Terese Løvås

Comparison of automatic reduction procedures for ignition chemistry

In this paper, we present a comparison between the reduced mechanisms obtained through a computational singular perturbation method (CSP) and the reduced mechanisms obtained through a lifetime analysis based only on the diagonal elements of the Jacobian matrix and a species sensitivity The two methods are used for the analysis of autoignition, which is an interesting test situation because of the sensitivity of ignition to the radical pool and the smaller range of timescales expected. It is found that the steady-state species selected by the two methods are in good agreement. The mechanisms are reduced to a 10-step mechanism when CSP is applied and an 11-step mechanism in the case of the simpler lifetime analysis. Both mechanisms are compared with the detailed mechanism and experimental data and are found to reproduce the physical and chemical parameters very well. This shows that for a large part of the timescale range, the system is close to linear. The comparison shows the advantage of the CSP method as being somewhat more accurate. However, the simpler lifetime analysis is of sufficient accuracy and of more convenience when applied to a system requiring a considerable reduction in computational time, as is the case when applying online reduction. (Less)

Development of adaptive kinetics for application in combustion systems

In this paper, an automatic method for reducing chemical mechanisms during run time based on the quasi-steady-state assumption (ASSA) is presented. The method uses a lifetime analysis of the chemical species which can be set to steady state according to a ranking procedure. Steady-state species concentrations are computed by algebraic rather than differential equations, thus yielding a significant reduction in the computational effort. In contrast to previous reduction schemes in which chemical species were selected only when they were in steady state throughout the whole process, the present method allows for species to be selected at each operating point separately generating an adaptive chemical kinetics scheme. The mechanism can change during the simulation run. This ensures that the optimal reduced mechanism is used at each time step leading to a very efficient and accurate procedure. The method is used for calculations of a natural gas fueled engine operating under homogeneous charge compression ignition (hCCI) conditions. We discuss criteria for selecting steady-state species and the influence of these criteria on the results, such as concentration profiles and temperature. A full mechanism with 53 species can be reduced to a minimun of 14 non-steady-state species while still reproducing the physical behavior of the detailed mechanism with good agreement.

Global Reaction Mechanism for Ethylene Flames with Preferential Diffusion

Two compact global mechanisms for ethylene (C2H4) diffusion flame combustion have been tailored to include important reaction steps for acetylene and benzene production. One mechanism (G11) contains 11 species with 10 reaction steps including acetylene (C2H2), and the other mechanism (G12) contains 12 species with a total of 11 reactions steps to include also the formation of benzene (C6H6). The reaction steps have been carefully selected to minimize the mechanism size for the use in large-scale computational fluid dynamics (CFD) simulations. Hence, the reaction constants have been optimized for the correct prediction of important radical concentrations. Particular focus has been on the mechanisms ability to reproduce important preferential diffusion effects and on the formation of H and C2H2 due to its importance to soot formation. The two global chemical models have been validated for a transient 1-dimensional diffusion flame configuration and show very good agreement with various detailed chemical schemes. The mechanisms are found to be nonstiff reducing typical computing time for a transient flamelet calculation (F. Mauss, 1998) from a few hours (171 species mechanism) to only a few minutes (G11).

The effect of turbulent clustering on particle reactivity

Abstract The effect of turbulence on the heterogeneous (solid–fluid) reactions of solid particles is studied numerically with Direct Numerical Simulations (DNS). A simplified reaction system is used, where the solid–fluid reaction is represented by a single isothermal reaction step. It is found that, due to the clustering of particles by the isotropic turbulence, the overall reaction rate is entirely controlled by the turbulence for large Damkohler numbers. The particle clustering significantly slows down the reaction rate for increasing Damkohler numbers which reaches an asymptotic limit that can be analytically derived. This implies that the effect of turbulence on heterogeneously reacting particles should be included in models that are used in CFD simulations of e.g. char burnout in combustors or gasifiers. Such a model, based on the chemical and turbulent time scales, is here proposed for the heterogeneous reaction rate in the presence of turbulence.

Combustion Properties of Norwegian Biomass: Wood Chips and Forest Residues

Flue gas emissions and particle size distribution were investigated during combustion experiments of wood, forest residue and mixtures of these two. The combustion experiments were carried out in a grate fired multi-fuel reactor with and without air staging at stable operation conditions and constant temperature of 850 °C. The overall excess air ratio was held at 1.6, and the primary excess air ratio was 0.8 during air staged experiments. NOx emissions are reduced by air staging. Fly ash particle concentration of forest residues in the flue gas is lower than wood. Aerosols number increased in the staged experiments for fuel blends.

Hydrochar slurry fuels and high-grade activated carbon for electricity production and storage Conceptual process design and analysis

This paper describes and analyzes a conceptual design of a bioenergy system for sustainable electricity production from low-grade biomass resources such as forest and agricultural residues, which is suitable for rural areas in developing regions susceptible to intermittent electricity supply. In order to make it a closed-loop system, the paper also identifies a bio-refining strategy focusing on production of high-grade activated carbons for energy storage using supercapacitor.

