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Article

Regulation Mechanism of Solid Waste on Ash Fusion Characteristics of Sorghum Straw under O2/CO2 Atmosphere

by
Ziqiang Yang
1,
Fenghai Li
1,2,3,*,
Mingjie Ma
1,
Xuefei Liu
1,
Hongli Fan
2,
Zhenzhu Li
4,
Yong Wang
5 and
Yitian Fang
3
1
College of Chemistry and Chemical Engineering, Henan Polytechnic University, Jiaozuo 454003, China
2
School of Chemistry and Chemical Engineering, Heze University, Heze 274015, China
3
Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China
4
Shandong Meiyu Engineering Consulting Co., Ltd., Heze 274700, China
5
Shandong Hongda Chemical Co., Ltd., Heze 274700, China
*
Author to whom correspondence should be addressed.
Energies 2023, 16(20), 7052; https://doi.org/10.3390/en16207052
Submission received: 14 September 2023 / Revised: 28 September 2023 / Accepted: 6 October 2023 / Published: 12 October 2023
(This article belongs to the Section B: Energy and Environment)
Browse Figures
Versions Notes

Abstract

:
Co-combustion of solid waste and biomass can alleviate biomass ash-related problems. To investigate the effects of solid waste on the ash fusion characteristics of biomass and its variation mechanisms under an oxidation atmosphere, an X-ray diffraction, thermogravimetric analyzer (TG), scanning electron microscope (SEM), and FactSage calculation were used to examine the ash fusion behaviors of sorghum straw (SS) with the addition of textile dyeing sludge (TDS) or chicken manure (CM). The ash fusion temperature (AFT) of SS increased gradually with the TDS ash addition; with CM ash addition, the AFT of SS mixtures increased rapidly (0–20%), decreased slightly (20–30%), and finally increased slowly (30–60%). The generations of high melting point (MP) minerals (e.g., KAlSi2O6, Fe2O3, and Fe3O4) led to an increase in the AFT of TDS-SS mixtures. The K+ in silicate was gradually replaced by Mg2+ or Ca2+, which caused the generations of high-MP minerals (e.g., Ca3MgSi2O8, Ca2MgSi2O7, and CaMgSiO4). The TG analysis showed that the additions of TDS or CM ash slowed down the weight loss of SS mixed ash due to the formation of high-MP minerals. The SEM and FactSage calculations were also explained with the AFT change and their variation mechanisms. The result provided effective references for the AFT regulation during the co-combustion of biomass and solid waste.

Graphical Abstract

1. Introduction

With the global economy developing quickly, the widespread use of fossil fuels results in increasingly serious problems of energy shortage and environmental pollution [1,2]. To alleviate these worsening problems, a “double carbon” goal has been proposed by the government in China [3]. The use of biomass is one of the most important methods to achieve the “double carbon” goal [4]. Biomass has gained international attention due to its carbon neutrality, environmentally friendly, and abundant reserves [5]. Therefore, biomass has become an important alternative energy.
Among biomass thermal conversion methods (combustion, gasification, and liquefaction), direct combustion is widely regarded as one of the most attractive solutions [6,7]. The advantages of biomass (e.g., low price, wide distribution, and easy access) encourage the high-speed increase in biomass power-plants [8,9]. Meanwhile, the high concentrations of alkali (Na and K), chlorine, and silicon in a biomass result in its low ash fusion temperatures (AFT) and accelerate low-temperature eutectic production [6,10]. This leads to some problems (e.g., alkali-induced corrosion, sintering, and slagging) during or after combustion, especially for the superheater slagging issue caused by alkali [11,12,13]. These issues result in poor combustion stability, low efficiency, and high probability of maintenance, and even cause the equipment to shut down in severe cases [14,15,16].
To solve these issues, conventional methods are mainly obtained by blending with coal or additives to change the ash composition. Zhang et al. investigated ash fusion behaviors of biomass by blending coal, and discovered that the higher coal ash content in the mixed ash, the more potassium retention in the solid-phase ash and the higher increase in biomass AFT [17,18,19]. Yuan et al. found that the combustibility and comprehensive combustion performance were gradually improved with the addition of coal compared to that of biomass mono-combustion [20]. In addition, the additives during biomass combustion were also an effective way to alleviate the slagging problem. Chin et al. conducted an AFT variation in palm oil biomass by adding calcite and kaolin, and found that both additives significantly improved biomass AFT, and kaolin was more effective than calcite to decrease the slagging degree [21]. The ash-related problems of biomass could be mitigated with phosphorus-containing compound additives [15,22,23]. Pan et al. found that sludge-based additives could effectively improve the potassium fixation ratio in biomass bottom ash by forming potassium aluminum silicate and K-Ca-P compounds, which reduced the bottom ash sintering tendency [15]. Although blending with coal or additives can mitigate the problem of biomass ash slagging and plugging, coal blending usually increases costs due to long-distance transportation, and additives also have issues such as being expensive and economically inefficient [24,25]. Therefore, it is particularly important to develop economic and environmentally friendly solid waste as an alternative to control biomass AFT.
Solid waste includes sewage sludge, animal manure, agricultural and forestry waste, food and beverage waste, radioactive hazardous waste, etc. [26,27,28]. Sewage sludge and animal manure are the main components of solid waste—the improper treatment of which can lead to water-quality deterioration, soil contamination, and human illnesses [29,30]. For a large amount of solid waste, combustion has become one of the most effective harmless treatments [31]. However, the industrial application of sewage sludge and manure is seriously hindered with their disadvantages of low calorific value, low combustible content, and poor combustion performance [32,33]. Zhang et al. found that the co-combustion of textile dyeing sludge and cattle manure decreased the AFT because of the generation of a low-temperature eutectic [34]. Li et al. found that during the co-combustion of sludge and sugarcane bagasse, the deformation temperature (DT) of bagasse increased due to the generation of high-MP minerals (e.g., KAlSi2O6 and KAlSi3O8), which alleviated the slagging tendency during combustion [35]. Consequently, the co-combustion with solid waste has become an economically viable, efficient, and environmentally friendly approach.
Although the co-combustion of biomass and solid waste has great potential for development, their modification mechanism of AFT has not been clarified due to the high alkali content in biomass and the complexity of different ash and mineral interactions. The AFT of biomass is an essential parameter for various types of boiler design and combustion parameter estimation [11,36], and the addition of solid waste (textile dyeing sludge and chicken manure) may increase the AFT of sorghum straw. However, most of the current investigations are dominated with biomass gasification (reducing atmosphere) [2,37,38,39,40]. Therefore, the aims of this paper were to investigate the AFT of sorghum straw and solid waste (textile dyeing sludge and chicken manure) during the co-combustion process, and to explore the AFT modification mechanism from mineral transformation (especially iron and calcium) under an oxidation atmosphere (O2/CO2). This provides references to alleviate the issues of slagging and blockage during biomass combustion, and to simultaneously recover the energy from solid waste.

