FIELD OF INVENTION
This invention relates generally to hot-gas cleanup systems used for cleaning the
coal gas at high pressure and high temperature for protecting the turbine and
downstream components in coal based combined cycles. More particularly the
invention relates to a method for removing Hydrogen sulphide from a dry hot gas
in a hot gas cleaning system.
BACKGROUND OF THE INVENTION
Coal is a cheaper source of fossil fuel energy abundantly used for electric power
generation. The present thermal power plants burn coal to generate steam and
use the steam to generate electric power at an efficiency of around 30% and
allow emissions of polluting gases like carbondioxide which causes global
warming, and acid gases like nitrogen oxides and sulphur oxides which cause
acid rain. Hence there is a necessity to burn coal in a more efficient way so as to
restrict emissions of gases polluting the environment. One of the current
concepts is to use coal for generating electric power at a higher efficiency for
example 60%, and allow only lower emissions. Such a concept further warrants
generating value added fuels like synthetic natural gas (CH4), syn gas (CO +
H2), hydrogen fuel, methanol etc. and value added chemicals like sulphur,
sulphuric acid, gypsum, fertilizers, from the byproduct.
According to the current concept, in coal based combined cycles, the coal is
burnt partially to gasify at high pressure and high temperature to generate coal
fuel gas, called syn gas (CO + H2). The generated coal fuel gas is used to drive
the topping Brayton gas turbine cycle and then a bottoming Rankine steam cycle
in the Integrated gasification combined cycle (IGCC). As a modification of the
IGCC, the Hydrogen produced by the gasification of coal and steam in the
gasifier and a shift reactor, can be used as a transportation fuel or used in fuel
cell to generate electric power. As a further alternative, Coal can also be hydro
gasified with Hydrogen to generate synthetic natural gas (CH4) which can be
used as the transportation fuel or can be burnt in the gas turbine to generate
electric power. In all the above coal transformation processes, the high pressure,
high temperature coal gas has ash particles or chemicals like Hydrogen sulphide
(H2S) which are harmful to the gas turbine or to the fuel cells or to the other
downstream components as they cause erosion and corrosion. The stringency of
emission specifically for protecting the turbine is far higher than the stack
emissions. Hence, there is a need for a high pressure high temperature gas
cleaning system, also known as hot gas cleanup system having atleast one hot
gas filter system to remove the fine particles and a hot gas H2S removal system
to remove the H2S to protect the turbines, the fuel cells and the downstream
components.
H2S is acidic and corrosive and can severely attack turbine blades, fuel cells and
all other components in the process circuit. The conventional method uses a wet
scrubbing system in which water is mixed with chemicals and sprayed into the
gas to remove H2S by dissolution or by a reaction. The disadvantage of this
method is that the hot coal gas is cooled off and leads to a loss in the cycle
efficiency. Further, the scrubbed acidic water solution is corrosive and the wet
scrubber therefore requires regular maintenance affecting the plant availability.
Again, the scrubbed acidic water solution has to be treated in a water treatment
plant before the water is reused or disposed to meet the statutory regulations.
In an existing hot gas H2S cleaning system, the high pressure high temperature
coal fuel gas is passed through a reactor having solid sorbents which react with
H2S gas, and form sulphides by a gas-solid reaction, and the emanating coal gas
is purified to meet the turbine requirements. When the sorbent bed is saturated,
it is regenerated by passing a stream of hot air through the bed to convert the
sorbent sulphides back into oxides. In general, sorbents based on Ferric oxide,
Zinc oxide and proprietary combinations are known to remove H2S in small
concentrations of < 10 ppm but these sorbents are not suitable for coal based
applications where the H2S concentration is around 2000 ppm for low sulphur
coals and around 15000 ppm for high sulphur coals. In the art described for coal
based applications in the Japanese Patent No. 07-256093, Zinc oxide containing
sorbent is used but this is useful only at higher temperatures and they work in a
narrow temperature range without process flexibility. But the optimized cycle
configuration in coal based combined cycles suggests an operating temperature
range of 300°C to 500°C for hot gas filter and hot gas H2S clean-up system.
