MECHANICAL
CFB refractory repair
Refractory, the heat-resistant materials that line the inside of most furnaces, are an important component of any steam generator because they protect expensive and vital parts of the generator from thermal shock, caustic chemicals, erosion from slag buildup, damage from falling slag, and catalytic heat. (See "Understanding Refractory Failures," p. 48, for an in-depth look at these phenomena.) Most refractories are more heat-resistant than metals and are needed in parts of the plant where temperatures exceed 1,000F.
According to the Refractories Institute, there are a wide variety of refractory compositions, but they fall into only two categories: brick and fired shapes, and monolithic refractories typically fabricated from plastics, castables, gunning mixes, or ramming mixes. Combinations of both types also are in use.
Although refractories may look like ordinary construction bricks, they are anything but. Refractory designs can have very complex or unusual geometries, and the finished products run the size gamut. Refractory is typically produced from combinations of compounds such as alumina, fireclays, bauxite, chromite, dolomite, magnesite, silicon carbide, and zirconia.
CFBC crucibles
ACC Refractories (Nagpur, Maharashtra), which has provided the refractories of 13 of the 20 circulating fluidized-bed combustion (CFBC) plants in India, notes that CFBC plants are good test sites for analyzing refractory performance because of their high temperatures and caustic chemical environment.
CFBC boilers have had great success in Europe and Japan because of their ability to burn low-grade fuels such as peat, wood waste, and pet coke, as well as hazardous wastes, very efficiently. NOx and SO2 emissions from CFBC plants are low due to the high operating temperature of the boiler. And although the high turbulence of the fluidized bed and the long residence time of coal in the cyclone improve the combustion efficiency of these plants, the high heat and caustic chemicals that are part of the CFBC process can cause refractories to be damaged.
Most of the fuel used in India for power generation is coal, lignite, or pet coke of highly variable quality. In particular, sulfur content can be all over the lot, making it very difficult to adjust feeder concentrations of limestone needed to counter the chemical. If levels of corrosive sulfur oxide gases generated are not kept in check, they can wreak havoc on refractory.
In harsh environments such as these, it is not unusual for refractories to be damaged beyond repair in less than a year. Based on ACC's experience, the most likely damage in CFBC plants will occur in the bullnose, the kick-off area of the combustor, and the loop seal (Figures 1 to 3). In the bullnose and combustor kick-off areas, the damage is typically due to abrasion; in the loop seal area, the cause is usually thermal shock.
1. Used and abused. This cyclone bullnose refractory in a circulating fluidized-bed combustor boiler has been significantly damaged.
Courtesy: ACC Refractories
2. Worn out. The refractory lining in this combustor kick-off area has experienced considerable erosion due to slag attack, abrasion, and thermal shock.
Courtesy: ACC Refractories
3. Unsealed. The loop seal in this CFBC boiler was damaged by thermal shock. The rapid heating and cooling inherent in CFBC boilers can be very hard on refractories.
Courtesy: ACC Refractories
Damaged goods
The usual culprit in CFBC refractory failures is calcium sulfate, which ACC has found to be the common denominator in all failed castable matrices it has analyzed. Apparently, sulfur prefers to attack the lime-bearing constituents of the castable matrix, which in turn reduces abrasion resistance and increases refractory wear and tear. The table shows which CFBC boiler refractory sections are affected, and why.
Source: ACC Refractories
What causes the damage? Thermal shock—resulting from a rapid change in refractory temperature—can be a big problem in the combustor and loop seal areas of CFBC plants. The area most susceptible to mechanical erosion is the cyclone inlet, where particles may be moving at 80 ft/sec or more and hit the refractory head-on.
Plastic to the rescue
To solve the problem, many CFB plants in India have tried a specially designed, phosphate-bonded aluminous plastic called Accplast 80, and the results have been excellent. Accplast 80 has better physical and thermal properties than typical refractory, and its chemical bonding deters slag from sticking to it. As a result, the refractory resists when slag is dislodged. In addition, Accplast 80 lasts significantly longer—sometimes as much as twice as long—than other refractory materials (Figure 4).
