Comprehensive reaction mechanism for n-butanol pyrolysis and combustion

A detailed reaction mechanism for n-butanol, consisting of 263 species and 3381 reactions, has been generated using the open-source software package, Reaction Mechanism Generator (RMG). The mechanism is tested against recently published data – jet-stirred reactor mole fraction profiles, opposed-flow diffusion flame mole fraction profiles, autoignition delay times, and doped methane diffusion flame mole fraction profiles – and newly acquired n-butanol pyrolysis experiments with very encouraging results. The chemistry of butanal is also validated against autoignition delay times obtained in shock tube experiments. A flux and sensitivity analysis for each simulated dataset is discussed and reveals important reactions where more accurate rate constant estimates were required. New rate constant expressions were computed using quantum chemistry and transition state theory calculations. Furthermore, in addition to comparing the proposed model with the eight datasets, the model is also compared with recently published n-butanol models for three of the datasets. Key differences between the proposed model and the published models are discussed.     

On the chemical kinetics of cyclopentadiene oxidation

The cyclopentadiene/cyclopentadienyl system forms a critical part in the oxidation chemistry of aromatic fuel components used in surrogate fuels and the importance of the cyclopentadienyl radical in poly-aromatic hydrocarbon (PAH) growth has also been noted due to its site dependent reactivity. The latter aspect has been subject to a number of studies along with the initial pyrolysis steps. By contrast, few studies have been performed of the corresponding oxidation chemistry under conditions of relevance to combustion applications. Thermochemical data for oxidation reactions featuring the cyclopentadienyl radical with O, OH, HO₂ and O₂ were determined at the G3B3 and G4/G4MP2 levels in combination with an analysis of internal rotations using density functional theory and with the Jahn–Teller effect treated as a pseudo-rotation. The calculated potential energy surfaces were subsequently used in a consistent manner for the determination of pressure dependent reaction rate parameters through the Rice–Ramsperger–Kassel–Marcus/master-equation approach with Eckart quantum tunnelling corrections applied to reactions involving hydrogen transfers. The accuracy of the method was further investigated by comparisons of computed rate parameters for pyrolysis reactions with alternative determinations. The resulting chemistry was incorporated into an evaluation framework for the study of cyclopentadiene oxidation using recent experimental flow reactor data and principal uncertainties in reaction pathways assessed.     

Kinetic and radiative extinctions of spherical burner-stabilized diffusion flames

Extinction of steady, spherical diffusion flames stabilized by a spherical porous burner was investigated by activation energy asymptotics. An optically-thin radiation model was employed to study the effect of radiation on flame extinction. Four model flames with the same adiabatic flame temperature and fuel consumption rate but different stoichiometric mixture fraction and flow direction, namely the flames with fuel issuing into air, diluted fuel issuing into oxygen, air issuing into fuel, and oxygen issuing into diluted fuel, were adopted to understand the relative importance of residence time and radiation intensity. Results show that for a specified flow rate emerging from the burner, only the kinetic extinction limit at low Damköhler numbers (low residence times) exists. In the presence of radiative heat loss, extinction is promoted so that it occurs at a larger Damköhler number. By keeping the radiation intensity constant while varying the flow rate, both the kinetic and radiative extinction limits, representing the smallest and largest flow rates, between which steady burning is possible, are exhibited. For flames with low radiation intensity, extinction is primarily dominated by residence time such that the high-flow rate flames are easier to be extinguished. The opposite is found for flames suffering strong radiative heat loss. The kinetic extinction limit might occur at mass flow rates lower than what is needed to keep the flame outside of the burner and not observable. An extinction state on the radiative extinction branch can be either kinetic or radiative depending on the process.     

Numerical simulation of the upward propagation of a flame in a vertical tube filled with a very lean mixture

Upward propagation of a premixed flame in a vertical tube filled with a very lean mixture is simulated numerically using a single irreversible Arrhenius reaction model with infinitely high activation energy. In the absence of heat losses and preferential diffusion effects, a curved flame with stationary shape and velocity close to those of an open bubble ascending in the same tube is found for values of the fuel mass fraction above a certain minimum that increases with the radius of the tube, while the numerical computations cease to converge to a stationary solution below this minimum mass fraction. The vortical flow of the gas behind the flame and in its transport region is described for tubes of different radii. It is argued that this flow may become unstable when the fuel mass fraction is decreased, and that this instability, together with the flame stretch due to the strong curvature of the flame tip in narrow tubes, may be responsible for the minimum fuel mass fraction. Radiation losses and a Lewis number of the fuel slightly above unity decrease the final combustion temperature at the flame tip and increase the minimum fuel mass fraction, while a Lewis number slightly below unity has the opposite effect.     

