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Introduction
In recent years, the cement industry has seen a tightening of both CO
and NO
X
emission limits that have essentially ‘boxed’ operators into a
very narrow operating window, requiring expensive end-of-pipe control
technologies, such as selective non-catalytic reduction (SNCR), selective
catalytic reduction (SCR), and others. The nature of some industrial
combustion processes makes the simultaneous reduction of NO
X
and CO
very difficult. A targeted injection of oxygen can be an effective tool to
reduce these air pollutants, providing operational flexibility by addressing
CO from the combustion process at the outlet of the system. Praxair’s
Hot Oxygen OPTILANCE™ technology (Figure 1) uses a specialised lance,
which injects a jet of high temperature, high momentum oxygen into
a process stream. The high velocity jet rapidly mixes with process gases.
High temperatures in the jet result in high concentrations of free radicals,
enabling rapid reactions at lower bulk temperatures. Due to the jet
mixing characteristics and reactivity achieved, OPTILANCE technology is
significantly more effective at initiating low temperature reactions than a
standard oxygen jet.
To reduce operating costs and to provide a very luminous hot flame,
most cement kilns use coal, petcoke, or refuse-derived fuels (RDFs) from
the recycling industry. Pollutant formation in such systems is inevitable.
NO
X
forms when N
2
in the preheated air stream is exposed to very high
flame temperatures. NO
X
may also form when nitrogen bound in the fuel
matrix is released during combustion, if sufficient excess O
2
is present.
The classic method to control CO and NO
X
from combustion
processes has been to control the amount of excess oxygen. Sufficient
excess oxygen ensures low CO emissions and complete combustion
(see operating point 1 in Figure 2). Without equipment changes, lower
NO
X
can only be achieved by lowering excess oxygen. As the amount of
Stefan Laux, Praxair Inc.,
discusses
the use of oxygen injection to reduce
CO and NO
X
emissions in cement kilns.