Power-to-Hydrogen in the German industry sector. Potential and impact on the energy system

Contents

Abstract

Statement of Academic Integrity

Declaration for the Transfer of the Thesis

1. Introduction
1.1. Decarbonization of non-energetic fossil fuel use
1.2. Importance of hydrogen in the industry sector
1.3. Power-to-X to advance decarbonization
1.4. Goal and approach
1.5. urbs optimization model
1.5.1. General
1.5.2. Model
1.5.2.1. Input
1.5.2.2. Output
1.5.3. German electricity model

2. Literary research
2.1. Hydrogen production technologies
2.1.1. Overview
2.1.2. Conventional hydrogen production
2.1.2.1. Steam reforming
2.1.2.2. Partial Oxidation
2.1.2.3. Autothermal reforming
2.1.2.4. Coal gasification
2.1.3. Electricity-based hydrogen production
2.1.3.1. Electrolysis
2.1.3.2. Low temperature electrolysis
2.1.3.3. High temperature electrolysis
2.2. Hydrogen storage
2.2.1. Storage technologies
2.2.2. Typical characteristics of salt cavern hydrogen storage
2.2.3. Potential for underground hydrogen storage in Germany

3. First results
3.1. Hydrogen demand in German industry sector
3.1.1. Method
3.1.2. Major hydrogen consumers in the industry
3.1.2.1. Chemical industry
3.1.2.2. Refineries
3.1.2.3. Steel industry
3.1.2.4. Glass industry
3.1.2.5. Overview of hydrogen consumer sites
3.1.3. Results on federal state level for 2016
3.1.4. General trends
3.1.5. Results on federal state level for 2030
3.1.6. Results on federal state level for 2050
3.2. Hydrogen production in Germany
3.2.1. Hydrogen as a by-product
3.2.2. Hydrogen production sites in Germany
3.3. Impact of Power-to-Hydrogen
3.3.1. Potential for decarbonization
3.3.2. Impact on CO2 emissions
3.3.2.1. Reforming of natural gas and naphtha
3.3.2.2. Partial oxidation of heavy oils
3.3.2.3. Savings in 2016
3.3.2.4. Savings in 2030 and 2050
3.3.3. Impact on fossil fuel use
3.3.4. Impact on energy use

4. Implementation in urbs
4.1. Input data
4.2. Scenarios
4.2.1. Results for 2016
4.2.2. Results for 2030
4.2.3. Results for 2050
4.3. Discussion of results and comparison with prior finding

5. Conclusion and outlook

6. List of Figures

7. List of Tables

Appendix A. Data for H2 use, consumption and storage

Appendix B. Data for urbs model of Germany

Appendix C. Results of the urbs model optimization

8. Bibliography

Abstract

In addition to the decarbonization of the electricity, heat and transport sectors, the non- energetic consumption of fossil fuels in the industry sector can also be replaced by renew- able resources and therefore holds great potential for decarbonization. In the larger con- cept of Power-to-X, Power-to-Hydrogen plays an important role: Power-to-Hydrogen can accelerate cross-sectoral electrification via electrolysis as conversion technology and via hydrogen as chemical storage.

The annual hydrogen demand for ammonia and methanol production, refining processes, steelmaking and float glass production in Germany is above 50 TWh. Today, this demand is largely covered by conventional reforming processes based on fossil fuels and associ- ated with considerable CO2 emissions. Power-to-Hydrogen has the potential to reduce fos- sil fuel use and CO2 emissions, while advancing the linkage between the energy and the industry sector.

From an environmental perspective, a necessary condition for the decarbonization of non- energetic fossil fuel use is an energy mix that is already primarily based on renewable energy sources. Even though this requirement will only be fulfilled in 2030 at earliest, a combination of conventional and power-based hydrogen production can be viable earlier through additional flexibilities including hydrogen storage in salt caverns. With increasing rates of renewable electricity generation, Power-to-Hydrogen gets more competitive as well. However, a hydrogen production entirely based on Power-to-Hydrogen increases the electricity demand of the energy system substantially, bringing about new challenges in terms of costs, grid stability and reliability of supply.

Nevertheless, on-site solutions that combine power-based hydrogen production with re- newable energy production on industry sites are first valid applications of Power-to-Hydro- gen allowing to cut costs and CO2 emissions, and ensuring a decarbonization of both en- ergetic and non-energetic consumption of resources.

Another concept that has an even wider range of possible applications than Power-to-Hy- drogen is Power-to-Syngas. It has the potential to further intensify the decarbonization of the non-energetic consumption of resources via the enhanced coupling of industry, energy and transport sectors.

Statement of Academic Integrity

Abbildung in dieser Leseprobe nicht enthalten

Hereby confirm that the attached thesis,

Power-to-Hydrogen in the German industry sector: Potential and impact on the energy system

Was written independently by me without use of any sources or aids beyond those cited, and all passages and ideas taken from other sources are indicated in the text and given the corresponding citation.

I confirm to respect the „Code of Conduct for Safeguarding Good Academic Practice and Procedures in Cases of Academic Misconduct at Technische Universität München, 2015”, as can be read on the website of the Equal Opportunity Office of TUM.

Tools provided by the chair and its staff, such as models or programmes, are also listed. These tools are property of the institute or of the individual staff members. I will not use them for any work beyond the attached thesis or make them available to third parties.

I agree to the further use of my work and its results (including programmes produced and methods used for research and instructional purposes.

