AlllOb 13fl71b

NBSIR 84-2825 (DOE)

Assessing the Credibility of the

Calorific Value of Municipal Solid

Waste

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Chemical Thermodynamics Division

Center for Chemical Physics

and the

National Measurement Laboratory*

and the

Temperature and Pressure Division**

Center for Basic Standards

Washington, DC 20234

February 1984

QC

/oo

/9 0 V

£

.

Prepared for U.S. Department of Energy

Office of Renewable Technology

Energy from Municipal Waste Division

Forrestal Building

)00 Independence Avenue, SW

ashington, DC 20585

NBSIR 84-2825 (DOE)

ASSESSING THE CREDIBILITY OF THE

CALORIFIC VALUE OF MUNICIPAL SOLID

WASTE

K. L. Churney, E. S. Domalski, A. E. Ledford, J. C. Colbert,

S. S. Bruce, T. J. Buckley, R. C. Paule*, and M. L. Reilly**

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Chemical Thermodynamics Division

Center for Chemical Physics

and the

National Measurement Laboratory*

and the

Temperature and Pressure Division**

Center for Basic Standards

Washington, DC 20234

February 1984

Prepared for U.S. Department of Energy

Office of Renewable Technology

Energy from Municipal Waste Division

Forrestal Building

1000 Independence Avenue, SW

Washington, DC 20585

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
! o~aO'
Assessing the Credibility of the Calorific Value of

Municipal Solid Waste

K.L. Churney, E.S. Domalski, A.E. Ledford, J.C. Colbert,

S.S. Bruce, T.J. Buckley,

Chemical Thermodynamics Division

Center for Chemical Physics

R. C. Paule,

National Measurement Laboratory

and

M.L. Reilly

Temperature and Pressure Division

Center for Basic Standards

National Bureau of Standards

Washington, D.C. 20234
PREFACE

This report gives the results of studies vhich were started in fiscal

year 1901, continued through fiscal year 1982, and were completed in

fiscal year 1983. Two important industrial/municipal queries required

response in this study. In response to the first, we compared the precision and accuracy of the calorific value of municipal solid waste (MSW)

which had been processed to 2 mm particle size or less and which had been

burned in both a gram-size bomb calorimeter and in the kilogram-size

flow calorimeter. In response to the second query, we determined whether

there would be a significant difference between the results of burning an

unprocessed and processed MSW sample in the kilogram-capacity combustion

flow calorimeter. The results of the calorimetric studies which respond

to these queries are p:

The chronology of

Oct 80 - Feb 81

Feb 81 - Apr 81

Apr 81 - Jun 81

Jun 81 - Jul 8l

'

Jul 81 - Feb 82

Feb 82 - Apr 82

Apr 82 - Dec 82

Jan 1983

Design of kilogram capacity flow calorimeter.

Critique and development of engineering drawings,

Competitive bidding.

Awarding of bid to vendors.

Fabrication period.

Assembly.

Preliminary testing, combustion runs

(jirst combustion May 82).

Baltimore County Sampling.

ii
Feb 83 Development of Measurement Protocol

(Statistically designed experiments).

Feb 82 - May 83 Construction and completion of product

gas analysis train.

Apr 83 - Oet 83 Calorimetric Measurements.

(A total of 26 calorimetric experiments

were carried out )

Sep 13, 1983 DoE Research Review on the Characterization

of Municipal Solid Waste.

Sep 83 - Oct 83 Calculation of Results.

iii
Table of Contents

Page

Abstract 1

1 . Introduction 2

2. Experimental Apparatus 5

2.1 Description of 2.5 Kg Flow Calorimeter and Auxiliary Apparatus _ 5

2.2 Description of a Typical Experiment 10

3. Sample Preparation and Measurement Protocol 15

4. Experimental Results 22

4.1 Combustion Bomb Calorimetry 22

4.2 Combustion Flow Calorimetry 27

4.2.1 Flow Calorimetry of RDF-4 27

4.2.2 Flow Calorimetry of RDF-2 32

4.3 Summary of Statistical Results 35

5. Discussion and Conclusions 36

6. References 39

7 . Acknowledgements 40

iv
List of Tables

Page

Table 1. Specifications for the 2.5 kg capacity flow calorimeter. 8

Table 2. Composition of municipal solid waste obtained Jan 10-21,

1983, from the Baltimore County Resource and Recovery

Facility. 17

Table 3» Measurement protocol. 19

Table 4. Ash and combustion bomb calorimetry results for two-gram test

samples. 23

Table 5. Summary of statistical analysis of ash and combustion bomb

calorimetry results for two-gram test samples. 25

Table 6. Flow and bomb calorimetry results on processed MSW (RDF-4). 28

Table 7. Mass Fractions of Bags E and E' contributed by Bags C and D. 29

Table 8. Analysis of flow and bomb calorimetry results on processed

MSW (RDF-4). 30

Table 9. Flow calorimetry results on unprocessed and processed MSW. 33

Table 10. Analysis of flow calorimetry results on kilogram-size samples

of processed and unprocessed MSW. 34

v
List of Figures

Page

Figure 1.

Figure 2.

Figure 3*

Figure 4.

Diagram of the 2.5 kg capacity flow calorimeter. 7

A plot of the concentrations of gaseous carbon dioxide,

water, and carbon monoxide as a function of time for

a typical experiment on RDF. 12

A plot of the temperature rise of the calorimeter water

plus the deviation of the observed data points from the

smooth curve fit to the data during the drift periods. 14

Diagram of the sample division technique used to prepare

gram-size test samples (C and D) for the bomb calorimeter

studies and kilogram-size samples (E and E') for the flow

calorimeter studies from 2.5 kilogram hand sorted and

reconstituted samples in bags C and D. 21
ASSESSING THE CREDIBILITY OF THE CALORIFIC VALUE OF MUNICIPAL SOLID WASTE

K.L. Churney, E.S. Domalski, A.E. Ledford, J.C. Colbert, S.S. Bruce,

T.J. Buckley, R. C. Paule, and M.L. Reilly

Abstract

A study has been made at the National Bureau of Standards to establish the

limits of reliability of the calorific value of municipal solid waste (MSW)

determined by the bomb calorimetric procedure currently used in commercial test

laboratories. This procedure involves using gram-size samples derived from MSW

that has been processed down to a particle size of 2 mm or less. Critics of

the procedure argue that gram-size samples are too small to be representative of

such a large quantity of so heterogeneous a material, and that processing MSW

may also alter its composition.

To test the bomb calorimetric procedure, a 2.5 kg capacity combustion flow

calorimeter was designed and constructed for the determination of the enthalpies

of combustion of kilogram-size samples of MSW in flowing oxygen near atmospheric

pressure.

Calorimetric data on processed MSW were obtained using both the kilogramsize flow and a gram-size bomb calorimeter. Intercomparison of results shows

that the calorific value (on a dry basis) of gram-size test samples agrees,

within the uncertainty of our experiments, with the corresponding values for

their kilogram-size parent samples provided that the sample division technique

used to obtain the gram-size samples is that described in this work. The

average difference of the parent minus gram-size sample values (on a dry basis)

1
is — 0.1 % with an imprecision (95% confidence interval) of +1.1% of the mean

calorific value.

The effects of processing on sample composition were determined by intercomparison of flow calorimetric results on kilogram-size samples of processed

and minimally processed MSW (150 mm or less particle size) that are nominally

identical. The average difference of the unprocessed minus processed values (on

a dry basis) is -0.5% with an imprecision (95% confidence interval) of +2.9% of

the mean calorific value.

1 . Introduction

The work described in this paper was performed to assist in resolving an

industrial problem formulated by members of the ASME Research Committee on

Industrial and Municipal Wastes, the ASME Performance Test Code Committee (PTC33) on Large Incinerators, and the ASTM Committee (E-38) on Resource Recovery.

