U.S. patent application number 10/714939 was filed with the patent office on 2005-05-19 for mercury reduction system and method in combustion flue gas using staging.
This patent application is currently assigned to General Electric Company. Invention is credited to Ho, Loc, Lissianski, Vitali Victor, Maly, Peter Martin, Payne, Roy, Seeker, William Randall, Zamansky, Vladimir M..
Application Number | 20050103243 10/714939 |
Document ID | / |
Family ID | 34435704 |
Filed Date | 2005-05-19 |
United States Patent
Application |
20050103243 |
Kind Code |
A1 |
Lissianski, Vitali Victor ;
et al. |
May 19, 2005 |
MERCURY REDUCTION SYSTEM AND METHOD IN COMBUSTION FLUE GAS USING
STAGING
Abstract
A method to reduce mercury in gas emissions from the combustion
of coal is disclosed. Mercury emissions can be reduced by staging
combustion process and/or reducing boiler excess oxygen. Fly ash
formed under combustion staging conditions is more reactive towards
mercury than fly ash formed under typical combustion conditions.
Reducing boiler excess oxygen can also improve ability of fly ash
to adsorb mercury.
Inventors: |
Lissianski, Vitali Victor;
(San Juan Capistrano, CA) ; Maly, Peter Martin;
(Lake Forest, CA) ; Seeker, William Randall; (San
Clemente, CA) ; Payne, Roy; (Mission Viejo, CA)
; Zamansky, Vladimir M.; (Oceanside, CA) ; Ho,
Loc; (Sparks, NV) |
Correspondence
Address: |
NIXON & VANDERHYE P.C./G.E.
1100 N. GLEBE RD.
SUITE 800
ARLINGTON
VA
22201
US
|
Assignee: |
General Electric Company
Schenectady
NY
|
Family ID: |
34435704 |
Appl. No.: |
10/714939 |
Filed: |
November 18, 2003 |
Current U.S.
Class: |
110/345 ;
110/342 |
Current CPC
Class: |
F23C 2201/101 20130101;
B01D 53/02 20130101; F23C 2201/30 20130101; F23J 2219/40 20130101;
B01D 2253/102 20130101; F23C 6/045 20130101; F23J 15/006 20130101;
F23J 15/022 20130101; B01D 53/10 20130101; B01D 2257/602 20130101;
B01D 53/64 20130101; F23J 2215/60 20130101 |
Class at
Publication: |
110/345 ;
110/342 |
International
Class: |
F23J 011/00; F23B
007/00 |
Claims
1-14. (canceled)
15. A method to reduce mercury in gas emissions from the combustion
of coal in a combustion unit, said method comprising: a. combusting
coal in a primary combustion zone of the combustion unit under
conditions of low or no excess oxygen during combustion in the zone
wherein the excess oxygen in the combustion zone is no greater than
two percent (2%); b. generating carbon rich fly ash during
combustion and entraining the fly ash into flue gas generated by
the combustion; c. releasing mercury during the combustion into the
flue gases; d. staging combustion air by injecting combustion air
in a post-combustion zone downstream of the combustion zone in the
combustion unit; e. injecting coal into a reburn zone in the
post-combustion zone and upstream of an overfire air burnout zone;
f. adsorbing the mercury in the flue gas with the fly ash; g.
collecting the fly ash with the adsorbed mercury in a combustion
waste treatment system.
16. A method to reduce mercury in gas emissions from the combustion
of coal in a combustion unit, said method comprising: a. combusting
coal in a primary combustion zone of the combustion unit under
conditions of low or no excess oxygen during combustion in the zone
wherein the excess oxygen in the combustion zone is no greater than
two percent (2%); b. generating carbon rich fly ash during
combustion and entraining the fly ash into flue gas generated by
the combustion; c. releasing mercury during the combustion into the
flue gases; d. staging combustion air by injecting combustion air
in a post-combustion zone downstream of the combustion zone in the
combustion unit, wherein an amount of reburning fuel in the
post-combustion zone is in a range of about 10 to about 30 percent
of a total heat input of fuel used for the combustion of coal; e.
adsorbing the mercury in the flue gas with the fly ash; f.
collecting the fly ash with the adsorbed mercury in a combustion
waste treatment system, and g. injecting activated carbon
downstream of the post-combustion zone and upstream of the
collection of fly ash.
