Gasification of grapevine pruning waste in an entrained-flow reactor: gas products, energy efficiency and gas conditioning alternatives

Abstract

Owing to its higher efficiency and versatility, gasification is seen as a necessary evolution in the development of biomass energy systems. This technology has been primarily tested in fixed bed (updraft and downdraft) and fluidised bed reaction systems, with less information available about the potential of entrained-flow reactors. This latter design benefits from a relatively simple mechanical structure, robustness against severe gasification conditions and also reduced investment and operating costs. This paper describes the development of a pilot scale entrained-flow reactor and evaluates its performance in the gasification of wood waste left over from the pruning of grapevine (Vitis vinifera). The original biomass was initially analysed for its chemical composition and thermal behaviour. A series of gasification trials were conducted to evaluate the effect of temperature and relative biomass/air ratio (Frg) on the yield, composition, heating value of the resulting syngas. The cold gas efficiency of the system was determined for different operating conditions from the heating value and yields of the resulting producer gas. The results showed that the use of higher temperatures caused a small increase in overall gas yields (from 1.76 Nm3 kg-1 at 750ºC to 1.96 Nm3 kg-1 at 1050ºC) and a notable rise in its heating value (from 3.65 MJ kg-1 at 750ºC to 4.95 MJ kg-1 at 1050ºC), primarily derived from an increase in the concentration of hydrogen. The experimental results show a reduction in the fuel properties of the producer gas when using biomass/air ratios (Frg) below 2.5, which was attributed to the partial combustion of the producer gas. However, this effect was largely counteracted by the production of higher gas yields (3.39 Nm3 kg-1 for Frg = 2.16 compared to 1.96 Nm3 kg-1 for Frg = 4.05), owing to the higher conversion of the fuel at low biomass/air ratios. Optimum gasification conditions (cold gas efficiency up to 83.06 %) were reached when using high reaction temperatures (1050ºC) and low Frg (2.19). This paper also provides a final review about the formation of unwanted tars and particulates in gasification processes, its effect in energy applications, and the use of alternative technologies (thermocatalytic cracking, reforming, water-gas shift) for the conditioning and upgrading of the resulting gas stream.