Advanced tools and techniques for optimizing boiler efficiency pay off.
For many operators, the annual energy bill for their boilers will run into millions or tens of millions of dollars.
Frequently, sophisticated monitoring and control processes are used to ensure the boilers are run in an optimal fashion, but over time the associated hardware and software can become degraded. When large energy bills are in play, measures to avoid such degradation and thus reduce energy consumption are very welcome—as chemical company Arkema found. Turning to an advanced analysis tool, the Arkema site in Calvert City, KY, improved its processes while achieving significant reductions in energy bills and associated carbon emissions.
Oil prices are a good indicator of general energy costs. From 1989 to 2003, the average price of a barrel of oil was around $20, eventually rising to $50 by 2005 and peaking at nearly $150 in mid-2008. Apart from the financial-planning headaches such volatility causes, the eye-watering energy bills landing on companies’ doormats bite deeply into profitability. For energy-intensive equipment like industrial boilers, the challenge is particularly acute: A 150 klb steam/hr (68,040 kg steam/hour) industrial boiler running on natural gas would have had an annual fuel bill of around $5 million between 1989 and 2002—rising to $10 million in 2007 and to $20 million in 2008. Where the trend is heading remains unclear.
One place to look for cost relief is in the hardware and software dedicated to optimizing boiler operation. Ensuring that these items are in good working order brings a number of benefits, including:
- Energy savings
- Better response to process steam demands
- Extended operating range for the boiler
- More reliability
- Improved safety
- Reduced carbon footprint
Like much of the equipment across industry, steam-producing boilers rely on PID (proportional-integral-derivative) controllers to regulate the process, reduce product instability and improve operations. In many operations where PID automation is used, however:
- PID loops are not being maintained
- PID loops have degraded
- PID loops are hampering production and performance
- Associated equipment is not performing properly
The search for solutions
Headquartered in France, Arkema is a leading manufacturer of chemicals. The company has production facilities worldwide, including 27 in the United States. Its Calvert City site—which includes the world’s largest HFC-32 refrigerant-production plant—worked with ABB to conduct a detailed “Fingerprint” analysis on its four boilers. The results show how powerful this type of analysis can be.
The Calvert City boilers produce steam at slightly different levels because they are different sizes and were installed at different times. The first two units, both installed in 1952, are brick-set with forced-draft (FD) intakes and induced-draft (ID) removal fans. Both are rated at 40 klb/hr. The third boiler, a 1965 economizer, has only an FD fan and is rated at 75 klb/hr—though it was typically operated at a maximum of 60 klb/hr. The fourth boiler, a 1996 economizer of the flue-gas-recirculation (FGR) type, was operated identically to the third. All four boilers produced steam at about 165 psi, but none were run at maximum load.
A Fingerprint analysis examines the state of hardware and controls, tests the stability and operation of the boiler, performs combustion load trials and executes dynamic step-response tests. In this process, boiler operations are first benchmarked to define existing performance levels and establish a basis for identifying and evaluating improvement opportunities. Recommended improvements are scrutinized to estimate return on investment (ROI) and prioritized according to payback. Subsequent actions then fix problems and sustain performance.
The Fingerprint work at Calvert City began with the second boiler. The unit that’s used most often, it was also the least efficient of the four. Initial examination revealed that the ID-fan-positioner movement was jerky, indicating a potentially faulty pneumatic cylinder or piston assembly. The FD fan was also found to have issues.
During the analysis, it was discovered that a loose hatch door near the oxygen sensor was leaking air into the ductwork before the ID fan. Furthermore, the two boiler oxygen sensors continued to read about 2% higher than a portable analyzer. The leakage meant that air was being added for an oil flow that was not really going into the boiler. Air-flow and fuel-flow measurements were thus going up and down. Both were exhibiting hysteresis, working against each other and creating variability.
In addition, a furnace-draft test showed that leakage air was being sucked in by the ID fan and emitted from the stack as wasted power. Based on load tests, the air/fuel ratio setting was updated. (The oxygen trim that fine-tunes the air/fuel-ratio had been underused in recent years, leading to suboptimal operation.)
An industry rule of thumb notes that, six months after installation, the performance of approximately 50% of process-control loops will be degraded to some degree. Accordingly, control loops were monitored using a Loopscan tool. As shown in Fig. 2, a number of deficiencies were found.
Making the case for improvements
The Fingerprint analysis resulted in a comprehensive to-do list. Recommendations for hardware improvements included:
- Repair FD and ID control drives.
- Resolve oxygen transmitter reading issues: check calibration, find leak, change location.
- Adequately seal all doors.
- Recalibrate steam flows.
- Add blowdown flow monitoring (blowdown removes solids buildup originating from the water/steam).
- Adjust, clean or replace sight glass for drum level.
Control logic recommendations included:
- Perform full combustion test to fine-tune steam-to-air curves, especially for oil.
- Update control logic to current implementation standards.
- Adjust logic to indicate when oil/gas is off.
- Update excess air calculation.
- Tuning improvements included:
- Retune loops to be less aggressive.
- Reduce output surge and ringing tendencies.
- Add a small filter to the level measurement to reduce feed-water chatter.
- Reduce filter on old steam flow measurement.
As a result of the remedial measures, oxygen readings—which had previously measured in the 6 to 7% range—were brought down to under 5%. This reduction in oxygen levels reflects less air being drawn in, less air heated up and less air blown out, which translates into substantial fuel savings. In fact, the approximate value in savings was $75,000 for the second boiler alone, and all without major capital investment.
The third boiler exhibited a problem in that it would trip inexplicably during storms. The project team traced the source of the issue to an FD fan with a roof intake. The exposed roof position rendered it susceptible to error because wind shear affected measurements from the Pitot tubes coming off the fan intake. To solve this problem, Arkema built a protective cover to guard against wind shear.
As a bonus, it was shown that it was safe to operate the boiler at higher loads, thus getting more out of the installed capital equipment.
In all, the Fingerprint analysis achieved a total annual plant energy savings of around $237,000. As the service cost about $25,000 per boiler, the payback time was short.
The Fingerprint analysis has been applied to other industrial boiler installations, with similar success. In addition to reducing energy consumption, the procedure can help clients reduce their greenhouse gas emissions. As these reduction targets become more important to industrial operations, tools like the Fingerprint analysis are expected be more widely used. MT&AP
Information for this article was supplied by ABB and adapted from the ABB Review.