Liquid hydrocarbon fuels are an essential component of our energy system.  They have unprecedented volumetric density of energy, roughly 32 MJ/liter for gasoline, compared to a lithium ion battery at 2.4 MJ/liter.  This means that for activities requiring high or sustained power delivery such as flying and shipping there are no current alternatives.  Compared to electric motors, internal combustion engines have significantly lower efficiency, and the recent improvements in the electric vehicle sector suggest that at least for light vehicle duty their use is not essential.  However, despite significant government action to promote electrification of the light duty fleet, there will still be a period of transition that could last well over a decade as the developing world increases its consumption of transportation services and the world builds the capacity to electrify personal transportation.  Therefore, there is an urgent need to develop pathways to produce renewable liquid hydrocarbon fuels as part of the energy transition to low carbon energy systems.

The global push for economy-wide decarbonization is fueling intense interest in the potential of hydrogen as a zero-carbon resource. Long coveted as a fuel of the future, hydrogen already is being used in a variety of applications to cut carbon emissions across the globe. This chapter details a case study from Mitsubishi Power in use of hydrogen in gas turbines to produce electricity. Currently, Mitsubishi Power’s largest and most advanced gas turbines make use of a dry low-NOx (DLN) combustion system that allows operation with up to 30% hydrogen in baseline configuration. Going forward, increasing the use of hydrogen as a percentage of a power station’s fuel mix – from a mixture of around 30% hydrogen all the way up to 100% hydrogen as an energy source – requires the need for innovative equipment modifications, such as a multi-cluster combustor.

The purpose of a process heater is to heat some type of fluid, usually a liquid hydrocarbon. Process heaters, also called fired heaters, consist of the heater itself, the burners used to generate the heat, the process fluid being heated, and the controls for monitoring and adjusting the system. This chapter is not intended to be exhaustive as there are entire books written on the subject of fired heaters. Rather, it is intended to be representative with a particular focus on the fuel including a discussion of renewable fuels. Unlike many other industrial combustion systems, such as glass melters and steel reheat furnaces, the fuel composition in a process heater varies considerably. It is commonly a waste product from the production of, for example, gasoline, diesel, and jet fuel. The fuel variability is an important parameter that significantly impacts the equipment design, particularly the burners which need to operate safely on all fuels and efficiently with minimal emissions on the design fuels.  Some common applications for process heaters in those industries include distillation/ fractionation, thermal cracking, catalytic cracking, hydrotreating, hydrocracking, and catalytic reforming.

Bio-based solid fuels are the oldest and most widely exploited energy sources. Bio-based solid fuels have the potential to approach, if not achieve, net-zero-carbon emissions. Biomass is a “net zero” fuel in that the same carbon atoms pulled from the atmosphere as CO2 are sequestered by the biomass as plant tissue and are re-released as CO2 upon burning. This prevents the addition of CO2 into the atmosphere beyond what is present in the natural carbon cycle. Today’s bio-based solid fuels encompass everything from the raw biomass our ancestors relied upon for heat to thermochemically and biochemically processed biomasses for use as solid fuels in a variety of commercial energy generation scenarios. This chapter summarizes solid biofuel sources, processing, and roles in a future net-zero energy system.

A confluence of environmental and supply security are driving the aviation community to consider alternatives to petroleum-derived jet fuels.  It was recognized early on in the process that the sheer size of the existing aircraft fleet and supporting jet fuel infrastructure, along with regulatory constraints, precluded the introduction of a chemical energy carrier requiring aircraft or fuel handling equipment modifications.  Consequently, the chosen path forward was focused on synthetic alternatives with essentially identical chemical compositions and physical properties, called drop-in fuels.  This chapter will describe the regulatory basis enabling the use of these fuels by the existing aircraft fleet and the technical approach used to validate the drop-in nature of these fuels.

