”
to
f c Fo
op r i
yr ns
ig pe
ht ct
ow ion
ne pu
r r rp
eq os
ui es
re o
d nl
fo y.
ra
ny
ns
en
-.
Co
_ . ,.
EPA Export 25-07-2013:16:43:21
se
ru
he
ot
.
se
.
A certainretentionor residence
timeisrequiredto transferheat
from the combustiongasesto the surfaceof the stoneand
then from the surfaceto the coreof the stone.Largerstones
requirelongertimeto calcinatethan smallerones.In principal,
calciningat highertemperatures
reducesthe retentiontime
needed.However,too high temperatures
will adverselyaffect
the reactivity of the product.The relationbetweenburning
temperatureand retentiontime requiredfor different stone
sizesis shownin the followingtable.
to
f c Fo
op r i
yr ns
ig pe
ht ct
ow ion
ne pu
r r rp
eq os
ui es
re o
d nl
fo y.
ra
ny
Lime is one of the key elements in life. This natural raw material is involved in the production of the majority of modem
products. The production of iron and steel, gold, silver, copper
and plastics as well as many chemical products and foodstuffs,
just to mention a few, requires lime and, to a lesser extent,
dolomite. The most important fields of application for lime and
dolomitic lime are:
lron and steel
Non-ferrous metals
Building industry
Pulp and paper
Chemical industry
PCC - Precipitated calcium carbonate
Sugar
Glass
Flue gas desulphurisation
Agriculture
Soil stabilisation
Water treatment
Sewage treatment.
tive heat transferto the core.A temperatureof 900 “C hasto
be reachedin the coreat leastfor a shortperiodof time since
the atmosphere
insidethe materialis pureCO,.Thestonesurface mustbe heatedto greaterthan 900 “C to maintainthe
requiredtemperaturegradient and overcomethe insulating
effect of the calcinedmaterialon the stonesurface.Whenproducing soft-burnt lime the surfacetemperaturemust not
exceed1100to 1150“C asotherwisere-crystallisation
of the
CaOwill occurandresultin lowerreactivity and thusreduced
slakingpropertiesof the burnt product.
ru
Lime ‘and Dolomite
he
1. Limestone,
FlOw Regenerative Lime Kiln
ot
The M&RZ@ Padlel
World wide more than 120 milliontonsper year of lime and
dolomiticlimeareproduced.Theiron andsteelindustryisthe
primaryconsumer
with an annualdemandof approx.40 million tons.
Stonesize Calciningtemperature Approx.residence
time
(hours]
b-4
[“Cl
50
1200
0.7
100
1000
1200
1000
2.1
2.9
8.3
Co
ns
en
High quality limestonecontains97 to 99%CaCO,.It requires Throughoutthispaper,the word“lime”is usedinterchangeably to mean“highcalciumlime” or “dolomiticlime”.
approximately1.75 tons of limestoneto produceone ton of
lime.Highqualitydolomitecontains40 to 43% MgCO,and57
to 60% CaCO,.It requiresapproximately2 tons of dolomitic
stoneto produceoneton of dolomiticlime.
2. Lime Production EauiDment
Thecalcinationor burningof limestoneanddolomiteis a sim- Two typesof kilnsareprimarilyusedto calcinelimestoneand
ple chemicalprocess.
Whenheatedthe carbonatedecomposes dolomitein today’slimeindustry:
accordingto its respectiveequation.
l
Rotarykilns,and
l Verticalshaftkilns.
CaCO,t approx.3180kJ [760kcal)= CaOt CO,
CaMg(CO,),t approx.3050kJ (725kcal)= CaO.MgOt 2 CO, Rotarykilns,with or without preheater,usuallyprocessgrain
sizesbetween6 and 50 mm.The heatbalanceof this type of
Thedecomposition
temperature
depends
on the partialpressu- kilnsis characterised
by ratherhigh losses
with the off-gases
re of the carbondioxidepresentin the processatmosphere.
In
andthroughthe kiln shell.Typicalfiguresfor off-gaslosses
are
a combustiongasatmosphereof normalpressureand 25%
in the rangeof 20 to 25%andfor kiln shelllosses15to 200/o
CO,,the dissociation
of limestonecommences
at 810 "C. In an of the total heat requirement.Only approx.60% of the fuel
atmosphere
of 100% CO,,the initial dissociation
temperature energyintroducedinto preheatertype kilnsis usedfor the calwouldbe 900 “C.Dolomitedecomposes
in two stagesstarting ciningprocess
itself.
at approx.550“C for the MgCO,portion andapprox.810 “C
for CaCO,.
