1

NBSIR 80-1968

Determination of the Calorific Value of

Refuse-Derived- Fuels by Large-Bomb

Calorimetry Summary of the 1 978

Fiscal Year Results

D. R. Kirklin, E. S. Domalski and D. J. Mitchell

Chemical Thermodynamics Division

and the

Thermal Processes Division

National Bureau of Standards

Washington, D.C. 20234

January 1980

Prepared for U.S. Department of Energy

Office of the Assistant Secretary

Conservation and Solar Applications

Division of Building and Community Systems

Urban Waste and Municipal Systems Branch

Washington, D.C. 20585

and

U.S. Environmental Protection Agency

Municipal Environmental Research Laboratory

'' nd Hazardous Waste Research Division

lati, OH 45268

QC I 100

.U56 ' NO.80-1968

1980
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NBSIR 80-1968

DETERMINATION OF THE CALORIFIC VALUE OF

REFUSE-DERIVED-FUELS BY LARGE-BOMB

CALORIMETRY SUMMARY OF THE 1978

FISCAL YEAR RESULTS

D. R. Kirklin, E. S. Domalski and D. J. Mitchell

Chemical Thermodynamics Division

and the

Thermal Processes Division

National Bureau of Standards

Washington, D.C. 20234

January 1980

Prepared for U.S. Department of Energy

Office of the Assistant Secretary

Conservation and Solar Applications

Divison of Building and Community Systems

Urban Waste and Municipal Systems Branch

Washington, D.C. 20585

and

U.S. Environmental Protection Agency

Municipal Environmental Research Laboratory

Solid and Hazardous Waste Research Division

Cincinnati, OH 45268

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistartt Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
1

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Table of Contents

Abstract v

Introduction 1

Experimental 3

Materials 3

Sample Preparation 4

Calorimetric Apparatus 5

Calorimetric Procedure 7

Discussion and Results 9

Calibration Experiments 9

RDF Combustion Experiment 12

Conclusion 17

References 21

ii
List of Tables

Table I Calibration Experiments of the Bomb Calorimeters 22

Table II

MAF Calorific Values in MJ*kg (Btu*lb~^)

of Teledyne National RDF 23

Table III

Moisture and Ash Contents of RDF Samples 24

iii
List of Figures

Figure 1.

Pellets of benzoic acid (left), processed

RDF (middle), and unprocessed densified

RDF (right) 25

Figure 2.

25-gram-capacity Bomb Calorimeter 26

Figure 3.

Inner arrangement of Large Combustion Bomb 27

Figure 4

Small and large combustion bombs 28

Figure 5.

Large calorimeter system (a view from the top) 29

Figure 6.

Time-temperature curve for a bomb calorimetric

experiment. 30

iv
Determination of the Calorific Value of Refuse-Derived-Fuels by

Large-Bomb Calorimetry

Summary of the 1978 Fiscal Year Results

Duane R. Kirklin, Eugene S. Domalski and David J. Mitchell

National Bureau of Standards

Washington, DC 20234

ABSTRACT

An oxygen bomb calorimeter which can accommodate a 25 gram sample

of refuse or a ref use^derived-fuel (RDF) has been designed and constructed

at the National Bureau of Standards for the purpose of studying the

effects of sample processing on the measured calorific value of such

material. This large calorimeter is an enlarged and modified version of

a conventional-size calorimeter also in use at NBS. The large calorimeter

can handle samples ten times larger than the conventional-size calorimeter

and therefore can be used to investigate RDF samples with either minimal

or no processing. Calorimetric results are presented for calorific

value measurements carried out on d(densif ied)-RDF in both calorimetric

systems. Moisture- and ash-free (MAF) calorific values were obtained in

the large calorimeter from six randomly chosen unprocessed RDF samples

and had a mean value of 24.99 MJ*kg ^ (10 742 Btu*lb ^). Another randomlychosen sample of unprocessed RDF was subjected to extensive processing

to obtain a "homogeneous" analysis sample for use in the conventionalsize calorimeter. Individual measurements in the conventional size

calorimeter on each of ten "homogeneous" sub-samples gave a mean calorific

value of 24.99 MJ*kg (10 743 Btu’lb ). The results of this investigation

indicate that the calorific value of d-RDF is unaffected by the sample

processing technique used at NBS.

V
INTRODUCTION

The Resource Conservation and Recovery Act of 1976, PL 94-580,

identifies solid waste as a potential source of oil, gas, or solid fuel

which can be converted into energy and mandates that the Department of

Commerce provide accurate specifications for recovered materials. The

National Bureau of Standards will furnish guidelines, as indicated in

this Act, for the development of specifications which pertain to the

physical and chemical properties, and characteristics of recovered

materials with a view toward their replacing virgin materials for various

applications.

The heating (or calorific) value of a fuel is perhaps its most

significant property. Heating values are used extensively to evaluate

the commercial potential of fuels and also to evaluate the performance

of incinerators and refuse-fired boilers. The E-38 Committee on Resource

Recovery of the American Society of Testing Materials (ASTM) is interested

in the development of standard methods of test required for the establishment of refuse-derived-fuels (RDF) as an article of commerce. The

American Society of Mechanical Engineers (ASME) Performance Test Code 33

Committee is interested in a method to evaluate the performance of

large incinerators and boilers. A laboratory measurement procedure

giving representative and reproducible heating values is necessary to

equip commercial laboratories to certify accurately the energy content

of RDF and also to evaluate the compliance of large incinerators and

refuse-fired boilers with their contract performance specifications.

