Process and apparatus for energy recovery from solid fossil inerts containing fuels

In an apparatus and process for energy recovery from solid fossil, inerts containing fuels the fuel is burned in a pressurized fluidized bed reactor for operating a gas turbine and a steam turbine. The flue gases of the fluidized bed are cleaned of injurious materials before introduction into the gas turbine. The gases are sharply cooled before material removal and warmed to gas turbine temperatures after removal.

The invention relates to a process and apparatus for recovering energy from 
solid, fossil fuels containing inert material. 
There is already known a so-called combiblock with pressure fluidized bed 
combustion, in which the hot flue gas emerging from the pressure fluidized 
bed combustion with a temperature of less 900.degree. C. laden with ash is 
supplied to a cyclone device or a E-filter for dust removal. In this 
connection, it is practical to discharge the entire amount of ash from the 
fluidized bed combustion, so that the subsequent filters are 
correspondingly loaded. The difficulties with this process lies, in 
particular, with the flue gas dust removal that must be carried out with 
high temperature, high pressure, and high ash loading. It has been pointed 
out that with this known process with the associated cyclone apparatus, 
the necessary cleanliness of the flue gases for a gas turbine operation 
with respect to dust content is not attainable and that with the high 
pressure losses corresponding to highest possible dust removal operation, 
such a dust removal apparatus is uneconomical for the reasons of overall 
thermal efficiency. An associated E-filter results in a very large 
construction size with high temperature and high pressure so that these 
E-filters are not suitable for large apparatuses. Also, the necessary flue 
gas cleanliness can not be obtained with an E-filter with respect to the 
dust content. 
Finally, a process is known (VDI-Report Number 322, 1978) by which a 
pressure free fluidized bed combustion is coupled with a hot air turbine 
and a steam turbine process. By contrast to the combiblock with pressure 
fired fluidized bed combustion, this known process possesses the 
disadvantage that it requires a very large overall construction size. 
Also, the flue gas cleaning occurs at the end of the device at discharge 
gas temperature so that almost the entire ash content must take part in 
the course of the flue gas cooling. This leads very quickly to 
considerable fouling of the heat transfer surfaces necessary for the steam 
process and therewith to an overall reduction of the thermal efficiency of 
the apparatus. 
The object of the present invention is to provide an apparatus and process 
of very high thermal efficiency in which solid, inert containing, fossil 
fuels may be used in a pressure fluidized bed reactor to drive electrical 
energy producing gas and steam turbines. The difficulties occurring with 
the cleaning of the gas generated in the fluidized bed are avoided. 
Another object consists further in the provision of an apparatus that makes 
possible a problem free gas cleaning with smallest possible construction 
volumes. 
According to the invention the flue gases generated in the fluidized bed 
are sharply cooled before removal of injurious material and reheated to 
the gas turbine inlet temperature after removal of the injurious material. 
The essence of the invention occurs in that the gas cleaning is carried out 
with correspondingly low flue gas temperatures so that conventional gas 
cleaning components can be used. This makes possible, in connection with 
the pressure operation of the device, the use of small size construction 
elements. The heat released by the cooling of the flue gases is added to 
the steam turbine process so that a portion of the heat is used for the 
reheating of the exhaust gases to gas turbine temperature. 
Through the combination of the gas turbine process and the steam turbine 
process a relatively high thermal efficiency is obtained. Preferably the 
flue gas is cooled before entrance in the gas cleaning to approximately 
30.degree. K. (Kelvin) above the condensation temperature so that at this 
temperature almost all conventional gas cleaning processes can be applied, 
such as fabric filter, E-filter, wet cleaning and the like, without the 
described difficulties of the initially described known processes with 
regard to the difficult dust separation and the relatively large 
construction sizes. 
The pressure washer preferably provided for the gas cleaning makes possible 
the washing out of many injurious materials and, in particular, chlorine, 
fluorine, as well as their hydrogen compounds, alkali metals, heavy 
metals, and the like. 
Moreover, it is advantageous if, after the injurious material cleaning, a 
controllable amount of compressed air is supplied to the flue gases heated 
by a portion of the heat released by the combustion before introduction in 
the gas turbine. Simultaneously the high temperatures occurring before the 
gas turbine are lowered so that a protected gas turbine process is the 
result. There thus results also lower temperatures in the waste heat 
boiler, so that no additional evaporation takes place in the waste heat 
boiler and thus the steam side conditions in the fluidized bed reactor are 
guaranteed also with part load. 
