Boiler cold start using pulverized coal in ignitor burners

An ignition, warm-up and low-load-stabilization system for furnaces fired by pulverized coal. In conjunction with a system in which pulverized coal is sent directly from a coal mill to a load-bearing nozzle and in which combustion air is brought to the nozzles from an air preheater that uses hot furnace gases to warm the combustion air, ignitor nozzles are provided that are supplied by pipes bearing coal from which the drying air has been separated. Combustion air for the ignitor nozzles is heated by an independent heat source that heats the combustion air or a portion thereof to a temperature higher than that of the air supplied by the air preheater. Such a coal-fired ignitor burner can replace oil or gas-fired ignitors and warm-up guns and thereby reduce the amount of oil or gas used in ignition, warm-up, and low-load stabilization.

BACKGROUND OF THE INVENTION 
The present invention relates to the field of coal-fired furnaces. It 
relates particularly to a furnace that can use coal for ignition, warm-up, 
and low-load stabilization. 
It is sometimes desirable to use coal rather than gas or oil in electrical 
generating facilities. In those situations, the utility will naturally 
have a coal-fired unit built rather than an oil-fired unit. However, even 
in coal-fired units, substantial quantities of gas or oil are often used. 
In a typical coal-fired unit, coal to be burned in the furnace is dried 
and pulverized in a coal mill and delivered directly from the coal mill to 
the load-carrying coal nozzles in the furnace. Operation of the coal mills 
requires that heated air be supplied to the mills for drying and conveying 
the coal. This air is supplied by a forced-draft fan that forces the air 
through an air preheater, a device that uses the hot products of 
combustion in the furnace to preheat the air. This preheated primary air, 
the air used for drying and conveying coal, is delivered with the coal to 
the coal nozzles and used to support combustion. The primary air is 
typically not sufficient in quantity to support combustion of all the 
coal, so secondary air is brought directly from the air preheater to the 
furnace to supply the rest of the air needed for combustion. The coal thus 
supplied with air is caused to burn due to ignition energy from the 
primary air, the secondary air, the heat in the coal itself, radiation and 
conduction from flame in the furnace, and radiation from furnace walls. 
It is to be noted that almost all of these combustion energy sources 
presuppose that the furnace has already been operating, and, in the large 
furnaces used in power generation, it presupposes that the furnace has 
been operating for a fairly long time. Accordingly, in order to cause and 
sustain combustion of the coal, it is necessary to use an auxiliary fuel 
for warming up the furnace walls, for providing ignition flame, and for 
warming up the air preheater. This is usually the function of oil- or 
gas-fired ignitors and warm-up guns. 
In a typical installation, a relatively high-capacity oil burner is started 
by an ignitor, and this starts the process or warming up the furnace walls 
and the heat-exchange surfaces of the air preheater. This can take some 
time, and the use of 70,000 gallons of oil in a 900-megawatt unit for one 
startup alone is not uncommon. In addition, there is considerable capital 
expense involved in providing the hardware that is used for supplying oil. 
Once the furnace has been brought up to temperature, the coal nozzles are 
ignited by oil- or gas-fired ignitors or by the warm-up guns themselves. 
The use of auiliary fuel is not necessarily over when the coal nozzles have 
started to supply coal. At higher boiler loads--that is, when the amount 
of coal supplied by the nozzles is great--the furnace can typically 
maintain stable combustion of the pulverized coal. However, when the load 
goes down and the coal supply is thereby decreased, the stability of the 
pulverized coal flame is also decreased, and it is therefore common 
practice to use the ignitors or warm-up guns to maintain flame in the 
furnace, thus avoiding the accumulation of unburned coal dust in the 
furnace and the associated danger of explosion. 
All of these functions of the oil- or gas-fired burners rely on the greater 
ease of ignition of these fuels; less heat is required, from whatever 
source, to liberate the volatiles and thereby initiate or sustain 
combustion. Conversely, the greater difficulty encountered in igniting 
coal is the reason why it has typically not been used for the ignition, 
warm-up and low-load-stabilization functions. An incidental advantage of 
oil and gas that also contributes to the greater desirability of their use 
for these functions is that it is possible to supply them in relatively 
small pipes, thereby keeping their contribution to the congestion in the 
fuel-nozzle area to a minimum. The usual method of supplying coal to 
nozzles has required rather large piping, and the addition of more 
large-size piping would not be welcome in the area immediately behind the 
fuel nozzles. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the present invention to accomplish the 
functions of ignition, warm-up, and low-load stabilization with the use of 
a minimum of auxiliary fuel. 
