Comparison of fly ash from co-combustion of coal/solid recovered fuel (SRF) and coal/refuse derived fuel (RDF).

The formation of ashes originating from the combustion of alternative fuels causes a need to find ways for their practical application and demands the knowledge about their properties. Therefore, the present work is devoted to studying the co-combustion of slternative fuel SRF/coal and RDF/coal. The major objectives of this paper is to present the detail characteristics of ash by using the advanced instrumental techniques (XRF, XRD, SEM, BET, TGA).


Introduction
According to the Regulation of the Polish Minister of Economy (1/1/2016) the landfill deposition of municipal waste with the higher heating value (HHV) than 6 MJ/kg is prohibited. Implementation of the requirements of this regulation is currently one of the biggest challenges for municipal waste management in Poland. Based on the statistical data, 12 485,4 thousand Mg of municipal waste was collected in Poland in 2018 [1] The application of waste derived fuels for energy production seems to be good option taking into account the environmental protection and the reduction of disposal. More waste fractions are characterized by high calorific value, that is why can be used as an alternative fuel. The use of waste as an energy source is an integral part of waste management and National Smart Specializations. The waste derived fuels from various types of wastes such as municipal solid wastes (MSW), industrial wastes or commercial wastes are commonly called refuse-derived fuel (RDF) and solid recovered fuel (SRF), depending on the fuel's characteristics [2][3][4][5].
RDF is generated from domestic wastes, which includes biodegradable materials as well as plastics. Non-combustible materials such as glass and metals are removed and the residual material is then removed. Whereas, solid recovered fuel (SRF) is produced from non-hazardous wastes including paper, wood, textiles and plastic with a calorific value up to 30 MJ/kg(depending on composition), and carbon and hydrogen contents c.a. 50 and 7 %, respectively. The net calorific value, chlorine and mercury contents of SRF are the main requirements which are classifying the SRF and they are described in SRF standard EN15359. [6][7][8]. Refuse derived fuel is used in combined heat and power facilities, many of them in Europe. With a moisture content of less than 15 % solid recovered fuel has a high calorific value and is used in facilities such as cement kilns.
Nowadays, the addition of alternative fuels (like SRF and RDF) in co-combustion has increased. Several authors have been studied the co-combustion of coal and SRF e.g. [9][10][11][12] The most important factor that influences this process is the composition of alternative fuels. It is worth to mentioned, that the using SRF, RDF instead of coal in co-combustion process helps to preserve natural resources and limiting the use of fossil fuels and reducing the impact on the environment through lower CO2 emissions [13][14][15] In be combusted. The fuel mixtures consist of RDF and coal are combusted. The amount of alternative fuel (RDF) can be up to 50 % of the total fuel mixture. The power plant will have a production capacity of 220 MW, including 145 MW of heat and 75 MW of electricity, and its annual production is estimated to be approximately 730 and 550 GWh of heat and electricity respectively.
Thus, the main aim of this paper is to study of RDF and SRF fly ashes. The morphological studies and the thermal behaviour of ashes were performed.

2.1.
Materials Fly ash samples examined in this study included three fly ashes from co-combustion of solid recovered fuel (SRF) (samples 1-2) and co-combustion of refuse-derived fuel (RDF) (samples 3).The coal used for the co-combustion of SRF experiments was a lignite coal. The co-combustion of studied fuel blends were carried out at 0.1 MWth fluidized bed combustor (at the Institute of Advanced Power Technologies of the Czestochowa University of Technology. In this study, coal + 10 % SRF and coal + 20 % SRF fuels were tested. During combustion experiments, the fluidized bed temperature was about 850 °C. The fly ashes from co-combustion RDF and hard coal were collected from combined-heat-and-power (CHP) plant in Zabrze. The fuel used was a mixture of 40% RDF and coal.

Instrumental methods
The samples of fly ashes were collected and prepared according to the standards BN-81-0623-01 (for slag, ash, and slag-ash mixtures). The investigation of the chemical composition included a determination of a basic chemical composition (SiO2, Al2O3, Fe2O3, CaO, MgO, Na2O, K2O, SO3, MnO2, TiO2, P2O5). An identification of crystalline phases present in the fly ashes, along with defining their relative amounts in the samples was carried out by means of the X-ray method on the D8 Advance (Bruker) powder diffractometer, equipped with a monochromatic device Ge (the length of radiation: CuKα1=1,5406 Å). Morphology investigation and sample textures were conducted using of the scanning microscope type Tesla BS-301-Satellite, equipped with spectrometer of energy dispersion (EDX), which allowed to determine the chemical composition of the selected samples of materials. Nitrogen adsorption/desorption was conducted using the Micrometrics ASAP 2010 (Micrometrics Instrument Corporation, Norcross, GA, USA) apparatus. A Mastersizer 2000 Particle Size Analyzer was used to measure the particle size distribution of the fly ash samples. Thermal analysis can provide important information about the thermal behaviour of the sample (phase transition, decomposition, etc.). TGA/DSC1 thermal analyzer (produced by Mettler Toledo was used in this study.

3.Results and Discussion
The chemical composition of studied ashes is presented Table 1.