2. Raw Materials and Experimental Methods

2.1. The Characteristics of Material

The sorghum, the fifth-largest grain in the world, has been extensively grown in many nations [41]. Therefore, sorghum straw (SS, from rural areas in Binzhou, Shandong, China) was selected as representative straw. The textile dyeing sludge (TDS) and chicken manure (CM) were selected, which came from textile dyeing factories and chicken farms in Heze, Shandong, China, respectively. Before analyses, the three air-dried samples were crushed to a size of less than 0.200 mm and maintained in a drying oven (101-2A, Tianjin Taisite Instrument Co., Ltd., Tianjin, China) at 105 °C for 24 h. According to Chinese standards GB/T 28731-2012 and GB/T212-2008, proximate analyses and ultimate analyses of three samples were carried out [42], and are presented in Table 1. The fixed-carbon content of TDS and CM was relatively low (much < 3%), while the fixed carbon of SS was higher (16.75%) than that of the other two samples. The ash yields of TDS and CM (59.58% and 32.54%, respectively) are higher than that of SS (4.54%), respectively.
An X-ray fluorescence spectrometer was used to determine ash composition in accordance with ASTM E1755-01 standards [36]. The relative standard deviation of the XRF measurements was <0.60%. The results are also shown in Table 1. The total content of Si, K, and Cl in SS ash was relatively high (>70%), which coincided with the ash composition characteristics of typical biomass. The iron content (Fe2O3: 46.32%) in TDS ash was high, which related to the usage of iron flocculants during sewage treatment [36], and the content of P and Ca in CM ash was high (P2O5: 16.53% and CaO: 46.91%, respectively).

2.2. Ash Sample Preparation

2.2.1. Preparation of Raw Material and Mixed Ash Samples

Alkaline elements (e.g., Na and K) were easily volatilized at high temperatures [43]. Therefore, the ashing temperature was chosen at 575 °C. The SS, TDS, and CM were put into a muffle furnace (SZ2-5-12TP, Jingmi Co., Ltd., Jinan, China) and their laboratory ashes were prepared [36], respectively. Firstly, the ash sample was heated at 10 °C/min from room temperature to 250 °C; secondly, it was heated to 575 °C within 30 min and kept for 2 h, to ensure that the material was completely ashed. The TDS or CM ash was added to SS ash in mass ratios of 10%, 20%, 30%, 40%, 50%, and 60%, respectively. Finally, the ash samples were ground and mixed thoroughly (<0.075 mm) with a mortar before being kept in a sample bag.

2.2.2. High-Temperature Ash Sample Preparation

The original or their mixed ash (~1.0 g) was kept in a small ceramic crucible, which was in a high-temperature tube furnace (Figure 1). To simulate the atmosphere during biomass combustion, the temperature was increased to the presetting temperature and kept for 10 min after the introduction of an oxidation atmosphere (O2/CO2, 3:7, volume ratio) at a rate of 15 mL/s. Then, the high-temperature ashes were quickly taken out and put into the ice water, the purpose of which was to prevent the crystal change under high temperatures [44]. The mixed ashes with different percentages were photographed to observe the macro morphology and to judge the fusion degree. The quenched sample was fully ground to <0.075 mm and dried for 24 h in a vacuum drying oven. Put it into the sample bag for various analyses.

