Patent Application
20110146547
The present invention relates to the field of particulate fuel combustion and the use of particulate fuel burners in industrial combustion furnaces.
The present invention relates in particular to the use of particulate solid fuel burners, such as particulate coal burners.
The present invention also relates to the use of particulate liquid fuel burners, whereby combustion is generated by injecting oxidant and particulate liquid fuel, i.e. droplets of liquid fuel, into a combustion zone. The present invention relates more particularly to the use of such particulate liquid fuel burners for heavy liquid fuels.
Coal is the most abundant fossil fuel currently available. Most of the power generated in the world uses coal as the fuel.
One way of generating heat or power is the use of coal burners.
In a coal burner, a conveying gas is often required to transport the solid fuel particles from a fuel storage or milling device (e.g. a coal pulveriser) to the burner for subsequent combustion with an oxidant. The oxidant for the combustion can be the conveying gas, a gas supplied separately from the conveying gas or a combination of the conveying gas and a separately supplied gas.
The combustion process of particulate solid fuel comprises several combustion steps which are described hereafter with reference to the combustion of particulate coal:
This multi-step combustion process distinguishes particulate solid fuel combustion from the gaseous fuel combustion process in which the gaseous fuel combusts directly with the oxidant.
The particulate liquid fuel combustion process, in which liquid fuel is injected into the combustion zone in the form of small particles or droplets, is also a multi-step process. In a first step, the injected liquid fuel droplets are heated to the evaporation temperature of the fuel when the fuel reaches its evaporation temperature, the liquid fuel evaporates to form inflammable fuel vapours and in the third step the inflammable fuel vapours combust with the oxidant and produce heat. For light fuels, such as domestic fuel oils, or No 1, 2 and 3 fuel oils, the evaporation temperature is relatively low and evaporation of the fuel into vapours takes place almost instantly following injection into the combustion zone at normal operational temperatures of most industrial furnaces. Consequently, the combustion of particulate light liquid fuels resembles that of gaseous fuels as far as rate of combustion following injection is concerned.
In the combustion of particulate medium heavy liquid fuels such as No 4 fuel oil and very heavy liquid fuels such as residual fuel oil, or No 5 and 6 fuel oils, the evaporation temperature is higher and evaporation of the liquid fuel takes place more slowly and more gradually. In this manner, the combustion of particulate heavy and especially very heavy liquid fuels resembles the multi-step process of particulate solid fuel combustion.
As a consequence, particulate solid fuel burners, such as particulate coal burners, and particulate heavy liquid fuel burners, are usually not suited for a narrow combustion chambers in which only short flames can be used for heat generation.
Indeed, when the length of the flame exceeds the width of the combustion chamber (the width being the free dimension of the combustion chamber along the flame axis), the flame impinges on the combustion chamber element opposite the burner, thereby causing incomplete fuel combustion and fouling with partial-combustion products such as soot as well as thermal damage to the impinged chamber element. The said chamber element can be a chamber wall positioned opposite the burner, for example in a glass feeder or forehearth or in a reheat furnace. The element can also be a chamber element to be heated such as a radiant heating panel or pipes, such as boiler pipes, positioned opposite the burner within the combustion chamber.
Air is traditionally used as the conveying gas and as the oxidant for particulate fuel burners, as the conveying gas for solid particulate fuels and as the pulverisation gas for particulate liquid fuel injectors. Burners using air as the oxidant for combustion are known as air-fuel burners.
In the case of oxy-fuel burners, the oxidant is an oxygen-rich gas (>25% vol O2) such as oxygen-enriched air or industrial oxygen having an oxygen content of at least 90% vol, preferably of at least 95% vol, and more preferably of at least 98% vol.
The advantages of oxy-fuel burners over air-fuel burners are multiple:
In the case of oxy-fuel burners, the risk of thermal damage to the installation in case of narrow combustion chambers is particularly important due to the higher flame temperature when compared to air-fuel burners.
Examples of narrow combustion chambers are side-fired tunnel or passage furnaces, such as cement passage kilns, glass feeders or forehearths.
Other examples of narrow combustion chambers are side- and/or cross-fired vertical boilers and cracking installations (FCC).
In view of the high availability of solid fuels such as coal, including low-grade coal, and of heavy fuels, including combustible industrial liquid waste, often at advantageous prices, it would be highly desirable to be able to use particulate fuel burners in narrow industrial combustion chambers.
This is made possible by the furnace according to the present invention and the process of operating same.
The combustion furnace of the present invention comprises a combustion chamber defining a combustion zone therewithin and having at least one chamber wall facing the combustion zone.
