Mechanism and Performance of Composite Phase Change Materials from the Direct Hydrolysis Residue of Municipal Sludge Loaded with Sodium Acetate Trihydrate.

The direct hydrolysis of municipal sludge for the production of oil and gas has become a key research focus, despite the application of hydrolysis residues presenting a challenge. In this study, municipal sludge was directly hydrolyzed in a high-pressure reaction kettle and the hydrolysis residue byproduct was used as a carrier to prepare a composite phase change heat storage material (CPCM), utilizing vacuum impregnation for sodium acetate trihydrate (SAT) loading. The results of Brunauer-Emmett-Teller (BET) and particle size analyses showed that the residue obtained by the hydrolysis of sludge and sawdust with a dry basis ratio of 4:1 had a higher pore volume and a uniform particle size. The adsorption capacity of the hydrolysis residue to SAT reached 600 wt %; the phase change temperature of the CPCM was 56.9 °C, and its latent heat reached 217.9 kJ/kg. The CPCM remained stable during 150 cycles of the melting-solidification process in a water bath and maintained excellent phase change characteristics. The hydrolysis residue can effectively improve the undercooling and phase separation of SAT without other additives.

Introduction

With the rapid increase of sludge production in municipal sewage treatment plants, the global demand for sludge recycling has increased. The disposal methods for residual sludge mainly include landfill, composting, natural drying, and incineration treatments.[1] Among these treatment technologies, landfill and composting can result in serious pollution of groundwater and soils, natural drying does not effectively remove harmful substances from sludge, and incineration treatment can produce a large amount of harmful gases that further pollute the environment.[2,3] Therefore, dry pyrolysis carbonization[4−6] and hydrothermal pyrolysis carbonization[7−9] have become new sludge disposal technologies. Municipal sludge has a high content of organic matter and has the potential to become an adsorbent after carbonization. Tang et al.[10] evaluated the potential use of biochar derived from the pyrolysis of digested sludge for the removal of ammonium nitrogen from water and indicated that biochar derived from the digester residue is a promising adsorbent for ammonia in municipal wastewater. Li et al.[11] added an antibiotic mycelium residue to sludge for copyrolysis, and the results showed that copyrolysis could significantly improve the performance of biochar. However, the pyrolysis of sludge needs drying pretreatment and wastes much more energy for water removal. Therefore, direct hydrolysis of sludge has become one of the most studied technologies. Sludge hydrolysis products such as pyrolyzed oil, gas, and pyrolysis residue can be reused to achieve a high degree of resource utilization.[12] There are a lot of studies on the utilization of oil and gas from sludge hydrolysis, but there are relatively few literature studies on the utilization of the hydrolysis residue. Some scholars use the residue from sludge hydrolysis as an adsorbent to deal with water pollution and soil problems.[13,14] Kim et al.[15] found that the main component of the residue obtained by hydrothermal treatment of sludge was a stable silicon dioxide crystal, which could realize the stabilization of heavy metals and harmlessness of the sludge residual. Due to the adsorption characteristics of the pyrolysis residue, it could be used as a carrier to adsorb hydrated salt to prepare composite phase change materials (CPCMs).

In 2019, China introduced the peak-valley tariff policy, which aims to resolve the mismatch between power production and supply in a timely manner and encourages people to use low-priced electricity at night.[16] Energy storage technology can effectively solve the imbalance between energy supply and demand in time and space. It can convert electricity into heat energy at night and store it in heat storage materials and then release it during the day when electricity consumption is large.[17] Sodium acetate trihydrate (CH3COONa·3H2O, SAT) is a representative inorganic salt phase change material having a melting point of about 58 °C, a latent heat of more than 256 kJ/kg, and the advantages of being nontoxic and low cost, with widespread availability. The hot water supply of urban floor heating in winter is between 40 and 50 °C, which is about 10 °C difference from the SAT melting temperature, so the energy storage of SAT is suitable to be a winter heating source.

