Method for operating a fluidized bed boiler

Abstract

The invention relates to a method for operating a fluidized bed boiler, comprising: a) setting the ratio of secondary oxygen containing gas to primary oxygen containing fluidizing gas to a value ranging from 0.0 to 0.8; b) carrying out the combustion of fuel with a fluidized bed comprising ilmenite particle; and to a fluidized bed boiler.

Claims

1. A method for operating a fluidized bed boiler, comprising: setting the ratio of secondary oxygen containing gas to primary oxygen containing fluidizing gas to a value ranging from 0.0 to 0.4, wherein the primary oxygen containing fluidizing gas is provided from below the fluidized bed, and wherein the secondary oxygen containing gas comprises all oxygen containing gas fed into the boiler for the combustion of fuel that is not primary fluidizing gas; carrying out the combustion of fuel with a fluidized bed comprising ilmenite particles; changing the fuel load; and adjusting the ratio of secondary oxygen containing gas to primary oxygen containing fluidizing gas in response to a change in the fuel load, wherein the amount of primary oxygen containing fluidizing gas is increased while the amount of secondary oxygen containing gas is decreased such that the total amount of oxygen containing gas that is fed into the furnace remains essentially constant.

2. The method of claim 1, wherein the fuel comprises biomass and/or waste.

3. The method of any one of claim 1, further comprising supplying oxygen to the furnace in an amount to keep the oxygen concentration in the flue gas above a lower value of 0.8 vol. % and below an upper value of 5.0 vol. %.

4. The method of claim 3, characterized by one of the following features: the fuel comprises coal and oxygen is supplied to the furnace in an amount to keep the oxygen concentration in the flue gas above a lower value of 0.8 vol. % and below an upper value of 2.5 vol. %; the fuel comprises biomass and oxygen is supplied to the furnace in an amount to keep the oxygen concentration in the flue gas above a lower value of 1.0 vol. % and below an upper value of 3.5 vol. %; the fuel comprises waste-based fuel and oxygen is supplied to the furnace in an amount to keep the oxygen concentration in the flue gas above a lower value of 2.5 vol. % and below an upper value of 5.0 vol. %.

5. The method of claim 1, wherein the ratio of secondary oxygen containing gas to primary oxygen containing fluidizing gas is lowered in response to a reduction in the fuel load; and/or the ratio of secondary oxygen containing gas to primary oxygen containing fluidizing gas is raised in response to an increase in the fuel load.

6. The method of claim 1, wherein the ratio of secondary oxygen containing gas to primary oxygen containing fluidizing gas is set to zero in response to a reduction in the fuel load.

7. The method of claim 1, wherein the fuel load is reduced by 10% to 70% and/or wherein the fuel load is increased by 10% to 300%; and/or wherein the fuel is biomass; and/or wherein the boiler is operated to generate heat and power.

8. The method of claim 1, characterized in that the ilmenite particles make up 10 wt. % to 100 wt. % of the bed material.

9. The method of claim 1, wherein the boiler is a bubbling fluidized bed (BFB) boiler and wherein the method is characterized by one or more of the following features: the fuel comprises waste and the ratio of secondary oxygen containing gas to primary oxygen containing fluidizing gas is set to a value ranging from 0.0 to 0.3; the ilmenite particles have an average particle size between 0.1 mm and 1.8 mm; wherein the average particle size of the ilmenite particles is 0.2 mm to 0.6 mm; and/or the ilmenite particles have a particle size in the range from 0.1 mm to 1.8 mm.

10. The method of claim 1, wherein the boiler is a circulating fluidized bed (CFB) boiler and wherein the method is characterized by one or more of the following features: the fuel comprises biomass and/or waste and the ratio of secondary oxygen containing gas to primary oxygen containing fluidizing gas is set to a value ranging from 0.0 to 0.4; the ilmenite particles have an average particle size between 50 μm and 400 μm.

11. The method of claim 1, wherein the oxygen containing gas is air.

Description

(1) It is shown in

(2) FIG. 1: a schematic drawing of the 12 MW.sub.th CFB boiler used for CFB experiments;

(3) FIG. 2: the mass flow of primary and secondary air and the oxygen concentration in the flue gases vs. time during operation in a 12 MW.sub.th CFB boiler;

(4) FIG. 3: the concentration of carbon monoxide and oxygen in the flue gases vs. time during operation in a 12 MW.sub.th CFB boiler;

(5) FIG. 4: carbon monoxide concentrations for rock ilmenite and silica sand as bed material during dynamic changes of the air to fuel ratio in a CFB boiler;

(6) FIG. 5: CO and CO.sub.2 concentration versus fluidization velocity in a BFB reactor with ilmenite and silica sand as bed material;

(7) FIG. 6: CO and CO.sub.2 concentration versus fuel load in a BFB reactor with ilmenite and silica sand as bed material;

(8) FIG. 7: a schematic drawing of a CFB boiler.

