FIELD OF THE INVENTION
[0001] The invention concerns a method in accordance with the preamble to claim 1 for reducing
nitrogen oxide emissions and corrosion in a bubbling fluidized bed boiler burning
biofuel or waste. The invention also concerns a bubbling fluidized bed boiler in accordance
with claim 5.
BACKGROUND OF THE INVENTION
[0002] There are two main types of fluidized bed boilers: bubbling fluidized bed boilers
(BFB) and circulating fluidized bed boilers (CFB). The main difference between these
two boiler types is in fluidization velocity. Low gas velocity is used in BFB boilers
and the position of the solids layer is relatively stationary. In CFB boilers higher
gases velocities are used sufficient to suspend the particle bed and entrain particles,
which are recirculated via an external loop back into the fluidized bed.
[0003] Bubbling fluidized bed boilers are in general use for producing energy from biofuels
of various kinds (peat, wood, industrial sewage sludge and other such) and from oil-based
waste (for example, plastics waste). Fuels of this type are characterized in that
the share of volatile matter is high in the dry matter of the fuel.
[0004] In fluidized bed combustion, nitrogen oxides (NO
x) are mainly the result of the oxidation of organic nitrogen in the fuel. Emission
standards for nitrogen oxides concerning bubbling fluidized bed boilers will become
significantly stricter in the next few years. The quantity of nitrogen oxides can
be reduced e.g. by methods to do with combustion technology, by injecting ammonia
into the furnace and by catalytic purification of the flue gases. Methods of combustion
technology are used to prevent formation of nitrogen oxides or to convert into other
compounds those nitrogen oxides, which are produced in earlier stages of combustion.
The present invention concerns a method of combustion technology and a related device
construction.
[0005] In a bubbling fluidized bed boiler, a bubbling fluidized bed, that is, a fluidized
bed is located in the lower part of the furnace and it consists of a fine incombustible
material, such as sand, which is fluidized by supplying fluidizing gas into the bed
from below. The fluidizing gas may consist solely of primary air or it may be a mixture
of primary air and an inert gas, such as flue gas. The fluidizing gas is set to flow
with such a velocity that the particles in the fluidized bed will not escape along
with the gas flow into the upper part of the boiler but they will remain in the lower
part of the boiler forming a fluidized bed, which is in continuous motion and which
efficiently mixes together the bed material and the fuel supplied into it.
[0006] The combustion air needed for burning the fuel is generally supplied in stages and
in several separate parts into the boiler furnace in such a way that a part of the
combustion air is formed by primary air, that is, fluidizing air, which is blown along
with the fluidizing gas, and a part is formed by secondary air supplied above the
fluidized bed, while the rest of the combustion air is supplied into the upper part
of the furnace as tertiary air.
[0007] The furnace of a bubbling fluidized bed boiler can be divided vertically into three
successive combustion zones based on the combustion air supply into the different
zones. The fluidized bed and the space above it, all the way to the level below the
secondary air nozzles, form the first combustion zone (I), into which primary air
is brought mainly along with the fluidizing gas supplied from below the fluidized
bed, besides which a minor quantity of primary air arrives in connection with the
fuel supply as carrier air and as cooling air for the start-up burners. The second
combustion zone (II) is located in the space between the secondary air supply level
and the tertiary air supply level. Secondary air is supplied into the second combustion
zone (II) mainly through secondary air nozzles located in the lower part of the zone.
If there are load carrying burners in the furnace, a small part of the secondary air
is supplied as cooling air for the load carrying burners. The third combustion zone
(III) begins from the tertiary air supply level and combustion air is brought into
it through tertiary air nozzles.
[0008] The fuel is supplied on top of the fluidized bed with the aid of carrier air, which
is used for, among other things, preventing clogging of the fuel supply chutes. In
the fluidized bed and above it fuel particles are dried, volatile matter is released,
that is, pyrolysis occurs, and the carbonized residue is incinerated. Drying and pyrolysis
are very fast events. The volatile matter released in pyrolysis comprises mainly methane
CH
4 and carbon monoxide CO as well as ammonia NH
3 and hydrogen cyanide HCN. The volatile matter rises upwards in the furnace and it
will burn when reaching an oxygen-containing region. In a fluidized bed boiler operating
with staged air supply, the combustion of volatile matter takes place mainly under
the effect of secondary air and partly also under the effect of tertiary air. The
combustion of carbonized residue of fuel particles takes place under the effect of
primary, secondary and tertiary air.
[0009] It is possible with the aid of staged air supply to reduce the formation of nitrogen
oxides in a bubbling fluidized bed boiler. In the presence of oxygen, NH
3 and HCN react into nitrogen monoxide NO. By staging the air supply, reducing substoichiometric
areas are formed in the furnace of the bubbling fluidized bed boiler. In these areas,
NH
3 and HCN formed from the fuel are reduced into molecular nitrogen in accordance with
reaction equations (1) and (2):
[0010] In addition, nitrogen oxides are reduced with the aid of an internal re-burning reaction,
in which hydrocarbon radicals formed in the pyrolysis will take part in reducing the
nitrogen oxides. An example of such a reaction is described in reaction equation (3),
in which -CH
i acts as the hydrocarbon radical:
[0011] Reducing areas are usually formed by adjusting the volume of primary and secondary
air. The furnace is kept substoichiometric as regards oxygen until the supply of tertiary
air, whereby the available dwell time for reactions (1) and (2) is maximized and the
quantity of NH
3 and HCN before the tertiary air level is minimized. The optimum total air coefficient
SR
tot for NO
x emissions before the tertiary air supply is just under 1, depending on the combustion
temperature. The air needed for complete combustion of volatile matter and of the
carbonized residue is brought into the furnace as tertiary air. NH
3 and HCN remaining in the flue gases will oxidize into nitrogen oxides after the tertiary
air supply.
[0012] With these conventional measures in accordance with staged air supply it is possible
to reduce nitrogen oxide emissions by approximately 30 % in comparison with non-staged
air supply. Fine and light fuels are problematic, because some fuel particles will
not end up in the fluidized bed, but they are carried away with the fluidizing gas
and the secondary air into the upper parts of the furnace. Thus, it is close to impossible
to bring about for such fuels any controlled combustion conditions in the furnace
which are favorable for the reduction of nitrogen oxides.
[0013] WO 2006084954 A1 describes a solution, which aims at reducing nitrogen oxide emissions of a bubbling
fluidized bed boiler by using staged air supply in such a way that a part of the primary
air is supplied into the furnace in connection with the fuel supply in the same direction
as the fuel itself. In the publication, this part of primary air is called combustion
air for volatile matter. Essentially all fuel supplied into the furnace is thus forced
on to the surface of the fluidized bed, whereby the fuel is made to mix into the fluidized
bed and to dry quickly. The pyrolysis following after the drying and the combustion
of volatile matter released in the pyrolysis also occurs almost immediately after
the fuel has mixed into the fluidized bed. Owing to the quick mixing together of fuel
and oxygen, a major part of the volatile matter released from the fuel can be burnt
above the fluidized bed before the secondary air supply level. The combustion of volatile
matter causes a high temperature, which maximizes the occurrence of hydrocarbon radicals
formed from the fuel and promotes reduction of released nitrogen oxides.
