CA1076165A - Heavy fuel oil combustion system - Google Patents

Abstract

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REMPLACEMENT
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Description

1076165 .. ..
This application is directed to an improved combustion system for heavy fuel oil specifically utili~ing viscosi.ty con-trol and to that system incorporating an improved A~omi~ing nozzle for the fuel oil. Parent application Seria~ ~'c~ 2?3,879 filed 6 June 1977 is directed to the improved atomi~ing fue.' oil no~le and to the nozzle incorporated in a general combustion system.
This invention pertains to liquid fuel combustion and in particular combustion of fuel:; having widely varying properties including so-called ~heavy" oil. The preferred system disclosed utili~es improved atomization through no~zle design and viscosity control to achieve successful combu~tion.
DESCRIPTION OF PRIOR ART
Historically, combusion of the so-called "heavy" oils has been extremely difficult due to a complex hydrocarbon structure and substantial variations in the properties and con-stituency of thc fuel. Conventional fuel oil is generally class- ;
ified by the API ~esignation t l to t6 with the 1 to 4 range provided somewhat variable but generally consistent combustion <~
properties. Oil designated as t5 or t 6 is classified AS re-sidual and therefore has a broad range of combustion properties.
Impurities of somewhat unknown value are also present in quant-ities which varv widely, and can include water. Recent efforts to conserve energy and dispose of by-products of industrial processes have led to the need for combustion of "waste" oil, ; which can include so-called cutting oil, exhausted automotive lubricaticn oil and other impurities. These waste oils exhibit many of the undesirable combustion characteristics of "heavy"
oil and therefore are considered equivalent to "heavy" oil in the ~
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. 10761~5 remainder of the disclosure. The variations which ?rovide the greatest barrier to efficient combustion include very hign vis-cosity (greater than 5000 SSU at 20 Centigrade), hish vapori-zation temperatures, non-uniform distiilation rates, and widely varying trace elements present as impurities which substantially influence combustion processes.
Examples of prior attempts to obtain satisfactory com-bustion of heavy oil are taught in U.5. Patents 3,185,202, and 3,301,305, assigned to the assignee of this application. These systems essentially utilizc the concept of increased residence time in the combustion chamber to overcome varying fuel properties and to insure complete combustion without dep~sition of carbon on the combustion chamber services. Whiie these approaches have been moderately successful, they ha~e includ~d various compli-cated devices in order to produce highly turbulent combustion gas and vapor flow patterns, and generally speaking do not pro-vide combustion in the type of relatively compact chamber dis-closed in this invention.
Other approaches to combustion of heavy oil utili~ing attempts to improve atomization through noz21e design include U.S. Patents 1,428,8~6, 3,770,209, and 3,8~0,183.
In general, these approaches have resulted in highly complicated nozzle geometries involving many internal passages and intricate air-oil intersections. These structures are sen- j sitive to variations in the oil characteristics and constituents indicated above resulting in combustion systems of relatively low reliability. Frequent cleaning of nozzles is required, and attemp~s to operate over long periods without substantial main-tenance have not generally been successful.
'" lo76l6s Prior art nozzles discussed above generally utilize atomizing fluids which generate fuel particles having asym-metrical velccity and acceleration compo.lents. Tnese particles tend to imringe on internal passages and agglomerate or re-combine, requiring additional atomizing air to re-shear or e-atomize the agglomerated fuel. The re-atomization necessity proeides non-uniform fuel/air mixtur~ nd results in poor or inefficient combustion.
In contrast, the invention disclosed in this application accomplishes proper atomization and good combustion as measured ' by accepted state of the art indicators s~ as absence o~ deposited j carbon and low bacharach smoke scale in t-~e cGmbustion gases using a relatively simple nozzle, which is easy to clean and is inherently -insensitivs to fuel property varlations.
SUMMARY OF THE INVENTION
Successful combustion of high viscosity or heavy oils is accomplished by the system of this invention in a preferred em~odiment through the use of a unique nozzle design in conjunction with self adjusting viscosity controi of the fusl. In particular, the nozzle utilizes a circulating oil flow contained in a cavity adjacent to the atomi-zing fluid source and exit orifices. Fuel exiting from the cavity is ~sheared" by the atomizing fluid passing through the cavity with re-combination of the fuel prevented by atomizing air passages which are coaxial with nozzle exit passages, containing critically sized exit and expansion orifices, Preheated fuel is withdrawn from a remote storage tank after which entrained air and/or vapors o; gases are separated and i ~', ., - :
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'~ 10761~5 additional -:utomatically controlled heat is supplied, in order to provide a relatively constant viscosity fuel J;o the burner described above. Combustion proceeds in a relatively small refractory cnamber ~ which utilizes recirculation zones to stabilize the combustion pro-cess prior to completed combustion gas exiting through the combustion chamber choke.
This system allows combustion of heavy or residual fueis in compact combustion chambers without deposition of carbon on the chamber inte~ior or significant reduction in combustion chamber life.
