Energy recovery from hydrogen combustion at elevated pressures

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 x x x ( 2 0 1 7 ) 1 e5

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Energy recovery from hydrogen combustion at elevated pressures Tuncay Yilmaz b, Alper Yilmaz a, Mehmet Tahir Erdinc¸ b,* a b

Department of Automotive Engineering, C¸ukurova University, 01330 Adana, Turkey Department of Mechanical Engineering, Osmaniye Korkut Ata University, 80000 Osmaniye, Turkey

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abstract

Article history:

Hydrogen is assumed as one of the most environmentally benign fuels. By the combustion

Received 24 November 2016

of hydrogen enormous amount of H2O; and therefore, latent heat is produced. Because,

Received in revised form

there is not so much need to heat at low temperatures, methods are needed to recover latent

30 December 2016

heat at higher temperatures. One of the methods to upgrade the latent heat at higher

Accepted 31 December 2016

temperatures is the method of combustion at elevated pressures. Increasing the pressure of

Available online xxx

the exhaust gas makes it possible to recover latent heat and higher extra exergy output from the system. In this work, both latent and sensible heat recoveries at elevated pressures are

Keywords:

investigated using environmentally friendly fuel hydrogen. It is shown that coefficient of

Combustion

performance (COP) and exergy efficiency (hex ) is very satisfactory even at high pressures.

Hydrogen

© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Latent heat Energy recovery Exergy Heat upgrading

Introduction In recent years, great effort is made for the use of non-fossil fuels because of the high impact of fossil fuels on temperature increase of earth's atmosphere. The fuel with the most desirable characteristics is hydrogen [1e3] because only water is produced by the combustion of hydrogen which is not harmful for the environment. Hydrogen is emphasized as fuel for internal combustion engines for future transportation [4e6]. Internal combustion engines can be combined with Rankine cycle to produce vehicles with high energy efficiency, that do not harm environment [7e9]. Aircraft engines fueled with hydrogen and heat recovery of exhaust gases can enhance the efficiency of the engine [10].

Because water vapor in the exhaust gas causes high amount of latent heat, this heat must be recovered. The importance of the latent heat recovery is explained by Xu et al. [11] and Shi et al. [12]. Normally, the need for heat at low temperatures is not so big and upgrading methods are used for the need for heat at high temperatures. Simple stage and advanced heat transformers are studied by Donnellan et al. [13]. Mechanical and absorption heat pumps are emphasized by many researchers for upgrading [14e17]. Recently Yılmaz et al. [18] recommended the use of pressurized combustion for obtaining heat recovery for high temperature applications using natural gas combustion. As explained above, there is no research work in literature on upgrading heat energy by combustion of hydrogen at high pressures. In this work, heat and exergy recovery by

* Corresponding author. Fax: þ90 328 825 0097. E-mail address: [email protected] (M.T. Erdinc¸). http://dx.doi.org/10.1016/j.ijhydene.2016.12.146 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Yilmaz T, et al., Energy recovery from hydrogen combustion at elevated pressures, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.146

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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 x x x ( 2 0 1 7 ) 1 e5

hci hc;m he hexp;m hsl 4

Nomenclature a Ar C cP COP ex ext h H2O kgf kgg M_ N2 O2 P Q_ q_ s T x w

ambient argon carbon specific heat, kJ kg1 K1 coefficient of performance exergy total exergy enthalpy, kJ kg1 water vapor kg fuel kg dry gas mass flow rate, kg s1 nitrogen oxygen pressure, kPa heat flow, kW specific heat, kJ/kgf 1 entropy, kJ kg1 g K  temperature, C absolute humidity, kgH2 O =kgg specific work, kJ/kgf

Greek h hexp;i

efficiency expander isentropic efficiency

compressor isentropic efficiency compressor mechanical efficiency electricity production efficiency expander mechanical efficiency second law efficiency relative humidity, %

Subscripts a air, atmospheric Ar Argon c combustion compressor 1 and compressor 2 c1, c2 d dry e electromotor, exchanger exp expander f fuel g gas, dry gas h heat water vapor H2O i isentropic m mechanical, mean Nitrogen N2 o dead state s isentropic p pressure

combustion of hydrogen is investigated to show high heat energy recovery at high temperatures.

