Note: Descriptions are shown in the official language in which they were submitted.
1 3 1 1 343
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This invention relates to method and apparatus
for scrubbing pollutants from an exhaust gas stream
whereby the materials reacted with the stream are rendered
at least benign and in many cases converted by the reac-
tion into useful products.sackground of the Invention
Scrubbing of exhaust gas pollutants is generally
costly, having significant environmental returns but lit-
tle or no economic returns unless it is possible to con-
vert by the scrubbing otherwise waste products into usefulproducts. The cost of the initial equipment is high.
Furthermore, the materials for scrubbing, such as the
oxides, carbonates, or hydroxldes of alkali and/or alka-
line earth metals are a continuing expense. In addition,
disposal of the reaction products derived from reaction of
the scrubbing materials with the exhaust gas adds to the
continuing cost, especially if the products contain toxic
components.
There are a growing number of boiler installa-
tions which are fueled by combustion of biologic materials(hereinafter collectively i'biomass~ including wood,
peat, or crop residue, where there is little or no produc-
tion of sulfur oxides and therefore no scrubbing of the
gaseous components of the exhaust is needed or done. On
the other hand, the ash produced from these operations
contains significant alkali and alkaline earth metal
salts, occurring commonly as the oxide, or, if wetted
and/or reacted with carbon dioxide, as the hydroxide or
the carbonate, or perhaps as hydrated salts of these.
I have discovered that the ash from boiler
installations, where alkali and alkaline earth metal
oxides, hydroxides, and/or carbonates are a significant
proportion of the ash, can be used in a process for scrub-
bing exhaust gas in place of the usual materials above
referred to, thus making use of an otherwise waste product
as a substitute for expensive materials which must be
purchased.
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Other materials are also usable for this purpose.
Industrial or municipal waste, incinerator ash or by-
products which contaln potassium or sodium or other sol-
uble salts which, when dissolved in water yield a basic
(high pH) solution and when recovered from the waste or
by-product, provide an economic benefit are also suitable.
In the following specification and claims, ash
derived from burning biomass material and industrial or
municipal wastes or other by-products, usable in the
process herein described and claimed, are collectively
identified by the generic term, "ash."
Unfortunately, the insoluble portion of reacted
ash must usually still be disposed of as waste, for
example, in most boiler applications, it will not be
lS usable for any other purpose. The waste material,
however, will no longer be a caustic material and should,
in most cases, be disposable as benlgn common fill or
could, in some landfills, be used as cover material. In
some cases, where the composltion of the residue and
transport costs permit, it may also be used as cement kiln
raw feed. In some other situations the insoluble portion
of the scrubbing material could be used, as it is now, for
production of calcium sulfate or gypsum, or as a mineral
filler.
In cases where the facility producing the ash
does not itself also have a boiler installation producing
an exhaust of high sulfur content in need of scrubbing,
the ash could be transported to other boiler installations
which have such a problem, or to installations where use
of a higher sulfur content fuel would provide an economic
benefit. In addition, because ash derived from biomass
material generally contains potassium and other alkali and
alkaline earth metal salts recoverable by utilization of
the exhaust gas heat or other waste heat sources, the
resulting alkali and alkaline earth metal salts can be a
valuable by-product of the process.
.
131 1343
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Brief Summary of the Invention
In accordance with the invention there is
provided a novel method of treatment of an exhaust gas
stream containing as pollutants, at least one of the
acidic oxides of sulphur, nitr~gen, carbon, and halogens
and acidic halogen compounds, whose temperature exceeds
the dew point of the gas, for producing scrubbed exhaust
and useful or benign by-products. The method comprises
providing a basic aqueous slurry of ash, the ash
containing alkali and alkaline earth metal salts. The
exhaust gas stream is contacted with the slurry in a
manner to scrub the exhaust gas stream and to cause the
pollutants therein to react with the water in the slurry-
to produce acids. The acids are allowed to react with
any oxides, hydroxides and carbonates of alkali and
alkaline metal salts in the slurry, thereby to produce a
solution with soluble alkali metal salts and a
precipitate of any insoluble alkali and alkaline earth
metal salts comprising at least one of halogen compounds,
carbonate, sulfate, sulfite, nitrate, and nitrite of
calcium, magnesium, potassium and sodium in the slurry.
The precipitate is recovered from the solution and the
solution, freed of the precipitate, is evaporated. Any
soluble alkali and alkaline earth metal salts present in
the solution are recovered in solid form and the scrubbed
exhaust gas stream is expelled.