The nozzle design employed Also provides for expulsion of impurities contained in the oil and allows them to be ejected into the -~m~
tion system where they can be utilized and in many cases become a ~-part of the combustion process.
However, in one broad aspect the invention provides in a device for atomizing heavy oil; an inner member having a plurality of passRoes for atomizing gas, each passag having first and second ends; an outer member having a plurality of passages, ach having first and second ends; a sharp edged orifice adjacent to the -,econd end of each of the outer member passages; an exit orifice abutting the sharp edged orifice; an expansion section adjacent the first end cf the outer member; means mounting said inner and outer members and the passages in spaced relationship defining an oil circulation cav-ity therebetween; the cavity communicating with the second end of the outer passage and first end of the inner passage; means supplying pressurized non-atomized oil to the circulating cavity; means supply- -ing pressurized atomizing fluid to the inner mstnber, ac a pressure less than that of the oil, wherein oil flowing in the circulating :a~ity is ~to=ized by flgld flowing in the inne~ passages, at the ::
'-- 1076165 sharp edged orifice, thereby e~pelling fluid entrained atomized oil from the exit orifice and throu~'a the expansion section.
The invention also provides in another aspect a combustion system for liquid fuel having in co~bination a combustion air source, an atornizing fluid source, a combustion chamber, a liquid fuel source and ; burner assembly with combustion air inlets ignition means and the atomizing device ger.erally as already described.
In yet another aspect the invencion provides a combustion system for liquid fuel having; a combustion chamber, a com~ustlon air source, means supplying pressurized atonizing fluid, a burner having primary and secondary air sources, ignition means, an atomizing fuel nozzle; and a liquid fuel viscosity control system comprising; a first fuel pump; a first fuel heater sùpplying preheated fuel from a reservoir; air separator meanli for deaerating 'he preheated fuel; and an orifice, second fuel pumping means and pressure control means causing flow of deareated, pressurized fuel through the orifice;
n~2ns continuously measuring the pressure arop caused by the fuel flowing through the orifice, second fuel heating means responsive to the pressure drop measurement so as to maintain maximum and minimum values. Preferably in this system the atomizing fuel nozzle is as already generally described.
DESCRIPTION OF THE DRAWIN~5 Figure 1 - Combustion system includins nozzle, burner assembly com-bustion chamber, and viscosity control system.
Figure 2 - ~urner assembly including nozzle pilot flame assembly and air induction means.
Figure 3 - Detail of nozzle design and salient component parts prior to 2ssembly of the invention.
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1()76165 . ~. ,~, Figure 4 - Additional sectional view of salient parts of the novel burner nozzle prior to assembly.
Figure 5 - Partial section of the nozzle in substantially increased detail showing salient feature_ of the invention, such as the exit orifice, the sharp edged orifice, and the oil circulating cavity.
Figure 6 - Schematic of fuel oil viscosity control.
In connection with a preferred embodiment it will be under-stood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alter-natives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the pendent claims.
The burner assembly preferred c~bodiment as shown in Figures 1 and 2 consists of a burner assembly 65 combustion chamber 86, and combustion air box and blower, 90. With reference to Figure 2 the burner assembly contains an atomizing nozzle 35 in-ternally mounted and coaxial with burner skirt 45, contained near the apex of the stabilizing cone 40, aIso mounted coaxial to the burner nozzle axis. Combustion air for the burner enters through primary air inlet 36 and passage 37 in the burner skirt. Second-ary air enters the peripheral passage 80 between the skirt 45 and combustion chamber refractory 85.
The burner nozzle consists of the nozzle holder 67 (Ref. Fig. 5) containing the atomizing ::luid inlet and nozzle inner-member 69 having a plurality of atomizing fluid orifices 30. A -~
nozzle outermember or shell 25 is mounted so as to encircle the nozzle holder and ~ontains a plurality of exit orifices 38, ex-pansion orifice 41 and sharp edged orifice 7 held in alignment !
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with the atomizing air inlet orifice 30 bv the nozzle retainer 125. The nozzle outer member 25 is supported at a shoulder 126 ~ on the nozzle holder 67 so as to maintain a circulating cavity 6 between the nozzle inner member and shell.
In operation, reference Figs 2 and 5, liquid fuel under pressure enters the oil inlet passing through orifice 8 of the nozzle inner member 69. Fuel is supplied through the inlet conduit Y66 (Fig 2) which terminates in the nozzle holder 67.
The fluid enters through the inlet 15 under pressure somewhat less than that of the fuel entering through fuel liquid passage 8.
The cavity 20 formed by nozzle inner member 69 and shell 25 pro-vldes passage for circulating oil flow within the cavity. The cavity design provides a radial "minimum gap" 40 which is cir- , cumferential and adjacent to both the atomizing air orifice 30 exit, and the sharp edged orifice 7 of the nozzle exit orifice 38.
This gap aligns certain solids which pass through the fuel filters and permits their expulsion by the atomizing air flowing through The alignment of these particles is crucial since the minimum orifice 40 and the flow passaae or cavity 6 cooperate to allow ':hese particles to move .into the exit orifice with an attitude which allows their expulsion and subsequent combustion.
