“Comparative production of hydrogen from natural gas by

“Comparative study of environmental aspects in the production of Hydrogen energy between natural gas steam reforming method and solar energy.”

Abstract

Identifying and building a sustainable energy system are perhaps two of the most critical issues that today’s society must address, hydrogen as energy carrier primarily delivered from water, can address issues of sustainability, environmental emissions, and energy security. A wide variety of processes are available for hydrogen production from conventional fuels and renewable energy sources (RES), the formation processes of hydrogen include steam reforming, partial oxidation, auto thermal, Electrolysis and other method to form hydrogen from water. Increasing impacts of global warming have made it a global priority to phase out the use of petrol and diesel as transportation fuel in favors of hydrogen energy. Production of hydrogen from conventional sources e.g. fossil fuels, natural gas etc involve in utilizing fossil fuel additionally  and CO2 emission to the environment, indeed as much as 2.5-5 tons of carbon is set free as a CO2 per ton of Hydrogen currently produced by conventional means. Though hydrogen produced by renewable energy resources particularly solar energy shown that these methods is considerably environment friendly having zero CO2 emission and by product in renewable energy resources  methods is mostly oxygen and can be later released in the atmosphere. Conclusively the production of hydrogen can be carbon free only if it is generated by renewable energy sources. The present comparison study will be discussing environmental aspects in the process of hydrogen energy production from natural gas e.g. methane, ethane, pentane and solar energy.

Introduction

Dramatic increment is predicted in the next decades for global energy consumption, impelled by mounting standard of living and population growth. According to International Energy Agency (IEA), this demand of more energy will require more growth in energy generation capacity and more secure and diversified energy sources,IEA, 2011, USDOE, 2009. In short term or long term scenario sustainable energy is becoming more important in the light of environmental degradation, global warming and diminishing of Non-renewable energy resources. Hydrogen in the capacity of energy carrier is expected to be optimum solution, (Kothari, Buddhi et al 2008). In the context of hydrogen as a energy source many researcher argue that it appears as one of the better option for mid and long terms, (J. Dufour et al 2009).

Hydrogen is produced by three different energy-supply system, namely, fossils fuel, nuclear reactor and renewable energy resources, (Kothari, Buddhi et al 2008). However, this article will be specifically concerned with Environmental impacts and aspects, in the production of hydrogen from natural gas by Steam methane reforming method (SMR) and electrolysis by using electricity source from solar panel.

SMR is endothermic conversion of methane and water vapor into hydrogen and carbon monoxide CO. The CO is further converted to CO2 and H2 through the water-gas shift reaction. SMR is 80-85% is world’s total hydrogen production source, even though one of the main problems associated with this method is its great impact on the environment, (Boyano, Blanco-Marigorta et al 2011).

Solar photovoltaic power is used in photo-electrolysis to directly convert water molecule into hydrogen and oxygen. Photo-electrolysis of water is the process whereby light is used to split water molecule, such system offer great potential for cost reduction and efficiency improvement of electrolytic hydrogen, (Bhandari, Trudewind et al 2012). Some photo catalysts are used here for absorption of visible light also to transmit appropriate wavelength.

 The environmental effect of hydrogen production from natural gas SMR, which is today the main path of production, is needed to be compared with environmental effects of production chain by the use of solar photovoltaic power, (Kothari, Buddhi et al 2008). The main focus of this article to show the environmental sustainability of the above mention methods to produce hydrogen. To do so several literature is been reviewed, additionally Life cycle assessments (LCA) study of the concerned production methods is been reviewed as well. On these bases we will get our desired results.

2. Environmental impacts and effects

2.1 Literature review and LCA for Steam methane reforming method.

SMR is endothermic conversion of methane and water vapour  into hydrogen and carbon monoxide, (eq. 1). Methane feed gas is combusted for the supply of heat, the process temperature requirement is 700-850 °C and pressures of 3 to 25 bar. Approximately 12% of CO is present as a product gas, (Bhandari & Trudewind,  2012). Water gas shift reaction is used to further convert CO into CO2 and H2 (eq. 2) . SMR constitutes rougly 50% of the global annual production of hydrogen of about 500 billion m3 Harrison & Ivy?Levene, 2008.

CH4 + H2O + Heat  ?   CO + 3H2                    (eq. 1)

CO + H2O ?  CO2 + H2 + Heat                       (eq. 2)

The process can be seen in a flow diagram in fig. 1. The produced hydrogen contains impurities, e.g CO2 and other traces, and it should be separated. Commonly used purification process is pressure swing absorption (Bhandari & Trudewind,  2012).       source; Gas technology insititute

The benefits this method have is, it is most viable approach to begin hydrogen market in near term, lowest current cost and existing feedstock infrastructure, however some challenges is also related which is high capitol cost, high operation and maintenance costs and definitely contribution to GHG emissions, (Bhandari & Trudewind,  2012).