[0014] According to
WO 2006084954 A1, the quantity of combustion air for volatile matter, supplied in connection with
the fuel supply, is adjusted to be such that the combustion of volatile matter released
from the fuel burnt at each time will take place in substoichiometric conditions in
relation to the volatile matter. The air coefficient for volatile matter, that is,
SR
vol, in the primary air zone is hereby as high as possible, however, less than 1, preferably
in a range of 0.75 - 0.97 and most preferably in a range of 0.90 - 0.95. The total
air coefficient, that is, SR
tot, on the same furnace level varies in a range of 0.50 - 0.80, being preferably 0.65.
The solution does not change the air volume supplied into the furnace and not either
the total air coefficient, but it changes the air distribution in the boiler, so that
the air coefficient for volatile matter is as high as possible at as low a level as
possible in the furnace and for as long a time as possible before the secondary air
level.
[0015] The air coefficient or the stoichiometric ratio SR tells how much air must be used
for the combustion in comparison with the theoretical (stoichiometric) volume of air
needed for complete combustion of the fuel. In substoichiometric combustion, the air
coefficient SR is under 1, and in superstoichiometric combustion the air coefficient
SR is over 1.
[0016] When the aim is to reduce nitrogen oxide emissions with means of combustion technology
in existing boilers, the temperature of flue gases should not usually rise from the
present-time values at the nose of the furnace. If in connection with changes in the
combustion technology the flue gas temperature becomes too high or if the temperature
distribution of flue gas is very uneven, this will lead to contamination of the heat
transfer surfaces of the boiler's second draft, especially when burning fuels that
contain lots of alkali metals. On the other hand, with chlorine-bearing fuels high
flue gas temperatures may cause strong corrosion in the super-heater area.
[0017] The size of a furnace can be characterized by furnace load, which can be obtained
by dividing the fuel power supplied into the boiler by the volume of the furnace.
The values of furnace load used in this application are calculated using furnace volume
up to the level of the nose of the boiler. Furnace load illustrates the residence
time of fuel gases in the furnace.
[0018] Bubbling fluidized bed boilers are typically manufactured for various furnace loads.
According to one classification, a low furnace load is about 90 - 110 kW/m
3, an average furnace load is about 110 - 130 kW/m
3 and a high furnace load is about 130 - 150 kW/m
3. In boilers with high furnace load, the combustion temperature is high, whereby fluidized
bed temperatures can easily exceed those temperatures, which are permissible from
the viewpoint of softening of the ash. Typically, the temperature of the fluidized
bed should be kept within a range of 800 - 900 °C. If the bed temperature rises much
above 900 °C and the fuel contains much alkali metals, this could lead to agglomeration
of ash particles in the fluidized bed.
PURPOSE OF THE INVENTION
[0019] The objective of the invention is to bring about a method of combustion technology
and a fluidized bed boiler, with which it is possible to reduce nitrogen oxide emissions
in the combustion of biofuel or waste and at the same time to prevent corrosion of
the furnace and the super-heater area.
SUMMARY
[0020] The method according to the invention is characterized by the features presented
in the characterizing part of claim 1.
[0021] Correspondingly, the bubbling fluidized bed boiler according to the invention is
characterized by the features presented in the characterizing part of claim 5.
[0022] The invention concerns a method for reducing nitrogen oxide emissions and corrosion
in a bubbling fluidized bed boiler burning biofuel and/or waste. In this method, air
required for fuel combustion is supplied in stages into the boiler furnace in such
a way that at least two substoichiometric zones (I) and (II) are brought about, which
have as one of their objectives to reduce the formation of nitrogen oxides, as well
as at least one superstoichiometric zone (III), in which the combustion is completed.
The combustion zones comprise a first combustion zone (I), which begins from the height
level of the primary air supply nozzles and extends above the fluidized bed up to
below the height level of secondary air nozzles; a second combustion zone (II), which
begins from the height level of the secondary air nozzles and extends up to below
the height level of tertiary air nozzles; and a third combustion zone (III), which
begins from the height level of the tertiary air nozzles.
[0023] In the method according to the invention, the length of the first combustion zone
(I) is optimized by placing the secondary air nozzles at a height h
s from the top surface of the bubbling fluidized bed, which height h
s is calculated from the formula:
wherein:
hS = the height of the secondary air level from the top surface of the bubbling fluidized
bed [m]
Qf = the boiler's full fuel power [MW]
A = the boiler's cross-sectional area [m2]
qf = the boiler's full furnace load under the nose [kW/m3]
LHVref = lower heat value of the fuel [MJ/kg]
pref = fuel density [kg/m3]
qf,ref = furnace load [kW/m3]
fa,ref = air requirement of volatile matter in the fuel [kg air/kg fuel]
tref = dwell time (in a normal state NTP; T = 0 oC, p = 1 atm) [s]
SRvol,ref = air coefficient for fuel volatile matter in the first combustion zone
(I),
in which formula some variables are furnace-specific, some variables are reference
values and thus constants, and some variables are design point-specific reference
values, which are chosen based on the furnace load of the bubbling fluidized bed boiler.
[0024] Some variables used in the calculation, such as the boiler's full fuel power Q
f, the boiler's cross-sectional area A and the boiler's full furnace load q
f under the nose, are variables specific to the furnace.
[0025] The reference values, which are the fuel's lower heat value LHV
ref, the fuel density pref, the furnace load q
f,ref and the air requirement of volatile matter in the fuel f
a,ref, are empirically chosen constants.
[0026] The dwell time t
ref and the air coefficient SR
vol, ref for fuel volatile matter in the first combustion zone (I) are design point-specific
reference values. The design point used at each time is chosen based on a furnace-specific
consideration. Typically, in a boiler with a high furnace load the dwell time t
ref is shorter and the air coefficient SR
vol, ref for volatile matter is lower than in a boiler with a low furnace load.
[0027] The design point-specific reference values t
ref and SR
vol, ref are preferably chosen based on the boiler's furnace load, so that in a boiler with
a high furnace load the values t
ref = 6 s and SR
vol, ref = 0.7 are used, and in a boiler with a low furnace load the values t
ref = 7 s and SR
vol, ref = 0.9 are used. A boiler with q
f higher than 130 kW/m
3 is regarded as a boiler of a high furnace load, and a boiler with q
f lower than 130 kW/m
3 is regarded as a boiler of a low furnace load. When required, it is possible to deviate
from the shown limit value of 130 kW/m
3, for example, when the fuel mainly used in the boiler is especially dry or especially
wet.
