EP3544723A1

EP3544723A1 - Process and plant for thermal treatment in a fluidized bed reactor

Info

Publication number: EP3544723A1
Application number: EP17808031.3A
Authority: EP
Inventors: Christian Binder; Edgar Gasafi; Jochen GRÜNIG; Maciej WROBEL
Original assignee: Outotec Finland Oy
Current assignee: Metso Metals Oy
Priority date: 2016-11-22
Filing date: 2017-11-17
Publication date: 2019-10-02
Also published as: BR112019009875A8; BR112019009875A2; AU2017365247B2; CN109982775B; WO2018095815A1; AU2017365247A1; CN109982775A

Abstract

The invention describes a process for calcining a mineral comprising the following steps feeding a mineral into a fluidized bed reactor (i), injecting biogas as fuel into the fluidized bed reactor (ii), burning the biogas for generating heat to calcine the mineral (iii), withdrawing calcined mineral from the fluidized bed in the reactor (iv) and withdrawing a mixture of gas and solid particles from a position above the fluidized bed (v).

Description

Process and plant for thermal treatment in a fluidized bed reactor The invention belongs to a process and the relating treatment in a fluidized bed reactor focusing on the use of biomass, which infects the feed system as well as the off-gas treatment.

Thermal treatments, especially calcining of minerals, are well-known processes, which is why the products of theses process are widely used. A good example for such a product is Kaolinite like it is shown in WO 2005/019349.

Kaolinite is a natural industrially used mineral with various applications (e.g. pigment, paper, polymer, cement, cosmetics industries, agriculture, and con- struction). Its calcination is a well-established process which is based on the thermal treatment of aluminium silicate minerals typically processed in a kiln or in hearth furnaces. As the calcination process improves the materials whiteness and hardness and its electrical properties depending on a certain exposure time at a certain temperature, it is crucial to be able to well control the temperature and residence time during the calcination process.

The use of a fluidized bed is known by principle from document EP 0 747 452 A2 which teaches a method of producing a meta-kaolin white pigment which involves thermal treatment of hydrated kaolinite in a fluidized bed reactor, supplied with fuel, oxygen-containing gas and granular kaolinite of particle size of dso between 0,1 to 8 mm, at 1000 to 1 100 °C to obtain, as withdrawn product, meta-kaolin white pigment with max. 2 wt.-% water of crystallization. Other known methods are the e.g. fast pyrolysis, which are commonly used for fine grained materials.

However, the quality of the calcined kaolin is a mixture of material with very good properties with a material of lower properties in a ratio that allows for reaching the minimum requested quality. Often, the residence times need to be rather long for large material amounts in order to ensure a high quality. This comes with an additional energy penalty, increasing the specific energy needed in the calcination process.

Therefore, the use of so called green energy sources is of particular importance to increase the ecological standards of such calcination processes. To ensure temperature control and residence time during the calcination process as a requirement for high product quality, the use of biomass as an energy sources was not possible unit now.

It is, therefore, the task underlying the invention to provide a process and the relating plant for calcining minerals to a product with high and constant quality using biogas as a fuel. It is also aim of the invention to provide a process cover- ing all necessary steps for calcining minerals environmental performance evaluation.

This task is fulfilled with a process according to claim 1 . In such a process, biogas is used as an energy source which improves the process performance regarding environmental balance. Therefore, a mineral is introduced into a fluidized bed reactor in form of particles, which can be fluid- ized. Further, the biogas is injected as fuel into the fluidized bed reactor where it is burnt for generating heat to calcine the mineral. Afterwards, the calcined mineral is withdrawn from the fluidized bed in the reactor and a mixture of gas and solid particles is withdrawn from a position above the fluidized bed.

The calcination step is the core of the process. Kaolin is fed to the reactor. The biogas is burned with air in the furnace in order to produce heat to raise the mineral particle temperature up to the desired level. The entrained solids are separated from the gas in a gas-solids separator (e.g. cyclone), which has an overflow and an underflow. The underflow contains the particles, which form the hot calcined product.

As a result, the heat needed for the calcination is provided by e.g. burning fuel gas derived from a gasification process, or combustion of fuel. This option allows for the utilization of alternative fuels for the required process energy through gasification of resources which minimize the environmental footprint. The invention is tackling the problem of residence time and temperature control occurring to the use of biogas by using a fluidized bed technology with energy recovery. Hereby, the residence time and the temperature can be controlled in a very narrow operating window and as such the overall efficiency of the process. It is preferred to operate this process as a continuous process. Most preferred is a steady state in the reactor.

