Patent Application
20240217873
The present invention relates to a method for producing lime or dolime, as well as to an installation for producing lime or dolime, particularly for carrying out such a method.
Such a method usually comprises a calcination of a downward moving calcareous or dolomitic material having a carbonate content CaCO3+MgCO3 higher than 90 wt % in contact with first fumes obtained by combustion of fuel in the presence of an oxidizing gas, a cooling of the downward moving calcined calcareous or dolomitic material with collection from bottom of a main value product under the form of lime or dolime and a release of a gaseous effluent containing CO2.
Said downward motion may be carried out according to a vertical direction, as for example in the shaft kilns, or according to a sloping direction, as for example in the rotary kilns.
So obtained lime or dolime consists of pure oxide products having a CaO+MgO content higher than 80 wt % with a certain content of impurities depending on the purity of the mother limestone or dolomitic limestone and the ash content of the fuel that is used in the kiln. It is important to note that no additives have to be mandatorily supplied with the raw calcareous or dolomitic material for producing lime or dolime. As a general rule, a marketable lime or dolime should have highly pure oxide content ranging between 98% and 80% by weight. A specific impurity that can be a “killer” for many lime or dolime applications is sulphur. For example, steel, refractory or lime slurry applications require low to very low sulphur content.
During the calcination, the starting calcareous or dolomitic material releases a large volume of CO2. In addition, to achieve this calcination, it is necessary to reach high temperatures and therefore to proceed with the combustion of fuels, which, in turn, causes a significant release of CO2. Overall, calcination processes have the disadvantage of actively participating in increasing the greenhouse effect.
Moreover, a classical calcination process has the disadvantage of providing combustion of fuel with air and cooling the calcined product with air. Therefrom results a release at the top of the furnace of a gaseous effluent having a high content of diatomic nitrogen, and a comparatively low content of CO2 (volume concentration from 20% to 27% on dry gas), which is expensive to capture due to the high presence of nitrogen.
A CO2 capture method associated to a cement clinker kiln is disclosed in the patent application US 2009/0255444. Such a method consists to thermically treat a raw material comprised of limestone, clay and iron ore for producing clinker. The gaseous effluents coming from the clinker kiln are introduced into a CaO-looping system for concentrating CO2 in the effluents. Clinker manufacturing accommodates with high ash fuels and raw material containing for example only 75 wt % of limestone. In this document the starting raw material contains limestone mandatorily mixed with high contents of clay and iron ore which are to be excluded from the production of lime or dolime. Moreover a residue is continuously purged from the CaO-looping system, said residue being directly recycled to the main cement clinker kiln. Thus the impurities in this residue are integrated in the final clinker product and must be taken into account in its recipe.
A CO2 capture method, with a so called “carbonate looping”, is also known in the power production industry using coal (see J. Hilz et al., Long-term pilot testing of the carbonate looping in 1 MWth scale, Fuel 210 (2017), p. 892-899). A CaO fluidized bed in the carbonate looping system captures CO2 present in the gaseous effluent of a coal combustor. Continuously fresh CaCO3 must be introduced in the carbonate looping system as make-up, and a residue is also continuously purged and removed from the loop.
From these prior documents, no information may be obtained about the production of marketable lime or dolime. Moreover they result in the continuous purge and removal of a waste product.
The object of the present invention is to produce lime or dolime of quality while allowing capture of the CO2 released during the calcination process carried out in a lime or dolime kiln, for use or sequestration, without modification of the kiln and of the process which is implemented therein and without continuous removal of an unusable waste product.
To solve this problem, the method indicated above further comprises
This method is a closed loop regenerative system for CO2 capture. During the carbonation, CO2 from for instance the flue gas of a lime kiln as gaseous effluent is captured by a CaO sorbent and submitted to the following exothermic reaction CaO+CO2→CaCO3. So, the CO2 content of the gaseous effluent is drastically reduced, when released in the atmosphere. According to the invention a CO2-depleted gaseous effluent means a gas having volume concentrations of CO2 lower than the concentrations of the gaseous effluent of the kiln, advantageously lower than 10% on dry gas, preferably lower than 5%.
