FR2958999A1 - method for combustion of fuel to heat fluid e.g. water, involves supplying oxygen enriched gas and fuel, heating fluid by using portion of released heat and injecting portion of liquid in combustion chamber implemented in heating step - Google Patents

Abstract

The method involves supplying an oxygen enriched gas (7) and a fuel (6) into a combustion chamber. Combustion in the combustion chamber is carried out to produce fumes (8) comprising water vapor and releasing heat. A fluid (1a) is heated by using a portion of the released heat. A water vapor part contained in the produced fumes is condensed by transfer of heat with another fluid (1b) with recuperation of liquid (12) and of the latter preheated fluid. A portion (13) of liquid is injected into the chamber implemented in a heating step. An independent claim is also included for a combustion installation comprising a combustion unit.

Description

The present invention relates to a combustion method for heating at least one fluid. This fluid may for example be water, for the purpose of producing steam and / or electricity, or more generally any fluid for the purpose of heating.

For environmental reasons, it is desired to limit the level of certain compounds present in the fumes produced by combustion, such as NOx (dioxide and nitric oxide in particular), toxic, and CO2 (carbon dioxide), suspected of cause global warming.

There are many methods in the state of the art to reduce NOx in fumes. One can cite the use of "low NOx" burners with staged combustion, but these create very long and sometimes unstable flames. We can also mention the injection of water or steam into the burner and / or into the combustion chamber, as well as the recycling of a portion of the fumes to the combustion zone. These techniques are effective, but have some disadvantages. The injection of water or steam consumes a large quantity of water and degrades the energy efficiency of the installation, this water having to be heated and being lost in the fumes. The recycling of some of the fumes involves installing flues and one or more fans for the transport and reinjection of fumes. The operation and maintenance of this equipment is expensive.

Moreover, these processes do not solve the question of CO2 emissions due to the discharge of fumes into the atmosphere. This issue is dealt with by some processes, such as oxycombustion (combustion with oxygen rather than air) followed by purification of fumes with more or less CO2, in order to bury these fumes or use as a product, not to release them into the atmosphere. But these methods do not solve the question of limiting the formation of NOx. On the contrary, oxyfuel combustion has a tendency to aggravate the formation of NOx, because of the intensity of combustion.

The problem is therefore to have a combustion process that both reduces the formation of NOx during this combustion and achieves a "capture of CO2" (in English: CO2 capture), that is to say to produce effluents concentrated in CO2 not to be released into the atmosphere (for use or more generally buried), without all or some of the disadvantages mentioned below.

To this end, the solution of the invention relates to a combustion process comprising the following steps: a) supply of at least one gas stream rich in oxygen and a fuel; b) combustion in a combustion chamber of the fuel and oxygen-rich gas stream provided in step a), said combustion producing flue gases comprising steam and generating heat; c) heating at least a first fluid using a portion of the heat evolved in step b); d) condensing a portion of the water vapor contained in the fumes produced in step b), by heat exchange with at least a second fluid, with recovery of liquid water and the second preheated fluid, which form all or part of the first fluid used in step c); and e) injecting at least a portion of the liquid water obtained in step d) into the combustion chamber implemented in step b).

The first fluid consists of a continuous or discontinuous flow of a liquid, a gas or a two-phase mixture to be heated. The heating can also be done continuously or batch (in English: batch). According to a particular mode, it is water that one wishes to transform into steam. This steam can be used as such or be used totally or partially to produce electricity or mechanical energy.

According to another particular embodiment, the first fluid may be a petroleum product that it is desired to heat for conversion (cracking or otherwise).

In step a) is provided by means known to those skilled in the art a gas flow rich in oxygen and a fuel. By "rich in", it is meant that the gaseous flow in question contains a volume concentration of the body in question greater than or equal to 50%, preferably greater than or equal to 90% and even more preferably greater than or equal to 99%. . The fuel may be any fuel used industrially, for example natural gas, fuel oil or a petroleum residue.

This oxygen-rich stream is generally obtained by cryogenic distillation. Alternatively, it can be obtained by separation of air based on selective adsorption on adsorbents or selective permeation through semi-permeable membranes. These techniques can be used alone or in combination. The production of the gas stream rich in oxygen can be continuous or discontinuous. In the latter case, it may be useful to have intermediate storage.

