Fuel costs and sustainability goals drive cement producers to use large amounts of secondary fuels. However, in comparison to fossil fuels these fuels typically have lower heating values and combustion characteristics that negatively impact the clinker forming process. The strategic use of oxygen provides a cost-effective means to combine high alternative fuel use targets with excellent operational results. The article describes the challenges posed by high alternative fuel rates and how oxygen is used to further increase the use of alternative fuels. A case study is presented that shows the beneficial impact of oxygen-enhanced combustion of alternative fuels.
The benefits of oxygen enrichment in cement kilns are well documented (1, 2). In the past, production increase was the main motivator for using oxygen if the plant's capacity to handle more flue gas was preventing further production increases. Injection of essentially pure oxygen removes nitrogen from the flue gas stream that is normally introduced with the combustion air. This frees up valuable flue gas capacity to be used for further production. In addition, enrichment with oxygen improves available heat for production and reduces flue gas losses.
Experience with oxygen injection shows demonstrated benefits to customers ranging from production increases of up to 25%, specific fuel savings of up to 5%, reduced specific dust losses and improved kiln stability as evidenced by clinker quality and kiln coating. Low investment costs and an easy implementation have made this technology an attractive solution for a short-term capacity deficit. Figure 1 shows kiln sizes and oxygen flow rates used successfully in various projects over the last 50 years.
Long-term experience with this application suggests that a yield of 3t to 4t of incremental clinker per ton 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 the purpose of production increase.
Oxygen for increased use of alternative fuels
Pressures to lower fuel costs are ever present for the global cement industry. Cement producers have been very successful in increasing alternative fuel utilisation to meet cost reduction goals. In many European markets, alternative fuels have replaced on average more than 50% of the fuel input3 with some plants averaging 60-70%. The use of high alternative fuel rates in the calciner is common and some plants have been very successful with the near complete replacement of fossil fuels. However, the properties of these fuels have posed 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 alternative fuels such as a cool burning zone, an unstable and/or long flame, insufficient burnout and high process variability. Such problems may ultimately result in off-spec clinker and poor kiln capacity. The extent of performance deterioration depends on the specifics of the fuel mixture and type of combustion equipment employed. The cement industry's recent progress in replacing fossil fuels has more clearly exposed the barriers mentioned above towards further increase of alternative fuel use.
The use of oxygen to improve combustion conditions with alternative fuels is not new to the cement industry (4,5). Historically, it was mainly limited to the combustion of hazardous waste liquids to ensure complete combustion of liquids with lower heating values and varying composition. However, oxygen can also be used to improve overall fuel costs by allowing stable and consistent combustion of low quality alternative fuels with low heating value and larger particle size. As an example, Figure 2 shows a refuse-derived fuel (RDF) from processed recycling streams that is commonly referred to as fluff. This fuel consists mainly of two-dimensional pieces of plastic and paper sheets of less than 20mm (3/4inch) in length, but can also contain pieces of plastic and wood with significant thickness that require longer burnout times.
The injection of oxygen into the flame root has proven to be very effective for these fuels as it enables more rapid heat-up, fuel devolatilisation and fuel ignition. The improved combustion conditions can compensate for flame cooling due to fuel moisture. In addition, the available residence time in the flame is used more effectively which enhances fuel burnout of fuels with larger particles. Large fuel particles are a concern for clinker quality and clinker sulphur retention, if they fall into the clinker bed and result in locally reducing conditions.
As an example of the effects, Figure 3 compares the results of a numerical simulation of a typical cement burner flame. The image on the left shows the temperature distribution for a specific fuel mix without the injection of oxygen and the image on the right shows the temperatures for the same conditions and the injection of oxygen. This CFD simulation, using 'Fluent,' modelled the combustion of coarse fluff particles in individual size classes that are tracked through heat-up, ignition and final burnout. The total burnout of all particle classes calculated shows a significant improvement in burnout due to the improved combustion conditions.
One frequently voiced concern about oxygen injection into cement kilns is the impact on NOx emissions. Our experience has shown in general that NOx emissions have not increased if the oxygen injection system is designed correctly. In the case of oxygen injection for alternative fuel combustion the oxygen raises combustion temperatures in the fuel-rich flame core where the overall fuel rich conditions favour the reduction of NOx to nitrogen. This effect has been successfully exploited using oxygen injection for NOx reduction in the power industry (6). In addition, the injection of oxygen into the flame compensates the cooling effects of moisture addition from the additional alternative fuel. Thus, oxygen brings the thermal condition in the burning zone back to where they are required to be for high quality clinker production.
