Fluidized Bed Combustion: Clean coal in a hot climate

By William Jarvis
Alstom Power, Windsor, CT, USA

As a government-owned electric utility, the Puerto Rico Electric Power Authority (Prepa) operates five oil-fired power plants that supply 4400 MW of electricity to roughly 1.3 million customers – spread evenly among residential, commercial and industrial users.

Figure 1. The AES Puerto Rico power project is scheduled to begin commercial operation in mid-2002
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With electricity consumption growing at three per cent annually and a need to maintain relatively high reserve margins on the island, Prepa began in the late 1990s to diversify its generation mix and modernize its infrastructure. As part of that plan, it decided to allow independent power production and cogeneration as a way to secure additional low cost energy for its customers.

One of the first independent power producers (IPPs) out of the blocks will also be the island’s first ever coal-fired power plant: the 454 MW AES Puerto Rico Project. And, in reaction to environmental concerns over burning coal, the plant will not only be powered by two circulating fluid bed (CFB) boilers, but will also use an assortment of back-end emission control technologies that will make it one of the cleanest coal-fired power plants in the world.

Figure 2. Based on CFB technology, AES Puerto Rico will be one of the cleanest coal-fired power plants in the world
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Specifically, CFB technology will be combined with selective non-catalytic reduction systems (SNCR), dry scrubbers, and electrostatic precipitators for particulate collection to produce emissions virtually unheard of in today’s power markets. These include 0.155 kg/MWh for NOx; 0.034 kg/MWh for SO2; particulate matter smaller than 10 microns (PM10) of 0.023 kg/MWh, CO emissions of 0.155 kg/MWh; and VOC emissions of 0.007 kg/MWh.

AES Puerto Rico

The AES Puerto Rico Power Project is being developed by US-based AES Corporation, the world’s largest global power company. AES either owns or has an interest in 160 power plants with over 54 000 MW of capacity in 23 countries. The company currently operates a number of plants powered by CFB technology.

The plant will be located on an 32 ha site approximately 5 km west of Guayama in the southeastern region of Puerto Rico. It is scheduled to begin commercial operation in mid-2002 burning low sulphur coal imported from South America and other sources. Duke/Fluor Daniel, a joint venture of Duke Energy Corp. and Fluor Corp., was chosen by AES to design, procure and construct the plant.

Upon completion, approximately 181 440 kg/h of process steam will be delivered to Phillips Puerto Rico Core, Inc., a nearby petrochemical plant that is a wholly-owned subsidiary of Phillips Petroleum Company. In addition, the facility will provide electricity and a percentage of its steam to Prepa as a qualifying facility (QF).

The plant’s CFB units are being supplied by Alstom Power, which based the design on its successful twin CFB Tonghae project in South Korea, which is rated at approximately 220 MWe per unit. Each of the AES Puerto Rico units will produce approximately 827 513 kg/h main steam and 711 970 kg/h of reheat steam.

The boilers will consist of a single cell design, with an internal evaporator surface and superheater, and reheater fluid bed heat exchangers. Four light oil, air atomized burners will be located within the furnace. The coal will be crushed and stored in silos, and then fed to the furnace via eight (per-unit) gravimetric feeders, with feed sloped chutes with a hot primary air assist.

Each unit will produce 165 bar steam, with 538à‚°C superheat and reheat steam temperatures. In addition to the low combustion temperatures, further NOx control will be accomplished by distributing secondary air to the combustor front and rear walls. Further NOx control will be achieved through the use of the SNCR.

The steam will be fed to two Alstom Power 255 MW steam turbine generator sets utilizing a top air generator rated at 300 MVA, 21 kV. Each turbine set features a two casing design utilizing a high pressure (HP) turbine and an intermediate/low pressure (IP/LP) turbine to optimize plant efficiencies.

The plant’s coal will be delivered via self-discharging vessels at Puerto Rico’s Las Mareas Port, just south of the plant site. A new dock facility is under construction in the harbour, where the coal will be unloaded and transported to the plant via covered conveyors.

Construction timetable

Since the plant is relatively close to the Las Mareas Harbour, Alstom shipped a number of major boiler components preassembled to reduce on-site construction times. For example, large sections (71 cm x 76 cm) of the cyclones were preassembled and off-loaded directly from ocean going vessels to the harbour unloading area.

Heavy haul transport trailers then brought the components up to the construction site. In addition, a number of other large components such as the three fluid bed heat exchangers (FBHE), weighing over 370 t each, were erected outside the boiler structure while steel was being erected, then placed inside the structure once the steel construction was completed. The boiler steam drums were lifted in December 2000 and January 2001, respectively. The remaining project milestones include:

  • Start commissioning August 2001
  • Hydrostatic test on both units, September 2001
  • Initial operation/sync T/G, March and April 2002
  • Commercial operation, June 2002.

Emissions control

A key driver in obtaining the necessary environmental permits for the plant involved its use of CFB technology. The technology has continued to advance from the initial clean coal and Purpa projects in the USA to essentially worldwide market penetration burning a variety of fuels. The technology’s fuel flexibility allows power plant owners to lower fuel costs by burning less expensive fuels like coal while still meeting stringent environmental requirements.

Figure 3. Major components were shipped pre-assembled to Las Mareas Harbour to reduce on-site construction times
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The environmentally attractive features of the CFB result from its inherent combustion characteristics. Sulfur dioxide (SO2) is captured in the combustion process by adding limestone to the fluidized bed. Because the combustion temperatures remain in the 815à‚°C to 871à‚°C range – as compared to 1648à‚°C for conventional technologies – NOx formation is also reduced during combustion.

