Preface

Clean energy is defined as energy obtained from renewable and zero-emission sources; it also refers to a range of environmentally friendly energy options derived from renewable, low-emission sources such as solar geothermal energy, hydropower, ocean power, and wind power. Unlike fossil energy sources, clean energy sources do not run out and constantly renew themselves. For this reason, clean energy is also called sustainable energy. Other natural resources such as plants and animal wastes and hydrogen are also among the elements from which clean energy is obtained. Unlike depleting fossil energy sources, clean energy is less costly. It does not harm the environment, contributes to the protection of nature, and is the most efficient use of nature. Today, countries adopt clean energy technologies and infrastructure, invest in renewable energy sources, and prioritize energy efficiency practices to accelerate the transition to an affordable, reliable, and sustainable energy system.

Clean energy is one of the most effective ways to combat climate change. While clean energy can be defined as energy obtained from sources that do not pollute the air, produce greenhouse gas emissions, or cause any harm to nature, green energy is the energy obtained from natural sources. On the other hand, renewable energy is produced from continuously renewable and inexhaustible sources. The direct use of renewable energy in reducing carbon emissions is more on the agenda today. Effective improvements in energy efficiency, the importance of which is often overlooked, are also considered. Issues such as the greater inclusion of electric vehicles in the transportation system, the widespread use of heat pumps in residences, the inclusion of clean hydrogen and its derivatives in the energy system, and the use of bioenergy, carbon capture, and storage technologies come to the fore.

Hydrogen, one of the clean energies, has the potential to solve the growing energy crisis today due to its clean, renewable, high-energy density, and non-carbon fuel properties. Hydrogen is a synthetic fuel that can be produced from various raw materials such as water, fossil fuels, and biomass using primary energy sources. During the production phase, there are many alternative production technologies such as steam recovery, waste gas purification, electrolysis, photo processes, thermochemical processes, and radiolysis. Hydrogen has a variety of uses, including nonpolluting vehicles, fuel cells, home heating systems, and aircraft. Furthermore, using hydrogen as an energy carrier is a long-term option for reducing global carbon dioxide emissions by obtaining high-value hydrocarbons through carbon dioxide hydrogenation. The costs of a hydrogen-based system, however, are still high. Therefore, research on hydrogen, which is abundant in nature, continues so that hydrogen-based systems, particularly for long-term energy storage, can become commercially attractive. The use of hydrogen energy as a complement to traditional energy sources also aids in the implementation of low-carbon solutions.

This edited collection consists of two sections and nine chapters. The first section includes case studies for hydrogen production research. The second section presents research on gasification, a process in which a limited amount of oxygen, air, air-water vapor mixture, or enriched oxygen-containing air is introduced to carbon-containing materials such as coal and biomass, which is used to obtain combustible gases such as methane and hydrogen.

Chapter 1 introduces the topic. Chapter 2 proposes a new photocatalyst to replace an organic photocatalyst for hydrogen generation from water and hydrogen sulfide. Chapter 3 discusses various routes to produce solar-based hydrogen, namely, photocatalytic, photo-electrocatalytic, and photobiological decomposition. It also discusses mitigation measures to prevent charge carriers from recombination during the photocatalyst process. Chapter 4 focuses on the use of hydrogen in aviation, the modifications needed to adapt an existing gas turbine to use hydrogen, and a CFD simulation of an existing gas turbine burning hydrogen. Chapter 5 discusses the use of hydrogen oxyfuel in glass and metal industries as a source of heat. The authors suggest an improvement of the exhaust gas stream for energy recovery and to decrease emissions. Moreover, they examine safety issues in the use of hydrogen energy and the acceptance of the community. Chapter 6 studies the effect of hydrogen and carbon monoxide on aluminosilicate refractory. Chapter 7 reviews recent advances in supercritical water gasification of pulping black liquor for hydrogen production. Chapter 8 criticizes minimizing carbon dioxide emissions from coal gasification processes. Finally, Chapter 9 discusses how to improve sustainable hydrogen production yield in hydrothermal gasification processes through novel metal catalysts.

> **Murat Eyvaz** Associate Professor, Department of Environmental Engineering, Gebze Technical University, Kocaeli, Turkey

> > **Dr. Yongseung Yun** Institute for Advanced Engineering, South Korea

Section 1

Hydrogen Energy Processes

**Dr. Ahmed Albahnasawi** Department of Environmental Engineering, Gebze Technical University, Kocaeli, Turkey Section 1
