Combustion efficiency improvement opportunities in CEMENT Sector'.ppt

COMBUSTION EFFICIENCY

1. 'Combustion efficiency
improvement opportunities in
CEMENT Sector'
 
J. Nagesh Kumar
Sr. Deputy Director
National Productivity Council
Chennai 600098

2. Cement manufacturing process

4. Cement process
•The clinker manufacturing process starts by
producing a fine powder containing strictly
controlled proportions of:
•limestone - to provide calcium carbonate; and
•clay - to provide silica, alumina and iron oxides.
•When the powder is homogenised and heated to
1450C in the kiln, the lime molecules combine
with all the silica, alumina and iron oxide
molecules to form clinker.

5. Thermal Energy Use in Cement plants
The thermal energy contributes nearly half of the
energy cost in a cement plant. The major
consumption is the fuel used in the kiln. The
other thermal energy consumers are
–Coal furnace - Used for drying of coal in some plants
–Raw mill furnace - Used for drying of raw meal during
starting of the plant
–Cement mill furnace - Used in some plants for drying
of slag

6. Precalciner Kilns
•Combustion air is taken from clinker cooler to precalciner
•Typically, 60% of the total fuel is burnt in the calciner, and the raw
meal is over 90% calcined before it reaches the rotary kiln section.
•Since the calciner operates at temperatures around the calcination
temperature of raw meal (800°C to 900°C), there may not be a
flame as such.
•The calciner efficiency is dependent on uniform air flow and uniform
dispersion of fuel and raw meal in the air.
•Typically, average residence times calculated on gas flow for early
units were about 1 to 2 seconds for coal and oil, and 2 to 3 seconds
for natural gas.
•Larger calciners are required for using lower grade fuels.
•Precalciner kilns can have very large outputs in excess of 10,000
tpd, with specific fuel consumption below 3 MJ/kg (700 kcal/kg).
•Precalciners can be in-line configuration or separate line
configuration.

7. Combustion in Kilns
•The raw meal is subjected to enough heat to allow the
clinkering reactions to proceed.
•The main chemical reactions to produce the calcium
silicates that later give cement its bonding strength
occur in the kiln system.
•There is a combination of endothermic and exothermic
reactions occurring in an extremely complicated
chemical reaction sequence.
•The raw material composition, mineralogical
composition and the time and temperature profile of
these materials in the kiln determine the ultimate
composition and mineralogy of the clinker, which in turn
determines the performance of the cement produced.

8. Kiln combustion

9. Where does all that heat come
from?
•The actual combustion process is an incredibly
complex series of chemical reactions that start
off with fuel, air, and an ignition source.
•In fact, there are actually more than one
thousand separate reactions involved from the
transition of fuel into the final combustion
products of carbon dioxide and water.
•Fuels used are mainly coal, petcoke and a few
incinerable materials such as tyres.

10. Where does all that heat go?
•The combustion process heats up
unburned fuel, feed, coating, refractory,
and the gases inside the kiln. That’s why
it’s so important to maintain stable
secondary air temperature. That can only
be accomplished by maintaining
consistent clinker cooler performance.

11. Material transformation in Kilns
1.Evaporating free water, at temperatures up to 100°C.
2.Removal of adsorbed water in clay materials 100° to
300°C.
3.Removal of chemically bound water 450° to 900°C.
4.Calcination of carbonate materials 700° to 850°C.
5.Formation of C2S, aluminates and ferrites 800° to
1,250°C.
6.Formation of liquid phase melt >1,250°C.
7.Formation of C3S 1,330° to 1,450°C.
8.Cooling of clinker to solidify liquid phase 1,300° to
1,240°C.
9.Final clinker microstructure frozen in clinker <1,200°C.
10.Clinker cooled in cooler 1,250° - 100°C.

