Imagine a box containing sand resting on a mesh. If air is blown very slowly upwards through the mesh, it percolates between the sand particles without disturbing them. When the velocity of the air stream is gradually increased, a point is reached when individual sand particles are forced upwards; they become supported by the air stream and begin to move about within a bed with a fairly well defined surface.
At still higher upward air velocities, an important change occurs; the bed becomes very turbulent with rapid mixing of the particles. Bubbles, similar to those in a briskly boiling liquid, pass through the bed and the surface is no longer well defined but becomes diffused. A bed of solid particles in this state is said to be 'fluidised', because it has not only the appearance, but also some of the properties, of a boiling fluid.
There are lower and upper limits of air velocity between which satisfactory fluidisation of sand, or any other granular substance, will take place. The velocity of the air stream causing fluidisation is termed 'fluidising velocity'. For a bed of any material, the larger the particles, the greater the velocity of the air (or other gas) that is required to fluidise it; for particles of a given size, the heavier they are, the greater the fluidising velocity needs to be.
In practice, a fluidised bed will contain particles of different sizes. The operating limits are set, on the one hand by the minimum air / gas velocity needed to keep the particles fluidised and, on the other hand, by the maximum velocity that can be used before an excessive quantity of bed particles are blown out of the bed containment box.

A fluidised bed of solids behaves in many ways like a liquid and has important characteristics:

• The bed finds its own level. If the vessel containing the fluidised bed of solids is tilted from a horizontal position, the surface of the bed remains level.
• Provided the fluidised state can be maintained, the bed can be transferred from one container to another as though it were a liquid.
• Solid particles in a fluidised bed are violently churned about; rapid mixing occurs and any added particles are quickly distributed throughout the bed.
• Objects can float or sink in a fluidised bed according to their density, as in a liquid.
• When a fluidised bed is heated, the thorough mixing enables heat to be rapidly transferred from one part to another, ensuring near uniformity of temperature, as in a boiling liquid. This is in contrast to conditions in a bed of stationary particles, in which heat is transferred by the much slower process of conduction from one layer of particles to another. Temperature differences in beds of stationary particles can be very high.
• Mixing in a fluidised bed causes heat to be rapidly transferred to a cooler surface (for example, a water tube) immersed in it. The constant movement brings a continuous supply of hot particles to this heat transfer surface.

Fluidised bed combustion (FBC) makes use of a fluidised bed of inert particles by the burning of fuel, usually (but certainly not exclusively) solid fuel e.g coal or biomass, within the fluidised solids. The fluidised solids dissipate and distribute the heat from the burning fuel particles such that the fluidised particles are virtually at a uniform temperature, both vertically and horizontally, everywhere within the bed. A typical fluidised combustion bed temperature is 850-900ºC. This uniformity usefully enables the combustion temperature to be confidently measured and controlled, such that some of the worst effects of conventional 'grate' combustion can be avoided. The fluidised bed also provides a high heat transfer rate to cooling surfaces e.g. boiler tubes, immersed within it. This provides means to reduce the size, and potentially the cost, of heat transfer equipment that uses solid fuel as its heat source. There are also potential environmental benefits from the use of FBC.

Fluidisation of solids
a) Sand particles resting on a mesh (left) become fluidised when air is blown through (right) and take on the appearance and some of the properties of a boiling fluid.

b) Granular solids remain in layers when one is poured on to another (left), but rapid mixing occurs on fluidisation (right).

c) A bed of stationary particles supports objects whatever their density (left). On fluidisation, an object of lower density (the green ball) floats while the higher density (red ball) sinks.

d) In a bed of stationary particles (left), heat is transferred slowly and there are big differences in temperature. In a fluidised bed (right), rapid mixing ensures uniformity of temperature.

The history of fluidised bed combustion (FBC) can be traced back to Fritz Winkler's fuel gas generator in Germany. During the late 1920s, Winkler and Lurgi were both working on the gasification of lignite, a fuel that could not be gasified by conventional coking methods that relied upon using externally heated retorts. The early Winkler process used a fluidised bed fed with lignite particles approximately 2mm in size, the gasifier bed being fluidised by a flow of steam and air (or oxygen).

Another early application of fluidisation was by Eugene Houdry who developed, around 1937, the use of fluidised catalytic cracking for petroleum refining in the USA, implemented by the Standard Oil Company (now Exxon) in 1942.

In France, Albert Godel had an idea for an 'lgnifluid' gasifier boiler around 1950. He worked with a young assistant, named Marcel Clovis Vaille, just graduated from the École Polytechnique. In the Ignifluid, an air-blown fluidised bed of coarse particles of coke generates a low heat content fuel gas, which is burned in secondary combustion within the free space above the fluidised bed.

The more familiar bubbling fluidised bed combustion originated in the mid-1950s from developments carried out in mainland Europe, the USA and Japan into methods of burning high ash coals without the heavy cost penalty of grinding them to a size suitable for pulverised fuel (pf) firing.

There seems general agreement that Douglas Elliot (who became Professor Elliot, but was always known as Doug Elliot; died June 1976) working for the Central Electricity Generating Board (CEGB) at their Marchwood Engineering Laboratory (MEL) in Southampton around 1960, first proposed the use of FBC in the UK. Doug Elliot's original idea was to apply the technology to recovering thermal energy from high carbon-in-ash residue remaining from anthracite burning power stations, but he later extended his idea to the possibility of applying FBC to coal-fired power generation in its entirety.

By around 1963 Doug Elliot's ideas had become an official part of the CEGB's research programme and around 1964 were laid before the Advisory Committee on Research & Development (ACORD) at the Ministry of Fuel & Power. It seems that this Committee found FBC to hold promise and suggested the technology be explored for use within all types of coal-fired boiler. This led to the involvement of the UK's National Coal Board (NCB; later British Coal Corporation, BCC) via its Coal Research Establishment (CRE) near Cheltenham and (especially in the first instance) the British Coal Utilisation Research Association (BCURA), located at Leatherhead, Surrey.

The following notes refer to coal as the fuel, since this was the fuel for which the development work was mainly carried out. However, the technology can also be applied to a wide range of biomass materials. This latter possibility is especially relevant now there is concern about global warming from the combustion of coal.

