Steam generator heated by combination of electric heat and condensation of contaminated process steam

A steam generator assembly, particularly for use in connection with the mechanical preparation of pulp, for generating pure steam required in a subsequent process, such as drying of proper in a paper machine, by utilizing the condensation heat of contaminated steam coming from a preceding process, such as a grinding process, includes a steam converter and electrical boiler arranged in a common housing to form a unified structure. The steam converter utilizes the condensation heat of the contaminated steam to generate pure steam from water supplied to the converter. The electric boiler is connected in parallel to the steam converter and includes electrodes adapted to generate steam when moistened by water supplied to the boiler from a separate water container in the housing. Selective control of the supply of water to the electric boiler and the extent to which the electrodes are moistened regulates the amount of pure steam generated by the electric boiler. The pure steam generated by the steam converter and electric boiler is discharged from the common housing through a steam outlet.

BACKGROUND AND SUMMARY OF THE INVENTION 
The present invention concerns a stream generator, in particular a steam 
generator used in connection with the mechanical preparation of pulp, for 
generating pure steam required in a subsequent process, e.g., in the 
drying of paper in a paper machine, by utilizing the condensation heat of 
contaminated steam coming from a preceding process, e.g., a grinding 
process. 
When wood chips are being ground by means of the so-called thermomechanical 
process (TMP) of prior art, wherein the wood chips are most usually 
introduced into a 1-phase or 2-phase system of a disc grinder, electrical 
energy is typically consumed at a rate of about 1.7 to 2.5 MWh per ton of 
ready paper pulp. 
In such processes, whose efficiency is, according to literature references, 
0.1 to 0.2%, almost all of the rest of the energy is recovered out of the 
process in the form of steam. In an integrated paper mill, this steam can 
be converted in a heat exchanger into pure steam and be used in the paper 
machine for drying the paper. The steam obtained in this way represents 50 
to 75% of the steam consumed by a paper machine. 
Such a process is now already in operation in many places around the world. 
A problem that remains is that, out of a TMP plant, quite varying 
quantities of steam are obtained to the paper machine, because these 
quantities depend on the degree of utilization of the grinders. In such a 
situation, if, for example, a grinding line, whose output may be of the 
order of 10 to 15 MW, falls off, a corresponding quantity of steam also 
becomes unavailable to the paper machine. In such a case, the power plant 
producing auxiliary steam must react to the altered situation rapidly. 
Today, in many parts of the world, the costs of electricity and fuel oil 
are rather close to each other and, moreover, often the marginal cost of 
electricity is still closer to the cost of the heat equivalent of fuel oil 
than the average cost of electricity. 
When a large grinding line is being started, said line having, e.g., two 
grinders in series and having a total output of the order of 10 to 15 MW, 
the grinders are loaded evenly while increasing the output continuously 
over 2 to 20 minutes, whereby steam, which is generated correspondingly, 
is obtained in proportion to the loading. 
It is also possible to combine a steam generator in which contaminated TMP 
steam is converted into pure steam with an electrical boiler which 
compensates for the output of grinders falling off. Such a boiler goes on 
with the same total electricity load while using part of its power for 
producing pulp and part for direct steam generation in the steam 
generator. 
In this procedure as well, there remains the problem that power cannot be 
shifted suddenly from the grinders to electricity-consuming and 
steam-producing electrodes without causing a violent fluctuation in the 
electricity supply network. 
Now it has been noticed that, in such a steam converter, in which part of 
the power is produced by means of power electrodes, the above-discussed 
difficulties can be overcome surprisingly easily in the way to be 
indicated below. 
The invention is based on the idea that the overall generation of steam is 
equalized by controlling the moistening area of the electrodes in the 
electrical boiler. More specifically, the steam generator in accordance 
with the present invention is characterized in that it comprises 
a steam converter; 
an electrical boiler connected in parallel with the steam converter; 
at least one power electrode arranged within the electrical boiler and 
comprising an outer face; 
means for moistening the outer face of said at least one electrode; and 
means for controlling the moistening area of the outer face of said at 
least one electrode.

