CA2064210A1

CA2064210A1 - Process for the purification of flue gases, and an apparatus for it

Info

Publication number: CA2064210A1
Application number: CA002064210A
Authority: CA
Inventors: Tuomo Pokki; Timo Kenakkala; Kari Kokkonen; Tarja Enwald; Sirpa Hamala
Original assignee: Individual
Current assignee: Tampella Oy AB
Priority date: 1989-07-19
Filing date: 1990-07-18
Publication date: 1991-01-20
Also published as: DE4091281T; IT9067565A1; DD300075A5; IT9067565A0; HU9200165D0; WO1991001174A1; HUT63073A; FI83167B; PL286126A1; FI893494A0; ES2038952A6; FI83167C

Abstract

The invention relates to a process for the purification of flue gases by feeding (7) limestone into the furnace (1) of a boiler, by separating (3) the dusts from the flue gases at a point after the furnace, by hydrogenating (14) the separated dust, which contains calcium oxide, by returning (12, 13) it to the flue-gas flow (11) at a point after the separation point but before the reaction zone (4), in which additional water or vapor is fed (15) into the dust-bearing flue gas, whereafter the dust which contains sulfated calcium hydroxide is separated (5) from the flue gases (6) thus purified. According to the invention, the hot, dust-bearing flue gas coming from the furnace is divided in the separation zone (3) into a dust-poor main flow (11) and a dust-rich secondary flow (12), vapor or water mist (14) being fed into the secondary flow at a point at which the temperature of the secondary flow is 300-600 ·C. The invention also relates to an apparatus, such as a cyclone, intended for use in this process and having nozzles (19) for feeding in steam or for atomizing water, and a connecting duct (12) between the separation device (3) and the flue-gas duct (11) for the purpose of making the activated dust-rich secondaryflow of flue gas to join the dust-poor main flow at a point after the cyclone (3).

Description

:
-~: -,VO 91/01174 2 ~ ~ 3 2 ~ ~ PCT/Fl90/00184 A process for the purification of flue gases, and an apparatus for it The present invention relates to a process for the purifica-tion of flue gases. It relates in particular to a process for the purification of flue gases wherein a dust-like substance which absorbs the gaseous sulfur compounds present in flue gases, primarily sulfur dioxide, or becomes absorbent while in - ~ .
the furnace, is fed into the furnace of a boiler, the partly reacted dust-like absorbent is separated from the flue gases at a point after the furnace, is hydrogenated and returned to the flue gas flow at a point after the separation point, but bèfore the reaction zone, in which additional water or steam is fed into the flue gas, whereafter the dust which contains solid sulfur compounds is ultimately separated from the flue gases thus purified.

The invention also relates to an apparatus for use in the process referred to above, for activating at a point before the reactor the partly reacted dust-like absorbent traveling together with the hot flue gases emerging from the boiler fur-nace, and in particular to a separation device, intended to be connected to the flue-gas duct between the boiler furnace and ~
the reactor, such as a cyclone having means for hydrogenating :
the partly reacted absorbent separated from the flue-gas duct and for returning it to the flue gas duct at a point after the separation apparatus, before the reactor.

In the prior art there are known numerous processes for puri-fying flue gases with respect to gaseous sulfur compounds pre-sent in them. In the so-called LIFAC sulfur removal process developed by Tampella Oy and described in GB Patents 2 169 887, 2 174 081 and 2 174 082, a limestone powder is injected in the form of a fine dust into the boiler furnace, in which the calcium carbonate present in the limestone breaks down to : . . :: ~,. . :
.. - . ~ - ,. , :
, ,. ~ : : :

, :
..

WO91/01174 -- PCT/F190/0018~ ~-2~ 2 calcium oxide, which reacts with the sulfur dioxide present in the flue gases, thereby forming a calcium sulfate crust on the surfaces of the calcium oxide particles. This crust may hamper the penetration of sulfur dioxide into a particle, thereby im-pairing the conversion of the lime.