At least one particulate fuel burner, hereafter referred to as “main particulate fuel burner”, is mounted in a chamber wall of said combustion chamber. Said main particulate fuel burner is adapted to generate in the combustion zone a flame, referred to as “main flame”, directed away from said chamber wall by injecting oxidant in gaseous form and fuel in particulate form into the combustion zone for combustion therein.
According to the invention, said main particulate fuel burner has associated therewith at least one auxiliary burner equally mounted in said burner wall; Said auxiliary burner is furthermore located in the vicinity of the main particulate fuel burner with which it is associated. The auxiliary burner is adapted to generate a flame, referred to as “auxiliary flame”, in the combustion zone located, said auxiliary flame being proximate said chamber wall and at least partially directed towards said main particulate fuel burner.
Said main particulate fuel burner may have at least two auxiliary burners associated therewith, for example, the main particulate fuel burner may be associated with two auxiliary burners, one on either side, or may be surrounded by four auxiliary burners.
As will be explained hereafter, an auxiliary burner may also be associated with more than one main particulate fuel burner. For example, an auxiliary burner may be associated with two main particulate fuel burners, one on either side of the auxiliary burner.
The nominal power of the main particulate fuel burner is advantageously greater than the nominal power of the auxiliary burner. Preferably the nominal power of the main particulate fuel burner is at least 50% greater than the nominal power of the auxiliary burner and more preferably at least twice the nominal power of the auxiliary burner.
It is also advantageous for the main particulate fuel burner and the auxiliary burner to be designed so that the momentum of the main flame generated by the main burner is higher than the momentum of the auxiliary flame generated by the auxiliary burner. Preferably, said main flame momentum is at least 50% greater than auxiliary flame momentum, and more preferably at least twice the auxiliary flame momentum.
The main particulate fuel burner is preferably adapted to generate a main flame, the axis of which is situated in a vertical plane perpendicular to the chamber wall in which the main particulate fuel burner is mounted. For many applications it is advantageous for said main flame axis itself to be perpendicular to said chamber wall.
The fuel in particulate form injected by the main particulate burner can be a particulate solid fuel or a particulate liquid fuel.
Examples of suitable particulate solid fuels are particulate coal, pet coke, combustible particulate solid waste. Different classes of particulate coal may be used depending on the process: lignite, bituminous coal or anthracite, from highly coking to non-coking coals, etc.
Particular examples of suitable liquid fuels are medium heavy liquid fuels, such as No 3 fuel oil, and heavy liquid fuels such as No 5 and No 6 fuel oils, furnace fuel oils (FFO). The present invention is particularly useful for the combustion of waste fuel or fuel waste, if appropriate for the process concerned, in particular with regard to any effects on the charge to be heated.
The main particulate fuel burner may be an oxy-fuel burner. In particular, the main particulate fuel burner may be an oxy-fuel burner operating with an oxidant, such as for example oxygen-enriched air, containing at least 50% by volume of oxygen, preferably at least 80% by volume, more preferable at least 90% by volume of oxygen, or industrial oxygen having an oxygen content of at least 95% by volume, and preferably of at least 98% by volume.
The auxiliary burner may or may not be a particulate fuel burner as described above. The auxiliary burner may be connected to the same fuel source as the main particulate fuel burner or to a different fuel source, for example a gaseous fuel source.
The auxiliary fuel burner may be connected to the same oxidant source as the main particulate fuel burner or to a different oxidant source.
The auxiliary fuel burner is advantageously an oxy-fuel burner. Preferably, both the main particulate fuel burner and the auxiliary burner are oxy-fuel burners.
The furnace typically comprises a multitude of main particulate fuel burners, each main particulate fuel burner having at least one auxiliary burner associated therewith. The furnace may thus comprise a multitude of main particulate fuel burners in one chamber wall.
In some cases, for example certain vertical boiler furnaces, the combustion furnace only has burners on one side of the combustion chamber.
In other cases, the combustion furnace has burners mounted in different chamber walls.
The invention thus also relates to a combustion furnace, having at least one main particulate fuel burner in a first chamber wall and at least one particulate fuel burner in a second chamber wall positioned opposite the first chamber wall across the combustion zone. Typically, such a combustion furnace will have a first multitude of main particulate fuel burners in the first chamber wall and a second multitude of main particulate fuel burners in the second chamber wall. Examples of such combustion furnaces include glass-melting furnaces, glass feeders or forehearths, tunnel calcination furnaces or reheat furnaces.
When several main particulate fuel burners are mounted in one wall, these burners are usually arranged in a geometric pattern.
For example, a furnace with a long narrow combustion chamber may have a substantially horizontal row of main particulate fuel burners mounted in one or both lateral chamber walls. A furnace with a vertical narrow combustion chamber may, for example, have main particulate fuel burners arranged in a checkerboard pattern in a chamber wall.
