Combustion process for the reduction of particulates in combustion fumes

Abstract

A combustion process wherein a fuel, a comburent and component A) are fed to a combustor, component A), comprising low-melting salts and/or oxides having a melting temperature 1,450 K, the ratio by moles A/(AA)0.01, being: A the sum by moles between the amount of metals, under the form of low-melting salts and/or low-melting oxides present in the component A) and the amount of metals under the form of the low-melting salts and/or low-melting oxides or their low-melting mixtures, contained in the fuel, A is the sum of the amount of all the metals contained in the fuel and of those contained in component A), in which the combustor is isothermal type and flameless.

Claims

1. A combustion process for removing particle size diameter lower than 2.5 m (PM 2.5) to values lower than 50 m/Nm.sup.3 from combustion fumes wherein a fuel, a comburent, optionally premixed with recycling fumes, and component A) are fed to a combustor, component A) comprising low-melting salts and/or oxides and/or mixtures thereof having a melting temperature, comprising a melting point <1,450 K, the ratio by moles A/(AA) is >0.01, being: A the sum by moles between an amount of metals, under the form of low-melting salts and/or oxides and/or mixtures thereof present in the component A) and an amount of metals under the form of the low-melting salts and/or oxides or mixtures thereof contained in the fuel, A is the sum of the amount of all the metals contained in the fuel and of those contained in component A), in which the combustor is isothermal type and flameless, wherein the comburent is oxygen and is used in molar excess with respect to a stoichiometric amount for combustion reaction with the fuel, wherein the titre of oxygen is at least 70% by volume, the complement to 100% being formed of inert gases and/or nitrogen; wherein metals present in the fuel and in component A) remain under a liquid state in the combustor and are removed from a bottom of the combustor; and wherein the pressure in the combustor is higher than the atmospheric pressure and up to 2,000 kPa and the temperature is comprised between 1,500 K (1,223 C.) and up to 2,100 K (1,827 C.).

2. A process according to claim 1, wherein the ratio by moles A/(AA) is at least 0.1.

3. A process according to claim 1, wherein the combustion gases at the combustor outlet are cooled at a temperature equal to or lower than 1,100 K.

4. A process according to claim 1, wherein as component A) a mixture having melting temperature <1,450 K is used consisting of one or more compounds as defined in A) and of salts and/or oxides having melting temperature above 1,450 K.

5. A process according to claim 1, wherein component A) is fed to the combustor separately from the fuel or in admixture with the fuel.

6. A process according to claim 1, wherein the fuel residence time in the combustor ranges from 0.5 seconds up to 30 minutes.

7. A process according to claim 1, wherein oxygen is premixed with recycling fumes, the recycling fume amount being higher than 10% by volume.

8. A process according to claim 7, wherein the recycling fumes contain water under the vapor form in an amount higher than 10% by volume calculated on the total volume of recycling fumes.

9. A process according to claim 1, wherein the fuel contains water/vapor in an amount, expressed as percent by weight, up to 80%.

10. Fumes obtainable according to a combustion process for removing particle size diameter lower than 2.5 m (PM 2.5) to values lower than 50 m/Nm.sup.3 from combustion fumes wherein a fuel, a comburent, optionally premixed with recycling fumes, and component A) are fed to a combustor, component A) comprising low-melting salts and/or oxides and/or mixtures thereof having a melting temperature, comprising a melting point <1,450 K, the ratio by moles A/(AA) is >0.01, being: A the sum by moles between an amount of metals, under the form of low-melting salts and/or oxides and/or mixtures thereof present in the component A) and an amount of metals under the form of the low-melting salts and/or oxides or mixtures thereof contained in the fuel, A is the sum of the amount of all the metals contained in the fuel and of those contained in component A), in which the combustor is isothermal type and flameless, wherein the comburent is oxygen and is used in molar excess with respect to a stoichiometric amount for combustion reaction with the fuel, wherein the titre of oxygen is at least 70% by volume, the complement to 100% being formed of inert gases and/or nitrogen; wherein metals present in the fuel and in component A) remain under a liquid state in the combustor and are removed from a bottom of the combustor; wherein the pressure in the combustor is higher than the atmospheric pressure and up to 2,000 kPa and the temperature is comprised between 1,500 K (1,223 C.) and up to 2,100 K (1,827 C.); and wherein a concentration of the PM 2.5 is lower than 50 g/Nm.sup.3.

11. A process according to claim 2, wherein the ratio by moles A/(AA) is at least 0.2.

12. A process according to claim 1, comprising: removing particles having particle size diameter lower than 2.5 m (PM 2.5) from the combustion fumes.

Description

EXAMPLES

Example 1

Characterization of Powders

(1) The particulate contained in combustion fumes is collected by an Andersen Mark III type impactor equipped with a pre-separator capable to remove the particles with aerodynamic diameter greater than 10 m and to separate PM 10, by using a sampling flow of 14 liters/min, and filters for granulometric fractions with aerodynamic diameter in the range 10-9 m; 9-5.8 m; 5.8-4.7 m; 4.7-3.3 m; 3.3-2.1 m; 2.1-1.1 m; 1.1-0.7 m; 0.7-0.4 m.

