Patent Description:
In known processes for producing cement clinker, raw meal is fed into a calcination device, in particular a rotary kiln, after it has been preheated and partially decarbonated in a multistage preheater system by using the heat of combustion gases exhausted from the rotary kiln. The preheated raw material is fed into the rotary kiln via the kiln inlet and travels to the kiln outlet while being calcined at temperatures of up to <NUM>.

Carbon dioxide (CO<NUM>) is the most significant long-lived greenhouse gas in the Earth's atmosphere. The use of fossil fuels and deforestation have rapidly increased its concentration in the atmosphere, leading to global warming. Carbon dioxide also causes ocean acidification, because it dissolves in water to form carbonic acid.

The cement industry is an important emitter of CO<NUM>. Within the cement production process, significant amounts of CO<NUM> are generated during the decarbonation of raw meal (containing CaCO<NUM>) to lime (CaO). During the production of Portland cement clinker about <NUM>,<NUM> tons of CO<NUM> per ton of Portland cement clinker are emitted by the calcination of the raw materials and from the fuel combustion in the rotary kiln.

The use of alternative fuels, in particular renewable fuels, in the rotary kiln burner may reduce the amounts of greenhouse gases. However, substantial amounts of CO<NUM> are still produced by the decarbonation of raw meal and emitted into the atmosphere.

It has been proposed to use carbon capture and sequestration methods in order to reduce or prevent the emission of CO<NUM> from industrial processes into the atmosphere. Such methods comprise capturing CO<NUM> from flue gases for storage or for use in other industrial applications. However, such methods require the separation of CO<NUM> form the flue gases, wherein respective separation plants involve high capital and operating expenditures.

<CIT> discloses a method for using the CO<NUM> contained in an exhaust gas of a cement manufacturing plant for carbonating carbonatable waste material, such as concrete demolition waste, fly ash or slag, wherein the carbonated waste material can be used as a supplemental cementitious material in cement compositions. The carbonation is carried out in a wet scrubber, a semi-dry scrubber or a dry scrubber, so that a separate installation is needed for the carbonation process. Therefore, a disadvantage of the method is that it requires high capital expenditures.

<CIT> discloses a method of manufacturing a supplementary cementitious material from recycled concrete fines, comprising the steps: providing recycled concrete fines as starting material, carbonation of the starting material in a carbonation atmosphere providing carbonated starting material, and de-agglomerating the carbonated starting material to form the supplementary cementitious material. The carbonation device can be placed between the cement plant exhaust gas filter and its stack. It might even be beneficial to place it before the filter to achieve some filtering of the exhaust gas. Therefore, the instant invention aims at further reducing the CO<NUM> footprint of a cement plant without significantly increasing the capital and operating expenditures.

In order to solve these objectives, the invention provides a method of producing a synthetic carbonated mineral component in a cement manufacturing plant, wherein the cement manufacturing plant comprises a calcination device for producing Portland clinker by decarbonating cement raw meal while releasing a CO<NUM> enriched exhaust gas and further comprising an exhaust gas installation for directing a flow of the CO<NUM> enriched exhaust gas from the calcination device to an exhaust stack of the cement manufacturing plant, wherein the method comprises the steps of:.

wherein the exhaust gas installation comprises a cement kiln bypass installation for extracting kiln bypass dust and exhaust gas from the calcination device, and a bypass filter connected to an exhaust end of the kiln bypass <NUM> installation, wherein the step of introducing the carbonatable substance into the exhaust gas installation comprises introducing the carbonatable substance into the kiln bypass installation upstream of the bypass filter, and wherein the step of removing the synthetic carbonated mineral component from the exhaust gas installation comprises separating the synthetic material from the exhaust gas by means of <NUM> said bypass filter and removing the synthetic material from the bypass filter.

