PRESSURISING OF BULK MATERIAL IN LOCK HOPPERS

Abstract

A process for pressurizing bulk material in a hopper; wherein the hopper is configured as a lock hopper (29) containing a bulk material, a source of pressurized gas, lines (22, 26, 28) to convey the pressurized gas from the source to one or more inlets (30) of the lock hopper, a valve arranged in the lines, wherein the opening position of said valve (34, 35) is controlled to provide pressurizing gas to the lock hopper at a preset constant gas volume flow rate.

Claims

1. A process for pressurizing bulk material in a hopper; wherein the hopper is configured as a lock hopper containing a bulk material, a source of pressurized gas, lines to convey the pressurized gas from the source to one or more inlets of the lock hopper, a valve arranged in the lines, wherein the opening position of said valve is controlled to provide pressurizing gas to the lock hopper at a preset constant gas volume flow rate.

2. The process as claimed in claim 1, wherein the opening position of the valve is controlled based on the actual gas volume flow rate of the pressurizing gas measured downstream the valve using a volume flow rate or velocity measurement device and/or on the gas volume flow rate of the pressurizing gas calculated based on actual upstream and downstream pressures measured using a pressure sensing device during the pressurizing.

3. The process as claimed in claim 1, wherein the source of pressurized gas is a gas supply main and/or an intermediate buffer vessel arranged between the gas supply main and said valve.

4.-9. (canceled)

10. An apparatus for pressurizing bulk material in a hopper; comprising a hopper configured as a lock hopper for containing a bulk material, a source of pressurized gas, lines configured to convey the pressurized gas from the source to one or more inlets of the lock hopper, a valve arranged in the lines, wherein the apparatus is characterized in that said valve is a controllable valve, the opening position of which is controllable by a control unit, the control unit being configured to control the opening position of the valve in order to provide pressurizing gas to the lock hopper at a preset constant gas volume flow rate.

11. The apparatus as claimed in claim 10, wherein the opening position of the valve is controllable by said control unit based on the actual volume flow rate of the pressurizing gas measured downstream the valve using a volume flow rate or velocity measurement device and/or on the volume flow rate of the pressurizing gas calculated based on actual upstream and downstream pressures measured using a pressure sensing device during the pressurizing.

12. The apparatus as claimed in claim 10, wherein the source of pressurized gas is a gas supply main and/or an intermediate buffer vessel arranged between the gas supply main and said valve.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawing in which:

[0034] FIG. 1 is a schematic diagram of an embodiment of a preferred apparatus of the present invention or useable in a process of the present invention,

[0035] FIG. 2 is a schematic diagram of a further embodiment of a preferred apparatus of the present invention or useable in a process of the present invention and

[0036] FIG. 3 is a graph showing lock hopper pressure and gas volume flow rate as a function of time during lock hopper pressurizing.

[0037] Further details and advantages of the present invention will be apparent from the following detailed description of several not limiting embodiments with reference to the attached drawing.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0038] With reference to FIG. 1, an apparatus for pressurizing bulk material in a hopper configured as a lock hopper 29 is provided for containing a bulk material, such as coal powder. A number of lines 22, 26, 28 are provided to convey the pressurized gas from the source (directly or via a buffer vessel) to one or more inlets 30 of the lock hopper 29. A controllable valve 34, 35 is arranged in the lines and its opening position may be controlled by a control unit (not represented separately). This control unit is programmed to control the opening position of the valve in order to provide pressurizing gas to the lock hopper at a preset constant gas volume flow rate, thus producing constant pressurizing gas actual velocities in the piping, the sintered metal discs 30 and the bulk material inside the lock hopper 29.

[0039] The gas flow rate control valve (controllable valve) may be operated in two different ways, both ways resulting in the constant gas volume flow rate aimed at (both ways are represented in the same figure for illustration only):

[0040] Either a volume flow rate (or velocity) measurement 31 is installed in the pressurizing gas pipeline connecting the buffer vessel 27 to the lock hopper 29. A constant volume flow rate set point value is fixed. The flow rate controller (control unit) acts on the control valve 34, 35 in such a way as to make the actual volume flow rate value measured in the measurement 31 fit with the set point value.

