MULAS, Aldo (Via Montixeddu 2/b, Cagliari, I-09123, IT)
MULAS, Guido (Via Montixeddu 2/b, Cagliari, I-09123, IT)
MULAS, Aldo (Via Montixeddu 2/b, Cagliari, I-09123, IT)
| CLAIMS 1. Method for removing SO2 from flue gases of boilers with fluidized- bed technology, said method comprising the direct introduction of CaO into the boiler. 2. Method according to Claim 1 , characterized in that the CaO has a granulometry between 0.001 and 500 μηι. 3. Method according to Claim 2, characterized in that the CaO has a granulometry between 0.01 and 400 μητι. 4. Method according to Claim 3, characterized in that the CaO has a granulometry between 0.1 and 300 μηι. 5. Method according to any one of Claims 1-4, characterized in that the CaO has, during operation, a specific absorption between 0.60 and 0.20 mol (SO2)/mol(CaO). 6. Method according to Claim 5, characterized in that the specific absorption is between 0.60 and 0.30 mol (SO2)/mol(CaO). 7. Method according to any one of Claims 1-4, characterized in that CaO is prepared as follows: a. Quarrying, crushing, and selection of limestone, essentially constituted of calcium carbonate (CaCO3), according to lump sizes suitable for subsequent calcination; b. Introduction of CaCO3, prepared as in the preceding point, in a lime kiln, in which the process of dissociation of CaCO3 is controlled to a temperature that can be from 850°C to 1100°C, for a residence time of the material in the kiln in the range from 12 to 24 hours. c. The CaO, obtained as described in the preceding point, is discharged from the kiln and is ground in suitable mills and is selected according to granulometric classes defined in relation to the site-specific requirements of the power station that receives the material; in the present case, the ideal granulometry, for any plant, will be defined according to a procedure that envisages adjustments, by successive approximations, of the granulometric distribution of the material to be introduced into the combustion chamber of the boiler that makes it possible to remove most of the SO2 resulting from combustion, at the same time minimizing or cancelling the amount of residual calcium oxide present in the ash that results from said combustion. 8. Method according to Claim 7, characterized in that the CaCO3 has lumps with a diameter of 40-100 mm. 9. Method according to Claim 7, characterized in that the calcining kiln is vertical or horizontal. 10. Method according to Claim 7, characterized in that the CaO is ground in hammer mills or ball mills and then is selected by screening for the desired granulometry. |
FIELD OF THE INVENTION
The present invention relates to the field of methods of purification of flue gases from boilers; in particular it relates to a method for the desulphurization of flue gases from steam generating boilers with fluidized-bed technology.
PRIOR ART
Currently, during operation, calcium carbonate (CaCO 3 ) is introduced into the combustion chamber of boilers with fluidized-bed technology, for the purpose of neutralizing pollutants resulting from said combustion, with particular reference to sulphur dioxide (SO 2 ).
The CaCO 3 used for this purpose is obtained by quarrying and subsequent stages of grinding in suitable mills (hammer mills or ball mills, etc.) and selection (screening) of limestone, constituted essentially of calcium carbonate (CaCOs), according to specific granulometric curves defined by the boiler manufacturer.
CaCO 3 of suitable granulometry is then introduced into the combustion chamber of the fluidized-bed boiler where it is submitted to a temperature of approx. 850°C, and dissociates into calcium oxide (CaO) and carbon dioxide (CO 2 ) according to the following reaction:
850°C
CaC0 3 CaO + CO 2
-42.5 Kcal
The calcium oxide (CaO), obtained as described above, binds to the sulphur dioxide (SO 2 ) that is present in the boiler flue gases, to produce the solid compound called calcium sulphate (CaSO ), according to the reaction:
CaO + SO 2 + 1/2 O 2 CaSO 4 The calcium sulphate (CaSO 4 ) thus formed is then removed from the combustion chamber of the boiler together with the ash.
However, the existing method known in the prior art, which has been used for years for removing SO 2 from flue gases from fluidized-bed boilers, has a number of drawbacks. The calcium carbonate (CaCO 3 ) to be fed into the combustion chamber of the fluidized-bed boiler must comply with strict granulometric curves specified by the manufacturer of said boiler, therefore in the stages of preparation thereof (grinding and granulometric selection), there is inevitably creation of processing wastes (pulverulent materials that do not comply with the specified granulometric curve), which prove difficult to use for various purposes.
