The present invention relates to a plant and method for regenerative combustion with low-calorific-value fuels.

In the steel industry, for example in industrial heating furnaces, high-temperature processes take place, which require high-calorific-value fuels for reaching the process temperature, wherein, for example, these temperatures indicatively reach temperatures of up to 1150°C - 1320°C in the furnaces.

It should be pointed out, however, that costly energy resources - such as noble chemical sources like natural gas - are specifically used in these apparatuses for facilitating variations in the production quantities.

It is also known that these large apparatuses consume significant quantities of thermal energy for their functioning. For this reason, attempts are being made in the steel industry to substitute noble energy sources, such as natural gases, with gases produced as by-products generated with other production apparatuses, such as, for example, blast furnaces or coke ovens. This tendency has the benefit of reducing the emission levels of CO ₂ on the part of the whole industrial plant. This is the case, as normally the process gases generated as by-products by modern and efficient equipment, generally have increasingly lower calorific powers. This means that these by-products cannot be easily used as fuels in process equipment operating at high temperatures. These by-products are consequently used with a low efficiency for the generation of electric energy or flare burnt before being released into the atmosphere.

The process equipment is consequently fed with nobler fuels, thus increasing the massive emissions of CO ₂ on the part of the whole plant, in addition to increasing the production costs. A more efficient use of by-products within the production plant, therefore, allows considerable benefits for the environment and for the profitability of industrial production, to be obtained .

All of these issues have resulted in a tendency, particularly in the steel production field, to promote the use of regenerative combustion plants and methods using fuels having a low calorific value. For the development of these plants and methods, for example, the use of regenerative burners has been envisaged, capable of recovering energy which would otherwise be lost and allowing these fuels having a low calorific value, to be used.

Current plants and methods, however, do not allow a complete exploitation of these fuels, nonetheless discharging fumes towards the outside which could release further heat that could be adopted for implementing an even greater saving through a plant and various methods of use of the combustion system of the plant .

FR 894658A describes a regenerative combustion plant with low-calorific-value fuels, according to the preamble of claim 1.

US 5823124 relates to a method and a system with reduced emissions of fuel and NO _x from a furnace.

The general objective of the present invention is to provide a plant and method for regenerative combustion with low-calorific-value fuels, capable of solving the drawbacks of the known art indicated above, in an extremely simple, economical and particularly functional manner.

A further objective of the present invention is to implement a plant and method for regenerative combustion with low-calorific-value fuels, which is technically simple to run and with extremely reduced and limited costs.

Another objective of the present invention is to implement a plant capable of drastically reducing possible emissions of carbon monoxide deriving from a combustion system developed with the regeneration of low-calorific-value fuel such as, for example, blast furnace gas .

The structural and functional characteristics of the present invention and its advantages with respect to the known art will appear even more evident from the following description, referring to the enclosed schematic drawings, which show an embodiment example of the invention. In the drawings: figure 1 is a scheme showing a plant configuration whereby a regenerative combustion method is effected, with low-calorific-value fuels, according to the present invention;

figure 2 shows an enlarged view of a part of the plant of figure 1.

With reference to the figures, these illustrate a plant capable of implementing a regenerative combustion method with low-calorific-value fuels, both plant and method according to the present invention.

The plant of the invention can be called a double regenerative combustion system and is associated with a furnace or combustion chamber 11 for high temperature thermal processes, around 1200 - 1300°C, equipped with regeneration units for both reagents, i.e. fuel and comburent .

In addition to the usual known feeding lines of the furnace or combustion chamber 11, for its correct functioning, an air feeding line 18 is also connected together with a feeding line of poor fuel 19 as described hereunder, for the treatment of the outgoing fumes from the same furnace. A non-regenerated 20 fume discharging line is also provided at the outlet of the furnace 11, operatively connected to one or more heat exchangers 16a, 16b, 16c, 16d, etc. which form a cascade heat recovery system, each operating within a different temperature range.

In the example shown and described, as reagents used in the plant for implementing the method, air coming from the air feeding line 18 can be used on the one hand, and on the other, poor fuel gases coming from the feeding line of poor fuel 19, which are treated in pre-heating in at least one air regeneration unit 12 and in at least one fuel regeneration unit 13, respectively, for their pre-heating. In the example described and illustrated, as can be seen, further air regeneration units 12 and fuel regeneration units 13 are provided, positioned in correspondence with consecutive control areas, 11a, lib, 11c, lid, etc.. of the chamber or furnace 11 for effecting the preheating .

