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Torrefaction is suggested to be an effective method to reduce the cost of biomass provision and improve the fuel properties. In this study, both raw and torrefied Miscanthus × giganteus (M×G) were gasified in an externally heated air-blown bubbling fluidized bed (BFB) gasifier using olivine as the bed material. The effects of equivalence ratio (ER) (0.18–0.32) and bed temperature (660–850 °C) on the gasification performance were investigated. Torrefied M×G has higher energy density primarily due to a higher ratio of lignin to cellulose and hemicellulose; it has lower bulk density, smaller particle diameter and lower reactivity than the original biomass. These properties affect its performance during gasification. The cold gas efficiency was on average 12% lower for torrefied than for raw M×G for the range of operating conditions studied. Within the same temperature range the carbon conversion was about 10% higher for raw than for torrefied biomass. The hydrogen conversion was higher for torrefied M×G since gasification of this feedstock results in higher yields of methane and ethane and lower yields of unreacted process water. The carbon loss with char elutriated from the gasifier for torrefied M×G was significantly higher than that of raw (5% vs 3%) and was driven by physical properties of torrefied M×G. The results obtained suggest that chemical composition expressed as lignin to cellulose and hemicellulose ratio has a pronounced effect on carbon conversion efficiency and tar production.

Gasification of Cynara cardunculus L. was performed in a bubbling fluidized bed (BFB) using air as the gasifying agent and magnesite and olivine as different bed materials. Temperature was varied during the experiments (700-800 ºC) with fixed biomass feeding and air flow rates.The effect of using the magnesite and olivine on the gas and tar composition, carbon and biomass conversion, and cold gas efficiency was investigated. The product gas showed high hydrogen content (13-16 %v/v) for both magnesite and olivine in the temperature range studied.Higher heating value and gas yield were improved with increasing the temperature from 700 to 800 ºC. Biomass and carbon conversion were greater than 75%, giving values higher than 90 % for both 700 and 800 ºC in magnesite and for 800 ºC in olivine. Indane and cresols were the main tar compounds at low temperature while naphthalene was the dominant tar species at the high temperatures. Gasification performance was better with magnesite at 700 ºC while olivine showed better properties at 800 ºC.

Gasification of Miscanthus x giganteus (MxG) was conducted in an air-blown bubbling fluidized bed (BFB) gasifier using magnesite as bed material and a moderate rate of biomass throughput (246.82–155.77 kg/m2h). The effect of equivalence ratio (ER) (0.234–0.372) and bed temperature (645–726 °C) on the performance of gasification was investigated. The results reveal that MxG is a promising candidate for energy production via BFB gasification; of the conditions tested, the optimal ER and temperature are approximately 0.262 and 645 °C, where no sign of agglomeration was found. The product gas from this condition has a higher heating value of 6.27 MJ/m3, a gas yield of 1.65 N m3/kgbiomass (39.5% of CO and 18.25% of H2 on N2 free basis), a carbon conversion efficiency of 94.81% and a hot gasification efficiency of 78.76%. Agglomeration was observed at some higher temperature conditions and believed to be initiated by the formation of fuel-ash derived low melting temperature K-rich (potassium) silicates (amorphous material that cannot be detected by XRD). It is suggested that relatively low temperature (650 °C) needs to be used for the gasification of MxG to avoid potential agglomeration.

Torrefaction is suggested to be an effective method to improve the fuel properties of biomass and gasification of torrefied biomass should provide a higher quality product gas than that from unprocessed biomass. In this study, both raw and torrefied Miscanthus × giganteus (M×G) were gasified in an air-blown bubbling fluidized bed (BFB) gasifier using olivine as the bed material. The effects of equivalence ratio (ER) (0.18-0.32) and bed temperature (660-850°C) on the gasification performance were investigated. The results obtained suggest the optimum gasification conditions for the torrefied M × G are ER 0.21 and 800°C. The product gas from these process conditions had a higher heating value (HHV) of 6.70 MJ/m(3), gas yield 2m(3)/kg biomass (H2 8.6%, CO 16.4% and CH4 4.4%) and cold gas efficiency 62.7%. The comparison between raw and torrefied M × G indicates that the torrefied M × G is more suitable BFB gasification.