2.3. Ash Fusion Temperature Test

An AFT analyzer (Keli Co., Ltd., Hebi, China) was used to test AFT under an oxidation atmosphere in accordance with the ASTM D1857 standard [45]. According to the ash cone, four characteristic temperatures (DT, softening temperature (ST), hemispherical temperature (HT), and flow temperature (FT)) were identified. According to the standard GB/T 219-2008 [45], the AFT error range does not exceed 40 °C. Since the AFT analyzer (Keli Co., Ltd., Hebi, China) used visual readings, the final result was the average of three parallel experiments to ensure its accuracy. As shown in Table 2, the AFT of the two solid wastes was higher than that of SS.

2.4. Ash Sample Analysis Methods

2.4.1. XRD Analysis

The mineral composition of the mixed ash was determined using an X-ray diffraction analyzer (XRD, Rigaku Corp., Tokyo, Japan) set to 40 kV, 40 mA, and Kα1 = 0.15408 nm. The scanning range was 10–70° 2-theta, the step size was 0.01°, and the scanning speed was 5° 2-theta/min. The diffraction data were analyzed using MDI Jade 6.0 software.

2.4.2. TG Analysis

A synchronous thermogravimetric analyzer (TG, STA-449-F3, NETZSCH, Selb, Germany) was used to explore the mineral decomposition and conversion characteristics during the ash fusion process. Approximately 8 ± 0.3 mg of ash samples was heated in a N2 atmosphere for each experiment. With a heating rate of 15 °C/min and a N2 flow rate of 60 mL/min, the temperature range was 35 to 1250 °C. A blank test was performed before testing to obtain a baseline. To reduce the impacts of buoyancy, each trial result was deducted from the baseline [34].

2.4.3. SEM Analysis

The change in ash sample microstructure and morphology analyzed with a scanning electron microscope (SEM; Thermo Scientific Apreo 2C, Waltham, MA, USA) that was used to judge the fusion degree. The sample was coated on a conductive adhesive to form a thin layer, and was then sprayed with gold to increase its conductivity. The voltage was set to 10 kV, and the measured area was magnified to a range of 5kx to obtain a clear image.

2.5. Thermodynamic Calculations

Under an atmospheric pressure (0.10 MPa) and oxidation atmosphere, the composition and liquid-phase concentration of minerals were estimated using the thermodynamic computation software FactSage (version 7.3). The calculated temperature range was set to 600–1500 °C with an interval of 30 °C. The FToxide and FactPS databases were selected in the Equilib module, and the SiO2-Al2O3-Cl2O-CaO-Fe2O3-MgO-K2O-Na2O-P2O5-SO3 tenfold system was selected for simulation calculation. The Gibbs free energies (ΔGs) of the major reactions at different temperatures during the process of ash fusion were calculated with the reaction module.

3. Results and Discussion

3.1. AFT Variations for SS as the TDS and CM Mass Ratio Increased

Different mass ratios of mixed ash were used to measure the AFT. According to Figure 2, the AFT of SS was improved overall with the additions of TDS or CM ash. High DT could mitigate the slagging problem caused by silicate melts during biomass thermal conversion [46]. Although the AFTs of the two groups of mixed ashes showed an upward trend, the increase rate was significantly different. When the TDS ash increased from 0 to 10%, DT increased rapidly. Subsequently, as shown in Figure 2a, the rate of increase decreased. According to Figure 2b, the DT increased rapidly (0–20%), then decreased slightly (20–30%), and finally increased slowly (30–60%) with the increasing CM ash mass ratio. In the range of 20–30%, the decrease in AFT was closely related to the formation of a low-MP eutectic. With the increasing percentages (0–60%) of TDS or CM ash, the improvement range of DT was 142 °C and 103 °C, respectively. Therefore, the addition of TDS or CM can increase SS AFT, and mitigate ash-related problems.

3.2. Mineral Evolution Analysis of Samples

3.2.1. Mineral Evolution Mechanism of SS Ash

The transformation and existence of minerals were directly connected to the AFT during the heating process. As a result, it is necessary to analyze the XRD spectrum of ash at a specific temperature. The SS ashes at different temperatures (600 °C, 700 °C, 800 °C, 900 °C, 1000 °C, and 1100 °C) were prepared in a high-temperature tube furnace (oxidation atmosphere), and the mineral compositions were tested with XRD. According to Figure 3, SS ash was mainly composed of sylvite (KCl) and potassium sulfate (K2SO4) at low temperatures (600–800 °C); they were mainly not only derived from inorganic potassium but also from some organic potassium migration [35]. With the increasing temperature, K2SO4 began to disappear, KCl transformed into the gas phase, CaMgSi2O6 generated, and the content of amorphous substances increased. When the temperature was at 1100 °C, KCl disappeared due to volatilization [46], and only CaMgSi2O6 existed. The KCl and K2SO4 were the main substances affecting ash-related problems. The MP of KCl and K2SO4 was about 770 °C and 850 °C, respectively, which resulted in SS with a low AFT [47].

