[0001] Embodiments as described herein relate generally to coal fired
boilers operating in conventional steam power plants. More particularly, a method
of and an arrangement for recovering heat from bottom ash of a combustion process
performed in a combustion device, from which bottom ash is removed at a high
temperature. The described embodiments are applicable to all types of coal fired
boilers, but more specifically applicable to pulverized fuel fired boilers.
BACKGROUND
[0002] A boiler typically includes a furnace in which fuel is burned to
generate heat to produce steam. The combustion of the fuel creates thermal energy
or heat, which is used to heat and vaporize a liquid, such as water, which makes
steam. The generated steam may be used to drive a turbine to generate electricity
or to provide heat for other purposes. Fossil fuels, such as pulverized coal, natural
gas and the like are typical fuels used in many combustion systems for boilers.
[0003] An ordinary boiler arrangement is comprised of a furnace to which
fuel mostly pulverized coal and combustion air are introduced. When combusting
the fuel, heat is generated and both bottom ash and flue gases are formed. The flue
gases are taken through different heat transfer pressure parts within boiler
arrangement and heat in flue gases used to generate steam. The heat is transferred
into use, such as, for instance, for generating electricity by means of steam turbines
and generators, in the form of high temperature steam. After the superheaters and
reheaters, the flue gases flow through an economizer that, again, collects heat from
the flue gases to boiler feed water, i.e., water or condensate that is returning to the
boiler from the use, for instance, from the turbines. Commonly, a final step of
collecting heat from the flue gases takes place in a combustion air preheater, where
the flue gas heat is used to preheat the air that is used as combustion air in the
furnace. The preheater is normally a rotary or tubular preheater. The combustion
air preheater is followed in the flue gas path by an electrostatic filter/precipitator
3
that separates any solid particles left in the flue gases before the flue gases are
vented to the atmosphere by means of a flue gas fan via a stack.
[0004] The boiler feed water entering the economizer originates, typically,
as already mentioned above, from the use in steam turbines and a condenser
downstream of the steam turbines. The condensate is getting collected in hotwell
after expansion in steam turbine and latent heat removal in condenser. The
condensate is first heated by gland steam condenser and then by steam extracted
from the steam turbines in one or more low-pressure preheaters until the condensate
is introduced into the feed water tank, which is used to deaerate the water, and
sometimes, to heat the water further, before pumping it towards the economizer.
The feed water pumped from the feed water tank by means of a pump may further
be heated by means of a high-pressure preheater before entering the economizer. In
addition to condensate return, make up system will add water in hot well to
compensate water lost in system leakages.
[0005] The common name for the material discharged from the boiler
furnace through the bottom ash hopper is bottom ash. It contains non-burning
material, clinker, unburnt fuel particles, etc. Normally, the bottom ash is discharged
in a water filled trough or to water- or air-cooled conveyors where it gets cooled.
The cooled bottom ash is then taken out of the plant to be dumped, or sometimes
used as construction material.
[0006] Thus, in conventional boilers, the loss of heat energy in the discharge
of the bottom ash forms a significant portion of boiler losses. This is even more so
with certain high ash content fuels, i.e., when the estimated bottom ash content of
the fuel is high. The reason for the high loss of energy is that the bottom ash to be
removed from the furnace is high in temperature, usually, about 1000°C to about
1200°C. For example, if the bottom ash flow from the boiler is 1 kg/s at a
temperature of 1100°C., using the reference temperature of 25°C. and the heat
capacity for ash of 1 KJ/kg, an energy loss of 1 MW while discharging bottom ash
can be expected.
4
[0007] What is needed is a simple efficient system for recovering heat from
bottom ash to improve the efficiency of power generation systems.
BRIEF DESCRIPTION
[0008] In an embodiment, described herein is a method of recovering heat
from bottom ash that is discharged from a combustion process in a combustion
system of a boiler. The method includes generating bottom ash in a combustion
system, by combusting fuel to produce heat energy in a boiler, thereby generating
bottom ash and discharging the bottom ash from the combustion system to a bottom
ash cooling circuit. The method also includes capturing heat, with a bottom ash
cooling circuit having a heat exchanger, from the bottom ash discharged from the
combustion system in order to utilize the recovered heat in a boiler water heating
circuit for preheating boiler water and exchanging the heat from the bottom ash
cooling circuit to the boiler water heating circuit and heating the boiler water.
[0009] Also described herein in yet another embodiment is a system for
recovering heat from bottom ash that is discharged from a combustion process of a
boiler. The system includes a combustion system for combusting fuel with
combustion air, in order to generate heat energy for the boiler, in which bottom ash
is generated, a discharge for discharging the bottom ash from the combustion
system, a bottom ash cooling circuit configured to recover heat from the bottom
ash, the bottom ash cooling circuit including at least a heat exchanger and operably
connected to a first path of the heat exchanger, and a boiler water heating circuit
operably connected to a second path of the heat exchanger, the boiler water heating
circuit receives heat via the heat exchanger of the bottom ash cooling circuit, the
boiler water heating circuit directing heated boiler water to the boiler.
