Expanded Natural Graphite Treated With Sulfuric Acid Research Articles

Microstructure and sorption performance of consolidated composites impregnated with LiCl

Desiccant materials play a crucial role in the whole performance of novel desiccant coated heat exchanger based systems which can handle sensible and latent loads simultaneously and separately within one single component. To improve sorption performance of widely utilized coated desiccant, i.e., silica gel, this paper introduces a new type of consolidated composite desiccant, by mixing silica gel with expanded natural graphite treated with sulfuric acid (ENG-TSA) and impregnating with LiCl. Consolidated samples with different densities of ENG-TSA were developed. Microstructure, sorption kinetics and sorption isotherms were investigated and compared with loose parent and composite silica gels. Results showed that both ENG-TSA and LiCl had negative influence on surface areas and pore volumes of consolidated composites. With proper choice of the density of ENG-TSA, both dynamic water uptake quantities and sorption rate coefficients of consolidated composites can be greatly enhanced than those of parent and composite silica gels. Calculated theoretical water uptakes for consolidated composites were approx. 20% less than those of composite silica gel, while measured water isotherms showed that consolidated composites possesses better adsorption capacity than composite silica gel. Fitting equations were developed based on the Polanyi theory for further system modeling.

Analysis of composite sorbents for ammonia storage to eliminate NOx emission at low temperatures

A new type of composite sorbent-SCR system has been designed for NOx elimination of diesel vehicles to promote the NOx conversion rate at low temperatures (50–250°C), with the sorption reaction temperature of 50–100°C for safety reason. NH3 is sorbed by composite sorbents with expanded natural graphite treated with sulfuric acid (ENG-TSA) matrix at room temperature, and released from composite sorbents by heating phase, which is driven by both electricity and exhaust waste heat. The composite sorbent is a porous composite of 80% chemical sorbents and 20% ENG-TSA. The NH3 sorption capacity of chemical sorbents (NH4Cl, NaBr, BaCl2, CaCl2 and SrCl2) have been evaluated and compared with traditional urea-SCR system with AdBlue, and the results are analyzed. NH3 cannot be pyrolyzed from AdBlue below 160°C, and the NOx conversion rate of the composite sorbent-SCR system is about 45% higher than the urea-SCR system (0%) at 50°C. NH4Cl and NaBr are not suitable for composite sorbent-SCR system for that the pressure is too high even at the environmental temperature. According to the NH3 sorption capacity, composite of CaCl2 with ENG-TSA and composite of SrCl2 with ENG-TSA are both optimal solid sorbents, and between them CaCl2 with ENG-TSA as matrix is the best choice for the small mass required and the low cost.

Performance analysis on a novel compact two-stage sorption refrigerator driven by low temperature heat source

A novel two-stage sorption refrigeration system is established and analyzed, which is driven by heat source with the temperature lower than 100 °C. CaCl2-BaCl2-NH3 is selected as working pair whereas matrix of expanded natural graphite treated with sulfuric acid (ENG-TSA) is used for the improved heat and mass transfer performance of composite sorbent. The non-fin filling technique is adopted to decrease mass and volume of sorption reactor, which further improves system compactness. Results show that two-stage sorption refrigeration system is flexibly adapted to different heat source with temperature below 100 °C. COP and SCP of the novel system range from 0.185 to 0.22 and from 50 W kg−1 to 76 W kg−1, respectively under the condition of 70 °C–90 °C heat source temperature and 5 °C–10 °C evaporation temperature. Performance of novel two-stage sorption refrigeration system is also compared with that of previous type by using the conventional fin tube sorption reactor based on mass and volume of the whole system. It is indicated that the highest improvement of SCPsys and VCPsys for the novel system are able to reach 28.1% and 32.5%, respectively when heat source temperature is 70 °C.

Experimental investigation on an innovative resorption system for energy storage and upgrade

Progress of efficient thermal energy storage (TES) has become a key technology for the development of energy conversion system. Among TES technologies, sorption thermal energy storage (STES) has drawn burgeoning attentions due to its advantages of high energy density, little heat loss and flexible working modes. Based on STES, this paper presents an innovative resorption sorption energy storage (RTES), and the experimental system is established and investigated for energy storage and upgrade. 4.8kg and 3.9kg MnCl2 and CaCl2 composite sorbents are separately filled in the sorption reactor, and expanded natural graphite treated with sulfuric acid (ENG-TSA) is integrated as the matrix for heat transfer intensification. It is indicated that the highest energy storage density are 662kJ/kg and 596kJ/kg when heat input temperature is 125°C and heat release temperature are 130°C and 135°C, respectively. For different heat input and release temperature, the energy efficiency and exergy efficiency range from 27.5% to 40.6% and from 32.5% to 47%, respectively. The novel RTES system verifies the feasibility for energy storage and upgrade, which shows the great potential for low grade heat utilization especially for industrial process.