The basic concept is that coal is supplied to a hot bed of particles (for example, coal ash or silica sand) and is fluidised by the upward passage of a stream of (combustion) air. Because of the thorough mixing, the coal is quickly distributed throughout the particle bed and is rapidly burnt, producing heat at a high rate for steam raising, water heating or other purposes e.g process drying. The temperature of the bed is uniform.

Coal is supplied continuously to the bed and the ash remaining after combustion is removed continuously to keep the volume of the bed solids to a near constant. Because of rapid mixing and the high rate of coal combustion, the amount of unburnt material in the bed is small; typically the bed will contain 0.5 % to 5 % of coal (depending upon the coal size being fed). For satisfactory operation the bed is kept below the temperature at which coal ash begins to fuse or sinter. The bed is therefore operated within the range 750°C to 950°C, being the temperature at which a soft, fine ash is produced.

Control of the bed temperature within close limits presents no difficulty. Excess heat can be transferred at a high rate from the bed to steam raising water tubes immersed in it; approximately half of the heat generated from the fuel can be extracted in these tubes. This is an important feature requiring a smaller heating surface and bringing about a reduction in boiler size. A wide range of coal sizes (either lump or crushed coal) can be used. Coal size, the bed material and combustor dimensions depend on the application of the system; the two main uses are for the production of steam and heat in general industry and for the generation of electricity on a large scale.

The fluidised bed combustion of coal has major advantages over other solid fuel heating systems:

• The high rate at which heat can be transferred from the bed for steam raising or other purposes enables smaller (and cheaper) boilers and furnaces to be used.
• Because the concentration of coal in the bed is small, combustion is hardly affected by coal type, ash or moisture contents. The burning coal is surrounded by inert material, so that there is no tendency for coal particles to stick together. For the same reason, coals of variable ash and moisture contents do not upset the process. Low-grade fuels can be used in a fluidised bed combustor. Biomass is especially suited.
• High moisture content biomass absorbs heat from the high thermal capacity bed particles, so that the biomass dries and burns. When it burns, it returns heat to the bed particles, enabling subsequent drying and combustion to continue. This has been likened to a thermal fly-wheel.

Fluidised bed combustors are designed as part of a matched unit with a particular type of boiler or furnace and may loosely be categorised as deep beds which use in-bed fuel feeding, shallow beds which use over bed fuel feeding, and fast or Circulating beds (CFBC). Other systems have been developed, for example, pressurised fluidised beds (PFBC).

Deep beds
The 'deep bed' makes use of crushed coal (<3mm) burning in a bed of coal ash and fired into the bed through a number of in-bed injection points. The fine fuel particles require a fairly deep bed of about 1m, plus further free-board above the bed, in order for the fuel to mostly burn before being lost from the bed containment.

When using a reasonable fluidising velocity of about 2m/s, a grit (unburnt particles) refiring system is necessary to prevent excessive carbon in ash loss. A 'deep bed' combustor is more effective when sulphur retention is required for the control of SO2 releases, because the added limestone has more time to react in the deeper bed. The deep bed is also more suitable for unreactive fuels. Otherwise, the deep bed is rather bulky for anything other than large boiler plant.

Shallow beds
The shallow fluidised bed combustor employs a bed only 50-300mm deep, thus making considerable savings in fluidising fan power. The uncrushed fuel is simply dropped or scattered onto the bed surface. Low ash fuels are burnt in a bed of graded particles such as sand (e.g. 0.7 to 1.3mm, average 1mm). The fuel particles (e.g coal or biomass) can be substantially larger than the sand particles, e.g. low ash content 'washed' coals up to 32 mm can be burnt in a shallow bed at high combustion efficiency. Higher ash fuels, like unwashed coal, start off using sand but gradually the sand is replaced by a bed of residual coal ash. With coals of >10% ash content its necessary to limit the top-size of coal to perhaps 15mm and also control the equilibrium ash bed particle size grading. Nonetheless, the shallow bed, over-bed fuel feeding combustion system, results in a relatively simple, low operating cost, compact arrangement favoured for application to industrial boilers and furnaces.

Circulating beds
Circulating fluidised bed combsution (CFBC) features a fluidised bed operating at a high fluidising velocity of about 10m/s, which causes the bed particles to become fully (or near fully) entrained. The hot gases and entrained solids pass upwards to a cyclone separator and, following separation, the hot gas is available for use within a downstream boiler. The entrained bed material and residual fuel particles are returned to the fluidised combustor.
These notes (and those by CSL available through the Bradford University link at the top of this page) were not written with the intention of describing a CFBC system and those who wish to pursue that technology are advised to research the topic elsewhere.

FOREWORD to:
Combustion Systems Limited, Fluidised Bed Combustion (FBC) Design Manual

The following notes have been written as a Foreword to the CSL Manual at a time (July 2005) when the document is being prepared for placing into the UK's public archive. It is more than 10 years since the demise of British Coal Corporation (BCC - December 1994) and a similar length of time since the company named Combustion Systems Ltd (CSL), being part of the intellectual property rights (IPR) vested within BCC's Coal Research Establishment (CRE), became privatised.

The CSL Design Manual gives both scientific and practical engineering information relevant to (predominantly) the design of atmospheric pressure bubbling fluidised bed coal fired combustion systems. Although primarily intended for application to coal firing, much of the information contained within the Manual is equally applicable to biomass combustion, waste incineration and thermal processing. One Section, No.8, deals with the early development of coal fired pressurised fluidised bed combustion (PFBC), but considerably more detailed information is available in archive material (elsewhere) that deals with the extensive large scale experimental work carried out at the Grimethorpe colliery site.

Archiving of the CSL Manual is being undertaken by the British Coal Utilisation Research Association (BCURA), a funding organisation promoting research and other activities concerned with the production, distribution, and use of coal and its derivatives. This funding organisation derives from the original BCURA laboratories situated at Leatherhead, in Surrey.