Presently preferred methods include one or more
of the following steps: contacting the exhaust gas
stream with the slurry by passing the same through said
slurry; separating the said solution of alkali and
alkaline earth metal salts from the said precipitate and
undissolved components; transferring the solution of
alkali and alkaline earth metal salts and the precipitate
and undissolved components to a separation system wherein
the æaid salt solution is separated from the said
precipitate and undissolved components; passing the
separated salt solution through a heat exchanger wherein
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131 ~343
it extracts heat from the exhaust gas stream before the
latter contacts the slurry; cooling the exhaust gas
stream to dehumidify it prior to the contacting step;
using the salt solution to cool and dehumidify the
exhaust gas stream; using heat from the exhaust gas
stream to remove water from the said salt solution and
deriving the said heat in part from one or more of (A)
the hot exhaust gas of said stream, (B) the latent heat
of vaporization of any moisture contained in said exhaust
gas stream, (C) the hydration reaction between ash and
water, and (D) compressing the said gas prior to the said
contacting step.
In another aspect of the invention there is
provided a method for treating ash and an exhaust gas
stream containing acid forming pollutants whose
temperature exceeds the dew point of the gas, to
chemically modify the ash and to scrub the exhaust.
Firstly, the gas stream is cooled in a manner to remove
and recover heat. Thereafter, an alkali-containing basic
scrubbing slurry comprising the ash and water is
provided. The cooled gas stream is contacted with the
slurry comprising the ash and water to cause pollutants
in the gas stream to react with water in the slurry to
form acids and allowing the acids to react with the
slurry to form a mixture of settleable solids and a
liquid solution containing dissolved solids. From the
resulting mixture, the settleable solids and liquid
solution containing dissolved solids are separated. The
liquid solution containing dissolved solids is heated
using only recovered heat collected during the cooling of
the gas stream. The dissolved solids are recovered in
solid form.
Still further objects, features and advantages
of the invention will become apparent from the following
detailed description of a presently preferred embodiment
thereof taken in conjunction with the accompanying
drawings.
C
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Brief Description of the Drawing
In the drawing, the sole figure is a schematic
representation of one apparatus for practising the
invention.
Detailed Description of Preferred Apparatus and Method
A slurry composed of ash and water exiting at 8
from an ash and water mixing tank (not shown), is pumped
via pipe 10 into the treatment tank 12, along with
additional water through inlet 14 from a suitable source
(not shown) to produce a dilute slurry 16. The ash is
trucked in from a source such as a generating plant fired
by biomass or an incinerator fired by waste material.
Exhaust gas from a cement kiln, incinerator or boiler
(not shown) containing one or more of the oxides of
sulfur, nitrogen, carbon, and/or compounds of halogens
and their oxides, enters heat exchanger 22 through inlet
18 from which it emerges as cooled exhaust. Condensed
exhaust gas moisture is collected in the heat exchanger
22 and conveyed to the treatment tank 12 through pipe 44.
The exhaust then travels to compressor 20 through pipe 23
and /-
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131 134''
is delivered via pipe 24 to distribution pipes 26 in the
bottom of the treatment tank 12. To prevent settling of
t:he sollds to the bottom of treatment tank 12, the slurry
may be stirred or recirculated by suitable means, for
example by recirculation pump 27.
The exhaust gas bubbles through slurry 16 of ash
and water to emerge from the tank top as scrubbed exhaust
28. Slurry 16, as a mixture of treated solid, water, and
dissolved materials, is pumped by pump 32 via pipe 30 to
the settling tank 34 where the settled solids 36 are
pumped out by pump 38 and the water 37 laden with dis-
solved salts is pumped to heat exchanger 22 to provide
cooling for the input exhaust gas. The water from salt
solution 37 is evaporated to a vapor and released via pipe
40 to the atmosphere, or the water is evaporated and then
condensed to a liquid to recapture the latent heat for
reuse. The æalts from the salt solution 37 are concen-
trated and/or preclpitated and collected from the heat
exchanger via plpe 42. The cationic components of the
collected salts are principally calcium, potassium, magne-
sium, and sodlum. The anionlc components of the salts are
prlnclpally sulfate, carbonate and nitrate. The actual
composition of the salts will depend on the initial compo-
sition of the ash to be treated and on the composltion of
the exhaust gas.
