Successes, Challenges, and Lessons Learned
By Joseph Klobucar, Brian Barth, Jeff Hoefer and Robert Newell
|The Columbia Energy Center near Pardeeville, Wisconsin.|
Interstate Power and Light Company and Wisconsin Power and Light Company, subsidiaries of Alliant Energy Corporation, along with their owner’s engineer, HDR Inc. (Omaha NE), have recently completed construction of air quality control systems (AQCS) on two major coal-fired generating stations in Iowa and Wisconsin. Both of these award-winning projects included rotary-atomized spray dryer (SDA) for SO2 control, pulse-jet fabric filter (PJFF) baghouses for particulate control, powdered activated carbon (PAC) injection for mercury control, draft system modifications, and associated balance of plant equipment.
While the same AQCS original equipment manufacturer (OEM) designed and supplied the major equipment for each project, the site configurations and additional work being performed at each site differed substantially.
This article summarizes the successes, challenges, and lessons learned in completing these two major projects. Successful system implementation, including performance, will be discussed in the context of project goals. The challenges associated with the systems will be discussed and the successful resolution of challenges will be presented. The lessons learned during project execution, commissioning, and commercial operation will be presented.
COLUMBIA ENERGY CENTER
The Columbia Energy Center (Columbia) is located near Pardeeville, Wisconsin.
Columbia Units 1 and 2 began operation in 1975 and 1978 and have nameplate generation capacities of 512 and 511 MW, respectively. The Units are jointly owned by Wisconsin Power and Light Company (WPL), Wisconsin Public Service Corporation (WPS), and Madison Gas and Electric (MGE). The Units are operated by WPL.
The units burn sub-bituminous Powder River Basin (PRB) coal from various mines. The Units use electrostatic precipitators (ESPs) to collect particulate matter (PM) or flyash. Unit 1 operates with a hot-side ESP and Unit 2 operates a cold-side ESP. Both Units operate low-NOx burners and overfire air combustion technology to reduce emissions of nitrogen oxides (NOx).
An ACI system was installed on Unit 2, upstream of the existing ESP in 2008 for mercury removal.
Alliant Energy has been moving toward a cleaner energy future by installing cost-effective environmental controls and utilizing state-of-the-art technology at Columbia.
These investments are designed to improve air quality and provide customers with competitive and reliable power. This emissions reduction project is referred to hereafter as the AQCS project and includes the following:
- Installation of dry FGD systems on both Units. Specifically, two SDA vessels and a downstream PJFF were installed on each unit (four SDA vessels and two PJFF in total) for SO2 and PM emissions reduction.
- Expansion of the existing ACI system to Unit 1 to provide carbon injection upstream of the new PJFF and SDA. The PJFF will capture spray dryer solids, residual flyash, and activated carbon, including bound mercury.
- Modification of the previously installed ACI system at Unit 2 to relocate the carbon injection point downstream of the existing ESP. Moving this injection point allows the plant to maintain beneficial reuse of the flyash.
The emissions reduction project included the following auxiliary equipment:
- Lime storage and lime slurry preparation equipment (common to Units 1 and 2 dry FGD)
- Activated carbon storage and handling (modification to the Unit 2 ACI system)
- Induced draft booster fans
- SDA byproduct material handling and recycle equipment
- Associated ductwork
- A new controls building and new electrical equipment.
Emission Reduction Performance
The following emission reduction goals for the unit were set forth at the start of the project.
At the conclusion of the project, an extensive performance testing procedure was undertaken to verify that all emission reduction goals were met.
In all cases, the emission reduction goals were exceeded by a wide margin. In addition, process performance guarantees for reliability, lime consumption, pressure drop, PAC consumption, water consumption, and auxiliary power consumption were verified during this testing and found to be well within the guarantees made.
The construction of the AQCS system included expenditure of over 1.9 million man-hours of craft labor on site. This major construction effort was accomplished with no lost-time injuries. This level of performance is extraordinary when compared to the OSHA reported average lost-time rate for 2014 of 1.1 incidents per 200,000 man-hours.
In general, the project execution proceeded smoothly with no major disruptions.
Unit 2 Tie-In During Extreme Winter of 2013-2014
The tie-in outages for the systems occurred in mid-January through the end of February 2014 for unit 2 and in mid-April through the end of May for unit 1. According to NOAA, the winter of 2013-2014 was one of the coldest on record for Wisconsin with the period of January/February 2014 being the second coldest on record for parts of the state. Despite this severe weather, the unit 2 tie-in outage was completed on time and with no significant safety incidents.
