A Scandinavian chemical wood pulp mill. Part 1. Energy audit aiming at efficiency measures

A Swedish wood-pulp mill is surveyed in terms of energy supply and use in order to determine the energy-saving potential. Conservation measures are of increasing interest to Swedish industry, as energy prices have continued to rise in recent years. The electricity price particularly increased after the deregulation of the Scandinavian electricity market in 1996. The deregulation expanded to all of the EU in July 2004, which may increase the Swedish electricity price further until it reaches the generally higher European price level. Furthermore, oil prices have increased and the emissions trading scheme for CO₂ adds to the incentive to reduce oil consumption. The energy system at the surveyed pulp mill is described in terms of electricity and process heat production and use. The total energy-saving potential is estimated and some saving points are identified. The heat that today is wasted at the mill has been surveyed in order to find potential for heat integration or heat export. The result shows that the mill probably could become self-sufficient in electricity. Particularly important in that endeavour is updating old pumps.     

Ash-forming elements in four Scandinavian wood species. Part 1: Summer harvest

Woody biomass in Finland and Sweden comprises mainly four wood species: spruce, pine, birch and aspen. To study the ash, which may cause problems for the combustion device, one tree of each species were cut down and prepared for comparisons with fuel samples. Well-defined samples of wood, bark and foliage were analyzed on 11 ash-forming elements: Si, Al, Fe, Ca, Mg, Mn, Na, K, P, S and Cl. The ash content in the wood tissues (0.2–0.7%) was low compared to the ash content in the bark tissues (1.9–6.4%) and the foliage (2.4–7.7%). The woods’ content of ash-forming elements was consequently low; the highest contents were of Ca (410–1340 ppm) and K (200–1310), followed by Mg (70–290), Mn (15–240) and P (0–350). Present in the wood was also Si (50–190), S (50–200) and Cl (30–110). The bark tissues showed much higher element contents; Ca (4800–19,100 ppm) and K (1600–6400) were the dominating elements, followed by Mg (210–2400), P (210–1200), Mn (110–1100) and S (310–750), but the Cl contents (40–330) were only moderately higher in the bark than in the wood. The young foliage (shoots and deciduous leaves) had the highest K (7100–25,000 ppm), P (1600–5300) and S (1100–2600) contents of all tissues, while the shoots of spruce had the highest Cl contents (820–1360) and its needles the highest Si content (5000–11,300). This paper presented a new approach in fuel characterization: the method excludes the presence of impurities, and focus on different categories of plant tissues. This made it possible to discuss the contents of ash element in a wide spectrum of fuel-types, which are of large importance for the energy production in Finland and Sweden.     

A new dynamic clothing model. Part 1: Heat and mass transfers

Beyond an insulating function, clothing constitutes a dynamic system managing the hygro-thermal exchanges between body and environment. The model built, presented here, and computed in the form of a user-friendly software, points out the interfacing function. The air confined under garment creates a micro-climate responsible for the felt comfort. The exchanges between the external side of clothing and the outer environment are lower than in a “static” insulating model, but are to be added to the transfers by renewal of the confined air. This model constitutes a tool giving a comprehensive valuation of the real transfers. It is useful for studies concerning thermal comfort and fabrics.     

A new turbulence model for porous media flows. Part I: Constitutive equations and model closure

A new model for turbulent flows in porous media is developed. The spatial- and time fluctuations in this new model are tied together and treated as a single quantity. This novel treatment of the fluctuations leads to a natural construction of the k and ε type equations for rigid and isotropic porous media in which all the kinetic energy filtered in the averaging process is modeled. The same terms as those found in the corresponding equations for clear flow, plus additional terms resulting from the interaction between solid walls in the porous media and the fluid characterize the model. These extra terms arise in a boundary integral form, facilitating their modeling. The model is closed by assuming the eddy viscosity approximation to be valid, and using simple models to represent the interaction between the walls in the porous media and the fluid.     