1
A method must be established for the precise and accurate measurement

of the heating value of refuse and RDF to evaluate adequately their

merit as fuels. Therefore, the National Bureau of Standards has entered

into a collaborative research agreement with the U.S. Environmental

Protection Agency and the U.S. Department of Energy to establish the

procedures necessary to determine the calorific value of refuse and RDF

by bomb calorimetry.

Refuse consists of combustible and non-combustible materials.

Unfortunately, heating values are determined on small portions of refuse

and considerable processing is necessary to obtain values representative

of the original collection of refuse. The first step of producing a

refuse-derived-fuel (RDF) is to remove the large non-combustible materials.

The remaining materials are then reduced to 15 cm (6in) particle size or

smaller and with the aid of air classifiers, the light combustible

fraction is separated from the heavy non-combustibles. This air classified,

combustible fraction of refuse is then reduced to a smaller particle

size to produce a more homogeneous material.

The objectives of this project are to determine the optimum particle

size of samples for combustion measurements and to establish whether a

conventional-size (2.5 gram capacity) calorimeter or a larger (25 gram

capacity) calorimeter will provide more representative calorific values

of RDF. This report which summarizes work carried out during fiscal

year 1978 provides a response to both objectives from the results obtained

for a Teledyne National RDF sample using a conventional-size (2.5 gram

2 i
capacity) bomb calorimeter and a recently constructed large (25 gram

capacity) bomb calorimeter. This report presents data that indicate

calorific values of unprocessed d-RDF (2.5 cm or 1 inch particle size

before densif ication) are identical to those of RDF processed to 0.2 cm

particle size.

EXPERIMENTAL

Materials

1. Benzoic Acid . NBS Standard Reference Material 39i, certified

energy of combustion of 26 434 +3 J*g ^ at standard bomb

conditions, was used to calibrate the conventional-size

calorimeter. All samples were drawn from the same bottle.

Fisher Scientific Company's certified benzoic acid with an

energy of combustion of 26 437 + 3 J*g ^ was utilized for the

large calorimeter under standard bomb conditions.

2. Oxygen . Ultra High Purity (UHP) grade of oxygen was supplied

by Matheson Gas Products. This oxygen is certified by the

manufacturer to contain combustible impurities not exceeding

0.002 percent and total impurities of less than 0.05 percent.

•k 3. d-RDF . In February 1977, a 20 kg (44 pound) sample of extruded,

d (densif ied)-RDF pellets was received from Teledyne National.

This laboratory sample was from the Baltimore County Resource

Recovery Plant located in Cockeysville, Maryland. The collection

of extruded pellets was contained in a plastic bag enclosed in

a cardboard box. The Teledyne National pellets are cylindrical

in shape, with a diameter of 2.5 cm (1 inch) and are brokenoff lengths of about 2.5 to 7.5 cm (1 to 3 inches).

The materials and equipment are specified to clarify the experimental

procedure, however, in no way does this imply an NBS endorsement of

these materials and equipment.

3
Sample Preparation

1. Sample Requirements . It is necessary that any set of analysis

samples prepared from a gross field sample of refuse or RDF be representative of the composition of the field sample and be unaffected by the

laboratory techniques utilized to produce these analysis samples. It is

assumed that the milling and blending of the gross field sample produces

a homogeneous product and the variational parameter which must be

measured and controlled is the moisture content. This study shows that

analysis samples equilibrated in a constant-humidity container at the

average relative humidity of the bomb-calorimetric laboratory increases

the precision of the measurements. The temperature and relative humidity

of this laboratory are maintained at 295 + 2 K and 45+10 percent,

respectively

.

2. Small Calorimeter Samples . A random group of extruded Teledyne

National pellets was removed from the field sample of RDF. The preparation from these pellets of RDF samples for the conventional-size

calorimeter has already been described fl] and is also provided below:

The pellets were ground in a Quaker City Mill (Model 4) to a particle

size of less than 1.3 cm (0.5 inch). The particles were then milled to

pass a 2 mm (10 mesh) screen in a Wylie Micro-Mill. The ground sample

was riffled and then thoroughly blended in a vee blender.

The ground material was then pressed into pellets with a pressure

of approximately 221 MPa (32 000 psi) . Each pellet weighed approximately

2.2 grams. These pellets were dried to constant weight at 105 °C in a

drying oven. The samples were then placed in the constant-liumidi ty

atmosphere for a minimum of 48 hours and moisture equilibration cliecked

by successive weighings.

4
3. Large Calorimeter Samples . Extruded RDF pellets were utilized

in the large calorimeter with no further processing by NBS laboratories.

The long extruded pellets were broken into pieces weighing between 20

and 25 grams and placed in the constant-humidity atmosphere.

