• Energy Research

Abstract The application of microwave energy in biomass pyrolysis has gained increasing interest due to its fast, volumetric, selective and efficient heating. However, the dominance may be levelled off if the energy and exergy efficiencies of overall system are taken into consideration. Hence, based on the lab-scale experimental data (at 600, 700 and 800 °C), energy and exergetic assessment of pyrolysis-derived gas, char and oil from gumwood under conventional and microwave heating were investigated in this work. The results showed that at each temperature, the corresponding energy and exergy rates of gas under microwave heating were found to be 23% and 26%, respectively, higher than those of conventional one. Meanwhile, the values for char were around 46%. This was mostly because microwave-derived pyrolysis product contained more gaseous and char products compared to the conventional process. In contrast, opposite trend was noticed for bio-oil due to the reduction of oil yield in pyrolysis product. It was demonstrated that the total energy and exergy value of individual pyrolysis products were significantly influenced by the energy and exergy content of individual component. For example, the increment rate of energy and exergy values of H2 and CH4 were sufficiently higher than the decrement rate of energy and exergy values of CO and CO2 with the increase of temperature; the total values were consequently increased at the elevated temperature. For the case of char and oil, the energy and exergy rate were decreased with temperature due to the reduction of individual product yield. Pyrolysis system efficiency (PSE), the overall performance indicator used in this study, of the microwave-assisted process was 13.5% higher than that of the conventional process. Moreover, the experimental results were further analysed with the conduct of a hydrogen plant simulation using Aspen Plus™ whose results confirmed that an improved performance of microwave heated system was achieved by producing 120 gH2/kg gumwood, 15% higher compared to that of conventional system.

Abstract Water scarcity is viewed as the top global risk for the next decade. One way to resolve water shortage is desalination which expends energy to derive fresh water from the vast amount of saline water. While the current desalination technologies are matured and reliable, desalinating seawater is still an energy intensive process, necessitating research for new generation desalination technology with lower energy consumption. Previously, our group proposed an innovative hydrate based desalination utilizing LNG cold energy (ColdEn-HyDesal) which reported a low specific energy consumption of 0.84 kWh/m3. In this work, we further evaluate the economic feasibility of ColdEn-HyDesal in Singapore. With a regasification rate of 200 t/h, the ColdEn-HyDesal facility simulated in this study produces 260 m3 water per hour. Through a comprehensive evaluation of the capital and operating costs, we observed significant reduction in the levelized cost of water (LCOW) from $9.31/m3 to $1.11/m3 with cold energy integration. The effect of water recovery rate, plant capacity and geological locations were analysed. Finally, the costs of water via the ColdEn-HyDesal process at various scales were benchmarked with matured desalination technologies, revealing that ColdEn-HyDesal technology is economically favourable at larger scale.

Abstract The utilization of unconventional natural gas is still a great challenge for China due to its distribution locations and small reserves. Thus, liquefying the unconventional natural gas by using small-scale LNG plant in skid-mount packages is a good choice with great economic benefits. A novel conceptual design of parallel nitrogen expansion liquefaction process for small-scale plant in skid-mount packages has been proposed. It first designs a process configuration. Then, thermodynamic analysis of the process is conducted. Next, an optimization model with genetic algorithm method is developed to optimize the process. Finally, the flexibilities of the process are tested by two different feed gases. In conclusion, the proposed parallel nitrogen expansion liquefaction process can be used in small-scale LNG plant in skid-mount packages with high exergy efficiency and great economic benefits.

Abstract A novel process for small-scale pipeline natural gas liquefaction is designed and presented. The novel process can utilize the pressure exergy of the pipeline to liquefy a part of natural gas without any energy consumption. The thermodynamic analysis including mass, energy balance and exergy analysis are adopted in this paper. The liquefaction rate and exergy utilization rate are chosen as the objective functions. Several key parameters are optimized to approach the maximum liquefaction rate and exergy utilization rate. The optimization results showed that the maximum liquefaction rate is 12.61% and the maximum exergy utilization rate is 0.1961. What is more, the economic performances of the process are also discussed and compared by using the maximum liquefaction rate and exergy utilization rate as indexes. In conclusion, the novel process is suitable for pressure exergy utilization due to its simplicity, zero energy consumption and short payback period.

Abstract Typical liquefaction processes are considered to be energy and cost-intensive. The dual mixed refrigerant (DMR) process (with two independent refrigeration cycles for cooling and subcooling) produces liquefied natural gas (LNG) at relatively high energy efficiency. However, it exhibits a high degree of configurational complexity and high sensitivity to operational conditions, and it also incurs a large capital investment. These factors eventually reduce the overall competitiveness of the liquefaction process, particularly for offshore applications. To address these issues, an energy- and cost-efficient dual-effect single mixed refrigerant (DSMR) process is proposed herein, and it employs a single loop refrigeration cycle to generate the dual cooling and subcooling effect, separately. The DMR process and the proposed DSMR process are simulated (with same design parameters) using well-known commercial simulator Aspen Hysys v10. Then, both processes are optimized using modified coordinate descent algorithm. The specific energy consumption of DSMR is 0.284 kWh/kg-NG, which is equivalent to an energy saving of 22.89% when compared to the conventional DMR process. The exergy efficiency of DSMR is 36.62%, which is 29.67% higher than that of the classical DMR process. Furthermore, the economic feasibility of the proposed DSMR process is evaluated in terms of its total annualized cost, which is 18.52% lower than that of the DMR process. Thus, the proposed DSMR process offers remarkable energy and exergy efficiencies with minimal capital investment. Therefore, DSMR could replace the classical DMR process, as well as other complex mixed refrigerant-based liquefaction processes.

Abstract The selection of liquefaction process is of great significance for distributed-scale LNG (liquefied natural gas) plant. This paper proposes four configuration strategies of expansion liquefaction cycle for distributed-scale LNG plant, namely multistage expanders, single precooling cycle, regeneration and mixture working fluid. FOM (figure of merit) is applied to evaluate the liquefaction cycles for distributed-scale LNG plant. Sixteen feasible liquefaction cycles are configured based on the configuration strategies and then optimized by genetic algorithm to maximum FOM for optimal synthesis. The cost analysis and exergy analysis of system are investigated. The optimized liquefaction process (Case 8) has two cycles, namely R410A precooling cycle and parallel nitrogen expansion cycle. The results show that the FOM of the optimized liquefaction cycle is 0.566 for distributed-scale LNG plant.