[0028] The invention also concerns a bubbling fluidized bed boiler suitable for burning
biofuel and waste, comprising a furnace, in the lower part of which there is a fluidized
bed into which primary air is supplied through primary air nozzles located under the
fluidized bed and fuel is supplied through fuel supply openings in the walls of the
furnace, which fuel supply openings are located at a first height above the fluidized
bed surface. The furnace also has a set of secondary air nozzles, which are located
at a second height above the fluidized bed surface, and a set of tertiary air nozzles,
which are located at a third height above the fluidized bed surface. Hereby a first
combustion zone (I) is formed in the furnace of the bubbling fluidized bed boiler,
which zone comprises a fluidized bed and above this a space up to under the height
level of the secondary air nozzles, a second combustion zone (II), which extends from
the height level of the secondary air nozzles up to under the height level of the
tertiary air nozzles, and a third combustion zone (III), which begins from the height
level of the tertiary air nozzles. Said first combustion zone (I) and second combustion
zone (II) are arranged to operate in a substoichiometric fashion so that the total
air coefficient SR
tot is less than 1 in each zone.
[0029] In a bubbling fluidized bed boiler according to the invention, the secondary air
nozzles are located at a certain height from the top surface of the bubbling fluidized
bed, which height is calculated with the aid of the following formula:
wherein:
hs = height of the secondary air level from the top surface of the bubbling fluidized
bed [m]
Qf = the boiler's full fuel power [MW]
A = the boiler's cross-sectional area [m2]
qf = the boiler's full furnace load under the nose [kW/m3]
LHVref = the fuel's lower heat value [MJ/kg]
ρref = fuel density [kg/m3]
qf, ref = furnace load [kW/m3]
fa, ref = air requirement of volatile matter in the fuel [kg air/kg fuel]
tref = dwell time (in a normal state NTP; T = 0 °C, p = 1 atm) [s]
SRvol, ref = air coefficient for fuel volatile matter in the first combustion zone (I),
in which formula some variables are furnace-specific, some variables are reference
values and thus constants, and some variables are design point-specific variables,
which are chosen based on the boiler's furnace load.
[0030] If it is desired in terms of combustion technology to minimize nitrogen oxide emissions
and at the same time to minimize the flue gas temperature at the furnace nose, the
bubbling fluidized bed boiler should always be operated in such a way that in the
first combustion zone (I) the air coefficient for volatile matter, that is, SR
vol, is in a range of 0.9 - 1.0. If SR
vol is raised close to a value of 1.0 by supplying into the first combustion zone (I)
a theoretical quantity of oxygen needed for the combustion of volatile matter, this
will result in a raised combustion temperature in the upper part of the fluidized
bed. An increased heat radiation will thus also raise the fluidized bed temperatures,
typically by about 20 - 50 °C. If the fluidized bed boiler is made with a high furnace
load and if the fuel is especially dry (water content less than 45 weight-%), this
may result in the rise of fluidized bed temperature above the permissible temperature
limits.
[0031] One way of lowering the fluidized bed temperature in a bubbling fluidized bed boiler
is by reducing the boiler's masonry surface in the region between the primary air
level and the secondary air level. In typical bubbling fluidized bed boilers of today,
the lower part of the furnace is made by laying bricks to a height of about 2.5 -
5 meters from the surface of the bubbling fluidized bed. The purpose of the masonry
is to protect the boiler's water pipes against corrosion and contamination, but at
the same time it also increases the temperatures in the part above the fluidized bed,
because the masonry prevents radiation heat transfer to the water pipes lining the
furnace.
[0032] In a solution according to the invention, it is recommended, especially in boilers
with a high furnace load, to change over to a masonry height of 1.8 - 2.4 meters from
the fluidized bed surface, preferably 1.8 - 2.0 meters from the fluidized bed surface.
[0033] If the fluidized bed temperatures would even after this be too high, nitrogen oxides
can be reduced by maintaining a low air coefficient SR
vol for volatile matter in the first combustion zone (I), for example, close to a value
of 0.7, and a short dwell time in the first combustion zone (I). A short dwell time
is achieved by locating the secondary air supply level relatively close to the fluidized
bed surface above it, which optimum distance can be calculated with the aid of the
mathematical formula presented in the claims. The second combustion zone (II) following
after the first combustion zone (I) can thus have a longer dwell time, whereby the
combustion of volatile matter and the reactions minimizing the quantity of nitrogen
oxides can be brought to an end in the second combustion zone (II).
[0034] The invention comprises two alternative embodiments for reducing nitrogen oxides
in the flue gases of a bubbling fluidized bed boiler. The embodiment to use at each
time is chosen primarily on the basis of the boiler's furnace load. When the bubbling
fluidized bed boiler has a high furnace load (for example, over 130 kW/m
3), and especially when burning a dry fuel (for example, moisture less than 45 weight-%),
then according to the invention an optimum air coefficient SR
vol = 0.9 - 1.0 for volatile matter in the fuel is not utilized in the first combustion
zone (I), but the operation is in a lower SR
vol range of 0.65 - 0.75. Instead, when the boiler has a low furnace load (for example,
less than 130 kW/m
3), a SR
vol range of 0.9 - 1.0, which is the optimum range for the nitrogen oxide reduction,
is used in the first combustion zone (I). The air coefficient SR
vol for volatile matter can be increased in the first combustion zone (I), for example,
in a manner presented in
WO 2006084954 A1 by supplying into the first combustion zone (I) along with the fuel supply a desired
volume of combustion air, which is preferably taken from the secondary air register,
where the air temperature is in a range of 165 - 330 °C.
[0035] WO 2006084954 A1 defines air coefficients for the various combustion zones, but it does not concern
itself with the grounds for design of the entire air distribution system or with the
optimum location of devices. Nor does the publication concern itself with limitations
of the method of combustion technology, such as high furnace load. The air coefficients
defined in
WO 2006084954 A1 for two different methods do not thus contain basic design information in sufficient
detail.
[0036] In a bubbling fluidized bed boiler according to the invention, the fuel used can
be e.g. peat, wood, industrial sludge, chlorine-bearing plastics waste, as well as
mixtures of these. The optimum air coefficient for fuel volatile matter, that is,
SRvol, in the first combustion zone (I) depends on the fuel used. For example, with
peat the optimum air coefficient is SR
vol = 1.0 and with wood SR
vol = 0.9.
[0037] The distance of the fuel supply openings from the surface of the bubbling fluidized
bed is also of significance. When the first combustion zone (I) functions in a range
of SR
vol = 0.9 - 1.0 and the combustion air for volatile matter is supplied together with
the fuel, the fuel supply openings must not be too close to the surface of the bubbling
fluidized bed and the vertical fuel supply angle must not be too steep to avoid the
risk that the combustion air for volatile matter supplied together with the fuel would
carry with it from the fluidized bed particles, which drift along with the flue gas
all the way to the boiler outlet.
