Cement Kilns Energy Recovery

Cement production demands major amounts of fuel. Energy costs can be upwards of 25% of their turnover. Therefore, many cement kilns in the UK, Belgium, the Netherlands, Switzerland and other countries have started to use pretreated waste streams as a fuel. This not only saves fuel input, but indeed often allows a charge for the treatment of waste. In principle, cement kilns can deal with the following types of waste  

For waste types (1), (3) and (4) the use in cement kilns can be regarded as a recovery operation. For waste type (2), however, there is no real benefit of using the material in the cement making process. The kiln is merely being employed as a means for a (thermic) waste disposal operation. 

There are two main processes for clinker production a so-called wet process and a dry process. In the dry process the alkali raw materials are introduced in dry form into the kiln. In the wet process, these materials are introduced in the form of a slurry. Among other things, the type of process depends on the source of the kiln s raw materials. If a kihi extracts alkali raw materials from lakes then a choice for a wet process is logical, despite the disadvantage of its rather high energy use (5,000 MJ/toime versus 3,600 MJ/toime clinker), as in the dry process no water has to be evaporated. 

The high temperatures ensure that inputs like MPW are effectively destroyed. Acidic substances such as HCl and SOx are neutralised by the alkali raw materials, which act in fact as a caustic scrubber. Metals are bound in the clinker or in the fly ash. Fly ash is captured with an electrofilter and subsequently added to the clinker. In general, no other flue gas cleaning is applied. 

Cement kilns can handle quite a lot of different materials. In most cases the input material should be chipped or shredded. Licenses often limit the PVC and chlorine input to l%-2% chlorine in waste. It is said that chlorine also has a negative impact on the quality of the clinker if it is available in too high quantities. The content in clinker seems to be limited to some 0.1% at maximum, and hence the average chlorine content of all fuels used combined may have to be somewhat lower. This implies that waste with a high chlorine 

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The matter of different investment criteria for energy users versus regulated utilities is exemplified by comparison of the feedwater heater project (41) to the cement kiln heat recovery project referred to above. The utility chose to replace the de-... 

Rotary kilns (e.g., cement kilns), metal-recovery and smelting furnaces, mobile incinerators, and industrial boilers are primarily used to incinerate hazardous wastes. The obvious benefits of combustion of waste as fuel are the recovery of energy from the waste and the conservation of fossil fuels. Because the kiln and furnace operators are paid to take in the waste, rather than having to pay for fuel, also create economic incentives. Mobile incinerators are most commonly used for soil decontamination projects, and can be moved from site to site once the job is completed. 

Coal used in power stations has the potential to be partly replaced by fuels derived from pre-treated plastics and paper waste, reducing both dependency on fossil fuels and reliance on landfill. APME reports on a project in the Netherlands which it co-sponsored to develop a substitute fuel from plastics. The environmental assessment of the project compared the environmental impacts of coal substitution with other plastics recovery methods, including gasification in feedstock recycling and energy recovery from plastics waste in cement kilns. The study also compared coal substitution with the generation of power from burning biomass. 

Carrasco, F., Bredin, N., Gningue, Y. Heitz, M. 1998. Environmental impact of the energy recovery of scrap tires in a cement kiln. Environmental Technology, 19, 461 -474. 

Many carpet manufacturers, fiber and chemical suppliers, recycling companies, and academic institutions are actively pursuing various methods to recycle fibrous waste. The approaches include chemical processes to depolymerize nylon and other polymers, recovery of plastic resins from carpet fibers, direct extrusion of mixed carpet waste, composites as wood substitutes, fibers for concrete and soil reinforcement, waste-to-energy conversion, and carpet as feedstock for cement kilns. 

The waste to energy recovery route covers energy via incineration or as a fuel source in smelters, cement kilns or chalk ovens as is often the ease with plastic waste today. Past problems with standards of operation at incinerators, such as operating temperature, residence times and emissions, led to concerns over the incineration of plastics waste in the past. However, sinee that time more stringent requirements for operation have been introduced, such as a minimum of 850 °C in the combustion chamber, minimum of 2 sec residence time for the resulting flue gases at 850 °C, in the presence of at least 6% oxygen. 

Petroleum wastes or other flammable liquid wastes are often burned for energy recovery. Relatively small amounts recovered from a spill can be shipped to a fuel blender, who mixes flammable waste liquids to achieve acceptable physical and chemical properties and markets the resulting material as fuel. Flammable wastes are sometimes burned at cement kilns for energy recovery because cement manufacture is highly energy-intensive. In the United States, facilities that recover energy from hazardous wastes are regulated under the RCRA Boiler and Industrial Furnace (BIF) Rules. 

HEAT RECOVERY ON THE SMOKE OF THE CEMENT KILN AND UTILIZATION OF THE RECOVERED ENERGY... 

Energy recovery from plastic waste can also be achieved in cement kilns (see sec. XV.D) [432]. 

The burning temperature for production of Portland cement clinker can be decreased by about 150°C through the use of fluxes, but opinions have differed as to the energy saving thereby obtainable, Klemm and Skalny (K52), who reviewed the subject, estimated it at 630kJkg" . Christensen and Johansen (C56) considered that this figure, while possibly realistic for an inefficient, wet process kiln, was unlikely to be so for a modern, precalciner-preheater kiln, in which heat recovery is efficient. They considered a value of lOSkJkg" more realistic. 

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