ABSTRACT
Each year, combustion at municipal solid waste incineration (MSWI) plants produces millions of tons of fly ash globally. This ash is characterized as a hazardous material and is mostly placed in landfills after a stabilization process or stored in hazardous waste sites. Thus, disposal of fly ash leaves a considerable social and environmental footprint and leads to high waste management costs. Thermochemical energy storage (TCES) systems are considered to be outstanding because of their high-energy density and near-zero energy loss over long periods of time. Calcium oxide (CaO), a main MSWI fly ash component, is a promising candidate for TCES. In this study, we investigate the potential of fly ash as a TCES material. To do so, we analyzed representative samples from different MSWIs using simultaneous thermal analysis (STA) under N2, CO2, and CO2/H2O atmospheres. These analyses were supported by additional techniques such as X-ray fluorescence (XRF) spectroscopy, inductively coupled plasma-optical emission spectroscopy (ICP-OES), and scanning electron microscopy (SEM). The STA results illustrate fly ash reactivity under different atmospheres. All samples could store heat through endothermic reactions and one sample was able to release stored heat under selected operating conditions. XRF analysis verified an average fly ash composition of 27% CaO, ICP-OES analysis demonstrated the presence of different heavy metals, and SEM analysis revealed the sintering and agglomeration of fly ash particles at high temperatures (1150 °C). This study shows that the use of fly ash as a TCES material is promising and that further investigation in the field is needed to corroborate this application.
INTRODUCTION
The rising demand for energy, limited availability of fossil fuels, growing environmental regulations, and increasing public awareness are the main factors driving the investigation of renewable energy sources. For example, using municipal solid waste to produce renewable energy has recently attracted significant attention because it is a good solution for energy recovery; however, it generates some waste. Municipal solid waste incineration (MSWI) improves disinfection, reduces the waste volume by 90%, and has the additional advantage of simultaneous energy recovery; however, MSWI still generates bottom and fly ash that need disposal.1
Different combustion processes at MSWI plants in Vienna produce approximately 48 000 tons of fly ash each year.2 The disposal of fly ash poses significant environmental risks owing to the presence of toxic metals and leaching of hazardous pollutants. To minimize environmental impact, researchers have investigated the recovery of metals and other materials from fly ash as well as its use as a filling material or in cement production.3−5 Although these research areas are beyond the scope of this study, they offer two general approaches for fly ash. The first approach involves the development of technologies, such as the FLUWA process to recover heavy metals from fly ash, to separate or reduce metals and toxic materials.5 The second approach involves the investigation of new fly ash applications.6,7 For example, Cherbanski et al.6 demonstrated the use of fly ash as a CO2 sorbent in a steam-methane reforming process, and Gutierrez et al.7 discussed the potential use of waste materials, such as MSWI fly ash, for thermal energy storage. Ceramic materials produced after the thermal treatment of fly ash was inert and had a thermal capacity of 0.714−1.112 [kJ kg⁻¹ K⁻¹].7 Thermal energy storage is used to store excess heat energy and positively impacts the environment by reducing fossil fuel consumption, thus mitigating global warming.8
Thermal energy storage falls into three categories: sensible, latent, and thermochemical heat storage (TCES). Compared with sensible and latent heat storage systems, TCES is characterized by high energy density and zero energy loss.8−10 The preliminary requirement for any material for use in a TCES system is its potential to store heat through endothermic reactions. For example, material A decomposes through an endothermic reaction into solid material B and gas component C (eq 1).
The secondary requirement for a TCES material is the possibility of an exothermic reaction of decomposed material B with the gas component C (eq 1). Excess heat, such as that from solar power plants (CSP), can be used in reactor 1 to decompose the powder generated from fly ash (A(s)) into a charged form (B(s)) and reactive gas (C(g)) by applying a temperature higher than the forward reaction’s equilibrium temperature (eq 1). Hence, the reaction energy 𝞓HR is stored in products B(s) and C(g).
