Cyclone fired wet bottom boilers

Cyclone-fired boilers are used for coals with a low ash fusion temperature which are difficult to use with PCC. 80-90% of the ash leaves the bottom of the boiler as a molten slag, thus reducing the load of fly ash passing through the heat transfer sections to the precipitator or fabric filter to just 10-20% of that present. As with PCC, units operate at close to atmospheric pressure, simplifying the passage of coal and air through the plant. Steam is generated in heat transfer tubes, driving a steam turbine and generator.

A crushed coal feed with 95% <5 µm size is typical. Combustion temperature in the external cyclone furnaces can be anything from 1650°C to over 2000°C.

Cyclone-fired boilers are suitable for coals with:

• a volatile matter content of more than 15% (dry basis);
• ash contents between 6 and 25% for bituminous coals, or 4 and 25% for subbituminous. The ash must have particular slag viscosity characteristics, and the maximum temperature at which the slag has a viscosity of 250 centipoise is 13401C for bituminous and 1260°C for subbituminous coals. Ash slag behaviour is critical to satisfactory operation; and
• a moisture content of less than 20% for bituminous and 30% for subbituminous coals.

There are over 100 units, mainly in the USA (operating on lower rank coals with low ash fusion temperature) and in Germany. Virtually none have been commissioned since 1980, because of the high combustion temperatures which result in high NOx formation.

Characteristics

The cyclone furnace chambers are mounted outside the main boiler shell which will have a narrow (or tapered) base, together with an arrangement for slag removal. Primary combustion air carries the particles into the furnace in which the relatively large coal/char particles are retained in the cyclone while the air passes through them, promoting reaction. Secondary air is injected tangentially into the cyclone. This creates a strong swirl, throwing the larger particles towards the furnace walls. Tertiary air enters the centre of the burner, along the cyclone axis, and directly into the central vortex. It is used to control the vortex vacuum, and hence the position of the main combustion zone which is the primary source of radiant heat. An increase in tertiary air moves that zone towards the furnace exit and the main boiler.

Larger particles are trapped in the molten and sticky layer which covers the entire surface of the cyclone interior, except for the area in front of the air inlets. The coal is fired under conditions of intense heat input, and while the finest particles may pass through the vortex in the centre, the larger ones are thrown towards the walls and are recirculated to achieve adequate burnout.

The combustion gases, residual char and fly ash pass into a boiler chamber where burnout is completed and there are various stages of heat exchange and heat recovery, producing steam to drive the turbine and generator. As a result of the intense combustion conditions, NOx formation tends to be considerably higher than in PCC.

Molten ash flows by gravity from the base of the cyclone furnaces, and is removed from the system at the bottom of the boiler. It drops out into a quench tank, thus losing a substantial amount of heat. Precautions against gas build-up and explosions are essential in and around the slag quench tank.

Design variations are necessary for firing coals with a high moisture content. Some of the moisture may be removed before feeding the coal to the furnace, and the water vapour bypasses the cyclone and goes into the main boiler area.

Unit size

As with PCC boilers, cyclone-fired units can be of almost any size. Existing units range in capacity from 33-1150 MWe, and there are more than twenty units sized between 400 and 900 MWe.

Thermal efficiency

As the units which were built mainly during the 1960s and 1970s, subcritical steam cycles are used, and the efficiencies of these units will be in the range 35-38%, similar to that for equivalent size PCC units.

Flue gas cleaning/emissions

The principles for flue gas cleaning will be similar to those for PCC boilers, except that in addition to the use of SCR for NOx reduction, various other combustion methods can be used, such as two-stage combustion, and a reburn concept in which 15-35% of the coal is pulverised, and added into the main boiler chamber, bypassing the cyclone furnace. Particulates removal and FGD are described elsewhere.

Residues

The residues consist of 70-90% of coarse slag, together with some fly ash.