Figure 1 shows the size of a benzoic acid pellet for the large

calorimeter, pellets for both the small and large calorimeters, and the

unprocessed RDF sample as-received from Teledyne National. These asreceived extruded RDF pellets appear heterogeneous while the milled and

blended RDF pellets have a more homogeneous appearance.

Calorimetric Apparatus

1. Large Isoperibol Bomb Calorimeter . The design of the large "isoperibol" (Isothermal-j acket) calorimeter which will accommodate the large

combustion bomb, described in the following section, is similar to that

of Coops et al. [2,3] and Gundry e_t al . [4], and is shown in Figure 2.

The calorimeter is constructed entirely of stainless steel. It consists

of a cylindrical calorimeter vessel (25.4 cm diam. , 45.7 cm height (10

inch diam., 18 inch height)) in which three rods support a concentric

cylindrical shield (20.3 cm diam., 30.5 cm height (8 inch diam., 12 inch

height)). The calorimeter lid supports a stirrer assembly and a shield

cover. The shield, shield cover, and stirrer assembly facilitate the

flow of water in the calorimeter so that water is moving (downward) in

the space between the bomb and shield, and (upward) between the shield

and wall of the calorimeter vessel. The combustion bomb is supported by

a foot which has been welded to the bottom of the vessel to insure that

the bomb will be positioned in the same manner in the calorimeter for

each experiment. The calorimeter vessel contains 19 liters of water for

5
each calorimetric measurement and is housed in a submarine vessel which

has a cover. The submarine cover is fastened to the vessel with six

bolts and has four vertical ports for: (1) fuse circuit leads, (2) a

calorimeter heater, (3) a central stirrer assembly and, (4) quartz

thermometer leads. The frequency of oscillation of a temperaturesensitive quartz crystal is used to determine the calorimeter temperature.

The NBS standard frequencies of 10 and 100 kHz are utilized. The entire

calorimeter system (bomb, calorimeter vessel, and submarine compartment)

is immersed in a constant temperature water bath maintained at 30° +

0.03°C. The overall volume of the bath is 280 liters and holds 235

liters of water with the calorimeter system immersed in it.

2. Large Combustion Bomb . The combustion bomb which accommodates a 25

gram sample of RDF was purchased from the Parr Instrument Company and is

shown schematically in Figure 3. The bomb has an overall height of 35

cm (13.75 in) and has a mass of 13.8 kg (30.5 lb) when assembled; the

internal and external volumes of the bomb are 1.85 and 3.62 liters,

3 respectively. A platinum crucible (^50 cm ) was used in the calibration

3 experiments with benzoic acid and a stainless steel crucible (^>^50 cm )

was used in the combustion experiments with RDF. The bomb body has a

wall thickness of 0.953 cm (0.375 in) and an outer diameter of 11.4 cm

(4.5 in).

A photograph of the small and the large combustion bombs is shown

in Figure 4. Figure 5 is a photograph of the large calorimeter viewed

from the top. It shows the large bomb immersed in the calorimeter

vessel with the submarine lid and calorimeter cover (inverted to display

the stirrer and shield cover).

6
Calorimetric Procedure

Bomb-calorimetric techniques are well established and are described

in detail elsewhere [5,6], but a brief description of the method used in

this work follows. In the thermochemical investigations, the heat

evolved by complete oxidation of a measured amount of RDF is compared

with the heat evolved by a measured amount of a selected standard reaction,

using a fixed calorimeter system with a specific temperature rise. The

standard reaction is the combustion of benzoic acid under standard bomb

conditions producing a temperature rise of 3 and 5 degrees in the small

and large calorimeters, respectively. The energy equivalent of the

calorimeter is the amount of energy produced by the standard reaction

and its accompanying side reactions divided by the corrected temperature

rise. The corrected temperature rise is derived from the observed

temperature rise by subtracting the contributions due to stirring energy

and thermal leakage.

Multiplication of the energy equivalent of the calorimeter (obtained

from the calibration experiments) by the corrected temperature rise

(measured in the RDF combustion experiment) gives the total energy

produced in the RDF combustion experiment. This total energy is then

corrected for known side reactions and divided by the mass of the RDF

sample to produce the gross calorific value. In a typical RDF experiment, a moisture equilibrated RDF pellet is weighed in a tared stainlesssteel crucible. The crucible and sample are supported inside the bomb.

The sample is in contact with a 10 cm length of 0.127 mm (.005 in)

3 diameter iron fuse wire. The bomb also contains 10 cm of H

2 O to

7
dissolve certain gaseous products of combustion and maintain an atmospliere

that is saturated with water. The sealed bomb is then charged with

high-purity oxygen at a pressure of 4.1 MPa (40.8 atm). The bomb is

lowered into the water-filled calorimeter and the covered calorimeter

then submerged in the constant temperature water bath (30°C) . The

calorimeter system is then heated to slightly below 25 °C, and frequent

measurement of temperature and time is begun. Figure 6 is an example of

a time-temperature curve for a typical bomb-calorimetric experiment (see

ref. 5, chap. 3 for more detail). The temperature is measured during

the period before the sample is ignited ("fore period", line ab in

Figure 6), during the reaction period immediately after the sample is

ignited ("reaction period", line be in Figure 6), and during the period

after the reaction is complete ("after period", line eh in Figure 6).

