Vortex-Generated Combustion Efficiency
Until recently, the resilience of our environment and unlimited availability of natural resources have been taken for granted and have not been seriously considered in the design of the machines that drive our industrialized world. The fact that the basic design of an industrial gas burner has not changed in over 100 years clearly demonstrates this.
Take for example the Bunsen burner that was first developed in the 1850s as an aspirated burner. During this period, the principal design criteria of an industrial gas burner was simply supply the gas and air in proportion and let it burn.
Over the next few decades as demand drove design, the basic configuration evolved into the typical burner design seen today on most industrial burners. In this design, gas is supplied by a centered pipe surrounded by numerous air-discharge ports as seen in Fig. 1.
The basic design criteria is still: supply the air and gas in proportion and let ‘em burn.
Historical ApproachIn today’s internationally competitive markets, no company can remain profitable for long and survive if they do not continually strive for improvement in both operations and technology. On a personal level, each of us has seen the price of a gallon of gasoline steadily increase over the past few years. The same is true for plant managers and the price of natural gas.
To make the challenge of operating a business even more difficult, regulatory requirements force companies to either curtail production or invest considerable amounts of capital in order to safeguard the environment. Higher energy costs and environmental protection are now and will continue to be essential elements in many business decisions.
There are several methods that have been developed in an attempt to increase fuel efficiency and/or reduce emissions. Some of the more prevalent are:
- Digital Control Systems – This method is characterized by monitoring exhaust stream with gas sensors and using a computer to evaluate the products of combustion while adjusting the only two controllable variables – the combustion-air control valve and the fuel-gas control valve. A conversion to this type of system can be expensive and time consuming to install.
- Premix – Premixing of the combustion air and the fuel gas has been around for some time and does, in fact, produce better fuel efficiency by burning at a higher temperature. The major drawback to this method is the danger of a combustible mixture leak and accidental ignition.
- Oxygen Enrichment – Injecting oxygen into the fuel or combustion air stream does result in a higher flame temperature as well as lowers undesirable emission by-products. The downside of this method is the initial capital cost and the continually rising cost of producing or purchasing the oxygen.
- Recuperative Furnaces – Heat exchangers are installed to elevate the combustion-air temperature, which decreases the amount of energy required to increase the process temperature. Significant fuel savings can be achieved, but the maintenance cost of the heat exchanger can be excessive.
- Regenerative Furnaces – This technology uses the hot combustion waste heat to preheat combustion air by alternate firing of burners. While this design produces significant fuel efficiency as well as reduction in NOx emissions, the cost is significant and the plant space required for installation is considerable.
Combustion Process 101With all industrial gas burners, like the accelerator on your automobile, an increase in the flow rate of the fuel increases the chemical-reaction rate, which translates to more energy.
All chemical reactions are accomplished by the absorption of energy (endothermic) or discharge of energy (exothermic). This phenomenon manifests itself as heat and can be measured by the temperature and composition of the final combustion products. This is usually stated as Q = DH where Q is the heat added or generated, H is the enthalpy or heat content that can be used to calculate useful work obtained from a system and DH is the net change in enthalpy. An endothermic reaction is stated as
Q = DH.
An exothermic reaction is stated as
Q = -DH.
Stoichiometric combustion is the ideal combustion process in which fuel is burned completely with the precise amount of combustion air and in which all the carbon (C) converts to CO2 (carbon dioxide), all hydrogen (H) converts to H2O (water vapor) and the nitrogen remains inert.
The key word here is “ideal.” The concept of perfect combustion assumes that each and every molecule of fuel has associated itself with the exact number of free oxygen molecules through natural affinity or some other form of mixing, either mechanical or chemical, and that no other chemical reactions take place.
The simplified combustion process is usually represented chemically as:
CH4 + 2O2 + 8N2 ® CO2 + 2H2O + 8N2 + 1,000 BTU Heat
(Methane + Air ® Carbon Dioxide + Water Vapor + Heat)
But is, in reality
CH4 + 2O2 + 7.52N2 ® CO2 + 2H2O + 7.52N2 + HEAT + Aldehydes + Alcohols + Acids + Products of Dissociation + Radicals
Obviously, this is a little more complex chemical process involving several hundred different species and thousands of associated chemical reactions that have been identified and modeled as a result of modern numerical combustion analysis. For a more detailed discussion of the chemical process please see the sidebar. Unfortunately, many of the associated chemical reactions are highly endothermic, each absorbing some of the available energy that would otherwise be available from perfect stoichiometric combustion, which lowers the flame temperature substantially.
This phenomenon is clearly demonstrated by the fact that methane, when burned in pure oxygen, produces a flame temperature of approximately 5150°F. But when burned in 16% excess air, the flame temperature drops to approximately 3300°F.
