U.S. patent application number 15/967663 was filed with the patent office on 2018-11-15 for controlling slagging and/or fouling in furnaces burning biomass.
This patent application is currently assigned to Fuel Tech, Inc.. The applicant listed for this patent is Fuel Tech, Inc.. Invention is credited to Ian Saratovsky, Christopher R. Smyrniotis.
Application Number | 20180327683 15/967663 |
Document ID | / |
Family ID | 63915517 |
Filed Date | 2018-11-15 |
United States Patent
Application |
20180327683 |
Kind Code |
A1 |
Smyrniotis; Christopher R. ;
et al. |
November 15, 2018 |
Controlling Slagging and/or Fouling in Furnaces Burning Biomass
Abstract
The description relates to controlling slagging and/or fouling
in biomass burning furnaces. Combustion of such a biomass the fuel
with air produces combustion gases containing sodium and/or
potassium compositions, and the combustion gases are treated by
contacting the combustion gases with kaolin and aluminum hydroxide.
At least one of the kaolin and aluminum hydroxide can be introduced
with the fuel, in a combustion chamber, with reburn fuel or with
overfire air. For fuels also high in zinc and/or heavy metals,
magnesium hydroxide is introduced into the combustion chamber or
following heat exchangers.
Inventors: |
Smyrniotis; Christopher R.;
(St. Charles, IL) ; Saratovsky; Ian; (Highland
Park, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Fuel Tech, Inc. |
Warrenville |
IL |
US |
|
|
Assignee: |
Fuel Tech, Inc.
Warrenville
IL
|
Family ID: |
63915517 |
Appl. No.: |
15/967663 |
Filed: |
May 1, 2018 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
62492485 |
May 1, 2017 |
|
|
|
15967663 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C10L 9/10 20130101; C10L
2200/0469 20130101; B01D 2251/604 20130101; B01D 2251/402 20130101;
C10L 5/48 20130101; B01D 2253/104 20130101; C10L 2200/0209
20130101; F23G 2200/00 20130101; C10L 10/06 20130101; F23G 7/10
20130101; B01D 2257/60 20130101; B01D 2258/0283 20130101; C10L
2200/0218 20130101; C10L 5/44 20130101; C10L 5/442 20130101; B01D
2253/106 20130101; F23G 2209/26 20130101; B01D 53/64 20130101; C10L
5/445 20130101; C10L 10/04 20130101; C10L 2200/0272 20130101; F23J
9/00 20130101; C10L 10/14 20130101; C10L 2200/0213 20130101; F23K
2201/501 20130101; C10L 2200/0484 20130101; Y02E 50/30 20130101;
C10L 5/46 20130101; F23J 7/00 20130101 |
International
Class: |
C10L 10/06 20060101
C10L010/06; F23J 9/00 20060101 F23J009/00; F23J 7/00 20060101
F23J007/00; C10L 10/14 20060101 C10L010/14 |
Claims
1. A process for controlling slagging and/or fouling in biomass
burning combustors, comprising: a. combusting the fuel comprising
biomass with air to produce combustion gases containing sodium
and/or potassium compositions; and b. contacting the combustion
gases with kaolin and aluminum hydroxide.
2. A process according to claim 1, wherein the introduction is into
the combustor at a temperature within the range of from 1500 to
300.degree. C.
3. A process according to claim 1, wherein at least one of the
kaolin and aluminum hydroxide are introduced with the fuel.
4. A process according to claim 1, wherein at least one of the
kaolin and aluminum hydroxide are introduced through ports in a
combustion chamber where the fuel is combusted.
5. A process according to any of claims 1-3, wherein at least one
of the kaolin and aluminum hydroxide are introduced into the
combustion chamber as an aqueous slurry.
6. A process according to claim 1, wherein biomass fuel is
introduced as a reburn fuel and at least one of the kaolin and
aluminum hydroxide are introduced with the fuel.
7. A process according to claim 1, wherein at least one of the
kaolin and aluminum hydroxide are introduced with overfire air.
8. A process according to claim 1, wherein the fuel is high in zinc
and/or heavy metals and magnesium hydroxide is introduced into the
combustion chamber or following heat exchangers.
