144
Water for Energy and Fuel Production
Coal impurities prohibit the use of CWF in turbines and diesel engines. Although
CWF with significantly higher specifications than coals used in previous turbine
and diesel engine tests is now possible with the development of the UCC product
described in Section 5.5.3.1.1. Several programs initiated by the Department of
Energy in the late 1970s help the development better injection systems for the use
of CWF in diesel engines and turbines. The major considerations for CWF use are
ignition timing, plugging, and sticking issues [125–129]. The use of CWF in diesel
engines and gas turbines are now possible. Direct firing of coal requires micronizing
to <20–30 μm for diesel engines and <10 μm for gas turbines and producing a CWF
containing around 50 wt% coal. In the past, CWF was largely used for the compression ignition (diesel) engine [125–129]. The characteristics of injection and combustion of CWF in diesel engines are significantly different to those for diesel fuels due
to the combined effects of poorer atomization and the time required to evaporate
the slurry water. However, combustion and thermal efficiencies matching diesel fuel
have been achieved for CWF at up to 1900 rpm [125–129].
The most researched area has been the design of the injectors, which gives the
optimum atomization of fuel for the best combustion and thermal efficiencies.
Coal particle size and rheology of coal–water mixture play a very important role
in efficient atomization. The engine modifications such as purged shuttle fuel pump
plunger, electronically timed injection, diamond compact injector tip nozzles, tungsten carbide-sprayed cylinder liner and top ring set, and pilot injection of diesel are
some of the engine modifications considered for the successful direct injection of
CWF [125–129]. The fate of mineral matter and its effect on the engine wear and
how to minimize coal agglomeration during the evaporation of individual CWF
droplets are also required further investigations [125–129] (Penn State’s coal-water
slurry fuel program, 2012, pers. comm.).
5.5.3.1 Production of CWF
The production of CWF requires sophisticated treatment of coal to remove mineral
matters and sulfur, and prepare a very fine particle size such that CWF can be used
as a replacement of heavy fuel oil not only in boilers but also in diesel engines and
gas turbines. The preparation of such coal follows multiple steps: physical cleaning,
advanced coal processing projects, advanced coal milling, and chemical cleaning.
Physical cleaning of coal is carried out by a wide array of solid–liquid and solid–
solid separation processes. Floatation technologies and various dewatering systems
are some of the processes used for this purpose. The objective is to remove ash
and other mineral matters without losing coal. Most of the current milling, separation, and dewashing techniques allowed coal particles to get down to 30–40-μm
size. Further removal of impurities and mineral matters required the applications of
chemical methods.
Advanced physical coal cleaning was developed by Bechtel and AMAx [130].
The primary objective was to produce UCCs suitable for conversion to stable and
highly loaded CWF. The main specification was an ash content of <1%–2%.The separation technologies were advanced column froth floatation and selective agglomeration. A more novel process for preparation of ultraclean micronized coal was
researched in China, based around high-pressure water jet milling [131,132]. It was
Water for Energy and Fuel Production
Coal impurities prohibit the use of CWF in turbines and diesel engines. Although
CWF with significantly higher specifications than coals used in previous turbine
and diesel engine tests is now possible with the development of the UCC product
described in Section 5.5.3.1.1. Several programs initiated by the Department of
Energy in the late 1970s help the development better injection systems for the use
of CWF in diesel engines and turbines. The major considerations for CWF use are
ignition timing, plugging, and sticking issues [125–129]. The use of CWF in diesel
engines and gas turbines are now possible. Direct firing of coal requires micronizing
to <20–30 μm for diesel engines and <10 μm for gas turbines and producing a CWF
containing around 50 wt% coal. In the past, CWF was largely used for the compression ignition (diesel) engine [125–129]. The characteristics of injection and combustion of CWF in diesel engines are significantly different to those for diesel fuels due
to the combined effects of poorer atomization and the time required to evaporate
the slurry water. However, combustion and thermal efficiencies matching diesel fuel
have been achieved for CWF at up to 1900 rpm [125–129].
The most researched area has been the design of the injectors, which gives the
optimum atomization of fuel for the best combustion and thermal efficiencies.
Coal particle size and rheology of coal–water mixture play a very important role
in efficient atomization. The engine modifications such as purged shuttle fuel pump
plunger, electronically timed injection, diamond compact injector tip nozzles, tungsten carbide-sprayed cylinder liner and top ring set, and pilot injection of diesel are
some of the engine modifications considered for the successful direct injection of
CWF [125–129]. The fate of mineral matter and its effect on the engine wear and
how to minimize coal agglomeration during the evaporation of individual CWF
droplets are also required further investigations [125–129] (Penn State’s coal-water
slurry fuel program, 2012, pers. comm.).
5.5.3.1 Production of CWF
The production of CWF requires sophisticated treatment of coal to remove mineral
matters and sulfur, and prepare a very fine particle size such that CWF can be used
as a replacement of heavy fuel oil not only in boilers but also in diesel engines and
gas turbines. The preparation of such coal follows multiple steps: physical cleaning,
advanced coal processing projects, advanced coal milling, and chemical cleaning.
Physical cleaning of coal is carried out by a wide array of solid–liquid and solid–
solid separation processes. Floatation technologies and various dewatering systems
are some of the processes used for this purpose. The objective is to remove ash
and other mineral matters without losing coal. Most of the current milling, separation, and dewashing techniques allowed coal particles to get down to 30–40-μm
size. Further removal of impurities and mineral matters required the applications of
chemical methods.
Advanced physical coal cleaning was developed by Bechtel and AMAx [130].
The primary objective was to produce UCCs suitable for conversion to stable and
highly loaded CWF. The main specification was an ash content of <1%–2%.The separation technologies were advanced column froth floatation and selective agglomeration. A more novel process for preparation of ultraclean micronized coal was
researched in China, based around high-pressure water jet milling [131,132]. It was