The granulometric curves specified by the manufacturer of a boiler may include the presence, more or less pronounced, of CaCO 3 particles characterized by an average diameter of less than 45pm, which, once introduced into the boiler, are easily removed by the air flowing through the combustion chamber, and go directly into the waste ash, without making any contribution to desulphurization of the flue gases (for example: the ash resulting from combustion in fluidized-bed boilers in which CaC0 3 was used as desulphurizing agent usually contains up to 7 wt.% CaCO 3 ).
When introduced into a boiler, CaC0 3 is subjected to thermal stress by sudden exposure to high temperatures (850°C). The thermal stress causes sintering of the particles, leading to a reduction of the specific surface of the material, limiting its reactivity and therefore the capacity for exchange with the surroundings (including SO 2 ) - (for example: 20 grams of CaC0 3 of granulometry between 200 and 300 pm, introduced into the combustion chamber of a fluidized-bed boiler with a diameter of 40 mm, in which there is: a fluidization velocity equal to 0.75 m/s, a temperature of 850°C, a bed constituted of 150 grams of silica sand of grain size 0.9-1 mm, and a concentration of S0 2 at inlet equal to 1800 ppm, can have, after a time elapsed since the moment of introduction equal to 6500 seconds, a specific capacity for absorption of S0 2 equal to 0.08 g(S0 2 )/g(CaCO 3 ), i.e. equal to 0.125 mol S0 2 /mol CaC0 3 .
The CaO particles that form directly in the boiler display mechanical characteristics that are not ideal, in particular, with respect to impact strength and abrasion resistance, and their consequent comminution so that they are, in their turn, immediately removed (elutriated) from the streams present in the boiler, without reacting with the sulphur dioxide (SO 2 ), and end up in the waste ash without making any contribution to absorption of the pollutants - (for example: 20 grams of CaCO 3 of granulometry between 200 and 300 μηι, introduced into a fluidized-bed boiler with a diameter of 40 mm, where there is: a fluidization velocity equal to 0.75 m/s, a temperature of 850°C, a bed constituted of 150 grams of silica sand of grain size 0.9-1 mm, and an inlet concentration of SO 2 equal to 1800 ppm, can have, after a time elapsed since the moment of introduction equal to 20 minutes, a rate of elutriation (removal by the streams present inside the boiler) of approx. 0.03 g/min).
Expenditure of thermal energy of the boiler that is not used for generating steam but for production of CaO, by the CaCO 3 that does not participate in removal of SO 2 ; for example: in a thermal power station, with electric power equal to 350 MWh, with a coal-fired fluidized-bed boiler, for which the annual production of ash is approx. 160 000 tonnes/year, where said ash is characterized by a CaO content of approx. 20 wt.% (average content found in analyses of the heavy ash produced in coal-fired fluidized-bed boilers), the amount of CaO is calculated to be 32 000 tonnes/year (160 000 tonnes/year of ash * 20% of CaO) which is disposed of in dumps. To obtain that amount of calcium oxide (CaO), which does not participate in removal of sulphur dioxide (SO 2 ), assuming use of coal with a calorific value of 6000 kcal/kg, and assuming that the heat required for obtaining 1 tonne of calcium oxide (CaO) is equal to 980 000 kcal, it is calculated that obtaining 1 tonne of calcium oxide (CaO) requires approx. 163 kg of coal; hence the annual amounts of calcium oxide (CaO) disposed of in dumps represents a fuel consumption equal to 5216 tonnes/year (32 000 tonnes of CaO * 0.163 tonnes of coal).
Emission of the quota of CO 2 , from combustion and process, resulting from the dissociation of CaCO 3 to give CaO that is then disposed of in dumps without contributing to desulphurization; (for example: in a thermal power station, with electric power equal to 350 MWh, with a coal-fired fluidized-bed boiler, for which the annual production of ash is approx. 160 000 tonnes/year, where said ash is characterized by a content of calcium oxide (CaO) of approx. 20 wt.% (average content found in analyses of the heavy ash produced in coal-fired fluidized-bed boilers), calculation gives an amount of calcium oxide (CaO) equal to 32 000 tonnes/year (160 000 tonnes/year of ash * 20% of CaO) that is disposed of in dumps. Production of this annual amount of calcium oxide requires, as already mentioned, approx. 5216 tonnes/year of fuel.
