approaches to coupling the design of resource recovery ... · ample, the properties of oil, wood,...
TRANSCRIPT
APPROACHES TO COUPLING THE DESIGN OF
RESOURCE RECOVERY FACILITIES TO PERFORMANCE
SPECIFICATIONS AND ACCEPTANCE TESTING
GEORGE M. SAVAGE and JOHN C. GLAUB Cal Recovery Systems, Inc.
Richmond, California
ABSTRACT
The definition of system performance specifications for resource recovery projects is an important aspect in the development of the contractual documents for securing project fmancing. Historically, little fundamental engineering consideration has been applied to the coupling of the performance specifications to the design of the facility, particularly in terms of the mass flows within the facility and available standard test methods for determining equipment and system performance. The upshot of the lack of coupling is manifested commonly in mismatches among subsystems comprising the overall facility design and in the difficulty of ascertaining equipment performance in terms of the limited number of available test standards. The paper reviews system performance specifications used by the industry, comments on their verification using existing test standards, and offers suggested improvements in defining equipment performance. As part of the review, consensus test standards for the resource recovery industry promulgated by the American Society of Mechanical Engineers (ASME) and by the American Society for Testing and Materials (ASTM) are discussed. Also as part of the presentation, a methodology is described for meshing the system mass balance with the performance specifications, thus coupling the performance specifications to the input waste composition.
INTRODUCTION
A critical aspect of a waste-to-energy project is the definition of performance guarantees and their incorporation into the construction contract. The guarantees serve
65
as an important form of protection for the bondholders and others holding a fmancial interest in the project. If the performance guarantees are met there is some assurance that the projected return on investment will in fact accrue to the lenders and concomitantly that the net disposal fee will be as projected. Consequently, it is important not only to have a well conceived facility design but to assure that the facility, as constructed and operated, performs to its design specifications.
In spite of the importance of meeting the performance projected for the facility, virtually no information exists in the open literature on the subject. A review of the contractual documents and official statements for various waste-to-energy facilities reveals a substantial (although by no means identical) degree of similarity among the performance criteria. Unfortunately, there is a dearth of available published information on the appropriateness and adequacy of the performance guarantees that are being used presently and those that have been used previously by the waste-to-energy industry. Similarly, information pertaining to the conformance of facility construction and operation to design specifications is virtually nonexistent in the published literature.
The specification of performance criteria in the case of waste-to-energy projects fulf1lls a rather large number of functions. The number of functions is large compared to other types of industrial projects due to the substantial degree of public involvement (in the form of ratepayers and bondholders) that normally attends a wasteto-energy project and to the ultimate function of the project, namely disposal and utilization of solid waste in an environmentally safe manner. The functions of the performance guarantees include assurances that:
• the facility is constructed and operates as designed • the facility can process, utilize, and dispose of
specified quantities of waste • the specified rate of recovery and the quality of
energy and of secondary materials will be as projected • the lenders' projected rate of return on their invest
ment in the project will be realized • the charges paid for waste disposal by municipal
ratepayers and priva�e haulers will be as forecasted • the facility meets the governing laws and regulations
at the federal, state, and local levels The performance guarantees for the project are inti
mately connected with the financing and the contractual aspects of the project. Meeting them lends assurance that the specified levels of performance will be met and consequently that the assumed set of circumstances (developed during the formation of the project) will be realized in terms of system capital cost, operating and maintenance costs, tax benefits, project revenues, disposal fees, etc. Moreover, a project that has contractual documents that contain well conceived performance guarantees is likely to secure a favorable bond rating, thus resulting in a lower cost of capital than would be required of a similar project with a poor set of guarantees. Performance guarantees are defined and documented in the facility construction contract, operating contract, or both and their adequacy is reviewed in the official statement when a bond issue is used as a financing instrument for the project.
