Tunable Diode Laser (TDL) based Hydrogen Chloride gas 

(HCl) Continuous Emission Monitoring System (CEMS) 

Development Challenges to Meet EPA’s Draft Performance 

Specification 

Keith Crabbe 

Cemtek Environmental, Santa Ana, CA 

Gervase Mckay

Unisearch Associates, Toronto, ON CAN 

Paul Tran 

Cemtek Environmental, Santa Ana, CA 

John T. Pisano and Thomas D. Durbin 

Department of Chemical and Environmental Engineering, Bourns College of 

Engineering, Center for Environmental Research and Technology (CE-CERT) 

University of California, Riverside, CA 

ABSTRACT 

New regulations and emissions limits are requiring the measurement and reporting of Hydrogen 

Chloride gas (HCl) levels. Both the Utility Boiler and Portland Cement Maximum Achievable 

Control Technology (MACT) rules for the first time require industry wide measurements for 

regulatory reporting requirements. Cemtek Environmental and Unisearch Associates have been 

involved in the HCl monitoring Performance Specification stakeholder’s group information 

exchange with the Environmental Protection Agency (EPA) for development of the new Draft 

Performance Specification for HCl CEMS. Cemtek has participated in a demonstration test at a 

North American cement manufacturing facility which investigated the performance of Tunable 

Diode Laser based HCl CEMS as outlined in the draft performance specification. This paper 

will present the demonstration field setup, the tests conducted including comparison with 

reference method, and obstacles and lessons learned in completing the tests along with additional 

laboratory trial results. 

 
INTRODUCTION

The United Sates Environmental Protection agency has put in place new rule regulating the 

emission of Hydrogen Chloride gas (HCl) from Stationary sources. These rules include the 

amendments to the NESHAP for Portland cement plants and to the NSPS for Portland cement 

plants and Coal fired Electric utility plants under the Mercury and Air Toxics Rule (MATS). 

These rules finalize standards that originated with Clean Air Act of 1990. At the time this study 

began the EPA regulations at 40CFR63, Subpart LLL which require installation, certification, 

operation and ongoing quality assurance of HCl CEMS to demonstrate continuous compliance 

with a 3 ppm HCl corrected to 7% O2, dry basis emission limit on a 30-day rolling average by 

not later than September 2013. 

Holcim Inc. developed, planned and executed a field evaluation project of four HCl CEMS at its 

St. Genevieve plant during the summer and fall of 2012. The general objectives of the study 

were to determine if contemporary HCl continuous emission monitoring systems (CEMS) have 

the capability, accuracy, precision and reliability. Cemtek Environmental using a Unisearch 

Associates HCl TDL analyzer was invited to participate in this evaluation. CEMTEK’s interest 

was to perform and practice some of the tests in the EPA draft performance specification 18. At 

the same time we were providing comments and feed back to the EPA team developing the PS as 

a stakeholder. The results of this effort are presented here along with some observations and 

issues in the conclusion of the report. (1) 

 

EXPERIMENTAL 

The study was conducted at the St. Genevieve plant under the direction of Glen Rosenhamer, 

Corporate Environmental Manager. Michelle Ferguson, Plant Environmental Manager, Aaron 

Dwyer (Electrical Superintendent), Rodney Miller (Instrumentation Specialist) provided 

extensive support for the project over many months. Jim Peeler, Emission Monitoring Inc. 

(EMI) provided consulting assistance during the entire project, participated in the field test, and 

prepared this report. Phil Kauppi, Prism Analytical Technologies, Inc. performed the FTIR 

testing for the stratification test, the Method 321 tests for the RATAs, and analyzed numerous 

compressed gas cylinders. (1) 

The Holcim St. Genevieve (GV) plant is a contemporary preheater precalciner kiln system which 

began operation during 2009. The kiln system has two inline raw mills and operating conditions 

include: (a) both raw mills operating, (b) one raw mill operating, and (c) no raw mills operating. 

These operating conditions affect the associated effluent HCl concentrations, the concentrations 

of other effluent components including NH3, as well as the effluent gas temperatures. HCl 

effluent concentrations varied over a range of 0-20 ppm, wet basis. Effluent gas temperatures 

also vary with operating condition and were expected to be approximately 205°F (two mills on), 

255°F (one mill on) and 320°F (no mills operating). (1) 

 
INSTRUMENTATION 

Tunable Diode Lasers (TDL’s) have special properties based upon small crystals (about 0.1 

mm2) made of a mixture of elements such as gallium, arsenic, antimony and phosphorus. The 

proper selection and proportion of these elements the crystal can be made to emit at wavelengths 

where the target gas, which is HCl in this application, absorbs radiation of a particular energy. 

Changing either the temperature or current through the laser permits the wavelength to be tuned 

over the selected absorption feature of the target molecule. When an electric current is passed 

through these crystals they emit very pure laser light in the near-infrared spectral region. The 

temperature of the laser, which is stabilized with a thermoelectric cooler, will roughly establish 

the wavelength near the target gas’ absorption feature. The laser current is then simultaneously 

modulated in the kilohertz region so that phase sensitive detection techniques may be used to 

improve sensitivity. One advantage of the TDL technology is that the presence of the measured 

component in the gas stream can be used as a reference point for the laser itself. This function is 

called “line locking” and consists of the absorption peak of the HCl being used to keep the laser 

at the desired frequency. However, in the absence of the measurement component, during bypass 

or scheduled shutdowns, the laser can drift, and be outside calibration when the measurement 

component returns. To avoid drift, this TDL is supplied with a sealed internal reference cell, 

which contains a known amount of the gas to be measured. The use of the reference cells allows 

for not only line locking of the analyzer, but also the ability to continuously monitor the 

instrument’s calibration. We conducted some testes using this cell to report calibration drift test 

results in the results section of this study. 