The invention is based on the idea to carbonate a carbonatable substance by injecting it directly into the gas streams of a Portland clinker manufacturing process that is enriched in carbon dioxide. In this way, the carbon dioxide generated within the clinker manufacturing process by the combustion of fuels and by the decarbonation of raw meal is directly used within the existing process and its content in the exhaust gas is thus reduced. In other words, an in-situ carbonation of carbonatable substances, such as waste materials, is integrated into the clinker manufacturing process of a cement plant. The in-situ carbonation process does not require any separate installations and therefore minimizes the capital expenditures needed for the carbonation process.

Rather, the existing exhaust gas installation is used for the carbonation process, i.e. the structural components of the cement plant that allow the CO<NUM> enriched exhaust gas to flow from the calcination device to an exhaust stack of the cement manufacturing plant. In most cement plants, two main CO<NUM> enriched exhaust gas streams are existing. The first exhaust gas stream is coming from the preheater of the cement plant, in which cement raw meal is preheated in counter-current flow to exhaust gas coming from the calcination device, i.e. the rotary kiln. When the preheater includes a Lepol heat exchanger, the preheating is done in a cross-current flow to the exhaust gas. This first stream is guided from the preheater exhaust gas outlet to an exhaust stack via a plurality of exhaust gas installation components, such as gas conditioning tower, raw meal mill, a filter unit and various ducts that interconnect said components. The second exhaust gas stream is coming from the kiln bypass that is used to extract kiln bypass dust from the calcination device, i.e. the rotary kiln. The second stream is guided from the kiln bypass to an exhaust stack via a plurality of exhaust gas installation components, such as a quenching chamber, a filter unit and various ducts that interconnect said components.

The carbonatable material may generally be added to the first stream and/or the second stream at any suitable point thereof. However, it is preferably avoided to add the carbonatable substance at a point, where the carbonatable substance could be mixed with a non-hydraulic dust.

The carbonation step may be carried out at various temperature ranges. The temperature of the CO<NUM> enriched exhaust gas decreases as it flows from the calcination device towards the exhaust stack. Therefore, the carbonation temperature depends on at which distance from the calcination device the carbonatable substance is introduced into the exhaust gas installation.

Two possible temperature ranges are preferred:.

The carbonation at the lower temperature range can be carried out in any point between the preheater gas outlet and the kiln production filter or before the chlorine bypass filter. In this atmosphere the content of carbon dioxide is around <NUM> to <NUM>%, and that of water is between <NUM>-<NUM>%. This environment enables the calcium silicate hydrates eventually contained in the carbonatable substance to recarbonate, providing calcium carbonates and fine silica. The carbonation at the higher temperature range can be achieved by injecting the carbonatable substance in the quenching air of a cement kiln bypass probe. This injection further contributes to the quenching and chlorine dust dilution, and reduces fresh air dilution, improving the carbon dioxide content of the exhaust gas and lowering the size of the bypass filter. The bypass can be adapted with a <NUM>-stage cooling step to increase and ensure residence time at this temperature, and increase the carbonation rate of the carbonatable material.

More specifically, the following preferred embodiments can be mentioned.

According to a preferred embodiment, the kiln bypass installation comprises a quenching chamber for cooling the kiln bypass dust and the exhaust gas, and wherein the carbonatable substance is introduced into the quenching chamber.

The carbonatable substance can be introduced into the quenching chamber together with the quenching air. In this connection, a preferred embodiment provides that a quenching air duct is connected to the quenching chamber for introducing quenching air into the quenching chamber, and wherein the carbonatable substance is introduced into the quenching chamber via the quenching air duct.

Generally speaking, the carbonation is conducted until the carbonatable substance has turned into a synthetic carbonated mineral component having pozzolanic properties. For example, the carbonation is conducted until the CaCO<NUM> content of the carbonatable substance has increased by least <NUM> wt. -%, preferably by at least <NUM> wt. -%, that is the CaCO<NUM> content of the carbonatable substance has increased from x% to (x+<NUM>)%, preferably from x% to (x+<NUM>)%.