In a particularly preferable manner, a controllable partial flow of the 
combustion air conducted to the fluidized bed reactor is supplied to the 
flue gas driving the gas turbine. At the same time with the proportioning 
of the air quantity supply to the boiler load, the adherence of the 
desired temperatures in the fluidized layer is also promoted by this 
means, so that a too high air excess in the fluidized layer by the 
compressor driven in the customary manner through the gas turbine is 
prevented. 
In an advantageous manner, the amount of compressed air supplied to the 
flue gas is controlled through the temperature of the exhaust gases ahead 
of the gas turbine and the temperature prevailing in the waste heat 
boiler. 
It is also preferable that the heat released by the combustion is used for 
the heating of the steam for the steam turbine process, whereby the steam 
outside of the fluidized bed is preheated through the heat contained in 
the flue gases. Through the pre-superheater and an injection cooler 
preferably acting on the steam supplied to the superheater, the 
temperature of the final superheater stressed particularly through heat in 
part load and peak load operation is lowered, so that these measures in 
connection with the supply of combustion air to the flue gas ahead of the 
gas turbine contributes to a reduction of the heat requirement of the 
apparatus components. 
If a controllable partial flow of the flue gas after injurious material 
cleaning without heat loading is by-passed around the fluidized bed, what 
can thus result through a by-pass connection, is that a heat displacement 
from the gas to the steam cycle and reverse is possible. Relevantly, the 
temperature of the flue gases ahead of the gas turbine can be regulated 
with the by-pass connection. Likewise in this connection, also an 
influencing of the temperature in the fluidized bed is possible. 
A simple realization of an apparatus for the carrying out of the process 
according to the invention is obtained in that between a compressor 
provided for the supply of the fluidized bed reactor with combustion air 
and the flue gas conduit leading from the reactor to the gas turbine, a 
by-pass connection is connected. In this manner also, a too high air 
excess in the fluidized layer through the air compressor customarily 
driven from the gas turbine is prevented, so that undesired heat transfers 
in the after connected heat transfer surfaces in the pressure fluidized 
bed reactor are precluded. 
A particularly simple solution in reference to construction is achieved in 
that the by-pass connection opens in the flue gas supplying inner pipe of 
a double jacket pipe, in the outer pipe of which is provided with 
combustion air to the reactor in flow opposition to the flue gas. 
A good regulating opportunity is guaranteed in that a valve is arranged in 
the by-pass connection. Preferably this valve is actuated by a temperature 
sensor that is arranged in the region of the fluidized bed, the gas 
turbine and/or the waste heat boiler. A careful treatment of the 
superheating surfaces particularly stressed with the transfer of the load 
from the gas to the steam cycle is achieved in that a preheater is 
connected to these. Preferably an injection cooler is arranged in the 
conduit leading from the preheater to the superheater so that the 
temperature in the final superheater can be lowered.

The device shown in FIG. 1 serves for the combustion of solid, fossil and 
inerts containing fuels, in particular coal, that is mixed with lime in a 
mixing device 1 in accordance with the necessary sulfur separation in the 
pressure fluidized bed reactor. The mixing apparatus is connected to a 
charging device, not further shown, through which the fuel is delivered to 
the pressure fluidized bed reactor 2 through a pneumatic conveying 
conduit. Beside the fuel addition, compressed combustion air is injected 
from the base of the pressure fluidized bed reactor through nozzles there. 
The combustion air which preferably has a temperature of approximately 
350.degree. C., is produced through a compressor 16 further explained 
below. In the pressurized bed of the reactor occurs the combustion of the 
fuel, through which flue gases are produced. 
The pressure fluidized bed reactor formed as a double jacketed construction 
3 comprises an outer pressure wall and an inner wall formed through fin 
and tube walls. The fin and tube walls are part of the evaporation system 
of the steam process. In the intermediate hollow space formed thereby 
combustion air is supplied through the compressor 16 with a temperature of 
approximately 350.degree. C. in an opposite flowing process further 
described below. 
