Characterisation of tars from biomass gasification: Effect of the operating conditions

Tars formation remains the main technical hurdle in the development of biomass gasification at commercial scale, due to its associated operating problems (condensation, catalyst deactivation, polymerisation). This work aims to study the effect of the operating conditions (biomass/air ratio, temperature and gasifying agent) on the production and the technical and environmental hazard (tar dew point and carcinogenic potential) of tars produced in a small-scale drop-tube gasifier, dealcoholised marc of grape being used as biomass fuel. An analytical HPLC method has been used for the detection of polycyclic aromatic hydrocarbons (PAHs), benzene, toluene, ethyl-benzene and xylenes (BTEX), phenol, and pyridine. Results show that an increase in the relative biomass/air ratio, a decrease in temperature, and higher steam content lead to a higher tar production. BTEX have been found as the main constituents of tars (60–70% wt.), whereas among PAHs, the most significant are the lighter ones (naphthalene, acenaphtylene, acenaphtene). Phenol production is favoured at lower temperatures or/and higher steam content (around 50% wt. of the tar mixture at 750 °C and steam gasification). A progressive aromatisation of tars has been observed when increasing the temperature, the effect of steam addition on tar composition being not significant at temperatures higher than ∼1000 °C.

► HPLC has been used to analyse tar composition from biomass gasification. ► Tar dew point and carcinogenic potential of tars have been estimated for different operating conditions. ► BTEX are the main constituents of tar for all operating conditions. Among PAHs, lighter ones are the most abundant. ► Frg ∼ 3–4 and steam contents ≤ 50% wt. lead to a trade-off between gasification performance and tar properties. ► A progressive aromatisation of tar occurs when increasing operating temperature.