Combustion-system control, component selection and accurate sizing of the combustion system are all important factors in providing accurate control of air and gas flows. Oversizing of burners and flow controls can cause turndown problems, overshooting of temperature setpoints and loss of temperature uniformity, resulting in higher fuel costs and lower product quality.
In today’s business environment, companies that utilize thermal-processing systems must focus on higher productivity, lower costs, increased product quality and reduced environmental impact. Therefore, increasing throughput while simultaneously reducing per-unit fuel consumption and emissions are the keys to present and future success. Meeting these challenges to achieve these objectives can in no small part be tied to improving the combustion system through the use of:
- Air/fuel ratio control
- Pulse-firing control
- Preheated air (recuperative and regenerative systems)
Air/Fuel RatioWhile improvements, upgrades and replacements of furnace systems, controls and burners can help to achieve the above objectives, an area that cannot be overlooked is simple air/fuel ratio control. Assuming a cold-air combustion system with 1900°F furnace exhaust temperature, simply tuning the burners to operate with 10% excess air rather than 30% excess air will result in a 15% fuel savings! Few companies would argue over saving 15% in what is often the highest cost of production.
MaintenanceBest maintenance practices such as repairing worn or damaged refractory or insulation, checking door seals, and ensuring positive furnace pressure via exhaust damper operation will go a long way toward increasing overall furnace efficiency. Maintaining proper air/fuel ratio of the combustion system and eliminating air infiltration into the furnace will not only maximize fuel efficiency, but they will also typically lower NOx emissions. A few hours spent on combustion-system tuning and maintenance every six months can result in significant, ongoing energy savings.
Pulse-Firing ControlMore heat-processing operations are utilizing pulse-fired combustion control systems, resulting in reduced fuel consumption, excellent temperature uniformity and, in many cases, lower emissions. Pulse firing has resulted in benefits for both indirect- and direct-fired applications. What is pulse firing?
Pulse firing utilizes multiple burners that take their turn firing either high-off or high-low to control temperature input rather than cross-connected (proportional) control via a zone air-control valve. Pulse fire utilizes individual burner air and gas valves that are controlled from a pulse controller or PLC. Temperature demand determines the on time and off time of the burners. The burners fire in a rotation sequence to optimize furnace uniformity. There is virtually infinite turndown with this firing method. Fuel savings of 10-20% are not uncommon when switching to pulse-fire control, depending on the condition and design of the existing combustion system.
On/Off Pulse-Fire Control vs. Proportional Control in Radiant Tubes
The benefits of on/off pulse-fire control for indirect-fired applications include improved tube uniformity, accurate burner ratio control and increased thermal efficiency since the burners are always fired at their maximum capacity. The flame length does not vary, and the heat release is uniform throughout the firing leg. Longer tube life results. The burners fire at the proper ratio every time, resulting in optimal energy release, and do not transition from ratio to rich or lean during turndown because they are at their maximum firing rate when “on.”
Using this firing method, carbon is less likely to form, minimizing hot spots in the tube. Thermal efficiency is gained with the absence of excess air at low fire. It is normally necessary to operate indirect-fired burners with 8-10% O 2at low fire. This reduces the risk of overheating, swelling of the radiant tube (at the firing end), premature tube failure and internal burner damage, all of which are associated with extended operation at low fire. Excess air is also necessary for burner turndown to keep from overshooting temperature setpoints.
High-Velocity Burners and Pulse Firing
Since high-velocity burners are at or near maximum firing rate when “on,” this yields the highest possible convective heat transfer. The products of combustion (POCs) are continuously stirred through the entire furnace, resulting in excellent temperature uniformity and, therefore, the best product quality possible. Furthermore, this “frequency” firing method reduces fuel consumption since the burners are only firing at their most efficient “high” fire rating, which maximizes the energy reaching the product. Pulse firing will actually reduce emissions like NOx since typical high-velocity burners produce higher concentrations of NOx at lower firing rates (with turndown).
Preheated AirWhile heat processes are implemented in differing furnace designs with a variety of control methodologies and atmospheres, the principles behind reducing per-unit cost via improved combustion-system efficiency (otherwise known as available heat) are similar. Preheating the combustion air is a well-known and effective technique to reduce fuel consumption. As carbon dioxide (CO2) emissions come under more scrutiny, additional benefits of preheating the combustion air include reduced CO2 emissions as well as increased productivity via increased furnace throughput. A graph of fuel savings versus air preheat temperature at a furnace temperature of 1900°F is shown in Figure 1.
How does preheated air save fuel?
- Preheating the combustion air reduces the heat required for POCs to reach exhaust-gas temperature, resulting in less fuel required to do the same work.