[0038] In a boiler with a low furnace load, the fuel supply openings are preferably arranged
at a height of 2.0 - 2.6 meters from the fluidized bed surface, and in a boiler with
a high furnace load they are at a height of 1.8 - 2.0 meters from the fluidized bed
surface. In a boiler with a low furnace load, the vertical fuel supply angle, which
in this context means the angle between the fuel chute and the normal of the furnace
wall, may be about 25° - 35°, and in a boiler with a high furnace load it may be about
35° - 40°.
[0039] There are usually two or three fuel supply chutes and supply openings side by side
at the same height on one or both side walls of the fluidized bed boiler. Seen from
above, the supply chutes are usually arranged perpendicular to the wall in question,
whereby the fuel flow will leave the fuel supply opening in the direction of the wall's
normal. Then there is a risk that from the fuel supply openings which are outermost,
that is, closest to the adjacent front or back wall of the furnace, the fuel may spread
on to the front or back wall of the furnace. This risk can be avoided by inclining
the outermost fuel supply chutes in the horizontal direction in such a way that the
direction of motion of the fuel discharging from the supply openings will be 5 - 15°
away from that boiler side wall which is closest to the supply opening. This angle
is called the fuel chute's horizontal supply angle. It is especially important to
guide the fuel flow away from the adjacent wall when chlorine- or sulfur-bearing fuel
is being burnt.
[0040] In an advantageous embodiment of the invention, a side air nozzle is mounted in between
a fuel chute located on a first wall of the furnace and an adjacent second wall, through
which nozzle airflow can be blown into the furnace to prevent the fuel released from
the fuel chute from attaching itself to the second wall.
[0041] In an advantageous embodiment of the invention, secondary air nozzles and/or tertiary
air nozzles are arranged in two opposite rows on two opposite walls of the furnace,
and in each row there are nozzles blowing a small air jet, nozzles blowing an air
jet of medium size and nozzles blowing a large air jet. The nozzles are arranged to
alternate so that every second nozzle is a nozzle blowing a small air jet and every
second nozzle is a nozzle blowing a medium-sized or a large air jet. In addition,
the nozzles are arranged in a staggered fashion so that opposite to each nozzle blowing
a small air jet there is on the opposite wall a nozzle blowing a medium-sized or large
air jet. Of the outermost nozzles in each row at least one is a nozzle blowing a large
air jet, and of the nozzles located in the central part of each row at least one is
a nozzle blowing a medium-sized air jet. With the aid of the outermost air jets the
adjacent walls are "swept" and an area with a high oxygen content is brought about
near the walls which are perpendicular to these nozzle walls. The small air jets have
a weak penetration, but their purpose is in fact to sweep the nozzle wall and to bring
about an area with high oxygen content close to the nozzle wall.
[0042] When the intention is to burn a fuel with a high content of chlorine and/or sulfur
in the bubbling fluidized bed boiler, the supply of fuel and combustion air is preferably
arranged in a way to efficiently prevent the furnace walls adjacent to the fuel supply
from being contaminated and from corroding. Hereby it is preferable to use in the
boiler all three solutions presented above: the outermost supply chutes are inclined
5-15° away from the side wall which is adjacent to the fuel supply spot, there are
side air nozzles between the outermost supply chutes and the side wall, and secondary
and/or tertiary air nozzles are arranged to blow air blasts in the manner described
above in such a way that such conditions are brought about near the furnace walls
that will prevent the walls from corroding and from being contaminated.
[0043] Reduction of nitrogen oxides by a two-staged substoichiometric combustion, which
is followed by the use of tertiary air in a third combustion zone, produces substoichiometric
conditions near the furnace walls. If the fuel contains much chlorine and/or sulfur,
strong corrosion of the evaporator walls will result. With the method according to
the invention it is possible to reduce the amount of nitrogen oxides in the flue gas,
however, at the same time keeping the corrosion of both the furnace and the super-heater
surfaces in the second draft within the permitted limits.
[0044] The tertiary air nozzles are preferably located 2 - 4 meters below the furnace nose.
[0045] At the present time, primary air supplied into the fluidized bed from under the bed
is distributed evenly over the entire fluidized bed area. Coarse wood fuel in particular
will fly from the fuel supply chute in point-form into the fluidized bed, and this
will result on the one hand in areas with much fuel and little air and on the other
hand in areas with much air and little fuel. This will lead to uneven combustion and
to high nitrogen oxide emissions. Primary air could advantageously be distributed
into the fluidized bed in such a way that more primary air than the average volume
is supplied to fuel-rich areas and, correspondingly, less primary air than the average
volume is supplied to fuel-poor areas. However, an uneven supply of primary air must
not cause uneven floating of the fluidized material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0046] The invention is described in the following by referring to the examples shown in
the appended figures, but there is no intention to restrict the invention narrowly
to the embodiments presented in the figures.
Figure 1 is a schematic perspective view of a bubbling fluidized bed boiler.
Figure 2 is a schematic perspective view of the lower part of a bubbling fluidized
bed boiler.
Figure 3 is a schematic sectional front view of the furnace of a bubbling fluidized
boiler designed for a high furnace load.
Figure 4 is a schematic sectional front view of the furnace of a bubbling fluidized
boiler designed for a low furnace load.
Figure 5 is a simplified sectional view of the cross-section of a furnace at the level
of fuel supply chutes.
Figure 6 is a side view of air jets supplied into a furnace.
Figure 7 is a schematic sectional view of a furnace at the air jet level.
DETAILED DESCRIPTION OF THE INVENTION
[0047] Figure 1 shows schematically a bubbling fluidized bed boiler and its furnace 1. The
lower part of the furnace 1 is provided with masonry 8, which protects the pipes (not
shown) of a water-steam circuit located on the walls A, B of furnace 1 from overheating.
Fuel is supplied into the furnace 1 through fuel supply openings 5 located in two
opposite walls A of the furnace. Air needed for fuel combustion is supplied through
primary air nozzles (not shown) located in the bottom of the furnace and through secondary
air nozzles 6 and tertiary air nozzles (not shown) located in the walls B of the furnace
1. In the upper part of the furnace 1 there is a nose 9 that reduces the boiler's
flow cross-sectional area, whereupon the flow of flue gases turns through super-heaters
(not shown) in the upper part of the boiler into a flue gas duct 13. Figure 1 also
shows start-up burners 10 near the fuel supply height as well as load bearing burners
11 at an upper position on the wall B of the furnace 1.
[0048] Figure 2 shows enlarged a part of the lower part of the furnace 1 of a bubbling fluidized
bed boiler. Fuel supply openings 5 are arranged on the furnace wall A. In between
each fuel supply opening 5 and the nearest wall B a side air nozzle 12 is fitted on
the wall A, from which nozzle air can be blown in between the fuel flow and the side
wall B parallel with the side wall B. The side air nozzles 12 are used to prevent
the side walls B from corroding and from being contaminated, especially when the fuel
contains much sulfur or chlorine.