For all typesof verticalsingleshaftkilnsthereis an imbalance
betweenthe heatavailablefrom theburningzoneandthe heat
In orderto fully calcinethe stoneandto haveno residualcore, requiredin the preheatingzone.Evenwith an idealcalcination
heatsuppliedto the stonesurfacemustpenetratevia conduc- process
(havinganexcessair factor of 1.0)a wastegastempe-
3
EPA Export 25-07-2013:16:43:22
EPA Export 25-07-2013:16:43:22
to
f c Fo
op r i
yr ns
ig pe
ht ct
ow ion
ne pu
r r rp
eq os
ui es
re o
d nl
fo y.
ra
ny
ns
en
Co
se
ru
he
ot
.
EPA Export 25-07-2013:16:43:22
f c Fo
op r i
yr ns
ig pe
ht ct
ow ion
ne pu
r r rp
eq os
ui es
re o
d nl
fo y.
ra
ny
en
to
Co
ns
se
.
ru
he
ot
EPA Export 25-07-2013:16:43:22
to
f c Fo
op r i
yr ns
ig pe
ht ct
ow ion
ne pu
r r rp
eq os
ui es
re o
d nl
fo y.
ra
ny
ns
en
Co
se
ru
he
ot
.
EPA Export 25-07-2013:16:43:22
f c Fo
op r i
yr ns
ig pe
ht ct
ow ion
ne pu
r r rp
eq os
ui es
re o
d nl
fo y.
ra
ny
en
to
Co
ns
se
.
ru
he
ot
EPA Export 25-07-2013:16:43:22
f c Fo
op r i
yr ns
ig pe
ht ct
ow ion
ne pu
r r rp
eq os
ui es
re o
d nl
fo y.
ra
ny
en
to
Co
ns
se
.
ru
he
ot
EPA Export 25-07-2013:16:43:22
f c Fo
op r i
yr ns
ig pe
ht ct
ow ion
ne pu
r r rp
eq os
ui es
re o
d nl
fo y.
ra
ny
en
to
Co
ns
se
.
ru
he
ot
EPA Export 25-07-2013:16:43:22
f c Fo
op r i
yr ns
ig pe
ht ct
ow ion
ne pu
r r rp
eq os
ui es
re o
d nl
fo y.
ra
ny
en
to
Co
ns
se
.
ru
he
ot
EPA Export 25-07-2013:16:43:23
to
f c Fo
op r i
yr ns
ig pe
ht ct
ow ion
ne pu
r r rp
eq os
ui es
re o
d nl
fo y.
ra
ny
ns
en
Co
se
ru
he
ot
.
EPA Export 25-07-2013:16:43:23
to
f c Fo
op r i
yr ns
ig pe
ht ct
ow ion
ne pu
r r rp
eq os
ui es
re o
d nl
fo y.
ra
ny
ns
en
Co
se
ru
he
ot
.
EPA Export 25-07-2013:16:43:23
to
f c Fo
op r i
yr ns
ig pe
ht ct
ow ion
ne pu
r r rp
eq os
ui es
re o
d nl
fo y.
ra
ny
ns
en
Co
se
ru
he
ot
.
EPA Export 25-07-2013:16:43:23
to
f c Fo
op r i
yr ns
ig pe
ht ct
ow ion
ne pu
r r rp
eq os
ui es
re o
d nl
fo y.
ra
ny
ns
en
Co
se
ru
he
ot
.
EPA Export 25-07-2013:16:43:23
to
f c Fo
op r i
yr ns
ig pe
ht ct
ow ion
ne pu
r r rp
eq os
ui es
re o
d nl
fo y.
ra
ny
ns
en
Co
se
ru
he
ot
.
7. Performance, Product Quality, Energy
Consumption, Maintenance
he
ru
se
.
7.1 Kiln Capacity
The trend in today’s market is to focus on large capacity kilns.
PFR-Kilns with a daily output of 600 tons have been in operation for years with up to 1000 tpd available. Small capacity
kilns are restricted by economic factors. The relation between
the cost to install a large kiln and a small kiln is not linear. It
is generally recognised and accepted that the investment costs
per ton of burnt lime are higher on small kilns than they are on
a larger kiln. Even so, under certain conditions, PFR-Kilns with
a daily output of 50-75 tons have been proven economical.
to
f c Fo
op r i
yr ns
ig pe
ht ct
ow ion
ne pu
r r rp
eq os
ui es
re o
d nl
fo y.
ra
ny
*
6.2 Grain Size of the Stone
A narrow range of grain size is ideal for any kiln, but, due to
the crushing properties of stone, a widely varying grain size is
the typical situation in the quarry. The PFR-Kiln is able to caltine a wide range of top to bottom stone size because of its
sophisticated charging system. The ideal range is 2:1, but operation using 4:l is still permissible. The top to bottom size
range is not the only criteria though as the shape of the grain
also plays a role. The minimum stone size for the standard type
PFR-Kiln is approximately 25 mm with a typical maximum
stone size of 125 mm. Upon customer request the maximum
size may be as high as 180 mm provided the burning zone as
well as the feeding and discharge equipment have been adequately designed for it.