The CO 2 emitted to atmosphere, for obtaining the "ineffective" CaO, which does not participate in desulphurization, and goes directly into the waste ash, can be calculated from the following formula:
32 000 tonnes/year "ineffective" CaO * 0.78 (stoichiometric coefficient) =
24 960 tonnes/year C0 2
5216 tonnes/year coal * 30.961 (PCI) * 97.66 (FC) * 0.98 (FO) =
15 456 tonnes/year C0 2
Production of residues and ash resulting from combustion, which are also constituted of CaCO 3 and of "ineffective" CaO, i.e. CaO that has not participated in removal of SO 2 . For example: in a thermal power station, with electric power equal to 350 MWh, with a coal-fired fluidized-bed boiler, the annual production of ash is approx. 160 000 tonnes/year, said ash being characterized by a CaCO 3 content of approx. 7 wt.%, and a CaO content of approx. 20 wt.% (average contents found in analyses of the heavy ash produced in coal-fired fluidized-bed boilers); calculation then gives an annual amount of CaCO 3 disposed of with the ash equal to 11 200 tonnes (160 000 tonnes/year ash * 7% CaCOs), and an annual amount of CaO equal to 32 000 tonnes (160 000*20% CaO).
It is therefore clear, despite the regular practice over many years of using CaCO 3 directly in the combustion chamber of fluidized-bed boilers, that a method is needed for removing SO 2 from flue gases that is more suitable in energy terms and provides better efficiency in absorption of SO 2 .
SUMMARY OF THE INVENTION
The present invention solves the aforementioned problems by means of a method for removing SO 2 from the flue gases of boilers with fluidized-bed technology, said method comprising the direct introduction of CaO into the boiler.
The method according to the invention therefore envisages the production of CaO in advance, which preferably should have a suitable granulometry, which must be assessed for each individual case based on site-specific requirements of the boiler in order to minimize the emission of SO 2 and at the same time minimize the presence of CaO in the ash. With the method of the invention, surprisingly, an increase in specific absorption was found relative to the values obtained with direct use of CaC0 3 in a boiler. For example, 20 grams of CaO of granulometry between 200 and 300 pm, introduced into a fluidized-bed boiler with a diameter of 40 mm, where there is: a fluidization velocity equal to 0.75 m/s, a temperature of 850°C, a bed constituted of 150 grams of silica sand of grain size 0.9-1 mm, and an inlet concentration of SO 2 equal to 1800 ppm, can have, after a time elapsed since the moment of introduction equal to 18000 seconds, a specific capacity for absorption of SO 2 equal to 0.37 g(SO 2 )/g(CaO) [equal to 0.34 mol (SO 2 )/mol(CaO)], versus 0.08 g(SO 2 )/g(CaCO 3 ) [0.125 mol SO 2 /mol CaCO 3 ] obtained, in the same experimental conditions, with introduction of 20 grams of CaCO 3 .
The CaO previously prepared with a suitable granulometry then has a specific molar absorption of approx. 2.7 times relative to that observed with induced formation, in the boiler, of CaO starting from CaCO 3 .
The present invention also offers other advantages, which are explained later. DETAILED DESCRIPTION OF THE INVENTION
Preferably, according to the invention, the granulometry of the CaO varies in the range from 0.001 to 500 μιτι; more preferably from 0.01 to 400 μηι; even more preferably from 0.1 to 300 μιη.
Preferably, according to the invention, the CaO has, during operation, a specific absorption between 0.60 and 0.20 mol (SO 2 )/mol(CaO); more preferably between 0.60 and 0.30 mol (SO 2 )/mol(CaO).