Being a synthetic carbonated mineral component and due to its pozzolanic properties, the carbonated material can be used in cementitious compositions as a replacement material for Portland cement. In this way, the Portland clinker content and thus the CO<NUM> footprint of the composition may be reduced.

Preferably, the synthetic carbonated mineral component obtained from the process of the invention may be mixed with the Portland clinker coming from the calcination device of the same plant and optionally co-ground in a cement mill.

Alternatively, the synthetic carbonated mineral component obtained from the process of the invention may be mixed with a Portland cement in a mixing process. In this case, the synthetic carbonated mineral component is preferentially separately ground beforehand to increase its fineness, and increase its pozzolanic activity.

Depending on the effective pozzolanic activity of the synthetic carbonated mineral component and other characteristics such as water demand, the clinker amount in the Portland cement can be adapted to produce the cement having the desired performance in terms of setting times and strength development.

The synthetic carbonated mineral component can be used as is and blended in a CEM I, or optionally pre-ground prior to blending to reach a specific fineness, or co-ground with the other cement components.

If the carbonation step occurs in the main filter, in the raw meal mill, or in the gas bypass filter, the carbonated material further contains chlorides and alkalis, which improve the reactivity of this material when used as a mineral component in cement compositions.

Additionally, the carbonation of these materials also ensures that the free lime in the material is carbonated, and therefore eliminated. This solves additional durability problems caused by excess free lime in binders.

The carbonatable substance may be any carbon dioxide reactive solid substance that can be carbonated when being contacted with the CO<NUM> enriched exhaust gas. According to the invention, the carbonatable substance is taken from an external source, which means that it is a material that is not inherent in a Portland clinker manufacturing process. Therefore, the carbonatable substance excludes any material flow usually occurring in a cement plant, such as raw meal, preheated raw meal, pre-calcined raw meal, Portland clinker and cement kiln dust.

Preferably, a waste material can be used as a source for the carbonatable material so that such waste material can be recycled. The carbonatable waste material can be selected from the fine fractions of recycled concrete, a concrete mud from a ready-mix plant, or any mineral component with a sufficient amount of metal oxide that is able to carbonate, or mixtures thereof. Preferably, the metal oxide is calcium oxide, or magnesium oxide, or mixtures thereof.

The carbonatable substance can be any material defined in the standard EN <NUM>-<NUM> published in April <NUM>, as long as the total metal oxide content, such as the CaO content, in the carbonatable substance is at least <NUM> wt. -%, preferably at least <NUM> wt. Blast furnace slag or fly ash having a high calcium oxide content are for example suitable for being used as the carbonatable substance.

Further, the carbonatable substance may be a concrete mud that is obtained by recuperating mud from a decantation basin in a concrete ready-mix plant and reducing the water content of the mud. The excess water of the concrete mud may be removed by known processes such as a filter press, or by making use of a heated screw integrated in the cement process. The concrete mud can be pre-dried in order to reduce its free water content to <NUM> wt. -% or lower, preferably <NUM> wt. -% or lower. Free water is defined by water that is able to evaporate at temperatures below <NUM>.

The concrete mud can be further dried in the raw meal mill or in a dedicated mill fed by exhaust gas coming from the preheater.

According to a further preferred embodiment, the carbonatable material is obtained from recycled concrete by crushing recycled concrete, separating a fine fraction of said crushed recycled concrete from a coarse fraction and using the fine fraction as said carbonatable material, wherein the fine fraction is preferably composed of particles having a particle size of <NUM>-<NUM>, preferably <NUM>-<NUM>, in particular <NUM>-<NUM>. This fine fraction is composed of hardened cement paste, and fine quartz or limestone sand coming from the sand in the recycled concrete.

The method of the present invention further contains an optional step of grinding and drying the carbonatable substance. The substance may be pre-ground in order to increase the specific surface area of the powder, and increase its potential to carbonate.