Explanation of hydrogen combustion and heat recovery system

Needed air mass flow rate and mass flow rates of combustion products are determined as follows: M_ a ¼ 34:27M_ f

(2)

M_ N2 ¼ 25:89M_ f

(3)

Hydrogen combustion and energy recovery system is shown schematically in Fig. 1. The system is shown in Tes diagram in Fig. 2. Ambient air mass fractions are given in Table 1 [19]. The molecular weights and specific gas constants of the gases needed and produced combustion products are presented in Table 2 [19,20]. Compressor 1 and expander are coupled to each other. Compressor 2 is used to elevate the pressure to the pressure P3 which is prescribed. After combustion, heat produced is used between points 4 and 5. At point 5, gas temperature is assumed equal to the temperature at point 3. At point 5d, gas has the pressure P3 with the corresponding saturation temperature. At point 6, gas has the saturation temperature at P ¼ P1.

Combustion analysis The analysis is similar to that given by Yilmaz et al. [18]. Theoretical complete combustion is assumed using ambient air. 2H2 þ O2 ¼ 2H2 O

(1)

Fig. 1 e Hydrogen combustion and energy recovery system.

Please cite this article in press as: Yilmaz T, et al., Energy recovery from hydrogen combustion at elevated pressures, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.146

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Using these values and compressor isentropic efficiencies one can calculate the properties at point 2 and 3 [18]. Work produced by the expander must be equal to the work needed by compressor 1. These works are given below: wexp ¼ 26:32ðh6  h7 Þhexp;m

(9)

wc1 ¼

34:27ðh2  h1 Þ hc1;m

(10)

wc2 ¼

34:27ðh3  h2 Þ hc2;m hc2;e

(11)

hc1;m , hc2;m and hexp;m are the mechanical efficiencies of the compressors 1, 2 and the expander, respectively. hc2;e is the electromotor efficiency of compressor 2. Recovered sensible and latent heats are determined as follows:

Fig. 2 e Pressurized hydrogen combustion and energy recovery system in Tes diagram.

qh1 ¼ 26:32ðh5  h5s Þ

(12)

qh2 ¼ 26:32ðh5s  h6 Þ

(13)

COP of the system is determined using the following equation: COP ¼

Table 1 e Mass fractions in ambient air [19]. Mass fraction N2 O2 Ar

0.75570 0.23161 0.01269

Table 2 e Molecular weights and specific gas constants of different materials [19,20]. Molecular weight (kg/kmol)

Special gas constant (J/kg K)

12.011 31.999 28.013 18.015 28.958 39.948 2.016

e 259.83 296.80 461.53 287.12 208.13 4124.24

C O2 N2 H2O Air Ar H2

qh1 þ qh2 h5  h6 ¼ 0:7680 wc2 h3  h2

(14)

The real exergies obtained from qh1 and qh2 are calculated as follows:  ex1 ¼ qh1 1 

 To T5 ln h T5s  T5 T5s sl

(15)

 ex2 ¼ qh2 1 

 To T5s ln h T5s  T6 T6 sl

(16)

ext ¼ ex1 þ ex2

(17)

hsl is the second law efficiency of mechanical energy production which is assumed as 0.5 in this work. Exergy efficiency is defined as follows: hex ¼

ext wc2

(18)

For the efficiencies of the compressors and expander mean values given by Yılmaz et al. [18] are presented in Table 3.