Description
FIELD OF THE INVENTION
[0001] The invention relates to controlling slagging and/or fouling
in biomass burning furnaces.
BACKGROUND OF THE INVENTION
[0002] Combustion of biomass is increasing because it is considered
to provide significant greenhouse gas reduction benefits to the
environment. Many advocate its use as being carbon neutral because
biomass consumes the same amount of CO.sub.2 from the atmosphere
during growth as is released during combustion. Advantageously, it
can be blended with high-sulfur fuels, such as some coals, to
achieve lower carbon and sulfur emissions than from the coal alone.
In addition, biomass co-firing can also result in lower NO.sub.x
because the flame temperature is typically lower and fuel nitrogen
in biomass is converted to NH radicals by combustion, and this can
reduce NO.sub.x by nonselective reduction.
[0003] In the exemplary situation of factories or other industrial
plants that produce large amounts of biomass waste and use it for
fuel in combustors to supply heat and/or electricity, there has
been noticed a tendency toward slagging or fouling, but mainly
fouling brought on by a mechanism known as sinter fouling. We note
that fouling is the nonmolten accumulation of combustion ash on
heat exchangers while slagging is the molten deposition on furnace
walls and high temperature heat exchangers cooling surfaces.
Fouling in these combustors typically occurs in bed fired biomass
furnaces but can also occur in the back passes of utility boilers
burning biomass fuels or biomass blends.
[0004] While biomass fuels have many advantages, they are usually
rich in potassium and/or sodium compositions, which can drastically
change the character of the ash. The ash is formed of the
noncombustible portion of the fuel, and its chemistry is important
to the formation and control of fouling and slagging. Potassium and
sodium containing ash presents problems that have not been
adequately controlled by existing technology. The capture and phase
state change of the potassium and sodium vapors is a problem for
proper operation of a biomass fueled furnace, either partially or
solely, because such reactions effectively cause a mechanism known
as sintering fouling.
[0005] In the sintering fouling process, biomass and biomass
mixtures containing other fuels, release potassium and sodium
vapors which travel with the combustion gases until the vapors
contact cooler objects in the furnace, such as heat transfer
equipment like, generating banks, reheats, economizers, feedwater
heaters, etc. In bubbling and circulating fluidized bed boilers and
furnaces, bed material may also be at the temperature zone required
to cause potassium and sodium vapor condensation to occur,
resulting in bed material agglomeration. This can lead to further
fouling.
[0006] When this happens, especially at temperatures of 870.degree.
C. to about 315.degree. C., the vapors are cooled and they condense
as a sticky liquid on heat exchanger tubes and/or bed material,
forming a film of glue-like substance. This film is a sticky liquid
material is comprised of condensed alkali metal (Na+K) compositions
and attracts particles of ash which have not melted and are not
otherwise sticky. These ash particles, many of which are high melt
point materials like calcium sulfate, stick to the heat exchangers
tubes and/or bed material, lining up edge to edge and corner to
corner, forming a thicker film, made from a dry ash layer that is
not easily removed by sootblowing or other ash removal
equipment.
[0007] Over time, the ash particles, which have much higher melt
points than the flowing combustion gas slowly fuse together, glued
by the condensed alkali metal compositions into a solidified mass
of high hardness and strength. This is the mechanism used to
manufacture lawn mower bases and automobile door handles, etc. In
the diagram in FIG. 1, potassium and/or sodium vapors 2 condense to
form a sticky liquid that acts as a glue to fuse ash particles 4
into a solid hard mass 6 with high adhesive properties.
[0008] There is a current need for a process for controlling
slagging and/or fouling in biomass burning combustors.
SUMMARY OF THE INVENTION
[0009] The present invention provides a process for controlling
slagging and/or fouling in biomass burning combustors. In one
preferred aspect, the process comprises: combusting a fuel
comprising biomass with air to produce combustion gases containing
sodium and/or potassium compositions; and contacting the combustion
gases with kaolin and aluminum hydroxide at an effective
temperature for controlling slagging and/or fouling, preferably
within the range of from 1100 to 300.degree. C.
