Combustion Application Experience with Highly Vitiated Air Burners

Paper from the AFRC 2013 conference titled Combustion Application Experience with Highly Vitiated Air Burners by David Schalles.

For those experienced in the art of industrial combustion, the concept of Flameless Combustion has forced us to revise our thinking about burners and furnaces. The primary benefits of minimized emissions and improved temperature uniformity are highly desirable. However, the combustion engineer must place a greater emphasis on the interactions of fluid flow, heating chamber geometry and product/load placement. Various techniques for achieving ‘highly vitiated' comburent streams which lead toward the ‘Flameless Combustion' operating regime have been developed. This paper will be limited to the general method of in-furnace self-induced recirculation of combustion products into the oxidizer and fuel streams. Several case-study industrial metals heating applications will be discussed to illustrate the importance of understanding and addressing the burner/furnace interactions.

1 "Combustion application experience with highly vitiated air burners" By David Schalles, Bloom Engineering Company, Inc. Presented at AFRC 2013 INDUSTRIAL COMBUSTION SYMPOSIUM "Safe and Responsible Development in the 21st Century" Sheraton Kauai, Hawaii, September 22-25, 2013 BACKGROUND Over the past 20 years or so, numerous researchers and inventors in the field of industrial fuel combustion have disclosed methods and technologies for achieving improved performance using the general concept of comburent dilution prior to the oxidation reaction. A variety of terms have been coined to describe this phenomenon. A precise definition remains elusive, but the general characteristics are now well known to those in the industrial combustion community. The key benefits include minimized emissions, improved temperature uniformity, reduced noise levels and potential for improved overall process cost-effectiveness. Controversy is evident from a reading of the recent literature. Some insist that the concept requires very high air preheat to function, while others have reported similar results with reduced air temperatures. ‘Flameless' conditions also are generally reported to exhibit almost no noise and lack of any visible flame. I am probably not alone in my initial reaction when observing this in a furnace; one's first instinct is that the flame is extinguished and someone should immediately turn off the fuel! Under these conditions the only evidence that oxidation reactions are occurring is that the chamber temperature is not falling off as would be expected. Furthermore, it has been argued that ‘Flameless Combustion' (defined in part as having no anchored stabilization point in space) is required for ultra-low NOx emissions in the case of high temperature industrial furnace applications with highly preheated combustion air1. DISCUSSION OF THE TECHNOLOGY Our vision for the practical development of these principles has led to a wide variety of ultra-low emission burner technologies which utilize the basic concept of high-level internal furnace products of combustion (aka POC) recirculation. POC Entrainment levels can be varied to suit the process requirements in a variety of ways including comburent velocities, flow passage and tile geometry. While others have reported a disconnect (i.e. instability region)2 between ‘conventional' anchored flames and fully detached Flameless oxidation reactions, we have developed a range of hybrid designs which exhibit some but rarely all of the defined ‘Flameless' characteristics. Some examples of our findings: 2 o Air Preheat While we agree that the ideal case for best temperature uniformity would include air preheat close to chamber temperature, we have certainly observed conditions of ultra-low NOx performance without visible flames utilizing reduced air temperatures (typical of recuperative heat recovery). Of course with lower preheat levels, the POC recirculation ratio does not need to be as high to achieve similar NOx levels. o Flame visibility Flame appearance of these ‘highly vitiated' flames can be affected by several factors such as:  Furnace temperature. We have observed natural gas burners to be operating with completely invisible flames at around 2000oF chamber temperature (Nox at less than 30ppm), as the furnace temperature was allowed to rise a substantially luminous flame developed, with little change in NOx or other variables. We believe this was due to thermal decomposition and soot particle formation of the natural gas at the higher chamber temperature. A similar report has been presented by Gupta, et al.2  Burner size/capacity. While a 1mm BTU/hr rated Bloom 1150 series regenerative burner exhibits an invisible flame at rated flow and high chamber temperature (for example 2400oF) a larger burner of 10 or 15mm BTU/hr size and scaled according to velocities and port area will exhibit a relatively luminous flame while emitting virtually identical NOx levels. Again we tend to attribute this to methane ‘cracking' which may be more pronounced in the larger diameter flames due to differences in radiation heat transfer from the flame root due to the optical path lengths. We note that much of the literature regarding flameless combustion phenomena is reported based on laboratory and ‘small' industrial scale burner designs. o Turndown Many ‘Flameless' burner designs are limited to pulse-firing control or very limited proportional turndown3. This is an expected consequence of operating in a fully detached flame condition, in which reduced velocities will quickly cause the flame speed to exceed the exit velocity and re-attach somewhere in the vicinity of the fuel nozzle exit. While pulse-firing may have additional process advantages in some applications, the installed and future maintenance costs should be clearly understood when committing to such systems. We tend to disagree with the parallel argument that the reduced velocities also result in reduced entrainment levels. From