12. Kiln Gas side changes
•Ambient air preheated by hot clinker from kiln 20°C up
to 600° to 1,100°C.
•Fuel burns in preheated combustion air in kiln 2,000°
to 2,400°C.
•Combustion gases and excess air travel along kiln,
transferring heat to kiln charge and kiln refractories
2,400° down to 1,000°C.
•Preheating system for further recovery of heat from
kiln gases into the material charge in the kiln system
1,000°C down to 350° to 100°C.
•Further heat recovery from gases for drying of raw
materials or coal.

13. Minimising Energy for Kilns
All kiln systems aspire to optimize heat
exchange between the gas streams and
material streams at various stages to
minimize waste heat and maximize thermal
efficiency.

  
                   
  
     

14. Kiln design factors
•Burning zone heat loading
•Secondary air velocity
•Burning zone gas velocity
•Kiln exit gas velocity
•Kiln exit gas temperature
•Preheater tower gas velocities
•Preheater tower pressure drops
•Preheater tower exit gas temperature
•Volatile concentrations
•Material residence time
•Cooler grate loading
•Cooler air supply
•Kiln dust cycles

15. Kiln
•Sloping at 3-4 % from horizontal
•1-4 rpm
•50 -100 m long
•Length to diameter 10-15
•Temperature 1450oC
•Residence time 10-15 mins
•Nodules of clinker 3-20mm dia are formed

16. The right size
•The key to burning any fuel is to make
sure that the particles are small enough
that they can be burned quickly and easily.
•Producing smaller particle sizes requires
using more grinding energy.
•The balance then becomes a trade-off
between how fine to grind and how much
more efficient the combustion process
becomes with a finer grind.

17. Coal Milling
•In plants using coal, coal mills are part of the system
to provide dried pulverized coal to kiln and
precalciner.
•The raw coal from stock yard is crushed in a hammer
crusher and fed to the coal mill.
•The coal mill can be an air swept ball mill or vertical
roller mill where the coal particles are collected in the
bag filter through a grit seperator.
•The required size is 80 % on 90 and less than 2%
on 212 . Hot air generated in a coal fired furnace or
hot air from clinker cooler is used in the drying of coal
in the mill.

18. Burners
•The burner system also plays a vital role, in determining the
thermal efficiency of the system.
•The present operating burners operate with, a primary air
ranging from 12% - 18%.
•The latest trend is to install multi-channel low primary air
burners. These burners have the following advantages
–Lower primary air requirement (7 - 8%)
–Sharper and shorter flame
–Better flame control
•In an operating plant, the replacement of a conventional burner
with a new burner (7 % primary air, including conveying air)
resulted in reduction of thermal energy consumption by nearly
15 kCal / kg of clinker.

19. Functions of a Kiln burner
•The burner must be able to fire coal, coke, fuel oil and natural gas or any
mixture thereof, ensuring complete combustion, low excess air and
minimum formation of carbon monoxide (CO) and nitrogen oxides (NOx).
•If relevant, the burner must be able to handle alternative fuels without
requiring change of its original design.
•In this way, only minor modifications to meet the special requirements
must be necessary.
•The burner must produce a short, narrow, strongly radiant flame, as this is
a condition for good heat transfer from the flame to the material in the
sintering zone of the kiln.
•Flame formation must be conducive to a dense, stable coating on the
refractory in the burning zone of the kiln as well as a nodular clinker with
low dust content and correctly developed clinker phases.
•The burner must use as little primary air as possible without compromising
stability during normal or upset operating conditions. Primary air is
basically false air, in other words air that has not been used for clinker
heat recuperation while passing through the clinker cooler.

20. Flame pattern

21. Flame stability
•Optimal flame length can promote rapid heating
and cooling of the clinker.
•Flame stability also means reduced back end
temperatures which in turn means lower heat
losses from exit gases and shell radiation.
•The right flame length also optimizes the ID fan
capability and reduces the potential for NOx
formation.
•Unstable flames on the other hand means a
varying ignition point, a variable stand off
distance from the burner tip, high risk of flame
out, and potential explosion risk.

