Waste Heat Recovery in the Cement sector

1. GLOBAL CEMENT: WASTE HEAT RECOVERY
Guillaume Jeangros, AQYLON
Waste heat recovery in the cement sector
through the organic Rankine cycle
In a typical cement production process, only
around 53% of the thermal energy used is actually
absorbed by the process. Of the 47% wasted heat,
around 35% could be recovered to generate up to
30% of the electrical needs of the factory.
Although waste heat recovery (WHR) is a proven
technology, it has not been widely implemented to
date, except in China. According to the International
Finance Corporation (IFC), there are around 850
cement-industry WHR installations worldwide, with
739 in China. The vast bulk of these use steam tur-
bine technology.
For the cement industry, WHR is very important,
as it can reduce the operating costs and increase
earnings before interest, tax, depreciation and am-
ortisation (EBITDA) by 10-15%. Electrical power
accounts for around 25% of operating costs. WHR
reduces the energy costs of the plant, reduces the
reliance on grid power supply, mitigates the impact
of future electrical price increases, increases plant
reliability, improves margin and market competitive-
ness, reduces CO2 emissions and decreases the plant’s
environmental footprint.
According to IFC, the market potential for WHR
in the cement industry in the 11 countries with the
most potential is estimated at US$5bn or 2GWe. India
accounts for US$1.4bn of this potential by itself. The
energy efficiency in the Indian cement industry is
already high, but the roadmap guided by the Cement
Sustainability Initiative (CSI) is very ambitious. It
suggests a target of reducing emissions by 45% by
2050. Turkey, Vietnam, Mexico, Egypt and Brazil are
also very interesting markets in terms of WHR po-
tential. In all of these countries, WHR is a major lever
to enable such reduction. See the potential extent of
future installations in Figure 1.
Electricity generation through WHR
Even for an optimised cement line, significant heat
losses occur. The CSI indicates that, in 2011, the
thermal energy consumed to produce clinker was
3610kJ/kg. Heat balance shows that almost 35% of
total heat consumed by the clinker could be recov-
ered - See Table 1.
Direct use of the waste heat, for raw material
drying purposes for example, is more efficient than
the transfer of heat into electricity. However, in most
plants, this is often impossible and the heat is lost
to the atmosphere. The >300kWh/t of wasted heat
corresponds to around US$1.35m of annual losses,
assuming a typical 2000t/day kiln working 300 days
per year, which fires coal with a calorific value of
7MWh/t at a coal price of US$45/t. Electricity gen-
eration is the best means to tackle these losses.
Issues with steam turbine heat recovery
The most common system for WHR in the cement
sector is the steam Rankine cycle (SRC), which is
used in 96.5% of installations. Despite wide avail-
ability and a relatively low cost basis in US$/W,
steam turbines were not designed for low-grade
heat and today face a number of issues.
22 GlobalCementMagazineDecember2015 www.GlobalCement.com
Power plants based on organic Rankine cycle (ORC) technology have been widely used in
the past, in particular for biomass and geothermal energy sources. However, steam turbine
technology has been preferred over ORC turbine technology for waste heat recovery
(WHR) applications in cement plants. Recent improvements in cement production are
making the conventional steam cycle less attractive, however, giving room for WHR via
ORC-based systems.
Below - Figure 1: Estimated
realised and remaining technical
potential and investment in
cement sectorWHR deployment.
Source: IFC.
kJ/kg %
Theoretical heat consumed 1913 53
Waste heat at preheater level 758 21
Exhaust air heat at cooler level 469 13
Wall heat losses 397 11
Heat discharged in clinker 72 2
TOTAL 3610 100
India
Turkey
Vietnam
Mexico
Egypt
Thailand
Brazil
Pakistan
Philippines
Nigeria
South Africa
0 100 200 300 400 500 600 700 800 900
1400
400
480
490
490
90
430
160
170
170
200
Total potential (MWe)
Requiredinvestment(US$m)
Far left -Table 1: Typical
cement plant heat balance.
Source: AQYLON.

