COMBUSTION OF BLACK LIQUOR AND PROCESSING OF LIME SLUDGE IN A RECOVERY BOILER

COMBUSTION DES LIQUEURS NOIRES ET TRAITEMENT DES BOUES DE CHAUX DANS UNE CHAUDIERE DE RECUPERATION

English Abstract

A method of modifying the course of combustion when evaporated black liquor is
burnt in the furnace cavity of a recovery boiler
is described. The method implies that lime sludge is supplied to the furnace
cavity whereby the course of combustion is modified so that
the temperature in the furnace cavity is raised. Also use of a recovery boiler
for simultaneous combustion of black liquor and processing
of lime sludge is described. In one embodiment of this use is disclosed that
the processing of lime sludge is performed so that increased
formation of calcium sulphide in the smelt of inorganic chemicals, which is
formed in the recovery boiler, is obtained and/or so that the
lime sludge is calcined.

French Abstract

L'invention concerne un procédé de modification du processus de combustion lorsque la liqueur noire évaporée est brûlée dans la cavité du four d'une chaudière de récupération. Le procédé consiste à envoyer les boues de caustification dans la cavité du four, le processus de combustion étant modifié de sorte que la température dans cette dernière augmente. L'invention porte également sur l'utilisation d'une chaudière de récupération pour la combustion simultanée de la liqueur noire et le traitement des boues de caustification. Dans un mode de réalisation, le traitement des boues résiduaires s'effectue de manière à augmenter la formation de sulfure de calcium dans le salin constitué de produits chimiques inorganiques formé dans la chaudière de récupération, et/ou à obtenir la calcination des boues résiduaires.

Claims

7
CLAIMS:
1. A method of modifying the course of combustion of
evaporated black liquor obtained from a sulphate pulp
cooking process in a recovery boiler having a furnace cavity
with an upper oxidation zone, a lower reduction zone and a
gas flue, which comprises supplying evaporated black liquor
and lime sludge to the furnace cavity of the recovery boiler
to regulate the temperature in the furnace cavity of the
recovery boiler so that the lime sludge is at least
partially calcined to calcium oxide and the lime sludge
gives increased formation of calcium sulphide in a smelt of
inorganic chemicals which is formed in the recovery boiler
and wherein the black liquor and the lime sludge is premixed
in an approximate dry solid content ratio of 15:1.
2. The method of claim 1, wherein the temperature of
the smelt is raised by approximately 100°C.
3. The method of claim 1 or 2, wherein the content of
particles in the flue gas before contacting with an
electrostatic precipitator are reduced by approximately 40%.
4. The method of claim 1, 2 or 3, wherein the
particles in the flue gas have a pH of about 10.4.
5. The method of any one of claims 1 to 4, wherein
approximately 65% of the supplied calcium carbonate lime
sludge is reacted to calcium oxide.
6. The method of any one of claims 1 to 5, wherein
flue gas particles reduce the clogging of flue gas passages
in the boiler, facilitate the cleaning of heated surfaces
and reduce the consumption of soot steam.

Description

CA 02164328 2004-06-18
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COMBUSTION OF BLACK LIQUOR AND PROCESSING OF
LIME SLUDGE IN A RECOVERY BOILER
The present invention relates to a method of modifying the course of
combustion
when evaporated black liquor is burnt. More precisely, the method involves
supplying of lime sludge to the furnace cavity in such an amount that the
course of
combustion is modified. Further, the invention relates to the use of a
recovering
boiler.for simultaneous combustion of.black liquor and processing of lime
sludge.
The processing of lime sludge implies a calcination of the lime sludge andlor
an
increase in the formation of calcium sulphide in the smelt of inorganic
chemicals
which is formed in the recovering boiler.
Background
in the production of woodpulp in accordance with the sulphate process, the
wood is
treated in the form of wood chips in an alkaline solution, which mainly
consists of
NaOH and NaHS, in a cooking process at a temperature which in the final stage
lies
in the range of 150-175°C.
Upon separation of the cellulose fibres which have been detached in the
cooking
process, a solution of a recycled liquor is obtained, which is also called
black liquor,
and which in addition to the inorganic chemicals charged in the cooking
process also
contains from the wood released organic substanses, which mainly consist of
lignin.
Thereafter, the black liquor is evaporated to a dry solids content which makes
it
possible to bum the organic contents without the need of supplying
supplementary
auxiliary fuel to the combustion plant.

