WO2023047346A1 - Heat recovery assemblies for glass furnaces - Google Patents

Abstract

A heat exchanger unit to recover and reuse at least part of the energy in the form of heat from flue gas produced in a combustion chamber of a glass furnace which comprises an energy recovery unit arranged to recover heat from the flue gas leaving the combustion chamber of said glass furnace and operating by means of a thermodynamic motor cycle, preferably operating with working fluid of an organic type, for the conversion into electrical energy of at least part of the energy contained in the form of heat in said flue gas.

Description

Heat recovery for glass furnaces

TEXT OF THE DESCRIPTION

The present invention relates to a heat exchanger assembly for a glass furnace which operates by combustion of a generic fuel, such as, byway of non-limiting example, natural gas coming from the distribution network.

Heat exchanger assemblies for glass furnaces are known, which allow to recover the thermal energy possessed by the flue gas coming out of the combustion chamber by preheating the combustion air that is conveyed inside the furnace.

For instance, patent application No. W02009093134 illustrates a heat exchanger unit having a first regeneration type exchanger and a second recovery type exchanger, arranged in series along the flue gas outlet path.

The regeneration exchanger has two regeneration chambers in refractory material. The heat exchange takes place alternately, according to a succession of cycles of a determined duration, typically twenty minutes: in a first cycle the flue gas pass into one of the two chambers, yielding heat to heat exchange elements; at the same time, in the other chamber, the combustion air directed towards the combustion chamber absorbs heat from similar elements, previously heated by the flue gas. In a second cycle, the flows are inverted through the use of valves: in the first chamber the heat exchange will take place between the elements heated in the previous cycle and the combustion air, in the second chamber the flue gas will release heat.

The combustion airthat enters the regeneration exchanger comes from the recovery type exchanger, where it is already heated thanks to the same flue gas coming out of the regeneration exchanger. In the specific solution of W02009093134, the recovery exchanger has two concentric and coaxial pipes and exploits the principle of counter-

1

SUBSTITUTE SHEET (RULE 26) current heat exchange. Downstream of the recovery exchanger, according to the direction of advancement of the flue gas, the latter are expelled from a chimney.

Thanks to this configuration, air enters the furnace at a temperature of about 1200-1300°C, while flue gas leaves the recovery exchanger at a temperature of about 450-600°C. It is estimated that the efficiency of this process is around 86%, which clearly determines the need to recover at least a part of the 14% of energy otherwise wasted.

Patent application 102017000073758 moves in this direction, introducing the known technique of the so-called "reforming" 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 water vapor and is also referred to as "steam reforming". 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.

Although it is an improvement in terms of energy efficiency and effectiveness of the combustion process inside the glass furnace, the known technique described up to now leaves room for further improvements both in terms of recovery of the energy contained in the exhaust flue gas and in terms of the quality of the combustion processes or the feeding of the glass furnace by making the most of the calorific value of the fuel and the cleaning of this chemical reaction also measured as combustion residues and therefore reducing the ordinary and extraordinary maintenance of all the parts that make up the furnace. The benefits that can be obtained, such as lower operating costs and lower environmental impacts, are evident.

The present invention therefore has the purpose of overcoming the technical problems known in the state of the art and of achieving these and other objectives and this purpose is achieved with a glass furnace equipped with a heat exchanger assembly as in claim 1. The invention therefore provides for energy recovery by means of a unit operating through a closed reversible thermodynamic motor cycle of the Rankine or Hirn type, i.e. a cycle composed of an adiabatic compression and expansion and two isobaric whose use advantageously allows to transform heat into work and then into electricity by means of a generator; this solution will now be referred to as the thermodynamic motor cycle or ROC (if the cycle is carried out with an organic fluid).

The thermodynamic motor cycle involves the action of a pump to raise the pressure, then an isobaric heating thanks to the heat recovered from the flue gas to obtain dry or superheated saturated steam, then expanded in the turbine and then condensed isothermo-barically. It therefore obtains the result of recovering heat from the flue gas with a simple and easy-to-make cycle.

Even more advantageously, it is possible to provide for the use of fluids other than water and water vapor and in particular of organic fluids (ROC), which have lower state change temperatures, and this, if on the 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 flue gas already partially treated by a regeneration chamber and/or a fuel reformer.

In the state of the art, it is known to use fluids such as toluene or other low- medium molecular weight hydrocarbons for this purpose.

Other variants and embodiments are an object of the independent claims.

