Chemical Engineering August 2014 - 39

to learn. Special optimum-seeking
algorithms are used during the
learning phase to find these interconnecting
strengths (weightings),
so that the network outputs that
result from training inputs would
best fit the target outputs that correspond
to the same inputs. If the
output of a combustion process will
be applied as training input to the
ANN, and the process input is applied
as a target output of the ANN,
the artificial neural network will
be trained as a controller. After the
training phase, the ANN will be able
to find the process input to match
the desired process output.
General experience with artificial
neural networks suggests that they
have excellent capability for learning
the corresponding input-output
pairs. However, in the case of inputs
that lie somewhat further from the
training-input data, error margins
from the ANN output may be surprisingly
large.
Advanced controller tuning. A
possible upgrade of traditional controllers,
such as PID controllers, is
the application of advanced methods
for optimizing their parameters,
or possibly changing their configurations.
Most of the configurations
on this basis belong to a wider theoretical
approach called " adaptive
control. " Most important for combustion
control are " gain scheduling "
and " multimode control. " Both
concepts require a " scheduling variable, "
the actual value of which essentially
characterizes the process
behavior. In the case of combustion
processes, a very good candidate for
this is the load signal. However, the
introduction of further variables
may result in two- or multi-dimensional
scheduling variables as well.
In the most frequently applied
case, the one-dimensional scheduling
variable will be divided into discrete
ranges, within which one set
of tuning parameters or controller
configurations will be applied. If the
parameter set of a controller will
only be changed while shifting from
one section to another one along
the scheduling variable range, the
system is called " gain scheduling, "
while in other cases, the entire controller
will be changed. The latter
case is called " multimode control, "
which has greater adaptivity but
also higher complexity.
The relative simplicity of these
techniques is a great advantage, although
assuring bumpless changes
between the operating ranges requires
significant designer effort
and knowledge.
Loop decoupling. Similar to the
previous controller tuning, loop
decoupling is not really an advanced
control method, but rather
an advanced extension to classical
control methods. This technology
can be applied to processes characterized
by strong internal crosscouplings.
That is, for processes
in which the independent, scalar
control loops strongly disturb each
other. Based on a process model, a
so-called decoupler can be designed.
A decoupler is a dynamic system
in the DCS, and its outputs will be
directed to the inputs of the actual
process. The aim of this design is
that the virtual system that results
from the decoupler plus the process
itself form a system free of internal
cross-couplings. And this type of
system can already be controlled by
a series of classical controllers. The
limitations of this method are evident,
however. If linear controllers
(like classical PIDs) are intended to
be used for the virtual system, and
the actual process is not linear, then
the method can get rather complicated.
Benefits
of advanced control
The advantages of advanced combustion
control are mainly higher
efficiency and lower pollutant levels,
although other goals such as
higher safety levels are possible.
Advanced control can be characterized
by fast response times, and allowing
a process to be run within a
narrower window of process parameters
(Figure 4).
When the deviations around the
setpoint are smaller, it is possible,
as the example shows, to increase
the average operation temperature
of the combustor (in this case a
boiler), and still avoid overheating
the furnace. By raising the operating
temperature, the overall thermal
efficiency can be augmented.
The example discussed earlier
illustrates a special characteristic
of the advanced combustion control.
Namely, that two goals can be
served simultaneously - in conventional
cases, these are in opposition
to each other. For example,
the introduction of carbon capture
facilities will significantly increase
fuel consumption, and by extension,
operating costs, while an efficiency
boost will decrease both emissions
and fuel cost. And this end can be
achieved through the use of advanced
combustion control, the
investment costs of which are far
lower than those of any other modifications
in the process itself.
It is likely that several advanced
combustion-diagnostic methods
currently used exclusively in laboratories
will eventually find their
way into industrial applications to
further optimize combustor performance
in various applications. And
existing combustors stand to benefit
as well, beause they can often be
upgraded with advanced combustion
control technology [7].
■
Edited by Scott Jenkins
Author
Pal Szentannai is a professor
at the Budapest University of
Technology and Economics
Department of Energy Engineering
(Muegyetem rkp. 9,
H-1111, Budapest, Hungary;
Phone: +36 1 463 1622; Email:
szentennai@energia.bme.hu)
He is also the executive committee
member designated by
Hungary in the IEA-FBC (International
Energy Agency-
Fluidized-Bed Conversion) Implementing Agreement.
He has authored and edited several books
on the topics of advanced power-plant process
control and fluidized-bed combustion. Szentannai
has several years of direct experience in the
energy industry as an engineer working on design
and commissioning of numerous thermal
and control systems.
Maximilian Lackner is a
chemical engineer who lectures
at Vienna University of
Technology (Getreidemarkt
9/166, 1060 Vienna, Austria;
Phone: +43 681 81 82 6762;
Email: maximilian.lackner@
tuwien.ac.at) and Johannes
Kepler University Linz. He
has developed in-situ laser
diagnostics for combustion
processes and carried out research
on laser ignition. Lackner has founded
five companies. He has written the textbook
" Combustion: From Basics to Applications "
(Wiley VCH, 2013). Lackner is also editor of the
five-volume reference work " Handbook of Combustion "
(Wiley VCH, 2010).
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