Furnaces – Process – Burning pulverized fuel
Patent
1996-10-09
1998-05-26
Bennett, Henry A.
Furnaces
Process
Burning pulverized fuel
110245, 110213, 110214, 60 30464, F23D 100
Patent
active
057551669
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method and a device for after-burning of more or less unburnt fuel particles in flue gases in a power plant preferably a PFBC power plant, which is fired with a particulate fuel. Further, the invention comprises a method and device, integrated with the afterburning, for separating coarser particles which are returned to a combustion chamber.
BACKGROUND OF THE INVENTION
In power plants which, in a primary combustion chamber are fired with a particulate fuel, for example coal, in a fluidized bed, the fuel particles reside for such a long time in the bed that all fuel in the particles is burnt before the particles leave the bed in the form of ash. It happens, however, that the flue gases leaving the bed and entering a freeboard above the surface bed bring with them material from the bed. This material may contain unburnt fuel particles, brought with the flue gases out of the combustor in which the combustion in the fluidized bed takes place. The flue gases from a power plant of the kind mentioned are cleaned in dust separators, usually of a cyclone type, before the gases are forwarded in clean form to a gas turbine for utilization of the energy in the flue gases.
The particles which contain unburnt fuel may, in any oxygen residues occurring, be burnt in the flue gases. This may take place in the form of fires downstream of the freeboard, for example in dust separators for the flue gases, which creates drawbacks in the system such as unbalances between different parallel-connected dust separators, erosion and sintering.
It is known to return material from dust separators to the bed to thus burn such unburnt fuel and hence increase the combustion efficiency. Examples of such return of coarse-separated particles are described in SE 451 501 (EP 233 630) and in U.S. Pat. No. 3,716,003.
Another possibility of solving the problem with unburnt particles entrained in the flue gases may be to locate an after-burner downstream of the primary combustion chamber. Such an after-burner is usually fired with a secondary fuel, for example gas or oil. Air or oxygen is supplied to the secondary combustion which allows a considerably higher temperature of the outflowing flue gases supplied to a gas turbine in the plant, whereby the efficiency in the gas cycle is increased, which is the main purpose of secondary combustion. At the same time, the secondary combustion contributes to non-burnt-out material from particles in the bed also being burnt out. A disadvantage in this connection, however, is that an additional fuel must be utilized. In addition, efforts for mechanical separation of dust particles in the flue gases downstream of the secondary combustion, at the high temperature then used, is difficult. As an example of after-burning may be mentioned the technique according to EP 144 172.
After-burning of unburnt fuel residues is also obtained with a device disclosed in U.S. Pat. No. 3,716,003, in which a vortex combustor of a cyclone type is used. The combustion takes place in the same chamber in which a vortex of flue gases is created. However, the mixing of unburnt fuel particles and the gas in this device is insufficient so that unburnt fuel particles do not encounter oxygen residues in the flue gases to a sufficient extent. This results in an incomplete after-burning. The aim is to achieve a combustion which is performed at a low gas speed and a high turbulence level. In a cyclone-type burner the burnable particles will be centrifuged out towards the shell surface of the cyclone and be burnt there, which results in the temperature of the cyclone wall becoming high. Since the combustion takes place inside a cyclone vortex, also the speed of combustion will also be high, which is undesirable.
A further method of reducing the quantity of unburnt particles, flowing with the flue gases out of the combustor, is to arrange firing with a complementary fuel in the freeboard above the bed surface, where nozzles for injection of a fuel are arranged, whereby the complementary fue
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patent: 4951612 (1990-08-01), Gorzegno
patent: 5024170 (1991-06-01), Santanam et al.
Combined Cycle Power Plants, Modern Power Systems, vol. 12, No. 5, pp. 53-55, May 1992.
ABB Carbon AB
Bennett Henry A.
Tinker Susanne C.
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