Simulation of a downdraft gasifier

Abstract

The Brazilian electric grid is managed by ELETROBRAS, a company controlled by the Brazilian Government. According to ELETROBRAS there are 1.2 million consumers in the Amazon region that are not connected to the grid. This is a low population density region, with 205 MW of off grid energy production systems installed and highly dependent of petrodiesel. Some authors showed that gasification systems with power below 5 MW have higher efficiencies than combustion systems with steam cycle. Moreover, gasification systems are very suitable to the needs of Amazonian communities: one because of the availability of biomass in the region, and two because of the higher efficiency compared to the systems using combustion and steam turbines. Diesel engines are already in operation in the Amazon region producing electric energy (as long as the small communities are not connected to the grid), and gasification systems would fit the actual infrastructure. However, the potential of biomass as energy resource for small Amazonian communities can only be evaluated through simulation models that can predict the gasification efficiency, the producer gas heating value and the overall electrical efficiency.

Apart from woody biomass residues, Açaí seed (Euterpe oleracea Mart.) is the biomass residues most wasted in Pará State (Brazil) due a production of 101 kton/year of that fruit. However, wastes of açaí seeds can be up to 70% moist and that could make the gasification process unviable. Again, numerical simulations can estimate the influence of the moisture content on the gasification process of açaí seeds.

The optimization of the gasification of açaí seeds and the resulting composition of the producer gas can be analyzed in two ways that occur simultaneously: first the influence of the air/fuel ratio, and second the influence of the moisture content. Once the producer gas heating value is a consequence of its composition, the optimization of the moisture content and air/fuel ratio would increase the gasification efficiency.

This paper presents the implementation and validation of a thermochemical equilibrium model for a downdraft gasifier. The numerical model is validated against experimental results. The reactor is a small scale, stratified downdraft open top gasifier fuelled with açaí seeds (Euterpe oleracea Mart.). Syngas compositions obtained for the simulation of several operating conditions is compared with the experimental results.