Natural gas internal combustion engines release over half of the fuel's energy as waste heat and emit pollution that harms human health and accelerates climate change. Enriching natural gas with hydrogen has been shown to mitigate these impacts by reducing emissions and increasing engine efficiency. Thermal energy in the exhaust gas from natural gas engines can be used to drive chemical reactions to reform a biomass-derived feedstock into a hydrogen-rich gas. This gas can be blended with the primary fuel to enhance combustion and displace some of the natural gas demand. Two types of chemical reformation processes, aqueous-phase reformation (APR) and vapor-phase reformation (VPR), have been identified which can convert a biomass-derived sugar feedstock solution into a hydrogen-rich gas by recovering waste heat from engine exhaust gas. VPR operates at higher temperatures than APR, which limits the amount of heat that can be transferred from the exhaust gas to the reaction temperature. This study used a thermodynamic pinch analysis to compare the performance of these two processes based on their respective process heat demands and the thermal energy available from engine exhaust gas to determine how many moles of feedstock can be reformed. The calculations were performed using specifications for eight natural gas engines with reactor conditions from fourteen APR and ten VPR experiments, using glycerol as a model compound.
Strict emissions legislation and energy security debates have spurred extensive research in alternative fuels and renewable energies. Literature research has shown the need for improvements in internal combustion engines (ICE) due to their low efficiencies. Significant gains in efficiency can be accomplished with the use of waste heat recovery (WHR) techniques. Organic rankine cycles (ORC) with turbocompounding harness the waste heat from an ICE to improve efficiency and fuel economy while reducing brake-specific emissions. A mathematical model was developed to explore the potential gains in 1st and 2nd law efficiencies. The model approaches the evaluations of the ORC from a practical and a theoretical method. The practical method in evaluation 1 limits the outlet exhaust gas temperatures from the evaporator to prevent the formation of condensation. The performance of the ORC is then evaluated and compared to the evaluation 2. In the theoretical method, in evaluation 2, the effect of pinch point on the evaporator and the entire cycle was analyzed. This analysis was conducted for R113, a dry fluid, and propane, a wet fluid, in order to analyze the differences in the two types of fluids. R113 showed a 13% -- 22% and a 6% -- 14.7% increase in 1st and 2nd law efficiencies, respectively. Propane showed a 9% -- 17.4% and a 2% -- 8.5% increase in 1st and 2nd law efficiencies, respectively. It was also shown that as the pinch point temperature decreases the 2nd law efficiencies increased. It was concluded that use of ORC with turbocompounding is an effective method for waste heat recovery in order to increase ICE efficiency.
We present a detailed thermodynamic analysis of thermochemical recuperation (TCR) applied to an idealized internal combustion engine with single-stage work extraction. Results for several different fuels are included. For a stoichiometric mixture of methanol and air, TCR can increase the estimated ideal engine Second Law efficiency by about 3% for constant pressure reforming and over 5% for constant volume reforming. For ethanol and isooctane the estimated Second Law efficiency increases for constant volume reforming are 9% and 11%, respectively. The Second Law efficiency improvements from TCR result primarily from the higher intrinsic exergy of the reformed fuel and pressure boost associated with gas mole increase. Reduced combustion irreversibility may also yield benefits for future implementations of combined cycle work extraction.
The Organic Flash Cycle (OFC) is proposed as a vapor power cycle that could potentially increase power generation and improve the utilization efficiency of renewable energy and waste heat recovery systems. A brief review of current advanced vapor power cycles including the Organic Rankine Cycle (ORC), the zeotropic Rankine cycle, the Kalina cycle, the transcritical cycle, and the trilateral flash cycle is presented. The premise and motivation for the OFC concept is that essentially by improving temperature matching to the energy reservoir stream during heat addition to the power cycle, less irreversibilities are generated and more power can be produced from a given finite thermal energy reservoir. In this study, modern equations of state explicit in Helmholtz energy such as the BACKONE equations, multi-parameter Span-Wagner equations, and the equations compiled in NIST REFPROP 8.0 were used to accurately determine thermodynamic property data for the working fluids considered. Though these equations of state tend to be significantly more complex than cubic equations both in form and computational schemes, modern Helmholtz equations provide much higher accuracy in the high pressure regions, liquid regions, and two-phase regions and also can be extended to accurately describe complex polar fluids. Calculated values of saturated liquid and vapor density and vapor pressure were then compared to values listed in the NIST Chemistry WebBook to ensure accuracy for the temperature range of interest. Deviations from the NIST WebBook were typically below 1%; a comparison of first law efficiencies for an ideal basic Rankine cycle yielded less than 0.4% difference between calculations using the Helmholtz-explicit equations of state and NIST REFPROP. Also by employing the BACKONE and Span-Wagner equations, the number of potential aromatic hydrocarbon and siloxane working fluids that are appropriate for high and intermediate temperature applications is expanded considerably. A theoretical investigation on the OFC is conducted using the aforementioned Helmholtz-explicit equations of state for 10 different aromatic hydrocarbon and siloxane working fluids for intermediate temperature finite thermal energy reservoirs (3̃00oC). Results showed that aromatic hydrocarbons to be the better suited working fluid for the ORC and OFC due to less "drying" behavior and also smaller turbine volumetric flow ratios resulting in simpler turbine designs. The single flash OFC is shown to achieve utilization efficiencies that are comparable to the optimized basic ORC (0̃.63) which is used as a baseline. It is shown that the advantage of improved temperature matching during heat addition was effectively negated by irreversibilities introduced into the OFC during flash evaporation. Several improvements to the basic OFC are proposed and analyzed such as introducing a secondary flash stage or replacing the throttling valve with a two-phase expander. Utilization efficiency gains of about 10% over the optimized basic ORC can be achieved by splitting the expansion process in the OFC into two steps and recombining the liquid stream from flash evaporation prior to the secondary, low pressure, expansion stage. Results show that the