The calculations which determine the actual gas concentration use Beer-Lambert Law. 

I = Ioe

-cl (1) 

Where: 

I = Intensity of light after absorption by target molecule 

Io = Intensity of light with no absorption by target molecule 

 = Absorption cross section (species specific) 

c = Concentration (number density) of target molecule 

l = Path length 

Beer’s Law is based on the relative absorption of light at wavelengths which are not absorbed 

versus light which is absorbed by the molecule of interest. The intensities I and Io are determined 

by scanning the wavelength using the characteristic tunability of diode lasers, which allows the 

emission wavelength to be adjusted over a span of a few nanometers. This is sufficient to span 

certain absorption features in molecules such as ammonia, carbon monoxide, methane, and 

others. The only important variable is the fraction of light which is absorbed at the middle of the 

scan (return power, I) vs. the extreme ends of the scan (initial power, Io). The laser is scanned at 

a frequency of approximately 4 kHz, or every 250 micro seconds (s) so that variations in Io due 

to external factors (such as changing dust levels) will be very small. The error introduced by 

averaging Io over this very short interval is negligible. The most important consequence of this is 

that the absolute light levels reaching the detector are not important in determining 
concentrations. It should be noted that this method actually responds to the total number of 

absorbing species present. The value is reported as a path averaged concentration, distributed 

along the entire measurement path. 

 A typical system configuration consists of the Analyzer unit in either stand alone or 19” rack 

mount configuration, which contains the TDL and associated electronics for signal transmittal 

and signal analysis. The Optical heads which contain the Launch and Receive components are 

mounted on the duct or stack and connected to the Analyzer via Fiber Optic/Coax cabling. This 

permits the analyzer to be placed in any suitable location, such as the CEM shelter or control 

room of the plant where it is not subjected to harsh environments and where it can be readily 

serviced as required. The optical signal is conveyed from the instrument to the measurement 

location by fiber optics and the return detected signal transported to the instrument by a separate 

coaxial cable. Thus, for example, continuous measurements can be made of the emissions in 

stacks and ducts which can be as much as 1500 feet (with a Photo Detector Amplifier or PDA) 

away from the instrument. 

MEASUREMENT LOCATION 

If the measurement is being made for stack emissions monitoring or HCl CEMS, it is on the 

stack or at another location that would be considered representative of stack emissions. The EPA 

is referring to the cross stack as Integrated Path or IP-CEMS in the draft PS-18. The 

measurement sensitivity is dependent upon the measuring path length (Beer–Lambert law) and 

the opaqueness (particulate matter density) of the gas stream. The path length distance between 

the light source and the detector can vary depending upon the sample stream’s particulate 

concentration. Monitoring locations that have conditions such as a fully saturated or condensing 

stack or duct that has a similar effect as high dust loading, which would extinguish the laser 

light, should be avoided. Therefore when choosing the measurement location, these factors along 

with the turbulence and stratification issues that arise must be considered. The measurements are 

usually made in-situ across the stack or duct. Various configurations can be chosen to define the 

process interface by the choice of flange sizes, materials, purging modes and purging media in 

order to adapt the sensors for process engineering measurements. In some cases, the optical path 

can be inside a probe. However, this will limit the path length. Alignment of the optical heads are 

of significant importance, and therefore the sight tubes to which the optics are attached to, are 

required to be aligned to specific tolerance typically within 1 degree and must be maintained at 

relative high temperatures. The optical heads must have the capability to adjust for any small 

misalignment errors. Another component that must be taken into account is the “thermal growth” 

of the duct or stack and material aging. Several engineered solutions are available to counteract 

this issue. 

Two methods are employed for measurements across stacks or ducts: 1) single-pass where the 

laser radiation is transmitted across the stack to the detector on the other side, and 2) dual-pass 

where the laser radiation is transmitted across the stack to a reflector and then back to the 

detector on the same side as the transmitter. In typical installations, instrument air is used at each 

of the optical lenses to keep them clear of flue gas contaminants. 
For this study we chose to use dual pass optics which would provide the best detection limit and 

sensitivity. The stack location was free of deformation which could cause optical alignment 

issues and the dust load in the stream was low during the test period. The windows never needed 

cleaning during the 4 months in continuous operation at this site.

1) Single-Pass Stack Configuration

2) Dual-Pass Stack Configuration 

FIGURES 1 & 2: CROSS DUCT SYSTEM DESIGN 

Optical 

Fiber 

Len

Detector

Coaxial 

Cable 

Stack

Air Air 

Retro

Reflector

Optical 

Fiber 

Len Detector

Coaxial

Cable

Stack

Air Air

Launching

Element

Receiving 

Element 
In special situations where long spool pieces or lagging is employed, especially in the prescience 

of negative pressure stacks or ducts, blowers are required to prevent the migration of HCl in the 

spool pieces. Such migration would cause a variable and inconsistent concentration of HCl in the 

laser path resulting in inconsistent and inaccurate measurements. Applying a positive pressure of 

ambient air to the spool pieces will assure that no stack gas HCl is migrating into the spool 

pieces. Particular attention must also be paid to the location of the measurement tools to limit the 

accumulation of particulate matter which can not only cause loss of signal due to light scattering 

but also can result in physical changes in the alignment of the laser light requiring realignment 

after cleaning. The blower system we used during this study kept the optics clean so instrument 

air was not needed.

SENSITIVITY AND DETECTION LIMIT 

The sensitivity and minimum detection limit of an absorption device are path length dependent. 

The longer the path length, the higher the absorption and the lower the sensitivity and detection 

limit. Therefore, longer path lengths result in better detect ability of low concentrations. 