17. A method to reduce mercury in gas emissions from the combustion
of coal in a combustion unit, said method comprising: a. combusting
coal in a primary combustion zone of the combustion unit under
conditions of low or no excess oxygen during combustion in the zone
wherein the excess oxygen in the combustion zone is no greater than
two percent (2%); b. generating carbon rich fly ash during
combustion and entraining the fly ash into flue gas generated by
the combustion; c. releasing mercury during the combustion into the
flue gases; d. staging combustion air by injecting combustion air
in a post-combustion zone downstream of the combustion zone in the
combustion unit, wherein an amount of reburning fuel in the
post-combustion zone is in a range of about 15 to about 25 percent
of a total heat input of fuel used for the combustion of coal; e.
adsorbing the mercury in the flue gas with the fly ash; f.
collecting the fly ash with the adsorbed mercury in a combustion
waste treatment system, and g. injecting activated carbon
downstream of the post-combustion zone and upstream of the
collection of fly ash.
18-25. (canceled)
26. A method to reduce mercury in gas emissions from the combustion
of coal in a combustion system, said method comprising: a.
combusting the coal in a primary combustion zone of the combustion
system, wherein elemental mercury (Hg.sup.0) is released in the
flue gas produced by the combustion; b. staging combustion air
supplied to the combustion system by adding a portion of the
combustion air to the primary combustion zone and a second portion
of the combustion air to an overfire air zone downstream of the
combustion zone to generate excessive active carbon in the fly ash;
c. maintaining a level of excess oxygen in the primary combustion
zone of no greater than 1.0 percent so as to release active carbon
in the fly ash generated by the combustion of coal; d. oxidizing
the elemental mercury by generating oxidized mercury (Hg.sup.+2);
e. adsorbing the elemental mercury in the flue gas by the active
carbon in the fly ash, and f. collecting the fly ash with adsorbed
mercury in a combustion waste treatment system.
27-28. (canceled)
29. A method to reduce mercury in gas emissions from the combustion
of coal in a combustion system, said method comprising: a.
combusting the coal in a primary combustion zone of the combustion
system, wherein elemental mercury (Hg.sup.0) is released in the
flue gas produced by the combustion; b. staging combustion air
supplied to the combustion system by adding a portion of the
combustion air to the primary combustion zone and a second portion
of the combustion air to an overfire air zone downstream of the
combustion zone; c. maintaining a level of excess oxygen in the
primary combustion zone of no greater than 1.0 percent so as to
release active carbon in the fly ash generated by the combustion of
coal and entrained in flue gases from the combustion; d. oxidizing
the elemental mercury by generating oxidized mercury (Hg.sup.+2);
e. reburning coal in the combustion system to generate additional
active carbon in the fly ash generated during combustion; f.
adsorbing the elemental mercury in the flue gas by the active
carbon in the fly ash, and g. collecting the fly ash with adsorbed
mercury in a combustion waste treatment system.
30. A method to reduce mercury in gas emissions from the combustion
of coal in a combustion system, said method comprising: a.
combusting the coal in a primary combustion zone of the combustion
system, wherein elemental mercury (Hg.sup.0) is released in the
flue gas produced by the combustion; b. staging combustion air
supplied to the combustion system by adding a portion of the
combustion air to the primary combustion zone and a second portion
of the combustion air to an overfire air zone downstream of the
combustion zone; c. maintaining a level of excess oxygen in the
primary combustion zone of no greater than 1.0 percent so as to
release active carbon in the fly ash generated by the combustion of
coal wherein a stoichiometric ratio (SR1) of the combustion of coal
in a primary combustion zone of the combustion system is in a range
of about 0.8 to about 1.05; d. oxidizing the elemental mercury by
generating oxidized mercury (Hg.sup.+2); e. adsorbing the elemental
mercury in the flue gas by the active carbon in the fly ash; f.
collecting the fly ash with adsorbed mercury in a combustion waste
treatment system; and g. adding an amount of reburning fuel in a
range of about 10 to about 30 percent of a total heat input of fuel
used for the combustion of coal.
31. A method to reduce mercury in gas emissions from the combustion
of coal in a combustion system, said method comprising: a.
combusting the coal in a primary combustion zone of the combustion
system, wherein elemental mercury (Hg.sup.0) is released in the
flue gas produced by the combustion; b. staging combustion air
supplied to the combustion system by adding a portion of the
combustion air to the primary combustion zone and a second portion
of the combustion air to an overfire air zone downstream of the
combustion zone; c. maintaining a level of excess oxygen in the
primary combustion zone of no greater than 1.0 percent so as to
release active carbon in the fly ash generated by the combustion of
coal wherein a stoichiometric ratio (SR1) of the combustion of coal
in a primary combustion zone of the combustion system is in a range
of about 0.8 to about 1.05; d. oxidizing the elemental mercury by
generating oxidized mercury (Hg.sup.+2); e. adsorbing the elemental
mercury in the flue gas by the active carbon in the fly ash; f.
collecting the fly ash with adsorbed mercury in a combustion waste
treatment system; and g. adding an amount of reburning fuel is in a
range of about 15 to about 25 percent of a total heat input of fuel
used for the combustion of coal.