Funding to enable the administrative activities of archiving the CSL Manual, including writing this Foreword and three Supplementary Sections (to Sections 5, 12 and 13), has been provided by Professor William G Kaye (Bill Kaye), a Council Member of BCURA and former Assistant Director of CRE. Professor Kaye was responsible for the development work into fluidised bed combustion that took place at CRE from the early 1970s, including the technical collaboration with UK coal-firing equipment manufacturers that resulted in around 120 prototype, demonstration and commercial coal-fired fluidised bed combustion (FBC) boilers and furnaces being installed at customer premises by the late 1980's. Professor Kaye donated his copy of the CSL Manual in order to facilitate archiving.

Maurice Fisher, previously a Team Leader working under Professor Kaye in CRE's Industrial Development (ID) Branch, undertook authorship of this Foreword and the Supplementary Sections. Part of Mr Fisher's responsibilities while employed by CRE was to provide technical support to CSL and ensure that up-to-date information was made available to Licensees in advance of such information being included within the Manual.

Professor Kaye and Mr Fisher wish to record their appreciation of the efforts of BCURA's Technical Officer, Dr David J A McCaffrey, who made possible the practical aspects involved in archiving the CSL Manual.

The historical text contained within Section 2 of this Foreword is provided for general information only; its accuracy cannot be entirely guaranteed. However, in pursuit of accuracy, Maurice Fisher gratefully acknowledges the contribution made by Mr John Highley, who held a scientific responsibility for FBC R&D at CRE from 1966 through into the 1980's. During the 1970's, John Highley was responsible for bringing the concept of 'Shallow Bed' FBC (see this Foreword, Section 4.2) to commercial reality. Some of John's recollections are supported by the books: Skinner D.G "The Fluidised Combustion of Coal", 1970, published by Mills & Boon, ISBN 0263317144 and Howard J.R (ed) "Fluidized Beds: Combustion and Applications", 1983, Applied Science Publishers, London & New York. Maurice Fisher also made reference to the book by Dr S J Wright (Steve Wright) entitled "Fluidised-Bed Combustion, A Clean Coal Combustion Process that Nearly Was", March 2001, printed by F M Repro Ltd (Tel. 01924 411011). Dr Wright was working at the BCURA laboratories when FBC was first considered, in 1964.

1. The Principles of Fluidised Bed Combustion (FBC)

Before proceeding to explain the derivation of CSL, the following notes are included for convenient guidance of anyone wishing to understand the fundamental technology covered by the CSL Manual.

When a packed bed of small granular free flowing particles are subjected to an upward gas flow, the particles initially remain static but they impose a resistance on the gas flow which increases as the gas flow rate increases. When the upward force created by the gas flow through the packed bed of particles equals the weight per unit area of the packed bed, the particles become suspended within the upward gas flow. The bed is then considered to be at minimum (or incipient) fluidisation and the upward gas velocity is termed the 'minimum fluidisation velocity'. Any further increase in the gas flow rate does not significantly affect the gas flow resistance, but any gas volume in excess of the minimum will (with the size of particles under consideration) result in the formation of bubbles rising through the bed of particles. The particles then take on the characteristics of a boiling fluid. Above the minimum fluidisation velocity, the upwards and sideways coalescing movement of these bubbles provides intense agitation and mixing of the bed particles. In this state the bed particles can transfer heat at very high rates (much greater than convective gas heat transfer) to any cooler surfaces in contact with them.

If the upward gas flow is turned off, the particles become static again i.e. they become defluidised or 'slumped', and settle down onto their supporting base plate, termed the gas (but commonly air) distributor plate. The purpose of this plate (though physically it may not take the form of a flat plate) is both to support the static particles and also to evenly distribute the fluidising gas flow across the whole base area of the particle containment.

In a FBC system, the particles are initially heated to above the ignition temperature of the fuel to be burned and then combustion takes place when the fuel is delivered into or onto the heated fluidised particles. The fuel burns by virtue of the oxygen within the fluidising gas (commonly atmospheric air), which is delivered by a fan (or blower) through the distributor plate and upwards through the bed particles.

Various refractory materials can be used to form the original 'bed' of particles, the most convenient being graded sand, around 1mm in mean diameter, enabling fluidising velocities in the range 1 to 3m/s. Alternatively, graded limestone or dolomite can be used if sulphur dioxide (SO2) capture is required. If SO2 capture is required on a continuous basis, limestone (or dolomite) has to be continuously fed, either mixed with the fuel or, more usually, via a separate feeder arrangement. When fresh particles are continuously being fed into the fluidised bed, excess have to be drained away, either through the air distributor plate or via an overflow weir. This maintains the design bed height (commonly measured by its gas flow resistance), which is important both with regard to preserving the design air flow resistance and also, depending upon circumstance, the turn-down characteristics of a FBC boiler.

Let us consider that the burning fuel is coal. Then, if the coal has only a very low, friable, ash content, the coal ash is mainly degraded by the action of the fluid bed, such that it is substantially carried away i.e. elutriated, within the emergent flue gases. Alternatively, if a high ash fuel is burned, especially one that leaves behind particles of adventitious stone or hard ash, some of the 'ash' remains in the bed. If such ash is of similar size to the original bed particles, the ash will fluidise and eventually replace the original bed particles. Excess may then need to be removed in order to maintain the design bed depth.

However, a coal that leaves behind hard ash particles that are significantly larger than the original bed particles will eventually cause the bed to defluidise, unless the concentration of large particles is controlled. Without complete fluidisation, heat released from the burning coal is unable to dissipate freely to the surrounding bed particles and high temperature clinkering ensues.

In order for a coal burning fluidised bed to operate efficiently, it must be smoothly fluidised throughout its volume and the bed particles need to be controlled in the range 850-950 C. During normal operation this temperature is achieved and stabilised by the opposing effects of the heat input from the burning fuel, versus outgoing heat in the flue gases and further heat transferred to any cool surfaces in contact with the bed particles. In the case of a FBC boiler, bed cooling surface is provided in the form of in-bed tubes and water cooled containment walls, integral with the boiler construction. Combustion gases leaving the surface of a FBC boiler's fluidised bed at (say) 900°C are then constrained to flow through or across additional conventional boiler heat transfer surface so that the gases are cooled to (say) 200 C before leaving the boiler and passing to atmosphere through the plant chimney.

