Page 45 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

18 1 Air Emissions

Fig. 1.3 Fate of combustion-related air emission control

and secondary air pollutants may enter built environments through building enve-
lopes and/or ventilation systems, and there is also an exchange between indoor and
outdoor environments.
According to the fate of the air emissions, technical approaches to effective air
emission control can be classified by contaminant species, if a specific compound is
tracked from its birth in combustion to air dispersion in the atmosphere. Alterna-
tively, they can also be classified by the point of control, as in
• Pre-combustion approaches,
• In-combustion approaches, and
• Post-combustion approaches.
In this book, we follow the latter classification (pre-combustion, in-combustion,
and post-combustion approaches), which is based on the location of air emission
control approaches with each compound at the second level. Pre-combustion
approaches are the most cost-effective, because elements that may be converted into
air pollutants in a combustion process are taken out of the fuels. Examples include
coal washing, crude oil refinery, and natural gas sweetening (more in-depth dis-
cussion later). Despite fuel cleaning prior to combustion, air emission forming
elements are still in the fuel.
In-combustion approaches include combustion process modification. For
example, lowering the combustion temperature can reduce the formation of nitric
oxides (NO x ), while combustion at high temperature promotes complete combus-
tion and consequently reduce the formation of hydrogen carbon (HC) and carbon
monoxide. Injection of a calcium-based sorbent into the furnace can reduce the
downstream concentration of sulfur dioxide (SO 2 ).
As a final attempt to capture air emissions, air pollutants and GHGs are separated
from the post-combustion gases, namely flue gas or engine exhaust, with air
cleaning devices. These air cleaning devices are usually designed for specific air
emission species. Air pollutants penetrating through these devices are discharged
into the atmosphere through the stacks of stationary facilities or the exhaust pipes of
engines.
The last step is called air dispersion. Effective air dispersion helps reduce the
immediate negative impact on local air quality; however, it does not reduce the total
amount of air pollutants or GHGs entering the atmosphere.