9
Introduction
amounts of other volatile hydrocarbons and nitrogen as well as impurities such as
sulfur, nitrogen and chlorinated compounds may also be present in the product gases.
For fossil fuels such as coal, shale oil, bitumen, tar sand, and crude oil, the gasification by steam alone has not been as effective as gasification by steam with oxygen,
carbon dioxide, and hydrogen. The thermodynamics of steam gasification of coal are
not very favorable [2]. Generally, such a gasification process predominantly generates
pure syngas at temperatures higher than 1000°C–1200°C. The required temperature
is, however, lower for biomass and low-rank coals. With excess steam and at high
temperatures, the most dominant product is hydrogen with some carbon dioxide.
When steam gasification is carried out in the presence of catalysts, such as alkalis, and Ni-based or other supported noble metal catalysts (e.g., Ru, Rh catalysts),
both gasification and reforming occur simultaneously. Along with steam gasification, steam reforming has been used for a long time to generate hydrogen needed
for the ammonia and urea productions, petroleum refining, and other hydrogenation
reactions producing chemicals. Ammonia is an important raw material for the fertilizer industry. Steam reforming is, to date, the most economical method for hydrogen
production. In the recent years, steam reforming has been carried out along with dry
reforming and partial oxidation reactions to generate syngas of various hydrogen–
carbon monoxide compositions.
Chapter 4 evaluates various aspects of steam gasification and reforming technologies (SGRT) such as (1) the mechanism and kinetics of steam gasification and
reforming processes in the presence and absence of other gases, (2) catalysis and
reactors for steam gasification and reforming processes, and (3) effects of feedstock
and operating conditions on the product distributions. The chapter also examines
underground gasification and combustion and multistage processes for steam gasification and reforming. Finally, the effects of water gas shift reaction and simultaneous presence of dry reforming and partial oxidation reactions (i.e., tri-reforming) on
the SGRT are also assessed.
In the recent years, novel approaches to steam reforming and gasification such
as solar reforming and gasification and microwave-assisted reforming have also
been investigated. Since steam reforming is an endothermic process, the use of solar
energy for heating makes the process more energy efficient. Chapter 4 examines this
and other novel steam gasification and reforming processes.
1.2.4 ChAPTer 5: SynTheTiC Fuel ProduCTion By
WATer under SuBCriTiCAl CondiTionS
In recent years, the use of water under high-temperature and high-pressure conditions (in subcritical region) to carry out various thermochemical transformations has
been increasing due to recognition that the properties of water change significantly
with increase in temperature and pressure. These changes allow a number of organic
reactions to occur in the aqueous medium. Water becomes nonpolar as temperature
increases. Chapter 5 describes in detail this new-found role of water as a reaction
medium for transformation of raw fuels such as coal, biomass, waste, and others to
produce a variety of gaseous, liquid, and solid synfuels.
Introduction
amounts of other volatile hydrocarbons and nitrogen as well as impurities such as
sulfur, nitrogen and chlorinated compounds may also be present in the product gases.
For fossil fuels such as coal, shale oil, bitumen, tar sand, and crude oil, the gasification by steam alone has not been as effective as gasification by steam with oxygen,
carbon dioxide, and hydrogen. The thermodynamics of steam gasification of coal are
not very favorable [2]. Generally, such a gasification process predominantly generates
pure syngas at temperatures higher than 1000°C–1200°C. The required temperature
is, however, lower for biomass and low-rank coals. With excess steam and at high
temperatures, the most dominant product is hydrogen with some carbon dioxide.
When steam gasification is carried out in the presence of catalysts, such as alkalis, and Ni-based or other supported noble metal catalysts (e.g., Ru, Rh catalysts),
both gasification and reforming occur simultaneously. Along with steam gasification, steam reforming has been used for a long time to generate hydrogen needed
for the ammonia and urea productions, petroleum refining, and other hydrogenation
reactions producing chemicals. Ammonia is an important raw material for the fertilizer industry. Steam reforming is, to date, the most economical method for hydrogen
production. In the recent years, steam reforming has been carried out along with dry
reforming and partial oxidation reactions to generate syngas of various hydrogen–
carbon monoxide compositions.
Chapter 4 evaluates various aspects of steam gasification and reforming technologies (SGRT) such as (1) the mechanism and kinetics of steam gasification and
reforming processes in the presence and absence of other gases, (2) catalysis and
reactors for steam gasification and reforming processes, and (3) effects of feedstock
and operating conditions on the product distributions. The chapter also examines
underground gasification and combustion and multistage processes for steam gasification and reforming. Finally, the effects of water gas shift reaction and simultaneous presence of dry reforming and partial oxidation reactions (i.e., tri-reforming) on
the SGRT are also assessed.
In the recent years, novel approaches to steam reforming and gasification such
as solar reforming and gasification and microwave-assisted reforming have also
been investigated. Since steam reforming is an endothermic process, the use of solar
energy for heating makes the process more energy efficient. Chapter 4 examines this
and other novel steam gasification and reforming processes.
1.2.4 ChAPTer 5: SynTheTiC Fuel ProduCTion By
WATer under SuBCriTiCAl CondiTionS
In recent years, the use of water under high-temperature and high-pressure conditions (in subcritical region) to carry out various thermochemical transformations has
been increasing due to recognition that the properties of water change significantly
with increase in temperature and pressure. These changes allow a number of organic
reactions to occur in the aqueous medium. Water becomes nonpolar as temperature
increases. Chapter 5 describes in detail this new-found role of water as a reaction
medium for transformation of raw fuels such as coal, biomass, waste, and others to
produce a variety of gaseous, liquid, and solid synfuels.