32-38. (canceled).
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to the combustion of coal and in
particular to the reduction of mercury (Hg) in flue gases generated
during coal combustion.
[0002] Mercury is a constituent part of coal mineral matter.
Mercury volatizes during coal combustion as elemental mercury
(Hg.sup.0). Mercury that remains as elemental mercury through the
furnace tends to remain in the flue gas. It is desirable to lower
the amount of mercury released in flue gases during coal
combustion.
[0003] Oxidized mercury is more easily collected by emission
control devices than is elemental mercury. Oxidization of mercury
in flue gases is a known technique to capture mercury and remove it
from flue gases. As flue gases cool, mercury is partially oxidized
by chlorine which is present in coal and is released during
combustion. It is believed that most oxidized mercury (Hg.sup.+2)
in flue gas is present as mercury chloride (HgCl.sub.2). Oxidation
of mercury occurs in combustion gas-phase reactions and on the
surface of fly ash. It is believed that mercury oxidation on the
surface of fly ash is a predominant channel of mercury
oxidation.
[0004] Oxidized mercury (HgCl.sub.2 or Hg.sup.+2) is water soluble
and is easily adsorbed on high carbon fly ash or activated carbon.
The mercury captured by fly ash may be collected with the ash and
removed via a particulate collection system. Oxidized mercury is
also easily removed by wet scrubbers that are used to control
sulfur dioxide (SO.sub.2) emissions. Mercury control is generally
most effective when the mercury in flue gas is mostly oxidized.
[0005] Bituminous coals typically have high chlorine content which
improves mercury oxidation. In addition, fly ash of bituminous has
a relatively high carbon content which promotes good mercury
oxidation. In contrast, low rank coals have a relatively low
chlorine content and have a high reactivity that results in low
carbon content in fly ash. Accordingly, elemental mercury from the
combustion of low rank coals oxidizes to a lesser extent than does
mercury from the combustion of bituminous or other high chlorine
coals.
SUMMARY OF THE INVENTION
[0006] The invention may be embodied as a method to reduce mercury
in gas emissions from the combustion of coal in a combustion unit
including the steps of: combusting coal in a primary combustion
zone of the combustion unit under conditions of low or no excess
oxygen during combustion in the zone; generating carbon rich fly
ash during combustion; releasing mercury during the combustion into
flue gases generated by the combustion; staging combustion air by
injecting combustion air in a post-combustion zone downstream of
the combustion zone in the combustion unit; adsorbing the mercury
in the flue gas with the fly ash, and collecting the fly ash with
the adsorbed mercury in a combustion waste treatment system.
[0007] The invention may also be embodied as a method to reduce
mercury in gas emissions from the combustion of coal in a
combustion system, said method comprising: combusting the coal in a
primary combustion zone of the combustion system, wherein elemental
mercury (Hg.sup.0) is released in the flue gas produced by the
combustion; staging combustion air supplied to the combustion
system by adding a portion of the combustion air to the primary
combustion zone and a second portion of the combustion air to an
overfire air zone downstream of the combustion zone; maintaining a
level of excess oxygen in the primary combustion zone of no greater
than 2 percent so as to release active carbon in the fly ash
generated by the combustion of coal; oxidizing the elemental
mercury by generating oxidized mercury (Hg.sup.+2); adsorbing at
least part of the oxidized mercury in the flue gas by the active
carbon in the fly ash, and collecting the fly ash with adsorbed
mercury in a combustion waste treatment system.
