TITLE:
Method for enhancing fluidized bed coal gasification performance by integrating solar
heat
FIELD OF INVENTION
The present invention generally relates to fluidized bed coal gasification of high ash
Indian coals for various applications like power generation, liquid fuels and chemicals
production. More particularly the invention relates to enhancement of calorific value of
synthesis gas (syngas) generated from fluidized bed coal gasification and cold gas
efficiency of gasifier by extracting heat from solar energy and admitting into gasifier
through fluidizing air.
BACKGROUND OF THE INVENTION & DISADVANTAGES
Gasification is a process of converting carbonaceous fuels into combustible gases also
called synthesis gas through partial oxidation of the fuel. Gasification process is
endothermic in nature and heat required for the conversion process is acquired from the

partial combustion of the fuel. The amount of fuel left after partial combustion will be
participated in gasification reactions, which leads to combustible gases generation. The
amount of fuel subjected to partial combustion depends on the desired gasification
conditions like operating temperature, fuel & reactants throughput and bed weight. For a
given conditions, one would prefer to subject more fuel into gasification reactions for a
minimal partial combustion of fuel. Equivalence Ratio (ER) is a parameter defined as
ratio of oxygen to carbon that is actually admitted into system to the stoichiometric ratio
of oxygen to carbon required for full combustion of fuel. ER parameter is a characteristic
of the amount of fuel subjected to partial combustion in order to maintain desired
gasification conditions. For a desired gasification conditions, ER should be optimized or
reduced in order to achieve better gasification performance. ER can be reduced by
providing energy required for gasification through external sources like preheated
fluidizing medium, preheated solid inert material and direct heating reaction zone with
solar radiation.
Fluidized bed gasification technology is the most suitable to gasify high ash Indian coals
with air and steam mixtures as reactants. The fluidizing air temperatures are generally
increased through heat exchange with process streams like synthesis gas and gas
turbine compressed air in order to achieve better gasification performance. Higher
calorific value of syngas and cold gas efficiency of gasification are highly suitable to

meet various applications like power generation, liquid fuels and chemical production. In
prior art, 2368/Del/2004 relates to pressurized fluidized bed coal gasification device for
producing an enhanced quality of coal gas with a mixture of air and steam as a fluidizing
medium admitted through the distributor located at the bottom of the device wherein the
invention is characterized by an additional steam supply system positioned at elevations
starting from 200 mm above the air distributor for admitting adequate quantity of steam.
The invention claims the process to have uniform temperature at different elevations
and to have quick conversion of coal gas, having improved chemical composition and
calorific value. Additional steam supplied to achieve higher calorific value in this
invention attracts the thermal loss in the overall efficiency of the process. Another prior
invention00301/kol/2008, relates to a method adaptable to enhance calorific value of
syngas in an IGCC process by employing the air blown fluidized bed gasification. Here,
the temperature of the fluidizing air is increased by two methods viz., (a) By tapping a
portion of a syngas before being admitted to the gas turbines and firing the syngas in a
burner, thus utilizing the heat produced by the extracted syngas and the air from the
forced draft fan (FD) in a tubular heat exchanger having a flue gas in a shell side while
cold air and steam mixture is passed through tubes (b) By using coal and air from FD
for the fluidized bed combustion which acts as heat generating source for heating
process air and steam mixture by admitting through tubes immersed in the bed.
Increasing fluidizing air temperature with other than the above mentioned methods, can
improve the

entire process efficiency as the heat of syngas or heat from coal combustion which is
earlier used for increasing fluidizing air would be available for process. The present
invention can easily provide an alternative solution by using abundant solar energy to
increase fluidizing air temperature without compromising the process efficiency as
Concentrated Solar Thermal (CST) energy can increase the fluidizing air temperature
up to 500°C.
In prior arts, solar energy has been utilized in variety of means for gasification of
carbonaceous material. One prior invention (US4290779), includes a solar gasifier
which heats the fluidizing gas and steam mixture through a high heat capacity refractory
honeycomb which is surrounding the entire reactor surface area and the honeycomb
being heated through solar concentrators solar window. But, these systems can work
up to limited capacities or sizes as the concentrator size cannot be expanded to suit the
requirements of commercial scale gasifier as the diameters of typical size ranges from 2
m to 3 m more over the alternate heating system to provide necessary heat for
gasification has to be set up to meet the requirement when solar energy is unavailable.
In another invention (US 4229184), relates to the apparatus for using focused solar
radiation to gasify coal and other carbonaceous materials. Invention is characterized by
incident solar radiation focused from an array of heliostats onto a tower-mounted