The performance guarantees that are commonly used in the waste-to-energy industry can be broken down into the general categories that are shown in Table 1. Example descriptions of each general category are included parenthetically. Specific projects may have additional or more rigorous performance criteria than those listed in the table. The listing, however, presents in general terms the performance guarantees that normally are addressed, defined, and specified in the contractual documents.
Attention is also brought to the fact that the category of environmental compliance (item 4) is a special case (or alternatively a subset of item 5 (i.e., compliance with federal, state, and local laws and regulations). However, due mostly to the attention given to the potential air pollution aspects of waste-to-energy facilities, environmental compliance is almost always addressed separately in the performance guarantees.
The performance of the facility is typically monitored (i.e., subjected to acceptance tests) for a period of time after the completion of construction. The time periods range from several days to perhaps as long as a month. The most prevalent period, however, is generally one to two weeks. There is no information available on what constitutes a reasonable testing period.
The history of the development of the general per-
66
formance criteria (i.e., those listed in Table 1) has not been documented in the available literature. In all likelihood the criteria have been borrowed from other publiclyfinanced projects and modified accordingly for solid waste conversion facilities. Subsequently, the criteria have been handed down from project to project, with only modest changes. Unfortunately, by far and away, most processing and thermal conversion facilities that handle non-MSW feedstocks have a relatively homogeneous feedstock; and perhaps most importantly, the properties of the feedstock are well-defined and either vary in a systematic pattern over time or are. virtually independent of time. For example, the properties of oil, wood, and coal can be ascertained with a great degree of certainty since they reside in reserviors in the case of oil and coal and in timber stands in the case of wood.
Solid waste conversion facilities do not share with most other industries the luxury of well-defined or timeindependent properties of the feedstock. This fact is well known and illustrated in Tables 2 and 3, but its implications with respect to defining, specifying, and measuring system performance has not been adequately studied or addressed.
Commonly cited performance guarantees for waste-toenergy projects sometimes neglect the inclusion of certain important criteria. For example, in the case of waste-fired thermal conversion systems, the maximum thermal rating of the boiler requires inclusion in the standard set of system performance guarantees. As will be discussed later, the reason is derived directly from the time varying nature of the composition of the waste. Another, more general need is a statement of the minimum acceptable limits of quality of the recovered products (e .g., allowable degree of contamination in recovered secondary materials, temperature and pressure of steam, etc.). The importance of maximum thermal rating and product quality will be addressed subsequently.
Conformance or nonconformance to the performance guarantees is substantiated through an acceptance test. The acceptance test is performed after the facility equipment has undergone shakedown. Typically, there is a time limit placed upon meeting the performance guarantees. An example is that the guarantees must be met within four years from the date of the initiation of constru.ction. The acceptance test is run by the operator of the facility and may or may not be overseen by the project's independent engineer or by an impartial third party.
Inasmuch as conformance to the performance guarantees must be confirmed through testing, it is obviously necessary that the performance criteria must be measurable, i.e., subject to verification through testing. Since relatively few standard test methods exist in the waste-toenergy industry, it is a difficult task to develop easily
TABLE 1 GENERIC CATEGORIES OF COMMONLY USED PERFORMANCE
GUARANTEES FOR WASTE-TO-ENERGY FACILITIES
1. System capacity (i.e., throughput of waste, tons per unit time).
2. Energy recovery (e.g., kilowatt-hours/ton of MSW, percent recovery of RDF).
3. Secondary material recovery (e.g., percent recovery of magnetic ferrous metal).
4. Environmental emissions).
compliance (e.g., wastewater discharges,
5. Compliance with federal, state, and local laws and regulations (e.g., noise, occupational health and safety).
measurable, and concomitantly meaningful, performance criteria.
In terms of the generic categories of performance guarantees that were mentioned previously, it must be realized that a test program for each of the performance criteria may consist of the application of a number of individual test methods. In the cases of energy recovery and of environmental compliance, a firm foundation of standard test methods exists. For those performance criteria for which no (or incomplete) standard test methods exist (e.g., throughput capacity), test methodologies must be developed for each waste-to-energy project. Needless to say, the latter case usually results in a less than desirable situation inasmuch as the equipment vendors are commonly the only source requested for supplying a test protocol, and obviously they are in a biased position.
