FI111608B - Flue gas cleaning process - Google Patents

Description

111608

The present invention relates to a process for the removal of gaseous sulfur compounds and, in particular, sulfur dioxide from the flue gases of a boiler for the combustion of sulfur fuels.

It is previously known to reduce the sulfur dioxide content of the boiler flue gases 10 by supplying calcium oxide, calcium carbonate or some other alkaline compound to the boiler combustion chamber. In a fluidized-bed furnace with a lriertopet, the addition of lime can reduce the sulfur dioxide content of the flue gases by up to 90% when the boiler operates at the best temperature range for chemical reactions, namely 800-1000 ° C. The sulfur dioxide thus absorbed leaves the cat-15 space with the dust in the form of gypsum.

In other boilers where higher temperatures are required and where the additive has a short residence time due to the nature of the combustion process, it is expected that the reduction in the sulfur dioxide content of the flue gas will be substantially lower, to about 50% or less. 1

On the other hand, it is known that the sulfur dioxide content of the flue gases can be reduced by various off-boiler absorption methods. One such method known per se is the so-called spray method, in which the flue gases from the boiler are introduced into a separate reactor, into which a small amount of calcium hydroxide aqueous slurry is sprayed through special nozzles. The reactor is typically a larger one. · A vessel where the flue gas velocity is allowed to fall and water slurry is sprayed downward from the top of the vessel.

! It has also been known to supply to the boiler combustion chamber, in addition to the sulfur-containing combustible material and the oxygen-containing gas, powdered calcium or magnesium hydroxide in excess of the sulfur dioxide gas produced in the combustion chamber. Water or steam is injected separately into the calcium or magnesium oxide-containing flue gases thus obtained in a separate phase from the combustion chamber. Alternatively, the powdered hydroxide is fed directly to the flue gases from the boiler either in the flue gas duct or in a subsequent reactor where the hydroxide is activated with water or steam. Such known methods are described, for example, in patent application FI851623.

There are two known ways to remove SO2 from flue gases by utilizing limestone injection. The first way is to use only a limestone injection into the kettle.

10 The following chemical reactions occur in the furnace using CaC03 injection:

CaCO3 -> CaO + C02 CaO + SO2 + 1/2 02 -> CaSO4 15 Another way is, for example, the LEFAC process known from FI 851623, in which, in addition to lime injection, water is injected into the flue gases in a humidification chamber or activation reactor. enhanced. The following reactions take place in the wetting chamber: 20 CaO + H 2 O → Ca (OH) 2

Ca (OH) 2 + SO 2 -> CaSO 3 + H 2 O CaSO 3 + / 402 CaSO 4

The invention is characterized in what is stated in the claims.

25

Normally, in a conventional LIFAC process, injection of CaCO 3 into the furnace is an essential feature of the process to achieve a sufficient Ca / S molar ratio (about 2.5). However, it has now been found that, under certain conditions, additional injection of CaCO 3 is not required to retain SO 2, but SO 2 can be achieved by simply using a humidification chamber. Thus, it has been found in the invention that CaCO3 injection is not required if the fuel itself contains sufficient alkali or alkaline earth metals, such as Na, Ca, K, and if the combustion temperature in the furnace is suitable (in the range 800 ° C to 1350 ° C). . Thus, all of the Ca, K and Na used in the process are fuel derived. Under these conditions, the capture of SO 2 is possible and sufficient; effective in the humidification chamber.

5

The process also achieves chlorine removal (HCl) which is readily removable with alkali and alkaline earth metals at lower temperatures (<150 ° C). Chlorine is only slightly removed by injection of limestone into the furnace.

The invention will now be described with reference to Figure 1 attached. Figure 1 shows an apparatus according to the invention in connection with a boiler. Boiler 10 is associated, for example, with a conventional steam power process in which the fuel is burned, the boiler feed water is vaporized and the steam is supplied to a steam turbine, which further rotates the electric generator to generate electricity. The boiler 10 may also function as such for heating district heating water or producing steam for industrial use. Thus, the use of boiler 10 is not limited here. According to Figure 1, fuel M is introduced into the furnace 11 of the boiler 10 where the combustion temperature is in the range of 800 ° C to 1350 ° C. Thus, the combustion temperature is within a specific range. In addition, fuel M is used which contains alkali or alkaline earth metals at the following stoichiometric molar ratio (Ca + 2K + 2Na) / (S + 2Cl)> 0.5.

The flue gases are further introduced into the humidification chamber 13 via a conduit 12. Water 25 is sprayed from the nozzle tube 14 into the humidification chamber 13. From the humidifying chamber 13, the flue gases are further directed through the electric filters 15 to the chimney 16. It is essential that all the water sprayed is evaporated by the flue gas's own heat. The fuel M can be, for example, peat, biomass, lignite or waste fuel.

