POWER February 2021 - 32

OPERATIONS & MAINTENANCE
Cleaning Boiler Superheaters
with a Mobile Shockwave
Method
One of the keys to boiler efficiency in thermal power plants is ensuring fouling and
clinkers formed on superheater bundles are routinely removed. A mobile shockwave
cleaning method offers one innovative solution.
Chien-Pang Yang, Chieh-Ming
Yang, Ching-Yun Yi, and Teng-Hui
Huang
I
n general, due to the complexity of
waste properties, using waste as fuel
tends to create a significant amount
of fouling and clinkers. Furthermore, an
ill-designed boiler with insufficient space
between superheater bundles is likely
to suffer from fouling and clinker problems
more often. This article discusses
a successful case employing a mobile
shockwave method to clean fouling
and clinkers from superheater bundles,
which lowered downtime for cleaning,
helped boost uptime by 200 hours, and
increased waste-handling capacity by
2,500 tons per boiler annually.
Fouling and Clinker Formations on
Superheaters
A superheater is one operation unit in
the boiler that serves to further heat
the steam to produce desired steam
for turning turbines. Fouling and clinker
buildup on superheaters occur from
two processes: initial fouling and clinker
due to inertia effect. Initial fouling
consists of a thin layer of chemically
active ash formed by volatile elements
and tiny ash particles that condense on
superheater bundles. Such sedimentation
has particles rich in alkali metal
oxides, chloride, and sulfate, which are
able to attach to the tube walls due to
Van de Waals force and electrostatic
force. The particles also react with
tube metals to become chemical compounds
with a low boiling point, which
in turn strengthens the attachment of
these tiny particles onto the walls. Due
to its excellent insulation property, fouling
will impede heat transfer.
Clinker due to inertia effect, on the
other hand, is the process of larger particles
attaching onto initial fouling due
to inertia effect. This is the main threat
to a boiler's safe operations, as fouling
32
and clinkers can quickly build up when
the sticky initial fouling attracts larger
particles. In particular, when the temperature
of these particles is so high
that they become molten, it aggravates
the sedimentation problem. The
thickness of fouling and clinkers may
vary depending on tube structure, air
dynamics, tube surface temperature,
tube location, and the physical and
chemical properties of fuels.
Compared to conventional fuels (such
as gas and coal), waste generally has a
lower heating value and higher water
content, making it a poorer form of fuel.
In addition, waste content varies widely,
which makes the burning process and
fouling problem more complicated.
Problem Faced and Cleaning Options
An energy-from-waste (EfW) plant in
Taiwan uses vertical passes of superheaters
and economizers. The spaces
between superheater bundles in this facility
are too narrow by design, such that,
when the flue gas passes through, fouling
and clinkers easily get stuck onto the
bundles (Figure 1).
Over time, they clog the passes and
hinder the smooth flow of flue gases,
making it difficult for induced draft fans
to suck out exhaust gases. This raises
flue gas temperatures, reduces heat
exchange efficiency, and reduces the
durability of boilers. Consequently, the
boilers needed to be shut down for
at least 48 hours just two and a half
months after annual outages, so that
fouling and clinkers could be cleaned
manually. This is equivalent to more
than 200 hours of downtime per boiler
per year, which is costly in terms of a
loss of waste that could have been incinerated,
a loss of power that could
have been generated, and a loss of
revenue. It also creates problems for
the local government in making waste
www.powermag.com
1. Energy-from-waste boilers can have
serious fouling issues due to the nature of
the fuel. In this case, inadequate spacing
in the superheater bundle, shown here,
also contributed to high fouling and clinker
formation. Courtesy: ECOVE Environment
Services Corp.
mobilization plans.
The most straightforward way to clean
fouling and clinkers is to send workers
inside the boiler to scrape them off with
simple tools. However, this is a laborious
task and requires boilers to go offline,
which is inefficient. There are several
other cleaning options that can be done
online. They include:
■ CO2
Shockwave. This method uses
the shockwave generated from the
detonation of heated, liquidized CO2
in a lance to implement cleaning.
The constraint in using this method
is that the lance needs to be fixed
to a mount on the boiler wall, which
limits the scope of areas that can be
cleaned.
■ Hydroblasting. The idea here is to
use a water lance to clean fouling
and clinkers. When water touches
the hot surface, the water droplet is
instantly absorbed and evaporates as
vapor, where the volume will expand
1,000 times and turn into an explosion,
shaking off fouling and clinkers.
However, the water lance cannot be
aimed at superheaters, otherwise,
it will create extreme thermal shock
that can seriously damage superheaters.
POWER
| February 2021
http://www.powermag.com

POWER February 2021

Table of Contents for the Digital Edition of POWER February 2021

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