In the step of CO2 depletion the CaO sorbent may advantageously consist in a fluidized bed or a moving bed.
The obtained circulating CaCO3—CaO based charge comprises CaCO3 and residual CaO, which has not captured CO2. The CaCO3 of this charge is submitted to the endothermic reaction of calcination CaCO3+heat→CaO+CO2. According to the invention the heat necessary for this calcination results from a combustion of fuel poor in impurities in the presence of dioxygen and CO2, as oxidizing gas. This oxidizing gas may preferably be a mixture of dioxygen and CO2. In such conditions the combustion has the effect of producing in the gas stream mainly CO2 with some impurities, optionally present only in the form of traces in the fuel, and oxygen not consumed by fuel combustion. This obviously results in a drastic increase in the CO2 content of the gas stream collected from the loop. By CO2-concentrated gas stream, it should be understood according to the invention that said gas stream has a CO2 content of at least 90%, particularly at least 95% by volume on dry gas. And this gas stream rich in CO2 becomes usable or sequestrable under favourable conditions, which makes it possible to radically reduce the contribution to the greenhouse effect of the lime or dolime production.
According to the invention, the used dioxygen (also called oxygen hereinafter) is a gas whose oxygen content exceeds 90% by volume, preferably 95%, advantageously 98%. The source of pure dioxygen can, for example, be an air separation unit which separates air into dioxygen and nitrogen, or an installed dioxygen tank.
According to the invention, the fuel of the step of calcination of the separated CaCO3—CaO based charge is preferably gaseous because such a fuel contains neither ash, nor sulphur. Such a fuel may be for example natural gas, hydrogen, biogas, coke oven gas or gasification gas. A liquid or solid fuel such as for example fuel oil, oil, liquid biofuel, petroleum coke, biomass, lignite, coal, may also be selected insofar the ash content of the fuel is <10%, particularly <7%, preferably <5%, most preferably <1% by weight and the fuel sulphur content is <1.5% by weight, preferably <1%, most preferably of 0.1% by weight. The natural gas is particularly preferred. In the following text, the wording fuel poor in impurities is sometimes used to summarize the fuels appropriate according to the invention.
In the step of calcination of the separated CaCO3—CaO based charge, this charge may advantageously consist in a fluidized bed or a moving bed.
The CaO-based sorbent material produced during the step of calcination is recycled to the step of CO2 depletion.
According to the invention the method comprises a continuous extraction of a fraction of said CaCO3—CaO based charge, before its calcination, and a compensatory introduction of fresh limestone having a CaCO3 content of at least 90 wt %, preferably 95 wt %, advantageously 98 wt %, into said step of calcination of the separated CaCO3—CaO based charge.
A particularity of a sorbent regenerative system is that the sorbent, here the CaO, becomes less and less active with an increasing number of looping cycles. This phenomenon results from an increased sintering and poisoning of the sorbent by impurities. Advantageously a CaO sorbent capture efficiency of 30% should be maintained during the step of carbonation in order to obtain continuously a CO2 capture rate of at least 90 vol % in the gaseous effluent coming from the lime or dolime kiln. In order to keep this stable performance, a certain amount of the CaCO3—CaO based charge circulating from the carbonation to the calcination is extracted. This extracted circulating charge is called the bleed. For compensation, as above indicated, fresh material in the form of CaCO3 of a purity higher than 90 wt % is added before or during the calcination of the CaCO3—CaO based charge. This added amount is called the make-up. According to the invention, the bleed is extracted before the calcination of the CaCO3—CaO based charge. So the collected product avoids a calcination energy cost for its production.
