Apparatus for transporting emissions from a stack

The apparatus and method of the present invention extract samples of emissions within a stack or duct through the use of a probe. The sample is transported from the probe through a transport device into a mass-monitoring device in which the amount of particulate matter contained in the sample of the emissions is continuously analyzed. The apparatus of the present invention includes a transport device which has a porous inner tube completely sealed inside a solid outer tube. The outer tube of the transport device is supplied with gas through a transpiration port in the outer tube. The gas permeates from the outer tube through the porous inner tube in order to reduce deposition by keeping the particulate matter suspended within the porous inner tube. The method of the present invention continuously analyzes the amount of particulate contained within a sample of the emissions collected from a stack or duct by (i) extracting a sample of particular matter from a stack or duct by using a probe, (ii) transporting the sample of the emissions from the probe through the aforementioned transport device into a mass-monitoring device, while simultaneously supplying a gas into the outside tube of the transport device such that gas flows from the outer tube through the porous inner tube to reduce deposition of particulate matter onto the porous inner tube, and (iii) continuously analyzing the amount of particulate contained in the emissions received from the transport device. Problems associated with deposition of the particulate matter contained within the sample of the emissions after collection by the probe and prior to analysis are reduced with the present invention resulting in greater accuracy when monitoring the amount of particulate matter emitted from a stack.

BACKGROUND OF THE INVENTION 
The present invention relates to an apparatus and method for continuously 
monitoring the amount of particulate contained in emissions which are 
sampled from a smoke stack and measured at a location outside of the 
stack. 
Emissions from smoke stacks are monitored in order to determine the nature 
and quantity of the matter which is emitted into the atmosphere. Such 
emissions have been, and will most likely continue to be, the subject of 
significant government regulation. At present, particulate mass is the 
only regulated variable of stack emissions for which there is no true 
real-time measurement. 
Emissions are sampled from the stack using a variety of well known devices, 
including probes, nozzles, and tubes which perform sampling according to 
standards established by the United States Environmental Protection Agency 
and the American National Standards Institute. 
Once extracted from the stack flow, the emissions, which include 
particulate, are transported outside of the stack to a measuring device 
which determines the amount of particulate matter contained within the 
emissions. The transport pipe used to carry the emissions is important 
because during transport of the sample from the stack to the measuring 
device for analysis, there is deposition of the particulate contained with 
the emissions to the transport pipe as the result of gravitational, 
thermal diffusion, and turbulent losses. These depositional losses prevent 
the particulate matter from being analyzed by the measuring device leading 
to incomplete measurement of the amount of particulate contained within 
the emissions. The rate of gravitational, diffusional and turbulent 
deposition losses depends on such factors as the flow rate of the 
emissions through the pipe, particle size and the pipe geometry. 
Complex calculations are required in order to predict the depositional 
losses of particulate matter. Software can be used to provide estimates on 
the amount of particle transmission and/or deposition for a range of 
particle diameters; however, on a real-time basis it is generally not 
possible to correct the amount of particulate matter transmitted through 
the system to take into account the losses. Any attempt to make such a 
correction leads to uncertainty in sampling results. 
There is a variety of conventional devices that are used to estimate the 
mass of particulate matter in emissions which are expelled from a smoke 
stack. 
One well-known apparatus is a light attenuation system that is based on the 
extinction of a light beam as it traverses the stack. Though this 
apparatus provides monitoring on a real-time basis, true mass correlation 
with light attenuation is not possible which can lead to inaccurate 
results. 
A second apparatus involves obtaining actual mass data by a batch sampling 
technique with a retrospective analysis. In this apparatus, a probe is 
inserted into a stack at a number of different locations in order to 
extract a composite sample of the particulate matter. This sample is 
analyzed at a later time in order to determine the average of the mass of 
particulate emitted from the stack. The procedure involves washing the 
inside of the probe and transport line to recover wall losses. This batch 
sampling, which can provide accurate values of mass concentration, is not 
capable of real-time measurement of the particulate mass within emissions. 