DETAILED DESCRIPTION OF THE INVENTION 
As illustrated in FIGS. 1 and 2, a housing 1 of unified structure is 
provided with an outlet pipe 3 for delivering pure steam. An inlet pipe 4 
is operatively connected to the housing 1 for supplying feed water. In 
addition, an inlet pipe 6 is provided for supplying impure waste steam. A 
steam converter 5 having a bottom 9 is positioned within the housing 1. A 
feed pump 16 supplies water from a container 8 positioned within the 
housing 1 to an inlet pipe 4. Further, a feed pump 25 (see FIG. 2) is 
provided for supplying water from a bottom portion of the electric boiler 
12 to a jet pipe 21. 
Let us assume that the steam converter 5 (FIG. 1) receives contaminated 
steam from TMP grinders of from any other process of mechanical pulp 
production, e.g., as a quantity corresponding to an output of 30 MW and, 
at the same time, the electric boiler 12 generates additional steam by 
means of electrodes 13 within the electric boiler 12 of the pure steam 
generator assembly comprising steam converter 5 and the electric boiler 12 
located in housing 1 and providing a unified structure, e.g., at a rate of 
15 MW. Let as assume further that more grinders are started at this power 
of 15 MW and that the starting takes place, e.g., during 3 minutes. Now 
the situation is arranged such that, out of the electrical boiler 12 in 
which the electrodes 13 are placed, water is pumped out, or allowed to 
flow out by means of the system's own pressure, from between the 
electrodes through the valve 14 as the grinders are taking more power, so 
that this additional power increases as much as the electrode power is 
reduced. In such a case, e.g., a paper machine using steam always, even as 
the grinders are being started, receives the same quantity of steam. When 
the grinders are under full load, the electrical boiler 12 is empty, 
having no water around the electrodes 13, and no steam is generated 
directly electrically. On the other hand, when grinders start being run 
down, the water level in the electrical boiler 12 is raised accordingly so 
that the power dropped off from the grinders is again shifted to the 
electrodes 13 and is directly converted into a corresponding quantity of 
pure steam. 
The electrical boiler 12 is connected in parallel with the steam converter 
5, and is provided with two electrodes 13 extending vertically. Moreover, 
it is provided with an inlet valve 11 and an outlet valve 14, by means of 
which the water level in the electrode space of the electrical boiler 12 
can be controlled so that the total output of the pure steam generator 
assembly comprising the electrical boiler 12 and of the steam converter 5 
can be maintained at a desired level, e.g., as of constant magnitude, 
irrespective of variations in the impure steam output coming from the 
grinder to the steam converter 5. In the example case, the electrode space 
of the electrical boiler 12 is connected via the inlet valve 11 to a 
separate water container 8. This water container 8 may be in direct feed 
connection with the heat transfer face 9 of the steam converter 5 by pump 
16 and separated from the electrode space of the electrical boiler 12 by 
means of a partition wall 10. The steam space of the electrical boiler 12 
bypasses the steam converter 5 via a channel 15 and is directly connected 
with the steam space 2 of the housing 1 enclosing the steam converter 5 of 
the electrical boiler 12 of the pure steam generator assembly. The 
separate water container 8 is preferably high enough so that the water 
contained therein may move into the electrical boiler 12 via the inlet 
valve 11 by the effect of gravitation. Alternatively, this movement of 
water may be arranged by means of a pump. 
The solution shown in FIG. 2 differs from that shown in FIG. 1 in the 
respect that therein the electrical boiler 12 includes a jet device 20 to 
26, by means of which the vertical electrodes 13 can be moistened. The jet 
device comprises a stationary, vertical jet pipe 21, 19, which is fitted 
between the electrodes 13 and which is supplied by the pump 25. The jet 
pipe 21, 19 is provided with nozzles 20, by means of which the inside 
faces the electrodes 13 can be sprayed with water. Between the electrodes 
13 and the jet pipe 21, 19, a covering means 22 is fitted, which can be 
shifted vertically by means of a lifting wire 24 and which, when facing 
the jet, prevents the jet from hitting against the electrode 13. Thus, the 
control takes place by varying the height of the covering means 22. 
Within the scope of the invention, it is also possible to conceive 
solutions differing from the exemplifying embodiment described above (FIG. 
1). Thus, the inlet and outlet valve may also be a single joint valve 
through which water can be shifted by means of a pump between the water 
container 8 and the electrical boiler 12 in both directions. The control 
proper, i.e., the opening and closing of the valves 11 and 14 is most 
appropriately operated by means of a computer in a way known per se. 
Differing from the example, the electrical boiler 12 may also be separate 
and, e.g., by means of a pipe, connected to the steam space 2 of the 
housing 1. 
As a more detailed description related to the drawings, it should be 
mentioned that a container 27 for contaminated condensate, an outlet pipe 
7 for contaminated condensate, a supply pipe 17 for circulation water, and 
a sealing means 23 for the lifting wire 24 are provided. In addition, a 
return pipe 26 for the jet device, a supply water pipe 28, a preheater 18 
for supply water and an exhaust pipe 29 for inert gases are provided. 
Further, a liquid distributor disk 30 of the steam converter and a return 
channel 31 for condensate are illustrated in FIG. 2. 
Finally, it should be stated that a solution in which electrical power is 
shifted between grinders and the electrical boiler by altering the liquid 
level in the electrode space is an alternative for the possibility that 
the shifting of the same electrical power takes place by means of 
switches.