E. Van den Bulck

Kinetics of the low-temperature pyrolysis of chromated copper arsenate-treated wood

Abstract In this paper, a kinetic evaluation of non-isothermal thermogravimetric (TG) data is carried out by the method of least squares for both untreated and chromated copper arsenate (CCA)-treated wood. The type of wood used is Pinus sylvestris and the slow-heating decomposition in an inert (nitrogen) atmosphere (low-temperature pyrolysis) is studied. In the case of untreated wood, best fit parameters are determined for the three independent parallel reactions scheme, proposed by several other researchers. For CCA treated wood a comparative study is performed, in which five different kinetic models are considered. These are based on the assumption of: (1) a single reaction; (2) independent parallel reactions; (3) competitive reactions; (4) a chain of successive reactions and (5) a combination of two independent parallel reactions and one subsequent reaction. The choice between which of these models offer the best fit, is based partly on chemical considerations and partly on the quality of the fit. Altogether, the DTG curve of CCA treated wood is well described by assuming two independent parallel reactions at low temperature, which may be catalyzed by the metals, followed by the successive decomposition of the residue. A comparison is made with the kinetic parameters derived for untreated wood, resulting in the following observations and conclusions drawn therefrom: hemicellulose decomposition is catalyzed by the metals (lower activation energy, pre-exponential factor and peak temperature and higher contribution to volatiles production); cellulose decomposition starts once the major part of chromium(III) arsenate is decomposed and volatilised (higher activation energy) and may then even be catalyzed by the metals (higher pre-exponential factor) and lignin decomposes very slowly at higher temperatures together with intermediate products from hemicellulose and cellulose decomposition (low pre-exponential factor and activation energy). During this slow charring process the metals act no more as catalysts. This hypothesis is confirmed by a TG analysis, performed on a chromium(III) arsenate precipitate, which is the major arsenic compound present in CCA treated wood, and by a study concerning the microdistribution of copper, chromium and arsenic in the pyrolysis residue of CCA treated wood.

Design theory for rotary heat and mass exchangers—I. Wave analysis of rotary heat and mass exchangers with infinite transfer coefficients

Abstract A theory is presented for modeling rotary heat and mass exchangers with infinite transfer coefficients. The continuity and energy conservation equations for one-dimensional transient flow are established and analyzed. Solutions to the equations are obtained by the method of characteristics and the shock wave method. Both methods provide a set of analytical equations that allow performance prediction of the heat and mass exchanger with infinite transfer coefficients for any inlet and operating conditions. A regenerator-operating chart is introduced which shows the fundamental modes of operation for rotary heat and mass exchangers. Part II presents correlations for the effectiveness of rotary dehumidifiers with finite transfer coefficients.

Low-temperature pyrolysis of CCA-treated wood: thermogravimetric analysis

Abstract A thermo-analytical study of untreated and chromated copper arsenate (CCA) treated wood samples is performed in order to obtain a better understanding of the low-temperature pyrolysis of CCA-treated wood waste in an inert atmosphere. The type of wood used in this study is Pinus sylvestris sapwood. The influence of the presence of CCA and the heating rate on the pyrolytic behaviour of wood samples is studied, as well as the release of volatile compounds and metals (Cr, Cu, As) during the pyrolysis process. This paper shows that CCA has a significant influence on the thermal behaviour of wood samples, which is more pronounced the higher the CCA concentration of the sample is. The temperature at the onset of pyrolysis, as well as the temperature where the maximum rate of decomposition occurs, are lowered by the CCA treatment. The final char yield (including the metals) is higher and the rate of weight loss is much more peaked for CCA-treated wood. It could be postulated that the CCA compounds act as promotors of the pyrolysis reactions favouring the formation of char. For higher heating rates, there is a shift of the DTG peak to higher temperatures for both untreated and CCA-treated wood samples. Within the accuracy of the evolved gas analysis (EGA) method applied, it is observed that the presence of CCA does not significantly influence the type and relative amount of measured volatiles. The volatilisation of metal compounds is shown to be strongly dependent on temperature and residence time of the wood sample at a given temperature. A critical point (10 min at 400°C) exists, below which the release of Cr and Cu is negligible and the release of As is below 10%. Above this critical point (longer times at 400°C), there is a dramatic increase in metal release for all three metals. The CCA concentration itself also has an influence in the sense that higher concentrations in the original sample give higher relative concentrations of metals in the resulting pyrolysis residue.

