METHOD OF REDUCING CORROSION OF A HEAT EXCHANGER OF AN INCINERATOR COMPRISING SAID HEAT EXCHANGER

Abstract

A method of reducing corrosion of a heat exchanger of an incinerator, said method comprising the steps ofintroducing oxygen-comprising gas and a particulate fuel into a combustion chamber,introducing an additive material comprising i) clay and ii) calcium carbonate into the incinerator,recuperating heat from the flue gas using a heat exchanger. For protecting the heat exchanger, the additive material is a powdery material that is introduced into the flue gas upstream of the heat exchanger, a powder particle of said powdery additive material comprising granules, each granule comprising a mixture of clay and calcium carbonate, at least 10% by weight relative to the calcium carbonate being calcium carbonate in a form that when characterized by means of Thermogravimetric Analysis under a nitrogen atmosphere with a rate of increase in temperature of 10 JC per minute has decomposed completely when a temperature of 875 C. has been reached.

Claims

1. A method of reducing corrosion of a heat exchanger (130) of an incinerator (100), said incinerator (100) comprising a chamber (110) for incinerating fuel in the presence of oxygen-comprising gas, a heat exchanger (130), and a flue gas channel (120) for passing flue gas emanating from the chamber (110) along the heat exchanger (130) for absorbing heat from the flue gas; the method comprising the steps of introducing oxygen-comprising gas and a particulate fuel into the chamber (110) to incinerate said particulate fuel resulting in a flue gas, introducing an additive material comprising i) clay and ii) calcium carbonate into the incinerator (100), recuperating heat from the flue gas using the heat exchanger (130); wherein the additive material is a powdery material that is introduced into the flue gas upstream of the heat exchanger (130), a powder particle of said powdery additive material comprising granules, each granule comprising a mixture of clay and calcium carbonate, at least 10% by weight relative to the calcium carbonate being calcium carbonate in a form that when characterized by means of Thermogravimetric Analysis under a nitrogen atmosphere with a rate of increase in temperature of 10 C. per minute has decomposed completely when a temperature of 875 C. has been reached.

2. The method according to claim 1, wherein at least 40% by weight and more preferably at least 70% relative to the calcium carbonate is calcium carbonate in a form that when characterized by means of Thermogravimetric Analysis under a nitrogen atmosphere with a rate of increase in temperature of 10 C. per minute has decomposed completely when a temperature of 875 C. has been reached.

3. The method according to claim 1 or 2, wherein the additive material is introduced in the flue gas where the flue gas has a temperature in a range from 875 C. to 1050 C., and preferably in a range from 900 C. to 1000 C.

4. The method according to any of the preceding claims, wherein the powdery additive material is introduced with a rate of at least 0.005% by mass relative to the flow of flue gas, preferably with a rate of at least 0.02% by mass and most preferably at least 0.04% by mass.

5. The method according to any of the preceding claims, wherein the incinerator (100) is part of a plant, said plant further comprising a unit for the thermal conversion of paper waste material comprising kaolin, wherein the kaolin is thermally treated in a fluidized bed having a freeboard in the presence of oxygenous gas, wherein the fluidized bed is operated at a temperature between 720 and 850 C. and the temperature of the freeboard is 850 C. or lower to result in the powdery additive material, which is introduced into the flue gas of the incinerator (100).

6. The method according to any of the preceding claims, wherein the weight/weight ratio of convertible calcium carbonate to the clay is in the range of 1 to 10, preferably 1 to 5 and more preferably 1 to 3.

7. The method according to any of the preceding claims, wherein the powdery material has a water content of less than 0.9% wt./wt. %, preferably less than 0.5% wt./wt.

8. The method according to any of the preceding claims, wherein additive-comprising material is collected from the flue gas downstream of the heat exchanger (130), and part of said particulate material is re-introduced into the flue gas upstream of the heat exchanger (130).

Description

[0043] The invention will now be illustrated with reference to the example section below, and with reference to the drawing wherein

[0044] FIG. 1 shows a schematic view of an incinerator; and

[0045] FIG. 2 shows a Thermogravimetric Analysis (TGA) graph for various calcium carbonate-comprising materials.