Investigation on performance of multi-salt composite sorbents for multilevel sorption thermal energy storage

Novel bi-salt and tri-salt composite sorbents are developed, and expanded natural graphite treated with sulfuric acid (ENG-TSA) is integrated as the matrix with different mass ratios for heat transfer intensification. Tri-salt composite sorbent is mainly composed of Manganese chloride (MnCl2), Calcium chloride (CaCl2) and Ammonium chloride (NH4Cl) whereas bi-salt composite sorbent comprises Calcium chloride (CaCl2) and Ammonium chloride (NH4Cl). Sorption characteristics under non-equilibrium condition are investigated and compared with that under equilibrium condition. Results show that the sorption hysteresis can be alleviated by bi-salt composite sorbent and even eliminated by tri-salt composite sorbent. Based on testing results, multilevel sorption thermal energy storage (STES) is analyzed, which can greatly enhance the versatility and working reliability. It is also worth noting that the highest energy storage density of reaction heat is 1802kJ/kg and 1949kJ/kg for tri-salt and bi-salt composite sorbents, respectively. Performance of bi-salt composite sorbent is relatively close to the theoretical data, which indicates three main stages. Comparably, performance of tri-salt composite sorbent shows continuous variation with the increment of reaction temperature. The promising multilevel STES reveals the great potential for energy utilization of variable heat source such as solar power when compared with conventional heat storage methods.

Solution to the sorption hysteresis by novel compact composite multi-salt sorbents

Two novel types of consolidated composite sorbents of NH4Cl/CaCl2/MnCl2 and NH4Cl/CaCl2 are developed by the matrix of expanded natural graphite treated with sulfuric acid (ENG-TSA). The Clapeyron curves and sorption/desorption properties of different sorbents are tested and compared under non-equilibrium conditions. Compared with single-salt sorbent, the multi-salt/ENG-TSA sorbents show the combining properties of each kind of metal chloride they contain. One novel phenomenon is discovered in the study is that the sorption hysteresis of single salts, which is regarded as one common phenomenon for chemisorption, can be changed by the composite sorbents of multi-salt. For the composite bi-salt sorbent of NH4Cl/CaCl2 the sorption hysteresis is much less significant than the single metal chlorides of NH4Cl and CaCl2. For the tri-salt composite sorbent of NH4Cl/CaCl2/MnCl2 the sorption hysteresis disappears. The reason is analyzed and it may be related to the resorption between the metal chlorides they contain, which leads to a reduction of reaction heat that is believed to be a typical reason for sorption hysteresis.

Development of SrBr2 composite sorbents for a sorption thermal energy storage system to store low-temperature heat

A composite material was developed as sorbent for sorption thermal energy storage (TES) which was used to recycle the low-temperature heat in industry and life fields in this study. The composite sorbent was formed by strontium bromide (SrBr2) and the additive of expanded natural graphite treated with sulfuric acid (ENG-TSA). Sorption characteristics, kinetic sorption performance, thermal conductivity and permeability of 15 samples were studied. The material test results indicated the following: (1) the composite SrBr2 is of high energy density and good mass transfer performance; (2) it can be regenerated below 100 °C; (3) the additive of ENG-TSA greatly improves the heat transfer performance, while no degradation is observed on sorption water uptake; (4) the optimal composite sorbent is of 743 kg/m3 and with 10 wt% ENG-TSA.An lab-scale sorption TES system with 1 kWh design capacity was established and investigated. Under the conditions in winter seasons: charging temperature Tcha is 80 °C, discharging temperature Tdis is 35 °C, condensing temperature Tc is 15 °C, evaporating temperature Te is 15 °C, the heat storage capacity can reach 1.02 kWh. The heat storage density obtained is 242 Wh/kg composite sorbent and the heat discharging power is 67.4 kW/m3 composite sorbent.

