POWER February 2021 - 33

OPERATIONS & MAINTENANCE
■ Steam Sootblower. This method uses
an ultra-high-speed compressed
stream blown out from a nozzle to
implement cleaning. A well-designed
nozzle can create an air stream
with speed exceeding 900 meters
per second (4.5 bar at 0.2 meter).
The constraint here is that the area
where the steam travels needs to be
covered to reduce the likelihood of
damage to superheater bundles.
■ Rapping. Using rappers that knock on
the header of bundles to implement
cleaning. This method is only suitable
for horizontal passes.
■ Stationary Shockwave. Methane and
oxygen are mixed and detonated to
create a pulse to implement cleaning.
However, there is a limitation on
the size of the area around the pulse
exit where no equipment or pipelines
can be placed. Therefore, this is better
suited for installation on preexisting
manholes.
■ Mobile Shockwave. Ethane and oxygen
are mixed and detonated to create
a pulse to implement cleaning.
Contrary to the stationary shockwave
method, this method is mobile and
allows precise position cleaning. The
main constraint is that the cleaning
outcome heavily depends on the operator's
techniques and experiences.
Because the EfW plant wanted a
cleaning solution that did not require any
modifications to the boiler, but at the
same time, it demanded the flexibility to
choose cleaning positions at will, the mobile
shockwave method was deemed the
best option and was therefore chosen after
careful studies and assessment.
Mobile Shockwave Method in
Practice
To clean fouling and clinkers on the superheater
bundles, a special container
is fixed at the front-end of the ignition
lance, which is then inserted into the
boiler. The ethane-oxygen mixture is
transferred into the container and detonated,
sending out a shockwave to
shake off fouling and clinkers (Figure
2). There are three issues that need to
be considered to ensure the success of
this method: operation safety, discharge
bridging, and lifecycle impact of boiler
tubes.
Operation
Safety.
Because this
method requires detonation of an ethane-oxygen
gas mixture, safety is a
high priority. Staff needs to be briefed
in advance about safety protocols. During
cleaning, waste must not be fed
into the boiler at the time of detonation.
After each detonation, flammable gas
must be aired out. Staff must wear personal
protective equipment, and they
should all be insured.
Discharge Bridging. A massive
amount of ash may fall off due to
shockwaves from detonation, which in
turn may possibly clog the boiler. Look
through the inspection hole (Figure 3)
to ensure bottom ash has been carried
away by bottom ash extractors before
the next detonation. Otherwise, the
boiler may again need to be shut down,
which renders such cleaning method
pointless.
Lifecycle Impact of Boiler Tubes.
Shockwave is the key mechanism of
cleaning, but it may also cause damage
or deformation to superheater bundles.
One factor that can ensure no damage
or deformation is done to superheater
3. Mobile shockwave cleaning can cause
significant amounts of ash to fall from the
superheater tubes. Therefore, the bottom ash
extractor must be monitored through inspection
covers to ensure it doesn't plug up, which
could otherwise require the boiler to be shut
down for remediation. Courtesy: ECOVE Environment
Services Corp.
bundles is to control the intensity of
shockwaves by selecting the appropriate
container and adjusting the pressure
of filled gas. Other factors include
the design strength of superheater bundles
and the level of shockwave pressure
these bundles bear.
In the case of the EfW plant, experts
agreed that the maximum pressure
at the point of detonation would fall
below the pressure that superheater
bundles with conventional design of
40 bar at 400C can take (about 52 bar).
The shockwave from detonation will become
weaker in pressure as it travels
from the center outward, so that when
it reaches superheater bundles, it is not
likely to damage them.
2. This diagram shows some of the components needed to implement the mobile shockwave
method. When activated, ethane and oxygen are mixed and detonated to create a pulse, which
shakes off fouling and clinkers from the superheater tubes. Courtesy: ECOVE Environment
Services Corp.
February 2021 | POWER
www.powermag.com
Outcome
The boiler differential pressure value
serves as a quantitative indicator of
the amount of fouling in a boiler. It also
shows whether the boiler needs to go
offline for fouling cleaning. The differential
pressure value is defined as the
pressure difference between the upper
boiler chamber and the entrance to the
economizer. The higher the pressure
difference, the higher the amount of
fouling, and vice versa. From the EfW
plant's No. 1 boiler differential pressure
chart, shown in Figure 4, it can be seen
how fouling affects boiler pressures,
as well as how the mobile shockwave
method contributes to lower differential
pressures.
As can be seen from the line chart,
the EfW plant underwent three major
outages, one in October, one the following
May, and one the following December.
The prominently low differential
pressures at these three annual outages
mean that fouling has been cleaned
33

http://www.powermag.com

POWER February 2021

Table of Contents for the Digital Edition of POWER February 2021