According to the invention the gaseous effluent submitted to the step of CO2 depletion, which exits from the lime or dolime kiln, should be conform to the normative environmental requirements and consequently is poor in impurities such as ash and sulphur, and moreover a fuel poor in such impurities is used during the step of calcination of the CaCO3—CaO based charge. Moreover, as above explained, the limestone of the make-up is also of high purity. Therefrom it results that the extracted fraction called bleed has advantageously a CaCO3+CaO content of at least 80 wt %, preferably of 90 wt %, particularly of 95 wt %. Said bleed is a pulverulent Ca-based material which may contain little or no ash and little or no sulphur and is of good quality and may consequently be exploited as an auxiliary value product in most of lime markets such as civil engineering, agriculture, waste water treatment, paper manufacturing, sludge treatment . . . . As such a bleed is no waste and may be industrially or commercially exploited. Moreover the amount of extracted bleed can be advantageously significant without penalizing the production of the main and auxiliary value products of the process (lime or dolime and bleed), while allowing to improve the purity content of the obtained bleed and the activity of the sorbent during the carbonation. Preferably, during said continuous extraction, a fraction of less than 15 wt %, preferably of 2 to 10 wt %, of said CaCO3—CaO based charge is extracted. It results from experimentation that increasing said fraction results in a significant decrease of the impurities in the bleed.
Consequently, according to the invention, in parallel to the production of lime or dolime, there is an additional production of a marketable Ca-product which is not to be removed as in the prior art.
Advantageously, the calcination of said downward moving calcareous or dolomitic material is carried out at a temperature of 750 to 1750° C., preferably 800 to 1350° C., depending on the searched properties of the final product.
According to an embodiment of the invention, the method comprises, during the CO2-depletion step, maintaining said carbonation at a temperature below 700° C., preferably from 600 to 670° C., in particular of about 650° ° C., by means of a first heat recovery from the transferred gaseous effluent. At a temperature of 700° C. calcination of CaCO3 may start. Thus, the carbonation is operated just under this temperature to get fast kinetics of carbonation while avoiding the reverse calcination reaction. Consequently, as the carbonation reaction is exothermic, it is necessary to extract heat from the reaction, particularly by means of a heat exchange with an external fluid. A second heat recovery from the CO2-depleted gaseous effluent is also possible after its removal from the carbonation since the temperature of this effluent is high, particularly of about 650° C.
According to a particular embodiment of the invention, the method comprises carrying out said step of calcination of the separated CaCO3—CaO based charge at a temperature from 850° C. to 1200° C., preferably from about 880 to 1050° C., more preferably from 900° ° C. to 1000° ° C., and for example around 920° C. or 950° ° C. and a third heat recovery from the collected CO2-concentrated gas stream. As this calcination is carried out under high CO2 partial pressure, such temperatures are maintained to accelerate the calcination while still producing a high specific surface CaO appropriate for capturing CO2.
Said first, second and/or third heat recoveries may consist of a conversion of calories into electrical power or of other heat recovery applications such as drying, district heating . . . .
The method according to the invention may advantageously comprise, for forming said oxidizing gas of said combustion of the calcination step, a step of mixing pure dioxygen with a fraction of the collected CO2-concentrated gas stream. The combustion of the fuel with pure oxygen would give rise to flame temperatures which are very high for the usual equipment. Also, advantageously, it is planned to introduce CO2 simultaneously for diluting the oxygen. Advantageously a fraction of the collected gas stream rich in CO2 is taken and mixed with oxygen. Instead of the usual oxidizing mixture O2+N2 of the air, a O2+CO2 mixture is thus obtained with the appropriate flame temperature, while producing during the calcination step a gas stream which is increasingly concentrated in CO2.
The present invention concerns also an installation for the production of lime or dolime, comprising at least one kiln, each of which comprises
According to the invention, said installation further comprises
In the installation according to the invention, the gaseous effluent containing CO2 may be generated by one or several kilns which are able to produce lime or dolime. Therein the raw material is supplied at the top of the kiln and the calcined material is discharged at the bottom, after being cooled. Such kilns are for example rotary furnaces, shaft(s) kilns, such as vertical shaft kilns, annular shaft kilns, parallel flow regenerative kilns, and so on, wherein the calcareous or dolomitic material moves downward according to a vertical direction or to a sloping direction.
