U.S. patent application number 11/992508 was filed with the patent office on 2009-10-01 for method and system for heating of water based on hot gases.
This patent application is currently assigned to Dall Energy Holding ApS. Invention is credited to Jens Dall Bentzen.
Application Number | 20090241814 11/992508 |
Document ID | / |
Family ID | 37593709 |
Filed Date | 2009-10-01 |
United States Patent
Application |
20090241814 |
Kind Code |
A1 |
Bentzen; Jens Dall |
October 1, 2009 |
Method and System for Heating of Water Based on Hot Gases
Abstract
Heat can be recovered from hot gas produced in a thermal reactor
(1), by injecting water into the gas at one or more injection zones
(4) in such an amount and in such a way that the gas temperature
due to water evaporation is reduced to below 400.degree. C.,
preferably below 300.degree. C., possibly below 150-200.degree. C.,
and the gas dew point becomes at least 60.degree. C., preferably at
least 70.degree. C., possibly 80 or 85.degree. C. The gas can then
be led through a condensing heat exchanger unit (8), where at least
some of the gas contents of water vapour are condensed, and the
condensing heat can be utilized for heating of a stream of fluid,
mainly water. Hereby, a method for production of hot water is
obtained, which is cheap and simple and has low maintenance costs,
and which moreover has a high efficiency degree and good
environmental qualities The method can be used for a broad spectrum
of fuels and conversion technologies.
Inventors: |
Bentzen; Jens Dall;
(Birkerod, DK) |
Correspondence
Address: |
LEYDIG VOIT & MAYER, LTD
TWO PRUDENTIAL PLAZA, SUITE 4900, 180 NORTH STETSON AVENUE
CHICAGO
IL
60601-6731
US
|
Assignee: |
Dall Energy Holding ApS
Birkerod
DK
|
Family ID: |
37593709 |
Appl. No.: |
11/992508 |
Filed: |
September 27, 2006 |
PCT Filed: |
September 27, 2006 |
PCT NO: |
PCT/DK2006/050049 |
371 Date: |
May 21, 2008 |
Current U.S.
Class: |
110/215 ;
110/345; 122/5.52 |
Current CPC
Class: |
F23G 2206/203 20130101;
F23G 5/46 20130101; F23G 7/10 20130101; Y02E 20/34 20130101; F23J
15/02 20130101; F23L 15/00 20130101; Y02E 20/12 20130101; F23G 5/30
20130101; F23L 7/002 20130101; Y02E 20/348 20130101; F23J 2219/80
20130101; F23J 2219/70 20130101; F23G 5/50 20130101; F23G 2207/50
20130101; F23C 10/28 20130101 |
Class at
Publication: |
110/215 ;
110/345; 122/5.52 |
International
Class: |
F23J 15/00 20060101
F23J015/00; F22B 37/00 20060101 F22B037/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 27, 2005 |
DK |
PA 2005 01345 |
Claims
1-18. (canceled)
19. A method for recovering heat from hot flue gas produced in a
thermal reactor fuelled with solid fuel, said method comprising the
steps of: injecting water into in the gas at one or more injection
zones in such an amount and such a way that due to evaporation of
injected water the flue gas temperature is reduced to below
400.degree. C., and the gas dew point becomes at least 60.degree.
C., subsequently passing the gas through a condensing heat
exchanger, where at least some of the water vapor in the gas is
condensed and condensing heat is released, and utilizing the
condensing heat for heating a stream of fluid, such as water, in
the heat exchanger.
20. A method according to claim 19, wherein the gas dew point
becomes at least 70.degree. C., preferably 80.degree. C. or
85.degree. C.
21. A method according to claim 19, wherein the flue gas
temperature is reduced to below 300.degree. C., preferably to
150-200.degree. C.
22. A method according to claim 19, wherein the injection zones are
located in one or more of the thermal reactor, zones downstream of
the thermal reactor in a gas flow direction, the fuel in the
thermal reactor and the heat exchanger.
23. A method according to claim 19, wherein impurities are removed
from the gas by means of a bag filter, a cyclone, an electro
filter, a scrubber or the like.