A confluence of environmental and supply security are driving the aviation community to consider alternatives to petroleum-derived jet fuels.  It was recognized early on in the process that the sheer size of the existing aircraft fleet and supporting jet fuel infrastructure, along with regulatory constraints, precluded the introduction of a chemical energy carrier requiring aircraft or fuel handling equipment modifications.  Consequently, the chosen path forward was focused on synthetic alternatives with essentially identical chemical compositions and physical properties, called drop-in fuels.  This chapter will describe the regulatory basis enabling the use of these fuels by the existing aircraft fleet and the technical approach used to validate the drop-in nature of these fuels.

Bio-based solid fuels are the oldest and most widely exploited energy sources. Bio-based solid fuels have the potential to approach, if not achieve, net-zero-carbon emissions. Biomass is a “net zero” fuel in that the same carbon atoms pulled from the atmosphere as CO2 are sequestered by the biomass as plant tissue and are re-released as CO2 upon burning. This prevents the addition of CO2 into the atmosphere beyond what is present in the natural carbon cycle. Today’s bio-based solid fuels encompass everything from the raw biomass our ancestors relied upon for heat to thermochemically and biochemically processed biomasses for use as solid fuels in a variety of commercial energy generation scenarios. This chapter summarizes solid biofuel sources, processing, and roles in a future net-zero energy system.

Nearly one-third of the energy produced in the US comes from liquid fuels derived from crude oils, natural gas plant liquids, and other condensates. Fuel atomization to produce spray(s) is necessary for practical combustion systems employing liquid fuels. This requirement stems directly from the high energy density of the liquid fuels. Despite the major changes underway in the portfolio of liquid fuels, fuel atomization and combustion systems have remained vastly unchanged. The current practice is to design drop-in liquid biofuels that can be used “as is” in existing combustion devices. However, such fuels can be energy intensive to produce and create wasteful byproducts, eroding the carbon footprint benefits of the liquid biofuels. Thus, it is imperative that the liquid fuel injection, atomization, and combustion systems of the future consider increased fuel flexibility to utilize both fossil and alternative fuels from multiple sources within the same combustor hardware. Fuel properties and fuel atomization and combustion hardware should be co-optimized to minimize the carbon footprint based on the life-cycle analysis of the fuel. This chapter discusses atomization of renewable liquid fuels, detailing the phenomenology and controlling physical processes.

In the search for carbon-free renewable and sustainable fuels, an underexplored option is the use of metals as recyclable energy carriers. Metals can be produced via electrolytic processes at efficiencies comparable to hydrogen- or carbon-based carriers; metals are energy-dense and stable solids that are easy to transport and store. The key limitation to the use of metals as recyclable fuels is the lack of any mature technology for power generation using metal fuels. This chapter will review the overall concept of metals as recyclable fuels, discuss the possible options for metal-fueled power-generation systems, and identify the remaining science and technology gaps.

Gaseous renewable fuel combustion is of primary interest for a range of applications including aircraft engines, ground power engines, reciprocating engines, and industrial furnaces, among others. While much of the combustion science and engineering that are needed to design and operate such devices is well developed and available in modern textbooks, the attainment of even higher efficiencies, greater performance, and reduced emissions for an ever-increasing array of new fuels and fuel blends requires an even deeper understanding of fundamental combustion concepts and the underlying physical and chemical phenomena. In many cases, these fundamental concepts are areas of much recent and ongoing research.  This chapter describes the basic combustion and chemical kinetic properties of the fuels, namely hydrogen, syngas, ammonia, methane, natural gas, and ethanol, considering the flame temperature, ignition delay time, flammability limit, laminar flame speed, and fuel stretch sensitivity.

The purpose of a process heater is to heat some type of fluid, usually a liquid hydrocarbon. Process heaters, also called fired heaters, consist of the heater itself, the burners used to generate the heat, the process fluid being heated, and the controls for monitoring and adjusting the system. This chapter is not intended to be exhaustive as there are entire books written on the subject of fired heaters. Rather, it is intended to be representative with a particular focus on the fuel including a discussion of renewable fuels. Unlike many other industrial combustion systems, such as glass melters and steel reheat furnaces, the fuel composition in a process heater varies considerably. It is commonly a waste product from the production of, for example, gasoline, diesel, and jet fuel. The fuel variability is an important parameter that significantly impacts the equipment design, particularly the burners which need to operate safely on all fuels and efficiently with minimal emissions on the design fuels.  Some common applications for process heaters in those industries include distillation/ fractionation, thermal cracking, catalytic cracking, hydrotreating, hydrocracking, and catalytic reforming.

