OCR Text |
Show 2.2 Fluidised bed combustion 2.2.1 Principles and advantages Fluidised bed combustion (fbc) is a relatively new combustion technology which has emerged during the last 20 years. The NCB has played (3) a leading role in its development . This method of firing utilises a bed of inert particles, such as ash or silica sand sized about 1 mm, through which evenly distributed air is passed. At a critical air velocity the upward drag forces on particles are such that particles become suspended in the air stream and are said to be 'fluidised'. In normal operation, with an air supply substantially above this velocity, most of the air passes through the bed as bubbles and the bed has an appearance similar to a boiling liquid. As with a boiling liquid there is a considerable amount of turbulence and this causes rapid mixing of the particles. To bring a fluidised bed into operation, the temperature of the sand bed is first raised to about 600 C using an auxiliary fuel such as gas. Coal is then fed and burns within the turbulent sand, raising the temperature to the normal operating value of between 800 C and 950 C. Because of the turbulence the coal is rapidly distributed throughout the bed. The coal combustion time is 0.5 to 5 minutes, depending on the coal size, and there is typically between 0.5% and 5% of fuel in the bed, the remainder being the inert material. Because the coal particles remain separate in the bed, the coal rank, which is a measure of the tendency of coal to stick together and form coke is not significant, and coal of any rank can be fired successfully. Also, coals of high ash and moisture content previously regarded as unsuitable for combustion can be used in fbc. However, in the UK it will continue to be economic to supply industrial customers with low-ash washed coals, to minimise coal transport costs. Low grade coals will only be fired immediately adjacent to the mine, except in countries where the coal is of a type such that the ash content can not be reduced by conventional techniques, e.g. India. For UK washed coals having low ash contents, uncrushed lump coal up to 50 mm in size can be fired directly to the bed, without drying or crushing. 20-8 |