USE OF PRE-OXIDIZED ILMENITE IN FLUIDIZED BED BOILERS

Abstract

The invention relates to a method for starting up a fluidized bed boiler, such as a circulating fluidized bed (CFB) or a bubbling fluidized bed (BFB) boiler, for operation with a predetermined concentration of ilmenite particles in the bed material. The invention also relates to a method for pre-oxidizing ilmenite, to pre-oxidized ilmenite and to the use of pre-oxidized ilmenite in a fluidized bed boiler.

Claims

1. A method comprising providing a predetermined concentration of ilmenite particles in a bed material in a fluidized bed boiler, and starting up the boiler for operation, characterized in that the predetermined concentration of ilmenite particles in the bed material comprises pre-oxidized ilmenite particles.

2. The method of claim 1, comprising: providing pre-oxidized ilmenite particles to the boiler; and/or providing fresh ilmenite particles to the boiler and pre-oxidizing said fresh ilmenite particles in the boiler.

3. The method of claim 1, further comprising the steps of a) providing bed material to the boiler; b) preheating the bed material; c) monitoring temperature in the bed; d) after the temperature in the bed has reached a predetermined fuel feeding temperature, batch-feeding fuel until ignition is achieved; e) after ignition is achieved, starting continuous feeding of fuel and increasing the fuel feeding rate until a predetermined operating temperature in the bed is reached.

4. The method of claim 3, characterized by one or more of the following features: the bed material provided in step a) comprises an inert bed material, preferably silica sand; the predetermined fuel feeding temperature in the bed is between 500° C. and 900° C., preferably between 500° C. and 600° C., preferably between 530° C. and 580° C., more preferably around 550° C.; the predetermined operating temperature in the bed is between 750° C. and 950° C., preferably be-tween 800° C. and 900° C.; more preferably between 850° C. and 900° C.

5. The method of claim 2, wherein: the pre-oxidized ilmenite particles are provided to the boiler before the bed material is preheated, preferably at the predetermined concentration of ilmenite particles in the bed material; and/or the fresh ilmenite particles are provided to the boiler after the predetermined operating temperature in the bed is reached.

6. The method of claim 2, comprising gradually providing the fresh ilmenite particles to the boiler, preferably at a rate to keep the temperature in the bed essentially constant.

7. The method of claim 3, comprising replacing bed material provided in step a) with ilmenite particles until the predetermined concentration of ilmenite particles in the bed material is reached, preferably by coordinating the feeding rate of ilmenite particles and the rate of bottom bed ash removal.

8. The method of claim 1, characterized by one or more of the following features: the predetermined concentration of ilmenite particles in the bed material is at least 10 wt. %, preferably at least 20 wt. %, further preferably at least 30 wt. %, further preferably at least 40 wt. %, further preferably at least 50 wt. %, further preferably at least 60 wt. %, further preferably at least 70 wt. %, further preferably at least 80 wt. %, further preferably at least 90 wt. %, most preferably 100 wt. % of the weight of the bed material; the fluidized bed boiler is selected from the group consisting of bubbling fluidized bed boilers and circulating fluidized bed boilers; the ilmenite particles are selected from the group consisting of rock ilmenite and sand ilmenite, preferably the ilmenite particles are rock ilmenite particles.

9. A method for producing pre-oxidized ilmenite particles inside a furnace, comprising the steps of: a) heating the furnace to a predetermined temperature; b) maintaining an oxidizing environment inside the furnace; c) continuously feeding fresh ilmenite particles to the furnace; d) pre-oxidizing the ilmenite particles by subjecting the ilmenite particles to the oxidizing environment inside the furnace at the predetermined temperature; e) continuously removing pre-oxidized ilmenite particles from the furnace.

10. The method of claim 9, wherein the predetermined temperature is between 500° C. and 1000° C., preferably between 700° C. and 950° C., more preferably between 750° C. and 900° C., most preferably between 800° C. and 900° C.; and/or the oxidizing environment inside the furnace is maintained by feeding oxygen containing gas into the furnace, wherein preferably the oxygen containing gas is air.

11. The method of claim 9, further comprising agitating the ilmenite particles inside the furnace, preferably by stirring, rotation or by passing a gas stream through the ilmenite particles; and/or monitoring a temperature inside the furnace and further preferably adjusting the feeding rate of fresh ilmenite particles to the furnace and/or the removal rate of pre-oxidized ilmenite particles from the furnace to keep the temperature in the furnace essentially constant.