[0049] Figures 3 and 4 show in principle the furnace 1 of a bubbling fluidized bed boiler
seen from the front. In Figure 3 the boiler is a bubbling fluidized bed boiler designed
for a high furnace load and in Figure 4 it is a fluidized bed boiler designed for
a low furnace load. Please note that the figures are just basic views and they are
not intended to present a bubbling fluidized boiler on its correct scale.
[0050] In the lower part of the furnace 1 there is a fluidized bed 2 consisting of fluidized
bed material, into which bed fluidizing gas is supplied through nozzles 3 arranged
in the bottom of the furnace 1, which gas makes the fluidized bed material float and
bubble. The fluidizing gas may be just air or it may be a mixture of air and a circulating
gas. The height h
B of the fluidized bed is usually about 800 mm, that is, the distance of the top surface
of the bubbling fluidized bed 2 from the height level of primary air nozzles 3 is
about 800 mm.
[0051] Fuel is supplied above the fluidized bed 2 surface by fuel supply devices 4, 5, which
comprise a fuel supply chute 4, which is directed obliquely downwards and ends in
a supply opening 5 located in the furnace wall A. A small amount of carrier air is
also usually brought along with the fuel into the boiler, its main function being
to prevent clogging of the supply chute 4. The fuel supply openings 5 are located
at an optimum height h
F from the top surface of fluidized bed 2, which optimum height h
F can be determined on the basis of the bubbling fluidized bed boiler's furnace load
and some other variables.
[0052] In boilers with a high furnace load, the distance h
F between the fuel supply openings 5 and the top surface of fluidized bed 2 is usually
in a range of 1.8 - 2.0 meters. An acute angle α is formed between the fuel supply
chute 4 and the normal N of the furnace wall A, which angle in a boiler with a high
furnace load is about 35° - 40°. This angle is called vertical supply angle α. Masonry
8 extends from the fluidized bed surface to a height h
R, which height is in a range of 1.8 - 2.0 meters in a boiler with a high furnace load.
[0053] Secondary air is supplied into the furnace 1 from secondary air nozzles 6 located
above the fuel supply level at height level h
s. The distance h
s of the secondary air nozzles 6 from the top surface of fluidized bed 2 can be optimized
on the basis of the bubbling fluidized bed boiler's furnace load and some other variables.
[0054] The space confined between the height level of primary air nozzles 3 and the secondary
air nozzles 6, which space contains the fluidized bed 2 and the volume above it all
the way up to a level under the secondary air nozzles 6, forms a first combustion
zone (I). Into the first combustion zone (I), fuel is supplied through supply openings
5 along with a small quantity of carrier air, and through primary air nozzles 3 is
supplied fluidizing gas, which can be, for example, air or a mixture of air and flue
gas.
[0055] Tertiary air is supplied above the secondary air nozzles 6 through tertiary air nozzles
7 arranged in the upper part of the furnace 1. The distance h
T of the tertiary air nozzles 7 from the top surface of the fluidized bed 2 can also
be optimized according to the furnace load and some other variables. The tertiary
air nozzles 7 are usually placed 2 - 4 meters below the furnace nose 9.
[0056] The space confined between the secondary air nozzles 6 and the tertiary air nozzles
7, which space begins from the height level h
s of the secondary air nozzles 6 and ends under the tertiary air nozzles 7, forms a
second combustion zone (II). Into the second combustion zone (II), secondary air is
supplied through the secondary air nozzles 6, to be mixed together with a flue gas
flow rising upwards from the first combustion zone (I) and containing non-combusted
gases and particles deriving from the fuel, which gases and particles will be further
combusted in the second combustion zone (II).
[0057] The burning of fuel continues further in a third combustion zone (III) beginning
from the height level h
T of the tertiary air nozzles 7. The first combustion zone (I) and the second combustion
zone (II) are substoichiometric zones, whereas the third combustion zone is a superstoichiometric
zone, that is, the total air coefficient SR
tot in the third combustion zone (III) is over 1.
[0058] The fine fuel supplied into the fluidized bed 2 dries immediately on coming into
contact with the hot fluidized bed material, and it undergoes pyrolysis essentially
in its entirety. The volatile matter released from the fuel in connection with the
pyrolysis will burn in and above the surface of the fluidized bed 2 with the aid of
primary air. The combustion of volatile matter takes place in the first combustion
zone (I) in conditions that are substoichiometric as regards the air coefficient SR
vol for volatile matter. The higher the air coefficient SR
vol for volatile matter, the more quickly the volatile matter will burn, at the same
time causing a high local temperature and forming a maximum quantity of hydrocarbon
radicals, which are needed for the reduction of nitrogen oxides formed from the fuel.
[0059] The reduction to molecular nitrogen of nitrogen oxides formed from the fuel is carried
out mainly in two stages. In the first substoichiometric combustion zone (I) a major
part of the volatile matter of the fuel and a part of the carbonization residue are
burnt. This takes place in conditions which are substoichiometric as regards the air
coefficient SR
vol for the volatile matter of the fuel, whereby a lot of hydrocarbon radicals will result.
In the embodiment according to Figure 3 (a boiler with high furnace load), the primary
air needed for this stage is brought into the furnace mainly through the primary air
nozzles 3 as fluidizing gas. A small quantity of air comes into the first combustion
zone (I) in connection with the fuel supply through the supply chutes 4 and as cooling
air for the start-up burners.
[0060] In the second combustion zone (II), combustion air is supplied into the furnace from
the secondary air nozzles 6 so much that substoichiometric combustion conditions are
maintained, whereby the total air coefficient SR
tot in the second combustion zone (II) is in a range of 0.75 - 0.85, preferably about
0.8.
[0061] Figure 4 shows a fluidized bed boiler with a low furnace load. In this embodiment,
primary air is supplied into the first combustion zone (I) mainly in two phases: on
the one hand, as fluidizing air through the primary air nozzles 3 and, on the other
hand, in connection with the fuel supply as combustion air for the volatile matter.
Air conducted into the first combustion zone (I) in connection with the fuel supply
is preferably taken from the secondary air register, whereby it reduces the quantity
of air to be supplied into the second combustion zone (II). A larger supply of air
into the first combustion zone (I) will result in such temperatures in the first combustion
zone (I), which are higher than in the boiler with a high furnace load shown in Figure
3. When air is supplied into the furnace 1 along with the fuel supply, the fuel is
made to ignite quickly and a major part of the fuel's volatile matter can be burnt
before the second combustion zone (II).
[0062] In a fluidized bed boiler with a low furnace load, the distance h
F of the fuel supply openings 5 from the top surface of the fluidized bed 2 is preferably
2.0 - 2.6 meters, and the height h
R of masonry 8 is preferably 2.5 - 4.0 meters. Although the refractory masonry 8 is
shown in Figures 3 and 4 as extending up to the height of the fuel supply openings
5 by and large, this need not necessarily be so, but the height h
R of masonry 8 may be different from the fuel supply height h
F. The vertical supply angle α formed between the supply chute 4 and the normal N of
the wall A in the furnace 1 is in an approximate range of 25° - 35° in a boiler with
low furnace load. When air intended for the combustion of volatile matter is supplied
along with the fuel into the fluidized bed, the higher fuel supply height h
F and the more obtuse supply angle α make sure that particles will not escape from
the fluidized bed 2 when the mixture of air and fuel hits the top surface of the bubbling
fluidized bed 2.