- The flame temperature is increased.
- More heat is available to heat the load or do useful work.
Two common ways to preheat the combustion air are recuperation and regeneration.
RecuperationRecuperation systems can come in a variety of forms:
- Central recuperative systems
- Self-recuperative burner systems (direct- and indirect-fired applications)
- Indirect-fired recuperative systems
This type of system typically uses a central-stack recuperator to exchange the heat from the POCs to the system’s main combustion air supply. Furnaces such as steel reheat furnaces typically utilize this technology. Mass-flow control of the air and fuel is the most common control methodology employed in these applications.
Self-Recuperative Burner Systems
Technology advancements in indirect- and direct-firing applications include a recuperator integrated into the burner design such that the flue products from the furnace are educted or drawn around the recuperator to preheat the combustion air to the burner.
Such designs are being widely implemented not only for their efficiency but for their simplicity of field piping and burner setup. These burners can be installed, set up, controlled and maintained more easily than burners fed from a central recuperator since the air up to the point of burner connection is still ambient, eliminating the need, expense and maintenance of hot-air piping.
Self-recuperative burners can be used for either indirect or direct firing and are pulse fired high-off. High-off pulse firing results in the benefits previously discussed with the added fuel efficiency of operating with hot air. Energy efficiency of a self-recuperative burner tends to be higher than indirect (plug-in) or central recuperators because heat losses are less than systems employing additional piping and hot-air control components.
Indirect-Fired Self-Recuperative Burners
Self-recuperating burners are available for use in single-ended radiant tubes and in P-shaped “closed” tube designs. The P tube takes the place of the conventional U tube. A self-recuperative burner (Fig. 2) can be used in alloy radiant tubes or silicon-carbide tubes.
The advantage of the silicon-carbide design is heat flux from the tube surface can typically be 110 btu/hr-in2, whereas the heat release of an alloy tube is typically limited to 55 btu/hr-in2. Therefore, roughly half the number of burners and tubes may be required when using silicon-carbide radiant tubes over alloy tubes. However, not all applications (like those where there may be mechanical interferences) lend themselves to silicon-carbide tubes. Nevertheless, whether the tube is alloy or ceramic, significant fuel savings are achieved, and NOx is automatically reduced by the recirculation of the flue-gas stream.
Self-recuperative burners for direct-fired applications require an eductor at the flue-gas end of the burner, which uses motive air to induce the flue gases over the integral recuperator, thereby heating the combustion air (Fig. 3). Added benefits to the heat-treating operation include better temperature uniformity of parts with potentially fewer rejects. In terms of fuel savings, a direct-fired self-recuperative burner with 1100°F air preheat in a 1900°F furnace would achieve 32% energy savings versus an ambient-air-fired burner.
Indirect-Fired Recuperative System
Plug-in-type recuperators are available that can yield greater than 60% efficiencies, depending on application temperature. These types of units are easily adaptable to existing U or W tubes with a cross between the recuperator and burner (Fig. 4). While plug-in recuperator-type systems are not as efficient as self-recuperative burner systems, they are a good choice to preheat combustion air and save energy.
Direct-Fired Regenerative BurnersFor high-temperature applications (1650-2350°F), the regenerative burner is the most efficient direct-fired burner. Exhaust gases are pulled through a built-in regenerative bed or media case via an exhaust fan. The air preheat temperature is considerably increased – up to 1800°F, depending on furnace temperature. Combustion efficiencies of more than 80% are possible at process temperatures of 2350°F. NOx levels that were historically very high from regenerative burners have been drastically reduced by today’s state-of-the-art, ultra-low-NOx burner technology. Even though this system requires additional components for control and higher maintenance, the cost can typically be recovered in a very short time due to the lower energy cost and higher productivity.
ConclusionAdvancements in burner designs, recuperative and regenerative systems, and pulse-fired control algorithms are just a few of the technological improvements available to the industrial heating industry. These advancements, when combined with good operating and maintenance practices, can lead to substantial cost savings, increased productivity, improved product quality with fewer rejects and reduced downtime.
As with any industry, there is no one burner, one control method or one system that will act as a magic wand for increased productivity, improved product quality and reduced fuel costs. There are benefits and consequences to all system types. In the final analysis, the question remains: What change, upgrade or improvement will be the best choice to achieve your goals?IH
For more information:Brian Kelly, technology and applications manager, Hauck Manufacturing Company/Elster Kromschroder, PO Box 90, Lebanon, PA 17042; tel: 717-272-3051; fax: 717-273-9882; e-mail: firstname.lastname@example.org; web: www.hauckburner.com