This document discusses how oxygen enrichment can help cement kilns increase production and usage of alternative fuels. It provides the following key points: 1) Oxygen enrichment can increase cement production by up to 25% and lower fuel costs by up to 5% while improving kiln stability and clinker quality. 2) Properly injected oxygen allows cement kilns to stably combust alternative fuels that have lower heating values and improve combustion, helping offset issues they may cause. 3) Oxygen injection systems for cement kilns can typically be installed within 3 months and require little capital while providing operational and cost benefits soon after startup.

1. T
his poses operating challenges such
as maintaining production levels and
product quality. The strategic use
of oxygen provides a cost-effective means
to combine high alternate fuel use targets
with excellent operational results.
Oxygen enrichment for
production increase
The benefits of oxygen enrichment in
cement kilns have been well documented.
Praxair’s own history with cement kiln
enrichment spans several decades with
demonstrated benefits to customers
ranging from production increases of up to
25 per cent, specific fuel savings of up to
five per cent, reduced specific dust losses,
and improved kiln stability as evidenced
by clinker quality, ring formation and kiln
coating. Such favourable results combined
with low investment costs, and an easy
and quick implementation may result in
high return on overall project investment.
Figure 1 shows kiln sizes and oxygen
flow rates used successfully in various
installations over the last 50 years.
Praxair’s long-term experience with this
application suggests that a yield of 3-3.5t
of incremental clinker per 1t of oxygen can
be achieved at most plants, if the flue gas
system capacity is preventing increase of
production and the plant has no further
bottlenecks towards processing more
material. Oxygen injection provides a high
degree of flexibility that can maximise
profits when cement market conditions are
favourable.
The use of oxygen for increased
production tends only to be justified in
periods of high industry kiln capacity
utilisation where high margins associated
with the incremental product cover the
cost of oxygen and enable the desired
return on the project investment. With
the exception of retiring older inefficient
clinker production lines and increasing
the production with oxygen injection at
modern low-cost facilities, present day
market conditions in most parts of the
world are generally not supportive of
oxygen use for a production increase.
Oxygen for increased
alternate fuel utilisation
Pressures to lower fuel costs are ever
present for the global cement industry.
Cement producers have been very
successful in increasing alternate fuel
utilisation to meet cost reduction goals.
In many European markets, alternate
fuels have replaced 50-70 per cent of
the fuel input to the plant. The use of
high alternate fuel rates in the calciner
is common and some plants have been
successful with the near complete
replacement of fossil fuels. However, the
properties of these fuels have posed new
challenges for the pyroprocessing section
in the kiln.
Preheater kilns that require firing most,
if not all fuels, through the main burner
are especially prone to issues associated
with expanded use of alternate fuels such
as a cool burning zone, an unstable and/or
long flame, insufficient burnout and high
process variability. Such negative outcomes
may ultimately result in off-spec clinker
and poor kiln utilisation, with the extent of
deterioration in performance dependent
on the specifics of the fuel mixture and
type of combustion equipment employed.
Oxygen addition at appropriate flow
rates and in the right location improves
the combustion process and allows stable
and consistent combustion of fuels with
low-heating value. Oxygen in the flame
root provides for more rapid heat-up, fuel
devolatilisation and fuel ignition. Thus,
IBenefitsofoxygeninjectionby Stefan Laux,
Jeffrey Mocsari, Markus Webel
and Stephen Hope
Praxair Inc, USA
The rising costs of fossil fuels and sustainability goals have driven
cement producers towards greater secondary fuel usage. However,
these alternatives typically have lower heating values and combustion
characteristics, which negatively impact the clinker burning process.
Figure 1: Praxair experience with
oxygen injection into cement kilns
White cement
Portland cement
Kiln capacity tpd*
Oxygenflowtpd
*Production level prior to oxygen use
Praxair is the largest industrial gases company in North and South America,
and one of the biggest worldwide, with 2007 sales of US$9.4bn. The
company produces, sells and distributes atmospheric and process gases,
and high-performance surface coatings. Praxair products, services and
technologies bring productivity and environmental benefits to a wide variety
of industries, including aerospace, chemicals, food and beverage, electronics,
energy, healthcare, manufacturing, metals and others.
