ENERGY CONSUMPTION
by Lawrie Evans, EmCem Ltd, UK
When it comes to achieving the best energy consumption, what
are the key factors a cement producer needs to address? In this
article, extracted from the newly published Cement Plant
Environmental Handbook (Second Edition), Lawrie Evans presents
a masterclass in understanding and optimising cement plant
energy consumption.
A
s control of sources, generation,
distribution and consumption of
energy is central to many current
world issues, controlling the industry’s
energy footprint is a matter of intense
interest to governments. This is recognised
in such initiatives as ISO 50001, the World
Business Council for Sustainable
Development’s Cement Sustainability
Initiative, Energy Star in the USA, PAT in
India and CO2 taxes/trading in Europe
and in other countries.
For the cement industry, there are three
main drivers to energy consumption:
• electrical power
• fuel
• customer demand for high-strength
products that require a significant
proportion of high-energy clinker as a
component.
For the producer, these factors have a
significant influence on cost competitiveness, usually accounting for over 50 per
cent of total production costs, so that
accurately and continuously monitoring
energy usage must be a way of life for any
producer’s technical team. The introduction of CO2 taxes in Europe and elsewhere
adds a further twist to the story. For major
groups, especially, decisions made in
balancing maintenance, investments,
operations and purchasing requirements all
have to take into account the impact on
their energy footprint.
Figure 1: electrical energy (MWh) index across 14 countries, 2013
scope. Most countries still have power
generation/distribution systems that are
effective monopolies and the cement
producer’s cost control capability is
usually limited to selecting the
appropriate contract and taking
opportunities offered in lower-cost offpeak power tariffs, where they exist.
Figure 1 illustrates the wide variation in
the cost of power across 14 countries.
The average country cost of electrical
power at an industrial level varies
enormously. When the added
complexity of on and off-peak power
costs, interruption clauses, supply
charges versus energy charges, etc., are
added, the evaluation of the benefits of
energy saving investment can become
very complex. Typical cement plant
power costs can range from EUR39 to
EUR170/MWh.
Mill designs
The most important first step in
controlling energy consumption is to be
aware of the relative importance of the
process areas where most energy is
consumed. Figure 2 shows a typical
breakdown of electrical energy
consumption at a cement plant. The
most obvious area for attention is that
Global scene
Globally a cement major such as
Italcementi consumes annually some
6000GWh of power and
35,500,000Gcal of heat for a total of
5Mtpe. This is the same total energy as
consumed by approximately 1.6m
Italians or 0.6m Americans per year. For
fuel-related energy costs, the worldwide
industry has largely moved to efficient
preheater/pre-calciner processes and
has found many options to switch to
cheaper fuels, with the global drive to
alternative fuels still proceeding. For
electrical energy, options to reduce
unitary costs are much more limited in
Figure 2: breakdown of electrical energy consumption at a typical cement plant
FEBRUARY 2015 ICR
ENERGY CONSUMPTION
of grinding, both raw and cement. In
either case, grinding is, by design, a very
inefficient process.
The ball mill has been the industry’s
workhorse for over a century and
despite its estimated meagre four per
cent efficiency, little has changed over
the years other than increases in the
wear resistance of mill internals and the
scale of the equipment. The addition of
closed circuiting and progressively
higher efficiency separators has
improved cement product quality and
produced higher outputs for a given mill
size, but the case for adding or
upgrading separators on energy saving
alone has proved to be poor, unless the
products are >4000Blaine. Starting from
the 1970s, a new generation of mills
appeared. Vertical mills (see Figure 3)
were common for solid fuel grinding,
generally with spring-loaded rollers. The
principle of the new generation of
vertical mill was to direct higher
pressure from the grinding element to
the material bed using hydraulic
systems. From this approach the roller
press, CKP (pre-grind vertical rollers)
and Horomill™ all developed.
Raw milling
The gas-swept vertical mill quickly
became the raw mill of choice. Grinding
energy was approximately 50 per cent
of the ball mill and the drying
capabilities allowed direct processing of
materials of up to 20 per cent moisture
content. The main energy issue was the
high-power consumption of mill fans,
with pressure drops of 100mbar not
uncommon with high nozzle ring
velocities (>70m/s) and internal mill
circulating loads of >1000 per cent.
