US20080006188A1

US20080006188A1 - Increasing boiler output with oxygen

Info

Publication number: US20080006188A1
Application number: US11/480,832
Authority: US
Inventors: Kuang Tsai Wu; Hisashi Kobayashi; Geoffrey Bruce Tuson; Lawrence E. Bool
Original assignee: Praxair Technology Inc
Current assignee: Praxair Technology Inc
Priority date: 2006-07-06
Filing date: 2006-07-06
Publication date: 2008-01-10
Also published as: WO2008005462A2; WO2008005462A3

Abstract

A fuel-fired steam generation apparatus such as a boiler can produce steam at a rate higher than the maximum rate at which it can produce steam using air as the only source of oxygen for combustion, by combusting the fuel with oxidant having a higher oxygen content than air but feeding the oxidant at a volumetric flow rate lower than the rate at which maximum steam production is obtained with air as the only source of oxygen for combustion.

Description

FIELD OF THE INVENTION

This invention relates to boilers and like devices in which fuel is combusted to generate heat which is transferred to water passing through the device.

BACKGROUND OF THE INVENTION

Fuel-fired boilers that combust fuel and transfer the heat generated by the combustion to water that is fed through the apparatus so as to heat the water, evaporate it into steam, and heat the steam, typically are limited in the maximum amount of steam that they can produce. The maximum amount of steam that can be produced is typically limited by the volume of the combustion chamber, and by the properties of the apparatus itself including the heat exchangers that are employed to heat the water and the steam. For example, a fuel-fired boiler typically faces at least the following limitations on the rate at which steam can be produced: limitations in the capacity of the fans which drive combustion air into the apparatus, and which assist passing flue gas out of the device; the temperature of the flue gas, particularly as it first exits the combustion chamber, since excessive temperatures risk degrading the material of construction of heat exchangers that come in contact with the hot flue gas, and risk raising the temperature of water or steam passing through the heat exchangers so high that the heat exchanger may rupture or otherwise fail from within; and the capacity of devices, such as those known as attemperators, which are used to reduce the temperature of the heated steam that is generated by the apparatus.
Thus, attempts to increase the mass flow rate of steam produced by the apparatus, simply by increasing the rate at which fuel is fed to the apparatus, or by increasing the amount of water fed to the apparatus, are eventually limited by one or more of the foregoing constraints, when air is employed as the source of oxygen for combustion of the fuel.
This situation is also affected by the fact that in many cases, the steam that is produced by the apparatus must be provided at a temperature which is within a given temperature range generally 5° F. above or below that given temperature. This limitation is imposed particularly in situations in which the steam produced by the apparatus is to be directed into turbines for generating electrical power. Such turbines are generally designed on the basis of several significant inputs, one of which is the temperature of the incoming steam into the turbine.
Other attempts have been made to provide increased steam generating capability, such as flue gas recirculation, and such as rebuilding all or part of the steam generating apparatus, but these attempts impose operational and economic burdens which operators want to avoid.
Accordingly, there remains a need for technology to increase the capability of a given, already existing steam generating apparatus to produce steam at higher mass flow rates, at the given desired steam temperature. The present invention provides this capability.

