[0.95]And Methane-Hydrogen Mixtures Pipeline Transmission

energies

Article Thermodynamic and Technical Issues of Hydrogen and Methane-Hydrogen Mixtures Pipeline Transmission

Szymon Kuczy ´nski,Mariusz Łaciak, Andrzej Olijnyk, Adam Szurlej and Tomasz Włodek * AGH University of Science and Technology, Drilling, Oil and Gas Faculty, Krakow PL30059, Poland; [email protected] (S.K.); [email protected] (M.Ł.); [email protected] (A.O.); [email protected] (A.S.) * Correspondence: [email protected]; Tel.: +48-12-617-3668

Received: 27 December 2018; Accepted: 4 February 2019; Published: 12 February 2019

Abstract: The use of hydrogen as a non-emission energy carrier is important for the innovative development of the power-generation industry. Transmission pipelines are the most efficient and economic method of transporting large quantities of hydrogen in a number of variants. A comprehensive hydraulic analysis of hydrogen transmission at a mass flow rate of 0.3 to 3.0 kg/s (volume flow rates from 12,000 Nm3/h to 120,000 Nm3/h) was performed. The methodology was based on flow simulation in a pipeline for assumed boundary conditions as well as modeling of fluid thermodynamic parameters for pure hydrogen and its mixtures with methane. The assumed outlet pressure was 24 bar (g). The pipeline diameter and required inlet pressure were calculated for these parameters. The change in temperature was analyzed as a function of the pipeline length for a given real heat transfer model; the assumed temperatures were 5 and 25 ◦C. The impact of hydrogen on natural gas transmission is another important issue. The performed analysis revealed that the maximum participation of hydrogen in natural gas should not exceed 15%–20%, or it has a negative impact on natural gas quality. In the case of a mixture of 85% methane and 15% hydrogen, the required outlet pressure is 10% lower than for pure methane. The obtained results present various possibilities of pipeline transmission of hydrogen at large distances. Moreover, the changes in basic thermodynamic parameters have been presented as a function of pipeline length for the adopted assumptions.

Keywords: hydrogen; hydrogen pipelines; hydrogen transmission; pipeline transmission; pressure drop; energy storage

1. Introduction Recent trends in modern economies are focused on greenhouse gas emissions reduction and mitigation of climate change effects. Countries around the world have begun to shift their energy production to renewable energy sources (RES). Energy from renewable sources may help mitigate emissions from traditional fossil fuel energy generation [1]. Implementation of EU energy policy requires investment in power technologies based on RES. The dynamics of RES development and application can be traced to the basis of its installed capacity. According to data from 2016 [2], the highest increase of installed power was observed for wind farms (i.e., 12,490 MW, or 51% of all new installed capacity in EU) and solar energy power plants (i.e., 6700 MW, or 27.4% of all new installed capacity in EU). By 2040, RES-based EU technologies will constitute 80% of new installed power, while after 2030, wind energy is predicted to become the leading electrical energy source [3]. Wind energy, with signiﬁcant growth in the RES share it represents, will cause problems associated with an uneven generation of electrical energy, resulting from variable atmospheric conditions [4]. Frequently, high electricity generation is possible during periods of low demand for electrical energy (e.g., days off), whereas during periods of higher demand (evening peak), production is much lower. Moreover,

Energies 2019, 12, 569; doi:10.3390/en12030569 www.mdpi.com/journal/energies Energies 2019, 12, 569 2 of 21 the use of renewable energy from solar and wind farms is connected with power transmission system problems because of the irregularity and instability of energy supply [5,6]. Accordingly, a significant development in energy storage technology is required to increase application of RES in electrical energy generation sector. Power-to-Gas is an example of such technology. By using this technology, electrical energy can be converted to gaseous fuel (hydrogen). Hydrogen as an energy carrier can store the largest quantities of energy and has high energy content per mass unit [4]. This makes hydrogen technology very advantageous from a technological point of view [7]. Hydrogen has great importance as a promising green energy carrier, but practically does not occur in nature freely, hence it is not the primary source of energy. Hydrogen is usually generated as a secondary energy carrier from primary sources such as natural gas or wind energy. Hydrogen will play an important role in the world energy mix in the future [8]. Requirements regarding the proportion of renewable energy sources in national electricity systems have been described in Directive 2009/28/EC. The general objective of achieving a 20% content of RES usage by 2020 in the European Union was included in this statement [7,9]. Most of the power generated in the field of renewable energy sources was comprised of onshore and offshore wind farms. Therefore, for the further development of renewable energy usage in the power generation sector, it will be necessary to make progress in the use of energy storage technologies. The use of hydrogen as an energy storage technology allows the largest amount of energy storage and is distinguished by having the highest power output. Thus, this technology is very beneficial in technical terms [10–12]. In the near future, the natural gas system will be used for transmission of the growing volume of alternative fuels (e.g., hydrogen and biomethane), which will be added to traditional natural gas mixtures. Recently, projects and concepts for the construction of energy storage sites in salt caverns have been the main focus related to the development of renewable energy sources. Hydrogen obtained during the withdrawal of salt caverns has to be transported to the place of its utilization. Pipeline transmission of hydrogen and the possibility of hydrogen addition to the natural gas transmission system are still new solutions requiring further research, and have been confirmed by real applications in a small number of cases. Specific thermodynamic analysis of methane–hydrogen mixtures is a main novelty of this research, in particular the use of hydrogen to improve natural gas flow parameters in the pipeline while maintaining quality requirements.

2. Basics of Hydrogen Transmission Hydrogen is presented as an effective energy carrier which can be used efficiently with minimum greenhouse gas emissions in the processes in which it is involved [13]. The use of hydrogen in the long term for the purpose of balancing energy production with the demand of the electrical energy market is a real solution to the problem of excessive amounts and shortages of energy on the market. Hydrogen is used for storing surplus electrical energy. Salt caverns, as an efficient source of hydrogen storage, provide good conditions for injection and production [14–17]. Salt caverns are located in sites which have specific geological conditions; therefore, in many cases hydrogen must be transported across long distances from the storage site. The idea of energy storage in salt caverns with the use of hydrogen is presented in Figure1. Over recent years the pipeline transport of hydrogen has been considered mainly as an integral and significant element of the renewable energy system [18]. On the other hand, the industrial use of hydrogen is widespread, where the production of hydrogen is closely associated with specific technological processes. Hydrogen is transmitted through pipelines of various diameters and lengths, depending on the way in which hydrogen will be used. Pipeline transmission is one of the cheapest methods for transporting large quantities of hydrogen [19,20]. Pipelines for the transmission of hydrogen at long distances exist in a few places worldwide, with a total length of 4500 km. Energies 2019, 12, 569 3 of 21 Energies 2019, 12 FOR PEER REVIEW 3

TECHNOLOGICAL PIPELINE INDUSTRIAL INSTALLATIONS

RENEWABLE ENERGY ELECTROLYSER GAS COMBUSTION Wind farm, ELECTRICITY ELECTRICITY UNIT solar farm H2O  H2 INJECTION WITHDRAWAL

SALT CAVERN

FigureFigure 1. 1.Scheme Scheme of ofhydrogen hydrogen en energyergy storage storage processes. processes.

