Cogasification of sewage sludge in an updraft gasifier

Sewage sludge is the residue produced by the domestic or industrial wastewaters treatment plants. According to the principles of sustainability, several restrictions have been imposed on the conventional methods currently adopted for its disposal. The consequent need to develop alternative processes for the use of sewage sludge for energy purposes such as gasification requires experimental tests in order to quantify the potential energy recover from this waste, as well as to evaluate the optimum conditions for its gasification. In the present study, the gasification with air of dehydrated sewage sludge (20wt.% moisture) mixed with conventional woody biomass was performed in a pilot scale updraft fixed-bed gasifier operating at atmospheric pressure. Attention was focused on the effect of the sewage sludge content and the equivalent ratio (ratio between the amount of air used and the stoichiometric air needed for combustion) on the product yields, gas composition and cold gas efficiency. The results obtained showed that it is possible to cogasify sewage sludge with wood pellets in pre-existing updraft fixed-bed gasification installations. However, at high sewage sludge content (⩾70w/w%) slagging and an excessive clinker formation may occur in the oxidation zone because of the high ash content and low ash fusion temperatures of the sludge making the gasification process unstable. In addition, the formation of an ash layer on the grate requires persistent ash discharge that introduces instabilities in the gasifier operating conditions. The rise in the equivalent ratio led to higher gas and carbon yields and cold gas efficiencies. At the optimal equivalent ratio of 0.25, compared with wood pellets gasification, the addition of sewage sludge with 20wt.% moisture up to 70w/w% gave rise to decreasing the dry gas yield from 1.56 to 1.07Nm³/kg of feedstock due to the higher moisture and ash content of sewage sludge and to moderate reductions of the gas lower heating value and cold gas efficiency from 5.8to4.9MJ/Nmdᵣy ³ and from 60% to 55%, respectively.