Fluidised Bed Combustion Technology – the past, present and future

Fluidised bed combustion (FBC) technology has received a wide acceptance in many industrial applications; for example, for use in burning fuels that contains a high moisture content. FBC technology offers many advantages, including: fuel flexibility; superior emissions performance; good applicability for low-grade and hard-to-burn fuels and application for newer energy concepts; high suitability for repowering; and FBC by-products are non-hazardous and can be reused. Circulating fluidised bed (CFB) technology has developed from being largely confined to industrial applications to utility scale applications.

Further improvements to the technology are currently being considered and large-scale demonstrations are now in progress.

Atmospheric CFB technology

Currently, there are about 300 atmospheric CFB units in operation or under construction. Commercial units up to 400 MWe are being offered.

Over the years, CFBs have been scaled up in size and have been shown to be very fuel flexible, being able to use low heating value hard-to-burn biomass and various ‘waste’ fuels. CFB combustors meet stringent emission level guarantees. CFB units now compete with stokers in the smaller industrial size, and with pulverised coal boilers for unit sizes up to 500 MWe.

Scale up

The first commercial size boiler with CFB combustion started up in 1979. CFB boilers gained acceptance for power generation during the early 1980s, mainly in cogeneration applications. One driving force for the development work was the dramatic increase in oil price during the oil crises of that time. The primary advantage of CFB technology is that it enables the substitution of expensive fuels with cheaper solid fuels.

These smaller boilers proved the readiness of CFB technology for use in coal-fired boiler applications. The simplicity of these boilers, without the in-line coal pulverisers and turndown capabilities without the need to use premium fuels generated more interest from utilities and IPPs.

The sequence of scale-up of Foster Wheeler boilers is shown in Table 1 below. The main considerations in the scale-up of CFB boilers have been as follows:

Fluidisation in large cross-sectional areas.

Distribution of air and fuel in the lower furnace.

Separation efficiency of large hot cyclone collectors.

Placement of steam-cooled heating surface.

Productive and effective R&D support to meet the scale-up concerns and an in-house capability to be able to design large-scale steam generators over many decades have been the cornerstones that have been required for this successful development and application of the CFB technology.

Fuel flexibility

There is a considerable diversity in the number and the diversity of CFB applications. In addition to large coal-fired CFB boilers, there are many smaller boilers that fire all types of low-grade, solid and solid-waste fuels, including municipal refuse, peat, industrial waste, coal-water slurries, sewage sludge, petroleum coke, oil shale, coal-mining waste, manure, wood and biomass, either alone or in various combinations and proportions.

Table 2 (page 23) highlights the variations that can occur in some of the fuels that are used as the main fuel feedstock for boilers of different capacities.

Because of the unique behaviour of each fuel in handling, pilot scale testing and commercial experience provided a sound platform for the development of successful designs for the fuel and the ash handling systems.

Environmental compliance

CFB furnaces burn fuel within a turbulent atmosphere under relatively uniform mixing conditions in order that they are able to achieve an efficient combustion. Operational experience has demonstrated that a 90 per cent SO2 capture target is routinely achieved with a calcium-sulphur molar ratio of 2.

In most cases, the sulphur retention efficiency is well above 90 per cent. NOx is nominally held to a value of less than 250 ppm, and well below 100 ppm with SNCR based ammonia injection. Several operating units have reported that they are able to achieve very low emission levels of CO, NOx, SO2 and particulate, generally well below permitted values with little difficulty.

Future emission level regulations will become increasingly stringent for NOx, SO2, CO, hydrocarbons, VOC, trace metals and particulates. The CFB system has been shown itself to be flexible and able to conform to the stricter pollution control needs that will be demanded.

Substantial improvements in pollution control are continually being achieved with CFB systems by changing the process conditions and/or by adding small subsystems. For example, the ammonia injection (SNCR for NOx reduction) system is added without changing floor space of the plant or ducting the gases to a separate reactor.

Beneficial use of by-products

CFB boilers generate two major waste streams – fly ash and bottom ash – which are a mixture of fuel ash, unburned carbon residues, and limestone particles coated with sulphate layers. Foster Wheeler has characterised residues from over 40 CFB boilers located throughout the world, from both an environmental impact and by-product utilization standpoint.