The use of ill-conceived performance criteria, poorly documented test methods, or both invites bickering, arbitration, or worse, litigation, during and after the conduct of the acceptance testing.
A complete acceptance test program consists of the development or definition of the test methods to be used and the procedures to be followed, the conduct of the tests, data reduction and analysis, interpretation of the test results, and a judgment of conformance or nonconformance to the performance specifications.
In terms of developing test methods the task is extremely difficult in the case of solid waste, primarily due to the nonhomogeneity and time dependent composition of the material. In particular, the gathering of representative samples of waste for weight determinations and analy-
67
ses is a poorly developed science and has been the object of considerable study and frustration within the solid waste industry.
With regard to conformance or nonconformance to the performance guarantees, some contracts provide for "middle ground" in that partial conformance to the guaranteed performance goals allows the facility to be accepted at a lower level of performance. Usually, penalty provisions or contract adjustments attend the acceptance of the plant on the basis of partial conformance. Among the synonyms for partial conformance are "essential" or "substantial" conformance.
SELECTION OF PERFORMANCE CRITERIA
The development and definition of performance criteria are based on the consideration of a number of factors. Table 4 presents a partial listing of the items that must be identified and judged as to their importance to the wasteto-energy project prior to formal adoption of specific performance guarantees and their incorporation into the contractual documents.
The selection and definition of performance criteria requires an understanding of the fundamental engineering principles governing equipment and system operation and performance and of methods of measurement. The need for an understanding of both cannot be overemphasized inasmuch as cases exist in the industry wherein the performance specifications defined in the contract could not be measured correctly. Reasons for the latter circum-
0- 00
TA
BL
E 2
A
VE
RA
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CH
AR
AC
TE
RIS
TIC
S O
F M
SW
FO
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IFF
ER
EN
T L
OC
AT
ION
S I
N T
HE
UN
ITE
D S
TA
TE
S
Si
te
Ta
co
ma
, W
A
Ba
lt
im
or
e,
MD
Ri
ch
mo
nd
, C
A
Lo
s
An
ge
le
s,
CA
Ak
ron
, O
H
Su
nn
yv
al
e,
CA
Ov
en
-D
ry
M
oi
st
ur
e
Co
nt
en
t
%
35
35
22
21
26
31
Pe
rc
en
t
Fe
rr
ou
s
Me
ta
ls
(a
s-
re
ce
iv
ed
)
6.4
4.0
5.7
3.3
5.7
4.1
Pe
rc
en
t
No
n-
Fe
rr
ou
s
Me
ta
ls
(a
s-
re
ce
iv
ed
)
1.5
1.1
1.2
0.9
1.7
1.0
Pe
rc
en
t
Pa
pe
r
an
d
Pla
st
ic
(a
s-
re
ce
iv
ed
)
39.5
37.9
45.5
26.2
41.5
51.3
Sa
mp
le
s
co
ll
ec
te
d
an
d
an
al
yz
ed
b
y
Ca
l
Rec
ov
er
y
Sy
st
em
s,
Inc
.
