**1. Introduction**

The rapid growing of population continuously increases the global demand for energy. This demand is actually mainly satisfied by the consumption of oil followed by coal, which remain the world's leading fuels, respectively accounting for 33% and 30% of global energy consumption [1]. Such an extensive use is leading to the depletion and ever-rising prices of these fossil non-renewable fuels [2]. On the other hand, greenhouse gases emission from burning of fossil fuels, mainly CO2, is the main cause of global warming [3]. These facts have motivated increasing research efforts regarding alternative and/or non-conventional energy resources. Among them, biomass, which may be categorized into first, second, and third generation according to its origin, has been recognized as a promising option, since it is sustainable, renewable, and less polluting [2–4].

Microalgae biomass may be considered to be a third generation biofuel, which holds several advantages, such as microalgae rapid growth rate, high oil content, high yield per area, no competition with crops for arable land or freshwater [5,6]. Furthermore, microalgae are considered as promising candidates for CO2 bio-sequestration [7]. However, the implementation of CO2 sequestration by microalgae is mostly limited by techno-economic constrains [8]. Microalgae may be cultivated in wastewater, allowing for simultaneous CO2 mitigation and wastewater treatment [7]. In order to

increase economic feasibility [7]. In this way, wastewater is used as source of nutrients and water, which allows for reducing the costs of microalgae culturing [5,9,10]. On the other hand, microalgae are efficient microorganisms for wastewater treatment, since they are able to remove not only nutrients, but also heavy metals [11] and emerging contaminants [12,13]. Still, during microalgae cultivation, residual biomass is generated and use should be given to this biomass within the actual circular economy context [14]. Therefore, the utilization of microalgae biomass as a third generation biofuel might be an option for closing the loop and increasing the sustainability of microalgae culture [8]. In this sense, integrating microalgae culture-wastewater treatment-biofuel production allows for carbon dioxide mitigation and wastewater treatment, while providing biofuel feedstock in a much cleaner manner [7,15].

Biomass thermochemical conversion is considered to be one of the most effective and promising routes aimed at the use of biomass for energy purposes. Thermochemical processes typically include [16]: pyrolysis, gasification and combustion, which is the most commonly used pathway to extract energy from biomass [17]. Algal biomass has lower decomposition temperatures during thermochemical conversion when compared to lignocellulosic biomass, which is due to differences in their major components and results in higher reactivity and lower operational costs [18]. In any case, a good understanding of microalgae behaviour during thermochemical conversion is essential in efficient processing.

The utilization of thermogravimetric analysis (TGA) for the characterization of thermal decomposition during the combustion of coal is well established, being more recent its use for biomassic fuels [19] and their co-processing with coal [20–22]. Such utilization is advantageous, since TGA offers a rapid evaluation of the thermal decomposition of any fuel, the initial and final temperatures of combustion, and other important features, such as maximum reactivity temperature or interaction between fuels during co-processing [19,23].

Comparatively with lignocellulosic biomass, only very recent and few works are concerned about the thermal analysis of microalgae combustion [18] and studies on the co-combustion of microalgae biomass with fossil fuels, such as coal, are even scarcer. However, co-combustion with fossil fuels is an interesting option that may help to reduce the consumption of non-renewable resources for power generation, while allowing for the utilization of existing infrastructures. Thus, in this manuscript, simultaneous TGA and differential scanning calorimetry (DSC) were used to assess the combustion behaviour of microalgae biomass and its blend with coal. The main aims were to evaluate the effect that blending with microalgae biomass has on the combustion of coal and its kinetics and to find out whether interactions between both fuels occur during their co-combustion.

#### **2. Materials and Methods**
