FIELD OF APPLICATION
The present invention relates to a method for recovering thermal energy from
liquid slags. In particular, it relates to a method for recovering thermal energy
from liquid slags using fluidized bed, while granulating the slag for use in cement
industry, manufacturing construction bricks, etc.
The invention also relates to an apparatus for recovering thermal energy from
liquid slags.
BACKGROUND OF THE INVENTION
In the steel making process, the impurities from the liquid pig iron are oxidized.
The oxides thus formed get collected on the surface of the metal bath and this
low density and high viscosity liquid is called slag. Slag is a waste generated
during the steelmaking process. But this mixture of oxides can be used in
cement industry, manufacturing of construction bricks, etc. In order to use the
slag produced during the various stages of steelmaking, the slag is granulated.

However, no worthwhile method has so far been suggested to recover the
enormous amount of heat possessed by the liquid slag. On an average, under
Indian conditions around 100-120 kg of slag is produced for every tonne of steel
produced in LD vessels. In the blast furnace where liquid pig iron is produced,
similar quantity of slag is produced. There are minor amount of slag produced in
other stages of secondary steelmaking areas. In India alone, 40-50 million tones
of steel per annum, is produced as of now and by 2015, this figure is expected to
be 110-115 million tones. The global steel production as of now is more than
1000 million tones per annum. It is very easy to estimate the amount of slag
produced in the steelmaking industry and the impact of the environment in terms
of energy wastage without recovering the heat from the liquid slag is obvious.
Earlier, there were some attempts to recover the waste heat from the slag.
However, no appropriate technology has been developed. It was considered to
use the thermal energy of the molten slag into chemical energy in the
endothermic reactions of the other industries such as cement industry. But such
an effort will call for the proximity of the two industries and it may not be
possible for the existing steel plants to use such a process.

The heat content of the liquid slag is very high and the quality of the heat is also
superior due to its high temperature. The recovery of the heat energy from
molten slag for producing steam can be considered.
The main problem in recovering the heat from the liquid slag is that upon the
removal of the heat, the slag will tend to stick to the heat transfer surfaces and
thereby preventing the next layer of liquid slag to come in contact with the heat
transfer surfaces. This seems to be a major problem in the thermal energy
recovery from the liquid slag for producing steam.
Thus in accordance with existing practice all over the world, this high amount of
quality heat at high temperature available with the liquid slag is not recovered.
The slag is dumped on the yard and water is poured over it. There are some
continuous slag granulation plants where the water is poured over the flowing
slag in the channel. In short, with the present steel industry, there is no system
of extracting the huge heat wasted in the form of quenching the liquid slag.

A need therefore, exists for developing a technology for recovering efficiently this
huge amount of quality heat energy from the liquid slag of steel plants.
SUMMARY OF THE INVENTION
The main object of the present invention therefore, is to provide a method and
an apparatus for recovering the thermal energy from liquid slags.
This and other objects of the present invention are achieved by using a bubbly
fluidized bed as the principal heat recovery apparatus.
The apparatus comprises a furnace having a bed charged with materials of a
particular predetermined particle size distribution and of very high fusion
temperature, like, alumina, silica and refractory materials. Hot air is supplied to
the bed through nozzles from an air plenum chamber. The temperature of the
bed material is built upto around 700 - 800° C and liquid slag is charged into the
system from a liquid slag distributor.

The hot gases coming from the air plenum chamber fluidizes the bed. The
passing gas will take away the heat from the falling liquid slag making it solid /
semi solid. Continuous bombardment of other particles from the fluidized bed on
the solidified slag will break the later to smaller particles or granules.
The gases containing the heat recovered from the liquid slag can, for example be
used, for generating steam.
Thus the present invention relates to a method for recovering thermal energy
from liquid slags, comprising the steps of: making a bed at the bottom of a
furnace using materials of high fusion temperature of pre-determined particle
size distribution; building up the temperature of the bed materials to around
700° to 800° C; fluidizing the bed by supplying hot air from the bottom of the
bed through an air plenum chamber; charging liquid slag into the fluidized bed;
allowing passing gases to take away the heat from the falling liquid slag making
the slag solid / semi-solid; breaking the solidified slag to smaller particles /
granules by continuous bombardment of particles from the fluidized bed; and
using the heat transfer red to the gases for generating high pressure steam in a
boiler.