Dust loading in the effluent stream has the effect of blocking and scattering the laser radiation 

such that the detection limit will be compromised, leading to a much higher minimum detection 

value, due to lower power levels of the laser radiation reaching the detector. While particulate 

matter affects the detection limit, most systems can tolerate laser radiation power reductions of 

up to 90% without affecting the accuracy of the measurement. Nevertheless, there is a trade off 

in path length considerations between detect ability and reliability of the measurement. A single 

pass system with a PDA is recommended for high dust applications. Water vapor is also active in 

the near-IR spectral region; however the HCl line selected was far from waters influence. 

Typical sensitivity and detection limits on coal fired sources dependent on the above criteria 

would be ±1 ppmv with a detection limit of 0.2 part per million by volume (ppmv) respectively. 

CALIBRATION REQUIREMENTS 

Prior to initial usage, the instrument must be calibrated for HCl using a gas standard. After the 

initial calibration is performed, system validation is performed on a regular basis as defined by 

the governing body, in this case the USEPA. A flow-through gas cell permanently in the optical 

path that allows for introduction of cylinder gases to be injected to check the analyzer response 

as an additive or dynamic spike should be part of the analyzer. After use the cell is purged with 

dry air or nitrogen. 

 
CALIBRATION FACTOR 

The TDL uses the Beer-Lambert Law to convert a quantity of absorption into a concentration as 

follows: 

L

VT I I C L ln( )  0  (2) 

Where: 

 is the line strength factor at STP 

I is the amount of measured light after passing through the gas medium 

0 I is the amount of initial light (i.e. transmitted light with no gas present that is to be 

measured) 

L is the length of the absorption medium (Path length) 

V is the volume correction 

TL is the line strength correction (a function of gas temperature) 

When we do the initial calibration of the instruments we use a certified cylinder of gas, and rearrange the equation to solve for . 

The test configuration shown in figure 3 has the double pass stack optics with a single blower at 

the stack. The analyzer is remotely located in a nearby CEMS shelter that is air conditioned and 

protected from the elements. 

FIGURE 3: SYSTEM DESIGN 
The power supply has an uninterruptable power supply (UPS) which provided power to the 

analyzer and programmable controller (PLC). The PLC is used to conduct the spike gas selection 

solenoid valves and sequences in the test configuration. The gases are conveyed to the flow-thru 

gas cell inside the analyzer with 1⁄4 Teflon tubing. A laptop PC captures the data form the 

analyzer and provides a convenient interface with the analyzer. For this test we used it to connect 

to a wireless internet connection for remote support. In permanent installation the laptop is only 

used for troubleshooting and changing configuration parameters 

SPIKE GAS DETERMINATION 

To determine the spike gas injections for the appropriate values need to perform the tests we 

need to account for the path length. The cylinder values when using the IP-CEMS are larger than 

the fully extractive CEMS since the concentrations is divided by the path length. Below is the 

formula used to calculate the expected analyzer response to a known HCl concentration with a 

zero HCl background: 

LSM

T

T

PL

PL C C

bottle room

stack

stack

cell

spike  bottle   

/

 (3) 

Where 

Cspike = Expected concentration of gas spike response in ppmv

Cbottle = Concentration of gas cylinder in ppmv

PLcell = Path length of flow-thru gas cell in meters 

PLstack = Path length of stack in meters 

Tstack = Temperature of stack in Kelvin 

Tbottle/room = Temperature of bottle/room in Kelvin 

LSM = Line strength multiplier 

The cell length used in this analyzer was measured to be 0.158 m and the stack path length was 

11.12 m.

TESTS CONDUCTED ARE FROM THE DRAFT PS 18 FOR IP-CEMS 

This section describes the performance specification 18 tests that are required for IP-CEMS to 

performance to demonstrate initial certification and in some cases includes our interpretation. 

Then we report our findings in the discussion and results section for ht testes we attempted. (3) 

 Interference Test 

 Limit of Detection (LOD) Determination 

 Response Time Test 

 Calibration Error (CE) Test 

 Seven Day Calibration Drift Test 
 Stratification Test 

 Relative Accuracy Test or Dynamic Spiking Test 

INTERFERENCE TEST 

This test is conducted either in a controlled environment or in the field during initial setup and 

qualification of the CEMS. If there are multiple measurement systems that are identical to each 

other, this check is only required on one system. Procedure: Inject HCl gas that is equivalent to 

20-40% of the lowest calibration span value anticipated. The balance of the cylinder 

concentration must conform to Table 1 under Section 17.0 of Performance Specification 18 

(PS18). Below is a copy of the table in question. 

TABLE 1: COPY OF TABLE 1 FROM SECTION 17.0 OF PS18 

1 Any of these specific gases can be tested at a lower level if the manufacturer has provided reliable means for limiting or 

scrubbing that gas to a specified level.

Potential Interferent Gas1 Approximate Concentration 

(balance N2)

CO2 15 ± 1 % 

CO 100 ± 20 ppm 

CH4 100 ± 20 ppm 

NO2 250 ± 50 ppm 

SO2 200 ± 20 ppm 

O2 3 ± 1 % 

H2O 10 ± 1 % 

Nitrogen Balance 

Other 

Each test must be evaluated in triplicate for each interfering gas noted in Table 1, first with HCl 

balance Nitrogen then individually with each interfering gas while maintaining constant HCl 

concentration. (e.g. 150 ppm HCl with 15% CO2 and balance N2.) Gas volume/rate will be 

documented to establish the error of blending HCl and interference gases. A gas blending system 

or manifold may be employed. Measure and record the HCl response and determine overall 

interference response using Table 2 of Section 17.0 of PS18. Calculate mean difference between 

measurement system response with and without the interference tests gasses. Calculate total 

interference. 