Pe
rc
en
t
As
h
(ov
en
-d
ry
)
28.9
24.9
27.8
30.0
25.8
33.1
He
at
in
g
Va
lu
e
(as
-r
ec
ei
ve
d)
Bt
u/
lb
(k
J/k
g)
3990
(9
280)
3000
(6
980)
3820
(8
890)
5580
(1
2980
)
4100
(9
540)
4210
(9
790)
0\
\D
Se
as
on
Au
tu
mn
Wi
nt
er
Sp
ri
ng
Su
mme
r
TA
BL
E 3
A
VE
RA
GE
SE
AS
ON
AL
CH
AR
AC
TE
RIS
TIC
S O
F M
SW
FO
R S
UN
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VA
LE
, C
AL
IFO
RN
IA
Ov
en
-D
ry
M
oi
st
ur
e
Co
nt
en
t
%
34.7
34.1
29.3
24.7
Per
ce
nt
F
er
ro
us
M
et
al
s
(as
-r
ec
ei
ve
d)
6.0
3.4
2.9
4.1
Pe
rc
en
t
No
n-
Fe
rr
ou
s
Me
ta
ls
(a
s-
re
ce
iv
ed
)
0.7
0.9
1.3
0.9
Pe
rc
en
t
Pa
pe
r
an
d
Pl
as
ti
c
(as
-r
ec
ei
ve
d)
57.9
53.8
47.3
46.1
Pe
rc
en
t
As
h
(ov
en
-d
ry
)
26.2
37.4
33.1
35.5
Sa
mp
le
s
co
ll
ec
te
d
an
d
an
al
yz
ed
by
C
al
Rec
ov
er
y
Sy
st
em
s,
Inc
.
He
at
in
g V
al
ue
(a
s-
re
ce
iv
ed
) B
tu
/l
b
(kJ/
kg
)
4510
(1
0490
)
3700
(8
606)
4300
(1
0000
)
4310
(1
0030
)
TABLE 4 CONSIDERATIONS IN THE DEVElOPMENT OF PERFORMANCE
GUARANTEES FOR WASTE-TO-ENERGY PROJECTS
o the technical objectives of the system
o the degree of financial risk to be assumed by those with a financial interest in the project and not in control of the construction and operation of the facility
o the quantities of waste to be handled by the facility
o the quantities of residues generated by the facility and their method(s) of disposal
o the rate of recovery and the quality of the recovered secondary materials
o the rate of energy production and its quality
o the performance of the pollution control equipment
o the conformance of the facility to federal, state, and local laws and regulations
o the temperament of those involved in the project
o the availability of adequate test methods and procedures for measuring equipment and system performance
o the length of time (i.e., the acceptance test period) over which the performance of the facility will be measured
o the limitations of time to be placed on the vendor for meeting the specified levels of system performance
stance include inadequately defined terms, no provisions
for sampling locations within the plant, and no existing
test methods. Another obvious but oftentimes overlooked
aspect of defining performance specifications is the defini
tion of the fundamental system parameters.
An industry-wide illustration of the poor understand
ing of system parameters is illustrated by the fact that
the performance guarantees of a number of facilities
specify a percentage of recovery of magnetic ferrous
metals (e.g., 80 percent of that in the MSW feedstock)
while no limitation is placed on the quality of recovered
material, e.g., the allowable level of contamination. (In
this discussion, quality is taken to include such param
eters as particle size and bulk density as well as allowable
levels of contamination.) For example, in the case of fer-
70
rous recovery the purity of the recovered material (i.e.,
lack of contamination) is fundamental to the market
price secured for the material. Purity is a fundamental
parameter and it must accompany the percent recovery
specification. Lacking a purity specification, it is possi
ble to recover a highly contaminated product yet meet
the recovery percentage.
AVAILABLE TEST METHODS
As mentioned earlier, the definition of the funda
mental performance parameters is only part of the devel
opment of a meaningful set of performance guarantees.