Total recycling of CCA treated wood waste by low-temperature pyrolysis

Abstract A system to turn a potentially harmful stream of solid waste into a set of substreams with either commercial value or highly concentrated residual streams is presented. The waste which is considered is metal impregnated (in particular Chromated Copper Arsenate (CCA) treated) wood waste and timber, such as telephone poles, railway sleepers, timber from landscape and cooling towers, wooden silos, hop-poles, cable drums and wooden playground equipment. These waste streams sum up to several 100,000 tons of material per year currently to be dumped in every major country of the European Community (EC). Technologies need to be developed to reduce this CCA treated wood waste, such that all of the metals are contained in a marketable product stream, and the pyrolysis gases and/or pyrolysis liquid are used to their maximum potential with respect to energy recuperation. Pyrolysing the CCA treated wood waste may be a good solution to the growing disposal problem since low temperatures and no oxidising agents are used, which result in lower loss of metals compared to combustion. An experimental labscale pyrolysis system has been developed to study the influence of the pyrolysis temperature and the duration of the pyrolysis process on the release of metals and the mass reduction. The macrodistribution and microdistribution of the metals in the solid pyrolysis residue is studied using Inductively Coupled Plasma Mass Spectrometry (ICP–MS) and Scanning Electron Microscopy coupled with Energy Dispersive X-ray Analysis (SEM–EDXA). Furthermore, a complete mass balance is calculated over the pyrolysis system. Based on these results a semi-industrial pyrolysis system (pilot plant scale) has been developed consisting of three stages: grinding, packed bed pyrolysis and metal separation. Special types of equipment have been developed to carry out the three stages. A new grinding system has been developed, based on a crushing mechanism rather than a cutting mechanism. The crushed wood is introduced by means of a screw feeding system into a reaction column. In this pyrolysis reactor the wood is heated by subjecting it to a flow of hot gases. This causes an adiabatic pyrolysis, which results in volatilisation of the volatile compounds whereas the mineral compounds (containing the metals) remain entrapped in a coal-type residue which is very rich in carbon. The condensable compounds in the pyrolysis gas condense while leaving the reaction zone due to the inverse temperature gradient. The pyrolysis gas leaving the reactor is used as fuel for the hot gas generator. The charcoal which is extracted at the bottom of the reactor, is cooled, compressed, removed and stored, ready to feed the subsequent stage. A specially developed grinder is used to remove the metal particles from the charcoal and the separation between metal and charcoal particles is accomplished in a pneumatic centrifuge as a result of the difference in density. Using this system the ultimate waste is less than 3% of the initial wood mass. Results obtained with a semi-industrial scale prototype confirm the effectiveness of the process.

Design theory for rotary heat and mass exchangers—II. Effective n ess-number-of-transferunits method for rotary heat and mass exchangers

Abstract Analytical performance correlations for rotary heat and mass exchangers with infinite transfer coefficients have been presented in part I. A finite-difference model for performance prediction of rotary dehumidifiers with finite transfer coefficients is used in combination with the ideal dehumidifier model to establish effectiveness correlations. Correlations for the humidity and enthalpy effectiveness for silica gel regenerators are given as functions of the dehumidifier number of transfer units (NTU). An Effectiveness-NTU model, incorporating the correlations for the effectiveness and ideal dehumidifier performance, allows rapid calculation of the dehumidifier performance. The correlations are valid for a wide range of operating conditions and account for the effect of unbalanced flow and high Lewis numbers.

Low-temperature pyrolysis of CCA-treated wood waste : Chemical determination and statistical analysis of metal input and output ; mass balances

Low-temperature pyrolysis is proposed as an alternative method to dispose of CCA-treated wood waste. In the frame of a study aiming at optimising the pyrolysis of CCA-treated wood, an experimental facility has been built to examine the influence of important process parameters (pyrolysis temperature, residence time, heating rate, particle size, …) on the release of metals and on the resultant mass reduction. In order to perform a mass balance calculation for the total system, a method for metal analysis was developed. Two leaching procedures and one dissolution procedure were tested and compared with each other, resulting in an optimal procedure to bring the metals into solution: “the BSI method” to determine the total amount of Cr, Cu and As in the dried wood and “the Reflux method” to determine the total amount of Cr, Cu and As in the pyrolysis residue. These results illustrate that Cr is more strongly bound in the pyrolysis residue compared to the CCA-treated wood. The analytical technique used was ICP-MS and the analytical problems like interferences and matrix effects were solved by using the appropriate isotope, an internal standard and mathematical corrections. The resulting optimal technique for CCA-treated wood (“the BSI method”) was applied to wood samples with different particle sizes. A statistical analysis of the Cr, Cu and As content in the CCA-treated wood shows the heterogeneous character of CCA-treated wood samples. Heterogeneity becomes less important when using samples with a small range of particle sizes. The smaller wood particles have significantly higher metal concentrations than the larger particles. Realistic mass balances for the metals were obtained and showed that most of the Cr, Cu and As remained in the pyrolysis residue.

