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Pulverizer Performance Upgrades Lower Fuel Costs

Issue 5 and Volume 111.

Teresa Hansen, Section Editor

St. Johns River Power Park (SJRPP), located on the St. Johns River in Jacksonville, Fla., is jointly owned by JEA (formerly Jacksonville Electric Authority) and Florida Power & Light (FPL). JEA is the plant operator. The plant consists of two coal-fired Foster Wheeler drum boilers and two General Electric turbine generators, each with an output of 640 MW. Each drum boiler is equipped with seven vertical spindle pulverizers, feeding 28 low NOx burners in an opposed, wall-fired configuration. The units, which were placed in commercial operation in 1987 and 1988, are able to burn a wide variety of coals from all over the world. Since 2005, the boilers typically fire a blend of 70 percent Colombian coal (or, to a lesser degree, domestic coal) and 30 percent delayed petroleum coke.

Beginning in Spring 2002, and concluding in Spring 2005, the plant owners upgraded its combustion equipment to lower nitrogen oxide (NOX), carbon monoxide (CO), and loss on ignition, or unburned carbon in ash, (LOI) and positioned itself more favorably for future compliance with the Clean Air Interstate Rule. The effort succeeded in obtaining the desired emission reductions and also allowed the plant to increase petroleum coke consumption from 20 percent to 30 percent; however, as fuel prices continued to climb, more emphasis was placed on burning lower grade fuels. Specifically, one of the plant’s long-term coal contracts was set to expire and SJRPP would be able to realize significant fuel savings if this coal could be replaced with lower grade coal.

Noting that the original pulverizers’ throughput was preventing this option, SJRPP invited Sure Alloy Steel Corp. (SAS) to evaluate the total fuel system. SAS’s comprehensive systemic evaluation consisted of data review, equipment inspections and operational observations. In addition, SAS conducted in-depth interviews with plant personnel, revealing recurring operational and maintenance problems as well as significant historical issues.

Based on the evaluation’s findings and the plant’s subsequent modifications, SJRPP increased pulverizer capacity by 14 percent (at constant fineness) and realized a $6.3 million annual fuel savings on its contract coal. It also stands to realize further savings on future spot-market purchases.

Background

From 1997 through 2004, SJRPP’s fuel supply consisted of 80 percent coal and 20 percent petroleum coke. The coal was supplied under two separate long-term contracts and the petroleum coke was purchased on the spot market. During the long-term contracts’ duration, SJRPP’s aggregate fuel cost remained largely unaffected by rising fuel prices in 2003 and 2004. Only the 20 percent component representing petroleum coke was affected by market forces.

The first of the two long-term coal contracts, however, was set for a market re-opener at the end of 2005. Because the low heating value of this coal challenged the fuel delivery system, SJRPP anticipated a substantial increase in fuel cost beyond 2005. To avoid or mitigate this expected fuel cost increase SJRPP would be required to burn a lower-grade coal (11,300 Btu/pound) than the coal it previously burned (11,800 Btu/pound). Although the lower-grade coal was substantially cheaper, a 7 percent increase in pulverizer coal throughput was required to make the substitute coal viable.

Equipment Description

SJRPP’s original mills are vertical spindle pulverizers with three static rotating grinding rollers and a rotary grinding zone. They were designed to be low-speed and high-volume with a capacity of 103.8 klb/hr at 70 percent fineness passing 200 mesh while grinding a 48 HGI (Hardgrove Grindibility Index) coal. The mills, however, never performed at this level and testing established a base capacity of 89.5 klb/hr (70 percent through 200 mesh, 48 HGI), which had historically been used for screening candidate fuels.

The mills were originally furnished with stationary air ports. To reduce maintenance costs, however, SJRPP later replaced these with aftermarket rotating air ports, which did not affect mill capacity. The original classifiers were designed to be externally adjustable, but that feature never worked properly, so the mill had to be shutdown to adjust the classifiers. The original discharge rings and classifier reject chutes were still being used. The grinding ring segments were original castings and the grinding rollers were aftermarket welded overlay material and included a spindle design modification. While the aftermarket modifications reduced maintenance costs, they did not affect mill capacity. Pulverizer capacity remained the obstacle to burning lower rank, less expensive fuels.

