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ADVANCED SENSOR DEVELOPMENT PROGRAM

FOR THE PULP AND PAPER INDUSTRY

Annual Report

By

J. R. Whetstone

H. G. Semerjian

C. Presser A. Gaigaias

j. Potzick

December 13, 1982

Work Performed Under Contract No. AI01-76PR06010

National Bureau of Standards

U. S. Department of Commerce

Washington, D, C.

and

National Engineering Laboratory

Boulder, Colorado

U. S. DEPARTMENT OF ENERGY

I\
DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or

process disclosed, or represents that its use would not infringe privately owned rights. Refer- ence herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

This report has been reproduced directly from the best available copy.

Available from the National Technical Information Service, U. S. Department of Commerce,

Springfield, Virginia 22161.

Price: Printed Copy AOS

Microfiche AOl

Codes are used for pricing all publications. The code is determined by the number of pages in the

publication. Information pertaining to the pricing codes can be found in the current issues of the

following publications, which are generally available in most libraries: Energy Research Abstracts

(ERA); Government Reports Announcements and Index (GRA and I); Scientific and Technical

Abstract Reports (STAR); and publication NTIS-PR-360 available from NTIS at the above

address.
DOE/PR/06010-T13

(DE83017094)

Distribution Category UC-95f

Advanced Sensor Development Program

For the Pulp and Paper Industry

Annual Report

December 13, 1982

REPORT NUMBER 1

National Bureau of Standards

U. S. Department of Commerce

Washington, D.C. 20234

Prepared for the

U.S. Department of Energy

Office of Industrial Programs

Washington, D.C. 20585

Under Contract No. DE-AI-1-76PR06010
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TABLE OF CONTENTS

Page

Table of Contents i

Executive Summary ii

1. INTRODUCTION 1

2. TECHNICAL PROGRAM 4

2.1 In-Situ Spectroscopic Flame Temperature Measurement . . 4

2.1.1 Analytical and Experimental Laboratory Phase . . 6

2.1.2 Field Experimental Phase 11

2.2 Consistency Measurement 18

2.2.1 Detection Techniques 19

2.2.2 Waveguide Approach 22

2.2.3 Analytical Model 23

2.2.4 Discrete Detection System 27

2.3 Steam Mass Flowmeter Project 28

2.3.1 Operational Description 28

2.3.2 Digital Processing Package 29

2.4 In-Solution Lignin Measurement Research 30

2.5 Evaluation of O 2 and CO Monitoring Systems

for Combustion Controls 31

2.6 Project Summaries 33

2.6.1 In-Situ Spectroscopic Flame Temperature

Measurement 33

2.6.2 Consistency Sensor 34

2.6.3 Steam Mass Flowmeter Project 34

2.6.4 In-Solution Lignin Measurement Research 35

3. PLANS FOR NEXT CALENDAR YEAR 36

List of Tables 38

List of Figures 38

References 39

Appendices 40

i
EXECUTIVE SUMMARY

The National Bureau of Standards is pursuing a basic research and

development program sponsored by the Office of Industrial Programs,

Department of Energy whose objective is the development of measurement

science and technology for on-line measurement of process variables in

the pulp and paper industry. Four research projects are currently in

progress; (1) in-situ flame temperature measurement, (2) pulp

consistency measurement, (3) steam flow measurement, and (4)

in-solution lignin measurement.

The objective of the in-situ flame temperature measurement

project is development of a spectroscopic technique for measurement of

flame temperatures and fuel /air mixture ratio in the combustion zone

of recovery boilers. The technique utilizes spectral analysis of the

emissions of diatomic species formed in the combustion process whose

spectral features are located in the ultraviolet and visible regions

of the spectrum and are easily distinguished from the intense

blackbody radiation emitted by the high concentration of hot

particulates in the furnace cavity. These spectral features are very

temperature dependent. Both laboratory and field activities are in

progress. Models have been developed describing the temperature

dependence of these emissions. The models are needed to interpret

observations of emissions from laboratory flames and from the recovery

boiler. Data from both have been collected. Current activities are directed toward improved field measurement of recovery boiler

emissions using high speed optical recording methods, and development

of high quality spectral data from combustion of black liquor, both in

the recovery boiler and in laboratory combustors. Such data will form

the basis for development of the algorithm needed for prototype sensor

design and operation.

The objective of the consistency measurement project is the

development of a method which is accurate under changing flow

conditions, non-intrusive, capable of real-time and on-line

measurement, and useful over the range of stock consistency (cellulose

concentration) and stream conductivity values found in the pulp and

paper mill. A new approach is being taken using radio frequency

excitation of circular waveguides. Initial experiments have been

completed using a static test cell. These have demonstrated the

feasibility of the approach. Current activities are directed toward

development of a sensing technique which allows pressurized operation

of the sensing section itself. An array of R.F. diode

detector/amplifier channels is under development for this purpose.

Upon completion this array will be used in a two-inch prototype sensor

for sensor response and performance testing on materials of known

properties. A two-inch flow loop has been constructed and will be

used for tests in paper pulp, and for development of pulp/water mixing

rules. The latter is required for determination of consistency values

from observed values of dielectric constant and conductivity.

Following testing of the two- inch prototype at NBS a six- inch

prototype will be constructed for tests at the Institute of Paper

Chemistry.
The objective of the steam flowmetering project is development of

a new measurement technique of improved accuracy (approximately 1%

uncertainty), of greater dynamic range, is non-intrusi ve to the flow

thereby reducing energy loss in metering stations, and measures the

average gas temperature. A long wavelength acoustic technique is

being developed which is expected to accomplish all of these

objectives. A patent application has been filed and non-exclusive

licenses have been granted to several private manufacturers. A

digital electronic processing package has been designed, fabricated

and tested. Initial tests on known air flows at atmospheric pressure

have been completed and a paper has been presented at the 1982

Instrument Society of America annual meeting. Currently development

of the steam prototype device is underway. Initial laboratory tests

in airflows and in steam are planned before the device will be tested

in a mill environment.

A new project has been started this year whose objective is the

development of a method for measurement of lignin concentration in the

liquor solutions found in the pulping process. The need here is a technique which is sensitive to lignin and lignin salts in the

presence of the highly conductive and caustic solutions found in the

digestion process. A new approach has been conceived which

investigates the non-linear coupling of the electric dipole moments of

lignin polymers with a radio frequency electric field. The project is

in the theoretical and experimental investigation stage to determine

the validity of the physical processes involved and the applicability

of the technique for on-line measurement of in-solution lignin

concentration measurement.

In addition to the four projects underway, one project has been

completed. An evaluation of CO and O 2 monitors used for combustion

controls has been completed which provides reliable data on their

performance, operating range and accuracy. The NBS experimental

furnace was used for these experiments. It is equipped with a complete on-line gas analysis system which is used as a reference.

Specific areas of interest included evaluation of the accuracy and

sensitivity of the measurement techniques over a wide range of

operating conditions, correlations of flame stoichiometry with stack

measurements, and the effect of stack gas temperature on composition

measurements. The O 2 stack gas monitor employed a Zr02 type

sensor. The system was found to be very sensitive to air leakage when

the furnace pressure was kept below atmospheric pressure. A

calibration system was also found to be necessary, especially for

aspirating type probes. The CO monitoring system is based on infrared

absorption across the stack. The CO measurements were found to be

quite sensitive to stack gas temperatures. This effect has been

quantified in a heated test cell. The relationship between the stack

gas CO levels and combustion zone stoichiometry was also found to be

sensitive to the energy extraction rate in the furnace cavity.

Experimental results have been obtained for both natural gas and No. 2

fuel oil fired systems. A final report has been prepared for

publication, and a paper was presented at the 1982 Instrument Society

of America annual meeting.
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I. INTRODUCTION

The Office of Industrial Programs of the Department of Energy,

Advanced Concentration and Evaporation Branch, has supported a basic

research and development program at the National Bureau of Standards

Center for Chemical Engineering, since July, 1980. The objective of

this program has been the development of new measurement science and

technologies which will ultimately be applicable to on-line measurement

of unit process variables in the pulp and papermaking industry. The

measurement techniques developed are such that they will find

additional application in measurement and control of unit processes of

other industries, as well as in the manufacture of pulp and paper.

This program utilizes the advice and assistance of a working group

composed of members of the various sectors of the pulp and paper

industry, i.e., academia, instrumentation and control system

manufacturers, and pulp and paper manufacturers. This group assisted

in development of a priority list of process variables for which

on-line measurement is currently either frought with severe systematic

errors or is not possible. The list is given in Table I. To broaden

the perspective this list provides, a survey was made of the industry

by NBS scientists and a report written [1]. The conclusions of this

report, which are in agreement with others of a more general nature

[2], are:

1. The industry penetration by automatic control systems is high

at the paper machine end of the process. This is due to the

availability of viable measurement techniques, i.e., sensors which give real-time or nearly real-time process variable

information for use by automatic control systems.

2. The pulping end of the process is, by contrast, much less

automated by virtue of the lack of reliable on-line process

variable measurement technologies (sensors).

3. Specific process variable measurement needs were identified in

the pulping end of the cycle. The unit processes of most

interest were the digestion process and the chemical recovery

cycle, the recovery boiler particularly.

This information and these recommendations have been used to

formulate the research program described here. Four research projects

are underway and are directed toward developing new measurement

technologies for observation of process variables associated with the

manufacture of paper pulp or with the preparation of the pulp as it

approaches the paper machine for the making of the paper or board

sheet. Specifically, these project objectives are: (1) in-situ

measurement of flame temperature and fuel /air mixture ratio in the

combustion zone of the recovery boiler; (2) measurement of the

consistency of pulp slurries; (3) simultaneous measurement of steam mass flowrate and steam temperature; and (4) measurement of lignin

1
Table 1

ON-LINE MEASUREMENT NEEDS

® Kappa number (lignin)

® Steam flowmeter

® Liquor combustion (UV sensor)

* Liquor characteristics

(defined by TE report)

® Percent solids black liquor

® Chip moisture

® Pulp quality (freeness)

® Active alkali

® Smelt bed parameters

® Lime kiln T profile

® Consistency (correlation

moment flowmeter)

® Bulk chip density

r

'

2
concentration in digestion liquors. An additional project has been

completed, having as the objective the evaluation of commercially

available CO and O2 sensors. The final report has been completed [3]

and a paper presented at the 1982 ISA meeting [4].

The four research projects are specific to the unit processes

found at the pulping end of the papermaking process. Two also have

application in the pulp preparation or blending operations that are found between the pulp mill and the paper machine itself. The

technical activities of these research projects are discussed in the

sections below. Some introductory material is included with each to

give a perspective of the work relative to the particular unit process

involved.

3
2. TECHNICAL PROGRAM

2.1 In-si tu Spectroscopic Flame Temperature Measurement

The objective of this research project is development of a new approach to combustion zone temperature measurement based upon

emissions from the combustion zone of a recovery boiler, which can then

be related to the combustion zone stoichiometry. The recovery boiler

is one of the most capital and energy intensive installations found in

the pulping process. It is a key part of the chemical recovery cycle

of the pulp making process and performs two tasks. Its primary task is

the reduction of the spent pulping chemical derived from the digestion

process, primarily the reduction of sodium sulfate to sodium sulfide.

Steam production is the secondary task. In many mills the recovery

boiler produces a large fraction of the total steam used for electrical

generation and process purposes. The recovery boiler employs a staged

combustion process. Figure 1 shows the major elements of one of the

two predominant types of boiler. Combustion air is introduced to the

furnace cavity at three levels. The fuel, black liquor, is injected

into the cavity between the secondary and tertiary air injection

levels. Black liquor is the concentrated spent cooking liquor of the

pulp digestion process. It is a slurry composed of inorganic salts,

lignin compounds leached from the wood during digestion, and water.

Currently, the normal firing condition is approximately 65% solids

concentration. The lignin compounds provide the fuel value of the

black liquor. The inorganic constituents, under the influence of the

combustion gases, ultimately form a char which falls to the furnace

floor forming a smelt bed in which the bulk of the chemical reduction

reactions are thought to occur. The resulting char forms a particulate

which pervades the furnace cavity and has sizes ranging from one micron

to several millimeters. Formation of the char begins with the black

liquor droplets issuing from the injection nozzle. These droplets dry

and undergo suspended pyrolysis and combustion in the furnace cavity

until they fall to the smelt bed. After combustion the char particles

remain at high temperatures and strongly emit blackbody radiation.

This radiation is located primarily in the infrared region of the

spectrum, although the high energy tail of the Planck distribution

function may extend into the visible region. Use of emissions from the

combustion products for the determination of the combustion temperature

requires an approach which is insensitive to this broad range of

blackbody radiation, and is specific to the combustion process.

The approach being taken in this project is to avoid the infrared

region of the spectrum where blackbody emissions are most intense.

Instead, the emissions of diatomic molecules formed as intermediates in

the combustion process will be utilized. These species emit radiation

in the ultraviolet and near visible region of the spectrum. The

concentration of these species is strongly dependent upon flame

4
stack

Figure 1. Black Liquor Recovery Boiler

5
temperature and, therefore, flame stoichiometry. The species of

interest include OH, CH and C2* The research direction of this

project is to: (1) quantify the relationships between these emissions

and flame stoichiometry; and (2) investigate the effect of continuum

emissions from particulates, spatial distribution of emissions, and the

effect of burner geometry.