Looking at the typical burner design as seen in Fig. 1, it is clear that at the moment of ignition, which is immediately past the plate in the combustion chamber, there is little diffusion or association of the fuel and air molecules. This can be seen by the gas and airflows depicted in Fig. 2 in the plane X0. This results in perfect stoichiometric combustion only at some imaginary boundary somewhere between the gas source and the air sources.
As illustrated, immediately upon combustion at the imaginary boundary layer between the gas and airflows, only water vapor and carbon dioxide are found with fuel and air molecules immediately adjacent. This is depicted in Fig. 2 at point X1.
The forward velocity of the air and the turbulence resulting from the combustion process create a mixture of air, fuel and combustion products depicted in Fig. 2 at point X3. The result is fewer oxygen molecules available for stoichiometric association with fuel molecules and more undesirable by-products of combustion reacting and disassociating with each reaction, reducing the net energy content that would otherwise be available for the heating process.
This is, of course, an oversimplification of the combustion process, which is complex and, to some degree, random. The objective of this oversimplification is to demonstrate that basic design of today’s burners does not take into consideration the actual combustion process and the need to consider stoichiometric proportioning and association of the fuel and air molecules.
Another finding as a result of computer modeling is that “a dominant factor in determining the burning rate of a premixed turbulent flame is the degree to which the flame front is wrinkled by turbulence. Higher turbulent intensities lead to greater wrinkling of the flame front and increase in the turbulent burning rate.”
The basic form for the calculated heat release used in computer modeling is
Q = r u Dh f ST A
where r u is the density of the unburned gas, h f is the heat of the reaction, ST is the turbulent flame velocity and A is the flame surface area. What this equation proposes is that an increase in the burning rate (the turbulent flame velocity ST) shows a direct correlation to the heat release of the flame as does the increase in the flame surface area (A). The key point to understand is that an increase in combustion efficiency can be achieved by increasing the flame surface area without increasing the fuel flow rate to the burner. Varying the turbulence in computer simulations indicates an increase in surface and flame speed, above flame speeds typically generated by diffusion burners, by 35% and 85% respectively.
New Hybrid Burner DesignA new industrial gas burner that utilizes the information obtained from computer modeling of various combustion schemes is currently being marketed and has achieved verifiable fuel savings typically in the range of 16%.
This new design is a hybrid of both a premix and diffusion burner that creates a low-velocity vortex by the injection of the gas fuel tangential to the combustion airflow. The rate of premixing increases as the vorticity (swirl) intensity increases and can be controlled by the angle of injection, volume of fuel gas being injected and fuel-gas pressure.
As shown in Fig. 3, combustion air flows along the axis of the burner and the fuel gas is injected into the combustion-air stream at a specific angle generating a low-velocity vortex. When the mixture emerges from the mixing tube, a premixed turbulent-flow condition exists.
As with the endothermic reactions in the combustion process that limit the upper temperature, the increased combustion rate serves to decrease the flame velocity. This decreases vorticity and thus the flame surface area, limiting the upper achievable temperature limit.
In summary, the advent of computerized fluid dynamics and the continued research into the combustion process have provided a much better understanding of the chemistry and physics of combustion than ever before. This increased understanding has and will continue to lead the way to more efficient industrial equipment that will have a significant impact on both the environment and business.
Today, industrial consumers of energy need to continue their search for newer available technologies that can provide both energy savings and emissions reductions in as many processes as possible. This will benefit both the company’s bottom line and our environment. IH
For more information: Contact Tibbs Golladay at Burner Dynamics, Inc., 5865 Old Leeds Rd., Birmingham, AL 35210; tel: 800-749-3058; fax: 205-879-3514; web: www.burnerdynamics.com
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: industrial gas burner, recuperative furnace, low NOx, endothermic, exothermic, stoichiometric combustion
SIDEBAR: Combustion ChemistryStated in mathematical terms, the combustion process could be written as:
Qp = [ å ni (DHf )(T0,i) – å nj (DHf)T0, j ]
Where Qp is the total process energy, ni is the concentration of species, nj is the number of reactants involved in the process and Hf is the heat of formation. Many detailed models of the oxidation of methane have been published and are all very complex involving hundreds of reactions.
One possible model of the combustion (oxidation) of methane (CH4) is reaction initiating
CH4 + O2 ® CH3 + HO2
CH3 + O2 ® CH2O + OH
and similar endothermic radical reactions
CH3 + O ® H2CO + H
CH3 + OH ® H2CO + H2
CH3 + OH ® CH3O + H
CH3 + H2CO ® CH3 + HCO
CH3 + HCO ® CH4 + CO
CH3 + HO2 ® CH3O + OH
Once a credible model is created, the total energy can be calculated by summing the given heats of formation (DHf ) of all of the products and all of the reactants using JANAF Thermo-Chemical Tables, Physical Reference Data or National Bureau of Standards, Circular C461.