Apparatus for the removal of injurious materials are connected downstream 
of the pressure fluidized bed reactor 2, namely a cyclone separator 11 for 
dust removal, a pressure washer 12 and a spray separator 13. The flue gas 
produced in the pressure fluidized bed reactor through combustion of the 
fuel is fed after conduction through the injurious material removal 
apparatus through a pressure increasing compressor 14, before it is 
delivered eventually to the gas turbine 15 subsequent to the reheating 
further described below. 
For the reheating of the cleaned gas, two gas heaters are provided in the 
pressure fluidized bed reactor, namely a preheater 8 which is arranged 
outside of the fluidized bed in a moderate temperature range of the flue 
gas in the pressure fluidized bed reactor of approximately 400.degree. to 
750.degree. C., and a final heater 5 arranged in the fluidized bed. 
Between the flue gas supply to the final heater and the flue gas discharge 
of the final heater to the gas turbine a by-pass conduit 6 is provided 
with one or more regulating valves. 
For the operation of the steam turbine, a superheater 4 is provided in the 
lower region of the fluidized bed, from which a conduit leads to a steam 
turbine 19. Outside of the fluidized bed, two further steam producing heat 
transfer surfaces are provided in the region of the pressure fluidized bed 
reactor, which are incorporated in the steam turbine process. In 
particular there is referred to in this connection an intermediate 
superheater 7 positioned between the preheater 8 and the final heater 5 
for the flue gases and a high pressure coil 9 and a lower pressure coil 
10. These latter steam producing heat transfer surfaces serve for the 
further cooling of the converted flue gases, as is further described 
subsequently. 
Between the walls of reactor 2 flows something above 80% of the compressed 
combustion air supplied through the compressor, that has a temperature of 
approximately 350.degree. C. 
The fuel mixture injected in the fluidized bed is burned with a temperature 
slightly below 900.degree. C. and at a pressure of approximately 10 bar, 
in the course of which the combustion air necessary in this connection is 
injected by the compressor beneath the fluidized layer through the nozzle 
base. 
Through the addition of the lime to the fuel, the sulfur released from the 
coal is combined into calcite so that a desulfurization occurs inside the 
pressure fluidized bed furnace. 
The heat released by the combustion is supplied through the superheater 4 
to the steam turbine process and through the final heater 5 serves for the 
warming of the flue gases to gas turbine temperature. The further cooling 
of the flue gases results inside the pressure fluidized bed reactor as by 
the intermediate superheater 7, the high pressure coil 9 and the lower 
pressure coil 10, as well as through the preheater 8 for the flue gases. 
Additionally, the water cooled walls 3 of the fluidized layer and the 
subsequent heat exchanger take up heat. At the same time, the flue gases 
are cooled to approximately 30.degree. K. (Kelvin) above the condensation 
temperature so that the flue gases exiting out of the pressure fluidized 
bed reactor possess a temperature of approximately 130.degree. C. (10 
bar). 
The cooled flue gases are then supplied to the cyclone separator 11 for 
course separation and then subjected to a wet cleaning in the pressure 
washer 12. In this connection the gas is cooled with simultaneous 
saturating out to approximately 100.degree. C. For improved injurious 
material removal, such as chlorine, fluorine, and their hydrogen 
compounds, the wash water can be treated with an alkali solution. As a 
result of the wet cleaning carried out under pressure, the apparatus can 
be maintained essentially smaller and is also given an improved reaction 
result during the wash. Finally, the flue gas cleaned of dust and 
injurious materials is delivered to the spray separator (13) serving as 
residual spray separator and subsequently to the pressure compressor 14. 
After passage through the spray separator 13, the flue gases are subjected 
to a secondary compression in the pressure increasing compressor 14 and 
finally supplied to the preheater 8. Through the imposition of the 
intermediate superheater 7 between the preheater 8 and the final heater 5, 
crack formation in the heat transfer surfaces of the preheater as a result 
of temperature shocks through entrained water particles are avoided. The 
flue gases warmed in the preheater 8 to a temperature of approximately 
400.degree. to 500.degree. C. are then supplied to the final heater 5, the 
heat transfer surfaces of which are fully embedded in the fluidized bed. 
There thus results an intensive heat transfer to the cleaned gas to be 
heated, that is enhanced through the fluidized bed firing under pressure. 