Incinerator-boiler systems which are used to dispose of municipal solid waste

(MSW) are bought and sold on the basis of thermal specifications. To determine

whether a system meets its thermal specifications, the calorific value of the

input waste stream fed to the system must be known. At present, the calorific

value is determined at commercial test laboratories using gram-size samples of

the waste.

It has been the opinion of many combustion engineers that one cannot

accurately sample a large, multiton quantity of MSW down to one or two grams and

obtain a representative test sample. The calorific value determined from such a

small sample would be suspect. Gram-size samples must be prepared from the bulk

MSW by separation and milling techniques to provide a powdered test sample of

approximately 2 mm or less particle size. Many combustion engineers feel that

this processing may significantly alter the composition of the test sample and,

hence, the calorific value. This might ocour because of changes in composition

2
due to excessive local heating associated with some milling operations or due to

sampling and handling a material which is inhomogeneous and segregates easily.

The general question of whether or not the calorific value of a multiton

quantity of MSW can be obtained by sampling down to one or two grams can be

broken into two parts. The first part iss How should one sample a multiton pile

or stream of raw waste in order to obtain increments of sufficient number and

size to characterize the calorific value with a given precision? The second

part is: How can one obtain representative gram-size samples that characterize

these increments? The various issues in the dispute about the validity of the

current method for determining the calorific value of MSW are primarily concerned with the second of these questions.

To address these issues, the National Bureau of Standards (NBS) initiated a

research program in collaboration with the U.S. Department of Energy to develop

test procedures for accurately determining the calorific values of refuse and

refuse-derived fuels (RDF). This research also was part of the NBS response to

the Resource Conservation and Recovery Act of 1976. Subtitle E of this legislation mandated NBS to provide guidelines for the development of specifications

for classification of materials destined for disposal.

NBS made a commitment to develop a calorimeter to accommodate a sample of

MSW which is larger than that currently used in commercial test laboratories.

In addition the MSW sample should be processed as little as possible. After

many discussions with combustion engineers and statisticians, we decided that an

acceptable sample size was about 2.5 kg and that the sample would be derived

from dried RDF-2^ + \ In terms of the general sampling question, cited

^

RDF-2 is MSW that has been processed to reduce the particle size so

that 95 mass-percent passes through a 15 cm square mesh screen.

3
previously, the sample size adopted for the oalorlmeter Is assumed to be a

oonvenlent fraotlon of a sampling increment for the waste stream.

For safety oonsiderations, we ohose to build a oalorlmeter for the oombustion of samples in flowing oxygen near atmospherlo pressure rather than to soale

up the conventional high pressure oxygen oombustion bomb. After the 1880's, the

development of the flow technique was discontinued in favor of the simpler

procedures and more quantitative results which could be obtained with the bomb

calorimeter. As a consequence, the first goal of this project was to demonstrate that the oxygen flow technique could be used to obtain complete combustion of MSW.

To test the oxygen flow technique, a combustion flow calorimeter [1,2] was

built and used to successfully combust 25 g pellets of RDF-4^ + \ Combustion of

RDF which has been compressed into a pellet rather than left in loose form was

adopted because this configuration simplifies quantitative collection of ash and

prevents dispersion of the sample away from the main reaction zone. The amount

of carbon in the ash and the uncertainty in the amount of CO in the combustion

products contributed errors of less than 0.1$ to the enthalpy of combustion.

After an initial study of the variability of MSW was carried out at NBS in

1980 [ 3 ], it was concluded that a calorimeter which could burn a kilogram-size

sample with a total uncertainty of less than one percent in the enthalpy of

combustion would satisfy our requirements.

Prior to designing a large-scale combustion flow calorimeter, the burning

characteristics of kilogram-size pellets of RDF-2 in oxygen were determined [4].

^

RDF-4 is MSW from which the metal has been removed and then processed

to reduce the particle size so that 95 mass-percent passes through a 2 mm

square mesh screen.

4
The burning characteristics were unknown and were expected to be different from

RDF-4. The physical heterogeneity, measured against sample size, and variability of composition of RDF-2 pellets are greater than those of RDF-4 pellets.

The goal of these experiments was to develop a method of controlled burning of a

2 to 2.5 kg pellet at a rate of 15 minutes or less per kilogram. This time limit

was the estimated maximum time which could guarantee that the imprecision

contributed by the calorimetric measurement would be less than one percent.

A series of eighteen trial experiments were carried out in a prototype

combustor [4]. Burning times of 15 min/kg for RDF-2 were obtained using the

following measures. The sample was enclosed by a crucible to reduce heat loss.

A small, secondary, flow of oxygen in the form of horizontal jets of preheated

oxygen was directed at the sides of the cylindrical pellet. The larger,

primary, oxygen flow was supplied as a diffuse flow that was directed at the

bottom of the sample.

After developing a suitable technique for burning a kilogram-size sample of

RDF-2, a 2.5 kg capacity flow calorimeter was designed, fabricated, and placed

in operation. Calorimetric measurements have been carried out in a study

consisting of 26 experiments designed to resolve part of the dispute about the

current method of determining the calorific value of MSW. A brief description

of the calorimeter, auxiliary measurement equipment, and the main features of a

typical experiment is given in section 2. The measurement protocol is discussed

in section 3* Results are analyzed in section 4. Conclusions are given in

section 5.

2. Experimental Apparatus

2.1 Description of 2.5 Kg Flow Calorimeter and Auxiliary Apparatus

The flow calorimeter consists of two basic parts: a constant temperature

jacket and the calorimeter proper. The latter contains the sample combustor.

5
Heat liberated in the oombuation reaotion is transferred to the stirred water in

the calorimeter vessel, the outermost container of the calorimeter proper. The

rise in temperature of the calorimeter water, after oorreotion for heat exchange

with the Jacket and for heat transport by the flowing gases, is proportional to

the enthalpy of combustion of the sample. The calorimeter is calibrated by

determining the temperature rise produced by combustion of a known mass of

microcrystalline cellulose whose enthalpy of combustion has been established by

bomb calorimetry.

A cross section of the flow calorimeter is shown in Figure 1. The important dimensions and calorimeter properties are listed in Table 1. All metal

components were fabricated from type 316 stainless steel.

The cylindrical compressed sample pellet (nominally 23 cm diameter and

15 cm high), A, is located inside and near the bottom of the combustor, B. The

pellet is supported on a horizontal lattice of nine alumina rods (6 mm diameter)

resting in notches cut in the top edge of a cup-shaped ash pan. The ash pan sits

on the access plate that forms the bottom of the combustor and retains the

residual ash from the burned MSW. An iron fuse wire, having a four turn coil

touching the top of the sample at the center, is stretched horizontally between

two electrodes (shown on either side of the sample in Figure 1). The sample is

ignited by passage of electrical current through the fuse wire.

Flowing oxygen is supplied to the sample by an array of nozzles arranged in

five horizontal tiers. Each tier contains six symmetrically spaced nozzles

located in the combustor wall. The bottom tier supplies oxygen through slots in

the ash pan to form a diffuse stream that is directed at the bottom of the

sample. The other tiers supply narrow jets of oxygen that are either directed

at the sides of the sample or into the space above the sample. The oxygen of

the top four tiers is preheated by passing it through coils wound around and

6
Figure 1. Diagram of the 2.5 kg capacity flow calorimeter. A denotes the

sample pellet, B the combustor, C the combustor enclosure, D the exhaust

coil, E the collector, F the flow shield, G the calorimeter vessel, H the

stirrer, I the submarine jacket, J the jacket water, K a quartz thermometer,

L the window, M the quick-cool heat exchange system, and N the horoscope.

