North American Review

Balancing the Costs
Armin Möck, Lechler GmbH, Germany,
explains why a spillback nozzle injection
system can create energy cost savings.
Introduction

In the second half of the 20th century, nearly all dust
emissions from cement, lime, steel, power and otherplants were reduced by installing dust filtration systems.
The most efficient and common way to separate the dust
from the hot gas at that time was to use electrostatic
precipitators (ESPs).
Many hundreds of ESPs have been built worldwide
in recent decades. For high dust separation efficiency
in the ESP, a certain level of gas humidity is essential.
Gas conditioning towers (GCT) were introduced for thispurpose, where water is injected and evaporated to cool
down the gas and to increase humidity.
Today, most of the ESPs built in this period are stillin existence worldwide, but some were replaced bybaghouse filters. Today, the most popular method ofcleaning gases is to use baghouse filters.
Nevertheless, all filter systems still need the GCT tocool down the hot gases from the cement kiln. Due tothe high dust loads in the hot gas, it is essential that theinjected water evaporates completely inside the GCT,
which is a large vertical tube with a downwards gas flow.
If the evaporation is not completed at the lower end of theGCT, build-ups of dust/mud will occur and will lead to theneed for plant stoppages and/or high maintenance costs.
Due to the limited space available in cement plants
and also the high price of steel, small GCTs are required
throughout the market. A small GCT can only be achievedby producing a fine water spray that evaporates faster thanlarge droplets. There are two main technologies on themarket to generate a fine water spray:l Spillback nozzles (also known as flowback nozzles,
hydraulic nozzles): these are systems that generate fine
droplet distribution with a high water pressure.
l Air atomising nozzles (otherwise called twin-fluid
nozzle, bi-fluid nozzles): these systems use lowpressurised water and compressed air as the mainenergy source to generate fine droplets.

Spillback systems (SB)SB systems operate at 35 bar at nozzle level. The main
operating features of this nozzle is a constant feed
pressure and a backflow line to the tank, where the control

Figure 1. GCT.

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North American Review
from the intense internal mixing of the compressed air andthe pre-atomised water.
The fine droplet spray exits the nozzle throughseveral openings to generate a specific spray angle. The
outstanding advantage of AA nozzles is the fact that thedroplet size can be changed (even during operation) by
varying the specific air consumption.
In the USA, AA-nozzles dominate the market becauseof the high presence of suppliers and low energy costs. In
all other parts of the world, the SB nozzle is still the mostused component for this application. The purpose of thisarticle is to focus on a comparison of operating costs ofthese two systems.

Case study

Figure 2. GCT principle.

valve is installed. In this way, the nozzle generates afine water spray combined with a flow rate turndown ratio of at least 10:1. The droplet size remains
unchanged over the turn-down ratio.
GCTs can be equipped with single nozzle lancesor, for larger diameters, with clusterhead lances toaccomplish a sufficient coverage of the GCTcross-section. Today, 75 - 80% of GCTs worldwide
are still equipped with the SB nozzles, which were
the standard nozzles supplied by most filter OEM
companies for many years.
The main disadvantage of wear caused by thehigh pressure no longer exists, since the use of wear
resistant materials like tungsten carbide for the wear
part is now common.

Air atomising system (AA)AA-systems run at a much lower water pressure of
approximately 5 - 6 bar. The fine water spray results

Table 1. Running cost evaluation
Energy costs

0.045 US$/kWh

Time frame

10 years (7500 hpa)

Mill operation

90%

Direct operation

10%

In past decades, new investments in cement plants weremainly decided on a return-of-investment-period of one to
two years. The attitude to focus
only on the size of the GCT islimiting the view for the optimum
solution.
With the unexpected andrapid increase in the price ofelectric energy in recent years,
the big cement producers are
now comparing investment,
maintenance and operating costs
over a period of 10 - 12 years
to find the most cost efficienttechnology.
As shown in Table 1, theinvestment costs for AA-systems
are higher than for SB-systems
because of the additional
investment for redundantFigure 3. Spillback nozzle
compressors. Nowadays, theprinciple.
operating costs are the main
parameter used to decide
between the SB or AA system. The
comparison made in this article is basedon a case study at an existing cement
plant in North America.