Experimental and numerical study of the accuracy of flame-speed measurements for methane/air combustion in a slot burner

Measuring the velocities of premixed laminar flames with precision remains a controversial issue in the combustion community. This paper studies the accuracy of such measurements in two-dimensional slot burners and shows that while methane/air flame speeds can be measured with reasonable accuracy, the method may lack precision for other mixtures such as hydrogen/air. Curvature at the flame tip, strain on the flame sides and local quenching at the flame base can modify local flame speeds and require corrections which are studied using two-dimensional DNS. Numerical simulations also provide stretch, displacement and consumption flame speeds along the flame front. For methane/air flames, DNS show that the local stretch remains small so that the local consumption speed is very close to the unstretched premixed flame speed. The only correction needed to correctly predict flame speeds in this case is due to the finite aspect ratio of the slot used to inject the premixed gases which induces a flow acceleration in the measurement region (this correction can be evaluated from velocity measurement in the slot section or from an analytical solution). The method is applied to methane/air flames with and without water addition and results are compared to experimental data found in the literature. The paper then discusses the limitations of the slot-burner method to measure flame speeds for other mixtures and shows that it is not well adapted to mixtures with a Lewis number far from unity, such as hydrogen/air flames.     

LES modeling of premixed combustion using a thickened flame approach coupled with FGM tabulated chemistry

Flamelet Generated Manifolds (FGM) tabulated chemistry is used in combination with a thickened flame approach to perform Large Eddy Simulation (LES) of premixed combustion. Two-dimensional manifolds are used to describe the chemistry by the mixture fraction and progress variable. Simulations of one-dimensional flames have been used to verify the coupling of the tabulated chemistry and the LES solver where important features like the grid dependence of flame propagation are carefully addressed. Finally, the method is applied to the turbulent flame of a premixed swirl burner including the complex geometry of the swirl nozzle. Results of the velocity, species and temperature are compared with experimental data. Thereby different efficiency functions are used to show the sensitivity related to this model parameter. Some aspects regarding dynamic thickening, numerical accuracy and computational efficiency are also addressed.     

Flame lift-off length and soot production of oxygenated fuels in relation with ignition delay in a DI heavy-duty diesel engine

The relation between ignition delay and flame lift-off length (based on OH* chemiluminescence) has been studied in an optically accessible heavy-duty diesel engine. Soot production has been studied as well, using both exhaust soot data and high-speed imaging of the in-cylinder natural luminosity. Since the luminosity is estimated to scale with T¹³ for our experimental conditions, the local temperature becomes a decisive factor in the interpretation of natural luminosity images. The fuels used include regular diesel and blends of various oxygenates (cyclohexanone, anisole, and dibutyl maleate) with synthetic diesel, to cover a wide range of cetane numbers and oxygen content.Based on a literature review, we hypothesize that the flame speed only determines an upper limit for the flame lift-off length (rather than its exact position). Under conditions that promote auto-ignition, the lift-off length may be shorter, and is likely governed by ignition chemistry.For the oxygenated fuels, the flame lift-off length is found to increase with ignition delay. An inverse correlation is found between the soot luminosity and the oxygen ratio at the lift-off length. The anisole-blend does not produce measurable amounts of exhaust soot as long as the injection duration is shorter than the ignition delay. Under cooled EGR-like conditions, the ignition delay is so large that anisole is considered a candidate fuel for soot-free operation of heavy-duty diesel engines up to medium load.     

A transport-flux-based directed relation graph method for the spatially inhomogeneous instantaneous reduction of chemical kinetic mechanisms

A novel, flux-based, directed relation graph (DRG) method is proposed for adaptive and automatic generation of skeletal chemical mechanisms. Unlike many previous approaches for static mechanism reduction, the proposed method does not rely on the complete solution of a set of test problems using the detailed mechanism, but rather it simplifies the model “on the fly,” analyzing the local reaction states with an extended directed relation graph. Compared to the original DRG method of Lu and Law [T. Lu, C.K. Law, Proc. Combust. Inst. 30 (2005)], the flux-based DRG explicitly considers the effect of transport fluxes, as they provide a way to quantify the coupling of the governing equations among adjacent grid cells. The resulting numerical scheme operates on a cell-by-cell basis, so that different chemical submodels are applied in different regions of the flame. This approach leads to very large reduction ratios, in which most of the flame structure is resolved using about 30% of the chemical species employed in the full mechanism, while a more detailed model that considers roughly 70–80% of the total species is used only at ignition. The flux-based DRG method has been employed in the two-dimensional simulation of axisymmetric coflow flames for a variety of fuels and chemical kinetic mechanisms (three steady flames and one oscillating flame), with a speedup factor of about 20 obtained for the largest problem — a JP-8 flame.     

