Page 57 - New Trends In Coal Conversion

Coal 25

is related with emissions to the atmosphere. During combustion, elements in the coal
are converted to their respective oxides. Some of these oxides are emitted in the form
of gases or aerosols (liquid droplets), or they may be adsorbed by particulates (fly ash)
and continue through the flue gas stream as emissions. Gaseous emissions include wa-
ter vapor, sulfur oxides (SOx), nitrogen oxides (NOx), carbon dioxide (CO 2 ), and they
may contain compounds and elements identified as hazardous air pollutants (HAPs)
such as mercury (Finkelman and Greb, 2008, and references therein).
Because current regulations in industrialized countries have placed limits on emis-
sions and waste products from power plants, clean coal technologies offer a series of
technological options for improving the environmental performance of coal, reducing
emissions while at the same time increasing the amount of useable energy derived from
each tonne of coal. These options include more efficient coal preparation, which serves
not only to increase the heating value of the coal and therefore the efficiency of the
combustion process but also reduces the levels of sulfur and ash-forming mineral mat-
ter. This may in turn help to reduce the amount of waste (another issue of environ-
mental concern) and SOx together with potentially harmful trace elements
associated with the combustion process. The release into the atmosphere of fine par-
ticulates from coal combustion is reduced, if not totally eliminated, by incorporation
of electrostatic precipitators (ESPs) or baghouses with fabric filters in the combustion
stream. These recover any suspended coal ash (fly ash) and other fine particulates with
an efficiency of up to at least 99.5%. As a result, coal power plants produce 80%e90%
less particulate, sulfur oxide (SOx), and nitrogen oxide (NOx) emissions than just a
few decades ago (National Coal Council, 2015).
In the following sections of this book modern coal combustion (and gasification)
technologies for avoiding negative environmental impacts and for generating more po-
wer using less coal are fully described.

1.6.2.2 Utilization of coal at coking plants

Coke is and will continue to be in the future an indispensable raw material for steel
production worldwide. As in the case of coal combustion, coke plants also have to
meet the legal environmental worldwide regulations, which are increasingly restric-
tive, especially with regard to the control of emissions.
During the process of coking coal at coke plants, several pollutants are generated
and released from the coke ovens to the atmosphere. The main harmful emissions
are NOx, SO 2 gases, and particulate matter whose source is the flue gas from the com-
bustion of cleaned coke oven gas in the battery. Among the modern emission control
measures introduced at coke plants, advanced processes for cleaning coke oven gas in
the by-product plant, such as scrubbing processes to desulfurize the coke oven gas, are
important measures directed at reducing emissions to the atmosphere. Also, filter
equipments are used to prevent the emissions of particulate matter during the process
of coal charging and coke pushing. Other issues of concern at coke plants are fugitive
emissions (that include VOCs, SVOCs, and PAHs, Aries et al., 2007) produced in the
batteries from leaking oven doors, lids, and ascension pipes, as well as fugitive emis-
sions in the by-product plant from leaking pipes, flanges, and valves, which can be