Another important aspect is the existence of methods and
procedures to measure the performance of the equipment
TA�LE 5 APPROVED STANDARD TEST METHODS AND SPECIFICATIONS AVAILABLE
TO THE RESOURCE RECOVERY INDUSTRY
Capacity Throughput
None
Energy Recovery
Title
ASTM
Test for Gross Calorific Value of Refuse-Derived Fuel (RDF-3) by Bomb Calorimeter
Tests for Total Sulfur in the Analysis Sample of RefuseDeri ved Fuel
Test for Forms of Chlorine in Refuse-Derived Fuel
Test for Carbon and Hydrogen in the Analysis Sample of Refuse-Derived Fuel
Test for Nitrogen in the Analysis Sample of RefuseDeri ved Fuel
Test for Residual Moisture in Refuse-Derived Fuel Analysis Sample
Calculating Refuse-Derived Fuel Analysis Data from As-Determined to Different Bases
Designating the Size of RDF-3 from its Sieve Analysis
Preparing Refuse-Derived Fuel-3 Laboratory Samples for Analysis
Ash in the Analysis Sample of Refuse-Derived Fuel ( RDF-3)
Physical and Chemical Characteristics of Refuse-Derived Fuel
Analysis of Metals in Refuse-Derived Fuel ( RDF) by Atomic Absorption Spectrophotometry
Dissolution of Refuse-Derived Fuel ( RDF-3) Ash Samples for Analysis of Metals
Silica in Refuse-Derived Fuel-3 (RDF-3) and RDF-3 Ash
Test for Volatile Matte.r in the Analysis Sample of RefuseDeri ved Fuel-3
71
Designation
E 711-81
E 775-81
E 776-81
E 777-81
E 778-81
E 790-81
E 791-81
E 828-81
E 829-81
E 830-81
E 856-82
E 885-82
E 886-82
E 887-82
E 897-82
TABLE 5 (Continued)
Test for Preparing Refuse-Derived Fuel (RDF-3) Samples for Analyses of Metals
Test Method for Total Moisture in a Refuse-Derived Fuel Laboratory Sample
Test Method for Fusibility of Refuse-Derived Fuel-3 Ash
Test Method for Packaging and Shipment of Laboratory Samples for RDF-3
Test Method for Thermal Characteristics of RDF-3 Macro Samples
ASME
Performance Test Code for Large Incinerators
Performance Test Code for Steam Generating Units
Performance Test Code for Steam Turbines
Secondary Material Recovery
Testing Waste Glass as a Raw Material for Manufacture of Glass Containers
Waste Glass as a Raw Material for the Manufacture of Glass Containers
Testing Municipal Ferrous Scrap
Specifications for Municipal Ferrous Scrap
Specifications for Municipal Aluminum Scrap (MAS)
Classification for Municipal Mixed Non-Ferrous Metals
Energy Recovery and/or Secondary Material Recovery
Conducting Performance Tests on Mechanical Conveying Equipment Used in Resource Recovery Systems
Test for Composition or Purity of a Solid Waste Materials Stream
Test Method for Measuring Electrical Energy Requirements of Processing Equipment
Test Method for Characterizing the Performance of Refuse Size Reduction Equipment
72
E 926-83
E 949-83
E 953-83
E 954-83
E 955-83
PTC 33
PTC 4.1
PTC 6
E 688-79
E 708-79
E 701-80
E 702-79
E 753-80
E 956-83
E 868-82
E 889-82
E 929-82
E 959-83
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and of the system. In this regard certain standard test methods and specifications have been promulgated by the American Society of Mechanical Engineers (ASME) and the American Society for Testing and Materials (ASTM). A listing of approved, consensus test standards and specifications, grouped by generic performance category, is given in Table 5. (The term "consensus" is used because the standards have been developed and approved by committees composed of producers, users, and those with a general interest in the standard under consideration.) As indicated in the table, there are no approved, consensus test methods for capacity (throughput) measurements. The lack of a test method for capacity, a fundamental system performance parameter, is a significant shortcoming which requires the attention of the solid waste industry.
It should be mentioned that a draft standard for the measurement of throughput is being considered by the ASTM. However, the method is directed towards RDF processing equipment and most likely would not prove useful for measuring the throughput of massburn or RDF firing facilities.
Not included in Table 5 are the categories of environmental compliance and compliance with applicable laws and regulations inasmuch as each of these categories has a number of test procedures that have been developed previously by federal and state agencies, e.g., the U.S. Environmental Protection Agency (EPA). For example, in the case of the measurement of particulate emissions, EPA Method 5 exists.