[00010] Also described herein, in another exemplary embodiment is a boiler
system having a bottom ash recovery system, the boiler system including a boiler
having a combustion system for combusting fuel with combustion air, in order to
generate heat energy for the boiler, in which bottom ash is generated, a discharge
for discharging the bottom ash from the combustion system, and a bottom ash
5
cooling circuit configured to recover heat from the bottom ash. The bottom ash
cooling circuit including at least a heat exchanger and operably connected to a first
path of the heat exchanger, and a boiler water heating circuit operably connected to
a second path of the heat exchanger. The boiler water heating circuit receives heat
via the heat exchanger of the bottom ash cooling circuit, the boiler water heating
circuit directing heated boiler water to the boiler.
[00011] Additional features and advantages are realized through the
techniques of the present disclosure. Other embodiments and aspects of the
disclosure are described in detail herein. For a better understanding of the disclosure
with the advantages and the features, refer to the description and to the drawings.
DRAWINGS
[00012] The described embodiments will be better understood from reading
the following description of non-limiting embodiments, with reference to the
attached drawings, wherein below:
[00013] FIG. 1 is a simplified schematic illustration of a power generation
system in accordance with an embodiment;
[00014] FIG. 2 is a schematic illustration of a boiler of the power generation
system of FIG. 1, in accordance with an embodiment;
[00015] FIG. 3 is a schematic illustration of a boiler of the power generation
system of FIGS. 1 and 2, in accordance with an embodiment; and
[00016] FIG. 4 is a block diagram illustration of a control routine for boiler
reheating in the power generation system in accordance with an embodiment.
DETAILED DESCRIPTION
[00017] Reference will be made below in detail to exemplary embodiments
as described herein, examples of which are illustrated in the accompanying
drawings. Wherever possible, the same reference characters used throughout the
6
drawings refer to the same or like parts. While the various embodiments as
described herein are suitable for use with heat recovery steam generation systems
that include combustion system, generally, a pulverized coal boiler such as for use
in a pulverized coal power plant has been selected and described for clarity of
illustration. Other systems may include other types of boilers, furnaces and fired
heaters that generate bottom ash. For example, contemplated boilers include, but
are not limited to, T-fired and wall fired pulverized coal boilers.
[00018] Embodiments as described herein relate to a power generation
system having a heat recovery steam generation system including a combustion
system and method and control scheme therefor that provides for improving and
reducing heat loss in boiler systems. In particular embodiments are related to a
system and method that provides method and system that utilizes the waste heat
from bottom ash of coal fired boiler by direct and indirect heating of make up and
condensate water.
[00019] FIG. 1 illustrates a power generation system 10 including a heat
recovery steam generation system with combustion system 11 having a boiler 12 as
may be employed in power generation applications. The boiler 12 may be a
tangentially fired boiler (also known as a T-fired boiler) or wall fired boiler. Fuel
and air are introduced into the boiler 12 via the burner assemblies 14 and/or nozzles
associated therewith. The combustion system 10 includes a fuel source such as, for
example, a pulverizer 16 that is configured to grind fuel such as coal to a desired
degree of fineness. The pulverized coal is passed from the pulverizer 16 to the
boiler 12 using primary air. An air source 18 provides a supply of secondary or
combustion air to the boiler 12 where it is mixed with the fuel and combusted, as
discussed in detail hereinafter. Where the boiler 12 is an oxy-fired boiler, the air
source 18 may be an air separation unit that extracts oxygen from an incoming air
stream, or directly from the atmosphere.
[00020] The boiler 12 includes a hopper zone 20 located below a main burner
zone 22 from which ash may be collected for subsequent removal as described later
7
herein. The material discharged from the boiler 12 following combustion through
the hopper 20 is commonly called bottom ash. The bottom ash contains non-burning
material, clinker, unburnt fuel ash particles, etc. The bottom of the boiler 12 is
provided with a hopper 20, for removing bottom ash and other debris from the boiler
12. In some systems, the bottom ash is discharged in a water filled trough 35 where
it gets cooled. The cooled bottom ash is then taken out of the bottom ash hopper
35 (water filled trough) to be dumped, or sometimes used as construction material.
In other combustion systems 11, the bottom ash is conveyed away for subsequent
uses. In an embodiment, a system and method is described for capturing the heat
from this bottom ash as part of the cooling process.
[00021] The boiler 12 also includes a main burner zone 22 (also referred to
as a windbox) where the air and an air-fuel mixture is introduced into the boiler 12,
a burnout zone 24 where any air or fuel that is not combusted in the main burner
zone 22 gets combusted, a superheater zone 26 with superheater 27 where steam
can be superheated by the combustion flue gases. The boiler 12 also includes an
economizer zone 28 with an economizer 31 where water can be preheated prior to
entering a mixing sphere or drum (25) to feed water to the waterwall 23. The boiler
feed water entering the economizer 31 originates from the use in the steam turbine
50 and a condenser 57 downstream of the steam turbines 50. The condensate is first
heated by a gland steam condenser 101 and then steam by means of one or more
low-pressure preheaters 102 until the condensate is introduced into the feed water
tank 103, which is used to deaerate the water, and, sometimes, to heat the water
before the water is pumped to the economizer 31. The feed water pumped from the
feed water tank 103 by means of a pump 104 may further be heated by means of a
high pressure preheater 105 before entering the economizer 31. Pumps 40 may be
employed to aid in circulating preheat water to the waterwall 23 and through boiler
12.