7
Table 1. Specifications for the 2.5 kg capacity flow calorimeter

Dimensions

Component Height

(cm)

Outside

Diameter

(cm)

Wall

Thickness

(mm)

Total

Mass

(kg)

Combustor 155 41 3.2 74

Combustor

Enclosure

163 61 4.8 196

Flow Shield 213 79 1.6 73

Calorimeter

Vessel

236 91 4.8 390

Submarine

Vessel

243 97 4.8 454

Additional Specifications

Calorimeter water 980 liters

Jaoket water 2160 liters

Heat capacity of

calorimeter 0.4 MJ/K

4.1 MJ/K

(assembly)

(water)

4.5 MJ/K (total)
welded to the outside of the combustor wall.

The combustor is surrounded by a concentric cylindrical vessel, the

combustor enclosure, C. The space between B and C contains tubes that supply

oxygen to the combustor, thermocouples mounted on the combustor wall, tubes (M)

of a quick-cool heat exchange system (also welded to the outside of the

combustor), and argon gas to reduoe corrosion of these components.

The product gas leaves the top of the combustor and then passes downward

through a ten-turn heat-exchange coil, D, then through the collector, E, and

finally through the exit tube out of the calorimeter. The heat exchange coil

transfers heat from the product gas to the calorimeter water. Most of the water

formed in the combustion reaction condenses and is trapped in the collector. The

calorimeter water is directed past the heat exchange coils by means of a flow

shield, F. Water is drawn through the bottom port (20 cm diameter) in the flow

shield, past the coils, and is mixed by stirrer blades, H, mounted in the top

port of the shield. Water returns to the bottom port via the annular space

between the calorimeter vessel, G, and flow shield at a nominal water flow rate

of 380 liters per minute (1pm). Thus, the calorimeter water makes a complete

circuit of the flow path every 2.5 minutes.

The calorimeter vessel is surrounded by a concentric cylindrical vessel,

the submarine vessel I, which constitutes the innermost portion of the constant

temperature jacket. The 2.5 cm air space between the vessels reduces the heat

exchange between the jacket and the calorimeter proper. The submarine vessel is

submerged in the stirred water, J, of the jacket. The calorimeter water

temperature is monitored by a quartz oscillator thermometer, K. A similar

thermometer monitors the temperature of the jacket water. Temperatures of the

inlet oxygen supply and the product gas are monitored with thermocouple probes

whose junctions are located in the gas tubes at the level of the calorimeter

9
vessel lid.

The combustion reaction is monitored visually through the window, L, sealed

to the inline port of a tee in the product gas line (at the top of the oombustor

in Figure 1) by means of the horoscope, N, which extends through the hollow

stirrer shaft. The horoscope is attached to a TV camera which is connected to a

video monitor-recording system.

From the calorimeter vessel, the product gas passes through a mixing

chamber and flowmeter. Just beyond the flowmeter, a portion of the gas is

continuously withdrawn for analysis. Part of this gas is dried and passed

through dedicated infrared detectors for CO and C02

. A second part is passed

through a scanning infrared detector used to monitor various trace components

(hydrocarbons, S0 2

, HC1, etc.). The remaining part of this gas is passed

through a cooled-mirror type of automatic dew point detector to monitor water

vapor. These detectors in combination with three manometers and six thermocouples (to monitor gas temperatures) are used to determine the composition of

the product gas.

A dual minicomputer based data logging and computation system is used to

monitor the outputs of a total of 33 thermocouples, two quartz thermometers,

eight flowmeters, and seven instruments in the product gas analysis train.

2.2 Description of a Typical Experiment

Before ignition of the sample, the calorimeter water temperature is

monitored for one hour after the drift rate becomes constant. During this

initial drift period and in a similar length final drift period after the

combustion reaction, the calorimeter water is pumped through the quick-cool heat

exchange tubes (M of Figure 1) in the combustor wall by a gear pump mounted in

the calorimeter water. This increases the rate of temperature equilibration of

the combustor with the calorimeter water and insures that the calorimeter proper

10
is more nearly isothermal during the drift periods.

Ten minutes prior to ignition, the supply of water to the quick-cool heat

exchange system is turned off (i.e. water from the gear pump is returned

directly to the calorimeter vessel) and the water remaining in the quick-cool

system is blown out with air. The combustor is flushed with oxygen. During the

combustion, oxygen is supplied from a manifold containing eight 6200 liter (STP)

standard oxygen tanks and six mass flow controllers. Oxygen is supplied to the

combustor, the underside of the viewing window (to prevent water condensation),

and the product gas as it leaves the collector. The last supply reduces the dew

point of the product gas below room temperature in order to prevent water from

condensing in the analysis train. Typical flow rates are 150 1pm to the

combustor, 10 1pm to the viewing window, and 110 1pm to dilute the product gas

for a total flow of 270 1pm.

Sample burning times were approximately 32 minutes for RDF-2 and microcrystalline cellulose, and about 42 minutes for RDF-4. The C02

, H

2

0, and CO

concentrations in the product gas during the combustion of an RDF-4 pellet are

shown in Figure 2. The peak and average C02 concentrations over the first ten

minutes of the combustion are 52 and 38 mole percent, respectively. The latter

concentration corresponds to an oxygen supply rate of about 2.5 times stoichiometry. The H

2o concentration reaches a maximum of 2.4 mole percent (i.e. dew

point of 16 °C) at 30 minutes; the abrupt fall in concentration at 42 minutes

indicates that the combustor has been completely flushed of product gas. The

occurrence of two peaks in the CO concentration versus time is typical of all

the RDF samples. In Figure 2, the initial and final maximum CO concentrations

are 0.016 and 0.027 mole percent, respectively. The initial peak always occurs

near the time of the maximum rate of production of C02

* the final peak occurs

near the end of the visual burning when the ash tends to inhibit the combustion.
Figure 2. carbon

CONCENTRATION / PPM

A plot of the concentrations of gaseous carbon dioxide, water, and

monoxide as a function of time for a typical experiment on RDF.

12

ELAPSED

TIME

FROM

IGNITION
The average CO concentration in a cellulose oombustion is about a faotor of

fifty smaller than that shown in Figure 2.

The temperature of the produot gas as it leaves the top of the oombustor

rises to about 400 °C in the first minute of the oombustion, gradually increases

to a maximum of about 450 °C between 12 and 15 minutes after ignition, and then

decays exponentially thereafter. The temperatures of the oombustor wall and of

the oxygen gas as it leaves the preheat ooils inorease more gradually to peak

values at about the same time and deoay similarly. The peak values of the

temperatures of the wall and adjaoent preheat coils within 25 om of the ash pan

are between 600 and 700 °c.

When the temperature of the oombustor base plate has decreased to 220 °C,

the calorimeter water is again oiroulated through the quiok-oool heat exohange

system. This safely removes the appreciable exoess heat stored in the

oombustor. A plot of the calorimeter water temperature vs elapsed time for the

experiment corresponding to Figure 2 is shown by the solid line of Figure 3.

The temperature of the calorimeter water had increased 8 °C after an elapsed

time of 60 minutes when the quiok-cool system was activated. During the next

10 minutes, when most of the exoess heat was removed, the temperature of the

calorimeter water increased by 1.2 °c. This inorease corresponded to 1 3% of the

total heat liberated by the combustion reaction.

After the drift rate of the calorimeter water attains a steady value,

temperature readings are made for an additional hour which corresponds to the

final drift period. The data points in Figure 3 are the deviations of the

calorimeter water temperature from the drift period equation. (The ordinate

scale is -30 to +30 mK.) The set of data points on the right of the figure

indicates that the final drift period begins about 150 to 180 minutes after

ignition. A correction is made for the heat exchange between the calorimeter

13
TOBS. - TCALC. / MILLIKELVIN

3 330 / 3aniVa3dH31 d3iVA a313Hia01V3

Figure 3. A plot of the temperature rise of the calorimeter water plus the

deviation of the observed data points from the smooth curve fit to the

data during the drift periods.