The chemical and physical properties of ashes have been evaluated to assess their potential utilisation. This data includes boilers firing such diverse fuels as anthracite culm, semi-anthracite, bituminous gob, bituminous coal, bituminous coal blends with biomass and shredded tyres, refuse derived fuels, and petroleum coke. Results show that the CFB by-products can be used in various applications without having any negative impact on the environment.

This study has demonstrated that ash streams from CFB boilers firing diverse fuels have the potential for use in one or more of these applications. The actual utilisation will depend upon the availability and price the CFB ash would locally displace for these applications (such as Portland cement, pulverised coal fly ash, lime or aggregate).

Future requirements for coal-based technologies will place great emphasis on ensuring that there is zero waste material generation. This is an area where more extensive research is still needed in order to develop methods to be able to use the CFB combustor by-products in various applications.

In new projects which use increasingly larger boilers (and consequent ash generation rates), better estimates of ash characteristics and utilisation need to be made in the early stages in order to increase the potential ash marketability.

Additional research in this area should address the release of various species from coal and sorbent and their subsequent partitioning between bottom ash/fly ash and gas streams. This partitioning of constituents can result in different fly ash and bottom ash compositions, and which can have a significant effect on their utilisation potential. As a result of this, provisions should often be made for using separate fly and bottom ash silos, since the blended ash may have only very limited uses.

Plant economics

Currently, a CFB costs less than a conventional pulverised coal (PC) unit with a scrubber for low quality fuels., though both systems may cost the same for good quality fuels.

In the current PC market, the once-through supercritical (OTSC) units are in demand. Foster Wheeler has completed designs for OTSC CFBs, and is currently in the process of looking to site the first OTSC CFB unit.

Foster Wheeler believes that OTSC CFBs will give the technology additional operating efficiency and will enhance the overall cost benefits of the technology.

Advanced CFB technologies

In the future, the key imperatives for power generation will be the three E’s: Efficiency, Environment and Economics. To respond to these imperatives, Foster Wheeler is participating in the development of a number of advanced power generation systems, including Advanced Circulating Fluidised Bed Combined Cycle (ACFBCC). The hybrid ACFBCC generates power at 45+ per cent efficiency based on HHV with lower emission and cost.

An ACFBCC plant integrates the best features of both coal gasification and of pressurized fluidized bed combustion (PCFB). The figure shown alongside shows a simplified process block diagram of an ACFBCC plant that has been designed for maximum efficiency.

In the plant, coal is fed to a pressurised carboniser that produces a low-Btu syngas and char. After passing through a cyclone and a high-temperature ceramic barrier filter to remove gas-entrained particulates and a packed bed of emathelite pellets in order to remove alkali vapours, the hot syngas is burned in a topping combustor to help produce the energy required to drive a gas turbine. The gas turbine then drives a generator and a compressor that feeds air to the carboniser, a PCFB combustor, and a fluidised bed heat exchanger (FBHE).

The carboniser char is burned in the PCFB and the hot exhaust gas passes through its own cyclone, ceramic barrier filter, and alkali getter and supports combustion of the syngas in the topping chamber. Steam generated in a heat recovery steam generator (HRSG) downstream of the gas turbine and in the FBHE associated with the PCFB drives the steam turbine generator that furnishes the balance of electric power delivered by the plant.

When operating with a large, commercially available gas turbine and a conventional 2400 psig/1000°F/1000°F/2.5" Hg steam cycle, studies have shown that ACFBCC plant can operate with efficiencies greater than 46 per cent.

Since the steam cycle is less efficient than the gas turbine cycle, the referenced high efficiency plant is specifically configured to minimise the size of the steam turbine; typically the gas and steam turbine power outputs are about equal and the plant operates with high excess air. If the customer wants additional power, he may either add a second gas turbine, or stay with one gas turbine and supplement the char feed to the PCFB with coal.

Since the gas turbine is in essence fully loaded, the PCFB coal heat release goes to the steam cycle.