Each of these regeneration units 12, 13, in a plant of the invention, is positioned on a respective duct for exhaust fumes to be regenerated 12e, 13e at the outlet of the furnace 11 and, according to the method of the invention, is operated periodically and selectively in two subsequent steps or different modes. The regeneration units 12 and 13 are in fact operated in a first regeneration mode, taking fumes of hot combusted products by means of the ducts 12e and 13e, from at least one control area 11a of the chamber 11. In order to effect this phase of the method of the invention, the plant provides fume regeneration valves 12a and 13a positioned on outlet ducts from regeneration means 12m, 13m, positioned inside the regeneration units 12 and 13, valves 12a and 13a being positioned on the ducts 12f and 13f downstream of the regeneration units 12 and 13 and which are both open.

In the same regeneration state, the method provides that air 12b and fuel 13b regeneration valves respectively - positioned on a respective air feeding duct 18a and poor fuel feeding duct 19a, at the inlet of the regeneration means 12m and 13m, but downstream of the regeneration units 12 and 13 - are respectively both closed in the plant.

When, on the contrary, the phase of the method is effected, in which the regeneration units 12 and 13 are operated in combustion mode, the air and fuel reagents are pre-heated by passing them through the hot regeneration means 12m and 13m inside the regeneration units 12 and 13. To do this, the fume regeneration valves 12a and 13a are both closed and the air regeneration valves 12b and fuel regeneration valves 13b are both open.

During the above-mentioned regeneration mode, the regeneration means 12m and 13m inside the regeneration units 12 and 13 were previously heated at a slightly lower temperature (for example 1200°) with respect to the temperature of the fumes leaving the control area 11a of the chamber 11 (for example 1250°) .

The fumes leave the regeneration units 12 and 13 passing through the fume regeneration valves 12a and 12b at a much lower temperature (for example about 200°C) with respect to the process temperature inside the control area 11a of the chamber 11.

This temperature is suitably limited from being too low to avoid the condensation of acid substances which would limit the life of the above-mentioned valves.

During the combustion mode, the regeneration means 12m and 13m, previously pre-heated in the previous regeneration mode, exchange their internal energy with the respective reagent (air and fuel) which is passed through said regeneration means 12m and 13m before reentering the specific control area of the furnace 11 and combusted. It should be noted that during the regeneration mode, the fumes leaving the air regeneration units 12, have the same composition as the fumes inside the chamber 11, with a slight increase in the concentration of the chemical species inside the combustion air. The fumes leaving the fuel regeneration unit 13 are otherwise mixed with a small quantity of fuel which remains inside dead volume areas of the regeneration unit 13 and fuel regeneration valve 13b during the time necessary for commuting from the combustion state to the regeneration state. This means that the fumes or exhaust gases exiting through the fume regeneration valve 13a contain a certain quantity of fuel in relation to both the commuting time between the regeneration mode and combustion mode and the volume of dead areas in which said fuel can be concealed. As this quantity of fuel is not used, it represents a loss of energy from the system, and also a risk for the safety and for the environment in which should be introduced.

This quantity of fuel may tend to be zero when the project parameters, such as, for example, the commutation time and volume of the dead areas are sufficiently small. These parameters, however, also depend on the size of the burner which is generally the result of considerations of a technical and economic nature. In order to limit the above-mentioned risk as much as possible, a post-combustion unit 14 is provided and operated according to the method and plant of the present invention, positioned on the route of regenerated fumes leaving the fuel regeneration units 13 discussed above.

It can therefore be seen that, according to the invention, said post-combustion unit 14 is fed first of all with all or at least a part of the regenerated fumes coming from the fume regeneration valve 13a, positioned on the duct 13f at the outlet of the fuel regeneration unit 13. Secondly, the post-combustion unit 14 is fed with a quantity of air coming from a valve 14c, situated on the air feeding line 18 and positioned at the inlet of the post-combustion unit 14. A flow of a certain quantity of fuel fed from a valve 13c of the feeding line of poor fuel 19, may also be necessary for completing the oxidation of the regenerated fumes coming from the fume regeneration valve 13a inside the post-combustion unit 14.

The provision of a post-combustion unit 14, according to the invention, is such as to eliminate the part of fuel contained in the regenerated fumes leaving the fuel regeneration unit 13 due to the continuous passage between the regeneration and combustion mode. The fumes thus oxidized leaving the post-combustion unit 14 through an outlet pipe 14d, have a temperature higher than the temperature at the inlet, when they passed through a fume valve 14a thanks to the exothermic oxidation process inside the post-combustion unit 14.

