Trial Combustions of

Kilogram-Size Samples of

Municipal Solid Waste

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Chemical Thermodynamics Division

and the

Temperature and Pressure Measurements and Standards Division

Washington, DC 20234

June 1 983

Prepared for U.S. Department of Energy

Office of Energy from Municipal Waste

Forrestal Building

1000 Independence Avenue, SW

Washington, DC 20585

JUi a 1 I jy 6

NBSIR 83-271 1 CL C loo

TRIAL COMBUSTIONS OF

KILOGRAM-SIZE SAMPLES OF

MUNICIPAL SOLID WASTE

A. E. Ledford, R. V. Ryan, M. L. Reilly, E. S. Domalski, and

K. L. Chu'rney

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Chemical Thermodynamics Division

and the

Temperature and Pressure Measurements and Standards Division

Washington, DC 20234

June 1 983

Prepared for U.S. Department of Energy

Office of Energy from Municipal Waste

Forrestal Building

1000 Independence Avenue, SW

Washington, DC 20585

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Trial Combustions of Kilogram-Size Samples of Municipal Solid Waste

A.E. Ledford, R.V. Ryan, M.L. Reilly, E.S. Domalski,

and K.L. Churney

Chemical Thermodynamics Division, Center for Chemical Physics

National Bureau of Standards, Washington D.C. 20234

Abstract

A new calorimeter is being developed at the National Bureau of Standards

to determine the enthalpies of combustion of kilogram-size samples of municipal solid waste (MSW) in flowing oxygen near atmospheric pressure. Experiments were Carried out to develop a prototype combustor in which pellets of

relatively unprocessed MSW can be rapidly and completely burned with minimal

scattering of ash. Pellets of up to 2.2 kg mass with ash contents between 20

and 35 percent have been successfully burned at a rate of 15 minutes per

kilogram initial mass with CO/CO2 ratios not greater than 0.1 percent.

Keywords: combustor; municipal solid waste; oxygen combustion; refuse

derived fuel

.

1

i
Introduction

The Resource Conservation and Recovery Act of 1976 mandates the National

Bureau of Standards (NBS) to publish guidelines for the development of specifications for the classification of materials which can be recovered and would

otherwise be destined for disposal. Of particular national interest is the

desire to produce useable energy from such sources as refuse and refusederived-fuel (RDF).

At present, the calorific value of RDF is determined at commercial laboratories with conventional combustion bomb calorimetric techniques using gramsize samples. Such samples are prepared by reducing the bulk RDF to a powder

of 0.5 mm particle size (RDF-4). Substantial processing is required to obtain

the gram-size sample and there is concern among some combustion engineers that

this processing may significantly alter its calorific value. There are also

questions about sampling procedures and how representative such a gram-size

sample can be of the waste stream.

To address these doubts, NBS has developed a calorimeter to determine the

calorific value of kilogram-size samples of dried RDF-2. RDF-2 is municipal

solid waste which has been processed to reduce the particle size so that 95

mass-percent passes through a 15 cm square mesh screen. Kilogram-size samples

of RDF-2 should represent the properties of the heterogeneous bulk material

more accurately and reliably than the highly processed gram-size samples

currently used in bomb calorimetric determinations.

For safety considerations, we chose to build a calorimeter for the com2
bustion of samples in flowing oxygen near atmospheric pressure rather than to

scale up the conventional high pressure oxygen combustion bomb. Unfortunately, after the 1880's the development of the flow technique to determine the

enthalpy of combustion of solids 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 the NBS project was to

demonstrate that the oxygen flow technique could be used to obtain complete

combustion of municipal solid waste (MSW).

A small combustion flow calorimeter has been sucessfully used to burn 25

gram pellets of the highly processed MSW prepared for combustion bomb experiments [1,2]. 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.

Before the design of a large-scale combustion flow calorimeter could be

undertaken, it was necessary to determine the burning characteristics of the

kilogram-size RDF-2 pellets in oxygen. The composition of the kilogram-size

pellet is more variable and the physical heterogeneity measured against the

sample size is greater than the 25 gram pellet of highly processed RDF-4. As

a consequence its combustion properties were expected to be different.