[0063] The optimum height level h
s for the secondary air nozzles 6 is calculated according to the boiler's furnace load
and some other parameters with the aid of the formula presented earlier.
[0064] In a boiler with a low furnace load, the air coefficient SR
vol for volatile matter in the fuel is in a range of 0.9 - 1.00, preferably about 0.95,
in the first combustion zone (I). In the second combustion zone (II), the total air
coefficient SR
tot is about 0.8, that is, of essentially the same magnitude as in a boiler with a high
furnace load. In the third combustion zone (III), the total air coefficient SR
tot is about 1.15 irrespective of whether the boiler is with a low furnace load or with
a high furnace load.
[0065] The solution according to the invention aims at optimizing the combustion of the
fuel's volatile matter in the two first substoichiometric combustion zones (I) and
(II) of the furnace. In a boiler with a low furnace load, the first combustion zone
(I) is longer and it operates with a higher air coefficient SR
vol for volatile matter than the corresponding zone (I) in a boiler with a high furnace
load. In a boiler with a low furnace load, the second combustion zone (II) is shorter
than in a boiler with a high furnace load, but the total air coefficient SR
tot in the second combustion zone (II) is essentially the same, about 0.8, in each boiler.
This means in practice that in a boiler with a low furnace load, the first combustion
zone (I) operates in a range of SR
vol = 0.95 - 1.00, which is an optimum range for the reduction of nitrogen oxides, whereby
a major part of the fuel's volatile matter will burn already in the first combustion
zone (I). In a boiler with a high furnace load, the temperature of the first combustion
zone (I) must be limited, for which reason the air coefficient for volatile matter
is preferably kept within a range of SR
vol = 0.65 - 0.75. In order to complete the reactions that aim at reduction of nitrogen
oxides before the third combustion zone (III), which operates substoichiometric, the
length (h
T - h
S) of the second combustion zone (II) is increased in comparison with the corresponding
length in a boiler with a low furnace load.
[0066] Figure 5 shows a cross-sectional view of the furnace 1 of a bubbling fluidized bed
boiler near the fuel supply height. On the two first walls A of the furnace there
are three supply chutes 4, 4a, 4b on each. Each fuel supply chute 4, 4a, 4b ends in
a fuel supply opening 5, from which a fuel flow F discharges into the furnace. The
central fuel supply chutes 4 are fitted perpendicular to the wall A as seen from above,
whereby the fuel flow F leaves from the supply openings 5 in the direction of the
normal N of the wall A. The fuel supply chutes 4a and 4b located nearest to a side
wall B are mounted at an angle β to the normal N of the first wall A. The size of
this so-called horizontal supply angle β is 5-15°, preferably about 10°. The purpose
of the angle β is to turn the fuel flows F discharging from the fuel supply chutes
4a, 4b, which are nearest to the furnace's side wall B, away from the adjacent wall
B, which is intended to reduce the contamination and/or corrosion of the side walls
B. This is especially important when fuel containing much chlorine and/or sulfur is
being burnt.
[0067] In addition to the fuel supply chutes 4a, 4b mounted in an oblique position, the
furnace in Figure 5 is equipped with side air nozzles 12 in the manner known from
Figure 2, each nozzle being fitted in between the outermost fuel supply chute 4a,
4b and the side wall B. The side air nozzles 12 are fitted to blow air in the direction
of the side wall B near the fuel supply height, whereby they prevent the fuel flow
discharging from the fuel supply chute 4a, 4b from ending up on the side wall B. It
is also possible to direct the side air nozzles 12 to blow air obliquely downwards
towards the fluidized bed at a suitable vertical angle (not shown). The solution is
advantageous especially when fuel containing much chlorine and/or sulfur is being
burnt in the bubbling fluidized bed boiler.
[0068] A third way of reducing contamination of furnace walls is by adjusting the momentum
of air jets blown from the secondary and/or tertiary air nozzles 6, 7 in such a way
that airflows are blown from the nozzles located nearest to the fuel supply wall A
with a higher momentum, on an average, than from the nozzles located nearest to the
central part of the wall B provided with air nozzles 6, 7. Figures 6 and 7 illustrate
this principle.
[0069] Figure 6 shows a side view of air jets S, L, which are blown from secondary or tertiary
air nozzles located on two opposite walls B
1 and B
2 of the furnace 1, that is, seen from the direction of the wall A. Figure 7 shows
the corresponding air jets S, M, L seen from above.
[0070] On each one of the two opposite walls B
1 and B
2 of the furnace 1 there are mounted eight air nozzles R
1, R
2 in one row, of which some nozzles are nozzles 6
S blowing a small air jet S, some are nozzles 6
M blowing an air jet M of medium size and some are nozzles 6
L blowing a large air jet L. Different momenta of the air jets S, M and L blown from
the different nozzles 6
S, 6
M, 6
L can be brought about with technology known as such, for example, with the aid of
valves (not shown). The air jets leaving the nozzles situated at different positions
have momenta differing from each other in such a way that air jets S with a small
momentum alternate with air jets M, L having a larger momentum. Opposite to each air
jet S having a small momentum there is on the opposite furnace wall either one of
the air jets M or L with a higher momentum. Air jets L with a high momentum are used
in the outermost position, that is, in the position nearest to the adjacent wall A,
and air jets M with a medium-sized momentum are used in a position near the central
part of the wall B. Thus, air jets discharging from the air nozzles located on the
nozzle walls B
1, B
2 will efficiently sweep the surfaces both of the nozzle walls B
1, B
2 and of the adjacent walls A. With the aid of the outermost large air jets L an oxygen-rich
area is provided near the walls A which are perpendicular to the nozzle walls B
1 and B
2. With the aid of the weak penetration of the small air jets S an oxygen-rich area
is provided near the nozzle walls B
1, B
2. The velocity of the small air jets S is preferably less than 15 m/s.
[0071] When burning fuels that are difficult in view of corrosion and contamination, it
is possible in the same furnace to use, as is shown in Figure 5, a horizontal inclination
by angle β for the outermost fuel chutes and side air blowers 12 as well as the staggered
positions of air jets shown in Figures 6 and 7, which provides an oxygen-rich zone
near to the furnace walls A, B.
[0072] In the following, the invention is described by referring to examples presented in
Tables 1 and 2. The tables show stage by stage the total air coefficients SR
tot in a boiler with a high furnace load (Table 1) and in a boiler with a low furnace
load (Table 2) when using peat or wood as fuel. The total air coefficient SR
tot increases in the vertical direction of the furnace as more air is supplied into the
furnace.
