Kenneth W. Richardson
Citgo Petroleum Corp.
Lake Charles, La.
Frank D. Taibi
Foster Wheeler Energy Corp.
Clinton, N.J.
During the mid-1980s, the unpredictability of electric power rates led two refiners and a chemical producer to consider installing their own power-generating facilities.
The outcome was the joint development of a cogenerating plant at Westlake, La.
The plant substantially reduces energy costs and emissions by burning petroleum coke in a state-of-the-art circulating fluidized bed (CFB) steam generator.
BACKGROUND
The $220-million cogenerating project began in 1988 with the formation of Nelson Industrial Steam Co. (Nisco), a four-partner joint venture consisting of Citgo Petroleum Corp., Conoco Inc., Gulf States Utilities (GSU), and Vista Chemical Co. The purpose of the venture was to develop a cogenerating facility to supply electricity and steam to the partners.
Nisco's two 100-mw CFB reheat steam generators started up smoothly in August of last year. This installation is the first commercial use of an advanced reheat CFB steam generator that combines an innovative heat exchanger with a CFB boiler. This extends the use of efficient CFB technology to large-capacity power generating units.
All four Nisco partners had diverse needs and economic interests, yet each recognized the mutual benefits a partnership could offer. The partnership provided a 200-mw cogenerating plant with two important features: the use of low-cost petroleum coke and the use of GSU's existing power plant at Westlake, which minimized capital costs.
Part of GSU's motivation for entering the project was the weakening of its baseload created by the three companies' collective 200-mw load. GSU sold two older generating units at its Roy S. Nelson generating station, north of Westlake, to Nisco in 1988.
Nisco then embarked on the $220-million construction project to replace the two conventional gas-fired boilers with two Foster Wheeler Energy Corp. (FWEC) coke-fired CFB reheat-steam generators. GSU's existing turbine generators and auxiliary plant-support systems are utilized in place.
LOW-COST FUEL
Citgo and Conoco supply the 1,800 tons/day of low-cost coke from their refineries in Lake Charles, La. The electricity is sold to GSU, which operates the cogeneration unit.
The use of petroleum coke as a CFB boiler fuel is a perfect solution to the problem of finding a value-added market for coke, a by-product of the refining process. At the same time, coke is an abundant, low-cost fuel, compared to natural gas or coal.
At current prices at the plant site, coke costs approximately $0.20/MMBTU, as compared to $2.00/MMBTU for natural gas. Coal would cost five to eight times as much as coke. GSU had used natural gas before the two CFB boilers were installed.
Coke's low volatile content and relatively high sulfur content initially called into question how well it would burn (Table 1). A test burn, however, demonstrated that coke sustained combustion in the CFB, with reduced air emissions.
Coke is becoming more abundant as refiners process increasingly sour and heavy crudes. Citgo and Conoco foresee a potentially large market for petroleum coke in CFB cogeneration plants.
A number of refiners, industrial steam/electricity users, and utilities are visiting the new plant to see it in operation.
CFB UNITS
The Nisco plant contains the world's largest circulating fluidized-bed reheat boilers constructed to date and uses innovative heat-exchanger technology developed by Foster Wheeler. Coke is burned in the CFB furnace, along with limestone, in a prolonged, multistage combustion process along the entire height of the furnace (Fig. 1).
Combustion air is injected into the bottom of the furnace to carry the solid particles to the top of the furnace (Fig. 2a). The solids leaving the furnace are collected by three steam-cooled cyclone separators. The captured solids drop into J-valves (Fig. 2b), which discharge them into an integrated recycle heat exchanger (Intrex), designed by FWEC to provide steam generation for utilities and large industrial applications. The Intrex is a unique, unfired heat exchanger that transfers heat from the recycled material to the boiler circuits.
NEW-GENERATION CFB
Reheat-steam generators are the workhorses of the utility industry. In this project, FWEC engineers applied CFB technology to the operating requirements of large-capacity steam generators which use the more-efficient reheat steam cycle.
