FIELD OF THE INVENTION
The present invention relates to a method of modifying sintering pattern of ash
generated, to reduce agglomeration, sintering and choking formation during
combustion of solid fuels in fuel firing systems.
BACKGROUND OF THE INVENTION
Sintering of the particles agglomeration and choking formation in boiler
components resulting from combustion of a certain type of solid carbonaceous
fuel material is known in the art. During combustion of such fuels in boiler
systems like FBC (Fluidized Bed Combustion) and CFBC (Circulated Fluidized Bed
Combustion), the ash particles generated from the solid fuel come in close
contact with each other in the furnace components at an operating temperature
range of 800 to 950°C. Solid carbonaceous fuel material comprises a combustible
portion and non-combustible portion. The non-combustion portion comprises
different forms of moisture, gases and minerals. The minerals originally present
in the fuel are converted into ash during combustion. The ash portion is
represented by oxides of Si, Fe, Ti, Ca, Mg, Na, K, S and other trace elements.
The chemistry of ash generated during combustion if related to a specified range
of oxide composition, shrinkage/sintering and subsequent forming of the ash
coking leads to forced shut down of the combustor. Depending on the chemistry

or chemical constituents of the ash, the ash particles undergo different
magnitude of agglomeration and sintering which interalia leads to choke
formation in the fuel path. Once a choke formation in the fuel/ash path of the
furnace components has started, the boiler has to be shutdown to clear the
choking.
The start temperature of sintering of the ash of the fuels could be anywhere
between 800 to 1100° C depending on the ash chemistry. For some fuel ashes,
the start temperature of sintering may be beyond 1100° C. For fuels which
exhibits the start temperature of sintering within the operating temperature
range of FBC/CFBC boiler systems and also if significant percentage of the
sintering falls within the above operating temperature range, then firing of such
fuels may result in agglomeration and choking formation in boilers. To overcome
the above said problems, either the operating temperature range of the boiler is
changed or modification of sintering profile of the ash using some chemical
additive or sometimes both are required.
Canadian Patent publication CA 1202485 (Kekish et al) discloses a method of
raising fusion point of slag by addition of 0.5 to 10.0% by weight of the slag of
a phosphate compound in fuels forming slag. The addition of phosphate
compound raises the fusion point of the slag from 1480 to 1540°C.
Lasse Hoist Sorensen, So et al (US 6,615,751 Bl) discloses a method for
reducing agglomeration, sintering and deposit formation in gasification and
combustion of biomass with high content of potassium and/or sodium by the
addition of a reactive phosphorous compound.

Lawrence et al (Application No. 1109/KOL/2011) discloses a method of addition
of silicon oxide based additive in sufficient amount to the fuels with highly
sintering characteristics for shifting the sintering profile to higher temperature
and reduction in total shrinkage percentage.
H.Haykiri et al disclosed that the addition of potassium rich hazelnut shell
reduced the sintering temperature lignite ash to 919 and 730 C for the blends of
5 and 10 wt % respectively (FUEL 89 (2010) pp3063-3068)
Verena et al showed that increased wood addition has a very poor effect on
sinter behavior of during combustion of wood-straw pellets and Calcium
hydroxide and Kaolin help to elevate the ash melting temperature of the ash of
wood-straw. (Fuel article in press, 2013).
Still the problem of sintering / agglomeration and choking of ash in furnace
components generated from combustion of some of the low rank fuels other
than biomass in the combustion temperature range of 800 to 950°C exists. There
is a need for an effective additive to shift the sintering pattern of ash
advantageously and modifying the rate of shrinkage / sintering of ash for
reducing agglomeration, sintering and choking formation.

OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose a method of
modifying sintering pattern of ash generated, to reduce agglomeration, sintering
and choking formation during combustion of solid fuels in fuel firing systems
which eliminates the disadvantages of prior art.
Another object of the present invention is to propose a method of modifying
sintering pattern of ash generation to reduce agglomeration, sintering and
choking formation during combustion of solid fuels in fuel firing systems in which
an additive is used to lower the starting temperature of sintering of ash
generation.
A further object of the present invention is to propose a method of modifying
sintering pattern of ash generation to reduce agglomeration, sintering and
choking formation during combustion of solid fuels in fuel firing systems in which
the rate of sintering/shrinkage is modified by injecting additive which allows a
selection of combustion operating temperature regime to reduce the choking
formation from combustion at critical locations of the fuel path.
SUMMARY OF THE INVENTION
According to the invention, a process is developed to change the sintering
pattern of specific type of solid fuel ashes which otherwise cause slow/rapid
sintering in the boiler operating temperature range of 800 to 950°C. The
sintering of at least four types of fuel ashes generated out of combustion of

