Technical and economic aspects of natural gas

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Relative threats for Earth sustainability

Threats are existing around this endless economic and energy consumption growth. What about the scarcity of resources? We live in a finite world with a finite amount of resource so that it is impossible to extract, produce and consume more and more resources, products and goods in the future (Barnett et Morse 2013).
Sharp and rapid oscillation in the price of oil have effects on companies, economies, and geopolitics. Indeed, oil price spikes could go against economic activity because people can have a reduction in their purchasing power due to higher budget dedicated to mobility or heating. For many countries, the oil price is a parameter they cannot really determine but that can change totally the government budgets possibilities and, then, the reforms they can or cannot implement. This is the case for Russia or Venezuela for instance where the national incomes are in majority due to fossils fuels exportations. It is also a main parameter at play in geopolitical relations. Oil price uncertainty is also a threat as it drives many other markets such as gas, fuels or food markets, etc. (Greenberg 2019).
Last but not least, global warming. The atmosphere of Earth is warming. In the 2010, the temperature was about 1°C higher than in the pre-industrial area (figure 2). It may seem to be very low but it is worth noticing that there is a difference of only 5°C in average between the last ice age (20000 years ago) and today. Europe was partly under a huge glacier and the ocean was 120 meters below its current level (Ehlers, Gibbard et Hughes 2011). This warming trend is accelerating as 2019 ended with a global average temperature of 1.1 ° C above estimated pre-industrial levels, which is only surpassed by the record set in 2016, due to a very strong El Niño episode. Moreover, the 2015-2019 period represents the five years and, 2010-2019, the hottest decade on record. Since the 1980s, each successive decade has been warmer than all previous decades since 1850 (World Meteorological Organization 2020).
It hides also local disparities where some regions are the subjects of higher warming such as mountains or islands and more severe sides effects for its ecosystems.
Nowadays, emissions of carbon dioxide (CO2) are twice the level that the natural environment can store through the carbon sinks of forests or oceans. CO2 in excess, about 3.5 billion tons a year (3GtCO2/year), is accumulating in the atmosphere, increasing its CO2 concentration (figure 3), and contributing to global warming reinforcement. The most alarming is the recent rise in continuous temperature increases since 1980. This is due to the CO2 concentration growth that is accelerating for several years, so that the warming is accelerating too.
Hence, environment is changing and deteriorating. The IPCC confirms the threats concerning global warming that causes a gradual deterioration of the environment whether it is air, water or soils. There is growing evidence on the probability of species extinction due to climate change (Kolbert 2015). The effects of climate change on species range from rising sea levels to habitat destruction to variations in the availability of food resources. Anthropogenic climate change is already affecting species in most latitudes and in most types of natural habitats, including coral reefs, forests, tundra, deserts, grasslands and wetlands. Climate change is a threat to nearly one-fifth of the species living in the world (Wheatley, et al. 2017). There is also threats about the acidification of oceans and the circulation of diseases, which is greatly increased by humans mobility. All that has an impact on human life and for the Earth sustainability.
This ascertainment implies strictly reduce the total amount of carbon emissions by the end of the century, requiring « unprecedented » efforts, a “change of growth paradigm” and « an ecological big bang, for policymakers and for humanity as a whole » (WWF 2018). The idea of an enforcement of a determined « carbon budget » linking a quantity of future warming to a total amount of CO2 emissions is based on the strong relationship between cumulative emissions and temperatures in reality and in climate models.

Decarbonisation, the most important challenge of XXIst century?

