Among the known methods and plants for injecting solid fuels into a tank furnace, two different types can essentially be distinguished. In the plants of the first type, the powdered coal is released under pressure from a distributing hopper into a series of metering apparatuses in which the mixture of powdered coal and propellent gas is formed and from which the mixture thus formed is conveyed towards the points for injection into the tank furnace. In these plants, one metering apparatus is generally employed for each tuyere stock or for each pair of tuyere stocks, with the result that the quantities injected through the tuyere stocks may be metered individually. These plants have the advantage of enabling a high degree of accuracy of the individual flow rates. On the other hand, they have the disadvantage that they require many relatively long pipes between the distribution hopper and the tank furnace. Furthermore, metering may be made difficult by the various variable parameters, in particular fluctuations in pressure inside the furnace, which have repercussions on the cost of the plant, because of the need for numerous measurements and regulations loops.
In the second type of plant, only one metering device is employed at or downstream from the distribution hopper. In this metering apparatus, the overall flow rate of the powdered coal is regulated so as to inject a predetermined quantity of powdered coal per unit time. These plants use the method described in the introduction, in other words the metered current containing the required quantity of coal is divided in a distributor into identical secondary currents. These plants have the advantage that the distributor is arranged in immediate proximity to the furnace so as to reduce the length of the secondary pipes and, consequently, the space requirement around the furnace.
On the other hand, these plants have the disadvantage that the variable parameters, in particular fluctuations in the pressure in the furnace, must be controlled, as well as the influence of the features of the various pipes, in order to obtain a uniform or determined distribution of the overall flow rate metered to the various tuyere stocks, as these parameters all influence the pressure gradient in the secondary pipes and, consequently, the flow rate of the powdered coal in these pipes. Various strategies have already been provided for this purpose. It has, for example, already been proposed to deliberately lengthen some pipes, or to provide them with calibrated flow orifices in order to compensate for the influences of the differences in the features of the pipes which are, for example, varying lengths, their cross-section of flow resulting from manufacturing tolerances or from wear and the influence of their layout (horizontal, vertical or oblique). However, both these differences and the measurements necessary to compensate for their influences are very difficult to detect and to determine. Furthermore, these compensatory or corrective measures do not enable the influence of the variations in pressure in the furnace to be eliminated.
Another solution is described in U.S. Pat. No. 4,702,182, the disclosure of which is incorporated by reference, and consists of providing a tuyere or a narrowed cross-section, either in the secondary pipes or in the distributor, where a pressure head is created necessary for acceleration of the pneumatic current up to the speed of sound. Acceleration of the pneumatic current up to the speed of sound has the advantage that the flow conditions upstream from the point where the supercritical speed is produced are no longer influenced by the flow conditions downstream from this point. This has, of course, the great advantage that it is no longer necessary to take into account the fluctuations in pressure and other variable parameters existing in the secondary pipes or in the chamber into which the powdered substances are injected. However, the tuyeres necessary for the acceleration up to the speed of sound require a high pressure head and a relatively small flow cross-section, which gives rise to a high pressure at the inlet and a high consumption of propellent gas and requires preliminary screening of the powdered coal in order to remove the coarse particles and to prevent risks of obstructing the tuyeres.