Characterization and pilot scale fluidized bed gasification of herbaceous biomass: A case study on alfalfa pellets

• Alfalfa pellets were characterized and introduced to fluidized bed gasification.
• ER varied between 0.25 and 0.30 by varying air flow, but solid flow kept constant.
• With the rise in ER, CO was the main contributor affecting combustible gas amount.
• Gas LHV, yield, efficiency and bed temperature all improved with increased ER.

Pilot-scale bubbling fluidized bed gasification tests of alfalfa pellets were performed at two different operational periods ranged on the basis of the two given equivalence ratio (ER) (the ratio between actual air and the stoichiometric air) namely as 0.25 and 0.30. During the test, the solid feeding rate was kept constant at 4.7 kg/h while the air input was varied and thus the ER. Increasing air from 4.16 to 4.99 N m3/h contributed to the evolution pattern of several parameters such as the rise in gas lower heating value (LHV) and gas yield, the average maximum of which were 4.2 MJ/N m3 and 1.5 N m3/kg respectively. Gas composition was mainly boosted by the concentration of CO, as the rest of the combustible components stayed rather unaffected due to the modified air flow rate. The steady state bed temperature agreed with the trend of air flow and ranged between 720 and 780 °C despite gasification start-up occurred at 800 °C. Total char (∼282 g/kg) and tar yield (∼1.1 g/N m3) showed reasonable values while tar composition was predominantly led by the amount of phenols.