HydrogenHydrogen is commonlyis commonly considered considered aa gasgas similarsimilar to to methane methane (i.e., (i.e., the the main main component component of of natural gas). Therefore, most technological requirements for hydrogen transmission pipelines are natural gas). Therefore, most technological requirements for hydrogen transmission pipelines are identical to those of natural gas pipelines, with certain modifications regarding safety, identicalinfrastructure, to those of and natural materials. gas pipelines,These conditions with certain must be modifications met before the regarding transmission safety, of hydrogen infrastructure, is and materials.initiated through These conditionsa pipeline mustnetwork. be met Hydrogen before thehastransmission its specific set of hydrogenof physical is and initiated chemical through a pipelineproperties network. which Hydrogen makes the has pipeline its specific transmission set of physical of this gas and significantly chemical properties different whichfrom that makes of the pipelinenatural transmission gas. Due ofto thisphysicochemi gas significantlycal properties, different hydrogen from that pipeline of natural transmission gas. Due tois physicochemicalmuch more properties,difficult hydrogen than natural pipeline gas. Even transmission compressed is hydrog much moreen can difficult provide thanonly naturalabout a third gas. Evenof the compressedenergy hydrogenwhen compared can provide to methane only about per unit a thirdof volume of the [21]. energy when compared to methane per unit of volume [21Natural]. gas transport and distribution networks are very well developed; therefore, the Naturalhydrogen gas pipeline transport system and is distribution directly compared networks to arethe verynatural well gas developed; transmission therefore, system [22]. the hydrogen The natural gas system consists of gas compression stations, pipelines, gas stations and gas storage pipeline system is directly compared to the natural gas transmission system [22]. The natural gas facilities, including caverns [23]. The gas compression station provides the energy needed to systemgenerate consists gas of flow gas at compression a given rate stations,and pressure. pipelines, In the gascase stations of natural and gas, gas gas storage networks facilities, are divided including cavernsinto [ 23high]. Thepressure gas compressiontransmission pipelines station provides and medium the energyor low-pressure needed distribu to generatetion pipelines. gas flow In at the a given rate andcase pressure.of hydrogen In thetransmission, case of natural the distribution gas, gas networksto smaller arereceiving divided centers into worldwide high pressure is quite transmission rare, pipelinesexcepting and mediumsome locations, or low-pressure such as the Leuna distribution industri pipelines.al district in In Germany. the case In of most hydrogen cases, hydrogen transmission, the distributiontransportation to smalleris considered receiving as a transmission centers worldwide from point is quiteA to point rare, B, excepting for a technologically some locations, justified such as the Leunapurpose. industrial Selected districtissues related in Germany. to hydrogen In mostpipeli cases,ne transmission hydrogen have transportation been presented is recently considered in a as a transmissionnumber of from papers point [24–29]. A to point B, for a technologically justified purpose. Selected issues related to hydrogen pipeline transmission have been presented recently in a number of papers [24–29]. 3. Model Development 3. Model Development 3.1. Basic Assumptions 3.1. BasicThe Assumptions hydraulic analysis of hydrogen transmission through the pipelines was based on a number of technological assumptions (e.g., mass flow rate from 0.3 to 3.0 kg/s, which corresponds to volume Theflow hydraulicrate from 12,000 analysis Nm of3/h hydrogen to 120,000 transmissionNm3/h). The recommended through the pipelinesoutlet pressure was based for an onexemplary a number of technologicaltechnological assumptions installation (e.g., was massassumed flow to rate be from24 bar 0.3 (g). to In 3.0 the kg/s, presented which exemplary corresponds case, to the volume 3 3 flow ratemedium from inlet 12,000 temperature Nm /h towas 120,000 set at Nm5 °C /h).and th Thee ambient recommended temperature outlet was pressure 15 °C. For for heat an exemplary flow technological installation was assumed to be 24 bar (g). In the presented exemplary case, the medium inlet temperature was set at 5 ◦C and the ambient temperature was 15 ◦C. For heat flow analysis, the pipeline was located one meter deep. The length of the exemplary pipeline chosen for analysis was 100 km. Energies 2019, 12, 569 4 of 21

3.2. Hydraulic Friction Factor The zone of turbulent flow in rough pipes consists of a transient flow zone and developed roughness zone. In the transient flow zone, the linear friction coefficient (λ) depends on the Reynolds number (Re) and relative roughness (ε): λ = f (Re, ε). Colebrook and White presented the following formula for the linear friction coefficient λ in this zone [30]:

1 2.51 ε √ = −2lg √ + (1) λ Re · λ 3.71

The Colebrook-White Equation (1) was systematically analyzed from a theoretical and experimental point of view and, as a result, considered to be the most accurate of all relationships determining λ coefficient in the transient flow zone. For the zone with a full roughness impact λ = f (ε), the linear friction coefficient is analyzed with the Prandtl-Nikuradse Equation (2) [31]:

1 ε √ = −2lg (2) λ 3.71

Equation (2) was established by Prandtl, based on experiments conducted by Nikuradse on pipes with artiﬁcial sand roughness. The Prandtl–Nikuradse equation could be used on the following assumed criterion: √ Re · ε · λ ≥ 200 (3)

3.3. Pressure Drop Analysis The pressure drop in gas pipelines can be determined with one of the hydraulic equations commonly applied for high pressure gas pipelines (e.g., Renouard equation, Panhandle equation). In this paper, authors applied the General Flow Equation (4), directly derived from the Bernoulli equation [32]: p 2 Q2 2 − 2 = · 5 · · b · n p1 p2 7.569 10 ZTin Ldλ 5 (4) Tb D where p1, p2—pressure at respectively inlet and outlet of pipeline, Z—compressibility factor, Tin—temperature in pipeline, L—length of pipeline, d—relative density, Qn—volume flow rate, D—inner diameter. The difference of levels on a given pipeline section has a significant impact on the flow hydraulics. In pressure drop equations, the influence of terrain elevation change is accounted for with the use of an equivalent length of the pipeline Le, which is the pipeline length corrected for the impact of terrain elevation with respect to the level of initial point of the pipeline or pipeline section [32].

L(exp(s) − 1) L = (5) e s where s—dimensionless parameter which determines the inﬂuence of terrain elevation, which depends on temperature (T), compressibility factor (Z), relative density (d) and elevation level difference (∆h). Dimensionless parameter which describes the terrain elevation impact is deﬁned as:

∆h s = 0.0684d (6) T · Z

If a pipeline of length L is divided into a number of sections (i.e., L1,L2,L3, etc.) in which the terrain level signiﬁcantly changes, then the parameter j should be introduced in such a way that the impact of the terrain elevation could be determined for each segment of the pipeline. Energies 2019, 12, 569 5 of 21

exp(s) − 1 j = (7) s

In this case, the pipeline equivalent length Le accounts for the effect of the terrain elevation for each pipeline section with the dependence:

Le = j1L1 + j2L2 exp(s1) + j3L3 exp(s2) + . . . (8)

3.4. Temperature Changes along the Pipeline The temperature change basic model as a function of pipeline length was determined with the • dependence combining the total heat transfer coefﬁcient (U), mass ﬂow rate (M) and isobaric heat capacity (Cp)[32]: T(x) Lr Z dT Z U · π · D · dx in = (9) − • Tin Tout · Tin 0 M Cp

For a pipeline with length L and diameter D, with the Joule–Thomson effect (µJT) change of temperature of the transmitted medium from T1 to T2 can be described with the equation [33]: −L·U·π·D M dp −L·U·π·D T2 = Tout + (T1 − Tout) exp • + π·U·D µJT · Cp · dL 1 − exp • (10) M·Cp M·Cp

3.5. Heat Transfer Analysis The basic principles of heat transfer analysis were formulated with heat flow laws based on conduction (Fourier law), convection (Newton law), and radiation (Stefan–Boltzman law) in association with the first law of thermodynamics. Conduction and convection are most the most important parameters for the heat flow transferred to the pipeline. Conduction, described with the Fourier law (according to cylindrical coordinates) for a pipeline in dynamic conditions, has the following form [34]:

1 ∂ ∂T ∂T k · r = ρ · Cp (11) r ∂r ∂r ∂τ where r—radius, k—thermal conductivity, ρ—density, τ—time, Cp—isobaric heat capacity. Under static conditions, the right side of Equation (11) equals zero; therefore, the total heat ﬂow per unit of length of the pipeline between the medium within the pipeline and the environment is

∂T Q = −2πr · L · k (12) ∂r After integration of Equation (12), we have

2πr · L · k · (Tn − Tn+1) Q = (13) ln rn+1 rn

The most important parameter which determines the ability of a particular cylindrical obstacle to heat transfer is the heat transfer coefﬁcient (U). Taking into account convection and conductivity effects for a pipeline with complex parameters, the heat transfer coefﬁcient equals [34,35]

