تجزیه و تحلیل عملکرد از گاسیفایر زیست توده ها

This paper presents an original modelling of the biomass gasification process and more particularly the wood gasification. Gasification is one of the more efficient ways to convert the energy embedded in the biomass. In the long term, the use of this rather new technology will reduce the too high pressure on the natural resources and that especially in the developing countries. The model based on the minimisation of the Gibbs free energy is performed in the ASPEN PLUS process simulator. The processes occurring in the gasification are here uncoupled in pyrolysis, combustion, Boudouard reaction, and gasification. In the first part of the paper the model is described and the physical meaning of the relevant parameters are given. In the second part, the results of a sensitivity analysis with respect to the oxygen factor, the air temperature, the oxygen content in air, the operating pressure and the injection of steam are presented. From this study, the following conclusions are derived: there exists a critical air temperature above which the preheating is no longer efficient, there is an optimum oxygen factor, the oxygen enrichment of air plays an efficient role under a certain value and the operating pressure has only a slight positive effect on the process efficiency.

Amongst the renewable energies, biomass is expected to be one of the most important in the near future. Indeed, biomass is a renewable fuel, which can be burnt or gasified similarly to other fossil fuels, using the same technologies with special adaptations. It can also be used in co-combustion with coal in large power plants [8] and [10], but its most suitable use is its gasification at small scale. If similar to conventional fossil fuels, it is different in its composition, its LHV, its corrosivity. Its big difference is its comparatively low impact on the environment; the content in N and S being very low, so are the emissions of NOx and of SO2. On top of that, biomass has a zero CO2 balance on a period of a few years. On the other hand, governments commit themselves to reduce their CO2 emissions in the international meetings about the global climate change like in Kyoto (1997), Buenos Aires (1998). In this framework the use of biomass in power generation is promoted. In this paper, we present one possible option, i.e. a small-scale gasifier of biomass and more particularly of wood. Here, an original model of the biomass gasification process is proposed. It is based on a minimisation of the Gibbs free energy [5] at equilibrium [4]. This implies that the residence time is long enough to allow the chemical reactions to reach an equilibrium state, this assumption of equilibrium is not usually reached in real gasifiers. The considered gasifier is a fluidised bed in which the temperature is supposed to remain constant and homogeneous. However, the higher the reaction temperature, the better is this assumption. The proposed model is valid when the system may be considered as adiabatic and produces results in particular the reaction temperature in the gasifier, independent of the geometry. Along the paper, a sensitivity analysis is carried out with respect to any influencing parameter.

From the sensitivity analysis, we can conclude that: 1. a preheating of the feeding air up to 300 °C increases ηG; 2. the best ηG is obtained when F is 25%; 3. the enrichment of the air in O2 up to 30% vol. increases ηG provided that this air is not preheated; 4. an increase of the pressure has only a slight positive effect on ηG when air is not preheated; 5. injection of steam degrades ηG. Finally, the gasifier considered here should be an atmospheric fluidised bed instead a pressurised one