Chemical Engineering August 2014 - 36

Combustion control:
Feature Report
vanced combustion-control systems
will always be chosen in accordance
with the expectations of the specific
application. These expectations can
vary widely among applications,
even in cases where the combustion
itself is basically similar. Gas
turbines are applied, for example,
in both stationary power plants and
in jet aircraft engines. The expectations
of the former case allow rackmounted
hardware elements in an
extra chamber, while special microcomputers
( " embedded systems " )
of minimal sizes and weights are
required in the aviation industry.
Rack-mounted hardware and embedded
systems can be considered
as the two ends of the spectrum of
the hardware that is generally used
for decision makers of combustion
control, and selecting among them
will be mostly determined by the
application area, rather than by the
combustion process itself.
Although the hardware platforms
of the control decision-maker can
be very different, a common requirement
is superior reliability.
The same is also true for the software
side of the decision makers.
In order to satisfy the need for high
reliability with an acceptable cost,
supplier companies deploy many
modern tools of quality control and
standardization on several levels.
Another consequence of the need
for high reliability may be some degree
of conservatism with regard to
new ideas for improving reliability.
This phenomenon can be observed
in many other similar situations
in which reliable operation is an
absolute priority. Several industry
branches, including oil-and-gas,
chemicals and food, are characterized
by the presence of high pressures
and temperatures, and the
involvement of hazardous materials
and expensive assets and raw materials.
This necessitates a high level
of security. Although for industrial
combustion control this conservatism
may be somewhat exaggerated,
numerous statistical analyses [18-
20] do suggest a definite lag in the
use of advanced control technologies
for combustion processes instead of
traditional ones, compared to the
stable power production,
less corrosion, higher
efficiency
DeNOx control:
less corrosion, optimized
NH3 consumption
Emission
control
O2 /temperature
NH3
HCl and or NH3
Filter control: minimized lime
consumption, less corrosion,
Figure 3. TDLS measurements are used in a municipal waste incinerator. The dust
load is 5-10 g per standard cubic meter, the path length of the measurement is 3-8 m.
Fast measurements with large temperature variations pose the main challenge [11]
use of advanced controllers in other
types of industrial processes (see
box, p. 37).
Built-in software tools realizing
some advanced control algorithms
as part of decision makers will be
provided by most suppliers of industrial
control systems. Although
very often, ranges and functionalities
of these tools are still rather
limited. In these cases, an option
for realizing them is an external,
general purpose (maybe PC-based)
computer, where the need for clear
interfaces between traditional and
advanced control systems will be
set up and realized. This approach
can be followed not only in the case
of new control systems for combustions
systems, but also while upgrading
existing plants.
All advanced methods offered
by the control theory to be realized
as software elements of decision
makers cannot be discussed
here, of course. The literature is
voluminous, but some textbooks are
available [25-29]. Some existing
directions and results of modern
control theory may have crucial significance
in other application areas
like robotics and flight stabilization
[30-32]. However, a brief overview
is included here of those methods
that (1) are mature enough, and
(2) show significant potential for
use in industrial combustion control
applications. Most of them are
model-based procedures, a fact
36 ChemiCal engineering www.Che.Com august 2014
that revalues all activities on modeling
combustion systems. Both
theoretical and empirical modeling
directions have their roles and
importance, and a rapid increase
of semi-empirical modeling can be
observed nowadays [33].
Model predictive control. Almost
all distributed control system (DCS)
vendors offer model predictive control
(MPC) as an advanced extension
to their standard control algorithms
[15], and this appears to be
the most frequently used advanced
control method currently. The reasons
for its relatively wide use come
from its relative simplicity and from
its inherent properties that fit well
with the general requirements of
industrial combustion control.
The history of MPC has resulted in
several variants, the most important
of which is DMC (digital matrix control).
It can be considered as a subset
to MPC, characterized by simpler algorithms
requiring less online computational
demand, and the lack of
some services of MPC, like handling
constraints inherently.
This is actually one of the most
important advantages of MPC in
many real industrial applications.
While theoretical control may disregard
constraints, real applications
definitely cannot. Examples in combustion
processes are actuators (for
instance, fuel-supply valves), which
are evidently characterized by their
limited operating ranges. Further
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Chemical Engineering August 2014

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