Calciner fuels,
transition to
Natural Gas
Summary
Summary
Natural Gas has many advantages however successful
implementation of a gas fired calciner requires a thorough
understanding of both the fuel and the process. A
case study is used to highlight the approach taken by a
Cement Plant and FCT Combustion using CFD to
identify and understand the existing limitations for
Natural Gas firing in the calciner of Kiln 1 and the
design and implementation of a solution.
is shown in Figure 1 as a function of the average carbon
chain length. Methane (CH4) has the shortest chain length
and therefore the highest auto-ignition temperature
whereas HFO with carbon chain lengths in the order of 19
has a very low auto-ignition temperature.
Region for
Natural Gas
Introduction
Natural Gas is an attractive fuel due to its high heating
value, no need to stockpile, store, blend, grind or preheat,
low carbon emissions and, in some regions, low price.
Technically however it can cause challenges such as a
high ignition temperature, high flue gas volume and low
heat transfer when compared with liquid and solid fuels.
Region for
HFO/Coal
So when converting a Calciner to Natural Gas what
burners should I use? And more importantly where should
they be located?
To answer these questions it is important to understand
both the fuel and the process in which it is to be used.
Fuel Ignition and Combustion Rate
Natural Gas consists predominately of methane and some
ethane which have very short carbon chains of 1 or 2
carbon atoms per molecule. Heavy fuel oil (HFO) and
coal on the other hand have very long carbon chain
lengths. The molecules with shorter chain lengths are
more stable and take more energy to break the bonds in the
fuel molecules, therefore higher temperatures are required
to commence and accelerate the combustion reactions.
The minimum auto-ignition temperature for hydrocarbons
Figure 1: Hydrocarbon minimum auto-ignition temperature versus
average carbon chain length.[1]
Therefore simply injecting Natural Gas in the same
temperature location as existing HFO or coal burners is no
guarantee of satisfactory ignition and combustion. This
point is further highlighted in Figure 2 where the
combustion reaction rate at 830°C for natural gas (CH4) is
only 25% of the reaction rate for HFO (C19H30). However
if the temperature is raised to 910°C the reaction rate of
Natural Gas is comparable with HFO.
to compensate and therefore more flue gas
produced.
Table 1: Combustion air and flue gas calculations
Figure 2: Arrhenius reaction rate constant for HFO and Natural Gas
Flue Gas Volume
For complete combustion in lean (Ø<1) or stoichiometric
(Ø=1) mixtures with atmospheric air the reaction can be
written:
𝑦
(𝑥 + )
2 (𝑂 + 0.79 𝑁 )
𝐶𝑥 𝐻𝑦 +
2
0.21 2
∅
𝑦
(𝑥 + ) 0.79
𝑦
𝑦 (1 − ∅)
2
→ 𝑥𝐶𝑂2 + 𝐻2 𝑂 + (𝑥 + )
𝑂2 +
𝑁
2
2
∅
∅
0.21 2
Where "𝐸𝑥𝑐𝑒𝑠𝑠 𝐴𝑖𝑟" =
(1−∅)
∅
Taking methane (CH4) to represent Natural Gas and
C19H30 to represent HFO the stoichiometric air
requirement for combustion is 17.2kg air per kg of CH4
compared with 14.2kg air per kg of C19H30. This
calculation is summarized in Table 1. However, the flue
gas volume per net unit of energy released is only 2.3%
higher for CH4 compared with C19H30. So why does
Natural Gas impact on exhaust fan capacity? There are a
two main reasons:


The rate of combustion for Natural Gas in the
calciner is slower releasing its energy further up
the calciner resulting in higher calciner exit and
therefore preheater exit temperatures. Higher
preheater exit temperatures reduce fan capacity.
The higher preheater exit temperature means that
more heat/energy is lost from the process and for
the same production rate more fuel must be burnt
If the specific fuel consumption for the kiln and calciner is
higher on Natural Gas (say 10% for example) then the flue
gas volume at 0°C is 12.5% higher and given the higher
preheater exit temperature is even higher again. However,
if CO2 from calcination is taken into account then the
exhaust gas volume is only 10% higher.
It is not so much that the volume of flue gas from Natural
Gas is higher than Oil but that the rate of combustion is
lower and the resulting heat transfer less efficient.
Therefore anything that can be done to improve the rate of
combustion and heat transfer to the meal will reduce the:



specific fuel consumption;
preheater exit temperature; and
flue gas volume.
Case Study – Cement
Carbon Emissions
Natural Gas has the inherent benefit over HFO and coal of
having lower carbon emissions per net unit of energy
released. As shown in Table 2, methane has 31% lower
carbon emissions per net unit of energy released when
compared with C19H30.. If the overall specific fuel
consumption is the same with both fuels then the total
carbon emission from fuel and raw meal is ~10% lower
for methane compared with C19H30. Even if the specific
fuel consumption for methane is 10% higher the total
carbon emission from fuel and raw meal is still ~8% lower.
Table 2: Flue gas carbon emissions
Figure 3: 4 Stage ILC, Kiln 1
The Cement Plant operates an FLS kiln (Kiln 1) with an
in-line calciner at their plant. The kiln and calciner
are primarily Natural Gas fired, supplemented by HFO.
The plant wanted to fire 100% Natural Gas, however, it
was not possible to do so without a significant
reduction in production rate.

When attempting to fire >80% Natural Gas in the
calciner excessively high temperatures were
obtained at the preheater exit.

