During combustion in a boiler, the primary pollutants nitrogen oxides (NOX), sulfur oxides (SOX) and mercury tend to form predominant species, but with trace to significant concentrations of alternate compounds. Both the major and minor species have a critical impact on the selection of control methods and how the control equipment performs in operation.
Basic chemistry tells us that very high temperatures will cause elemental nitrogen (N2) and oxygen (O2) to react to produce nitrogen oxides. For example, lightning from a thunderstorm produces such
use of overfire air, where initial combustion with a slightly lean air mixture allows a significant portion of the coal’s nitrogen atoms to self-react to produce N2. Final combustion is achieved with additional air injection above the main flame front. This nitrogen chemistry is by far the most thermodynamically stable reaction, as the triple bond between nitrogen atoms in N2 is very strong. New developments in overfire air technology are further improving NOX reduction capabilities. Suffice it to say that the process alone can often reduce NOX concentrations to well below 0.20 lb/MMBtu.
For additional NOX reductions, selective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR) are the choices, with
oxides, which in turn enter the soil where they become nitrogen sources for plant fertilization. However, what is often not realized is that of the NOX produced in a coal-fired utility boiler (or other coal furnaces, for that matter) generally only 25 percent comes from nitrogen and oxygen reacting in the combustion air. The other three-quarters come from nitrogen in the fuel, which, as the coal particles decompose and combust, reacts with oxygen.
The type of furnace greatly influences this latter mechanism. Cyclone boilers, where the pebble-sized coal particles take longer to decompose, generally produce NO in quantities exceeding 1.0
X
lb/MMBtu, while pulverized coal units may only produce half that amount, or less. So from just a boiler design standpoint, the type of boiler has a large impact on the amount of NO to be removed.
X
The other major factor is the speciation of NO . Typically, 90
X
percent or so of the initial product is nitrogen monoxide (NO), with
most of the remainder consisting of nitrogen dioxide (NO ). While
2
NO is partially soluble in water (and even more so with further
2
oxidation to N O ), nitrogen monoxide is not. This eliminates wet
25
scrubbing as a method for NO removal and has led to development
of other techniques for NO reduction. One of the earliest was the
X
the latter able to reduce NOX concentrations to 0.10 lb/MMBtu or lower. In both mechanisms, ammonia (NH3) or urea (N2H4CO) is injected into the flue gas stream to react with NOx. The equations below are representative of the process.
4NO + 4NH3 + O2 ➔ 4N2 + 6H2O 2NO +4NH +O ➔3N +6H O
2 3222
By the far the most prominent sulfur species that arises from
combustion is sulfur dioxide (SO ). It is this compound that
2
is removed in scrubbers. However, small amounts of sulfur
trioxide (SO ) also form in the furnace and a small percentage
3
of additional SO may form due to sulfur dioxide oxidation
3
in SCR catalyst beds. It is well known that sulfur trioxide,
when added in controlled amounts to flue gas, will lower the
resistivity of sub-bituminous fly ash such as that generated
from Powder River Basin (PRB) coals. But excess SO induces a
3
blue plume in stack emissions and, perhaps more importantly,
reacts with atmospheric moisture to produce sulfuric acid
(H SO ) aerosols.
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