[0010] In one aspect, at least one of the kaolin and aluminum
hydroxide are introduced with the fuel. In another, at least one of
the kaolin and aluminum hydroxide is introduced through ports in a
combustion chamber where the fuel is combusted. In another, at
least one of the kaolin and aluminum hydroxide is introduced into
the combustion chamber as an aqueous slurry.
[0011] In some embodiments a biomass fuel is introduced as a reburn
fuel and at least one of the kaolin and aluminum hydroxide is
introduced with the fuel.
[0012] In one alternative, at least one of the kaolin and aluminum
hydroxide is introduced with overfire air.
[0013] Other preferred aspects, including preferred conditions and
equipment and their advantages, are set out in the description
which follows.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The invention will be better understood and its advantages
will become more apparent when the following detailed description
is read in conjunction with the accompanying drawings, in
which:
[0015] FIG. 1 is a diagram showing the sinter fouling mechanism on
a heat transfer surface.
[0016] FIG. 2 is a schematic diagram showing a combustor with a
feed of biomass fuel as one exemplary simplified embodiment of the
invention.
[0017] FIG. 3 is a graph showing a ternary phase diagram in a
K.sub.2O--SiO.sub.2--Al.sub.2O.sub.3 system.
[0018] FIG. 4 is a graph showing the effectiveness of an exemplary
treatment regimen according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0019] FIG. 1 is a diagram presented to illustrate that over time,
ash particles, which have much higher melt points than the flowing
combustion gas slowly fuse together and are glued by condensed
alkali metal compositions into a solidified mass of high hardness
and strength. The material produced is very strong and difficult to
remove from heat transfer surfaces 8. In FIG. 1, potassium and/or
sodium vapors 2 condense to form a sticky liquid that acts as a
glue to fuse ash particles 4 into a solid hard mass 6 with high
adhesive properties.
[0020] Reference will first be made to FIG. 2, which is a schematic
diagram showing a combustor with a feed of biomass fuel as one
exemplary simplified embodiment of the invention showing a
combustor 10 being fed a fuel comprising biomass 12 via a conveyor
14 above an air supply 16 for combustion in chamber 18.
[0021] Biomass is a broad term that covers vegetative waste or
dedicated growth as well as organic matter such as refuse from
domestic and industrial wastes. The key criteria are that it have a
significant cellulose content and a suitable BTU value. In general,
any organic fuel can be considered a biomass fuel. For the context
of this description, the term "biomass" is used to describe waste
products and dedicated energy crops. Waste products include wood
waste material (e.g., saw dust, wood chips, used wood from
reclamation, etc.), crop residues (e.g., corn husks, wheat chaff,
nut shells, olive oil and wine pressings, etc.), and municipal,
animal and industrial wastes (e.g., sewage sludge, manure, etc.).
Dedicated energy crops, including short-rotation woody crops like
hard wood trees and herbaceous crops like switchgrass, are
agricultural crops that are solely grown for use as biomass fuels.
These crops have very fast growth rates and can therefore be used
as a regular supply of fuel.
[0022] The biomass can be employed alone or as a blend, e.g., a
fuel comprising biomass and coal. It will be understood that the
principles of the invention can be applied to other carbonaceous
fuels and fuel mixtures (any other fuel of choice, typically a
carbonaceous thermal fuel or refuse). Biomass is interesting,
especially as a blending component, because it is considered
environmentally friendly and can help keep NO.sub.x and SO.sub.x on
the positive side, but can add to fouling and slagging due to its
significant alkali metal contents.
[0023] In one embodiment, the fouling and/or slagging problems
resulting from the combustion of a fuel comprising biomass are
greatly moderated by combusting the fuel comprising biomass 12 with
air to produce combustion gases containing sodium and/or potassium
compositions and contacting the combustion gases with kaolin and
aluminum hydroxide. The noted kaolin and aluminum hydroxide can be
added with the fuel, into the combustion chamber 18 above the
combustion of the fuel, such as carried on a conveyor 14, into
overfire air (not shown) or onto a portion of the fuel used as a
reburn fuel (not shown).
[0024] FIG. 2 illustrates an embodiment wherein at least one of the
kaolin and aluminum hydroxide are introduced with the fuel,
preferably as a sprayable slurry, from suit applicator, e.g., spray
20 from supply tank 22. In this embodiment, the kaolin and aluminum
hydroxide can be supplied from a common source, as shown, or from
separate sources.
