FI86767B - Procedure for combustion of fuel in a fluidized bed, and fluidized bed arrangement for carrying out the procedure - Google Patents

Description

1 86767

A method of burning fuel in a fluidized bed and a fluidized bed apparatus for performing the method

Fluidized bed boilers that burn high sulfur coal are known for their type. These boilers operate with a conventional bubbling technique, in which the fluidized bed operates at a surface speed of 1.21-3.66 m / s and in which the mattress consists of particles with an average diameter of approximately 1000 μτη. The coal is burned in a bubbling mattress and limestone or dolomite sorbent is added to reduce the emission of sulfur dioxide. The grain size of the sorbent to be added is 1000-2000 μπι and the mattress consists mainly of coal ash, spent sorbent and partly used sorbent, and partly burnt fuel particles. The bubbling mattress has tubes to transfer heat to the steam. Pipes are also installed above the mattress in the free chamber to transfer heat from the combustion gases and thus cool these. In use, the mattress slurries fine particles such as coal, ash and partially used sorbent. Many of these particles are captured in a recycle cyclone that follows the heat exchanger. These particles return to the mattress to burn the fuel particles and promote further absorption of unused sorbent sulfur dioxide. Very fine particles escape from the recycle cyclone and remain in the filter system. The flow rate in recycling corresponds approximately to the total flow rate of solid fuels and sorbent fed to the boiler.

Conventional fluidized bed boilers have numerous disadvantages. One disadvantage is the low fuel efficiency, about 97%, because small unburned fuel particles escape from their combustion systems. This problem would be greatly exacerbated if an attempt were made to use the boiler to burn fuel with low volatile components, such as petroleum coke with 90% non-volatile carbon. Compare with coal with 42% 2 86767 non-volatile carbon. Another disadvantage is the poor utilization of the sorbent. The molar ratio of calcium to sulfur should be at least 3: 1 in order to achieve a 90% reduction in sulfur dioxide, which corresponds to standard air pollution regulations. This is due to the fact that relatively large sorbent particles, when absorbing sulfur dioxide on their surfaces, leave the inner part of the material largely unused. The third disadvantage is that these boilers emit polluting nitrogen dioxide, which is generated from the nitrogen contained in the fuel. In many parts of the country, nitrogen oxide emissions do not exceed local regulations, but in some areas, such as Southern California, they do. U.S. Patent No. 4,177,741 proposes to increase the efficiency of conventional fluidized bed boilers by compressing the recycled powder before it is returned to the bubbling mattress. This prevents the powder from flying out of the mattress and promotes its combustion in the mattress. U.S. Patent No. 4,259,911 discloses compressing coal powder and recycled material prior to injection into a mattress. U.S. Patent No. 4,329,234 proposes to improve the utilization of sorbent to remove part of the fluidized bed and to grind the sorbent particles to a diameter of 50 microns, thereby developing a new surface for the sorption of sulfur dioxide. The broken particles are introduced into the mattress compressed with the fuel carbon. All of these methods are variations on the classic bubble mattress boiler described earlier.

DE Pat. No. 3,023,480 describes a different way of achieving good utilization of the sorbent in the reduction of sulfur oxide from combustion gases. According to this patent, flue gas is passed through a fluidized bed with sorbent particles of 30 to 200 μm and a surface velocity of 0.9 to 9.1 m / s. This gives a fluidized mass with a particle density of 0.1-10 kg / m and a solid which is captured according to the fluidizing gas. Particles that are entrained due to the high gas velocity are removed by a recycle cyclone and returned to a mass of 700-167 ° C. The amount recycled per hour is approximately equal to five times the weight of the fluidized mass. This method makes it possible to effectively reduce sulfur dioxide when using fine and highly mixed particles with a large surface area. However, this patent does not disclose combustion in the transport reactor described above by recovering heat through the tubes in the reactor.

U.S. Patent No. 4,111,158 discloses a fluidized bed boiler based on the fluidized bed principle that provides improvements in fuel efficiency, sulfur oxide removal, and nitrogen oxide control and reduction. When boilers using the bubbling technique use surface speeds of 1.21-3.66 m / s and when they have a clearly defined upper surface, combustion boilers with a mattress operate at surface speeds of 4.5-13.7 m / s. These do not have a clearly definable top surface but rather a gradation of the particle density from the bottom of the boiler to its surface. The particle enters the reactor in a gas stream and is separated from it in a recycle cyclone located downstream of the reactor, whereby the particle is directed to the bottom of the reactor.