Conventional utility boilers burning pulverized coal operate at temperatures of about 2,500 F. These high temperatures result in higher SOx and NOx stack emissions compared to the CFB, with its relatively low furnace temperature of 1,550-1,650 F.
A conventional CFB does not provide sufficient heat to the flue gas entering the heat-recovery area (HRA) to achieve the required super-heat and reheat temperatures. The solution to this problem is to use the furnace solids-circulation loop in the CFB to generate superheated steam (main steam) via the Intrex heat-exchanger section, thus allowing the flue gas to provide heat for the reheat-steam cycle.
INTREX SYSTEM
The Intrex design simply and efficiently transfers part of the furnace heat-absorption duty to an unfired bubbling-bed heat exchanger. The system eliminates both the need for an unnecessarily tall furnace, and the need to install furnace panels that may erode excessively.
Placing only superheater tube coils in the Intrex beds increases superheater heat-transfer surface. This increase enables the entire reheater to be placed in a conventional parallel-pass heat recovery area.
The Nisco cogen project consists of two CFB steam-generator/reheat-turbine-generator sets, each designed to generate 100 mw (net) when firing delayed petroleum coke in suspension with limestone. The two units can be operated without or with reheat extraction steam, the latter being an alternate mode of operation required to meet the process requirements of the industrial partners (Table 2).
At maximum continuous rating (MCR) without extraction steam, each unit will produce 825,000 lb/hr of superheated steam at 1,625 psig and 1,005 F., and 727,300 lb/hr of reheat steam at 464 psig and 1,005 F. Table 3 lists other important performance parameters.
In the Nisco furnace, delayed coke sized to 0.25 in. is fed into the lower portion of the furnace through the front wall. Crushed limestone is fed through ports adjacent to the coke-feed openings.
Primary air, heated by flue gas in a tubular air heater, passes up through the air distributor and densely suspends the fuel and sorbent particles at the bottom of the furnace. Heated secondary air is introduced above the fuel feed points. This air provides sufficient gas velocity to entrain the fine particulate matter, which is carried up through the furnace.
Entrained solids leaving the furnace are collected by three steam-cooled cyclone separators which discharge the flue gas into the HRA. The collected solids drop into J-valves, which provide a seal between the positive pressure in the lower furnace, where solids are returned, and the negative pressure in the cyclones.
The J-valves are shop-fabricated with vibration-cast, abrasion-resistant refractory. This refractory can withstand rapid thermal transients and does not limit the start-up or shutdown rate of the unit.
The J-valves discharge solids into the Intrex. The Intrex diverts solids nonmechanically by using five adjacent fluidized-bed cells (three inlet channels and two main cells) and one solids-return channel (Fig. 2c).
This solids-diversion method takes advantage of the high heat-transfer rates found in bubbling, fluidized-bed heat exchangers that operate with fine (about 200 m) material. Because of the fine material and low fluid-particle velocities (0.5-1.5 fps), tube wear is not a concern.
SPECIAL FEATURES
The Intrex design offers a number of features that enhance operational efficiency and reduce maintenance requirements:
- Integrated water-cooled configuration. The Intrex is formed by FWEC's Monowall-the same water-cooled membrane construction used for the furnace. The furnace rear wall is in-line with the Intrex front wall (Fig. 1).
The Intrex is top-supported and expands downward at the same rate as all the other water-cooled pressure parts. Load is picked up from the furnace rear wall and from the downcomers and risers that service the Intrex from the steam drum. Air plenums and distributors are all water-cooled using the same method of construction as the furnace air distributor and plenum.
Conventional external heat exchangers typically are large, refractory-lined boxes set apart from the furnace. Consequently, they must be bottom-supported. They also require large, refractory-lined ducts for return of solids to the furnace and a hanger arrangement for tube support.
In conventional-type units, the furnace and cyclone are top-supported and expand downward, while the heat exchanger is bottom-supported and expands upward. As a result, significant flexibility must be designed into the large refractory lines extending from the heat exchanger.
- Nonmechanical solids diversion. With the Intrex configuration, all solids collected by the cyclones are passed through the steam-cooled beds under normal operating conditions. This is accomplished by fluidizing the main cells at a higher superficial velocity than the inlet channels.