1 4

ELAPSED

TIME

FROM

IGNITION
proper and the jacket during the three hour time interval between the two drift

periods. This correction amounts to about 4f of the observed calorimeter water

temperature rise which was between 6.0 and 9.5 °C for all experiments.

3. Sample Preparation and Measurement Protocol

To resolve the dispute about the validity of the current method of

determining the calorific value of MSW, two problems were addressed. The first

is the sampling problem: How does one obtain a representative gram-size sample

from a kilogram-size quantity of RDF-4? The second is the processing problem:

Does processing MSW down to 2 mm particle size (RDF-4) alter its calorific value

significantly (i.e. one percent or more)?

Both the sampling and processing problems were examined in context of the

variability of MSW for two weeks, a time period appropriate to a specification

test of an incinerator-boiler system. Thirteen to twenty-three kilograms of

RDF-2 were obtained on each of ten consecutive working days (5 days per week for

a 2 week period from January 10 to 21, 1983) from the Baltimore County Resource

and Recovery Facility in Cockeysville, Maryland. Each sample was removed from

the conveyor belt immediately beyond the primary shredder before any further

processing occurred. The material was brought to NBS each day, dried for

12 hours at 105 °C to determine its initial moisture content, and then stored at

5 °C.

Ideally, the processing problem could be solved by comparing the calorific

value of identical pairs of kilogram-size samples of dried RDF-2, one

unprocessed and the other processed. We attempted to prepare such identical

kilogram-size samples by sorting each day's material into eight categories and

then reconstituting the material into 2.5 kg samples having the same relative

mass composition as the whole.

A summary of the results of hand sorting plus the initial moisture content

15
are given in Table 2. The material was hand sorted into metal, oombustible, and

noncombustible fractions. Metals were separated into magnetio and nonmagnetic

components. The oombustible fraotion was separated into wood/vegetable matter,

textiles, plastics, and paper. The noncombustible fraotion was divided into

glass/ceramics and "fines" (primarily grit and sand). The compositions according to main groups are given in lines 10 through 12 of Table 2. Metals were

omitted from the reconstituted material. The combustible and noncombustible

compositions of the samples actually burned in the flow and bomb calorimeters

are listed in the last two rows of Table 2. The sorting and reconstituting of

the material for each day required the efforts of three or more people for

approximately one day.

Four of the ten day's material were selected for our study: days 6 and 9,

which had the largest and smallest noncombustible content, respectively, and

days 4 and 7, which had intermediate noncombustible contents. The bottom row of

each of these days is underlined in Table 2.

Processed MSW consists of a low density component, fluff, which resembles

chopped up cotton fibers, and a high density component which resembles sand or

metal particles. Some high density material is entrained in the fluff; this

entrainment is not uniform. Sampling problems arise because of segregation of

the high and low density components. The sampling technique used in the first

NBS study [3] of the variability of MSW for a two week period in January 1980

consisted of blending (in drums filled with steel balls), coning and quartering,

and taking gram-size grab samples. This method was found to be unsatisfactory.

In that study, the peroent coefficient of variation^, JtCV, of the enthalpy of

( + ) 100 x ( s / average ) , where s is the standard deviation of a single

measurement
Table 2. Composition of municipal solid waste obtained Jan 10-21, 1983, from

the Baltimore County Resource and Recovery Facility.

Mass Percent (Dry)

Day 1 2 3 4 5 6 7 8 9 10

1) Initial Moisture 35.0 28.9 52.7 27.6 29.3 34.3 25.1 30.8 22.0 26.4

2) Metal (Magnetic) 9.1 8.6 6.4 5.0 5.7 6.0 2.6 0.3 2.2 3.9

3) Metal (Nonmagnetic) 1.6 1.3 1.4 1.3 1.9 1.5 1.3 0.9 0.6 1.5

4) Wood, Vegetable 1.4 0.5 0.7 0.8 2.2 1.3 0.4 0.2 2.5 1.5

5) Textiles 2.8 0.6 0.5 1.6 3.2 3.2 5.2 11.4 13.8 4.9

6) Plastics 5.3 4.6 4.6 3.3 5.4 6.2 8.2 12.8 6.6 6.2

7) Paper 58.2 63.0 62.2 66.2 57.9 55.8 67.7 58.6 64.3 69.8

8) Glass, Ceramics 1.0 3.4 2.6 2.6 3.5 2.4 0.8 0.5 0.3 0.9

9) "Fines" 20.6 18.0 21.6 19.2 20.2 23.6 13.8 15.3 9.7 11.3

10) Metals 10.7 9.9 7.8 6.3 7.6 7.5 3.9 1.2 2.8 5.4

11) Combustible Content 67.7 68.7 68.0 71.9 68.7 66.5 81.5 83.0 87.2 82.4

12) Noncombustible Content 21.6 21.4 24.2 21.8 23.7 26.0 14.6 15.8 10.0 12.2

- - - Samples as 1Burned - - - -

13) Combustible Content 75.8 76.2 73.8 76.7 74.3 71.9 84.8 84.0 89.7 87.1

14) Noncombustible Content 24.2 23.8 26.2 23.3 25.7 28.1 15.2 16.0 10.3 12.9

17
combustion on a dry basis was 3*2J for supposed identical gram-size samples of

RDF-4 burned in a bomb calorimeter. The corresponding quantity for a pure (or

truly homogeneous) material should be of the order of 0.151.

For this study, we introduced a new technique for obtaining gram-size test

samples. A Brinkmann Co. Sample Divider PTZ^ was used. It consists of a

central cup from whose bottom surface tubes extend outward and downward to

collector bottles. In the model we used there are eight tubes and eight

bottles. The cup, tubes, and bottles rotate around the central vertical axis of

the cup. The sample is introduced into the divider through a funnel feeding a

horizontal vibrating trough which teases the sample into the divider cup.

An initial assessment of the new sample division technique showed this

method to be superior to the coning and quartering procedure used in the

variability study [3]. This conclusion is confirmed by the results cited in

section 4.

The measurement protocol adopted for our study is summarized in Table 3*

Four bags of the reconstituted kilogram-size samples of RDF-2 were selected from

each of the four days. The contents of two of the bags, designated A and B,

were burned in the large flow calorimeter (Step 2C, Table 3) without further

processing after drying to determine the residual moisture (Step 2k). A known

amount of water was added to each of the samples and then each sample was

compacted to form a pellet (Step 2B). The water was added to improve the

cohesion of the pellet.

^

The commercial sources cited in this paper are included to adequately

describe the experimental procedures. Suoh identification does not imply

recommendation or endorsement by the National Bureau of Standards.

18
Table 3. Measurement protocol

1. Select four 2.5 kg samples from each day.

For example:

Day 4, label samples A4, B4, C4, D4.

2. A4, B4 A) Dry.

B) Add water and pelletize.

C) Burn in flow calorimeter.

3. C4, D4: A) Reduce to 2 mm particle size.

B) Blend, take grab sample (40 grams).

C) Divide to obtain test samples (2 grams each)

D) Recombine remainder of C4 and D4 to obtain

two identical kilogram-size samples.

Label E4, E'4.

E) Test samples: two sets of

two combustion bomb calorimeter,

two residual moisture, and

two furnace ash determinations

(one set for each bag).

4. E4, E'4: A) Pelletize.

B) Burn in flow calorimeter.