The charged form component (B(s)) of the initial material (A(s)) can react with gas component C(g) in reactor 2 to form the discharged form (A(s)) through the release of stored heat 𝞓HR by applying a temperature lower than the back reaction’s equilibrium temperature (eq 1). Then, the released heat can be removed from the reactor using either a reactive or inert gas. Further, this charging and discharging process can take place in a single reactor; however, a storage system is required in this case. Figure 1 shows a simplified representation of procedures within a TCES system.11
A(s) + 𝞓HR ↔ B(s) + C(g) (1)
Figure 1. TCES system.11
To illustrate the proposed concept, Figure 2 emphasizes a suggested integrative consisting of a TCES system to a combustion process. Fly ash is collected from the filter system, and heat can be provided and collected in the boiler’s heat exchanger system. Additionally, flue gas may be introduced as a CO2 source for discharging the TCES materials (fly ash). The TCES system includes reactors for heat transfer and storage facilities for charged and discharged material, respectively. Aged energy storage material can be used in the same way as the original fly ash.
Figure 2. Schematic of the TCES system connected to a combustion facility.
A TCES system requires suitable materials for operation, and previous studies have found that calcium oxide (CaO) is a promising material for use in TCES systems.11−16 CaO is a main component of fly ash, along with silicon oxide (SiO2) and aluminum oxide (Al2O3).7 Two reversible exothermic and endothermic reactions are possible with CaO: it can react with water vapor (H2O(g)) and carbon dioxide (CO2) to produce calcium hydroxide Ca(OH)₂ (eq 2) and calcium carbonate (CaCO3) (eq 3), respectively.
CaO(s) + H₂O(g) ↔ Ca(OH)₂ (s) + 𝞓HR
𝞓HR = −109 kJ/mol (2)
CaO(s) + CO₂ (g) ↔ CaCO₃ (s) + 𝞓HR
𝞓HR = −178 kJ/mol (3)
Additionally, owing to its chemical composition, MSWI fly ash may have an alternative use as a TCES material. To demonstrate this potential use, this study chemically characterized fly ash samples sourced from the grate furnaces of three MSWI plants in Austria. To achieve representative results, samples containing a mixture of seasonal collections from each plant over the course of 4 years were used. We analyzed these samples through X-ray fluorescence spectros-copy (XRF) and inductively coupled plasma-optical emission spectroscopy (ICP-OES) to detect matrix and nonmatrix elements and examined each sample’s reactivity under different atmospheres through simultaneous thermal analysis (STA). In addition, we performed a leachability test and scanning electron microscopy (SEM) analysis.
MATERIALS AND METHODS
To detect matrix elements in our XRF analysis, we milled and grained the fly ash samples to a particle size of 250 μm. The samples were embedded in aqua regia in accordance with the standard EN 13657 (2002) and then analyzed via the ICP-OES test in accordance with the standard EN 11885 (2009).
We used an STA 449 Jupiter instrument (Netzsch, Germany) to conduct the STA of fly ash under different atmospheres. We set a 30 K min⁻¹ heating rate up to a sample temperature of 1150 °C and used aluminum oxide crucibles of 125-μL volume for all the experiments. For each experimental run, we measured a sample mass of approximately 30 mg. The different gases used for STA had a minimum purity of 99.999% (v/v). To identify the effect of thermal treatment in STA on the fly ash, we analyzed all the samples before and after the STA experiments through SEM analysis (FEI Quanta 250 FEG SEM).
We performed a leachability test on the original samples in accordance with the standard EN 12457-4 (2002) and analyzed leachates using ICP-OES in accordance with the standard EN 11885 (2009).
RESULTS AND DISCUSSION
XRF and ICP-OES. Table 1 presents XRF analysis results for fly ash from the different plants. Fly ash samples A and B exhibit similar elemental contents, with a maximum 2.1 mass percent difference. However, fly ash sample C significantly differs in SiO2, Na2O, and Cl− content. Additionally, the CaO contents, where CaO exists freely or as part of compounds such as silicates, aluminates, sulfates, and carbonates,17,18 varies between 25 and 32.3 mass percent.