3.2.2. Mineral Evolution Mechanism of Mixed Ash

The type and quantity of minerals were altered by the interaction between different minerals during the heating process, and the mineral composition could be used to estimate its AFT. The temperature involved in preparing ash samples was usually 100–150 °C lower than FT to ensure the operation feasibility of taking high-temperature ash samples from the porcelain boat [25]. Therefore, to explore the modification mechanism of adding TDS ash or CM ash at different percentages (10%, 20%, 30%, 40%, 50%, and 60%) on the ash fusion behaviors of SS, the mixed ash was prepared in a high-temperature tube furnace at 1000 and 1100 °C (the variety and content of minerals at this temperature were moderate, which could illustrate the change of AFT).
According to Figure 4a, the minerals in SS ash were mainly composed of sylvite (KCl), clinopyroxene (CaMgSi2O6), and ferric oxide (Fe2O3) at 1000 °C. When the TDS mass ratio was at 50%, the leucite (KAlSi2O6, MP: 1510 °C) appeared. When the temperature was 1100 °C (Figure 4b), the high-MP mullite (Al6Si2O13, MP: 1840 °C) and kaliophilite (KAlSiO4, MP: 1750 °C) were generated. As the TDS mass ratio increased, the mixed ash was mainly composed of high-MP minerals such as ferric oxide (Fe2O3, MP: 1565 °C), leucite (KAlSi2O6, MP: 1510 °C), and mullite (Al6Si2O13, MP: 1840 °C). With the addition of CM ash (Figure 4c,d), the clinopyroxene (CaMgSi2O6, MP: 1391 °C) transformed into akermanite (Ca2MgSi2O7, MP: 1450 °C). Only a small amount of KAlSi3O8 was discovered in the mixed ash since the K concentration significantly decreased with CM ash addition. It was possible that K+ was replaced by Ca2+ in potassium salts because Ca2+ has a higher ionic potential than that of K+ (7.5 nm−1) [14]. According to Figure 4a–d, the addition of TDS ash (Al2O3: 12.17%) or CM ash increased the Al concentration, which led to the generation of minerals containing Al (e.g., KAlSiO4, KAlSi2O6, and KAlSi3O8). This would alleviate the slagging problem caused by K. The following chemical reactions in the TDS-SS and CM-SS ashes might occur under an oxidation atmosphere.
K2O (s) + SO3 (s) → K2SO4 (s) ΔG (1100 °C) = −627.54 kJ/mol
CaO (s) + MgO (s) + 2SiO2 (s) → CaMgSi2O6 (s) ΔG (1100 °C) = −134.22 kJ/mol
CaMgSi2O6 (s) + CaO (s) → Ca2MgSi2O7 (s) ΔG (1100 °C) = −64.18 kJ/mol
CaO (s) + Al2O3 (s) + 2SiO2 (s) → CaAl2Si2O8 (s) ΔG (1100 °C) = −128.20 kJ/mol
K2O (s) + Al2O3 (s) + 2SiO2 (s) → 2KAlSiO4 (s) ΔG (1100 °C) = −413.91 kJ/mol
KAlSiO4 (s) + SiO2 (s) → KAlSi2O6 (s) ΔG (1100 °C) = −33.12 kJ/mol

