Enviromix_split_phase_applying_flameless_combustion_principles_to_pyrolysis_furnaces

As the drive towards reduced NOx emissions accelerates, the need to promote synergy between all furnace components becomes paramount. One of the most exciting methods of reducing these emissions is through the application of the principles of flameless combustion. Through high rates of flue gas recirculation into feed combustion air and/or fuel, and high combustion reactant injection velocities, the traditional stable flame front of a burner may be removed and from this a low peak temperature, distributed combustion results. This significantly reduces the formation of thermal and prompt NOx to levels not attainable with conventional staged combustion methods. Achieving this, however, is far simpler on pilot furnaces and smaller single burner operations where the intricate fluid dynamics of the interaction are more easily influenced. The application in large scale, multiple burner, industrial furnaces such as those used in the pyrolysis of ethane and naphtha, is far more difficult. In spite of these challenges, steps are being taken to move traditionally accepted technology in this direction. A significant move has been made by Hamworthy Combustion Engineering Ltd (HCEL) utilising its Enviromix Split Phase® pyrolysis furnace burner (ESP®). While not a true flameless combustion burner the design of the ESP® integrates many of the key aspects of flameless combustion to provide a unique solution to the current requirements of pyrolysis furnace licensors and end users. The final move to flameless combustion, or indeed to an accepted pseudoflameless combustion scenario, will rely significantly on the furnace to burner synergy. The goal, therefore, of this investigation is to promote discussion between all parties, licensor, EPC and burner vendor around integration of the technology and the pooling of resources to find a mutually beneficial outcome.

E n v iro m ix S p lit P h ase® : A p p ly in g C o m b u s tio n P rin c ip le s to F la m e le s s P y ro ly s is F u rn a c e s S i m o n Kretzschmar H a m w o r t h y C o m b u s t i o n Engineering Ltd ( H C E L ) Fleets Corner, P o o l e Dorset, E n g l a n d H C E L P r ocess Sales D e p a r t m e n t 2 0 1 1 Illllllllllll HAMWORTHY C O M B U S T I O N NO TICE- TE R M S OF USE This document contains confidential and proprietary information and know how belonging to Hamworthy Combustion Engineering Ltd. W e grant you permission to retain the document on your files and to have access to the information and know-how contained herein upon condition that you will not permit the document or its contents to be available to persons outside your company and employment. W e may withdraw this licence at any time whereupon you will destroy all copies of this document in your possession or control. By accessing this document you accept this condition. Enviromix Split Phase® and ESP® are registered trademarks of Hamworthy Combustion Engineering Ltd. in the UK and other foreign countries around the world © 2011 Hamworthy Combustion Engineering Ltd all rights reserved Enviromix Split Phase®: Applying Flameless Combustion Principles to Pyrolysis Furnaces By Simon K retzschm ar H am w orthy C om bustion E ngineering L td Fleets C orner, Poole D orset, E ngland A bstra ct: As the drive towards reduced NOx emissions accelerates, the need to promote synergy between a ll furnace components becomes paramount. One o f the most exciting methods o f reducing these emissions is through the application o f the principles o f flameless combustion. Through high rates o f flue gas recirculation into feed combustion air and/or fuel, and high combustion reactant injection velocities, the traditional stable flame fron t o f a burner may be removed and from this a low peak temperature, distributed combustion results. This significantly reduces the form ation o f thermal and prom pt NOx to levels not attainable w ith conventional staged combustion methods. Achieving this, however, is far sim pler on p ilo t furnaces and smaller single burner operations where the intricate flu id dynamics o f the interaction are more easily influenced. The application in large scale, m ultiple burner, industrial furnaces such as those used in the pyrolysis o f ethane and naphtha, is far more d iffic u lt. In spite o f these challenges, steps are being taken to move traditionally accepted technology in this direction. A significant move has been made by Hamworthy Combustion Engineering Ltd (H C E L) u tilisin g its E nvirom ix S p lit Phase® pyrolysis furnace burner (ESP®). W hile not a true flameless combustion burner the