Accordingly, there is a need for providing a suitable sorbent and an optimized
process to work within this temperature regime.
Thus, an object of the present invention is to provide a method for removing
hydrogen sulphide from a dry hot gas in a hot gas cleaning system which use a
sorbent capable of removing H2S in concentration atleast between about 2000 to
15000 ppm and an optimized thermal process operable atleast around 300 to
500°C temperature range to improve the cycle efficiency of coal transformation
processes.
Another object of the invention is to provide a method for removing hydrogen
sulphide from a dry hot gas in a hot gas cleaning system which use a sorbent
capable to effect H2S cleaning from a dry hot gas in all coal transformation
processes.
A further object of the invention is to provide a method for removing hydrogen
sulphide from a dry hot gas in a hot gas cleaning system which operates under
an optimized flexible thermal process regime to match the temperature regimes
envisaged in the overall cycle configuration of coal transformation processes.
An yet another object of the invention is to provide a method for removing
hydrogen sulphide from a dry hot gas in a hot gas cleaning system which
eliminates the disadvantages of the prior art wet scrubbing process.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1 - Shows an arrangement of atleast two sorbent reactors filled with
sorbents and a regeneration hot air supplying heat exchanger in a
Hot gas H2S cleaning system according to the present invention.
SUMMARY OF THE INVENTION
Accordingly there is provided a method for removing hydrogen sulphide from a
dry hot gas in a hot gas cleaning system which replaces the existing wet
scrubbing system for improving the cycle efficiency of coal based combined cycle
power generation and production of value added fuels and chemicals in all coal
transformation processes. The method generally consists of: Using sorbents
containing copper oxide or its combinations with any other matrix such as ferric
oxide, for removing H2S gas from coal gas by a gas-solid reaction to reduce the
H2S concentration to meet the stringent requirements for protection of turbines,
fuel cells, down stream components; using an optimized flexible thermal process
regime for effective absorption of H2S and adaptable to the optimized cycle
configuration and regeneration of sorbent bed hot air, and using the
sorbents in suitable sorption reactors to ensure continuous supply of H2S
removed coai gas for further coal transformation processes.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF
THE PRESENT INVENTION
Figure 1 shows a Hot gas H2S cleaning system for removing H2S from coal gas.
The system mainly comprises atleast two Sorbent Reactors (1 & 2) and a
regenerating hot air supplying heat exchanger (3). The sorbent reactor is a fixed
bed reactor and comprises an insulated steel vessel designed for the required
operating pressure. A sorbent material (4) in one of the form of a solid grain, a
spherical shape; a cylindrical shape and a hollow cylinder shape, is distributed in
plurality of tiers in one of the atleast two sorbent reactors, to allow flow
stabilization of the coal gas at each tier and thereby prevents selective
channeling of gas without wetting the sorbents. Apart from the fixed bed, it is
also possible to use other kinds of reactors like fluidized bed or entrained bed or
any other gas - solid mixing methods. The sorbent material (4) is made of a
combination of ferric oxide and copper oxide, the proper combination depending
on the H2S concentration in the coal gas stream. Ferric oxide is a cheap sorbent
but is not capable of fully absorbing H2S at the level of 2000 ppm normally
present in Indian coal gas. Copper oxide is an effective sorbent to absorb all H2S
at high levels of concentration but it is costlier than ferric oxide. An effective
ratio of copper oxide and ferric oxide can absorb H2S to the required
concentration and further emerges as a cost effective sorbent. A typical
formulation of 80% Ferric oxide and 20% copper oxide can reduce H2S from a
level of 15000 ppm to less than 15 ppm which is sufficient to protect the turbine
and the downstream components. The temperature of the bed is important in
promoting the gas to solid sorbent reaction for removing the H2S gas. The coal
fuel gas is a low calorific value gas and burning of the gas in the turbine requires
a minimum temperature of 280°C for ignition and flame stability, and hence the
hot gas H?S cleanup system which is preceding disposed, should work above