[0073] Into the first combustion zone (I) of the furnace air is supplied mainly together
with the fluidizing gas as fluidizing air and in connection with the fuel supply as
carrier air. The small air volume used for cooling start-up burners has only a minor
effect on the total air coefficient SR
tot of the first combustion zone (I).
Table 1. Total air coefficients SR
tot in a fluidized bed boiler with a high furnace load.
|
Wood SRtot |
Wood SRVol |
Peat SRtot |
Peat SRVol |
Fluidizing air |
0.40 |
0.56 |
0.35 |
0.60 |
Carrier air |
0.45 |
0.62 |
0.40 |
0.68 |
Cooling air for start-up burners |
0.47 |
0.66 |
0.43 |
0.73 |
Secondary air |
0.80 |
1.11 |
0.80 |
1.36 |
Cooling air for load bearing burners |
0.83 |
1.15 |
0.83 |
1.41 |
Tertiary air |
1.15 |
1.59 |
1.15 |
1.95 |
[0074] Addition of secondary air at the beginning of the second combustion zone (II) and
addition of tertiary air at the beginning of the third combustion zone (III) clearly
raise the total air coefficient SR
tot.
[0075] In a boiler with a high furnace load, the fuel supply openings are preferably situated
at a height of about 1.8 - 2.0 meters from the surface of the bubbling fluidized bed,
and the vertical supply angle for the fuel supply chutes is in a range of 35° - 40°.
[0076] The air distribution according to Table 1 is suitable for use in boilers with a high
furnace load and when the fuel is especially dry (under 45 weight-% water in the fuel).
In this air distribution, the air coefficient for volatile matter of the fuel in the
first combustion zone (I), that is, SR
vol, is in a range of 0.65 - 0.75, whereby the combustion temperatures are low in the
lower part of the furnace.
[0077] Table 2 shows the air distribution in a bubbling fluidized bed boiler with a low
furnace load, where additional air taken from the secondary air register and intended
for the combustion of fuel's volatile matter in the first combustion zone (I) is supplied
into the boiler furnace in connection with the fuel supply. The air coefficient SR
vol for volatile matter should be kept within an optimum range for the reduction of nitrogen
oxides, which range is 0.9 - 1.0.
Table 2. Total air coefficients SR
tot in a fluidized bed boiler with a low furnace load.
|
Wood SRtot |
Wood SRvol |
Peat SRtot |
Peat SRvol |
Fluidizing air |
0.40 |
0.56 |
0.44 |
0.74 |
Carrier air |
0.45 |
0.62 |
0.48 |
0.82 |
Combustion air for volatile matter |
0.62 |
0.86 |
0.56 |
0.95 |
Cooling air for start-up burners |
0.65 |
0.90 |
0.59 |
1.00 |
Secondary air |
0.80 |
1.11 |
0.80 |
1.36 |
Cooling air for load bearing burners |
0.83 |
1.15 |
0.83 |
1.41 |
Tertiary air |
1.15 |
1.59 |
1.15 |
1.95 |
[0078] In this case, the combustion air for volatile matter supplied into the first combustion
zone (I) in connection with the fuel supply clearly raises the total air coefficient.
However, after the supply of secondary air, the total air coefficient is at the same
level as in Table 1. Thus, the total air volume to be supplied into the bubbling fluidized
bed boiler is the same as in the case shown in Table 1, but the air distribution is
different, when in the solution according to Table 2 a part of the secondary air of
Table 1 is supplied into the first combustion zone (I) along with the fuel supply.
[0079] In a boiler with a low furnace load, the fuel supply openings are preferably situated
at a height of about 2.0 - 2.6 meters from the surface of the bubbling fluidized bed
and the vertical supply angle of the fuel supply chutes is 25° - 35°. With a sufficient
distance of the supply openings from the fluidized bed and with a sufficiently obtuse
supply angle the combustion air for volatile matter is prevented from carrying along
particles from the fluidized bed.
[0080] The combustion air for volatile matter supplied into the furnace in connection with
the fuel supply is preferably taken from the secondary air, whereby the temperature
of the supplied air is in a range of 165 - 330 °C. A sufficiently high temperature
of the combustion air for volatile matter will keep the fuel supply chute dry and
clean, and no separate "fluidizing air" is needed in the supply chute for cleaning
the chute. The flow rate of the combustion air for volatile matter is at an optimum
level of 10 - 15 m/s in the fuel supply chutes. This sufficiently low velocity will
also for its own part prevent the combustion air for volatile matter from crashing
aggressively into the fluidized bed and thus prevents the resulting escape of sand.
[0081] The tertiary air nozzles are preferably located about 2 - 4 meters below the nose
of the furnace.
[0082] It is recommendable always when possible to use the method intended for boilers with
a low furnace load for reducing nitrogen oxides.
[0083] Many different modifications of the invention are possible within the scope defined
by the claims presented in the following.
1. A method for reducing nitrogen oxide emissions and corrosion in a bubbling fluidized
bed boiler burning biofuel and/or waste, in which method air needed for burning the
fuel is supplied in stages into the boiler furnace (1), so that at least two substoichiometric
zones (I), (II) are brought about, which have as one objective to reduce the formation
of nitrogen oxides, and at least one superstoichiometric zone (III), in which the
combustion is completed, which combustion zones comprise:
- a first combustion zone (I), which begins from the height level of primary air nozzles
(3) and extends above the fluidized bed (2), ending under the height level (hS) of secondary air nozzles (6);
- a second combustion zone (II), which begins from the height level (hS) of the secondary air nozzles (6) and ends under the height level (hT) of tertiary air nozzles (7); and
- a third combustion zone (III), which begins from the height level (hT) of the tertiary air nozzles,
characterized in that the length of the first combustion zone (I) is optimized by locating the secondary
air nozzles (6) at a certain height (hS) from the top surface of the bubbling fluidized bed (2), which height (hS) is calculated from the formula:
wherein:
hS = height of the secondary air level from the top surface of the bubbling fluidized
bed [m]
Qf = full fuel power of the boiler [MW]
A = cross-sectional area of the boiler [m2]
qf = boiler's full furnace load below the nose [kW/m3]
LHVref = lower heat value of the fuel [MJ/kg]
ρref = fuel density [kg/m3]
qf, ref = furnace load [kW/m3]
fa, ref = air requirement of volatile matter in the fuel [kg air/kg fuel]
tref = dwell time (in a normal state NTP) [s]
SRvol, ref = air coefficient for fuel volatile matter in the first combustion zone (I),
in which formula some variables are furnace-specific variables, some variables are
reference values and thus constants, and some variables are design point-specific
reference values, which are chosen according to the boiler's furnace load.
2. Method according to claim 1, characterized in that the design point-specific reference values tref and SRvol, ref are chosen according to the boiler's furnace load so that in a boiler with a high
furnace load, having a furnace load higher than 130 kW/m3, the values tref = 6 s and SRvol, ref = 0.7 are used, and in a boiler with a low furnace load, having a furnace load lower
than 130 kW/m3, the values tref = 7 s and SRvol, ref = 0.9 are used.