This optional grinding step of the fine fraction can for example be carried out using a ball or a vertical mill, either fed by atmospheric gas for drying only, or by exhaust gas to also carbonate, like a raw meal mill. Cement paste would be deagglomerated from aggregates by attrition. The finer ground material may be separated by means of a dynamic or a static separator. The rejects of the separator may be removed from the system, and be for example reused as a silica additive in the cement plant for the production of clinker.

When the carbonatable substance is construction demolition waste, it may further contain chlorine bypass dust that is produced at the cement plant. The carbonatable substance may contain chlorine bypass dust in an amount of between <NUM>,<NUM> and <NUM> wt. -%, preferably between <NUM> and <NUM> wt. -%, of the carbonatable substance.

The invention will now be described in more detail with reference to the attached drawings. Therein, <FIG> shows a layout of a cement plant, <FIG> shows a second layout of a cement plant, <FIG> shows a layout of a cement plant for carrying out the method of the invention, and <FIG> shows another layout of a cement plant.

<FIG> schematically illustrates a cement plant.

In the cement clinker production plant <NUM> raw meal <NUM> is ground in a raw meal mill <NUM> and the ground raw meal is charged into a preheater string <NUM>, where it is preheated in counter-current to the hot exhaust gases <NUM> coming from a rotary clinker kiln <NUM>. The preheater string <NUM> comprises a plurality of interconnected preheaters, such as cyclone suspension-type preheaters. The preheated and optionally pre-calcined raw meal is then introduced into the rotary kiln <NUM>, where it is calcined to obtain cement clinker. The clinker leaves the rotary kiln <NUM> and is cooled in a clinker cooler <NUM>. The cooled clinker is charged into a cement mill <NUM>, where the clinker is ground to a desired fineness, optionally together with other components of the final product, such as supplementary cementitious substances and gypsum.

In <FIG>, the flow of solid material is shown with solid lines, while the flow of gasses is shown with dotted lines. It can be seen that cooling air <NUM> is introduced into the clinker cooler <NUM>, where the air is heated in heat exchange with the clinker. The heated air leaving the clinker cooler <NUM> is introduced into the rotary kiln <NUM>, where the preheated raw meal is calcined, i.e. decarbonated, while releasing CO<NUM>. The CO<NUM> enriched exhaust gas <NUM> is introduced into the preheater string <NUM> in order to preheat the raw meal. The exhaust gas withdrawn from the preheater string <NUM> is introduced into a gas conditioning tower <NUM>, where water may be injected in order to cool the exhaust gas. In a typical operation mode, the cooled exhaust gas may be introduced into the raw meal mill <NUM> via the line <NUM> for preheating the raw meal and further cooling the exhaust gas. The exhaust gas leaving the raw meal mill is loaded with fine particles of raw meal and is introduced into the main filter <NUM> for separating said fine particles from the exhaust gas. The exhaust gas is withdrawn from the main filter <NUM> at <NUM> and directed to an exhaust stack (not shown).

If the raw meal mill <NUM> is not in operation, the cooled exhaust gas coming from the gas conditioning tower <NUM> is directly led to the main filter <NUM> via the line <NUM>, where cement kiln dust entrained from rotary kiln <NUM> is separated from the exhaust gas. The separated particles collected in the main filter <NUM> may be introduced into the cement mill <NUM> via the line <NUM> to be co-ground with the clinker.

In order to adapt such a typical configuration of a cement manufacturing plant for recycling waste material and for reducing the CO<NUM> footprint of the cement manufacturing process, the method provides for the introduction of carbonatable waste substances coming from an external source into the process. A storage container for the carbonatable substance is denoted by reference numeral <NUM>. The storage container <NUM> may, e.g., contain a fine fraction obtained from crushed recycled concrete. The carbonatable material may optionally be introduced into a mill <NUM>, such as a rotary mill via the line <NUM>, in order to reduce the particle size of the carbonatable substance. The mill <NUM> may be fed by atmospheric gas or preferably by exhaust gas coming from the gas conditioning tower <NUM> via line <NUM>, in order to perform carbonation within the mill <NUM>.