The difference between the first point in the after-period and the last

point in the fore-period gives the observed temperature rise. The

slopes with respect to time of the temperature-time curve during the

fore- and after-periods allow one to calculate the portion of the

temperature rise due to stirring energy and thermal leakage. The

submerged calorimeter must be stirred at a constant rate to obtain a

uniform and meaningful temperature vs. time curve. A preliminary

experiment was performed to determine the amount of RDF sample necessary

to produce about the same temperature rise in the calorimeters as that

produced in our calibration reaction with benzoic acid, (a three and

five Kelvin temperature rise in the small and large calorimeter, respectively).
DISCUSSION AND RESULTS

Calibration Experiments

It is necessary to determine in preliminary calibrations the energy

equivalent for both the large and conventional-size calorimeter systems.

The results of these calibration experiments are presented in Table 1.

Experiments number 1046 through 1050 were performed in the conventionalsize calorimeter and 2006, 2010 and 2011 were performed in the large

calorimeter. In a calibration experiment, one accounts for all energy

supplied to the calorimeter system and measures the resulting temperature

rise. The row identifiers in rows 4 through 10 of column 1 of Table I

are the different sources of energy in a typical calorimetric experiment.

A comparison of the small- and large-calorimeter entries will point out

some of the differences between the two calorimeters. AU °,t, is the

c h

energy of combustion of benzoic acid at the specified reference temperature,

tj^. The small calorimeter is referenced to 28 °C and the large calorimeter

is referenced to 30 °C. The heat of combustion of the calorimetric

standard, benzoic acid, is known to approximately one part in ten

thousand. From the mass of benzoic acid, shown in row 3, the

energy supplied by combustion of benzoic acid can be calculated and is

presented in row 4, q-BA. Row 5 shows the electrical energy that was

supplied to ignite the sample, q-ign. The platinum fuse wire of the

small calorimeter is heated electrically to its melting point to ignite

the samples. The molten platinum fuse solidifies and therefore supplies

no detectable energy to the small calorimeter. In the large calorimeter

an iron fuse wire was used. The electrical energy necessary to melt the

iron fuse wire must be accounted for, but in addition, the molten iron

9
reacts to produce iron oxides. The quantity of iron fuse wire burned

and the energy of combustion of the iron fuse wire are used to calculate

the energy supplied by the combustion of iron, q-Fe. Some of the nitrogen

impurities in the oxygen are converted to nitrogen oxides which react

with water to form nitric acid. The quantity of nitric acid produced is

measured and from its energy of formation, q-HNO^ is calculated. In

high-precision combustion calorimetry, it has become standard practice

to apply corrections proposed by Washburn [7] that produce calorimetric

data referenced to a standard set of conditions. These corrections to

standard states for the isothermal bomb process at 28 °C and unit

fugacity are known as Washburn Corrections and are presented as q-wc

.

The small calorimeter is designed to operate between 25 °C and 28 °C

while the large calorimeter operates between 25 °C and 30 °C. In

actual practice, the final temperature of our experimental reaction is

not exactly 28 °C or 30 °C and therefore, a small correction must be

applied to the calorimetric data for high precision work and is shown as

q-Tj corr. in Table I. Due to the limited quantity of oxygen in the

large bomb, combustion is incomplete and a few tenths of a milligram of

carbon residue were found in the combustion crucible. The energy of

combustion of benzoic acid assumes complete combustion and therefore a

small correction for unburned carbon, q-C , was included in the ^ corr. energy summation. The algebraic sum of the energies in rows 4 through

10 from various sources yields the total energy supplied to the calorimeter, Q-Total, corrected to 28 or 30 °C.

Some additional discussion is warranted in regards to the unburned

carbon residue. Stoichiometrically , 2 grams of oxygen are required for

10
the combustion of 1 gram of benzoic acid. Normally, 10 grams of oxygen

are used for each gram of benzoic acid to insure complete combustion in

the conventional-size bomb. It has been found in practice that 6.5 g of

O

2 per gram of benzoic acid is the minimum requirement for complete

combustion in the present design of bomb calorimeters. The large bomb,

utilizing a 16.3 gram benzoic acid sample and 4.1 MPa (40 atm) of

oxygen contains about 100 grams of O

2

. This is on the borderline of the

minimum amount of oxygen necessary for complete combustion in bomb

calorimetry. However, in our experiments the total correction for

unburned carbon is usually less than 40 joules for a 400 000 joule

experiment (1 part/10 000) and even if completely neglected, the error

thereby introduced would be acceptable for precision combustion calorimetry

of benzoic acid. The correction for unburned carbon, q-C , is given

in Table 1 for the large calorimeter data.

The corrected temperature rise of the calorimeter, AT-corr, is

calculated and divided into the total energy supplied to the calorimeter,

Q- total, to yield the energy equivalent, E , , of the calorimeter.

C3J. The mass of water introduced into the bomb, the crucible and the sample

may change from experiment to experiment. The product of the mass of

these materials and their specific heats are subtracted from the energy

equivalent of the calorimeter and the energy equivalent of the "empty"

calorimeter, E , , is obtained.