Pass/Fail Criteria: Combined interference response must not be greater than 3.0 percent of the 

calibration span used for the test. 
LIMIT OF DETECTION (LOD) TEST 

See note on the Interference Test for determining when the LOC test may be conducted. The 

challenge gas must consist of the interferences listed in Table 1 and HCl at a concentration 

between 2-5 times the estimated LOD. Procedure: Spike the gas mixture into the system 

calibration cell. Collect seven (7) independent measurements. LOD units must be determined 

and reported on a ppm-meter basis or on the actual measurement path length for a specific site 

installation. Calculate standard deviation of measured values and estimate the LOD at 3 times the 

standard deviation. 

RESPONSE TIME DETERMINATION TEST 

Procedure: Inject an HCl reference gas and record the time to reach 95% of the reference gas 

concentration. Inject a zero gas and record the time to reach a reading that is 5% of the upscale 

gas reading from the reference gas concentration. Repeat steps 1 and 2 for a total of 3 repetitions. 

Calculate the average upscale and downscale response time from three repetition of each test. 

The greater average of the upscale or downscale response time is the response time of the 

system. Pass fail criteria is apparently to report the results. 

CALIBRATION ERROR (CE) TEST 

Mean difference between the reference gas value (R) and CEMS response at each calibration 

point (A). This is conducted on a daily basis. Procedure: Do a 3-point calibration error test with a 

low, mid and high gas concentration range, in sequential order, by injecting gas into an inline 

calibration cell (must know temperature, pressure, and path length of cell). Concentrations of the 

low, mid, and high gas are as tabulated in Table 2. 

TABLE 2: REQUIRED LEVELS OF HCL FOR CE TEST IN RELATION TO 

ANALYZER SPAN 

HCl Calibration 

Concentration Level 

Range (% of span) 

Zero < LOD 

Low Level 10 – 50 

Mid Level 80 – 100 

High Level 190 – 200 

Measure and calculate the relative equivalent concentration of HCl gas at stack conditions (i.e. 

correct to stack conditions) Repeat steps 1 and 2 for a total of three independent CEMS 

measurement responses such that each calibration gas injection is at least 2 minutes apart. 

Pass/Fail Criteria: Calculated CE must be less than 5%. Calibration intercept must be equal to or 

less than 15% of the instrument’s span. If failed, then take corrective action and repeat test until 

the CE test results are passing. 
SEVEN-DAY CALIBRATION DRIFT (CD) TEST 

Purpose is to verify the ability of the CEMS to maintain calibration for seven consecutive 24-

hour operating periods/days. The seven operating days do not need to be 7 consecutive calendar 

days. If there are manual or automatic adjustments are made to the zero/span response settings, a 

daily CE test must be conducted prior to the adjustments. Procedure: Inject a certified zero. 

Continue to inject zero gas until two consecutive measurements take at least 2 minutes apart are 

within 5% [of each other]. Record the average CEMS response to zero gas. The measured 

concentrations must be corrected for calibration cell temperature, pressure, and path length [to 

stack conditions]. Inject a certified calibration gas standard. Continue to inject this gas until two 

consecutive measurements take at least 2 minutes apart are within 5% [of each other]. 

Record the average CEMS response to the calibration standard. The measured concentrations 

must be corrected for calibration cell temperature, pressure, and path length [to stack conditions]. 

Subtract the average CEMS zero and calibration standard response from the [calculated] 

calibration standard gas value [corrected to stack conditions] and express as an absolute percent 

different of the span value. Pass/Fail Criteria: Calculated CD must be less than 5% per day. 

RELATIVE ACCURACY (SA) TEST FOR IP-CEMS 

Six or more sets of independent measurements—of at least a 1 minute average—are used to 

calculate the average and standard deviation to determine CEMS accuracy and to compare to the 

limitations as noted in Section 13.0 of PS18. Target level of spiking is 20 to 40 percent of the 

[analyzer] span value. Procedure: Inject a certified zero. Continue to inject zero gas until two 

consecutive measurements take at least 2 minutes apart are within 5% [of each other]. 

Record the average CEMS response to zero gas. The measured concentrations must be corrected 

for calibration cell temperature, pressure, and path length [to stack conditions]. 

Inject a certified calibration gas standard. Continue to inject this gas until two consecutive 

measurements take at least 2 minutes apart are within 5% [of each other]. 

Record the average CEMS response to the calibration standard. The measured concentrations 

must be corrected for calibration cell temperature, pressure, and path length [to stack conditions]. 

Re-inject a certified zero. Continue to inject zero gas until two consecutive measurements take at 

least 2 minutes apart are within 5% [of each other]. 

Record the average CEMS response to zero gas. The measured concentrations must be corrected 

for calibration cell temperature, pressure, and path length [to stack conditions]. 

Repeat 6 times for 6 sets of data. Calculate the spike recoveries and its relative accuracy per 

Section 12.13 of PS18. Pass/Fail Criteria: Calculated recoveries (SR) must be less than 20% for 

each of the independent measurements. The relative accuracy over the 6 or more independent 

measurements may not exceed 15 percent. 

 
RESULTS AND DISCUSSION

INTERFERENCE TEST 

This test was conducted in the Cemtek Environmental facility and completed a partial test using 

the compressed gases we had on hand we use for testing our other CEMS we produce. Following 

the PS18 procedure the results are shown in table 3 below demonstrating. 

TABLE 3: INTERFERENCE TEST RESULTS 

Test Gas Interference 

(ppmv) 

CO2 0.079 ± 0.13 

CO -0.0053 ± 0.0036 

CH4 -0.13 ± 0.05 

NO2 Not Tested 

SO2 0.071 ± 0.057 

O2 0.06 ± 0.09 

H2O Not Tested 

Nitrogen 

The interference test conducted in the above was done at the condition where there is no HCl in 

the ambient air or in the test cell. The draft Performance Specification 18 (further references to 

this Specification will be noted as PS18) Interference Test calls out for HCl gas, at a constant 

level to be tested with and without the above gas compositions noted in Table 1. Thus the 

intended purpose to informally conduct the Interference Test in accordance to PS18 is 

incomplete. A follow-up test would be prudent to test all interfering gases in the presence of 

HCl. This would allow one to quantify the interference bias due to the given gases for a constant 

level of HCl. 

