Combustion Science and Technology Abstract
Combustion Science and Technology Predictions made by climate researchers are highly worrisome and demand rapid action to avoid the threat of climate catastrophe. Global energy systems must be transformed as quickly as possible by minimising or avoiding net greenhouse gas emissions. There is broad agreement on this goal, demonstrated by international treaties such as the Sustainable Development Goals of the United Nations and the European Green Deal presented in 2019.
However, the practical measures required for the transition are the subject of heated discussion. Consensus on the goal, dissent on the pathway is how the situation can be summarized.
This opinion article aims to bring engineering sciences into the centre of the discussion. We are concerned that technological options that are important for our society from an ecological and economic point of view are being neglected. We plead for competition between all technological solutions to reach the goals in the best possible way and to consider feasibility, ease of transition, and economical and societal aspects.
We are convinced that the thermochemical utilisation of chemical energy carriers is an important component of future energy systems and is key to enabling climate neutrality. Biogenic and synthetic carbonaceous and carbon-free chemical energy carriers will be indispensable for reliable power generation and energy supply for mobility, industry, and buildings.
This opinion article is the result of intensive discussions between a group of more than fifty internationally renowned researchers who are scientifically engaged in thermofluids and energy process engineering. With this article we express our plea: Let us consider all options and explore new ideas that will move us towards a climate-neutral energy system!
Combustion Science and Technology Introduction
A key challenge of the 21st century is transforming the energy industry into a climate-neutral circular economy. To meet this global challenge, energy must come from renewable energy sources and emission of greenhouse gases must be avoided. Governments mostly agree about this goal but not about how to achieve it. To meet climate targets, a combustion science and technology agnostic approach is indispensable, since the energy mix of the future will be more diverse and will include technologies that have not yet been well researched. Gradual, manageable transformation of energy systems towards climate neutrality will be made possible by including chemical energy carriers and processes for thermochemical and electrochemical energy conversion. These topics will be discussed using Germany as a case study as most German examples are similar to other parts of Europe and the world despite differences in energy mix, resources, and policy.
Realizing a climate-neutral circular economy requires electrification of the mobility, industry, and building sectors1, as well as energy carriers from non-fossil fuel sources for energy storage and transport, which will largely depend on renewable electricity with limited energy contribution from biomass sources. Consequently, the worldwide demand for electrical energy will increase sharply in the coming years. However, meeting this demand is not the only challenge faced by the energy sector. Zero Impact Combustion Science and Technology Energy Transformation Processes
Depending on requirements, energy can be stored electrically, electrochemically, mechanically, thermally, or chemically. These methods differ in their storage capacity and discharge duration. As chemical energy carriers combine a high storage capacity with the longest discharge durations.
Chemical Energy Carriers
Chemical energy carriers are distinguished by their origin. Significant fossil fuel chemical energy carriers are natural gas, crude oil, and coal, which were formed from organic material in geological prehistoric times. Synthetic chemical energy carriers are produced using renewable energy. To produce so-called e-fuels, electrical energy can be converted into hydrogen by electrolysis, then used and stored directly, or converted into liquid or gaseous synthetic fuels (by catalysis), which is more easily stored and higher in volumetric energy density. Synthetic chemical energy carriers include biofuels, such as ethanol.
Synthetic chemical energy carriers are carbonaceous or carbon-free. Carbonaceous energy carriers include synthetic natural gas and synthetic fuels such as e-fuels or biofuels. When consuming these energy carriers, the greenhouse gas carbon dioxide (CO2) is released. However, if the carbon in the fuel originates from biomass, CO2 that was removed from the atmosphere, or power and industry plant exhaust, there are zero carbon dioxide net emissions.
Thermochemical Energy Conversion
The energy stored in chemical energy carriers from renewable sources can be used by employing electrochemical and thermochemical processes. Electrochemical processes, such as fuel cells and redox flow batteries, are slowly becoming established on the market despite enormous development efforts, whereas thermochemical processes are widespread and particularly reliable. Thermochemical energy conversion of carbonaceous chemical energy carriers from renewable sources is of great importance, particularly during a transitional phase, which can last for decades. There is no time limit for the thermochemical energy conversion of carbon-free chemical energy sources. In many sectors, this approach will be important to energy supply in the long-term.
The Combustion Science and Technology Phases
Development of a climate-neutral energy system needs time and resources. Worldwide, infrastructure must be rebuilt, new plants built, and innovative technologies conceived and developed.
To keep technological and economic risks manageable, we argue for a transformation that is as continuous as possible. In our opinion it makes sense to proceed rapidly, steadily, and with plannable steps that we classify in three higher-ranking phases:
- Short-term: Focus on drop-in technologies that use existing infrastructure and enable smooth transition. For example, natural gas can increasingly be replaced by synthetic natural gas. In this way, time is gained to prepare for parallel combustion science and technology changes.
- Medium-term: Focus on technologies in which infrastructure is supplemented or newly built. By this time, there will be power plants that can be operated with 100 % carbon-neutral energy carriers, such as green hydrogen as a substitute for natural gas (whereas only admixtures of up to a maximum of approximately 20 % are possible in the short-term). Other technologies will be under development.
- Long-term: Focus on future technologies whose potential is apparent now but which needs to be explored in greater depth.
There is a great need for research in combustion science and technology development (R&D) in all three phases. Research and development of drop-in technologies is naturally more applied, while future technologies require more fundamental investigations. Research on future technologies should pursue many different angles so as to identify the best solutions over time. Among the variety of technological options, thermochemical energy conversion based on chemical energy carriers can make a significant contribution at all three transformation stages of the energy system.
Currently, the use of electricity, fuels, and other types of energy is relatively inexpensive: In many countries, it costs little to nothing to release climate-damaging gases into the earth’s atmosphere. A global CO2-neutral and largely CO2-free economy and lifestyle entails significantly higher costs, as long as the damage caused by CO2 does not have a quantifiable economic value. These costs arise from the recovery and storage of the greenhouse gas from the atmosphere or its recovery from waste gas streams. Renewable energy is also not available for free: For example, there are considerable infrastructure costs for photovoltaics and wind power plants, as well as transport and conversion into electricity or e-fuels. The real cost of combustion science and technology energy conversion processes must be charged within the framework of international agreements. The current CO2 tax in Germany is EUR 25 per ton of CO2, rising to EUR 55 per ton in 2025, which slowly becomes comparable to the price of CO2 direct air capture of between USD 92 and 232.
- A. Electrical power
In Germany, nearly all recent science-based studies conclude that CO2-free technologies must be based primarily on renewable power generation. There is political consensus in Germany to end mining and use of coal as a fossil fuel by 2038. This is in agreement with the goals of the European Green Deal and the Sustainable Development Goals of the United Nations.
The generation of electricity from hydropower and biogenic fuels is already contributing significantly to avoiding CO2 emissions. However, both of these resources have limited availability as opposed to wind and solar energy, which will therefore play an even more important role in the future.
- Fluctuating Renewable Electricity
A central challenge with using renewable energy sources comes from the large fluctuations in generated power. Illustrate this for an annual cycle and a monthly cycle, respectively. The maximum and minimum power output from the fluctuating wind and sun as hourly averages in 2019 for Germany. The values fluctuate between almost zero and less than half of the installed capacity of renewable energies. The fluctuations of the hourly mean power output are particularly pronounced in the winter months. At this time, nearly no solar energy is available, so it cannot compensate for fluctuations in wind energy.
Hourly averaged electricity generation from wind power and photovoltaics in Germany in 2019. Highlighted are mean, maximum, and minimum power output. Data from Fluctuations are particularly pronounced in the winter months.