Page 180 - New Trends In Coal Conversion

Technologies for control of sulfur and nitrogen compounds and particulates 143

from the burning of coal and other fossil fuels. The most important emissions to air
from the combustion of coal are NO x ,SO x , particulate matter, and GHGs such as
CO 2 . Other substances such as CO, heavy metal, hydrogen fluoride, halide com-
pounds, unburned hydrocarbons, nonmethane volatile organic compounds
(NMVOCs), and dioxins are emitted in smaller quantities but may have a significant
influence on the environment due to their toxicity or their persistence.
Options of control techniques for the main pollutants are reviewed in the following
sections.

6.2.1 NO x removal technology
6.2.1.1 What is NO x ?

The term NO x is shorthand in chemistry for molecules containing nitrogen and one or
more oxygen atoms. In atmospheric chemistry, NO x is a generic term for the nitrogen
oxides that are most relevant to air pollution, namely nitric oxide (NO) and nitrogen
dioxide (NO 2 ). NO and NO 2 are major pollutants in the atmosphere, precursors to
acid rain, photochemical smog, and tropospheric ozone accumulation. They are corro-
sive and hazardous to health. NO x gases are usually produced from the reaction be-
tween nitrogen and oxygen during the combustion of fossil fuels in air, especially at
high temperatures (Mollenhauer and Tsch€ oke, 2010; Omidvarborna et al., 2015;
Annamalai and Puri, 2007).

6.2.1.2 NO x formation and types
There are three types of NO x : thermal, fuel, and prompt. The first one is highly tem-
perature dependent and comes mainly from natural gas combustion, whereas the sec-
ond one comes from fuel combustion. Prompt NO x is usually accepted as negligible.
• Thermal NO x : Thermal NO x is formed when the high temperature oxidation of diatomic ni-

trogen finds air during the combustion process (Beychok, 1973). Above 1600 C, molecular
nitrogen and oxygen are disassociated to atoms and take part in different reactions.
• Fuel NO x : Some estimations affirm that 54% of anthropogenic NO x is caused by transpor-
tation fuel. This type of NO x could reach 50% of total emissions when combusting oil
and 80% when combusting coal (Beychok, 1973).
• Prompt NO x : Prompt NO x is formed when atmospheric N 2 reacts with some emitted frag-
ments from fuel, such as carbon or methane, acting as radicals. This kind of NO x contributes
to a minimal percentage of the total (Omidvarborna et al., 2015).

6.2.1.3 General techniques for NO x emission control

The control of NO x emissions is generally achieved using the following three kinds of
techniques (Fig. 6.1):
• Precombustion controls (primary techniques)
• In-combustion controls (primary techniques)
• Postcombustion controls (secondary techniques)