Evaluation of the influence of the fuel fines content on the stratified downdraft gasification of wood chips

Downdraft gasifiers are known to be sensitive to the fines content of the used fuel, which is why commercially available gasifiers typically require fuel with less than 5 m% fines content. Therefore, thoroughly sieved fuels are necessary, which increases the fuel costs. Despite being one main limitation of downdraft gasification, there is hardly any research investigating the impact of the fuel fines content in this process. Therefore, the aim of this work was to systematically analyze the effects of the fuel fines content on the gasification process and the gas quality. This work evaluated 34 operating points with 0 m% to 28.5 m% fuel fines content in an 85–kW stratified downdraft gasifier, with a detailed monitoring of the gasifier bed temperatures and the gas quality. The results revealed three main novelties: (1) Contrary to expectations, an increase of the fuel fines content up to 27.1 m% did not affect the temperature profile and the process stability. However, more grate shaking was required. (2) The increasing fuel fines content significantly reduced the H2 concentration in the product gas, and increased the amounts of CH4 and tars. Hence, an increase of the fuel fines worsened the gas quality. (3) The operation with 28.5 m% fines content was not able anymore, due to massive fluctuations in the gas quality. Summing up, the results of this work open the door for process optimizations of real commercial small-scale stratified downdraft gasifiers towards enhanced fuel fines tolerance.