Combustion – Process of combustion or burner operation – Flame shaping – or distributing components in combustion zone
Reexamination Certificate
2003-06-26
2004-08-17
Yeung, James C. (Department: 3749)
Combustion
Process of combustion or burner operation
Flame shaping, or distributing components in combustion zone
C431S012000, C431S115000, C431S116000
Reexamination Certificate
active
06776609
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns an improved method of operating burners that use flue gas recirculation (FGR). The improved method measures changes in air and air-flue gas mixture temperatures to estimate density variations, and then uses this information to optimize the performance of a combustion air fan over a range of air and recirculated flue gas mixture ratios and temperature ranges. The method can be applied to any burner that uses FGR, as long as the amount of flue gas that is used can be varied at some or all operating conditions without extinguishing the flame. Advantages of this technique are smaller fan size with associated lower first cost for a given maximum burner thermal capacity, reduced electrical power usage over the full range of burner operation, and a lower peak power requirement at a maximum burner capacity.
2. Description of Related Art
Over the past 30 years, the cost of owning and operating combustion equipment, e.g. boilers, has increased due to more stringent environmental regulations and, more recently, due to higher electricity prices. One impact of environmental regulations has been the requirement to use ultra-low NO
x
burners (hereinafter ULNBs) to achieve sub-9 ppm NO
x
emissions requirements. While the best ULNBs can match the thermal efficiency of standard burners, electric power usage is typically higher. Conventional ULNBs now often require over twice the fan power that was required of previous generation 30-ppm low NO
x
burners (hereafter LNBs).
Most fans used in industrial boilers and process heaters are constant speed fans, and a constant speed fan moves air at a fixed maximum volumetric flow rate. As air density changes, the mass flow of air changes while the volumetric flow stays the same.
Power consumption of a combustion air fan is proportional to the pressure rise created by the blower multiplied by the volumetric flow rate. With pressure rise measured in units of lb
f
/ft
2
and flow measured in units of ft
3
/sec it is apparent that the product of these two numbers has units of lb
f
-ft/sec, which are units of power.
Power∝&Dgr;P·Q (1)
In a fixed boiler and burner geometry, the pressure requirements of the combustion air fan will vary in proportion to the product of density and the square of the velocity. Therefore, if density remains constant the pressure drop will increase with the square of an increase in mass flow.
&Dgr;P∝&rgr;u
2
(2)
If mass flow remains constant and density changes, then {dot over (m)}∝&rgr;u remains constant, and pressure will change linearly with the inverse of density.
Δ
P
∝
m
·
2
ρ
(
fixed
geometry
)
Volumetric flow will change linearly with mass flow if density is constant, and linearly with specific volume (inverse of density) if mass flow is constant.
Q
=
m
·
ρ
Increasing mass flow at constant air density therefore increases fan power requirements in proportion to the mass flow ratio raised to the third power.
Power
∝
m
3
ρ
2
(
fixed
geometry
)
by
equations
(
1
)
,
(
3
)
,
and
(
4
)
For a fixed mass flow, decreasing air density by increasing the temperature (for example by replacing cool ambient air with the equivalent mass of warm flue gas) will increase fan power requirements in proportion the square of the specific volume change, equation (5). Lower density air uses more fan power for a fixed mass flow.
While the fan behaves as a constant volume device, the burner is not. At a fixed heat input, a burner is more accurately described as a constant mass flow device. In order to maintain a fixed fuel-air ratio (or fixed “dilution level”) at a fixed fuel flow, a constant mass flow of air is required, not a constant volume flow. Many ULNB's require that a nearly constant “dilution level” be maintained in order to maintain a given NO
x
emissions level.
Combustion systems use excess air to ensure complete combustion of the fuel, and importantly in some burners, to lower the combustion temperature to minimize NO
x
formation. Excess air is conventionally defined as the amount of air that is in excess of the stoichiometric requirement of the fuel with which it is mixed. Good practice calls for an excess air level of 15% or greater. For burners operating at 9 ppm (parts per million on a volumetric basis) or lower NO
x
emissions, the excess air level may be 65% or higher. Most of the excess air serves to lower the combustion temperature and hence its oxygen content acts as an inert like nitrogen to lower combustion temperature.
Flue gas is warmer than air, and it is thermally more efficient to recirculate some flue gas in place of some of the excess air in high-excess-air burners. This can be done as long as the oxygen-depleted flue gas is not mixed with air in a proportion that makes the mixture have insufficient oxygen for complete combustion of the fuel. If flue gas is substituted for an equivalent amount of air on a mass basis, it has been shown that similar burner emissions with be achieved. This is because the primary effect of the addition of either flue gas or excess air to the burner is to quench the flame and to reduce NO
x
formation. Therefore, controlling the “total dilution” of the burner, where “total dilution” is defined as the total mass flow of air and flue gas in excess of stoichiometric air, can be an effective way of controlling emissions.
From the above summary, it is seen that increasing mass flow and increasing the temperature of the mass flowing through a burner will increase the power required to achieve a specific firing rate at a fixed fuel-air or dilution level. Large increases in mass flow associated with ULNBs utilizing FGR have had a significant detrimental impact on fan power requirements as will be show below.
Power requirements of burners with different NO
x
emissions, different FGR levels, and different dilution levels are discussed in the following paragraphs and summarized in Table 1. To put the issue of fan power into perspective, one starts with an uncontrolled gas-fired burner with about 100 ppm NO
x
emissions, before evaluating a typical 30 ppm burner and then lastly two different 9 ppm ULNBs.
A standard natural gas fired burner with uncontrolled NO
x
emissions is designed for optimum performance at about 15% excess combustion air. When installed in a standard industrial boiler, a 100 MMBtu/hr burner operating at 15% excess air with 100 ppm NO
x
emissions would require approximately a 50 hp fan, or 0.5 fan hp per MMBtu/hr of boiler capacity.
In the late 1980's and early 1990's, 30 ppm NO
x
regulations were introduced, and the method of choice for achieving this emissions level was to add typically 15% FGR to a burner operating at 15% excess combustion air. Burners that operate at these, or similar, FGR and excess air levels have now been in widespread use for over 20 years. While this approach has proven to be a good method of achieving 30 ppm emissions, larger combustion air fans and fan motors were required. The combined mass flow of air and flue gas was increased by 15% (with the addition of 15% FGR to the air). In addition the volumetric flow of gas through the fan and burner was increased due to the decrease in density. With 15% FGR, the temperature increase was on the order of 40° F., which corresponds to a density decrease of 8%. Fan power requirements increased by 1.15
3
/0.92
2
(equation 5) or a factor of 1.80. Where a 50 hp fan worked for a 100 ppm burner, now the end user needed about 90 hp (0.90 fan hp per MMBtu/hr of capacity).
More recently, 9 ppm and lower emissions regulations have been introduced for natural gas fired burners in boilers and process heaters. Alzeta Corp. of Santa Clara, Calif. sells a burner for use in food processing and other industries that utilizes only excess combustion air (no FGR) to achieve the flame dilution nec
Morales Luis H.
Nickeson Robert W.
Sullivan John D.
Alzeta Corporation
Peters Howard M.
Peters, Verny, Jones & Schmitt, L.L.P.
Yeung James C.
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