EP4656995A1

EP4656995A1 - High-efficiency oxy-combustion ceramic roller kiln

Info

Publication number: EP4656995A1
Application number: EP25154321.1A
Authority: EP
Inventors: Ramón Nebot Forés; Luis JIMÉNEZ ROMERO; Joaquín VILLAR FONT DE MORA
Original assignee: Ingeneo Equipos Industriales SL
Current assignee: Ingeneo Equipos Industriales SL
Priority date: 2024-05-31
Filing date: 2025-01-28
Publication date: 2025-12-03
Also published as: ES1311232Y; ES1311232U

Abstract

The invention is based on harvesting latent heat from the exhaust fumes of the kiln to heat the fuel and the oxidizing agent, decreasing the necessary gas flow rate, with high fuel savings. To that end, based on the conventional structure of a ceramic roller kiln, it is envisaged that the burners are embodied as oxy-combustion burners (2.7), fed by oxygen and a fuel such as natural gas, wherein the oxygen and fuel are preheated by means of an airflow forced by means of a heat-carrying air blower fan (2.1) in which the air is previously passed through a fume heat recovery unit (2.2) associated with the fume exhaust stack of the kiln and/or a rapid cooling recovery unit (2.3) established in the cooling area (1.7) of the kiln.

Description

TECHNICAL FIELD

The present invention belongs to the sector of manufacturing ceramic tiles and claddings, and within this sector, more specifically to the single-channel and double-channel roller kilns currently using air or preheated air to react with a gaseous fuel such as natural gas, and more recently hydrogen.
The present invention relates to a new combustion system in which a kiln is completely transformed from air/fuel combustion to oxygen/fuel combustion entirely or partially, in which nitrogen is almost completely eliminated since it is not involved in the combustion reaction. In this way, the consumption of natural gas or the fuel used is drastically reduced. Furthermore, a gas and oxygen preheating system is used, harnessing the hot fumes coming from the combustion or from the cooling area to increase energy efficiency thereof.
The invention is based on the harnessing latent heat in the exhaust fumes from the kiln to heat the fuel and the oxidizing agent. In this way the adiabatic flame temperature is increased from about 2200°C to 3080°C, improving the energy transfer to the roller plane by the radiation phenomenon; therefore, since the piece can be overheated, the gas flow is reduced, thereby entailing a saving of about 50%.
The invention therefore provides for the complete transformation of a ceramic kiln to oxy-combustion technology with preheating of the fuel and oxidizing agent. This can be installed in the combustion fume outlet, in the rapid cooling area or a combination thereof.
The object of the invention is to reduce fuel consumption by also reducing the carbon emissions produced during the combustion reactions of the hydrocarbons used up until now.

BACKGROUND OF THE INVENTION

Taking into account the current situation in which the reduction of the use of greenhouse effect fuels is pursued, is of vital importance to optimize the energy in kilns that use mainly natural gas as fuel, and even hydrogen, since the latter does not always come from completely renewable sources. In addition, the result is an improvement in production costs, less dependence on fuels and, therefore, an improvement in the competitiveness of the companies that adopt this technology.
Ceramic roller kilns with air/gas technology with the preheating of oxidizing agent for firing ceramic tiles or even with self-recovery burners are already known today.
Oxy-combustion technology in glass melting furnaces, ceramic frit kilns even in color calcination, in which nitrogen is almost completely eliminated from the oxidizing agent to reduce fuel consumption, is also known.
To a lesser extent, the preheating of the gaseous fuel and the oxidizing agent in oxy-combustion kilns for glass production and melting ceramic frits for the optimization of gas and oxygen consumption is also known.
The novelty of this invention is summarized in two important actions:

First action.

The kiln is transformed from air-gas technology to oxy-combustion, eliminating almost entirely the nitrogen from the air that is not involved in combustion, therefore reducing fuel consumption. For this purpose, the combustion air lines are completely eliminated and replaced with a new oxygen line with all the regulating and control equipment thereof; this oxygen can be supplied by means of a liquid oxygen tank or by means of an oxygen production plant operating with molecular sieves.

Second action

The technology for preheating gaseous fuels, either natural gas (or the like), hydrogen or any mixture thereof, and the corresponding oxygen as an oxidizing agent, in ceramic kilns, taking into account its challenges and peculiarities, is adapted. For said adaptation, it has been necessary to develop a double jacket heat recovery unit capable of working with dirty and corrosive fumes, a particularity of ceramic kilns.
It has also been necessary to develop shell and tube recovery units with working temperatures close to 400°C and with materials resistant to extreme oxidation by high-temperature oxygen, as well as a new design of oxy-combustion burners capable of withstanding the extreme radiation produced by oxy-combustion with preheated fuels and oxidizing agents and, of course, the extreme oxidation experienced by the burner elements, such as the body itself, which is heat-insulated, the nozzles and the duct, furthermore taking into account the large number of burners since a kiln of this type has hundreds of burners, to be able to distribute the heat well according to the phases of heating and firing of the pieces, since the quality of the finished product is of utmost importance as the flame falls directly on the face of the product.