respective fuels and four types of ashes including additive mixtures generated
are required and studied. The review of the results enables development of the
process which solves the prior art. Problems related to sintering, agglomeration
and choking.
According to the present invention, an additive is added along with fuels to alter
the ash chemistry so that the pattern of shrinkage/sintering of the ash is
modified which allows the selection of suitable combustion operating
temperature regime and thereby the choking formation is avoided.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - shows shrinkage profiles of fuel A (with and without additive)
Figure 2 - shows shrinkage profiles of fuel B (with and without additive)
Figure 3 - shows shrinkage profiles of fuel C (with and without additive)
Figure 4 - shows shrinkage profiles of fuel D (with and without additive)
Table 1 Chemical composition of the additive.
Table 2 Shrinkage characteristic data of the ashes.

DETAILED DESCRIPTION OF THE INVENTION
The present invention discloses a method for reducing ash particle
agglomeration/shrinkage/sintering and choking formation resulting from
combustion of solid carbonaceous fuels like lignite, sub-bituminous, bituminous
and anthracite comprising a combustible portion and non-combustible portion.
The ash generated from the fuels by standard ASTM D3174-04 at 750°C is used
to prepare ash pellet. When the ash pellet is subjected to further heat treatment,
the ash pellet tends to start shrinking around 850-900°C and sinters gradually
(either slowly or quickly based on ash constituents) as the temperature
increases. The sintering pattern of the ash specimen is illustrated by the area
shrinkage profile and maximum shrinkage is reached around 950 to 1100°C.
Depending on the ash chemistry, the rate of ash shrinkage and total shrinkage
percentage compared to original ash pellet specimen varies for different fuel
ashes. The shrinkage pattern for fuel ashes A, B, C and D are shown in
Figures 1-4.
The start temperature of sintering of the ash of the fuels could be anywhere
between the temperature of 800 to 1100°C depending on the ash chemistry. The
sintering of ash "A" and "B" starts around 850°C, whereas ash D starts 950°C.
For ash A and B, the sintering pattern exhibit the start temperature of sintering

which falls within the operating temperature range and it is also observed that
significant percentage of sintering (30 to 35% ) falls within the above operating
temperature range. It clearly indicates that firing of fuel A and B will result in
agglomeration and choking formation in the combustor.
In the same way the ash is generated from the mixture of fuel 95% by weight
and additive 5% by weight at 750°C according to the procedure described in
standard ASTM D-3174:04. The composition of the chemical additive is depicted
at Table 1.
New ash pellets (prepared from the fuel with additive mixture) are also subjected
to similar heat treatment and the sintering profiles are observed. The changes in
sintering phenomenon of the ash with and without addition of chemical modifier
is shown for all the fuel ashes A, B, C and D. Figures 1-4. The sintering data
obtained from the profile viz start of sintering, end of sintering, % of sintering
and rate of shrinkage for the ashes both with additive and without additive, are
shown in Table 2. The shrinkage profiles with additive clearly show that start of
sintering shift to lower temperature. For ashes with additive B and C, the start of
sintering shifted to lower temperature 810°C and 800°C respectively compared to
850°C and 900°C for the ashes without additive. Both for the ashes A and D also,
it is observed that the start of sintering shifted to lower temperature with
additive. The total quantum of shrinkages is reduced for ashes with additive and
the rate of shrinkage (% shrinkage /°C) was observed to be increased two times
for ashes with additive compared to ashes without additive.