While conventional systems can either provide real-time data or mass 
emission data, they cannot provide both. For the foregoing reasons a need 
exists for an apparatus and method for extracting representative samples 
from a stack, and transporting the sample to a mass-monitoring device in 
order to continuously analyze the amount of particulate contained within 
the emissions without significant loss of particulate matter due to 
deposition. 
SUMMARY OF THE INVENTION 
The present invention provides an apparatus and method for continuously 
monitoring the amount of particulate matter contained within emissions 
from a smoke stack. The apparatus and method both provide for the transfer 
of emissions from a smoke stack to a mass-monitoring device with minimal 
loss of particulate due to deposition. Minimizing particulate deposition 
is important in order to accurately and continuously measure the amount of 
particulate contained within the emissions. 
The apparatus of the present invention includes (i) a probe for extracting 
a sample of the emissions from within the smoke stack, (ii) a transport 
device for transporting the emissions from the probe to a mass-monitoring 
device located outside the stack, and (iii) the mass-monitoring device. 
The transport device within the apparatus includes a porous inner tube 
having an inlet that is placed in open communication with the probe and an 
outlet that is placed in open communication with the mass-monitoring 
device or with a flow splitter placed between the porous inner tube and 
the mass monitoring device. The transport device further includes an outer 
tube which completely surrounds the porous inner tube and is sealed to the 
porous inner tube by a sealing means. The sealing means ensures that a 
gas, which is supplied into the outer tube, flows from the interior of the 
outer tube through and into the porous inner tube. The gas is supplied 
into the outer tube through one or more ports. The flow of gas from the 
outer tube into the porous inner tube minimizes particulate matter contact 
with, and deposition on, the internal wall of the porous inner tube. Using 
the transport device to minimize particulate deposition during transport 
of the emissions improves the accuracy of the results obtained from 
continuously monitoring the amount of particulate contained within 
emissions. 
The transport device may also include at least one support for the porous 
inner tube which bears against the outer tube to provide reinforcement to 
the porous inner tube. The support design does not significantly restrict 
the flow of gas throughout the entire length of the outer tube so that gas 
flows about, and then permeates through, the entire length of the porous 
inner tube in order to prevent the particulate matter contained within the 
emissions from depositing on the porous inner tube. 
The apparatus may also include a thermal control device for maintaining the 
temperature of the gas supplied through the port in the outer tube to a 
level above the dew point temperature of the emissions collected from the 
smoke stack in order to prevent condensation on the internal wall of the 
porous inner tube. 
The apparatus may further include temperature sensors attached to the probe 
in order to determine the temperature of the emissions entering the probe. 
Knowing the temperature of the emissions establishes the minimum level for 
the temperature of the gas which flows from the outer tube through the 
porous inner tube. 
The apparatus may also include a velocity sensor (e.g., an S-type pitot 
tube) attached in the vicinity of the probe in order to determine the 
free-stream velocity of the emissions entering the probe. 
The present invention also includes a method for continuously analyzing the 
amount of particulate matter in smoke stack emissions. The method 
comprises the steps of (i) extracting a sample of the emissions from a 
smoke stack using a probe, (ii) transporting the sample from the probe 
through the porous inner tube of the aforementioned transport device into 
a mass-monitoring device located outside the stack, while simultaneously 
supplying a gas into the outer tube of the transport device such that the 
gas permeates from the outer tube through the porous inner tube of the 
transport device, and (iii) continuously analyzing the amount of 
particulate contained in the emissions received from the transport device 
through use of a mass-monitoring device.

DETAILED DESCRIPTION OF INVENTION 
Similar reference characters denote corresponding features consistently 
through the attached drawings. Various items of equipment such as 
fasteners, fittings, etc., are omitted so as to simplify the description. 
However, those skilled in the art will realize that such conventional 
equipment can be employed as desired. 