Preferably the CaO to be used in fluidized-bed boilers for removing SO 2 , according to the method of the invention, is prepared as follows:
a) Quarrying, crushing, and selection of limestone, essentially constituted of calcium carbonate (CaCO 3 ), to lump sizes suitable for subsequent calcination (for example: 40-100 mm in diameter);
b) Feeding the CaCO 3 , prepared as in the preceding point, into suitable lime kilns (vertical or horizontal), in which the process of dissociation of CaCO 3 is controlled optimally at a temperature that can be from 850°C to 1100°C, for a residence time of the material in the kiln in the range from 12 to 24 hours. c) The CaO, obtained as described in the preceding point, is discharged from the kiln and is ground in suitable mills (hammer mills or ball mills, etc.) and selected (screening) according to granulometric classes defined in relation to site-specific requirements of the power station that receives the material; in the present case the ideal granulometry, for each plant, will be defined according to a procedure that envisages adjustments, by successive approximations, of the granulometric distribution of the material to be introduced into the combustion chamber of the boiler. These adjustments can be carried out by introducing CaO directly, in the combustion chamber of the fluidized-bed boiler in operation, according to granulometric distributions to a first approximation, and, at the same time, measuring: the concentration of SO 2 at the outlet of the boiler chimney (which must show values below the legally stipulated limits), and the amount of residual CaO in the ash. In essence, the ideal granulometric curve, for a specific boiler, is the one that makes it possible to remove the largest amount of SO 2) resulting from combustion, at the same time minimizing or cancelling the amount of residual calcium oxide present in the ash resulting from said combustion.
Once the "ideal" granulometric curve of calcium oxide (CaO), described in the preceding point, is established, the regime of the procedure for removal of SO 2 is entered by direct introduction of CaO of suitable granulometry (according to the "ideal" granulometric curve as previously established) in the combustion chamber of the fluidized-bed boiler.
The invention, described in the preceding points, comprises, with respect to the procedure currently used, the following advantages:
- The waste (particles not complying with the "ideal" granulometric curve) resulting from the preparation of CaO, with particular reference to the grinding process (described above as point "c"), is immediately usable for the production of derivatives with high added value (for example Ca(OH) 2) a compound that is widely used in the building industry, for water purification, for soil stabilization, in agriculture for controlling soil pH);
- Introduction, in the fluidized-bed boiler, of a material that is thermodynamically inert and does not require, for reaction with SO2, temperature-induced transformations, therefore without affecting the thermal efficiency of the system (in contrast, when using CaCO 3 , the dissociation induced inside the combustion chamber of the boiler, by the production of CaO, comprises a subtraction of heat equal to 980 000 kcal per tonne of CaO produced, including what ends up in the waste ash);
- All the CaO introduced into the combustion chamber of the boiler is immediately available for reaction with SO 2 (in contrast, when using CaCO 3 , there is dispersion of said material, which is volatilized, without being converted to CaO, by a percentage that amounts to 7 wt.% of the residual ash from combustion);
- No thermal stress is induced by the temperature present in the combustion chamber of the boiler, on the particles of CaO introduced there directly, which therefore do not undergo any change in mechanical characteristics, with drastic reduction (relative to the use of calcium carbonate) of the comminution processes induced by abrasion and impacts between particles, and the consequent greater persistence thereof in the system (for example, 20 grams of CaO of granulometry between 200 and 300 pm, introduced into a fluidized-bed boiler with a diameter of 40 mm, where there is: a fluidization velocity equal to 0.75 m/s, a temperature of 850°C, a bed constituted of 150 grams of silica sand of grain size 0.9-1 mm, and an inlet concentration of SO2 equal to 1800 ppm, can have, after a time elapsed since the moment of introduction equal to 20 minutes, a rate of elutriation (removal induced by the streams present inside the boiler) of approx. 0.003 g/min, compared with 0.03 g/min obtained in the same experimental conditions with introduction of 20 grams of CaCO 3 );
- Increase, relative to the use of CaCO 3 , in the specific absorption (amount of SO2 absorbed per gram of sorbent fed into the combustion chamber of the boiler), with consequent reduction of the consumption of sorbent material;
- Reduction of the ash discharged from the boiler, relative to that produced when using calcium carbonate (CaCOs), since the better specific absorption and lower susceptibility to crushing of the particles of CaO represents a drastic reduction of material that ends up in the ash without binding to the SO 2 ;
- Optimization in the procedures for thermodynamic management and efficiency of fluidized-bed boilers. The smaller amount of sorbent that is currently used (calcium carbonate) produces a reduction of the volumes occupied by it inside the combustion chamber, with improvement of heat exchange of the fuel with the surroundings;
- Reduction in emissions of CO 2 defined as "ineffective". Control in the preparation of the sorbent CaO makes it possible to optimize and rationalize the emission of CO 2 emitted for producing it. Currently the production of calcium oxide, which takes place in a manner that is induced and uncontrolled inside the combustion chamber of the fluidized-bed boiler, produces a sorbent CaO, 20% of which is lost in the ash without contributing to the absorption of SO 2 , although producing process and combustion CO 2 .