3.3. Thermogravimetric Analysis of Mixed Ash Samples

Thermogravimetric and differential scanning calorimetry (TG-DSC) can be used to measure the variations in the thermal behavior of the mixed ash. The fusion, evaporation, and decomposition of minerals can cause changes in their quality during the ash heating process [45,48]. The first derivative of the TG (DTG) curve may be used to determine the mass loss rate. Thus, the ash fusion behaviors can be analyzed with the TG-DTG-DSC curve. Figure 5 shows the TG-DTG-DSC curve of SS ash and its mixtures. There were two obvious weight loss peaks in the SS ash fusion process; the first weight loss peak was located at 710–805 °C with a weight loss rate of 3.29%, and the decomposition of K2SO4 and CaCO3 resulted in a weak endothermic peak appearing on the DSC curve, while the smaller weight loss peak indicated a low CaCO3 content in SS ash [42,45]. The second stage of weight loss took place between 820 and 918 °C, and the weight loss rate was 11.77%. This stage mainly was responsible for the evaporation of sulfate and carbonate, which corresponded to the XRD pattern in Figure 3.
The biomass combustion was usually divided into four stages [42,49,50]: (1) water evaporation (room temperature to 200 °C); (2) pyrolysis (partial lignin, and cellulose) releases a large amount of volatile matter (200–370 °C); (3) combustion and carbonization of residue (lignin pyrolysis; 370–530 °C); and (4) decomposition and stability of inorganic substances. According to Section 2.2.1, SS ash was prepared at 575 °C, and the first three stages were completed before 575 °C, so it was summarized as stage 1 in Figure 5. As shown in Figure 5a,d, the total mass loss of the mixed ash gradually decreased with the increasing mass ratio of TDS or CM ash. Compared with that of TDS, the initial temperature (CM: 605 °C; TDS: 705 °C) and end temperature (CM: 750 °C; TDS: 820 °C) in stage 2 of CM were lower, which indicated that CM was easier to release volatiles than TDS. As shown in Figure 5b,f, there are two obvious peaks in the CM-SS mixed ash, while TDS-SS has only one peak. The Ca content in CM was high (CaO: 46.91%), while in TDS, it was relatively low (CaO: 5.38%). On the DTG curve of CM-SS ashes, a mass loss peak occurred between 600 °C and 750 °C. At these temperatures, the decomposition of CaCO3 (CaCO3 → CaO + CO2) led to the mass loss. The CaO and CO2 produced with decomposition could reduce NOx and SO2 emissions, which was conducive to the combustion of clean biomass [51,52]. In stage 2 (Figure 5b), the DTG curves indicated the obvious effect of the CM on the SS ash fusion behavior. The CM ash increased maximum mass loss temperatures by 16 °C and decreased DTG peak value (CM: 60%, −1.27%/min). The CM ash significantly accelerated CaCO3 decomposition and caused the DTG curves to move backward. Due to the presence of alkali ions in the ashes, the decomposition for pure CaCO3 was shifted to low temperatures and was detected at about 800 °C [42]. As seen from Figure 5c, the endothermic peaks in the range of 600–750 °C resulted from the decomposition of K2SO3 and CaCO3; the decomposition of CaCO3 and the formation of CaMgSi2O6 accelerated the weight loss of mixed ash. In stage 3, the mass loss trend of the two ashes was roughly the same. The addition of TDS ash or CM ash slowed down the weight loss of SS ash due to the generation of high-MP minerals (e.g., Ca2MgSi2O7, Fe2O3, KAlSi2O6, and KAlSi3O8). In stage 4, the minerals gradually stabilized with the increasing temperature, resulting in weight loss reaching equilibrium.

3.4. Ash Morphology

3.4.1. Macroscopic Morphology Analysis

As shown in Figure 6a, the ash shrunk and its color changed with increasing temperature. When the temperature increased (1100 °C), the adhesion and slagging occurred between the ash and porcelain boat. The ash volume gradually increased and the color tended to be reddish brown (Figure 6b) as the mass ratio of TDS ash increased because of its relatively high Fe2O3 content (46.32%). With the increasing CM ash mass ratio, the volume of the ash sample increased and there was no slagging. When TDS ash or CM ash were added to SS, the fusion disappearance indicated that the ash fusion characteristics were improved even at high temperatures.

3.4.2. Microscopic Morphology Analysis

SEM can be used to illustrate the fusion phenomenon of the mixed ash surface and to analyze the degree of sintering and slagging with its microsurface morphology [53]. From Figure 7a, the surface of SS ash was compact and smooth with very few particles and almost no porous structure, indicating that ash fusion occurred and ash sintering happened. With the addition of TDS ash, a small amount of a porous structure began to appear on the surface and the particles increased, as shown in Figure 7b. When the percentage of TDS ash reached 50% (Figure 7d), the ash surface was transformed into a large area of loose particles with rough surface distribution. After the addition of CM ash, the surface of the ash changed from compact and smooth into loose and porous with the appearance of a porous and columnar structure. According to Figure 7g, when the mass ratio of CM ash reached 50%, a large area of a rough porous structure appeared on the ash surface. The above phenomenon was the same as the macro surface structure change (Figure 6). The results showed that AFT was improved with the addition of TDS ash or CM ash; the sintering phenomenon was alleviated [14,15,54].

3.5. Mineral Transformation Analyses Based on FactSage Software

3.5.1. Thermodynamic Calculation Composition of SS

FactSage software is one of the largest fully integrated databases for chemical thermodynamics calculation systems. It offers information on the mineral or liquid-phase content at a certain temperature as well as the trend of the phase content with increasing temperature [55,56]. To further investigate the regulation mechanism of TDS or CM ash on the ash fusion characteristic of SS, FactSage thermodynamic software was selected to explain the AFT variation in SS mixtures. The normalized major ash components of mixtures are displayed in Table 3.
The SS ash composition (Table 3) was input into the FactSage software for calculation, the results of which are shown in Figure 8. The SS ashes were mainly composed of potassium sulfate (K2SO4), forsterite (Mg2SiO4), clinopyroxene (CaMgSi2O6), sylvite (KCl), and potassium silicate (K2Si2O5). When the temperature reached ~1000 °C, the main minerals (total about 75%) such as KCl, CaMgSi2O6, and K2Si2O5 disappeared. This explained the SS with low AFT and the temperature at which all minerals changed into the liquid phase (Tliq, 1450 °C). And a small amount of akermanite (Ca2MgSi2O7) appeared in the range of 1000–1200 °C; this was further validated with reaction 3 (CaMgSi2O6(s) + CaO(s) → Ca2MgSi2O7(s)).