SI

The important parameters to compare between the two calorimeters

are the energy equivalents and the total energy capacities of the two

calorimeters. The energy equivalent of the small calorimeter is 14 555

joules per Kelvin while that of the large calorimeter is 86 162 joules

11
per Kelvin. The product of the energy equivalent and the specified

temperature rise gives the total energy capacity. The large calorimeter,

has a total energy capacity of 430 810 joules compared to 43 665 joules -

of the small calorimeter. Therefore, a sample which has a mass ten

times larger and less particle size reduction can be used in the large

calorimeter. This is extremely important since the calorific value of a

solid fuel can now be determined with only minimal processing and the

possibility of chemical changes imposed upon the refuse or RDF analysis

sample as a result of size reduction, can now be investigated quantitatively.

RDF Combustion Experiment

In a calorimetric experiment, the corrected temperature rise of the

calorimeter multiplied by the mean energy equivalent of the empty

calorimeter plus its contents yields the total energy supplied to the ,

calorimeter.

A correction is subtracted for the formation of sulfuric acid from

the small amount of sulfur ('^>0.1%) contained in the d-RDF sample. Of

the six energy correction types contained in rows 5 through 10 of table

I, neither the Washburn correction nor the carbon corrections are

applied. The Washburn correction (used in high-precision calorimetry)

cannot be applied since the actual reactants and products in the RDF

experiments are unknown. In addition, complete combustion was assumed

in the RDF experiments. The total energy measured, minus the algebraicsum of all corrections yielded the energy of combustion of the analysis

sample. The calorific value of each RDF sample is simply ttie energy of

combustion per unit of mass (weighed in air).

12
Table II presents the calorific values obtained on several analysis

samples of an RDF from Teledyne National. The values presented from the

conventional-size calorimeter were obtained on samples which were subjected

to considerable processing as outlined in the experimental section

earlier. The values determined in the large calorimeter were obtained

on the as-received densified RDF with no further processing.

Table II presents moisture- and ash-free (MAF) calorific values

obtained from these two types of calorimetric samples. Typical RDF

samples have a moisture- and ash-free calorific value of between 20.93

and 23.23 MJ*kg"^ (9 000 and 10 000 Btu'lb"^). The Teledyne National

samples studied are obviously atypical and exhibit an unusually high

heating value.

Ten experiments in the small calorimeter on "homogeneous" RDF

yielded a mean calorific value of 24.99 MJ*kg ^ (10 743 Btu'lb

and a standard deviation of a measurement of 0.80 percent while six

experiments on as-received RDF samples with no further processing yielded

a mean calorific value of 24.99 MJ*kg ^ (10 742 Btu*lb and a standard

deviation of 2.54 percent in the large calorimeter. The agreement of

these mean calorific values is significant when one considers the

radically different treatment of the two samples.

The range of calorific values is much larger on the large, unprocessed

samples than on the small, processed ones. Calorific values were dispersed over the range from 24.51 to 25.20 MJ*kg ^ (10 539 to 10 835

Btu*lb for the small calorimeter samples while a range of 24.39

to 26.09 MJ*kg (10 485 to 11 216 Btu'lb was observed for the large

calorimeter samples. As expected, these results suggest that the unprocessed

13
I

samples are more heterogeneous and that the calorific values would

scatter widely. A closer look at the heating values obtained in the

large calorimeter reveals that five of the six values are close together

and that one value, 26.09 MJ*kg ^ (11 216 Btu*lb ^) , lies further from

the mean. The 26.09 MJ*kg ^ (11 216 Btu*lb value is 1.7 standard

deviations from the mean and cannot be traced to any instrumental

malfunctions. Therefore, it is included as part of the sample population.

If the high calorific value were to be omitted, the mean calorific

value would be 10 647 Btu*lb ^ which is less than one percent from the

mean obtained when 11 216 Btu*lb ^ is included. This omission would

change the standard deviation from 2.54 percent to 1.51 percent.

Several things could account for the spread of calorific values

from the large calorimeter. In addition to the sampling problems

associated with RDF, the moisture and ash contents are very important

parameters. Table III contains the moisture and ash contents used to

calculate the moisture- and ash-free results from the as-determined

calorimetric data which are presented in Table II. We attribute the

fluctuations in the moisture content of analysis samples 1052, 1053 and

1054, to the relative humidity changes of the room. Therefore, sample

1056 through 1062 were equilibrated in a constant humidity atmosphere

and all had virtually the same moisture content. Water was absorbed or

evaporated from the samples during the weighing process but always at a

rate which was not readily reflected in the corresponding calorific

value. This was accomplished by maintaining the constant humidity

container at the average relative humidity of the room ('^45 percent).