Manufacturers have countered this
generally satisfactorily with pressure
drops reduced by lower nozzle ring
velocities and the addition of external
Figure 3: vertical roller mill
ICR FEBRUARY 2015
spillage elevator recirculation systems
plus higher-efficiency separators.
Better seal designs for mill roller
assemblies and pull rods have reduced
the inevitable in leaking air issue and its
impact on power consumption.
However, it remains a design where
issues of wear and reliability are more
challenging than for ball mills, and these
issues have not diminished with
increased scale. For raw grinding with
relatively dry raw materials, the
combination of the roller press and V
separator is a viable alternative with far
lower mill fan power.
grinding power reduction is countered
by the very high power required by mill
fans. In addition, the absence of the
heat generated in a ball mill and the
high volume of air required by the
vertical mill have required the provision
of waste heat from cooler exhausts
and/or auxiliary furnaces to dry raw
materials and achieve a limited
dehydration of gypsum.
A typical comparison of three
competing technologies is given in Table
1, demonstrating that an efficient ball
mill/third-generation separator,
CKP/ball mill/third-generation separator
Table 1: grinding technology comparison – power use (mill + auxiliaries)
Plant 1 –
Closed-circuit
ball mill with
thirdgeneration
Plant 2 – CKP
(closed circuit
with thirdgeneration
separator)
Plant 3 –
Vertical mill
(kWh/t cement)
CPJ 35
30.8
29.8
30.4
CPJ 45
32.5
30.3
34.8
CPA (J) 55
44.3
–
–
Cement grinding
For cement grinding, the technology
development away from ball mills has
taken a different route. The
development of roller presses in the
1980s took advantage of the benefits of
higher-pressure grinding and many
presses were retrofitted to ball mills as
pre-grinders. The main benefit was seen
at lower Blaines as the first generation
of presses suffered from stability
problems when attempts were made to
grind more finely by recirculating
separator rejects. These problems are
now largely resolved and the
combination of a V and third-generation
dynamic classifier separators
together with a roller press can
produce finished cement with
high energy efficiency.
The Horomill and CKP systems
have also enjoyed some market
success and have provided good
energy efficiency levels compared
to ball mills. The vertical mill
option has been slower to enter
the cement grinding market.
Grinding bed stability problems
offered a challenge which the
major manufacturers battled with,
until finally a significant number
of mills began to be installed in
the late 1990s, and this has
multiplied in the past decade.
However, in pure energy
efficiency terms, the benefit of
and vertical mill on a typical 4000Blaine
limestone cement show little overall
difference in energy consumption.
Considering the higher capital cost, and
more demanding maintenance and
operating regime, there is no clear
energy case to favour some of the
modern variants.
Other mill debates
Even for solid fuel grinding, there has
been a minor trend back to ball mills. This
is most evident for pet-coke grinding,
where the demand for very low residues,
and the very hard and sometimes
abrasive nature of high-sulphur cokes has
resulted in ball mill selection.
Many of the grinding design issues,
which are still under debate, are usually
very clear in other areas of process
selection:
• high-efficiency process fans and lowpressure drop preheaters
• adequately-sized bag filters for the
main exhaust to avoid high pressure
drops and poor bag life
• avoidance of pneumatic transport
systems
• low-energy raw meal homogenisation
silos.
The main continued discussions are
those of two- or three-fan systems for
the raw mill/kiln or single filter for kiln
and cooler, precipitator or bag filter for
the cooler exhaust and two or three tier
ENERGY CONSUMPTION
kiln. For a bag filter on a separate cooler
the main equipment energy efficiency
issue is the air-to-air heat exchanger, but
this is often substituted with a water
spray in the cooler or more recently, by
using a ceramic filter capable of
operating at above 400°C.
Finally, in design terms, the most
difficult decision is to avoid overdesign by
applying too many safety factors. Postcommissioning audits often uncover a
high contribution to poor energy efficiency
from under-run equipment operating
where it cannot perform efficiently.
In normal operations maintenance also
plays a major part in ensuring energy
efficiency. The impact of poor plant
reliability upon overall electrical energy
consumption is often under-estimated. In
the kiln area, 100 short/medium stops (30
minutes to eight hours) per year can cost
up to 5kWh/t clinker. The avoidance of in
leaking air, correct alignment of motors,
stopping compressed air leaks, etc., are all
part of the value of good maintenance.