our testing results, we find this claim to be highly design-dependent. In many cases where a staged-combustion-plus-internal -vitiation design is employed, we find that proportional turndown can be achieved without sacrificing ultra-low NOx performance. Neglecting surface friction effects, our findings suggest that the entrainment ratios tend to remain reasonably constant through turndowns of as much as 5:1 or better. Additionally, momentum-maintaining lances can be added to achieve additional thermal turndown of a burner. o Uv Sensing Most reports of flameless combustion phenomena indicate that, due to the detached and diluted oxidation reactions, there is insufficient UV energy emitted to allow flame supervision via UV scanners. While the conditions for flameless combustion always include chamber temperature above the normal natural gas self-ignition 3 temperature, and hence avoid the need for flame supervision to meet safety codes, most manufacturers will include some additional safety control features. For example, for batch heat-up furnaces it may be necessary to include multiple temperature sensing probes with exposed tips, to assure that the chamber wall hot face temperature is adequate to support flameless operation throughout the chamber. While the lack of supervisibility can be overcome with proper system design, we feel that designs which can achieve UV supervision without sacrificing other benefits of the highly vitiated combustion can provide an additional level of operational safety. o NOx One of the most desirable outcomes from this type of combustion is minimization of NOx formation via the Thermal mechanism while allowing for the use of maximized combustion air temperature for best efficiency. We have developed several combustor designs which can achieve Nox and other emission levels comparable to those reported for Flameless designs1, while maintaining at least some degree of attached and visible flames. These designs typically employ a combination of high levels of internal POC recirculation as well as staging of air, fuel or both. o Furnace atmosphere In ‘highly vitiated combustion' discussions, it is often stated that the furnace atmosphere becomes homogenous and uniform from both a temperature and chemical species atmosphere standpoint, with combustion or oxidation reactions occurring throughout the furnace4. Taken to the extreme, the mixing model for the furnace would be one of a well-stirred tank reactor. While this would be useful in providing highly uniform temperature field, the downsides could include:  Potential for contamination of the product being heated. Metals and alloys can be susceptible to surface reactions with reacting species, for example hydrogen embrittlement or surface appearance problems.  Low levels of combustibles remaining in the POC exiting the furnace zone where the heat release was intended. This could negatively impact emissions as well as fuel consumption depending on the concentrations of the unreacted species and whether the downstream conditions allow the reactions to complete. The furnace designer should take this into account when determining the combustion chamber geometry and POC exhaust location(s). o Furnace effects Chamber size, burner placement, flow patterns and obstructions cannot be overlooked when evaluating burner performance. With high levels of internal POC entrainment required for the ‘Flameless' operating regime, these factors become more critical. For example, closely spaced burners may experience interactions and POC exchange, while reducing the amount of inert ‘POC' entrainment. Obviously the location of wall surfaces can affect the ability of the comburent streams to entrain POC. In a simple example, a single burner applied to a small diameter chamber may achieve close to a ‘plug flow' condition, where there is little volume available for recirculation eddies to be generated. The same burner installed in a larger chamber may behave quite differently, especially when designed to operate with high internal-recirculation levels. Therefore, the question of "what is your burner NOx emission?" cannot be properly answered without evaluating these geometry and flow pattern parameters. Other factors including flame appearance, stability and heat release profile may likewise be affected. 4 The burner supplier and/or furnace designer can use a number of tools to predict the furnace effect on burner performance including CFD, physical modeling and data analysis from previous installations. In summary, the general technology of ‘highly vitiated combustion' is therefore difficult to put in a well-defined ‘box'. There is the possibility of many variations in utilizing the basic principles, and customization for specific applications will continue for the foreseeable future. APPLICATION OF THE TECHNOLOGY- 3 Examples Natural Gas Fired Steel Reheat A typical application of our patented5 1610-series ‘Cyclops' burners began with the requirement for Best Available Control Technology for a natural-gas fired reheat furnace in the US Midwest region. The furnace was to heat square-section steel billets to approximately 2150oF feeding a merchant bar mill which produces small sectional shapes such as angles and channels. Burners are arranged to fire across the width of the furnace from both sides. CFD modeling was applied to help optimize the burner locations and orientations. In this case, the furnace builder specified that the combustion air would be supplied to the burners from a recuperator at approximately 950F. Bloom 1610-series Cyclops burner on Steel Reheat Furnace:Velocity vectors colored by magnitude Resulting emission performance has been verified to meet the following EPA permit requirements: - NOx below 40ppmvd at 3% O2 - CO below 100ppmvd at 3% O2 (typically observed much lower) Furnace throughput and fuel consumption are considered confidential by the client, but met the contractual requirements. The ‘flames' are invisible at normal operation but become somewhat luminous under conditions of turndown: Three sidewall burners; the first is cascaded off. There is a temperature gradient in the furnace caused by the temperature of the incoming billets. Temperature uniformity across this width is very good. Oxy-Enriched Gas-fired Steel Reheat It seems counterintuitive to consider the possibility of adding oxygen to the combustion air in a ‘flameless combustion' burner. However, we have found that by applying the basic principles of POC dilution into the comburents at the flame root, it is still possible to achieve most of the benefits associated with ‘flameless combustion'. Our basic 1610-series Cyclops burner can be outfitted with a patented6 oxygen sparging system into the burner air stream and provide an Ultra-Low NOx mode to at 5 least 28-30% O2 equivalent in the air/O2 mixture. The test data clearly indicate where the thermal NOx formation as per the Zeldovich mechanism begins the expected exponential increase: A steel mill in the US Midwest has an on-site oxygen plant associated with its steelmaking facility. An excess of oxygen is available in close proximity to the rolling mill reheat furnace. Furthermore, the mill operation is often able to handle more throughput than provided in the original furnace design. The furnace flue system is limited to the point where little additional burner power could be applied. While regenerative burners could have been applied, the economics in this case favored the use of oxy-combustion. Previous to our involvement, some straight ‘oxy-fuel' burners had been added as a booster zone. Despite being touted as ‘low Nox', there was a substantial increase in furnace NOx whenever these were operated. The 1610-series Oxy-Cyclops burners have now been fitted to the largest firing zone of the furnace with very good results in terms of productivity, fuel consumption, temperature uniformity and NOx emissions. The construction of the furnace makes it very difficult to observe or photograph the ‘flames' but it seems to be a moot point as to whether they are invisible or exhibit some luminosity, so long as the desired performance is being achieved. Alternate Fuels: Regenerative Steel Reheat on Low CV Fuel example The vast majority of literature regarding Flameless combustion has been limited to natural gas fuel. In some industries, alternate fuel sources are utilized for industrial heating. The application of Flameless technology can be significantly affected by the fuel source. While it is often possible to operate in or close to the Flameless regime, such fuels generally require detailed studies (scale-model studies, CFD modeling, etc.) to develop suitable combustor designs. Fuel characteristics which affect how the Flameless mode is achieved include: density, viscosity, flame speed, sooting potential, contaminant level and supply pressure and temperature. A good example of how such fuels can be utilized to achieve a ‘highly-vitiated' Flameless combustion condition has been installed in multiple locations in hot strip rolling mills in China. Utilizing side-fired burners equipped with regenerative air preheating, we have been able to achieve the required performance goals of high fuel efficiency (as low as about 250 kcal/kg), very low NOx (typically under 50ppm including fuel NOx) and good heating uniformity. In some cases the fuel was preheated to as much as 600oF to further improve combustion efficiency. 6 Bloom 1150-series Ultra Low NOxRegenerative side-fired Hot Strip Mill furnace operating on mixed gas. Typical BFG+COG ‘mix gas' analysis: species mol% hydrogen 12.100 oxygen 0.100 nitrogen 42.000 carbon monoxide 22.200 carbon dioxide 16.300 methane 4.400 ethane 2.900 propane 0.000 Available supply pressure <0.5 psig Trace amounts of tar, naphthalene, NH3, HCN and non-combustible dust Of particular note here is that the fuel already contains almost 60% inerts (a 1.5 ‘built-in' entrainment ratio). On the down-side, the very low available pressure limits the amount of in-furnace POC entrainment that can be achieved. Also, the organic nitrogen compounds contribute to emissions via the fuel NOx pathway. Through lab-scale testing plus CFD modeling to examine the effects of furnace geometry and multi-burner interaction studies, we developed a variant of our 1150-series Ultra low Nox regenerative burner which functions very well on this type of fuel. Furthermore, we have simplified the design such that in many areas of the furnace, the burners are designated as ‘hot operation only', thus eliminating the need for cold-start stabilizing control commonly required with Flameless technologies. CONCLUSION The concept of Flameless Combustion has forced us to revise our thinking about burners, furnaces and related control systems. A variety of techniques for achieving ‘highly vitiated' comburent streams which lead toward the ‘Flameless Combustion' operating regime have been developed by combustion equipment suppliers to suit a wide variety of applications. This paper provides a glimpse into the process design considerations for some specific areas of interest in our base market areas. We believe that there is much more to learn about Flameless Combustion in general, and how the concepts can be applied to a broad variety of additional process applications. 7 1 "Advanced Combustion Equipment for Continuous Furnace" , A. Milani and J.G. Wunning, IFRF Combustion Journal May 2004 2 "High Temperature Air Combustion Technology", A. Gupta presented at IFRF TOTeM 25, October 2003 3 "How do I achieve flameless combustion in practice?" A. Milani and J.G. Wunning, IFRF Online Combustion File No. 215, October 2003 4 "On emerging furnace design methodology that provides substantial energy savings and drastic reductions in CO2, CO and NOx emissions", R. Weber et al, 2nd International Seminar on High Temperature Combustion in Industrial Furnaces, Jan. 2000, Stockholm, Sweden 5 "Burner for Non-symmetrical Combustion and Method", H. Finke et al, US patent 6,471,508, Oct 2002 6 "Burner for Non-symmetrical Combustion and Method", H. Finke et al, US patent 6,793,486, Sep 2004