proposed enhancements had a more pronounced effect for the OFC using aromatic hydrocarbon working fluids (5-20% utilization efficiency improvement) than for siloxane working fluids (2-4%). The proposed modifications were aimed towards reducing irreversibility in flash evaporation; it was observed for siloxanes that the primary source of irreversibility was due to high superheat at the turbine exhaust because of the highly "drying" nature of the fluid. Though an order of magnitude analysis, results showed that the OFC and ORC to require similar heat transfer surface areas. For low temperature thermal energy reservoirs (80-150oC) applicable to non-concentrated solar thermal, geothermal, and low grade industrial waste heat energy, alkane and refrigerant working fluids possess more appropriate vapor pressures. The optimized single flash OFC was again shown to generate comparable power per unit flow rate of the thermal energy reservoir than the optimized basic ORC. With some of the previously proposed design modifications though, the OFC can produce over 60% more power than the optimized ORC. For low temperature applications, the minimum temperature difference between streams in the heat exchanger, or pinch temperature, becomes an important design parameter. Reduction of the pinch temperature even slightly can yield significantly higher gains in power output, but will also increase required heat exchanger surface area and subsequently capital costs. A high-level design of a liquid-fluoride salt (NaF-NaBF4) cooled solar power tower plant is presented; liquid-fluoride salt is used rather than current molten nitrate salts to increase the receiver temperature and subsequently allow for higher efficiency gas power cycles to be used. Graphite or direct energy storage in the salt itself is proposed. The power block component of this heliostat-central receiver plant is a combined cycle system consisting of a topping Brayton cycle with intercooling, reheat, and regeneration and a bottoming low-temperature modified OFC. The combined cycle is designed with dry cooling in mind, such that operation in desert climates are more suitable. The combined cycle design is shown to increase power block efficiencies by 6%-8% over the Brayton cycle with intercooling, reheat, and regeneration alone. An estimated 30% annual average total solar-to-electric conversion efficiency is possible with this system design, which is comparable to some of the most efficient high temperature solar power tower designs to date. Theoretically, power block efficiencies over 60% are possible; however, emission losses from the isothermal central receiver would limit the plant's operational temperature range. Results show that for high efficiency solar power towers to be realized, high temperature non-isothermal, or partitioned, receivers operating efficiently above 1000oC are necessary. Other potential areas of renewable energy system integration for the OFC include a co-generation solar thermal-photovoltaic system that employs highly concentrated, densely packed photovoltaic cells using single-phase or two-phase cooling. The thermal energy absorbed by that coolant could then be used as the working fluid in a separate OFC to further produce power in co-generation with the concentrated photovoltaics.
Also in modern combustion engines, the maximal energetic efficiency is lower than 45%, which means that 55% of the supplied energy is lost and released to the environment. Automobile manufacturers and R&D partner suppliers are taking a special concern in investigating heat energy from exhaust gases, with the aim of recovering part of the heat by means of a Rankine Process and using it for energy cogeneration in the automobile. In the context of this Master Thesis, a new calculation software of the different thermodynamic states of the waste heat recovery system has to be programmed and used together with the rest of the available calculation tools. Another objective of this Master Thesis is the set up of valid simulation models for the different components of the heat recovery system based on measured testing data. This technical conditions and on the other hand the cycle requirements with the final objective of finding optimized states for each component. Finally, in order to reflect the behaviour of the system as a whole, the integration of the different simulated components of the waste heat recovery model would come in very useful. A complete simulation of the system would enable the estimation of optimization issues of the different parameters and operating points of the cycle.
This book is for chemical engineers, fuel technologists, agricultural engineers and chemists in the world-wide energy industry and in academic, research and government institutions. It provides a thorough review of, and entry to, the primary and review literature surrounding the subject. The authors are internationally recognised experts in their field and combine to provide both commercial relevance and academic rigour. Contributions are based on papers delivered to the Fifth International Conference sponsored by the IEA Bioenergy Agreement.
This book offers comprehensive coverage of the design, analysis, and operational aspects of biomass gasification, the key technology enabling the production of biofuels from all viable sources--some examples being sugar cane and switchgrass. This versatile resource not only explains the basic principles of energy conversion systems, but also provides valuable insight into the design of biomass gasifiers. The author provides many worked out design problems, step-by-step design procedures and real data on commercially operating systems. After fossil fuels, biomass is the most widely used fuel in the world. Biomass resources show a considerable potential in the long term if residues are properly handled and dedicated energy crops are grown. Includes step-by-step design procedures and case studies for Biomass GasificationProvides worked process flow diagrams for gasifier design. Covers integration with other technologies (e.g. gas turbine, engine, fuel cells)
This Intergovernmental Panel on Climate Change Special Report (IPCC-SRREN) assesses the potential role of renewable energy in the mitigation of climate change. It covers the six most important renewable energy sources - bioenergy, solar, geothermal, hydropower, ocean and wind energy - as well as their integration into present and future energy systems. It considers the environmental and social consequences associated with the deployment of these technologies, and presents strategies to overcome technical as well as non-technical obstacles to their application and diffusion. SRREN brings a broad spectrum of technology-specific experts together with scientists studying energy systems as a whole. Prepared following strict IPCC procedures, it presents an impartial assessment of the current state of knowledge: it is policy relevant but not policy prescriptive. SRREN is an invaluable assessment of the potential role of renewable energy for the mitigation of climate change for policymakers, the private sector, and academic researchers.
