- Written by Martin Demuth, Davor Spoljaric, Johannes Rauch & Michael Potesser, Messer Group GmbH, Christoph Hochenauer, Institute of Thermal Engineering, Graz University of Technology
Oxygen-enrichment can lower production costs for many processes because fuel costs are continuously on the rise. Enrichment is used more and more frequently in combustion systems technologies like oxygen burners, oxygen lancing and other oxygen enrichment systems in order to raise productivity, save energy or enable the use of lower quality fuels. Messer Oxipyr® technologies are used to provide discrete oxygen-rich areas in furnaces to allow complete combustion of fuels or lowering of emission values. This article determines the possibilities for and the advantages of oxygen application in rotary kiln burners for cement clinker production based on Messer testing results and installations that include a unique way of atomising liquid waste fuels with oxygen.
- Written by M. R. Karim, M. F. M. Zain & M. Jamil, Universiti Kebangsaan Malaysia; F.C. Lai, Sika Kimia Sdn Bhd; M. N. Islam, Dhaka University of Engineering and Technology
Modern constructions demand a lot of cement, but OPC has a large negative impact on the environment, producing large amounts of CO2. With increasing demand, raw material prices increase and fuel resources are falling. As there is currently no viable alternative to OPC, research centres around the use of alternative fuels and supplementary cementitious materials. Here researchers from Malaysia and Bangladesh present ways to use biogenic wastes as an alternative fuel and as a novel binder.
OPC is a versatile and widely-used construction material but burning CO2-emitting materials at 1450°C produces a lot of CO2,1 which contributes to global warming.2 To get a sustainable solution to this problem, much research has been conducted towards the use of biogenic wastes in cement production. Examples include palm oil fuel ash (POFA)3,4, rice husk ash (RHA),5,6 sawdust ash/ash from timber (AFT)7 and bagasse ash8 as supplementary constituents of cement and concrete.
RHA is generated from the rice husk processing industry. POFA is a waste from the palm oil milling industry. AFT is a waste from wood mills and the fibreboard industry. All of these types of ash can be produced by burning the relevant material in a kiln or furnace at 500-600°C. Large amounts of POFA, RHA and AFT are produced in Malaysia every year but they are not used to a significant extent at present. In spite of the potential financial, technical and environmental benefits, these materials are currently dumped locally with no commercial return. It has already been proven that all of these waste materials contain a high amount of amorphous silicon dioxide (SiO2). This means that they could be used as pozzolanic materials in cement and concrete production.
Additionally, these biogenic agricultural wastes contain different proportions of cellulose, hemicellulose, lignin, ash, resin, wax/oils, water-soluble substances and moisture. Carbon, nitrogen, oxygen and hydrogen are the major constituent elements of these compounds. Small amounts of sulphur and chlorine and traces of others element are also present. Their volatile matter content is around 60%-75%. High ash content lowers the calorific value of residues while the presence of oil, resins and wax raises the calorific value. The heat potential of some of the selected agricultural wastes from the biomass are given in the literature.9-12
Energy from biomass plays a big role in energy demand worldwide, supplying 10% of the total energy demand.13 In addition, in this study it is presented that the proper direct burning of the biogenic waste can be used as alternative fuel in the cement industry without quality problems or performance deterioration. This has the potential to save energy compared to OPC production.
- Written by Raine Isaksson and Neil Taylor, Isaksson-Taylor, Australia
While using alternative fuels is desirable, occasionally they cannot be used due to processing issues, lack of permits or poor availability. This article looks at how higher-cost traditional fuels can best be used to achieve improved kiln performance. With properly prepared fuel, the correct feed chemistry, a suitable burner and good operation, the clinker melt profile and nodule size can be fine tuned to provide better heat recovery, improved clinker grindability and increased operating efficiency with reduced CO2 emissions.
Introduction - The advantages and disadvantages of alternative fuels
The cost benefits and environmental advantages associated with alternative fuels make them highly desirable for many companies, although limited availability, high entry costs, potential process issues and quality concerns mean they are not an option for every plant. While the cost savings associated with alternative fuels can be significant, there are usually a number of drawbacks that can adversely affect output and product quality to varying degrees. For example, most alternative fuels are usually associated with increased amounts of water and false air, which increase kiln exhaust gas volumes and reduce capacity when kilns are draft restricted (1).
These fuels can also contain excessive amounts of ballast, which can reduce heat transfer in the burning zone and they often introduce circulating species, which can contribute to build-ups and blockages. They can also introduce species which significantly affect clinker nodulisation, mineralogy and ultimately clinker quality (2). The overall result is that while there are significant cost benefits, there are usually also penalties in reduced kiln capacity, more variable quality and environmental performance due to the increased variability associated with the use of alternative fuels.
- Written by Dirk Lechtenberg, MVW Lechtenberg & Partner
The use of alternative fuels and raw material is common practice in the European cement industry. Dirk Lechtenberg gives an overview of scrap tyres as an alternative fuel in this excerpt from MVW Lechtenberg & Partner's Alternative Fuels & Raw Materials Handbook. Due to be published in the summer of 2011, the handbook will give an insight into over 80 different types of alternative fuels and raw materials with detailed descriptions of the availability, common use and practice in the cement industry. This includes processing considerations, the influence on the environment, clinker production and the economics of the various alternative fuels.
Scrap tyres are tyres that are no longer suitable or legal for their intended use due to damage, brittleness or low tread depth. By the time it is scrapped, a tyre generally loses around 20% of its weight. Tyres comprise natural rubber, synthetic rubber, carbon black, oil and various reinforcers.
- Written by Stefan Laux & Jeff Mocsari, Praxair Inc., Tonawanda, USA
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.