Causes, consequences and impacts of climate change

There are already observed changes in the climate system. The atmosphere and oceans are warming, sea is rising and acidifying and there is a diminution of amounts of snow and ice. Recent climate changes have had widespread impacts on human and natural systems. Besides, there are also more extreme weather and climate events since 1950: increases in warm temperature extremes in all continents, in extreme high sea levels and in the number of heavy precipitation events. Droughts, floods, storms, hurricanes, fires are more frequent or more intense all around the world. Furthermore, many aspects of global warming and associated climate impacts will continue for years and centuries, even if anthropogenic emissions of greenhouse gases are reduced because it is a slow and a long-term process. Species extinctions are also huge consequences of climate change. The risks of abrupt or irreversible changes in the long term increase as the magnitude of the warming increases (IPCC 2014).
Global warming acceleration is mainly due to human activities and fossil fuels consumption. This consumption releases GHG that reinforce greenhouse effect and warm the climate. CO2 is the largest source of GHG emissions. Energy-related CO2 emissions account for 59% of the world’s equivalent CO2 emissions. Total CO2 emissions, which also include industrial processes and emissions related to the land sector, represent 76% of global GHG emissions (IEA 2020). Huge efforts have to be made to release less GHG and important threats for human life and the Earth are coming if we do not change our fossil fuels consumption habits. Coal consumption starts to increase in 1850 to supply the first steam engines. Then oil in the XXth century during the second industrial revolution and then gas. It is worth noticing that these different consumptions do not substitute but add up to each other, which leads to more and more emissions as seen the evolution of CO2 concentration in figure 3 (IEA 2019).
Global CO2 emissions increased by 64% between 1990 and 2017. They account for three-quarters of GHG emissions, which reached 53.5 GtCO2eq in 2017 (I4CE et Ministère de la transition écologique et solidaire 2019). The IPCC has just launched a new warning signal: in its last report of October 2018, it implies that the increase of 1.5°C in the global temperature will probably be reached between 2030 and 2050 (IPCC 2018).
As well, the atmospheric methane content has risen steadily since the pre-industrial era, from 720 parts per billion (ppb) in 1750 to 1850 ppb in 2017. However, it seems to undergo through a very strong growth since 2014 at an unprecedented rate. It is therefore a growing problem as methane has a 100 years Global Warming Power (GWP) between 25 to 30 times higher than carbon dioxide that means that one molecule of CH4 has the same effect on Earth warming than 25-30 molecules of CO2 during one century (Reisinger, Meinshausen et Manning 2011). Researchers believe that if this increase continues at this rate, it would have unexpected effects on the climate. In the RCP2.6 IPCC scenario, limiting the global warming less than 2°C, the atmospheric methane content should decrease by 6 ppb between 2010 and 2050, then by 4 ppb between 2050 and 2100, when it would reach 1250 ppb. Nevertheless, the current trend is completely the opposite: at the current rate, the methane content could reach 2400 ppb in 2100 that is the double of the objective. This trend could jeopardize compliance with the Paris Agreement, even with massive CO2 emissions reduction. Methane’s increase since 2007 was not expected in future greenhouse gas scenarios compliant with the targets of the Paris Agreement, and if the increase continues at the same rates, it may become very difficult to meet the Paris goals (Nisbet, et al. 2019). Besides, there may have been a chronic underestimation (-25% to – 40%) of methane emissions from the combustion of fossil fuels, so that balance sheets may be misjudged raising another problem for GHG accountings (Schwietzke, et al. 2016).
This increase of human energy consumption presented above drives to increased GHG concentration in the air since the pre-industrial era. Average concentration remained stable around 280ppm before 1850. Then, concentration began to increase after 1850 because of the start of coal consumption. The growth becomes stronger after 1950 with the acceleration of fossils fuels consumption and the large development of oil and gas (WEO 2018). Cumulative emissions of CO2 largely determine global mean surface warming by the late XXIst century and beyond (IPCC 2014).