SDA Byproduct Bin Vent Filter Damage
During commissioning, a relief valve on one SDA byproducts waste storage silo was frozen shut while the material handling system was being tested. This resulted in a build-up of pressure in the silo. The excess pressure deformed the casing of the bin vent filter for the silo. There was no damage to the silo proper or other equipment. The bin vent filter was replaced and there was no significant impact to project schedule as the other silo was available for use.
Pebble lime was used as a reagent for this project in truck-load quantities. The lime reagent was received in pneumatic trailers and these trailers were unloaded into a pair of lime silos for use in the process. Because of the height of these silos (top height 129′ above grade), the on-truck blowers were not capable of unloading the trucks in acceptable time. Therefore a system was designed with external blowers to assist the on-truck blowers in transporting the lime into the silos.
The procedure that was initially used for this lime unloading was to transport the lime out of the truck using the on-truck blower to inject the transported material into the line leading to the silo while an external blower added additional transport air into the same line. While this method was capable of the required unload time, it resulted in excessive velocity in the transport line (owing to the fact that two blowers were used, the on-truck blower and the fixed blower). This excessive transport velocity resulted in excessive breakdown of the pebble lime particles leading to handling difficulty downstream. It also resulted in accelerated wear of the transport line, resulting in several premature failures of elbows in this line.
The procedure was reviewed after the difficulties were identified and it was revised so that only the fixed transport blower was used both for truck evacuation and transport of the material to the silo. Since the revised procedure was implemented, lime breakdown and accelerated wear problems have abated.
SDA Byproduct Silo Unload Building Dusting
SDA byproduct material from the process was pneumatically transported to a pair of silos. From there, it was conditioned by the addition of water and loaded into trucks for on-site landfill disposal. Because of concerns about fugitive dust emissions, a fully enclosed truck loading facility was provided as part of this system. This facility included roll-up doors for truck entry and was intended to be operated with the doors closed to prevent fugitive emissions of dust from the truck-loading process.
Shortly after start-up, it was recognized that if the system was operated as designed, the levels of dust and steam generated by the process were in excess of the capacity of the HVAC system. Experimentation with various setting and procedures for operating this process were not effective in remedying these problems. Eventually, the HVAC system was re-designed with the exhaust from the system ducted to the PJFF inlet. This largely resolved the issues and enabled operation of the loading facility as designed.
During winter months after tie-in of the system, some ice accumulation at the stack outlets occurred. Pieces of this ice broke free from the stack tops and cause some damage to nearby ductwork cladding. The exact mechanism of this icing is not well understood as there are numerous variables in play, including process flow rate, ambient temperature, wind speed, and humidity, however, it was determined that a solution was required to prevent future recurrence. As a result, electric heaters were installed at the top of each stack to prevent ice from forming and adhering to the stack caps. This modification was completed in the spring of 2016 and will be monitored for effectiveness during upcoming cold weather.
|The Ottumwa Generating Station near Chillicothe, Iowa.|
Fouling of Atomizer Gearbox Coolers
The SDA process uses high-speed rotary atomizers to achieve proper atomization of the feed slurry into the flue gas and to achieve the emission performance required. The system uses two atomizers per unit with each atomizer driven by a 1000 hp medium voltage electric motor. A drive gearbox is required to achieve the proper atomizer rotation speed. This drive is equipped with integral oil cooler that uses a thermostatically controlled mechanism to maintain the oil temperature within the required range. The oil is cooled by a shell-and-tube heat exchanger that is cooled (on the tube side) by process water from the plant. Process water for the plant is sourced from Lake Columbia, a man-made body of water that is fed from the Wisconsin River and used primarily for plant cooling.
After start of operations, it was found that the water (tube) side of the atomizer cooler was becoming fouled. This fouling resulted in an excessive need to clean the atomizer coolers, with cleaning required on a nearly monthly basis during some periods. It was believed that the fouling was primarily caused by biological growth in the cooler. A chlorination system was added to the service water. Since the chlorination system was started-up in 2015, no fouling issues have been encountered.
Spray Dryer Motor and Spindle Vibration
Upon initial startup of the system, the SDA rotary atomizers experienced motor trips on high vibration with unusually high frequency. These trips were found to happen as often as 50 times per month. Following instructions from the equipment OEM, each atomizer was removed from service and replaced with a spare after three motor trips. This was causing the owner excessive labor for atomizer maintenance.
