• Energy Research

Municipal solid waste (MSW) pyrolysis and gasification are in development, stimulated by a more sustainable waste-to-energy (WtE) option. Since comprehensive comparisons of the existing WtE technologies are fairly rare, this study aims to conduct a life cycle assessment (LCA) using two sets of data: theoretical analysis, and case studies of large-scale commercial plants. Seven systems involving thermal conversion (pyrolysis, gasification, incineration) and energy utilization (steam cycle, gas turbine/combined cycle, internal combustion engine) are modeled. Theoretical analysis results show that pyrolysis and gasification, in particular coupled with a gas turbine/combined cycle, have the potential to lessen the environmental loadings. The benefits derive from an improved energy efficiency leading to less fossil-based energy consumption, and the reduced process emissions by syngas combustion. Comparison among the four operating plants (incineration, pyrolysis, gasification, gasification-melting) confirms a preferable performance of the gasification plant attributed to syngas cleaning. The modern incineration is superior over pyrolysis and gasification-melting at present, due to the effectiveness of modern flue gas cleaning, use of combined heat and power (CHP) cycle, and ash recycling. The sensitivity analysis highlights a crucial role of the plant efficiency and pyrolysis char land utilization. The study indicates that the heterogeneity of MSW and syngas purification technologies are the most relevant impediments for the current pyrolysis/gasification-based WtE. Potential development should incorporate into all process aspects to boost the energy efficiency, improve incoming waste quality, and achieve efficient residues management.

Abstract The recent development of potable power sources is of considerable interest. A mesoscale combustor-powered thermoelectric generator integrated with a mesoscale stagnation-point reverse-flow combustor and a built-in heat exchanger was designed and optimized in this work to augment electric power and overall efficiency. The mesoscale combustor can run at a volumetric heat load of 72 MW/m3. The thickness of heat spreader, input power, equivalent ratio, fuel types, and heat exchanger channel types were explored to optimize the system performance. The optimized electric power of 30.7 W obtained at an overall efficiency of 3.21% was larger than the previous record of 18.1 W at an overall efficiency of 3.01% in 2015. The proposed thermoelectric generator, which was conducted with 13.2 kg of methane, reached an energy density of 300 Wh/kg. The novel design provided a concrete way to solve the contradiction between the availability of Bi2Te3-based thermoelectric module and its relatively low working temperature. Additionally, a new metrics, which is defined as (EFS/efTE,max) at Pin, was firstly proposed to evaluate various thermoelectric generators, in which different thermoelectric materials, combustion types, and input powers could be involved. The proposed metrics correlates the overall, running thermoelectric, maximum thermoelectric, combustion (reaction), and heat collection efficiencies under different input powers. The power of this metrics was comprehensively discussed by deducing the running thermoelectric efficiencies of various previous works and presenting the performance comparisons of different previous thermoelectric generators. The obtained metrics, that is, (75.9%/4.23%) at 957 W in this work was higher than those in previous studies.

Abstract The enhanced geothermal system (EGS) has been regarded as a promising means to exploit the abundant and low-carbon hot dry rock geothermal resources. In this work, two-stage EGSs with/without one-stage recuperative cycle for power generation based on organic Rankine cycle (ORC) were proposed, and their thermo-economic performance was evaluated, comparing to the one-stage EGS-ORC. The recuperative cycle improved the thermal performance of two-stage EGS and it had the highest thermal efficiency of 16.48%. Intermediate pressure and steam extraction ratio were important parameters that impacted the thermal performance of the two-stage EGS. The higher thermal efficiency was achieved at the intermediate pressure of 0.22 MPa and steam extraction ratio of 0.1. The two-stage EGS with one-stage recuperative cycle also had the lowest levelized cost of energy (LCOE) of $0.1895 kWh−1. According to the sensitivity analysis, the temperature of hot water and prices of electricity had the greatest impact on LCOE. The LCOE could be as low as $0.1704 kWh−1 if the temperature of hot water was 165.33 C. On the other hand, if the temperature of hot water was 149.58°C, the LCOE would be increased to $0.2179 kWh−1. The results from this study could provide the possible strategies to improve the overall thermo-economic performance of EGS-ORC.

