Thermal Processing of CCRs

摘要

An on-going problem for the U.S. electric power industry remains the coal combustion residuals (CCRs) that are byproducts that result from coal's combustion to generate electricity. Currently, there are 676 coal combustion waste impoundments that are adjacent to existing coal-fired power plants and receive much of the 60 million tons that is produced annually. The average coal-fired power plant produces more than 540 tons per day (tpd) of coal ash that are mainly coal fly ash (47.9%), bottom ash (13.3%), flue gas desulfurization ("FGD") sludge (29.5%), and fluidized bed combustion ("FBC") ash (9.3%). Depending upon the source and make-up of the coal being burned (i.e. bituminous, lignite, etc.), the main chemical composition can vary considerably. Both fly ash and bottom ash include substantial amounts of silicon dioxide (SiO_2), both amorphous and crystalline, calcium oxide (CaO), which are both endemic in many coal-bearing strata, as well as aluminum oxide (Al_2O_3), and iron (III) oxide (Fe_2O_3). Further, certain CCRs contain more than 25% FE_2O_3 Significant quantities of magnesium oxide (MgO) are also present in most fly ash. Coal ash, depending on the specific coal bed source, can contain toxic constituents in varying amounts that include arsenic, beryllium, boron, cadmium, hexavalent chromium, cobalt, lead, manganese, mercury, molybdenum, selenium, strontium, thallium, and vanadium, along with dioxins and polycyclic aromatic hydrocarbons (PAH). Technologies International Corporation (TIC) has developed a novel process for the conversion of wet CCRs into saleable products by a thermal process, based on a modified electric arc furnace (EAF). This process allows for an efficient and cost effective method for processing the CCRs at a cost of about $22 per ton. The thermal process converts the carbon into a syngas, which can be used to generate some of the energy used by the plant. The process also recovers the iron and other metals that may be found in the ash and can convert them to ingots for sale in the metals market. At these very high temperatures, inorganic portions of the coal fly ash are converted into slag, a glassy ceramic/basaltic non-leaching, sand-like material, which immobilizes the heavy metals and prevents them from leaching out. In fact, the toxicity characteristic leaching procedure (TCLP) levels are several orders of magnitude below regulatory levels (previous tests show the material to be five times less leaching than bottle glass). The slag can be used as concrete aggregate and also is the building block of rock wool, which is used to produce insulation for buildings/homes (blown-in insulation and used in the production of ceiling tiles), the dashboards of automobiles as well as used in agriculture, as a growth medium in indoor farming/hydroponics. Rock wool is a chemically neutral substrate that has an optimum air to water ratio, making it a great growing medium. TIC has been in existence for 10 years and holds 18 related patents. The process has zero air emissions. Nearly $9 million invested in TIC, including $1.25 million funding from DOE's NETL. This process can convert and dispose of all of the ash in an environmentally safe and cost effective ($22/ton) manner.