Characteristics and interactions between coal and carbonaceous wastes during co-combustion
Introduction
Co-processing of biomass with coal is a cost-effective and environmentally benign approach to mitigate the emission of gaseous pollutants of CO2, SO2 and NOx from fossil fuel combustion [1], [2], [3], and is commonly adopted by coal-firing power plants in western countries as an option to reduce carbon emissions [4]. It is estimated that [5], the substitution of 10% of coal with biomass for the generation of electricity at coal-fired power stations would result in approximately 150 GW biomass power capacity, which is 2.5 times of the biomass power capacity installed nowadays worldwide. At coal-fired power stations, there are two commonly used approaches to replace a portion of coal with biomass, i.e. direct and indirect methods [6], [7]. In terms of direct co-firing process, renewable fuel and coal are burned together in the same furnace where milling process is conducted together or separately. On the other hand, indirect co-firing is based on thermal conversion of renewable to gaseous or liquid fuel and the co-firing of this converted fuel together with the base fuel [6], [7]. Direct co-firing based on co-milling is normally preferred due to its advantages such as high efficiency (36–44%) [8], low retrofitting costs [9], flexibility in biomass supply [10], etc. However, co-firing also presents technical challenges associated with slagging [11], [12], deposition and corrosion in boiler [13], [14], poor handleability [15], adverse impacts on electrostatic precipitation and potential poisoning of SCR catalyst [8], etc. Hence, more research is necessary in order to maximise the benefits as well as to overcome the challenges.
Although coal is originated from carbonaceous materials, its pyrolysis, combustion and gasification characteristics are significantly different from other carbonaceous materials, such as biomass and municipal solid wastes. Thus, to minimise the detrimental effect of co-firing on utility boilers, it is essential to understand how combustion characteristics is affected by the blending of different carbonaceous wastes with coal especially when a variety of carbonaceous wastes are to be used to replace a large portion of coal in pulverised fuel boilers [4], [16], [17], [18].
Over the past two decades, much work has been conducted to understand combustion characteristics of different fuels using thermogravimetric analyser (TGA) [19], [20], [21], [22], [23], aiming at a better understanding of possible interactions with respect to combustion behaviours [22], [24], [25], [26], [27], ash residue [28], [29], [30] and emission issues [28], [29], [31], [32]. However, co-processing still presents some technical challenges in industrial applications due to the change in the type of carbonaceous wastes to be co-fired and the variation in their thermochemical properties. To date, many researchers have investigated the interactions between coal and biomass during co-combustion [4], [16], [22], [27], [29], [32] and other carbonaceous wastes [21], [22], [33], but controversial results on the presence of interactions were reported. Recently, due to the significant amount of tyre and waste electrical and electronic equipment (WEEE) generated annually, there are some studies on the co-firing of coal and tyre scraps being reported [21], [22], [34]. However, not much effort has been made so far to understand the interactions during co-combustion of coal and tyre scraps, which is the same for non-metallic part of waste printed circuit boards (NMPCBs), one of the main components of WEEE, and for waste Polystyrene. Therefore, there is still a need to study the co-combustion behaviours when coal is co-fired with these carbonaceous wastes, which are of significant potential to be used as alternative fuels, and to understand the interactions between samples during co-combustion.
In this paper, the ignition and combustion behaviours of coal, a suite of carbonaceous wastes and their blends were studied using thermogravimetric analysis. The influence of blending on combustion characteristics was examined. The interactions between coal and carbonaceous materials during co-firing were also investigated in detail. The degree of interaction was quantified.
Section snippets
Sample preparation
In this work, an Australian coal (AC) and oat straw (OS) were selected as the coal and biomass samples. Other carbonaceous wastes studied include non-metallic part of printed circuit boards (NMPCBs) and waste tyre (tyre) together with waste polystyrene (PS) as a representative of plastic waste.
All the samples were air-dried prior to size reduction. Samples were grounded to <106 μm (Endecott 106 μm sieve) following standard milling procedures (CEN/TS 15443:2006). Blends were prepared at
Characterisation of materials
The proximate and ultimate analysis results and the calorific values of the individual samples are listed in Table 1. A significant difference in volatile content in the studied samples was observed. Hence, it will make a big impact on combustion properties such as ignition characteristics and burnout rate of the samples [22]. Among all samples, NMPCBs had the highest percentage of ash, which was mainly due to the presence of glass fibre. On the contrary, negligible amount of ash was detected
Conclusions
In this study, the combustion characteristics of coal, carbonaceous wastes and their blends were investigated at different blending ratios. Polystyrene was found to be the most reactive fuel whereas NMPCBs showed the least reactivity among all the materials investigated. The results revealed that ignition temperature decreased with the addition of carbonaceous materials, which would enhance boiler performance if these materials were to be co-fired with coal in a utility boiler. Burnout profiles
Acknowledgements
This work is partially sponsored by Municipal Innovation Team on WEEE Recycling Technologies (2012B82011), Major Research Scheme (2012B10042), and Zhejiang Provincial Innovation Team on SOx and NOx Removal Technologies (2011R50017). The University of Nottingham Ningbo China is acknowledged for providing scholarship to the first author.
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