Table 1. Total Content of Matrix Elements in the Different Fly Ash Samples As Determined through XRF Spectroscopy (Mass Percent)19
Table 2. Total Content of Nonmatrix Elements in the Different Fly Ash Samples Determined through Inductively Coupled Plasma-Optical Emission Spectroscopy (mg/kg)19
Table 2 presents the ICP-OES analysis results and shows the different metals comprised in the fly ash samples. High levels (greater than 1 000 ppm) of barium, lead (Pb), copper, and zinc are noted, making fly ash a potential source of these metals. Consequently, the FLUREC process has been used to recycle zinc from fly ash in Switzerland on a large scale.20
STA. To investigate the thermal activity of the samples, we decomposed all the fly ash samples under an N2 atmosphere using a heating rate of 30 K min⁻¹ up to a sample temperature of 1150 °C.
In Figures 3−10, the green, blue, and dotted red lines show the percentage of mass loss based on TG, DSC, and the temperature profile, respectively. Three mass loss steps can be observed in the TG signals for fly ash samples A and B. The first mass loss (approximately 1.8%) occurs between 44 and 500 °C and is related to water and organic material evaporation. The second mass loss (greater than 4%) occurs between 500 and 750 °C and is related to the dehydration of metal hydroxides (e.g., calcium hydroxide or magnesium hydroxide) in the fly ash. The third and final mass loss (approximately 34% and 26% for fly ash samples A and B) occurs between 750 and 1150 °C and is related to decarbonation and decomposition of sulfates and chloride.21,22 The endothermic DSC signals of fly ash samples A and B, which occur in the same temperature ranges as the mass losses, are depicted in Figures 3 and 4, respectively.
Figure 5 illustrates the decomposition of fly ash sample C under an N2 atmosphere. Unlike fly ash samples A and B, fly ash sample C exhibits four mass change steps in the relevant endothermic DSC signals. Additionally, its total mass change is significantly lower (∼12%) than the mass changes of fly ash samples A (∼40%) and B (∼33%). A major reason for the reduced mass loss is the thermally inert behavior of Al2O3 and SiO2, which are the main components of sand and in higher concentrations in fly ash sample C.
Table 3 presents the results of thermal decomposition of fly ash samples A, B, and C, including information regarding the reaction type (exothermic or endothermic), based on mass and DSC signals.
We obtained the fly ash energy content by integrating the endothermic peaks generated by the DSC signals (Figures 3−5) calculated with linear baselines, and the results are summarized in Table 4. Fly ash sample C exhibits the highest energy content; it is approximately 4 times greater than those of fly ash samples A and B, which may be explained by higher free lime (Ca(OH)₂) content in fly ash sample C (see CaO content in Table 1).
Table 3. Results of Decomposition of Fly Ash Samples in N2 Atmosphere (Mass Percent)
Figure 3. Thermal decomposition of fly ash sample A under an N2 atmosphere. The green, blue, and dotted red lines show the percentage of mass loss based on TG, DSC, and temperature profile, respectively.
Figure 4. Thermal decomposition of fly ash sample B under an N2 atmosphere. The green, blue, and dotted red lines show the percentage of mass loss based on TG, DSC, and the temperature profile, respectively.
Figures 6 and 7 illustrate the thermal reactivities of fly ash samples A and C, respectively, under a CO2 atmosphere. Fly ash samples A and C have the same total mass change as that under an N2 atmosphere; however, their DSC signals show significant differences. The reaction of fly ash samples with CO2 is clear, particularly for fly ash sample C (Figure 7). The mass of fly ash sample C increases by approximately 2.62% at a temperature of 292 °C through an exothermic reaction and decreases through an endothermic reaction at approximately 700 °C. These changes are related to adsorption or other chemical reactions of fly ash sample C with CO2 under those operating conditions. Fly ash sample C exhibits a small increase in mass (0.38%) during the temperature profile’s cooling phase, which could be a sign of a reverse reaction.
Figure 5. Thermal decomposition of fly ash sample C under an N2 atmosphere. The green, blue, and dotted red lines show the percentage of mass loss based on TG, DSC, and the temperature profile, respectively.
