By Ed Malone, Plant Engineer, Athens Generating Plant
A new type of mounting system for process transmitters is helping to improve control system maintenance at Athens Generating Plant’s combined-cycle power generation facility in Athens, N.Y. The system replaces traditional impulse line assemblies with an integrated manifold assembly that allows the plant’s instrumentation and control engineers to “clip” an instrument on or off in minutes, simplifying routine calibration and emergency changeover in the event of a failure.
Ed Malone, plant engineer at the Athens Generating station where two of the turbine HRSGs have been modified and re-equipped with close coupled instrument mounting solution, or CCIMS.
Athen’s Generating is a 1,080 MW combined-cycle generating facility that sells its electricity on a competitive basis and is positioned to boost general electric reliability in the mid-Hudson Valley, southeastern New York and New York City regions. The facility has three Siemens 501G combustion turbine generators. Because it is powered by natural gas combined-cycle technology, the plant delivers a much higher electricity generating efficiency than other older plants.
The three gas turbines are fitted with an HRSG (heat recovery steam generator) that converts heat from the exhaust into steam to drive a secondary steam turbine. Each HRSG has 12 differential pressure transmitters configured to provide a precision flow and level measurement system. The configuration ensures that if one instrument fails another reading can be used temporarily to ensure the turbine does not need to be shut down unnecessarily, as the reliability of operation is vital.
Athens Generating experienced a number of maintenance issues with its HRSG instrumentation and planned to upgrade the systems during each generator’s scheduled shutdown. Dating from the original construction layout when the plant first came online in 2003, many HRSG instruments were located in exposed and difficult-to-access positions. What’s more, they had inadequate heat tracing to deal with the area’s cold weather. This led to maintenance problems, including frozen media in impulse lines and instrument memory glitches that, on occasion, required urgent attention.
In the weeks leading to the scheduled shutdown of the first of three generators, Athens Generating’s distributor ACI Controls hosted a demonstration of a number of new fluid instrumentation components. Athens Generating expressed interest in an integrated instrument mounting and manifold system from Parker Instrumentation called CCIMS, or close coupled instrument mounting solution. It offered a means of interfacing differential pressure transmitters that avoided many of the usual problems with impulse lines. It also incorporated a “clip on/clip off” mechanism for the process transmitter.
As part of the planned HRSG upgrade, which included relocating the instrumentation into much more accessible positions, Athens Generating also decided to interface the instruments via CCIMS units. The primary benefit for the operator was the ability to swap an instrument in a few minutes for regular calibration tasks, thus allowing calibration to take place in the plant’s instrumentation workshop.
Previously, removing an instrument had involved dismantling tubing, a time-consuming process. Following the upgrades, a spare instrument can now be pre-mounted on the removable portion of CCIMS in the workshop, allowing the actual field changeover to be accomplished in minutes. The fast changeover facility also provides the plant’s instrumentation and control staff with additional security against any unforeseen control system problems, allowing an instrument to be replaced rapidly in the event of a failure.
The CCIMS units also helped to speed the instrumentation refurbishment for Athens Generating, as it eliminated the normal time-consuming process when installing a conventional hand-built impulse line arrangement. Instead, engineers were able to work on several aspects of the installation in parallel to help ensure the upgrades happened swiftly and efficiently.
So far, two of the turbine HRSGs have been modified and re-equipped with CCIMS. The third will be upgraded during a future shutdown.
Manifold suppliers have introduced a number of integrated mounting systems for process transmitters over the years. CCIMS not only solves the mounting problem of the pipework orifice plates (which can generate mechanical tolerance problems) but it also offers new features for the instrument engineer-side of the problem too, such as the clip-on mechanism. Athens Generating is using this latter feature to particularly good effect.
Parker Instrumentation’s CCIMS is an integrated mounting and manifold block that provides a direct connection of differential pressure transmitters to process pipework, eliminating long impulse lines. CCIMS provides a standardized, bolt-together connection to an orifice plate that assembles in minutes, typically replacing hand-crafted assemblies of discrete tubing, joint and valve components that can take many man-days to fabricate.
CCIMS integrates double block-and-bleed valve facilities in various configurations to suit different applications and also features a two-part construction that allows the instrument to be “clipped” on or off the pipe in seconds. With no need to undo threaded connections, tubing joints, or even welds, calibration and repairs can be completed rapidly.
Further gains are derived from the close coupled nature of the connection. A traditional connection, or “hook up,” for a differential pressure transmitter can involve 20 or more joints, every one of which is a potential leak path. CCIMS reduces this to five, an improvement in integrity that helps to avoid both the human and environmental safety issues caused by leakages or emissions.