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After evaporation of the black liquor, it is transported to a liquor
combustion plant,
which normally is a so called recovery boiler, but can also be a liquor
gasification
plant.
In a recovery boiler the burning of the black liquor is mainly effected in the
furnace
cavity which comprises an upper oxidation zone and a lower reduction zone, the
temperature being higher in the oxidation zone than in the reduction zone.
In the combustion of the organic part of the black liquor the chemical energy
is taken
care of by the transformation to heat energy and electrical energy, and the
inorganic
chemicals are recovered. Normally, the sulphur content is converted to an
active form
for the cooking process, i. e. to sulphide. The portion of sodium which is not
bound to
the sulphur exists mainly in the form of carbonate.
The inorganic chemicals leave the combustion process mainly in melted form
(e.g. at
750-850°C) and are further processed for recovery of the chemicals.
In such a further processing there is i.a. formed sparingly soluble CaC03,
which is
called lime sludge. Said lime sludge is washed with water prior to returning
in a
rotary kiln, whereby a weakly alkaline solution (weak liquor) is obtained. The
returning of the lime sludge, i, e. the conversion of CaC03 into CaO, is
energy-
consuming and puts high demands on the quality of the fuel, so that a high
flashing
point can be maintained in the calcination zone of the rotary kiln. Normally
approximately 40-45 % of the effective heat of the fuel is consumed by the
calcination reaction, 15-20 % of the consumed fuel being lost by heat
radiation from
the mantle surface of the kiln and the lime cooler.
In experiments aiming at an improvement of the utilization of a recovery
boiler it has
now surprisingly been found that the recovery boiler may be used for
simultaneous
combustion of the black liquor and the processing of lime sludge. Most
surprising i s
however that the course of combustion is modified in the furnace cavity when
blacf;

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3
liquor is burnt in the presence of lime sludge, so that the temperature in
said furnace
cavity is raised even though the conversion of CaC03 to Ca0 is endothermic.
One aspect of the invention is directed to the use of a recovery boiler for
simultaneous burning of black liquor and processing of lime sludge.
One embodiment of this aspect of the invention implies that the processing of
the
lime sludge is calcination of lime sludge and/or the processing of lime sludge
is
conducted so that increased formation of calcium sulphide in the smelt of
inorganic
chemicals, which is formed in the recovery boiler, is obtained.
The utilization of such calcium sulphide rich smelt of inorganic chemicals for
the
production of white liquor with enhanced and/or reduced sulphidity has been
disclosed in the simultaneously filed patent application having the title
"Preparation
of white liquor".
Another aspect of the invention is directed to a method of modifying the
course of
combustion when evaporated black liquor is burnt in the furnace cavity of a
recovery
boiler by supplying lime sludge to the furnace cavity, whereby the course of
combustion is modfied so that the temperature in the furnace cavity is raised,
and
at the same time the supplied lime sludge is processed.
In one embodiment of the invention the type of processing of the supplied lime
sludge is controlled by regulation of the temperature in the furnace cavity.
The
temperature may then be regulated so that the lime sludge will be at least
partially
calcined. Moreover, the temperature may be regulated so that the lime sludge
will
give increased formation of calcium sulphide in the smelt of inorganic
chemicals
which has been formed in the recovery boiler.

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In a specific method aspect, the invention
provides a method of modifying the course of combustion of
evaporated black liquor obtained from a sulphate pulp
cooking process in a recovery boiler having a furnace cavity
with an upper oxidation zone, a lower reduction zone and a
gas flue, which comprises supplying evaporated black liquor
and lime sludge to the furnace cavity of the recovery boiler
to regulate the temperature in the furnace cavity of the
recovery boiler so that the lime sludge is at least
partially calcined to calcium oxide and the lime sludge
gives increased formation of calcium sulphide in a smelt of
inorganic chemicals which is formed in the recovery boiler
and wherein the black liquor and the lime sludge is premixed
in an approximate dry solid content ratio of 15:1.

