Patent Application
20110146543
The present invention relates to the field of particulate fuel combustion and the use of particulate fuel burners in industrial 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 structure (such as a chamber wall) 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 structure.
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 advantage 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 and/or cross-fired tunnel or passage furnaces, such as cement passage kilns, glass feeders or forehearths.
In view of the high availability of solid fuels such as coal, including low-grade coal, and of heavy fuels, often at advantageous prices, it would be highly desirable to be able to use particulate fuel burners in industrial narrow combustion chambers.
This is accomplished by the method of operating a furnace of the present invention.
The furnace used in said method comprises a combustion chamber having walls defining a combustion zone within the combustion chamber.
The furnace further comprises at least one pair of particulate fuel burners. In the present context, each pair of burners consists of a first particulate fuel burner and a second particulate fuel burner, both said burners being equipped to inject fuel and oxidant into the combustion chamber and comprise a burner block having at least one passage for transporting fuel and/or oxidant through the burner block towards the combustion chamber.
The burner blocks of the first and second particulate fuel burners of each pair are mounted in the walls of the combustion chamber so as to be positioned opposite one another across the combustion zone.
Both burners of each pair are adapted to generate a flame in the combustion zone by:
The method according to the invention comprises alternating first and second steps.
In the first step, the second particulate fuel burner of each pair does not generate a flame in the combustion zone, whereas the first particulate fuel burner of each pair generates a flame in the combustion zone directed at the second burner block so as to cause local overheating of the second burner block of said pair.
In the second step, the first particulate fuel burner of each pair does not generate a flame in the combustion zone and the second particulate fuel burner of each pair generates a flame in the combustion zone directed at the first burner block so as to cause local overheating of the first burner block of said pair.
According to a preferred embodiment of the present invention, which even further reduces the risk of thermal damage to the installation in case of narrow combustion chambers:
A number of gases can be used as the deflecting gas. The deflecting gas can, for example, be an oxidant. The deflecting gas can advantageously be flue gas, in particular flue gas recycled from the combustion chamber. Depending on the nature of the process, it may be indicated to partially or totally remove water vapour from the flue gas, e.g. by condensation, before the flue gas is thus recycled. For other applications, the use of steam as the deflecting may be beneficial.
The fuel injected by the first and second particulate fuel burners of the/each pair can be a particulate solid fuel or a particulate liquid fuel.
Examples of suitable particulate solid fuel are particulate coal, pet coke, combustible solid particulate 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) and certain combustible liquid industrial wastes. 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.
Suitable particulate solid fuel burners are notably known from WO200603296, WO2007063386, and from co-pending European patent application EP 09174622.2. Suitable particulate liquid fuel burners are known from EP 1750057 and WO03006879.
The first and second particulate fuel burners of one pair, of several pairs or of each pair may be oxy-fuel burners. In particular, the first and second particulate fuel burners of one pair, of several pairs or of each pair may be oxy-fuel burners 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.
In many instances, the combustion chamber has a first wall and a second wall, the first wall being positioned opposite the second wall across the combustion zone.
In that case, the burner block of the first particulate fuel burner of a pair can for example be mounted in the first wall and the burner block of the second particulate burner of the pair in the second wall.
In this manner, when the furnace comprises a multitude of pairs of particulate fuel burners, the first particulate fuel burner of each pair may be mounted in the first wall and the burner block of the second particulate burner of each pair in the second wall. In particular, the first wall of the combustion chamber may comprises a row of burner blocks of first particulate fuel burners and the second wall a row of burner blocks of second particulate fuel burners. According to this embodiment of the invention, in the first step, flames are generated by the burners mounted in the first wall and, in the second step, flames are generated by the burners mounted in the second wall of the combustion chamber.
Alternatively, some (i.e. a first portion) of the multiple pairs can have the burner block of the first particulate fuel burner mounted in the first wall and the second particulate fuel burner mounted in the second wall, while the remaining portion of the multitude of pairs has the burner block of the second particulate fuel burner mounted in the first wall and the first particulate fuel burner mounted in the second wall. In particular, the first wall may have a first row of burner blocks mounted therein and the second wall a second row of burner blocks mounted therein, so that said first and second rows are alternating rows of burner blocks of first particulate fuel burners and burner blocks of second particulate fuel burners, i.e. a block of a first burner followed by a block of a second burner followed by a block of a first burner, etc. According to this embodiment, flames are generated by some burners mounted in the first wall and by some burners mounted in the second wall during both the first and the second step.
The first and second walls are advantageously lateral walls of the combustion chamber, in particular when the furnace has a long and narrow combustion chamber as is usually the case with glass feeders or forehearths and reheat furnaces.
According to a first simple embodiment of the method according to the invention, the first and second steps have a predetermined duration. The duration of the first and second step is/are selected so as to provide, on the one hand, sufficient local overheating of the burner block so as to be beneficial for the subsequent step, while, on the other hand, preventing thermal damage to the burner block or other constituent burner parts.