Returning now to tha oil under pressure circulating in the cavity 6, cavity geometry and the pressure differentials between 6 and the atomizing air inlet15 is such that oil flows in a path which is radial to the sharp edged orifice 7, where it is sheared by the atomizing fluid flow from the atomizing fluid ori-fice 30 forming particles of oil which move through the exit o i-fice 38. This action, produced by the radial oil flow at the sharp edged orifice 7 and the atomizing fluid flow through the ' ~ . i~76165 .:
orifice 30 results in generating a stream of fluid entra;ned fuel particles which pass rapidly through the exit orifice 38 without agglomeration, and into the expansion orifice 41 where they under-go additional expansion and are then further entrained by the primary air flowing past the nozzle. Radial flow is essential in the formation of fuel particles which are repelled by fuel flowing from the counterparts location on the opposite side of the critical gap. This essentially neutralizes radial velocity components, resulting in fuel particles which flow essentially in a direction parallel to the exit orifice axis, thereby mini-mizing wall attachment. The length of the exit orifice 38 has also been found co be significant relative to the amount of agglomeration of the particles sheared by sharp edged orifice 7 and in the amount of recombination of the sheared oil particles which might occur in the time of their passage between the sharp edged orifice the exit orifice 35 and resulting flame shape.
The minimum amcunt of agglomeration accompanying the structure disclosed and claimed here has resulted in a functional and re-liable burner usable in small combustion chambers.
Combustion of the atomized fuel now entrained by the primary air adjacent to the nozzle shell 25 proceeds as a spinning - ~
action is imparted by the secondary air passing through the per- ~¦
ipheral passage 80 and containing spinning vanes. Ignition and combustion occurs in the region just outside the stabilizing cone !
40 and is accomplished by the ignitor and pilot assembly 50.
Although a gaseous pilot which is electrically ignited is disclosed it will be realized by tnose skilled in the art that any other means of ignition such as direct electric arc or other pilot systems can be utilized.
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The now ignited mixture of primary, secondary, and atomized fuel droplets proceed into the com~ustion chamber 86 where the circulation zones 152 and 153, 154 and 151 are esta-blished to stabilize the complex combustion phenomen 3 . Combustion gases formed by the process then proceed through th~ circular combustion chamber choke or exit 155 where they pro~eed to scrub the heat exchange surfaces of ar.y particular or des red config-uration (not shown).
Control of the fuel viscosity as supplied to the fuel nozzle 35 is accomplished through the system depicted in Figs 1 and 6. In particular reference to Fig 6, the system disclosed provides for proper oil flow through the r.ozzle for a wide range of oil characteristics usually encountered. In operation, oil stored in a remote tank is preheated and pumped to the separator 100 by fuel supply pump 101. The separator maintains a reservoir of deaerated oil and its reservoir lOS, and also provides for re-turning excess oil and entrained gases and/or vapors to the fuel storage tank.
Preheated deaerated oil is now supplied to che fuel pump 104 whose output is monitored by a by-pass type fuel pressura relief valve 102, whereby excessive fuel which causes the pressure ' to exceed a preset value is returned to the reservoir 105.
Preheated and deaerated oil now operating at a pressure controlled by the combir.ation of fuel pressure relief valve 102 is now pumped into the optionll fuel steam heater 106. The func-tion of the heater 106 and 108 are identical and both are only I disclosed for completeness. The following description will in-i volve a system where the elcctric fuel heater has baen selected and provides ~he major source of viscosity control. The fuel oil ' - 10-..
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is r,umped through the electric heater 108 and continues on through fixed orifice 112. A differential pressure switch llO is con-nected to monitor the fuel pressure drop across the orifice 112 and aiso controls the application of heat to the fuel heater 108, in a manner which continues to apply heat until the pressure drop is less ~han a certain preset value. The pressure of the heated fuel oil is further monitored by pre~sure regulating valve 114, prior to passing through the filter 116. The l:OW correct vis-cosity and filtered fuel is pumped through the fuel metering valve 120 whose throughput (volume flow) is controlled by the demand for heat on the overall combustion system and therefore forms a capacity control for the burner. The pressure of fuel exiting the metering valve 120 is monitored by differential pressure valve 122 which also monitors the pressure of t..e incoming atomizina fluid. The funotion of differential regulator 122 is to maintain a proper pressure differential between the atomizing fluid ar.d the fuel inlet to the nozzle 35. As discussed above, it is desirable to maint3in a fuel preqsure slightly in excess of that of the atomizing fluid in order to irsure the radial flow of fuel through -the sharp edged orifice 7 and exit orifice 30 of the nozzle. Gther ~ ;
pressure t~mperature and flow control components, namely, the low fuel temperat~re switch 121, dial thermometer ll9, bypass sole-r.oid valve ll~l and the burner safety valve assembly 123 and check valve 126 do not form part of this invention and are only included as part of the disclos~re of a complete combustion system.
The system described above comprising the burner a~sembly, combustion chamber and fuel viscosity control provide reliable com-bustion of heavy fuel in small combustion over a wide variety of fuel characteristics. In practice it has been found that the I

