IEA Clean Coal Centre

Circulating fluidized bed combustion (CFBC) at atmospheric pressure

FBC in boilers at atmospheric pressure can be particularly useful for high ash coals, and/or those with variable characteristics. Relatively coarse particles at around 3 mm size are fed into the combustion chamber. Two formats are used, bubbling beds (BFBC) and circulating beds (CFBC).

There was rapid growth in the coal-fired power generation capacity using FBC between 1985 and 1995, but it still represents less than 2% of the world total.

Characteristics

Combustion takes place at temperatures from 800-900°C resulting in reduced NOx formation compared with PCC. N2O formation is, however, increased. SO2 emissions can be reduced by the injection of sorbent into the bed, and the subsequent removal of ash together with reacted sorbent.

Circulating beds use a higher fluidizing velocity, so the particles are constantly held in the flue gases, and pass through the main combustion chamber and into a cyclone, from which the larger particles are extracted and returned to the combustion chamber. Individual particles may recycle anything from 10 to 50 times, depending on their size, and how quickly the char burns away. Combustion conditions are relatively uniform through the combustor, although the bed is somewhat denser near the bottom of the combustion chamber. There is a great deal of mixing, and residence time during one pass is very short.

CFBCs are designed for the particular coal to be used. The method is principally of value for low grade, high ash coals which are difficult to pulverise, and which may have variable combustion characteristics. It is also suitable for co-firing coal with low grade fuels, including some waste materials. The direct injection of limestone into the bed offers the possibility of economic SO2 removal without the need for flue gas desulphurisation. The advantage of fuel flexibility often mentioned in connection with FBC units can be misleading. Once the unit is built, it will operate most efficiently with whatever design fuel is specified.

The design must take into account ash quantities, and ash properties. While combustion temperatures are low enough to allow much of the mineral matter to retain its original properties, particle surface temperatures can be as much as 200°C above the nominal bed temperature. If any softening takes place on the surface of either the mineral matter or the sorbent, then there is a risk of agglomeration or of fouling.

Various CFBC designs are used. The fluidizing velocity is high enough to entrain a substantial proportion of the material, and the solids are separated from the flue gases in a cyclone operating at a temperature near that of the exhaust gas. Ash and unburned carbon are recirculated, probably many times. Even though the solids inventory is distributed throughout the unit, a dense bed is required in the lower furnace to mix the fuel during combustion. Because of recirculation of the bed material, particle residence times are relatively long compared with the gas residence time, and can be measured in tens of seconds. For a bed burning a bituminous coal, the carbon content of the bed is only around 1%, with the rest of the bed made up of ash, together with sand (if needed), and/or lime and calcium sulphate. Overall carbon conversion efficiencies should be over 98%, leaving only a small proportion of unburned char in the residues.

Larger boilers will have several cyclones in parallel to remove the solids for recirculation. One design characteristic is the need for heat recovery from the bottom ash, some of which is removed. This is part of the basic design in some units. Ash coolers are prone to plugging, hence the use of some fluidizing air, and the heat transfer tubes in them are prone to erosion, which may be exacerbated by the air flow.

In one design, there are wall heating tubes, and then a heat exchanger with the flue gases in an external chamber. In a second design, there are platen heat exchangers in the combustion chamber in addition to the wall tubes, although further heat exchange is also needed for efficient operation. In a third arrangement, the upper part of the furnace has a considerable number of heat exchange tubes, such that the exit flue gases are substantially cooled before leaving for the cyclone. The returning ash cools the base of the combustor. Where there are heat exchange tubes in the path of the recirculating solids the possibilities of erosion are considerably increased. In all cases, the finest fly-ash leaves the cyclone with the flue gases, and is normally separated by using an ESP. This can contain quite high proportions of carbon, possibly up to 15%.

Flue gas cleaning/emissions

Combustion takes place at temperatures from 800-900°C resulting in reduced NOx formation compared with PCC. N2O formation is, however, increased. SO2 emissions can be reduced by the injection of sorbent into the bed, and the subsequent removal of ash together with reacted sorbent. Limestone or dolomite are commonly used for this purpose. Commonly only particulates removal is required in order to meet emissions limits, but in some places NOx reduction is also required. The technologies are described in the appropriate sections.

Residues

The residues consist of the original mineral matter, most of which does not melt at the combustion temperatures used. Where sorbent is added for SO2 removal, there will be additional CaO/MgO, CaSO4 and CaCO3 present. There may be a high free lime content and leachates will be strongly alkaline. Carbon-in-ash levels are higher in FBC residues that in those from PCC.

Unit size

Atmospheric CFBC is used in a number of units around 250-300 MWe size, and there are a number of commercially operating plants. There are designs for units up to 600 MWe size. A 460 MWe supercritical unit is under construction at Lagisza, Poland and is due to be commissioned in 2006.

CFBC boilers represent the market for relatively small units, in terms of utility requirements. They are used more extensively by industrial and commercial operators in smaller sizes, both for the production of process heat, and for on-site power supply. A few are used by independent power producers, mainly in sizes in the 50 to 100 MWe range.

Thermal efficiency

In the 100-200 MWe range, the thermal efficiency of FBC units is commonly a little lower than that for equivalent size PCC units by 3 to 4 percentage points. In CFBC, the heat losses from the cyclone/s are considerable. This results in reduced thermal efficiency, and even with ash heat recovery systems, there tend to be high heat losses associated with the removal of both ash and spent sorbent from the system. The use of a low grade coal with variable characteristics tends to result in lower efficiency, and the addition of sorbent and subsequent removal with the ash results in heat losses. The supercritical unit at Lagisza referred in above is expected to have a thermal efficiency of over 40%.

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