19
The contents of the remaining two bags, designated C and D, were processed

to 6 mm sieve size using a Williams Hammer Mill and then to 2 mm sieve size

using a Wiley Mill (Step 3A). A diagram of the measurement protocol for bags C

and D is given in Figure 4. Powdered dry ice was mixed with the material in the

throat of the Wiley Mill to prevent excessive local heating. The resulting

RDF-4 of each bag was then blended in a vee blender and a forty-gram grab sample

was removed (Step 3B). (The grab sample was saved for future use in another

study.) Gram-size test samples were then obtained from the milled, blended

material of bag C and bag D using the Brinkmann Sample Divider (Step 3C). In

Step 3D, the remainder of each bag was divided into two equal parts. One part

of bag C and one part of bag D were combined to form the sample of bag E and

similarly for identical bag E'. Kilogram-size pellets of RDF-2 (Step 2B) and

RDF-4 (Step 4 a) were prepared by placing the sample in a die piece and

compressing it with a total force of 534 kN.

Masses of samples were recorded before, after, and during each step of the

measurement protocol to account for sample loss. Individual measurements in

sets made with either the bomb or flow calorimeter were carried out in a random

order.

In subsequent sections of this paper, the labels C and D refer to the gramsize test samples withdrawn from bags C and D, respectively, before step 3D in

which the remainder of bags C and D were split and recombined to obtain bags E

and E'. The strategy behind step 3D was to take further advantage of the

improved capability of the Brinkmann Sample Divider to divide the sample into

identical parts. Having prepared identical gram-size samples in Step 3C, we

prepared identical kilogram-size samples in Step 3D. Hence, differences in the

enthalpy of combustion of each pair of E and E' samples for a given day afford a

direct measure of the measurement precision of the new calorimeter on an actual

20
| Bag C, RDF

| Bag D, RDF

|

(3A)

J

(3A)

| RDF4 | RDF4 |

I '”1

blended RDF

(2.5 kg)

blended RDF

(2.5 kg)

(3B)

[

(3B)

]

grab sample

(40 grams) j remainder

j (30

1

j remainder| grab sample

(40 grams)

1

( •ar'i

1

test samples

(2 grams) 2 each:

calorific value

residual moisture

furnace ash

|

remainder

|

j"

(3D)

remainder

(3D)

test samples

(2 grams) 2 each:

calorific value

residual moisture

furnace ash

[ pellet E |<-

(4B)

(4A) t --T T-- T (4A)

sample E

~2. 5 kg

sample E'

-2.5 kg

>| pellet E'j

(4B)

flow calorimeter flow calorimeter

Figure 4. Diagram of the sample division technique used to prepare gramsize test samples (C and D) for the bomb calorimeter studies and

kilogram-size samples (E and E') for the flow calorimeter studies from

2.5 kilogram hand sorted and reconstituted samples in bags C and D.

The steps given in Table 3 are indicated in the parentheses along the

arrows

21
sample of RDF (as distinct from a sample of pure cellulose which does not

contain noncombustible components). While labels E, E', A and B refer to kilogram-size samples, the samples of bags E and E' differ from those of bags A and

B in two important respects. Bags E and E' contain processed material (i.e.

RDF-4) while bags A and B contain unprocessed MSW (i.e. RDF-2). The samples in

bags E and E' are essentially exact replicates while those in bags A and B are

only approximately identical (i.e. to the extent that hand sorting into general

classifications given in Table 2 permits).

4. Experimental Results

The experimental results are given in Tables 4, 6, 7, and 9. Three important conclusions are drawn from these results. The first is concerned with the

improvement in results obtained with the new sampling technique as compared to

the coning and quartering technique used in the earlier variability study [3]»

This is discussed in the analysis of the gram-size test results in Section 4.1.

The second conclusion concerns whether or not gram-size test samples represent

their kilogram-size parent samples of RFD-4. This is discussed in the comparison of flow and bomb calorimetry results in Section 4.2.1. The final conclusion

concerns whether or not processing of RDF-2 to RDF-4 alters the calorific value.

This is discussed in Section 4.2.2.

4.1 Combustion Bomb Calorimetry

The gram-size test sample results are listed in Table 4 for sampling days

4, 6, 7, and 9. They consist of duplicate results for each sample of the

enthalpy of combustion (ASTM Method E7 11-81), bomb ash (dried residue from a

combustion bomb experiment) and furnace ash (ASTM Method E830-81). All results

were converted to a dry basis using moisture contents obtained with ASTM Method

E790-81. In the furnace ash test, the sample is combusted in air in a furnace

at 575 °c for two hours.

22
Table 4.

Sample

C4

D4

C6

D6

C7

D7

C9

D9

Ash and combustion bomb calorimetry results for two-gram

test samples.

Ash (Dry Basis)

Furnace

%

Bomb

%

HHV2

MJ/kg

HHV3-B

MJ/kg

34.45 33.62 14,619 22.022

34.54 33.50 14.555 21.886

32.73 31.54 14.867 21.716

32.30 31.21 14.857 21.598

37.82 34.58 14.678 22.436

37.19 35.67 14.546 22.612

37.67 36.18 14.509 22.735

36.89 35.51 14.714 22.817

27.76 26.78 16.273 22.224

28.30 26.58 16.506 22.480

25.04 24.19 17.254 22.760

25.49 23.13 17.253 22.443

17.89 16.41 18.200 21.772

17.68 16.31 18.221 21.773

17.43 15.84 18.336 21.787

17.55 16.57 18.325 21.966

23
Table 5 summarizes the statistical analysis of the results; the methods are

summarized in references [5] and [6]. The percent coefficient of variation,

$CV, for the between-day, within-day, and within-bag variability are calculated

for furnace ash, bomb ash and calorific values. HHV2 and HHV3 are the negative

of the enthalpy of combustion on a moisture free and a moisture-ash free basis,

respectively. HHV3-B refers to the use of the bomb ash value in the calculations; HHV3-F refers to the use of the furnace ash value in the calculations.

For sake of comparison, the corresponding results for the 1980 variability study

are given in the lower half of Table 5. The total %CV in column 6 is the square

root of the sum in quadrature of the $CV's given in the preceding three columns.

In this work, the within-bag $CV, which corresponds to the combined measurement and sampling error for gram-size samples, is substantially less for all

properties than in the prior variability study [3]. The small value for the

HHV2 within-bag $CV and its close agreement with the within-bag values for

HHV3-B and HHV3-F indicates that the new sample division procedure has reduced

our sampling error for gram-size samples to less than 0.8/6. The within-day $CV

for HHV2, 2.0$, suggests that the extra error introduced in failing to prepare

equivalent kilogram-size samples (C and D) for each day by hand sorting and

recombination is somewhat larger in size than the combined measurement and

sampling error. (The label "within-day $CV" is the between-bags $CV estimated

within days.)

The fact that the within-day and within-bag $CV for HHV3-B are about the

same while the corresponding values for HHV2 are not indicates that the hand

sorting and reconstituting procedure failed to yield equivalent samples for each

day primarily because the initial noncombustible contents were different. The

lower between-day %CV for HHV3-B (or HHV3-F) as contrasted to that for HHV2

suggests that the primary variability in the MSW stream is in its ash (and

24
Table 5. Summary of statistical analysis of ash and combustion bomb

calorimetry results for two-gram test samples.

Components of Variability, $CV

Property

(Dry basis) Average

Within

Bag

Within

Day

Between

Days Total

January 1983 Study (4 days , this work)

Furnace Ash , $ 28.8 1.2 4.1 29-9 30.2

Bomb Ash , % 27.4 1.7 4.8 31.1 31.5

HHV2, MJ/kg 16.108 0.5 2.0 10.9 11.1

HHV3-B, MJ/kg 22.189 0.6 0.6 1.9 2.1

HHV3-F, MJ/kg 22.649 0.8 0.3 2.5 2.6

January 1980 Study (10 days , [3])

Furnace Ash , % 31.8 6.5 23.0 29.8 38.2

Bomb Ash , % 28.9 10.6 15.6 33.7 38.6

HHV2, MJ/kg 15.393 3.2 6.6 11.2 13-4

HHV3-B, MJ/kg 21.664 1.6 1.5 3.7 4.3

HHV3-F, MJ/kg 22.523 5.1 5.4 3.1 8.0

25
initial moisture) content and not the calorific value of its combustible

content. The latter was also a conclusion of the first variability study [3].

