The investigation of co-combustion process for synergistic effects using thermogravimetric and kinetic analysis with combustion index
Introduction
The global economic growth and population explosion in our society have caused an increase in energy demand [1], resulting in the immense consumption of fossil fuels. Especially, negative environmental impacts and increasing concerns about socio-political problems have impelled our civilization to constrain the excessive use of coal in the near future [2]. The particulate matter, NOx, and SOx concentrations released to the environment stemmed from the coal combustion process are at higher levels compared to other fossil fuel utilization [3], [4]. According to the International Energy Agency (IEA), 45% of the CO2 emission caused by the use of coal [5], [6], which is a significant factor causing climate change. Moreover, increasing high-rank coal consumption day by day [7] has urged researchers for effective utilization of low-rank coals such as lignite in coal combustion-based power generation plants [8]. Turkey has 15 billion tons of lignite reserve and 60 percent of this reserve is identified by the low heating values (<6300 kJ/kg), further, only 8 percent of this reserve is taking place with a high calorific values (~12,500 kJ/kg) [9]. Furthermore, these low-rank coals have high moisture and ash content. Ultimately, direct combustion of these low-quality lignites is not an effective utilization method and great efforts are made to achieve sustainable development considering energy, environmental, and economic concerns altogether [10].
Biomass energy seems to offer an alternative solution to fossil fuels and has huge potential towards the path for sustainable energy production [11], [12], [13]. Generating power and heat from biomass by using direct combustion is the major application technique. Nevertheless, some deficiencies such as high moisture content, the variability of heating value and quality, the necessity of big area or volume for storage, comparatively low energy density, economic problems associated with distribution and transportation, limits the application of biomass as a fuel [14], [15], [16], and it meets only a small fraction of our energy demand in the current power plants. Therefore, co-combustion is deemed necessary for biomass utilization to eliminate these disadvantages and looks a promising, probable, and easy technique as energy products for heating, power generation, or other purposes.
The extremely high ignition temperature of Imbat coal makes it difficult to use it in the mono-combustion process as a fuel. Hence, in order to boost the applicable combustion of Imbat coal, some supporting fuel like biomass is essential to trigger the firing of Imbat coal. The combustion characteristics of the almond shell are quite different from those of Imbat coal. Almond shell has low ash and high volatile content compared to Imbat coal, so the addition of almond shell to Imbat coal can improve the performance and influence the entire combustion process. Among biomass sources, the almond shell is exclusive in terms of high organic fraction and huge volume, locally and widely availability [17], which make its co-firing process with Imbat coal more economical and technically preferable [18], [19], [20]. The blend can also relieve alkali-brought problems in the mono-combustion process of almond shells.
Thermogravimetric analysis (TGA) is the most common method used to characterize the combustion process [21]. The literature is abundant in resources regarding the combustion process of Turkish coal [22], [23], [24], [25], [26], [27], [28]. However, research about co-combustion characteristics of Turkish coal has been rarely reported. Magalhães et al. [29] investigated the combustion behavior and kinetics of Turkish lignites (Soma and Tunçbilek) and olive residue by using TGA. However, the researchers examined all solid fuels in the mono-combustion process and did not investigate the effect of biomass/coal ratio. Varol et al. [1] investigated combustion and co-combustion characteristics of three biomass fuels (wood chips, olive cake, and hazelnut shells) and three Turkish lignites (Tunçbilek, Seyitömer, and Orhaneli). The authors performed their experiments using three different blend ratios (25%, 50% and 75% by wt.) at a single heating rate. Moreover, there are no findings of combustion performance index in their study. Kanca et al. [30] examined pyrolysis and combustion characteristics of Turkish lignite, cotton wastes, and their blends in five different proportions at a single heating rate. The author did not report any kinetic analysis results and combustion performance index results about co-utilization processes. Additionally, the co-combustion process of Imbat coal with almond shells and their interaction has not been studied thoroughly. Along with the thermogravimetric analysis of biomass and coal, kinetic modeling of the combustion process is vital for industrial-scale production [31]. Successful implementation of kinetic characterization helps to minimize errors in the design of industrial equipment and increases product yield [32]. Kinetic parameters can be obtained with quantitative methods applied to TGA curves. Kinetic parameters typically include the order of reaction, pre-exponential factor, and activation energy [33]. However, the kinetic parameters are extremely dependent on experimental conditions such as size and moisture content of the sample, heating rates, and the medium. These parameters naturally lead to publications with many various kinetic parameters in the literature that would make it difficult to compare [33].
This study has two folds. First of all, thermal behavior of Imbat coal and almond shell blends including different blend ratios in the co-combustion process employing the TGA technique to describe quantitatively the extent of synergetic mechanisms and associated threshold through determining ignition temperatures, combustion zones, and percentage mass loss were investigated. This paper, on the other hand, has an objective to examine non-isothermal kinetics like the pre-exponential factor and activation energy calculation together with the combustion index. The kinetic parameters during the co-combustion process were calculated by using the Coats-Redfern model.
Section snippets
Sample characterization
The feedstocks used in this study were provided by local growers in Turkey and Turkish Coal Enterprises Institution; almond shells (AS) from the Aegean region and Imbat coal (IC) from Soma, Manisa. Almond shell, Imbat coal, and their different blends (20%:80%, 30%:70%, 40%:60%, 50%:50%, 60%:40%, 70%:30% and 80%:20% by wt.) were used in the experiments. The feedstocks were dried, crushed, and sieved to reduce the particle size down 250 μm for experimental study. The grinding process was carried
Fuel characterization
The amount of elements in the biomass and coal structure that create them with different characteristics. Blending coal and biomass in different ratios can significantly change the properties of the fuel. The elemental and proximate analysis results of almond shel and Imbat coal sample blends has been influenced by the its ratios in the mixture. Elemental analysis results are converted to atomic H,C and O and plot of O/C versus H/C of Imbat coal, almond sheld and their blends are characterized
Conclusions
In this study, the synergistic processes of the co-combustion have been investigated to reveal the influences resulted from intrinsic characteristics using different almond shells and Imbat Coal blend ratios. The main conclusions are as follow:
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The blending ratios have a significant effect on the reaction rate of the co-combustion process but the kinetic characteristics of AS, IC, and their blends has no linear relation with blending ratios due to interactions between the almond shell and Imbat
CRediT authorship contribution statement
Senem Sezer: Data curation, Investigation, Software, Validation, Visualization, Writing - original draft, Writing - review & editing. Furkan Kartal: Data curation, Investigation, Software, Validation, Visualization, Writing - original draft, Writing - review & editing. Uğur Özveren: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing - original draft, Writing - review &
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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