In this process the removal of sulfur dioxide and the conver-sion of lime are improved by moistening the lime and by in-creasing the moisture content of the flue gases by spraying either steam or water into them. Thus a calcium hydroxide forming reactlon is triggered, which improves the degree of sulfur removal and the degree of utilization of the limestone.
It is known that the formation of calcium hydroxide begins at a temperature below 570 C, and that the rate of formation is greater the higher the temperature. In practice, steam or water has been sprayed into the flue gases after the flue gases have first been used for preheating, by indirect heat exchange, the air to be ~ed into the boiler furnace, whereupon the temperature of the flue gases has usually been below lS0 C, and often even below 120 C. This system has been found to be the most advantageous in terms of the heat economy of the boiler.
~, From FI Lay-open Print 76931 there is additionally known a process for the purification of flue gases, wherein a dust in which the partly reacted dust-like calcium oxide has concen-trated is separated from the hot flue gases emerging from the boiler furnace, whereafter the dust thus separated is direct-ed, together with flue gases, into a separate reactor, in which the flue gases are moistened either with water or with a - slurry which has been produced when moistening in a separate moistening system the above-mentioned dust separated from the flue gases. In terms of apparatus this process is inconven-ient, since its implementation requlres a large number of de-vices which include both moving par.s and parts which wear, and may in practice cause stoppages.

': ' . ~ , -' :

, ' , ~ '; . .. : ~

;- ~091/01174 PCT/F190/00184 3 2~ 2 ~

The object of the present invention is therefore to provide a process for the purification of flue gases, and an apparatus for use in it, by means of which an absorbent present in the dust traveling together with the flue gases emerging from the boiler furnace is activated before it is fed into the actual reactor. By the use of the method and apparatus according to the invention, the removal of sulfur dioxide and the degree of utilization of the absorbent are improved.

According to the present invention, this is accomplished by dividing, in a separation zone, the hot, dust-bearing flue gas emerging from the furnace into a dust-poor main flow and a dust-rich secondary flow, into which steam and/or water mist is fed at a point at which the temperature of the secondary flow is still so high that the reaction among the sulfur co~-pounds, the absorbent and water is still drastic enough to break up the surface layer of the partly reacted dust par- -ticles, thereby activating them.

.
In the method according to the present invention, the flue gases are thus not entirely separated from the dust at a point before the actual reactor, as is done in the above-mentioned FI Lay-open Print 76931; instead, the hot flue gases which contain partly reacted dust-like absorbent are divided into two fractions, namely a dust-poor main flow which, in a manner known from the LIFAC process, is directed, possibly via an air preheater, into the actual reactor, and a dust-rich secondary flow into which steam and/or water mist is fed. Thereby the problematic solids-moistening systems which include moving parts (cf. FI Lay-open Print 76931) are entirely avoided, and the entire flue gas volume need not be moistened with steam or water at a point before the actual reactor, as is necessary in the above-mentioned LIFAC process.

According to the present invent'on, an oxide, hydroxide or ,. ' ~ ' ~ :'.
~: !' . !.",~', 4 ~ PCT/F190/00184 ..~
carbonate of calcium or magnesium, preferably limestone, which is known as an inexpensive raw material, can be fed into the furnace of a boiler. In the furnace the limestone breaks down into calcium oxide, which partly reacts with the gaseous sul-fur compounds, mainly sulfur dioxide, present in the flue gases, whereby a gypsum crust is formed on the calcium oxide particles, preventing the calcium oxide in the inner part of the particle from reacting with sulfur dioxide. In order to activate the calcium oxide partlcles, their surface layer is broken according to the invention by rapidly increasing the moisture content of the flue gas at a temperature at which the - chemical equilibrium is on the side of calcium hydroxide for-mation, but nevertheless so high that the calcium hydroxide and hydrous calcium sulfate forming reactions will take place.
The spraying of steam or atomization of water into the dust-rich secondary flow of flue gases makes possible an effectiye mixing and a rapid formation of calcium hydroxide and hydrous calcium sulfate, causing a sudden swelling of the particles and their breaking up into small particles which have reactive calcium hydroxide on their planes of fracture.

