Raw Material andor Fuel Substitution

Substitution of limestone with raw materials containing non-carbonate calcium and using fuels with lower carbon contents can provide CO2 reductions. These options are discussed in this section.

8.4.2.1 Non-Carbonate Calcium Containing Raw Materials

Raw mix often contains industrial byproducts such as iron and steelmaking wastes and certain coal ashes, when available. Although the use of slags and fly ashes as an iron source in clinker raw material is commonplace world wide, addition of components for the purpose of minimizing CO2 emissions has become a focus within the last decade. Alternative raw materials capable of supplying significant amounts of calcium, without requiring calcination, have been used to reduce specific fuel energy consumption and increase kiln system capacity in some cement kilns [26, 27]. One of the more commonly used raw material substitutes is blast furnace slag (BFS), which may contain approximately 40% carbonate-free CaO [26, 27]. In addition, Class C fly ashes, which typically contain 25% CaO, are also a potential source of carbonate-free CaO [28].

Reductions in CO2 emissions associated with use of reduced carbonate raw materials occur due to reduced calcination and thermal energy requirements. A secondary, but important, related benefit associated with the use of reduced carbonate raw materials is potentially increased kiln system capacity.

A commercially available technology called CemStar has been used in some U.S. kilns. In this technology, a portion of the limestone feed is replaced with BFS. The amount of slag used varies between about 5% and 10% of the clinker output. Use of CemStar potentially can provide several benefits, including reductions in NOx (about 30%, ton/ton of clinker basis) and CO2 (about 4%, ton/ton of clinker basis), and increase in kiln output (about 7.5%) [29].

8.4.2.2 Biomass Fuel Substitution

Biomass is "material that comes from plants" [30]. Sources of biomass fuels may include primary wood, wood products, and wood-related wastes. Most of these materials are not widely used in U.S. cement kilns. Another source of biomass is scrap tires. Vehicle tires contain between 14 and 27% natural rubber [31]. As of 2006, approximately one third of all U.S. cement kilns used scrap tire derived fuel (TDF) as a kiln system fuel [15]. TDF represents approximately 5% of the thermal energy consumed by the U.S. cement industry [25]. Also as of 2003, approximately 53 million of the 130 million scrap tires generated in the U.S. were consumed in cement kilns [31]. The extent to which cement kilns use biomass has a bearing on the magnitude of CO2 emissions from these kilns because of the difference in the carbon contents of biomass materials and traditional fuels like coal.

A technology called midkiln firing (MKF) facilitates the firing of tires (MKF-tires), coal, or other fuels in the mid-kiln region of a kiln. This results in less intense firing at the primary burners and consequently about 35% lower NOx formation in the kiln. Tires or other waste fuels are often used at the mid-kiln location because high temperatures and residence time in the kiln permit good combustion. In particular, the use of MKF-tires results in reduced fossil fuel consumption (by about 15%) [29].

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