SAT, as an important phase change heat storage material in recent years, has inherent phase separation and undercooling problems. Hu et al. used synthetic aluminum nitride nanoparticles for the nucleation of SAT and carboxymethyl cellulose (CMC) as a thickener of SAT, effectively inhibiting SAT undercooling.[18] Furbo et al.[19] added 1% carboxymethyl cellulose to SAT as a thickener, with results showing that the energy discharged from the SAT unit was stable at 205 kJ/kg over six test cycles. Wang et al.[20] conducted a study to overcome the problems of SAT undercooling and phase separation. By adding the nucleating agent Na4P2O7·10H2O and polyacrylamide as a thickening agent, the degrees of phase separation and undercooling were effectively reduced while maintaining a high heat storage capacity. Liu et al.[21] used vermiculite as a carrier, finding that after adsorption of SAT in vermiculite micropores, the cyclic melting–solidification stability of phase change materials was improved and the appearance of phase separation was significantly weakened. Most of the mentioned works reduce the undercooling degree and phase separation of SAT by adding nucleating agents and thickening agents, which increases the preparation cost of CPCMs. Using the hydrolysis residue of municipal sludge as a carrier is a novel way to prepare CPCMs without adding nucleating agents and thickening agents.

After the municipal sludge mixture was directly hydrolyzed in a high-pressure reactor, the hydrolysis residue was used to adsorb SAT and prepare CPCMs. The effect of the hydrolysis residue on the subcooling degree and phase separation of SAT was investigated by thermal cycling, assessing the CPCMs’ heat storage and heat release capability to prompt the utilization of novel CPCMs.

Materials and Preparation

Experiment Materials

The municipal sludge used in experiments was taken from a sewage treatment plant in Qingdao City of China. SAT (Shanghai Eppie Chemical Reagent Co., Ltd., China) was utilized as a phase change material, exhibiting a phase change at 58 °C and a latent heat of 256.4 kJ/kg.

Preparation of the Hydrolysis Residue and CPCMs

Preparation of the Hydrolysis Residue

As the carbon content of sludge (SL) was low, a certain quality of sawdust (SD) was mixed with SL to increase the carbon content of the hydrolysis residue.

As shown in Figure S1, the mixed SL–SD samples with a dry basis mass ratio of 1:1, 2:1, or 4:1 (its residual named 1SL1SD, 2SL1SD, or 4SL1SD, respectively) were directly pyrolyzed in a 500 ml autoclave. The reaction termination temperature was set to 350 °C (when the reaction kettle reached this temperature, it stopped heating and cooled to room temperature naturally). Due to the high moisture content of the initial sludge, moisture was vaporized in the reactor, increasing the pressure within the reactor. The reaction termination pressure could be as high as 16 MPa, and during the reaction, the remaining moisture was involved in the microporous reforming of the hydrolysis residue.

After the reaction and gas–solid–liquid separation, the hydrolysis residue was dried at 105 °C for 8h and three hydrolysis residues (1SL1SD, 2SL1SD, and 4SL1SD) were then used as carriers to adsorb SAT to prepare the CPCM.

Preparation of CPCMs

As seen from Figure S1, the weighed SAT was placed in a triangular flask and heated in a water bath at 70 °C until fully melted. Then, hydrolysis residues with different mass ratios were added to the mixture, with continuous magnetic stirring at a constant temperature until the mixture was thoroughly integrated (when the sludge hydrolysis residue and SAT were completely mixed, the hydrolysis residue was infiltrated into the SAT solution and evenly distributed). Finally, the vacuum adsorption process was carried out. To fully extract the air adsorbed in the residue micropores, the vacuum degree was increased while ensuring that the crystalline SAT solution was not vaporized and lost during the process. The vacuum degree was selected according to the formula of evaporation pressure corresponding to saturated water. The formula is the Antoine formula, as shown in eq where T is in the range of 290–500 K and p indicates the pressure in MPa. The calculated saturation evaporation pressure of water at 70 °C is 0.032 MPa, so the vacuum must be controlled to greater than 0.032 MPa to prevent vaporization. The pressure vacuum could be between 0.050 and 0.060 MPa during the CPCM preparation process, as the saturation temperature had not been reached and no evaporation of crystallization water was occurring at this stage.

