Cement kilns fuel types used

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 ... 

Because cement kilns are so good at destroying organic chemical wastes, emissions of dioxins - or any other type of products of incomplete combustion (PIC) - are so low they pose no danger to the environment. In the case where some of the hazardous waste fuels used contain toxic dioxin, the cement kiln temperatures of 1650°F will destroy dioxins in less than one second. Because cement kilns operate at much higher temperatures (at least 2450°F), and because the burning wastes have an average residence time in the kiln of at least two seconds, any dioxins are destroyed. However, dioxin waste is never accepted by Southdown for use in its cement kilns. 

Health risks to residents near cement kilns may actually decrease when hazardous waste fuels are used. This is because the permit needed to recycle hazardous waste fuels requires more stringent emissions controls than those for cement kilns using only fossil fuels. Also, fossil fuels contain natural impurities that are reduced or no longer emitted when some types of hazardous waste fuels are used. 

Since tires have a Btu value comparable to the best coal, they would be expected to be an economically attractive fuel in some situations. Recent U.S. experience has shown economic feasibility for tire-to-energy power plants and for tdf used in cement kilns and pulp and paper mills. The economic barriers facing these types of tire combustion will be discussed below. Despite these economic barriers, the use of tdf has increased over the last year, and this trend is expected to continue. 

There are two types of incinerators used predominantly for incineration of hazardous waste. The first type is the rotary kiln, which operates at temperatures of approximately 2300 "F. More hazardous wastes are burned in this type of kiln than any other and it is the one for which most tests on destruction efficiencies have been done. A second type is the cement kiln, which is now being used more often. The temperature of the cement kiln reaches 3000 "F. The hazardous waste is blended with the fuel and used in the making of cement. By modifying the fuel blend, it is possible to obtain efficient destruction, even for chlorinated solvents. 

Sources of CO2 associated with Portland cement manufacture include (i) the decarbonization of limestone (ii) the exhausts of kiln fuel combustion and (iii) the exhausts of the vehicles used in cement plants and distribution. Of these sources, the first produces a minimum of about 0.47 kg CO2 kg cement, whilst production via the second source varies with the plant efficiency. For example, an efficient precalciner plant will produce 0.24kg CO2 kg cement, while a low-efficiency wet process may produce up to 0.65 kg CO2 kg . The production of CO2 via the third source is almost insignificant (0.002-0.005 kg CO2 kg cement). Hence, the typical total CO2 footprint is around 0.80 kg CO2 kg finished cement This leaves aside the CO2 associated with electric power consumption, which varies according to the local generation type and efficiency. Typical electrical energy consumption is of the order of 90-150 kWh per metric ton of cement this is equivalent to 0.09-0.15 kg CO2 kg finished cement if the electricity is coalgenerated. All of this amounts to about 7% of the total CO2 generated worldwide (Malhotra, 1999). 

Two types of hazardous wastes typically have been introduced into opportunity fuel commerce spent solvents and other relatively light organic fluids, and waste oils from automotive and manufacturing sources. Both are widely used in boilers and kilns— particularly cement kilns—throughout the USA and the world. 

Measurement. The need for cost-effective pollution prevention, efficiency improvements, and emissions meastmement has created a need for new types of integrated control and meastirement systems. One of the best methods to improve environmental performance initially is to maximize efficiency through optimization in their manufacturing and energy processes. Many cement kilns now use continuous NOx measurement at the kiln exit to feed back information to the process control to optimize combustion. Capital costs are usually small, especially if PEM/ optimization software systems can be included at the time of a control system u rade. The increased operator awareness of the plant is a benefit, and fuel savings, qualify control, and maintenance planning can quickly pay back initial costs. 

Dry process cement production facilities often have several other types of manufacturing equipment designed to increase fuel efficiency. First, many dry process kilns add a preheater to the feed end of the kiln to begin heating of the feed prior to its entrance to the kiln. Two main types of preheaters exist, the suspension preheater and the traveling grate preheater both use hot, exiting kiln air to facilitate a more efficient heat transfer to the feed than could occur in the feed end of the kiln itself.1 This... 

All three types can be found in use in cement works control laboratories, but the preferred system is the multichannel type fitted with channels for the simultaneous determination of the eight elements Fe, Ca, K, S, Si, Al, Mg, and Na. Depending on the local geology, a chaimel for the determination of fluorine may be fitted if the limestone deposit is close to a source of fluorspar (CaF2). (The determination of fluorine is beyond the scope of an energy-dispersive XRF spectrometer.) At some works a channel is added to monitor the chlorine content of the clinker, which can be introduced from the fuel used to fire the kiln. 

The major part of the fuel energy consumption is used up for the binning of the cement clinker, which in the Federal Republic of Germany is mainly prodnced in three types of kilns, namely... 

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