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.

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. 

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.

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.

An alternative fuel project is a complicated endeavour. Among the numerous issues that must be considered, some will inevitably take a lower priority. Selecting which airlock valve to use on the system should not be one of them.

The consequences of the valve choices will directly affect the success of the project in several areas including:

• Cost effectiveness;
• Production output;
• Safety and fail safe back-up systems.

The purpose of this article is to highlight some of the most important things to consider when selecting valves for your project, as well as open the door to alternative ways of using fuels to create heat for various processes.