Process of controlling the starting up of the gasification of solid fuels in a fluidized state

Fuels are gasified in a fluidized state by a treatment with oxygen-containing gas and water vapor in a gasifying reactor. A solids mixture which contains ash and fine-grained fuels is combusted in a heating-up phase, which precedes the gasification and in which the temperature in the reactor is increased approximately to the temperature desired for the gasification. In a succeeding inertizing phase the supply rate of oxygen-containing gas is decreased and an inert gas is fed to the reactor until the product gas no longer contains free oxygen whereas the temperature is maintained virtually constant. In the succeeding gasification the fuel supply rate is increased and, after an adjusting time, the temperature is maintained virtually constant at the value desired for the gasification in the range from 600.degree. to 1500.degree. C. The gasification temperature is controlled by a change of the fuel supply rate.

This invention relates to a process of controlling the starting up of the 
gasification of fine-grained solid fuels, which are treated in a fluidized 
state with oxygen-containing gas and water vapor in a gasifying reactor, 
which is provided at its end with a duct for discharging product gas and 
at its bottom portion with means for withdrawing ash. The gasification is 
effected under a pressure of 1 to 100 bars. 
A process of that kind is described in U.S. Pat. No. 4,594,967 and involves 
a controllable cooperation of several portions of the fluidized bed. The 
operation is initiated by means of a heating-up burner and the fuel is 
subsequently supplied with oxygen at a substoichiometric rate until 
steady-state gasification conditions have been established. 
It is an object of the invention to start up the gasifying reactor in an 
easily controllable manner and to permit the use of a reactor having a 
simple design. In the process described first herein-before this is 
accomplished in accordance with the invention in that during the 
heating-up phase preceding the gasification a mixture of solids comprising 
ash and fine-grained fuels is combusted in a fluidized state in the 
reactor with a supply of oxygen-containing gas to provide a 
hyperstoichiometric supply of oxygen and the temperature in the reactor in 
thus increased approximately to the temperature desired for the 
gasification, the heating-up phase is immediately succeeded by an 
inertizing phase, in which the supply rate of oxygen-containing gas is 
decreased and an inert gas is supplied to the reactor and the content of 
free oxygen in the product gas is decreased virtually to zero whereas the 
temperature is maintained virtually constant, and the inertizing phase is 
succeeded by the gasification, in which oxygen or oxygen-containing gas 
and optionally steam are fed to the reactor, the fuel supply rate is 
increased and the temperature desired for the gasification, which when 
measured in the top portion of the reactor or in the discharge duct lies 
in the range from 600.degree. to 1500.degree. C., is maintained virtually 
constant after a time for temperature adjustment and in which the supply 
rate of solid fuel is decreased when the temperature is too low and the 
supply rate of fuel is increased when the temperature is too high. During 
a steady-state gasification a decrease of the temperature will be avoided 
because the product gas would otherwise contain undersired products of 
carbonization. 
During the heating-up phase the temperature is gradually increased. In a 
reactor having a refractory lining it is recommendable to increase the 
temperature at a rate of about 40.degree. to 120.degree. C. per hour. The 
supply rate of solid fuel will be decreased when the temperature is too 
high and the supply rate of solid fuel will be increased when the 
temperature is too low because the oxygen will then be present in a 
hyperstoichiometric proportion in the gasifying reactor. Particularly for 
the sake of economy it will be desirable during the heating-up phase to 
supply air as an oxygen-containing gas into the reactor. Approximately at 
the time when the temperature desired for the gasification has been 
reached at the end of the heating-up phase the supply rate of 
oxygen-containing gas is decreased and in an inertizing phase the reactor 
is supplied with inert gas at a progressively increasing rate. The total 
rate at which gas is supplied will initially remain approximately 
constant. That inert gas usually consists of recycled product gas, 
nitrogen or carbon dioxide. 
When the reactor has sufficiently been purged with inert gas during the 
inertizing phase so that the product gas no longer contains oxygen, the 
gasification may be initiated. For that purpose the reactor is supplied 
with a gasifying agent, which consists mainly of oxygen (e.g., of air) and 
of more or less steam. At the beginning of the gasification, during the 
time for temperature adjustment, the reactor is supplied with inert gas 
(e.g., N.sub.2 or CO.sub.2) at a progressively decreasing rate and the 
reactor is simultaneously supplied with fuel at a higher rate whereas the 
supply rate of ash is decreased to zero. If the fuel, such as brown coal, 
has a high water content in itself, the proportion of steam in the mixed 
gasifying agents may be decreased, possibly to zero. When the gasification 
has reached a steady state, the temperature is maintained constant within 
a fluctuation range of .+-.40.degree. C. This is accomplished by a control 
of the oxygen supply. 
As an additional measure for controlling the temperature in the reactor the 
supply rate of steam may be varied during the heating-up phase, during the 
inertizing and during the gasification.