Pulverizer Performance Evaluation

The problem of restricted capacity due to air port and classifier limitations, which was revealed during the SAS evaluation, is relatively common. Mill capacity was SJRPP’s main constraint to reducing fuel costs. When the plant exceeded the feed rate and, consequently, the mill’s capacity to grind, the mill differential and motor amps gradually increased, leading to an increase in fuel use. The mill table differential pressure would increase and begin to fluctuate as the fuel mass exiting the pulverizer cycled, creating thermal boiler control upsets. This often required the operators to keep the pulverizer’s feeders and air controls in manual mode.

The classifier reject discharge chutes were also an issue. The weighted trap doors tended to bind or lock in a throttled position, reducing the return of oversized, rejected particles to the grinding zone. As additional chutes became bound and the path for oversized fuel returning to the grinding zone became restricted, the classifier capacity diminished. Ultimately, fuel would fill the classifier cone and force oversized fuel particles to exit the mill prematurely. This resulted in reduced fineness, poor flame quality, high CO production and high LOI. Correcting the problem required personnel entry, which rendered the mill unavailable for a day. This problem was exacerbated by wet fuel conditions as the elevated primary air temperature caused the classifier’s content to cake or char into a solid mass.

In Spring 2004, SAS built and SJRPP installed a prototypical rotating air port in the 1B Mill. SAS and SJRPP selected this mill as the prototype because it had a history of being the worst of the 14 mills with regard to mill bowl differential pressure and the aforementioned “surging.” Consequently, it had the lowest fuel loading and the remaining mills were required to make up for its deficiency.

Once the air port installation was complete, the operators increased the 1B Mill’s participation and immediately noticed improvements. The SAS rotating air port’s effectiveness at reducing classifier duty through primary classification resulted in additional capacity from the mill. Based on this success, SJRPP decided to add another prototype rotating air port to another mill.

Adjustable SAS Rotating Air Port

The air directional vanes of the rotating air port face the direction of grinding table rotation at a precise angle. Combined with the throat annulus area, these vanes vector the air port velocity at approximately 10,200 feet per minute (fpm). These vanes are also adjustable to accommodate fuel and/or operating changes. Through the rotation of the air port with the grinding table, the primary air circulates circumferentially-an efficient fuel drying method. The vanes’ cantilever design allows the radial seal to use the total air port area. The deflectors, immediately above the vanes, move the coal/air mixture toward the mill’s center, providing primary coal classification and depositing larger particle sizes back onto the grinding segments. The 50 mesh fineness levels, which are largely responsible for LOI, are significantly improved at this mill level and the reduction in classifier turndown contributes to the grinding capacity increase.

High Spin Static Classifier

SAS replaced the original cage assembly with a high spin static classifier blade, cage and adjustable cycling mechanism (Figure 1). The blades are shifted 180 degrees from the original arrangement and cycle around a pivot point for blade adjustment to the discharge ring tangent. The blades are formed to deflect the flow downward and they extend below the classifier cone top. This design takes advantage of the rotational forces imparted on the dense phase flow within the pulverizer at the primary air porting elevation. The conical form, along with the blades’ depth and direction, accelerate the circumferential movement of the dense phase flow around the classifier cone, which acts as a cyclone separator. Centrifugal force controls the fineness level and the blades can be easily rotated from the mill’s exterior. SAS completely redesigned the original adjustment mechanism using stainless steel gimball bushings, turnbuckles and pivot arms. The original actuation gearbox and connecting arm were re-used. This device provides control of the 200 mesh fineness as well as classifier differential, and it aids in lowering system resistance by eliminating restriction. The classifier cone was replaced and built in three segments to facilitate any future maintenance requirements.

