Data processing: measuring – calibrating – or testing – Measurement system – Performance or efficiency evaluation
Reexamination Certificate
2000-08-02
2003-06-24
Hoff, Marc S. (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system
Performance or efficiency evaluation
C702S031000, C374S043000, C700S274000
Reexamination Certificate
active
06584429
ABSTRACT:
This invention relates to a fossil-fired boiler, and, more particularly, to a method for determining its thermal efficiency to a high accuracy from its basic operating parameters.
CROSS REFERENCES
This application is related to U.S. Pat. Nos. 5,367,470 and 5,790,420 which patents are incorporated herein by reference in their entirely. Performance Test Codes 4.1 and 4 published by the American Society of Mechanical Engineers (ASME) are incorporated herein by reference in their entirely.
BACKGROUND OF THE INVENTION
The importance of accurately determining boiler efficiency is critical to any thermal system which heats a fluid by combustion of a fossil fuel. If practical day-to-day improvements in thermal efficiency are to be made, and/or problems in thermally degraded equipment are to be found and corrected, then accuracy in efficiency is a necessity.
The importance of accurately determining boiler efficiency is also critical to the Input/Loss Method. The Input/Loss Method is a patented process which allows for complete thermal understanding of a steam generator through explicit determinations of fuel and effluent flows, fuel chemistry including ash, fuel heating value and thermal efficiency. Fuel and effluent flows are not directly measured. The Method is designed for on-line monitoring, and hence continuous improvement of system heat rate.
The tracking of the efficiency of any thermal system, from a classical industrial view-point, lies in measuring its useful thermal output, BBTC, and the inflow of fuel energy, m
AF
(HHVP+HBC). m
AF
is the mass flow of fuel, HHVP is the fuel's heating value, and HBC is the Firing Correction term. For example, the useful output from a fossil-fired steam generator is its production of steam energy flow. Boiler efficiency (&eegr;
B-HHV
) is given by: &eegr;
B-HHV
=BBTC/[m
AF
(HHVP+HBC)]. The measuring of the useful output of thermal systems is highly developed and involves the direct determination of useful thermal energy flow. Determining thermal energy flow generally involves measurement of the inlet and outlet pressures, temperatures and/or qualities of the fluids being heated, as well as measurement of the fluid's mass flow rates (m
stm
). From this information specific enthalpies (h) may be determined, and thus the total thermal energy flow, BBTC=&Sgr;m
stm
(h
outlet
−h
inlet
), delivered from the combustion gases may be determined.
However, when evaluating the total inflow of fuel energy, problems frequently arise when measuring the flow rate (m
AF
) of a bulk fuel such as coal. Further, the energy content of coal, its heating value (HHV), is often not known with sufficient accuracy. When such difficulties arise, it is common practice to evaluate boiler efficiency based on thermal losses per unit mass flow of As-Fired fuel (i.e., Btu/lbm
AF
); where: &eegr;
B-HHV
=1.0−(&Sgr;Losses/m
AF
)/(HHVP+HBC). For evaluating the individual terms comprising boiler efficiency, such as the specific loss term (&Sgr;Losses/m
AF
), there are available numerous methods developed over the past 100 years. One of the most encompassing is offered by the American Society of Mechanical Engineers (ASME), published in their Performance Test Codes (PTC).
INTRODUCTION TO NEW APPROACH
This invention teaches the determination of boiler efficiency having enhanced accuracy. Boiler efficiency, if thermodynamically accurate, will guarantee consistent system mass/energy balances. From such consistencies, fuel flow and effluent flow then may be determined, having greater accuracy than prior art, and greater accuracy than obtained from direct measurements of these flows.
Before discussing details of the present invention it is useful to examine ASME's PTC 4.1, Steam Generating Units, and PTC 4, Fired Steam Generators. Both PTC study a boiler efficiency based on the higher heating value (&eegr;
B-HHV
), no mention is made of a lower heating value based efficiency (&eegr;
B-LHV
). Using PTC 4.1's Heat-Loss Method, higher heating value efficiency is defined by the following. For Eq. (1A), HHV, if determined from a constant volume bomb calorimeter, is corrected for a constant pressure process, termed HHVP. Gaseous fuel heating values, normally determined assuming a constant pressure process, need no such correction, HHVP=HHV.
