Coal gasification plant slag tapping process

An automatic arrangement in which the intermittent tapping of liquid molten slag from the hearth of a slagging coal gasifier vessel (30) through a tap orifice (32) into a slag quench chamber (31) is selectively controlled by a system of timers (T1, T2, T3), the differential pressure between the gasifier vessel (30) and the quench chamber (31) being measured (22) and deviations from the expected measurements caused by malfunctions being used to override the timer system until the malfunctions have been corrected.

This invention relates to coal gasification plant, and in particular to a 
method of and apparatus for controlling the level of molten slag in the 
hearth of slagging coal gasifiers. 
It is known to gasify coal or other carbonaceous fuel in slagging coal 
gasifiers of the kind (hereinafter referred to as the kind specified) in 
which coal or other carbonaceous fuel is introduced into the top of a 
column-like gasifying vessel and is gasified under high pressure and 
temperature by means of oxygen and steam introduced near the fuel bed 
through tuyeres. The residual ash collects as a molten slag and iron in 
the hearth of the gasifier vessel from which it is periodically discharged 
(commonly known as slag-tapping) downwardly through a slag tap outlet or 
orifice in the hearth into water contained in a quenching chamber. 
Usually, the pool of molten slag and iron is maintained in the hearth by 
directing hot combustion products from a burner located beneath the slag 
tap orifice up the tap orifice to retain the slag and iron in the hearth 
by maintaining a positive differential pressure between the quench chamber 
and the gasifying vessel, that is, a higher pressure in the quench 
chamber. The tapping of the molten slag and iron being initiated and 
controlled by stopping or reducing the burner output and reducing the 
pressure in the quenching chamber by controlled venting to atmosphere 
through a venting system so as to reduce the differential pressure between 
the quenching chamber and the gasifier vessel. 
Examples of such slagging gasifier plant are those disclosed in United 
Kingdom patent specification No. 977,122, the Gas Council Research 
Communication No. GC 50 and GC 112. 
The problems which the present Invention seeks to overcome are those 
experienced with known methods of slag level control in a vessel with 
continuous liquid flow at varying rate and intermittent outflow. The 
particular difficulties which the system overcomes are: 
1. The lack of a consistent measurement of slag pool level. 
2. The need to keep below the maximum permissible negative differential 
during taps. Above this differential, spraying occurs and the tap tends to 
block. 
3. The need for rapid sensing and response to tap blockages to give quick 
clearance. 
4. The need to avoid emptying the hearth, as this can cause a blocked tap. 
5. The need for rapid system control if an irredeemably blocked tap is 
experienced. 
According to the one aspect of the Invention, there is provided a method of 
controlling the level of molten slag in a slagging coal gasifier of the 
kind specified comprising the steps of, periodically tapping the slag from 
the hearth of the gasifier vessel through the slag tap orifice into the 
quench chamber using a system of timing means which set a maximum and a 
minimum predetermined period of time between slag tapping operations and 
which set the duration time of actual slag tapping, measuring the 
differential pressure between the gasifier vessel and the quench chamber 
whereby to give an indication of the level of slag in the hearth of the 
vessel and to cause initiation of a slag tapping operation when the slag 
reaches a predetermined level or when any deviations from the expected 
differential pressure measurements caused by malfunctions being detected 
and used to override the timer system until the malfunctions have been 
corrected. 
According to another aspect of the Invention, there is provided an 
apparatus for carrying out the method described above comprising a system 
of timing means for setting said maximum and minimum periods and for 
setting the said slag tapping duration time, means for measuring said 
differential pressure and for producing a signal representative of said 
measurement, and means responsive to said signal for initiating a slag 
tapping operation when the slag reaches said predetermined level or when 
any said deviations from the expected differential pressure measurement is 
detected.