The larger average value for HHV3-F as compared to HHV3-B is a reflection

of the fact that the percentage of furnace ash is greater than the percentage of

bomb ash. The data of Table 4 can be used to calculate a value of furnace ash

minus bomb ash of 1.44 + 0.33$. The uncertainty, .33$, is the imprecision of

the mean at the 95$ confidence level

It should be noted that the total $CV reflects primarily the day-to-day

variability and not the variability of the measurement processes being studied

here. The latter is characterized by the within-bag and, to a certain extent,

the within-day $CV. The fact that the between-day $CV is much larger than the

within-bag or within-day $CV's in Table 5 does not mean that the between-day $CV

is the limit to which a two-week supply of MSW (i.e. an incinerator test sample)

can be characterized. Rather it means that each day's properties must be

characterized separately and/or samples should be composited as described by

Mandel and Paule [5].

It should also be noted that neither the within-day or between-day $CV of

this work are strictly comparable to the corresponding parameters of the earlier

variability study [ 3 ] - The values of the within-day $CV of this work have been

deliberately altered by the hand sort and reconstitution procedure. The

between-day $CV is for only four of ten days samples of MSW and the four days

were not selected at random.

^

For this and all other statements labeled "imprecision", we are using

the product of the standard deviation of the mean and the appropriate

Student t factor at the 95$ confidence level.

26
4 . 2 Combustion Flow Calorimetry

Six determinations of the energy equivalent (i.e. apparent heat capacity)

of the flow calorimeter gave a %CV for a single measurement of 0.26%. The

average energy equivalent was 4.509 MJ/K with an imprecision of + 0.012 MJ which

is 0.27% (0.26% x Student t // n ) of the mean.

In the calculation of the flow calorimetry results, we have omitted

several minor calormetric corrections (e.g. for small amounts of CO in the

product gas, for small amounts of unburned sample in the ash, etc.). We do not

expect the absence of these corrections to affect our results or conclusions in

a practically significant manner. For convenience the results of the flow

calorimetry will, henceforth, be referred to as flow results.

4.2.1 Flow Calorimetry of RDF-4.

The flow and bomb results on processed MSW (RDF-4) are summarized in

Table 6. The columns labeled furnace ash, bomb ash, bomb HHV2, bomb HHV3 are

calculated from the averages of the gram-size test measurements on C and D

weighted by the amounts of sample from bags C and D which form the samples in

bags E and E'(.See Table 7.)

The analysis of the data in Table 6 is given in Table 8. The first four

rows show the average and imprecision of the paired differences for values of

flow ash minus furnace ash and flow minus bomb values for ash, HHV2 and HHV3.

The day average rather than each of the paired differences of flow minus bomb

values for ash, HHV2, and HHV3 were used to evaluate imprecisions as the

differences tend to group themselves into within-day sets.

The first two rows of Table 8 show that the average flow ash is 2.7 +1.5$

greater than the average bomb ash and is 2.5 +, 1»9% smaller than the average

furnace ash. These differences are consistent with the relation between the

combustion zone temperatures (i.e. bomb > flow > furnace). They are also

27
Table 6. Flow and bomb calorimetry results on processed MSW (RDF- 4) •

Sample

Ash (Dry Basis), %

Furnace a ' Flow Bomb^ a ' HHV2,

Flow

MJ/kg

Bomb^ a ' HHV3,

Flow

MJ/kg

Bomb' a

E4 33.51 32.78 32.48 14.732 14.723 21.918 21.804

E'4 33.50 32.94 32.47 14.838 14.725 22.126 21.804

E6 37.43 36.17 35.49 14.458 14.612 22.650 22.650

E'6 37.43 36.32 35.49 14.489 14.612 22.752 22.650

E7 26.63 26.23 25.17 16.725 16.821 22.671 22.479

E'7 26.62 26.11 25.17 16.735 16.821 22.648 22.479

E9 17.63 17.20 16.28 18.425 18.272 22.253 21.826

E'9 17.64 17.11 16.28 18.295 18.271 22.070 21.825

v ' Calculated from averages of bomb calorimetry measurements on bags C and D and known mass fractions of C and D used to make

up bags E and E'.

(b) HHV3-B

28
Table 7. Mass Fractions of Bags E and E' contributed by Bags C and D.

Components (Dry Basis)

Bag Bag Mass Fraction Bag Mass Fraction

E4 C4 0.50333 (a) D4 0.49667

E'4 C4 0.49891 D4 0.50109

E6 C6 0.49901 D6 0.50099

E'6 C6 0.49862 D6 0.50138

E7 C7 0.50078 D7 0.49922

E'7 C7 0.50036 D7 0.49964

E9 C9 0.49043 D9 0.50957

E'9 C9 0.49478 D9 0.50522

( a ) Fifth significant figure is carried for computational purposes.

29
Table 8. Analysis of flow and bomb calorimetry results on processed

MSW (RDF- 4).

Property Difference

(Dry Basis)

1) Flow Ash-Bomb Ash, $

2) Flow Ash-Furnace Ash, $

3) HHV2 F10W-HHV2 Bomb, MJ/kg

4) HHV3 Flow-HHV3 (c) Bomb, MJ/kg

Property

(Dry basis) Average

5) Ash, Flow, % 28.11

6) HHV2, Flow, MJ/kg 16.087

7) HHV3, Flow, MJ/kg 22.386

8) Ash , Bomb , % 27.35

9) Ash, Furnace, % 28.80

10) HHV2, Bomb, MJ/kg 16.107

11) HHV3, Bomb, MJ/kg 22.189

^

product of Student t for the 95$

deviation of the average.

^

percent of flow value

(c) HHV3-B

Average Imprecision'1

+0.76 (+2.7$) (b) 0.41 (1.5$)

-0.69 (-2.5$) 0.54 (1.9$)

-0.020 (-0.1$) 0.178 (1.1$)

+0.197 (+0.9$) 0.187 (0.8$)

Components of Variability, $CV

Measurement Between

Error Day

0.3 30.0

0.4 11.3

0.5 1.5

confidence level and the standard

30
consistent with the observed variation in the amount of furnace ash formed as a

function of furnace temperature [7]. The average difference of the flow minus

bomb values for HHV2 is -0.1$ of the average HHV2 flow value. This difference

is not statistically different from zero as the 95$ confidence interval is from

-1.2 to +1.0$. The average difference of the flow minus bomb values for HHV3 is

+0.9$ of the average HHV3 flow value. The difference is statistically different

from zero as the 95$ confidence interval is from +0.1 to +1.7$. However, the

interval misses zero by such a small amount that we do not regard the nonoverlap to be practically important.

The average difference of the flow minus bomb values for HHV3 can be

different from zero even if the HHV2 difference were zero, because, in this work

the average difference of the flow minus bomb ash values is +0.8$ which is 2.7$

of the mean ash value. This causes the HHV3 difference to be +1.1$ even if the

HHV2 difference were zero. This is approximately what is observed in rows 3 and

4 of Table 8 (e.g. the average difference of the flow minus bomb values for HHV3

is 1.0$ larger than the average difference of the flow minus bomb values for

HHV2)

The difference of the flow minus bomb values for HHV2 completes the evidence for the conclusion, drawn in the previous section, that the new sample

division technique provides representative gram-size test samples. The results

of this section show that HHV2 is the same for the kilogram-size parent and the

gram-size test samples; the results of the previous section showed that gramsize test samples from the same bag were practically the same.