The solids density in the inlet channels is greater than in the main cells, so the bed level does not extend up to the inlet channel weir ports, which are positioned slightly higher than the weir ports in the main cells. As a result, all the solids flow through the screen openings in the bottom area of the partition walls, under and up through the serpentine superheater tube coils, and into the solids-return channel (SRC).
Differential fluidization and a simple pressure balance between adjacent fluidized beds direct all solids through the steam-cooled Intrex cells. During shutdown or start-up, solids collected by the cyclones are passed through the inlet channels and directly into the SRC, bypassing the main steam-cooled beds.
The system's advanced concept eliminates the need for high-temperature mechanical valves. These valves-typical on a conventional heat exchanger-divert and control the flow of solids to the heat exchanger or furnace.
Because of the high-temperature particulate environment in which the mechanical valves operate, they pose significant maintenance and reliability problems. Intrex's nonmechanical method of diverting solids eliminates these problems.
- Solids-return channel. The primary function of the SRC is to uniformly return the recycled solids to the furnace.
The SRC is a single fluidized bed that is common to all Intrex cells. If flow is unbalanced between the cyclones or Intrex cells, aeration of the SRC will uniformly distribute the solids through each of the ports in the furnace rear wall.
Another significant function of the SRC is that of a buffer zone between the furnace and the Intrex, to prevent coarse particle spit-back into the Intrex beds.
- Passive design. During normal operation, no attempt is made to control solids flow rate; that is, all solids flow through the main cells. Differential pressure is measured to monitor bed level and to ensure that this mode of operation is maintained.
A spray-water attemperator upstream of the primary superheater controls the final steam temperature. Another spray-water attemperator is provided upstream of the finishing superheater for emergency conditions.
If required for thermal-balance fine tuning, spray-water attemperation can be shifted between these locations to change the effective steam inlet temperature. This, in turn, changes the temperature differential for heat transfer and the overall effectiveness of the heat-transfer surface. Steam-temperature control by spray attemperation is fast, effective, and has been proven over many years.
The two CFB reheat-steam generators in operation at Nisco's Westlake cogen facility were designed as an integrated system of proven components. The focus of this system is on:
- Wear resistance
- Rapid start-up and shutdown
- Minimal differential thermal movement
- Simple control of main and reheat-steam temperature
- Elimination of high-temperature mechanical devices
- Optimum heat recovery
- Low maintenance requirements
- Efficient use of the reheat-steam cycle
- The ability to burn low-volatility, high-sulfur fuel in an efficient, reliable, and environmentally sound manner.
ENVIRONMENTAL BENEFITS
CFB combustion is a proven method of producing steam economically by burning fuel in the presence of limestone. CFB boilers operate at a relatively low furnace temperature, which permits combustion below the ash-fusion temperature while providing an optimal temperature for capturing limestone sulfur and reducing NOx formation.
Staged combustion of a CFB further reduces NOx emissions. And the limestone absorbs at least 90% of the sulfur emissions from coke.
NISCO PLANT
Overall, environmental emissions from the plant are significantly less than those allowed by the U.S. 1990 Clean Air Act Amendment standards. And these standards are likely to become even more stringent in the future.
Aside from environmental considerations, the Nisco project demonstrated that an older utility boiler can be replaced successfully. Operators of utilities and industrial plants with older units of this size have found this interesting.
From the perspective of operating flexibility, the CFB boilers offer another advantage because the Nisco operator can achieve a hot restart in 20 min. And coke-a waste product-has not been used as a primary fuel in the U.S. for power generation.
The Nisco facility contains the first CFB designed specifically to use coke. Petroleum coke is viewed as an "opportunity fuel," an up-and-coming fuel source. The key is being able to handle coke, despite its sulfur and vanadium content, which the circulating fluidized bed can do successfully.
The Nisco project is effectively demonstrating that coke can be used with CFB technology, to reduce the cost of fuel, stabilize power costs, and at the same time operate in an environmentally sound manner.
Copyright 1993 Oil & Gas Journal. All Rights Reserved.