During a period, in which the raw meal mill <NUM> is not operating, the ground carbonatable substance is introduced into the flow of CO<NUM> enriched exhaust gas via the line <NUM>. More specifically, the ground carbonatable substance is introduced into the exhausts gas duct that connects the gas conditioning tower <NUM> with the main filter <NUM>. Alternatively, the carbonatable substance is introduced into said duct directly via the line <NUM>, i.e. without having been ground.

The carbonatable substance is entrained by the exhaust gas and is transported through the line <NUM> to enter the main filter <NUM>. During its residence time in the duct that connects the gas conditioning tower <NUM> and the main filter <NUM> and during its residence time in the main filter <NUM>, the carbonatable substance gets carbonated by reacting with the CO<NUM> contained in the exhaust gas, thereby reducing the CO<NUM> content of the exhaust gas. By carbonating the carbonatable substance, a synthetic carbonated mineral component is obtained that is withdrawn from the main filter <NUM> together with the other fine particles that are retained by the filter. The synthetic material component may be introduced into the cement mill <NUM> together with the other fine particles that are retained by the filter <NUM>.

In order to increase the residence time of the carbonatable material in the CO<NUM> enriched exhaust gas so as to increase the carbonation rate, the material removed from the main filter <NUM> can be recirculated into the exhaust gas via the line <NUM>.

In order to avoid that the synthetic carbonated mineral component is mixed with non-hydraulic dust, such as fine particles of raw meal, the carbonation process as described above is only carried out when the raw meal mill <NUM> is not in operation with cement raw meal. Introducing the carbonatable substance into the exhaust gas during mill shutdown enables to lower the exhaust gas temperature and reduce the water injection into the gas conditioning tower <NUM>, and further enables the trapping of pollutants, typically mercury, sulfates, chlorides, or organic pollutants such as dioxins and furans, from the exhaust gas by absorbing the pollutants on the particles of the carbonatable substance.

Alternatively, during a period, in which the raw meal mill <NUM> is not operating to grind cement raw meal, it may be used to grind and carbonate the carbonatable substance. To this end, the carbonatable substance from container <NUM> is fed via line <NUM> into the exhausts gas duct that connects the gas conditioning tower <NUM> with the main filter <NUM> and is entrained via line <NUM> into the raw meal mill <NUM>, in order to dry, grind and carbonate the carbonatable substance.

One part of the carbonated material is collected in the finished product circuit of the raw meal mill <NUM> and another part of the carbonated material that is entrained by the exhaust gas to the main filter <NUM> is collected there. The collected carbonated material is either transported directly to cement mill <NUM> or recirculated via line <NUM> to the main filter <NUM> or recirculated into the raw meal mill <NUM> or recirculated into the mill <NUM>, in order to pursue carbonation.

In another method shown in <FIG>, the raw meal mill <NUM> has a separate filter <NUM>, so that a simultaneous operation of a) the raw meal mill <NUM> for grinding cement raw meal and of b) the carbonation process by entraining the carbonatable substance originating from container <NUM> into the main filter <NUM> may be realized.

<FIG> shows an embodiment, wherein the carbonatable substance is introduced into a kiln bypass duct. As far as the same reference numerals are used as in <FIG>, the same structural components are concerned. In the embodiment of <FIG> the carbonatable substance is introduced into the flow of exhaust gas in the kiln bypass duct <NUM>, which is a duct for withdrawing a partial amount of the cement kiln dust loaded atmosphere at a location between the exhaust gas outlet of the rotary kiln <NUM> and the preheater string <NUM>. The exhaust gas is cooled in a quenching chamber by injecting cooling air <NUM> into the kiln bypass duct <NUM>. Further downstream, a second cooling installation <NUM> is optionally provided. The cooled mixture of cement kiln dust and exhaust gas is introduced into a bypass filter <NUM>, where the cement kiln dust is separated from the exhaust gas <NUM>. The exhaust gas is fed to an exhaust stack (not shown), while the cement kiln dust is conveyed to the cement mill <NUM> via the line <NUM>.