22. Flame momentum
•The crucial parameter of flame formation is the primary air momentum which
may be expressed as the primary air percentage (% of stoichiometric air
requirement) multiplied by the injection velocity.
•Consequently, if the velocity is doubled, the primary air percentage may be
reduced to half.
•The primary air consumption will normally be in the range of 6-8 %,
•The burner in a rotary kiln functions as an injector, the purpose of which is to
draw the secondary air coming from the cooler into the flame in order to make
the fuel burn as close as possible to the centre line of the kiln.
•This explains why the momentum of the burner is the parameter that
determines flame formation.
•A higher momentum means faster mixing and a shorter and hotter flame.
•Good coating formation is only possible if the inner surface is cold enough for
the liquid to solidify upon contact. The use of a narrow flame in a cement rotary
kiln is extremely important since a divergent flame that impinges upon the
lining will strip off the coating, resulting in very high kiln shell temperature and
short refractory life. Flame impingement upon the material charge will increase
the evaporation of sulphates, which usually leads to increased coating
formation in the kiln riser duct.

23. The flame in the burner
•The clinker must be correctly burned to
minimise its free lime content with
minimum fuel
•Ash must be uniformly absorbed by clinker
•For normal portland cement the conditions
must be sufficiently oxidising that the iron
present in the cooled clinker is almost fe3
+
•Proper flame control extends the life of
refractory

24. Multi-channel burners
•Compared to a simple single-tube burner modern multi-channel
burners offer much better possibilities for flame shape control
because of their separate primary air channels, allowing for
adjustment of primary air amount and injection velocity
independently of the coal meal injection.
•The most important flame control parameters are primary air
momentum (primary air amount multiplied by discharge velocity) and
amount of swirl (tangentially air discharge).
•A high momentum will give a short, hard flame whereas a low
momentum will make the flame longer and lazier.
•Swirl will help creating recirculation in the central part of the flame.
This will stabilise the flame and give a short ignition distance.
•Too much swirl however can cause high kiln shell temperatures due
to flame impingement on the burning zone refractory. A good swirl
control system is therefore
•important. The best solution would be a system where swirl could be
adjusted independent of the momentum.

25. NOx and SOx…SOx and NOx.
•In a perfect world combustion products would be limited to just
water and carbon dioxide.
•Thermal NOx is generated in and around the flame at
temperatures greater than 1200° C.
•A short hot burning zone can reduce the formation of thermal NOx.
•SO2 is formed as sulfide or elemental sulfur is oxidized at
temperatures of 300 to 600° C.
•Limiting the source of sulfur or the necessary oxygen can limit the
potential for SO2 formation.
•CO is either formed because of incomplete combustion or the
rapid cooling of combustion products below the ignition
temperature of CO (610° C). Either situation is detrimental to
optimizing the process.
•Optimizing the combustion process is the key to optimizing kiln
operations.

26. GHG from cement kilns
•5CaCO3 + 2SiO2 --> (3CaO,SiO2) +
(2CaO,SiO2) + 5CO2
•1 metric tonne of cement generates 1
metric tonne of CO2 greenhouse gas.

27. Process Control & Management
Systems
•Heat from the kiln may be lost through non-optimal process
conditions or process management.
•Automated computer control systems may help to optimize the
combustion process and conditions.
•Improved process control will also help to improve the product
quality and grindability, e.g. reactivity and hardness of the
produced clinker, which may lead to more efficient clinker
grinding.
•In cement plants across the world, different systems are used,
marketed by different manufacturers.
•Most modern systems use so-called 'fuzzy logic' or expert
control, or rule-based control strategies.
•Expert control systems do not use a modeled process to
control process conditions, but try to simulate the best human
operator, using information from various stages in the process.

