Page 112 - New Trends In Coal Conversion

Minimization of Hg and trace elements during coal combustion and gasification processes 75

from iron, steel, and nonferrous metal production (71%) and the combustion of fossil
fuels (18%). The largest anthropogenic source of Hg emissions to air on a global scale
is the combustion of coal and other fossil fuels (54.9%). Other sources include metal
production (17.5%) and cement production (11.5%).
These emissions have been notably decreased since 1990. A combination of
targeted legislation, improved controls, and abatement techniques have, in general,
led to significant progress being made in most countries to reduce heavy metal emis-
sions. Changes in fuel use within this sector are mainly responsible for decreases in
emissions from energy production sector, particularly fuel switching from coal to
gas and other energy sources in many countries, the closure of older, inefficient
coal-burning plants, and improved pollution abatement equipment (EEA, 2017).
However, predicting future emissions can also be vital for preparing reduction stra-
tegies that aim to reduce emissions consistently in the future. Mercury emission inven-
tories are proving to be a major challenge even for those countries with extensive data
on coal use. Pacyna et al. (2006) produced three future scenarios for Europe for 2010
and 2020. The first was based on a Business as Usual (BAU) scenario, assuming
economic development and new control technologies but without the pressure of
new environmental laws. The Policy Target (POT) scenario assumes that all existing
EU directives are implemented fully by the relevant target years. The Deep Green
(DEG) scenario assumed implementation of all solutions/measures leading to the
maximum degree of reduction of mercury emissions. The predicted results ranged
from around 20% mercury reduction under the BAU scenario to 40% under the
POT scenario and 80% with the DEG scenario. This type of emission prediction allows
governments and other agencies to consider how much could be achieved in terms of
pollution reduction with different approaches. Their next step would likely be to deter-
mine the most economic option of obtaining a result somewhere between the POT and
DEG scenario results while emissions in Europe were predicted to continue to decrease
under all scenarios (Sloss, 2009).

3.5 Technologies for mercury and TE control

TE concentrations in coal are variable and their behavior during coal combustion and
gasification processes quite complex. However, due to the emerging legislation,
mainly on mercury, many researchers and companies evaluate different control
options.
One of the simplest ways for reducing TE emissions from coal conversion pro-
cesses is to burn less coal or to burn the coal more efficiently. Some coal plants are
moving to cofiring biomass or other materials with coal as a move toward lowering
CO 2 emissions. This can have an effect on TE emissions, usually achieving a reduction
due to the lower concentration of TEs, especially mercury, in most biomass materials
(Table 3.3) and due to the change in combustion and ash conditions (Sloss, 2009). On
another hand, coal washing is standard at many mines and plants to reduce the ash con-
tent and increase the calorific value of the coal. Conventional coal washing can remove
mercury associated with mineral materials but will not remove any mercury associated