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Saturday, March 21, 2009

Concrete, Cement, Energy, Air, Environment & Fly Ash

Concrete is a construction material composed of cement (commonly Portland cement) as well as other cementitious materials such as fly ash gravel, limestone, or granite, plus a fine aggregate such as sand, water, and chemical admixtures. A typical process of manufacturing cement consists of grinding a mixture of limestone and clay or shale to make a fine "rawmix," heating the rawmix to sintering (almost melting) temperature in a cement kiln and grinding the resulting clinker, right, to make cement. The rawmix is fed into the kiln and gradually heated by contact with the hot gases from combustion of the kiln fuel. Typically, a peak temperature of 1400-1450 °C is required to complete the reaction. In order to achieve the desired setting qualities in the finished product, a quantity (2-8%, but typically 5%) of calcium sulfate is added to the clinker and the mixture is finely ground to form the finished cement powder.


The rotary kiln (picture above diagram below) consists of a tube made from steel plate, and lined with firebrick. The tube slopes slightly (1–4°) and slowly rotates on its axis at between 30 and 250 revolutions per hour. Rawmix is fed in at the upper end, and the rotation of the kiln causes it gradually to move downhill to the other end of the kiln. At the other end fuel, in the form of gas, oil, or pulverized solid fuel, is blown in through the "burner pipe", producing a large concentric flame in the lower part of the kiln tube. As material moves under the flame, it reaches its peak temperature, before dropping out of the kiln tube into the cooler. Air is drawn first through the cooler and then through the kiln for combustion of the fuel. In the cooler the air is heated by the cooling clinker, so that it may be 400 to 800 °C before it enters the kiln, thus causing intense and rapid combustion of the fuel.


Fly ash is used as a replacement for some of the Portland cement content of concrete. Fly ash is one of the residues generated in the combustion of coal. Fly ash is generally captured from the chimneys of coal-fired power plants, and is one of two types of ash that jointly are known as coal ash; the other, bottom, is removed from the bottom of coal furnaces. Toxic constituents include arsenic, beryllium, boron, cadmium, chromium, chromium VI, cobalt, lead, manganese, mercury, molybdenum, selenium, strontium, thallium, and vanadium, along with dioxins, and PAH compounds. In the past, fly ash was generally released into the atomosphere, but pollution control equipment mandated in recent decades now require that it be captured prior to release. In the U.S., fly ash is generally stored at coal power plants, stored in retention ponds or placed in landfills. About 43 percent is recycled, often used to supplement Portland cement in concrete production.

Again, in making concrete much of the Portland cement can be replaced with two industrial waste products — fly ash, left over from burning coal in power plants, and blast-furnace slag, which can help make the concrete stronger. Portland cement is at the heart of concrete’s environmental problems. About a ton of CO2 is emitted for every ton of cement produced. The basic manufacturing process involves burning limestone and other minerals at about 2,700 degrees Fahrenheit to create an intermediate product called clinker.

The CO2 associated with Portland cement manufacture falls into 3 categories: (1) CO2 derived from decarbonation of limestone, (2) CO2 from kiln fuel combustion, 3) CO2 produced by vehicles in cement plants and distribution. Nearly a ton of carbon dioxide is emitted to produce each ton of portland cement, while fly ash is a byproduct of energy generation. Mixes in which up to 25% of the cement is replaced by fly ash are quite common, and some designers are specifying over 50% substitution for certain applications. Use of fly ash in concrete in the U.S. is governed largely by ASTM Standard C618.

Cement kiln emissions:

Carbon dioxide
Dust
Nitrogen oxides (NOx)
Sulfur dioxide (SO2)
Carbon monoxide (CO)
Dioxins and furans (PCDD/F)
Polychlorinated biphenyls (PCB)
Polycyclic aromatic hydrocarbons
Benzene, toluene, ethylbenzene, xylene (BTEX)
Gaseous inorganic chlorine compounds
Chlorides
Gaseous inorganic fluorine compounds

(BuildingGreen.com, Wiki: Concrete, Portland Cement, Cement Kiln, Fly Ash)

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