Physical and chemical comparison of soot in hydrocarbon and biodiesel fuel diffusion flames: A study of model and commercial fuels

Data are presented to compare soot formation in both surrogate and practical fatty acid methyl ester biodiesel and petroleum fuel diffusion flames. The approach here uses differential mobility analysis to follow the size distributions and electrical charge of soot particles as they evolve in the flame, and laser ablation particle mass spectrometry to elucidate their composition. Qualitatively, these soot properties exhibit a remarkably similar development along the flames. The size distributions begin as a single mode of precursor nanoparticles, evolve through a bimodal phase marking the onset of aggregate formation, and end in a self preserving mode of fractal-like particles. Both biodiesel and hydrocarbon fuels yield a common soot composition dominated by C_x ions, stabilomer PAHs, and fullerenes in the positive ion mass spectrum, and and C_2xH⁻ in the negative ion spectrum. These ion intensities initially grow with height in the diffusion flames, but then decline during later stages, consistent with soot carbonization. There are important quantitative differences between fuels. The surrogate biodiesel fuel methyl butanoate substantially reduces soot levels, but soot formation and evolution in this flame are delayed relative to both soy and petroleum fuels. In contrast, soots from soy and hexadecane flames exhibit nearly quantitative agreement in their size distribution and composition profiles with height, suggesting similar soot precursor chemistry.     

A Computational Model for Two—stage 4K—Pulse Tube Cooler：part I.Theoretical Model and Numerical Method

IntroduedonThe need for ctyogenic coOling of magnehcresonance imaging (MRI), superconduchng quantUminterfernce devices (SQUIDs) and the associatedsuperconduchng devices has brought about a strOngdemand for high reliability and efficiency in coolers.SWg and GM coolers, which are very compact andreliable machines and have already commercial products,are widely used for such aPplications in the last decades.However, they have a tnoving pistOn at the cold end,which caused evidenily mechotal …     

A comparison of electricity and hydrogen production systems with CO2 capture and storage. Part A: Review and selection of promising conversion and capture technologies

We performed a consistent comparison of state-of-the-art and advanced electricity and hydrogen production technologies with CO₂ capture using coal and natural gas, inspired by the large number of studies, of which the results can in fact not be compared due to specific assumptions made. After literature review, a standardisation and selection exercise has been performed to get figures on conversion efficiency, energy production costs and CO₂ avoidance costs of different technologies, the main parameters for comparison. On the short term, electricity can be produced with 85–90% CO₂ capture by means of NGCC and PC with chemical absorption and IGCC with physical absorption at 4.7–6.9 €ct/kWh, assuming a coal and natural gas price of 1.7 and 4.7 €/GJ. CO₂ avoidance costs are between 15 and 50 €/t CO₂ for IGCC and NGCC, respectively. On the longer term, both improvements in existing conversion and capture technologies are foreseen as well as new power cycles integrating advanced turbines, fuel cells and novel (high-temperature) separation technologies. Electricity production costs might be reduced to 4.5–5.3 €ct/kWh with advanced technologies. However, no clear ranking can be made due to large uncertainties pertaining to investment and O&M costs. Hydrogen production is more attractive for low-cost CO₂ capture than electricity production. Costs of large-scale hydrogen production by means of steam methane reforming and coal gasification with CO₂ capture from the shifted syngas are estimated at 9.5 and 7 €/GJ, respectively. Advanced autothermal reforming and coal gasification deploying ion transport membranes might further reduce production costs to 8.1 and 6.4 €/GJ. Membrane reformers enable small-scale hydrogen production at nearly 17 €/GJ with relatively low-cost CO₂ capture.     

A computational model for two-stage 4K-pulse tube cooler: Part II. Predicted results

The internal physical processes and performance of a two-stage pulse tube cooler operating at 4 K-temperature region are numerically analyzed by a new mixed Eulerian-Lagrangian computational model. The detailed time-variations of gas temperature, pressure, mass flow rate, enthalpy flow in a cycle, in the first and the second-stage regenerators are presented in the paper. The behavior of the various gas elements, which enter the pulse tube from its cold end has been revealed and discussed. More attention is paid to the effects of different regenerative materials on the performance of the 4 K two-stage pulse tube cooler.     