[0008] The invention may be further embodied as a system to treat
mercury in flue gas emissions from a coal fired furnace comprising:
a primary combustion zone receiving combustion air and having a
downstream passage for flue gases and fly ash generated during
combustion; a coal injector adapted to inject coal into the primary
combustion zone; an air injector adapted to introduce combustion
oxygen into the combustion zone, wherein an amount of excess oxygen
in the zone is no greater than 2 percent so as to release active
carbon in the fly ash generated by the combustion of coal; an
overfire air burnout zone downstream of the combustion zone and
included in the downstream passage, wherein combustion air is
injected into the burnout zone; a combustion treatment waste system
coupled to the flue gas output and a discharge for captured
particulate waste, and wherein said primary combustion zone burns
the coal such that the fly ash has active carbon to oxidize and
adsorb the mercury released in the flue gas.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a schematic diagram of a coal fired power plant
having a primary combustion zone with a low NOx burner (LNB), a
coal reburn zone, and an overfire air (OFA) zone.
[0010] FIG. 2 is a schematic diagram of coal fired power plant
similar to the plant shown in FIG. 1 and also an activated carbon
injector and a wet scrubber.
[0011] FIG. 3 illustrates a boiler simulation facility.
[0012] FIG. 4 is a chart of the effects of mercury, NO.sub.x and CO
emissions with respect to the stoichiometric air to fuel ratio
(SR1) in the primary combustion zone combusting a bituminous
coal.
[0013] FIG. 5 is a chart of the effects of deep air staging on
mercury removal where the staging was adjusted by varying SR.sub.2
(OFA) for constant SR.sub.1 values of 0.5 and 0.7.
[0014] FIG. 6 is a chart of mercury removal with respect to loss of
ignition (LOI) at levels of excess oxygen of 3% and below 0.5% for
a constant SR.sub.1 value of 0.5 and a PRB coal.
[0015] FIG. 7 shows the effect of excess oxygen (O.sub.2) on Hg
removal.
[0016] FIG. 8 is a chart of the effects of staging on fly ash
surface area and the oxidation of mercury for SR.sub.1 values of
1.132 and 1.0.
DETAILED DESCRIPTION OF THE INVENTION
[0017] Reactive fly ash formed in situ in the coal combustion
process can be effective at reducing mercury (Hg) emissions from
coal-fired power plants. Reactive fly ash can improve mercury
oxidation through catalytic reactions on the surface of the fly
ash. Oxidized mercury is easily adsorbed on carbon containing
particles such as in-situ formed high carbon fly ash or activated
carbon, or can be removed in wet scrubbers.
[0018] Although mercury oxidation on fly ash takes place in the
post-combustion zone of a boiler, the reactivity of fly ash is
affected by combustion conditions in the combustion zone of the
boiler. In particular, it has been found that the reactivity of fly
ash is influenced by the amount of excess air in the combustion
zone.
[0019] A method has been developed for effective mercury control in
a stationary coal-firing combustion systems by applying deeply
staged combustion to generate highly reactive fly ash and by
reducing boiler excess oxygen in the combustion zone of a boiler.
The combustion air is staged such that some air is introduced
downstream of the combustion zone and air introduced into the
combustion zone is at or near stoichiometric conditions.
[0020] Staging and/or operating the combustion zone under reduced
excess oxygen conditions, e.g., where the combustion is at or near
stoichiometric conditions, increases the reactivity of fly ash in
the flue gases. Fly ash formed under combustion staging conditions
is more reactive with mercury than is fly ash formed under typical
combustion conditions. Similarly, fly ash formed under reduced
boiler excess air conditions adsorbs oxidized mercury more
effectively that does fly ash generated from combustion with
greater amounts of excess air.
[0021] Increasing the staging during combustion and/or reducing
excess combustion air increases the portion of released mercury
that is captured in the flue gas by making the fly ash more
reactive. Under typical excess air combustion conditions, fly ash
with low carbon content (less than 1%) is not an effective mercury
sorbent in full-scale boilers. However, by increasing the staging
of air and by reducing the amount of excess air in the combustion
zone low carbon content carbon fly ash can be effective in
improving mercury oxidation.
[0022] Fly ash formed under combustion staging conditions has a
higher surface area than does fly ash formed under normal
combustion air conditions. Fly ash with high surface area is more
effective at mercury oxidation than is the relatively low surface
area fly ash generated from combustion using conventional amounts
of excess combustion air. Accordingly, increasing the degree of
combustion staging can increase the capacity of fly ash to capture
mercury.
[0023] Staged combustion diverts a portion of the combustion air
from a primary combustion zone to a secondary zone, e.g., an
overfire air (OFA) zone. Combustion modifications, such as,
NO.sub.x control technology, can be made to existing boilers to
utilize combustion staging to reduce NO.sub.x and mercury
emissions. Deeper staging is traditionally applied to achieve more
significant NO.sub.x control. Conventional optimum deep staging for
effective NO.sub.x reduction may require different operating
conditions than does the deep staging discussed here for effective
mercury oxidation on fly ash.