When FBC is used other than in a boiler e.g. a hot gas furnace where there are no water-cooled surfaces, the bed temperature is stabilised (most simply) by passing excess fluidising air through the bed. Hot gases from the fluidised bed within a furnace are mostly used for process drying applications.

In order to understand the range of FBC technology marketed by CSL, it is necessary to appreciate that several FBC technologies appeared during the 25 year period 1965-1990, developed by various organisations world-wide. Technologies included Pressurised Fluidised Bed Combustion (PFBC) and Circulating Fluidised Bed Combustion (CFBC). The prospects for Pressurised Circulating Fluidised Bed Combustion (PCFBC) also received some attention.

In order to distinguish the original atmospheric pressure FBC technology from the more complex forms that emerged later, the earliest and simplest form of FBC became known as Bubbling FBC or BFBC. It existed in two forms, though without a hard distinction between them. From 1974 CRE applied BFBC to a system called 'Shallow Bed' BFBC, as distinct from the earlier work which, from thence onward, became known as 'Deep Bed' BFBC. The main difference between the two was that 'Shallow Beds' operated with beds of particles generally below 350mm deep but mostly only 150mm; sometimes only 75mm. Such beds burned low ash 'washed' coal, typically 25mm top-size. 'Deep Beds' utilised beds up to 1m deep and burned crushed coal with an ash content often up to 20%. The CSL Manual very predominantly deals with BFBC, both 'Deep Bed' and 'Shallow Bed' technologies, though it also includes early PFBC work, as carried out at BCURA from around 1967. At that time, and for the next two decades, the UK was a world leader in BFBC & PFBC 'know-how'.

For convenience in the following Sections, the term FBC implies atmospheric pressure BFBC. The only other variant considered is pressurised FBC, which is referred to as PFBC.

2. The UK History of Fluidised Bed Combustion

There seems unanimous agreement that Douglas Elliot* (Doug Elliot) working for the Central Electricity Generating Board (CEGB) at their Marchwood Engineering Laboratory (MEL) in Southampton around 1960, first proposed the use of FBC in the UK. Doug Elliot's original idea was to apply the technology to recovering thermal energy from high carbon-in-ash residue from anthracite burning power stations, but he later extended his idea to the possibility of applying FBC to coal-fired power generation in its entirety.

[*Doug Elliot subsequently became Professor Elliot of the Mechanical Engineering Department of The University of Aston in Birmingham.]

By c1963 Doug Elliot's ideas had become an official part of the CEGB's research programme and c1964 were laid before the Advisory Committee on Research & Development (ACORD) at the Ministry of Fuel & Power. It seems that this Committee found FBC to hold promise and suggested the technology be explored for use within all types of coal-fired boiler. An outcome was that BCURA were commissioned to carry out a literature survey and make recommendations for further action.

The National Coal Board (NCB - predecessor to BCC) subsequently requested CRE to similarly conduct a FBC review by applying CRE's expertise from many years work on fluidised bed carbonisation (the manufacture of Homefire and Roomheat smokeless fuels). Although the BCURA report was published 12 months in advance of that by CRE, it seems likely that both organisations were familiarising themselves on a near parallel time scale.

BCURA may have taken the initial lead by being given responsibility by ACORD for applying FBC to industrial boilers, whereas CRE's involvement commenced somewhat later when the CEGB requested that the NCB take over development of FBC for power generation. Although BCURA had done sufficient cold studies to enable them to commission their 27" (685mm) hot test rig by 1966, CRE also had a small team working on FBC by that time and commissioned their own FBC combustion rig by early 1967. Initially CRE converted a fluid-bed carboniser to FBC by installing in-bed cooling coils, but this arrangement subsequently gave way to a purpose designed 3ft (915mm) test rig.

From these dates, the activities at BCURA and CRE diverged along different paths, though inevitably with some cross-fertilisation of accrued information.

BCURA applied its R&D experiences to the design, installation and hot testing of a commercially procured (Cochran) vertical shell steam boiler, rated at 3.7tph of steam (from and at 100°C). The fluidised bed material was contained within the boiler's vertically orientated 1.22m diameter firetube. The overall objective of the programme was to evolve a design of industrial boiler that would be significantly cheaper than conventional coal fired boilers and hence improve the competitive position of coal in the packaged boiler market. Installation of the boiler was completed by the beginning of July 1969 and over 400 hours of development programme was achieved by March 1970. At that time the unfavourable relative cost of coal vs fuel oil in the UK industrial market caused the work to be suspended (Ref: "Final Report on the Development of a Shell-Type Fluidised-Bed Boiler Burning Coal", Report No. DHB 010172, January 1972).

At around the same time that BCURA began their design of a vertical shell industrial boiler, another part of BCURA began work on an advanced power generating FBC variant. This was pressurised fluidised bed combustion (PFBC). The motivation for this novel concept was two fold (i) more compact power generation boilers than are feasible using PF or conventional atmospheric pressure FBC firing (thus saving on capital costs), and (ii) higher power generating efficiency (10% or more) by utilising a combined cycle involving both gas and steam turbines. Although the NCB intended CRE to carry out the development of FBC for power generation, PFBC research was conveniently carried out at BCURA because they had a suitable pressure vessel from a previous programme (work into Magneto Hydro Dynamics, MHD, on behalf of the CEGB).

The enormous physical size of power generation boilers precludes full-scale trials. Hence, the approach at CRE was necessarily one of building flexible hot and cold test rigs in support of their scientific research into power generation using atmospheric pressure FBC. The test facilities were especially concerned with investigating the effect of various operating conditions on coal combustion efficiency and ascertaining heat transfer coefficients to different arrangements of in-bed heat transfer (bed cooling) tubing. CRE also sponsored research on heat transfer at Birmingham University and on the combustion process at Cambridge University.

It should be noted that 1967 pre-dates the concept of environmentally beneficial "clean coal" combustion (and gasification) technology. The aim when CRE began their R&D work into power generation FBC was only to develop technology that would reduce the capital and operating costs associated with conventional combustion technology, being pulverised fuel (PF) coal firing. PF firing suffers from high operating costs associated with grinding coal to face powder fineness and maintenance costs caused by molten ash deposits forming on the boiler tubing (an inevitable occurrence with the flame temperature of pulverised fuel burning) causing a reduction in boiler thermal output and metal wastage due to corrosion.