12. The method of claim 9, characterized by one or more of the following features: the ilmenite particles are subjected to the oxidizing environment for: i) not more than 12 hours, preferably not more than 10 hours, further preferably not more than 8 hours, further preferably not more than 5 hours, further preferably not more than 3 hours, further preferably not more than 2 hours, most preferably not more than 60 minutes; and/or ii) at least 5 minutes, preferably at least 10 minutes, more preferably at least 20 minutes, most preferably at least 30 minutes; the concentration of oxygen in the oxygen containing gas is between 0.5 vol. % and 30 vol. %, preferably between 2 vol. % and 21 vol. %, further preferably between 2 vol. % and 10 vol. %, more preferably between 3 vol. % and 9 vol. %, most preferably between 3 vol. % and 8 vol. %; the ilmenite particles are selected from the group consisting of rock ilmenite and sand ilmenite, preferably the ilmenite particles are rock ilmenite particles; the temperature inside the furnace is monitored and preferably the feeding rate of fresh ilmenite particles to the furnace and/or the removal rate of pre-oxidized ilmenite particles is adjusted to keep the temperature in the furnace essentially constant.

13. The method of claim 9, wherein: the method is carried out using a fluidized bed boiler, preferably a bubbling fluidized bed boiler or a circulating fluidized bed boiler; or the method is carried out using a rotary kiln.

14. Pre-oxidized ilmenite particles, obtainable by the method of claim 9.

15. Use of pre-oxidized ilmenite particles, preferably as defined in claim 14, as bed material in a fluidized bed boiler, preferably a bubbling fluidized bed boiler, more preferably a circulating fluidized bed boiler.

Description

[0060] In the following, advantageous embodiments will be explained by way of example.

[0061] It is shown in

[0062] FIG. 1: a schematic drawing of a CFB boiler;

[0063] FIG. 2: a schematic drawing of the 12 MW.sub.th CFB boiler used for CFB experiments;

[0064] FIG. 3: temperature profile in the bottom bed and mass flow of fuel fed during the startup sequence using fresh rock ilmenite in the Chalmers 12 MW.sub.th CFB boiler;

[0065] FIG. 4: bottom bed temperature and top temperature as a function of operating time during the startup sequence using fresh rock ilmenite in the Chalmers 12 MW.sub.th CFB boiler;

[0066] FIG. 5: bottom bed temperature and top temperature as a function of operating time during silica sand operation and during the pre-oxidation procedure for ilmenite in a commercially fired CFB boiler;

[0067] FIG. 6: boiler load as a function of operating time during silica sand operation and during the pre-oxidation procedure for ilmenite in a commercially fired CFB boiler.

COMPARATIVE EXAMPLE

Normal Startup Sequence for Fluidized Bed Boilers

[0068] The normal startup procedure for fluidized bed combustors is composed for operation with silica-sand as bed material. This procedure is initiated by preheating the primary air which is used for the fluidization of the bed via a start burner which is placed in the wind box. The heated air is flowing through the bottom nozzles and into the silica-sand bed, heat is accumulated in the bed and the temperature of the bed is monitored. When the bed temperature reaches around 550° C. a batch of fuel is injected, usually by starting the fuel feeding system with a pulse. The sequence of feeding fuel batch-wise is usually carried out until a so called ignition is achieved. The ignition is usually reached when the temperature starts to increase more rapidly in the bed in contrast to when only the start burner is used for heating, which generates a smoother temperature profile. The start burner is turned off and the fuel feeding is put into continuous feeding mode with increasing mass flow of fuel until the normal operating temperature in the bed is reached, which may be around 850-900° C.

Example 1

[0069] Startup of the Chalmers Boiler using Ilmenite as Bed Material

[0070] The Chalmers 12 MW.sub.th CFB-boiler is shown in FIG. 2. Reference numerals denote:

[0071] 10 furnace

[0072] 11 fuel feeding (furnace)

[0073] 12 wind box

[0074] 13 cyclone

[0075] 14 convection path

[0076] 15 secondary cyclone

[0077] 16 textile filter

[0078] 17 fluegas fan

[0079] 18 particle distributor

[0080] 19 particle cooler

[0081] 20 asifier

[0082] 21 particle seal 1

[0083] 22 particle seal 2

[0084] 23 fuel feeding (gasifier)

[0085] 24 fuel hopper (gasifier)