The complexity of developing and subsequently using a test method is illustrated in the case of measuring the composition or purity of a solid waste stream, ASTM E889-82. The method cites seven other documents for the conduct of the measurements, namely:
• C566 Test Method for Total Moisture Content of Aggregate by Drying
• C702 Methods for Reducing Field Samples of Aggregates to Testing Size
• D75 Practice for S�inpling Aggregates • D644 Test Method for Moisture Content of Paper
and Paperboard by Oven Drying • D2013 Method of Preparing Coal Samples for
Analysis • D2234 Methods for Collection of a Gross Sample
of Coal • E380 Metric Practice
EXAMPLES OF FACILITY PERFORMANCE
GUARANTEES
As an insight into the categories of performance guarantees and into the performance criteria that are used
74
by the waste-to-energy industry, it is illustrative to examine a sampling of guarantees that are representative of the industry. A summary of key performance guarantees for massburn and RDF recovery facilities is shown in Table 6. Inasmuch as the point of the illustration is to give the reader an understanding of the extent and magnitude of typical performance guarantees, the facilities are identified alphanumerically and the quantities have been modified.
A review of the guarantees for capacity illustrates the lack of standardization in that the time period for the test ranges from undefined in the case of facility B. l to as short a period as 72 hr for facility B.3. In the opinion of the authors it is difficult to rationalize a time period as short as 72 hr for a waste-to-energy facility that may cost upwards of 100 to 200 million dollars.
A perusal of the guarantees for secondary material recovery shows that with the exception of facility A.2 there are no levels of quality placed upon the recovered products. The need for a quality specification was addressed earlier. Also, as briefly mentioned earlier, the requirement for a guaranteed maximum thermal rating of the boilers (in terms of steam production) has not bee included in the performance guarantees in five out of the six facilities. The lone exception is the set of guarantees for facility A.l .
MAXIMUM THERMA L RATING
The importance of specifying and subsequently measuring the maximum thermal rating of the boilers is derived from the fact that over the life of a waste-to-energy project (perhaps 20 to 30 years) there will almost assuredly be a change in the composition of the waste and, therefore, in its heating value. Data in support of the above assertion comes in the form of the measured increase in refuse heating value experienced in Europe during the 30-year period from 1950 to 1980. Dirilgen and Luthy reported that the increase in heating value averaged approximately 1.7 percent per annum, or cumulatively a gain of about 65 percent over the 30-year period.* Feindler and Thoemen reported a 49 percent increase in heating value over a 15 -year period, or approxima tely 3.3 percent per annum.**
*Dirilgen, N. and R. M. Luthy, "The Wheelabrator-Frye/Von
Roll Approach to Refuse-to-Energy Systems," International
Conference on European Waste-to-Energy Technology, Reston,
Virginia, October 1980. **Feindler, K. S. and K. H. Thoemen, "Completion of the Dues
seldorf Refuse Power Plant," Proceedings of the 1982 National Waste Processing Conference, American Society of Mechanical
Engineers, New York, May 1982.
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o I
Throughput Ratio (m/m*)
FIG.l IDEALIZED RELATION OF REFUSE
HEATING VALUE AND THROUGHPUT CAPACITY
FOR A TYPICAL BOILER
Inasmuch as a boiler is limited in terms of its ability to absorb heat as well as in terms of its throughput capacity, a change in heating value of the waste has repercussions with respect to the operation and performance of the facility. Given a design maximum for the boiler's thermal capability (i.e., steam flow at a specified temperature and pressure), there is a reciprocal relation between refuse heating value and mass flow through the boiler. In other words, if there is an increase in the heating value of the wastes, the mass throughput must be reduced. On the other hand, if the heating value decreases, theoretically the mass throughput would have to be increased to maintain the design level of steam generation. However, the physical limitations on the throughput capability of the boiler may preclude increasing the rate of refuse firing. The upshot of the latter situation is reduced steam production and an increase in material requiring ultimate disposal. Concomitantly, energy revenues decrease and operating costs increase. Moreover, if the facility operator is tied to supplying a specified quantity of steam production, another boiler may be required in addition to the need for acquiring more refuse.