The goal of the trial experiments was to develop a method for controlled

burning of the sample pellets at a rate of 15 minutes per kilogram or less.

This time limit was estimated to be the maximum time which would guarantee

that the imprecision contributed by the calorimetric measurements would be

less than one percent.

3
Apparatus and Samples

Trial combustions of samples up to 2.2 kg of RDF-2 were carried out in a

burner cooled by convective and radiant heat losses to ambient temperature.

The burner was mounted in a large exhaust hood. The burner design was similar

to that used in the 25 gram experiments in that thermal shields were employed

to keep the hot, reacting, combustion product gases from being cooled by

contact with the outer burner walls. The combustor differed from that used in

the 25 gram experiments in that all of the oxygen was supplied locally to the

sample; no oxygen flowed between the thermal shield and the cool combustor

walls. Oxygen was supplied in the form of high velocity jets which were

directed either at the top of or the side of the sample or both, depending on

the experiment. A diffuse, slower, flow of oxygen was directed upward at the

bottom of the sample.

The combustor, as it appeared in the first of the trial burns, is shown

in cross section in Fig. 1. The unit consists of two chambers. The lower

chamber, which was the cylindrical region enclosed by the lower burner jacket

(E of Fig. 1), contained the sample and two oxygen supply inlets. The sample

was supported on vertical stainless steel pins which fit into the holes of a

perforated stainless steel plate (H). Primary oxygen was supplied to the

bottom of the sample from the inlet I. Secondary oxygen was directed at the

top of the sample by three jets aimed radially and horizontally by the inlet

F. The intent of the sample-oxygen supply arrangement was to mimic as far as

possible the 25 gram flow experiments while maintaining an unobstructed view

of the combustion. Hence, no crucible was used.

4
The thermal shield (B) of the upper chamber and upper burner jacket (C)

sat on a annular steel plate (D) which was supported by three rods (G) that

projected through the lower chamber to the concrete floor. The annular plate

also supported the lower burner jacket which was bolted to the plate around

two-thirds of its circumference.

For access to the lower chamber, one-third of the circumference of the

lower burner jacket was a semi-cylindrical door. Two Pyrex glass viewing

ports (not shown in Fig. 1) were installed in the door so that the course of a

combustion could be observed.

Oxygen was supplied to the primary and secondary inlets in the lower

burner jacket by independent sources, each consisting of three standard 6,200

liter (STP) oxygen tanks equipped with reducing valves and connected in parallel. Flow rates were measured with variable orifice flowmeters. A product

gas analysis train similar to that used in the 25 gram experiments was used to

monitor the CO and CO2 production. The product gases were continuously sampled at a constant flow rate of 5 L/min through a stainless line whose entry

port was centrally located in the top of the upper combustion chamber. The

gases were cooled to room temperature by passing them through a helical heat

exchanger immersed in cold water and then dried by by passing them through a

drying tube containing anhydrous CaSO^. CO and CO 2 concentrations were

measured using two dedicated infrared detectors which were equipped with

appropriate interference filters. The detectors were calibrated with standard

gas mixtures of 0.1, 0.3, 1.6, 1.0 and 2.0 mol% CO in nitrogen and 10 and 20

mol% CO 2 in oxygen respectively. The pressure in the product gas line at the

CO and CO? detectors was measured with a digital capacitance manometer.

5
Temperatures were monitored at as many as twelve different locations

using 18 gauge Type K (Chromel P - Alumel) thermocouples.

The RDF-2 from which the sample pellets were made was obtained from the

Teledyne National Resource Recovery Facility in Cockeysvi 1 le, Maryland where

municipal solid waste of Baltimore County is processed. Lots were withdrawn

at random from the conveyer belt leaving the primary shredder. At NBS, the

large noncombustibles were removed and the remainder was aried in air at 105°C

for 12 to 16 hours. Sample pellets were made by compressing the dried RDF-2

in a cylindrical die piece with a force ranging from 265 to 625 kN (30 to 70

tons). A single compression yielded a pellet with reasonably good adhesion of

the various heterogeneous horizontal layers. This adhesion was not improved

by wetting the material with water prior to pressing. The finished pellet had

a diameter of 22 cm and a height of about 6 cm/kg of sample mass. To test the

effect of increased surface area, three or seven vertical holes were drilled

in some of the pellets using a metal drill and jig to hold the sample. One

cellulose pellet was made by pressing pure cellulose fluff using the same

technique.