The research effort has taken two parallel directions to date.

The first is an analytical and experimental laboratory effort through

which a scientific knowledge base is being established. The second

effort is a field effort consisting of two phases; the first phase is

the gathering of spectroscopic data from operating recovery boilers, and

the second phase is testing a prototype device which measures

combustion temperatures in various regions of the recovery boiler

cavity.

2.1.1 Analytical and Experimental Laboratory Phase

Initial experiments and analytical efforts had the objective of

characterizing and quantifying the emissions from OH species formed in

methane and fuel oil flames. The experimental part of this work was performed in the NBS experimental furnace and in laboratory flames.

Initial spectral measurements were made using a .275 meter spectrometer

which has a spectral range of 190 to 600 nm, with a resolution of 0.5

nm. Preliminary experiments were done both in natural gas and oil

flames. Spectra were obtained in the 250-500 nm range. In natural gas

flames, black body emissions are almost negligible; the OH emissions

from the band located at 310 nm ^ transitions) are very

strong; CH emissions both at 390 nm X^tt) and 431 nm

(A^A -^XS) are also observed very clearly. Both the OH emissions

at 310 nm and the CH emissions at 431 nm exhibit strong temperature

dependence, and prospects for utilization of these bands for

measurement of combustion temperature appear promising.

Similar spectra have been obtained in oil flames. In this case,

the continuum contribution from particulates is much stronger and

increases in intensity towards the longer wavelengths. These initial

experiments were carried out at a single axial location, very near the

fuel injection point of the experimental furnace. The preliminary

experiments indicated the need for instrumentation with higher spectral

resolution to resolve individual lines in the OH and CH spectra. To

this end, a 0.5 meter spectrometer was obtained which has a spectral

resolution of better than 0.02 nm, with a 1180 line/mm grating. This

resolution was demonstrated by resolving the mercury emission doublet

at 313.1 nm. Spectra was again obtained in the experimental furnace

with both natural gas and oil flames; the quality of the spectra was excellent and a number of very strong lines were identifiable (see

Figure 2). These lines were subsequently shown to have strong

temperature dependence. The sharp bandhead should also facilitate

identification of, and compensation for, blackbody emissions.

6
Natural

Gas

Flame

OH

Emissions

(0,0)

A^Ex^n 7

Figure

2.

OH

Emissions

from a

Natural

Gas

Flame
Observation of the emissions confirmed their dependence on the

equivalence ratio/stoichiometry of the flames and has provided the

initial experimental basis for further investigation of these as a means for flame temperature measurement.

Identification of the spectral lines contained within the observed

emission bands is critical since each band is composed of several

groups or series of emission lines. Each line and series has a characteristic dependence of emission intensity upon temperature. A

literature search was completed of published data on OH emission

intensities at various temperatures. These data provide the input for

an algorithm developed to calculate the temperature dependence of the

emission intensity for lines occurring in the 310 nm band. A computer

program was initially developed using this algorithm to compute the

temperature dependence for individual emission lines, and some typical

results are shown in Figure 3. As the four curves indicate, emission

intensity may either increase or decrease for a particular line with

temperature. Utilization of appropriate combinations of line

intensities is expected to result in an accurate, in-si tu flame

temperature and combustion zone stoichiometry measurement technique.

However, to realize this objective, further development of the

laboratory data and the analytical algorithm are needed.

Further analytical developments were directed toward the

development of spectral maps or diagrams which simulate the

characteristics of the actual spectrometric data. In order to develop

a reliable algorithm for temperature measurement, each line observed in

a particular band must be unambiguously identifiable in order to know

the dependence of emission intensity upon temperature. The initial

computer program was expanded to include a plotting capability which

results in the spectral map shown in Figure 4. This diagram shows the

PI, Q1 , and R1 bands of the OH spectrum in the 310 nm region at a single temperature value. A set of diagrams such as these is

necessary for each emitting species for each of its emission bands of

interest in order to identify the individual emission line intensities

and, therefore, obtain temperature information from them.

Models which predict the intensity and the temperature dependence

of individual emission lines are useful in identifying each observed

spectral line. However, the utility of such models is limited when

closely spaced lines overlap in such a way that they cannot be resolved

by the spectrometer. In addition, a separate model is needed to

predict the spectra of each emitting species. We have, therefore,

undertaken an effort to develop a more generalized scheme to predict

the spectra of diatomic molecules, including OH, CH and C 2

* The

model provides the capability to integrate over small spectral regions,

simulating a spectral resolution typical of the spectrometer/detector

system, using a slit function. The model can also predict emissions

from non-uniform temperature and composition fields. A relatively

8
EMISSION

INTENSITY

0 500 woo 1500 2000 2500 3000 3500

TEMPERATURE (°K)

Figure 3. Temperature Dependence of Emission Intensity

for the OH P, R, and Q Bonds Near 306 nm

9
A1ISN31NI

. I 10

Figure

4.

Spectral

Diagram

-

PI,

R1

,

and

Q1

Bonds

in

306

nm

Region
simple model to account for self-absorption is also included. However,

line broadening is not taken into account, which would become

especially important where self-absorption is appreciable. Present

efforts are focused on the extension of the model to include line

broadening. Spectroscopic data on CH and C 2 have also been gathered,

and the model will be modified to include these species, in addition to

OH. An example of typical calculated spectra is presented in Figure 5

for the (0,0) -> Xu transitions of OH at 310 nm for 2000®K.

Comparison of Figures 5 and 2 shows the similarity between the observed

and predicted spectra. Such spectral diagrams will be utilized in

interpretation of the experimental observations, taken with the

spectrometric systems being used in this research effort. It will also

aid in the selection of the spectral regions that will be utilized for

the final sensor configuration.

The combination of spectral models and laboratory experiments in

flames will yield the necessary information for quantification of the

relationships between emission intensity and flame stoichiometry.

2.1.2 Field Experimental Phase

The parallel field experimental effort has two objectives, (1) to

obtain spectral data from operating recovery boilers and (2) to test

prototype devices on recovery boilers. Work has begun on the first

objective. Such a task differs from that possible in a laboratory

environment in three respects; most obvious is the situation of a poor

immediate environment. Next it is necessary to develop a method to

bring the combustion zone emissions to the spectrometer in such a way

that sufficient intensity is available at its entrance slit. Finally,

the device must be protected from the rather harsh conditions occurring

near the wall penetrations of the recovery boiler. To satisfy these

requirements a special optical system was designed and constructed.

Its function is to view a particular region in the recovery boiler

cavity using one of the boiler's access ports, and to transmit the

light gathered from a portion of the combustion zone via an optical

fiber to an imaging system mounted at the entrance slit of the

spectrometer. This system was built and tested using the MBS

experimental furnace. Modifications were made to the initial design to

increase the intensity falling upon the entrance slit of the

spectrometer.

Subsequently, data was obtained from the recovery boiler of the

Westvaco Mill located at Luke, Maryland. These data were not of the

quality that are normally obtained in a laboratory environment due to

the large and rapid intensity fluctuations of combustion in the

recovery boiler cavity. These fluctuations were of such a magnitude

and time duration (periods of a few seconds) that only qualitative data

could be obtained. The large scale fluctuations were due to the

extended time period necessary to record the data taken by the

spectrometer. Under normal conditions, i.e., a constant intensity of

input light, recording of the spectrometer data is obtained by

11
Predicted

OH

Emissions

iL't 00 0,

St

S

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Figure

5.

Predicted

Emission

Spectra

for

OH

in

the

306

nm

Region

for

2000°C
detection of the light intensity passing through the spectrometer'

dispersive element and falling on its exit slit behind which a photodetector is placed to measure the intensity of the light passing

through the system. The dispersive element is scanned at a constant

rate causing the wavelength of the light falling on the exit slit to

increase or decrease proportionally with the amount of optical

intensity contained in the wavelength region selected by the dispersive

element and the entrance and exit slit widths. Recording of the

spectral data is accomplished using a strip chart recorder which

records the exit slit photodetector output as a function of time. Ten

to sixty minutes is necessary to scan a wavelength region of 10 to 100 nm under constant input intensity conditions. As an example, the

spectra recorded in Figure 2 required several minutes to record using a constant intensity flame. In order to obtain a useful record of the

spectral data the input intensity must remain constant during the

recording time. Such conditions can be obtained in laboratory

experiments. As mentioned above, the intensity of the visible

emissions from the boiler cavity varied considerably over the course of

5 to 10 seconds. These intensity variations carry through to the exit

slit of the spectrometer. In all of the observations taken, variations

in the input intensity caused such large fluctuations in the

spectrometer output intensity that only the presence of gross spectral

features could be discerned.

To develop reliable data of the general spectral features from

recovery boiler emissions, a data recording technique is necessary

which can record the spectrometer output intensity over a wide

wavelength range (1 to 25 nm) in a sufficiently short time period (0.10

seconds or better). Recent developments in distributed element optical

detector systems form the basis for this new type of readout device and

are known as Optical Multichannel Analyzers (OMA). Two of these

systems were obtained for evaluation as a detector for the OMA detector

which has a sensitive length of approximately one inch with a spatial

resolution of approximately .001 inch. Use of the OMA removes the

requirement of scanning the dispersive element of the spectrometer.

Instead a portion of the spectrum falls on the sensitive region of the

OMA's detector, the intensity is digitized and the microprocessor based

controller displays the detected spectral region. Such data can be

taken quickly; temporal resolutions of 100 nanoseconds are possible.

Data collection times of 20 to 100 milliseconds provide good signal-tonoise ratios with the intensities provided by laboratory flames.

Typical spectra obtained in a laboratory flame, seeded with black

liquor particles, are shown in Figures 6-9. The OMA/spectrometer

combination when delivered, will be used for future data acquisition

from recovery boilers, and for laboratory experiments needed for

quantification of the relationship between flame stoichiometry and

emission spectra characteristics.

13
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2.2 Consistency Measurement

The papermaking process involves many unit processes in which the

process stream is the paper pulp slurry. Pulp slurries are predominately water in which cellulose concentrations range up to 12 to 15

percent to form the solid phase fraction of the slurry. Consistency is

the terminology used in the industry to denote solid phase fraction,

defined as the mass fraction of dry cellulose in the slurry or paper

stock. On-line measurement of consistency in operating mills is

largely done using two techniques. The first utilizes the change in

reflectance of the pulp with change in consistency. Use of this device

in low consistency slurries (up to 2%) slurries is claimed to be

effective, sensitive and accurate, especially in the blending processes

occurring as the stock is blended with white water to produce stock of

the final consistency for use on the paper machine. For higher

consistencies however, due to a large decrease in the change in

reflectance with change in consistency at higher consistency values,

the sensitivity and accuracy of this technique may be considerably

degraded. The second technique employs a hydrodynamical ly shaped body

which is immersed in the flow. The contribution to the drag on the

body from the cellulose fibers in the slurry is used to infer the

consistency value for the pulp. Devices of this type are claimed to be

flow dependent, sensitive but inaccurate, and require frequent

maintenance.

The objective of this research project is the development of a consistency measurement technique which can be used for real-time

measurement, is preferably non-intrusive, not flow property dependent,

has a broad range of response, and an accuracy of a few percent in the

consistency value. The approach that has been taken is to use the

effects of cellulose in stock slurries to modify the propagation

characteristics of radio frequency or microwave radiation in circular

waveguides relative to the response obtained in water-filled

waveguides.

In the past, attempts have been made to measure consistency using

the change in the electrical properties of the stock due to changes in

the cellulose concentration of the pulp slurry [5]. The electrical

property most affected by changing the quantity of cellulose in the

stock is the dielectric constant of the mixture. The addition of

cellulose (dielectric constant of 2-3) to water (dielectric constant of

approximately 80) causes the mixture's dielectric constant to change

significantly. Therefore changes in the cellulose concentration in

stock slurries may be determined sensitively through measurement of its

dielectric constant. A common approach has used low frequency bridge

circuits to sense changes in the capacitance value of a parallel plate

capacitor filled with stock. In this technique the capacitance is not

measured directly but inferred from measurement of the total impedance

of the sensing capacitor. Since the total impedance also includes a strong contribution from non-zero resistance values, the conductivity

or resistivity of the stock contained between the plates of the sensing

capacitor strongly influence the value of the total impedance. Changes

18
in the total impedance of the sensing element may be the result of

either conductivity or dielectric constant changes. Generally, the

conductivity is assumed constant, and not measured, either conductivity

or dielectric constant changes. Generally, the changes in conductivity

are interpreted as dielectric constant, e.g., consistency changes,

causing systematic errors in the measurement.

The initial approach of this research project was to use radio

frequency techniques to take advantage of the large difference in the

values of the dielectric constants of water (approx. 80) and cellulose

(approx. 2-3). To reduce or eliminate the effects of changing values

of the process stream conductivity the technique preferable should

simultaneously measure the conductivity. Given a successful detection

scheme for the dielectric constant and conductivity of the stock

passing through the sensing volume of the instrument, the development

of the appropriate mixing rules for cellulose and water will yield an algorithm for the inference of consistency values from measured

dielectric constant values of the stock. This is the approach

previously taken by the low frequency bridge technique. The essential

difference in our approach is the capability to simultaneously measure

the dielectric constant and the conductivity of the stock passing

through the sensitive region of the device. Measurement of the

conductivity of the process stream is essential since determination of

the bulk dielectric constant of the stock is dependent on it at the

frequencies of interest.