280°C. The upper limit of the gas cleaning temperature is set at 500°C dictated
by the stability of the tube sheet of the hot gas filter preceding the hot gas H2S
cleaning system. Above the limit of 500°C, the sulphides of sorbents is unstable
and the H2S absorption is low. Hence the optimum cycle configuration suggests
that the operation of the hot gas cleanup system be maintained in the range of
300°C to 500°C. The copper oxide sorbents alone or in combination with ferric
oxide for the cost effectiveness, respond very well in the temperature range of
200°C to 500°C in removing the H2S and reducing the H2S concentration from
15000 ppm to less than 15 ppm which is the acceptable turbine limit. The
sorbent bed is also effectively regenerated by hot air in the temperature range of
200°C to 500°C. Thus the process temperature is flexible with copper oxide
containing sorbents and matches well with the required temperature regimes of
300°C to 500°C dictated by the optimized overall cycle configuration. After the
bed is active for considerable length of time, sulphides are formed due to solid
state reaction and the bed reaches saturation stage. At this time the bed is
optimally regenerated with a current of hot air or oxygen from a heat exchanger
(3), at a temperature above 200°C to convert the sulphides formed back into
oxides. There is a necessity for atleast two fixed bed sorbent reactors so that
when one reactor is in the sorption cleaning mode, the other reactor is in
regeneration mode and the two reactors alternatively ensure continuous supply
of the clean gas. The system has valves and necessary diagnostic control and
instrumentation accessories to control the process effectively.
WE CLAIM
1. A method for removing hydrogen sulphide from a dry hot coal gas in a hot:
gas clearing system; the system comprising atleast two sorbent reactor (1,2)
having multiple tiers of hot beds operable in a temperature range of 200°C to
500°C and a heat exchanger (3) for generating the active beds of one of the
atleast two sorbent reactors (1,2) via periodical supply of one of hot air,
oxygen and a combination of air and oxygen, the method comprising the step
of
• Disposing a sorbent material (4) of ferric oxide, copper oxide or
combination of those in the multiple tiers of the hot bedded reactors (1,2)'
• Passing the dry gas via the sorbent reactor (1,2) when the temperature of
the bed reaches between 300°C to 500°C;
• Passing hot air or oxygen or a combination of air and oxygen via the heat
exchanger (3) when the bed of the reactor (1,2) reaches a saturation
stage thereby reinstating the reactors (1,2) in a regeneration mode so as
to receive continuous supply of the dry gas for cleanin.
2. The method as claimed in claim 1, where the sorbent material (4) is copper
oxide.
3. The method as claimed in claim 1, wherein the sorbent material (4) is
Ferric Oxide
4. The method as claimed in claim 1 wherein the sorbent material (4) is a
formulation of ferric oxide and copper oxide
5. The method as claimed in claim 1 wherein the sorbent material (4)
constitutes 80% ferric oxide and 20% copper oxide.
6. A method for removing hydrogen sulphide from a dry hot gas in a hot gas
cleaning system as substantially herein described and illustrated with the
accompanying drawing which shows two sorbent reactors, and a heat
exchanger for periodical supply of hot air, oxygen and a combination of air
and oxygen to regenerate the active bed of the sorbent reactors
alternatively to ensure continuous supply of cleaned process gas.

A method for removing hydrogen sulphide from a dry hot coal In a hot gas cleaning system comprising
A method for removing hydrogen sulphide from dry hot gas in a hot gas cleaning system for replacing the existing wet scrubbing system for improving the efficiency of clean coal combined cycle power generation. The method consists
of using sorbent containing copper oxide in combination with ferric oxide for removing H2S from coal gas from concentrations around 15,00ppm to 15 ppm to meet the stringent requirement of turbine. The copper oxide and ferric oxide mixture of typical formulation 20% copper oxide and 80% ferric oxide are used
in sorption reactors in a temperature range of 200 - 500 C for absorbing and removing H2S. The sorbents form sulphide due to reaction with H2S and hence the sorbent bed is periodically regenerated with a supply of hot air from a heat
exchanger. For the continuous supply of cleaned process gas, two sorption reactors and one heat exchanger are required as shown in the drawing.