24. A method according to claim 19, wherein impurities are removed
from the condensed water.
25. A method according to claim 19, wherein pH of the condensed
water is adjusted.
26. A method according to claim 19, wherein at least some of the
water injected in to the flue gas is vaporized using a nozzle.
27. A method according to claim 19, wherein part of the injected
water is injected at a speed above 20 m/s in a gas flow
direction.
28. A method according to claim 19, wherein the fluid heated in the
heat exchanger is further heated, e.g. by means of a fluid cooled
feeder, a fluid cooled grate, fluid cooled surfaces in the reactor
or other cooled surfaces around the thermal reactor.
29. A method according to claim 19, wherein water vapor and heat
are transferred to combustion air, which is led to the thermal
reactor.
30. A method according to claim 19, wherein the thermal reactor is
of the fluid bed type, and injection of water into the bed is used
to regulate temperature and flow conditions in the bed.
31. A method according to claim 19, wherein condensed water from
the heat exchanger is injected in the one or more injection
zones.
32. A system for decomposition of solid fuel and for production of
hot fluid, said system comprising a thermal reactor for decomposing
the fuel and producing hot flue gas from the fuel, an evaporative
cooler with water injection devices, e.g. in the form of nozzles,
for injection of water into the flue gas so that the injected water
evaporates, a control system for controlling the water injection
into the gas so that, as a result of evaporation of injected water,
the gas temperature is reduced to below 400.degree. C. and the gas
dew point becomes at least 60.degree. C., and a condensing heat
exchanger for condensing at least some of the water vapor in the
gas and utilizing condensing heat for heating a stream of fluid,
such as water.
33. A system according to claim 32 further comprising one or more
nozzles for injecting water into one or more of the fuel in the
thermal reactor, the thermal reactor and the flue gas in connection
with the heat exchanger.
34. A system according to claim 32, further comprising a gas
cleaning unit in the form of a bag filter, electro filter, scrubber
or the like.
35. A system according to claim 32 including means for leading at
least some of the fluid from the heat exchanger to another unit for
further heating.
36. A system according to claim 32, further comprising an enthalpy
exchanger, where water vapor and heat are transferred to combustion
air to be added to the reactor.
37. A system according to claim 32 including a fluid bed reactor
with means for injection of water into the bed in order to adjust
temperature, emissions (NOx) and flow conditions.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to a method and a
system for heat recovery from hot gas, e.g. flue gas, produced in a
thermal reactor, or--more precisely--for heating of water by means
of the hot gases that are released by thermal conversion
(gasification or combustion) of solid fuels e.g. biomass, waste or
coal.
BACKGROUND OF THE INVENTION
[0002] Heating of water from hot gases that are released during
thermal conversion of fuels is well known. The hot water can be
used for heating purposes, e.g. in houses, apartment houses,
offices, in industries etc. and for domestic water. Installations
for such purposes are produced in very different sizes, approx. 1
kW-250 MW input effect.
[0003] Reference is made to "Varme stabi", Nyt teknisk Forlag, 4th
ed., 2004, ordering No. 44031-1, ISBN 87-571-2546-5, Ullmann's
Encyclopedia of Industrial Chemistry Release 2005, 7.sup.th
Edition, "User friendly it tool for biomass heating plants" in
proceedings of "2nd world conference and technological exhibition
on biomass for energy, industry and climate protection" and DE
3544502 A1.
[0004] The water is usually heated in a closed circuit and led to a
point of consumption, after which the water is returned to the heat
production unit after release of the thermal energy. When the water
leaves the production unit (supply), the water temperature usually
is 60-90.degree. C. The temperature of the water returning to the
heat-production unit after cooling at the consumer (return) is
about 30-50.degree. C.
[0005] Concurrently with the technological development and the
attention to energy savings, there has been a tendency to reduce
the supply and return temperatures, as the heat loss from the
distribution pipes is reduced in that way.
[0006] The hot water can be produced close to the required
locations or be sent to the consumer via a district heating
network.