According to the present invention, steam or water mist is fed into the dust-rich secondary flow of flue gases, either after it has been separated from the main flow or, preferably, into the separation zone itself, at a point at which the flue gases already have a significant concentration of dust but at which there, nevertheless, still prevails effective mixing.

The dust-rich secondary flow is made to join the dust-poor main flow of flue gases, either in the reactor or at a point before it, but preferably only after the main flow has been brought into an indirect heat-exchange contact with the air to - be fed into the boiler furnace. Part of the secondary flow can, however, be introduced into the main flow even before the above-mentioned indirect heat-exchange contact.

. - - - , . . ~ ~ . ~ -, - ' . . ' '' . ' :' : ' '- ~, . .

, ~ .,. : ., . -'. . ; ,.

oll74 PCT/F190/00184 29g~2~

A portion of the dust-poor main flow separated from the flue-gas flow can be introduced into the tail end of the actual re-actor in order to raise its temperature. Likewise, a portion of the preheated air may be introduced into this section of the reactor.

In the process according to the present invention, the separa-tion device used is preferably a cyclone, the structure of which is, however, such that the dust traveling together with the flue gases emerging from the boiler furnace will not sepa-rate out from the gas phase as a separate solids phase, but the flue gases will divide into two separate gas fractions, i.e. a dust-poor main flow and a dust-rich secondary flow, which secondary flow is removed from the lower end of the cy-clone, there being in the lower section of the cyclone, in the vicinity of the secondary-flow outlet, nozzles for feeding in steam and/or for atomizing water into the cyclone, the -~
secondary-flow outlet in the lower end of the cyclone being connected by means of a connecting duct to the flue-gas duct which is between the cyclone and the actual reactor and in which the main flow travels, in order to make the activated, dust-rich secondary flow to join the dust-poor main flow at a point after the separation device.

The invention is described below in greater detail with refer-ence to the accompanying drawing, which depicts a partly cross sectional side elevation of an apparatus suitable for the im-plementation of the process according to the invention.
.
In the drawing, the furnace of the boiler is generally indi-cated by reference numeral l. A sulfur-containing fuel 8 is burned in the boiler furnace l, usually with air 9. The flue gases, which contain gaseous sulfur compounds, mainly sulfur dioxide, cool on the heat surfaces 20 in the upper section of the boiler and in the air 9 preheater lO located at a point after the boiler. At a point after the boiler and the pre-- . : .
,..... . . . :: . . :
: ,.. , . , . -: , ... . . . . : .: : . : . ., . , :

.

WO91/01174 PCT/F190/00184 :

2a6 ~ 6 heater lO the flue gases are directed into an oblong reactor 4, which is followed by a dust-separation apparatus 5 and a flue 6.

The finely-divided reagent for absorbing the gaseous sulfur compounds present in the flue gases, or its precursor, prefer-ably calcium carbonate, is fed from a storage container 2 by means of batching feeders 17 along a pipe 16 into the air flow of a pneumatic air-conveying blower 18, and it travels to-gether with this air flow along a pipe 7 to the upper section of the boiler. In addition, water is sprayed into the oblong reactor 4 via nozzles 15, which are installed at the beginning of the reactor 4. In addition, it is possible to feed into the reactor 4, at an appropriate point, preferably its tail end, warm air 9' or hot flue gases ll' derived from the boiler, in order to raise the temperature at the tail end of the reactor, at a point before the dust separator 5.

According to the present invention, the hot, dust-bearing flue gases emerging from the boiler are divided in a cyclone 3 into two fractions, namely a dust-poor main flow ll of flue gases, which is directed via an air preheater lO into the reactor ~, and a dust-rich secondary flow 12, which is made to join the main flow at a point after the air preheater lO, preferably at a point before the reactor 4. However, a portion of this dust-rich secondary flow 12 can be directed along a pipe 13 branch-ing from the pipe 12 and be introduced into the main flow even at a point before the air preheater lO.