With reference to the attached drawing tables, there it is reported:

In Fig. 1 a block diagram of a generic system according to an executive and nonlimiting embodiment of the invention;

In Fig. 2 a schematic plan view of an embodiment of the heat exchanger assembly for a glass furnace detailing the components of the regeneration chamber, a thermodynamic exchanger for the thermodynamic motor cycle and a recovery exchanger.

With reference to figure 1, there is a glass furnace 60 provided with a combustion chamber 2 and fueled by one or more burners by means of one or more nozzles 56 (shown in figure 2) positioned inside of the aforementioned combustion chamber. In general terms, the furnace can operate with oxy-combustion (Oxyfuel type furnace) or a combustion technique through the use of pure oxygen, or with ambient air possibly enriched with substances capable of improving the combustion process. In this second case it is possible and commonly expected to use the technique with regeneration chambers: the air or the gas mixture intended to support the combustion enters the combustion chamber 2 passing alternately through one of the two regeneration chambers 11 which, as known, are designed to heat the oxidant giving it heat previously accumulated by cooling the exhaust flue gas 5 (Flue Gas) previously travelled in the same chamber.

As better described below, the alternation of the two re-generation chambers 11, i.e. the combined operation in such a way that when a first chamber transfers heat to the oxidant a second chamber acquires heat from the exhaust flue gas and the inversion of the operating mode between the first and second chamber is coordinated by a control unit 20 which controls, among other things, a set of valves 19a, 19b, 19c whose opening or closing allows the fuel, the oxidant and the flue gas to follow the programmed operating paths of the furnace.

The exhaust gases 5 are then conveyed to a reforming unit 53 configured in such a way as to obtain, according to the "reforming" technique, an endothermic reaction between fuel molecules and water molecules, which causes the breaking of the bonds between carbon-hydrogen atoms and the formation of fuel molecules with a higher energy content to increase the calorific value of the fuel flow 51.

A residual part of the heat still available in the flue gas 5 is then used to generate electric current by a thermodynamic unit 110 (ROC) with corresponding electric generator 120. In the embodiment according to the functional diagram of figure 1, the presence of both the thermodynamic unit 110, 120 and the reforming unit 53 is provided; however the latter needs not to be necessarily present and the choice of arranging it or not must be evaluated by the skilled technician according to the overall engineering of the furnace. In this embodiment, the flue gas leaving the regeneration chamber pass through the reformer and the ROC and then flow into a chimney for discharge or treatment.

The use of organic fluids allows to take advantage of small enthalpy jumps at medium-low temperatures and therefore to be advantageous even in the presence of flue gas already partially cooled by treatments in the regeneration chambers and optionally by the reforming unit.

In the case of the oxyfuel technique, as indicated above, the regeneration chambers may not be used and therefore it is even more advantageous to recover the energy of the exhaust flue gas with other methods.

According to another embodiment, the energy produced by the electric generator 120 is used for the operation of an electrolyser 140 from which, by a known electrolysis process, the separation of water molecules into molecules of dihydrogen (Hz) and oxygen (Oz) is obtained. This configuration is possible and advantageous in any combination with the other embodiments of the invention and even more favorable in the case of oxyfuel furnaces since the oxygen necessary for combustion is produced directly by the electrolyser 140 powered at least in part by the energy recovered by the thermodynamic unit 110.

The electricity coming from the generator 120 can be integrated by external energy sources 150, preferably of renewable origin such as wind and/or photovoltaic generators also positioned on site and/or other energy harvesting devices alternatively or in combination of two or more of these devices. This electrical energy can also be used for the operation of electrical parts of the furnace such as for instance fusion electrodes.

On the other hand, the electrolyser 140 produces molecules of dihydrogen (Hz) which can integrate the fuel (typically methane CH4 already originally mixed with other gases including dihydrogen) by adding gas with a high calorific value obtained from the electrolysis of water, electrolysis which is powered by the energy recovered from the heat exchanger unit object of the invention. In an embodiment of the invention, a fuel desulphurization unit used in the furnace is introduced. Desulphurization is particularly effective when applied to the fuel before any reforming operation. In the specific case wherein methane gas is used as a fuel, the chemical steam reforming reactor of the same contains a catalyst that is poisoned by the sulfur compounds present in the gas (typically: the odoriser) and which would have a very short life without the process of upstream de-sulfurization. The excess hydrogen produced by the electrolyser and not used by the desulfurizer can then be fed directly to the furnace, for instance mixed with the mixture coming out of the reforming unit.

In the specific case wherein methane or natural gas is used as fuel and therefore the reforming process takes place with the use of steam, the reformer requires a supply of water vapor, with a steam-to-carbon (S/C) ratio between 1.5 and 4, preferably around 1.9-2.3.

