113
5
Hydrothermal Processes
in Subcritical Water
5.1 intrOdUCtiOn
As discussed in Chapter 4, gaseous water, steam, at high temperature and pressure
is a powerful reactant to produce gaseous synthetic fuels from a variety of carbonaceous feedstock. While the products formed depend on the operating conditions,
the catalyst, and the nature of feedstock, steam plays a very powerful role in the
gasification and reforming processes. In recent years, significant efforts have been
placed to explore the role of liquid water as a reactant and/or a reaction medium at
high temperature and pressure for the production of a variety of synthetic gaseous,
liquid, and solid fuels [1–10].
The properties of water at high temperature and pressure are significantly
different from those at room temperature and pressure [5,11–13]. For many different
types of carbohydrate feedstock, water provides an environment such that at a temperature of 180°C–250°C, a residence time of 1–12 h, and a pressure above saturation pressure at this temperature range, hydrothermal carbonization (HTC) occurs
producing mainly (50%–80%) solid char with about 5%–20% liquid dissolved in
water and a small amount of gas (2%–5%). As the temperature increases to about
270°C–390°C, the residence time of few hours, and the pressure below the critical
pressure of about 213 atm, carbohydrates are converted to liquids by the process
called hydrothermal liquefaction (HTL). The amount and nature of liquid produced
depend on the nature of feedstock, the operating conditions, and the nature of the
catalyst (if present). At much higher temperatures (>300°C), hydrothermal gasification (HTG) occurs.
The chapter addresses this hydrothermal biomass–water conversion chemistry
under subcritical conditions. The issues of purification, upgrading, and utilization of
the products obtained from the three processes—HTC, HTL, and HTG—are also
examined. In particular, a hydrothermal upgrading (HTU) process to upgrade the
products from the HTL is briefly described.
The chapter also addresses various aspects of coal–water chemistry under hightemperature and high-pressure conditions. While water does not have as much affinity
for coal as it has for biomass, water can also play an important role in coal liquefaction.
The weathered coal created by pretreatment with water can have a significant negative
effect on the yield and products of coal liquefaction. Water can also act as a hydrogen
donor for the coal liquefaction process under high-temperature and high-pressure conditions. Finally, coal–water slurry, if prepared properly, can be a good feedstock for
combustion in boilers, diesel engine, or gas turbines. The chapter briefly examines
these three roles of water in coal–water interactions under subcritical conditions.
5
Hydrothermal Processes
in Subcritical Water
5.1 intrOdUCtiOn
As discussed in Chapter 4, gaseous water, steam, at high temperature and pressure
is a powerful reactant to produce gaseous synthetic fuels from a variety of carbonaceous feedstock. While the products formed depend on the operating conditions,
the catalyst, and the nature of feedstock, steam plays a very powerful role in the
gasification and reforming processes. In recent years, significant efforts have been
placed to explore the role of liquid water as a reactant and/or a reaction medium at
high temperature and pressure for the production of a variety of synthetic gaseous,
liquid, and solid fuels [1–10].
The properties of water at high temperature and pressure are significantly
different from those at room temperature and pressure [5,11–13]. For many different
types of carbohydrate feedstock, water provides an environment such that at a temperature of 180°C–250°C, a residence time of 1–12 h, and a pressure above saturation pressure at this temperature range, hydrothermal carbonization (HTC) occurs
producing mainly (50%–80%) solid char with about 5%–20% liquid dissolved in
water and a small amount of gas (2%–5%). As the temperature increases to about
270°C–390°C, the residence time of few hours, and the pressure below the critical
pressure of about 213 atm, carbohydrates are converted to liquids by the process
called hydrothermal liquefaction (HTL). The amount and nature of liquid produced
depend on the nature of feedstock, the operating conditions, and the nature of the
catalyst (if present). At much higher temperatures (>300°C), hydrothermal gasification (HTG) occurs.
The chapter addresses this hydrothermal biomass–water conversion chemistry
under subcritical conditions. The issues of purification, upgrading, and utilization of
the products obtained from the three processes—HTC, HTL, and HTG—are also
examined. In particular, a hydrothermal upgrading (HTU) process to upgrade the
products from the HTL is briefly described.
The chapter also addresses various aspects of coal–water chemistry under hightemperature and high-pressure conditions. While water does not have as much affinity
for coal as it has for biomass, water can also play an important role in coal liquefaction.
The weathered coal created by pretreatment with water can have a significant negative
effect on the yield and products of coal liquefaction. Water can also act as a hydrogen
donor for the coal liquefaction process under high-temperature and high-pressure conditions. Finally, coal–water slurry, if prepared properly, can be a good feedstock for
combustion in boilers, diesel engine, or gas turbines. The chapter briefly examines
these three roles of water in coal–water interactions under subcritical conditions.