The three decarbonisation pillars

Some strategies have to be set in order to fight climate change and limit as much as possible this warming. No matter what each country did in the past, three groups of decarbonisation strategies can be gathered: improve energy sobriety and energy efficiency (pillar 1), lower the carbon content of energy carriers (pillar 2) and switch fuel to decarbonise energy uses (pillar 3) (DDPP 2015). These three pillars of energy system transformation has been highlighted to decarbonise energy systems. A decomposition of CO2 emissions based on KAYA equation (equation 1) allows us to structure the factors of CO2 emissions.
POP reflects the population effect (POP) on the total CO2 emissions. The term GDP/POP is called « production per person », that translates the wealth of the inhabitants and the effect of GDP growth. Energy/GDP is named « energy intensity of the economy »: it is the amount of energy used to produce a dollar or a euro of goods or services. Last, CO2/Energy means that CO2 emissions will depend a lot on the distribution of the various sources we decide to use for a given energy consumption.
Population growth without any changes for the other parameters implies directly emissions growth. The trend is the same for the production per person factor. Actions levers are existing concerning these two first parameters to reduce the emissions but it implicates a large political will and a deep social acceptance that are not obvious currently. A bit outside of our scope, we will not open this discussion in this thesis.
The three pillars mentioned for decarbonisation would tackle the two others terms in the equation that play directly on the energy.
– Energy efficiency and sobriety play on Energy/GDP in equation 1. For reaching CO2 emissions reduction, the goal is to lower the energy consumed per unit of GDP. The possibilities are coming from technical improvements of products and processes. There is also a work to be done to reduce wastes in processes. For example, it concerns the improvement in processes or devices efficiencies. Energy efficiency improvement represents the absolute urgency to reduce global energy demand and relative emissions. The increase of 1,2% in energy efficiency was far too low in 2018, compared to the 3% annual increase required to achieve an essential energy transition (IEA 2019).
– Decarbonizing energy carriers and decarbonising the end-uses play on CO2/Energy. While pillar two aims at reducing the carbon content of all transformed energies (electricity, H2, heat, liquids and gases), pillar three deals with the switch for decarbonised energy to fulfil demand. We could separate the fraction into two terms, one (CO2 (Well –> Tank)) / Energy that would deal with the vector decarbonisation and the fuel switch, for example shifting from coal to natural gas and even renewables in the power sector. Another term (CO2 (Tank–> Wheel)) / Energy would focus on the uses decarbonisation, for instance shifting from conventional to electric vehicles or gas to electric heating systems.
Sobriety is a concept in which consumption sectors reduce their needs. They have to think about another paradigm of consumption in order to reduce their needs without degrading their quality of service. The entry is through the energy services, that is to say the analysis of services rendered by energy consumption: heating, movement, operation of equipment, industrial processes, etc. In order to bring the need for energy services closer to their utility, sobriety acts on parameters as diverse as the dimensioning of equipment, their duration of use and their degree of pooling resources, the filling rate or the maximal speed of vehicles. It can be translated concretely by switching off night lighting, unplugging household appliances when not used, reducing packaging, reducing our mobility needs, reducing the power of vehicle engines, developing shared mobility, etc.
Efficiency represents the reduction of the energy quantity needed to meet one demand. It consists in seeking to minimize the losses associated with the energy chain supplying these services through different vectors such as gas, electricity or heat, themselves derived from primary energy resources. Therefore, efficiency is a way to reduce the energy consumption for an equal service. It can be achieved by isolating buildings to reduce heating power, by changing lighting appliances with lower consumption ones, by increasing efficiency in vehicles or in industrial processes, etc. This involves also taking into account the reduction of the energy consumption required for their manufacturing, called gray energy that represents a not negligible energy content and associated relative carbon footprint (Fourcroy, Gallouj et Decellas 2015).
Renewables are energy sources whose natural renewal is fast enough so that they can be considered as inexhaustible at the scale of human time. Renewables is the short and common word of the terms « renewable energy sources » or « renewable energies » that are more precise from a physics point of view. They come from cyclical or constant natural phenomena: solar power for the heat and the light, wind power created by atmosphere waves, geothermal energy, hydropower and bioenergies. Their renewable nature depends partly on the speed at which the source is consumed and on the other hand on the rate at which it is renewed. The priority choice of renewable energies to replace fuels releasing GHG to cover residual needs is justified by their inexhaustible nature (they are flux energies, in contrast to the stock energies based on finite reserves of coal, oil, fossil gas and uranium) and their much lesser impact on the environment, either locally or globally. Nevertheless, these resources must be exploited in a sustainable manner so as not to cause environmental damage such as droughts with misuse of water or deforestation. Most of the new renewables are integrated in the electricity production, an energy carrier relatively easy to transport thanks to the grids built since decades in developed countries, with lots of end uses applications.