28. Kiln Stops
•The yearly average heat consumption not only
depends on the operation of the kiln under
stable conditions but also on the number of kiln
stops per year.
•In a well operated and maintained plant kiln
stops should not exceed the numbers given
below:
–short stops - 2 h: 30
–medium stops 2h - 24 h: 10
–long stops ca.- 24 h: 5

29. False Air (Air Infiltration)
•In the kiln system (main sources$ kiln hood, kiln
seals, preheater) false air drastically affects the
heat and power consumption of the exhaust gas
fans.
•For instance an increase of 1% in the oxygen
content of the exhaust gases as a result of false
air infiltration at the kiln inlet area (seal, housing)
causes
–an increase in heat consumption of about 25 kcal/kg
cli and
–an increase in the range of 1.5 - 2 kWh/t clinker in the
power consumption of the kiln exhaust gas fan

30. Seals
•Seals may start leaking, increasing the heat requirement of
the kiln.
•Most often pneumatic and lamella-type seals are used,
although other designs are available (e.g. spring-type).
•Although seals can last up to 10,000 to 20,000 hours, regular
inspection may be needed to reduce leaks.
•Energy losses resulting from leaking seals may vary, but are
generally relatively small.
•Philips Kiln Services reports that upgrading the inlet
pneumatic seals at a relatively modern plant in India (Maihar
cement), reduced fuel consumption in the kiln by 0.4% (or
0.01 MBtu/ton clinker) (Philips Kiln Services, 2001).
•The payback period for improved maintenance of kiln seals is
estimated at 6 months or less (Canadian Lime Institute,
2001).

31. Kiln - inlet & outlet seals
•The inlet & outlet seals of the kiln are important, as it helps
reduce the air infiltration into the system. The inlet seal is
more important, as the infiltration chances are higher. Based
on the experiences of the Indian cement plants, it is
recommend to install
–Pneumatic seal for kiln inlet
–Spring loaded mechanical seal for kiln outlet
•The pre-heater flanges, poking holes & inspection holes are
all potential sources of leakage during operation of the plant.
Hence the number of flanges should be reduced to a bare
minimum. The inspection & poking holes in the pre-heaters
should be provided with air tight covers, so that, the air
infiltration into the system is minimum.

33. Clinker Quality Control
•The main parameters are the "literweight"
and the "Cao free" , which should be kept
constant and within certain limits.
•Otherwise, negative impacts on clinker
quality (e.g., under- or overburning), high
fuel consumption and the effects of poor
cement grinding are the result of quality
variations.

34. Refractory
•Kilns are lined throughout with refractory bricks
providing varying degrees of insulation to the
steel shell.
•Brick compositions vary from around 25% to
30% alumina at the cooler back end, to 45%
alumina in the calcining zone and rising to 70%
alumina as the burning zone is approached
•Dense magnesite or dolomite bricks are used in
the burning zone.
•Heat transfer within the kiln system results from
a complex interchange between the gas, inner
kiln walls and feed surface.

35. Oxygen Enrichment
•Oxygen enrichment in the kiln will increase
production capacity.
•Production increases of around 3-7% have been
found on the basis of annual production
•Any cost savings will depend on the electricity
consumed for oxygen generation (approximately
0.01 kWh/scf)
•Oxygen enrichment may result in higher NOx
emissions, if the injection process is not carefully
managed
•Oxygen enrichment is unlikely to result in net
energy savings.

36. Heat balances (mJ/kg clinker)

37. Typical energy consumption for a
cement plant
•Electrical energy consumption - 75
units / ton of cement
•Thermal energy consumption - 715 kCal
/ kg of clinker

38. Summary of Critical Data Information
on Different Kiln Systems

39. Thermal profile of the kiln

40. Precalciner

41. Effect of Precalciner
•Co-current heat transfer
•Raw material residence time is
less than a minute
•50 – 65 % of the fuel is fired in
the preheater
•Heat transfer is entirely
convective
•90 - 95 % calcination take place
•Helps to increase the kiln
throughput
•Increases kiln refractory life
•Less Nox formation

42. Pre-heater - stages & sizing
•The number of stages in the pre-heater system has a major bearing on
the thermal energy consumption of the kiln.
•The more the number of stages in the preheater, the higher the thermal
efficiency of the system.
•Presently, it is recommended to install a 6-stage / 5-stage pre-heater
system.
•One additional stage, will on one hand reduce the thermal energy
consumption by 10 - 15 kcal / kg of clinker.
•On the other hand it increases the pre-heater fan power consumption by 1
unit / ton of clinker.
•The net benefit is to be quantified before taking the decision.
•It is also important to note that, the pre-heater outlet temperature reduces
by about 25oC, when an additional stage is added. This reduces the heat
available for drying the limestone and coal. This aspect also has to be
borne in mind, while deciding the number of stages in the pre-heater.