[0086] 25 hopper

[0087] 26 fuel hopper 1

[0088] 27 fuel hopper 2

[0089] 28 fuel hopper 3

[0090] 29 sludge pump

[0091] 30 hopper

[0092] 31 ash removal

[0093] 32 measurement ports

[0094] The influence of the bed temperature during oxidation of fresh rock ilmenite is illustrated in FIG. 3. FIG. 3 shows the temperature profile in the bottom bed and the amount of fuel fed from the startup sequence until normal operation is reached in the Chalmers 12 MW.sub.th CFB-boiler using fresh rock ilmenite as bed material. The temperature in the bed is slowly increased by the preheated primary air stream, similar to ordinary silica-sand startup (1). When the bed temperature of 550° C. is reached (2), a very small amount of fuel is fed to the furnace. The bed temperature is starting to increase more rapidly and the start burner is turned off and as the temperature in this case is quickly increasing further the fuel is also completely turned off (3). It is not a normal procedure to turn off the fuel feeding at this time, usually there is a need for increasing the fuel feed to reach a higher bed temperature. However, as can be seen in FIG. 3, the temperature is increasing drastically even though the fuel is turned off. At this stage, recirculated flue gases are fed to the bottom nozzles to cool the bed. Without wishing to be bound by theory, it is contemplated that this phenomenon of temperature increase is strongly coupled to the exothermic oxidation of the fresh rock ilmenite bed (cf. Eq. 1). After a while, the temperature drops 150° C., the fuel feeding is restarted (4), however, this time the temperature decreases too much and no ignition is achieved and the start burner is restarted. At around 600° C., the fuel feeding is restarted and ignition is reached and the temperature starts to increase (5). This time the fuel feeding has to be increased continuously to reach the operating temperature of the bed. The second startup clearly follows the normal startup procedure for ordinary silica-sand. Without wishing to be bound by theory, the conclusion is that in this case the pre-oxidation happened during the first startup-sequence. During the second start-up attempt the ilmenite particles were already pre-oxidized, which is why the usual startup sequence could be followed, leading to the conclusion that if the ilmenite is pre-oxidized the exothermic oxidation can be avoided. At around 400 minutes after the first startup trial the boiler is running under normal temperature and fuel conditions (6).

[0095] The oxidation of the rock ilmenite resulting in a local heat release can be seen in FIG. 4, where the bottom bed temperature and top temperature of the boiler is shown as a function of operating minutes. There is a drastic temperature increase within the bed whereas the temperature above the bed is only increasing moderately. This indicates that the oxidation of the fresh rock ilmenite occurs locally in the bed leading to a very rapid temperature increase. During this part of the startup procedure for a CFB boiler, the bed is a stationary bubbling bed where there are usually few or commonly no heat transferring surfaces present. These data indicate that the oxidation has to be controlled to enable a safe startup of a fluidized bed boiler when rock ilmenite is used as bed material.

Example 2

Pre-oxidation by Gradual Feeding of Rock Ilmenite During Startup

[0096] A safe startup procedure for using rock ilmenite in fluidized bed boilers has been developed and tested in a commercially fired boiler. This procedure is based on a gradual increase of the rock ilmenite concentration in the boiler, so that the exothermic oxidation reaction and the resulting heat formation can be controlled. The 75 MW.sub.th CFB boiler used for the test is equipped with two storing silos (one for silica-sand and one for rock ilmenite) and separate lines for introducing the bed materials to the boiler. This setup allows the feeding of two different bed materials independent of each other. The startup procedure of the boiler is initiated with 100 wt. % of the ordinarily used silica-sand as bed material. This means that the boiler is first started according to the sequence in Comparative example 1. When a stable operating temperature is achieved, a continuous mass flow of rock ilmenite is initiated to the boiler using the second feeding line. FIG. 5 shows the temperature profile in the bottom bed and in the top of the boiler during operation with solely silica-sand and during operation with gradual increase of ilmenite. As can be seen in FIG. 5 there is no clear changes in either bottom bed or top temperatures when the ilmenite is introduced, with the exception at around 16:00. This is due to a standard procedure for water sooting of the convection path and the boiler load is reduced by the operators. This can also be seen in FIG. 6 where the boiler load is plotted as a function of the operating time. This shows in comparison to the operation with solely ilmenite in the CTH-boiler that the temperature in the bottom bed can be controlled when using this pre-oxidizing method during the startup of the boiler. The mass flow of rock ilmenite to reach safe operation is site dependent and is in this procedure calculated according to the boiler dimensions, fluid dynamics, boiler bed pressure and heat transferring surfaces. The mass flow of rock ilmenite can also be adjusted so that the operating temperature in the bed is kept essentially constant. The rock ilmenite concentration in the bed can be increased up to 100 wt. % by compensating the feeding rate of rock ilmenite with the bottom bed ash removal system.