[0025] FIG. 3 is a graph showing a ternary phase diagram in a
K.sub.2O--SiO.sub.2--Al.sub.2O.sub.3 system. Addition of
Al.sub.2O.sub.3 to decrease the aluminum-to-silica ratio results in
an increase in melting points from less than 1000.degree. C. to
greater than 1300.degree. C. The solid black arrow denotes the
direction of melting temperature increase as Al.sub.2O.sub.3
content in the deposit increases. The kaolin and/or aluminum
hydroxide can be introduced with the fuel or into the combustion
chamber 18 or following heat exchangers 24 as is suitable for
access and temperature for reaction. A preferred temperature range
for reaction with the kaolin is from 1100.degree. to 300.degree.
C., and for the aluminum hydroxide is from 1500.degree. to
300.degree. C.
[0026] Kaolin acts to break up the dominant mechanism where
entrained potassium and sodium vapors condense to a sticky liquid
on colder tube surfaces 8 causing solid ash particles to stick to
the thin liquid film, particles fuse to form very hard strong
deposit, known as sintering with no melting taking place. The name
kaolin derives from the village in China where it has been used for
millennia as a potting clay. Kaolinite is a clay mineral and is
part of the group of industrial minerals having a chemical
composition represented as Al.sub.2Si.sub.2O.sub.5(OH).sub.4. It is
a layered silicate mineral, with one tetrahedral sheet of silica
(SiO.sub.4) linked through oxygen atoms to one octahedral sheet of
alumina (AlO.sub.6) octahedra. Rocks rich in kaolinite are known as
kaolin or "china clay".
[0027] The kaolin reacts with alkali metals (e.g., potassium and
sodium) to form high melting temperature aluminosilicates and
thereby lowers their availability to act as glue for the ash in the
sintering process. The formed particles have locally higher melting
temperatures due to the presence of leucite and kalsilite and blend
in with the growing deposit. As such, vaporous and molten alkali
metal compounds are consumed and lower their tendency to glue ash
particles to heat transfer surfaces 8.
[0028] In addition to gaseous and molten alkali metal compounds
binding together fly ash particles, the ash from many biomass fuels
and fuel blends have silica to alumina ratios (the weight %
SiO.sub.2 in the ash divided by weight % Al.sub.2O.sub.3 in the
Ash, i.e., .dbd.SiO.sub.2/Al.sub.2O.sub.3) in excess of 2, which
results in the formation of compounds with melting temperatures
below 1,000.degree. C. and results in increased deposition
potential.
[0029] The melting temperatures of high silica content deposits is
increased according to the invention by the addition of an
aluminum-rich compound, aluminum hydroxide (aluminum trihydrate,
ATH). According to references dealing with the nomenclature of ATH,
the naming for the different forms of aluminum hydroxide is
ambiguous and there is no universal standard. However, as used
herein, it includes all four polymorphs, which have a chemical
composition of aluminum trihydroxide (an aluminum atom attached to
three hydroxide groups). The injection of ATH adjusts the silica to
alumina ratio in the resulting ash in the furnace. This further
works with the effect of the kaolin to discourage deposition caused
by high silica to alumina imbalances and helps maintain cleaner
heat transfer surfaces 8. The combination of kaolin and ATH is
synergistic and permits cleaner heat transfer surfaces, higher
furnace efficiencies and longer run times than either chemical
applied alone. It is unexpected that the combination of kaolin
addition to minimize the impact of alkali metals on melting
temperature (via chemical reaction) by effective reducing the fly
ash bonding agent and simultaneous addition of aluminum-rich
compounds to increase the melting temperature of the bulk deposit,
results in significantly cleaner boiler surfaces.
[0030] In another embodiment, at least one of the kaolin and
aluminum hydroxide is introduced through ports (not shown) in a
combustion chamber where the fuel is combusted.
[0031] In another, at least one of the kaolin and aluminum
hydroxide is introduced into the combustion chamber as an aqueous
slurry.
[0032] In some embodiments, a biomass fuel is introduced as a
reburn fuel and at least one of the kaolin and aluminum hydroxide
is introduced with the fuel.