14
One reason for the good precision of the small calorimeter results is

that the moisture was virtually constant for all analysis samples and

was determined on the calorimetric analysis sample directly. This was

not true for the large calorimeter analysis samples. With the small

analysis samples, 48 hours were sufficient for equilibration, but the

as-received extruded pellets for the large calorimeter had approximately

13 percent moisture and equilibrated very slowly in our constant humidity

atmosphere. Since equilibrated analysis samples of the same size have

the same moisture content, we feel that separate moisture experiments

would give an average moisture value indicative of all the equilibrated

RDF analysis samples. Two experiments were carried out on several

extruded pellet fragments each weighing about 5 grams, and yielded

moisture contents of 5.71 and 5.92 percent. Two experiments were performed

on extruded pellets of approximately the same size (^25 grams) as the

combustion samples and yielded moisture contents of 6.82 and 6.30 percent.

We feel that none of the extruded pellets had equilibrated completely

and that the equilibration was very slow and largely dependent upon the

size of extruded pellets chosen for equilibration. The moisture measurements were done on different days during the period that the calorimetric

experiments were being performed. We feel that the moisture measurements

on extruded pellets of approximately the same size ('^>25 grams) as our

calorimetric analysis samples were more representative of the calorimetric

samples used in the combustion experiments. We, therefore, used an

average moisture of 6.56 percent to calculate calorific values on a

moisture-free basis. The uncertainty of the actual moisture of each

calorimetric sample may be part of the cause for the larger range of

heating values measured in the large calorimeter.

15
Our earlier experiences in determining MAF heating values of RDF

indicated that the precision can be enhanced by determining the ash

content from the combustion bomb residue for each sample burned. For

the small calorimeter samples, the percent standard deviation of a

measurement is less than one percent. The bomb combustion residue

method is especially applicable to the "unprocessed" large calorimeter

samples because of the heterogeneity of these samples. The lower

precision obtained with these "unprocessed" samples is probably caused

by their heterogeneity and not by variability of their ash content.

16
CONCLUSION

We have designed and constructed a bomb calorimeter which is

capable of handling a sample with ten times the mass of that handled in

our conventional-size bomb calorimeter. Variance in the calorific value

of RDF samples because of sampling or preparation procedures (e.g.,

size reduction) can be determined from heat measurements in this large

calorimeter. This isoperibol (isothermal-jacket) calorimeter is an

enlarged and modified version of our conventional-size calorimeter with

the same basic principles of operation. The large calorimeter has been

calibrated and exhibits a precision better than one part in ten thousand.

It can be considered a precise calorimeter like our conventional-size

one. However, the internal volume (1.85 liters) of the bomb can contain

only the minimum amount of oxygen usually required for complete combustion

of solid samples with ten times the mass of conventional 2.5 gram samples

(A.l MPa of oxygen for a 25 gram RDF sample).

One objective of this research project is to decide whether or not

the calorific value of refuse is altered by the amount of particle size

reduction and if so, whether there is an optimum size. From a limited

number of experiments in both the large and small calorimeters, we found

that the calorific value of a given sample of Teledyne National RDF was

unaffected by particle size reduction in our laboratories. As-received,

extruded RDF pellets were found to have the same mean calorific value as

pellets prepared from 2 mm (10 mesh) particles which were obtained by

the milling of as-received, extruded RDF pellets followed by several

riffling and blending operations.

Our calorific values, which were measured in two different bomb

calorimeter, suggests that either system is applicable to processed

17
RDF samples. However, a considerable amount of time and effort can be

saved with the 25 gram capacity calorimeter since the larger sample size

requires less sample processing.

The precision (standard deviation) of calorific value measurements

on pellets prepared from 2 mm particles was three times better than that

obtained with pellets where no processing was done in our laboratory.

The reason for this difference in precision is not known but we suspect

that it is the result of a greater heterogeneity of the unprocessed

sample and the variance in the moisture content of our analysis samples.

We believe that more precise moisture data can be obtained if as-received,

extruded RDF pellets are equilibrated in a constant humidity atmosphere

and for each calorimetric sample, a moisture sample also is selected.

If the calorimetric and moisture samples have the same history and are

weighed soon after each other, then the moisture contents should be

comparable. In addition, it is worth noting that the moisture content

of pellets of identical size prepared from 2 mm (10 mesh) particles was

found to be the same after the pellets were air-dried and equilibrated

at constant humidity.

We found that the measurements of the ash content of RDF by conventional dry-ashing techniques yields a range of values for "homogeneous,"

milled and blended 2 mm (10 mesh) particles of RDF. We suggest that the

non-combustible fraction of RDF clings to the fibrous RDF particles and

is not evenly distributed by the blending process. Therefore, we

adopted a procedure of determining the ash from the combustion residue

remaining in the bomb which is a characteristic of the actual combustion

sample

.

18
In summary, it appears that the calorific value of the Teledyne

National RDF is unaffected by the particle size reduction procedures

carried out in our laboratory. Also, to obtain good precision, one must

correct the measured calorific value for ash residue remaining in the

combustion bomb after an experiment. To avoid any sample processing for

the large calorimeter experiments, a "twin" moisture-determination

pellet must be selected and its moisture content measured while it still

may reasonably be expected to have the same moisture content as the

calorimetric sample. One can draw the general inference from the

calorimetric results that processing of a sample, which has been extruded

from minus 1.9 cm (0.75 inch) pieces of RDF, down to a particle size

below 0.2 cm (0.08 inch), can be effected without changes in chemical

composition or representativeness.