In addition, all of the test gas interferences measured is all below the calculated detection limit of 

1.8 ppmv for the test setup and cannot be measured with sufficient confidence. Even at an 

effective path length of 11 meters, the interference would be still well below the detection limit. 

The detection limit was determined at Holcim to be 0.2 ppmv with an operating temperature 

range of 250-300°F (394-422 K). It may be necessary to increase the integration time to increase 

the sensitivity of the analyzer. 

Even though the inputted temperatures and pressures are at the elevated levels for stack gas 

conditions, the correction factor would have been applied uniformly for the detection limit and 

the tabulated values in Table 3. Further testing, however, should have the proper test conditions 

inputted into the analyzer. 

Follow-up / Additional Testing 

 Re-test the conditions above with the proper temperature and pressure conditions. 

 Re-test to include the omitted test gases as they were not available on hand to measure. 
 Increase the integration time to 1 minute or more from 10 seconds to improve the 

detection limit. This will not be required if the path length is increased such that the 

detection limit is lowered sufficiently. 

 Have a method of automating the switching of the calibration gases via a manifold. Will 

speed up the data acquisition portion of the test. 

LIMIT OF DETECTION (LOD) TEST 

This test was conducted in the field at the Holcim trial and the results. Successively decreasing 

spike gas concentrations were introduced to the in-line audit cell by means of diluting the 318 

ppm audit gas with nitrogen using the EMI gas dilution manifold. The diluted spike 

concentration of 9.9 ppm is equivalent to an effluent HCl concentration of 0.165 ppm and was 

clearly distinguishable from the zero HCl concentration response of -0.005 ppm observed for the 

effluent after the spike was discontinued. Therefore the LOS for the TDL system as installed 

at St. Genevieve under real world conditions while actively monitoring cement kiln effluent 

is ≤ 0.16 ppm. Lower concentrations were spiked into the audit cell, however it was difficult to 

distinguish the presence of the response from the apparent zero value. (1) 

RESPONSE TIME TEST 

Response Time Test Results were taken from the data collected performing calibration error tests 

using a bottle of HCl and Nitrogen supplied by Holcim and conducted after the completion of the 

Trial period. The results are show below. The optical setup was set up to have an effective path 

length of 11.12 meters. The temperature and pressure was dynamically inputted from the 

customer DCS. Integration time was left at 10 seconds. Calibration gas was introduced via a 

solenoid manifold assembly controlled by a PLC. The cylinder concentration employed was a 

318.5 ppmv bottle. The average effective analyzer response is roughly 5 ppmv depending on the 

temperature conditions at the time of test. 

Procedure 

Gases were injected into the flow-through gas cell via solenoids. The sequence is automated by a 

PLC and is as follows. 

1. 15 minute (900 seconds) delay after internal zero/audit check 

2. Flow N2 gas for 120 seconds. Purge sequence 

3. Flow HCl gas for 300 seconds 

4. Flow N2 gas for 240 seconds. Purge/Zero sequence 

The analyzer records the response in real-time as a flat text file. The data will be then pulled and 

analyzed accordingly. 

Results 

Tables 2 and 3 tabulate the raw data for the rise and fall of the HCl analyzer response average for 

each test run. Averages of the background data where no HCl was present were taken before and 

after to determine the background levels. The Analyzer Reponses takes this into account by 
subtracting the background levels. Also included is the average temperature and expected 

analyzer response. 

Additionally, a non-linear total least squares fit has been applied to the data to determine the 

experimental time-constant k, which is noted below as Equation [1]. This was done via the 

Solver add-in in Excel. From the fitted time-constant, the time to reach 95% of the analyzer 

response was determined. This can be seen in Table 3 and graphically in Figures 4. 

FIGURE 4: RESPONSE TIME TEST RESULTS 

Rise and Fall time graphs for Tests 1, 2, and 3. Shows the best fit to the data as solved 

‐20

30

80

0 50 100

% Response

Time (s)

Test #1, Rise Time

y

Fit

‐20

0

20

40

60

80

100

120

0 50 100

% Response

Time (s)

Test #1, Fall Time

y

Fit

‐20

0

20

40

60

80

100

120

0 50 100

% Response

Time (s)

Test #2, Rise Time

y

Fit

‐20

0

20

40

60

80

100

120

0 50 100

% Response

Time (s)

Test #2, Fall Time

y

Fit

0

20

40

60

80

100

120

0 50 100

% Response

Time (s)

Test #3, Rise Time

y

Fit

‐50

0

50

100

150

0 50 100

% Response

Time (s)

Test #3, Fall TIme

y

Fit
TABLE 4: CALCULATED RESPONSE TIME FOR RISE AND FALL OF HCL-TDL 

ANALYZER IN RESPONSE TO ZERO AND SPIKE GAS. 

Test Run Rise Time 

(s) 

Fall Time 

(s) 

1 25.10 16.24 

2 25.14 17.20 

3 19.74 12.81 

Average 23 15 

Standard Deviation 3 2 

CALIBRATION ERROR TEST 

Calibration Error Test Results was not performed due to timely request for the cylinder values 

need for this study. 