The invention also provides an apparatus for recovering thermal energy from
liquid slag, comprising a furnace provided with a bed at the bottom charged
with materials of high fusion temperature of pre-determined particles size
distribution; an air plenum chamber for supplying hot air to the bed for fluidizing
the bed; an arrangement for charging liquid slag into the fluidized bed; and a
boiler for generating steam using the passing gases from the fluidized bed
carrying the heat recovered from the falling liquid slags.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention can now be explained with reference to the figures of the
accompanying drawings, where
Figure 1 shows an apparatus for recovery of
thermal energy from liquid slag
Figure 2 shows in schematic representation
the liquid slag distributor arrangement

DETAILED DESCRIPTION
The apparatus used in the present invention for recovering the heat energy from
liquid slag is shown schematically in Figure 1. The apparatus 1 comprises a
furnace 1' at the bottom of which an air plenum chamber 2 is provided. The air
plenum chamber 2 is provided with a hot air inlet 3 for supplying hot air to the
fluidized bed 4. Porous nozzles 3' can be used for supplying air to the fluidized
bed 4. The nozzles 3' can be made up of refractory material with metallic
skeleton to withstand the high temperature of the liquid slag. To start the
furnace, initially, a bed material 4 is charged with a particular particle size
distribution. This bed material 4 is typically made up of alumina, silica and
refractory material with very high fusion temperature. With low fusion
temperature, the materials will fuse together and the fluidization will stop as
soon as the liquid slag is charged.

It is preferred to build-up the temperature of the bed material to around 700° C -
800° C before the liquid slag is charged into the system. So, the furnace 1' must
be run with some kind of conventional fuel, such as coal. But, the fluidized bed
combustion technology has the advantage of its ability to handle any kind of
solid fuel including urban wastes. Even a fuel of very low calorific value can be
easily and effectively burnt in the fluidized bed. With one such fuel, obviously
the urban solid waste is preferred, the furnace can be started.
The hot gases coming through the plenum chamber 2 make the bed 4 behave
like a fluid and due to the combustion the temperature of the bed will rise.
When the optimum temperature of the bed is reached, charging of liquid slag
through a distributor 10 can be started.
Since, liquid slag has very high temperature, direct pouring over the porous
nozzles 3' might damage them and it may affect the fluidization also due to its
high viscosity. So it is preferred to use a distributor 10 located at a certain
elevation above the bottom of the fluidized bed 4.

The suggested liquid slag distributor arrangement is shown schematically in
Figure 2. The distributor comprises cups 12 with a connecting channel 13
supported on elevation columns 14, When liquid slag overflows from cups 12 of
distributor 10, it will have higher retention time in the fluidized bed 4 due to the
elevation. The bed 4 and the passing gasses through an air distribution plate 15
of the bed 4 will take away the heat from the liquid slag and it will become solid
or semi-solid. As illustrated in Figure 2 the continuous bombardment by the
other particles in the fluidized bed on the newly solidified slag will break it into
much smaller particles, i.e. granulation will take place.
The supply of slag from the steel plant will not be continuous. However, for the
effective operation of the fluidized bed, a continuous feed is preferred. It is
therefore preferable to have a buffer tank (not shown). Liquid slag can be
poured onto this buffer tank and then taken into the furnace 1' in small
quantities from there through a liquid slag inlet chute 11. The heat thus
transferred to the hot gases can be used to generate high pressure steam which

may be used to run a turbine. The continuous supply of steam from the boiler
can be ensured by substituting the waste heat of liquid slag with other solid fuels
like coal or municipal solid waste when the supply of slag is limited. After using
the heat from the hot gases the exhaust gases form the furnace can be further
used in economiser or air preheater 9.
Due to the continuous charging of liquid slag, the bed height will continue to rise
and this will have an adverse effect on the fluidized bed. So, frequent draining
of the bed to remove the excess granulated slag is needed. The granulated slag
thus drained through granulated slag drain 7 will have a lot of sensible heat due
to its high temperature. A vapour absorption air-conditioning system can be run
with this waste heat and the heat wasted in the reasonably high temperature
exhaust gases. In this vapour absorption system, the operating cost is very less
and the electrical energy needed is much less as compared to the conventional
vapour compression systems. Moreover, the energy requirement can be fulfilled
with the waste heat and this system will result in huge recurring cost savings.

The fluidized bed heat recovery apparatus of the present invention can also be
used to promote endothermic chemical reactions. The fluidized bed will ensure
huge contact area between the heat source and the chemical constituents,
thereby increasing the effectiveness of the chemical reaction.
Thus the invention provides a novel method of recovering the thermal energy
from liquid slag, while granulating the slag for use in cement industry,
manufacturing construction bricks, etc. The thermal energy of liquid slag which
otherwise would have been wasted can be recovered during granulation and
used for generating high pressure steam. The hot fluidized bed can also be used
to incinerate the municipal waste to produce more steam and electric power.
Further, the same bed can be used for combustion of coal or other solid fuels
effectively. Some of the endothermic chemical reactions that require very high
temperature and heat can also be promoted in this setup.