As shown in FIG. 1, the apparatus of the present invention includes probe 
20 which collects a sample X of the emissions from a location within smoke 
stack 21 where the particulate mass concentration at the said location is 
representative of the average mass concentration across the stack cross 
sectional area. A sample of the emissions Z, including any particulate 
matter contained therein, flows from probe 20 through inlet 14 into porous 
inner tube 12 and then from porous inner tube 12 through outlet 16 into 
mass-monitoring device 25. By using transport device 10 to transfer the 
sample X from probe 20 to mass-monitoring device 25, the amount of 
particulate matter, which is lost due to deposition before the sample X 
can be analyzed, is reduced. This reduction in deposition of the 
particulate matter leads to increased accuracy when continuously 
monitoring the amount of particulate contained within the sample X. 
FIG. 2 shows one embodiment of transport device 10. Transport device 10 
includes solid, nonporous outer tube 11 which is sealed about porous inner 
tube 12, preferably using seals 13A, 13B. Seals 13A, 13B are preferably 
configured using flanges and O-rings to maintain a dust tight seal between 
porous inner tube 12 and outer tube 11. A preferred design configuration 
allows apparatus 10 to be disassembled for cleaning and maintenance among 
other reasons. Porous inner tube 12 and outer tube 11 may also be welded 
together, or be part of an integrally molded unit. In addition, porous 
inner tube 12 and outer tube 11 may be fabricated such that they are 
crimped or otherwise joined during assembly to form a seal. 
The length and shape of transport device 10 may vary depending on the size 
of the stack and physical arrangement of the location where the 
mass-monitoring device is located. Transport device 10 may have one or 
more bends 22A, 22B (as shown in FIGS. 1 and 4), which may be required to 
change the direction of the sample stream as it passes from the probe 20 
to the outlet 16. Bends 22A, 22B preferably have a radius of curvature 
that is at least two, and even more preferably three, times the outside 
diameter of porous inner tube 12. Porous inner tube 12 and outer tube 11 
may have circular, oval, square and rectangular cross sections among 
others, and each may be different from the other, e.g. the porous inner 
tube can have a circular cross-sectional area while the nonporous outer 
tube has a polygonal cross-sectional area. The material and relative 
diameters of porous inner tube 12 and outer tube 11 as well as the size of 
the pores in porous inner tube 12 will vary depending such design factors 
as the flow rate and temperature of the sample X as well as the average 
size of the particulate matter contained with the the sample. Porous inner 
tube 12 is typically made of stainless steel and has pores which are 
between about 0.2-0.8 .mu.m, and preferably about 0.4-0.6 .mu.m. Outer 
tube 11 is preferably a high nickel alloy which is typically used in high 
temperature, corrosive environments. A typical high nickel alloy tube is 
manufactured by Haynes-Stellite Company and is sold under the trademark 
HASTALLOY. 
Gas Y is supplied into outer tube 11 through port 17. Port 17 is preferably 
located near the center of the axial length of outer tube 11, and 
additional transpiration ports may be placed in outer tube 11 in order to 
more evenly distribute the flow of gas Y to porous inner tube 12. 
Seals 13A, 13B ensure that gas Y flows through porous inner tube 12 
minimizing contact between any particulate matter contained in sample X 
and porous inner tube 12 reducing the amount of deposition of particulate 
onto the inner wall of porous inner tube 12. Gas Y is preferably dry 
filtered air, but may be recirculated stack emissions from which the 
particulate matter has been removed. 
Gas Y is preferably under sufficient pressure to ensure that the velocity 
of gas Y, when flowing through porous inner tube 12, is greater than the 
deposition velocity of a selected size of the particulate matter which is 
directed toward the inside of porous inner tube 12 in order to force the 
particulate matter to remain suspended within porous inner tube 12. 