design o f the ESP® integrates many o f the key aspects o f flameless combustion to provide a unique solution to the current requirements o f pyrolysis furnace licensors and end users. The fin a l move to flameless combustion, or indeed to an accepted pseudoflameless combustion scenario, w ill rely significantly on the furnace to burner synergy. The goal, therefore, o f this investigation is to promote discussion between a ll parties, licensor, EPC and burner vendor around integration o f the technology and the pooling o f resources to fin d a m utually beneficial outcome. K eyw ords: Flameless Combustion, Split Phase Burners, Pyrolysis Furnaces, N O x Reduction 1. Introduction In the w orld o f process furnace design, a fine line is usually trodden during energy efficiency optim isation between optimum operating conditions and the ever reducing N O x emission levels allowed from the burners. W hile the furnace designers' specific requirements remain, the burners are required to be: 1) 2) 3) 4) 5) 6) Easy to maintain. Provide the required heat flux. Operate on a high turndown range. Operate on a wide fuel range. Capable o f providing stable and safe operation. Able to meet local emissions legislation In many applications the traditional air and fuel staging techniques required fo r low N O x operation are no longer feasible as the alignment o f reducing N O x and high furnace temperatures create a situation where meeting the emissions levels are no longer possible. U ltra-low N O x burners requiring extreme staging and other techniques such as internal flue gas recirculation may s till meet the current emission requirements however they have their own disadvantages w ith regard to com plexity o f operation and instability at turndown. The lim itation fo r the current technology is being reached, w hile N O x emission lim its steadily sink as countries endeavour to adhere to the principles o f the ‘Kyoto Protocol' M ila n i et al (2001)1. W hat is needed is a fundamental mindset change fo r certain combustion applications. This could either be in the form o f de-NOx units attached to the flue stream or a complete redesign o f the burners themselves. W a lke r J. et al (2004)9 Significant research has been conducted in academia and in some industrial applications into new modes o f combustion. From in itia l FLO X® recuperative and regenerative burners utilised in furnaces in the m etallurgical industry in the early 90's, to their later incarnations, the development o f the technology has continued W unning (2003)2. However, the mainstream application to the petrochemical and refining process industry has been slow. This may be due to the low er operating temperatures o f many process heaters (approximately 800 oC) whereas this technology is more suited to higher temperature applications such as in the cracking and reform ing furnaces (> 1000 oC). The purpose, therefore, o f this paper is to illustrate the developments that have been achieved by Hamworthy Combustion Engineering Ltd (HCEL) as a burner manufacturer in w orking towards these alternative technologies and to the promote open discussion between a ll parties involved, that is needed to fu lly develop and integrate this technology into furnaces o f the process industry. 2. Background: N O x fo rm a tio n and tra d itio n a l reduction techniques In order to discuss the redesign o f the existing technology it is w orthw hile review ing the traditional NO x reduction techniques. The oxides o f nitrogen are formed from the NO produced in the flame. NO is further oxidised to NO 2 in the presence o f additional O2, either in the furnace, stack or atmosphere. The form ation o f NO in flames is generally accepted to fo llo w one o f three paths. Table(1): M ethods o f N O x F orm ation NO x F orm ation Type Thermal D escription Formed from atmospheric nitrogen and oxygen and is a function o f the combustion temperature. Formed from the nitrogenous compounds in the fuel and is a function o f the fuel composition Formed from atmospheric nitrogen and fuel and is a function o f stoichiometry. Fuel Prompt Fuel N O x is not a factor in pyrolysis furnaces and prompt N O x is generally a lim ited contributor. As such the main focus o f N O x reduction is on the most significant contributor: thermal NO x 3. Thermal N O x is influenced by the furnace and flame temperatures and is dependent on O 2 concentration. M ost methods o f emissions reduction in the latest generation burners o f today involve peak flame temperature reduction11. Flame temperature reduction has been achieved prim arily via air staging, fuel staging and flue gas recirculation (FGR). A ir and fuel