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Plant furnaces employ radiant and convective heat recovery from the flue gases of fired fuels to maximize maximize fuel efficiency. efficiency. The temperature level of flue gases at the furnace stack has decreased dramatically by design as energy costs have gradually risen. Plants of several decades ago had stack flue gases operating at or above !! degrees ". #odern and upgraded plants$ in today%s energy environment$ have furnace stack flue gases operating at or belo& '!! degrees ". "urnace stack temperature increases due to gradual build(up of fouling over time$ as &ell as due to increased thermal load from higher plant rates than original design. )t is desirable to keep furnace stack flue gases as cool as practical from periodic coil cleaning and maintenance and occasionally by replacement of overloaded or &orn out convection coils &ith improved designs. "urnace convection coils reduced performance and fouling occurs for a variety variety of reasons. The most serious fouling occurs on the outside of convection coils. *o&ever$ sometimes fouling does occur on the inside tube surface$ such as &hen thermal cracking of the process stream in a coil occurs$ producing solid product$ such as carbon. The outside surface of convection coils may also become fouled from carbon due incomplete combustion and also from refractory dust resulting from flame erosion of burner blocks and from furnace casing brick&ork and insulation. The gradual build(up of fouling materials deteriorates convection coil heat transfer$ causing +leakage+ of potential heat recovered as hotter flue gas flo&ing to do&nstream convection coils. This can result in overheating of some coils not rated for the operating temperature$ and certainly increased heat losses in furnace stack gases.
,oil designs may be bare tube surface$ or they may be enhanced &ith extended finned surface$ or a combination of the t&o. -enerally those coils in hotter flue gas service above 0!!(6!! 1eg "2 are constructed of bare tubes. 3hen a large duty has to be accomplished$ some finned tubes are used in the convection coil design$ sometimes in combination &ith bare tubes in the hottest flue gases. "ouling of convection coils is such a gradual process that it is not really noticed over many years of service. ,ertain convection coils are more dramatically impacted in terms of lost heat transfer than others. 4t times$ only a very thin coating of refractory dust is re5uired to deteriorate the performance of bare surfaces in high temperature flue gases. )n such high temperature flue gases$ the buildup of only !.!! to !.!! inch !.!!0(!.!0 cm2 of fouling thickness on the outside of bare tubes can significantly reduce heat transfer. "inned convection coil heat transfer is also dramatically influenced from refractory and airborne dust material build(up and there are more crevices for the dust to settle in.
Plant furnaces employ radiant and convective heat recovery from the flue gases of fired fuels to maximize fuel efficiency. The temperature level of flue gases at the furnace stack has decreased dramatically by design as energy costs have gradually risen. Plants of several decades ago had stack flue gases operating at or above !! degrees ". #odern and upgraded plants$ in today%s energy environment$ have furnace stack flue gases operating at or belo& '!! degrees ". "urnace stack temperature increases due to gradual build(up of fouling over time$ as &ell as due to increased thermal load from higher plant rates than original design. )t is desirable to keep furnace stack flue gases as cool as practical from periodic coil cleaning and maintenance and occasionally by replacement of overloaded or &orn out convection coils &ith improved designs.
"urnace convection coils reduced performance and fouling occurs for a variety of reasons. The most serious fouling occurs on the outside of convection coils. *o&ever$ sometimes fouling does occur on the inside tube surface$ such as &hen thermal cracking of the process stream in a coil occurs$ producing solid product$ such as carbon. The outside surface of convection coils may also become fouled from carbon due incomplete combustion and also from refractory dust resulting from flame erosion of burner blocks and from furnace casing brick&ork and insulation. The gradual build(up of fouling materials deteriorates convection coil heat transfer$ causing +leakage+ of potential heat recovered as hotter flue gas flo&ing to do&nstream convection coils. This can result in overheating of some coils not rated for the operating temperature$ and certainly increased heat losses in furnace stack gases. ,oil designs may be bare tube surface$ or they may be enhanced &ith extended finned surface$ or a combination of the t&o. -enerally those coils in hotter flue gas service above 0!!(6!! 1eg "2 are constructed of bare tubes. 3hen a large duty has to be accomplished$ some finned tubes are used in the convection coil design$ sometimes in combination &ith bare tubes in the hottest flue gases. "ouling of convection coils is such a gradual process that it is not really noticed over many years of service. ,ertain convection coils are more dramatically impacted in terms of lost heat transfer than others. 4t times$ only a very thin coating of refractory dust is re5uired to deteriorate the performance of bare surfaces in high temperature flue gases. )n such high temperature flue gases$ the buildup of only !.!! to !.!! inch !.!!0(!.!0 cm2 of fouling thickness on the outside of bare tubes can significantly reduce heat transfer. "inned convection coil heat transfer is also dramatically influenced from refractory and airborne dust material build(up and there are more crevices for the dust to settle in.