3.5.2. Thermodynamic Calculation Composition of Mixed Ash

Figure 9 shows the mineral composition of SS mixed ashes at different temperatures calculated with FactSage simulation. The findings of the XRD tests and the minerals predicted with simulation were almost consistent. The existence of the small difference mainly resulted from the FactSage calculations being performed in ideal chemical equilibrium, and the thermodynamic calculations were not taken into account (e.g., kinetic limitations, mass transfer, and other unknown reactions) [56,57]. With the increase in the TDS or CM ash mass ratio, high-MP minerals such as magnetite (Fe3O4, MP: 1594 °C), leucite (KAlSi2O6, MP: 1510 °C), and ferric oxide (Fe2O3, MP: 1565 °C) gradually increase as shown in Figure 9a–f, while low-MP minerals such as sylvite (KCl, MP: 770 °C), andradite (Ca3Fe2Si3O12), potassium silicate (K2Si2O5), and forsterite (Mg2SiO4) gradually decrease or even disappear. This was almost the same as the XRD patterns in Figure 4a,b, whereas the presence of Fe3O4 was not detected in the XRD. In the ideal state, the Fe mainly exists in the form of Fe3O4 in the oxidation atmosphere [36]. This led to the increase in AFT in mixed ash samples. The high-MP mineral Fe2O3 existed at the same time, and its structure was similar to that of corundum (Fe3+: 54.54/nm; Al3+: 59.00/nm calculated with (z/r)) [36]. When TDS ash was at 50%, the total high-MP mineral (Fe3O4 and KAlSi2O6) contents reached 75%. The nepheline (NaAlSiO4), sylvite (KCl), ferric oxide (Fe2O3), whitlockite (Ca3P2O8), forsterite (Mg2SiO4), and calcium sulfate (CaSO4) disappeared at ~975 °C because of the generation of their low-MP eutectics, which accelerated a decrease in the mineral contents. With the addition of CM ash (10–30 %), the high-MP mineral monticellite (CaMgSiO4, MP: 1390 °C) gradually increased (Figure 9a′–c′). This may be explained with the high content of CaO (46.91%) in CM, and the ionic potentials of Ca2+ and Mg2+ are higher than that of K+; K+ in silicate was replaced by Ca2+ or Mg2+. When the content of CM ash reached 40–60% (Figure 9d′–f′), CaMgSiO4 began to gradually decrease and even disappear, while the high-MP mineral wollastonite (Ca3MgSi2O8, MP: 1550 °C) and whitlockite (Ca3P2O8) gradually increased. The continuous increase in CaO led to the reaction of CaMgSiO4 with CaO to form Ca3MgSi2O8. At the same time, the P2O5 derived from CM ash (P2O5: 16.53%) reacted with CaO and formed Ca3P2O8 at low temperatures (600–700 °C), and then Ca3P2O8 gradually fused with an increasing temperature. Si4+ has a lower ionic potential (95/nm) than that of P5+ (147/nm), and Ca2+ has a strong tendency to react with P2O5 [36,58]. Low-MP mineral content was connected to DT, and the skeleton structure of high-MP minerals mainly dictated FT [59]. The increase in the content of Ca3MgSi2O8 and CaMgSiO4 led to an obvious increase in DT compared to that of FT. According to thermodynamic calculations (ΔG), it might be speculated that the reaction occurred as follows [8,9,36,60]:
2SiO2 (s)+ 3Al2O3 (s) → Al6Si2O13 (s) ΔG (1100 °C) = −10.60 kJ/mol
3Fe2O3 (s) + CO (H2) (g)→2Fe3O4 (s) + CO2 (H2O) (g)ΔG (1100 °C) = −93.59 kJ/mol
2MgO (s) +SiO2(s) → Mg2SiO4(s) ΔG (1100 °C) = −61.10 kJ/mol
K2CO3 (s) + 2SiO2 (s) → K2Si2O5 (s) + CO2 (g) ΔG (1100 °C) = −107.73 kJ/mol
CaO (s) + P2O5 (s) → Ca3P2O8 (s) ΔG (1100 °C) = −816.81 kJ/mol
CaMgSiO4 (s) + 2CaO (s) + SiO2 (s) → Ca3MgSi2O8 (s) ΔG (1100 °C) = −120.91 kJ/mol