[00022] Combustion of the fuel with the primary and secondary air within
the boiler 12 produces a stream of flue gases that are ultimately treated and
exhausted through a stack downstream from the economizer zone 28. The often
8
final step of collecting heat from the flue gases takes place in the combustion air
preheater 17, where the flue gas heat is used to heat the air that is used as
combustion air in the combustion system 11. The air preheater 17 is followed in the
flue gas path by an electrostatic filter/precipitator or a bag filter (not shown) that
separates any solid particles left in the flue gases before the flue gases are vented to
the atmosphere via a stack. As used herein, directions such as “downstream” means
in the general direction of the flue gas flow. Similarly, the term “upstream” is
opposite the direction of “downstream” going opposite the direction of flue gas flow.
[00023] Generally, in operation of the power generation system 10 and
combustion system 11, the combustion of fuel in the boiler 12 heats water in the
waterwalls 23 of the boiler 12, which then passes through the steam drum (or
equivalent), hereinafter referred to as drum 25 to the superheater 27 in the
superheater zone 26 where additional heat is imparted to the steam by the flue gases.
The superheated steam from the superheater 27 is then directed via a piping system
shown generally as 60 to a high pressure section 52 of turbine 50, where the steam
is expanded and cooled to drive turbine 50 and thereby turn a generator 58 to
generate electricity. The expanded steam from the high pressure section 52 of the
turbine 50 may then be returned to a reheater 29 downstream from the superheater
27 to reheat the steam, which is then directed to an intermediate pressure section 54
of turbine 50, and ultimately a low pressure section 56 of the turbine 50 where the
steam is successively expanded and cooled to drive turbine 50.
[00024] As illustrated in FIG. 1, the combustion system 11 includes an array
of sensors, actuators and monitoring devices to monitor and control the combustion
process and the resulting consequences with respect to boiler operation. For
example, temperature and pressures monitors shown generally as 36 are employed
throughout the system to ensure proper control, operation and ensure that
operational limits are not exceeded. In another example, the combustion system 11
may include a plurality of fluid flow control devices 30 that supply secondary air
for combustion to each fuel introduction nozzle associated with the burner
assemblies 14. In an embodiment, the fluid flow control devices 30 may be
9
electrically actuated air dampers that can be adjusted to vary the amount of air that
is provided to each fuel introduction nozzle associated with each burner assembly
14. The boiler 12 may also include other individually controllable air dampers or
fluid flow control devices (not shown) at various spatial locations around the
furnace. Each of the flow control devices 30 is individually controllable by a
control unit 100 to ensure that desired air/fuel ratios and flame temperature are
achieved for each nozzle location.
[00025] FIG. 1 also illustrates that the backpass 33 of the boiler 12
downstream from the superheater 27, reheater 29, and economizer 31 in economizer
section 28 is fitted with a monitoring device 37. The monitoring device 37 is
configured for measurement and assessment of gas species such as carbon
monoxide (CO), carbon dioxide (CO2), mercury (Hg), sulfur dioxide (SO2), sulfur
trioxide (SO3), nitrogen dioxide (NO2), nitric oxide (NO) and oxygen (O2) within
the backpass 33. SO2 and SO3 are collectively referred to as SOx. Similarly, NO2
and NO are collectively referred to as NOx.
[00026] Continuing with the operation of the boiler 12, in operation, a
predetermined ratio of fuel and air is provided to each of the burner assemblies 14
for combustion. As the fuel/air mixture is combusted within the furnace and flue
gases are generated, the combustion process and flue gases are monitored. In
particular, various parameters of the fireball and flame, conditions on the walls of
the furnace, and various parameters of the flue gas are sensed and monitored. These
parameters are transmitted or otherwise communicated to the combustion control
unit 100 where they are analyzed and processed according to a control algorithm
stored in memory and executed by a processor. The control unit 100 is configured
to control the fuel provided to the boiler 12 and/or the air provided to the boiler 12,
in dependence upon the one or more monitored combustion and flue gas parameters
and furnace wall conditions.
[00027] Furthermore, the power generation system 10 also includes an array
of sensors, actuators and monitoring devices to monitor and control the heating
10
processes associated with steam generation, and reheating in accordance with the
described embodiments. For example, the power generation system 10 may include
a plurality of fluid flow control devices (not shown), that control the flow of water
or steam in the system 10. In an embodiment, the fluid flow control devices 30 may
be electrically actuated valves that can be adjusted to vary the amount of flow there
through. Each of the flow control devices e.g., 66 is individually controllable by a
control unit 100. The power generation system 10 may also include a plurality of
sensors operable to monitor various other operational parameters of the power
generation system 10 for example temperature and pressure sensors may by
employed as needed to monitor the operation and effect in numerous parts of the
system 10. In an embodiment, the temperature and pressure sensors may each be
operably connected to the control unit 100 or another controller as needed to
implement the methodologies and functions described herein.
[00028] FIGS. 2-3 schematically illustrate several variations of the
associated with the described embodiments, in connection with a boiler, though the
bottom ash cooling arrangement of the described embodiments may as well be used
in other boiler types in which bottom ash is discharged. A common feature to all
embodiments is that the bottom ash is discharged from the hopper 20 to a bottom
ash cooling device 35, for example, water-cooled scraper. The water cooled scraper
is normally provided with cold water from a cooling water source (not shown) e.g.,
pond or similar storage facility where heat is dissipated to ambient. Other applicable
bottom ash cooling device(s) 35are water-cooled drum coolers, or a cooled transport
screw arrangement driven by means of a motor just to name a couple of alternatives.