Experimental study on a resorption system for power and refrigeration cogeneration

Energy conversion technologies, especially for power generation and refrigeration technologies driven by the low temperature heat, are gathering the momentum recently. This paper presents a novel resorption system for electricity and refrigeration cogeneraion. Compared with adsorption refrigeration system, resorption refrigeration is characterized as safety and simple structure since there is no ammonia liquid in the system. The cogeneration system is mainly composed of three HTS (high temperature salt) unit beds; three LTS (low temperature salts) unit beds, one expander, three ammonia valves, two oil valves, four water valves and connection pipes. Chemical working pair of MnCl2–CaCl2–NH3 is selected. Since scroll expander is suitable for small type power generation system, it is chosen for expansion process. 4.8 kg MnCl2 and 3.9 kg CaCl2 impregnated in expanded natural graphite treated with sulfuric acid (ENG-TSA) are filled in the cogeneration system. Experimental results show that maximum cooling power 2.98 kW is able to be obtained while maximum shaft power is about 253 W with 82.3 W average value. The cogeneration system can be utilized for the heat source temperature lower than 170 °C. Total energy efficiency increases from 0.293 to 0.417 then decreases to 0.407 while exergy efficiency increases from 0.12 to 0.16.

Study on MnCl2/CaCl2–NH3 two-stage solid sorption freezing cycle for refrigerated trucks at low engine load in summer

A novel MnCl2/CaCl2–NH3 two-stage solid sorption freezing cycle is designed and established for the refrigerated truck with the rated power of 80kW. The conventional sorption/desorption process and the resorption process are combined in the two-stage cycle. Theoretical analysis shows that such a cycle could adapt to the low heat source temperature and the high cooling temperature of the sorption beds very well, which is quite essential for the truck when the running speed and the load are low in summer. The expanded natural graphite treated with sulfuric acid (ENG-TSA) is chosen as the matrix, and composite adsorbents of MnCl2/ENG-TSA and CaCl2/ENG-TSA are developed. The hot air heated by the electric heater is used to simulate the engine exhaust gas to drive the system. When the hot air, the ambient air and the refrigerating temperature are 230°C, 30°C and −5°C, respectively, the average refrigerating capacity is 2.2kW in the sorption process. Correspondingly, the COP and SCP are 0.13 and 91.7W/kg, respectively. The average refrigerating capacity of 1.1kW in one cycle is gotten, which could meet the required refrigerating capacity of the light refrigerated truck at the low engine load engine in summer.

Study on consolidated composite sorbents impregnated with LiCl for thermal energy storage

LiCl/H2O shows a great potential for sorption thermal energy storage with its large water sorption capacity. However, as a highly hygroscopic salt, LiCl is likely to turn into liquid solution when it contacts with water vapor. A new type of consolidated composite sorbent was developed, by using activated carbon (AC) as a porous host matrix to carry LiCl, mixing with expanded natural graphite treated with sulfuric acid (ENG-TSA) to increase heat transfer and adding silica solution (SS) as a binder to enhance mechanical strength. Samples with different densities and varied mass ratios between different elements were prepared and related characteristics including equilibrium water uptake, thermal conductivity, TGA/DSC simultaneous thermal analysis and kinetic performance were investigated. Results reveal the existence of an obvious sorption plateau corresponding to formation of LiCl⋅H2O, as well as two sharp increase of water uptake related to hydration reaction and three-phase deliquescence processes respectively. Activated carbon is a better choice of matrix compared with silica gel, with a water uptake up to 0.97g/g at 30°C and 1660Pa. It is proved that among the four elements in the consolidated composite (AC, LiCl, ENG-TSA, silica solution), LiCl is responsible for almost all the water uptake effect. Addition of other elements do not influence the equilibrium water uptake of LiCl. Variation of bulk densities from 462 to 820kg/m3 displays thermal conductivities from 2.0 to 2.83W/(mK), improving that of loose-packed AC–LiCl composite by at least 14 times.

Experimental study on working pairs for two-stage chemisorption freezing cycle

Recently CaCl2–BaCl2–NH3 chemisorption freezing cycle driven by low temperature heat source has been successfully developed. In order to develop one working pair with more desirable performance than CaCl2–BaCl2–NH3, working pairs of CaCl2–NaBr–NH3, CaCl2–BaCl2–NH3, SrCl2–BaCl2–NH3 and SrCl2–NH4Cl–NH3 were investigated and compared. Most of composite adsorbents were developed with new type matrix of expanded natural graphite treated with sulfuric acid (ENG-TSA), leaving one working pair with expanded natural graphite (ENG) for comparison. For SrCl2–NH4Cl–NH3 and CaCl2–NaBr–NH3, experimental results show that the maximum adsorption quantities are 95.4% and 88.6% of theoretical values, respectively. Simulation results show that for different working pairs coefficient of performance (COP), cooling capacity, and specific cooling power (SCP) range from 0.215 to 0.285, 2 to 3.65 kW and 161.4–260.74 W kg−1, separately. The best results are obtained from CaCl2–NaBr–NH3, and its SCP and COP are as high as 260.74 W kg−1and 0.285, which are improved by 15.1% and 5.6% if compared with the values for CaCl2–BaCl2–NH3. Comparisons also show that the matrix of ENG-TSA improves SCP effectively. Taking CaCl2–BaCl2–NH3 as an example, its SCP is improved by 76.9% if compared with the result obtained from working pair with the matrix of ENG.