4. Conclusions

Co-combustion of biomass and solid waste is considered to be a very promising approach. The ash fusion behaviors of solid waste (textile dyeing sludge (TDS) and chicken manure (CM)) and the effects on sorghum straw (SS) were investigated. With the increasing TDS ash mass ratio, the four characteristic temperatures of the SS mixtures also increased. However, with the increase in the CM ash mass ratio, DT increased rapidly (0–20%), then decreased slightly (20–30%), and finally increased slowly (30–60%). DT was increased by 142 °C and 103 °C, when TDS and CM were added at 60%, respectively.
With the addition of TDS ash, the generation of high-MP minerals (e.g., KAlSi2O6, Fe2O3, and Fe3O4) resulted in an increase in the AFT of SS mixtures. Under the oxidation atmosphere, with the increasing CM ash mass ratio, K+ in the silicate was gradually replaced by Ca2+ or Mg2+, which caused the formations of high-MP minerals (e.g., Ca3MgSi2O8, Ca2MgSi2O7, and CaMgSiO4). At a certain temperature, the addition of TDS or CM ash slowed down the weight loss of SS ash. With the addition of TDS ash or CM ash, the AFT of SS increased. The SEM and FactSage calculations were also explained with the AFT change and their variation mechanisms. Although a certain gap exists between the laboratory-scale experiment and industrial application, this may provide the references for the modification of the biomass ash fusion temperature under a combustion condition, solid waste clean conversion, and its energy utilization.

Future Work

In the future, the feasibility of co-combustion biomass with other solid waste (e.g., kitchen waste and municipal solid waste) will be explored. Thermodynamics software (FactSage 7.3) can be used to predict the ash-related problems in the biomass combustion process and reduce the experiment cost. At the same time, the laboratory conclusions will be applied to the industrialization stage for verification. This is of great significance for finding alternatives to fossil energy and achieving the goal of carbon neutrality and a carbon peak.

Author Contributions

Software, X.L. and H.F.; Investigation, M.M. and Y.F.; Resources, M.M., H.F., Z.L., Y.W. and Y.F.; Writing—original draft, Z.Y.; Writing—review & editing, Z.Y. and F.L.; Supervision, F.L. All authors have read and agreed to the published version of the manuscript.

Funding

The National Natural Sciences Foundation of China (21875059) and Natural Science Foundation of Shandong Province, China (ZR2018MB037), provided funding for this work.