Typically the selected particle size is 10 .mu.m aerodynamic diameter, AD, 
although other sizes may be selected as well. The particle deposition 
velocity includes the effects of both sedimentation and turbulent 
diffusion. The flow of gas Y through porous inner tube 12 tends to create 
a buffer zone between sample X and porous inner tube 12, which reduces the 
probability that aerosol particles of not only the selected size, but of 
other sizes as well, will contact the wall of the porous inner tube 12. 
Another factor leading to reduced sedimentation losses of particulate 
matter is the addition of gas Y to sample stream X which causes the gas 
velocity inside the porous inner tube 12 to increase as the sample stream 
flows from inlet 14 to exit 16. The increased velocity of sample X within 
transport device 10 leads to shorter residence time of sample X within 
transport device 10. Because the particulate matter contained in sample X 
spends less time within transport device 10, the potential for losses by 
gravitational setting of the particulate matter to porous inner wall 12 is 
reduced. 
Transport device 10 may further include at least one structural support 
which reinforces porous inner tube 12 by bearing against outer tube 11 to 
ensure that porous inner tube 12 is properly aligned within outer tube 11. 
In a preferred embodiment, supports 15A, 15B provide support to porous 
inner tube 12 and have openings 18A, 18B, 18C, 18D which permit gas Y to 
flow from transpiration port 17 throughout the entire length of outer tube 
11. 
Another embodiment of transport device 10 is shown in FIG. 3. Ends 19A, 19B 
of porous inner tube 12 may protrude between seals 13A, 13B from outer 
tube 11 and extend into a bend, a probe, a flow splitter, or a 
mass-monitoring device in order to facilitate mating transport device 10 
with these devices depending on their design. 
Mass-monitoring device 25 is preferably a beta attenuation monitor which 
extracts particulate from emissions X and gas Y on a filter strip. The 
mass of the particulate matter deposited on an area on the filter strip is 
measured continuously to provide a real time measurement of the amount of 
particulate contained within sample X. The beta attenuation monitor draws 
in either a representative portion of sample X and gas Y, or all of the 
sample X and gas Y with the aid of pump 30. A preferred beta attenuation 
monitor is GRASEBY-ANDERSEN PM-10 Beta Gauge Automated Dust Measuring 
Instrument Model No. FH62 I-N, which is distributed by Graseby Andersen 
Incorporated of Smyrna, Ga. 
A sample of emissions Z is preferably collected using a single probe 20 
from one location within a stack 21, or a duct (not shown). The opening of 
probe 20 where sample X enters is preferably axially aligned with the flow 
of emissions Z within the smoke stack 21. Modifications based on such 
considerations as the velocity of gas containing emissions Z, the flow 
rate of sample X and the anticipated aerosol particle size can be made to 
the design of probe 20 to facilitate the transmission of particulate 
matter through probe 20. 
Referring now to FIG. 4, a shrouded probe 50 is preferably used in the 
apparatus of the present invention. A shroud placed about the probe 
decelerates the flow of emissions Z which results in lower deposition of 
particulate matter within the probe. Use of shrouded probe 50 results in 
approximately constant transmission of aerosol in sample X even when the 
velocity of emissions Z within stack 21 varies. A preferred shrouded probe 
which may be used for sampling is disclosed in U.S. Pat. No. 4,942,774, 
which is incorporated herein by reference, and is manufactured by Graseby 
Andersen, Incorporated of Smyrna, Ga. 
S-type pitot tube 51 may also be incorporated into the apparatus of the 
present invention to determine the free-stream velocity of emissions Z in 
the region of smoke stack 21 where shrouded probe 50 is collecting sample 
X. The design of S-type pitot tube 51, and the location of S-type pitot 
tube 51 within smoke stack 21 relative to shrouded probe 50, is specified 
in Appendix A of 40 CFR .sctn. 60 (1995). 
Temperature sensor 52 may be attached to shrouded probe 50 to monitor the 
temperature of sample X entering the apparatus. The temperature of sample 
X should be determined so that the temperature of gas Y which is supplied 
to outer tube 12 can be maintained at a level equal to, or above, the 
temperature of sample X to prevent vapor condensation onto porous inner 
tube 12. 