According to the present invention, addition of an additive in sufficient amounts
to the fuels, increases the rate of sintering during combustion. Such additives
also shift the sintering pattern to a lower temperature. The shrinkage almost
comes to an end at lower temperature compared to temperature at end of
sintering for fuel ashes without additive.
It is clear that mixing of the additives with fuels modifies the sintering pattern of
ash. The shifting of sintering pattern to lower temperature enable the boiler
operator/designer to select suitable combustion temperature range, so that
sintering/shrinkage related choking can be minimized. For example, for the fuel A
and C, the combustion temperature range of 850 to 920°C with additive allows
completion of sintering in the combustor and thereby minimizing the choking in
the dense phase areas of the fuel path. Since the sintering pattern extended over
a wide temperature range without additive, it may not be possible to select
suitable temperature range to minimize the shrinkage/sintering. The inventive
method provides clear indication that major portion of sintering/shrinkage can be
completed within the combustor in a particular temperature range using
additives so that the further shrinkage/sintering and related choking is minimized
in the fuel path.
The additive may be either added along with the fuel in required proportion
before preparing the fuel, in a device so that the fuel/additive mixture enters the
combustor through the fuel feed system or can be added separately into the
combustor from a separate bunker where the prepared additive is stored and fed
in the required rate corresponding to the fuel feed rate to attain the desired
fuel/additive proportion.

While, this invention has been described with respect to particular embodiments
thereof, it is apparent that numerous other forms and modifications of this
invention will be obvious to those skilled in the art. The appended claims and this
invention generally should be construed to cover all such obvious forms and
modifications which are within the true spirit and scope of the present invention.

WE CLAIM :
1. A method of modifying sintering pattern of ash generated, to reduce
agglomeration, sintering and choking formation during combustion of solid
fuels in fuel firing systems, the method comprising the steps of :
- adding at an effective amount an additive to the solid fuels wherein the
chemical composition of the additive by weight percentage of CaO, MgO,
BaO, SrO, SiO, Al203, Fe203, Ti02„ Na20, K20, ignition loss respectively is 35-
56, 0-4, 0-1, 0-2, 0-10, 0-5, 0-10, 0-2, 0-1, 0-1 and 30-44;
- modifying the sintering pattern of the ash of the fuel including the additive
mixture;
- shifting the start of sintering temperature of the ashes with additive to a
lower temperature;
- increasing the rate of shrinkage of the ashes with the additive mixture; and
- selecting, an operating temperature range based on modified sintering
pattern, wherein the additive is one of calcium based or any materials
having said chemical composition and wherein the effective amount of the
additive varies between 0.1% to 10%.

2. The method as claimed in claim 1, wherein the effective amount of
additive is mixed with the fuel before combustion.
3. The method as claimed in claim 1, wherein the fuel firing systems is one
of circulating fluidized bed combustion (CFBC) and fluidized bed
combustion (FBC).
4. The method as claimed in claim 1, wherein the choking is formed at any
location in the fuel/ash path of the firing system.
5. The method as claimed in any of the preceding claims, wherein the solid
fuels is one of lignite, sub-bituminous coal, bituminous coal and
anthracite.
6. The method as claimed in claim 1, wherein the additive is added along
with the fuel or separately into the combustor.
7. The method as claimed in any of the preceding claims, wherein the
increase of the rate of shrinkage with additive is achieved up to 3 times
the rate of shrinkage of ashes with out additive.

8. The method as claimed in any of the proceeding claims, wherein the
shifting of sintering/shrinkage pattern is achieved by shifting the
shrinkage profile to at least 30 to 100°C lower than the profiles for fuel
without additive and wherein the quantum of shrinkage reduction is
achieved up to 2 to 10% at the operating temperature range of (800 to
950°C) of the CFBC/FBC.

ABSTRACT

The invention relates to a method of modifying sintering pattern of ash
generated, to reduce agglomeration, sintering and choking formation during
combustion of solid fuels in fuel firing systems, the method comprising the steps
of adding at an effective amount an additive to the solid fuels wherein the
chemical composition of the additive by weight percentage of CaO, Mg0, Ba0,
Sr0, Si0, Al203, Fe203, Ti02„ IMa20, K20, ignition loss respectively is 35-56, 0-4, 0-
1, 0-2, 0-10, 0-5, 0-10, 0-2, 0-1, 0-1 and 30-44; modifying the sintering pattern
of the ash of the fuel including the additive mixture; shifting the start of sintering
temperature of the ashes with additive to a lower temperature; increasing the
rate of shrinkage of the ashes with the additive mixture; and selecting an
operating temperature range based on modified sintering pattern, wherein the
additive is one of calcium based or any materials having said chemical
composition and wherein the effective amount of the additive varies between
0.1% to 10%.