This work was carried out in order to quantify the impact of the pyrolysis heating rate both on the properties of the residual charcoal and on the behaviour during gasification by H 2 O of the charcoal. The experiments were conducted on 10 mm diameter beech wood spheres, pyrolysed at atmospheric pressure under heating rates covering the range from very slow, 2.6 K min K1 , to very rapid, over 900 K min K1 , i.e. the highest value that can be reached. When charcoal is submitted to gasification at 20% H 2 O i nN 2 at 1200 K, the ratio of the times for complete conversion reach 2.6. Such a difference is considerable as far as an industrial application is concerned. The initial properties of the charcoal such as apparent density, porosity, and pore surface area obtained by N 2 or Ar adsorption were measured in order to explain the differences in gasification kinetics within the charcoal. The charcoal particles exhibit densities as different as 219-511 kg m K3 and porosities between 87 and 70% for charcoal prepared at 900 and 2.6 K min K1 respectively. The specific surface area is higher than 600 m 2 g K1 for three charcoals. Influence of ash content of the initial charcoals, at 1.6-2.7%, is also regarded with particular attention to explain the observed differences in gasification kinetics.

The present work deals with a study coupling experiments and modeling of charcoal gasification by steam at large particle scale. A reliable set of experiments was first established using a specially developed "macro-TG" apparatus where a particle was suspended and continuously weighed during its gasification. The main control parameters of a fixed-bed process were modified separately: steam gasification of beech charcoal spheres of different diameters (10 to 30 mm) was studied at different temperatures (830 to 1030 • C), different steam partial pressures (0.1 to 0.4 atm H 2 O), and different gas velocities around the particle (0.09 to 0.30 m/s). Simulations with the particle model were performed for each case. Confrontations with experimental data indicate that the model predictions are both qualitatively and quantitatively satisfactory, with an accuracy of 7%, until 60% of conversion, despite the fact that the phenomena of reactive surface evolution and particle fracturing are not well understood. Anisotropy and peripheral fragmentation make the end of the process difficult to simulate. Finally, an analysis of the thermochemical situation is proposed: it is demonstrated that the usual homogeneous or shrinking core particle models are not satisfying and that only the assumption of thermal equilibrium between the particle and the surrounding gas is valid for a model at bed scale.

Two-stage fixed bed gasification is one of the most promising technologies for low and medium energy production from biomass. In industrial processes, control and optimisation is often based on constructor know-how rather than on an understanding of the mechanisms involved. We present a new original tool, the Continuous Fixed Bed Reactor (CFiBR), which was specifically designed and built to enable a fine understanding of the limiting stage of a gasifier: the char bed gasification zone. The reactor, the instrumentation, the operating procedure and set-up tests are described in detail. The potential of the reactor is demonstrated through the characterisation of the gasification of a continuous wood char bed. Temperature profiles and gas concentrations along the 65 cm bed were established and showed that the most reactive zone was the first 10 cm of the char bed. Accurate energy and mass balances provided relevant information regarding the contributions of the main reactions involved in the fixed char bed gasification process. (Résumé d'auteur

This article presents the monitoring results of an existing large scale system that combines heat pumps with unglazed solar collectors (used for heat production or as heat source for the heat pumps). The system provides space heating and domestic hot water to a new housing complex ( 10,000 heated m2) in Geneva, Switzerland. Detailed monitoring of one of the blocks ( 1000 heated m2, 32 inhabitants) enables to characterise the behaviour of the system (building demand, control strategy, temperature levels) and to determine the energy flows as well as the performance of the system. The results show a very low space heating demand for Switzerland ( 20 kWh/m2/yr), and an unusually high domestic hot water consumption ( 50 kWh/m2/yr). The measured seasonal performance factor of the system, including backup electric heating and heat source circulation pump, is 2.9 for 2012 (average of 2.5 in winter and 4.4 in summer). This result can partly be explained by the high domestic hot water consumption, which implies a heat production at high temperature. This project is part of IEA SHC Task 44 “Solar and Heat Pump Systems”

The present work analyses the potential of a combined solar thermal and heat pump (HP) system on new and existing multifamily buildings. The study uses numerical simulation as a complement to a monitored case study. After a description of the case study and a summary of the monitoring results, we present the numerical model developed for this study. Simulation results are validated with the monitored values, at component and system level, in terms of monthly profiles and yearly integrals. On this basis, we carry out an extensive sensitivity analysis concerning the principal sizing parameters of the system. Finally, we investigate the sensitivity of the system to space heating (SH) and domestic hot water (DHW) demands, in particular concerning the applicability of the analysed system in the case of building retrofit. For Geneva's weather conditions, a sizing factor of 3 m 2 solar collector per kW of HP capacity is a good compromise between system size and system performance, resulting in a system seasonal performance factor (SPF sys) between 3.1 and 4.1, depending on the SH distribution temperature. The associated electricity consumption (ranging from 12 kWh/m 2 for a new low-energy building, up to 45 kWh/m 2 for a non-retrofitted building) strongly depends on the heat demand. Such is also the case for the collector area (from 0.08 m 2 per m 2 heated area for a new low-energy building, up to 0.20 for a non-retrofitted building). Finally, a SPF sys of 5 could potentially be achieved, but only in newly constructed buildings with a high efficient envelope, a low SH distribution temperature, and with a collector area of at least 0.20-0.25 m 2 per m 2 heated area. However, the related investment may not be worthwhile, given the rather small associated electricity saving, not to mention that such a collector area would not fit on buildings with more than 4 storeys.

This study focuses on the effect of peripheral fragmentation during gasification of an isolated wood charcoal particle in the regime of diffusional limitations. In fact, all the models available in the literature fail to reproduce the specific changes in biomass charcoals for which conversion increases quasi linearly versus time, whatever the particle size. We assumed this discrepancy is partly due to the fragmentation phenomenon, i.e. the detachment of fragments from the surface of the particle, which is not usually taken into account in models. The classical assumption in percolation models is a critical value of porosity beyond which fragmentation occurs. We compared our model results with experimental data. We demonstrated that in the diffusion-limited regime, the direct extrapolation of such a method to a continuum model is not satisfying. A new criterion of fragmentation based on a critical porosity and a critical porosity gradient is proposed which considerably improves modelling of gasification charcoal particles.

This article presents the behavior of an existing system combining solar collectors and heat pumps at large scale (10'000 living m², more than 1'000 m² solar collectors) for space heating and domestic hot water production, focusing on summer period. Ongoing detailed monitoring enables to measure its energy performance. The monitoring results for 2012 show a system SPF of 2.9 (2.6 in winter and 4.4 in summer). The direct solar fraction in summer is lower than 50%, which is low considering the oversizing of the solar collector area for domestic hot water production. The high domestic hot water demand (50 kWh/m²/yr whereas the usual value is around 20) can partly explain this low value, but other factors should also be considered. The results presented in this article are part of a research project aiming to assess the relevance of the concept of coupling solar thermal and heat pumps in various types of building (especially existing buildings with low efficient thermal envelope)

No abstract