[0007] The energy released by thermal conversion of a fuel can be
transferred to hot water in stages, e.g.:
[0008] 1. By cooling of the area around the place where the thermal
conversion takes place, e.g. a water-cooled feeder, a water-cooled
grate, water-cooled areas in the reactor or other cooled surfaces
where the thermal conversion takes place.
[0009] 2. Cooling of the (dry) hot gases
[0010] 3. Further cooling of the gases, by which vapours in the gas
is condensed.
[0011] Re 2. Cooling of the (Dry) Hot Gases
[0012] The gas leaving the thermal unit is usually around
700-1000.degree. C., depending on technology, fuel and operation
conditions. It is well known, e.g. at CHP stations, that the
temperature in the thermal unit can be adjusted or controlled by
water injection in order to protect materials, e.g. the
superheater, against a too high temperature. The amount of water
injected in order to adjust the temperature in the boiler room is,
however, very limited; the temperature of the gas remains high
(above 600.degree. C.), and the characteristics of the gas, e.g.
the water dew point, are not changed substantially.
[0013] Usually, the energy from the hot gas is transferred to
another medium, e.g. water, by using a heat exchanger where the hot
gas is flowing at one side while another colder medium (e.g. water)
is flowing at the other side. Thus, the water is heated whereas the
gas is cooled. In some plants, more heat exchangers are used, e.g.
air preheating and/or steam superheating and/or hot water
production.
[0014] These heat exchangers are usually of the convection heat
exchanger type, as the energy mainly is transferred from the gas
via convection. Usually, steel pipes are used. When solid fuels are
converted, the gas contains particles. These particles result in
several problems in this heat exchanger: fouling, corrosion, low
heat exchange rates etc. and often a device is mounted to keep the
gas tubes clean, e.g. soot blowing or mechanical cleaning.
[0015] The heat exchanger used for transferring energy from the dry
hot gas is made of materials matching the qualities of the gas,
usually heat-proof steel.
[0016] Usually, the gas is cooled in the "convection part" to
around 150.degree. C., as the temperature of the gas then is above
the acid dew point and the water dew point. If the gas is cooled to
or below the acid or water dew points, severe corrosion may occur
in the heat-proof material of the heat exchanger.
[0017] Ammonia, chlorine, sulphur, particles, salts etc. is often
removed from the gas, for instance by a dry or semi-dry cleaning
process. In this way, the materials causing problems for the
environment or the materials blocking and/or corroding during the
subsequent process stages can be removed.
[0018] Re 3. Further Cooling of the Gases by Which Vapour in the
Gas is Condensed
[0019] In order to utilize more of the heat energy, the gas can be
further cooled, by which vapours, including water vapour in the
gas, are condensing. The composition of the gas depends on the fuel
conversed and of the conditions in the thermal reactor. With high
moisture content in the fuel and a low amount of excess air in the
thermal unit, a high water dew point is obtained. Usually, the
water dew point in the gas will be approx. 35-60.degree. C., if the
gas has atmospheric pressure. If the gas is cooled below the water
dew point, water vapour will condense, and condensation energy is
released which can be used for further heat production. Depending
on the fuel and the conditions in the thermal process, the energy
utilization can be increased by up to about 30%.
[0020] By condensing of water vapour, other materials are released
from the gas too, e.g. ammonia, chlorine, sulphur, particles, salts
etc. As some of these substances may spoil, e.g. corrode the
materials used for cooling the dry gas (the convection part), the
condensing part is usually made of other materials. In the
condensing part, e.g. glass fibre, plastic material, glass,
acid-proof stainless steel, titanium etc. are used.
[0021] As the gas which is led to the condensing unit is cooled to
e.g. 150.degree. C. and has a water dew point of around
35-60.degree. C., the temperature of the water heated in the
condensing unit becomes too low to be used for supply. Therefore,
the water from the condensing unit must be further heated.
[0022] The energy in the gas after the condensing unit can be
further utilized, for instance by transferring water vapour and
heat to the combustion air that is added to the thermal process, or
by means of a heat pump.
