Abstract

The method of simultaneously reducing carbon dioxide (CO₂) emissions and sulfur dioxide (SO₂) emissions produced by the combustion of carbon-containing matter in a hearth consists in injecting into the hearth a calcium-based agent, a fraction of which absorbs SO₂ after decarbonization, and then, after the flue gases have been subjected to intermediate cooling, in causing them to transit via a first reactor and in putting them in contact therein with the other fraction of the absorbant that has not reacted with SO₂ so as to capture CO₂ from the flue gases by carbonization, then, in a separator, in extracting the solids contained in the flue gases output from the first reactor so as to subject them to heat treatment in a second reactor in order to extract CO₂ therefrom by decarbonization and in order to recycle the resulting regenerated CO₂ absorbant to the first reactor.

The invention relates to installations for burning carbon-containing matter, e.g. fossil fuels or waste, in a hearth, in particular a hearth operating as a fluidized bed. More particularly, it relates to a method of reducing both carbon dioxide (CO₂) emissions and also sulfur dioxide (SO₂) emissions in the flue gases produced by this type of installation.

Background of the invention

It is known that the combustion of fossil fuels such as coal or waste produces gaseous emissions of CO₂ and of SO₂, and that injecting calcium carbonate (CaCO3) into the hearth makes it possible to reduce SO₂ emissions in situ. Unfortunately, such reduction in SO₂ is accompanied by production of CO₂ in addition to the CO₂ coming from the combustion of the carbon-containing matter. CO₂ is a 'greenhouse' gas which would appear to contribute to global warming.

In addition, injecting calcium carbonate for desulfurizing the flue gases suffers from the drawback of producing, in the hearth, a large quantity of ash that is too rich in calcium sulfate (CaSO₄) and in lime (CaO) to be easy to recycle.

Objects and summary of the invention

An object of the invention is to remedy those drawbacks.

To this end, the invention provides a method of simultaneously reducing carbon dioxide emissions and sulfur dioxide emissions produced by the combustion of carbon-containing matter in a hearth, said method consisting in injecting into the hearth a calcium-based agent, a fraction of which absorbs SO₂ after decarbonization, and then, after the flue gases have been subjected to intermediate cooling, in causing them to transit via a first reactor and in putting them in contact therein with the other fraction of the absorbant that has not reacted with SO₂ so as to capture CO₂ from the flue gases by carbonization, then, in a separator, in extracting the solids contained in the flue gases output from the first reactor so as to subject them to heat treatment in a second reactor in order to extract CO₂ therefrom by decarbonization and in order to recycle the resulting regenerated CO₂ absorbent to the first reactor.

More detailed description

Combustion of carbon-containing matter such as fossil fuels or waste produces gaseous emissions of CO₂ and of SO₂. In known manner, combustion in a fluidized bed, for example, makes it possible to obtain effective desulfurization of the flue gases when an absorbant based on calcium such as calcium carbonate is injected into the hearth, such desulfurization taking place by means of the following reactions:

Such fluidized bed combustion thus suffers from the drawback that it generates CO₂ in addition to the CO₂ that results from burning the organic carbon of the fuel.

It is also known that CO₂ is a 'greenhouse' gas whose concentration in the atmosphere is increasing, which could contribute to global warming. A second drawback of such fluidized bed combustion lies in the quantity of ash produced that is rich in CaSO4 and in CaO, which could limit the use of such ash.

The invention makes it possible to minimize those drawbacks firstly by significantly reducing CO₂ emissions coming from the decarbonization of the calcium carbonate, and from the oxidation of the organic carbon, and secondly by making it possible for the absorbant to be regenerated. The basic idea of the invention is to use the surplus CaO in the ash to capture CO₂ from the flue gases.

