Characterization of Ash Melting of Reed and Wheat Straw Blend.

Ash melting could cause severe problems in boiler operation, such as agglomeration of the fluidized bed. Our previous work has shown that the ash melting behavior of the blend of reed and wheat straw is complex and needs further investigation. The ash melting behavior was studied using different laboratory methods such as simultaneous thermal analysis, heating microscope, scanning electron microscopy with energy dispersive X-ray (SEM/EDX) analysis, and X-ray diffraction (XRD). In the thermodynamic modeling, we used FactSage software, which supplements well the results obtained by other methods and vice versa. The results indicated that melting started at 660-680 °C when Na and K salts were melted and molten K2SO4 covered the ash material; the content of liquid mass fraction was 13.8%, revealing that the studied ash blend could already be sticky at 680 °C. Intensive melting took place in the temperature range of 800-980 °C. The rapid melting between 950 and 980 °C was caused by the melting of SiO2 and K2MgSi5O12; the ash material became glassy and amorphous. We propose an alternative distribution of ash melting stages for the studied blend.

Introduction

The increased worldwide energy demand has decreased fossil fuel reserves, and concerns with climate change have paved the way for renewable energy resources.[1,2] Biomass fuel is an abundant and readily accessible alternative energy; it is receiving increasing attention because of its renewability and carbon-neutral nature.[3] Biomass combustion units were originally designed for wood firing.[4,5] However, during recent decades, more attention has been paid to multifuel firing boilers.[6] A number of low-grade biomass fuels have been considered as potential fuels.[7] Biomass ashes contain a variety of inorganic elements that form complex compounds in gaseous, liquid, and solid phases during thermal conversion.[8] Agglomeration is one of the key issues in terms of co-firing of biomass in the fluidized bed boiler.[9] The prediction of ash-related problems requires detailed knowledge of the fuel being fired.[10]

When fuels are blended, the properties of the formed ash cannot be predicted according to the properties of the ash of a single fuel, i.e., the behavior of inorganic and some organic constituents is not necessarily linear with blends. In addition, ashes originating from mixtures of several parent fuels have shown a complex melting behavior depending on the mineral composition. Therefore, knowing the impact of a certain blend on the properties of the final ash enables us to avoid fuel combinations with unwanted properties or even to design an ash material for a certain application.[11−13] A number of studies have focused on the behavior of ash melting of biomass-to-biomass or biomass-to-coal blends.[14−17]

Wheat straw wastes are an herbaceous agricultural residue composed of leaves, bran, and straw, which are usually incinerated or landfilled.[2] Common reed (Phragmites australis) is widely distributed worldwide,[18] where its availability and usage for energetic purposes have become attractive.[19−22] Reed and wheat straw are both complex fuels for thermochemical conversion due to their chemical composition. Nevertheless, these fuels are available worldwide and have been used for energetic purposes. Reed and wheat straw differ by their mineral composition. Due to high silicon content, reed ash is considered as ash with a high melting temperature. Conversely, wheat straw ash has low melting temperature influenced by potassium and chlorine.[23−27]

Numerous methods and techniques are available to study the ash melting behavior—ash melting tests, determination of the chemical and mineralogical composition, morphology analyses, and prediction of the ash melting behavior by modeling.[28−39] However, no detailed stepwise studies of the biomass ash melting process combined with various laboratory methods have been reported.

The fireside fly ash deposit accumulation in boilers is the main concern of the plant operators, and the temperature at which the particle changes from nonsticky to sticky is critically important in boiler design and operation. It has been found that fly ashes could be sticky, containing at least 15% of melt, i.e., liquid fraction.[40] However, regarding the stickiness of the ash particle, the percentage of the molten phase of ash allows only a coarse estimation.[41] Modeling programs such as FactSage allow for estimating the ash chemistry and molten fraction according to the given composition and the Gibbs free energy minimization principle. The modeling results, however, do not necessarily represent real life scenarios, though their aim is to evaluate the nature of the whole process.[42,43]

According to our previous work,[44] the blend of reed and wheat straw showed the most complex ash melting behavior among the studied blends, and blending with certain amounts could result in eutectics. The behavior of the blend of reed and wheat straw differs a great deal from that of the parent fuels; it needs detailed analysis regarding its suitability as a boiler fuel.