Another common feature of the described embodiments is that the water used for
cooling the bottom ash in the cooling device(s) 35 is used for preheating water for
the boiler 12.
[00029] FIG. 2 illustrates a simplified diagram of a bottom ash heat recovery
system 60 in accordance with an embodiment. The bottom ash recovery system 60
includes the bottom ash cooling device 35, which comprises a portion of a bottom
ash cooling circuit 42, which further comprises at least a \slurry circulation pump
11
44, and a heat exchanger 46. The bottom ash cooling circuit 42 includes the water
cooled scraper 35. The water cooled scraper 35 is normally provided with cold
water from a cooling water source (not shown) e.g., pond or similar storage facility
where heat is dissipated to ambient. In an embodiment, the cooling water is mixed
with the hot bottom ash. Cooled ash is separated by the scraper 35 for removal and
disposal. Hot slurry (heated water with some remaining ash) is then pumped via
pump 44 to heat exchanger 46. Heat exchanger 46 transfers heat from the slurry to
the boiler makeup water circuit, shown generally as 39. Thus, the heat from the
bottom ash cooling water circulating in the cooling circuit 42 is transferred via heat
exchanger 46 to the boiler makeup water via closed boiler water circuit 39. In an
embodiment, the boiler makeup water circuit 39, includes, but is not limited to, in
addition to the heat exchanger 46, at least a circulation pump 41. In an embodiment
the heat exchanger 46 is positioned between pump 41 and water storage tank 43.
The water storage tank 43 then feeds heated water to the hot well 48, which also
receives condensate from the condenser 57.
[00030] FIG. 2 also shows additional optional equipment arranged in both
the cooling circuit 42, and the boiler makeup water heating circuit 39. Optionally,
the bottom ash cooling circuit 42 is provided with a second heat exchanger 46a
connected to the side of (or in parallel with) the first heat exchanger 46. In practice,
the cooling water coming from the bottom ash cooling device 35 is divided, in this
additional embodiment, by means of a valve (not shown) between the two heat
exchangers 46 and 46a, depending on the amount of cooling required to cool the
bottom ash, or the desired heating of the makeup water. Whenever more heat from
the bottom ash is recovered, or is needed to be captured to ensure a low enough
temperature of the bottom ash in its discharge than that needed for heating the
makeup water, a portion of the cooling water flow may be directed to the second
heat exchanger 46a, so that the excess heat is transferred by the second heat
exchanger 46a from the cooling water circuit 42 to another cooling water circuit 62.
In other words, the operation of the valve (not shown) opening the cooling water
flow path to the second heat exchanger 46a is, preferably, controlled at least in part
12
by at least one of the temperature of the bottom ash in its discharge from the scraper
35, the temperature of the makeup water exiting the heat exchanger 46, the
temperature of the cooling water after the heat exchanger 46. This connection
ensures a low enough temperature for the bottom ash discharged by the scraper 35.
In yet another embodiment, the boiler water heating circuit 39 may also be provided
with a second heat exchanger 64, in addition to the first one 46, for heating the
makeup water. The second heat exchanger 64 is arranged in series with the first
heat exchanger 46 for providing the boiler water heating circuit 39 with additional
heat for heating the makeup water. The heat exchanger 64 may, for instance, be
used in start-up or partial load situations when neither the flue gas heat recovery
nor the bottom ash heat recovery, nor such together, are able to provide a sufficient
amount of heat for the heating of the makeup water. The use of the second heat
exchanger 64, i.e., the steam flow therein, may preferably, but not necessarily, be
controlled at least in part based on at least one of the temperature of the flue gases
upstream and/or downstream of the combustion air preheater 17, the temperature of
the bottom ash in its discharge from the scraper 35, the temperature of the partially
heated water exiting the heat exchanger 64, and the temperature of the feed water
exiting the economizer 31.
[00031] Another option for arranging the heat exchanger in the cooling
circuit 42 is to position the heat exchanger 46a of the second cooling circuit 62
described above in series with (and not in parallel to) the heat exchanger 46 of the
first cooling circuit 42. This connection functions, in practice, so that the hot water
from the bottom ash heat recovery system 60 first heats the water in the boiler water
heating circuit 39 by the heat exchanger 46, and then, further flows to the heat
exchanger 46a to be cooled further, the cooled water/slurry is then returned to the
bottom ash heat recovery system 60 and the pond/storage by means of flow
produced by the pump 44.