EXAMPLE 1

(9) CFB Boiler Operation

(10) By way of example, FIG. 7 shows a typical CFB boiler. The reference numerals denote: 1 Fuel Bunker 2 Fuel Chute 3 Primary Combustion Air Fan 4 Nozzle Bottom 5 Primary Air Distributor 6 Secondary Air Ports 7 Fluidized Bed 8 Furnace 9 Cyclone 10 Loop seal 11 Immersed Superheater 12 Return Leg 13 Heat Exchangers 14 Flue Gas Treatment Plant 15 Flue Gas Recirculation Fan 16 Stack

(11) 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 means for fuel feeding (not shown) are for example screw feeders and rotary valves, not excluding others. 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) can be 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 (upper part of the furnace).

(12) The CFB boiler can be operated using the inventive method, by a) setting the ratio of secondary oxygen containing gas to primary oxygen containing fluidizing gas to a value ranging from 0.0 to 0.8; b) carrying out the combustion of fuel with a fluidized bed comprising ilmenite particles.

EXAMPLE 2

(13) Operating the Chalmers Boiler without Secondary Air Feeding

(14) The Chalmers 12 MWth CFB-boiler setup is shown in FIG. 1, wherein reference numerals indicate: 10 furnace 11 fuel feeding (furnace) 12 wind box 13 cyclone 14 convection path 15 secondary cyclone 16 textile filter 17 fluegas fan 18 particle distributor 19 particle cooler 20 gasifier 21 particle seal 1 22 particle seal 2 23 fuel feeding (gasifier) 24 fuel hopper (gasifier) 25 hopper 26 fuel hopper 1 27 fuel hopper 2 28 fuel hopper 3 29 sludge pump 30 hopper 31 ash removal 32 measurement ports

(15) The boiler is operated using rock ilmenite as bed material with only primary air for more than 500 minutes during dynamic changes in the air-to-fuel-ratio as indicated by the oxygen content in the flue gas. This experiment is initiated during ordinary CFB air feeding conditions, i.e. both primary and secondary air is fed to the furnace. FIG. 2 shows the mass flows of primary and secondary air and the oxygen concentration in the flue gases during 600 minutes of operation using wood-chips as fuel. The O2-concentration in the flue gases is measured by two separate standard online gas analysis instruments, using paramagnetic sensors.

(16) During the start of this experiment, the air for the fuel spreader is turned off and the secondary/primary air ratio is around 0.24 and the boiler is operated slightly below 4 vol. % of oxygen (O.sub.2) in the flue gases, as can be seen from FIG. 2. After around 50 minutes of operation the secondary air is reduced in two steps until the valve for secondary air feeding is closed. The changes are clearly shown in FIG. 2, where the primary air increases with equal amount as the removed flow of total secondary air. In this experiment the mass flow of primary air should be kept the same as during the start of the experiment to yield the same gas velocity and bed material circulation. This is done by decreasing the total air and compensating the fuel feeding to reach the same O.sub.2 concentration in the flue gases as in the start of the experiment. These settings are kept for around 20 minutes before any changes are made.

(17) FIG. 3 shows the concentrations of CO and O.sub.2 during the experiment. As can be seen no carbon monoxide (CO) is detected at the O.sub.2 concentration of slightly below 4 vol. % even though no secondary air is fed to the boiler (i.e., the secondary air to primary air ratio is 0.0). From FIGS. 2 and 3 it can be seen that the O.sub.2 in the flue gases can be lowered to around 2 vol. % without any constant CO concentrations detected in the flue gases. This proves that the oxygen-carrying properties of ilmenite are sufficient to enable the total removal of secondary air even at a lower air-to-fuel-ratio. It should be noted that non oxygen-carrying bed material such as e.g. silica-sand, does not allow operation at such low oxygen concentrations even if the operation is conducted with both primary and secondary air.