In this study, the ash melting behavior and chemistry of the reed and wheat straw blend were investigated in detail with different methods and an alternative approach. As is different from previous studies, we applied a new approach and explored the ash melting mechanism using stepwise thermal treatment. Variations in mineralogical properties were determined by both laboratory methods and FactSage software. The obtained data will provide valuable information on the usage of this blend, e.g., in boilers and gasifiers.

Results and Discussion

Characterization of Materials

Table shows that the ash content is higher for WSP (4.5% dry basis) compared to reed (3.8% dry basis). WSP contains more S and Cl as well.

Table 1

Proximate Analysis, Ultimate Analysis, and Cross-Calorific Value of the Parent Fuels

fuel	moisture	proximate analysis, dry basis (wt %)			ultimate analysis, dry basis (wt %)						gross calorific value(MJ/kg)
		ash	volatile matter	fixed carbon	C	H	N	O	S	Cl
WSP	8.2	4.5	77.3	18.2	46.6	6.2	0.4	41.9	0.1	0.3	18.8
R	6.3	3.8	80.9	15.3	47.5	6.2	0.2	42.2			18.9

Table gives the composition and the loss of ignition (LOI) of the parent fuel ashes as suggested by the XRF analyses. SiO2 content is very high in reed ash, e.g., four times higher than in WSP ash. WSP ash has the highest P2O5, SO3, Cl, and K2O contents. In the ash of WSP, both K2O and Cl contents are very high with values of 32.4 and 7.7 wt %, respectively. The relatively high LOI value of WSP reveals the presence of carbonates besides other volatile species, e.g., KCl.

Table 2

Chemical Composition of Parent Fuel Ashes and the Blend of Ashes (wt %)

	R	WSP
Na2O	1.7	0.1
MgO	1.7	4.7
Al2O3	0.1	0.3
SiO2	80.2	15.0
P2O5	1.0	3.3
SO3	2.8	3.5
Cl	1.0	5.7
K2O	6.1	24.1
CaO	2.2	17.1
Fe2O3	0.1	0.1
LOI	3.0	25.7

Standard Ash Melting Tests

The results of the ash melting test of 50R/50WSP according to the standard method of CEN/TS 15370 are presented in Figures and 2.

Figure 1

Ash melting temperatures according to CEN/TS 15370.

Figure 2

Ash melting curves at different end temperatures.

The curves of a series of heat treatment tests are shown in Figure . The heat treatment procedure of the standard method of CEN/TS 15370 was applied for HSM tests using different end temperatures. We can observe a similar path of heat treatment curves for all the HSM tests, which gives evidence of a relatively good repeatability of the process.

In Figure , the ash melting path of the selected sample material could be divided into four regions:[45] 1 – unaffected by heat treatment; 2 – sintering stage, densified mass by the end; 3 – expanding stage, porous medium; 4 – flowing liquidous mass by the end. The Tsp (“sintering point,” and in our case, Tsp = 688 °C) in region 2 indicates the temperature where the swelling behavior changed to height decrease. Texp (“expansion point,” and in our case, Texp = 780 °C) shows the temperature where due to melting, the height decrease of the sample body turns to height increase. Temp (“excessive melting point,” and in our case, Temp = 933 °C) expresses the temperature where the excessive melting starts. The expansion in region 3 could be attributed to several effects such as the thermal expansion of ash particles and primary pores trapping air in the ash particle as well as the evolution of gases during the decomposition of ash components.[14,45] Region 4 shows the last step of the ash melting process; the fluctuations (seen as noise or oscillations in the plot) in height are the result of bubbling.

Results of SEM/EDX Analysis

The SEM images and the EDX results of the initial ash sample and the ash samples after heat treatment with HSM are shown in Figure and Tables and 4.