[0024] During staged combustion, some carbon in coal does not burn
out as completely as it would in a boiler environment with a high
level of excess air in the combustion zone. Staged combustion tends
to increase the level of unburned carbon in the fly ash, especially
when bituminous coals are being burned. Increasing the unburned
carbon in fly ash by under deeply staged combustion conditions
improves mercury removal.
[0025] There is a relatively small increase in fly ash when staging
low rank coals. The lack of a large increase in fly ash suggests
that any increase in mercury adsorption on fly ash for the low rank
coals may not be significant. Increasing the depth of combustion
staging, e.g., the amount of oxygen introduced far downstream in
the flue gases (such as at an OFA injector), improves mercury
oxidation on the surface of fly ash of low rank coals even when
staging does not increase significantly the amount of carbon in fly
ash.
[0026] Fly ash formed during combustion of coal is more reactive
towards mercury when boiler excess oxygen is reduced. The level of
boiler excess air affects boiler NO.sub.x and CO emissions and
typically is set to minimize these emissions and unburned carbon.
This is achieved by adjusting air flow to the combustion zone and
to the overfire air (OFA) zone.
[0027] It has been discovered that reducing the boiler excess air
results in an increase in the efficiency of mercury removal by fly
ash. Minimization of excess air (O.sub.2) can be achieved by
adjusting the air flow to the primary combustion, reburn and OFA
zones. Excess air in the primary combustion zone may also be
reduced by blocking leaks in the boiler back pass section. Reducing
these air leaks should improve efficiency of mercury oxidation and
adsorption on fly ash without affecting boiler NO.sub.x and CO
emissions.
[0028] Reactive fly ash may have a composition of 1 to 30 percent
(%) carbon by weight, and preferably 3 to 20% by weight. The
combustion conditions in the boiler are modified to promote a
higher amount of active carbon in the fly ash than would otherwise
occur. By increasing fly ash reactivity, the efficiency of mercury
removal by fly ash is improved. The combustion conditions are
optimized by applying a deeply staged combustion process that
generates reactive fly ash and by reducing excess air in the
boiler. Reducing excess oxygen in a combustion process is
complementary with low nitrogen oxide(s) (NOx) combustion control.
Accordingly, mercury emissions may be reduced by operating at very
low levels of excess oxygen and deeply staging, while also
minimizing NO.sub.x emissions.
[0029] FIG. 1 shows a coal-fired power plant 10 comprising a coal
combustion furnace 12, e.g., a boiler, having a coal fuel injection
system 14, primary air injectors 16, reburn coal injectors 18 and
overfire air (OFA) injectors 20. An exterior air duct 22 may
distribute air provided by an air source 24, e.g., ambient air, to
the primary combustion air injectors 16 and overfire air injectors
20. The coal injection system 14 and combustion air injectors 16
may be included in a low NOx burner (LNB) system.
[0030] The furnace 12 includes a combustion zone 26 and a post
combustion zone 28, which includes a convective pass 30. The power
plant 10 further includes a particulate control device (PCD) 32,
ash burnout unit 34 and a mercury collection unit 36 comprising a
bed of activated carbon or other reagent. Most of the coal is
burned in a primary combustion zone 26 of the boiler 12. The
remaining coal is injected downstream through reburn injectors 18
to provide a fuel-rich reburning zone 40. Overfire air is injected
into a OFA burnout zone 42 to complete combustion.
[0031] Most mercury content of the coal is transferred to a flue
gas phase during combustion in the primary combustion zone 26. In
the reburning zone 40, carbon in the reburning coal does not burn
out as completely as in a primary combustion zone that has excess
air. Therefore, coal reburning increases the level of unburned
carbon in the flue gas. By selecting coal type and specific
conditions for injection of coal fuel and air, the combustion
process can be controlled to produce a flue gas with increased
carbon-containing active fly ash. The flue gas is cooled in the
convective pass 30 where mercury is absorbed by the fly ash carbon.
The fly ash with mercury is then collected in the PCD 32. Fly ash
collected in the PCD 32 is treated in the ash treatment unit 36.
Ash treatment unit can be a burnout unit or the like. If a burnout
unit is used, then excess heat can be partially recovered, for
example by the plant by preheating water used for boiler heat
exchange. Mercury released from the fly ash carbon is absorbed by
activated carbon as the ash burnout products pass through mercury
collection unit 36.