1 U = (14) 1 + rin ln rout + rin ln riso + rin ln 2zx + rin αin kp rin kiso rout kground riso zxαout Energies 2019, 12 FOR PEER REVIEW 6

Energies 2019, 12, 569 6 of 21 1 U = 1 r  r  r  r  r  2z  r + in ln out  + in ln iso  + in ln x  + in (14) In order to determine theα heat transfer  in the presented  case (Figure  2), theα thermal conductivity in k p  rin  kiso  rout  kground  riso  zx out coefficient was analyzed for the pipeline wall (kp), thermal insulation (kiso), and ground in which In order to determine the heat transfer in the presented case (Figure 2), the thermal conductivity the pipeline was buried (kground). Rin and Rout represent the inner and outer radius of the pipeline, coefficient was analyzed for the pipeline wall (kp), thermal insulation (kiso), and ground in which the respectively. riso is the radius including thermal insulation, and zx is the depth of pipeline deposition pipeline was buried (kground). Rin and Rout represent the inner and outer radius of the pipeline, in the ground. Convection effects are described by α —inner coefficient of heat penetration (assumed respectively. riso is the radius including thermal insulation,in and zx is the depth of pipeline deposition or determinedin the ground. with, Convection for example, effects the are Dittus–Boelter described by formula)αin—inner andcoefficientαout—outer of heat coefficient penetration of heat penetration(assumed defined or determined with Equation with, for (15) exam forple, a pipelinethe Dittus–Boelter seated informula) the ground and αout at—outer a depth coefficientzx [35]: of heat penetration defined with Equation (15) for a pipeline seated in the ground at a depth zx [35]: · 2 k⋅ ground αout = 2 k ground√ (15) α = 2 2 out 2zx+ 4z −D D · ln + x2 − tot2  tot 2z x D4z x Dtot (15) D ⋅ ln  tot  tot D  tot  where Dtot is the total diameter of pipeline with thermal insulation. Calculationswhere Dtot is the of thetotaloverall diameter heat of pipeline transfer with coefficient thermal insulation for pure hydrogen and pure methane as a Calculations of the overall heat transfer coefficient for pure hydrogen and pure methane as a function of the pipeline diameter are presented in Figure3. Other parameters, for example, thermal function of the pipeline diameter are presented in Figure 3. Other parameters, for example, thermal insulationinsulation or burial or burial depth depth of theof the gas gas pipeline, pipeline, havehave remained unchanged. unchanged. Calculations Calculations were made were made 3 for thefor same the same volume volume flow flow rates rates inin three three variants variants specified specified in the inpresented the presented case (12,000 case Nm (12,0003/h, 40,000 Nm /h, 40,000Nm 3/h,3/h, 120,000 120,000 Nm Nm3/h).3 Obtained/h). Obtained results are results similar are for similar methane for and methane hydrogen and for hydrogen assumed volume for assumed volumeflow flow rates. rates.

ambient air

Tout = Tamb

ground x Z

Tin

FigureFigure 2. 2.Cross Cross section section of an exemplary exemplary pipeline. pipeline. Energies 2019, 12 FOR PEER REVIEW 7

2 Methane 12000 Nm³/h K)

2 Methane 40000 Nm³/h Methane 120000 Nm³/h Hydrogen 12000 Nm³/h 1.8 Hydrogen 40000 Nm³/h Hydrogen 120000 Nm³/h

1.6

1.4 Overall heat transfer coefficient, W/(m

1.2 100 150 200 250 300 Pipeline inner diameter, mm

Figure 3. OverallFigure heat 3. transferOverall heat coefﬁcienttransfer coefficient as as a functiona function of ofpipeline pipeline inner diameter inner for diameter different flow for different ﬂow rates of pure methane and pure hydrogen. rates of pure methane and pure hydrogen. The final equation for total heat flux has the following form: 2πrLT (− T ) Q = in in out 12rr r  r r  z  r ++inln out in ln iso + in ln x + in (16) αα    inkrkrk p in iso  out ground  r iso  z x out

3.6. Real Gas Behavior and Thermodynamic Parameters Description Thermodynamic parameters of transmitted hydrogen and methane/hydrogen mixtures as real gas are calculated with the Peng–Robinson equation of state commonly applied in the oil and gas industry [36]: RT a p = − m − + + − (17) v bm v(v bm ) b(v bm )

The parameters of the equation of state am and bm are based on classic mixing rules. They depend on the critical parameters of the analyzed gas. Compressibility factor Z is determined on the basis of a polynomial form of the Peng–Robinson equation of state: Z 3 + (B −1)Z 2 + (A − 3B2 − 2B)Z + (B3 + B2 − AB) = 0 (18) where A and B are dimensionless parameters of equation of state, depending on the present temperature and pressure conditions: a p = m A 2 2 (19a) R T b p B = m (19b) RT The compressibility factor is a key parameter while determining the pressure drop in the pipeline, and changes in most of the thermodynamic parameters of the analyzed gas as well as

Energies 2019, 12, 569 7 of 21

The ﬁnal equation for total heat ﬂux has the following form:

2πrin L(Tin − Tout) Q = (16) 1 + rin ln rout + rin ln riso + rin ln 2zx + rin αin kp rin kiso rout kground riso zxαout

Z3 + (B − 1)Z2 + (A − 3B2 − 2B)Z + (B3 + B2 − AB) = 0 (18) where A and B are dimensionless parameters of equation of state, depending on the present temperature and pressure conditions: a p A = m (19a) R2T2 b p B = m (19b) RT The compressibility factor is a key parameter while determining the pressure drop in the pipeline, and changes in most of the thermodynamic parameters of the analyzed gas as well as changes in temperature is a function of the pipeline length. Another important parameter is the density of the transmitted gas, which is determined with the general form of the equation of state: p ρ = (20) ZRT Equation (10) also makes use of specific heat capacity at a constant pressure (Cp) and the Joule–Thomson coefficient (µJT), which have a significant impact on temperature changes of the transmitted hydrogen [37,38]: T dam −a ∂Z dT√ m Cp = Cpid + Cpr = Cpid + R · T + Z − 1 + · ∂T p 2 2bm √ √ ∂Z + + ∂B ∂Z − − ∂B d2am √ (21) ( ∂T ) (1 2)( ∂T ) ( ∂T ) ( 2 1)( ∂T ) T +( + ) p √ p − p √ p + √dT2 · ln Z 1 √2 B Z+(1+ 2)B Z−( 2−1)B 2 2bm Z+(1− 2)B where Cpid—isobaric heat capacity for ideal gas, Cpr—residual part of isobaric heat capacity. The Joule–Thomson coefficient can be expressed with specific heat capacity at constant pressure: " # 1 ∂v µJT = T − v (22) Cp ∂T p Energies 2019, 12, 569 8 of 21

Using the real gas Law (18), Equation (22) can be written as " # 1 T ∂Z µJT = (23) Cp Z · ρ ∂T p where ∂A ∂B 2 2 (B − Z) + 6BZ + 2Z − 3B − 2B + A − Z ∂Z ∂T p ∂T p = 2 (24) ∂T p 3Z2 + 2(B − 1) + (A − 2B − 3B2) In the case of hydrogen, the description of the Joule–Thomson effect has a special meaning. Unlike for natural gas, the Joule–Thomson coefﬁcient for hydrogen is negative, which means that hydrogen temperature increases with isenthalpic expansion.

