WO2003004136A1

WO2003004136A1 - Flue gas purification process

Info

Publication number: WO2003004136A1
Application number: PCT/FI2002/000602
Authority: WO
Inventors: Ilari Ekman; Pauli Dernjatin; Marika RYYPPÖ; Marko Fabritius
Original assignee: Fortum Oyj
Current assignee: Fortum Oyj
Priority date: 2001-07-05
Filing date: 2002-07-03
Publication date: 2003-01-16
Anticipated expiration: 2004-01-05
Also published as: FI20011469A0; FI111608B; FI20011469L

Abstract

The invention concerns a flue gas purification process. The fuel (M) is burnt in the fire chamber (11) of a boiler (10). From the boiler (10) the flue gases are conducted through a duct (12) into a humidification chamber (13) and further out through a chimney (16). All the Ca, K and Na used in the process derive from the fuel. The fuel (M) is burnt in the fire chamber (11) of the boiler (10) within a temperature range of 800°C - 1350°C. After the boiler the flue gases are conducted along a duct (12) into the humidification chamber (13), into which water is sprayed from a nozzle pipe (14) or such.

Description

Flue gas purification process

This invention concerns a method for removing gaseous sulphur compounds and especially sulphur dioxide from the flue gases of a boiler burning sulphur- containing fuels. In this application the method is called a Bio-LIFAC process, and it is especially suitable for purification of the flue gases from power plant processes.

It is previously known in the art to reduce the sulphur dioxide content of flue gases by feeding calcium oxide, calcium carbonate or some other alkaline compound into the fire chamber of the boiler. In a fluidised-bed furnace provided with a circulating fluidised bed, lime may be added to reduce the sulphur dioxide content of flue gases by as much as 90 %, when the boiler is working in the temperature range, which is the best possible for chemical reactions, which is between 800 and 1000°C. The sulphur dioxide thus absorbed will leave the boiler together with the fly ash in the form of gypsum.

In other boilers where temperatures higher than the above-mentioned ones must be used and where the residence time of additive is short due to the character of the combustion event, it can be expected that the reduction in sulphur dioxide content of flue gases will remain essentially less, at approximately 50 % or less.

It is known, on the other hand, that the sulphur dioxide content of flue gases can be reduced by various absoφtion methods outside the boiler. One such method known as such is the so-called spray method, wherein the flue gases from the boiler are conducted into a separate reactor, into which aqueous slurry of calcium hydroxide is sprayed in the form of droplets through special nozzles. The reactor is typically a rather big vessel, wherein the velocity of flue gases is allowed to slow down and aqueous slurry is sprayed downwards from the upper part of the vessel. It has also been known to feed into the fire chamber of the boiler powdery calcium or magnesium hydroxide excessively in relation to the sulphur dioxide gas formed in the fire chamber, besides the feeding of the sulphur-containing material to be burnt and the oxygen-containing gas. Water or steam is sprayed separately in a separate phase from the fire chamber into the calcium or magnesium oxide- containing flue gases thus obtained. Alternatively, powdery hydroxide has been fed directly into the flue gases coming from the boiler either in the flue gas duct or in the following reactor, wherein the hydroxide is activated by water or steam. Such a known method is described e.g. in FI patent 78846.

There are two known ways of removing SO₂ from flue gases using limestone injection. The first way is by using only limestone injection into the boiler. By using CaCO₃ injection, the following chemical reactions will take place in the fire chamber:

CaCO₃ -> CaO + CO₂ CaO + SO₂ + 'ΛOi -> CaSO₄

The other way is the LIFAC process, which is known e.g. from FI patent 78846 and wherein, besides the injection of alkali or alkali earth metal hydroxide, water is sprayed into the flue gases in a humidification chamber or in an activation reactor in order to boost the SO2 reaction. The following reactions take place in the humidification chamber:

CaO + H₂O -> Ca(OH)₂ Ca(OH)₂ + SO₂ -> CaSO₃ + H₂O

The characteristic features of the method according to the invention are presented in the claims. Normally, in a conventional LIFAC process the injection of CaCO into the fire chamber belongs to the process as an essential feature in order to achieve a sufficient molar ratio of Ca/S (about 2.5). However, it has now been found that given certain preconditions there is no need for any additional injection of CaCO₃ for the retention of SO₂, but the SO₂ retention may take place simply by using the humidification chamber. It has thus been found in the invention that no CaCO₃ injection is needed, if the fuel as such contains a sufficient quantity of alkali or alkali earth metals, such as calcium, potassium and sodium, and if the combustion temperature in the fire chamber is suitable (in a range of 800 - 1350°C). Under these circumstances, all Ca, K and Na used in the process originate in the fuel. Under the mentioned circumstances retention of SO2 is possible and sufficiently effective in the humidification chamber.

In the process removal of chlorine (HC1) is also achieved. Chlorine can be removed easily with alkali and alkali earth metals at lower temperatures (< 150°C). Chlorine is removed only slightly in limestone injection into the fire chamber.