secondary mirror which redirects the focused solar radiation down through a window
onto the surface of a vertically-moving bed of coal, or a fluidized bed of coal, contained
within a gasification reactor. The reactor is designed to minimize contact between the
window and solids in the reactor. Heat and temperatures required for gasification is
provided by heating solid coal particles. Focusing solar radiation over the reaction zone
to heat the moving coal particles is a challenging task and applicability of this method to
commercial scale reactors has limited applicability due to larger reaction zones.
The present invention can overcome the above mention drawbacks of prior arts. This
invention focuses on using the solar energy to pre heat the fluidizing air instead of using
syngas or coal. This invention proposes solar collector field or system for indirect
heating of fluidizing medium which can be expanded depending on the quantity of
medium to be heated, this kind of arrangement avoids the scale up issues of gasifier for
commercial aspects. Thus, there is a provision to expand the capacity of the solar
collector field thereby the temperature of the air-steam mixture can be controlled to suit
the requirements of the plants. Also the present invention allows the system to be self-
sustained to meet the heat and temperature required for gasification by subjecting more
carbon towards combustion when the solar radiation falls (during nights and cloudy
days).

OBJECTS OF THE INVENTION
It is therefore, an object of the present invention is to propose a process for enhancing
the performance of fluidized bed coal gasification process of high ash Indian coals by
using concentrated solar thermal technology (CST) energy.
Another object of present invention is to propose a process for enhancing the
performance of fluidized bed coal gasification process of high ash Indian coals by using
solar energy, in which the fluidizing air temperature is preheated up to 500°Cthrough
heat exchanger using heat transfer fluids (HTFs) namely water, thermal fluid or salt that
is heated by passing through the solar field comprising any one of the three CST
technologies namely Linear Fresnel Reflector (LFR) system, Parabolic Trough Collector
(PTC) and Power Tower (PT) technology.
A further object of present invention is to propose a process for enhancing the
performance of fluidized bed coal gasification process of high ash Indian coals, in which
the Equivalence Ratio (ER) has to be decreased which leads to enhancement in
gasification performance for a given conditions by admitting fluidizing air preheated by
using CST energy.

A still another object of the present invention is enhance the overall thermal efficiency of
the complete process with coal gasification byutilizing & integrating the equivalent heat
available with the process stream hat has been used earlier to pre heat fluidizing air.
BRIEF DESCRIPTION OF THE INVENTION
According to this invention, there is provided a process for enhancing the performance
of fluidized bed coal gasification comprising:
subjecting the fluidized bed air leading to the fluidized bed gasifier to the step of
preheating to a temperature up to 500°C using concentrated solar thermal energy,
reducing the equivalence ratio by introducing fluidized air up to 500 °C & steam as
fluidizing medium and high ash coal into fluidized bed coal gasifier,
increasing the calorific value of syngas and increase in the cold gas efficiency,
increasing coal flow rate and decreasing air flow rate so that both are adjusted
simultaneously within the specified limits to maintain the equivalence ratio.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
In the following, present invention will be explained in more detail with reference to the
following drawing.
Fig.1 schematically illustrates the Fluidized bed coal gasification process scheme with
fluidizing air preheating using solar energy
DETAILED DESCRIPTION OF THE INVENTION:
This invention relates to a process for enhancing the performance of fluidized bed coal
gasification process by using solar heat.
The performance parameters of coal gasification are characterized by calorific value of
syngas and cold gas efficiency. The fluidizing air is preheated using solar energy and
admitted into gasifier which leads to improve fluidized bed coal gasification process
parameters. Wherein the invention is characterized by the reduction in equivalence ratio
in the range of 12-15 % which leads to increment in lower calorific value of syngas
(kcal/kg) in the range of 20-23% and in cold gas efficiency in the range of 8.5-9.5% by

admitting the fluidizing air into fluidized bed coal gasifier which is preheated up to 500
°C using CST energy. The fluidizing air is generally preheated by exchanging heat with
process streams such as (compressed hot air, steam and hot syngas) and in this way
the maximum temperature that could be attained by fluidizing air is based on process
streams limitation and in addition the heat available with process stream is not fully
utilized for process application. Whereas temperatures up to 350° C can be achieved
using LFR systems and PTCs and 600° C using PT technology. This abundantly
available solar energy is made of use to preheat fluidizing air up to 500 °C to gain
advantage over the normal process followed.
The desired operating temperature of the fluidized bed coal gasifier which is normally
around 1025 °C is maintained by regulating air to coal ratio for a given process
parameters. By increasing the temperature of fluidizing air admitted into fluidized bed
coal gasifier, the operating temperature of gasifier increases and in order to control the
temperature either air flow rate is decreased or coal flow rate is increased. The
reduction in air flow rate reduces nitrogen content in syngas which leads to
improvement in calorific value of syngas followed by cold gas efficiency. The increment
in coal flow rate increases the amount of carbon subjecting to gasification reactions
constitutes to improvement in CO and H2 contents in syngas which leads to