In a preferred embodiment as shown in Figure 1, a desulfurizer 130 (HDS unit also known as hydrotreater) is arranged to treat a methane-based fuel mixture. Using the known hydrodesulphurization processes, it is therefore possible to neutralize or otherwise 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 be advantageously fed by the dihydrogen molecules coming from the electrolyser 140 which, as mentioned, is powered at least in part by the heat recovered from the flue gas 5. For the correct working of the hydrodesulphurization process and of the equipment involved in it, it is essential to use high purity hydrogen which cannot be guaranteed by primary or secondary reforming processes but which can be guaranteed with water electrolysis processes. The configuration according to the invention is therefore extremely advantageous, according to which the energy recovered from the flue gas is reused to power an electrolyzer that is able to produce the hydrogen H2 necessary for the correct operation of the plant. This makes it possible to make the plant itself autonomous from external sources and further reduces the operating costs in ecological and economic terms resulting from the reduction of the supply sources of dihydrogen.

As regards the electrolysis process, in the electrolyser 140, this process can be carried out in exothermic mode and therefore produce heat, in thermally neutral mode, i.e. in the absence of generation/transfer and/or absorption of thermal energy, or in endothermic mode and therefore, in particular in the exothermic mode, the heat generated can be used to heat the oxygen and/or hydrogen produced and/or to feed by thermal energy, for instance, the thermodynamic system for generating electricity 110 or other units which operate for the recovery of thermal energy. In the endothermic mode, any heat absorbed by the electrolyser 140 can be supplied, for instance, by the exhaust flue gas deriving from upstream the reformer 53 and/or outgoing from it and/or from the thermodynamic unit 110.

With reference to Figure 2, a different embodiment of the invention is shown, also to be considered a non limiting embodiment, wherein a glass furnace fed with gas and air as oxidant is implemented, said furnace being combined with a heat exchanger assembly comprising a regeneration unit near the combustion chamber, a recovery unit in the terminal part of the flue gas path and a thermodynamic unit interposed between the two, again with reference to the flue gas path leaving the combustion chamber. Compared to the conceptual scheme of figure 1, where possible, the numerical references have been reused to indicate similar parts of the system.

1 indicates a heat exchanger assembly, schematically illustrated, for a glass furnace 60 having a combustion chamber 2, which is fed by a flow of combustion air 3 and a flow of fuel 51, defined for instance as methane gas. The airflow 3 enters the assembly 1 through an inlet 4 from the external environment at room temperature, for instance at a temperature of about 25°C, and is preheated by the assembly 1 through heat exchange with a flow of exhausted flue gas 5 coming from the combustion chamber 2.

The assembly 1 comprises a regeneration-type heat exchanger 10 and a recovery-type heat exchanger 25, arranged in series with each other. The exchanger 10 has two regeneration chambers 11 made of refractory material and provided with respective upper openings 12, communicating with the combustion chamber 2 through respective ducts 13, called turrets, also made of refractory material. In the specific case, the chambers 11 are arranged on a rear side of the furnace 60, so that the furnace 60 is commonly referred to as "endport" or "furnace with U-flame". Alternatively, the chambers 11 are arranged on opposite sides of the furnace 60 (in which case, the furnace is commonly referred to as "sideport"). The regeneration exchanger 10 operates according to a series of cycles of determined duration, between which the flows of air 3 and flue gas 5 passing inside the two chambers 11 are inverted. In other words, in any given cycle, one of the chambers 11 operates as a supply of combustion air and the other chamber 11 operates as an exhaust for the flue gas, and then inverts their function at each cycle. Each of the chambers 11 houses respective bundles 22 of thermal storage elements (partially and schematically shown), of a known type and not described in detail, which absorb heat during the cycles wherein the corresponding chamber 11 is crossed by the flow of flue gas 5, and transfer the accumulated heat to the air flow 3 during the other cycles.

The regeneration exchanger 10 further comprises a outlet 14 through which the flue gas flow 5 exits from the regeneration exchanger 10 towards the recovery exchanger 25; and an inlet 16 through which the air flows from the recovery exchanger 25 to the exchanger 10.

The outlet 14 is connected to the chambers 11 by means of a channel or duct 15, in particular shaped in a T or Y shape.

The duct 15 comprises two portions 57 which respectively end up in the chambers 11 and, in particular, flow into a portion 18 ending up in the outlet 14.