staging methods (Figure (1.1) and Figure (1.2)) are based around introducing either the air or fuel in sub or super stoichiom etric quantities into different regions o f the flame. This creates a fuel or air (depending on the staging method) lean prim ary combustion zone. The remaining air or fuel is introduced into the secondary combustion zone. By distributing the combustion the volume o f the exothermic combustion reaction is increased reducing the peak flame temperature therefore reducing the thermal N O x11. F igure (1): T ra d itio n a l Staging M ethodology (1) - Staged A ir ; (2) - Staged Fuel ; (3) Staged Fuel w ith FG R (1 ) i * (2 ) (5) '% % i /'V't i ■ _ k1 *I ;* j »if * 'T' * Tf‘ i1i*r 'i ■ 1 i |L -i ‘ 4 i v , ■s 1 i* > 1 1 *» * * I m i L1 4 1 J i X' a 4 J Li *1 1.1 ft* 1‘ •M i )m i» i1 i' H* lU I * ‘t A further method is that o f flue gas recirculation. Here the furnace flue gases are induced into the flame. This method is usually combined w ith other methods o f NO x reduction, such as the staged fuel in Figure (1.3) The high percentage o f inerts chills the flame further by absorbing the heat o f reaction. The more extreme the staging the low er the N O x emissions, however the price fo r this is flame stability, turndown a b ility and the risk o f CO form ation. The stability o f the flame is therefore the lim itin g factor in how much cooling (and thus N O x reduction) can be achieved. W hile a ll this is considered, the stability o f the flame is an im portant aspect o f combustion, not only ensuring functionality but also fo r safety and efficiency ensuring that there is no release o f incomplete combusted fuels W unning J. (2000)4. Eventually a lim it is reached where no more N O x reduction can be achieved w ith a stable flame maintained. 3. Flameless C om bustion: D e fin itio n and principles Lean combustion through staging and internal flue gas recirculation principles has become common practice. The issue o f flame stability s till, however, remains and w ith this comes the issue o f a m inim um flame temperature. The ideal case would be to remove this lim itation. In order to overcome this considerable w ork has been conducted in the realm o f flameless combustion, an application firs t applied in high temperature air combustion furnaces (H IT A C ' s) as long ago as the early 90s, Awosope et al (2005)5. Also known as mild combustion and flameless oxidation M ila n i et al (2002) 6this method o f combustion allows the extreme staging o f combustion to previously unconsidered degrees. Flameless combustion is defined, according to W unning J. (2005)7, as " ... stable combustion without a flame and with defined recirculation of combustion products' A guile F et a l.8 summarise the method o f flameless combustion as being achieved by the injection o f the combustion air and the fuel at high velocities into the furnace chamber. In order to achieve the correct aerodynamics and high recirculation o f the combustion products, the design o f both the burner and the chamber are then considered critical. These high temperature products are required to m aintain combustion w ithout the need fo r a stabilised, defined flame. M ila n i et al (2001)1 describe this as volume combustion, as opposed to typical flame front combustion. Due to the homogeneous nature o f the temperature and composition in the chamber a defined flame boundary can no longer be seen and combustion is distributed throughout the chamber hence the term flameless combustion. Some o f the key features o f flameless combustion according to Awosope et al (2005)5 are: 1) H igh velocity injection o f the fuel and combustion air into a furnace designed to facilitate significant flue gas recirculation. 2) Any methods that are designed to facilitate flame form ation at the burner (e.g. b lu ff bodies and swirlers) are noticeably absent. 3) Significant quantities o f high temperature flue gas are recirculated either internally or externally. 4) Heat removal. The injection o f the reactants at high velocity and the removal o f b lu ff bodies ensure that combustion is distributed and the correct flo w regime is developed w ith in the furnace w ithout the establishment o f a flame front. The recirculation o f the high temperature flue gas ensures the stability o f the reaction. This is required considering that the consequent dilution o f the reactants can result in local O2 concentrations o f anywhere between 3 and 15% by volume A guile F et al8. The removal o f the heat is a requirement to ensure non-adiabatic combustion. The effects on N O x reduction are thus significant as a result o f two factors that come into play simultaneously: 1) The very high internal recirculation leads to dilution o f the combustion air w ith the flue gas. This reduces the local concentration o f O2 w hilst increasing the quantities o f inerts, thus reducing the adiabatic flame temperature o f the m ixture. W unning J. (2000)4 2) The distribution o f combustion again contributes to the avoidance o f peak temperatures. The result o f these two factors is that the thermal N O x form ation w hich is significantly influenced by the peak temperatures is therefore suppressed. 