DESCRIPTION

The present invention relates to a system and method for making the combustion processes which take place in industrial plants such as furnaces for metals or glass, ceramics, cement products or similar applications which require thermal energy generated by combustion of fossil gas such as methane in a combustion chamber more efficient. The invention is mainly addressed to applications of the OXY-Fuel type where the combustion agent is pure oxygen instead of atmospheric air and the thermodynamics of the combustion process causes the combustion fumes to be released from the combustion chamber at high temperatures (of the order of 1500°C) and are mostly composed of carbon dioxide and steam.

Although the invention can also be advantageous when applied to combustion processes using air as an oxidizer, the primary use is envisaged with pure oxygen power plants as it has been found that there is a large margin for improvement in the efficiency and the characteristics of the fumes are suitable for the treatment of the fumes themselves both in terms of exploitation of the enthalpy content and recovery of at least part of the substances contained therein by recirculation and mixing with the fuel.

In the specific case of a glass processing furnace, the applicant has observed how the energy profile in the combustion chamber is such that, by operating with the best possible mix of methane and oxygen at room temperature (25°C), the process efficiency is about 67% against the production of high temperature fumes and particular composition as anticipated above.

Units and systems for recovery of the enthalpy content of fumes are known in the state of the art which exploit, for instance, a unit operating by means of a closed reversible thermodynamic cycle engine of the Rankine or Hirn type, or a cycle composed by a compression and an adiabatic expansion and two isobars, the use of which advantageously allows the heat to be transformed into work and therefore into electrical energy by means of a generator; this solution is also referred to as an engine thermodynamic cycle or, if the cycle is carried out with an organic fluid, Organic Rankine Cycle (ORC).

The engine thermodynamic cycle involves the action of a pump to raise the pressure, then an isobaric heating thanks to the heat recovered from the fumes until dry or overheated saturated steam is obtained, then expanded in a turbine and then iso thermally-iso barically condensed. The result is therefore obtained of recovering heat from the fumes with a simple and easy to implement cycle.

Even more advantageously, it is possible to envisage the use of fluids other than water and steam and in particular of organic fluids (ORC), which have lower change-of-state temperatures, and this, if on one hand allows a lower thermal difference, and therefore a lower extractable energy, also allows to potentially use lower level thermal sources, such as the heat collected from fumes already partially treated by other heat recovery units.

The constructive and functional characteristics of these heat recovery systems are such as not to suggest their use with exhaust fumes as emitted from the combustion chamber as they have excessively high temperatures; it is therefore advisable to introduce one or more intermediate heat recovery stages with simpler devices suitable for operating with high temperature fumes. The term "intermediate" refers to the path of the combustion fumes which follow a path starting from one or more combustion chamber outlets and branching off to one or more expulsion points outside the plant.

Still with the aim of improving the quality of combustion and therefore its efficiency, the so-called "re-forming" technique is known which, through an endothermic reaction between fuel molecules and water molecules, causes the breaking of the bonds between carbon-hydrogen atoms and the formation of molecules with a higher energy content (H2, CO). In this way, part of the waste heat is reused to increase the calorific value of the fuel.

In the specific case wherein the fuel is natural gas, the reforming process takes place using steam and is also referred to as "steam reforming" or Steam Methane Reforming (SMR) in the case the fossil gas is methane. However, this is only one of the operating modes that fall within the scope of this patent application which is therefore not limited by this specific configuration. Steam reforming is commonly carried out by mixing a certain quantity of steam from an external source with methane: it will be seen later how the invention described here also allows the steam contained in the combustion fumes to be reused without resorting or resorting only in part to an external source and therefore also limiting the environmental impact resulting from the plant operation.

Furthermore, the invention provides for feeding the steam reforming process with a percentage of fumes mixed with fuel gas and steam: this has a double positive value:

- the CO2 contained in the fumes participates in the steam re-forming reactions, increasing the endothermic effect;

- the fumes contain a high percentage of H2O already in the form of steam useful for guaranteeing the steam to carbon (S/C) ratio of the process fluid entering the SMR reactor. In the specific case wherein methane is used as fuel, the reformer requires a supply of steam, with a steam-to-carbon (S/C) ratio between 1 .5 and 4, preferably around 1 .9-2.3 and again more preferably equal to 2.

The present invention therefore has the object of overcoming the technical problems known to the state of the art and of achieving these and other objectives and this object is achieved with a combustion process efficiency unit of a combustion chamber of industrial plants operating by oxy-combustion of natural gas in accordance with claim 1 .