[0032] The concentrations of nitrogen oxides, mercury, and carbon
in fly ash are reduced by a three-step process. In the first step,
the concentration of NOx is decreased in the fuel-rich zone of coal
reburning (in other embodiments this step can be accomplished by
LNB or by another fuel/air staging low NOx Combustion Modification
technology). The combustion zone of the particular technology is
controlled to form enhanced carbon in fly ash. The enhanced carbon
in fly ash can be formed by optimizing the fuel staging and air
staging conditions and combustion conditions, for example, by
changing the amount of the reburning fuel, temperature of flue gas
at the location of reburning fuel and/or OFA injection. Also, more
active carbon in fly ash can be formed by selecting a coal type or
coal particle size. Further, enhanced carbon can be controlled by
adjusting LNB flow, by selecting a specific LNB design, by
regulating excess air in the main combustion zone 26, adjusting the
stoichiometric ratio of fuel and adjusting fuel/air mixing in
primary 26 and secondary 28 combustion zones.
[0033] In the second step, the carbon-containing fly ash is cooled
to below 450.degree. F., and desirably to below 400.degree. F. and
preferably below 350.degree. F. At these fly ash temperatures, NOx
is further reduced in a reaction with carbon, and mercury is
absorbed by the enhanced carbon in the fly ash. The PCD 32 can
collect the ash with carbon and absorbed mercury.
[0034] In the third step, the carbon is burned out from the fly
ash. At the same time, mercury is desorbed from fly ash and
collected in an activated carbon bed or a bed of other reagents,
for example, gold or other metals, that form amalgams. Carbon
burnout reactors are designed for effective removal of carbon. The
burnout reactor can be used in combination with a mercury capture
reactor.
[0035] Since the stream of gas through the carbon burnout reactor
is much smaller than the stream of flue gas, the amount of
activated carbon needed to collect mercury can be about two orders
of magnitude lower than the amount of injected activated carbon to
accomplish the same result.
[0036] It is beneficial to use in-situ formed carbon, i.e., formed
in the boiler, in fly ash for mercury removal. Enhanced carbon in
fly ash can be produced in a matter of seconds at combustion
temperatures in the combustion zone 26. Additionally, the cost of
controlling conditions to optimize production of enhanced carbon in
fly ash from a coal-fired boiler typically, on a mass basis, is
much less than the cost of injected activated carbon. Since the
carbon is produced "in situ," no extra costs are incurred in
respect of handling of the activated carbon and delivering it to
the boiler.
[0037] Flue gases, fly ash, unburned coal particles and other
particulate material (collectively referred to as "combustion
products") flow upwards through the furnace 12. In general, flue
gas consists of fly ash and various gasses and volatile compounds,
such as nitrogen, oxygen, carbon dioxide, nitrogen oxides, water,
carbon monoxide, sulfur dioxide, and various acid gasses. The
precise composition of the flue gas is determined by the nature of
the process generating the flue gas and can vary significantly in
time. Some combustion products fall to the bottom of the furnace
and are discharged as waste.
[0038] Coal combustion in conventional coal-fired furnaces is
usually not totally complete, and generates fly ash with some
carbon content. Moreover, active carbon fly ash is generated during
conventional low Nitrogen oxide (NO.sub.x) processes such as Low
NO.sub.x Burner (LNB), in overfire air (OFA) injection, coal
reburning and in connection with other conventional low NO.sub.x
combustion technologies.
[0039] In the furnaces disclosed herein, the primary combustion
zone 26 is configured such that carbon rich fly ash is formed by
maintaining fuel-rich conditions. For example, the amount of excess
air in the primary combustion zone 26 is less than two percent, and
preferably 0.3 percent or less. The overfire air zone 42 is
configured to operate fuel-lean. However, since temperature in the
OFA zone is lower than that in the main combustion zone, the carbon
in the fly ash in the flue gases does not completely burnout.
Accordingly, the reactivity of the fly ash flowing downstream of
the furnace and to the conductive pass 30 is greater than would
otherwise be expected in an efficient coal-fired furnace.
[0040] The combustion is staged by applying coal reburn. Reburn is
a two-stage fuel injection technology in which part of the coal
fuel (usually 15-25% of the total furnace heat input) is diverted
above the existing primary combustion zone 26 to a reburn zone 40
to produce a slightly fuel-rich environment in the reburn zone. A
portion of the combustion air is diverted downstream of the reburn
zone to the overfire air injectors 20 and the OFA burnout zone 42.