Advanced Overfire Air/SNCR and Sorbent Injection System
By Lars Capener, Vice President Marketing, Nalco Mobotec
Many utilities must reduce nitrogen oxide (NOX) and sulfur dioxide (SO2) emissions from their boilers, but may not have the budget to install and operate large, complicated control systems, such as selective catalytic reduction control and wet-alkali scrubbing. A multi-pollutant control technology operates in-furnace without most of the extensive backend equipment in other scenarios. It is applicable to virtually all types of solid fuel-fired boilers including wall- and tangential fired, stokers, circulating fluidized-beds and boilers designed to co- or completely combust biomass. And an important and fundamental feature of the process is improved combustion dynamics, which in turn lowers particulate emissions and improves boiler efficiency.
Researchers have discovered over the last several decades that if primary combustion is performed at a slightly sub-stoichiometric ratio of oxygen to fuel, the reducing environment generated at the burner level will significantly inhibit NOX formation. The chemistry is complicated. But in a nutshell, the process allows many of the fuel-bound nitrogen atoms to combine with each other to produce N2 rather than NOX . This discovery led to the development of overfire air (OFA) techniques, where a portion of the incoming combustion air is taken off before the burners and is injected above the primary combustion zone.
Conventional OFA can successfully reduce NOX concentrations by perhaps 30 percent. A side result often is lowered combustion efficiency in the furnace. This can lead to higher unburned carbon in the fly ash and greater emissions of carbon monoxide (CO) from the stack. Rotating opposed fired air (ROFA), as shown in Figure 1, is an advanced overfire air technique that, like conventional OFA, reduces NOX by providing a fuel-rich initial combustion zone via diversion of a portion of the inlet combustion air to points above the burner level. The diverted air is given a pressure boost by an auxiliary fan and then is injected at locations determined by computational fluid dynamics (CFD). Individual CFD analyses are performed in advance for every steam generator retrofitted with the ROFA system.
The equipment has been installed on more than 50 industrial and power boilers worldwide, including more than 30 in the U.S., with a number of additional installations contracted this year. Many units have been in operation for five years or more. Results have shown that ROFA alone can reduce flue gas NOX concentrations by 40 percent to as much as 60 percent. Also of significant importance is that the enhanced mixing improves combustion efficiency such that unburned carbon concentrations and carbon monoxide levels of the fly ash and flue gas exiting the furnace are typically very low, often on the order of perhaps 5 percent and 20 parts-per-million by volume (ppmv), respectively.
A side benefit is a reduction in furnace hotspots, which otherwise sometimes lead to premature waterwall tube failures.
Supplemental NOX Control
Additional NOX reduction is possible with Rotamix, the patented process of ammonia or urea injection into the upper furnace. Rotamix is identical, chemistry-wise, to conventional selective non-catalytic reduction (SNCR), where ammonia (NH3) or urea [CO(NH2)2] and NOX react to produce elemental nitrogen and water as outlined by the following equations.
4NO + 4NH3 + O2 4N2 + 6H2O Eq. 1
2NO2 + 4NH3 + O2 3N2 + 6H2O Eq. 2
However, when Rotamix is combined with ROFA, ammonia or urea/NOX reaction efficiency is enhanced by the air/flue gas mixing generated by the ROFA system. Rotamix operates independently of ROFA, with its own chemical feed tank and pumps.
As with ROFA, the injection point locations are determined for each individual boiler by a team of experts. Rotamix has the potential to reduce NOX in half as compared to ROFA alone. In fact, in the example outlined in reference 1, the ROFA/Rotamix combination reduced full-load NOX on an 82 MW (gross) power boiler from 0.58 lb/MMBtu to 0.10 lb/MMBtu. The latter value is at the threshold of what more complicated and costly SCRs can achieve.
For decades, sulfur dioxide reduction technology in coal-fired boilers has been based upon reacting the SO2 with an alkaline material to produce a benign salt. For large units in particular this involves installing and operating a large, backend scrubber that typically employs limestone (CaCO3) or hydrated lime [Ca(OH)2] slurries to react with the SO2. Wet flue gas desulfurization (FGD) equipment is large, expensive and complicated to operate, although well-designed and operated scrubbers can do a great job at removing sulfur dioxide.
An extension of ROFA is the technique known as fine sorbent injection (FSI), which uses the overfire air system for enhanced mixing of either limestone or lime in the furnace. These two products calcine or dehydrate to quicklime (CaO), which then reacts with sulfur dioxide (SO2) to form a benign material of calcium sulfate (CaSO4).