In the following, the invention will be described with reference to the appended Figure 1. Figure 1 shows equipment according to the invention in connection with a boiler. The boiler 10 is associated e.g. with a conventional steam power process, wherein fuel is burnt, supply water of the boiler is vaporized and the steam is conducted into a steam turbine, which further rotates an electric generator in order to produce electricity. The boiler 10 may also function as such e.g. to heat district heating water or to produce steam for industrial use. This means that the puφose of use of the boiler 10 is not restricted herein. According to Figure 1, fuel M is conducted into the fire chamber 11 of the boiler 10, and the combustion temperature in the fire chamber is in a range of 800 - 1350°C. Thus, the combustion temperature is within a certain exact range. In addition, fuel M is used, which contains alkali or alkali earth metals in the following stoichiometric molar ratio: (Ca + 2K + 2Na) / (S + 2C1) > 0.5

The flue gases are conducted further into a humidification chamber 13 by way of a duct 12. Water is sprayed from a nozzle pipe 14 into the humidification chamber 13. From the humidification chamber 13 the flue gases are conducted further through electric filters 15 into a chimney 16. The essential thing is that all the sprayed water is vaporized by the flue gases' own heat. Fuel M may be e.g. peat, biomass, lignite or waste-derived fuel.

Thus, the main conditions of the invention are the following:

1) The fuel to be burnt must have a certain composition; the stoichiometric molar ratio must be (Ca + 2K + 2Na) / (S + 2C1) > 0.5.

2) The combustion temperature in the fire chamber must be in a range of 800 - 1350°C.

In the method, water is sprayed into the flue gases after the fire chamber 11 in a separate humidification chamber 13, in order to convert the alkali and alkali earth metal oxides contained in the fuel into hydroxides, which will react further with sulphur dioxide or chlorine.

The following main reactions take place in the humidification chamber 13:

CaO + H₂O -> Ca(OH)₂

Ca(OH)₂ + SO₂ -> CaS0₃ + H₂O

Ca(OH)₂ + 2 HC1 -> CaCk + 2 H₂O

The Bio-LIFAC process according to the invention is suitable for fuels containing sufficiently alkali and alkali earth metals in themselves. Such fuels are lignite, peat and bio fuels. The first requirement is that the fuel in itself contains alkali and alkali earth metals in the following molar ratio:

(Ca + 2K + 2Na) / (S + 2Cl) > 0.5.

In peat power plants, the molar ratio of fuel is typically in a class of 2.5, due to the natural occurrence of alkali and alkali earth metals in the fuel.

The following requirement on the Bio-LIFAC process according to the invention is a low combustion temperature. The temperature window required for the process is in a range of 800 - 1350°C. At these temperatures the alkali metals are released as oxides in the fire chamber. The residence time of oxides and sulphur dioxide is very short especially in power plants using peat and bio fuel. The probable reason for this is the long combustion time of these fuels in the fire chamber, whereby oxides and dioxides occur only in the upper part of the fire chamber and they have only a short time to react in the right temperature window.

The remaining alkali metals bound to the fuel, which have been released as oxides in the fire chamber, leave the fire chamber together with the flue gases and move on to the humidification reactor.

If the temperature in the boiler is above 1350°C, the risk exists that sintering or dead burning of the alkali metal oxides will occur. In case sintering of CaO occurs, the SO2 retention becomes ineffective both in the fire chamber and in the humidification reactor.

Explicitly due to the sintering characteristic of CaO at higher temperatures, the

Bio-LIFAC process according to the invention is suitable for combustion processes at lower temperatures and for such boilers, wherein the combustion temperature is within the above-mentioned window. Such combustion devices are, for example, Bubbling Fluidized Bed boilers (BFB) and Circulating Fludized Bed boilers (CFB) and lignite boilers.

Examining the individual fuel particle, the following happens:

As the fuel particle arrives in the fire chamber it will first dry as the humidity is vaporized and it is pyrolized as the other volatile substances leave. Solid carbon and ash will hereby remain in the fuel particle. Depending on the temperature and distribution of air the volatile substances and the solid carbon will burn partly and will become partly gaseous with the aid of the bubbling/carrying air. The sulphur in the fuel will at this stage be mainly converted into hydrogen sulphide (FhS).

Complete combustion of the solid carbon and volatile substances takes place in the fire chamber with the aid of secondary and tertiary airflows. During the burnout the hydrogen sulphide will be oxidised into oxides of sulphur (SO2 and SO₃).

Calcination into oxide (CaO) of the calcium carbonate (CaCO₃) in the fuel ash takes place when the temperature of the ash particle exceeds 800°C. On the other hand, the sodium and potassium in the ash will form sodium and potassium carbonates (Na2CO₃ and K2CO₃), which do not calcinate.

Sodium and potassium carbonates react with sulphur oxides in the upper part of the fire chamber forming sodium and potassium sulphates (Na2SO₄ and K2SO ). Calcium oxide reacts partly with sulphur oxides in the upper part of the fire chamber forming calcium sulphate (CaSO₄).