gasification performance improvement. Equivalence Ratio(ER) explained as ratio of
actual oxygen to carbon in the gasifier to the stoichiometric oxygen to carbon for
complete combustion is directly proportional to air to coal ratio. Due to reduction in air
flow rate and increment in coal flow rate, ER decreases which leads to improvement in
gasification performance by increasing fluidizing air temperature by solar energy. The
heat available with process stream which was earlier used to pre heat fluidizing air is
used for process application which leads to overall thermal efficiency of the complete
process with coal gasification.
Coal (18) after crushing & sieving is transported through conveying system into coal
feeding system (6) consisting of locks, hoppers and feeders. Coal from coal feeding
system (6) is admitted through coal transport line (19) into fluidized bed coal gasifier (8)
at a location which is above distributor (9). Fluidizing medium consisting of a mixture of
air & steam or oxygen enriched air & steam is admitted through fluidizing medium
transport line (17) at a location which is below distributor (9) into fluidized bed coal
gasifier (8). Coal and fluidizing medium admitted into gasifier (8) undergoes gasification
process for a provided operating conditions such as temperature, pressure, coal flow
rate, air & steam flow rates, bottom ash flow rates and bed inventory &

residence time. Synthesis gas (Syngas) consisting mainly of CO, C02, CH4, H2, H20
and N2 is generated through coal gasification process along with the ash & unburnt
carbon formation. Syngas and fine ash particles are removed from the top of the gasifier
exit line (23) and sent to gas clean up system (10) consisting of series of heat
exchangers to reduce syngas temperatures, particulates separators to remove fine ash
particles in the syngas and absorbers to remove contaminants such as NH3, H2S &
alkalis from syngas to ensure the syngas composition & condition meets the end use
application requirement. Coarser ash particles and unburnt carbon are removed from
ash discharge line (20) into ash discharge system (7) consisting of extractors and locks
& receivers. Constant desired operating temperature is maintained by regulating air to
coal flow rate ratio and constant bed inventory is maintained by regulating bottom ash
flow rate. Bubbling fluidized bed condition is maintained by admitting sufficient fluidizing
medium flow rate into fluidized bed coal gasifier (8). Calorific value of syngas and cold
gas efficiency (defined as ratio of chemical energy stored in syngas to the energy of
coal supplied) are the two important parameters to evaluate the coal gasification
performance. The present invention proposes method to improve gasification
performance by preheating fluidizing air using solar energy.
The fluidizing air (14) from air source (3) is preheated in a heat exchanger (4) where
heat transfer fluid (12) coming from solar heating system (2) exchanges heat with cold

medium which is fluidizing air. Solar heating system(2) consists of CST collector field
can be in three configurations namely PTC system (2a) which uses thermal fluid as
HTF, LFR system (2b) which can take both thermal fluid and water as HTF and PT
system (2c) which can use fluid, water and thermal salts as HTF.
The preheated fluidizing air (15) mixed with steam (16) supplied from steam source (5)is
admitted into gasifier through fluidizing medium transport line (17). >
The proposed solar heating system (2) combined with heat exchanger (4) is capable to
vary the fluidizing air temperature from ambient to 500 °C. In prior art, fluidizing air is
preheated using hot process streams and the maximum temperature attainable by
fluidizing air is limited to the availability of the process streams. By using solar energy to
preheat fluidizing air instead of process stream, there are two advantages. One is to use
*
the heat available with process stream that has been used to preheat fluidizing air to
improve overall process efficiency. Other is enhancement in coal gasification
performance.
By admitting fluidizing air preheated up to 500 °C into fluidized bed coal gasifier, the
operating temperature of the gasifier increased by 17%for a given process conditions.
To reduce and maintain the gasifier at desired temperature, either coal flow rate has to
be increased or air flow rate has to be decreased.