Air and air-steam gasification of poultry litter was experimentally studied in a laboratory scale bubbling fluidized bed gasifier at atmospheric pressure using silica sand as the bed material. The effects of equivalence ratio (ER), gasifier temperature, steam-to-biomass ratio (SBR), and addition of limestone blended with the poultry litter, on product gas species yields and process efficiency, are discussed. The optimum conditions (maximum carbon conversion, gas yield, heating value, and cold gas efficiency) were achieved at an ER 0.25 and 800°C, using air (SBR = 0) and poultry litter blended with 8% w/w limestone, yielding a product gas with a lower heating value (LHV) of 4.52 MJ/Nm 3 and an average product gas composition (dry basis) of H 2 : 10.78%, CO: 9.38%, CH 4 : 2.61, and CO 2 : 13.13. Under these optimum processing conditions, the cold gas efficiency, carbon conversion efficiency, and hydrogen conversion efficiency were 89, 73, and 43% respectively. The reported NH 3 measurement at an ER of 0.28 and 750°C is 2.7% (equivalent to 19,300 mg/Nm 3) with 14.7 mg/Nm 3 of HCl observed as the dry product gas. High temperature and steam injection favor production of CO and H 2 , while their effect on CH 4 was almost negligible. It is demonstrated that poultry litter can be gasified by blending with limestone, making it possible to overcome the fluidization problems caused by the mineral composition of poultry litter ash (high K and P content), yielding a gas with a similar heating value compared to gasifying without limestone addition, but with a significantly lower tar content.

Thermochemical gasification offers an attractive solution for the conversion of low-grade biomass and waste. However, practical experiences of the gasification processes reveal that the formation of tar is troublesome to continuous operation. Therefore, tar measurement protocols and tar reduction systems are priorities in the development of effective biomass gasification. Results of tar measurements often raise questions regarding their reliability and accuracy, because of calibration, sampling, and discrimination issues. The present work evaluates the solid phase adsorption (SPA)−gas chromatography (GC) measurement system for tar in product gas by comparing the mass spectroscopy detector (MSD) and flame ionization detector (FID) and their associated measurement uncertainty. The measurand is defined as the total GC detectable tar in a normal cubic meter of dry product gas when employing the common quantitation method, "quantitation as naphthalene". The GC-FID measurements were significantly higher than the GC-MSD measurements. Their overall uncertainties also vary by a significant margin. The measurement uncertainty analysis shows that this difference is taken into account by the uncertainty induced by the particulars of the GC-MSD and GC-FID measurement systems, where the relative expanded uncertainty is shown to be 109.4% and 35.0%, respectively. While a quantitative method based on a single calibration curve offers significant advantages, in terms of speed and simple quantitation of total GC detectable tar, such an approach introduces greater uncertainty within the reported results.

The current study investigates the effect of temperature, equivalence ratio, and biomass composition on tar yields and composition. Torrefied and raw Miscanthus x giganteus (M×G) were used as biomass feedstocks in an atmospheric bubbling fluidized bed gasifier for experiments undertaken between 660 and 850°C and equivalence ratios from 0.18 to 0.32. Tar was sampled according to the solid phase adsorption method and analyzed by gas chromatography. There is an indication that torrefied M×G produces higher amounts of total GC-detectable tar as well as higher yields of 20 individually quantified tar compounds compared with those of raw M×G. Under similar gasification conditions (800°C and an equivalence ratio of 0.21), the total GC-detectable tar for torrefied M×G is approximately 42% higher than that for raw M×G. Higher tar yields are observed to be related to higher lignin and lower moisture content of torrefied M×G. The effect of temperature on tar yields is in good agreement with the literature. The highest yield of total GC-detectable tar, secondary tars, and tertiary-alkyl tars is observed between 750 and 800°C, followed by a decrease at higher temperature, whereas tertiary-polycyclic aromatics increase with the temperature over the range tested. The effect of equivalence ratio on total GC-detectable tar is not clear because data points vary significantly (up to 47%) over the range of equivalence ratios tested. Temperature is the main driver for tar production and its chemical composition; however, this study indicates that tar yields depend significantly on biomass composition.