As shown in the Picture 1, calcium carbonate 2 is injected as an absorbing agent into the hearth 1, which, in this example is a circulating fluidized bed hearth. This hearth, which operates at a temperature of about 850° C., is provided with a particle separator of the cyclone type which returns the particles to the bottom of the hearth. The flue gases produced at the output of the hearth are charged with CaO and they pass through a recuperator boiler 3 so as to lower their temperature to within the range 800° C. to 400° C., and typically to 650° C. The resulting cooled flue gases charged with CaO penetrate into the reactor 4 into which the regenerated absorbing agent 5 is re-injected as indicated below. The regenerated absorbing agent 5 is made up in part by the fraction of the absorbing agent 2 that has not reacted with SO₂ in the hearth 1. While in the reactor 4, the flue gases are put in contact with the regenerated absorbing agent and with the CaO contained in the incoming flue gases so as to capture CO₂ contained in the flue gases, by means of the following reaction:

The solids present in the outgoing flue gases that leave the reactor 4 are extracted in part in a gas/solid separator 6, e.g. a cyclone.

For example, the reactor 4 and the associated separator 6 may be a re-circulating fast fluidized bed reactor characterized by a gas transit time dependent on the desired CO₂ capture efficiency. Typically the CO₂ capture efficiency lies in the range 20% to 80%, with gas transit times shorter than 10 seconds, and with solid transit times of several minutes.

Under the separator 6, the reactor 4 may include a particle heat exchanger 18 and an outlet valve 19 having an adjustable flow rate, making it possible to adjust the temperature in the reactor 4 so as to optimize the exothermic CO₂ absorption reaction and the temperature in the reactor 11, the re-circulation to the reactor 4 serving to increase the time during which the CaO transits through the reactor 4. More particularly, the valve 19 is disposed between the separator 6 and the particle heat exchanger 18 in which a fraction of the solids output from the separator 6 is cooled before being re-injected into the reactor 4.

On leaving the separator 6, the flue gases with part of the CO₂ content removed go through a second recuperator boiler 8 in which they are cooled conventionally to about 120° C., and they then go through a final dust filter 9 before they are released into the atmosphere via a chimney at C. The solids 10 collected in the dust filter 9 are in part recycled and mixed with the solids 7 output by the separator 6, in a reactor 11 heated to a temperature higher than the temperature of the reactor 4, and typically higher than 800° C., so as to enable CO₂ to be released by means of the following reactions:

By further increasing the temperature in the reactor 11, it is possible to cause SO₂ to be released as well.

In a variant, by pre-treating the solids 7 and 10, e.g. with water in an enclosure 17, before they are sent to the reactor 11, it is possible to reduce the operating temperature of the reactor 11.

The gas 12 produced by the reactor 11 is essentially a mixture of CO₂ and of SO₂. These two components can be separated from each other at 12 for subsequent use or for underground storage.

The solids 13 extracted from the reactor 11 and containing a high proportion of CaO are optionally treated in an enclosure for the purpose of improving their reactivity for CO₂ and SO₂ capture, e.g. by adding water or water vapor or by adding a reaction promoter such as sodium salts, before they are returned to the reactor 4 or to the hearth 1. In addition, in the enclosure 14, the solids may be cooled so as to facilitate transporting them. It is therefore in the reactor 11 that the CO₂ absorbant is regenerated so as to be recycled to the reactor 4.

In order to minimize the consumption of calcium carbonate 2, a small fraction of the solids 13 is recycled to the hearth 1. A fraction 16 of the ash usually extracted from the fluidized bed at the bottom of the hearth 1 may be re-injected into the reactor 11.

The reactor 11 is preferably equipped with a bleed outlet 20 making it possible to extract the surplus solids if necessary in order to regulate the solids content of the reactor 4. The grain-size of the absorbing agent 2 is adjusted by grinding so as to optimize the quantity of absorbing agent contained in the flue gases that are released from the hearth 1.

The absorbing agent 2 injected into the hearth 1 may advantageously be dolomite containing magnesium carbonate, which makes it possible to reduce the operating temperature of the reactor 11, and to reduce the decarbonization energy required. More generally, the agent 2 may be an alkaline earth which behaves in the same way as calcium carbonate but at different temperatures.

The invention is not limited to fluidized bed combustion. It is applicable to various combustion modes, in particular using powdered coal, and more generally to treating flue gases containing CO₂ produced by a combustion hearth into which an absorbant is injected.