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combustion obtained with this combustion require minimal main-tenance and operates with good efficiency over a ratio of burner demand in excess of 6 to 1. Deposits of carbon or tho refractor~
of the combustion chamber have been essentially eliminated and oper-a~ion OL the nozzle has been made substantially more reliable than ailable units through the ability of the burner no~zle to pass relatively large amounts of unfilterable solids normally found in fuels of this type. This has been accomplished without restor-ing to combustion assists such as ultra-sonic atomi~ation or water injection and provides a simple and economic way to efficiently utili~e the large potential of fuel energy available in the so-called heavy or residu;l oils, and waste oil. Combustion of - , lighter distillates is of course easily accomplished since many of the above mentioned difficulties do not exist.
Thus, it is apparent that there has been pro~ided in accordance with the invention a novel combustion system that fully satisfies the objects, aims and ac~ antages set forth above. While the invention has been described in conjunction with specific ' embodiments it is evident that many alternatives, modifications ! 20 and variations will he apparent to those skilled in tr.e art in light of the foregoing description. Accordingly, it i3 intended to embrace all such alternatives, modifications, and variations ~ as included in the s~irit and broad scope of the following claims.
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Claims (5)

THE EMDODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a combustion system for liquid fuel having;
a combustion chamber, a combustion air source, means supplying pressurized atomizing fluid, a burner having primary and secondary air sources, ignition means, an atomizing fuel nozzle;
and a liquid fuel viscosity control system comprising;
a first fuel pump;
a first fuel heater supplying preheated fuel from a reservoir;
air separator means for deaerating said preheated fuel;
and an orifice;
second fuel pumping means and pressure control means causing flow of deaereated, pressurized fuel through said orifice;
means continuously measuring the pressure drop caused by said fuel flowing through said orifice, second fuel heating means responsive to said pressure drop measurement so as to maintain maximum and minimum value.

2. The combustion system of claim 1 wherein the atomi-zing fuel nozzle comprises;
an inner member having a plurality of inner passages with first and second ends, an outer member having a plurality of outer passages having first and seconds ends, each outer passage containing a sharp edged orifice adjacent said second end, and an exit orifice abutting said sharp edged orifice, and an expansion section adjacent to said outer member first end; means mounting said inner and outer members and inner and outer passages in spaced relationship de-fining a liquid fuel circulation cavity therebetween, said cavity communicating with the second end of the outer passage and the first end of the inner passage, a minimum gap defined by said cavity adjacent to the second end of said first passage and the first end of said second passage, means supplying pressurized fuel to said circulating cavity, means supplying pressurized atomizing fluid to said inner member at a pressure less than that of the fuel, wherein liquid flowing in said circulating cavity is atomized by fluid flowing in said inner passage, at said sharp edged orifice, causing air entrained particles of liquid to be expelled from said exit orifice and through said expansion section of said outer passage.

3. The combustion system of claim 2 wherein said mininum gap acts to align fuel impurities causing them to pass through said outer passage and be expelled from said expansion orifice.

4. The nozzle of claim 2 wherein the liquid flow in said cavity is essentially radial to said passages.

5. The combustion system of claim 1 wherein the com-bustion chamber comprises;
a cylinder terminated by first and second truncated conoidal ends, said cylinder and conoid internally intersecting in an obtuse angle, and said first truncated conoidal end defining a combustion gas choke, and said second truncated conoidal end defining an aperture, means mounting a combustor in said aperture said combustor supplying atomized fuel, primary air, secondary air and ignition, wherein said choke combustor and cylinder conoid intersection cooperate to establish recirculation zones of said fuel and air in the vicinity of said intersections to enhance combustion within the chamber.