EXAMPLE 3

(18) FIG. 4 shows the result from a similar experiment where the air-to-fuel-ratio has been dynamically changed during operation with solely rock ilmenite and during operation with solely silica-sand as bed material in the Chalmers CFB-boiler. During this experiment, both primary and secondary air is fed to the boiler. As can be seen from FIG. 3 when silica-sand is used as bed material there is CO present already when the O.sub.2 in the flue gases is around 3 vol. % and at 2.5 vol. % of O.sub.2 the CO concentration is already higher than the restrictions for normal CO emissions in this boiler. This can be compared with the rock ilmenite operation, where the restrictions for CO emissions is violated first at around 1.3 vol. % O.sub.2 in the flue gases.

(19) This shows that even though the boiler operation with silica sand as bed material is conducted with both primary and secondary air, silica-sand as bed material does not allow the boiler to be run at the low O.sub.2 concentrations in the flue gas that can be reached with ilmenite as bed material and no secondary air feeding, as shown in Example 1.

EXAMPLE 4

(20) 1) Setup Used for BFB Experiments

(21) A 2-4 MW.sub.th gasifier system at Chalmers University of Technology was used for BFB combustion experiments with ilmenite. It is of the type indirect gasification. In this technique, the actual gasification reactions are separated from the combustion reactions and the heat needed for the endothermic gasification reactions is supplied by a hot circulating bed material. The bubbling fluidized bed gasifier is connected to the 12 MW.sub.th circulating fluidized bed boiler and the two reactors are communicating via the bed material, see FIG. 1. Fuel is fed on top of the bed in the gasifier and the gasifier is fluidized with pure steam. Usually the system is operated with silica-sand and the gasifier is operated in the temperature interval of 750-830° C. FIG. 1 shows the boiler and gasifier setup.

(22) 2) Ilmenite Operation in the Gasifier

(23) Variations in Fluidization Velocity at Constant Fuel Feed

(24) With the aim of investigating gas/solid contact between the volatiles and the bed material, the gasifier was operated with 100 wt. % of ilmenite with an average particle size of 0.14 mm as bed material for a few days. The first experiment was conducted at four different steam flows yielding a variety in gas velocities: 0.13, 0.19, 0.25 and 0.28 m/s, which corresponds to 5, 7, 9 and 11 times the minimum fluidization velocity of the ilmenite fraction. During this experiment the gasifier was continuously fed with 300 kg of fuel (wood-pellets) per hour and the bed temperature was kept at 820-830° C. FIG. 5 shows the analyzed gas components CO.sub.2 and CO in the outlet of the gasifier during ilmenite operation. Data for ordinary silica-sand during normal gasification conditions (Ref, sand, marker color red) has been added in the figure for comparison with the ilmenite. As can be seen in FIG. 5, the CO concentration is clearly decreased and the CO.sub.2 concentration is increased by almost a factor 4 when ilmenite is used in comparison to the silica-sand operation. As the gasifier is fluidized with pure steam all the extra oxygen supplied for the increased oxidation of hydrocarbons and CO is coupled to the oxygen-carrying properties of the ilmenite. This further shows the oxygen buffering effects that ilmenite possesses and the ability to transport oxygen from oxygen rich to oxygen depleted zones during fuel conversion. The fluidization conditions and gas solid contact in the gasifier can be compared to the conditions in a BFB-boiler and it is therefore likely that ilmenite will contribute with increasing oxygen transport also in a BFB boiler.

(25) Variation in Fuel Feed During Constant Fluidization Velocity

(26) The second experiment was conducted during a constant steam flow of 200 kg/h (yielding a gas velocity of 0.19 m/s, corresponding to 7 times the minimum fluidization velocity) and a variation in fuel feed: 200, 300 and 400 kg.sub.fuel/hour (wood pellets). FIG. 6 shows the measured gas concentrations of CO and CO.sub.2 in the outlet of the gasifier. The trend is very similar to the one in FIG. 5, a clearly decreasing CO concentration as a function of the oxygen transport via the ilmenite. The CO.sub.2 concentration also reveals that hydrocarbons are combusted and not only CO is oxidized. This result shows that even though the fuel feed is increased from 200 to 400 kg/h there is still oxygen enough to support the oxidation of CO and hydrocarbons.

(27) During combustion in a fluidized bed boiler, air is usually supplied both as primary air via nozzles below the bed and as secondary air in the freeboard of the furnace. The experiments in the gasifier show that a high fuel conversion can be achieved via the buffered oxygen in the ilmenite bed, i.e. without any addition of air at all. This means that a high degree of oxidation of the volatiles is conducted already in/or close to the bed and suggests the operation of a BFB boiler with less or no secondary air.

(28) The preliminary tests indicate that an excess air ratio of 1.23 or less can be achieved for waste. It is suggested that an excess air ratio of 1.19 or less can be achieved for biomass fuel.