A new turbulence model for porous media flows. Part II: Analysis and validation using microscopic simulations

A new model for turbulent flows in porous media developed in an accompanying paper [F.E. Teruel, Rizwan-uddin, A new turbulence model for porous media flows. Part I: Constitutive equations and model closure, Int. J. Heat Mass Transfer (2009), doi:10.1016/j.ijheatmasstransfer.2009.04.017.], in which new definitions of the macroscopic turbulence quantities are introduced, is analyzed and validated. The model is validated using a simple but often used porous medium consisting of a staggered arrangement of square cylinders. Theoretically predicted values of the newly defined turbulence variables, under fully developed conditions, are compared with corresponding variables used in existing turbulence models. Additionally, evolution of the macroscopic turbulence quantities obtained numerically using the model developed here are compared with reference results, obtained by averaging over space the microscopic level solution of the RANS equations. Comparison exercise for the 75% porosity case is carried out for a range of turbulence intensity at the entrance of the porous medium. Comparison of results shows very good agreement. The spatial evolution of the dispersive kinetic energy, which is included in the definition of the macroscopic turbulent kinetic energy introduced here, is computed using the microscopic solution. Its magnitude relative to the conventional turbulent kinetic energy shows the importance of this quantity in the representation of turbulence effects in porous media flows.     

Dark zones of solid propellant flames: Critically assessed datasets, quantitative model comparison, and detailed chemical analysis

The formation of propellant dark zone (DZ) structures in the gaseous flames above many solid propellants has been a subject of recurrent interest to us for about 20 years. The DZ structure is controlled by small molecule chemistry. The DZ chemistry is very important in controlling both the flame structure at low pressure (10–100 atm) and burning rates at high pressure (above ∼500 atm), even though DZs collapse at higher pressures. We developed a detailed, frequently updated mechanism to model it. This report reviews prior work, introduces our most recent modeling results, and documents the first quantitative tests of several key assumptions. All relevant experimental literature is critically assessed to identify datasets for testing our model. Comparison of predictions and experimental results shows reasonable agreement, and thus we advocate use of our mechanism. But the precision of both experiments and predictions is not tight, nor are there many test datasets. Further experimentation is needed, and types of study that would be of greatest benefit are suggested. A detailed discussion of the chemistry that controls DZ structure is presented.     

A comprehensive chemical kinetic combustion model for the four butanol isomers

Alcohols, such as butanol, are a class of molecules that have been proposed as a bio-derived alternative or blending agent for conventional petroleum derived fuels. The structural isomer in traditional “bio-butanol” fuel is 1-butanol, but newer conversion technologies produce iso-butanol and 2-butanol as fuels. Biological pathways to higher molecular weight alcohols have also been identified. In order to better understand the combustion chemistry of linear and branched alcohols, this study presents a comprehensive chemical kinetic model for all the four isomers of butanol (e.g., 1-, 2-, iso- and tert-butanol). The proposed model includes detailed high-temperature and low-temperature reaction pathways with reaction rates assigned to describe the unique oxidation features of linear and branched alcohols. Experimental validation targets for the model include low pressure premixed flat flame species profiles obtained using molecular beam mass spectrometry (MBMS), premixed laminar flame velocity, rapid compression machine and shock tube ignition delay, and jet-stirred reactor species profiles. The agreement with these various data sets spanning a wide range of temperatures and pressures is reasonably good. The validated chemical kinetic model is used to elucidate the dominant reaction pathways at the various pressures and temperatures studied. At low-temperature conditions, the reaction of 1-hydroxybutyl with O₂ was important in controlling the reactivity of the system, and for correctly predicting C₄ aldehyde profiles in low pressure premixed flames and jet-stirred reactors. Enol–keto isomerization reactions assisted by radicals and formic acid were also found to be important in converting enols to aldehydes and ketones under certain conditions. Structural features of the four different butanol isomers leading to differences in the combustion properties of each isomer are thoroughly discussed.     

Strength tests of axially symmetric perforated plates for chemical reactors: Part 2—Experiments

The aim of the present paper is to elaborate the methodology of empirical studies, which would allow establishing stress histories in perforated plates centrally loaded by a concentrated force. The other objective is to empirically verify the mathematical model proposed in a companion paper (Part 1). The diameter of the holes, as well as the geometry of their distribution in the plate prepared for the study, provided for the constancy of section-weakening coefficients. Some resistance strain gauges were placed in between the holes in order to measure the strains. The axially symmetric perforated plate was subjected to bending with a concentrated force applied centrally. Additionally, bending of the plate was monitored with a dial gauge during the measurements of the strains. On the basis of the measured values of the strains we were able to establish radial, circumferential and equivalent stresses in the plate. The empirical research corroborated the correctness of the adopted model of calculating the stress states in the perforated plate.     