The present invention also relates to a process of operating a combustion furnace. Said furnace comprises a combustion chamber defining a combustion zone within the combustion chamber. The combustion chamber has at least one chamber wall facing the combustion zone. At least one main particulate fuel burner is mounted in a chamber wall of the combustion chamber. Said main particulate fuel burner is adapted to generate in the combustion zone a main flame directed away from the chamber wall into which said main burner is mounted by injecting oxidant in gaseous form and fuel in particulate form into the combustion zone for combustion therein. The main particulate fuel burner is associated with at least one auxiliary burner mounted in said chamber wall.
In the process of the invention, oxidant in gaseous form and fuel in particulate form are injected into the combustion zone for combustion therein by means of the main particulate fuel burner so as to generate a main flame in the combustion zone. Said main flame is directed away from said chamber wall and has a flame root adjacent said main particulate fuel burner. Furthermore, according to said process, an auxiliary flame is generated in the combustion zone by means of the at least one auxiliary burner associated with the main particulate fuel burner. In contrast to the main flame, the auxiliary flame is located proximate said chamber wall and at is at least partially directed towards the root of the main flame.
In this manner, the auxiliary flame provides additional heating for the particulate fuel injected by the main particulate fuel burner. As a consequence, the residence time required for said fuel particles to reach their devolatilization, respectively their evaporisation temperature after their injection into the combustion zone is reduced, thus shortening the distance to be travelled by the fuel particles in the combustion zone before combustion of the volatiles, respectively of the fuel vapours commences, which in term leads to a shortening of the main flame.
As already mentioned with respect to the furnace of the invention, the main particulate fuel burner may be associated with at least two auxiliary burners.
According to one advantages embodiment, the auxiliary flame is a highly divergent flame. For example, the auxiliary flame may have a flame axis substantially perpendicular to the chamber wall a flame opening of at least 90°, preferably of at least 110° and more preferably of at least 130°. The flame opening is the cone angle α of the (truncated) cone defined by the root of the flame as shown in
According to an alternative embodiment of the process, the auxiliary flame has a flame axis forming a sharp angle with said chamber wall. The angle between said flame axis and the chamber wall is not more than 60° and more preferably not more than 30°. This is particularly suited for the embodiment of the invention, according to which more than one auxiliary burner is associated with one main particulate fuel burner. Indeed, several such auxiliary burners can be positioned around and in the vicinity of one main particulate fuel burner, whereby each said auxiliary burner generates an auxiliary flame directed at the root of the main flame generated by said main particulate fuel burner.
The main flame typically has a flame axis situated in a vertical plane perpendicular to said chamber wall. In many cases, it is advantageous for the flame axis of the main flame to be substantially perpendicular to said chamber wall.
The purpose of the auxiliary burners is to accelerate the heating of the particulate fuel by increasing the temperature in the vicinity of the root of the main flame.
The auxiliary burner is usually operated continuously, as is the main particulate fuel burner.
In some cases, it is not necessary for the auxiliary burners to operate constantly for the required shorter main flame length to be obtained. In that case, the auxiliary burner may be operated to generate the auxiliary flame intermittently.
The transfer of energy to the charge to be heated in the furnace is mainly effected by the one or more main flames, which are directed away from the chamber wall towards the charge. It is consequently advantageous for the main particulate fuel burner to operate at a power which is greater than the power at which the auxiliary burner operates. Preferably the power of the main particulate fuel burner is at least 50% greater than the power of the auxiliary burner. More preferably the power of the main particulate fuel burner is at least twice the power of the auxiliary burner.
In order to limit any deflection or disturbance of the main flame by the auxiliary flame, it is also advantageous for the momentum of the main flame generated by the main burner to be higher than the momentum of the auxiliary flame generated by the auxiliary burner. Preferably, said main flame momentum is at least 50% greater than the auxiliary flame momentum, and more preferably at least twice the auxiliary flame momentum.
As described above with respect to the furnace of the invention, the fuel injected by the main particulate fuel burner can be a particulate solid fuel. It can also be a particulate liquid fuel.
The main particulate fuel burner is advantageously an oxy-fuel burner.
The auxiliary burner may or may not be a particulate fuel burner as described above. The auxiliary burner may generate the auxiliary flame by injecting the same fuel as the main particulate fuel burner or by injecting a different fuel, for example a gaseous fuel source.
The auxiliary fuel burner may generate the auxiliary flame using the same oxidant as the main particulate fuel burner or using a different oxidant.
