Page 252 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

228 8 Pre-combustion Air Emission Control

compounds, and trace elements. Basic principles of coal cleaning and related
technologies have been well documented (e.g., [27]), and they are briefly introduced
as follows.
Coal cleaning methods may be classified into conventional physical cleaning and
advanced cleaning methods. Advanced cleaning methods include advanced phys-
ical cleaning, aqueous phase pretreatment, selective agglomeration, and organic
phase pretreatment. Of these alternatives, conventional physical cleaning is widely
used for the sake of low cost and relatively high efficiency.
Conventional coal cleaning involves the following steps:

(1) Crushing of the coal: Grinding into smaller particles with diameters less than
50 mm. It also liberates ash-forming minerals and inorganically bound sulfur.
(2) Particle screening: The crushed coal particles are screened into three modes;
coarse, intermediate, and fine.
(3) Floatation: It involves the separation of ash and sulfur compounds from the
coal before it is pulverized and introduced into combustion chamber. The
lighter coal particles float on top while the heavier minerals sink to the bottom
in a stream of water.
(4) Drying: In this step wet coal particles are dried using a dewatering device,
generally a vacuum filter, centrifuge, or a cyclone, to separate water from the
solid, followed by further drying in hot air.
Coal washing can remove about 60 % of ash-forming materials. It is often
necessary to reduce the excessive amount of ash-forming materials from the coal
especially for steel processing applications. For example, the lignite from India and
Greece may contain more than 50 % wt ash-forming material. A significant amount
of Pyrite (FeS x ), As, Se, and Hg can be removed by coal pre-cleaning.
Coal washing alone can remove up to 50 % of the pyretic sulfur, which is
equivalent to 10–25 % removal of the total sulfur content of the coal. Ninety
percent of the inorganic fuel sulfur, especially pyritic sulfur, FeS 2 , can be easily
removed by coal washing. The organically bound sulfur cannot be removed by
physical cleaning methods; however, it can be removed with biological or chemical
methods.
Biological treatment can remove both inorganic and organic sulfur. For example,
a bacteria, Thiobacillus ferrooxidans is capable of converting FeS 2 into water-
soluble FeSO 4 , whereas a mutant of Pseudomonas, called Coal bug 1 (CB 1 ) can
consume organic sulphur in thiophene groups. However, these techniques are time-
consuming and may require very small coal particles.
Chemical methods can remove organically bound sulfur and involves treatment
with alkaline or caustic solutions, oxidative leaching, and chlorinolysis with
chlorine-based chemicals. However, these chemical methods may change the
property of the coal and reduce its potential for use as a fuel.
Like any other process, there are pros and cons for physical coal cleaning. The
advantages are as follows