BRIEF SUMMARY OF THE INVENTION

One aspect of the present invention is a method of modifying a steam generation apparatus, comprising
(A) providing a steam generation apparatus comprising
(1) a combustion chamber into which gaseous oxidant and fuel are fed, and in which said fuel and said gaseous oxidant are combusted thereby producing heat and hot flue gas,
(2) a preheater into which and through which said hot flue gas passes for transferring heat to said gaseous oxidant before said gaseous oxidant is fed to said combustion chamber, and
(3) a pathway into which water is fed, from which heated steam is obtained at an outlet, and within which the water is evaporated and converted to said heated steam by heat transferred thereto in one or more heat exchangers in said pathway and within said steam generation apparatus to which heat of said combustion is radiated and in one or more convective heat exchangers in said pathway and within said steam generation apparatus to which heat in said hot flue gas is transferred by convective heat exchange,
the pathway further comprising one or more devices that can pass coolant into steam in the pathway to reduce the temperature of said steam,
wherein the highest mass flow rate at which said apparatus can provide steam at said outlet at a temperature within a given temperature range, when said gaseous oxidant is air and said air is the only source of oxygen for said combustion, is limited by the maximum flue gas temperature to which said heat exchangers can be exposed, by maximum rates at which said fuel can be fed into said combustion chamber and at which said flue gas can be withdrawn therefrom, and by the maximum rate at which said coolant can be passed into said steam, and
(B) increasing the rate at which steam is provided at said outlet at a temperature within said given temperature range to a mass flow rate higher than said highest mass flow rate, by
(1) increasing the mass flow rate into said pathway of water by an amount equal to the desired increased flow rate of steam and increasing the mass flow rate of fuel into said combustion chamber by an amount corresponding to the increased amount of heat needed to produce said steam at said increased rate, while
(2) increasing the oxygen content of said gaseous oxidant with which said fuel is combusted but decreasing the volumetric flow rate at which said gaseous oxidant having increased oxygen content with which said fuel is combusted is fed into said combustion chamber to values effective to provide said increased mass flow rate of steam without exceeding any of said maxima, and
(3) combusting said fuel in said combustion chamber with said gaseous oxidant having said increased oxygen content fed at said decreased volumetric flow rate,
without decreasing or increasing the heat transfer area of any of said heat exchangers,
wherein flue gas produced in said combustion chamber that exits from said apparatus is not fed back into said apparatus, and the temperature of said flue gas entering said preheater is lower than when said flue gas is produced by combustion in said apparatus when air is the only source of oxygen for combustion.
Another aspect of the present invention is a method of operating a steam generation apparatus, comprising
(A) providing a steam generation apparatus comprising
(1) a combustion chamber into which gaseous oxidant and fuel are fed, and in which said fuel and said gaseous oxidant are combusted thereby producing heat and hot flue gas,
(2) a preheater into which and through which said hot flue gas passes for transferring heat to said gaseous oxidant before said gaseous oxidant is fed to said combustion chamber, and
(3) a pathway into which water is fed, from which heated steam is obtained at an outlet, and within which the water is evaporated and converted to said heated steam by heat transferred thereto in one or more heat exchangers in said pathway and within said steam generation apparatus to which heat of said combustion is radiated and in one or more convective heat exchangers in said pathway and within said steam generation apparatus to which heat in said hot flue gas is transferred by convective heat exchange,
the pathway further comprising one or more devices that can pass coolant into steam in the pathway to reduce the temperature of said steam,
wherein the highest mass flow rate at which said apparatus can provide steam at said outlet at a temperature within a given temperature range, when said gaseous oxidant is air and said air is the only source of oxygen for said combustion, is limited by the maximum flue gas temperature to which said heat exchangers can be exposed, by maximum rates at which said fuel and gaseous oxidant can be fed into said combustion chamber and at which said flue gas can be withdrawn therefrom, and by the maximum rate at which said coolant can be passed into said steam, and
(B) providing steam at said outlet at a temperature within said given temperature range at a desired mass flow rate which is higher than said highest mass flow rate, by
(1) feeding water into said pathway at a mass flow rate equal to the desired mass flow rate of steam and feeding fuel into said combustion chamber at a mass flow rate corresponding to the amount of heat needed to produce said steam at said desired mass flow rate, while
(2) feeding into said combustion chamber gaseous oxidant with which said fuel is combusted having a higher oxygen content than air, at a volumetric flow rate lower than the volumetric flow rate of air at which said highest mass flow rate of steam is provided when air is the only source of oxygen for said combustion, at values of said oxygen content and said volumetric flow rate which are effective to provide said desired mass flow rate of steam without exceeding any of said maxima, and
(3) combusting said fuel in said combustion chamber with said air and said gaseous oxidant having said increased oxygen content fed at said lower volumetric flow rate,
wherein the heat transfer area of said heat exchangers is the same as when air is the only source of oxygen for said combustion, and wherein flue gas produced in said combustion chamber that exits from said apparatus is not fed back into said apparatus, and the temperature of said flue gas entering said preheater is lower than when said flue gas is produced by combustion in said apparatus when air is the only source of oxygen for combustion.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic side cross-section of a steam generating apparatus with which the present invention is useful.

