Internal recirculation CFB demonstrated at Carbondale22 January 1998
Internal recirculation CFB boilers have been under investigation for many years. The first practical application of the Babcock & Wilcox IR-CFB, which supplies heat and power for Southern Illinois University in Carbondale, has now been operating long enough to report on the initial operating experience.
The first Babcock & Wilcox (B&W) internal recirculation CFB (IR-CFB) particle separation system of the 46 te/hr (101 500 lb/hr) coal-fired circulating fluidized bed (CFB) boiler supplied by B&W for Southern Illinois University (SIU) has been started up and initial operating experience has been encouraging. The facility burns high sulphur Illinois coal to produce steam for electricity generation, heating and chilling. The design relies on a solids collection/ recycle system consisting of a primary U-beam impact collector and a multicyclone secondary collector similar to previous B&W CFB units.
However, with the IR-CFB design, all particles collected by the U-beam impact separator are recirculated directly back to the furnace without the use of L-valve return legs. Solids inventory in the furnace is controlled by varying the recirculation rate from the multicyclone dust collector to the furnace.
Steam produced at the SIU's power plant is used to heat and cool 70 buildings totaling approximately 500 000 m2 (5 000 000 ft2) of floor space. As the university continued to expand in size and environmental requirements became more de-manding, the need to upgrade the power plant became evident.
Southern Illinois University is located in Carbondale, Illinois approximately 80 km (50 miles) north of the confluence of the Mississippi and Ohio Rivers. Illinois has the largest reserves of bituminous coal of any state in the nation, and the university itself is located in, and adjacent to, the largest reserves in the state. The university has relied on this fuel source since its origin in 1869.
A feasibility study prepared for the Illinois State Capital Development Board (CDB) recommended that the combustion technology used for the power plant should use Illinois coal as the fuel source. The CDB, as part of a programme to stimulate the development of coal technologies for the commercial, industrial and institutional sectors, provided the funding for the new IR-CFB boiler project.
IR-CFB boiler process
Most of the entrained solids in IR-CFB boilers recirculate within the furnace or are captured and returned directly to the furnace by the U-beam separators. The fines collected by the secondary multicyclone separator are also recirculated to the lower furnace. Solids recirculation inside the furnace and secondary solids recirculation loops provide intensive solids mixing. Because of slip velocity between flue gas and solids, solids residence time is increased to maximize the fuel burnout and sorbent utilization.
IR-CFB boilers operate at relatively high solids densities in the upper furnace. This provides a high rate of gas-solids reaction for combustion, good sulphur capture at relatively low calcium to sulphur molar ratios, low NOx emissions, a high rate of heat transfer to the furnace walls and predictable temperature profile for the entire furnace height.
The SIU boiler arrangement is typical of the IR-CFB boiler design. The IR-CFB design features include:Two stage solids separation for higher carbon burn-out efficiencies, better limestone utilization and higher solids residence time. Controllable solids recirculation (better load change response and wider turndown ratio). Less refractory in the boiler (than previous CFB designs) for quick start-up and less maintenance for lower operating cost. Low, uniform velocities at the furnace exit, and U-beams and superheater to significantly reduce erosion. Gravity fuel feed and air assisted gravity flyash recycle system.
The SIU unit is an atmospheric IR-CFB boiler designed to generate 46 te/hr (101 500 lb/hr) of steam, at 47 bar(g) (675 psig) and 399°C (750°F) with a feedwater temperature entering the economizer of 109°C (228°F).
The crushed coal, sized between 12.7 mm (0.5 in) x zero size, is introduced from a coal silo via a gravimetric belt feeder through the furnace right hand side wall.
A single fuel feed point is used to feed the coal into the lower furnace, or the primary zone. One secondary solids reinjection point is located in the boiler rear wall in the lower furnace. The boiler has one bed drain. An inert material system is used to provide start-up inventory in the furnace and can also be used to assist in bed temperature control during load changes. Make-up for the inert material system can be supplied by outside truck delivery or by the ash handling system when the unit is in operation.
The boiler has two gas fired over-bed burners, located in the furnace rear wall, and two gas fired, in-bed lance burners, located in the front wall. The over-bed burners are capable of heating the bed during start-up prior to the introduction of coal. The rating of these burners is 17.6 MW (60 MBtu/h) each, and steam generation while firing natural gas is limited to 70 per cent MCR. The lance burners are used for stabilizing the bed once coal firing has been initiated.
Bubble caps for primary air distribution are installed between the tubes in the membrane construction furnace floor. The balance of combustion air is admitted as overfire air at two levels on the front and rear walls of the furnace through special nozzles for staged combustion. Both the primary and the secondary air streams are supplied by separate primary and secondary air fans.
The boiler has a vertical pendant type superheater bank. A terminal spray attemperator (condensed saturated steam supply) is used for steam temperature control over the load range. The superheater is located within the water cooled, gas-tight membrane enclosure. Downstream of the superheater is a two-drum generating bank followed by a multicyclone dust collector and finally a horizontal tube economizer. A baghouse is provided for final particulate control prior to the flue gas entering the induced draft fan and exiting through the stack.
The IR-CFB unit is equipped with a Westinghouse digital control system (DCS) to monitor and operate the unit.