I have not previously submitted this thesis for academic credit.

Munich, 16.02.2018. (Szujo, Anna)

Declaration for the Transfer of the Thesis

I agree to the transfer of this thesis to:

- Students currently or in future writing their thesis at the chair:
- Flat rate by students
- Only after particular prior consultation.
- Present or future employees at the chair:
- Flat rate by employees
- Only after particular prior consultation.

My copyright and personal right of use remain unaffected.

Munich, 16.02.2018. (Szujo, Anna)

1. Introduction

1.1. Decarbonization of non-energetic fossil fuel use

With increasing efforts of decarbonization, major energy sectors such as the power, heat and mobility sector are less dependent on fossil fuels and reversely more relying on renew- able energies. In Germany, the part of renewable energy sources is 31,5% in the power sector, 13,5% in the heat sector and 5,2% in the mobility sector in 2015 (Figure 1) 1.

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Figure 1 End-use energy consumption by sectors, 2015 1

However, in addition to the necessary decarbonization of the electricity, heat and mobility sectors, the non-energetic consumption of fossil fuels in the industry also needs to be re- placed by renewable raw materials and therefore holds great potential for decarbonization.

The non-energy use of fuels is the part of the energy balance that covers fuels used for material purposes in different sectors such as the chemical and petrochemical industry. This part of the energy balance is not consumed as fuels or transformed into other fuels. In the energy balance, non-energy use is shown separately in final consumption under the heading non-energy use (Figure 2). 2

While non-energetic energy use of resources accounts for 890 PJ (equal to 247 TWh), almost 99% of that energy is used for material purposes in the industry. The total in indus- try, amounting to 876 PJ (243,3 TWh) corresponds to the energy use for chemical feed- stock and processes in the mineral and metal industry. Compared to that, the energetic use of fuels in the industry sector is around 2315 PJ (643 TWh). 3

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Figure 2 Total final consumption in Germany including non-energy use, 2015 3

The non-energetic consumption of resources is almost entirely covered by fossil fuels (nat- ural gas, crude oil, coal), corresponding to 15,9 million tonnes and representing 4% of total fossil fuel use (currently 397 million tonnes). Crude oil accounts for 15,3 million tonnes thereof. 4

One chemical feedstock, that is most commonly produced via the conversion of fossil re- sources, is hydrogen. Annually, approximately 19 billion Nm³1 of hydrogen are produced in Germany for material, non-energetic purposes 5.

1.2. Importance of hydrogen in the industry sector

Besides its considerable relevance as a chemical and petrochemical raw material and in- dustrial chemical, hydrogen gained importance as a practically inexhaustible (secondary) energy source complementing and even substituting the electric current 6. In recent years there has been an increased focus on hydrogen as an alternative energy carrier because of limited fossil fuel sources, increasing oil prices and environmental considerations 5.

1 A standard cubic meter is a volume measure unit for gases. It is used for comparison Gas quantities used at different temperatures and pressures. The gases are in the physical standard state at a standard temperature of Tn = 273.15 K = 0 ° C and a standard pressure of pn = 1.01325 bar compared. 5

The reasons for this are divers, including 5

- the high gravimetric energy density of 33,3 kWh/kg (in contrast to the low volumetric energy density of 3 kWh/m³)
- the abundance: hydrogen is the most frequent element of the earth
- the high environmental impact: clean combustion, since only water is produced
- the non-toxicity of hydrogen
- good transport and storage options
- the flexible conversion to other energy sources and therefore the application in electric- ity, heat, transport and industry sectors.

Hydrogen is a feedstock for ammonia and methanol and is used in refineries for petro- chemical processes such as hydrocracking and hydrotreating. Further, hydrogen is in- volved in reduction processes in the steel and metal industry, float glass production, and in the food industry for margarine production. Hydrogen is used as a doting gas in the semi- conductor industry and as a coolant for super-conductors as well. 7

Of the 19 billion Nm³ hydrogen that is produced every year, approximately 71% stems from fossil resources, based on natural gas, naphtha, heavy oils and coal 8.

Therefore, one of the pillars of the decarbonization of the non-energetic consumption of fossil fuels is the electricity-based, renewable production of hydrogen. Decarbonizing hy- drogen production by a sustainable production based on renewable sources has great po- tential to decrease the consumption of non-energetic fossil resources considerably. Power- based hydrogen production is made possible by Power-to-Hydrogen, which is one key el- ement of the larger concept of Power-to-X.

1.3. Power-to-X to advance decarbonization

The common basis of all Power-to-X concepts is the transfer of electrical energy into an- other energy form. This concept describes technologies that transform electricity from re- newable sources into material energy storage, energy carriers and energy-intensive chem- ical products. 9

Power-to-X thus contributes to the decarbonization of the energy systems, while at the same time reducing the share of fossil raw materials in the key heat and transport markets as well as the (chemical) industry sector, thus allowing the integration of renewable ener- gies in other sectors than the energy sector. Moreover, the concept ensures the more flex- ible use of electricity from volatile renewable energies and even the integration of surplus power in different sectors while providing negative controlling power to the power grid. In addition, Power-to-X helps the provision of energy sources or raw materials for industrial use, implicating less dependence on external raw material imports. 10

The “X” serves here as a placeholder for other forms of energy and comprises the following applications 10:

