POWER July 2015 - 38

ENVIRONMENTAL ISSUES
1. The PCME STACK 181 WS. Courtesy:
Altech PCME
2. Schematic of the PCME 181 WS sampling system. Courtesy: Altech
PCME
T1
300C
Heating
chamber
Heater 1
T2
300C
Heater 2
LMS181
T5
dP
Eductor
Venturi
Analyzer
enclosure
Check
valve
T6 > 190C
Measurement
cell
toring device, the PM CPMS. According to
the rule, a PM CPMS:
Isolation
valve
Gas flow
■ Must be based on an operating principle
based on in-stack or extractive light scatter,
light scintillation, beta attenuation,
mass accumulation detection of the exhaust
gas, or representative exhaust.
■ Must have a cycle time (a period required
to complete sampling, measurement, and
reporting for each measurement) no longer
than 60 minutes.
■ Must be capable of detecting PM concentrations
of 0.5 mg/m3.
Since initial MATS final rule promulgation
in 2011, the PM CPMS has found its way
into other rules, such as the Portland Cement
(PC) and ICI Boiler maximum achievable
control technology (MACT). Some sources
affected by the PC MACT have reportedly
adopted the PM CPMS as an alternative to
PM CEMS, and the difficulties associated
with PS-11 correlation testing and have established
operating limits as per the description
in the rule.
The Technology
Altech PCME is one of very few companies
with a proven technology that will provide
reliable and continuous monitoring after a
wet FGD.
PCME has developed an extractive optical
wet PM CEMS-the STACK 181 WS-based
on the STACK 181, a light scatter instrument
utilizing PCME's ProScatter technology (Figure
1). This extractive instrument takes a representative
continuous sample from the stack
(under fixed flow or isokinetic conditions),
heats it far above dew point, and evaporates
any water droplets present to enable measurement
of the particulate concentration by an
optical technique. The approach overcomes
38
T3 internal ambient temperature
T4 external ambient temperature
the problem of interference from condensation
and water droplets when using an in-situ
particulate monitor after a wet process.
The PCME STACK 181 WS uses an eductor
to extract a continuous sample from the
stack via a probe. The sample flow rate is measured
by a differential pressure technique and
then controlled with a microcontroller and a
variable speed pump, thus enabling flow to be
set at a constant value or varied over a limited
range in unison with a 4-20 mA or Modbus
external stack flow meter signal.
The sample is kept at an elevated temperature
as it transits a short length of heated line
and passes directly into a heated vaporizing
chamber to evaporate water droplets and elevate
the sample temperature above the dew
point. Within the vaporizing chamber, the
sample flow becomes cyclonic to maximize
the contact with the chamber wall and flash
vaporize the water droplets.
The fact that the sampling system is powered
by an eductor driven with an integrated
sampling pump means there are no moving
parts that contact the stack effluent (Figure
2). This reduces the possibility of blockage or
Main
blower
contamination of the pump mechanism. Flue
gas is returned to the stack by the same sampling
port through which the sample is taken.
The key measurement part of the system
is provided by the PCME ProScatter Forward
Scatter particulate monitor (PCME STACK
181), which benefits from using a narrow
forward angle of scatter that minimizes the
effect of changing particle size and refractive
index (Figure 3). While the calibration of the
instrument is sensitive to different particle
sizes, the instrument has reduced sensitivity
compared to light scattering sensors using
angles of scatter further from the angle
of incidence. The instrument benefits from a
powerful graphics user interface, suitable for
the set-up and automatic control and measurement
of the light scatter sensor, heater
systems, and sample line flow.
The PCME STACK 181 Forward Scatter
analyzer built into the wet stack system has automatic
zero and span checks on the measurement
instrument to ensure good quality measurements
and to permit early diagnosis of any deterioration
in system performance. Instrument self-checks
automatically deploy a reference scattering body
3. Scattered. The orange rod, shown here in the raised position, turns the diffuser into the
active position. When the laser beam strikes a particle, these electrons will scatter through the
sample within a defined area called the interaction volume. Source: Altech PCME
Laser
Rod turns diffuser into
active position
Detector
Quartz rod
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POWER | July 2015
Scattered light
Collecting mirror Beam dump

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POWER July 2015

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