The first heat exchanger is chosen by the person skilled in the art with characteristics suitable for the temperature and flow rate of the incoming combustion fumes and can be a device such as for instance a boiler.

In a first embodiment, the invention also comprises a second heat exchanger for transferring part of the energy contained by said fumes in the form of heat to increase the temperature of an oxygen flow aimed for said combustion chamber, preferably arranged downstream of the reforming unit according to a path of the fumes which from the combustion chamber go towards a discharge or an external collection unit.

The invention also relates to a method for improving the efficiency of combustion chambers operating in industrial plants such as furnaces for metals or glass, ceramics, cement products, said method comprising the steps of:

- having an OXY Fuel type combustion chamber;

- having a source of steam, a source of fossil gas and a source of diatomic oxygen (O2);

- withdrawing the combustion fumes leaving said chamber;

- mixing said steam together with the fossil gas and a part of the combustion fumes to obtain a fuel mixture;

- purifying said fuel mixture;

- subjecting the purified fuel mixture to an endothermic steam reforming process to obtain the final fuel mixture;

- performing the combustion in the combustion chamber using the heated diatomic oxygen and the final fuel mixture.

In a embodiment the method further comprises one or more of the following steps:

- using part of the heat of the combustion fumes leaving the combustion chamber to generate steam at a known temperature from a water source; - using part of the heat of the combustion fumes leaving the combustion chamber to support said endothermic steam reforming process;

- using part of the heat of the combustion fumes leaving the combustion chamber to raise the diatomic oxygen to a specific temperature.

Preferably the three steps are performed simultaneously with the path of the combustion fumes emitted from the combustion chamber towards the exhaust or towards the collection point outside the plant and which initially enter a first heat exchanger to then exit and be conveyed towards a reforming unit and finally towards a second heat exchanger.

In a embodiment, said water source comprises at least in part liquid water obtained by condensation of the combustion fumes.

In another embodiment of the invention, the method comprises the further step of recovering part of the thermal energy of the fumes through a closed reversible thermodynamic cycle, preferably of the Rankine or Hirn type and even more preferably using organic type working fluids.

The attached drawing tables from 1 to 3 show, respectively in each figure, the block diagrams of generic glass processing plants according to three different embodiments of the invention which implement possible embodiments of the invention itself but are not to be considered as limiting the invention. Figure 4 illustrates the plant of figure 1 in a specific configuration of the operating parameters.

With reference to figure 1 , a glass furnace is shown provided with a combustion chamber 2 and fed by fuel intended for one or more burners by means of one or more burner nozzles (not visible in the figure) positioned inside said combustion chamber according to the prior art and not further described here. In this embodiment the furnace operates with oxy-combustion (Oxyfuel type furnace) or a combustion technique using pure oxygen, although the use with ambient air possibly enriched by substances capable of improving the combustion process is not precluded. Techniques for recovering the heat contained in the exhaust fumes are known, for instance what is described in patent applications number 102017000073758 and 102021000024635 which introduce the technique with regenerative chambers: the air or the gas mixture intended to support combustion enters the combustion chamber passing alternately through one of two regenerative chambers which are designed to heat the comburent by transferring previously accumulated heat by cooling the exhaust fumes which previously passed through the same chamber.

However, this solution brings limited advantages in terms of heat recovery from the fumes and does not address the technical problem of recovering the substances contained in these fumes which, in the case of oxy-combustion, mainly consist of carbon dioxide and dihydrogen monoxide or water in steam form. The invention proposes and, as will be seen, achieves the objective of at least partially reusing these substances as well as lowering the temperature of the combustion fumes by recovering an important part of their energy thanks to heat exchangers and thermo-dynamic machines for the generation of electricity.

Thus a combustion process efficiency unit is illustrated for a glass furnace having a combustion chamber 2, which is fed by a flow of combustion oxygen 22 and by a flow of fuel 21 , composed of a mixture based on methane gas. The combustion flow 22, coming from a source 13 external to the plant and made available in gaseous form and at room temperature (for instance at a temperature of about 25°C), is heated by heat exchange by a heat exchanger 7 which withdraws energy in the form of heat from the combustion fumes 202 (already subjected to recovery treatment) and, once heated, it is conveyed into the combustion chamber 2.