(2) At the end of the sampling procedure, the collected particulate fractions have been subjected to chemico-physical analysis by scanning electronic microscopy (SEM) and X-ray analysis.

(3) The particle chemical analysis has been carried out with a SEM Philips XL30 microscope, equipped with a thin window EDX system for the microanalysis by energy dispersion spectrometry, by using an automatic system capable to automatically detect the particles when a predetermined threshold is exceeded.

(4) The morphological parameters and the composition have been determined for each of the identified particles by measuring the intensities of the lines characteristic of the X-ray spectrum, then converted into the corresponding atomic concentrations.

(5) The particulate with sizes smaller than 0.4 m, that escapes from the last stage of the Andersen impactor, has been collected on mica supports for the analysis by atomic force microscope by a pneumatic actuator capable to collect, by thermophoretic effect, a sufficient and statistically significant number of particles. The gaseous current coming out from the impactor is then sent to a condensation system of the combustion steam. The condensed phase has then been subjected to spectroscopic analysis for determining the concentration of the nanometric particulate (<0.4 m).

(6) The analysis for metals is carried out by induction-plasma spectroscopy by using the ICP-OES instrument by Thermo Electron Corporation.

(7) Ashes of fuels are determined according to the ISO 1171 test. The fuels are pyrolyzed at 600 C. until constant weight.

(8) Ash melting temperature is determined according to the ASTM D 1857-87 test.

(9) The low-melting fraction of the fuel ashes is determined by using a melting pot with a hole at the bottom having a diameter of 5 mm. The fuel ash sample is heated at 1,450 K and left at this temperature for at least 2 hours. Then the weight of the melted fraction which flows from the bottom of the melting pot is determined. On said fraction metal determination is carried out.

Example 2

(10) An isothermal and flameless 5 MW combustor, operated at 1,650 K and at the pressure of 5 bar and by using oxygen as comburent at 92% by volume, in excess on the stoichiometric amount, so to have an oxygen concentration in the fumes coming out from the combustor comprised between 1% and 3% by volume. An industrial waste is also fed at a rate of 11 kg/min, constituted of a mixture of exhausted solvents, water and a solid insoluble residue in an amount of 1.4% by weight on the total waste weight.

(11) The analysis of the waste has given an incombustible ash amount equal to 1.04% by weight. By the optical ICP analysis (inductive coupled plasma: ICP-OES) it is found that the ashes are formed mainly of alumina, silica and calcium (calcium sulphate). The ashes further contain heavy metals, among which Nickel, Manganese, Cobalt, Chromium, Copper, Lead, in a total concentration by weight of 370 ppm weight.

(12) The melting temperature of the incombustible ashes is of 1,712 K.

(13) 0.5 l/min of an aqueous suspension of the following composition is fed into the combustor: 10% by weight of commercial bentonite in powder having composition about Al.sub.2O.sub.3.4SiO.sub.2, melting temperature 1,590 K, 3% by weight of commercial potassium pyrophosphate, having melting temperature 1,363 K. By optical ICP analysis the metals in component A), in the optional component bentonite and in the fuel are determined. It has been found that the molar ratio A/(AA) is equal to 0.1.

(14) Total powders in the fumes at the outlet of the combustor are found to be 0.02 mg/Nm.sup.3.

(15) After filtration of the fumes on flue filter sleeve, in the fumes emitted into the air it is found that PM 2.5 is 8 g/Nm.sup.3. It is noted that both the above values are very low.

(16) In the fumes emitted into the air the normalized concentration values of heavy metals are lower than 1 g/Nm.sup.3.

Example 3 Comparative

(17) Example 2 is repeated but omitting the addition of the aqueous suspension of bentonite and potassium pyrophosphate.

(18) It is found that in the fumes emitted into the air the particulate PM 2.5 is 3 mg/Nm.sup.3, and the heavy metal content is of 0.15 mg/Nm.sup.3.

Example 4 Comparative

(19) In a prior art thermal 6 MW combustor, operated at atmospheric pressure and using air as comburent, 13 kg/min of the same industrial waste used in example 2 are fed. The walls of the combustor are maintained at a temperature higher than 1,150 K. The fumes leave the combustion chamber at the temperature of 1.310 K.

(20) After filtration of the combustion fumes on a sleeve filter and on an electrostatic filter, the fumes emitted into the air contain an amount of powders of 9 mg/Nm.sup.3. PM 2.5 is 6 mg/Nm.sup.3, heavy metals 0.44 mg/Nm.sup.3.

(21) By comparing the data obtained in the example of the invention with those of the comparative examples it is noticed that the powders in the fumes discharged into the atmosphere in the process of the invention are much lower than those obtained in the prior art processes. The PM 2.5 is lower of two-three orders of magnitude and the heavy metal content lower of two orders of magnitude than in the comparative examples.

(22) Therefore with the process of the present invention a remarkable improvement in the reduction of emitted powders and heavy metals is obtained in comparison with the prior art processes.