3.7. Pipeline Diameter Selection Prior to the hydraulic analysis of the pipeline, the optimum diameter of the pipeline should be determined for parameters such as assumed working pressure, length of the pipeline, roughness, etc. The selection of the diameter is also important for determination of the inlet pressure to the pipeline (includes associated costs) and possibility to compress the medium. In the analyzed case, the diameter was determined with a function of inlet pressure at the beginning of the pipeline for the assumed outlet pressure at the end of the pipeline of 24 bar (g). Calculations were based on the General Flow Equation (4), which directly stems from the Bernoulli law. The presented equation also contains an element (∆h) referring to the change of the elevation of the pipeline with respect to the assumed reference level [32,39]: v u • u5 16λ · Z2 · R2 · T2 · L · M2 D = t (25) 2 2 2 2 π · Z · R · T · p1 − p2 − 2 · g · Pav · ∆h Calculations were performed for pure hydrogen (Figure4) and methane/hydrogen mixtures with a maximum hydrogen content of 15% mol (Figure5). It should be noted that from the perspective of mass ﬂow rate, the recommended diameters for the pure hydrogen are much smaller than for methane/hydrogen mixtures, which results from the low mass of hydrogen (Figures4 and5). The recommended diameters for the methane/hydrogen mixture are much larger, as the density of pure hydrogen under normal conditions equals 0.0898 kg/m3, while the density of a mixture of methane and 15% hydrogen is 0.6223 kg/m3. Recommended pipeline diameters for the assumed ﬂow rates of pure hydrogen and methane/(15%)hydrogen mixtures are presented in Table1. Evidently, transmission of the same volume of methane in a mixture with 15% hydrogen content required much larger diameters.

Table 1. Recommended pipeline diameters for pure hydrogen and methane/hydrogen mixture transmission.

Pure Hydrogen Methane (85%) Hydrogen (15%) Volume Flow Rate, Nm3/h Diameter, mm Diameter, mm 12,000 100–150 125–200 40,000 150–250 250–300 80,000 200–300 300–400 120,000 250–400 350–500 Energies 2019, 12 FOR PEER REVIEW 9

It should be noted that from the perspective of mass flow rate, the recommended diameters for the pure hydrogen are much smaller than for methane/hydrogen mixtures, which results from the low mass of hydrogen (Figures 4 and 5). The recommended diameters for the methane/hydrogen mixture are much larger, as the density of pure hydrogen under normal conditions equals 0.0898 kg/m3, while the density of a mixture of methane and 15% hydrogen is 0.6223 kg/m3. Recommended pipeline diameters for the assumed flow rates of pure hydrogen and methane/(15%)hydrogen mixtures are presented in Table 1. Evidently, transmission of the same volume of methane in a mixture with 15% hydrogen content required much larger diameters.

Table 1. Recommended pipeline diameters for pure hydrogen and methane/hydrogen mixture transmission.

Volume Flow Rate, Pure Hydrogen Methane (85%) Hydrogen (15%) Nm3/h Diameter, mm Diameter, mm 12,000 100–150 125–200 40,000 150–250 250–300 Energies 2019, 12, 569 80,000 200–300 300–400 9 of 21 120,000 250–400 350–500

450 0.3 kg/s (12000 Nm³/h) 400 1.0 kg/s (40000 Nm³/h) 350 2.0 kg/s (80000 Nm³/h) 300 3.0 kg/s (120000 Nm³/h) 250

200

150 Calculated diameter, Calculated mm diameter, 100

0 2.533.544.555.566.577.5 Pipeline inlet pressure, MPa

FigureEnergies 4.FigureDiameter 2019, 124. DiameterFOR calculation PEER REVIEWcalculationfor for pure pure hydrogen hydrogen pipe pipelineline as asa function a function of pipeline of pipeline inlet pressure. inlet pressure. 10

700 2.08 kg/s (12000 Nm³/h) 600 6.93 kg/s (40000 Nm³/h) 13.86 kg/s (80000 Nm³/h) 500 20.79 kg/s (120000 Nm³/h)

400

300

200 Calculated diameter, Calculated mm diameter,

100

0 2.533.544.555.566.577.5 Pipeline inlet pressure, MPa

Figure 5. DiameterFigure 5. Diameter calculation calculation for methane–hydrogen for methane–hydrogen (15%) (15%) mixture mixture pipeline pipeline asas aa functionfunction of of pipeline inlet pressure.pipeline inlet pressure.

4. Calculation4. Calculation Results Results Flow modeling in a hydrogen transmission pipeline covers the analysis of changes in pressure Flowand modeling temperature in a as hydrogen a function transmission of length of the pipeline pipeline. coversThe changes the analysis of compressibility of changes factor, in pressure and temperaturedensity, asspecific a function heat, and of length the Joule–Thomson of the pipeline. effect The as changesparameters of especially compressibility important factor, for density, speciﬁc heat,hydrogen and the pipeline Joule–Thomson transmission were effect presented as parameters for one of especiallythe variants. importantA comparative for analysis hydrogen was pipeline 3 transmissionalso were performed presented for the methane/hydrogen for one of the variants. mixture and A mass comparative flow rate of analysis 40,000 Nm was/h. also performed for 3 the methane/hydrogen4.1. Pressure Drop mixture and mass ﬂow rate of 40,000 Nm /h.

4.1. Pressure DropThe profiles of pressure and temperature changes for recommended pipeline diameters, assumed mass flow rates of 0.3, 1.0, 2.0 and 3.0 kg/s, (volume flow rates: 12,000 Nm3/h, 40,000 Nm3/h, The profiles80,000 Nm of3 pressure/h and 120,000 and Nm temperature3/h) and hydrogen changes inlet fortemperature recommended of 5 °C are pipeline presented diameters, in Figures assumed 6–9. Results of modelling the pressure drop profiles confirmed the preliminary calculations of the mass flow rates of 0.3, 1.0, 2.0 and 3.0 kg/s, (volume flow rates: 12,000 Nm3/h, 40,000 Nm3/h, pipeline diameter. For the smallest recommended diameters for a selected flow rate, the inlet 3 3 ◦ 80,000 Nmpressure/h and in 120,000 the pipeline Nm was/h) from and 6.01 hydrogen to 6.76 MPa inlet (Figures temperature 6–9), while of for 5 theC arelargest presented recommended in Figures 6–9. Results of modellingdiameters the the inlet pressure pressure ranged drop profilesfrom 2.91 confirmedto 3.2 MPa (Figures the preliminary 6–9). The analysis calculations of temperature of the pipeline diameter. Forchanges the revealed smallest that recommended the transmitted hydrogen diameters warms for aup selected more slowly flow for rate, smaller the diameters inlet pressure and in the higher flow rates. pipeline was from 6.01 to 6.76 MPa (Figures6–9), while for the largest recommended diameters the inlet pressure ranged from 2.91 to 3.2 MPa (Figures6–9). The analysis of temperature changes revealed that the transmitted hydrogen warms up more slowly for smaller diameters and higher flow rates.

Energies 2019, 12 FOR PEER REVIEW 11

Energies 2019, 12 FOR PEER REVIEW 11 Energies 2019, 12, 5696.5 16 10 of 21

6 p, D=100mm 6.5 1416 p, D=125mm 5.5 p, D=150mm 6 p, D=100mm 12 p, D=200mm 14 p, D=125mm 5 T, D=100mm 5.5 p, D=150mm T, D=125mm 1012 p, D=200mm 4.5 T, D=150mm 5 T, D=100mm T, D=200mm 8 T, D=125mm 10 4 4.5 T, D=150mm T, D=200mm 68 Temperature, Temperature, °C Pressure, MPa(g) 3.5 4 4 3 6 Temperature, Temperature, °C Pressure, MPa(g) 3.5 2.5 24 3 2 0 2.5 2 0 20406080100

2 Pipeline length, km 0 0 20406080100 Pipeline length, km Figure 6. Pressure (p) (continuous lines) and temperature (T) (dashed lines) changes for hydrogen

massFigure flow 6. Pressurerate of 0.3 ( pkg/s) (continuous (12,000 Nm lines)3/h) and and different temperature diameters. (T) (dashed lines) changes for hydrogen mass ﬂow rate of 0.3 kg/s (12,000 Nm3/h) and different diameters. Figure 6. Pressure (p) (continuous lines) and temperature (T) (dashed lines) changes for hydrogen mass flow rate7 of 0.3 kg/s (12,000 Nm3/h) and different diameters. 16

