Combustors design considerations

The concept of distribution of available propellant may be of aid in determining the performance characteristics of any combustor under consideration and may thereby prove valuable in the design of future combustors. Most combustors will fit neither of the above cases exactly, as these cases represent extremes. However, any combustion system using atomized propellants can be represented adequately by some combination of the two cases considered above. 

Basu, P. Design Consideration for Circulating Fluidized Bed Combustors, J. Inst. Energy. vol. 59, p. 179 (1986). 

During the last five years, several successful pilot- and full-scale demonstrations of catalytic combustors for gas turbine applications have been presented. Here, we have divided these systems into five different classes the large- and small-size fully catalytic combustor (designs la and b) and the hybrid designs with partially inactive catalyst, with secondary fuel and with secondary air (designs Ila, b and c). The first part of this section is devoted to fundamental gas turbine considerations, which will be followed by a summary of demonstrations of catalytic combustors. 

For an efficient absorption tower design, the plant should emit tail gases at less than 1000 ppm of nitrogen oxides. This level is about half the current emissions limit in Western Australia. Should emissions exceed this figure, then consideration must be given to the installation of a catalytic tail-gas combustor which enables emission levels to be lowered below 400 ppm. The plant does not normally produce any liquid waste streams (see Section 5.4.7 Environmental Impact Analysis). 

Aerodynamic valves employ the fluid mechanical properties of specially designed inlets to act as a physical barrier to the backflow of combustion products out of the combustor through the inlet section. The main advantage of aerodynamic conflgurations is lack of valves and moving parts so the risk of mechanical breakdown or failure is eliminated. This is a key consideration for heavy-duty pulse combustion burners where the inlet section undergoes severe operating conditions. 

The gas velocity is considerably higher than the terminal velocity of the individual bed particles. Two-stage combustion of coal takes place in a CFB furnace. Depending on the design, the bottom section will either work as a gasifier or combustor. 

The basic consideration in the design of the flapper (membrane) valves, as shown schematically in Fig. 2.6a, is the ability to rapidly open and close the openings in the base plate according to the combustor operating frequency. Moreover, the valves must withstand the maximum pressure in the combustion chamber, yet open with only a small pressure difference. 

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