The fuel 21 is also made available to the same combustion chamber 2, which in the context of the invention is a mixture based on fossil gases (preferably Methane CH4), part of the exhaust fumes 203 and superheated steam 14.

Again with reference to figure 1 , the exhaust fumes, or more simply the fumes, are addressed towards a first heat exchanger or boiler 3 intended to raise the temperature of the water coming from a water source 11 , vaporising it and bringing it to a known temperature thus obtaining superheated steam 14. In a preferred configuration (figure 4) this boiler has been sized in such a way that, with water from the source 1 1 at a temperature of 25°C, the steam 14 is overheated to a temperature of about 500°C.

This superheated steam 14 is then mixed by a mixer 4 which also receives as input the content of a part 203 of the exhaust fumes 201 and a methane gas flow coming from a source 12.

In the preferred configuration of figure 4, the methane gas source delivers a flow of gas at 25°C while about 30% of the fumes is conveyed to the mixer 3. This allows, in addition to mixing with steam at 500°C, to obtain a mixture of fuel at a temperature of about 720°C whose composition is mostly made up of methane, carbon dioxide and water with the exception of residual impurities in a percentage normally lower than 1 %.

The fuel mixture thus obtained is subjected to a gas treatment process by means of a unit 5, aimed at purifying the fuel mixture and preparatory to maintaining the operating life of the reformer 6 which is located downstream in the path of the fuel mixture towards the combustion chamber 2. This treatment has as its objective the reduction or neutralization of the chemical substances dispersed in the fumes and/or in the gas entering the mixer 4 such as for instance the desulphurisation of the gas, the reduction of hydracids such as hydrochloric acid or hydrogen sulphide commonly present as a result of the production processes of fossil gases or syngas. In the context of plants operating by oxy-com- bustion there is no need to purify the fuel mixture from Nox nitrogen compounds as instead required in ovens that use air as a comburent. As previously illustrated, with the aim of improving the combustion quality and therefore its efficiency, the reforming technique is known which, through an endothermic reaction between fuel molecules and water molecules, causes the breaking of the bonds between carbon-hydrogen atoms and the formation of molecules with a higher energy content (H2, CO) to increase the calorific value of the fuel flow 21. This technique is implemented in the reformer 6 and supported in terms of energy requirements (providing heat to the reformer for the endothermic reaction) thanks to a heat exchange which cools the exhaust fumes once they have been conveyed into the reformer.

In a preferred embodiment (figure 4) and validated by various calculation simulations, the fumes 204 entering the reformer 6 (downstream of the withdrawal of the fumes intended for the mixer 4) have a temperature of about 1280°C and release heat until they drop to about 605°C at the reformer outlet. The retained heat is effectively reused to increase the calorific value of the fuel.

Again, as already introduced, such a configuration allows the steam reforming process to be fed with a percentage of fumes mixed with fuel gas and steam and has a double positive value:

- the CO2 contained in the fumes participates in the steam re-forming reactions by inhibiting the water-gas shift reaction (exothermic reaction) thus increasing the endothermic effect;

- the fumes contain a high percentage of H2O already in the form of steam useful for guaranteeing the steam to carbon (S/C) ratio of the process fluid entering the SMR reactor.

In the specific case wherein methane is used as fuel, the reformer requires a supply of steam, with a steam-to-carbon (S/C) ratio between 1 .5 and 4, preferably around 1 .9-2.3 and again more preferably equal to 2.

The final fuel mixture, indicated with 21 , is then conveyed to feed the combustion in the combustion chamber 2 using the gaseous oxygen previously heated by the second heat exchanger 7 as comburent. In the configuration of Figure 4 it has successfully being verified that the temperature of the oxygen leaving the exchanger 7 is raised from 25°C to about 600°C while the exhaust fumes 203 are lowered to about 400°C starting from an inlet temperature of 605°C to the exchanger 7. The temperature of the superheated oxygen may be a few units of degree lower considering the inefficiencies of the actual devices.

Finally, a residual part of the heat still available in the fumes 205 is then used to generate electric current by a thermodynamic unit 1 10 (ORC) with relative electric generator 120. In the embodiment according to the functional diagram of figure 1 , the fumes from the combustion chamber 2 not reused in the fuel mixture, pass first into the reformer 6 and then into the thermodynamic unit 110 to then flow into a chimney for discharge or treatment. This residual part, with reference to the ambient temperature and pressure conditions, is approximately 26% of the calorific value entering the furnace, and is available for the ORC system, which manages to recover about a further 10% of the calorific value entering the furnace.