The OEM made various efforts to resolve this issue including re-piping of the slurry feed system, but their efforts were not effective. Eventually, two changes were made which were largely effective in resolving these issues. First, the vibration monitoring software was modified through the installation of a filtering algorythem. Second, the trip logic was changed so that instead of tripping the medium voltage motor on high vibration, the slurry feed would be tripped. This resulted in the motor and atomizer continuing to rotate through a trip and the slurry feed being shut-off. With this arrangement, it was found that the slurry feed could be re-established after a water flush and this was sufficient to resolve many trip events.
Recycle Slurry Tank Material Accumulation
Shortly after tie-in, an agitator failed in the recycle slurry mix tank. This failure was investigated, including sending slurry samples out to an agitator manufacturer for testing. Based on the result of this testing, it was determined that an improper agitator design was the root cause of the failure. The agitator was not adequate to maintain suspension of solids in the recycle slurry tank. As a result, the impellor, motor, and tank-baffles were replaced on both recycle slurry tanks. This repair work required a forced outage of one unit to accomplish. The repairs on the other unit were coordinated to coincide with a planned plant outage.
OTTUMWA GENERATING STATION
The Ottumwa Generating Station (Ottumwa) is located near Chillicothe, Iowa. Ottumwa Unit 1 began operation in 1981 and has a nameplate generation capacity of 726 MWg. The Unit is jointly owned by Interstate Power and Light (IPL) and Mid-American Energy. The Unit is operated by IPL.
The Unit burns sub-bituminous Powder River Basin (PRB) coal from various mines. The Unit uses hot-side ESPs to collect PM. The unit operates low-NOx burners and overfire air combustion technology to reduce emissions of nitrogen oxides (NOx).
New AQCS System
Alliant Energy has been moving toward a cleaner energy future by installing cost-effective environmental controls and utilizing state-of-the-art technology at Ottumwa. These investments are designed to improve air quality and provide customers with competitive and reliable power. This emissions reduction project is referred to hereafter as the AQCS project includes the following:
- Installation of a dry FGD system. Specifically, two SDA vessels and downstream PJFFs (two SDA vessels and two PJFF in total) for SO2 and PM emissions reduction.
- Installation of an ACI system to remove mercury from the flue gas.
The emissions reduction project included the following auxiliary equipment:
- Lime storage and lime slurry preparation equipment
- Activated carbon storage and handling
- New induced draft fans
- SDA byproduct handling and recycle equipment
- Associated ductwork
- A new controls building and new electrical equipment.
The AQCS system was designed for extended operation both with and without ESP in service. This dual-mode capability allows the operators flexibility to either use the ESP to gain beneficial use of flyash sales or shut down the ESP to avoid maintenance cost and power consumption.
Efficiency / Output Improvements
In addition to the AQCS project, efficiency and output improvements were concurrently implemented at the plant. These improvements included a new turbine, new high pressure heater and added a convection surface area to the boiler (reheat and economizer).
Emission Reduction Performance
The following emission reduction goals were set forth at the start of the project.
At the conclusion of the project, extensive performance testing was undertaken to verify that all emission reduction goals were met.
In all cases, the emission performance goals were exceeded by a wide margin. In addition, process performance guarantees for reliability, lime consumption, pressure drop, PAC consumption, water consumption, and auxiliary power consumption were verified during this testing and found to be within the guarantees made.
Plant Efficiency/Output Improvements
Modifications were also made to the plant to improve output and efficiency. These modifications had the goals as stated:
After the completion of the project, testing was conducted to verify that these performance gains were realized. The results found that all performance goals were exceeded by the new system.
Turbine/Boiler Work Coordination
Implementation of the efficiency and output improvements during the same outage as the AQCS project resulted in several challenges to the project. First, the AQCS system needed to be designed to accommodate the changes to flue gas condition of the revised boiler system. The efficiency / output improvement projects resulted in a flue gas flow that was approximately 25% higher and 25 to 50 degrees cooler that the pre-modification conditions. This was accommodated by careful design of the new AQCS system.
A second challenge of this project was to coordinate the tie-in (fall of 2014) and commissioning of the new AQCS system, the new turbine, and major boiler work during the same outage. This meant that major work was being performed on the boiler, turbine/generator, and AQCS system simultaneously before and during the outage.