Table 4. Energy Content in kJ/kg of Fly Ash Samples A, B, and C
Figure 8 presents the results of an STA of fly ash sample C under a mixed CO2/H2O/N2 atmosphere with a 30 min temperature plateau at 900 °C and partial pressure of water of 0.25 bar. The total mass loss is 10.85%, with the mass signal increasing by 2.78% and 1.19% during the heating and cooling (30 K min⁻¹) phases, respectively. This further confirms fly ash sample C’s reaction with CO2/H2O. The mass increase during the cooling phase is notable compared with the previous higher temperature (1150 °C) run. Higher temper-atures increase the sintering effect between particles, reducing the reactivity of fly ash with CO2. Additionally, this experiment shows the positive influence of water vapor on the reaction of CO2 and fly ash sample C.
In another experimental run, we decomposed fly ash sample C at 880 °C for 30 min under an inert N2 atmosphere, which was then changed to pure CO2 30 min before a cooling phase (10 K min⁻¹). We observed a 1.81% mass increase, indicating that the fly ash starts reacting with CO2 at 873 °C. Reducing the decomposition temperature from 1150 to 880 °C and the cooling rate from 30 to 10 K min⁻¹ increases the mass gain from 0.38% (Figure 7) to 1.81% (Figure 9). This illustrates the influence of high temperatures on fly ash sample C’s reactivity through sintering, which is further confirmed through SEM analysis (Figure 11). Lower decomposition temperatures and cooling rates yield higher adsorption or chemical reactions of fly ash sample C and CO2.
Figure 6. Thermal behavior of fly ash sample A under a pure CO2 atmosphere. The green, blue, and dotted red lines show the percentage of mass loss based on TG, DSC, and the temperature profile, respectively.
Figure 7. Thermal behavior of fly ash sample C under a CO2/N2 atmosphere. The green, blue, and dotted red lines show the percentage of mass loss based on TG, DSC, and the temperature profile, respectively.
Figure 8. Thermal behavior of fly ash sample C under CO2, N2, and H2O atmospheres at 900 °C. The green, blue, and dotted red lines show the percentage of mass loss based on TG, DSC, and the temperature profile, respectively.
To optimize the reverse reaction of fly ash sample C, we performed the next experiment under the same conditions as the previous one (Figure 9), barring the addition of water vapor when switching to a CO2 atmosphere for the carbonation phase starting at 91 min (Figure 10). In Figure 10, the mass signal shows a gain of approximately 4.58% during the cooling (carbonation) phase and the DSC signal confirms the reaction’s exothermic behavior (see DSC peak at 92 min). The mass and DSC signal peaks seen between 60 and 75 min (temperature plateau at 880 °C) were most likely due to a water droplet being accidentally introduced during vapor switching, triggering a temporary reaction of CO2/H2O with fly ash and its decomposition under these operating conditions. The integration of two endothermic peaks in the DSC signal (charging) with a linear baseline indicates that up to 900 °C, 240-kJ/kg energy can be stored in this fly ash with an 11% mass loss. Furthermore, more than 40% (∼99 kJ/kg) of the stored energy can be released under these conditions.
SEM Analysis. Figure 11 illustrates the results of the SEM analysis of the three fly ash samples before and after thermal treatment via the STA process (30 min at 1150 °C). As expected, samples have different shapes and sizes because small particles adhere to larger particles to create different particle size distributions in each sample. The thermal treatment causes strong sintering and agglomeration; thus, particles with larger surface areas become more visible. Agglomeration within the fly ash and sintering could negatively impact the stability of the cycle with respect to potential use as a TCES material; however, all the samples remained in a compact powder form and could easily be removed from the crucibles. Moreover, thermal treatment through STA at 1150 °C could be advantageous for fixing heavy metals in the matrix, thus reducing their concentration in leachates. Haiying et al.21 and Liu et al.23 reported the same trend for thermally treated fly ash.