2.2.1. Detection Techniques

A resonant transmission line technique was chosen for the initial

experiments. (This preceded turning of attention to the circular

waveguide approach.) A theoretical description of the characteristics

of parallel conductor transmission lines was developed which describes

the effect of variation in the dielectric and conductivity properties

of the medium separating the conductors. The transmission line

consists of two parallel wires co-1 inear with the axis of the pipe

carrying the pulp slurry. The circuit shown in Figure 10 was used to

measure the response of the transmission line to various excitation

frequencies. The detected signals are shown in Figure 11, with

distilled water filling the space between the wires. The in-phase

signal has a minimum at a frequency fo» while the quadrature signal

goes through zero at another frequency, fgo* For these two

conditions the conductivity and dielectric values of the medium

separating the conductors can be obtained analytically.

Preliminary data was obtained for distilled water which forms the

reference response condition. Also water-methanol mixtures were used

to vary the dielectric properties but not the conductivity of the

system. Data were also taken on pulp-water mixtures which showed a direct correlation between consistency changes and variation in the

frequencies fo and fgQ. These data demonstrated the general

19
20

Figure

10.

Parallel

Conductor

Transmission

Line

Circuit

Diagram
lanindMv loels

sanindwv (ols

21

Figure

11.

In-Phase

and

Quadrature

Signals

-

Parallel

Transmission

Line
validity of the theoretical model of the resonant transmission line

system.

The model of the resonant transmission line system describes the

interference effects which occur between two electromagnetic waves

propagating in opposite directions along the transmission line. One wave travels from the source toward the end of the transmission line.

The second wave is produced by the reflection of the initial wave at

the end of the line back toward the source. For the interference to be

detected, sufficient signal must be reflected to produce a measurable

interference effect. A minimum of one wavelength is required.

This initial work used parallel wire transmission lines operating

near 40 MHz. This configuration operates well for relatively low

conductivities. However, as the conductivity values approach 0.15

mhos/meter the attenuation of the signal is so strong that the strength

of the reflected wave is reduced severely, causing interference between

the two waves to diminish. The signal is, therefore, reduced below the

point of detection. The level of conductivities at which this occurs

is comparable to the maximum conductivity encountered in the pulp

handling processes of the industry. The maximum concentration of

inorganic salts expected to be encountered in process streams is

equivalent to approximately 1.2 parts per thousand of sodium sulfate in

water [6]. The conductivity of such a mixture is approximately 0.15

mhos/meter, the upper bound of conductivity at which the parallel

conductor configuration is useful. Two factors primarily contribute to

the undesirability of this approach for pulp consistency measurement:

(1) The attenuation of electromagnetic waves increases with

increasing conductivity.

(2) The wavelength at 40 MHz is such that the interference pattern

extends over distances of 25 centimeters or more which

enhances attenuation of the electromagnetic field, thereby

decreasing the intensity of the interference signal beyond the

limits of detection.

2.2.2. Waveguide Approach

To overcome the limits encountered with the parallel conductor

transmission line approach it was necessary to operate at higher

frequencies, reducing the effects of ionic conductivity. The

attenuation of the electromagnetic field is proportional to the ratio

of the conductivity, s, to the frequency, f. In addition, at higher

frequencies the wavelength is shorter and the interference pattern

occurs over a smaller distance, further minimizing the effects of

attenuation. To realize these benefits the frequency must be increased

significantly. Use of frequencies above 100 megahertz reduces the

effects of attenuation, and the use of circular waveguides for

propagation of the electromagnetic field provides a non-intrusive means

22
of measurement. Circular waveguides, of course, lend themselves very

well to process piping. As an indication of the operating frequency

ranges available with this approach the cutoff frequencies for pipes of one- and two-inch diameter (water filled) are listed below;

Pipe Diameter TM(MHz) TE(MHz)

2.8 cm 710 927

5.0 cm 398 520

The cutoff frequencies are those frequencies below which

electromagnetic waves of a given mode cannot propagate in the

waveguide. Two modes of propagation are possible for these waveguide

geometries, transverse magnetic (TM) and transverse electric (TE). For

circular pipe geometries the TM mode propagates at the lowest

frequency. By using excitation frequencies below the lowest cutoff

frequencies, one ensures that only one excitation frequency propagates

and that a simple interference pattern is produced. Experiments were performed in the 2.8 cm (1 inch) diameter pipe at a frequency of 900

MHz and in the 5 cm (2 inch) diameter pipe at 490 MHz. This choice

produces the shortest wavelength interference pattern, and at the same time guarantees single frequency operation.

The measurement required to determine values for the dielectric

constant and conductivity of the stock filling the pipe, or waveguide,

are the wavelength and attenuation of the radio frequency

electromagnetic field. Measurements of the axial electromagnetic field

strength at the inner surface of the waveguide were made by inserting a small wire through a slit (~ 1 mm wide) cut lengthwise through the pipe

wall. The small wire was in turn connected to an r.f. diode mounted on

a movable track. Figure 12 shows the voltage output as a function of

the location of the detector along the waveguide which is filled with

tap water. A strong interference pattern is evident. A theoretical

fit was made to the data in Figure 12 assuming that the detector output

is proportional to the square of the sum of two waves traveling in

opposite directions. The result of the fit is also shown in Figure 12;

although there are small discrepancies, the overall agreement between

the data and the fit is very good. The results of the fit were used to

estimate the wavelength (Ag) and the attenuation (a). Figure 13

shows the interference pattern obtained with the pipe filled with a 1.3

gram/liter aqueous solution of Na2

S04 . This concentration produces

a conductivity which is somewhat larger than that encountered in most

mill streams. The effect of attenuation is much more evident than in

Figure 12. The interference effects are sufficiently large for a measurement of Ag but with poor results.

2.2.3. Analytical Model

To determine dielectric constant and the conductivity values from

the interference pattern measurements, an analytical description or

23
TAPWATER FREQUENCY

=

490

MHz

DISTANCE

ALONG

THE

WAVEGUIDE
FREQUENCY

=

490

MHz

25

DISTANCE

ALONG

THE

WAVEGUIDE
model is needed describing the physical phenomena governing the

stock-filled, waveguide system. If one assumes that the substance

filling the waveguide is homogeneous and its response to the

electromagnetic field linear, it is possible to relate the measured

quantities, the frequency, f, ^nd a to the dielectric constant (K)

and the conductivity (c^). The relationship is given by

K

=

a =

10

11

4390 f

2533

(b2 - .2 . (L84i)2)

3a,

( 1 )

( 2 )

where 3 = 27r/Ag, f is the frequency in MHz, and a is the pipe radius

in cm. Both a^and 3 are in units of cm"\

To test this theory, measurements were made using a 2- inch I.D.

copper tube as the waveguide with an excitation frequency of 490 MHz.

Results are shown below. The values of the dielectric constant, K,

have been corrected for temperature, and represent the dielectric

constant at a reference temperature of 25®C.

Solution Dielectric Constant

Tap Water 76.69

10% Methanol and Tap Water 73.80

The value of K for tap water should be 78.57 at 25®C. The deviation is

most likely due to a small uncertainty in the value of a, the tube

radius (5.08 cm was used for calculation). The radius of the waveguide

was not highly uniform. An uncertainty or error in the value of the

diameter of 0.6% (.012 inches) used in the computation of K would

account for the difference between the measured value of the dielectric

constant for tap water and the correct value of 78.57. The tube used

for the waveguide was egg-shaped by at least this amount. For the

methanol -water mixture the decrease in the value of the dielectric

constant reflects the decrease in K due to the displacement of polar

water molecules by the non-polar methanol much as cellulose fibers

would do in the paper stock.

These experiments have demonstrated the feasibility of this

approach in detecting changes in the dielectric properties of the

medium filling the pipe for low conductivity solutions. Additional

experiments were conducted using aqueous solutions of sodium sulfate

with a concentration of 1.3 grams/liter to investigate the performance

of the technique at high conductivities. Again, the strength of the

interference pattern was reduced to the point of non-detectability.

These measurements were performed with the radio frequency field

injected into the waveguide a small distance from the detection slot of

26
a static test cell. Reflection of the field from the test cell ends

served to generate the counter traveling wave which interfered with the

wave emitted by the transmitter. With a more highly conductive fluid

in the waveguide the field strength of the initial and the reflected

waves were more rapidly reduced causing the interference pattern

amplitude in the detection region to decrease.

To increase the strength of the interference pattern a second

transmitting station was placed opposite the first with the detection

region located in-between. This configuration gives two oppositely

directed electromagnetic fields of high intensity traveling through the

detection volume of the waveguide. The resulting interference pattern

developed between the two counter- travel ing initial waves produce an easily detected pattern with a good signal to noise ratio using aqueous

solutions of sodium sulfate exceeding concentrations of 1.3

grams/liter. These measurements have demonstrated the feasibility of

the approach of simultaneous measurement of conductivity and bulk

dielectric constant of the material filling the waveguide. However,-

questions concerning the sensitivity and accuracy of the technique when

applied to paper stock remain. Some measurements were made with a pulp

slurry but gave poor results due to flocculation and inhomogeneities of

the pulp in the static test cell. These effects are expected to be

overcome by flowing stock through the detection region to eliminate

flocculation and stratification, and to time average inhomogeneities on the stock flow.

2.2.4. Discrete Detection System

To achieve operation in a pressurized, flowing environment, a system is required for detection of the electromagnetic field strength

at the waveguide wall at enough positions to allow reconstruction of

the interference pattern with sufficient fidelity to determine the

wavelength, A and the attenuation, a. This requirement must be met

in a way which maintains the pressure integrity of the piping in which

the sensor is installed. The approach taken is to use a set of 31

equally-spaced holes through which wires insulated from the waveguide

pipe wall pass. The tips of the wires are flush with the wall. A RF

diode is used as a non-linear detector at each location. The detected

signals are filtered, amplified, and multiplexed to an analog-to-digital converter interfaced to a microprocessor. The

microprocessor stores the data for several digitization periods and

processes it to obtain values for the dielectric constant and the

conductivity of the material in the waveguide using the model discussed

above.

Development of this approach has begun. The initial detector

electronic components have been designed, constructed, and tested, and

are operational from the detection diodes to the microprocessor. The

sensitivities of the individual detection diodes are not identical.

Therefore, a normalization technique is under development to determine

the relative sensitivities of the diodes.

27
2.3 Steam Flow Meter Project

Steam is used throughout the pulp and papermaking process, from

the digestion of the wood chips to form pulp to the drying of the paper

sheet on the paper machine and the generation of electricity. Most

fully integrated paper mills have power generating plants comparable in

size to those serving small to moderately sized municipalities. For

the most part, metering of steam is accomplished with orifice meters

which have a dynamic range of approximately 3 to 1 when used with a single range pressure transducer. The accuracy of this metering

technique is dependent upon the condition of the orifice plate itself

and the performance of the pressure transducer. The accuracy of

determination of the volumetric flowrate is 1 to 5%. The accuracy of

the mass flow is dependent upon temperature and total pressure

measurement capabilities.

The objective of this project is development of a measurement

technique for gases, to be applied to steam, of improved accuracy

(approximately 1%) in the mass flowrate with a dynamic range of at

least 10 to 1. The approach taken uses a long wavelength acoustic

technique which simultaneously measures the volumetric flowrate and the

gas temperature. These values combined with geometric dimensions of

the meter tube assembly and the total pressure of the gas (steam) give

the mass flowrate. This measurement technique is new. It is

physically non-intrusi ve to the flow and is applicable to gases of all

types and theoretically applicable to liquid flows, although no experimental work has been done on this application of the technique.

The application of the technique to steam flows represents one of the

most difficult applications from the point of view of hardiness of the

acoustic devices (microphones and sound sources) used.

2.3.1 Operational Description

This approach to the measurement of the volume or mass flowrate of

a fluid utilizes long wavelength acoustic waves. The frequency is far

below the cutoff frequency for the pipe diameter used, therefore the wave propagation along the axis of the pipe is essentially that of a plane wave. As with the consistency measurement approach, two

oppositely directed waves are used. The experimental arrangement

consists of a loud speaker, which is the sound source, placed upstream

(or downstream) of two microphones which are mounted in the pipewall

such that they are flush with the inner wall of the pipe. The

microphone farthest from the loud speaker is placed a fixed distance

from the end of the pipe which is open to the atmosphere. When the

sound waves propagating down the pipe from the loud speaker encounter

this open end, a large fraction of the acoustic intensity is reflected

back into the pipe. The result is two oppositely directed acoustic waves in the microphone region of the pipe. These form a standing

wave. For these conditions it can be shown that if the frequency of

28
the sound field generated by the loud speaker is adjusted until

one-half wavelength of the sound field equals the microphone spacing.

The velocity of the two waves and, therefore, the velocity of the gas

flowing in the pipe, can be calculated from values of the frequency,

microphone separation distance, and the pressure of the flowing gas.

The type and molecular weight of the gas must be known also.

Calculation of the gas velocity requires a measurement of the

difference in the phase of the two acoustic waves. The phase

difference is the result of the difference in their respective sound

speeds as observed from the pipe wall. The mathematical expression

describing the standing wave contains terms of the form c-v (upstream

travelling wave) and c+v (downstream traveling wave), where c is the

sound speed in the stationary gas and v is the velocity of the gas.