In order to enhance fluidizability for the fluidized bed, it is preferred that the mineral is granulated before being fed into the fluidized bed reactor, e.g. with addition of water). In some cases, the available moisture in the natural resource might be sufficient for a good granulation result.

However, often it is preferred to dry the granulated solids to water content below 10 wt.-%, even more preferred below 5 wt.-% for better stability. For the subsequent drying, low calorific heat from the process may be used. For preferred operating conditions, reaction temperature lies between 800 and 1300 °C. Preferably, Kaolin is calcined at a temperature between 1050 °C +/- 50 °C.

Compared to a rotary kiln or a hearth furnace, the fluidized bed has an excellent heat and mass transfer yielding a stable operation with a well defined and homogenous operating temperature. In contrast to systems requiring burners for heat provision, the fuel can be injected through lances and combusted without flame formation in the fluidized bed.

This results in a lower maximum calcination temperature minimizing fuel consumption thereby keeping the product quality at a uniform high level. During calcination, crystalline water is removed.

Looking at kaolin as an educt, its whiteness is increased. Due to the mineral phase changes upon heating up to approximately 1 100 °C, the structure becomes denser and the hardness increases, yielding a product with improved quality suitable for the pigment industry. If the fluidized-bed process is designed well, energy efficiency can be significantly increased by recovering large fractions of sensitive heat from the off-gas and solid product by applying pre-heating and cooling stages respectively.

In a preferred embodiment the fluidized bed is a circulating fluidized bed which shows especially good heat and mass transfer rates.

It is of particular importance that it is possible to feed fresh solid particles into the fluidized bed. Thereby, a more stable fluidized bed is obtained. Moreover, the gas fraction gained from the gas-solid-mixture can be treated such that sulphuric gases in the flue gas are removed.

Preferably, the mineral contains kaolin, diatomaceous earth and/or perlite. It is especially preferred that the used educt shows at least one of the three named minerals with a content of at least 50 wt-%, even more preferred more than 80 wt.-%

Kaolin granules were calcined at 1050 °C +/- 100 °C without encountering major problems during calcination tests. Especially a process dealing with kaolin process is promising.

For diatomaceous earth, the production of granules required a very high amount of water increasing fuel consumption for calcination and reducing the energy efficiency of the considered calcination process. However, SO2 in the fuel and flue gas is partially fixed by the calcium contained in diatomaceous earth.

For perlite, the retention time in the circulating fluidized bed has to be very short in comparison to other minerals.

Further, heat can be recovered by pre-heating the feed for by pre-heating the feed for the reactor and/or by applying a fluid-bed cooler for cooling the hot calcined product. Heat recovery through pre-heating is achieved by contacting the hot off-gas from the reactor in a counter-current mode with the cold raw mineral feed. In addition to this step(s), the heat remaining in the gas stream is used for drying the feed material. In order to maximize the contacting time for a better heat exchange, up to three pre-heating stages can be foreseen. The two-stage pre-heating starts with a moist feed, which is conveyed into the pre-heater of the first preheating stage. By mixing the solids with the combustion off-gas coming from the second preheating stage, the moisture is evaporated and the solids temperature is raised. The gases leaving the pre-heater enter the cyclone for separation of the entrained solids. The off-gas is further cleaned in an electrostatic precipitator before being vented through the stack.

In the second pre-heating stage, the dried solids are heated to a temperature above 500 °C further reducing fuel consumption in the calcination step. The second stage applies the same counter-current flow of gas and solids with cooling of the gases by preheating the solids. A heat transfer vessel allows for the heat exchange, while the cyclone separates the solids from the gas.

A third pre-heating step can be applied if the moisture is not too high. By this way, even higher efficiency can be achieved by making use of the sensitive heat contained in the solids and the gas.

The first cooling of the calcined mineral is performed in multiple cyclones downstream of the furnace (bed product discharge) allowing for pre-heating of air coming from the so called fluid-bed cooler. In the fluid-bed cooler consisting of consecutive fluidized beds, the hot solids obtained as a product directly and indirectly pre-heat the air required for fluidization and establishment of process conditions allowing for complete combustion of the fuel in the reactor. Final cooling of the product is accomplished by water cooling (e.g. by heating water from 35 to 55 °C) allowing for adjustment of a solids temperature below 90 °C.