Figure 9. Decomposition of fly ash sample C under inert N2 conditions at 880 °C and switching to pure CO2 atmosphere. The green, blue, and dotted red lines show the percentage of mass loss based on TG, DSC, and the temperature profile, respectively.
Figure 10. Decomposition of fly ash sample C under inert N2 atmosphere at 880 °C and switched to a CO2/H2O atmosphere. The green, blue, and dotted red lines show the percentage of mass loss based on TG, DSC, and the temperature profile, respectively.
Leachability Test. Table 5 shows the results of the leaching test. The limit values are the lower and (in brackets) highest limits for nonhazardous waste landfill as set by the Austrian Landfill Ordinance.22 The most notable difference between the leaching behaviors of the fly ashes is the leachate Pb content of each sample: they are 304, 52, and 16 mg/kg for fly ash samples A, B, and C, respectively. No other heavy metals were leached in such significant amounts. Because fly ash sample C’s Pb content does not exceed the typical leachate limits for disposal in standard above-ground landfills, this material can be considered nonhazardous. Contrastingly, fly ash sample A has a leachable Pb content that exceeds the typical leachate limits in Austria,24 Spain,25 Belgium,25 and France26 by a factor of 30 or more, meaning that fly ash sample A would be considered hazardous waste in most countries. This does not necessarily preclude its use, but it increases the requirements to be met and weaken its acceptance among the public considerably.
CONCLUSIONS
The results of this investigation present a new application for MSWI fly ash as a TCES material. XRF analysis results show that CaO, which exists freely or in compounds such as silicates, aluminates, carbonates, and sulfates, is a major fly ash component.
Figure 11. SEM images showing particle changes before (left) and after (right) thermal treatment at 1150 °C for 30 min.
The preliminary requirement to qualify as a TCES material, which is the ability to store heat (charging), was satisfied by all three fly ash samples through endothermic decompositions under an inert atmosphere during STA. We observed three mass loss steps for fly ash samples A and B and four mass loss steps for fly ash sample C. DSC signals indicated endothermic reactions for each mass loss step. In the temperature range between 30 and 1150 °C, fly ash samples A, B, and C achieved total mass losses of 40%, 32%, and 13%, respectively. Through thermal treatment up to a temperature of 1150 °C, fly ash samples A, B, and C possessed energy contents of 90, 98, and 394 kJ/kg, respectively.
Exothermic reverse reactions (discharging) of fly ash sample C with CO2 and CO2/H2O were demonstrated through STA. The conversion of the reaction increased as temperature decreased from 1150 to 880 °C and the cooling rate decreased from 30 to 10 K min⁻¹; however, complete conversion was not achieved under these experimental conditions. The stored energy of fly ash sample C up to 880 °C was 240 kJ/kg, approximately 99 kJ/kg of which could be released under the selected operational circumstances. This finding indicates that fly ash sample C meets the second necessary requirement of TCES materials: an exothermic reverse reaction of decom-posed material (fly ash) with gaseous components such as CO2 and H2O.
Although SEM analysis revealed the sintering and agglomeration of particles in all the samples after STA, they were all still in powder form and could easily be removed from the crucibles. With respect to the potential applicability of fly ash as a TCES material, this would provide a benefit to the stability of its cycle; this issue will be part of further study.
Leaching test results indicated that some varieties of fly ash may be considered as nonhazardous waste, making the use of such material in TCES systems more feasible. Nevertheless, further research is necessary to improve and demonstrate the potential use of MSWI fly ash as a TCES material.
Table 5. Leachate Contaminant Content in the Fly Ash Samples Determined by Inductively-Coupled Plasma-Optical Emission Spectroscopy (mg/kg)
- Indo Tambangraya Megah (ITMG)
- Bukit Asam (PTBA)
- Baramulti Sukses Sarana (BSSR)
- Harum Energy (HRUM)
- Mitrabara Adiperdana (MBAP)
- Adaro Energy (ADRO)
- Bumi Resouces (BUMI)
- Samindo Resources (MYOH)
- United Tractors (UNTR)
- Berau Coal