In all, eighteen experiments were carried out to test the effectiveness

of: (1) various arrangements of primary and secondary oxygen inlets, (2) preheating the oxygen, and (3) reducing heat losses from the sample by the use of

a crucible and a radiation shield. Thirteen experiments were run with RDF-2

pellets. Five experiments were run with pure cellulose or its equivalent (a

stack of unglazed paper plates).

Changes in the apparatus made as a result of these tests are illustrated

6
by Fig. 2 which shows the configuration of the lower chamber of the combustor

used in the final two experiments. An RDF-2 sample with seven vertical 2.5 cm

diameter holes sat on a horizontal lattice of alumina rods which rested on a

stainless steel support (D of Fig. 2). Two tiers of secondary oxygen inlets

(B and C) were aimed radially inward and horizontally at the side of the

pellet. The lower tier of six inlets was supplied with oxygen that was

preheated by passing it through coiled tubing (F) which was wound on the

outside of the crucible. The upper tier of three inlets was supplied with

oxygen that was preheated by passing it through the coil (I) which was inside

of the crucible. Oxygen was supplied to the bottom of the sample from the

primary oxygen inlet (E). Most of the ash falls through the center of the

inlet to the bottom of the crucible. A radiation shield which just fits the

inside diameter of the annular steel plate (D of Fig. 2) has been placed

© around the crucible to reduce heat losses.

Back flow of the product gases between the crucible side and the lower

thermal shield was prevented by a low upward flow of diffuse oxygen from the

multiport ring inlet (G of Fig. 2). Rectangular openings were cut in the

crucible and thermal shield in order to observe the combustion. A Pyrex

window covered the opening in the thermal shield.

Description of a Typical Experiment

In a typical experiment, the combustor was flushed with oxygen for ten

minutes. The sample was then ignited by passing electrical current through an

iron fuse wire (H of Fig. 2) touching the top of the sample. The flow rates

were adjusted to preselected initial values for the experiment. Primary and

7
secondary oxygen flows were varied to study the effectiveness of different

rates and inlet arrangements. The ratio of diffuse to directed flow was of

the order of 3:1 or greater and the total flow rate ranged between 3.5 and 11

moles per minute. At the end of the burn, the combustor was flushed with

oxygen and allowed to cool. The ash was collected and weighed. The ash

contents of RDF-2 pellets ranged from 15 to 32% of the initial mass and no

unburned organic material was identified in the ash.

In general, the most rapid burning of the sample occurred on the areas

where the secondary jets of oxygen struck the sample. As the secondary flow

velocity was increased, these areas became white hot and the flame became ever

more turbulent. Above a critical flow rate, the sample began to fragment

vigorously with significant scattering of burning matter and ash. As the

temperature of the preheated secondary oxygen was increased, the flow rate at

which these changes occurred was lowered. Intense burning occurred in the

vertical holes, when present. Bright columns of flame were observed above the

holes.

CO production tended to be larger during the combustion of the last

quarter of the sample. The fraction combusted at any time was assumed to be

the ratio of CO 2 produced up to that time divided by the total CO2 production.

The ratio of total CO to CO 2 ranged from 2 to less than 0.1 mol%. The time

required to burn the last quarter of all RDF-2 samples was always longer than

that required to burn the first three quarters of the sample. This was due to

the presence of noncombustibles which tended to inhibit the combustion of the

last quarter of the unburned material. Peak temperatures of the exhaust gases

at the exit stack ranged from 365 to 500°C, depending upon the experiment, and

8
occurred before half of sample had been combusted. The flame temperature

above the sample was determined to be in excess of 1400°C. The peak temperature of the combustor components nearest the sample ranged from 500 to 1200°C

for the top of the primary oxygen supply and 440 to 500°C for varous parts of

the crucible and lower thermal shield. For the most part, only minor surface

corrosion of the these components occurred. Actual ignition of the combustor

components only took place when the burning sample fell from its support and

came into contact with the oxygen inlets.