Accordingly, in a furnace system that includes a furnace, a main coal 
nozzle arranged to direct coal into the furnace, an air preheater having a 
flue-gas inlet, an air inlet, and an air outlet and being positioned to 
receive flue gases from the furnace and transfer heat from the flue gases 
to the air entering the air inlet and leaving the air outlet, a main 
pulverizer, a conduit positioned to conduct coal from the pulverizer 
outlet to the main coal nozzle, means for forcing a first air stream from 
the preheater outlet, through the pulverizer, and into the nozzle, and 
means for forcing a second air stream from the preheater outlet into the 
furnace, there is provided according to the present invention an ignitor, 
warm-up, and low-load-stabilization system comprising: an ignitor nozzle 
positioned for ignition of coal leaving the main coal nozzle, and ignitor 
pulverizer for pulverizing coal, a separator for separating coal from air, 
means for conveying coal mixed with air from the ignitor pulverizer to the 
separator, means for conveying coal from the separator to the ignitor 
nozzle, means for causing a third air stream having a temperature higher 
than the temperature of either the first or the second air stream to flow 
to the ingitor nozzle, and a lighter, positionable near the outlet of the 
ignitor nozzle, for igniting coal issuing from the ignitor nozzle.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows the system for supplying air and fuel to the load-carrying 
nozzles of a pulverized-coal boiler. The furnace is generally shown at 10. 
A conduit 36 connects the outlet of the furnace to the flue-gas inlet of 
air preheater 38. Conduit 40 connects the flue-gas outlet to a stack, not 
shown, that releases the products of combustion to the atmosphere. A fan 
42 draws from the atmosphere and blows air through the air inlet of air 
preheater 38. Conduit 34 connects the air outlet of air preheater 38 to 
windboxes 12 and 30 located on either side of the furnace. The typical 
furnace would actually have four windboxes, one at each corner, but, for 
the sake of simplicity, only two are shown. Another conduit 32 conducts 
air from conduit 34 to the air inlet of pulverizer 22. The outlet of 
pulverizer 22 is connected by conduit 21 to exhauster 20, whose outlet 
communicates with several conduits. Conduits 18 and 24 lead from the 
exhauster outlet to coal nozzles 19 and 25, which are arranged so as to 
direct coal fed to them into the interior of furnace 10. Nozzels 16 and 26 
are fed by a second pulverizer-exhauster combination that is not shown in 
the drawing, while a third pulverizer-exhauster combination, also not 
shown, feeds nozzles 14 and 28. Again, for each pair of nozzles shown 
there is typically another pair of nozzles not shown that is fed by the 
same pulverizer. 
Windboxes 12 and 30 communicate with the interior of the furnace through 
openings in the vicinity of the nozzles. Dampers, not shown in the 
drawing, control the allocation of air from the windbox among the 
openings. 
In normal operation, coal and air enters furnace 10 through one or more 
elevations of nozzles. Combustion takes place in the interior of furnace 
10, producing hot flue gases that flow out conduit 36, through air 
preheater 38, and through conduit 40 to a stack. Air preheater 38 has 
moving heat-exchange surfaces that alternately contact the hot flue gases 
and the air entering the preheater from fan 42. The surfaces thus absorb 
heat from the flue gases and release it to the air from fan 42. Part of 
the heated air leaving air preheater 38 passes through conduit 32 and into 
pulverizer 22. Pulverizer 22 is an apparatus for drying and crushing coal, 
and the hot air brought by conduit 32 is used to dry the coal. The air 
stream flowing in conduit 32 and pulverizer 22 also flows through conduit 
21, exhauster 20, and conduits 18 and 24 to the associated nozzles. In 
flowing through pulverizer 22, the air stream entrains the coal that has 
been sufficiently pulverized and carries it to nozzles 19 and 25. Since 
fan 42 and exhauster 20 both provide motivating force for this motion, it 
can be seen that together they constitute means for forcing a first air 
stream from the air outlet of preheater 38, through pulverizer 22, and 
into either nozzle 19 or nozzle 25. 
The air forced by fan 42 and exhauster 20 through pulverizer 22 is referred 
to as primary air and is delivered with the coal to main coal nozzles 19 
and 25. However, there is not usually enough primary air to support 
combustion of all of the coal, so some of the air leaving air preheater 38 
goes through conduit 34 to windboxes 12 and 30. Windboxes 12 and 30 supply 
the secondary air, the remainder of the air required to support combustion 
of all the coal. 