The measurement errors given in the fifth through seventh rows of Table 8

are obtained from the analysis of the flow ash, HHV2, and HHV3 using paired

differences from replicate samples E and E' for each day. Between-day variability was computed in the usual manner [6]. The precision of a calorimetric

31
measurement on kilogram-size samples of dry, processed MSW (0.4 to 0.5$) and on

dry cellulose (0.3$) are comparable in magnitude. Confirmation of the assertion

that E and E' are replicate samples is obtained from the fact that the measurement error from the flow ash is small.

4.2.2 Flow Calorimetry of RDF-2

The flow results on unprocessed MSW (RDF-2) are summarized and compared

with those on processed MSW (RDF-4) in Table 9. Results on the unprocessed

samples A and then B for each day are given in columns headed "UNPR"; those on

the processed samples E and then E' for each day are given in columns headed

"PR". An analysis of the data in Table 9 is given in Table 10. Average values

and imprecisions in the differences of the unprocessed minus processed values

for ash, HHV2, and HHV3 are given in the first three rows of Table 10. All

differences have 95$ confidence intervals that overlap zero and, thus, are not

significantly different from zero (at this confidence level). The average

effect of processing kilogram-size samples of RDF-2 to RDF-4 is such as to

increase HHV2 by 0.5$ and leave HHV3 unchanged. The 95$ uncertainty intervals

for the HHV2 difference, -3*4 to +2.4$, and the HHV3 difference, -2.0 to +2.0$,

are larger than those for the differences of the flow minus bomb values given in

Table 8 because hand sorting and reconstituting, as we carried it out, did not

produce completely identical bags.

Our hand sorting process was subjective because personnel assisting in this

task varied and composite materials (e.g. insulated wire or some types of shoes)

were not separated into components prior to being assigned a category. Moreover, finer distinctions that might be significant calorimetrically were not

made (e.g. distinction between hard and soft plastics or cotton and artificial

fiber textiles). It is interesting to note that the day-averages of the percent

ash content listed in Table 6 bear the same relation to each other as the total

32
Table 9. Flow calorimetry results on unprocessed (UNPR) and processed (PR)

MSW.

Ash (Dry Basis), % HHV2, MJ/kg HHV3, MJ/kg

UNPR (a) PR (b) UNPR (a) PR (b) UNPR (a ) pR (b)

33.19 32.78 14.370 14.732 21.507 21.918

34.39 32.94 14.306 14.838 21.806 22.126

36.06 36.17 14.432 14.458 22.570 22.650

39.00 36.32 14.315 14.489 23.466 22.752

26.07 26.23 16.976 16.725 22.961 22.671

24.00 26.11 17.039 16.735 22.418 22.648

16.97 17.20 18.350 18.425 22.100 22.253

18.06 17.11 18.250 18.295 22.272 22.070

^

results

^

results

for each day are sample A and

for each day are sample E and

then B.

then E'.

33
Table 10. Analysis of flow calorimetry results on kilogram-size samples of

processed and unprocessed MSW.

Unprocessed-Processed

Property Differences

(Dry Basis) Average Imprecision^

1) Ash, $ +0.36 (+1.3$) (b) 1.70 (6.0$)

2) HHV2,. MJ/kg -0.083 (-0.5$) 0.471 (2.9$)

3) HHV3, MJ/kg +0.001 (+0.0$) 0.446 (2.0$)

Components of Variability, $CV

Unprocessed Property

(Dry Basis) Average

Within

Day

Between

Day

4) Ash, $ 28.47 4.9 31.4

5) HHV2, MJ/kg 16.005 0.3. 12.3

6) HHV3, MJ/kg 22.388 1.7 2.3

^

product of Student

and the standard

t for the 95% confidence level at 3 degrees of freedom

deviation of the average

^

percent of average

34
percent of nominally noncombustible contents listed in Table 2, line 14. However, the percent ash content is larger in absolute magnitude than the value

given in line 14 by 9*1 + 2.4$. This is consistent with the fact that all

components of our nominally combustible category produce some ash.

Within-day and between-day components of variability of the data for unprocessed samples are given in rows four to six of Table 10. Comparing these

values with the corresponding bomb values listed in Table 5 shows the within-day

$CV for HHV2 and HHV3 are in general agreement.

4.3 Summary of Statistical Results

(1) Replicate kilogram-size samples (i.e. samples E and E') of processed

MSW (RDF-4) have been prepared for the first time by a new sample division

technique.

(2) The differences in the enthalpy of combustion (on a dry basis) of

replicate samples E and E' provided a $CV for a single measurement with the new

flow calorimeter on an actual sample of RDF of 0.4$. (See Table 8.)

(3) The new sample division technique provided gram-size test samples, from

the same kilogram-size parent sample, that have a $CV for a single measurement

of 0.5$ for the enthalpy of combustion (on a dry basis). (See Table 5.)

(4) The new sample division technique provided gram-size test samples from

kilogram-size samples of RDF-4 which have the same average enthalpy of combustion (on a dry basis) as the kilogram-size parent sample to within 0.1$. The

95$ confidence interval for the average difference in the enthalpy of combustion

of parent minus gram-size test samples ranges from -1.2 to +1.0$. (See Table 8,

line 3»)

(5) Processing of kilogram-size samples of MSW having a sieve size of minus

15 cm (RDF-2) down to a 2 mm sieve size (RDF-4) alters the enthalpy of combustion of the material (on a dry basis) by less than 0.5$. The 95$ confidence

35
interval for the average difference of the unprocessed minus processed values is

from -3*4 to 2.4$. The main contribution to this confidence interval is the

variability in the supposed identical samples prepared by hand sorting. (See

Table 10.)

(6) Kilogram-size samples of dried RDF-2 having the same enthalpy of combustion (on a dry basis) to about 2% can be prepared by matching the relative

mass compositions according to the following categories of: metals, textiles,

plastics, wood/vegetable matter, paper, glass/ceramics, and "fines" (residual

grit and sand). (See Tables 5 and 10.)

5. Discussion and Conclusions

As indicated in the introduction, this work was performed as part of the

effort to resolve the general question of whether or not the calorific value of

a multiton quantity of MSW can be obtained by sampling down to one or two grams

and obtain representative test samples. This general question can be divided

into two parts. The first is: How should one sample a multiton pile or stream

of raw waste to obtain a number of kilogram-size increments of sufficient number

and amount to characterize the calorific value with a given precision? The

second is: How can one obtain representative gram-size test samples that characterize these increments?

The first question is currently being investigated by ASTM Committee

E-38^ and is beyond the scope of this paper.

The sample preparation and measurement protocol of this work were designed

The subject and principles of the design of a sampling protocol, based

on experimental test, are addressed in ASTM Method D 2234-76, Standard

Method for the Collection of a Gross Sample of Coal. A variant of this

method, as applied to MSW, is under study by the ASTM Committee E-38.

36
primarily to answer the second question. We assumed that the sample increment

was, or could be, conveniently divided into bags containing between 2 and 2.5 kg

of minimally processed MSW (RDF-2). To answer the second question, two problems

were studied.

The first problem was the processing problem: Does processing of RDF-2 (MSW

of 150 mm or less particle size) down to RDF-4 (MSW of 2 mm or less particle

size) alter the calorific value of the waste?

The second problem was the kilogram-to-gram sampling problem: How does one

obtain a representative gram-size test sample from a kilogram quantity of RDF-4?

Our results show that processing from 150 mm to 2 mm particle size does not

alter the calorific value of the MSW within the uncertainty of our experiments.

The average of the unprocessed minus processed difference in the calorific value

on a dry basis is -0.5$ and on a dry, ash-free basis is 0.0$ of the corresponding mean calorific value. The imprecision in these differences are 2.9 and

2.0$, respectively of the corresponding mean calorific value. Most of the

imprecision is due to the fact that completely identical pairs of samples were

not prepared by our hand sorting and reconstituting of RDF-2. An important

qualification is that excessive local heating of the RDF-2 during milling was

avoided. In our case, powdered dry ice was added to the sample as is it was fed

to a Wiley Mill in the final step of processing.

