy consumption that results from human-related activities causes the depletion of fossil fuel and the global warming problem. Thus, searching for clean and sustainable energy source is necessary for the future. Hydrogen is an important alternative fuel that is expected to replace fossil fuels because it is clean and environmentally friendly energy source [
Dimethyl ether (DME) is an excellent resource for hydrogen production with its high H/C ratio and high- energy volume density. DME does not contain harmful materials and it burns without producing NOx, and particulates. DME was liquid in the low pressure similar to liquefied petroleum gas (LPG), thus could be stored and transported using the facilities providing LPG. DME is an ideal vehicle fuel, but also has capability of chemical hydrogen storage. Therefore, many scholars have been studied to the DME reforming processes. Steam Reforming (SR) is the most commonly used process of hydrocarbon reforming techniques of DME because it provides high yields of hydrogen production. However, it also requires a large amount of external heat source. The exhaust gas of the vehicle is wasted after clarification by an after-treatment system, but the heat resource and steam from the exhaust gas can be efficient by SR reaction [
PEMFC exhibits special advantages including fast start-up, low working temperature, high specific energy density, simple structure, convenient operation and great durability [
In this paper, the hydrogen production system for PEMFC was studied. The hydrogen production process and operating parameters were calculated and simulated by using commercial process simulation software Aspen Plus. The effects of mass ratio of DME and steam and energy efficiency of the PEMFC power system are analyzed.
The system of hydrogen production mainly includes the following reactions:
Overall reaction of steam reforming of DME:
And steam reforming of DME process involves following four main reactions:
Water-gas shift:
DME hydrolysis:
Steam reforming of methanol:
Methanol decomposition:
In addition, Partial oxidation reaction (PROX):
Streams | |
---|---|
DME H2O ENGINE O2 PEMFC OFFGAS | DME for reforming process Deionized water for reforming process Engine for reforming process Oxygen for PROX reaction Product for PEMFC Off gas of system |
Blocks | |
MIXER SPLITER SR HTWGS LTWGS PROX HEATER1 HEATER2 HEATER3 HEATER4 HEATER5 HEATER6 HEATER7 | Mixer for DME and steams Spliter for H2 and Off gas Yield reactor used for isothermal SR reaction Gibbs reactor used for isothermal HTWGS reaction Gibbs reactor used for isothermal LTWGS reaction Stoic reactor used for isothermal PROX reaction Heater preheating deionized water Heater preheating DME Cooler between SR and HTWGS reactors Cooler between HTWGS and LTWGS reactors Cooler between LTWGS and PROX reactors Heater preheating O2 Cooler between PROX and Spliter |
Q_rest can provide heat for reaction of dimethyl ether steam reforming, and H2_rest can be reused by PEMFC. It indicates that the system has a good energy cycle, and the energy can be used to the maximum extent. This can also improve the energy efficiency of the whole system.
The hydrogen production process mainly consists of three parts: SR part, purification part and supply part. SR part is the core part of the hydrogen production process, it mainly include DME stream reforming reaction (Equation 1) and water-gas reaction (Equation 2). These two reactions occur simultaneously in the reactor and their conversions are considered to be close the equilibrium. H2O and DME are preheated by heater 1 and heater 2.
Their temperature must be up to the temperature of the SR reaction. The SR reactor temperature is constant, so the external heat supply should be maintain isothermal. Purification part includes high-temperature water-gas reaction (HTWGS), low-temperature water-gas reaction (LTWGS) and PROX (Equation 6-7). HTWGS and LTWGS use the Gibbs reactor and PROX uses the Stoic reactor. Those three reactions also should be maintained constant temperature. The isothermal conditions mean that the heat produced by the reactions must be removed and can be used as hot streams for energy recovery [
reduction unit needed to decrease CO to levels below 10 ppm [
FDME,in is the molar flow rates of DME at the inlet. QLHV,DME is the low heat values of DME. PH2,PEMFC is the energy produced of PEMFC with supplying hydrogen.