Second Law Analysis of Solid Desiccant Rotary Dehumidifiers

This paper presents a second law analysis of solid desiccant rotary dehumidifiers. The equations for entropy generation for adiabatic flow of humid air over a solid desiccant are developed. The generation of entropy during operation of a rotary dehumidifier with infinite transfer coefficients is investigated and the various sources of irreversibility are identified and quantified. As they pass through the dehumidifier, both the process and regeneration air streams acquire nonuniform outlet states, and mixing both of these air streams to deliver homogeneous outlet streams is irreversible. Transfer of mass and energy between the regeneration air stream and the desiccant matrix occurs across finite differences in vapor pressure and temperature and these transfer processes generate entropy. The second law efficiency of the dehumidifier is given as a function of operating conditions and the effect of finite transfer coefficients for an actual dehumidifier is discussed. It is shown that operating the rotary dehumidifier at conditions that minimize regeneration energy also yields a local maximum for the second law efficiency.

The Use of Dehumidifiers in Desiccant Cooling and Dehumidification Systems

The use of rotary dehumidifiers in gas-fired open-cycle desiccant cooling systems is investigated by analyzing the performance of the rotary heat exchanger-rotary dehumidifier subsystem. For a given cooling load, the required regeneration heat supply can be minimized by choosing appropriate values for the regeneration air mass flow rate and the wheel rotation speed. A map is presented showing optimal values for rotational speed and regeneration flow rate as functions of the regeneration air inlet temperature and the process air inlet humidity ratio. This regeneration temperature is further optimized as a function of the process humidity ratio. In the analysis, the control strategy adjusts the process air mass flow rate to provide the required cooling load. Additional control options are considered and the sensitivity of the regeneration heat required to the wheel speed, regeneration air mass flow rate, and inlet temperature is discussed. Experimental data reported in the literature are compared with the analytical results and indicate good agreement.

Model predictive control of automotive powertrains - first experimental results

This paper illustrates the capabilities of model predictive control for the control of automotive powertrains. We consider the minimization of the fuel consumption of a gasoline engine through dynamic optimization. The minimization uses a mean value model of the powertrain and vehicle. This model has two state variables: the pressure in the engine manifold and the engine speed. The control input is the throttle valve angle. The model is identified on a universal dynamometer. Optimal state and control trajectories are calculated using Bock¿s direct multiple shooting method implemented in the software MUSCOD-II. The developed approach is illustrated both in simulation and experimentally for a test case where a vehicle accelerates from 1100 rpm to 3700 rpm in 30 s. The optimized trajectories yield minimal fuel consumption. The experiments show that the optimal engine speed trajectory yields a reduction of the fuel consumption of 12% when compared to a linear trajectory. Thus, it is shown that, even with a simple model, a significant amount of fuel can be saved without loss of the fun-to-drive.

Influence of swirl on the initial merging zone of a turbulent annular jet

This paper presents an extensive study of the influence of swirl on the initial region of an annular jet. A total of five different swirl numbers S are investigated: one at zero swirl, one at low swirl (S=0.18), two at intermediate swirl (S=0.37 and 0.57), and one at high swirl (S=0.74). The flow fields are measured using the stereoscopic particle image velocimetry (PIV) technique. A detailed study on the accuracy of the PIV measurements is presented, including a validation with laser Doppler anemometry data. In this way a detailed set of accurate data is presented of the three components of velocity and the root-mean square value of their fluctuations in a plane through the central axis of the geometry. Despite its simple geometry, the immediate flow field of an annular jet is very complex. The concentric central tube of the nozzle acts as a bluff body to the flow, thus creating a central recirculation zone (CRZ) behind it. At low swirl numbers the swirl induced pressure gradients alter the structure of ...