*-CFB boiler combustion
The concertation of the main oxides is different, because of different origin of studied fuels. The chemical position reported in Table 1 shows that fly ashes in mainly composed of silicon (SiO2), aluminium (Al2O3) and iron (Fe2O3). In addition, the RDF ash is rich in calcium (CaO). The concertation of potassium and sodium are not high and comparable, this can suggest that slagging of these ashes should not to appear. Figure 1 presents the TG and DTG curves during heating the studied ashes under the nitrogen atmosphere. TG results show that thermal conversion up to 1000 °C of studied ashes have different course. For 10 % SRF ash the mass loss above 200 °C goes different than for other ash samples. Up to 750 °C the thermal decomposition takes place with nearly the same rate. Above 750 °C the mass loss is The major addition of SRF (20 %) to coal influences on its ash thermal behaviour. The significant mass loss is observed in the temperature range 400°C and 600 °C. Above 600 °C the mass loss in nearly constant. The weight loss is observed between c.a. 352 to 670 °C with the total loss of 2.74 % with maximum weight loss rate at 544 °C, which was indicated to the oxidation of coal. The second weight loss between 670°C to 777 °C was the result of the oxidation of carbon by the iron oxides with the total loss of 0.83 %. The residue at c.a. 1000 °C was about 83,57% of the original sample weight. For 40% RDF ash the mass loss goes into three stages from c.a. 340°C to 840 °C. The weight loss is observed between c.a. 340 to 420 °C. The second weight loss is observed between c.a. 420°C to 680 °C with maximum weight loss rate at 590 °C, which was indicated to the oxidation of coal. The next weight loss between 700°C to 900 °C was the result of the oxidation of carbon by the iron oxides. Due to several overlapping peaks, the minerals are difficult to identify. The most of the effects detected on the TGA diagrams are related to the loss of humidity between room temperature and approximately 300 °C. Thermal reactions between 275-450 °C was oxidation of the surface (Fe3O4) and between 480°C -1000 °C was oxidation of the bulk 2Fe3O4 → 3Fe2O3. Between 100°C and 200 °C: escape of adsorbed and interlayer water. About 550 °C was dehydroxylation of illite and at ca. 900 °C was destruction of the lattice and formation of spinel (illite). CaSO4 is decomposed only at temperatures higher than 1000 °C. In a case of hematite in temperature 400°C -500°C shows some weight loss and oxide undergoing decomposition and dehydration of hematite [16].

Thermogravimetric experiments
Concluding, the weight losses of studied ashes with the increase of temperature take place due to moisture loss and the decomposition of some minerals. Table 2 shows the specific surface area (BET) of studied ashes. The specific surface area (SB.E.T.) of studied ashes is in the range from 17.92 m 2 /g to 6.88 m 2 /g for SRF ashes. The ash adsorption capacity is a major property for the beneficial utilization of ash. The adsorption capacity of fly ash mainly indicates from amount of carbon content but also from other properties such as the particle size, surface chemistry, and positioning of carbon in the fly ash particle. The values of sorption capacity of studied ashes decrease with the share of alternative fuels in fuel mixture. Figure 2 shows the N2 adsorption isotherm at 77K for studied ashes.   N2 adsorption isotherms for all ashes were classified according to IUPAC regulations as isotherms of type II. This type of isotherms is typical for macro-pores materials, and is connected to situations in which low relative partial pressures of an adsorptive on a surface of an adsorbent result in occurrence of some monomolecular mini-layer of the adsorbed substance (N2), whereas in the case of high relative partial pressures some multi-molecular layer of adsorbent on the surface of adsorbent is created. In the isotherms of the fly ashes were observed that the loop of hysteresis (type H3), begins with relative low pressures that prove a low content of micropores in that structure. The loop of H3 hysteresis is typical for adsorbents with pores formed in the shape of gaps.

Texture analysis
In order to determine the morphology and the porous structure, the samples of fly ashes were subjected to an analysis of SEM-EDX scanning microscopy. The results were presented in Figure 3. The morphology of the co-combustion ashes were quite similar to each other. The dominant particles of all ashes were contained mainly coarse and angular, flaky, drossy, and irregular particles with a broad particle size range. The morphology of a fly ash particle was controlled by combustion temperature and cooling rate. The multi-mineral, subangular particles of fly ash often were consisted of a core of quartz or aluminosilicate that were reacted with calcium to produce a calcium-rich aluminosilicate followed by calcium and iron oxides. In all fly ashes were also observed irregularly shaped unburned carbon particles. The grains of CFB ashes had irregular shapes because in the temperature of the CFB boiler, mineral substances accompanying the coal were not subject to partial melting.

Mineralogical composition
The mineralogical characteristics of the fly ashes were analysed by XRD ( Figure 4). Quartz, anhydrite and illite were identified in all studied ashes. These minerals are usually present in coal fly ashes.  In addition, magnetite and iron oxide were also present. In the fly ash several peaks were present that could not be related to specific XRD-patterns of minerals. The presence of the background in the diffraction pattern points to amorphous phases, which was represented a wide peak from 15° to 35 o 2Θ. The position of the background is influenced by the composition of the amorphous phase.