This 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 increaseThe 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 utilisationPressures 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,

IBenefits of oxygen injectionby Stefan Laux, Jeffrey Mocsari, Markus Webel and Stephen HopePraxair 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 cementPortland cement

Kiln capacity tpd*

Oxy

gen

flow

tpd

*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

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66 ICR JANUARY 2009

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.

ImplementationOxygen 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

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