Step b) burns at least one fuel and at least a portion of the oxygen-rich gas stream produced in step a). The combustion usually takes place in the combustion chamber of a unit, typically a boiler or an oven. Combustion produces fumes that include CO2, water vapor and NOx. It produces heat in the form of hot fumes and heat radiation. The use of a gas stream rich in oxygen as an oxidant, rather than air, reduces the amount of nitrogen that is found in the fumes, which promotes the capture of CO2 in these fumes. Another consequence of the oxyfuel combustion is that the quantity of water contained in the fumes is, in proportion, much larger, the nitrogen being little or not present in the fumes.

In step c), at least one first fluid is heated by the combustion carried out in step b) and at least a portion of this first heated fluid is obtained. Heat transfer is by radiation from the combustion flame (s) and hot gases, as well as by convection and conduction to exchangers in which this first fluid is passed. It is also possible to introduce other fluids to be heated. The heating can be done with or without changing the state of the fluids. For example, if the unit is a boiler, it can, from industrial water at room temperature, produce hot water and steam at different pressure levels. This vapor can be used as such or to produce mechanical and / or electrical energy. The boiler can also, mainly or in addition, produce hot oil from oil at room temperature.

In step d), a heat exchange is carried out between the fumes produced by the combustion in step b) and a second fluid. This exchange can take place in a system of one or more heat exchangers with extraction of liquid water. This preheats the second fluid, which forms all or part of the first fluid heated in step c). This also makes it possible to condense part of the water contained in the fumes produced in step b). In general, a substantial fraction of this water is condensed, generally at least 95% by weight. It is therefore possible to recover large quantities of liquid water and still relatively humid fumes, which can contain up to a few percent of water by volume.

The fact of having burned with oxygen-rich oxidant, and not with air, reduces the volume of smoke and increases their water concentration. The condensation operation is thus facilitated and / or the equipment required for condensation is smaller (investment gain). At the end of the condensation, the fumes are already very enriched in CO2. They include in particular a little water vapor and impurities.

In step e), part of the liquid water recovered in step d) is injected into the combustion chamber of the unit. This reduces the level of NOx present in the fumes produced by combustion by about 50 to 80% compared to a situation without water injection. The amount of water obtained by condensation in step d) is sufficient to cover the water requirements in step e), so that it is not necessary to have an external water source for injection. On the contrary, we usually end up with a surplus of water, equal to the amount produced by condensation minus the quantity injected to lower the NOx level.

The combination of a combustion using a gas rich in oxygen, a condensation of the water followed by a water injection in the combustion chamber makes it possible to lower the level of NOx without consume make-up water (other than to compensate for possible purges of the system) and without reinjecting a part of the flue gases into the combustion chamber (no flue gas recirculation). In addition, from an energy point of view, the heat produced by the condensation of water is not lost, since it serves to preheat the second fluid. The overall efficiency of the installation is thus improved compared to a situation where the water present in the flue gas would not be condensed. The fumes, at the end of step d), are very rich in CO2. Their conditioning for a CO2 capture (use, landfill) is already very advanced. Depending on the specifications to be achieved, other processing steps may be necessary.

Furthermore, according to particular embodiments, the invention may include one or more of the following features:

in step a), the supply of the oxygen-rich gas stream implements a separation of ambient air. This separation can be done in particular by cryogenic distillation, selective adsorption and / or selective permeation.

in step a), said ambient air separation further produces a nitrogen-rich gas stream and said process further comprises a step f) of heating at least a portion of the nitrogen-rich gas stream obtained in step a) by heat exchange with at least a portion of the fumes produced in step b), so as to obtain a gas stream rich in nitrogen and hot; and wet fumes being produced in step d), drying the wet fumes obtained in step d) using all or part of said gas stream rich in nitrogen and hot.

- In step a), the fuel and / or the oxygen-rich gas stream are preheated by heat exchange with at least a portion of the fumes produced by the combustion implemented in step b). This preheating is useful for the conditioning of certain fuels, such as fuel oil. In addition, it recovers a portion of the heat contained in the fumes and improves the overall energy efficiency of the process. The heat exchange generally takes place upstream of the water condensation of step d) and before or after the exchange involving the nitrogen-rich gas in step f). in step e), the liquid water is injected in the form of droplets. This makes the water injection more efficient from the point of view of reducing the NOx level in the fumes produced by the combustion. More particularly, it is intended to inject droplets with a diameter of between 20 and 40 micrometers. for a given duration, a specific mass flow ql of liquid water being injected into the combustion chamber in step e) and a specific mass flow q2 of said fuel being burned in step b), said determined flow rates ql and q2 are in a ratio ql / q2 between 1/10 and 4/10. In other words, the amount of water injected in step e) represents, by weight, 10% to 40% of that of the fuel burned in step b). This range of relative amounts of water injected into the combustion unit makes the injection more efficient from the point of view of reducing the NOx content in the fumes produced by the combustion. in step e), the liquid water is injected into said combustion chamber at a distance from a burner of said combustion chamber of between 0.5 and 2 meters, preferably 0.8 and 1.3 meters; . These distances between the water injection point (s) and the nearest burner make the injection of water more efficient from the point of view of reducing the NOx content in the fumes.