Energy transition and decarbonisation options

IPCC scenarios compatible with the Paris Agreements define the temporality of a transition where CO2 emissions would be reduced by 75% or almost carbon neutrality by 2050 to reach zero or even become negative by 2100. An “energy transition” from a toxic reliance on fossil fuels to low-CO2 production methods is needed. The term « energy transition » is used to describe a major change in our energy infrastructure. Several definitions exist in the literature. They represent a shift in energy sources and fuels used in a society and an economic system. In our case, it represents the shift from a fossil fuel based economy to a low carbon and efficient based economy (Fouquet et Pearson 2012) (Hirsh et Jones 2014) (Miller, Richter et O’Leary 2015) (Smil 2016).
80% of the energy consumed in 2015 in the world came from fossil fuels. Electricity generation represents around 40% of world GHG emissions because more than 60% of world electricity is produced from coal, oil or gas combustion. Industries and transportation sectors account for 21% emissions each. The last 18% are due to buildings and other sectors like agriculture or energy sector self-consumption (IEA, World energy balances 2018 2018) (Munteam, et al. 2018).
Therefore, low carbon electricity is expected to expand to cut off emissions for this largest emitting sector, but also by being an option in the decarbonisation of other sectors. An increase of electrical use is expected in the transport sector with the development of plug-in-hybrid or full-electric vehicles (IEA 2018) and the heating sector for buildings or industries thanks to heat pumps. The power sector will undertake tremendous transformations, related to the increase of the share of renewable energy sources, as discussed above. Literature is expanding concerning electric mix as in some countries where studies focus on the technological, economic and political conditions to reach 100% renewable electricity (Friedrich 2012) (ADEME 2016) (Bogdanov et Breyer 2016) (Ram 2017) (Blakers, Lu et Stocks 2017) (Lu, Blakers et Stocks 2017) (Zappa, Junginger et Van der Broek 2018) (Brown, et al. 2018).
Nevertheless, low carbon electricity does not represent a silver bullet to decarbonise the entire energy system. Some uses are not “electricity compatible” and have to rely on other energy vectors. Mobility can be switched partially to electric motor but it is still not possible for long road transportation or aviation because of the size, the weight and the cost of batteries needed. Likewise, some industrial processes do not use electricity so they have to use other decarbonisation options as cement factories, chemicals processes. Besides, some electric devices can be used but are a non-sense from the efficiency point of view such as convectors heating. Therefore, other energy carriers like heat, gas, fuels, mandatory for some use applications, have to be decarbonised too. Hence, production of these carriers have to switch from fossil fuels that emits lots of GHG and pollutants to renewables sources, which cannot be produced with renewable electricity. For example, heat can be produced thanks to solar energy with thermic panels or also thanks to a heat network supplied by renewable sources such as wood or wastes. Biofuels produced by biomass helps also to use less fossil fuels.
Next section will develop the multiple possible use of biomass, a renewable energy sources, for different energetic purposes.

