Down-Draught Combustion

However, it is now known that volatile matter will only bum in these circumstances provided it (a) is mixed with oxygen, (b) is maintained at temperatures above 600°C (1110°F), and (c) has sufficient residence time (ca. 0.5 s) for the oxidation to go to completion. Thus, it is essential that the system have sufficient turbulence, temperature, and time, and successful exploitation of the principle has been achieved in the form of a variety of mechanical stokers used in industrial steam raising. 

Volatile matter from the coal mixes with the air and ignites as it passes through the incandescent top layer of the bed thereby effectively controlling smoke emission. 

Strongly caking coals can cause problems with chain-grate units but not with underfeed or spreader units. Coal size is important with chain-grate stokers since fines lead to excessive caking and high unburned carbon losses. The amount of coal smaller than 0.6 cm (1.4 in.) is usually limited spreader and underfeed stokers are less sensitive to particle size. 

FIGURE 14.16 Pressure drop across a bed of particle as a function of gas velocity—corresponds to the minimum fluidization velocity. 

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The aim of the present work is to further analyse the influence of primary airflow and fuel particle parameters on the combustion process. The work ranges from sub-stoichiometric to over-stoichiometric supply of primary air to the bed, to cover the influence of the parameters studied. Experiments were performed with the combustion front moving both upward (as in the studies referred to) and downward (as in a modem boiler with down-draught combustion). The work aims at forming a basis for modelling of wood burning, as well as for design of small boilers. The experiments were conducted so that the results can be used for both batch and continuously fired plants. 

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