Zur Vergasung feuchter Biomasse unter indirekter Wärmezufuhr (English shop)

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Description

The energetic utilization of solid biomass represents one way of using domestic resources. Because of the low energy density of biogenic solid fuels, it is advantageous to convert them into a combustible gas by thermochemical conversion in decentralized plants. For the gasification of partially dried or undried biomass, indirectly heated and comparable processes are employed. These show deficiencies in the conversion by gasification and by secondary degradation of the higher hydrocarbons, so that the producer gas generated is essentially determined by pyrolysis.

In this work, an indirectly heated tubular reactor for the gasification of moist biogenic solid fuels is designed, built and experimentally investigated. The investigation of moist beech wood chips with a water content in the range of 27 % to 55 % at temperatures of 835 °C to 940 °C yields a producer gas in which the calorific value of the dry gas lies at 11.1 MJ/m³ ≤± 6 % over the entire range investigated. The volume fraction of hydrogen, the most abundant component in the dry producer gas, is also nearly constant at 52.4 %. This also applies, with a greater range of variation, when different biomasses are used, provided that these are suitable for achieving a sufficient conversion by gasification.

The radial temperature gradient in the gasification zone, of an order of magnitude of 1 K/mm, is considerably lower than would be expected if only the endothermic gasification reactions were considered. It can therefore be assumed that the exothermic reactions of secondary degradation make a substantial contribution to the energy balance of the process.

The modeling of the stationary process is based on the energy balance. Drying and pyrolysis are calculated and balanced for fixed temperatures. The product composition of the pyrolysis is assumed on the basis of experimental results from the low temperature distillation test according to ISO 647. For the calculation of the temperature of the secondary degradation, a kinetic estimation of the reaction conversion by gasification is carried out.

The calculation of the gasification zone is performed iteratively. For the gas phase, the chemical equilibrium is assumed to be established, while the conversion of the solid phase carbon is determined kinetically. On the basis of the equilibrium composition, the gasification zone is balanced and the gasification temperature is redetermined by means of heat transfer calculation.

With the model, the stationary state of the indirectly heated gasification can be calculated with short computing times. Besides the representation of the gas composition, the result of the modeling agrees in order of magnitude with the experimental results for the radial temperature gradients and the throughput.

Description

The energetic utilization of solid biomass provides a way to use local resources. It is an advantage to convert biomass thermo chemically into a combustible gas. Decentralized systems are preferred due to the low energy density of the biogenic solid fuels. Indirectly heated and similar processes are used for the gasification of partially dried or non dried biomass. These processes exhibit problems during the conversion via gasification and secondary degradation of higher hydrocarbons, so that the producer gas is determined significantly the by pyrolysis.

An indirectly heated tubular reactor for the gasification of wet biogenic solid fuels is designed, built and experimentally investigated. The experimental reactor is electrically heated. The study of moist beech wood chips with a water content in a range of 27 % to 55 % at temperatures of 835 °C to 940 °C provides a producer gas, in which the lower calorific value of the dry gas in the entire range of investigation, is of about 11.1 MJ/ m³ ≤± 6 %The percentage (by volume) of hydrogen, which is the main product, is ca. 52.4 % and nearly constant in all investigations.. This applies with greater variation in the use of different biomass where it is appropriate, to achieve a sufficient conversion through gasification.

The radial temperature gradient in the gasification zone has an order of magnitude of 1 K/mm, significantly less than this would be expected due to the endothermic gasification reactions. It can be assumed that the exothermic reactions of secondary degradation provide a significant contribution to the energy balance of the process.

The modeling of stationary process is based on the energy balance. Drying and pyrolysis are calculated for fixed temperatures. The composition of the products of pyrolysis is assumed on the basis of experimental results from the low temperature distillation after ISO 647. The temperature for the secondary degradation is calculated via a kinetic approach of conversion for the gasification.

The gasification zone is calculated iteratively. For the gas phase is assumed a chemical equilibrium, while the conversion of fixed carbon is determined kinetically. The equilibrium composition of the gas is assumed for the energy balance and to determine the heat transfer.

The model is suitable to calculate the stationary state of the indirectly heated gasification by short computing times. Next to the representation of the gas composition, the result of modeling compared with the experimental results is of the same order of magnitude.