According to the ash or sulphur content of the gaseous effluent containing CO2 released at the top exit of said at least one kiln, said means for transferring said gaseous effluent may consist only in a duct connecting said top exit to the carbonation reactor or may additionally comprise appropriate anti-pollution equipment.
The installation comprises any separation device able to separate a particular solid material from a gas, as for example a cyclone.
In the carbonation reactor the sorbent material may advantageously be in the form of a fluidized bed or a moving bed. In the calcination reactor the CaCO3—CaO based charge may also advantageously be in the form of a fluidized bed or a moving bed.
Preferably the installation may further comprise a first heat exchanger which is arranged within the carbonation reactor to allow recovery by an external fluid of calories released during carbonation. The temperature within the carbonation reactor may so be maintained at a temperature lower than 700° C. At least one second heat exchanger may be arranged to allow a heat recovery by an external fluid from the CO2-depleted gaseous effluent which is removed from the first separation device. At least one third heat exchanger may be arranged to allow a heat recovery by an external fluid from the CO2-concentrated gas stream collected from the second separation device. Said external fluid is particularly water which, in said first, second and/or third heat exchangers, passes to the vapor state and may be supplied to steam turbines for producing electricity.
Other details and features of the method and the installation according to the invention may result from the claims.
An installation according to the invention is now disclosed by means of
The illustrated installation comprises a conventional lime kiln 1, wherein 175 tpd (ton per day) of lime are produced. 12.5 tph (ton per hour) of limestone having a CaCO3 content of 96 wt % are introduced through the top supply 2 and are calcined into lime in contact with fumes obtained by combustion of 1.6 tph of biomass supplied in 3 in the presence of primary air as carrier gas and of secondary air supplied in 4. 7.3 tph of lime, cooled by a cooling air introduced in 6 and having a CaO content of 93 wt % are discharged through the bottom discharge 5. A gaseous effluent is released from the kiln through the top exit 7 and, by means of the connecting duct 9, is transferred to a carbonation reactor 8 via a purification system 16, which comprises a dust collector, a dryer and/or a desulphurization unit.
As it results from table 1, the CO2 volume concentration in the gaseous effluent is very low with respect to the N2 concentration (65%). In such condition a separation of both components is not easily feasible and would be costly due to the large gas volume to be treated.
The carbonation reactor 8 is provided with a fluidized bed of a sorbent material based on CaO supplied by a recycling duct 10. There 90% of the CO2 of the gaseous effluent is captured by CaO, which is carbonated into CaCO3 according to an exothermic reaction. Within the carbonation reactor 8 the temperature of the gaseous effluent must be maintained at a value of about 650° C., under the start of the reverse calcination reaction, by means of a heat exchanger 11 which communicates with a turbine 12 in order to convert heat into electrical power. A power of 2.3 MWe is so obtained.
From the carbonation reactor 8, the gaseous effluent carrying a CaCO3—CaO based charge is supplied via a transfer duct 13 to a cyclone 14 from the top of which a CO2-depleted gaseous effluent is released.
Now the gaseous effluent exiting from the top of the cyclone 14 contains only traces of CO2 and may be removed in the atmosphere. Before this removal the gas passes through a heat exchanger 15 which communicates with a turbine 17 in order to convert heat into electrical power. A power of 1.5 MWe is so obtained.
The solid particles of the separated CaCO3—CaO based charge exit from the bottom of the cyclone 14 and are supplied to the bottom of the calcination reactor 19 by means of the transfer duct 18.