WE CLAIM
1. A method for recovering thermal energy from liquid slags, comprising the
steps of:
- making a bed at the bottom of a furnace using materials of high
fusion temperature of pre-determined particles size distribution;
- building up the temperature of the bed materials to around 700° to
800° C;
- fluidizing the bed by supplying hot air from the bottom of the bed
through an air plenum chamber;
- charging liquid slag into the fluidized bed;
- allowing passing gases to take away the heat from the falling liquid
slag making the slag solid / semi-solid;
- breaking the solidified slag to smaller particles / granules by
continuous bombardment of particles from the fluidized bed; and
- using the heat transfer red to the gases for generating high pressure
steam in a boiler.

2. The method as claimed in claim 1, wherein said materials of high fusion
temperature are selected from alumina, silica, refractory materials, etc.
3. The method as claimed in claims 1 or 2, wherein the step of charging liquid
slag comprises taking into a liquid slag distributor placed inside the furnace
quantities of liquid slag through a liquid slag inlet chute.
4. The method as claimed in claim 1, wherein the furnace is run with a
conventional fuel like coal, for raising the bed materials temperature to
around 700° to 800° C.
5. The method as claimed in claim 4, wherein the furnace is started using
urban solid wastes as fuel.

6. The method as claimed in claim 5, wherein the heat from the hot gases
exhausted from the furnace is additionally utilized in an economizer or air
preheater of the boiler.
7. The apparatus for recovering thermal energy from liquid slag, comprising:
- a furnace provided with a bed at the bottom charged with high
materials of fusion temperature of pre-determined particles size
distribution;
- an air plenum chamber for supplying hot air to the bed for fluidizing
the bed;
- an arrangement for charging liquid slag into the fluidized bed; and
- a boiler for generating steam using the passing gases from the
fluidized bed carrying the heat recovered from the falling liquid
slags.

8. The apparatus as claimed in claim 6, wherein said arrangement for charging
liquid slag comprises a distributor having cups connected by a channel, said
distributor supported on elevation columns inside the furnace.
9. The apparatus as claimed in claim 6, wherein said air plenum chamber is
provided with a hot air inlet for supplying hot air to the fluidized bed
through nozzles.
10. The apparatus as claimed in claim 9, wherein said nozzles are made of
refractory materials with metallic skeleton to withstand the impact from high
temperature liquid slag.
11. The apparatus for recovering thermal energy from liquid slag, comprising a
furnace provided with a bed at the bottom charged with materials of high
fusion temperature of pre-determined particles size distribution; an air
plenum chamber for supplying hot air to the bed for fluidizing the bed; an
arrangement for charging liquid slag into the fluidized bed; and a vessel
containing chemical constituents for promoting endothermic chemical
reactions utilizing the passing gases from the fluidized bed carrying the heat
recovered from the falling liquid slags.

12. A vapour absorption air-conditioning system configured to run with the
waste heat of the granulated slag and with the heat wasted in the
reasonably high temperature exhaust gases from the apparatus as claimed
in claim 7.
13. A method for recovering thermal energy from liquid slags, substantially as
herein described and illustrated in the accompanying drawings.
14. An apparatus for recovering thermal energy from liquid slag, substantially
as herein described and illustrated in the accompanying drawings.

The main object of the present invention therefore, is to provide a method and
an apparatus for recovering the thermal energy from liquid slags. This and other objects of the present invention are achieved by using a bubbly
fluidized bed as the principal heat recovery apparatus. The apparatus comprises a furnace having a bed charged with materials of a
particular predetermined particle size distribution and of very high fusion
temperature, like, alumina, silica and refractory materials. Hot air is supplied to
the bed through nozzles from an air plenum chamber. The temperature of the bed material is built upto around 700 - 800° C and liquid slag is charged into the system from a liquid slag distributor.
The hot gases coming from the air plenum chamber fluidizes the bed. The
passing gas will take away the heat from the falling liquid slag making it solid /
semi solid. Continuous bombardment of other particles from the fluidized bed on
the solidified slag will break the later to smaller particles or granules.
The gases containing the heat recovered from the liquid slag can, for example be
used, for generating steam.
Thus the present invention relates to a method for recovering thermal energy
from liquid slags, comprising the steps of: making a bed at the bottom of a
furnace using materials of high fusion temperature of pre-determined particle
size distribution; building up the temperature of the bed materials to around
700° to 800° C; fluidizing the bed by supplying hot air from the bottom of the
bed through an air plenum chamber; charging liquid slag into the fluidized bed;
allowing passing gases to take away the heat from the falling liquid slag making
the slag solid / semi-solid; breaking the solidified slag to smaller particles /
granules by continuous bombardment of particles from the fluidized bed; and
using the heat transfer red to the gases for generating high pressure steam in a
boiler. The invention also provides an apparatus for recovering thermal energy from
liquid slag, comprising a furnace provided with a bed at the bottom charged with materials of high fusion temperature of pre-determined particles size distribution; an air plenum chamber for supplying hot air to the bed for fluidizing the bed; an arrangement for charging liquid slag into the fluidized bed; and a
boiler for generating steam using the passing gases from the fluidized bed
carrying the heat recovered from the falling liquid slags.