The relation of throughput requirement and heating value of the waste is shown in Fig. 1. The relation shown is an idealized case only and does not include the effect of operational changes that might be .necessary in order to combust waste of a different composition and heating value (e.g., changes in excess air, etc.) The diagonal
75
line represents the limit of the steam raising capability of the boiler. The "*,, superscript refers to the heating value (HV) and throughput (m) corresponding to the maximum design condition of the boiler. The design condition is met when HV /HV* and m/m* are equal to unity. The shaded area corresponds to the possible range of operation of the boiler. If the refuse heating value is greater than HV* (as shown by the dashed line), the m
•
must be less than m*. Conversely, if HV is less than HV* (as shown by the dotted line), then m is limited to the
•
value of m*. To protect the system designer, the guarantees for a
thermal conversion system should include a stipulation that the facility will perform up to a specified thermal rating and up to a specified level of throughput. Should a change in refuse heating value occur, the contract documentation should specify that the throughput specifica� tion governs in the case of a reduction of refuse heating value and that the steam generating specification governs in the case of an increase in the heating value. Consequently, it is important to confirm maximum thermal rating during the acceptance period. If the heating value of the waste used during the acceptance test is lower than the design value, the unit �ould be fired with a supplemental fuel (e.g., oil, gas, or wood waste) in order to confirm the capability of the boiler to meet the design level of steam generation.
The agreements must also specify the manner in which any capacity shortfall will be handled in the event of an increase in the heating value of the refuse.
SPECIFYING PERFORMANCE OF RDF
PRODUCTION FACILITIES
To date, no widely accepted methods or procedures exist for specifying the performance of RDF production facilities nor of their unit operations. In the past, the lack of knowledge of fundamental operating and performance parameters for unit operations has manifested itself in a number of ways. The following are illustrative of common problems:
• operation of equipment under design capacity • greater than projected operating costs • poor separation efficiency • inability of the equipment to perform adequately
under variations in waste composition In an RDF recovery facility, where a number of unit operations exist in series, the poor performance of any piece of equipment is likely to produce a "ripple-effect" of poor performance throughout the remainder of the processing line.
From the standpoint of defining performance guarantees for RDF production facilities, perhaps the single-most
exasperating challenge is to design performance criteria that assures compatibility of the different pieces of processing equipment un,der conditions of varying composition of the input waste. Historically, the solid waste industry has considered refuse as a material in and of itself where in actuality it is a mixture of different material categories and of different particle sizes. Consequently, the properties of solid waste vary with its makeup, and therefore processing equipment must be designed to accommodate or tolerate this fundamental fact. The attitude that solid waste is a singular material is partially responsible for the poor defmition of performance parameters in the past for refuse processing equipment.
One of the classic examples of ill-defmed equipment performance criteria occurred previously in the case of air classification of refuse, where routinely specifications called for a minimum percentage of input material (typically 70 to 80 percent) to report to the "light fraction", i.e., RDF fraction. The preceding criteria has implicit in it the understanding that the input material contains on the order of 70 ,to 80 percent combustible material. A better criteria would require a minimum recovery percentage of various components, e.g, paper, plastic, etc. The primary reason that the latter performance cr-iteria is the appropriate one is the fact that if a desirable component is not present, one cannot expect a piece of equipment to recover it. The former performance criteria (i.e., the requirement that 70 to 80 percent report to the light fraction) presupposes that a given percentage of the waste stream is light fraction in character, a situation that an equipment designer cannot assume if he or she expects their deSign to work under the reality that refuse varies in composition.
If one accepts the fact that solid waste is a mixture of different materials and that its composition affects the performance of processing equipment, then one is drawn to the conclusion that equipment performance must be specified in terms of its effect on various refuse components. Moreover, the definition of performance guarantees also has to be fashioned in terms of the preceding rationale.