Although the CEGB could see the potential advantages in FBC technology compared to PF firing, the incentive for them to contribute to the R&D effort was not strong. From the mid-1950's and throughout the 1960's was an era of cheap oil in the West and, having failed (during the mid-1960's) in their effort to develop coal based MHD generating technology, the CEGB saw oil fired power stations as a simpler alternative to developing a new coal burning technology. There was also confidence in an expanding future for nuclear energy in electrical power generation, further militating against the CEGB developing coal fired FBC. The NCB, fearing that without a new power generating technology they might lose their coal supply market to oil and nuclear, continued funding FBC R&D both at CRE and at BCURA until 1971, the year in which BCURA substantially closed (a small contingent remained at Leatherhead for a few years thereafter completing external contracts).

3. Formation of CSL

By the early 1970's sufficient FBC R&D had been completed to enable the NCB to approach the CEGB to jointly fund a prototype coal fired power generating FBC demonstration plant. The NCB wished to build a prototype FBC boiler to demonstrate the future application of the technology to power generation, but the CEGB was not convinced of the value of such an exercise. Although plans were made, and tenders procured, for installation of a 20MWe (electrical) output FBC boiler to be located at the NCB's Grimethorpe Colliery in South Yorkshire, this prototype was never built. (It is important not to confuse this conceptual FBC prototype with the PFBC research facility that was eventually constructed at Grimethorpe in the late 1970's.)

The reluctance of the CEGB to contribute to the prototype 20MWe FBC boiler led to a (temporary) halt in UK FBC research. However, with over 5 years of practical R&D information available from both BCURA and CRE, there was concern that this store of 'know-how' should not be wasted. Hence, the UK Government required its National Research and Development Corporation (NRDC) to take the lead in protecting the UK's FBC technology know-how. This it did by setting up a licensing organisation to market the UK's FBC technology world-wide. This organisation, named Combustion Systems Ltd (CSL), marketed its 'know-how' via a Design Manual, the 'Final' (though in fact only the first) version of which was approved in March 1972. At that time, all UK FBC R&D work came to an end (apart from some ongoing contract work at BCURA).

Apart from the NRDC, CSL's commercial partners were British Petroleum (BP) and the NCB. BP's involvement was prompted by the ability of FBC to cheaply capture acidic sulphur dioxide gas (SO2) emitted during the combustion of heavy and residual fuel oils. The subject of 'Acid Rain' was emerging as a world-wide concern around 1970 and in recognition of this BP carried out preliminary work at their Sunbury laboratory into the application of FBC. By utilising low cost limestone (calcium carbonate) as the fluidised bed material (at this time all coal-fired FBC used residual coal ash as the bed material), BP confirmed that they could 'annex' SO2 as solid calcium sulphate (the use of limestone for SO2 capture had already been demonstrated in the US). From 1970 and for the next several years, BP contracted BCURA to carry out similar, but larger scale, work using the Cochran vertical shell boiler. This BP contract led to the development of novel 'climbing film oil' nozzles (CFNs - see Sections 5 and 15 of the CSL Manual) as a means of efficiently burning liquid fuels in FBC boilers.

Apart from the environmental potential of FBC to cheaply capture acidic SO2 from the flue gases of sulphur bearing fuels (solid, liquid or gas), the awaking need to control acidic stack emissions also focussed on oxides of nitrogen (NOx). Nitric oxide (NO) and nitrogen dioxide (NO2) both contribute to 'Acid Rain' by the formation of nitric acid. NOx also contributes to crop damage and to health damaging photo-chemical smog. Since NOx emissions are generally greater when combustion temperature is high (as with PF coal firing), low temperature FBC combustion was seen to have a (previously unvoiced) further advantage of inherently lower NOx emissions. Thus, around 1970 FBC first became seen as a potential environmentally 'clean coal' technology (and equally a 'clean' means of burning residual fuel oils).

3.1 FBC Revival

UK (and world-wide) interest in FBC was revived by the sharp increase in oil price following the 4th Arab-Israeli War during late 1973. In October 1973, members of the Organisation of Petroleum Exporting Countries (OPEC) reduced supplies of oil and increased prices sharply. Comparing 1974 with 1973, the price of oil imported into the UK almost trebled.

With British industry heavily reliant upon what was then high cost fuel oil, a Labour government strategy was evolved with the NCB in 1974, called 'Plan for Coal'. This sought to re-expand the use of coal in industry. However, during the previous 20 years UK industry had become used to the physically clean, automatic operation inherent when burning fuel oil and so a new coal burning technology was required that would compete by providing similarly clean, efficient and automatic combustion.

Another important requirement to the NCB was that the new combustion system should be less selective of coal type. Conventional industrial coal burning systems prefer the higher volatile, more reactive, free burning (none coking) coals, which limited the market demand for coals from certain pits and increased transport costs because of the need to supply coals from other mining areas.

CRE identified FBC as a prime candidate to satisfy both the coal customer and coal supplier, but it needed to be in a new format and not merely an adaptation of the techniques developed for FBC power generation. The resulting design became known as 'Shallow Bed' FBC and was a UK innovation developed predominantly through the efforts of CRE sponsored by the NCB (later BCC). Commercial proliferation was accomplished by CRE collaborating with many UK equipment manufacturers.

'Shallow Bed' FBC was optimised specifically to the needs of industrial coal burning applications. It used washed, low ash, coals (as conventionally supplied to industry by the NCB) in low height combustion chambers and used (relatively) low-pressure fans. There was no intention to include sulphur retention because low-sulphur coals were available (adequate to the industrial environmental legislation of the time).

The development and commercialisation of 'Shallow Bed' FBC was carried out at CRE from about 1974 to the late 1980's. Starting with an extensive R&D programme involving laboratory scale studies, it progressed through the installation of full-scale prototype boilers and hot-gas furnaces at industrial premises and culminated in extensive collaboration with many UK coal-firing equipment manufacturers. The whole activity was supported by the NCB / BCC Marketing Department. It became a £multi-million development and demonstration programme which resulted in substantial market exploitation of the newly emerged FBC technology. Individual equipment manufacturers applied their own design skills to their commercial FBC contracts and the proprietary 'know-how' remained the property of these individual manufacturers. However, the fundamental knowledge and research results, which came from the collaborative efforts, were made available to CSL and formed a substantial addition to the Manual.