Figure 3

Images of the ash samples. (a1) Initial ash sample at 550 °C, SEM picture at 30× magnification; (a2) Initial ash sample at 550 °C, SEM picture at 1000× magnification; (a3) Initial ash sample at 550 °C, SEM picture at 1000× magnification; (b1) Ash sample at 600 °C, regular photo at 1× magnification; (b2) Ash sample at 600 °, SEM picture at 30× magnification; (b3) Ash sample at 600 °, SEM picture at 1000× magnification; (c1) Ash sample at 700 °C, regular photo at 1× magnification; (c2) Ash sample at 700 °C, SEM picture at 30× magnification; (c3) Ash sample at 700 °C, SEM picture at 1000× magnification; (d1) Ash sample at 800 °C, regular photo at 1× magnification; (d2) Ash sample at 800 °C, SEM picture at 30× magnification; (d3) Ash sample at 800 °C, SEM picture at 1000× magnification; (e1) Ash sample at 900 °C, regular photo at 1× magnification; (e2) Ash sample at 900 °C, SEM picture at 30× magnification; (e3) Ash sample at 900 °C, SEM picture at 1000× magnification; (f1) Ash sample at 1000 °C, regular photo at 1× magnification; (f2) Ash sample at 1000 °C, SEM picture at 30× magnification; (f3) Ash sample at 1000 °C, SEM picture at 1000× magnification; (g1) Ash sample at 1050 °C, regular photo at 1× magnification; (g2) Ash sample at 1050 °C, SEM picture at 30× magnification; (g3) Ash sample at 1050 °C, SEM picture at 1000× magnification; (h1) Ash sample at 1099 °C, regular photo at 1× magnification; (h2) Ash sample at 1099 °C, SEM picture at 30× magnification; (h3) Ash sample at 1099 °C, SEM picture at 1000× magnification.

Table 3

Chemical Composition of the Initial Ash and Ash Samples after Heat Treatment up to 1000 °C Obtained by EDX Analysis

area	initial (Figure 3a1)	600 °C (Figure 3b2)	700 °C (Figure 3c2)	800 °C (Figure 3d2)	900 °C (Figure 3e2)	1000 °C (Figure 3f2)
SiO2	59	50	42	18	25	14
K2O	17	20	25	39	34	38
Na2O	1.4	1.1	1.2	0.9	1.4	1.7
CaO	11	14	12	4.9	5.7	4.8
MgO	2.7	3.3	2.9	0.8	1.25	0.8
SO3	3.7	4.3	13	35	32	40
Cl	3.4	4.1	1.8	0.4
P2O5	2.0	2.8	2.6	0.8	0.9	0.6

Table 4

Chemical Composition of Ash Samples after Heat Treatment at 1050 °C (Figure g2) and Flow Temperature (Figure h2) Obtained by EDX Analysis

	1050 °C (1)	1050 °C (2)	1050 °C (3)	FT (1)	FT (2)	FT (3)
SiO2	5	39	0.3	3.8	48	2.6
K2O	46	25	46	43	21	45
Na2O	1.0	1.4	2.2	1.3	1.7	2.4
CaO		13		3.8	12
MgO		2.3			2.6
SO3	47	17	51	47	10	49
Cl
P2O5	0.8	2.7		1.4	2.9	0.5

According to their ash composition, Saidur et al.[46] have divided biomass fuels into three categories:

Ca- and K-rich, Si-lean ashes (generally woody biomass)

Si-rich, Ca- and K-lean ashes (generally herbaceous or agricultural biomass)

Ca-, K-, and P-rich ashes (sunflower and rapeseed)

The composition of 50R/50WSP ash (see also Table , initial) does not match well with the classification above; in our case, it is Si-, K-, and Ca-rich ash. The ash composition of the biomass blend could result in a new category of ash material. Therefore, general trends of the ash melting behavior of a certain type of single biomass are not valid for biomass blends.