[0033] In one alternative, at least one of the kaolin and aluminum
hydroxide is introduced with overfire air.
[0034] The kaolin and aluminum hydroxide are introduced at a
temperature suitable into the combustion chamber 18 or following
heat exchangers 24 at a dosage of from 1 to 10 pounds of reagent
per ton of fuel.
[0035] It is an advantage of the invention that when combusting
fuels high in zinc and/or heavy metals, in addition to alkali metal
compositions, the resulting slagging problems can be addressed by
introducing magnesium hydroxide into the combustion chamber 18 or
following heat exchangers 24. The dosage will be from 0.5 to 7.5
pounds of magnesium hydroxide reagent per ton of fuel.
[0036] The following examples are presented to further explain and
illustrate the invention and are not to be taken as limiting in any
regard. Unless otherwise indicated, all parts and percentages are
by weight.
Example 1
[0037] This example describes the use of the invention to control
problems for a manufacturer that uses biomass-based materials to
construct specialty wood-based materials and burns waste. Results
are described for a prior art process and that of the
invention.
[0038] Waste including residual material (sawdust) along with other
biomass waste and woody biomass materials are fed to a furnace to
generate hot gases that are used in the manufacturing process. The
furnace is a bubbling bed type that utilizes a floating suspended
bed in the bottom consisting of fine rock and coarse sand about a
quarter inch in diameter. Various fuel streams are conveyed or
blown into the furnace both into the bed and above the bed. Firing
the furnace to produce gas for process causes fouling in the small
heat exchangers, which can readily be cleaned. The problem is the
bed material gets coated with condensed potassium and sodium vapors
due to fluctuating temperatures as part of the process. The bed
agglomerates and causes clinkers several feet long by several feet
wide to form and stick to the furnace. When these growing clinkers
break off of the walls due to their increasing weights, the large
clinkers fall into the suspended bed and collapse the bed so the
furnace cannot continue to fire in this collapsed state.
[0039] The unit has to be shut down, cooled to a safe temperature,
opened and a team of people in protective suits have to go in with
jack hammers and break up the clinkers to get them out of the
furnace. This results in significant downtime that was expensive as
it shut down the entire manufacturing operation and idles the
entire plant during cleaning.
[0040] A chemical program was in use during this time but appeared
to have limited impact on keeping the boiler clean and reducing
downtime. Due to a lack of success with a prior art program, the
present invention was tested. Based on the potassium content and
the silicon-to-aluminum ratio, kaolin and ATH (aluminum trihydrate)
were identified as deposit mitigants in order to simultaneously
capture potassium to minimize inter-fly ash particle bonding and
ATH to decrease the silicon-to-aluminum ratio in the bulk fly ash
deposit. As such kaolin and ATH were injected into the furnace as
dry powders, using dry kaolin at from 0.5 lbs/ton of fuel to 10
lbs/ton of fuel with a preferred rate of 3-5 lbs/ton of fuel.
[0041] In contrast to the prior art program, the invention provided
a positive impact on furnace cleanliness. This was observed
immediately as the unit ran longer between outages and cleaned
faster, saving valuable downtime to keep the plant running longer
with shorter downtimes. The previously observed large clinkers were
replaced by significantly smaller and more brittle agglomerates and
ash that could easily be raked out of the bed in a fraction of the
time with less manpower.
[0042] The graph in FIG. 4, below shows the difference in the
percentage of maintenance downtime for cleaning in the prior art
chemical program and the process of the invention.
[0043] Maintenance downtime hours for cleaning were reduced by 40%
from the prior art versus use of the invention. Results continued
for the invention and most recently show a 70% drop in maintenance
downtime hours for cleaning as the program has been tuned for
better performance.
[0044] The above description is for the purpose of teaching the
person of ordinary skill in the art how to practice the invention.
It is not intended to detail all of those obvious modifications and
variations, which will become apparent to the skilled worker upon
reading the description. It is intended, however, that all such
obvious modifications and variations be included within the scope
of the invention which is defined by the following claims. The
claims are meant to cover the claimed components and steps in any
sequence that is effective to meet the objectives there intended,
unless the context specifically indicates the contrary.
* * * * *