Summary of Results

The total burn time of RDF-2 pellets was reduced from 77 minutes per

kilogram initial mass in the early runs with the experimental arrangement

illustrated in Fig. 1 to an acceptable 15 min/kg using the arrangement of

Fig. 2. As more than one change in sample and/or combustor configuration was

made in each trial experiment, interpretation of the effects due to the individual changes tends to be somewhat ambiguous. However, we draw the following

conclusions: (1) The introduction of a crucible (having a side wall one half

the height of that shown in Fig. 2) caused a reduction in burn times for RDF-2

pellets of 33%. (2) The introduction of vertical holes reduced burn times

between 10 and 50%. (3) Burn times for a single tier of lower secondary

oxygen inlets (equivalent to C of Fig. 2) were about 50% less than those

obtained using a single tier of upper secondary inlets (which were directed

downward at a 45° angle toward the top edge of the pellet). (4) We found that

cellulose pellets were easily ignited and burned smoothly leaving negligible

ash. Burn times for cellulose samples were up to 50% shorter than those for

RDF-2. Cellulose appears to be a satisfactory potential calibrant.

9
Using the results of this and earlier work [1,2], the design of the final

kilogram-capacity calorimeter has been completed. Assembly started in

January, 1982.

10
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. Res. and Cons.

8: 147-157; 1982.

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

and Thornton, D.D. 25 gram capacity combustion flow calorimeter, NBSIR

82-2457, March 1982.
Fig. 1. The 2.5 Kilogram Combustor. A denotes the stack, B the upper thermal

shield, C the upper burner jacket, D the annular plate, E the lower

burner jacket, F the secondary oxygen inlets, G the support rod, H the

sample support and I the primary oxygen inlet.

Fig. 2. The Configuration of the Lower Chamber of the Combustor for Experiments 17 and 18. A denotes the lower thermal shield, B the upper

tier of secondary oxygen inlets, C the lower tier of secondary oxygen

inlets, D the sample support, E the primary oxygen inlet, F the lower

tier preheat coil, G the multiport ring oxygen inlet, H the iron

fuse, I the upper tier preheat coil and J the crucible lid.
13

Fig. 1.
14 Fig. 2
NBS-114A (rev. 2 -ec) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

4. TITLE AND SUBTITLE

1. PUBLICATION OR

REPORT NO.

NBSIR 83-2711

2. Performing Organ. Report NoJ 3. Publication Date

June 1983

Trial Combustions of Kilogram-Size Samples of Municipal Solid Waste

5. AUTHOR(S)

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

6'. PERFORMING ORGANIZATION (If joint or ocher thanNBS. see instructions) 7. Con tract/ Grant No. national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

EA-77-A-01 -601

0

8. Type of Report & Period Covered

Interim

1 April 1980-31 March 1981

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

U.S. Department* of Energy

Office of Energy from Municipal Waste

10. SUPPLEMENTARY NOTES

Funding equally shared by the NBS Office of Recycled Materials and the DoE Office of

Energy from Municipal Waste

| | 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 new calorimeter is being developed at the National Bureau of Standards to

determine the enthalpies of combustion of kilogram-size samples of municipal solid

waste (MSW) in flowing oxygen near atmospheric pressure. Experiments were carried out

to develop a prototype combustor in which pellets of relatively unprocessed MSW can be

rapidly and completely burned with minimal scattering of ash. Pellets of up to 2.2 kg mass with ash content between 20 and 35 percent have been successfully burned at a rate of 15 minutes per kilogram initial mass with C0/C0

2

ratios not greater than

0.1 percent.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

Combustor; kilogram-size samples; municipal solid waste; oxygen flow combustion

;

refuse-derived fuel.

13. AVAILABILITY 14. NO. OF

XXI Unlimited

1 | For Official Distribution. Do Not Release to NTIS

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