The initial ash sample (see also Figure a1) exhibits particles with irregular shapes. The three main elements detected were Si, Ca, and K (see also Table ). The ash particle in Figure a2 has porous structure, and the composition is SiO2 (56%), K2O (16%), CaO (12%), Cl (5%), SO3 (4%), P2O5 (3%), MgO (3%), and Na2O (2%), which supports the composition of the total sample as shown in Table marked as “initial.” White particles in Figure b contain mainly K2O, Cl, and SiO2. The gray outer layer contains mainly SiO2 (90–92%) and less K2O (5%), Na2O (1%), and Cl (1%). Both reed and wheat straw stem particles have circular plate-like shapes with white particles on the surface. However, based on the surface morphology (SEM images), the particle shown in Figure a3 could be attributed to reed.[47,48] On this particle, the circular “plates” contain 99% of SiO2 and 1% of K2O, and the gray outer layer contains SiO2 (92%) and less K2O (4%) and CaO (2%). At 600 °C, the surface of the ash sample (Figure b1,b2) has holes, and the inorganic particles are neither softened nor stuck together (see also Figure b3). By increasing the end temperature, the surface of the sample body became more even. At 700 °C (see Figure c1–c3), the first transition from rough surface toward smooth surface is observed, and from the temperature of 800 °C, a glassy surface appears (see Figure d1–d3). As can be seen, the number of pores visible on the surface decreased with the increase in temperature. The molten and glassy surface has filled the pores. This could also explain the expansion of the sample body, while the evolved gases have limited pass-through from the inside of the sample body to the outside. At 900 °C (see also Figure e1–e3), the sample body is curved and has a scabrous surface, which is most probably caused by bubbling and penetration of evolved gases. After the excessive melting point (933 °C), it started to collapse and build glassy material (as seen in Figure f1–h3). The melted material (Figure g2 and h2) divided into three zones differs by composition (see also Table ). The middle zone (marked as number 1 in Figure g2 and h2) contains mainly K and S, revealing molten K2SO4. The next zone around the middle area (marked as number 2 in Figure g2 and h2) contains Si, K, Ca, Mg, S, and P. The outer zone (marked as number 3 in Figure g2 and h2) again contains mainly K and S. As can be seen, the content of Na increases from the middle to the outside (see also Figures g2,h2 and Table ). The reason is that compounds containing Na and K were the first that started to melt, and the melted fraction has flown away from the center of the sample body.

In general, the chemical composition of the surface changes along with the temperature increase (see also Tables and 4). With higher temperatures, the Si and Ca contents of the surface are decreasing, and the contents of K and S are increasing. This reveals that a K and S compound (e.g., K2SO4) covers the surface.

Results of XRD Analysis

XRD analysis was performed for the initial ash material gathered after the standard ashing procedure (CEN/TS 14775:2004) at 550 °C and the ashes with end temperatures of 800, 1000 and 1100 °C. The XRD analysis enabled us to detect crystalline species at 550 and 800 °C (as seen in Figure ) but basically not at 1000 and 1099 °C.

Figure 4

XRD patterns. (a) Initial ash sample at 550 °C; (b) ash sample at 800 °C; (c) ash sample at 1000 °C; (d) ash sample at 1099 °C.

K2SO4 was detected also by XRD, which confirms the existence of the K and S detected by SEM as K2SO4. Du et al.[37] have also found K2SO4 in wheat straw ash at the final heat treatment temperatures of 815 and 1000 °C, and the other two main crystalline compounds were SiO2 and KAlSi3O8. At the lower heat treatment temperature of 600 °C, they have detected CaCO3, KCl, SiO2, CaMg(CO3)2, and KAlSiO4. Our results showed brianite, dmitryivanovite, nepheline, and KCl as crystalline compounds present in the initial ash material. At higher temperatures, the two main compounds were KCl and K2SO4.

K and Si are the two main elements in the ash. According to Ma et al.,[33] only part of the K species in the biomass ash is present in the crystalline phase. Other types of K species may be present as amorphous phases, such as potassium silicates. The ash does not contain all the chemical compounds in the crystalline phase. According to Uibu et al.,[49] the estimated amorphous phase content of oil shale ashes could be up to 21–40%. In the studied ash material, the ash started to turn glassy and therefore amorphous from 800 °C.