- Power-to-Heat: conversion of electricity to heat (via electric resistance heating) to be used in the local and district heating networks
- Power-to-Gas: conversion of electricity in gaseous energy sources. The hydrogen cre- ated through electrolysis can be used as a stand-alone energy source or be transferred to other gases, e.g., methane via methanation which can serve as a substitute of natural gas. The fuel gas produced in this manner can be fed into the public gas network, stored temporarily in caverns, or used in the transport sector (fuel cell vehicles, propulsion of gas vehicles) or for the generation of electrical energy (gas-fired power stations, cogen- eration plants).
- Power-to-Liquid / Power-to-Fuel: conversion of electricity to liquid energy sources. These processes are also based on electrolysis followed by different chemical proce- dures, e.g., Fischer-Tropsch method. This also includes the storage of hydrogen as Liq- uid Organic Hydrogen Carriers (LOHC).
- Power-to-Chemicals, including Power-to-Hydrogen: conversion of electricity for the non-energetic but material use of hydrogen, these are all processes involving hydrogen as raw material.

Electricity serves as a common basis for all Power-to-X technologies and can intensify the coupling of sectors electricity, heat, transport and industry as well. Figure 3 shows the ap- plications and potential of sector coupling via different Power-to-X technologies.

The strong dependency on fossil fuels of non-energetic use in the industry sector comes along with significant opportunities of decarbonization, especially through the renewable supply of hydrogen.

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Figure 3 Sector coupling through Power-to-X, after 5

1.4. Goal and approach

The goal of this thesis is to determine the potential of power-based hydrogen production (Power-to-Hydrogen) in the German industry sector and to study and discuss the impact and the additional flexibilities that are thereby being created for the time periods 2016, 2030 and 2050.

The thesis is divided into the following sections (Figure 4):

- Determining the potential of Power-to-Hydrogen including a literary research on hy- drogen production and storage technologies and the largest hydrogen consumers in the German industry. This is followed by the calculation of hourly hydrogen demand for in- dustry purposes for the years 2016, 2030, and 2050 on federal state level. The potential of electricity-based hydrogen production can be derived by a comparison with current hydrogen-generating processes and a deeper understanding of the hydrogen market.

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Assessing the impact of Power-to-Hydrogen on CO 2 emissions, energy use and con- sumption of fossil resources based on the chemical reactions of the relevant processes and the German energy mix for the years 2016, 2030 and 2050.

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2 CO2 emissions and equivalents

- Incorporating hydrogen demand and production in the urbs energy model of Ger- many and developing different scenarios as a function of hydrogen demand (time-de- pendent) and production (dependent on current energy mix and climate protection sce- narios).
- Discussing and comparing the results to prior findings, examining the impact of Power-to-Hydrogen on the economic and environmental costs of the electricity system and identifying flexibilities through hydrogen as an additional commodity in the German energy system.

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Figure 4 Approach taken for Master's thesis

1.5. urbs optimization model

1.5.1. General

The urbs model is a linear programming optimization model for capacity expansion and unit commitment of distributed energy systems. “Urbs” means city in latin and had origi- nally the purpose to optimize urban energy systems. Today the application has been ex- panded to larger scales. 11

Demand is located in vertices (called <sites>) that represent distinct locations which can be of various scales. The vertex is only determined by the labelling and scaling of input data and is characterized by processes, e.g., energy conversion, transmission etc. 11

The urbs model (Figure 5) converts the input data, including energy demand and supply, economic and technical characteristics of power plant portfolio (commodities, processes, transmission, storage) to a cost-optimized allocation of capacities for conversion, trans- mission and storage 11. The calculations of the model are based on linear optimization.

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Figure 5 Input/Output model flow chart of urbs model, after 11

Linear optimization) is a special case of mathematical optimization in which the objective and all constraint functions are given by linear relationships. The following formula shows the canonical form of linear optimization 12:

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With xu as an optimization variable and cu as a cost vector and Au, b′ as constraints. The optimal solution x’ has the smallest value of cx among all vectors that satisfy the constraints. 12

1.5.2. Model

The models’ objective is to minimize total costs to satisfy a given energy demand and supply for a set of commodities. The optimization includes use of resources q, investment in capacities n and scheduling of conversion units s and transmission n for the available technologies 11:

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The cost types are 4:

- Investment costs: they are the costs associated with an investment in newly built ca- pacities for conversion processes, storage size/power, and energy transmission. To compare investments with different economic lifetimes (i.e., depreciation), investment costs are annualized, i.e., multiplied by the annuity factors derived from wacc3 and de- preciation.
- Fixed costs: in contrast to investment costs, fixed costs are linked to the total capacities of all technologies and are independent of operation. They are given per throughput of MWh for processes and of MW for transmission. For storage, they can be given in both dimensions, depending on the storage technology.
- Variable Cost: they are calculated per modelled time step for usage of conversion pro- cesses, transmission and storage. For energy conversion, costs are calculated based on the process throughput in MWh. For energy transmission, the in-going power flow deter- mines the costs (MWh as well). Storage technologies can have costs both MWh and MW for maintaining a certain storage level as well as charging and discharging costs.
- Fuel Costs: those are the costs associated with stock commodity purchase.
- Revenues: this cost function is the sum of all revenues/expenses made by selling com- modities. Since this revenue is income thus positive, the cost function is multiplied by-1.
- Purchase Costs: this cost function is the sum of all expenses made by purchasing com- modities.

1.5.2.1. Input

The input data of the model is gathered in an .xls sheet covering several model entities. These are sites, commodities, processes, transmission and storage. Demand and inter- mittent commodity supply and buy-sell prices are modelled through time series datasets 13. In the following all input data is briefly covered.