[0023] In some, especially chemical plants, chilling of hot gases
by massive water injection into a "quench" is used. A "quench" is
thus wet, as there is a surplus of water. In these plants, no
considerable evaporations will take place of the injected water, as
the water amount is very large in order to secure cooling of the
gases. Similarly, no significant change of the gas characteristics
(e.g. the dew point) will take place. The nozzles used in a quench
are of the type generating large water drops and delivering a large
amount of water. Thus, in a quench the heat capacity (approx. 4.16
J/g/.degree. C.) of water is used to cool the gas.
[0024] In some, especially chemical plants, chilling of hot gases
by water injection into an "evaporative cooler" is used. In an
"evaporative cooler" the cooled gas can be dry and thus dry gas
cleaning systems can be used for cleaning the gases, which is
necessary due to environmental legislation. One example of such
plants is cement production plants. The water vapour in the gas
from "evaporative coolers" is not condensated and used for
production of hot water.
[0025] In some plants, fuelled with gas or oil, the combustion
chamber is very compact and followed by an injector which is used
as a gas pump. The ejector can then be followed by a heat exchanger
where water vapours condensate and energy hereby is be retrieved.
However such systems can not be used for several reasons, for
example:
[0026] A. Feeding systems and combustion chambers for solid fuels
are very different from feeding systems and combustion chambers for
gaseous fuels.
[0027] The following the condensing heat will corrode and/or block
up with particles if solid fuels are used.
OBJECTS AND SUMMARY OF THE INVENTION
[0028] The invention provides a method and a plant allowing
transfer of energy from hot gases to water or another fluid by
means of considerably fewer heat transfer units, as the heat
transfer from hot gases can be gathered in a single condensing
unit. Moreover, a more simple water circuit is obtained as coupling
and control of water circuit for a condensing unit as well as a
convection part are avoided.
[0029] Thus, the invention provides a method for heat recovery from
hot flue gas, produced in a thermal reactor. According to the
method, water is injected at one or more injection zones in such an
amount and in such a way that the flue gas temperature is reduced
to below 400.degree. C. and the gas dew point is at least
60.degree. C. due to water evaporation. Subsequently, the gas is
led through a condensing heat exchanger unit (8), where at least
some of the water vapour is condensed, and the condensation heat is
used for heating a liquid stream, mainly water.
[0030] In this way, the comprehensive evaporation heat of water
(approx. 2.2 MJ/kg) is utilized twice:
[0031] 1. By injection of water and its evaporation, the amount of
water vapour in the gas is increased, and thus the dew point of the
gas is increased. As an example could be mentioned that injection
of water into a flue gas from combustion of biomass in such an
amount that the gas is cooled to 150.degree. C. will increase the
dew point for the flue gas to approx. 85.degree. C. The dew point
in flue gas is usually 35-60.degree. C. without water
injection.
[0032] 2. The cooled gas containing a large amount of water vapour
can then produce the amount of energy in the condensing heat
exchanger unit which was previously produced in at least two units,
i.e. a dry and hot convection part and a condensing part. Besides,
the dew point of the flue gas has increased considerably due to the
water injection, which means that the condensing heat exchanger
unit can heat water or another liquid to a temperature suitable for
using the water directly as supply.
[0033] At least a part of the water injected into the hot gases
will atomize in a nozzle, by which the water will evaporate more
quickly.
[0034] Water injection into the hot gas may take place in several
injection zones, which may comprise the fuel, the thermal reactor,
a gas cleaning unit and/or the condensing heat exchanger unit. By
water injection into fuel and/or the thermal reactor, a number of
advantages are obtained:
[0035] If the plant is designed for wet fuels, the same plant can
be used for dry fuels by water injection into the fuel and/or the
thermal reactor. Thus, a fuel flexible plant is obtained.
[0036] NOx-formation can be controlled and reduced, as NOx
formation is independent of temperature.
[0037] The thermal reactor and the gas pipes to the condensing heat
exchanger unit may be separated or be built together in one unit,
as the thermal conversion then takes place in one zone, whereas
water injection may take place in that reactor zone and possible
also somewhere else in a subsequent zone.