Thermodynamic Modeling

According to FactSage, the initial composition of the ash blend at 550 °C consists mainly of K2MgSiO12, CaSiO3, SiO2, KCl, K2SO4, Na2Ca3Si6O16, and Ca3P2O8 as well as less amounts of KalSi3O8 and Na2SO4 as seen in Figure . At 660 °C, the first melting takes place, and KCl and K2SO4 (see also Figure ) as well as NaCl and Na2SO4 tend to melt. While the content of liquid mass fraction in the two latter compounds is relatively low (<1%), they are not shown in Figure . Arvelakis et al.[50] have found that the mixture of 50% KCl and 50% K2SO4 starts to melt at 688 °C, i.e., the mixture of compounds starts melting earlier than one separate compound. The two main liquid compounds were KCl and K2SO4, and the liquid mass of KCl/K2SO4 comprises 12.5% of the total initial mass of the ash blend sample together with NaCl and Na2SO4; the liquid mass fraction is 13.8%. This reveals that the studied blend ash could already be sticky at 680 °C.[40] Our modeling reveals that the temperature range of 660–680 °C could be considered as the starting point of melting.

Figure 5

Solid materials according to FactSage, 550–1500 °C.

Figure 6

Liquid materials according to FactSage, 550–1500 °C.

In the temperature range of 680–780 °C, no additional liquid mass fraction was added, although in the eutectic melting of KCl and K2SO4, liquid fraction increased with increasing temperature. According to calculations with FactSage, in that temperature range, solid K2Ca2Si9O21 was composed as an intermediate solid compound, but at 800 °C, K2Ca2Si9O21 and Na2CaP2O7 were totally decomposed. Also, the solid fraction of K2MgSi5O12 showed a moderate decrease at 800 °C, but the content of CaSiO3, Ca3P2O8, and SiO2 had increased moderately. Additionally, the molten fraction of SiO2 and K2O was detected.

The temperature of 800 °C could be considered as the start of the intensive stage of melting, which ends at 980 °C. In this range, an intensive melting of the ash material was observed, i.e., by the end of this temperature range, almost 65% of ash was melted. At the temperature of 860 °C, the content of solid K2MgSi5O12 started to decline and reached zero at 980 °C. The SEM pictures indicate that the glassy material was formed from 800 °C and above. The glass transition of K2O–MgO–SiO2 compounds could start from ∼900 °C.[51] At this temperature, a small amount of solid CaMgSi2O6 was composed as an intermediate product, which started to decompose and melt immediately after it was built. Three Ca-containing compounds, Ca3P2O8, CaSiO3, and CaMgSi2O6, were the only solid fractions left at this temperature. In terms of liquid fractions, the contents of K2O, SiO2, CaO, and MgO were increased. Among the liquid compounds, the molten SiO2 is the main mass fraction, which constitutes 37.5 wt % of the total initial amount of the ash blend. Consequently, this region is a temperature region where the decomposition and extensive melting of such main solid compounds as K2MgSi5O12 and SiO2 occur.

At the temperature of 980 °C, the final stage of melting starts and then ends at 1220 °C. From the temperature of 980 °C forward, the content of solid CaSiO3 tends to decrease and reaches zero at 1220 °C. The solid fraction of Ca3P2O8 is the only solid material that remains until the end of the process, i.e., up to 1500 °C. Rizvi et al.[35] have presented ash melting thermodynamic modeling by FactSage for miscanthus, peanut, sunflower, and pine wood ashes, and for all related ashes, Ca3P2O8 was solid up to 1500 °C as well. Similarly to our results, they showed liquid SiO2, K2O, Na2O, and CaO above 700 °C.

FactSage Modeling Compared to Other Methods

The height curve of HSM, the mass loss curve of TGA, the DTA curve, and the melt fraction curve obtained by simulation using FactSage are shown in Figure .

Figure 7

HSM, STA, and melt fraction curves.