"ouling$ ho&ever is causing a serious penalty in the furnace performance and efficiency in this example.
"igure on Page 2 sho&s the improved plant performance from successful cleaning of all of the convection section coils and reduction of their original base case operating fouling factors by 0!. 8xcess oxygen &as maintained the same as the base case. The furnace actually operates at fairly efficient levels of excess oxygen from the ,old ,onvection 9ection at .7$ 1ry #olal2$ ho&ever the combustion air preheater being of the rotary regenerative type has a fairly high leakage$ due to the large pressure differential across the seals bet&een the "1/)1 streams$ as a result of the high plant rate. !. of the total "1 "an air flo&2
4s a result of the total cleaning of the convection section to 0! reduction in original fouling factors$ the furnace total fired fuel has decreased by .07 belo& the base case$ e5ual to 6. #illion ;tu/*r$ **< savings$ or !.7 #illion ;tu/9T =*' **<. The furnace stack temperture has been reduced from '7! 1eg " to ' 1eg " as a result of the cleaning and the improved heat transfer and efficiency of the convection section coils. The furnace efficiency has improved from >>.7! to !.' ?*<2.
;ased on @0.!!/## ;tu **< energy and '0! days/year operation$ the fuel savings &ould e5ual @6>!$!!! per year. Thus$ improving convection section performance from cleaning and reduction of fouling offers substantial energy savings e5ual to the benefits of fairly large ne& capital proAects.
The Heat-r-Rate-r and simulation soft&are take into account the changing performance of the "eed -as preheat and *igh Temperature 9team preheat coils and the resulting impact on the inlet temperature to the #ixed "eed -as coil$ after the blending of "eed -as and #edium Pressure 9team$ let do&n from the *igh Pressure 9team header. Thus$ the #ixed "eed ,oil inlet temperatures change in the convection coil cleaning scenarios. The results of the as cleaned convection coil rating indicate a substantial energy savings for cleaning$ but the as fouled operating rating gives another uni5ue perspective. ,hem(8ngineering 9ervices has devised a systematic approach for selection of the convection coils that &ould provide the greatest energy savings benefit from cleaning. Befer to Table $ &hich summarizes the method and the optimal selection of coils that &ould benefit by providing the greatest energy savings from cleaning. )n Table $ the original existing e5uipment rated fouling factors are multiplied by the rated coil duties to define a +"ouling )mpact "actor+. The "ouling )mpact "actors are then summed and normalized to provide a +Percent Potential 8nergy 9avings+$ upon cleaning.
Table
,oil "ouling 1uty Percent Potentlal 9elected for 9avings
"actor ,leaning
## ;tu/*r
"ouling )mpact "actor
8nergy
#ixed "eed '.0
!.!'60
0.>0
!.0
'.0
*T Proc 4ir
!.!0
0.
!.>'>
.>
*T 9tm 9pht !.!0!> '6.
.67
.'0
'6.
?T 9tm 9pht !.!!>7> 7.
70.7!
!.660'
7.
"eed Prht
!.!!770
.!
!.'>
6.!
;"3 Prht .>
!.!!>6>
77.07
!.7600
.>
"uel Prht
!.!!77
>.6!
!.!!
.
?T Proc 4ir
!.!!'!6
.6
Total >6.7
!.!0' '.>07'
. !!.!
Those coils that indicate the greatest Percent Potential 8nergy 9avings should be cleaned. The remainder of the coils are of lesser significance and the cost of cleaning does not result in as great a benefit. Thus$ it is apparent that the cleaning of the #ixed "eed$ *igh Temperature 9team 9uperheat$ ?o& Temperature 9team 9uperheat and ;oiler "eed&ater Preheat coils for this plant example should achieve about >7 of the impact of cleaning all of the furnace coils. "igure ' sho&s the results from *eat(r(Bate(r and the simulation soft&are$ follo&ing cleaning of the four coils recommended for cleaning$ based on percent potential energy savings.