The activation reactor or the humidification chamber is an essential part of the Bio-LIFAC process according to the invention. Water is sprayed into the flue gases in the humidification chamber. The alkali metal oxides in the flue gas react quickly, and the following reactions take place in the humidification chamber 13: CaO + H₂O -> Ca(OH)₂ Ca(OH) + SO₂ -> CaSO₃ + H₂O

The higher the humidity content of the flue gas is as it leaves the reactor the more efficient is the retention of sulphur dioxide. The sprayed quantity of water will vaporize in the reactor and the final product is separated in the solid phase together with the fly ash, as is the case in the original LIFAC process.

Besides the retention of sulphur dioxide, the gaseous chlorine exits at a lower temperature (< 150°C) according to the following formula:

Ca(OH)₂ + 2 HC1 -> CaCl₂ + 2 H₂O

A consequence of this is that in addition to the SO2 emissions also HC1 emissions can be removed from the flue gases with the aid of the humidification reactor without any injections of additives.

In the following test results are presented, which were obtained at a Finnish power plant.

The following test run was carried out at the power plant. The peat used as fuel had a sulphur content of 0.14 w-% of the dry matter and a chlorine content of 0.04 w-%. The fuel contained 0.46 w-% of calcium, 0.02 w-% of sodium and 0.03 w-% of potassium. In this case, the main alkali metal was calcium and the molar ratio Ca/S of the fuel was 2.6 and its Ca/Cb molar ratio was 5.1.

The fuel was burnt in a 295 MW BFB boiler. From the flue gas a secondary flow was taken into a 1000 nm³/h test humidification reactor. Test results

Under these circumstances, in the test run almost all of the SO2 was removed from the flue gases after the humidification reactor. The test indicates that the calcium derived from the fuel is considerably more reactive than the calcium derived from limestone injection. With the above-mentioned molar ratio of 2.6 a sulphur retention of less than 70 % would be achieved by limestone injection in a humidification reactor, and taking the sulphur retention of the injection into

10 account the total sulphur retention would typically be less than 80 %. Now the sulphur retention was in fact over 90 %.

In the following, the advantages of the Bio-LIFAC process according to the invention will be examined.

15

Due to the suitable alkali and alkali earth metals content of the fuel, there is no need for any CaCO₃ injection, neither for injection of any other absorbing material into the fire chamber. This results in many advantages: - Those costs are avoided, which in known methods are caused by the injection of CaCO₃ or by injections of other absorbing materials.

- Less problems than before result from dirt in and clogging of the flue gas ducts. In the known methods, an additional CaCO₃ injection may block e.g. the air pre- heaters.

The simple Bio-LIFAC process is suitable for burning taking place at a low temperature, such as is normally the case e.g. when burning peat, lignite, biomass and waste-derived fuel, due to the burning technology and/or the low heat value of the fuel. At the same time these fuels often contain sufficient alkalis and alkali earth metals. Under these circumstances, the Bio-LIFAC process according to the invention may well be used in certain processes.

The invention and its technology may be utilised advantageously to meet the stricter emission standards, which can be expected within the EU area and outside it. At the present time only a moderate SO2 retention (less than 50 %) is achieved by injecting only limestone into the fire chamber. This is due to the fact that the particles have too short a residence time in the correct temperature window in the fire chamber. Should the emission limit of 200 mg/nm come into force for SO2, then limestone injection alone is not sufficient and wet desulphurisation would cause additional costs in the power plant process. Since the costs of the humidification reactor system are low, the Bio-LIFAC process according to the invention would be one of the best exploitable techniques when aiming at low emission values.

Another advantage of the process is dechlorination (HC1), which is achieved easily with the aid of alkali and alkali earth metals at low temperatures (< 150°C). In the traditional purification method using limestone injection into the fire chamber, chlorine is removed only to a slight extent.

Claims

1. Flue gas purification process, wherein the fuel (M) is burnt in the fire chamber (11) of a boiler (10), from which the flue gases are conducted through a duct (12) into a humidification chamber (13) and further out through a chimney (16), characterised in that

- the fuel (M) for burning has the following molar ratio (Ca + 2K + 2Na) / (S + 2C1) > 0.5,

- all the Ca, K and Na used in the process derive from the fuel, - the fuel (M) is burnt in the fire chamber (11) of the boiler (10) in a temperature range of 800 - 1350°C, and

- after the boiler the flue gases are conducted along a duct (12) into the humidification chamber (13), into which water is sprayed from a nozzle pipe (14) or such.

2. Process according to claim 1, characterised in that the sulphur retention is > 90 % and the HC1 retention correspondingly is > 80 %, when the Ca/S molar ratio is > 2 and the Ca/Ch molar ratio is > 5.

3. Process according to claim 1, characterised in that the following reactions take place in the humidification chamber (13): CaO + H₂O -> Ca(OH)₂ Ca(OH)₂ + SO₂ -> CaSO₃ + H₂O

4. Process according to claim 1, characterised in that the following reactions take place in the humidification chamber (13): Ca(OH)₂ + 2 HC1 -> CaCl₂ + 2 H₂O.

5. Process according to any one of the preceding claims, characterised in that so much water is sprayed into the humidification chamber (13) that all water will be evaporated by the own heat of the flue gases.

6. Process according to any one of the preceding claims, characterised in that the fuel (M) is peat, lignite, biomass or waste-derived fuel.