4. Tougher than plastic. Accplast 80—a specially designed, phosphate-bonded aluminous plastic—was used in this resuction duct. After more than two years of service, it is still in excellent condition, showing few signs of wear.
Courtesy: ACC Refractories
"The first thing we had to do was extend the unit's foundation by 70 inches on both ends to make room for the clutches," says Ron Haglind, a project manager for Great River. "We could do that while the unit was still on-line, since the work didn't affect the unit's operation."
The foundation work was completed two weeks before the outage began. Once the turbines were taken off-line, GRE removed the generator enclosure, the exhaust stacks, and the turbines themselves and set them off to the side. Next, engineers reworked the existing foundations to accommodate the clutches and the new turbine positions, including moving all anchor bolts. After installing the clutches, they moved the turbines to their new locations along with the enclosure and the stacks. Aligning the turbines took a few days each.
Most of the outage went according to schedule—except for one unexpected issue at the 26-year-old plant.
"When we took the stacks off, we found there was a lot of corrosion on their metal, so we had to make extensive repairs," says Haglind. "That was a big surprise." Despite the setback, the unit was back on-line only a day behind schedule. Domyahn recommends that anyone doing a similar upgrade inspect the exhaust stacks and inlets ahead of time to avoid any surprises.
Quick payback
With the clutches in place, GRE now uses the turbines to bring the generator up to speed and synchronize it to the grid. At that point, fuel to the turbines is cut off and the clutches disconnect the turbines from the generator. The generator then draws power from the grid to keep turning, meeting the utility's requirements for spinning reserve. Because the turbines no longer spin 18 hours a day on standby, the TwinPac's power consumption has dropped by 85%. When peaking power is needed, the turbines fire back up. Once they match speed with the already synchronized generator, the clutches engage and the generator starts providing the needed power.
"By installing the clutches, we took what used to be a seldom-used peaking plant and turned it into a baseload spinning reserve plant that lets us sell an extra 50 MW of very cheap coal-fired electricity," says Domyahn. "The payback has been phenomenal."
However, it isn't easy to install any phosphate-bonded plastic such as Accplast 80 on a CFBC boiler. One of the biggest challenges is keeping the material in position until it is hardened by heating to 572F. Until it reaches that temperature, Accplast 80 can lose its shape and slump. To prevent that from happening, ACC Refractories recommends holding it in place with ceramic anchors until it hardens.
Following are several other steps that should be taken when installing plastic refractories:
- Provide retainers at predetermined intervals, especially in sloped areas.
- Ensure that any spaces between ceramic anchors are completely filled.
- Equally space the ceramic anchors, use wooden wedges to give them a tight fit, and properly seat the anchors on the support collar to avoid point loading.
- Replace any misaligned anchors.
- Completely fill the shuttering with plastic before placing the next set of forms.
- Remove the shuttering piece by piece after checking for slumping.
- Remember to remove any wooden forms before reheating the boiler.
- Re-ram the plastic after removing the shuttering to ensure that it is properly compacted around anchors.
An ounce of prevention
Attention to maintenance increases the reliability of most power plant equipment, and refractories are no different. During shutdowns, conduct a walk-through and inspect every area protected by refractory. Don't be troubled by the small gaps you may note around refractory panels; they provide for expansion during reheating.
Accordingly, there's no need to fill these gaps unless they have widened since the last shutdown. If the gaps have expanded, they should be packed with ceramic fiber to allow for compression during unit startup. Also note gaps around refractory panels that seem too narrow. If they have become packed with ash and/or bed material, the lining may be unable to provide for sufficient thermal expansion.
Finally, check refractory for diagonal cracks that can foster erosion of the material. If cracks have developed and need to be fixed, consider taking the following steps:
- Replace any anchors that have deformed or oxidized.
- Always use at least two anchors per piece of dislodged refractory.
- Remove any old material and replace it with fresh monolithic material. Install this new material without shuttering.
- To repair refractory, try using Accplast 80.
- During every shutdown, clean the expansion joints with compressed air and fill them with a ceramic blanket or wool.
—Contributed by C. Sur, A. Nagar, D.K. Singh, and I.N. Chakraborty, ACC Refractories, India
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