Carbon Reduction in Flyash Using Microwaves

The ratio of furnace bottom ash (FBA) to flyash produced in a coal-fired boiler was historically around 25:75 with utility boilers operating with an unstaged combustion regime. With the advent of low NOx burner systems this ratio has changed and is now 15:85 or even lower. This is unfortunate since the demand for FBA is high, with 100% of is sold to the construction industry, whereas only 50% of flyash is used. Currently 6 million tonnes of flyash is produced annually in the UK alone. There is currently 250 million tonnes of flyash stockpiled with no foreseeable use. Carbon content in flyash can dictate its potential saleability with the potential market for flyash with a carbon content greater than 5% being relatively poor. Needless to say, the carbon is present in flyash as a result of incomplete burnout in the boiler and the low NOx burners (that produce more flyash relative to FBA) operate at lower temperatures than other systems, which inevitably leads to higher carbon levels in ash. Burnout efficiency also relates to many factors such as grind size, residence time in the boiler, coal type and boiler conditions. Regardless of the causes of carbon in ash, its removal will increase the value of the flyash as an ash product. The use of microwaves to remove carbon from flyash is ideal, in that the carbon responds rapidly to microwave heating whilst the ash remains relatively transparent at lower temperatures. This has been shown by measuring the dielectric properties of concentrated carbon and pure ash samples. The ash in flyash has a loss factor (e’’) of around 0.05, which is particularly low when compared with the pure carbon from the flyash at around 5. The carbon is therefore an excellent candidate for rapid heating in microwaves. Under conventional heating the flyash is a bulk powder with low thermal conductivity thus heating costs are high. Microwave heating is a far more efficient way of raising the temperature as only the carbon needs heating and only the carbon particles are responsive to microwaves. The reason for carbon being such a responsive material relates to its chemical structure. Preliminary tests carried at Nottingham have shown that the ‘ordering’ of the carbon, in terms of aromatic to aliphatic structuring, increases its response to microwaves. The carbon in flyash has been ‘graphitised’ during combustion in the boiler (at temperatures in excess of 1500C), and therefore has a more ordered structure than that of its parent coal. This ordering leads to a high response to microwaves and a rapid heating rate. A notable increase in loss factor was seen when comparing the coal with the intermediate char and the final flyash char.