[0038] Before and/or after the condensing unit, the gas can be
cleaned of undesirable materials such as e.g. ammonia, heavy
metals, acids, chlorine, sulphur, particles, salts, etc. This may
for instance be done in a bag filter, a cyclone, and electrofilter
or in a scrubber, possibly combined with addition of absorbents
such as active carbon, lime, bicarbonate etc. As long as the gas
temperature is above the water dew point, dry gas cleaning
technologies can be used, e.g. bag filter or electrofilter. If the
gas is wet, scrubbers and/or wet electrofilters can be used.
[0039] A part of the water injected into the gas can advantageously
be injected at great speed in the direction of the gas flow. By
this, kinetic energy from the water can be transferred to the gas,
and the water injection may then act as a gas pump (ejector).
[0040] If an especially high supply temperature is desired, the
water heated in the condensing heat exchanger unit can be further
heated, e.g. via a water-cooled feeder, a water-cooled grate
water-cooled areas in the reactor and/or other cooled surfaces
around the thermal conversion area or via another thermal
production.
[0041] After the condensing heat exchanger unit, a certain energy
amount will be left in the gas in the form of heat and water
vapour. Some of that energy can be utilized by transfer to the
combustion air via an enthalpy exchanger. In an enthalpy exchanger,
water vapour and heat are transferred to the combustion air,
implying an even higher water vapour amount in the gas and thus a
higher efficiency of the condensing unit. Enthalpy exchangers can
be designed in different ways, e.g. as rotating units, where
combustion air flows on one side and hot gas on the other, or as a
system where the gas after the condensing heat exchanger unit
changes with cold water, whereby the water is heated. The heated
water can then be used for heating and humidifying the combustion
air.
[0042] By combustion of solid fuels, e.g. straw or waste
sedimentation of particles will often occur on the convection part,
as the hot particles are sticky due to a low ash melting point. By
water injection and corresponding reduction of the gas temperature,
this problem is eliminated.
[0043] The hot water can be produced close to the consumption place
or be sent to the consumer via a district heating network. Plants
designed according to the invention can be built in a very wide
spectrum of sizes, approx. 1 kW-250 MW input effect.
[0044] The thermal unit may have other purposes than only heat
production, e.g. production of gas and electricity among others.
Among technologies relevant for the invention can be mentioned:
Combustion plants for solid fuel (biomass, waste and coal) for mere
heat production as well as CHP production, gas and oil fired
boilers, motors, gas turbines, gasification plants etc.
[0045] If the thermal unit is of the fluid bed type, water
injection into the bed can be used for adjusting the temperature in
the bed, by which operational (e.g. slag formation) and
environmental (e.g. reduction of NOx) advantages can be obtained.
Water injection into the bed will further contribute to
fluidization of the bed. This kind of temperature adjustment is
considerably more robust than the traditional technique in the form
of cooling coils which are quickly worn down of the bed
material.
[0046] The condensed water can be cleaned of particles, salts,
heavy metals etc. and be adjusted for pH, before it is used or led
away.
[0047] The water injected into the fuel in the thermal unit, in the
gases or in the condenser may be condensate, segregated in the
condensing unit, or water added from outside.
[0048] In the thermal unit, the condensing unit and in the
connecting gas duct there may be atmospheric pressure, or pressures
above or below the atmosphere.
[0049] The invention further provides a system for decomposition of
fuel and production of hot water, and comprising a thermal reactor,
a flue gas duct, one or more water injection devices e.g. in the
form of nozzles and a condensing heat exchanger unit connected to
the flue gas duct. Here at least some of the water vapour of the
gas is condensed, and the condensation heat is used for heating of
a flow of fluid, preferably water, and means for control of the
water injection into the flue gas in order that the flue gas
temperature is reduced to below 400.degree. C., and the gas dew
point becomes at least 60.degree. C. due to the evaporation of
water.