The reactor 1 shown in FIG. 1 is used for the gasification of solid fuels 
in a fluidized state. The fuels are fed by a feeder conveyor 2. Coal, 
brown coal or peat, e.g., may be used as solid fuels. The fuel and inert 
material are fed from a supply bin 3 via metering means 4 consisting, 
e.g., of a star wheel feeder. A container 6 for the fuels to be gasified 
and a container 7 for inert material, particularly ash or sand, are 
provided over the supply bin 3. For the sake of simplicity, the fuel to be 
gasified is said to consist of coal and the inert material is said to 
consist of ash in the following explanations. 
The reactor 1 contains in its bottom portion a chamber 9 for distributing 
gases and/or water vapor, which enter through line 10 and pass through a 
grate 11 into the reactor 1. A branch line 12 provided with a valve 13 
permits the supply of such fluids at a metered rate also to a region above 
the grate 11 at the same time. 
During a steady-state gasification, a circulating fluidized bed is 
maintained in the reactor 1. In that case a mixture of product gas and 
solids is conducted through the discharge duct 15 into a cyclone 16 and is 
separated therein. The product gas flows through line 17 to a waste-heat 
boiler 18 and is available in line 19 for further use. Because the product 
gas has high contents of H.sub.2 and CO, it may be processed further to 
form, e.g., a synthesis gas. Solids collected in the cyclone 16 are 
recycled to the reactor 1 in line 20. Through a pipe 22, which extends 
centrally through the distributing chamber, low-carbon ash enters the ash 
chamber 23 and is periodically withdrawn through line 24. 
A steam line 26, an oxygen line 27, an air line 28 and an inert gas line 29 
are connected to the manifold 10. Each of the lines 26 to 29 is provided 
with a control valve 30 and with a sensor 31 for measuring the flow rate. 
The control valves 30 are controlled by a controller 35 via signal lines 
32. Each sensor 31 indicates the flow rate in the associated line to the 
controller 35 via a signal line 33. The temperature in the discharge duct 
15 is detected by a temperature sensor 34, which delivers corresponding 
data via the signal line 36 to the controller 35. In a manner to be 
described hereinafter the controller 35 effects a semiautomatic or 
automatic control of the temperature. The supply rate of coal to the 
reactor 1 is controlled by the control line 37. Details of methods by 
which that control may be effected will be explained with reference to 
FIGS. 2a and 2b. 
In FIG. 2a the temperature in .degree.C. is plotted along the vertical axis 
T and the horizontal axis t is the time axis in both FIGS. 2a and 2b 
(values, e.g., in hours). Rates (e.g., in kg/h) of substances which are 
fed to the reactor 1, which rates vary with time, are plotted along the 
vertical axis M in FIG. 2b. The solid line a indicates the course of the 
rate at which air is supplied through line 28. The line b represents the 
rate of inert gas supplied through line 29. The dot-and-dash line c 
represents the coal supply rate, and the dotted line d represents the rate 
of steam flow through line 26. 
For the initial warming up, ash in fed to the reactor 1 and is fluidized by 
means of hot air. A start-up burner 40 is started at a later time and is 
supplied through line 41 with gaseous or liquid fuel, such as natural gas 
or fuel oil, whereas air is supplied through line 42. As a result, a 
gradually increasing temperature is sensed by the sensor 34 until at the 
time A coal from the bin 3 is supplied to the reactor 1 via the star wheel 
feeder 4 at a controlled rate. During the now ensuing heating-up phase, 
coal is supplied and is fluidized by a supply of air and is combusted in 
the reactor in the presence of an excess of oxygen so that the temperature 
is increased further. The start-up burner 40 can now be shut off and the 
proportion of the ash supplied is decreased toward zero. When the 
temperature rise is too steep, the supply rate of coal to the reactor will 
be decreased and will be increased when the rate of temperature rise is 
lower than desired. An excessively high temperature may be corrected by a 
supply of steam to the reactor. The controller 35 may be adjusted to the 
desired temperature manually or as a result of an automatic computation. 
The temperature rise in the heating-up phase is continued until the 
temperature has reached or slightly exceeds the temperature desired for 
the gasification. This is achieved at the time B in FIG. 2a. The 
inertizing phase is now initiated to eliminate the oxygen content of the 
product gas. Whereas the temperature is kept constant, the rate at which 
air is supplied in line 28 to the reactor 1 is decreased and the supply 
rate of inert gas is increased at the same time. Care is taken to maintain 
the total rate of air and inert gas approximately constant. In FIG. 2a, C 
indicates the time at which the oxygen content of the product gas has been 
decreased to zero and at which the inertizing phase is terminated. An 
analyzer, not shown, is used to determine the oxygen content of the 
product gas in the duct 15. 
The gasifying operation can now be initiated. For this purpose a starting 
phase, called adjusting time, is first required between times C and D. 