CALIBRATION DRIFT TEST 

Calibration Error Test Results were taken from the data collected using a bottle of HCl and 

Nitrogen supplied by Holcim and conducted after the completion of the Trial period. The results 

are show below in table 5. 
TABLE 5: CALIBRATION DRIFT TEST RESULTS 

Daily Spiking Summary

Day Date

#

Of

readings

Pre-Spike

Bkg 

Average

(ppm)

Post-Spike

Bkg 

Average

(ppm)

HCl 

Baseline

(ppm)

Average

Analyzer

Response

(ppm)

Average

Analyzer

Spike 

Reponse

(ppm)

Tavg (K)

Expected 

Response

at Tavg

(ppm)

S

(

0.0 10/16 13 0.00062 -0.00064 -0.00001 5.322 5.322 357.22 5.195 100.1 10/16 13 -0.00032 -0.00040 -0.00036 4.959 4.959 359.12 5.215 90.2 10/16 13 0.00053 0.00000 0.00027 5.455 5.455 386.72 5.522 90.3 10/16 13 0.00286 0.00002 0.00144 5.847 5.846 383.50 5.486 101.0 10/17 13 5.29874 5.60349 5.45112 11.888 6.437 442.42 6.170 102.0 10/18 13 -0.00017 -0.00051 -0.00034 5.585 5.585 372.74 5.366 103.0 10/19 13 -0.00018 -0.00023 -0.00021 5.640 5.640 376.29 5.405 104.0 10/20 13 -0.00092 -0.00118 -0.00105 5.596 5.597 376.75 5.410 105.0 10/21 13 -0.01857 -0.00504 -0.01180 5.277 5.289 362.12 5.248 106.0 10/22 13 0.56555 0.35426 0.45991 7.136 6.676 419.30 5.896 117.0 10/23 13 0.00342 0.00412 0.00377 5.787 5.783 373.52 5.374 108.0 10/24 13 0.00009 0.00126 0.00068 5.778 5.778 378.99 5.435 10
TABLE 6: SPIKE TEST PARAMETERS 

RELATIVE ACCURACY TEST 

The traditional Part 60 relative accuracy specification for SO2, NOx and most other pollutant gas 

CEMS is ≤20% of the reference measurement value or ≤10% of the emission standard, 

whichever is least restrictive. This specification is used for the Holcim project as a tentative 

specification. The relative accuracy is calculated as the sum of the absolute value of the 

differences and the 95% confidence coefficient, divided by either the mean reference 

measurement result or the emission standard, to express the result as a percentage. The emission 

standard is 3 ppm at 7% O2, dry basis. For the purposes of this comparison, we will simply use 3 

ppm HCl as the emission standard. (1) 

TABLE 7 CEMTEK TDL TWO MILLS ON RATA CALCULATIONS

 

Run Date Time CEMS (ppmw) Ref (ppmw) Difference Difference 2

1 18-Sep 9:00 - 9:30 0.02 0.12 -0.10 0.01

2 18-Sep 10:11 - 10:40 0.07 0.15 -0.08 0.01

3 18-Sep 11:16-11:46 0.04 0.14 -0.10 0.01

4 18-Sep 11:47 - 12:17 0.02 0.11 -0.09 0.01

5 18-Sep 12:18 - 12:48 0.03 0.09 -0.06 0.00

6 18-Sep 13:19 - 13:48 0.01 0.17 -0.16 0.03

7 18-Sep 13:50 - 14:19 0.06 0.11 -0.05 0.00

8 18-Sep 14:21 - 14:50 -0.01 0.10 -0.11 0.01

9 18-Sep 15:16 - 15:46 -0.01 0.16 -0.17 0.03

SUM 0.23 1.15 -0.92 0.11

AVG 0.03 0.13 -0.10 0.01

 RATA RESULTS 

Average Difference -0.10

Standard Deviation 0.04

Confidence Coefficient 0.03

Relative Accuracy % 104%

Relative Accuracy ppm 0.13 ppm

Emiswsion Standard 3 ppm

 Relative Accuracy % of STD 4.4% % of STD

The two mills on RATA results are shown in Table 7. For this low concentration test, the TDL 

CEMS relative accuracy was 4.4% of the emission standard. The CEMS meets the RA 

requirement. The small mean difference of -0.1 ppm is attributed to the fact that the FTIR 

reference system does not measure values below 0.1 ppm. (1) 

MISC Parameters 

Bottle 

Concentration SPAN Ref/Room Temp PLCell PLStack t-value

318.5 30 73 0.158 11.12 n-1

ppm ppm °F meters meters 12

295.9 t-value @ n-1 = 12

K 2.179
TABLE 8: CEMTEK TDL TWO MILLS OFF RATA CALCULATIONS (18 RUNS) 

Run Date Time CEMS (ppmw) Ref (ppmw) Difference Difference 2

1 19-Sep 6:58 - 7:26 13.06 13.86 -0.80 0.64

2 19-Sep 7:28 - 7:58 13.43 14.18 -0.75 0.56

3 19-Sep 7:59 - 8:29 13.93 14.68 -0.75 0.56

4 19-Sep 8:38 - 9:07 14.08 15.29 -1.21 1.46

5 19-Sep 9:09 - 9:39 14.38 15.43 -1.05 1.10

6 19-Sep 9:40 - 10:10 14.30 15.59 -1.29 1.66

7 19-Sep 10:16 - 10:47 15.20 16.08 -0.88 0.77

8 19-Sep 10:47 - 11:16 15.44 16.46 -1.02 1.04

9 19-Sep 11:16 - 11:46 14.92 16.59 -1.67 2.79

10 19-Sep 11:53 - 12:23 13.91 15.74 -1.83 3.35

11 19-Sep 12:24 - 12:54 7.72 8.99 -1.27 1.61

12 19-Sep 12:55 - 13:25 5.77 6.40 -0.63 0.40

13 19-Sep 13:38 - 14:08 4.40 5.31 -0.91 0.83

14 19-Sep 14:09 - 14:39 4.32 4.86 -0.54 0.29

15 19-Sep 14:40 - 15:10 4.11 4.61 -0.50 0.25

16 19-Sep 15:18 - 15:46 4.01 4.41 -0.40 0.16

17 19-Sep 15:58 - 16:18 3.92 4.30 -0.38 0.14

18 19-Sep 16:19 - 16:49 3.83 4.12 -0.29 0.08

SUM 180.73 196.90 -16.17 17.72

AVG 10.04 10.94 -0.90 0.98

RATA RESULTS 

Average Difference -0.90

Standard Deviation 0.43

Confidence Coefficient 0.22 t = 2.11

Relative Accuracy 10.2%

The two mills off RATA results for the TDL CEMS are shown in Table 8 based on all 18 runs 