CaO + SO2 + ½O2 CaSO4 Eq. 3
Components of an FSI system include a materials storage silo, solid materials feeders and a pneumatic conveying system to move the reagent to the furnace.
FSI is one process that may require some backend modifications. In many cases with simple coal combustion, an excellent device for removing flyash from the flue gas before it exits the stack is an electrostatic precipitator (ESP). However, sorbent injection to remove pollutants alters the quality of the flyash, sometimes adversely affecting ESP performance. The increasingly popular alternative to ESPs on units with sorbent injection (and particularly those using activated carbon for mercury removal) is the fabric filter device commonly known as a baghouse. These use thousands of bags to mechanically filter ash particles.
An advantage of a baghouse with the FSI system is that partially-reacted particles collect on the bags and continue to react with residual SO2 as it passes by. At plants equipped with FSI and a baghouse, more than 60 percent SO2 removal has been achieved. Work continues on increasing removal efficiencies to 90 percent without excessive reagent consumption.
The Mercury Issue
Currently, the leading technology for mercury removal from flue gas is activated carbon injection (ACI) into the boiler backpass after the air heater. Mercury adsorbs onto the carbon, which, in the simplest scenario, is removed along with the flyash in a baghouse. Alternative designs to separate the flyash from the mercury-impregnated carbon are available.
The ability of activated carbon to adsorb mercury appears to be in large part a function of whether the mercury exits the furnace in an elemental form (Hg0) or oxidized state (Hg+2). This speciation in turn is largely determined by the coal’s chlorine content. In the U.S. at least, eastern bituminous coals have more chlorine than western coals, such as those from the Powder River Basin, so Hg+2 is often the predominant species in bituminous-fired boilers. Thus, mercury removal in these units can be quite high with both activated carbon and non-carbon sorbents, as opposed to those boilers firing western coals. Regarding the latter, considerable research is underway to find cost-effective reagents that will oxidize the mercury in the flue gas to provide a more reactive product. Initial tests have shown that mercury removal by non-carbon sorbents may exceed 90 percent if the mercury is oxidized.
Where the ROFA process offers a significant advantage is the excellent mixing of the mercury removal agent with the flue gas as provided by ROFA. Among the processes being evaluated by company personnel via full-scale tests sre mercury removal by both carbon and non-carbon adsorbents, and evaluations of conditioning reagents that maximize mercury’s affinity for adsorption. Of particular interest is the effectiveness of non-carbon based sorbents that will not affect flyash quality or cause baghouse fires, as activated carbon can.
1. Coombs, K.A., Sr., Crilley, J.S., Shilling, M., and B. Higgins, “SCR Levels of NOX Reduction with ROFA and Rotamix (SNCR) at Dynegy’s Vermillion Power Station”; presented at the 2004 Stack Emissions Symposium, July 28-30, 2004, Clearwater Beach, Fla.
2. Haddad, E., Ralston, J., Green, G., and S. Castagnero, “Full-Scale Evaluation of a Multi-Pollutant Reduction Technology: SO2, Hg, and NOX ”; presented at the EPA/EPRI Mega-Symposium, 2003.
Multi-Element Water Strainer Technology
By Ed Sullivan, freelance writer
Bulk raw water users, such as PPL Electric Utilities, protect process and downstream equipment by selecting multi-element water strainer technology
Whether used for cooling or for the process itself, the raw water drawn from lakes, rivers and reservoirs must first be strained to create acceptable water for use. In many instances this means continuously straining tens of thousands of gallons of water per minute to remove dirt and debris that can wreak havoc on critical process systems and equipment.
In essence, the raw water strainers that accomplish this task are the first lines of defense for the entire plant’s system. Choosing an inadequate strainer can lead to high maintenance and operating costs, periods of insufficient water supply, damage to process equipment and expensive downtime. Worse, the straining media of an overwhelmed water strainer can rupture or collapse, permitting debris to compromise critical plant operating components. In the power industry clean water is crucial for a variety of tasks including extending the service life of turbine seals and protecting spray nozzles and heat transfer equipment.
Unfortunately, such failures are not unusual, particularly when the strainer design does not allow for sufficient straining surface area. In applications using raw water from rivers, single basket strainers sometimes become overwhelmed and clogged during periods of high debris volumes in the water due to seasonal conditions.
“You never really know what you’re going to experience with river water,” said Sang Partington, a senior engineer with PPL’s Generation Technical Group. During autumn and high water flow a power plant may have a lot of debris such as tree branches, leaves and other solids in the water. “Your water strainer has to be able to handle the solids and still maintain a continuous volume of water flow,” Partington said.