A methodology for calculating the levelized cost of electricity in nuclear power systems with fuel recycling

In this paper we show how the traditional definition of the levelized cost of electricity (LCOE) can be extended to alternative nuclear fuel cycles in which elements of the fuel are recycled. In particular, we define the LCOE for a cycle with full actinide recycling in fast reactors in which elements of the fuel are reused an indefinite number of times. To our knowledge, ours is the first LCOE formula for this cycle. Others have approached the task of evaluating this cycle using an ‘equilibrium cost’ concept that is different from a levelized cost. We also show how the LCOE implies a unique price for the recycled elements. This price reflects the ultimate cost of waste disposal postponed through the recycling, as well as other costs in the cycle. We demonstrate the methodology by estimating the LCOE for three classic nuclear fuel cycles: (i) the traditional Once-Through Cycle, (ii) a Twice-Through Cycle, and (iii) a Fast Reactor Recycle. Given our chosen input parameters, we show that the ‘equilibrium cost’ is typically larger than the levelized cost, and we explain why.     

Residential energy consumption across different population groups: comparative analysis for Latino and non-Latino households in USA

Residential energy cost is an important part of the household budget and could vary significantly across different population groups in many countries. In the United States, many studies have analyzed household fuel consumption by fuel type, including electricity, natural gas, fuel oil, and liquefied petroleum gas (LPG), and by geographic areas. Past research has also demonstrated significant variation in residential energy use across various population groups, including white, black, and Latino. However, our research shows that residential energy demand by fuel type for Latinos, the fastest growing population group, has not been explained by economic and non-economic factors in any statistical model in public domain. The purpose of this paper was to discuss energy demand and expenditure patterns for Latino and non-Latino households in hhe United States as a case example of analyzing residential energy consumption across different population groups in a country. The linear expenditure system model developed by Stone and Geary is the basis of the statistical model developed to explain fuel consumption and expenditures for Latino households. For comparison, the models are also developed for non-Latino, black, and non-black households. These models estimate energy consumption of and expenditures for electricity, natural gas, fuel oil, and LPG by various households at the national level. Significant variations in the patterns of these fuels consumption for Latinos and non-Latinos are highlighted. The model methodology and results of this research should be useful to energy policymakers in government and industry, researches, and academicians who are concerned with economic and energy issues related to various population groups in their country.     

Consumers' preference for renewable energy in the southwest USA

The southwestern part of the US has abundant supply of renewable energy resources but little is known about the consumers' preferences for renewable energy in this region. This paper investigates households' willingness to pay for a renewable energy program in a southwestern state, New Mexico (NM). Using the contingent valuation method, we provide different scenarios that include provision of 10% and 20% of renewable energy supply, to elicit households' willingness to pay (WTP) for the renewable energy. We estimate the WTP for specific shares of renewable energy in the total energy mix as it is a key factor in affecting the price of the energy portfolio in the market. The survey design also allows us to check the scope sensitivity of renewable energy which can help guide the future renewable energy policy. We hope results from this study will offer useful insights to energy regulators and utility companies and help them increase the share of renewable energy supply.     

Indices for comparing greenhouse gas emissions: integrating science and economics

Abatement of greenhouse gases is a key element of possible policy responses to global warming. Comprehensive abatement strategies view the greenhouse abatement issue as one involving multiple gases and not CO₂ alone. These strategies require the formulation of greenhouse gas indices that will allow for an evaluation of the trade offs involved. This paper uses an optimal control methodology to determine greenhouse gas indices. Analytical expressions for the indices are determined using cost-benefit and cost-effectiveness framings. An integrated framework that includes the relevant scientific and economic processes is used to evaluate indices for methane, nitrous oxide and HCFC-22. Additionally, the optimal control problem for a two gas abatement strategy involving methane and carbon dioxide is numerically solved. In the case of methane, indices were calculated for direct as well as indirect radiative forcing effects. In addition to the numerical results, the analysis allows us to make several qualitative observations regarding the dependence of trace gas indices on the scientific and economic uncertainties involved in climate change policy.     

Stock-Watson dynamic OLS (DOLS) and error-correction modelling approaches to estimating long- and short-run elasticities in a demand function: new evidence and methodological implications from an application to the demand for coal in mainland China

In this paper robust elasticity estimates of coal demand for China are derived using annual data 1953–1992. In so doing, we illustrate the use of a powerful yet practically convenient and recently developed modelling procedure devised by Stock and Watson (known as Dynamic OLS (DOLS)), who provide evidence, based on Monte Carlo simulations, of this estimator being superior in small samples compared to a number of alternative estimators, as well as being able not only to accommodate higher orders of integration but also to account for possible simultaneity within regressors of a potential demand system. Furthermore, cointegration and error-correction methods are employed to derive short-run price and income elasticities. Estimated results are quite robust not only in terms of statistical prowess but also in terms of economic intuition and indicate that, over the long run, both price and income elasticities are close to unity. While short-run price and income elasticities are less (in absolute value) than their long-run counterparts, there seems to be some divergence in short-run parameters from a subsample analysis. Overall, results seem to imply that for China, coal consumption should remain relatively constant as future modernization strategies for economic development are pursued. In addition, the study has clear methodological implications for estimating the long- and short-run elasticities in a demand function in general, and in a wide variety of fields in future applied research.     