2. Firstly, due to continuous improvements in
cement production technology and grate cooler tech-
nology, the gas temperature, both at preheater and
cooler levels, has dropped dramatically in the past
decade. At the same time, the potential efficiencies of
the conventional steam cycles have dropped signifi-
cantly. To operate optimally, the steam turbines need
at least 280°C, but lower pressures and temperatures
lead to condensation in the turbine, causing erosion
of the blades.
To compensate for the low temperature of the
gases, the temperature has, in some cases, been
raised by burning additional fuel, increasing the fuel
cost to dramatic levels and introducing additional
inefficiencies. Also, in order to increase the waste gas
temperature, the recovery point is set at the middle of
the air cooler exhaust, resulting in WHR of only 50%
of the available power in some cases.
One of the main issues for the steam turbine is
to overcome the varying temperature of the exhaust
gas, for example ranging from 265°C to 500°C at the
Jordan Qatrana Cement plant near Amman. Indeed,
at partial load, the small steam turbines are both
inefficient and unstable as no steam turbine under
10MW was ever specifically designed to overcome
flow fluctuations.
In SRC systems, the condensing pressure is below
atmospheric pressure, resulting in the need for com-
plex and expensive vacuum and purging equipment.
This and several of the other factors listed above
mean that maintenance costs are often increased by
the requirement to have a 24/7 operator present.
The steam turbine’s requirement for a cooling
water tower is also an issue, as there is always a risk
of blowdown. Cooling towers also need chemical
pre-treatment as well as make-up water, leading
to further increases in maintenance bills. In these
parts of the plant, a shutdown can bring on the risk
of freezing in some countries and the system can be
compromised by a failure of the water cooling tower,
regardless of the weather.
The ORC solution
The organic Rankine cycle (ORC) can be an answer
to many of the issues listed above. Organic fluids have
lower boiling points than water. As a consequence,
vapour from an organic fluid can be generated at a
lower temperature.
ORC systems can therefore be used for lower tem-
perature heat sources, down as far as just 80°C. The
condensing pressure is above atmospheric pressure,
so there is no need for a vacuum system. In addition,
ORC fluids have extremely low freezing tempera-
tures. Higher organic fluid density makes the design
of the turbine and piping small compared to those
required of SCR turbines.
AQYLON was established in 2009. It began pro-
ducing power units of over 100kW for heat recovery
on small biogas engines in France before entering
the WHR market, with solutions of 0.5-5MW. Since
then, the company has developed a specific tool
to design a whole WHR unit for any kind of heat
– single source or multi-source. The proprietary
software is able to match the ORC module design
to the specific characteristics of the heat source by:
1. Selecting the best fluid option from a database of
over 15,000 fluids; 2. Optimising the thermodynamic
cycle; 3. Designing a custom turbine rotor, stator,
and nozzle design for excellent isentropic efficiency;
4. Designing the heat exchanger on the hot source;
5. Designing the best available cold source - direct
aerocondensation or indirect condensation through
an air chiller.
ORC WHR in the cement sector
For industrial WHR applications (cement and other
industrial sectors) standard products from 500kW
to 5MW have been designed for three classes of
temperatures: H for high temperature (>260°C),
M for intermediate temperature (120-220°C), L for
low temperature (90-120°C). Each class uses its own
working fluid: hydrocarbon, siloxane or refrigerant.
All are available in standard 20ft and 40ft contain-
ers, ensuring rapid implementation and low erection
cost. The systems have >98% availability, isentropic
efficiency of the turbine of >85% and retain efficiency
at partial load levels, even at very low partial loads.
For cement plants, the thermal heat for an ORC
installation could be recovered from both the clinker
cooler and the preheater. For example, exhaust gas
from the preheater could be cooled from 330°C to
180°C and exhaust gas from the cooler from 275°C
down to 180°C, to feed a first heat exchanger. The
heat is transferred to the ORC module by a closed
thermal oil loop, usually Therminol 66. The ORC
module uses this thermal oil to evaporate a hydro-
carbon fluid: toluene, cyclopentane or pentane,
depending on the temperature of the exhaust gases.