[00032] FIG. 3 illustrates a bottom ash heat recovery system, now identified
as 160 in accordance with another embodiment. In fact, the embodiment of FIG. 3
is a modification and expansion of the embodiments depicted in FIG. 2, wherein
13
like components are referenced alike, and where reference numerals are
incremented by 100 to indicate the additional embodiment(s). The bottom ash
recovery system 160 includes the bottom ash cooling device 135, which comprises
a portion of a bottom ash cooling circuit 142, which further comprises at least a
slurry circulation pump 44, and a heat exchanger 146. The bottom ash cooling
circuit 142 includes the water cooled scraper 35 as described herein. Hot slurry
(heated water with some remaining ash) is then pumped via pump 44 to heat
exchanger 146. Once again, heat exchanger 146 transfers heat from the slurry to
the boiler water circuit, shown generally as 139. In an embodiment, the boiler water
circuit 139, includes, but is not limited to, in addition to the heat exchanger 146, at
least a circulation pump 41. In this embodiment, the heat exchanger 146 is
advantageously positioned in the boiler 12 water circuit downstream of the pump
141, the hot well 48 and condenser 57. Advantageously in this embodiment, all
water drawn by the boiler 12 (and not just makeup water) including the condensate
from the turbine 50 is heated by the bottom ash recovery system 160. Such a
configuration is highly beneficial in that with the heat exchanger 146 directly in the
boiler water flow, all water and condensate heated by the boiler 12 is preheated by
the heat exchanger 146 and the bottom ash recovery system 160. As a result, much
greater amounts of heat can be recovered from the bottom ash recovery system. For
example one benefit comes from reduction in extraction steam flow, thereby excess
steam will pass through the turbine & generate extra power for same heat input thus
increasing the overall plant level heat rate.
[00033] FIG. 3 once again also depicts additional optional equipment
arranged in both the cooling circuit 42, and the boiler water heating circuit 39.
Optionally, the bottom ash cooling circuit 142 is provided with a second heat
exchanger 146a connected to the side of (or in parallel with) the first heat exchanger
146. In practice, the cooling water coming from the bottom ash cooling device 135
is divided, in this additional embodiment, by means of a valve (not shown) between
the two heat exchangers 146 and 146a, depending on the amount of cooling required
to cool the bottom ash, or the desired heating of the boiler water. Whenever more
14
heat from the bottom ash is recovered, or is needed to be captured to ensure a low
enough temperature of the bottom ash when discharged than needed for heating the
makeup water, a portion of the cooling water flow may be directed to the second
heat exchanger 146a, so that the excess heat is transferred by the second heat
exchanger 146a from the cooling water circuit 142 to another cooling water circuit
162. In other words, the operation of the valve (not shown) opening the cooling
water flow path to the second heat exchanger 146a is, preferably, controlled at least
in part by at least one of the temperature of the bottom ash in its discharge from the
scraper 135, the temperature of the boiler water exiting the heat exchanger 146, the
temperature of the cooling water after the heat exchanger 146. This connection
ensures a low enough temperature for the bottom ash discharged by the scraper 135.
[00034] Another option for arranging the heat exchanger in the cooling
circuit 142 is to position the heat exchanger 146a of the second cooling circuit 162
described above in series with (and not in parallel to) the heat exchanger 146 of the
first cooling circuit 142. This connection functions, in practice, so that the hot water
from the bottom ash heat recovery arrangement 135 first heats the water in the boiler
water heating circuit 139 by the heat exchanger 146, and then, further flows to the
heat exchanger 146a to be cooled further, such that the water may be returned to
storage pond of the bottom ash heat recovery system 135 by means of the pump 44.
[00035] The embodiments of the present invention discussed above offer a
number of different ways to control both the temperature of the bottom ash exiting
the cooling system and the heating of the boiler water/makeup water.
[00036] Further, it has to be understood that the heat exchangers e.g., 46,
46a, and the like used for heating the boiler water/makeup water may be positioned
upstream of the pumps 44, 41, and not necessarily thereafter as depicted. And,
finally, it should be understood that the word “condensate” should be understood
broadly also to cover such water returning to the boiler arrangement that has not
been in a gaseous state (i.e., steam), but has been hot water that has been cooled
down in use.
15
[00037] It will be appreciated that while the examples provided are described
with respect to a controlled circulation boiler, such descriptions are merely
illustrative. Other configurations for the boiler 12 as are employed in steam
generation heat recovery systems are possible, including, but not limited to natural
circulation boilers, and supercritical boilers.
[00038] FIG. 4 depicts a method 200 for capturing heat from the bottom ash
of a boiler. The method initiates with directing cold water to bottom ash scraper as
part of a bottom ash cooling circuit 35 as depicted at process step 210. As depicted
at process step 220, cooled ash is separated from a hot slurry for disposal. The hot
slurry is directed to a fluid to fluid heat exchanger 46 as depicted at process step
230. The method 200 continues at process step 240 where boiler water/makeup
water is directed to the heat exchanger 46 in a boiler water heating circuit 39 in a
heat exchanging relation with the hot slurry. Having heated the boiler
water/makeup water, the cooled slurry is returned to the storage pond for
recirculation. Likewise, the heated boiler water/makeup water is directed to at least
one of the boiler 12, the hot well 48, and/or a storage tank 43 for consumption by
the boiler 12 as needed as depicted at process step 250. Optionally as depicted at
process step 260. it should be understood, that the, the bottom ash heat recovery
system 60 may be configured to place the heat exchanger 46 in series with the
condenser and hot well to heat all condensate and boiler makeup water for the boiler
12. In yet another option, as depicted at process step 270, an additional heat
exchanger 46a may be employed in parallel or series with heat exchanger 46 to
further facilitate cooling the bottom ash. In another option, as depicted at process
step 280, an additional heat exchanger 64, 164may be employed in parallel or series
with heat exchanger 46 to further facilitate heating the boiler water/makeup water.