The processing carried out at NBS on the RDF samples represents

only a small portion of the processing necessary to prepare calorimetric

samples from municipal solid waste (MSW) . Our results show no change in

the calorific values as a consequence of NBS processing on d-RDF.

Resource recovery facilities (e.g. Teledyne National) start out with

MSW (i.e. raw refuse) and process it into an RDF. We have no assessment

of the possible changes in the calorific value which could take place as

a result of the processing by the RDF manufacturer.

Determination of calorific values of MSW which have received minimal

or no processing requires large samples. Experts in resource recovery

intuitively feel that kilogram-size samples (1-25 kg) may approach the

kind of sampling representation desired for a large array of MSW.

19
Kilogram-size samples which have minimal processing (i.e., to minus 6

inch material) would be desirable. Statistical studies must be

performed on MSW to establish the actual size of sample which is truly

representative.

Conventional bomb calorimetry is limited to gram-size samples

which have experienced considerable processing (i.e. particle size

reduction) . Since representative calorific values of MSW will probably

require kilogram-size samples which have received minimal processing,

a combustion calorimeter which can accommodate such large samples must

be developed. Therefore, our future research efforts will be directed

toward statistical studies needed to derive a representative sample

from MSW and the development of a kilogram-capacity combustion calorimeter.

20
REFERENCES

[1] Kirklin, D. R. , Mitchell, D. J., Cohen, J., Domalski, E. S., and

Abramowitz, S. , NBSIR 78-1494, December 1978.

[2] Coops, J., van Nes, K. , Kentee, A., and Dienske, J. W. , Rec. Trav.

Chim. 113-130 (1947).

[3] Coops, d., and van Nes, K. , Rec. Trav. Chim. 6^, 131-141 (1947).

[4] Gundry, H. A., Harrop, D. , Head, A. J., and Lewis, G. B., J. Chem.

Thermodynam. 1^, 321-332 (1969).

[5] Prosen, E. J., Experimental Thermochemistry , Chapter 6, F. D.

Rossini, Editor, Interscience Publishers, New York, 1956.

[6] Jessup, R. S., NBS Monograph 7, 1960, "Precise Measurement of Heat

of Combustion with a Bomb Calorimeter," (U.S. Government Printing

Office, Washington, DC 20420).

[7] Washburn, E. W. , J. Research NBS 10, 525 (1933), RP546.

21
Table

I.

Calibration

Experiments

of

the

Bomb

Calorimeters 1

vX) (T> <3N •H CO m vO vO CO o •<r « 00 • (N CO vO iH CO • i-H <7^ vO CN 00 00 m o • rH • crt 1—

00 fO (J^ • • • CN • m -O' 00 NO o CN CN • 00 r'' O • CJ^ CM ON iH • rH CN • O fH CO m f-H tH rH 00 a^ CN CT\ NO /T'S CM vO CO in 1 1 o • vO -O' 00 ^s S'?

w rH <r CO 00 00 <r

(U <1* tH in CO

4-1 v£> 00 00 CO o o

(U cn C3^ iH rH v£) vO » o o a • • o f'' m o ON Mf m • CN « • •H o CM m o> 00 00 00 m • CN • 00 vO vO o o

f-H i-H 00 UO • • • CTt • CO CM vO 00 UO tH

O o <r CN • 00

O • CT\ vO ON o « tH vO o o\

iH cvj vD • CT\ CO

m rH 1 CN CO ON CN ON vO 00 o 00

CO CN CN

m 1 O • N? 00 • < O iH CO 'd* 00 m CN

(UO fO 00 CO

oo cn 00 <} >d- rH 00

^4 • CTi • m CO vO

O

m ON » CO rH 00 CO C7^ o 'O' rH 'O' vO • 00 • 00 m O rH 'O' r—

• • • 00 • CN rH \o O <r <Ti rH • vO CO CO • rH O o\ • t—

CM • vO CN

m 00 00m o vO vO tH iH v£) CN 'O’ o\ 1—

CO rH vO • vO CO 00

tH CO 1 a\ 00 CO

tH . 00 o o o> Ov O

vOm -d- 00 * o • o • iH 1 o o 1 • 1^ • 00 cn UO o o <T O 1 CO 00 CN 1 m f'' C3V 00 m O H tH • 1 • • o 1 m f'' in • lO H 'd' NO CO rH 1 in 'd' • 1 CO crt m vO <f

vO • CO CO CO • •<r rH

CN I—

<d- CN

H

ON

00 00 tH 00 C'' <r

vD 00 o o^ CO • o^ • rH • ON 1 o m 1 • o • 00 CO

'cr o CTn tH »H 1 CN 00 CO 1 CO VO o 00 uo o H CO ON • 1 • • o 1 CO vO r uo H 'd' NO CN rH 1 m 'd' • 1 CO cjv m vO