Quantitative comparison of different chemical pretreatment methods on chemical structure and pyrolysis characteristics of corncobs

In order to identify the effects of different chemical pretreatment methods on structure changes of corncobs and their subsequent pyrolysis characteristics, the chemical pretreatment of corncobs was performed using different concentrations of sodium hydroxide (NaOH), sulfuric acid (H₂SO₄), and hydrogen peroxide (H₂O₂) solutions. The structure changes of corncobs were characterized by elemental analysis, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD). The pyrolysis characteristics of raw and pretreated corncobs were conducted on thermogravimetric analyzer (TGA) and pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS). TG/DTG analysis of raw and pretreated corncobs demonstrated that the rank order for the ease of pyrolysis was untreated < 1% NaOH < 1% H₂O₂ < 2% NaOH < 2% H₂SO₄ < 1% H₂SO_4. Py–GC/MS analysis showed that chemical pretreatment can effectively promote the production of furans and levoglucosan (LG) and inhibit the formation of acetic acid, ketones and phenols. The rank order of LG yields from untreated and pretreated corncobs was untreated < 1% NaOH < 2% NaOH < 1% H₂O₂ < 1% H₂SO₄ < 2% H₂SO_4. The maximum yield of LG (15.01%) was obtained by fast pyrolysis of corncobs pretreated using 2% H₂SO₄. It was 18.53 times the yield of LG from untreated corncobs. The results could be mainly attributed to the passivation of alkali metal and alkali earth metal and the removal of hemicellulose and lignin fractions during pretreatment.     

A hierarchical model for automotive PEM fuel cell system comprehensive evaluation and comparison

This paper proposes a hierarchical model for automotive fuel cell system evaluation, comparison and applicability assessment through Analytic Hierarchy Process. A dual-layer criterion based hierarchical model is proposed, which features 9 sub-performances and 18 indices. Criterion weights and scoring function are both established. Experimental application is carried out to illustrate the evaluation process and how the proposed model operates. Results show that the sub-performance derived from automotive powertrain requirements represented the overall characteristics of the PEMFCs. The selected 18 most significant indices also made each sub-performance quantitatively assessable.     

Excess heat from kraft pulp mills: Trade-offs between internal and external use in the case of Sweden—Part 2: Results for future energy market scenarios

In this paper the trade-off between internal and external use of excess heat from a kraft pulp mill is investigated for four different future energy market scenarios. The work follows the methodology described in Svensson et al. [2008. Excess heat from kraft pulp mills: trade-offs between internal and external use in the case of Sweden—Part 1: methodology. Energy Policy, submitted for publication], where a systematic approach is proposed for investigating the potential for profitable excess heat cooperation. The trade-off is analyzed by economic optimization of an energy system model consisting of a pulp mill and an energy company (ECO). In the model, investments can be made, which increase the system's energy efficiency by utilization of the mill's excess heat, as well as investments that increase the electricity production. The results show that the trade-off depends on energy market prices, the district heating demand and the type of existing heat production. From an economic point of view, external use of the excess heat is preferred for all investigated energy market scenarios if the mill is studied together with an ECO with a small heat load. For the cases with medium or large district heating loads, the optimal use of excess heat varies with the energy market price scenarios. However, from a CO₂ emissions perspective, external use is preferred, giving the largest reduction of global emissions in most cases.     

High temperature metal hydrides for energy systems Part A: Numerical model validation and calibration

Metal hydrides have become more and more significant both as hydrogen storage devices and as basic elements in energy conversion systems. Magnesium hydride (MgH₂) shows very interesting characteristics – i.e. high enthalpy of reaction and hydrogen gravimetric density – making it capable of being employed in the field of energy production. So there is interest in investigating the behaviour of this kind of hydride, placed in an energy system. In order to examine MgH₂ in depth, its main kinetic parameters must be found and this can be accomplished by means of the calibration of a model previously realised by author Marco Gambini in his 1989–1994 works.In the light of this, an assessment of the activation energies (E_a = 72,200 kJ/kmol, E_d = 89,000 kJ/kmol) and kinetic constants (k_0a = 15,000 s^?1, k_0d = 198,500 s^?1) in both absorption and desorption phase of magnesium hydride has been provided. Thanks to these values, the behaviour of MgH₂ can be modelled, making the performance of a magnesium hydride-based energy system estimable.     

