Combustion process

Abstract

A combustion process wherein a fuel, a comburent and a component B), sulphur or sulphur containing compounds, are fed to the combuster in an amount to have a molar ratio B/A.sup.I0.5, wherein: B is the sum by moles between the amount of sulphur present in component B)+the amount of sulphur (component B.sup.II)) contained in the fuel, A.sup.I is the sum by moles between the amount of alkaline and/or alkaline-earth metals (component A.sup.II)) contained in the fuel+the amount of the alkaline and/or alkaline earth metals (component A)) in the form of salts and/or oxides contained in component B), being the combustor isothermal and flameless.

Claims

1. A combustion process for reducing basic ashes in combustion fumes, comprising feeding into a combustor a fuel, a comburent and a component B), wherein combustion fumes comprising SO.sub.2 are output from the combustor, wherein component B) is selected from sulfur or sulfur containing compounds, and component B) is added in an amount to have a molar ratio B/A.sup.I0.5, wherein: B is the sum by moles between the amount of sulfur present in component B)+the amount of sulfur contained in the fuel, A.sup.I is the sum by moles between the amount of alkaline and/or alkaline-earth metals contained in the fuel+the amount of the alkaline and/or alkaline-earth metals contained in component B), being the combustor is isothermal and flameless, the comburent being in molar excess with respect to the stoichiometric amount required for the combustion reaction with the fuel, and wherein the partial pressure of SO.sub.2 ranges from 40 to 300 Pa.

2. A process according to claim 1, wherein sulfur in the fuel is present under the form of elementary sulfur or of organic and/or inorganic compounds containing sulfur.

3. A process according to claim 1, wherein in the combustor the pressure is comprised between values higher than 101.3 kPa and up to 2,000 kPa, and the temperature is comprised between 1,500 K up to 2,100 K.

4. A process according to claim 1, wherein the comburent comprises oxygen having a titer of at least 70% by volume, the complement to 100% being formed of an inert gas and/or nitrogen.

5. A process according to claim 1, wherein the molar ratio B/A.sup.I is at least 1.

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

7. A process according to claim 1, wherein the feeding of component B) to the combustor is carried out by feeding the component B) separately from the fuel or in admixture with the fuel.

8. A process according to claim 1, wherein component B) is elementary sulfur and is fed to the combustor with aqueous dispersion containing surfactants.

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

10. A process according to claim 4, wherein the combustion fumes are recycled.

11. A process according to claim 10, wherein the fed oxygen is premixed with the recycled fumes, the amount of combustion fumes being higher than 10% by volume, preferably higher than 50% by volume.

12. A process according to claim 10, wherein the recycling fumes contain water in the vapor form, the water amount, calculated with respect to the total volume of the recycling fumes, being higher than 10% by volume.

13. A process according to claim 12, wherein the fuel contains an amount of water up to 80% by weight.

Description

EXAMPLES

Example 1

(1) Analytical Methods

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

(3) The basic ashes are determined as alkaline and earth-alkaline metals.

(4) Total ashes are determined as the weight residue after igniting at 600 C. according to conventional analytical procedures.

(5) Sulphur or sulphate are determined by chemical analysis.

(6) Moisture was determined according to conventional analytical procedures, for instance by using Karl Fischer instrument.

Example 2

(7) The combustor is a isothermal and flameless 5 MW combustor with walls coated by refractory, operated at 1,650 C. and 400 kPa. The comburent used is oxygen at 90% by volume and is fed 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.

(8) The fuel is olive husk having a content of sulfur, total ashes and humidity as it follows (% by weight):

(9) TABLE-US-00001 sulphur 0.1 total ashes (residue at 600 C.) 5 humidity 9

(10) Metal k, Na, Ca, Mg, calculated as % by weight on the total ashes, are in the following amounts.

(11) TABLE-US-00002 K 18.3 Na 1.1 Ca 12.8 Mg 1.2

(12) Alkaline and earth-alkaline metal represent the basic ashes.