In the final heater the flue gases are heated to a suitable temperature 
for the gas turbine process, namely a temperature of slightly below 
900.degree. C. or equal to 900.degree. C. 
A portion of the flue gases can be by-passed around the final heater 5 
through the by-pass conduit 6 provided with one or more regulating valves. 
In this way the temperature of the flue gases supplied in the gas turbine 
can preferably be regulated. Also, a control of the heating surfaces in 
the fluidized layer is thus possible. On the basis of the by-pass conduit 
is also a measure of influence on the temperature relationships inside the 
fluidized bed possible according to the partial amount of the flue gases 
supplied in the by-pass. There can thus be undertaken a heat displacement 
from the steam turbine process to the gas turbine process and reverse. 
The flue gas heated to the gas turbine temperature is supplied in a double 
pipe construction 21 to the gas turbine 15, whereby combustion air in the 
opposite flow process is supplied from the compressor 16 to the pressure 
fluidized bed reactor. 
Preferably the pressure of the air supplied to the reactor in the outer 
pipe of the double wall pipe 21 develped through the compressor 16 is 
somewhat greater than the pressure of the secondarily compressed flue 
gases through the compressor 14, which is advantageous on grounds of 
facility of the double pipe construction acted upon in the inner pipe with 
the hot flue gases. On the basis of the double pipe construction, it is 
also prevented that, with a defective inner pipe, hot gases can emerge in 
the environment, to the contrary the pressurized air would flow into the 
inner pipe. As a result of the gas sided pressure operation, the 
mechanical pressure strains are likewise largely precluded on the side of 
the gas heaters 5 and 8. The heat transfer surfaces of the gas heaters are 
stressed thus solely by temperature. 
The flue gas conducted in the gas turbine is expanded and cooled in this 
connection to approximately 450.degree. C. The gas turbine 15 drives the 
air compressor 16, that supplies the air necessary for the combustion and 
a generator 17 that produces the electrical energy. The flue gas cooled in 
the gas turbine arrives eventually in the waste heat boiler 18, where it 
is used for the feed water heating for the steam process and is thereby 
cooled to approximately 110.degree. C. 
Waste heat boiler 18, evaporator 3, superheater 4, intermediate superheater 
7, high pressure coil 9 and low pressure coil 10 thus form the heat 
transfer installation for the steam process. The steam turbine drives a 
generator 20 for the production of electrical energy. 
With the exemplary embodiments according to FIGS. 2 and 3, a presuperheater 
4a is arranged between the intermediate-superheater 7 and the flue gas 
preheater 8. 
The heat transfer surfaces for the steam lead to the steam turbine, so that 
the presuperheater 4a is connected after the superheater 4 and an 
injection cooler 4c is provided in the connection conduit. 
From FIGS. 2 and 3 it is apparent that a connection leads from compressor 
16 to the base of the pressure fluid bed reactor, through which fuel 
carrier air is injected in the reactor. A further connection leads from 
the compressor to the double jacket pipe 21, the connection point being 
more precisely seen in FIG. 2. A by-pass conduit 25 leads as well from the 
compressor 16 to the double jacket pipe 21 and opens in the inner pipe 23 
of the double jacket pipe 21 ahead of the gas turbine 15. In this 
connection, the pressure produced through the compressor 16 is somewhat 
greater than the pressure in the flue gas. In the outer pipe 24 flows 
compressed combustion air from the compressor 16 into the double wall 
intermediate space of the reactor 2 and from there out to the base of the 
fluidized bed and is there injected through nozzles in the fluidized 
layer. 
In the by-pass connection 25 a valve 26 is arranged. The valve 26 can be 
controlled through with temperature sensors 27, 27' or 28 which are 
arranged in the region of the gas turbine 15, the fluidized bed reactor, 
or the waste heat boiler 18. 
Through the by-pass connection 25 compressed combustion air can be supplied 
to the heated flue gas before the introduction in the gas turbine 15 so 
that the gas turbine 15 can be operated with a controllable amount of gas 
and the temperature ahead of the gas turbine can be reduced. 
Simultaneously in this connection, the combustion process and the 
temperature condition in the heat transfer surfaces in the fluidized bed 
reactor can be influenced through the remaining air amount supplied to the 
fluidized bed reactor.