In order to activate the adsorbent traveling together with the dust-rich secondary flow 12, 13, water as a finely-divided mist is sprayed through nozzles l9 installed in the lower sec-tion of the cyclone 3 and connected to a water pipe l4. A
strong turbulence prevails in the lower section of the cyclone 3, and thus the finely-divided drops of water will mix well with the dust-enriched flue gas before it flows out along the : .. ,. .,...... . ~

--:~O9l/01174 PCT/Fl90/00184 7 .~5~2~ ~

pipe 12 connected to the lower section of the cyclone 3. Al-ternatively, or in addition to this, water can be atomized directly into the pipe 12 at a point before the branch pipe 13. Instead of water, or in addition to water, the reagent present in the dust can be activated using steam. At the point of spraying water or steam, the temperature of the dust-rich flue gases is approx. 200-600 C, at which temperature the formation of calcium hydroxide and hydrous calcium sulfate is very drastic, causing the dust particles to swell rapidly and to break up into smaller particles with reactive calcium hy-droxide on their planes of fracture.

By using the process and apparatus according to the invention, the following advantages are gained in comparison with the state of the art: the need for limestone is reduced, the de-gree of sulfur removal is improved, the steam quantity re-quired for the moistening is low and can well be replaced by water spraying, and in addition, the apparatus required is simple and does not include parts which move and are therefore -prone to wear and disturbances.

The invention is described below with the aid of an example.
ExamPle Limestone powder is fed into the furnace of a boiler in order to remove sulfur dioxide from the flue gases by using a lime-to-sulfur ratio of two. In order to improve the removal of sulfur dioxide, the moisture content of the flue gases leaving the boiler is increased, whereby the reactivity of the lime remaining in the flue gases is improved through the formation of calcium hydroxide.
.
According to the present invention, a secondary flow is sepa-rated by means of a solids-separating apparatus from the flue gas flow leaving the boiler, 50-80 % of the dust present in the flue gases concentrating in this secondary flow. This dust :

.

,, . . , .~ , - ., . ~ .

~ : . : .

:
~: . . . : . . : .
., :

- . . :
.. . . . ~ .
. ~

WO91/01174 PCT/F190/00184-;

~ 8 consists of fly ash and of lime which has formed from the limestone powder blown into the boiler and on the surface-of which a calcium sulfate layer has formed in the boiler fur-nace. The temperature t(in) of the flue gas entering the dust separation system is usually approx. 400 C after the heat surfaces of the boiler. No substantial cooling takes place in the dust-separation apparatus, and thus the main flow of the flue gases returns at the same temperature to the flue-gas duct. Water and/or low-pressure steam is atomized into the secondary flow of the flue gases, and its temperature is low-ered. Under the effect of the moistening, the calcium sulfate layer on the particle surface, preventing the binding of sul-fur dioxide, breaks up, and the more active lime returning to the process together with the secondary flow of the flue gas improves the capacity of the entire process to bind sulfur dioxide.

Table V(tot) V(in) Ca/S H20 t(in) CaO(tot) CaO(in) XSO2 XSO2 Nm3/s Nm3/s g/Nm3 C 5/Nm3 g/Nm3 %t65-t60C
l00 20 2.0 - 400 l0 40 70 75 l00 20 2.0 l00 200 l0 40 75 80 l00 l0 2.0 l00 200 l0 80 75 80 l00 20 2.0 l50 120 l0 40 85 90 l00 l0 2.0 150 120 l0 80 85 90 l00 20 l.5 - 400 7.5 30 60 65 l00 20 l.5 l00 200 7.5 30 65 70 l00 20 l.5 150 120 7.5 30 70 75 l00 l0 l.5 150 120 7.5 30 70 75 In can be seen from the table that by moistening the dust-rich secondary flow of flue gas, the sulfur-binding capacity of the LIFAC process can be improved substantially or the consumption of lime can be decreased.