the first fluid heated in step c) is water. By "water" is meant industrial water, having possibly undergone treatments. It is not necessarily absolutely pure. the second fluid used in step d) is obtained by mixing and treating a make-up water and a part of the liquid water produced in step d). Mixing and processing can be performed in any order. In the particular case where the first fluid is water, in fact, it is advantageous to use the excess water evolved by the condensation carried out in step d) to form part of the fluid to be heated. This water of "industrial" quality must generally be treated (demineralization, degassing, etc.), just like make-up water, before it can be admitted to the unit intended to heat it, according to methods known to those skilled in the art.

Stage f) makes it possible to remove a large part of the water remaining in the humid fumes at the end of the stage d) of condensation. The drying will be more or less pushed depending on the specifications to be achieved. For example, extensive drying is required to treat the fumes by cryogenic distillation to remove remaining impurities. Using a drier whose regeneration is done thanks to the nitrogen-rich gas obtained in step a) and heated in step f) makes it possible to recover even more heat contained in the fumes and further improves the overall energy efficiency of the process. In general, this step f) of heat exchange between the fumes and the nitrogen-rich gas is carried out before the step d) of condensation of the water. In some cases, this exchange may be accompanied by partial condensation of water. This water can be used in the same way as that produced in step d).

The invention also relates to a combustion plant comprising at least: a combustion unit comprising a combustion chamber fluidly connected to a fuel source and to a source of oxygen-rich gaseous flow, an outlet intended for fumes and at least one an inlet for a first fluid to be heated in said combustion unit; and a condenser having an inlet in fluid connection with said combustion unit outlet for smoke, an inlet for a second fluid, an outlet fluidly connected to said combustion unit inlet for said first fluid, and an outlet for the recovery of liquid water; and means for injecting liquid water into said combustion chamber, said means being fluidly connected to said outlet of the condenser intended for the recovery of liquid water.

Furthermore, according to particular embodiments, the invention may comprise one or more of the following characteristics: said source of oxygen-rich gas flow comprises an ambient air separation unit comprising an outlet intended for said rich gas stream; in oxygen and an outlet for a gas stream rich in nitrogen.

The installation furthermore comprises: a dryer having at least one inlet in fluid connection with at least one outlet of said condenser intended for fumes, said dryer being moreover designed so as to be at least partially regenerated by using a flow gas rich in nitrogen; and a heat exchanger comprising: an inlet in fluid connection with said combustion unit outlet for fumes, an outlet for fumes and fluidly connected to said fume condenser inlet, an inlet fluidly connected to said outlet of the ambient air separation unit for a nitrogen-rich gas stream, and an outlet for a nitrogen-rich gas stream fluidly connected to an inlet of said dryer. said installation further comprises: at least one reservoir intended to receive liquid water coming from said condenser; and a water producing unit comprising an inlet in fluid connection with said tank and an outlet in fluid connection with said condenser inlet for said second fluid.

By "fluidically connected to" or "in fluid connection with", it is meant that there is a pipe connection so that a fluid can pass into these pipes without undergoing a change in its chemical nature or its physical state. . The pipes may comprise, for example, buffer capacities or pumps, but not chemical reactors, condensers or evaporators.

The ambient air separation unit is adapted to and designed to produce at least one oxygen rich stream and at least one nitrogen rich stream. The condenser is capable of and designed to condense a large portion of the water contained in the flue gases produced by the combustion unit. It carries out the combustion of at least one fuel with at least one flow rich in oxygen. The dryer is suitable for and designed to dry fumes after they pass through the condenser. It can be at least partly regenerated by a nitrogen-rich stream.

The means for injecting water into the combustion unit may for example be nozzles or lances equipped with an atomization system so that the water is injected in the form of droplets, preferably a diameter of between 20 and 40 micrometers.

Other features and advantages will appear on reading the following description, made with reference to Figure 1, which shows an installation according to the invention and adapted to implement a method according to the invention.

