Removing high volatile trace elements in coal power station by injecting high carbon fly ash

Coal as available source of energy is consisting of carbon and a mixture of various materials including trace elements. Thus value of coal is partially offset by environmental impacts of its combustion. In coal-fired power station, pulverised coal consisting 10% to 30% minerals (depending type of the coal) is burned at temperatures higher than 1600°C. In combustion zone, minerals and trace elements go under different processes including decomposition, diffusion and agglomeration. Trace elements in the coal are redistributed into vapour phase, bottom ash and fly ash during and after combustion. Coal fly ash, or pulverised fuel ash (PFA) is the well known coal combustion by-product which has been used for many years for a wide range of construction applications (Sear, 2001). The main constituents of fly ash are glassy aluminosilicate phases with lesser of mullite, quartz, magnetite, hematite and carbon (Swaine, 1995). However, composition of fly ash depends on several factors including:
- coal composition;
- furnace configuration; and
- combustion conditions.

In a 500 MW power plant, the coal feed is about 5000 t/d (using a typical UK bituminous coal). If the ash content is 15%, then some 750 t/d of ash passes through the boiler (Couch, 1994). In a recent study, fate of trace elements from a 1050 MW power station has been studied (Llorens et al., 2001). In order of magnitude, the transfer of trace elements from the power plant studied to be surrounding environment exceeds more than 10 t/year of As, Cr, pb, V, Li and B through atmospheric emissions, while more than 1 t/year of Li, Ba, Cr, Sr, V, Mo and B from fly ash and Sr and B from bottom ash was estimated to be potentially mobilised by leaching of solid combustion by-products. Hence, a typical power plant produces large amount of waste per day and it only needs a tiny proportion of this contaminates environments of power plants. In a modern power station burning pulverised coal, more than 99% of the fly ash is removed from the flue gases by some means of particle attenuation, usually electrostatic precipitators or the use of fabric filters. This means that most trace elements are also removed, the separation efficiency being 95 to 99% (Godbeer and Swaine, 1995). Trace elements with high volatility leave power station with gaseous phase. Albeit the proper operation of electrostatic precipitators or fabric filters ensures that most particles larger than 10 mm are removed from flue gases, there is possibility some large particles (larger than 10 mm diameter) escape power stations. These particles would return to earth’s surface promptly and hence near the source. However, fine particles are removed form the atmosphere by rain, snow and fog depends on local climate of power station.
There are several methods to control concentration high volatile elements such as mercury in exit of power station. One of these methods is adding additives like activated carbon. Activated carbon may be injected in top of furnace and then removed by electrostatic precipitators. Although activated carbon is an excellent adsorbent, it is expensive. Another alternative adsorbent is high carbon fly ash. It is reported that there is always some carbon in fly ash (Swaine 1995, Watt and Throne 1965, Roy et al. 1981 and Suloway et al, 1983). Amount of this carbon in fly ash depends on combustion condition in power station. Tighter emissions standards have meant the fitting of low NOx burners in power stations. These changes have lead to higher carbon in ash and also deposition pattern in the boiler. This high carbon fly ash can be injected into top of the furnace after air heater in duct and adsorb high volatile elements and then collected by electrostatic precipitators or flue gas clean up section.
In order to model adsorption of mercury by high carbon fly ash, Langmuir hypothesis were developed. Assuming that the adsorption process is not diffusion limited and concentration of mercury in top of furnace is 1 mg/m3 (average concentration of mercury in UK coals is 1 ppm). Therefore ω can be calculated for residence time in duct. At adsorbent injection rate of 1000 g high carbon fly ash per unit volume of gas, 18.9% mercury would be removed.