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In a preferred embodiment of the invention the black liquor and the lime
sludge are
mixed before they are supplied to the furnace cavity.
The amount of lime sludge which is to be supplied in order to modify the
course of
combustion depends on several process parameters which may vary between
different recovery boilers. However, a man of ordinary skill in the art may
easily adapt
the amount of lime sludge that is required to modify the course of combustion
in the
furnace cavity of a recovery boiler when the present invention is used, with
the
guidance of the present description of the invention.
The temperature in the furnace cavity may be controlled so that lime sludge
wilt be
at least partially calcined and/or so that lime sludge gives increased
formation of
calcium sulphide in the smelt of inorganic chemicals which is formed in the
combustion plant. The determining factor is whether the temperature in the
furnace
cavity reaches calcination temperature for lime sludge, or whether the
temperature is
kept therebelow. In the formula below, which describe the reaction with lime
sludge
(CaC03) in the combustion of black liquor, the calcination temperature has
been set
at 800°C, but it can vary considerably e.g. by ~ 70°G.
1. CaC03 + Na2S > CaS + Na2C03 above 600°C
2. CaS + Na2C03 > Ca0 + NaZS + C02 above 800°C
3. Ca0 + NaZS > CaS + NazO below 800°C
The following relationship was established at a mill trial:
A
CaC03 > CaS ------- Ca0
B
0% (100-A+B)% (A-B)%
In the mill trial described below, A was =64.7 and B was =44.9, i.e. CaS=80.2
% and
Ca0=19.8 %. In a laboratory experiment at 750°C, 15 minutes, 10 g smelt
+ 1.9 g
lime sludge, A=0 and B=0, was obtained with 75 % of Ca as CaS.

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At a mill trial the black liquor was premixed with lime sludge in an
approximate dry
solids content ratio of 15:1, (which corresponds to 20-30 % of the amount of
lime
5 sludge that is produced in the process) and was supplied to the furnace
cavity of the
recovery boiler at normal capacity. The extensive follow-up of the analysis of
the trial
gave in summary the following results:
- The temperature of the smelt raised by approximately 100°C, which
indicates a
considerable raise of the temperature in the furnace cavity.
- The content of particles in the flue gases before the electrostatic
precipitator
was reduced by approximately 40%.
- Particles' pH increased from 9.8 to 10.4 despite a decreased content of
Na2C03
in the particles.
- The temperature of outgoing superheated steam raised by approximately
15°C
during the trial, and returned thereafter to a tower level.
- Approximately 65 % of supplied CaC03 had reacted to Ca0 according to the
formula 1 'above.
The reason for the modification of the course of combustion, which is
expressed by
the above observations, is not clarified, but the carbon dioxide formed at the
simultaneous combustion of black liquor and processing of lime sludge
(according to
formula 2 above) is probably of importance for the modification.
Raised temperatures in the furnace cavity of a recovery boiler gives the
following
advantages for liquor combustion:
The heat recovery in the lower part of the furnace cavity is markedly
improved.
The reduction of sulphur is improved.
The emission of particles with the flue gases from the level of the furnace
cavity
is considerably reduced (in the above example by approximately 40 %).

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- The nature of the particles is changed to the better, which reduces the
clogging
of the flue gas passages in the boiler and facilitates the cleaning of the
heat
surfaces and reduces the consumption of soot steam.
- Reduced amounts of deposits on the superheater surfaces give higher
outgoing steam temperature and thus increased production of electricity.
- Increased calcination in the recovery boiler.
- An increase in the capacity of the liquor combustion.
Other advantages which can be obtained by conducting the processing of lime
sludge in the recovery boiler is that the calcination can be done with a
cheaper fuel
and with lower energy consumption due to the fact that the high radiation
losses that
arise in a rotary kiln process are avoided. Moreover, the temperature in the
furnace
cavity may be regulated so that the processing of lime sludge is controlled so
that
increased amounts of calcium sulphide are formed in the smelt of inorganic
chemicals, if this is considered to be desirable.