While additional production may not be a primary goal of oxygen use for alternative fuels, the reduced flue-gas volume can compensate for the water ballast that negatively impacts the capacity of the flue gas system. Thus, lost production can be recovered or if desired, clinker production can be increased by combining the goals of alternative fuel increase and production increase.
Oxygen use has proven to be flexible and straightforward from an operational point of view. Kiln operators quickly realise the potential to stabilise the combustion during kiln upsets. Our experience indicates that oxygen often promotes better kiln coating as the temperature distribution in the burning zone is favourably affected by the additional flame stability. None of the applications has led to a reduction in refractory life and many kilns have benefited from a stronger coating.
Case study: Increase in fluff combustion at Lafarge Karsdorf
The injection of oxygen to increase the use of alternative fuels was implemented at Lafarge's Karsdorf plant in Germany in 2008. The plant has three kilns, of which kiln lines three and four are four-stage cyclone preheater kilns with a nominal daily production of 2000 t/d. These kilns fire dried lignite dust, low quality waste oils, animal meal and shredded plastic fluff in the main burner and whole tyres through the feed shelf. To allow for higher chlorine input resulting from alternative fuel combustion, each kiln was retrofitted with a 5% bypass. Details on the plant history and the equipment can be found in the references (7).
The oxygen injection equipment was initially installed on kiln line four and since then has been expanded to line three. A supply system for liquid oxygen with a cryogenic tank and atmospheric vaporisers was installed next to the clinker cooler. Oxygen is delivered in liquid bulk form to the storage tank where it is evaporated for use and the gas is injected into the kiln to support the combustion process. Figure 4 shows the initial test installation and delivery of oxygen next to kiln line four. This temporary installation will be replaced by vertical tanks at a different location. 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.
A control skid on the burner deck is used to meter and control the oxygen flow. In addition, it provides the safety shut-off function for the oxygen. This control system is interfaced with the kiln control system for transfer of oxygen flow setpoints and actual 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.
The oxygen is injected via a proprietary stainless steel lance through the burner into the flame root and through a second lance installed in the kiln hood. The lance in the kiln hood is installed to maximise oxygen injection flexibility for both alternative fuels rate improvement and production increase.
Initial optimisation of the oxygen injection system had the following goals:
- Increase of solid alternative fuel (fluff).
- Minimisation of dried lignite use.
- Maximisation of tyre use.
- Maintenance of the clinker quality.
A recovery of approximately 10% production capacity that was lost due to the combustion of alternative fuels was desirable. The reduction of the fuel costs as the main purpose of the oxygen injection is reflected in these goals. Specifics of the fuel cost situation are proprietary information of the customer, but any replacement of expensive fossil fuels with less expensive or free alternative fuels is of economic interest if clinker quality can be maintained. In addition to the costs of alternative fuel streams, the availability as well as the production constraints discussed below play an important role in finding the optimum fuel balance. After an optimisation phase the plant arrived at a typical fuel mix displayed in Table 1 as a percentage of heat input. Baseline data without oxygen injection is shown for comparison. Details about the optimisation results and experience are presented in the references (8).
| Baseline | With oxygen injection |
|
| Total alternate fuel rate (% heat input) | 66& | 75% |
| Liquids AF |
36% | 21% |
| Plastic fluff | 21% | 35% |
| Other AF |
9% | 9% |
| Production | 100% | 105% |
Table 1: Typical fuel mix arrived at after fuel mixture optimisation process.
Even at the baseline conditions the plant was already using an impressive 66% alternative fuels. As the table shows, these were mostly liquid alternative fuels. Although these liquids usually combust very well due to their high volatile content, general availability (at a reasonable cost) did not allow a further increase of this fuel group. Instead, with injection of oxygen the amount of low cost fluff could be increased significantly. The percentage of tyres and animal meal stayed nearly constant. However overall economic success of the oxygen injection is mainly due to the reduction of heat input from dried lignite from 34% to 25% as this fuel is clearly the most expensive.
In addition to the installation of the oxygen delivery system only a few minor changes were necessary to make the above possible. The original kiln control approach had to be modified as it did not allow the reduction of lignite as the control fuel below a certain minimum value and the fluff dosing system had to be upgraded to allow higher flow rates. Apart from permitting considerations, further increase of alternative fuels will likely face the following process restrictions.
1. Further decrease of dried lignite will remove the major source of sulphur for the clinker. This would require a new low cost sulphur source to be added to the fuel mix.