It is evident that the above discussion presupposes that hot flue gases are 
flowing through conduit 36. Of course, at the beginning of furnace 
operation, the gases flowing through the conduit 36 are relatively cool. A 
typical coal-fired unit includes supplementary burners that burn oil or 
natural gas, and it is the function of these burners to operate when the 
gases coming through conduit 36 are relatively cool. This is because 
pulverized coal is relatively difficult to ignite, and stable combustion 
cannot be guaranteed unless significant amounts of heat energy are present 
in the combustion area. This heat energy that is used to start or maintain 
combustion comes from many sources. It could come directly by radiation 
from flame that is already in the furnace, by radiation from the walls of 
the furnace, by conduction from the generally hot gases in the furnace, or 
by conduction from the primary and secondary air flowing into the the 
furnace. In actuality, all of these sources contribute to the ignition 
energy, and at high-load conditions they all add up to a sufficient amount 
of ignition energy for stable combustion of the coal. However, in many 
situations the combustion of these energy sources is not sufficient to 
guarantee stable combustion. One of these situations is that of a cold 
furnace, in which there is little radiation from the furnace walls and 
little energy transferred to the primary and secondary air by the air 
preheater. In such cases the supplementary burners are used. Another 
situation in which supplementary burners are used is the case in which the 
furnace is operated at a relatively low load, when the amount of reactants 
burning is sufficiently low to cause a reduction in the energy derived 
from the various sources. In this case again, supplementary burners are 
used to maintain stable combustion. In the past, these supplementary 
burners have all burned oil or natural gas. This is a natural choice, 
since oil and natural gas are much easier to light than pulverized coal 
is. 
FIG. 2 shows a system that enables the supplementary burners to be fired by 
pulverized coal. A ignitor pulverizer 110 receives air at inlet 112 from 
air preheater 38 of FIG. 1. Conduit 100 conducts the coal-air mixture 
leaving pulverizer 110 to exhauster 102, and conduit 98 connects the 
outlet of exhauster 102 to further conduits 96. Conduits 96 lead to 
cyclone separators such as separator 65. The number of such separators 
depends on the designer; only one is necessary, but more could be used. 
The outlet of separator 65 is connected by an air line 62 to a point in 
the interior of the furnace remote from the fuel nozzles. Bin 66 is 
positioned to receive the coal leaving separator 65, and the outlet of the 
bin is controlled by valve 67. Coal from bin 66 is fed through coal pipe 
70 to approximately valved coal pipes 74, 78, and 82, each of which 
terminates in coal nozzles not shown in FIG. 2. Similar coal pipes 86, 90, 
and 94 also receive coal either from coal bin 66 or another coal bin not 
shown and feed it to nozzles positioned at their exits. 
Those skilled in the art will recognize that it is not essential that 
pulverizer 110 be a separate pulverizer. The functions of pulverizer 110 
and pulverizer 22 could be combined in the same pulverizer, the output 
being divided between a direct connection to the furnace and a connection 
to a separator 65. Accordingly, the main pulverizer and the ignitor 
pulverizer in the claims can be embodied in the same hardware. 
Fan 118 draws air from the air preheater shown in FIG. 1, and this air 
stream is divided among conduits 119, 120 and 122. Conduit 119 feed an 
in-duct air heater, possibly an electric heater, and the output of air 
heater 116 is sent by means of conduit 114 to the ignitor nozzles at the 
ends of coal pipes 82 and 94. The temperature of the air leaving air 
heater 116 is preferably between 300.degree. F. and 1000.degree. F. A 
similar heater and similar connections exist between conduit 120 and the 
nozzles at the end of coal pipes 78 and 90 and between conduit 122 and the 
nozzles at the ends of coal pipes 74 and 86. 
FIG. 3 shows an ignitor nozzle of the type that would be fed by coal pipe 
82. The ignitor nozzle is actually made of three concentric nozzles 128, 
130 and 134. Nozzles 128 and 134 are both fed by conduit 80, which is 
attached to nozzle 128 by flexible connector 126. Coal pipe 82 is 
connected through ball joint 138 to coal-pipe extension 144. Interior to 
and concentric with coal pipe 82 and coal-pipe extension 144 is lighter 
142. Lighter 142 may be a small version of an ordinary coal-/or gas-fired 
ignitor, or it may be a high-energy arc ignitor. In either case, the 
ignitor is flexible at least through the area of the ball joint in order 
to allow it to move with coal-pipe extension 144. Air conduit 124 
communicates with windbox 12 of FIG. 1 and has nozzle 130 fitted on its 
exit. Accordingly, nozzle 130 is in communication with windbox 12. A 
typical unit would have a discriminating flame detector 132 of any desired 
type in order to determine whether or not there is flame at the end of the 
ignitor nozzle. 
To start up the furnace when it is cold, pulverizer 110 is started, 
receiving coal at its inlet and crushing it. The air inlet of pulverizer 
110 receives air that has been blown through air preheater 38 by fan 42. 