The use of fluids of organic nature makes it possible to exploit small enthalpy changes at medium-low temperatures and therefore be advantageous in this case wherein the fumes are already partially cooled by the treatments from the heat exchangers 3 and 7 and from the reforming unit 6.

According to a further embodiment of the invention, schematically shown in Figure 2, a further stage is provided for treating the combustion fumes leaving the second exchanger 7 before they are conveyed to the thermodynamic recovery unit 110; this treatment, carried out according to known techniques, has as its objective the reduction or neutralization of the chemical substances dispersed in the combustion fumes coming from the combustion chamber 2 and also the reduction of the polluting and harmful agents for the operation of the unit 1 10 such as for instance acidic gases, such as hydrochloric acid or hydrogen sulphide or other compounds of the exhaust fumes which are generated by combustion in chamber 2.

In a further embodiment of the invention, shown in Figure 3 as a embodiment of Figure 2 but which can also be freely combined with one or more of the other possible embodiments, a condensation unit 9 is provided which is arranged downstream of the thermodynamic unit 110 and in any case in the terminal of the path of the combustion fumes from the combustion chamber 9 towards the exhaust; advantageously, this unit allows the recovery of water dispersed in the form of steam in the exhaust fumes so as to be made available as a source of water addressed to the first exchanger 3 according to one or more of the embodiments described up to now. It is evident how this further step is extremely advantageous in terms of efficient use of the combustion process also in favor of reusing the combustion products, not only because part of the fumes is recirculated as a fuel mixture but also because it is possible to extract a part of the other components of the combustible gas mixture from the non-reused part of the fumes. The use of two heat exchangers, of the further transfer of heat to the reformer 6 and to the thermodynamic unit 110 leads to an advantageous lowering of the temperature of the fumes which can therefore be treated by condensation by working at relatively low temperatures. In one embodiment, these temperatures are close to 100°C and the condenser 9 is able to generate a quantity of water condensed at 25°C about 40% of which is used at the inlet to the first heat exchanger 4.

Thanks to these expedients it has been verified how the initial efficiency of an oxy-fuel type furnace with 100 MW of Higher Caloric Power has been raised from 67% (without energy recovery) to about 77% which corresponds to an efficiency increase of about 10% compared to the basic case and in line with that of the configurations, at the current state of the art, more efficient than the EndPort furnaces with air combustion. According to a further embodiment, not shown in the figures, the energy produced by the electric generator 120 is used for the operation of an electrolyser from which, by a known electrolysis process, the separation of water molecules into dihydrogen molecules (H2) and oxygen (O2) is obtained. This configuration is possible and advantageous in any combination with the other embodiments of the invention since part of the oxygen necessary for combustion can be produced directly by the electrolyser energized at least in part by the energy recovered from the thermodynamic unit 110.

Furthermore, the electrolyser produces molecules of dihydrogen (H2) which can integrate the fuel (in the mixing phase in mixer 4), effectively adding gas with a high calorific value obtained from the electrolysis of water, electrolysis which is fed by the energy recovered by the heat exchanger unit object of the invention.

In an embodiment of the invention a unit for the desulphurisation of the fuel used in the furnace is introduced. The desulphurizer operates being powered by hydrogen and the desulphurization process is particularly effective when applied to the fuel before any reforming operation. In the specific case wherein methane gas is used as fuel, the chemical steam reforming reactor of the same contains a catalyst which is poisoned by the sulfur compounds present in the gas (typically: the odorizer) and which would have a very short life without the upstream desulfurization process. The excess hydrogen produced by the electrolyser and not used by the desulphurizer can then be fed directly to the furnace, for instance mixed with the mixture leaving the reforming unit.

In the case of methane gas treatment, using the known hydrodesulfurization processes it is possible to neutralize or in any case reduce the harmful effects on the plant, in particular on the catalyst of the reforming reactor, and on the environment, linked to the presence of sulfur and its compounds in the combustion process.

The hydrodesulphurization, which requires dihydrogen to be carried out, can advantageously be fed by the dihydrogen molecules coming from the electrolyser which, as mentioned, is energized at least in part indirectly by the heat recovered from the fumes 205.

However, it is evident that the invention must not be considered limited to the particular arrangements illustrated above, which constitute only exemplary embodiments thereof, but that different embodiments are possible, all within the reach of a person skilled in the art, without thereby departing from the scope of protection of the invention itself, which is defined by the following claims.