Due to the sensitivities associated with burning coal, the air emissions control system for AES Puerto Rico also includes an SNCR for additional NOx emission control, a dry scrubber for additional SO2 emission control, and an ESP for particulate emission control.

Each piece of equipment, except the dry scrubber, has been successfully used on other CFB applications. The dry scrubber, which has provided SO2 emission reduction on other types of boilers, was considered to be among the best equipment for pollutant emission control at this particular plant. The overall system design has been chosen because each technology has successfully demonstrated its ability to produce emissions levels similar to those required for this project.

Figure 4. The CFB boilers are based on the design of those installed at Alstom’s Tonghae power project in South Korea. Each unit will produce 165 bar steam, with 538à‚°C superheat and reheat steam temperatures
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ESPs are an appropriate technology to follow the dry scrubber because of the high concentration of dust in the scrubber outlet gases. The collected particulate is immediately removed from the flue gas stream. The low temperatures and high humidity of the CFB gases help maximize the ESP’s capabilities to handle multiple fuels.

The overall, projected emissions for the AES Puerto Rico Plant are as follows:

  • Total NOx emissions in the flue gas will be limited to 57 ppmvd at seven per cent O2, 0.155 kg/MWh, or 111.5 kg/h, which ever is more stringent. The SNCR system using urea reagent will reduce the NOx emissions to compliance levels, with the emissions measured and determined over a 24-hour period using a rolling average.
  • Total SO2 emissions in the flue gas will not exceed either 9.0 ppmvd at seven per cent O2, 0.034 kg/MWh, or 245 kg/h, or which ever is tighter. About 70 per cent of the SO2 formed during the combustion of sulphur in the fuel will be captured internally by the calcium oxide generated from the calcination of the limestone in the CFB combustor. In addition, the dry scrubber will use water and dry lime to reduce the flue gas temperature and react with SO2 to form calcium sulphite. The SO2 emissions will be determined over an eight-hour period.
  • The ESP will control particulates leaving the flue gas stream from both the CFB and the dry scrubber. This equipment is capable of meeting the emission rate for both particulate matter (PM) and PM smaller than 10 microns (PM10) of 0.023 kg/MWh and providing plume opacity of less than 20 per cent based on a six minute average. The plant’s environmental permit also requires a removal efficiency of at least 99 per cent.
  • Total carbon monoxide (CO) emissions will be limited to 94 ppmvd at seven per cent O2, 0.155 kg/MWh, or 111.5 kg/h, whichever is more stringent. Performance of existing Alstom CFB boiler installations indicates this level of CO emissions is achievable without supplemental or post-combustion CO reduction equipment.
  • Volatile organic compound (VOC) emissions in the flue gas are limited to either 7.7 ppmvd at seven per cent O2, 0.007 kg/MWh, or 5.2 kg/h. Performance at existing Alstom CFB installations indicates this VOC emissions level is also achievable without supplemental or post-combustion VOC reduction equipment.
  • Sulphuric acid mist emissions will be limited to 0.64 ppmvd at seven per cent O2, 0.037 kg/MWh, or 0.01 kg/h, whichever is more stringent.
  • Fluorides emissions will be limited to 0.0007 kg/MWh, or 0.5 kg/h, which ever is more stringent.
  • Emissions of ammonia will be limited to 10 ppmvd at seven per cent O2. Limiting the ammonia injection for SNCR NOx control will limit the ammonia emissions.
  • A continuous emissions monitoring system (CEMS) will allow authorities to measure compliance/non-compliance as required. The system monitors SO2, CO, O2, NOx and NH3, using an extraction-type system with dilution-type probes located in the ducts at the scrubber entrances and the ESP outlets.

Figure 5. Steam will be fed to two Alstom Power 255 MW steam turbine generator sets using a top air generator rated at 300 MVA, 21 kV
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Each boiler will have an independent set of analyzers. The system is configured to allow either a bank of analyzers to operate in a “time share” configuration in the event of an analyzer failure. The CEMS will provide process data information to the control room via the plant’s distributed control system. Also a single opacity monitor per unit will be located in each flue of the stack, with the data acquisition computer located in the control room.

Reduced water usage

Due to the premium placed on Puerto Rico’s water supply, a number of steps have also been taken to minimize the plant’s use of fresh water that will keep the prices low and help the local environment. The AES Puerto Rico Power Project is designed for a zero discharge of water used in the process and/or runoff water from the fuel storage area. The plant will use treated effluent from the regional sewage treatment plant in Guayama for cooling tower makeup and cooling the flue gas in the dry scrubber, thus eliminating most of the water that is currently being discharged into the sea by this treatment facility. This also reduces the dependency on local fresh water sources and eliminates the need to use sea water.

Figure 6. The AES Puerto Rico power project will supply process steam to Phillips Puerto Rico Core, Inc.
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Steam will be recycled as 181 440 kg/h of steam will be pumped into the Phillips Puerto Rico Core petrochemical plant before returning as condensate and other makeup water – via a specially designed piping system – to be used again.

The AES Puerto Rico Project will clearly allow Prepa to meet its corporate objectives:

  • Increasing its system-wide electrical capacity
  • Providing its customers with relatively stable electricity rates
  • Maintaining its relatively high reserve margins.

Figure 7. AES Puerto Rico will be the island’s first coal-fired power plant
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Through the use of CFB boilers and other emissions control technologies, the project also demonstrates the continued viability of coal as a power generation fuel – particularly on an island that is dependent upon much more expensive imported fuel oil.

Upon completion next year, this facility will help drive the island’s economic development by generating electricity in a cost effective and environmentally friendly manner – all of which will help Puerto Rico maintain its reputation as one of the Caribbean’s major tourist destinations.

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