FIG. 2 is a schematic side cross-section of another steam generating apparatus with which the present invention is useful.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a simplified schematic view is shown of a steam generation apparatus 1, a boiler, with which the present invention can be practiced.
Fuel 3 and gaseous oxidant 5 are fed to burner 7. The fuel and the oxidant combust as flame 9 in combustion chamber 10. The combustion forms hot flue gas 12 which passes out of combustion chamber 10 and flows in convective heat exchange contact with heat exchangers as described below. Preferably, before the gaseous oxidant is fed into burner 7 it is heated in preheater 15 by heat transfer there from hot flue gas 12 entering preheater 15.
The fuel can be any combustible solid, liquid or gas, or mixtures thereof. The preferred fuel is coal Other preferred fuels with which this invention can be practiced include other combustible solid matter, liquids such as fuel oil, and gases such as methane and natural gas.
Solid fuel such as coal is typically fed as a stream of pulverized solids carried in a stream of transport air through the burner into the combustion chamber. Liquid fuels are typically atomized, and the atomizing can be carried out by a stream of air which thus enters into the combustion chamber together with the liquid fuel.
Burners employed for combustion of any of these fuels typically include inlets in close proximity to the point at which the fuel enters into the combustion chamber, through which combustion air (known as “primary air”) enters into the combustion chamber and to provide oxygen for combustion with the fuel. As is known, some burner designs contain additional passages in which additional air, known as “secondary air”, can also enter into the combustion chamber and participate in the combustion of the fuel.
Typically, combustion air is provided to the burner air inlets, and to every burner in apparatus having more than one burner, from a common source by appropriate feed lines. Another typical arrangement is that the combustion air is fed from its source to a windbox from which the air flows directly to each burner.
The boilers and other steam generation devices to which this invention applies can be configured so that the fuel and all gaseous oxidant (e.g. combustion air) needed for complete combustion of the fuel enters through burners such as burner 7 installed in the radiative section of the combustion chamber 10. Alternatively, the devices can be configured so that only a portion of the gaseous oxidant (e.g. combustion air) enters through burners such as burner 7, and the balance thereof (typically 10% up to 25% of the total oxygen for combustion fed to the apparatus) enters through one or more overfire ports 17 located vertically above said burners, between the radiative zone burners and the convective section heat exchangers.
Feed water 21 that is to be converted to steam follows a pathway (through appropriate piping and heat exchangers) from a suitable source, through optional but preferred heat exchanger 22 (known as an “economizer”) in which it is heated by heat transferred from hot flue gas 12, and through one or more heat exchangers 24 where it is heated by heat radiated from the combustion in combustion chamber 10, and through one or more convective heat exchangers (such as at 26 and 28, also known as “superheaters”) which transfer heat by convective heat exchange from the hot flue gas. In a typical steam generation apparatus, there is one or more device 40 (commonly known as an “attemperator”) which can feed coolant such as water into the stream of hot steam in order to reduce the temperature of the steam as desired by the operator. As described above, reducing the temperature of the steam can become necessary in order to provide the steam at its outlet 30 at a temperature in the desired temperature range appropriate for further use of the steam as in a turbine. Typically, depending on the apparatus, the desired temperature may be 800° F. to 1200° F., often about 1000° F., so the desired range being plus or minus 5° F. from the given desired temperature. It should be understood that outlet 30 is not confined to a nozzle or the like, but can be a point in a conduit that conveys the steam to another piece of equipment such as a turbine.
Steam generating apparatus of this type can also be used to heat steam received from other sources. In one such embodiment, the apparatus heats steam (termed “reheat steam”) that has been exhausted from a power turbine, especially where the power turbine is a power turbine to which steam from outlet point 30 was fed. FIG. 2 illustrates such an embodiment, wherein steam 50 is passed through heat exchangers 52 and 54 in which the steam is heated by heat transfer from flue gas 12, and is provided point 60 at a desired temperature. Device 56, which is typically a controllable supply of attemperating water, can inject coolant into the steam in order to reduce the temperature of that steam so that its temperature at outlet point 60 is as desired.
The present invention is also useful in steam generating apparatus wherein burners are arrayed in several rows, typically 2 to 10 rows, with each row containing 2 to 10 or more burners side by side in each row. In these embodiments, at least one burner is at a higher elevation than at least one other of said burners, with the flue gas exiting the combustion chamber at a higher elevation than even the uppermost of all the burners. Preferably, each burner at a given elevation is one of a plurality of burners in a given row of burners at the same elevation. In these embodiments, the same amount of fuel and oxidant can be fed to every burner. Preferably, though, to a “lower group” of burners which contains half or more than half of the total number of burners (but less than all of the burners), and in which all the burners in this lower group are at lower elevations than all of the burners that are not in this lower group; more than half of the total amount of fuel that is fed to all burners of the apparatus is fed to burners that are in this “lower group”. (The “lower group”, or the group of burners not in this “lower group”, or both groups, may contain one or more than one burner.) In this embodiment, illustrated in Case 5 in the Example, the rate of production of steam is increased, and operation is so efficient that no cooling fluid (such as attemperating water) at all is fed into the steam that is produced in this embodiment.
It has been determined by simulations of the operation of a coal-fired steam generating apparatus of the type depicted in FIG. 1, that steam can be produced at a mass flow rate higher than the maximum mass flow rate attainable using air as the only source of oxygen for combustion of the fuel, but at the same given target temperature as attained using air as the only source of oxygen for combustion, as follows:
Additional water is fed to the apparatus, so that the total amount of water fed equals the total amount of steam that is to be provided at the outlet. Additional fuel is provided into the apparatus, in an amount corresponding to the additional amount of steam that is to be provided at the given temperature, so that the total amount of fuel fed to the apparatus corresponds to the amount of steam to be provided under these new operating conditions.
The amounts of fuel and oxidant to be provided can be determined for any given operation from straightforward stoichiometric and thermodynamic calculations.
In addition, two adjustments are made to the oxidant with which the fuel is to be combusted.
One adjustment is that the oxygen content of the oxidant is increased. If the apparatus had been operating with air as the only source of oxygen for combustion, then the oxygen content of the oxidant is increased above that of air. However, even if the overall oxygen content of oxidant provided to the apparatus to be combusted with the fuel was at a value already above that of air, then the oxygen content is increased above that value.
The oxygen content can be increased in any of several ways. One way is to premix oxygen, or highly-oxygen-enriched air, with the incoming air, before or after the air passes through the preheater, so that oxidant containing the desired higher oxygen content reaches all burners. Another technique is to feed oxygen, or highly-oxygen-enriched air, into the windbox when a windbox is employed with the apparatus as described above. A third technique is to inject, with a lance or other suitable apparatus, high purity oxygen directly into the burner, or into each burner if more than one burner is employed in the apparatus.
While in principle any higher oxygen content is effective in conjunction with the other adjustments described herein, the final oxygen content of the oxidant can typically be up to 30 vol. %, but the advantages of the present invention can be realized if the oxygen content of the oxidant is up to 25 vol. %, or less.
Another adjustment that is made in the supply of oxidant to the combustion is that the overall volumetric flow rate of the oxidant is reduced, typically by up to 20 percent from the volumetric flow rate that had been employed in obtaining the maximum mass flow rate of steam obtainable using air as the only source of oxygen for combustion.
However, the volumetric flow rate of oxidant may be reduced from a lesser volumetric flow rate as well, particularly in the course of balancing that flow rate with the increased oxygen content of the oxidant being fed at that reduced volumetric flow rate.
A preferred technique is to replace a given volume of air from the oxidant with that same volume of oxygen of at least 90 vol. %, and preferably at least 99.9 vol. %, purity. This results in reducing the total volume of oxidant being fed, while increasing the oxygen content of that oxidant.
The steam generating apparatus is then operated with combustion carried out using oxidant having an oxygen content higher than that of air, but which is fed at a volumetric flow rate lower than the rate at which air was being fed to the apparatus when the apparatus was generating its maximum possible mass flow rate of steam at the aforesaid given temperature with air as the only source of oxygen for combustion.
Under these conditions, surprisingly, steam is provided at a higher mass flow rate than the maximum mass flow rate of steam that could be obtained using combustion with air as the only source of oxygen, but the aforesaid operational maximums are not exceeded. That is, the maximum flue gas temperature that can be tolerated by the apparatus, including particularly by the heat exchangers with which the flue gas comes into contact, is not exceeded, even though more heat is generated and steam is produced at a higher mass flow rate.
This discovery is surprising in that one would normally expect that feeding additional amounts of fuel, and feeding oxidant containing more oxygen than air, into the same apparatus without changing the heat transfer surface areas of any of the heat exchangers and without otherwise changing the physical dimensions of the apparatus itself, would lead to generation of such higher amounts of heat and such higher temperatures that the apparatus would not be able to tolerate such conditions. However, the results described in the following Example indicate that the aforementioned maxima are not exceeded, and indeed higher rates of steam production are attained. It is even more surprising, and beneficial, that the amount of attemperating water or other coolant to add to the steam are reduced and even completely eliminated.
Without intending to be bound by any particular explanation for these observations, the data reported in the Example are consistent with the proposition that enough heat produced by combustion of the fuel and the oxidant having a higher oxygen content than air is conveyed by radiative heat transfer to the one or more heat exchangers to which heat of combustion is radiated, and is thus transferred in those heat exchangers to the water, that the temperature and heat content of the flue gas exiting the combustion chamber do not exceed the maximum that is imposed by the equipment including the other heat exchangers with which the hot flue gas comes into contact; and the subsequent heat exchange from the flue gas to those other heat exchangers raises the temperature of the steam even higher, as the water had already absorbed that additional amount of radiative heat from the combustion.
In the embodiments in which some combustion air is provided into the combustion chamber as the aforementioned overfire air, the oxygen content of that air provided as overfire air should be less than the oxygen content of the oxidant supplied to the burner.
Among other advantages, the flue gas temperature at the point at which the flue gas enters the aforementioned preheater is lower than is the case when the apparatus is operated using air as the only source of oxygen for combustion. This is advantageous in that it reduces the further need to cool the flue gas. This also confirms that more of the heat contained in the flue gas has been transferred to the production of steam.