[0050] Other objects and advantages of the invention will become
apparent upon reading the following detailed description and upon
reference to the drawings, in which:
BRIEF DESCRIPTION OF THE DRAWINGS
[0051] FIG. 1 schematically depicts a first design of a plant
according to the invention;
[0052] FIG. 2 schematically depicts a second design of the plant
according to the invention, where solid fuel is burned in a
grate-fired boiler, and where particles are removed from the flue
gas in a bag filter before condensing;
[0053] FIG. 3 schematically depicts a third design of the plant
according to the invention, where solid fuel is burned in a
grate-fired boiler, and where water is added by means of an
ejector;
[0054] FIG. 4 schematically depicts a fifth design of the plant,
where fuel is gasified and the heat energy in the gas is
utilized;
[0055] FIG. 5 is a diagram of the flue gas output from cooling,
with and without preceding water injection and evaporation; and
[0056] FIG. 6 are two tables with energy calculations, where wet
and dry fuel, respectively, are converted. The calculations show
results for today's standard technology and for the invention with
and without moistening of combustion air.
[0057] In the following, corresponding parts in the different
designs will have the same reference terms.
[0058] While the invention is susceptible of various modifications
and alternative constructions, a certain illustrative embodiment
thereof has been shown in the drawings and will be described below
in detail. It should be understood, however, that there is no
intention to limit the invention to the specific form disclosed,
but on the contrary, the intention is to cover all modifications,
alternative constructions, and equivalents falling within the
spirit and scope of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0059] Referring now more particularly to FIG. 1 of the drawings,
there is shown a unit or reactor 1, to which fuel is added. The
fuel is converted thermally by addition of air (and/or oxygen).
Thus, a warm gas is produced in the thermal unit 1. The fuel added
to unit 1 is solid e.g. biomass, waste or coal. If the thermal unit
1 is designed for fuels with low calorific power, e.g. wet fuel,
and if the added fuel has a higher calorific power, the temperature
in the unit or in the generator 1 can be adjusted by adding water
to the fuel at 2 and/or by adding water at 3 within the thermal
unit 1.
[0060] At 4, water is injected into the hot gases leaving the
thermal unit 1. The water evaporates and cools the gases
considerably, as the evaporation energy from water is very high.
The unit in which injection 4 is placed can be built of heat-proof
steel, bricks, castings and/or other materials. The amount of water
dosed at 4 can be controlled on basis of the gas temperature and/or
the dew point by means of adequate control means S, placed in a
position after 4, where the injected water has evaporated.
[0061] If the cooled gas contains impurities, e.g. particles, a gas
cleaning unit 5 can remove these impurities from the dry gas. Via a
gas blower or pump 6, the gas can be pumped on to a condensing heat
exchanger unit 8, where the heat in the gases, including the
condensation heat in the water vapour, can be transferred to the
water to be heated. In the condensing unit, water can also be
injected at 7.
[0062] The gas sucker 6 can also be placed after the condensing
unit 8, where the gas flow is lower due to the cooling of the gas
and the condensing of the water vapours.
[0063] In and/or after the condensing unit 8, more impurities can
be removed from the gas at 9 and/or from the produced condensate at
12. After the condensing unit 8, some of the energy left in the gas
in the form of heat and moist can be transferred, at 10, to the
combustion air which is added to the thermal unit 1. The humidified
air can be further heated in a heat exchanger 11, before the air is
added to the thermal unit 1, whereby the supply lines are kept
dry.
[0064] This type of plant can be produced in many different sizes,
from a few kW (villa boilers) to large plants above 100 MW.
[0065] FIG. 2 shows a combustion plant for production of district
heating, and where the gas is cleaned before condensing and
combustion air is moisturized. 1 is a burner for combustion of
solid fuel. The plant is brick-lined so that it can burn fuels with
a high water content (up to 60% water) or which otherwise have a
low calorific value (below 10 MJ/kg). Fuels with a higher calorific
value can also burn in such a plant, as water can be added to the
fuel at 2, or in the boiler room at 3. Further, at 4 water is added
to the hot gases leaving the burner 1. The water evaporates and
cools the gases to ca. 150-200.degree. C. Subsequently, the gas is
cleaned of particles in a bag filter 5. If other substances are to
be removed from the gas, absorbents can be added before the filter,
e.g. lime, active carbon, bicarbonate etc.