Ash derived from biomass burning systems may con-
tain ~mburned carbon which, in some situations, will float
in water. The process illustrated can be modified, if
desired, as shown in the Figure to allow removal of the
carbon. Water 37 having unburned carbon is pumped from
the surface of the settling tank to be filtered or other-
wise treated to remove the carbon and is then returned to
the process. If necessary, the solution containing
dissolved alkali and alkaline earth metal salts may be
removed by plping (not shown) to be filtered or otherwise
cleansed of particulate matter at a particulate removal
13113~3
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component. The solution is then conveyed to the heat
exchanger 22.
The heat exchanger 22 is a dual purpose heat
exchanger-crystallization unit of a known type which will
extract heat from the exhaust gas and use that heat,
lncluding latent heat derived from condensation of the
exhaust gas moisture to evaporate water.
The Apparatus
The whole system is created from well known parts
combined by standard methods. For example, typically the
treatment tank may have a volume of one million gallons
(3,800,000 liters) and be provided with gas distribution
and stirring means; the settling tank may have a volume of
100 thousand gallons (380,000 liters), both being con-
structed from stainless steel, or other suitablematerials, such as rubber, which can tolerate highly alka-
llne or acidic solutions.
Workin~ Principle
The basic working prlnciple in this invention ls
recomblnation and reactlon of two wastes produced during
burning to provide mutual neutralization of the wastes.
One waste stream comprlses the gasses and gaseous oxides
which produce acidic solutions in water and the other par-
ticulate matter, namely ash from a biomass or lndustrial
or municipal waste burning facility, which produces basic
solutions in water.
After partial dissolution in water, the two
wastes react to neutralize each other. In the case of the
ash, the process provides for reaction of or removal of
the caustic components, thus rendering what remains as
neutral solids suitable for disposal as non-hazardous
waste. At the same time, the exhaust gas passing through
the slurry in the treatment tank ls cleansed of a slgnlfl-
cant portion of the compounds of the halogens and oxldes
of sulfur, nitrogen and halogens by forming salts of these
components.
131 1343
cant portion of the compounds of the halogens and oxides
of sulfur, nitrogen and halogens by forming salts of these
components.
Example
S Exhaust gas from, e.g. a boiler, may be fed
through duct 18, to heat exchanger 22 at a rate of 200,000
cubic feet (6000 m3) per minute by compressor 20. The
exhaust gas is variable in composition, but may contain
roughly 10% water, 15% carbon dioxide, 65% nitrogen, 10%
oxygen and 500 to 1000 ppm nitrogen oxides and 100 to 1000
ppm sulfur dioxide. In heat exchanger 22 the exhaust gas
is cooled and water is condensed, resulting in a decrease
in flow volume. The exhaust gas is then drawn by compres-
sor 20 through pipe 23 for delivery through plpe 24 to
dlstrlbutlon pipes 26 and allowed to react with slurry 16
where the alogens and the oxides of sulfur, nitrogen, car-
bon and halogens are reacted.
Ash may be lntroduced to treatment tank 12, for
example, at a rate of elght to twelve tons (7200 to 10,800
kg) per hour dry welght. Water ls added to produce a
dllute slurry of up to 95% water content. The slurry
water content ls determlned by the initial concentration
of alkali and alkallne earth metal salts or other metal
salts in the ash and on the desired degree of removal of
~5 these salts from the residue.
After reaction wlth the exhaust gas, the slurry
of treated ash ls pumped at a rate of approxlmately 200
gallons (760 llters) per mlnute to settllng tank 34. In
thls tank the sollds settle to form a slurry of approxl-
mately 35% water and 65% sollds, beneath a solutlon ofwater and soluble salts dlssolved during treatment. The
water solution is pumped through outlet 37 to heat
exchanger 22 at approximately 200 gallons (760 llters) per
minute to provide cooling for the exhaust gas and to evap-
orate the water therefrom to produce the by-product
salts. Any floatlng carbon can be removed as prevlously
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explained. The by-product salts, removed via pipe 42, are
produced at a rate of approximately five to twenty tons
(4500 to 18,000 kg) per day. The by-product salts include
potassium sulfate, calclum carbonate, and other salts with
cationlc components incuding potassium, calcium, magne-
sium, and sodium and anionic components including car-
bonate, sulfate, and nitrate. A portion of the nitrate
oxidizes the sulfite to sulfate.
While there has herein been disclosed and
described a presently preferred method and apparatus for
practicing the invention, it will nevertheless be under-
stood that the same is by way of illustration and not by
way of limitation, and it is intended that the scope of
the invention be limited only by the proper interpretation
to be afforded the appended claims.
