Keeping the Circulation Going
With the lack of reliable electricity supply and problems with setting up IPP projects, Indian industrialists have been forced to supply their own power. Although coal is often the best choice in terms of economy for these captive power plants, the choice of coal burning technology can vary. Circulating fluidized bed systems are proving to be a popular choice.
Circulating fluidized bed (CFB) boilers have been in operation for many years in industrial steam and power generation applications to burn a wide range of solid fuels, demonstrating the required low emissions.
In India, power sector reforms in 1991 saw the introduction of captive power plants; and the size of these plants and necessary fuel flexibility has also seen CFB become a good option for captive power generation here as well.
A good example is a new B&W internal recirculation circulating fluidized-bed (IR-CFB) boiler, which is operating at Kanoria Chemicals & Industries Ltd. (KCIL) in Renukoot, Uttar Pradesh, India. The unit is designed for 81 MWth output for captive power requirements, firing high ash, low sulphur coal. This boiler was supplied by Thermax Babcock & Wilcox Ltd. (TBW), a joint venture company of Babcock & Wilcox (B&W) and Thermax in India.
KCIL is located in the town of Renukoot, in the state of Uttar Pradesh, India, which is very close to the Singaroli coal mine. The steam generator is designed to burn high-ash, low sulphur coal. The coal is supplied from the Singaroli surface coal mine which is 65 km (40 miles) from KCIL. The raw coal 250 mm x 0 (10 in. x 0) is delivered by truck to the plant site.
It is fed onto the receiving hopper and then loaded into the coal crusher via vibrating feeders and conveyors. The crushed coal is fed into the coal silo. Table 1 shows the fuel analysis actually coming to the boiler. The coal, in general, is erosive, medium-volatile, low-sulphur, and high-ash (>45 per cent).
The IR-CFB boiler design is unique, with U-beam impact type particle separators as opposed to hot cyclone separators.
In the B&W CFB boiler, most of the entrained solids recirculate within the furnace and are captured and returned directly to the furnace by the U-beam impact separators. The fines passing the U-beams are collected by the secondary multi-cyclone dust collector/separator (MDC) or by the first field(s) of the electrostatic precipitator (ESP) and are also recirculated to the furnace.
The two-stage solids recirculation provides increased residence time to maximize fuel burnout and sorbent utilization. This provides a high rate of gas-solids reaction for combustion, good sulphur capture if required at relatively low calcium-to-sulphur molar ratios (Ca/S), low NOx emissions, a high rate of heat transfer to the furnace walls and predictable temperature profile for the entire furnace height.
The latest B&W IR-CFB boiler design features:
Two-stage solids separation for high carbon burnout efficiencies and also for better limestone utilization
Controllable solids recirculation (better load change response and wider turndown ratio)
Use of in-furnace surfaces (division and wing walls) for furnace temperature control
Less refractory in the boiler for quick start-up and less maintenance for lower operating cost
Low and uniform velocities at the furnace exit to eliminate or significantly reduce erosion in the upper furnace and primary solids separator
Gravity fuel feed and simplified secondary ash recycle system.
The KCIL boiler is designed with two fuel feed points and four secondary solids re-injection points that are located on the boiler rear wall in the primary zone.
The boiler has two furnace bed drains. One sand (less than 250 microns-average) feed system is used to provide make-up (for emergency use only) and start-up inventory in the furnace. The unit has two 15.12 Mkcal/hr (60 MBtu/hr) over-bed oil-fired burners, located at the boiler rear wall, capable of heating the bed for start-up on coal.
Boiler furnace: The KCIL furnace cross section dimensions are 4.32 m wide x 4.57 m deep (14 ft 2 in. x 15 ft). The furnace is made of gas-tight membrane enclosure water-cooled walls with 76 mm (3 in.) tube diameter on 102 mm (4 in.) centres. In addition to the enclosure walls, one division wall, reaching about 40 per cent of the furnace depth, is installed to achieve the desired furnace temperatures. The furnace plan area at the primary zone is reduced to provide good solids mixing, promote solids entrainment, and operate the boiler at low loads with reasonable flue gas velocities. The upper furnace superficial velocity is 6 m/s (20 ft/s). The primary air flow is about 60 per cent of the total air flow.
A thin layer of refractory is applied to all lower furnace wall surfaces (including division wall) to protect against corrosion and erosion.
Based on previous operating experience, an ultra high strength, abrasion resistant low cement alumina refractory is used for the lower furnace up to 7.3 m (24 ft). Refractory 16 mm (5/8 in.) thick is installed over a dense pin stud pattern.
Refractory is also installed at the furnace roof panel, U-beam enclosure side walls and U-beam transfer hopper.
To reduce the erosion commonly experienced at the refractory interface in the lower furnace, a 450 to 600 mm (18 to 24 in.) band of metal spraying has been applied including the division wall.
U-beam solids separators: The solids separation system is a key element of any CFB boiler design, influencing both capital and operating costs of the unit. The boiler has two stages of primary solids separators: in-furnace U-beam separators and external U-beam separators.