The ambient air 14 is separated into a cryogenic separation unit 2. In particular, an oxygen-rich stream 7 and a nitrogen-rich stream 15, both having a purity of at least 99.5% by volume, are produced. Stream 7 is 11.7 ton / h. Stream 15 is 3.6 ton / h and is at 10 ° C. Unit 2 is capable of producing other fluids or a larger amount of the aforementioned fluids.

In a combustion unit 1, a steam boiler, is burned a stream 6 of residual oil type No. 6 with the stream 7 rich in oxygen in a combustion chamber equipped with one or more burners. The quantity of fuel oil is 2.9 tons / h; it is 115 ° C at the entrance to the boiler 1. This corresponds to an energy of about 32 MW thermal heating value lower. The combustion produces 14.9 tons / h of fumes 8 at 316 ° C and radiation. 40 ton / h of water is heated initially to 44 ° C at the entrance to the boiler, to recover 40 ton / h of steam 1d at 40 bar and 450 ° C.

The fumes 8 produced by the combustion comprise, by volume, 1.3% of O 2, 57.8% of water in the form of steam, 40.7% of CO 2 and approximately 250 ppm of NOx. In a first heat exchanger system 5a, the heat contained in the flue gases 8 is used to preheat the fuel oil 6a from 50 ° C. to 115 ° C., prior to its injection into the boiler 1 and, on the other hand, heat the nitrogen-rich stream, which becomes a stream at 150 ° C. At the outlet of these exchanges, the fumes 8a are at 225 ° C.

After these exchanges, the cooled fumes 8a are admitted into a condenser 5 which carries out a heat exchange between the flue gases 8a at 225 ° C. and water 1b at 21 ° C. so as to preheat the latter at 44 ° C. its entry into the boiler 1. A large part of the water vapor contained in the fumes 8a condenses in a stream of 5.3 ton / h of liquid water 12 at about 60 ° C collected in a tank 18.

A portion 13 of the liquid water of the reservoir 18 is injected into the combustion chamber of the boiler 1 to lower the level of NOx in the fumes 8. The water is injected in the form of droplets with a diameter of about 30 micrometers. The water flow 13 is between 0.29 and 1.15 ton / h, ie 10% to 40% of the burned fuel rate 6. The water is injected into the combustion chamber of the boiler 1 at one or more points, these points being located approximately one meter from the nearest burner.

Without injection of water, all things being equal, the concentration of NOx in the fumes 8 is reduced by about 75%.

The flow of water lb is constituted by water 13a from the reservoir 18 and which is not injected into the boiler 1 and by the make-up water 1c. The water Ib undergoes conventional treatments 19 (demineralization, etc.). It is then preheated from 21 ° C. to 44 ° C. in the condenser 5 before it is admitted into the boiler 1.

At the outlet of the condenser 5, the fumes 9 are at about 60 ° C and contain only 2% water by volume. They are dried in a dryer 4, comprising, for example, silica gel beds. To regenerate these beds, the stream 16 rich in nitrogen at 150 ° C. is used.

At the outlet of the dryer 4, 9.5 ton / h of dry smoke 10 is recovered, comprising 3% of O 2 and 96.7% of CO2 by volume. Depending on the intended use, they may be subjected to other treatments 3 aimed, for example, at removing oxygen and / or other impurities. We obtain a flow 11 suitable for being buried or used as is.

An overall thermal efficiency can be defined as the ratio of the power consumed (32 MW) minus the losses by the fumes 10, all divided by the power consumed. Thanks to the elements 5 (condenser for preheating the water to be heated in the boiler 1) and 4 (dryer using a portion of the heat contained in the fumes 8 for its regeneration), it passes about 90% (typical value for a steam boiler) at 94.4%.

In addition, no make-up water is used to perform the water injection 13 (between 0.29 and 1.15 ton / h) in the combustion chamber. In fact, the condensation, producing 5.3 tons / h of water, gives off a surplus of 4 to 5 tons / h of water which can be used to constitute a part of the 40 tons / h of water or 1 lb transformed. in steam ld by the boiler 1.