EXAMPLE

The invention is described in detail for ten cases of boiler operation simulated by a computer model. The boiler is assumed to be of the type illustrated in FIG. 2, with the feature that reheat steam exhausted from a power turbine is passed through the apparatus to be reheated by heat exchange with flue gas 12. The boiler computer model is based on zone methods of analysis. The model divides the boiler furnace into hundreds or thousands of volume and surface zones. It then calculates local and overall heat transfer, gas and wall temperature profiles, and volatile and char burn out, depending on coal properties and boiler operating conditions. Boiler steam production is calculated using computerized steam tables after the net heat transfer to various radiative and convective heat exchangers are known. This modeling approach for fuel combustion and flame radiation had been employed to model air- and oxy- fuel fired glass furnaces with success and are described in Wu, K. T. and Kobayashi, H., “Three-Dimensional Modeling of Alkali Volatilization/Crown Corrosion in Oxy-Fired Glass Furnaces”, 98^thAnnual Meeting of the American Ceramic Society, Indianapolis, Ind., 1996.
The simulations assumed a 278 MW thermal input wall-fired boiler fired with bituminous coal using four rows of burners, spaced one row above the other, with four burners in each row. Table 1 summarizes properties of the coal used in the modeling simulation.

TABLE 1

Proximate Analysis (%, wet)

	Moisture	7.9
	Volatile Matter	37.1
	Fixed Carbon	43.6
	Ash	11.4

HHV (Btu/lb, wet) 12885

Ultimate Analysis (%, dry)

	C	72.3
	H	4.6
	N	1.3
	O	7.7
	S	1.7
	Ash	12.4

Table 2 summarizes five simulated cases in which the boiler is operated without overfire air. Case 1 is based on data taken from operation of an actual power generation boiler. For simulated Cases 2 through 5, maximum coal flow and therefore maximum steam generating capacity is reached when the temperature of the flue gas entering the screen tubes (the point at which the flue gas first contacts a convective heat exchanger) reaches 2375±3 F. The five cases in Table 2 are:
Case 1. Baseline operation with air only. All burners fired at the same firing rate.
Case 2. Maximum steam generation capacity operation with air only. All burners are fired equally.
Case 3. Steam generation rate increased by oxygen enrichment of combustion air to 22.09% O₂in air. All burners are fired equally.
Case 4. Steam generation rate increased by oxygen enrichment of combustion air to 22.80% O₂in air. All burners are fired equally.

Case 5. Same as Case 4, but heat input biased so that 35% of the fuel is fed and combusted in each of the bottom two rows of burners, and 15% of the fuel is fed and combusted in each of the top two rows of burners.