4. Incorporating Flameless Combustion Principles into a Burner Design: The ESP® 4.1 M eeting a m arket requirem ent The movement away from traditional staging technology came fo r Hamworthy Combustion during the development o f a burner fo r the pyrolysis furnace market. F igure (2): Furnace 14 (C ourtesy H C E L R & D D epartm ent) N O x reduction on these units is a matter fo r considerable debate. The high operating temperature (1200 deg C +) together w ith the high hydrogen (and therefore high adiabatic flame temperature) fuels that can be associated w ith the burners on these furnaces result in high N O x emissions. The compounding issue is that many o f these units are located in countries where the emissions legislations are stringent. The data in Table(2) below illustrates typical emissions from burners in these furnaces. Table (2): T yp ica l Technology Associated Em ission L im its B u rn e r Type Conventional Low N O x Technology T yp ica l N O x Em ission mg/Nm 3 * 430 135 Required <90 * Corrected to 3% O2, Fuel Composition 80% H2 / 20% CH4. W a lke r J. et al (2004)9 discuss in depth the d iffering methods that may be considered to reduce N O x emissions. These include: 1. 2. 3. U n it Turndown Fuel Switching Steam Injection However, a ll o f these methods impact the operation o f the furnace reducing productivity and constraining p ro fit margins. W a lke r J. et al (2004)9 in particular point out that switching from waste fuel gas to natural gas can double the costs o f ethylene production. The requirement therefore falls either to the use o f catalytic deNO x units or finding a burner capable o f emissions typical o f ultra-low N O x technology but which has stable flames, high turndown ratios, and low susceptibility to coking, W a lk e r J. et al (2004)9 , w hilst providing the highly specific heat flu x distribution required o f these furnaces C olannino J. (2008) 10 . The firs t phase o f this new burner development began in 2001, w ith the construction o f furnace 14 Figure(2) at the HCEL Advanced Technology Centre in Poole. This fu ll sized section o f a site furnace is rated to 6 M W and capable o f firin g m ultiple burners, w hilst measuring the effects o f the heat flu x from the burners via arranged water tubes. Thus the burners were developed under conditions that replicated what was to be expected on site. 4.2 The ESP®: Burner Design and Operation W ith the requirements on emissions, stability and operational ease that have already been outlined, the E nvirom ix S plit Phase® (ESP®) burner was developed. W hilst the fu ll design methodology o f the ESP® fa lls out o f the scope o f this paper, the most significant points to note were the complete absence o f a flame stabilising body and the u tilisation o f significant flue gas recirculation. However it can be seen that the design o f this burner is aided by its application: that o f a high temperature pyrolyis cracking furnace. The high operating temperatures typical o f these furnaces are ideal in supporting the stability o f distributed combustion. F igure (3): ESP B u rn e r Layout Isom etric C om bustion A ir T h roat V e n tu ri E x it Ports Fuel Gas / Flue Gas M ix in g Chambers Flue Gas V e n tu ri In le t P orts Com bustion A ir In le t P ilo t / S ta rt-u p Lance Gas In le t N orm al O peration Gas In le t F igure (4): ESP B u rn e r Layout Sectioned Isom etric P ilo t / S ta rt up Lance Gas Nozzle N orm al O peration Gas Nozzle The key to the burner' s design is the asymmetrical refractory block, w hich takes advantage o f the positioning o f these burners against the vertical w alls o f the furnace. This refractory block incorporates two adj acent venturi ports facing the furnace w alls and w hich are open to the furnace chamber. On the forw ard section o f the burner, facing the process tubes, is the main combustion a ir throat w hich is continuous to the combustion a ir w indbox o f the burner. A p ilo t / start up lance is located in the combustion air throat. The normal operation gas lances extend through the furnace flo o r and into the flue gas venturi ports. The burner lights up w ith the