[0046] FIG. 1 shows a plant comprising an incinerator 100 comprising a combustion chamber 110, a flue gas channel 120, a heat exchanger 130 and an exhaust pipe 140.

[0047] A mixture of household and industry derived waste materials was fed from a fuel storage via a hopper on a grate 170. Air is introduced into the combustion chamber 110 via an air supply conduit 180.

[0048] Additive material is introduced into the flue gas channel 120 via lances 150.

[0049] Downstream of the heat exchanger, the additive material is separated from the cooled down flue gas from the heat exchanger 130 using a conventional filter system before the cleaned flue gas is vented to the atmosphere via the exhaust pipe 140.

EXPERIMENTAL SECTION

[0050] 1. Characterization of Additive Material

[0051] The following materials were used for incineration experiments, and characterised as discussed below.

[0052] Powder Size

[0053] Laser diffraction was used to measure particle size in the range of 0.1-600 m. Typically, a solid-state, diode laser is focused by an automatic alignment system through the measurement cell. Light is scattered by sample particles to a multi-element detector system including high-angle and backscatter detectors, for a full angular light intensity distribution. In a typical test, 10 mg of a sample was added to the liquid dispersing medium. The recommended dispersing medium for the samples is isopropyl alcohol. 95% by weight of the particles of the samples A to F described below had a size of less than 100 m.

[0054] Additive material suitable for use in the present invention

[0055] ACalcium carbonate-containing material produced from deinking paper sludge prepared in accordance with WO0009256.

[0056] The material's composition was determined by means of X-ray fluorescence. The material contained 30 mass % of calcium carbonate; 25 mass % of calcium oxide; and 36% of silica-alumina clay in the form of meta-kaolin.

[0057] Reference Materials:

[0058] BLaboratory grade calcium carbonate (laboratory grade calcium carbonate, Perkin Elmer Corporation, Waltham, Mass., USA)

[0059] CGround limestone (mercury sorbent, sample obtained from the Chemical Lime Company in St. Genevieve, Mo., USA)

[0060] DGround limestone (sample obtained from the Mercury Research Center at 19 Gulf Utility, Pensacola, Fla., USA)

[0061] EGround dolomite stone (sample obtained from the USA National Institute of Standards and Technology (NIST) denoted as standard reference material (SRM) 88b))

[0062] FGround limestone (sample obtained from the USA National Institute of Standards and Technology (NIST) denoted as standard reference material (SRM) 1d. SRM 1d is composed of argillaceous limestone)

[0063] Material Decomposition

[0064] TGA measurements were carried out in a nitrogen atmosphere and at a heating rate of 10 C. per minute using a Setaram Labsys EVO TGA apparatus (Setaram Company, Caluire, France).

[0065] As can be seen in FIG. 2, where the curves A-F correspond to the calcium carbonate-comprising materials listed above, the decomposition of calcium carbonate occurs at different temperatures. For curve E, it is the second steep downward slope at about 950 C. that relates to the decomposition of calcium carbonate, the first steep slope at about 800 C. relating to the decomposition of magnesium carbonate.

[0066] EDX Measurements

[0067] Individual particles of the additive material (A) produced in accordance with WO0009256 contain both clay and calcium compounds as can be observed from Energy Dispersive X-ray spectroscopy (EDX) applied in conjunction with Electron Microscopy (EM), both methods are considered known to someone skilled in the art. EDX measurements on even the smallest particulates visible in the EM, typically having dimensions of a few micrometers, show that in each particulate both calcium- and silica/alumina species are present. The calcium represents the calcium and calcium carbonates present in the additive material, whereas the silica/alumina species represent the clay fraction present in the additive material.

[0068] 2. Incineration Experiment

[0069] Experiments were performed using the incinerator 100 substantially as shown in FIG. 1.