It should be appreciated that while various steps of the method 200 are depicted in
a particular order, they need not be, and are described in such order merely for the
purposes of illustrating the examples of the embodiments. Some steps may of
discussion, some steps may readily be conducted in different order. In addition to
operational savings, the power generation system of the described embodiments
16
provides for capital cost savings on new plant or boiler design and constructions.
In particular, with the control system disclosed herein, it is possible to design/plan
equipment for lower boiler restart constraints. Furthermore, the power generation
system of the described embodiments provides for capital and recurring cost
savings on existing retrofitted plant or boiler designs and constructions. In
particular, with the system and methodology disclosed herein, it is possible to
modify existing equipment for lower restart constraints while achieving faster
restarts.
[00039] In an embodiment, described herein is a method of recovering heat
from bottom ash that is discharged from a combustion process in a combustion
system of a boiler. The method includes generating bottom ash in a combustion
system, by combusting fuel to produce heat energy in a boiler, thereby generating
bottom ash and discharging the bottom ash from the combustion system to a bottom
ash cooling circuit. The method also includes capturing heat, with a bottom ash
cooling circuit having a heat exchanger, from the bottom ash discharged from the
combustion system in order to utilize the recovered heat in a boiler water heating
circuit for preheating boiler water and exchanging the heat from the bottom ash
cooling circuit to the boiler water heating circuit and heating the boiler water.
[00040] In addition to one or more of the features described above, or as an
alternative, further embodiments of the method of recovering heat from bottom ash
may further include directing the heated boiler water to the boiler for heating with
heat energy generated by combusting fuel in the combustion system.
[00041] In addition to one or more of the features described above, or as an
alternative, further embodiments of the method of recovering heat from bottom ash
may include directing, in the bottom ash cooling circuit, cold water to the bottom
ash, separating cooled ash to yield a hot slurry, and directing the hot slurry to the
heat exchanger.
17
[00042] In addition to one or more of the features described above, or as an
alternative, further embodiments of the method of recovering heat from bottom ash
may include recovering the heat from the hot slurry, for heating the boiler water.
[00043] In addition to one or more of the features described above, or as an
alternative, further embodiments of the method of recovering heat from bottom ash
may include storing, in a storage tank, the heated boiler water.
[00044] In addition to one or more of the features described above, or as an
alternative, further embodiments of the method of recovering heat from bottom ash
may include receiving condensate resultant from condensing the steam, directing
the condensate to the heat exchanger, and heating the condensate with heat captured
in the bottom ash cooling circuit.
[00045] In addition to one or more of the features described above, or as an
alternative, further embodiments of the method of recovering heat from bottom ash
may include arranging an additional cooling device in the bottom ash cooling
circuit.
[00046] In addition to one or more of the features described above, or as an
alternative, further embodiments of the method of recovering heat from bottom ash
may include arranging an additional heating device in the boiler water heating
circuit.
[00047] In addition to one or more of the features described above, or as an
alternative, further embodiments of the method of recovering heat from bottom ash
may include that the boiler water comprises at least one of boiler water condensate
and boiler makeup water.
[00048] Also described herein in yet another embodiment is a system for
recovering heat from bottom ash that is discharged from a combustion process of a
boiler. The system includes a combustion system for combusting fuel with
combustion air, in order to generate heat energy for the boiler, in which bottom ash
18
is generated, a discharge for discharging the bottom ash from the combustion
system, a bottom ash cooling circuit configured to recover heat from the bottom
ash, the bottom ash cooling circuit including at least a heat exchanger and operably
connected to a first path of the heat exchanger, and a boiler water heating circuit
operably connected to a second path of the heat exchanger, the boiler water heating
circuit receives heat via the heat exchanger of the bottom ash cooling circuit, the
boiler water heating circuit directing heated boiler water to the boiler.
[00049] In addition to one or more of the features described above, or as an
alternative, further embodiments of the system for recovering heat from bottom ash
may include that the boiler water heating circuit comprises at least one of, the heat
exchanger, a storage device for storing heated boiler water, and a pump operable to
distribute the boiler water through the boiler water heating circuit.
[00050] In addition to one or more of the features described above, or as an
alternative, further embodiments of the system for recovering heat from bottom ash
may include that the bottom ash cooling circuit comprises at least one of the heat
exchanger, a bottom ash scraper, the bottom ash scraper operable to remove cooled
bottom ash from a mixture of cold water yielding a hot slurry and a pump operable
to distribute a hot slurry through at least the heat exchanger in the bottom ash
heating circuit. The heat exchanger is operable to exchange heat between the hot
slurry and the boiler water.
[00051] In addition to one or more of the features described above, or as an
alternative, further embodiments of the system for recovering heat from bottom ash
may further include an additional heat exchanger, the additional heat exchanger
arranged in the bottom ash cooling circuit for recovering heat from the hot slurry.
[00052] In addition to one or more of the features described above, or as an
alternative, further embodiments of the system for recovering heat from bottom ash
may include that the additional heat exchanger is at least one of arranged in parallel
with the heat exchanger, arranged in series with the heat exchanger, and operable
to exchange heat with another cooling source.