PPL upgraded raw water strainers at its Brunner Island and Montour plants
Manufacturers offer a variety of water strainer designs, including those that operate automatically. One of the more significant advances in strainer design occurred in the 1960s when the first multi-element, automatic self-cleaning strainer design was developed.
This strainer design was particularly significant because it provided a durable and reliable alternative to the classic basket-type strainer that may also be limited by its strainer surface area, which can become clogged and force excessive cleaning cycles (backwashing) and reduced water for process requirements.
By replacing the basket with multiple tubular elements, the design provides three to four times the straining surface area of a typical basket strainer. As a result, debris and solids, including from seasonal peaks, are efficiently removed without downtime. The increased surface area of the multi-element design allows for fewer backwashes. This equates with lower operational costs, less maintenance and greater overall efficiency.
Optimizing Water Flow at PPL
About five years ago, PPL’s Partington noticed that the old, basket-type water strainers at the Brunner Island plant required high maintenance. “The old system was constantly backwashing,” said Partington.
PPL began to upgrade the raw water strainers at its Brunner Island and Montour plants, which feed off the Susquehanna River in central Pennsylvania. Both are large generating facilities with approximately 1,500 MW of capacity.
According to Partington, the outflow of clean, filtered water through the strainers was also at lower-than-optimal volume when backwashing was taking place, so he began to look for a more advanced and efficient strainer technology. After reviewing several more advanced designs, Partington selected the multi-element strainer from R.P. Adams.
Although initially designed for raw water applications, the R. P. Adams multi-element strainer can remove solids as small as 25 microns. This means the multi-element strainer can be used as the “first line of defense” in water filtration, or it can be installed at a point of use for critical plant operations requiring fine levels of separation.
Another feature of the multi-element design is in the backwash mechanism’s engineering. With basket strainers, the backwash mechanism comes into direct contact with the straining media. This can be problematic as large, suspended solids often found in raw water can become lodged between the straining media and the backwash arm. The result may be straining media damage and even a rupture that can compromise plant operations.
The multi-element design uses a tube sheet to separate the straining media from the backwash mechanism. This prevents the backwash mechanism from coming into contact with the media and damaging the elements. And the greater surface area of the multi-element design means less frequent backwashing occurs so less water goes to waste, less power is consumed and less maintenance is required. Partington’s decision was also based on the fact that these strainers could provide the necessary plant water requirements even in backwash mode.
“The new units will not backwash unless the differential pressure of the strainer is high enough to activate backwashing automatically or by the timer, saving us money on the power consumption,” said Partington. “We should also save significant money on maintenance, too.” Exactly how much remains to be seen because the units are so new.
“The element exchange program allowed us to go for greater straining efficiency, which helped us optimize the raw water system,” Partington said. The decision to exchange the original elements for a smaller micron size has worked well, so the plant is planning to stay with that size for its particular installation. As the company continues to upgrade water strainers at various locations it can effectively fine-tune the solid removal and water flow as the situation warrants.
To date, PPL has upgraded to eight R. P. Adams strainers at the Brunner Island plant and has installed the first unit at its Montour site. PPL intends to standardize on the R. P. Adams design and will phase-in new strainers as opportunities arise.
Big Hoists Lift a Big Duct
Apollution control project at the Plant Scherer Power generating station in Juliette, Ga., required ingenuity, experience and knowhow. Not to mention some nifty lifting equipment. The duct work needed to be positioned within a congested area between the bag house and chimney. But there was no room in the area for the size and capabilities needed to use a conventional crane, said Michael Watson, BE&K Construction’s Heavy Rigging Superintendent.
The lift would be a four point pick. Duct sections ranged in weight from 150 tons with physical dimensions 125’ long X 45” wide and 28’ tall. The job’s second phase had sections that were up to 250 tons and with increased physical size as well. Fourteen sections in all were to be positioned over the 12-month duration of the job scope.
Chain hoists have a lift capacity of 110 tons per unit.
The chain hoists are pneumatic powered and have a lifting capacity of 110 tons per unit. The hoists have 85 feet of travel lift. Hoists were provided by Delta Rigging & Tools Inc. The hoists have an incredible size-to-weight lifting ratio, said James Kowalik President of Delta Rigging & Tools. Their compact size made these hoist the best choice for the project. The hoists run on compressed air and have a vane driven motor and spring applied disc brake which provides smooth operating characteristics. These Hoist function at 85psi and require a minimal 355CFM. Lift and lowering speed is variable and is extremely precise so keeping the 4 units in sync is easy.
Using the hoists for power plant work has become the preferred method by the riggers who plan these onetime lifts or for repeated maintenance work such as tube panel repalcment, pre-heater baskets and fan motor change outs.