The particle size varies between 30-250 μm and the particle density 3 20-40 kg / m at the top of the reactor. No heat is recovered from particles or gases in the reactor or during recycling. The reactor tubes would be subject to severe erosion and would not be efficient in heat transfer because the particle density is low compared to a bubbling mattress (500 kg / m 2).

The heat is recovered by passing part of the bed from the bottom of the reactor and cooling it in a separate fluidized bed heat exchanger optimized for the process. A high fuel efficiency ratio is achieved with fully combustible small fuel particles in a highly turbulent reactor and hot recycle. Efficient use of the sorbent is also achieved by using fine particles and keeping them fine at the effective temperature through the reactor and recycling. Limited control of nitric oxide is obtained by continuously passing combustion air 4,86767 along the entire length of the reactor. The disadvantage of the system is the need for a separate fluidized bed heat exchanger and a large recycling cyclone.

Injection of ammonia to reduce nitric oxides is proposed in U.S. Patent 3,900,554. la 2. On the other hand, the advantage of good mixing is not mentioned, as in a recycling cyclone which produces 95% nitric oxide at the same molar ratio 2.

EP-A-112237 describes an apparatus for fluidized bed combustion in which solids are collected from a burner and then stored in a separate container. These solids are stored for energy recovery. When the storage tank is filled, the primary fuel stream fed to the burner is cut off and the burner is heated by injecting the material collected therein. Thus, this system uses intermittent re-injection rather than continuous recycling.

EP-A-118931 describes a dilute fluidized bed, i.e. a layer which lacks an upper limit, i.e. the entire reactor space is filled with a fluidized material and there is no free chamber. The system appears to be isothermal with respect to the cyclone and reactor, although cooling is accomplished by removing the systemic solid and cooling it before feeding it back. The reaction is known to continue throughout the recycling process.

It is an object of the present invention to provide the advantages of high combustion efficiency and efficient sorbent utilization 86767 without the use of a separate fluidized bed heat exchanger with a large recycle cyclone. This object is now achieved by a method and an apparatus which are mainly characterized by what is stated in the characterizing parts of claims 1 and 7.

The present invention uses a bubbling fluidized bed boiler with intra-mattress tubes for heat transfer. The average particle size of the mattress is between 100-800 μιη, of which 20-40% are smaller than 200 μιη. The surface speed of the mattress is 0.9-2.13 m / s, ie much lower than the fluidized bed 4.5-13.7 m / s. The result of the relatively low surface velocity, together with the small particle size of the mattress, is the formation of a bubbling mattress at a high slurry rate of the powder portions of the mattress. At the top of the mattress, a particle density of 0.5 kg / m3 is achieved. This is comparable to a particle density of 10-40 kg / m3 of a typical entratted mattress. However, the present invention uses particle sizes typical of an entrapped mattress, but with significantly lower surface velocities, but still forms a bubbling mattress with substantially less migration of mattress material.

Compared to a conventional fluidized bed boiler, this uses a much smaller average particle size (500 μιη compared to 6,86767 1000 μm) and a significantly higher mattress displacement. The recirculation rate of a conventional fluidized bed boiler corresponds approximately to the combined solids feed rate, with the recirculation rate of the present invention being 20 times this value when the mattress is changed every 40 minutes. Unlike bubble boilers, the present invention does not use any heat exchange surface between the mattress and the recycle cyclone to cool the gas and particles, which in turn are used in entrail boilers. This invention uses isothermal recycling to operate the combustion or desulfurization at an ideal temperature. Unlike a bubbling or entraining boiler, the present invention uses ammonia injection at the recirculation cyclone inlet to control nitric oxide emission. Additional advantages are an increase in the heat transfer coefficient of 100-300% in the tubes in the mattress due to the small particle size and a reduction in the erosion of the tubes (compared to conventional bubbling mattresses) due to the low surface velocities. Another interesting point is the range of fluidization rates I0: 1, which allows a fully fluidized start with a low flow-through.

It has been shown that without the use of ammonia, an 86% reduction in nitrogen oxides can be achieved by operating at a low temperature, 788 ° C, which produces less nitrogen oxides from fuel-derived nitrogen. In addition, it is known that tar reduces nitrogen oxides in the presence of oxygen at a temperature of 788 ° C. At 788 ° C, a large amount of tar is slurried from the mattress and recycled. Part of the 86% nitric oxide removal is due to its reaction with hot tar, but the magnitude of this effect is unknown.

The present invention thus relates to a method and apparatus for burning fuel having excellent performance and efficiency.

The invention will now be described in more detail with reference to the drawing, in which Fig. 1 is a vertical sectional view of a device according to the invention.