The three hydrolysis residues 1SL1SD, 2SL1SD, and 4SL1SD were all loaded with 600 wt % SAT to the residual for the preparation of CPCMs, which were named CPCM-1SL1SD-6, CPCM-2SL1SD-6, and CPCM-4SL1SD-6, respectively. When the hydrolysis residue 4SL1SD was loaded with 700 wt % SAT and 800 wt % SAT, the resulting CPCMs were named CPCM-4SL1SD-7 and CPCM-4SL1SD-8, respectively.

Thermal Performance Tests

The five CPCMs were assessed using multiple melting–solidification cycles, as shown in Figure S2. The CPCM was encapsulated in a test tube (15 mm diameter) and sealed with aluminum foil. Thermocouple wires were fixed in the center of the CPCM in the test tube, and the thermocouple was connected to a data acquisition instrument to record the temperature change during the heat release/storage process, allowing analysis of the evolution of the CPCM phase change process over each cycle. The temperature of the water bath was set at 70 °C for the melting process and 40 °C for the solidification process.

Differential scanning calorimetry (DSC) was performed using a synchronous thermal analyzer (Netzsch, STA 449F5, Germany) to determine the latent heat value and the melting point of the CPCMs during each cycle.

Residue Particle Tests

The pore volume and pore size distribution of the hydrolysis residue were characterized by Brunauer–Emmett–Teller (BET) analysis. The particle size distribution of hydrolysis residues was characterized using a laser particle size analyzer (RISE-2002, RISE Company, China). The microstructure of the CPCMs was characterized by scanning electron microscopy (SEM, JSM-6390LV, Nippon Electronics Corporation, Japan). The chemical composition compatibility of CPCMs before and after cycling was detected by X-ray diffraction (Rigaku, MiniFlex600, Japan).

Results and Discussion

Basic Properties of the Sludge and Hydrolysis Residual

The basic analysis of the sludge is shown in Table . The moisture content of the sludge was as high as 76.7%, while the calorific value of the dry sludge sample was very low, making it unsuitable for direct combustion.

Table 1

Composition Analysis of Municipal Sludge

	proximate analysis (%, ar)					ultimate analysis (%, ad)
sample	M	A	V	FC	HHV (MJ/kg, ad)	C	H	O	N	S
sludge	76.7	9.4	12.6	1.3	11.433	36.05	8.82	27.46	7.76	1.33

In the table, ar, as-received basis; ad, air-dried basis; HHV, high heat value; M, moisture; A, ash; V, volatiles; and FC, fixed carbon.

As for the pure SD of as-received basis, its moisture content (Mar) is 9.7%, ash content is 1%, volatile content is 76%, and fixed carbon content is 13.3%. The proximate analysis of the three hydrolysis residues is shown in Table .

Table 2

Proximate Analysis of the Three Hydrolysis Residues

proximate analysis (wt %, dry basis)	1SL1SD	2SL1SD	4SL1SD
ash	39.90	39.85	59.65
volatile	30.20	38.80	28.55
fixed carbon	29.90	21.35	11.80
HHV (MJ/kg)	15.02	20.29	18.51