TABLE 2

Boiler Operations without overfired oxidant

	Case 1	Case 2	Case 3	Case 4	Case 5

Furnace Operation:
Coal flow (lb/hr)	73551	84584	84584	84768	88262
Firing rate (MMBtu/hr, HHV)	947	1089	1089	1092	1137
Oxidant flow (SCFH)	11016934	12667944	11687674	11274010	11738744
O2% in oxidant	20.67	20.67	22.09	22.80	22.80
Flue gas flow (SCFH)	11518689	13244975	12264705	11852286	12340857
Burner load	equal	equal	equal	equal	distributed
Air preheat (F.)	600	631	616	610	617
Flue Gas Temperatures (F.):
Entering screen tubes	2271	2374	2378	2373	2375
Entering finishing superheater	2093	2198	2191	2183	2190
Entering economizer	829	882	855	842	853
Entering air preheater	696	740	716	706	715
Heat Absorptions (MMBtu/hr):
Waterwalls 24	470	509	544	561	585
Finishing superheater 28	51	60	58	59	59
Reheaters 52 and 54	106	128	122	118	122
Steam Production:
Superheated steam (lb/hr)	693396	784238	798178	809028	843005
Reheated steam (lb/hr)	623938	712958	717869	722858	752400
Superheat attemperation (lb/hr)	10755	36622	9171	0	0
Reheat attemperation (lb/hr)	6803	15008	7437	2827	2178
Superheat attemperation (%	1.6	4.7	1.1	0.0	0.0
superheated steam)
Reheat attemperation (%	1.1	2.1	1.0	0.4	0.3
reheat steam)
Carbon in Ash (%):	6.81	7.82	6.95	6.64	6.54
Boiler Efficiency:
Gross (% of HHV coal heat	89.17	88.43	89.22	89.41	89.25
input)

Table 3 summarizes five simulated cases in which combustion is staged by operating the boiler with overfire air. In the simulations, maximum coal flow and therefore maximum boiler steam generating capacity is reached when the flue gas temperature entering the screen tubes reached 2375±3 F. The five cases in the table are:
Case 6. Baseline operation with air. All burners are fired equally. Burner zone SR (stoichiometric ratio, the ratio of oxygen fed to the amount of oxygen needed to completely combust all combustible components in the fuel) is 0.9 and only air is used in the overfire zone.
Case 7. Maximum steam generation capacity operation with air only. All burners are fired equally. Burner zone SR is 0.9 and only air is used in the overfire zone.
Case 8. Steam generation rate increase by oxygen enrichment of combustion air to 22.09% O₂in air. All burners are fired equally. Burner zone SR is 0.9 and oxygen enrichment is applied to both the burner combustion air and the overfire air.
Case 9. Same as Case 8 except the burners are fired at different loads and SR levels. Oxygen enrichment is applied to both the burner combustion air and the overfire air.
Case 10. Same as Case 8 except the burners are fired at different firing rates and SR levels. Oxygen enrichment is applied only to the burner combustion air, not the overfire air.

TABLE 3

Boiler operations with overfired oxidant

	Case 6	Case 7	Case 8	Case 9	Case 10

Furnace Operation:
Coal flow (lb/hr)	73551	84510	83113	82230	85320
Firing rate (MMBtu/hr, HHV)	947	1088	1070	1059	1099
Oxidant flow (SCFH)	11016934	12656909	11484452	11362430	12194070
O2% in oxidant	20.67	20.67	22.09	22.09	22.09/20.67
Flue gas flow (SCFH)	11518689	13233444	12051452	11923404	12685502
Burner load	equal	equal	Equal	distributed	distributed
SR1/SRt	0.90/	0.90/	0.90/1.22	0.75 to 1.0/	0.75 to 1.0/
	1.24	1.24		1.22	1.23
Air preheat (F.)	602	633	616	613	631
Flue Gas Temperatures (F.):
Entering screen tubes	2262	2374	2374	2376	2377
Entering finishing superheater	2083	2200	2189	2186	2191
Entering economizer	833	886	854	850	870
Entering air preheater	698	743	714	711	729
Heat Absorptions (MMBtu/hr):
Waterwalls 24	459	495	522	519	523
Finishing superheater 28	51	61	59	58	60
Reheaters 52 and 54	107	131	123	122	127
Steam Production:
Superheated steam (lb/hr)	685476	775051	775130	768240	790733
Reheated steam (lb/hr)	619106	708602	701237	694822	716681
Superheat attemperation (lb/hr)	16561	45881	17479	15286	27174
Reheat attemperation (lb/hr)	9005	18802	11373	11136	12941
Superheat attemperation (%	2.4	5.9	2.3	2.0	3.4
superheated steam)
Reheat attemperation (%	1.5	2.7	1.6	1.6	1.8
reheat steam)
Carbon in Ash (%):	11.20	11.66	10.30	10.33	11.20
Boiler Efficiency:
Gross (% of HHV coal heat	88.42	87.88	88.60	88.74	88.18
inputs)