In a cold start-up, this air is still relatively cool. The cool air is 
blown through pulverizer 110, conduit 100, exhauster 102, and conduits 98 
and 96 to separator 65. Separator 65 removes the coal that has been 
entrained by the air blown through pulverizer 110, and it drops it into 
bin 66. Simultaneously, the air separated from the coal is exhausted into 
the furnace through line 62. Alternately, bin 66 could be a storage bin 
large enough to hold the amount of coal needed for a startup. In such a 
case, the pulverized coal left in bin 66 from previous operation of the 
furnace would fuel the operation until the furnace has heated up. Inerting 
line 64 is used to maintain an atmosphere in bin 66 during storage that 
discourages spontaneous combustion. After the furnace has heated up, 
ignitor pulverizer 110 starts to work, replenishing the supply of stored 
coal in bin 66. 
Whichever method is used, coal is supplied by bin 66. Valve 67 regulates 
the amount of coal that is allowed to fall from bin 66, and this coal is 
forced by appropriate means through conduits 70 and 82 and out the ignitor 
nozzle. Similarly, coal is also forced through coal pipe 94 and through 
the nozzle fitted at its exit. Due to the fact that the coal is sent to 
conduits 82 and 94 with almost no air, coal pipes 82 and 94 can be made 
relatively small, so they do not contribute to the congestion in the 
furnace corners. At the same time that the coal is being delivered to the 
ignitor nozzles, air from preheater 38 is forced by fan 118 through 
conduit 119 to heater 116. Heater 116 heats the air to a temperature high 
enough to provide stable combustion. Without heater 116, the only heat in 
the air would be that imparted to it by air preheater 38, and on a cold 
start this is not very much heat. The hot air leaving heater 116 is fed by 
conduit 114 to conduits 80 and 92. Part of the air flowing through conduit 
80 passes through nozzle 134 of FIG. 3. According to the present state of 
the art, nozzle 134 may have vanes 136 to properly direct the air flow, 
and this air flow imparts an appropriate flow pattern to the coal that 
leaves the openings of coal-pipe extension 144. 
It is to be noted that the present system allows the amount of heat 
introduced by air heater 116 to be kept to a minimum. Since the air that 
is heated is used only to add to the ignition energy at the ignitor 
nozzle, the necessity of adding heat to the entire volume of air flowing 
through preheater 38 is avoided. Furthermore, since the inert water vapor 
that results from the drying of the coal has been separated from the coal 
before the coal reaches the ignitor nozzles, none of the energy supplied 
by air heater 116 is used up in heating inerts. The rest of the air that 
flows through conduit 80 is conducted through nozzle 128 and past vanes 
140, which also impart a flow pattern appropriate for stable combustion. 
Though the amount of air heated by heater 116 will normally be kept as low 
as possible, system designs may provide sufficient capacity to heat 100 
percent stoichiometric air if required. Thus, the amount of air supplied 
through nozzles 128 and 134 may be stoichiometrically sufficient for 
combustion of the coal. If it is not, windbox air will be introduced 
through nozzle 130. Even if the amount of heated air introduced through 
nozzles 128 and 134 is sufficient for combustion of all the coal, however, 
it may be desirable, depending on the characteristics of nozzles 128 and 
134 and vanes 136 and 140, to introduce windbox air in order to cause a 
flow pattern adapted to feeding hot combustion products back into the 
combustion zone, thereby contributing to ignition energy and the stability 
of the ignitor flame. 
Typically, the coal leaving coal-pipe extension 144 would have its 
volatiles liberated by lighter 142, and combustion of some of the coal 
would also be started in the presence of the air flowing through ignitor 
134. The remainder of the air needed for combustion would be supplied by 
nozzle 128, so combustion is completed after the coal and air leaving 
nozzle 134 meets the air in nozzle 128. As was noted before, the air 
coming through conduit 80 is hot enough so that its contribution to 
ignition energy provides for a stable flame. 
It is to be understood that the nozzle of FIG. 3 is merely illustrative; it 
merely shows the functions that would typically be performed by a nozzle 
used with the present invention. 
The stable flame at the outlet of the ignitor nozzle begins to warm the 
furnace walls and steam pipes, and as they warm up, the flue-gas 
temperature increases. Eventually, the air preheater becomes hot enough 
for operation of the main coal nozzles, and their pulverizers are started. 
The coal issuing from the main coal nozzles is ignited by flame from the 
ignitor nozzles, and normal operation beings. If the furnace is operating 
at low loads, the ignitor nozzles remain on, providing low-load 
stabilization. It may be determined that the cost penalty in leaving the 
ignitors in operation is minor, so they may be left operating even at high 
loads. 
While the invention has been described in terms of a specific embodiment, 
the use of a specific embodiment is by no means meant as a limitation. 
Accordingly, any modification within the scope of the appended claims that 
is apparent to those skilled in the art in light of the foregoing 
description is meant to be included in the invention.