Abstract Geothermal resources are well-recognized as a clean and low-carbon emission energy resource for power generation and heat supply. However, CO2 emissions occur during the construction, operation, and decommission stages of geothermal power plants. In this work, the life cycle CO2 emission characteristics of a geothermal power plant based on the organic Rankine cycle are systematically evaluated. The effect of the organic working medium and the recuperative cycle on CO2 emissions of the whole system are analyzed. Sensitivity analysis is conducted by varying the parameters of output power and the sources of the substituted electricity. Results reveal that a considerable amount of CO2 would be released due to the construction of geothermal wells and plants; however, the production of electricity could offset a much larger amount of CO2 emission. The net CO2 emission of the considered geothermal system during its lifespan reaches approximately −300 thousand tCO2e. In addition, the total amount of CO2 emission reduction relies heavily on the output power and the substituted electricity sources.

Abstract The clean and efficient energy production from municipal solid waste (MSW) is highly desirable due to increasing energy demand and environmental concerns. In this study, four incineration- (S1) and gasification-based (S2 with combustion boiler, S3 with gas turbine/combined cycle and S4 with internal combustion engine) MSW treatments were compared using methods of environmental life cycle assessment (LCA) and exergetic life cycle assessment (ELCA). LCA was applied to measure the environmental performances and ELCA was supplemented to reflect the thermodynamics efficiencies. Afterwards, cumulative degree of perfection (CDP) and abatement exergy (AbatCExC) efficiency of the considered systems were also calculated to determine the imperfection and environmental sustainability of the processes. Results showed that gasification-based systems were effective to mitigate the environmental impacts of acidification, nutrient enrichment, and photochemical ozone formation potential, but caused higher global warming impacts. The S3 system exhibited the best performance from both environmental and exergetic perspective, due to its high net efficiency of electricity generation and low exhaust emission into air. Results from ELCA indicated that the rest two gasification-based systems (S2 and S4) were inefficient as compared to MSW direct incineration, mainly due to auxiliary energy consumption for MSW pretreatment and more conversion steps. The CDP and AbatCExC of the systems in descending order was expressed as S3 system > S1 system > S2 system > S4 system.

Pyrolysis/gasification-based waste-to-energy (WtE) techniques, comprising partial oxidation of waste and subsequent syngas combustion, show potential benefits over direct incineration. To facilitate their development under the specific conditions of China, pyrolysis and gasification of typical municipal solid waste (MSW) are investigated in a fluidized bed reactor. The effect of the equivalence ratio (ER), reaction temperature, and moisture content on MSW conversion is studied. A rising ER increases the syngas yield but decreases the syngas heating value. The combustible gas yield is strengthened at lower ERs and later drops when the ER exceeds 0.4 as a result of the continuously enhanced oxidation reactions. A higher temperature favors pyrolysis reactions but causes an evident decrease in the syngas heating value during gasification. When the ER is at 0.4 and the temperature is at 650 °C, an optimum operating performance is obtained under the specific input simulated MSW (S-MSW) and test conditions, with...

The production of clean and efficient energy from municipal solid waste (MSW) is extremely urgent matter due to an increasing energy demand and environmental concerns. In this study, a high steam parameter (520 °C, 7.9 MPa) circulating fluidized bed (CFB) MSW incineration system, equipped with a mechanical, biological treatment and external heat exchanger systems, was introduced and a comparative study with a typical mechanical grate (450 °C, 5.3 MPa) incineration system and conventional CFB (485 °C, 5.3 MPa) incineration system was carried out from a life-cycle, environmental and exergetic perspective which could assess different energy and material outputs based on real operating data. Moreover, the potential system optimization of this advanced CFB system was proposed. The results showed that the advanced CFB system was more environmentally friendly and resource-efficient than conventional MSW incineration systems. The recovery of material should be given priority over energy recovery. According to the assessment of the environment, and energy and material recovery, a process improvement with an incinerated refuse-derived fuel and a semi-compost produced by MBT as a soil conditioner was highly recommended.