6.5 7 16 p, D=150mm 14 6 6.5 p, D=200mm p, D=250mm 5.5 p, D=150mm 14 6 T, D=150mm 12 p, D=200mm 5 T, D=200mm 5.5 p, D=250mm T, D=250mm 12 4.5 T, D=150mm 10 5 T, D=200mm 4 T, D=250mm 4.5 10 Pressure, MPa(g) 8 Temperature, °C 3.5 4 Pressure, MPa(g) 3 8 Temperature, °C 3.5 6 2.5 3 6 2 4 2.5 0 20406080100

2 Pipeline length, km 4 0 20406080100 Figure 7. Pressure (p) (continuous lines) and temperature (T) (dashed lines) changes for hydrogen Figuremass ﬂow 7. Pressure rate of 1.0 (p) kg/s (continuous (40,000 Nm lines)3/h)Pipeline and for temperature different length, diameters. km(T) (dashed lines) changes for hydrogen mass flow rate of 1.0 kg/s (40,000 Nm3/h) for different diameters. Figure 7. Pressure (p) (continuous lines) and temperature (T) (dashed lines) changes for hydrogen mass flow rate of 1.0 kg/s (40,000 Nm3/h) for different diameters.

Energies 2019, 12 FOR PEER REVIEW 12

Energies 2019, 12, 569 11 of 21 Energies 2019, 12 FOR PEER REVIEW 12 7 16

6.57 16 14 6.56 p, D=200mm 14 5.56 p, D=250mm p,p, D=200mm D=300mm 12 5.55 p,T, D=250mm D=200mm p,T, D=300mm D=250mm 12 4.55 T,T, D=200mm D=300mm 10 T, D=250mm 4.54 T, D=300mm 10 Pressure, MPa(g) 8 Temperature, °C 3.54 Pressure, MPa(g) 8 Temperature, °C 3.53 6 2.53 6 2.52 4 0 20406080100 2 4 0 20406080100Pipeline length, km

Pipeline length, km Figure 8. Pressure (p) (continuous lines) and temperature (T) (dashed lines) changes for hydrogen Figuremass flow 8. Pressure rate of 2.0 (p )kg/s (continuous (80,000 Nm lines)3/h) andfor different temperature diameters. (T) (dashed lines) changes for hydrogen Figure 8. Pressure (p) (continuous lines) and temperature (T) (dashed lines) changes for hydrogen mass ﬂow rate of 2.0 kg/s (80,000 Nm3/h) for different diameters. mass flow rate of 2.0 kg/s (80,000 Nm3/h) for different diameters. 6 16

5.56 16 14 5.5 5 p, D=250mm 14 p, D=300mm 12 4.55 p,p, D=250mm D=400mm p,T, D=300mm D=250mm T, D=300mm 12 4.5 p, D=400mm 4 T,T, D=250mm D=400mm 10 T, D=300mm 4 T, D=400mm 10 Pressure, MPa(g) 3.5 Temperature, °C 8 Pressure, MPa(g) 3.5 Temperature, °C 3 8 6 2.53 6 2.52 4 0 20406080100 2 Pipeline length, km 4 0 20406080100 Figure 9. Pressure (p) (continuous lines)Pipeline and temperature length, km (T) (dashed lines) changes for hydrogen Figure 9. Pressure (p) (continuous lines) and temperature (T) (dashed lines) changes for hydrogen mass flow rate of 3.0 kg/s (120,000 Nm3/h) for different diameters. mass flow rate of 3.0 kg/s (120,000 Nm3/h) for different diameters. 4.2. ThermodynamicFigure 9. Pressure Parameters (p) (continuous Analysis lines) and temperature (T) (dashed lines) changes for hydrogen 4.2. massThermodynamic flow rate of Parameters3.0 kg/s (120,000 Analysis Nm 3/h) for different diameters. The analysis of selected thermodynamic parameters, such as density, flow rate, compressibility 4.2.factor, ThermodynamicThe isobaric analysis heat of Parameters capacity selected and thermodynamicAnalysis Joule–Thomson parameters, coefficient weresuch as presented density, for flow hydrogen rate, compressibility transmission atfactor, a flow isobaric rate of 40,000 heat Nmcapacity3/h (1.0 and kg/s). Joule–Thomson Figure 10 illustrates coefficient the changes were in presented hydrogen densityfor hydrogen and its The analysis of selected thermodynamic parameters, such as density, flow rate, compressibility flow rate as a function of pipeline length for selected pipeline diameters. factor, isobaric heat capacity and Joule–Thomson coefficient were presented for hydrogen

Energies 2019, 12 FOR PEER REVIEW 13 Energies 2019, 12 FOR PEER REVIEW 13

transmission at a flow rate of 40,000 Nm3/h (1.0 kg/s). Figure 10 illustrates the changes in hydrogen transmission at a flow rate of 40,000 Nm3/h (1.0 kg/s). Figure 10 illustrates the changes in hydrogen Energiesdensity2019 and, 12, 569its flow rate as a function of pipeline length for selected pipeline diameters. 12 of 21 density and its flow rate as a function of pipeline length for selected pipeline diameters. 6 30 6 ρ, D=150mm 30 ρ, D=150mm ρ, D=200mm 5.5 ρ, D=200mm 5.5 ρ, D=250mm ρ, D=250mm w, D=150mm 25 5 w, D=150mm 25 5 w, D=200mm w, D=200mm

3 w, D=250mm

3 4.5 w, D=250mm 4.5 20 20 4 4 15 3.5 15 3.5Density,kg/m Density,kg/m Flow velocity, m/s Flow velocity, Flow velocity, m/s Flow velocity, 3 3 10 10 2.5 2.5

2 5 2 5 0 20406080100 0 20406080100 Pipeline length, km Pipeline length, km

Figure 10. Density (ρ) (continuous lines) and ﬂow velocity (w) (dashed lines) changes for hydrogen Figure 10. Density (ρ) (continuous lines) and flow velocity (w) (dashed lines) changes for hydrogen Figuremass ﬂow 10. Density rate 1.0 kg/s(ρ) (continuous (40,000 Nm lines)3/h). and flow velocity (w) (dashed lines) changes for hydrogen mass flow rate 1.0 kg/s (40,000 Nm3/h). mass flow rate 1.0 kg/s (40,000 Nm3/h). The density change is directly connected to compressibility factor Z change, as presented in The density change is directly connected to compressibility factor Z change, as presented in FigureThe 11 density. Compressibility change is factordirectlyZ for co hydrogennnected to is compressibility higher than unity factor even Z for change, relatively as presented low pressure in Figure 11. Compressibility factor Z for hydrogen is higher than unity even for relatively low Figurevalues. 11. This Compressibility is one of the most factor characteristic Z for hydrogen properties is higher of hydrogen, than unity which even distinguishes for relatively it fromlow pressure values. This is one of the most characteristic properties of hydrogen, which distinguishes it pressuremost of thevalues. real This gases. is one For of instance, the most compressibility characteristic properties factor Z > of 1 forhydrogen, natural which gas is distinguishes usually only atit from most of the real gases. For instance, compressibility factor Z > 1 for natural gas is usually only frompressures most equal of the or real greater gases. than For 40 instance, MPa. compressibility factor Z > 1 for natural gas is usually only at pressures equal or greater than 40 MPa. at pressures equal or greater than 40 MPa. 1.045 1.045 D=150mm D=150mm D=200mm 1.04 D=200mm 1.04 D=250mm D=250mm 1.035 1.035 1.03 1.03 1.025 1.025 1.02 1.02 Compressibility factor Z Compressibility factor Z 1.015 1.015 1.01 1.01 0 20406080100 0 20406080100 Pipeline length, km Pipeline length, km