The two rows of in-furnace U-beams are able to collect more than 75 per cent of the solids entering the primary separators. The U-beams are made of TP309H material. The flue gas velocity across the U-beams is around 8 m/s (26.5 ft/s), reducing the gas-side pressure drop (less than 25 mm or 1 in. of water column) as compared with cyclone-type separators (less than 150 mm or 6 in. wg).
Four rows of external U-beams, installed behind the furnace rear wall plane, collect most of the solids passing the in-furnace U-beams. A particle storage hopper is located at the bottom of the external U-beams. The separated solids are recycled internally into the furnace via discharge ports from the transfer hopper.
Secondary ash recycle system: The fraction of solids passing through U-beams is collected at three locations such as air heater hopper, ESP knock-out chamber and ESP I&II Pass, with the major portion collected at the ESP. The collected fines are moved to an ash hopper by a dense phase conveying system. Two variable drive rotary feeders are used to control the ash recycle rates from the ash hopper to the furnace. The ash is dropped onto the two air assisted conveyors and diverted into four streams, injected by gravity into the lower furnace through the rear wall.
Bed drain and ash coolers: The purpose of draining the bed material from the furnace is to control the bed solids inventory and remove oversized material accumulated during operation. For KCIL, two 203 mm (8 in.) diameter bed drain pipes are used to drain the bed material. Horizontal slide valves are used to move the bed drain solids to the ash coolers. The drained material is at bed temperature and carries a considerable amount of sensible heat. The bed drain material is cooled to 205 degreesC (400 degreesF) which is an acceptable temperature to enter the ash disposal system.
A system for controllable draining and cooling is provided by the two fluidized-bed ash coolers. The bed drain solids can be cooled in the fluidized-bed ash coolers and stripped of the fine material (less than 250 microns). The finer material fractions are returned to the furnace with fluidizing hot air. The coarse material is removed at the bottom of the ash cooler via variable rotary feeders. The amount of bed drain solids is controlled in order to maintain the bed solids inventory in the furnace.
Water-cooled air plenum and bubble cap nozzles: The windbox or air plenum is completely made of water-cooled panels except at the rear wall. The bubble caps, 102 mm x 115 mm spacing (4 in. x 4.5 in.), are fitted on the distributor water-cooled floor panel. The B&W bubble cap pressure drop at full load is approximately 406 mm (16 in.) of water column. The bubble caps are designed to distribute the air uniformly, preventing the back sifting of solids even at low load operation, create good turbulence, and promote fuel, limestone and bed material mixing in the primary zone.
The KCIL boiler erection was completed in July 1996. Initial pre-commissioning activities began in August 1996, 20 months after full release of detailed boiler drawings. The overall project was delayed six months due to the late delivery of some of the major equipment such as structural steel, turbine-generator (TG) set, coal handling and crushing system, and also a few vendor equipment items.
The boiler hydrotest was performed in May 1996. The refractory installation and curing were done in September 1996. The steam blow operation was completed in October 1996 with oil firing, utilizing over-bed start-up burners. The turbine rolling, stabilization, and TG synchronization were completed in December 1996 with oil firing. The first coal firing was established in January 1997 and commercial operation began in February 1997. The boiler performance test was completed in September 1997.
Performance and maintenance
KCIL boiler performance test data are given in Table 1. The boiler operating data indicates that the boiler has been successfully operational for the past one and a half years. The boiler efficiency (88.9 per cent on HHV basis) and combustion efficiency (>99 per cent) are higher than predicted. This is mainly due to the very low unburned carbon (less than one per cent) and low flue gas outlet temperature (130 degreesC or 266 degreesF). NOx emission is less than 75 ppm. The sulphur content in KCIL fuel is negligible and therefore, no limestone injection system is added.
The boiler availability was 91.8 per cent for year 1997 and is 95.8 per cent for 1998.
During the first stage of boiler operation, the furnace temperature often exceeded the design value. This was mainly due to the insufficient upper furnace inventory which was caused by failures of the ESP first fields and pneumatic ash conveying system. The ESP problem was solved by implementing proper adjustments to its rectifier. Operation of the pneumatic ash conveyor was fixed by adjusting frequency of ash dump valves and correcting the ash silo back pressure.
KCIL fuel ash is highly erosive, which caused waterwall tube leaks to occur at the refractory interface. This problem was solved by applying the proper thickness of metal spray with a proper interface refractory angle.
Economizer tube erosion was the reason for forced outages. This erosion was caused mainly by flue gas flow obstruction at the tube shield and the strap. After removing the tube shield and the back strap, no significant erosion was observed.
The KCIL boiler incorporates a number of proven design features to reduce maintenance. IR-CFB boilers are anticipating very low maintenance compared to hot cyclone-based CFB boilers. Some of the following key areas for the boilers that will require virtually no maintenance are:
U-beam solids separators
Air-assisted gravity fuel feed system
Secondary ash re-injection system
Bed ash cooler.
With such positive experience, CFB technology will continue to provide a good solution for India`s industrial needs.
Figure 1. The IR-CFB boiler at Kanoria
Figure 2.Cross section of the IR-CFB boiler
Figure 3. Particle collection system
Figure 4. Detailed view of particle collection system
Figure 5. Plan view of the U-beam impact separator