DESCRIPTION OF THE INVENTION

The new kiln or modification of the existing kiln that is proposed fully solves the problems set forth above in a fully satisfactory manner as it is embodied in a countercurrent-type radiation heat recovery unit, which in a first step is responsible for preheating an air stream up to about 400°C, using the residual heat or energy from the exhaust fumes of the ceramic kiln, or from a radiation recovery unit located in the rapid cooling area, or a combination of both.
This heat recovery unit is installed at the fume outlet of the kiln in question and has a passage for the primary fluid with the minimum possible obstruction. An air dilution controlled by means of a servomotor or the like can be installed to protect the heat recovery unit from temperatures exceeding 500°C.
Once the air or secondary fluid has been preheated to about 400°C, it is led to the first heat exchanger, which will mainly be the oxygen heat exchanger, since, for safety reasons, it can work at higher temperatures than in the case of natural gas or hydrogen. This convection-type heat exchanger is conceived as a shell and tube countercurrent type using, as the primary fluid, air in the "tube" area and, as the secondary fluid, oxygen in the "shell" area, although other arrangements may be valid to obtain the desired oxygen temperature. With this second step, it goes from having a preheated air stream of up to about 400°C to having an air stream of about 400°C and an oxygen stream of about 300°C. The latter, i.e., oxygen, is ready to be introduced into the burners to subsequently oxidize the fuel, thereby producing the combustion reaction.
Moreover, the excess hot air at about 350°C will be used to preheat mainly the fuel, using a heat exchanger similar to the one used for preheating the oxygen. This second exchanger is mainly by convection and is conceived as a shell and tube countercurrent type using, as the primary fluid, air in the "tube" area and, as the secondary fluid, fuel in the "shell" area, although other arrangements may be valid to obtain the desired fuel temperature. With this third step, it goes from having a preheated air stream to having an air stream at about 300°C and a gaseous fuel stream at about 250°C. The latter, i.e., fuel, is ready to be introduced into the burners to be subsequently oxidized with the oxidizing agent and produce the combustion reaction.
For this last step, which basically corresponds to the combustion reaction, a plurality of oxy-combustion burners particularly prepared to work with the oxygen and the fuel already preheated up to temperatures of 350°C and 250°C, respectively, are conceived.
For this purpose, the burners are, basically, in two preferably concentric conduits having a variable section, in which the fuel gas circulates through the central tube and the oxygen through the outer tube. These conduits are made of a material resistant to high temperatures, with a low coefficient of expansion and resistant, above all, to oxidation. Furthermore, the burner has an external heat insulation to reduce insulation losses, a feature that is only found in this technology, since in conventional oxy-combustion burners it is not necessary to insulate the conduits because the gas and oxygen are at room temperature. These concentric conduits transport both reagents to the refractory material duct where they meet and the combustion reaction takes place. The main peculiarity of this new burner with respect to the existing ones is that, due to the increase in the temperatures of the reagents, there is an increase in the reaction kinetics and, therefore, the distance between the burner and the point of maximum temperature is closer to the burner wall, this being an undesirable effect, since the point of maximum temperature must be distributed inside the kiln such that it reaches all the surfaces with the smallest possible gradient to prevent different degrees of firing on the tile plane and thereby maintain a uniform quality in the finished product.
To solve this, the diameters, angles and arrangement of the nozzles are optimized to achieve uniformity in the flame temperature profile for the even distribution of energy across the width of the kiln.
Last but not least is the heat-carrying air fan. This equipment is responsible for drawing in outside air through a grating near the kiln and then blowing and passing it through the fume heat recovery unit or the rapid cooling radiation recovery unit, then through the oxygen exchanger and finally through the gas heat exchanger with sufficient energy to maintain a minimum working flow rate and both a dynamic and static pressure, high enough to withstand the relatively high pressure drops of recovery units and exchangers, so as to be able to transfer heat with the smallest surface. Once this air leaves the last exchanger, it still has some residual heat as it is at a temperature of about 150°C, which is why, finally, a system of synchronized or electrically interlocked motorized valves has been designed to regulate the amount of heat that is recirculated to the system.
As for everything else, the rest of the conventional elements that make up a current kiln of this type are maintained, thereby defining a new system capable of taking advantage of oxy-combustion technology together with the technology of harnessing heat from fumes.
Based on this structuring, fuel savings of close to 50% are achieved, 45% of which is due to the change from air-gas to oxy-combustion and the remaining 5% is due to the preheating of gas and oxygen.

BRIEF DESCRIPTION OF THE DRAWINGS

To complement the description that will be made below and for the purpose of helping to better understand the features of the invention according to preferred practical embodiment thereof, a set of drawings is attached as an integral part of said description, wherein the following is depicted in an illustrative and non-limiting manner:

Figure 1 shows a schematic profile view of a single-layer kiln for tiles, which includes the system for preheating gas and oxygen according to the objective of the present invention.
Figure 2 shows a scaled up cross-section view comparing, in the upper part of the figure, a conventional single-layer ceramic kiln to the same kiln, in the lower part of the figure, modified with substituted oxy-combustion burners according to the object of the invention.