The most general illustration of this invention as applied to a boiler where all combustion air is supplied through the burners (that is, a boiler without overfire air) is made by comparing modeling Cases 3 and 4 to Case 2. These comparisons show that the mass flow rate of steam and the efficiency can be increased by increasing the oxygen content of the gaseous oxidant, such as by enriching the combustion air with oxygen, or by introducing oxygen into the combustion zone of the boiler by some means other than general enrichment (such as lancing). They also show that using oxygen to increase the rate of steam production brings added benefits of reduced attemperation coolant flows and reduced flue gas temperature at the inlet to the air preheater relative to maximum capacity operation with air as the only source of oxygen for combustion. The modeling of this invention indicates capacity and efficiency gains can be achieved with oxygen enrichment without problems arising related to heat transfer imbalance and therefore without having to resort to flue gas recirculation to the combustion chamber, and the complications and drawbacks associated with flue gas recirculation.
Referring to Table 2, in Case 3, oxygen is used to increase combustion air oxygen concentration from 20.67% oxygen to 22.09% oxygen. This enrichment results in the ability to produce more superheat (and, if desired, reheat) steam without exceeding the screen tube temperature limit than is possible in the cases without oxygen enrichment as in Case 2. It also results in an increase in boiler fuel efficiency (89.22% efficiency in Case 3 versus 88.43% efficiency in Case 2).
Other benefits of using oxygen are lower attemperation coolant flow rates and reduced flue gas temperature at its inlet to the air preheater. In Case 2 attemperation spray flow and air preheater gas inlet temperature are increased relative to Base Case 1, due to the increase in flue gas flow rate that results from increasing the firing rate but using only air as the source of oxygen for combustion. The increased flue gas flow carries more heat to the convective heat exchangers of the boiler. This raises the gas temperature at the air preheater inlet, and increases the steam temperature which in turn necessitates increased attemperation coolant flow rates for cooling the steam. As noted above, attemperation coolant flow rates and the air preheater flue gas inlet temperature can be capacity limiting factors in a boiler. For Case 3, which employs oxygen enrichment, attemperation coolant flow rates and the air preheater flue gas inlet temperature are lower than in either Case 1 or Case 2, possibly because of the reduced flue gas volume and heat content that results from replacement of a portion of the combustion air with oxygen. As shown in Table 4, the reduced requirement for steam temperature reduction (reduced “desuperheating load”) results in reduced attemperation coolant flows.

TABLE 4

Steam Production Rate Increase

	Case 2	Case 3

Superheated Steam
Temperature (F.) of steam to	955	898
which attemperating water is
added at 40
Temperature (F.) of steam	874	879
after addition of
attemperating water at 40
Amount of de-superheating, F.	81	19
Amount of attemperating	36622	9171
water flow, lb/h
Rate of production of	784238	798178
superheated steam at outlet
30, lb/h
Reheat Steam
Temperature (F.) of steam to	826	818
which attemperating water is
added at 56
Temperature (F.) of steam	786	798
after addition of
attemperating water at 56
Amount of de-superheating, F.	40	20
Amount of attemperating	15008	7437
water flow, lb/h
Rate of production of	712958	717869
reheated steam at outlet 60,
lb/h

Case 4 shows the result of further oxygen enrichment of the oxidant, to an oxygen content of 22.8 vol. %. The steam generation rate and boiler efficiency are increased further versus Case 3 at this higher level of enrichment, but still without exceeding the temperature limit for the flue gas at the screen tubes, and the attemperation coolant flow rates and air preheater flue gas inlet temperature are further reduced.
Another embodiment of using lower flow rates of oxidant having an oxygen content higher than that of air, in a boiler without overfire air, is illustrated by Case 5. In Case 5, oxygen addition to the oxidant is combined with distributing a greater portion of total fuel input to the lower rows of burners. This fuel redistribution allows the total firing rate to be increased further and allows the steam generation rate to be increased without exceeding the temperature limit for the flue gas entering the screen tubes. Replacing a portion of the combustion air with oxygen allows greater fuel redistribution than is possible with air as the only source of oxygen.
Oxygen addition results in flame stability at a wider range of velocity and mixing conditions, and also results in reduced pressure drop through the burner combustion air passages thereby allowing higher firing rate operation.
In Case 5, the oxygen content of the combustion air is 22.9 vol. % oxygen. Additionally, fuel input is redistributed so the bottom two rows of burners each supply 35% of total heat input, and the upper two rows of burners each supply 15% of total heat input. The maximum steam generation rate, as determined by reaching the maximum allowable flue gas temperature at the screen tubes, is greater than that in any of Cases 2, 3 and 4, all of which have equal distribution of fuel to all rows of burners. Boiler efficiency in Case 5 is higher than the efficiency in Case 2 which is the maximum capacity case for operation with air as the only source of oxygen for combustion. The attemperation coolant flow rate is low relative to Cases 2, 3 and 4, and the flue gas temperature at its inlet to the air preheater is low relative to Cases 2 and 3.
Cases 6 through 10 apply to a boiler in which a portion of the combustion air flow is diverted from the burners to a separate overfire air injection port for injection of air into the boiler above the top row of burners and below the convective heat exchangers.
In Case 6, the firing rate matches that of Base Case 1. 90% of the amount of air required for complete stoichiometric combustion is supplied through the burners (at which the SR=0.9) and additional air is supplied as overfire air to bring the total boiler stoichiometric ratio to 1.24.
Case 7 calculates the maximum steam generation rate for the boiler when it is operated with overfire air, determined as the rate when the maximum permissible flue gas temperature entering the screen tubes is reached. The same burner zone and stoichiometric ratios apply as in Case 6. Attemperation coolant flow rates and the flue gas temperature at the inlet to the air preheater are increased substantially relative to Case 6, and could prevent actual achievement of this steam generation rate in an actual boiler.
In Case 8, the oxygen concentration in the oxidant for combustion, and in the oxidant fed through the overfire air port, is 22.09% oxygen. The higher oxygen content in the combustion oxidant and at the overfire air port does not yield an increase in the steam generating rate. The production rate of superheated steam changes insignificantly, but about 1% less reheat steam is produced relative to Case 7. There is an increase in boiler fuel efficiency (88.6% efficiency in Case 8 versus 87.88% efficiency in case 7), but the objective of increased steam production rate is not achieved. However very close to the same capacity as in Case 7 is achieved with significantly lower attemperation coolant flows and with lower temperature at the flue gas inlet to the air preheater.
In Case 9, burner and overfire combustion air are again 22.09 vol. % oxygen. Additionally, fuel input is redistributed so that the top three rows of burners each supply 22.2% of total fuel input, and the bottom row of burners supplies 33.3% of total fuel input. The stoichiometric ratio of the lowest row of burners is 1.0, and of the remaining burners is 0.75. The oxygen in the overfire air flow brings the total boiler stoichiometric ratio to 1.22. With this oxygen addition and fuel distribution, the maximum steam generation rate as determined by reaching the maximum allowable flue gas temperature at the screen tubes is reduced relative to Case 7 (which had no oxygen enrichment).
Case 10 is identical to Case 9 with the exception that the oxygen content of the oxidant is higher only in the oxidant fed to the burners, not in the air fed through the overfire port. In this embodiment, both the steam generation rate and boiler efficiency are increased relative to Case 7, the case of maximum steam production without oxygen addition. The increased steam generation rate in Case 10 is achieved with less attemperation coolant flow, and with a lower flue gas temperature at the air preheater inlet, than in Case 7.