In USA, According to National Renewable Energy Laboratory (NREL) block flow diagram of the SMR method is given, whereby before steam reforming, the CH4 is pretreated in a hydrogenation vessel in order to convert any sulfur compounds to H2S, the H2S is then removed in a ZnO bed. After pretreatment, the natural gas and steam are fed to the steam reformer. The produced gas is then fed to high temperature shift (HTS) and LTS reactors where the water gas shift reaction converts 92% of the CO into H2. Pressure swing adsorption (PSA) unit is used to purify the hydrogen. PSA off gas is primarily used to fueled reformer and also some amount of natural gas, (Span & Mann, 2001). Figure 2 shows the flow diagram.

     Figure 2                                Hydrogen plant block flow diagram

        Source NREL, US

2.2 Photovoltaic-electrolysis system

Photolysis is process When water molecules absorb energy at a rate of 285.57 kj/mole of water from ultraviolet radiation, hydrogen in principle is produced, (Kothari, Buddhi et al 2008).Photocatalysts are used to absorb visible light and transmit the energy of appropriate wavelengths, (eq. 3). Sunlight has been directly converted into electricity by photovoltaic cell. Photoelectrolyzer is a device which is made by the combination of photovoltaic cell and electrolyzer to generate hydrogen. Hydrogen generation begins when the photoelectrolyzer is placed is water and exposed to sunlight. The hydrogen is then collected and stored, (Fig. 2). Electricity obtain from other sources can also be used in the electrolysis process to get hydrogen. The basic arrangement to use for electrolysis is Hoffman Voltammeter apparatus, (Kothari, Buddhi et al 2008).

 

Figure 2. Photovoltaic/electrolysis production system block diagram.  

Source Kothari & Buddhi, 2008).

 

H2O + X + Light  ?  Reduced X + 2H+ + 1/2 O2                (eq. 3)

Reduced X +  2H?     ?  X + H2

                                                                                         .

H2O+X+Light?H2+1/2O2+X

 

3. Comparative  environmental aspects and impacts ( literature review and life cycle assessment)

Kothari & Buddhi, 2008 have reported a big disadvantage of the SMR processes is that the production of hydrogen is accompanied by the emission of large quantities of CO2 into atmosphere as it uses fossil fuels both in manufacturing process and as the heat source. The quantity of CO2 emitted per kg of hydrogen production were calculated with natural gas (Methane, Ethane, Pentane, Naptha) feed at 75% average efficiency and are show in the table1. Sulfur oxide has been ignored because it is negligible. Comparison is shown in table 1 between SMR and photovoltaic electrolysis, emission of CO2 is varied from 7.33 to 9.46 kg of per kg hydrogen produced using natural gas. The CO2 emissions are zero, if renewable energy source like solar is used.

 

 

 

Process

Reactions

H2
 from H2O (%)

CO2/H2
(Nm3/Nm3)

CO2 and CO emission at per kg of H2production at 75% System efficiency
CO2(kg)

Steam reforming

CnHm+nH2O?nCO+(n+m/2) H2

+CO conversion

nCO+nH2O?nCO2+nH2

From

Methane

CH4+2H2O?CO2+4H2

50.0

0.25

7.33

Ethane

C2H6+4H2O?2CO2+7H2

57.1

0.29

8.38

Pentane

C5H12+10H2O?5CO2+16H2

62.5

0.31

9.17

Naphtha

C10H22+20H2O?10CO2+31H2

64.51

0.32

9.46

Electrolysis (PV)

2H2O?2H2+O2

        100          0

0

 

 

Source Kothari & Buddhi, 2008

Cetinkaya et al, 2012 have reported the LCA for above two methods of hydrogen productions, productions capacity of PV electrolysis is relatively higher than SMR. It is also reported that Global Worming Potential (GWP) of PV is much less than the SMR. Table 2 show a summary.

Table 2

        
               H2 production methods

H2 production capacity
       (kgH2/day)

GWP (kgCO2
      eq./kgH2)

      Steam reforming of natural gas

            111,200

          11.9

       Water electrolysis via PV energy

               160

           2.4

  Source cetinkaya, 2012 

Cetinkaya, 2012 also assessed energy equivalent and Carbon dioxide equivalent for both production methods. Figure 3 shows the predicted energy equivalent and CO2 equialent emission percentage for Natural Gas Steam Reforming (NGSR), 96% of the energy is used in power plant construction and decommissioning, however the main source for CO2 emissions is caused by the plant operation. The overall energy equivalent and CO2 emissions of the four main process for the NGSR utility are given in table 3, the transportation data were calculated for chosen location.

Table 3 Energy and carbon dioxide equivalents of the processes for NGSR utility

Process

Energy equivalent kJ/kg H2

CO2 equivalent emissions g CO2-e/kg H2

Construction and decommissioning of the plant

159,600

2972.00

Natural gas production and transport

4150

52.84

Electricity generation (steam export excluded)

1910

273.42

Operation

8595.02

Total

165,660

11,893.28

                                                                                                                                   . 