Commodities

Commodities are the resources relevant to the modelled energy system, including the fol- lowing four types 13:

- Stock: can be bought at any time for a given price, e.g., coal, gas, biomass and uranium 3 weighted average cost of capital
- Supply Intermittent (SupIm): represents the production of each intermittent commod- ity, e.g., wind, solar.
- Demand: given in time series, demand to be satisfied by output of process or storage.
- Environmental (Env): greenhouse gas emissions, e.g., CO2, given in tonnes.

Commodities are defined over the tuple (site, commodity, type) and are given by MWh.

Processes

Processes represent conversion technologies from one set of commodities to an- other. For the urbs model, the processes are MIMO4 which means that one (or more) in- puts can be converted to multiple outputs. They are defined over two tuples, the first tu- ple (site, process) and the second tuple (process, commodity, direction) 13.

Transmission

Transmission represents the transfer from one site to another and is defined over the tu- ple (site in, site out, transmission, commodity) 13.

Storage

Storage processes offer the possibility to store commodities with the purpose of retrieving them on a later step, thus representing a temporal shift of commodities. Storage is de- fined over the tuple (site, storage, stored commodity) 13.

Variables and parameters

Besides model entities, urbs makes use of different variables and parameters to illustrate the energy model. The variables used in the model are related to different areas of use: costs, commodity, process, storage, transmission and DSM5. They are determined during optimization. Technical and economical parameters are provided before the optimization takes place and define the specification of the modelled energy system.

1.5.2.2. Output

The output of the model depends on the settings of the model. In general, there are two output types:

- Detailed reporting of the optimization results as an Excel workbook

4 multiple in and multiple out

5 Demand side management

- Plotted representation of optimal energy model showing the power (MW) and en- ergy (MWh) allocation of the site

The report on the optimization differs depending on which scenarios the program runs with. Besides the base scenario where the input data is not changed, there are other sce- narios that are defined by the changes that are made on specific parameters, e.g., on CO2 emissions. The resulting workbook compiles all data in terms of costs and capacities(process, transmission and storage). In the commodity sums section are compiled the commodity balance along with the created, consumed, stored and shifted (DSM) com- modities and export/import as well. The same information is gathered as a time series for the different sites for all demand types. Plots can be created for every site.

1.5.3. German electricity model

To understand the impact of Power-to-Hydrogen on the electricity system in Germany, the urbs model of the German electricity system serves as a basis. The goal is to incorpo- rate all hydrogen-related data in the model. In the following, a short overview of the Ger- man electricity model is given.

Geographic settings

For the model, a regional resolution has been chosen, represented by the 16 federal states of Germany. It provides detailed information on federal state level including elec- tricity generation commodities and processes, storage and transmission. Germany is con- sidered as an isolated system which means that there is no energy exchange with neigh- boring countries.

Temporal framework

The optimization problem is calculated for one year with a time-step resolution of 1 hour. Due to the high complexity of the calculation model and the associated computer capacity requirements, only 4380 hours are calculated at a time.

Commodities

In accordance with the general urbs mode, commodities are classified in demand (<De- mand>), stocks (<Stock>), environmental (<Env>), intermittent Supply (<SupIm>):

- Demand: the Germany model primarily takes into account electrical demand. For this purpose, the total German electricity demand was weighted using the total energy con- sumption of the individual federal states. In addition to electricity, heat and accordingly geothermal plants are included as well.
- Stock: for power plant data in Germany the energy sources include coal, lignite, gas, gas (renewable), nuclear, geothermal, biomass, garbage (non-renewable) and slack. Slack is included to ensure additional capacities the system can resort to in the event of missing production capacities.
- Supply intermittent: intermittent supply covers time series of wind, solar and run-of- river.
- Environment: this section contains the CO2 constraints of the German energy system. It limits the sum of all created (as calculated by commodity_balance) CO2 in all sites.

For the datasheets Buy-Sell-Price, and DSM the values are irrelevant, because they are neglected for the optimization problem.

Transmission

The transmission lines in the model are all HVAC lines connecting federal states to one another. No gas or heat transmission is included.

Storage

Two major storage technologies are included that are currently in use: Hydro and pumped hydro energy storage.

2. Literary research

The aim of this part of the thesis is to lay the foundations of an integration of Power-to- Hydrogen in the German energy system via the replacement of conventional hydrogen production by power-based production.

2.1. Hydrogen production technologies

2.1.1. Overview

In the industry sector there is a considerable demand for hydrogen for chemical and petro- chemical processes. Even though some of the hydrogen is produced as a by-product in industrial processes, the major part of the demand is typically covered by conventional hydrogen production processes that are based on fossil fuels, such as naphtha, natural gas and coal.

When producing hydrogen, there is a distinction between two major paths: conventional thermal conversion of fossil fuels and the production by electrolysis of water, whereby the water is split by means of electricity into hydrogen and oxygen. The electricity is either renewable or comes from fossil energy sources. There are also processes based on bio- mass, however, due to their limited occurrence, the deployment of hydrogen from biomass is factored out for the purpose of this thesis.

The clear majority of today's hydrogen production is from fossil fuels, and only 5 % of hy- drogen production is based on electricity, as a by-product of chlorine production 8.

