Slagging is the formation of molten, partially fused or re-solidified deposits on furnace walls and other surfaces exposed to radiant heat. Over a period of time, a base deposit of slag may also form on boiler tubes. The base deposit may be initiated by the settling of fine ash particles or the gradual accumulation of particles with very low melting-point constituents. As the base deposit thickens, the temperature at its outside face increases significantly above the tube surface temperature. Eventually, the melting point of the ash constituents is exceeded and the deposit surface becomes molten. The process then becomes self-accelerating, with the molten slag trapping essentially all of the impinging ash particles.
Steam- or air-driven sootblowers are commonly used to remove ash and slag deposits from external tube surfaces, but their effectiveness varies. Also, sootblowing may cause localized erosion and corrosion in areas swept too clean by the blowing medium. This problem is often mitigated by installing shields on all tubes adjacent to sootblowers. However, sometimes the slag formation can’t be reached using conventional cleaning techniques.
Boiler Slag—It’s a Blast to Deal With
Slag removal can be a constant battle at many solid fuel-fired power plants. Conventional weapons include picks, jackhammers, shotguns fired through portholes, hydro-blasting, and CO2-blasting. In many boilers, sootblowers, and rapping systems only keep slag deposits in check until the next major outage, when more vigorous removal methods—such as explosives—can be applied.
But each of these methods is labor intensive, consumes substantial amounts of downtime, and may not dislodge severe deposits. In the l960s, a Midwestern plant superintendent, short of personnel because of a labor strike and a frustrated with seemingly immovable slagging, resorted to dynamite! In recent years, explosive deslagging has become more widely accepted as a state-of-the-art combat technique, and several hundred power plants throughout the U.S. now make use of it during annual outages.
Some plant owners worry that the industry has expanded too fast, and unqualified blasters are being allowed into the power plant. Although contractors must obtain a slew of licenses, permits, bonds, and certificates of competency, the regulations and tests are geared toward large-scale civil engineering jobs. Carving out roadways and tunnels, the veterans point out, does not ensure that a contractor can use controlled explosives near delicate boiler tubes.
Perhaps such concerns by the seasoned experts can be chalked up to competitive posturing, but clearly, using explosives to remove "Greyhound bus-size" clinkers is risky business. Extensive damage has been done to boiler refractory, tube bundles, even nearby plant buildings, when explosive deslagging was improperly performed. One Texas utility, for example, reportedly spent $5 million replacing boiler tubes when the efforts of a new, lower priced contractor went awry.
Explosives have been used in units ranging from packaged boilers up to 1,300-MW central stations. Early applications were limited to large clinkers that bridged the throat and blocked the bottom hoppers. After building their confidence in the bottom hoppers, contractors began to use blasting more selectively, moving up into the firebox and ultimately to all sections of tube bundles. Explosive deslagging of the boiler has even been performed online, with the unit turned down to low load. Check out these videos of actual power plant boiler deslagging operations: video 1, video 2, and video 3.
Outside the boiler, explosives have been used to clean such power plant components as scrubbers, fly ash silos, and precipitators. Every winter, trainloads of frozen coal are blasted to speed up the unloading process.
The technique isn’t limited to coal-fired plants. Crude oil-fired boilers, with vanadium-laden slag deposits, also have benefitted from explosive deslagging. So have recovery boilers at pulp and paper plants, which burn a concentrated black-liquor fuel capable of producing severe ash deposits.
Perhaps the largest application outside coal-fired power plants is in waste-fired systems. Waste-to-energy plants, which were built in increasing numbers in the 1970s and 1980s, create particularly tough slag deposits that often are impervious to conventional removal methods. Complicating the issue is the fact that the composition of waste-fired fly ash can vary greatly from plant to plant and from month to month, depending on the amount of paper, plastics, glass, metals, food scraps, and so on that constitute the waste fuel. For example, the average heating value of municipal solid waste has been steadily increasing over the past decade, and the moisture content has been decreasing, because of expanded recycling programs.
Explosive deslagging not only dislodges the severe deposits in waste-to-energy plants, but it also typically reduces them to a fine dust that can be easily swept away. In contrast, explosive deslagging of coal ash might still leave chunks of slag that must be carted off.
Long Fuse Necessary
Originally, explosive deslagging was performed with dynamite. Today, most contractors use binary explosives for the firebox areas and linear explosives—or detonation cords—for tube passes.
Binary explosives comprise two separate parts—a solid and a liquid—which are not explosive until mixed together. Because they are mixed on-site just prior to use, and because they require both heat and shock to ignite, most experts consider binary explosives to be safer to store, transport, and use than dynamite.
But dynamiters point out that contractors using binary compounds are, in essence, manufacturing explosives—an entirely different and more risky business than merely using them. Plus, the dynamiters say, binary explosives have more cutting power, and are therefore more likely to destroy boiler internals.
Firebox deslagging can be achieved without a worker even entering the boiler. Charges are placed on the end of a telescopic pole and inserted through inspection ports and manway doors. The charges initiate a concussion wave, sonic energy, and vibrations that together fracture the slag deposits and allow them to fall into ash extractors in manageable chunks. Firebox deslagging can begin while the boiler is still cooling down around 300F, which by itself cuts a substantial amount of plant downtime.
In tube-bundle regions, remotely placed binary or dynamite charges aren’t precise enough. Instead, detonation cords, filled with granular explosives in calculated concentrations, are draped throughout the gas path and are ignited in a sequence of time-delayed blasts. If set properly, the detonation cords effectively remove slag from the gas path, as well as from the horizontal surfaces between the tubes, with no tube damage.
Talking to the Plants
One Midwestern utility has been relying on explosive deslagging for more than 20 years. During each major outage, the technique is used on the firebox areas of its two 500-MW-class coal-fired units. The station formerly tried hydroblasting and shotgun-blasting, which adequately removed the slag but took an exorbitant amount of time. In contrast, one firebox was completely deslagged in just four hours using explosives.
A waste-to-energy facility located in the Northeast first used explosive deslagging in 1991, and it now relies on the technique almost exclusively. The facility uses the technique on all tube passes, as well as in the firebox area.
The plant’s operations manager reports that hand-poking and CO2-blasting were tried during the plant’s first two years of operation, but both failed to remove the heavy slagging deposits formed by the waste fuel. The explosives make a "clean sweep" of the entire boiler, he says, reducing the heavy slag to mere dust. The facility typically conducts a six- to eight-day annual outage, and only about 48 hours of it are taken up by the entire deslagging operation, including the removal of all dust. The third pass, which includes the primary superheater and evaporator sections, experiences the heaviest slagging, and requires about 75% of the deslagging time.
The plant reports that explosive deslagging has not damaged any boiler tubes at the facility, although some easily replaceable heat tiles have fallen off. The facility has also used explosives to deliberately dislodge damaged refractory, saving repair time and money.
Portions of this article first appeared in the March 1996 issue of POWER.
—Dr. Robert Peltier, PE is POWER’s editor-in-chief.