Achieving Effective Control of Process Heating Combustion Systems
Careful selection of combustion control methods and equipment helps optimize the control of industrial heating systems.
According to Process Heating:
Power is nothing without control. You can have the best, most technically advanced burner. If the burner is not properly controlled, and the heat is not applied appropriately during the production process, the result will be disappointing — or worse.
Producing high quality, heat processed products such as metals and ceramics, efficiently firing process heaters, or drying materials at the lowest cost depends on the quality and integrity of the combustion system. Control methods such as parallel positioning, ratio regulation and mass flow — together with burner-management systems, logic controllers and the accurate sizing of air- and fuel-control components — influence the accuracy by which air and gas flows are controlled. The oversizing of burners and flow controls, or inadequate control methodology, can cause turndown problems, overshooting of temperature setpoints and loss of temperature uniformity. All of these can result in higher fuel costs, lower product quality and emissions issues.
Manufacturers continually seek competitive advantages, and more effective or lower cost thermal processes can provide that. In response, thermal process manufacturers are striving to achieve higher productivity, lower costs, greater product quality and reduced environmental impact. Increasing throughput while reducing per-unit fuel consumption and emissions is critical to success. Achieving these objectives, however, depends on the precise control of the combustion system.
This article will look at some of the key considerations for effective burner control.
Air-Fuel Ratio Control
Improvements, upgrades and replacements of furnace systems, controls and burners, and air-fuel ratio control all can help achieve increased throughput and reduced fuel consumption and emissions. For example, in a cold-air, high temperature combustion system with a 1,900°F (1,038°C) furnace exhaust temperature, simply tuning the burners to operate with 10 percent excess air rather than 30 percent excess air can result in approximately 15 percent fuel savings.
Low emissions burners require precise air-fuel control to maintain low emissions, so flexibility and adjustability are required to accommodate both the burner and process conditions. This often entails the use of parallel-positioning-type systems or mass-flow systems to control air and fuel flows and help ensure emissions levels are met.
Harnessing parallel positioning in single burner and low emissions applications can optimize air-fuel ratio through the burner’s operational range to maintain the required low emissions. By adjusting the air and fuel curves, a system can be tuned to the process requirements. Moreover, the quality and precision of the actuators in such a system are critical: If the valve positions are not repeatable, then neither is the air-fuel ratio.
The actuators used in this type of control solution, for instance, experience almost no hysteresis, providing for precise control of the heating system.
Applications with varying process conditions often use mass-flow control systems to improve burner operational performance as well as decrease emissions. Different flow elements can be used in these systems depending on the range and turndown required. Air preheat also is a factor in controlling the air-fuel ratio.
Ratio regulators or pressure-balanced (proportional) systems frequently are used in multiple-burner applications because they provide a simple, cost-effective way to maintain the burners’ air-fuel ratio. Ratio regulators will maintain a constant gas-air ratio with a cross connection to a burner air header. Double-diaphragm regulators also can maintain air-fuel ratio on a flow basis, making them suitable for self-recuperative burner applications. In addition, combination devices — incorporating the ratio control functions plus an integrated safety shutoff valve that provides a compact solution for delivering precise control in a reduced footprint — are available.
An increasing number of heat processing operations are using pulse-fired combustion control systems to deliver reduced fuel consumption, improved temperature uniformity and reduced emissions for indirect- and direct-fired applications.
Pulse firing is a specialized technique that normally incorporates ratio regulators. In this method, multiple burners take turns firing either high-off or high-low to control temperature input rather than across connected (proportional/modulating) control that uses a single-zone air control valve. Pulse fire uses individual burner air and gas valves that are controlled by a pulse controller or a PLC. Temperature demand determines the on time and off time of the burners, and the burners fire in rotation to optimize furnace uniformity. Virtually infinite turndown is possible with this firing method.
On/Off Pulse-Fire Control vs. Modulating Control in Radiant Tubes. The benefits of on/off pulse-fire control for indirectly fired applications include:
- Improved tube uniformity.
- Accurate burner ratio control.
- Higher thermal efficiency. (The burners are always fired at their maximum capacity.)
- Consistent flame length. (It does not vary.)
- Uniform heat release throughout the firing leg, resulting in longer tube life.
- Optimal energy release because the burners fire at the proper ratio every time.
- Consistent burner operating profile. The burners do not transition from ratio to rich or lean during turndown because the burners are at their maximum firing rate when on.