According to a particularly preferred embodiment of the present invention, a system consisting of a fluidized bed boiler 10 with a combustion chamber 11 with further fluidized beds and a B on the application level 12 is used. The boiler 10 comprises cooling pipes 13 located in the fluidized bed B, a fuel supply pipe 14, a sorbent supply pipe 15 and a mattress downcomer 16. The fluidizing air is led to the bottom 17 of the boiler 10. The hot gas and slurry particles leaving the surface of the mattress B pass through the free chamber 18 and are passed through a channel 19 to a circulating cyclone 21 mounted above the mattress so that fine particles can flow freely back into the mattress through a straight drip tube 22 with a corresponding drip tube 22. bulk. The recirculation cyclone has a split point of approximately 12 microns.

Ammonia is injected into the hot gas stream through a passage 19 located just before the recycle cyclone into the ammonia by means of an injector 23, whereby nitrogen oxides are reduced as the fuel burns with the nitrogen from the fuel. Ammonia is obtained from a storage tank 24.

The hot gas leaves the recycle cyclone 21 through the duct 25 and passes through a convection heater 26 where the remaining heat of the gas is transferred. After the convection heater 26, the gas passes through a filter system 27, such as a hose filter, where dust is removed from the gas before it is driven out of the barrel 28.

According to the present invention, there is provided a fluidized bed burner method and apparatus having a bubbling fluidized bed burner having a surface velocity in the range of 0.15-2.13 m / s, but having a grain size of the mattress material in the range of 45-2000 μm. 20% of the mattress material is less than 200 yum in diameter. When using dolomite, a large fraction comes from the size of the dolomite raw material fed. The bubbling mattress has 8,867,767 tubes for transferring heat from the hot fluidized bed; the heat transfer coefficient on the outer surface of these tubes is between 567-1136 2 W / (m K) due to the fine mattress particles. Solid or liquid fuel is fed directly into the mattress with solid fuel. If a sulfur sorbent such as limestone (CaCO 2) or dolomite is used, it is also fed directly into the mattress.

The fluidized bed burner has a hot free chamber with no heat exchange surface. Much of the powder is slurried from the fluidized bed in a hot state, resulting in excellent mixing between the particles and the gas and sufficient precipitation time for chemical reactions. Most of the particles slurried from the mattress into the free chamber fall back into the mattress, but a significant portion travels with the hot gas to the recycle cyclone, where it is separated from the gas and returned to the mattress, all at substantially the same temperature as the fluidized bed. The amount of particles entering the cyclone in the gas is approximately 0.5 kg / m 3. The extended precipitation time and the excellent mixing of fine carbon particles and oxygen-rich combustion gas in both the free chamber and the recycle cyclone result in complete combustion of the carbon particles, so they cannot leave the recycle cyclone. Recycled carbon is believed to have the advantage of increasing the removal of nitric oxide at temperatures below 760 ° C. Similarly, the extended precipitation time and the excellent mixing of fine sorbent particles and sulfur oxides in the hot free chamber flue gas and the recycle cyclone promote good sulfur oxide uptake by the sorbent. The fine particles are about is in the mattress and 3 s in the free chamber and the recycling cyclone. A 5 μm split point and a highly efficient drip tube are designed for the recycle cyclone, allowing a large proportion of particles larger than 5 μm to easily circulate back into the escape filter system while being blocked. The flow rate of the captured particles in the recycle is about 20 times the combined flow rate of the sorbent and the fuel.

Its flow per hour corresponds to twice the weight of the fluidized bed.

9 86767

If there is nitrogen from the fuel in the fuel and a reduction in nitric oxide is required, ammonia is injected into the hot flue gas stream just before the inlet to the recycle cyclone. Although ammonia selectively and very effectively reduces nitric oxide without catalysis between 950-1000 ° C, in this invention efficient removal is normally achieved by using a fluidized bed at a temperature between 788-900 ° C and achieving an increase of 0-84 ° C above the mattress after combustion. When ammonia is injected at an ammonia / nitric oxide molar ratio of 1.5-2, 80-90% removal of nitric oxide is obtained due to the excellent mixing in the recycle cyclone. Under certain conditions, nitrogen oxides are removed even without the use of ammonia injection. When operating at a low temperature of 788 ° C and using a fuel with a high percentage of non-volatile carbon, a significant proportion of the recycled particles are carbon. These hot fine particles reduce nitrogen oxides so that 86% removal is achieved by this invention.

The hot combustion gas and dust leaving the recycle cyclone pass through a heat exchanger where the gases cool to their outlet temperature. Eventually, the filter system removes dust before the flue gases escape.