The calcination reactor 19 is also supplied with a fuel containing almost no impurities. In the illustrated case 1857 Nm3/h of natural gas (i.e. a fuel containing no ash and no sulphur) are introduced into the calcination reactor 19 via the inlet 20 and 23 tph of an oxidizing gas containing dioxygen and CO2 are supplied via the introduction duct 21. The calcination reactor is operated at a temperature of about 900° C. in order to accelerate the calcination and produce a high specific surface CaO during the calcination of the CaCO3—CaO based charge.
From the calcination reactor 19, the gaseous effluent carrying active CaO is supplied via a transfer duct 22 to a cyclone 23 from the top of which a CO2-concentrated gas stream is collected.
The CO2 concentration in the gas stream exiting from the cyclone 23 is extremely high. Such a gas may be industrially valorized, for example for technical CO2 production, or for sequestration. Before collection, the gas stream passes through a heat exchanger 24 which communicates with a turbine 25 in order to convert heat into electrical power. A power of 3.17 MWe is so obtained.
The active CaO-based sorbent material exits separately from the bottom of the cyclone 23 and is recycled to the bottom of the carbonation reactor 8 by means of the recycling duct 10.
Before said combustion of a fuel poor in impurities in the presence of an oxidizing gas containing dioxygen and CO2, dioxygen is mixed with a fraction of the collected CO2-concentrated gas stream. 5 tph of oxygen produced at a concentration of 90% by an air separation unit 26 and 18 tph of CO2-concentrated gas recycled by means of the recirculation duct 27 are mixed and introduced in the calcination reactor 19 by means of the introduction duct 21, as oxidizing gas. Obviously recirculated CO2-concentrated gas and pure dioxygen may be fed separately to the calcination reactor wherein their mixture takes place in situ.
During the capture of CO2 in the carbonation reactor 8, there is formation of CaCO3 as above explained, but CaO of the fluidized bed participates only partially to the carbonation. Consequently, the CaCO3—CaO based charge which circulates between the carbonation reactor 8 and the calcination reactor 19 contains not only particles of CaCO3 but also particles of CaO.
CaO becomes less and less active with increasing cycles. There is an increased sintering of the particles. And, in order to keep a CO2 capture efficacy of at least 30% of active CaO in the CaO-based sorbent material, a bleed flowrate of 0.8 tph of CaCO3—CaO based charge (2 wt % of the CaCO3—CaO based charge) is extracted from the transfer duct 18 via the extraction duct 28. For compensation, a make-up of 1.06 tph of fresh limestone having a CaCO3 content of 96 wt % is introduced into the calcination reactor via the entrance 29. As the fuel used in the calcination reactor 19 does not contain any ash or sulphur and the compensatory limestone of the make-up has a high purity degree, the recycled CaO-based sorbent material is very pure as well as the circulating CaCO3—CaO based charge which contains only Ca-based components. Consequently the bleed is no waste and may be used in several fields, such as the gas or water epuration, the agriculture, the paper manufacture, the civil engineering, etc.
The captured CO2 in the gas stream collected from the calcination reactor is summarized in Table 5
Simultaneously the gas stream collected from the calcination reactor is very concentrated in CO2 and exploitable or sequestrable, the bleed is a valuable Ca product manufactured in parallel to the production of lime or dolime and the need of electricity of the installation, particularly the air separation unit, is satisfied by the production of the turbines.
The method according to the invention will now be disclosed in a lime plant comprising several furnaces and producing 2000 tpd of lime, the fuel being lignite. The gaseous effluents of all furnaces are collected together and sent into a carbonator-calcinator system as illustrated on
66 tph of oxygen produced at a concentration of 90% by the air separation unit 26 and 222 tph of CO2-concentrated gas recycled by means of the recirculation duct 27 are mixed as oxidizing gas and introduced in the calcination reactor. 23 014 Nm3/h of natural gas (i.e. a fuel without ash or sulphur) are also supplied to this reactor, as fuel.