The graph in Figure 6 represents the current hydrogen production technologies according to primary and secondary energy sources (own representation, after 14). The path from electricity to hydrogen (via the production of syngas) corresponds to the concept of Power- to-Hydrogen and connects the power sector to the industry sector.

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Figure 6 Hydrogen production technologies, after 14

2.1.2. Conventional hydrogen production

In thermal conversion, there are mainly reforming processes, which are by far the most widely used technologies. Reforming is the conversion of hydrocarbons into hydrogen, with by-products of water vapor and carbon monoxide. The reaction proceeds at high tempera- tures (between approximately 700 and 900 °C). Catalyst helps to realize the implementa- tion. 16

The conversion always takes place with the addition of air and/ or water vapor as the oxi- dizing agent6. When applied to gaseous or liquid fuels, we distinguish between three differ- ent technologies depending on the oxidizing agent: steam (methane) reforming, partial ox- idation and autothermal reforming. When applied to a solid fuel, the process is called gas- ification.

Reforming usually produces a syngas7, which is a mixture of carbon monoxide and hydro- gen. This synthesis gas is converted to hydrogen by gas treatment, whereby the carbon monoxide content decreases. The carbon monoxide from the synthesis gas is decreased by a Water Gas Shift reaction. 16

6 Oxidizing agents can oxidize other substances by removing their electron numbers and thereby reduce themselves (and thus increase their own electron numbers), while educing agents work in the opposite direc- tion 17.

7 Syngas is a mixture of carbon monoxide and hydrogen and an intermediate resource for chemical sub- stances (hydrogen, ammonia and methanol) and petrochemical processes (synthetic natural gas, hydro- formylation) 15.

2.1.2.1. Steam reforming

In the steam reforming process, the oxidant is pure water vapor. The reaction is endother- mic and requires the supply of heat which is provided to the partial burning of secondary fuels. Starting materials of steam reforming are mostly natural gas and water; but in princi- ple also other light hydrocarbons such as LPG8 or naphtha can be used. 18

The general reaction is 19

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This is followed by the Water Gas Shift reaction, where carbon monoxide and water react to produce carbon dioxide and hydrogen 19:

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The most common reforming process is steam methane reforming, where hydrogen is pro- duced from a methane source (such as natural gas) using high temperature steam. The first step is the sulphur removal of the hydrocarbon. Subsequently, in the steam methane reforming reaction steam reacts with a catalyst and produces hydrogen and carbon mon- oxide (at 800 ° C) 2021:

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The carbon monoxide content can be further reduced through processes like selective CO oxidation (at 100 ° C) 20:

8 Liquified petroleum gas

9 The standard enthalpy change of a reaction is the enthalpy change which occurs when equation quantities of materials react under standard conditions, and with everything in its standard state 22.

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It is followed by the Water Gas Shift reaction (refer to 2.1.1.1.). For other starting materi- als, such as heavy oils, the process runs analogously 25.

2.1.2.2. Partial Oxidation

Partial oxidation refers to the conversion of primarily heavy hydrocarbons (as coal and heavy oils) by means of oxygen (typically from air) or air as an oxidizing agent 14. In the case of oxygen, the hydrocarbons oxidize to carbon monoxide and hydrogen. If the reaction is conducted with air instead of pure oxygen, nitrogen is also a reaction product 16. The reaction takes place under high pressure and temperature between 1250 °C and 1400 °C 43. It is an exothermic process, i.e., there is no need of an external heat source, the heat required for the reaction is supplied by the partial combustion of the fuel 19. Partial oxida- tion mostly occurs in refineries 16.

A distinction is made between thermal partial oxidation (TPOX) and catalytic partial oxida- tion (CPOX). While TPOX occurs at temperatures between 400 and 600 °C and pressures between 250 and 300 bars, the CPOX makes use of a catalyst and thus allows to reduce the reaction temperature 16. The general reaction is 19

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The net reaction is 19:

10 Naphtha is the term for distillates in the gasoline range, which serve as a raw material of petrochemistry, in particular to produce ethylene and co-products (propylene, butylene, butadiene) 23.

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Generally, the longer the hydrocarbon, the lower the corresponding hydrogen yield 18. The reaction involving methane is the following:

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In contrast to steam reforming, the partial oxidation process takes less time and needs a smaller vessel for the reaction. However, the produced hydrogen quantity for the same fuel is smaller (3 H2 vs. 2 H2) and thus also of a lower purification level. 25

The oxygen that is used is supplied by an ASU11, which significantly increases energy con- sumption in comparison to steam methane reforming. However, this loss is partly compen- sated by the additional heat release of the process and given that pure oxygen is used as an oxidizing agent, practically no nitrogen is involved in the reaction which means that the energy consumption of the Water Gas Shift is lower. 19

Overall, partial oxidation is still less efficient than steam reforming, however it offers the advantage to use a wide range of starting products instead of only exclusively methane 25. The basic reaction for partial oxidation of heavy hydrocarbons is 7

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The reaction can change according to the starting fuel. In refineries, partial oxidation is most commonly used with heavy oils 20.

2.1.2.3. Autothermal reforming

Autothermal reforming is a combination of the steam methane reforming and partial oxida- tion processes, the oxidizer is a mixture of air and water vapor 19.