The original slim A4 soft back CSL Design Manual written in 1972 grew over the subsequent 20+ years into a substantial (1200+ pages) edition, supplied to CSL licensees in several stiff-backed ring binders. This format enabled the simple addition of new, and replacement of updated sections, as they were approved and made available.

It is this multi-volume Design Manual, together with this Foreword and three Supplementary Sections, which is now being archived.

4. FBC Technologies Within the CSL Manual

4.1 Deep Bed FBC

At the time when CRE commenced their research into coal fired FBC for power generation, the CEGB were interested in a lower cost, lower maintenance alternative to 50 year old PF combustion technology.

The new combustion system was required to:

• Equal the efficiency performance of PF coal firing,
• Eliminate heat transfer surface fouling and corrosion,
• Reduce the high electrical power consumption and maintenance costs associated with grinding coal to PF powder (typically 70% below 76microns and 98-99% <245microns).
  The environmental 'clean coal' benefits of low cost SO2 and NOx emission reduction, as identified later in the development, were not priorities at the outset. Concern over 'Acid Rain' did not start to impact on the CEGB until around 1970.

Historically, the NCB (and later the British Coal Corporation, BCC) supplied the electricity generating industry with coal termed 'part treated smalls', meaning that the coal was essentially 'raw' run-of-mine coal with a quantity of washed (low ash) lump coal blended back into the 'raw' coal in order to control the final average ash content to typically 18%. The evolving 1960's FBC technology was expected to receive similar coal and burn it to at least 99% combustion efficiency, as is achievable with PF firing. High combustion efficiency in PF firing is obtained by virtue of the coal being milled to fine powder and burned in large furnaces at flame temperatures approaching 2000 C. However, the former is undesirable because of its high power consumption, while the latter creates the ash slagging conditions which create undesirable fouling and corrosion of boiler tube heat transfer surfaces.

The FBC arrangement which emerged to challenge PF firing consisted of crushing the coal to a top-size of some 2-3mm and pneumatically injecting it via a number of feed points into the base of a deep fluidised bed consisting of coal ash particles averaging around 1mm. Coal crushing was necessary in order that the considerable quantity of residual coal ash should not cause the bed to lose fluidisation due to contamination by oversize particles. Although coal crushing introduced a power consumption penalty, it was only 20% (or less) of the power required to grind coal to PF size.

Coal crushing satisfactorily resolved the problem of burning coal and maintaining the desired bed particle size inventory, but crushing had the disadvantage of producing additional coal fines which were difficult to burn because they tended to be quickly conveyed out of the bed by the rising fluidisation bubbles (termed 'elutriation'). Thus, these coal fines carried away significant unburned carbon. To reduce the proportion of carbon fines escaping unburned, concepts studied included:

• A deeper bed. Bed depths became progressively deeper as the development proceeded, eventually nearing 1m static i.e. unfluidised, with a commensurate fluidising air flow resistance of up to 0.1bar. The air distribution system, requiring an air flow resistance proportionate to the bed resistance, added to the mounting uneconomic requirement of high fan power.
• The fluidising velocity was limited to 0.9m/s to minimise the loss of unburned coal fines. This implied a very large bed area for full-size power generation boilers and conceptual designs became known as 'ranch style' due to their required plan areas. Subsequently, the 'stacked bed' concept appeared more realistic, where several beds were placed one above the other within a single, tall, combustion chamber. This was not seen as a disadvantage for future power plant applications, since PF boilers are inherently tall for the reason of achieving sufficient residence time for coal burn-out.
• Pneumatically feeding the crushed coal, with its fines content, to the base of the fluidised bed, maximising its opportunity for burn-out. This involved complex designs of flow splitters to ensure the coal was evenly distributed to many feed points across the whole plan area of the bed (or beds).

Even including all the above sophistication, achieving 99% combustion efficiency in order to compete with existing PF technology, proved elusive. Thus, fines recycle cyclones were added to the combustor exit in order to enable recycling of gas borne carbon fines back to the fluid-bed. Since inert ash was inevitably recycled along with the unburned carbon, the recycle loop substantially increased the quantity of solids needing to be injected into the base of the fluid-bed along with the coal. This added further to existing concerns about the complex design of the pneumatic coal feeding system.

As the development proceeded and concern over 'Acid Rain' grew, CRE also studied how limestone can be fed into a fluid-bed and provide low cost SO2 capture. On entering the fluid-bed, operating at 850-900°C, limestone (CaCO3) calcines to lime (CaO) which annexes gaseous SO2 as gypsum (CaSO4).

4.1.1 The Vertical Shell Boiler at BCURA

The original design concept for the Cochran 3.7tph steam industrial boiler at BCURA was based upon a similar coal crushing strategy as was applied to FBC for power generation. FBC know-how was still in its infancy and there was a presumption that coal crushing was a basic requirement in order that coal particles would be buoyant and mobile within the fluidised bed.

There was no specific requirement to achieve 99% coal combustion efficiency for industrial boiler applications. The needs of the industrial coal boiler market were (are) more modest, and even 95% combustion efficiency can be acceptable if the industrial coal price is competitive. Hence, the coal was crushed to a coarser size, 3-6mm, which gave a coarser residual bed ash. This enabled higher heat release rates per unit of bed area by virtue of the coarser bed material accommodating fluidising velocities up to 3m/s, rather than the 0.9m/s necessary to prevent excessive unburned carbon losses in power generation research. This helped the concept by reducing the size of boiler relative to its steaming rate.

Initially, the same arrangement of pneumatically injecting crushed coal into the base of a deep (0.5m) fluidised bed was employed as in the research for power generation FBC. However, just before the boiler development ended, a few tests were undertaken using conditions that were novel at the time.