This sound velocity difference results in a shift in phase between the

two waves. The microphones sense the acoustic field strength and these

electrical signals are decoded by the processing electronics to

determine phase difference. Solution of this expression under the

conditions mentioned above yields a value for the gas velocity, v,

dependent upon the difference in phase between the two waves and the

properties of the gas. The mass and volume flowrates of the gas can be

calculated from the gas velocity. A theoretical model has been

developed which shows that for the acoustic conditions existing in the

pipe, this measurement technique is independent of the velocity profile

of the flowing gas, and that the gas temperature, averaged over the

cross-sectional area of the pipe, can be obtained from the measurement

of the frequency [7]. A complete description of the technique was given in a paper presented at the 1982 Annual Meeting of the Instrument

Society of America [8]. The paper is reproduced here as Appendix A.

2.3.2 Digital Processing Package

This year has seen the development of a digital electronic

processing package for the flowmeter. Initial tests of the device

which demonstrated the feasibility of the concept were performed using

an analog processor. This processor employed several phase-locked loop

circuits to maintain the frequencies of the two sine waves fed to the

loudspeaker, and to detect the signals from the microphones. These

required constant adjustment for proper operation. More importantly

tests of the flowmeter against the NBS secondary gas flow standards

showed a temperature dependent offset. The source of these errors was shown to be the processor. To overcome this temperature dependent

offset, and to obtain a stable processing method for use in continued

developments of the technique, a digital processor was designed and has

been commissioned. This electronic processor uses a 16 bit

microprocessor which has been programmed to control a set of specially

designed measurement and control circuits necessary for measurement of

the phase shift between the two oppositely directed waves in the flow

tube and control of the signal fed to the loudspeaker. The

microprocessor only performs those tasks which are necessary to

maintain the various feedback loops of the flowmeter and provide the

29
necessary information to a second microprocessor for calculation of the

flowrate and gas temperature. Flow tests using the NBS secondary gas

flow standards have shown that the magnitude of the offset in the

flowrate values has been reduced but not eliminated. Additional tests

and improvements of the system will be performed to reduce the offset

to a level below 1%.

Incorporation of this technique into pipelines must be

accomplished for the technique it to be useful for the great majority

of the flows encountered in industrial situations whether these are steam or gas flows. Preliminary experiments have been performed to

test several schemes to generate the reflected acoustic wave traveling

counter to the flow direction. One technique will be selected and

implemented in the coming year.

2.4 In-Solution Lignin Measurement Research

A new area of research was begun in the latter part of this year.

The general objective is to measure the concentration of lignin and

lignin salts which leach into the caustic solutions used in the kraft

digestion process. If such a measurement technology can be developed,

a new era of control of the digestion process is possible. The degree

of removal of lignin from the wood would provide a direct measure of

the progress of the digestion process. Desirable characteristics

of such a measurement are that (1) it be sensitive to lignin and

lignin-salt polymers, (2) it be minimally affected by the high

conductivity solutions used in the digester, and (3) that it be capable

of on-line measurement. The third requirement need not be so stringent, as we have learned from industry sources. Measurement of

the final lignin concentration, in either the spent digestion liquor or

in the cooked pulp immediately after, or as the digester is blown down

to the brown stock holding tank, is a useful initial objective. A

reliable measurement at this point would allow the adjustment of the

charge of white liquor to the next batch of chips placed in the

digester. Such a technique is useful for batch digester operation, but

not so much for continuous digester operation. The full realization of

an on-line measure holds the prospect for automatic control of this

critical process of the pulp and paper industry, both for batch and

continuous digestion.

The initial concept involves a spectroscopic or frequency

dependent technique which would observe a unique property of lignin.

In this way, the effects of the digestion liquor may be avoided. The

initial concept involves the use of the highly polar nature of the

lignin molecule. A technique has been proposed which uses non-linear

coupling between the lignin molecule and an applied electric field.

This coupling mechanism manifests resonant behavior which allows one to

discriminate between the high molecular weight lignin molecules and the

much lower molecular weight inorganic salt molecules. (Sodium sulfide

and sodium hydroxide form bulk of the cooking liquor.) The difference

30
in molecular weight changes the frequencies at which the applied

electric field couples with the dipole moments of the two types of

molecules. Coupling frequencies in the range of 10 to 100 megahertz

are expected for the lignin polymers, while those for inorganic salts

are much higher. Such a large anticipated spread between the resonant

frequencies of the constituents of the liquor is expected to allow the

lignin constituent to be detected with a good signal-to-noise ratio.

Initial tests have shown that the non-linear coupling mechanism exists

and is observable in small molecules.

This project is still very much in the exploratory research stage.

During the coming year, it is expected to progress to the level of a feasibility testing phase.

2.5. Evaluation of O 2 and CO Monitoring Systems for Combustion

Controls

Industrial efforts to improve the efficiency of boilers and

furnaces have lead to the development of a number of automatic control

systems, which regulate the fuel and air flow rates and try to optimize

the combustion and energy transfer processes. Most of the combustion

control systems rely on measurement of the composition of flue gases,

which is then used to adjust the fuel /air mixture in the combustion

zone. In the past, gases have been sampled out of the exhaust stream,

either in a continuous manner or in batches, and analyzed for a number

of species. More recently, in-situ gas analyzers have been developed

to provide a continuous reading of species concentration in the flue

gases with a faster response as compared to gas sampling systems. The

general goal is to keep the fuel /air mixture as close to the

stoichiometric mixture as possible, where the combustion and energy

transfer processes are optimized, without generating excessive amounts

of gaseous and particulate emissions.

The approach generally used for combustion control is to monitor

the level of carbon monoxide (CO) and/or excess oxygen in the flue

gases, which are used as an indication of deviation from

stoichiometric conditions. Optimum combustion performance is usually

obtained with slightly fuel -lean flames, due to non-uniform fuel /air

mixing. The CO analyzers are based on the measurement of infrared

absorption across the stack gases. The concentration of excess O 2

is determined by the electrochemical reaction occurring across a test-tube shaped ceramic probe, made up of yttria stabilized zirconium

oxide (Zr02). The current practice is to locate these monitoring

systems in the boiler stack, far downstream of the combustion zone,

where the gas temperatures are low and chemical reactions are assumed

to have been frozen or completed. At this point the wall and gas

temperatures are low enough, so that any interference due to wall or

gas radiation is not expected, and the lifetime of the instruments is

substantially increased.

31
The objective of this study was to provide data on the accuracy

and sensitivity of the two measurement techniques over a wide range of

operating parameters, and the effect of heat transfer processes on the

relationship between the combustion zone properties and stack gas

measurements. Performance of the two stack gas monitoring systems, a Zr02 type O 2 sensor and an in-si tu CO monitor based on infrared

absorption, has been evaluated in the MBS experimental furnace. These

instruments are typical of in- situ flue gas analyzers that are used

for combustion control in boilers and furnaces.

The experiments were designed to compare the measurements

obtained with the in-si tu CO and O2 analyzers to those obtained with

the gas sampling system and to theoretical equilibrium calculations.

Furnace conditions varied from a hot highly radiating environment

representative of industrial furnaces, to a cooler environment

representative of industrial boilers. Data were recorded over the

range of equivalence ratios 0.75 to 1.05 for both natural gas and No. 2

fuel oil. Interest was centered around the desired region for

automatic controls; viz., 100£C0<300 ppm, 0<p2£2% (excess) and

0.95_<(()£l .0.

The Zr02 type O 2 monitor performed reliably over a long

period of time, and provided quite repeatable measurements with an accuracy of 0.25% O 2 over the full range and a sensitivity of about

2% O 2 per 0.1 equivalence ratio. The instrument, however, did not

have a provision for on-line calibration, and the 0^ measurements

were found to be quite sensitive to the air aspiration rate, which

controlled the gas sampling rate from the stack. The Zr02 sensor

provides the capability for operating at high temperatures, nominally

2800®F and below, which is valuable especially for high temperature

furnace applications. It also means that this sensor could be located

closer to the combustion zone, and provide better time response.

Measurements with the in-si tu CO monitoring system could only be

carried out where the gas temperatures were more moderate, in the

order of 700®F. In general, the readings obtained with the in-situ CO

monitor were less than half of those obtained with the gas sampling

system. This deviation was attributed primarily to the effect of gas

temperature on the CO absorption coefficient. This effect was quantified in a set of experiments carried out in a heated cell.

These findings indicated that any accurate measurement of CO

concentration in the stack gases, using infrared absorption, will

require a simultaneous measurement of gas temperature and compensation

of the CO measurements. Additional effects due to CO2 absorption

may also be important.

Gas composition measurements obtained in the stack, as the

furnace heated up over a long period of time, also displayed a definite effect of furnace temperature and heat transfer processes on the stack gas composition. These results indicate that, for identical

32
combustion zone parameters, different O 2 and CO measurements could

be obtained in the stack; hence, the relationship between combustion

fuel /air mixture and stack gas composition is not unique. It can be

concluded that the application of stack monitoring systems for

combustion control will require a careful study of the characteristics

of the particular installation, before a control system can be

implemented which will provide reliable control over the narrow range

of operation desired for efficient combustion.

These observations also point towards the need for combustion

control systems, which do not necessarily rely on stack gas

measurements, but rather on measurements made directly in the

combustion zone.

Results of this study have been presented at the 1982 Annual

Meeting of the Instrument Society of America [3], and a copy of this

paper has been attached in Appendix B. A more detailed report has also

been prepared on this work, which is in its final stage of preparation

and will be published shortly [4].

2.6 Project Summaries

2.6.1 In-Si tu Spectroscopic Flame Temperature Measurement

A basic research project is in progress with the objective of

developing an in-si tu flame temperature measurement technique, which

could also be utilized for the control of fuel /air mixture ratio in a combustion system. The approach is based on analysis of spectral

characteristics of emitted radiation in the visible and ultraviolet

regions of the spectrum. These regions are selected to minimize the

contribution of blackbody radiation which is especially intense in the

recovery boiler. This project has a laboratory/analytical phase and a field testing phase. In the field experiments data are being obtained

on the spectral features of radiation emitted from operating recovery

boilers. Future experiments are planned on the spectral features of

radiation emitted from operating recovery boilers. Future observations

are planned to utilize high speed optical recording techniques. The

laboratory/analytical phase of the project consists of analytical work,

which is developing information on the temperature response of emission

bands of the diatomic species OH, C2

* and CH, which are formed in the

combustion process itself. A model has been developed which generates

spectral diagrams of emission bands for temperatures of interest.

These are essential in analyzing the data obtained in both laboratory

and field experiments. Laboratory data has been developed on OH and CH

emission spectra from natural gas and oil flames, also data has been

obtained from laboratory burners in which pulverized black liquor

particles have been burned. Spectra have been taken from 300 to 600 nm

in these flames. These efforts are the initial phase of the study

necessary to accumulate a sufficiently broad knowledge base for

development of an in- situ flame temperature measurement and combustion

control technique.

33
2.6.2 Consistency Measurement

A basic research effort is underway with the objective of on-line

measurement of paper pulp consistency. The technique is

electromagnetic in nature and is minimally affected by changes in

process stream flow conditions, i.e., velocity profiles, velocity, or

swirl number. Two approaches have been attempted. Initial activities

focused upon twin conductor transmission lines. This method provided

initial insights into the technique but was severely limited by

attenuation of the detected signal which was reduced below the

threshold of detection by the conductivity of the slurry occupying the

space between the two conductors. Successful operation was not

possible at the levels of conductivity normally encountered in process

streams in the industry. To deal with this problem another approach

was taken which uses circular waveguides. In this case, the

interference pattern of oppositely directed radio frequency waves is

observed. Both the conductivity and the dielectric constant of the

medium filling the waveguide can be calculated from these observations.

High conductivity values are attainable through the use of dual

transmitting points in the waveguide. A prototype device is under

development for use in pressurized piping systems. Current efforts are directed toward development of a technique for diode response

normalization needed to correctly reconstruct interference pattern

characteristics. Once these response characteristics are characterized, the mixing rules for various pulps will be developed

using a small flow loop (2-inch diameter, approximately 10 gallons of

volume). This loop will be used for extensive testing of the 2-inch

prototype and the development of cellulose-water mixing rules. To

demonstrate the capability to scale the device up to sizes similar to

those normally used in an operating pulp and paper mill a six- inch

prototype is planned. It will be tested at the Institute of Paper

Chemi stry

.

2.6.3 Steam Mass Flowmeter Project

A new approach to the metering of gases is being developed which

employs a longwave acoustic technique. This technique allows the flow

of any gas, such as steam, to be measured using a non- intrusive method

which simultaneously measures the mass or volume flowrate and the

average gas temperature. Measurement of these parameters allows one to

calculate the flow of energy in a steam delivery system. The concept

of this unique measurement has been shown to be sound. A new digital

processing technique has been developed and tested. Flow tests of the

flowmeter against the MBS secondary flow standards on room temperature

air flows have shown it to be very linear in its response, but to have

a systematic offset which is temperature dependent. The digital

processor has eliminated a portion of the offset. Continued

developments will be made for closed pipe configuration of the meter

and for steam testing.