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SAS Cone Extension

SAS installed its cone extension kit (Figure 2) to eliminate the existing classifier rejects discharge chutes, and thus eliminate the mechanical trap doors and their potential to bind. The feed pipe extends lower in the mill and a cone extension is bolted to the bottom of the classifier cone, extending it approximately 48 inches. A series of pipe spools are then bolted to the bottom of the classifier cone extension. This classifier component returns the oversized, rejected particles back into the grinding zone, approximately 32 inches above the table. By eliminating the discharge doors and returning the fuel directly into the grinding zone, mill differential was reduced, fineness was improved and plugging or “surging” was eliminated. In addition, the changes extended the classifier components’ service lives.

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SAS Multi-Outlet Diffuser

The SAS multi-outlet diffuser (MOD) creates a homogenous mixture of fuel and air in the pulverizer’s turret section (Figure 3). This allows a balanced fuel flow thru each of the pipes exiting the mill. The stimulation of flow characteristics enhances particle size distribution, dense phase flow mixing and coal pipe fuel distribution. Fuel distribution is critical for effectively reducing NOX and LOI concurrently.

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Evaluation of Complete SAS Upgrade

SAS and SJRPP completely upgraded the 2G Mill because its grinding surfaces were restored in August 2004. SAS performed baseline testing in September 2004 and the upgrade in October 2004. It retested the mill in November 2004.

For these tests, SJRPP slowly raised the mill output until its maximum capacity was established. Unending and increasing mill bowl differential pressure prohibited additional fuel input beyond this point; otherwise, the surging effects previously mentioned would have occurred. All tests were performed with coal from the lower grade supplier.

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SJRPP performed the baseline testing in September 2004 with the restored grinding surfaces and prior to the SAS upgrade. The mill’s baseline capacity was 90.2 klb/hr when corrected to standard fineness of 70 percent through 200 mesh. These test conditions are presented in Table 1 and mill capacity corrected to standard fineness is shown in Figure 4.

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This result was favorable to the previously established base capacity of 89.5 klb/hr that had been the historic basis for fuel procurement.

Upon completing the upgrade in October 2004, SJRPP returned the mill to service and retested in November 2004. At the onset of the retesting, it became immediately apparent that the mill’s new capacity exceeded the 100 klb/hr capacity of the coal feeder. SAS reconfigured the feeders and testing resumed.

Because this was the first test performed since the upgrade, SAS did not know the mill’s fineness at the time of the test. The classifier was in a “neutral” position based on SAS’s experience and no problems were noted with flame stability, CO or LOI. Test conditions from the first test following the complete SAS upgrade are provided in Table 1 and mill capacity corrected to standard fineness is shown in Figure 4.

Following the first test, SAS adjusted the classifier to produce a finer product. SAS adjusted the mill while it was in service so that it could monitor mill motor amps, feeder speed and differential pressure to ensure the desired classifier position. Test conditions from this test are provided in Table 1 and mill capacity corrected to standard fineness is shown in Figure 4.

Table 1 illustrates that fineness was greatly improved in the second test and the corrected capacity confirms the capacity increase observed during the first test. The two tests demonstrated an average corrected capacity of 103.0 klb/hr after the complete SAS upgrade versus 90.5 klb/hr prior to the upgrade. The 14 percent capacity increase was double SJRPP’s requirement of 7 percent required for the lower quality coal.

Before and After SAS Upgrade

Based on these results and favorable operating experience from October 2004 to April 2005, SJRPP accepted the 11,300 Btu/pound offer in lieu of the December 2005 market re-opener. This mitigated the cost increase expected from the re-opener by $6.3 million annually beginning in 2006.

Additionally, in April, 2005, SJRPP placed an order with SAS to upgrade the remaining fleet of 13 mills and an aggressive schedule was developed for implementation. This allowed SJRPP to substitute the lower grade coal for a fuel savings of more than $2.5 million in 2005. In January of 2005, SJRPP received its last shipment of the higher grade 11,800 Btu/pound coal from this supplier. Circle 200