For increased safety of operation, the burner blocks of the first and second burner of at least one pair may be equipped with a temperature detector. Said temperature detector may include a temperature sensor mounted inside the burner block or may detect the temperature of a surface of a burner block facing away from the combustion zone.
In that case, the duration of the first and second steps may be determined as follows:
According to an alternative embodiment:
Advantageously, the method according to the invention combines two or more of said approaches and the advantages thereof.
According to one such embodiment:
Alternatively:
According to a further advantageous mode of operation:
In general, the predetermined duration of the first and second step will be identical. Likewise, the first and second predetermined upper temperature limits will normally be the same. Similarly, the first and second predetermined lower temperature limits will be the same. However, different predetermined durations, different upper temperature limits and/or different lower temperature limits may be selected when the furnace configuration or the process parameters justify same, for example when different burners are used as first burners and as second burners.
Suitable particulate solid fuel burners, are notably known from WO200603296, WO2007063386, and from co-pending European patent application EP 09174622.2. Suitable particulate liquid fuel burners are known from EP 1750057 and WO03006879.
The method of the present invention is particularly advantageous for narrow furnaces and in particular for tunnel furnaces. The benefits of the invention are particularly apparent when the furnace is a glass melting furnace, a glass feeder or forehearth, a tunnel calcinations furnace or a reheat furnace.
The mechanisms and advantages of the methods according to the present invention are described in more detail hereafter, reference being made to
The present invention 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 vaporisation 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.
During the first step, the flames 10 generated by the first burner 100 of each pair is directed at the burner block 201 of the “inactive” second burner 200 of said pair so as to cause local overheating of said burner block 201.
In the present context, “local overheating” a burner block refers to the heating of the burner block 101, 201 to a temperature higher than the temperature of the furnace wall 150, 250 surrounding the burner block. The maximum temperature to which the burner block can thus be locally overheated is determined by the thermal resistance properties of the burner block and of other burner constituent parts, such as metallic injectors mounted in or connected to said burner block.
In the respect, a preferred material for the burner block is AZS.
Fused cost AZS blocks have an important thermal inertia and high thermal resistance.
In order to prevent thermal damage to metallic parts of the burner block, these metallic parts are advantageously restricted to the rear half of the burner block, i.e. the metallic parts do not extend beyond half the width of the burner block starting from the surface of the block facing away from the combustion zone.
A suitable choice of metal for (part of) the metallic parts can also help prevent thermal damage. Examples of such suitable metals are heat resistant alloys such as Inconel® 600 and Kantal®.
The burner block can typically be heated to a temperature (on the side of the block facing the combustion zone) of at least 1400° C., preferably at least 1500° C. The burner block can advantageously be heated to temperatures of up to 1700° C. (on the side of the block facing the combustion zone), in particular in the absence of metallic parts in the burner block on the side of the block facing the combustion zone.
In the present context, a burner is said to be “active” when it injects fuel and oxidant into the combustion zone and thereby generates a flame in the combustion zone and “inactive” when it does not generate a flame in the combustion zone.
During the first step, the burner block 201 of the second burner 200 acts as a heat accumulator.
As illustrated in
As the at least one passage 202 is situated in the burner block 201 of the second burner 200, the heat accumulated in said burner block 201 during the first step is released to progressively raise the temperature of the reactant or reactants flowing therethrough towards the combustion zone 1 (fuel and/or oxidant preheating). As a consequence, the reactant is injected into the combustion zone 1 at a higher temperature than would have been the case without local overheating of said block 201 in the preceding step. As a consequence, the injected fuel particles reach their devolatilization/evaporation temperature more rapidly, i.e. after a shorter travel distance in the combustion zone.
This in turn leads to the start of combustion of the volatiles/fuel vapours after a shorter residence time and shorter travel distance so that the overall flame length is likewise shortened. The flames generated by the method according to the invention can therefore safely be used for heating narrower furnaces.
During said second step, the flame 20 generated by the “active” second burner 200 of each pair is directed at the burner block 101 of the “inactive” first burner 100 of said pair, so as to cause local overheating of said burner block 101.
In this manner, analogous flame-shortening effects are obtained for the flame 10 generated by the first burner 100 of each pair in the subsequent first step in which the first burner 100 becomes “active” again (see
In that the local overheating by the burner flame generated by the “active” burner is limited to the burner block of the “inactive” burner of the pair, excessive temperature and therefore thermal damage to the furnaces wall surrounding the “inactive” burner is provided.
In that the reactant preheating takes place in the one or more passages of the burner block directly upstream of the combustion zone, premature devolatilization/evaporation and ignition of the fuel in the burner supply system is prevented.
In some cases, the flame shortening achieved in the above manner may not suffice to shorter the flame to less than the width of the furnace.