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Structural diagram of high-temperature tube furnace.
Figure 1. Structural diagram of high-temperature tube furnace.
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Figure 2. Effects of (a) TDS ash and (b) CM ash addition on the AFT of SS.
Figure 2. Effects of (a) TDS ash and (b) CM ash addition on the AFT of SS.
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Figure 3. The XRD quantitative pattern analysis of SS ash at different temperatures. (1: sylvite (KCl); 2: potassium sulfate (K2SO4); 3: clinopyroxene (CaMgSi2O6)).
Figure 3. The XRD quantitative pattern analysis of SS ash at different temperatures. (1: sylvite (KCl); 2: potassium sulfate (K2SO4); 3: clinopyroxene (CaMgSi2O6)).
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Figure 4. The XRD quantitative pattern analysis of mixed ash sample. (a): TDS + SS 1000 °C; (b): TDS + SS 1100 °C; (c): CM + SS 1000 °C; (d): CM + SS 1100 °C; 1: sylvite (KCl); 2: potassium sulfate (K2SO4); 3: clinopyroxene (CaMgSi2O6); 4: ferric oxide (Fe2O3); 5: quartz (SiO2); 6: leucite (KAlSi2O6); 7: anorthite (CaAl2Si2O8); 8: mullite (Al6Si2O13); 9: kaliophilite (KAlSiO4); 10: akermanite (Ca2MgSi2O7); 11: potassium feldspar (KAlSi3O8); 12: andalusite (Al2SiO5).
Figure 4. The XRD quantitative pattern analysis of mixed ash sample. (a): TDS + SS 1000 °C; (b): TDS + SS 1100 °C; (c): CM + SS 1000 °C; (d): CM + SS 1100 °C; 1: sylvite (KCl); 2: potassium sulfate (K2SO4); 3: clinopyroxene (CaMgSi2O6); 4: ferric oxide (Fe2O3); 5: quartz (SiO2); 6: leucite (KAlSi2O6); 7: anorthite (CaAl2Si2O8); 8: mullite (Al6Si2O13); 9: kaliophilite (KAlSiO4); 10: akermanite (Ca2MgSi2O7); 11: potassium feldspar (KAlSi3O8); 12: andalusite (Al2SiO5).
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Figure 5. TG-DTG-DSC analysis of mixed ash samples. (a): CM mixed TG curves (b): CM mixed DTG curves (c): CM mixed DSC curves (d): TDS mixed TG cueves (e): TDS mixed DTG curves (f): TDS mixed DSC cueves
Figure 5. TG-DTG-DSC analysis of mixed ash samples. (a): CM mixed TG curves (b): CM mixed DTG curves (c): CM mixed DSC curves (d): TDS mixed TG cueves (e): TDS mixed DTG curves (f): TDS mixed DSC cueves
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Figure 6. Macro morphology of ash. (a): SS; (b): TDS of different ratios at 1100 °C; (c): CM of different ratios at 1100 °C.
Figure 6. Macro morphology of ash. (a): SS; (b): TDS of different ratios at 1100 °C; (c): CM of different ratios at 1100 °C.
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Figure 7. SEM microscopic morphology of ash with different percentages at 1100 °C. (a): SS (b): +10%TDS; (c): +30%TDS; (d): +50%TDS; (e): +10%CM; (f): +30%CM; (g): +50%CM.
Figure 7. SEM microscopic morphology of ash with different percentages at 1100 °C. (a): SS (b): +10%TDS; (c): +30%TDS; (d): +50%TDS; (e): +10%CM; (f): +30%CM; (g): +50%CM.
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Figure 8. Composition of SS ash under thermodynamic calculation.
Figure 8. Composition of SS ash under thermodynamic calculation.
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Figure 9. Composition of mixed ash under thermodynamic calculation. (a): +10% TDS; (b): +20% TDS; (c): +30% TDS; (d): +40% TDS; (e): +50% TDS; (f): +60% TDS; (a′): +10% CM; (b′): +20% CM; (c′): +30% CM; (d′): +40% CM; (e′): +50% CM; (f′): +60% CM.
Figure 9. Composition of mixed ash under thermodynamic calculation. (a): +10% TDS; (b): +20% TDS; (c): +30% TDS; (d): +40% TDS; (e): +50% TDS; (f): +60% TDS; (a′): +10% CM; (b′): +20% CM; (c′): +30% CM; (d′): +40% CM; (e′): +50% CM; (f′): +60% CM.
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Table 1. The characteristics of the three samples.
Table 1. The characteristics of the three samples.
SampleProximate Analysis, wt/% Ultimate Analysis, wdaf/%
MadVadAadFCad CHONS
SS7.7370.984.5416.75 48.315.7144.210.131.64
TDS11.2526.3259.582.85 20.322.871.089.212.52
CM2.9261.9232.542.62 41.908.3143.844.891.06
SampleAsh Composition
SiO2Al2O3Cl2OP2O5SO3Fe2O3CaONa2OK2OMgO
SS32.400.6710.313.304.440.477.630.9628.5311.27
TDS15.5119.830.803.433.9746.325.382.400.471.88
CM10.052.910.0316.533.932.0846.912.2514.960.35
Remark: ad—air-dried basis; daf—dried as free basis; M—moisture; A—ash; V—volatile matter; FC—fixed carbon.
Table 2. AFT of three samples.
Table 2. AFT of three samples.
SampleAFT (°C)
DTSTHTFT
SS1056112111311147
TDS1202138313981436
CM13221469>1500>1500
Table 3. Oxide composition of SS mixed ash sample.
Table 3. Oxide composition of SS mixed ash sample.
SamplesAsh Composition (wt./%)
SiO2Al2O3Cl2OP2O5SO3Fe2O3CaONa2OK2OMgO
SS32.400.6710.313.304.440.477.630.9628.5311.27
10%TDS + 90%SS30.712.599.363.314.405.067.411.1125.7210.34
20%TDS + 80%SS29.024.508.413.334.359.647.181.2522.929.40
30%TDS + 70%SS27.336.427.463.344.3014.236.961.3920.118.46
40%TDS + 60%SS25.658.346.513.354.2518.816.731.5417.317.52
50%TDS + 50%SS23.9610.255.553.374.2123.406.511.6814.506.58
60%TDS + 40%SS22.2712.174.603.384.1627.986.281.8311.705.64
10%CM + 90%SS30.170.909.284.624.390.6311.561.0927.1710.18
20%CM + 80%SS27.931.128.255.954.340.7915.491.2225.819.09
30%CM + 70%SS25.691.347.237.274.290.9519.421.3524.468.00
40%CM + 60%SS23.461.576.208.594.241.1123.341.4823.106.91
50%CM + 50%SS21.221.795.179.914.191.2827.271.6121.745.81
60%CM + 40%SS18.992.014.1411.244.141.4431.201.7420.394.72
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Yang, Z.; Li, F.; Ma, M.; Liu, X.; Fan, H.; Li, Z.; Wang, Y.; Fang, Y. Regulation Mechanism of Solid Waste on Ash Fusion Characteristics of Sorghum Straw under O2/CO2 Atmosphere. Energies 2023, 16, 7052. https://doi.org/10.3390/en16207052

AMA Style

Yang Z, Li F, Ma M, Liu X, Fan H, Li Z, Wang Y, Fang Y. Regulation Mechanism of Solid Waste on Ash Fusion Characteristics of Sorghum Straw under O2/CO2 Atmosphere. Energies. 2023; 16(20):7052. https://doi.org/10.3390/en16207052

Chicago/Turabian Style

Yang, Ziqiang, Fenghai Li, Mingjie Ma, Xuefei Liu, Hongli Fan, Zhenzhu Li, Yong Wang, and Yitian Fang. 2023. "Regulation Mechanism of Solid Waste on Ash Fusion Characteristics of Sorghum Straw under O2/CO2 Atmosphere" Energies 16, no. 20: 7052. https://doi.org/10.3390/en16207052

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