Referring to the drawings, the invention relates to a system for 
controlling the level of molten slag in the hearth of a slagging coal 
gasifier of the kind specified, and for removing the slag at a fixed rate 
for a fixed period at intervals determined by the rate of slag build-up. 
The characteristics of the differential pressure between the gasifier 
vessel 30 (FIG. 2) and the quench chamber 31 are used as an indication of 
the molten slag level existing in the hearth. Hot burner gases from the 
burner 21 bubble up through the slag tap orifice 32 and through the head 
of molten slag in the hearth of the vessel 30, creating a differential 
pressure which is indicative of the slag level. The differential pressure 
is said to be positive when the quench chamber is at a higher pressure 
than the gasifier vessel. 
Although the differential pressure can be used as a form of slag level 
measurement, it is envisaged that it is within the scope of the invention 
that any method of level measurement could be used, for example a radio 
active source. 
During a slag-tapping operation the burner 21 located below the slag-tap 
orifice 32 is turned down to avoid any splattering or diffusion of the 
slag stream, and to avoid high temperature build-up in the quench chamber 
31. 
A slag-tapping operation is achieved in the gasifier by reducing the 
differential pressure between the quench chamber and the gasifier vessel. 
When this happens the quantity of slag, which previously had been held in 
the hearth and slag tap above the quench chamber by the positive 
differential pressure, runs through the slag tap orifice 32 into the 
quench chamber water (not shown) and is quenched and removed as frit 
through a removal means (not shown). The quench chamber pressure is 
lowered by a venting system, as described in UK patent specification No. 
1,562,790. As explained eariler, the burner is turned down to a required 
burning rate to avoid any slag being diffused around the quench chamber 
and to avoid high temperature in the chamber. 
With particular reference to FIG. 1, the system for obtaining an automatic 
slag-tapping operation to control the slag level in the hearth comprises 
three automatic timers, an enable period timer T1, a tap period timer T2 
and a run period timer T3. The enable period timer T1 has a differential 
pressure override trip 5 which can over-ride the enable period timer 
period and start the tap period timer T2 to commence a slag tapping 
operation. The system is versatile and can be run in a number of ways 
according to the level of the slag in the hearth. 
In a normal operation for a slag tap to take place the run timer T3, which 
is set to a minimum predetermined time between taps dependent upon the 
rate of slag production, is allowed to run out which then starts the 
enable time T1 to being its time period. The run period timer T3 re-sets 
to the pre-determined time. The end of the enable timer T1 period, or upon 
the operation of its high differential pressure trip 5, then starts the 
tap timer T2. The tap timer mechanism initiates the actuation of the 
valving means of the quench chamber venting system and a tap is completed 
at the end of the tap timer T2 period or upon the operation of its low 
differential pressure trip 6. The run timer T3 is then triggered so that 
the cycle can be repeated. 
Referring also to FIG. 2 the low differential pressure trip 6 is activated 
during the tap period on the occurance of a blocked tap orifice to 
terminate the tapping operation. This would ensure a rise in quench 
chamber pressure due to re-establishment of the burner rate and closure of 
the venting valve means, with consequential clearing of the blocked 
orifice. 
If the orifice becomes blocked during a run period, it will cause a rapid 
rise in pressure in the quench chamber and this high pressure is detected 
by a blocked-tap-orifice-detector 7. This automatically turns down the 
burner rate and initiates the actuation of the quench chamber venting 
valve means 27 to reduce the differential pressure to a safe level to 
enable an operator to take remedial action. 
Referring also to FIG. 3, in a normal slag tapping cycle, the slag level in 
the hearth is rising during the run period controlled by the run timer T3. 
When the T3 timer period runs out, the enable timer T1 is started and if 
at any time during its timing period the high level differential pressure 
trip 5 is tripped, the timer T1 enables the trip 5 to initiate a tapping 
period, i.e. it starts timer T2. Should the high level differential 
pressure trip 5 not be tripped during the enable period, the tapping 
operation will begin at the end of the enable period. At the start of the 
tap period, in order to enable a tapping of the slag to take place, the 
burner rate is turned down and the quench chamber is vented to atmosphere 
in order to reduce the differential pressure for this purpose. 
The rate of the burner 21 is turned down by reducing the gas and oxygen 
flows. The gas flow 19 through control valve 14 is closed by an automatic 
valve 15, but a sufficient amount of gas is fed to the burner via a needle 
valve 16 so as to maintain a small flame for relighting the burner. The 
oxygen flow 26 through the control valve 17 is cut off by an automatic 
valve 28. The set points of gas and oxygen flow controllers 8 and 9 
respectively are switched from a burner-rate-manual-control 11 to manual 
slag tapping-rate-set-point-controls 12 and 13 which may be set to zero. 
The air flow through a control valve 18 remains constant and is controlled 
by controller 10, and its set point is derived from the manual control 11. 
The differential pressure is measured by a detector 22 whose output signal 
is fed to the high and low differential pressure trips 5 and 6, the 
blocked tap detector 7 and the differential pressure controller 25. The 
output signal from controller 25 operates the valve 27 to achieve the 
differential pressure required by the controller's set point. During a 
slag tapping operation this set point is set by a pneumatic signal 
generator 23. During the run period this set point is set by a pneumatic 
signal generator 24 which is always set to a high differential to ensure 
that the valve 27 is closed. 
If during a slag tapping operation a blocked slag orifice occurs giving 
rise to a momentary drop in differential pressure, this will be detected 
by trip 6 and the tapping will be terminated and the run period 
recommenced. This rapid re-establishment of the run condition will tend to 
blow the obstruction clear of the orifice. 
If there is no interruption of tapping, the run period recommences after 
the tapping period is completed.