Classification of carbon in Canadian fly ashes and their implications in the capture of mercury

Abstract

Fly ashes produced from Canadian power plants using pulverized coal and fluidized bed combustors were examined for their carbon content to determine their ability to capture mercury. The feed coal used in these power plants were lignite, subbituminous, high and medium volatile bituminous, their blends, and also blends of coal with petroleum coke (Petcoke). The carbon and mercury content of the coals and fly ashes were determined using the ASTM standard method and by the cold vapour atomic absorption spectrometry method. The carbon content of the fly ash was concentrated by strong acid digestion using HCl and HF. The quantitative and qualitative analyses of the carbon concentrate were made by using a reflected light microscope. The results show that the carbon content of fly ash appears to be partially related to depositional environment during coalification and to the rank of the coal. The Hg captured by the fly ash depends on the rank and blend of the feed coals and the type of carbon in the fly ash. The isotropic vitrinitic char is mostly responsible for the capture of most Hg in fly ash. The inadvertent increase in carbon content due to the blending of coal with petroleum coke did not increase the amount Hg captured by the fly ash. The fly ash collected by the hot side electrostatic precipitator has a low Hg content and no relation between the Hg and carbon content of the ash was observed. These results indicate that the quantity of carbon in the fly ash alone does not determine the amount Hg captured. The types of carbon present (isotropic and anisotropic vitrinitic, isotropic inertinitic and anisotropic Petcoke), the halogen content, the types of fly ash control devices, and the temperatures of the fly ash control devices all play major roles in the capture of Hg.

Introduction

Characterizing particles of ash emitted from anthropogenic sources such as coal-fired power plants is very important for assessing the possible impacts on human health and the environment [1], [2], [3], [4], [5], [6], [7], [8], [9], as well as the ability for elements such as mercury to be captured [10]. Most macroscopic and microscopic particles are removed by particle control devices such as electrostatic precipitators (ESP), fabric filters (FF), and mechanical cyclone separators (MCS). These devices can achieve up to a 99.8% rate of efficiency in removing fly ash from the flue gas [11], [12]. Fly ash consists mainly of aluminosilicate particles and small amounts of carbon, which consists of char, unburnt carbon and coke [9]. Pervious studies [10], [13] have found relatively low amounts of carbon in fly ash from Canadian coal-fired power plants.

Some carbon in fly ash can capture flue gas mercury through adsorption [10], [14], [15]. However, not all carbon in fly ash has the same ability to capture mercury [16], [17]; therefore, it is important to understand the nature of fly ash carbon. In general, carbon in fly ash from bituminous coal shows evidence of optical anisotropy in polarized light, where as carbon from lignite-subbituminous coal does not go through a thermoplastic stage [18] and are optically isotropic [15], [16], [19], [20], [21]. For the purpose of this study, the isotropic carbon will be designated as char and the anisotropic carbon will be designated as coke. These two different carbons have different physical properties. For example, the surface area increases from isotropic inertinite derived char to isotropic vitrinitic char to anisotropic vitrinitic coke [15], [17]. Although the surface area of char is less significant than the presence of halogen species for mercury capture [21], a variable retention capacity for mercury has been observed for different carbons in simulated flue gas [22]. Even though these carbons are exposed to the same flue gas composition, at the same temperature, for the same amount of time, the difference in their capacity to capture mercury suggests that this is due to the nature of the carbon. To better understand the possible influence of char composition on mercury capture we use reflected light microscopy to identify and count the different kinds of char in various fly ashes, and compare these results to the mercury captured by the fly ashes.

Many attempts have been made to link the various type of carbon in fly ash with parent coal properties, such as rank and maceral composition [19], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32]. Several classification systems were introduced to differentiate the combustion carbon based on morphological characteristics [19], [32], [33], [34], [35], [36], [37], [38]. Some of these systems are based on observations of carbon that were produced in the laboratory and may not be applicable to chars found in fly ash collected from coal-fired power plants. Furthermore, some of the classifications are too complicated for every day reporting with regard to environmental and industrial compliance. Accordingly, we use a simple, three-component, char classification system.

Carbon in coal (macerals) can be divided into three groups. The first group is vitrinite, which formed mainly from wood and often is the most abundant maceral in coal [39].

The second group is inertinite, which is generally the second most abundant maceral in coal [39]. The inertinite group is comprised mainly of natural char coal that formed due to forest fires during coal formation [40]. The third group, liptinite is derived from hydrogen rich waxes, spores, pollens and resins [39].

There are also associations of coal macerals such as bimacerals, when a inertinitic fragment is found in a vitrinitic matrix [39].

Combustion of coal generally produces a mixture of vitrinitic and inertinitic carbon, since liptinite is completely burnt during combustion without leaving any char particles [10].

In general heat treated coal produces isotropic and anisotropic carbon [18], [41]. Low rank coal (lignite to high volatile bituminous C) produces isotropic char and higher rank bituminous coal produces anisotropic coke with various granular optical textures [18], [42].

However, the inertinite is mostly formed at high temperatures during coalification due to forest fires and peat fires (temperatures up to 600–700 °C) [43], [44], [45], therefore it may escape the total combustion and be carried away by flue gas to the ESP, where it contributes to the unburnt carbon found in fly ash collected by the ESP.

Vitrinite char is mostly volatilised during combustion and therefore is a reactive component of coal. Weathering can reduce the reactivity of vitrinite [46]. Vitrinitic char in fly ash from Canadian lignite to high volatile C bituminous feed coals consists of cenospheric isotropic char that may not contain microscopic ribs or interconnected lattices within its core (Fig. 1a). The vitrinitic char are often large (up to 200 μm) and can form subangular spongy structures (Fig. 1b). These types of char generally have large surface areas of 200–400 m² g⁻¹[15], [17] and are able to capture volatile elements such as Hg from flue gas particle control devises, such as cold side ESP’s and FF’s [15], [47], [48]. On the other hand, vitrinitic carbon formed from higher rank bituminous coal (above high volatile A bituminous) consists of thick and thin anisotropic coke cenospheres (Fig. 1c and d). Thick-walled coke cenosphere often contain devolatilization vacuoles and mosaic structures within their walls (Fig. 1c). In general, the size of the mosaic structures increases with the rank of feed coal for anisotropic char. The anisotropic char with mosaic textures have a lower surface area (35–60 m² g⁻¹) [15], [17], [22]. Both vitrinitic char and coke may contain fragments of inertinitic char.

Inertinite are natural chars [44], [49], [50]. Fly ash inertinite have small surface areas (15–25 m² g⁻¹), compared to isotropic chars (25–35 m² g⁻¹) and anisotropic chars (35–60 m² g⁻¹) [15], [17]. Inertinite are the lightest of the fly ash carbons, separating at a specific gravity of 1.8 [51]. Inertinite char is isotropic and may show a combustion front. The inertinitic chars burn less-readily than the vitrinite particles. Inertinite macerals are friable and brittle, and may become fragmented into smaller particles during the milling of coal and also during combustion [13]. The inertinitic chars in fly ash may also show a typical cellular morphology (Fig. 2a,b,d,e) similar to activated carbon, or sub-angular solid char of various sizes (Fig. 2f). The fly ashes from some subbituminous coal contain pyrolytic carbon that was present in the feed coals [44], [49].

Petroleum coke (Pet coke) is sometimes blended with coal as feed. The petroleum coke is anisotropic with a medium to coarse mosaic texture (Fig. 3c,e).