Keep the boiler, replace the burner
Today’s efficient natural gas boiler is an amazing blend of traditional heat exchange technology and sophisticated design improvements. With metallurgical improvements and computer-aided design tools, the boiler structure itself has been made more efficient and reliable. But the most striking advancements have been made with the burners and their associated controls. In many cases, a sound older boiler can be dramatically improved in efficiency and turndown capability by replacing an obsolete burner and control system. Further, taking this step can reduce boiler emissions.
Opportunity for Savings
According to the DOE, there are more than 45,000 industrial and commercial boilers larger than 10 MMBtu/hr in the United States with a total fuel input capacity of 2.7 million MMBtu/hr. A report by DOE’s Federal Energy Management Program states, “Efficiency of existing boilers can be improved in three ways; replacement with new boilers, replacement of the burner, or installation of a combustion control system. While installation of a new boiler or replacement of the burner can lead to the greatest efficiency gains, the higher costs associated with these measures typically leads to longer payback periods than installing a combustion control system.”
The report notes that many combustion control systems lack continuous oxygen, carbon monoxide and nitrogen oxide emissions monitoring capability as well as the automated controls needed to improve fuel performance. This prevents continuous adjustments of the fuel-to-air mixture to maximize efficiency while controlling operation to ensure emissions levels meet regulatory requirements.”
Linkage Slop Hurts Efficiency
Older boilers use mechanical sensors and linkages to adjust fuel and air flow to the burner. Commonly, these don’t allow a turndown to much more than 35% load. For steam or hot water demands below that, the boiler needs to cycle. With the required purges and reheat cycles, this can be highly inefficient.
Further, where the fuel-air mixture is controlled by mechanical linkages, the response is slow and often inaccurate. The system needs to be regularly checked and rebalanced to account for changes in the linkage controls (linkage joints can wear and set screws can loosen). Alan Silver from SCC Inc. (Siemens) was recently a presenter at a Technology & Market Assessment Forum sponsored by the Energy Solutions Center.
Silver notes, “Eliminating mechanical linkages minimizes the “hysteresis” associated with the burner operation. Hysteresis, or “linkage slop” as it is commonly referred to, can cause burners to operate in a rich condition or a lean condition or both. These conditions impose inefficiencies in fuel consumption.” As the controls incur mechanical wear, hysteresis increases, even where the control is calibrated as closely as possible.
Parallel Positioning Advantages
He explains, “Reducing this hysteresis allows the burner technician to set the burner operating condition closest to its design, thus maximizing efficiency.” Siemens, like many other current control providers, uses a parallel positioning system for boiler firing control. With this feature, a positioning control mechanism, usually servo activated, moves to a preset position in response to system needs. The system uses a master control signal to control the boiler system, causing a change in both the flow of fuel and air when there is a needed change in steam flow.
Silver notes, “Parallel positioning systems typically allow for greatly increased repeatability and much less hysteresis than linkage-based systems due to direct coupling of the actuators with the drive motors. A typical parallel positioning system will have 900 positions of movement through 90 degrees of rotation.” With this level of precision, getting an ideal match of fuel and air for the required steam load is possible.
Stack Oxygen Monitoring
Another aspect of newer control system is their adaptability to stack oxygen monitoring. This allows for digital feedback of stack oxygen levels, allowing oxygen trim for optimum combustion even with changing atmospheric conditions or fuel pressure variations. Silver explains, “This is similar to what happens in newer automobiles with oxygen sensors in the vehicle exhaust.”
With newer controls that have parallel positioning features and exhaust oxygen feedback, it is possible to achieve stable lower firing levels. This allows greater turndown ratios and reduced need for boiler cycling. This in turn promotes energy savings and reduced emissions.
Getting the Flue Gas Right
Cleaver Brooks offers a broad line of both boilers and boiler burners with digital controls. According to Rakesh Zala from that company, flue gas monitoring is important for burner efficiency, and accurate oxygen monitoring is critical. “A lot of the burner controls now include an O2 sensor. Typically, the sensor requires periodic calibration. Control systems are sometimes designed to perform automatic calibration. Check with your control system provider to confirm this.”
Most boilers have a “sweet spot”, where they operate most efficiently. Especially if your plant has multiple boilers, it is valuable to stage them to keep as many as possible operating at peak efficiency. Modern controls with stack gas monitoring can help you find that sweet spot and hold the boiler in that area. Zala says, “Typically steam boilers operate at peak efficiency when firing at 60%-80% rate. Condensing hot water boilers operate at peak efficiency when at lower firing rates.”
Range of Retrofit Options
Cleaver Brooks and other manufacturers offer retrofit digital control packages for boilers as well as other control options. In addition to the boiler burner controls, owners often choose to install packaged boiler water controls to monitor and maintain condition of feedwater and makeup water. Another important offering is master boiler controls. These tie together individual boilers and other boiler room equipment for optimum operation together. These controls can be remotely located, allowing regular adjustment and observation of boiler operations without a trip to the boiler room.
Paybacks Can Be Short
Because boilers are such significant users of energy, the payback for a boiler burner/control upgrade can be short – sometimes less than a year. If the boiler structure is sound but the controls predate the digital control age or are otherwise inadequate, it will pay to look into a control or burner upgrade. Be sure to include stack gas monitoring and oxygen trim if available. It could be a brilliant investment.
– This story appeared in the Summer 2013 Gas & Technology supplement. See additional stories below.