Page 39 - Industrial Plant 2015

the sugar is extracted from beets and concentrated in
various steps until the final stage of crystallization, use between MP and LP steam headers.
large amounts of steam and electrical energy. As a
result, each Sugar Beet plant features an integrated
Powerhouse to produce all the steam necessary to
sustain the main process.
The overall process shows strong, mutual, interactions
between multiple areas of the plant, especially between
the refining process and the powerhouse, providing an
edge to the use of APC technology.
ABB implemented an APC system to a Sugar Beet
plant in the Mediterranean region in 2013. The
application covers the evaporation section and the
Powerhouse, closely interconnected and continuously
interacting among them, to maximize energy efficiency
for the overall plant.
The APC system directly control production quality (Brix
– sugar density) by modulating the main production
parameters, like steam usage, in a proper way. In
addition, while maintaining production quality and
quantity targets, the APC system optimizes
Powerhouse, so to minimize its production costs.
This article will focus on the powerhouse section given
its widespread use in industrial plants.
Powerhouse configuration
The plant Powerhouse, despite having some peculiar
characteristics, has configuration that is relatively
common in industrial environments.
The overall layout is based a backpressure configuration,
with two separate boilers, generating steam at two
separate pressure levels, 65 and 40 bar, made available
on two headers, referred to as HP and MP steam
headers in this article.
Each steam header has an interconnected steam
turbine, with both turbines outlet connected to a
common LP steam header. Attemperators are also
available on both the boiler outlets and on the LP steam
header and Pressure Reducing Valves (PRVs) are
located between HP and MP steam headers and, also, Fig. 2 - Powerhouse
simplified
The figure 2 shows a simplified diagram of Powerhouse. configuration
Steam turbines drive two separate electrical generators,
used to provide power supply to the entire plant, for a
total generation of up to 25 MW. The internal electrical
grid operates as an electrical island with no electrical
interconnection to the national grid.
As known, when the interconnection to the electrical
grid is in place, the grid acts as a prevalent source of
energy and hence stabilizes frequency at 50 Hz.
However, in an electrical island configuration, the local
power production and consumption must be balanced
at all times to avoid frequency fluctuations. This implies
that turbine control needs to trim
steam inlet in the turbines so to balance In normal cases, the
power production following load grid acts as a prevalent
source of energy
changes.
In this configuration, any change in and hence stabilizing
frequency at 50 Hz.
electrical loads causes an action by In an electrical island
turbines control to counterbalance the configuration, the local
change; this action, in turn, affects power production and
steam demand to the boilers, causing consumption must be
a continuous trimming of boilers and balanced at all times to
steam. avoid frequency changes
In addition to that, crystallization ovens,
a key equipment in a Sugar Beet plant, in the local grid
are an important and large LP steam
user. These ovens follow specific production sequences/
batches, with steam usage that is highly variable over
time. These constant changes in LP steam usage,
combined with the need for frequency control performed
by turbines, create a strong and continuous disturbance
to the powerhouse.
Additional factors contribute powerhouse management
complexity in this site. Equipment efficiency is quite
different for the two boilers and, especially, for the two
turbines. The HP turbine is newer than the MP turbine and
its specific production, expressed as power produced
divided by the steam flow inlet, is significantly higher.
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