43. Pre-heater - monitoring
•The continuous monitoring of the pre-
heater is very essential, to maintain the
pre-heater system leak proof.
•Hence, it is recommended to install
oxygen and CO analysers at the kiln inlet,
pre-heater outlet to enable continuous
monitoring of the system.

44. Coolers
•The cooler is a critical equipment used for cooling and
also recuperate the heat back into the kiln system.
•In majority of the existing plants, conventional grate
coolers are being used.
•These coolers have lower recuperation efficiency,
occupy more space & need more cooling air.
•The latest high efficiency coolers, have higher
recuperation efficiency, need lesser cooling air, are
compact in size and has lower radiation loss.
•Typically, the high efficiency cooler can reduce the
thermal energy consumption by 30 - 40 kCal/kg of
clinker.

45. Conventional Cooler vs high efficiency
Cooler

46. Refractory lining - kiln, pre-heaters
& coolers
•The refractory lining in the kiln, pre-heaters & coolers are very vital for
conserving thermal energy within the system and protect the outside
supporting metallic part.
•The older systems used to utilise all along only high alumina bricks.
The present trend is to install basic bricks in the kiln, which have
comparatively higher life (more than twice ) and lower consumption.
•Since this reduces the stoppages of the plant, the runnability of the
plant improves, resulting in lower thermal and electrical energy
consumption.
•In all other areas of the pre-heaters and the cooler, it is
recommended to install alumina bricks (30 - 40%), preceded by
insulation bricks.
•Refractory choice is also a function of insulating qualities of the brick
and the ability to develop and maintain a coating.
•The coating helps to reduce heat losses and to protect the burning
zone refractory bricks.

47. Use of Waste-Derived Fuels
•Waste fuels can be substituted for traditional commercial fuels in the
kiln.
•In 1999 tires accounted for almost 5% of total fuel inputs in the U.S.
cement industry, while all wastes total about 17% of all fuel inputs.
•The trend towards increased waste use will likely increase after
successful tests with different wastes in Europe and North America.
•New waste streams include carpet and plastic wastes, filter cake,
paint residue and (dewatered) sewage sludge
•Cement kilns also use hazardous wastes. Since the early 1990’s
cement kilns burn annually almost 1 million tons of hazardous waste
(CKRC, 2002).
•The revenues from waste intake have helped to reduce the
production costs of all waste-burning cement kilns,
•The high temperatures and long residence times in the kiln destroy
virtually all organic compounds, while efficient dust filters may
reduce any potential emissions to safe levels

49. Waste fuel firing in Kiln

50. Waste fuel Firing in Precalciner

51. Alternate fuel firing

52. PREHEATER EXIT GAS
300 - 400
o
C, 180 - 250 KCAL/KG
COOLER EXIT GAS
200 - 300
o
C, 80 - 130 KCAL/KG

53. Typical Thermal Energy Consumption
for different types of plants
Kiln process Heat consumption (kcal per kg
clinker)
Wet process with internals 1400-1500
Long dry process with internals 1100
1-stage cyclone preheater 1000
2-stage cyclone preheater 900
4-stage cyclone preheater 800
4-stage cyclone preheater plus calciner 750
5- stage preheater plus calciner plus high
efficiency cooler
720
6-stage preheater plus calciner plus high
efficiency cooler
less than 700

54. Energy balance (OPC)

55. Heat requirement in dry kiln with
suspension preheater (KJ/kg clinker)

56. Heat balance results of a kiln