In regards to the kilogram-to-gram sampling problem, our results show that

it is possible to obtain gram-size test samples that have the same calorific

value as the kilogram parent of RDF-4 if one uses the new sample division

technique described in this work. We show that different gram-size test samples

taken from a single parent kilogram-size RDF-4 sample have the same calorific

value as the parent within the uncertainty of our experiments. The average

difference of the parent minus gram-size sample calorific values is -0.1$ on a

37
dry basis and +0.9J on a dry, ash-free basis of the corresponding mean calorific

value. The imprecisions of the differences are 1.1 and 0.8J, respectively, of

the corresponding mean calorific values.

The new sample division technique was successful because it is insensitive

to the effects of segregation whereas our earlier oonlng and quartering method

was not. Beoause the technique employed in this study is time consuming, it is

reoommended only as a reference procedure. An apparatus with more rapid

throughput is needed for routine testing. We definitely do not recommend coning

and quartering plus taking of grab samples as a technique for the preparation of

gram-size test samples of RDF-4. However, it can be used if a within-bag JCV in

the calorific value (on a dry basis) of three percent or more is tolerable.

In summary, this work shows that if 2.5 kg increments are collected to

characterize the calorific value of a MSW stream, each increment can be

processed (provided that excessive local heating is avoided) and then be sampled

using the sample division technique cited here (or any other procedure

sufficiently insensitive to segregation) to obtain representative gram-size test

samples. Combining^ the results on the processing to small particle size (see

Table 10) and kilogram-to-gram sampling problem (see Table 8), we show that the

average difference in the calorific values of the minimally processed (RDF-2)

Averages of the results on each basis (i.e. dry or dry, ash-free) were

obtained by adding; the imprecisions were obtained as the square root of

the sum of the squares, i.e.

Dry:

Dry, ash free:

38
kilogram-size sample minus the processed (RDF-4) gram-size sample is -0.651 on a

dry basis and +0.956 on a dry, ash-free basis of the corresponding mean calorific

values. The imprecisions in these values are 3.1 and 2.256, respectively, of the

mean calorific values.

6. References

[1] Reilly, M.L., Churney, K.L., Kirklin, D.R., Ledford, A.E., and Domalski,

E.S., "An oxygen flow calorimeter for kilogram-size samples of municipal

solid waste. Part I. A 25 gram capacity combustion flow calorimeter for

determining the calorific value of refuse-derived fuels", Resources and

Conservation, Q, 147 (1982).

[2] Domalski, E.S., Churney, K.L., Reilly, M.L., Kirklin, D.R., Ledford, A.E.,

and Thorton, D.D., "25 gram capacity combustion flow calorimeter",

NBSIR 84-2457, U.S. National Bureau of Standards, Washington, D.C. (1982).

[3] Kirklin, D.R., Colbert, J.C., Decker, P.H., Ledford, A.E., Ryan, R.V., and

Domalski, E.S., "The variability of municipal solid waste and its relationship to the determination of the calorific value of refuse-derived fuels",

Part A of NBSIR 82-2491, U.S. National Bureau of Standards, Washington, D.C.

( 1982 ).

[4] Ledford, A.E., Ryan, R.V., Reilly, M.L., Domalski, E.S., and Churney, K.L.,

"An oxygen flow calorimeter for kilogram-size samples of municipal solid

waste. Part II. Trial combustions of kilogram-size samples of municipal

solid waste", Resources and Conservation, 8, 159 (1982).

[5] Mandel, J., and Paule, R.C., "Statistical analysis of sampling and measurement errors in the characterization of refuse-derived fuel", Proceedings of

the Second International Conference on Materials and Energy from Refuse,

Antwerp, Belgium, p6.25 (1981).
[6] Mandel. J., and Paule, R.C., "Interlaboratory evaluation of a material with

unequal numbers of replicates", Analytical Chemistry, 1+2:119^ (1970) and

Correction, Analytical Chemistry, U3, 128? (1971).

[7] Kirklin, D.R.

,

Domalski, E.S., Kelly, R.V.

,

and Robbins, C.R., "Ash content

and X-ray analysis of selected RDF and coal samples as a function of temperature", Resources and Conservation, 9, 2^3 (1982).

7. Acknowledgements

Funding for this project was provided by the U. S. Department of Energy

(formerly Urban Waste Technology Branch) Office of Renewable Technology,

Energy from Municipal Waste Division (during FY 1981, FY 1982, and FY1983)

and the NBS Office of Recycled Materials (during FYI98I and FY1982).

Uo
NBS-114A (rev. 2-<0)

S. Publication D»t*

February 1981

U.3. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See Instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 84-2825 (DoE)

2. Performing Organ. Report No,

4. TITLE AND SUBTITLE 1 Assessing the Credibility of the Calorific Value of Municipal. Solid Waste

5 AUTHQR(S)

K.L. Cnurney, E.S. Domalski, A. E. Ledford, Jr., J.C. Colbert, S.S. Bruce, T.J. Buckley,

JL£ Pauls, M. L. Reilly

6. PERFORMING ORGANIZATION (If joint or other than NBS. see Instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON. D.C. 20234

7. Contract/Grant Nolask Order

DE-AI01 -76PR0601 0 , A146

I. Type of Report & Period Covered

INTERIM

I Oct 1980 - 30 Sep 1983

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

U.S. Department of Energy - Office of Renewable Technology,

Energy from Municipal Waste Division - Forrestal Building

1000 Independence Avenue, S.W.

Washington, D.C. 20585

Fundign provided by NBS Office of Recycled Materials (FY1981 and FY1982) and

U.S. Department of Energy, Office of Renewable Technology, Energy from Municipal

Solid Waste Division (FY19B1, FY19B2, and FY19831.

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant Information. If document Includes a significant

bibliography or literature survey, mention, it here) - * A study has been made at the National Bureau of Standards to establish the limits

of reliability of the calorific value of municipal solid waste (MSW) by the bomb calorimetric procedure currently used in commercial test laboratories. This procedure involves using gram-size samples derived from MSW that has been processed down to a particle size of 2 mm or less. Critics of the procedure argue that gram-size samples are too small to be representative of such a heterogeneous material, and that processing MSU

alters its composition.

To test the bomb calorimetric procedure, a 2.5 kg capacity combustion flow calorimeter was designed and constructed for the determination of the enthalpies of combustion

of kilogram-size samples of MSW in flowing oxygen near atmospheric pressure.

Calorimetric data on processed MSW were obtained using both the large flow and a gram-size bomb calorimeter. Intercomparison of results shows that the calorific value

(on a dry basis) of gram-size test samples agrees, within the uncertainty of our experiments, with the corresponding values for their kilogram-size parent samples provided

that the sample division technique used to obtain the gram-size samples is that describe^

in this work. The average difference of the parent minus gram-size sample values (on a dry basis) is -0.1% with an imprecision (95% confidence level) of + 1.1% of the mean calorific value.

The effects of processing on sample composition were determined by (continued)

12. K£Y WORDS (Six to twelve ep tries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) calorimetry; calorific value; enthalpy of combustion; higher heating value, municipal

solid waste; refuse derived fuel

13. AVAILABILITY

PCI Unlimited

| | For Official Distribution. Do Not Release to NTIS

| | Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

[XI Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

50

15. Price

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USCOMM-OC S043-PS0
intercomparison of flow calorimetric results on kilogram-size samples of

processed and minimally processed MSW (150 mm or less particle size) that

are nominally identical. The average difference of the unprocessed minus

processed values (on a dry basis) is -0.5% with an imprecision (95% confidence

level) of + 2.9% of the mean calorific value.