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WHY A NEW COMBUSTION SYSTEM ?
a) Body quality (white, red, wet or dry ground)
This tendency is not only reductive on the evaluation of what we
mean for “Good Burner” but also negative in order
to have the maximum of :
a) Quality
The utilization consequently of only one type of burner in all roller
kiln zones is definitely to be rejected.
FOR PRELIMINARY PHASE WE MEAN THE
ONE BETWEEN 20 and 650 °C
Where we have to keep in mind that the product, at the kiln entrance,
is at ambient temperature and has:
1) “Free Water” between 1-1,5%
And as we intend to run at the maximum possible productivity, the time
available to heat the product from 20 to 650 °C which permit elimination
of the present “waters”, should be kept at the very minimum.
For products in single firing, with thickness of max. 9 mm, and size
400x400 we have seen that the minimum transit time is so divided
:
a) Pre-Heating : 12-15’ minimum
Considering now the pre-heating, we suggest to divide it into two important
phases, such as :
Phase 1 - Free Water Elimination
Phase 2 - Organic Compound Elimination
PHASE 1
FREE WATER ELIMINATION
The time variable assume a great importance when You have to compare
the rapidity at which certain processes must take place and the advancing
speed of the pieces inside the kiln .
In order to avoid such a problems we don’t have anything to do except
to reduce the advancing speed and therefore lose productivity.
To completely eliminate explosion problems or microfractures on the
tiles’ edge TE.CO.MA. has developed the “RADIANT SYSTEM”
For a better understanding and clearness see our report :
“NEW TECHNOLOGY TO PRODUCE TILES...”
Once the free water is elimination, that as demonstrated in our
a.m. report, takes place in 3’ (6 mt. of kiln) without damaging the product,
we’ll have to quickly remove the constitution water that, as we know, takes
place between 450-650 °C .
PHASE 2
ORGANIC COMPOUND ELIMINATION
In a total time of 9’ ( 18 mt. of kiln ) we’ll have reached the temperature
of 650-700 °C eliminating completely the constitution water.
All the burners usually installed by the kilns manufacturers do
not allow the introduction of an adequate quantity of air (due to the burner
shut-off) and therefore the oxygen for a complete oxidation of the
organic is usually inadequate.
For this purpose TE.CO.MA. installs burners called
“CALIBRATING BURNERS” able to avoid high shrinkage of
the central tiles and leaving the two side tiles shrink properly, having
in this way only 1 calibre on the cross-section.
The burners alignment on the side walls is done through
a pocket computer, made by TE.CO.MA., which permits to control:
1 - Gas quantity in Ncm/h each burner
Require our “BURNER TESTER” manual
Basically 4 are the kinds of burners necessary to realise
the targets that an updated and technologically advanced kiln as
to reach the target of:
QUALITY, QUANTITY, ENERGY SAVING.
The four kind of burners are apparently all the same except for the
“AUTOMAX type”.
Each burner is equipped with ignition and flame control device, security
electrovalve for gas, everything as per CEE regulation and supplied with
documentation .
The rate at which the ceramic technology developed, from the traditional
system of MUFFLED or SEMI-MUFFLED tunnel kilns,
to the update rollers heart kilns operating with “Open Flame System”
absolutely didn’t give enough time to the manufacturers to adequately
study and adjust the “Combustion System” of their kilns and to suit
the new requirements of a growing energy-conscious ceramic industry.
As time has become so precious in such a kind of stiffening competition,
many kiln manufacturers have overlooked the importance of a new fuel
burner project and they have opted to retain the traditional design-concepts
of the old “oil-gas burners” and chose, in the immediate, the way
of employing those burners, which are more adaptable to tunnel kilns, to
a newly developed roller-heart kilns.
Many industry-users themselves felt the effects of an energy and fuel
rising costs, thus, they saw in this new technology the possibility to
cutdown deeply their production costs in order to be more competitive in
the international market .
Consequently, as this “rising technology” gradually gained popular
acceptance among the ceramic manufacturers, the immediate results of their
practical use in the industry have been far below the common expectations
. The intended benefits in order to achieve high thermal efficiency
and improved cost-effectiveness were left with much to be desired .
Looking now at the magnitude of problems existing in this particular
industry sector employing roller-kilns, it has been observed that most
of the causes of these problems arise from inadequate and/or
poor fuel-combustion.
To produce a burner for natural and other aeriform fuel combustion,
is relatively easy;
what is more difficult is the “Applied combustion” or the burner
utilization to a system for the production of a material which has different
difficulties based on the type of technology in use, as :
Or different bodies, as :
The general tendency is to build a burner that, on the test desk, grant
a stable working, within a certain delivery of gas and air, then use it
on a roller kiln for whatever typology and/or requested productivity.
We have to carefully consider that, as previously said, the roller
kilns are at open flame, this means with “exhaust gas direct inlet”
and that during the firing process we have different reaction based
on :
b) Typology (single firing, double firing Etc.)
b) Quantity
c) Energy saving
In order to underline the reason of our statement we invite
the reader to consider the single-firing.
These products, it doesn’t matter if white or red body, dry or wet
ground, during the preliminary firing phase, depending by the different
moment of the process, show different requirements.
2) “Colloidal and Hygroscopic water”
The present experience confirm us the necessity to operate on the firing
process, introducing a new concept “ The Transit Time“ .
b) Firing : 5-6’ ( 10’ for
granite )
For a better understanding, we have to remind that in the rollers kiln,
in this first phase, we use “Convection” to transmit the heat, present
in the hot air coming from the firing zone and from the burners, to the
pieces introduced inside the kiln .
This system is so slow that to enhance the temperature of the pieces
at 120 °C and eliminate the free water, we need at least 12’
with an air temperature around 250 °C .
Since the average advancing speed of the product in a 80 mt. kiln is
abt. 2 mt/l’, we would need at least 24 mt. of machine to get rid of this
water and so, to avoid this inconvenience, people try to elevate
the temperature of the air around 500 °C in the first 6
meters of kiln.
The result would be acceptable, if were not for the colloidal water
present which, at such a temperature, produce microfractures on the tiles
and very often explosion of the product due to the speed exit of the steam
generated by this kind of water.
utilising burners type “AUTOMAX” which grant the controlled
heating of the material and corresponding water content elimination avoiding
at the same time to slow down the cycle:
This happen in further 6’ utilizing in zone a set of burners
type “PULSAR” or “ASTERIX” able to supply heat at low temperature
( 300 -700 °C ) , without flame out of the cone, based on the quantity
produced in terms of Kg/hour.
Once increased the piece temperature over 300 °C , 6’ time ( 12
m of kiln ) , the firing process can proceed without damages and with very
high gradients , in many cases up to 225 °C/minute.
Over 850 °C starts the oxidation of the organic compounds
and therefore the black core elimination.
In this phase are used burners type “ASTERIX” or
“PULSAR”, able to work with quantity of air up to 200 Cm./hour and
to supply the oxygen for a complete and quick oxidation of the organic
compound present.
To overcome this problem, they force the utilizer to reduce kiln
speed in order to get rid of black core.
Once the pieces reach the firing zone, by this system, we’ll have supplied
to the product 80% of the needed heat for the total
Silica-Alluminate. transformation.
At this point of the firing cycle the problem is that, having
large section kilns like ( 5 pcs. 400x400 or 6 pcs. 330x330
) , the size of the pieces on the whole useful cross-section
doesn’t reach the wanted range of 5/10 mm.
2 - Air quantity in Ncm/h “
“
3 - Exhaust gas temperature