Development of highly conductive KNO3/NaNO3 composite for TES (thermal energy storage)

PCM (phase change material) is one of the most popular media in TES (thermal energy storage) system for its high energy storage density and suitable melting temperature. However, its low heat transfer performance leads to low rate of heat stored and released. In this paper, a highly thermal conductive additive of ENG-TSA (expanded natural graphite treated with sulfuric acid) had been used to improve the thermal conductivity of the KNO3/NaNO3 nitrate mixture. Composite samples with different densities and mass fractions of ENG-TSA were produced by the cold-compression method. The thermal conductivities, phase transition properties, thermal stability and micro structures of different samples were evaluated, and results showed that the highest effective thermal conductivity of composite PCM was 50.78 W/(mK), which was 110 times higher than that of salt powder. The additive of ENG-TSA in the PCM caused a slight decrease in latent heat, but there was no remarkable variation in phase change temperature. The salt segregation from the consolidated composite PCM blocks could be controlled at a small value when the densities and mass fractions of ENG-TSA were optimized.

Thermal conductivity, pore structure and adsorption performance of compact composite silica gel

A novel composite solid desiccant material is proposed in this study to enhance thermal conductivity and adsorption performance of silica gel. This desiccant material is fabricated by combining silica gel with expanded natural graphite treated with sulfuric acid (ENG-TSA) as a host matrix. The performance of this composite material with respect to different densities and different silica gel ratios was investigated. Experimental results showed that the highest thermal conductivity of consolidated composite adsorbents is 19.1Wm−1K−1. This is an increase of more than 270 times as compared to that of pure silica gel. Analysis on pore parameters demonstrated that the compression of simple mixture of ENG-TSA with silica gel does not destroy the morphology of silica gel. In addition, both non-equilibrium and equilibrium adsorption performance of composite silica gel and pure silica gel were evaluated. Non-equilibrium adsorption performance indicated prominent enhancement of heat transfer, whilst equilibrium adsorption performance indicated a reasonable mass transfer.

Study on consolidated activated carbon: Choice of optimal adsorbent for refrigeration application

For refrigeration condition in order to choose an optimal consolidated composite activated carbon (AC), the thermal conductivity, permeability, equilibrium adsorption performance, and non-equilibrium adsorption performance were studied. Six samples of three types of adsorbents (consolidated AC with expanded natural graphite treated with sulfuric acid (ENG-TSA), consolidated AC with expanded natural graphite (ENG) and granular AC) with different density and different grain size were produced and compared. Experiments showed that the thermal conductivity and permeability of consolidated AC with ENG-TSA are between 6.75 to 7.45W/mK and 2.00×10−12 to 1.00×10−11m2, respectively, when the density is ranged between 350kg/m3 and 450kg/m3. The study on the equilibrium and non-equilibrium adsorption performance indicated that the addition of ENG-TSA helps to improve the concentration swing. The cycle time of consolidated AC with ENG-TSA decreases significantly compared with that of consolidated AC with ENG and granular AC. For the optimal results from the matrix of ENG-TSA although the bulk density of AC decreases, the volume cooling power is 30% improved in comparison with the results of granular AC because the cycle time decreases significantly.

Development of thermal conductive consolidated activated carbon for adsorption refrigeration

A new type consolidated composite activated carbon (AC) was developed with a host matrix of expanded natural graphite treated with sulfuric acid (ENG-TSA). Samples with different density, different grain size of AC, and different proportion of AC were produced and thermo-physical properties were evaluated. Results show that the highest effective thermal conductivity and thermal diffusivity of consolidated composite AC were 34.2 W m −1 K −1 and 3.89 × 10 −5 m 2 s −1, which were 150 times and 72 times higher, respectively, than ordinary granular AC. The permeability of adsorbents ranged between 1.24 × 10 −14 and 7.81 × 10 −10 m 2 while the density ranged between 215 and 448 kg m −3. The adsorption performance for composite AC and granular AC were evaluated by fitting experimental data with the equilibrium Dubinin–Astakhov (D–A) equations. Results show that the addition of ENG-TSA can improve the performance of adsorption refrigeration machines by increasing the concentration swing.