The present invention thus offers the possibility of burning a purely wide range of solid and liquid fuels, some of which can be very difficult to burn (such as petroleum coke with 90% non-volatile carbon, etc. poorly volatile constituents) or fuels which may contain sulfur or nitrogen or a combination of them, all of which cause air pollution. The present invention burns these fuels using a conventional bubbling mattress with a fine particle composition and back-recycling of this powder largely through a hot recycling loop above the bubbling mattress. petroleum coke with 90% non-volatile carbon gives an efficiency of 99.4% and a removal of 98% sulfur, 0 86767 oxides with a calcium / sulfur molar ratio of 1.8. A 95% reduction in nitrogen oxides is achieved with an ammonia / nitric oxide molar ratio of 2. All this takes place within the fluidized bed recycling system and simultaneously with this operation.

A further advantage of the present invention is the large 15: 1 liquefaction scale. Because the bubbling fluidized bed is composed of small particles, its minimum fluidization rate is as low as 0.15 m / s.

Example - Oil Coke Oil coke was burned with air in a fluidized bed boiler, the structure of which is illustrated in Figure 1. The fluidized bed boiler was 0.91 m in diameter and 3.66 m high, including a recycled cyclone built above it. The fluidized bed boiler had a fireproof lining. The bubbling mattress works at a depth of 1.0-1.22 m and included air tubes to transfer heat out of the mattress.

The petroleum coke used in the test had the following composition and calorific value:

Non-volatile carbon 89.7% by weight

Nitrogen 1.9

Broken 2.1

Other volatile substances 4.4

Ash 0.3

Humidity 1.6

Upper combustion value 33 190 kJ / kg This fuel is difficult to burn due to its high content of non-volatile carbon and low volatile components. It also contains nitrogen and sulfur, which form air-polluting nitrogen oxides and sulfur oxides. The fuel was led to the fluidized bed from the fuel supply port, with most of the material being 50-400 μm in diameter. Dolomite, a sulfur sorbent, was introduced into the mattress from the sorbent feed. Its composition was: ,, 86767

Calcium carbonate 56.6% by weight

Magnesium carbonate 45.5

Inert ingredients 0.9

Its size ranged from 4700 to 1200 yiim. This particular dolomite was comminuted by heating in a mattress into small particles.

The fluidized bed was originally made of crushed dolomite with an average size of 800 yum.

After about 500 hours of testing, the mattress consisted of ash, spent sorbent, and partially used sorbent; the average grain size had stabilized at about 300 μm. The fluidized bed operated at an average surface speed of 1.22 m / s. It was necessary to reduce the mattress from time to time to maintain a constant level.

The recycle cyclone was designed to keep most of the particles larger than 5 in a fluidized bed boiler and a drip tube was designed with minimal resistance to particle backflow. The result was a high recycling rate of the powder per hour of recycling, approximately twice the weight of the mattress and 20 times the combined solids feed. Fuel particles and sorbent particles that were unable to leave the fluidized bed with the gas stream before reaching a very small size remained in the mattress and comminuted as the mattress functioned. The fuel particles that were prevented from leaving the fluidized bed boiler burned perfectly, guaranteeing a high fuel efficiency even for the heavy fuel, which contains about 90% non-volatile carbon. The combustion efficiency was further improved by the isothermality of the recycling track. The fuel particle is heated to its full combustion temperature in the mattress and is not cooled in a free chamber or recirculation cyclone. When operating at a mattress temperature of 971 ° C with an excess of 20-30% air, 99.4% combustion efficiencies were achieved. Afterburning above the mattress ranged from 27 to 56 ° C. The fine grinding and maintenance of the sorbent particles allowed the sorbent to absorb sulfur from the gas over a large area in a fluidized bed boiler.

98% sulfur oxide removal was achieved with a calcium / sulfur molar ratio of 1.8. An additional advantage provided by the fine particle size in the boiler was the increase in the heat transfer coefficient on the surface of the tubes inside the mattress. It was observed that the heat transfer coefficients varied between 567-1136 W / (m K) on the outer surface of the tubes when 2 it is in a conventional fluidized bed boiler 227-341 W / (m K).

To reduce nitrogen oxides to comply with local pollution control laws in Southern California, ammonia was injected into the flue gas prior to the cyclone, thereby selectively reducing nitric oxide according to known reactions to nitrogen and water. The NH 4 / NO molar ratio of 2 reduced about 95% of the nitrogen oxides.