During that phase the supply rates of coal and oxygen-containing gas are 
increased whereas the supply of inert gas is gradually shut off. Finally 
steam at progressively increasing rates can be supplied to the gasifying 
process; see the dotted line d in FIG. 2b. Such controls may be effected 
automatically or by hand. Care is taken at the same time to maintain the 
temperature virtually constant or to permit only a slight temperature drop 
during the adjusting time, whereafter the temperature remains constant; 
see the lines m and n in FIG. 2a. 
During the steady-state gasification beginning at the time D, coal, steam 
and oxygen (e.g., as air) are supplied to the reactor 1 ideally at 
constant rates. For instance, 1 kg steam may be used per sm.sup.3 of 
oxygen (sm.sup.3 =standard cubic meter). During gasification of brown coal 
or peat, which have a very high water content, the rate of steam may be 
reduced or the supply of steam may be omitted. 
During the gasifying operation the temperature is controlled by control of 
the supply of coal through the star wheel feeder 4. More coal will be fed 
to the reactor 1 when the temperature is too high and less coal when the 
temperature is too low. It is recommendable to maintain the temperature 
constant during the gasification within a fluctuation range of 
.+-.40.degree. C., preferably .+-.30.degree. C. 
EXAMPLE 
In a plant as shown on the drawing, 21,318 kg coal are gasified per hour. 
The reactor 1 is 2.5 m in diameter and above the grate 11 has a height of 
15 m. The coal to be gasified is a coal mixture having a lower calorific 
value of 5579 kcal/kg, a water content of 24% by weight and an ash content 
of 8.3% by weight. The coal has the following elementary analysis 
(calculated without water and ash): 
______________________________________ 
C 79% by weight 
H 5.4% by weight 
O 12.1% by weight 
N 3.5% by weight 
100.0% by weight 
______________________________________ 
The combustion and gasification are effected without commercially pure 
oxygen, only with air, nitrogen and water vapor. No secondary air is 
supplied through line 12. 
For the first heating up up to about 350.degree. C., hot air at 420.degree. 
C. is fed into the reactor, which contains ash in an increasing quantity 
up to 1000 kg. Thereafter the burner 40 is additionally operated and is 
fed with fuel oil at a progressively increasing rate of up to 361 kg/h. 
After a heating up for 8 hours, a temperature of 600.degree. C. has been 
reached in the duct 15. At that temperature the supply of coal into the 
reactor begins; this corresponds to the point A in FIGS. 2a and 2b. The 
rates at which coal and auxiliary substances are supplied at various times 
are stated (in kg/h) in the following table, as well as the temperatures 
in the duct 15. Points A to D refer to FIGS. 2a and 2b and the timing of 
the rates of supply to the reactor is also apparent from FIG. 2b. 
______________________________________ 
Time (h) 
8 13 13.5 14 15 16 
Duration (h) 
Time A B C D 
______________________________________ 
Coal 0 1764 1764 1764 21318 21318 
Air 38767 38767 14853 14853 38767 38767 
Nitrogen 0 0 23914 23914 0 0 
Steam 0 0 0 0 0 2000 
Fuel oil 361 0 0 0 0 0 
Temperature 
600 950 950 950 950 920 
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The composition of the gas in the duct 15 is at different times: 
______________________________________ 
Time A B C 
______________________________________ 
CO.sub.2 (% by vol.) 
1.9 6.69 6.69 
H.sub.2 O (% by vol.) 
1.9 2.74 2.74 
O.sub.2 (% by vol.) 
17.9 13.13 
0 
N.sub.2 (% by vol.) 
78.3 77.44 
90.57 
______________________________________ 
When the steady-state gasification begins at time D, a product gas having 
the following composition is produced: 
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CH.sub.4 2.5% by vol. 
H.sub.2 14.7% by vol. 
CO 20.8% by vol. 
CO.sub.2 7.0% by vol. 
N.sub.2 48.8% by vol. 
H.sub.2 O 6.2% by vol. 
______________________________________ 
For the control of the temperature in the interval of time between times A 
and B, during which combustion air is supplied at a rate of 38,767 
sm.sup.3 /h, it must be borne in mind that the coal supply rate must be 
decreased or increased by 20 kg/h in case of an increase or decrease of 
the temperature by 10.degree. C. relative to the desired value in order to 
bring the temperature to the desired value. 
The steady-state gasification is carried out at the desired temperature of 
920.degree. C., a coal supply rate of 21,318 kg/h and an air supply rate 
of 38,767 kg/h. The coal supply rate must be changed by 150 kg/h in case 
of a temperature change by 10.degree. C. in order to restore the desired 
temperature. 
It will be appreciated that the instant specification and claims are set 
forth by way of illustration and not limitation, and that various 
modifications and changes may be made without departing from the spirit 
and scope of the present invention.