The auxiliary fuel burner is advantageously an oxy-fuel burner. Preferably, both the main particulate fuel burner and the auxiliary burner are oxy-fuel burners.
The invention is particularly useful for heating a charge in a narrow furnace.
The process of the invention can, in particular be used to heat a charge in a furnace which only has burners on one side of the combustion chamber, as is the case in certain vertical boiler furnaces.
The process of the invention is also useful for heating a charge in a furnace having burners mounted in different chamber walls, for example in combustion furnaces having at least one main particulate fuel burner in a first chamber wall and at least one particulate fuel burner in a second chamber wall positioned opposite the first chamber wall across the combustion zone.
For many applications, the first and second walls are advantageously lateral walls of the combustion chamber.
The process of the invention can in particular advantageously be used to heat a charge in glass-melting furnaces, in glass feeders or forehearths, in tunnel calcination furnaces and in reheat furnaces.
The main particulate fuel burner preferably comprises a burner block defining a passage therethrough for the injection of fuel and/or oxidant therethrough into the combustion zone for generating the main flame. The burner block, which is mounted in the chamber wall, is typically made out of refractory material, such as AZS.
The auxiliary burner may likewise comprise a burner block defining an injection passage therethrough.
According to a specific embodiment of the furnace and the process of the present invention, the auxiliary burner is mounted in the same burner block as the main particulate fuel burner with which it is associated.
The present invention and its advantages are illustrated in more detail hereafter, reference being made to the non-limiting
In state-of-the-art combustion furnaces, the flame generated by said burners is generally directed away from the furnace wall towards the charge (for example, in the case of a vertical boiler furnace) or substantially parallel to the charge towards the opposite chamber wall (for example in a melting, fining or reheat furnace). In these state-of-the-art furnaces, a limited amount of heat is generated in the vicinity of the chamber walls. Indeed, high temperatures close to the chamber walls are generally avoided in order to prevent thermal damage to the refractory walls of the combustion chamber.
In the case of particulate fuel burners, and more specifically in the case of solid particulate fuel burners and particulate fuel burners for heavier liquid fuel fractions, this method of operating a furnace results in long flames and/or incomplete fuel combustion due to the long residence time/travel distance of the particulate fuel from its point of injection until it reaches its devolatilization/evaporation temperature. This makes these state-of-the-art furnaces badly suited for certain particulate solid and liquid fuels, in particular when the furnace is a narrow one.
The present invention now makes it possible to obtain relatively short particulate fuel flames by reducing the residence time required for the fuel particles to reach their devolatilization, respectively their evaporisation temperature after their injection into the combustion zone, thus shortening the distance travelled by the fuel particles in the combustion zone before combustion of the volatiles, respectively of the fuel vapours commences, which in term leads to a shortening of the flame.
According to the present invention, this is achieved as follows.
The particulate fuel burner 100 of
The central fuel injector may be a particulate solid fuel injector through which particulate solid fuel is transported by means of a conveyor gas. The central fuel injector may also be a liquid fuel pulverizer, also known as “atomiser”, by means of which liquid fuel is sprayed into the combustion zone in the form of particulate liquid fuel. The mechanism of injecting particulate liquid fuel used by the central fuel injector may be mechanical spraying, gas assisted spraying or a combination of the two. Suitable known spray devices are described in EP-A-1750057 and WO-A-03006879.
The oxidant used contains 90% vol O2.
Burner block 110 is mounted in chamber wall 10.
Main particular fuel burner 100 injects particulate fuel and oxidant through passage 120 into combustion zone 20 to generate main flame 130 directed away from the chamber wall 10 towards the charge (for example, in the case of a vertical boiler furnace) or substantially parallel to the charge towards the opposite chamber wall (for example in a melting, fining or reheat furnace). Main flame axis 131 is perpendicular to furnace wall 10.
Auxiliary burner 200 is also a particulate fuel oxy-burner. Burner block 210 is mounted in chamber wall 10 between two successive main particulate fuel burners 100.
The auxiliary flame 230 generated by auxiliary burner 200 is a very wide flame with low momentum. Auxiliary burner 200 furthermore operates at substantially less than the power of main particulate fuel burner 100.
As shown in
Due to the low power and momentum of the auxiliary flame, this is achieved without significant disturbance to the main flame's form and flow pattern and without causing thermal damage to chamber wall 10 and main burner block 110 due to overheating. Excellent overall furnace efficiency can thus be obtained.
In the embodiment shown in
Again, the flame length of the main flame is shortened and substantially complete fuel combustion can be achieved in the main flame, even in narrow furnaces, and this with good overall furnace efficiency and without significant disturbance to the main flame's form and flow pattern and without thermal damage to the chamber wall.