p ilo t/ start up lance firin g into the combustion air. This allows fo r stable operation u n til a safe auto-ignition temperature o f 750 oC is reached in the furnace. The p ilo t is monitored w ith an ionisation detection lance. The start-up flames in Figure (5) are typical o f conventional burner operation. Figure(5): ESP Burners firin g on start-u p (C ourtesy O M V ) Once a bulk autoignition temperature has been reached in the furnace the normal operating fuel gas lances are fired. The lances are fitted w ith nozzles that have a single large diameter d rillin g (see Figure(4)). This significantly reduces the possibility for coking in the nozzles and thus allows the firin g o f a wide range o f gas m ixtures, even those containing components susceptible to coking such as olefinic and aromatic compounds. Figure(6): ESP Burners firin g d u rin g norm al operation. (C ourtesy O M V ) The firin g o f these lances switches the burner over to non burner stabilised lean combustion and therefore ultra-low NO x operation. The position o f these lances in the venturi ports has been optimised to entrain the maximum amount o f flue gas w hile m aintaining stable operation. These recirculated gases m ix w ith the fuel gas to create the lean, low calorific value fuel/ flue gas m ixture. This m ixture is then contacted w ith the combustion air from the combustion air throat to form an extended zone o f combustion as can be see in Figure(7). The com bination o f the injection o f significant quantities o f inerts and high fuel gas velocities results in the highly extended reduced peak temperature combustion zone. The combustion in this zone is stabilised by the high temperature o f the furnace and the high temperature o f the fuel / flue gas m ixture. The flame front is d iffic u lt to determine in the distributed combustion as can be seen in Figure(6). A fte r extensive m atrix testing and scale up work, Ham worthy Combustion Engineering Ltd has produced a range o f burners that can operate on a range o f parameters outlined below in Table (3). The high content o f the H 2 in the fuel (80%) represents the typical desired fuel to be fired in these furnaces. Table(3): Sum m arised O perating Param eters B u rn e r Burner Liberation Range [M W ] Turndown [ratio] Excess A ir [% ] Combustion A ir RD L [mmH2O] Fuel Pressure [barg] Noise [dB (A )] NO x [mg/Nm 3] ESP® 0.5 - 4.0 8:1 10 - 15 5 - 40 1.5-3 < 80 < 90 * ** ^Corrected to 3% O2 dry, 1200 0C furnace temperature, 10% excess air ** Fuel composition 80% H2 / 20% CH4 All this is done w hile still m eeting the application requirem ents of: 1. 2. 3. 4. 5. Required Heat Flux U ltra-low N O x Emissions H igh turndown Stable Operation W ide range o f operation fuel gases V alidation o f the operation o f the burners both in site installed furnaces and in the HCEL test furnace 14 w ith CFD, u tilisin g ANSYS Fluent, has been an im portant aspect o f the development o f the ESP burner. The peak predicted temperature shown below in Figure(7) o f 1752 K ( 1479 0C) is considerably below the approximately 1960 0C adiabatic flame temperature o f the 80% H 2 / 20% CH 4 m ixture and shows the efficiency o f the temperature reduction techniques. F igure (7): ANSYS CFD S im ulation - Tem perature and V e locity P rofiles As can be clearly seen in the CFD validation, true flameless combustion has not yet been achieved on this burner, evident by the isolated hotspots Figure (7). Indeed it cannot be i f the heat flu x requirements o f this ethylene furnace application, together w ith the current aerodynamic design o f the chamber remain. However the main achievement o f this burner is the meeting o f stringent emissions legislation u tilisin g techniques associated w ith flameless combustion. 5. Current Installations and References The success o f the burner can be seen in the three installations Table(4) w hich are in operation. It must be noted that CFD m odelling has become integral to the design and optim isation o f these burners, and has been utilised on each installation. Table(4): ESP® Reference L is t C ontract No Year GUSG0488 2007 GUSG0408 2006 GUSG0297 2004 EPC C ou ntry SFPC USA Selas Linde Germany Selas Linde Germany End User C ountry Borouge Ruwais Abu Dhabi BP Geslenkirchen Germany O M V Schwechat Austria E quipm ent Service 420 ESP® Ethylene Cracker 180 ESP® Ethylene Cracker 48 ESP® Ethylene Cracker Equipm ent Service 420 ESP® Ethylene Cracker A fourth site is awaiting the installation o f the burners. Table(5): ESP® Reference L is t C ontract No Year GUSG0644 2009 EPC C ou ntry SFPC USA End User C ountry Borouge Ruwais Abu Dhabi 6. Conclusions The principles associated