[0066] The flue gas is sucked through the gas sucker or the pump 6
and is cooled in the condensing unit 8, comprising two cooling
towers placed above each other, designated respectively "Kol. 1"
and "Kol. 2", and a heat exchanger 13, as the flue gas flows
counter-flow with the cooled condensate 7a. As the condensing unit
8 is built of glass fibre, it is important that the gas is cooled
to below ca. 150.degree. C., before the inlet. Addition of water in
the nozzle in 7b protects the condenser inlet 14 from becoming too
warm. In the cooling tower "Kol. 1" cooling water is added at 7a.
Hereby, steam in the flue gas flow is condensed, and the condensate
is gathered in a room 15 under the cooling towers and the inlet 14.
The hot condensate is heat exchanged in the heat exchanger 13 by
water in a district heating system which is not shown, as the cold
district heating water is added via a return pipe, whereas the hot
water is led back to the system via a supply pipe. As the flue gas
dew point is high, e.g. ca. 85.degree. C., the temperature of the
produced condensate can be about 85-90.degree. C. Thus, the
district heating water can be heated from the condensate at one
single stage.
[0067] The combustion air added to the burner 1 can be heated in a
humidifier 17, where hot water is added at 18, or by means of a
heater device 11, ensuring that the air ducts are kept dry. The
water added at 2-4, 7a, 7b and 18, may--as shown--be the cooled
condensate that leaves the heat exchanger 13, and any surplus
condensate can be led away at 19. Condensate gathered at the bottom
of the humidifier 17 can be used for addition to the cooling tower
"Kol. 2".
[0068] When the flue gas has been cooled by the condensate in the
tower "Kol. 1", it is led through another section, "Kol. 2", where
the gas is cooled by water having been cooled by the combustion
air. The cooling of the flue gas and humidifying of the combustion
air together form an enthalpy exchanger 10, which increases the
energy efficiency.
[0069] FIG. 3 shows a combustion plant for production of district
heating. The gas is led through the plant by means of an ejector
pump. 1 is a burner for combustion of solid fuel. At 4, water is
added to the hot gases leaving the burner 1. The water evaporates
and cools the gases. At 7a water is injected at great speed in the
direction of the gas flow through a pipe 20, the cross section of
which is increased in the flow direction. Thus, the water injection
at 7a through the pipe 20 acts as an ejector.
[0070] In a condensing heat exchanger 8, heat energy is transferred
from the flue gas to the district heating water. The heat exchanger
in 8 may be made of glass, plastic or acid-proof stainless steel,
but needs not be heat-proof. The exchanger can be cleaned of
particles by means of water injected at 7b, but this needs not be a
continuous cleaning. The produced condensate can be cleaned of
particles etc. at 12, before it is used as injection water at 4a,
4b and 7 or drained off to a drain at 19.
[0071] FIG. 4 shows a preferred design of a gasifier plant 1, where
the produced gas firstly is cooled by being used for preheating of
combustion air in a heat exchanger 21, and then is cooled by water
injection at 4. The drafted gasifier is of the type "staged fixed
bed", but can in principle be other gasifier types, e.g. a fluid
bed gasifier.
[0072] After water injection at 4, the gas is cleaned of particles
(and possibly tars) e.g. in a bag filter and/or an active carbon
filter 5, after which the gas in a heat exchanger 8 is cooled
during condensing of water. By means of a gas blower or pump 6 the
gas is blown to a conversion unit, here illustrated by an engine,
but there could also be other conversion units, e.g. a gas turbine,
liquefaction equipment for conversion of the gas to fluid fuel
etc.
[0073] The flue gas energy from the conversion unit can be utilized
e.g. for heat production. Thus, the invention can be utilized
twice.
[0074] In FIG. 5 is a diagram showing the calculation of the output
from cooling of flue gas from respectively a traditional boiler and
by water injection according to the invention, cf. FIGS. 2 and 3.
Common data for the two calculations are:
[0075] amount of fuel (waste/wood chip) of 3000 kg/hour
[0076] humidity content in the fuel is 45%
[0077] O.sub.2 in the flue gas is 5% (dry)
[0078] the temperature of the flue gas out of boiler/after water
injection=150.degree. C.
