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Why PBC?

M. Radovanovic
Faculty of Mechanical Engineering, University of Belgrade, 27. marta 80, Belgrade, Serbia, Serbia and Montenegro


The use of coal as an alternative and more economical source of energy is limited by:

    - its solid physical form (disadvantages in transportation, handling, storage and combustion), and
    - the environmental problems (fume emission and ash disposal).

It seems that environmental problems will be the most important in the future. Let me show you in illustration only one old, but most impressive figure.
Apart from the very serious situation in Europe you have seen, one can conclude that pollution of air, water and soil and consequences of "acid-rains" is, and will be an international problem.
Now we come to the answer to the question: why FBC? The primary advantage of fluldised bed combustion is a large reduction of sulphur oxides emissions during the combustion process and low emission of nitrogen oxides absolutely and relatively, compared with conventional coal fired systems.

The reason, of course, is the advantage in general over conventional combustion. I am sure that many of you in this room are aware of this, but for the interest of those who may not, I will briefly review the FBC advantages as we see them:
    - The furnace temperature is kept low and uniform. This means that:
      - NOv formation is low,
      - ash fusion does not occur,
      - less fouling and corrosion exists,

    - Sulphur oxides can be reduced by adding limestone: an expensive and efficiency consuming flue gas desulphurlsation plant is not required.
    - Heat transfer coefficients are high. This means, that the heat exchanger areas and consequently the boilers, can be smaller than in pulverized fuel firing. This means also lower investments costs.
    - Low grade fuels and waste materials, both of them with an extreme high content of ash and/or moisture can be burned, because of excellent multi-fuel properties of this kind of technology. Low grade, cheaper fuel means also more economical heat production. On the other hand, burning waste materials, a fluldised bed combustor can be used as a very good incinerator, (3T criteria).
    - High heat release up to 3 MW/m2 bed area (AFBC).
    - Related to the particle size of coal, there are lower costs for fuel preparation.
    - About the same combustion efficiency.

However, there are also disadvantages:
    - it is necessary to use significant fan power because of pressure drop across the distributor plate and the bed itself,
    - limited range of control and low control speed related to steam production.

A number of basic questions need to be answered before the fluidised bed technique of coal combustion can be completely adopted.

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