Example 3

Start-Up Using Pre-Oxidized Ilmenite

[0097] Pre-oxidized ilmenite, for example pre-oxidized rock ilmenite, is provided as the sole bed material to a conventional CFB boiler as shown in FIG. 1. Then the bed particles are preheated, for example by an overbed burner or by preheating the primary air via a start burner which is placed in the wind box. The heated air is flowing through the bottom nozzles and into the ilmenite bed, heat is accumulated in the bed and the temperature of the bed is monitored by means of shielded thermocouples installed in the bed. When the bed temperature reaches around 550° C. a batch of biomass fuel is injected by starting the fuel feeding system with a pulse. The sequence of feeding fuel batch-wise is carried out until ignition is achieved. Then the start burner is turned off and the fuel feeding is put into continuous feeding mode with increasing mass flow of fuel until the normal operating temperature in the bed is reached, which in this case is selected to be around 850-900° C.

Example 4

Pre-Oxidation of Ilmenite Using a Rotary Kiln

[0098] A rotary kiln is put into operation and the furnace is heated to a predetermined temperature of 800-900° C. in the reaction zone of the kiln during air excess. Air is continued to be supplied to maintain an oxidizing environment inside the furnace. Fresh ilmenite particles, for example rock ilmenite particles, are continuously fed from one side of the furnace and subjected to the oxidizing atmosphere inside the furnace. Pre-oxidized ilmenite particles are continuously removed from the other side of the furnace. The speed of rotation is adjusted to allow for a residence time of the ilmenite particles in the furnace of 1 to 2 hours.

Example 5

Pre-Oxidation of Ilmenite Using a CFB Boiler

[0099] By way of example, FIG. 1 shows a typical CFB boiler, which can be used for the production of pre-oxidized ilmenite particles. The reference numerals denote:

[0100] 1 Fuel Bunker

[0101] 2 Fuel Chute

[0102] 3 Primary Combustion Air Fan

[0103] 4 Nozzle Bottom

[0104] 5 Primary Air Distributor

[0105] 6 Secondary Air Ports

[0106] 7 Fluidized Bed

[0107] 8 Furnace

[0108] 9 Cyclone

[0109] 10 Loop seal

[0110] 11 Immersed Superheater

[0111] 12 Return Leg

[0112] 13 Heat Exchangers

[0113] 14 Flue Gas Treatment Plant

[0114] 15 Flue Gas Recirculation Fan

[0115] 16 Stack

[0116] During normal operation, fuel is stored in the fuel bunker (1) and can be fed to the furnace (8) via a fuel chute (2). Alternative methods, such as pneumatic feeding and screw feeding (not shown) can also be used. The fluidization gas, in this case for example air, is fed to the furnace (8) as primary combustion air via the primary air distributor (5) from below the bed. Entrained particles are carried away by the fluidization gas stream and are then separated from the gas stream using a cyclone (9) and circulated back into the furnace (8) via a loop seal (10). Additional combustion air (so called secondary air) is fed into the furnace to enhance the mixing of oxygen and fuel. To this end, secondary air ports (6) are located throughout the furnace, in particular the freeboard (the part of the furnace above the dense bottom bed).

[0117] The CFB boiler can be utilized for producing pre-oxidized ilmenite particles. To this end, the boiler is started up and the furnace is heated to a predetermined operating temperature (800° C. to 900° C.). An oxidizing environment is maintained inside the furnace (8) by feeding of oxygen-containing gas (in this case for example air) via the primary air distributor (5) and preferably also the secondary air ports (6). When the operating temperature is reached, the continuous feeding of fresh ilmenite particles, preferably fresh rock ilmenite particles, is started via the fuel chute (2). The ilmenite particles are pre-oxidized by subjecting them to the oxidizing environment inside the furnace (8) at the predetermined temperature and pre-oxidized particles are continuously removed from the bottom of the boiler using the ordinary screw feeders for bottom ash removal (not shown).