It is one part of the invention that the inventors found out that this requires a very constant and similar temperature of each particle fed into the fluidized bed. As a result, it is also an object of the invention to provide a process and the relating plant which ensure a constant pre-heated feed into the reactor, which was defined as a fluidized bed reactor due to the very good heat and mass transfer.

At typical reaction conditions, e.g. pressure between 0 and 20 bar and/or temperature between 700 and 1500 °C, the solids are treated in a fluidized bed for a defined residence time as a thermal treatment. Preferably, said thermal treatment is a calcining.

After a defined (average) residence time, the solid particles are withdrawn from the fluidized bed in the reactor through a withdrawing conduit which is arranged such that during operation it is in the area of the established fluidized bed. Further, a mixture of gas and solid particles is withdrawn from a position above the fluidized bed often called free board. The gas normally contains the fluidizing gas, which often contains oxygen. In most of the processes fluidizing gas is air. Particles in such a mixture often show a diameter below the average diameter of the particles in the fluidized bed, so they are transported higher than these bigger particles.

To separate a gas fraction from a solid fraction the gas-solid mixture is passed through at least one cyclone. Thereby, a solid fraction (preferably containing at least 70 wt.-% of the solid particles of the gas-solid mixture withdrawn from the reactor) is obtained and at least partly in the fluidized bed reactor.

It is the essential idea underlying this part of the process that at least parts of the fresh solid particles are fed into the at least one cyclone. Thereby, they are mixing with the solid-gas-mixture from the freeboard, and together with the separated solids they are fed back into the reactor. Thereby, it is possible to feed in particles with only one conduit instead of two (fresh feed and recirculation feed). So, the established fluidized bed is less disturbed and, therefore, operation conditions are more stable. This enables up- and downturns on other process positions and so makes it possible to use biogas as an energy source.

Further, this process design is much easier and, therefore, reduces investment and operating costs.

It is preferred to operate this process as a continuous process. Most preferred is a steady state in the reactor.

In a preferred embodiment of this idea, at least two cyclones are arranged in series which ensures a better separating of the solid particles from the gas-solid mixture.

Using a number of series-connected cyclones, it is preferred to introduce the fresh solid particles into a suspension preheater in front of the last cyclone of the line, e.g. the second cyclone. Thereby, the introducing of the fresh solids does not infect the separating step to much.

It is also preferred the the fresh solid particles are preheated by the gas fraction obtained in one of the cyclones, Thereby, the particles come in direct or indirect contact with the gas fraction obtained in at least one cyclone. Thereby, energy efficiency of the process can be increased since the fresh particles show a higher inlet temperature entering the reactor.

Such a preheating is most efficient if the preheating is performed in countercur- rents. Further a second feed for feeding fresh solid particles directly into the reactor may be foreseen. Such a second position for introducing particles is especially helpful in cases of starting the process or to stabilized operating conditions. In a preferred embodiment, said second feed is preheated.

Another problem is that the use of biomass as a green energy source will lead to high SO2 and/or SO3 content(s) in the off-gas due to the sulphur content in biomass. Using a fluidized bed reactor, this question is of particular importance since the off-gas streams are very large due to the fact that the gas stream withdrawn of the reactor contains off-gas and fluidizing gas.

Therefore, an idea how to overcome the problem of SO2 and/or SO3 in the reactor's flue gas is provided.

In such a process for thermal treatment of solid particles in a reactor featuring a fluidized bed, a gas-solid mixture is withdrawn from the reactor, preferably from a position above the fluidized bed, out of the reactor. Such gas-solid-mixture is passed to at least one device to separate the gas fraction from a solid fraction which comprises at least 60 wt.-% of the solid particles contained in the gas- solid mixture.

The obtained gas fraction containing SO2 and/or SO3 is passed into a packed bed to absorb SO2 and/or SO3 and withdrawing the solution with the absorbed SO₂ and/or the absorbed SO₃.

Thereby, sulphuric residues can be removed from the gas stream to a value below legal limits. However, said gas stream often also contains further impurities, which is why it is preferred to clean it in at least one further treatment step. Preferably, at least one treatment step is situated downstream of the absorption step. In one embodiment, at least one step of the further treatment is an electrostatic precipitator (ESP). So it is possible to reduce dust content of the gas stream to a minimum.

In another embodiment of the invention, the gas-solid-mixture is separated in a cyclone. Thereby, good separation rates are achieved. Further, it is possible to use this cyclone for the feed system into the reacto-.