performed. For this high concentration test, the TDL CEMS relative accuracy was 10.2% which 

meets the 20% relative accuracy specification. The mean difference was -0.90 ppm reflecting 

that the TDL values were lower than the reference FTIR measurements. (1) 
TABLE 9 CEMTEK TDL TWO MILLS OFF RATA CALCULATIONS (15 RUNS) 

Run Date Time CEMS (ppmw) Ref (ppmw) Difference Difference 2

1 19-Sep 6:58 - 7:26 13.06 13.86 -0.80 0.64

2 19-Sep 7:28 - 7:58 13.43 14.18 -0.75 0.56

3 19-Sep 7:59 - 8:29 13.93 14.68 -0.75 0.56

4 19-Sep 8:38 - 9:07 14.08 15.29 -1.21 1.46

5 19-Sep 9:09 - 9:39 14.38 15.43 -1.05 1.10

7 19-Sep 10:16 - 10:47 15.20 16.08 -0.88 0.77

8 19-Sep 10:47 - 11:16 15.44 16.46 -1.02 1.04

11 19-Sep 12:24 - 12:54 7.72 8.99 -1.27 1.61

12 19-Sep 12:55 - 13:25 5.77 6.40 -0.63 0.40

13 19-Sep 13:38 - 14:08 4.40 5.31 -0.91 0.83

14 19-Sep 14:09 - 14:39 4.32 4.86 -0.54 0.29

15 19-Sep 14:40 - 15:10 4.11 4.61 -0.50 0.25

16 19-Sep 15:18 - 15:46 4.01 4.41 -0.40 0.16

17 19-Sep 15:58 - 16:18 3.92 4.30 -0.38 0.14

18 19-Sep 16:19 - 16:49 3.83 4.12 -0.29 0.08

SUM 137.60 148.98 -11.38 9.91

AVG 9.17 9.93 -0.76 0.66

RATA RESULTS 

Average Difference -0.76

Standard Deviation 0.30

Confidence Coefficient 0.17 t = 2.145

Relative Accuracy 9.3%

The relative accuracy test procedures allow for the rejection of three test runs, as long as nine test 

runs are reported. The two mills off RATA results were recalculated based on 15 test runs 

(omitting runs 6, 9, and 10) as shown in Table 9. The relative accuracy based on 15 test runs was 

9.3% of the reference value. (1) 

 
TABLE 10: CEMTEK TDL ONE MILL ON RATA CALCULATIONS (17 RUNS) 

Run Date Time CEMS (ppmw) Ref (ppmw) Difference Difference 2

1 20-Sep 8:00 - 8:30 2.56 3.21 -0.65 0.42

2 20-Sep 8:30 - 9:00 2.21 2.35 -0.14 0.02

3 20-Sep 9:00-9:30 2.14 2.05 0.09 0.01

4 20-Sep 9:41-10:11 1.54 1.84 -0.30 0.09

5 20-Sep 10:12-10:42 1.33 1.66 -0.33 0.11

6 20-Sep 10:42 - 11:12 1.35 1.53 -0.18 0.03

7 20-Sep 11:22 - 11:52 1.26 1.44 -0.18 0.03

8 20-Sep 11:52 - 12:22 1.45 1.35 0.10 0.01

9 20-Sep 12:22 - 12:52 1.19 1.30 -0.11 0.01

10 20-Sep 13:06 - 13:36 1.05 1.21 -0.16 0.03

11 20-Sep 13:36 - 14:06 0.94 1.06 -0.12 0.01

12 20-Sep 14:06 - 14:36 0.74 0.85 -0.11 0.01

13 20-Sep 15:00 - 15:30 0.36 0.49 -0.13 0.02

14 20-Sep 15:30 - 16:00 0.73 0.54 0.19 0.04

15 20-Sep 16:00 - 16:30 2.19 1.05 1.14 1.30

16 20-Sep 16:59 - 17:29 0.59 0.71 -0.12 0.01

17 20-Sep 17:29 - 17:59 0.00 0.07 -0.07 0.00

18 SUM 21.63 22.71 -1.08 2.16

AVG 1.27 1.34 -0.06 0.13

RATA RESULTS 

Average Difference -0.06

Standard Deviation 0.36

Confidence Coefficient 0.19 t =2.12

Relative Accuracy 18.7%

Relative Accuracy ppm 0.25 Ppm

Emiswsion Standard 3 Ppm

Relative Accuracy % of STD 8.3% % of STD

 
TABLE 11: CEMTEK TDL ONE MILL ON RATA CALCULATIONS (14 RUNS) 