During thermal treatment, FactSage modeling revealed that the melting of KCl, NaCl, K2SO4, and Na2SO4 takes place between 640 and 680 °C. An endothermic peak at 660 °C of the DTA curve (see also Figure ) also supports this melting behavior. The chemical composition of the surface of the sample bodies from HSM tests at different temperatures (see also Tables and 4) indicates that the contents of K and S increased remarkably at 700 °C, which supports the FactSage calculations. Molten K2SO4 was formed on the surface. In the temperature range of 680–780 °C, almost 15% of the ash material is in the molten phase and the height of the ash sample body of the HSM test tends to decrease to 85% of the initial height due to melting (see also Figure ). Between 800 and 980 °C, an extensive increase in the height of the sample body observed by the HSM test is coupled with a moderate mass loss revealed by the TGA curve. The DTA curve did not indicate any clear endothermic peak in this temperature range. Consequently, as indicated by modeling, this region could be described as a temperature region where the decomposition and extensive melting of such main solid compounds as K2MgSi5O12 and SiO2 occur. This extensive melting of Si and K compounds, evaporation of liquids, and the trapped gases in the molten mass resulted in the swelling (increase in height) of the HSM sample body. The start of this melting stage fits well with the Texp or “expansion point” in Figure . In general, the extensive melting started at 800 °C and continued up to 980 °C, when the content of the molten phase was 65% from the initial mass of the ash. The molten fraction reached the critical content where the swelling turned to collapsing of the sample body as indicated by the HSM test. According to the FactSage modeling, the critical content of the molten phase is 25% from the initial sample mass in our case.

As an alternative to Pang et al.,[45] we staged the melting process (shown in Figure ) for the studied blend as follows: Region 1 – unaffected by heat treatment; region 2 – the first stage of melting and Tsp is a “starting point” of melting; region 3 – intensive melting stage, Temp is an “extensive melting point,” and Tcr is a “critical point” where the sample body starts to diminish; region 4 – flowing liquid mass by the end. Of course, such a behavior is not valid for all biomasses or their blends; it is rather exceptional.

Figure 8

Proposed staging of the ash melting process.

After extensive melting, the liquid mass fraction increases quite linearly in the temperature range of 980 and 1200 °C and achieves its maximum value of 70%, counting from the initial mass of the ash sample.

The EDX results of the ash samples after thermal treatment at 1050 °C and flow temperature (see Figures g2,h2 and Table ) show that the K and S are the main components at the edge and on the top of the sample body. According to FactSage results, the K and S are in the form of molten compounds. The results reveal that the Si, Mg, Ca, and P containing solid and melted fractions are surrounded by melted potassium compounds, which are more available on the top and on the edges of the sample body.

From 1240 °C onward, the melting process is stabilized, i.e., solid, liquid, and gaseous masses remain unchanged.

Conclusions

Our focus has been on the melting behavior of the ash of the biomass blend of 50% reed and 50% wheat straw pellet as a potential fuel for boiler houses. For this purpose, we used a number of laboratory methods such as simultaneous thermal analysis, heating microscopy, scanning electron microscopy with energy dispersive X-ray (SEM/EDX) analysis, and X-ray powder diffraction (XRD). FactSage software was used for thermodynamic modeling of the ash melting process.

Our results showed that the ash started to melt at 660–680 °C with the melting of KCl, NaCl, K2SO4, and Na2SO4. The content of the liquid mass fraction was 13.8%, revealing that the studied ash blend could already be sticky at 680 °C. The results demonstrated that after the first melting, the liquid K2SO4 covers the surface. The melted K2SO4 could cover the single particle in the thermal units and could cause stickiness of the ash particle.

Intensive melting started at 800 and at 980 °C; the content of the molten phase was 65% from the initial mass of the ash. During the intensive melting, the studied ash material started to swell, and when the liquid mass fraction was 25% (950 °C) from the initial mass, the ash material started to dwindle. Rapid melting between 950 and 980 °C was caused by the melting of SiO2 and K2MgSi5O12, and the material became glassy and amorphous. K2MgSi5O12 was decomposed to oxides according to FactSage modeling. After 980 °C, the remaining 5% of the ash material was melted, and the process ended with a flowing material.

The FactSage modeling is a helpful tool for interpreting the results of the ash melting behavior and supplements well the results obtained by other methods used in this study, especially regarding the stickiness of the ash material. The alternative distribution of the melting stages for the studied ash blend was proposed.

Experimental Section

Parent Materials and Characterization

In our study, reed (R) and wheat straw pellets (WSP) were used as parent fuels. WSP originates from North Lithuania and R from the west-coast areas of Estonia. The parent fuels were prepared and characterized by standard analysis (according to the standard methods of CEN/TS 14774-1:2004, CEN/TS 14775:2004, and CEN/TS 15148:2005), elemental analysis (according to the standard methods of EN 15104:2011 and EN 15289:2011) by a Vario EL CHNOS elemental analyzer, and gross calorific value (according to the standard method of EN 14918) by an IKA C 5000 calorimetric bomb.

