Èñòî÷íèê: Nano Dispersions Technology, Inc., Bldg. 231, City of Knowledge, Clayton, Panama. Department of Aerospace Engineering and Mechanics, University of Minnesota, Minneapolis, Minnesota 55455, USA and Department of Mechanical and Aerospace Engineering, University of California, Irvine, California 92617, USA.
http://www.aem.umn.edu/people/faculty/joseph/archive/docs/390.pdf
In this paper we discuss the possible clean coal applications of colloidal dispersions of coal in
water (CCW) that we manufacture with a proprietary wet-comminution device. These dispersions
are a new material because the coal particles do not settle but are held in suspension by Brownian
motions. The closest coal water slurries used previously are dispersions of micronized coal with
mean particle sizes greater than fifteen microns; these dispersions are not colloidal because the
particles settle rapidly under gravity leaving clear water behind. A new material like CCW has a
possibly vast but unknown field of applications in improvements and strategies for new clean coal
technologies.
In this paper we discuss the possible clean coal applications of colloidal dispersions of
coal in water (hereafter called CCW), which we are able to manufacture cheaply and in
large quantities with a proprietary wet-communition device. These dispersions are a new
material because the coal particles do not settle but are held in suspension by Brownian
motions (see Fig. 6 in section IV). The closest coal water slurries used previously are
dispersions of micronized coal with mean particle sizes greater than fifteen microns;
these dispersions are not colloidal because the particles settle rapidly under gravity
leaving clear water behind. A new material like CCW has a possibly vast but unknown
field of applications in improvements and strategies for new clean coal technologies.
Because coal is cheap and abundant (per million BTU, compared with natural gas and
oil), its global consumption will likely increase under any foreseeable scenario as
described in Ansolabehere et al. (2007). Its low cost and wide availability also make it
particularly attractive in major developing economies for meeting their energy needs.
However, the combustion of coal introduces pollutants harmful to the environment; the
reduction of harmful pollutants in the combustion of coal is the goal of clean coal
technologies. Our study targets the possible applications of CCW in these technologies.
In the work that follows, we will be describing the development and production of a
colloidal or nano suspension of coal in water. While at first glance this could be
considered a coal water fuel, its attributes are entirely different from any coal slurry
known thus far and it can be considered a new material, yet to be tested. The fact that this
material has a large population of colloidal particles conveys properties never seen before
in coal water fuels, such a non-settling behavior even after several months of storage.
Brownian motions and particle entanglements prevent settling.
The macro-view of the fuel is that of a uniform, non-particulate liquid. This allows
for various things. Flow can be controlled with great precision (this is also a plus in
applications such as gasification) and the time scale of particle combustion is reduced
along with an increase in particle reactivity (Davies et al., 1999).
As will be further described later, this material is different from other types of milled
coal, such as micronized coal (DOE-Report NETL, 2001). The latter has no significant
submicron particles, as opposed to the material advanced here. A colloidal suspension of
coal in water can address the issues associated with firing coal in LSD and CCGT.
The first step of the road to an ultra clean coal water fuel has to do with cleaning. There are well-established commercial coal-cleaning technologies (Harrison et al., 2002) with many different approaches. Physical cleaning is typically used to reduce ash levels to around 8% and coal preparation plants incorporate a wide array of solid-liquid separation equipment. The type of process may vary considerably from coal to coal. In general, however, coal cleaning involves the separation of carbonaceous material from mineral matter by settling or by froth flotation. The key of the process is the degree of liberation of the coal from the mineral. The process described in this work mills coal to colloidal sizes by comminution, in the presence of water. This process can help improve mineral separation and enhance both physical and chemical cleaning procedures that are sensitive to particle size. Froth flotation and ash removal by chemical means are examples of this type of operations where reaction rates and efficiencies, as well as flotation, can be influenced by size.
Large hard coal particles have been known to induce wear in cylinder walls of diesel
engines and turbine blades (Wibberley et al., 2008). This means that coal cleaning for
mineral matter is not enough to prevent wear. The coal particles must be very small. The
colloidal suspension of coal advanced here has a large population of very small particles.
This is an important and useful feature not only in terms of settling properties and overall
stability. It can also help establish combustion patterns such that whatever particle outside
the submicron domain would burn in a space heavily populated by very small burning
particles that in turn aid the combustion of the larger particle (this is still a research issue
that will be addressed later). Another property of polydisperse CCW is that the colloidal
population acts as a “pseudo” fluid that impedes the settling of larger particles.
Another important advantage of the colloidal particles is related to the possibility of
reducing the water content of the fuel. Traditional slurries prepared with say, a grind of
200 mesh (less than 75 ?m), are very difficult to make with less than 40% water due to
particle packing limits and particle interactions that strongly increase the viscosity. When
a large fraction of submicron particles exists, the addition of specific amounts of larger
particles allow for a reduction in water content without an associated viscosity increase,
by modifying the maximum packing fraction of the particulate system.