Figure 11. Compressibility factor changes for hydrogen mass ﬂow rate 1.0 kg/s (40,000 Nm3/h). Figure 11. Compressibility factor changes for hydrogen mass flow rate 1.0 kg/s (40,000 Nm3/h). Figure 11. Compressibility factor changes for hydrogen mass flow rate 1.0 kg/s (40,000 Nm3/h). Another characteristic parameter of hydrogen is the Joule–Thomson coefficient. The Another characteristic parameter of hydrogen is the Joule–Thomson coefficient. The Joule–Thomson effect occurs in temperature change of the gas during isenthalpic pressure drop. For Joule–Thomson effect occurs in temperature change of the gas during isenthalpic pressure drop. For

Energies 2019, 12, 569 13 of 21

Another characteristic parameter of hydrogen is the Joule–Thomson coefficient. The Joule–Thomson effectEnergies occurs 2019, in12 FOR temperature PEER REVIEW change of the gas during isenthalpic pressure drop. For most14 real gases, the Joule–Thomson effect is positive (i.e., gas temperature decreases with pressure reduction). In themost case real of gases, hydrogen the Joule–Thomson (Figure 12), the effect opposite is positive effect (i.e., occurs gas temperature (i.e., the Joule–Thomson decreases with coefficientpressure is negative):reduction). with In a the rapid case change of hydrogen in pressure, (Figure the 12), hydrogen the opposite temperature effect occurs increases. (i.e., the ThisJoule–Thomson is theoretically possiblecoefficient for the is hydrogennegative): with transmission a rapid change pipelines, in pressure, with considerable the hydrogen pressure temperature drops increases. per unit ofThis pipeline is theoretically possible for the hydrogen transmission pipelines, with considerable pressure drops per length (e.g., with pipeline diameters which are too small or very high flow rates). unit of pipeline length (e.g., with pipeline diameters which are too small or very high flow rates).

14.44 -0.27 Cp, D=150mm Cp, D=200mm -0.275 14.42 Cp, D=250mm μJT, D=150mm μJT, D=200mm -0.28 14.4 μJT, D=250mm -0.285

14.38 -0.29

14.36 -0.295 -0.3 14.34 Isobaric heat capacity, kJ/(kg·K) -0.305 coefficient, K/MPa Joule-Thomson 14.32 -0.31

14.3 -0.315 0 20406080100 Pipeline length, km Figure 12. Isobaric heat capacity (Cp) (continuous lines) and Joule–Thomson coefficient (µ ) Figure 12. Isobaric heat capacity (Cp) (continuous lines) and Joule–Thomson coefficient (μJT) (dashed JT 3 (dashedlines) lines) changes changes for hydrogen for hydrogen mass flow mass rate flow 1.0 kg/s rate (40,000 1.0 kg/s Nm (40,0003/h). Nm /h). 4.3. Methane-Hydrogen Mixtures 4.3. Methane-Hydrogen Mixtures HydrogenHydrogen can can also also be be transmitted transmitted throughthrough natural natural gas gas pipelines pipelines as asan anadditive additive to natural to natural gas. gas. ThisThis is one is one of theof the alternative alternative methods methods of of hydrogenhydrogen transportation. Numerous Numerous analyses analyses and andstudies studies devoteddevoted to thisto issuethis haveissue been have performed been performed recently [21recently,29,40]. [21,29,40]. Hydrogen hasHydrogen different has thermodynamic different parametersthermodynamic compared parameters to methane, compared which to is methane, the main which component is the main of natural component gas. Thisof natural causes gas. significant This changescauses in significant the flow conditions changes in ofthe natural flow conditions gas that contains of natural hydrogen. gas that contains Industrial hydrogen. practice Industrial and scientific publicationspractice and indicate scientific that publications the maximum indicate admissible that the molar maximum fraction admissible of hydrogen molar in afraction mixture of with naturalhydrogen gas should in a mixture not exceed with natu 15%.ral The gas maximumshould not hydrogenexceed 15%. content The maximum in the natural hydrogen gas content transmission in systemthe suggestednatural gas in transmission the United Statessystem should suggested be in in the the range United of 5%–15%.States should Several be Europeanin the range countries of 5%–15%. Several European countries have introduced limits on the content of hydrogen in natural have introduced limits on the content of hydrogen in natural gas pipeline systems from 0.1% to gas pipeline systems from 0.1% to 12% by volume. The maximum hydrogen content usually 12% by volume. The maximum hydrogen content usually depends on the technical conditions for a depends on the technical conditions for a given pipeline [41]. Hydrogen significantly influences givennatural pipeline gas transmission [41]. Hydrogen conditions. significantly The basic influences advantage natural is lowering gas transmission the pressure conditions. drop of natural The basic advantagegas transmitted is lowering with the a hydrogen pressure admixture, drop of natural and po gasssibility transmitted to transmit with natural a hydrogen gas across admixture, longer and possibilitydistances to transmitwithout additional natural gas gas across compression longer distances stations. withoutUnfortunately, additional the gashydrogen compression content stations. in Unfortunately,natural gas significantly the hydrogen deteriorates content in the natural energy gas parameters significantly and calorific deteriorates value theof natural energy gas. parameters The andchanges calorific in value higher of heating natural value gas. The(HHV) changes and lower in higher heating heating value value(LHV), (HHV) calorific and value lower of heatingthe valuemixture, (LHV), and calorific Wobbe value index of as the a function mixture, of and hydrogen Wobbe content index asin the a function methane of mixture hydrogen are contentpresented in the methanein Figure mixture 13. are presented in Figure 13.

EnergiesEnergies 20192019,, 12 ,FOR 569 PEER REVIEW 14 of15 21 Energies 2019, 12 FOR PEER REVIEW 15

55 55 50 50 45 45 40 40 3

3 35 35

MJ/Nm 30

MJ/Nm 30 25 HHV 25 HHV 20 LHV 20 LHV Wobbe H 15 Wobbe H 15 Wobbe L 10 Wobbe L 100 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 Molar fraction of hydrogen in mixture with methane Molar fraction of hydrogen in mixture with methane

FigureFigure 13. Changes of higher heatingheating valuevalue (HHV),(HHV),lower lower heating heating value value (LHV) (LHV) and and Wobbe Wobbe indexes indexes as Figure 13. Changes of higher heating value (HHV), lower heating value (LHV) and Wobbe indexes asa functiona function of of hydrogen hydrogen molar molar fraction fraction in in mixture mixture with with methane. methane. as a function of hydrogen molar fraction in mixture with methane. TheThe pressure pressure and and temperature temperature changes as a a function function of of pipeline pipeline length length for for a a mixture mixture of of methane methane The pressure and temperature changes as a function of3 pipeline length for a mixture of methane andand 15% 15% hydrogen transmitted at a flow ﬂow rate of 40,000 NmNm3/h are are presented presented in in Figure Figure 1414 forfor three three and 15% hydrogen transmitted at a flow rate of 40,000 Nm3/h are presented in Figure 14 for three recommendedrecommended pipeline pipeline diameters. diameters. The The change change in in th thermodynamicermodynamic parameters parameters wa wass also also analyzed analyzed as as a a recommended pipeline diameters. The change in thermodynamic parameters was also analyzed as a functionfunction of of molar molar fraction fraction of of hydrogen hydrogen in the mixture for a 250 mm diameter. function of molar fraction of hydrogen in the mixture for a 250 mm diameter. 5 16 5 16

4.5 14 4.5 14 p, D=250mm p,p, D=300mm D=250mm 4 p,p, D=400mm D=300mm 12 4 T,p, D=250mm D=400mm 12 T,T, D=300mm D=250mm 3.5 T,T, D=400mm D=300mm 10 3.5 T, D=400mm 10 Pressure, MPa(g) 3 8 Temperature, °C Pressure, MPa(g) 3 8 Temperature, °C