34
2.6.4 In-Solution Lignin Measurement Research

A new project has been started this year whose objective is the

development of a method for measurement of lignin concentration in the

liquor solutions found in the pulping process. The need here is a technique which is sensitive to lignin and lignin salts in the presence

of the highly conductive and caustic solutions found in the digestion

process. A new approach has been conceived in which the non-linear

coupling of the electric dipole moments of lignin polymers with a radio

frequency electric field. The project is in the theoretical and

experimental investigation stage to determine the validity of the

physical processes involved and the applicability of the technique for

on-line measurement of in-solution lignin concentration.

35
3.

PLANS FOR NEXT CALENDAR YEAR

The four major projects will continue in the coming year. Two,

the consistency measurement and the steam flowmetering project, are expected to reach the field or mill testing stage. At this point, NBS

will have completed the research and development work on these

measurement technologies. It is hoped that these developments will be

utilized by the pulp and paper industry either directly or through

purchase from instrumentation suppliers who have developed commercially

available products from them. The other two projects will continue in

their development. Completion of the in-si tu flame temperature

measurement technique is expected in calendar year 84. The in-solution

lignin concentration measurement project is sufficiently new that the

coming year is expected to yield progress in developing the basis for

physical understanding of the measurement approach and determination of

its ability to detect lignins in aqueous solutions. Specific

objectives for each of the projects are listed below.

In-Situ Flame Temperature Measurement -

1. Obtain preliminary data from laboratory flames into which

powdered black liquor is injected.

2. Receive the Optical Multichannel Analyzer system and mate it

with the 0.5 meter optical spectrometer.

3. Obtain spectra from an operating black liquor recovery boiler.

4. Present a paper at the 14th Annual Symposium on Instrumentation

for the Pulp and Paper Industry.

5. Complete laboratory experiments using powdered black liquor

particles in gas flames to study the effect of equivalence/

temperature ratio on the emission characteristics of

black-liquor combustion.

6. Publish an interim report on emission algorithm development and

comparison with laboratory and recovery boiler spectral data.

7. Complete conceptual design of prototype sensor.

Steam Flowmeter Project -

1. Complete experiments to determine source of systematic errors

in the flowmeter indication and reduce error contribution as much as is feasible. Test two- inch prototype against NBS gas

flow standards.

2. Design and construct closed-pipe meter tube, and test against

gas flow standards.

36
2. Rent a package boiler and install at NBS for steam testing of

the three- inch prototype of the flowmeter.

3. Construct a three-inch prototype device compatible with mill

pressure code requirements and test on air flows against NBS

secondary gas flow standards.

4. Conduct mill tests.

5. Publish final report.

Consistency Measurement Project -

1. Complete development of diode response normalization

technique.

2. Develop mixing rules for cellulose-water systems.

3. Design and construct six-inch prototype sensor for tests at the

Institute of Paper Chemistry (IPC).

4. Complete tests at IPC.

5. Prepare final report.

In-Solution Lignin Measurement Research -

1. Complete design of linear-component test cell and perform

tests using known organic solutions.

2. Obtain samples of lignins and perform experiments to determine

the sensitivity of the measurement technique.

3. Extend model of the physical mechanism observed.

4. Prepare interim report on the physical model and experimental

results.

37
List of Tables

1.

On-line Measurement Needs

List of Figures

1. Black Liquor Recovery Boiler

2. OH Emissions from a Natural Gas Flame

3. Temperature Dependence of Emission Intensity for the OH, P, R, and Q

Bonds Near 306 nm

4. Spectral Diagram - PI, R1 , and Q1 Bonds in 306 nm Region

5. Predicted Emission Spectra for OH in the 306 nm Region for 2000®C

6 . Black Liquor Emissions in the 306 nm Region

7. Black Liquor Emissions in the 431 nm Region

8 . Black Liquor Emissions in the 470 nm Region

9. Black Liquor Emissions in the 590 nm Region

10. Parallel Conductor Transmission Line Circuit Diagram

11. In-Phase and Quadrature Signals - Parallel Transmission Line

12. Waveguide Interference Pattern in 2-inch Static Test Cell - Tap

Water-Filled

13. Waveguide Interference Pattern in 2-inch Static Test Cell - Aqueous

Solution of 1.3 g/1 of N 2SO4

38
References

[1] Whetstone, J.R., and Johnsen, E.G., "Sensors for Efficient Energy

Utilization in the Paper Industry," to be published as an NBSIR.

[2] Fan, S.S., "Impact on Energy Conservation of Automatic Control

Systems Utilization in the U.S. Pulp and Paper Industry,"

Thermo-electron Report #TE4265-20.

[3] Presser, C., Semerjian, H.G., and Juberts, M., "Evaluation of

O2 and CO Monitoring Systems for Combustion Controls," ISA

Proceedings, October 18-21, 1982, Yol . 37, Part 3.

[4] Presser, C., Semerjian, H.G., and Juberts, M., to be published.

[5] Bently, R.G., and Tasman, J.E., "Consistency Measurement, 1. The

Dielectric Behavior of Pulp Suspensions," Report of the Pulp and

Paper Research Institute of Canada, PPR/82.

[6] Private Communication, R. Bareiss, St. Regis, Incorporated.

[7] Robertson, B., "Effect of Arbitrary Temperature and Flow Profiles

on the Speed of Sound in a Pipe," J. Acoust. Soc. Am. , Vol. 62,

No. 4, October 1977, p. 813.

[8] Potzick, J.E., and Robertson, B., "Longwave Acoustic Flowmeter",

ISA Proceedings, October 18-21, 1982, Yol. 37, Part 1.

39
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ISA

PREPRINT

ADVANCES IN INSTRUMENTATION

Volume 37 Part 1

LONG WAVE ACOUSTIC FLOWMETER

J. E. Potzick

B. Robertson

Chemical Process Metrology Division

Proceedings of the

ISA International Conference and Exhibit

Philadelphia, Pennsylvania

October 18-21, 1982

INSTRUMENT SOCIETY OF AMERICA

67 Alexander Drive

P.O. Box 12277

Research Triangle Park

N.C. 27709

U.S.A.

LONG-WAVE ACOUSTIC FLOWMETER

J. E. Potzick

B. Robertson

Fluid Engineering Division

National Bureau of Standards

Washington, D.C. 20234

ABSTRACT

An acoustic flowmeter has been developed for measuring the flow of an arbitrary single-phase

fluid in a pipe. Sound waves are induced in the fluid at two frequencies, one twice the

other. The phases and amplitudes of the waves are detected by two microphones located in the

wall of the pipe, one downstream from the other a distance of six diameters or more. The

frequencies of the sound are automatically adjusted so that the shorter wavelength is equal

to the distance between microphones. The instrument then measures in real time the volume

flowrate of, and the sound speed in, the fluid, independent of fluid composition or temperature. If the fluid is a gas and its molecular weight and specific heat ratio are given, the

instrument calculates the temperature. Also, given an independent measurement of the pressure in the pipe, the instrument calculates gas density and mass flowrate. The device is nonintrusive, bidirectional, operates in a noisy environment, and responds to rapid changes

in flow and temperature. Because the waves are much longer than the pipe diameter, the measurements are independent of the flow, temperature, or density profiles.

INTRODUCTION

Ultrasonic flowmeters^^ ^ are not entirely successful in measuring gas flowrate in a pipe.

Those based on a Doppler principle will not work unless there are particles to shift the

frequency of the ultrasound. Those measuring the time of flight of a short ultrasound pulse

do not work well in flows containing even a moderate level of noise because producing a jitter-free pulse that is loud enough to be reliably detected over the noise is too difficult. Those using a continuous wave are not entirely satisfactory for a more complicated

reason. Ultrasound in a pipe consists of many high-order spatial waves. These modes superpose at a microphone to form a sinusoidal function of time whose phase depends on the phases

and amplitudes of all the spatial modes. Unavoidable temperature changes in the gas cause

these amplitudes and phases to vary in an extremely complicated and uncontrolled way. The

result is a shift in the phase of the detected sinusoid that depends on the temperature in

an unpredictable way. This phase shift leads to a shift in the flowrate indication that is

indistinguishable from that caused by the flow to be measured. On the other hand, ultrasonic flowmeters are available for measuring liquids flowrates in a pipe, but they all give an indication that is flow-profile dependent.

41
( 2 ) We have developed a long-wave acoustic flowineter' ' that attempts to avoid these problems.

In contrast to ultrasonic flowmeters, this instrument uses sound whose wavelength is much

longer than the cutoff wavelength of the pipe so that only the fundamental or plane-wave

f 31 mode propagates' . This eliminates the strong temperature dependence of the flow indication offset that occurs with ultrasonic flowmeters. It also results in measurements that

are averages over the cross section of the pipe, independent of any nonuniformity in flow,

(41 density, or temperature . No parts of the flowmeter protrude into the fluid being

measured, so the device produces minimal pressure drop.

In what follows, a brief description is given of the flowmeter's principle of operation, its

physical design, and some preliminary test results. An analog prototype flowmeter is described which, however, displays such deficiencies as a zero offset as large as 8 percent

of full scale over a gas temperature range of 100°C, and an offset drift as the circuits warm up. A digital design is then described which should overcome these difficulties,

but at this time has not yet been fully tested. A complete mathematical description of the

(51 operation of the flowmeter will be presented in another paper' .

PRINCIPLE OF OPERATION

A schematic diagram of the acoustic flowmeter is shown in Fig. 1. Two microphones, small

compared to the diameter of the tube, are placed at the top of the meter tube and spaced

longitudinally six diameters apart from each other. The microphones are mounted in the wall

with their diaphrams flush with the inside surface of the tube. A loudspeaker is connected

to a branch from the tube at least two diameters upstream or downstream from both microphones. The sound generated by the loudspeaker has a wavelength that is much longer than

the diameter of the tube, and so only the fundamental spatial mode or plane-wave mode

propagates. Higher order spatial modes decay exponentially with a decay distance that is shorter than 0.275 times the pipe diameter. Thus, at the microphones the sound is a plane wave with the pressure essentially constant across the cross section of the pipe, and the

(3) microphones detect that pressure' . The sound undergoes multiple reflections by elbows,

tees, valves, and open ends, both upstream and downstream of the microphones and loudspeaker. The reflected acoustic waves combine to form two sine waves at the microphones,

one wave travelling upstream and one downstream, with velocities c-v and c+v, respectively,

where v is the velocity of the fluid and c is the sound speed in the fluid when it is stationary. The amplitudes of these waves will not in general be equal. If the loudspeaker

is upstream of the microphones, the downstream wave in the vicinity of the microphones is the superposition of waves direct from the loudspeaker and from all upstream reflectors. It

is incident on the downstream reflectors, and the reflected waves superpose to form the

upstream wave in the vicinity of the microphones.

The superposition of the upstream and downstream travelling plane waves forms a standing

wave whose amplitude does not quite vanish at the nodes (or minima) as shown in Fig. 1. Care must be taken to avoid placing the microphones near nodes where the acoustic amplitude

is too small to give an adequate signal-to-noise ratio. This may involve some testing of

microphone locations to ensure adequate amplitudes in an existing closed piping system

before a suitable location is chosen.

42
Because the signal detected by each microphone is a superposition of sine functions of the same frequency, it will be sinusoidal in time. The difference in the phases of the sinusoids detected by the two microphones is in general a complicated function of the sound

wavelength, the piping geometry, and the average Mach number M = v/c. We limit the following discussion to a Mach number less than 0.1. Then if the distance between microphones is an integer multiple of one half the wavelength, the phase difference becomes considerably

simpler^^^. The wavelength will satisfy this condition, = 2L/n, when the frequency f^ of the wave satisfies

f„ = nc/2L. (1)

Here L is the microphone spacing and n is a whole number. Since a long wavelength is desired,

n is generally 1 or 2, so the microphones are one-half wavelength or one wavelength apart.

When Eq. (1) is satisfied, the phase difference can be shown to be

= nitM, (2)

independent of the relative amplitude of the waves. This phase difference arises because

the flowing fluid drags the sound downstream, increasing the sound speed in the downstream

direction and decreasing it in the upstream direction. When the flow changes direction,

the phase difference changes sign. The relation between phase difference and Mach number

turns out to be exact for any Mach number less than 1 and is valid for all single-phase

fluids of low acoustic attenuation.

The amplitudes of the sinusoids at each microphone are used to set the operating frequency to achieve the above results as follows. When the distance between microphones is an integer

multiple of one half the wavelength, the amplitude at that frequency will be the same at the

two microphones. This can be seen in Fig. 1. Thus the ratio of amplitudes at the two microphones will be one. This ratio is unity only at the operating frequency and at other

specific frequencies that are far enough away from the operating frequency that no confusion

results when using this ratio to determine the operating frequency. The difference between

this ratio and unity can be used with integral feedback to control the frequency. The time

integral of this difference can be used to control the frequency of an oscillator which

generates a frequency equal to the correct operating frequency.

Once the frequency is set, the speed of sound can be determined by measuring the frequency

and using the known distance between microphones. The average velocity of the fluid is then

the average Mach number times the speed of sound. The total volume flowrate is this

velocity times the cross sectional area. These quantities are directly measurable without

knowledge of the type of fluid in the pipe, provided the fluid has low sound attenuation.

These results are summarized in Fig. 2.