Bioenergy role in decarbonisation

Biomass feedstocks overview

Biomass feedstock can be separated in three main categories. First generation biomass gathers food production relative cultures: sugarcane, beetroot, corns, colza, maize, sunflower, palm or even soya.
These resources aim at feeding the planet but are also convertible in biofuels as biodiesel for oil seeds and bioethanol for sugar seeds. It is their only possibility of energetic valorization. Second generation biomass is referring to lignocellulosic resource: forests, plants for energetic purpose such as Miscanthus and agricultural wastes. Several energetic conversions are possible: direct use in heat, biofuels (Diesel Fisher-Tropsch and Ethanol) and electricity or gas conversion. This generation of biomass will be the center of this thesis because of its high potential and advanced end-uses possibilities. Third generation is composed of algae and microalgae. For the moment, this energetic conversion process are only at an early step of R&D for all valorization possibilities: electricity, fuels, gas and heat. Their maturity is still limited compared to other generations, so that we will not discuss this resource in this work as mentioned in the last section. In the end, if it will develop in the future, this resource will boost the green gas production field (Solagro, Afterres2050 2016). Bioenergy is the only source of renewable energy able to provide electricity, but also heat, fuels for transport and gas.
In theory, this resource represents a way to store carbon thanks to photosynthesis. Indeed, all the carbon released during energetic valorization has been stored during the growth of the biomass, so that the carbon footprint in the entire life cycle is neutral or negative. However, concerns about biomass sustainability and biomass breeding plans have to be paid attention, in order to be a sustainable carbon sink. For decarbonisation purpose, the release of other GHG, such as nitrogen oxides due to biomass exploitation and the changes that could occur through land use, land-use change and forestry (LULUCF) have to be considered and their negatives side effects for emissions minimized (United Nations 2020).
In addition, it is worth creating technological revolutions as reorienting crops to pull even more carbon dioxide out of the atmosphere and store that carbon underground. For example, crop engineering aims at breeding plants to store more carbon in their roots and then growing them with no-till methods that leave carbon undisturbed (IRENA 2014).

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Biomass, a renewable and polyvalent source of energy conversion

Bioenergy means energy content in solid, liquid and gaseous products derived from biomass feedstocks as animal by products, vegetable by products, household or industrial wastes, energy crops or woody biomass. All types of biomass use the CO2 in the air, in the soils or in the water in order to grow through photosynthesis process (Solagro, Afterres2050 2016).
As well as renewable energies like solar, wind or hydro, bioenergy represents an important feedstock for GHG emissions reductions. Bioenergy would play a major role in two of the three pillars in all deep decarbonization project scenarios where bioenergy will reduce the carbon content of all transformed energies and will switch energy end-uses to low or zero-carbon energy carriers (DDPP 2015). Because this resource represents a way to store carbon, the main strength of biomass is its carbon neutrality as carbon released during combustion had been captured during biomass growth. While other renewables are only used to make green electricity, bioenergy can be used in lots of other energy sectors.
This flexibility makes bioenergy and bioenergy technologies valuable for the decarbonisation of energy use and for reaching political objectives. In Europe, bioenergy represents almost 10% of total EU energy consumption, and 60% of its renewable energy. Used for heat at 74%, electricity at 14% and fuels at 13%, it is a key source of renewable energy to reach the European political objectives known as 3×20 with 20% of renewable energy in the energy mix in 2020, whose checking will be done at the end of the year (European Environment Agency 2017).
World bioenergy consumption has almost doubled since 2000 from 55 Mtoe to 112 Mtoe in 2015 with an annual growth rate of 5%. However, trends are different: while bioheat and bioelectricity grow by 6-7% per year, biofuels stagnate, due to European changes of regulations for biofuels in the early 2010s (REN21 2019). Indeed, biomass is one of renewable and low carbon feedstocks that represents many sources of energy that are storable and compatible with many valorization processes.
Biogas produced by biomass upgraded to biomethane has emerged as a good alternative to the use of food-based crops biofuels to replace fossil fuels for transport, due to its reduced environmental impact, reduced indirect effects and lower GHG emissions. Beyond this, biomethane is an energy carrier with a strong benefit because it is flexible in use as a gas and offers a large and seasonal storage capacity. Biomethane could even reach negative GHG emissions when it is produced from feedstocks which otherwise would emit methane during decomposition, such as manure or when the CO2 emissions associated with its use are captured and stored. Various biomethane support schemes in European countries have led to significant improvements in biogas upgrading technology. Markets have been recently developing, in Sweden for example or in France where the biomethane injected almost doubled each year since 2015 (Scarlat, Dallemand et Fahl 2018) (GRDF, GRTgaz, et al. 2019).
This biomass valorization in green gas will be at the center of our work in this thesis and next section will introduce the features of green gas in the decarbonisation context.