When firing 100% Natural Gas the feed rate was
limited to 200t/h.
For this Cement Plant, HFO is a significantly more
expensive fuel than Natural Gas and consequently the
operating costs are higher if HFO firing is required.
FCT Combustion was engaged to study their calciner
and determine what was limiting the substitution of
Natural Gas and find a solution.
On examining the meal calcination it was also evident that
the temperature in the calciner was not well distributed
resulting in some meal passing through the calciner almost
uncalcined. This can be seen as the red meal particle path
lines in the right hand image of Figure 6.
CFD as a design tool
The arrangement of the calciner, shown in Figure 4, is
such that the tertiary air duct (TAD) enters the kiln riser at
an angle and all the existing burners (4xHFO and
4xNatural Gas) are arranged in a ring around the riser duct
above the TAD entry and just below the calciner cone.
Figure 5: Calciner Fuel Burnout as a function of distance above the
original ring of burners determined by CFD modelling.
Figure 4: Arrangement of kiln and calciner.
CFD modelling (see Figure 5) showed that with 100%
HFO the fuel burnout was more than 60% complete only
1m above the ring of burners and 100% complete by 9m
above, in a 15m calciner vessel. However even at
80%NG:20%HFO the fuel burnout was not complete at
the calciner exit and combustion continued on into the
bottom stage cyclone raising the calciner exit temperature
and therefore also the preheater exit temperature.
Figure 6: Meal particle path lines coloured by mass fraction of CaCO3
100% HFO
Existing
Burners Only
20%HFO:
80%NG
Existing Burner
Only
100%NG
With addition
of FCT Burners
The solution proposed was to install two FCT Natural Gas
burners in the exit of the TAD with the aim of raising the
temperature of the tertiary air and therefore accelerating
the combustion of the Natural Gas from the existing
burners (as shown in Figure 2).
The result of the FCT burners in the TAD on fuel burnout
with 100% Natural Gas is compared with the original
burners in Figure 7. The burnout performance on 100%
Natural Gas with the proposed burners is better than the
existing 20%HFO-80%NG and approaches that of the
100%HFO case.
Existing Burners
FCT
Burners
Figure 8: Temperature field in the calciner for the original burners
and with the addition of the FCT tertiary air duct burners.
Figure 7: Fuel burnout comparison of the existing burners with the
addition of the FCT burners in the TAD.
The result on the calciner temperature field is shown in
Figure 8. While the existing oil burners provide an even
heat distribution throughout the calciner, the existing
Natural Gas burners result in poor heat distribution and
lower temperatures due to the incomplete fuel burnout.
With the addition of the FCT burners in the TAD
combustion of Natural Gas is accelerated and the heat
more evenly distributed.
The impact on calcination is shown in Figure 9. The
improved fuel burnout and temperature distribution results
in fewer meal particles passing through the uncalcined.
Figure 9: Meal particle path lines coloured by mass fraction of
CaCO3 comparing the existing condition with the addition of the FCT
burners in the TAD.
Given the new burners are in a small circular duct it is
important that the flame does not impinge on the
refractory. Therefore a highly swirled gas burner was
opted for and CFD (see Figure 10) showed that the
refractory temperatures in the vicinity of the burners was
acceptable.
Flow
Direction
Figure 11: Two new FCT burners installed in the TAD, one visible
and one on the opposite side of the TAD.
Burner
TAD
Figure 10: Refractory hot face temperature predicted by CFD (max.
1022°C)
Implementation
Two Natural Gas burners (designed for only ~10% of the
total gas to the calciner) were supplied by FCT
and installed in opposing positions in the TAD by
the Plant as shown in Figure 11, Figure 12 and Figure
13.
During the commissioning period the kiln was
initially operated on only the original burners firing
100% Natural Gas achieving a feed rate of only 200t/h.
Within seconds of turning on the additional FCT gas
burners the preheater exit temperature dropped 16°C
even with an increase in the total calciner gas flow.
Figure 12: Installation of the FCT burners in the TAD.
Figure 14: Preheater Exit Gas Temperature and Kiln Feed Rate before and
after FCT burner optimisation.
Figure 13: Closeup of the FCT burner nozzle and view of the nozzle in
the refractory.
Results
In the days following the kiln feed rate was increased 15%
to 230t/h whilst maintaining the preheater exit temperature
at the level previously attained at 200t/h. In addition the
specific fuel consumption dropped 4% and clinker quality
was maintained.
Figure 15: Calciner Gas Firing Rate and Kiln Feed Rate before and after
FCT burner optimisation.
Conclusion
When changing the calciner fuel from HFO to Natural Gas
it is not just a matter of installing the Natural Gas burners
in the same location as the as the existing HFO burners.
There are many factors that need to be considered and
understood.

The inherent differences between the fuels and
how they disperse, mix and burn.

The aerodynamics of the calciner vessel.

The temperature of the tertiary air.

The location of the meal entry point(s).

The impact on refractory.

The resultant heat transfer to the meal and degree
of calcination.
Figure 16: Freelime and Kiln Feed Rate before and after FCT burner
optimisation.
Only by thorough understanding of these factors and the
use of CFD can effective solutions be developed and
verified as demonstrated in this project.
References
[1] Coward, H.F. and Jones, G.W., “Limits of
flammability of Gases and Vapours”, Bulletin 503, U.S.
Department of the Interior, Bureau of Mines, 1952.