If the fluid is a real gas its equation of state can be written

mP = PRZT, (3)

where m is the molecular weight of the gas, P is the absolute pressure in the pipe, p is the

43
gas density, R is the molar gas constant, Z(P,T) is the compressibility factor for the gas^^\

and T is the average temperature of the gas. The compressibility factor Z, which is 1 for an ideal gas, becomes quite significant at pressures and temperatures near the critical point.

The relationship between the gas temperature and the sound speed can now be determined^^^

and the temperature, density, and mass flowrate can be calculated from phase and frequency

measurements, as shown in Fig. 2, where it has been assumed that Z = 1. This assumption

leads to a -0.04% error for dry air at 0.05 atm and 33°C, a 0.03% error at 1 atm and 25°C,

and a -1.64% error at 40 atm and -3°C^®^.

The total mass flowrate can be obtained by multiplying the total volume flowrate by the gas density. The result is

G = yPAAt/L (4)

where G is the mass flowrate, y is the specific heat ratio, P is the absolute pressure, A is the cross-sectional area. At = 4((i^/(2Tif^) is the time difference between sinewaves, and L is the distance between microphones.

This formula is correct to within 1 percent for air at atmospheric pressure flowing with a Mach number less than 0.1. There will be other sources of error which have not yet been

identified or quantified.

The response time of the acoustic flowmeter to changes in flowrate can be on the order of

the period of the sound frequency used. In practice, however, many periods usually are averaged to smooth the effects of flow fluctuation and turbulence, but the response may still

be considered to be real-time for many applications.

In addition, since the phase measurement is bidirectional, the flow indication is also

bidirectional, with no preferred direction. If the flow changes direction, the flowmeter

response will pass linearly and smoothly through zero. The uncertainty as a percent of

reading may increase, though, at very low flowrates. The magnitude of the zero offset has yet to be measured.

The flowmeter is easily scaled to different pipe diameters. The only requirement is that the

microphone spacing must be maintained at least as large as six diameters. For small pipes,

the spacing need not be as short as six diameters. For large pipes, the spacing must

increase with the pipe diameter. Increasing this spacing decreases the operating frequency.

This may require changing the loudspeaker so that it is better matched for the new frequency.

PHYSICAL DESIGN

A prototype acoustic flowmeter has been fabricated for use in air at atmospheric pressure and

temperatures from about 0° to more than 100°C. It uses a 5.08 cm diameter brass meter tube

about 70 cm long, as shown in Fig. 3. The metal wall of the meter tube must not transmit

sound directly to the microphones at the operating frequency. So a rubber vibration isolation coupling is used between the loudspeaker section and the meter tube, and the microphones

are mounted elastically as shown in Fig. 3. One way of helping to minimize the generation of

44
bending modes of the pipe may be to use two loudspeakers mounted symmetrically on opposite

sides of the pipe. However, even with this symmetric mount, elbows or tees elsewhere would

generate bending modes. Thus other efforts may be necessary to discourage sound propagation

in the pipe walls, especially the bending modes, but also modes that distort the pipe cross

section. A meter tube of a material with high intrinsic damping, such as polyvinyl chloride

or a fiber composite may help.

The microphones used are 1/4- inch quartz dynamic pressure transducers with a resonant frequency of 250 kHz. Wide bandwidth is necessary in order to reduce the phase shifts

(9l of the microphones to negligible values' , If the sensitivities of the two microphones

are not exactly equal, the voltage amplitude ratio used to determine the operating frequency

will not equal the acoustic amplitude ratio. As a result the feedback will not set the

frequency generated by the loudspeaker equal to the correct operating frequency. To avoid

this error, the flowmeter uses two frequencies, one twice the other, at f and 2f. The correct frequency is obtained only when the amplitude ratio at one frequency equals the

ratio at the other, even though these ratios may not equal 1. The difference in these

ratios is taken as the frequency error signal, and the time integral of this quantity

is the frequency input to the oscillator that generates the required sine waves.

For the 30.5-cm spacing between microphones, the lower frequency is about 560 Hz at room

temperature. This frequency is clearly audible, and can be heard to increase as the

temperature of the gas increases. This meter may be inappropriate in applications where

a low ambient sound level is required.

Phase measurements can be made for either frequency. However, in the following, values at the lower of the two frequencies are stated.

We choose full-scale flowrate to correspond to the Mach number equaling 0.1. The most

recent prototype can measure flows nearly five times as fast, but the smaller number gives

the range over which the linear formula in Eq. (4) is correct within 1 percent. The sensitivity of the measurement of the velocity v with respect to the measurement of the phase

difference A41 is Acti-j/v = n/c. This is independent of the flowmeter dimensions. The phase

difference at full scale is easily seen to be 0.1 tt or 18 degrees. The full scale value for

the time difference At is the above divided by 2Trf, or 0.1/(2f^), which is 88 microseconds

at 25°C. The full scale value for the volume flowrate at 25®C in a 5-cm diameter pipe is 70.2 liters/second. The full scale value for the mass flowrate at atmospheric pressure In a 5-cm diameter pipe is 83 grams/second (g/s). Pertinent data for the prototype flowmeter is

summarized in Fig. 2.

If we require 1 percent resolution of a reading of 10 percent of full scale, then we must be

able to measure At with 88 ns resolution. The acoustic noise that is always present in

flowing fluids complicates this task. In addition, the microphones, their amplifiers, and

any following electronic circuits must not introduce phase uncertainty which leads, to a time

delay greater than 88 ns, even as the operating frequency changes. Hence the need for

wideband microphones and electronics mentioned earlier.

45
An early version of the acoustic flowmeter used mostly analog circuits to process the

microphone signals. Voltage-controlled phase shifters and synchronous detectors were used

in a feedback arrangement to measure the phases and amplitudes of the microphone signals

at each frequency The ratio difference used for the frequency error signal was obtained

with analog dividers and a differential integrator. The time- integrated frequency error then

set a voltage-controlled oscillator to 2f. This frequency was divided by two, and the two frequencies were shaped into sine waves, added, and amplified to drive the loudspeaker. The

time difference at was determined from the phase measurements of the microphone signals at frequency f and used with a measurement of the frequency f to calculate the temperature and mass flowrate. A more complete description is given in Ref. (2).

This analog version was tested on the air flow calibration facilities at the National Bureau

of Standards (NBS), and the results are shown in Fig. 3. Here the indicated mass flowrate

calculated from Eq. (4) using the parameter values for air at atmospheric pressure. The

straight line is the result of a least squares fit to the data below full scale, which is Mach 0.1. The slope of this line is within 1 percent of the expected value of 1, but

there is an offset of 0.25 g/s, which is about 0.3 percent of full scale. The two data

points above Mach 0.1 fall below the line. This is most likely due to a temperature dependence of the offset, since the temperature of the air flow decreased from room temperature at lower frequencies to 0“C at the highest flowrate.

The analog flowmeter has also been tested at temperatures up to 100®C. Over this temperature

range, an offset as large as 8 percent of full scale has been observed. In addition, the

analog flowmeter had a flow- indication offset which varied in time as the electronic circuitry

warmed up.

In order to overcome the latter offset and to make changing design parameters easier, a new

digital version of the acoustic flowmeter has been constructed, but not yet tested or debugged. A Fourier coefficient technique is used to measure the amplitudes and phases at the microphones. This technique provides the required resolution in the presence of wideband and harmonic noise without the use of narrow band filters. The latter could introduce

a phase shift that is indistinguishable from the phase shift to be measured.

Processing of the microphone signals is done in three stages in this digital prototype as shown in Fig. 5. The stages in order are a hard-wired parallel processor, a serial processor

or microcomputer progratrmed in assembly language, and a small desktop computer programmed in

a high level language. The earlier stages provide greater speed of computation, and the

later stages provide easier design and programming with the possibility of changing parameters

to fit different conditions.

In the first stage, the parallel processor computes Fourier sine and cosine coefficients at each of two frequencies for each microphone, eight poefficients in all, and sends them to the serial processor at the end of each acoustical period. A block diagram of the parallel

processor is shown in Fig. 6. The microphone signals are amplified and digitized with 8

bits resolution, 256 times each acoustical period. These digital signals are then applied

to one input of each of four multiplier-accimulators operating in parallel for each microphone. A master clock, whose frequency is set by the serial processor to 256 f, toggles an

46
8-bit counter whose output cycles sequentially through 256 addresses each 8 bits long. This

output is connected to the address inputs of four read-only memories so that they are addressed

simultaneously. These four memories have been programmed with the functions sin f, sin 2f, cos f, and cos 2f, respectively. Since the memories are always enabled, they output these

functions in a periodic manner. These data outputs then go to the other inputs of the

multiplier-accumulators as shown. Two hundred fifty-six products are then accumulated on each of the multiplier-accumulators during the acoustic period, thus forming the eight Fourier

coefficients. This integration provides some of the discrimination against nonsynchronous

noise. At the end of the acoustic period, the coefficients are loaded into registers that can be read by the serial processor, and an interrupt it generated. The outputs from the

sin f and sin 2f memories are also converted to analog form, mixed, and amplified by a 70-watt audio amplifier to drive the loudspeaker.

On interrupt, the microcomputer, which is an AmZ8002 evaluation board, fetches the eight

Fourier coefficients. It uses them to calculate the amplitudes of the signals detected by

the microphones and adjust the amplitudes of the signals sent to the loudspeaker so that the

detected amplitudes remain within the correct range. The gain of the microphone amplifiers

can be set to obtain any desired signal-to-noise ratio within the power limitations of the

loudspeaker and its power amplifier. The microcomputer then calculates the difference of

amplitude ratios to get the frequency error, integrates it, and uses the result to increment

or decrement the operating frequency f as necessary. When the amplitude and frequency errors

are sufficiently small, the microcomputer uses trigonometric identities and a look-up table

to calculate the phase difference between the microphone signals. It averages this phase

difference over a user selectable number of terms. This provides further noise discrimination.

When the average is complete, the microcomputer sends the frequency f and the average phase

difference through a serial port to the desktop computer.

In the third stage, the desktop computer calculates and displays the gas temperature, the

volume flowrate, and the mass flowrate in engineering units. This computer can also send

new parameters to the microcomputer to change such things as ranges and number of terms in

averages.

CONCLUSION

An acoustic flowmeter has been designed which has many advantages over conventional flowmeters. Such a meter has been built with analog circuits and tested and has demonstrated

the soundness of the principle, although some deficiencies in the implementation were

revealed. A second flowmeter is being constructed with digital circuits. We expect this

digital version of the acoustic flowmeter to perform better than the earlier analog version.

Testing to quantify its performance is planned.

REFERENCES

1. Lynnworth, L. C., "Ultrasonic flowmeters," published in "Physical Acoustics," Vol. 14 (Academic Press, 1979), pp. 407-525.

47
2.

U.S. Patent Application SN 300,830; foreign patent applications being filed.

3. Morse, Philip M. , and Ingard, Uno, "Sound Waves in Ducts and Rooms," Theoretical

Acoustics , McGraw-Hill Book Co., New York, N.Y., 1968, p. 467-607.

4. Robertson, B. "Effect of arbitrary temperature and flow profiles on the speed of sound

in a pipe," J. Acoust. Soc. Am. , Vol. 62, No. 4, October 1977, p. 813.

5. Robertson, B. , and Potzick, 0., "A long-wave acoustical technique for measuring gas flow in a tube," in preparation.

6. Streeter, Victor L., "Fluid Properties," Handbook in Fluid Dynamics , McGraw-Hill Book

Co., Inc., New York, N.Y., 1961.

7. Ref. 3, "Acoustic Wave Motion," pp. 227-305.

8. Wexler, Arnold, and Hyland, Richard W., "Formulations for the Thermodynamic Properties

of Dry Air from 173.15 to 473. 15K, and of Saturated Moist Air from 173.15 to 372. 15K,

at Pressures to 5 MPa," May 1980, as yet unpublished.

9. Potzick, J., and Robertson, B. , "Synchronous marker for measuring phase in the presence

of noise," Rev. Sci. Instrum. , Vol. 48, 1977, p. 1290-1294.

10. Potzick, J., and Robertson, B., "Voltage-controlled phase shifter for measuring transfer

function in the presence of noise," Rev. Sci. Instrum. , Vol. 52, 1981. p. 280.

Acknowledgement

This work is supported by the Office of Industrial Programs, Conservation & Renewable

Energy, U. S. Dept-of-Energy, Mr. Stanley F. Sobczynski, Program Manager, under Interagency

Agreement #EA-77-A-01-6010.

48
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The line is fit of all data below Mach 0.1

52
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ISA

PREPRINT

C.l. 82 — 920

EVALUATION OF 0-2 AND CO MONITORING SYSTEMS

FOR COMBUSTION CONTROLS

C. Preiser, H. G. Semerjian, M. Juberts

International Conference and Exhibit

Philadelphia, Pennsylvania

October 18-21, 1982

INSTRUMENT SOCIETY OF AMERICA

67 Alexander Drive

P.O. Box 12277

Research Triangle Park

N.C. 27709

U.S.A.