8conomic optimization can be accomplished by cleaning Aust those coils that result in the most energy loss and not expending cleaning efforts on the remainder of the convection coils$ thereby controlling maintenance costs. ,onvection coil rating studies are valuable tools for the plant understand the extent of convection coil fouling$ the benefits of coil cleaning and for designating those critical coils that can really improve plant performance and eliminate excessive energy use. Table ' sho&s a summary of the energy saving benefits from cleaning the entire convection section of the 4mmonia plant$
compared &ith cleaning selected coils that provide maximum potential energy savings. Table3 1790 TPD Ammonia Plant Furnace Convection Section Coil Cleaning
,ase
;ase
,lean 8ntire
,lean #ixed(
,onvection "eed$ *T/?T 9tm 9ection ( 0! and ;"3 ,oils
Beduced 0! Beduced
"ouling "urnace "ired "uels
"ouling
=at -as "uel$ 9,"* '> =- ;tu/9," ?*>!0 ,ryo "uel ;tu/9," ?*>.> ?o& Press Purge$ 9,"* 0! ?P Purge ;tu/9," ?*
'7>7 '. / !'. >>!0 0.0 / >>.> 0! '0.6 / !.>
"ired "uels *eat Belease$ ?*< ## ;tu/*r =atural -as >7.> >6.> ,ryo "uel
.>> .>>
7 '. / !'. >>!0 0.0 / >>.> 0! '0.6 / !.>
>6!.'>
.>>
?P Purge 6.70 Total '.7 "urnace 8ff$ $ ?*< >. "ired "uel Beduction$ (.
6.70
6.70
6.6
.!
>>.7!
!.'
;ase
(.07
8nergy 9avings$ based on @0.!!/## ;tu **<$ '0! 1/Cr ## ;tu/*r$ ?*< '. ## ;tu/*r$ **< .07 ## ;tu/Ton =*'$ **< !.0 4nnual 9avings$ @/Cear @6$!!
;ase
.6!
;ase
6.
;ase
!.7
;ase
@6>!$!!!
4s indicated from the rigorous thermodynamic rating results$ the energy savings for 0! reduction in fouling factors for cleaning of the designated coils is !$ &hich is very close$ but slightly greater than the >7 proAected savings from the "ouling )mpact "actor cleaning benefit analysis. 3hen partial or complete convection section coil cleaning is completed$ flue gases throughout the convection section &ill be at lo&er temperature levels$ &hile achieving overall heat transfer re5uirements for all of the coils in the convection section. Those coils that &ere not cleaned after cleaning critically fouled coils &ill under(perform compared &ith ho& they previously performed$ because of the lo&er temperature driving force. This can be seen in "igure '$ &here the Process 4ir Preheat$ "eed Preheat and "uel Preheat coils &ere not selected for cleaning. -enerally$ this does not cause any serious e5uipment limitations or problems$ but it does point out the desirability of
careful evaluations and coil cleaning decisions. 9ince the flue gases run cooler after cleaning$ previous overheating of certain coils is not a significant issue. 3hat can be of great importance in Bating studies of convection coils is the careful determination of the extremely fouled coils &hich are leading to overheating conditions for do&nstream coils. The specific elimination of this local fouling can improve the life and reliability of the do&nstream overheated coils.
Dpgrade of Dnderperforming ,onvection ,oils
)n the example sho&n after cleaning in "igure '$ by replacing the #ixed "eed ,oil and *igh Temperature Process 4ir Preheat ,oil$ additional fuel savings can be achieved$ &hile improving process performance. Befer to "igure . The #ixed "eed coil has been upgraded &ith a ne& design$ increasing the surface area from >6 "t to !' "t ;are ,oils2$ &hile reducing #ixed "eed pressure drop from 0 Psi to ' Psi. The *igh Temperature Process 4ir Preheat coil has been upgraded &ith a ne& design$ increasing surface area from ! "t to 0 "t ;are ,oils2$ &hile reducing Process 4ir pressure drop from .' Psi to .' Psi. The improved thermal performance of the #ixed "eed ,oil lo&ers the Badiant Beforming duty$ &hile the improved thermal performance of the ne& *igh Temperature Process 4ir ,oil increases the heat recovery from the 3aste *eat ;oilers after 9econdary Beforming$ thereby lo&ering the 4uxiliary ;oiler duty$ thus saving natural gas fuel. The fuel savings of the combined coil retrofit is 7. #illion ;tu/*r$ **< or !.!6 #illion ;tu **
The example illustrated in "igure sho&s dramatically ho& upgrade of older underperforming ,onvection 9ection coils can can have attractive benefits$ contributing significant savings in energy for the older plants.