[0070] The incinerator processed an averaged amount of fuel of 4.2 kg/s consisting of a mixture of household and industrial derived waste materials. The incineration resulted in an averaged flue gas flow of 30.5 kg/s. The additive applied in this example was produced from a mixture of paper residue and composted sewage sludge in a weight ratio of 85% to 15%, using the method descried in WO9606057. The additive is injected into the flue gas of the incinerator leaving the incineration chamber at a height of 19 meters measured from the lowest point of the incineration grate. During the experiment it was observed that no flames reached this height for more than 90% of the duration of the experiment. The first heat exchanger internalboiler tubeprotruding into the flue gas flow, is located at more than 30 meters downstream of the additive injection location. The temperature of the flue gas at the location of the additive injection varied with the particulate fuel and the energy production in the incinerator, being between 950 and 1050 C. Typically 0.02 kg/s of additive was injected into the flue gas by means of pneumatic injection through four steel injection lances (right-pointing arrow in FIG. 1) of 32 mm internal diameter, resulting in a ratio of additive to flue gas of 0.06-0.07% wt./wt. The averaged velocity of the injection air was 15 m/s. Injection of the additive was continued for nine months in a full calendar year of operating the incinerator. After this one year period, the incineration was stopped for regular maintenance during which stop the boiler tubes were inspected for corrosion. The decay of the thickness of the walls of the heat exchanger boiler tubes was used as indication of corrosion, because the thickness of the walls of these tubes is what determines the longevity of these tubes for their duty in the heat exchanger, as well as the risk of boiler tube failure during operation. Boiler tube wall thickness measurements were carried out by means of ultrasonic measurement on a multitude of individual boiler tubes, resulting in several hundreds of wall thickness measurements on tubes located in variousdocumentedlocations of the incinerator heat exchanging section. Comparison of these wall thickness measurements to those carried out in previous years at the same locations, was carried out by expressing the measured decay of wall thickness on a per million ton of processed fuel basis (mm decay per million tons), thus correcting for non-equal intervals between wall thickness measurements in different years. Comparison of wall thickness decay of boiler tubes in the year in which the additive was applied for a period of nine months to the observed wall thickness decay of boiler tubes in the two preceding years indicated that at hot flue gas sections with boiler tube wall temperatures of 600 C., the decay of wall thickness was reduced by over 60%. The decay in slightly cooler sections with boiler tube wall temperatures of 500 C. was reduced by over 40%. Both results demonstrate a significant reduction in high-temperature corrosion when applying the additive. Application of the additive in consecutive years with additive injection applied during the entire year resulted in a further decrease of high-temperature corrosion, as witnessed from an almost unmeasurable decay of wall thickness of boiler tubes.

[0071] It was further observed that deposits of partially molten materials originating from the fuel on the heat exchanger boiler tubes had become more brittle displaying a reduced degree of melting of these deposits.

[0072] The above data suggest that introducing the additive as specified in the appended main claim is also suitable for an equivalent method for the gasification of a material, and in particular a method of operating a gasifier, said gasifier comprising [0073] a chamber for gasifying fuel in the presence of oxygen-comprising gas by incomplete conversion of the fuel, [0074] a heat exchanger, and [0075] a flue gas channel for passing flue gas emanating from the chamber along the heat exchanger for absorbing heat from the flue gas;

[0076] the method comprising the steps of [0077] introducing oxygen-comprising gas and a particulate fuel into the chamber for gasifying the particular fuel resulting in gas containing at least 5% by vol. of CO and typically more than 10% by vol., [0078] introducing an additive material comprising i) clay and ii) calcium carbonate into the gasifier, [0079] recuperating heat from the flue gas using the heat exchanger;

[0080] characterized in that the additive material is a powdery material that is introduced into the flue gas upstream of the heat exchanger, a powder particle of said powdery additive material comprising granules, each granule comprising a mixture of clay and calcium carbonate, at least 10% by weight relative to the calcium carbonate being calcium carbonate in a form that when characterized by means of Thermogravimetric Analysis under a nitrogen atmosphere with a rate of increase in temperature of 10 C. per minute has decomposed completely when a temperature of 875 C. has been reached.

[0081] Preferably, the additive material will be added to the flue gas at a flue gas temperature of less than 1200 C.

[0082] Preferred embodiments correspond to the dependent claims of the method of incinerating listed below.