The novelty was to feed uncrushed, washed i.e. low ash 'smalls' coal (13mm to zero), onto the fluidised bed from above, by gravity, using a simple variable speed screw conveyor. With no coal crushing this drastically simplified the overall coal preparation and feeding system. The bed depth was only some 0.3m deep at this stage in the development in order to reduce the amount of cooling surface contacting the bed. Combustion efficiency immediately improved to around the target 95%. This predominantly occurred because the uncrushed coal lost significantly less heat as unburnt coal fines.

The concept of over-bed feeding lump, washed, coal to fluidised beds was, as will be read below in Section 4.2, extensively applied by CRE to commercial industrial boilers and furnaces from 1974 through to the late 1980s. However, CRE evolved this technique independently of BCURA. The work on the vertical shell Cochran boiler at BCURA ended in 1969, several years before the NCB requested CRE to develop a sophisticated new industrial coal firing concept suited to the second half of the 20th Century. By this time BCURA had closed (1971). The BCURA report containing the few over-bed washed coal tests had not been widely circulated, nor had the full significance and promise of the washed coal tests been appreciated at the time the report had been written.

In subsequent years CRE management acknowledged the work at BCURA as the first practical demonstration of feeding lump coal by gravity to a fluidised bed, but the concept of applying this to a truly shallow bed (down to 75mm) very much originated at CRE.

4.2 Shallow Bed FBC

Industrial boilers need to be physically compact, operationally simple and use low power fans. To satisfy these requirements CRE evolved the concept of 'Shallow Bed' FBC where uncrushed 'lump' coal is fed to the surface of a fluidised bed typically only 0.15m (static) depth. Combustion efficiency was found to be >96% with reactive industrial coals, comparable to that obtained from conventional industrial boilers burning similar coal on mechanical stoker grates. Apart from the adoption of beds typically only 0.15m deep, CRE also demonstrated that lump coal, even up to 50mm, remained buoyant and mobile within the 1mm fluidised bed material. This is because a fluidised bed behaves in many ways like a liquid and lump of coal will float, providing its density is lower than the effective density of the bed. Hence, the lump coal burned within the finer bed material without sinking or agglomerating.

The main starting point for the adaptation of FBC to UK industrial boilers was the ready availability of sized and washed (low ash) coals from the NCB. Washed coal had, for some long time, been supplied to UK industry for burning on conventional mechanical grate stokers e.g. chain grates. It was considered unreasonable that a relatively small user of coal should pay the additional penalties involved in using high ash 'part treated' power station coals, where the high ash content is brought to site as part of the coal delivery and then taken away again as combustion discard.

The two main types of washed and size graded coals were:

• 'Singles' coal, being typically 25mm top-size down to a nominal 10mm, and
• 'Smalls' coal, being typically 13mm top-size but containing up to 30% below 3mm.

'Singles' coal has predictable flow characteristics i.e. a predictable angle of repose, which made it best suited to the smaller industrial coal customer. 'Singles' could be relied upon to have <7% (often <5%) ash content. 'Smalls' coal, while of lower thermal cost, required more sophisticated handling equipment and had an ash content typically 7 10%. 'Smalls' were supplied to the medium to large coal consuming industrial customer.

With so little ash in these industrial coals there was insufficient residual to form a bed of inert particles. Hence, silica sand, sized around 1mm, became the standard bed material for 'Shallow Bed' FBC. This sand size permitted fluidising velocities in the range 1-3m/s. With the coal being in 'lump' form, a few large dense ash (mostly stone) particles remained after coal combustion, with a tendency to sink to the base of the bed. Hence, bed cleaning systems were developed which enabled oversize ash particles to be separated from the sand, while the bed remained in hot, fluidised, operation.

Since the coal was washed and uncrushed, the quantity of coal 'fines' within the coal feedstock was minimal, thus eliminating the need to inject the coal into the base of a deep bed to avoid the fines escaping unburned. In consequence, simplified over-bed feeding techniques could be applied in conjunction with a much shallower bed. This, in turn, required only a low height boiler combustion chamber to contain the fluidised bed material, which meant that the 'Shallow Bed' concept suited conventional height industrial boilers and boiler houses.

Over-bed coal feeding is far simpler than any in-bed arrangement and enables a variety of mechanical feeding methods. For the largest industrial boilers it was possible to apply commercially available spreader thrower feeders to distribute the coal across a substantial bed area. Two such feeders in tandem could feed coal to a 30MWth (thermal) output boiler.

By 1976 the first generation of 'Shallow Bed' FBC prototypes were installed at various UK premises. These included boilers and hot gas generators (furnaces), the latter being mainly for grass drying in cattle food manufacture. Between 1976 and 1985 the majority of the commercial coal equipment suppliers within the UK adopted the technology and some 90 boilers plus 30 furnaces were installed, totalling over 1,200MW thermal. Individual units were generally small; many were below 5MWth, but a few boilers reached 35MWth and furnaces reached 40MWth.

The photograph shown here is of the 2MWthermal vertical shell hot water boiler, as installed to heat the commercial tomato growing greenhouses at the Co-operative WholeSale Society (CWS) premises, at Marden near Hereford. It was in operation from June 1977 until 1990.

4.2.1 Shallow Bed BFBC with High Ash Coals

Although the use of washed, low ash, coals substantially avoided the problem of large ash particles being left behind after coal combustion, washed coals did leave occasional large ash particles and required the bed to be 'cleaned' in order to avoid gradual defluidisation. This was achieved by the incorporation of occasional drain ports into the air distributor that connected to relatively simple sieving systems, using either mechanical or aerodynamic principles.

Although UK 'Shallow Bed' FBCs operated (almost) exclusively on washed coals, there was pressure from collaborating manufacturers and CSL's overseas licensees to extend the range of coal types suited to shallow bed combustion. High ash coal, fed in lump form, over-loaded the fine bed material with oversize ash because the natural migration of the residual ash was insufficient to enable an adequate removal rate via the air distributor's drain ports.

One solution was to use a hybrid arrangement between the earlier crushed coal deep-bed system and the newer shallow bed design. By partial crushing of a high ash coal to, say, 10mm, in conjunction with added moisture to 'bind' the resultant coal fines to the larger coal lumps, the coal could still be fed over-bed and the resulting large ash would be limited to the 10mm top-size of the coal. The 10mm ash exhibited enhanced migration to off-take ports in the air distributor such that, even at a high rate of large ash production, sufficient could be removed by the sort of bed cleaning system developed for washed coals (see CSL Manual Section 15, Figure 15.5).