(13) The sum of the amounts of alkaline and earth-alkaline metal constitute 33.4% by weight of the total ashes.

(14) On the total ashes, sulphate determination has been carried out. The amount found was 4.7% by weight.

(15) The comburent oxygen was fed to the combustor in an amount in excess with respect to the stoichiometric value, so to have an oxygen concentration in the fumes coming out from the combustor comprised between 1% and 3% by volume.

(16) In a reactor olive husk was admixed with water, under stirring, to form a slurry containing water in an amount equal to 60% by weight of water on the dry olive husk.

(17) In a reactor, under stirring, a water dispersion was prepared by adding a solid mixture formed by sulphur in powder with sodium alkylarylsulphonate surfactant to water.

(18) The olive husk slurry was fed to the combustor at a rate of 1,200 kg/hour, calculated on the dry olive husk.

(19) The aqueous dispersion of sulphur was fed to the combustor in an amount of 18 kg of sulfur/hour.

(20) The ratio by moles B/A.sup.I is 1.1. The combustor has worked for 480 hours.

(21) Every 8 hours about 550 kg of vitrified slags, containing the metals K, Na, Ca and Mg in the same ratios and amounts as in the ashes of the fed fuel, are discharged from the combustor.

(22) It is found that the fumes emitted into the air contain an ash amount lower than 3 mg/Nm.sup.3.

(23) At the end of the combustion process no corrosion of the refractory of the combustor walls is noticed. Furthermore, in the vapour superheater made of AISI 304H material and operating with fumes at 800 C. and at wall temperatures of 570 C., that is located in the part of the heat recovery plant downhill of the combustor, no surface modification of the wall material can be found.

Example 3 Comparative

(24) The combustor is operated as in example 2 but without feeding sulphur. The combustor is runned for 72 hours.

(25) In the fumes coming out from the combustor, SO.sub.2 is in concentrations<30 ppv, corresponding to 0.00014 bar, 14 Pa.

(26) At the end of the running period the combustor and the fume pipe at the outlet, both having the interior walls protected by refractories, are visually inspected. It is noticed that the refractory material made of high-fired bricks (high purity alumina containing 9% by weight of chromium, 6% by weight of zirconium), has been corroded on the surface. Further the slags discharged from the combustor contain a chromium and zirconium concentration higher than that present in the ashes of the fed fuel. Therefore the excess of chromium and zirconium derives from the corrosion of the refractory material of the combustor.

Example 4 Comparative

(27) The combustor is operated as in example 2. The fuel consists of scrap phenolic pitches originating from a petrochemical plant producing bisphenol A, and is fed by a melter. The pitches contain basic ashes in an amount of 0.8% by weight as sodium, and do not contain sulphur. The melted pitches are fed at a flow rate of 500 liters/hour (pitch density about 0.98 g/cm.sup.3). After 2 running hours melted slags came out at the bottom of the combustor. The plant is stopped and melted slags analyzed. They are constituted of sodium aluminate. Metal analysis has given the following results: sodium 6% by weight, chromium 8% by weight, zirconium 6% by weight. The composition aluminum/chromium/zirconium is similar to the composition of the refractory bricks of the combustor. Therefore the refractory of the combustor walls has been leached by the basic ashes contained in the fuel.

Example 5

(28) Example 4 Comparative is repeated but using phenolic pitches contain basic ashes in an amount of 0.4% by weight as sodium and not containing sulphur.

(29) The combustor temperature was of 1,550 K.

(30) The comburent used is oxygen at 90% by volume and is fed in excess on the stoichiometric amount. The process lasted 5 running days and sulphur was fed as an aqueous dispersion using alkylarylsulphonate surfactant at a rate of 5.5 Kg/hour.

(31) The molar ratio B/A.sup.I is 2/1.

(32) During the running time, at the bottom of the combustor slags are obtained. They were analyzed and shown to contain the basic ashes introduced in the combustor under the form of sodium sulphate. The plant is stopped and the combustor is inspected. It is observed that the refractory of the combustor walls does not show any corrosion.