Run Date Time CEMS (ppmw) Ref (ppmw) Difference Difference 2

2 20-Sep 8:30 - 9:00 2.21 2.35 -0.14 0.02

3 20-Sep 9:00-9:30 2.14 2.05 0.09 0.01

4 20-Sep 9:41-10:11 1.54 1.84 -0.30 0.09

6 20-Sep 10:42 - 11:12 1.35 1.53 -0.18 0.03

7 20-Sep 11:22 - 11:52 1.26 1.44 -0.18 0.03

8 20-Sep 11:52 - 12:22 1.45 1.35 0.10 0.01

9 20-Sep 12:22 - 12:52 1.19 1.30 -0.11 0.01

10 20-Sep 13:06 - 13:36 1.05 1.21 -0.16 0.03

11 20-Sep 13:36 - 14:06 0.94 1.06 -0.12 0.01

12 20-Sep 14:06 - 14:36 0.74 0.85 -0.11 0.01

13 20-Sep 15:00 - 15:30 0.36 0.49 -0.13 0.02

14 20-Sep 15:30 - 16:00 0.73 0.54 0.19 0.04

16 20-Sep 16:59 - 17:29 0.59 0.71 -0.12 0.01

17 20-Sep 17:29 - 17:59 0.00 0.07 -0.07 0.00

18 SUM 15.55 16.79 -1.24 0.33

AVG 1.11 1.20 -0.09 0.02

RATA RESULTS 

Average Difference -0.09

Standard Deviation 0.13

Confidence Coefficient 0.07 t =2.16

Relative Accuracy 13.6%

Relative Accuracy ppm 0.16 ppm

Emiswsion Standard 3 ppm

Relative Accuracy % of STD 5.5% % of STD

For this test 17 reference test runs were completed. HCl concentrations began above 3 ppm and 

decreased throughout the day as lime inject was increased on several occasions. The TDL CEMS 

relative accuracy was calculated for runs 1-17 and the results were 18.7% of the reference value 

as shown in Table 10. This result meets the 20% relative accuracy specification. The mean 

difference was -0.06 ppm reflecting that the TDL values were virtually equal to the reference 

FTIR measurements. The one mill on RATA results were recalculated based on 14 “best” test 

runs (omitting runs 1, 5, and 15) as shown in Table 11. The relative accuracy based on 14 test 

runs was 13.6% of the reference value. (1) 

ISSUES ENCOUNTERED DURING THE TEST PERIOD 

‐ Optical locks not secured caused unrepeatable gas injections. 

‐ Calibration Gas regulator and transport tubing needed conditioning for several hours to 

come to equilibrium. 

‐ EPA had questions about how to check zero in IP-CEMS and presented to them in 

response novel way of spectral subtraction. Testing it in the field and show good results. 

Also showed the internal Span Check results of time. 

‐ Implement a residual background calculation for correctly measuring when no HCl 

present. 
‐ Implementation of dynamic temperature inputs to analyzer. This employed was to 

account for the potentially wide swings in temperature at site. The expected error in the 

measurements would be in the range of 6 to 7% if a fixed temperature of 126.6°C was 

employed—thus a fixed calibration factor. 

‐ Automated daily injections to the Cal Error test near the end of the trial period with very 

good results. 

‐ Found better results by turning off low correlation R-value <0.7 correction. 

‐ Difference found between the factory calibration factor as compared to specialty gas 

cylinders of HCl. The Cal factor was set by an external audit module that was over 5 

years old. 

‐ Measurement by TDL is wet basis and compliance limit is 3 ppmvd at 7% O2 on a 30 –

day rolling average. 

CONCLUSIONS 

The results obtained for the tests conducted for this study provide some of the first data from 

HCl CEMS while following the test procedures in Draft PS 18. The EPA has received this data 

to further their PS development. 

The interference test was partially completed and the results showed that for the interfering gases 

tested the TDL would pass the test for those gases. The responses were all below the LOD. This 

test will need to be performed for the complete list of gases and concentration ranges added to a 

base HCl concentration as the test calls out in future studies. An interference test was conducted 

by the University of California, Riverside with similar results. (3) 

The limit of detection test showed that he TDL is a sensitive measurement technique and the 

result of this test exceeded the expectation of the manufacturer. The lab LOD was expected to be 

0.2 ppmv and the measured LOD installed was better than that measuring 0.16 ppmv. Further 

installation test need to done to demonstrate repeat performance. (3) 

The response time test used the spike test data and showed the response to the spike gas and zero 

gas was less than 30 seconds. Normal CEMS response time limits are 15 minutes maximum so 

this a very rapid repose time and can be useful for process control at this speed of analysis. 

The calibration error test was not conducted during this study and will be conducted at another 

installation. We did prepare in time to select the gases needed before demobilizing form the field 

The seven day drift results showed that using the internal Cal cell as well as the gas flow though 

demonstrated the stability and repeatability needed to pass this test. All testes were 2.6% and 

less. 

The stratification test was not conducted during this study. 
The relative accuracy test was evaluated as a 40CFRPart 60 CEMS would be evaluated. The 

TDL passed all the test conditions presented. Several conditions to generate low and high levels 

of HCl stack gas and all were within passing tolerances. 

During the study period we also learned a number of things to make the analyzer perform better. 

Some were in the form of signal processing analysis. Some improvements is startup and 

commissioning procedure checklist were made. Over all the TDL was very simple to startup and 

maintain with very little attention need to obtain excellent results. 

ACKNOWLEDGEMENT 

Holcim Inc. and the staff at St. Genevieve plant. 

REFERENCES 

1. Peeler. Jim - Emission Monitoring Inc., Rosenhamer, Glen - Holcim Inc., Field Evaluation of 

HCl CEMS at Holcim St. Genevieve Plant, December 19, 2012 

2. USEPA, Draft Version 9-19-2012, Draft Performance Specification 18, 

Performance Specifications and Test Procedures for HCl Continuous Emission 

Monitoring Systems in Stationary Sources

3. Dene, Chuck – EPRI, Continuous Emission Monitor For Hydrochloric Acid, 

Tunable Diode Laser Test Report, Unisearch LasIR S Series, 1022083, Final 

Report, December 2011 EPRI, Palo Alto, CA: 2011 1022083).