It is also preferred that the gas fraction is passed to a postcombustion before the packed bed. So, contained H₂S is oxidized to SO2 and/or SO3 which is absorbed in the packed bed. Thereby, it is also possible to remove H₂S present in the initial biogas.

Moreover, the idea also covers a plant with the features of claim 14. Especially, it possible to operate a process with feature of any of claims 1 to 13 in said plant. Such a plant comprises a reactor wherein under operating conditions a fluidized bed is established. Further, it features a feed conduit to feed at least one mineral into the reactor, a conduit to inject biogas as fuel into the fluidized bed reactor, a conduit for withdrawing calcined mineral from the fluidized bed in the reactor and a conduit for withdrawing a mixture of gas and solid particles from a position above the fluidized bed. So, , biogas can be used in a calcining process without infecting product quality.

Such a plant comprises a reactor featuring under operating conditions a fluid- ized bed. Such plant further comprises two conduits, at least one cyclone and one recirculation line.

The first conduit is able to withdraw particles from the fluidized bed since it is positioned such that during normal operation it is in the area of the fluidized bed. The second conduit is able to withdraw a mixture of gas and solid particles from a position above the fluidized bed and feed the mixture into at least one cyclone through which the gas-solid mixture is guided to separate a gas fraction and a solid fraction which comprises at least 60 wt. % of the solid particles contained in the withdrawn gas-solid mixture. Via the recirculation line, solid particles from the cyclone can be re-fed into the reactor, especially in a position where the fluidized bed is established under operating conditions.

As the core of this idea a conduit is foreseen which leads fresh solid particles into the at least one cyclone. Thereby, the number of conduits leading into the reactor can be reduced, which stabilized reactor conditions. Moreover, investment and operating costs are reduced.

Preferably, the plant for thermal treatment of solid particles comprising a reactor featuring under operating conditions a fluidized bed. Further, said plant shows at least one device to separate the gas fraction from a solid fraction whereby the solid fraction comprises at least 60 wt.-% of the solid particles contained in the gas-solid mixture. Moreover, the plant features a packed bed, wherein the gas fraction containing SO2 and/or SO3 is passed to absorb SO2 and/or SO3 and a conduit is withdrawing the solution with the absorbed SO2 and/or the absorbed SO₃. As a result, it is possible to use biogas as a green energy source without any problems concerning sulphur content in the process' off-gas. Developments and advantages in application possibilities of the invention also emerge from the following description of the process. All features described and/or illustrated in the drawings form the subject matter of the invention are per se or in any combination independent of their inclusion in the claims or their back references.

In the figure:

Fig. 1 shows the principle of the invention for calcining a mineral Fig. 1 shows a process for calcining a mineral using fuel as a biogas as well as the new injecting system. Gas of desired composition is entering via conduit 1 1 through the nozzle grate 12 and fluidizes the material in the reactor 10. Before entering the nozzle grate 12, the air can be heated by an electric pre-heating system 13 to the desired temperature. Temperature is measured at various locations and data logged.

Via lance 13, biogas is injected as a fuel into reactor 10. Therein fuel is burned whereby the solid particles in the fluidized bed 10 are thermally treated. Solids entrained with the reactor's off-gas in a gas-solid mixture are recirculated to the fluidized bed 10a by conduit 31 and cyclone 20. The separated solid fraction containing at least 60 wt.-% of the solids contained in the withdrawn gas-solid mixture is recirculated into the reactor 10 via conduit 26. The dustladen gas leaving the cyclone 20 passes via conduit 22 through a pre- heater 23 which can be applied for pre-drying and pre-heating the feed material which is fed into via conduit 24.

The off-gas is then passing via conduit 25 a secondary cyclone 30 for further dust separation. Further separated parts are recycled via conduit 38 into reactor 10.

Final dedusting of the off-gas is accomplished by passing the gas via conduit 31 in a candle filter 32 from where solid particles are withdrawn via conduit 33. If required, e.g. if H₂S or CO are in the off-gas, the gas is bypassed through or can be treated in a post combustion chamber 36 before entering the packed bed operated as a SO2 scrubber 40. After scrubbing and passing a subsequent wet electrostatic precipitator 42 for removal of scrubbing liquid droplets and SO3, the gas leaves the system via a stack 45. The absolute pressure inside the reactor is close to atmospheric pressure.

A cooling screw 50 continuously discharges material via conduits 51 , 52 from reactor bed providing the actual calcined product. Via conduit 60, it is possible to add further fresh solids into the reactor 10. Also, steam can be introduced via conduit 61 and electric pre-heating system 62.