PREFERRED EMBODIMENT OF THE INVENTION

In Figure 1, the following reference numbers have been used, separated into two groups for better understanding of what is represented by the invention with respect to what already exists.
Group 1: corresponding to the elements already existing in a conventional kiln (some of which can be refitted):

Ceramic rollers (1.1).
Combustion fume outlet (1.2).
Fume fan (1.3).
Refractory insulation (1.4).
Heating and firing area (1.5).
Firewall (1.6).
Cooling area (1.7).
Rapid cooling fan (1.8).
Indirect cooling (1.9).
Cooling air suction fan (1.10).
Final cooling (1.11).
Air-gas burners (1.12).

Group 2: Corresponding to the elements making up the essence of the invention, which are not included in the current state of the art and therefore are novel.

Heat-carrying air fan (2.1).
Fume heat recovery unit (2.2).
Rapid cooling heat recovery unit (2.3).
Oxygen heat exchanger (2.4).
Gas heat exchanger (2.5).
Recirculation valves (2.6).
Oxy-combustion burners (2.7).
Finally, reference is made to the following fluids:
- Fumes (H).
- Oxygen (O).
- Fuel (F).
- Air (A).

In view of Figure 1 described, it can be seen how the kiln of the invention is based on the conventional structuring of a kiln for firing ceramic tiles, in which the pieces enter by means of the ceramic rollers (1.1) through the area where there is located the combustion fume outlet (1.2) for the exit of fumes by draft induced as a result of a fume fan (1.3) expelling the gases produced by combustion into the atmosphere or into the purification system.
The pieces start out in the heating and firing area (1.5) and end up in the cooling area (1.7), said areas being separated by the firewall (1.6), and everything is in turn surrounded by the refractory insulation (1.4) to prevent energy losses to the outside.
Once they are in the heating and firing area, the ceramic pieces go through a plurality of air-gas burners (1.12) to the refrigerating area where rapid cooling area is located, where cooling takes place with air blown by the rapid cooling fan (1.8). After that is the indirect cooling area, where a less abrupt temperature drop occurs as a result of the cooling air suction fan (1:10). Lastly, the pieces go to the end of the final cooling area (1.11), where they achieve the temperature necessary so that they can be handled.
Therefore, according to the essence of the invention, a new heat-carrying air blower fan (2.1), which is connected through its blowing mouth to a fume heat recovery unit (2.2) or to a rapid cooling recovery unit (2.3), or to both, has been provided.
In the event that a fume recovery unit (2.2) is used, it is intercalated in the current stack or combustion fume outlet (1.2), and in the event that the cooling recovery unit (2.3) is used, it is located in the rapid cooling area after the firewall (1.6) in the final cooling area (1.7)
Once the energy of the kiln is recovered from at least one of the two heat sources mentioned in the form of hot air, this hot airflow is connected to one or mas oxygen heat exchangers (2.4) along the kiln and on same to heat oxygen.
And then along the same kiln and generally on top of same, the outlet for the hot air going through said oxygen heat exchangers (2.4) is connected to a gas exchanger (2.5) to preheat both the oxidizing agent (oxygen) and the fuel (natural gas). Finally, the air with residual heat is conveyed to the recirculation valves (2.6), where it is decided whether the residual heat is recirculated to the system or, conversely, the heat is expelled to the outside, in its entirety or partially.
Last but not least, the hot gas and oxygen enter the oxy-combustion burners (2.7), located transversely to the direction of the ceramic tiles arranged in a generally staggered manner both in the upper chamber of the ceramic rollers and in the lower part thereof, thereby comprising the heating and firing area (1.5).

Claims

A high-efficiency oxy-combustion ceramic roller kiln which, based the conventional structuring of a kiln for firing ceramic tiles, in which kiln there is a refractory insulation (1,4) and in which the pieces are movable by means of ceramic rollers (1.1) through the area of the kiln where there is located the combustion fume outlet (1.2) for the exit of fumes by draft induced as a result of a fume fan (1.3) expelling the gases produced by combustion, which rollers move said pieces through a heating and firing area (1.5) as well as through a cooling area (1.7) separated by a firewall (1.6), wherein in the heating and firing area (1.5) a plurality of burners are established, whereas the refrigerating area includes one or more cooling steps by means of the use of fans, characterized in that the burners are embodied as oxy-combustion burners (2.7), fed by oxygen and a fuel such as natural gas, wherein the oxygen and fuel are preheated by means of an airflow forced by means of a heat-carrying air blower fan (2.1) in which the air is previously passed through a fume heat recovery unit (2.2) associated with the fume exhaust stack of the kiln and/or a rapid cooling recovery unit (2.3) established in the cooling area (1.7) of the kiln.
The high-efficiency oxy-combustion ceramic roller kiln according to claim 1, wherein the circuit of air forced by means of the heat-carrying air blower fan (2.1) includes a complete or partial recirculation system for recirculating said air by means of recirculation valves (2.6).
The high-efficiency oxy-combustion ceramic roller kiln according to claims 1 and 2, wherein the complete or partial recirculation means for recirculating the air are tared for said air to reach a temperature of the order of 400°C.