w ith flameless combustion may be applied readily in the design o f burners fo r the process pyrolysis industry, as can be seen w ith the ESP® burner. The ESP® thus represents a significant mindset change away from conventional staging technology. W hile true flameless combustion has not yet been achieved the pseudo nature o f this burner can be seen as a result o f applying these combustion principles to a conventional application. As can be seen in Table(2) and Table(3), the success o f this design is reflected in the typical emissions lim its that are expected from these burners compared w ith traditional staging and conventional technology. These emissions are typical o f ultra-low N O x operation in these furnaces. It must be noted however that the burner has a ll the benefits o f ultra-low N O x technology w ithout the lim itations o f instability, low turndown and complex operation. The high turndown satisfies operator control, w hilst the simple robust design allows fo r easy maintenance. The large nozzle d rillin gs prevent coke form ation and allow the burner to operate on wide ranges o f fuel gas. A significant cost saving both in fuel duties (due to not having to im port natural gas) and maintenance may therefore be realised. CFD validation is s till a significant aspect o f burner integration into pyrolysis furnaces, both in the positioning o f the burner, and also in m inor m odifications to the burner its e lf to achieve optim al performance. Burners no longer are sim ply catalogue additions to a furnace, rather they are bespoke designs that require complex burner / furnace synergy investigations to meet the stringent requirements o f today' s process industry. The key point now is what can be done to integrate burner / furnace design further. O nly by taking advantage o f the combustion aerodynamics o f the chamber can true flameless combustion be approached. This is the next step in furnace optim isation as only by further distributing combustion and reducing the NO x emissions can increased efficiency through alternative fuels and operating characteristics be achieved w ithout compromising N O x lim its. Further movement towards more distributed combustion w ill alter the sensitive heat flu x profiles provided by the burners. This w ill require reassessment o f the furnace design to ensure that productivity losses are not encountered. This leads back to the fin a l purpose o f this paper; namely to promote discussion w ith across the industry, from academia to burner vendors, EPCs and licensors about the best direction to take in the application o f this technology to the process industry. 7. Acknowledgements This paper is an expression o f the efforts o f many individuals at Hamworthy Combustion Engineering Ltd, however special credit must be given to the fo llo w in g fo r their invaluable advice and contribution. H am w orthy C om bustion Engineering L im ite d - Poole , Dorset U K : M r Richard W ithnall M r M ichael Booth M r Darren Gordon D r Anand Odedra M r B ill Gooderham M r M ark Young - Sales D irector (Process) Regional Sales Manager (Process) Senior Sales Manager (Process) CFD Engineer Group Development Manager (Process) Technical Manager (Process) John Z in k Com pany - Tulsa, O klahom a USA: M r Erw in Platvoet - D irector Process Burner Engineering 8. References 1) Milani A. and Saponaro A., Diluted Combustion Technologies, IFRF Combustion Journal Article Number 200101 ISSN 1562-479X, 2001 2) Wunning J., FLOX® - Flameless Combustion, Thermprocess Symposium, 2003 3) Mullinger P. and Jenkins B., Industrial and Process Furnace Principles, Design and Operation, Butterworth -Heinemann, 2008 4) Wunning J., Flameless Combustion in Thermal Process Technology, Second International Seminar on High Temperature Combustion - Stockholm(Sweden), 2000 5) Awosope IO. and Lockwood F.C, Prediction of Combustion and NOx Emission Characteristics of Flameless Oxidation Combustion, IFRF Combustion Journal Article Number 200501 ISSN 1562-479X, 2005 6) Milani A. and Wunning J., What is Flameless Combustion, IFRF Online Combustion Handbook ISSN 1607-9116, 2002, 7) Wunning J., Flameless Oxidation, 6th HiTACG Symposium, 2005 8) Aguile F. et al, Studies of Flameless Oxidation for Steel Industry Applications, www.kgu.or.kr/download.php, Access date 02/03/2011 9) Walker J. and Salbilla D., Analysis of NOx Reduction Techniques on an Ethylene Cracking Furnace NPRA, 2004 10) Colannino J., Ethylene Furnace Heat Flux Correlations , PTQ Q1, 2008 11) Baukal C., John Zink Combustion Handbook, CRC Press, 2001 12) Poe R. et al, Advanced Combustion Sytems for Cracking Furnaces, Fifteenth Ethylene Forum, 2006