3. Method according to claim 1 or 2, characterized in that in a boiler with a high furnace load the air coefficient SRvol for volatile matter in the first combustion zone (I) is kept within a range of 0.65
- 0.75, preferably at a value of about 0.7, by adjusting the supply of primary air.
4. Method according to claim 1 or 2, characterized in that in a boiler with a low furnace load the air coefficient SRvol for volatile matter in the first combustion zone (I) is kept within a range of 0.9
- 1.0, preferably within a range of 0.95 - 1.0, by supplying into the first combustion
zone (I) besides the primary air also additional air intended for the combustion of
the fuel's volatile matter, which additional air is supplied to the furnace along
with the fuel supply and preferably taken from a secondary air register.
5. Bubbling fluidized bed boiler suitable for burning biofuel and/or waste, which comprises
a furnace (1), in which there are
- a fluidized bed (2) into which primary air is supplied through primary air nozzles
(3) located under the fluidized bed and fuel is supplied through fuel supply openings
(5) located in the walls of the furnace (1), which fuel supply openings (5) are located
at a first height (hF) above the surface of the fluidized bed (2),
- a set of secondary air nozzles (6), which are located at a second height (hS) above the surface of the fluidized bed (2),
- a set of tertiary air nozzles (7), which are located at a third height (hT) above the surface of the fluidized bed (2),
whereby the following zones are formed in the furnace (1) of the bubbling fluidized
bed boiler:
- a first combustion zone (I), which comprises a fluidized bed (2) and a space above
it all the way up to a level under the height level (hS) of the secondary air nozzles (6),
- a second combustion zone (II), which extends from the height level (hS) of the secondary air nozzles (6) all the way up to a level under the height level
(hT) of the tertiary air nozzles (7),
- a third combustion zone (III), which begins from the height level (hT) of the tertiary air nozzles (7),
which first combustion zone (I) and second combustion zone (II) are arranged to operate
substoichiometric in such a way that the total air coefficient SR
tot is under 1 in each zone,
characterized in that the secondary air nozzles (6) are located at a certain height (h
S) from the top surface of the bubbling fluidized bed (2), which height is calculated
with the aid of the following formula:
wherein:
hS = height of the secondary air level from the top surface of the bubbling fluidized
bed [m]
Qf = full fuel power of the boiler [MW]
A = cross-sectional area of the boiler [m2]
qf = full furnace load of boiler under the nose [kW/m3] LHVref = lower heat value of fuel [MJ/kg]
ρref = fuel density [kg/m3]
qf, ref = furnace load [kW/m3]
fa, ref = air requirement of volatile matter in the fuel [kg air/kg fuel]
tref = dwell time (in a normal state NTP) [s]
SRvol, ref = air coefficient for fuel volatile matter in the first combustion zone (I),
in which formula some variables are furnace-specific variables, some variables are
reference values and thus constants, and some variables are design point-specific
reference values, which are chosen according to the boiler's furnace load.
6. Bubbling fluidized bed boiler according to claim 5, characterized in that the height hS of the secondary air level is calculated by using the values tref = 6 s and SRvol, ref = 0.7 for a boiler with a high furnace load having a furnace load higher than 130
kW/m3.
7. Bubbling fluidized bed boiler according to claim 5, characterized in that the height hS of the secondary air level is calculated by using the values tref = 7 s and SRvol, ref = 0.9 for a boiler with a low furnace load having a furnace load lower than 130 kW/m3 .
8. Bubbling fluidized bed boiler according to any one of claims 5-7, characterized in that the lower part of the furnace (1) is equipped with masonry (8), which extends to
a height of 1.8 - 2.4 meters from the surface of the fluidized bed (2), preferably
to a height of 1.8 - 2.0 meters from the surface of the fluidized bed (2).
9. Bubbling fluidized bed boiler of low furnace load according to claim 7, characterized in that the fuel supply openings (5) are fitted at a height (hF) of 2.0 - 2.6 meters from the surface of the fluidized bed (2) and the vertical supply
angle (α) of the fuel chutes (4) is 25° - 35°.
10. Bubbling fluidized bed boiler of high furnace load according to claim 6, characterized in that the fuel supply openings (5) are fitted at a height (hF) of 1.8 - 2.0 meters from the surface of the fluidized bed (2) and the vertical supply
angle (α) of the fuel chutes (4) is 35° - 40°.
11. Bubbling fluidized bed boiler according to any one of claims 5-10, characterized in that the fuel supply chutes (4, 4a, 4b) are placed in connection with a first furnace
wall (A) and that the fuel supply chutes (4a, 4b) located nearest to a second furnace
wall (B), which is perpendicular to the first wall (A), are turned in the horizontal
direction away from said second wall (B) by a horizontal supply angle (β), which horizontal
supply angle (β) is in a range of 5-15°, preferably about 10°.
12. Bubbling fluidized bed boiler according to any one of claims 5-11, characterized in that between the fuel supply chute (4a, 4b) on the first furnace wall (A) and the nearest
second wall (B) a side air nozzle (12) is mounted, through which an airflow can be
blown into the furnace (1) to prevent the fuel released from the fuel supply chute
(4a, 4b) from attaching to the nearest second wall (B).
13. Bubbling fluidized bed boiler according to any one of claims 5-12,
characterized in that
- the secondary air nozzles (6) and/or the tertiary air nozzles (7) are arranged in
two opposite rows (R1, R2) on two opposite walls (B1, B2) of the furnace (1),
- in each row (R1, R2) there are nozzles (6S) blowing a small air jet (S), nozzles (6M) blowing a medium-sized air jet (M) and nozzles (6L) blowing a large air jet (L), which nozzles are arranged to alternate in such a way
that every second nozzle is a nozzle (6S) blowing a small air jet (S) and every second nozzle is a nozzle (6M, 6L) blowing a medium-sized (M) or a large air jet (L),
- the nozzles are arranged in a staggered fashion, so that opposite to each nozzle
(6S) blowing a small air jet (S) there is on the opposite wall (B1, B2) a nozzle (6M, 6L) blowing a medium-sized (M) or a large air jet (L),
- of the outermost nozzles in each row (R1, R2) at least one is a nozzle (6L) blowing a large air jet (L) and of the nozzles located in the central area of each
row (R1, R2) at least one is a nozzle (6M) blowing a medium-sized air jet (M).
14. Bubbling fluidized bed boiler according to claim 5, characterized in that it comprises outermost fuel supply chutes (4a, 4b) inclined inwards in the manner
described in claim 11, side air nozzles (12) arranged in the manner described in claim
12 and secondary and/or tertiary air supply nozzles (6, 7) arranged in the manner
described in claim 13, whereby the boiler is especially suitable for burning fuel
containing chlorine and/or sulfur.
15. Bubbling fluidized bed boiler according to any one of claims 5-14, characterized in that the tertiary air nozzles (7) are located 2 - 4 meters below the nose (9) of the furnace
(1).