Gas Y delivered through transpiration port 17 may be heated to prevent 
condensation of sample X onto porous inner tube 12, especially when 
analyzing emissions with high dew point temperatures. A thermal control 
apparatus is preferably used to heat gas Y to a temperature equal to, or 
greater than, sample X collected from the smoke stack. The thermal control 
apparatus is typically a heating apparatus which is preferably located 
either in close proximity to outer tube 12 near transpiration port 17, or 
even more preferably cartridge heater 54 placed in the annular region 
between porous inner tube 12 and outer tube 11 of transport device 10. 
The present invention may also comprise splitter 34 because of the limited 
air handling capacity of mass-monitoring device 25. As stated previously, 
mass-monitoring device 25 is preferably a beta attenuation monitor, and 
beta attenuation monitors typically have limited air handling capacity. 
Splitter 34 is therefore used to reduce the amount of sample X and gas Y 
which enters the beta attenuation monitor, or some other mass-monitoring 
device. 
A sampling portion consisting of sample X and gas Y is drawn from splitter 
34 by pump 30 through primary outlet 33 into mass-monitoring device 25 for 
analysis, while the remaining portion also consisting of sample X and gas 
Y is drawn from splitter 34 by pump 31 into secondary outlet 32. 
A preferred embodiment of splitter 34 is shown in more detail in FIG. 5. A 
non-porous extension tube 46 of porous inner tube 12 extends into splitter 
34 and is secured within inlet hub 40 such that sample X and gas Y flow 
from outlet 16 into drift tube 41. As sample X and gas Y flow through 
drift tube 41 the flow is divided into sampling portion B which is drawn 
through primary outlet 33 into mass-monitoring device 25, and remaining 
portion A, which is drawn through one or more openings 45A, 45B in 
manifold 42, and then through opening 44 in outlet hub 43 before entering 
secondary outlet 32. Remaining portion A is then either discharged, or 
filtered and recirculated for use as gas Y to be feed into outer tube 11 
of transport device 10. Length of drift tube 41 is selected so that 
sampling portion B is representative of the mixture of sample X and gas 
flow Y. 
Porous inner tube 12, inlet hub 40, drift tube 41, manifold 42, outlet hub 
43, primary outlet 33 and secondary outlet 32 may be joined together in a 
variety of air-tight design configurations, including but not limited to, 
welding, molding and press fitting, and may be further secured through the 
use of seals, gaskets and/or O-rings. 
The relative percentage of sample X and gas Y which divide into sampling 
portion B and remaining portion A are defined by the magnitude of the 
cross-sectional area of the interior opening of primary outlet 33 relative 
to the magnitude of the cross-sectional area of the interior opening of 
drift tube 41. Sampling portion B is representative of the mixture of 
sample X and gas flow Y. Splitter 34 is preferably designed such that the 
flow rate of sampling portion B from splitter 34 is no greater than the 
maximum air handling capacity of mass-monitoring device 25. 
The present invention also includes a method for continuously analyzing the 
amount of particulate contained within sample X or emissions Z collected 
from smoke stack 21. First, sample X is extracted from emissions Z by 
probe 20. Second, sample X is transported from probe 20 through transport 
device 10 into mass-monitoring device 25, while simultaneously supplying 
gas Y through transpiration port 17 into outer tube 11 of transport device 
10 such that gas Y permeates from inside outer tube 11 into porous inner 
tube 12. Finally, sample X of emissions Z is continuously analyzed in 
mass-monitoring device 25 in order to estimate the amount of particulate 
matter contained within emissions Z. 
Although the present invention has been described in considerable detail 
with reference to certain preferred versions thereof, other versions are 
possible. The transport device may contain various designs for the porous 
inner tube, outer tube and transpiration port, including various sizes, 
shapes and materials. In addition, the present invention may use a variety 
of conventional probes, heaters, splitters and mass-monitoring devices.