In Figure 2, the organic fluid is vaporised in the
evaporator (1) and then supplied to the ORC turbine
(2) that drives the generator (3), which produces
clean and reliable power. The vaporised fluid enters
the regenerator (4) to give its remaining energy to the
liquid organic fluid. The steam is then condensed in
a condenser (5) which is supplied with cooling water.
The condensed fluid is then pumped (6) into the re-
generator (4) and finally into the evaporator (1) to
close the circuit.
Today, there are only a dozen ORC turbines in-
stalled in the world. Ormat has installed a 1.2MW
unit in Germany and a 4.8MW unit in India; Tur-
boden has a 2MW unit in Morocco, a 4MW unit in
Romania, a 5MW in Slovakia and a 7MW in USA;
ABB has two 2MW units in Switzerland.
Factors to consider in cement plants
Each cement plant has its specific operating condi-
tions, which need to be taken into account when
GLOBAL CEMENT: WASTE HEAT RECOVERY
www.GlobalCement.com GlobalCementMagazineDecember2015 23

3. GLOBAL CEMENT: WASTE HEAT RECOVERY
24 GlobalCementMagazineDecember2015 www.GlobalCement.com
looking for a ‘final answer,’ as to whether or not any
particular WHR project is economically viable. The
following parameters need to be considered, includ-
ing: Exhaust gas temperature; Exhaust gas flow rate
and flow fluctuation; Dust content; Local cost of elec-
tricity; Other factors, such as on-site space, distance
between heat source and ORC module, ambient tem-
perature, among others.
In general terms, higher temperatures are pre-
ferred as well as higher flow rates. This leads to larger
ORC power units, which are generally cheaper in
terms of US$/W than smaller units.
Gases with high dust content imply expensive heat
exchangers. The heat source recovery point needs to
be chosen, either before or after the electrostatic pre-
cipitator or the bag filter.
The efficiency of the installation will depend on
the temperature differential between evaporation
and condensation of the ORC fluid, themselves
determined by the temperature of the hot and cold
sources.
Installed cost could vary widely, as well as the
pinch points across the heat exchangers in the cycle.
Typically, prices will be US$2-4/W. This is much
higher than a reciprocating engine but fuel costs are
zero and maintenance costs are very low. This results
in an electricity cost per kWh that could drop sharply
below that of other power generation methods.
With electricity prices around US$100/MWh
(including power generation, delivery and taxes), the
payback time of an ORC module is usually around
three years - See Tables 2-6.
Tables 2 - 6: Calculation of
potential savings and payback
from a typical cement plant
ORCWHR installation.
Source: AQYLON.
Clinker cooler (Table 2)
Flow rate 75,000Nm­3/hr
Temperature input 375°C
Temperature output 150°C
Thermal power available 6.4MWth
Preaheater (Table 3)
Flow rate 150,000Nm­3/hr
Temperature input 300°C
Temperature output 200°C
Thermal power available 5.7MWth
Thermal oil loop (Table 4)
Fluid Therminol 66
Temperature input 290°C
Temperature output 145°C
Thermal power recovered 12.1MWth
ORC on thermal oil loop (Table 5)
Gross efficiency 21.3%
Gross ORC power 2.6MW
Net efficiency @ 20°C 20.4%
ORC Power 2.5MW
Savings and payback (Table 6)
Total Power 2500KWe
Electricity price US$100/MWhr
Annual working hours 8000hr
Annual revenue US$2m
ORC Capital expenditure US$6m
US$/W 2.4
Operating expenditure US$100,000
Payback time 3 years
ORCTurbine (2) and
Turbo Generator (3)
Regenerator - Condenser (4/5)
Water
Pump (6)
Evaporator (1)
Thermal Oil
Right - Figure 2: Schematic
of 2MWe ORCWHR system from
AQYLON.The system is 5.2m
high, 2.5m wide and 18.2m
long. All modules are based on
20ft and 40ft isoframes.
Source: AQYLON.
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