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Thus, the main conditions of the invention are as follows: f 1) A particular composition is required for the fuel to be burned; the stoichiometric molar ratio should be (Ca + 2K + 2Na) / (S + 2Cl)> 0.5.

2) The combustion temperature in the furnace should be in the range 800 - 1350 ° C.

The method involves spraying water into the flue gases after the furnace 11 in a separate humidification reactor 13 to convert the alkali or alkaline earth metal oxides 10 contained in the fuel into hydroxides which further react with sulfur dioxide or chlorine.

The main reactions in the wetting chamber 13 are as follows: 15 CaO + H 2 O → Ca (OH) 2

Ca (OH) 2 + S02 -> CaS03 + H20 CaS03 + V2O2 -> CaSO4 Ca (OH) 2 + 2 HCl -> CaCl2 + 2 H2O 20 Process description and process requirements

The Bio-LIFAC process is suitable for fuels that contain enough alkali or alkaline earth metals per se. Such fuels include lignite, peat and biofuels. The first requirement is that the fuel itself contains alkali-25 or alkaline-earth metals in the following molar ratio: (Ca + 2K + 2Na) / (S + 2Cl)> 0.5.

Peat power plants typically have a molar ratio of about 2.5 due to the natural presence of alkali and alkaline earth metals in the fuel.

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The next requirement for the Bio-LIFAC process is the low combustion temperature. The required temperature window for the Bio-LI-FAC process is in the range of 800 ° C to 1350 ° C. At these temperatures, alkali is released as oxides in the furnace. The residence time of oxides and sulfur dioxide is very short, especially in power plants using peat and biofuel. The reason for this is probably the long combustion time of these fuels in the furnace, whereby the oxides and dioxins are only present in the upper parts of the furnace and have only a short time to react in the correct temperature window.

The remainder of the fuel-bound alkali released into the furnace 10 as oxides is removed by the flue gases from the furnace and transferred to the humidification reactor.

If the temperature in the furnace is higher than 1350 ° C, there is a risk of alkali oxide sintering or dead burning. In the case of CaO sintering, the SO 2 retention becomes ineffective in both the furnace and the humidification reactor.

Specifically, due to the sintering property of CaO at higher temperatures, the Bio-LIFAC process is suitable for lower temperature incineration processes and boilers where the combustion temperature is within the above window. Such combustion processes include, for example, Bubbling Fluidized Bed (BFB) Boilers and Circulating Fluidized Bed (CFB) Boilers and Rus Carbon Boilers.

25 When inspecting a single fuel particle, the following occurs:

When a fuel particle enters the furnace, it first dries, i.e. the moisture evaporates and pyrolyses, i.e. the volatiles are removed. This leaves solid carbon and ash in the fuel particle. Depending on temperature and air distribution, volatiles and solid carbon 30 are partially burned, partially gasified by fluidized / carrier air. At this stage, the sulfur in the fuel is mainly converted to hydrogen sulphide (H2S).

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The combustion of solid carbon and volatiles occurs in the furnace through secondary and tertiary atmospheres. During combustion, hydrogen sulfide is oxidized to sulfur oxides (SO2 and SO3).

5

Calcination of calcium carbonate (CaCOs) in fuel ash to oxide (CaO) occurs when the temperature of the ash particle rises above 800 ° C. Instead, Na and K in the ash form sodium carbonates and potassium carbonates (Na 2 CO 3 and K 2 CO 3) which do not calculate.

10

Sodium and potassium carbonates react with sulfur oxides in the upper part of the furnace to form potassium sulphates and sodium sulphates (K2SO4 and Na2SO4). The calcium oxide reacts partially with sulfur oxides in the upper part of the furnace to form calcium sulfate (CaSO 4).

15

The activation reactor

The activation reactor or wetting tank is an essential part of the Bio-LIFAC process. Water is sprayed into the flue gases in a humidification chamber. The alkali oxides contained in the flue gas react rapidly and the following reactions occur in the humidification chamber 13:

Ca0 + H20-> Ca (0H) 2 Ca (OH) 2 + SO2 -> CaSO3 + H20 CaSOa + V ^ -> CaSO4

The higher the moisture content of the flue gas when it leaves the reactor, the more effective the trap is to retain sulfur dioxide. The amount of water that is sprayed evaporates 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.

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In addition to the capture of sulfur dioxide, the gaseous chlorine is removed at a lower temperature (<150 ° C) according to the following formula:

Ca (OH) 2 + 2 HCl-> CaCl2 + 2 H2O 5 As a result, in addition to SO2 emissions, HCl emissions are also removed from the flue gases by means of a humidification reactor without any additive injections.