[0041] Or, the characteristics of the control valve 34, 35, i.e. the mass flow rate produced by the valve depending on the pressure level measured upstream 32, the pressure level measured downstream 33 and the opening position of the valve, are included into the controller. For a fixed pressurizing gas volume flow rate and thus fixed pressurizing gas velocities, the corresponding mass flow rate value, conditioned by the pressure level measured in 33, is continuously computed. The controller positions the valve accordingly, in such a way as to produce a (continuously increasing) mass flow rate, the set point value for the valve position resulting from the pressure levels upstream 32 and downstream 33 and the mass flow rate value.

[0042] The pressurizing gas volume flow rate directly supplied from the process gas supply main 21 may be controlled in an equivalent way using controllable valve 35, the pressure upstream now being the pressure level in that process gas supply main. Though, whenever a buffer vessel for pressurizing gas is installed, the lowest pressure levels and thus the best effect of the pressurizing gas volume flow rate control as described is achieved when pressurizing gas is supplied from the buffer vessel.

[0043] FIG. 2 shows an embodiment similar to that of FIG. 1, wherein valve 35 within line 22 from the main supply is however configured as a simple on/off (open/closed) valve with associated Laval tuyere. Such a configuration is especially useful if the lock hopper 29 is meant to be pressurized a priori/mainly from the buffer vessel 27. In case of failure of the buffer vessel circuit, the overall operation of pressurizing may then be done from the supply main in a conventional way.

[0044] In any case, valve 35 (be it the version of FIG. 1 or of FIG. 2) may be (controllably) opened by the end of the pressurizing cycle when the pressure inside the buffer vessel drops to lower pressures.

[0045] As an illustration of one of the embodiments of the present invention, the following calculation can be made: p.sub.1 being the initial (absolute) pressure level in the lock hopper, p.sub.2 the final (absolute) pressure level in the lock hopper, and pressurizing being performed either with constant mass flow rate (by means of a Laval tuyere with constant upstream pressure level) or with constant volume flow rate (by means of a valve controlled as described herein), the maximum actual gas velocity in case of constant mass flow rate pressurizing equaling the actual constant gas velocity in case of constant volume flow rate pressurizing, the ratio of pressurizing time durations of constant volume flow rate pressurizing versus constant mass flow rate pressurizing equals In(p.sub.2/p.sub.1)/[(p.sub.2-p.sub.1)/p.sub.a], p.sub.a being the (absolute) atmospheric pressure, In the natural logarithm. Example: p.sub.1=0 bar g=1 bar a, p.sub.2=9 bar g=10 bar a, p.sub.a=0 bar g=1 bar a. The ratio of pressurizing time durations becomes In(p.sub.2/p.sub.1)/[(p.sub.2-p.sub.1)/p.sub.a]=0,256, i.e. the pressurizing time duration may be reduced by up to about 74% while the maximum actual gas velocity and the maximum pressure losses in the sintered metal discs as well as in the bulk material remain constant and the noise level remains close to constant (the noise level is primarily conditioned by the actual gas velocity and slightly increases with the logarithm of the pressure level).

[0046] A corresponding illustrative example is also depicted in the graph of FIG. 3, wherein the results in terms of hopper pressure and gas volume flow rate are shown as a function of time for a pressurizing operation according to the present invention with a constant volume flow rate (VAC), curves (2) and (4), compared to a conventional operation using a valve and a Laval tuyere configuration (1YD), curves (1) and (3). The shown examples are based on the following assumptions: pressurizing gas vessel of 65 m.sup.3 useful volume and an initial pressure of 17 bar a; lock hopper to be pressurized having an available gas volume of 22 m.sup.3 and a final pressure of 12 bar a, maximum gas volume flow rate of 3,54 m.sup.3/s to be respected. As can be seen in FIG. 3, the reduction in time needed for pressurizing a lock hopper is hugely significant and surprising, from conventionally 76 s to merely 15.4 s, all the more so because these reductions may be achieved with only reduced modifications to existing apparatuses.

[0047] The present invention is not restricted to embodiments and to specific applications relating to injecting coal into a blast furnace. It may also be applied to other installations comprising pressurized hoppers containing powder materials and requiring periodic pressurization of said hoppers.