Operating cost evaluation

Spillback power consumption

AA system power consumption

Mill operation

128 kW

Mill operation

447 kW

Direct operation

108 kW

Direct operation

652 kW

Running costs
Spillback system

US$425 250 (total in 10 years)

Twin-fluid system

US$1 577 813 (total in 10 years)

Running cost difference

US$1 152 563

New compressors

US$350 000(estimate)

Total cost difference

US$1 502 563

Investment costs
SBAA

110%

100%

-plus compressors

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At the customer’s cement plant, thetemperature after the 4-stage preheating
tower is 370 - 450 °C. The raw millruns for 90% of the time. Based on thehigh humidity of the raw material, thelocation of the raw mill downstreamof the GCT (4.9 m dia., active height28 m) and the capacity of the mill, therequired temperature after the GCT at milloperation is 220 °C.
During direct operation (raw mill off),
the GCT outlet temperature reached170 - 180 °C. Tests to reduce the outlet
temperature resulted in mud in the GCT
dust hopper. The GCT is equippedwith AA-nozzles, and the compressed
air is supplied by a 350 hp (=260 kW)
compressor that runs constantly, evenNorth American Review
that offers both injection systems (the results of which
are published in Table 1). The worksheet enables plantoperators to enter variables such as:l Energy costs.
l Number of years to focus on.
l Number of operating hours per year.

Figure 4. Spillback clusterhead lance.

l Percentage of mill and direct operation.
It also allows individual local figures to be entered toget a realistic picture of the situation.
Based on the savings of US$1.15 million, plus
the saving of the unnecessary investment in thecompressors, it became clear that any investment
in a new GCT would be paid off quickly and would
save a lot more money over the years to come.
Taking into account that the life cycle of a GCT and
an injection system is at least 20 years, and theincrease of costs for electricalenergy are unpredictable for the
coming decades, the real savings
might even be higher.

during mill operation. A second
identical back-up-compressor isalso installed. Maintenance andrunning costs for the compressors
are high, and the life cycle of thecompressors is limited.
Facing lower emission limits
for mercury in the near future,
the outlet temperature after GCT
has to be dropped to 130 °C.
Simultaneously, clinker production
will be increased from 3200
to 3800 tpd. This leads to anincreased injected water quantity
from 20 to 56 m3/h.
This fact, plus the company’s
experience with the existing GCT
and its limits, as well as the newFigure 5. Air atomising spray nozzle
gas quantities and the lower outlet
principle.
temperature, forced the customer toimprove the performance of the gas
cooling process.
At first glance, the simplest solution would be toinject more water into the existing GCT. This is technicallypossible, but the relative water load per area (m2/hwater per m2 GCT cross-section) would then be at theupper limit. Two parameters indicate a severely higher
compressed air necessity:

Conclusion

It became clear in the above
and specific comparison of theoperating costs that the SBsystem is in favour. However,
AA-nozzles are still the only
possible solution for many
projects where a finer water
spray is needed, where the GCTexists and an extension is notpossible (no space, foundation
limits, etc.) or because the raw
mill runs 98% of the time and thehigh compressed air consumption
during 2% running time isacceptable.

l Finer water spray to overcome the known process
limits regarding the wet bottom.
l A larger quantity of water required to cool down theincreased gas quantity to a lower outlet temperature.
The investment costs for a water injection systemthat comprises nozzle lances, a pump and control skid,a PLC, and ring mains is comparable, and knowing theadditional costs for larger compressors, the main focus
was the comparison of the operating costs. The goal wasto find out if the difference of the operating cost within
a 10 year period between the SB and the AA injection
systems and the cancellation of the compressors would
be equal to purchasing a new and larger GCT.
The cement plant generated a flexible cost
evaluation worksheet using Excel, with a nozzle supplier

Figure 6. Pump skid.

Only the long term comparison of all costs based on
the individual parameters of each cement plant (% milloperation, raw material humidity, energy prices etc.) will
give the cement companies a good position to decide
for the best system. _________________________________l
Reprinted from April 2007 World CemenT
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