This challenge was successfully handled by thorough planning and careful execution.
Byproduct Handling System Plugging
The SDA process causes the reaction of SO2 with pebble lime to produce SDA byproducts which becomes mixed with flyash from the boiler in the PJFF. This mixed byproduct material is transported by a pneumatic transport system and either used for recycle into the process (increasing the efficiency of lime utilization) or sent to a silo for disposal. Plugging of the pneumatic transport system was encountered.
This required changes to baghouse discharge valves and transport switching valves to a design that was less prone to pluggage. In addition, rubber elbows were installed in place of hard pipe elbows at several points in the transport system. These rubber elbows are capable of some flexing that allows accumulated material to break-away during normal operation. These modifications were found to substantially eliminate buildup problems in the pneumatic transport system and increase transport capability.
Recycle Silo Plugging
The system design incorporated a recycle injection system to increase lime utilization.
In this system, byproduct material is retained in a separate silo and mixed with water to create slurry. This slurry is metered to the atomizers along with fresh lime slurry to accomplish the SO2 reduction requirements. This enabled use of residual lime and alkalinity in the byproduct material to reduce the amount of fresh lime required, increasing system lime utilization efficiency.
Shortly after the project tie-in (during tuning) the entire AQCS system was voluntarily shut down for several weeks for the year-end holidays. During this time, byproduct material was retained in the recycle silo and continued to be fluidized.
At the conclusion of this period, it was found that this byproduct material could not be withdrawn from the silo.
Further investigation determined that the byproduct material had solidified in the silo.
The byproduct material also was found to have entered an auto-thermal event in some areas, reaching temperatures high enough to burn the paint off the silo walls and sinter some byproduct material in the silo.
Material had to be manually removed from the silo, a time consuming and expensive process.
An investigation of this event determined that because the byproduct silo was not insulated, condensation of water in the silo occurred during the winter down-time which lead to the solidification and most likely initiated the auto-thermal reaction that damaged the silo paint.
The remedy for this problem was to insulate the silo and adopt operating procedures to prevent retention of non-moving byproduct material in the silo for extended periods.
These solutions were implemented and no repeat events of this type have been encountered.
Byproduct Handling System Issues
During commissioning, a number of problems were encountered with the byproduct handing system. The byproduct handing system was a vacuum type system with transport blowers and collectors pulling byproduct to the destination. After start-up, a vacuum blower failed in service prompting a thorough inspection of the system.
With this inspection, it was found that bags and cages had failed in some filter separators leading to dust in the clean air plenums and blowers. A root cause investigation revealed a number of contributing causes, including improper setup, failed pulse valves, failed equalizer valves, incorrect level switches, and some bag fit-up issues.
All of the issues were corrected and the system has operated normally since that time.
Fugitive Dust at SDA Flop Gates
At the bottom if each SDA vessel there is a “flop gate.” This is a simple gravity valve that allows any solid material that accumulates in the SDA vessel to exit under the influence of gravity. Below each flop gate is concrete bunker with a roll up door to contain any material produced. Experience has shown two problems with this arrangement.
First, there is no way to lock the flop gate so when working in this bunker solids can emerge from the SDA and present a hazard to workers in this vicinity. Second, the enclosure is too small to accommodate a truck, so loading has to be done by withdrawing material by loader and loading a truck external to the bunker.
Because this material can be dry and friable, some dust can be generated in this process, creating a potential fugitive dust emission problem. A fix or work-around for these situations is under development at the current time.
After start-up, it was found that access to the unit was not satisfactory for service of the systems and equipment. Among the issues noted were a) the system included a single elevator and if the elevator was down for service, long stair climbs were required to the atomizers and related equipment on the top of the SDA and b) the top of the lime and recycle silos were accessible by stairway only. As a remedy to this problem, access platforms connecting the top of the SDA to the top of the silos were added. In addition, access platforms connecting the existing ESP elevator to the SDA platforms were added so some redundancy now exists on the elevator.
Slurry Feed Line Failures
During operation, some slurry transport elbows failed resulting in spillage of significant quantities of slurry. These failures were related to rubber elbows and connections incorporated into the transport lines and excessive flexing of some of the lines as a result of their pipe support design.
As a result of these spills, the lines were analyzed and most of the rubber elbows were replaced with steel elbows. In addition, pipe supports were upgraded in certain areas to reduce movement of the lines.
It is believed that these modifications will eliminate any lime piping failures in the future.