Part one: A novel model of HTPEM-based micro-combined heat and power fuel cell system

Fuel cell-based combined heat and power (CHP) systems have received increasing attention during the last decade. This is mainly due to the high efficiencies obtainable on even a small-scale system basis. The current paper focuses on the development of a complete model of a system consisting of the high-temperature proton exchange membrane (HTPEM) fuel cell stack based on PBI membranes, steam-reforming reactor, burner, heat reservoir and other auxiliary equipment included in a typical reforming-based fuel cell system. The model is implemented in the Matlab®Simulink$\mathrm{Matlab}\circledR \mathrm{Simulink}$ environment enabling both static system integration as well as dynamical control strategies to be evaluated. All results of the submodels correspond well with experimental results obtained. Additionally, a novel system integration of an HTPEM fuel cell and a steam reforming-based fuel processing unit is presented. The total energy utilization efficiency of the system modeled is as high as 90–100_%LHV$90–100{\%}_{\mathrm{LHV}}$ depending on the operating point chosen, whereas the electrical efficiency can be up to 45_%LHV$45{\%}_{\mathrm{LHV}}$. This is more than 30% better than the best low-temperature PEM-based systems demonstrated today.     

Bio-butanol: Combustion properties and detailed chemical kinetic model

Autoignition delay time measurements were performed at equivalence ratios of 0.5, 1 and 2 for butan-1-ol at reflected shock pressures of 1, 2.6 and 8 atm at temperatures from 1100 to 1800 K. High-level ab initio calculations were used to determine enthalpies of formation and consequently bond dissociation energies for each bond in the alcohol. A detailed chemical kinetic model consisting of 1399 reactions involving 234 species was constructed and tested against the delay times and also against recent jet-stirred reactor speciation data with encouraging results. The importance of enol chemistry is highlighted.     

A generalized cryo-adsorber model and 2-D refueling results

Our earlier work [Int J Hyd Energy 2010; 35: 3598-3609], describes a 3-D model for the cryo-adsorber and the 1-D results for the isobaric refueling period using quasi-static adsorption approximation. Herein the isobaric constraint is relaxed by solving the Darcy equation for computing the pressure drop in the bed, the quasi-static approximation is relaxed by introducing the Langmuir adsorption kinetics, and the 2-D refueling results are compared with the 1-D quasi-static isobaric refueling results. In spite of the significant differences in formulation, the two results compare well with each other. The 2-D refueling results show that the pressure transients equilibrate quickly and a nearly steady pressure profile gets established in the bed. This observation justifies the isobaric approximation used earlier. The Langmuir kinetics used here has a desorption rate constant which could vary depending on the diffusional resistance at adsorbent particle level. A sensitivity analysis of this parameter shows that the refueling rate varies negligibly while this parameter is varied over many orders of magnitude. This observation shows that as long as the pellets are small enough, refueling is controlled by macroscopic processes like simultaneous cooling/adsorption in the bed and the movement of the adsorption front out of the bed, rather than the molecular processes like sorption at an adsorbent site or diffusion through the adsorbent lattice. This observation justifies the quasi-static approximation used earlier. The above two approximations offer significant computational advantage to the design and optimization of cryo-adsorber beds with complex geometry.     

A review of thin-film crystalline silicon for solar cell applications. Part 2: Foreign substrates

One of the most promising ways to reduce the cost of photovoltaics is thin-film crystalline silicon solar cells. This paper, together with part 1, reviews the current state of research in thin-film crystalline silicon solar cells. Deposition on silicon, novel techniques which use a high-quality, reusable silicon substrate and light trapping have been described in part 1 of this paper. This paper describes deposition on glass and ceramics and discusses cell designs for thin-film crystalline silicon solar cells.