Biomass gasification at temperatures below 1300 °C yields producer gas with a range of heavy hydrocarbons. These compounds, collectively known as tar, cause fouling and emission problems in equipment using the producer gas. This paper gives an overview of the work performed at the Energy research Centre of The Netherlands (ECN) on tar measurement, tar prevention, tar cracking, and tar removal. Much of the work has been performed in cooperation with partner institutes and industry. Measurement techniques discussed are the tar guideline, solid-phase adsorption (SPA) method, and tar dew point analyzer. On the subject of tar prevention, the effects of operating conditions, fuel composition, and bed materials in fluidized-bed gasifiers are covered. Tar cracking results are presented for catalytic materials, hightemperature treatment, and the use of plasma. ECN research on tar removal involves among others the development of the water-based GASREIP system and the oil-based OLGA technique.

No abstract

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.

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.

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.

No abstract

We report the performance of a standard commercial micro gas turbine on biomass producer gas and mixtures of biomass producer gas with natural gas. The micro gas turbine delivers full power on gas mixtures with a net heating value of at least 15 MJ/Nm 3. For gas of lower heating value, the maximum fuel gas flow limits the attainable power. At reduced power, the lower limit for stable operation is a net heating value of about 8 MJ/Nm 3. The gross efficiency of the micro gas turbine depends on output power but not on the gas heating value, within our measurement accuracy. Above 70% of full power, emissions of CO, unburned hydrocarbons and NO are over 200 times, 15 times and 3 times lower than those of a gas engine of similar size. At part load below 70% of full power, the micro gas turbine burner switches to a different operating mode which produces higher CO and NO levels. The CO emission remains far below that of a gas engine, the NO emission becomes similar.