Hydrogen rich fuel gas production from the pyrolysis of wet sewage sludge at high temperature

The present work explores an alternative method to the one generally employed for the pyrolysis of sewage sludge. This method consists in carrying out the pyrolysis of a wet sewage sludge as it is produced in the water treatment plant, i.e. with a typical moisture content of between 70 and 80 wt%, without subjecting it to any previous thermal drying process. In order to maximize gas production, the treatment is carried out at elevated temperatures (i.e. 1000 °C) using long gas residence times and high heating rates. Under these conditions the natural moisture of the sludge is converted during the process into steam, which gives rise to the partial gasification of the sludge and the reforming of the organic vapours at an early stage. In addition, homogeneous reactions between non-condensable gases are also favoured, especially in the water gas shift reaction. In order to establish the most favourable conditions for the production of a hydrogen rich fuel gas, the influence of the moisture content, heating rate and soaking time on the process was studied. It was found that subjecting the vapours to long residence times at high temperature combined with high heating rates and, above all, the presence of water in the raw sewage sludge, give rise to a process that combines both pyrolysis and gasification. As a result of this process, not only is a higher gaseous fraction produced but also gases of a much higher hydrogen content than in the pyrolysis of dry sludge.