About Praxair
OXYGEN ENRICHMENT
66 ICR JANUARY 2009

2. the improved combustion conditions can
compensate for flame cooling due to
water or ash. In addition, the available
residence time in the flame is utilised more
effectively which enhances fuel burnout.
Furthermore, the reduced flue-gas volume
that results from oxygen combustion
can compensate for the water ballast
that impacts the capacity of the flue gas
system. Praxair’s experience indicates that
oxygen can often promote better kiln
coating and can actually extend refractory
lining life as the temperature distribution in
the burning zone is favourably affected by
the additional flame stability.
Praxair’s experience in the cement
industry includes the use of liquid and
solid alternate fuels at the main burner.
The operational experience in this regard
spans TDF, RDF or ‘fluff’, liquid waste
and numerous other post consumer/post
industrial waste streams with variable
calorific value and geometry (see Figure 2
for a typical fuel that benefits from oxygen
use). The recovery of lost production due
to alternate fuel use is additional value
that oxygen can provide.
The injection of oxygen typically allows
increasing alternate fuel utilisation rates by
10 per cent of the total heat input while
shifting the fuel mix towards lower costs.
The oxygen is delivered by a specially
designed oxygen lance to the base of the
burner flame. From experience in cement
kilns and other solid fuel-fired systems,
Praxair has developed engineering
expertise to optimise the quantity of
oxygen and the injection location. This
results in a number of benefits including an
increase in the utilisation of alternate fuels
or switching to fuels with lower calorific
values without negatively impacting NOx
emissions. Experience also indicates that
oxygen can have a beneficial impact on
SOx and CO emissions levels.
Implementation
Oxygen is typically delivered in liquid bulk
form into on-site storage tanks where it is
evaporated and the gas is injected into the
kiln or calciner to support the combustion
process.
The supply system shown in Figure 3
consists of cryogenic storage tanks where
the oxygen is stored at a temperature
of -183°C and ambient air evaporators
that convert the liquid to oxygen gas at
ambient temperature. A control skid (see
Figure 4) is used to meter and control the
oxygen flow. In addition, it provides the
safety shutoff function for the oxygen.
This control system is interfaced with
the kiln control system for transfer of
setpoints and flow rates. It also receives
digital permissives from the kiln control
that oxygen can be used in the process.
A rigorous safety analysis is performed
prior to start-up to ensure a safe and
successful integration of oxygen into the
kiln combustion process. In addition, all
operators and engineers are trained in the
use of oxygen.
Alternatively, a dedicated on-site oxygen
generation system known as Vacuum
Pressure Swing Adsorption (VPSA) system
may be constructed. The decision to
receive oxygen in bulk shipment or install
a VPSA is customer specific with key
factors being oxygen volume, proximity to
a Praxair liquid source and the expected
duration of oxygen enrichment. The best
solution is found after considering the
most appropriate oxygen supply mode,
the best injection technique and optimised
oxygen flow rates, the alternate fuel rate
and performance of the kiln system.
Oxygen can typically start flowing
from a liquid supply system within three
months of a customer agreement. The
site preparation includes a foundation
for the liquid tanks and the oxygen
piping between the supply and the kiln
before the rented tanks are installed
and commissioned. This oxygen system
installation requires little capital and
changes to equipment and operations
tend to be minor. If the desired additional
amount of alternate fuel can be physically
fed to the combustion process, operational
and cost improvements are obtainable
within days.
Oxygen enrichment can combine low
capital cost and ease of installation with
operational and cost benefits. It allows
lowering fuel costs by firing alternate
fuels with lower heating values while
maintaining the burning zone conditions
necessary for high quality clinker.
____________________________I
Figure 3 (below): oxygen
delivery by liquid bulk
truck into site storage
Figure 4 (right): example
of a prefabricated
control skid to
monitor and control
oxygen flow
Figure 2: RDF is a typical fuel that benefits
from oxygen injection
OXYGEN ENRICHMENT
JANUARY 2009 ICR 67