U cO • CO CO CO • 'd- tH

0) CN 1—

'd- <d CN rH

4J ed

<u CO a CN m

•H 00

mmO VOm tH o

V4 'd*H r'' C3^ o 1"'O o o 00 • »H • 'd' 1 r" vo 1 • CN • a\ • • • • 1—

'd'O

O rH ON 1 CJ\ CN 1 COm CO 00 rs. 'd o o

tC) o H

m 00 • 1 • • O 1 CN C3V vO • m

m

u 1—

-d- sOm o 1 m -d- • 1 vO CTt

m vO

m

m CN 00 vD • cn CO 1 CO • d- dH CN rH 'd' •d- CN t—

rH 1—

• • t—

tH o

COa

CO 00 00 cO rH vO CO

m vO ON

r'' • o • CN 1 o a\ 1 CJ\

m C3V c

<r o CN CN 1 cy^

m d- 1 » o • r'' • o H CO O • 1 * • O 1 d- CO o 00 t—

<d' nD

H 1—

1 m 'd' • 1 d' vO vO • uo vO • m CO H CdV m vO UO CN iH CO • -d- <f

-d- CNH tH

00 iH

m

ON CO 00

H o • • 00 1 o CO 1 CO 00 o> iH

'd- o m CN o 1 00

H CN 1 • vO • • o H CO O • 1 • • O 1 <JV CN 00 NO H 'd' NO CN iH 1 vO f" • 1 'd' vO vO • UO

'£> • CO CO CN CTvm vO uo

CN rH 'd CO • 'd' 'd' CN H iH 1—

H . t—

1 t 3 O H 1 • • cd z 1 00 1—

1 Ui•

•-)

H <u • • t—1 4J • 1 C • * a •n M • cd > >H X *o M V4 H U 4-1 • OJ 0) OJ 1 u 4-t •“3 *-) CO >-) O id u • c S P O 3 - 00 c O '''—

<J o 4J o H o 0 < oo (U 2 o iw o O o CO u •H • O < CO •H Ui 33 5 H CJ H 1 U 1 W C/3 T3 'C3 - 3 CP 1 1 \ 1 1 1 1 1 H 1 •H 1 1 iJ 4-J < B cr a* O' cr cr cr cr <dy <3 w w w w C/) CO 22
Table II. MAF Calorific Values in MJ'kg ^ (Btu •Ib"^) of

Teledyne National RDF

Small Calorimeter Large Calorimeter

Expt. No. Q Expt. No. " V

1052 24.95 2030 25.27

(10726) (10866)

1053 24.51 2032 26.09

(10539) (11216)

1054 25.01 2033 24.96

(10752) (10732)

1056 24.78 2034 24.42

(10656) (10501)

1057 25.06 2038 24.77

(10776) (10650)

1058 25.06 2039 24.39

(10776) (10485)

1059 25.20

(10835)

1060 25.12

(10799)

1061 25.09

(10788)

1062 25.08

(10781)

Range (10539-10835) (10485-11216)

Mean (10743) (10742)

Std. Dev. (86) (273)

%Std. Dev. 0.80% 2.54%

23
Table III. Moisture and Ash Contents^ of RDF Samples

Small Calorimeter Samples Large Calorimeter Samples

Expt. No. % Moisture % Ash Expt. No. %. Moisture % Ash

1052 3.74^ 11.57 1 5.71

1053 4.15^ 11.50 2 5.92

1054 3.75^ 13.94 3 6.82

1056 3.89 11.94 4 6.30

10.57 3.82 11.87 2030 15.01

1058 3.85 12.25 2032 13.37

1059 3.88 11.84 2033 14.90

1060 3.96 12.08 2034 14.46

1061 3.93 11.76 2038 13.32

1062 3.88 12.04 2039 14.18

These samples were not equilibrated in a constant-humidity atmosphere

prior to weighing,

b) All percent by weight

24
"/at

</(><>«/

Nat»on«t

Bureau

of

Sfancfards

25

Pellets

of

benzoic

acid

(left)

,

processed

RDF

(middle)

,

and

unprocessed

denslf

led

RDF

(right)

Figure

1.
25-gram capacity Bomb Calorimeter

A, pulley to stirrer motor; B,one of four stacks attached to the submarine lid to

accommodate (1) fuse circuit, (2) heater, (3) temperature sensor, and (4) stirrer,

C, submarine lid; D, submarine flange with “O” ring-, E,calorimeter vessel lid; stirrer

shaft; G,shield cover; H,stirrer, I, calorimeter vessel supports; J,shield; K,combustion

bomb; L, submarine vessel; M, calorimeter vessel; N,bomb foot.

Figure 2.

26
A

Inner Arrangement of Large Combustion Bomb

A Grounded Electrode

B Valve

C Split Ring

D Compression Ring

E Drop Band

F Buna-N 0-Ring

G Bomb Head

H Bomb Body

I Handle

J Gas Inlet Tube

K Ungrounded Electrode

L Fuse

M Fuse Attachment Hook

N Ring Holder

0 Ring Support

P Sample Pellet

Q Crucible

I

I <

Figure 3. 27
Small and large combustion bombs

J

Figure 4

28
Figure 5.

29
o o o

0)

o

E

X4^

ajn^ejadujai

30

Figure
NBS*114A (REV. 0-78)

USCOMM-DC
1

Trr
/.’W), lil

\

:h

'•I

'•m

i*‘r

N -A I