Ashing, Characterization, and Blending of Ashes

The ashing procedure was carried out according to the standard method of CEN/TS 14775:2004: (a) from room temperature to 250 °C with a heating rate of 4.5 °C/min, (b) 60 min at 250 °C, (c) heating to 550 °C with a heating rate of 10 °C/min, (d) 120 min at 550 °C, and (e) cooling down to room temperature.

The X-ray fluorescence (XRF) technique was applied to determine the elemental composition of the ashes of the parent fuels using a Rigaku ZSX Primus II WDXRF device with a rhodium anode (4 kW) and a 30 μm tube.

The ash blend of R and WSP was prepared as follows: (a) the fuels were ashed separately, and (b) after ashing of a single fuel, the ashes were blended according to the proportions of the parent fuels of 50% of R and 50% of WSP (50R/50WSP), i.e., it was not 50% reed ash and 50% wheat straw ash but the relevant ash amounts that a single parent fuel contains, which depicts the ash blend left after the thermal treatment of the parent fuel blend mixed by ratios of 50/50.

Ash Melting Behavior

Ash fusion characteristics were determined according to the standard method of CEN/TS 15370-1:2006 using a Misura 3 HSM ODHT 1600-5008 (HSM, hot stage microscope). Shrinkage temperature (ST), deformation temperature (DT), hemispherical temperature (HT), and flow temperature (FT) were measured. In addition to the standard ash melting test, a number of end temperatures were chosen (600, 700, 800, 900, 1000, and 1050 °C), and the ash material was heated up to the desired end temperature. The aim of various end temperatures is to collect ash material for further analysis and investigate the chemical composition stepwise along the thermal treatment process as described in Section . The tests were performed under the atmosphere of air.

A TA Instruments SDT Q600 device was used for simultaneous thermal analysis (STA) of the ash blend, i.e., thermogravimetric analysis (TGA) coupled with differential thermal analysis (DTA). The sample was heated from room temperature up to the final temperature with a heating rate of 10 °C/min, and an alumina crucible was used. Synthetic air (a mixture of 21 mL/min O2 and 79 mL/min N2) was used as carrier gas.

Morphology and Inorganic Content of Ash Material

The morphology of the ash material after heat treatment in HSM was examined by scanning electron microscopy (SEM); energy dispersive X-ray (EDX) spectroscopy was used to determine the inorganic matter on the surface of the treated ash material. All SEM/EDX spectroscopy work was performed with a Leo 1530 Gemini at a voltage of 15 kV.

The mineralogy analysis was carried out by means of a Rigaku Ultima IV diffractometer using the X-ray powder diffraction (XRD) method with Cu Kα radiation (λ = 1.5406 Å, 40 kV at 40 mA) and a silicon strip detector D/teX Ultra.

Thermodynamic Calculations

In our study, the thermodynamic equilibrium was calculated to determine the composition of the mineral matter and the molten fraction of the selected ash sample. We used the FactSage 7.3 software package and the following thermodynamic databases: FToxid-SLAGA (P2O5 and P2O3S2 were excluded because the phosphorus content is moderate and the calculations were inaccurate), FTsalt-SALTF, FTsalt-B1 (NaCl and KCl were selected), FTsalt-oP28D, FTsalt-hP22 (Na2SO4, K2SO4, Na2CO3 and K2CO3, were selected), and FTsalt-hP14. The input for the calculations was the chemical composition of the fuel ashes shown in Table . The equilibrium calculation was made for the temperature range of 550–1500 °C with a step of 20 °C under pressure of 1 bar.

The gas atmosphere was not constrained in the calculations, and the data for the gas phase was taken from the FactPS database. The formation of the gas phase in the calculations was mainly due to the predicted decomposition of CO2 from carbonates. In all the calculations, the conditions were consistent with an either inert or oxidizing gas atmosphere. The effect of reducing or oxidizing atmospheres can lead to variations in the behavior of Fe- or S-containing species,[52−54] but as the ash composition showed very low Fe contents and sulfur has been predicted as mainly composed of sulfates, the effect of the gas atmosphere is not considered in any further detail.