55
EVALUATION OF 0^ AND CO MONITORING SYSTEMS

FOR COMBUSTION CONTROLS

C. Presser, H.G. Semerjlan and M. Juberts

Thermal Processes Division

Center for Chemical Engineering

National Engineering Laboratory National Bureau of Standards

Washington, D.C. 20234

ABSTRACT

A study has been carried out to provide an evaluation of CO and O2 monitors used for combustion controls, and to provide reliable data on their performance, operating range and accuracy. The experiments have been carried out in the NBS experimental furnace, which is equipped with a complete on-line gas analysis system used as a reference system. Specific

areas of interest included evaluation of the accuracy and sensitivity of the measurement

techniques over a wide range of operating conditions, correlation of flame stoichiometry

with stack measurements, and the effect of stack gas temperaturq on composition measurements. The O2 stack gas monitor has a Zr02 type sensor. The system was found

to be very sensitive to air leakage when the furnace pressure was kept below atmospheric,

A calibration system was also found to be necessary, especially for aspirating type probes. The CO monitoring system is based on infrared absorption across the stack. The CO measurements were found to be quite sensitive to stack gas temperatures, and this effect

has been quantified in a heated test cell. The relationship between the stack gas CO

levels and combustion zone stoichiometry was also found to be sensitive to the energy extraction rate in the furnace cavity. Experimental results are presented for both natural

gas and No. 2 oil fired systems.

I. INTRODUCTION

Industrial efforts to improve the efficiency of boilers and furnaces have lead to the development of a number of automatic control systems, which regulate the fuel and air flow

rates and try to optimize combustion and energy transfer processes [1]. The approach used

for combustion control is to monitor the level of CO and/or excess O2 in Che flue gases.

For typical boiler configurations, automatic control is desirable over the concentration

range of 100-300 ppmv CO and 0-3Z excess O2 [2-5]. In-situ CO monitors are based on the measurement of infrared absorption across the stack gases [6]. Excess O2 is determined

by the electrochemical reaction occurring across a test-tube shaped zirconium oxide

(Zr02) probe (7-9).

The installation of these analyzers in the stack may present problems since stack conditions may not be representative of conditions inside the combustion chamber. Air infiltration can significantly influence the O2 concentration, and to a lesser degree the CO level [10]. The relative advantages of using CO and O2 monitoring systems, or both,

for combustion control have been debated in the literature [11-14]. Several studies have

identified automatic on-line control of combustion processes as a generic technology, which

can provide energy savings and an improvement in process efficiency for a wide variety of industries [15-19]

This study evaluates an in-situ CO and an O2 monitor, in the NBS experimental furnace, which is equipped with a complete on-line gas analysis system. Specific areas of interest were evaluation of the accuracy and sensitivity of the measurement techniques over a wide

range of operating conditions; effect of higher inlet temperatures (for installations with

heat recovery systems) on the stack gas composition; effect of furnace cavity (or stack) pressure level on the CO and O2 measurements; and the effect of variations in the stack

gas temperature.

56
II. DESCRIPTION OF THE EXPERIMENTAL SETUP

a) Experimental Furnace

The experimental furnace (Figure 1) has a combustion chamber, with the internal dimensions

of 80 cm (W) X 110 cm (H) x 240 cm (L) . A single burner fires horizontally through one

end into the combustion chamber. The combustion products exhaust from the opposite end of

the chamber, past a recuperator, and then up through the stack. The furnace has a rated capacity of 300 kW (1x10° BTU/hr). Several ports are located along both furnace side walls and the downstream end wall (Figure 1). These ports permit visual observation of

the flame, the introduction of intrusive probes and provides an optical path across the furnace. The burner [20] is designed to recirculate and entrain combustion products with

incoming fresh combustion air, preheated up to 900*C, thereby enhancing flame stability. Either natural gas or No. 2 fuel oil can be supplied to the burner. Atomizing compressed

air is mixed with the liquid fuel, externally, imned lately downstream of the nozzle head. Cooling air is passed around the fuel injector in order to reduce thermal decomposition of

the fuel. A refractory stabilizer plate provides a separation between the preheated combustion air and the fuel supply line. The recuperator is designed to preheat incoming combustion air to 1550*C before reaching the burner. A high temperature damper located

inside the stack is used to maintain positive furnace pressure during experiments. A 4 kW

(5Hp) turbo blower supplies combustion air to the burner, having a rated maximum capacity

of 23,100 1/min (49,100 ft^/hr). A variable thermal load heat exchanger consisting of two water-cooled 25.4 cm (10 in) diameter stainless steel tubes is used to remove 30-S0Z

of the furnace thermal energy.

b) Gas Sampling and Analysis System

A complete on-line gas analysis system is used for combustion gas sampling and analysis

(Figure 2). A water-cooled sampling probe supplies sample gas to a heated bellows vacuum

pump through a 460 cm (15 ft) section of heated line. The gas sample is then delivered

through a 1070 cm (35 ft) section of heated line to the analyzer manifold. The heated

lines and the pump are preheated to 150*C. Gas sample is delivered to the analyzers,

entering first a FID type total hydrocarbon analyzer which provides a wet reading. The manifold then leads into a gas dryer which removes water vapor in the sample by permeation

distillation. The resulting dry gas is then introduced into the following detectors; Beckman* CO, CO2 and O2 analyzers, and TECO NO/NOj^ and SO2 analyzers. Unused

sample gas is bypassed into the sample dump manifold. All analyzers are checked and calibrated prior to start of and during a test sequence.

c) Data Acquisition System (DAS)

A Digital Equipment Corp. (DEC) PDF 11/60 minicomputer based data acquisition and control system is used to monitor and record the many instrument variables involved in operating

the furnace. Since the computer facility is located 610 m (2000 ft) away from the furnace, a Remote Industrial Control (ICR) I/O subsystem is used to interface the process

instrumentation and control signals to the minicomputer. The following list presents

those furnace input parameters which are converted and entered into the computer mass

storage: coolant water flow, air flow, natural gas and oil flows and stack emissions by

the gas sampling system (CO ,C02 ,HC,N0,02 ,S02 ,N0/N0^) . A Fluke data logger is used

to process and record the various furnace temperatures, being compatible with T,K,R and S type thermocouples. The data logger has a data acquisition rate of approximately 13-15 channels/sec. A Kennedy 75 ips , 9 track, 800 or 1600 bpi magnetic tape transport system

is used for storing raw and processed data on tape. A Versatec high speed printer/plotter

is used for hard copy listings and for plotting.

d) In-situ Oxygen Analyzer

An in-situ O2 monitor with an yttria stabilized zirconia tube probe (Figure 3) was

located at the far end wall of the furnace. The probe consists of zirconium oxide

(Zr02) high temperature solid electrolyte ceramic material shaped like a test tube. Two

porous platinum electrodes coat the inside and outside surfaces of the zirconia substrate

and conduct electricity by means of oxygen ion transport across the cell. The sensor temperature is set at 700*C in order to ensure good ion conduction (voltage output) across

the cell. The high temperature also serves to keep the flue gas hot and to burn away any

*Certain commercial equipment, instruments or materials are identified in this report in order to adequately specify the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards,

nor does it imply that the material or equipment is necessarily the best available for the purpose. 57
combustible material. The sensor probe uses an aspirating extraction technique to transport the sample gas from the probe inlet to the sensing cell. The monitor is calibrated by matching the O2 concentration with atmospheric air (i.e., 20. 9Z O2

).

e) In-situ Carbon Monoxide Analyzer

The in-situ CO monitor (Figure 4) is an infrared device that uses a high resolution

absorption spectroscopy technique to measure CO in the range of 0-1000 ppmv. An IR light source emitting a wide beam of polychromatic light is chopped at a frequency of 63 Hz and collimated before being transmitted through a sapphire window interface and across the furnace stack to a duplicate sapphire window and collecting mirror on the detector side. Blowers protect the sapphire windows from combustion products, soot and high stack temperatures. The detector cell collecting mirror directs the attenuated beam into a wavelength discrimination module which employs a gas cell correlation technique. A narrow

band filter (at 4.7pm) located in front of an electronically cooled lead-selenide detector

allows only the CO sensitive portion of the infrared spectrum that is not absorbed by the stack emissions to pass. Each signal is amplified and transmitted via a signal cable to the control room module. The instrument calibration is determined by comparing the monitored CO level to a preset check value of 780 ppmv CO which is based on a 91 cm (3 ft) path length. When the signal voltage drops below IV the opacity in the stack is 80Z or greater and insufficient light is being transmitted across the stack.

III. RESULTS

The experiments were designed so that the concentrations recorded by the O2 and CO in-situ monitors could be compared with those measured with the gas sampling system. The

gas sampling/analysis system has a much slower response and is inadequate for control

purposes; however, it provides accurate and reliable measurements. Experiments were conducted with both natural gas and No. 2 fuel oil. The fuel/air equivalence ratio <t> was

varied from about 0.75 to 1.05, the region around i))”1.0 being the main area of interest.

a) Measurements Below the Recuperator

For the first set of experiments, both the O2 and CO in-situ monitoring systems were

located in the region below the recuperator. The in-situ CO monitor was located at the same height with the O2 probe. The stack gas sampling probe was inserted into the stack

through a port which enabled gases to be sampled from the area iniiiediately above the region where the infrared beam traverses the stack cavity. The furnace pressure was kept

positive, thereby preventing air infiltration into the furnace. The furnace was preheated

for 3-4 hours (to about 900*C) before data acquisition was initiated. Included are results obtained with the gas sampling/analysis system, with the in-situ O2 monitor, and from equilibrium calculations based on stack temperature measurements. It should be pointed out that the CO2 , O2 and CO measurements obtained from the gas analysis system

are "dry" measurements. These readings are then corrected using a flue gas concentration

carbon balance and are then reported as the "wet" values. These are, of course, the appropriate values for comparison with the in-situ measurements and the equilibrium

calculat ions

.

Typical results for No. 2 fuel oil are shown in Figure 5. The general trends show that as the fuel/air mixture becomes richer (i.e. as $ increases) and approaches a stoichiometric

mixture, the maximum theoretical value of CO2 concentration (Figure 5A) is attained

(9.5Z for natural gas and 15. 3Z for No. 2 oil) at approximately i^l , and then starts decreasing beyond that point due to lack of available oxygen. The excess O2 (Figure 5B) falls steadily, with increasing 41 , until the stoichiometric mixture is reached, beyond which point no more oxygen is detected. The CO concentration (Figure 50 remains below 20 ppmv for fuel lean mixtures, and rapidly rises beyond 4^1. The equilibrium calculations

are included in each case, and the general trends agree with the measurements.

Experimental data obtained in the furnace over the range 0.95<4i<1.04 is expanded and shown

in Figure 6 for No. 2 oil. Upon closer examination of Figure 6 it can be seen that the CO

and excess O2 curves intersect outside the limits proposed by Gilbert [5] (i.e., less than 100 ppmv CO and 0.12 O2

). These results tend to indicate that, under certain

circumstances, the relationship between the conditions existing in the combustion zone and those monitored in the stack could be altered drastically. They also suggest that the range of the control parameters (i.e., CO and O2 concentration) to be utilized may strongly depend on the particular combustion system configuration and heat extraction

profile.

The O2 measurements obtained with the Zr02 probe are also shorn in Figure 5B. For the

fuel lean flames, these readings are observed to deviate from the gas sampling measurements by more than 0.52 O2

. Part of this discrepancy is attributed to a lack of

a calibration procedure, except at ambient conditions. It was also realized that the

58
readings could be affected by varying the aspiration flow rate. A second set of tneasurem:nts made after calibration of the Oj monitor are also shown in Figure SB (solid

symbols). The difference between the two sets of measurements is indicative of the uncertainty present in the oxygen measurements using this particular Zr02 sensor.

Measurements with the in-situ CO monitoring system were also attempted at this location below the recuperator, where the gas temperatures are at or above S00*C. This was considered to be representative of typical industrial furnace-type applications. The nominal optical path across the stack was 91 cm (3 ft). This corresponded to the inner wall dimensions, since the cooling air for the sapphire windows also purged the connecting

flanges between the external (source or analyzer) units and the furnace Inner wall. The blower air flow was assumed to be small enough not to penetrate the central part of the stack, and simply flow up along the walls with a minimum amount of influence on the CO readings. This indeed may be the case for industrial applications, where the in-situ CO monitoring system is usually placed on a boiler stack with a diameter several times that of the experimental furnace, with correspondingly larger flow rates. Unfortunately, because of the smaller size of the NBS furnace, the cooling air was found to penetrate into the stack gases, effecting not only the CO readings, but also the measurements obtained with the Zr02 probe and the gas sampling probe. Both of these probes were located near the center of the stack, indicating substantial air penetration.

Attempts were made to minimize the cooling air; however, the readings obtained from the in-situ CO monitor were still far lower than those obtained from the gas analysis system.

In addition, it was also observed that radiation emitted from the hot furnace walls,

especially after several hours of heating, effected the detector signal. This radiant

energy, however, is emitted as a continuum and effects only the d.c. level. Therefore,

the frequency discrimination capability of the analyzer should compensate for this effect

.