M_ Ar ¼ 0:43M_ f

(4)

M_ H2 O;c ¼ 8:936M_ f

(5)

M_ g;d ¼ 26:32M_ f

(6)

x ¼ 0:3397 þ 1:3017xa

(7)

Results and discussions It is explained in Fig. 2 that the temperature at point 5 (T5), is assumed equal to the temperature at point 3 (T3). Therefore latent heat recovery is not possible by combustion at the atmospheric pressure Pa, because T5 ¼ Ta in this case. Besides this, point 6 is assumed as saturation temperature at

Calculation method Table 3 e Mean vales of efficiencies. Inlet air properties are assumed as follows: Pa ¼ 101:325 kPa; Ta ¼ 293:15 K; 4a ¼ 0:50

(8)

hc1 ; hc2

hexp

hc1;m ; hc2;m

hexp;m

hc2;e

0.88

0.89

0.98

0.98

0.92

Please cite this article in press as: Yilmaz T, et al., Energy recovery from hydrogen combustion at elevated pressures, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.146

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Fig. 3 e Variation P2 with P3.

atmospheric pressure Pa. Therefore, no sensible heat recovery is foreseen below this temperature. So, the sensible heat (qh1) and latent heat (qh2) are zero for combustion at pressure Pa. In Fig. 3, variation of P2 from P3 is demonstrated. It is seen that P2 increases with P3 rapidly at first, but it increases very slowly after approximately 700 kPa. Variation of different temperatures in the system with P3 is shown in Fig. 4. Inlet temperature (T1) and the saturation temperature at 1 bar (T6) are constant. As expected, saturation temperature T5s at P3 increases slowly; however, temperature after compressor 2, T5 ¼ T3 increases to higher temperatures with the increase in pressure P3. As seen from this figure, temperature T7 decreases below at 0  C, if the pressure P3 is higher than 500 kPa. For this case, the exhaust gas from the expander can also be used for cooling purposes. Sensible and latent heats recovered in the system are presented in Fig. 5. The amount of the recovered latent heat is very high, but it does not increase too much with the pressure P3 after P3 ¼ 500 kPa. The works necessary for compressors 1 and 2 are shown in Fig. 6. The work for the second compressor increases nearly linearly with pressure P3; whereas the work for the compressor 1 does not increase much after P3 ¼ 500 kPa.

Fig. 4 e Variation of different temperatures with P3.

Fig. 5 e Variation of recovered sensible and latent heat with P3.

Fig. 6 e Work necessary for compressors 1 and 2 dependent from P3.

The absolute humidities are given in Fig. 7. The absolute humidity at point 5 is very high because of the hydrogen combustion. x6 and x7 decrease very rapidly with pressure P3. COP and hex of the system decrease with pressure P3 as seen in Fig. 8. Values of the coefficient of performance are higher

Fig. 7 e Absolute humidities dependent from P3.

Please cite this article in press as: Yilmaz T, et al., Energy recovery from hydrogen combustion at elevated pressures, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.146

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 x x x ( 2 0 1 7 ) 1 e5

Fig. 8 e Variation of COP and hex with P3.

than 5 even at high pressures till P3 ¼ 1500 kPa, which can be considered as very good. Exergy efficiency hex , which contains a second law efficiency of 50% is also high till P3 ¼ 1500 kPa. Especially at lower pressure till P3 ¼ 300 kPa, this efficiency is higher than 2, which can be considered as very good.

Conclusion Heat and exergy recovery by pressurized combustion is investigated using environmentally friendly fuel hydrogen. Energy recovery from exhaust gas by hydrogen gas combustion at elevated pressure can be very high, especially by the energy recovery of latent heat. This recovery can be considered as very good till pressures 500 kPa and good till the pressures 1500 kPa. The coefficient of performance (COP) and exergy efficiencies (hex ) are also very high. Because of them, this method of combustion at elevated pressures can be emphasized for heat upgrading.

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Please cite this article in press as: Yilmaz T, et al., Energy recovery from hydrogen combustion at elevated pressures, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.146