According to a specific embodiment of the present invention illustrated in
In accordance with the invention, the momentum of the deflecting jets 21 must however be small enough so as to enable local overheating of the burner block 201 of the “inactive” burner 200 by means of the flame 10 generated by the “active” burner 100. This embodiment therefore still allows efficient heating of the narrow chamber (total fuel combustion) without excessive fouling or thermal damage to the burner block 201 of the “inactive” burner 200 and surrounding wall 250.
The deflecting gas can, for example, be an oxidant. In that case, the amount of oxidant injected by the “active” burner 100 can be reduced accordingly, while still enabling total fuel combustion.
The deflecting gas can also be recycled flue gas. When hot flue gas is injected as deflecting gas through the burner block 201 of the “inactive” burner 100, the hot flue gas can contribute to the heating up of said burner block 201, in addition to the local overheating caused by the flame 10 of the “active burner” 100.
As mentioned above, the advantages of oxy-fuel burners over air-fuel burners are generally recognized in the art.
Oxy-fuel flames are generally shorter than air-fuel flames, due to the higher oxygen-concentration in the oxidant and the lower oxidant volumes injected. Oxy-fuel flames are therefore in theory particularly suited for heating narrow furnaces. In practice however, due to the generally higher temperature of oxy-fuel flames and higher radiant heat transfer, particular care has to be taken to prevent thermal damage to the walls 250 and to the burner block 201 of the “inactive” burners 200.
Consequently, the use of the embodiment of the present invention whereby the “inactive” burner 200 of the pair of burners injects one or more jets of gas 21 to deflect the flame 10 back into the combustion zone 1 is particularly useful in the case of oxy-fuel burners.
The timing of change-over from step 1 to step 2 and from step 2 to step 1 is determined in function of the required flame length and the thermal resistance of the burner block. This timing, i.e. the duration of step 1 and of step 2, can be predetermined or can be controlled by measurements conducted during the process.
According to a preferred embodiment, the duration of each step is predetermined. However, the burner block of at least one pair of burners, and preferably of each pair of burners is preferably equipped with a temperature sensor, and the method is controlled so that change-over takes place when the temperature of an “inactive” burner block reaches a critical level, even when the predetermined duration of the on-going step has not yet been reached.
The present invention is particularly suited for cross-fired furnaces comprising multiple pairs of particulate fuel burners.
According to one embodiment of the method of the present invention, the first burners 100 of the pairs are all positioned on one side of the combustion chamber and the second burners 200 of the pairs are all positioned on the opposite side of the combustion chamber so that at any one time, the “active” burners are all on one side of the combustion chamber and all “inactive” burners are on the opposite side of the combustion chamber.
According to an alternative embodiment of the method of the invention illustrated in
According to a preferred embodiment shown in
A calcination furnace of the tunnel type for the production of clinker is equipped with a dozen pairs of particulate coal oxy-burners. Each lateral wall of the furnace (taken in the direction of travel of the product to be calcined) presents an alternating row of first and second burners. The first burner of each pair is positioned directly opposite the second burner of said pair. The burner blocks are made of AZS. The metal reactant injectors are made of the heat resistant alloy Inconel® 600 and are positioned in the rear three quarters of the burner blocks.
Tests were conducted with particulate liquid fuel known as heavy fuel FO 2 and with particulate bituminous coal.
In a first test, only the first burner of each pair was operated at its nominal power to generate a flame in the combustion zone situated between the lateral walls. The flames so generated clearly impacted the burner blocks of the second burners causing rapid overheating thereof. The test was terminated before the burner blocks of the second burners and the surrounding chamber walls suffered thermal damage.
In a second test, the furnace was operated according to the embodiment of the invention whereby the “inactive” burners do not inject a deflecting gas. Change-over between steps took place when the burner block of the “inactive” burners reached a predetermined safe upper limit at which the “inactive” burner block does not suffer thermal damage.
At nominal burner power, the flames generated by the “active” burners were found to impinge on the burner blocks of the “inactive” burners. Significant deposition of partial combustion products (soot) on the “inactive” burners was observed and non-negligible overheating of the chamber wall in the vicinity of the blocks of the inactive burners was detected. No such problems were observed when the burners were operated below their nominal power.
In a third test, the furnace was operated according to the embodiment of the invention whereby the “inactive” burners inject hot recycled flue gas into the combustion chamber so as to deflect the tip of the flame generated by the corresponding “active” burner away from the burner block and back into the combustion zone.
The burners were operated at nominal burner power.
Change-over between steps took place when the burner block of the “inactive” burners reached the predetermined safe upper limit at which the “inactive” burner block does not suffer thermal damage.
Apparently complete fuel combustion was achieved. No substantial deposition of partial combustion products on the “inactive” burners was observed and overheating of the burner blocks of the “inactive” burners remained localized and did not cause a substantial temperature increase of the chamber wall in the vicinity of said blocks.