This method minimizes hot spots in the tube. Thermal efficiency is gained with the absence of excess air at low fire. Normally, it is necessary to operate indirectly fired burners with 8 to 10 percent O2 at low fire. This reduces the risk of overheating, swelling of the radiant tube at the firing end, premature tube failure and internal burner damage. Excess air is necessary for burner turndown to keep from overshooting temperature setpoints in many processes.
High Velocity Burners and Pulse Firing. Because high velocity burners are normally at or near maximum firing rate when on, this method of firing yields the highest possible convective heat transfer. The products of combustion are stirred continuously through the entire furnace, resulting in improved temperature uniformity and product quality.
With the burners firing at their most efficient high fire rating, maximum energy reaches the product, and fuel consumption is reduced. Normally, pulse firing will help reduce NOX emissions because typical high velocity burners produce higher concentrations of NOX when they are turned down in a modulated system. The high velocity jet entrains the cooler furnace atmosphere that reduces peak flame temperatures, resulting in lower NOX.
Burner Management/System Control
Once the burners, combustion components and control method are defined, a user then needs to select controls for running the furnace and managing the burners.
Whether it is a simple air-heating application that needs basic PID logic, or a heat-treating furnace that requires multiple ramps, soaks or controlled cooling to achieve correct heat processing, there is a temperature controller suitable for every application.
Sometimes PLCs incorporate the PID temperature control functionality, but this must be applied and programmed correctly to achieve proper control and results.
Burner management and flame safeguards relate to the safety of the system. Ignition sequencing, flame sensing and logic all contribute to the safe operation of burners. Flame safeguards can range from simple flame switches to advanced units that incorporate flame safeguards, an ignition transformer, parameter customization for applications, diagnostics and fieldbus communication connectivity. Such features can simplify and reduce installation costs while improving the ability to relay critical burner status to the furnace control system.
Solutions are available that integrate configurable safety and programmable logic in a single modular platform. Process temperature control, burner control, flame amp, fuel-air ratio control, limits control, annunciation, and digital and analog I/O all can be incorporated into the combustion system design. Some are available as a single platform that can be configured for simple to complex applications.
In effect, a variety of control options are available. Your combustion system design — together with your desired functions and goals of the heating system — will dictate which control mechanism to choose.
Most maintenance managers are acquainted with the panic associated with handling a critical thermal process failure and investigating potential causes after the fact. In many cases, the issues may require repairs or lead to unplanned downtime.
In addition, to achieve greater productivity, many thermal process manufacturers are looking for ways to improve the visibility of asset and production issues. Experience has shown that unplanned downtime can result in tens of thousands of dollars of lost revenue per hour. Shutdowns also can lead to substantial response and recovery costs, labor and overhead costs, customer service impact and more.
An effective way to view and share data before they get to the equipment can help factory personnel minimize the downtime required for repairs. This includes mobility tools that enable them to receive real-time alerts when operating parameters exceed limits. Such tools also can display historical data to see when and why issues occurred.
In response, technology providers have developed tools that can optimize thermal processes and connect production assets in the cloud. Cloud-based technologies allow for enhanced control and performance monitoring, and they make critical asset data continuously available in a secure manner.
A remote-monitoring solution connects combustion equipment to the cloud in a safe way, making critical thermal process data normally trapped at equipment level available on any smart device or desktop. Using the data, users can:
- Closely monitor thermal processes without being on site.
- Get real-time alerts when parameters stray outside normal limits.
- Track historical data over time to identify when and why something happened.
- Provide actionable recommendations.
In other words, a remote-monitoring system turns thermal process data into actionable information.
Don’t Overlook Maintenance
Employing best maintenance practices will go a long way toward increasing your process heating system efficiency. These include repairing worn or damaged refractory or insulation, checking door seals and keeping positive furnace pressure via exhaust damper operation.
Maintaining a proper air-fuel ratio of the combustion system and eliminating air infiltration into a furnace maximize fuel efficiency and lower NOX emissions. Just a few hours spent on combustion system tuning and maintenance every six months can result in ongoing energy savings.
In conclusion, advancements in burner designs, firing and control techniques and connectivity are just a few of the technological improvements that can be leveraged by the industrial heating industry today. When combined with best operating and maintenance practices, these advancements can lead to substantial cost savings, greater productivity, enhanced product quality with fewer rejects and reduced downtime.
As with any industry, no one burner, control method or system is a magic wand. There are benefits and consequences to each. In the final analysis, the question is, what change, upgrade or improvement will help you to best achieve your goals?