GAS COMPOSITION AND EXERGY EFFICIENCY DETERMINATION AT CARBON BOUNDARY POINT IN THE DOWNDRAFT BIOMASS GASIFICATION PROCESS

Abstract

Biomass as a renewable energy resource has a great energetic potential. There are a lot of incentives all over the world, such as for example CDM program, for implementation of waste biomass. This paper focuses on the wood chips gasification in an idealized downdraft equilibrium gasifier. The model for determination of carbon boundary temperature and other important parameters at carbon boundary temperature such as gas composition, exergy efficiency of the process was developed. The purpose of this paper is to determine upper limit for exergy efficiency that should be expected from the downdraft gasification process in dependace of moisture content in wood chips. In addition to this, to diagnoze if and what kind of measures should be taken with wood chips’ moisture prior to the gasification process. In the model idealized gasifier is used in which chemical equilibrium is achieved, ashes are not taken into consideration and heat losses are ignored.The gasification efficiencies are calculated at carbon boundary temperature, where exactly enough air is added to achive complete gasification and avoid carbon in the solid residual. It is shown that the exergy efficiencies are lower than the energy efficiencies, and also, that exergy efficincies decrease with the increase of moisture content in the biomass. Also it is shown that overall exergy efficiency of the process decrase with slower rate than exergy efficiency based only on chemical exergy. For wood chips with high moisture content exergy efficiency could be improved by drying the biomass prior to gasification process.