Green gas seen as a decarbonisation option

Gas can be a key energy for the energy transition

Gas has attracted a growing interest in decarbonizing the energy sector. Gas, as an energy carrier, has two key strong points in order to reach global emissions reductions that are required to keep the rise in global average temperatures below 2°C and to improve the world’s air quality. First, versatility: gas can play multiple roles across the energy system in a way that no other fuel or technology can match. Gas can generate electric power, can heat residential and tertiary buildings, can be used in industrial boilers and as raw material for chemistry, and be used for long distance transportation (IEA 2018). Second, the environmental dimension: combustion of gas does produce some nitrogen oxides (NOX), but emissions of the other GHG or major sources of poor air quality, particulate matter and sulfur dioxide (SOX), are negligible compared to coal or oil releases. Gas, as an energy source, releases also approximately 20-30% less CO2 than oil, 40-50% less than coal for the same energy produced. Indeed, efficient combined gas cycle power plants emit on average 440gCO2eq/kWh while oil power plants emit around 780gCO2eq/kWh and coal power plants emit almost 1000gCO2eq/kWh. Even if these rough estimates depend of the type and quality of the gas, oil and coal used, gas anyway produces less GHG releases compared to other fossils (ADEME 2010).
Gas contributes to a reliable electricity supply and can facilitate the insertion of variable renewables (VRE) in the electric network by playing a “back-up” role. As wind and solar powers are intermittent, electricity has still to be produced in the case of limited VRE production. Gas power plants, whose production variations and rampings performance are good, provide this service efficiently. Gas has also a well-developed network in many countries, for example in Europe, so that gas power plants are sure to be supplied with the resource (IRENA 2013). Gas can also be used in the transportation sector and reduce emissions from this sector by replacing petroleum products as well as fighting against local air pollution thanks to the absence of fine particles releases. It can also be used for heating in buildings or industrial processes because, although it is still a carbon energy carrier, it can reduce emissions related to heating compared to oil based fuel or high carbon electricity (Cornot-Gandolphe 2018).
For several political strategies, gas is said to be a relevant transition energy option to switch from fossils consumption to an almost 100% renewable world (European Commission 2011). At the same time, the associated underground storage infrastructures of gas are a relevant solution to the inter-seasonal storage requirements inherent to a highly renewable electricity system. This network is currently well developed with a large storage capacity, around 2.8 million kilometers of pipelines worldwide, so that gas supply is relatively reliable in many countries. For example, there are 130 TWh of gas storage capacity in France representing 3 to 4 months of national gas consumption, and 1580 TWh on a European scale. Decisions on the future of gas networks need to consider their potential to deliver different types of gas in a low emissions future, as well as their role in ensuring energy security. As gas infrastructures and networks are well developed worldwide, this energy carrier would play a major role in energetic transition thanks to its storage capacity.
Gas infrastructure will continue to play an important role under the effect of several factors: the growth of gaseous mobility uses, and a contribution of gas for electricity production that remains important for the passage of the winter peak in addition to the decarbonised electricity. However, the use of gas should deeply decarbonize itself if gas wants to play a role in achieving ambitious goals such as carbon neutrality or zero emissions that would be needed to reach the 2°C or 1,5°C Paris agreement objectives (The United Nations 2015).

