The invention relates to thermal dryers and more particularly concerns an adaptive control for a thermal dryer enabling it to respond to a change in thermal demand in the drying of bulk solid material. The invention will be specifically disclosed by way of example, in connection with a thermal dryer for removing moisture from fine particles of washed coal.
In the processing of coal, it is desirable, and often mandatory, to remove a number of impurities from the coal prior to use. The need to remove these impurities has increased in recent years with the advent of modern mining techniques. Although these modern mining techniques are highly efficient and permit high production rates, they tend to be less descriminating than older mining techniques in the collection of material from the ground. As a result, coal mined by many of these modern techniques tends to have an even greater amount of impurities than was common several years ago. As a result, the need to process coal through a cleaning process to remove impurities has been accentuated in recent years.
There are a number of processes for removing impurities from the coal. In general, these are washing type processes which leave the coal wet. It is then necessary to remove moisture from the cleaned coal through a thermal dryer or the like.
The most common approach to drying wet coal is to use a fluidized dryer where hot gases are forced through an orifice plate within the dryer resulting in a large pressure drop and fluidizing a bed of material being moved through the dryer on top of the plate. Control is a major problem in the prior art drying systems of this type. Most controlled prior art systems use the discharge gas temperature as the controlled variable on the assumption that this parameter provides a good indication of moisture content of the material being dried. If conditions of infeed to the system remain constant, this assumption would generally hold true. However, infeed conditions do not remain constant in many applications and changes in the mass flow rate into the system alter the thermal demand placed on the dryer. This alters the operating set point on these types of dryer systems and requires skill and experience on the part of operators to judge the level of a new set point. The set point is generally based upon the subjective impression of the operator as to appearance of the coal feeding into the system. As a result, coal is frequently dried beyond specification to insure that it is acceptable. This overdrying of the coal results in a considerable waste of energy and money, especially when oil or gas fuel is being used to supply the drying heat.
Another problem resulting from poor control, and especially downstream control, is that sudden swings in the thermal load on the dryer are not accommodated by the control system. As a result, the coal is frequently either underdried, resulting in a wet product, or overdried, resulting in a waste of energy and possibly a fire in the dryer. These swings or fluctuations in the thermal load normally result from breakdown or blockages in the system upstream of the dryer.