The enthalpy in the fumes treated at the exit of the outlet pipe 14d, resulting from the oxidation process inside the post-combustion unit 14, can be recovered by means of a heat exchanger 16c connected to the same and being part of the heat recovery group mentioned above. It should be remembered that the non- regenerated fraction of fumes sent to the discharge line of non-regenerated fumes 20 is at times necessary for controlling the pressure in the combustion chamber 11. This fraction of fumes is normally at a high temperature and its enthalpy can be recovered only through the above-mentioned heat exchangers made of special materials. It should also be noted that the flow-rate associated with the flow of fumes in the discharge line of non-regenerated fumes 20 at the outlet of the furnace 11, is normally a small fraction of the flow-rate of the regenerated fumes processed through the regeneration units 12 and 13 and therefore a small quantity of enthalpy is associated with the same . Consequently, by mixing the flow-rate at the outlet of the non-regenerated fume discharge line 20 with the flow-rate of the oxidized fumes in the outlet pipe 14d of the post-combustion unit 14, a greater flow-rate will be obtained in a pipe 20b at medium temperatures. As its temperature is intermediate between the temperature present in the flows of fumes in the pipes 14d and 20, a standard heat exchanger can be used as heat exchanger 16c.

Said heat exchanger 16c is connected at the other side to a generic heat recovery system or an ORC (Organic Rankine Cycle) system or to a co-generation system, schematized in 17, with further heat recovery.

As it may be possible to process only a portion of regenerated fumes coming from the fuel regeneration unit 13 through the fume valve 14a towards the post- combustion unit 14, a remaining portion of these fumes can be sent, through a valve 14b positioned on a pipe 14k, to be possibly mixed downstream of the heat exchanger 16c with cooled exhaust fumes of said heat exchanger 16c having a similar temperature. This ensures a limited or no reduction in efficiency for the heat recovery process.

This flow of non-oxidized exhaust fumes can be possibly mixed with a flow of fumes representing a part or all of the regenerated fumes passing through the valve 12a at the outlet of the air-regeneration unit of 12. This flow of fumes passing through a valve 12d is normally at the same temperature as the fumes regenerated by the fuel-regeneration units 13. Furthermore, if the dead volume mentioned above, containing a few fractions of fuel, is sufficiently small as to mix the flow of non-oxidized fumes not passing through the valve 14a and which reach the valve 14b with the flow of fumes or exhaust gases passing through the valve 12d, a means is provided for reducing concentrations of fuel species to such an extent that they do not represent any risk for safety or for the environment .

The flow of fumes or exhaust gases reaching a pipe 20c downstream of the heat exchanger 16c, possibly obtained by mixing the fumes introduced and processed by the heat exchanger 16c with the flow of fumes or non-oxidized exhaust gases coming from the valve 14b and from the valve 12d, can be further processed in a second heat exchanger 16d positioned downstream of the first heat exchanger 16c.

This last exchanger 16d can also be connected to the same or another recovery system 17 as previously indicated for the heat exchanger 16c.

Finally, the flow of treated fumes leaving the exchanger 16d through a pipe 20d is then sent to a chimney or to a fume post-treatment plant.

Other heat exchanger units 16a and 16b can be installed upstream of the outlet of the outlet pipe 14d from the post-combustion unit 14.

In particular, a first heat exchanger 16a is required if the recovery of the heat of the non- regenerated exhaust gases coming from line 20 is effectively feasible from a technical and economic point of view. It is connected to the same or another heat-recovery system such as the above-mentioned heat exchangers 16c and 16d. A second heat exchanger 16b can be installed if the flow of fumes or exhaust gases reaching a valve 12c, which represents a part or all of the exhaust gases regenerated by the air-regeneration units 12, is at a higher temperature than that of the oxidized fumes leaving the pipe 14d from the post- combustion units 14.

Alternatively, the heat exchanger 16b can be installed in place of the heat exchanger 16a when the flow of non-regenerated exhaust gases coming from line 20, is at such low temperatures that no heat recovery is technically and economically feasible. In these cases, it is possibly necessary that non-regenerated exhaust gases or fumes, coming from line 20, be mixed with the exhaust gases reaching the valve 12c from the air-regeneration units 12, in order to limit the temperature of the hot fluid in an inlet pipe 20a of the heat exchanger 16b.

The double regenerative combustion system described above can naturally comprise all or some of the various provisions or equipment described. The final solution depends on the technical and economic analysis of the specific plant conditions such as the availability of heat-recovery systems, the type of fuel, the plant dimensions, possible nearby utilities, and so forth.

It can thus be seen that with a plant and method as defined with double regenerative combustion, it is possible to use fuels with a very low calorific value in high-temperature thermal processes, with evident economic and operational benefits, in addition to a decrease in pollution.

This is obtained by pre-heating both reagents by means of regeneration units, which allow pre-heating temperatures about 100 °C lower than the process temperature, to be reached. This means that the combusted gases or fumes processed by this regeneration plant connected to a chamber at 1250°C, at the outlet of the plant are at less than 200°C.

The forms of the structure for the implementation of a plant and method of the invention, as also the materials and assembly modes, can naturally differ from those shown for purely illustrative and non-limiting purposes in the drawings, .

The objective mentioned in the preamble of the description has thus been achieved.

The protection scope of the present invention is defined by the enclosed claims.