Residue Particle Analysis

Adsorption Performance

BET tests were conducted on the dried hydrolysis residues, as shown in Figure . The results show that the micropore size of the three hydrolysis residue particles is mainly distributed between 2 and 20 nm. The pore size distribution of the three carrier materials was similar, exhibiting a single peak, although the pore volume of 4SL1SD was the highest. Since the molecular diameter of SAT was about 0.656 nm, the hydrolysis residue micropores became the microelement space for the adsorption of SAT, with the SAT molecular micelle potentially locked into the microelement space. The pore volume of hydrolysis residues 1SL1SD, 2SL1SD, and 4SL1SD were 0.015995, 0.016356, and 0.039346 cm3/g, respectively, with corresponding specific surface areas of 3.6999, 3.7065, and 7.3578 m2/g. With the increase in mixed proportion of sludge, the pore volume and specific surface area of the prepared hydrolysis residue gradually increased, while a continuous increase in the sludge mass ratio (e.g., 1SL0SD in Figure ) led to a decrease in adsorption performance. Therefore, the addition of sawdust at different mass ratios has a varying effect on sludge residue reforming, with results indicating that an excessive mixing proportion should be avoided, as shown by the hydrolysis residue 4SL1SD exhibiting the strongest adsorption capacity.

Figure 1

Pore size distribution of hydrolysis residues 1SL1SD, 2SL1SD, and 4SL1SD.

Figure 2

Particle size distribution of different hydrolysis residues.

Size of the Residue Particle

The particle size of hydrolysis residues 1SL0SD, 4SL1SD, and 0SL1SD prepared by SL–SD mixing at dry basis ratios of 1:0, 4:1, and 0:1 was analyzed, with the results shown in Figure .

In Figure , the particle size distribution of 1SL0SD exhibited two major peaks at 18.9 and 32.5 μm, with a median particle size (D50) of 28.760 μm. The particle size distribution of 0SL1SD exhibited a major peak and a smaller peak. The main peak value was 38.9 μm, with a D50 of 39.381 μm. The particle size distribution of 4SL1SD exhibited a major peak and a smaller peak. The main peak value was 15.8 μm, and the D50 was 14.795 μm. The particle size distribution of 4SL1SD was relatively concentrated and its particle size was the smallest.

Multicycle Performance of the CPCM

Characteristics of the SAT Melting–Solidification Cycle

To compare the improvement of the subcooling and phase separation by residues on the melting–solidification cycle, seven melting–solidification cycles of pure SAT were carried out under the same experimental conditions, as shown in Figure . As seen from Figure , the multicycle characteristics of the PCM were analyzed. The results of the synchronous thermal analysis show that the phase change temperature of SAT is 58.8 °C, with a latent heat of 256.4 kJ/kg. SAT exhibits an obvious undercooling phenomenon, with a maximum undercooling temperature (MST) of 7.3 °C, while the exothermic section is very unstable.

Figure 3

Thermal cycle of sodium acetate trihydrate. Note: MST is the maximum undercooling temperature.

Determination of the Optimal Carrier Components

CPCMs with the same mass adsorption ratio of 1:6 between the three hydrolysis residues and SAT were placed in the test system (Figure S, Supporting Information) and subjected to 150 melting–solidification cycles. The melting–solidification curve is shown in Figure . The melting change temperatures of CPCM-1SL1SD-6, CPCM-2SL1SD-6, and CPCM-4SL1SD-6 were 57.2, 58.3, and 56.9 °C, respectively, as measured by a synchronous thermal analyzer.

Figure 4

Thermocycling with 150 cycles of the melting–solidification process of the three CPCMs: (a) CPCM-1SL1SD-6, (b) CPCM-2SL1SD-6, and (c) CPCM-4SL1SD-6.

As shown in Figure a, after 60 cycles, the exothermic section of the phase change characteristics of CPCM-1SL1SD-6 rapidly shortened and the exothermic temperature fluctuated. After 100 cycles, there was almost no constant temperature observed in the phase change exothermic phase. For CPCM-2SL1SD-6 (Figure b), after 75 cycles, the exothermic section of the phase change characteristics rapidly shortened and the degree of undercooling increased gradually, with the maximum degree of undercooling reaching 6.5 °C. After 150 cycles (Figure c), the exothermic section of the phase change characteristics of CPCM-4SL1SD-6 remained very stable, with a characteristic phase change temperature of about 55.5 °C. The maximum subcooling temperature (MST) of 3.4 °C appeared in the 15th cycle, while the degree of undercooling gradually decreased in subsequent cycles. Wang et al.[20] made the undercooling temperature of SAT less than 5 °C and avoided the phenomenon of phase separation by adding a nucleating agent and a thickener. Here, the sludge hydrolysis residue greatly reduced the undercooling of SAT (3.4 °C) and overcame the phenomenon of phase separation of SAT without additives.