The fumes are freed of about 75% of the NOx they contain in the absence of water injection and comprise 96.7% of CO2. A capture (or purification) of CO2 is therefore performed. Complementary treatments 3, for example by cryogenic distillation, can be carried out to remove all or part of the remaining impurities (3.3%) and obtain an even purer CO2 stream 11 for specific use or burial .5

Claims (13)

Revendications1. A method of combustion comprising the steps of: a) providing at least one gas stream (7) rich in oxygen and a fuel (6); b) combustion in a combustion chamber of the fuel (6) and oxygen-rich gas stream (7) provided in step a), said combustion producing fumes (8) comprising water vapor and releasing heat; c) heating at least a first fluid (1a) using a portion of the heat evolved in step b); d) condensing (5) a portion of the water vapor contained in the fumes (8) produced in step b), by heat exchange with at least a second fluid (1b), with recovery of water liquid (12) and the second preheated fluid, which forms all or part of the first fluid (la) implemented in step c); and e) injecting at least a portion (13) of the liquid water (12) obtained in step d) into the combustion chamber implemented in step b). 2. Method according to claim 1, characterized in that, in step a), the supply of the gas stream (7) rich in oxygen implements a separation (2) of ambient air (14). 3. Method according to claim 2, characterized in that, in step a), said separation (2) of ambient air (14) further produces a gaseous stream (15) rich in nitrogen and in that said process further comprises a step f): - heating at least a portion of the nitrogen-rich gas stream (15) obtained in step a) by heat exchange (5a) with at least a portion of the fumes (8) produced in step b), so as to obtain a gas stream rich in nitrogen and hot (16); and - wet fumes (9) being produced in step d), drying wet fumes (9) obtained in step d) using all or part of said gas stream rich in nitrogen and hot (16). 4. Method according to any one of claims 1 to 3, characterized in that, in step a), the fuel (6) and / or the gas stream (7) rich in oxygen are preheated by heat exchange ( 5a) with at least a portion of the fumes (8) produced by the combustion implemented in step b). 5. Method according to any one of claims 1 to 4, characterized in that, in step e), the liquid water (13) is injected in the form of droplets. 6. Method according to any one of claims 1 to 5, characterized in that, for a given duration, a specific mass flow ql liquid water (13) being injected into the combustion chamber in step e) and a specific mass flow q2 said fuel (6) being burned in step b), said determined flow ql and q2 are in a ratio ql / q2 between 1/10 and 4/10. 7. Method according to any one of claims 1 to 6, characterized in that, in step e), the liquid water (13) is injected into said combustion chamber at a distance from a burner of said chamber of combustion between 0.5 and 2 meters, preferably 0.8 and 1.3 meters. 8. Method according to any one of claims 1 to 7, characterized in that the first fluid (la) heated in step c) is water. 9. Process according to claim 8, characterized in that the second fluid (1b) used in step d) is obtained by mixing and treating (19) a make-up water (1c) and a portion (13a) of the liquid water (12) produced in step d). Combustion plant comprising at least: a combustion unit (1) comprising a combustion chamber (30) fluidly connected to a source (6a) of fuel and to a source (2) of gas flow rich in oxygen, an outlet ( 8) for fumes and at least one inlet (1a) for a first fluid to be heated in said combustion unit (1); and a condenser (5) having an inlet (8a) in fluid connection with said smoke unit outlet (8), an inlet (1b) for a second fluid, an outlet fluidly connected to said inlet (1a) of the combustion unit (1) for said first fluid and an outlet (12) for the recovery of liquid water; and - means (13) for injecting liquid water into said combustion chamber, said means (13) being fluidly connected to said outlet (12) of the condenser (5) intended for the recovery of liquid water. 11. Installation according to claim 10, characterized in that said source (2) of gaseous flow rich in oxygen comprises an ambient air separation unit comprising an outlet (7) for said gas flow rich in oxygen and an outlet (15). ) for a gas stream rich in nitrogen. 12. Installation according to claim 11, characterized in that it further comprises: - a dryer (4) having at least one inlet (9) in fluid connection with at least one outlet of said condenser (5) for fumes, said dryer (4) being further designed to be at least partially regenerated using a nitrogen-rich gas stream; and - a heat exchanger (5a) comprising: an inlet in fluidic connection (8) with said outlet of the combustion unit (1) for fumes, an outlet for fumes and fluidly connected to said inlet (8a) of the condenser (5) for fumes, an inlet fluidly connected to said outlet (15) of the ambient air separation unit for a nitrogen-rich gas stream, and an outlet for a nitrogen-rich gas stream fluidically connected (16) to an inlet of said dryer (4). 13. Installation according to any one of claims 10 to 12, characterized in that it further comprises: - at least one reservoir (18) for receiving liquid water (12) from said condenser (5) ; and a water producing unit (19) comprising an inlet (13a) in fluid connection with said reservoir (18) and an outlet in fluid connection with said inlet (1b) of the condenser (5) for said second fluid .