The test results obtained with the Rauhalahti power plant 10 are presented below.

The following test run was performed at the Rauhalahti power plant. The fuel peat contained the following fuel composition: Ca 0.46 wt%, Na 0.02 wt% and K 0.03 wt%. In the test run, calcium (Ca) was the main element and the molar ratio Ca / S in the fuel was 2.615 and the Ca / Cl2 molar ratio was 5.1.

The fuel was peat with a sulfur content of 0.14% by weight in the dry state and the fuel was burned in a 295 MW BFB boiler. The flue gas had been flushed to a 1000 nm / h test humidification reactor. The Cl content of the fuel in the fuel when dry was 20 0.04%.

Test results

Ca / S 2.6

Reactor Inlet Temperature_143 ° C

Reactor outlet temperature_12__ ° C_ .. SO2 input__105 ppm, dry, 6% 02 SO2 outlet__7_ppm, dry, 6% 02

Cl Input 331.4 mg / nmJ, dry

Cl outlet__52_mg / nnri dry ~ (S02 removed after reactor 94% of total ___

Cl removed after reactor 83% of total ___ 8 111608 Thus, during test run, almost all SC 2 has been removed from the flue gases after the wetting reactor. The experiment shows that fuel derived Ca is significantly more reactive than limestone injection Ca. With the aforementioned mo-5 ratio of 2.6, limestone injection in a humidification reactor would yield less than 70% sulfur retention, and, given the sulfur retention of the injection, total sulfur retention would typically be less than 80%. Now the sulfur retention was over 90%.

The advantages of the Bio-LIFAC process are as follows: 1) Due to the appropriate alkaline and alkaline content of the fuel, injection of CaCC3 is not necessary and no other injection of absorbent material into the furnace is required. This has many benefits: - no cost of injection of CaCC3 or other absorbent agent (cf. known methods), - less problems with fouling and clogging of flue gas ducts; an additional injection of CaCC> 3 may clog the air preheaters, for example.

20 2) A simple Bio-LIFAC process is suitable for low-temperature combustion, such as is typically the case for peat, lignite, biomass and waste fuel, due to combustion technology and / or low calorific value of the fuel. At the same time, these fuels often contain enough alkali and alkaline earth metals. Thus, the Bio-LEFAC process can be used well in certain processes.

• · 3) The invention and its technology can be advantageously utilized to meet the stringent emission standards expected within and outside the EU.

At present, only moderate (less than 50%) SCV retention is achieved by mere injection of limestone into the furnace. This is because the particles have an excessively short residence time of 9,111,608 in the correct temperature window in the furnace. If the SO2 emission limit of 200 mg / hm3 comes into force, limestone injection alone will not be sufficient and wet desulphurisation would cause additional costs in the power plant process. Due to the low cost of the humidification reactor system, the Bio-LIFAC-5 process would be one of the best available techniques for achieving low emission values.

4) The process also achieves chlorine removal (HCl), which is easily removed with alkali and alkaline earth metals at lower temperatures (<150 ° C). 10 Chlorine is only slightly eliminated by limestone injection into the furnace. 1 · -

Claims (6)

A process for purifying flue gases, wherein a fuel (M) is burned in a fireplace (11) in a boiler (10) and the flue gases are discharged from the boiler (10) through a duct 5 (12) into a humidification chamber (13) and further out through a chimney (16), characterized in that - the fuel (M) burned has the following molar ratio (Ca + 2K + 2Na) / (S + 2C1)> 0.5 - everything Ca, K and Na used in the process originates from the fuel 10. the fuel (M) is burned in the fireplace (11) in the boiler (10) within the temperature range 800 ° C-1350 ° C - the flue gases are conducted after the boiler through the duct (12) into the humidification chamber (13) into which water is sprayed through a nozzle tube (14) or equivalent. 15 Process according to Claim 1, characterized in that the retention of sulfur is> 90% resp. the retention of HCl is> 80% when the Ca / S molar ratio is> 2 and the Ca / Cl 2 molar ratio is> 5. Process according to Claim 1, characterized in that the following reactions take place in the wetting chamber (13): CaO + H2O -> Ca (OH) 2 Ca (OH) 2 + SO2 -> CaSO3 + H2O CaSO3 + 1/202 -> CaSO4 25 Process according to claim 1, characterized in that the following reaction takes place. The wetting chamber (13): Ca (OH) 2 + 2HCl -> CaCl2 + 2H20 111608 Process according to any of the preceding claims, characterized in that water is injected into the humidification chamber (13) to the extent that all water evaporates through the flue gas's own heat. Process according to any of the preceding claims, characterized in that the fuel (M) is peat, lignite, biomass or waste-containing fuel. ψ *>, ·