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The process for the reaction including methane is 21:

4 CH4 + 02 + H20 ↔ 4 C0 + 10 H2

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Autothermal reforming regroups the advantages of both processes: the high hydrogen yield of steam methane reforming and the process heat supplied by partial oxidation (Figure 7). However, autothermal reforming is also associated with higher investment and operating costs for air separation and a more elaborate flue gas cleaning. 19

Figure 7 Overview of reforming processes, after 27

2.1.2.4. Coal gasification

Coal gasification is referred to as the conversion of coal with oxygen or an oxygen-contain- ing gas to a syngas. For the process, the raw material is first dried and then broken down in hydrogen and carbon compounds under exclusion of air. Subsequently, these com- pounds are partially combusted. The heated carbon and vapor reacts to carbon monoxide and hydrogen, wherein carbon monoxide is reduced to CO2 via Water Gas Shift reaction. 21

Typically, the coal gasification process is endothermic. Possible oxidizers are air or a mix- ture of oxygen and water vapor or carbon dioxide. Similar to partial oxidation, the product gas purity increases with the use of oxygen because the use of air leads to an introduction of nitrogen into the process 14. The general gasification process is the following 24:

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There are different versions of this process, depending on the starting product, e.g., 2628:

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2.1.3. Electricity-based hydrogen production

Power-to-Hydrogen describes the concept of converting electricity from renewable ener- gies to hydrogen for the use in chemical and petrochemicals processes in the industry. The underlying technology is electrolysis of water.

2.1.3.1. Electrolysis

In the electrolysis process, water is split into hydrogen and oxygen with the help of electric current. When using electricity from renewable energy for electrolysis the result is an emis- sion-free energy source. In the further conversion into thermal or electrical energy, no emis- sions are released. The basic reaction of electrolysis is 24

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The electrolyzer consists of a DC source and two with precious metals coated electrodes separated by an electrolyte. The electrolyte which is an ionic conductor can be a liquid, e.g., conductive potassium hydroxide in the alkaline electrolyzer. 21

The electrochemical reaction of water electrolysis can be segmented into two steps. The reduction reaction takes place at the negatively charged cathode 29

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and the oxidation reaction occurs at the positively charged anode

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Depending on the technology, the charge carrier can be 0H–, H30+, or 02–29.

Electrolysis cells are characterized by their electrolyte type. Among the most common tech- nologies of water electrolysis is the alkaline electrolysis (AEL) with a liquid basic electrolyte, the proton exchange membrane electrolysis (PEMEL) with a polymer solid electrolyte and the high-temperature electrolysis (HTEL) with a solid oxide as the electrolyte 30.

In order to couple water electrolyzers with renewable energies, there are special require- ments for the plants. This applies above all to the dynamics, the partial load behavior, the switching on and off and the stand-by operation of the electrolyzers 29. In the following is an overview of these electrolysis technologies along with an assessment of the linkage with fluctuating energy supply by renewables.

2.1.3.2. Low temperature electrolysis

Low-temperature electrolysis takes place at temperatures below approximately 300 °C 5. Generally, temperature is determined by the cell types. The two types of cells at low tem- peratures are alkaline electrolysis (AEL) cells and PEM cells.

Alkaline electrolysis (AEL)

The alkaline electrolysis is the most sophisticated technology among electrolyzers and al- ready commercially available. In AEL, the electrolyte is an aqueous alkaline solution (KOH or NaOH) 45. AEL works either atmospherically or under elevated pressure. While pres- sured AEL produces hydrogen of lower purity and has lower efficiency compared to atmos- pheric AEL, the efficiency losses are compensated by the fact that compressed hydrogen is directly produced, overall leading to lower energy demand than atmospheric AEL 29.

Typical module sizes range from 5 kW to 3,4 MW, with specific energy consumption of 4,2

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According to manufacturers, AEL electrolyzers can be operated between 20 and 100% of the design capacity, and overload operation up to 150% is possible 31. However, remain- ing at partial load ranges of 20-40% for longer periods leads to contaminations of the gas. Higher rates of the corresponding foreign gas can result in a safety-related, automatic shut- down of the electrolyzer. In addition, shutting down the system often entails the enrichment of foreign gases through the diffusion of the product gases. Consequently, restart can need time-consuming rinsing, thus frequent startups and shutdowns lower the efficiency of the system. Regarding the dynamics of the system, commonly peripheral components are the bottlenecks as they are not always able to follow intermittent power supply. 29

Polymer electrolyte membrane electrolysis (PEMEL)

The PEM electrolysis is based on the technology of solid polymer membranes. In compar- ison to AEL it is a new and less developed technology 31.

At the module level, PEMEL systems currently range from 0,06 to 30 Nm³/h, with a maxi- mum electrical power consumption of up to approx. 150 kW per module. This is very small compared to the power range of alkaline electrolysis. Specific energy consumption amounts to 4.5 – 7.5 kWh/Nm³. 31

The main advantages of PEM include faster cold start, higher flexibility, and better coupling with dynamic and intermittent systems. Furthermore, the purity of the produced hydrogen is very high. However, costs are considerably higher. 29

In general, the PEM electrolysis allows a larger partial load range compared to the AEL. At cell or stack level, the lowest part load range is often specified as 0%, meaning that the foreign gas concentration does not reach critical values at that level. In addition, PEM elec- trolyzers also tolerate short-term overloads. A major advantage of PEM electrolysis is fast dynamic behavior. At the cellular level, transients in the electrical power consumption are followed virtually without delay. The system periphery (circulation pump, liquid gas separa- tors) has higher time constants but lower than the ones of AEL. This allows the operating point to follow an intermittent power input well. Furthermore, in the starting phase a PEM electrolyzer reaches quickly the operating temperature. Overall, PEM seems to be better suited for the integration of renewable energy sources. 29

2.1.3.3. High temperature electrolysis

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The energy equilibrium is given by the relationship of Gibbs free energy ∆G0 and the equi- librium cell voltage E0, describing the relationship between thermodynamics and electricity where n is the number of transferred electrons in moles and F is the Faraday constant.