Reference numbers

10 reactor

10a fluidized bed

1 1 conduit

12 nozzle grate

13 electric pre-heating system

14 lance

20 cyclone

21 ,22 conduit

23 preheater

24-26 conduit

30 cyclone

31 conduit

32 candle filter

33.34 conduit

35 blower

36 post combustion chamber

37 conduit

40 packed bed

41 conduit

42 electrostatic precipitator

43 conduit

44 blower

45 stack

46 conduit

50 cooling screw

51 ,52 conduit

61 ,61 conduit electrostatic precipitator

Claims

1 . Process for calcining a mineral comprising the following steps: i) feeding a mineral into a fluidized bed reactor, ii) injecting biogas as fuel into the fluidized bed reactor, iii) burning the biogas for generating heat to calcine the mineral, iv) withdrawing calcined mineral from the fluidized bed in the reactor and v) withdrawing a mixture of gas and solid particles from a position above the fluidized bed.

2. Process according to claim 1 , characterized in that the mineral is granulated before being fed into the fluidized bed reactor.

3. Process according to claim 1 or 2, characterized in that the granulated solids are dried to a water content below 10 wt.-%.

4. Process according to any of the previous claims, characterized in that the reaction temperature is between 800 and 1300 °C.

5. Process according to any of the previous claims, characterized in that the fluidized bed is a circulating fluidized bed.

6. Process according to any of the previous claims, characterized in that the mineral contains kaolin, diatomaceous earth and/or perlite.

7. Process according to any of the previous claims, characterized in that the gas-solid mixture is passed through at least one cyclone to separate the gas fraction from a solid fraction which comprises at least 60 wt.-% of the solid particles contained in the gas-solid mixture, that at least parts of the solid fraction are recirculated in the fluidized bed reactor and that at least parts of the mineral is fed into the at least one cyclone.

8. Process according to any of the previous claims, characterized in that two cyclones are arranged in series and the mineral is fed via a suspension preheater into a second cyclone.

9. Process according to any of claims 7 or 8, characterized in that the mineral is preheated by the gas fraction obtained in one of the cyclones.

10. Process according to any of the preceding claims, characterized in that the preheating is performed in countercurrents and/ or a second feed line is feeding mineral directly into the reactor.

1 1 . Process according to claim 10, characterized in that the second feed is preheated.

12. Process according to any of the preceding claims, characterized in that the gas-solid mixture is passed out of the reactor through at least one separating device to separate the gas fraction from a solid fraction which comprises at least 60 wt.-% of the solid particles contained in the gas-solid mixture, and that and that the gas fraction containing SO2 and/or SO3 is passed into a packed bed to absorb SO2 and/or SO3 from which a solution with the absorbed SO2 and/or the absorbed SO3 is withdrawn.

13. Process according to any of the previous claims, characterized in that the gas stream from the packed bed is cleaned in a further treatment and/or that before passing the packed bed, the solid gas stream is passing an after-burning to convert H₂S to SO2 and/or SO3.

14. Plant for calcining a mineral comprising a reactor (10) wherein under operating conditions a fluidized bed (10a) is established, wherein a feed conduit

(26,38,60) to feed a mineral into the reactor (10), a device (13) to inject biogas as fuel into the reactor (10), a conduit (51 ) for withdrawing calcined mineral from the fluidized bed (10a) in the reactor (10) and a conduit (21 ) for withdrawing a mixture of gas and solid particles from a position above the fluidized bed (10a).

15. Plant according to claim 14, characterized by at least one cyclone (20,30) through which the gas-solid mixture is guided to separate a gas fraction and a solid fraction which comprises at least 60 wt.-% of the solid particles contained in the gas-solid mixture, a recirculation line (26,38) to recirculate solid particles from the cyclone (20,30) in the reactor (10) and a feed conduit (24) for the mineral into the at least one cyclone (20,30).

16. Plant according to claim 14 or 15, for thermal treatment of solid particles comprising reactor (10) featuring under operating conditions a fluidized bed (10a), at least one separating device (20,30,32) to separate the gas fraction from a solid fraction which comprises at least 60 wt.-% of the solid particles contained in the gas-solid mixture, a packed bed (40), wherein the gas fraction containing SO2 and/or SO3 is passed to absorb SO2 and/or SO3 and a conduit (46) for withdrawing the solution with the absorbed SO2 and/or the absorbed SO₃ from the packed bed (40).