The benefits of going gravimetric1 October 2003
Gordon Gardiner, Redler (Division of Stock Equipment Co), Stroud, Gloucs, UK
Because of the heterogeneity of coal, gravimetric feed systems have a number of advantages over volumetric systems when it comes to achieving optimised combustion in pulverised coal fired power plants, with lower emissions. Gravimetric systems are widely used in the USA, but volumetric feeders predominate in Europe. With the growing pressure to reduce NOx and carbon dioxide emissions this is likely to change in the coming years
Most large capacity power stations fired by coal in the UK and throughout the world are PF (pulverised fuel) type, where the coal is pulverised to an extremely small grain size before being blown to the boiler for combustion.
Somewhere in the coal delivery system to the boiler has to be a device which measures and controls the amount of coal being delivered. This device is the 'coal feeder'.
Volumetric methods of feeding coal have for many years been quite acceptable to combustion and performance engineers. However, due to efficiency and environmental issues becoming more and more important, optimising the combustion of coal is receiving increasing attention.
In any combustion process, there are two primary requirements:
• feed just enough fuel to meet demand for heat input;
• provide just enough air to burn the fuel.
If coal were a uniform material, which did not vary in heating value, there would be no requirement for gravimetric feeders. A good reliable volumetric feeder would do the job.
Gravimetric vs volumetric
However, coal is a heterogeneous material. It is a most variable bulk solid consisting of coarse and fine particles over a wide range of total moistures, and the bulk flowing density changes significantly with any change in surface moisture, total moisture, particle size and ash characteristics.
When firing coal, there is a currently no effective way to measure kcal flow, so this value must be inferred from either volume or weight flow. The volume measurement will be in error by the variation in both density and heating value, whereas the weight flow signal will be in error only to the extent of the variation in heating value. The gravimetric feeder is vital if an accurate flow rate is to be maintained irrespective of density changes.
The feeding error with a volumetric system can be up to 23%, but the maximum error with a gravimetric system is reduced to around 3%.
The efficiency improvements and benefits of gravimetric feeders over chain type volumetric feeders can be summarised as follows:
• vastly improved coal/air ratio, reducing excess air significantly;
• reduction in excess air lowers the PF velocity through the mill, improving both classification and distribution;
• less modulation of coal flow through the mill;
• less slagging;
• better tuning of low NOx burners due to lower PF velocity (such burners are much less forgiving regards 'carbon in ash');
• fuel accounting becomes much more factual.
These advantages are mainly to do with the positive impact of gravimetric feeders on the combustion process. But gravimetric feeders also have an impact on the feed out of the coal bunkers, which in turn has a substantial effect on the efficient performance of the boiler.
A volumetric feeder arrangement, which is usually in the form of a drag chain feeder, assumes a given volume of coal is being extracted. Many stations are experiencing bunker discharge problems due in part to the more substantial use of imported coals for which the bunkers were not designed. A gravimetric feeder, being a belt feeder device, weighing the coal actually passing over the belt, will react to any reduced feed out of the bunker.
The feeders must preserve the entire downward force of coal from the bunker and redirect it in the direction of flow. A belt feeder does this better than a drag chain feeder and gives a more efficient and regular discharge.
In many existing coal-fired plants equipped with volumetric feeders (such as those in the UK), there is a gradually growing appreciation of the economic and environmental benefits of gravimetric feeders and undoubtedly conversions will eventually occur.
The gravimetric feeder can be adapted to meet the dimensional constraints associated with an existing volumetric feeder layout. There are a number of possible solutions, depending on the layout of the existing feeder. Generally conversion will either involve fitting a secondary belt conveyor to feed coal to the discharge point, or enhancing the chain type cleanout conveyor to convey coal to the discharge point.
There are thousands of gravimetric feeders in use throughout the world, but very few in Europe. These devices are potentially an important piece of the combustion process and their increased use could contribute to a reduction in NOx and CO2 emissions. As there are targets for the reduction of these emissions over the coming years it is likely that the use of gravimetric feeders in coal-fired plants will steadily increase.