Realized volatility and the influence of market measures on predictability: Analysis of Nord Pool forward electricity data

This is the first paper to utilize intra-daily high-frequency data and to apply known market measures for the prediction of volatility in the Nord Pool electricity forward market. The work is based on recent methods of separating realized volatility into two components: continuous and jump volatilities. In addition, the link between future price volatility and current observable economic variables is examined. The measures—trading volume, time-to-maturity, asymmetric effect from negative shocks, and intra-week seasonality—are assessed to identify improvements in day-ahead predictions. The model where the total variation is separated into its continuous and jump components is compared with the simpler heterogeneous autoregressive model of realized variation both in- and out-of-sample. The results show a strong degree of persistence in realized volatility, and significant impacts from the mentioned market measures when predicting Nord Pool forward price volatility. Hence, there is a clear preference for models accounting for the systematic impact of market measures to improve volatility assessment for tomorrow. Moreover, separating the total variation into continuous and jump components seems potentially useful when predicting day-ahead volatility.     

Trade and energy consumption in the Middle East

Over the past 30 years many economies have experienced large increases in economic trade, income and energy consumption. This brings up an interesting question. How do increases in trade affect energy consumption? This study uses panel cointegration data estimation techniques to examine the impact of trade on energy consumption in a sample of 8 Middle Eastern countries covering the period 1980 to 2007. Short-run dynamics show Granger causality from exports to energy consumption, and a bi-directional feedback relationship between imports and energy consumption. Long run elasticities estimated from FMOLS show that a 1% increase in per capita exports increases per capita energy consumption by 0.11% while a one percent increase in per capita imports increases per capita energy consumption by 0.04%. These results are important in establishing that increased trade affects energy demand in the Middle East in both the short and long-run. This has implications for energy policy and environmental policy.     

Outage costs in Chilean industry

This paper estimates the cost of energy shortages to Chilean industry. The information was obtained from a sample survey carried out shortly after the 1989 electricity restriction. The results obtained show that a 10%, one-month, equiproportional restriction involves an average net outage cost of US¢7.7 (kW h)⁻¹. In the same scenario, the average outage cost drops to US¢3.2 (kW h)⁻¹ when a limited, selective restriction is applied. We show that, under current legislation, the outage cost in fact lies between the costs of an equiproportional and a selective restriction. However, subtle modifications to legislation would make it possible to allocate the scarce supply efficiently.     

An hour-ahead prediction model for heavy-tailed spot prices

We propose an hour-ahead prediction model for electricity prices that capture the heavy tailed behavior that we observe in the hourly spot market in the Ercot (Texas) and the PJM West hub grids. We present a model according to which we separate the price process into a thin-tailed trailing median process and a heavy-tailed residual process whose probability distribution can be approximated by a Cauchy distribution. We show empirical evidence that supports our model.     

Oil prices and the impact of the financial crisis of 2007–2009

Oil prices increased dramatically during 2004–2006. Industry experts initially attributed these price increases to fundamental factors such as the rise in global demand, but also because of disruptions in the supply of oil. The price increases however were so substantial that additional factors are needed to explain such dramatic changes. We propose that the decline in the value of the U.S. dollar measured both by the appreciation of the Euro and of gold prices, played an important role as oil suppliers demanded compensation for the declining value of the dollar. Using a Markov switching regime methodology we find evidence that this hypothesis is true prior to the financial crisis, but its validity does not hold after the crisis when oil prices crashed and the dollar rallied.     

Effect of pharmaceutical agent degradation on axonal transport drug delivery: An analytical solution for a transient situation

This paper is motivated by recent experimental research that demonstrated pharmacological efficiency of axonal transport drug delivery. The purpose is to develop a model of this process and to study how the rate of destruction of pharmaceutical agent complexes (PACs) affects their transport in the axon. The model includes two populations of PACs: PACs in the state when they are driven retrogradely (from the axon terminal toward the neuron soma) by dynein motors and PACs residing in the accumulated state (but can still be re-released to the dynein-driven state). The coupling between the kinetic states is accounted for by first-order reactions. Utilizing Laplace transform, analytical solutions for concentrations of these two populations of PACs are obtained. The effect of PAC destruction is investigated for different values of other parameters. It is shown that the shapes of the waves describing the PAC concentrations can be significantly affected by transport parameters.