19
[00053] In addition to one or more of the features described above, or as an
alternative, further embodiments of the system for recovering heat from bottom ash
may include a third heat exchanger, the third heat exchanger arranged in the boiler
water heating circuit and operable to provide additional heating to the boiler water
heating circuit.
[00054] In addition to one or more of the features described above, or as an
alternative, further embodiments of the system for recovering heat from bottom ash
may include that the boiler water is at least one of boiler makeup water and
condensate condensed from steam generated in the boiler.
[00055] In addition to one or more of the features described above, or as an
alternative, further embodiments of the system for recovering heat from bottom ash
may include that the bottom ash cooling circuit includes one of a cooled transport
screw, a water-cooled drum cooler, and a water-cooled scraper conveyer.
[00056] Also described herein, in yet another exemplary embodiment is a
boiler system having a bottom ash recovery system, the boiler system including a
boiler having a combustion system for combusting fuel with combustion air, in
order to generate heat energy for the boiler, in which bottom ash is generated, a
discharge for discharging the bottom ash from the combustion system, and a bottom
ash cooling circuit configured to recover heat from the bottom ash. The bottom ash
cooling circuit including at least a heat exchanger and operably connected to a first
path of the heat exchanger, and a boiler water heating circuit operably connected to
a second path of the heat exchanger. The boiler water heating circuit receives heat
via the heat exchanger of the bottom ash cooling circuit, the boiler water heating
circuit directing heated boiler water to the boiler.
[00057] Finally, it is also to be understood that the system 10 and control unit
100 may include the necessary electronics, software, memory, storage, databases,
firmware, logic/state machines, microprocessors, communication links, displays or
other visual or audio user interfaces, printing devices, and any other input/output
interfaces to perform the functions described herein and/or to achieve the results
20
described herein. For example, as previously mentioned, the system may include
at least one processor and system memory/data storage structures, which may
include random access memory (RAM) and read-only memory (ROM). The at least
one processor of the system 10 may include one or more conventional
microprocessors and one or more supplementary co-processors such as math coprocessors or the like. The data storage structures discussed herein may include an
appropriate combination of magnetic, optical and/or semiconductor memory, and
may include, for example, RAM, ROM, flash drive, an optical disc such as a
compact disc and/or a hard disk or drive.
[00058] Additionally, a software application that adapts the controller to
perform the methods disclosed herein may be read into a main memory of the at
least one processor from a computer-readable medium. Thus, embodiments of the
present invention may perform the methods disclosed herein in real-time. The term
“computer-readable medium,” as used herein, refers to any medium that provides
or participates in providing instructions to the at least one processor of the system
10 (or any other processor of a device described herein) for execution. Such a
medium may take many forms, including but not limited to, non-volatile media and
volatile media. Non-volatile media include, for example, optical, magnetic, or optomagnetic disks, such as memory. Volatile media include dynamic random access
memory (DRAM), which typically constitutes the main memory. Common forms
of computer-readable media include, for example, a floppy disk, a flexible disk,
hard disk, solid state drive (SSD), magnetic tape, any other magnetic medium, a
CD-ROM, DVD, any other optical medium, a RAM, a PROM, an EPROM or
EEPROM (electronically erasable programmable read-only memory), a FLASHEEPROM, any other memory chip or cartridge, or any other medium from which a
computer can read.
[00059] While in embodiments, the execution of sequences of instructions in
the software application causes at least one processor to perform the
methods/processes described herein, hard-wired circuitry may be used in place of,
or in combination with, software instructions for implementation of the described
21
methods/processes. Therefore, embodiments as described herein are not limited to
any specific combination of hardware and/or software.
[00060] As used herein, “electrical communication” or “electrically
coupled” means that certain components are configured to communicate with one
another through direct or indirect signaling by way of direct or indirect electrical
connections. As used herein, “mechanically coupled” refers to any coupling
method capable of supporting the necessary forces for transmitting torque between
components. As used herein, "operatively coupled" refers to a connection, which
may be direct or indirect. The connection is not necessarily being a mechanical
attachment.
[00061] As used herein, an element or step recited in the singular and
proceeded with the word “a” or “an” should be understood as not excluding plural
of said elements or steps, unless such exclusion is explicitly stated. Furthermore,
references to “one embodiment” of the described embodiments are not intended to
be interpreted as excluding the existence of additional embodiments that also
incorporate the recited features. Moreover, unless explicitly stated to the contrary,
embodiments “comprising,” “including,” or “having” an element or a plurality of
elements having a particular property may include additional such elements not
having that property.
[00062] Additionally, while the dimensions and types of materials described
herein are intended to define the parameters associated with the described
embodiments, they are by no means limiting and are exemplary embodiments.
Many other embodiments will be apparent to those of skill in the art upon reviewing
the above description. The scope of the invention should, therefore, be determined
with reference to the appended claims. Such description may include other
examples that occur to one of ordinary skill in the art and such other examples are
intended to be within the scope of the claims if they have structural elements that
do not differ from the literal language of the claims, or if they include equivalent
structural elements with insubstantial differences from the literal languages of the
22
claim. In the appended claims, the terms “including” and “in which” are used as
the plain-English equivalents of the respective terms “comprising” and “wherein.”