Claims

1. A method of modifying a steam generation apparatus, comprising

(A) providing a steam generation apparatus comprising

(1) a combustion chamber into which gaseous oxidant and fuel are fed, and in which said fuel and said gaseous oxidant are combusted thereby producing heat and hot flue gas,

(2) a preheater into which and through which said hot flue gas passes for transferring heat to said gaseous oxidant before said gaseous oxidant is fed to said combustion chamber, and

(3) a pathway into which water is fed, from which heated steam is obtained at an outlet, and within which the water is evaporated and converted to said heated steam by heat transferred thereto in one or more heat exchangers in said pathway and within said steam generation apparatus to which heat of said combustion is radiated and in one or more convective heat exchangers in said pathway and within said steam generation apparatus to which heat in said hot flue gas is transferred by convective heat exchange,

the pathway further comprising one or more devices that can pass coolant into steam in the pathway to reduce the temperature of said steam,

wherein the highest mass flow rate at which said apparatus can provide steam at said outlet at a temperature within a given temperature range, when said gaseous oxidant is air and said air is the only source of oxygen for said combustion, is limited by the maximum flue gas temperature to which said heat exchangers can be exposed, by maximum rates at which said fuel can be fed into said combustion chamber and at which said flue gas can be withdrawn therefrom, and by the maximum rate at which said coolant can be passed into said steam, and

(B) increasing the rate at which steam is provided at said outlet at a temperature within said given temperature range to a mass flow rate higher than said highest mass flow rate, by

(1) increasing the mass flow rate into said pathway of water by an amount equal to the desired increased flow rate of steam and increasing the mass flow rate of fuel into said combustion chamber by an amount corresponding to the increased amount of heat needed to produce said steam at said increased rate, while

(2) increasing the oxygen content of said gaseous oxidant with which said fuel is combusted but decreasing the volumetric flow rate at which said gaseous oxidant having increased oxygen content with which said fuel is combusted is fed into said combustion chamber to values effective to provide said increased mass flow rate of steam without exceeding any of said maxima, and

(3) combusting said fuel in said combustion chamber with said gaseous oxidant having said increased oxygen content fed at said decreased volumetric flow rate,

without decreasing or increasing the heat transfer area of any of said heat exchangers,

wherein flue gas produced in said combustion chamber that exits from said apparatus is not fed back into said apparatus, and the temperature of said flue gas entering said preheater is lower than when said flue gas is produced by combustion in said apparatus when air is the only source of oxygen for combustion.

2. A method according to claim 1 wherein said gaseous oxidant fed into said combustion chamber in step (A)(1) is air.

3. A method according to claim 1 wherein said fuel is coal.

4. A method according to claim 1 wherein steam is provided at said outlet at a temperature within said given temperature range at said increased oxygen content and decreased volumetric flow rate of said oxidant without passing any coolant into said steam.

5. A method according to claim 1 wherein said gaseous oxidant and fuel are combusted in said combustion chamber at a plurality of burners, and at least one of said burners is at a higher elevation than at least one other burner, the method further comprising

feeding more than half of the total amount of fuel that is fed to all of said plurality of burners to a group of burners which contains at least half but fewer than all of said burners, in which group all burners are at a lower elevation than all burners not in said group.

6. A method according to claim 1 wherein said gaseous oxidant and fuel are combusted in said combustion chamber in one or more burners, and all oxidant necessary for complete combustion of said fuel is fed through said burners.

7. A method according to claim 1 wherein said gaseous oxidant and fuel are combusted in said combustion chamber in one or more burners, wherein a portion of the oxidant necessary for complete combustion of said fuel is fed through said burners and the balance of the oxidant necessary for complete combustion of said fuel is fed into said combustion chamber through one or more overfire ports separate from and vertically above said one or more burners, provided that the oxygen content of the oxidant fed through said one or more overfire ports is less than the oxygen content of the oxidant fed through said one or more burners.