[0079] It appears from FIG. 5 that about 1700 kW can be produced in
the condensing unit by cooling of the flue gas to ca. 45.degree. C.
with standard technology, whereas 8500 kW can be produced by using
the invention. The temperatures of the produced water are very
different too. With standard technology water can be produced at
about 65.degree. C. However, by using the invention, water can be
produced at 85-90.degree. C. In most cases, a supply temperature of
85.degree. C. will be satisfactory, but if this is not enough, a
radiation section/grate cooling can be incorporated for boosting
the temperature. If e.g. 95.degree. C. supply temperature is
desired, ca. 10-20% of the energy must be produced in the radiation
section/grate cooling.
[0080] FIG. 6 shows two tables with key figures for selected
calculations for district heating plants. It appears from the key
figures that the efficiency by use of wet fuels will be the same
for a standard design with condensing operation and with "water
injection".
[0081] The calculations concerning the invention are
"conservative", i.e. the fact that the invention allows for better
control of the plant and thus for less surplus of air, giving a
higher efficiency degree, has not been taken into account in the
calculation.
[0082] As condensing operation on dry fuels is not standard, the
new method gives a higher efficiency degree by use of dry fuels. It
should be noted that in case of high return temperature (above
45.degree. C.) and dry fuel, the process will be water consuming,
unless moistening of combustion air is used.
[0083] Further, moistening will be able to increase the efficiency
degree considerably, especially at higher return temperatures. Due
to water injection, the amount of flue gas is increased during
cooling of the flue gas. The condensing unit and belonging pipes
must of course be dimensioned for this.
[0084] Summarization of the most important advantages of the
invention:
[0085] Simpler and Cheaper Plant
[0086] The most important advantage of the concept is that the
construction becomes considerably simpler and cheaper than for
traditional condensing plants with both a convection part and a
condensing unit. By use of the invention, a convection boiler and
belonging boiler circuit with shunt and heat exchanger can be
saved, and the water circuit and the control of the heat
productions become much simpler and thus cheaper. However, there
will be an extra cost of water dosing and a larger condensing
plant, but that will be very small compared to the savings.
[0087] Compact Plant
[0088] The principles used for transferring heat from gas to water
in the concept (evaporation of water in a hot gas and
scrubber+plate exchanger/condensing pipe cooler) are very effective
(compared to dry convection) and thus compact.
[0089] As the number of units is reduced, and as the principles for
heat transfer are more effective, the total plant becomes more
compact.
[0090] Lower Maintenance Costs
[0091] Maintenance costs of a water injection system become
considerably lower than the present maintenance costs of "boiler
operation".
[0092] By use of fluid bed and by use of water injection to adjust
the bed temperature, savings are also obtained for maintenance, as
the traditional cooling pipes, which will be worn out of the bed
material, are avoided.
[0093] Fuel Flexibility
[0094] Up to now, it has been necessary to construct plants for
either wet or dry fuel. Wet fuel necessitates brick lining in the
combustion chamber to obtain a good combustion. If dry fuel is used
in brick-lined plants, the combustion temperature will be too high.
With the water injection concept, the combustion chamber can be
used for wet fuel, and in case of combustion of dry fuel, an
adequate amount of water will be added in order to keep the
temperature down.
[0095] Higher Efficiency by Better Control of Air
[0096] The efficiency is increased by lower air consumption, as the
flue gas loss becomes smaller. With careful positioning and control
of the water nozzles, the air consumption can be reduced compared
to plants with "boiler operation", which will give a better
efficiency.
[0097] Higher Efficiency by Moistening of Combustion Air
[0098] The efficiency is further increased by 5-15% by moistening
of combustion air.
[0099] Lower Emissions
[0100] Thermal NOx can be reduced by water injection in and around
the combustion chamber, especially in case of gas and coal
combustion.
[0101] Emissions of HCl, SO2, Dioxins etc, will be reduced when the
water in the condensing unit is neutralized e.g. with NaOH.
[0102] Particle emissions will be reduced when filters are used
e.g. bag filters.
[0103] It should be understood that numerous changes and
modifications of the embodiments of the invention described above
could be made within the scope of the appended claims. Furthermore,
the use of solid fuel in the method and system defined by the
claims could be replaced by or supplemented by the use of gaseous
and/or liquid fuel.
* * * * *