Example - Utah coal

Utah coal was burned in the same fluidized bed boiler described above. The composition and calorific value of the carbon were as follows:

Non-volatile carbon 43%

Nitrogen 1.3%

Sulfur 0.6%

Other volatile substances 37.1%

Ash 8.0%

Humidity 10.0%

Upper combustion value 26,750 kJ / kg

Utah coal had significantly less non-volatile coal and significantly more volatiles than oil coke and was thus easier to burn. The particle size of the coal was -41.7 mm. The sulfur sorbent was the same dolomite used in the previous example. Its composition was as follows: li 13 86767

Calcium carbonate 56.6% by weight

Magnesium carbonate 45.5% by weight

Inert substances 0.9% by weight

It ranged in size from 1200 to 4700 μm but was comminuted by heating into fine mattress particles.

The combustion efficiency with coal was 99.8% with a 20% excess air at a mattress temperature of 871 ° C. With coal, the boiler can be operated at temperatures as low as 760 ° C with only a 20% excess of air while maintaining good combustion properties, with petroleum coke the acceptable combustion properties could be maintained at 788 ° C only by increasing the excess air to 60%. When coal was used at 871 ° C, the afterburning above the mattress was reduced to 5.6-11.1 ° C. The reductions in sulfur and nitrogen oxides were the same as for petroleum coke.

Claims (10)

14 86767 A method of burning solid or dirty liquid fuels in an atmospheric fluidized bed combustion chamber (10) to recover useful energy, comprising: maintaining a fluidized bed (B) of inert particles, ash and partially burned fuel particles by feeding gas upward through the bed; collecting substantially all of the separated fines in the bed exhaust gas into a recycle cyclone (21) and returning it to the bed; and recovering heat from the exhaust gas downstream of the recycle cyclone (21); wherein substantially all of the combustion of the fuels takes place in the fluidized bed and not in the free chamber (18) or in the recycle cyclone (21) above the fluidized bed; recovering thermal energy from the combustion of the fluidized bed (B) and not from the free chamber (18) or the cyclone (21); maintaining a particle recycle path comprising a fluidized bed (B), a free chamber (18) and a cyclone (21) at a substantially constant temperature; and maintaining the bed at a surface velocity that causes substantial but controlled removal of fine particles from the bed; characterized in that the average particle size is between 100 and 500 μπί; maintaining a surface velocity of 0.15-2.2 m / s in the bed, resulting in significant but controlled removal of fine particles from the bed; the circulation of the finely divided material of the fluidized bed being continuous and the hourly return flow through the recycling cyclone (21) corresponding to 1 to 5 times the weight of the material in the bed. A method according to claim 1, characterized in that the combustion of the fuel particles above the fluidized bed is limited to a maximum temperature of 38 ° C and preferably to a temperature of about -6.7 ° C. A method according to claim 1, characterized in that the temperature of the fluidized bed (B) is maintained between 788 ° C and 900 ° C and the afterburning above the fluidized bed of fuel particles is limited to 65.5 ° C to prevent nitrous oxide emissions from bound nitrogen-containing fuel. in the presence of ammonia without the aid of a catalyst. A method according to any one of the preceding claims, characterized in that the fluidized bed burner is maintained at a temperature of 760-816 ° C to minimize the formation of nitrogen oxides from fuel combustion and to promote carbon formation, whereby hot carbon reduces nitrogen oxide in the bed and recirculation bend. Method according to one of the preceding claims, characterized in that a sulfur oxide sorbent (15) is added to the atmospheric fluidized bed burner (10) in order to reduce sulfur oxide emissions. Method according to one of the preceding claims, characterized in that ammonia is introduced into the flue gas stream at a point immediately upstream of the recycle cyclone (23) in order to prevent nitrogen oxide emissions. A fluidized bed incinerator for carrying out the method according to any one of the preceding claims, which device comprises the following combination: an atmospheric fluidized bed burner (10); means for supplying solid or dirty liquid fuel and inert particles to the burner; means (17) for introducing fluidized air to the bottom of the burner (10); a recycling cyclone (21) for collecting substantially all of the effluent material escaping from the outlet of the fluidized bed burner (10) and returning the collected material to the fluidized bed (B); means for collecting heat from the bed (13) and the exhaust gas stream (26), but not from the free chamber (18) or the cyclone; and means for collecting dust (27) from the cooled exhaust stream; characterized in that the cyclone is arranged to recycle the finely divided material on a continuous basis. Device according to claim 7, characterized in that it comprises means (15) for feeding 16 86767 sulfur oxide sorbents to an atmospheric fluidized bed burner (10) simultaneously with the fuel. Device according to Claim 7 or 8, characterized in that it has means (23) for supplying ammonia to the hot flue gas stream immediately upstream of the recycle cyclone (21). Device according to one of Claims 7 to 9, characterized in that the means (26) for recovering heat from the exhaust gas stream are located downstream of the means (21) for collecting and recovering the exiting material.