Thermodynamic Modeling of Allothermal Steam Gasification in a Downdraft Fixed-bed Gasifier

A process of converting a solid carbonaceous fuel into a gaseous energy carrier in presence of a gasifying medium at high temperature is called gasification. The resulting gaseous energy carrier, known as producer gas, is more versatile in its use than the original solid fuel. Gasification is widely considered as a more efficient and less polluting initial thermochemical upstream process of converting biomass to electricity. The objective of this study was to investigate the process of allothermal steam gasification in a fixed-bed downdraft gasifier for improved quality (HHV, high hydrogen content) of the producer gas generated. The study involved thermodynamic equilibrium modeling based on equilibrium approach in which the concentrations of the gaseous components in the producer gas at equilibrium temperature are determined based on balancing the moles in the overall gasification equation. The results obtained suggest that the maximum equilibrium yield of producer gas with high energy density is attained at a gasification temperature of around 820°C and a steam/biomass ratio of 0.825 mol/mol. The equilibrium yield was richer in hydrogen at 52.23%vol, and with a higher heating value of 11.6 MJ/Nm3. Preliminary validation of the model results using experimental data from literature shows a close relationship. The study has further shown the advantage of using steam as a gasifying medium towards the improved quality of the producer gas generated.

Evaluation of Test Bench Engine Performance Measurements in Relation to Vehicle Measurements on Chassis Dynamometer

A 1600 cc direct injected turbocharged Euro 5 diesel engine was operated on standard diesel fuel from a gas station in Denmark for evaluation of the test bench procedure. The NEDC (New European Driving Cycle), FTP-75 (Federal Test Procedure) and WLTP (World Harmonized Light Vehicle Test Procedure) driving cycles were simulated in the engine test bench in two ways: 1) by transient engine operation were the inertia of the vehicle during deceleration was simulated by addition of power from an electric motor mounted on the crank shaft, and 2) by steady state measurements where the total driving pattern was simulated from an integration of multiple steady state measurements. The mathematical model that calculates equivalent NEDC driving cycle vehicle emissions from the engine steady state measurements in the test bench, starting with warm engine, is presented. By applying this model any driving cycle emissions can be calculated from the presented tabulated steady state measurements, starting with warm or cold engine.Both engine test methods showed acceptable agreement with measurement in an NEDC vehicle test on chassis dynamometer where the vehicle was equipped with a similar engine as the test bench engine. The two engine test bench methods gave very similar results, but the transient engine test procedure showed a little higher emission of CO2 and NOx, results that were closest to the vehicle measurements. This is interpreted as a result of extra emissions when the engine adjusts from one operating point to the next during transient operation. These extra emissions are not caught in the steady state method. Application of the two engine test procedures on the FTP-75 procedure and the newer WLTP showed that the steady state engine test method gave significantly lower emissions of NOx and a little lower CO2 emissions compared to the transient engine test. The results indicated that this was mainly an effect of the time delay on the engines EGR system adjustment, which is not caught in the steady state method.The advantages and disadvantages of applying the different measurement methods and test procedures are discussed in relation to introduction of new test procedures in order to reduce engine/vehicle emissions.Copyright

Comparison of Bio-Fischer-Tropsch Fuel and Commercial Diesel Fuel Application in a 1600 CC Euro 5 Diesel Engine

A direct injected and turbocharged Euro 5 diesel engine has been set up in a test bench where the vehicle driving conditions of the European NEDC (New European Driving Cycle) test can be simulated. The engine is operated as the engine of a corresponding vehicle, equipped with a similar engine and driving through the NEDC cycle. The regulated gaseous emissions, carbon monoxide, hydrocarbons and nitrogen oxides, as well as particulate numbers and size distributions where measured in 5 selected steady state operating points during the engine test. Fuel consumptions and carbon dioxide emissions where measured as well. The steady state operating conditions were chosen within the engine operating range during a vehicle NEDC test and representing as broad an operating range as possible during the NEDC test. A method is presented in which the NEDC test emissions are calculated from the 5 steady state measurements. It is shown that the method gives emission results that agree well with values that can be expected from the vehicle in question during an NEDC test. In this way a limited number of steady state measurements can be used to simulate vehicle emissions. The reason for carrying out engine experiments instead of vehicle measurements was to obtain well controlled engine conditions and thus better insight in the operation of the engine in the individual phases of operation, and thereby enable evaluation of the possibilities for improving engine performance with respect to emission and fuel consumption reduction.Two different fuels where tested. These were a Fischer-Tropsch fuel, produced from biomass at the Gussing gasification plant in Austria and a commercial diesel from a fuel station in Denmark. The results of the measurements and engine modification considerations showed that bio Fischer-Tropsch fuel does have advantages with respect to particulate and also small advantages with carbon monoxide and carbon dioxide emissions. However, NOx emissions are rather a question of the injection timing of the fuel, and the NOx emissions can be adjusted to give the same level of emissions by changing the injection timing with ordinary diesel. The injection strategy was changed in order to attempt to reduce NOx emissions below the limits in the Euro 6 regulations.Copyright

Towards dynamically reduced mechanisms based on domain splitting

We present a method for developing dynamically reduced chemical mechanisms based on the quasi steady state assumption and a domain splitting procedure. This procedure carries out the reduction within each domain separately according to a steady state selection parameter valid for the corresponding domain. The domains are defined according to a clustering algorithm. The method is applied to calculations of ethene diffusion flamelets.