Based on these observations, it was concluded that our inability to obtain reliable data

at this location was primarily due to the higher stack gas temperatures, which resulted in

a reduced absorption coefficient, and hence reduced the indicated CO levels. It was decided to relocate the in-situ CO monitoring system in the stack above the recuperator,

where the gas temperatures are substantially lower.

b) Measurements Above the Recuperator

The in-situ CO analyzer was fixed to the tapering portion of the stack above the recuperator (Figure 1). The gas sampling probe drew effluent at a location above the infrared beam so that one could detect dilution effects by the CO monitor cooling blowers. The initial tests produced a large increase in the excess O2. After restricting the blower output with several pieces of shim stock, it was found that when the opening was

too small the monitor output signal voltage would drop until the beam opacity reached beyond 80Z. Insufficient air cooling allowed flue gases and soot particles to enter the instrument connecting flanges. The increased path length permitted the CO level to grow rapidly until the analyzer could not continue to operate. The best results obtained are shown in Figure 7, producing a O.SZ increase in the stack excess O2 and a reduction in the detector electronic signal from 2V to 1-1. 5V. It was impossible to produce a full 2V signal without increasing significantly the stack excess O2 . The inability to obtain a strong detector signal without excess O2 dilution may indicate that the cooling air does not pass through the connecting flanges uniformly, and permits effluents to move into the flanges at the same time. The air may pass into the flanges tangentially and develop a swirling motion with a central recirculating zone which draws flue gases toward the sapphire windows.

The results indicate that the CO2 peak (Figure 7A) is shifted to slightly larger than ^~1 because of the low furnace temperature (data was recorded at 871*0. The maximum

value attained is lower than the theoretical maximum due to the 0.5Z excess O2. The CO level (Figure 7C) will consequently rise when the flame is slightly richer than expected

and the excess O2 (Figure 7B) reaches its minimum past the stoichiometric point.

Comparison with the in-situ CO monitor in Figure 7C shows that past stoichiometric

conditions both CO readings rise simultaneously, but the in-situ analyzer readings

generally remain at less than half the gas sampling value (orginally, without the restriction, this ratio was 1:3-4). For automatic control purposes the in-situ instrument

could be used as an indicator of the presence of CO, but not necessarily to provide an accurate measurement of the CO concentration. Good agreement between in-situ O2 monitor

and the gas sampling data (Figure 7B) is primarily due to the modified setting of the air aspiration rate.

For stack temperatures below 540*C along with heat removal by the furnace heat exchangers

and the recuperator, conditions in the stack may be considered similar to that of a boiler except for the effect of the high temperature flame and furnace wall radiation on

59
completion of combustion processes. In order to simulate boiler conditions where wall incident energy is negligible data were recorded for lean flames under quasi-steady

conditions, Figure 8, during the furnace transient start-up. The results presented in Figure 8 are for 4>~0.5 with gas sampling in the stack. The length of each test was approximately 30 minutes which depended upon keeping the stack temperature between

150-190*0. The flame gas temperature rose to 685*0 during this trial. Figure 80 shows

that there is an initial surge of 00 due to poor fuel/air mixing and the cold furnace

walls, and then the 00 level peaks after several minutes, dropping exponentially as the stack gas temperature increaaes. During the first 100 seconds the OO2 concentration

(Figure 8A) increases and the O2 concentration (Figure 8B) decreases rapidly. Afterwards the OO2 level gradually increases and the excess O2 drops slowly as the 00

level decreases. The 00 concentrations recorded from the in-situ device are also shown in Figure 80. It is again observed that the in-situ 00 monitor provides readings which are less than half the value obtained with the gas analysis system, throughout its operating

range

.

c) In-situ 00 Monitor: Investigation of the Effect of Stack Gas Temperatures

It was pointed out earlier that the relatively low 00 concentrations measured in the stack

gases, with the in-situ 00 monitor, were primarily attributed to the higher temperatures encountered in the stack. In order to clearly identify and quantify this effect, an absorption cell was fabricated, where the gas temperature, pressure and composition could

be carefully controlled, and 00 measurements could be made using the 00 monitoring

system.

The cell was 91.5 cm (3 ft.) long and was encased by high vacuum flanges. Two sapphire

windows, located on the end flanges, allowed optical access into the cell. The sapphire windows were identical to the ones used in the 00 monitoring system, so that the spectral response of the system would not be altered. The source and analyzer units of the 00 monitoring system were then mounted on the two sides of the cell. A calibration gas mixture of 779 ppm OO/N2 was introduced into the cell from one end, and the gases exiting from the other end were fed into the gas sampling/analysis system, where the 00 concentration was continuously monitored with the nondispersive (NDIR) 00 analyzer.

The cell was then gradually heated, and the results obtained with both the in-situ monitor

and the NDIR analyzer are shown in Figure 9. It can be seen that, while the reading from

the NDIR analyzer stayed constant, the measurements obtained with the in-situ monitor

decreased by more than two fold as temperature increased from 20*0 to 260*0. The results obtained during these experiments document the sensitivity of the in-situ monitor

measurements of 00 concentration on the stack gas temperature. This dependence on temperature is due to the variation of the absorption coefficient K • N'Q, which then affects the infrared light transmission through Beer's Law:

T - I/Iq - exp ( - K-L)

where T is the transmittance, I and Iq are transmitted and incident intensities,

respectively, and L the optical path. The absorption coefficient could change either due

to changes in the molecular number density N (cm“^) or the absorption cross section Q

(cm^). The observed reduction in measured absorption, with increasing temperature,

could be to a large extent attributed to a decrease in the number density, since the pressure was kept constant. However, this does not account completely for the observed

effect, and the remaining change has to be attributed to changes in Q, with temperature,

over the spectral bandwidth of the absorption measurements. Results obtained with this simple experiment suggest that accurate measurements of CO concentration in stack gases, using line-of-s ight infrared absorption techniques, will require an appropriate correction

for changes in the stack gas temperature.

IV. CONCLUSIONS

Performance of two stack gas monitoring systems, a Zr02 type O2 sensor and an in-situ CO monitor based on infrared absorption, has been evaluated in the NBS experimental

furnace. Interest was centered around the desired region for automatic controls; viz., 100£C0O00 ppm, 0£02£2Z and 0.95£4ia.0.

The in-situ O2 measurements were found to be quite sensitive to the air aspiration rate, which controlled the gas sampling rate from the stack. The Zr02 sensor provides the capability for operating at high temperatures, which is valuable especially for furnace applications. The O2 sensor, however, was found to be quite sensitive to air infiltration into the furnace from the surroundings, which is quite typical of industrial

furnaces and boilers.

60
Measurements with the in-situ CO monitoring system could be carried out only above the recuperator. Below the recuperator, both the high gas temperature and radiation from the walls appeared to interfere with the measurements. The cooling air proved to cause a substantial amount of dilution effect. In general, the readings obtained with the in-situ

CO monitor were less than half of those obtained with the gas sampling system. This deviation was attributed primarily to the effect of gas temperature on the CO absorption

coefficient

.

It can be concluded that the application of stack monitoring systems for combustion

control will require a careful study of the characteristics of the particular

installation, before a control system can be implemented which will provide reliable control over the narrow range of operation desired for efficient combustion. These observations also point towards the need for combustion control systems, which do not necessarily rely on stack gas measurements, but rather on measurements made directly in the combustion zone.

ACKNOWLEDGEMENT

Support received from the U.S. Department of Energy, Office of Industrial Programs, under Interagency Agreement No. EA-77-A-01-6010 is gratefully acknowledged.

REFERENCES

1.

Robertson, J.L. and Clarke, P.R. , "Microprocessors and Instrumentation in Furnace Control," Industrial Heating , Vol . 48, No. 5, May, 1981, pp. 9-12.

2. Gilbert, L.F., "CO Control Heightens Furnace Efficiency," Chemical Engineering , Vol. 87, No. 15, July, 1980, pp. 69-73.

3. Spanbauer, J.P., "Energy Savings Through Advanced Boiler Control," paper presented at the 1980 Annual Meeting of the Technical Association of the Pulp and Paper Industry, 1980.

4. Jutila, J.M., "Guide to Selecting Stack Gas Monitors," Industrial Technology , Vol. 27, No. 11, Nov., 1980, pp. 43-54.

5. Gilbert, L.F. , "Precise Combustion-Control Saves Fuel and Power," Chemical Engineering , Vol. 83, No. 13, June, 1976, pp. 145-150.

6. McArthur, L. , Lord, H.C. and Gilbert, L.F., "Efficient Combustion Through

Boiler Air-Fuel Ratio Control," Final Control Elements, Vol. 3, 3rd Control Valve Symposium and Process Control Technology, Instrument Society of America, Pittsburgh, 1977, pp. 101-106.

7. Hoffman, A.R. , "Nature of Platinum-Zirconia Oxygen Sensor," Industrial Heating , Vol. 47, No. 7, July, 1980, pp. 32-33.

8. "Considerations for Energy Savings in Design and Use of Continuous Heat Treating Furnaces - Part I - Role of Controls," Industrial Heating,

Vol. 48, No. 6, June, 1981, pp. 33-37.

9. Shafer, R.V., "Oxygen Measurement and Control for More Efficient Operation

of Soaking Pits," Industrial Heating, Vol. 48, No. 8. August. 1981, pp. 14-19.

10. Opel, A.E. and Gilbert, L.F., "Closed Loop Source Monitoring Saves Energy

and Money," ASME Paper No. 78-WA/APC-6, Dec., 1978.

11. Molloy, R.C., "Microprocessor Based Combustion Monitoring and Control

Systems Utilizing In-Situ Opacity, Oxygen, and CO Measurements," presented

at the 4th World Energy Engineering Conference in Atlanta, Georgia, Oct. 12-15, 1981 and also presented at the 3rd Annual Industrial Energy Conservation Technology Conference and Exhibition, Houston, Texas, April

26-29, 1981.

12. Richardson, D.M., "Combustion Efficiency through Use of Flue Gas Analyzers," Industrial Heating , Vol. 45, No. 5, May, 1978, pp. 20-21.

61
13. Anson, D., Clarke, W.H.N., Cunningham, A.T.S. and Todd, P., "Carbon

Monoxide as a Combustion Control Parameter," Combustion , Vol . A3, No. 9, March, 1972, pp. 17-20.

lA. Pruce, L.M., "Maximum Boiler Efficiency and Save Fuel with CO Monitoring,

Power , Vol. 12A, No. 1, Jan., 1980, p. 70-72.

15. Hall, E.H. et al, "Evaluation of the Theoretical Potential for Energy

Conservation in Seven Basic Industries," Battelle Columbus Laboratory,

Report to FEA, January, 1975.

16. Hamm, J.R. et al , "Energy Conversion Alternatives Study (ECAS) - Vol. Ill Combustors, Furnaces and Low Btu Gasifiers," Westinghouse Electric Corp. Reserach Laboratories, NASA CR-13A9A1, February, 1976.

17. Resource Planning Associates, "Implementation of Energy Conservation

Technology in the Pulp and Paper Industry," Report for Office of Industrial Programs, Dept, of Energy, September, 1980.

18. Fam, S.S., "Impact on Energy Conservation of Automatic Control Systems Utilization in the U.S. Pulp and Paper Industry," Thermo Electron Rpt.

No. TEA265-20-80, 1980.

19. "Basic Research in Engineering - Workshop on Fluid Dynamics and Thermal Processes," Engineering Societies Commission on Energy, Inc., February,

1980.

20. Young, S.B., "High Temperature Waste Heat Recovery Systems for Forge

and Other Furnaces," Industrial Heating, Vol. A7, No. 7, July, 1980, pp. 16-18.

62
7

Figure 1 - NBS ExPERI^ENTAL Furnace. 1) Biwjer; 2) Recuperator; 3) Location OF In-situ O2 Monitor; 4) Location of In-situ CO Monitor Beuow

Recuperator; 5) Porth)les; 6)Heat Exchanger Probes 7) Location

OF In-situ CO Monitor in the Stack.

63
64

Figure 2

-

Schematic

of

Gas

/Analysis

and

SA^PLING

System.
NOZZLE

I

I

65
1

f

66

i

Figure

4

-

ScHEmiic

of

1n-situ

CO

Monitoring

System.
o 1®:'

j -

”

' r t ;

’ H ! ; ;

f

-

o u

^

t) :

1

.

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;

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,

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,

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.

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too

oo

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(X) ^

67

Figife 5A

-

Variation

of

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Concbitration

with

Equivalence

RationNo. 2

Ficl

Oil;

SAm.iNG

Beuow

tecuPERATOR.
(2) k

68

Figure

5B

-

Variation

of O2

Concentration

with

Equivalence

Ratio;

No. 2

Fuel

Oiu

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Beuow

Recuperator.
00 • 00 ': OO'OSS 00’002 OO'OSI OO'OOl

(j-OT X Wdd) 00

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69

Figure

5C

-

Variation

of

CO

Concentration

with

Equivalence

Ratio;

No. 2

Fuel

Oil;

Sampling

Below

Recuperator.
(^-or X ijdd) 03

70

I'i

i'

i

ij

I j I i It

Figure 6

-

Variation

of

CO

and O2

Concentration

with

Equivalence

Ratio;

Expanded

Scale;

No.2

Fuel

Oil;

SA^PLING

Below

Recuperator.
I

No. 2

Fuel

Oil;

Sampling

Above

tecuPERATOR.

73

No. 2

Fuel

Oil;

Sampling

Above

Recuperator.
74

Figure

8A

-

Start-up

Transiemt

Test;

CO^

Variation

with

Time;

Natwial

Gas;

Sampling

Above

Recuperator,
Il

’LING

Above

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o

o

o

76

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8C

-

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with

Time;

Natural

Gas;

Sampling

Above

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o

o

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(l^dd) 3QIXONOl^ Noaavo

d

77

TEMPERATURE

(«C)