Moreover, in the following claims, terms such as “first,” “second,” “third,” “upper,”
“lower,” “bottom,” “top,” etc. are used merely as labels, and are not intended to
impose numerical or positional requirements on their objects. Further, the
limitations of the following claims are not written in means-plus-function format
are not intended to be interpreted as such, unless and until such claim limitations
expressly use the phrase “means for” followed by a statement of function void of
further structure.

WE CLAIM:

1. A method of recovering heat from bottom ash that is discharged from a
combustion process in a combustion system of a boiler, the method comprising:
generating bottom ash in a combustion system, by combusting fuel to
produce heat energy in a boiler, thereby generating bottom ash;
discharging the bottom ash from the combustion system to a bottom ash
cooling circuit;
capturing heat, with a bottom ash cooling circuit having a heat exchanger,
from the bottom ash discharged from the combustion system in order to utilize the
recovered heat in a boiler water heating circuit for preheating boiler water; and
exchanging the heat from the bottom ash cooling circuit to the boiler water
heating circuit and heating the boiler water.
2. The method as recited in claim 1, further comprising directing the heated boiler
water to the boiler for heating with heat energy generated by combusting fuel in the
combustion system.
3. The method as recited in claim 1, further comprising:
directing, in the bottom ash cooling circuit, cold water to the bottom ash;
separating cooled ash to yield a hot slurry; and
directing the hot slurry to the heat exchanger.
4. The method as recited in claim 3, further comprising recovering the heat from
the hot slurry, for heating the boiler water.
5. The method as recited in claim 1, further comprising: storing, in a storage tank,
the heated boiler water.
6. The method as recited in claim 1, further comprising:
receiving condensate resultant from condensing the steam;
directing the condensate to the heat exchanger; and
heating the condensate with heat captured in the bottom ash cooling circuit.
24
7. The method as recited in claim 1, further comprising arranging an additional
cooling device in the bottom ash cooling circuit.
8. The method as recited in claim 7, wherein the arranging an additional cooling
device in the bottom ash cooling circuit is in parallel with the heat exchanger.
9. The method as recited in claim 1, further comprising arranging an additional
heating device in the boiler water heating circuit.
10. The method as recited in claim 1, wherein the boiler water comprises at least
one of boiler water condensate and boiler makeup water.
11. A system for recovering heat from bottom ash that is discharged from a
combustion process of a boiler, the system comprising:
a combustion system for combusting fuel with combustion air, in order to generate
heat energy for the boiler, in which bottom ash is generated;
a discharge for discharging the bottom ash from the combustion system;
a bottom ash cooling circuit configured to recover heat from the bottom ash, the
bottom ash cooling circuit including at least a heat exchanger and operably
connected to a first path of the heat exchanger; and
a boiler water heating circuit operably connected to a second path of the heat
exchanger, the boiler water heating circuit receives heat via the heat exchanger of
the bottom ash cooling circuit, the boiler water heating circuit directing heated
boiler water to the boiler.
12. The system as recited in claim 11, wherein the boiler water heating circuit
comprises at least one of:
the heat exchanger;
a storage device for storing heated boiler water
a pump operable to distribute the boiler water through the boiler water
heating circuit.
25
13. The system as recited in claim 11, wherein the bottom ash cooling circuit
comprises at least one of:
the heat exchanger;
a bottom ash scraper, the bottom ash scraper operable to remove cooled
bottom ash from a mixture of cold water yielding a hot slurry;
a pump operable to distribute a hot slurry through at least the heat exchanger
in the bottom ash heating circuit,
wherein the heat exchanger is operable to exchange heat between the hot
slurry and the boiler water.
14. The system as recited in claim 11, further comprising an additional heat
exchanger, the additional heat exchanger arranged in the bottom ash cooling circuit
for recovering heat from the hot slurry.
15. The system as recited in claim 14, wherein the additional heat exchanger is at
least one of:
arranged in parallel with the heat exchanger;
arranged in series with the heat exchanger; and
operable to exchange heat with another cooling source.
16. The system as recited in claim 11, further comprising a third heat exchanger,
the third heat exchanger arranged in the boiler water heating circuit and operable to
provide additional heating to the boiler water heating circuit.
17. The system as recited in claim 16, wherein the third heat exchanger is at least
one of:
arranged in parallel with the heat exchanger;
arranged in series with the heat exchanger; and
operable to exchange heat with another heating source.
18. The system as recited in claim 11, wherein the boiler water is at least one of
boiler makeup water and condensate condensed from steam generated in the boiler.
26
19. The arrangement as recited in claim 18, wherein the bottom ash cooling circuit
includes one of a cooled transport screw, a water-cooled drum cooler, and a watercooled scraper conveyer.
20. A boiler system having a bottom ash recovery system, the boiler system
comprising:
a boiler having a combustion system for combusting fuel with combustion air, in
order to generate heat energy for the boiler, in which bottom ash is generated;
a discharge for discharging the bottom ash from the combustion system;
a bottom ash cooling circuit configured to recover heat from the bottom ash, the
bottom ash cooling circuit including;
at least a heat exchanger and operably connected to a first path of the heat
exchanger, and
a boiler water heating circuit operably connected to a second path of the heat
exchanger, the boiler water heating circuit receives heat via the heat
exchanger of the bottom ash cooling circuit, the boiler water heating circuit
directing heated boiler water to the boiler.