New boiler life with a new burner
Perhaps you’ve been unhappy with the performance of your boiler plant. It seems to be operating inefficiently, especially at part load. The controls may be sluggish, or your exhaust stack monitoring indicates incomplete combustion or excessive emissions. Is it time for a boiler replacement? Maybe a controls and burner replacement will solve the problem.
Is the boiler the problem?
In certain cases, boiler replacement is warranted. If you’ve had a lot of tube leaks or major corrosion problems, it’s probably time to replace. Or perhaps the boiler no longer meets your steam or hot water requirements. It might be too large or too small for current needs. It may be supplying steam at the wrong temperature or pressure. There’s not much you can do about that except replace it with the right equipment.
But boiler design is a mature technology and most boilers have a potential life of many decades if properly maintained. However there have been many recent advances in boiler burners and controls. By replacing an obsolete or deteriorated burner or burner control system, the life of a sound boiler can be extended by many years.
Achieving ideal fuel mixture
A report by the U.S. DOE describes the role of the burner. “A power burner mechanically mixes fuel and combustion air and injects the mixture into the combustion chamber. All power burners essentially provide complete combustion while maintaining flame stabilization over a range of firing rates. Different burners, however, require different amounts of excess air and have different turndown ratios.”
The report adds that an efficient natural gas burner requires only 10% to 15% excess air in the flue gas to burn fuel without forming excessive carbon monoxide. It states that most modern gas burners today exhibit turndown ratios of 10:1 or 12:1 with little or no loss in combustion efficiency. This is important because a higher turndown ratio reduces the number of burner starts and provides better boiler load control. It also reduces stresses on the boiler and burner and provides significant energy savings. New burners and controls can help you achieve that high turndown characteristic.
Certain cases are obvious
In some cases, older boilers that have been converted from oil firing to natural gas are still using features such as rotary cup burners. These are inefficient and often have emission levels that are unacceptable by modern standards. If the boiler has significant remaining life, these burners should almost always be replaced.
Another common situation is where the boiler burner is modulated using a mechanical linkage control. In some cases, there are also additional linkages for an oil valve (for dual fuel boilers) and for dampers for flue gas recirculation (FGR).
Linkage controls are sloppy
Mechanical linkage controls are problematical because they are difficult to keep in calibration, and under the best conditions inevitably have some slip. Even changing ambient temperatures can take the control out of calibration. In some cases, it’s practical to keep an existing burner and simply replace the linkage control with a modern parallel positioning control system, where individual servo motors precisely modulate the valves and dampers to maintain optimum fire conditions. In other cases, replacement of both the burner and control may be needed.
Roger Perstein is Vice President of Sales and Marketing for the Burner Systems Group of Cleaver Brooks, a major provider of boilers and burners. He points out, “Burner replacements frequently offer opportunities to improve the efficiency and/or reduce emissions.” He explains that modern controls technology allows far more precise combustion ratio control and greater responsiveness to season or operational variability in steam or hot water demand. “Modern burners can deliver as much as 5% to 10% in energy savings and up to an 80% reduction in emissions.”
Features of new systems
Perlstein describes the several features of modern burner and control systems that improve efficiency and reduce emission. “The big ones are parallel positioning [using servomotor drives], flue gas recirculation, [increased] turndown and staged combustion. The government refers to this group of features as Maximum Achievable Control Technology (MACT) and they enable higher efficiency. These advances can also have a positive impact on emissions. And they have the potential to pay for the burner upgrade purely from fuel and energy cost reductions.”
Burner and control replacement is usually done at a time of low steam or hot water demand and can take from a little as 2-3 days to as long as several weeks. If it is a plant with multiple boilers, having one out of service may be no problem. In some cases, it may be necessary to rent a skid-mounted or trailer-mounted temporary boiler while the regular boiler is out of service.
Remote monitoring and alarming
Another feature that is available in new burners and control is remote monitoring, including internet access technology. Controls can monitor for “out of tune” conditions and can send alerts for undesirable conditions. They can also provide logging for flue gas monitoring, boiler demand history, high flue temperature or many other conditions.
Smaller boilers can benefit too
The advantages of updating boiler burners and controls are not limited to large industrial or institutional boilers: Smaller boilers –1,000 bhp and less – can also benefit. Companies like Carlin Combustion Technology offer solutions for these smaller boilers. Carlin specializes in replacement burners for residential, commercial and light industrial boilers. Facilities like clinics, retail stores, schools, bakeries, churches and many others need a reliable and efficient source of steam or hot water. If properly maintained, their older boilers may be great candidates for burner replacement.
According to Carlin spokesperson Jim Jones, burners for smaller boilers have made great strides since 2000. Improvements have included linkageless controls, step-firing options and much more precise fuel metering and air damper management. Jones indicates the first step is an assessment of the current boiler and controls. “If that boiler has been maintained well, it may have a long life into the future.”
He explains that many older boilers –both cast iron and steel — were very stout machines, with rugged designs that give them a long life. “This is where burner replacement makes a lot of sense.” Jones suggests getting help from a local or regional boiler specialist first to do the assessment, and if appropriate, to recommend type and size of the replacement burner. “Size is important. Too often the burner is oversized, which leads to a lot of stop-and start-operations. Correct burner sizing and step-firing can eliminate much of this.”
Evaluate the potential
Regardless of the size of the boiler, if your unit has older controls, particularly if it is an oil conversion or has mechanical linkages, now is a good time to consider a burner or control upgrade. Perlstein from Cleaver Brooks agrees that the evaluation of the boiler and burner be done by conducting an operating efficiency and emissions audit.
He notes, “This should be done by a qualified service company that has access to emissions gas analysis equipment. They must take fuel savings and energy savings into account in determining return on investment. Also, carbon taxes should be factored in since lowering emissions can also reduce operating costs.” Every application is different and it is impossible to project potential savings without this kind of analysis. But these are steps worth investigating for any operator of older boiler equipment.