2. Further increase of fluff will result in additional chlorine to the system. The current bypass design is at its limit. While this restriction can be removed it would require further investment.
These plant-specific limits to a further increase of alternative fuels can only be an example. Each kiln has its specific challenges that require careful consideration as to whether oxygen is beneficial for production costs.
Figure 5 shows a summary of the NOx emissions measured at the kiln inlet (feed shelf) for two oxygen injection locations and multiple oxygen flow rates and fuel mixes. Each dot represents a NOx average calculated for constant test conditions between six and 32 hours. The baseline data without oxygen on the left is an average of 166 hours. Most of the NOx averages displayed in the graph are at or below the baseline without oxygen. Only three averages are above the baseline. In general, the kiln NOx emissions do not trend well with oxygen consumption. Kiln temperatures, operational variations, fuel mix and fuel composition govern NOx to a far greater extent. For example, the optimisation has shown that NOx is reduced with increased fluff rates. The reasons for this trend are likely to relate to flame cooling due to fluff and the reduction in fuel nitrogen content when lignite fuel is reduced. However, it should be noted that the NOx emissions at the plant stack are below those at the kiln inlet and are controlled if necessary by an SNCR system. The high fluff rates in the kiln NOx concentrations have resulted in a reduced consumption of urea at Karsdorf.
The emissions of SO2 and CO are mainly driven by the ability of the plant to maintain sufficient excess oxygen in the flue gas leaving the kiln. The parametric oxygen injection tests at Karsdorf support this general finding. Overall, oxygen injection provides additional oxygen for combustion and can improve the emissions of SOx and CO. Our experience with oxygen injection over the clinker bed confirms that the localised higher oxygen concentrations can favour the retention of sulphur in the clinker. It should be noted that stack emissions from precalciner cement plants are dominated by the combustion conditions in the calciner and oxygen injection into the kiln has not shown any increase in stack emissions at these plants.
The results of oxygen injection at Karsdorf are very positive:
- The goals mentioned above were achieved within the optimisation potential possible by the current equipment and further improvement is possible with systematic improvement of bottlenecks. Oxygen use has proven to be without disadvantages from an operational or process technology point of view.
- All clinker analyses showed normal values and very high alternative fuel rates used at Karsdorf do not result in clinker quality problems.
- Plant emissions did not increase and higher fluff rates result in lower NOx concentrations from the kiln and lower urea consumption for NOx control.
- A very strong coating in the sinter zone of the kiln was observed during the annual kiln overhaul.
These positive results led to the plant's decision to continue the use of oxygen and to expand it to kiln three. The improvement of alternative fuel use rates results in a substantial reduction of fuel costs and a reduction in fossil fuel CO2 emissions.
References
1. Wrampe, P., Rolseth, H.C.: 'The Effect of Oxygen upon the Rotary Kiln's Production and Fuel Efficiency: Theory and Practice,' IEEE Transactions on Industry Applications, Vol. IA-12, No. 6. Nov/Dec 1976.
2. Leger, C.B.; Friday, J.: 'Oxygen Enrichment for Cement Kiln Firing,' 43rd IEEE-IAS/PCA Cement Industry Technical Conference, Vancouver, British Columbia, April 29-May 3, 2001.
3. Verein Deutscher Zementwerke e.V.; 'Activity Report 2005 - 2007,' Düsseldorf, January 2008. www.vdz-online.de
4. Hansen, E.R., Leger, C.B., & Ho, M.: 'Theory and Practice with Oxygen Enrichment in a Cement Kiln Firing Waste Derived Fuels,' International Incineration Conference, Houston, Texas, May 1994.
5. Conveney, D. F. & Hicks, J. K: 'Oxygen-enrichment for improvements in emissions control while burning waste fuels,' Portland Cement Association Mill Session Paper M223-5, 1994.
6. Bradley, J., Bool, L. & Kobayashi, H.: 'NOx Reduction from a 125-MW Wall-Fired Boiler Utilising Oxygen Enhanced Combustion,' 29th International Technical Conference on Coal Utilisation and Fuel Systems, Clearwater, FL, April 2004.
7. '80 years at the Karsdorf cement works - an interview covering past and present,' Cement International, Vol. 5, June 2007.
8. Ehrenberg, C., Meissner, T., Laux, S., Webel, M., Oberschelp, D., Schäfer, R., & Wrampe, P.: 'Optimisation of the combustion of secondary fuels by O2-enrichment,' 6th International VDZ Congress, Düsseldorf, 30 September - 2 October 2009.