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Further, the minimum electric energy supply required for the electrolysis process is equal to the change in the Gibbs free energy (∆Gr) 5:

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where ∆Hr is the enthalpy change, T the temperature and ∆S the entropy change. The electrical energy demand ∆Gr, and correspondingly the equilibrium cell voltage E0 de- crease with increasing temperature, leading to higher efficiencies, which makes the com- bination with fluctuating renewable systems advantageous 32. However, in contrast to electricity demand, heat demand increases with higher temperature. This leads to increas- ing cost due to larger specific cell areas and higher cost per hydrogen unit 31.

Even though total energy demand is increasing, operating at higher temperature can lead to a considerable decrease in the cost of hydrogen production, as over two thirds of the cost of electrolytic hydrogen comes from the use of electricity. More cost-savings can be made by covering the additional heat demand by an external heat supply such as renewa- ble energy or waste from industrial processes. 33

HT-electrolysis via SOEC

High-temperature electrolysis takes place at temperatures between 700 and 1000 °C 31. One of the most common cell types for high temperature electrolysis are Solid Oxide Elec- trolysis Cells (SOEC). Solid Oxide Electrolysis is the most recently developed electrolysis technology and is still at a basic stage of research. At present there are no commercial facilities. Currently the largest laboratory system has a hydrogen production rate of 5,7 Nm³/h with a power of 18 kW 31.

At higher temperatures and current densities, degradation rates increase and lifetime of the cell decreases drastically. Solid Oxid Electrolysis (SOEL) follows electrical load changes very quickly, however, the dynamics are limited by the mechanical, lifetime reduc- ing problems. This applies also for jumps in the partial load range. When starting and stop- ping the SOEL, temperature-related mechanical stresses occur. Therefore, the shutdown and start must be very slow. Due to the limited dynamics from a technical view renewable power sources for HT electrolysis appear to be less well suited despite the increase in efficiency. 31

High temperature co-electrolysis

The co-electrolysis takes place in Solid Oxid Electrolysis Cells. High temperature co-elec- trolysis reduces water and carbon dioxide simultaneously, transforming them to hydrogen and carbon monoxide using electric power. The product is syngas, a mix of hydrogen and carbon monoxide that is feedstock for chemical products. 34

The chemical reaction looks as follows 32:

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If the energy source does not emit carbon monoxide and dioxide and the carbon dioxide source is from biomass, the co-electrolysis can be carbon-neutral.

Syngas, which is the product of the co-electrolysis, is an essential intermediate resource of chemical (hydrogen, ammonia, methanol, aldehydes) and petrochemical processes (synfuels, synthetic natural gas, hydroformylation) for the three main sectors transportation, energy storage and chemical industry 15.

Current conventional hydrogen production technologies generally produce syngas, hydro- gen can subsequently be obtained via purification processes. In addition to that, syngas is deployed on a broader basis than hydrogen (Figure 8), the application area of syngas de- pends on the H2/CO ratio. Therefore, renewable syngas (Power-to-Syngas) has an even larger decarbonization potential than renewable hydrogen (Power-to-Hydrogen).

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Figure 8 Application areas of syngas in different sectors, after 15

2.2. Hydrogen storage

The hydrogen produced during electrolysis can be stored directly. From an energetic point of view, this is more efficient than the conversion to methane, since the conversion losses are lower overall. In addition, the hydrogen can directly be used for industry purposes.

2.2.1. Storage technologies

There are different technologies for storing hydrogen – ranging from small size storage such as gas trailers, liquid tanks and metal hydrides to large size storage as it is the case for underground cavities.

Hydrogen can be stored either as 35

- a compressed gas in carbon fibers composite pressure vessels or in metal pressure vessels,
- a refrigerated liquefied gas when high gravimetric storage performance is needed (also referred to as cryogenic storage),
- a cryo-compressed gas in cryogenic vessels at low temperatures and high pressures to ensure high volumetric and gravimetric performance
- a solid form in hydrides.

Cryogenic and compressed storage are the most mature technology 36. Currently, hydro- gen for industry uses is stored either in compressed or in liquid form (as a refrigerated liquefied gas) 37.

However, while the storage capacity of aboveground storage is limited by realizable volume (with capacities far below 1 million m³), pressure and material costs, in the geological un- derground much larger storage volumes of almost 500 million m³ can be realized with lower costs, lower land consumption and higher pressures (up to 200 bar and above) 38. Hy- drogen can be stored in salt caverns, i.e., artificial cavities created in the salt rock under- ground by the injection of water, or in porous storage facilities. These are natural porous deposits whose tightness is ensured by gas-tight rock formations. 39

In comparison to porous deposits, salt caverns are the preferred option for the following reasons, according to 39:

- salt caverns are technically tight for hydrogen,
- hydrogen is not contaminated by rock salt because it is inert to gases,
- no contamination by residual hydrocarbons are to be feared,
- due to the low surface contact between gas and brine and to the high level of salinity of brine chemical reactions are negligible,
- caverns are well suited for high storage and retrieval rates, respectively short charge and discharge periods and frequent load changes and are therefore suited for the inte- gration of volatile renewable energy sources.