Increasing the PCFB coal flow increases the steam turbine power output. In the extreme case, the steam turbine size can be tripled and the plant excess air/unused oxygen brought to a minimum. The latter represents the maximum power configuration, and the steam turbine power is about three times larger than the gas turbine. With the plant output now doubled with the same size gas turbine, plant costs on a dollar per kilowatt basis are reduced; and despite a lower overall operating efficiency, the plant cost of electricity remains approximately the same.

Depending upon the owner’s power needs and economics, an ACFBCC plant can be designed for peak efficiency, maximum power output, or anything in between. In all cases, the larger the gas turbine, the higher the gas efficiency, and economics are best for larger size (250 to 500 MWe) plants.

For small size plants and repowerings (100 to 400 MWe), smaller gas turbine outputs are involved and the plant can be operated in a non-topped mode, which is frequently referred to as a First Generation PFB. With the gas turbine driven by the PCFB 1560°F exhaust, gas turbine power is reduced and the plant efficiency drops to approximately 39-40 per cent. Gas turbine and plant costs, however, also reduce, and the resulting cost of electricity is similar to that of the peak efficiency plant.

Lakeland demonstration plant

In December 1997, the City of Lakeland, Florida, signed a Cooperative Agreement with the US Department of Energy (DoE) that will facilitate the demonstration of the ACFBCC technology. The project will be conducted under the DoE Clean Coal Technology Programme at the City of Lakeland’s McIntosh Power Station.

The DoE funding results from a combination of two previous Clean Coal awards. The DMEC-1 PCFB Repowering Project selected under round 3, and the Four Rivers Energy Modernization Plant (FREMP) selected under round 5.

The DMEC-1 project was intended to demonstrate non-topping PCFB technology (gas turbine temperature is essentially the same as the PCFB temperature), while the FREMP project was planned to demonstrate topped PCFB technology (gas turbine inlet temperature is markedly higher than the PCFB temperature). As a result, the plant will be tested in both the non-topping and topping operating modes. This was the first time that creating one project from two was carried out under the Clean Coal Technology Programme.

In August 1998, Lakeland released Foster Wheeler to begin preliminary engineering and permitting support of the demonstration plant. Lakeland is in the process of initiating permitting support of the demonstration plant. Lakeland is in the process of initiating permitting and licensing activities that are now expected to take 2 years. The plant startup is scheduled for 2003-2004.

The Lakeland McIntosh 4 plant will be constructed on undeveloped land located adjacent to the existing Unit 3.

The plant will be designed to burn a range of coals, including low-priced, high-ash, high-sulphur coals that are available on the open market. Limestone will be procured from Florida sources while the ash will be disposed in landfill or marketed.

The Lakeland project will be the world’s first commercial demonstration project of the Foster Wheeler PCFB combustion technology. Pressurised operation will mean that smaller units can be largely shop-fabricated, and that as a result, the system’s modular design will allow the utility to add economic increments of unit in order to match load growth.

The future of CFB technology

Fluidised bed combustion technologies – bubbling and circulating – have been accepted worldwide as advanced technologies for burning various fuels.

To insure the continued application of this technology in both atmospheric and pressurised combustion and gasification modes, it is necessary to address and to meet the trend drivers, namely environment, efficiency and economics. Some of the key issues involved are as follows:

Improve plant efficiency to 45 per cent near-term and to 50 per cent in about 10 years time.

Improvement of the combustion performance of hard-to-burn fuels.

Combustion efficiency improvement to over 99.5 per cent.

Generation and destruction/removal of trace elements/species, in addition to conventional gaseous pollutants such as SOx, NOx, and CO2.

Use of by-products with goal of zero waste generation.

System scale-up and impact on process behaviour.

Fluidised bed technology is making an impact as a technology of choice for hard-to-burn fuels. Its scale-up to utility size is being demonstrated at plants such as Turow.

The successful demonstration of the 300 MWe Jacksonville units will provide further evidence of viability. The pressurized hybrid combined-cycle circulating fluidised bed demonstration project at Lakeland, Florida, will take this technology to the next level of efficiency for the 21st century.