The inlet 16 is defined, in particular, by a duct connected to the portions 57 through respective ducts or pipes 17. According to an embodiment not shown, the pipes 17 are connected separately and directly to the exchanger 25 through respective inlets

16. The exchanger 10 further comprises a set of reversing valves 19a, 19b. The valves 19a are arranged, respectively, in the pipes 17, while the valves 19b are respectively arranged in the portions 57, in an intermediate position between the outlet 14 and the connection of the pipes 17 with the portions 57.

The valves 19a, 19b are controlled by a control and command unit 20 in an automatic and programmed way, to be switched in a synchronized manner with each other and to invert the flow of air 3 and the flow of flue gas 5 according to the aforementioned cycles (duration, for instance, twenty minutes).

The recovery exchanger 25 is preferably made of metallic material and is operated continuously, i.e. without inverting the flue gas flow 5 coming from the regeneration exchanger 10 and the combustion airflow 3 coming from the external environment.

The recovery exchanger 25 comprises two ducts 26, 27, concentric with each other for at least a part of their path; the duct 27 receives the flow of flue gas 5 from the outlet 14 and conveys this flow of flue gas 5 towards an outlet 31 of the exchanger 25, communicating with a chimney, not shown, through which the flue gas are discharged into the external environment.

At the same time, the duct 26 conveys the flow of air 3 towards the inlet 16. The air and flue gas, inside the ducts 26,27, preferably flow countercurrently and exchange heat through the metal walls that separate the ducts 26,27.

The flue gas generally have a temperature of about 1500°C in correspondence with the outlets 12. In the specific example illustrated, the extent of the heat exchange between flue gas and air in the exchangers 10 is set according to the project so as to obtain, for the flue gas, a temperature of about 450-600°C at the outlet of the chambers 11, and of about 200°C at the outlet 31, and for the air a temperature of about 300- 500°C at the inlet 16, and of about 1250°C at the mouths 12.

The assembly 1 also comprises a feed line 55 (schematically represented) for conveying the flow of fuel 51 to the glass furnace 60. In particular, the feed line 55 comprises two nozzles 56, arranged in the combustion chamber 2 in close proximity to the ducts 13 respectively. The flow outgoing from the nozzles 56 is controlled by means of respective valves 19c, which are controlled by the unit 20 in a synchronized mode with the valves 19a, 19b to inject fuel concurrently with the outflow of air from the ducts 13.

According to a preferred embodiment of the present invention, the assembly comprises a thermodynamic recovery unit 110 and an electric generator 120 (schematically illustrated) configured in such a way as to obtain, by exploiting a thermodynamic motor cycle preferably operating with organic fluids (Organic Rankine Cycle), the production of electric energy intended to power other units of the same furnace such as for instance an electrolyser (not shown) configured as in figure 1.

Preferably, the thermodynamic recovery unit 110 is housed at least partially in the duct 15 so as to be directly lapped by the flue gas flowing in the duct 15.

In the preferred example shown in Figure 2, a single thermodynamic recovery unit 110 is provided, arranged in correspondence with portion 18. Alternatively, it is possible to conceive two independent units 110' and 110" and distinct from each other and respective electrical generators coupled thereto, arranged in correspondence with a respective portion 57 of the duct 15, between the chambers 11 and the valves 19b.

In the aforementioned embodiment with two thermodynamic recovery units, the flue gas reach the units at a temperature higher than that reached at the single unit in the embodiment of figure 1 as they are arranged near the combustion chambers and before the inversion of the flows, consequently they are subjected to alternating cycles of operation at different temperatures linked to the exchange between the outgoing flue gas and the inlet air to the corresponding chamber. The greater complexity of construction between double units 110' and 110" is compensated by the possibility of using fluids, including non-organic fluids in the Rankine cycle, and the choice between these two configurations must be made in the design of each individual plant.

In an alternative embodiment, the assembly comprises at least one thermal inertial unit made at least partially with phase change materials or PCM (Phase Change Materials) aimed at accumulating the heat of the flue gas and the subsequent transfer of accumulated heat to the oxidant intended for combustion in said furnace. The operating cycle of the PCM system composed of the heating (latent heat of liquefaction) and cooling (latent heat of solidification) phases has much slower times (hours) than the inversion cycle, in the case of using regeneration chambers (typically twenty minutes). The thermal inertia of the system with PCM can therefore constitute a reserve (source) of almost constant heat that can be used for instance for the preheating of the fuel (natural gas) or for the generation of the water vapor necessary for the reforming process in the specific case of methane reforming.