The furnace cross section dimensions are 13.1 m x 3.66 m (10 ft 2 in x 12 ft) deep. The furnace is made of gas-tight membraned enclosure water-cooled walls with 76 mm (3 in) tube diameter on 102 mm (4 in) centers. The upper furnace superficial flue gas velocity is approximately 4.9 m/s (16 ft/s). The cross sectional dimensions of the lower furnace, or primary zone, are reduced to provide good solids mixing, to promote solids entrainment and to enable operation of the boiler at low loads with reasonable bed flue gas velocities. The primary air flow accounts for approximately 60 per cent of the total air flow.
A thin layer of refractory is applied to all lower furnace wall surfaces 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 from the floor to an elevation of 7.3 m (24 ft).
The solids separation system is a key element to any CFB boiler design, influencing overall performance as well as the capital and operating costs of the boiler. The primary solids collection device is an impact type separator consisting of an array of U-shaped, stainless steel elements (U-beams) in a labyrinth arrangement. This primary collector consists of two stages. The first stage is an in-furnace separator consisting of two rows of U-beams located across the furnace exit and discharging collected solids directly back into the furnace along the rear wall. The design collection efficiency of the in-furnace U-beams is in excess of 75 per cent.
The small fraction of fine solids which pass through the U-beam separators enters the high-efficiency multicyclone dust collector (MDC) installed upstream of the economizer. The MDC acts as the secondary solids separator and captures more than 90 per cent of the solids entering the device. The combined collection efficiency of the MDC and U-beams is in excess of 99.75 per cent.
The collected fines are deposited into the MDC ash hopper and a variable speed rotary feeder is used to control the ash recycle rate from the ash hopper back to the furnace. The ash passing through the rotary feeder is dropped into an air slide conveyor (air-assisted gravity flow) and is transported by gravity into the lower furnace through a spout in the furnace rear wall.
Coal handling: The steam generator is designed to burn high sulphur coal supplied from the Old Ben II coal mine which is approximately 80 km (50 miles) from SIU. The raw coal is delivered by truck to the plant site where it is stockpiled in the coal yard and then fed onto the fuel feed conveyor system by front-end loaders.
Coal is transported from the coal yard via an inclined belt to a transfer chute which feeds both the CFB boiler and two existing stoker-fired boilers. A drag chain conveyor moves the coal to the crusher and two bucket elevators deliver the coal from the crusher to the silo. The silo capacity is designed for 27 hours of continuous operation at full load. The coal is classified as erosive, medium-volatile and high-sulphur.
Commissioning and start-up: The boiler erection was completed in February 1996. Initial pre-commissioning activities began in June of 1996, 25 months after the start of the unit design. The final boiler hydro test was performed in August 1996. The refractory installation and curing were completed in January 1997.
The final refractory dry out was done with the over-bed burners, and first coal fire was established in December 1996. The steam line blowing operation was completed in January 1997 with gas firing, utilizing the over-bed start-up burners. From January 1997 to July 1997, the boiler was not fired due to circumstances unrelated to the CFB boiler. Turbine rolling, stabilization and turbine generator synchronization are expected to be completed in February 1998.
The success of the commissioning and start-up activities was the result of several factors. Considerable time was spent with operator training and in the check out of the control systems and components. All of the boiler subsystems were run and tested prior to firing the unit. Reliable input/output indications were obtained from both the burner management and combustion control systems.
Once first fire on coal was obtained, the unit operated for 29 consecutive days before tripping. As part of the start-up plan, SIU also recruited experienced operating personnel from other CFB units. Three operators (each to cover a shift), an instrumentation technician and a chief engineer greatly contributed to the successes at SIU, often eliminating problems before they ever fully developed.
Boiler Performance: The initial coal firing was successfully conducted at 70 per cent MCR boiler load. The over-bed start-up burner performance is proven and operated as predicted. Coal firing experience has confirmed the following equipment performance:Gravity coal feed system Reduced number of U-beam rows Internal solids recirculation Refractory performance Fan performance, bubble caps, over bed burners, etc. IR-CFB recycle system in comparison with MDC ash recycle system Overall boiler performance
The boiler formal performance and acceptance test results indicate the SIU IR-CFB boiler has met all guarantees for steam conditions, auxiliary power consumption, boiler efficiency, NOx, and particulate emissions.
The original design of the plant included one continuous emission monitor (CEM) for monitoring emissions from both the IR-CFB and the existing stoker units. During start-up, the need for a dedicated emission monitor on the IR-CFB unit was determined and modifications made to both the controls and the boiler to incorporate a stand alone monitor for this unit. Sootblowers were also added in the field to the economizer section to control fouling.
Maintenance areas: The SIU IR-CFB boiler incorporates a number of proven design features to reduce maintenance in the following areas:U-beam solid separators Refractory (lower furnace, furnace roof, U-beam enclosure, transfer hopper, and so on) Air-assisted gravity fuel feed system Pendant superheaters Ash reinjection system Bed drain water-cooled ash cooler
The SIU IR-CFB boiler has been installed and commissioned, and has established coal and gas firing operation at 100 per cent MCR and intermediate loads. The final acceptance test on coal firing at MCR conditions was completed in September 1997. The boiler passed all performance guarantees.
The CFB boiler has proven to be a viable technology to burn high sulphur coal in a manner that combines simplicity and lower operating cost with higher availability and lower maintenance. The SIU IR-CFB is expected to meet the campus electric power generation and heating/cooling load requirements, utilizing high sulphur coal with high efficiency while meeting the local and state government emissions regulations.
TablesTable 1. Boiler design and predicted performance data