Selection of a certain type of 1.8 L engine, power 80 kW, indicated thermal efficiency of 35%, fuel consumption 8.0 L/100 km. And Selection of ChaoYue 3 PEMFC fuel cell vehicle, power 50 KW, fuel consumption 1.12 kg/100 km.
The calculated results are as shown in
Variables | Initial value |
---|---|
SR temperature Pressure of all Reactors DME/Steam mass ratio of SR HTWGS LTWGS PROX HEATER1 HEATER2 HEATER3 HEATER4 HEATER5 HEATER6 HEATER7 | 750 K 1 atm 0.3/0.7, 0.35/0.65, 0.4/0.6 723 K 423 K 400 K 373 K 750 K 723 K 423 K 400 K 400 K 343 K |
Mass ratio of DME and steam | Conversion rate of DME |
---|---|
DME:Steam = 0.3:0.7 DME:Steam = 0.35:0.65 DME:Steam = 0.4:0.6 | 50% 43.14286% 14.57143% |
H2 (kmol/hr) | |||
---|---|---|---|
Model A Model B | 0.3:0.7 140.538726 140.538783 | 0.35:0.65 163.859110 163.859577 | 0.4:0.6 186.871870 186.878333 |
REACTOR | Enthalpy (Gcal/hr) | ||
---|---|---|---|
SR HEATER1 HEATER2 HEATER3 HEATER4 HEATER5 HEATER6 HEATER7 HTWGS LTWGS PROX | 0.3:0.7 0 1.61658318 0.24175101 −0.055205 −0.6259349 −0.0465996 4.26E−05 −0.1141745 0.55356551 −0.0804862 −0.0080375 | 0.35:0.65 0 1.50111295 0.28204284 −0.0562453 −0.6417521 −0.0480511 8.38E−05 −0.1176629 0.67638246 −0.1259303 −0.0158039 | 0.4:0.6 0 1.38564273 0.32233468 −0.0548379 −0.6562922 −0.0494898 1.97E−04 −0.1211142 1.13663565 −0.1887269 −0.0368286 |
REACTOR | Enthalpy (Gcal/hr) | ||
---|---|---|---|
SR HEATER1 HEATER2 HEATER3 HEATER4 HEATER5 HEATER6 HEATER7 HTWGS LTWGS PROX | 0.3:0.7 0.87764865 1.61658318 0.24175101 −0.0556454 −0.6259289 −0.0465988 4.26E−05 −0.1140853 0.49594954 −0.0804862 −0.0079886 | 0.35:0.65 0.84802104 1.50111295 0.28204284 −0.0562449 −0.6417473 −0.0480505 8.38E−05 −0.1175975 0.67637782 −0.1259284 −0.0157064 | 0.4:0.6 0.55390332 1.38564273 0.32233468 −0.0548373 −0.6562884 −0.0494898 1.97E−04 −0.1211137 1.13662693 −0.1886922 −0.0368286 |
DME/steam mass ratio | Model A | Model B |
---|---|---|
0.3:0.7 0.35:0.65 0.4:0.6 | 54.4913% 60.11311% 66.25342% | 57.96117% 63.89651% 69.0002% |
69.0002%) is higher than that of model A (54.4913%, 60.11311%, 66.25342%), because the engine has energy losses of model A, while model B has no energy loss by engine working.
In this study, the simulation of hydrogen production via dimethyl ether steam reforming for PEMFC was studied by using Aspen Plus software. It shows that the whole system has good energy cycle utilization. Both of yield of H2 and energy efficiency of system increased with the increase of the mass ratio of DME and steam, and the energy efficiency of hydrogen production system using reactor as heat source is higher than that hydrogen production system using engine exhaust gas as heat source. Finally, this article can provide important theoretical references to the hydrogen production process for PEMFC.
Zhenguo Luo,Cong Li, (2016) Simulation and Analysis of Hydrogen Production by Dimethyl Ether Steam Reforming for PEMFC. Journal of Power and Energy Engineering,04,25-30. doi: 10.4236/jpee.2016.45004