In an executive embodiment, this inertial thermal unit is arranged between said reversing valves 19b and the corresponding regeneration chambers 11; in light of the above, by virtue of the phase change times much higher than the normal operating cycles of a regeneration chamber (tens of hours instead of tens of minutes), this inertial unit is preferably used to extract energy to be reused outside the heating of the combustion air (the latter a peculiar feature of the regeneration chamber).

In an executive embodiment, this inertial unit has a substantially cylindrical configuration, composed of several coaxial elements and arranged coaxially to the fitting that opens into the regeneration chamber where the innermost coaxial element is in contact with the outer surface of the said fitting while one or several outermost coaxial elements are used to transfer the heat taken from the flue gas to other fluids involved in the combustion process such as, as anticipated, the fuel and/or the substances involved in the reforming process.

However, it is clear that the invention must not be considered limited to the particular arrangements illustrated above, which are merely exemplary embodiments thereof, but that various 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.

Claims

1. A heat exchanger assembly (1) to recover and reuse at least part of the energy in the form of heat from flue gas produced in a combustion chamber (2) of a glass furnace, the assembly (1) characterized in that it comprises an energy recovery unit arranged to recover heat from the flue gas leaving the combustion chamber of said glass furnace and operating by means of a thermodynamic motor cycle, preferably operating with an organic working fluid, for conversion into electrical energy of at least part of the energy contained in the form of heat in said flue gas.

2. The assembly according to claim 1 wherein said recovered electrical energy contributes to the activation of an electrolyser for the production of dihydrogen (H2) and oxygen (O2) molecules as a consequence of the electrolytic processes applied to water molecules, said dihydrogen (H2) and oxygen (O2) molecules being intended to constitute at least part of the fuel and oxidant respectively for combustion inside said glass furnace.

3. The assembly according to claim 1 or 2 further comprising: a first regeneration type heat exchanger (10), equipped with: a) two regeneration chambers (11); b) a set of medium temperature ducts (15,17) communicating with said regeneration chambers for the passage of flue gas and combustion air, and having at least one outlet (14) for the exit of flue gas from said first exchanger (10), and at least one inlet (16) for the entry of combustion air into said first exchanger (10); c) reversing valves (19a, 19b), arranged in said ducts (15,17) and controllable so as to reverse a flow of combustion air (3) and a flow of flue gases (5) between said regeneration chambers (11) with cycles of specific duration; said regeneration chambers being positioned near the combustion chamber (2) and communicating with it through appropriate high temperature ducts (15, 17'). e assembly according to one or more of the preceding claims further comprising a second heat exchanger of the recovery type (25), comprising a first and a second duct (26,27) separated by metal heat exchange walls; said second duct (27) communicating with said outlet (14) for the passage of flue gas from the first to the second exchanger; and said first duct (26) communicating with said inlet (16) for the passage of heated air from the second to the first exchanger. e assembly according to one or more of the preceding claims further comprising at least a reforming unit (53) suitable for obtaining an endothermic reaction of the fuel by exploiting the heat of the flue gas taken downstream of said regeneration chambers (11), considering the gases feed path. e assembly according to one or more of the preceding claims, wherein part of the molecules of dihydrogen (H2) produced by said electrolyser is used in a desulphurization unit for the preventive desulphurization of the fuel intended to feed the said glass furnace along said feeding line passing inside the reforming unit. e assembly according to one or more of the preceding claims, characterized in that said reforming unit (53) and/or said thermodynamic energy recovery unit are arranged in correspondence with one of said ducts (15) and preferably arranged at least partially so as to exchange heat with the flue gas passing through one of said ducts (15). e assembly according to one or more of the preceding claims, characterized by the fact that it comprises at least one thermal inertial unit arranged between said reversing valves and the corresponding regeneration chambers for storing flue gases heat and the subsequent release of accumulated heat to other modules of said assembly such as for instance the line for feeding the fuel or for treating the substances involved in the fuel reforming process. e assembly according to the preceding claim wherein said at least one thermal inertial unit is made at least partially with phase change materials or PCM-Phase Change Materials. he assembly according to the preceding claims 8 or 9 wherein said thermal inertial unit is arranged externally to the portions (57) of the high temperature duct (15) terminating respectively in the chambers (11) and in contact with at least part of the surface of the same, said inertial unit further comprising a zone or portion intended for the transfer of the heat taken from the flue gas to at least a second fluid involved in the combustion process of the glass furnace. he assembly according to any one of the preceding claims, characterized in that it comprises at least one injection device configured so as to introduce a chemical agent suitable for reducing nitrogen oxides in the flue gas.

14