The objective of defining meaningful performance specifications for processing equipment while accommodating the variations in refuse composition, can be met in part by defining equipment performance in terms of percent of recovery of particular components. For example, considering a piece of equipment assigned to segregate feed material into two streams (e.g., air classifier lights and heavies or screen oversize and undersize), the specification is a predetermined percentage of recovery of various components of the feedstock. In the case of an air classifier, the specification might be 90 percent recovery of the paper and plastic components in the light fraction.
76
Alternatively, the specification for a screen might be the recovery of 90 percent of the glass in the undersize fraction. The above examples are for illustration and are not meant to be all inclusive performance criteria for air classifiers and screens.
The utility of percentage recovery as a performance parameter is derived from its simplicity. In this regard the following hold:
(1) The purveyor of the equipment has a well-defmed and measurable level of performance which must be attained.
(2) Inasmuch as the specification is oriented towards performance, judgments of operating parameters (e.g., rpm, air velocity, size of screen opening, etc.) fall on the equipment designer, where the responsibility should lie.
(3) All parties are relieved of the uncertainties that arise as a consequence of not being able to guarantee the composition (and likewise heating value) of the waste.
(4) By meeting the percent recovery values for the specified components in the product streams, the composition of the material inputs to downstream processes ar-e well-defined.
(5) The performance specifications are essentially independent of variations in the composition of the waste.
(6) The percent recovery of components can be measured easily and unambiguously. Presently, there is a draft test standard "Determination of the Recovery of a Product by a Materials Separation Device" under consideration by the ASTM.
(7) A system mass balance can be reliably predicted if the percent recovery specifications are met. Hence, quality of the product streams (in terms of contamination, heating value, ash content, etc.) can be predicted as a function of input waste composition.
With regard to the last item (7), the performance specifications for the equipment actually become an integral and easily comprehendilble aspect of the facility design. As an example of the foregOing, a typical RDF facility design in the form of a computerized mass balance is shown in Fig. 2. The mass balance illustrates the use of percent recovery of particular waste components for individual unit processes. Choosing the trommel screen as an example, the following recovery percentages (expresses as mass fractions) are specified for the components reporting to the oversize fraction (see listing on next page).
The intimate tie between the performance specifications (in terms of percent recovery of components) for various pieces of processing equipment and a complete mass balance is clearly shown in Fig. 2. With regard to the selection of performance guarantees for the above equipment, the values of the percent recovery factors
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FIG
. 2
PROC
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AM
SPL
ITS
(MA
SS F
RA
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BA
SIS)
Component
Ferrous Aluminum Glass Mixpaper Newspaper Corrugated Plastic O. Plastic Yard Waste Food Waste Inorganics Organics
Mass Fraction Solid Water
0.7 0.7 0.6 0.6 0.15 0.15 0.95 0.95 0.95 0.95 0.95 0.95 0.8 0.8 0.7 0.7 0.4 0.4 0.2 0.2 0.2 0.2 0.4 0.4
shown in the matrices become the specifications of the equipment.
CONCLUSION
The definition and development of performance criteria and of performance guarantees requires an understanding of the fundamental parameters that govern equipment operation and performance and, equally im-
78
portant, an understanding of test methods and procedures; that are available for measuring them. A review of performance' guarantees and test methodologies used by the solid waste industry has shown that additional guarantees are needed in some cases in order to fully characterize the performance and capability of the equipment. In the case of refuse-fired boilers, the maximum thermal rating must be included in the performance guarantees and confirmed during the acceptance test period. For secondary material recovery, the quality (e.g., purity) of the recovered material must be specified in addition to its percent recovery.
Performance guarantees for RDF production facilities can be tied to the design of the plant through the use of percent recovery values for the refuse components of interest. The use of percent recovery factors circumvents the problems inherent in the performance specifications commonly being employed by the solid waste industry in the case of RDF processing equipment, i.e., the dependence of the performance parameters and the resultant guarantees upon composition of the wastes.
Inasmuch as conformance to performance guarantees is essential in terms of assuring reliable and economical operation of the facility, due diligence must be given to their definition, development, and confirmation.