η
B
-
HHV
=
HHVP
+
HBC
-
∑
Losses
/
m
AF
HHVP
+
HBC
(1A)
Using PTC 4's Heat-Balance Method, higher heating value efficiency is defined as:
η
B
-
HHV
/
fuel
=
HHVP
-
∑
Losses
/
m
AF
HHVP
(1B)
The above are considered indirect means of determining boiler efficiency. Eq. (1A) implies that the input energy in fuel & Firing Correction m
AF
(HHVP+HBC) less &Sgr;Losses, describes the “Energy Flow Delivered” from the thermal system, the term BBTC. The newer PTC 4 (1998, but first released in 2000) advocates only the use of heating value in the denominator, developing a so-called “fuel” efficiency, &eegr;
B-HHV/fuel
. It is important to recognize that once efficiency is determined using an indirect means, fuel flow may be back-calculated using the classic definition provided BBTC is determinable: m
AF
=BBTC/[&eegr;
B-HHV
(HHVP+HBC)]; or m
AF
=BBTC/[&eegr;
B-HHV/fuel
HHVP].
The concept of the Enthalpies of Products and Reactants is now introduced as important to this invention. These terms both define heating value and justify the Firing Correction term (HBC) as being intrinsically required in Eq. (1A) Higher heating value is the amount of energy released given complete, or “ideal”, combustion at a defined “calorimetric temperature”. For a solid fuel such as coal, evaluated in a constant volume bomb, the combustion process typically heats a water jacket about, and is corrected to, the calorimetric temperature. Any such ideal combustion process is the difference between the enthalpy of ideal products (HPR
Ideal
) less reactants (HRX
Cal
) both evaluated at the calorimetric temperature, T
Cal
. Correction from a constant volume process (HHV) associated with a bomb calorimeter, if applicable, to a constant pressure process (HHVP) associated with the As-Fired condition is made with the &Dgr;H
V/P
term, see Eq. (37B).
&dgr;Q
T-Cal
=−HHV=−HHVP+&Dgr;H
V/P
(2A)
HHVP≡=−HPR
Ideal
+HRX
Cal
(2B)
This invention teaches that only when fuel is actually fired at exactly T
Cal
, and whose combustion products are cooled to exactly T
Cal
, is the thermodynamic definition of heating value strictly conserved. At any other firing and cooling temperatures, Firing Correction and sensible heat losses must be applied. At any other temperature the so-called “fuel” efficiency (which ignores the HBC correction), is thermodynamically inconsistent. At any other temperature, evaluation of the HRX
Cal
term must be corrected to the actual As-Fired condition through a Firing Correction referenced to T
Cal
. The HPR
Ideal
term is corrected to the actual via loss terms referenced to T
Cal
where appropriate (that is, anywhere a &Dgr;energy term is applicable).
When a fossil fuel is fired at a temperature other than T
Cal
, the Firing Correction term HBC must be added to each side of Eq. (2B):
HHVP+HBC=−HPR
Ideal
+HRX
Cal
+HBC
(3A)
Eq. (3A) implies that for any As-Fired condition, the systems' thermal efficiency is unity, provided the HPR
Ideal
term is conserved (i.e., system losses are zero, and ideal products being produced at T
Cal
). For an actual combustion process, the HPR
Ideal
term of Eq. (3A) is then corrected for system losses, forming the basis of boiler efficiency:
&eegr;
B-HHV
(
HHVP+HBC
)=−
HPR
Ideal
−&Sgr;Losses/
m
AF
+HRX
Cal
+HBC
(3B)
This invention recognizes that the HPR
Ideal
term of Eqs. (2B) & (3A), and thus Eq. (3B), is key in accurately computing boiler efficiency stemming from Eq. (3B). This invention teaches that all terms comprising Eq. (3B) must be evalu
Barbee Manuel L.
Exergetic Systems LLC
Hoenig Gary
Hoff Marc S.
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