Coal flow rate has been increased by 27.3 % to reduce gasifier temperature increased
by admitting preheated fluidizing air at 500 °C but carbon conversion and cold gas
efficiency decreases due to lower solid residence time and higher coal throughput
respectively. In order to improve carbon conversion and cold gas efficiency, residence
time has been increased by 46% which allows the increment in coal flow rate by 15% to
maintain the desired operating temperature and carbon conversion. By subjecting more
coal into gasifier for a given air flow rate, more carbon is participated in gasification
reaction which increases carbon monoxide (CO) and hydrogen (H2) mole fractions in
syngas. Due to increase in CO and H2 composition, calorific value of syngas increased
by 23% which leads to increment in cold gas efficiency by 9.7%. Equivalence Ratio (ER)
has decreased from 42.3 % to 36.7% by increasing coal flow rate by 15% in order to
maintain desired gasifier temperature that has increased due to increasing fluidizing air
temperature to 500 °C which has enhanced coal gasification performance. In the
process of enhancing gasification performance by increasing coal flow rate and
residence time, bed inventory has been increased by 70 % which may lead to
operational problems like clinkering and de-fluidization.
Airflow rate has been decreased by 16.5 % to reduce gasifier temperature increased by
admitting preheated fluidizing air at 500 °C but carbon conversion and operating velocity
decreases due to lower amount of C02 available for gasification reaction and lower

volumetric flow rate of fluidizing medium respectively. In order to improve carbon
conversion and operating velocity, residence time has been increased by 26% which
allows the decrement in air flow rate by 12.5% to maintain the desired operating
temperature and carbon conversion. By subjecting less amount of air flow rate into
gasifier for a given coal flow rate, nitrogen (N2) content in syngas is decreased which
increases carbon monoxide (CO) and hydrogen (H2) mole fractions in syngas. Due to
increase in CO and H2 composition, calorific value of syngas increased by 20.7% which
leads to increment in cold gas efficiency by 8.5%. Equivalence Ratio (ER) has
decreased from 42.3 % to 37% by decreasing air flow rate by 12.5% in order to maintain
desired gasifier temperature that has increased due to increasing fluidizing air
temperature to 500 °C which has enhanced coal gasification performance. In the
process of enhancing gasification performance by decreasing air flow rate and
increasing residence time, bed inventory has been increased by 30 % and operating
velocity has been decreased by 10 % which may lead to operational problems like
clinkering and de-fluidization.
In the above two cases, ER has been decreased from 42.3 % to 37% in order to reduce
the gasifier temperature increased by admitting preheated fluidizing air at 500 °C and to
attain enhanced coal gasification performance. But the chances to end up with
operational problems are more likely to occur in the process of regulating the gasifier
temperature either by increasing coal flow rate alone or decreasing air flow rate alone.

In order to overcome the above said operational problems, coal and air flow rates are
adjusted simultaneously by maintaining ER at 37% in order to reduce the gasifier
temperature increased by admitting preheated fluidizing air at 500 °C and to attain
enhanced coal gasification performance. Enhanced coal gasification performance is
characterized by increment in the calorific value of syngas in the range of 20-23% and
increment in the cold gas efficiency in the range of 8.5-9.7%. While varying coal and air
flow rates simultaneously, the maximum limit that the coal flow rate has to be increased
is 15 % and the maximum limit that the air flow rate has to be decreased is 12.5 %.
The scope of the invention as narrated herein is defined in the appended claims

WE CLAIM:
1. A process for enhancing the performance of fluidized bed coal gasification
comprising:
subjecting the fluidized bed air leading to the fluidized bed gasifier to the step of
preheating to a temperature up to 500°C using concentrated solar thermal energy,
reducing the equivalence ratio by introducing fluidized air up to 500 °C & steam as
fluidizing medium and high ash coal into fluidized bed coal gasifier,
increasing the calorific value of syngas and increase in the cold gas efficiency,
increasing coal flow rate and decreasing air flow rate so that both are adjusted
simultaneously within the specified limits to maintain the equivalence ratio.
2. The process as claimed in claim 1, wherein the said equivalence ratio (ER) is
reduced from 42.3% to 37%.
3. The process as claimed in claim 1, wherein coal flow rate increased to the limit of
15%, air flow rate decreased to the limit of 12.5% and the equivalence ratio is to be
maintained at 37%.

4. The process as claimed in claim 1, wherein the said gasification performance is
enhanced by increment in the calorific value of syngas in the range of 20-23% and
increment in the cold gas efficiency in the range of 8.5-9.7%.
5. The process as claimed in claim 1, wherein the thermal efficiency of the entire
process of fluidized bed coal gasification increases by utilizing heat available with
process stream that has been used earlier to preheat fluidizing air and by integrating
within the process.