In order to keep a CO2 capture efficacy of at least 30% of active CaO in the CaO-based sorbent material, a bleed flowrate of 10 tph (2 wt %) of CaCO3—CaO based charge is extracted from the transfer duct 18 via the extraction duct 28. For compensation, a make-up of 13 tph of fresh limestone having a CaCO3 content of 98 wt % is introduced into the calcination reactor.
The electrical power produced with the steam turbines is: 30 MWe for the turbine 12, 21 MWe for the turbine 17 and 39 MWe for the turbine 25.
The captured CO2 in the gas stream collected from the calcination reactor is summarized in Table 10
The method according to the invention will now be disclosed in the same lime plant as in Example 2. The gaseous effluents of all furnaces are collected together and sent into a carbonator-calcinator system as illustrated on
Obviously the gaseous effluent which penetrates into the carbonation reactor 8 and the CO2-depleted gaseous effluent which exits from the cyclone 14 show the same features as in the tables 6 and respectively 7 of Example 2.
65 tph of oxygen produced at a concentration of 90% by the air separation unit 26 and 220 tph of CO2-concentrated gas recycled by means of the recirculation duct 27 are mixed as oxidizing gas and introduced in the calcination reactor. 40 tph of the above-mentioned lignite are also supplied to this reactor, as fuel.
In order to keep a CO2 capture efficacy of at least 30% of active CaO in the CaO-based sorbent material, a bleed flowrate of 16 tph (3 wt %) of CaCO3—CaO based charge is extracted from the transfer duct 18 via the extraction duct 28. For compensation, a make-up of 20 tph of fresh limestone having a CaCO3 content of 98% is introduced into the calcination reactor. The bleed contains 16 wt % of impurities and is still a valuable product.
The electrical power produced with the steam turbines is: 31 MWe for the turbine 12, 21 MWe for the turbine 17 and 39 MWe for the turbine 25.
The captured CO2 in the gas stream collected from the calcination reactor is summarized in Table 13.
The method according to the invention will now be disclosed in the same lime plant as in Example 2. The gaseous effluents of all furnaces are collected together and sent into a carbonator-calcinator system as illustrated on
Obviously the gaseous effluent which penetrates into the carbonation reactor 8 and the CO2-depleted gaseous effluent which exits from the cyclone 14 show the same features as in the tables 6 and respectively 7 of Example 2.
78 tph of oxygen produced at a concentration of 90% by the air separation unit 26 and 265 tph of CO2-concentrated gas recycled by means of the recirculation duct 27 are mixed as oxidizing gas and introduced in the calcination reactor. 48 tph of the above-mentioned lignite are also supplied to this reactor, as fuel.
In order to keep a CO2 capture efficacy of at least 30% of active CaO in the CaO-based sorbent material, a bleed flowrate of 50 tph (10 wt %) of CaCO3—CaO based charge is extracted from the transfer duct 18 via the extraction duct 28. For compensation, a make-up of 66 tph of fresh limestone having a CaCO3 content of 98% is introduced into the calcination reactor. The bleed contains 8.25 wt % of impurities and is a valuable product.
The electrical power produced with the steam turbines is: 32 MWe for the turbine 12, 21 MWe for the turbine 17 and 47 MWe for the turbine 25.
The captured CO2 in the gas stream collected from the calcination reactor is summarized in Table 13.
A comparison between Example 3 and Example 4 shows that increasing extraction of the bleed rate from 3% to 10% of CaCO3—CaO based charge results in a significant decrease of the bleed impurities (ash+CaSO4+other impurities) from 16.43% to 8.25%.
Other embodiments and variants of the present invention may be taken into account within the scope of the claims.
For example the heat recovery may be of any type, not only electrical.
In summary, such plants avoid a high participation to the greenhouse effect and the mass flows and power production are at a favourable level making the supply of make-up possible locally from the plant quarry, the bleed highly valuable and the power production profiting to local communities.
| Number | Date | Country | Kind |
|---|---|---|---|
| 21173090.8 | May 2021 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/062544 | 5/10/2022 | WO |