The introduction of new fuels into the market is a unique opportunity to take advantage of new fuel compositions to improve the efficiency and emissions of internal combustion reciprocating engines and alternative fuel feedstocks. However, there are numerous challenges that introductions of new fuels face before they can become first legal, then ubiquitous. This chapter reviews four different case studies related to changing fuel composition. In some circumstances, the fuel formulation was changed in seemingly minor ways, and resulted in the unanticipated consequences. In other cases, a fuel change was desired, but an unexpected barrier slowed the introduction of the fuel change. These case studies should be viewed as opportunities to understand the interdependencies that exist and factors that need to be considered when trying to change the fuel in the marketplace.

In the search for carbon-free renewable and sustainable fuels, an underexplored option is the use of metals as recyclable energy carriers. Metals can be produced via electrolytic processes at efficiencies comparable to hydrogen- or carbon-based carriers; metals are energy-dense and stable solids that are easy to transport and store. The key limitation to the use of metals as recyclable fuels is the lack of any mature technology for power generation using metal fuels. This chapter will review the overall concept of metals as recyclable fuels, discuss the possible options for metal-fueled power-generation systems, and identify the remaining science and technology gaps.

Ammonia is the second most transported chemical in the world today, with a global annual trade of around 180 Mtons. The history of the chemical’s generation and widespread utilization is based around demand from global food production, resulting in rapid expansion of the fertilizer industry through the twentieth century. Current widespread utilization of ammonia facilitated by global transportation has been enabled through the significant breakthrough of two German Nobel prizewinners (Fritz Haber and Carl Bosch) in the early twentieth century. Their catalytic Haber–Bosch process enabled the creation of ammonia from its constituent elements on industrial scale for the first time. The chemical can be utilized as a fuel via two main routes: first, by cracking ammonia to recover hydrogen prior to utilization in a combustion system or fuel cell, or secondly by direct ammonia use. Whereas the former requires an additional process penalty, the latter is less well publicized to the inherent difficulties associated with direct ammonia/air utilization, excessive NOx production when unproperly burned, and slow reaction kinetics, resulting in challenges associated with ignition and flame stability. Recent advances on enhanced ammonia combustion strategies have increased the potential of directly fired ammonia utilization or ammonia/fuel mixtures.

Ammonia is the second most transported chemical in the world today, with a global annual trade of around 180 Mtons. The history of the chemical’s generation and widespread utilization is based around demand from global food production, resulting in rapid expansion of the fertilizer industry through the twentieth century. Current widespread utilization of ammonia facilitated by global transportation has been enabled through the significant breakthrough of two German Nobel prizewinners (Fritz Haber and Carl Bosch) in the early twentieth century. Their catalytic Haber–Bosch process enabled the creation of ammonia from its constituent elements on industrial scale for the first time. The chemical can be utilized as a fuel via two main routes: first, by cracking ammonia to recover hydrogen prior to utilization in a combustion system or fuel cell, or secondly by direct ammonia use. Whereas the former requires an additional process penalty, the latter is less well publicized to the inherent difficulties associated with direct ammonia/air utilization, excessive NOx production when unproperly burned, and slow reaction kinetics, resulting in challenges associated with ignition and flame stability. Recent advances on enhanced ammonia combustion strategies have increased the potential of directly fired ammonia utilization or ammonia/fuel mixtures.

Hydrogen will play an increasingly important role in the push toward greater use of renewable energy and the reduction in carbon emissions from the transportation sector, electrical energy generation and transmission, and the production of commodity chemicals, such as ammonia and polyolefins. In this chapter, the operating principles of fuel cells and electrolyzers are detailed. The main function of these devices is the interconversion of electrical and chemical energy.