2.5 6 2.5 6

2 4 20 204060801004 0 20406080100 Pipeline length, km Pipeline length, km Figure 14. Pressure (p) (continuous lines) and temperature (T) (dashed lines) changes for methane–15% Figurehydrogen 14. mixture Pressure mass (p ﬂow) (continuous rate of 6.93 lines) kg/s (40,000and temperature Nm3/h) for different(T) (dashed diameters. lines) changes for Figure 14. Pressure (p) (continuous lines) and temperature (T) (dashed lines) changes for methane–15% hydrogen mixture mass flow rate of 6.93 kg/s (40,000 Nm3/h) for different diameters. methane–15% hydrogen mixture mass flow rate of 6.93 kg/s (40,000 Nm3/h) for different diameters. This analysis of the hydrogen molar fraction in the hydrogen/methane mixture confirms the This analysis of the hydrogen molar fraction in the hydrogen/methane mixture confirms the effect of pressure drop in the analyzed pipeline. The required inlet pipeline pressure, for a 15% effect of pressure drop in the analyzed pipeline. The required inlet pipeline pressure, for a 15%

Energies 2019, 12, 569 15 of 21

EnergiesThis 2019 analysis, 12 FOR PEER ofthe REVIEW hydrogen molar fraction in the hydrogen/methane mixture confirms the16 effect of pressure drop in the analyzed pipeline. The required inlet pipeline pressure, for a 15% hydrogenhydrogen content content in in the the gas gas mixture, mixture, is is approxim approximatelyately 10% 10% lower lower when when compared compared to to pure pure methane methane (Figure(Figure 15).15). An An increase increase in in hydrogen hydrogen molar molar cont contentent caused caused the the temperature temperature of of the the analyzed analyzed gas gas mixturemixture to to more more rapidly rapidly approach approach the the ambient ambient te temperature.mperature. On On the the other other hand, hand, the the Joule–Thomson Joule–Thomson effecteffect for for methane methane caused caused slight slight cooling cooling of of the the anal analyzedyzed mixtures, mixtures, and and for for pure pure hydrogen hydrogen this this effect effect diddid not not take take place. place. TheThe variability variability of of analyzed thermodynamic para parametersmeters of of a methane/hydrogen mixture mixture as as a a functionfunction of of hydrogen hydrogen molar molar fraction fraction is is shown shown in in Figures Figures 16–18. 16–18 The. The analyzed analyzed mixtures mixtures contained contained a maximuma maximum 15% 15% of ofhydrogen; hydrogen; studies studies and and practice practice have have shown shown that thatthis thisamount amount of hydrogen of hydrogen in the in methanethe methane or natural or natural gas gasmixture mixture does does not not significantly significantly affect affect the the transmission transmission parameters parameters of of the the pipeline.pipeline. However, However, it it should should be be noted noted that that a a change change in in density density is is important: important: it it can can be be altered altered by by up up to to 20%20% for for a a 15% 15% content content of of hydrogen hydrogen in in the the mixture mixture when when compared compared to to pure pure methane. methane. Apart Apart from from a considerablea considerable lowering lowering of ofdensity, density, the the Joule–Thomson Joule–Thomson effect effect is isalso also lowered lowered with with an an increase increase in in hydrogenhydrogen content. content. With With increased increased hydrogen hydrogen content, content, it itss specific specific heat heat also also incr increaseseases for for a a unit unit of of mass mass duedue to to the the high high calorific calorific value value of of hydrogen hydrogen per per unit unit of of mass. mass. The The compressibility compressibility factor factor has has been been also also observedobserved to to grow grow significantly. significantly. The The addition addition of of hydrogen hydrogen improves improves to to some some extent extent the the natural natural gas gas transmissiontransmission conditionsconditions byby pressure pressure drop drop reduction reduction in thein the pipeline. pipeline. However, However, hydrogen hydrogen content content above above15%–20% 15%–20% in the in gas the mixture gas mixture significantly significantly influences influences the calorific the calorific value of value natural of natural gas. The gas. change The changein thermodynamic in thermodynamic conditions conditions with an increasedwith an in hydrogencreased hydrogen content may content also affectmay also the naturalaffect the gas naturaltransmission gas transmission system (i.e., gassystem compression (i.e., gas stationscompressi oron gas stations reduction or stations).gas reduction Another stations). important Another issue importantis selection issue of the is material selection for pipelineof the material construction for pipeline in light of construction the hydrogen in corrosion light of casethe [hydrogen42], which corrosionaffects the case cost [42], of its which construction affects the [20 ,cost43]. of its construction [20,43].

5.5 16

5 14

12 4.5 10 4 8 3.5 6 Pressure, MPa(g) Temperature, Temperature, °C 3 4 p, Pure CH4 p, 5% of H2 2.5 p, 10% of H2 p, 15% of H2 2 T, Pure CH4 T, 5% of H2 T, 10% of H2 T, 15% of H2 2 0 0 20406080100 Pipeline length, km

Figure 15. Pressure (p) (continuous line) and temperature (T) (dashed line) changes as a function of Figurehydrogen 15. contentPressure in (p mixture) (continuous with methane line) and and temperature pipeline length. (T) (dashed line) changes as a function of hydrogen content in mixture with methane and pipeline length.

Energies 2019, 12 FOR PEER REVIEW 17 Energies 2019, 12, 569 16 of 21 Energies 2019, 12 FOR PEER REVIEW 17 40 10 40 10 35 9 35 9 30 8

3 30 8

3 25 7 25 7 20 6 20 6 Density,kg/m Flow velocity, m/s Flow velocity,

Density,kg/m 15 5 Flow velocity, m/s Flow velocity, 15 ρ, Pure CH4 ρ, 5% of H2 5 10 ρ, Pure10% CH4of H2 ρ, 5%15% of of H2 H2 4 10 ρ,w, 10%Pure ofCH4 H2 ρ,w, 15%5% of of H2 H2 4 w, Pure10% CH4of H2 w, 5%15% of of H2 H2 5 3 w, 10% of H2 w, 15% of H2 5 0 204060801003 0 20406080100Pipeline length, km Pipeline length, km Figure 16. Density (ρ) (continuous lines) and flow velocity (w) (dashed lines) changes as a function of Figure 16. Density (ρ) (continuous lines) and ﬂow velocity (w) (dashed lines) changes as a function of Figurehydrogen 16. contentDensity and (ρ) (continuouspipeline length. lines) and flow velocity (w) (dashed lines) changes as a function of hydrogen content and pipeline length. hydrogen content and pipeline length. 2.8 5 2.8 5 2.75 2.75 2.7 4.5 2.7 4.5 2.65 2.65 2.6 4 2.6 4 2.55 2.55 2.5 3.5 2.5 3.5 2.45 2.45 2.4 Cp, Pure CH4 Cp, 5% of H2 3

Isobaric heat capacity, kJ/(kgK) Cp, 10%Pure ofCH4 H2 Cp, 5%15% of of H2 H2

2.4 3 Joule-Thomson coefficient, K/MPa 2.35 Isobaric heat capacity, kJ/(kgK) Cp,μJT, 10% Pure of CH4 H2 Cp,μJT, 15% 5% ofof H2H2 Joule-Thomson coefficient, K/MPa 2.35 μJT, 10%Pure ofCH4 H2 μJT, 5%15% of of H2 H2 2.3 2.5 μJT, 10% of H2 μJT, 15% of H2 2.3 0 204060801002.5 0 20406080100Pipeline length, km Pipeline length, km Figure 17. Isobaric heat capacity (Cp) (continuous lines) and Joule–Thomson coefﬁcient (µJT) Figure 17. Isobaric heat capacity (Cp) (continuous lines) and Joule–Thomson coefficient (μJT) (dashed (dashed lines) changes as a function of hydrogen content and pipeline length. lines)Figure changes 17. Isobaric as a functionheat capacity of hy (drogenCp) (continuous content and lines) pipeline and Jo length.ule–Thomson coefficient (μJT) (dashed lines) changes as a function of hydrogen content and pipeline length.