Biomass and green gas: a linkage to decarbonise the gas and the energy system

In the medium term, the issue of the decarbonisation of energy systems favors the substitution of coal and fuel oil for other less carbon-based primary sources, including natural gas. However, in the longer term, the need in deep decarbonisation and almost carbon neutrality is a lot more ambitious. The fossil carbon from natural gas would be an obstacle to achieving these objectives. That is why we deal with “green gas”.
In order to consider the gas produced as “green”, these fields take in input a low carbon source of energy. Produced from renewable sources as biomass or green electricity, biomethane or green gas can complete the gas production side in order to produce low carbon gas. Although its use releases as much CO2 as any gas combustion, green gas represents a substitute for natural gas consumption limitation. It would reduce the insertion into the atmosphere of fossil carbon, based on a short carbon cycle.
Therefore, greening the gas production aims at reducing the carbon content of gas during the life cycle of this energy carrier. It allows a deep decarbonisation of the energy system thanks to the decarbonisation of its fossil source and use as an energy carrier only. Indeed, the benefits of green gas production are that it requires no change to existing equipment for natural gas boilers and gas power plants, limited adaptation for cars and that it is compatible with the current natural gas network after purification. Another benefit is also to use the value of existing assets already built through the world.
Nowadays, some installations are already producing green gas thanks to the methanisation process. This process uses biomass in input, which undergo an anaerobic digestion natural process, and has a large potential for development. However, there are arbitrations and prioritization needed for land use and management because of the competition for the resource with food needs attention. Land management should be done with care in order to avoid the overexploitation of lands to produce bioenergy. Other processes exist and are developing to produce green gas: biomass gasification and power to gas. Gasification uses a thermal process to convert lignocellulosic resource into gas, with another considerable resource potential to decarbonise the gas supply. Power to gas should also develop thanks to VRE development. Electricity that would be produced when there is low demand could go into this process to convert electricity in gas. That is also a promising technology to decarbonise gas supply (OIES 2017) (GRDF 2013).
Many uncertainties are existing concerning the development of green gas. First is the role of fossil or green gas in the future. As it is a source of energy that releases CO2, gas would not be an option in the long-term in high constraint scenarios for GHG emissions. Some scenarios shows that gas consumption should not increase a lot in 2050 and 2100 milestones to reach 2°C objectives (IEA 2019). In this way, short-term decisions on whether to invest in gas grids will have major long-term implications, in particular the need in gas infrastructures adaptations to incorporate local green gas sources. Green gas production costs are still more expensive than natural prices even with quite high CO2 prices (it would be discussed further in chapter 5), so that policy incentives have to be set to launch these fields with development uncertainties in the future and with a lot of work in R&D needed to improve processes. Besides, questions about the relative importance and roles of electricity and gas networks are central to the design of energy transitions to a low emissions future in the political and economic efforts prioritization.
Questions about the quantities and the maximum capacities for green gas production and injection are tackled as well as a focus on the already existing potential. This work tries also to define some threshold costs and possible subsidies for which technologies would develop or not in different CO2 taxations contexts for decarbonisation purpose. Then, as the energy system is complex with many interactions between productions, conversions and consumptions, we will deal with several technologies and energy carriers. They are as good competitors in some cases but they act in synergies in other cases. That is why it is worth studying the development of these green gas fields, its incorporation in gas markets and the linkages existing with other vectors, carriers and networks in the future of energy.

Table of contents :