In the key area of grinding there are
important factors to control. For ball
mills, ball charge level, lining and
diaphragm condition must be monitored
and maintained in near-optimum
condition. Mill stops, defined as mill
motor off, and measured by mean time
between failures (MTBF), are frequently
poorly recorded and the resolution of
underlying issues is frequently not
addressed.
Instability, where ball mill feed is
stopped and the mill ground out, is also
infrequently recorded or acted upon.
When it comes to mill control, operators
rarely concentrate on pushing mill
production when the kiln is regarded as
the key. Expert systems on mills should
be universal and well-tuned.
Grinding aids can give benefits of 515 per cent in production but need to be
continuously evaluated for cost
effectiveness. Unfortunately, their cost
has risen more rapidly than the cost of
energy in recent years and the economic
balance has to be re-evaluated. The
benefit of aids on cement flowability has
to be considered, along with the added
scope for reduction of cement clinker
content with some modern additives.
Correct timing on the maintenance of a
first chamber cement mill lining and the
successful implementation of an expert
system on a cement mill both offer
benefits in terms of power consumption
(see case studies panel). Accurate process
measurements are also key to energy
saving opportunities. Air compressors are
another area for attention. Often, these
Table 2: energy consumption comparison of fixed-speed vs variablespeed plant air compressors
Previous situation with three fixed-speed compressors
Energy consumed (kWh)
106,319
Measurement time (h)
168
Previous situation with two fixed-speed compressors
Energy consumed (kWh)
97,725
Measurement time (h)
168
Saving (%)
8.1
Annual saving in energy
consumption (kWh)
445,479
Annual cost saving (EUR)
44,548
are multiple units operating on a cycle of
on- and off-load. Replacement of one (of
three) with a variable-speed type (see
Table 2) can provide rapid payback. Even
lighting and buildings offer excellent
opportunities for power savings. Table 3
shows the 40-80 per cent energy savings
that can be achieved by simply replacing
old lighting systems. Buildings such as
the new Italcementi Group Research and
Innovation Centre (i.lab) in Bergamo,
Italy, demonstrate that good building
design creates significant savings.
Power generation
A major change has occurred in the
last 20 years in the area of in-house
generation of electrical energy by
cement manufacturers, most
significantly using waste heat recovery
(WHR) from the pyro-processing line.
Figure 4 shows the areas suited to heat
recovery for power generation, and
WHR technology is already applied to
preheater and cooler exhausts.
The modern technology originated in
Japan in the 1980s, where high power
prices and large-scale operations
combined to produce useful economic
returns, with most applications using
steam boilers at the preheater exhaust.
Little further development happened
outside Japan until the turn of the
century, when a combination of lower
capital cost, Chinese equipment, and
the idea to improve recovery by splitting
cooler exhausts into higher and lower
temperature streams combined to offer
the paybacks necessary for the
technology to expand, first inside China
and then beyond.
The results of WHR have been
impressive, e.g., with the 19MW net
achieved from a combined installation on
two five-stage pre-calciner kilns
(5500tpd and 7500tpd) in Thailand being
typical. Options for the technology are
evolving with other thermodynamic
cycles being applied:
Figure 4: opportunities for waste heat recovery from the kiln and cooler
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ENERGY CONSUMPTION
• steam Rankine cycle with various
enhancements – the most widely
applied technology
• organic Rankine cycle – a variety of
organic fluids applied and favoured at
• lower gas temperatures
• Kalina (ammonia/water) cycle
• supercritical CO2 cycle.
There are also further developments
which can increase the power recovered,
including recycling the lower temperature
cooler exhaust, meal bypassing preheater
stages to boost exit temperatures and the
use of alternative fuels and excess air,
also to boost preheater exit temperature
and energy recovery. Other options for
power generation can use the land
owned by the cement plant for raw
material reserves. These include wind
farms photovoltaics, concentrated solar
panels or growing and burning biomass
either to boost power in a WHR system
or for use in an internal, stand-alone
power generation plant.
It is clear that the issues surrounding
optimum electrical energy efficiency for a
cement plant continue to be an active
and exciting area for future
development.
ICR FEBRUARY 2015