As shown in Figure , the right half of each curve (heat storage) exhibited similar trends to the solidification processes. The temperature of the isothermal phase change in the endothermic section was slightly higher than that in the solidification process, while the temperature of all three materials in the phase change section was similar to that measured in the thermal analyzer. The maximum melting temperature deviation was observed in CPCM-2SL1SD-6, at only 1 °C difference between the DSC and multicycle test results. Therefore, the multicycle processes established from CPCM tests are credible.

A comparison of the above performances indicates that CPCM-4SL1SD-6 exhibits better multicycle stability, mainly due to the characteristics of no phase separation and the small degree of undercooling. The advantage is due to two particular factors. First, after six cycles of SAT loading in the micropores of the hydrolysis residue, no crystal water was lost. For the other two materials, the shortening of the phase change stage resulted in the irreversible evaporation of crystal water and a gradual decrease in the mass of trihydrate in micropores. Second, the 4SL1SD carrier had a smaller particle size and a more uniform distribution, resulting in a larger CPCM packing density. When the hydrate melted, the solutions inside and outside of the residue particles were uniformly connected, with the residue acting as a thickener, resulting in crystal water and salt not easily being stratified, solidified, and crystallized reversibly, which increased the stability of thermal cycles. Therefore, 4SL1SD was selected as the optimal carrier, as its unique particle size and micropore structure made the addition of nucleating agents and thickening agents unnecessary for the preparation of CPCMs.

Determination of the Adsorption Ratio

The heat storage capacity of CPCMs mainly comes from the latent heat of hydrated salts, with a higher proportion of SAT increasing the heat storage capacity. Therefore, CPCM-4SL1SD-7 and CPCM-4SL1SD-8 were subjected to multiple melting–solidification cycles, and their performances were compared with the performance of CPCM-4SL1SD-6, as shown in Figure .

Figure 5

Performance comparison of (a) CPCM-4SL1SD-7 and (b) CPCM-4SL1SD-8 after multiple thermal cycles.

After more than 10 thermal cycles, the two CPCMs exhibited differences in multicycle stability. As seen from Figure a, the maximum undercooling degree of CPCM-4SL1SD-7 was 5.3 °C, which was larger than that of CPCM-4SL1SD-6 of 3.1 °C, although the exothermic section and the exothermic temperature were not stable. After several cycles, the subcooling degree of CPCM-4SL1SD-8 (Figure b) gradually increased, reaching a maximum subcooling degree of 15.3 °C, with an unstable exothermic section. Therefore, the optimum mass ratio of the hydrolysis residue to SAT was determined to be 1:6 to maintain a high heat storage capacity, to reduce the degree of undercooling, and to ensure stability over multiple cycles.

Appearance of CPCM-4SL1SD-6

Figure a shows the liquid state of CPCM-4SL1SD-6. It can be seen that CPCM-4SL1SD-6 is a black viscous mixture with a certain liquidity. Figure b shows the solid state of CPCM-4SL1SD-6 after 150 times of melting and solidification in a test tube to be very uniform. So, there is no significant phase separation for CPCM-4SL1SD-6 after 150 cycles. 4SL1SD with a uniform and fine particle size can effectively stick to the SAT solution to prohibit the phase separation of SAT.

Figure 6

Appearance characteristics of CPCM-4SL1SD-6 after 150 thermal cycles: (a) liquid phase and (b) solid phase.

Micromorphology of CPCM-4SL1SD-6

The microstructure of CPCM-4SL1SD-6 before and after preparation was characterized by SEM. As shown in Figure a, the hydrolysis residue still contained carbon and exhibited a relatively regular micropore structure, with 2–20 nm sub-micropores distributed on the surface of the micropores according to the BET analysis. Figure b shows the micromorphology of the adsorbed SAT, showing that SAT is filled in the micropores of the hydrolysis residue, with some overflow, which is mixed with sludge ash and adheres around the micropores. Therefore, there is almost no unfilled space in the microstructure and the space utilization rate is high, indicating that vacuum adsorption achieves a high heat storage capacity.