A storage container for the carbonatable substance is denoted by reference numeral <NUM>. The storage container <NUM> may, e.g., contain a fine fraction obtained from crushed recycled concrete. The carbonatable material may optionally be introduced into a mill <NUM>, such as a rotary mill, in order to reduce the particle size of the carbonatable substance. The carbonatable substance coming from the mill <NUM> is added to the flow of cooling air <NUM> and introduced into the kiln bypass duct <NUM>. Alternatively, the carbonatable substance is directly fed from the storage container <NUM> into the flow of cooling air <NUM>, bypassing the mill <NUM>.

The carbonatable substance is entrained by the exhaust gas and enters the bypass filter <NUM>. During its residence time in the kiln bypass duct <NUM> and during its residence time in the bypass filter <NUM>, the carbonatable substance gets carbonated by reacting with the CO<NUM> contained in the exhaust gas, thereby reducing the CO<NUM> content of the exhaust gas. By carbonating the carbonatable substance, a synthetic carbonated mineral component is obtained that is withdrawn from the bypass filter <NUM> together with the cement kiln dust that is retained by the filter. The synthetic material component may be introduced into the cement mill <NUM> together with the cement kiln dust that is retained by the filter <NUM>.

In order to increase the residence time of the carbonatable material in the CO<NUM> enriched exhaust gas so as to increase the carbonation rate, the material removed from the bypass filter <NUM> can be recirculated into the exhaust gas via the line <NUM>.

The method described with reference to <FIG> has the effect that the carbonatable substance is first carbonated at a high temperature of <NUM> to <NUM> when being in the first cooling stage (cooling air <NUM>) and is then carbonated at a low temperature of <NUM>-<NUM> when travelling through the second cooling stage <NUM>. Therefore, both temperature ranges that are optimal for the carbonation process can be used.

Additional benefits of adding the carbonatable substance into the kiln bypass duct include: the reduction of cooling/quenching air due the cooling effect brought about by the introduction of the carbonatable substance, and the reduction of the alkali and chloride content by dilution.

Claim 1:
A method of producing a synthetic carbonated mineral component in a cement manufacturing plant, wherein the cement manufacturing plant comprises a calcination device for producing Portland clinker by decarbonating cement raw meal while releasing a CO<NUM> enriched exhaust gas and further comprising an exhaust gas installation for directing a flow of the CO<NUM> enriched exhaust gas from the calcination device to an exhaust stack of the cement manufacturing plant, wherein the method comprises the steps of:
a) providing a carbonatable substance from an external source,
b) introducing the carbonatable substance into the exhaust gas installation for contacting the carbonatable substance with the CO<NUM> enriched exhaust gas,
c) carbonating the carbonatable substance by reacting the carbonatable substance with CO<NUM> contained in the CO<NUM> enriched exhaust gas, thereby obtaining the synthetic carbonated mineral component,
d) removing the synthetic carbonated mineral component from the exhaust gas installation, wherein the exhaust gas installation comprises a cement kiln bypass installation for extracting kiln bypass dust and exhaust gas from the calcination device, and a bypass filter connected to an exhaust end of the kiln bypass installation, wherein the step of introducing the carbonatable substance into the exhaust gas installation comprises introducing the carbonatable substance into the kiln bypass installation upstream of the bypass filter, and wherein the step of removing the synthetic carbonated mineral component from the exhaust gas installation comprises separating the synthetic material from the exhaust gas by means of said bypass filter and removing the synthetic material from the bypass filter.