Energies 2019, 12, 569 17 of 21 Energies 2019, 12 FOR PEER REVIEW 18

0.98

0.97

0.96

0.95

0.94

0.93

0.92

0.91 Compressibility factor 0.9 Pure methane 5% of hydrogen 0.89 10% of hydrogen 15% of hydrogen 0.88 0 20406080100 Pipeline length, km

Figure 18. Compressibility factor changes as a function of hydrogen content and pipeline length. Figure 18. Compressibility factor changes as a function of hydrogen content and pipeline length. 5. Discussion 5. Discussion Hydrogen pipeline transmission analysis discussed an example of 100 km long pipeline and the requiredHydrogen outlet pressure pipeline of transmission 24 bar (g). Pressure analysis drop discussed and temperature an example change of 100 profileskm long were pipeline determined and the forrequired selected outlet pipeline pressure diameters of and24 bar mass (g). flow Pressu ratesre from drop 0.3 toand 3.0 temperature kg/s, which correspondschange profiles to volume were flowdetermined rates from for 12,000 selected to 120,000pipeline Nm diameters3/h of hydrogen. and mass An flow analogous rates analysisfrom 0.3 has to been3.0 kg/s, performed which 3 forcorresponds a mixture to of volume methane flow and rates hydrogen from 12,000 with a to maximum 120,000 Nm hydrogen/h of hydrogen. content of An 15%. analogous Due to theanalysis fact thathas hydrogenbeen performed has a significantly for a mixture lower of methane mass than and methane, hydrogen the with pipeline a maximum transmission hydrogen of methane content and of hydrogen15%. Due mixtures to the fact requires that largerhydrogen pipeline has diametersa significantly for similar lower volumemass than flow methane, rates. On the the basispipeline of thetransmission determined of profiles methane of pressureand hydrogen and temperature mixtures requires changes larger in the pipeline,pipeline adiameters detailed analysisfor similar of thermodynamicvolume flow rates. parameters On the basis was of performed. the determined This profiles analysis of was pressure important and temperature from a hydrogen changes and in methane–hydrogenthe pipeline, a detailed mixture analysis pipeline of thermodynami transmission perspective.c parameters In was particular, performe thed. analysisThis analysis included was density,important flow from velocity, a hydrogen isobaric and heat methane–hydrogen capacity, the compressibility mixture factor,pipeline and transmi Joule-Thomsonssion perspective. coefficient. In Theparticular, last two the parameters, analysis whichincluded significantly density, flow affect velocity, the conditions isobaric of transport,heat capacity, are specific the compressibility to hydrogen. Hydrogenfactor, and has Joule-Thomson negative values coefficient. of Joule-Thomson The last coefficienttwo parameters, and the which compressibility significantly factor affect value the exceedsconditions 1.0 of in transport, the range ofare relatively specific to low hydrogen pressures.. Hydrogen Due to differenthas negative thermodynamic values of Joule-Thomson parameters, thecoefficient hydrogen and content the compressibility in a mixture withfactor methane value exceeds causes 1.0 significant in the range changes of relatively in natural low gas pressures. pipeline transportDue to different conditions. thermodynamic The most important parameters, change the is hydrogen the reduction content of pressure in a mixture drop, whichwith methane allows ancauses increase significant in the distancechanges atin whichnatural it gas is possiblepipeline totransport transport conditions. the assumed The volumemost important of natural change gas. Significantis the reduction differences of pressure also occur drop, in which the temperature allows an increase changeprofiles in the distance in the pipeline, at which as it the is possible hydrogen to contenttransport reduces the assumed the positive volume Joule-Thomson of natural gas. effect Significant for natural differ gas.ences It also should occur be in noted the temperature that in the simulationchange profiles it was in assumedthe pipeline, that as full the heat hydrogen transfer content between reduces the transported the positive gasJoule-Thomson and the ambient effect environmentfor natural gas. may It occur. should The be molar noted fraction that in ofthe hydrogen simulation in ait mixture was assumed with natural that full gas heat may transfer have a beneficialbetween influencethe transported on the conditionsgas and the of ambient its transmission. environment However, may hydrogenoccur. The content molar shouldfraction not of exceedhydrogen 15%–20% in a mixture in the mixture.with natura Withl gas a hydrogen may have content a beneficial above 20%influence in the on mixture the conditions with methane, of its thetransmission. higher heating However, value (HHV)hydrogen drops content below should 35 MJ/Nm not 3exceed; thus, natural15%–20% gas in loses the its mixture. calorific With value a (belowhydrogen values content described above by 20% norms in andthe mixture standards), with even methane, though the the higher upper Wobbeheating index value remains (HHV) within drops 3 acceptedbelow 35 norms. MJ/Nm ; thus, natural gas loses its calorific value (below values described by norms and standards), even though the upper Wobbe index remains within accepted norms.

Energies 2019, 12, 569 18 of 21

6. Conclusions The main objective of this paper was to analyze the possibilities of pipeline transmission of hydrogen and methane/hydrogen mixtures. This analysis has been performed to determine the impact of hydrogen content on the conditions of natural gas transmission, the main component of which is methane. It should be emphasized that hydrogen will play an increasingly important role as an energy carrier in the global economy, particularly for energy storage. The economic environment for the use of hydrogen should be favorable in the coming years. A steady increase of renewable energy content in the total energy balance in all regions of the world, and the growing irregularities in power generation and usage, will have an important role in the field of hydrogen utilization. Hydrogen pipeline transmission is the most effective method for transporting significant amounts of hydrogen over long distances, in particular for its storage, when the appropriate storage site for hydrogen is located at a considerable distance from the source of its generation due to geological conditions (suitable locations for salt cavern). In addition, technological and technical issues related to pipeline transmission of natural gas with an increased hydrogen content should be considered, where a significant change in thermodynamic parameters may also affect the operational conditions of installations associated with the transmission system. An additional problem is the impact of the increased molar fraction of hydrogen on the pipe material.

Author Contributions: Conceptualization, T.W. and M.Ł.; formal analysis, T.W.; funding acquisition, M.Ł. and A.S.; investigation, T.W. and S.K.; methodology, T.W.; supervision, M.Ł. and A.S.; validation, T.W. and A.O.; visualization, T.W.; writing—original draft, T.W.; writing—review and editing, S.K. and T.W. Funding: This work received funding from the Statutory Research of Natural Gas Department at Drilling Oil & Gas Faculty, no. 11.11.190.555. Conﬂicts of Interest: The authors declare no conﬂict of interest.

Nomenclature

4 2 am Peng Robinson EOS parameter, N·m /mol 3 bm Peng Robinson EOS parameter (co-volume), m /mol A, B Dimensionless Peng Robinson EOS parameters Cp Isobaric heat capacity, J/(kg·K) or J/(mol·K) Cpid Ideal gas isobaric heat capacity, J/(kg·K) or J/(mol·K) Cpr Real gas (residual) isobaric heat capacity, J/(kg·K) or J/(mol·K) d Relative density of gas D Pipeline inner diameter, m g Gravity constant, m/s2 ∆h Elevation level difference, m k Thermal conductivity, W/(m·K) L Pipeline segment length, m Le Equivalent pipeline segment length, m • M Mass ﬂow rate, kg/s P Pressure, Pa p1 Pipeline inlet pressure, Pa p2 Pipeline outlet pressure, Pa pav Pipeline average pressure, Pa pb Base pressure, Pa 3 Qn Volume ﬂow rate under normal conditions, Nm /s R Pipeline inner radius, m R Gas constant, J/(mol·K) Re Reynolds number Energies 2019, 12, 569 19 of 21

T Fluid temperature, K Tb Base temperature, K Tin Temperature in pipeline, K Tout Ambient temperature, K U Overall heat transfer coefficient, W/(m2·K) V Molar volume, m3/mol Z Compressibility factor, zx Depth of pipeline burial, m A Convective heat transfer coefficient, W/(m2·K) E Relative pipeline roughness Λ Linear friction factor µJT Joule–Thomson coefficient, K/Pa P Fluid density, kg/m3

Abbreviations

RES Renewable energy sources HHV Higher heating value LHV Lower heating value J–T Joule–Thomson

References

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