Figure 7

Micromorphology of hydrate adsorption by hydrolysis residues: (a) SEM image of the hydrolysis residue and (b) SEM image of 4SL1SD-6.

Thermal Performance of CPCM-4SL1SD-6

The stable latent heat in CPCMs after multiple cycles is key to effective practical application. The latent heat value of SAT is 256.4 kJ/kg, while the latent heat value of CPCM-4SL1SD-6 is 219.8 kJ/kg (according to a weight theory mass of 6/7). For CPCM-4SL1SD-6 subjected to 150 thermal cycles, samples were collected every 15 cycles. The DSC test was performed on CPCM samples (Figure ). As shown in Figure a, the endothermic melting process of the sample in the synchronous heat analyzer shows that the heat flux of CPCM-4SL1SD-6 in the phase change process after multiple cycles had little effect on the heat storage capacity. Therefore, CPCM-4SL1SD-6 retained good heat storage performance after 150 cycles. The melting point and latent heat of CPCM-4SL1SD-6 are shown in Figure b.

Figure 8

DSC test of CPCM-4SL1SD-6 through a multicycle process of (a) CPCM-4SL1SD-6 and (b) the melting point and latent heat of CPCM-4SL1SD-6 in the multicycle process.

The melting point of CPCM-4SL1SD-6 was between −1.4 and −0.1 °C different from that of SAT. The difference between the latent heat value and the theoretical weighted calculation value is typically −3.1 to 3.2%. Such a fluctuation is within the allowable range of experimental errors and can be regarded as stable. Therefore, CPCM-4SL1SD-6 can provide a long service period, with the thermal properties of the material remaining stable after repeated melting–solidification heat cycles. Compared with the CPCM prepared by Liu et al.,[21] there was no need to add the nucleating agent borax or a thickening agent, with the thermal performance remaining stable after more than 100 cycles, greatly reducing the cost of preparing CPCMs.

Chemical Compatibility of CPCM-4SL1SD-6

Figure shows the XRD patterns of 4SL1SD and CPCM-4SL1SD-6 during 150 melting and solidification cycles. The characteristic diffraction peaks included those of CH3COONa·3H2O, SiO2, and Al2O3, with the main components of the hydrolysis residue being SiO2 and Al2O3. For CPCM-4SL1SD-6, the diffraction peak after 150 cycles was consistent with that of the first cycle, indicating that no chemical reactions occurred between the hydrolysis residue and SAT, with only physical infiltration and adsorption occurring. The component stability also verifies that CPCM-4SL1SD-6 exhibits good thermal compatibility and chemical stability.

Figure 9

XRD patterns of 4SL1SD and CPCM-4SL1SD-6 during 150 cycles.

When the CPCM becomes invalid after many cycles and crystallization water evaporation, the recycling of the waste CPCM is also very critical. For sodium acetate, which is the most widely used as a carbon source supplement and for adjusting the pH value in sewage treatment, a CPCM can be used as an additive for sewage treatment after the CPCM loses efficiency.

Conclusions

The major conclusions can be drawn as follows:

The 4SL1SD hydrolysis residue has a higher pore volume and uniform finer particle size, resulting in a stable adsorption capacity.

CPCM-4SL1SD-6, which does not require the addition of thickeners or nucleating agents, overcomes the problems of undercooling and phase separation, with multicycle thermal stability.

The melting temperature of CPCM-4SL1SD-6 is 56.9 °C, with a latent heat value of 219.8 kJ/kg. After 150 melting–solidification cycles, little change was observed with no chemical reaction occurring between components, indicating good chemical compatibility.

The novel findings on the effective recycling of sludge hydrolysis residues provide an economic way for the production of composite phase change heat storage materials.