6.4 Excess Combustion Air
Excess air has a considerable influence on fuel consumption in
the typical counter flow shaft kiln. But this is not the case in
the parallel flow regenerative kiln where the excess air factor
has hardly any effect. The same amount of heat is recovered in
the stone of the non-burning shaft regardless of the introduced excess combustion air. Therefore the air volume can be
adjusted to produce a short or long flame and adapt the burning zone temperature to produce the desired product. Lime
cooling air does not take part in the combustion and dilutes
the combustion gases thereby making the CO, content in the
off-gas of PFR-Kilns lower than in a conventional single shaft
kiln.
ot
perature in the calcining zone in a consistent, uniform manner.
Temperaturewithin this zone can vary due the stone grain size,
the chemical analysis of the stone, variations in the amount
and distribution of air flow, and variations of the heat value of
the fuel. Fuel input has to be controlled as a function of the
calcining temperature. As the temperature in the crossover
channel is an excellent indicator for the calcining temperature
an accurate measurement of this temperature by optical pyrometer is required.
Co
ns
en
6.3 Quality of the Stone
As for all types of vertical shaft kilns the use of hard, nondecrepitating, high purity limestone is an ideal condition for
trouble-free operation of the PFR-Kiln. Nevertheless, due to
the fact that the shafts of the PFR-Kiln are virtually a pipe
without any devices which could obstruct the free flow of
limestone and lime the movement of the material column is
slow and uniform minimising abrasion and formation of fines.
This means that also soft limestone can be calcined in the PFRKiln.
l
In case the limestone has a tendency to decrepitate during the
calcining process an increased percentage of fines will be generated. The installation of so called air cannons in the crossover
channel area where dust particles could stick to the refractory
lining facilitates the calcination of soft and decrepitating
stone.
High quality limestone and dolomite with consistent chemical
properties is often not available or is scarce. Varying contents
of carbonates and impurities can result in the production of
overbumt or underbumt product with inconsistent values for
residual CO, and loss on ignition. For such cases a folly automatic temperature control system of the Maerz PFR-Kiln may
be implemented to adjust the heat input to maintain uniform
quality of the calcined product.
16
The output of a PFR-Kiln can be varied within a wide range: it
is quite possible to operate the kiln at only one half of the
nominal capacity without considerable influence on the specific fuel and power consumption.
7.2 Product Quality
7.2.1 Residual CO,
The PFR-Kiln allows the production of lime and dolomitic lime
with residual CO, figures as low as 0.5%, in certain cases even
lower. The steel industry, the biggest consumer of lime and
dolomitic lime, generally asks for residual CO, contents of less
than 2%.
7.2.2 Reactivity
The parallel flow of material and combustion gases during the
calcining process is the ideal condition to produce high reactive lime and dolomitic lime as required for most applications.
For special applications such as the production of porous conCrete,lime with medium or low reactivity is required. By adapting operating parameters, such as excess air ratio and heat
input, medium burnt lime can be produced in the PFR-Kiln
with adequate quality of the raw stone. The production of hard
burnt lime, however, is in general not possible in this type of
kiln.
.
EPA Export 25-07-2013:16:43:23
EPA Export 25-07-2013:16:43:23
to
f c Fo
op r i
yr ns
ig pe
ht ct
ow ion
ne pu
r r rp
eq os
ui es
re o
d nl
fo y.
ra
ny
ns
en
Co
se
ru
he
ot
.
EPA Export 25-07-2013:16:43:23
f c Fo
op r i
yr ns
ig pe
ht ct
ow ion
ne pu
r r rp
eq os
ui es
re o
d nl
fo y.
ra
ny
en
to
Co
ns
se
.
ru
he
ot
EPA Export 25-07-2013:16:43:23
to
f c Fo
op r i
yr ns
ig pe
ht ct
ow ion
ne pu
r r rp
eq os
ui es
re o
d nl
fo y.
ra
ny
ns
en
Co
se
ru
he
ot
.
to
f c Fo
op r i
yr ns
ig pe
ht ct
ow ion
ne pu
r r rp
eq os
ui es
re o
d nl
fo y.
ra
ny
ns
en
Co
se
ru
he
ot
.
!
/
‘.
EPA Export 25-07-2013:16:43:23