.' ':
': .
. .

. ,. . ,.: . . , : , j - ~ :

... : . : . , , .. . . - - .. ~ . .

Claims

1. A process for purifying flue gases by feeding (7) into the furnace (1) of a boiler a dust-like substance which ab-sorbs the gaseous sulfur compounds, in particular sulfur diox-ide, present in the flue gases, by separating (3) the partly reacted dust-like absorbent from the flue gases at a point after the furnace, by hydrogenating (19) the separated absorb-ent, by returning (12, 13) it to the flue-gas flow (11) at a point after the separation point, into which flue-gas flow there is fed (15) additional water and/or steam in the reac-tion zone (4), whereafter the dust which contains solid sulfur compounds is separated (5) from the thus purified flue gases (6), characterized in that the hot, dust-bearing flue gas emerging from the furnace is divided in the separation zone (3) into a dust-poor main flow (11) and a dust-rich secondary flow (12) into which steam and/or water mist (14) is fed at a point at which the temperature of the secondary flow is still so high that the reaction among the gaseous sulfur compounds, the absorbent and water will be drastic enough to break the surface layer of the partly reacted dust particles, thereby activating them.

2. A process according to Claim 1, characterized in that an oxide, hydroxide and/or carbonate of calcium and/or mag-nesium, preferably limestone, is fed into the boiler furnace (1).

3. A process according to the Claim 1 or 2, characterized in that steam and/or water mist (14) is introduced into the dust-rich secondary flow at a point at which the temperature of this flow is approx. 200-600 °C.

4. A process according to any of the above claims, charac-terized in that steam and/or water mist (14) is fed into the separation zone (3), at a dust-rich point in it.

5. A process according to any of the above claims, wherein the air (9) to be fed into the boiler furnace (1) is preheated by indirect heat exchange (10) with the hot, dust-bearing flue gases coming from the boiler, characterized in that the ac-tivated dust-rich secondary flow (12) is made to join the dust-poor main flow (11) of the flue gases only after the lat-ter has first been brought into the said indirect heat-exchange contact (10) with the air (9) to be fed into the boiler furnace.

6. A process according to Claim 5, characterized in that a portion of the activated dust-rich secondary flow is made to join the main flow (11) of flue gases before the latter is brought into an indirect heat-exchange contact (10) with the air (9) to be fed into the boiler furnace (1).

7. A process according to any of the above claims, charac-terized in that a portion (11') of the dust-poor main flow (11) separated from the hot, dust-bearing flue gases is fed directly into the tail end of the reaction zone (4) in order to raise its temperature.

8. A process according to any of the above claims, charac-terized in that a portion (9') of the preheated air (9) is directed to the tail end of the reaction zone (4) in order to raise its temperature.

9. An apparatus for activating, at a point before the actual reactor (4), a partly reacted dust-like absorbent trav-eling together with the hot flue gases coming from the furnace (1) of a boiler, the apparatus having a separation device (3), such as a cyclone, intended to be connected to the flue-gas duct (11) between the boiler furnace (1) and the reactor (4), and means (12-14, 19) for hydrogenating the partly reacted absorbent separated from the flue-gas duct and for returning it to the flue-gas duct (11) at a point after the separation device (3), characterized by nozzles (19) for feeding vapor and/or atomizing water into the separation device (3) and by a connecting duct (12) between the separation device (3) and the flue-gas duct (11) in order to make the activated secondary flow of the flue gas to join the dust-poor main flow at a point after the separation device (3).

10. A device according to Claim 9 connected to a flue-gas duct (11) having a heat exchanger (10) for preheating the air to be fed into the boiler furnace (1), characterized in that the separation device (3) connecting duct (12) for the ac-tivated dust-rich secondary flow is connected to the flue-gas duct (11) at a point after the heat exchanger (10), and that the connecting duct (12) possibly has a branch (13) which is connected to the flue-gas duct (11) at a point before the heat exchanger (10).