According to 40, the use of salt caverns as seasonal hydrogen storage leads to higher flexibilities in the power system and indicates lower CO2 emissions by enabling the produc- tion of hydrogen from different renewable sources, also compared to on-site electrolyzers and storage. With increasing decarbonization and further hydrogen penetration in the mo- bility (hydrogen as a fuel), natural gas and electricity (reconversion of hydrogen to electric- ity) market besides the industry market, hydrogen storage in salt cavern is a cost-efficient large-scale alternative to other current hydrogen storage technologies 40.

2.2.2. Typical characteristics of salt cavern hydrogen storage

Typical dimensions for a storage cavern are a volume of 500,000 m³, a pressure range between 60 and 180 bars at a depth of over 1000 m. This results in a storage capacity for hydrogen of about 140 GWh based on the calorific value or 85 GWh taking into account the losses at the reconversion in a gas turbine power plant. The input and output power is about 700 MW 41. Table 1 gives an overview of the most important techno-economic parameters of salt cavern storage:

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Table 1 Techno-economic parameters of a salt cavern 4243

2.2.3. Potential for underground hydrogen storage in Germany

Currently, there are three salt cavern storage facilities for hydrogen, on in Teesside, UK and the other two near Houston, Texas 43. There is no hydrogen underground storage in Germany. The map in Figure 9 shows the locations of all German storage cavern projects 36, distinguishing between salt caverns, porous storage (aquifers) and depleted storage facilities. The total technical volume for gas storage in salt caverns, aquifers and depleted storage facilities is 273,8 TWh, among which 153,7 are salt caverns (Table 2) 36.

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Table 2 Existing salt caverns in Germany, after 36

According to 40, the most cost and time-effective ways to establish underground hydrogen storage in (salt) caverns are either to convert existing natural gas caverns to hydrogen caverns, or to construct additional caverns in already explored and approved cavern fields with an existing infrastructure. Often there are existing wells or smaller caverns that cannot be used for natural gas due to their small volumes 43. Moreover, additional natural gas storage capacities could become available for conversion in the long term, with decarbon- ization intensifying.

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Figure 9 Underground gas storage facilities in Germany, after 36

Northern Germany has a considerable salt cavern storage potential, while overlapping with fluctuating energy demand coming from on- and offshore wind turbines 40. The most suit- able regions for hydrogen storage in salt caverns are 40:

- northern Nordrhein-Westfalen at the Epe and Xanten locations,
- northwest Germany with the storages in Bremen-Lesum, Etzel, Harsefeld, Huntorf, Jem- gum, Krummhorn and Nüttermoor,
- central Germany with natural gas caverns in Bernburg, Stabfurt and Bad Läuchstadt.

However, in southern Germany, that is characterized by a large penetration of renewable energy, there are no salt deposits suitable for cavern construction 40. Therefore, other concepts of hydrogen storage such as aboveground tanks and vessels will need to be ad- dressed in addition.

Table 3 compiles the current storage capacities for natural gas (working gas volumes) of each region according to 40, as well as the estimated potential for hydrogen storage in volumes and energy content.

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Table 3 Storage capacities for natural gas and potential for hydrogen storage 40

For salt caverns, a significant expansion is in preparation. Considering the ecological re- quirements, about 400 additional salt caverns can be tapped by 2050. Including all existing and potential projects, the storage capacity of caverns will amount to 43 billion Nm³ in 2050, that can both be used for hydrogen or methane storage. For methane, there is additional pore storage potential, resulting in 53,7 billion Nm³ total storage capacity (refer to Table 4). 44

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Table 4 Total working volumes of caverns and aquifers in 2050 44

For a yearly hydrogen production of 19 billion Nm³ hydrogen, approximately 407 salt cav- erns of the dimension of 500.000 m³ are needed. However, given that most hydrogen for industry uses is produced captively or as by-product, the need for large scale storage is going to be limited by the increasing transportation efforts. Therefore, for industry uses small scale hydrogen storage is going to prevail. Large scale storage can get more attrac- tive in combination with hydrogen for mobility uses.

3. First results

3.1. Hydrogen demand in German industry sector

3.1.1. Method

In order to determine the hydrogen demand in Germany at a federal state level on hourly basis for the urbs model, the procedure is as follows:

i. Determine major hydrogen consumers in the German industry sector
ii. Based on chemical reactions and stoichiometric equivalents, calculate the specific hydrogen demand per product
iii. Locate hydrogen consumers sites such as chemical parks, refineries etc. on federal state level and calculate hydrogen demand per consumer site
iv. Determine total hydrogen consumption per Federal state
v. Calculate hourly hydrogen consumption per Federal state

In the following, the results corresponding to these steps are going to be elaborated for the German industry sector.

3.1.2. Major hydrogen consumers in the industry

The largest hydrogen consumers in the industry sector are the chemical industry (ammonia and methanol production), refineries, steel industry and glass industry.

3.1.2.1. Chemical industry

In chemical industry, hydrogen is a feedstock for chemical base products, mainly ammonia and methanol.

Ammonia production

Ammonia is a basic material for nitrogen-based fertilizers, which make up 80 % of ammo- nia, while the other 20% are being used to produce explosives, plastic materials and refrig- erants in cooling systems 45. In general, ammonia is produced based on the Haber-Bosch procedure. To produce ammonia, nitrogen and hydrogen are needed 46:

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