Combustion occurs in the primary combustion zone 26, in the reburn
zone 40 and is completed in the OFA zone 42. The efficiency of
mercury removal by fly ash is increased by reducing excess air in
the primary combustion zone 26. This is achieved by reducing the
total amount of the combustion air, diverting more fuel to the
reburn zone, and diverting more air into OFA zone, than would occur
in a conventional furnace having a reburn zone and an OFA zone.
[0041] FIG. 2 depicts a coal power plant 40 similar to the plant 10
shown in FIG. 1. The same reference numbers have been used to label
the components of the coal power plant 44 shown in FIG. 2 that are
the same as the components of the plant 10 shown in FIG. 1. The
power plant 44 includes an activated carbon injection system 46 and
a web scrubber 48.
[0042] The benefits and effectiveness of reducing excess oxygen to
generate high carbon fly ash are evident from the following
description of coal combustion tests. Tests were performed in a 1.0
MMBTU/hr Boiler Simulator Facility (BSF) 60. The BSF facility is
shown schematically in FIG. 4. The BSF provides sub-scale
simulation of the flue gas temperatures and compositions found in a
full-scale boiler of a utility power plant.
[0043] As shown in FIG. 3, the BSF 60 includes a burner 62, a
vertically down-fired radiant furnace 64, a cooling section 66, a
horizontal convective pass 68 extending from the furnace, an
electrostatic precipitator (ESP) 69, and a stack 70 with flue gas
sampling instruments in communication with the convective pass. The
burner 62 is a variable swirl diffusion burner with an axial fuel
injector. Primary air is injected axially into the combustion zone
of the furnace. Secondary air is injected radially through swirl
vanes (not shown) to provide controlled fuel/air mixing in the
combustion zone. The swirl number can be controlled by adjusting
the angle of the swirl vanes. Numerous ports located along the axis
of the furnace allow access for supplementary equipment such as
reburn injectors, additive injectors, overfire air injectors, and
sampling probes.
[0044] The radiant furnace 64 has eight modular refractory lined
sections with an inside diameter of 22 inches and a total height of
20 feet. The convective pass 68 is also refractory lined, and
contains air cooled tube bundles to simulate the superheater and
reheater sections of a utility boiler. Heat extraction in radiant
furnace and convective pass can be controlled such that the
residence time-temperature profile matches that of a typical
full-scale boiler. A suction pyrometer (not shown) measures furnace
gas temperatures.
[0045] The ESP 69 for the BSF is a single-field unit consisting of
12 tubes with axial corona electrodes. Mercury concentration was
measured at the ESP outlet using an online Hg analyzer. The
analyzer is capable of measuring both elemental (Hg.sup.0) and
total mercury in flue gas. Oxidized (Hg.sup.+2) mercury can de
determined as a differences between total mercury and Hg.sup.0. The
average temperature across the ESP was 350.degree. F.
[0046] Seven types of coals were tested: Western coal (#l--the
reference numeral appears in FIGS. 6 to 8); and Eastern Bituminous
coal (#2), three types of U.S. Powered River Basin Coal (PRB)
(#3-#5), and two lignite coals (#6 and 7). The BSF was fired on
coal, and the carbon in ash content (also characterized as LOI) was
controlled by staging combustion air. The amount of unburned carbon
(UBC) in the fly ash is indicated by the loss-on-ignition (LOI)
value of the ash.
[0047] By air staging, part of the combustion air is diverted away
from the primary combustion zone and injected downstream in the OFA
zone. Combustion of fuel in the primary combustion zone 26 occurs
in fuel-rich or near stoichiometric conditions as characterized by
the air to fuel stoichiometric ratio (SR.sub.1). Conditions after
injection of overfire (OFA) are fuel-lean and characterized by the
air to fuel stoichiometric ratio (SR.sub.2) at the OFA zone.
[0048] By reducing the amount of oxygen, the primary combustion
zone 26 operates under fuel rich conditions. At these fuel-rich
conditions, the stoichiometric air-fuel ratio (SR.sub.1) in the
primary combustion zone is below, at or slightly above 1.0. At an
SR.sub.1 of 1.0, ideally all of the coal is burned in the primary
combustion zone. In real-world conditions, some active carbon does
not burn when the SR.sub.1 is equal 1.0. Conventional furnaces
generally operated where the SR.sub.1 is above 1.1 in order to
complete combustion.