Another solution entailed the development of new air distributor designs to enable high ash coals to be fed at 25-30mm top-size. This work commenced around 1982. The basic idea was to modify the previous flat plate air distributors to provide for downward extraction of bed material over the whole area of the bed, removing the reliance on large ash migration to specific off-takes. However, this was not a popular solution as it had the disadvantage of imposing additional height on the combustion system, off-setting one of the prime advantages of the compact shallow bed arrangement (see CSL Manual Section 15, Figure 15.7).

The most likely contender began development too late to be thoroughly tested, though short-term trials on test facilities and a small commercial FBC showed encouraging results. This design, termed the 'directional air distributor', involved the fluidising air entering through a flat plate distributor but via standpipes that encouraged cross movement of oversize ash to conventional bed cleaning drain ports (see CSL Manual Section 15, Figure 15.6).

It is unfortunate that distributor designs which promised to extend the application of shallow bed BFBC to higher ash, lower cost, coals did not become ready for full-scale commercial trial until around 1985. This coincided with a year long miner's strike and curtailed development. Thus, their potential was never realised.

4.3 Pressurised Fluidised Bed Combustion (PFBC)

Research into PFBC, as a possible combined cycle high efficiency power generating system, commenced at BCURA in 1967. By 1969 a 1.2m x 0.6m bed area test facility was in operation at BCURA. Several successful 100h test runs were completed using this facility at pressures up to 6bar.g, studying, amongst other things, wear rates and ash deposition on a static gas turbine blade cascade. When BCURA formally closed in 1971, operation of this PFBC facility continued for several years, sponsored by contracts from the US.

In 1975, under the auspices of the International Energy Agency (IEA), the governments of the USA, West Germany and the UK agreed to build and operate a PFBC of 80MWth capacity and 12bar operating pressure, located at Grimethorpe Colliery, South Yorkshire. Construction started in 1977 and commissioning was complete by 1980. The experimental IEA programme lasted from September 1980 to March 1984. A supporting PFBC programme, studying boiler tube and turbine blade metallurgy, was carried out by CRE in conjunction with the Central Electricity Research Laboratory (CERL; based at Leatherhead) during the latter 1970's.

Subsequent to the IEA Grimethorpe programme, a further period of testwork was carried out at Grimethorpe from 1984 to 1988, funded by BCC and the CEGB. Supplementary funding came from the US Department of Energy, the US Electric Power Research Institute (EPRI), the EC and residual IEA finds. Work at Grimethorpe finally ended in 1992.

Although the CSL Manual has Section 8 devoted to PFBC, the information within Section 8 predates all of the Grimethorpe experimentation. Anyone wishing to research PFBC in depth should consider Section 8 only as an introduction to the technology. The Grimethorpe information is archived separately to this CSL Manual.

5. CSL Licensees

By the latter 1980's, CSL publicity shows FBC technology to be a well established commercial technology for steam raising (boilers) and industrial heating (furnaces) in small and medium sized plants. As well as being extensively used in the UK, the UK's technology was available on a world-wide basis to CSL licensees. In total, some 1,200MWth capacity utilising this technology was in operation at the time of peak interest.

The CSL publicity literature from the mid-1980's lists Licensees as:

In the UK:
Babcock Power (applying the technology to PFBC); Babcock also had commercial 'links' with organisations in Sweden and the USA; EMS Thermplant (applying FBC technology to fire-tube and packaged water-tube boilers); Motherwell Bridge (applying FBC technology to waste incineration); ME Boilers (applying FBC technology to 'Coil' boilers) and Stone Danks (applying FBC technology to fire-tube and packaged water-tube boilers).

In Italy:
Kinetics Technology International (applying the technology to petroleum and chemical processing furnaces and waste incineration).

In Belgium:
Seghers Engineering (applying the technology to calciners).

In Japan:
Hitachi Zosen (applying the technology to waste incineration and fire-tube boilers, including petroleum coke combustion); Yoshimine Boiler (applying the technology to water-tube boilers).

In Korea:
Hyundai Heavy Industries (applying the technology to industrial boilers).

In India:
IAEC (applying the technology to packaged boilers).

In USA:
Stone-Johnston Corporation (applying the technology to fire-tube and packaged water-tube boilers).

6. Demise of UK FBC

It is unfortunate that, by the time of archiving the CSL Manual (2005), most of the coal fired FBC boilers and furnaces designed using the CSL technology, were either closed or converted to gas and / or oil burning. The once rapid proliferation of 'Shallow Bed' FBC technology was stalled by changing circumstances.

• During 1984-85, a year long UK coal miners strike damaged customer confidence in coal, exacerbated by the anticipated privatisation of BCC.
• Oil and gas prices became more competitively priced relative to coal.
• EU Directives imposed stricter environmental legislation on industrial boiler emissions.
• Climate change fears discouraged the use of coal more than oil or gas.

Many industrial coal users were tempted to convert away from coal and join the so-called 'Dash for Gas' in order to make savings in manning levels, maintenance costs and achieve surer compliance with environmental requirements. In consequence, there was a slow but inexorable demise of all forms of industrial coal burning in the UK and with that demise went many of the 'Shallow Bed' FBC installations. The same factors that contributed to closure of the FBC boilers and furnaces also contributed to the eventual closure of BCC, in December 1994.

Nonetheless, during the 20 years that the 'Shallow Bed' FBC concept was on-going, in-depth performance monitoring and design evaluation exercises were undertaken and much of the obtained knowledge now resides within the CSL Design Manual. It is a storehouse of collated information. Supplementary sections have been written and appended (to Sections 5, 12 and 13) where the Manual was considered in need of bringing to the level of knowledge that existed up to the time when authorship of the CSL Manual ceased.

The use of FBC is still ongoing in 2005 (and in 2013), though mostly in the form of CFBC. Only a few examples of bubbling 'Shallow Bed' technology now exist in the UK. BFBC is now mostly applied to waste and biomass incineration, though a few of the 'Shallow Bed' furnaces for process drying still remain from the 1980's.

Notes prepared by M J Fisher, July 2005