Introduction
Climate, energy and institutional context
The 3 decarbonisation pillars
Low carbon electricity, a key option to decarbonise energy systems, but not the silver
Bioenergy: another alternative to decarbonise almost all energy systems?
Renewable gas: the missing piece of the decarbonisation puzzle?
Objectives of the thesis
Outline of the thesis
Keywords
Chapter 1: The emergence of greening the gas in energy sector
1. The energy and climate context
1.1. A recent strong growth in world energy consumption
1.2. Relative threats for Earth sustainability
2. Decarbonisation, the most important challenge of XXIst century?
2.1. Causes, consequences and impacts of climate change
2.2. The three decarbonisation pillars
2.3. Energy transition and decarbonisation options
3. Bioenergy role in decarbonisation
3.1. Biomass feedstocks overview
3.2. Biomass, a renewable and polyvalent source of energy conversion
4. Green gas seen as a decarbonisation option
4.1. Gas can be a key energy for the energy transition
4.2. Biomass and green gas: a linkage to decarbonise the gas and the energy system
Chapter 2: State of the art of greening the gas production
1. Technical and economic aspects of natural gas
1.1. Gas physical properties
1.2. Natural gas value chain
1.3. Gas consumption
1.3.1. World mapping
1.3.2. Gas uses
1.4. Gas relative emissions
1.5. Gas trade and prices
1.5.1. International exchanges
1.5.2. Prices settlement
1.6. Projections of the future of gas
2. Presentation of renewable gas
2.1. History of biogas and technological introduction
2.2. Green gas technologies portfolio presentation
2.2.1. Methanisation
2.2.1.1. Process description
2.2.1.2. Type of inputs
2.2.2. Gasification
2.2.2.1. Process description
2.2.2.2. Type of gasifiers
2.2.3. Electricity electrolysis
2.2.4. Summary
2.2.4.1. Technological development
2.2.4.2. Green gas technologies benefits
2.2.4.3. Green gas technologies concerns
3. Green gas system details
3.1. Historical production
3.1.1. Biogas production
3.1.1.1. World
3.1.1.2. Europe
3.1.2. Biomethane production
3.1.2.1. World trends
3.1.2.2. Europe
3.1.2.3. France
3.2. Biogas and biomethane potentials
3.2.1. World
3.2.2. Europe
3.2.3. France
3.3. Biogas and biomethane costs
3.4. Political instruments to incentivize green gas production
Chapter 3: Energy system modelling and thesis methodology
1. POLES Model presentation
1.1. General aspects
1.2. Biomass energy modelling
1.2.1. Biomass potential
1.2.2. Bioenergy routes
2. Overview of the gas system modelling approach
3. Green gas energy module in POLES
3.1. General structure
3.2. Detailed code mechanisms
3.2.1. Methanisation
3.2.2. Gasification
3.2.3. Power to gas
3.2.4. Technology modelling
4. Database structure
4.1. Green gas production
4.2. Resource potentials and marginal costs
4.3. Technological costs and performances
5. Scenarios considered
5.1. Climate policies scenarios
5.2. Technological scenarios
5.3. The importance of CCS
5.4. Summary
Chapter 4: Green gas in sectoral decarbonisation
1. Increasing green gas in the gas supply
1.1. Main features of the gas supply
1.2. Regional specificities for green gas production
1.3. Scenarios sensitivity for green gas and fossil gas production
1.4. Conclusion for greening gas supply
2. Gas role in transport decarbonisation
2.1. Current road transportation structure
2.1.1. The actual park
2.1.2. Road transportation emissions mitigation solutions
2.1.3. Transportation related policies
2.2. The gaseous mobility
2.3. Expected future for road mobility
2.3.1. Benchmark
2.3.1.1. Future of mobility patterns
2.3.1.2. Future of gaseous mobility
2.3.2. Mobility module in POLES and analysis
2.4. Discussion
2.4.1. The need of reducing mobility environmental footprint
2.4.2. The gas mobility role in mobility mixes
3. Other sectors decarbonisation
3.1. Buildings
3.1.1. Overview of energy consumption in buildings
3.1.2. Results and analysis
3.1.3. Discussion
3.2. Industrial decarbonisation feedstock
3.2.1. Overview of energy consumption in industry
3.2.2. Results and analysis
3.3. Agriculture decarbonisation
3.3.1. Overview of energy consumption in agriculture
3.3.2. Results and analysis
Chapter 5: electricity and gas networks interactions
1. The electricity system in the decarbonisation context
1.1. The electricty generation mix
1.1.1. Current electricity production overview
1.1.2. The merit order mechanism for power plants allocation
1.2. The need of different flexibility solutions
1.2.1. Dispatchable production plants.
1.2.2. Flexibility brought by demand
1.2.3. Storages systems
1.3. The different time-horizons of flexibility
2. Interactions between gas and electricity networks
2.1. Limits of electrification and high rates of renewables incorporation
2.2. Power to gas technological costs
2.3. Power to gas business model assessment
2.3.1. Methodology
2.3.2. P2G business model discussion
2.3.3. The question of CO2 cost accounting
2.4. Conclusion
2.4.1. P2G business model
2.4.2. Demonstration projects and challenges
Conclusion and perspectives
Green gas: an important option for energy system decarbonisation
Green gas: a long term electricity storage option
Greening gas supply: still a prospective field?
Threats and concerns with gas and green gas use and development
Perspectives of this work
Appendix
Appendix 1: Biomethane detailed techno economic parameters
Appendix 2: Biomethane feed in tariffs in France
Appendix 3: Green gas production database
Appendix 4: Electric mixes for P2G business model study
Glossary
Index of figures
Index of tables
Bibliography
Executive summary

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