[0049] FIG. 4 presents a chart of the effect of excess oxygen on Hg
and NOx reduction and carbon monoxide (CO) emissions for Bituminous
coal #2. Pilot scale BSF tests demonstrated that reducing excess
oxygen had a significant effect on NOx, LOI and mercury removal.
Mercury removal was defined as a difference between total
introduced mercury with fuel and the amount of mercury measured in
the gas phase at the ESP 68 outlet. As the value of SR1 decreased
from 1.16 to 1.0, excess oxygen decreased from 2.7% to almost 0%.
The amount of mercury and NOx emissions (left hand side of FIG. 4
chart) was reduced at low SR.sub.1 values, e.g., below 1.05.
However, at low SR.sub.1 values, e.g. below 1.04, the amount of CO
emissions (right hand side of chart) and LOI increased. The LOI of
the fly ash also increased as the SR.sub.1 value decreased.
[0050] FIG. 4 indicates that reducing excess oxygen (O.sub.2) such
that the SR.sub.1 in the primary combustion zone is below 1.05
increases mercury removal by fly ash adsorption and PCD
removal.
[0051] With deep staging, combustion air, e.g., 20% to 35%, was
diverted to the OFA zone 26. To avoid excessive oxygen in the OFA
zone, the stoichiometric air-fuel ratio (SR.sub.2) for the OFA zone
was controlled by metering the amount of overfire air. In one
example, SR.sub.2 was maintained at a value of 1.16. Maintaining
the level of excess oxygen in the OFA zone to 0.4% and preferably
to less than 0.2% was found to increase the amount of mercury
removal.
[0052] FIG. 5 shows the effect of reducing excess oxygen in the OFA
zone on mercury removal for lignite coal #6 at SR.sub.1 values of
0.5 and 0.7. FIG. 6 shows the effect of excess oxygen in the OFA
zone on mercury removal for PRB coal combustion at an SR.sub.1 of
0.5. Mercury removal increased as excess oxygen in the OFA zone
decreased, and Hg removal approached 70% to 80% at values of excess
oxygen in the range of 0.2% to 0.4%. The increase in mercury
removal was substantial at very low excess oxygen levels, e.g.,
less than 1%, in the OFA zone. A 5% improvement in mercury removal
efficiency was achieved as the excess oxygen levels in the OFA zone
were reduced from 3% to 2%.
[0053] FIG. 7 is a chart that shows the effect of excess oxygen in
the OFA zone on mercury removal for a coal blend of 70% PRB coal
and 30% Western Bituminous coal. The mercury removal efficiency
increased from 60% at excess oxygen levels of 3% to above 80% for
low levels of excess oxygen, e.g., below 0.5%. FIG. 7 demonstrate
that up to 90% mercury removal can be achieved for coal blends at
excess oxygen of about 0.3%. FIG. 7 also demonstrates that about
10% improvement in mercury removal can be achieved by reducing
excess O.sub.2 from 3% to about 2%.
[0054] FIG. 8 shows surface area of fly ash and amount of oxidized
mercury in flue gas at conventions combustion conditions and air
staging. Conventional conditions (bars on the left hand side of
FIG. 8) corresponded to SR.sub.1=1.132 and did not include staging.
Air staging conditions (bars on the right hand side of FIG. 8)
corresponded to SR.sub.1=1.0 and SR.sub.2=1.132. As shown in FIG.
8, the BSR pilot scale tests demonstrated that deep staging
increased efficiency of mercury oxidation even when the amount of
carbon in the fly ash did not increase due to deep staging. Testing
PRB coal #5 demonstrated that because of high reactivity of this
coal air staging had a small effect on carbon in fly ash content.
Carbon in fly ash content at all staging conditions remained in the
range of 0.3% to 0.4%.
[0055] Air staging, however, increased surface area of fly ash.
Under deep staging conditions more elemental mercury was oxidized
than under conventional combustion. FIG. 8 demonstrates that
applying deep staging increased the percent of oxidized mercury in
flue gas from about 30% to about 60% and increased fly ash surface
area from 1.48 m.sup.2/g (meters square to gram of fly ash) to 1.74
m.sup.2/g. In summary, the BSR pilot-scale results show that deeper
staging and very low excess O.sub.2 improve efficiency of mercury
removal by fly ash.
[0056] While the invention has been described in connection with
what is presently considered to be the most practical and preferred
embodiment, it is to be understood that the invention is not to be
limited to the disclosed embodiment, but on the contrary, is
intended to cover various modifications and equivalent arrangements
included within the spirit and scope of the appended claims.
* * * * *