 Fig. 3. (a) Energy equivalent and (b) Carbon dioxide equivalent percentages for NGSR                                                                                    

Source cetinkaya, 2012 

According to Cetinkaya, 2012, Table 4 shows the solar power plant energy equivalents and CO2 equivalent emissions for the production of hydrogen. Energy and CO2 equivalent percentage for solar PV electrolysis are shown in figure 4.    

Figure 4; Source cetinkaya, 2012 

Processes

Energy equivalent    kJ/kg H2

CO2 equivalent emissions  g CO2-e/kg H2

Materials and manufacturing of PV modules

25,550.48

1519.53

Transportation

602.53

461.36

Inverters

830.91

110.93

Wiring

602.41

60.24

Installation

2679.68

37.18

Operation and maintenance

2285.00

161.20

Decommissioning and disposal

893.23

61.70

Total

33,444.24

2412.13        

Figure 4. Source cetinkaya, 2012 

Granovskii et al., 2006 examined various hydrogen production methods, their analysis include the energy demand and GWP. They concluded that solar electrolysis is advantageous by resulting in less air pollution compared to natural gas reforming method. Corresponding emissions for natural gas SMR and Solar electrolysis are reported to be about 85 and 30 gCO2 eq./MJH2 (based on low heating value LHV).

Koroneos et al,. 2004 performed LCA study to compare the environmental impact of hydrogen production methods. Their study is based on the Global Emission Model for Integrated Systems (GEMIS) database. The examined impact categories are GWP, AP, EP and solid particulate matter SPM. SMR method is reported to have negative impacts on environment by releasing high equivalent emissions of CO2 and SO2. Methane (CH4) emissions, which primarily come from natural gas losses to the atmosphere during production and distribution, have large effect on the GWP of the system. It is also reported that CO2 equivalent emission for natural gas option is double to that for PV system (0.08 vs. 0.04 kg/MJH2).

Ozbilen et al., 2011a studied LCA of commercial production methods including all the major process steps involved in every method. Using both CML 2001 and Eco-indicator 95 methods they examined the energy consumption and the CO2 equivalent emissions of each production method. The results showed that steam reforming of natural gas has the highest environmental impact. The study considered only GWP and Acidification potential AP. The results for concerned production methods is shown in figure 5.

Figure 5.  

H2 production method

         GWP
     (gCO2eq./kgH2)

            AP
    (gSO2eq./kgH2)

Natural gas steam reforming

         12,000

             14.52

Solar based electrolysis

         2,000

             8.07

 Source Ozbilen et al., 2011a

 

Wulf & Kaltschmitt, 2012 have analyzed the overall life cycle hydrogen production at a hydrogen refueling station in Germany. Their results shows that the hydrogen production based on water electrolysis fed by German grid electricity mix of 2010 ( 16% renewable, 22% nuclear and rest fossil)should be avoided to reduce GHG emissions from life cycle perspective. SMR method is better than grid based electrolysis in terms of GHG reduction because of efficiency losses in fossil fuel, electricity and electrolysis route.

4. Results  

Environmental hazardous effects can be seen when hydrogen is produced from SMR method by releasing carbon dioxide equivalent to the atmosphere. Table 3 and 4 shows that CO2 emissions equivalent is 11,893.28 g CO2-e/kg H2 and 2412.13 g CO2-e/kg H2 for SMR method and PV electrolysis respectively.

Global warming potential (GWP) is reported to be eminent in steam reforming method than by electrolysis from solar energy. LCA study done by Cetinkaya, 2012 shows that GWP of SMR is 11.9kgCO2eq./kgH2, whereby GWP for PV electrolysis is 2.4 kgCO2eq./kgH2.

Acidification potential (AP) in SMR method is violent from solar energy production method, shown in table 5. According to LCA study by Ozbilen 2011a, it is reported that AP for natural gas steam reforming is 14.52 gSO2eq./kgH2 and AP value for solar based electrolysis is 8.07gSO2eq./kgH2. GWP is also stated by Ozbilen et al., 2011a. SMR is reported to have GWP of 12,000 gCO2eq./kgH2 and electrolysis from solar panel is reported to have 2,000gCO2eq./kgH2.

 

5. Conclusion

Analysis of various literature and life cycle assessment (LCA) associated with present and advanced technology of hydrogen production from steam reforming methane and electrolysis by using solar energy, conclude that

Hydrogen production from Steam reforming methane is relatively more hazardous to environment by releasing various pollutants to the atmosphere. GWP and AP are reported to be relatively high.

PV electrolysis reported to have high potential for H2 production from environmental point of view, although its efficiency is quite low. It produces almost 100% clean hydrogen and shows the lowest greenhouse gases emissions.

Nevertheless PV electrolysis is relatively expensive process tell now. In near future scenario SMR is relatively better option for commercial production of H2. However carbon capture and storage CCS technique installation in production facility can be used to minimize the hazardous effects to the environment.