8. A method of operating a steam generation apparatus, comprising

(A) providing a steam generation apparatus comprising

wherein the highest mass flow rate at which said apparatus can provide steam at said outlet at a temperature within a given temperature range, when said gaseous oxidant is air and said air is the only source of oxygen for said combustion, is limited by the maximum flue gas temperature to which said heat exchangers can be exposed, by maximum rates at which said fuel and gaseous oxidant can be fed into said combustion chamber and at which said flue gas can be withdrawn therefrom, and by the maximum rate at which said coolant can be passed into said steam, and

(B) providing steam at said outlet at a temperature within said given temperature range at a desired mass flow rate which is higher than said highest mass flow rate, by

(1) feeding water into said pathway at a mass flow rate equal to the desired mass flow rate of steam and feeding fuel into said combustion chamber at a mass flow rate corresponding to the amount of heat needed to produce said steam at said desired mass flow rate, while

(2) feeding into said combustion chamber gaseous oxidant with which said fuel is combusted having a higher oxygen content than air, at a volumetric flow rate lower than the volumetric flow rate of air at which said highest mass flow rate of steam is provided when air is the only source of oxygen for said combustion, at values of said oxygen content and said volumetric flow rate which are effective to provide said desired mass flow rate of steam without exceeding any of said maxima, and

(3) combusting said fuel in said combustion chamber with said air and said gaseous oxidant having said increased oxygen content fed at said lower volumetric flow rate,

wherein the heat transfer area of said heat exchangers is the same as when air is the only source of oxygen for said combustion, and wherein flue gas produced in said combustion chamber that exits from said apparatus is not fed back into said apparatus, and the temperature of said flue gas entering said preheater is lower than when said flue gas is produced by combustion in said apparatus when air is the only source of oxygen for combustion.

9. A method according to claim 8 wherein said fuel is coal.

10. A method according to claim 8 wherein steam is provided at said outlet at a temperature within said given temperature range at said increased oxygen content and decreased volumetric flow rate of said oxidant without passing any coolant into said steam.

11. A method according to claim 8 wherein said gaseous oxidant and fuel are combusted in said combustion chamber at a plurality of burners, and at least one of said burners is at a higher elevation than at least one other burner, the method further comprising

12. A method according to claim 8 wherein said gaseous oxidant and fuel are combusted in said combustion chamber in one or more burners, and all oxidant necessary for complete combustion of said fuel is fed through said burners.

13. A method according to claim 8 wherein said gaseous oxidant and fuel are combusted in said combustion chamber in one or more burners, wherein a portion of the oxidant necessary for complete combustion of said fuel is fed through said burners and the balance of the oxidant necessary for complete combustion of said fuel is fed into said combustion chamber through one or more overfire ports separate from and vertically above said one or more burners, provided that the oxygen content of the oxidant fed through said one or more overfire ports is less than the oxygen content of the oxidant fed through said one or more burners.

14. A method of modifying a steam generation apparatus, comprising

(A) providing a steam generation apparatus comprising

(2) a preheater into which and through which said hot flue gas passes for transferring heat to said gaseous oxidant before said gaseous oxidant is fed to said combustion chamber,

the pathway further comprising one or more devices that can pass coolant into steam in the pathway to reduce the temperature of said steam, and

(4) a second pathway into which steam is fed, from which said steam is obtained at an outlet, and within which said steam is heated by heat transferred thereto in one or more additional heat exchangers in said second pathway and within said steam generation apparatus to which heat in said hot flue gas is transferred, the second pathway further comprising one or more devices that can pass coolant into steam in said second pathway to reduce the temperature of said steam therein,

15. A method according to claim 14 wherein said gaseous oxidant fed into said combustion chamber in step (A)(1) is air.

16. A method according to claim 14 wherein said fuel is coal.

17. A method according to claim 14 wherein steam is provided at said outlet at a temperature within said given temperature range at said increased oxygen content and decreased volumetric flow rate of said oxidant without passing any coolant into said steam.

18. A method according to claim 14 wherein said gaseous oxidant and fuel are combusted in said combustion chamber at a plurality of burners, and at least one of said burners is at a higher elevation than at least one other burner, the method further comprising

19. A method according to claim 14 wherein said gaseous oxidant and fuel are combusted in said combustion chamber in one or more burners, and all oxidant necessary for complete combustion of said fuel is fed through said burners.

20. A method according to claim 14 wherein said gaseous oxidant and fuel are combusted in said combustion chamber in one or more burners, wherein a portion of the oxidant necessary for complete combustion of said fuel is fed through said burners and the balance of the oxidant necessary for complete combustion of said fuel is fed into said combustion chamber through one or more overfire ports separate from and vertically above said one or more burners, provided that the oxygen content of the oxidant fed through said one or more overfire ports is less than the oxygen content of the oxidant fed through said one or more burners.

21. A method of operating a steam generation apparatus, comprising

(A) providing a steam generation apparatus comprising

22. A method according to claim 21 wherein said fuel is coal.

23. A method according to claim 21 wherein steam is provided at said outlet at a temperature within said given temperature range at said increased oxygen content and decreased volumetric flow rate of said oxidant without passing any coolant into said steam.

24. A method according to claim 21 wherein said gaseous oxidant and fuel are combusted in said combustion chamber at a plurality of burners, and at least one of said burners is at a higher elevation than at least one other burner, the method further comprising

25. A method according to claim 21 wherein said gaseous oxidant and fuel are combusted in said combustion chamber in one or more burners, and all oxidant necessary for complete combustion of said fuel is fed through said burners.

26. A method according to claim 21 wherein said gaseous oxidant and fuel are combusted in said combustion chamber in one or more burners, wherein a portion of the oxidant necessary for complete combustion of said fuel is fed through said burners and the balance of the oxidant necessary for complete combustion of said fuel is fed into said combustion chamber through one or more overfire ports separate from and vertically above said one or more burners, provided that the oxygen content of the oxidant fed through said one or more overfire ports is less than the oxygen content of the oxidant fed through said one or more burners.